Reducing the water absorption of thermoplastic starch. processed by extrusion. Philip Oakley. A thesis submitted in conformity with the requirements

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1 Reducing the water absorption of thermoplastic starch processed by extrusion by Philip Oakley A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Chemical Engineering and Applied Chemistry University of Toronto Copyright by Philip Oakley 2010

2 Reducing the water absorption of thermoplastic starch processed by extrusion Master of Applied Science 2010 Philip Oakley Graduate Department of Chemical Engineering and Applied Chemistry University of Toronto Abstract Novel plastics that are biodegradable, environmentally benign, and made from renewable natural resources are currently being researched as alternatives to traditional petroleum-based plastics. One such plastic, thermoplastic starch (TPS) is produced from starch processed at high temperatures in the presence of plasticizers, such as water and glycerol. However, because of its hydrophilic nature, TPS exhibits poor mechanical properties when exposed to environmental conditions, such as rain or humidity. The overall objective of this thesis was to produce a thermoplastic starch based material with low water absorption that may be used to replace petroleum-based plastics. Three different methods for reducing water absorption were investigated, including the following: extrusion of starch with hydrophobic polymers, starch modifying chemicals, and citric acid/sorbitol as plasticizers. It was found that all methods reduced the water absorption of TPS. TPS blended with polyethylene and sorbitol/glycerol plasticized starch samples exhibited the lowest water absorption of all samples tested. ii

3 Acknowledgements Firstly, I would like to acknowledge the support and guidance provided by my supervisor, Dr. Mohini Sain. I d also like to acknowledge the members of Dr. Sain s research group for the training and help they provided throughout the completing of this thesis. Thank you to Casco Inc. for generously supplying the cornstarch used for the experiments in this thesis. Finally, I thank my parents for their continual support of my education. iii

4 TABLE OF CONTENTS Abstract...ii Acknowledgements... iii TABLE OF CONTENTS...iv LIST OF TABLES... vii LIST OF FIGURES... x Chapter 1 : Introduction and Literature Review Introduction Literature Review Starch Thermoplastic starch Water absorption Reducing TPS water absorption Problem Statement Objective Specific Objectives Research Approach References Chapter 2 : Extrusion of starch with paper sizing agents Introduction Experimental Materials Plasticization SEM Water Absorption Results and Discussion SEM Water Absorption Conclusions References Chapter 3 : Extrusion of starch with maleated polyethylene, green polyethylene, and green polyethylene compatibilized with maleic anhydride iv

5 3.1 Introduction Experimental Materials Plasticization FTIR SEM TGA Water Absorption Results and Discussion FTIR SEM TGA Water Absorption Conclusions References Chapter 4 : Extrusion of starch with beeswax, paraffin wax, and paraffin wax compatibilized with maleic anhydride Introduction Experimental Materials Plasticization FTIR SEM TGA Water Absorption Results and Discussion Plasticization FTIR SEM TGA Water Absorption Conclusions References v

6 Chapter 5 : Extrusion of citric acid/glycerol and sorbitol/glycerol co-plasticized starch Introduction Experimental Materials Plasticization SEM Water Absorption Mechanical Testing Results and Discussion Plasticization SEM Water Absorption Mechanical Testing Conclusions References Chapter 6 : Conclusions and Recommendations References Appendix A : Chapter 2 Data and Statistics A.1 Water Absorption Data A.2 Statistical Analysis Appendix B : Chapter 3 Data and Statistics B.1 Water Absorption Data B.2 Statistical Analysis Appendix C : Chapter 4 Data and Statistics C.1 Water Absorption Data C.2 Statistical Analysis Appendix D : Chapter 5 Data and Statistics D.1 Water Absorption Data D.2 Mechanical Testing Data D.3 Statistical Analysis vi

7 LIST OF TABLES Table 1.1: Water uptake at equilibrium in plasticized maize starch Table 1.2: Technical substitution potential of bioplastics Table 1.3: Commercial starch plastic producers Table 2.1: Used symbols and corresponding sample compositions Table 2.2: Temperature profile used for extrusion Table 3.1: Used symbols and corresponding sample compositions Table 3.2: Temperature profile used for extrusion Table 3.3: Data from derivative TGA curves Table 4.1: Used symbols and corresponding sample compositions Table 4.2: Data from derivative TGA curves Table 5.1: Used symbols and corresponding sample compositions Table 5.2: Mechanical properties of TPS blends and pure polymers with literature values for comparison Table A.1: Water absorption weight data for TPS sample Table A.2: Calculated water absorption values for TPS sample Table A.3: Water absorption weight data for AKD sample Table A.4: Calculated water absorption values for AKD sample Table A.5: Water absorption weight data for BSO sample Table A.6: Calculated water absorption values for BSO sample Table A.7: Average water absorption values with confidence limits Table B.1: Water absorption weight data for TPS sample Table B.2: Calculated water absorption values for TPS sample Table B.3: Water absorption weight data for 5GPE sample Table B.4: Calculated water absorption values for 5GPE sample Table B.5: Water absorption weight data for 10GPE sample Table B.6: Calculated water absorption values for 10GPE sample Table B.7: Water absorption weight data for 20GPE sample Table B.8: Calculated water absorption values for 20GPE sample Table B.9: Water absorption weight data for 5MGPE sample Table B.10: Calculated water absorption values for 5MGPE sample Table B.11: Water absorption weight data for 10MGPE sample Table B.12: Calculated water absorption values for 10MGPE sample vii

8 Table B.13: Water absorption weight data for 20MGPE sample Table B.14: Calculated water absorption values for 20MGPE sample Table B.15: Water absorption weight data for 5MPE sample Table B.16: Calculated water absorption values for 5MPE sample Table B.17: Water absorption weight data for 10MPE sample Table B.18: Calculated water absorption values for 10MPE sample Table B.19: Water absorption weight data for 20MPE sample Table B.20: Calculated water absorption values for 20MPE sample Table B.21: Average water absorption values with confidence limits Table C.1: Water absorption weight data for 5BW sample Table C.2: Calculated water absorption values for 5BW sample Table C.3: Water absorption weight data for 10BW sample Table C.4: Calculated water absorption values for 10BW sample Table C.5: Water absorption weight data for 5PW sample Table C.6: Calculated water absorption values for 5PW sample Table C.7: Water absorption weight data for 10PW sample Table C.8: Calculated water absorption values for 10PW sample Table C.9: Water absorption weight data for 5MPW sample Table C.10: Calculated water absorption values for 5MPW sample Table C.11: Water absorption weight data for 10MPW sample Table C.12: Calculated water absorption values for 10MPW sample Table C.13: Average water absorption values with confidence limits Table D.1: Water absorption weight data for 20SOR sample Table D.2: Calculated water absorption values for 20SOR sample Table D.3: Water absorption weight data for SORBLEND sample Table D.4: Calculated water absorption values for SORBLEND sample Table D.5: Mechanical testing data for TPS sample Table D.6: Mechanical testing data for MPE sample Table D.7: Mechanical testing data for 20SOR sample Table D.8: Mechanical testing data for SORBLEND sample Table D.9: Average water absorption values with confidence limits Table D.10: Average max stress values with confidence limits Table D.11: Average modulus values with confidence limits viii

9 Table D.12: Average elongation values with confidence limits ix

10 LIST OF FIGURES Figure 1.1: The Carbon cycle of biodegradable polymers Figure 1.2: Structure of the amylose polymer Figure 1.3: Structure of the amylopectin polymer Figure 1.4: Helical structure of α-amylose molecule Figure 1.5: SEM image of potato starch granules Figure 1.6: SEM Images (2500x magnification) of tapioca starch at the temperature ( o C) below each image Figure 1.7: T g of TPS as a function of plasticizer content and type Figure 1.8: Strain at break versus water content, W, for various plasticized starches Figure 1.9: Twin-screw extruder used for compounding starch Figure 1.10: Strength (a), strain at break (b), and elastic modulus (c), versus water absorbed for TPS Figure 1.11: Starch modification reactions Figure 1.12: Chemical structure of (a) glycerol, (b) xylitol, (c) sorbitol, (d) maltitol Figure 2.1: Chemical reaction of ASAs with starch Figure 2.2: Chemical reaction of AKD with starch Figure 2.3: Chemical structure of styrene/butyl acrylate copolymer Figure 2.4: SEM images of extruded samples at 200X magnification: (a) ASA (b) AKD (c) TPS (d) BSO Figure 2.5: Water absorption profile for AKD, BSO, and TPS samples Figure 2.6: Water absorption at equilibrium for the TPS and BSO samples with 95% confidence intervals Figure 2.7: Water absorption at 10 days for the TPS and AKD samples with 95% confidence intervals Figure 3.1: Reaction of MAH with PE initiated by DCP or BPO Figure 3.2: Reaction of maleated PE with starch Figure 3.3: FTIR spectrum for GPE and MGPE Figure 3.4: SEM image of TPS sample Figure 3.5: SEM images of extruded samples: (a) 5GPE, (b) 10GPE, (c) 20GPE Figure 3.6: SEM images of extruded samples: (a) 5MGPE, (b) 10MGPE, (c) 20MGPE 37 Figure 3.7: SEM images of MPE samples: (a) 5MPE (b) 10MPE (c) 20MPE Figure 3.8: TGA curves for pure GPE, TPS, and blends of TPS/GPE Figure 3.9: TGA curves for pure MGPE, TPS, and blends of TPS/MGPE x

11 Figure 3.10: TGA curves for pure MPE, TPS, and blends of TPS/MPE Figure 3.11: Derivative TGA curves for pure GPE, TPS, and blends of TPS/GPE Figure 3.12: Derivative TGA curves for pure MGPE, TPS, and blends of TPS/MGPE.. 41 Figure 3.13: Derivative TGA curves for pure MPE, TPS, and blends of TPS/MPE Figure 3.14: Water absorption at equilibrium for MGPE samples with 95% confidence intervals Figure 3.15: Water absorption at equilibrium for GPE samples with 95% confidence intervals Figure 3.16: Water absorption at equilibrium for MPE samples with 95% confidence intervals Figure 4.1: Reaction scheme for grafting maleic anhydride onto paraffin wax Figure 4.2: FTIR spectrum for PW and MPW Figure 4.3: SEM images of extruded samples: (a) 5BW (b) 10BW Figure 4.4: SEM images of extruded samples: (a) 5PW (b) 10PW (c) 5MPW (d) 10MPW Figure 4.5: TGA curves for pure BW, TPS, and blends of TPS/BW Figure 4.6: TGA curves for pure PW, TPS, and blends of TPS/PW Figure 4.7: TGA curves for pure MPW, TPS, and blends of TPS/MPW Figure 4.8: Derivative TGA curves for pure BW, TPS, and blends of TPS/BW Figure 4.9: Derivative TGA curves for pure PW, TPS, and blends of TPS/PW Figure 4.10: Derivative TGA curves for pure MPW, TPS, and blends of TPS/MPW Figure 4.11: Water absorption at equilibrium for BW samples with 95% confidence intervals Figure 4.12: Water absorption at equilibrium for PW samples with 95% confidence intervals Figure 4.13: Water absorption at equilibrium for MPW samples with 95% confidence intervals Figure 5.1: Reaction scheme of citric acid with starch Figure 5.2: SEM images of extruded samples: (a) 20CA (b) 30CA (c) 45CA Figure 5.3: SEM images of extruded samples: (a) 20SOR (b) 30SOR (c) 45SOR (d) SORBLEND Figure 5.4: Water absorption at equilibrium for sorbitol plasticized samples with 95% confidence intervals Figure 5.5: Stress-strain curves for extruded samples xi

12 Chapter 1 : Introduction and Literature Review 1.1 Introduction Over the past century the petroleum-based polymer industry has grown rapidly by creating materials that are cheap, easy to transform, hydrophobic, and biologically inert. However, several decades of using and disposing of non-biodegradable plastic has caused an accumulation of plastic waste in landfills, polluted maritime environments, and contributed to the depletion of our limited reserves of fossil fuels 1. One alternative to disposing these plastics in landfills is burning them for energy production. However, this merely moves pollution from ground level into the atmosphere in the form of carbon dioxide and other gases. Recycling also has the problem of being energy intensive and requiring selective sorting out and cleaning of waste plastic 2. A better alternative is to minimize the quantities of non-degradable plastics used by substituting them with biodegradable plastics. Biodegradable plastics are often made from renewable natural polymers, such as starch, proteins, and cellulose. In contrast to petroleum-based plastics, these so called bioplastics do not drain our limited supply of fossil-based resources and have a small carbon footprint. However, the most attractive feature of bioplastics is their total biodegradability. As a result, they fit in perfectly well with our ecosystems and have a closed loop carbon cycle where no waste is generated 3. This principle is demonstrated below in Figure 1.1 where plant material is processed into plastic which later degrades and may form new plant material. Starch has attracted attention as a suitable material for the production of biodegradable plastics due to its natural abundance and low cost 4,5. Starch is a renewable biomaterial; therefore, it may be used to produce plastics without depleting fossil fuel resources when sustainable (or carbon neutral) farming techniques are used. Plasticized starch or 1

13 thermoplastic starch (TPS) is prepared under specific extrusion conditions and in the presence of plasticizers, such as glycerol and water 6,7,8,9. However, four problems hinder TPS from becoming a commonly used plastic, including the following 10 : 1. hydrophilic nature of TPS and poor water resistance 2. deterioration of mechanical properties upon exposure to environmental conditions like humidity 3. brittleness in the absence of suitable plasticizers 4. soft and weak nature in the presence of some plasticizers Figure 1.1: The Carbon cycle of biodegradable polymers Literature Review Starch Starch is a polymer which occurs widely in plants 11. It is produced during photosynthesis and functions as the principal polysaccharide reserve material 12. The principle crops used for production of starch include potatoes, corn, and rice 11. In all of these plants, starch is deposited in the form of complex structures called granules, with varying shapes and sizes depending on the botanical origin. 2

14 Chemistry The two main constituents of starch granules are the polysaccharides amylopectin and amylose. Both of these polymers are composed of repeating units of α-d-glucose. The major component of most starches is amylopectin and it constitutes about 70% of the polysaccharide content 13. The structure of amylose and amylopectin are shown below in Figure 1.2 and Figure 1.3, respectively. Figure 1.2: Structure of the amylose polymer 14. Figure 1.3: Structure of the amylopectin polymer 14. Amylose is essentially a linear molecule consisting of α-1,4-linkages between glucose monomers. The polymer strands of α-amylose in starch adopt a helical structure (shown below in Figure 1.4) similar to that found in nucleic acids. Typical chain lengths for α- amylose units are approximately 1000 monomer units 11. 3

15 Figure 1.4: Helical structure of α-amylose molecule 15. The second polymer in starch, amylopectin, is an extensively branched macromolecule. Like amylose, the glucose units are connected through α-1,4-linkages. The difference between amylose and amylopectin is that at irregular intervals there are branch points where a secondary polysaccharide chain is connected to the main chain by α-1,6- linkages. Each starch molecule has two important functional groups. The OH group is susceptible to substitution reactions and has a high affinity for water, causing much of the problem of water absorption in TPS. Also, the C-O-C bond is susceptible to chain breakage 11. A number of chemical reactions with these two groups have been studied in the literature. These reactions have a number of purposes, such as increasing the hydrophobicity of starch Physical structure Amylose and amylopectin in starch are organized into granules as alternating semicrystalline and amorphous layers. As revealed by SEM studies 17, starch granules are smooth with a spherical or ellipsoid shape. The granule surface is smooth with only 4

16 minor ripples and bumps at the nanometer scale. SEM images of potato starch granules are shown below in Figure 1.5. Figure 1.5: SEM image of potato starch granules 17. Starch granules consist of both semi-crystalline and amorphous phases. The semicrystalline regions are composed of double helices formed by short amylopectin branches and the amorphous regions are composed of amylose and non-ordered amylopectin branches Thermoplastic starch Thermoplastic starch (TPS) is the material produced when starch is heated in the presence of plasticizers. It is a plastic material with poor elongation and high tensile strength properties Plasticization of starch Starch plasticization is a three stage process during which the following events take place 20 : 1. Plasticizers are absorbed by starch granules and form hydrogen bonds with amylose and non-ordered amylopectin. This facilitates increased amylose and amylopectin mobility in the amorphous regions. Amylose and amylopectin rearrange, forming new intermolecular interactions. 3. Amylose and amylopectin become more mobile and lose their intermolecular interactions and granular structure when heat and shearing forces are applied. Energy absorbed by the granules melts their crystallite structures and facilitates the formation of new bonds among starch and plasticizers. SEM images of this process are shown 5

17 below in Figure 1.6. Starch restructured in this way acquires flowing properties close to thermoplastics such as polyethylene and polypropylene. Figure 1.6: SEM Images (2500x magnification) of tapioca starch at the temperature ( o C) below each image A thermoplastic material is formed as starch is cooled back below the melting temperature of the starch crystallites. Starch chains re-associate and a new crystalline order is created, different from the original granule structures. The re-organizing of amylose molecules is rapid due to the greater mobility of amylose molecules compared to amylopectin molecules. On the other hand, the restructuring of amylopectin molecules proceeds slowly over several days. The final material contains amorphous regions as well as crystalline regions Plasticizers Plasticizers are required to form hydrogen bonds with amylose and amylopectin chains during the plasticization of starch. This increases amylose and amylopectin chain mobility which in turn lowers the melting temperature of starch. Therefore, plasticizers are used in order to process starch into a thermoplastic material at a temperature below its degradation temperature, but above its melting temperature 21. Glycerol is the most 6

18 common plasticizer used for starch, but many others have been studied. Other common plasticizers include the following: xylitol 22, sorbitol 22, maltitol 22, urea 23, formamide 23, and sugars 24. Figure 1.7: T g of TPS as a function of plasticizer content and type 19. The amount and type of plasticizer used influence the properties of TPS, such as its water absorption (discussed later), glass transition temperature (T g ), and modulus 25. Shown above in Figure 1.7 is the relationship between T g and plasticizer amount and type. In general, an increase in the concentration of plasticizer results in a lower T g because more plasticizer groups are available to bond with starch, thus increasing starch chain mobility. However, the intensity of this decrease is dependent on the nature of the plasticizer used 19. A general rule is that the glass transition temperature decreases as molecular weight decreases because smaller molecules are better able to penetrate the starch granule structure, promoting amylose and amylopectin chain mobility 22. 7

19 Role of water during plasticization Plasticization of starch is aided by water in addition to other plasticizers. In a study by Hulleman et al. 26, it was found that as the amount of water added to a starch/glycerol mixture increased, polysaccharides increasingly migrated out of the starch granules during plasticization. Due to this larger fraction of non-granular and interacting polysaccharides, the material became more deformable without losing coherence. This affected the strain at break, as shown below in Figure 1.8. Figure 1.8: Strain at break versus water content, W, for various plasticized starches 26. However, above specific water contents the strain at break started to decrease because the interaction or entanglement of polysaccharides is limited at relatively high water contents. The formation of an even more coherent polysaccharide system is hindered, possibly due to the formation of inter and intramolecular contacts within granules, instead of the formation of a non-granular polysaccharide network Plasticization by extrusion Starch may be plasticized using small scale laboratory methods such as solution casting; however, plasticization of starch by extrusion is a more realistic approach to the industrial preparation of TPS. Extrusion is the method of choice for producing TPS in large quantities; therefore, it was the method used in this thesis. Shown below in Figure 1.9 is a diagram of a twin-screw extruder used for processing starch into TPS. 8

20 Figure 1.9: Twin-screw extruder used for compounding starch 27. Starch, plasticizers, and other additives are placed in the hopper from where they are fed into the barrel of the extruder. The heat and shear required to plasticize starch granules are supplied by external heaters and by the compression and shearing action of the screws, respectively. As starch is pushed through the barrel of the extruder it restructures into a free-flowing material until it emerges from the die and is cooled back below the melting temperature. The material that emerges from the die is TPS and will ideally contain no intact or unplasticized starch granules Water absorption Theory Water absorption in TPS is a diffusion process driven by a concentration gradient and complicated by swelling of the material. Water molecules in the air present as humidity or rain will diffuse into a hydrophilic polymer such as TPS to produce a swollen material. Dissolution of the material is prevented if the bonding between neighbouring polymer molecules is strong as a result of crosslinking or hydrogen bonding. Swelling of the material continues until the forces due to swelling of the polymer balance the osmotic pressure. The polymer swelling required to accommodate water absorbed from the surface is initially constricted by the internal glassy material. This causes compressive forces to build up in the plane of the sample surface, and initial swelling occurs predominantly perpendicular to the surface resulting in a thickness increase without a corresponding increase in the longitudinal dimension 28. 9

21 An experiment by Russo, et al. 29 was conducted in order to determine which theoretical model of diffusion applies to the case of water diffusion into TPS. They found that a Fickian diffusion model accounting for polymer swelling described their data most accurately. Therefore, Fick s law of diffusion, shown below in equation (1.1), may be used to model water absorption by TPS. (1.1) Where C is water concentration, t is time, D is the diffusion coefficient, and x is the path length. In the study by Russo, et al. 29, the best fit to the experimental data could only be made by applying an exponential dependence on water concentration. Therefore, the diffusion coefficient is modeled by equation (1.2). (1.2) Where C is the concentration at a point, C 0 is the concentration at the surface, and D 0 and A are constants. The swelling during water absorption was taken into account by modifying the diffusion coefficient and rescaling the linear distances. The new equation for the diffusion coefficient is equation (1.3). (1.3) Where S is the degree of swelling calculated as the maximum thickness of the sample at full swelling divided by the initial thickness of the polymer. By combining equation (1.3) with equation (1.1), a model for water absorption in TPS was produced, shown in equation (1.4). [ ] (1.4) 10

22 Water absorbed by TPS over time is a function of all terms on the right side of equation (1.4). The parameters which are a function of TPS material properties and thus may be engineered are D 0, S, and x. However, S, the amount of swelling must be close to zero for all applications; therefore, it was not considered an engineer-able variable parameter. Also, the method by which the path length, x, is varied involves adding clay or nanoparticles that have a detrimental effect on the strain at break of TPS 30. Therefore, this method was not investigated. Lowering the diffusion constant, D 0, by modifying the material properties of TPS was examined in this thesis Water absorption effects on mechanical properties Water absorption in TPS promotes two separate processes an increase in plasticization, and an increase in crystallinity known as retrogradation. Absorbed water behaves as a plasticizer just as it does during the granule melting process and leads to a decrease in T g and hence an increase in the strain at break (Figure 1.10b). However, the increase in crystallinity accompanied by this causes a decrease in strength (Figure 1.10a) and modulus (Figure 1.10c) 30 because the crystalline regions are less efficient than the amorphous regions at transferring stresses between polymer chains 22. Figure 1.10: Strength (a), strain at break (b), and elastic modulus (c), versus water absorbed for TPS

23 These immediate detrimental effects on the mechanical properties of TPS are a hindrance to its commercialization as a commodity plastic. For TPS to be widely used in many applications it must be water resistant and able to maintain its mechanical properties over a long period of time Reducing TPS water absorption Chemical modification of starch Starch may be modified via many different chemical reactions, as shown below in Figure Modified starches are the products of glucosidic bond cleavage (acid modification to dextrins), forming new functional groups (carbonyl group formation during oxidation), substitution of free available hydroxyl groups (by etherification or esterification), and bridging of molecular chains by cross-linking reactions 16. By replacing OH groups or cross-linking starch molecules, chemical modification reduces the diffusion coefficient and hence water absorption of TPS. Figure 1.11: Starch modification reactions

24 Blending with hydrophobic polymers Some authors have tried to improve the water resistance of TPS by melt-blending starch with hydrophobic polymers such as poly(e-caprolactone) 31, cellulose acetate 32, poly(butylene adipate-co-terephthalate) 33, polylactides 34 and polyethylene 35. When a hydrophobic polymer is blended with a hydrophilic polymer, the diffusion coefficient for the blend is lower than that for the hydrophilic component. Therefore, the blend will be more water resistant than the hydrophilic component alone. A theoretical explanation for this observation is provided by analysis of the equation for the diffusion coefficient of miscible polymer blends, shown below as equation (1.5) 36. (1.5) and are the respective volume fractions and D 1 and D 2 are the respective diffusion coefficients of polymer 1 and 2 in the blend. ΔE 12 is a measure of the thermodynamic interaction energy of the blend and is given by equation (1.6). (1.6) E db, E d1, and E d2 are the activation energies of the blend and the two unblended components. Solving equation (1.5) for D b gives equation (1.7). (1.7) Therefore, the diffusion coefficient and hence water absorption of the polymer blend is dependent on the weight fraction of TPS, weight fraction of hydrophobic polymer, the thermodynamic interaction energy between TPS and the hydrophobic polymer, and their respective diffusion coefficients. As the weight fraction of TPS decreases in the blend, so too will the diffusion coefficient and water absorbed since the hydrophobic polymer will have a lower diffusion coefficient than TPS Plasticizer Choice Plasticizer type has an influence on water absorption in TPS since it is largely responsible for its material properties, including the diffusion coefficient. In a study by 13

25 Mathew and Dufresne 22 it was found that for the commonly used group of polyol plasticizers, a continuous decrease in water uptake and diffusion coefficient was observed as molecular weight increased (shown below in Table 1.1). An explanation for this was proposed based on the chemical structure of the plasticizers, shown below in Figure The amount of end hydroxyl groups is greater for the low molecular weight compounds and these groups have an affinity for and are more accessible to water 22. Figure 1.12: Chemical structure of (a) glycerol, (b) xylitol, (c) sorbitol, (d) maltitol 22. Table 1.1: Water uptake at equilibrium in plasticized maize starch 22. Plasticizer Diffusion Coefficient (cm 2 /sx10 8 ) Equilibrium Water Absorption (%) Xylitol Sorbitol Maltitol

26 1.3 Problem Statement Water absorption in thermoplastic starch causes deterioration of its strength properties, limiting its applications in any environment exposed to humidity or water. In a survey of bioplastic industry experts, shown below in Table 1.2, starch based plastics were found to have the lowest technical substitution potential for petroleum-based plastics 37. Technical substitution potential refers to the ability of a novel plastic to replace the common petroleum-based plastics, based solely on material properties. Therefore, starch based plastics currently do not have strong enough material properties to replace petroleum-based plastics in a wide array of applications. The most problematic material property of starch plastics their tendency to absorb water from their surroundings must be minimized in order to increase their technical substitution potential. Table 1.2: Technical substitution potential of bioplastics 37. Industry currently minimizes water absorption and improves mechanical properties of starch plastics by blending with hydrophobic polymers. As shown below in Table 1.3, almost every commercially available starch plastic is a blend with other polymers, with the exceptions of partially fermented starch from Solanyl and starch composites from PaperFoam 38. However, these commercial products are included in the low technical substitution estimate above. Therefore, their material properties are not ideal for substituting petroleum-based plastics. Novel starch based plastics with better material 15

27 properties than the currently available products must be prepared and studied if starch plastics are to replace petroleum-based plastics to a high degree. Table 1.3: Commercial starch plastic producers Objective The overall objective of this thesis was to produce a thermoplastic starch based material with low water absorption that may be used to replace petroleum-based plastics. 16

28 1.4.1 Specific Objectives The specific objectives for this thesis were the following: 1. Reduce water absorption in TPS by chemical modification with paper sizing agents. 2. Reduce water absorption in TPS by blending with the hydrophobic polymers polyethylene, paraffin wax, and beeswax. 3. Reduce water absorption in TPS by using sorbitol and citric acid as plasticizers. 1.5 Research Approach Chemical modification of starch has been well studied in the literature and many hydrophobicity increasing reactions are known 16. However, none of the current starch based plastics on the market make use of hydrophobicity increasing chemical reactions 38. Also, many literature studies on starch chemical modification make use of TPS preparation methods, such as solution casting, which are unsuitable to industrial scale plastic production. Therefore, an experiment was undertaken in Chapter 2 of this thesis with the objective of reducing the water absorption of TPS by extruding starch with paper sizing agents chemicals known to increase starch hydrophobicity. Blending TPS with hydrophobic polymers is performed to improve TPS properties in most of the commercially available starch plastics. However, some new plastics are commercially available that have not been blended with TPS, such as green polyethylene. Therefore, in Chapter 3 green polyethylene was blended with starch in order to reduce water absorption. Also, waxes are commonly used to reduce water absorption in wood products, but have not been blended with TPS. Therefore, in Chapter 4 paraffin wax and beeswax were blended with starch in order to reduce water absorption. Plasticizers used in the preparation of TPS have an effect on material properties, such as water absorption. Sorbitol has been shown to produce TPS with lower water absorption than glycerol plasticized TPS. Also, citric acid has been used as a plasticizer 17

29 for TPS, but water absorption tests have not been published. Therefore, in Chapter 5 sorbitol and citric acid were used as plasticizers for starch in order to reduce water absorption. 18

30 1.6 References 1. Showmura, R.S. (1990). Second International Conference of Marine Debris. US Department of Commerce, Honolulu. 2. Rouilly, A. and L. Rigal (2002). Agro-materials: a bibliographic review. Polymer Reviews. 42:4, Tharanathan, R.N. (2003). Biodegradable films and composite coatings: past, present and future. Trends in Food Science & Technology. 14, Gross, R. A. and B. Kalra. (2002). Biodegradable polymers for the environment. Science. 297, Reddy, C.S.K., Ghai, R. and V.C. Kalia. (2003). Polyhydroxyalkanoates: an overview. Bioresource Technology. 87, de Graff, R.A., Karman, A.P. and L. Janssen. (2003). Material properties and glass transition temperatures of different thermoplastic starches after extrusion processing. Starch. 55, Forssell, P., Mikkilä, J., Suortti, T., Seppäl, J. and K. Poutanen. (1996). Plasticization of barley starch with glycerol and water. Journal of Macromolecular Science, Part A. 33: 5, Shogren, R.L., Fanta, G.F. and W.M. Doane. (1993). Development of starch based plastics a re-examination of selected polymer systems in historical perspective. Starch. 45:8, R.F.T. Stepto. (2003). The processing of starch as a thermoplastic. Macromolecular Symposium. 201, Kalambur, S. and S. Rivzi. (2006). An overview of starch-based plastic blends from reactive extrusion. Journal of Plastic Film and Sheeting. 22:39, R., Chandra and R. Rustgi. (1998). Biodegradable Polymers. Progress in Polymer Science. 23, H. Cornell. (2003). Starch in food; structure, function, and applications. Woodhead Publishing Limited, CRC Press A. M. Donald. (2003). Starch in food; structure, function, and applications. Woodhead Publishing Limited, CRC Press Royal Society of Chemistry. (2008). Carbohydrates. Available at Accessed on July 5, T. A. Newton. (2003). Glucanes containing alpha glycosidic linkages. Available at Accessed on July 5, R. N. Tharanathan. (2005). Starch value addition by modification. Critical reviews in Food Science and Nutrition. 45, Glaring, M.A., Koch, C.B. and A. Blennow. (2006). Genotype-specific special distribution of starch molecules in the starch granule: a combined clsm and sem approach. Biomacromolecules. 7:8, Ray, S.S. and M. Bousmina. (2005). Biodegradable polymers and their layered silicate nanocomposites: in greening the 21st century materials world. Progress in Materials Science. 50, Lourdin, D., Coignard, L., Bizot, H. and P. Colonna. (1997). Influence of equilibrium relative humidity and plasticizer concentration on the water content and glass transition of starch materials. Polymer. 38:21,

31 20. Ratnayake, W.S. and D.S. Jackson. (2007). A new insight into the gelatinization process of native starches. Carbohydrate Polymers. 67, R.F.T. Stepto. (2000). Thermoplastic Starch. Macromolecular Symposia. 152, Mathew, A.P. and A. Dufresne. (2002). Plasticized waxy maize starch: effect of polyols and relative humidity on material properties. Biomacromolecules. 3, Ma, X. and J. Yu. (2004). The plasticizers containing amide groups for thermoplastic starch. Carbohydrate Polymers. 57, Barrett, A., Kaletunc, G., Rosenburg, S. and K. Breslauer. (1995). The effect of sucrose on the structure, mechanical strength and thermal properties of corn extrudates. Carbohydrate Polymers. 26, Da Roz, A.L., Carvalho, A.J.F., Gandini, A. and A.A.S. Curvelo. (2006). The effect of plasticizers on thermoplastic starch compositions obtained by melt processing. Carbohydrate Polymers. 63, Hulleman, S.H.D., Janssen, F.H.P. and H. Feil. (1998). The role of water during plasticization of native starches. Polymer. 39:10, AIPMA. (Year Unknown). Plastic Process. Available at: Accessed July 20, Thomas, N.L. and A.H. Windle. (1981). Diffusion mechanics of the system pmmamethanol. Polymer. 22:5, M.A.L. Russo, et al. (2007). A study of water diffusion into a high-amylose starch blend: the effect of moisture content and temperature. Biomacromolecules. 8, N. Lilichenko, et al. (2008). A biodegradable polymer nanocomposite: mechanical and barrier properties. Mechanics of Composite Materials. 44:1, Averous, L., Moro, L., Dole, P. and C. Fringant. (2000). Properties of thermoplastic blends: starch polycaprolactone. Polymer. 41, R.L. Shogren. (1996). Preparation, thermal properties, and extrusion of highamylose starch acetates. Carbohydrate Polymers. 29:1, Nabar, Y., Raquez, J.M., Dubois, P. and R. Narayan. (2005). Production of starch foams by twin-screw extrusion: effect of maleated poly(butylene adipate-coterephthalate) as a compatibilizer. Biomacromolecules. 6, Dubois, P. and R. Narayan. (2003). Biodegradable compositions by reactive processing of aliphatic polyester/polysaccharide blends. Macromolecular Symposium. 198, Shujun, W., Jiugao, Y. and Y. Jinglin. (2005). Preparation and characterization of compatible thermoplastic starch/polyethylene blends. Polymer Degradation and Stability. 87, D.R. Paul. (1984). Gas transport in homogenous multicomponent polymers. Journal of Membrane Science. 18, Shen, L., Worrell, E. and M. Patel. (2010). Present and future development in plastics from biomass. Biofuels, Bioproducts and Biorefining. 4, Shen L., Haufe, J. and M. Patel. (2009). Product overview and market projection of emerging biobased plastics. Copernicus Institute for Sustainable Development and Innovation, Utrecht University, Netherlands. Report No: NWS-E

32 Chapter 2 : Extrusion of starch with paper sizing agents 2.1 Introduction Chemical modifications to starch are often carried out for a variety of reasons, including increasing its hydrophobicity. A group of chemicals known as sizing agents are used to hydrophobically modify the starch applied to paper in a procedure known as paper sizing. In this procedure, cellulose paper fibres are covered with a thin film of starch modified by a sizing agent, creating a water repellent surface. Reactions between starch and sizing agents have been well studied and are generally carried out at alkaline conditions and moderate temperatures 1,2. However, little is known about how these chemicals affect the water absorption and mechanical properties of thermoplastic starch processed by high temperature extrusion at neutral ph. Styrene maleic anhydride (SMA) is the only paper sizing agent to have been extruded with starch and reported in the literature 3,4,5. In a study by Vaidya and Bhattacharya 4, starch was successfully extruded and reacted with SMA at neutral ph. TPS/SMA blends showed improved water resistance over TPS; however, a significant drop in tensile strength was observed in a high humidity environment. Different paper sizing agents were examined in this chapter. Alkenyl succinic anhydride (ASA), alkyl ketene dimer (AKD), and a styrene/butyl acrylate copolymer (BSO) were studied. ASAs react with the OH groups of starch by an esterification reaction, as shown below in Figure 2.1. The length of the alkenyl group ultimately determines the extent of hydrophobic character in the modified starch, with longer chains increasing hydrophobicity more than shorter chains 6. 21

33 Figure 2.1: Chemical reaction of ASAs with starch 2. The reaction of starch with ASA is generally carried out in aqueous medium and under alkaline conditions. A study of the modification of starch with ASA found the optimal reaction conditions to be ph 8.5-9, 23 o C, and 5% ASA concentration. Previous studies on the ASA modification of starch use a solution casting method to prepare films 7 ; therefore, the effects of high temperature extrusion on this reaction are unknown. AKD is another common commercial chemical used for paper sizing and is classified as non-hazardous 1. It has a reactive β-lactone functionality that can react with hydroxyl or amino groups under mild reaction conditions. The reaction scheme whereby AKD replaces an OH group on a starch glucose unit is shown below in Figure 2.2. Figure 2.2: Chemical reaction of AKD with starch 1. The reaction between AKD and starch may be enzyme catalyzed or performed under alkaline conditions with or without the use of a solvent. It is generally carried out at ph at a temperature of o C. The effect of carrying this reaction out in an extruder at higher temperatures is currently unknown, as AKD has only been used to modify starch in a batch reaction 8. 22

34 Basoplast (BSO) is a polymeric paper sizing agent consisting of a styrene/butyl acrylate copolymer 9. The structure of this chemical is shown below in Figure 2.3. Figure 2.3: Chemical structure of styrene/butyl acrylate copolymer. Butyl acrylate reacts with starch through a graft copolymerization reaction 10 ; however the reaction mechanism for Basoplast paper sizing is unknown due to a lack of information provided by the manufacturer. The objective of this chapter was to reduce the water absorption of TPS by chemical modification with ASA, AKD, and BSO. Reactions between these chemicals and starch were carried out at neutral ph in an extruder, despite having maximum efficiencies in alkaline solution. The rationale behind this decision was to minimize energy use and eliminate unit operations such as batch reaction, filtration, and drying of starch that would be required were the reaction to take place outside of an extruder in alkaline solution. By performing the reaction inside an extruder, the plasticization of starch and reaction with sizing agent are accomplished in a single step. 2.2 Experimental Materials Industrial grade cornstarch (11% moisture) was obtained from Casco Inc. (Cardinal, ON). Glycerol was purchased from ACP Chemicals Inc. (Montreal, QC). ASA was purchased in the form of Eka SA 220 from Akzo Nobel (Amsterdam, NL). AKD was purchased from Hercules Inc. (Wilmington, DE) as Hercon 115 and Basoplast 400 DS was obtained from BASF Corporation (Ludwigshafen, Germany). 23

35 2.2.2 Plasticization Starch, glycerol, and any additives were mixed with a high speed kitchen mixer for 30min. The compositions of four samples prepared are listed below in Table 2.1. Table 2.1: Used symbols and corresponding sample compositions. Weight Proportion Sample Starch Glycerol Additive TPS N/A ASA % Eka SA 220 AKD % Hercon 115 BSO % Basoplast 400DS Starch mixtures were compounded using a twin screw extruder, the ONYX TEC-25/40, supplied by ONYX P.M. Inc. (Toronto, ON, Canada). The extruder had a screw diameter of 25mm and L/D ratio of 40; screw speed was 125RPM and feeder speed was 12RPM. The temperature profile along the extruder barrel (from feed zone to die) is shown below in Table 2.2. Table 2.2: Temperature profile used for extrusion. Zone Temperature ( o C) Plastic emerged from the extruder out of a circular die and was reduced to small fragments using the rotating knife on the ONYX TEC-25/ SEM Specimens were fractured with a knife and the exposed surfaces were observed with a JEOL JSM-840 scanning electron microscope (Tokyo, Japan). All surfaces were coated with gold to avoid charging under the electron beam. The electron gun voltage was set at 15 kv. The micrographs of samples were taken at magnifications of 200 to identify cracks, holes and other changes on the surface of the samples. 24

36 2.2.4 Water Absorption Water absorption (WA) of each sample was measured by first preparing 2x2sqinch thin film specimens using a model ARG-450 hydraulic press supplied by Dieffenbacher N.A. Inc. (Windsor, ON). Samples were pressed for 4min at 160 o C and 500kPa, cut into square specimens, and dried overnight in a desiccator. Dried specimens were placed in a desiccator containing distilled water at room temperature (23 o C, 100% RH) and weighed every 24h. WA of each specimen was calculated by the following equation: (1) where W a is the weight of the specimen at a specific time interval and W i is the initial dry weight of the specimen. Equilibrium was assumed to be reached when the difference between successive WA values was less than 1%. The result of each sample represents the average of five specimens. 2.3 Results and Discussion SEM Images for all extruded samples at 200X magnification are shown below in Figure 2.4. The TPS, BSO, and AKD samples have smooth surfaces with some roughness caused by the physical slicing of the samples in preparation for SEM and some starch granules remaining unplasticized. The image for the TPS sample is similar to images from previous studies which show smooth surfaces with some roughness or pits caused by unplasticized granules 11. The ASA sample image in Figure 2.4b shows only partially melted granules. This indicates that complete plasticization of starch was prevented by the addition of 15% ASA in the form of Eka SA 220. This is a novel finding since previous studies have modified TPS with ASA after plasticization by dipping samples into an ASA solution 7. In this experiment, it appeared that modification of starch with ASA prior to plasticization had the effect of hindering the plasticization of starch. A possible explanation for this result is that ASA reacted with starch, replacing hydroxyl groups with hydrocarbon chains. This modified starch was then less capable of forming hydrogen bonds with glycerol through its hydroxyl groups. Therefore, the plasticization 25

37 of starch was hindered because glycerol was less capable of aiding in starch polymer chain mobility through hydrogen bonding. Figure 2.4: SEM images of extruded samples at 200X magnification: (a) ASA (b) AKD (c) TPS (d) BSO Water Absorption Shown below in Figure 2.5 are the water absorption profiles for the AKD, BSO, and TPS samples (data in Appendix A). An ASA sample was not prepared because the extruded material was very brittle due to incomplete starch plasticization. TPS and modified TPS samples typically demonstrate the water absorption phenomenon shown below, whereby the samples absorb water quickly during the first days immersed in a high humidity environment. Then the rate of water absorption slows until equilibrium is reached at which point the samples stop absorbing more water 12. The AKD sample absorbed less water than both the TPS and BSO samples; however, the specimens fell apart after 10 days, before equilibrium was reached. Also, there was no significant 26

38 Eq. Water Absorption (%) Water Absorption (%) difference in water absorption between the BSO and TPS samples. These results are discussed further in the sections below AKD BSO TPS Time (days) Figure 2.5: Water absorption profile for AKD, BSO, and TPS samples BSO TPS Sample BSO Figure 2.6: Water absorption at equilibrium for the TPS and BSO samples with 95% confidence intervals. Shown above in Figure 2.6 are the water absorption results at equilibrium for the BSO sample with TPS as a reference. The absorption value for TPS (46%) is comparable to 27

39 Water Absorption, 10d (%) the literature values which range from 46-62% 12,14, depending on starch type and glycerol content. There was no significant difference in water absorption between the BSO and TPS samples. A possible explanation for this observation is that Basoplast did not react with starch under the conditions of this experiment. Therefore, starch was not hydrophobically modified and the water absorption for the TPS and BSO samples was equal. Additional experiments must be undertaken in order to determine the reaction mechanism for Basoplast with starch and the optimal conditions for this reaction. Once this is known, another experiment may be designed to produce Basoplast modified TPS AKD TPS Sample AKD Figure 2.7: Water absorption at 10 days for the TPS and AKD samples with 95% confidence intervals. Shown above in Figure 2.7 are the water absorption results for the AKD sample with TPS as a reference. The AKD sample absorbed less water (35%) than the TPS sample (48%) with 95% confidence (calculated by t-test; see Appendix A). However, the AKD sample fell apart after 10 days, before equilibrium was reached. This suggests that the strength of the AKD reacted sample was lower than the TPS sample, especially under high relative humidity conditions. These observations are corroborated by a previous study that showed the tensile strength of cornstarch films was reduced and the hydrophobicity increased with the addition of AKD 13. Therefore, AKD modified TPS 28

40 prepared by extrusion exhibits similar water absorption and strength properties as AKD modified TPS prepared by solution casting. 2.4 Conclusions An experiment was conducted in order to reduce the water absorption of TPS by chemical modification with ASA, AKD, and BSO. A large number of unplasticized starch granules were visible in SEM images of the ASA sample; therefore, ASA inhibited starch plasticization. Water absorption for the BSO and TPS samples was statistically equal at 46%. Starch extruded with AKD absorbed 35% water compared with 48% for the TPS sample after 10 days. However, the strength of the AKD sample was low under high humidity, and the specimens fell apart before an equilibrium water absorption value was reached. 29

41 2.5 References 1. El-Tahlawy, K., Venditti, R. and J. Pawlak. (2008). Effect of alkyl ketene dimer reacted starch on the properties of starch microcellular foam using a solvent exchange technique. Carbohydrate Polymers. 73, Angellier, H., Molina-Boisseau, S., Belgacem, M.N. and A. Dufresne. (2005). Surface chemical modification of waxy maize starch nanocrystals. Langmuir. 21, Bhattacharya, M., Vaidya, U.R., Zhanc, D. and R. Narayan. (1995). Properties of starch and synthetic polymers containing anhydride groups. II. effect of amylopectin to amylose ratio in starch. Journal of Applied Polymer Science. 57, Vaidya, U.R. and M. Bhattacharya. (1994). Properties of blends of starch and synthetic polymers containing anhydride groups. Journal of Applied Polymer Science. 52: Seethamraju, K., Bhattacharya, M., Vaidya, U.R. and R.G. Fulcher. (1994). Rheology and morphology of starch/synthetic polymer blends. Rheologica Acta. 33, Jeon, Y., Viswanathan, A. and R.A. Gross. (1999). Studies of starch esterification: reactions with alkenyl-succinates in aqueous slurry systems. Starch. 51, L. Ren et al. (2010). Influence of surface esterification with alkenyl succinic anhydrides on mechanical properties of corn starch films. Carbohydrate Polymers. In Press. 8. Qiao, L., Gu, Q. and H.N. Cheng. (2006). Enzyme-catalyzed synthesis of hydrophobically modified starch. Carbohydrate Polymers. 66, R.V. Lauzon. (2002). Method for preparing aqueous size composition. US Patent No. 6,414,055 B Athawale, V.D. and S.C. Rathi. (1999). Graft polymerization: starch as a model substrate. Polymer Reviews. 39: 3, Mathew, A.P. and A. Dufresne. (2002). Plasticized waxy maize starch: effect of polyols and relative humidity on material properties. Biomacromolecules. 3, Yu, J., Wang, N. and X. Ma. (2005). The effects of citric acid on the properties of thermoplastic starch plasticized by glycerol. Starch. 57, Li, X., Shen, Y., Li, G. and X. Lai. (2010). Preparation and properties of hydrophobic starch based biodegradable composite films. Polymeric Materials Science and Engineering. 26:5, Mathew, A.P. and A. Dufresne. (2002). Plasticized waxy maize starch: effect of polyols and relative humidity on material properties. Biomacromolecules. 3,

42 Chapter 3 : Extrusion of starch with maleated polyethylene, green polyethylene, and green polyethylene compatibilized with maleic anhydride 3.1 Introduction Some authors have tried to improve the water resistance of TPS by melt-blending starch with hydrophobic polymers, while maintaining the biodegradability of the overall product. Hydrophobic and biodegradable polymers such as poly(e-caprolactone) 1, cellulose acetate 2, poly(butylene adipate-co-terephthalate) 3, and polylactides 4 have shown to be valuable candidates for melt-blending with TPS; however, use of these biodegradable polymers in commercial applications is restricted by their relatively high cost and poor mechanical properties compared to commodity plastics such as polyethylene (PE) and polypropylene. Melt blending of starch with non-biodegradable polymers such as PE is a good way of improving TPS properties, but PE contents must be kept low to ensure biodegradability of the blend 5. A new form of PE, called green polyethylene, has recently become available on the market. It is produced from the renewable material sugarcane; therefore, it seems to be a suitable material to meltblend with starch from a marketing ( green materials) and environmental perspective. Developing melt-blends with satisfactory properties depends on the ability to generate a small dispersed phase size with strong interfacial adhesion, thereby improving the stress transfer between the component phases 6. This is accomplished by using a compatibilizer that reacts with the hydroxyl groups of starch to form covalent bonds, providing interfacial adhesion 7. Common compatibilizers used in starch mixtures are ethyleneeacrylic acid (EAA), maleic anhydride (MAH), and ethylene vinyl alcohol (EVA). MAH is the most suitable of these compatibilizers because EVA is hydrophilic and the moisture it attracts is detrimental to TPS s mechanical properties. Also, a large amount of EAA is required for compatibilization which becomes costly 8. The reactions whereby MAH reacts with PE to form maleated PE which then reacts with starch are shown below in Figure 3.1 and Figure 3.2, respectively. 31

43 Figure 3.1: Reaction of MAH with PE initiated by DCP or BPO 5. Figure 3.2: Reaction of maleated PE with starch 5. Reactive extrusion is the simplest and most cost effective method for carrying out this two-step reaction. It is a process whereby the reactions in Figure 3.1 and Figure 3.2 are carried out in a single step at high temperature using an extruder. Combining these two reactions into a single step eliminates the need for two separate extrusion steps and therefore reduces energy use and processing costs 9. Previous authors have studied the extrusion of starch with PE and found that blends containing MAH showed higher tensile strength, elongation at break, and thermal stability than those of blends without MAH 8. However, these authors did not compare extruded starch/pe/mah with starch/maleated PE blends and water absorption tests were not conducted. 32

44 The objective of this chapter was to reduce water absorption in TPS by blending with polyethylene. PE was melt blended with starch in three different ways, including the following: reactive extrusion of green polyethylene and starch facilitated by MAH and DCP, melt blending of green polyethylene and starch by extrusion, and melt blending of maleated polyethylene and starch by extrusion. 3.2 Experimental Materials Industrial grade cornstarch (11% moisture) was obtained from Casco Inc. (Cardinal, ON). Glycerol was purchased from ACP Chemicals Inc. (Montreal, QC). Green polyethylene DA-5800 (GPE) was supplied by Braskem S.A (Sao Paulo, Brazil) and maleated polyethylene (MPE) by Arkema Inc. (Philadelphia, PA). Maleic anhydride (MAH) and dicumyl peroxide (DCP) were purchased from Sigma-Aldrich (Oakville, ON) Plasticization Starch, glycerol, and water were mixed with a high speed kitchen mixer for 30min. MPE or GPE, MA, and DCP were added and mixed for an additional 10min. Compositions of the ten samples prepared are listed below in Table 3.1. Table 3.1: Used symbols and corresponding sample compositions. Weight Proportion Sample Starch Glycerol Water GPE MPE MAH DCP TPS GPE GPE GPE MGPE MGPE MGPE MPE MPE MPE

45 The temperature profile along the extruder barrel (from feed zone to die) is shown below in Table 3.2. Other extrusion parameters were the same as previously described, see section Table 3.2: Temperature profile used for extrusion. Zone Temperature ( o C) FTIR An experiment was conducted in order to determine if maleic anhydride reacted with green polyethylene. GPE was extruded separately at the same processing conditions and in the presence of MAH and DCP. The extrudate was purified to remove any unreacted MAH and the purification method was as follows: dissolution of PE in xylene followed by precipitation in acetone SEM Testing was the same as the previous experiments; see section TGA The thermal properties of the blends were measured with a TGA Q500 type thermal analyzer purchased from TA Instruments (New Castle, DE). Sample weight varied from 1 to 5 mg. Samples were heated on a platinum pan from ambient temperature to 600 C at a heating rate of 15 C/min. Results shown for each sample are from a single measurement. Derivatives of TGA thermograms were obtained using TA Instruments Universal Analysis software Water Absorption Testing was the same as the previous experiments; see section

46 Absorbance Units 3.3 Results and Discussion FTIR Maleic anhydride, after grafting onto PE, exists in the form of succinic anhydride. In FTIR spectra the presence of 5 membered anhydride rings, such as succinic anhydride is shown by a peak in the area of 1790cm -1. If any unreacted MAH remains after the purification it is shown by the peak at 698cm -1, attributed to the C=C bond in MAH 8. MGPE GPE 1790cm cm Wavenumber (cm -1 ) Figure 3.3: FTIR spectrum for GPE and MGPE. Shown above in Figure 3.3 are the spectra for unreacted green polyethylene and green polyethylene extruded with MAH and DCP. In the reacted sample, a peak at 1790cm -1 caused by the asymmetrical stretching vibration bond of anhydride groups is present. This verified the fact that MAH grafted onto GPE in the presence of DCP, and may be used as compatibilizer in TPS/GPE blends during extrusion. The spectrum for the unreacted GPE shows no peaks at 1790cm -1 and 698cm -1. This indicates that there are no anhydride groups in the sample, as expected. 35

47 3.3.2 SEM Figure 3.4: SEM image of TPS sample. Shown above in Figure 3.4 is the SEM image for the TPS sample at 100X magnification. The sample has a smooth surface with some roughness caused by the physical slicing of the sample in preparation for SEM and some starch granules remaining unplasticized. The image for the TPS sample is similar to images from previous studies which show smooth surfaces with some roughness or pits caused by unplasticized granules 10. Shown below in Figure 3.5 are SEM images of the GPE samples. Some starch particles remained unplasticized and were removed from the surface of the blends during the fracture of the specimen, leaving some pits in the fracture surface. These pits are most clearly visible at 500X magnification in Figure 3.5b. There appears to be phase separation between starch and GPE in all samples and this is most clearly visible at a higher GPE content, in Figure 3.5c. Therefore, interfacial adhesion between TPS and GPE was poor, since a continuous phase was not formed. This result was expected because PE and starch are structurally dissimilar polymers known to form incompatible blends 5. 36

48 Figure 3.5: SEM images of extruded samples: (a) 5GPE, (b) 10GPE, (c) 20GPE Figure 3.6: SEM images of extruded samples: (a) 5MGPE, (b) 10MGPE, (c) 20MGPE 37

49 Shown above in Figure 3.6 are SEM images of the MGPE samples. As with the GPE samples, some starch particles remained unplasticized and are visible in the images, especially in Figure 3.6c. This may be caused by GPE interfering with starch plasticization during extrusion and could be remedied by feeding GPE from a side feeder at a later stage on the extruder. In this way, starch will plasticize in the earlier stages without interference from GPE. With the addition of MAH, TPS and GPE combined a continuous phase in which the phase interface between TPS and GPE disappeared. This result shows that the morphology of the blends with MAH was improved due to the increased compatibility between TPS and GPE. Figure 3.7: SEM images of MPE samples: (a) 5MPE (b) 10MPE (c) 20MPE Shown above in Figure 3.7 are SEM images of the MPE samples. Again, some starch particles remained unplasticized likely from MPE interference as explained above. TPS and MPE combined in a mostly continuous phase as with the MGPE samples. Again, this was a result of maleic anhydride improving the compatibility between TPS and PE. 38

50 3.3.3 TGA TGA curves for all PE/TPS samples are shown below in Figure 3.8, Figure 3.9, and Figure 3.10 with PE and TPS as references. In the case of all GPE, MGPE, and MPE samples, there were three well defined shifts in the TGA curve. First, at around 100 o C, water evaporation and unreacted MAH sublimation (in the case of MGPE) caused the initial weight loss. Weight loss continued gradually as water continued to evaporate along with glycerol (starting at 150 o C). A second major shift occurred from 300 o C to 350 o C where the thermal degradation of starch occurred. Finally, the third shift was a result of PE degradation beginning at 450 o C. Figure 3.8: TGA curves for pure GPE, TPS, and blends of TPS/GPE. 39

51 Figure 3.9: TGA curves for pure MGPE, TPS, and blends of TPS/MGPE. Figure 3.10: TGA curves for pure MPE, TPS, and blends of TPS/MPE. Derivative TGA curves for all PE/TPS samples are shown below in Figure 3.11, Figure 3.12, and Figure 3.13 with PE and TPS as references. 40

52 Figure 3.11: Derivative TGA curves for pure GPE, TPS, and blends of TPS/GPE. Figure 3.12: Derivative TGA curves for pure MGPE, TPS, and blends of TPS/MGPE. 41

53 Figure 3.13: Derivative TGA curves for pure MPE, TPS, and blends of TPS/MPE. Shown below in Table 3.3 are the data extracted from the derivative TGA curves for all pure polymers and TPS blends tested. Table 3.3: Data from derivative TGA curves. Sample T 5% ( o C) Starch T max ( o C) PE T max ( o C) TPS N/A GPE 337 N/A 388 MGPE 304 N/A 381 MPE 435 N/A 476 5GPE GPE GPE MGPE MGPE MGPE MPE MPE MPE

54 T 5%, the temperature corresponding to 5% weight loss of the sample showed no discernable trend with respect to amount or type of PE for the GPE and MPE samples. However, for the MGPE samples the T 5% values increased with the amount of PE, indicating an increase in stability of the blends. Perhaps this trend was not seen for the GPE and MPE samples because these blends were not as compatible; therefore, increasing the amount of PE in these samples did not further increase the stability of the blends because the PE portion was not strongly associated with the less thermally stable starch portion. Starch T max and PE T max values shown above in Table 3.3 correspond to the maximum rate of degradation for starch and PE, respectively. There was no trend evident in the starch T max values for the blended samples, which all have a T max value close to that of TPS. Therefore, it does not appear that any significant starch degradation occurred in all three types of TPS/PE blends. PE T max values for the MPE blended samples are similar to the value for pure MPE, indicating no PE degradation occurred. An interesting phenomenon was observed in the derivative TGA curves for the GPE and MGPE blended samples. The derivative TGA curves for pure GPE and MGPE displayed a wide degradation temperature range as a result of the composition of GPE. GPE is a branched co-polymer of ethene and butane and since it is produced from a natural source the chain length and branching amount vary considerably, resulting in a wide range of degradation temperatures. However, in the case of the GPE and MGPE samples blended with TPS, the degradation temperature range was reduced and shifted to approximately 480 o C from 380 o C. One possible explanation is that starch char remaining in the sample absorbed heat and limited heat transfer to PE, thus increasing the temperature at which PE degraded. 43

55 Eq. Water Absorption (%) Water Absorption Results from the water absorption testing at 100% RH and room temperature for all PE samples are shown below in Figure 3.14, Figure 3.15, and Figure Raw data and confidence interval calculations are contained in Appendix B. TPS is shown as a reference on the plots and the value for TPS water absorption (66%) is comparable to the literature value (62%) for glycerol plasticized (50wt%) TPS 10. Adding polyethylene had the effect of significantly reducing water absorption below the value of TPS for all PE samples. This result is explained by the theory for a polymer blend that predicts the diffusion coefficient and water absorption of a hydrophilic polymer will be reduced when blended with a hydrophobic polymer (equation 1.7). Also, the water absorption value for all samples decreased as the amount of PE increased from 5% to 20%, except for the GMPE sample (discussed below). Again, this observation is explained by equation (1.7) which predicts the diffusion coefficient and water absorption of a polymer blend will decrease as the weight fraction of hydrophobic polymer increases TPS 5MGPE 10MGPE 20MGPE Sample Figure 3.14: Water absorption at equilibrium for MGPE samples with 95% confidence intervals. The MGPE samples displayed higher water absorption than the GPE and MPE samples. A number of possible explanations exist for this observation. It is known that MAH causes starch destruction at high temperatures 8 ; however, no evidence of starch destruction was found in the TGA results. Another possible explanation is that MGPE 44

56 Eq. Water Absorption (%) Eq. Water Absorption (%) interfered with starch plasticization during extrusion, leaving pits and unplasticized granules where water can pass through. Evidence for this explanation is provided by the SEM images which contain more unplasticized granules for the MGPE samples than the MPE and GPE samples. Also, it is possible that MAH bonded with starch, preventing starch plasticization. This effect was the most pronounced in the 20MGPE sample since more PE and MAH was available to interfere with starch plasticization and this may explain why the 20MGPE sample had higher water absorption than the 10MGPE sample TPS 5GPE 10GPE 20GPE Sample Figure 3.15: Water absorption at equilibrium for GPE samples with 95% confidence intervals TPS 5MPE 10MPE 20MPE Sample Figure 3.16: Water absorption at equilibrium for MPE samples with 95% confidence intervals. 45

57 The MPE samples displayed the lowest average water absorption of all three sample types tested. These samples exhibited better plasticization and thus lower water absorption than the MGPE samples. Also, the MPE samples showed better interfacial adhesion than the GPE samples. Therefore, their water absorption was lower since the diffusion coefficient equation for polymer blends (equation 1.7) contains a term for the activation energy (related to interfacial adhesion) of the blend. When the activation energy of the blend is reduced, the interfacial adhesion and water resistivity will improve. 3.4 Conclusions An experiment was conducted in order to determine which method of melt blending TPS with PE (reactive extrusion of GPE and TPS facilitated by MAH and DCP, melt blending of GPE and TPS by extrusion, and melt blending of MPE and TPS by extrusion) was the most effective at reducing water absorption. It was found that all methods reduced the water absorption of TPS significantly. However, the GPE/TPS samples showed poor interfacial adhesion and the MGPE/TPS samples had higher water absorption due to some starch remaining unplasticized. 46

58 3.5 References 1. Averous, L., Moro, L., Dole, P. and C. Fringant. (2000). Properties of thermoplastic blends: starch polycaprolactone. Polymer. 41, R.L. Shogren. (1996). Preparation, thermal properties, and extrusion of high-amylose starch acetates. Carbohydrate Polymers. 29:1, Nabar, Y., Raquez, J.M., Dubois, P. and R. Narayan. (2005). Production of starch foams by twin-screw extrusion: effect of maleated poly(butylene adipate-coterephthalate) as a compatibilizer. Biomacromolecules. 6, Dubois, P. and R. Narayan. (2003). Biodegradable compositions by reactive processing of aliphatic polyester/polysaccharide blends. Macromolecular Symposium. 198, Kalambur, S. and S. Rivzi. (2006). An overview of starch-based plastic blends from reactive extrusion. Journal of Plastic Film and Sheeting. 22:39, Barlow, J.W. and D. R. Paul. (1984). Mechanical compatibilization of immiscible blends. Polymer Engineering and Science. 24:8, J.M. Raquez et al. (2008). Maleated thermoplastic starch by reactive extrusion. Carbohydrate Polymers. 74, Shujun, W., Jiugao, Y. and Y. Jinglin. (2005). Preparation and characterization of compatible thermoplastic starch/polyethylene blends. Polymer Degradation and Stability. 87, J.M. Raquez et al. (2006). Biodegradable materials by reactive extrusion: from catalyzed polymerization to functionalization and blend compatibilization. C. R. Chimie. 9, Mathew, A.P. and A. Dufresne. (2002). Plasticized waxy maize starch: effect of polyols and relative humidity on material properties. Biomacromolecules. 3,

59 Chapter 4 : Extrusion of starch with beeswax, paraffin wax, and paraffin wax compatibilized with maleic anhydride 4.1 Introduction Waxes are hydrophobic organic compounds often used for waterproofing purposes in applications such as wax paper and wood composites 1. However, melt-blending of waxes with TPS has not been investigated in the literature, despite the existence of research into melt-blending TPS with other hydrophobic polymers, such as poly(butylene adipate-co-terephthalate) 2, and polylactides 3, polyethylene 4, etc. This chapter investigates the potential for two common waxes, paraffin wax and beeswax, to be melt-blended with TPS in order to improve its water resistance. Beeswax is a natural, hydrophobic, and biodegradable wax produced in the beehive by honey bees. The chemical nature of beeswax is basically lipoid, with the major components being 14% hydrocarbons, 35% monoesters, 3% diesters, and 12% free acids 5. Previous studies on melt-blending esters with TPS have found that the mixtures are immiscible; however, they form compatible blends as a result of the hydrogen bonding interaction between the ester carbonyl group and the OH groups on starch 6. Therefore, beeswax may form a compatible blend with TPS as a result of hydrogen bonding interaction and compatibilizers such as maleic anhydride (MAH) are not required. Paraffin wax is a petroleum derived wax composed of a mixture of alkanes ranging from 20 to 40 carbon chain-length. In order to improve the compatibility between starch and paraffin wax, a compatibilizer such as MAH must be used when they are melt-blended. The maleation reaction of paraffin wax has been studied in the literature 7 and is shown below in Figure 4.1. Once the MAH group is attached, the maleated paraffin wax will react with starch in a similar fashion to maleated polyethylene, as shown in Chapter 3, Figure

60 Figure 4.1: Reaction scheme for grafting maleic anhydride onto paraffin wax 7. In this chapter, beeswax was melt-blended with starch by extrusion. Also, paraffin wax was melt-blended with starch in two different ways, including reactive extrusion with starch facilitated by MAH and DCP, and melt blending with starch by extrusion. The objective of this chapter was to determine which type of wax (paraffin or beeswax) and which method of melt blending starch/paraffin wax most effectively reduces the water absorption of TPS. 4.2 Experimental Materials Industrial grade cornstarch (11% moisture) was obtained from Casco Inc. (Cardinal, ON). Glycerol was purchased from ACP Chemicals Inc. (Montreal, QC). Beeswax (BW), paraffin wax (PW, T m = o C), maleic anhydride (MAH), and dicumyl peroxide (DCP) were purchased from Sigma-Aldrich (Oakville, ON). 49

61 4.2.2 Plasticization Starch and glycerol were mixed with a high speed kitchen mixer for 30min. BW or PW or PW, MAH, and DCP were added and mixed for an additional 10min. The compositions of ten samples prepared are listed below in Table 4.1. Table 4.1: Used symbols and corresponding sample compositions. Weight Proportion Sample Starch Glycerol Water BW PW MAH DCP TPS BW BW BW PW PW PW MPW MPW MPW Extrusion parameters and conditions were the same as the previous experiment, see section FTIR An experiment was conducted in order to determine if maleic anhydride reacted with paraffin wax. Paraffin wax was extruded separately at the same processing conditions and in the presence of MAH and DCP. The extrudate was purified to remove any unreacted MA and the purification method was as follows: dissolution of PW in boiling water for 10min, followed by vacuum filtration SEM Testing was the same as the previous experiments; see section TGA Testing was the same as the previous experiment; see section

62 Absorbance Units Water Absorption Testing was the same as the previous experiments; see section Results and Discussion Plasticization During the extrusion of starch with wax at 20% concentration, wax leaked from the extruder at the vacuum ports and side feeder port. At the temperatures used for extrusion, the viscosity of wax is very low; therefore, it leaked from any point on the extruder open to the atmosphere. This was not a problem for the 5% and 10% wax samples so the characterization was completed for those samples FTIR Maleic anhydride, after grafting onto the polymer, exists in the form of succinic anhydride. In FTIR spectra the presence of 5 membered anhydride rings, such as succinic anhydride is shown by a peak in the area of 1790cm -1. If any unreacted MAH remains after the purification it is shown by the peak at 698cm -1, attributed to the C=C bond in MAH PW MPW cm cm Wavenumber (cm -1 ) Figure 4.2: FTIR spectrum for PW and MPW. Shown above in Figure 4.2 are the spectra for unreacted paraffin wax and maleated paraffin wax. The spectrum for the unreacted wax shows no peaks at 1790cm -1 and 51

63 698cm -1. This indicates that there was no succinic anhydride or maleic anhydride in the sample, as expected. The spectrum for reacted wax shown above contains a peak at 1790cm -1 and no peak at 698 cm -1. This indicates that MAH grafted onto the wax in the form of succinic anhydride and any excess MAH was successfully removed by purification SEM Figure 4.3: SEM images of extruded samples: (a) 5BW (b) 10BW Shown above in Figure 4.3 are SEM images of the BW samples. Some starch particles remained unplasticized and were removed from the surface of the blends during the fracture of the specimen, leaving some pits in the fracture surface (visible in Figure 4.3a). Therefore, BW may have interfered with starch plasticization in the same way PE did in Chapter 3. There appears to be phase separation between TPS and BW in both samples and this is most clearly visible at a higher BW content, in Figure 4.3b. Therefore, interfacial adhesion between TPS and BW was poor, since a continuous phase was not formed. A compatibilizer may be required in order to improve the interfacial adhesion between BW and TPS, and hence the morphology. 52

64 Figure 4.4: SEM images of extruded samples: (a) 5PW (b) 10PW (c) 5MPW (d) 10MPW Shown above in Figure 4.4 are SEM images of the PW and MPW samples. Some starch particles remained unplasticized and are visible in the images which may be caused by wax interfering with starch plasticization during extrusion. Also, cracks caused by retrogradation or damage by the electron beam are visible in all samples. It appears that TPS and wax combined in a continuous phase with no interface visible in both the PW and MPW samples. Therefore, adding MAH as a compatibilizer does not have an effect on the morphology of the blends TGA TGA curves for all wax/tps samples are shown below in Figure 4.5, Figure 4.6, and Figure 4.7 with wax and TPS as references. In the case of all BW, PW, and MPW samples, there were two well defined shifts in the TGA curve. First, at around 100 o C, water evaporation caused the initial weight loss. Weight loss continued gradually as water continued to evaporate along with glycerol (starting at 150 o C), and then wax (at 53

65 o C). A second major shift occurred from 300 o C to 350 o C where the thermal degradation of starch occurred. Figure 4.5: TGA curves for pure BW, TPS, and blends of TPS/BW. Figure 4.6: TGA curves for pure PW, TPS, and blends of TPS/PW. 54

66 Figure 4.7: TGA curves for pure MPW, TPS, and blends of TPS/MPW. Derivative TGA curves for all wax/tps samples are shown below in Figure 4.8, Figure 4.9, Figure 4.10 and with wax and TPS as references. Figure 4.8: Derivative TGA curves for pure BW, TPS, and blends of TPS/BW. 55

67 Figure 4.9: Derivative TGA curves for pure PW, TPS, and blends of TPS/PW. Figure 4.10: Derivative TGA curves for pure MPW, TPS, and blends of TPS/MPW. Shown below in Table 4.2 are the data extracted from the derivative TGA curves for all pure polymers and TPS blends tested. 56

68 Table 4.2: Data from derivative TGA curves. Sample T 5% ( o C) Starch T max ( o C) Wax T max ( o C) TPS N/A BW 235 N/A 372 PW 179 N/A 232 MPW 209 N/A 260 5BW N/A 10BW N/A 5PW N/A 10PW N/A 5MPW N/A 10MPW N/A T 5%, the temperature corresponding to 5% weight loss of the sample was greater for the PW and MPW blends than the BW blends, despite the values for pure PW and MPW being lower than that of BW. This indicates that these samples were more stable than the BW samples because they form more compatible blends with TPS, as shown in the SEM images. There was no trend evident in the starch T max values for the blended samples, which all have a T max value close to that of TPS. Therefore, it does not appear that any significant starch degradation occurred in the BW, PW, or MPW samples. It was not possible to analyze wax T max values because of degradation overlapping with TPS Water Absorption Shown below in are Figure 4.11, Figure 4.12, and Figure 4.13 are the water absorption results for all TPS/wax blended samples with TPS as a reference. Raw data and confidence interval calculations are contained in Appendix C. The value for TPS water absorption (66%) is comparable to the literature value (62%) for glycerol plasticized (50wt%) TPS 8. 57

69 Eq. Water Absorption (%) TPS 5BW 10BW Sample Figure 4.11: Water absorption at equilibrium for BW samples with 95% confidence intervals. Shown above in Figure 4.11 are the water absorption results for TPS blended with beeswax. The average water absorption values decrease when beeswax is added to TPS and continue to decrease as the concentration of beeswax increases from 5 to 10%. This result is explained by the diffusion coefficient theory for polymer blends (equation 1.7) which predicts the water absorption for a hydrophilic polymer will decrease when a hydrophobic polymer is added and when the concentration of that hydrophobic polymer increases. However, the reduction in water absorption is not statistically significant for either the 5BW or 10BW sample. This may be a result of the sample preparation method used. When the samples were pressed at high temperature, some of the wax migrated out of the samples because of its low viscosity. Therefore, the water absorption of the extruded blends may be lower than the values reported here for the hot pressed samples. 58

70 Eq. Water Absorption (%) Eq. Water Absorption (%) TPS 5PW 10PW Sample Figure 4.12: Water absorption at equilibrium for PW samples with 95% confidence intervals TPS 5MPW 10MPW Sample Figure 4.13: Water absorption at equilibrium for MPW samples with 95% confidence intervals. Shown above in Figure 4.12 and Figure 4.13 are the water absorption results for TPS blended with paraffin wax and paraffin wax with MAH, respectively. The average water absorption values decrease when paraffin wax is added to TPS as explained by the diffusion coefficient theory for polymer blends (equation 1.7). Contrary to the BW samples, the reduction in water absorption is statistically significant for both the PW and MPW samples. This indicates that paraffin wax more effectively reduces water 59

71 absorption in TPS. An explanation for this comes from the chemical structures of paraffin wax and beeswax. Paraffin wax contains only hydrocarbon chains, whereas beeswax contains water attracting groups such as esters. Therefore, beeswax and its blends with TPS will be more hydrophilic than paraffin wax and its blends with TPS. The water absorption values for all PW and MPW samples are statistically equal with 95% confidence. Therefore, using MAH as a compatibilizer had no effect on the water absorption of the blends since paraffin wax formed a compatible blend with TPS when no compatibilizer was used. Also, increasing the concentration of wax from 5 to 10% did not reduce the water absorption of the samples. Again, this may be a result of the sample preparation method used or loss of wax during extrusion. 4.4 Conclusions An experiment was conducted in order to determine whether beeswax or paraffin wax (with and without MAH) most effectively reduced water absorption when melt-blended with TPS. It was found that paraffin wax blends with TPS absorbed less water than beeswax blends due to paraffin wax s greater hydrophobicity than beeswax. Also, the use of MAH as a compatibilizer between paraffin wax and TPS did not improve the morphology or water absorption. Wax loss from openings on the extruder became a problem for greater than 10% wax content samples. Additionally, the 10% wax content samples showed no significant improvement in water absorption values compared with 5% wax content samples. 60

72 4.5 References 1. Kamke, F.A. and T.R. Miller. (2006). Enhancing composite durability using resins and waxes a review. Wood Protection Conference. New Orleans, LA. 2. Nabar, Y., Raquez, J.M., Dubois, P. and R. Narayan. (2005). Production of starch foams by twin-screw extrusion: effect of maleated poly(butylene adipate-coterephthalate) as a compatibilizer. Biomacromolecules. 6, Dubois, P. and R. Narayan. (2003). Biodegradable compositions by reactive processing of aliphatic polyester/polysaccharide blends. Macromolecular Symposium. 198, Shujun, W., Jiugao, Y. and Y. Jinglin. (2005). Preparation and characterization of compatible thermoplastic starch/polyethylene blends. Polymer Degradation and Stability. 87, J.J. Jiminez et al. (2004). Quality assurance of commercial beeswax. Part I. Gas chromatography electron impact ionization mass spectrometry of hydrocarbons and monoesters. Journal of Chromatography A. 1024: B.Y. Shin et al. (2004). Rheological, mechanical and biodegradation studies on blends of thermoplastic starch and polycaprolactone. Polymer Engineering and Science. 44:8, Krump, H., Alexy, P. and A.S. Luyt. (2005). Preparation of a maleated fischer-tropsch paraffin wax and ftir analysis of grafted maleic anhydride. Polymer Testing. 24, Mathew, A.P. and A. Dufresne. (2002). Plasticized waxy maize starch: effect of polyols and relative humidity on material properties. Biomacromolecules. 3,

73 Chapter 5 : Extrusion of citric acid/glycerol and sorbitol/glycerol coplasticized starch 5.1 Introduction Plasticizers are essential for processing starch into a thermoplastic material and have an effect on material properties such as water resistivity, glass transition temperature, strength, and elongation. Glycerol is the most common plasticizer used for starch; however, it contains a high amount of end hydroxyl groups, making glycerol plasticized starch very hydrophilic. Alternative plasticizers such as xylitol 1, sorbitol 1, maltitol 1, urea 2, formamide 2, and sugars 3 have been studied in order to improve water resistivity and other material properties of TPS. In this chapter, sorbitol and citric acid were investigated as alternative plasticizers to glycerol. Sorbitol is a biodegradable sugar alcohol commonly used as a sugar substitute and has been studied as a plasticizer for starch. Sorbitol plasticized starch was found to have reduced water absorption and greater tensile strength compared with glycerol plasticized starch. However, at high relative humidity a significant drop in tensile strength properties was observed 4. Therefore, in this chapter sorbitol plasticized starch was blended with polyethylene and paraffin wax in order to improve its strength properties at a high relative humidity. Citric acid is a natural and biodegradable organic acid found in a variety of fruits and vegetables. It is commonly used to impart hydrophobicity to starch by a cross-linking reaction, shown below in Figure 5.1. Yu et al. 5 studied the properties of glycerol plasticized TPS modified by citric acid and found that the addition of a small amount (0.6-3% w/w starch) of citric acid improved the water resistance of TPS at a high relative humidity. 62

74 Figure 5.1: Reaction scheme of citric acid with starch 6. Citric acid has also been added to starch as plasticizer (up to 40% w/w starch) in glycerol/citric acid co-plasticized TPS 7. It forms hydrogen bonds with starch molecules and in the same way as polyols. However, the effect of adding a high percentage of citric acid on the water absorption and tensile properties of TPS is currently unknown. In this chapter, starch was plasticized with glycerol, sorbitol, citric acid, and mixtures of these plasticizers. Also, a blend of glycerol/sorbitol plasticized starch with hydrophobic polymers (polyethylene and paraffin wax) was prepared. The objective of this chapter was to determine which plasticizer was most capable of reducing water absorption in TPS. 5.2 Experimental Materials Industrial grade cornstarch (11% moisture) was obtained from Casco Inc. (Cardinal, ON). Glycerol was purchased from ACP Chemicals Inc. (Montreal, QC) and maleated 63

75 polyethylene (MPE) from Arkema Inc. (Philadelphia, PA). Paraffin wax (PW) and sorbitol (SOR) were purchased from Sigma-Aldrich (Oakville, ON) and citric acid (CA) from Caledon Labs (Georgetown, ON) Plasticization Starch, water, and plasticizers were mixed with a high speed kitchen mixer for 30min. Where citric acid and/or sorbitol were used, they were first dissolved in the water. When BW and MPE were added, they were mixed in for an additional 10min. The compositions of ten samples prepared are listed below in Table 5.1. Table 5.1: Used symbols and corresponding sample compositions. Weight Proportion Sample Starch Glycerol Water CA SOR MPE PW TPS SOR SOR SOR CA CA CA SORBLEND Extrusion parameters and conditions were the same as the previous experiment, see section SEM Testing was the same as the previous experiments; see section Water Absorption Testing was the same as the previous experiments; see section Mechanical Testing The mechanical behaviour of the TPS samples was analyzed according to ASTM D-638 using an Instron 3367 testing machine in tensile mode, with a load cell of 1kN capacity. Thin film specimens were prepared by hot pressing for 4min at 160 o C and 500kPa using 64

76 a model ARG-450 hydraulic press supplied by Dieffenbacher N.A. Inc. (Windsor, ON). Next, the specimens were cut using a die machined to the specifications for type V samples in ASTM D-638. Samples were conditioned at 0% relative humidity for 48h. The gap between pneumatic jaws at the start of each test was adjusted to 25mm and all samples were strained at 2.5mm/min. The average values of the Young s modulus, strength, and elongation at break were calculated from at least 5 measurements. 5.3 Results and Discussion Plasticization Samples TPS, 20SOR, 20CA, and SORBLEND were plasticized well in the extruder at the conditions listed in the experimental section above. However, it was only possible to extrude the other samples at very high temperatures (~200 o C). This was due to a number of reasons. Firstly, citric acid and sorbitol have been shown to be less effective plasticizers than glycerol because they have less OH groups per unit weight. Therefore, the samples were not as well plasticized and did not flow as well in the extruder and the temperature was increased to reduce their viscosity. At these temperatures, starch degradation was a problem and most of the extruded plastic was burned. For this and reason, the samples 30SOR, 45SOR, 30CA, and 45CA were not tested for water absorption and mechanical properties SEM Images for the citric acid/glycerol and sorbitol/glycerol plasticized samples are shown below in Figure 5.2 and Figure 5.3, respectively. The 20CA, 20SOR, 30CA, and 30SOR samples have smooth surfaces with some unplasticized granules visible. These images are similar to images from previous studies for glycerol plasticized starch 1. Therefore, both citric acid and sorbitol were effective plasticizers when combined with glycerol in the proportions tested. However, starch particles remained mostly unplasticized when only citric acid or sorbitol was used as plasticizer, as shown in Figure 5.2c and Figure 5.3c. Starch was not fully plasticized in these samples because both sorbitol and citric acid are less effective plasticizers than glycerol. They are less effective because they have less OH groups per unit mass than glycerol and some of these groups are less 65

77 accessible because of their chemical structure. Therefore, complete plasticization of starch was not possible with 45% citric acid or sorbitol used as plasticizers. Sorbitol/glycerol plasticized starch was extruded with paraffin wax and maleated polyethylene in order to determine how well it melt-blended with hydrophobic polymers. A SEM image of the extruded sample is shown below in Figure 5.3d. The sample surface was smooth with no unplasticized starch particles visible. Also, there was no phase separation visible; therefore, the blend appears to be compatible. The image is similar to those from compatible blends of glycerol plasticized starch and polyethylene 8. Figure 5.2: SEM images of extruded samples: (a) 20CA (b) 30CA (c) 45CA 66

78 Figure 5.3: SEM images of extruded samples: (a) 20SOR (b) 30SOR (c) 45SOR (d) SORBLEND Water Absorption One citric acid plasticized sample was prepared for water absorption testing, 20CA. After 48 hours all specimens turned into a paste that could not be removed from trays in the desiccator in order to take weight measurements. Previous studies show that both the tensile strength of TPS and the molecular weight of starch decrease as the percentage of citric acid increases 7. For these samples with a high percentage of citric acid, it is likely that starch polymer-polymer interactions were weakened by chain shortening. Placing the samples in a high relative humidity environment further reduced the strength of the samples to the point where they were no longer able to remain coherent. Shown below in Figure 5.4 are the water absorption results for the sorbitol/glycerol plasticized samples with glycerol plasticized TPS as a reference. Raw data and confidence interval calculations are contained in Appendix D. The value for TPS water 67