Application of Glycerine Carbonate in Novel One-Component Polyurethane Blocked Systems

Transcription

1 Application of Glycerine Carbonate in Novel One-Component Polyurethane Blocked Systems Nicholas Kob,, USA Aisa Sendijarevic, Nirali Lathia, Troy Polymers, Inc., USA Abstract Glycerin carbonate (GC) is a new product having 73% bio-content, and is a unique hydroxylfunctional carbonate having carbonate and hydroxyl reactive sites. Each reactive site opens pathways to utilize GC in polyurethanes. One component PU systems blocked with GC are fully polymerized when reacted with amines. This is an advantage to conventional blocked systems which release blocking group when polymerized. In this study, blocked one component polyurethane systems of carbonate (GC) functional urethane adduct and amines were prepared and polymerized into films. In addition to bio-content, the hydroxyl group in the hard segment enhances hydrogen bonding and improves strength of the cured material. The curing of GCadduct with amines and properties of the films, including stress-stain properties and glass transition, were studied. Introduction Coating compositions containing blocked isocyanates are commonly used in the coating industry for the production of one-component coatings (1). Because the isocyanate groups are blocked the formulation is stable under storage conditions enabling a one component package. Blocked isocyanate formulations find use in many coatings applications such as electrodeposition coating, powder coating, electrical wire, and automotive coatings. Blocked isocyanates offer flexibility and can be used in both solventborne and waterborne systems. In the simplest terms a blocking agents reacts with an isocyanate conventional blocking agents preventing the isocyanate from reacting; then by heating and presence of co-reactants and catalyst a release of the blocking agent from the isocyanate occurs and the coating is formed that contains free blocking group. This liberation of the blocking agent is one of the disadvantages of use of conventional blocked isocyanate formulations. The temperature at which deblocking occurs depends on several variables and is greatly influenced by the choice of blocking group, Figure 1. Figure. 1. DeblockingTemperatures for Blocked Aliphatic Isocyanates Temperature in ºC ε-caprolactam MEKO 1,2,4-Triazole DIPA DMP Diethyl malonate Catalyzed with 1 % DBTL No catalyst

2 It is desirable to have blocked coating systems which cure with minimal energy (lower temperature) and do not release the blocking agent upon cure. Glycerine carbonate (GC) is a new 5-membered cyclic carbonate which is made from renewable glycerine and has the structure shown in Figure 2: Figure 2: Glycerine Carbonate Glycerine carbonate is readily biodegradable, non-toxic, and has a renewable content of 76% (2). It is unique compared to other cyclic carbonates in that is has two reactive sites (carbonate ring and pendant hydroxyl group). Each reactive site as well as their combination opens numerous possibilities to utilize glycerine carbonate in polyurethane applications. This study focus on the application of glycerine carbonate in a one component (blocked type) polyurethane coating system where it is reacted with isocyante to form a carbonate functional adduct (blocking agent). This carbonate functional adduct is reacted with amines to cure and form a urethane coating as shown in Figure 3 (3,4,5). This novel system is shown to have several advantages as compared to conventional blocked systems: GC system does not de-block, rather it becomes part of the urethane matrix. Glycerine carbonate system cures at lower temperature below 100 o C. Glycerine carbonate cured system contains pendant hydroxyl group in hard segment which can improve coating adhesion or be used as a grafting site. Figure 3: GC Urethane Chemistry

3 Experimental Chemicals The chemicals used in this study are shown in Table 1. The polyols were demoisturized at 1-3 mmhg with continuous mixing at 70 C. The solvent ( ethyl acetate) used in the film preparation was dried over molecular sieves. Table 1. Raw Materials Designation Identification Supplier POLYOLS Desmophen S Polyester diol, Bayer eq.wt.490 Millester Polyester polyol (diol) eq.wt.984 Jeffol PPG-2000 Polyoxypropylene glycol, eq.wt.1000 Jeffol PPG-3706 EO-capped PPG-diol, eq.wt.1934 Jeffsol Glycerine Carbonate Glycerin carbonate, (1.5% Glycerine) Jeffsol Glycerine Carbonate Glycerin carbonate, (3.0% Glycerine) Poly-G Polyoxypropylene glycol Arch Chemicals eq.wt.500 Polycin D-140 Castor oil-based diol, Vertellus eq.wt.400 Arcol PPG-4000 Polyoxypropylene glycol, Bayer eq.wt.2000 AMINES EDA Ethylene diamine, eq.wt.30 TEPA Tetraethylenepentamine, 37.9 TETA Triethylenetetramine, eq.wt.36.7 ISOCYANATES Lupranate T-80 TDI (2,4/2,6) 80/20-TDI, BASF eq.wt.87.1 IPDI Isophorone di-isocyanate Bayer Eq.wt.112 Airthane PET-70D TDI- polyether prepolymer, Air Products eq.wt.500 Rubinate 3050 MDI (2,4 /4,4 ) eq.wt.125 CATALYSTS Dabco T-12 Dibutyltin dilaurate Air Products PTB Potassium tert-butoxide Alfa Aesar

4 Synthesis of glycerine carbonate (GC) adducts The NCO-prepolymers were prepared by reacting isocyanates and polyols at 70 C to 80 C in a heated reaction kettle which was equipped with stirrer under continuous flow of dry nitrogen. Prepolymer synthesis was monitored via NCO% determination (di-n-butyl amine titration method, ASTM D-2572). When synthesis was completed glycerin carbonate was added to the prepolymer (in equimolar ratio) via dropping funnel and reaction was continued at 70 C to 80 C. The reaction was completed when NCO% was reached zero or very close to zero. All bis-urethane adducts (GCadducts) formed in this reaction were liquid at 70 C. The viscosity of adducts was measured via Rheometrics. Infra-red analysis (IR) was utilized to confirm conversion of isocyanate prepolymers into adducts. Curing study Potassium tert-butoxide (PTB) catalyst solution was prepared by dissolving 5g of solid PTB into 20g of methanol yielding 25% solution (6). Curing of AD-10 will be described as an example: 100g of adduct AD-10 solution in ethyl acetate (81% solid) was mixed with 62g of ethyl acetate to obtain 50% solution. 48.3g of 50% solution, which contains 24.1g (0.028eq.) of AD-10 was added into 200g cup suited for Speed Mixer. Afterwards, 1g of PTB catalyst solution and 1.06g (0.028eq.) of tetraethylenepentamine (TEPA) were added into cup. The cup with catalyst solution was mixed with Speed Mixer machine for 3 minute ml of this solution was coated into thin film of mil thickness via Dr. Blade. Initial sample (zero time) for IR analysis was taken from solution just after mixing. The films were left at room temperature or placed in the oven at selected curing temperature. A piece of film was taken periodically, the sample dissolved in chloroform, spread over KBr plate and IR-spectra recorded. The ratio of absorbance of carbonate group at 1800 cm-1 divided by absorbance of aromatic group at 1600cm-1 was calculated as a quantitative measure of conversion of carbonate group in the reaction with amine. Film preparation and testing Polyurethane film based on PC-carbonate were prepared as above and cured in air-circulated oven at 70 C or 100 C.Thicker films about 1mm, were cast into rectangular Teflon coated mold of 5x5 inches and cured in air-circulated oven at 100 C for 24 hours. The films were then remelted at 270 F using Laboratory Carver press to create sh eets free of bubbles. Bulk polymerization: 72g of 50% solution of GC-adduct AD-11 and 2.76g of propylene carbonate were added into Speed Mixer cup, mixed and placed in vacuum oven at 70 C for 16 hours to remove solvent. 2.04g of TEPA was added to the cup and mixed via Speed mixer for 30 seconds. The resin was cast into 1mm Teflon coated mold preheated at 100 C and placed in hydraulic Carver press to cure at 100 C for 24 hours. The she et was elastomeric, opaque and free of bubbles. DSC spectra were measured on DSC calorimeter Universal V4.1D TA instrument at the heating rate of 10 C. The DSC spectra were taken in tempera ture interval -70 C to 300 C. Tensile strength and elongation at break of selected films were measured via Instron Tester according to ASTM D 412.

5 Results and Discussion GC- Adducts The list and composition of adducts prepared in this study is shown in Table 2. The adducts (AD) were based on aromatic isocyanates (TDI and MDI) and cycloaliphatic isocyanate (IPDI). Polyether diols of different molecular weights (1000, 2000 and 4000 MW) and polyester polyol of 1000 and 2000 MW were used as a flexible segment. All adducts were liquids at C, and some of the adducts were viscous liquids at room temperature with decreasing viscosity at increasing temperature. They can be used without solvent in the application that allow or require elevated temperatures. The adducts are soluble in commonly used solvents that are used in coatings application. It should be noted that the synthesis off the GC-adduct was controlled well via NCO% determination. The synthetic procedure was similar to that in preparation of urethane NCO-prepolymers. Adducts AD-1 and AD-9 (shown in Table 2) were prepared using the same prepolymer, but using two grades of GC that contain different concentration of glycerin. This was done to observe what if any effects are observed over the full glycerine carbonate product glycerine specification range (with 3% being the upper glycerine spec in glycerine carbonate). As expected, the adduct AD-9 that contains 3% of glycerin had higher viscosity as compared to adduct AD-1 prepared with GC that contains 1.5% glycerin. In Adducts 10 and 11 small amount of solvent was added during the second step of synthesis due to their high viscosity. It could be expected that viscosity of MDI-based adducts is higher than those based on TDI or IPDI. FTIR was used in characterization of the GC-adducts. The absorption of NCO-group of prepolymer is at 2300 cm-1 and urethane at 1730cm-1. Carbonate absorption of adduct is recorded at about 1800 cm-1 and urethane groups at 1730 cm-1. The formation of the GC adduct can be easily followed by disappearance of the isocyanate stretch and appearance of the carbonate stretch. Curing study FTIR method was used as a method to study curing, the carbonate stretch at 1800cm-1 is lost as the GC adduct cures. The curing of GC-adduct is taking place at 70 C that is significantly lower than with other conventional types of blocked isocyanates, which cure at C, Figure 1. Quantitative data of curing of AD-3 (TDI/PPG 1000) was studied by following the ratio of carbonate absorption to aromatic absorption as a quantitative measure of curing. The curing at room temperature was sluggish. However, the curing was good with high functional, low equivalent weight TETA and TEPA as shown in Figure 3. This indicates that increased crosslinking enhances curing of GC-adducts. This could be possible associated with the nature of polymer hard segment that is long and flexible. The hard segment has two -O- groups, and pendant hydroxyl group. The curing of AD-10 that is based on MDI/PPG1000 is shown in Figure 4. The curing of AD-10 with TEPA crosslinker appears to be faster than that with AD-3 (TDI based PPG 1000 system). The curing rate increased with increasing PTB-catalyst concentration from 1% to 2%. The results of the study indicate that curing of GC-adducts appears to be controlled by the type of amine-cross linker and catalyst and by the type of isocyanate used to prepare adducts. It should be noted that cured films were transparent and colorless. If necessary, antioxidants and other additives can be added to resin to stabilize color by time. Limited curing evaluation was carried out with cycloaliphatic isocyanate-based adducts ( AD-1 and AD-4). Screening tests with EDA indicated slow cure. Other types of catalyst were tested, including T-12 and Dabco T-45, but these failed to accelerate curing of GC-adducts.

6 Adduct identification First step prepolymer formation Table 2: Composition of GC-Adduct (AD) AD-1 AD-2 AD-3 AD-4 AD-5 AD-8 AD-9 AD-10 AD-11 TDI (2,4/2,6) (g) IPDI (g) Rubinate 3050 (MD)(g) Poly G R (g) Jeffol PPG 2000 (g) Jeffol 3706 (g) 400 Desmophen S , (g) 400 Millester (g) Polycin D-140 (g) 0.01% Dabco T-12 Time of reaction (h) Temperature ( C) NCO% 0.01% 0.01% 4 h; 3.5 h; 3 h; 1.5 h; 4 h; 4 h; 80 C 75 C 75 C 115 C 80 C 80 C 3 h; 80 C h; 70 C Consistency at RT Liquid Liquid Liquid Gel Liquid Liquid Liquid liquid liquid Second step- GC adduct formation Prepolymer ( g) g prepol g IPDI GC (1.5% glycerine), ( g) GC (3% glycerine), (g) 18.1 Time of reaction (h) Temperature ( C) 6 h 80 C 16 h 80 C 3h 75 C 16h 80 C 3h 3 h 70 o C h 70 o C 2 h 70 o C Consistency at RT Viscous liquid Viscous liquid Solid Solid Very viscous Adduct viscosity, RT( cps) ND Viscosity, 50 C, (cps) (flow) Very viscous Solid Viscous Liquid at 70 C Solid Viscous Liquid at 70 C

7 Figure 3: TDI-based GC- Adduct Curing Figure 4: MDI-based GC-Adduct Curing

8 DSC DSC spectra of GC-adducts showed a glass transition temperature, Tg, of flexible segment at about -25 C, depending on the composition of GC-add uct. The second transition temperature was at about 150 C and higher and is associated wit h softening or melting temperature. Overall, DSC spectra indicate two phase morphology, similar as other types of polyurethanes. Stress-strain properties Stress-strain properties of cured GC-adducts are shown in Table 3 and are all TEPA cured. MDI/polyester-based AD-11 films exhibited higher strength than MDI/polyether-based AD-10 films, as could be expected. Cured samples exhibited relatively high elasticity, as measured by elongation at break. MDI-based systems were prepared also with propylene carbonate (PC) as co-reactant. The addition of (PC) to the adduct prior cure could have two benefits: (1) PC can be used as a plasticizer to lower viscosity of GC-adducts; (2) propylene carbonate will increase urethane concentration in hard segment and should increase strength of cured polymer. The tensile strength improved in the samples that contain PC, due to increased hard segment concentration. AD-11 cured at 100 C exhibited high er tensile strength than those cured at 70 C and 150 C. AD-11 that was cured in bulk was slightl y stronger than that cured in solvent. These results indicate that the tensile strength of cured GC-adducts could be improved by altering formulation and curing condition of GC-adducts. The optimization, as needed, could be considered in future studies. Table 3. Stress-strain properties of GC-adducts cured with TEPA Sample ID Temperature ( o C) Tensile Strength (psi) % Strain at Max Load % Strain at Break AD-10* AD AD-11* AD-11* AD-11* AD-11* 100 bulk * contains propylene carbonate prior curing Conclusions GC-adducts were prepared by reacting glycerin carbonate with NCO-prepolymers. Their structure was confirmed with IR-spectroscopy. GC-adducts based on aromatic and aliphatic isocyanates and polyols (polyether and polyester) of different molecular weight were prepared. The consistency of adducts depends on their composition. All adduct were liquid at 70 C. Some adducts are viscous liquid at room temperature. The viscosity decreased by temperature. The adducts are cured with polyamines of high functionality and low equivalent weight. The curing increased from RT to 70 C and 100 C. There was none to sluggish cure at room temperature. The curing is accelerated with DBA catalyst. Strength properties of cured adduct depend on their composition.

9 References 1. G.M. Carlson at all.: Cure Kinetics Characterization of Blocked Isocyanate Containing Coatings by FT-IR and Thermal-Mechanical Analysis, Advances in Urethane Science and Technology, Vol. 9, p.47 (1984). 2. JEFFSOL R GLYCERINE CARBONATE, Technical Bulletin, Corporation. 3. John H. Clements: Reactive Application of Cyclic Alkylene carbonates. Ind. Eng. Chem. Res. 42, 4, (2003) 4. John M. Whelan, Jr., at all.: Multiple Cyclic Carbonate Polymers, U.S. Pat. 3, , Aldo Bosetti, Vercelli at all.: Process for Synthesis of Aromatic Urethanes, U.S. Pat. 6,034, 265, Albert Gordon Anderson: Preparation of Hydroxy Urethanes, U.S. Pat. 5, 977,262,