ANTIOXIDANT OPTIMIZATION IN PRESSURE SENSITIVE ADHESIVES USING MODULATED THERMOGRAVIMETRIC ANALYSIS

Transcription

1 ANTIOXIDANT OPTIMIZATION IN PRESSURE SENSITIVE ADHESIVES USING MODULATED THERMOGRAVIMETRIC ANALYSIS Cathy D. Stewart, Analytical Chemist, Intertape Polymer Group, Marysville, MI Introduction Antioxidant optimization is important in pressure sensitive adhesives, not only because of the high cost of additives, but also due to the adverse effects excessive antioxidant can cause (such as yellowing of the adhesive due to quinone formation). Previously, thermogravimetric analysis has been used to determine antidegradent optimization in tire rubber formulations by measuring the change in activation energy with increasing heating rates. This procedure, based on Flynn and Wall method [1], requires multiple measurements for each level of antidegradent followed by several complex mathematical equations. A new procedure, developed by TA Instruments, uses Modulated TGA to determine kinetic parameters on a continuous basis, making determination of activation energy simple and easy. This paper will describe how this method was used to determine optimum antioxidant levels in a natural rubber adhesive. Theory: Why Antioxidants are Added to Adhesives Adhesives, and the raw materials that go into adhesives, can react with oxygen in the air to form free radicals. This process is called auto-oxidation. Auto-oxidation can initiated by heat, UV light, stress from mixing, rolling or extruding, or a reaction with impurities (metal ions in particular will promote auto-oxidation). The oxidation of the adhesive can result in discoloration, viscosity changes, char formation, cracking and loss of adhesion. In simple terms, a free radical is generated when a molecule of rubber, resin, oil, etc. loses a hydrogen ion. When these free radicals are exposed to oxygen, the reaction is propagated and new oxidized free radicals are formed. Initiation RH! R" + H" Propagation R" + O 2! ROO" ROO" + RH! ROOH + R" ROOH! RO" + OH" RO " + RH! ROH + R" And so on. Antioxidants put a stop to auto-oxidation by neutralizing the free radicals. The antioxidant sacrifices its own hydrogen ion, becoming in effect a free radical itself. R" + AH! RH + A" RO " + AH! ROH + A " ROO" + AH! ROOH + A" 113

2 The effect of oxidation on a smoke rubber sample can be seen in Figure 1. Two samples of smoke sheet rubber were analyzed by thermogravimetric analysis (TGA). The samples were heated at a steady 150 C for 5 hours. One sample was heated under nitrogen gas, the other under oxygen. The smoke sheet rubber heated under nitrogen lost weight at a slow rate. The sample heated under oxygen initially gained weight as the rubber began to auto-oxidize, and then began to lose weight at a rapid rate as the molecular bonding between the rubber molecules begin to fail. Antioxidants help prevent the breakdown of polymers due to auto-oxidation. Smoke Sheet Oxidation Test min ANA Smoke Sheet in O2 ANA Smoke Sheet in N Temp = 150 C Weight (%) Time (min) Universal V4.5A TA Instruments Figure 1. Smoke Rubber Oxidation Test by TGA Problems with Antioxidants There are two major problems with antioxidants: cost and antioxidant yellowing. Cost is simple enough to understand. Add too little antioxidant and it will be consumed before all the free radicals are neutralized. Add too much and you end up wasting money on raw materials. But too much antioxidant is more than just a waste of money. The A" radical formed when the antioxidant sacrifices a hydrogen is not completely benign. Many antioxidants, particularly the class of antioxidants known as hindered phenols, can react with themselves to form bright yellow quinones, which can discolor not only the adhesive, but anything that comes in contact with the adhesive as well. BHT (Butylated Hydroxytoluene), one of the most common antioxidants, is notorious for this. And because BHT is volatile, the yellow discoloration can transfer to whatever the BHT comes into contact with. Using TGA to Measure Antioxidant Efficiency TGA decomposition kinetics can provide useful information on aging stability and lifetime predictions. The TGA decomposition kinetics method uses the data from experiments run at several heating rates to calculate kinetic parameters including activation energy and the specific rate constant. The theory behind this method was originally described in a paper by Flynn and Wall [1] and won t be discussed 114

3 here, except as necessary. Numerous researchers have used this method to determine the activation energy of natural rubber, synthetic polymers and resins [2-7]. The experiments described in this paper were used to determine the activation energy of a natural rubber-resin adhesive. Experiment Standard TGA Method Samples of a natural rubber adhesive were formulated with increasing levels of antioxidant. The antioxidant chosen was a common hindered phenol-type of antioxidant. The antioxidant was added to the adhesive at levels of 0%, 0.1%, 0.25%, 0.5%, 1%, 1.5%, and 2.0%. Multiple TGA analyses were run on each formulation at increasing temperature ramp rates of 10 C/min, 15 C/min, 20 C/min and 30 C/min. Figures 2 and 3 show how increasing the amount of antioxidant causes the decomposition thermograms to shift to a higher temperature. Figure 2. Overlay of Thermograms Figure 3. Close-up of Thermograms 115

4 The temperatures at which each sample had decomposed by 25% and 50% were then determined as shown in Figure 4. Sample: ANA % BLX1836 Size: mg Method: Hi-Res - Dynamic Comment: NR Adhesive % BLX TGA File: C:...\ANA to19999\ANA Operator: CDS Run Date: 20-Oct :13 Instrument: TGA Q5000 V3.8 Build Weight (%) C 75.00% C 50.00% 0.5 Deriv. Weight (%/ C) Temperature ( C) Universal V4.5A TA Instruments Figure 4. Temperatures at 25% and 50% Decomposition Assuming everything works, the plot of ln! (where! is the heating rate) vs 1000/T (T is the temperature in degrees Kelvin) should generate a straight line. The slope of this line is used to calculate activation energy according to the following equation: Equation 1 Slope = Ea/R Where: Ea is the activation energy R is the gas constant ( K -1 mol -1 ) The plot of the addition of 0.5% antioxidant to the adhesive is shown in Figure

5 0.5% Antioxidant % Degraded 25% Degraded ln! /T K Figure 5. Calculating Slope So for this example, the slope of the 25% decomposed sample is -20.4, and for the 50% decomposed sample Applying the equation above, you get activation energy for 0.5% antioxidant of KJ/mol at 25% decomposition, and KJ/mol at 50% decomposition. Problems Limitations of this traditional kinetics method include the amount of time required for several experiments, and the fact that kinetic parameters are calculated as a single value. This is based on the assumption that a single decomposition mechanism controls the entire decomposition range of the material. In this case, because this was an adhesive and not a pure polymer sample, the results were complicated due to the rubber and the resin having independent decomposition mechanisms. Slight differences in sample weight will also affect the decomposition behavior of the sample. As a result, the methodology for this type of analysis can start to fall apart when running multiple samples multiple times. The primary difficulty is in ensuring accurate and consistent weights and run conditions. Small errors make big differences. In this experiment, seven samples were run in triplicate at four different ramp rates for a total of 84 separate analyses. Each sample had to be analyzed, converted and plotted to calculate activation energy, and at the end of the experiment the data showed only slight correlation to antioxidant values. Figure 6 shows the plot of Antioxidant Levels vs. Activation Energy. While there does appear to be a significant increase in the activation energy between 0.5% and 1.0% antioxidant, the graph is not as clear-cut as it could be. 117

6 Activation energy (Ea) % AO 0.1% AO 0.25% AO 0.5% AO 1% AO 1.5% AO 2% AO Figure 6. Calculated Activation Energy at 25% Decomposition Modulated TGA Modulated TGA (MTGA) superimposes a sinusoidal temperature modulation on the traditional underlying heating profile. The use of discrete Fourier transformation allows kinetic parameters, such as activation energy, to be calculated on a continuous basis. So rather than run each sample at 10 C/min, 15 C/min, 20 C/min and 30 C/min, each sample need only be analyzed once, and the activation energy can be calculated anywhere along the decomposition thermogram. In recent years, Modulated TGA has become a common method for determining Activation Energy in polymer samples [8, 9]. MTGA is not only faster, it eliminates the problem with duplicating very small weights on multiple samples. Figure 7 shows the set-up of the MTGA run on the TA Instruments 5000IR TGA. Figure 7. MTGA Set-Up 118

7 Figure 8 shows a typical TGA plot: % Weight Loss overlain with the Derivative Weight curve. Figure 8. Normal TGA Figure 9 shows a Modulated TGA plot. Note the modulations in the Derivative Weight curve. Figure 9. Modulated TGA With modulated TGA the Activation Energy can be calculated anywhere along the thermogram as seen in Figure

8 Figure 10. Plot of Activation Energy Figure 11 shows the calculation of the Activation Energy at 25% and 50% decomposition for the 1.0% Antioxidant sample. Figure 11. Activation Energy at 25% and 50% Decomposition 120

9 Table 1 shows the Activation Energies calculated based on the MTGA analysis. Table 1. Activation Energies based on MTGA Sample 25% Reduction 50% Reduction 0.00% AO KJ/mole KJ/mole 0.25% AO KJ/mole KJ/mole 0.50% AO KJ/mole KJ/mole 1.00% AO KJ/mole KJ/mole 1.50% AO KJ/mole KJ/mole 2.00% AO KJ/mole KJ/mole Figure 12 shows the plot of these activation energies. As with the standard TGA method for calculating Ea, the activation energy shows a jump between 0.5% and 1.0% AO. Only here the results are much more obvious and straightforward. In order to be effective in this formulation, the adhesive should be blended with a minimum of 0.5% AO, but no more than 1.0% AO. Additional AO has no further effect on activation energy. AO Optimization by Modulated TGA Activation Energy (kj/mol) % AO 0.25% AO 0.5% AO 1% AO 1.5% AO 2% AO 25% 50% % Antioxidant Figure 12. Chart of Activation Energies Modulated TGA vs. DSC OIT Another method of measuring the effectiveness of antioxidants is by using a High Pressure Oxidative Induction Time Analyzer coupled to a Differential Scanning Calorimeter (OIT/DSC). OIT is an accelerated thermal-aging test used to measure the resistance of a substance to oxidative decomposition. Since antioxidants increase oxidative resistance, OIT is a good indirect measurement of antioxidant content. An experiment was carried out on samples of adhesive collected from a trial that showed different mechanical behavior when tested using Dynamic Mechanical Analysis. Looking at the DMA 121

10 data, it was suspected that the adhesive was losing antioxidant during processing due to prolonged exposure to heat. The samples were analyzed using both OIT/DSC and Modulated TGA. As can be seen in Table 2 there was a good correlation between the Oxidative Induction Time and the Activation Energy measured at 25% and 50% degradation of the adhesive. Where OIT is unavailable, MTGA can be a useful way to measure oxidative decomposition. Table 2: OIT vs. MTGA OIT 25% 50% 1 st Bypass Sample 9.0 minutes 97.3 KJ/mol KJ/mol Final Die Sample 10.7 minutes KJ/mol KJ/mol Conclusions Thermogravimetric analysis can be used to determine the appropriate amount of antioxidant to use in an adhesive formulation. Modulated TGA is a great time saver for this type of analysis and provides data as good, if not better, than the traditional TGA method of analysis. Acknowledgments I would like to thank the staff of the Intertape Polymer Group R&D Lab for all of their help. I would especially like to thank my manager, Mr. Rich St. Coeur, for encouraging me to write this paper and enter the world of the PSTC. References 1. Flynn, Joseph H. and Leo A. Wall (1966), A Quick Direct Method for the Determination of Activation Energy from TGA Data, Journal of Polymer Science Part B: Polymer Letters, Vol. 4, No. 5, pp Chakraborty, S., M. Debnath, S. Dasgupta, R. Mukhopadhyay, and S. Bandyopadhyay (2008), Antidegradant Dose Optimization through Thermogravimetric Decomposition Kinetics Study, Rubber World, July, pp 18-20, Mathew, Aji P., S. Packirisamy and Sabu Thomas, (2001), Studies on the Thermal Stability of Natural Rubber/Polystyrene Interpenetrating Polymer Networks: Thermogravimetic Analysis, Polymer Degradation and Stability, 72, pp Ferrand, Damien, Willi Schwotzer, Fabian Käser, and Bertrand Roduit, (2008), Simulating the Aging of Adhesives, Adhesives and Sealants Industry, February, pp

11 5. Sauerbrunn, S. and P. Gill, Decomposition Kinetics Using TGA, TA Instruments Application Note TA ASTM Standard E (2007), Test Method for Decomposition Kinetics by Thermogravimetry, ASTM International, West Conshohocken, PA, DOI: /E Moreno, R. M. B., E. S. de Medeiros, F. C. Ferreira, N. Alves, P. S. Gonçalves and L. H. C. Mattoso, (2006), Thermogravimetric Studies of Decomposition Kinetics of Six Different IAC Hevea Rubber Clones using Flynn-Wall-Ozawa Approach, Plastics, Rubber and Composites, Vol. 35 No. 1, pp Gracia-Fernández, C. A., S. Gómez-Barreiro, R. Ruíz-Salvador, and R. Blaine, (2005), Study of the Degradation of a Thermoset System using TGA and Modulated TGA, Progress in Organic Coatings, 54, pp Blaine, R. L. and B. K. Hahn, (1998), Obtaining Kinetic Parameters by Modulated Thermogravimetry, Journal of Thermal Analysis, 54, pp

12 TECH 32 Technical Seminar Speaker Antioxidant Optimization in Pressure Sensitive Adhesives using Modulated Thermogravimetric Analysis Cathy D. Stewart, Intertape Polymer Group Cathy D. Stewart is a chemist at Intertape Polymer Group s R&D facility in Marysville, MI. With over 30 years of experience as an analytical chemist, her current areas of expertise include GPC, TGA, GC/MS, FT-IR and DMA analysis. Since joining IPG 2 ½ years ago, she has developed interests in the reverse engineering of adhesives and the pyrolysis GC/MS of adhesives, polymers and resins. Prior to IPG, she spent 16 years at National Steel as an oil chemist and as an expert in microbiological corrosion. It was here that she developed an interest in antioxidant behavior. She received her Bachelor of Science degree in biology/chemistry from Central Michigan University in She can be reached at cstewart@itape.com. 111