CURE KINETICS OF UV CURABLE PRESSURE SENSITIVE ADHESIVES CHARACTERIZED USING MULTILAYER LAP-SHEAR GEOMETRY BY DMA

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1 CURE KINETICS OF UV CURABLE PRESSURE SENSITIVE ADHESIVES CHARACTERIZED USING MULTILAYER LAP-SHEAR GEOMETRY BY DMA Wei Zhang, Research Scientist, Advanced and Applied Polymer Processing Institute, Institute for Advanced Learning and Research, Danville, VA Hailing Yang, Research Associate, Advanced and Applied Polymer Processing Institute, Institute for Advanced Learning and Research, Danville, VA Ronald D, Moffitt, Director and Research Associate Professor, Advanced and Applied Polymer Processing Institute, Virginia Polytechnic Institute and State University, Danville, VA Introduction Recently radiation (such as ultraviolet (UV) light) curable pressure-sensitive adhesives (PSAs) are one of the fastest growing products in the tape industry. It is known that UV radiation, using new and evolving technology, eliminates solvents or dispersing media for the tape s manufacturing cycle -. When preparing the radiation-curable PSAs, together with other components a radiation-sensitive chemical (photoinitiator) is incorporated into the adhesive system. Depending upon The formulated PSA is coated onto a substrate using a suitable coating technique dependent upon the viscosity and processing conditions, and is followed by radiation curing. UV curable PSAs are divided into two type based on their processing conditions: hot-melt UV PSAs and room temperature (RT)-coatable UV PSAs. At the molecular level, the main difference between these two PSA systems is that the primary curing reaction in a hot melt UV PSA is the formation of crosslinks whereas in a RT-coatable UV PSA oligomer polymerization promotes chain extension during curing. These curing reactions increase the average molecular weight (chain length) and improve the mechanical and thermal properties to the levels required for the final products. However, the cured PSA properties may vary significantly if under-cured or over-cured and the processing window can be very narrow. The UV curing process of a RT-coatable UV PSA is illustrated in Figure. The RT-coatable UV PSAs have low viscosities that allow for the use of conventional coating equipment at ambient temperature,. A RT-coatable UV PSA may include oligomeric thermoplastic elastomers, oligomeric acrylated acrylate polymers and oligomeric acrylated urethane in acrylic monomers. Under UV irradiation, free radicals are produced from the photoinitiator and they initiate the polymerization reaction of the acrylic monomer and the oligomers to form a polymer that becomes an integral part of the adhesive. The commonly used reactive diluents are -ethylhexylacrylate, ethoxylated nonylphenol acrylate, and ethoxylated ethylacrylated -. Viscosities of RT-coatable UV PSAs are also well suited for screen-printing. The most common applications are for inline adhesives coating and printing to nameplates and decals. In our previous study,5, a multilayer lap-shear geometry has been designed and proven as a reliable testing method for evaluating the dynamic mechanical properties of polyacrylic pressure sensitive adhesives. This method can be developed and utilized as a standard method for testing PSA adhesives in the thin layer laminates or coating forms. A multilayer geometry with five to ten layers could be an appropriate structure to produce enhanced responses. This lap-shear test determines the shear strength of adhesives in actually bonded bonding materials. The factors that control the performance of a PSA such as bonding, debonding, cohesive, and peel can be derived from the storage/loss modulus, compliances

2 and most importantly the enhanced loss tangent (tan δ). This test method is a comparative method for determining adhesive strengths, surface preparation parameters and adhesive environmental durability. UV Light Figure. Illustration of the curing mechanism of UV curable pressure sensitive adhesives (adapted from Reference ). The objectives of present work are: () to utilize the new multilayer lap-shear geometry to characterize the UV curable PSAs after curing, () to study the curing kinetics of the UV curable PSAs based the parameters such as glass transition temperatures and the activation energy obtained from the new testing method, and () to predict the long term adhesion performance from the time-temperature superposition curves generated from DMA tests at different temperatures and frequencies. Experimental Approach Material Three UV curable PSA samples were provided by Rahn USA Corporation and comprise four essential components: aliphatic urethane acrylate oligomeric elastomers, tackifier, diluent and photoinitiators. The three UV curable PSAs are a high tack UV curable PSA (R), a high shear/saft UV curable PSA (R) and a well balanced UV curable PSA (R). All these samples contain photoinitiators loaded to to 6 wt% with UV absorption maxima at 5, 07 and 68 nm. Coating, Curing and Testing Procedures The UV curable PSAs were coated onto the substrates (paper and transparency) using a roller pin. The coating process was followed by the curing process using a Fusion UV System conveyer system with a Q-bulb installed. The dosage of UV light was controlled by the number of passes (,, and ) under the lamp at fixed line speed (0 ft/min). Because the thickness of the coated PSA tapes could not be controlled well during the coating process, the UV dosage was normalized using the coating thickness to correct for coating thickness variations. The resulting PSA tapes were then characterized using a TA Instruments Q00 differential scanning calorimeter (DSC), and were compared to uncured PSA samples. The cured PSA tapes were then used to make the multilayer lap shearing structures for dynamical mechanical analysis (DMA) testing using a TA Instruments Q800 DMA. The experimental procedures are demonstrated in Figure.

3 There were two type of tests carried out using DMA: frequency sweep/temperature ramp test and frequency sweep/temperature step test. The temperature ranged from -50 to 60 o C and the frequency spanned 0. Hz to 00 Hz. The temperature ramp test heating rate was o C/min. For the temperature step tests, 5 o C increments and 5 minutes soaking time for each temperature step was used before frenquency testing began to ensure sample thermal homogeneity at each temperature step. Processed and Provided by Rahn USA Additives or Reactive Diluents Monomers Pre-Polymerization Pre-Polymer Mixing or Compounding RT-Coatable UV PSAs PSA Tape Intermediate Coating on a Paper or Transparency UV Light DSC Cure Testing Multilayers Lap-shear DMA Testing Figure. Experimental procedures of coating, curing and testing of UV curable PSAs (adapted from Reference ). Multilayer Lap-Shear Testing using DMA As discussed in our earlier works,5, multilayer lap-shearing geometry enhanced the viscoelastic responses of the coated PSA as compared to the traditional lap-shear structure. Multilayer structures with five to eight layers were the best geometry for the DMA testing, in which eight-layer were used. In this study, however, a five-layer structure, which is the lower limit of the recommended structures, was used because the home-made coatings were usually thicker than the commercially-produced PSA coatings. The size of overlap area is about mm (length) 6mm (width). The thickness varied between 0.8 and. mm for the five-layer structures. The DMA testing procedures were discussed in detail earlier. In brief, for such a five-layer structure, there are only four layers of PSA coating being sheared but recorded as tensile properties. The shearing properties were converted from tensile properties by,5 ( N ) t h G = E () N l

4 where l is the overlapped length of the five-layer PSA-paper sample, t is the overlapped thickness of the five-layer PSA-paper sample, E is the measured Young s modulus in tensile mode in DMA for the five-layer PSA-paper sample, G is the shear modulus of the five-layer PSA-paper sample after converted, h is the total PSA thickness of the five-layer PSA-paper sample, and N is the total number of layers of sample. Results and Discussion Thermal Properties by DSC The DSC thermograph of the uncured PSAs and the UV-cured PSAs at different dosages are shown in Figures through 5. Since the uncured PSAs are mixtures of oligomers and additives, the glass transitions are not observed clearly for all three PSAs. But the glass transitions of the cured PSAs are shown in these figures, which are at around -5 o C for R, -5 o C for R and -5 o C for R. Depending on the dosage of UV light, there are slight changes in the glass transition temperatures, but no significant changes were observed, which will be discussed in more detail later in this paper. T g s measured from the DSC thermograph are listed in Table. The glass transition temperatures of the PSAs can also be estimated based on the compositions using the Fox Equation 6 T g w = T g, w + T g, w + T g, + L () where T g is the glass transition temperature (absolute) of the mixture or copolymer and T g,, T g,, T g,, are the glass transition temperatures of components,,, present at weight fractions w, w, w, in the blend. The T g s estimated with Fox Equation are listed in Table. Table. Glass transition temperatures PSAs Glass Transition Temperature, o C Uncured Pass Pass Pass Pass R Fox Equation -6.8 R DSC R DMA, Hz R Fox Equation -. R DSC R DMA, Hz R Fox Equation -0. R DSC R DMA, Hz

5 Heat Flow (W/g) R uncured RP RP 0.05 RP RP Exo Down Temperature ( C) Universal V.D TA Instruments Figure. DSC thermograph for uncured PSA R (Rahn UV Curable PSA Sample ), cured after pass (RP), passes (RP), passes (RP), and passes (RP) under the Q-bulb UV lamp. 0.0 Heat Flow (W/g) R uncured RP RP RP RP Exo Down Temperature ( C) Universal V.D TA Instruments Figure. DSC thermograph for uncured PSA R (Rahn UV Curable PSA sample ), cured after pass (RP), passes (RP), passes (RP), and passes (RP)) under the Q-bulb UV lamp.

6 0.0 Heat Flow (W/g) R uncured RP RP RP RP Exo Down Temperature ( C) Universal V.D TA Instruments Figure 5. DSC thermograph for uncured PSA R (Rahn UV Curable PSA sample ), cured after pass (RP), passes (RP), passes (RP), and passes (RP) ) under the Q-bulb UV lamp. Dynamical Mechanical Analysis (DMA) Results Figures 6 through 8 show the variation of shearing storage modulus G, shearing loss modulus G, and loss tangent (tan δ) with temperature during the frequency sweep/temperature ramp test at four selected frequencies (0.,, 0, 00 Hz). These figures are for the cured PSAs with UV dosage of three passes through UV lamp at 0 ft/min. The curves of the cured PSAs subjected to other curing dosages are not shown in this paper, but they are very similar to the figures included in this text. These figures showed that the cured PSAs have three temperature regions depending on the responses of modulus and tan δ, which are the glassy, transition and flow regions. The glass regions extend up to about 0 o C for R, -0 o C for R and 0 o C for R, where the storage modulus is about 0 MPa. The glassy regions are followed by the transition region where the modulus is in the order MPa and it extends to about 5 o C. The higher temperature region is the flow region where the storage modulus is in the order of 0. MPa. Tan δ, which is defined by the ration of loss modulus and storage modulus, constitutes the ratio of elastically stored energy to viscously dissipated energy per deformation cycle. For PSA, tan δ can be correlated to the cohesive strength. The tan δ value increased from less than 0. in the glassy region to about 0.8 in the transition region and remained flat or decreased slightly in the flow region depending on the frequency. The shape of the curves at different frequencies is very similar to each other. But the curves are shifted to higher temperatures at higher frequency, as expected for a viscoelastic polymer. This is because the material behaves stiffer at higher frequency (or lower temperatures) and softer at lower frequency (or higher temperatures). The glass transition temperatures can be obtained from these curves as the peak temperature of the loss modulus or tan δ. Even difference temperatures can be found at different frequencies, the peak temperatures of loss modulus at Hz are reported in Table as the T g for the corresponding cured PSAs at a certain UV dosage.

7 00 0 UV Curable PSA (R) UV dosage: Passes.5 G' & G", MPa l Hz Hz 0Hz 00Hz Temperature, o C 0 Figure 6. Variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature ramp test at four selected frequencies (0.,, 0, 00 Hz). The R UV curable PSA were cured with UV dosage of three passes at 0 ft/min line speed UV Curable PSA (R) UV dosage: Passes.9 G' & G", MPa l Hz Hz 0Hz 00Hz Temperature, o C Figure 7. Variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature ramp test at four selected frequencies (0.,, 0, 00 Hz). The RUV curable PSA were cured with UV dosage of three passes at 0 ft/min line speed. -0.

8 0 UV Curable PSA (R) UV dosage: Passes..9 G' & G", MPa l 0. 0.Hz Hz 0Hz 00Hz Temperature, o C -0. Figure 8. Variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature ramp test at four selected frequencies (0.,, 0, 00 Hz). The R UV curable PSA were cured with UV dosage of three passes at 0 ft/min line speed. Each of the three PSA samples has been exposed to the UV light at different UV dosages by passing the coated PSA laminates through the conveyer system a number of times at fixed line speed. The UV dosages are expressed in this paper by two ways, the number of passes or the normalized dosages. The later was calculated based on the total bulb output power and the exposed area, which is.95 J/cm at 0 ft/min. This UV light intensity was then normalized by the thickness of PSA coatings, which is then shown to be in the range of 0 to 00 J/cm depending on the actual thickness of individual coatings. Figures 9 through show the variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature step test at 0 Hz and four UV dosages in term of number of passes. It is obvious that the temperature step testing results are very similar to those using temperature ramp. The features of the curves did not show significant differences with increasing the UV dosage. The glass transition has some shifting at different dosages, but not necessarily shifting to one direction. It has been shown from the DSC thermograph that the variations in the glass transition temperatures at different UV dosage are small. This may indicate that there is little dependence of the glass transition temperature on the UV dosage, and the degree of chain extension or degree of crosslinking during the curing. In another word, the uncured oligomeric components of these PSAs had molecular weights high enough to show negligible effects on the glass transition temperatures when the curing process extends the polymer chain lengths. There are no or very little trifunctional oligomers in these PSAs so that the crosslinking effects during curing are not significant. The little dependence of T g on UV dosage may also due to the fact that the PSAs are mixtures of several components and they may make mixed contributions to the overall glass transition of the cured PSAs. Figure showed a summary plot of all the glass transition temperatures measured from DSC and DMA at different dosages.

9 00. UV Curable PSA (R) Frequency: 0 Hz. 0 Moduli, MPa Pass Pass Pass 0. Pass Temperature, o C Figure 9. Variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature step test at 0 Hz. The R UV curable PSA were cured with UV dosage of,, or passes at 0 ft/min line speed UV Curable PSA (R) Frequency: 0 Hz Moduli, MPa Pass Pass Pass 0. Pass Temperature, o C Figure 0. Variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature step test at 0 Hz. The R UV curable PSA were cured with UV dosage of,, or passes at 0 ft/min line speed.

10 0. UV Curable PSA (R) Frequency: 0 Hz 0.8 Moduli, MPa Pass Pass Pass Pass Temperature, o C Figure. Variation of shearing storage modulus G, shearing loss modulus G, and tan δ with temperature during the frequency sweep/temperature step test at 0 Hz. The R UV curable PSA were cured with UV dosage of,, or passes at 0 ft/min line speed. 0 5 Glass Transition Temperature - UV Dosage 0 5 T g, o C 0-5 Tg R by DMA -0 Tg R by DSC Tg R by DMA Tg R by DSC -5 Tg R by DMA Tg R by DSC UV dosage, J/cm Figure. Summary of all the glass transition temperatures measured from DSC and DMA at different dosages for R, R and R. We have mentioned above that the curves of the moduli and tan δ are very similar to each other at different frequencies but they are shifted to higher temperatures at higher frequency, which is typical for

11 a viscoelastic polymer. Therefore, the frequency sweeps at each temperature were collapsed into a single master curve using time-temperature superposition (tts) referenced to a defined temperature as shown in Figure a. Vice versa, the temperature profile at each frequency can be shifted along the temperature axis to collapse into a single master curve at the reference frequency. Figure b shows one of the master curves constructed in the temperature space and corresponds to Hz as the reference frequency by using a computer program based on Williams-Landel-Ferry (WLF) equation 7. The WLF equation considered the equivalency of time and temperature in the context of free volume theory for an activated flow process in viscoelastic materials such as pressure sensitive adhesives. The WLF equation yields an equivalent frequency for a given temperature relative to a reference temperature and experimental frequency: ln a T C( T To ) = () C + ( T T ) o where a T is the shifting factor at T, C and C are constants for a given polymer, and T 0 is the reference temperature. This time-temperature superposition shift factor a T follows an Arrhenius-type temperature dependence according to the expression: E a a T = exp () R T T0 with R being the gas constant and E a being the activation energy. The activation energy can be calculated from the slope of a log(a T ) versus the inverse of temperature plot as shown in Figure. By reversing Equation, the shifting factors in the temperature space can be derived from: T S EaT = RT ln a + E T a EaT = RT ln F / F 0 + E a where T S is the shifted temperature and T is the experimental temperature, F and F 0 are the experimental frequency and the reference frequency, respectively. The resulted master curve is shown in Figure b. These master curves can be used to predict either the short-term/long-term performances or high-/lowtemperature performances of these PSAs in application where the testing at these conditions is not possible. The activation energies calculated from time-temperature superposition are shown in Figure 5a. It shows a clear dependence of the activation energy on the UV dosage. For R and R a big jump in the activation energy at around 0 J/cm, while the significant change for R is about 80 J/cm. Even the data collapse very well overall, especially the glassy and transition region, a closer examination of Figure showed that the data at the high temperature end are somehow scattered. Since this is the flow region, the activation energy governing the flow is different from the activation energy controlling the glass transition. Therefore, the data at high temperatures were shifted with a different shifting factor to get better tts results. These tts shifting resulted in different activation energies, which are shown in Figure 5b. Similarly, when the tts shifting is focused more on the transition region instead of the overall curve, the activation energy is also found out to be lower than that derived from the whole data range as shown in Figure 5c. All the activation energies are shown to increase with increasing UV dosage. This suggests it is harder to move a polymer chain (flow region) or a chain segment (glass transition) as the PSAs are cured at a larger degree. The activation energy dependence on the UV dosage relationship can be used to define the cure windows which are traditionally done by other parameters (5)

12 such as peel adhesion, tack, cohesion, and shear adhesion fail temperature (SAFT). Further works by combining the multilayer lap shear and the SAFT testing will be carried out to correlate the multilayer lap-shear method with other testing methods. 0 a UV dosage: Passes Reference T: 5 o C.8.6. Moduli, MPa Time-Temperature Superposition Master Curve RP E-06.E-0.E+0.E+06.E+0.E Frenquency, Hz 00 b. UV Curable PSA (R) G' and G", Mpa o UV dosage: Passes Reference F: Hz Temperature, K -0. Figure. The tts master curves of shear deformation versus temperature of UV curable PSA R in frequency space (a) and temperature space (b).

13 Time-Temperature Superposition Shifting Factor RP Log a T TTS Arrhenius Ea:.9 kj/mol Tref: 98. K Tref: 5. C standard error: 0.7 End condition: Finished normally Shifting Factor Linear (Shifting Factor) /Temperature, K - Figure. Variation of time-temperature superposition shifting factor as a function of temperature. The activation energy is calculated from the slope of the linear fit of log(a T ) versus /T. Conclusions UV curable PSAs were characterized using the multilayer lap-shearing geometry with DMA and DSC after cured at different dosages of UV irradiation. The glass transition temperatures from both methods were observed to not depend on the UV dosage significantly. However, time-temperature superposition analysis using the data from the dynamical mechanical analysis revealed that increasing UV dosage increases the energy needed to move a chain segment (glass transition) or a whole chain (flow). Timetemperature superposition analysis can be used to predict either the short-term/long-term performances or high/low temperature performances of these PSAs in application where the testing at these conditions is not possible. This study provides a new method to characterize pressure sensitive adhesives and study the curing kinetics of radiation-cured PSAs. Further studies combining the traditional testing methods for PSAs and this new multilayer lap-shear method may be required to correlate the new methods with other methods. References. Ranjit Malik, Radtech Report, 00, 5.. Victor Lu, Jeffrey Wang, Brian Maxwell, Sarah Shinkwin, and Jim Stockhausen, Proceedings of the Pressure Sensitive Tape Council s 9th Annual Technical Meeting TECH XXIX, Las Vegas, NV, May -5, 006. Sean Des Roches and Wendy Murphy, Adhesives and Sealants Industry (006), 6-.

14 . Hailing Yang, Wei Zhang, Ronald D. Moffitt, Thomas C. Ward, David A. Dillard Nancy R. Sutherland and Lynne Thomas, Proceedings of the Pressure Sensitive Tape Council s 9 th Annual Technical Meeting TECH XXIX, Las Vegas, NV, May -5, Hailing Yang, Wei Zhang, Ronald D. Moffitt, Thomas C. Ward, and David A. Dillard, The International Journal of Adhesion and Adhesive. In press. 6. a. Schneider, Hans Adam, Journal of Research of the National Institute of Standards and Technology, 0 (997), 9. b. T. G. Fox, Bull. Am. Phys. Soc. (965), (Session J.). 7. Class, J. B. and Chu, S.G., Journal of Applied Polymer Science, 0(985), 805. Acknowledgements We thank Rahn USA Corporation for providing UV curable PSA samples. We also appreciate the assistances on this work from our colleagues Mr. Oscar Robertson and Jeff Forlines.

15 80 Activation Energy, kj/mol l Activation Energy - UV Dosage 60 R R 0 R a UV dosage, J/cm Activation Energy, kj/mol l 00 Activation Energy (Flow) - UV Dosage 90 R 80 R R b UV dosage, J/cm Activation Energy (Glass Transition) - UV Dosage Activation Energy, kj/mol l 80 R 70 R R c UV dosage, J/cm Figure 5. Variation of activation energy a T with dosage using the frequency sweep/temperature step tests of the whole data sets (a), only the data points in the flow region (b), and only the data points in the glass transition region (c)

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