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Solventless UV crosslinkable acrylic pressure sensitive adhesives
International Journal of Adhesion & Adhesives 31 (2011) 822–831
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Solventless UV crosslinkable acrylic pressure sensitive adhesives J. Kajtna a,b, M. Krajnc b,n a b
Aero d.d., Ipavcˇeva ulica 32, 3000 Celje, Slovenia Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aˇskercˇeva cesta 5, 1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Accepted 3 August 2011 Available online 7 August 2011
The synthesis and characterization of solventless acrylic UV crosslinkable pressure sensitive adhesives are presented. Different prepolymers were synthesized using bulk polymerization procedure. The reaction mixture consisted of acrylic monomers (2-ethylhexyl acrylate, acrylic acid and t-butyl acrylate), azobisisobutyronitrile initiator, chain transfer agent n-dodecylmercaptan and unsaturated UV photoinitiator 4-acryloyloxybezophenone, which was copolymerized into polymer backbone. Different formulations were tested and the prepolymer was characterized by viscosity measurements and final monomer conversions. The prepolymers were coated onto PET foil and crosslinked by application of UV light source. Peel adhesion at 1801 on glass plate was measured. Gel phase was determined using the Soxhlet extraction and copolymer glass transition temperatures (Tg) were analyzed by differential scanning calorimetry (DSC). Results showed that the final monomer conversions in highly exothermic bulk polymerization reached a level between 75% and 90%. Prepolymer viscosity was highly influenced by change in polymer molecular weight and by addition of acrylic acid as a comonomer. On the other hand, the viscosity remained at the low level when t-butyl acrylate was used. The amount of gel phase for all adhesives was above 60 wt.%. Peel strength measurements showed decrease in peel strength with decreasing polymer molecular weight and increase of peel, when acrylic acid was used as a comonomer. All adhesive coatings with t-butyl acrylate comonomer showed cohesive failure. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Pressure sensitive Peel Solventless
1. Introduction Due to increased environmental concerns mainly water based adhesives are used in today’s production of self adhesive materials. More recently, additional demands have been placed on adhesives production. They include reductions in volatile organic compounds (VOC), wastewater and reduction of energy. Due to this reason a lot of interest is already placed in the development of new radiation curable pressure sensitive adhesives (PSA), especially ultraviolet (UV) curable PSA. UV PSA technology is subdivided into hot melt UV and room temperature (RT) coatable UV PSAs. Hot melt UV PSAs require heating to facilitate the coating process, while the RT coatable UV PSAs are liquid at room temperature. The latter represents a novelty in the field of PSA adhesives and are therefore very interesting from applied, scientific and research point of view. Their preparation processes, applications and correlations between chemical structure and applied properties are still insufficiently known and understood. RT coatable UV PSAs have low molecular masses, which give low viscosities and enable coating at ambient temperature using conventional roll-coating equipment. The appropriate applied properties of the adhesives can be obtained only after curing reaction, which must be fast and highly efficient.
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Corresponding author. Fax: þ386 1 24 19 541. E-mail address: [email protected] (M. Krajnc).
0143-7496/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.08.002
The scientific background of radiation curing adhesives and the relationship between chemical composition and adhesive properties is poorly covered considering relatively small number of published scientific articles. According to recent papers, the most work in this field of research covers the synthesis and characterization of UV crosslinkable PSAs synthesized in organic solvents (solution polymerization). Czech [1] synthesized a copolymer of 2-ethylhexyl acrylate and 4-acryloyloxybenzophenone (range from 0.1 to 3 wt%) in ethyl acetate and acetone. He found that the amount of unsaturated photoinitiator, molecular weight and UV reactivity affected the adhesive properties and the viscosity of the product. Czech [2] also published a study dealing with a synthesis of solvent based PSA used for removable products. In the study, the influence of different crosslinker types on adhesive removability is shown. He showed that the increased amount of crosslinkers lowers the peel strength of the adhesive and improves the removability, which was at a constant level during ageing. In another article published by Czech [3] and Czech and Wesolowska [4] the development of solvent free acrylic PSA is presented. Three different methods of acrylic prepolymer synthesis were performed: a polymerization in an extruder, polymerization in reactor with subsequent solvent removal and polymerization of sirup-type PSA directly on the carrier. They found that the most difficult method is polymerization in an extruder but they were able to synthesize a product with acceptable adhesive properties. They also determined the influence of UV light intensity on the adhesive properties. Czech et al. also developed UV
J. Kajtna, M. Krajnc / International Journal of Adhesion & Adhesives 31 (2011) 822–831
crosslinkable solvent based PSAs with very low shrinkage [5]. They found, that the used 4-acryloyloxybezophenone photoinitiator was the most efficient among all of the tested photoinitiators. It was also observed, that the increased amount of unsaturated photoinitiator increased the viscosity and polymer molecular weight. The optimal shrinkage performance was observed beneath 0.5 wt% of added photoinitiator. UV induced crosslinking process is much faster as compared to crosslinking with commercial crosslinking agents such as aluminum acetylacetonate or titanium acetylacetonate. Do et al. [6] used an unsaturated photoinitiator (2-(acryloyoxy)ethyl 4-(4-chlorobenzoyl) benzoate) for the syntheses of UV crosslinkable acrylic PSAs. The unsaturated photoinitiator was incorporated into the polymer backbone via the solution polymerization process in ethyl acetate with different monomer ratios. Subsequently they also prepared different blends with hydrogenated rosin epoxy methacrylate (HREM), which was used as a tackifier. They found, that gel fraction increased by increasing the hydrogenated rosin content and by increasing UV dosage. Hydrogenated rosin added in the PSA influenced (retarded) the UV reaction, while tack increased with increasing HREM content. Do et al. [7] evaluated the effect of electron donor comonomer (2-hydroxyethyl methacrylate) on photoreaction efficiency and on adhesive properties (probe tack, peel and shear adhesion failure temperature). They found, that 2-hydroxyethyl methacrylate acted as a good hydrogen donor and caused an increase in photoreaction efficiency. However, at high amounts of added 2-hydroxyethyl methacrylate, tack and peel strength rapidly decreased due to increased crosslinking density. Ozawa et al. [8] investigated the PSA properties of non UV cured and UV cured blends of acrylic adhesive polymer and urethane oligomer. The latter was used as a photo polymerization initiator with peak absorption at 327 nm. They found that peel adhesion and probe tack values of UV irradiated samples were reduced, what was also correlated with increase of Tg and moduli increase (DMA analysis). The low molecular weight acrylic adhesive prepolymers either in solution or in bulk, can be crosslinked by addition of metal chelate additive [9] or by addition of unsaturated UV reactive photoinitiator in the prepolymer mixture. Descriptions of various types of photoinitiators are published in a study by Allen [10]. The photoreactivity of the adhesive prepolymers can be adjusted in different ways [11]. New types of copolymerizable photoinitiators were tested in a research published by Czech [12]. All of the tested photoinitiators contained vinyloxycarbonyl groups, which are also known as organic carbonate or carbamate. In this work, effects of photoinitiator addition on viscosity, molecular weight and on adhesive properties are presented. It is shown, that all of the tested photoinitiators can be used for synthesis of UV crosslinkable PSAs. They have negligible effect on viscosity and molecular weight of the solvent based PSAs. In all cases, tack and peel decrease with increasing amount of added photoinitiator, while cohesion of the adhesive increases with increasing crosslinking density.
2. Experimental procedure 2.1. Materials Monomers 2-ethylhexyl acrylate (2-EHA), acrylic acid (AA), and t-butyl acrylate (t-BA) were purified by conventional methods. They were washed three times with dilute sodium hydroxide solution, then washed three times with distilled water and dried over calcium chloride and then subjected to vacuum distillation under nitrogen. Initiator azobisisobutyronitrile (AIBN), chain transfer agent n-dodecylmercaptan (CTA), unsaturated photoinitiator 4-acryloyloxybezophenone (4-ABF-Chemitec Company) were used in the commercially available form without further purification.
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2.2. Polymerization procedure—prepolymer synthesis The solventless UV crosslinkable adhesives were synthesized in a 250 ml glass reactor equipped with a reflux condenser, an anchor impeller, N2 purge and thermometer. Different formulations were tested and the polymerizations were carried out at equal process parameters. The total amount of reaction mixture was 100 g. The anchor impeller was operating at stirring rate of 100 min 1 and was identical for all experiments. After 30 min of mixing and nitrogen purging, the reaction mixture was heated to 83 1C at heating rate of 2.75 1C/min. Due to highly exothermic effect of bulk polymerization, the temperature of the reaction mixture reached approximately 180 1C. After the exothermic phase of the polymerization, the reaction mixture was stirred at 80 1C for another 60 min. Different formulations were tested (Table 1) in order to determine the influence of different chemical parameters on prepolymer and adhesive coating properties. In the first part of experiments, AA was used as a comonomer. Runs 1–5 were performed to investigate the influence of prepolymer molecular weight and Runs 6–9 to evaluate the influence of AA addition on prepolymer properties. In the third part of experiments (Runs 10–13) amount of CTA was increased at constant level of acrylic acid, again to determine the influence of prepolymer molecular weight on viscosity. The most important prepolymer property is, in this case, the prepolymer viscosity since the viscosity is the key parameter in selection of appropriate coating system. In Runs 14–19, t-BA was used as a comonomer instead of AA, which should influence the prepolymer viscosity due to steric effects of monomers t-butyl group in polymer backbone. Experiments were performed with two different levels of CTA agent (0.3 and 0.7 wt%). 2.3. Prepolymer characterization Viscosities of synthesized prepolymers were measured using Brookfield viscosimeter with spindle no. 3 at 20 1C. The overall gravimetrical monomer conversion monitoring was performed for synthesis Run 1. Ten samples (each 2 g) were removed from the reaction mixture at different stages of polymerization. The removed samples were immediately cooled and then first dried in Table 1 Reaction mixture formulations. Run
Syntheses with acrylic acid (AA) as comonomer 2-EHA (wt%)
AA (wt%)
AIBN (wt%) CTA (wt%)
4-ABF (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13
93.3 93.3 93.3 93.3 93.3 98.1 93.1 88.1 83.1 93.8 93.4 93.1 92.8
5.0 5.0 5.0 5.0 5.0 0 5.0 10.0 15.0 5.0 5.0 5.0 5.0
0.1 0.2 0.3 0.4 0.5 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Run
Syntheses with t-butylacrylate (t-BA) as comonomer
14 15 16 17 18 19
0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0 0.4 0.7 1
2-EHA (wt%)
t-BA (wt%)
AIBN (wt%) CTA (wt%)
4-ABF (wt%)
92.1 86.1 80.1 92.5 86.5 80.5
6.0 12.0 18.0 6.0 12.0 18.0
0.2 0.2 0.2 0.2 0.2 0.2
1.0 1.0 1.0 1.0 1.0 1.0
0.7 0.7 0.7 0.3 0.3 0.3
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vacuum dryer at room temperature for 12 h, and then at 70 1C for 12 h. Final conversion of monomers for all syntheses was determined gravimetrically by drying 2 g of PSA in vacuum dryer at 70 1C for 24 h. 2.4. Adhesive coating characterization The samples for Soxhlet extraction were prepared by UV crosslinking under two 9 W lamps, where approximately 2 g of synthesized prepolymer was exposed to UV light for 72 h. The amount of formed gel in all syntheses was determined by Soxhlet extraction with tetrahydrofuran (Merck) under reflux during 24 h. Synthesized prepolymer mixtures were coated on PET foil using a Mayer bar coating system (wire thickness was 0.3 mm). The coating weight of adhesive was approximately 2072 g/m2. Then, the coatings were subjected to UV light (medium pressure Hg vapor lamp, 400 W/in). For the adhesive performance characterization of the produced PSAs, the peel adhesion at 1801 (FTM 1—Finat test method 1) was used. DSC analyzing technique with Mettler Toledo DSC821 was employed to determine Tg value. Standard 40 ml alumina DSC pans with perforated lids were used and the samples were subjected to two heating-cooling cycles from 100 to 10 1C at heating rate of 10 1C/min along with an empty reference pan in a DSC furnace. The result of the second cycle was used for determination of Tg while the first cycle was used only to remove the previous thermal history.
3. Results and discussion
polymerizations, where no solvents are added in the reaction mixture. Due to increased viscosity, the mobility of macro radicals at higher monomer conversions becomes limited. The rate of termination decreases while the rate of propagation remains at the same level and the termination process becomes diffusion controlled, which also causes increase in polymerization temperature. The increase in polymerization rate and increase of polymer kinetic chain length may lead to autoacceleration or gel effect, which is quite common phenomenon for radical polymerization processes. The diffusion controlled regime of termination process followed by the diffusion controlled propagation process at high conversion leads also to limited monomer conversion. As may be seen in Fig. 2, final overall monomer conversion reached approximately 80%. The time plot of overall monomer conversion shows typical conversion curve for vinyl monomers undergoing radical polymerization at high monomer concentrations. The highest polymerization rate was in range between 6 and 10 min, which also coincides with the highest measured polymerization temperature. The results of gravimetrically determined final overall monomer conversion for all other syntheses are presented in Fig. 3. As can be seen in Fig. 3 all final monomer conversions are in range from 80% to 90%. No evident trend in change of overall conversions can be seen from the results. The level of overall conversion for all syntheses was quite high. The desired overall monomer conversion for PSA applications should be as high as possible in order to prevent unwanted low molecular constituents migration to substrate-adhesive interface layer.
3.1. Prepolymer synthesis 3.2. Prepolymer viscosity Due to the absence of solvents in reaction mixture, all syntheses showed high exothermic effect as expected. Maximal values of polymerization temperatures were in range from 180 to almost 200 1C. Time plots of temperature reaction mixture monitoring for some of the syntheses are presented in Fig. 1. During the propagation stage, the polymer molecular weight increases, which also leads to increase in reaction mixture viscosity. This phenomenon is especially noticeable in bulk
The main emphasis of this part of the work was given to study the influence of different parameters on prepolymer viscosity. The first set of experiments was performed to determine the effect of initiator (AIBN) and chain transfer agent (CTA) concentration on prepolymer viscosity. Both additives influence polymer molecular weight, hence the prepolymer viscosity. The results are gathered in Fig. 4.
Fig. 1. Time plot of reaction mixture temperatures.
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Fig. 2. Overall monomer conversion time plot for Run 1.
Fig. 3. Final overall monomer conversion for Runs 1–19.
The prepolymer viscosity in case of synthesis with 0 wt% of CTA was too high to measure. Regarding the influence of CTA additive on viscosity, following results were adopted. The viscosity decreased as a function of increased CTA additive, which was also expected. The polymer molecular weight as well as polymer kinetic chain length decreased with increasing CTA concentration, which influences the entanglement of polymer molecules in bulk and decreases the viscosity. Similar results were also expected for syntheses with variable AIBN concentrations. Surprisingly, the viscosity reached the maximal value at 0.4 wt% of AIBN. The
prepolymer viscosity should decrease with increasing AIBN concentration due to reduction of polymer molecular weight. Beside the influence of polymer entanglement on prepolymer viscosity, the formation of intermolecular hydrogen bonds (addition of acrylic acid—AA) has an influence on viscosity. It can be speculated, that at 0.4 wt% of AIBN the kinetic polymer chain length reaches a level, which enables optimal arrangement of polymer molecules in bulk, accompanied by high hydrogen bond effect. At higher concentrations of AIBN the effect of shorter kinetic chain length prevails, which also lowers the viscosity similar as with
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Fig. 4. Prepolymer viscosity as a function of AIBN and CTA concentration (Runs 1–5 and 10–13).
Fig. 5. Prepolymer viscosity for syntheses with acrylic acid—AA (Runs 6–9).
CTA addition. The effect of hydrogen bonds formation on viscosity is especially noticeable for syntheses with increasing AA addition (Runs 6–9), which are presented in Fig. 5. Plot of viscosity in Fig. 5 reveals exponential increase of viscosity with increasing AA addition. Again, this effect may be attributed to the hydrogen bonds formation between polymer molecules due to increased amount of AA in the reaction mixture. In syntheses 14–19, t-BA monomer was used as comonomer in polymerization procedure in order to decrease prepolymer viscosity. t-BA functional group in polymer should reduce the
viscosity. The syntheses were performed at two different CTA concentrations (0.7 and 0.3 wt%) again to evaluate the effect of polymer molecular weight on viscosity. As can be seen in Fig. 6, the application of t-BA comonomer on viscosity meets our expectations. The prepolymer viscosity is almost independent from the added amount of t-BA comonomer. Because of the spacious group in the acrylic backbone, the polymer chains cannot entangle strongly, which results in a lower prepolymer viscosity as compared to AA comonomer (Fig. 5). On the other hand, the effect of
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Fig. 6. Prepolymer viscosity for syntheses with t-BA comonomer.
Fig. 7. Gel phase amount as a function of AIBN and CTA addition.
reduced polymer molecular weight on viscosity can be seen. The higher addition of CTA reduces the polymer molecular weight, which is exerted in lower prepolymer viscosity. The latter set of results supplements the results in Fig. 4, where the amount of CTA reagent was changed in 2-EHA and AA monomer combination. 3.3. Gel phase determination The amount of gel phase in crosslinked adhesive coating is a function of polymer molecular weight, molecular entanglement,
crosslinking density and intermolecular hydrogen bonds. The crosslinking density in this study does not influence the amount of gel phase due to equal amounts of 4-ABF photoinitiator and identical UV crosslinking parameters. The first set of results in Fig. 7 represents the influence of polymer molecular weight on gel phase amount, which derive from variable addition of CTA and AIBN. The measured amounts of gel phase in Fig. 7 show, that the gel phase was reduced with increasing amount of both additives. Note, that the amount of gel phase for synthesis 10 (0 wt% of CTA)
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Fig. 8. Gel phase amount for syntheses with AA and t-BA at 0.7 and 0.3 wt%.
was not measured because of the high prepolymer viscosity. The reduction in gel phase may be attributed to the less intense polymer molecules entanglement, which caused better extraction to THF solvent. The next set of results gathered in Fig. 8 represents the influence of comonomers on gel phase amount. It was expected that in case of AA addition and increased amount of hydrogen bonds, higher gel phase amount would be measured. It appears, that effect of hydrogen bond formation in coating does not influence on the amount of gel phase. The crosslinking reactions have a predominant effect on gel phase amount as compared to intermolecular hydrogen bond formation, since the amount of gel phase remained almost unaffected by the increasing amount of AA. Similar results were determined also for syntheses with t-BA comonomer. Again, the effect of high and low amount of CTA agent and subsequent reduction of polymer molecules entanglement on gel phase may be seen. Measured values of gel phase at coatings prepared from prepolymer with 0.3 wt% of CTA are approximately 15% lower as those from prepolymer synthesized with 0.7 wt%. However, increasing amount of added t-BA comonomer does not have any influence on gel phase amount. 3.4. Glass transition temperature determination—DCS analysis DSC analysis of crosslinked adhesive coatings was employed in order to determine the glass transition temperatures (Tg). The Tg of the polymer depends on a number of factors. The most common method to change the polymer Tg is by addition of comonomers in reaction mixture. Polymer Tg exert influence upon the adhesion properties of the particular PSA by determining the softness of the PSA, which is needed for flow of the adhesive and bonding of the PSA with the surface. Monomers, whose polymers have low Tg values provide tack and flexibility of the adhesive while monomers, whose polymers have higher Tg values are used as secondary monomers to enhance cohesion of the adhesive. One of factors that influence the polymer Tg is also the polymer
Table 2 Tg values for synthesized adhesives. Run 1 2 3 4 5 Run 6 7 8 9 Run 10 11 12 13
AIBN (wt%) 0.1 0.2 0.3 0.4 0.5 AA (wt%) 0 5 10.0 15.0 CTA (wt%) 0 0.4 0.7 1
Tg (1C) 49.3 52.3 56.8 66.0 62.7 Tg (1C) 66.9 52.3 45.4 25.0 Tg (1C) – 49.7 52.3 –
Run
t-BA (wt%)
CTA (wt%)
Tg (1C)
14 15 16
6.0 12.0 18.0
0.7 0.7 0.7
– – –
Run
t-BA (wt%)
CTA (wt%)
Tg (1C)
17 18 19
6.0 12.0 18.0
0.3 0.3 0.3
63.2 56.3 49.9
molecular weight. Due to the introduction of CTA in the monomer mixture, the change in Tg may be expected. The results of the Tg measurements are presented in Table 2. In the DSC thermogram only one transition was observed, which indicates that a homogenous composition was synthesized. In case of Tg determination for syntheses, where the polymer molecular weight was changed via variable AIBN and CTA amounts (Runs 1–5
J. Kajtna, M. Krajnc / International Journal of Adhesion & Adhesives 31 (2011) 822–831
and 10–13) a decrease of Tg may be seen. This is likely due to decrease in polymer molecular weight, which influences the polymer Tg. Moreover, when the t-BA comonomer at 0.7 wt% CTA was used (Runs 14–16), the glass temperature transition zone was undefined, which indicates, that the molecular weight was in fact too low for Tg determination. In other syntheses where AA and t-BA comonomers were used (Runs 6–9 and 17–19), results meets our expectations. Homopolymers of both monomers have higher Tg values as compared to 2-EHA, therefore the Tg should increase, which may also be seen from the results in Table 2.
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3.5. Adhesive properties The first set of peel strength measurements in Fig. 9 shows the influence of polymer molecular weight on peel strength. Regardless to the type of additive used in syntheses, a substantial decrease in peel strength can be seen. In fact a partial cohesive failure was observed for synthesis with 1 wt% of CTA. This effect is typical for either under crosslinked coatings, or adhesive coatings consisting of low molecular polymers. This adhesive had also the lowest gel phase amount, as it was shown in Fig. 7.
Fig. 9. Peel strength-syntheses with AIBN (Runs 1–5) and CTA (Runs 11–13).
Fig. 10. Peel strength as a function of AA addition.
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Fig. 11. Peel strength for syntheses with t-BA (Runs 14–19).
In Fig. 10, the influence of AA on peel strength is shown. The peel values increase with increasing AA amount and reaches the maximal value at 15 wt% of AA. This increase of peel strength is likely due to hydrogen bonds effect, which plays an important role in adhesion process. Unfortunately, the viscosity of adhesive with 15 wt% was extremely high as it was shown in Fig. 5. The last set of results in Fig. 11 represents the influence of t-BA comonomer on peel strength. Despite the fact, that the prepolymer viscosity was at low level, the conversion at high level and gel phase amount above 60%, all coatings showed low peel strength with cohesive failure. It can be speculated, that the polymer molecular weights in these syntheses were too low.
4. Conclusions In this study of solventless pressure sensitive adhesives, three different monomers were used: 2-ethyl hexyl acrylate (provides tack and flexibility of the adhesive), acrylic acid (increases the Tg, enhances cohesion and improves adhesion through hydrogen bonds) and t-butyl acrylate (increases the Tg, reduces the prepolymer viscosity). The overall monomer conversion of highly exothermic bulk polymerization procedure for production of solventless PSA reached a level from 77 to almost 93 wt%. The main emphasis of this study was given to study the influence of different parameters on prepolymer viscosity. By addition of CTA agent, the prepolymer viscosity decreases due to the reduction of polymer molecular weight. The same effect was also expected for syntheses with increasing initiator AIBN concentration. Surprisingly, the prepolymer viscosity reached a maximal value at 0.4 wt% of AIBN, presumably due to optimal arrangement of polymer molecules in bulk, accompanied by high hydrogen bond effect. Increased amounts AA exerted exponentially increase of prepolymer viscosity due to hydrogen bonds formation between polymer molecules. Application of t-BA comonomer showed expected effect on prepolymer viscosity. The values remain in the same range regardless to the added amount of t-BA comonomer. The measured amounts of gel phase show, that the gel phase was reduced with increasing amount of AIBN and CTA.
The effect is especially noticeable for syntheses where CTA was added. The reduction in gel phase may be attributed to the less intense polymer molecules entanglement. In case of AA comonomer application the amount of gel phase remained almost unaffected by the increasing amount of AA. It seems that the crosslinking reactions have a predominant effect on gel phase amount as compared to intermolecular hydrogen bond formation. Similar results regarding the gel phase amount were determined also for syntheses with t-BA comonomer. In the DSC thermogram only one transition was observed, which indicates that a homogenous composition was synthesized. In case of Tg determination for syntheses where the polymer molecular weight was changed via variable AIBN and CTA addition a decrease of Tg can be seen. This is likely due to decrease of polymer molecular weight, which influences the polymer Tg. Otherwise, the Tg values increase when AA and t-BA comonomers were used. Results of peel strength measurements reveal, that a substantial decrease in peel strength can be seen when amount of AIBN initiator and CTA additive were increased. The peel values increase with increasing AA amount and reaches the maximal value at 15 wt% of AA. This increase of peel strength occurs due to hydrogen bonds effect. In case of t-BA comonomer, low peel strengths with additional cohesive failure were determined.
References [1] Czech Z. 2-Ethylhexyl acrylate/4-acryloyloxy benzophenone copolymers as UV-crosslinkable pressure-sensitive adhesives. Polym Bull 2004;52:283–8. [2] Czech Z. Solvent-based pressure-sensitive adhesives for removable products. Int J Adhes Adhes 2006;26:414–8. [3] Czech Z. Development of solvent-free pressure-sensitive adhesive acrylics. Int J Adhes Adhes 2004;24:119–25. [4] Czech Z, Wesolowska M. Development of solvent-free acrylic pressuresensitive adhesives. Eur Polym J 2007;43:3604–12. [5] Czech Z. Development in the area of UV-crosslinkable solvent-based pressure-sensitive adhesives with excellent shrinkage resistance. Eur Polym J 2004;40:2221–7. [6] Do HS, Park JH, Kim HJ. UV-curing behavior and adhesion performance of polymeric photoinitiators blended with hydrogenated rosin epoxy methacrylate for UV-crosslinkable acrylic pressure sensitive adhesives. Eur Polym J 2008;44:3871–82.
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[7] Do HS, Kim SE, Kim HJ. Adhesion–current research and application. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 2005. [8] Ozawa T, Ishiwata S, Kano Y. Adhesive properties of ultraviolet curable pressure-sensitive adhesive tape for semiconductor processing (I)—interpretation via rheological viewpoint. Furukawa Rev 2001;20:83–8. [9] Chech Z, Wojciechowicz M. The crosslinking reaction of acrylic PSA using chelate metal acetylacetonates. Eur Polym J 2006;42:2153–60.
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[10] Allen NS. Photoinitiators for UV and visible curing of coatings: mechanisms and properties. J Photochem Photobiol A 1996;100:101–7. [11] Czech Z, Loclair H, Wesolowska M. Photoreactivity adjustment of acrylic PSA. Rev Adv Mater Sci 2007;14:141–50. [12] Czech Z. New copolymerizable photoinitiators for radiation curing of acrylic PSA. Int J Adhes Adhes 2007;27:195–9.
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Solventless UV crosslinkable acrylic pressure sensitive adhesives J. Kajtna a,b, M. Krajnc b,n a b
Aero d.d., Ipavcˇeva ulica 32, 3000 Celje, Slovenia Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aˇskercˇeva cesta 5, 1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Accepted 3 August 2011 Available online 7 August 2011
The synthesis and characterization of solventless acrylic UV crosslinkable pressure sensitive adhesives are presented. Different prepolymers were synthesized using bulk polymerization procedure. The reaction mixture consisted of acrylic monomers (2-ethylhexyl acrylate, acrylic acid and t-butyl acrylate), azobisisobutyronitrile initiator, chain transfer agent n-dodecylmercaptan and unsaturated UV photoinitiator 4-acryloyloxybezophenone, which was copolymerized into polymer backbone. Different formulations were tested and the prepolymer was characterized by viscosity measurements and final monomer conversions. The prepolymers were coated onto PET foil and crosslinked by application of UV light source. Peel adhesion at 1801 on glass plate was measured. Gel phase was determined using the Soxhlet extraction and copolymer glass transition temperatures (Tg) were analyzed by differential scanning calorimetry (DSC). Results showed that the final monomer conversions in highly exothermic bulk polymerization reached a level between 75% and 90%. Prepolymer viscosity was highly influenced by change in polymer molecular weight and by addition of acrylic acid as a comonomer. On the other hand, the viscosity remained at the low level when t-butyl acrylate was used. The amount of gel phase for all adhesives was above 60 wt.%. Peel strength measurements showed decrease in peel strength with decreasing polymer molecular weight and increase of peel, when acrylic acid was used as a comonomer. All adhesive coatings with t-butyl acrylate comonomer showed cohesive failure. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Pressure sensitive Peel Solventless
1. Introduction Due to increased environmental concerns mainly water based adhesives are used in today’s production of self adhesive materials. More recently, additional demands have been placed on adhesives production. They include reductions in volatile organic compounds (VOC), wastewater and reduction of energy. Due to this reason a lot of interest is already placed in the development of new radiation curable pressure sensitive adhesives (PSA), especially ultraviolet (UV) curable PSA. UV PSA technology is subdivided into hot melt UV and room temperature (RT) coatable UV PSAs. Hot melt UV PSAs require heating to facilitate the coating process, while the RT coatable UV PSAs are liquid at room temperature. The latter represents a novelty in the field of PSA adhesives and are therefore very interesting from applied, scientific and research point of view. Their preparation processes, applications and correlations between chemical structure and applied properties are still insufficiently known and understood. RT coatable UV PSAs have low molecular masses, which give low viscosities and enable coating at ambient temperature using conventional roll-coating equipment. The appropriate applied properties of the adhesives can be obtained only after curing reaction, which must be fast and highly efficient.
n
Corresponding author. Fax: þ386 1 24 19 541. E-mail address: [email protected] (M. Krajnc).
0143-7496/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.08.002
The scientific background of radiation curing adhesives and the relationship between chemical composition and adhesive properties is poorly covered considering relatively small number of published scientific articles. According to recent papers, the most work in this field of research covers the synthesis and characterization of UV crosslinkable PSAs synthesized in organic solvents (solution polymerization). Czech [1] synthesized a copolymer of 2-ethylhexyl acrylate and 4-acryloyloxybenzophenone (range from 0.1 to 3 wt%) in ethyl acetate and acetone. He found that the amount of unsaturated photoinitiator, molecular weight and UV reactivity affected the adhesive properties and the viscosity of the product. Czech [2] also published a study dealing with a synthesis of solvent based PSA used for removable products. In the study, the influence of different crosslinker types on adhesive removability is shown. He showed that the increased amount of crosslinkers lowers the peel strength of the adhesive and improves the removability, which was at a constant level during ageing. In another article published by Czech [3] and Czech and Wesolowska [4] the development of solvent free acrylic PSA is presented. Three different methods of acrylic prepolymer synthesis were performed: a polymerization in an extruder, polymerization in reactor with subsequent solvent removal and polymerization of sirup-type PSA directly on the carrier. They found that the most difficult method is polymerization in an extruder but they were able to synthesize a product with acceptable adhesive properties. They also determined the influence of UV light intensity on the adhesive properties. Czech et al. also developed UV
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crosslinkable solvent based PSAs with very low shrinkage [5]. They found, that the used 4-acryloyloxybezophenone photoinitiator was the most efficient among all of the tested photoinitiators. It was also observed, that the increased amount of unsaturated photoinitiator increased the viscosity and polymer molecular weight. The optimal shrinkage performance was observed beneath 0.5 wt% of added photoinitiator. UV induced crosslinking process is much faster as compared to crosslinking with commercial crosslinking agents such as aluminum acetylacetonate or titanium acetylacetonate. Do et al. [6] used an unsaturated photoinitiator (2-(acryloyoxy)ethyl 4-(4-chlorobenzoyl) benzoate) for the syntheses of UV crosslinkable acrylic PSAs. The unsaturated photoinitiator was incorporated into the polymer backbone via the solution polymerization process in ethyl acetate with different monomer ratios. Subsequently they also prepared different blends with hydrogenated rosin epoxy methacrylate (HREM), which was used as a tackifier. They found, that gel fraction increased by increasing the hydrogenated rosin content and by increasing UV dosage. Hydrogenated rosin added in the PSA influenced (retarded) the UV reaction, while tack increased with increasing HREM content. Do et al. [7] evaluated the effect of electron donor comonomer (2-hydroxyethyl methacrylate) on photoreaction efficiency and on adhesive properties (probe tack, peel and shear adhesion failure temperature). They found, that 2-hydroxyethyl methacrylate acted as a good hydrogen donor and caused an increase in photoreaction efficiency. However, at high amounts of added 2-hydroxyethyl methacrylate, tack and peel strength rapidly decreased due to increased crosslinking density. Ozawa et al. [8] investigated the PSA properties of non UV cured and UV cured blends of acrylic adhesive polymer and urethane oligomer. The latter was used as a photo polymerization initiator with peak absorption at 327 nm. They found that peel adhesion and probe tack values of UV irradiated samples were reduced, what was also correlated with increase of Tg and moduli increase (DMA analysis). The low molecular weight acrylic adhesive prepolymers either in solution or in bulk, can be crosslinked by addition of metal chelate additive [9] or by addition of unsaturated UV reactive photoinitiator in the prepolymer mixture. Descriptions of various types of photoinitiators are published in a study by Allen [10]. The photoreactivity of the adhesive prepolymers can be adjusted in different ways [11]. New types of copolymerizable photoinitiators were tested in a research published by Czech [12]. All of the tested photoinitiators contained vinyloxycarbonyl groups, which are also known as organic carbonate or carbamate. In this work, effects of photoinitiator addition on viscosity, molecular weight and on adhesive properties are presented. It is shown, that all of the tested photoinitiators can be used for synthesis of UV crosslinkable PSAs. They have negligible effect on viscosity and molecular weight of the solvent based PSAs. In all cases, tack and peel decrease with increasing amount of added photoinitiator, while cohesion of the adhesive increases with increasing crosslinking density.
2. Experimental procedure 2.1. Materials Monomers 2-ethylhexyl acrylate (2-EHA), acrylic acid (AA), and t-butyl acrylate (t-BA) were purified by conventional methods. They were washed three times with dilute sodium hydroxide solution, then washed three times with distilled water and dried over calcium chloride and then subjected to vacuum distillation under nitrogen. Initiator azobisisobutyronitrile (AIBN), chain transfer agent n-dodecylmercaptan (CTA), unsaturated photoinitiator 4-acryloyloxybezophenone (4-ABF-Chemitec Company) were used in the commercially available form without further purification.
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2.2. Polymerization procedure—prepolymer synthesis The solventless UV crosslinkable adhesives were synthesized in a 250 ml glass reactor equipped with a reflux condenser, an anchor impeller, N2 purge and thermometer. Different formulations were tested and the polymerizations were carried out at equal process parameters. The total amount of reaction mixture was 100 g. The anchor impeller was operating at stirring rate of 100 min 1 and was identical for all experiments. After 30 min of mixing and nitrogen purging, the reaction mixture was heated to 83 1C at heating rate of 2.75 1C/min. Due to highly exothermic effect of bulk polymerization, the temperature of the reaction mixture reached approximately 180 1C. After the exothermic phase of the polymerization, the reaction mixture was stirred at 80 1C for another 60 min. Different formulations were tested (Table 1) in order to determine the influence of different chemical parameters on prepolymer and adhesive coating properties. In the first part of experiments, AA was used as a comonomer. Runs 1–5 were performed to investigate the influence of prepolymer molecular weight and Runs 6–9 to evaluate the influence of AA addition on prepolymer properties. In the third part of experiments (Runs 10–13) amount of CTA was increased at constant level of acrylic acid, again to determine the influence of prepolymer molecular weight on viscosity. The most important prepolymer property is, in this case, the prepolymer viscosity since the viscosity is the key parameter in selection of appropriate coating system. In Runs 14–19, t-BA was used as a comonomer instead of AA, which should influence the prepolymer viscosity due to steric effects of monomers t-butyl group in polymer backbone. Experiments were performed with two different levels of CTA agent (0.3 and 0.7 wt%). 2.3. Prepolymer characterization Viscosities of synthesized prepolymers were measured using Brookfield viscosimeter with spindle no. 3 at 20 1C. The overall gravimetrical monomer conversion monitoring was performed for synthesis Run 1. Ten samples (each 2 g) were removed from the reaction mixture at different stages of polymerization. The removed samples were immediately cooled and then first dried in Table 1 Reaction mixture formulations. Run
Syntheses with acrylic acid (AA) as comonomer 2-EHA (wt%)
AA (wt%)
AIBN (wt%) CTA (wt%)
4-ABF (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13
93.3 93.3 93.3 93.3 93.3 98.1 93.1 88.1 83.1 93.8 93.4 93.1 92.8
5.0 5.0 5.0 5.0 5.0 0 5.0 10.0 15.0 5.0 5.0 5.0 5.0
0.1 0.2 0.3 0.4 0.5 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Run
Syntheses with t-butylacrylate (t-BA) as comonomer
14 15 16 17 18 19
0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0 0.4 0.7 1
2-EHA (wt%)
t-BA (wt%)
AIBN (wt%) CTA (wt%)
4-ABF (wt%)
92.1 86.1 80.1 92.5 86.5 80.5
6.0 12.0 18.0 6.0 12.0 18.0
0.2 0.2 0.2 0.2 0.2 0.2
1.0 1.0 1.0 1.0 1.0 1.0
0.7 0.7 0.7 0.3 0.3 0.3
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vacuum dryer at room temperature for 12 h, and then at 70 1C for 12 h. Final conversion of monomers for all syntheses was determined gravimetrically by drying 2 g of PSA in vacuum dryer at 70 1C for 24 h. 2.4. Adhesive coating characterization The samples for Soxhlet extraction were prepared by UV crosslinking under two 9 W lamps, where approximately 2 g of synthesized prepolymer was exposed to UV light for 72 h. The amount of formed gel in all syntheses was determined by Soxhlet extraction with tetrahydrofuran (Merck) under reflux during 24 h. Synthesized prepolymer mixtures were coated on PET foil using a Mayer bar coating system (wire thickness was 0.3 mm). The coating weight of adhesive was approximately 2072 g/m2. Then, the coatings were subjected to UV light (medium pressure Hg vapor lamp, 400 W/in). For the adhesive performance characterization of the produced PSAs, the peel adhesion at 1801 (FTM 1—Finat test method 1) was used. DSC analyzing technique with Mettler Toledo DSC821 was employed to determine Tg value. Standard 40 ml alumina DSC pans with perforated lids were used and the samples were subjected to two heating-cooling cycles from 100 to 10 1C at heating rate of 10 1C/min along with an empty reference pan in a DSC furnace. The result of the second cycle was used for determination of Tg while the first cycle was used only to remove the previous thermal history.
3. Results and discussion
polymerizations, where no solvents are added in the reaction mixture. Due to increased viscosity, the mobility of macro radicals at higher monomer conversions becomes limited. The rate of termination decreases while the rate of propagation remains at the same level and the termination process becomes diffusion controlled, which also causes increase in polymerization temperature. The increase in polymerization rate and increase of polymer kinetic chain length may lead to autoacceleration or gel effect, which is quite common phenomenon for radical polymerization processes. The diffusion controlled regime of termination process followed by the diffusion controlled propagation process at high conversion leads also to limited monomer conversion. As may be seen in Fig. 2, final overall monomer conversion reached approximately 80%. The time plot of overall monomer conversion shows typical conversion curve for vinyl monomers undergoing radical polymerization at high monomer concentrations. The highest polymerization rate was in range between 6 and 10 min, which also coincides with the highest measured polymerization temperature. The results of gravimetrically determined final overall monomer conversion for all other syntheses are presented in Fig. 3. As can be seen in Fig. 3 all final monomer conversions are in range from 80% to 90%. No evident trend in change of overall conversions can be seen from the results. The level of overall conversion for all syntheses was quite high. The desired overall monomer conversion for PSA applications should be as high as possible in order to prevent unwanted low molecular constituents migration to substrate-adhesive interface layer.
3.1. Prepolymer synthesis 3.2. Prepolymer viscosity Due to the absence of solvents in reaction mixture, all syntheses showed high exothermic effect as expected. Maximal values of polymerization temperatures were in range from 180 to almost 200 1C. Time plots of temperature reaction mixture monitoring for some of the syntheses are presented in Fig. 1. During the propagation stage, the polymer molecular weight increases, which also leads to increase in reaction mixture viscosity. This phenomenon is especially noticeable in bulk
The main emphasis of this part of the work was given to study the influence of different parameters on prepolymer viscosity. The first set of experiments was performed to determine the effect of initiator (AIBN) and chain transfer agent (CTA) concentration on prepolymer viscosity. Both additives influence polymer molecular weight, hence the prepolymer viscosity. The results are gathered in Fig. 4.
Fig. 1. Time plot of reaction mixture temperatures.
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Fig. 2. Overall monomer conversion time plot for Run 1.
Fig. 3. Final overall monomer conversion for Runs 1–19.
The prepolymer viscosity in case of synthesis with 0 wt% of CTA was too high to measure. Regarding the influence of CTA additive on viscosity, following results were adopted. The viscosity decreased as a function of increased CTA additive, which was also expected. The polymer molecular weight as well as polymer kinetic chain length decreased with increasing CTA concentration, which influences the entanglement of polymer molecules in bulk and decreases the viscosity. Similar results were also expected for syntheses with variable AIBN concentrations. Surprisingly, the viscosity reached the maximal value at 0.4 wt% of AIBN. The
prepolymer viscosity should decrease with increasing AIBN concentration due to reduction of polymer molecular weight. Beside the influence of polymer entanglement on prepolymer viscosity, the formation of intermolecular hydrogen bonds (addition of acrylic acid—AA) has an influence on viscosity. It can be speculated, that at 0.4 wt% of AIBN the kinetic polymer chain length reaches a level, which enables optimal arrangement of polymer molecules in bulk, accompanied by high hydrogen bond effect. At higher concentrations of AIBN the effect of shorter kinetic chain length prevails, which also lowers the viscosity similar as with
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Fig. 4. Prepolymer viscosity as a function of AIBN and CTA concentration (Runs 1–5 and 10–13).
Fig. 5. Prepolymer viscosity for syntheses with acrylic acid—AA (Runs 6–9).
CTA addition. The effect of hydrogen bonds formation on viscosity is especially noticeable for syntheses with increasing AA addition (Runs 6–9), which are presented in Fig. 5. Plot of viscosity in Fig. 5 reveals exponential increase of viscosity with increasing AA addition. Again, this effect may be attributed to the hydrogen bonds formation between polymer molecules due to increased amount of AA in the reaction mixture. In syntheses 14–19, t-BA monomer was used as comonomer in polymerization procedure in order to decrease prepolymer viscosity. t-BA functional group in polymer should reduce the
viscosity. The syntheses were performed at two different CTA concentrations (0.7 and 0.3 wt%) again to evaluate the effect of polymer molecular weight on viscosity. As can be seen in Fig. 6, the application of t-BA comonomer on viscosity meets our expectations. The prepolymer viscosity is almost independent from the added amount of t-BA comonomer. Because of the spacious group in the acrylic backbone, the polymer chains cannot entangle strongly, which results in a lower prepolymer viscosity as compared to AA comonomer (Fig. 5). On the other hand, the effect of
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Fig. 6. Prepolymer viscosity for syntheses with t-BA comonomer.
Fig. 7. Gel phase amount as a function of AIBN and CTA addition.
reduced polymer molecular weight on viscosity can be seen. The higher addition of CTA reduces the polymer molecular weight, which is exerted in lower prepolymer viscosity. The latter set of results supplements the results in Fig. 4, where the amount of CTA reagent was changed in 2-EHA and AA monomer combination. 3.3. Gel phase determination The amount of gel phase in crosslinked adhesive coating is a function of polymer molecular weight, molecular entanglement,
crosslinking density and intermolecular hydrogen bonds. The crosslinking density in this study does not influence the amount of gel phase due to equal amounts of 4-ABF photoinitiator and identical UV crosslinking parameters. The first set of results in Fig. 7 represents the influence of polymer molecular weight on gel phase amount, which derive from variable addition of CTA and AIBN. The measured amounts of gel phase in Fig. 7 show, that the gel phase was reduced with increasing amount of both additives. Note, that the amount of gel phase for synthesis 10 (0 wt% of CTA)
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Fig. 8. Gel phase amount for syntheses with AA and t-BA at 0.7 and 0.3 wt%.
was not measured because of the high prepolymer viscosity. The reduction in gel phase may be attributed to the less intense polymer molecules entanglement, which caused better extraction to THF solvent. The next set of results gathered in Fig. 8 represents the influence of comonomers on gel phase amount. It was expected that in case of AA addition and increased amount of hydrogen bonds, higher gel phase amount would be measured. It appears, that effect of hydrogen bond formation in coating does not influence on the amount of gel phase. The crosslinking reactions have a predominant effect on gel phase amount as compared to intermolecular hydrogen bond formation, since the amount of gel phase remained almost unaffected by the increasing amount of AA. Similar results were determined also for syntheses with t-BA comonomer. Again, the effect of high and low amount of CTA agent and subsequent reduction of polymer molecules entanglement on gel phase may be seen. Measured values of gel phase at coatings prepared from prepolymer with 0.3 wt% of CTA are approximately 15% lower as those from prepolymer synthesized with 0.7 wt%. However, increasing amount of added t-BA comonomer does not have any influence on gel phase amount. 3.4. Glass transition temperature determination—DCS analysis DSC analysis of crosslinked adhesive coatings was employed in order to determine the glass transition temperatures (Tg). The Tg of the polymer depends on a number of factors. The most common method to change the polymer Tg is by addition of comonomers in reaction mixture. Polymer Tg exert influence upon the adhesion properties of the particular PSA by determining the softness of the PSA, which is needed for flow of the adhesive and bonding of the PSA with the surface. Monomers, whose polymers have low Tg values provide tack and flexibility of the adhesive while monomers, whose polymers have higher Tg values are used as secondary monomers to enhance cohesion of the adhesive. One of factors that influence the polymer Tg is also the polymer
Table 2 Tg values for synthesized adhesives. Run 1 2 3 4 5 Run 6 7 8 9 Run 10 11 12 13
AIBN (wt%) 0.1 0.2 0.3 0.4 0.5 AA (wt%) 0 5 10.0 15.0 CTA (wt%) 0 0.4 0.7 1
Tg (1C) 49.3 52.3 56.8 66.0 62.7 Tg (1C) 66.9 52.3 45.4 25.0 Tg (1C) – 49.7 52.3 –
Run
t-BA (wt%)
CTA (wt%)
Tg (1C)
14 15 16
6.0 12.0 18.0
0.7 0.7 0.7
– – –
Run
t-BA (wt%)
CTA (wt%)
Tg (1C)
17 18 19
6.0 12.0 18.0
0.3 0.3 0.3
63.2 56.3 49.9
molecular weight. Due to the introduction of CTA in the monomer mixture, the change in Tg may be expected. The results of the Tg measurements are presented in Table 2. In the DSC thermogram only one transition was observed, which indicates that a homogenous composition was synthesized. In case of Tg determination for syntheses, where the polymer molecular weight was changed via variable AIBN and CTA amounts (Runs 1–5
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and 10–13) a decrease of Tg may be seen. This is likely due to decrease in polymer molecular weight, which influences the polymer Tg. Moreover, when the t-BA comonomer at 0.7 wt% CTA was used (Runs 14–16), the glass temperature transition zone was undefined, which indicates, that the molecular weight was in fact too low for Tg determination. In other syntheses where AA and t-BA comonomers were used (Runs 6–9 and 17–19), results meets our expectations. Homopolymers of both monomers have higher Tg values as compared to 2-EHA, therefore the Tg should increase, which may also be seen from the results in Table 2.
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3.5. Adhesive properties The first set of peel strength measurements in Fig. 9 shows the influence of polymer molecular weight on peel strength. Regardless to the type of additive used in syntheses, a substantial decrease in peel strength can be seen. In fact a partial cohesive failure was observed for synthesis with 1 wt% of CTA. This effect is typical for either under crosslinked coatings, or adhesive coatings consisting of low molecular polymers. This adhesive had also the lowest gel phase amount, as it was shown in Fig. 7.
Fig. 9. Peel strength-syntheses with AIBN (Runs 1–5) and CTA (Runs 11–13).
Fig. 10. Peel strength as a function of AA addition.
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Fig. 11. Peel strength for syntheses with t-BA (Runs 14–19).
In Fig. 10, the influence of AA on peel strength is shown. The peel values increase with increasing AA amount and reaches the maximal value at 15 wt% of AA. This increase of peel strength is likely due to hydrogen bonds effect, which plays an important role in adhesion process. Unfortunately, the viscosity of adhesive with 15 wt% was extremely high as it was shown in Fig. 5. The last set of results in Fig. 11 represents the influence of t-BA comonomer on peel strength. Despite the fact, that the prepolymer viscosity was at low level, the conversion at high level and gel phase amount above 60%, all coatings showed low peel strength with cohesive failure. It can be speculated, that the polymer molecular weights in these syntheses were too low.
4. Conclusions In this study of solventless pressure sensitive adhesives, three different monomers were used: 2-ethyl hexyl acrylate (provides tack and flexibility of the adhesive), acrylic acid (increases the Tg, enhances cohesion and improves adhesion through hydrogen bonds) and t-butyl acrylate (increases the Tg, reduces the prepolymer viscosity). The overall monomer conversion of highly exothermic bulk polymerization procedure for production of solventless PSA reached a level from 77 to almost 93 wt%. The main emphasis of this study was given to study the influence of different parameters on prepolymer viscosity. By addition of CTA agent, the prepolymer viscosity decreases due to the reduction of polymer molecular weight. The same effect was also expected for syntheses with increasing initiator AIBN concentration. Surprisingly, the prepolymer viscosity reached a maximal value at 0.4 wt% of AIBN, presumably due to optimal arrangement of polymer molecules in bulk, accompanied by high hydrogen bond effect. Increased amounts AA exerted exponentially increase of prepolymer viscosity due to hydrogen bonds formation between polymer molecules. Application of t-BA comonomer showed expected effect on prepolymer viscosity. The values remain in the same range regardless to the added amount of t-BA comonomer. The measured amounts of gel phase show, that the gel phase was reduced with increasing amount of AIBN and CTA.
The effect is especially noticeable for syntheses where CTA was added. The reduction in gel phase may be attributed to the less intense polymer molecules entanglement. In case of AA comonomer application the amount of gel phase remained almost unaffected by the increasing amount of AA. It seems that the crosslinking reactions have a predominant effect on gel phase amount as compared to intermolecular hydrogen bond formation. Similar results regarding the gel phase amount were determined also for syntheses with t-BA comonomer. In the DSC thermogram only one transition was observed, which indicates that a homogenous composition was synthesized. In case of Tg determination for syntheses where the polymer molecular weight was changed via variable AIBN and CTA addition a decrease of Tg can be seen. This is likely due to decrease of polymer molecular weight, which influences the polymer Tg. Otherwise, the Tg values increase when AA and t-BA comonomers were used. Results of peel strength measurements reveal, that a substantial decrease in peel strength can be seen when amount of AIBN initiator and CTA additive were increased. The peel values increase with increasing AA amount and reaches the maximal value at 15 wt% of AA. This increase of peel strength occurs due to hydrogen bonds effect. In case of t-BA comonomer, low peel strengths with additional cohesive failure were determined.
References [1] Czech Z. 2-Ethylhexyl acrylate/4-acryloyloxy benzophenone copolymers as UV-crosslinkable pressure-sensitive adhesives. Polym Bull 2004;52:283–8. [2] Czech Z. Solvent-based pressure-sensitive adhesives for removable products. Int J Adhes Adhes 2006;26:414–8. [3] Czech Z. Development of solvent-free pressure-sensitive adhesive acrylics. Int J Adhes Adhes 2004;24:119–25. [4] Czech Z, Wesolowska M. Development of solvent-free acrylic pressuresensitive adhesives. Eur Polym J 2007;43:3604–12. [5] Czech Z. Development in the area of UV-crosslinkable solvent-based pressure-sensitive adhesives with excellent shrinkage resistance. Eur Polym J 2004;40:2221–7. [6] Do HS, Park JH, Kim HJ. UV-curing behavior and adhesion performance of polymeric photoinitiators blended with hydrogenated rosin epoxy methacrylate for UV-crosslinkable acrylic pressure sensitive adhesives. Eur Polym J 2008;44:3871–82.
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[7] Do HS, Kim SE, Kim HJ. Adhesion–current research and application. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 2005. [8] Ozawa T, Ishiwata S, Kano Y. Adhesive properties of ultraviolet curable pressure-sensitive adhesive tape for semiconductor processing (I)—interpretation via rheological viewpoint. Furukawa Rev 2001;20:83–8. [9] Chech Z, Wojciechowicz M. The crosslinking reaction of acrylic PSA using chelate metal acetylacetonates. Eur Polym J 2006;42:2153–60.
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[10] Allen NS. Photoinitiators for UV and visible curing of coatings: mechanisms and properties. J Photochem Photobiol A 1996;100:101–7. [11] Czech Z, Loclair H, Wesolowska M. Photoreactivity adjustment of acrylic PSA. Rev Adv Mater Sci 2007;14:141–50. [12] Czech Z. New copolymerizable photoinitiators for radiation curing of acrylic PSA. Int J Adhes Adhes 2007;27:195–9.