High solids solution acrylics: Controlled architecture hybrid crosslinking pressure sensitive adhesives

Transcription

1 High solids solution acrylics: Controlled architecture hybrid crosslinking pressure sensitive adhesives Christopher L. Lester, Ph.D. Performance Adhesive Center, Avery Dennison William L. Bottorf, Performance Adhesive Center, Avery Dennison Kyle R. Heimbach, Performance Adhesive Center, Avery Dennison

2 Abstract As a continuation of work reported at the 2009 PTC Technical eminar we report herein the synthesis of acrylic polymers with controlled molecular weight, architecture and placement of reactive functional groups. In particular, acrylic polymers useful as pressure sensitive adhesives are described that utilize hybrid cross-linking technology. The hybrid cross-linking technology described is acid metal chelate used in conjunction with alkoxysilane sol-gel reactions. The influence of type, amount, and placement of alkoxy-silane functionalities on viscoelastic properties and corresponding pressure sensitive adhesive attributes are discussed. Additionally, hybrid cross-linking, controlled architecture pressure sensitive adhesives are reported with varying glass transition temperature and solubility parameter. The novel controlled architecture acrylic polymers allow for the development of high solids solution adhesives at low viscosities and 100% solids warm melt compositions with processable rheology. Furthermore, the controlled architectured polymers display enhancements in adhesive performance relative to random copolymers of the same composition.

3 Introduction Polymer architecture and micro-structure have been shown historically to dramatically influence material and, in particular, adhesive properties. Control of acrylic polymer architecture and micro-structure has largely consisted of modulating molecular weight and branching through polymerization temperature, initiator type, and in-process monomer concentration. Also, some ability to modulate composition spatially along the polymer chain could be afforded via selection of monomers with reactivity ratios different from the primary backbone monomers. While a considerable span of adhesive performance can be attained with the aforementioned methods, much finer controls are possible with different polymerization techniques. Controlling polymer architecture in a finer sense has been a subject of significant research over the past fifty years. It has been demonstrated widely in the literature that exerting finer control over the polymer architecture results in different and often enhanced adhesive performance. In some cases, countercurrent properties can often be decoupled. Previously reported architectures include block copolymers, telechelic polymers, and random polymers of controlled molecular weight. While, the aforementioned architectures all provide unique properties, they also have disadvantages. Random copolymers either require high molecular weight to attain certain balances of properties or require high degrees of cross-linking which can yield a poor balance of properties. Telechelic polymers by definition have reactive functional groups placed exactly at the end-groups and nowhere else in the backbone. The functional groups then serve solely to increase linear molecular weight and/or form networks in which free polymer chain ends are eliminated. Telechelic polymers consequently yield high strength elastomeric materials but lack the viscous liquid character critical to pressure sensitive adhesive (PA) performance and require further formulation for good PA characteristics. Phase separated block copolymers when formulated appropriately are known to yield a wide range of adhesive performances. However, due to the nature of the physical cross-links in phase separated systems the thermal and solvent resistance can be poor.

4 In approximately the last 20 years, a variety of pseudo-living or controlled radical polymerization techniques have been developed to afford good architectural control of (meth)acrylic monomers. These techniques are more tolerant to a wider variety of functional groups when compared to living anionic, cationic, and catalytic techniques. A substantial amount of fundamental research has been performed to understand these types of polymerization and a thorough review has been edited by Matyjewski. 1 Reversible Addition Fragmentation chain Transfer (RAFT) polymerization is one such technique that has been shown to work exceedingly well with a wide variety of (meth)acrylic monomers yielding excellent control of molecular weight and polydispersity. 2 The RAFT mechanism for controlled polymerization is well understood and reported extensively. 1-3 It was previously reported at the 2009 PTC Technical eminar that controlled placement of cross-linkable functional groups could be readily afforded by controlled radical polymerization. 4 These novel polymers allowed for the ability to synthesize polymers to be of modest to low molecular weight and correspondingly to display low solution viscosities at high solids content and to also display low viscosities in the melt. In addition to the desirable solution and melt properties, it was found that the performance of the resulting adhesives was comparable to high molecular weight controls and in some cases the adhesive performance was markedly improved. This study details the synthesis of controlled architecture acrylic polymers with controlled placement of reactive alkoxy-silane functionalities. These types of polymers are described as hybrid cross-linking pressure sensitive adhesives. The influence of the type and amount of alkoxy-silane monomers is described with regards to visco-elastic properties and corresponding pressure sensitive adhesive performance. Formulated systems using the hybrid cross-linked materials are described as well as polymers in which the glass transition and solubility parameter have been varied to modulate performance. Experimental

5 Base acrylic esters such as 2-Ethylhexyl Acrylate (EHA), Butyl Acrylate (BA), Acrylic Acid (AA) and Isobornyl Acrylate (IBA) were obtained from various commercial suppliers and used as received. Methacryloxypropyl Tri-methoxysilane (MPtM) and Methacryloxymethyl Tri-ethoxysilane (MMtE) were all obtained from Wacker Chemical and used as is. Dibenzyl trithiocarbonate (DBTTC) was obtained from Arkema France and used as received and is shown in cheme 1. Also in cheme 1 is a depiction of how cheme 1. Chemical structure of dibenzyl trithiocarbonate (DBTTC) and polymers after a single monomer addition followed by a subsequent monomer addition. monomers are incorporated upon sequential addition. All of the polymerizations were initiated with Azobis(isobutyronitrile) (AIBN). The polymers were all made in organic solvents, most typically Ethyl Acetate. Unless otherwise stated all of the polymers were formulated with aluminum acetoacetonate (AAA) at 0.5% by weight based on polymer solids. All samples were coated at approximately 2.0 mil adhesive thickness onto 2.0mil mylar. The coatings were all air dried for 10 minutes and placed in a forced air oven for

6 5 minutes at 130 o C and closed with 100% solids platinum cured silicone paper liner. The laminates were all aged in a controlled climate room for 24 hours prior to testing. Molecular weights were measured using a Polymer tandards ervices GPC outfitted with a refractive index detector and calibrated using polystyrene standards. olution viscosities were measured using a Brookfield RVT viscometer. pindle and spindle speeds were selected such that a torque value of 40-80% was achieved for optimal accuracy. Dynamic mechanical analysis (DMA) was performed on a TA Instrument AR-1000 rheometer using parallel plate clamps. 1.0mm thick samples were placed in the clamp and annealed at 75 o C for 10 minutes to ensure good adhesion. The samples were then cooled to -80 o C for 10 minutes and ramped at 3 o C per minute up to 150 o C. During the temperature ramp the sample was oscillated at a frequency of 10 rad/s. Unless otherwise noted, the following test methods were used for evaluating the adhesive properties of the acrylic polymers. PA PERFRMANCE TET METHD Test Condition 180 Peel a, b, 15 Minute Dwell 72 Hour Dwell hear trength c hear Adhesion Failure Temp.(AFT) d (a) (b) (c) (d) Peel, sample applied to a stainless steel panel with a 5 pound roller with 1 pass in each direction. amples conditioned and tested at 23 C. Peel, sample applied to a high-density polyethylene or polypropylene panel with a 5 pound roller with 5 passes in each direction. amples conditioned and tested at 23 C. hear: 1 kg weight with a 1/2 inch by 1/2 inch overlap. ample applied to a stainless steel panel with a 10 pound roller with 5 passes in each direction. amples conditioned and tested at 23 C. AFT: 1000 gram weight, 1 inch by 1 inch overlap (2.2 pounds/square inch). ample applied to a stainless steel panel with a 10 pound roller with 5 passes in each direction. amples conditioned for 1 hour at 23 C and 15 minutes at 40 C. Temperature increased by 0.5 C/min. until failure.

7 Results and Discussion cheme 2 depicts polymer architectures that are possible through the use a. b. c. Alkoxy- ilane Alkoxy- ilane Alkoxy cheme 2. Varying RAFT/DBTTC mediated architectures including: a. Random, b. end functional acid, and c. end functional alkoxy-silane. of well controlled RAFT polymerizations. These architectures were reported previously, but briefly it was found that segregating the cross-linkable functionalities such as carboxylic acids or alkoxysilane moities can afford dramatically different material properties that can be very desirable for pressure sensitive adhesives. 4,5 In particular it has been shown that low molecular weight architectured polymers are advantageous for processing in that they can be at high concentrations in organic solvents at a low viscosity or even melt processable in the absence of solvent. In addition to being advantageous for processing, the low molecular weight architectured polymers were found to yield pressure sensitive adhesive performance comparable to high molecular weight low solids analogues. It was also previously reported that placing cross-linkable sites that react

8 independently from other cross-linkable functional groups can provide significant performance enhancements in addition to having enhanced processability through lower molecular weight. This type of system is shown in cheme 3 in which alkoxy-silane groups are positioned in the end regions of a Alkoxy- ilane Alkoxy- ilane CH 3 CH 3 CH 3 Water 2 i CH i 3 Lewis acid i + CH3 CH 3 CH 3 CH 3 cheme 3. Depiction of Hybrid-crosslinked RAFT derived architectured PA. pressure sensitive adhesive acid containing random copolymer. This polymer can be cross-linked with AAA which also serves as a Lewis acid catalyst for the sol-gel condensation reaction of the alkoxy-silane moieties. This type of material was previously compared to a commercial random copolymer of the same composition. To expand on this work, additional copolymer controls were made and characterized. In all cases, identical copolymer compositions consisting of 2-EHA, BA and acrylic acid were used and the architecture and presence of alkoxy silane monomers was varied. Table 1 details the various polymers molecular weight, solids, and solution viscosities. All of the RAFT derived materials exhibit similar measured molecular weights with narrow polydispersities which is indicative of a well controlled polymerization. As a result of the molecular weights being fairly low, the solids and viscosities of these polymers are all

9 Table 1 51 EHA 45 BA 4 AA Type RAFT Architectured RAFT Random Commercial Control Commercial Control MPtM Y N Y N Mn 80, ,531 Mw 127, ,961 PDI olids olution Viscosity 14,000cps 11600cps 4700cps 5,000cps >67.0% and less than 14,000 cps. The commercial controls are higher in molecular weight with broad polydisperities and correspondingly display lower solids contents to be at a reasonable viscosity. Figure 1 is a plot of storage modulus as a function of temperature for the different polymers described in Table 1. All of the polymers exhibit identical glass transition temperatures (T g ) because of the identical base compositions. However, at temperatures above the T g there are marked differences between the materials. The RAFT polymer with MPtM displays a very flat plateau modulus while the RAFT copolymer 1.000E E E8 RAFT with MPtM RAFT no MPtM Commerical with MPtM Commerical no MPtM 1.000E E6 G' (dyne/cm^2) 1.000E Temperature ( C) Figure 1. torage modulus as a function of temperature for EHA/BA/AA copolymers of varying architecture and MPtM amount. without MPtM does not to the extent that the material actually displays some flow

10 characteristics at elevated temperature. The commercial control without MPtM displays similar behavior to that of the silane-free RAFT polymer but with overall higher modulus as a function of temperature which results from the materials higher molecular weight. When adding an equivalent amount of MPtM, the commercial control displays a flat plateau modulus but with overall higher values than the RAFT polymer containing MPtM. The pressure sensitive adhesive performance is very reflective of the DMA data as can be seen in Table 2. For example, the RAFT copolymer containing alkoxy-silane monomer exhibits Table 2 51 EHA 45 BA 4 AA Type RAFT RAFT Commercia Commercial Control l Control MPtM Y N Y N tainless teel 15 min Dwell (Lbs/in) tainless teel 72 hr Dwell (Lbs/in) Polypropylene Lbs/in) AFT, 1kg/q. In (Failure Temp o C) hear, 2kg/ q. In (Failure Time, Mins) > > adhesive adhesive 41.0 what could be characterized as the best overall balance of PA performance. It displays high ultimate adhesion to stainless steel, moderate adhesion to polypropylene, with relatively high shear values and >200 o C AFT. The RAFT copolymer without alkoxysilane is a low strength material that displays significant failures in peel adhesion coupled with low shear and AFT values. The commercial control without alkoxy-silane was better performing than the RAFT analogue in that it displayed high adhesion values but it still displayed lower shear and AFT values. The commercial control with alkoxy-silane is a high material in that it has high AFT values and shears that did not fail ly but did not display the high adhesion values of the RAFT copolymer containing alkoxy-silane. This is a result of the random incorporation

11 of the MPtM in the polymer back-bone which would produce lower molecular weight between cross-links that yields an overall higher modulus. Also it is important to note that even if there were equivalent performance between the RAFT and commercial control analogues there would still remain the processing advantage of the RAFT materials afforded by high solids at coatable viscosities. It is important to note that the coating and drying conditions for MPtM containing polymers were 10 minutes at 130 o C in a forced air oven in order to ensure complete cure. While it is possible to reach these kinds of conditions in some coating assets it may be difficult in others. Additionally, when coating thermally-sensitive substrates the high temperatures may present difficulties. cheme 4 displays various alkoxy-silane methacrylate monomers and relative a. Increasing Reactivity b. cheme 4. Reactivity of varying alkoxy-silane monomers: a. Methacryloxypropyltrimethoxysilane (MPtM), b. Methacryloxymethyl triethoxysilane (MMtE) propyl analogues. hown in Figure 2 is a plot of Williams Plasticity Index (WPI) as a function of temperature for MPtM and MMtE containing pressure sensitive adhesives. The MPtM containing PA exhibits substantially lower plasticities at all temperatures when compared to the MMtE containing PAs. The MMtE exhibits higher plasticities and of note is the flatter response to

12 Williams Plasticity Index 5 MPtM 4.5 MMtM Figure 2. Williams Plasticity as a function of drying temperature for varying alkoxysilane 1.5 monomer Temp (oc) temperature over the MPtM materials. Figure 3 is a plot of storage modulus as a function of temperature for the PAs with varying alkoxy-silane types and one can see that the materials when fully cured are remarkably similar. The similar rheology is

13 manifested in the PA performance displayed in Table 3 with the 1.000E E 8 MPtM MMtE DE V -8670A DE V -8670A New ilane 1.000E 7 G' (Pa) 1.000E E temperature ( C) Figure 3. torage modulus as a function of temperature for EHA/BA/AA RAFT polymers with varying alkoxy-silane monomers.

14 Table 3 51 EHA 45 BA 4 AA Type MPtM MMtE tainless teel 15 min Dwell (Lbs/in) tainless mixed teel 72 hr Dwell (Lbs/in) Polypropylene Lbs/in) AFT, 1kg/q. In (Failure Temp o C) hear, 1kg/0.25 q. In (Failure Time, Mins) >200 > adhesive 135 adhesive primary difference observed in slightly lower peel performance of the MMtE containing PA which is attributable to a slightly higher modulus resulting in mixed adhesive/ failure modes. This difference in peel can be modulated in a variety of ways including varying the MMtE content as well as varying AAA cross-linker level in the same fashion one would modify a standard solution acrylic. In order to demonstrate how to tune the performance of these types of PAs, a series of RAFT polymers was made of the same composition described previously in which the statistical number of MMtE was varied from per end region. The molecular weights and physical characteristics of the wet adhesives are shown in Table 4. The molecular weights and polydispersities are all approximately the same. Correspondingly, the solids and solution viscosities are similar in that they are all >68% and <15000cps. It should be noted that at higher levels of MMtE some increase in polydispersity occurred which resulted in higher viscosities. The higher polydispersity and corresponding increase in viscosity is likely due to some reaction of the MMtE during the polymerization. Figure 4 is a plot of storage modulus as a function of MMtE monomers per chain end. All of the samples display the same glass transition temperature that

15 Table 4 RAFT Architectured 51EHA/45BA/4AA # MMtE Mn Mw PDI olids Viscosity one would expect from polymers of the same composition but differ markedly in the rubbery plateau. In every case the rubbery plateau is extremely flat with the modulus increasing as the number of MMtE monomers increases. Interestingly, upon raising the MMtE level from 2 to 2.5 per end region the modulus stays the same. This means that there are very few unfunctionalized chain ends. The 1.000E E G ' ( d yn e /cm ^ 2 ) 1.000E E E E temperature ( C) Figure 4. torage modulus as a function of temperature for EHA/BA/AA RAFT polymers with varying amount of MMtE per polymer end region.

16 dramatic difference in rheology can be observed in the pressure sensitive adhesive performance. Table 5 displays broad range of adhesive performance can be attained by modulating the theoretical number of MMtE per chain. For Table 5 51 EHA 45 BA 4 AA # MMtM tainless teel 15 min Dwell (Lbs/in) tainless teel 72 hr Dwell (Lbs/in) Polypropylene (Lbs/in) AFT, 1kg/q. In (Failure Temp o C) hear, 1kg/0.25 q. In (Failure Time, Mins) mixed zip >200 >200 >200 > Mixed 155 Mixed 224 Adhesive 195 Adhesive Example, at lower levels of MMtE the adhesives exhibit permanent PA performance profiles in that the materials are very high in peel adhesion with failures. In particular, the 0.5 MMtE per end material displays permanent adhesive performance but is accompanied by lower shear and AFT values. The increase to 1.0 MMtE per end yielded lower initial peel values but higher ultimate peels with greatly enhanced shear and AFTs. Further increasing the MMtE level then results in reductions in peel performance and higher strength materials as evidenced by higher shear values. It is important to note that the shear values are not dramatically higher for samples with higher MMtE levels but this is due not to a lack of strength but from a shift in failure mode from to adhesive. Ultimately, at the highest levels of MMtE one can observe performance that is characteristic of removable adhesives.

17 The high strength of the higher MMtE levels would lend itself to formulation with tackifiers. In Figure 5, storage modulus as a function of temperature is plotted for the 2 MMtE polymer that has been tackified with two levels of a hydrogenated rosin ester. A classic tackifier response is observed in that the glass transition temperature has been increased with an accompanying lowering of plateau modulus. Naturally the magnitude of change in the rheology 1.000E E9 0% 20% 40% G ' ( d yn e /cm ^ 2 ) 1.000E E E E temperature ( C) Figure 5. torage modulus as a function of temperature for EHA/BA/AA polymer with 2 MMtE per end with varying level of rosin ester tackifier. is a direct function of level of tackier. The PA performance of the formulated adhesives is shown in Table 6. As one would expect the peel adhesions of the tackified systems are substantially higher relative to the unformulated base polymer. In particular the adhesion to olefin substrates with polypropylene used as an example is dramatically improved. In addition to the greatly enhanced peel adhesions what is remarkable to note is that the materials exhibit excellent shear values that is a result of the high strength of the base polymer.

18 Table 6 51EHA/45BA/4AA % Tackifier 0% 20% 40% tainless teel 15 min Dwell (Lbs/in) tainless teel 72 hr Dwell (Lbs/in) HDPE (Lbs/in) Polypropylene (Lbs/in) AFT, 1kg/q. In (Failure Temp o C) hear, 1kg/0.25 q. In (Failure Time, Mins) >200 > adhesive 597 mixed In order to modulate adhesion of these systems on various substrates without the use of tackifiers one common approach is to use T g modifying monomers and in particular monomers that are lower in solubility parameter. In this case polymers are made of the architecture described previously but are varied in composition to yield a copolymers with a calculated Fox T g s of -50 o C and -40 o C. In this circumstance the polymer composition was primarily 2-EHA with the high T g low solubility parameter monomer IBA being used. In this case, 0.5 MMtE per end was utilized in order to generate materials that would be permanent high peel adhesives. These materials are directly comparable to the EHA BA copolymer with 0.5 MMtE per end described previously. Table 7 displays the ultimate peel adhesions of these materials on stainless steel and a variety of plastic substrates. The control material displayed moderately high peels on stainless steel and the more polar plastics with relatively low shear. When the Tg is increased to -50 o C very high peels are observed on stainless steel and the polar plastics in some cases peel adhesions close to 10 lbs/in are observed. The moderate T g material also displayed a significant enhancement

19 Table MMtE per chain end Fox T g ( o C) tainless teel 72 hr Dwell (Lbs/in) LDPE 72 hr Dwell (Lbs/in) HDPE 72 hr (Lbs/in) PET 72 hr (Lbs/in) mixed zipping zipping zipping zipping AB 72 hr (Lbs/in) hear, 1kg/0.5 q. In (Failure Time, Mins) in peel adhesion on lower surface energy plastics relative to the low Tg control. Additionally, it is important to note that the -50 o C material displayed higher shear than the control. The higher T g material exhibited moderate peel adhesion on stainless steel and polar plastics with higher shear. n the low surface energy surfaces, the material exhibited low bond strength with zippy or slipstick failure modes. This data indicates that the higher T g yielded a higher modulus material that had more difficulty wetting low polarity surfaces. This can be considered a typical response with a simple increase in T g of a pressure sensitive adhesive system. This paper has detailed the influence of concentration of alkoxy-silane functionalities in a hybrid cross-linked high solids solution acrylic pressure sensitive system. pecifically, it has been shown that the use of alkoxy-silane functionalities in the manner described yield pressure sensitive adhesives that maintain modulus over a wide range of temperatures. The unique rheology of the pressure sensitive adhesives yields good performance with regards to balance of adhesion and cohesion as well as excellent temperature resistance. It has been found that increases in the number of alkoxy-silanes per polymer end region modulate the absolute value of the plateau modulus in a dramatic fashion. The resulting change in modulus yields a wide range of pressure sensitive adhesive performance from permanent to removable. Furthermore it has been shown that

20 these materials can serve as excellent formulating base polymers such that high adhesion to plastics can be attained while maintaining high strength. Additionally, it has been demonstrated that the architectures described can be used with a variety of monomer compositions to afford varying desirable properties such as adhesion to low surface energy substrates.

21 References 1. Matyjaszewski, K., Ed. AC ymposium eries, Controlled Radical Polymerization: Progress in ATRP, NMP and RAFT; American Chemical ociety: Washington DC, 2000; Vol Le, T.P.; Moad, G.; Rizzardo, E.; Thang,.H. Int. Patent Appl. W , Moad, G.; olomon, D. H. The Chemistry of Free Radical Polymerization; Elsevier: Amsterdam, Lester, C. L.; Bottorf, W. L., PTC Tech PCT application W 2009/117654