Acrylate ester modifications of isobutylene/para methylstyrene copolymer.
The reaction mechanisms of this benzylic bromine functional group in the vulcanization process was also given considerable attention in earlier presentations (refs. 2-4). The resultant vulcanizates combine the permeability resistance and vibration dampening characteristics of butyl rubber with the complete ozone resistance and chemical inertness of the EPDM elastomers. These attributes reflect the completely saturated nature of the new polymer molecular backbone.
Other reactions of the benzylic bromine can give a variety of functional derivatives which were also outlined in the earlier publications. These included ester and ether modifications, carboxylation and hydroxyl functional derivatives, ionomers and a variety of alternative grafted polymer compositions. This article will focus on the synthesis and characterizations of acrylate ester and benzophenone modifications of the isobutylene/p-methylstyrene copolymers.
The acrylate double bond provides a very reactive moiety for further latitude in crosslinking reactions. The benzophenone ether coupled with the acrylate provides an internally stabilized polymer system. Particular attention is paid to free radical driven vulcanization processes and the covulcanization of these new derivatives with general purpose elastomers.
Materials used were: Acrylic acid, methacrylic acid, sorbic acid, cinnamic acid, 4-hydroxy-benzophenone and tetrabutylamonium hydroxide (1.0M in methanol) were purchased from Aldrich and used without purification, brominated poly (isobutylene-co-p-methylstyrene was obtained from Exxon Chemical Co. All rubber curatives are commercial rubber chemicals.
A NMR and FT-IR were used for 'H NMR and IR measurement. An oscillating disc cure rheometer (ODR, described in ASTM D2084) was used for cure studies. The physical properties of the crosslinked compounds were measured by various ASTM methods. The UV testing was carried out in an American Ultraviolet Model C1006A Mini Laboratory Conveyor equipped with a variable intensity medium pressure mercury lamp capable of 300 watts per inch. The unit was housed in a nitrogen-purged box for safety reasons. In these studies the UV dose was measured with a high energy UV integrating radiometer, which measures the integrated value of energy in a pass under the UV lamp in J/[cm.sup.2]. UV exposure was determined by passing the radiometer through the unit before and after sample runs. Doses employed for this radiation cure work were between 0.025 J/[cm.sup.2] and 0.2 J/[cm.sup.2]. Multiple passes were frequently employed. Electronbeam processing was carried out. Dose was controlled by either varying the amps supplied to the electron gun, or the exposure time as determined by conveyor speed.
Preparation of methylacrylate modified poly
A 5-1 glass jacketed reaction vessel, fitted with an overhead compressed air driven mechanical stirrer, and a hose connector was purged with nitrogen. At room temperature under nitrogen, the vessel was charged with 3,100 ml toluene and 475 g poly (isobutylene-co-p-methylstyrene-co-p-bromomethylstyrene) which comprising 1.73% mol. p-methylstyrene and 1.25% mol. p-bromomethylstyrene (106 mmol. Br) and having Mooney viscosity of 65, 1 + 8 min., @ 125[degrees]C. The polymer was dissolved by stirring at room temperature overnight. A tetrabutylamonium salt of methacrylic acid was prepared in a flask by charging 123.6 ml of 1.0 M tetrabutylamonium hydroxide in methanol, 9.6 g methacrylic acid (113 mmol.), and 100 ml isopropanol to the flask and swirling the contents of the flask at room temperature, giving a water white clear solution. This solution was then added to the vessel containing the dissolved polymer, a circulating bath temperature at 83[degrees]C. After 45 minutes, the bath temperature was raised to 95[degrees]C and let the reaction to proceed for 7 hours. Then the bath temperature was lowered to 70[degrees]C, and after a 2.5 hour period, the reactor was let to cool. The yellow viscous solution was quenched and washed with 10 ml concentrated HCI in 1 liter distilled water, and subsequently washed with [H.sub.2]O/isopropanol (80/20, volume) 5 to 6 times. The polymer was isolated by precipitated into 0.1% 2,5 - di-t-butyl-4-methylphenol (BHT) in isopropanol and dried in vacuo for 48 hours at 1 mm Hg 65[degrees]C. Solution viscosity of the recovered polymer was same to the starting material. 'H NMR and IR analysis of the polymer showed the complete conversion of the benzyl bromide to benzyl ester. Sorbate or cinnamate modified copolymer was prepared using the similar procedure described above.
Preparation of methacrylate modified copolymer
The procedure for the preparation of methylacrylate modified copolymer was repeated as set forth therein using the same starting terpolymer except the charge to the second flask was 126.3 ml of tetrabutylamonium hydroxide (1.0 M in methanol), 9.6 g of methacrylic acid (113 mmol.) 1.02 g of 4-hydroxybenzophenone (5.15 mmol.) and 100 ml isopropanol. The final washed modified polymer was isolated by precipitation into isopropanol without BHT. The acrylate-benzophenone modified polymer was prepared using a similar procedure. The resulting polymers were characterized using 'H NMR and IR spectroscopy.
Results and discussions
The acrylates modified poly(isobutylene-co-p-methylstyrene) may be prepared from the brominated poly(isobutylene-co-p-methylstyrene) which contains reactive p-bromomethyl styrene moiety via nucleophilic displacement reactions as shown in figure 1. The benzylic bromines of the brominated copolymer can be fully or partially substituted by acrylic acids and/or 4-hydroxy-benzophenone under basic conditions, i.e. in the presence of tetrabutylamonium hydroxide and inorganic basic salts. The rates of the substitution reactions become faster as the temperature rises and the concentration of the quaternary amonium hydroxide increases. The reactions normally are over about four to six hours at 80[degrees]C in toluene/isopropanol (95/10, volume). The 'H NMR (400 MHz) spectra of the starting brominated copolymer and modified final copolymers in [CDCl.sub.3] show the disappearance of the bromomethyl proton resonance of [-C.sub.6] [H.sub.5] ?? Br at 4.45 ppm and the appearance of benzyl ester proton resonance of [C.sub.6] [H.sub.5] ?? OCO- at 5.15 ppm and phenylmethoxy proton resonance of [-C.sub.6] [H.sub.5] ?? [OC.sub.6] [H.sub.5]s CO- at 5.08 ppm. The IR spectra of the starting polymer and final products also indicate the disappearance of benzyl bromide absorbance at 615 [cm.sup.-1] and the appearance of benzylic ester absorbance at carbonyl region of 1723 [cm.sup.-1].
The acrylates modified copolymer without adding stabilizer tend to gel or crosslink when subjected to elevated temperature as expected. However, it has been discovered that the thermally induced crosslinking or gel formation can be substantially inhibited by grafting a minute amount (< 0.1% mol.) of thermostabilizer, e.g. benzophenone onto the copolymer. Thus, when the modified copolymer is to be subjected to elevated temperature during synthesis drying process or application compound mixing, the use of an additional extractable and volatile low molecular weight thermostabilizers such as a hindered phenol, a hindered amine or a hydroquinone compound can be eliminated. The high temperature (180[degrees]C) tests of the modified copolymers using Brabender Plastograph and Mooney viscometer show the efficient stabilization effect of the grafted benzophenone. The simple methacrylate modified copolymer crosslinks after 10 minutes while the methacrylate-benzophenone modified copolymer with 0.05% mol. benzophenone begins to gel only after 60 minutes of heating.
Isobutylene based polymers have been known to be unstable to high energy electron-beam and ultraviolet (UV) radiation primarily by degradation. The degree of breakdown of the isobutylene based polymers were reported by the G value for scission G'(s), representing the number of bond breaking events take place per 100 eV (refs. 5 and 6). Some of the G'(s) for isobutylene based polymers are listed in table 1 and indicate that the copolymer with higher the p-methylstyrene content is more stable under the radiation exposure. The acrylate modified and acrylate-benzophenone modified polymer are crosslinkable readily under UV or electron-beam radiation without volatile low molecular weight additives, e.g., photosensitizers or co-crosslinking agents. The gel formations of the irradiated film are shown in table 2. The 0.5 mm thick pads of the acrylate-benzophenone modified polymers with 50 phr N660 black were irradiated under high energy (10-20 M rad) electron-beam radiation. The tensile properties of the crosslinked pads are shown in tables 3 and 4. The data indicate the high radiation response and the stability of the cure compounds under excess dose of radiation energy.
Table 1 - G'(s) for isobutylene based polymers
G'(s) Polyisobutylene 4.1 Poly(isobutylene-co-isoprene), 1.5% 2.2 mol, isoprene 2.3% mol, p-methylstyrene 2.3 5.3% mol, p-methylstyrene 1.8 7.0% mol, p-methylstyrene 1.3
[TABULAR DATA OMITTED]
Table 3 - tensile properties of electron beam cured
methacrylate-benzophenone modified polymer compound(*)
Dose, M rad Tensile strength Elongation, % at break, MPa 5 8.3 200 10 8.5 205 20 9.6 215 40 8.3 215
(*)32 ML, (1 + 8, 125[degrees]C), 0.4% (mold) methyacrylate, 0.1% (mol) benzophenone; 50 phr N660 black; 05 mm thickness
Table 4 - tensile properties of electron-beam cured acrylate-benzophenone
modified polymer compound(*)
Dose, M rad Tensile strength Elongation, % at break, MPa 5 7.1 505 10 7.6 490 20 8.1 375 40 7.0 275
(*)32 ML, 1 + 8, 125[degrees]C, 0.35% (mol) acrylate, 01% (mol) benzophenone; 50 phr N660 black; 0.5 mm thickness
Peroxide cure systems have the advantage of fast cure reactions, excellent heat resistance, high resistance to set and absence of other inorganic and organic substances inherent in the other cure systems. Butyl rubber and isobutylene based polymers in general undergo rapid chain scission in the presence of peroxides. The acrylate modified poly (isobutylene-co-p methylstyrene), unlike butyl rubbers, does not degrade with peroxide at elevated curing temperatures. The low acrylates functionality in this material can be cured efficiently with low peroxide level. The cure states dependence on peroxide level are shown in table 5.
Table 5 - cure states vs. peroxide level for methacrylate benzophenone
Polymers Di-cup 40 [M.sub.H]-[M.sub.L], KE, phr dN[multiplied by]m(2) 0.36% mol. methacrylate 1.5 40.4 0.05% mol benzophenone 2.0 40.9 2.5 41.2 0.88% mol. methacrylate 1.5 59.7 0.07% mol. benzophenone 2.0 64.2 2.5 65.2
(1.)With 50 phr N660 black
(2.)Monsanto OCD, 170[degrees]C, 3[degrees] arc, 60 min.
This material also can be co-cured with high unsaturated rubbers, e.g., natural rubber using peroxide system. The cure states of the different blend composition are shown in table 6. The rheometer torque ([M.sub.H]-[M.sub.L]) vs. composition data for the blends shows rheometer torque increases that are consistently above the tie-line between the data points for the individual blend components which indicates the interpolymer and intrapolymer co-cure of the two rubbery components.
[TABULAR DATA OMITTED]
The only commercially available peroxide curable isobutylene based polymer is poly (isobutylene-co-isoprene-co-divinylbenzene) (ref. 7). This material has high gel content and cannot be processed easily. The acrylate modified copolymer is gel free and responds to peroxide cure efficiently, giving heat stable and oxidation resistance compositions (table 7). Extractables of the crosslinked composition are low by virtue of the relatively simple cure system.
[TABULAR DATA OMITTED]
The acrylate modified polymers are sensitive to radiation and peroxide cure systems, but are less efficient to a conventional accelerated sulfur vulcanization system. The modified polymer with low methacrylate functionality (0.35% mol.) can be cured slightly with Santocure/sulfur but the polymer with higher functionality (0.7% mol.) can be cured efficiently with a more active system used for butyl rubber. The curing rate is slower than butyl rubber (table 8). The slowness of the initiation using sulfur system is probably the reason for the low curability of the modified polymers. The slow initiation possibly due to the low level of allylic hydrogens (0 per acrylic, 3 per methacrylic) and the lower reactivity of the type of allylic hydrogens which are conjugated to the carbonyl group of the ester and different from the ordinary allylic hydrogens in butyl rubber or highly unsaturated general purpose rubbers.
[TABULAR DATA OMITTED]
Vulcanizates of rubber blends which contain low unsaturated rubber, e.g., butyl, EPDM with more highly unsaturated rubber, i.e. general purpose rubbers, are of interest in the rubber industry, but one of the problems associated with the curing of the blends compositions is the inability to achieve a balance cure of the individual components and also a truly intervulcanized composition, i.e. a composition where predominant interpolymer crosslinking take place between different polymer molecules in the different phases. For example, in sulfur curable systems containing a blend of NR is cured much faster than the IIR resulting in a highly cured NR phase and an undercured, curative starved IIR phase, with little or no interpolymer crosslinking taking place at the interphase (figure 2). As a consequence of this lack of curing balance, the vulcanizates may exhibit inferior mechanical properties. One technique used to minimize the problem of vulcanization in balance is the use of low unsaturated rubber with functional groups is susceptible to crosslinking mechanisms independent of the sulfur curing system used to crosslink the high unsaturated rubber. For example, blends of brominated butyl rubber can be vulcanized along with NR by including an independent curing system for each type of rubber into the compound, e.g. a ZnO -- base system for curing halobutyl and an accelerated sulfur system for NR (ref. 8). However, even in these systems there may be a drop-off of important physical properties. This drop-off most likely the result of the lack of significant interpolymer crosslinking. It has been found that the sulfur system curable compositions containing highly unsaturated rubber, e.g., NR, SBR, BR and the acrylate modified poly (isobutylene-co-p-methylstyrene) has improved curing efficiency. Figure 4 is a plot of curing behavior as shown by rheometer torque ([M.sub.H]-[M.sub.L]) as blend composition of the methacry-late-benzophenone modified isobutylene based copolymer and natural rubber. The [M.sub.H]-[M.sub.L] values are above the tie-line of the individual rubber components which indicates the possible existence of the interpolymer crosslinking between NR and this modified isobutylene copolymer. The free radicals can be generated with sulfur cure system in NR phase, results not only in the individual crosslinking of the NR and modified isobutylene copolymer themselves but also interpolymer crosslinking involving the free radical generated in the NR and the acrylate group on the modified isobutylene copolymer serve as free radical acceptors (terminators). The resulting vulcanizates with interpolymer and intrapolymer crosslinks possibly will have enhanced ozone resistance, chemical resistance, temperature stability and improved dynamic response, as well as improved tensile properties as compared with conventional covulcanizates with low extent of interpolymer crosslinking.
The development of functionalized derivatives of brominated isobutylene/p-methylstyrene copolymers extends the cure and chemical reactivity for isobutylene based polymers. The acrylate and methacrylate derivatives of this new elastomer system substitutes the active olefin ester for the benzyl bromide in the brominated polymer providing a vulcanizable polymer base which is essentially halogen free.
An internally stabilized system was developed through the benzophenone modification of the acrylate derivatives. Thus, eliminating the need for further stabilizer additives. Detailed synthesis reactions were described together with polymer characterizations to confirm resultant structures.
Free radicals driven curing reactions such as peroxides with the acrylate derivative were demonstrated to give fast and effective crosslink densities at very low levels of acrylate functionality. Radiation cure with electron beam sources proceed equally efficiently and effectively to give useful and practically cured products. Sulfur vulcanizates with conventional cure systems require higher levels of acrylate functionality for efficient cure response. Sulfur cures are less efficient than peroxide but in blends with GPR rubbers synergistic cure reactions occur to give very effective crosslinking and vulcanizate properties.
These newly functionalized polymers are the first essentially halogen free isobutylene copolymers shown to be covulcanizable with general purpose rubbers. Also, they are the first copolymers capable of electron beam crosslinking without the typical polyisobutylene degradation reaction.
(1 (a).)K.W. Powers and H-C Wang, "Functionalized P-methylstryrene/isobutylene copolymers," presented to ACS Rubber Division, Toronto, Ontario, Canada, May 1991.
(1 (b).)H-C Wang and K.W. Powers, Elastomerics, January and February, 1992.
(2.)I.J. Gardner, H-C Wang, J.V. Fusco and P. Hous, "Crosslinking of brominated P-methylstyrene/isobutylene copolymers and in blends with general purpose rubbers."
(3.)R. Bielski, J.M.J. Frechet, J.V. Fusco, K.W. Powers and H-C Wang, J. Poly. Sci., Part A: Poly. Chem., 31, 755 (1993).
(4.)J.M.J. Frechet, R. Bielski, H-C Wang, J.V. Fusco and K.W. Powers, Rubber Chem. Tech. 66, 98 (1993).
(5.)G.G.A. Bohm and J.O. Tweskyem, Rubber Chem. Tech. 55, 574 (1982).
(6.)J. Pacansky and R.J. Waltham, J. Rad. Curing, 15, 12 (1988).
(7.)XL10000 marketed by Polysar Division, Bayer AG.
(8.)Exxon bromobutyl rubber compounding and applications, Exxon Chemical Co., 1992, Sec. 5.
This article is based on a paper given at the October 1993 Rubber Division meeting.
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|Date:||Oct 1, 1994|
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