Acrylic rubbers ageing

FIGURE 3.24 Plots of volume fraction of the filled rubber in the swollen gel (Vrf) against aging temperature for acrylic rubber (ACM)-silica and epoxidized natural rubber (ENR)-silica hybrid nanocomposites. (From Bandyopadhyay, A. and Bhowmick, A.K., Plastic Rubber Comp. Macromol. Eng., 35, 210, 2006. Courtesy of Money Publishers.)... 

Jha, A. and Bhowmick, A.K., Thermal degradation and ageing behaviour of novel thermoplastic elastomeric nylon-6/acrylate rubber reactive blends, Polym. Degrad. Stab., 62, 575, 1998. 

In order to produce a resin with excellent impact strength and at the same time excellent weather resistance and aging resistance, it is essential to eliminate the unsaturated ethylene polymer from the graft copolymer. Therefore, ASA polymers that are crosslinked with the alkyl acrylate rubber polymer are preferred (17). 

In contrast, the butyl acrylate rubber of ASA is free of C—C double bonds which gives ASA clear advantages in terms of weatherability [29,30] (Figures 16.10 and 16.11) and resistance against heat ageing (Figure 16.12). 

Acrylate rubbers such as poly (butyl acrylate) or poly (ethyl hexylacrylate) are characterized by better aging characteristics than polydienes. Ethyl hexylacrylate and acrylonitrile were grafted onto PVC in solution by R. G. Bauer and M. S. Guillord. They observed that this graft copolymer was transparent in contrast to a mere polyblend of PVC and an AN/acrylate copolymer. 

The above illustrated crosslinking reactions of homopolymers, however, form elastomers with poor aging properties. Commercial acrylic rubbers are therefore copolymers of ethyl or butyl acrylate with small quantities of comonomers that carry special functional groups for crosslinking. Such comonomers are 2-chloroethylvinyl ether or vinyl chloroacetate, used in small quantities (about 5%). These copolymers crosslink through reactions with polyamines. 

None of these rubbers has carbon-carbon double bonds. Consequently, they have relatively good aging properties, but, on the other hand, they cannot be vulcanized by the classical sulfur process. For this reason, some of these rubbers are cross-linked with the aid of peroxides, and, in this case, by polymerization of vinyl groups in the case of some silicone rubbers or by free radical transfer reactions in the case of ethylene/vinyl acetate or acrylic rubbers. Other speciality elastomers are cross-linked by reaction with diamines, for example, in the cases of acrylic, epichlorohydrin and fluorine rubbers. 

The obvious solution to the ageing problem is to choose intrinsically resistant materials for outdoor applications. For example, window frames are made from extruded sections of rigid PVC toughened with an acrylate rubber (in preference to BR or other rubbers containing the easily oxidized carbon-carbon double bond). Further protection is afforded by adding ... 

Extrapolated unaged and 40 years natural ageing data Compound S (acrylate rubber)... 

The important characteristics of acrylate rubbers are good oil resistance and good heat resistance carbon black reinforced materials age very well in air up to about 200 C. It is these properties which account for the major use of acrylate rubbers, i.e., in oil seals for automobiles. Polyacrylates are fairly readily hydrolyzed to poly(sodium acrylate) by heating with aqueous sodium hydroxide for a few hours. 

M class elastomers contain only C atoms and no double bonds in their backbones this improves aging resistance. Examples ethylene-propylene rubber (EPDM), acrylate rubber (ACM), fluoro rubber (EKM)... 

The values for the loss factor tan <5 of the j -maximum (corresponding to the glass transition temperature of rubber) are a measure of the active rubber content. After four months of aging, this content has decreased to approx. 75% and after one year to approx. 50% in ABS. Simultaneously, the glass transition temperature increases this indicates crosslinking. For the acrylate rubber in ASA, neither the level of land... 

These acrylic or acrylate rubbers are produced from different acrylate esters. Usually the ACM polymers are made from a basic acrylate monomer and a second, specific curing monomer, as shown in Figure 4.24. Also, ethyl-acrylate-based ACM polymers possess very good heat aging resistance while butyl-acrylate-based ACM polymers possess better low-temperature flexibility. Other alkyl acrylates are used to manufacture unique grades of ACM. 

In general, however, vulcanizates of straight polyacrylates are weak and do not have good ageing properties. Consequently, commercial acrylate rubbers... 

The demands on insulating materials in soil and fresh water are relatively low. Anodically evolved oxygen makes the use of aging-resistant insulating materials necessary. These consist of special types of rubber (neoprene) and stabilized plastics of polyethylene, and polyvinylchloride, as well as cast resins such as acrylate, epoxy, polyester resin and many others. 

The carboxylated types (XNBR) contain one, or more, acrylic type of acid as a terpolymer, the resultant chain being similar to nitrile except for the presence of carboxyl groups which occur about every 100 to 200 carbon atoms. This modification gives the polymer vastly improved abrasion resistance, higher hardness, higher tensile and tear strength, better low temperature brittleness, and better retention of physical properties after hot-oil and air ageing when compared to ordinary nitrile rubber. 

Pure polyvinyl chloride alone It a rigid plastic of high volume resistivity. Addition of monomeric liquid plasticizer makes It flexible but lowers volume resistivity seriously. This loss of volume resistivity was not prevented by pre-purification of commercial resin and plasticizer, though It could be worsened by addition of Ionic soluble Impurities. Volume resistivity was surprisingly Increased by heat aging. It was not improved by use of polymeric liquid plasticizers, nor even, surprisingly, by use of nitrile rubber as plasticizer. Flexlblllzatlon without serious loss of volume resistivity was best achieved by internal plasticization by copolymerization with 2-ethylhexyl acrylate. Further studies are needed to explain these observations and to optimize the use of Internal plasticization In this way. 

In the United States, Du Pont and PPG had a long-established reputation in industrial and consumer paints. W. R. Grace since buying Dewey Almy, and Rohm Haas because of its age-old tradition in acrylics, drew substantial profits from their specialties. This was also true of American Cyanamid (additives for plastics, cosmetics) and of Monsanto (products for rubber, special polymers). Since its withdrawal from the tire business, BF Goodrich, aside from its PVC lines, is concentrating now on specialties. 

The final dispenser evaluation with COMB pheromone again compared dispensers made of 3-mil acrylic film with those of 16-mil polymeric PVC the 13 x 13 mm laminate dispensers each contained 1 mg of racemic pheromone. Also included in this test were rubber septa baited with 1 mg of compound. All of the dispensers were aged outdoors in California at temperatures ranging from 32-39°C in the daytime to 15-21°C overnight. Analyses of residual lure contents showed a tof just 2 days for the rubber septa and approximately 15 days for the 16-mil polymeric PVC laminate the 3-mil acrylic dispensers lost pheromone so slowly that tj/2 could not be measured, but it was much greater than 70 days. On the basis of these tests, laminate dispensers made of 16-mil polymeric PVC were selected for the COMB monitoring program and have been successfully used in 1980 and 1981. 

In order to improve both the thickness sensitivity and low temperature sensitivity of the impact strength, polycarbonate has been blended with a variety of low T, elastomeric impact modifiers. More important among these are the core-shell rubbers like PMMA-g-polybutadiene, PMMA-g-SBR (MBS), PMMA-g-n-butylacrylate (acrylic core-shell), all normally composed of 0.1/core particles. These modifiers improve both the thick-section (6.4 mm) and low temperature notched Izod impact properties of polycarbonate [Witman, 1981 Neuray and Ott, 1981 Bussink et al., 1977] (Figures 15.13 and 15.14). The blends are of course opaque. Impact modified polycarbonate also shows better retention of impact strength with heat-aging. 

The polyacrylate and ethylene-acrylic copolymers and one of the ethylene-propylene terpolymers (Nordel) were the best of the Intermediate temperature elastomers. Except for resistance to compression set, these materials were Inferior to the silicones in thermal stability as measured by their retention of tensile properties. The other EPDM compounds and butyl rubber were considerably inferior to the above-mentioned elastomers. It is not expected that the service life of the tested materials will be limited solely by their ability to resist hydrolytic degradation. The only caulking compositions which retained moderate physical integrity on thermal aging were the silicones. 

---