Butyl-type Rubbers

Plastics, such as PE, PP, polystyrene (PS), polyester, and nylon, etc., and elastomers such as natural rubber, EPDM, butyl rubber, NR, and styrene butadiene rubber (SBR), etc., are usually used as blend components in making thermoplastic elastomers. Such blends have certain advantages over the other type of TPEs. The desired properties are achieved by suitable elasto-mers/plastic selection and their proportion in the blend. 

In an acetone extract from a neoprene/SBR hose compound, Lattimer et al. [92] distinguished dioctylph-thalate (m/z 390), di(r-octyl)diphenylamine (m/z 393), 1,3,5-tris(3,5-di-f-butyl-4-hydroxybenzyl)-isocyanurate m/z 783), hydrocarbon oil and a paraffin wax (numerous molecular ions in the m/z range of 200-500) by means of FD-MS. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out (Chapter 2). The method of Dinsmore and Smith [257], or a modification thereof, is normally used. Mass spectrometry (and other analytical techniques) is then used to characterise the various rubber fractions. The mass-spectral identification of numerous antioxidants (hindered phenols and aromatic amines, e.g. phenyl-/ -naphthyl-amine, 6-dodecyl-2,2,4-trimethyl-l,2-dihydroquinoline, butylated bisphenol-A, HPPD, poly-TMDQ, di-(t-octyl)diphenylamine) in rubber extracts by means of direct probe EI-MS with programmed heating, has been reported [252]. The main problem reported consisted of the numerous ions arising from hydrocarbon oil in the recipe. In older work, mass spectrometry has been used to qualitatively identify volatile AOs in sheet samples of SBR and rubber-type vulcanisates after extraction of the polymer with acetone [51,246]. 

Figure 19.13 shows the dynamic mechanical properties of such a blend of sPS with a mixture of Kraton G 1651 (15 %) and microsuspension rubber particles (20%) consisting of 60% butyl acrylate (BA) core grafted with 40% styrene shell (S//BA). The glass transition temperatures of the Kraton (-60 °C) and the butyl acrylate (-45 °C) phases can be easily distinguished from one another. The TEM image of such a product after deformation is shown in Figure 19.14. The annealed specimen is shown since the two rubber types are better discernible than in the nonannealed sample. As expected, crazing and voiding in the rubber particles dominate. The product had the following notched impact strengths (ISO 179/eA) injection moulded (80 °C mould temperature) 6.3, injection moulded (140 °C) 4.0 and annealed 3.7kJ/m2.

FIG U RE 8.4 Pyrograms of vinyl acetate, acrylic, aUcyd enamel, epoxy, and chlorinated rubber type architectnral paints. 1 = benzene, 2 = isooctene, 3 = acetic acid, 4 = 2-ethylhexyl acrylate, 5 = 2,2,4-trimethyl 1,3-pentanediol mono-isobutyrate, 6 = methyl methacrylate, 7 = butyl methacrylate, 8 = acrolein, 9 = methacrolein, 10 = hexanal, 11 = phthaUc anhydride, 12 = phenol, 13 = isopropenylphenol, 14 = bisphenol A, 15 = xylenes, 16 = trimethylbenzenes. 

Limitations Does not bond well to natural rubber or butyl rubber Strength characteristics poor tendency to creep, lack of tack requires a tackifier for use in adhesives Poor resistance to hydrolytic degradation (reversion), even in the polyether type... 

In contrast, the level of sulfur migration is much less from natural rubber to butyl rubber. The solubility of sulfur in butyl is much less than the solubility of sulfur in natural rubber (Figure 4.11). The diffusion of MBTS is similar to sulfur diffusion, but it occurs more slowly due to the large size of the MBTS molecule. MBTS is also less soluble than sulfur in each rubber type (Figure 4.12). 

The thermal resistance of chloroprene-rubber is considerably better than that of natural rubber. However, it does not reach the resistance of ethylene-vinyl acetate copolymers, hydrated nitrile rubber, ethylene-propylene ter-rubber, and butyl rubber, respectively. Long-term thermal resistance is listed at approx. 80 °C, Figure 5.237. Types with special formulation can be used short-term up to 120 °C [697]. 

Hilton and Altenau and Hayes and Altenau used mass spectrometry to qualitatively identify volatile antioxidants in sheet samples of synthetic styrene-butadiene rubbers and rubber type vulcanizates. They extracted the polymer with acetone in a Sohxlet apparatus, removed excess solvent and dissolved the residue in benzene. Substances identified and determined by this procedure include N-phenyl-fi-napthylamine, 6-dodecyl-2,2,4-trimethyl 1,2-dihydroquinolines, trisnonyl-phenyl-phosphite, isobutylene - bisphenol A reaction product, 2-mercaptobenzothiazole sulphen-amide (accelerator) N-cyclohexyl-2-benzothiazole sulphenamide, N-tert-butyl-2-benzo-thiazole sulphenamide, 2-(4-morpolinothio) benzothiazole, 2-(2,6-dimethyl-morphal-inothio) benzothiazole, N,N -diisopropyl 2-benzothiazoles, 2-mercaptobenzo-thiazole and N,N -dicyclohexyl-2-benzothiazole sulphamide. 

Considerable effort has been made in the area of snail control in utilizing slow-release incorporated tri-n-butyl fluoride within elastomeric substrates.Using mostly rubber-type substrates, at least five different formulations are cited in controlling seven species of snails under a variety of conditions. Although all of these are physically entrapped, a variety of shapes and densities are now available to allow quite a range of use in the field. The citations in reference 26 showed complete control under a variety of conditions for periods up to 19 months long. 

Tackifiers (A.S.C., 1986 Dick, 1987) - Tackifiers are used in certain types of adhesives and sealants to greatly improve initial adhesive strength (tack strength) on contact with an adherend surface before a stronger bond is formed later during the cure process. Elastomers such as natural rubber and butyl rubber, and thermoplastic hot melt systems all have low tack and require tackifiers to adhere well to polar substrates such as glass, ceramics, masonry and metal surfaces. 

In addition to compatibility studies, DSC is often used to characterise the properties of a blend. For example. Van dyke etal [66] have used it to investigate a number of variables (e.g. rubber type, rubber content and degree of cure) on the properties of polyamide 12/butyl rubber blends. More examples of the application of DSC to the characterisation of blends are given in Section 6.4.3. 

Van Dyke ID, Gnatowski M, Koutsandreas A, Burczyk A. Effect of butyl rubber type on properties of polyamide and butyl rubber blends. / Appl Polym Sci 2004 93(3) 1423-1435. 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

The polymerization system is of the cationic type, usiag coinitiators such as AlCl and water at very low temperatures (—100° C) and leading to an almost instantaneous polymeriza tion (see Elastomers, SYNTHETIC-BUTYL RUBBER). 

Rubber blends with cure rate mismatch is a burning issue for elastomer sandwich products. For example, in a conveyor belt composite structure there is always a combination of two to three special purpose rubbers and, depending on the rubber composition, the curatives are different. Hence, those composite rubber formulations need special processing and formulation to avoid a gross dissimilarity in their cure rate. Recent research in this area indicated that the modification of one or more rubbers with the same cure sites would be a possible solution. Thus, chlorosulfonated polyethylene (CSP) rubber was modified in laboratory scale with 10 wt% of 93% active meta-phenylene bismaleimide (BMI) and 0.5 wt% of dimethyl-di-(/ r/-butyl-peroxy) hexane (catalyst). Mixing was carried out in an oil heated Banbury-type mixer at 150-160°C. The addition of a catalyst was very critical. After 2 min high-shear dispersive melt mix-... 

It is possible to distinguish between SBR and butyl rubber (BR), NR and isoprene rubber (IR) in a vulcan-izate by enthalpy determination. In plastic-elastomer blends, the existence of high Tg and low Tg components eases the problems of experimental differentiation by different types of thermal methods. For a compatible blend, even though the component polymers have different Tg values, sometimes a single Tg is observed, which may be verified with the help of the following equation ... 

Generally, systems developed in the USA favour a combination of polyethylene with either butyl-rubber or hot-applied mastic adhesives, the latter consisting of a blend of rubber, asphalt and high molecular weight resins. In European and Far East coating plants, epoxy type primers and hard ethylene copolymer adhesives have been successfully employed. 

The materials selected for evaluation included three materials currently being used in these applications Biomer (Thoratec Laboratories Corporation, Emeryville, CA), representative of segmented ether-type polyurethanes Avcothane-51 (Avco Everett Research Laboratory, Inc., Everett, MA), a block copolymer of 10% silicone rubber and 90% polyurethane and Hexsyn (Goodyear Tire and Rubber Company, Akron, OH), a sulfur vulcanized hydrocarbon rubber that is essentially a polyhexene. Also selected, because of their easy availability, were Pellethane (Upjohn Company, North Haven, CT), an ether-type of polyurethane capable of being extruded in sheet form, and a butyl rubber formulation, compounded and molded at the National Bureau of Standards. The material thickness varied, but the sheets were generally about 1 mm thick. 

Diffusion of a gas or liquid through a semi-permeable material. The permeability of elastomers to gases varies with the elastomer type and with the gas. Butyl rubber is much less permeable to air than is natural rubber hence its use in tyre inner tubes and similar apphcations. The rate of permeation is generally related to the size of gas molecule, i.e., the smaller the molecule the higher the rate. The exception is C02 which has a rate 10 to 100 times greater than that of nitrogen. 

Chemical Type Viton Butyl Rubber PVA Neoprene Nitrile PVC Latex... 

The saturated rubbers such as butyl or EPDM cannot be cross linked by sulphur and accelerators. Organic peroxides are necessary for the vulcanization of these rubbers. When the peroxides decompose, free radicals are formed on the polymer chains and these chains can then combine to form cross links of the type where only carbon-carbon bonds are formed, unlike in sulphur vulcanization. These carbon-carbon bonds are quite stable ones. Such bonds are also formed by vulcanization using gamma or x-ray radiation of compounded rubbers. Some rubbers can be vulcanized by the use of certain bisfunctional compounds which form bridge type cross... 

Rubber antioxidants are commonly of an aromatic amine type, such as dibeta-naphthyl-para-phenylenediamine and phenyl-beta-naphthylamine. Usually, only a small fraction of a percent affords adequate protection. Some antioxidants arc substitute phenolic compounds (butylatcd hydro -vamsole, di-tert-butyl-para-cresol, and propyl gallate). 

Vinyl-type addition polymerization can also be carried out with acidic catalysis such as boron tnfluoride or tin tetrachloride and with basic catalysis such as alkali melals or alkali alkyls. An example of the first ease is the low-temperature polymerization of isobutene, which gives Vistanex" and butyl rubber an example of the second type is the polymerization of butadiene with sodium, which leads to buna rubber. 

Other thermoplastic elastomer combinations, in which the elastomer phase may or may not be cross-linked, include blends of polypropylene with nitrile (30,31), butyl (33), and natural (34) rubbers, blends of PVC with nitrile mbber (35,36), and blends of halogenated polyolefins with ethylene interpolymers (29). Collectively, thermoplastic elastomers of this type are referred to herein as hard polymer/elastomer combinations. Some of the more important examples of the various types are shown in Table 3. 

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