Butyl-type elastomers

Subsequently, there are also TPOs made from butyl-type elastomers such as butyl modified polypropylene, butyl-low-density polyethylene (LDPE), and butyl-high-density polyethylene (HDPE), as well as EP elastomer and various polyethylenes. One distinctive incident of filler addition to a PP-EP-elastomer-based TPO is that the filler preferentially goes onto the elastomer phase of the blend. Thus, the addition of filler to a TPO actually lowers the flexural modulus. 

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

Ozonc-rcsjstant elastomers which have no unsaturation are an exceUent choice when their physical properties suit the appHcation, for example, polyacrylates, polysulfides, siHcones, polyesters, and chlorosulfonated polyethylene (38). Such polymers are also used where high ozone concentrations are encountered. Elastomers with pendant, but not backbone, unsaturation are likewise ozone-resistant. Elastomers of this type are the ethylene—propylene—diene (EPDM) mbbers, which possess a weathering resistance that is not dependent on environmentally sensitive stabilizers. Other elastomers, such as butyl mbber (HR) with low double-bond content, are fairly resistant to ozone. As unsaturation increases, ozone resistance decreases. Chloroprene mbber (CR) is also quite ozone-resistant. 

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). 

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

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-... 

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. 

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 ... 

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. 

Elastomers must be crosslinked to hold their final form. The crosslinking reaction takes place through generation of free radicals that promote bonding at sites of unsaturation. The most common crosslinking agents for this include reactive peroxides, such as dicumyl peroxide, diacetyl peroxide, di-tert butyl peroxide, and others. Since each has a different temperature at which thermal decomposition initiates, curing conditions vary with the peroxide type. 

Significant developments in synthetic rubber began at this time. Outstanding were the introduction of polychloroprene (neoprene) by Carothers, and of the oil-resistant polysulfide rubber Thiokol by Patrick. These were soon followed by styrene-butadiene copolymers, nitrile rubber, butyl rubber, and various other types, some of which were rushed into production for the war effort in the early 1940s. The stereospecific catalysts researched by Ziegler and Natta aided this development, including synthesis of true rubber hydrocarbon (polyisoprene). Since 1935 synthetic rubbers have been referred to as elastomers. 

Measurements were made in apparatus depicted in Figure 4. T = 100°C elastomer in the initial styrene solution, 10%. EPDM, type B initiator, tert-butyl peracetate, 0.09% modifier, tert-dodecyl mercaptan, 0.02%. Polybutadiene, lntene initiator, tert -butyl peracetate, 0.02% modifier, tert-... 

EPDM elastomer, type C and elastomer in starting solution, 10%. Initial polymerization T = 100°C styrene conversion, 30% and initiator, tert-butyl peracetate, 0.09%. Suspension polymerization suspending agent, 0.6% ferf-butyl peroxide, 0.4% and water/(rubber -f monomer) = 1.2. Suspension cycle 2 hrs at 120°C, l hr at 140°C, and 2 hrs at 155°C. 

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