Polymer processing polychloroprene rubber

In the vulcanization of polychloroprene rubber, the zinc oxide is even more essential. It must always be used, accompanied by magnesium oxide (MgO). Vulcanization of trans-l,4-polychloroprene, the main constituent of the polychloroprene rubbers, proceeds much more sluggishly than is the case with natural rubber or SBR. This is because the chlorine substituent inhibits the activity of the carbon atoms in the alpha position to the double bond. The vulcanization sites for practical purposes are confined to the small amount of 1,2 polymer. Fig. 20.9, which forms adventitiously during the polymerization process. This polymer has a more active tertiary allylic site, indicated in Fig. 20.9, and it is here that crosslinking occurs. 

Polymer Chemical Stabilization Processes. Cellular rubber and ebonite are produced by chemical stabilization processes. Most elastomers can be made into either open-celled or closed-celled materials. Natural rubber, SBR, nitrile rubber, polychloroprene, chlorosulfonated polyethylene, ethylene-propylene terpolymers, butyl rubbers, and polyacrylates have been successfully used (110-112). 

Commercial polychloroprene rubber is manufactured by aqueous free-radical emulsion polymerization followed by isolation of the solid polymer by one of several processes freeze roll isolation, drum drying (76), extruder isolation (77), precipitation and dr5dng or spray drying (78,79). Isolation of powdered polychloroprene has been reviewed (80). Of the methods cited, freeze roll and drum drying isolation are commercially important (Fig. 3). 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

During emulsion polymerization, a high conversion of monomer to polymer produces cross-linked rubber which is insoluble. To obtain a high conversion in the polymerization reaction and a processable polymer, suitable polymer modification should be made. The use of sulphur moieties allows this goal to be reached [2]. Sulphur-modified polychloroprenes contain di- and polysulphide sequences in the polymer chains. After the polymerization reaches the desired degree, reaction is stopped by adding thiuram disulphide ... 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

Natural Rubber and Synthetic Polyisoprene Polybutadiene and Its Copolymers Polyisobutylene and Its Copolymers Ethylene-Propylene Copolymers and Terpolymers Polychloroprene Silicone Elastomers Fluorocarbon Elastomers Fluorosilicone Elastomers Electron Beam Processing of Liquid Systems Grafting and Other Polymer Modifications... 

In addition, some rubbers, for example natural rubber and polychloroprene, stiffen at low temperatures by partial crystallisation. This is a gradual process continuing over many days or weeks and is most rapid at a particular temperature characteristic of each polymer, for example -25°C for natural rubber. Hence, tests intended to measure the effect of crystallisation must detect changes in stiffness or recovery after periods of ageing at a low temperature. 

Adhesives on the basis of a rubber are applied as watery dispersions, as solvents, or as solvent-free fluids. Sometimes the rubber is vulcanised after the gluing process, sometimes it remains uncured. Polymers often used are butyl rubber, polyisobutylene, and polychloroprene. A more recent development is the use of... 

Of the 17 billion lb of butadiene consumed in 1999, almost two thirds went into the production of elastomers (styrene-butadiene latex rubber (SBR), polybutadiene, nitrile, and polychloroprene). Adiponitrile, ABS resins, styrene-butadiene latex, styrene block copolymers, and other smaller polymer uses accounted for the remainder. The largest single use was for styrene-butadiene copolymers (SBR and latex). Most of it was made by an emulsion process using a free-radical initiator and a styrene-butadiene ratio of about 1 3. More detailed description of the rubber and polymer used can be found in Chapters 16 and 15. 

Pressure sensitive and contact adhesives are made from a variety of polymers including acrylic acid esters, polyisobutylene, polyesters, polychloroprene, polyurethane, silicone, styrene-butadiene copolymer and natural rubber. With the exception of acrylic acid ester adhesives which can be processed as solutions, emulsions, UV curable 100% solids and silicones (which may contain only traces of solvents), all remaining rubbers are primarily formulated with substantial amounts of solvents such as hydrocarbon solvents (mainly heptane, hexane, naphtha), ketones (mainly acetone and methyl ethyl ketone), and aromatic solvents (mainly toluene and xylene). 

The critical links between the raw polymer and finished vulcanized product used by the global consumer involve the compounding, processing, and curing or vulcanization. There are several major considerations in designing a rubber composition based on polychloroprene dry polymers (Table 5). 

Crystallization. Some elastomers crystallize at temperatures that can significantly impact processing and vulcanizate behavior. Thus it is necessary to account for these behaviors when developing rubber compound formulations and processes. Crystallization is manifested by stiffening and hardening of the raw polymer, uncured compounded polymer, and the vulcanized polymer. Elastomers that crystallize will do so on stretching and thereby exhibit increased tensile strength. Those elastomers (eg polychloroprene and natural rubber) will... 

The iodometrical analysis of active oxygen in the ozonized Denka M40 solutions shows that the amount of 0-0 groups is ca. 43%. It is of interest to note that the HI reaction with ozonized polychloroprene solutions occurs quantitatively for 3—4 h, while in SKD the same proceeds only to 20% after 24 h. The above data, however, provide insufficient information for the preferable route of the zwitterions deactivation via dimerization, polymerization of zwitterions or secondary processes). The DSC analysis of the products of Denka M40 ozonolyis reveals that the chloroprene rubber ozonolyis yields polyperoxide as the enthalpy of its decomposition is found to be very elose to that of dicumeneperoxide (DCP), The higher value of (ca. two times of that of DCP) testifies the possible formation of polymer peroxides [4],... 

Polychloroprenes give vulcanizates which are broadly similar to those of natural rubber in physical strength and elasticity. (See Table 18.1.) However, the polychloroprenes show much better heat resistance in that these physical properties are reasonably well maintained up to about 150°C in air. The heat resistance and compression set resistance of the G type polymers are inferior to those of the W type polymers due to the presence of labile polysulphide bonds. (On the other hand, for the same reason, the G types are easier to process.) As might be expected from the highly regular structure of polychloroprene, normal grades readily crystallize and become stiff when cooled below -10°C. 

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