Polychloroprene oxidation

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

Processing ndProperties. Neoprene has a variety of uses, both in latex and dry mbber form. The uses of the latex for dipping and coating have already been indicated. The dry mbber can be handled in the usual equipment, ie, mbber mills and Banbury mixers, to prepare various compounds. In addition to its excellent solvent resistance, polychloroprene is also much more resistant to oxidation or ozone attack than natural mbber. It is also more resistant to chemicals and has the additional property of flame resistance from the chlorine atoms. It exhibits good resiUence at room temperature, but has poor low temperature properties (crystallization). An interesting feature is its high density (1.23) resulting from the presence of chlorine in the chain this increases the price on a volume basis. 

Curing Systems. Polychloroprene can be cured with many combiaations of metallic oxides, organic accelerators, and retarders (114). The G family of polymers, containing residual thiuram disulfide, can be cured with metallic oxides alone, although certain properties, for example compression set, can be enhanced by addition of an organic accelerator. The W, T, and xanthate modified families require addition of an organic accelerator, often ia combination with a cure retarder, for practical cures. 

Two propylene oxide elastomers have been commercialized, PO—AGE and ECH—PO—AGE. These polymers show excellent low temperature flexibihty and low gas permeabihty. After compounding, PO—AGE copolymer is highly resiUent, and shows excellent flex life and flexibiUty at extremely low temperatures (ca —65°C). It is slightly better than natural mbber in these characteristics. Resistance to oil, fuels, and solvents is moderate to poor. Wear resistance is also poor. Unlike natural mbber, PO—AGE is ozone resistant and resistant to aging at high temperatures. The properties of compounded ECH—PO—AGE he somewhere between those of ECH—EO copolymer and PO—AGE copolymer (22). As the ECH content of the terpolymer increases, fuel resistance increases while low temperature flexibihty decreases. Heat resistance is similar to ECH—EO fuel resistance is similar to polychloroprene. The uncured mbber is soluble in aromatic solvents and ketones. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

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 World War II, polychloroprene was chosen as a replacement for natural rubber because of its availability. Two copolymers of chloroprene and sulphur which contain thiuram disulphide were available (Neoprene GN and CG). One of the first successful applications of these polychloroprene adhesives was for temporary and permanent sole attachment in the shoe industry. However, these polychloroprene cements show a decrease in viscosity on ageing and a black discolouration appears during storage in steel drums. Discolouration was produced by trace amounts of hydrochloric acid produced by oxidation of polychloroprene... 

Neoprene AF ( 963). It is a polychloroprene modified with methacrylic acid. Although it is a slow-crystallizing elastomer, the cohesive strength develops very rapidly and it has improved creep resistance at high temperature compared with Neoprene AC or AD. The improved properties of Neoprene AF are derived from the interaction between the carboxyl functionality with the metal oxides added in the solvent-borne polychloroprene adhesives. 

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

Metal oxides. Magnesium oxide is used to cure polychloroprene by converting its few active allylic chloride from 1,2 addition into ether cross-links. There is a synergistic effect when magnesium oxide is used in combination with t-butyl phenolic resins in solvent-borne polychloroprene adhesives. When solvent is removed, the phenolic group in the resin reacts with the magnesium oxide to cross-link [49]. 

Most rubbers used in adhesives are not resistant to oxidation. Because the degree of unsaturation present in the polymer backbone of natural rubber, styrene-butadiene rubber, nitrile rubber and polychloroprene rubber, they can easily react with oxygen. Butyl rubber, however, possesses small degree of unsaturation and is quite resistant to oxidation. The effects of oxidation in rubber base adhesives after some years of service life can be assessed using FTIR spectroscopy. The ratio of the intensities of the absorption bands at 1740 cm" (carbonyl group) and at 2900 cm" (carbon-hydrogen bonds) significantly increases when the elastomer has been oxidized [50]. 

Formulation of a solvent-borne CR. A typical formulation of a solvent-borne CR adhesive may include the following components (fillers are not commonly added and curing agents are added to improve heat resistance) (1) polychloroprene elastomer (2) metal oxides (3) resins (4) antioxidants (5) solvents (6) fillers (7) curing agents (8) other modifiers. 

Metal oxides. Metal oxides provide several functions in solvent-borne polychloroprene adhesives. 

Acid acceptor. This is the main function of metal oxides in CR adhesive formulations. Upon age, small amounts of hydrochloric acid are released which may cause discolouration and substrate degradation. Magnesium oxide (4 phr) and zinc oxide (5 phr) act synergistically in the stabilization of solvent-borne polychloroprene adhesives against dehydrochlorination. 

Scorch retarder. Magnesium oxide retards scorch during mill processing of polychloroprene adhesives. 

Solutions of polychloroprene adhesives containing metal oxides and r-butyl phenolic resin may show phasing (e.g. clear upper layer and flocculated lower layer of metal oxides) on standing for days or months. To recover the full utility... 

Antioxidants. Similar antioxidants as for solvent-borne CR adhesives can be used. Addition of antioxidants is important when resins sensitive to oxidation are included in polychloroprene latex formulations. 2 phr is the common amount of antioxidant in latex adhesives. 

The most common use of curing agents is with carboxylic latices. Isocyanates and melamines can be used but zinc oxide is the most common curing agent. Zinc oxide cross-links carboxylated latices and improves bond strength by ionomer formation [78]. Carboxylated polychloroprene reacts slowly with zinc oxide in dispersed form, causing a gradual increase in adhesive gel content. This can lead to restricted adhesive shelf life. Resin acid sites compete with the polymer acid sites for Zn(II). The more resin acid sites, the more stable the adhesive. 

Solvent-borne polychloroprene adhesives are unsuitable for bonding low-energy substrates, such as PVC. However, water-borne polychloroprene adhesives display good peel adhesion to vinyl substrates. Addition of an accelerator such as zinc oxide is essential for improved hot bond strength. 

Polychloroprene and Hypalon are usually cross-linked with metal oxides which combine with reactive side groups in the polymer chain. 

Polychloroprene CR Satisfactory resistance organics fair poor with oxidative chemicals and hydrocarbon solvents Fair 0 to 100... 

FIGURE 4.8 Comparative tensile stress-strain plot of polychloroprene-ordinary zinc oxide (ZnO) and poly-chloroprene-nano-ZnO system. (From Sahoo, S., Kar, S., Ganguly, A., Maiti, M., and Bhowmick, A.K., Polym. Polym. Compos., 2007 (in press). Courtesy of Smithers Rapra Technology Ltd.)... 

A small amount of the bis-azo compound—sufficient to react with only 1 or 2 percent of the isoprene units—converts the specimen to a material having all of the physical properties characteristic of vulcanized rubber. Polychloroprene may be vulcanized by the action of metal oxides... 

Polychloroprene rubbers are not efficiently vulcanized by sulfur. The chlorine atoms deactivate the double bonds toward reaction with sulfur. Vulcanization is achieved by heating with zinc and magnesium oxides. Crosslinking involves the loss of... 

An activator in rubber compounds containing organic accelerators. In polychloroprene, zinc oxide is considered to be the accelerator rather than the activator. The use of zinc oxide as a reinforcing agent and as a white colouring agent is obsolescent. Zinc oxide is manufactured by either the French (or indirect) process or by the American (or direct) process. It can be used as a filler to impart high thermal conductivity. 

The chlorine atom in the repeat unit has a tendency to deactivate the double bond in the main chain, thus polychloroprene tends to resist oxidation, ozone and UV light to a higher degree than the other unsaturated rubbers, although they still require protection if the maximum performance is to be obtained. Unfortunately, this deactivation of the double bond means that the polymer cannot be crosslinked by sulphur. 

Heat, See also High temperature entries Hot entries Temperature entries Therm- entries effect on rubber aging, 27 785 in ethylene oxidation, 70 650 exponents of dimensions, 8 585t external resistance to, 25 312-316 in industrial hygiene, 74 221 wood reaction to, 26 348-351 Heat aging, of polychloroprene polymers, 79 844-845... 

These steps are typical for most of the synthetic elastomers. The use of sulfur for vulcanization is common for the production of most elastomers. Magnesium and zinc oxides are often used for the cross-linking of polychloroprene (CR). Saturated materials such as EPM and fluoroelastomers are cross-linked using typical organic cross-linking agents such as peroxides. 

DuPont was looking for a synthetic rubber (SR). Carothers assigned Arnold Collins to carry out this research. Collin s initial task was to produce pure divinylacetylene. While performing the distillation of an acetylene reaction, in 1930, he obtained a small amount of an unknown liquid, which he set aside in a test tube. After several days the liquid turned to a solid. The solid bounced and eventually was shown to be a SR polychloroprene, whose properties were similar to those of vulcanized rubber but was superior in its resistance to ozone, ordinary oxidation, and most organic liquids. It was sold under its generic name neoprene and the trade name Duprene. ... 

The vulcanization of polychloroprene (Neoprene) is carried out in different ways. Vulcanization by sulfur, even with an accelerator, is not practiced to a large extent. Vulcanizations by metal oxides (without diamine), either alone or in combination with sulfur (sometimes together with an accelerator), give the best physical properties for the crosslinked product. Halogenated butyl rubber is crosslinked in a similar manner. The mechanism for crosslinking by metal oxide alone is not established [Stewart et al., 1985 Vukov, 1984]. 

The molecular weight distribution of peroxide formed at 4% oxidation was determined with a Waters gel permeation chromatograph. The peroxide was prepared as a 0.7% (w./v.) solution in tetrahydrofuran, and the molecular weight distribution then obtained is shown in Figure 2. By analogy with polychloroprene count 25 is equivalent to about 140 monomer units in the peroxide, and the peak maximum is at about 18 units—i.e., a molecular weight of 2000. The incipient peaks at counts 34, 36, and between 32 and 33 result from products of peroxide decomposition. 

Metal Oxides. Halogen-containing elastomers such as polychloroprene and chlorosulfonated polyethylene are cross-linked by their reaction with metal oxides, typically zinc oxide. The metal oxide reacts with halogen groups in the polymer to produce an active intermediate which then reacts further to produce carbon—carbon cross-links. Zinc chloride is liberated as a by-product and it serves as an autocatalyst for this reaction. Magnesium oxide is typically used with ZnCl to control the cure rate and minimize premature cross-linking (scorch). 

Chloroprene is a monomer used almost exclusively for the production of polychloroprene elastomers and latexes. It readily forms dimers and oxidizes at room temperature. Occupational exposures occur in the polymerization of chloroprene and possibly in the manufacture of products from polychloroprene latexes. 

The oxidation of other rubbers has been studied by FT-IR including polychloro-prenes >. These results suggest that the thermal oxidation of polychloroprenes involves the 1,2 and 3,4-structural irregularities in the initial stage. In particular, it is felt that the initial step is the abstraction of a tertiary allylic chlorine or hydrogen from the 1,2 or 3,4 units yielding a tertiary carbon radical. 

Polychloroprene. Polychloroprene dispersions have a range of qualities similar to those of solvent-based polychloroprene adhesives and a similar range of uses. As an example, the bonding of vinyl materials with phenolic resin/paper decorative laminates often is carried out with these products. It is necessary to incorporate acid-acceptor dispersions of metallic oxides, and the dispersions in general do not provide such long open times as solvent-based polychloroprene adhesives. 

Poly(methyl acrylate) Poly(methyl methacrylate) Polyacrylonitrile Polymethacrylonitrile Polybutadiene Polyisoprene Polychloroprene Poly(methylene oxide) Poly(ethylene oxide) Poly(tetramethylene oxide) Poly(propylene oxide) Poly(hexamethylene succinate) Poly(hexamethylene sebacate) Poly(ethylene terephthalate) Nylon 6 Polycarbonate... 

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