Polychloroprene (CR)

Repeat Unit 2

CH

C

(

CH

₎

n

CH

Cl

This polymer is frequently, but incorrectly, referred to as Neoprene which is a trade name.

Polychloroprene is produced by emulsion polymerisation, during which the following forms of addition are possible:

C CH₂ Cl C CH₂ H C CH₂ Cl C CH2 H - 1,4 addition

cis trans - 1,4 addition

C CH₂ Cl CH₂ 1,2 addition CH C CH Cl CH2 3,4 addition CH₂

Since it is not possible to commercially produce a polymer that is based on the cis 1,4 form, commercial polymers are based on the trans 1,4 form which has a crystalline melting point, Tm, of

+75 °C and a Tg of –45 °C. Pure 1,4 trans polychloroprene thus crystallises readily and would

normally be considered to be of limited use for a rubber. Such a polymer, however, does not crystallise when dissolved in a solvent, but will do so when the solvent evaporates. This feature is used to good effect in the production of contact adhesives.

The temperature of polymerisation, however, influences how closely the polymer attains the trans 1,4 form. Raising the polymerisation temperature from –40 °C to +40 °C increases the percentage of 1,2 and 3,4 forms both of which reduce the regularity and hence the tendency to crystallise. Thus chloroprene based polymers that are intended to be rubbery are polymerised at higher temperatures. The 1,2 grouping in the main chain is the site of crosslinking reactions during cure. The ability to crystallise can also be controlled by copolymerising chloroprene with small amounts of other monomers.

Two different mechanisms are used to control the molecular weight of the polymer during polymerisation:

^ In the so-called G types, sulphur is copolymerised with the chloroprene to yield a product as

shown schematically below:

The G types are stabilised with TETD, with the result that the G types do not require further acceleration to cure.

^ In the so-called W types, the molecular weight is controlled by the use of a mercaptan.

The following differences are apparent between the G and W types:

G types

G types can break down during mixing or milling via cleavage at the Sx group; this decreases molecular weight and hence reduces the elasticity, or nerve, during processing. The extent of breakdown is somewhat dependent on the exact grade, Neoprene GW being virtually unaffected by milling. Cleavage at the Sx group can also occur during long-term storage, and the G types therefore have the disadvantage of a limited storage life.

The G types do not require further acceleration during cure, but exhibit slightly inferior ageing characteristics. Resilience and tack are generally better than with the W types.

W Types

These types exhibit superior storage life, and ageing characteristics, but require the addition of accelerators to achieve an acceptable rate of cure. They do not break down during mixing. During processing they are less prone to scorch, and will accept higher loadings of filler. The cured compound generally exhibits a lower compression set, and a greater ability to resist heat ageing. 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.

The chlorine atom also confers an increased level of resistance to oils, so that the oil resistance of polychloroprene is roughly intermediate between natural rubber and nitrile rubber, and is often sufficient for many applications. Polychloroprene is also self-extinguishing in flame tests.

Metal oxides are principally used for curing these materials; peroxides are generally not used. The most widely used cure system is based on magnesium oxide/zinc oxide, the cured properties achieved being dependent on the ratio of the two; the most common ratio is magnesium oxide 4.0 and zinc oxide 5.0. As the zinc oxide tends to promote scorch it is added late in the mixing cycle, whilst magnesium oxide is added early. One drawback of the MgO/ZnO cure system is that chlorine liberated during cure reacts with the oxides to yield chlorides which are hydrophilic and compounds containing this cure system can swell in hot water; even in cold water swell can be progressive and eventually large.

Lead oxide (PbO or Pb3O4) up to levels of 20 phr can be used to improve resistance to water as the

chloride formed during cure is insoluble.

The W types require additional acceleration and ethylene thiourea (ETU), gives the best balance of all properties. However, the use of this accelerator is increasingly being restricted due to fears of its effects on pregnant women, and more recently men. DETU, thiurams and guanidines can also be used. Sulphur is sometimes used to increase the degree of cure in the W types, but this detracts from the ageing performance of the vulcanisate.

Uses

Due to its balance of strength, oil resistance, inflammability, increased resistance to ozone, ageing and weathering, polychloroprene finds widespread industrial use. Typical uses are V-belts, conveyor belts, wire and cable jacketing, footwear, wet suit applications, coated fabrics, inflatables, hoses, extrusions and many other goods. Adhesives are also a strong market area.