Chloprene Rubber (CR) is the earliest commercially successful synthetic rubber. This was introduced by Dupont under the trade name ‘Duprene’ in 1932. Later is was sold under the trade name ‘Neoprene’. During the second World War, US Government designated this as GR-M (Government Rubber - Mono Vinyl Acetylene). This is now being commercially produced by several large companies including Dupont.

Chloprene rubber provides a very useful all-round balance of properties. The monomeric unit of
CR is 2-chloro-1,3-butadiene.

CR is a sterio regular rubber, being in fact largely trans - 1, 4 Poly Chloprene (88%). Cis accounts for 9.5% 1,2 around 1.5% and 3,4 addition around 1%. The 1, 2 addition makes Chlorine available in the principle site of vulcanization. The trans content attributes crystalline
nature on stretching. The gum vulcanization of Chloroprene have high tensile strength. It has moderate resistance to swelling in oils/solvents combined with good resistance to low temperature softening. It shows good resistance to weathering, heat ageing, oxidative and ozone ageing.

Production of Chloroprene Rubber

CR is produced free-radical emulsion polymerisation of chloroprene. Chloprene is produced from butadiene.

Two types of processess are usually used to emulsion polymerisation of Chloroprene :

(1) Sulphur modified or Thiuram modified

The control of the polymer molecular weight is achieved by copolymerisation. Polymer chains are cleaved at the sites where the Sulphur is copolymerised.

(2) Mercaptan modified

The control of the molecular weight is achieved by the inclusion within the polymerisation receipe of chain transfer agent.

When Chloprene is emulsion polymerised in the absence of any above modifiers the product form even at low conversion is a tough, insoluble, non-plastic material. Addition of 0.2 - 1.5% of Sulphur is dissolved form in the monomer is still not suitable as an elastomer. If polymer in the latex form is heated in an alkaline medium with a ‘thiophilic’ agent, such as Tetraethyl Thiuram Disulphide the polymer becomes plasticised to a soft, soluble Polychloroprene which is suitable as an elastomer.

Outline of Process

(1) Sulphur and Rosin-acid soap dissolved in Chloroprene monomer.
(2) Emulsification of solution in water in which NaOH and sodium salt of the naphthalene sulphonic acid - formaldehyde condensation product have been dissolved.
(3) Insitu formation of soap-sodium resinate at the interface between hte Chloroprene drop lets and the aqueous phase. This results in the formation of a stable emulsion of Chloroprene in water.
(4) Emulsification achieved by re-circulation the Chloroprene-Water mixture through centrifugal pump.
(5) Polymerisation in an agitator at controlled temperature. Extend of polymerisation measured by the specific gravity of reaction mixture.
(6) Polymerisation stopped by addition of Tetraethy Thiuram disulphide. This acts as the stabiliser also.
(7) Acidification of alkaline latex to a pH of 5.5 - 5.8 by addition of acetic acid. This also has the effect of arresting the petising action of Thiuram disulphide, conversion of Sodium resinate back to Rosin acid and also prepares the latex for the subsequent separation of the polymer.
(8) Separation of polymer by continuous cogaulation of a polymer film, followed by washing and drying.

Typical Polymerisation Receipe

Grading of Chloroprene Rubber

Chloroprenes are classified as general purpose or adhesive types. The general purpose types are divided into three classes : G, W and T. G types are copolymerised with sulpur and thiuram disulphide. The W types are modified during manufacture with a mercaptan which gives them greater storage stability and are more crystallization resistant. These grades normally require organic accelerators to cure reasonably fast. The T types are closely related to W types and contain a polymer gel fraction to imporve behaviour. These types have very low nerve and shrinkage. The G, W and T types are further classified into different grades. The general purpose types are used in a variety of applications-particularly molded and extruded goods, hose helts, wrie, cable, heets and soles, coated fabrics, gaskets, etc.

Adhesive types

Different grades of adhesive types of Neoprene is available-Neoprenes AC, AD and CG show a high degree of crystallinity. Neoprene AF shows a low degree of crystallinity.

Toluene is usually used as a solvent for dissolving AC, AD or CG in either chip or milled forms. With AF, some polar solvent should be used with toluene to increase polymer solubility.

Compounding

In curing Neoprenes, metallic oxides are considered essential with 4 parts of light calcined MgO and 5 parts of ZnO standard. The W and T grades do require organic acceleration and thiourea at 0.5-1.0 part is probably the most economical. All Neoprene compounds require 1-2 Phr antioxidant for good ageing resistance. For reinforcement blacks, blacks, silica or hydrated calcium silicate may be used. Whiting is an extender in neoprene. Petroleum oils are used as softeners in Neoprene, usually aromatic. Processing aids include low molecular weight Polyethylene, Microcrystalline waxes and palm oil. CI resins and hydrogenated rosin esters are used as tackifiers.

When water resistance is required red lead (Pb3O4) is used as the curing agent in place of MgO and ZnO.