Neoprene

Chloroprene rubber, polychloroprene or Chloroprene rubber is a synthetic rubber, which is used in automotive engineering and for insulating sportswear. In the German -speaking world, he is known under the brand name neoprene. Neoprene is a trademark of the DuPont company, names of others are, for example Baypren Lanxess. They are prepared by polymerization of 2- chloro-1 ,3-butadiene ( chloroprene ). The abbreviation according to ISO 1043 (1975) for chloroprene rubber CR.

Structural formula

The double bonds are found both in trans and in cis position (cis, trans isomerism ). The ratio of trans / cis is about 9:1. The chlorine atoms are very slow to react and contribute to the stability and resistance of polychloroprene with. To a very small extent, an incorporation of monomers occurs in the 1,2 - or 3,4- position on. The resulting structures are 1,2 - and 3,4- units known or a function of the polymerization temperature on the order of one percent in the polymer chain.

1,2-structure unit

3,4 structural unit

1,2- unit is due to the much more reactive chloro group essential for the crosslinking ( vulcanization) of the polymer.

History

1930 by Arnold Collins, Wallace Hume Carothers in the team of, among other things, for the first time polychloroprene polymerized under economically favorable conditions in the emulsion process. In 1932, the U.S. company DuPont brought the polymer under the name Duprene, 1938, as neoprene on the market. In the following decades there were several improvements in the manufacturing process and polymer properties:

• Copolymers with sulfur ( Neoprene GN) for improved processability 1939 ( DuPont);
• Mercaptan - controlled variants in the 1950s (DuPont ) for improved heat resistance and solubility;
• Xanthate -modified chloroprene rubber in the Eighties ( Bayer AG ) for improved properties of the vulcanizates and reduced tendency to crystallize.

Production

Chloroprene rubber is produced on a large scale in the process of emulsion polymerization. The resulting dispersion was precipitated by addition of acid and subsequent cooling, dried and usually in the form of chips for the processing industry put on the market. To prevent adhesion of the chips, they are dusted with talc. However, the polymer dispersion itself is used as an adhesive, a well-known commercial form are eg the crystallizing types under the trade name Dispercoll ® C.

By the regular structure, and high content of 1,4- trans linkages of the monomer during the polymerization, polychloroprene tends, more or less crystallized, which performs some of the processing time after the hardening of the material. For adhesives this is desirable, but less so for rubber products. By suitable choice of the polymerization of the comonomers and by using regulators to adjust the molar mass, the crystallization tendency can be influenced according to the desired direction.

Properties and Applications

Dissolved in organic solvents is polychloroprene, as well as the polymer dispersion itself, suitable due to their good resistance for various adhesives. Vulcanizates are characterized by chemical resistance, good resistance to embrittlement, weathering, ozone attack and flame resistance.

• Good resistance to swelling in mineral oils with high aniline point, greases, many refrigerants and water (at a special mix design).
• Medium resistance to swelling in mineral oils, low molecular weight aliphatic hydrocarbons ( benzene or iso-octane ).
• Strongly swelling in aromatics, such as benzene, toluene, chlorinated hydrocarbons, esters, ethers, ketones.
• Operating temperature range about -45 ° C to 100 ° C depending on the mixture composition ( briefly up to 130 ° C).

Hoses, cable sheathing, extruded profiles, gaskets and belts based on chloroprene rubber can be found by the favorable combination of properties especially in the automotive industry again.

A known application, such as a material for wet suits, the foamed vulcanizate. Through the use of chemical blowing agents which release below the vulcanization gases can be a pressure-resistant foam or foam or sponge rubber with excellent insulating properties win.

The worldwide consumption of chloroprene rubber adhesives including estimated at about 300,000 tons per year.

Vulcanization

Chloroprene rubber can not be vulcanized with sulfur, in contrast to most other unsaturated elastomers. Usually, for the vulcanization of polychloroprene metal oxides such as zinc oxide (ZnO) and magnesium oxide (MgO ) is used. For improved water resistance can - for reasons of environmental protection to a limited extent - lead oxide are used. A typical vulcanization accelerator is ethylene thiourea ( ETU also = Ethylene thio urea ), considered the sulfur donor ( sulfur donor ). The chemical structures, resulting in the presence of ETU in the vulcanization of polychloroprene by ZnO and MgO, are only on the reaction of allylic bound chlorine ( see above: 1.2 unit) due to which present only a few percent in the polymer chain is. Present in the plurality of vinylic bound chlorine atoms (see above: 1.4 unit) do not react under the vulcanization ( 160 ° C), practically. For this reason, the crosslinking density of polychloroprene can not increase due to higher additions of the vulcanizing agent.

Polychloroprene can be processed with various other polymers to form polymer blends: with natural rubber (NR ) or polybutadiene ( BR) to reduce costs and improve the low- temperature flexibility, with styrene - butadiene rubber ( SBR), to reduce costs and reduce the tendency to crystallize as well as with nitrile rubber ( NBR) to improve the oil resistance.

Foamed neoprene

In foamed neoprene, many small gas bubbles are evenly distributed, which makes it has excellent thermal insulation properties. Best known, this variant is by wetsuits for water sports ( diving suits, surfing suits). However, bottle cooler, sport supports and protective covers of any kind, Noise absorbing for stairways or other pads are made of foamed neoprene.

For the use of sportswear neoprene is manufactured in different strengths depending on the desired heat insulation. Thicker material insulates better, but is also less ductile and has a higher lift.

Typically neoprene covered on both sides with fabric ( nylon or Lycra), whereby the surface is closed and is less susceptible to damage. Smooth skin neoprene is laminated on one side only and has unilaterally a closed, smooth rubber surface. This material is suitable for sealing strips within the neoprene clothing. In addition to the suit -clad versions, there are also non-laminated. They are particularly resilient and because of its close fit also water and thus heat insulating. The advantage lies in particular in the flexibility, which allows great freedom of movement. One drawback is their sensitivity to mechanical influences.

In the manufacture of clothes, the neoprene material is glued together edge to edge.