The unstoppable rise of chloroprene rubber
The idea behind chloroprene rubber came from the American botanist and chemist Julius Arthur Nieuweland, who worked alongside Wallace Hume Carothers, head of research for DuPont. In 1930, Arnold Collins, a colleague of Carothers, polymerised polychloroprene for the first time under economically viable conditions using the emulsion method. Two years later, DuPont launched the polymer under the name Duprene, renaming it neoprene in 1938. Over the decades, numerous improvements were made in the chemical engineering sector to the manufacturing process and to the properties of the polymer. These resulted in:
- Sulfur-containing copolymers for improved processability (1939)
- Mercaptan-regulated variants for improved solubility and heat resistance (1950s)
- Xanthogenate-modified chloroprene rubbers for improved vulcanisate properties and a lower tendency to crystallise
Production of chloroprene rubber
In the chemical industry, chloroprene rubber is always produced by emulsion polymerisation. The resulting dispersion is precipitated by addition of acid and subsequent cooling, dried and made into chips. These are dusted with talc to prevent them from sticking together. An exception is strongly crystallising variants, which can be made into adhesives. As well as distinguishing between strongly, moderately and weakly crystallising variants, chloroprene rubber can also be divided into sulfur-modified and mercaptan types, depending on the manufacturing process.
Modifying the production parameters gives rise to various different types, some of which differ considerably in terms of their properties. For instance, processing characteristics and elastic properties are very much dependent on the polymerisation temperature. As the temperature rises, the chain structure becomes less uniform, causing the rate of crystallisation of the polymers to fall.
At low temperatures, polymerised chloroprene rubber grades have a strong and rapid tendency to crystallise. This is particularly important in adhesives, which need to have a high initial strength. Their high level of hardness and low elasticity means that these chloroprene rubber polymers are not suitable for manufacturing rubber products. Instead, polychloroprene grades polymerised at elevated temperature are used for this purpose.
The tendency to crystallise can also be reduced by copolymerisation, with acrylonitrile or styrene for example.
Vulcanisation of chloroprene rubber
Polychloroprene is processed by means of the usual rubber shaping methods. However, chloroprene rubber cannot be cured with sulfur. Metal oxides such as magnesium oxide (MgO) or zinc oxide (ZnO) are generally used instead. The water resistance can be improved using lead oxide, although the use of this substance is restricted for environmental protection reasons.
A typical vulcanisation accelerator for chloroprene rubber is ethylene thiourea (ETU). The chemical structures that are formed by vulcanising polychloroprene with MgO and ZnO with addition of ETU can only be attributed to the reaction of the allyl-bound chlorine, only small amounts of which are found in the polymer chain. The more prolific vinyl-bound chlorine atoms are largely unreactive under the vulcanising conditions (approx. 160°C). Because of that, the crosslink density cannot be increased even by adding more vulcanising agents.
New approaches in chloroprene rubber production
ETU-based activators are currently the industry standard for accelerating the vulcanising process. However, a number of European bodies have now classified ethylene thiourea as being potentially carcinogenic. For that reason it is likely that the use of this substance in the chemical industry will be restricted or banned altogether in the near future. Scientists involved in the SAFERUBBER project have succeeded in developing an environmentally friendly and low-cost alternative.
The additive offers significant advantages over ETU. For example, chloroprene rubbers produced with additive have better flow properties and can therefore be poured into moulds more easily. This enables the amount of rubber used, and the associated waste, to be reduced. The proportion of zinc oxide used as a vulcanisation activator can also be lowered in this way.
Chloroprene rubber can be blended with various other polymers, for example:
- with polybutadiene (BR) or natural rubber (NR) to improve the low-temperature flexibility
- with nitrile rubber (NBR) to improve the oil resistance, or
- with styrene-butadiene rubber (SBR) to reduce the tendency to crystallise.
Neoprene – foamed chloroprene rubber
The many evenly distributed gas bubbles in neoprene give it outstanding thermal insulation properties. This variant of chloroprene rubber is best known for its use for immersion suits for water sports like diving and surfing. However, sports bandages, sound-absorbing layers for staircases, bottle coolers and various types of protective covers are also made from neoprene.
For use in sports clothing, neoprene is foamed in different thicknesses, according to the level of thermal insulation required. Thicker material has better insulating qualities but greater buoyancy and less stretch. Neoprene is generally coated on both sides with a textile fabric (Lycra or Nylon) to make the surface less susceptible to damage. However, there is also smooth-skin neoprene, which is coated on one side but has a smooth, closed rubber surface on the other and is suitable for use as sealing strips inside the neoprene clothing. Uncoated suits are also available; these are exceptionally elastic and thus allow good freedom of movement. The disadvantage of this type is that they are susceptible to mechanical influences.
The individual elements of neoprene clothing are bonded together.
Properties of polychloroprene
Owing to its strain crystallisation and associated self-reinforcement, chloroprene rubber has better mechanical properties than most other synthetic rubbers, so it is a popular material in chemical technology. Its high chlorine content gives CR good flame resistance. Although it burns within a flame, it is extinguished as soon as this is removed. The degree of flame resistance is very much dependent on the plasticiser used, however.
The tensile strength and also the tear resistance and tear propagation resistance can be further improved by adding highly active carbon blacks. The values obtained in this way are similar to those for comparable natural rubber vulcanisates.
Other specific properties of polychloroprene include:
- Good resistance to embrittlement, ozone and weathering influences, although light-coloured grades have a tendency to discolour when exposed to strong light
- Good swelling resistance to mineral, vegetable and animal fats, mineral oils with a high aniline point, many refrigerants and water (depending on the compound formulation)
- Highly swelling in aromatics such as benzene, toluene, esters, ethers, ketones and chlorinated hydrocarbons
- Resistant to dilute acids and salt solutions
- Thermal application range, depending on composition: approx. -45 to +100°C (up to 130°C for short periods)
- Flame resistance
- Outstanding dynamic properties
- Moderate oil resistance
The electrical insulating properties of polychloroprene are poorer than those of natural rubber or SBR because of its polarity. However, adding certain fillers can produce mixes which are suitable for the low-voltage range.
Key applications of polychloroprene
Chloroprene rubber grades with a low to moderate tendency to crystallise are used in chemical engineering for technical rubber products requiring resistance to oils and fats, flame resistance or weather and ozone resistance.
In the automotive industry, for example, the material is found in drive belts, hoses, seals and (anti-vibration) bearings. Favourable properties such as flame resistance and good insulating properties also make polychloroprene a popular material for window profiles, construction elements, cladding and cable sheathing.
In foamed form, as neoprene, the vulcanisate is used for diving suits. Through the use of chemical blowing agents, which release gases below the vulcanising temperature, chloroprene rubber can also be expanded to make a pressure-resistant foam or sponge or foam rubber, which also offers good flame resistance. Contact adhesives can also be manufactured from polychloroprene grades with a strong tendency to crystallise.