Glossary of Terms
Chemicals which are added to rubbers to accelerate the rate of vulcanisation. Rubber without accelerators takes twenty or thirty times longer to cure.
Mostly used in neoprenes to absorb the acid produced by the chemical reactions which take place during curing. Acid acceptors are usually metal oxides.
Chemicals which are added to rubbers to activate curing.
A “breaking up” of the rubber normally observed around tool split lines. It results from the rubber shrinking during cure and tearing away at points where the moulding is held.
An internal mixer commonly used in the rubber industry. It consists of two shaped rotors which turn inside a sealed chamber.
The exudation of droplets of excess plasticiser onto the surface of the rubber. This happens when rubbers contain unsuitable levels of plasticisers which are not fully compatible.
A white or light coloured crystalline deposit on the surface of the rubber. Many chemicals added to rubber have a limited solubility. If they are present in a cured rubber at levels higher than their solubility, they crystallise out onto the surface.
Term used to describe the ability of a rubber to stick to itself in the uncured state, so allowing pre-building of the material before curing.
Producing rubber in continuous sheets by processing it through a series of rollers. Harboro does not specialise in this sector of the industry.
Commonly used as an “extender” in rubber. Treated clays reinforce the rubber and are termed “hard” clays. Those which do not provide reinforcement are called “soft” clays.
A general word used to describe a particular formulation or “recipe”, or as a descriptive term for unvulcanised rubber in general. It is used in such applications as “compounding ingredients”, “compound no. Xyz”, etc.
The degree to which a rubber does not recover fully to its original state after it has been compressed for a long period of time. (See also “measurement of rubber properties” on pages 28-29).
Rubber materials tend to crystallise on standing, resulting in an increase in hardness. This increase is usually small, but can be more significant in the case of some neoprenes (polychloroprenes).
The chemicals involved in curing the rubber, e.g. Accelerators, vulcanising agents, activators etc.
Also known as vulcanisation, this is the permanent change which the rubber undergoes during moulding. When mixed, rubber compounds are thermoplastic and will melt and deform easily. After curing, the rubber is heat stable.
General term used to describe all natural and synthetic polymeric materials which have rubbery or “elastic” properties.
Describes the ability of a rubber to withstand high voltages. It is usually quoted in kv per mm of rubber thickness. (See also measurement of rubber properties)
Producing continuous lengths of rubber with a constant profile by forcing it through a die under pressure and vulcanising it.
General term describing the bulk materials which are added to the rubber (usually as powders).
Excess rubber found on mouldings as a result of the moulding process, but which does not form a part of the moulding itself. This does not usually include feed gates but refers particularly to thin films of rubber formed at tool split lines etc.
The tendency of some materials to crack as a result of repeated bending or stressing at the same point.
The term used to describe a rubber “recipe”.
Also known as “accelerated ageing”. Rubber materials are often tested for shorter times at higher temperatures to predict what will happen over longer times and at lower temperatures.
The difference between the amount of energy absorbed when a rubber is stretched and the amount of energy released when the rubber is relaxed. High hysteresis indicates a high loss of energy (and so is good for energy absorbing applications). Low hysteresis rubbers are more resilient.
The set of rubbers resulting from a single mould cycle or pressing. Also called a “heat”.
The relationship between stress (force) and strain (extended length). Modulus is usually given as the stress (or force) required to extend a rubber (e.g. 100%, 200% Extended).
Liquids which are incorporated into rubber, generally described as plasticisers because of their softening effect.
General term used to describe all rubbers and plastics. In fact, it is the chemical term used to describe all organic materials which are formed from chains of repeated chemical units.
The technical term used to describe “bounce” or “snap”. Increasing resilience means an increasing “bounciness”.
The onset of cure, when chemical reactions begin to take place in the rubber as it is being heated. A “scorched” rubber is no longer processable.
The degree to which a rubber does not fully recover to its original shape after it has been deformed for a long period of time. Also called “permanent set”.
The change in length of a rubber under tension or compression. A rubber stretched to double its original length would be at 100% strain.
An applied force or load. A load of 1kg applied to a test piece would be a stress of 1kgf.
Any material which melts on heating and resets on cooling. This melting and refreezing can be repeated indefinitely.
Any material which melts on heating but then undergoes a permanent chemical change after which it is heat stable (i.e. it will degrade on further heating rather than “melt”).
Removing rubber from the mould before the vulcanisation process has been completed. Often results in porosity in the rubber or unusually poor compression set.
The permanent chemical change that a rubber undergoes on heating together with chemicals. The chemicals which accomplish this change are called “vulcanising agents” and crosslink the molecular rubber chains.