WHAT ARE ELASTOMERS ?
Polyurethane Elastomers?are materials offering a number of unique properties which enable products to be manufactured to meet a range of demanding applications.
It is not claimed that Polyurethanes are the answer to all problems, but they are extremely versatile and this is the key to their widespread and growing use.
There are many different types of Polyurethanes. The four basic types are:
Each has its own performance advantages. As a general guide:?Polyethers?are recommended for applications where parts undergo dynamic stress ie. they incur lower heat build-up. They also have advantages in resilience, low temperature performance and resistance to water attack (hydrolosis).?Polyethers?also have lower viscosity and specific gravity.
Polyester?based urethanes have superior cut, tear, abrasion and oil solvent resistance.
MDI?based products have lower isocyanate odour than similar?TDI?types and have superior hydrolysis resistance and higher resilience.
TDI?based products are less sensitive to moisture, have shorter demould times and lower cure temperature requirements compared toMDI?types.
Properties of Polyurethane Elastomers
Polyurethanes compete against many other materials including rubber, plastic and metals. The most common method of classifying Polyurethanes is according to their hardness. The table below shows how Polyurethanes rate in hardness to other materials.
Elastomers Queensland offer a very wide hardness range in which items can be produced. They range from 20 shore A which is softer than a gum eraser to over 80 Shore D which is much harder then a golf ball.
In applications where severe wear is a problem, Polyurethane Elastomers offer outstanding durability when compared with rubber, plastics or even metals. It should be emphasised, however that abrasion resistance is a complex property. Selection of the proper formulation of Polyurethane Elastomer should be based on actual experience or simulated service tests.
Polyurethane Elastomers have greater load-bearing capability than do conventional elastomers of equal hardness. This capacity is a highly important advantage in some designs, eg. stripper springs, whilst in others it is the very factor that makes them possible. In addition to high load bearing properties both tension and compression polyurethanes have a high load bearing capacity in shear.
At lower hardness levels, practically all elastomeric materials, including Polyurethane Elastomers, merely bend under impact. As conventional elastomers are compounded up to higher hardness they tend to lose elasticity and crack under impact. On the other hand, Polyurethane Elastomers when at their highest hardness levels, have significantly better impact resistance than almost all plastics. Such great toughness, combined with the many other outstanding properties associated with the high hardness Polyurethane leads to many applications in engineering.
Low Temperature Properties
Many Polyurethane Elastomers remain flexible at very low temperatures and possess outstanding resistance to thermal shock. The low temperature resistance of Polyurethane has lead to many applications in below zero degrees celcius conditions.
Dry Heat Resistance
Whilst many Polyurethane Elastomers are only suitable for continuous operation up to 80 degrees celcius, intermittent use of up to 100 degrees celcius is possible.
Oxygen and Ozone Resistance
Products made from Polyurethanes are highly resistant to degradation by atmospheric oxygen and ozone.
Oil, Grease and Chemical Resistance
Whilst many rubbers and plastics have excellence to one or more specific solvents, oils or chemicals, the wide resistance of Polyurethanes to chemical attack means that they can be used in a multitude of environments with the exception of strong acids and alkalis and certain strong solvents. As with all materials being examined for oil and chemical resistance, it is best to place a sample of the material in actual service. If this is not practical, tests should be devised which simulate actual service conditions as closely as possible.
Mould, Mildew, Fungus Resistance
Suitably compounded Polyurethane Elastomers, usually polyether based, do not support fungus growth and are generally resistant to such attack. This makes them particularly suitable for tropical environments.
Polyurethane Elastomers can be machined using conventional equipment but you should consult us for specific recommendations and procedures.
Bonding to Other Materials
During the initial moulding process and under controlled conditions, Polyurethanes can be bonded to a wide variety of materials. High strength bonds can be obtained to most metals, wood and many plastics. Bond strengths often exceed the tear strength of the Polyurethane itself.
Polyurethanes are characterised by high elongation, high tensile strength and high modulus. This provides a combination of toughness and durability in fabricated parts.
Especially high split tear, is a strong indicator of toughness. In actual use, especially in impingement abrasion applications, high tear strength is important. Polyurethanes in this regard have a distinct advantage over conventional rubber.
Resilience in conventional rubbers is generally a function of hardness. This often undesirable relationship does not hold true with Polyurethanes. Formulations are available in a very wide range of resilience. For shock-absorbing uses, low rebound compounds are usually used ie. rebound value of 10-40%. For high frequency vibrations or where quick recovery is required, compounds in the 45-65% rebound value, are used. In general, toughness is enhanced by high resilience.
Polyurethane Elastomers resist cracking under repeated flexing. As with any elastomer, the rate of cut growth under flexing may be reduced by decreasing the thickness of the part. Unlike other elastomers, Polyurethane Elastomers can be used practically in very thin sections because of their strength and toughness.
Depending on the type, Polyurethane Polyether Elastomers are very resistant to the effects of water immersion and have excellent long-term stability in water up to 50 degrees celcius. Continuous use in hot water over 80 degrees celcius is not recommended. Water absorption is very low, in the range of 0.3-1% by weight and volume swell is negligible.
Some Polyurethanes are very good insulating properties and are used in many potting and encapsulating applications.
Polyurethane Elastomers are considered to have the best resistance to gamma radiation of all elastomers. They retain a high proportion of their original flexibility and toughness whilst exposed to gamma radiation.
Special compounds containing flame retardants can be formulated to meet several self-extinguishing or non-burning specifications.
Polyurethanes resemble most plastics and elastomers in that friction against non-lubricated surfaces generally decrease with increasing hardness. A high co-efficient of friction is valuable for such products as solid industrial tyres, feed rollers, drive rollers etc. High hardness compounds have the lowest co-efficient of friction, and formulations having very low values are available. Such formulations are widely used for bushings, bearings and wear strips.
Special formulations are available for sound deadening and anti-vibration applications.