Specification

Polyurethanes compete against many others materials including rubber, plastics and metal. The most common method for classifying polyurethanes is a according to their hardness. The below shows how polyurethanes rate in hard to other materials.

HARDNESS

SIGMATHANE offers a very wide hardness range in which items can be produced. They range from 10-15 Shore A which is softer than a gum eraser to over 75 Shore D which is much harder than a golf ball. For those unfamiliar with this method of measuring hardness, the illustration above, and the photos below may be of help.

hardness

ABRASION RESISTANCE

In applications where severe wear is a problem, SIGMATHANE offers outstanding durability when compared with rubber plastics or even metals. In many applications the unusual combination of properties has made it possible to design and fabricate products from SIGMATHANE using less material than has been possible with other elastomers. It should be emphasised, however, that abrasion resistance is a complex property. Selection of the proper formulation of SIGMATHANE should be based on actual experience or simulated service tests. For comparative abrasion data please consult abrasion physical data.

ABRASION-RESISTANCE

COMPRESSION PROPERTIES

SIGMATHANE 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 which makes them possible. In addition to high load bearing properties in both tension and compression, SIGMATHANE have a high load bearing capacity in shear.

MECHANICAL PROPERTIES

At lower hardness levels practically all elastomeric materials, including SIGMATHANE 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, SIGMATHANE even 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 SIGMATHANE, leads to many applications in engineering.

TENSILE PROPERTIES

SIGMATHANE are characterised by high elongation, high tensile strength and high modulus. This provides a combination of toughness and durability in fabricated parts. The tensile test is normally carried out on a tensile testing machine. In this test we are interested not just in the overall stress-strain curve. The shape of the curve and the ultimate elongation provide valuable information eg. High elongation at break, a long rubbery plateau followed by a steep rise to break indicates a high toughness.

TENSILE-PROPERTIES

TEAR

Tear strength together with a high split tear is a strong indication of toughness. In actual use, especially in impingement abrasion application, high tear strength is important SIGMATHANE in this regard have a distinct advantage over conventional rubber.

TEAR-STRENGTH

RESILIENCE

Resilience in conventional rubbers is generally a function of hardness. This often undesirable relationship does not hold true with SIGMATHANE Formulations are available in a very wide range of resilience.

For very 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 40-65% rebound value, are used.

In general, toughness is enhanced by resilience.

FLEX PROPERTIES

SIGMATHANE 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 however, SIGMATHANE can be used practically in very thin sections because of their strength and toughness.

LOW TEMPERATURE PROPERTIES

Many SIGMATHANE remain flexible at very low temperatures and possess outstanding resistance to thermal shock The low temperature resistance of SIGMATHANE has led to many applications in below 0oC conditions.

DRY HEAT RESISTANCE

WATER RESISTANCE

Depending on the type, SIGMATHANE elastomers are very resistant to the effects of water immersion and have excellent long-term stability in water up to 50°C. Continuous use in hot water over 80°C is not recommended.

Water absorption is very low, in the range of 0.3-1% by weight and volume swell is negligible. This means, for example, that SIGMATHANE can operate at close tolerance in water lubricated bearings without fear of seizure.

The moisture vapour transmission rate of SIGMATHANE is relatively high and advantage is taken of this fact in some applications, eg. Poromeric shoe upper materials. However, where this property might be disadvantageous. the advice of our Technical Service personnel should be sought on the suitability of SIGMATHANE for any particular application.

ELECTRICAL PROPERTIES

Some SIGMATHANE have very good insulating properties and are used in many potting and encapsulating applications.

OXYGEN AND OZONE RESISTANCE

Products made from SIGMATHANE are highly resistant to degradation by atmospheric oxygen and ozone. Tests on samples, aged over 500 hours in an atmosphere containing 3ppm ozone, show no attack even while under 20% strain. Past experience has shown that materials which resist this concentration for several hundred hours are virtually immune to attack by normal atmospheric concentrations.

This makes SIGMATHANE highly successful when employed around electrical equipment, without the hardening and cracking often experienced with conventional elastomers, and indeed, many plastics.

OIL GREASE AND CHEMICAL RESISTANCE

Whilst many rubbers and plastics have excellent resistance to one or more specific solvents, oils or chemicals, the wide resistance of SIGMATHANE elatomers 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.

For specific information on chemical resistance, please consult the Chemical Resistance Chart in the Physical Data section.

RADIATION RESISTANCE

SIGMATHANE are considered to have the best resistance to gamma ray radiation of all elastomers. They retain a high proportion of their original flexibility and toughness whilst exposed to gamma radiation.

FLAME RESISTANCE

Special compounds containing flame retardants can be formulated to meet several self-extinguishing or non burning specifications.

MOULD, MILDEW, FUNGUS RESISTANCE

Suitably compounded SIGMATHANE, usually polyether based, do not support fungus growth and are generally resistant to such attack. This makes them particularly suitable for tropical environments. Advice on the most suitable material should be discussed with our Technical Service personnel.

FRICTIONAL PROPERTIES

SIGMATHANE resemble most plastics and elastomers in that friction against non-lubricated surfaces generally decreases with increasing hardness. A high coefficient of friction is valuable for such products as solid industrial tyres, feed rollers, drive rollers etc.

High hardness compounds have the lowest coefficient of friction, and formulations having very low values are available. Such formulations are widely used for bushings, bearings and wear strips.

Wear of shafts and mating surfaces is minimal, and usually considerably less than with conventional plastic materials.

MACHINEABILITY

SIGMATHANE can be machined using conventional equipment but you should consult us for specific recommendations and procedures.

BONDING TO OTHER MATERIALS

During the initial molding process and under controlled conditions, SIGMATHANE 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 Erapol itself. These bond strengths usually exceed the bond of rubber-metal substrate by several times.

It is more difficult to bond cured Sigma Elastomers sheet or moldings to other materials but special techniques have been developed to satisfy most requirements. We supply a complete range of bonding agents and primers to meet specific requirements.

NOISE REDUCTION

Special formulations are available for sound deadening and anti-vibration applications.