Polyurethane has earned a reputation as a super-material for its incredible mechanical properties that allows it to absorb impacts

Polyurethane has earned a reputation as a super-material for its incredible mechanical properties that allows it to absorb impacts, fight abrasion and resist chemical attack (among others) while being relatively easy to mould and, above all, remaining comparatively inexpensive.

These properties have made it a first choice in aggressive applications to replace either rubber or steel parts and improve the characteristics of the equipment in question. Uses range from wheel loader and tractor parts to lifting equipment and other wear parts, as well as in various applications to fight vibration or to dampen impacts on heavy equipment.

Heavy trucks and yellow machines are increasingly making use of polyurethane suspension parts to outlast the harshest conditions found in Africa. A good example of the strength and durability of the materials is to be found in Africa’s cross-border truck fleet, where polyurethane has largely become a standard for these trucks in order to minimise the possibility of breakdowns because of the heavy loads and bad roads.

Unfortunately, however, not all polyurethanes are created equal. Different grades exist and formula may be mixed to give different properties to the material. Perhaps most importantly, the biggest influence in the strength of the material is the method in which it is moulded and formed to take on its final shape.

Thermoset and thermoplastic urethanes Fred Evans, Managing Director of Polyflex in Johannesburg, has been formulating different types of polyurethanes since the early 1990s for products as diverse as hydraulic seals, OEM suspension parts for trucks and various custom parts for yellow machines.

“Polyurethane is the generic name for a range of plastic and elastonomeric materials originating from a chemical called Isocynate…”

He says the difference between thermoset and thermoplastic urethane is vast. “At the start of this discussion I stress that Polyflex only manufactures thermosetting urethanes. We do not manufacture thermoplastic urethanes, which are generally of lower mechanical properties.

“Polyurethane is the generic name for a range of plastic and elastonomeric materials originating from a chemical called Isocynate. These can be formulated in many ways, using different formulations, raw materials and recipes. However, the basic manufacturing technique is supposedly very simple which has led many inept operators to start manufacturing components out of urethane and start supplying sub-standard products to the market,” says Evans.

Getting it right Evans explains that it is not a simple process and although certain basics do exist, they are just the basics and nowhere near the actual requirements to produce a saleable, industrialised product.

“Before elaborating on the mechanical properties it is important to understand the dynamics of the material and the range of properties possible with the same material…”

With nearly 40 years of experience in engineering plastics and vast international experience, he has applied the globally accepted best practice in his manufacturing plant. “Polyflex uses sophisticated urethane dispensing and ancillary machinery that is used to produce carefully formulated types of urethanes required for different products.

The formulation is tricky and has to be carefully monitored and controlled in order to obtain the best results. At Polyflex the two main products are called our normal service and our extreme service materials.

“Before elaborating on the mechanical properties it is important to understand the dynamics of the material and the range of properties possible with the same material, but using different manufacturing techniques and ratios.

“Using the same materials, but varying the mix ratios the converter has the ability to produce material with a widely varying range of physical properties. Processing technique and material selection play a large part in obtaining the desired product.

While competent converters often tailor the properties to suit specific applications, the effects of inadvertently mixing incorrect ratios can have an enormous bearing on the end product.

On the lower end of the market, the polymers are used to manufacture shoe soles, foam rubber, and imitation beer barrels for pubs and picture frames

By comparison a calibrated machine-mixed product ensures consistent results. Fit for purpose “Now we can add in the fact that there are many types of urethane raw material available to the converter – all offering different properties and with prices that range between R33 per kg to over R150 per kg.

On the lower end of the market, the polymers are used to manufacture shoe soles, foam rubber, and imitation beer barrels for pubs and picture frames. At the upper end of the price range, polymers are specifically designed for high-performance duties such as industrial tyres.

Each of these materials is able to be cross-linked with a variety of cross-linking agents also varying in price from R41 per kg to R130 per kg. “Again the end user will probably not be able to differentiate between the various materials and all too often the cheaper low-performance materials are used in the manufacture of engineered products, which may only last long enough to cover the guarantee.

Its called percentage-of-theory

But, due to the smallest mistake the mix can end-up wrong. Its called percentage-of-theory, which is a chemical term used by the converter in order to tailor the final mechanical properties to the desired specification.

Similar to the way a chef might decide how much baking powder to put in a cake so that it rises just right without collapsing in to middle – not too much and not to little. The introduction of small quantities of inferior product or exclusion of too much curative, etc., can throw the mix out and lead to poor quality urethanes.”

“If your experienced supplier is slightly more expensive there may be a very good reason for this

Evans concludes that all these factors point to the fact that engineers and technical experts need to be cautious when selecting products and rather use established and competent suppliers of polyurethane.

“If your experienced supplier is slightly more expensive there may be a very good reason for this – due to a better quality of materials used, capital expenditure on technologically advanced machines and trained staff.

It is never worth the risk of going the ‘bucket and broomstick’ route, nor taking a chance on products where the originals cannot be traced.” Fred Evans, Managing Director of Polyflex.