PU Foams in Sport and Leisure Applications
by Duncan Geddes
Several kinds of polyurethanes are of commercial significance and can be conveniently classified as follows – flexible foams, rigid foams, elastomers, fibres and moulding compositions, surface coatings and adhesives. Today polyurethane foams are used in a wide variety of applications due to their versatility, light weight, durability, ease of processing and cost efficiency. One area that takes advantage of the intrinsic properties of polyurethane foams is the sport and leisure market.
Common properties of PU foams
The first commercial use of viscoelastic/memory foam was when it was developed during the mid 1960’s as a result of NASA’s AMES research technology where it was used as a cushioning material to reduce the G forces experienced by astronauts during take-off and landing. Nowadays due to the foams ability to dampen vibrations as well as shock it is used in racing car cockpits as a noise and vibration harshness (NVH) foam and as the padded foam in racing driver’s helmets.
Other types of foam that can be used in motor racing applications are reticulated polyurethane foams, the open cell structure of reticulated foam makes it particularly suitable for air filter applications and facilitates optimum engine performance in combination with maximum airflow/minimum pressure drop. The open cell nature of the foams allows them to be post treated with latices containing flame retardant additives to protect them against large ignition sources. Specially formulated reticulated foam is also used for fuel explosion suppression within fuel tanks of racing cars.
PU foam for helmets and safety mats
Motorcycle helmets and cycling helmets designed to protect the rider’s head during impact consist of two main components, a hard outer protective shell and an inner energy absorbing layer. The hard shell spreads the initial impact whilst the energy absorbing layer, which is often polyurethane foam, cushions and absorbs energy so that less impact energy is transferred to the rider’s skull and hence the brain.
In contact sports such as American football, lacrosse, hockey and cricket hard protective helmets made from thermoplastic polymers such as polycarbonate are often lined with shock absorbing foams such as polyurethane or synthetic rubber foams. The cushioning and energy absorption properties of flexible polyurethane foam can be readily modified by adjusting the formulation used to manufacture the foam, making it suitable as a personal protection material for various types of sports headgear and for other body parts.
Polyurethane foam is also widely used as a cushioning material for various types of gymnastic mats, martial arts mats, judo mats, wrestling mats and agility mats for use in schools, sports halls, leisure centres and climbing centres etc. The main type of foam used is combustion modified rebond foam (a re-cycled foam) made from scrap foam generated from block foam production and conversion operations, which has been chipped and bonded together with a polyurethane binder. Mats for high jumping and pole vaulting are filled with good quality low density polyurethane foam for impact protection. BS 12503-1:2001 (Parts 1-7) is the European Standard for classifying all types of sports mats by their use and their safety requirements such as shock absorption.
Other uses for PU foam
Bouldering is a form of rock climbing which takes place on boulders or small rock formations where no ropes are used to aid the climber. Boulder mats made from combinations of polyurethane foam are used for protection against falling. The mats are generally made of either two or three layers of polyurethane foam to provide different levels of support and impact cushioning. Higher density combustion modified rebond foam is used for the top layer (130-160kg/m3) with lower density combustion modified polyurethane foam being used for the lower layers.
Polyurethane foam logs are used in dismount pits in gymnastics centres and in large trampoline parks which have started to become popular in the UK. Different qualities of polyurethane foam can be used depending on how often the pits are used. Where the usage of the pits is high such as trampoline parks, higher density foams having better tear and abrasion resistance increases the longevity of the foam before it needs replacing.
Low to medium density high load bearing polyurethane packaging foams which are designed to provide mechanical damping of unexpected impacts are used as the foam core in rugby post protectors, and also in tackle and rucking shields. For tackle bag’s which allows the player to practice tackling high resilient foam is utilised.
Shoes and trainers
Today, sports shoes are generally required to have as low a weight as possible, should be comfortable and have a resilient sole with good abrasion resistance. Polyurethane integral foams are able to meet these requirements. Nowadays running shoes normally consist of a thin running surface of solid or microcellular polyurethane with a centre/mid sole of softer foam of approximately 400kg/m3. For the insole viscoelastic/memory foam or high load bearing polyurethane foam’s are utilised to provide cushioning, the foams can also contain antibacterial additives to prevent problems with foot infections.
The characteristics of flexible polyurethane foams are predominantly determined by the starting materials and the formulations used in their manufacture, along with additives which can modify the polymer chemistry and other properties such as flammability, antibacterial, antistatic and acoustic properties etc. It is not surprising therefore that polyurethane foam is found in a variety of sport applications from personal protective equipment through to athletic footwear and as a sound absorbing material in sports halls.