Smooth with the rough

Researchers develop polyurethane foam production process that is claimed to make tougher, cleaner and more environmentally friendly plastics. Siobhan Wagner reports

Polyurethane-based products ranging from wheelchair, wheelbarrow and bicycle tyres to building insulation foam are claimed to be more environmentally friendly as a result of new production processes from Bayer MaterialScience.

One example is the collaboration between Bayer and Middlesborough company Greentyre to develop a tailor-made system for the production of the company’s micro-cellular polyurethane tyres.

The tubeless tyre, claimed to be able to run over nails, metal shards or even broken glass without puncturing, are now being sold for wheelchairs, bicycles and light industrial use around the world. The tyres are produced in the UK and, according to the developer, created using a process that is clean, non-carcinogenic, chlorofluorocarbon-free and which releases no harmful toxins to damage the atmosphere.

The manufacturing process begins when a mould is filled with a liquid reaction mix before being rotated during the foaming process. The rotation ensures the material is distributed evenly across the whole tyre circumference.

‘The rotation method also creates a special integral skin foam structure in which the density of the foam increases towards the outside of the tyre,’ said Daniel Seidlitz, a specialist in polyurethane foam systems at Bayer. ‘The material is virtually solid at the tyre’s outer circumference, and its even surface makes for excellent rolling properties,’ he added.

The high elasticity of the micro-cellular material also ensures that the tyre’s rolling resistance is almost as good as that of an air-filled tyre.

‘Products made of this material are longer-lasting and much lighter than solid rubber tyres,’ claimed Colin Scarsi, managing director of Greentyre. When the tyre does eventually wear down it can be recycled.

Currently being sold for bikes, the tyres are also available in two sizes for wheelbarrows at DIY superstores.

The tyres are not just useful for leisure activities and gardening. Under the brand name Greencare Mobility, Greentyre has designed a special wheelchair. One key feature is its modular structure, which enables users to choose from an array of options to meet individual clinical need. It also allows them to choose from a range of tyres, depending on the type of use.

More environmentally friendly production practices and products are becoming a growing trend in many sectors, and both the UK and the US have developed guidelines that require products, including construction materials, to contain a minimum content of renewable raw materials. This is in an effort to move away from petrochemical raw materials.

To help meet these new guidelines, Bayer has developed a technology for the production of rigid polyurethane foams based on renewable raw materials. The foams could be used for insulating buildings, refrigerators and pipelines.

Natural materials such as sugar have been used for many years in the production of polyols — one of the two basic components in polyurethanes alongside isocyanates, a compound consisting of a nitrogen, carbon and oxygen atom.

However, developers are working on a growing number of natural oil polyols (NOPs) that contain vegetable oils as well as the other raw materials. The researchers believe these NOPs, which contain a higher proportion of renewable raw materials, will continue to grow in importance as they are regarded as more sustainable than conventional materials based on fossil fuels.

Yet, the NOPs now available on the market also have a number of drawbacks that limit the extent to which they can be used. ‘Very often the NOPs available in the market are not sufficiently compatible with conventional polyurethane raw materials, the ones that are based on fossil sources,’ said Klaus Lorenz, senior principle scientist with the Polyether Process Development at Bayer. ‘So in many cases there remain little degrees of freedom to combine them with other raw materials for the purpose of getting an optimised formulation.’

Lorenz said it is also difficult to find NOPs that are compatible with the blowing agents — substances such as compressed gas or a chemical agent capable of producing a cellular structure in a plastic — used in the rigid foam industry.

Lorenz said that in Bayer’s new production process, structural features of both the conventional raw materials (polyols) and natural oils such as soybean or canola oil are combined into a new NOP.

The NOP is then compatible with conventional polyols and it can help increase the compatibility between the conventional polyols and the blowing agent.

‘The new process uses an ingenious method to merge two different chemical reactions in a single step,’ said Lorenz. It combines alkoxylation, which is a common process for the production of glycols used for manufacturing polyester fibres and resins, and transesterification, which is used to produce biodiesel.

‘By combining these processes we can produce polyols with a high content of vegetable oil components and yet deliver the same range of properties and the structural diversity of conventional polyols,’ he said.

When using various popular blowing agents, rigid foams based on the NOPs produced with this new technology exhibit properties that not only match, but in some cases even surpass those of conventional products.

When the compatibility with conventional polyols is improved, it means the quantity of NOPs specified in formulations can be increased.

While polyether polyols based on sugar may contain up to about 30 per cent renewable raw materials, this new technology enables the production of NOPs containing between 40 and 70 per cent renewable raw materials. Rigid foams made with these NOPs may contain in the range of 10-15 per cent bio-based content.

Peter Seifert of Global Polyether Product Management at Bayer said this new production technology will put Bayer at an advantage in the raw materials market used in polyurethane manufacture.

‘The process enables us to formulate rigid foams with a significantly higher renewable raw material content, which can help protect our environment not only by cutting energy consumption, but also through the careful selection of raw materials,’ he said.