By Liz White, UT staffGlasgow, Scotland-Researchers at Strathclyde University in Scotland have developed a laboratory scale method of making flexible polyurethane foam fire retardant using nano-clay: this is a major advance since current fire retardants are rapidly becoming less and less acceptable on environmental and health grounds. And the Strathclyde team is now looking hard for an industrial partner to help take the process up to commercial scale for further evaluation, according to Dr John Liggat, who, along with Professor Richard Pethridge, led the work on making high-quality furniture foams flame retardant, at Strathclyde's Department of Pure and Applied Chemistry. Scotland has a bad record with regard to fire safety, with 110 deaths a year, according to a Strathclyde University press report. Soft furnishings containing foam are often implicated in these deaths, said the report-despite the fact that the UK has the toughest regulations in Europe on furniture fire safety and foam fire performance. As the Strathclyde press statement notes, effective fire retardant foam formulations are available. Liggat and co had, however, been working with companies for some time to develop new flame-retardant PU foam formulations, to eliminate chemicals which are environmentally questionable, the researcher said in a 17 Dec telephone interview. His colleague Pethrick, meanwhile, had been using nano-clays to modify the mechanical and barrier properties of epoxy resins. "Some initial funding from the Engineering and Physical Sciences Research Council allowed us to develop the work on epoxies and especially to look at nano-clays to develop flame resistance," he said. By happy coincidence the two themes came together: "We thought it would be an inventive step to try and extend the work into foams," Liggat continued. This was a step which many people said, "would be very difficult to do because they felt addition of clay would upset the rheology of the system," he explained. Intuitively it seemed unlikely to work: the general opinion was that some amorphous 'gloop ' would be the outcome, not foam. Nevertheless, "We got funding for a year from EPSRC back in 2001," and also had some aid from the flexible foam group within the BRMA (now the British Rubber and Polyurethane Manufacturers' Association), Liggat said. And the work proved successful. Initial fire tests on the nano-clay filled foam have been promising and need scaling up further to full-scale fire tests when larger samples of the material are available, he added. Foaming easy-if you account for rheologyMaking nano-clay filled foam is now an easy process as far as Liggat is concerned, with the caveat that you need, " full and proper understanding of the rheology to enable the foaming process to continue."The skeptics in the BRMA group were converted when they saw the good quality of the first samples of foam that the process came up with, he said. Now the group has gained £163,900 from the Scottish Executive's Proof of Concept fund, which will allow Liggat's team to further investigate their initial findings, scale up the process and take it through to the shop floor. Liggat and his team have made foam in a bench-top tray and now need to take it to a slabstock producer. Hence the group has issued a "tech alert in Europe for partners," via Innovation Relay, a European wide organization to facilitate knowledge transfer, Liggat said. He wants a major or specialist foam manufacturer to take the process further, with a view to negotiating a licence agreement for manufacture, he added. Strathclyde is currently filing an international patent on the technology. Existing fire retardants contain halogens, phosphorus, nitrogen or antimony and are under scrutiny from environmental and health angles, Liggat said adding that there is a major move towards products which have no volatile or leachable constituents, for example. A 2003 meeting of major UK PU foam producers, organised via Faraday Plastics, "laid out a nine-point wish list for the flame retarded foam of the future." The foam must keep its existing performance and cost benefits whilst achieving further improvements in fire performance. Both halogen or phosphorus should be reduced and preferably eliminated. Faraday Plastics' report noted that, "A radical rethink in PU formulation development is needed to achieve these requirements." Will it be economic? Is nano-clay cheap? Not necessarily, Liggat said, but he stressed the difference between using 2 parts of nano-clay per hundred polymer in the fire retardant PU foams, and the heavy loading-or perhaps 10 times this level-for conventional fire retardants. "We see activity at very low levels," Liggat said, adding that he is "not in a position to comment on the full economic costs." "We need to fully cost it as part of the commercialisation process," Liggat commented. "Tonne for tonne the cost may be high," for the nanoclay, he agreed, while pointing out that the level of use is low. The clay works by forming a homogenously dispersed matrix within the foam, throughout the cell walls. This acts as a barrier to prevent combustion products dispersing, and also produces char which prevents the spread of fire. One of the difficulties with controlling the burning of PU foams is that the open-cell structure allows easy passage of air and combustion products, and that the high level of polyether materials "can readily fuel the combustion process," according to Pethrick's report. Also, he said, the low aromatic content makes char formation difficult. With nano-clay as filler, char formation is higher. The nanoclay dispersion throughout the cell walls can be clearly seem using electron microscopy, Liggat said. Nano-clay produces its good fire-retardance largely without affecting the mechanical properties and hence the comfort and flexibility of the foam. This is in contrast to melamine, which is widely used as an effective fire retardant filler for flexible PU foam in airplane seats, for example, but is is expensive and is well known to detract from foam comfort. . Is Liggat confident that a scale up of the process will work? "After a fashion," the Scotsman said cautiously. "The critical aspect is the mixing," which involves ultrasonics, he pointed out. Currently the Strathclyde team is using an industrial foaming machine with only minor modifications, he added. "Its also worth emphasisng," said Liggat, that the method is not just for flexible foams and that it is feasible to make fire retardant rigid insulating foams for building products in the same way. "We use a package of additives to control the rheology"
