Passing the acid test

A new type of latent acid catalyst promises to extend shelf-life – and speed up throughput – for a number of composite processes. Lou Reade reports.
Developing a conductive composite sounds like a potential goldmine. That’s certainly how it seemed to Bac2. The company, based in Southampton, was set up in 2002. At the time it had its sights fixed firmly on a growing and lucrative market: fuel cells. “We started with a conductive polymer that had some interesting properties,” begins Graham Murray, the company’s founder. “We used it to make bipolar plates for fuel cell stacks, and that led us to the composites industry.” The company’s bulk moulding compound (BMC) was augmented with graphite, rather than the traditional glass fibre. But developing a technology in this industry was less lucrative than it might appear. “The fuel cell market was – and still is – in its infancy,” says Murray. “We weren’t getting a return on the work we were doing.” However, while developing the conductive material, the company had stumbled upon a potentially more practical technology. Composites like BMC and sheet moulding compound (SMC) are cured under acidic conditions. In order to give them a shelf life, they are compounded with both acid and alkali and kept at 0°C. These retarding agents are called latent acid catalysts. When the compound is heated in a mould, the alkali decomposes and the residual acid cures the resin. “We were seeing that once we’d make the compound we only had 30 minutes to use it,” says Murray. “So we had to mix and mould it almost immediately.” Murray, who has a 20-year background in phenolic resins, began searching for an alternative acid catalyst that would give a longer shelf life, but also a lower activation temperature. The retardant role is traditionally filled by ammonium salts, and primary or secondary amines. The acidic element is usually a strong acid such as sulphuric or hydrochloric acid. “We wanted an active BMC, but with a storage life of a few weeks or months,” he adds. The search revealed a surprising candidate in the shape of hydroxylamine – whose main use is as a precursor in the production of polyamide (nylon). Not only did it act as an excellent buffer, but it decomposed at 120°C – much lower than existing alternatives. “We couldn’t find any previous use of hydroxylamine as a retarding agent for acid catalysis,” he says. “We saw now that we could build our business around this, rather than conductive composites.” Making the grade Bac2 currently offers four catalyst grades, which can be blended by the customer to deliver various degrees of buffering. CSR20 is a one-to-one ratio of hydroxylamine and acid, while CSR100 has a 1.8:1 ratio. Two further grades, CSR150 and CSR250 have higher ratios of hydroxylamine, and will deliver greater stability – mopping up any stray acid. These are targeted at urea formaldehyde resins, which are very sensitive to acid. “It’s a balance between reactivity and storage life,” says Murray. Local supply has also dictated the nature of the catalyst. Bac2’s nearby supplier of hydroxylamine supplies it as hydroxylamine nitrate – which means that the acid used is nitric acid. “Hydroxylamine nitrate is used in the nuclear fuel industry, for cleaning up nuclear waste,” says Murray. Bac2 has also begun to develop a non-aqueous grade, for materials that cannot tolerate water. “We got some requests for it,” notes Murray. “We only had phenolic and formaldehyde resins at first, which are water-based. We found a specific adduct that could be crystallised out of aqueous solution, and used with a number of solvents. That was the birth of CSR 800.” The grade is still in its early stages: Bac2 has had meetings with “one big company”, to whom it would supply the catalyst. So far, demand is low so it is still being made in-house in “kilogramme quantities”, though Murray has identified two manufacturers in Europe who could produce it on a larger scale if necessary. “Until we know what the volumes are, and where it’s going to be used, we can’t do much more,” he says. The push of pultrusion The main users of CSR up to now have been in pultrusion. In terms of process advantage, he points to the spec sheets of typical suppliers: these quote the need for pultrusion ovens at 180°C. “With CSR, once the part is through the die, we’ve started at 90°C and gone up to 120°C. Even at these low temperatures we are getting fast pull-through speeds,” he explains. In these cases, Bac2 is supplying the catalyst, working with the resin supplier to create a blend with the correct reaction conditions. The company also supplies SMC and BMC formulations that contain the catalyst. “Composites are probably the most interesting application, but they are not the most commercially attractive because the volumes are quite low,” he says. For this reason, Murray sees the abrasives and coatings sector as very attractive, and is working with a number of companies in this field. The main advantage, he feels, is the ability to decrease energy costs. Murray says that increasingly stringent fire regulations create an opportunity for Bac2. He explains that CSR catalysts help to make phenolic resins – with their excellent fire performance – easy to process in SMC form. “Mould companies are used to moulding polyester SMC,” he states. “When we made our phenolic SMC, they did not treat it any differently. So if a company wants a fire safe product, rather than a smoky polyester, it will not be too hard to switch over.” Bac2 tested the fire performance of its phenolic SMC, and found it passed the highest ‘HL3’ level. The catalyst also helps to slow down a potentially explosive reaction. “If you take the temperature up to 120°C, the acid is released almost immediately. That’s fine for BMC and SMC, which are under pressure, but it won’t work for processes like abrasives and coatings.” In these cases, he says, an activation temperature of around 50°C is enough to get the reaction started. “A controlled temperature increase gives a smooth coating – and ensures that you don’t foam the resin,” he clarifies. For now, he says, no scale-up is necessary. For SMC and composites, there is no need because volumes are relatively low. However, he does not rule out the prospect of the need for increased production. “If we pick up in the abrasives and laminates sector, volumes could become quite big,” he says. And interest in the product is not just local, as Bac2 is also supplying customers in Scandinavia, India and US. “This is for the catalyst,” he concludes. “But at some point, it would be great if we could export composite SMC and BMC.” www.bac2.co.uk