Radiation as a cure for adhesives?

Wood Focus magazine
1 Dec 2010

Radiation could be used to cure adhesives in the manufacture of wood composite panels in a way that could reduce energy consumption and offer an alternative to formaldehyde-emitting adhesives.

Dr David Harper, Associate Professor at the Center for Renewable Carbon at the University of Tennessee, USA, and collaborators at Oak Ridge National Laboratory, struck upon the idea when trying to reduce the number of heating steps that the wood normally goes through during panel manufacture.

Harper and his team aimed to develop a radiation curable thermoplastic hybrid adhesive, which has tack and strength once cooled, and can hold substrates together until completely cured by electron beam radiation treatment.

‘[There are] four options with radiation curing – ultraviolet, gamma, X-ray and electron beam. For panel products it looked like electron beam radiation gives us the best balance of penetration and throughput of the options,’ he says. The method includes creating a window into the pressing process for the composite to go into a shielded vault. E-beams are then applied after the adhesive has been added and composite consolidated. The adhesive can then be cured virtually instantaneously compared to hot pressing.

One of the main advantages to the method, Harper claims, is that it comprises an adhesive without any volatile-organic compounds (VOCs), in particular, formaldehyde. With both the European Union and the Environmental Protection Agency in the USA investigating the use of formaldehyde in industry, any way of reducing its occurrence has become a pressing issue. He explains, ‘There are a whole host of different regulations between the different countries, basically moving to lower formaldehyde emission levels, and this is an alternative adhesive where you don’t have any formaldehyde or VOC emissions.’

However, Harper notes one of the problems with using radiation is ‘the way you cure is through free radical polymerisation. If you’re going to hit something with a bunch of free radicals, you’re going to start to break down the structure. You have to be able to cure the adhesive at a low enough levels so that you are not going to be breaking down the wood.’

After testing, his team has found that significant breakdown of wood occurs at about 80kGy, so the scientists have been able to cure the adhesive effectively by working at about 40kGy.

Harper adds that using radiation as a way of curing adhesives has financial strings attached. ‘Whenever you use electron beam you are generating X-rays – that’s your immediate concern. One of the biggest expenses for using electron beam radiation is adequate shielding. The advantage of electron beam radiation is that whenever you turn the beam off, the radiation dissipates quickly,’ he adds.

Harper hopes that the method could offer a more economical way of curing adhesives, but a full-life cycle and economic analysis of the process needs to be completed.

He added that long-term savings would have to be properly proven, if manufacturers are likely to convert to the method. ‘If you were to implement this, you would have to look at [the] adhesive costs, which is where it gets scariest for most of the panel manufacturers because it might seem as though you are going to be locked into purchasing a more expensive adhesive. You have to get away from thinking of that and look at your energy costs, VOC emissions, product improvements and increase in production.’

Dr. Mark Irle, Research Director at Ecole Supérieure du Bois, Nantes, France, comments ‘This futuristic research requires many years of further development, but offers the potential of cold pressing panel products with uniform density profiles or pressing relatively wet mattresses, either of which could result in products with novel properties.’