Proteins extracted from oilseed rape (canola), soy or castor meals using a new technique are yielding formaldehyde-free structural adhesives suitable for use in wood composites, say scientists at Advanced Biopolymer Technologies, in Mantua, USA.

The team says its work in the field is novel in providing adhesives that can be tailored to provide different cure rates, properties and viscosities for varying applications such as plywood, I-beams, finger joints, furniture construction, and particle, medium density and orientated strand board. The adhesives are said to have ‘at least equal or better performance compared to current commercial technologies’, claims Principal Scientist Dr Joseph Marcinko.

‘We are able to produce particleboard [that] outperforms urea formaldehyde (UF) adhesives in physical property performance and moisture resistance. Particleboard made with our technology can survive two hours in boiling water and remain in-tact with minimal swelling – UF resin cannot do this.’

Tests at Washington State University, USA, for example, have shown one of the new adhesives formulations to have a modulus of rupture of 2,112psi and modulus of elasticity of 344,330psi, compared to 1,216 and 270,620 for the UF adhesive, respectively.

‘The essence of our patent pending technology is being able to isolate different fractions from the protein sources, for example, water-soluble and water insoluble fractions, and being able to disperse oils in [them],’ explains Marcinko. The reactive oils, such as polymeric methylenediphenyl diisocyanate [PMDI], epoxy compounds, or other reactive molecules, have a crosslinking function. (see image below).

The adhesives are prepared differently based on the needs of the application to yield materials with varying degrees of oil dispersing capability, crosslinking and moisture resistance. The agricultural meal can be washed with water to remove the water-soluble fractions, or denatured using caustic and enzyme digestion, followed by acid neutralisation and water washing.

He continues, ‘We are separating and characterising these fractions and using them in the appropriate relative concentrations based on the performance needed. That is, the more the water insoluble fraction is purified, the more oil we can disperse and the higher the moisture resistance of the composite.’ The water-soluble separated fraction is reused by either recombining it back into the adhesive or using it in other applications, such as to prepare polyurethane foams (see image below). There is said to be little or no waste.

Furthermore, the researchers have found that the ‘aqueous dispersions of these proteins “protect” the reactive functionality of the crosslinking molecule [PMDI] and keep it stable for long periods of time in an aqueous environment’.

John Williams, Polymer and Materials Manager at the UK National Non-Food Crops Centre, believes the work may well address a key limiting factor of using natural adhesives – variability in quality. ‘Previously, it was not possible to obtain derived fractions – you get what you get and you could not get any better. It is interesting that they are characterising better materials through fractionation. This allows for specification.’

Marcinko adds that using castor and canola, in addition to conventional research into soy, expands the portfolio of protein sources in time for passing of a federal law in the USA that restricts formaldehyde emissions from wood composites. ‘We are interested in utilising raw material sources that have little other use after the extraction of oil from the plant, like canola and castor. The value of having multiple sources of protein is that resources differ in different parts of the world and the ability to utilise different sources also has an economic benefit.’

Williams outlines, however, the need to look beyond reactive oils such as PMDI to crosslink the proteins. He says, ‘Internal cross-linking would be extremely interesting to the industry. Also, what can they achieve by combining the different fractions?’

Advanced Biopolymer Technologies is negotiating worldwide license agreements for its products.