Sustainability is a buzzword of our times, becoming an increasingly important aim for companies and governments alike. That can even be seen in the adhesive industry, with building standards like Leadership in Energy & Environmental Design (LEED) and environmental laws playing an ever greater role. Sustainability is usually synonymous with the idea of replacing petrochemical-based resources with plant-based alternatives. Thats because the former are effectively finite due to the length of time it has taken for the supplies we use to accumulate, and therefore could run out. Meanwhile, with favourable conditions, new plants can be grown every few months. But how sustainable is moving from petrochemicals to chemicals derived from plants in reality?

Within the adhesive industry, plant-derived raw materials are already used to great effect, for example in starch-based adhesives, which are important products for manufacturing. They are deployed especially extensively in the packaging industry, applied primarily in bonding paper products. If you ever buy goods in a corrugated boxboard carton, it will probably have been manufactured with starch based adhesives. They are easy to apply from water dispersions and have been readily, economically, available for decades.¹

Recent years have seen increased efforts to bring more biological materials into the adhesive industry. Much of this effort focuses on vegetable oils and animal fats. They can be epoxidised to make raw materials for epoxy adhesives, or the oils can be used as polyols that react with polyisocyanates to make polyurethane polymers. Its industry thats been taking the lead on these approaches, but some governments are starting to view the general idea of using plants as chemical raw materials as worth investing in. For example, within the French governments "Investments for the Future" program, its estimated that up to ?00 million of projects focussed on such "green chemistry" could be funded.²

Figure 1 : Gum rosin prices/tonne from January 2009 to July 2011 Credit: Flint Group/Mintec

But the adhesive industry can also demonstrate some of the main downsides of relying on plants for raw materials. Take gum rosin, a key tackifier for hot-melt adhesives that is extracted from pine trees, with around 60 per cent of global supply coming from China. The price of gum rosin increased from $600/tonne to $3,400/tonne between January 2009 and February 2011, although it had fallen back to around $2,600/tonne by July 2011.³ While there were some issues with the crop, the price rise has been attributed simply to increased demand, and is not expected to return to pre-2009 prices soon.⁴

Figure 2 : Year-over-year food price rises 2010-2011 Credit: UN Food and Agriculture Organisation

The agricultural resources that might allow increased use of plant-based chemicals are already under pressure. In February 2011, the UN Food and Agriculture Organisations Food Price Index hit an all-time high. Some of this has been attributed to use of crops like corn and rapeseed for energy purposes, as bioethanol and biodiesel respectively, for example. By June 2011, cereals like had seen the largest year-over-year rise in prices of any food class, at 71 per cent, and oils and fats prices had increased 53 per cent.⁵ These are the same feedstocks sectors like the adhesive industry need to replace petrochemicals, demonstrating how unintended costs of exploiting crops for non-food purposes could quickly mount.

One way to avoid this conflict is by using materials as feedstocks that would otherwise be lost as waste. Here, learning to convert lignocellulose, the most abundant plant cell wall component and the highest-volume human waste material, to useful materials could pay dividends.⁶ In adhesives cellulose from waste paper has been used to make polyols, for example. Most plants also contain some silica which can be used to make water glass binders that can replace urea/formaldehyde adhesives in wood composite manufacturing.⁷

The idea of sustainability has flooded society with its various meanings, cascading down from ensuring that life on Earth can continue. Among its other branches are encompassed sustaining jobs, company revenues, and economic growth, as well as ensuring people can feed themselves. However, current limited agricultural output looks set to ultimately thwart adhesive producers trying to gain long-term business viability in the face of concerns about the security of petrochemical supply. Perhaps the largest sustainability question is, then, within the adhesive industry, can true long-term sustainability be found?