Author: Technical

The adhesives industry is far from stagnant, like in any other industry the popularity of different technologies rises and falls. Some technologies stick around for a prolonged period of time where as others are merely the ‘flavour of the month’ before fading into obscurity, finding use in only very niche applications.

From the first recorded use of adhesives in 70,000 BC, all the way up until the 18^th century, glue was typically produced for only personal use. Adhesives were made from animal products such as bone, blood, skin or casein (a milk-based protein). Adhesives made from plant matter such as tree resin or starch were also used. In 1700 the first commercial glue plant started operating, producing glue from horses’ hooves. An industry was born.

In 1841 rubber vulcanization was discovered and marked a turning point for the Adhesive industry. The beginning of synthetic adhesives. Phenolic adhesives were the first commercial synthetic adhesives and sales begun in 1902. Phenolic adhesives were a revelation and use began for many applications such as a binder for plywood. There are still many cases of phenolic adhesives being used today, however, in recent years there has been a significant movement to the use alternatives due to the hazards associated with phenol formaldehyde.

The onset of the second world war created a demand for more durable adhesives that could withstand very harsh conditions. Chemists eagerly took up the challenge and structural adhesive technologies such as epoxy and polyurethane were quickly developed. Such technologies have since found use in a wide variety of industries such as aerospace, construction and electronics, in many cases replacing mechanical fixings. These technologies have been immensely popular due to good mechanical properties, good heat resistance and resistance to chemicals and it is likely that their popularity will continue far into the future.  

It’s interesting to look back at the development of adhesive technologies over time and the factors that drove these developments. We can find some indication of the future of the adhesives industry by considering the challenges of today. With the outset of REACH the spotlight has become even more firmly set on the health and safety of products. We are already beginning to see a change in the adhesives market towards products with less hazards and it seems this will become more pronounced in the years to come. Structural Adhesives Ltd are conscious of this and have recently developed an STP adhesive for the construction industry that is significantly less hazardous than typical construction adhesives. Additionally, society is becoming more aware of the safety materials from a fire point of view and we are likely to see a shift to adhesives that are less combustible, for this reason Structural Adhesives Ltd have ongoing development projects for A1 and A2 rated adhesives. Furthermore, the ever-pressing matter of dwindling resources and the volatility of petrochemical prices promises a shift to more biobased adhesives and adhesives made from recycled materials. On the subject of recycling, the amount of material salvaged from products at the end of their lives would be much higher if the products could be easily de-bonded. We could see possibilities of a future where adhesives could be turned on or off on demand.

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The raw materials that make up almost all classes of structural adhesives including epoxies and polyurethanes are derived from fossil fuels. Fossil fuels are not going to last forever and it’s up to us as a community to find sustainable alternatives before it’s too late. Plants present a renewable source of aromatic hydrocarbons that can be converted into a variety of chemicals so its inferable that these will be a source of adhesive raw materials in the future.

The conversion of crops such as corn and sugar cane into the necessary raw materials is the cheapest and easiest way to produce biobased adhesives. However, with a growing population there’s billions of hungry mouths to feed and the use of food stuff in applications such as these is ethically questionable. Using trees as a renewable resource rather than agriculture may be a more suitable option, but even this is not without complications. Firstly, deforestation is already a huge contributor to climate change and the level of deforestation would only increase if wood was also required for the production of petrochemical alternatives. In addition to climate change, we must consider the effect deforestation has on the prevalence of precious habitats and endangered species.

Of the trees that are already harvested for existing purposes not 100% of the material is currently utilised, there is some waste. Lignin makes up approximately a quarter to a third of the mass of vascular plants. In the paper-making process the paper is delignified as lignin weakens the paper, is coloured and yellows in air. Lignin is an unwanted by-product and almost all the lignin produced from paper making is burnt. Therefore, in lignin there is an abundance of material that can be used as a substitute for petroleum based aromatic chemicals, that would otherwise have been wasted.

For lignin based raw materials to compete commercially with petrochemical based chemicals more research and development is required. The structure of lignin is complex and hence the downstream processing is significant and energy costs are high. However, work is being carried out and the process is becoming steadily more efficient. The cost of producing aromatic fine chemicals is steadily decreasing and the cost of petrochemicals will start to increase due to lack of availability in the not too distant future. It is likely lignin-based bio adhesives will be common place before too long.

The technical team at Structural Adhesives Ltd are researching into biobased raw materials currently on the market and keeping a keen eye on developments in this sector.

Recent bouts of unseasonal weather got me thinking about the effect of atmospheric conditions on the use of adhesives. Over the past few years the UK has seen snow in April and a heatwave in October both of which would have an effect on adhesives that may have been unexpected.

The industry accepted temperature for epoxy adhesive use is above 10°C. At temperatures below this the reaction becomes slow or may not proceed at all. A general rule for many epoxies is that for every 8-10°C increase in temperature the cure rate is halved and for every 8-10°C decrease in temperature the cure rate is doubled. For example, if an adhesive has a pot life of 10 minutes and a cure time of an hour at 23°C, at 33°C the pot life would be just 5 minutes and the cure time 30 minutes. This could cause practical issues on site as a short pot-life may result in material going off before it can be used, resulting in waste. Equally low temperatures may also cause a practical issue. Taking the previous example, if the temperature was 13°C the pot-life would be 20 minutes and the cure time 2 hours, which would mean the bond would have to be left longer before it could be moved which would decrease productivity. Epoxy adhesives that cure at low temperatures are available and should be considered by those looking to use adhesives outdoors or in poorly heated warehouses.

Another climate factor that effects the use of adhesives (particularly epoxies) in the construction industry is rainfall. Generally, contamination of adhesives with water will have a detrimental effect on the cure rate and degree of cure. Therefore, most adhesives cannot be used outside whilst there is precipitation. Furthermore, some adhesives are intolerable of damp substrates, in these cases materials that are outside during wet weather must be left until completely dry before they can be bonded, which will hold up construction significantly. In extreme or prolonged cases of bad weather construction output overall decreases and this has a profound effect on the use of adhesives.