JEC COMPOSITES MAGAZINE - Issue #61 - November/December 2010 - 35

Adding value

What is a “green” structural adhesive?

Examples of where a structural adhesive can add value include applications such as metal bonding where an aesthetic appearance is important and bond strength is critical, e.g. locker doors on coaches and rub rails on buses. Structural integrity is vital in the marine industry; by using bonding in systems such as stringer grids, the overall weight of the boat can be reduced together with the VOC emissions. Structural adhesives can also add value by allowing the joining of dissimilar materials, again opening up design options for customers, which can result in reducing the overall weight of the final component. Today’s new engineered composites provide the designer with a wealth of options and challenges. Again, weight can be reduced and VOC emissions significantly decreased by using a structural adhesive.

Other joining methods
Urethanes

Environment

One and two component urethane systems are typically 100% solids with little or no VOC emissions. However, solvents can sometimes be used as carriers in non structural versions which would release VOC’s at around <25g/l. One must also consider the isocyanate content and, as with epoxies, there is the potential for off-gassing if mixing off ratio.

A green adhesive is one which will eliminate toxins to the greatest extent possible. This means developing products which focus as much on human health as on performance. This can be done by reducing “off-gassing” to almost no detectable levels and therefore producing a product that will prevent problems with indoor and outdoor air quality. Another part of being green is to enable the product to be recycled and/or reground. ITW Plexus believes the adhesive manufacturer has an obligation to develop products which focus on human health and recyclability. The customer who is using the product must determine which product provides the ‘safest’ option for their operators whilst providing the best performance for their product. Green issues are constantly being monitored by independent organisations and industry bodies which look at current and future legislation. For example, government agencies such as the HSE (UK) or AFSSET in France and industry groups like BASA in the UK and FEICA in Europe.

There is a wide variety of structural adhesives in the market today and each one offers the user different levels of ‘greenness’. Let’s compare three of the main types with more traditional joining methods:

Epoxies

Reactive two component epoxies are typically 100% solids with little or no VOC emissions. Some activator systems can contain, for example, butyl acetate which would release less than 25g/l of VOC. Other things to consider with these products are the amine content and the sensitivity of being mixed off ratio which can then increase the possibility of off-gassing.

No61 November - December 2010 / jec composites magazine

Green case study
Methyl methacrylates

Two part reactive methyl methacrylate adhesives (MMA’s) vary depending on formulation but typically have between 40-60% reactive VOC components which results in greater than 99.5% conversion and therefore typically VOC emissions are less than 0.5%. The products are off ratio tolerant compared to other systems and there are no off-gassing issues. These products do have a distinct odour.

The more traditional ways of joining composite materials include the use of glass tabbing or polyester putties and there are drawbacks associated with both of these technologies. The application method used for glass tabbing can greatly effect the VOC emissions. If the application is a closed mould this will suppress the emissions given off. Older technology can be 40-50% styrene with new technology reducing this to <35% styrene. VOC emissions can be up to 15%. Off-gassing is also a matter of concern with this bonding technique. Polyester putty emissions are again application and techniquerelated. They are typically very high in VOC’s, around 224 g/l. New lower VOC technologies are available but the levels are not reduced considerably. The use of these materials is very application dependent and once again off-gassing is a matter of concern.

There follows an example of how a structurally green adhesive recently resolved a recycling issue for a major automotive OEM. Within the automotive industry there is a huge commitment to plastic recovery and re-use programmes. A problem often arises concerning the incompatibility of the structural adhesive with the plastic recycle recovery process. It is the intrinsic nature of most structural adhesives which poses a serious regrind problem, and therefore the adhesive must be cut out of parts and disposed of in a non recyclable waste stream. This process is not just labourintensive but is wasteful for the recycling program since a large portion of plastic is often cut out with the adhesive.

In this case study, the application was a thermoplastic bumper which needed to be structurally joined. The main challenge was the customer’s need to regrind and recycle waste material which arose

JEC COMPOSITES MAGAZINE - Issue #61 - November/December 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #61 - November/December 2010