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Environmental conditions and anything that can affect issues such as global warming and heavy metals are gaining considerable attention from both consumers and regulatory agencies. The more governments and consumers become educated and aware, the more demanding they become for materials and processes that will address their environmental concerns. As a result, we see increased demands from these groups for improved technologies that will help reduce hazardous air and ground pollutants, whilst affording some level of reusability without sacrificing performance.
SALLY SINAI TECHNICAL MANAGER, ITW PLEXUS
Ultimately, it is up to both manufacturers and suppliers to develop and implement products that meet new criteria imposed on all industries, while helping to come up with more green products. Reactive structural adhesives such as methylmethacrylate-based systems help composite structure manufacturers and assemblers reduce their emissions to negligible levels when compared with other traditional joining methods, while providing a degree of regrindability for recycling. This paper compares various joining methods, showing their ability to provide a greener solution for the environment while still providing endusers with efficient and durable structural joints.
As the trend in the market place is green, green, green, manufacturers are looking for new and innovative ways to reduce emissions and volatile organic compounds (VOCs). The automotive market is without exception.
Since 1990, some reports suggest that the replacement of automotive steel has avoided burning more than 22.2 billion gallons of fuel globally - an astonishing figure by any measure. Furthermore, it has been estimated that this ongoing move to lighter materials by the automotive industry will cut the world’s greenhouse gas emissions by 20%.
In order to facilitate this growing move away to lighter materials, designers had to identify an appropriate method of joining new and dissimilar materials. It was essential that the new joining method would be fast and easy to achieve. The resultant joints had to be strong and durable – at least on a par with the performance of welding and other traditional methods of joining steel components. This is where structural adhesives are helping make a big difference (Figure 1).
The use of structurally green adhesives
The different joining methods currently used on these alternative materials are glass tabbing, putties, bolts, rivets and structural adhesives. The latter are used either as single part moisture cure systems or as two-part reactive cure systems.
The benefits of using a structural adhesive are many for the customer. They offer continued advances in technology, cost savings in labour and capital investment – and they distribute stress and load over a wider area. The correct adhesive choice is critical and factors which should be considered include VOC content, surface preparation, working and fixture times, requirements of the application and dynamic fatigue resistance.
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.
What is a “green” structural adhesive?
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:
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.
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
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.
Other joining methods
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
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.
Green case study
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 during the production process. Adhesive would therefore remain both on the bonded part and cut offs.
A Plexus two component reactive MMA adhesive system was able to resolve the problem for the OEM. The nature of the MMA adhesive means it is compatible with many types of thermoplastic regrind materials, and there is no need to cut the adhesive from the bonded part. Test data clearly showed that there were minimal effects on the injection moulding parameters of the plastic and that there was minimal loss of properties when compared to the virgin material (figure 2 & 3).
In conclusion I believe I have shown that structural adhesives can provide an excellent alternative to traditional assembly methods. Various structural systems can offer low VOC emissions which in turn can reduce the overall emissions for the customer. Many products can offer little or no off-gassing which makes them safer to use in both indoor and closed environments. The choice of adhesive is critical and a balance between performance and human health will often be determined by the customer. Adhesive manufacturers are looking to develop products which can address this balance and enable the choice for the customer to be an easy one with no compromise on product performance or welfare of their workers.
ITW Plexus specialises in the design and manufacture of sophisticated structural adhesives for the bonding of materials used in such diverse markets as transportation, marine, wind energy, automotive, engineering and construction and the company is a division of Performance Polymers Europe (PPE), headquartered in Rushden England.