Written by: Marek Bernaciak
Adhesives simplify composite construction
New MA adhesives for bonding plastics and metals
Introduction
Structural adhesives can help reduce emissions, increase assembly speed and improve productivity. The article explains how these adhesives have simplified the construction process in many composite applications in the architectural, transportation, marine and construction industries.
The use of structural adhesives has increased significantly over the last few decades. Among other things, these adhesives have been used to bond the Shuttle’s thermal shield tiles and have replaced mechanical fasteners in well-known bombers and stealth fighters. But what are structural adhesives? How do they simplify composite structures?
Simply put, ‘structural adhesives’ are those adhesives that, regardless of chemical structure, are used in load-bearing joints. Applications include the bonding of large or expensive parts, or joints where failure of the bonded joint could be very dangerous. The use of structural adhesives, particularly methacrylate structural adhesives, speeds up the manufacture of glass reinforced plastic (GRP) structures by allowing such parts to be bonded together without costly surface treatment or sandblasting. Labour costs are reduced and production is simplified.
Methacrylate adhesives
One of the major advantages of methacrylate adhesives is their ability to bond most materials with little or no surface preparation. In particular, methacrylate adhesives have a strong solvent action on almost all polyester resins and gelcoats. As a result, GRP parts made in open or closed moulds can usually be bonded without sanding or roughening.
Almost all resins can be successfully bonded. In the same way that methyl methacrylate monomer solvates polyester resin, it also solvates most of the waxes used in many other resins. On the other hand, if the parts are more than a few weeks old, some surface preparation may be required. In all cases, a laboratory test should be carried out to determine whether such preparation is necessary.
Architectural applications
The use of composite materials in architectural applications is growing rapidly. Architects are quickly learning that highly complex structures can be built quickly and easily from GRP. These structures are not only cost effective but also extremely durable.
The main consideration with such structures is how they are attached to the building. In general, this also means that such a structure can be attached to the building at a later date. The high strength and elongation of methacrylate adhesives give them the rigidity and durability to do this job. In addition, these adhesives work well in bonding materials with different coefficients of thermal expansion.
Bonding metal brackets
There are two important things to remember when gluing brackets to GRP. The first is to use toggle clamps to ensure the correct amount of pressure is applied as the glue cures. Secondly, the use of buttonholes provides a quick visual check that there is enough glue in the right place under the bracket.
When attaching metal brackets to composite structures, care must be taken to eliminate the ‘penetration’/transparency effect of the joint on the face. This effect can occur when the adhesive shrinks too much during curing, causing indentations on the face, and is particularly common when the laminate is very fresh or very thin. Fortunately, advances in adhesive technology have led to the development of new low-shrinkage adhesives that eliminate the penetration effect caused by adhesive shrinkage.
Stress and deformation of mismatched parts is another phenomenon that can also cause ‘voids’. Figure 1 shows a simple sketch of an aluminium reinforcement (actual application) whose curvature does not match the contour of the GRP part. Although the aluminium bracket is very rigid, the fitter presses the parts together in a clamp. Of course, once the glue had cured, the aluminium bracket returned to its original shape, causing the part to deform (Figure 2). As the assembly to be bonded was part of the façade of a large leisure centre, such play was unacceptable.
Marine
The marine industry has been very quick to embrace the benefits of bonding. Applications that were typically done with glass mats (bonding structural components to resin and glass) are now routinely done with structural adhesives. The main advantage of structural adhesives in these applications is that they are faster and lighter than resin and glass. Another increasingly important advantage is the significantly reduced emissions of volatile organic compounds.
Figure 3 shows the hull of a 10 metre motor yacht after glue application. The deck is waiting above the hull to be lowered into place as soon as the workers have finished applying the adhesive. No expensive dispensing equipment is required for the application, as construction adhesives can be dispensed with most available dispensers. Photo 4 shows a worker using a simple plastic bag to dispense the mixed adhesive. The glue application time for a boat of this size is approximately 15 minutes.
There are two common types of deck to hull joints in boat building. Figure 5 shows a “knuckle-joint”, a pattern typical of Europe, Australia and New Zealand.
Figure 6 shows a joint known as a ‘shoe box joint’, which is more common in the USA.
From a technological point of view, a bolt joint may require additional time and material to produce sufficiently large flange surfaces for bonding. From an adhesive point of view, such a joint is superior to a ‘shoebox’ joint in every respect. Firstly, the horizontal surface of the flanges allows easier and more consistent application of adhesive by the installer. Secondly, a consistent thread of glue directly translates into a stronger and more leak-proof joint. Finally, the matching flat surfaces of the pin joint allow for easier alignment of the deck to the hull.
Once bonded, the joints can be shortened with bolts or riveted at regular intervals, depending on the manufacturer’s assumed design parameters and whether the boat is to be certified to Lloyds of London, Det Norske Veritas (DNV) or American Bureau of Shipping (ABS) requirements. In all cases the joint is covered by a bulwark railing).
Another construction technique that is increasingly being used in the marine industry is the use of stringers made entirely from GRP composites.
For decades, boatbuilders had used plywood stringer systems as an internal framework to reinforce the hull and floor of the boat. Such systems were easy to make and very economical. On the other hand, water and moisture could attack wooden stringers and cause them to rot. In the age-old battle against seawater, many builders are now designing their stringers entirely from GRP composites. Despite the initial increase in tooling and material costs, the increased productivity and durability provide a very quick payback for builders. Studies show that gluing in GRP stringers increases bottom strength by 20 per cent and lateral strength by 10 per cent, without adding weight or labour.
As well as the efficiency of gluing the stringers, customers and dealers also notice an improvement in quality. When you open the boot, you find a beautiful gelcoat room, not just a bare deck or old wood.
Transport applications
Today, many of the cabs of large trucks, their accommodation parts and fairings are made from GRP. The use of composite components is increasing as designers strive to make their trucks more aerodynamic. In addition, as interiors are designed to increase comfort for the driver, more GRP components are being used inside cabs for storage and rest (and entertainment) areas.
Figure 9 shows the bonding of the stringer to the roof of the sleeper cab of a large truck. The stringer, which is glued to the roof, has flanges approximately 20 mm wide for gluing. The operator places the stringer on the roof and makes an outline of it with a red marker. He then applies glue to the inside of the roof and places the stringer in the correct position. A clamp is then used to hold the stringer in place for 80 to 90 minutes before the adhesive reaches its initial strength.
The use of structural methacrylate adhesive means that no sanding or roughening is required prior to bonding. In addition, adhesive application and assembly can be completed in less than half the time required for resin and glass mat bonding.
The benefits of structural bonding in large truck applications go beyond the elimination of simple surface preparation. The use of adhesives eliminates the need for fasteners such as rivets. Even today, most tractor trailers have metal panels joined by thousands of rivets. Each rivet creates a potential leak path through the cladding. The use of adhesives eliminates potential leaks and also creates an aesthetically pleasing appearance. In addition, where parts are made of GRP, adhesives reduce point loads, thus reducing potential cracking of the gelcoat.
Volatile organic compound (VOC) emissions
When composites are bonded using glass mat and resin, the amount of adhesive required is typically one third of the amount of resin.
Table 1.
Volatile Organic Compound (VOC) emissions for a general purpose styrene resin compared to a typical methacrylate adhesive
| Polyester resin | ||
| Quantity required | 1500g | 500g |
| Curing emissions | 6% (styren) | 1% (monomer MMA) |
| Calculation of emissions | 1500 x 6% = | 500g x 1% = |
| Net emissions | 90g | 5g |
Table 1 shows the emission calculations for polyester resin and a typical methacrylate adhesive. It is clear from the table that emissions of volatile organic compounds can be reduced by more than 90%!
Simplified installation
The applications listed in this article show how structural adhesives can simplify assembly in composite structures.
The two marine applications shown – bonding the deck to the hull and bonding the stringers – are gaining increasing acceptance worldwide. Using methacrylate structural adhesives for these purposes allows boatbuilders to increase productivity without sacrificing quality.
In recent years, the construction of large truck cabins has evolved from a sheet metal flap fastened with thousands of rivets to the use of resin-glass composites (GRP) bonded with structural adhesives. The use of structural adhesives allows for faster assembly, reduced point loads on joints under load, and almost no translucency in the cab structure, bonnet or skin. In other words, the bonded part is stronger, more durable and more aesthetically pleasing.
Finally, the use of structural adhesives in place of polyester resins can also benefit the environment by reducing emissions of volatile organic compounds (VOCs) during composite assembly by up to more than 90%.
See also: PLEXUS Adhesives
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