JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010 - 58

KNOW-HOW
Bonding

Composite bonders – meeting the requirements of modern engineering
Today’s structural adhesives not only need to combine good adhesion onto lightweight composite structures with long-term durability, but also comply with modern healthand-safety standards.
In addition to the above mentioned categories, other chemistries such as one and two component silyl terminated polymers gain increasing importance. This is partly due to their high adhesion properties and flexibility, but as well due to their favourable labelling in terms of health and safety.

DR. BODO WIXMERTEN TECHNOLOGY MANAGER ADHESIVES, COMPOSITES & TOOLING HUNTSMAN

Composite substrates

dhesives have numerous advantages over mechanical joining techniques. This is even more obvious when it comes to the assembly of dissimilar substrates and lightweight composite parts. Bonding allows a uniform distribution of strain and stress across the joint, which makes it the ideal technique to join composite materials of different constituents. Adhesives also maintain the integrity and strength of the substrates as there are no holes, rivets or fastening elements to weaken the structure. They supersede the use of bolts, nuts or screws, protecting the symmetry of the finished piece and allowing wider design and aesthetic possibilities. Major advances in adhesive technology started in the 1940’s when improved synthesized polymers replaced natural materials. This new polymer chemistry led the way for the rapid development of a new era of structural adhesives and composites.

The substrate class which has been used as a reference for the development of high-end composite bonders is carbon-fibrereinforced polymer (CFRP). In contrast to glass-fibre-reinforced plastics, CFRP is used in applications requiring very high strength combined with low weight. The matrix in glass-fibre-reinforced structures is often based on polyester and vinylesters, whereas the polymer in carbon-fibre-based materials is usually epoxy-based, although other chemistries can also be used. The reason is that polyester resins offer relatively poor adhesion properties, combined with high water absorption, shrinkage and high volatile organic contents (VOCs). They are also non-tough and fracture easily, which means such polymers are not ideal for highperformance applications. Vinylesters offer characteristics which fall between polyester- and epoxy-based materials in terms of strength and cost, combined with a low viscosity. They are manufactured by the esterification of epoxy resin with an unsaturated monocarboxylic acid, and dissolved in styrene as reactive diluent. In demanding applications, epoxy matrices are mainly being used due to their adhesion properties onto the reinforcement and overall superior mechanical properties in terms of strength and stiffness. This results in improved resistance to fatigue and micro-cracking, as well as excellent durability when exposed to chemicals or high temperatures.

Structural adhesives
There are three main types of structural adhesives which can be distinguished by their chemistry, each with different structures and specific physical characteristics. The three main modern day adhesive categories are epoxy-, polyurethane- and methylmethacrylate-based systems. - Epoxy-based (EP) adhesives are generally known and used for their excellent adhesion to metals and rigid substrates such as thermoset plastics or composites. They provide excellent durability, chemical and temperature resistance with low shrinkage. - Polyurethane-based (PU) adhesives are more suitable for bonding thermoplastics, and are perfectly suited to softer substrates due to their flexibility. - Methylmethacrylate-based (MMA) adhesives provide excellent adhesion onto metals, composites and thermoplastics. They are generally faster setting products, thus enabling significant productivity gains.

Advances in bonding CFRP structures
The obvious thought when looking for an adhesive to bond carbon fibre structures based on an epoxy matrix is to evaluate an epoxy-based adhesive. In this way, good compatibility between the adhesive and the substrate can be achieved so as to produce a stable bondline and therefore high durability under

jec composites magazine / No59 August - September 2010

JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010