Polypropylene (PP) is an attractive option in manufacturing because it is significantly lower in cost compared to other engineering plastics, such as polyamide. What’s more, the performance capabilities of these fibreglass-reinforced Polypropylene materials are considerably better when compared to those without reinforcement. In terms of higher modulus of elasticity, for example, the reinforced materials achieve a reading of around 4000 N/mm2 against approximately 1500 N/mm2 for unreinforced PP materials.
Beyond that, injection moulded plastic components offer substantial design freedom, allowing almost any shape to be produced. For instance, the benefits of state-of-the-art front-end carrier assemblies on car bodies may include the integration of numerous sub-assemblies, including cooling fan systems, bumpers, headlamps, and hood latches with the integral safety hook. Such fibreglass-reinforced polypropylene / metal structures reduce weight and optimise both the torsional stiffness of vehicle bodies and the energy absorption capacities for pedestrian safety.
Joined up thinking
But how do you join these structures? Adhesives are eminently suitable because they produce a unitised, continuous joint, and allow localised stress concentrations - such as occurs when rivets or bolts are used - to be distributed more uniformly over the entire surface. However, joining low surface energy materials has traditionally presented problems, as Henkel Loctite’s Colin Chapman explains.
“Some plastics have never been easy to bond, so we set out to discover a formulation that cures at room temperature, is easy to handle and achieves strengths up to substrate failure of the fibreglass-reinforced polyolefin materials. It needed to reach these strength criteria without any time-consuming and costly pre-actions, such as flame treatment, etching, corona or plasma treatment. Further, the adhesive had to work on flexible polyolefin substrates that were joined both to themselves and to metal substrates.” The result of this development is Loctite 3038 polyolefin adhesive, which enables low surface energy plastics such as PP, LDPE and HDPE to be readily bonded. So, how does it work?
The trick is to ensure that the forces of adhesion between substrate surface and the adhesive exceed the forces of cohesion between the adhesive’s molecules. When that occurs, the individual adhesive molecules will attach to the substrate. In fact, the higher the force of adhesion, the more the adhesive molecules will come in direct contact with the substrate surface. When the drop of adhesive comes into contact with that surface, it loses its spherical form and spreads out.
“In order to achieve good wetting and, therefore, high-quality adhesion to a substrate, the material needs a higher surface energy than the adhesive. If the surface energy of the substrate is lower than that of the adhesive, optimum wetting of the surface by the adhesive cannot be achieved,” says Mr Chapman. “Adhesives have a surface energy of 35 - 40mN/m. With polyolefin substrates, such as polypropylene, the surface energy is much lower. Previously, to make such materials capable of being bonded or painted, a variety of different chemical or physical surface treatment methods had to be utilised. But the patented formulation of Loctite 3038 provides adhesion to low-energy substrates without any pre-treatment.
“This is a vital step forward for designers, allowing them the possibility of bonding plastics such as PP, LDPE and HDPE to each other, but also to metal. It not only means that cost and weight savings can be achieved, but also there is great scope for innovation when it comes to creating new designs.”