Investigation and demonstration of the durability of air plasma pre-treatment on polypropylene automotive bumpers

Abstract

Higher utilisation of low-density materials such as polymers and polymer composites is a pre-requisite for the lightweight vehicle of the future. Of the commodity polymers polypropylene (PP) is by far the most attractive for the automotive industry. Additionally, PP can be utilised as a glass or mineral filled composite which may be used for semi-structural applications.A major problem is that PP (along with other polyolefins) has a non-polar surface chemistry which means the wetting characteristics of components made from this material are poor. Ultimately, this will result in poor adhesion of paints, coatings or adhesive bonding products. This problem has been overcome in the majority of instances by treating the surface of the substrate in order to alter the surface chemistry.PP has found extensive use as films and flat sheets and hence certain techniques such as flame and corona have been favoured despite both having problems of heterogeneous or patchy treatment across a surface. However, for complex 3-D automotive shapes, such as bumpers, these methods are less useful. While flame treatment for example is widely used it has several disadvantages in a commercial volume production environment which all centre around the potential for the part to undergo overtreatment, incipient melting or melting during machine stops etc., as well as the hazards associated with combustible gas. An alternative method is atmospheric plasma pre-treatment.This work has investigated the effect of a forced air plasma pre-treatment on surface chemistry and bond strength of a commercial grade polypropylene material.The plasma head was attached to a robot arm which makes it highly suited to continuous production environments and can treat complex surfaces. A range of translation speeds were investigated and the surface chemistry and topography of the treated surfaces were examined using atomic force microscopy and X-ray photoelectron spectroscopy. The processing was further optimised by single lap-shear testing.An optimised set of parameters was used to pre-treat and bond a full size automotive bumper assembly with a polyurethane (PU) adhesive. Bumpers were then subjected to a standard automotive range of climate conditioning as well as soaking at −20°C and 70°C before front centre impact testing. Parts pre-treated and bonded using this pre-treatment and adhesive system, successfully passed all the required standard automotive impact tests.For added benefit, it was also found that the open time of the pre-treatment was 1 week depending on storage conditions.