Laser surface pre-treatment of polyolefin substrates for adhesive bonding

Adhesive bonding offers many advantages over mechanical fastening, but requires accurate surface preparation, which is widely recognized as the key step to producing reliable and durable adhesive bonds. The use of laser cleaning processes helps to increase the reactivity of the topmost layers of substrates, without affecting the bulk material properties. These methods are often applied to treat polymers, which are generally characterized by high chemical inertia, very low surface energy values and, consequently, poor adhesive properties and this is particularly true for polyolefins. Furthermore, laser treatment provides an alternative to the polluting and less accurate practices such as manual abrasion and primer. In this paper the effect of different laser pre-treatments on polypropylene (PP) and high density polyethylene (HDPE) substrates was studied, by microscopic investigation and single lap- shear tests. As a comparison, untreated and primer treated specimens were also included in the experimental campaign. The results show that laser treatment significantly affected the substrates' surface and the mechanical properties of adhesive bonded joints. Finding the optimal combination of the working parameters, it is possible to enhance the strength of both untreated and primer-treated joints.