Polyolefins are considered among the most difficult polymeric materials to treat because they have poor adhesive properties and high chemical barrier responses. In this paper, an in-depth study is reported for the low pressure plasma (LPP) treatment of neutral polypropylene to improve adhesion properties. Changes in wettability, chemical species, surface morphology and roughness of the polypropylene surfaces were evaluated by water contact angle measurement, X-ray photoelectron spectroscopy and, furthermore, atomic force microscopy (AFM). Finally, the bonded joints were subjected to tensile tests, in order to evaluate the practical effect of changes in adhesion properties. The results indicate that plasma is an effective treatment for the surface preparation of polypropylene for the creation of bonded joints: contact angles decreased significantly depending on the plasma-parameter setup, surface morphology was also found to vary with plasma power, exposure time and working gas.
The use of adhesives in place of traditional joining techniques as welding and riveting is becoming increasingly common in structural design. Compared to other conventional joining processes, adhesive bonding offers several advantages: including acoustic insulation, vibration attenuation, structure lightening, corrosion reduction and uniform stress distribution. These benefits can be offset if the surfaces to be bonded are not carefully cleaned, degreased and prepared. Many approaches to surface treatments based on physical or chemical modifications have been developed in the years in order to improve the surface activity. Although widely used and very efficient, these techniques present several disadvantages. Physical methods based on mechanical abrasion are supposed to extend bonding area as they increase the roughness, but cause an extensive degradation to the specimens and they are often not easy to reproduce. On the other hand, chemical treatments are typically used with the aim of modifying both morphology and chemical structure of the substrates layers, but they present serious environmental problems of waste disposal, which has moved investigations to an industrial alternative to these processes. According, the use of energetic methods of surface cleaning, such as laser and plasma cleaning processes have been recently consolidated since they are useful to modify the topmost layers of the substrates, increasing their reactivity, without affecting the bulk material properties. These treatments offer an effective and environment-friendly processing of different materials, providing strong and durable bonds with adhesives through modification of the surface morphology and functionalities. In this study, the effects of laser and low pressure plasma treatments on mechanical properties of adhesive bonded joints has been investigated and compared performing lap-shear tests
For an effective application of polymers, it is essential to have good adhesion behaviour to ensure good mechanical properties and durable components. Unfortunately, in general terms, polymers are characterized by high chemical inertness, which leads to very low surface energy values and, consequently, poor adhesive properties; this is particularly true for polyolefins. In this study, the effects of low pressure plasma treatment on surface roughness of polyethylene and polypropylene samples and on shear properties of adhesive bonded joints based on these substrates have been investigated. In particular, the optimization of three plasma process parameters, exposure time, voltage and working gas, were studied performing roughness measurement, contact angle evaluation and lap-shear tests. The experimental results show that the optimized plasma process may remarkably change the surface morphology, increasing wettability properties of the surfaces and shear strength of the bonded joints. These good properties remain almost unchanged even after some days of storage in the laboratory.
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