ABSTRACT
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.
ABSTRACT
Friction stir welding (FSW) is a solid-state technique, which has assumed an increasingly important role in automotive, naval, and aeronautical industry over the years. Nowadays, thanks to its several benefits, FSW is used to weld any type of metallic, polymeric, or composite material. In recent decades, adhesive bonding has also enhanced relevance due to a request for much lighter structures to increase performance without increasing fuel consumption. From a mechanical perspective, welding has a high tensile strength despite a low fatigue resistance through the lack of joint elasticity. Therefore, the aim of this study is to investigate and compare static and dynamic behavior of welded, weld-bonded, and adhesive-bonded joints. After choosing the most suitable adhesive, surface preparation, consisting of sandblasting, was carried out. First of all, on the basis of previous experience in FSW, the process parameters of hybrid welding were determined. Both quasi-static and dynamic behavior of welded, adhesive-bonded, and weld-bonded joints, made in overlapped configuration, were then compared. Experimental tests showed that the adhesive limits the negative effect, due to the presence of the structural notch of FSW overlapped joints.
ABSTRACT
We investigated the viscoelastic response of direct and indirect dental restorative composites by the novel technique of AM-FM atomic force microscopy. We selected four composites for direct restorations (Adonis, Optifil, EPH, CME) and three composites for indirect restorations (Gradia, Estenia, Signum). Scanning electron microscopy with micro-analysis was also used to support the results. The mean storage modulus of all composites was in the range of 10.2-15.2 GPa. EPH was the stiffest (p<0.05 vs. all other composites but Adonis and Estenia), while no significant difference was observed between direct and indirect group (p≥0.05). For the loss tangent, Gradia had the highest value (~0.3), different (p<0.05) from Optifil (~0.01) and EPH (~0.04) despite the large coefficient of variation (24%), and the direct composites showed higher loss tangent (p<0.01) than the indirect composites. All composites exhibited minor contrast at the edge of fillers, showing that these are pre-polymerized, as confirmed by EDS.
