In the present paper, the following topics are reviewed in detail: (a) the available adhesives, as well as their recent advances, (b) thermodynamic factors affecting the surface pretreatments including adhesion theories, wettability, surface energy, (c) bonding mechanisms in the adhesive joints, (d) surface pretreatment methods for the adhesively bonded joints, and as well as their recent advances, and (e) combined effects of surface pretreatments and environmental conditions on the joint durability and performance. Surface pretreatment is, perhaps, the most important process step governing the quality of an adhesively bonded joint. An adhesive is defined as a polymeric substance with viscoelastic behavior, capable of holding adherends together by surface attachment to produce a joint with a high shear strength. Adhesive bonding is the most suitable method of joining both for metallic and non-metallic structures where strength, stiffness and fatigue life must be maximized at a minimum weight. Polymeric adhesives may be used to join a large variety of materials combinations including metal-metal, metal-plastic, metal-composite, composite-composite, plastic-plastic, metal-ceramic systems. Wetting and adhesion are also studied in some detail in the present paper since the successful surface pretreatments of the adherends for the short- and long-term durability and performance of the adhesive joints mostly depend on these factors. Wetting of the adherends by the adhesive is critical to the formation of secondary bonds in the adsorption theory. It has been theoretically verified that for complete wetting (i.e., for a contact angle equal to zero), the surface energy of the adhesive must be lower than the surface energy of the adherend. Therefore, the primary objective of a surface pretreatment is to increase the surface energy of the adherend as much as possible. The influence of surface pretreatment and aging conditions on the short- and long-term strength of adhesive bonds should be taken into account for durability design. Some form of substrate pretreatment is always necessary to achieve a satisfactory level of long-term bond strength. In order to improve the performance of adhesive bonds, the adherends surfaces (i.e., metallic or non-metallic) are generally pretretead using the (a) physical, (b) mechanical, (c) chemical, (d) photochemical, (e) thermal, or (e) plasma method. Almost all pretreatment methods do bring some degree of change in surface roughness but mechanical surface pretreatment such as grit-blasting is usually considered as one of the most effective methods to control the desired level of surface roughness and joint strength. Moreover, the overall effect of mechanical surface treatment is not limited to the removal of contamination or to an increase in surface area. This also relates to changes in the surface chemistry of adherends and to inherent drawbacks of surface roughness, such as void formations and reduced wetting. Suitable surface pretreatment increases the bond strength by altering the substrate surface in a number of ways including (a) increasing surface tension by producing a surface free from contaminants (i.e., surface contamination may cause insufficient wetting by the adhesive in the liquid state for the creating of a durable bond) or removal of the weak cohesion layer or of the pollution present at the surface, (b) increasing surface roughness on changing surface chemistry and producing of a macro/microscopically rough surface, (c) production of a fresh stable oxide layer, and (d) introducing suitable chemical composition of the oxide, and (e) introduction of new or an increased number of chemical functions. All these parameters can contribute to an improvement of the wettability and/or of the adhesive properties of the surface.

This study concerns the surface and adhesive properties of isotactic polypropylene (iPP) modified by an electric discharge plasma and affected by long-term hydrophobic recovery of the polymer surface after modification. The investigations were focused on the change in polarity of the modified polymer expressed by the polar fraction as well as on the decrease in the surface free energy, its polar component and mechanical work of adhesion (A _m) to polyvinyl acetate. A _m of modified iPP to polyvinyl acetate as a function of polar fraction can be described by a mathematical formula. It has been confirmed that the most intensive decrease in the surface and adhesive properties investigated is produced by the long-term hydrophobic recovery of the polymer appears in the course of the first 30 days after its modification. During subsequent aging the process of polymer hydrophobic recovery proceeds more slowly. It has been found that the value of surface and adhesive properties of iPP as well as the dynamics of the decrease in these properties during hydrophobic recovery of the surface after modification is, in the main, dependent on the iPP crystallinity.

A study of the durability of corona discharge plasma effects on a polymer surface was investigated in this work. We used the corona discharge plasma technique to modify the wettability properties of low density polyethylene (LDPE) film and evaluated the influence of relative humidity and temperature on the aging process with three different storage conditions. The effects of the aging process on the plasma-treated surface of LDPE film were quantified by contact angle measurements, Fourier-transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. The results obtained with these techniques have allowed us to determine how the aging process promotes changes in the plasma-treated surface by decreasing its wettability and taking place a remarkable hydrophobic recovery process.

Nitrogen-based plasma systems such as N₂, NH₃, Ar/NH₃, and O₂/NH₃ were used to modify microporous polyethersulfone membranes. Treatments were designed to alter the surface chemistry of the membranes to create permanently hydrophilic surfaces. Contact angle measurements taken initially, as well as 1 year post-treatment confirmed that treatments using O₂/NH₃ plasmas (with a 5:3 gas flow ratio) were successful in achieving our designed goals. Analyses by FT-IR and XPS established the incorporation of NH_x and OH species in the PES membranes. Moreover, the plasma penetrates the thickness of the membrane, thereby modifying the entire membrane cross-section. Optical emission spectroscopy studies of excited state species present in the modifying gases revealed the presence of OH^*, which was not present in a 100% ammonia plasma, suggesting OH^* must play a critical role in the membrane modification process. Investigations using bubble point analysis, differential scanning calorimetry, and scanning electron microscopy demonstrate there is no damage occurring under these specific treatment conditions. The usefulness of this treatment is revealed by increased water flux, reduced protein fouling, and greater flux recovery after gentle cleaning when compared to an untreated membrane.

The effect of Ar, Ar/C2H5OH, O2 and Ar/O2 RF (13.56 MHz) plasma treatments on surface free energy and morphology, optical properties and adhesion of polycarbonate (PC) substrates has been studied. Changes in the surface properties were followed as a function of the plasma treatment time. The polar and dispersion components of the polymer free surface energy were determined on the basis of the theory of Owens, Wendt, Kaelble and Uy. It was found that all RF plasma treatments led to an increase in the polar component of PC, mainly due to an increased hydrogen bonding ability. The increase in surface free energy reached its maximum at short plasma treatment with 3:1 gas mixture of Ar/O2. This treatment also led to pronounced improvement of the adhesion of thin SiO2 films plasma deposited on modified PC substrates, while the treatments with pure oxygen or Ar/ethanol plasma had negative effect on the adhesion.