Improvement of adhesion and paint ability of EVA copolymers with different vinyl acetate contents by treatment with UV-ozone

The surface modifications produced by UV-ozone treatment of two ethylene-vinyl acetate (EVA) copolymers containing 12 and 20 wt% vinyl acetate (EVA12 and EVA20 respectively) were studied. The treatment with UV-ozone improved the wettability of both EVAs due to the creation of new carbon–oxygen moieties. The extent of these modifications increased with increasing length of the treatment and the modifications produced in EVA20 were produced for shorter lengths of treatment. The UV-ozone treatment also created roughness and heterogeneities on the EVA surfaces. Whereas roughness formation prevailed on the UV-ozone treated EVA12, important ablation was dominant on the treated EVA20. T-peel strength values in joints made with polychloroprene adhesive increased when the EVAs were treated with UV-ozone. Short length of UV-ozone treatment (1 min) produced higher T-peel strength in joints made with EVA20 whereas higher T-peel strength values in joints made with EVA12 were obtained after treatment for 5–7.5 min in which a cohesive failure into a weak boundary layer on the treated EVA surface was found. Furthermore, the adhesion of UV-ozone treated EVA20 to acrylic paint increased. Finally, the ageing resistance of the treated EVA/polychloroprene adhesive joints was good and the surface modifications on the UV-ozone treated EVAs lasted for 24 h after treatment at least.     

Improved adhesion of LDPE films to polyolefin foams for automotive industry using low-pressure plasma

The use of polymer films for technical applications has increased considerably in the last years, since they offer good balanced properties. Polymer films find many applications as individual materials or as laminates with other films, foams, membranes, etc. In these cases it is necessary to improve the low intrinsic surface energy of polymer films to ensure their optimum mechanical performance. In this work, low-pressure glow discharge plasma with different gases is used to improve the adhesive properties of a low-density polyethylene (LDPE) film, to obtain the optimum mechanical response of laminates with polyolefin foam for automotive applications (steering wheels). The results show a remarkable increase in T-peel strength of the adhesive joints. Furthermore, since automotive industry is characterized by high technical requirements, the evaluation of the durability of the adhesive joints (in terms of storage conditions: temperature and relative humidity) shows that the T-peel strength of adhesive joints is subjected to an aging process that slightly decreases their mechanical performance, but does not restrict the use of these laminates in automotive uses.     

UV-cured fluorinated coatings for plastics: effect of the photoinitiator and of the substrate filler on adhesion

A high-performance fluorinated acrylic coating, containing perfluoropolyether chains, was applied to polyethylene and polypropylene by means of the UV-curing technique. The conditions for a good adhesion onto the plastic substrates were studied. A chemical bonding between the cured network and both the substrates was achieved when the photopolymerization was performed in the presence of benzophenone as a photoinitiator: by hydrogen abstraction from the substrate, polymer radicals were formed and could then initiate the polymerization of the acrylic species; grafting experiments of hydroxyethylacrylate confirmed this mechanism. However, the grafting reaction did not take place when the plastic substrates contained carbon black as a filler, which acted as a radical scavenger. In this case, in fact the adhesion results are not good, similar to those obtained in presence of a cleavage-type photoinitiator.     

Viscoelastic and adhesion properties of hot-melts made with blends of ethylene-co-n-butyl acrylate (EBA) and ethylene-co-vinyl acetate (EVA) copolymers

Several hot-melts (HMAs) were prepared by using blends of ethylene-co-n-butyl acrylate (EBA) and ethylene-co-vinyl acetate (EVA) copolymers - EBA/EVA. HMAs were prepared with mixtures of EVA copolymers with 18 (EVA18) and 27 (EVA27) wt% vinyl acetate contents and EBA copolymer with 27 wt% n-butyl acrylate, polyterpene resin and mixture of microcrystalline and Fischer-Tropsch waxes. HMAs made with EBA/EVA blends showed lower viscosities and reduced shear thinning than the ones made with EBA or EVA due to differences in compatibility, but both the set time and the open time were not affected as they depended mainly on the wax nature and amount. The increase of the vinyl acetate (VA) content in EVA copolymer reduced the crystallinity of the EBA/EVA blends. Even EBA copolymer was more compatible with EVA27 than with EVA18 (the α- and β-transitions shown in DMTA plots were closer) and the compatibility did not vary with the EBA content in the blends. The addition of polyterpene resin and the mixture of waxes decreased the compatibility of the EBA/EVA blends, the higher compatibility was observed for the HMAs made with only one copolymer. The tack of the HMAs depended on their EBA/EVA contents, EBA/EVA27 HMAs showed broader temperature interval with higher tack, while the tack of EBA/EVA18 HMAs blend decreased and the temperature interval with tack was shortened and shifted to lower temperatures. Adhesion to polypropylene film was improved in HMAs made with 75 wt% EBA/25 wt% EVA18 and 50–75 wt% EBA/50-25 wt% EVA27. The adhesion to aluminum film of EBA or EVA hot melts was improved only in the joints made with EBA/EVA 27 HMAs, more noticeably when they contained higher EBA content.     

Interface states and MWS polarization contributions to the dielectric response of low voltage ZnO varistor

The main purpose of this work is to study the dielectric response of commercial low voltage ZnO varistors by means of dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) in a wide temperature range. Four relaxation processes have been studied here and all of them demonstrate Cole-Davidson behaviour. The first two faster relaxation mechanisms are known processes in ZnO varistors and those are related to the ZnO bulk traps. The next one faster relaxation mechanism attributed to the MWS polarization which should be related to the intergranular Bi-rich microregions. The remaining slower relaxation mechanism is associated to the grain boundaries interfaces. Based on the block model for the ZnO varistor a gradual reduction in the total depletion width is observed at a temperature about 330 K, which can be considered that is due to the gradual decrease of the interface states density at this temperature region.     

Influence of the interfacial composition on the adhesion between aggregates and bitumen: Investigations by EDX, XPS and peel tests

The study of the adhesion between aggregates and bitumen is necessary to enhance the lifetime of the roads. The purpose of this work concerns the interaction between the mineralogy of the aggregates and the adhesion force measured at the interface between bitumen and aggregate. The adhesion of bitumen was studied according to the mineralogy of the aggregates, which were made of dolomite rock or granite. A method was developed to measure the fracture energy during the peeling of the bitumen layer from the aggregate surface. The specific manufacturing of the samples ensured reproducible measurements using a constant thickness of the bitumen layer and by introducing a strengthened and flexible membrane into the bulk of bitumen. The peeling results demonstrated that the locus of the failure varied according to the mineralogy of the aggregate. The failure was cohesive during the peeling of the dolomite/bitumen system while the failure was partly interfacial concerning the granite/bitumen system. The interface between bitumen and minerals was characterized, before and after peeling. In case of the granite, the detection of sulfur by X-ray Photoelectron Spectroscopy (XPS) highlighted only the bitumen residues and allowed identifying the mineral compounds that weaken the interface between bitumen and granite. Finally, XPS analyses showed that the alkali feldspars of the granite induced a weak interface with bitumen.     

The effects of phenolic resin-derived PyC interlayers on microstructure and mechanical properties of Cf/SiC composites

A novel, easy and cost-effective way, infiltration and pyrolysis of phenolic resin solution, was exploited to prepare pyrolytic carbon (PyC) interlayers for carbon fiber/silicon carbide (Cf/SiC) mini-composites. X-ray photoelectron spectroscopy, dynamic contact angle measurement and scanning electron microscope were carried out to characterize chemical structure of carbon fibers (CFs), wetting properties between CFs and phenolic resin solution and microstructure of CFs and their composites, respectively. Remarkably, SEM results showed regulation of uniformity and thicknesses of PyC interlayer could be achieved through controlling the concentration of phenolic resin solution and oxidation condition of CFs. When CFs were treated by 10?min' oxidation with 40?mg/L ozone followed by dip-coating with 4?wt% phenolic solution, uniform PyC interlayer with approximately 120?nm were prepared on CFs. The corresponding Cf/SiC specimens had the largest increase in tensile strength and work of fracture with the improvement of 26.2% and 71.6% from the PyC-free case.     

Shrinkage behavior and interfacial diffusion in Ni-based internal electrodes with BaTiO3 additive

The effects of particle size of starting materials and amount of a BaTiO₃ additive on the shrinkage behavior and elemental diffusion in Ni-based internal electrodes have been investigated in order to control the shrinkage of the internal electrode in multilayer ceramic capacitors (MLCCs). Two kinds of Ni and BaTiO₃ powders were used with different particle sizes. Volume shrinkage over the range of 700–1300 °C at 150 °C intervals and linear shrinkage during sintering were measured for starting materials and composites in a reducing atmosphere. The interfaces of Ni/BaTiO₃ composites with 90:10 and 70:30 volume ratios, respectively, were investigated using TEM. Composites with bimodal Ni powder show less shrinkage than those with monomodal Ni powder, showing less shrinkage in monolith Ni of bimodal particle size. The shrinkage behavior is changed during sintering with increasing amounts of BaTiO₃ additives in both Ni-based composites. The particle size of the BaTiO₃ additive affects the shrinkage behavior of composites, without the additional amount affecting the final shrinkage. A reaction layer of about 300 nm wide is observed at the interface between the Ni and BaTiO₃ powders in composites, in which elemental Ni diffuses into the BaTiO₃ without counterdiffusion.     

The effects of grit-blasting on surface properties for adhesion

A detailed study of the effects of grit blasting with different alumina grits on the surface characteristics of mild steel and aluminium alloy substractes is reported. Non-contacting 3D-laser profilometry was used to characterise surface texture, and surface energy was measured by static contact angle techniques. The chemical composition of the surface was determined by XPS analysis. Adhesion characteristics were investigated by the measurement of strength of lap shear and tensile butt joints using a two-part room temperature curing epoxy adhesive. As initial joint strengths were relatively insensitive to the changes in grit-blasting parameters, further studies were based on joint response to accelerated ageing conditions. The results indicate that the changes in joint properties associated with roughened surfaces cannot be explained simply by the increased roughness characteristics, such as mechanical keying and increased effective bond area. It is evident that changes in physical and chemical properties of the surfaces, arising from the grit-blasting process contributed significantly to the joint behaviour.     

Joining of Al2O3 ceramic to Cu using refractory metal foil

Joining of Al₂O₃ ceramic to Cu has been conducted with Ag-26.7Cu-4.5Ti braze and refractory metal (W or Ta) foil. The interfacial microstructure in the joint with W foil is similar to that with Ta foil. The joining region in the joint consists of a reaction zone, braze zone I, refractory metal layer and braze zone II. The reaction zone of Cu₃Ti₃O with a thickness of about 5 μm develops close to Al₂O₃ side due to the reactions of Ti and Cu in the braze with Al₂O₃ substrate. The braze zones I and II are mainly composed of Ag- and Cu-based solid solutions. For the joint with W foil, the adsorption of Ti at the braze/W interfaces followed by the Ti diffusion into W foil occurs, whilst slight dissolution and diffusion of Ta into the brazes take place in the joint with Ta foil. The average shear strengths of joints with W and Ta foils are much higher than those without refractory metal foil, indicating the contribution of the refractory metal foil to the improvement of joint mechanical strength. Introduction of refractory metal foil in Al₂O₃/Cu joining is beneficial for the shift of joint residual stress distribution and the decrease of stress concentration in the joint since the coefficient of thermal expansion (CTE) of refractory metal layer approximately approaches that of Al₂O₃. Furthermore, a slight thickness increase of the Cu₃Ti₃O reaction zone in the joint with refractory metal foil may also give rise to the joint strength promotion.     

Cyclic oxidation in burner rig of TiAlN coating deposited on Ti-48Al-2Cr-2Nb by reactive HiPIMS

The Ti-48Al-2Cr-2Nb intermetallic alloy was coated by using a mixed titanium-aluminum nitride with the aim of improving its oxidation resistance. The protective TiAlN coatings were obtained by reactive High Power Impulse Magnetron Sputtering (HiPIMS) technique. The surface of the Ti-Al alloy was prepared for HiPIMS according to different methods; the deposition of an intermediate Ti-Al interlayer was also investigated. A cyclic oxidation test (100 and 200 cycles) was performed up to 950 °C in burner rig apparatus adopting severe heating and cooling rates. The microstructure and composition of samples before and after oxidation were investigated by several techniques: microscopy (optical and SEM-EDS), X-ray photoelectron spectrometry (XPS) and X-ray diffraction (XRD). The oxide layers formed on the sample surface showed different thickness and composition depending on the presence of the protective coating and the processing path adopted for its deposition. The nitride coatings appreciably enhanced the oxidation resistance and sustained repeated thermal shocks without showing damage or spallation.     

The effect of interfacial bonding of calcium phosphate cements containing bio-mineralized multi-walled carbon nanotube and bovine serum albumin on the mechanical properties of calcium phosphate cements

The aim of this study was to investigate the effect of adding bio-mineralized hydroxyl functionalized multi-walled carbon nanotubes (MWCNTs–OH) on the compressive strength of calcium phosphate cements (CPCs). Bovine serum albumin (BSA) was also incorporated as a protein which acts as promoter of hydroxyapatite (HA) crystal growth when bounded to CPC granules. The results show that the strong interfacial bonding of CPC/MWCNTs–OH is essential to improve the mechanical properties of CPC/bio-mineralized MWCNTs–OH/BSA composite.     

Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite

SiC/SiC composites reinforced with near stoichiometric SiC ceramic fibres (Hi-Nicalon S and SA3 Tyranno fibres) are attractive materials to be used in nuclear environment. Netherless, their mechanical properties must be improved and controlled. For example, SA3 Tyranno fibres (TSA3) -reinforced composites exhibit a brittle behaviour whereas composites reinforced by Hi-Nicalon S (HNS) fibres exhibit a conventional damage tolerant response. This difference is related to the nature of the fibre/matrix (F/M) coupling. The aim of this work was to identify the SiC fibres characteristics influencing the F/M coupling and consequently the mechanical properties of the composites. The experimental results point out that the TSA3 fibres exhibit a granular and rough surface leading to an increase of the residual stress and the interfacial shear stress in the SiC/SiC composites. Beside the roughness, the experimental results also point out that the surface chemistry of the SiC fibres significantly influence the F/M bonding.     

Effect of surface roughness on interlaminar peel and shear strength of CFRP/Mg laminates

In this study, grit blasting with different abrasive particle sizes was carried out on magnesium alloy sheets, then the carbon fiber reinforced polymer (CFRP)/magnesium alloys laminates were prepared using a hot-press process. The surface characteristics of magnesium alloy, and the interlaminar strength of CFRP/Mg laminates were examined, in order to investigate comprehensively the effect of surface roughness on interlaminar strength of laminates under peel and shear loading conditions. The results show that the rougher surface significantly improves the peel strength of laminates, while the shear strength of laminates increases only slightly with increasing surface roughness. Hence, the rougher surface exhibits a good overall interlaminar strength under peel and shear loading when compared to the smoother surfaces.     

Phase transformation of nonstoichiometric cubic tungsten carbide on the surface of carbon nanotubes during high-temperature annealing of aluminum matrix composites

In this work, metal matrix composites based on 5049 aluminum alloy reinforced with multi-walled carbon nanotubes (CNTs) coated with nonstoichiometric cubic tungsten carbide were obtained by powder metallurgy. For the first time for this system, high-temperature annealing of the synthesized composites at 500–600 °C for 0.5 h was carried out. Effect of the annealing on the evolution of the structural-phase composition and the change in the microhardness and Young's modulus of the bulk composite was studied. Characterization of the structure shows that despite the growth of the matrix grains, the structural heterogeneity inherent for composites in the as-synthesized state is retained after heat treatment. Along with coarse grains, fine grains still remain, which indicates an increased resistance to recrystallization of the composite even at a temperature of ～0.9T_m. XRD analysis shows that annealing at temperatures above 525 °C leads to a solid-state interfacial reaction of the ceramic coating on the CNTs surface with the matrix material, resulting in the in-situ formation of the WAl₁₂ intermetallic compound around the reinforcing particles and Al₄C₃ nanorods. At the same time, the structural integrity of the CNTs is preserved. An increase in the annealing temperature contributed to an increase in the intensity of phase transformations and an increase in the fraction of the in-situ phases. The microhardness and Young's modulus of the composites decreased by ～20% and ～27%, respectively. Nevertheless, despite the increase in the grain size, the level of these properties remained quite high and equal to 141 HV and 80 GPa, correspondingly, due to the formation of a larger fraction of the in-situ WAl₁₂ and Al₄C₃ phases. The obtained results are applicable for varying the mechanical properties of the composite by controlling the degree of in-situ reaction between the matrix alloy and the ceramic coating on the CNTs.     

Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics

Damage resistance and R-curve behavior of multilayer Al₂O₃/SiC ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods. Due to the crack deflection at the Al₂O₃/SiC interfaces, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m^1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior.     

Effects of the oxygen pressure on the crystalline orientation and strains of YSZ thin films prepared by E-beam PVD

Yttria−stabilized zirconia, YSZ, thin films were prepared by E-beam physical vapor deposition (PVD) at 200 °C under oxygen pressure of 1 × 10⁻³∼1 × 10⁻⁵ Torr. Observations by Field Emission Scanning Electron Microscope (FESEM) proved that different oxygen pressures influenced the thickness of interfacial SiO_x layer formed between the YSZ thin films and Si(100)-substrate. X-ray diffraction (XRD) patterns were used to determine the crystalline structure and calculate the surface grain size of deposited YSZ thin films. XRD patterns also showed that the peaks corresponding to planes (111), (200), (220), and (311) were found and the YSZ thin films revealed the fluorite structure. At lower oxygen pressure (1 × 10⁻⁵∼1 × 10⁻⁴ Torr) YSZ thin films revealed the (111) preferred orientation and at higher oxygen pressure (5 × 10⁻⁴∼1 × 10⁻³ Torr) YSZ thin films revealed the (200) preferred orientation. The effects of oxygen pressure on the lattice constants and the internal strains of YSZ thin films were also investigated.     

Role of interface on electrical conductivity of carbon nanotube/alumina nanocomposite

Interface of multiwalled carbon nanotube (MWCNT)/alumina (Al₂O₃) nanocomposites have been studied using TEM. At low sintering temperature (T_sin=1500 °C), a 3–5 nm thick amorphous interface region was noticed. Nanocomposite sintered at 1700 °C possessed a well-defined graphene layer coating on matrix grains as the interface between CNT and Al₂O₃. A mechanism of such layered interface formation has been proposed. No traceable chemical reaction product was observed at the interface even after sintering at 1700 °C. It was noticed that while DC electrical conductivity (σ_DC) of 1500 °C sintered 2.4 vol% MWCNT/Al₂O₃ nanocomposite was only~0.02 S/m, it raised to ~21 S/m when sintering was done at 1700 °C. Such 10³ times increase in σ_DC of present nanocomposite at a constant CNT loading was not only resulted from the exceptionally high electron mobility of CNT but the well-crystallized graphene interface on insulating type Al₂O₃ grains also significantly contributed in the overall increase of electrical performance of the nanocomposite, especially, when sintering was done at 1700 °C.     

Using the 3-point bending method to study failure initiation in epoxide-aluminum joints

The increasing use of adhesives in industry has boosted the search for tests which allow the adherence level to be defined. These tests, depending on the type of load, examine different stresses, failure modes and mixed modes. Furthermore, these tests can be focused either on initiation or propagation of adhesive failure. The subject of this study is the initiation of adhesive failure. The initiation of failure can be determined with the 3-point bending test. Trials of 3-point bending tests were conducted on an aluminum 2024 substrate, with two different thicknesses, in order to understand the impact of the adherend thickness. The aluminum substrate received different types of surface pre-treatment: acetone cleaning, hydrochloric acid etching or aminopropyltriethoxysilane coating. Two adhesives were used: the first one was a mixture of epoxy pre-polymer DGEBA and DETA amine, whereas the second was a commercially formulated adhesive, ELECOLIT 6604. The initiation of adhesive failure was obtained by 3-point bending test and verified with SEM analysis. The failure loads measured enabled the effect of surface treatment on adhesive failure to be revealed: the results indicate that the surface treatment with silane is the most efficient in comparison to hydrochloric acid etching and of course to the simple acetone degreasing. It was assumed that the scatter of the results obtained for each series is due to the variation of the “intrinsic” adherence between the adhesive and the substrate. Furthermore, it was noted that the failure loads for different substrate thicknesses cannot be compared due to the effect of the thickness: it was therefore not possible to simply compare results obtained for different thicknesses of the same substrate for a given substrate-adhesive system. The energy approach proposed in this study makes it possible. The energy requested to initiate the adhesive failure for a given system can then be known, whatever the initial geometry of the 3-point bending test is. However, it was also shown that the thickness of the substrate must be correctly chosen. A thick substrate increases the dispersion and a thin substrate may induce local unwelcome plastic strain. In conclusion, this study allows to define an energy criteria for adhesives failure initiation.