Surface treatment of wood polymer composites for adhesive bonding

An experimental study was undertaken to evaluate different surface treatment techniques for adhesive bonding of a Wood Polymer Composite (WPC) material. The surface treatment methods were flame, corona discharge treatment (CDT), mechanical abrasion (MA) and combination treatment of MA followed by the CDT. Surface analytical techniques used were contact angle analysis, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), Atomic Force Microscopy (AFM) and 2 dimensional profilometry. Adhesion pull-off test was used to assess the effects of surface treatments. The combination surface treatment, i.e. MA+CDT, was particularly effective in improving bonded joint strength. The adhesion pull-off strengths increased from 0.2 MPa for untreated substrate to 3.4 MPa for MA+CDT treated substrates. The carbonyl, hyroxyl and wood indices obtained from the ATR-FTIR spectra were valuable indicators for studying the nature of WPC substrates.     

On adhesion mechanisms and interfacial strength in acrylonitrile–butadiene–styrene/wood sawdust composites

Adhesion mechanisms and interfacial strengths of acrylonitrile–butadiene–styrene (ABS) copolymer/wood sawdust composites containing two different silane coupling agents [3-Methacryloxypropyl trimethoxysilane (KBM503) and N-2(aminoethyl)3-aminopropyl trimethoxysilane (KBM603)] were studied using the Fourier Transform Infrared (FTIR) technique and mechanical testing. The results suggested that increasing wood sawdust content tended to increase the modulus of the composites, but decreased the strength of the composites. Concentrations of 1.0 wt% KBM503 and 0.5 wt% KBM603 were recommended for the optimization of the mechanical properties of the composites, respectively. Up to the recommended dosages, KBM603 was more effective in terms of the improved interfacial strength of the composites. The adhesion mechanism performed by KBM503 involved dipole–dipole interaction at the ABS/sawdust interface, whereas that by KBM603 was associated with covalent chemical bonds at the interface. The improved interfacial strength of the composites was reduced by the increased amounts of wood sawdust particles.     

表面、界面的作用与粘接机理(一)

表面、界面的作用，对粘接过程有很大的贡献。本文介绍了材料表面、界面的湿润与吸附作用对粘接的影响，内容包括表面张力与界面张力，接触角，固体材料表面能的测算，湿润与湿润速度，粘附功与粘附界面的稳定性，粘附与吸附作用的对照等。     

Covalent bonding of wood through chemical activation

The reaction of softwood thermomechanically pulped fibres with succinic anhydride (SA) and its subsequent reaction with hexamethylene diamine (HMDA) were examined, the latter employing the coupling agents dicyclohexylcarbodiimide, or diisopropylcarbodiimide. FTIR and ¹³C NMR studies showed amide linkages were produced between SA modified fibres and the diamine. Wood veneers were covalently bonded using this technology. It was found that during hot pressing bonding occurs between SA treated veneers and HMDA without the need for a coupling agent.     

Atmospheric plasma modification of polyimide sheet for joining to titanium with high temperature adhesive

This investigation highlights the rationale of adhesive bonding of atmospheric pressure plasma treated high temperature resistant polymeric sheet such as polyimide sheet (Meldin 7001), with titanium sheets. The surface of polyimide (PI) sheet was treated with atmospheric pressure plasma for different exposure times. The surface energy was found to increase with increase in exposure time. However, longer exposure time of plasma, results in deterioration of the surface layer of PI resulting in degradation and embrittlement.Contact angle measurements with sessile drop technique were carried out for estimation of surface energy. SEM (EDS) and AFM analyses of treated and untreated specimens were carried out to examine the surface characteristics and understanding morphological changes following surface treatment. Untreated samples and atmospheric pressure plasma treated samples of polyimide Meldin 7001 sheet were bonded together as well as with titanium substrates to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to measure bond strength and to investigate the effect of surface treatment on adhesive bond strength. An optimized plasma treatment time results in maximum adhesive bond strength and consequently, this technology is highly acceptable for aviation and space applications.     

Correlation between physical bonding and mechanical properties of wood–plastic composites: part 2: effect of thermodynamic factors on interfacial bonding at wood–polymer interface

Abstract

The main objective of this study was to find out if there is any significant correlation between physical properties and interfacial bonding of interphases in wood–plastic composites. To this end, high-density polyethylene (HDPE), mixture of 3% maleic anhydride grafted polyethylene (MAPE) and HDPE (coded as MHDPE) and polylactic acid (PLA) were separately interacted with veneers to identify factors underlying interfaces. Plastics were first melted at 180?°C and dispensed on wood surfaces so that the contact angle (CA) could be directly measured. Wood sanding moderately decreased the CAs of plastics in order of PLA, MHDPE, and HDPE. The treatment of veneers with MAPE comprehensively improved wetting, as the CA of HDPE was significantly reduced on the wood surface after the treatment. Thereafter, the interfacial shear strengths (IFSS) of the wood–polymer interface were determined using the automated bonding evaluation system. PLA had the highest IFSS both for unsanded and sanded veneers. Comparing both parts of this research finally revealed that applying sanding or/and MAPE treatments resulted in lower surface free energy and higher IFSS at the wood–polymer interface. However, our observations support the idea that, at higher temperatures, wetting of composites is mainly influenced by polymer properties rather than interfacial tension at the wood–polymer interface.     

Chain Scission upon Fracture of Autoadhesive Joints Formed from Glassy Poly(phenylene oxide)

By employing the ion mechanoemission technique, the fracture process of lap shear autoadhesive joints formed from the two identical samples of 2,6-dimethyl-1,4-phenylene oxide (PPO) at constant healing temperatures (T) lower than the PPO bulk glass transition temperature by 60–92 K has been investigated in high vacuum at ambient temperature. For the first time, the ion emission due to the PPO chain scission has been observed during fracture of both the PPO–PPO autoadhesive joints and the PPO bulk sample. It has been found that the number of ions emitted upon the PPO–PPO joints fracture and the lap shear strength of the PPO–PPO joints increases with T. This behavior is attributed to an increase in the number of broken chains provided by an increase in the number of intermolecular physical bonds and of topological entanglements evolved from the chains segments interdiffused across the PPO–PPO interface with an increase in T. The kinetics of the PPO–PPO joints fracture has been discussed.     

The effect of surface energetics and molecular interdiffusion on adhesion in multicomponent polymer systems

The interfacial region of coated plastics is an example of a multicomponent polymer system. Practical adhesion, as determined by the peel test, has been found to be strongly dependent on the composition of the system and the degree of interaction between its components. Several interactions are possible during the coating process of polypropylene (PP)/ethylenepropylene-diene-monomer (EPDM) blends with chlorinated polyolefin (primer) and polyurethane (PUR) paint. Wettability, a necessary but not sufficient condition alone for molecular interdiffusion, was found to be good in all cases. The lack of interfacial adhesion between PP and PUR and between EPDM and PUR was explained by high interfacial tensions calculated from surface energetics, which, in turn, were determined by contact angle and inverse gas chromatography (IGC) measurements. The improvement of interfacial adhesion between PUR and PP by chlorinated polyolefin was explained by acid-base interactions detected by IGC. The creation of surface topography by extraction of low molecular weight fractions during the coating process does not influence the adhesion. Molecular interdiffusion was shown to be facilitated by solvents.     

Mechanism of adhesion of low surface energy materials treated with trialkylborane

Excellent adhesion to low surface energy materials without surface pretreatment was obtained by acrylates polymerization initiated by trialkylborane at room temperature. The adhesion mechanism was elucidated by electron spin resonance (ESR) and gas chromatography-mass spectroscopy (GC-MS) analysis based on model compound and chain transferring agent technologies. On the other hand, polypropylene (as one kind of low energy surface materials) was treated with trialkylborane and its effects were also studied by attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) and Raman spectroscopy, static contact angle measurements. The result provided further supports the elucidated mechanism.The results showed the adhesion resulted from the monomer’s graft on the surface of low surface energy materials. The graft resulted from the hydrogen abstraction reaction of materials (not less than three hydrogen sites) by alkoxyl and other radicals. These radicals came from oxidation of trialkylborane through the cleavage of O−O bond of R₂BOOR. The adhesion was caused neither by the materials’ surface energy improvement nor by the copolymerization of polymerizable monomers and unsaturated groups on the surface of materials.     

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.     

Design for improved adhesion of fluorine-incorporated hydrogenated amorphous carbon on metallic stent: Three-layered structure with controlled surface free energy

Hydrogenated amorphous carbon (a-C:H) or element-incorporated a-C:H have attracted much attention as coating materials on coronary artery stents owing to their outstanding properties. However, their applications have been limited because of poor adhesion to metallic materials. The present work was thus aimed at improving the adhesive property of a-C:H-based film on a stent by introducing interlayers with controlled surface free energies.Here we propose a three-layered coating for a SUS316L stent, comprising fluorine-incorporated a-C:H (a-C:H:F), silicon-incorporated a-C:H (a-C:H:Si), and hydrogenated amorphous silicon carbide (a-SiC:H) layers (from top to bottom). Each layer was deposited using a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method, and the surface free energy of each layer was controlled by the experimental parameters. Thereafter, the three-layer-coated stent was expanded and evaluated under a scanning electron microscope (SEM) in order to determine whether or not cracking or delamination had occurred. It can be seen from the SEM images that the occurrence of cracks or delamination was markedly suppressed even in the bent region of the stent after expansion, where the plastic deformation is highly localized when stents are expanded. This indicates that the method we applied in this work can serve as one approach to overcoming the limitation described above.     

Improving the adhesion strength of polymers: effect of surface treatments

Abstract

To improve their adhesion strength, polymeric surfaces are usually modified through different treatments. This study investigates the effect of mechanical, chemical, and energetic treatments on the bonding strength of ethylene propylene diene methylene (EPDM), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS) materials. Three adhesives based on different chemical compositions, namely silicone, polyurethane, and modified-silane (MS) polymer, were considered. Results show that the effect of the applied treatments on the adhesion strength of EPDM surfaces is insignificant. Only a slight improvement is obtained in the case of polyurethane-based adhesive, while the failure modes remained adhesive. As for PVC, most treatments were effective in the case of the silicone-based adhesive, especially grit blasting, primer, and UV/ozone treatments. Only UV/ozone treatment improved the adhesion strength and altered the failure mechanisms of this material when polyurethane and MS-based adhesives are used. The adhesion of ABS increased and the failure modes changed from adhesive to cohesive for most treatments. Particularly, a significant improvement is obtained when primer coating and UV/ozone radiation are applied. This comparative study paves the way for the design of polymeric joints with highly enhanced adhesion performance.     

The effect of post surface silanization and luting agents on the push-out bond strengths of adhesively inserted fiber reinforced posts

The effect of silane treatment on the push-out bond strengths of three different luting agents to fiber-reinforced composite (FRC) posts after thermocycling was evaluated.Sixty single-rooted human maxillary central incisors were sectioned below the cemento-enamel junction, and the roots were endodontically treated. RelyX Fiber Posts (size #2) were inserted using etch-and-rinse, self-etch, and self-adhesive luting agents (cementing agents). For half of the specimen in each group, the fiber posts were treated with a silane coupling agent. Bonded specimens were cut (2-mm-thick sections) and push-out tests were performed (crosshead speed, 0.5 mm/min). Failure modes were evaluated using a stereomicroscope at original magnification ×40.For each luting agent the use of silane did not result in any statistically significant difference at any level of the root compared to those of the control groups except for Variolink II and RelyX Unicem luting agents in apical root section (p<0.05; one-way ANOVA). The post hoc analysis showed that regardless of the pre-treatment procedures, Variolink II achieved significantly higher bond strengths than Panavia F 2.0 and RelyX Unicem in all root sections (p<0.05).The use of a silane coupling agent had no influence on bond strengths depending on the luting agent used, whereas the type of luting agent (etch-and-rinse, self-etch, and self-adhesive) appeared to be a significant influence on the push-out bond strength values independent of the pre-treatment used. Therefore, pre-treatment of fiber posts with a silane coupling agent does not seem to be mandatory, which saves time in the clinical situation.     

Effect of surface roughness on the adhesion of electrolessly plated platinum to poly(ethylene terephthalate) films

Poly(ethylene terephthalate) films were treated with aqueous sodium hydroxide solutions of different concentrations for various times. The rate of weight loss increased with the addition of a swelling agent (methylene chloride) or a cationic surfactant. The surface roughness of the treated films was determined from atomic force microscopy (AFM) and pore diameter was obtained from scanning electron microscopy (SEM). In general, surface roughness was found to increase with increasing weight loss for the treated films. A maximum roughness was obtained for samples with a weight loss of approximately 15-20%, beyond which the roughness of the samples decreased. The addition of methylene chloride and surfactant resulted in an almost two-fold increase in the roughness for all treatment times investigated. The adhesion of electrolessly plated platinum film was dependent on the contact area produced by chemical treatment. Treatments producing smaller diameter pores of greater depth gave better adhesion.     

Atmospheric plasma treatment of CFRP composites to enhance structural bonding investigated using surface analytical techniques

This paper considers the effects of an atmospheric plasma treatment (APT) on the surface properties of an amine cured carbon fibre/epoxy resin composite, and how those effects manifest over time. In particular, the ability of the APT to remove a thin layer of silicone-containing, proprietary, release agent (Chemlease^® 41 EZ), typically used in the production of composite components, has been investigated. It was concluded that the reduction in water contact angle (WCA) after APT for both the solvent wiped and contaminated surfaces was as a result of an increase in oxygen containing species at the surface, as determined through X-ray photoelectron spectroscopy (XPS). Further, it was found that the APT slightly reduced the failure strength of lap shear specimens for solvent wiped surfaces, whereas an increase in failure strength was observed for silicone contaminated samples. WCA and XPS results suggest that the contaminant layer was not removed, but instead transformed to a more stable inorganic form.     

Plasma surface modification of polyimide for improving adhesion to electroless copper coatings

The influences of oxygen plasma treatment of polyimide (PI) films on the adhesion of electroless copper coatings as well as on the chemical composition of the film surface and the PI surface morphology were investigated. The plasma operating parameters were 1800 W forward power with O₂ flowing at a rate of 300 cm³/min at a pressure of 200 mTorr. The peel strength increased with decreasing plasma treatment temperature. However, extension of the treatment time at higher temperatures had a positive effect on adhesion. A correlation between the enhancement in peel strength and the content of oxygen-containing groups at the PI surface (investigated using XPS) was observed. A change in the morphology as a result of plasma etching was also observed, in the formation of pits in the film surface. The pits ranged from 3 to 6 μm in depth and the diameter varied from 10 to 200 μm. Comparison of the data obtained after plasma treatment with the results of chemical etching in alkaline solutions of permanganate showed approximately the same adhesion increase (to 0.6 kN/m) in both cases. However, chemical etching did not affect the surface morphology and increased the oxygen content at the PI surface less than the plasma treatment.     

Fabrication of surface-treated BN/ETDS composites for enhanced thermal and mechanical properties

The ceramic/polymer composites based on epoxy-terminated dimethylsiloxane (ETDS) and boron nitride (BN) were prepared for use as thermal interface materials (TIMs). 250 µm-sized BN was used as a filler to achieve high-thermal-conductivity composites. To improve the interfacial adhesion between the BN particles and the ETDS matrix, the surface of BN particles were modified with silica via the sol–gel method with tetraethyl orthosilicate (TEOS). The interfacial adhesion properties of the composites were determined by the surface free energy of the particles using a contact angle test. The surface-modified BN/ETDS composites exhibited thermal conductivities ranging from 0.2 W/m K to 3.1 W/m K, exceeding those of raw BN/ETDS composites at the same weight fractions. Agari׳s model was used to analyze the measured thermal conductivity as a function of the SiO₂-BN concentration. Moreover, the storage modulus of the BN/ETDS composites was found to increase with surface modification of the BN particles.     

Comparative study of zein- and gluten-based wood adhesives containing cellulose nanofibers and crosslinking agent for improved bond strength

Many of the currently used wood adhesives contain chemicals that are harmful to human health and the environment. Increasing environmental and human health concerns have made the development of safe biobased adhesives a priority. In this study, two plant proteins, i.e., zein and wheat gluten, were used to develop wood adhesives and their performance was compared through simple lap shear tests and plywood flexural/internal bond tests in dry and wet conditions. To increase their bond strength, cellulose nanofibers were added to create nanocomposite adhesives and glutaraldehyde was also used to crosslink the proteins. Single-lap shear test was performed to measure the bond strength of different adhesive formulations and determine the optimal formulations and processing conditions. Fractured bond surfaces were studied using optical observation and scanning electron microscopy to determine bond failure mechanisms. Thermal and chemical properties of the adhesives were evaluated using thermogravimetric analysis and Fourier transform infrared spectroscopy, respectively. The bond strength of both zein and gluten adhesives was significantly increased by the addition of the cellulose nanofibers and/or glutaraldehyde, although the two adhesives responded differently to the two reinforcement materials due to the different solvents used to prepare the adhesives. The bond failure mode changed from cohesive failure of the adhesive to structural failure of the adherent for the gluten adhesive containing CNFs and glutaraldehyde. Potential zein and gluten adhesive formulations were used to produce plywood samples and their performance was assessed under different conditions. The formulations with industrial potential were discovered through this study.     

The interactions of amphiphilic latexes with surfaces: the effect of surface modifications and ionic strength

The effect of surface modifications brought about by a polymeric stabilizer on the interactions between polymer colloid particles and various substrates in aqueous media are directly measured using atomic force microscopy. The interactions of polystyrene particles with grafted hydrophilic ‘hairs’ of hydroxypropyl cellulose (denoted HPC/PS), of molecular weight ∼10⁵, with mica, silica and graphite substrates are measured. HPC/PS is found to be compatibilized so that it will interact with both hydrophobic and hydrophilic substrates. The observed jump-to contact between HPC/PS and silica is characteristic of polymer solutions and is the result of the grafted hairy layer. Further direct evidence of HPC-substrate interaction is seen in a secondary adhesion with mica. The adhesion of the particles was found to follow the order silica>graphite>mica. The magnitudes of these interactions are rationalized in terms of the interactions of each of the substrate, core polymer and surface modification. It is concluded that the combined effects of surface roughness and hairy layer collapse due to compression give rise to the observed trend.