Adhesion characteristics of plasma-surface-treated carbon fiber-epoxy composite with respect to release films used during demolding

The load capabilities of carbon fiber-epoxy composite adhesive joints are affected by surface characteristics of the composite adherends such as surface free energy and chemical composition, which can be altered by plasma surface treatment and the type of release film for demolding carbon fiber-epoxy composites from metal molds. In this paper, suitable plasma surface treatment conditions for carbon fiber-epoxy composite adherends were investigated to enhance the strength of carbon fiber-epoxy composite adhesive joints using dielectric barrier discharges of atmospheric pressure plasmas. The effects of plasma surface treatment on the surface free energy and adhesion strength of carbon fiber-epoxy composites were experimentally investigated with respect to surface treatment time. Also, the surface and adhesion characteristics of carbon fiber-epoxy composites were investigated with respect to release films such as fluorinated ethylene propylene (FEP), high density polyethylene (PE) and Nylon 6.6. Quantitative chemical bonding analysis with X-ray photoelectron spectroscopy (XPS) was also performed to understand the load capabilities of composite adhesive joints with respect to plasma treatment time and release films. From the experimental results, it was found that plasma treatment of carbon fiber-epoxy composites did enhance its adhesion strength, irrespective of the type of release film. Regarding adhesion strength, Nylon 6.6 was found to be the most suitable release film for these composites when no plasma treatment could be applied. From the XPS measurements on carbon fiber-epoxy composites, it was found that the carbon bond ratio of C=O to C-C and C-H reached a maximum at around 10 s treatment time, which corresponded well with the load transmission capability of the composite adhesive joint.     

Ammonia plasma treatment of polyolefins for adhesive bonding with a cyanoacrylate adhesive

Polypropylene (PP), low-density polyethylene (LDPE), and high-density polyethylene (HDPE) sheets were surface-modified by radio-frequency ammonia plasmas in order to improve the strength and durability of adhesive bonding, particularly under hot and humid conditions. Surface analyses by contact angle measurements, XPS (X-ray photoelectron spectroscopy), and FTIR-ATR (Fourier transform infraredattennuated total reflection) showed incorporation, upon plasma treatment, of both nitrogen- and oxygen-containing functional groups on the polyolefin surfaces, with similar surface compositions on modified LDPE and PP. Plasma-treated polyolefin samples bonded with a cyanoacrylate adhesive possessed a high shear bond strength in 'dry' conditions. On exposure to hot and humid conditions (immersion in 60°C water), the bond strength decreased with time in some cases while for other samples the lap shear strength was the same after exposure to the humid environment for 1 month compared with that under 'dry' conditions. Ammonia-plasma-treated HDPE specimens displayed the best strength retention upon water immersion. The excellent durability of the bond strength under humid conditions is indicative of covalent bonding between the cyanoacrylate adhesive and amine groups, which unlike physical bonding (e.g. van der Waals interactions) is not disrupted by the ingress of water molecules. It is also possible that the structure of the interphase is in the form of an interpenetrating network, obtained through penetration of the adhesive into the plasma-modified laycr, followed by covalent bonding and curing of the penetrated adhesive.     

Effects of water on the curing and properties of epoxy adhesive used for bonding FRP composite sheet to concrete

When a concrete surface is contaminated by water due to rain, saline water, ground water, and water jetty treatment, water, alkalis, and other contaminants on the concrete surface may interact with an epoxy adhesive used for bonding fiber‐reinforced polymer composite sheets to concrete. This can influence both the curing rate and the degree of cure of the curing reaction. This in turn can affect the time required for field application. It can also influence the mechanical properties and durability of epoxy adhesives. In this paper, water effects on the curing and properties of two kinds of commercial adhesives were evaluated. Curing kinetics were studied using isothermal DSC analysis. Results showed that water accelerated the curing reaction. However, excess water offsets part of the accelerating effect. While water is typically considered to be harmful to properties of adhesives, it was seen that a small amount (less than 2%) of water improved degree of cure, mechanical properties, and durability of adhesives. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2261–2268, 2004     

Comparative determination of TiO2 surface free energies for adhesive bonding application

The development of durable bonds using titanium adherens has been investigated from the point of view of surface energy theoretical models measurements. The traditional Chromium Acid Anodization, which provides excellent durability, has to be phased out due to the use of hazardous Cr (VI) in the bath and as a result, special attention is paid to the sodium hydroxide anodizing and other alkaline chemical etchers. There are hardly any references on the surface free energy of adhesive titanium oxide coatings and therefore the objective of this work was to evaluate the surface and interface energy parameters of the various types of alkaline chromate free surface treatments using Neumann, Fowkes and van Oss–Chaudhury–Good methods in order to determine which method provides greatest differentiation between the coatings. Results show that Fowkes method produced the greatest variance in surface energies of the compared surface treatments and hence can be considered as better suited for more accurate discrimination between the oxide surface treatments on Ti–6Al–4V alloy. Although, in the case of model liquids, i.e. water and diiodomethane, the trends obtained for contact angles, surface energies, works of adhesion and solid/liquid interface energies all correlated between each other, a disagreement between the trends of solid/liquid interface energies calculated using Fowkes and van Oss–Chaudhury–Good methods for surface treatment/adhesive resin was obtained. In case of real adhesive systems, the use of work of adhesion appears more adequate in order to discriminate the surface treatments. Based on these findings the anodization in the tested alkaline bath after a previous alkali etching in the same bath is recommended, although adhesion test has to be still performed.     

Strength and Durability of Epoxy-Aluminum Joints

The adhesive strength and durability of adhesively-bonded aluminum joints in wet environments was analyzed. A2024-T4 alloy was subjected to two different surface treatments based on etching with chromic-sulfuric acid (FPL) and with sulfuric acid-ferric sulfate (P2). Small differences were observed in the lap shear strength as a function of the applied surface treatment. However, durability in humid environments was higher for the joints whose adherends were treated with P2.

Although the amount of water absorbed by the epoxy adhesive is lower in saline environments, the effects on the glass transition temperature of the epoxy adhesive and on the lap shear strength of the joints are more marked than the effects caused by aging with distilled water.

Finally, a new epoxy adhesive with a siloxanic hardener was tested, obtaining good mechanical properties, high glass transition temperature, moderate values of lap shear strength, and high durability in wet environments.     

Silanisation of adhesively bonded aluminium alloy AA6060 with γ-glycidoxypropyltrimethoxysilane. I. Durability investigation

The wedge test was used to determine the durability of adhesively bonded joints of pretreated aluminium alloy AA6060 in a hydrothermal environment. Testing of joints bonded with the one-component epoxy adhesive XD4600 showed that the durability was higher for surfaces that were grit-blasted with alumina than for alkaline etched, FPL-etched, and sulphuric acid anodised surfaces. All these surfaces performed much better than those abraded with ScotchBrite®. It was discovered that increased surface roughness improved the durability, while increased surface contamination reduced the durability of the bonded joints. On a very rough surface such as the grit-blasted, the effects of surface contamination were more than outweighed by the effects of surface roughness. Treatment of some of the pretreated surfaces with a 1% aqueous solution of γ-glycidoxypropyltrimethoxysilane significantly improved the durability, but the ranking between the pretreatments was the same as before the silane treatment. The silane treatment also reduced the initial crack lengths of the wedge test specimens. The best performance was seen by the grit-blasting plus silane treatment, which performed much better than the well-established FPL-etch.     

STUDY OF BONDED PLASMA-TREATED POLYETHERIMIDE COMPONENTS FOR POWER INTEGRATION: DURABILITY IN A HOT/WET ENVIRONMENT

This work deals with the study of the durability, in a hot/wet environment, of structural adhesively bonded polyetherimide (PEI) assemblies used in power electronics packaging technology. An overall approach is proposed, for which the epoxy joint-PEI substrates assembly on the one hand, and the adhesive system components (substrate surface and bulk adhesive) on the other hand, are studied separately with different analytical techniques. The first part of this work was devoted to the substrate surface state and to its modification using a cold plasma treatment of the PEI surface. Then for chosen parameters (power, duration) contact angle measurements indicated an increased surface tension resulting from surface decontamination (removal of release agent and carbon contaminants) and from the creation of polar species, such as esters or carboxylic acid groups, on the PEI surface (XPS analyses). The second part of this study concerned the bulk adhesive ageing in an ethylene glycol-water solution at 70°C. Mass uptake measurements versus time showed the liquid diffusion in the bulk adhesive associated with a microscopic damage of the epoxy system. An overall plasticizing of the adhesive with a considerable decay of the α-transition temperature of one of the two adhesive epoxy-amine networks (TGDDM-BAPP) was also highlighted using rheometry. However, in these ageing conditions, the adhesive glassy modulus decreases slighty because of the thermomechanical stability of the other epoxy network. In the third part, the asymmetric wedge test showed the beneficial effect of the cold plasma treatment on the epoxy/PEI interface durability in the aggressive medium.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

Modification of glass surfaces adhesion properties by atmospheric pressure plasma torch

This work is focused in the surface modification of glass samples by an atmospheric pressure plasma torch (APPT), with the aim of properly characterizing every chemical surface modification which could contribute to the assessment of reliable adhesive bonding with polyurethane, silicone, epoxy and cyanoacrylate adhesives.Characterization of glass surfaces both pristine and plasma treated is achieved by surface energy determination through contact angle measurements, Fourier transform infrared spectroscopy in attenuated total multiple reflection mode (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and surface roughness determination. Moreover, the hydrophobic recovery processes suffered by the samples throughout a period of 30 days of storage after APPT treatment is evaluated. Finally, adherence tests are carried out following the ASTM D4541 standard. Results show an increased surface energy with treatment, which partially recovered its initial value with aging. For the adhesive bonding, only the polyurethane shows an improvement on resistance among all the tested adhesive fluids. It is possible to conclude a cleaning, etching and activation effects on the glass surfaces after plasma treatment, with a change in the locus of failure from adhesive to cohesive in some cases, and a reduction in data standard deviation.     

Effect of Ellman's Reagent and Dithiothreitol on the Curing of the Spore Adhesive Glycoprotein of the Green Alga Ulva

Green algae of the genus Ulva (syn. Enteromorpha) are common, green macroalgae found throughout the world in the upper intertidal zone of seashores and as a fouling organism on a variety of man-made structures including ships' hulls. Adhesion of motile spores is achieved via the secretion of an adhesive, which is present in spores in highly condensed form within membrane-bound vesicles. The adhesive is initially liquid and displays a hydrogel-like behavior on release. It then starts to undergo “curing reactions,” becoming progressively less soluble with time in anionic detergents, less sensitive to proteolysis, and less viscoelastic, which suggests that extensive cross-linking occurs. Spores also become progressively more difficult to detach from a surface. However, the nature of this adhesive curing process is totally unknown. In the present article we have tested the hypothesis that thiol cross-linking may be involved. We show that nontoxic concentrations of the thiol-capping reagent (Ellman's reagent) or thiol-reducing agent (dithiothreitol) effectively inhibit the time-dependent development of adhesive spore strength after attachment to a surface. Furthermore, we show by SDS-PAGE immunoblot analysis of extracted adhesive proteins that the major adhesive antigen retains solubility in the presence of these reagents, after release from spores, which suggests that cross-linking had been inhibited.     

The influence of hydroxyl group concentration on epoxy–aluminium bond durability

The influence of hydroxyl group (OH) concentration on the durability of adhesive bonds formed between an epoxy resin and aluminium adherend has been examined. Initially, surface analysis in combination with chemical derivatisation was employed to characterise the OH and epoxy functional groups present in FM-73, a structural epoxy adhesive. Bulk FM-73 indicated a higher degree of cure than the surface of FM-73 present at the interface of an epoxy–aluminium adhesive joint. Plasma and water treatment of the aluminium adherend was employed to alter the metal oxide's surface OH concentration. Despite a several-fold difference of aluminium surface OH concentrations for the different metal pre-treatments, there was no significant variation in the adhesive joint fracture toughness in a humid environment, G _Iscc. In contrast, grit-blasting the aluminium prior to bonding increased G _Iscc almost 15-fold. Simple calculations indicate that the aluminium surfaces used in the bonding experiments would have a large excess of OH groups available to react with a standard epoxy resin and that the influence of surface roughness on adhesion durability is not insignificant.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

Surface modification of ultra‐high‐molecular‐weight polyethylene. I. Characterization and sintering studies

We performed surface modification of ultra‐high‐molecular‐weight polyethylene (UHMWPE) through chromic acid etching with the aim of improving the performance of UHMWPE's composites with poly(ethylene terephthalate) fibers. In part I of this study, we evaluated the effects of chemical modification on the surface properties of UHMWPE with X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and contact angle measurements. The thermal properties, rheology, and sintering behavior of the modified UHMWPE were compared to those of the base material. XPS and FTIR analysis confirmed the presence of carboxyl and hydroxyl groups on the surface of the modified powders. The substitution of polar groups into the backbone of the polymer decreased its contact angles with water and hexadecane and increased its surface energy, as evidenced by contact angle measurements. The modified UHMWPE was more crystalline than the base resin and less prone to thermal degradation. Although the rheological properties were virtually identical, the modified powders sintered more readily, presumably due to their higher surface energy, which suggested enhanced processability by compression molding. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Durability of adhesives based on different epoxy/aliphatic amine networks

The mechanical and adhesives properties of epoxy formulations based on diglycidyl ether of bisphenol A cured with various aliphatic amines were evaluated in the glass state. Impact tests were used to determine the impact energy. The adhesive properties have been evaluated in terms single lap shear using steel adherends. Its durability in water at ambient temperature (24 °C) and at 80 °C has also been analyzed. The fracture mechanisms were determined by optical microscopy. It was observed a strong participation of the cohesive fracture mechanisms in all epoxy system studied. The 1-(2-aminoethyl)piperazine epoxy adhesive and piperidine epoxy adhesive presents the best adhesive strength and the largest impact energy. The durability in water causes less damage to piperidine epoxy networks. This behavior appears to be associated with the lower water uptake tendency of homopolymerised resins due to its lower hydroxyl group concentration.     

A Surface Treatment for Ti-6AI-4V

A simple nonchromate surface treatment that was developed for aluminum alloys (STAB (3)) has been found to provide Ti-6A1-4V, bonded with epoxy adhesives, with good hydrothermal endurance. The simplest form of the treatment is to degrease, dip in caustic soda, hard [i. e., forceful) spray-rinse and dry. Although STAB (3) for aluminum was satisfactory with a room temperature dip in caustic soda, the solution temperature should be greater than 40°C for Ti-6A1-4V. A steel wool scrub prior to the NaOH dip enhances durability. Surface property measurements following the surface treatment indicate that a porous oxide layer is formed on top of a 300A barrier layer. The durability of this treatment is attributed to mechanical interlocking between the adhesive and the porous oxide (hydroxide) and to the chemical stability of the oxide (hydroxide) under hydrothermal stress.     

Role of acid-base interfacial bonding in adhesion

The strength of macroscopic adhesive bonds of polymers is known to be directly proportional to the microscopic exothermic interfacial energy changes of bond formation, as measured by Dupre's 'work of adhesion'. Since the work of adhesion can be very appreciably increased by interfacial acid-base bonding with concomitant increases in adhesive bond strength, it is important to understand the acid-base character of polymers and of the surface sites of substrates or of the reinforcing fillers of polymer composites. The best known acid-base bonds are the hydrogen bonds; these are typical of acid-base bonds, with interaction energies dependent on the acidity of the hydrogen donor and on the basicity of the hydrogen acceptor. The strengths of the acidic or basic sites of polymers and of inorganic substrates can be easily determined by spectroscopic or calorimetric methods, and from this information one can start to predict the strengths of adhesive bonds. An important application of the new knowledge of interfacial acid-base bonding is the predictable enhancement of interfacial bonding accomplished by surface modification of inorganic surfaces to enhance the interfacial acid-base interactions.     

Hygrothermal ageing of adhesive joints with nanoreinforced adhesives and different surface treatments of carbon fibre/epoxy substrates

The effect of water absorption on the strength of single lap adhesive joints subjected to accelerated hygrothermal ageing (55 °C, 95% relative humidity, 800 h) was analysed. Two different variables were studied: the surface treatment of the carbon fibre/epoxy laminates (peel ply, grit blasting and atmospheric pressure plasma) and the addition of carbon nanofillers (0.5 wt% nanofibres and 0.25 wt% nanotubes) to the epoxy adhesive. The joint strength and the failure mode of the joints were investigated. Furthermore, the amount of water absorbed by the adhesive was determined.Adhesive joints with peel ply-treated laminates exhibit an increase in their strength, which is attributed to a relaxation of stresses in the adhesive/laminate interface; with grit blasting, this property remains almost constant. Plasma treatment provides the worst ageing behaviour because this treatment results in a surface with a higher surface free energy, which is more susceptible to environmental attack. The nanoreinforcement of the adhesive has a beneficial effect: it decreases the amount of absorbed water.     

Investigation on the effectiveness of silane-based field level surface treatments of aluminum substrates for on-aircraft bonded repairs

The aim of this study was to assess the role of silane-based field level surface treatment processes on aluminum substrate with a film-type epoxy adhesive. Two silane-based surface treatment compositions based on a dilute aqueous solution of GPS (3-glycidoxypropyltrimethoxy silane) and a hybrid sol-gel solution of TPOZ (zirconium n-propaxine) and GPS were used. The surface morphology of the treated aluminum substrates was characterized by profilometry. Contact angle measurement and X-ray photoelectron spectroscopy were carried out to analyze surface wettability, which in turn is related to the surface chemistry and cleaning efficiency for bond performances. Quantitative evaluation of the joint strength and environmental durability presented that two GPS- and TPOZ-GPS based sol-gel coatings improved the initial adhesion and environmental durability, and hence can be considered promising alternative surface treatment techniques to the existing on-aircraft anodizing process for bonded repairs. Finally, observation of the fracture surfaces revealed that a loss of interfacial integrity between the adhesive and aluminum substrate was the dominant mechanism behind the permanent loss of adhesion; the loss of interfacial integrity induced the low-strength interfacial adhesion failure mode.     

Structural Adhesive Bonds to Primers Electrodeposited on Steel

Adhesive bond strength and durability were investigated for steel substrates which had been cathodically electroprimed before bonding. Lap shear and torsional impact strengths of two model epoxy adhesives were evaluated. Very poor strengths and durability were found for one adhesive, which was cured with a mixture of three amine curing agents. Scanning electron microscopy and analysis of primer susceptibility to interaction with the curing agents suggested that, for the high concentrations of curing agent in the amine-cured adhesive, chemical and physical degradation of the primer occurred during cure at elevated temperature.

For the second adhesive, which was cured with a single imidazole catalyst, excellent strength and durability were obtained, with no evidence of primer degradation. Surprisingly, for this adhesive, strengths to primed steel were up to 88% higher than to cleaned (i.e., degreased) bare steel. The concurrent improvements in environmental durability over bare steel, as assessed by water immersion and salt spray accelerated exposures, were attributed to the more favourable surface energetics of the adhesive/primer interface.     

Cracks at adhesive interfaces

The application of Griffith's energy balance argument to cracks at adhesive interfaces is studied. Adhesive interfaces are generally brittle, representing the simplest form of fracture mechanics geometry because cracks are constrained to travel along the interface, giving a defined crack path which eases analysis. Experimentally, such cracks may be propagated along the interface between optically smooth rubber pieces, and measured through the transparent material. The development of adhesive fracture test-pieces since Griffith's time reveals difficulties in his reasoning, and allows improved understanding of the energy balance method. The most important conclusion is that stress does not normally enter the cracking criterion. It is demonstrated experimentally that stress may remain constant while the crack criterion changes. The strength of an adhesive interface is shown to be a meaningless parameter; instead, the work of adhesion, or adhesive energy, which is the work of adhesion together with energy losses, should be used to define the behaviour of cracks at interfaces.