Influence of surface treatment on adhesion of polyethylene fibres

Abstract

A detailed examination has been undertaken of the influence of surface treatment on the adhesion of polyethylene fibres to epoxy resin. The pull-out adhesion has been determined for untreated, chromic acid treated, and plasma etched monofilaments with different draw ratios and thermal annealing treatments. In a few cases, additional chemical treatments were applied to plasma treated fibres before the pull-out test. The polyethylene surface energy also has been determined by measurement of contact angle. The results, taken together, suggest that the adhesion depends on three factors: (i) the wettability (or physicochemical interactions), which is affected by the extent and nature of the surface treatment as well as the fibre draw ratio; (ii) the surface roughness, after plasma etching only, where a honeycomb structure of pits permits mechanical keying between the fibre and the resin (this structure has been examined by scanning electron microscopy); and (iii) the number of chemical bonds per unit area between the fibre and the resin. It is concluded that these three factors can be regarded as additive and that optimum results are obtained when their respective pull-out strengths reach their maximum values, ~2, ~3, and ~1·7 MN m^?2.

MST/640     

Hybrid composites based on polyethylene and carbon fibres Part 2: influence of composition and adhesion level of polyethylene fibres on mechanical properties

Combining high performance polyethylene (HP-PE) and carbon fibres as reinforcing elements in so-called hybrid composite structures results in a unique class of structural materials possessing high damping and impact resistance. Mechanical properties of unidirectional HP-PE/carbon-epoxy hybrids have been studied, emphasizing basic mechanical characterization such as tensile, compressive and shear strength, initial as well as long-term modulus, vibrational damping and impact response. This paper describes the influence of overall composition and adhesion level of the HP-PE fibres on the mechanical properties of such hybrids.     

Contributions of chemical and mechanical surface properties and temperature effect on the adhesion at the nanoscale

The atomic force microscope (AFM) is a powerful tool to investigate surface properties of model systems at the nanoscale. However, to get semi-quantitative and reproducible data with the AFM, it is necessary to establish a rigorous experimental procedure. In particular, a systematic calibration procedure of AFM measurements is necessary before producing reliable semi-quantitative data. In this paper, we study the contributions of the chemical and mechanical surface properties or the temperature influence on the adhesion energy at a local scale. To reach this objective, two types of model systems were considered. The first one is composed of rigid substrates (silicon wafers or AFM tips covered with gold) which were chemically modified by molecular self-assembling monolayers to display different surface properties (methyl and hydroxyl functional groups). The second one consists of model polymer networks (cross-linked polydimethylsiloxane) of variable mechanical properties. The comparison of the force curves obtained from the two model systems shows that the viscoelastic contributions dominate for the adhesion with polymer substrates, whereas, chemical contributions dominate for the rigid substrates. The temperature effect on the adhesion energy is also reported. Finally, we propose a relation for the adhesion energy at the nanoscale. This relation relates the energy measured during the separation of the contact to the three parameters: the surface properties of the polymer, the energy dissipated within the contact zone and the temperature.     

Relationship between surface properties and adhesion for etched ultra-high-molecular-weight polyethylene fibers

Chemical etching is an established and popular method of increasing the adhesion to such materials as polyethylene. Ultra-high-molecular-weight polyethylene (UHMWPE) fibers are exceptional candidates for composite materials except for their poor adhesion. In this research, the bulk, surface and adhesive properties of as-received and chromic acid etched UHMWPE fibers have been examined. The fiber tensile properties, surface chemistry and wettability have been characterized. The adhesion of epoxy has been characterized by the interfacial shear strength of a droplet microbond. The more than six-fold increase in interfacial shear strength observed in this work is related to the etching process. The removal of an oxygen-rich weak boundary layer, surface roughening and oxidation of the UHMWPE contribute to the enhanced adhesion.     

Anodic oxidation of pitch-precursor carbon fibres in ammonium sulphate solutions: the effect of fibre surface treatment on composite mechanical properties

Tonen HM pitch-based carbon fibres were electrochemically treated in solutions of ammonium sulphate using a pilot plant surface treatment apparatus. Embedded single fibre and short beam shear specimens prepared with these treated fibres exhibited strengths over 300% greater than those made with untreated fibres. The surface treatment did not, however, result in improvements in the longitudinal compressive strength. This suggests that the compressive strength is not limited by the shear strength of the fibre/matrix interface.     

Interfacial polycondensation of nylon-6,6 at the glass fibre surface and its effect on fibre–matrix adhesion

The surface of glass fibres was modified using chemical treatments to improve fibre–matrix interface properties. Interfacial polycondensation was performed with the fibre acting as the interface, and nylon-6,6 chains were grafted on the free hydroxyl groups located at the fibre surface. Grafted nylon was observed through the scanning electron microscope. The effect of the treatment on the fibre-matrix adhesion was investigated by measuring the interfacial shear strength in fragmentation micromechanical tests. The two-parameter Weibull distribution was used to analyse the experimental results. Polarized optical microscopy showed the existence of a transcrystalline layer in treated samples, indicating better fibre wettability by the matrix. Scanning electron microscopy confirmed the presence of an excellent bonding between fibre and matrix in treated samples, whereas in untreated samples, fibre pull-out was predominant, indicating poorer fibre–matrix adhesion. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electrochemical oxidation of carbon fibres: surface chemistry and adhesion

PAN based carbon fibres were subjected to electrochemical oxidation under a wide variety of conditions. Sodium hydroxide, ammonium hydrogen carbonate, ammonium carbonate, sulphuric acid and nitric acid were used as electrolytes in four concentrations changing from 2 to 20 wt%. The applied voltage varied from 0.5 to 5 V in four steps. The surface chemistry of the fibres was studied by diffuse reflectance infrared spectroscopy (DRIFT). The strength of adhesion between the fibre and an epoxy matrix was characterised by the interfacial shear stress (IFSS), which was determined by fragmentation. The functional groups formed on the surface of the fibres depend very much on the type of the electrolyte used, while their number on its concentration, as well as on the voltage of oxidation. A close correlation was found between surface chemistry and fibre/matrix adhesion, the concentration of certain functional groups could be related quantitatively to IFSS. Some of the functional groups formed during oxidation could not be identified by the DRIFT technique used for analysis.     

电晕处理对超高分子量聚乙烯纤维表面性能的影响

对超高分子量聚乙烯（UHMWPE）纤维表面进行了电晕处理,用XPS,FT-IR和SEM研究了处理前后纤维表面化学结构及物理结构的变化,通过单丝拔出试验和短梁剪切试验评价了UHMWPE纤维与树脂基体的微宏观界面粘接性能,结果表明：经电晕处理后,UHMWPE纤维表面含氧量增多,含氧基团数量与种类增加,表面浸润性得到改善,纤维与基体的界面粘结强度（Ts）提高幅度可达535％,短梁剪切强度TNOL提高了40％以上．     

The relevance of the surface structure and surface chemistry of carbon fibres in their adhesion to high temperature thermoplastics

This paper has three principle thrusts: (i) the fabrication of unidirectionally reinforced composites made from carbon fibres subjected to different surface treatments in combination with various (polycarbonate, polyethersulphone and epoxy) matrices, (ii) a study of some of the mechanical properties (that is, the interlaminar-shear strength (ILSS) and the failure behaviour) of these composites; and (iii) determination of the correlations between the adhesion of the matrix polymers as measured by the ILSS and the surface structure as well as the surface chemistry of the various fibres. It will be shown that the surface structure of the fibres has a minor effect, while the surface chemistry appears to have an extraordinarily great influence on the adhesion of the fibres to high-temperature thermoplastics. The data clearly show that, depending on the processing temperature during the fabrication of the composites, chemical bonds can be formed at the fibre-polymer interface. This bond formation is initiated by the decomposition of carboxylic groups and, as a consequence, dangling carbon atoms are free to react with the functional groups of the polymer.     

The relevance of the surface structure and surface chemistry of carbon fibres in their adhesion to high temperature thermoplastics

The paper is concerned with the surface chemistry of several different carbon fibres subjected to various surface treatments. The microstructure and nanostructures of these fibres were investigated in the Part I of this series of papers. For analysis of the surface chemistry of the fibres, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) were employed; the first method was used for identification and semi-quantitative determination of functional surface groups, while the second method was used for a quantitative determination of these groups. The possible interactions of the various carbon-fibre surfaces due to different surface treatments (and therefore to different functional groups) were analysed by wetting studies using the Wilhelmy technique and aqueous solutions of different pH values as test liquids. By variation of the pH value of the test liquids, the distinct acid-base complexes that formed with the functional groups were identified. The same test liquids were used for characterization of the surface chemistry of the high-temperature thermoplastics (polycarbonate and polyethersulphone) used as matrix materials in the fabrication of the composites in this study. Acid-base interactions at the carbon-fibre surfaces are mainly determined by carboxylic groups of different acidity. The concentration of these groups as determined by desorption of carbon dioxide up to 500 °C is shown to be directly proportional to the measured work of adhesion of each group.     

In vitro evaluation of artificial ageing on surface properties and early Candida albicans adhesion to prosthetic resins

Objective The aim of this in vitro study was to examine potential changes and influences of prosthetic resin surface properties on Candida albicans adhesion after surface treatment or artificial ageing. Materials and methods Standardized specimens of a denture base resin and a veneering composite were prepared, polished, and randomly subjected to different surface roughness treatments or artificial ageing protocols (storage in ethanol or artificial saliva for 7/90 d, thermocycling). Surface roughness (Ra) and surface free energy were determined prior and after each treatment. Specimens were incubated with phosphate buffered saline or whole saliva for 2 h at 37°C, and later with Candida albicans suspension (2.5 h, 37°C). Adherent viable fungi were quantified using a bioluminescence assay. Results Artifical ageing did not affect substratum surface roughness, yet slight increases in substratum surface free energy and significant increases in Candida albicans adhesion were observed. Saliva coating marginally influenced Candida albicans adherence to reference and surface treated specimens, yet more pronounced differences in Candida albicans adhesion between the various artificially aged specimens were found. Conclusion No correlation between substratum surface roughness or surface free energy and Candida albicans adhesion could be established.     

The effect of fibre diameter on the mechanical properties of graphite fibres manufactured from polyacrylonitrile and rayon

A relationship between diameter and both Young's modulus and tensile strength of graphite fibres, manufactured from both polyacrylonitrile and rayon precursors is demonstrated. Thin fibres exhibit higher values of Young's modulus and tensile strength than thick fibres. The observed relationship is deduced to be a consequence of the “sheath” and “core” type structure which is characteristic of these fibres.     

The effect of interfacial adhesion and morphology on the mechanical properties of polypropylene composites containing different acid surface treated sepiolites

The study of the microstructure of polymeric composites and its relationship to mechanical properties, are of great importance. In the present study vv8 have carried out a study of the microstructure of polymeric composites of polypropylene and different sepiolites treated with organic acids, in order to determine the mesophase produced around We filler particles and its relationship with the mechanical properties of the composites. This study was made using scanning electron microscopy, differential Scanning calorimetry and mechanical tests.     

Strain rate and temperature effects on energy absorption of polyethylene fibres and composites

The influence of strain rate and temperature on modulus, strength and work of fracture of high-performance polyethylene (HP-PE) fibres and composites is investigated. Results showed that an increase in strain rate and/or decrease in temperature leads to a reduction in work of fracture. At high strain rates or low temperatures a constant minimum level for the fracture energy is reached. The energy absorption capacity of HP-PE/epoxy laminates is investigated using full penetrating dart-impact tests and showed similar trends than observed for HP-PE fibres. The impact energy of these laminates could be described quantitatively in terms of fibre, matrix and delamination effects by combining the tensile test results on fibres and unidirectional composites with fracture toughness experiments on laminates.     

Surface properties and adhesion of maleinized polyethylene films

The wettability and the adhesion of polyethylene films were improved by introducing polar groups in the polymer chains. The surface properties of films grafted with maleic anhydride (MA) were investigated. The wettability was found to be dependent on the MA content, on the film preparation conditions and on the hydrolytic process of the anhydride groups. The kinetics of the hydrolysis indicated a restructuring of the polymeric surface due to the movement of the polar groups towards the surface; it did not influence the adhesion properties. The behaviour of the maleinized films was compared with oxygen plasma treated materials, which showed a better wettability, but a worse adhesion on polar substrates than the maleinized polyethylene. These results were explained on the basis of X-ray photoelectron spectroscopic analyses, by which the main functional groups present at the surface were identified and quantitatively determined. © 1998 Chapman & Hall     

高温热处理对活性炭纤维微孔及表面性能的影响

研究了1173K高温改性处理对沥青基活性炭纤维吸附性能、孔径分布、微孔结构和表面化学的影响。低温(77K)N2吸附结果表明热处理后活性炭纤维比表面积略有下降，通过密度函数理论解析活性炭纤维全孔范围的孔分布得出活性炭纤维表面孔径大于1．0nm的微孔明显减少，微孔孔径更加集中于0．5nm～1．0nm，从而提高了活性炭纤维的碘吸附值。X射线衍射分析表明活性炭纤维是乱层石墨结构，热处理使活性炭纤维类石墨微晶碳层面的层间距下降，X光电子能谱分析表明热处理后活性炭纤维表面的含氧官能团C=O和COOH的含量变化不大，而呈碱性酚羟基C—OH含量的明显下降使活性炭纤维表面碱性降低。     

The effect of fibre sizing on fibres and bundle strength in hybrid glass carbon fibre composites

Tests have been carried out on single carbon fibres supplied in the sized and unsized conditions, as well as impregnated tows and tows in a glass–carbon fibre hybrid composite of the same fibre. The results were analysed using a Weibull distribution for the strengths of the reinforcing fibres and composites. The tensile strength of the single fibres appeared to be unaffected by the sizing of the filaments. In the case of the impregnated tows, an increase in characteristic strength of 7% was observed for the unsized fibres. The strength of the impregnated tows in hybrid composites was seen to be 15% higher than those tested in air. This can be attributed to the “hybrid effect”. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of low-temperature-plasma surface treatment on the adhesion of ultra-high-molecular-weight-polyethylene fibres

The poor adhesion of ultra-high-molecular-weight polyethylene (UHMW-PE) fibres to epoxy resin (due to characteristics of their surface structure) makes it difficult to develop light composite materials with good mechanical properties. In our experiments a low-temperature plasma was applied to treat the surfaces of the fibres. The results showed that the surface energy of the treated fibres was increased greatly so that the fibres could be wetted very well by resin, which was an essential prerequisite in improving the adhesion of the fibre/epoxy resin systems; and the results also showed that the pull-out adhesion strength of the treated fibres to resin was increased significantly by about 10 times. This paper discusses the mechanism of the improvement of the adhesion. Two reasons for improvement are that: varieties of polaroxygen-containing groups are produced in the non-polar surface layer, forming chemical bonds; and plasma-etched pits spread all over the surface of the fibres into which the resin penetrates to form a mechanical interlock between fibre and resin. A synthetic analysis and discussion of these two factors and of non-polar interactions influencing the adhesion is given, and a preliminary relationship between them is presented.     

The adhesion behaviour of high modulus polyethylene fibres following plasma and chemical treatment

Previously published pull-out adhesion results have been substantiated by more extensive studies of chemical and plasma treatment. Particular attention has been paid to the affect of geometrical variables on the values of adhesion obtained. The effect of strain rate has also been examined. Most of the results can be understood on a semi-quantitative basis by a simple extension of lap joint theory.     

Jute fibre/epoxy composites: Surface properties and interfacial adhesion

Jute fibres were surface treated in order to enhance the interfacial interaction between jute natural fibres and an epoxy matrix. The fibres are exposed to alkali treatment in combination with organosilane coupling agents and aqueous epoxy dispersions. The surface topography and surface energy influenced by the treatments were characterized. Single fibre pull-out tests combined with SEM and AFM characterization of the fracture surfaces were used to identify the interfacial strengths and to reveal the mechanisms of failure.