F.R. Jones

A review of particulate reinforcement theories for polymer composites

A concerted survey is presented of the existing theories for predicting the strength and modulus of particulate-filled polymeric composites. The macroscopic behaviour of particulate composites is affected by the size, shape, and the distribution of the inclusions. The interfacial adhesion between the matrix and inclusion is also important. The limitation of theoretical models in describing these parameters and expressing the experimental data on the macroscopic behaviour is demonstrated.

A study of HMDSO/O2 plasma deposits using a high-sensitivity and -energy resolution XPS instrument: curve fitting of the Si 2p core level

A quantitative X-ray Photoelectron Spectroscopy (XPS) analysis of deposits formed from a microwave sustained hexamethyl disiloxane (HMDSO) plasma is undertaken. Curve fitting of the Si 2p core level has been achieved using component peak binding energies determined from standard compounds. The pure HMDSO plasma deposit was dominated by Si(–O)2 (44%) environments indicating a large proportion of siloxane bond formation in the plasma environment. The introduction of 200 sccm (standard cubic centimetres per minute) of oxygen to the plasma produced a deposit in which half the silicon atoms were co-ordinated with four oxygen atoms while the majority of the remaining silicon was co-ordinated to three.

Single fibre fragmentation test for assessing adhesion in fibre reinforced composites

The single fibre fragmentation test for measuring the properties of the fibre–matrix interface in fibre-reinforced composites is reviewed. Special emphasis has been paid to the recent stress transfer models in single fibre composites and its application to the development of a suitable data reduction technique for the fragmentation test. The complexities of the correlation of the micromechanical results to the properties of the macrocomposites have been highlighted.

Plasma polymerisation for molecular engineering of carbon-fibre surfaces for optimised composites

Abstract This paper explores the relationship between fibre surface chemistry and interfacial bond formation in carbon-fibre/epoxy composites. Plasma copolymerisation of acrylic-acid/hexane, allyl-alcohol/hexane and allylamine/octadiene gas mixtures is used to obtain molecularly thin, conformai coatings on Type A carbon-fibre surfaces. Since the microporosity and chemical functionality of the untreated fibre surface can be concealed, any functionality incorporated into the film can be considered to provide the principal adhesion mechanism. The single-filament fragmentation test has been employed to estimate the adhesion of these modified fibres to an epoxy resin. Coatings of a hydrocarbon nature inhibit chemical interaction between the fibre and matrix. Improvements in the level of adhesion may be attributed to the introduction of oxygen- and nitrogen-containing functionalities which have known reactivity to epoxy groups. Thus, carboxylic acid and amine groups are shown to be more effective than hydroxyl groups.

A Review of Interphase Formation and Design in Fibre-Reinforced Composites

This paper reviews the surface treatment and sizing of reinforcing fibres used for manufacturing composites. Carbon fibre surface treatment and coating is discussed primarily to identify the mechanism of interphase formation. In this case, adsorption of sizing polymers is shown to be an integral part of the interaction with matrix polymers. ToFSIMS imaging was used to identify the locus of failure and confirm the nature of the interphase. In the case of glass fibres the hydrolysis of the silane coupling agent is shown to be critical. The surface chemistry of the glass controls the degree of polymerisation of the polysiloxane and hence the interaction with the matrix polymer whether it be thermoplastic or thermoset. For completeness a brief review of the surface treatments of advanced polymer fibres is also included. The role of the interphase in the micromechanics of the failure of fibre composites is also modelled and discussed in an attempt to provide design guidelines for composite manufacture.

Effect of fibre conditioning on the interfacial shear strength of glass-fibre composites

Abstract A study of the interaction of water-sized E glass fibres, supplied with and without an aminopropylsilane coupling agent, with vinyl and epoxy resins is reported. Interfacial shear strength measurements, made by means of the multifragmentation technique, have demonstrated that molecularly thin layers are effective adhesion promoters, as indicated by (a) the silane contamination on the nominally non-coupled fibres and (b) the aqueous extraction of the coupled fibres. Epoxy resins adhere through amino coupling reactions, but for the vinyl ester resin the maximum adhesion probably occurs through aluminium hydroxide sites exposed through extractive hydrolysis, and acidic residues in the resin.

Plasma copolymerization as a route to the fabrication of new surfaces with controlled amounts of specific chemical functionality

Abstract The plasma copolymerization of allyl amine with 1,7-octadiene and of acrylic acid with hexane has been investigated. Plasma copolymerization is shown to be a promising route to the fabrication of new surfaces with controlled concentrations of specific surface functionalities; in this case, amine and carboxylic acid.

An X-ray photoelectron spectroscopic investigation into the chemical structure of deposits formed from hexamethyldisiloxane/oxygen plasmas

The effect of oxygen addition to microwave-sustained plasmas of hexamethyldisiloxane (HMDSO) has been investigated. Attention was directed to the solid products formed on aluminium substrates (plasma deposits). To enable a quantitative analysis of these, X-ray photoelectron spectroscopy (XPS) of standard silicon-containing materials was carried out. When suitable charge correction is applied to the XP spectra of HMDSO/O2 plasma deposits, a number of very clear trends emerge. From changes in elemental composition, core line binding energies (Si2p, C 1s, and O 1s) and widths, we show how oxygen addition to the plasma affects the chemical nature of the plasma deposit. The data reported also provide (some limited) information on the reactions taking place in the plasma.

Radiofrequency-induced plasma polymerisation of propenoic acid and propanoic acid

Inductively coupled, radiofrequency-induced plasmas of propenoic (‘acrylic’) acid and propanoic acid, operated at a low electrical power (1–10 W), have been investigated using a combination of mass spectrometry (MS) and deposition-rate measurements. Thin films of plasma polymers of both compounds were deposited onto silicon substrates and analysed by X-ray photoelectron spectroscopy (XPS). The positive-ion MS data obtained from both compounds indicate the presence of species of the form (M+ H)+, (2M+ H)+ and (3M+ H)+, where M represents the molecular weight of the starting material. No neutral oligomeric species were detected. XPS analysis reveals an inverse relationship between the electrical power supplied to the plasma and the degree of retention of the carboxylic acid functionality in the solid product. Comparison of the MS and XPS results suggests that the above cationic species are responsible for the carboxylic-acid functionalisation of the plasma-polymer product, whereas fragmentation processes lead to the introduction of other functional groups such as alcohol and ketone. The thin film which featured the highest degree of retention of carboxylic acid (65%) was obtained from a plasma of propanoic acid operated at 1 W, and was deposited at the lowest rate (0.90 ng s–1).

The Effect of Matrix Plasticity on the Stress Fields in a Single Filament Composite and the Value of Interfacial Shear Strength Obtained from the Fragmentation Test

An axisymmetrical finite-element model has been used to study the effect of matrix properties (elastic modulus, yield and/or cold draw strengths and yield strain) on the interfacial shear stress in a short embedded fibre and, consequently, the value of interfacial shear strength obtained from the fragmentation test. It is observed that the maximum shear stress at the fibre-matrix interface is related to matrix yield strength. The maximum shear stress at the interface is limited only to a very small portion of the fibre which is not the fibre end. However, at higher applied strains, a major portion of the fibre is subjected to a slightly lower value of interfacial shear stress, defined as ‘plateau shear stress’, which corresponds to the cold draw strength of the matrix. Matrix yield strain is observed to be the major parameter controlling the fibre fragmentation process and the number of fibre fragments at saturation. It has been shown that the use of the elastic theories, such as the shear lag and finite difference models, for the normalization of the value of interfacial shear strength obtained from the fragmentation test is not appropriate since the data reduction technique for the fragmentation test assumes a perfectly plastic matrix. The value of the plateau shear stress is compared with the fragmentation test results and it is observed that the interfacial shear strength calculated from the fragmentation test can exceed the plateau value of the interfacial shear stress in certain cases. This discrepancy can be explained on the basis of limitations of the constant shear model. Further, the stress field developed around a short fibre embedded in a matrix is compared with existing one-dimensional and bi-dimensional models. It has been observed that one of the serious limitations of the various micromechanical models is to predict the area of influence caused by the presence of the fibre. Finite-element analysis is used to study the area of influence.