Erika Fekete

Adhesion and Surface Modification

This review emphasizes the role of interactions in particulate filled composites. In an introductory section a general view is given about the factors influencing composite properties. Two basic type of interactions must be considered: particle/particle and matrix/filler interaction. The effect of the former is detrimental to composite properties, it decreases strength and impact resistance. The occurrence and extent of aggregation is determined by the relative adhesion and shear forces during homogenization. Due to matrix/filler interaction an interphase forms spontaneously in the composite with properties different from those of both components. The amount and characteristics of the interphase strongly influence composite properties. The strength of adhesion between the components can be characterized by thermodynamic quantities, mainly by the reversible work of adhesion. The most important techniques used for the estimation of the strength of interaction and the properties of the interphase are briefly reviewed. The modification of interactions is achieved through the surface treatment of the filler. Surface treatments are divided into four arbitrary groups and are discussed accordingly, i.e. non-reactive and reactive treatment, application of functionalized polymers and introduction of a soft interlayer around the particles. The practical relevance of interactions and their modification is also mentioned in the last section.

Surface modification and characterization of particulate mineral fillers

Abstract The efficacy of the surface treatment of particulate fillers depends on the chemical character of the components, on the method and conditions of the treatment, and on the amount of treating agent. To achieve maximum efficacy the coupling agent must be chosen specifically for each polymer/filler pair. Because of its ionic character CaCO3 can be effectively treated with stearic acid, which forms a basic salt on the filler surface. One stearic acid molecule is attached to each Ca2+ ion and in the complete monolayer the molecules are vertically oriented to the surface. Maximum efficacy is expected at the monomolecular coverage of the surface, but the methods utilized to determine this concentration yielded different results. Stearic acid surface treatment of CaCO3 decreases its surface tension and the effect is more pronounced on the polar than on the dispersion component. The change in the surface characteristics of the filler influences also the properties of the composites in which they are used.

Interaction, miscibility and phase inversion in PBI/PI blends

Abstract The miscibility of polybenzimidazole {poly[2,2′-( m -phenylene)-5,5′-bibenzimidazole], (PBI)} and a polyimide {poly[2,2′-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane- m -phenylenediimine] (PI)} was studied in the entire composition range. Films were cast from solution by a procedure developed in a preliminary work. The blends were studied by FT-IR spectroscopy, thermal and thermomechanical methods, and by measuring vapour absorption at an ambient temperature. Direct relationships were found between the strength of hydrogen-bonding of the components, the glass transition temperature of the blends, and the solvent diffusion rate (water and 1,2,4-trichlorobenzene). The results of the experiments indicate partial miscibility between the two polymers. Phase inversion occurs between 0.4 and 0.6 PI content.

Acid-base interactions and interphase formation in particulate-filled polymers

Particulate-filled composites were prepared from CaCO 3 and polymer matrices of various acid-base characters. Interfacial interaction of the components was characterized by the reversible work of adhesion, which was calculated either from dipole-dipole or acid-base interactions. The thickness of the spontaneously formed interlayer was derived from the tensile strength of the composites. The results proved that acid-base interactions play an important role in interphase formation. The strength of interfacial adhesion is determined by the joint effect of dispersion forces and acid-base interactions. Stronger interaction leads to a thicker interphase with decreased mobility. Treatment of CaCO 3 with an aliphatic fatty acid leads to a decrease in the strength of interaction, and to changes both in the thickness and properties of the interphase. In composites containing coated fillers, acid-base interactions influence composite properties less due to the neutral character of the surface.

An interphase with changing properties and the mechanism of deformation in particulate-filled polymers

Abstract The comparison of the results of calculations based on experimental data with those derived from a simple, two phase, elastic model proved the existence of a hard interphase in particulate-filled composites. Moreover, beside elastic properties, also other mechanical characteristics of the interphase, including yield stress, are different from those of the components. An energy analysis showed that the relationship between the yield stress of the matrix and the debonding stress determines the mechanism of deformation. Strong adhesion leads to matrix yielding, while decreased interaction leads to debonding, with a corresponding dependence of composite yield stress on filler content. Particle size, interaction and interphase properties determine the stress necessary to separate the matrix/filler interface. The thickness of the interphase depends on the strength of the interaction; a linear correlation was found between the size of the interlayer and the reversible work of adhesion.

Surface changes of corona-discharge-treated polyethylene films

Morphological and chemical changes of the surface of low-density polyethylene (LDPE), linear middle-density polyethylene (L-MDPE), and their 80/20 blend were studied by different techniques after corona-discharge treatment in air and subsequent annealing. The surface tension was determined by wetting; the roughness was measured by atomic force microscope (AFM), and the surface chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS), whereas the low-molecular-mass fraction washed off by chloroform by FTIR. The surface tension of the films increases with the electrode current. The surface roughness depends primarily on the polymer type and is less affected by the corona treatment. At the initial stage of annealing, posttreatment-type oxidation and hydrophobic recovery are competing. The former is more pronounced in L-MDPE, the latter in LDPE. After annealing at 50°C for 160 days, hydrophobic recovery becomes predominant in each film studied, which is accompanied by significant smoothening of the surface. According to XPS and FTIR results, this is due to the migration of low-molecular-mass components (oligomers, oxidized polymer fractions, and additives) to the surface.

Chemical reactions during the processing of stabilized PE: 2. Structure/property correlations

Interesting correlations were observed in an optimization project directed towards the development of a recipe for blown film production. Analysis of the data and additional experiments indicated that some reactions take place during the processing of stabilized PE which also change the structure of the polymer. Modification of the rheological properties of the polymer and the mechanical characteristics of blown films can be related to these chemical processes. A close correlation was found among basically all rheological and mechanical properties. A tentative explanation was given which relates chemistry to the structure of the polymer and the properties of the product. According to the hypothesis long chain branches are formed during processing which decrease MFI, change the orientation of the film in the perpendicular direction and result in a considerable decrease in the strength of the blown films.

Chemical reactions during the processing of stabilized PE: 1. Discolouration and stabilizer consumption

A series of experiments was carried out to develop an additive package for HDPE blown films. The evaluation of the results yielded interesting correlations relating the colour of the polymer to its chain structure and to the properties of the films. Additional experiments proved that all reactions are related to each other. The discolouration of the polymer could be described by simple first order reaction kinetics and a linear correlation was found between stability and a quantity calculated from the kinetic model. The reaction of the vinyl functionality of the polymer seems to be related to the changes in its rheological properties. In spite of the good general correlations found, some details remained unexplained and need further investigation.

Microstructure and surface properties of fibrous and ground cellulosic substrates

Cotton and linen fibers were ground in a ball-mill, and the effect of grinding on the microstructure and surface properties of the fibers was determined by combining a couple of simple tests with powerful techniques of surface and structure analysis. Results clearly proved that the effect of grinding on cotton fiber was much less severe than on linen. For both fibers, the degree of polymerization reduced (by 14.5% and 30.5% for cotton and linen, respectively) with a simultaneous increase in copper number. The increased water sorption capacity of the ground substrates was in good agreement with the X-ray results, which proved a less perfect crystalline structure in the ground samples. Data from XPS and SEM-EDS methods revealed that the concentration of oxygen atoms (bonded especially in acetal and/or carbonyl groups) on the ground surfaces increased significantly, resulting in an increase in oxygen/carbon atomic ratio (XPS data: from 0.11 to 0.14 and from 0.16 to 0.29 for cotton and linen, respectively). Although grinding created new surfaces rich in O atoms, the probable higher energy of the surface could not be measured by IGC, most likely due to the limited adsorption of the n-alkane probes on the less perfect crystalline surfaces.

Miscibility‐property correlations in blends of glassy amorphous polymers

Blends were prepared from seven polymers in various combinations in the entire composition range. The Flory-Huggins interaction parameter (χ 12 ) was used for the quantitative estimation of miscibility. The determination of χ 12 was attempted by several experimental techniques including the measurement of transparency, glass transition temperature, solvent diffusion and mechanical properties. The relatively simple methods used for the estimation of miscibility work surprisingly well. Solvent absorption can be determined easily for practically all blends, thus the method offers a quantitative measure of component interaction if the solvent is selected properly. After appropriate data reduction, the composition dependence of mechanical properties also supplies a quantitative estimate of compatibility. Although the approach presented in the paper reflects well the general correlation between miscibility and properties, it must be refined and improved in order to obtain a reliable estimate of blend performance.