Béla Pukánszky

Evaluation of interfacial interaction in polypropylene/surface treated CaCO3 composites

Abstract Interfacial interactions significantly influence the properties of particulate filled polymers. The adhesion between the particles and the polymer matrix depends on the size (area) of the interface and the strength of the interaction. While the former quantity is related to the specific surface area of the filler, the strength of the interaction can be modified by its surface treatment. In the current experiments polypropylene composites were prepared which contained different amounts of filler treated with eight functional trialkoxy silane coupling agents and the routinely used stearic acid, for comparison. Tensile properties of the composites were determined and the effect of interfacial interaction was evaluated by semi-empirical equations developed previously. Significant difference was detected in the effect of the various silane coupling agents. Amino functional silanes increase the strength of the interaction considerably. The other coupling agents reduce the surface tension of the filler, which leads to a decrease in the reversible work of adhesion between the filler and the polymer. These changes result in a decrease in the tensile strength of the composite. The results also prove the validity of the model equations applied. Parameter B, a measure of the strength of the interaction, was calculated for each silane coupling agent thus quantitatively characterizing their effectiveness.

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.

Possible coupling reactions of functional silanes and polypropylene

Abstract Apparent reactive coupling of CaCO3 to polypropylene (PP) was achieved with the application of aminofunctional silane coupling agents. The strong coupling effect could be attributed to increased CaCO3/silane and PP/silane interaction. A detailed study has shown that aminofunctional silanes bond strongly to the surface of the filler, but in the case of multilayer coverage also to the previous layers. Experiments were carried out to determine the mechanism of interaction between the silane coupling agents and the apolar polypropylene, which does not contain reactive groups. Model reactions followed by (Fourier Transform Infrared Spectroscopy) FTIR analysis demonstrated that during processing at elevated temperatures, oxidation of the polymer takes place in spite of the presence of stabilizers. Reactive carbonyl groups form as a result, which enter into chemical reactions with the amino functionality of the silane, resulting in strong amide bonds.

Nucleating effect of montmorillonite nanoparticles in polypropylene

The nucleation effect of two layered montmorillonite silicates of different origin was studied in polypropylene (PP). Composites were prepared as a function of composition in an internal mixer or by homogenization in a twin-screw compounder. Melting and crystallization characteristics were determined by differential scanning calorimetry. The gallery distance of the silicates and the structure of the composites were studied by WAXS. The results prove that montmorillonite may considerably influence the crystallization of PP. The effect depends very much on the origin and treatment of the filler. Both treatment and composite preparation lead to considerable changes in the separation distance of the silicate layers. Even though the composites contain fillers with more than one population of layer distances, the nucleating effect is related to the completely collapsed galleries of 1 nm distance. Besides gallery distance, organophillization also modifies the surface tension of the filler, but this apparently does not influence its nucleating efficiency. The efficient gallery distance is twice as large as the characteristic matching lattice dimension determined by Lotz et al. Although a few observations could not be explained and the tentative explanation given earlier may need further verification, the results prove that nucleation does not occur at the flat surface of the filler but between its galleries, where polymer molecules may adsorb preferentially.

Polymer nanocomposites: structure, interaction, and functionality

This feature article discusses the main factors determining the properties of polymer nanocomposites with special attention paid to structure and interactions. Usually more complicated structure develops in nanocomposites than in traditional particulate filled polymers, and that is especially valid for composites prepared from plate-like nanofillers. Besides the usually assumed exfoliated/intercalated morphology, i.e. individual platelets and tactoids, such nanocomposites often contain large particles, and a network structure developing at large extent of exfoliation. Aggregation and orientation are the most important structural phenomena in nanotube or nanofiber reinforced composites, and ag-gregation is a major problem also in composites prepared with spherical particles. The surface characteristics of nanofillers and interactions are rarely determined or known; the related problems are discussed in the paper in detail. The surface of these reinforcements is modified practically always. The goal of the modification is to improve dispersion and/or adhesion in nanotube and spherical particle reinforced composites, and to help exfoliation in nanocomposites containing platelets. However, modification decreases surface energy often leading to decreased interaction with the matrix. Very limited information exists about interphase formation and the properties of the interphase in nanocomposites, although they must influence properties considerably. The properties of nanocomposites are usually far from the expectations, the main reason being insufficient homogeneity, undefined structure and improper adhesion. In spite of considerable difficulties nanocomposites have great potentials especially in functional applications. Several nanocomposite products are already used in industrial practice demonstrated by a few examples in the article.

Determination of the surface characteristics of particulate fillers by inverse gas chromatography at infinite dilution: a critical approach.

CaCO3 fillers were investigated by inverse gas chromatography (IGC) to determine the dispersion component of their surface tension as well as their acid-base character. Because of the high energy of the filler surface, it readily adsorbs water, thus the parameters measured by IGC depend on the conditioning temperature, as well as on the measurement conditions. As a consequence, the determined surface characteristics are not material constants; different fillers or the effect of coating can be compared only under standard conditions. The use of the same conditioning and measurement temperature eliminates the effect of measurement time. Under appropriate standard conditions the acid-base characteristics of the filler can be determined reliably. However, the accuracy of the determination and the value of the derived parameters depend very much on the selected approach and on the acid-base constants used for the probe molecules. A critical analysis of the approaches used in the current literature pointed out those that yield the most reasonable and accurate values. The results prove that the surface of CaCO3 is strongly basic in character. Coating significantly reduces basicity. Surprisingly, the filler coated with an amount of stearic acid resulting in minimum surface tension showed relatively strong acidity, which indicates a coating exceeding monolayer coverage and/or the uneven distribution of the surfactant on the surface.

Mechanism of interfacial interactions in particulate filled composites

A stress analysis was carried out in order to determine the local stress distribution around spherical particles in particulate filled composites. The results were introduced into the criteria for micromechanical deformations and the conditions for the initiation of each mechanism were calculated. Results of the calculations have shown that shear yielding, and possibly crazing, do not depend on thermal stresses and adhesion, but debonding does. The results of the analysis were in accordance with literature and experimental data. The calculated debonding stresses correspond to the observed values, but both debonding and yield stress depend strongly on composition, a phenomenon the analysis does not account for. The discrepancy can be explained with matrix/filler interaction and the role of the interphase. The results indicate that although stress analysis helps to predict deformation and failure in particulate filled composites, it must be further refined to find more exact solutions and the effect of inte...

Thermoplastic starch/layered silicate composites: structure, interaction, properties

Composites were prepared from thermoplastic starch and layered silicates organophilized with different surfactants. Sodium montmorillonite and silicates treated with aminododecanoic acid, stearyl dihydroxyethyl ammonium chloride and distearyl dimethyl ammonium chloride surfactants, respectively, were added to plasticized starch in amounts of 0–7 vol%. Changes in the structure of the composites were followed by X-ray diffraction. Dynamic mechanical analysis and tensile tests were carried out to obtain information about the mechanical properties of the composites. The degradation of starch was followed by the measurement of colour, and water absorption was also determined on selected samples. The results revealed that, in spite of the wide variety of organophilization, no or only limited exfoliation took place during the homogenization of the composites. On the other hand, practically all components of the system entered into interaction with each other. Competitive dissolution and adsorption of the surfactant and the plasticizer took place in the composites. The occurrence of chemical reactions cannot be excluded either in the case of the silicate modified with the amino acid. The type and strength of interaction determine the properties of the composites. The reinforcing effect of the silicates varies in a wide range: the amino acid modified clay improves strength almost ten times as much as the silicate treated with the aliphatic amine surfactant. Changes in all other properties including viscosity, colour and water adsorption correspond to those observed in strength and are dominated by interactions.

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.