Model filled rubber Part V Mechanical properties of rubbery composites

Monodisperse size crosslinked polymeric particles of specific chemical compositions, synthesized by emulsifier-free emulsion polymerization, were used as model fillers to study the effect of filler chemical composition on stress-strain behavior of rubbery composites. The modulus, E or G of filled composites increased while the stress and the strain at break decreased with increasing filler-matrix interactions. Physical crosslinking, either due to particle clustering or a network of filler particles with an adsorbed polymer layer supplemented chemical crosslinking. As a result, the overall crosslink density(chemical and physical) was effectively enhanced. The strength of the physical networks, and hence the stiffness of the composites increases with increasing particle-matrix interactions. However, excessively strong matrix-filler interaction would cause a loss of polymer flexibility at the particle-matrix interface, resulting in a decreased stress and elongation at break of the particle filled composites in the order PS > PMMA > PSVP.     

Particulate-fibre-reinforced glass matrix hybrid composites

This study extends the concept of hybridization of glass matrix composites to fibre and particulate reinforcements. Hybridization of Nicalon^®-glass matrix composites with silicon carbide particulates of mean diameters of 0.77, 1.5 and 6 m was studied. The microcrack stress, transverse strength and interlaminar strengths improved significantly on hybridization. At the optimum loading of 7.5 wt % for 6 m particle size the microcrack stress increased by 62%, transverse strength by 650% and interlaminar shear strength by 200%. The ultimate strength declined for this composite by 6%. The decline in ultimate properties was attributed to the damaged graphitic skin at the particle-fibre contact points.     

Effect of ultra fine particles on the elastic properties of oriented polypropylene composites

Fine spherical particles with various diameters (70, 160 and 40 nm, and 35, 65 and 125m) were mixed with isotactic polypropylene (PP). For the oriented composites having hexagonal symmetry produced by drawing, the elastic properties were determined by five compliances,S, or stiffness constants,C. Four of these, namely,S ₃₃,S ₁₁,S ₁₃ andS ₄₄ (orC ₃₃,C ₁₁,C ₁₃ andC ₄₄) were determined for the oriented composites filled with particles whose average diameters were 7 nm and 65m. For the composites filled with the smaller particles (7 nm), all the stiffness constants (C ₃₃,C ₁₁,C ₁₃ andC ₄₄) increased with the filler content, whereas for those with larger particles (65m), this relation was reversed. The Young's moduli of the oriented composites filled with relatively small particles (7, 16 and 40 nm) in each re-stretching direction increased with increasing filler content and with decreasing filler size, whereas those of the composites filled with larger filler (35, 65 and 125m) decreased with increasing filler content and size. It was concluded that the modulus of the oriented composite is determined by three factors, namely: (1) molecular orientation of matrix polymer; (2) the volume-fraction and size of filler; and (3) the fraction of void introduced by drawing. The moduli observed for the oriented composites are well explained by an equation derived on an assumption of the independence of the three effects. It was also concluded that extremely small fillers with particle sizes comparable to that of the crystalline region in PP matrix have a prominent reinforcing effect in the oriented polymer matrix.     

Pressure infiltration of sol/gel processed short fibre ceramic matrix composites

Using pressure infiltration short fibre-reinforced ceramic matrix composites with uniform and dense microstructure can be made. Based upon this processing technique, composites composed of silica sol, alumina particles, and alumina short fibres were fabricated. The related processing parameters studied in this work include infiltration rate, fibre volume fraction Vf, particle size, and infiltrate viscosity. An optimal infiltration rate was 4 mm min-1, at which rate the composite with Vf of 8.1% and particle size of 3 m has the highest green density. An equilibrium between particle packing strength and applied load must be obtained during the infiltration to obtain high green density and composite strength. The influence of fibre volume fraction and particle size on composite green density is in a synergistic manner because it involves particle–fibre interactions, fibre–fibre interactions, and sedimentation. Furthermore, the increase of sol viscosity results in more sedimentation in the infiltrate and lower composite green density. The fracture toughness of composites is 38% higher than that of monolithic alumina. © 1998 Chapman & Hall     

Preparation of barium titanate ultrafine particles from amorphous titania by a hydrothermal method and specific dielectric constants of sintered discs of the prepared particles

Barium titanate ultrafine particles were synthesized from amorphous titania by a hydrothermal method. The mean size of the barium titanate particles prepared at a hydrothermal treatment time of 4 h, was nearly equal to 0.04 5 m in the range of barium-to-titanium molar ratio (BT) 2, and approximately agreed well with the crystallite size. At a BT molar ratio of 1.0, the mean particle size increased to 0.2 m, while the crystallite size remained constant at 0.045 m. When the particle size ranged from 0.12–0.20 m, prepared for the BT molar ratio of 1.0–1.4, the specific dielectric constant for a sintered disc composed of these particles attained a value of 5000 or more. As the BT molar ratio increased to exceed 1.5, when the mean particle size decreased from 0.13 m to 0.045 m, the specific dielectric constant for the sintered disc was decreased greatly. The specific dielectric constant for the sintered disc can be correlated well with the size of the composing particles.     

PVC-Cu composites with chemically deposited ultrafine copper particles

PVC-Cu composites with chemically deposited ultrafine (0.1 to 0.3 m diameter) copper particles were prepared by hot-pressing copper-coated PVC powder (–106, +150 m) at 120° C. Metallic copper in fine-particle form was deposited on the PVC particles by reducing an ammoniacal cupric acetate solution with hydrazine at 85° C. The electrical resistivity (d.c.) and transverse rupture strength of these composites were measured. Measurement of electrical resistivity indicated that in these composites copper particle network formation was initiated at a copper content of about 0.2 vol%; with further increase of copper content the resistivity dropped sharply from about 10¹⁴ (for pure PVC) to about 10⁵ MN m^–2 cm at a copper content of about 0.5 vol%. Increase of copper loading beyond 0.5 vol% did not decrease resistivity significantly whereas the rupture strength increased continuously from 120 MN m^–2 (for pure PVC) to a value of about 300 MN m^–2 with 4 vol% copper loading. This high value of resistivity even after copper particle chain formation and the continuous increase of rupture strength, is thought to be due to formation of a thin layer of polymer film between the copper particles introducing a quasi-random character to the otherwise segregated network of copper particles.     

Spray forming of ultra-fine SiC particle reinforced 5182 Al-Mg

This paper describes the spray forming of SiC particle reinforced Al metal matrix composites (MMCs) with particular emphasis on microstructure characterization of SiC particle distribution. A 5182 Al-Mg alloy was used as matrix material, and SiC particles with a mean diameter of 1.2 m and 2.0 m as reinforcement. The reinforcing particle distribution and microstructural characteristics of MMCs were analyzed in the current study using TEM, SEM and optical microscopy. The distribution of SiC particles in the as-spray deposited and hot-extruded conditions was characterized. SEM results indicate that the SiC particles are homogeneously distributed although some clustering was evident in the matrix. TEM and OM examinations show that most of SiC particles are present intergranularly in the Al matrix. EDS analysis indicated that Mg tends to segregate and form oxide phases in the vicinity of SiC particles and that there is no compositional variation of Mg across grain boundaries in the Al matrix.     

Effect of particle size on the sintering of Li2O-Al2O3-4SiO2-borosilicate glass composites

The effect of spodumene particle-size on the sintering of spodumene-borosilicate glass composites was investigated. The results can be explained by a particle-size dependence of the extent of reaction between spodumene and glass. For samples fabricated with fine spodumene particles (1.5–3 m), a reactive liquid-phase sintering behaviour was observed. Smaller particle size resulted in a higher sintered density and coarser microstructure. For samples fabricated with coarse spodumene particles (3–7 m), a non-reactive liquid-phase sintering behaviour was observed when a high-viscosity glass, a lower initial glass content, and a lower sintering temperature were used. A larger particle size resulted in a higher sintered density and coarser microstructure. If a low sintering temperature or a coarse (e.g., 7 m) particle size were used, the dissolution of the glass into spodumene was reduced which resulted in an increased amount of glass crystallization.     

Fabrication of polystyrene/agave particle biocomposites using compression molding technique: evaluation of flammability,biodegradability, mechanical and thermal behaviour

Polystyrene (PS) composites reinforced with ungrafted and acrylonitrile (AN) grafted agave particles (AgP) have been prepared with 10–30% particle content by weight using compression molding technique. The composite specimens thus prepared were subjected to the evaluation of mechanical, chemical, flammability and biodegradability properties. PS composites with 20% particle loading exhibited optimum mechanical properties. AN grafted AgP/PS composites exhibited higher mechanical strength as compared to ungrafted AgP/PS composites. Further AN grafted AgP/PS composites exhibited better thermal properties and biodegradability as compared to PS matrix. Addition of fire retardant fillers such as magnesium hydroxide (Mg(OH) $_{2})$ and zinc borate lowered burning rate of PS composites considerably. Scanning electron microscopy (SEM) of tensile fracture surfaces of AN grafted AgP/PS composites showed better particle/matrix adhesion.     

Microstructure and mechanical properties of Cr3C2 particulate reinforced Al2O3 matrix composites

Al₂O₃ matrix with three grades of Cr₃C₂ particle size (0.5, 1.5 and 7.5 m) composites were fabricated by a hot-pressing technique. Fully dense compacts with Cr₃C₂ content up to 40 vol % can be acquired at 1400 °C under 30 MPa pressure for 1 h. The flexural strength increases from 595 to 785 Mpa for fine Cr₃C₂ particle (0.5 m) reinforced Al₂O₃ matrix composites. The fracture strength is significantly dependent on the fracture modes of matrix (intergranular or transgranular). The transgranular fracture with a compressive residual stress gives a high fracture strength of composites. At the same time, the fracture toughness increases from 5.2 MPa m^1/2 (10 vol % Cr₃C₂) to 8.0 MPa m^1/2 (30 vol % Cr₃C₂) for the coarse Cr₃C₂ particle (7.5 n) reinforced Al₂O₃ matrix composites. The toughening effects of incorporating Cr₃C₂ particles into Al₂O₃ matrix originate from crack bridging and deflection. The electrical conductivity and the possibility of electrical discharge machining of these composites were also investigated.     

Effects of chemical modification of wood particles with glutaraldehyde and 1,3-dimethylol-4,5-dihydroxyethyleneurea on properties of the resulting polypropylene composites

Both glutaraldehyde (GA) and 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) can crosslink the cell wall polymers and dimensionally stabilize wood particles and the treated wood particles are thus expected to enhance the properties of the resulting wood particle/polypropylene composites. Compared to the composites filled with untreated particles, treatments of wood particles with both GA and DMDHEU showed a great reduction in water uptake and dimensional swelling of the resulting composites up to 39% and 46%, respectively. Both the flexural and tensile moduli increased due to wood particles treatments with GA and DMDHEU. Treatments of wood particles improved the tensile strength but moderately weakened the flexural strength and Charpy impact strength of the composites. Dynamic mechanical analysis and microscopy suggested an improved interfacial compatibility between wood particles and matrix due to GA and DMDHEU treatments. Chemical treatment resulted in smaller particle sizes and altered microscopic fracture appearance after composite production as compared to untreated particles. Morphological changes were attributed to embrittlement of wood particles, which may negatively influence the mechanical properties of the resulting composites.     

Effect of filler content and size to properties of composite resins on microwave curing

The effect of filler content and size to property of composite resin using high purity spherical silica particles on microwave curing was examined by mechanical means and electron microscope observation to develop microwave-curing composite resin inlay. Increasing filler content in three kinds of filler particles (0.45, 0.96 and 1.46 m) resulted in increasing compressive strength, diametral tensile strength and knoop hardness and indicated the highest values at the operative mixing limit. The effect of particle size was that the mechanical property of the 0.96 m filled resin was highest, followed by 1.46 m and 0.45 m on the same filler content. The 0.96 m filled resin had about the same knoop hardness on both surfaces of the cured sample, which means that uniform polymerization occurred in the cured sample.With electron microscope observation, the bubbles of about 1.0 m and the cracks between the fillers and the matrix were observed in each cured resin. This is caused by the excessive absorption of microwave energy in fillers and surface treatment materials of fillers. Therefore, improvement of the composition of filler and surface treatment material of filler are needed, which makes it harder to absorb the microwave energy. ©©1999©Kluwer Academic Publishers     

Production of SiC particulate reinforced aluminium composites by melt spinning

Melt spinning is successfully used for the preparation of a rapidly solidified SiC particle reinforced AlSi7Mg0.3 alloy. The composites are prepared by introducing SiC particles in a semi-solid matrix slurry (SiC volume fractions up to 0.15, particle size 10 or 20 m). Duralcan material (SiC volume fraction 0.20, particle size 12 m) was also used. After stirring in the semi-solid state the composites are heated above the liquidus temperature and subsequently melt-spun. Featureless, columnar and dendritic zones can be identified in the ribbons. A finer dendritic structure is found around the SiC particles. The SiC particles tend to segregate to the air side of the ribbons and the segregation effect is influenced by particle size and volume fraction. As interface velocities are higher than the critical velocities predicted by models on interface pushing, it is concluded that fluid flow in the melt puddle is responsible for the segregation effect.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Microstructures of in situ Al/TiB2 MMCs prepared by a casting route

In situ Al/TiB₂ metal matrix composites (MMCs) have been successfully produced by Salt-Metal reactions. This is a novel low-cost reactive approach, which involves adding Ti and B bearing salts to molten Al. The reactions between the salts lead to the formation of the reinforcing TiB₂ particles in the Al matrix. The in situ formed TiB₂ particles are very fine (below 1 m in size). Strings and clusters of particle agglomerates are distinct microstructural features of all the composites with pure Al as the matrix. The effects of processing parameters on the kinetics of TiB₂ formation and on the final microstructures are studied in detail. Besides, efforts are made to improve the distribution of TiB₂ particles in the Al matrix by means of chemical additions; it is found that a homogeneous distribution is obtained by using a eutectic Al-Si alloy as the matrix material.     

Production of rapidly solidified Al/SiC composites

Rapidly solidified metal matrix composites have been produced on a laboratory scale either by (1) melt spinning a composite after introduction of the ceramic phase and extrusion of the flakes obtained, or (2) blending melt-spun powder (basic alloy) with the ceramic phase and subsequent extrusion. AlMg(Si) alloys were used as matrix material while SiC particles with diameters of 3 or 20 m were used as the ceramic phase. For the composites prepared by route 1 it was found that the basic alloy was reinforced by the addition of 3 m particles whereas for the 20m particles reinforcement was observed only for very ductile matrices. The bond between SiC particles and matrix was good. A diffusion and wetting bond was formed. For the composites prepared by route 2 it was found that reinforcement did not occur and that the bond between particles and matrix was weak. Debonding of the particles took place in the case of tensile fracture. The advantage of a rapidly solidified matrix for these composites is that relatively high ductilities are combined with good reinforcement effects. Prior contact of the ceramic phase and the aluminium melt is needed for a good bond between SiC and the matrix material. It is concluded that route 1 should be preferred for the production of rapidly solidified aluminium matrix composites.     

Preparation of cast aluminium alloy-mica particle composites

The optimum conditions for producing cast aluminium alloy-mica particle composites, by stirring mica particles (40 to 120 m) in molten aluminium alloys (above their liquidus temperatures), followed by casting in permanent moulds, are described. Addition of magnesium either as pieces along with mica particles on the surface of the melts or as a previously added alloying element was found to be necessary to disperse appreciable quantities (1.5 to 2 wt.%) of mica particles in the melts and retain them as uniform dispersions in castings under the conditions of present investigation. These castings can be remelted and degassed with nitrogen at least once with the retention of about 80% mica particles in the castings. Electron probe micro-analysis of these cast composites showed that magnesium added to the surface of the melt along with mica has a tendency to segregate around the mica particles, apparently improving the dispersability for mica particles in liquid aluminium alloys.The mechanical properties of the aluminium alloy-mica particle composite decrease with an increase in mica content, however, even at 2.2% the composite has a tensile strength of 14.22 kg mm^–2 with 1.1% elongation, a compression strength of 42.61 kg mm^–2, and an impact strength of 0.30 kgm cm^–2. The properties are adequate for certain bearing applications, and the aluminium-mica composite bearings were found to run under boundary lubrication, semi-dry and dry friction conditions whereas the matrix alloy (without mica) bearings seized or showed stick slip under the same conditions.     

Development and characterization of novel EPDM/NR prophylactic waste composites

Ethylene propylene diene rubber (EPDM) is a well-known versatile polymer, which is frequently used in the production of rubber goods based on conventional and specialty polymers. The present paper investigates the role of recycled natural rubber prophylactic waste compared to virgin natural rubber in the development of novel ethylene propylene diene rubber composites. The processing characteristics have been evaluated using a Monsanto rheometer at three different temperatures 150, 160 and 170°C. The cure curves of EPDM compounds have been found to be the resultant of slow curing or marching cure curve of EPDM and that of fast curing S shaped cure curve of natural rubber. The curing properties such as optimum cure time, scorch time and induction time have been found to be decreasing with the loading of prophylactic filler. For most of the cases, the values obtained for compositions with virgin natural rubber (ISNR-5) have been found to be lower than that with prophylactic filler.The cure activating nature of the prophylactic waste in EPDM is higher at higher temperatures. The unaged tensile strength has been increased with the loading of prophylactic filler up to 30 phr. The aged tensile strength and unaged/aged elongation at break have been found to be a maximum at 20 phr prophylactic filler loading. The tear strength has been found to be a maximum at 40 phr. Better performance has been noted in the case of virgin natural rubber filled samples for unaged/aged tensile strength, elongation break and tear strength except at 40 phr loading. The diffusion process in EPDM vulcanizates is found to be anomalous. Crosslink density values determined using Mooney-Rivlin equation agree with the tensile strength values for most of the cases. An increase in the crosslink density has been noted with the thermal aging of the samples.     

Comparison of the response of three human monocytic cell lines to challenge with polyethylene particles of known size and dose

The response of three human monocytic cell lines (Monomac-1, U937 and THP-1) to challenge with polyethylene particles of known size and dose was evaluated. Particles with a mean size of 0.21, 0.49, 4.3, 7.2, and 88 m were co-cultured with the cells for 24 hours prior to the assessment of cell viability and production of the pro-inflammatory cytokines, IL-1, IL-6 and TNF. Additionally, GM-CSF and prostaglandin E₂ were measured in culture supernatants from particle stimulated U937 cells. All particle fractions were evaluated at particle volume (m³) to cell number ratios of 100 : 1, 10 : 1, 1 : 1 and 0.1 : 1. None of the test fractions had any effect on cell viability. Only the response of the U937 cell line was demonstrated to be comparable to that of primary macrophages as determined in a previous study. Furthermore only particle volume to cell number ratios of 10 : 1 or greater consistently stimulated significantly enhanced levels of cytokine secretion with particles within the phagocytosable size range (0.1 to 10 m) being the most biologically active. No response was observed when U937 macrophages were stimulated with the largest (88 m) particles at any of the volume ratios tested in this study. These results suggest that the size and volume of polyethylene particles are critical factors in macrophage activation. In addition, the U937 cell line has been shown to be a suitable model for the in vitro study of macrophage-particle interactions. © 2001 Kluwer Academic Publishers     

Processing and properties of a bioactive glass-ceramic reinforced with ductile silver particles

In order to overcome the problem of an adverse interfacial reaction which occurs when bioactive Apoceram glass-ceramic is reinforced with titanium particles, we have investigated employing silver as the reinforcement. Composites reinforced with 10vol%silver were successfully produced by two routes, namely hot pressing and cold forming followed by sintering and crystallisation. There was no difference in the microstructure of the matrices other than the presence of large (10 m) pores in the materials produced by the cold formed route. The matrices were free from microcracks and no reaction was observed between the matrix and the silver particles. The flexural strength and single edge notched bend toughness were determined at room temperature for the composites and the corresponding monolithic materials. Although the strength of cold formed monolithic material was poor compared with that produced via hot pressing, there was no difference in the mechanical properties of the composites produced by the two routes. It is therefore recommended that future development of the composites should concentrate on the less expensive cold-forming route.