Graham C. Lees

Effect of silane-based filler surface treatment formulation on the interfacial properties of impact modified polypropylene/magnesium hydroxide composites

Abstract Magnesium hydroxide (Mg(OH) 2 ) flame retardant filler has been treated with combinations of vinyltriethoxysilane (VS), dicumyl peroxide (DCP) and trimethylolpropane triacrylate (TMPTA) and melt blended to V f =0.14 and 0.41, (30% w/w and 65% w/w) with an impact modified polypropylene matrix. The VS/DCP combination afforded the best balance of strength and toughness at V f =0.41. The superiority of this system was found to be due to encapsulation of the filler particles by the crosslinked elastomeric phase of the matrix which was confirmed by secant modulus measurements and low temperature fractography. The data obtained from this study form the basis of a proposed mechanism for this encapsulation effect.

The effect of surface roughness on the adhesion of Candida albicans to acrylic

The upper fitting surface of the maxillary denture is frequently a reservoir of infection for Candida albicans. The surface is not smooth, yet the majority of studies on Candida adhesion to denture materials have used smooth, often polished surfaces. In the present work, the effect of surface roughness on adhesion of C. albicans was studied. Sheets of polished, clear acrylic were abraded using emery paper of increasing grit size. Measurements describing roughness were calculated from a magnified trace of the surface. Standardised cell suspensions were incubated with the acrylic at 37°C for 1 h. Attachment was measured in terms of the percentage area of a microscopic field covered by cells, using an automated image analysis system, or by counting the number of adherent cells per field. Cells adhered in significantly (P <0.001) higher numbers to roughened acrylic than to smooth acrylic, but maximal adhesion was achieved on acrylic surfaces roughened with medium grit size emery paper. Scanning electron micro...

Bacterial adhesion to inert thermoplastic surfaces

The adhesion of four bacterial species, two strains of each, to four hydrophobic thermoplastics was observed. Image analysis of adherent cells stained with acridine orange provided a rapid, direct and objective means of measuring adhesion to clear, translucent and opaque surfaces by calculating the percentage area of a microscopic field covered by cells (percentage coverage). There was a highly significant correlation (P>0.05) between percentage coverage value and adherent cell count (obtained manually) for both rods and cocci. Bacterial adhesion to thermoplastics appeared to be strain specific and was not related to polymer composition. Highest percentage coverage values were obtained using hydrophobic bacteria, and lowest using hydrophilic bacteria. There was no relationship between the origin of the organisms (culture collections or isolates from biomaterial-associated infections) and their ability to adhere, after cultivation in brain heart infusion broth. Many factors influence this ability: an awareness of all experimental variables is essential.

Flow micro-calorimetry and FTIR studies on the adsorption of saturated and unsaturated carboxylic acids onto metal hydroxide flame-retardant fillers

Carboxylic acids, particularly fatty acids, are by far the most important surface modification systems for inorganic fillers used in the plastics and rubber industries. However, relatively little is understood regarding factors that affect their adsorption behaviour, including the interplay between acid molecular structure, adsorption conditions, and substrate chemistry. In this study, the adsorption (from n-heptane and toluene) of a range of saturated and unsaturated fatty acids, together with acrylic acid, onto aluminium and magnesium hydroxides has been investigated using flow micro-calorimetry and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). With C18 saturated fatty acids, the overall heat of adsorption per unit surface area of substrate was found to be affected by the structure of the alkyl tail. The orientation of the adsorbed unsaturated fatty acids was found to be affected by the number of double bonds in the molecule. Acrylic acid was found to be very strongly adsorbing and could displace isostearic acid from the filler surface. However, isostearic acid was found to adsorb on top of the layer of aluminium or magnesium acrylate. These findings have important implications particularly for mixed fatty acid treatment methods and the choice of fatty acid treatment for metal hydroxide flame-retardant fillers.

Influence of surface modification of magnesium hydroxide on the processing and mechanical properties of composites of magnesium hydroxide and an ethylene vinyl acetate copolymer

Magnesium hydroxide is of increasing interest as an environmentally friendly flame retardant filler for ethylene vinyl acetate (EVA) copolymers which are widely used in cable applications. In this paper, magnesium hydroxide (Premier Periclase DP393) was used at a flame retardant loading of 60% w/w in an EVA copolymer with a vinyl acetate content of 18% (molar). The particular modifiers used in this study were stearic acid (converted to ammonium stearate (AS)) and γ-aminopropyltriethoxysilane (APS). The effect of surface-treated magnesium hydroxide on the processing, mechanical and the flame retardant properties were determined, together with characterisation of the interaction between filler and polymer. These studies have shown that, relative to the untreated filler, use of AS coating leads to a decrease in tensile strength to a limiting value at monolayer coverage, but an increase in elongation at break, which reaches a maximum at monolayer coverage. However, the APS treatment enhanced both tensile stre...

Magnesium hydroxide filled EVA: The effects of filler surface modification on the strength of filler/matrix adhesion and the consequences for composite structure and properties

The interaction of ammonium stearate (AS) and γ-aminopropyltriethoxysilane (APS) treatments with a magnesium hydroxide flame retardant filler and their effects on its use in an ethylene vinyl acetate copolymer compound have been investigated. The work has shown clear evidence of changes in the structure of the surface layers formed on the filler as the amount of additive is increased and the levels at which these occur can be correlated with theoretical monolayer quantities. Infrared (IR) spectroscopy data suggest that the stearate coating changes from a half salt to a full salt as the coating level is increased and that the APS coating on the filler initially has a significant bicarbonate content, presumably due to reaction with atmospheric carbon dioxide. The effect of coating level on the melt flow rate, insoluble matrix content, crystallisation behaviour, tensile properties, limiting oxygen index, and ageing of the filled compound has been studied. Distinct trends have been observed, many of which can be correlated with the structure of the filler surface layers. Of particular importance is the observation that, unlike APS, excess stearate appears to promote detrimental ageing effects.

The use of flow micro-calorimetry and FTIR techniques for characterising filler/organic acid interactions

Reaction of filler grade magnesium hydroxide (Martinswerke Magnifin H 10) with oleic acid (cis 9-octadecenoic acid) and stearic acid (octadecanoic acid) has been investigated using flow micro-calorimetry (FMC) and solution adsorption isotherm methods (both from heptane). Diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), sedimentation volume measurements and X-ray diffraction were also carried out on the isolated and dried adsorption isotherm treated samples. Stearic acid adsorbed to a greater extent (24 mg g-1) than oleic acid (12 mg g-1 ) and this is likely to correspond to vertical adsorption of both acids based on measured filler surface area and adsorbate molecular area. It is considered that stearic acid forms a layer more than one molecule in thickness and was shown by X-ray diffraction, in the dry state at least, to exhibit structural order. Oleic acid adsorbs as a single molecular layer with any additional (i.e. excess) acid present probably reacting fully with the magnesium ion, thu...

Prediction of Mechanical Properties Following the Dispersion Assessment of Flame Retardant Filler/Polymer Composites Based on the Multifractal Analysis of SEM Images

Backscattered electron scanning electron microscope (SEM) imaging of the distribution of metal hydroxide filler particles in cryo‐cut surfaces of poly(ethylene‐ran‐vinyl acetate)/metal hydroxide composites followed by multifractal analysis of the images has led to a numerical measure of dispersion quality, i.e. the width (D0–D4) of the multifractal spectrum. The D0–D4 values were then determined for a series of composites that formed a quadratic algorithm screening experimental design where the following four factors: type of filler (magnesium hydroxide or aluminium hydroxide), filler loading (30% w/w–60% w/w), vinyl acetate content (20% w/w and 28% w/w), and isostearic acid coating of the filler, were investigated. Two multiple regression models were constructed to examine the dependence of tensile strength and elongation at break on dispersion quality and composite formulation. Predictions of tensile strength and elongation at break were forecast and the results compared with experimentally measured data. A positive correlation was found between the as measured data and predictions from the multiple regression model. Examination of the models indicated that composite tensile strength and elongation at break decreased as a function of decreasing dispersion quality confirming the importance of dispersion quality in defining ultimate material properties. Dedicated to Professor John L. Stanford on the occasion of his 60th birthday.

Evolution of reactive unsaturated interfacial modifiers for polyolefin based composites

The development of a novel reactive unsaturated interfacial modifier for filled (magnesium hydroxide, aluminium hydroxide and calcium carbonate) polyolefins is reviewed. Initial work explored maleimide functional candidate molecules including 1,3-phenylene dimaleimide (1,3-PDM) and a range of linear alkyl dimaleimides. Most of the dimaleimides were very effective interfacial modifiers in polypropylene and poly(ethylene-co-vinylacetate) (18% w/w vinylacetate) based composites. However, the dimaleimides caused the composites to have a distinct pink/brown colouration apart from when the maleimide groups were spaced by a C12 alkyl chain. The problem with colour and high effective dosage prompted a switch to unsaturated carboxylic acid based systems. The performance of a range of unsaturated carboxylic acids and the hexafunctional monomer trimethylolpropane triacrylate are reviewed, together with a novel oligomeric unsaturated carboxylic acid (OUCA) based system developed by the Noveon Division of Lubrizol Limited. The data obtained from this series of interfacial modifiers, and adsorption studies on 1,3-PDM and OUCA, afforded some insight into the competing factors (such as the strength of adsorption versus ability to form a polymerised interfacial layer and the ability to react with matrix chains) that significantly affect the coupling performance of interfacial modifiers.

Evaluation of an alternative modification route for layered silicates and synthesis of poly(styrene) layered silicate nanocomposites by in-situ suspension polymerization

The organically modified montmorillonites (o-MMT) used in this study were prepared in a semi-solid state in molten long chain alkyl (hydrogenated tallow (HT) or stearyl) dimethyl/aryl ammonium chloride intercalant (quat), within a Brabender Plasticorder W50E chamber. The effect of quat level and structure was investigated using WAXS, FTIR (DRIFTS) and solvent swelling/dispersion viscosity studies. It was found that mono-stearyl (or HT) quats were the most suitable intercalants for the in-situ polymerised PS matrix nanocomposites produced. The distearyl (or HT) quats generally led to reduced interfacial effects in the composites and reduced toluene dispersion viscosity due to the close proximity of long alkyl tails within the molecules facilitating their self-assembly into ordered arrays, which were difficult for toluene to penetrate (toluene was used as a probe to gauge compatibility with styrene). Substitution of a benzyl group (for a methyl) led to increased compatibility with styrene/toluene, though the detrimental effect of two long alkyl groups was not overcome. PS matrix nanocomposites have been formed via in-situ free radical suspension polymerisation of styrene/organo-montmorillonite (o-MMT) dispersions. These composites displayed evidence of large interfacial area relative to the volume fraction of montorillonite added; this was manifested as a reduction in melt flow rate, broadening of the molar mass distribution (increase in M w) and an increase in thermal stability, relative to the unfilled matrix. However, wide angle X-ray scattering (WAXS) patterns of the composites revealed a strong (001) reflection (d = 3.3–3.4 nm) together with clear (002) and (003) reflections. Therefore a mixed intercalated/flocculated morphology, with no significant exfoliation into single platelets, was indicated.