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New methods for higher loading of fillers without causing dispersion related problems


Increasing polymer prices, driven by rising crude oil prices are driving development of newer methods of addition of higher levels of fillers and glass fibers. Plastics additive suppliers and equipment manufacturers have developed new technologies that enable processors to maximize levels of fillers in their formulations and minimize some of the problems that can result from high filler levels.
In PVC pipes, calcium carbonate is added to lower costs, increase stiffness and improve toughness and processing properties. At filler levels of 20-30 phr (parts per hundred parts) problems begin to arise during processing. These include separation of calcium carbonate and PVC particles during pneumatic conveying of dry blends to intermediate storage silos, as well as formation of calcium carbonate deposits on the walls of hot mixers. In a collaborative effort, calcium carbonate supplier Omya International AG and plastics machinery manufacturer Rollepaal recently found a way to overcome these difficulties. Their method consisted of mixing calcium carbonate uniformly with a PVC-calcium carbonate dry blend in a cold immediately prior to feeding the mixture into the extruder. This procedure, which was demonstrated at the K 2007 show, eliminated the particle segregation problem, as well as the deposits of calcium carbonate that had separated from the PVC in the hot mixer. The result was that compact PVC pipes (160 mm dia and 4 mm wall thickness) could be extruded with calcium carbonate loadings of up to 45 phr. In addition, 200 mm foam-core PVC pipes with 5.9 mm-thick walls could be extruded containing up to 40 phr calcium carbonate.
Glass-reinforced plastics (GRP) are another area in which higher loadings of fillers can be employed as a result of technological advances. One of the recent trends in GRP - including resin transfer molding (RTP) and vacuum film infusion is the use of resins filled with glass microspheres. These formulations decrease consumption of expensive resin, reduce part weight, and improve surface finish and thermal and acoustic properties. However, because GRP is a closed-mold process, the required reinforcing fabrics often get highly compressed, limiting the flow of highly filled resins. One solution to this problem is to use a reinforcing fabric design that resists deformation during molding so that even highly filled or viscous resins can permeate it freely. One such high-flow fabric has been developed by Scott & Fyfe Ltd. (Fife, U.K.) and its subsidiary Flemings Textiles Ltd. It consists of 300 gsm chopped-glass fibers stitched-bonded to an engineered core designed to maximize resin blow. The chopped fibers, which are 50 mm in length, have a silane finish to promote good wetting of fibers. The core is intended for laminates in the 2-4 mm range. The new reinforcing fiber, known as Polymat Hi-Flow, allows the use of highly filled unsaturated polyester loaded with 10% glass microspheres and 60% chalk by weight that could not be used with conventional reinforcing fabrics. Use of this formulation with the new mat reduces resin infusion time by 50%. Meanwhile, the 10% loading of microspheres reduces part weight by 40%. In a recent technical report, Scott & Fyfe estimated that replacement of unsaturated polyester with even 5% of glass microspheres by volume could save up to 15% on resin costs.
Wood-plastic composites have also benefited from new processing methods that increase loadings of fillers. Generally, the wood flour or particles used in these composites is incorporated at levels between 40-60% by weight, with some applications using up to 70%. While there are good economic incentives to use even higher levels of wood in some composites, problems arise with conventional extruders when the content is over 70%. At lower levels of wood, the wood and polymers can first be pelletized or agglomerated prior to feeding into the machine. For higher wood levels, new systems have been introduced over the past few years that allow direct addition to the extruder of wood fiber, chips or powder, along with the carrier resins. Specially configured counter-rotating twin-screw designs in these machines reduce the shear forces that can damage wood fibers. The units also include venting systems for the volatiles emitted by the wood. Such systems have made it possible to process highly filled materials. The driving force for adding more filler to polymers remains primarily economic. But the resulting improvements in quality, durability and appearance and a host of novel properties are also promoting this trend.
(Source : Omnexus)

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