Plastics at the cutting edge

Teeth, nanoparticles and bullet-proof vests were just some of the subjects up for discussion at this year’s Antec – the annual technical conference of the Society of Plastics Engineers in the US. The event, which pitches up in a different city each year, is host to around 700 technical papers that cover the range of emerging plastics technologies – from machinery and processing to new formulations, polymers and additives. Shape memory polymers (SMPs), which consistently return to their original shape, could soon be seen in a range of dental applications. A team from New Ortho Polymer in the US is studying how one SMP measures up against traditional orthodontic plastics such as thermoplastic polyurethane. The material, developed by Patrick Mather at Case Western Reserve University, is a co-polymer of butyl methacrylate and methyl methacrylate. It has a glass transition temperature of 50ºC. One type of orthodontic device is the self-ligating bracket, which forces teeth to grow in the right direction. The brackets are often made from metal-based shape memory alloys, but the researchers believe that SMPs are more easily fabricated into complex shapes, more aesthetically pleasing and less expensive. Mather and New Ortho Polymer are looking to develop ‘intelligent orthodontic appliances’ – which, Mather says “change shape, configuration or mechanical properties as conditions change during orthodontic treatment”. He says that the material could make elements of a visit to the dentist easier – for both dentist and patient. A conventional ‘O-ring’, for example, is normally stretched onto a bracket assembly using a special pliers-like tool. Mather says that O-rings made from his material slip easily into place. They are then ‘shrunk’ by heat activation “The main idea is to save chair time, which is good for both orthodontists and patients,” he said. The properties of SMPs might be improved further by adding nanomaterials – substances with very small particle size. A team at Akron University has used a nano-sized clay filler to improve the recovery force of a polyurethane-based SMP. The nanoparticles bond chemically to the PU matrix, helping to increase its storage modulus. Previous attempts to create SMPs from polyurethene – by adding fibres or other particles – were unsuccessful, say the researchers. “The materials will be used in shoes, as stents in biomedical applications, and as structural and insulation foams,” said the paper’s co-author, Sadhan Jana. XHEAD: Surface effects Weld lines are ugly and unsightly, especially for certain coloured mouldings. The effect is particularly likely to show up when using pigments with a platelet morphology. But now researchers at the Wolfson Centre for Materials Processing at Brunel University in the UK say they can overcome the problem with a double modification of the injection moulding machine. The weld lines are visible because the platelets of the pigment – aluminium flakes, in the case of this experiment – align with the melt flow during mould filling. The surface alignment of the platelets changes at the site of the weld lines, which alters reflectivity – and causes a clearly visible mark. Peter Allan described how selective heating of the mould, combined with melt shearing in the cavity, can eliminate these marks. With the mould open, a robot arm moves an induction coil next to the cavity surface and heats it to the required temperature. The mould then closes and the part is injection moulded. While the melt is still in the cavity, it is sheared using Brunel’s own Scorim process. This uses two pistons to apply a shear force to the melt. Once the part is cooled it is ejected from the mould. “The technology could be applied to components in the automotive industry and consumer goods that require good visual properties of an exposed surface,” says Allan. “The successful use of the technique will eliminate the need to paint the surfaces after moulding – a procedure that the industry would like to avoid.” The trials made moulded parts from polypropylene, which was filled with 2% aluminium flake with a median particle size of 33 microns. Brunel says it is crucial to apply both modifications together, or the weld lines will remain. For example, the ‘unheated’ surface of the mould cavity will still exhibit weld lines. Staying with the subject of surfaces, scientists at Ohio State University have developed a faster way of producing micro-structured plastic substrates. These structures are crucial to emerging applications such as micro-fluidic devices – which analyse tiny biological samples. Current production methods, such as hot embossing and micro injection moulding, typically take 1-10 minutes to produce a device. Avraham Benatar, of OSU’s plastics and composites joining laboratory, told the conference that micro-embossing using ultrasonics could shrink production time to around 10 seconds. This is mainly because the ultrasonic method only heats the surface of the substrate. The other methods heat the entire plastic part. The experiment used an ultrasonic welder to heat the surface of a square PMMA plate. It was then embossed with a surface structure of two fluid wells connected by a micro-channel. The depth of the structure was 0.24mm. There is one main drawback, said Benatar: after embossing, the device tends to stick to the mould. However, the team believes that a final ultrasonic ‘afterburst’ will help to loosen the part from the mould. XHEAD: Mixing it Two Korean universities have developed an improved method of mixing polymers. The teams, from Soonchunhyang University and Kongju National University, call their new design of microcompounder an extensional batch mixer (EBM). They say it mixes more quickly and efficiently, and at lower temperatures, than conventional small-scale mixers. Plastic granules are ordinarily melted using a rotating screw, but this does not happen in the EBM. Instead, it has two cylindrical reservoirs that are connected by a narrow rectangular channel called the shear channel. Each reservoir is closed off by a piston. Molten material is introduced into one reservoir and pressure is applied to the piston. The material is forced through the shear channel and into the second chamber. The process is reversed, and repeated, over a number of cycles. An exit valve then opens and the material is extruded out of the system by depressing both pistons. Younggon Son, of Kongju National University, told delegates that the EBM produced blends of polystyrene and polyethylene with finer morphology and narrower particle size distribution than traditional mixing methods. He said this was because the EBM relies on extensional flow to mix the materials, while the other mixers rely on shear flow. Son claimed that extensional flow blends immiscible fluids more efficiently, and is more appropriate for temperature sensitive polymers. “Extensional flow occurs when the flow is accelerating,” Son explained. He said that this happens in the EBM when the polymer melt moves from the reservoir into the narrow shear channel. XHEAD: Armour improvement Finally, researchers at Case Western Reserve University in the US are studying how biaxially oriented polypropylene (BOPP) – the material used to make crisp packets – might improve the performance of body armour. There is a key difference: while crisp bags use BOPP film – which has been stretched in two directions to improve strength – this experiment produced cross-rolled PP. This compresses PP sheet between rollers. The researchers produced and tested two cross-rolled samples, each 4.5mm thick: one had been compressed from an original thickness of 9mm, the second from 18mm. According to Yankai Yang, of Case Western’s Center for Applied Polymer Research, cross rolling can improve the tensile strength of PP by a factor of 10, compared with unoriented material. Samples were subjected to a ballistic test of at least four shots. The oriented sheets showed improved resistance to higher velocity shots – but more importantly, they absorbed the energy of the projectile by delaminating. The unoriented PP sample shattered under the test. Next year, Antec is held in Cincinnati – the heartland of the US plastics industry.