HIGH IMPACT POLYSTYRENE : HIPPS MATERIAL PROPERTIES.

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1 HIGH IMPACT POLYSTYRENE : HIPPS MATERIAL PROPERTIES. This material can be regarded as styrene toughened with rubber. To do this polybutadiene (BR) is dissolved in styrene and the solution is polymerized with a normal catalyst. Styrene butadiene rubber (SBR) was originally used but was replaced for technical reasons by As the styrene polymerises the rubber comes out of solution in the form of fine particles suspended in polystyrene. The rubber particles are very effective "shock absorbers" for the brittle polystyrene because many of the rubber chains have been chemically grafted to the polystyrene. The amount of rubber incorporated is quite small. Normal grades contain 3 to 7% of low-cis polybutadiene but high impact grades contain about 9% of high-cis polybutadiene. Very high impact grades are often made by compounding more rubber in to standard grades as it is not necessary to have all the rubber grafted to the polystyrene chains. Because of the rubber content HIPPS is translucent and also softer and easier to cut than GPPS. A characteristic feature of HIPPS is that when it is deformed or bent it will exhibit a stress whitening effect at the point of deformation. The effects of the rubber on the properties of polystyrene are; * Impact strength is greatly increased as more rubber is incorporated. * The transparency and some of the surface gloss of GPPS are inevitably lost as there are two solid phases present. * Tensile strength, stiffness, hardness and Vicat softening point fall as the rubber content Increases. Heat distortion temperatures are about 15 C lower than GPPS. * The density of the material decreases slightly as rubber content increases. Very high impact grades can have densities of only 1.02 g/cm3 compared to 1.05 g/cm3 for GPPS. * Resistance to light, solvents and chemicals is worse than that of GPPS. * The electrical insulating properties are reduced compared to GPPS. In general high impact grades are used for injection moulding. These may be supplied as compounded materials or additional rubber and color may be incorporated via masterbatches at the moulding shop. Standard grades used for sheet extrusion are usually processed as supplied although colored compounds are available. The properties of HIPPS and ABS overlap in some ways although, in general, ABS is harder, stronger and more chemically resistant. On the other hand HIPPS exhibits a lower water absorption than ABS ( 0.05 to 0.08% in 24 hours versus 0.2 to 0.35%. for ABS ) and need not usually be dried before processing. HIPPS is also cheaper than ABS. In the past the low gloss of HIPPS was a major weakness versus ABS but high gloss, high impact grades are now available providing the required balance between impact strength and gloss. These grades give a brilliant finish thus meeting the needs of many domestic products. The gloss ratings are comparable to and in some cases better than ABS and they are now being used for products previously made from ABS.

2 In general HIPPS is used where a certain impact resistance is required and where the relatively poor chemical resistance is not an issue. Some profiled products are produced but the major application is for sheet for subsequent thermoforming. A wide variety of cups and small containers for food are made in this way. By coextrusion the product range can be extended. Combinations include a HIPPS main layer with a GPPS gloss layer; a HIPPS main layer with a GPPS antistatic layer; layers of different grades of HIPPS and regrind HIPPS as the center layer of a sandwich construction for margarine tubs. CHEMICAL PROPERTIES. HIPPS is not affected by aqueous solutions of salts, acids, alkalis and weak oxidising agents. It is not resistant to aromatic and chlorinated hydrocarbons; esters, ethers, aldehydes, ketones and various chlorinated hydrocarbons, for example, methylene chloride, ethylene dichloride and trichlorethylene. Aliphatic hydrocarbons cause swelling. Although HIPS is resistant to most foodstuffs and beverages, some spices and essential oils can cause swelling or rnoulding failure. For example it is not resistant to ground caraway, cedar-wood oil, ground cloves and ground ginger. It has limited resistance to curry. IHIPPS burns easily and forms burning drops giving off large amounts of sooty smoke and smells of burning rubber as well as styrene. COLORING. As the natural color of the material is an off-white a wide color range is possible. Often seen as white sheet or, as white thermoforrned sheet but can be readily colored by a range of techniques, for example, dry coloring, masterbatches and liquid coloring. Dry colors are widely used with HIPPS. Colorants are normally supplied in unit packs, or in bulk, and the amount used is dependant upon the strength of the dyestuff used or, the density of the final color required. As the color of the base polymer can vary from supplier to supplier, it is preferable when color matching to use the actual material that is to be processed, as a change in base color can affect the amount, or proportions, of pigments that are to be used. If there is a possibility of water absorption occurring with HIPPS, then dry the material before adding the colorant. However this is not always possible then, in order to reduce the amount of contamination occurring during the drying process, it is advisable that the full wetting agent procedure is used as advised. To assist dispersion of colorant in the polymer melt the use of mixing sections in the extruder is recommended. The masterbatches used for HIPPS are of the 'universal' or of the styrene based types, with a typical concentration of 1% i.e. 25 grams. per 25 kilo bag of polymer.

3 Higher concentrations may be necessary depending upon the intensity of the color required in the finished product. Color matching can easily be carried out using solid masterbatches. Liquid colour is used as an alternative to solid masterbatches and is well suited when long runs on one color is required. MATERIALS HANDLING. HIPPS exhibits low water absorption and so normally does not need to be dried before use However, moisture forming on the granules through condensation will affect the surface finish of extruded sheet. If the granules are kept in a cold environment, opening the 25 kilo bags in the warm atmosphere of the extrusion shop can cause condensation, as a result of the temperature difference. During periods of high humility, it is advisable to store the day's requirements in the extrusion shop. The moisture pickup problem can also be encountered when using a bulk supply material handling system and therefore should be catered for within the design of the handling system. If it does prove necessary then dry in a hot air oven for 3 hours at 70 C or in a dessicant drier for 1-2 hours. As with GPPS, antistatic grades may need 30 % longer drying times. The level of residual monomer in styrene based plastics has dropped but nevertheless, the workplace should be well ventilated and extruder vents should be fitted with an exhaust system. The polymer is supplied in pellet form. For extrusion, the pellets should not be externally lubricated. Gravity feed hoppers are adequate FLOW PROPERTIES. For extrusion HIPPS grades have MFI values measured at 200 C and 5Kg of between 2 and 10. For the production of sheet, the MFI value is usually 2 to 4, the lowest values being for sheet for thin walled vacuum moldings. It is common to blend HIPPS with GPPS to get the required balance of toughness, stiffness and price. In this case blend grades with equal MFI values if possible. If two grades (1 and 2 ) with different MFI s are blended remember that the MFI of the blend will be; MFI blend = antilog ( log ( MFI 1) * % grade 1/ log (MFI 2 ) * % grade 2 / 100 ) The viscosity values of a reasonably stiff extrusion grade are given by; Viscosity (Pa/s) = antilog [ * temperature (c) * log (shear rate (s-1))]

4 SCREW AND BARREL DESIGN. HIPPS can be extruded with single stage screws and unvented barrels but best results are obtained using vented extruders with a barrel L/D ratio of about 30:1. If single stage screws are used they should have a compression ratio around 3 and an L/D ratio of 24 or more. They should preferably be fitted with a mixing zone. This is especially important when using blends of HIPS and GPPS or when blending in color. Dimensions of a range of vented screws is given below. TWO STAGE VENTED SCREW DESIGN. Zone lengths: 1st Feed 5D, 1st Compression 5D, 1st Metering 5D, Decompression 1D, Vent 4D, 2nd Compression 2D, 2nd Metering 5D and Mixing 3D. SCREW DIAMETER (mm) st feed depth st metering depth Vent zone depth nd metering depth BARREL AND DIE TEMPERATURES. The target melt temperature is between 180 and 220 C. Temperature settings vary according to the material, speed and screw design. The following table gives a starting point. ZONE TEMPERATURE (C) Zone Zone Zone Zone Zone Adapter & Die 200 DIE DESIGN AND CONSTRUCTION. Dies for profile and sheet are similar to those used for ABS. Coextrusion of sheet containing HIPPS has been employed. It is possible for the different materials to be joined before entering a standard die provided their viscosities are not very different from each other. If one melt has a much lower viscosity then it will tend to rnigrate to the outside of the flow channel to minimize the work done in flow. In these cases, dies are designed with separate flow channels for each material. Usually they are provided with separate adjustments for the thickness distribution in each layer. DOWNSTREAM OPERATIONS. Three roll polishing stacks are used for sheet extrusion. They are temperature controlled by

5 circulating water or oil and flow channels for the fluid should be close to the roll surface and extend over the whole width of the sheet. Rolls are either driven by a common drive or with individual DC drives which are linked by thyristor based electronics for synchronisation. The degree of wrap-round can be controlled by changing the roll arrangement. In vertical stacks, there is 560 o of wrap-round. With an upstack where the material enters between the bottom two rolls, the top surface of the sheet can be highly polished and it is possible to laminate on the underside of the sheet. The reverse is the case for the downstack. If the rolls are set at 45 o to the vertical, wrap-round can be either 315 or 405 o. In another arrangement, the feed is into the bottom two rolls which are at 45 o to the vertical. The third roll is mounted on the same horizontal plane as the middle roll. This gives a wrap-round of 225 o. Horizontally disposed rolls give the best view of the material as it enters the first nip and the possibility of varying die to nip distance without the problem of sag. The haul-off consists of two rubber covered rolls driven by a thyristor controlled DC motor. Before stacking, sheet can be edge trimmed using smooth rotating circular blades for thicknesses up to 2.5 mm and a saw for thicker products. Again for transverse cutting, the choice is between guillotine and travelling saw. Extraction systems must he provided when saws are used. A cyclone separator and filter is used to allow the swarf to be reused. Note that sheet temperature will affect the choice and performance of the cutting and sawing processes. Lines can be equipped with automatic thickness monitoring devices which traverse the width of the film on C-frames. The output from these devices can be used to control line speed and to perform automatic die adjustment. EXTRUSION CAPACITY. Typical maximum outputs for sheet are shown in the table below. EXTRUDER SIZE (mm) SHEET WIDTH (mm) MAXIMUM OUTPUT (Kg/hr) , , ,200-2, ,800-2,700 1, ,400-3,800 1,350 The output from most sheet lines is usually below the figures above because of cooling limitations for the sheet. The thinner the sheet, the easier it is to cool and hence outputs should be closer to the maximum in these situations. STARTING UP HIPPS presents no special problems and the procedures given for ABS should be used.

6 SHUTTING DOWN. The extrusion plant can be shut down with HIPPS inside it provided the temperatures are reduced to below 150 C as quickly as possible. For cleaning it can be advantageous to purge with GPPS. REPROCESSING. Large proportions of regrind can be used, however it is good practice not to use more than 20% in blends with virgin material. FINISHING. HIPPS can be machined without difficulty but keep the surfaces being cut well cooled. It may be joined together by welding (for example, ultrasonic) and by solvents. The strength of the joint between GPPS and GPPS or between HIPPS and HIPPS is greater than that between GPPS and HIPPS. The material be printed by processes such as screen printing and dry offset. Components can be decorated by hot foiling and painting but care must be taken to select the correct type of paint, as some solvent based types have a tendency to attack or stress crack the moulding where the paint has been applied.