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6 ROTATIONAL MOULDING Colouring Pigments play a major role in many rotationally moulded articles. Indeed there would be a much lower demand for products if they could not be provided in a multitude of colours. One of the benefits of polyethylene is that it can be readily pigmented to provide a very wide range of colours. Apart from aesthetic appeal, pigments can play a vital role by nature of their ability to absorb and reflect light and UV radiation from the sun. The ability to block light is essential for applications such as water tanks. A tank produced from unpigmented polyethylene would allow the transmission of light and provide a haven for algae to grow. Pigments added at sufficient levels can block light sufficiently to stop algae growth. As different pigments will allow different levels of light to pass through polyethylene, the quantity or loading of the pigment will need to be different to achieve the same level of blocking. This is true also when considering the absorption of UV radiation. Most pigments used in the colouration of polyethylene for rotational moulding will assist in resistance to weathering by absorbing UV radiation. It is UV radiation that attacks polyethylene and causes it to become brittle, leading to cracks. Some pigments can have quite a significant benefit. The best example of this is carbon black, the pigment typically used to colour polyethylene black. The right type of carbon black when added at a rate of 2 – 2.5% and well dispersed can enable UV life spans in polyethylene of greater than 20 years. Other pigments in combination with UV stabilisers can provide UV life spans beyond 10 years. For a pigment to be suitable for rotational moulding it needs to possess certain properties. It needs to be safe to use. This is particularly important when considering articles that come in contact with food or water. There is also the question of environmental effect when the article has reached the end of its useful life. During processing the pigment needs to be thermally stable, and in the final application it needs to resist fading. They need to be “light-fast”. Pigments may also have negative effects upon polyethylene. If they are poorly dispersed the resulting concentrated areas of pigment may cause embrittlement of the polyethylene. Due to their nature, pigments can affect the rate at which the material heats and cools. The result is that optimum cooking and cooling times may differ from colour to colour. Different pigments may also affect shrinkage to varying degrees. This may be an important factor when tight dimensional tolerances are required. Some organic pigments increase the crystallization rate by acting as nucleating agents. They may, as a result, contribute to warpage and brittle behaviour. The ability of pigments to absorb radiation from the sun may have another unwanted outcome, a change in dimensions. Radiation absorbed by a pigment may be converted in to heat and raise the temperature of a moulding. This in turn leads to expansion. Such expansion may lead to increased stress in a moulding and reduce its useful life. Other unwanted outcomes may be reduced rigidity and increased creep. The choice of pigment used for colouring polyethylene can play an important role in the performance of mouldings. Pigments can be incorporated in to polyethylene in a number of ways. Dry blending of powder with pigments can give good results, however this practice is not recommended as the pigment may not be properly dispersed and this will reduce the impact resistance and tensile strength of the moulding. High-speed blending in, for example, a Henschel-type mixer, will give better results than simple tumble blending of the powder components. Coloured powders obtained by grinding compounds into which the pigment has been incorporated will lead to mouldings with the best balance of physical properties. This is particularly so for mouldings which will be subject to stress during their use. In the melt compounding process the pigment is premixed in a masterbatch form, which is then mixed with the polymer granules as it passes through an extruder which melts and mixes the two materials together to form a uniform coloured melt. An extruder consists of a long screw which rotates inside a steel barrel. The barrel is heated by electrical heater bands which circle the barrel, and each heater is controlled separately so we can vary the amount of heat in each one. There is a hole in the top of the barrel at the back end, to which the hopper containing the material is fixed. The barrel is open at the front allowing the melted plastic to be pumped out. Attached to the front of the barrel is a die with holes in it and the plastic material passes out of these holes. Typically it is cut into pellets with an underwater chamber called a pelletiser like shown in Figure 3. 6 Qenos Technical GuidesPDF Image | ROTATIONAL MOULDING Guide
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