Making things out of plastics is like playing a game with molecules. The aim is to re-organise them into new shapes without their changing colour, sticking to the mould, or doing anything that could spoil the finished article. Additives help with all these problems. In fact, processing plastics without additives is virtually impossible.
Consider the range of plastics available and the wide variety of plastic objects in every day use. From fizzy drinks bottles and carrier bags to window frames and computers, it is clear that there are many different ways of forming plastics. Most of these processes involve melting polymer powder or granules inside a heated tube. This 'melt' is forced through a shaped die, injected into a mould, or rolled or blown into flat film. The ease with which this is done depends on the physical and chemical properties of each plastic material, and can be improved through the use of certain additives known as PROCESS AIDS.
Process aids become liquid during the moulding process and form a liquid around colour particles so that they mix better. Other additives make the individual polymer particles adhere more to each other inside the tube so that they melt quicker. This means that the moulding temperature can be lower which saves energy and prevents or minimises heat damage to the plastics.
Certain plastics can be difficult to process because they become viscous and sticky when they melt. LUBRICANTS help reduce viscosity by creating a film between the mould and the polymer melt, and by lubricating the polymer particles against each other. More intricate shapes can be moulded and the moulding temperature can be lowered. Most plastics have to be processed at over 180oC, a temperature which unfortunately can sometimes spoil the colour and weaken or embrittle the plastic. However, these effects can be prevented or minimised by additives known as ANTIOXIDANTS, which are special compounds such as Vitamin E, which help protect the plastic under hostile conditions. Other additives called HEAT STABILISERS help stop plastics from decomposing during processing.
What is it about plastics objects that catches our attention? Its shape? Its usefulness? Its colour? Plastics are coloured using two main methods. The surface can be painted or printed after moulding, or additives called PIGMENTS can be incorporated before or during moulding. With this method, colour pigments can create all sorts of decorative effects that go all the way through the object and of course, will never wear off. This property, coupled with the range of moulding techniques available, gives designers today a tremendous freedom when working with plastics.
Pigments are tiny particles which have to be evenly mixed into the polymer in its molten state. Colourists specialise in scientifically devising shades suitable for all types of situations. Through a skilful manipulation of additives, plastics components can be colour matched with parts made from other materials such as wood, metal and fabrics, cars radios and kitchen appliances all use this technique.
Different pigment formulations co-ordinate plastics with other materials, as in the Land Rover Discovery (BASF).
There are two vacuum cleaners in a shop. They both look attractive and offer the same attachments, but one is on special offer. Unknown to the customer, the cheaper appliance has been made from plastics components that contain no additives. Does this make any difference? What are additives for anyway? Is the cheaper machine better value?
Without an IMPACT MODIFIER, the vacuum cleaner will crack if it is bumped around the furniture and skirting boards in the usual way. But since it would contain no pigments, it would already look drab and dirty. More worrying would be the lack of FLAME RETARDANTS, for if an electrical spark causes the plastics housing to catch fire, it will burn dangerously without being able to extinguish itself. And this vacuum cleaner is the one on special offer? The other model may cost more, but in the end will prove to be more cost effective. The additives that assist the moulding of plastics, such as lubricants, process aids and heat stabilisers, can cost many times more than the polymeric raw material, and although only small amounts are used, they are nevertheless essential and greatly enhance the final performance of the finished article.
Other additives such as MINERAL FILLERS like chalk, talc and clay, are naturally occurring substances and are cheaper then the raw polymer. However, they are not necessarily used to reduce cost but for the benefits they give to the basic material: talc and chalk increase rigidity, clay improves electrical properties. Mineral fillers also increase the thermal conductivity of plastics so that they heat up and cool down quickly, meaning shorter mould cycle times and more articles produced at lower cost. When a small saving of 0.5p per moulding may not sound much, but if it involves producing several injection mouldings every few seconds this 'small' saving can amount to many tens of thousands of pounds over a year. There is a wide variety of additives available to help reduce costs.Good design in plastics includes the art of combining the inherent safety properties of plastics, such as unbreakable materials, with appropriately designed product features. Rounded edges, child resistant closures and tamper evident seals are examples. Even greater safety factors can be built in through the use of additives.
Most people are probably not aware that all our major plastics are similar in composition to natural polymers such as wood, wool, silk or cotton. They are all based on organic molecules which can catch fire and burn. In some cases this is not a problem, but in other situations it could mean life or death. the building materials that are used to construct our homes, schools and public buildings must be protected against fire by law, which means they must nor ignite or spread flame. Depending on the type of plastics material and the likely hazard, there are many FLAME RETARDANT additives available to help satisfy this requirement.
Tough crash helmets and colour coded occurred regularly when pulleys overheated, causing serious warning lights are produced with additives (Shell).
An excellent example of lives saved by flame retardants in plastics is the conveyor belt in coal mines. For many years fires occurred regularly when pulleys overheated, causing serious accidents and deaths. But when belting made from PVC containing high levels of flame retardants was introduced in the mid-1950s, these accidents stopped.
An increasing amount of plastics is found in cars today. These man made materials are often selected for their safety value. Dashboards and bumper not only have to look stylish and work well, but must also prevent injury by absorbing and dissipating the force of any impact. The additive type used in this case would be an impact modifier. Pigments are additives usually chosen to make plastics look more attractive, but they can also increase the safety factor, such as the special colour-coding for electrical wiring. Designer often use colours to accentuate the controls on machines and day-glow pigments prevent many road accidents: runner and cyclists wear reflective fabrics and strips, while road, rail and building site workers can easily be seen in their fluorescent helmets and jackets.
Hand in hand with advances in medical science, plastics have become an important medium for raising standards of hygiene to the high level we now enjoy. Throughout our lives, plastics help prevent disease and prolong active healthiness. PVC, for example, with its low toxicity, flexibility, clarity and sealing properties, all achieved through additives, has become one of the most important plastics in medicine.PVC tubing, blood transfusion sets and disposable packs of equipment have helped create an easier and healthier lifestyle. Additives have allowed the development of a sealed system of PVC tubing and bags which separates blood into its constituents without the need to open the equipment. This has led to a whole new field of blood component therapy. Plasticsed PVC medical bags help save lives.
To prevent plastics materials from becoming hard and brittle at low temperatures, or soft and sticky at high temperatures, additives are used to 'design' the plastics for its specific end use. Today plastics containers can be transferred from freezer to microwave and remain virtually unbreakable and safe in all conditions.
Polythene, and later polypropylene, inspired a post-war revolution in kitchenware. Now we have something even more versatile and good-looking - Clingfilm - which can prevent micro-organisms spoiling fresh or cooked foods. To do this the polymer is formulated with suitable additives such as plasticisers and non-toxic stabiliser systems. The plasticiser makes the film very clingy so that bacteria cannot get into the food, yet the film can also be permeable so that oxygen and water vapour can pass through for safer storage.Most plastics are by nature insulators, and this property has encouraged the development of a myriad of electrical products which are safe and a pleasure to use: telephones, shavers, hairdryers, radios, TV sets and food processors. Their plugs, sockets and wires are of course all insulated with plastics as well. However, being non-conductive, static can build up in plastics which attracts annoying dirt and dust. The additive that helps to counteract that problem is known as an ANTI-STATIC AGENT.
Many natural materials decay with age - even granite erodes over the years. In the world of plastics measures are being taken to protect products from the effects of time. In extending the life and service of plastics, the unseen protection given by additives can result in materials which lead to new products for even more demanding situations. Imagine the conditions that plastics are subjected to - heat light, electric current, water weathering, cold, and knocks and kicks of continual use in the home, office factory or field. Effectiveness is critical.
Cars are good examples. The next time you are out in a car, take a look at the exterior and interior and under the bonnet. Not only will you find more components than ever before made of plastics, but many of them - from bumpers and wheel trim to steering wheels and door panels - have to endure long exposure to the elements, as well as flying stones, scrapes, knocks and wear. Look at the engine with its plastics mouldings, cables and tubes. Conditions are tough, temperatures are high, yet plastics components still have to - and do - work effectively. In all these cases the longer life additives are silently at work.
Non-fading pigments will help these Spectrum stadium seats to hold their colour for many years (Hille).
Children's toys and garden furniture, packaging and flooring are just some of the products that form the backdrop to our lives, and it is hard to overestimate the rough treatment they have to endure. In sports stadiums, more and more spectator seating is moulded in brightly coloured plastics, and playing surfaces are often made of synthetic fibres. All of these are exposed to the weather, day and night, summer and winter, but a combination of LIGHT STABILISERS, UV ABSORBERS and ANTIOXIDANTS ensure constant high performance. Natural materials usually have to be finished off after manufacture with paints and lacquers. plastics enjoy the advantage of incorporating before or during the moulding process the additives that prolong their useful lives for many years.We all tend to think that plastics consume energy. In fact plastics actually help to save energy in many different ways. For example plastics in cars save European motorists some six billion litres of fuel per year. This is because plastics have replaced heavier metal parts and inspired designers to create wind-cheating aero-dynamic shapes that cut down on fuel consumption. Without additives to give plastics strength and durability, this would not be possible.
When certain plastics are moulded at around 220oC, particular additives called BLOWING AGENTS break down to form gases such as nitrogen, carbon dioxide and water. These gases, trapped in the plastics, turn the material into foam, thus increasing the insulation and energy absorption properties and reducing weight. These foams can be seen in everyday use such as protective food packaging, cushioning in sports shoes and in automobile parts where lower weight saves fuel.
Three stages in the life cycle of time controlled degradable mulching film: begin the growing season, partlydegraded film after harvesting and the film after ploughing. (Prof. G Scott, University of Aston)
Throughout the world crop yields are boosted by plastics film laid over the soil to trap heat and moisture. Tomato production, for example, can be increased by 300%. But what happens to the plastic sheet at the end of the growing season? Additives have been developed that allow the sheet to capture the sun's warmth during the growing season but to break up as soon as the harvest arrives. The sheets disintegrates due to the action of sunlight and the fragments can be ploughed into the soil where the soil bacteria quickly breaks them down into carbon dioxide and water. In areas of predictable climate, this process can be timed to an accuracy of within seven days. Where plastics cannot be reused or recycled, biodegradation could offer a clean, safe method of disposal.
Plastics waste disposal can cause problems, especially as plastics are usually mixed up with other types of waste such as paper, metals and food. For recycling they really need to be sorted into individual polymer types such as polythene, polystyrene or polyvinyl chloride, otherwise they have no strength if remoulded and may literally fall apart. This is an area in which additives called COMPATIBILISERS can help. They act like chemical adhesives, sticking the different waste plastics material together so that a reasonable amount of cross-blending can be accepted. Mixed plastics waste can be remoulded into fencing, pallets and road markers, thus saving valuable timber. All this comes from a waste product that would other wise have been buried in a landfill site. Additives are vital for reprocessing waste plastics into useful second generation products.
4) Additives: A Buyers Guide
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