Rheology
Rheology is the study of deformation and flow. Consider the following examples of flow 1. Squeezing toothpaste through a tube, 2. Tipping honey from a spoon, and The toothpaste, honey and water all have different viscosities. The toothpaste is the most viscous, the water the least. Viscosity means the resistance to flow. In viscous flow, a material continues to deform as long as a stress is applied. To put it more simply, in the case of the toothpaste tube, the toothpaste is squeezed out only...
Case Study Packaging
Recycling rates vary greatly across Europe, due to different legislative and environmental measures adopted by member states. The country at the forefront of implementing plastics recycling schemes is Germany. Packaging waste concepts introduced in Germany in June 1991 have provided much valuable data for the rest of the EU. The target set was 64 to be recycled by July 1995, based on a system of collection, sorting and recycling technology. The high environmental awareness present in the German...
ShortTerm Mechanical Testing
Experiments for measuring the effects of reprocessing have now been discussed. MFI as a method of monitoring degradation has already been mentioned, next the methods of mechanical evaluation will be considered namely tensile and impact properties. Tensile testing can be thought of as a stretching test. Tensile properties are a method used widely to analyse the short-term stress-strain response of a material. Computer controlled tensile test machines are commonly employed, allowing one simple...
References Ive
1. Plastics Recycling in Perspective, APME Technical and Environmental Centre, APME, Brussels, Belgium, 2000. 2. Assessing The Eco-Efficiency of Plastics Packaging Waste Recovery, New Insights Into European Waste Management Choices, Summary Report, APME, Brussels, 2001. 3. L. Bontoux, F. Leone, M. Nicolai and D. Papameletiou, The Recycling Industry in The European Union Impediments and Prospects, EUR 17271 EN, Report Prepared by IPTS for the Committee for Environment, Public Health and Consumer...
Case Study Plastic Lumber
Recycled plastic is finding use as a wood replacement product. It does not splinter, rot or warp like conventional wood. It does not require preservation treatments and its lifetime can exceed over 50 years. There are now a number of plastic lumber type products available ranging from those made of entirely of one material, generally HDPE, to mixtures of plastic, wood, rubber and other materials. A number of factors are pushing the use of plastic lumber, especially in the USA where alternatives...
Polymers
Polymers take their name from two Greek words poli which means many and meros which means parts. This is because they are made up of a number of repeat units, called monomers. The simplest and most commonly used monomer is ethylene. Chemically it consists of two carbon atoms C and four hydrogen atoms H . It can be represented in the two ways shown in Figure 2.1. The lines shown in Figure 2.1 a represent the bonds that exist between the atoms to form a molecule. b CH2CH2 Figure 2.1 An ethylene...
Agglomeration
Agglomeration is a process stage to be carried out after the material is sorted into a fairly homogeneous stream. The aim of the agglomeration is to increase the bulk density of the waste, which is necessary to ensure good feeding for some plastication units. It finds most application in waste plastics with film flake, which can have a bulk density of just 100 to 150 kg m3. This can make both feeding and storage cumbersome. Examples of film waste before and after agglomeration are shown in...
Intrusion Moulding
The intrusion process is suited to mixed plastics. Generally, they are finely ground before processing to aid dispersion. The process has elements of extrusion in that a plasticating unit is used to soften the mixture, which is then fed into a mould and this is then cooled. On rotating systems a number of tools can be filled as others are cooled usually by submerging them in water . Once the plastic has cooled it can be ejected or removed depending on the type of mould that is used. The process...
Rise of the Biopolymers Recycling versus Degradation
In Chapter 7, the conflict between recycling plastics and causing them to degrade in the environment was briefly discussed in the context of oil based plastics. This chapter is devoted to another naturally biodegradable set of materials called biopolymers. The polymers discussed up until now have all been made synthetically, however, polymers also occur naturally and are produced by microorganisms, plants and animals. Polymers that are produced by living organisms are called biopolymers. The...
Physical and Chemical Changes
As well as the effects of heat on the polymer, the effects of shear also need to be considered. Shear is a type of force that involves exerting stress across the surface of the polymer, while in effect, the base of the polymer is untouched. This action is shown in Figure 3.3. The action of shear, like temperature, causes a decrease in viscosity. This is due to the shearing action causing mechanical damage and breakage to the polymer chains. At very high levels of shear this leads to...
Thermal Conversion Technologies
Thermal processing can be defined as the conversion of solid wastes into conversion products with a release of heat energy. It can serve two purposes volume reduction and energy recovery. There are a number of different categories usually distinguished by their air requirements. 1. Pyrolysis thermal processing in the absence of oxygen, 2. Hydrogenation pyrolysis but in a high hydrogen or carbon monoxide environment, and 3. Gasification partial combustion in which a fuel is deliberately...
References
1. Association of Plastic Manufacturers in Europe, http www.plasticseurope.org, accessed September 2007. 2. British Plastics Federation, http www.bpf.co.uk, accessed September 2007 3. WRAP, material plastic index.html accessed September 2007. 4. Recycling and Recovery of Plastics, Eds., J. Brandrup, M. Bittner, G. Menges and W. Michaeli, Carl Hanser Verlag, Munich, Germany, 1996.
Conclusion
An overview of the processes discussed in this chapter is given in Table 6.1. Whilst primary mechanical recycling will continue to be common practice amongst plastics processors, secondary recycling is subject to both practical and economic limits for use. Success depends primarily upon the economics of sorting to obtain single polymer materials, and knowledge of material provenance and degradation history. Processes adapted from conventional machinery to take more highly contaminated feedstock...
Introduction
It would be hard to imagine a modern society today without plastics. Plastics have found a myriad of uses in fields as diverse as household appliances, packaging, construction, medicine, electronics, and automotive and aerospace components. As can be seen from this list, plastic technology can be applied with great success in a variety of ways. So what makes plastic such a versatile material The reason for its success in replacing traditional materials such as metals, wood and glass in such a...
Weld Lines
Whenever molten flows come together a weld line will be produced. Weld lines are common to almost all processing operations as often on passing through extruders or moulds extruder dies, injection moulding tools, blow moulding tools , molten flows will meet obstructions which split material into different streams, this is shown in Figure 6.15. How well the materials can 'knit' back together, will affect the weld line strength. Figure 6.15 Molten material flowing around obstruction Figure 6.15...
Gasification
Gasification technology is based on the use of partial combustion. It was originally developed for use with coal and oil. There are a number of variations, depending upon the type of gases used. These include oxygen in the form of air, steam, pure oxygen, oxygen enriched air or carbon dioxide. The temperature required also depends on the type of fuel. It generally falls in the range 800-1600 C. Gasification is favoured for fuel gas production since a single gaseous product is formed at high...
Chemical Recycling
One other method is chemical recycling, where the polymer is broken down into smaller molecules that can be easily separated from impurities. In chemical recycling, also known as feedstock recycling, plastics are used to make raw materials for petrochemical processes. Common examples of such processes are cracking and hydrogenation. Processes like these place restrictions on the types of materials they can handle and each method has slightly different restrictions and tolerances. Generally,...
TwinScrew Extruders
A twin-screw extruder can have either counter-rotating or co-rotating screws. Co-rotating twin-screw extruders are used mainly for compounding. Counter-rotating screws are generally used in the processing of heat sensitive materials, such as polyvinylchloride PVC pipes and sheets. They are especially suited to heat sensitive materials, because they allow greater temperature control. The aim of both types of screw is plastication, that is, to produce an easily formable melt stream. The...
The Case of Thermosets
In Chapter 2 the reasons that thermosets could not be reprocessed in the same way as thermoplastics were introduced. Thermosets are crosslinked, meaning they cannot be re-melted and reprocessed in the same way as thermoplastics. Until relatively recently thermosets were considered non-recyclable but with recyclability becoming an ever increasing criteria for material selection, the thermoset business faced a serious threat if ways could not be found to overcome this hindrance to market...
Assessing Property Deterioration Caused by Repeated Cycling by Injection
Simple experiments can be performed to assess the effects of processing on properties. Mouldings should be produced using the same conditions as used for full production to get useful and representative results. The following three experimental procedures explore the recycling limitations of a plastic material. They identify the limits of reprocessing, as regards retention of mechanical and rheological properties, and allow the limits of recyclability to be assessed in terms of comparisons of...
Suggested Further Reading Bjz
3. E. S. Stevens, Green Plastics, Princeton University Press, Princeton, NJ, USA, 2002. Association of Plastics Manufacturers in Europe American Society for Testing Materials International Organization for Standardization Method Recycling and Recovery from Composite Materials SPI Society of the Plastics Industry WEEE Waste Electrical and Electronic Equipment Directive WKR Wormser Kunststoff Recycling GmbH, Germany WRAP Waste and Resources Action Program
References Rtu
1. ASTM D5033-00, Standard Guide for Development of ASTM Standards Relating to Recycling and Use of Recycled Plastics, 2000. 2. V. Mattus, M. Neitzel, R. Dittmann, H. Hoberg and H. Wallentowitz, Kunststoffe Plast Europe, 1998, SS, 1, 18. 3. M.J. Bevis, C.E. Bream, P.R. Hornsby, K. Taverdi and K.S. Williams In Proceedings of SPE Antec '98, Atlanta, GA, USA,Volume III, 1998, p.2932. 4. BASF AG, Plastics Recycling - Activities of BASF, BASF Ludwigshafen, Germany, Technical Information for Experts,...
Recycling Techniques
Despite increasing research efforts it is not yet economical to separate the wide variety of plastics that end up in the waste stream. The ideal is streams of single, clean and homogeneous recyclates, which present little technical effort for recycling. Of course, streams of this type, mainly industrial, can and are recycled. However, mixed waste streams present more of a technical and economic challenge to recover. Consider your household rubbish, perhaps even rummage in your bin What plastics...
Compatibilisers
The major problem with mixing plastics is the fact that they are generally immiscible with each other. However useful combinations of immiscible materials are not uncommon. Blending of existing polymers to create new ones is a cost effective method to create new materials, since synthesising new polymers is both expensive and difficult. Blends were first introduced in Chapter 3. A common commercial example of a blend is high impact polystyrene, which is a blend of polystyrene and polybutadiene...