Basic Principles of Plastics

Key Questions What types of polymeric materials (plastics and 

How do they behave imder the influence of heat How does melt viscosity vary with temperature Which plastics are used in injection molding  

Contents i.i Classification and Identification of Polymeric Materials (Plastics and Elastomers) 

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It may be useful to recall some facts concerning the environmental impact of plastics. Sustainability, pollution minimization, recycling and safe elimination of wastes are basic principles of environment policy. 

Chapter 14 provides the basic principles of polymer science, and addresses the importance of this subject. This chapter aims to give a broad and imified description of the subject matter—describing the polymerization reactions, structures, properties, and applications of commercially important polymers, including those used as plastics, fibers, and elastomers. This chapter focuses on synthetic polymers because of the great commercial importance of these materials. The chemical reactions by which polymer molecules are synthesized are addressed along with the process conditions that can be used to carry them out. This chapter also discusses topics on degradation, stability, and environmental issues associated with the use of polymers. 

Polymer-dispersed liquid crystals are the next generation of display materials. Initial research has outlined the basic principles of their operation The PDLC films combine the properties of plastics and liquid crystals producing display devices impossible with conventional materials. They wfil be used in applications ranging from architectural glass to projection TV and optical computing. 

Theory of rolling The basic principle of compaction of particulate solids between two counter-currently rotating rollers (Figure 230) is similar to that used in calenders for plastic foils or in rolling mills for metals. The first can be adjusted to extremely narrow gap tolerances across press rollers with face widths of up to 2 meters and production speeds of approximately lOOm/min in the latter enormous pressing forces can handle ingots of more than 35 tons weight. 

When articles hitherto produced from classic materials (such as wood or metal) are to be made by the injection molding process, it is advisable, to give serious consideration to the special properties of plastics materials, as well as the more salient points of injection molding as a production method. This plastics-oriented procedure requires an understanding of the basic principles of the manufacturing and production processes involved, and also of the plastics material s behaviour. The book provides a comprehensive overview of the principles involved in injection molding. 

Below, we consider first the basic principles of toughening of brittle glassy polymers that clarify the role of the different particle morphologies, and of their flexibility and size, as well as volume fraction, that act together in maximizing craze plasticity at levels of flow stress that avoid craze-matter breakdown. 

There are approximately nine companies producing lumber from mixed plastics collection programs, with three in the midwest area. While most companies possess a proprietary machine or license, the basic principles of each machine are the same. Postconsumer bottles or rigid plastics must first be ground and films must be densified and ground. The material is fed into an extruder barrel, forced through a die and cooled. Additives may be provided in the feed to enhance properties or set the color. LDPE,... 

Before reviewing in detail the fundamental aspects of elastomer blends, it would be appropriate to first review the basic principles of polymer science. Polymers fall into three basic classes plastics, fibers, and elastomers. Elastomers are generally unsaturated (though can be saturated as in the case of ethylene-propylene copolymers or polyisobutylene) and operate above their glass transition temperature (Tg). The International Institute of Synthetic Rubber Producers has prepared a list of abbreviations for all elastomers [3], For example, BR denotes polybutadiene, IRis synthetic polyisoprene, and NBR is acrylonitrile-butadiene rubber (Table 4.1). There are also several definitions that merit discussion. The glass transition temperature (Tg) defines the temperature at which an elastomer undergoes a transition from a rubbery to a glassy state at the molecular level. This transition is due to a cessation of molecular motion as temperature drops. An increase in the Tg, also known as the second-order transition temperature, leads to an increase in compound hysteretic properties, and in tires to an improvement in tire traction... 

Polymerization. To understand the principles of plastics engineering, one should have an appreciation of the basic principles of polymerization. Polymers begin with specific molecules that are referred to as monomers. These contain a functional structure that allows them to form chemical bonds successively between individual molecules in a chain reaction. In a chain reaction, a molecule forms a bond to a second molecule, enabling the second to bond to a third, the third to bond to a fourth, and so on. Successive bondforming reactions typically occur several thousand times before some condition is encountered that terminates the progress of the reaction chain. In linear polymers, this reaction process forms molecules that have a structure essentially as simple as that of their monomers but are much larger in size. 

You will learn the basic principles of injection molding and how to design and make your own molds. You Will start by making a simple mold to test the machine. Then you will mold a plastic knob for the machine itself. Next, you ll progress to a mold that creates a small plastic container with a snap lid. 

Fabric, fibres and filaments are obviously important forms and uses of plastics materials. Although the basic principles of bonding plastics apply to fibres and fabrics, the other factors involved in bonding them are a subject in themselves and will not be discussed further. 

The basic principle of the installation of buried plastic piping is to embed it in a soil of such quality that the resultant ultimate pipe deflection is controlled to an acceptable value that is limited by either the pipe performance requirements or the pipe material capabilities. The former generally permits deflection up to about 10 percent (some engineers may set conservatively lower values) while the latter is determined by the maxunum allowable stress, or strain, in the pipe wall for the given pipe material and construction.The pipe supplier can provide the limiting deflection values for a given pipe material and construction. These values may depend on the fluids being handled. With thermoplastic pipe of solid wall construction deflection will seldom be limited by material performance constraints. 

The basic principles of rheology and the various experimental methods that can be applied to investigate these complex systems of food colloids have been discussed in detail in Chapter 7. Only a brief summary is given here. Two main types of measurements are required (1) Steady-state measurements of the shear stress versus shear rate relationship, to distinguish between the various responses Newtonian, plastic, pseudo-plastic and dilatant. Particular attention should be given to time effects during flow (thixotropy and negative thixotropy). (2) Viscoelastic behaviour, stress relaxation, constant stress (creep) and oscillatory measurements. 

The basic principles of solvent extraction, etc., for the analysis of plastics formulations are well established and documented [1 ] and are still valid today. However, changes and improvements have been made, particularly in the fields of separation science and spectroscopy, to incorporate new ideas and instrumentation and to accommodate new additives. Despite such trends as robotics, the need for an experienced practical plastics analyst working at a bench using solvents, extraction flasks and beakers still exists. 

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