Technology & Engineering
Classes of Polymers
Polymers can be classified into several categories based on their structure, properties, and applications. These classes include thermoplastics, thermosetting polymers, elastomers, and fibers. Thermoplastics can be melted and reshaped, while thermosetting polymers become permanently hardened after curing. Elastomers are known for their elasticity, and fibers are long, thread-like polymers used in textiles and composites.
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12 Key excerpts on "Classes of Polymers"
eBook - PDFFundamentals of Modern Manufacturing
Materials, Processes, and Systems
- Mikell P. Groover(Author)
- 2016(Publication Date)
- Wiley(Publisher)
The word is derived from the Greek words poly, meaning “many,” and meros (reduced to mer), meaning “part.” Most polymers are based on carbon and are therefore organic chemicals. As engineering materials, polymers are relatively new compared to metals and ceramics (see Historical Note 8.1 at www.wiley.com/college/groover). Polymers can be classified into three types: (1) thermoplastic polymers, (2) thermosetting polymers, and (3) elastomers. Thermoplastic polymers (TP), also called thermoplastics, are solid materials at room temperature, but they become viscous liquids when heated to temperatures of only a few hundred degrees. This characteristic allows them to be easily and economically shaped into products. They can be subjected to this heating and cooling cycle repeat- edly without significant degradation. Thermosetting polymers (TS), or thermosets, cannot tolerate repeated heating cycles as thermoplastics can; when initially heated, they soften and flow for mold- ing, but the elevated temperatures also produce a chemical reaction that hardens the material into an infusible solid. If reheated, thermosetting polymers degrade and char rather than soften. Elastomers (E) are polymers that exhibit extreme elastic extensibility when subjected to relatively low mechan- ical stress. Some elastomers can be stretched by a factor of 10 and yet completely recover to their original shape. Although their properties are quite different from thermosets, they have a similar molecular structure that is different from the thermoplastics. In popular nomenclature, thermoplas- tics and thermosets are known as plastics and elastomers are known as rubbers. Thermoplastics are commercially the most important of the three types, constituting around 70% of the tonnage of all synthetic polymers produced. Thermosets and elastomers share the remaining 30% about evenly. Common TP polymers include polyethylene, polyvinylchloride, polypropylene, polystyrene, and nylon.
eBook - PDF- Carlos J. Hilado(Author)
- 1998(Publication Date)
- CRC Press(Publisher)
SECTION 1.4. CHEMICAL CLASSIFICATION Polymers have traditionally been divided into two general groups: thermoplastic and thermosetting. 4 MATERIALS FOR THE PLASTICS INDUSTRY Thennoolastic polymers are capable of being repeatedly softened by increase of temperature and hardened by decrease of temperature, the change upon heating being physical rather than chemical. Thennosening polymers are capable of being changed into a substantially infusible or insoluble product when cured by application of heat or chemical means . The division between thennoplastic and thennosening polymers is now less clearly defined, because polymers are manufactured and modified and blended to give an increasing range of properties. There are thennoplastic and thennosetting polyurethanes, thermoplastic and thennosening polyesters, and thennoplastic and thennosening polyimides. There are thennoplastic elastomers, which bring together the traditionally separate disciplines of thennoplastics and thennoset rubber. The major types of thennoplastic materials are olefm, including polyethylene (PE) and polypropylene (PP); vinyl, including polyvinyl chloride (PVC), polystyrene (PS), acrylonitrile-butadiene-styrene (ABS), and acrylic; cellulosic, including cellulose nitrate, acetate, butyrate, and propionate; and aromatic, including polycarbonate (PC), phenoxy, polysulfone, and polyimide. Other thennoplastic materials are polyamide (nylon), acetal, and thennoplastic polyester. The major types of thennosetting materials are polyurethane; phenolic (including phenol-formaldehyde); amino (melamine-formaldehyde and urea-formaldehyde); epoxy; and unsaturated polyester. Estimated production by region of the different types of plastics is shown in Table 1.3. Some major markets for the different types of plastics are shown in Table 1.4. SECTION l.S. OLEFIN POLYMERS Olefin polymers are the plastics produced in the largest quantity, and are produced all over the world.
eBook - PDF- Saeed Farrokhpay(Author)
- 2023(Publication Date)
- Arcler Press(Publisher)
In the next section, we will look at some instances that are of commercial relevance, albeit we will focus on their characteristics instead of the complete chemistry of its industrial manufacture. The thermosets and thermoplastics are the two primary types of polymers classified by their properties. As a result of the categorization previously indicated, it has also been discovered that the polymers of most economic relevance are thermoplastics or addition polymers instead of thermosets, and it would be talked 1 st about such polymers (Ringsdorf, 1975; Garbarczyk and Paukszta, 1985). 4.5. THERMOPLASTIC POLYMERS Several thermoplastic polymers are explored in-depth in the subsections (Novikov and Kozlov, 2000). 4.5.1. Poly(Ethylene) Such polymers contain one of the most basic molecular configurations ([CH 2 CH 2 —] n ), which is now the greatest tonnage plastic substance, have been commercially produced in 1939 for usage in electrical insulation. A polymer’s nomenclature is hard to comprehend. Ethane, instead of ethylene, is the IUPAC-recommended term for the monomer. As a result, the polymer’s IUPAC name is poly(ethene) (Aranda and Ruiz-Hitzky, 1992; Israelachvili, 1997). Consequently, while working with the substance, scientists have never utilized this nomenclature; consequently, this polymer would be referred to by its more comprehensive designation, poly(ethylene) (Figure 4.4). Figure 4.4. The structural formula of polythene. Source: https://omnexus.specialchem.com/selection-guide/polyethylene- plastic. The Fundamentals of Materials Chemistry 94 There are four major industrial ways for preparing poly(ethylene), each of which produces products with somewhat various qualities. The four techniques are (Nucci et al., 1991; Busico and Cipullo, 2001): • Ziegler procedures; • High-pressure procedures; • The standard oil (Indiana) procedures; • The Phillips procedures. The Ziegler procedures are based on metal alkyl complexes catalyzing coordination interactions.
eBook - PDF- J Maxwell(Author)
- 1994(Publication Date)
- Woodhead Publishing(Publisher)
Understanding plastics Classifying plastics The term ‘plastics’ covers a broader variety of species than the term ‘metals’. Hence, without effective classification, the possibilities for over- simplification and confusion are endless. In terms of mechanical performance in automotive applications, and in particular the response to stress, probably the most important distinction is between unfilled plastics and composites. In automotive plastics terms, ‘composite’ means plastics reinforced with fibres, usually glass. Nowadays there is a very extensive range of distinct materials answering to this description; their significance is discussed later. Perhaps the most confusing aspect of plastics to the non-specialist is the profusion of raw material forms and processes involved. These are classified and reviewed in this chapter, together with the all-important questions of economics and ‘The numbers game’. Polymers can themselves be classified in a number of ways: - Thermoplastics and thermosets. - Chemical structure. - Crystalline and amorphous thermoplastics. Thermoplastics and thermosets Essentially, polymers can be defined as either thermoplastic or thermo- setting. Although they often display similar properties, and indeed in the automotive industry frequently compete for the same applications, there are fundamental differences in structure and processing methods. These differ- ences have implications for production costs and feasibility. Thermoplastics are softened and melted by heat, and shaped or formed before being allowed to freeze again. The heating and freezing processes can be repeated many times without significant chemical change (although not without some deterioration, usually attributable to the breaking of molecular chains). 13 14 Plastics in the automotive industry Thermosetting materials, when heated above a critical temperature, undergo chemical reaction involving cross-linking between chains.
eBook - PDF- P.E., Philip A. Schweitzer(Authors)
- 2006(Publication Date)
- CRC Press(Publisher)
1 Introduction to Polymers Plastics are an important group of raw materials for a wide array of manufacturing operations. Applications range from small food containers to large chemical storage tanks, from domestic water piping systems to industrial piping systems that handle highly corrosive chemicals, from toys to boat hulls, from plastic wrap to incubators, and a multitude of other products. When properly designed and applied, plastic provides light weight, sturdy/economic/resistant, and corrosion products. Plastics are polymers. The term plastic is defined as “capable of being easily molded,” such as putty or wet clay. The term plastics was originally adopted to describe the early polymeric materials because they could be easily molded. Unfortunately, many current polymers are quite brittle, and once they are formed they cannot be molded. In view of this, the term polymer will be used throughout the book. There are three general categories of polymers: thermoplastic polymers called thermoplasts , thermosetting polymers called thermosets , and elastomers called rubbers . Thermoplasts are long-chain linear molecules that can be easily formed by heat and pressures at temperatures above a critical temperature referred to as the glass temperature . This term was originally applied to glass and was the temperature where glass became plastic and formed. The glass temperatures for many polymers are above room temperature; therefore, these polymers are brittle at room temperature. However, they can be reheated and reformed into new shapes and can be recycled. Thermosets are polymers that assume a permanent shape or set when heated; although, some will set at room temperature. The thermosets begin as liquids or powders that are reacted with a second material or that through catalyzed polymerization result in a new product whose properties differ from those of either starting material.
eBook - PDF- George Murray, Charles V. White, Wolfgang Weise(Authors)
- 2007(Publication Date)
- CRC Press(Publisher)
1 1 Classification of Materials 1.1 INTRODUCTION Advancements in technology in most industries have been associated with the development of new materials and processes as well as advances in the state of the art of existing materials and combinations thereof. It is estimated that currently about 85,000 materials are available for industrial applications. In considering which materials to use for a particular structure or device, the selection process is further complicated by the wide variation in properties of materials with the manner in which the material is processed, for example, the heat treatment time, temperature, and cooling rate used for certain alloys. Some type of materials classification is an essential part of the selection process and an important element of engineering education. In the following sections, materials will be classified in the broad categories of metals, polymers, ceramics, composites, and semiconductors. In subsequent chapters, these materials and their associated processing methods will be pre-sented in more detail. In the design and material selection procedures, items such as the recycling potential of the material and environmental problems must be considered. 1.1.1 R ECYCLING Recycling is generally considered to be a part of solid waste management strat-egies. Any strategy devised must be incorporated in the material selection and design steps. Polymers are of much concern in recycling because many are nonbiodegrad-able and consist of about 20% of municipal solid waste. The thermoplastic polymers, which are easily formed and are abundant in packaging materials, have been given the most attention in terms of recycling technology. Representing about 90% of all plastics sold, they consist primarily of polyethylene, polyeth-ylene terephthalate (PET), polystyrene, polypropylene, and polyvinyl chloride (PVC).
eBook - PDF- Michael R Fisch(Author)
- 2004(Publication Date)
- WSPC(Publisher)
When we discuss polymers in more detail, you will see the role carbon plays in polymers. We will begin by discussing the basic chemistry of polymers. An impor- tant difference between polymers and materials such as fluids or metals is that polymers are long-chain or network type molecules. We will illustrate this using cartoons. We will then discuss the major types of polymers. From the materials viewpoint, there are two main types of polymers: ther- moplastic polymers that become less rigid when heated, and thermosetting polymers that become more rigid when heated. Both types of polymers are often used with additives that improve color, strength, and other physical and chemical properties. 9.3 What is a polymer? The term polymer literally means “many parts.” It is derived from Greek where poly means many and meros means unit or part; that is the simple, repeated building block of the chain or network. Thus, a polymer is a large molecule made up of many smaller and simpler chemical units covalently bonded together. For example, polyethylene (CH3-(CHz),-CH3) is a long chain molecule composed of ethylene molecules (CHz=CH2), Notice how- ever that the terminal groups are CH3. This is a fairly standard occurrence - the terminal moieties2 are frequently different from the central moieties that make up the polymer. Moreover, the two terminal groups need not be the same. Molecules with these general properties, that is, long chain molecules consisting of three different moities, a distinct one at each end and many repetitions of the central moiety, are ubiquitous. Such molecules occur in nature, and are also synthetically produced. This chapter will primarily focus on artificially produced polymers. Syn- thetic polymers often have a central structure of the form: -A-A-A-A-A-A-or -A-B-A-B-A-B-. –B—–B– .... ‘A plastic deformation is a permanent deformation that does not change upon re- ’A moiety is a sub-section of a molecule that has characteristic properties.
- Donald Askeland, Wendelin Wright, Donald Askeland(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Polymers also have many useful physical properties. Some polymers, such as acrylics like Plexiglas™ and Lucite™, are transparent and can be substituted for glasses. Although most polymers are electrical insulators, special polymers (such as the acetals) and polymer-based composites possess useful electrical conductivity. Teflon has a low coefficient of friction and is the coating for nonstick cookware. Polymers also resist corrosion and chemical attack. 16-1 Classification of Polymers Polymers are classified in several ways: by how the molecules are synthesized, by their molecular structure, or by their chemical family. One way to classify polymers is to state if the polymer is a linear polymer or a branched polymer (Figure 16-1). A linear polymer consists of spaghetti-like molecular chains. In a branched polymer, there are primary polymer chains and secondary offshoots of smaller chains that stem from these main chains. Note that even though we say “linear,” the chains are actually not in the form of Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 573 16-1 Classification of Polymers Figure 16-1 Schematic showing linear and branched polymers. Note that branching can occur in any type of polymer (thermoplastics, thermosets, and elastomers). (a) Linear unbranched polymer: notice chains are not straight lines and not connected. Different polymer chains are shown using different shades designed to show clearly that each chain is not connected to another. (b) Linear branched poly- mer: chains are not connected; however, they have branches.
eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
They have very high molecular weights (10 4 –10 6 ) and hence are called macromolecules. 9. Based on chemical composition (i) Organic polymers A polymer whose backbone chain is made mainly of carbon atoms and the side chains consist of oxygen, nitrogen, sulphur, etc. Examples include polythene, PVC, nylon, etc. (ii) Inorganic polymers These polymers do not have a carbon backbone chain. Examples include silicone rubbers, phosphazene, etc. The classification of polymers is summarised in Figure 6.1. Polymers 357 Classification of Polymers Based on Origin Based on Monomer Composition Based on Chain Structure Based on Chain Composition Based on Polymerisation Mechanism Based on Molecular Forces Based on Tacticity Based on Degree of Polymerisation Based on Chemical Composition 1. Homopolymer 2. Copolymer (i) Random copolymers (ii) Alternating copolymers (iii) Block copolymers (iv) Graft copolymers 1. Linear polymers, e.g., polyethene 2. Branched polymers, e.g., low density polyethene 3. Cross-linked or network polymers, e.g., bakelite 1. Homochain polymer, e.g., Polyethene, PVC 2. Heterochain polymer, e.g., nylon terylene 1. Addition polymers, e.g., Polyethene PVC 2. Condensation polymer, e.g., nylon 1. Elastomers, e.g., rubber 2. Fibres, e.g., nylon 3. Thermoplastics, e.g., polyethene 4. Thermosetting plastic, e.g., bakelite 1. Isotactic 2. Syndiotactic 3. Atactic 1. Oligopolymers 2. Macromoleules 1. Organic polymers, e.g., polythene PVC 2. Inorganic polymers, e.g., Silicone rubbers Phosphazene, etc. Natural polymers, e.g., starch cellulose Synthetic polymers polyethene, e.g., PVC, nylon, etc. Figure 6.1 Classification of polymers 358 Engineering Chemistry: Fundamentals and Applications 6.3 Functionality The number of reactive sites in a molecule is termed as its functionality. For a molecule to act as a monomer it must have at least two reactive sites, that is, its functionality should be two. For example, ethylene can add two molecules of hydrogen or halogen.
- A.A. Collyer(Author)
- 2016(Publication Date)
- Elsevier Science(Publisher)
polymerization process. The molecules in a given plastic material are most unlikely to be Table 1.1: Comparison of the mechanical properties of several types of engineering materials [2-4] Material Specific gravity (gem 1 ) Modulus (GNm 2 ) Specific modulus (MNmkg 1 ) Strength (MNm 2 ) Specific strength (kNmkg 1 ) Aluminium Mild Steel Brass (70 C u / 3 0 Zn) Nylon 66 Polycarbonate Nylon 6 6 / 30% glass 2.7 7.86 8.5 1.14 1.24 1.38 71 210 100 3 2.3 8 26 27 12 2.6 1.9 5.8 80 460 550 80 60 160 30 59 65 70 48 116 2 Thermoplastics as Engineering Materials of exactly the same length and a molecular weight average, Mw, is generally quoted for a given grade. The molecular weight average has an important eflfect on the particular polymer: if Mw is very large the mechanical properties of the polymer will be enhanced but it will be difficult to process whereas, if Mw is low, there will be inferior mechanical properties, but good processing characteristics. A compromise is necessary here. In a plastic material the polymer chains can coil and uncoil, entangle and align. When a stress is applied to a polymer sample, the chains uncoil and align over a period of time (time-dependent properties) and, if the temperature is high enough, the molecules will slip over one another (flow). The above is true only of thermoplastic materials, which can be softened repeatedly by raising the temperature. Thermosetting materials start as thermoplastics but the first time they are heated crosslinks are formed. These give a three-dimensional network structure that is not broken down by further heating. Elastomers too behave like thermosets but are distinguished from them by their large capacity for extension. Further discussion will be limited to thermoplastics. In Figure 1.1, the lower curve shows how the elastic modulus of an amorphous thermoplastic material varies with temperature.
eBook - PDFFundamentals of Modern Manufacturing
Materials, Processes, and Systems
- Mikell P. Groover(Author)
- 2019(Publication Date)
- Wiley(Publisher)
The reason for this property is that TP polymers consist of linear (and/or branched) macromolecules that do not cross-link when heated. By contrast, thermosets and elastomers undergo a chemical change when heated, which cross-links their mole- cules and permanently sets these polymers. In truth, thermoplastics do deteriorate chemically with repeated heating and cooling. In plas- tic molding, a distinction is made between new or virgin material, and plastic that has been previously molded (e.g., sprues, defective parts) and therefore has experienced thermal cycling. For some applications, only virgin material is acceptable. TP polymers also degrade gradually when subjected to continuous elevated temperatures below T m . This long-term effect is called thermal aging and involves slow chemical deterioration. Some TP polymers are more suscepti- ble to thermal aging than others, and for a given material the rate of deterioration depends on temperature. MECHANICAL PROPERTIES In the discussion of mechanical properties in Chapter 3, polymers were compared with metals and ceramics. The typical thermoplastic at room temperature is charac- terized by the following: (1) much lower stiffness, the modulus of elasticity being two or more orders of magnitude lower than metals and ceramics; (2) lower tensile strength, about 10% of the metals; (3) much lower hardness; and (4) greater ductility on average, but there is a tremendous range of values, from 1% elongation for polystyrene to 500% or more for polypropylene. Mechanical properties of thermoplastics depend on temperature. The functional relationships must be discussed in the context of amorphous and crystalline structures. An amorphous thermo- plastic is rigid and glass-like below its glass-transition temperature T g and flexible or rubber-like just above it.
eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
Typical medical applications include Polymers 377 • Use in orthopaedic fixation devices. Biodegradable polymers are used to join fractured bones. They provide the required strength to the bone and after the bone heals and can take the load, they harmlessly degrade over time. This prevents the patient from an additional operation for removal, as would be required if a metallic device was used. • In controlled drug delivery, the polymer slowly degrades into smaller fragments releasing the drug gradually and in a controlled manner. • It is used in dental devices, biodegradable vascular stents, biodegradable soft tissue anchors, etc. Biodegradable polymers are also being increasingly used in the manufacture of plastic bags, toys and other plastic products. These polymers gradually degrade in the environment (or in landfills) and leave the environment ‘greener’. 6.12 Properties of Polymers The properties of a polymer largely depend on their structural characteristics. Properties like tensile strength, elasticity, crystallinity, chemical reactivity, electrical properties, etc, depend upon various structural characteristics like chemical composition, sequence of monomer units, spatial arrangement of side groups, nature of packing, molecular weight, etc. The various properties of polymers in light of their structural variations are discussed below 1. Mechanical properties Properties like strength, elasticity and hardness depend on the crystallinity of the polymer, whereas tensile strength and impact resistance are related to the molecular weight of the polymer. (i) Strength Strength of a polymer depends upon the magnitude and distribution of intermolecular forces of attraction between the polymeric chains. Branched and straight chain polymers are held together by weak intermolecular forces of attraction. The magnitude of these forces increases with increase in length of the polymeric chain or the degree of polymerization.
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