Polymeric materials: elastomers, plastics, fibers, composites, nanocomposites and blends

3 Polymeric materials: elastomers, plastics, fibers, composites, nanocomposites and blends A T S U N N Y and S T H O M A S, Mahatma Gandhi University, India

Abstract: The advances in the area of polymer science and technology provide a rich set of materials useful for probing the fundamental nature of matter. This chapter presents a brief overview of various polymeric materials including elastomers, plastics, fibers, their blends and composites, their major processing techniques and applications. A new generation of multicomponent polymeric materials is emerging; it now remains to be seen how the practical application of these materials will grow from the seeds over the coming years. Key words: polymer processing, polymer blends, composites, nanocomposites.

3.1 3.1.1

Overview of polymeric materials Introduction

If an era is known by the kinds of materials its people use to build the world they live in, such as the Stone Age, Bronze Age, etc., our own could be called the Polymer Age. Polymers form much of our packing and wrapping materials, bottles, containers, textiles, building materials, furniture, glues and adhesives, automobile parts, parts of electrical equipment and finally personal items including pens, razors, toothbrushes, etc. We even use polymer bags to discard our polymer trash. Almost every ordinary thing we come into contact with each day contains some kind of polymer somewhere in or around it, or comes to us wrapped in a polymer. The combination of the Greek words poly meaning ‘many’ and meros, ‘parts’, gives us the word for the molecule that composes these substances, polymer. All the substances referred to as polymers are giant molecules with molar masses ranging from several thousands to several millions, composed of a great many much smaller parts joined together through chemical bonds in any conceivable pattern. The individual parts that combine to form them, monomers, from the Greek word mono, join to each other in enormously large numbers to produce polymers with molecular weights ranging from tens of thousands to millions 47

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of atomic mass units. These can be in the form of chains, sometimes as sheets, sometimes as intricate, three-dimensional lattices. The essential requirement for a small molecule to quantify as a monomer or ‘building block’ is the possession of two or more bonding sites, through which each can be linked to other monomers to form the polymer chain. The number of bonding sites is referred to as the functionality. Table 3.1 lists examples of monofunctional as well as multifunctional monomers [1]. A process used to convert monomer molecules into a polymer is called polymerization and the two most important groups are step-growth and addition. A step-growth polymerization is used for monomers with functional groups such as –OH, –COOH, –COCl, etc., and is normally, but not always, a succession of condensation reactions. Consequently the majority of the polymers formed in this way differ slightly from the original monomers because a small molecule is eliminated in the reaction, e.g., the reaction between ethylene glycol and terephthalic acid produces a polyester better known as terylene. Depending on its processing and thermal history, it may exist both as an amorphous (transparent) and as a semicrystalline (opaque and white) material. Its monomer can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or the transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a by-product. Polymerization is through a polycondensation reaction of the monomers (done immediately after esterification/transesterification) with ethylene glycol as the by-product (the ethylene glycol is recycled in production) [2]. The addition polymerizations are chain reactions which convert the monomers into polymers by stimulating the opening of the double bond with a free radical or ionic initiator. The product then has the same chemical composition as the starting material, e.g. acrylonitrile CH2=CH(CN) produces polyacrylonitrile (PAN) –[CH2–CH(CN)]– without the elimination of a small molecule. The length of molecular chains, which will depend on the reaction conditions, can be obtained from measurements of molar masses [1, 2]. One of the most important features that distinguishes a polymer from a simple molecule is the inability to assign an exact molar mass to a polymer. This is a consequence of the fact that in a polymerization reaction the length of the chain formed depends entirely on random events. In the condensation reaction it depends on the availability of a suitable reactive group, and in an addition reaction on the lifetime of the chain carrier. Because of the random nature of the chain growth process, the product is a mixture of chains of differing length – a distribution of chain lengths which in many cases can be calculated statistically. While computing the molecular weight of a polymer we can use either the number fraction or the weight fraction for the averaging method [3, 4]. There are different methods for measuring average molecular weight, and they do not all give the same answer. The number average

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Table 3.1 Functionality and chemical formula of monomers of some common polymers Functionality of monomers

Chemical formula

Functional groups present per molecule

Monofunctional

Acetic acid Ethyl alcohol Hexyl amine Methyl isocyanate

—COOH —OH —NH2 —NCO

Bifunctional

Hexamethylene diamine

—NH2 —NH2

Ethylene glycol

—OH —OH

Terephthalic acid

—COOH —COOH

Glycine

—COOH, —NH2

Tricarballylic acid

—COOH —COOH —COOH

Trimethylol propane

—OH —OH —OH

Lysine

—COOH, —NH2 —NH2

Glutamic acid

—COOH —COOH —NH2

Pentaerythritol

—OH —OH —OH —OH

Tartaric acid

—COOH —OH —OH —COOH

Gallic acid

—COOH —OH —OH —OH

Hydroxyglutamic acid

—COOH —OH —NH2 —COOH

Trifunctional

Tetrafunctional

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molecular weight, Mn, is the definition of molecular weight that would be expected from the common definition for ‘mean’ or ‘average’. The weight average molecular weight, Mw, gives a result that is greater than Mn, but this is of importance because light scattering experiments give results that follow the formula for weight average molecular weight, rather than number average molecular weight [3, 4]. Realistically a polymer chain is better represented by a loosely coiled structure (Fig. 3.1) than an extended rod. The term conformation is now used when referring to a three-dimensional geometric arrangement of the polymer which changes easily when the bonds are rotated, while arrangements fixed by the chemical bonding in the molecule such as cis and trans isomers or d and l forms are referred to as configurations. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed [2]. Properties like durability, fragility, rigidity, flexibility, toughness, elasticity, resilience, optical clarity, chemical and solvent resistivity, etc., are inherently built into polymeric materials and can be further modified by adding plasticizers, fillers, coloring agents, stabilizers, etc. Thus polymers provide us with materials of virtually unlimited properties.

3.1 A random coil arrangement of polymer chains.

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3.1.2

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Classification of polymers

Polymers exist in countless forms and numbers because of the very large number and types of atoms present in their molecules. Polymers possess different chemical structures, physical properties, mechanical behavior and thermal characteristics and can be classified in different ways. Table 3.2 lists examples for different categories of polymers. Natural and synthetic polymers Based on their origin, polymers are classified into natural and synthetic. Polymers obtained from natural sources are called natural polymers, e.g. natural rubber, cotton, silk, wool, cellulose, etc. Polymers synthesized from low molecular weight materials are called synthetic polymers, e.g. nylons, polyesters, epoxies, etc. Organic and inorganic polymers Polymers are also classified into organic and inorganic polymers based on their chemical structure. A polymer whose backbone is made essentially of carbon–carbon links (–C–C–) is termed an organic polymer, e.g. natural rubber, polyethylene, etc. On the other hand, the main chain of inorganic polymers will contain non-carbon atoms such as silicon, e.g. glass, silicone rubbers, polygermane, etc. Elastomers, fibers and plastics Depending on their ultimate properties, polymeric materials can be broadly classified into elastomers, fibers and plastics. In a simplistic manner we can say that elastomeric materials exhibit elasticity, e.g. natural rubber, silicone rubber, etc.; fiber-forming materials possess rigidity and stiffness, e.g. nylon, terylene, etc.; and plastic materials are generally strong and tough, e.g. polystyrene, polyvinyl chloride, etc.; but there is no firm dividing line between the groups. Rigid plastics and fibers are resistant to deformation and are characterized by high modulus and low percentage elongations, while elastomers readily undergo deformation and exhibit reversible elongations under small applied stress, i.e., they exhibit elasticity. Thermosets, thermoplastics and elastomers The polymers that soften or melt upon heating, called thermoplastic polymers, consist of linear or branched chain molecules having strong intramolecular bonds but weak intermolecular bonds. Melting and solidification of these

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Table 3.2 Some important categories of polymers with examples Type of polymer

Examples

Thermoplastics

• • • • • • • • • • • • • • • • • • • •

Thermosets

• • • • • • • • • • • • • • • • • • • • • • •

Acrylonitrile butadiene styrene (ABS) Cellulose acetate Ethylene vinyl acetate (EVA) Ethylene vinyl alcohol (EVAL) Fluoroplastics (PTFE, along with FEP, PFA, CTFE, ECTFE, ETFE) Liquid crystal polymer (LCP) Polyacetal (POM or acetal) Polyacrylates (acrylic) Polyacrylonitrile (PAN) Polyamide (PA or nylon) Polyamide-imide (PAI) Polyaryletherketone (PAEK or ketone) Polybutadiene (PBD) Polybutylene (PB) Polybutylene terephthalate (PBT) Polycaprolactone (PCL) Polycarbonate (PC) Polychlorotrifluoroethylene (PCTFE) Polycyclohexylene dimethylene terephthalate (PCT) Polyester Polyether ether ketone (PEEK) Polyetherimide (PEI) Polyethersulfone (PES) Polyethylene (PE) Polyethylenechlorinates (PEC) Polyethylene terephthalate (PET) Polyhydroxyalkanoates (PHAs) Polyimide (PI) Polyketone (PK) Polylactic acid (PLA) Polymethylpentene (PMP) Polyphenylene oxide (PPO) Polyphenylene sulfide (PPS) Polyphthalamide (PPA) Polypropylene (PP) Polystyrene (PS) Polysulfone (PSU) Polyurethane (PU) Polyvinyl acetate (PVA) Polyvinyl chloride (PVC) Polyvinylidene chloride (PVDC) Styrene-acrylonitrile (SAN)

• • • • •

Epoxies (EP) Phenol formaldehyde (PF) Cresol formaldehyde resins (CF) Urea formaldehyde (UF) Melamine formaldehyde (MF)

Polymeric materials: elastomers, plastics, fibers, composites Table 3.2 Cont’d Type of polymer

Examples • Unsaturated polyester resins (UP) • Polyurethanes (PU)

Elastomers

• • • • • • • • • •

Natural rubber (NR) Polybutadiene (BR) Neoprene (CR) Nitrile rubber (NBR) Styrene butadiene rubber (SBR) Butyl rubber (IIR) Ethylene propylene rubber (EPM) Ethylene propylene diene rubber (EPDM) Propylene oxide rubber (POR) Halogenated rubbers

Crystalline polymers

• • • • • •

Nylon Polyethylene (PE) Polyacetal Polytetrafluoroethylene (PTFE) Polyethylene terephthalate (PET) Polybutylene terephthalate (PBT)

Amorphous polymers

• • • • •

Polycarbonate (PC) Polymethyl methacrylate (PMMA) Epoxies (EP) Polyvinyl chloride (PVC) Polystyrene (PS)

Linear polymers

• • • • • • • • • •

Polyethylene (PE) Polyisobutylene Polyisoprene (PIS) Polystyrene (PS) Polymethyl methacrylate (PMMA) Polyvinyl acetate (PVA) Polyvinyl chloride (PVC) Polyethylene terephthalate (PET) Polyhexamethylene adipamide Polyacrylonitrile

Liquid crystalline polymers

• Poly(chloro-1,4-phenylene terephthalimide) • Polyterephthalic hydrazide • Thermo tropic polyesters (copolymers of p-hydroxybenzoic acid, p,p’- bisphenol and terephthalic acid)

Copolymers

• • • •

Acrylo nitrile butadiene styrene (ABS) Ethylene vinyl acetate (EVA) Ethylene-propylene copolymer Styrene acrylonitrile (SAN)

Homochain polymers

• • • •

Polyethylene (PE) Polypropylene (PP) Polystyrene (PS) Polyvinyl chloride (PVC)

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Table 3.2 Cont’d Type of polymer

Examples

Heterochain polymers

• • • • •

Nylon 6 Polycarbonate (PC) Polyacetal Polyphenylene sulfide (PPS) Polyether ether ketone (PEEK)

Organic polymers

• • • • • •

Polyethylene (PE) Polypropylene (PP) Polystyrene (PS) Polyvinyl chloride (PVC) Styrene acrylonitrile (SAN) Acrylo nitrile butadiene styrene (ABS)

Commodity plastics

• • • •

Low density polyethylene (LDPE) High density polyethylene (HDPE) Polystyrene (PS) Polypropylene (PP)

Transition plastics

• • • •

Styrene acrylonitrile (SAN) Acrylo nitrile butadiene styrene (ABS) Polymethyl methacrylate (PMMA) Polyacetals

Engineering plastics

• • • • • •

Nylon 6 Polyethylene terephthalate (PET) Polycarbonate (PC) Polyphenylene oxide Polysulfones Styrene acrylonitrile (SAN)

Specialty thermoplastics

• • • • •

Polyphenylene sulfide Polytetrafluoroethylene (PTFE) Polyether ether ketone (PEEK) Polyether imide (PEI) Polyimides

Thermoplastic elastomers

• Thermoplastic polyurethane • Ethylene propylene diene terpolymer • Styrene isoprene styrene (SIS)

polymers are reversible and they can be reshaped by application of heat and pressure. They are either semicrystalline or amorphous in structure. Examples include polyethylene (PE), polystyrene (PS), nylons, polycarbonate (PC), polyacetals, polyamide-imide, polyether-ether ketone (PEEK), polysulfone polyphenylene sulfide, polyether imide, etc. On the other hand, thermosetting plastics have crosslinked or network structures with covalent bonds between all molecules. They do not soften but decompose on heating. Once solidified by crosslinking processes, they cannot be reshaped. Common examples of thermosetting polymers include epoxies, polyesters, phenol formaldehyde, etc.

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Crystalline and amorphous polymers Crystalline polymers have a regularly arranged molecular structure. In these polymers the chemical structure allows the polymer chains to fold on themselves and pack together in an organized manner. The resulting organized regions show the characteristics of crystals, e.g. polyamides, cellulose, polyvinyl chloride, etc., while amorphous polymers possess a random coily structure, e.g. polystyrene, polymethyl methacrylate, etc. [2]. Homopolymers and copolymers Based on the type of monomers from which they are formed, polymers are also classified into homopolymers and copolymers. Homopolymers are formed by polymerizing only one type of monomer, whereas copolymers are formed by polymerizing two or more types of monomers so that an individual polymer chain will contain residues of each of the monomers. In a copolymer the arrangement of the monomer units can take many forms. Many important commercial materials approximate to a random copolymer (–ABAABBAAAAAABABBABBBAAB–) arrangement with the monomer units randomly distributed along the chain. In a block copolymer (–AAAABBBBAAAABBBBAAAAB–) the sequence or block of one repeat unit is followed by a block of another repeat unit, which in turn is followed by a block of the first repeat unit and so on. Graft copolymers, on the other hand, are branched molecules where the main chain is made entirely of one repeat unit while the branch chains are made of yet another repeat unit.

3.2 3.2.1

Elastomers Introduction

The term elastomer is often used interchangeably with the term rubber, and is preferred when referring to vulcanizates. Elastomer comes from two terms, elastic (describing the ability of a material to return to its original shape when a load is removed) and mer (from polymer, in which poly means many and mer means parts). Each link of the chain is the ‘-mer’ or basic unit that is usually made of carbon, hydrogen, oxygen and/or silicon. To make the chain, many links or ‘-mers’ are hooked or polymerized together. An elastomer can be stretched 300% without breaking, and when released returns to within 10% of the original gage length within 10 seconds. Any low-crystallinity or amorphous, high molecular weight polymer with sufficiently flexible chains can in principle be made into an elastomer by crosslinking the chains. Such a polymer gel is an elastomer above its glass transition temperature Tg. Crosslinks can be chemical (thermoset elastomers) or physical (thermoplastic elastomers).

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The chain molecules undergo uncoiling and recoiling upon the application and release of a small force. In the unstrained state, they should tend to take up the more probable randomly coiled conformation such that the entropy factor is the highest at its normal state. When strained, the chains should be able to be extended and be brought to more ordered conformations. An ordered arrangement of the chain molecules give rise to partial crystallinity and also a decrease in the entropy factor. It is thus the entropy factor that favors the recoiling on release of the force. There should also be sufficient interchain free volume to provide for unhindered segmental mobility during coiling and uncoiling. Further interchain cohesive force should also be low enough so as not to hinder the free segmental mobility. Chain flexibility and segmental mobility can be obtained by selecting repeat units made of the C–C and C–O linkages and by avoiding bulky side groups on the repeat units. By avoiding aromatic and cyclic structures in the chain backbone, chain stiffening can be eliminated. In fact, it has been established that rubber elasticity can only be exhibited by such polymeric systems consisting of long flexible chain coils with very weak interchain cohesion and interconnected here and there through crosslinks to form a three-dimensional network [5–7]. A polymeric material can be made to possess the elastomeric property over a wide range of temperature, or its ‘use’ temperature as an elastomeric material can be widened by pushing down the glass transition temperature (Tg) as low as possible and pushing up the flow temperature (Tf) as high as possible. The glass transition temperature can be lowered by copolymerization with small quantities of a suitable comonomer. This is internal plasticization. External plasticization involves compounding the polymer with a mutually compatible high boiling liquid called a plasticizer. The plasticizer reduces the interchain cohesion in the polymer and favors segmental mobility. Milling or mastication of rubber, by which the molecular weight and hence the Tg of the rubber is reduced considerably, also provides a method of plasticization. The elevation of the Tf is accomplished by cross linking or vulcanization. In a crosslinked material the Tf is generally so high that the material on heating starts decomposing before its Tf is reached [5–7].

3.2.2

General purpose and specialty elastomers

From the early twentieth century, chemists have been attempting to synthesize materials whose properties duplicate or at least simulate those of natural rubber, and this has led to the production of a wide variety of synthetic elastomers. Now there are many different types of high performance elastomers. Their unique properties are essential in hostile environments, and application areas include the petrochemical and refining industries, automotive, aerospace, defense, wire and cable, construction, chemical plants, nuclear, medical, food and seals. Correct material selection, compounding and processing are

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essential for the preparation of specialty elastomers [8]. Table 3.3 shows examples for various general purpose and specialty elastomers for different applications.

3.3

Plastics

A plastic can be defined as an organic high molecular weight polymer capable of changing its shape on the application of a force and retaining this shape on removal of this force, i.e., a material in which a stress produces a nonreversible strain. The main criterion is that plastic material can be formed into complex shapes and possesses good tensile strength and rigidity with or without elongation or impact strength. These materials can be either fully amorphous or partially crystalline.

3.3.1

Thermoplastics

A thermoplastic is a plastic that melts to a liquid when heated and freezes to a brittle, very glassy state when cooled sufficiently. Most thermoplastics are high molecular weight polymers whose chains associate through weak van der Waals forces (polyethylene), stronger dipole–dipole interactions and hydrogen bonding (nylon), or even stacking of aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers (Bakelite, vulcanized rubber) as, unlike thermosetting polymers, they can be remelted and remolded. Many thermoplastic materials are addition polymers, e.g. vinyl chain-growth polymers such as polyethylene and polypropylene. Thermoplastics are elastic and flexible above a glass transition temperature Tg, specific for each one – the midpoint of a temperature range in contrast to the sharp freezing point of a pure crystalline substance like water. Below a second, higher melting temperature, Tm, also the midpoint of a range, most thermoplastics have crystalline regions alternating with amorphous regions in which the chains approximate random coils. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity, as is also the case for non-thermoplastic fibrous proteins such as silk. (Elasticity does not mean they are particularly stretchy examples being nylon rope and fishing line.) Above Tm all crystalline structure disappears and the chains become randomly interdispersed. As the temperature increases above Tm, viscosity gradually decreases without any distinct phase change. Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name. This quality makes thermoplastics recyclable. The processes required for recycling vary with the thermoplastic. The plastics used for soft drink bottles are a common example of thermoplastics that can be and are widely recycled. Animal horn, made of the protein α-keratin, softens on heating, is somewhat

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Table 3.3 Abbreviations, starting materials, repeating units and uses of some commercially important general purpose and specialty elastomers Polymer (abbreviation)

Starting materials

Repeating unit

Uses

Polyisoprene (NR/PIP)

Natural rubber/ isoprene

–CH ( 2–C(CH3)==CH–CH2)–n

General purposes

Styrene butadiene Styrene and rubber (SBR) butadiene

–CH ( 2 –CH==CH–CH2–CH2– CH2–CH(C6H5))–n

Tires, general purposes

Polybutadiene (BR)

Butadiene

–CH ( 2–CH==CH–CH2)–n

Tire treads

Butyl rubber (IIR)

Isobutylene and –CH ( 2–C(CH3)2)–n small amounts of isoprene

Inner tubes, cable sheathing, roofing, tank liners

Chlorobutyl rubber (CIIR)

Halogenated butyl rubbers

–CHX–C(CH ( 3)2)–n

Inner tubes, cable sheathing, roofing, tank liners

Bromobutyl rubber (BIIR)

Halogenated butyl rubbers

–CHX–C(CH ( 3)2)–n

Inner tubes, cable sheathing, roofing, tank liners

Acrylonitrilebutadienestyrene rubber (ABS)

Acrylonitrile, butadiene and styrene

–CH ( 2–CH(CN)–CH2–CH(C6H5))–n Oil hoses, flexible fuel tanks, gaskets

Nitrile rubber (NBR)

Acrylonitrile and butadiene

–CH ( 2–CH==CH–CH)–m –CH ( 2–CH(CN))–n

–CH ( 2–CH==CH–CH)–m Hydrogenated nitrile rubber (HNBR)

Hydrogenated nitrile rubber

–CH ( 2–CH(CN))–n

Poly chloroprene/ neoprene (CR)

Chloroprene

–CH ( 2–CH==CH–CH2)–n

–CH ( 2–CH2–CH2–CH)–m

Cl

Silicones Fluorosilicone rubber

Dialkyl silanols Fluorinated silanols

Polyurethanes (PUR)

Polyisocyanates –R ( 1–NHCOOR2OOCHN)–n

R | –O–Si– | R n

Gasoline hoses, fuel tanks, creamery equipment, adhesives Gasoline hoses, fuel tanks, creamery equipment, adhesives Used when oil resistance, good weathering and inflammability characteristics are required Medical applications, gaskets, door seals, flexible molds Printing rollers, sealing and joining

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Table 3.3 Cont’d Polymer (abbreviation)

Starting materials

Repeating unit

Uses

Ethylene propylene rubber (EPM)

Ethylene and propylene

–(CH ( 2–CH2)m–(CH2–CH)p)–n | CH3

Window strips and channeling

Ethylene propylene diene monomer (EPDM)

Ethylene, propylene and 1,4-hexa diene/ dicyclo pentadiene

High ozone, oxygen and heat resistance

Chloro sulfonated PE, SO2 and Cl2 polyethylene (CSM)

–CH ( 2–CH)–n | X

High chemical resistance with good oxidative and heat resistance

Polysulfide elastomers

Ethylene dihalides and alkali sulfides

–R–S ( y)–

Gasoline hoses, tanks, diaphragms and balloon fabrics

Fluorinated rubbers (FKM, FEPM)

Fluorine containing polyacrylates

–CX ( 2–CH(COOH))–n

Special sealings and membranes in cars and planes, and in the chemical industry

Perfluoro elastomers (FFKM) Epichlorohydrin rubber (ECO)

Epichloro –OCH ( 2CHCH2X)–n hydrin–ethylene oxide copolymers

Excellent adhesion properties

Propylene oxide rubber (POR)

Propylene oxide and allyl glycidyl ether

Good adhesion properties

Polyacrylic rubber

Ethyl acrylate, methyl methacrylate with small amounts of hydroxyl, carboxyl, amine comonomers

–CH ( 2–CH(COOH))–n

Pressure sensitive adhesives, high quality latex paints, automotive coatings

Ethylene vinyl acetate (EVA)

Polyethene and vinyl acetate

–CH ( 2–CH2–CH2–CH (OOCCH3))–n

Good chemical and solvent resistance

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reshapable, and may be regarded as a natural, quasi-thermoplastic material. Polymers, especially plastics, may be converted into products in a wide variety of ways. Whatever the process chosen, it may be divided into two stages, viz. getting the shape and setting the shape. The shaping operation can be carried out with the polymer existing as a: • • • • • •

melt (as in compression, injection and blow molding and extrusion) rubbery state (as in vacuum forming) solution (as when casting film or fiber spinning) suspension (as in latex technology or PVC paste processes) liquid monomer or low molecular weight polymer (as in casting and laminate production) rigid solid (in machining operations).

When processing polymer melts, the factors that should be taken into account in order both to process than efficiently and to obtain quality products are: • • • • • • • •

water absorption of raw materials physical form of raw materials thermal stability of the polymer flow properties of the molten plastic material adhesion of melt to mold thermal properties affecting heating and cooling of melt compressibility and shrinkage frozen-in orientation. Commonly used methods of molding thermoplastics are as follows:

• • • • •

Injection molding (most thermoplastic materials are molded by the process of injection molding because of the speed with which finished can be produced) Blow molding (used for the manufacture of bottles and containers) Rotational molding (used for producing large hollow parts and can be used to produce multiwall constructions by successive steps) Calendering (used for the continuous manufacture of sheet or film) Forming of thermoplastic sheets (vacuum forming is widely used for the mamipulation of cellulose acetate and acrylic resin sheeting).

More methods of processing are given in Table 3.4. Some thermoplastics normally do not crystallize: they are termed ‘amorphous’ plastics and are useful at temperatures below the Tg. They are frequently used in applications where clarity is important. Some typical examples of amorphous thermoplastics are PMMA, PS and PC. Generally, amorphous thermoplastics are less chemically resistant and can be subject to stress cracking. Thermoplastics will crystallize to a certain extent and are

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Table 3.4 Some common polymer processing methods (all but the last two are suitable for all kinds of composite materials) Processing technique

Purposes

Calendering

For producing continuous films and sheets

Die casting

For converting a liquid pre-polymer to a solid object with a desired shape

Rotational casting

For producing hollow articles

Film casting

For producing polymeric films

Compression molding

For producing articles from thermosetting materials

Injection molding

For producing articles from thermoplastic materials

Blow molding

For producing hollow containers from thermoplastics, glasses, etc.

Extrusion molding

For producing films, filaments, pipes, rods, hoses, etc., all in continuous lengths

Thermofoaming

For fabricating three-dimensional articles from plastics

Foaming

For producing expanded or spongy materials

Pultrusion

For producing articles such as tubing or fishing rods from continuous strands of a fiber

Spinning • Melt spinning • Dry spinning • Wet spinning

For producing long fibers

Open molding • Hand layup • Spray-up

For producing sheets/laminates

Vacuum molding

For producing sheets/laminates

Resin transfer molding • Melt intercalation or melt blending method • In-situ intercalative polymerization method

For producing sheets/laminates For the preparation of nanocomposites For the preparation of nanocomposites

called ‘semi-crystalline’ for this reason. Typical semi-crystalline thermoplastics are PE, PP, PBT and PET. The speed and extent to which crystallization can occur depends in part on the flexibility of the polymer chain. Semi-crystalline thermoplastics are more resistant to solvents and other chemicals. If the crystallites are larger than the wavelength of light, the thermoplastic is hazy or opaque. Semi-crystalline thermoplastics become less brittle above Tg. If a plastic with otherwise desirable properties has too high a Tg, it can often be lowered by adding a low-molecular-weight plasticizer to the melt before forming (plastics extrusion; molding) and cooling. A similar result can sometimes be achieved by adding non-reactive side chains to the monomers

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before polymerization. Both methods make the polymer chains stand off slightly from one another. Before the introduction of plasticizers, plastic automobile parts often cracked in cold winter weather. Another method of lowering Tg (or raising Tm) is to incorporate the original plastic into a copolymer, as with graft copolymers of polystyrene, or into a composite material. Lowering Tg is not the only way to reduce brittleness. Drawing (and similar processes that stretch or orient the molecules) or increasing the length of the polymer chains also decreases brittleness [9–11].

3.3.2

Thermosetting plastics

Thermosetting plastics (thermosets) are polymer materials that irreversibly cure to a stronger form. The cure may be done through heat (generally above 200°C), through a chemical reaction (two-part epoxy, for example), or by irradiation such as electron beam processing. Thermoset materials are usually liquid or malleable prior to curing and are designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors, and integrated circuits (ICs). The curing process transforms the resin into a plastic or rubber by a crosslinking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The crosslinking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, when the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material. Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and reshaped after it is cured. This implies that thermosets cannot be recycled, except as filler material. Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature of the material [10, 11]. Some methods of molding thermosets are: • • • •

Reactive injection molding (used for objects like milk bottle crates) Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables) Compression molding (used to shape most thermosetting plastics) Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines and emblems as well as production and replacement parts).

More methods of processing are given in Table 3.4.

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3.4

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Fibers

Fibers are a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread. They are very important in the biology of both plants and animals, for holding tissues together. Human uses for fibers are diverse. They can be spun into filaments, string or rope, used as a component of composite materials, or matted into sheets to make products such as paper or felt. Fibers are often used in the manufacture of other materials. Synthetic fibers can be produced very cheaply and in large amounts compared to natural fibers, but natural fibers enjoy some benefits, such as comfort, over their synthetic counterparts.

3.4.1

Natural fibers

Natural fibers include those produced by plants, animals and geological processes [12–14]. They are biodegradable over time. They can be classified according to their origin: •

•

• •

Vegetable fibers are generally based on arrangements of cellulose, often with lignin: examples include cotton, hemp, jute, flax, ramie and sisal. Plant fibers are employed in the manufacture of paper and textile (cloth), and dietary fiber is an important component of human nutrition. Wood fiber, distinguished from vegetable fiber, is from tree sources. Forms include groundwood, thermomechanical pulp (TMP) and bleached or unbleached kraft or sulfite pulps. Kraft and sulfite refer to the type of pulping process used to remove the lignin bonding the original wood structure, thus freeing the fibers for use in paper and engineered wood products such as fiberboard. Animal fibers consist largely of particular proteins. Instances are spider silk, sinew, catgut, wool and hair such as cashmere, mohair and angora, fur such as sheepskin, rabbit, mink, fox, beaver, etc. Mineral fibers comprise asbestos. Asbestos is the only naturally occurring long mineral fiber. Short, fiber-like minerals include wollastinite, attapulgite and halloysite.

3.4.2

Synthetic fibers

Synthetic or artificial fibers generally come from synthetic materials such as petrochemicals. But some types of synthetic fibers are manufactured from natural cellulose, including rayon, modal and the more recently developed Lyocell [13]. Cellulose-based fibers are of two types: regenerated or pure cellulose such as from the cupro-ammonium process, and modified or derivatized cellulose such as the cellulose acetates. Figure 3.2 gives a schematic representation of a wide variety of fibers.

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Artificial

Natural

Regenerated Mineral

Plant

Animal

Wool Leaf

Bast

Synthetic

Mohair

Silk

Fruit

3.2 Scheme showing the wide variety of fibers.

Fibers are made from polymers by a process called spinning. There are three principal spinning methods: • • •

Melt spinning (the polymer is in the melt stage; this is the simplest method) Dry spinning (the polymer is dissolved in an appropriate solution; many common polymers like PAN, PVC, etc., can be converted to fibers on a large scale) Wet spinning (employs a fairly concentrated polymer solution; slowest compared to the above two processes and employed commercially to make fibers from cellulose, viscose rayon, etc.).

More methods of processing are given in Table 3.4.

3.5

Composites (macro, micro and nanocomposites)

3.5.1

Background

Composite materials (or composites for short) are engineered materials [14] made from two or more constituent materials with significantly different physical or chemical properties and which remain separate and distinct on a macroscopic level within the finished structure. A better or unique combination of properties is realized when different materials (or phases) are combined. The primary needs for all the advanced composites are light weight, higher operating temperatures, greater stiffness, higher reliability and affordability. The most visible applications pave our roadways in the form of either steel and aggregate reinforced ‘portland cement’ or ‘asphalt concrete’, shower stalls and bath tubs made of fiberglass, aerospace components (tails, wings, fuselages, propellers), boat and scull hulls, bicycle frames and racing car bodies. Other uses include fishing rods and storage tanks. The new Boeing

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787 Dreamliner structure, including the wings and fuselage, is composed of over 50% composites. Carbon composite is a key material in today’s launch vehicles and spacecraft. It is widely used in solar panel substrates, antenna reflectors and yokes of spacecraft. It is also used in payload adapters, interstage structures and heat shields of launch vehicles. There are two categories of constituent materials: matrix and reinforcement. At least one portion of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials, while the wide variety of matrix and strengthening materials allows the designer of the product or structure to choose an optimum combination. Engineered composite materials must be formed to shape. The matrix material can be introduced to the reinforcement before or after the reinforcement material is placed into the mold cavity or onto the mold surface. The matrix material experiences a melding event, after which the part shape is essentially set. Depending upon the nature of the matrix material, this melding event can occur in various ways such as chemical polymerization or solidification from the melted state.

3.5.2

Categories of composite materials

Most commercially produced composites use a polymer matrix material, often called a resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, polypropylene, PEEK, and others. The composites are classified according to their constituent materials [15–17] as • • •

natural composite materials – wood, bone, bamboo, tissues, etc. macro composites – steel, reinforced concrete, etc. micro composites – alloys, toughened thermoplastics, reinforced thermoplastics, etc.

The polymeric composites are mainly micro composites. Polymeric composites can be futher classified into particulate reinforced, fiber reinforced and laminar composites depending on the type of reinforcements. Particulate reinforced composites are further classified, based on the particle size of the dispersed phase, as micro and nanocomposites. More recently, advances in synthesis techniques and the ability to readily characterize materials on an atomic scale have led to interest in nanometer-sized materials. Since nanometer-sized grains, fibers and plates have a dramatically increased surface

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area compared to their conventional-sized materials, the chemistry of these nanosized materials is altered compared to conventional materials [15–17]. Polymer nanocomposites are polymers that have been reinforced with small quantities (less than 10%) of nanosized filler particles of size below 100 nm. The dispersed phase can be inorganic particles, minerals, modified clays, carbon nanotubes, etc. [16–18]. Nanocomposites have been found to exemplify even more positive attributes than their predecessors do, and thus an understanding of what occurs when nanocomposites of a polymer and inorganic components are produced is significant. Although particle-filled polymer composites have been extensively studied because of their widespread applications in the automobile, household and electrical industries, recently nanocomposites have generated much interest among various scientists, principally because of the potential they offer for applications in high performance coatings, catalysis, electronics, magnetic and biomedical materials. In polymer nanocomposites research, the primary goal is to enhance the strength and toughness of polymeric components using molecular or nanoscale fillers [14–19]. Most notable are increased modulus, increased gas barrier, increased heat distortion temperature, resistance to small molecule permeation, improved ablative resistance, increase in atomic oxygen resistance, retention of impact strength, etc. Interestingly, these performance improvements are achieved without increasing the density of the base polymer, without degrading its optical qualities and without making it any less recyclable. It is a remarkable fact that in addition to the profound changes in physical properties that materials display when they are nanometric in scale, their chemical behavior is profoundly altered as well. When an inorganic solid is composed of only a few thousands of atoms, it has a great deal of surface area. Polymer nanocomposites contain a rigid filler component (this can be fiber, filler or nanoscopic organic component) dispersed within a flexible polymer matrix on a nanoscale level. The rigid portion, with high modulus and high strength, usually has a high melting temperature and is insoluble in organic solvents, and combining with the flexible polymer is thermodynamically unfavorable. Therefore it is very difficult to prepare a nanocomposite, and phases may undergo segregation during processing and end use. Hydrodynamic effects and physical or chemical sorption of matrix at the filler surface governs the reinforcement [19–25]. Fiber reinforced composite materials can be divided into two main categories normally referred to as short fiber reinforced materials and continuous fiber reinforced materials. Continuous reinforced materials will often constitute a layered or laminated structure. The woven and continuous fiber styles are typically available in a variety of forms: pre-impregnated with the given matrix (resin), dry, unidirectional tapes of various widths, plain weave, harness satins, braided, and stitched. The short and long fibers are typically employed in compression molding and sheet molding operations. These come in the

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form of flakes, chips, and random mate (which can also be made from a continuous fiber laid in random fashion until the desired thickness of the ply/ laminate is achieved) [26]. Depending on the origin of the fiber, these can be again classified as natural fiber reinforced composites and synthetic fiber reinforced composites. Natural fiber reinforced composites offer good mechanical performance and ecofriendliness. Currently the application of natural fiber based composites is increasing rapidly. This is especially related to certain problems concerning the use of synthetic fiber reinforced composites. As far as synthetic polymer composites are concerned, waste disposal and recycling are major issues worldwide. Landfill disposal is being increasingly excluded around the world due to growing environmental sensitivity. Therefore, in recent years environmentally compatible alternatives are being looked at and examined by researchers. This research covers factors such as efficient, cost-effective and environmentally friendly recovery of raw materials, CO2 neutral thermal utilization or biodegradation in certain circumstances. That is why composites based on renewable resources consisting of either natural fibers or so-called biopolymers, or both, are economically and ecologically acceptable. Natural fibers like flax, hemp, banana, sisal, oil palm, jute, etc., have a number of techno-economic and ecological advantages over synthetic fibers like glass fibers. The combination of interesting mechanical and physical properties together with their environmentally friendly character has aroused interest in a number of industrial sectors, notably the automotive industry. The lignocellulosic fibers have an advantage over synthetic ones since they buckle rather than break during processing and fabrication. In addition, cellulose possesses a flattened oval cross-section that enhances stress transfer by presenting an effectively higher aspect ratio [25–31]. Laminar composites are composed of two or more layers held together by the matrix binder. These have two of their dimensions much larger than the third, e.g., wooden laminates, glasses and plastics.

3.6

Blends (micro and nano blends)

Polymer blends are mixtures of two or more polymers and/or copolymers in which the minor component contributes at least 2 wt%. It is now a truism that, in recent years, polymer blends have experienced an important renaissance. Academic and industrial research in this field is flourishing, and the input of research papers, reviews and patents is growing exponentially. The exorbitant use and ubiquitous nature of these materials in modern life can be evidenced by noting the fact that polymer blends constitute ca. 36 wt% of total polymer consumption, and their pertinence continues to increase. Polymer blends have gained significant commercial growth in the last two decades, outpacing the growth rate of existing polymers by at least 2–5%. The current worldwide

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market volume for polymer blends and alloys is estimated to be more than 700 000 metric tonnes per year, with an average growth rate of 6–7% [32]. The most important factors that should be addressed as far as a multiphase polymer system is concerned are miscibility as well as compatibility between the component polymers. When two immiscible polymers are mixed, the size, shape and relative distribution of one phase into the other depend on material parameters (blend composition, viscosity ratio, elasticity ratio and interfacial tension) as well as processing conditions (temperature, time, intensity and type of mixing and nature of flow). Therefore the greatest challenge in the field of multiphase polymer blend research is the manipulation of polymer structure via judicious control of the melt flow during processing and the interfacial interactions. The mechanism of development of morphology from pellet-sized or powder-sized particles in polymer blends is directly derived from the complex interplay of material parameters and processing conditions. As a result of this, for a given blend, different types of morphologies are possible. However, from the point of view of performance, they may be divided into two categories: blends with discrete phase structure (droplet or drop in matrix) and blends with bicontinuous phase structure (co-continuous). Other important types of morphologies include fibrillar, composite laminar, core shell, onion ring like, etc. The size, shape, uniformity and distribution of the dispersed particles depend on several factors including the viscosity ratio, composition, elasticity ratio, shear stress and interfacial modification. Polymer blends have gained significant commercial growth in the last two decades, outpacing the growth rate of existing polymers by at least 2–5%. The important renaissance in recent times has occurred mainly because of the developments of microfibrillar composites (MFC), electrically conducting polymer blends, nanostructured polymer blends, biodegradable polymer blends, high temperature polymer blends, and polymer blends as biomaterials [32–39]. Nanostructured blends very often exhibit unique properties, which are directly attributed to the presence of structural entities having dimensions in the nanometer range. The idealized morphology of these polymer blend systems is characterized by the molecular-level dispersion of the phases, which leads to a considerable enhancement in the mechanical, electrical and optical properties. Nanostructured polymer blends can be prepared by reactive blending (in-situ polymerization and in-situ compatibilization), micro-emulsion polymerization, solvent casting, spin casting, controlled evaporation, or thermal treatment of an initially miscible system. It is well established that block copolymers (bcps) self-assemble to form a variety of morphologies such as spherical, cylindrical, lamellar and gyroid phase [39–41]. Furthermore, blending bcps with thermoplastic homopolymers has been widely employed to produce polymeric materials with different

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nanoscale structures. However, it should be noted that the preparation of nanostructured polymer blends for immiscible polymers, with a phase size of less than 100 nm, is very challenging using normal processing methods currently available. Very recently, nanostructured blends have been produced from block copolymers by using conventional melt processing, but the method shows obvious limitation for practical application. Therefore the main challenge facing polymer scientists as far as nanostructured polymer blends are concerned is to design an easy, economic and efficient method to develop polymer blends with nanostructured morphology, as these materials find a wide range of applications. For example, Leibler and co-workers [39] have recently developed nanostructured transparent PS/SBS blends that can replace PS, which is often too brittle, and HIPS, which is opaque in packaging applications [38–41].

3.7

Conclusions

The advances in the area of polymer science and technology provide a rich set of materials useful for probing the fundamental nature of matter. As we have learnt, matter behaves differently below 100 nm than it does at macroscale. Nanosized materials have unique structures and tunable properties, making them suitable for many real-world applications. The goals of this chapter are to give a brief overview of various polymeric materials including elastomers, plastics, fibers, their blends and composites. A new generation of solid synthetic materials is emerging; it now remains to be seen how practical application of these materials will grow from the seeds over the coming years.

3.8

References

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