Lecture 4 - Polymers and Composites PDF

Polymers and Composites I- Polymers Polymer: A compound consisting of long-chain molecules, each molecule made up of repeating units connected together  There may be thousands, even millions of units in a single polymer molecule ...

Polymers and Composites I- Polymers Polymer: A compound consisting of long-chain molecules, each molecule made up of repeating units connected together  There may be thousands, even millions of units in a single polymer molecule  The word polymer 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 considered organic chemicals ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Types of Polymers  Polymers can be separated into plastics and rubbers  As engineering materials, it is appropriate to divide them into the following three categories: 1. Thermoplastic polymers 2. Thermosetting polymers 3. Elastomers: Rubber- like Onl (1) and (2) which Only hich are plastics are considered in this lecture ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 1 Thermoplastic Polymers - Thermoplastics Solid materials at room temperature but 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 heating and cooling cycles repeatedly without significant degradation  Symbolized by TP ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Thermosetting Polymers - Thermosets  Cannot tolerate repeated heating cycles as thermoplastics can  When initially heated, they soften and flow for molding  Elevated temperatures also produce a chemical reaction that hardens the material into an infusible solid  If reheated, e eated, tthermosets e osets deg degrade ade aand d cchar a rather than soften  Symbolized by TS ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 2  Market Shares  Thermoplastics are commercially the most important of the three types  About 70% of the tonnage of all synthetic polymers produced  Thermosets and elastomers share the remaining 30% about evenly, with a slight edge for the former  O On a volumetric o u et c bas basis, s, cu current e taannual ua usage oof polymers exceeds that of metals ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Examples of Polymers  Thermoplastics:  Polyethylene, polyvinylchloride, polypropylene, polystyrene, and nylon  Thermosets:  Phenolics, epoxies, and certain polyesters ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 3 Reasons Why Polymers are Important:  Plastics can be molded into intricate part shapes, usually with no further processing  Very compatible with net shape processing  On a volumetric basis, polymers:  Are cost competitive with metals  Generally require less energy to produce than metals  Certain plastics are translucent and/or transparent, which makes them competitive with glass in some applications ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Linear Structure Figure 8.7 Various structures of polymer molecules: (a) linear, characteristic of thermoplastics. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 4  Branched Structure Figure 8.7 Various structures of polymer molecules: (b) branched, also characteristic of thermoplastics. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tightly Cross-linked Structure Figure 8.7 Various structures of polymer molecules: (d) tightly cross-linked or networked structure as in a thermoset. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 5  Effect of Branching on Properties  Thermoplastic polymers always possess linear or branched structures, or a mixture of the two  Branches increase entanglement among the molecules, which makes the polymer  Stronger in the solid state  More viscous at a given temperature in the plastic or liquid state ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Effect of Cross-Linking on Properties  Thermosets possess a high degree of cross-linking, while elastomers possess a low degree of cross-linking  Thermosets are hard and brittle, while elastomers are elastic and resilient  Cross-linking causes the polymer to become chemically set  Thee reaction eact o ca cannot ot be reversed e e sed  The polymer structure is permanently changed; if heated, it degrades or burns rather than melt ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 6  Thermoplastic Polymers (TP)  A thermoplastic polymer can be heated from a solid state to a viscous liquid state and then cooled back down to solid  Heating and cooling can be repeated many times without degrading the polymer  The reason is that TP polymers consist of linear (and/or branched) macromolecules that do not cross-link upon heating  By B contrast, t t thermosets th t and d elastomers l t change chemically when heated, which cross-links their molecules and permanently sets these polymers ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Mechanical Properties of Thermoplastics  Low modulus of elasticity (stiffness)  E is two or three orders of magnitude g lower than metals and ceramics  Low tensile strength  TS is about 10% of the metal  Much lower hardness than metals or ceramics  Greater ductility on average  Tremendous T d range off values, l ffrom 1% elongation for polystyrene to 500% or more for polypropylene ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 7  Physical Properties of Thermoplastics  Lower densities than metals or ceramics  Typical yp specific p g gravity y for p polymers y are 1.2  Ceramics specific gravity = 2.5  Metals specific gravity = 7.0  Much higher coefficient of thermal expansion  Roughly five times the value for metals and 10 times the value for ceramics  Much M h llower melting l i temperatures  Insulating electrical properties  Higher specific heats than metals and ceramics ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Commercial Thermoplastic Products and Raw Materials  Thermoplastic products include  Molded and extruded items  Fibers and filaments  Films and sheets  Packaging materials  Paints and varnishes  The starting plastic materials are normally supplied li d to the h ffabricator bi iin the h fform off powders or pellets in bags, drums, or larger loads by truck or rail car ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 8  Thermosetting Polymers (TS)  TS polymers are distinguished by their highly cross-linked three-dimensional, covalently-bonded structure  Chemical reactions associated with cross-linking are called curing or setting  In effect, the formed part (e.g., pot handle, electrical switch cover, etc.) becomes one large macromolecule  Always amorphous and exhibits no glass transition temperature ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e General Properties of Thermosets  Rigid - modulus of elasticity is two to three times greater than thermoplastics  Brittle, virtually no ductility  Less soluble in common solvents than thermoplastics  Capable of higher service temperatures than thermoplastics  Cannot be remelted - instead they degrade or burn ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 9  TS vs. TP Polymers  TS plastics are not as widely used as the TP  One reason is the added p processingg costs and complications involved in curing  Largest market share of TS = phenolic resins with  6% of the total plastics market  Compare polyethylene with  35% market share  TS Products: countertops, countertops plywood adhesives adhesives, paints, molded parts, printed circuit boards and other fiber reinforced plastics ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e II- Composite Materials Composite Material Defined:A materials system y composed p of two or more distinct phases whose combination produces aggregate properties that are different from those of its constituents  Examples:  Cemented carbides (WC with Co binder)  Plastic molding compounds with fillers  Rubber mixed with carbon black  Wood (a natural composite as distinguished from a synthesized composite) ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 10  Components in a Composite Material Most composite materials consist of two phases: 1. Primaryy p phase - forms the matrix within which the secondary phase is imbedded 2. Secondary phase - imbedded phase sometimes referred to as a reinforcing agent, because it usually strengthens the composite material  The e reinforcing e oc gp phase ase mayay be in tthe e form o of fibers, particles, or various other geometries ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Our Classification of Composite Materials 1. Metal Matrix Composites (MMCs) - mixtures of ceramics and metals, such as cemented carbides and other cermets 2. Ceramic Matrix Composites (CMCs) - Al2O3 and SiC imbedded with fibers to improve properties 3. Polymer Matrix Composites (PMCs) - polymer resins imbedded with filler or reinforcing agent  Examples: epoxy and polyester with fiber reinforcement, and phenolic with powders ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 11  Functions of the Matrix Material  Primary phase provides the bulk form of the part or product made of the composite material  Holds the imbedded phase in place, usually enclosing and often concealing it  When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e The Reinforcing Phase  Function is to reinforce the primary phase  Imbedded phase p is most commonly y one of the following shapes:  Fibers  Particles  Flakes ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 12  Physical Shapes of Imbedded Phase Figure 9.1 Possible physical shapes of imbedded phases in composite materials: (a) fiber, fiber (b) particle, particle and (c) flake. flake ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Properties of Composite Materials  In selecting a composite material, an optimum combination of properties is often sought rather than one particular property sought,  Example: fuselage and wings of an aircraft must be lightweight, strong, stiff, and tough ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 13 Three Factors that Determine Properties 1. Materials used as component phases in the composite 2. Geometric shapes of the constituents and resulting structure of the composite system 3. How the phases interact with one another ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Other Composite Structures  Laminar composite structure – conventional  Sandwich structure  Honeycomb sandwich structure ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 14  Laminar Composite Structure Two or more layers bonded together in an integral piece  Example: plywood, plywood in which layers are the same wood, but grains are oriented differently to increase overall strength Figure 9.7 Laminar composite structures: (a) conventional laminar structure. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Other Laminar Composite Structures  Automotive tires - multiple layers of rubber bonded together with reinforcing agent  FRPs - multi-layered, fiber-reinforced plastic panels for aircraft, boat hulls, other products  Printed circuit boards - layers of reinforced copper and plastic for electrical conductivity and insulation, respectively  Snow skis - layers of metals, particle board, and phenolic plastic  Windshield glass - two layers of glass on either side of a sheet of tough plastic ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 15  Sandwich Structure: Foam Core Relatively thick core of low density foam bonded on both faces to thin sheets of a different material Figure 9.7 Laminar composite p structures: (b) sandwich structure using foam core. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Sandwich Structure: Honeycomb Core Alternative to foam core  Foam or honeycomb achieve high ratios of strength-to-weight and stiffness-to-weight Figure 9.7 Laminar composite structures: ((c)) sandwich structure using honeycomb core. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 16 Figure 9.11 Composite materials in the Boeing 757 (photo courtesy of Boeing Commercial Airplane Co.). ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 17