SCM-503/703: Structure & Properties of Polymeric Materials 603-1.1 Natural Polymers Synthetic Polymers Polymers - Macromolecules Proteins polyamides, enzymes, muscles, tissue, air, wool, silk Carbohydrates polysaccharides, starch, cellulose, rayon(reconstituted cellulose) Nucleic Acids - DNA, RNA Plastics - DPE, LDPE, Lexan, plexiglass, teflon, polystyrene Fibers - nylon, orlon, kevlar, rayon Elastomers - rubber, spandex, poly(urethane) foam, silly putty Adhesives - glues, cement, "scotch tape", epoxy, air spray Coatings - paints, varnishes, enamels, formica Poly(urethane) Specialty Polymers - graphite (lubricant), toners, Diamond (insulator), polypyrrole (conductor), poly(vinylidene fluoride) (piezoelectric), photoresists (novolac resins, PMMA)
603-1.2 Polymer Production 12000 80000 70000 10000 60000 8000 50000 Millions of lb. 6000 40000 4000 30000 2000 Thermoset Resins - Total Synthetic Rubber - Total Synthetic Fiber - Total Thermoplastic - Total (right) 20000 10000 0 0 1988 1990 1992 1994 1996 1998 2000 Year
603-1.3 Key Concepts Q: What makes polymers unique? Polymers are chain molecules that are characterized by a molecular length that is much greater than the other dimensions of the molecule. Since the bonding in the chain direction is covalent, and the bonding between chains is weak, polymers can exhibit highly anisotropic properties. Polymeric material will also exhibit isotropic properties when the chain segments are oriented randomly within the material. If the random orientation occurs down to a molecular length scale, the material is classified as being amorphous. Most amorphous polymers have chains that are long enough for them to entangle with one another. The molecular weight required for chain entanglement to occur is referred to as the critical molecular weight for the particular polymer. Polymers with average molecular weights that are greater than the critical molecular weight exhibit polymeric properties. An important and unique polymeric property is viscoelasticity.
603-1.4 Plastics Plastics are mainly structural materials that are capable of being formed into different "plastic states" during processing. Plastics are categorized as thermoplastic and thermosetting depending on whether the plastic state is temporary or permanent. Thermoplastics The shape of these materials can be changed through the application of heat and pressure. Although they are processed as viscous melts, these materials are generally hard at their use temperature. The thermo-mechanical behavior of thermoplastics is represented by the viscoelastic curve. Viscoelastic Curve 1.00E+10 B 1.00E+09 A semi-crystalline 1.00E+08 log(e) -{E in Pa) 1.00E+07 amorphous C D 1.00E+06 1.00E+05 low molecular weight E 1.00E+04 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 Reduced Temperature - T/Tg
603-1.5 Thermo-Mechanical Transitions The viscoelastic curve represents the relationship between a sample's temperature and its modulus. The modulus is a measure of the sample's resistance to being deformed by an imposed stress. The viscoelastic curve is divided into five distinct regions, these have been labeled A - E on the previous curve. A - Glass. The material is rigid, yet brittle if not reinforced by chemical cross-links or crystallites. Polystyrene and plexiglass are amorphous glasses at room temperature. B - Glass Transition Zone. The material starts to become compliant over time (at constant temperature), or in a narrow temperature range. The glass transition temperature, T g, identifies this zone. C - Rubber. The material is very flexible, capable of being stretched to several times its original dimensions without breaking. Commercial rubbers are chemically cross linked to keep them from softening at elevated temperatures. D - Rubbery Flow Zone. The material becomes tacky, and will spread like a liquid if pressure is applied. Pressure-sensitive adhesives are designed to exhibit this behavior at room temperature. E - Polymer Melt. As the polymeric material is heated beyond the rubbery flow zone its viscosity steadily decreases. As the viscosity (resistance to flow) decreases, so does the materials modulus. This is the processing region of the viscoelastic curve. The region between the two transition zones is affected by the molecular weight of the polymer and by the degree of crystallinity of the solid. Semi-crystalline polymers can absorbed a considerable amount of energy in this region without fracturing. This is referred to as toughness.
603-1.6 Molecular Weight & Viscoelasticity The effect of molecular weight on the thermo-mechanical properties of polymeric materials is represented on the figure given below. Transition Temperatures for Thermoplastics 3 2.5 Reduced Transition Temperatures ----- T/Tg 2 1.5 1 0.5 Liquid Glassy Waxy Gas Glass Transition, Tg Rubber-Liquid Transition, Tr Melt Temperature, Tm Rubbery Leathery Thermal Decomposition, Td 0 0.01 0.1 1 10 100 Reduced Molecular Weight M/Mcr Characteristic Thermo-Mechanical Behavior ard, Rigid - Polymer is below its Tg at its use temperature. For small stresses, the behavior is independent of molecular weight. Soft/Waxy - Polymer has a molecular weight below its critical molecular weight. The material exhibits some crystallinity. Tough/Leathery - The high degree of crystallinity leads to an increase in the modulus. The crystallites are covalently linked by polymer chains. Therefore, covalent bonds need to be broken to fracture the material. Rubbery - The material can be stretched to large strains.
Production of Thermoplastics 603-1.7 Thermoplastics are by far the largest class of synthetic polymers when measured based on annual production. Production of the 6 main recyclable polymers amounted to 70 billion pounds in 1999, with polyethylene leading the way. Thermoplastic Resin Production 35000 30000 25000 Polyethylene (2,4) Polypropylene (5) PVC & Copolymers (3) Styrene Polymers (6) Thermoplastic polyester (1) Millions of lb. 20000 15000 10000 5000 0 1988 1990 1992 1994 1996 1998 2000 Year These materials are referred to as commodity plastics because of their relatively low production cost. PET (recycle #1) is often considered to be an engineering plastic because it combines good strength with a reasonable cost. The combination of low cost/volume and high strength is referred to as the materials performance. igh-performance plastics combine reasonable cost with high strength. Many of these latter materials, such as kevlar, exhibit thermotropic liquid crystal behavior.
Thermoplastic Processing 603-1.8 The most common processing technique for thermoplastics is extrusion. Solid polymer is added to a feed hopper (A). It then fills the spaces between the vanes of the extrusion screw. As the material is forced down the barrel of the extruder it is compressed and heated. Much of the heating results from the friction between the particles. When the polymer reaches the metering and mixing section of the extruder (B) it is a viscous melt. The melt is then continuously forced through a narrow opening called a die. The shape of the die orifice determines the cross-section of the final polymer product. Thermoplastic sheet, film and fiber are produced in this way.
Fibers 603-1.9 Although fiber-forming polymers could certainly be classified as thermoplastics, the fiber industry is separate from the plastics industry. Fibers are defined as elongated structures that are considerably longer than they are thick. Fibers exhibit highly anisotropic properties resulting from the alignment of chains and crystalline regions along the fiber axis. The amorphous regions between crystalline domains exhibit rubbery behavior. As a consequence, fibers are flexible, yet tough. Since many of the chains are aligned along the fiber axis, fibers exhibit high tensile strength. The trade-off is poor shear strength, leading to fraying. Annual fiber production is around 10 billion pounds. The biggest growth in fiber production has been in polyolefins, particularly polypropylene. Synthetic Fiber Production 4500 4000 3500 3000 Millions of lb. 2500 2000 1500 1000 Polyester Nylon Olefin Acrylic Acetate & Rayon 500 0 1988 1990 1992 1994 1996 1998 2000 Year
Fiber Classification 603-1.10 The Federal Trade Commission lists the following classification scheme for commercial fibers. Acetate > 92% acetate substituted cellulose - introduced by Celanese in 1924. Acrylic > 85% polyacrylonitrile - introduced by DuPont in 1950. Anidex > 50% polyacrylates - introduced by Rohn & aas in 1970. Aramid > 85% polyarylamide - introduced by DuPont in 1961. Azlon Lyocell Melamine regenerated protein - marketed by Azlon. solvent extruded cellulose - introduced by Acordis Cellulosics in 1992. > 50% crosslinked melamine resin. Modacrylic 35%-85% polyacrylonitrile - introduced by Union Carbide in 1943. Nylon > 85% aliphatic polyamide - introduced by DuPont in 1939. Olefin > 85% polyolefin (PE and PP) - introduced by ercules in 1961. PBI fiber containing polybenzimidazole - introduced by Celanese in 1983. Polyester > 85% PET or a polyester of p-hydroxybenzoate - introduced by DuPont in 1953. Rayon regenerated cellulose - introduced by American Viscose in 1910. Saran > 80% poly(vinylidene chloride) - introduced by Firestone in 1941. Spandex > 85% segmented polyurethane - introduced by DuPont in 1959. Sulfar > 85% poly(phenylene sulfide) - introduced by Phillips in 1983. Vinal > 85% poly(vinyl alcohol)/poly(vinyl acetate). Vinyon > 85% poly(vinyl chloride) - introduced by FMC.
Elastomers 603-1.11 Elastomers and rubbers are flexible materials that can be stretched, reversibly, to several times their original dimensions. Most commercial elastomers are cross linked after polymerization. The degree of cross-linking determines the classification of the rubber. The cross-linking in elastomers prevents them from being stretched to more than double their original length. The term rubber is reserved for more lightly cross-linked material. Most elastomers are loaded with carbon black, or similar additives. The annual production of elastomers is about half that of synthetic fibers. The main elastomer product is styrene-butadiene rubber. Synthetic Rubber Production 3000 2500 Styrene-Butadiene Other Nitrile Polybutadiene Ethylene-Propylene Polychloroprene 2000 Millions of lb. 1500 1000 500 0 1988 1990 1992 1994 1996 1998 2000 Year
Thermosetting Resin 603-1.12 Thermosetting resins are polymeric materials that form a covalently bonded network as the final product. The polymerization reactions involved in network formation are generally of the step-growth type. The point at which a continuous mesh of covalent bonds spans the entire part or film is called the gel point. The conversion of monomer at the gel point is controlled by the stoichiometry of bifunctional and multi-functional monomers. Because the shape is unalterable once the polymerization is complete, thermoset resins are generally produced in oligomeric, prepolymer form. These prepolymers are then used as molding and casting compounds for thermosetting plastics, or as part of an adhesive or coating formulation. The Thermoset Resin Production 5000 4500 4000 3500 Phenolic Polyester Melamine Urea Epoxy Millions of lb. 3000 2500 2000 1500 1000 500 0 1988 1990 1992 1994 1996 1998 2000 Year production of thermoset resins is about 10 billion pounds per year.
Adhesives 603-1.13 Adhesives are polymer-based formulations used to bond two surfaces together. Adhesives can be either reactive (form chemical bonds) or adhere by physical (van der Waals, ionic, etc) bonds. In either case, it is important that the adhesive can be reliably delivered to the joint that needs to be made, and that the adhesion and cohesion of the final joint are optimized for the particular use. Adhesion refers to the bonding between dissimilar surfaces. Cohesion is the bonding within the polymer film itself. Modern adhesives are classified either by the way that they are used or by their chemical constituents. Anaerobics - used to bond metal surfaces together when air is excluded. They are generally based on acrylic resins. Cyanoacrylates - reactive cyanoacrylic adhesives that cure in the presence of moisture. They solidify in seconds. Toughened Acrylics - general use, strong acrylic-based adhesive. It's applied as a two-part, resin/catalyst system. Epoxies - adhesives consisting of epoxy resin and hardener. These onepart, or two-part adhesives are extremely strong and versatile. Polyurethanes - fast-cure, two-part adhesives used for bonding glassreinforced plastics. Modified Phenolics - phenolic resins for bonding metal to metal, or metal to wood. They require pressure and heat to cure. ot Melts - semi-crystalline polymers that bond physically. Joints form quickly, but are not very strong. Plastisols - poly(vinyl chloride) based dispersions that require heat to harden. Once set, the joint is tough and resilient. Rubber Adhesives - solutions or latexes of rubber that solidify by loss of solvent. Do not give load-bearing joints. Poly(vinyl acetates) - PVA emulsions for use in bonding porous surfaces. Used in the packaging industry. Pressure Sensitive Adhesives - used for labels and tapes. They don't solidify, rather stay in the rubbery flow regime.
Characterization Polymer Design Testing 603-1.14 Microstructure Morphology Properties Applications Synthesis + Processing monomer microstructure polymer helix (ordered) random coil (disordered) morphology crystalline phase amorphous phase