POLYMER TYPES, APPLICATIONS AND FABRICATION/PROCESSING
Types of Polymer : Plastic • Have some structural rigidity under load, and are used in general purpose application. • Have a wide variety combination of properties: • Can be brittle • Flexible exhibit elastic and plastic deformation • May have any degree of crystallinity, and all molecular structures and configurations (linear, branch, isotactic, etc.) • May be either thermoplastic or thermosetting
Polymer Types: Elastomers Elastomers – rubber • Crosslinked materials • Natural rubber • Synthetic rubber and thermoplastic elastomers • SBR- styrene-butadiene rubber styrene butadiene – Silicone rubber
Polymer Types: Fibers Fibers - length/diameter >100 • Textiles are main use • Must have high tensile strength: subjected to a variety of mechanical deformation such as stretching, shearing and abrasion • Usually highly crystalline • Tensile strength increase with degree of crystallinity • Have linear and unbranched chains that are symmetrical and have regular repeat units. • Highly polar groups • Improve the fiber-forming properties by increasing both crystallinity and the intermolecular forces between the chain. • Must exhibit chemical stability to a rather extensive variety of environments, nonflammable and amenable to drying.
Example: • LycraTM • is a synthetic fibre known for its exceptional elasticity • It is a polyurethane-polyurea copolymer • KevlarTM • Applications, ranging from bicycle tires and racing sails to body armor because of its high tensile strength-to-weight ratio—famously: "...5 times stronger than steel on an equal weight basis..." • When used as a woven material, it is suitable for mooring lines and other underwater applications.
Polymer Types: Fibers • Formed by spinning • ex: extrude polymer through a spinnerette • Pt plate with 1000’s of holes for nylon • ex: rayon – dissolved in solvent then pumped through die head to make fibers • the fibers are drawn • leads to highly aligned chains- fibrillar structure
MISCELLANEOUS APPLICATIONS • Coatings – thin film on surface – i.e. paint, varnish • To protect item • Improve appearance • Electrical insulation • Adhesives – produce bond between two adherents • Usually bonded by: • Secondary bonds • Mechanical bonding
MISCELLANEOUS APPLICATIONS • Films • Films having thickness between 0.025 and 0.125 mm are fabricated and used extensively as bags for packaging food products and other merchandise • Important characteristic of the materials produced and used as films: • Low density • High degree of flexibility • High tensile and tear strength • Resistance to attack by moisture • Some polymers meet these criteria: • Polyethylene, polypropylene, cellophane & cellulose acetate • Foams – gas bubbles in plastic
MISCELLANEOUS APPLICATIONS • Foams – • plastics materials that contain high volume percentage of small pores and trapped gas bubbles. • Both thermoplastic and thermosetting materials are used as foams. • commonly used as cushion in cushions in automobiles and furnitures
Advanced Polymers • Ultrahigh molecular weight polyethylene (UHMWPE) • Molecular weight ca. 4x106 g/mol • Excellent properties for variety of applications • bullet-proof vest, golf ball covers, hip joints, etc. • Extraordinary characteristic; • An extremely high impact resistance • Outstanding resistance to wear and abrasion • Very good chemical resistance to normally encountered solvent • A self-lubricating and nonstick surface UHMWPE Adapted from chapter-opening photograph, Chapter 22, Callister 7e.
POLYMERIZATION • Polymerization = the synthesis process of polymers by which monomers are linked together to generate long chains composed of repeat unit. • The raw materials for synthetic polymer are derived from coal, natural gas and petroleum products. • The reaction by which polymerization occur are grouped into two general classification: • Addition • Condensation
Propagation • Termination POLYMERIZATION: Addition (Chain) Polymerization • Initiation R· = active initiator, · is an unpaired electron Sequential addition of monomer unit Two propagating chain may link together to form one molecule Two growing molecules that react to form two “dead chain”
Condensation (Step) Polymerization • The formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species. • Small molecular weight byproduct such as water that is eliminated.
Polymer Additives Foreign substance called additives are introduce to Improve mechanical properties, processability, durability, etc. • Fillers • Added to improve tensile strength & abrasion resistance, toughness & decrease cost • ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. • Plasticizers • Produces reduction in hardness and stiffness • Added to reduce the glass transition • temperature Tg • commonly added to PVC - otherwise it is brittle
Polymer Additives • Stabilizers • Antioxidants • UV protectants • Lubricants • Added to allow easier processing • “slides” through dies easier – ex: Na stearate • Colorants • Dyes or pigments • Flame Retardants • The flammability resistance of the polymer may be enhanced by additives called flame retardants.
Forming Technique of Plastics • Thermoplastic – • can be reversibly cooled & reheated, i.e. recycled • heat till soft, shape as desired, then cool • ex: polyethylene, polypropylene, polystyrene, etc. • Thermoset • when heated forms a network • degrades (not melts) when heated • mold the prepolymer then allow further reaction • ex: urethane, epoxy
Processing Plastics - Molding • Compression and transfer molding • thermoplastic or thermosetting Adapted from Fig. 15.23, Callister 7e. (Fig. 15.23 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984. )
Processing Plastics - Molding • Injection molding • thermoplastic & some thermosetting Adapted from Fig. 15.24, Callister 7e. (Fig. 15.24 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971. )
Processing Plastics – Extrusion • An extruder is a device that used a large screw to melt a thermoplastic polymer, compress it, & force it into a extrudate. Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)
Blown-Film Extrusion Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)
Summary • General drawbacks to polymers: -- E, sy, Kc, Tapplication are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement. • Thermoplastics (PE, PS, PP, PC): -- Smaller E, sy, Tapplication -- Larger Kc -- Easier to form and recycle • Elastomers (rubber): -- Large reversible strains! • Thermosets (epoxies, polyesters): -- Larger E, sy, Tapplication -- Smaller Kc Table 15.3 Callister 7e: Good overview of applications and trade names of polymers.