POLYMER

1. POLYMER TYPES, APPLICATIONS AND FABRICATION/PROCESSING

2. 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

5. Polymer Types: Elastomers Elastomers – rubber • Crosslinked materials • Natural rubber • Synthetic rubber and thermoplastic elastomers • SBR- styrene-butadiene rubber styrene butadiene – Silicone rubber

7. 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.

8. 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..."[3] • When used as a woven material, it is suitable for mooring lines and other underwater applications.

9. 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

10. 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

11. 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

12. 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

13. 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.

14. POLYMER SYNTHESIS AND FABRICATION

15. 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

16. 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”

17. 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.

18. 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

19. 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.

20. 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

21. 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. )

22. 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. )

23. 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.)

24. Blown-Film Extrusion Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)

25. 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.