Classes of Polymeric Materials Chapter 3: Thermosets

1. Classes of Polymeric MaterialsChapter 3: Thermosets Professor Joe Greene CSU, CHICO

2. Thermosetting Resins (thermosets) • Introduction • Thermoplastics are supplied as pellets, powders, or granules and do not undergo a chemical reaction. • Thermoplastics have large molecular weights & long molecules • The high viscosities are reduced by high temperatures • Thermoset resins are supplied as liquid chemicals (low MW and low viscosity) and undergo a chemical reaction that features polymerization and crosslinking. • Liquid chemicals have short chains that polymerized into long chains and high molecular weights and high viscosity. • The chains are crosslinked (attached) to each other to make a stiff molecule • Rubbers involve cross-linking of already polymerized molecules to stiffen the molecules together in Vulcanization • Heat is needed to cause polymerization to build MW and to cause stiffening of molecule through cross-linking • Heat reduces the viscosity of the chemicals until the reaction occurs and then causes the viscosity to get very large during crosslinking.

3. Thermosetting Resins (thermosets) • Types of thermosets • Temperature activated • Catalyst activated • Mixing-activated • Temperature activated Fig 3.84 • All thermosets require heat to undergo chemical reaction • Lower temperature thermosets (room temperature cure) react to a more rubbery polymer that gets stiffer upon additional heat. • Pot life: time that it takes for the thermosets to react to a solid after mixed. • Gel time: time it takes for two liquid thermoset polymers that are mixed to form a gel or skin (and stop flowing) • Several thermosets are supplied as powder or granular form. • Heat reduces the viscosity and melts the polymer to allow it to flow & mold • Additional heat triggers a chemical reaction which forms a cross-linked 3D • Common heat activated polymers • Formaldehyde (FOR), phenoplasts (PF), amnioplasts (UF), polyester, vinyl ester, alkyd, allyl, furan, some epoxies, and polyimides

4. Thermosetting Resins (thermosets) • Catalyst activated: Fig 3.85 • Some thermosets supplied as stable liquid form • Small amount of liquid (catalyst) is added which starts a chemical reaction and leads to formation of 3D structure. • Chemical type and amount of catalyst controls the extent of reaction and the speed of polymerization. • Many systems can set at room temperature. • Useful for casting resins and for glass fiber reinforced composites. • Common polymer is unsaturated polyester resin (UPR)

5. Thermosetting Resins (thermosets) • Mixing activated systems: Fig 3.86 • Some thermosets supplied as two stable liquids. • When the two are added together, a chemical reaction starts and forms a 3D structure. • Ratio of the two chemicals and temperature controls the extent of reaction and the speed of polymerization. • Many systems can set at room temperature. • Useful for casting resins and for glass fiber reinforced composites. • Common polymers are polyurethane and epoxies. • Polyurethane can be mixed at high speeds in a Reaction Injection Molding (RIM) process.

6. Commercial Thermosets H O = C H OH HNH C N O N C H C H HN- C- NH N N N H H H H • Formaldehyde Systems: Functional Groups • Formaldehyde plus one of the three hydrogen containing chemicals to form a 3D molecular network • Phenol, • Melamine, or • Urea. • Condensation reaction involving the oxygen and two hydrogens from two different molecules, Phenol, Urea, or formaldehyde. • One stage systems with resols • Two stage systems with novolacs prepolymers, or precursers • Usually have large amounts of filler, e.g., wood flour, cellulose fibers and minerals. • Supplied as powder or granual form or pills (compacted preforms) • Molding temperatures (125°C – 200°C) and molding pressures of 2000 to 8,000 psi for compression molding and 18,000 psi for injection molding

7. Commercial Thermosets • Formaldehyde Systems: Functional Groups • Phenoplasts (phenolics) are based on phenol and formaldehyde and were one of the first commercial polymers, Bakelite, and were used for billiard balls. • Used with other materials to act as a binder, adhesive, coatings, surface treatments, etc. • Applications • Temperature resistant insulating parts for appliances (handles, knobs), electrical components (connectors, distributor caps) and bottle closures. • Abrasive binder for grinding wheels and brakes. • Decorative laminates (counter tops or table tops) • Fire resistant rigid foams.

8. Commercial Thermosets • Formaldehyde Systems: Functional Groups • Aminoplasts (amino resins) are based on urea and formaldehyde or melamine and formaldehyde. • Can be made translucent or in light colors for aesthetics • Urea-formaldehyde resins are used for many of the same applications as phenolics if have color requirements • Castable foam system is used for home insulation • Melamine-formaldehyde resins are based on melamine and formaldehyde • Noted for their excellent water resistance. • Used for dishwater safe dinner ware which can be decorated with molded-in paper overlays. • Form the surface layer for decorative laminates (Formica) • Used as an adhesives for water resistant plywood.

9. Commercial Thermosets O H C C H H C C H • Furan Systems • Feature a ring structure which can be opened cleaved to yield polymeric molecules which have 3-D molecular networks. • Combined with fomaldehyde related thermosets. • Used as binder for sand and foundry work or abrasive particles in grinding wheels. • Used as adhesives and matrix for reinforced plastics where corrosion resistance is important. • Allyl systems (Pg 171) • Manufacture involves the reaction of a monofunctional unsaturated alcohol, allyl alcohol (AA) with a difunctional acid. • Ester linkages are formed though not a polymer • 2 unsaturated C=C per monomer permits formation of 3-D molecule with the use of catalysts and elevated temperatures. • DAP (diallylphthalate) is most common allyl monomer • Thermoplastic pre-polymers are available that are cured with little shrinkage • Applications include high performance molding compounds for electrical

10. Commercial Thermosets • Alkyd Systems • Alkyd comes from alcohol (alk) and acid (yd) • Reaction of difunctional alcohol and difunctional acids or anhydrides forms a polyester which is what alkyd is. • Used as coatings (paints, coatings, varnishes) • Unsaturated Polyesters • Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol) • At least some of the acid (or anhydride) features double bonds between adjacent carbon atoms for unsaturation. • Characteristic ester linkages are formed, hence the name Polyester

11. Polyester Chemistry O O • Unsaturated Polyesters • Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol) C6H4(COOH)2 + (CH2)2(OH)2 -[(CH2)2 -O- C - C-O]- terephthalic acid + ethylene glycol Polyethylene terephthalate (PET) • Acids include: maleic, fumaric, isophthalic, terphthalic, adipic, etc. • Anhydrides include: maleic, phthalic • Glycols include ethylene glycol, diethylene glycol, propylene glycol

12. Polyester Chemistry • Heat or radiation can trigger the cross linking reaction • Catalyst is used • Starts reaction but is not consumed and is retrieved at end of reaction. • Initiator • Methyl ethyl ketone (MEK) peroxide, benzoyl peroxide, and cumene hydroperoxide • Starts reaction, then is consumed in reaction. • Accelerators (or promoters) speed up the reaction. • Inhibitors extend shelf life (hydroquinone, tertiary butyl catechol) • Condensation Reaction results in CO2 and H2O • Monomer required to polymerize, e.g., Styrene, to react with the unsaturations in the polyester molecules to form 3-D network. • Styrene at 30% to 50% in commercial polyester systems for polyester • vinyl toluene for vinyl ester resins • methyl methacrylate

13. Polyester Chemistry • Step 1: Create polymer and build MW of polymer chain • Condensation Polymerization of Di-ACID and Di-ALCOHOL • Fig 2.: Condensation reaction • Connects one end of acid with one end of alcohol to form polyester bond. • The opposite end of acid reacts with another free end of alcohol, and so on . • Have water as a by-product means condensation. • Still have unsaturated polymer. The Carbon atom has double bonds:

14. Polyester Chemistry • Step 2: Crosslink polyester polymer with unsaturated styrene. • Addition (free radical) reaction to connect polyester with styrene • Use a peroxide (free radical) to open the unsaturated bond to form saturation • One reaction starts, the other unsaturated bonds open up and react with the styrene to form a saturated polymer. • The ends of the polyester-styrene crosslinked polymer has peroxide end-groups. • Peroxide is an initiator and not a catalyst since it is consumed in reaction. Catalysts are not consumed in the reaction and can be retrieved at the end of it.

15. Sheet Molding Compound (SMC) • SMC is the paste that is compression molded • 33% polyester resin and stryrene, which polymerizes and crosslinks • 33% glass fibers (1” fibers) • 33% Calcium Carbonate

16. Epoxy Chemistry • Epoxy: O H H C C H + H2N (C) N (C) NH2 H H H H epoxide group + amines (DETA) epoxy • Other epoxy resins • diglycidyl ether of bisphenol A (DGEBRA) • tetraglycidyl methylene dianiline (TGMDA • epoxy phenol cresol novolac • cycloaliphatic epoxies (CA) • Curing agents (hardeners, catalysts, cross-linking agents) • aliphatic or aromatic amines (DETA, TETA, hexamethylene tetramine,etc.) • acid anhydrides (phthalic anhydride, pyromellitic dianhydride, etc.) • Active hydrogen react with epoxide groups. • As much as 15% hardener is needed

17. Polyurethane Chemistry • Reaction between isocyanate and alcohol (polyol). • Crosslinking occurs between isocyanate groups (-NCO) and the polyol’s hydroxyl end-groups (-OH) • Thermoplastic PU (TPU) have some crosslinking, but purely by physical means. • These bonds can be broken reversibly by raising the material’s temperature, as in molding or extrusion. • Ratio between the two give a range of properties between a flexible foam (some crosslinking) to a rigid urethane (high degree of crosslinking). • In PUR foams density can range from 1 lb/ft3 to 70 lb/ft3. • Foams are produced by chemical blowing agents. • Catalyst are used to initiate reaction. • RIM process is used to produce fenders and bumper covers

18. Other Thermosets • Polyimides • Bismaleimide • Polybenzimidazoles • Phenolics • Carbon Matrices • Thermoplastic matrices • Polyamides • Polypropylene • PEEK • Polysulfone • PPS

19. Polyimides • For temperature stability up to 600 F • Polyimides or polybenzimidazole (PBI) rather than epoxy • Aerospace applications due to high cost • Chemical Structure • Polyimides • Characterized by cyclic group containing a nitrogen and two carbonyl groups (C with double bond with oxygen) • PBI • Characterized by a five member ring containing two nitrogens and is attached to a benzene ring. • Polyimids and PBI are structurally planar and very rigid. Large aromatic groups are added into polymer to make stiffer.

20. Polyimides • Formed with two step condensation. Fig 2-5 • First step: An aromatic dianhydride is reacted with an aromatic diamine to form polyamic (polamide) acid. • Second step: Curing of the polyamic acid. • Formation of imide group by closing of 5-member ring • Condensation step of solvent molecules: water, alcohol, solvents • Chain extension • Cross-linking • High viscosities of polyamid acids require use of prepregs. • Impregnating the fiber mat with monomer solutions of diamines and diester acids. • Long times and gradual increase in temperature are needed.

21. Polyimides • Major condensation polyimids, Dupont’s Avimid N & K • are marketed as Prepreg polyimids • Avimid N Tg = 675F (360C), and • Avimid K: Tg = 490F (254C) • Linear polyimids are produced which have thermoplastic behavior above the Tg. • They process like thermoplastics for a few heat cycles. • Advantages of thermoplastic nature • Tractable nature of resins when hot facilitates the removal of volatiles. • Voids, formed as result of the evolution of gases, can be eliminated by applying pressure while heating the resins above Tg. • Applications • Wing skins for high performance aircraft.

22. Polyimides • Addition Polyimides • Many polyimids are cross linked with an addition reaction • Two general cross-linking reactions are widely used • End group reactions • Bismaleimide reactions • Reactive End Group Resin Fig 2-6 • First phase (imidization): results in the formation of the oligomeric (small polymer) imide • Second phase (consolation): is when the oligomer melts and flows to fill voids that were created from volatiles depart. • Third phase (crosslinking): oligomer builds MW & crosslinks • MW = 1500 • Shorter polymer chains gave lower viscosity and better wet-out • Wet-out is defined as uniform coating and soaking of resin in fiber. • Commercial end group resin (PMR) is PMR 11, PMR 15 and PMR 20 • PMR-11 has more end groups and higher cross-linking density and higher stiffness • PMR-20 gave better thermal stability. • PMR-15 has the best physical properties balanced.

23. Polyimides • Second type of endgroup crosslinking has acetylene endgroups and is called Thermid 600 • Crosslinking • First step: joining two polyimid oligomers to form a butadiene linkage which results in chain extension. Each double bond can react with double or triple bonds to form highly crosslinked. • Addition reaction • Problems is with too fast a cure and chain extension competing with cross-linking mechanism thus causing MW to build too fast. • Alleviated with proper solvents. • Disadvantage is the loss of tackiness in prepregs as the solvent evaporates.

24. Polyimides • Bismaleimide (BMI) resins • Addition polymerization • Reactions involving bismaleimide (BMI) derivatives: Fig 2-8 • Case 1 • Carbon-Carbon double bond in the maleimide group reacts with the carbon-carbon double bond in the olefin co-reactant (similar to maleic acid is crosslinked with styrene in polyester) • Case 2 • An aromatic diamine adds to the carbon-carbon double bond of the maleimide in what is called Michaels Reaction. • Both cases: the coreactants (olefin or diamine) form bridges between the imide molecules to form a crosslinked structure • Commericial products • Ciba-Geigy uses an olefinic compound with two olefins

25. Polyimides • Bismaleimide (BMI) resins • Advantages • Low processing temperature versus polyimides (Cured at 350F) • Standard epoxy processing equipment can be used since same T. • Postcure of 475 F is required to complete polymerization. • BMI are fully formed polyimides when reacted to form composite • Thus, no volatiles are removed and no consolidation problems • Tack and drape are quite good because of the liquid component of the reactants

26. Polyimides • Polybenzimidazole (PBI) resins • Less prevalent than the polyimides, PBI have equivalent and sometimes superior physical and thermal properties • Formation reaction- fig 2-9 • Five member ring containing two nitrogens is formed with accompanying aromatic groups. • Groups are flat and stiff leading to good physical properties and aromatics result in high thermal. • Problems are expensive, difficult process, toxicity • Some have been alleviated and is commercially available • Resin is thermoplastic with a Tg over 800F (427C) • It does not burn, contribute fuel to flames or produce smoke • Forms a tough char • Resins are toxic and need to be handles with care.

27. Phenolics • Phenolics is an old thermoset resin • Used for general purpose, unreinforced plastic • electrical switches • junction boxes • automotive molded parts • consumer appliance parts, handles, billiard balls • Fillers are required due to high shrinkage and brittle nature. • Sawdust, nut shells, talc, or carbon black • Fiber reinforced Phenolics have aerospace applications • Rocket nozzles, nose cones due to ablative nature (Goes from solid to gas during burning) • High temperature aircraft ducts, wings, fins, and muffler repair kits

28. Phenolics • Phenolic chemical structure- • Formed by reaction between phenol and formaldehyde • Condensation reaction releases water as a byproduct. • Initially low molecular weight, soluble and fusible, A-Stage resin • Condensation reaction involves more and more phenol molecules that causes the resin to pass through a rubbery, thermoplastic state that is only partially soluble phase called B stage. • Resin is cured and cross-linked thermoset resin, C- Stage. • Other terms describing phenolic formation • Resole: If phenol/formaldehyde reaction is carried out in excess formaldehyde and base catalyst is called resole at low molecular weight stage. Requires just heat to convert to C-stage (1 step) • Novolac: If phenol/formaldehyde reaction is carried out in excess phenol with an acid catalyst is called novolac. • Requires addition of a hardener (hexamethylenen tetramine) to achieve C- Stage in 2 steps. It provides acid to both reactants which speeds up reaction. • Reinforcements are mixed with novolacs for composites. Bstaging is when any other resin is cured to an intermediate stageand cured by heating

29. Thermoplastic Composites • Plastics are reinforced with glass and a few with carbon fiber • Nylon, PP, PBT, PEEK and PEK, and Polysulphone • Advantages • Requires less processing time since it is heated and not cured. • Thermoplastic pre-preg sheets have infinite shelf life versus thermoset • Disadvantages • Have lower thermal resistance than most thermoset composites • Have lower strength and modulus than some thermoset composites • Have difficulty wetting out high fiber loading composites.

30. Thermoplastic Matrices • Two types of thermoplastic composites: Discontinuous and continuous reinforcements • Discontinuous fiber- Conventional thermoplastics and short (3mm) or long fibers (6mm) • Polypropylene, nylon, PET, PBT, Polysulphone, PE, ABS, PC, HIPS, PPO • Short Glass or Carbon fiber increases • Tensile strength, modulus, impact strength, cost, thermal properties • Short Glass or carbon fiber decreases • Elongation, • CLTE, • Moisture sensitivity

31. Thermoplastic Matrices O O O O C n O C n • Several types of resin types • Conventional plastics: Less expensive (< $2 per pound) • Commodity plastics : PP, PE, PVC, PS, etc. (<$1 per pound) • Engineering resins: PC, PET, PBT, Nylon, ABS etc. (>$1pp) • High Performance Plastics: High Costs (> $10 per pound) and High Thermal Properties • PEEK, PEK, LCP, PPS, Polyaryle Sulfone, Polysulfone, Polyether sulfone, Polyimid • PEEK and PEK = $30 per pound • Polyarylesters • Repeat units feature only aromatic-type groups (phenyl or aryl groups) between ester linkages. Called wholly aromatic polyesters PolyEther-Ether-Ketone (PEEK) PolyEther-Ketone (PEK)

32. Properties of Reinforced PEEK

33. Advantages and Disadvantages of Polyketones • Advantages • High continuous use temperature (480F) • High toughness, especially at high temperatures. • Outstanding wear resistance • Excellent water resistance and better than thermoset composites • Excellent mechanical properties • Very low flammability and smoke generation • Resistant to high levels of gamma radiation • Higher Elongation (30%-100%) versus thermosets (1%-10%) • Disadvantages • High material cost and long processing times • High processing temperatures due to high viscosities (1 Million poise) versus thermoset composites (Epoxy = 10 poise). Syrup = 1000 poise • Moderate or poor resistance to hot oils • Difficult to have high fiber loadings due to high viscosity • Need special processing techniques; comingle plastic powder with fiber sheet and consolidate (impregnate resin in fiber bundle) through heated rollers.

34. Thermoset Reacting Polymers • Process Window • Temperature and pressure must be set to produce chemical reaction without excess flash (too low a viscosity), short shot (too high a viscosity), degradation (too much heat)

35. Compression Molding of Polyesters • Compression molding was specifically developed for replacement of metal components with composite parts. • Materials can be either thermosets (SMC) or thermoplastics (GMT) • Most applications today use thermoset POLYESTER polymers, e.g., SMC or BMC. In fact,compression molding is the most common method of processing thermosets.

36. Resin Transfer Molding of Polyester or Epoxy • In the RTM process, dry (i.e.,unimpregnated ) reinforcement is pre-shaped and oriented into skeleton of the actual part known as the preform which is inserted into a matched die mold. • The heated mold is closed and the liquid resin is injected • The part is cured in mold. • The mold is opened and part is removed from mold.

37. Open Mold Processing of Composites • Open Mold processes of Polyester or Epoxy • Vacuum bag, pressure bag, SCRIMP • Autoclave: Apply Vacuum Pressure and Heat in an oven which can be 5 feet to 300 feet long

38. Polyurethane Processing • Polyurethane can be processed by • Casting, painting, foaming • Reaction Injection Molding (RIM)

39. Structural RIM for Urethanes (Fast RTM) • Fiber preform is placed into mold. • Polyol and Isocyanate liquids are injected into a closed mold and reacted to form a urethane.

40. Composite Reinforcement Classifications • Reinforcement Type • Discontinuous (fibers are chopped and dispersed in matrix resin) • Short fibers: fiber lengths 3mm or less (glass filled plastics, GF-Nylon) • Long fibers: fiber lengths greater than 6 mm. (Some injection molded materials with 6mm fibers, Sheet Molding Compound (SMC) with 1” fibers, DFP Directed Fiber Preforms for RTM and SRIM) • Particulates: fibers is forms as spheres, plates, ellipsoids (some injection molded materials reinforced with mineral fibers) • Continuous (fibers are throughout structure with no break points) • Glass roving: glass bundles are wound up in a packet similar to yarn. • Roving is woven into several weaves using a loom machine like in apparel. • Mat products: random swirl glass pattern. • Woven product: roving is woven into machine direction (warp) and cross direction (weft) • Uni product: roving is woven in one direction with a cross thread given to hold mat together.

41. Processing of Fiber Reinforcements • Carbon fiber or glass fiber • Hand lay-up and Spray-up • Filament winding

42. Injection Molding Glass Reinforced Composites Glass filled resin pellets • Plastic pellets with glass fibers are melted in screw, injected into a cold mold, and then ejected.

43. Composites Can Have a Fiber Preform • Fiber type • Roving form that can be sprayed into a 3-D preform • Roving form that is woven into a glass sheet and then formed to shape (preform)

44. Glass Fibers • Properties of Glass Fibers: (Table 3-1)

45. Carbon/Graphite Fibers • Need for reinforcement fibers with strength and moduli higher than those of glass fibers has led to development of carbon • Thomas Edison used carbon fibers as a filament for electric light bulb • High modulus carbon fibers first used in the 1950s • Carbon and graphite are based on layered structures of hexagonal rings of carbon • Graphite fibers are carbon fibers that • Have been heat treated to above 3000°F that causes 3 dimensional ordering of the atoms and • Have carbon contents GREATER than 99% • Have tensile modulus of 344 Gpa (50Mpsi)

46. Carbon Fiber Mechanical Properties Note: 1Mpsi = Mpa

47. Organic Fiber- Kevlar Properties • Properties- Table 3-3 • Kevlar has high heat resistance, though less than carbon fiber. • Kevlar has exceptional exposure limits to temperature • No degradation in properties after 7 days at 300 F. • 50% reduction in properties after 7 days at 480F. • 50% reduction in properties after 12 months of sunlight exposure in Florida • Kevlar are hygroscopic and are susceptible to moisture and need to be dried • Aramids do not bond well to matrices as do glass and carbon fibers • The ILSS (interlaminar Short beam shear) values are lower.