Classes of Polymeric Materials Chapter 3: Thermosets Professor Joe Greene CSU, CHICO

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.

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

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)

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.

Commercial Thermosets O = C 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 C N HNH H OH HN- C- NH O H

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.

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.

Commercial Thermosets 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 raction 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 prepolymers are available that are cured with little shrinkage Applications include high performance molding compounds for electrical

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

Polyester Chemistry 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 O

Polyester Chemistry Heat or radiation can trigger the cross linking reaction Catalyst (or initiator) is used. Methyl ethyl ketone (MEK) peroxide, benzoyl peroxide, and cumene hydroperoxide 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

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:

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.

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

Epoxy Chemistry Epoxy: O H H Other epoxy resins 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

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

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)

Compression Molding of Polyesters Compression molding was specifically developed for replacement of metal components with composite parts. T 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.

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.

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

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

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.

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

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

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

Thermoplastic Resins Several types of resin types Conventional plastics: Less expensive (< $2.00 per pound) Commodity plastics : PP, PE, PVC, PS, ABS, etc. Engineering resins: PC, PET, PBT, Nylon, etc. 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) O n C O n C

Properties of Reinforced PEEK

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.

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)

Glass Fibers Properties of Glass Fibers: (Table 3-1)

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)

Carbon Fiber Mechanical Properties Note: 1Mpsi = Mpa

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.