Handbook of Plastics Technologies Part 5 ppt

Reinforcing fibers greatly increase epoxy modulus,strength, impact strength, heat deflection temperature, and dimensional stabilityTable 3.35.. Polarity, reactivity, low shrinkage, high

3.1.4.3 Other Formulating Ingredients. A number of classes of additives are oftenused by individual formulators to modify or introduce new properties into the epoxy sys-tem.

3.1.4.3.1 Diluents. When epoxy resins are too viscous or too exothermic for a ular process, they can be modified by addition of low-molecular-weight aliphatic ep-oxides Diepoxides can copolymerize directly into the curing process without reducingcross-linking Monoepoxides can also copolymerize but do reduce the degree of cross-linking and thus soften properties The literature also mentions nonreactive diluents such

partic-as plpartic-asticizers, but these would raise serious questions about degradation of properties

3.1.4.3.2 Polymer Blends. A number of polymers are mentioned as modifiers for oxy resins Coal tar, phenol-formaldehyde, and polyurethane combine readily to produceintermediate properties Silicones can add more unique properties Polyesters andmelamine-formaldehyde are also mentioned in the literature

ep-3.1.4.3.3 Flame Retardants. Flame retardance can be built into the epoxy resin byuse of tetrabromobisphenol A or anhydride curing agents containing phosphorus or halo-gen It can also be helped by nonreactive additives such as alumina trihydrate or organo-halogens + antimony oxide

3.1.4.3.4 Functional Fillers. A variety of fillers can be used to add specific ties Metals, and beryllium and aluminum oxides, can be added to increase thermal con-ductivity (Table 3.33) Metals can be added to increase electrical conductivity(Table 3.34) Graphite increases lubricity and electrical conductivity Mica increases elec-

proper-TABLE 3.32 Pot Life of Epoxy/Curing Agent Systems

Aliphatic amines 1 hrAmine-terminated polyamides 3 hrAromatic amines 18 hrAcid anhydrides 84 hr

FIGURE 3.35 Flexible curing agents for epoxy resins

trical resistance Alumina trihydrate increases arc resistance Microballoons producestructural foam of high compressive strength

3.1.4.3.5 Reinforcing Fibers. Reinforcing fibers greatly increase epoxy modulus,strength, impact strength, heat deflection temperature, and dimensional stability(Table 3.35)

TABLE 3.33 Thermal Conductivity of Filled Epoxy Resins, Btu/[(ft2-hr-ºF)/ft]

3.1.4.4 Markets and Applications. The largest use of epoxy resins is in coatings, prising 53 percent of the total U.S market (Table 3.36) They do not require solvents, sothey protect the environment They have high adhesion and chemical resistance, so theygive durable protection They are particularly useful in marine maintenance.

com-Reinforced epoxy resins are the basis of printed circuit boards, tanks, pipes, and space materials Cast epoxies are very useful in electrical potting and encapsulation oftransistors, switches, coils, integrated circuits, transformers, and switchgears

aero-Performance in adhesives is outstanding Polarity, reactivity, low shrinkage, high ulus and strength, heat and chemical resistance all contribute to wide use in auto, aero-

mod-TABLE 3.35 Properties of Reinforced Epoxy Resins

Reinforcing fiber None Glass Graphite Graphite

15%

1174214Printed circuit boards 12

Flooring and paving 8Reinforced plastics 7Tooling, casting, molding 4

space, appliance, and mechanical construction The total U.S market is 600 million lb/yr,and growth rate has still not reached maturity.

3.1.5 Silicones

Silicone chemistry is a marriage of organic polymers and inorganic ceramics, which hasproduced synergistic benefits in abhesion, low-temperature flexibility, high-temperaturestability, flame-retardance, electrical resistance, water resistance, and physiological inert-ness, leading to a family of elastomers and thermoset plastics with a wide variety of spe-cialized applications

3.1.5.1 Chemistry. Silica sand is electrothermally reduced to silicon metal

SiO2 + C → Si + CO2This is mixed with copper catalyst and reacted with methyl chloride at 250 to 280°C toproduce a mixture of methyl chlorosilanes

9% CH3SiCl3 b.p 66°C designated T for trifunctional

74% (CH3)2SiCl2 b.p 70°C designated D for difunctional

6% (CH3)3SiCl b.p 57°C designated M for monofunctional

These are separated by fractional distillation

The chlorosilane Si-Cl bond hydrolyzes rapidly in water to form silanol Si-OH, whichcondenses instantly to form siloxane Si-O-Si (Fig 3.36) Thus, (CH3)2SiCl2 (D) produceslinear silicone rubber Introducing CH3SiCl3 (T) produces branching and cross-linking; athigh concentrations, it produces a thermoset plastic Conversely, introducing (CH3)3SiCl(M) caps the ends of the growing chains and lowers the molecular weight of the rubber

The most common alkyl group is methyl Introducing some phenyl groups preventscrystallization at low temperatures and thus keeps silicone rubber flexible down to lowertemperatures; phenyl groups also increase heat stability at high temperatures, thus creating

a wider useful temperature range for silicone rubber CF3CH2CH2- and NCCH2CH2groups are used to increase resistance to fuels, oils, and organic solvents CH2=CH-groups provide reactivity for vulcanization/cure of the rubber CH3O- and CH3CO2-groups hydrolyze more slowly than Cl- and are used to provide controlled reactivity forcross-linking, coating, and adhesive bonding

-3.1.5.2 Properties. Unlike most elastomers, silicone rubber does not contain C=Cgroups, so it is much more resistant to oxygen and ozone

FIGURE 3.36 Silicone synthesis

The Si-O and Si-C bonds in silicones are very stable, giving them high resistance toheat, electrical, and chemical attack.

The large size of the Si atom, and the oblique (150o) angle of the Si-O-Si bonds, givevery little steric hindrance and very free rotation This makes the silicone molecule veryflexible and rubbery, even down to very low temperatures On the down side, it also pro-duces low mechanical strength and low solvent resistance

The sheath of primary hydrogen atoms, on the methyl groups surrounding the polymermain-chain, gives low intermolecular attraction, which also contributes to rubbery behav-ior and low mechanical strength, and especially to low surface energy and low surface ten-sion, which produce abhesion (nonstick) and water-repellent performance

3.1.5.3 Rubber. Silicone rubber can be heat-cured by fairly conventional techniques Itcan also be cast and cured at room temperature, producing what is called room-tempera-ture vulcanized (RTV) rubber

3.1.5.3.1 Heat-Cured Rubber. High-molecular-weight (500,000) linear silicone ber is very soft and has no strength or creep resistance It can be cross-linked by heatingwith peroxides (Table 3.37) The reaction of peroxide with the methyl group (Fig 3.37) isnot very efficient and levels off at 0.4 to 0.7 cross-links per 1000 Si atoms—too low togive good strength and resistance to compression set Therefore, the rubber is usuallymade with a fraction of a percent of vinyl side-groups; these react readily with peroxide,giving a 90 percent yield of predicted cross-links and much better strength and compres-sion-set resistance If vinyl side-groups are increased up to 4 to 5 percent, silicone rubbercan even be cured by conventional sulfur vulcanization

rub-Most rubber is reinforced by carbon black; silicone rubber is not Instead, it is forced by fine-particle-size fumed silica This definitely improves tensile strength, though

reit still cannot equal most other types of elastomers (Table 3.38) Other fillers do not crease strength but may be used to improve processability, increase hardness and reducetack and compression set Carbon black is used to increase electrical conductivity

in-Small production runs are processed by compression or transfer molding at 800 to3,000 psi and 104 to 188°C; mold shrinkage is 2 to 4 percent Long production runs aremore economical by injection molding at 5,000 to 20,000 psi, 188 to 252°C, and a 25 to

90 sec cycle Extrusion requires post-cure in a 316 to 427°C hot-air oven, typically

60 ft/min; steam post-cure can run 1200 ft/min Calendering typically runs 5 to 10 ft/min

Specific formulations can aim at various product needs (Table 3.39) Particularly standing is their wide useful temperature range (Table 3.40)

out-3.1.5.3.2 Room-Temperature Vulcanized (RTV) Silicones. Low-molecular-weightliquid silicone oligomers, with reactive functional groups, can be poured or spread with

TABLE 3.37 Peroxides for Cross-Linking Silicone RubberBis(2,4-dichlorobenzoyl) peroxide 104–132oC

2,5-dimethyl-2,5-di(t-butylperoxy) hexane 166–182°C

little or no equipment and cross-linked (cured) at room temperature without damage todelicate electronics or other systems They are very useful in caulking, sealants, adhesives,and arts and crafts They are available as one- or two-part systems

One-part systems are packaged in dry sealed cans and are perfectly stable in this state.When they are poured or spread to form products, they are activated by atmospheric mois-ture, and the cross-linking reaction occurs The stable packaged oligomer has acetoxy ormethoxy end-groups When these are exposed to atmospheric moisture, they hydrolyze tohydroxyl end-groups, which condense with each other very rapidly to polymerize to highmolecular weight and cross-link to thermoset rubbery products (Fig 3.38) Acetoxy ismore reactive, becoming tack-free in 1/4 to 1/2 hr and fully-cured in 12 to 24 hr; but it re-leases acetic acid, which can corrode copper and steel Methoxy is slower, becoming tack-free in 2 to 4 hr and fully-cured in 24 to 72 hr; it does not cause corrosion, and it giveshigher-strength products (Table 3.41) Since one-part systems depend on diffusion of at-mospheric moisture, they are limited to 1/4-in thickness; thicker products require two-partsystems

Two-part systems are stable until they are mixed The pairs are very specific cally and must be mixed in the proper stoichiometric ratio, so the supplier specifies theprocedure, and the processor simply needs to follow it The two parts may react by con-

chemi-TABLE 3.38 Fillers for Silicone Rubber

Filler Particle size, µm Tensile strength, psi

Precipitated silica 18–20 600–1100Diatomaceous silica 1–5 400–800

densation or addition (Fig 3.38) In condensation cure, the hydroxyl-terminated siliconeoligomer is cross-linked by tetraethyl silicate, catalyzed by dibutyl tin dilaurate or faster

by stannous octoate, and liberates alcohol, so it can be used only in an open system In dition cure, a silicone oligomer containing vinyl CH2=CH- groups reacts with a siliconeoligomer containing silane Si-H groups, catalyzed by platinum; since no volatiles are lib-erated, this can be done in a closed system, and it gives higher strength products(Table 3.42)

ad-More recently, this has led to the development of liquid injection molding (LIM), inwhich the reactive silicone oligomer system is injection molded at 200 to 250°C and cures

in a few seconds, a great advance over conventional vulcanization systems

TABLE 3.39 Properties of Heat-Cured Silicone Rubbers

Grade High.-temp

High-strengthLow-temp

resistant

Solvent-Wire andcable

TABLE 3.41 Properties of Cured Methoxy RTV Silicone

Working time 30 minTack-free time 2–3 hrCure time (1/8 in thick) 24 hrShore A hardness 28Tensile strength 150 psi

Adhesion: lap shear 100 psiAdhesion: peel 20 lb/inVolume resistivity 4.7 × 1014Ω-cmDielectric constant 3.6

Dissipation factor 0.002

FIGURE 3.38 RTV silicone chemistry

3.1.5.3.3 Silicone Resins. Hydrolysis of (CH3)2SiCl2 produces linear flexible cules for rubber Hydrolysis of CH3SiCl3 produces highly cross-linked molecules for ther-moset plastics These are too cross-linked and brittle for most purposes Useful thermosetplastics are prepared by copolymerizing difunctional and trifunctional monomers In com-mercial practice, the ratio of difunctional to trifunctional is generally 80/20 to 40/60 Forsome products, methyl silicon may be partly replaced by phenyl silicon.

mole-The mixed monomers are dissolved in organic solvent and stirred with water to duce hydrolysis and condensation to low-molecular-weight oligomers Methyl silicon istoo reactive and exothermic and must be cooled to control the A-stage reaction Phenyl sil-icon is less reactive and may be heated to 70 to 75°C to promote the reaction

pro-The oligomer solution is then catalyzed by triethanol amine, metal octoates, or dibutyltin diacetate and heated to increase the viscosity At this point, it is cooled and can bestored until used These silicone oligomers are used to make glass fabric laminates and re-inforced molding powders Phenyl silicon is compatible with epoxy, alkyd, urea,melamine, and phenolic resins and may be blended with them to increase their resistance

to heat, flame, water, and weather

Glass fabric laminates are made by dipping the glass fabric into the oligomer solution,impregnating it with 25 to 45 percent silicone resin, and evaporating the solvent Layers ofimpregnated fabric are then plied to the desired thickness and press-cured Flat sheets arecured 30 to 60 minutes at 1000 psi and 170°C Complex shapes can be made by lower-

TABLE 3.42 Properties of Cured Two-Part RTV Silicones

Cure

Condensationcure Addition

pressure techniques such as vacuum-bag molding These laminates are 20 to 40 percentweaker mechanically than epoxy, melamine, and phenolic but superior in electrical insula-tion properties, especially at high temperatures and in moist conditions (Table 3.43) Theyare used in electric motors, terminal boards, printed circuit boards, and transformers Theyare also used for fire-resistance in aircraft firewalls and ducts

Molding powders are B-stage silicone resin plus glass fiber and catalyst They are pression molded 5 to 20 min at 1000 to 4000 psi and 160°C and then post-cured severalhours to achieve optimum properties Electrical insulation and resistance to heat and mois-ture are outstanding (Table 3.44) Molded parts are used in electric motors and switches

com-3.1.5.3.4 Coatings. Silicone resin solutions are baked to produce release coatingsthat are resistant to heat, water, and weather These are used in cooking and baking and forwater-repellent masonry They are also copolymerized with other thermosetting coatings

to increase their heat and weather resistance

3.1.6 Polyimides

New high-tech industries such as aerospace and electronics have created growing needsfor lightweight, strong materials with increased resistance to heat, oxygen, and corrosion.Organic polymer chemists have spent the past half century developing new polymers withhigher and higher performance The guiding general principle has been the use of hetero-cyclic resonance to provide molecular rigidity and thermal-oxidative stability There have

TABLE 3.43 Electrical Properties of Silicone-Glass Cloth Laminates

Matrix resin Phenolic Melamine Silicone

Dielectric strength, V/mil 150–200 150–200 250–300

Insulation resistance, Ω, dry

wet

10,00010

20,00010

50,00010,000

TABLE 3.44 Silicone Resin MoldingsSpecific gravity 1.65Flexural modulus, 23°C

200°C

1,800,000 psi900,000 psiFlexural strength, 23°C

200°C

14,000 psi5,000 psiTensile strength, 23°C

200°C

4,400 psi1,300 psiDielectric constant 3.6

been two persistent problems: (1) the syntheses are expensive, and (2) the molecular ity that gives heat resistance also makes processing very difficult The most successfulcandidates so far have been the polyimides (Fig 3.39).

rigid-Research has developed three synthetic routes to processability (1) Thermoplasticpolyimides contain enough single bonds in the polymer backbone to provide a certainamount of molecular flexibility and therefore processability (2) Two-stage condensationpolymerization leaves single bonds in the first stage to permit processability and thencloses them to heterocyclic imide rings in the final stage of processing (3) Second-stageaddition polymerization begins with synthesis of imide-containing vinyl or acetylenicmonomers in the first stage and then reacts the vinyl or acetylenic groups in the secondstage to produce cross-linking cure without liberating volatile by-products

3.1.6.1 Thermoplastic Polyimides. Several types of linear high-molecular-weight imides have been developed, which contain enough single bonds in the polymer backbone

poly-to make them somewhat flexible and therefore usable in conventional thermoplastic meltprocessing (Fig 3.40) This does, of course, sacrifice some of the inherent thermal stabil-ity of polyimides (Table 3.45)

The best-known are General Electric Ultem poly(ether imides); these offer heat tion temperatures of 207 to 221°C and continuous service temperatures of 170 to 180°C.Also popular are Amoco Torlon polyamide-imides, with heat deflection temperatures of

deflec-278 to 282°C More specialized are Ciba-Geigy trimethyl phenyl indane polyimides, withheat deflection temperatures of 232 to 257°C, embrittlement times of >2000 hr at 200°C

FIGURE 3.40 Thermoplastic polyimides

and 250 hr at 250°C, and decomposition temperatures of 450 to 510°C And fluorinatedpolyimides containing the hexafluoroisopropylidene group have been reported with tem-

peratures like T g = 340°C and continuous service temperature 371°C

3.1.6.2 Two-Stage Condensation Polyimides. Imides are produced by condensation action of amines with dibasic acids (Fig 3.41) Diamines plus tetrabasic acids producepolyimides When the reaction is run to completion, the highly cyclic structure is such arigid molecule that melt processing is impossible In fact, intramolecular cyclization com-petes with intermolecular cross-linking, so the cured polymer may actually be thermoset.However, the reaction can be run in stages by controlling temperature and time In the firststage, it produces a polyamic acid, which still has enough single bonds in the polymerbackbone to be a flexible molecule that is soluble and melt processable When the first-stage polymer has been impregnated into reinforcing fabric and/or melt processed into theshape of the finished product, then increasing the temperature and reaction time drives thecondensation cyclization reaction to the final imide structure Since the condensation reac-tion liberates water or alcohol, special techniques are required to remove the volatiles andavoid bubbles and cracks in the solidifying polymer

re-DuPont uses oxydianiline and pyromellitic dianhydride (Fig 3.42) to produce a series

of Kapton films (Table 3.46), Vespel sintered moldings (Table 3.47), and Pyralin lacquers

TABLE 3.45 Thermoplastic Polyimide Temperature Limits

FIGURE 3.41 Two-stage condensation of polyimides

FIGURE 3.42 DuPont polyimide

(Table 3.48) Monsanto (Skybond) and American Cyanamid (FM-34) used m-phenylenediamine and benzophenone tetracarboxylic dianhydride (Fig 3.43) to produce glass clothlaminates (Table 3.49) General Electric silicone polyimides (SiPI) are block copolymers

of benzophenone tetracarboxylic dianhydride with methylene dianiline and pyl) tetramethyl disiloxane (Fig 3.44), designed primarily for high-temperature electricalinsulation (Table 3.50)

bis(aminopro-3.1.6.3 Second-Stage Addition Polymerization Cure of Polyimides. To cure ting polyimides without the problem of volatile by-products, the cross-linking reaction isbased on addition polymerization instead of condensation polymerization This again is atwo-stage process In the first stage, a low-molecular-weight oligomer is prepared contain-ing finished imide groups; since it is low-molecular-weight, it is still easily processable,even though it contains aromatic and heterocyclic rings Then, in the second stage, reactivegroups in the oligomer are polymerized by addition reactions, building to high molecularweight and a high degree of cross-linking as well Several types of reactive groups havebeen developed

thermoset-TABLE 3.46 Kapton Polyimide Films

Tensile modulus, kpsi, 23°C200°C

430260Tensile strength, kpsi, 23°C

200°C

2517Elongation, %, 23°C

200°C

7090Impact strength, J/mm 23Folding endurance, cycles 10,000Initial tear strength, g 510Tear propagation, g 8Volume resistivity, Ω-cm 1015Dielectric constant 3.6Dissipation factor 0.0025Dielectric strength, V/mil 5,400

FIGURE 3.43 Monsanto and American mid polyimides

Cyana-3.1.6.3.1 Bis-Maleimides. Reaction of maleic anhydride with diamines leads to tworeactions First, the amine reacts with the dianhydride groups and produces bis-maleim-ides (Fig 3.45) Then, the amine adds across the double bonds (“Michael reaction”), thuslengthening the oligomer chain These oligomers are easily impregnated into glass cloth,

“catalyzed” by high-temperature peroxide such as dicumyl peroxide, stacked to the sired thickness, and press-cured or vacuum-bag cured, for example at 75 to 210 psi and

de-TABLE 3.47 Vespel Polyimide Moldings

Flexural modulus, 23oC, kpsi

260oC

550305Flexural strength, 23oC, kpsi

260oC

158.3Tensile strength, 23oC, kpsi

260oC

8.84.6Elongation, %, 23oC

260oC

64Compressive modulus, kpsi 386Notched impact strength, fpi 1.1Heat deflection temperature, oC 360

Volume Resistivity, Ω-cm 1014Dielectric constant 3.6Dissipation factor 0.003Water absorption, % 0.2

TABLE 3.48 Pyralin Lacquer Properties

Tensile strength, kpsi 18

Decomposition temperature, °C 560Volume resistivity, Ω-cm 1016Dielectric constant 3.5Dissipation factor 0.002Dielectric strength, V/mil 4000

200 to 250°C, followed by oven post-cure 12 to 24 hr to complete the cross-linking tion This produces excellent mechanical properties and heat resistance (Table 3.51).

reac-3.1.6.3.2 Acetylene-Terminated Imide Oligomers. Oligomers containing finished ide groups can be synthesized with terminal acetylenic (ethynyl) groups (Fig 3.46) Whenthese are impregnated into reinforcing fabrics and heat-cured, for example 500 hr/288 to

im-TABLE 3.49 Skybond Polyimide LaminatesFlexural modulus, kpsi

335 hr/299°C

3,1203,120Flexural strength, kpsi

30 min/407°C

8053Tensile strength, kpsi

335 hr/299°C

5742Volume resistivity, Ω-cm 2.47 × 1015Dielectric constant 4.15Dissipation factor 0.00445Dilectric strength, V/mil 179Water absorption, % 0.7

TABLE 3.50 Silicone Polyimide Electrical Properties

Bulk resistivity 1017Ω-cmDielectric constant 3.0Dielectric strength 5.5 MV/cm

FIGURE 3.44 General Electric silicone polyimides

FIGURE 3.45 Bis-maleimides

316°C, they give laminates with extreme heat resistance (Table 3.52) The mechanism ofthe cure reaction is complex, probably producing a variety of aromatic and fused-ringstructures (Fig 3.47).

3.1.6.3.3 Nadimide-Terminated Oligomers. Research at NASA, the U.S Air Force,and industrial laboratories has developed a series of thermoset polyimdes that are made byimpregnating the monomers into laminating fabric and then polymerizing and cross-link-

ing them in situ The body of the polyimide oligomer is made from benzophenone

tetra-carboxylic acid ester or bisphenyl hexfluoropropene tetratetra-carboxylic acid ester reactingwith an aromatic diamine such as phenylene diamine or methylene dianiline (Fig 3.48).The end-groups of the oligomer are made by end-capping with norbornene dicarboxylicacid ester And thermosetting cross-linking cure occurs by addition polymerization of theC=C bonds in the norbornene ring Laminate properties are very good (Table 3.53), andheat aging resistance is promising (Table 3.54) More recently, dinadimide end-capping(Fig 3.49) has reached use temperatures of 260 to 290°C

3.1.6.4 Polyimide Applications. Polyimides are used where their lubricity, low cient of thermal expansion, heat resistance, and radiation resistance are required Typicaluses include bearings and piston rings in jet engines, appliances, office equipment, com-

coeffi-TABLE 3.51 Bis-Maleimide Cured PropertiesFlexural modulus, 25°C

250°CAged 3000 hr/250°C

4000 kpsi

3200 kpsi

2600 kpsiFlexural strength, 25°C

250°CAged 3000 hr/250°C

70 kpsi

50 kpsi

26 kpsiTensile strength 50 kpsiCompressive strength 50 kpsiNotched impact strength 13 kpsi

Volume resistivity 6 × 1014Ω-cmDielectric constant 4.5Dissipation factor 0.012Dielectric strength 25 kV/mm

FIGURE 3.46 Acetylene-terminated imide oligomers

pressors, and automotive transmissions; seals and insulators in nuclear applications; tric motors, wire and cable, and magnet wire; printed circuit boards; and high-temperatureadhesives

elec-3.1.7 Miscellaneous Cross-Linking Reactions

Beyond the major thermoset plastics described above, research, development, and ized production have explored a number of other cross-linking reactions for producing

special-TABLE 3.52 Acetylene-Terminated Polyimide Cured Properties

Laminate flexural modulus, 23°C316°C

4,600,000 psi3,000,000 psiLaminate flexural strength, 23°C

316°C

69,000 psi45,000 psiShear strength, room temperature

Aged 500 hr/200°CAged 500 hr/288°CAged 500 hr/316°C

12,000 psi8,700 psi7,400 psi6,000 psiWeight loss, 1000 hr/351°C 4%

Dielectric constant, 10 MHz

12 GHz

5.383.12Loss tangent, 10 MHz

12 GHz

0.00060.0048

FIGURE 3.47 Cross-linking acetylene-terminated polyimides

TABLE 3.53 Nadimide-Terminated Polyimide Laminate Properties

Tensile modulus 21,700,000 psiTensile strength 180,000 psiFlexural modulus 17,600,000 psiFlexural strength 206,000 psi

Coefficient of thermal expansion 0

TABLE 3.54 Nadimide-Terminated Polyimide AgingShear strength at 316°C before aging

After 400 hrAfter 800 hrAfter 1200 hr

thermoset plastics, particularly addition reactions that do not produce volatile by-products.These may be grouped as (1) reactions of hydrocarbons, (2) triazine and other heterocyclicring formation, and (3) polyphenylene sulfide.

3.1.7.1 Reactions of Hydrocarbons. Several types of reactive hydrocarbon functionalgroups can be used to polymerize and cross-link monomers and oligomers into thermosetplastics These include addition polymerization of acetylene-terminated molecules andring-opening polymerization of strained carbon rings They also include Friedel-Craftscondensation to form hydrocarbon polymers

3.1.7.1.1 Acetylene-Terminated Monomers and Oligomers

Addition polymerization of acetylene nyl) groups can occur at high temperatures, forexample 500 hr at 288 to 316°C followed bycure 4 to 15 hr/407 to 434°C With monofunc-tional monomers, a major product is trimeriza-tion to form new aromatic rings (Fig 3.50)—but with difunctional monomers, a great vari-ety of cross-linked structures have been identi-fied and/or theorized Practically, many ofthese give thermoset plastics of high heat and moisture resistance, superior to epoxy res-ins Since there are no volatile by-products, this offers processing advantages over manycondensation-cured thermosets

(ethy-Polyimides have been cured by synthesizing acetylene-terminated oligomers ing finished imide groups, and these have shown excellent heat resistance, as discussedabove (Sec 3.1.6.3.2)

contain-Polysulfones have been made from acetylene-terminated sulfone monomers

(Fig 3.51), and cured graphite-fiber laminates have shown T g = 300°C and good cal properties at 170°C before and after heat and humid aging Semi-interpenetrating poly-mer networks with linear thermoplastic polysulfones showed promise of combining theheat deflection temperature and solvent-resistance of the thermoset polymer with the im-pact resistance of the thermoplastic

mechani-Polyphenylquinoxalines were cross-linked by acetylenic end-groups (Fig 3.52), giving

T g = 321°C and good resistance to hot humid aging, but the addition of aliphatic bon structure apparently sacrificed heat-aging resistance Propargyl ether of bisphenol A

hydrocar-(Fig 3.53) was cured to a thermoset plastic with T g = 360°C

FIGURE 3.49 Dinadimide end-capped polyimides

FIGURE 3.50 Addition polymerization of

acetylenic monomers

Phenylethynyl end-capping of polyimide oligomers (Fig 3.54) has shown promise for

high-temperature plastics and adhesives, with T g > 300°C and high adhesive strength, hotstrength, and oil resistance (Table 3.55)

3.1.7.1.2 Ring-Opening Polymerization of Strained Carbon Rings. The carbon atom

is tetrahedral, which means that normal C-C-C bond angles are about 109o In small ringstructures, the bond angles are much smaller than this, so they are under considerablestrain, unstable, and reactive When they break open into dienes or diradicals, they can po-lymerize Several such ring-opening reactions have been suggested for cross-linking cure

of thermoset plastics

Benzocyclobutene. Polyimide oligomers with benzocyclobutene end-groups(Fig 3.55) have been cured by electrocyclic ring-opening at 250°C The opening of thecyclobutene ring can lead to homopolymerization, or it can copolymerize with C=C bonds

in maleimides or with acetylene-terminated oligomers (Fig 3.56), all of which lead to

cross-linking and thermosetting cure Cured samples had T gs from 240 to 400°C or more;after 200 hr/350°C aging, they still retained 85 to 93 percent of their original weight Sim-ilarly, a benzocyclobutene-terminated diketone (Fig 3.57) cured to a thermoset plastic

FIGURE 3.51 Acetylene-terminated sulfone

FIGURE 3.52 Acetylene-terminated noxaline

polyphenylqui-FIGURE 3.53 Propargyl ether of bisphenol A

FIGURE 3.54 Phenylethynyl end-capped polyimide oligomer