Handbook of Plastics, Elastomers and Composites Part 4 docx

Other monomers, in conjunction with poly-styrene, such as alpha methyl styrene, methyl methacrylate, vinyl toluene, diallyl phtha- Downloaded from Digital Engineering Library @ McGraw-Hi

Trang 1

2.5.2 Potting Resin Selection

Tables 2.2 through 2.5 provide helpful information for guidance in the selection processfor potting resins

2.5.3.1 Cast epoxies. Cast epoxy resins have proven to be very popular in a wide

va-riety of applications because of their versatility, excellent adhesion, low cure shrinkage,good electrical properties, compatibility with many other materials, resistance to weather-ing and chemicals, dependability, and ability to cure under adverse conditions Some oftheir application ﬁelds are adhesives, coatings, castings, pottings, building construction,chemical resistant equipment, and marine applications The most widely used epoxy res-ins in the casting ﬁeld are the epi-bis and cycloaliphatic epoxies Table 2.6 lists the proper-ties of typical cured epi-bis resins with a variety of curing agents, and Table 2.7 providesinformation on the properties of blends of cycloaliphatic epoxy resins

Novolac epoxy resins, phenolic or cresol novolacs, are reacted with epichiorohydrin to

produce these novolac epoxy resins which cure more rapidly than the epi-bis epoxies andhave higher exotherms These cured novolacs have higher heat-deﬂection temperaturesthan the epi-bis resins as shown in Table 2.8 The novolacs also have excellent resistance

to solvents and chemicals when compared with that of an epi-bis resin as seen in Table 2.9

2.5.3.2 Cast polyesters. General purpose polyester, when blended with a monomersuch as polystyrene and then cured, will produce rigid, rapidly curing transparent castingsexhibiting the properties shown in Table 2.10 Other monomers, in conjunction with poly-styrene, such as alpha methyl styrene, methyl methacrylate, vinyl toluene, diallyl phtha-

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)

Any use is subject to the Terms of Use as given at the website.

Trang 3

Trang 5

124 Chapter Two

Trang 7

Trang 9

128 Chapter Two

late, triallyl cyanurate, divinyl benzene, and chlorostyrene, can be blended to achievespeciﬁc property enhancements The reactivity of the polyester used, as well as the conﬁg-uration of the product, affect the choice of systems

Flexible polyester resins are available that are tougher and slower curing and producelower exotherms and less cure shrinkage They absorb more water and are more easilyscratched but show more abrasion resistance than the rigid type Their property proﬁle isshown in Table 2.11 The two types, rigid and ﬂexible, can be blended to produce interme-diate properties, as shown in Table 2.12

2.5.3.3 Cast polyurethanes. Polyurethanes are reaction products of an isocyanate, a

polyol, and a curing agent Because of the hazards involved in handling free isocyanate,prepolymers of the isocyanate and the polyol are generally used

Trang 10

The choice of curing agent influences the curing characteristics and final properties amines are the best general-purpose curing agent, as shown by Table 2.13 The highestphysical properties are produced using MOCA 4,4-methyl-bis (2chloroaniline) The othermajor class of curing agents, the polyols, are more convenient to use, but the final productshave lower physical properties By providing good abrasion resistance and a low coeffi-cient of friction, polyurethanes find application in roller coatings and press pads as well asgaskets, casting molds, timing belts, wear strips, liners, and heels and soles.

Di-2.5.3.4 Cast phenolics. Phenolic casting resins are available as syrupy liquids

pro-duced in huge kettles by the condensation of formaldehyde and phenol at high temperature

in the presence of a catalyst and the removal of excess moisture by vacuum distillation.These resins, when blended with a chemically active hardener, can be cast and cured solid

Trang 11

130 Chapter Two

Trang 12

in molds constructed from various materials and of a variety of mold designs They willexhibit a broad-based property proﬁle as described in Table 2.14 The mold designs avail-able are draw molds, split molds, cored molds, ﬂexible molds, and plaster molds.

2.5.3.5 Cast allylics 11 The allylic ester resins possess excellent clarity, hardness,

and color stability and thus can be cast to form optical parts These castings can be eitherhomo or co-polymers The free radical addition polymerization of the allylic ester presentssome casting difﬁculties such as exotherm control, monomer shrinkage during curing andthe interaction between the exotherm, the free radical source, and the environmental heatrequired to decompose the peroxide and initiate the reaction

A simple casting formulation is as follows:

Prepolymer: 60 parts/wt

Trang 13

132 Chapter Two

Trang 14

Tert-butyl perbenzoate: 2 parts/wt

Tert-butylcatechol: 0.1 parts/wt

The function of the catechol is to retard the polymerization and the exothermic heatover a longer time period, allowing the heat to dissipate and minimize cracking Monomercatalyzed with benzoyl peroxide or tert-butyl perbenzoate may be stored at room tempera-ture from 2 weeks to 1 year, but, at 120°F (49°C), the catalyzed resin will gel in a fewhours

Molds for casting allylics may be ground and polished metal or glass, with glass beingthe preferred type, since it is scratch resistant and able to take a high polish

Cast allylics are noted for their hardness, heat resistance, electrical properties, andchemical resistance as shown in Tables 2.15 and 2.16 They do lack strength, so their us-age is conﬁned to optical parts and some small electrical insulators

Allylic monomers are sometimes used with alkyds to produce polyesters, with the phthalate resin being the most widely used because of its lower cost and very low water va-por pressures Alkyd-diallylphthalate copolymers have signiﬁcantly lower exotherm than analkyd-styrene copolymer The electrical properties of allylic resins are excellent, and thevariations of dissipation factor, dielectric constant, and dielectric strength with temperatureand frequency are given in Figs 2.12 and 2.13 The surface and volume resistivities remainhigh after prolonged exposure to high humidity Resistance to solvents and acids is excel-lent, along with good resistance to alkalies Tables 2.17 and 2.18 compare the chemical re-sistance of some plastics with the chemical resistance of the DAIP formulations

ortho-2.6 Laminates 12

2.6.1 Laminates

Laminates can be deﬁned as combinations of liquid thermosetting resins with reinforcing

materials that are bonded together by the application of heat and pressure, forming an fusible matrix Plywood is a good example of a thermosetting laminate with the phenolicresin serving as the binder to bond the layers of wood sheets together when compressedwith heat in a molding press

in-2.6.2 Resins

The resin systems primarily used for laminates are bismaleimides, epoxies, melamines,polyesters, polyimides, silicones, and phenolics and cyanate esters These resins are de-scribed in Sec 2.7.8, and the reinforcements are described in Sec 2.6.3

2.6.3 Reinforcements

Glass ﬁbers are the most commonly used laminate reinforcement and are available in

some six formulations, with the E-glass providing excellent moisture resistance, which, inturn, results in superior electrical properties along with other valuable properties Proper-ties of reinforced plastics using a variety of reinforcing fibers are shown in Table 2.19.Compositions of the major types of glass fibers are shown in Table 2.20 The glass fibersare available in filaments, chopped strands, mats, and fabrics in a wide variety of diame-ters, as described in Table 2.21

Glass fabrics: Many different fabrics are made for reinforced plastics, with E-glass

be-ing the most common Filament laminates usbe-ing glass types of D, G, H, and K are also

Trang 15

Trang 16

common with the ﬁlaments combined into strands, and the strands plied into yarns Theseyarns can be woven into fabrics on looms.

2.6.4 Processes

The liquid resins are poured onto the reinforcement material and the combined forcement sheet is then placed in a “horizontal treater,” as shown in Fig 2.12, where theresin is dissolved in a solvent to achieve optimum wetting or saturation of the resin intothe reinforcement The sheets come out of the treater and are sheared to size and stacked

Trang 17

resin-rein-136 Chapter Two

and stored in temperature- and humidity-controlled rooms The preimpregnated, stackedsheets are further stacked into packs with each pack containing 10 laminates The stackedunits are then placed in between two polished steel plates, and the press is closed Thisprocess involves molding temperatures ranging from 250 to 400°F (122 to 204°C), withmolding pressures in the 200- to 3000-lb/in2range

The press may have 24 platens, making a press load of some 240 laminates each cycle.After molding, the laminates are trimmed to ﬁnal size and sometimes postcured by heatingthem in ovens

2.6.5 Properties

2.6.5.1 Physical and mechanical. The use of reinforcements in combination withthermosetting resins will produce laminates exhibiting higher tensile, compressive, and

Figure 2.12 (a) Horizontal treater and (b) decorative laminate treater (Source: Charles A.

Harper, Handbook of Plastics, Elastomers, and Composites, 3d ed., McGraw-Hill, New York,

1996, p 2.2)

Trang 18

ﬂexural strengths due to the polymerization of the resin with its property enhancementcontribution to the matrix The impact strength sometimes improves by a factor of 10, andthe laminates have no clear yield point, as shown in Fig 2.13 (stress and strain).

Reinforced thermosetting laminates are anisotropic, with their properties differing pending upon the direction of measurement The properties of a fabric laminate are con-trolled by the weave of the fabric and the number and density of the threads in the warp

de-and woof directions, with these values differing from the values in the z of thickness

direc-tion Both thermal expansion and conductivity properties are also anisotropic, as displayed

in Table 2.22

2.6.5.2 Electrical. The dielectric strength of laminates will decrease with increasingthickness and is highly dependent upon the direction of the electric ﬁeld stress Thisproperty will show higher when tested across the sample’s thickness, whereas end-to-end testing will show lower values Laminates with higher resin content will show bet-ter electrical properties but poorer physical properties than laminates with lower resincontent

2.6.5.3 Dimensional stability. Laminates using thermosetting resins are superior instability, thermal resistance, and electrical properties, with dimensional stability the mostimportant due to stresses incurred within the laminate during the molding and curing pro-

Figure 2.13 Stress-strain curves for various materials (a)

Rein-forced plastics, (b) wood and most metals, and (c) steel (Source:

Charles A Harper, Handbook of Plastics, Elastomers, and

Compos-ites, 3d ed., McGraw-Hill, New York, 1996, p 2.3)

Trang 19

Trang 20

cess Laminate thickness is limited by the thermal conductivity of the polymerizing resinand the thermal conductivity of the cured laminate The laminate derives its heat from thepress platens to begin the curing action, and the resin, as it polymerizes, will produce sub-stantial reactive heat, making it difﬁcult to produce very thick laminates The normalthickness range for laminates will range from 0.002 to about 2.000 in with special gradesproduced at 4 to 10 in.

2.7 Molding Compounds

2.7.1 Resin Systems

Thermosetting resin systems are the backbone of a large, versatile, and important family

of molding compounds This family provides the industrial, military, and commercial kets with plastic molding materials exhibiting exceptional electrical, mechanical, thermal,and chemical properties These important property values enable the product designer,manufacturing engineer, and research and design engineer to select from a wide choice ofproducts, and enable them to choose the most suitable molding compound to meet theirspeciﬁc needs and requirements These versatile materials are covered by military, indus-trial, and commercial speciﬁcations that are designed to ensure the quality of the moldedarticles utilizing thermosetting molding compounds

Trang 21

Trang 22

The following sections will take each of the family members, in alphabetical order, andoutline their history; speciﬁcations; and reinforcements, ﬁllers, and data sheet value appli-cations.

2.7.2 AIlyls: Diallyl-ortho-phthalate (DAP) and Diallyl-iso-phthalate (DAIP)

2.7.2.1 History 13 The most sophisticated work on saturated polyesters is usuallytraced to W H Carothers, who, from 1928 to 1935 with DuPont, studied polyhydroxycondensates of carbolic acids Unable to achieve suitable heat and chemical resistancefrom these esters, he turned to polyamide-carboxylic acid reaction products, which later

became nylon Carothers’ concepts on saturated polyesters yielded several other polyester

products such as Terylene, which became patented and introduced in the United States as

dacron and mylar.

In 1937, Carlton Ellis found that unsaturated polyesters, which are condensation ucts of unsaturated dicarboxylic acids and dihydroxy alcohols, would freely co-polymerizewith monomers that contained double bond unsaturation, yielding rigid thermosetting res-ins The allylic resins became commercial resins suitable for compounding into a broadrange of products with exceptional electrical, mechanical, thermal, and chemical properties

prod-2.7.2.2 Physical forms. Molding compounds using either the DAP or the DAIP resinsystems are available in free-ﬂowing granular form and also in high-bulk factor ﬂake form.The resins are in a white powder form, which makes it possible to provide a broad opaquecolor range Both compounds in their granular form are readily molded, preformed, or pre-plasticated automatically, whereas the high-bulk-factor compounds generally require aux-iliary equipment for such operations

2.7.2.3 Reinforcements. Compounds of either DAP or DAIP resin systems utilize avariety of reinforcement materials ranging from the granular compounds with mineral,

Định dạng
Số trang	40
Dung lượng	0,92 MB