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2.7 Incorporation of fibers into matrix

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chichi's version from 2018-02-28 21:13

Section 1

Question Answer
Ready-to-mold fiber-reinforced polymer sheets are available in two basic forms:prepregs and sheet-molding compounds
Prepregs These are thin sheets of fibers impregnated with predetermined amounts of uniformly distributed polymer matrix. Fibers may be in the form of continuous rovings, mat, or woven fabric
______ is the primary matrix material in prepreg sheets, although other thermoset and thermoplastic polymers have also been used.Epoxy
The normal shelf life for epoxy prepregs is ______ at room temperature however, it can be prolonged up to 6 months of more if stored at -18oC.6-8 days
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Section 2

Question Answer
Sheet-molding compoundsthin sheets of fibers precomponds with a thermoset resin and are used primarily in compression molding process
Common thermoset resins for SMC sheets are...polyesters an vinyl esters
SMC-Rcontaining randomly oriented discontinous fibers
The nominal fiber content (by weight percent) is usually indicated by two-digit numbers after the letter R. For example, the nominal fiber content in SMC-R30 is...30% by weight
SMC-CRcontaining a layer of unidirectional continous fibers on top of a layer of randomly oriented discontinous fibers.
The nomial fiber contents are usually indicated by two-digit numbers after the letters C and R. For example, the nominal fiber in SMC-CR40R30 are...40% by weight of unidirectional continuous fibers and 30% by weight of randomly oriented discontinous fibers
XMCcontaining continuous fibers arranged in an X pattern, where the angle between the interlaced fibers is between 5o and 7o
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Section 3

Question Answer
Commercial surface treatments for carbon fibers are of two types:oxidative or nonoxidative
Oxidative surface treamentsproduce acidic functional groups, such as carboxylic, phenolic, and hydroxylic, on the carbon fiber surface. They may be carried out either in an oxygen-containing gas (air, oxygen, carbon dioxide, ozone, etc.) or in a liquid (nitric acid, sodium hypochloride, etc.).
Gas-phase oxidationconducted at 250oc or above and often in the presence of a catalyst
Oxidation at very high temperature causes...excessive pitting on the carbon fiber surface and reduces the fiber strength
Nitric acidis the most common liquid used for the liquid-phase oxidation. The effectiveness of treatment in improving the surface properties depends on the acid concentration, treatment time, and temperature, as well as the fiber type
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Section 4

Question Answer
Nonoxidative surface treatementshave been developed for carbon fibers. In one of these treatments, the carbon fiber surface is coated with an organic polymer that has functional groups capable of reacting with the resin matrix
Examples of polymer coatings arestyrene maleic, anhydride copolymers, methyl acrylate-acrylonitrile copolymer, and polyamides.
The preferred method of coating the fiber surface is electropolymerization
Electropolymerizationin which carbon fibers are used as one of the electrodes in an acidic solution of monomers or monomer mixtures
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Section 5

Question Answer
Fillers are added to a polymer matrix for one or more of the following reasons:reduce cost, increase modulus, reduce mold shrinkage, control viscosity, produce smoother surface
Most common filler for polyester and vinyl ester resins iscalcium carbonate, which is used to reduce cost as well as mold shrinkage
Example of other fillers areclay, mica, and glass microspheres
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Section 6

Question Answer
Hot-melt impregnationis used mainly for semicrystallin thermoplastics, such as PEEK and PPS, for which there are no suitable solvents available for solution impregnation
Solution impregnationis used for polymers that can be dissolved in a suitable solvent, which usually means an amorphous polymer, such as polysulfone and PEI
Liquid impregnationUses low-molecular-weight monomers or prepolymers (precursors) to coat the fibers
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