Shrinkage reducing during cure

The ability to incorporate a large amount of the bulky and highly aliphatic DCPD moiety into a UPR polymer achieved two things. First, because DCPD is even less expensive than phthalic anhydride, a very cost-effective UPR is created. Secondly, the bulkiness of the DCPD moiety prevents the polymers stacking-up too closely, and consequently, reduces the shrinkage during curing. These two factors make such resins ideal for uses such as boat construction and tub and shower applications. 

The primary resin of interest is epoxy. Carbon-fiber-epoxy composites represent about 90% of CFRP production. The attractions of epoxy resins are that they polymerize without the generation of condensation products that can cause porosity, they exhibit little volumetric shrinkage during cure which reduces internal stresses, and they are resistant to most chemical environments. Other matrix resins of interest for carbon fibers include the thermosetting phenolics, polyimides, and polybismaleimides, as well as high-temperature thermoplastics such as polyether ether ketone (PEEK), polyethersulfone (PES), and polyphenylene sulfide. 

Ebonite compounds shrinks to about 6% during cure and as such due allowance for a change of dimensions has to be made for this during manufacture. The ebonite dust and the mineral fillers added for controlling the exothermic reaction serves to reduce this shrinkage. 

Thermal conductivity also helps to improve heat transfer during cure. This reduces exothermic temperatures and extends pot life, particularly at high filler loadings. Shrinkage during cure is also reduced, as explained in the sections above. 

Shrinkage is the reduction in volume during cure. Epoxies have a low curing shrinkage. Shrinkage may be reduced through the use of fillers. 

The fillers are added so that the adhesives are sag-free, motionless and form shapes that suit the components to be bonded, reduce shrinkage during cure, increase thermal conductivity, improve corrosion resistance and reduce costs. The concentration of these curatives, fillers and conducting agents is usually monitored using ICP-OES. 

Low profile plastics are added to reduce shrinkage during cure. They are normally thermoplastics that include polyvinyl acetates, polymethyl methacrylate, and copolymers with other acrylate, vinyl chloride-vinyl acetate copolymers, polyurethane, polystyrene, polycaprolactone, cellulose acetate butyrate, saturated polyester, and styrene butadiene copolymers. More details about the low profile additive (LPA) mechanism are published in the literature. ... 

Most material studies reported in medical journals are of interest to those involved in mainstream plastic applications. Some medical plastics must perform under constant water immersion. It was reported that absorption of 1 % water reduces the fatigue life of PMMA by a factor of four, since bone cement can only be replaced by a surgical operation such a performance is clearly unacceptable. The use of silane to treat the hydroxyapatite filler in this material reduced water uptake. The water uptake increased with increased concentration of hydroxyapatite. In applications, such as dental fillings, increased water uptake is considered helpful since it compensates for the loss of volume due to shrinkage of the filling during curing. 

Reduced shrinkage and stresses during cure Improved bond strengths Improved flow properties Extended pot life Reduced cost. 

Fillers and other additives are used to make molded objects from phenolic resins to reduce shrinkage during cure, improve strength, and provide flow control. Representative materials added are mineral flour, wood flour, nutshell flour, pressed olive stones, lignite, and natural rubber. Crosslinked phenolics containing inorganic or organic fillers are schematically illustrated in Scheme 35. 

Mica belongs to the fillers that are added to phenolic resins in order to reduce their shrinkage during cure, to improve their strength and to provide flow control. Furthermore, mica adds outstanding dielectric properties, high thennal and chemical resistance to phenolics, and it lows their water absorption. 

Fillers are inorganic particulates that can be added to the resin to reduce shrinkage, to reduce the peak exotherm during cure, to increase viscosity, to increase local hardness, to reduce flammability and to reduce cost. They can increase modulus and compressive strength and may be... 

Solvent cements should be chosen with approximately the same solubility parameter as the plastic to be bonded. Table 7.19 lists typical solvents used to bond major plastics. The solvent cement can be bodied to 25 percent by weight with the parent plastic to fill gaps and reduce shrinkage and internal stress during cure. 

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