Shrinkage During Cure

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. 

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In some moulding compositions other special purpose ingredients may be incorporated. For example, naphthalene, furfural and dibutyl phthalate are occasionally used as plasticisers or more strictly as flow promoters. They are particularly useful where powders with a low moulding shrinkage are required. In such formulations a highly condensed resin is used so that there will be less reaction, and hence less shrinkage, during cure. The plasticiser is incorporated to... 

Warping, difficulty of moulding to close tolerances and wavy or fibre-patterned surfaces or faults arising from the high shrinkage 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. 

Epoxies show superior adhesion to metals and glass, and have limited shrinkage during cure. They are used for surface coatings, as adhesives and rc>r laminating to produce plastic tanks, pipe, and aircraft parts, etc (Ref 3, p 445-R). They also have been used in manuf of plastics (Ref 1), construction of rockets St missiles (Ref 2) and in manuf of explosive compositions of improved mechanical and thermal stability (Ref 4)... 

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. 

Radiant cured epoxy systems also provide high glass transition temperatures, low shrinkage during cure, and relatively low residual stresses. They have no volatile emissions and excellent shelf life. The good physical and adhesive properties of conventional epoxy adhesives systems are maintained with the radiant cured systems. 

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. ... 

It should be noted that carbamide resins contain a large amount of tree water (30%-40%), which leads to more shrinkage during curing of the composition. In some cases this leads to cracking of the material [7],... 

Selecting an adhesive that has a low shrinkage during cure (<3%). 

Finite-element analysis has also been applied to predict internal stress distributions and to help explain stress transfer mechanisms such as shrinkage during cure. ... 

Stresses due to large mismatches in the CTEs of the adherends or residual stresses due to shrinkage during cure. 

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