Shrinkage index

Physical Properties. Physical properties include specific gravity, water absorption, mold shrinkage, transmittance, ha2e, and refractive index. Specific gravity affects performance and has commercial implications. The price of the material divided by the specific gravity gives the yield in cost per unit volume. Comparison of yields gives an evaluation of raw material costs. 

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

Water absorption 24 h at 23°C 30 d at 23°C Shrinkage on moulding Refractive index... 

Long chain alcohol esters have lower softening temperature and lower shrinkage on moulding. Polycyclohexyl methacrylate has refractor index similar to crown glass. The esters are prepared by alcoholysis of methyl ester. 

The increase in refractive index of fluorinated polyimide is thought to be the result of two main factors an increase in polarizability because of the elimination of fluorine atoms and an increase in the number of molecules per unit volume because of volume shrinkage. 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

Melting point, crystalline, °C Specific gravity Refractive index nf15 Mold shrinkage, average, %... 

Cross-linked pre-polymers having fractionalized molecular weights between 35,000 and 70,000 daltons with polydispersity indexes of less than 3.4 have been prepared by the free radical addition of 2-hydroxethyl methacrylate, HEMA, with 2% methacrylic acid or glycerol methacrylate. Once crosslinked, these materials are particularly useful as contact lenses because of their limited shrinkage and expansion. 

The level of sinkage/shrinkage is also influenced by other factors such as fillers, pigments, melt flow index, density and cooling rate. It should be noted that materials which are oriented, either purposely or accidentally (e.g. in a moulding, operation), will shrink significantly if they are heated above the temperature used for the orientation process. Amorphous materials show less shrinkage than crystalline polymers. 

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