Flexural moduli

The flexural moduli of a range of virgin and glass fibre reinforced engineering polymers are given in Table 2.9 

Polymers Unreinforced polymer Glass fibre reinforced polymer  

It is seen in Table 2.9, that flexural moduli improve considerably upon incorporation of 25 to 50% of glass fibre in the formulation. Epoxy resins are an exception, carbon fibre also improves the flexural modulus of PC from 2.8 to 13 GPa and with polyacetal the addition of carbon fibre improved the flexural modulus from 2.6 to 17.2 MPa. For PP the addition of 20% calcium carbonate produced hardly any change in flexural modulus. 

No improvement in flexural modulus resulted from the incorporation of minerals, aluminium powder, silica or silicones as shown in Table 2.10. 

Substances that have been used in this context include glass fiber (occasionally glass beads), carbon fiber, carbon nanotubes, carbon black, graphite, fuUerenes, graphite chemically modified clays and montmorillonites, silica, and mineral alumina. Other additions have been included in polymer formulations, including calcium carbonate, barium sulfate, and various miscellaneous agents, such as aluminum metal, oak husks, cocoa shells, basalt fiber, silicone, rubbery elastomers, and polyamide powders. The effects of such additions of polymer properties are discussed next. 

The effect of fiberglass reinforcement on the mechanical properties of polymers is reviewed in Table 3.1. Generally speaking, glass fiber is incorporated in the 20%-50% range. This has an appreciated effect on tensile strength and flexural modulus. 

It is seen in Table 3.1 that polyamide 6 improves its tensile strength from a value of 40 MPa to 145 MPa, that is, by a factor of about 3.6. The improvement obtained by the incorporation of glass fiber into polybutylene terephthalate was even more dramatic, from 52 to 80-196 MPa. Very useful improvements in tensile strength result from the incorporation of glass fiber into the formulation. Such improvements ranged from 17 to 41 MPa on unreinforced polymer to 180 MPa on glass-reinforced polytetrafluoroethylene. 

Such enhancements in tensile properties by the incorporation of glass fiber into the polymers have been important in the introduction of plastics into engineering applications. 

With the exception of epoxies and perfluoroalkoxy, the incorporation of 25%-50% glass fiber reinforcement leads to an improvement in flexural modulus. As shown in Table 3.2, the outstanding polymers are polyethylene terephthalate, where 

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Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

The TPX experimental product of Mitsubishi Petrochemical Ind. (221) is an amorphous, transparent polyolefin with very low water absorption (0.01%) and a glass-transition temperature comparable to that of BPA-PC (ca 150°C). Birefringence (<20 nm/mm), flexural modulus, and elongation at break are on the same level as PMMA (221). The vacuum time, the time in minutes to reach a pressure of 0.13 mPa (10 torr), is similarly short like that of cychc polyolefins. Typical values of TPX are fisted in Table 11. A commercial application of TPX is not known as of this writing. 

Resin Fillei Density, g/cm Tensile strength, MPa Flexural modulus, GPa" Notched Izod, J/m HDT at 1.82 MPa", °C... 

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