Sunday, December 12, 2010

Rubber Properties: Thermal Expansivity , Pressure

The coefficient of thermal expansion for elastomer is approximately 4.8 x 10E-04 /K similar to a hydrocarbon liquid. Addition of fillers reduces the value slightly. Comparing this expansivity to that of steel (3.5 x E-05 /K) ,a tenfold difference, one begins to understand the built-in interfacial strain in a bonded rubber-metal or composite structure.

The high coefficient of expansion couple with the high bulk modulus mean that a potential exists for subtantial thermal pressure .If the elastomer is confined and subsequently heated. This describes the mouldind enveronment, with thermal pressure a neccessity for replicating mould contours and surface finish. The moulding procedure of “bumping” vent excess rubber create by this thermal expansion. If not relieved, the thermal pressure could exceed the press clamping force. This can make the mould open at the parting line, creating a condition referred to as “Backrinding”.

Beerbower has calculate these pressure using the ratio of thermal expansion at constant pressure to compressibility at constant temperature .Using Beerbower's Equation. It is possible to predict the pressure developed .

= 1.13 for saturate backbone hydrocarbon that identify according to ASTM D 1418 designation "M" ; e.g., EPDM and 1.22 for unsaturated elastomer ASTM D1418 designation " R" ; e.g., NBR , SBR , NR

T is temperature (K)

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