TABLE OF CONTENTS Polymeric materials are used over a wide range of temperatures and the effects of temperature on the physical properties must be established before a plastic material can be used for designing a particular product.
The nature of viscoelasticity is such that temperature is of fundamental importance. In polymeric materials the primary bonds are strong covalent bonds along the molecular chains, and these bonds are not affected by temperature unless degradation caused by temperature occurs. However, the secondary forces that cause the bonding chains to stay together, restricting their relative movement, may be overcome by increasing the temperature. This increases the thermal motion of the molecules, changing the physical properties of the polymeric materials.
When polymeric materials are subjected to elevated temperatures for prolonged periods of time in the presence of air, oxidative degradation will occur, the rate and extent of which depends upon the family type and formulation of the polymeric materials, the temperature, and the length of exposure. This degradation reduces the tensile strength properties and toughness, and can eventually lead to surface cracking and brittleness. Tensile strength and modulus of elasticity properties of polymeric materials decrease proportionally to the rate of temperature increase, while the elongation rate is increased as the temperature increases.
Natural and light colored plastic resins exposed to elevated melt temperatures and long residence time inside the plastifying unit may discolor the molded part to yellow, brown, or black, depending on the thermal resistance of the...
