The physical and mechanical properties of plastics are closely related to the temperature. When the temperature changes, the mechanical behavior of the plastic changes, showing different physical states and exhibiting the mechanical properties in stages. The physical state and mechanical properties of plastic when heated The molding process has a very important significance.
Plastic thermodynamic properties
The molecular thermal motion of plastics at different temperatures is called the physical state of the polymer.
The physical state of the thermoplastic is divided into the glassy state (crystalline polymer also known as crystalline state), high elastic state and viscous flow state.Figure 1 shows the linear amorphous polymer under constant pressure deformation degree and temperature curve , Also known as thermodynamic curve.
1) glassy state
Plastics in the state below the temperature Tg is a hard solid, called in the glass state, it is the state of use of most plastic parts in this state of the plastic under the action of the external force can only be a small chain of small elastic deformation and flexibility Deformation obeys Hooke's law.
Tg is called the glass transition temperature, which is the critical temperature at which the polymer transitions from the glassy state to the high-elastic state (or the high-elastic state to the glassy state). It is the maximum temperature for most plastics and is an important parameter for the reasonable selection of plastics.
The polymer also has an embrittlement temperature Tx below Tg, and the polymer is easily broken at this temperature so that Tx is the minimum temperature at which the plastic is used. The wider range of Tx to Tg indicates a wider range of operating temperatures for plastics .
2) high elasticity
When the plastic is heated to a temperature above Tf, the plastic enters the high-elastic state due to the movement of the polymer segments. The plastic, rubber-like elastomer in this state exhibits significantly greater deformability but reversibly deformable properties.
3) viscous flow
When the plastic heated temperature exceeds Tf, due to the overall movement of the molecular chain, the plastic began to flow significantly, the plastic began to enter the viscous flow into a viscous flow of liquid, also known as the melt in this state, the plastic melt The body can cause macroscopic flow under the condition of not too big external force. At this moment, the deformation is mainly irreversible plastic deformation. Once formed and cooled, its deformation will be maintained forever.
Tf is called the viscous flow temperature, which is the critical temperature at which the polymer transitions from a high-elastic state to a viscous fluid (or from a viscous fluid to an elasto-plastic one). As the plastic continues to heat up to the temperature Td, the polymer begins to decompose and discolor.
Td is called the thermal decomposition temperature, which is the critical temperature at which the polymer begins to decompose at high temperature. Degradation of the polymer can reduce the physical properties, mechanical properties or poor appearance of the product. Tf is an important reference temperature for plastic molding, The wider the range of Tf-Td, the easier it is to mold the plastic.