Introduction

Plastics are derived from organic products derived from natural products such as cellulose, coal, natural gas, salt and petroleum.
Every motor vehicle now contains many plastic components. So you can improve energy efficiency without loss of impact resistance. The materials also can not rust, are elastic and shock absorbing. Plastics consist of macromolecules. Since these are composed of repeating units called monomers, they are called polymers. Due to their mechanical-thermal behavior, plastics can be divided into three polymer families: thermosets, elastomers and thermoplastics.

1. thermosets (for example bakelite, epoxy)
• rigid
• Shape can not be changed by applying temperature
• Not weldable

2. elastomers (e.g., synthetic rubber, silicone)
• Elastic
• Shape can not be changed by applying temperature
• Shape can not be changed permanently
• Not weldable

3. Thermoplastics
• Can change the shape as often as you like by applying heat
• Most are weldable (equal to equal and a few compatible)


Thermoplastics

By supplying heat, thermoplastics can be converted into different states of aggregation. Depending on the material, different temperatures are required. When cold, the thermoplastics are solid. With increasing heat, they go into the thermoelastic state, in which a change in shape is possible, the original form is maintained. Upon further application of heat, the material reaches the thermoplastic state. Only then is it soft or unstable in its shape and thus weldable. Overheating can degrade the thermoplastic to its basic constituents. The temperature supply in combination with appropriate pressure creates a homogeneous connection. In principle, only the same plastics can be welded together, and there are some compatible thermoplastics that can also be bonded, such as PMMA with ABS and PVC with PVC-U. However, this must be tested in the specific case. There are also substances that can not be welded well due to their extremely high molecular weight. These include ultra high molecular weight PE-HD (UHMW), cast PMMA and PTFE. Since plastics (as well as metals) oxidize, although the oxidation layer is not visible in the thermoplastics, they must be machined before welding. The materials can be repaired or welded if damaged without affecting the strength.

Advantages:
• Low weight
• Good electrical insulation
• Excellent resistance to chemicals and weather
• Low thermal conductivity

Disadvantage:
• Low heat resistance
• Large thermal expansion
• Low elasticity
• The mechanical properties change over time
• Gas permeability
• Increasing thermal load leads to loss of dimensional stability and strength


Crystalline and semi-crystalline

Thermoplastics can also be classified into non-crystalline (amorphous) and semi-crystalline thermoplastics, with semicrystalline substances containing both crystalline and amorphous regions.
a.) Non-crystalline (amorphous) (PC, PMMA, PS, PVC)
b.) Partially crystalline (PP, PE, POM, PA)

Propertiesnot crystallineamorphous partially crystalline
Colortransparentopaque
mechanicallytoughhard brittle
shrinkinglowhigh
wearhighlow
slidinglowhigh
flexibilityhighlow