BASICS OF PLASTICS
THE CLASSiFiCATion oF PLASTiCS ACCorDinG To THEir DEGrEE oF CroSS-LinKinG
3.7 Amorphous thermoplastic: the polymer chains form a random, unordered structure. 3.8 Semicrystalline thermoplastic: randomly oriented, amorphous regions alongside regular crystalline regions. 3.9 Elastomer: amorphous tangle of polymer chains which are interconnected at larger intervals with atomic bonds. 3.10 Thermoset: amorphous polymer chains tightly cross-linked by atomic bonds. 3.11 Thermoplastic elastomer: example of a polymer blend.
Plastics are generally differentiated according to their degree of cross-linking. This classification is helpful in that fundamental material properties such as strength,
heat distortion temperature, workability and thermoplastic formability are directly related to the degree of cross-linking of the polymers.
With regard to the kind and degree of cross-linking, there are four groups: thermosets (also known as duromers), elastomers, thermoplastics and thermoplastic elastomers. The boundaries between them are indistinct and in some cases it 3.4
is not always clear to which group a plastic belongs. Materials with a semi-crystalline structure are uncross-linked and belong to the thermoplastics. Plastics with amorphous structures can be found in all groups and can be both uncrosslinked (thermoplastics) as well exhibit different degrees of cross-linking (thermosets, elastomers). THERMOPLASTICS Thermoplastics are uncross-linked and consist of polymer
chains that can be linear or branched. They are heat-deformable because the polymer chains do not form atomic bonds between each other but are linked only by secondary valence forces. The process of heat deformation is repeatable. Thermoplastics can be amorphous or semi-crystalline. in the case of amorphous thermoplastics the linear or branched molecule chains are randomly oriented and tangled. Because of their brittle nature they are particularly prone to stress cracking. Their appearance can be opaque or transparent. Amorphous thermoplastics can 3.7
be dissolved with an appropriate solvent. Examples of amorphous thermoplastics include PMMA, polystyrene (PS) and polyvinyl chloride (PVC). Semi-crystalline thermoplastics, by contrast, exhibit at least in parts a regular three-dimensional structure to the molecule chains. The higher density of the crystalline state compared with the amorphous condition means that heat input causes the volume to expand. Semi-crystalline thermoplastics are harder and
more resistant to solvents than amorphous structures. Polyethylene (PE), polypropylene (PP) and polyamide (PA) are examples of semi-crystalline thermoplastics. ELASTOMERS Elastomers exhibit a three-dimensional amorphous structure with
slight cross-linking that cannot be loosened through heat without the material decomposing. For this reason they cannot be heat-deformed, melted or welded. The tangled structure of the polymer chains is the reason for its exceptional elasticity and the fact that once the stress has been removed it returns to its original 3.9
condition. EPDM and the large family of rubber materials are examples of elastomeric plastics.
Published on Oct 1, 2010
This book seeks to fill that gap by providing an introduction to the structural and design possibilities of plastic. It introduces the mater...