11:43 AM, 5/7/2018 +60 12-350 1402: The Bronze Age of Bioplastics
Bioplastics date back to the Bronze Age, but they are due for a comeback as our modern hunger for petroleum exceeds the available supply. As people cut back on the use of petroleum based products, a new solution will need to be found. By using natural, abundant, biodegradable materials (such as gelatin or starch), new non-petroleum based plastics can be made. The term bioplastic comes from the need to mark these types of plastics separately from their more common oil-based cousins.
Polymer Structure
Polymers can be classified as fibers, elastomers, or plastics. The classification is dependent upon how the atoms in a molecule are aligned or hooked together. In primary valence bonding, the atoms are hooked together using their valence electrons. An example of valence bonding is covalent bonding. The secondary bonding of the atoms does not involve the valence electrons. The secondary bonding is called secondary valence forces. It is responsible for the force of attraction between individual molecules. Examples of secondary valence forces are hydrogen bonds and dipole – dipole bonding. Secondary bonding is weaker than primary bonding; therefore, the molecules must come close together for secondary bonds to have an effect. The close alignment of the molecules depends on the structure and how the structure aligns itself for the secondary bonding forces. The alignment distinguishes a polymer from a fiber, elastomer or a plastic. Fibers are linear and have high symmetry. Elastomers have irregular structure and are flexible. Plastics are between fibers and elastomers. There are some structural differences between plastics, fibers, and elastomers, but some polymers can be used as plastics and fibers. Some examples are polypropylene and polyamides. The way these polymers are processed makes the distinction between a fiber and a plastic. If polypropylene is extruded then it is used as a fiber, but if the process is an injection molding then it is used as a plastic.
11:43 AM, 5/7/2018 +60 12-350 1402: Physical Properties
The physical properties of plastics help determine the applications and use of that plastic. The typical tests for polymers to use to compare one polymer to another are: tensile strength, glass transition temperature (Tg), elongation, and tear resistance. Precise equipment is used to measure properties such as tensile strength and glass transition temperature, but these properties can also be approximated with simple equipment.
Crystalline polymers have individual chains that are folded and packed regularly in an ordered fashion. The polymer chains are usually long; therefore the ordered arrangement is not perfect. Examples of crystalline polymers are polyethylene and polypropylene. Polypropylene is used in yarn for carpet, knitted fabrics and liners for disposable diapers. Polyethylene is used in packaging materials, in wire and cable insulation and milk cartons. Amorphous polymers have unordered regions. The polymer chains are arranged randomly and can be entangled. Polystyrene and polycarbonate are examples of amorphous polymers. Polystyrene is used in fast food packaging, egg containers, and mirror and picture frames. Polycarbonate is used in automobile taillight lenses, bumpers, and drapery fixtures. Figure 1 shows the point where the glass
11:43 AM, 5/7/2018 +60 12-350 1402: transition temperature occurs for an amorphous polymer. Figure 2 shows the point where the glass transition temperature and melting temperature occurs for a crystalline polymer. By knowing the glass transition temperature of a polymer and the state of the polymer, then you will know at what temperature you should operate industrial processes or at what temperature you should blend two polymers together. If you try to blend a polymer below its Tg you will not be able to blend it because the polymer chains will not be able to move into new positions to relieve the stress that is being put in them.
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