THERMOPLASTICS AND THERMOSETS
Plastics are carbon based materials made up of very long molecules. Plastics are also called polymers; they are manufactured by modification of natural products or by synthesis from intermediates. Plastics can be divided into thermoplastics and thermosets.
Thermoplastics are plastics that soften or melt and flow as a thick fluid when heated above a certain temperature. In this state, the material is often referred to as a plastics melt. It is also in this state that the material is usually formed or shaped into a product. Upon cooling thermoplastics harden and behave as a solid. After a thermoplastics product has been formed, it can be reheated and softened to be shaped again. Thus, thermoplastics can be processed several times and this is what makes them suitable for recycling.
Thermosets are plastics that harden when heated above a certain temperature. The hardening is due to a curing or crosslinking reaction that connects the individual molecules and causes the formation of a three-dimensional molecular network. The shaping of thermosets usually occurs before the crosslinking sets in, thus, at a temperature below the curing temperature. The crosslinking reaction is not reversible; a thermoset cannot be softened again like a thermoplastics. It is more difficult, therefore, to recycle a thermoset than a thermoplastics. Examples of thermosets are phenolics, ureas, certain polyesters, melamines, and epoxies.
Amorphous and Semi-Crystalline Plastics
Thermoplastics can be further divided into amorphous and semi-crystalline plastics.
Amorphous plastics have a random, irregular molecular structure without crystalline regions. Examples of amorphous plastics are polystyrene (PS), polycarbonate (PC), acrylic (PMMA), acrylonitirile butadiene styrene (ABS), and polyvinylchloride (PVC).
Semi-crystalline plastics can form highly regular regions where the molecules form crystals; these crystalline regions are referred to as crystallites. The ability to form crystals is determined to a large extent by the shape of the plastics molecule. Plastics that have linear molecules without large sidegroups usually have the ability to form crystallites. An example is high density polyethylene (HDPE), which can achieve high levels of crystallinity, as high as ninety percent. Other polymers that can form crystalline regions are acetal (POM), nylon (PA), polyester terephthalate (PETP), low density polyethylene (LDPE), and polypropylene (PP).
Plastics with bulky side groups often cannot form crystallites and, therefore, are amorphous. An example is polystyrene.
The crystalline regions in thermoplastics have different properties than the amorphous regions, for instance the density and the optical properties are different. As a result, the light transmission through a plastics changes when crystallites are present; the crystallites act as a filler and make the material opaque or translucent below the melting point [slide].
Above the melting point, the crystallites disappear and the material is transparent. Since amorphous plastics have no crystallites, they are often transparent [slide] - unless of course they contain fillers or other materials that alter the optical properties. It is interesting to note that “crystal polystyrene” is an amorphous plastics. It is called “crystal” because it is transparent, not because it is crystalline.
Semi-crystalline plastics are never completely crystalline; the highest level of crystallinity occurs in high density polyethylene with a crystallinity of up to 90 percent [slide]. Despite this, semi-crystalline plastics are often referred to as crystalline material. It should be remembered though, that the term semi-crystalline is more appropriate. Some plastics crystallise rapidly, e.g. high density polyethylene, others crystallise slowly, e.g. polyethylene terephthalate (PET). In fact, if PET is quenched rapidly after melt forming, it may cool down in completely amorphous state. In general, the morphology that develops in a plastics will depend on how fast it is cooled during and after the shaping process [slide]. The morphology is also affected by the stresses exerted on the plastics during and after the shaping process.
Thus, the flow and temperatures in the die and downstream of the die play an important role in the morphology that ultimately develops in the plastics part. The part properties are strongly determined by the morphology of the part. Therefore, the extruded product properties are affected by the flow and temperatures in the die and downstream of the die.
3.1.2 Liquid Crystalline Polymers
Next, we will discuss liquid crystalline plastics or LCPs. The molecules of LCPs are rod-like structures organised in large parallel domains; this is true not only in the solid state but also in the melt state [slide]. The large, ordered domains give LCPs unique characteristics compared to amorphous and semi-crystalline plastics.
Differences in mechanical and physical properties between plastics can often be attributed to their structure. The order in semi-crystalline plastics and LCPs make them stiffer, stronger, and less resistant to impact than amorphous plastics. Semi-crystalline and liquid crystalline plastics tend to be more resistant to creep, heat, and chemicals; however, they tend to require higher melt temperatures in processing.
Despite the higher processing temperatures, LCPs shrink less during cooling than amorphous plastics.
When amorphous plastics are heated they soften gradually, while semi-crystalline plastics tend to soften more abruptly. In melt processing, amorphous plastics usually do not flow as easily as semi-crystalline plastics. LCPs have the high melt temperature of semi-crystalline plastics, but they soften gradually like amorphous plastics. LCPs have the lowest viscosity and shrinkage of all thermoplastics.
Elastomers
Elastomers are materials capable of large elastic deformations. There are three types of elastomers: conventional (vulcanizable) elastomers, reactive system elastomers, and thermoplastics elastomers.
Conventional elastomers become elastic by creating a three-dimensional network of cross-links between the molecules. The formation of chemical cross-links is called “vulcanisation” or “curing.” Examples of conventional elastomers are polyisoprene and polybutadiene. Elastomers can also be produced from low-molecular-weight reactive chemicals. Some polyurethane and silicone elastomers fall into this category. In thermoplastics elastomers or TPEs there is no chemical crosslinking. The links between the molecules are formed by physical links rather than chemical links. Examples are urethane TPE and styrenic TPE.
