This section will be covering the chemical composition of plastic, elements, and resources are found within its creation, and makeup.
Undoubtedly, the consumer world is ripe with complaints, and fears of using plastic products due to their chemical composition; although some of these fears are warranted, many are exaggerated, and are a result of a lack of research, here we will dive into the heart of what exactly plastic is made up of. There are two basic chemical categories that plastics can be divided into, which are based upon how the plastic is built chemically: aliphatic, or linear, polymers and heterochain polymers. Aliphatic polymers are lined linearly with carbon atoms within their backbone chemical chains. Heterochain polymers house oxygen, nitrogen, or sulfur atoms with carbon. Plastic however, is manufactured differently by various industrial manufacturers, and the general chemical compositions may have dozens of spinoff variations, for the same product, what will be described within this text, will be the general concept of chemical compositions.
A list of Carbon chain aliphatic polymers are:
- HDPE
- LDPE
- PP
- PS
- ABS - acrylonitrile-butadiene-styrene – appliance housings, helmets, pipe fittings, boat hulls
- PVC
- PMMA – polymethyl methacrylate – impact resistant windows, skylights, canopies
- PTFE - polytetrafluoroethylene – self-lubricating bearings, nonstick cookware
A list of heterochain polymers are:
- PET
- PC – polycarbonate – compact discs, safety glasses, sporting goods
- Polyacetal – bearings, gears, shower heads, zippers
- PEEK – polyetheretherketone – machine, automotive, and aerospace
- PPS – polyphenylene sulfide – machine parts, appliances, electrical equipment
- Cellulose diacetate – photographic film
- Polycaprolactam (nylon 6) – bearings, pullies, gears
Plastics are not necessarily classified by their chemical compositions, discussed above, but their behavior, which will bring them into two categories of: thermoplastic and thermosetting resins. We can breakdown the molecular code of plastics, by their polymers. Polymers are compounds that are engineered chemically, with the defining trait of large molecules; ranging from thousands, to even millions of atomic mass units. Having such large molecules within their makeup in combination with their physical state (and structure), give polymers the ability to be highly malleable (Rodriguez, F.). Diving further into the state of plastics, as referenced, the polymers can be classified into thermoplastics, and thermosets. Thermoplastics have the ability to be remolded extensively, an example being polystyrene (a foam cup or dish) which has the capability of being melted and formed into a new product. Thermoplastics are comprised structurally by individual molecules that are separate from each other, allowing more room and flexibility for the molecules to travel between one another. Thermoplastics can have varying molecular weight and can be branched in a linear structure. Thermosets cannot be processed again after being reheated and molded. This is because the thermosets go through a complex chemical reaction when they are originally manufactured, and the network of the molecular structure as a result is insoluble (Rodriguez, F.).
Plastic and Molecular Structuring
Physically, plastics and how they behave, is influenced by how their molecules are arranged, in mass, during their conception. Morphology is the technical terminology utilized to describe the process and is defined as the arrangement of molecules on a large scale. Plastic morphology can be one of two traits: amorphous or crystalline(Rodriguez, F.).
Amorphous are molecules that are structured randomly and are intertwined with one another. Crystalline molecules are those which are arranged close together and in an order, which is apparent. Breaking this down further from a categorization and structural perspective, thermosets are generally amorphous, while thermoplastics can be amorphous or semi crystalline (Rodriguez, F.). Thermoplastics can have their molded shapes until a certain melting point is reached, like most materials. All melting points are set by a term coined as the glass transition temperature which defines the melting point of any polymer, and the glassy state of a polymer. In regards to the glass transition temperature, when molecules are not reaching their particular melting point, they are rested in a state known as the glassy state; when a polymer is in this state, the molecules have little to no movement, and to the human perception it is stiff, and hard. Above the glass transition temperature, polymers will enter into the rubbery state where molecular mobility increases, and material is stretchable, and holds elasticity. This process differs depending on the polymer (Rodriguez, F.). Non-crystalline polymers like polystyrene, will not even pass into the rubbery stage, and go directly towards the melting phase when the appropriate temperature is reached. Partly crystalline polymers, like LDPE, the liquid state will not be reached until the melting point has fully been passed. When any melting point is reached in these instances, the crystalline composition which was originally found within the glassy state will no longer be stable. Once instability is reached, the liquid or rubbery state polymers can be then molded or extruded. Thermosets, however, are on the opposite end of the spectrum, and will not be stable to a specific temperature, once this temperature is reached chemical degradation will commence; these polymers will not melt upon reheating (Rodriguez, F.).
Properties
Plastic properties are easily visible during manufacturing and when the polymer is stressed to its maximum capacity, during this, the behavior of the plastic is easily recognizable, and distinguishable. Examples would include polystyrene, glassy structure, which requires high amounts of stress to make it stretch. While polyethylene are crystalline and can be elongated to high percentages past normal once their temperature is reached. PET is yet another great example, a plastic favored for water and beverage bottles and is crystalline, which has high dimensional stability and stiffness under stress.
Various properties that apply to all plastics are: stiffness, breaking stress (flexural modulus and tensile strength respectively), and toughness – energy absorbed by a polymer until failure.
Polymer Additives
When plastic is manufactured, it is simply, not just plastic alone that gives it the properties, or the category of plastic that make it different from its other, and many, various cousins; what can also determine plastics traits are the additives that are put into it during the manufacturing process.
Among these various additives are plasticizers. Plasticizers are instrumental in manipulating the glass transition temperature of polymers. PVC, or polyvinyl chloride, is mixed with liquids that are non-volatile. For instance, a plastic garden hose is required to be flexible for consumer use, but it is not merely just plastic PVC. A mixture of various applications, upwards of 30, are used to make the hose maintain its flexibility to withstand extreme, within reason, temperatures. A mixture of the following is used to achieve this: di(2-ethylhexyl) phthalate, with 70 parts PVC will have a glass transition temperature, or Tg, of about −10 °C (15 °F) (Rodriguez, F.). Colorants are another additive that alters the appearance of plastics. Color is a large marketing application of the consumer world when it comes to plastics. Color additives are extremely simple to apply to the plastic molding and manufacturing process. The following, frequently used colors are applied with the following elements: black (carbon), and white (zinc oxide and titanium dioxide). There are other oxides that are used for coloring as well which are: iron, chromium (which are inorganic); there are also organic additives that can be used to apply colors as pigments and dyes. A 3rd additive that is applied to plastics are reinforcements. Reinforcements are used to increase the mechanical properties of the polymer. Fillers, for example, is a type of reinforcer which can increase the stiffness. Types of fillers that can be used within the manufacturing process are: divided silica, carbon black, talc, mica, and calcium carbonate, and various other materials. Stabilizer are the 4th category of additive which can be applied to plastics. Stabilizers are added to the manufacturing process to increase the useful life of plastics and counter the aging process. Stabilizers are used are antioxidants to counter the oxidation process that plastics are subject to. Categories of stabilizers are as follows: phenols, and tertiary amines. Butylated hydroxytoluene is another example and is used in polyolefin packing for foods and pharmaceuticals. Furthermore, zinc, calcium soaps, organotin mercaptides, and organic phosphites (Rodriguez, F.).
Citations:
Rodriguez, F. (2018, October 22). Plastic. Retrieved from https://www.britannica.com/science/plastic#ref82466