Plastic is one of the elements most used by human beings for the production of a large number of products, such as bottles, toys, containers and others. However, there is not just one type of them, but there is an extensive classification of plastics that we will show you in this article. We invite you to continue reading.
Plastic Classification
At first glance, if you take a tour of all the places in your home or office, you will see a wide variety of products made from plastic. In itself, plastic, as previously mentioned, is one of the most used materials in the world for the production of all kinds of products that can be imagined. It's easy to classify everything as just "plastic." However, the classification of plastics usually touches on about 7 types of these that you should know about. The full list of plastics includes:
Polyethylene terephthalate (PET or PETE)
It is produced by the polymerization of ethylene glycol and terephthalic acid. Ethylene glycol is a colorless liquid obtained from ethylene, and terephthalic acid is a crystalline solid obtained from xylene. When heated together under the influence of chemical catalysts, ethylene glycol and terephthalic acid produce PET in the form of a viscous molten mass that can be spun directly into fibers or solidified for further processing as a plastic.
Chemically speaking, ethylene glycol is a diol, an alcohol with a molecular structure containing two hydroxyl (OH) groups, and terephthalic acid is an aromatic dicarboxylic acid, an acid with a molecular structure containing a large six-sided carbon (or aromatic) and two carboxyl groups (CO 2 H). Under the influence of heat and catalysts, hydroxyl and carboxyl groups react to form esters (CO-O), which serve as chemical bonds connecting various PET units in long-chain polymers. Water is also a byproduct.
It is the most recycled plastic. In the United States, however, only about 20 percent of the material is recycled. PET bottles and containers are typically melted down and spun into fibers for upholstery or rugs. When collected in a suitably pure state, PET can be recycled to its original uses, and methods have been devised to break the polymer down into its chemical precursors in order to resynthesize it into PET. The recycling code number for PET is 1.
High density polyethylene (HDPE)
It is manufactured at low temperatures and pressures with Ziegler-Natta impellers and activated metallocene or chromium oxide (known as Phillips catalyst). The lack of branching in its structure allows the polymer chains to be closely packed, resulting in a dense, highly crystalline material with high strength and moderate stiffness. With a melting point over 20°C (36°F) higher than LDPE, it can withstand repeated exposure to 120°C (250°F) so it can be sterilized.
On the other hand, it can also be mentioned that the products manufactured by this type of plastic classification include molded bottles for milk and household cleaners; shot blasting of extruded food bags, construction films and agricultural pulp; and injection molded buckets, lids, appliance cases and toys. Within the recycling section, this type of plastic can be defined with the following enumeration: 2.
Polyvinyl chloride (PVC)
It is a kind of derivative produced by the polymerization of vinyl chloride. Second only to polyethylene among plastics in production and consumption. It is used in a wide range of household and industrial products, from raincoats and shower curtains to window frames and internal plumbing. A rigid, lightweight plastic in its pure form, it is also made in a flexible "plasticized" form. Within the nomenclature used for recycling, this type of classification of plastics ranks third.
It was prepared by the French chemist Henri Victor Regnault in 1835 and then by the German chemist Eugen Baumann in 1872, but it was only patented in 1912, when another German chemist, Friedrich Heinrich August Klatte, used sunlight to initiate the polymerization of vinyl chloride. . The commercial application of plastic was initially limited by its extreme stiffness; however, in 1926, attempts were made to dehydrohalogenate it in a high-boiling solvent to obtain an unsaturated polymer that could bond rubber to metal.
On the other hand, it was Waldo Lunsbury Semon who developed plasticized PVC, whose inert and flexible product was responsible for the commercial success of the polymer. Under the Koroseal brand name, he transformed plastic into shock-absorbing seals, insulation for electrical cables, and coated fabric products. One of the best-known applications of plastic began in 1930, when Union Carbide and Carbon Corporation released Vinylite, a derivative that became the standard material for phonograph records in the United States.
Pure PVC finds application in the building trades, where its stiffness, strength, and fire resistance are useful in pipes, ducts, siding, door and window frames. It is also blow molded into clear and transparent bottles. Due to its rigidity, it must be extruded or molded above 100 °C, a temperature high enough to initiate chemical decomposition (in particular, the emission of hydrogen chloride). Decomposition can be reduced by adding stabilizers, which are mainly metal compounds such as cadmium, zinc, tin, or lead.
Low Density Polyethylene (LDPE)
It is made from ethylene gas at very high pressures (up to about 350 megapascals or 50.000 pounds per square inch) and high temperatures (up to about 350°C or 660°F) in the presence of rust initiators. These processes produce a polymer structure with long and short branches. Because the branches prevent the polyethylene molecules from clumping together into hard, rigid crystalline arrangements, LDPE is a very flexible material. Its melting point is about 110°C or 230°F.
When mentioning the main utilities or products that can be made through this type within the classification of plastics, it can be said that the main uses are packaging films, garbage and shopping bags, agricultural mulch, cable insulation. and wires, squeeze bottles, toys and household items. Like most of all plastics within the classification, when they are recycled they have a number, in this case it is 4.
Polypropylene (PP)
It is a gaseous compound obtained by thermal cracking of ethane, propane, butane and the naphtha fraction of petroleum. Like ethylene, it belongs to the "lower olefins," a class of hydrocarbons whose molecules contain a single pair of carbon atoms joined by a double bond. The chemical structure of the propylene molecule is CH2 = CHCH3. However, under the action of polymerization catalysts, the double bond can be broken and thousands of propylene molecules join together to form a polymer chain (a large multi-unit molecule).
Much of the production of polypropylene is spun by fusion into fibers, which is used in home furnishings, such as upholstery and indoor and outdoor rugs. There are also many industrial end uses, including rope and twine, disposable nonwovens for diapers and medical applications, and nonwovens for stabilizing and strengthening soil in construction and road construction. paving. These applications take advantage of the polymer's hardness, elasticity, water resistance and chemical inertness. In the world of recycling, this member of the plastics classification is 5.
Polystyrene or Styrofoam (PS)
This relatively brittle and rigid thermoplastic resin is polymerized from styrene (CH2 = CHC6H5). Styrene, also known as phenylethylene, is made by reacting ethylene with benzene in the presence of aluminum chloride to produce ethylbenzene, which is then dehydrogenated to produce clear liquid styrene. Styrene monomer is polymerized using free radical initiators primarily in bulk and suspension processes, although emulsion and solution methods are also employed.
Foamed polystyrene is made into insulation, food packaging, and containers such as beverage cups, egg cartons, and disposable plates and trays. Solid polystyrene products include injection molded cookware, audio cassette holders and compact disc packaging cases. Many foods are stored in transparent trays of this vacuum derivative, due to the material's high gas permeability and good water vapor transmission. It is ranked number 6 in the ranking of plastics in recycling.
various plastics
This last classification of plastics, we will find all those types that have not been able to be classified within the guidelines of the aforementioned. In these cases we can find some examples such as Acrylonitrile Butadiene Styrene, fiberglass, nylon, among many others. All these plastics that make up this large group are usually identified within the recycling parameters with the number 7.
Polycarbonate (PC)
The PC was introduced in 1958 by Bayer AG in Germany and in 1960 by the General Electric Company in the United States. As developed by these companies, PC is produced through a polymerization reaction between bisphenol A, a volatile liquid derived from benzene, and phosgene, a highly reactive and toxic gas obtained by reacting carbon monoxide with chlorine. The resulting polymers (long molecules with several units) consist of repeating units that contain two aromatic rings (benzene) and are connected with ester groups (CO-O).
Mainly due to the aromatic rings embedded in the polymer chain, PC has exceptional rigidity. It is also highly transparent, allowing about 90 percent of visible light to pass through. Since the mid-1980s, this property, combined with the excellent flow properties of the molten polymer, has found increasing use in CD injection molding. Due to its significantly higher impact resistance than most plastics, large water bottles, shatterproof windows, crash shields, and crash helmets are also made.
Polylactic acid (PLA)
It is the most used ecological polymer and its biodegradability is the key point in the market. It is currently used in short-lived packaging films, containers, foodservice products and bottles. It is also a promising biopolymer for various applications in the biomedical field, used to manufacture implants and devices based on biological sources, as well as sutures, screws and scaffolds. Also as a drug carrier. It has great potential to replace polycarbonate (PC), especially in the manufacture of electrical enclosures, since its cost is lower.
Acrylics
Another of the types that fall within the classification of plastics are all those that derive from acrylic compounds. These are any of a class of synthetic plastics, resins, and oils used to make many products. By varying the starting reagents and the formation process, a material can be produced that is hard and transparent, soft and elastic, or a viscous liquid. Acrylic compounds are used to make optical and structural castings, jewelry, adhesives, coating compounds, and textile fibers.
Acrylonitrile Butadiene Styrene (ABS)
It is a type of the plastic classification that is made by emulsion or bulk polymerization of acrylonitrile and styrene in the presence of polybutadiene. The most important properties it has are its impact resistance and toughness. Furthermore, it is often defined by three main properties: fluidity, heat resistance, and impact resistance. Styrene monomer gives ABS good machinability, acrylonitrile gives it stiffness, chemical and heat resistance, while butadiene makes the product harder and more elastic even at low temperatures.
Changes in the proportions of the elements that make up this type of plastic material or the addition of specific additives allow different degrees of specific properties to be developed. On the other hand, it can be said that it has little resistance to weathering and that is why it is recommended to apply it only indoors. Additionally, it should be noted that if you want to work with this derivative, you need temperatures that can range from -20 °C to + 80 °C.
Fiberglass
Glass fibers were little more than a novelty until the 1930s, when their thermal and electrical insulating properties were appreciated and methods were developed to produce continuous glass filaments. Modern manufacturing begins with liquid glass obtained directly from a glass melting furnace or by remelting preformed glass beads. To produce a continuous fiber, the liquid is introduced into a ferrule, a receptacle that is perforated with hundreds of fine nozzles through which the liquid comes out in fine jets.
The solidification currents are collected in a single strand, which is wound on a coil. The strands may be twisted or coiled into threads, woven into cloth, or cut into small pieces and then tied into mats. Staple fibers are most often made in a rotary process, in which fine streams of glass are thrown through holes on a turntable, then broken up and blown away with a jet of air or steam. The fibers accumulate on a moving conveyor and are turned into wool, mats or sheets.
An excellent thermal and acoustic insulator, fiberglass wool is commonly used in buildings, appliances, and plumbing. Glass filaments and strands add electrical strength and resistivity to molded plastic products such as recreational boat hulls, automobile body parts, and housings for a variety of consumer electronics. Glass fabrics are used as electrical insulators and as reinforcing belts in automobile tires.
Nylon
It is any synthetic plastic material that is made from high molecular weight polyamides and is usually, but not always, made as a fiber. It was developed in the 1930s by a research team led by American chemist Wallace H. Carothers, who worked for EI du Pont de Nemours & Company. The successful manufacture of a useful fiber by chemical synthesis from compounds readily available in air, water, and coal or oil stimulated the expansion of polymer research and resulted in a rapidly growing family of plastics.
Nylon can be drawn, molded, or extruded through rows of a melt or solution to form fibers, filaments, bristles, or sheets to make yarn, fabric, and cord; and can be formed into molded products. It has a high resistance to wear, heat and chemical products. Stretched by the cold, it is hard, elastic and strong. Most commonly known in the form of thick and thin filaments in items such as socks, parachutes, and bristles, nylon is also used in the molding trade, particularly injection molding.
They can be prepared from a dicarboxylic acid and a diamine or an amino acid that can undergo self-condensation, characterized by the "CONH" functional group on a ring, such as ε-caprolactam. By varying the acid and amine, it is possible to make rubber-like products. Whether made as wires or in molds, they are characterized by a high degree of crystallinity. Under tension, the orientation of the molecules continues until the sample is stretched to about four times its original length, a property that is particularly important in filaments.
Bioplastic
It is a plastic and malleable material formed by chemical compounds derived or synthesized by microbes such as bacteria or genetically modified plants. Unlike most of the types we mention when classifying plastics that are made from petroleum, bioplastics are made from renewable resources, and some bioplastics are biodegradable. Since the beginning of the XNUMXth century, the development and use of plastics has skyrocketed and their usefulness and importance have grown so much that it is difficult to imagine modern life without them.
Today, almost all plastics are obtained from petroleum through chemical extraction and synthesis. Because petroleum-based plastics are generally non-biodegradable, plastic waste is very durable and its disposal has become a serious problem. Despite efforts to encourage and support recycling, landfills are filled with plastic waste, which also accumulates in the environment.
An additional problem with petroleum-derived plastics is that petroleum-derived resources are running out. Conservative sources believe that, at the current rate of consumption, all known sources of oil on Earth will have been depleted by the end of the XNUMXst century. Since modern life depends on plastics, oil is a non-renewable resource, and oil-derived plastic waste pollutes the environment, bioplastics may find a long-term sustainable solution.
The first known bioplastic, polyhydroxybutyrate (PHB), was discovered in 1926 by a French researcher, Maurice Lemoigne, from his work on the bacterium Bacillus megaterium. The importance of the Lemoigne discovery was overlooked for many decades, largely because oil was cheap and plentiful at the time. The oil crisis of the mid-1970s sparked interest in finding alternatives to petroleum products.
The rise of molecular genetics and recombinant DNA technology after this period fueled research, so that by the beginning of the XNUMXst century the structures, production methods and applications of many types of bioplastics had been established. Bioplastics that were used or under study included PHB and polyhydroxyalkanoate (PHA), both of which are synthesized in specialized microbes, as well as polylactic acid (PLA), which polymerizes from lactic acid monomers produced by microbial fermentation of plant-derived sugars. and starch.
Again, it can be said that the degradation of the chemical bonds between the monomers in these plastics is caused by microorganisms or water, which makes bioplastics highly desirable materials for making biodegradable bottles and packaging films. Because the degradation products are natural metabolites, the polymers are of interest for medical applications such as controlled-release drug packaging and absorbable surgical sutures.
It should be noted that this classification of plastics currently represents an insignificant part of the total world production of plastics. Commercial manufacturing processes are associated with low throughput and high costs. However, improvements in genetic engineering and metabolism have resulted in strains of microbes and plants that can significantly improve yields and production capabilities while lowering overall costs. These factors could expand the bioplastics market in the future.
PDK-Plastic
By now, you've probably heard the statistic that only about 9% of plastics are actually recycled, with the rest ending up in landfills and the oceans. To combat the plastic crisis, many environmentally conscious people are moving away from the material, but a group of scientists in Berkeley, California are trying to change the material. According to a study, scientists have developed a new form of plastic that actually enables a closed-loop recycling process with no waste.
The plastic is called polydiketoenamine or PDK. In general terms, it is a kind of plastic that allows it to be worked from a molecular level, separating each one of the elements and rejoining them, in order to do it a large number of times and thus making the plastic almost usable. indefinitely. Its properties are very similar to other types of plastics such as nylon and therefore, it can be used to make containers to store all kinds of food. In short, a perfect candidate to be within the classification of plastics.
SPI codes for plastic classification
In 1988, the Society for the Plastics Industry (SPI) established a classification system to help people recycle and dispose of plastic correctly. Today, manufacturers follow this coding system and put a number, or SPI code, on each product, usually at the bottom. Plastic marked with an SPI 1 code is made from polyethylene terephthalate. These containers sometimes absorb odors and flavors from foods and beverages stored inside. However, this is still a common plastic for many household items and supplies.
The SPI code of 2 identifies plastic made from high-density polyethylene. These products are very safe and there is no evidence of chemicals leaching into food or drinks. However, due to the risk of contamination, it is not safe to reuse an HDPE bottle as a food or beverage container if it did not originally contain any type of edible substance. For example, it is not convenient to reuse shampoo or soap containers to store tomato sauce.
Plastic labeled with an SPI code of 3 is made with polyvinyl chloride. This type of plastic should not come into contact with food, as it is a toxic and dangerous chemical. PVC is found in many everyday objects, but is primarily intended for industrial use in the plumbing and construction industries. On the other hand, the one with the SPI code of 4 is made of low-density polyethylene. This plastic tends to be durable and flexible. It also doesn't release harmful chemicals on objects, making it a safe choice for food storage.
You'll find the SPI 5 code on plastic items made from polypropylene. Plastic marked with an SPI code of 6 is made from polystyrene, which can be recycled, but is not efficient. Recycling requires a lot of energy, which means few places accept it. Finally, the SPI code 7 is used to indicate different types of plastic that are not defined by the other six codes. Think of these varieties as plastics, but they don't fit the rules of society.
If you liked this article on the types and classification of plastics and want to learn more about other interesting topics, you can check the following links: