Plastic
- Identifier codes:
PET
HDPE
PVC
LDPE
PP
PS
Plastic is a material made up of organic or synthetic compounds that have the property of being malleable and therefore can be molded into solid objects of various shapes. This property gives plastics a wide variety of applications. Its name derives from plasticity, a property of materials, which refers to the ability to deform without actually breaking.
Plastics are usually high molecular weight organic polymers. They are usually synthesized, commonly from petroleum chemical derivatives (petrochemicals). However, there are also a very small number of plastics made from renewable sources, such as polylactic acid derived from corn starch or cellulose derived from cotton. There are also plastics produced by bacteria such as polyhydroxyalkanoates.
The plastics derived from petrochemicals are easy to manufacture and their costs are very low. Therefore, its applications are multiple and on different scales. 51% of the world's plastics are produced in Asia; China alone is responsible for 31% of world production. Latin America, on the other hand, produces 4%. In addition, plastics are used in building and construction, mobility and transportation, electrical and electronic devices, agriculture, health care, and other fields. The most important types of plastics used in European trade They are polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET) and polyurethane (PU).
History
The use of polymers dates back to the 16th century BC. C., when ancient Mesoamerican cultures first processed natural rubber into solid objects such as balls, human figurines, bands for tying ax heads to wooden handles, and other objects. Ancient Mesoamericans obtained the raw material to make objects from latex rubber produced by the tree Castilla elastica. This species is native to the tropical lowlands of Mexico and Central America. Latex is a sticky white liquid that when dry is a brittle solid that retains its shape. Spanish chroniclers reported that the Mesoamerican Indians processed the latex of C. elastica mixing it with the juice of another species, Ipomoea alba, coagulating the resin. Thus, these discoveries precede the vulcanization process by 3,500 years.
In 1839 Goodyear in the United States and Hancock in England developed in parallel the vulcanization of rubber, that is, the hardening of rubber and its greater resistance to cold. Thus began the commercial success of thermosetting polymers.
The plastics industry begins with the development of the first thermosetting plastics by Baekeland in 1909. Baekeland produces the first synthetic polymer and also develops the plastic molding process, which allowed it to produce various articles of commerce. These early plastics were called bakelite after their discoverer. Bakelite is formed by a condensation reaction of phenol with formaldehyde.
Between the years 1926 and 1928, alkyd thermoplastics and amine resins emerged, respectively. Alkyds are polyesters modified by the addition of fatty acids and other components, they are derivatives of polyols and dicarboxylic acids or anhydrous carboxylic acids. Among the amine resins is urea-formaldehyde, also known as urea-methane, which is a non-transparent synthetic thermoplastic resin with applications in adhesives. Table 1 summarizes other important events in the historical development of thermoplastics.
Timeline
These are the dates of some historical milestones in the development of thermosetting plastics, those that do not change shape when heat is applied to them.)
1600 before our era. Mesoamerican cultures processed natural rubber into various solid objects for the first time.
1839 Goodyear develops the vulcanization of rubber.
1909 Baekeland obtains the first synthetic phenolic resin, Bakelite.
1926 Alkyds and amine resins are introduced. Aniline-formaldehyde is introduced to the US.
1928 Urea-formaldehyde is introduced commercially.
1931 Hyde begins research on organosilicon polymers.
1933 Ellis patents unsaturated polyester resins.
1935 Henkel manufactures melamine-formaldehyde resins.
1937 A compression molding system for plastics is introduced for the first time. Polyurethanes are produced for the first time.
1938 Melamine is introduced commercially.
1939 First patent (in Germany) for epoxy resin.
1941 A urethane-type polyester is introduced in Germany.
1942 Dow Corning manufactures silicone industrially.
1943 Castan patents an epoxy resin.
1946 Polyurethane elastomers are introduced.
1947 Epoxy resin is introduced commercially.
1954 Polyurethane is introduced in the US.
1957 Urethane-type polyether is introduced in the US.
1964 Polyimides are introduced as a manufactured product.
Properties and characteristics
Plastics are synthetic chemical substances, called polymers, with a macromolecular structure that can be molded by heat or pressure and whose main component is carbon. These polymers are large groupings of monomers joined together through a chemical process called polymerization. Plastics provide the necessary balance of properties that cannot be achieved with other materials, for example: color, light weight, pleasant touch, and resistance to environmental and biological degradation.
In fact, "plastic" refers to a state of the material, but not to the material itself: synthetic polymers, usually called plastics, are actually synthetic materials that can reach the plastic state, that is, when the material is viscous or fluid and has no resistance properties to mechanical stress. This state is reached when the solid state material is transformed into a plastic state, generally by heating, in which it is ideal for the different production processes since it is when the material can be manipulated in different ways. So the word "plastic" is a way of referring to synthetic materials capable of entering a plastic state, but "plastic" It is not necessarily the group of materials that this word refers to on a daily basis.
The properties and characteristics of most plastics (although they are not always fulfilled in certain special plastics) are these:
- are easy to work and mould,
- have a low cost of production,
- have low density,
- They're usually waterproof.
- good electric insulators,
- acceptable acoustic insulation,
- good thermal insulation, although most do not resist very high temperatures,
- corrosion resistant and many chemical factors,
- some are neither biodegradable nor easy to recycle and, if burned, are very polluting.
Production process
The first part of the production of plastics consists of the elaboration of polymers in the chemical industry. Today the recovery of post-consumer plastics is essential as well. Part of the plastics used by the industry are used directly in the form of grain or resin. More often there are various ways of processing plastics. One of them is the extrusion of profiles or threads, which allows the generation of an extensive and continuous product. Another form of processing is molding (by injection, compression, rotational inflation, etc.). There is also thermoforming, a process that uses a thermoplastic material previously produced through extrusion processing. This type of processing has different variants: vacuum thermoforming, pressure and mechanical thermoforming.
Classification of plastics
Depending on the base monomer
This classification considers the origin of the monomer from which the production of the polymer starts.
- Natural: They are polymers whose monomers are derived from natural products with certain characteristics such as cellulose, casein and rubber. Within two of these there are other plastics from which they come:
- The derivatives of cellulose are celluloid, jealousy and cellon.
- Rubber derivatives are rubber and ebony.
- Synthetics: These are those that originate in products produced by humans, mainly petroleum derivatives, such as polyethylene bags
According to its behavior in the face of heat
Thermoplastics
A thermoplastic is a plastic that, being plastic or deformable at room temperature, becomes a liquid when heated and hardens into a glassy state when cooled sufficiently. Most thermoplastics are high molecular weight polymers, which have chains associated by weak Van der Waals forces (polyethylene), strong dipole-dipole interactions and hydrogen bonding, or even stacked aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers in that, after being heated and molded, they can be reheated and form other objects, since in the case of thermosets or thermosets, their shape after cooling does not change.
Their physical properties gradually change if they are melted and molded several times.
The main thermoplastics are:
- Cellulose resins: obtained from cellulose, the constituent material of the woody part of the plants. This group belongs to the Rayon.
- Polyethylenes and derivatives: They use as raw material the ethylene obtained from the oil cracheum which, subsequently treated, allows to obtain different monomers such as vinyl acetate, vinyl alcohol, vinyl chloride, etc. PVC, polystyrene, methacrylate and others belong to this group.
- Derivatives of proteins: The nylon and the lining, obtained from the diamids, belong to this group.
- Rubber derivatives: These are the examples of this group called commercially pliofilm, rubber hydrochloride obtained by adding hydrochloric acid to rubber polymers.
Thermostable
Thermosetting plastics are materials that, once they have undergone the heating-melting and formation-solidification process, become rigid materials that do not melt again. Frequently to obtain it the precursor is an aldehyde.
- Phenol polymers: They are hard, insoluble and infusible plastics but, if during their manufacture an excess of phenol is used, thermoplastics are obtained.
- Epoxy resins.
- Melamine resins.
- Baquelita.
- Aminoplastics: Urea and derivative polymers. Melamine belongs to this group.
- Polyesters: Resins from the sterification of polyalcohols that are usually used in varnishes. If they contain excess acid, thermoplastics are obtained.
According to the synthesis reaction
They can also be classified according to the reaction that produced the polymer:
Addition polymers
They always involve the breaking or opening of a monomer bond to allow the formation of a chain. As the molecules are longer and heavier, paraffin wax becomes harder and more tenacious. Example:
- 2n H2C=CH2 → [-CH2]2-CH2-CH2-CH2-]n
Condensation polymers
They are those where the monomers must have at least two reactive groups per monomer to give continuity to the chain. Example:
- R-COOH + R'-OH → R-CO-OR' + H2O
Polymers formed in stages
The polymer chain gradually grows as long as monomers are available, adding one monomer at a time. This category includes all Carothers condensation polymers plus some others that do not release small molecules but do form gradually, such as polyurethanes.
According to its molecular structure
Amorphous
Amorphous plastics are those in which the molecules are arranged disorderly and do not present any type of order. As there is no order between chains, holes are created through which light passes, which is why amorphous polymers are transparent.
Semi-crystalline
Semi-crystalline polymers have areas with a certain type of order along with amorphous areas. In this case, having an order, there are fewer gaps between chains, so light does not pass through unless they have a small thickness.
Crystallizables
Depending on the cooling speed, the percentage of crystallinity of a semi-crystalline polymer can be decreased (rapid cooling) or increased (slow cooling), however, an amorphous polymer will not present crystallinity even if its cooling speed is extremely slow.
Commodities
They are those that have global manufacturing, availability and demand, an international price range and do not require great technology for their manufacture and processing.
Engineering
They are the materials that are used in a very specific way, practically created to fulfill a certain function; they require specialized technology for their manufacture or processing and are relatively expensive.
Elastomers or rubbers
Elastomers are characterized by their great elasticity and ability to stretch and rebound, recovering their original shape once the force that deformed them is removed. They include natural rubbers obtained from natural and synthetic latex; among the latter are neoprene and polybutadiene.
Elastomers are materials with large molecules, which after being deformed at room temperature, recover to a greater extent their size and geometry when the force that deformed them is released.
Resin codes
There are a wide variety of plastics and to classify them, a coding system is used as shown in Table 1. Products carry a mark consisting of the international recycling symbol ♻ with the corresponding code in the middle depending on the material specific. The main objective of this code is to identify the type of polymer from which the plastic is made for its correct recycling.
The number present in the code is arbitrarily designated for the identification of the polymer from which the plastic is made and has nothing to do with the difficulty of recycling or the hardness of the plastic in question.
| Type of plastic: | Polyethylene Tereftalato | High density polyethylene | Vinyl polychloride | Low density polyethylene | Polypropylene | Polystyrene | Other |
|---|---|---|---|---|---|---|---|
| Acronym | PET (also PETE) | HDPE (tb. PEHD or PEAD) | PVC | LDPE (tb. PEBD or PEBD) | P | PS | Other |
| Code | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
Most common uses
- Applications in the industrial sector: motor parts, electrical and electronic devices, bodywork, electric insulation, etc.
- In construction: pipes, waterproofing, polystyrene insulating foams, etc.
- Consumer industries and others: wrappers, toys, toy wrappers, suitcases, sports items, textile fibres, furniture, trash bags, etc.
Recycled
Plastic waste is not likely to be assimilated back into nature. Due to this, the recycling of plastic products has been established, which basically consists of collecting them, cleaning them, selecting them by type of material and melting them again for use as an additional, alternative or substitute raw material, for the molding of other materials. products.
In this way, humanity has found an adequate way to fight against the contamination of products that, due to their composition, materials or components, are not easy to dispose of conventionally. Its effectiveness and social acceptance can be considered debatable.
Large amounts of non-renewable natural resources can be saved when "recycled" materials are used in production processes. When used correctly, these recycled materials can prevent the overexploitation of resources that are still considered renewable, such as forests, avoiding serious impacts on ecosystems such as deforestation, erosion, and desertification. The use of recycled products reduces energy consumption. When fewer fossil fuels are consumed, less carbon dioxide is generated and the greenhouse effect is prevented. In addition, the production of other harmful gases from said combustion is also reduced, such as sulfur and nitrogen oxides that produce acid rain or tropospheric ozone pollution.
From a financial point of view, a good recycling process is capable of generating income. Due to the above, it is unavoidable to improve and establish new technologies in terms of plastic recovery processes and seek a solution to this problem that is so harmful to society and that is increasing day by day, deteriorating the environment. In the following sections, the design of a smelter for low-density polyethylene, its use, its characteristics, recommendation, and the positive impact it will provide to the community are discussed.
Some plastics are not recoverable, such as crystal polystyrene or bakelite.
Biodegradable plastics
At the end of the XX century, the price of oil fell and interest in biodegradable plastics also waned. In recent years this trend has been reversed; In addition to an increase in the price of oil, there has been a greater awareness that oil reserves are being depleted at an alarming rate. Within this context, there is a marked increase in scientific and industrial interest in research for the production of biodegradable plastics or EDPs (environmentally degradable polymers and plastics). The manufacture of biodegradable plastics from natural materials is one of the great challenges in different sectors, industrial, agricultural and materials for various services. Given this perspective, research involving plastics obtained from other sources has gained new momentum and polyhydroxyalkanoates appear as a highly promising alternative.
The substitution of current plastics for biodegradable plastics is a way by which the polluting effect of those would be diminished in the environment. Biodegradable plastic waste can be treated as organic waste and disposed of in sanitary deposits, where its degradation takes place in meager periods of time.
Biodegradable polymers can be classified as follows:
Polymers extracted or removed directly from biomass
They are in the research phase to produce edible films and coatings, intended for food preservation.
Diverse polysaccharides and hydrocolloids of a protein nature are being studied, such as cassava starch plasticized with glycerol, polyethylene glycol, standard and pre-gelatinized corn starch, carboxymethylcellulose, pectin, pectin mixed with sodium alginate, gum tragacanth, guar gum, ethylcellulose, gelatin added with glycerol, gelatin-casein cross-linked with transglutaminase, sorbitol and sucrose, casein (a protein present in milk), whey, soy and wheat gluten.
Gluten is toxic to people with gluten-related disorders (such as celiac disease and non-celiac gluten sensitivity), which affect up to 15% of the general population and whose numbers are steadily increasing. The health impact of the use of gluten in these products is a matter of concern that needs to be assessed and regulated. (See also Gluten-Related Neurological Disorders)
Polymers produced by classical chemical synthesis
They use biological monomers from renewable sources.
Polymers produced by microorganisms, native or genetically modified producing bacteria
In this category are biodegradable plastics produced by bacteria, which include polyhydroxyalkanoates (PHA) and polylactic acid (PLA). PHAs, due to their origin from renewable sources and the fact that they are biodegradable, are called “double green polymers”. PLA, a natural monomer produced by fermentative processes from elements rich in sugars, cellulose and starch, polymerized by man.
Polylactic acid (PLA)
Starch is a natural polymer, a large carbohydrate that plants synthesize during photosynthesis and that serves as an energy reserve. Cereals such as corn and wheat contain a large amount of starch and are the main source for the production of PLA. The bioplastics produced from this polymer have the characteristics of a resin that can be injected, extruded and thermoformed.
The production of this biopolymer begins with the starch that is extracted from corn, then microorganisms transform it into a smaller molecule of lactic or 2-hydroxy-propionic acid (monomer), which is the raw material that polymerizes forming chains, with a molecular structure similar to products of petrochemical origin, that are joined together to form PLA.
PLA is one of the most studied biodegradable plastics currently and has been available on the market since 1990. It is used in the manufacture of transparent bottles for cold drinks, food packaging trays, and numerous other applications.
Polyhydroxyalkanoates
PHAs are generally produced by Gram-negative bacteria, although there are Gram-positive bacteria that also produce them on a smaller scale. The first PHA discovered was PHB, which was described at the Pasteur Institute in 1925 by the microbiologist Lemoigne who observed the production of PHB by Bacillus megaterium. Subsequently, in 1958 Macrae and Wildinson observed that Bacillus megaterium accumulated the polymer when the glucose/nitrogen ratio in the culture medium was not in equilibrium and observed its degradation when there was a lack or deficiency of carbon or energy sources. From this fact, PHA inclusions were found in a wide variety of bacterial species. At present approximately 150 different polyhydroxyalkanoates are known.
The first patent for PHB was filed in the United States by J. N. Baptist in 1962. Two important events occurred in 1983: first was the discovery by De Smet of a PHB-producing strain of Pseudomonas oleovorans (ATCC 29347), and consecutively, the first production of the first biopolyester for commercial use took place. A copolymer formed by monomers of four and five carbons, called PHB and PHV, respectively called "Biopol" commercially and is produced using Ralstonia eutropha, from glucose and propionic acid. This bioplastic is currently already synthesized from a single carbon source in recombinant bacteria; it exhibits a high biodegradability potential and better thermomechanical properties than pure PHB.
In general, PHAs are insoluble in water, biodegradable, and non-toxic, which is why one of the main benefits obtained from the application of PHAs is environmental. The use of these products reduces the dependence on oil by the plastic industry, causes a decrease in solid waste and a reduction in the emission of gases that cause the greenhouse effect would be observed.
The points of interest regarding the applications of bioplastics, according to the IBAW (International Association and Working Group of Biodegradable Polymers), focus on the sectors of packaging, medicine, agriculture and disposable products. However, with the advancement of this industry, the use of biomaterials has expanded, which are applied to cell phones, computers, or audio and video devices. According to this information, it has been established that 10% of the plastics currently used in the electronics industry can be replaced by biopolymers.
Plastic related problems
In modern life, plastic has been a phenomenon of unquestionable importance. Today people live surrounded by plastic objects that in previous centuries were not necessary for daily life. Plastics have been manufactured to meet the demands of a wide variety of uses, giving rise to a vast industry where civilization should be called the plastic civilization, due to the determining role that this material has played in its development, in improving the people's living conditions and the accelerated growth of science and technology.
In general, people have very little knowledge about what a plastic is, how it is obtained, what are the types of plastic and their applications, and what are the processes for transforming it. This information is important for those who work in the commercialization of plastics, and plastic production or transformation industries, or who are just curious about the subject.
Environmental issues
Currently, these plastics are widely used as containers or wrappers for substances or food items that, when discarded without control, after use, have created gigantic marine garbage dumps, such as the so-called Garbage Island, the largest landfill in the world. In Argentina, during the 2021 Provincial Census of Marine Coastal Litter, 84.5% of the debris found was plastic.
In this way, a problem associated with environmental pollution arises, often the product of the waste of high and low density plastics. The molecular characteristics (types of polymers) of plastic contribute to its great resistance to environmental degradation and even more so to biodegradation. UV radiation from the sun is the only form of natural degradation that makes its effects felt in plastic in the medium term, destroying polymeric bonds and making it brittle and brittle.
As is evident, the cumulative waste of these plastics in the environment brings serious consequences to the communities, such as diseases among which is dengue, produced by the accumulation of garbage and the stagnation of sewage, serving as breeding grounds of the white-legged mosquito. Among other important consequences we can mention the obstructions of the sewage pipes. In addition to this, the disposal of these plastic materials into the environment causes a decrease in the beautification of some areas, establishments, municipalities, cities or states.
The buoyant plastics thrown into the sea are a big problem in the equatorial calm zones, since they gather in those sectors, accumulating in large quantities.
In Chile, during a severe drought produced in 1967 in the IV region of La Serena, a large number of goats from the rural ranches bordering the Pan-American Highway fed on the plastic remains (polyethylene bags) that were discarded washed ashore by users, causing mass death within a few hours of ingestion.
Many of the advantages of plastic products become disadvantages when we throw them away, either the packaging because it is disposable or when we throw away plastic objects because they have broken.
Although the vast majority of plastics could be reused or recycled, the truth is that today this waste is a problem that is difficult to solve, mainly in large cities. It is really an expensive and complex task for the municipalities in charge of the collection and final disposal of waste, since the volume they represent must be added to the number of containers.
Due to their characteristics, plastics generate problems in collection, transfer and final disposal. Some data alert us to this. For example, a truck with a capacity to transport 12 tons of common waste will only transport five or six tons of compacted plastic, and barely two of uncompacted plastic.
Among the total number of disposable plastics that go to the trash today, the sustained increase in PET containers stands out in recent years, mainly coming from disposable bottles of table water, oils, and alcoholic and non-alcoholic beverages. Companies have been substituting glass containers for returnable plastic ones at first, and not returnable later. This decision implies a permanent change in the composition of the garbage. In Uruguay, this process has accelerated since mid-1996, worsening in 1997 when, in addition, many returnable glass containers were transformed into disposable glass.
In this way, it is clear that the abandonment of these materials to the environment represents a serious environmental problem.
Therefore, there is concern about developing a team with the capacity to recover said plastics that have been discarded by society, which are considered non-reusable.
In this way, the purpose arises to design an equipment that uses thermal energy by induction melting the low density polyethylene that is deposited in it, once melted, agglomerates and in a liquid state they are poured into a mold to elaborate other products that will be used in other applications.
A candidate material to replace petroleum is hemp, usable for all petrochemical uses, but also 100% biodegradable and highly recyclable.
Plastic wood
Another of the solutions that have been proposed to the accumulation of plastic waste has been plastic wood. This has been an innovation for a decade now, arising from the abandonment of wood waste such as cargo pallets, deteriorated furniture and of course the accumulation of plastic waste in our landfills. Wood and plastic composite materials (MCM) are materials generally made up of recycled plastic and wood such as pine, cedar, etc. Its composition has a continuous plastic mixture called matrix (includes PE, PP, PVC, etc.) and another made of fiber or wood dust. Both are built in ovens at 230 °C for the fusion of both. In addition to wood and plastic fibers, they may contain other fillers (ligno-cellulosic or inorganic). On the other hand, some fibers that can replace a percentage of wood or/and plastic can be fiber-based fillers, for example cellulose fibers, peanut shells, bamboo, straw, etc.
In addition, it should be noted that the felling of trees for the construction of furniture for the home and kitchen has decreased, making these materials ecological and more durable compared to those made entirely of wood.
This type of wood is used to make bases for school blackboards, desks, etc.
Regarding natural wood, it has advantages such as:
- It is not attacked by xylophagus insects.
- Do not rot with moisture.
However, it also has the disadvantage that certain solvents such as benzene, hexane and some ketones (varnish thinners) can attack it.
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