THERMOPLASTICS

Before addressing the individual study of plastics, it's worth clarifying certain terminological clarifications that often cause confusion. In the technical field, the most rigorous term for these materials is "polymer," which raises the question of the most appropriate way to describe them depending on the context. The answer, although it may seem ambiguous, is quite clear: all plastics—understood as synthetic materials for everyday use, such as polypropylene (PP) or acrylonitrile butadiene styrene (ABS)—are indeed polymers, but they belong to a specific subcategory within this family. 

Specifically, most of the plastics that are part of our daily lives are thermoplastics, that is, polymers that can be melted and molded repeatedly without altering their fundamental chemical structure. Although it is convenient to refer to them simply as polymers, it is necessary to distinguish between synthetic polymers—such as those mentioned above—and natural polymers, which include amber, silk, or even deoxyribonucleic acid (DNA). The latter, although they have similar macromolecular structures, do not have significant applications as structural materials, justifying their exclusion in the context of materials engineering. A polymer, strictly speaking, is a macromolecule composed of repeating monomeric units, whose chemical formula can reach considerable levels of complexity and whose molar mass can easily exceed 5×10⁵ u (atomic mass units). 

To establish an illustrative comparison, silicon carbide (SiC), widely used in industrial applications, has a molar mass of approximately 40.10 u, and its synthesis is much simpler compared to the production of most plastics, which require complex and highly controlled chemical reaction chains. From a popular perspective, the term "plastic" immediately evokes objects such as polypropylene bottles or action figures made of polyvinyl chloride (PVC), demonstrating that the technical definition does not always coincide with the general perception. In reality, a plastic is any substance that exhibits malleability, deformability, or moldability under specific conditions of temperature and pressure. This definition includes the most common synthetic polymers, but also encompasses other materials that, for economic or technical reasons, are not widely used. Therefore, the focus of this analysis will be on thermoplastics, which constitute a relatively small—albeit enormously important—group within the universe of plastic polymers. 

They all share an essential characteristic: they are based on carbon (C), an element whose chemical versatility allows the formation of an overwhelming number of organic and inorganic compounds. This ability to form multiple bonds and complex structures is precisely what makes carbon the undisputed protagonist of polymer chemistry, although it is also responsible for why the study of these materials can sometimes be so intricate.

Thermoplastics constitute the essential core of the family of synthetic, inorganic, or semi-inorganic polymers—some derived from natural polymers such as cellulose—with which we live daily. Their presence is so widespread that even in regions with severe economic constraints, their use is ubiquitous. This proliferation responds to a universal pattern that allows for no exceptions: thermoplastics are inexpensive, easy to manufacture, and extraordinarily versatile. Each of them has particular properties that make it more or less suitable for a specific application, which has allowed their integration into sectors as diverse as electronics, automotive, food, leisure, and packaging. The mass production of these materials has been key to understanding the contemporary world, in which hundreds of thousands of devices, vehicles, toys, and containers are manufactured daily. While they sometimes do not match the quality of traditional materials, they fulfill their function at a huge cost difference.

However, this material revolution is not without consequences. Plastics, despite their many virtues, pose serious environmental challenges. Although some pieces can achieve a useful life of up to 150 years, this figure pales in comparison to the longevity of materials such as glass, steel, or ceramics. Furthermore, most plastic products are designed to be disposable, which has generated a global crisis surrounding their recycling and disposal. For many people, the term "plastic" is directly associated with pollution, a perception that, although simplified, reflects legitimate concerns about the ecological impact of these materials. If they were not so harmful to the environment, their production would be even more rampant, and they would likely have completely displaced older materials, thanks to their excellent mechanical, chemical, and thermal properties.

A curious characteristic of plastics—and one they share with certain nickel (Ni) and cobalt (Co) superalloys—is that they are often better known by their trade names than by their scientific designations. This phenomenon can be illustrated with an example from the pharmaceutical industry: when the German company Bayer successfully developed acetylsalicylic acid, it marketed it under the name "Aspirin," which is simply a registered trademark. Similarly, alprazolam, a benzodiazepine used to treat anxiety disorders, is sold as "Xanax" in the United States and as "Tranquimizin" in Spain. In both cases, the active ingredient is the same, but the brand name varies depending on the marketing strategy and intellectual property protection.

This parallel applies perfectly to the world of plastics. The brand name of a thermoplastic is often a tool of differentiation and pride for the manufacturer, who uses it to protect the formula and facilitate its identification in the market. Furthermore, brand names are designed to be easily memorable, which simplifies communication between suppliers and customers. It is much more practical to request "40 units of model XXXXX ABS" than to say "40 units of model XXXXX Acrylonitrile Butadiene Styrene," even though both terms refer to the same material. This practice responds both to the need to protect the investment in research and development—which can involve years of work and millions in funding—and to the desire to effectively position the product in the market.

Therefore, in developing this content, both the technical and commercial names of each thermoplastic will be provided whenever possible, although in some cases the scientific name may be excessively long or complex. With these preliminary clarifications, it's time to delve into the common properties that define this fascinating family of materials.

Thermoplastics get their name from their ability to be molded through the application of heat, a process that softens them without altering their chemical composition. This thermal behavior, which distinguishes them from other types of polymers, enables them to be formed using techniques such as injection molding, compression molding, extrusion, or even direct precipitation. Despite the popular belief that plastics burn easily, many thermoplastics have considerable thermal tolerance, even being used as sealants between metal components in applications requiring resistance up to 100°C. In terms of density, thermoplastics are extremely lightweight materials, surpassing alloys such as Al-Mg and technical fibers such as Si and C. Most have densities between 1.00 and 1.30 g cm⁻³, allowing them to float in water if their density is less than 1. Polytetrafluoroethylene (PTFE), commercially known as "Teflon," with a density of 2.17 g cm⁻³, is a notable exception within this family. In terms of mechanical behavior, thermoplastics stand out for their flexibility—not to be confused with plasticity—and can be marketed in rigid or flexible versions, as is the case with polyvinyl chloride (PVC), which is suitable for uses as diverse as pipes, toys, and clothing.

One of the most outstanding properties of thermoplastics is their impact resistance. This characteristic is due to the relatively loose arrangement of their molecular chains, which allows them to absorb kinetic energy without fracturing or permanently deforming. Unlike malleable or brittle metals such as Cr or Bi, thermoplastics tend to return to their original shape after the application of force, making them ideal materials for applications where shock tolerance is a priority. However, this advantage is offset by limited tensile and compressive strength, which restricts their use in structures subjected to prolonged stress.

From a thermal perspective, thermoplastics have moderate strength, lower than that of ceramics and metals, although they compensate for this weakness with their insulating capacity. Their high specific heat—the amount of energy required to increase the temperature of a substance by 1°C—and their low electrical conductivity make them especially useful in environments where thermal and electrical insulation is crucial for operator safety. Regarding their chemical resistance, this varies considerably between compounds. Some thermoplastics outperform steel and certain Cu-Zn alloys in this regard, although they fall behind materials such as Ti or those that develop passivating oxide layers. PTFE, for example, is one of the most corrosion-resistant materials known, while polyoxymethylene (POM) is vulnerable to strong acids such as HCl or H₂SO₄.

The melting point of thermoplastics is generally low, with exceptions such as PTFE, which reaches 327°C. When directly exposed to flame, they tend to melt and release halogen elements such as Cl or F, which are highly corrosive and dangerous, especially in the case of fluorine. However, this same chemical inertness allows their use in medical applications, such as the manufacture of prosthetics. A little-known aspect is their sensitivity to ultraviolet (UV) radiation, which alters the long chains of atoms centered on carbon, causing color changes, structural weakening, and progressive decomposition. To counteract this effect, stabilizers such as Cd or Pb are used, which can also act as plasticizers, although the two functions are independent of each other.

Although thermoplastics are flammable, they are considered less combustible than other polymers, and heat is precisely the agent necessary for their formation. Their advantages include low density, good chemical resistance, thermal and electrical insulation capacity, excellent impact tolerance, flexibility, and plasticity. On the other hand, their disadvantages include high prices in some cases, low melting points that limit their use in critical applications, recycling difficulties, a relatively short lifespan, and generally poor mechanical properties—except for impacts. The environmental challenge they pose is considerable, especially due to the massive accumulation of plastic waste in marine ecosystems. The true cost of plastics lies not in their production price, but in how society disposes of them. Fortunately, most are recyclable, and more and more educational institutions are promoting responsible environmental awareness among children and adolescents.

Thermoplastics, commonly known as plastics, are obtained from two main sources that define their origin and determine their chemical and environmental properties. The first group includes those substances that exist in nature and serve as the basis for the synthesis of new polymers. In this case, we are talking about semi-synthetic plastics, since they are based on natural materials such as cellulose, plant resins, resinous gums, or certain organic fibers. Although these compounds have a natural component, the final product—after chemical transformation—does not differ in environmental impact from fully synthetic plastics, so their "semi-natural" nature does not necessarily imply less pollution.

The second group is made up of plastics derived from hydrocarbons, primarily petroleum and natural gas. Although these sources can be considered organic byproducts, the transformation process they undergo to become polymers involves a complete rupture of their original molecular structure. Therefore, plastics obtained through this process are considered 100% synthetic. This distinction, however, does not imply a significant difference in terms of their polluting potential, as both types of plastics—whether semi-synthetic or synthetic—present similar challenges in terms of recycling and final disposal.

In both cases, polymer synthesis requires a series of complex chemical reactions, which take place under controlled conditions of pressure and temperature. Specific catalysts are used to facilitate the separation and distillation of the base compounds, as well as to promote the formation of macromolecules by joining smaller units. These polymer chains, whose length and complexity determine the properties of the final material, are obtained from relatively abundant raw materials, allowing for mass production and, in many cases, at a reduced cost. However, there are specialized plastics whose production involves more expensive and demanding processes, resulting in significantly higher prices.

Once synthesized, polymers are usually presented in the form of granules, which constitute the primary state of the material before processing. Unlike other materials where the color is merely superficial, in plastics, the color is intrinsic, meaning it spreads evenly throughout the entire volume of the material. This is because the color responds to modifications in the polymer's chemical structure, allowing for homogeneous pigmentation even in cuts or internal fractures.

Some plastics, such as PVC, can be manufactured in rigid or flexible versions, depending on the additives incorporated during their synthesis. The addition of elements such as cadmium (Cd) or lead (Pb) in small quantities acts as a molecular stabilizer, securing the polymer chains and improving the material's mechanical properties. When these metals are incorporated into the process, the plastic is said to be doped, and the added elements are called dopants. Furthermore, to counteract the sensitivity of many plastics to ultraviolet radiation—present in sunlight—substances with barrier properties are introduced, which prevent the degradation of molecular chains due to the effect of light.

Other additives are used to improve thermal resistance, oxidation stability—remembering that plastics are derived from carbon (C)—and other specific properties depending on the desired application. Together, these modification processes allow thermoplastics to be adapted to a wide range of industrial, medical, domestic, and technological uses. However, it should be emphasized that the above constitutes only an introductory summary, as the production of synthetic polymers is a highly complex discipline that falls within the field of chemistry—primarily inorganic, although with important contributions from organic chemistry and materials engineering.