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Plastic is an organic material consisting of a wide range of synthetic or semi-synthetic organic compounds, mainly from pure polymers or blended (high molecular weight) with additives. Its malleability allows solid objects of any shape to be made with different molding methods.
Plastics are high molecular weight organic materials, that is, made up of molecules with a very long chain (macromolecules), which essentially determine the specific framework of the characteristics of the materials themselves.
They can be made of pure polymers or mixed with additives or various fillers. Filled plastics are composite materials in which the matrix is precisely the chosen plastic material, within which are embedded carbon fibers, glass, Kevlar or even wood. The most common polymers are synthetics produced from substances derived from petroleum, but there are also plastics developed from other sources.
The IUPAC (International Union of Pure and Applied Chemistry) in defining plastics as “polymeric materials that may contain other substances designed to improve their properties or reduce costs”, recommends the use of the term “polymers” instead of the generic term “plastics”.
Because of its low cost, ease of manufacture, versatility and impermeability to water, plastic is used in a multitude of products both in industry and in the home (thus replacing some materials such as wood, metals, ceramics, etc.). The success and dominance of plastics since the early 20th century has led to environmental concerns regarding their slow rate of decomposition after being discarded as trash due to their molecular composition. Toward the end of the century, an approach to this problem was addressed with extensive efforts toward recycling.
Polymeric chains of plastic materials can be linear, that is independent from each other, or they can be connected to form a three-dimensional network. In the first case polymers, and therefore the plastic materials they are made of, are called thermoplastic and are characterized by the property of melting at a certain temperature, regaining the solid state if brought to a temperature lower than the melting point; in the second case they are called thermosetting because, subjected to the action of heat, they do not melt but with the progressive increase of temperature they tend to decompose.
Linear polymers often have random ramifications, due to unwanted cross-linking that takes place during polymerization: this does not affect their thermoplasticity, but it modifies their mechanical, thermal and electrical characteristics; for valuable products, however, it is possible, with appropriate stereospecific catalysts, to obtain perfectly linear polymers. The latter have a parallel distribution of the chains, but in some areas there are geometric organizations of parallelism visible to X-rays (crystalline areas or crystallites), while in some others this organization is very poor (amorphous areas).
The preponderance or not of crystals on the amorphous depends on the type of polymerization, the catalysts used and the monomers themselves and involves for the polymer different mechanical characteristics: more precisely, a monoriented set bears a greater load than a dispersed one. In this regard, it was noted that, mechanically stretching an amorphous polymer, artificially originate crystallites with the result of increasing the tensile strength while decreasing the elongation, this phenomenon is used in the manufacture of synthetic fibers.
Thermosetting polymers, on the other hand, have a high spatial cross-linking and therefore high viscosity, which explains their characteristics of high mechanical strength with low elongation, insolubility and infusibility. Compared to metals, the temperature level at which the plastic behavior varies (becoming elastic or viscous) of plastics is very low; moreover, they have a very high coefficient of thermal expansion, so they have dimensional instability as the temperature varies. All good insulators, they have a breaking load generally equal to 1/10 of that of metals except in thin and crystalline fibers.
Important is the resistance to heat, which in most polymers is below 100 ºC, an essential condition for a product to retain its geometric characteristics and therefore can be used practically. The melting point of a given plastic material, called the first-order transition point, is defined only for those polymers with a high degree of crystallinity, which therefore behave like substances with a low molecular weight; amorphous polymers, on the other hand, have a melting range in which their characteristics pass from a plastic stage to a viscous one; in addition to the melting point, there is also a transition point of the second order, below which the plastics have a glassy behavior, characterized by a sharp drop in elongation at break and the loss of resilience. This point exists for each polymer and is defined as a function of the external pressure and mechanical load to which it is subjected.
The glass transition temperature can be varied by copolymerization or by the addition of a plasticizer which, by depressing the viscosity between the polymer chains, decreases the temperature at which the chains themselves are prevented from sliding on each other.
As far as classification is concerned, phenoplasts, aminoplasts, epoxy resins and unsaturated polyesters belong to thermosetting plastics; celluloid, polyvinyl, polystyrene, acrylic resins, polyamides, etc. are thermoplastic. The polymer, however, is almost never used alone to be transformed into products, but it is added with substances that give it resistance to external agents, structural modifications, coloration, etc..
Phenolic resins (now almost totally replaced by other thermosetting resins) were generally loaded with wood sawdust, cotton cloth or paper, up to 50%, both to improve the mechanical characteristics and to reduce the cost of the printed piece. It is preferred to use as filler, for all thermoplastic materials and for some thermosetting materials (such as polyester), glass fiber, in a percentage varying between 15 and 50%. A considerable increase in impact resistance and yield strength is obtained; the greatest advantage is given by the increase in stiffness, which makes some materials (polyamides, polypropylene, ABS) suitable for the production of parts that were previously made exclusively of metal, for example technical precision parts, car bumpers.
Other substances, added in small percentages, make plastics used for furniture or in electrical equipment self-extinguishing. The external plasticizers facilitate the forming of the product by decreasing friction and cohesion between the particles, while the internal plasticizers make the flexibility of the polymer chains permanent.
The dyes, mostly organic, give aesthetic properties; the cross-linking agents, used both for thermosets and thermoplastics, carry out the cross-linking during moulding or extrusion (with cross-linked polyethylene we obtain pipes resistant to high pressures, used for water distribution); the lubricants prevent the adhesion of the moulded piece to the mould during moulding; the light and heat stabilizers prevent the degradation of the polymer subjected to external energy sources.
The mutual proportion of the components depends on the type of polymer, but above all on the application and processing technique. The main types of processing of plastic materials are: molding, extrusion (for the production of pipes up to 1 m in diameter, and profiles of various shapes), extrusion-blow molding for the formation of both containers and hollow bodies in general, and thin films.
With calendering, films and sheets of various thicknesses (even less than 1 mm) are obtained, which can then be transformed, with vacuum forming, into objects of the most varied shapes: typical are packaging for fruit. Sheets a few mm thick, flat or corrugated and often reinforced with fiberglass and colored, are widely used in construction, for waterproof roofing and resilient flooring.
Some materials (in particular polystyrene, polyurethanes, and, to a lesser extent, polyethylene) can be expanded; these materials are widely used as packaging, or sheets used in construction for insulation and soundproofing of buildings and plants.
Characteristics and technological properties of plastics
The main properties of a thermoplastic and thermosetting product are:
- Fair hardness and solidity at room temperature.
- High molecular weight.
- Excellent electrical insulation
- Easy to accumulate electrostatic charges.
- Excellent thermal and acoustic insulation.
- Transportability and ease of use thanks to the light weight of the material.
- Resistance to the action of time.
- Good resistance to stress and impact.
- Good elasticity.
- Ease of coloring.
- Resistance to atmospheric agents.
- Impermeability to liquids and gases.
- Excellent resistance of thermoplastic products to acids.
- Excellent resistance of thermosetting products to solvents.
- Thermoplastic resins can be melted and molded several times, while thermosetting resins can be molded only once.
- Excellent plasticity, workability, ductility and malleability.
Classification of plastics
The most common types of plastics are produced from petroleum, but in recent years, other plastics have been developed from other sources. In principle, three typical behaviors of plastics can be distinguished:
- thermoplastic (thermoplastic polymers or plastomers)
- thermosetting (thermosetting polymers)
- elastomeric (elastomers)
Plastics production and processing techniques
Among the processes to which plastics are subjected are:
- Molding (by compression, transfer, injection, blowing)
- Forming by extrusion or blow molding and thermoforming
History of plastics
The world’s first fully synthetic plastic was Bakelite, invented in New York in 1907, by Leo Baekeland who coined the term “plastic”. Many chemists over time have contributed to the development of plastics, including Nobel laureate Hermann Staudingerwho has been called “the father of polymer chemistry” and Herman Mark, known as “the father of polymer physics.”
- 1855: Swiss chemist Georges Audemars produces rayon in the laboratory, obtained from cellulose and used as an artificial fiber.
- 1861: Alexander Parkes patents a first plastic material, named parkesine (later known as xylonite), from some chemical processes between nitrocellulose and camphor. It is a first type of celluloid, used for the production of handles and boxes, but also of flexible items such as cuffs and collars of shirts.
- 1869: the American John Wesley Hyatt perfected the parkesine and, with the addition of nitrogen, patented cellulose nitrate, or celluloid, with the aim of replacing the expensive and rare ivory in the production of billiard balls, except meeting with immediate success among dentists as a material to be used for dental impressions. Chemically, celluloid was still cellulose nitrate and was unsuitable for high-temperature molding techniques because it was very flammable. The problem was overcome with the advent of the new century, when cellulose acetate, or celluloid, was developed, which was sufficiently fireproof to reinforce and waterproof the wings and fuselage of early airplanes or to produce motion picture film.
- 1907: the Belgian-American chemist Leo Hendrik Baekeland obtained by condensation between phenol and formaldehyde the first thermosetting resin of synthetic origin, which will patent in 1910 under the name of Bakelite. This new material has an overwhelming success and Bakelite becomes in a short time and for many years the most widely used plastic material.
- 1912: the German chemist Fritz Klatte discovers the process for the production of polyvinyl chloride (PVC), which will have great industrial developments only many years later.
- 1913: the Swiss Jacques Edwin Brandenberger invents Cellophane, the first cellulose-based material produced in very thin and flexible, transparent and waterproof sheets, which is immediately applied in the field of packaging.
- 1920: Hermann Staudinger, a German chemist from the University of Freiburg, began to study the structure and properties of natural and synthetic polymers, proposing open-chain formulas for the synthetic polymers of styrene and formaldehyde and for natural rubber, and attributing the colloidal properties of high polymers exclusively to the high weight of their molecules (called macromolecules for this reason); this research allowed him to obtain the Nobel Prize for Chemistry in 1953.
- 1926: Waldo Semon, of BF Goodrich, introduced the use of plasticizers for the synthesis of polyvinyl chloride (PVC), starting from previous experiments carried out, but never perfected, on vinyl chloride by Henri Victor Regnault in 1835 and Eugen Baumann in 1872. PVC is still used today in countless industrial, domestic and food applications.
- 1928: Polymethyl methacrylate (PMMA) is developed and used in countless applications.
- 1930s: urea resins are marketed; oil becomes the “raw material” for the production of plastics. At the same time, production and processing techniques improve, starting with molding.
- 1935: Wallace Carothers of DuPont synthesizes nylon (polyamide), a material that will spread with the Second World War in the wake of American troops, finding a number of applications, thanks to its characteristics that make it absolutely functional to the textile industry: from women’s stockings to parachutes, the rise of “synthetic fibers” begins.
- 1937: polystyrene resins are put on the market.
- 1938: polytetrafluoroethylene (or PTFE, patented and marketed as Teflon in 1950) is synthesized.
- 1939: the first vinyl chloride-acetate copolymers are industrialized.
- 1941: starting from Carothers’ work, Rex Whinfield and James Tennant Dickson patented polyethylene terephthalate (PET), in collaboration with the Calico Printers’ Association of Manchester. After the war, this polyester had great success in the production of artificial textile fibers (Terylene), a sector in which it is still widely used today (an example is PET, a fabric known as fleece). Also in this year, polyurethane is synthesized by William Hanford and Donald Holmes.
- 1950s: Melamine-formaldehyde resins are discovered (known under the commercial name of “pharma”), which make it possible to produce laminates for furniture and to print low-cost tableware, while synthetic fibers (polyester, nylon) experience their first boom as a modern and practical alternative to natural fibers.
- 1953: German chemist Karl Ziegler synthesizes polyethylene (PE), which finds full success only two decades after its invention, thanks to its high melting point, allowing applications unthinkable until then.
- 1954: Italian chemist Giulio Natta produces isotactic polypropylene (marketed under the name Moplen) as the culmination of his studies on ethylene polymerization catalysts, which in 1963 earned him the Nobel Prize together with German Karl Ziegler, who had isolated polyethylene the previous year.
- 1963: Ziegler and Natta were awarded the Nobel Prize for Chemistry in recognition of their studies on polymers.
- 1973: Nathaniel Wyeth (Du Pont) patented the PET bottle as a container for carbonated drinks. Light, shock-resistant and transparent, the bottle invented by Wyeth is today the standard for packaging mineral water and soft drinks.