What are the properties of Baeklite polymer



As plastic (Coll. plastic) is the name given to a solid, the basic component of which is synthetically or semi-synthetically produced polymers with organic groups.

A plastic workpiece consists of millions of very long, intertwined molecular chains(Polymers), made up of repetitive basic units (Monomers) are composed. For example, the plastic polypropylene consists of repeatedly repeating propylene units (see picture on the right).

An outstanding feature of plastics is that their technical properties, such as formability, hardness, elasticity, breaking strength, temperature and heat resistance and chemical resistance, can be varied within wide limits through the selection of raw materials, manufacturing processes and the addition of additives.

Plastics are processed into molded parts, semi-finished products, fibers or films. They are used as packaging materials, textile fibers, thermal insulation, pipes, floor coverings, components of paints, adhesives and cosmetics, in electrical engineering as a material for insulation, circuit boards, housings, in vehicle construction as a material for tires, upholstery, dashboards, petrol tanks and much more.

Synthetic plastics are produced from monomers through polymerization (polyaddition, polycondensation, etc.). The raw material is mostly cracked naphtha. Semi-synthetic plastics are created by modifying natural polymers (e.g. cellulose to celluloid).

 

Development history of plastics

 

Preliminary stages

Biopolymers and naturally occurring polymers have been used by humans since ancient times. People use wood as a building material and wool as clothing. In Arabia, water basins and canals were sealed with natural asphalt. Certain tree resins were also used there as gum arabic and exported to Europe. Amber is known from Eastern Europe as a fossil resin for use in arrowheads and jewelry. In the Middle Ages, animal horn was transformed into a plastically deformable material through certain process steps. Already around 1530 the Fugger's house was made according to a recipe of the Bavarian Benedictine monk Wolfgang Seidel [1] transparent artificial horn made from goat cheese and sold.

Development of a plastics industry

In the 17th and 18th centuries, naturalists from Malaysia and Brazil brought with them elastic masses obtained from milky tree sap. For this in Germany the term rubber introduced. A rapidly growing rubber industry has developed since the middle of the 19th century. Victor Regnault discovered vinyl chloride from which polyvinyl chloride could be made. The inventor Charles Goodyear discovered in 1839 that rubber vulcanized with the addition of sulfur, i. H. can be made permanently elastic. He first made rubber gloves from the new material. Around 1850 he also discovered hard rubber, a natural rubber hardened by heating in the presence of sulfur, which was initially marketed as ebonite. From this, for example, jewelry, fountain pens, piano keys, tobacco pipes and parts of telephones were made. This first thermoset started the development of plastics as a material in the human environment.

Cellulose nitrate was later developed for the impregnation of textiles in England and shellac in the USA. In 1869 John Wesley Hyatt invented the celluloid and three years later the first injection molding machine. In 1872, Adolf von Baeyer described the polycondensation of phenol and formaldehyde. The material galalith (made from casein) was invented in 1897 and is very similar to animal horn or ivory. For example, buttons, pins, housings for radios, cigarette boxes, toys, handles for umbrellas and much more in a wide variety of colors were made from them.

Little was known about the exact structures of polymeric materials until the late 19th century. It was only known from vapor pressure and osmosis measurements that the molecules had to be very large and with a high molar mass. However, it was wrongly believed that these were colloidal structures.

The German chemist Hermann Staudinger is considered the father of polymer chemistry. As early as 1917, he told the Swiss Chemical Society that “high molecular compounds” consist of covalently bound, long-chain molecules. In 1920 he published in the Reports of the German Chemical Society an article considered the foundation of modern polymer science. [2] In the years from 1924 to 1928 in particular, other important theories about the structure of plastics followed, which form the basis for today's understanding of this class of materials. [3][4][5] Staudinger received the Nobel Prize in 1953 for this work.

Staudinger's work enabled chemistry to develop rapidly in the field of polymer chemistry on the basis of secure scientific foundations. At the beginning of the 20th century, the Belgian chemist Leo Hendrik Baekeland - based on the work by Adolf von Baeyer described above - developed a process for the production and further processing of a phenolic resin. This plastic, which he baptized Bakelite, was the first synthetic thermoset to be produced industrially in large quantities. Due to its good electrical properties, it was used, among other things, in the up-and-coming electrical industry.

The Munich chemist Dr. In 1910, Ernst Richard Escales named the group of materials "Kunststoffe". The magazine of the same name, which he founded, first appeared in 1911. In 1912, Fritz Klatte developed an industrial process for the production of polyvinyl chloride (PVC). [6] The patent for polymethyl methacrylate (PMMA, trade name "Plexiglas"), registered by Otto Röhm in 1928, started an era that continues to this day.

"PS" production began in Ludwigshafen, and in 1931 ICI in Great Britain produced polyethylene for the first time. The production of epoxy resins also began in Ludwigshafen in 1934 using a process developed by Paul Schlack. In 1935, Henkel (Mainkur) and Ciba (Switzerland) simultaneously described the development of melamine-formaldehyde resin and DuPont the development of polyamide 66 (nylon). The polyamide 6 based on caprolactam produced by Paul Schlack in 1937 was christened Perlon. At about the same time, the I.G. Colors with the production of Buna S and Buna N as a synthetic rubber substitute. Otto Bayer developed polyurethane in Ludwigshafen that year. The plastic polytetrafluoroethylene (Teflon) was developed at DuPont in 1938. Low-Density Polyethylene (PE-LD) followed at ICI in 1939. The material polyethylene terephthalate (PET) was invented by J. R. Whinfielt and J. T. Dickson at Calico Printers in 1941. In 1942 Harry Coover (USA) discovered the “superglue” methyl cyanoacrylate at Eastman Kodak.

year World production (jato)
1930 10.000
1949 1.000.000
1976 50.000.000
2003 200.000.000

In the early 1950s, the German chemist Karl Ziegler discovered that catalysts made from aluminum alkyls and titanium tetrachloride allow the polymerization of ethene to form polyethylene at room temperature. [7][8][9] Until now, polyethylene had to be polymerized under high pressure in steel autoclaves. The polymers produced according to Ziegler also showed a significantly higher degree of order and completely different material properties with regard to their chain structure (see here). Based on the work of Ziegler, the Italian chemist Giulio Natta successfully researched a similar process for the production of polypropylene. [10] Today, the polyethylenes (PE) and polypropylene (PP) produced in this way are, alongside polystyrene (PS), the plastics most frequently used as packaging materials for food, shampoos, cosmetics, etc. Ziegler and Natta received the Nobel Prize in Chemistry in 1963 for their work.

Especially after 1950, the production of plastics increased enormously due to the numerous successes in the field of polymer chemistry. Thanks to the development of thermoplastics and, in particular, corresponding processing methods, molded parts could now be produced in an unbeatably cheap way. Plastic has changed from being a substitute material of particular importance to a material for industrial mass production. As a result, the proportion of thermosets fell steadily and in 2000 was only 15%. The per capita consumption of plastics in 2000 was 92 kg in Western Europe, 13 kg in Eastern Europe, 130 kg in North America, 19 kg in Latin America, 86 kg in Japan, 13 kg in Southeast Asia and 8 kg in the Middle East / Africa .

The plastics industry is still a growth sector today, with manufacturing capacities in Asia expected to overtake the leading and roughly equally strong regions of Europe and North / South America between 2006 and 2008.

Classification

Plastics can be divided into three main groups:

Thermoplastics

Thermoplastics are plastics that consist of long linear molecules. By supplying energy, these materials become malleable to plastic and finally melt. They can be brought into the desired shape using various primary and forming processes. After each part has cooled, it retains its shape. This process is reversible (French: reversible).
Most of the plastics used today fall under this group. They are just as frequently used for simple consumer goods, packaging, etc. as they are for technical parts in the automotive and electrical industries or in the construction industry, especially for roofing membranes, window profiles and pipes.
In order to create new properties that have not yet existed, two or more thermoplastics can be mixed (polyblend).

 

Thermosets

Thermosets are polymers that are produced in a curing process from a melt or solution of the components through a crosslinking reaction. This irreversible reaction is usually caused by heating (hence the English technical term thermosets), but can also be initiated or accelerated by oxidizing agents, high-energy radiation or the use of catalysts. Heating thermosets does not lead to plastic deformability, but only to their decomposition. Hardened thermosets are usually hard and brittle and can only be processed mechanically in the further manufacturing process.
Because of their mechanical and chemical resistance, even at elevated temperatures, they are often used for electrical installations. One of the most common and oldest plastics in this class is Bakelite. This group also includes polyesters (PES), polyurethane resins for paints and surface coatings and practically all synthetic resins such as epoxies.

 

Elastomers

Elastomers include all types of crosslinked rubber. The crosslinking takes place, for example, by vulcanization with sulfur, by means of peroxides, metal oxides or irradiation.
The elastomers are wide-meshed and therefore flexible. They do not soften when heated and are not soluble in most solvents. Therefore, they are used for hygiene items or chemical gloves. The rubber compound of car tires is also an elastomer that gets its properties through vulcanization.
Examples of elastomers are natural rubber (NR), acrylonitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), butadiene rubber (BR) and ethylene-propylene-diene rubber (EPDM ).

Individual plastics are designated according to a globally standardized system of abbreviations, which is described for Germany in DIN EN ISO 1043 Part 1 and DIN ISO 1629 (rubbers).

properties

Compared to ceramic or metallic materials, plastics are characterized by a number of unusual properties:

Density and strength

The density of most plastics is between 800 and 2200 kg / m3. This makes them considerably lighter than metals or ceramic materials.

In terms of mechanical properties, plastics are often inferior to other classes of materials. Their strength and rigidity usually do not match that of metals or ceramics. Due to the low density, however, this can be partially compensated for with structural means (higher wall thicknesses) or the use of fiber-reinforced plastics. One example is the use of Kevlar in aircraft wings.

Although the strengths are comparatively low, plastic parts break less easily than, for example, ceramic or glass. They mostly have good toughness. Therefore, everyday objects for children and toys are often made of plastic.

Chemical resistance

In contrast to metals, many plastics are resistant to inorganic media due to their organic nature. This includes mineral acids, alkalis and aqueous salt solutions. Therefore, plastic materials are preferred for the production of easy-care household and electrical appliances, vehicle equipment, toys, etc.

In contrast to metals, however, they are sensitive to organic solvents such as alcohol, acetone and petrol. Nevertheless, it was also possible to develop durable plastics in this area. One example is the polyethylene car tank in the VW Polo. It is extremely resistant to corrosion and yet insensitive to petrol.

Low processing temperatures

The usual processing temperatures for plastics are in the range from 250 to 300 ° C. While metals have to be cast at high temperatures and there are restrictions with regard to the casting molds, thermoplastics can also be used to manufacture more complex molded parts with comparatively little effort. At the same time, additives such as color pigments or fibers can be incorporated into the material in one processing step, which would decompose at the high temperatures of metal casting or sintering of ceramics.

Low conductivities

The thermal conductivity of plastics is significantly lower than that of metals. That is why many plastics, especially foams, are used as insulation materials.

The electrical conductivity of plastics is 15 orders of magnitude smaller than that of metals. Many plastics are used to insulate electrical lines and cables.

Manufacturing

  see also main article polymerization

Plastics are generally produced by gradually joining together monomers to form long chains - the polymers, with basically between Chain polymerization (also chain reaction) and Step polymerization (also step reaction) is differentiated.

Chain polymerizations

In chain polymerization, growth begins with a molecule to which further monomers are successively added. The molecule that starts the polymerization is called initiatorwhich is called growing up on this Monomer. The number of monomers that ultimately make up the polymer is the degree of polymerization. The degree of polymerization can be adjusted through the ratio of monomer to initiator. Mathematically it is estimated by the Mayo equation.[11]

Free radical polymerization

In radical polymerization, the growth reactions are initiated and propagated by radicals. Compared to other chain reactions, it is insensitive, easy to control and provides a high degree of polymerization even with very small conversions. It is therefore mainly used in the production of cheap plastics such as LD-PE, PS or PVC.

One danger in this process is the one that is released Heat of polymerization The radical polymerization is exothermic, that is, heat is released during the reaction. This heat, if not dissipated, generates further radicals so that the reaction can accelerate itself. In extreme cases, such a “self-acceleration” can overload the reactor material and thus lead to a thermal explosion to lead.[11]

Ionic polymerization

In ionic polymerizations, the growth reactions are initiated and propagated by ionic species. The growing chains are more durable (several hours to days) than their radical analogues (lifespan around 10-3s), one speaks in this context of so-called living polymers. Therefore, one can still use the living, i.e. chains capable of polymerization, give up another monomer and thus continue new growth.[12][13]

Polymers whose chains consist of two or more different types of monomers are called Copolymers. If long blocks of one monomer are found in a copolymer, followed by blocks of the other, one speaks of Block copolymers. Ionic polymerisation is used for such special applications. One example is the synthetic rubbers acrylonitrile butadiene rubber (NBR) and styrene butadiene rubber (SBR), which are used in the manufacture of car tires. The disadvantage of this process is its high sensitivity to impurities, water and oxygen.[11] Ionic polymerizations are therefore more complex and costly than radical polymerization.

Organometallic catalysts

These polymerizations take place in the presence of catalysts. The catalyst is a metal complex (compound of metal atoms surrounded by other species) that is able to bind the growing chain. The addition of further monomers takes place through insertion (Insertion) of the monomer between growing chain and catalyst species. The result is a higher degree of order of the resulting polymers (see tacticity) and a lower degree of branching. Due to this more regular structure, the individual chains are packed more efficiently in the solid, and the plastic becomes more dense. The most important class of catalysts in industry at the moment is that of the Ziegler-Natta catalysts. They play a role, for example, in the manufacture of polyethylene.[14]

Low-density polyethylene (LD-PE) is ethene polymerized in the gas phase with a low degree of order, many side branches and low density. This plastic is mainly found as transparent or colored packaging film for beverage bottles, books, CDs, etc.

High-density polyethylene is produced with an organometallic catalyst in the Ziegler-Natta process. The result is a polymer with a high degree of order, few branches and high density. This plastic is used, for example, as a material for car tanks, petrol cans, etc.

Step polymerizations

In contrast to chain polymerizations, the formation of the polymers in step polymerizations does not take place through initiation of a growing chain, which further adds successive monomers, but through direct reaction of the monomers with one another. This reaction can release a by-product such as water (Polycondensation) or by simply adding the monomers to a new species (Polyaddition) respectively.

Polycondensation

In the case of polycondensations, the linear chain is formed by an intermolecular reaction of bifunctional polymers with the elimination of a smaller species, such as water.

Example polyamides:

Carboxylic acids react with amines to form amides. If you use molecules that carry two carboxylic acid groups, one of these molecules can react with two amines. The result is a polymer made up of three monomers (one carboxylic acid unit, two amines). If the amines used also have two amine groups, the previously created species can in turn react with two carboxylic acid molecules, etc. The resulting polymers can then further bond with one another, so that the degree of polymerization depends crucially on the duration of the reaction (see Carothers equation).

Polyesters are produced by reacting dicarboxylic acids with diols (dialcohol). Among the most important plastics made by polycondensation are polyethylene terephthalate (PET), a polyester, nylon, a polyamide and Bakelite, a thermoset.

Polyaddition

In the case of polyadditions, the polymer is formed by adding the individual monomers to one another, without the formation of by-products.

Example of polyurethanes:

Isocyanates react with alcohols in an addition reaction to form so-called urethanes. Here, too, the following applies: if bifunctional monomers are used, long linear chains are formed. Polyurethane produced in this way is used for dashboards, paints, adhesives, etc. If water is added to the polymerization mixture, it reacts with the isocyanates to form amines and carbon dioxide. The CO released in the mixture2 is enclosed in the plastic in the form of bubbles, so that a foam is obtained. Polyurethane foam is used for mattresses, seating, sponges, etc.

Additives

So-called additives are added to plastics during the manufacturing process (Compounding). They are used to precisely adjust the material properties to the needs of the respective application and to improve the chemical, electrical and mechanical properties. Such molding compounds provided with additives are labeled in accordance with DIN EN ISO 1043 (thermoplastics) and DIN 7708 (thermosets).

Plasticizers

Around two thirds of the additives manufactured worldwide are used for the production of polyvinyl chloride, almost three fifths of the additives manufactured are Plasticizers.[15] They reduce the brittleness, hardness and glass transition temperature of a plastic and thus make it easier to shape and process. These are substances that are able to penetrate the plastic on a molecular level and thus increase the mobility of the chains against each other. Qualitatively, it can be understood as a “molecular lubricant”. Until a few years ago, diethylhexyl phthalate (DEHP) (synonym: dioctyl phthalate DOP) was the most frequently used plasticizer. However, this turned out to be harmful to the environment and health, which is why European industry now largely wants to forego its use.

Extenders also improve the processability, which is why it is also referred to as secondary plasticizers. Important extenders are epoxidized oils, high-boiling mineral oils and paraffins.[16]

Stabilizers

Stabilizers serve to improve the chemical properties. They increase the service life of the plastic and protect it from damaging influences (oxidation, radiation and heat or fire) in its area of ​​application.

The plastic can discolour through reaction with atmospheric oxygen and the polymer chains can decompose or re-crosslink. This can be prevented by adding Antioxidantswhich scavenge the free radicals produced during the reaction (Radical chain terminator), or prevent the formation of radicals (Deactivators).[16] Phenols or amines, for example, are used as breakers, and phosphanes and amines serve as deactivators.

Sunscreens protect against damage from ultraviolet light. Double bonds between carbon atoms are able to absorb light of this wavelength, which is why plastics in particular that contain this structural element (e.g. polyisoprene) are at risk from UV light. However, due to catalyst residues, structural defects and side reactions during processing, practically all polymers can show an absorption capacity for UV radiation. This induces the formation of free radicals in the material, which initiate side reactions such as chain breakdown and crosslinking. There are basically three ways to prevent damage: reflection of light, addition of substances that absorb the light and addition of radical scavengers. Important light stabilizers are carbon black, which absorbs the radiation, σ-hydroxybenzophenone, which converts the energy into infrared radiation, and dialkyldithiocarbamates, which absorb UV light and act as free radical scavengers.[17]

Plastics are sensitive to the effects of heat. Above a temperature characteristic of the material (Decomposition temperature) the breakdown of the molecular structure begins. Heat stabilizers should prevent this. These are essential for polyvinyl chloride, which would otherwise lose its mechanical stability with the formation of HCl and possibly harmful decomposition products.[18] The disintegration mechanism takes place via the formation of double bonds. Organic barium, zinc, tin and cadmium compounds and inorganic lead salts complex these and thus interrupt the disintegration mechanism.[17] In particular, the lead compounds represent a not inconsiderable environmental problem with regard to the disposal of the plastic. Currently 80% of the heat stabilizers are based on lead.[15] However, the chemical industry is currently trying to replace them. At Cognis, a subsidiary of the Henkel Group, a stabilizer based on calcium and zinc was developed especially for window profiles.[15]

In the event of a fire, plastics pose a great danger because they are able to sustain the fires on the one hand and toxic or caustic gases such as hydrogen cyanide, carbon monoxide, hydrogen chloride and polyhalogenated dibenzodioxins and dibenzofurans are released in the event of uncontrolled combustion. Flame retardants either prevent oxygen from entering the fire or disrupt the chemical reactions (radical chain mechanisms) of the combustion.[19] Polycarbonates often do not require flame retardants, since carbon dioxide, which acts as an extinguishing agent, is a decomposition product of the polymer.

Important flame retardants are:[16]

  • Polybrominated diphenyl ethers (PBDEs): set free radicals that intercept the intermediate products of the burning process
  • Aluminum hydroxide (Al (OH)3): releases water molecules
  • Phosphorus-containing compounds: form phosphoric acids that catalyze the splitting off of water
  • Aluminum trihydrate (ATH)

Colorants

Most polymers are colorless in their pure form; they only become colored by adding Colorants. One distinguishes between Dyes (dissolve at the molecular level in the polymer or adsorb on the surface) and Pigments (insoluble, mostly inorganic aggregates).[17] Textiles are almost exclusively colored with dyes.[17] However, the vast majority of plastics are colored with pigments, as these are more lightfast and usually cheaper.[17] Important pigments in this area are rutile (white), carbon black (black), cobalt or ultramarine blue, and chrome oxide green.[17] Meanwhile, the use of Effect pigments possible, e.g. B. Strontium aluminates doped with rare earths show an intense night glow.[20] Areas of application for plastics colored in this way are safety markings that are easier to find in the dark, light switches or flashlights.

Fillers

Fillers are classic extenders that make the production of plastic cheaper. “Active fillers” also improve the mechanical properties of the material. Important fillers include: chalk, sand, kieselguhr, glass fibers and spheres, zinc oxide, quartz, wood flour, starch, graphite, carbon black and talc.

Plastics industry

The plastics manufacturing industry is an important branch of the chemical industry. In 2006, 3570 companies with around 372,900 employees achieved total sales of 79.4 billion euros in this area in Germany.[21] They are active in the partially overlapping areas

  • Plastics production
  • Plastics processing
  • Plastics engineering

Plastics are largely produced by global chemical groups such as Basell, BASF, Bayer, Dow Chemical, DSM, DuPont and Solvay. They supply a limited range of plastics in quantities of sometimes several hundred kt per year.

processing

See main article plastics processing.

Plastics processing is the subject of an independent branch of industry. Primary molding processes are mainly used, which, in contrast to metallic materials, run at significantly lower processing temperatures (up to 350 ° C). This enables the production facilities (so-called. Tools) can be used several times and thus allow cost-effective production.

A large number of processes are used, some of which have their origins in much older metalworking and have been tailored to the properties of plastics and developed further. For example, injection molding for plastics is very similar to die casting for metals. Other processes, such as extrusion or blow molding, are only used for plastics.

The foaming process, in turn, has its origins in plastics, but is now also used for other classes of materials, such as metal foam. They can be further subdivided into chemical, physical or mechanical blowing processes.

For all of these processes, special machines and tools are required that are made available by plastics machine construction.

Important bulk plastics

See also the list of plastics.

Around 90% of global production (around 150 million tons annually) are made up of the following six plastics in the order of their share:[22]

 

Polyethylene

Polyethylene is mainly produced in three different qualities: HD-PE (High-density PE), LLD-PE (Linear low density PE), LD-PE (Low density PE). HD-PE is synthesized using Ziegler-Natta catalysts; its chains show a very high degree of order and a low degree of branching. These can therefore arrange themselves efficiently in the solid, so that a crystalline material is created, the density of which is higher than that of LD-PE (but both have a density that is lower than that of water). It is used to manufacture bottles, beverage crates, barrels, battery cases, buckets, bowls, etc. LD-PE is polymerized in the gas phase under high pressure, 1-butene, 1-hexene and 1-octene are polymerized into LLD-PE in order to produce a controlled degree of branching. Both variants have a low crystalline content and a high or medium degree of branching. The material has excellent film-forming properties and is mainly used for the production of packaging films for cigarette packets, CDs, books, paper handkerchiefs, etc., as well as carrier bags.

 

Polypropylene

Polypropylene is produced almost exclusively by metal catalytic methods, since only the crystalline material obtained in this way has commercially useful properties. It is a very hard, strong and mechanically resilient plastic with the lowest density of all mass-produced plastics. Due to these properties, it has in some cases already displaced metal materials. As with the lid shown on the right, it also shows the so-called Film hinge effect, d. H. it can connect the housing and cover to one another through a thin film without breaking due to the bending load. A significant part of the polypropylene produced worldwide is used for food packaging, other areas of application are:

  • Automotive industry: as a material for air filter housings, spoilers, headlamp housings, seat covers and gas pedals.
  • Construction: garden furniture, toilet lids, artificial turf, furniture hinges, etc.
  • Other: glasses cases, suitcases, satchels, sterilizable medical devices.

 

Polyvinyl chloride

For a long time, polyvinyl chloride was considered to be the most environmentally damaging plastic due to its unusually high chlorine content and the by-products that are created during combustion. In addition, the vinyl chloride required for production is carcinogenic. In the meantime, however, the chlorine content is also cited as a positive aspect (saving on crude oil). A general distinction is made between hard polyvinyl chloride and soft polyvinyl chloride made with the addition of plasticizers. Rigid PVC is an amorphous thermoplastic and has a high degree of rigidity and hardness. It is extremely flame retardant, but in the heat of an existing fire it can release hydrogen chloride and dioxins. It shows very good resistance to acids, bases, fats, alcohols and oils. For this reason, it is also mainly used to manufacture drain pipes and window profiles. Serious disadvantages are its very low heat resistance, it can only be used permanently up to 65 ° C and briefly up to 75 ° C, and its tendency to "stress whitening" when bending. Soft PVC is a rubber-elastic, leather-like thermoplastic. Important applications are the production of floor coverings, seals, hoses, artificial leather, wallpaper, roofing membranes, etc.

 

Polystyrene

Polystyrene is mainly produced as an amorphous thermoplastic, but thanks to recent developments there is now also crystalline polystyrene, but this is of less importance. Both variants are characterized by low moisture absorption, good processability and very good electrical properties. They differ in their impact resistance. Disadvantages are its tendency to form stress cracks, its poor heat resistance, flammability and its sensitivity to organic solvents. Polystyrene expanded by means of carbon dioxide during the polymerization is used, among other things, as Styrofoam expelled.

Application areas:

  • Electrical engineering: as insulation of electrical cables, material for housing, (as HIPS), Switches, etc.
  • Construction industry: as insulating material (foam polystyrene)
  • Packaging: expanded polystyrene, packaging films, yoghurt pots, etc.

 

Polyurethane

The properties of polyurethanes can be varied very widely through the choice of monomer components. This is how very elastic textile fibers become (Elastane) Made of PUR, it is also used as an additive for paint and material for circuit boards (Bectron). The best-known application is likely to be polyurethane foams. They are used as mattresses, in car seats, seating, insulating material, sponges, etc. Here, too, the exact material properties can be set by choosing the monomers.

Polyethylene terephthalate

Polyethylene terephthalate is a polyester made from terephthalic acid and ethylene glycol. It crystallizes very slowly, so that here too, depending on the area of ​​application, amorphous and partially crystalline (C-PET) Can produce material.

C-PET has high rigidity, hardness, abrasion resistance and is resistant to diluted acids, oils, fats and alcohols. However, it is sensitive to hot water vapor.

Application examples:

  • Electrical engineering: parts for household and kitchen appliances, computers, etc.
  • Mechanical engineering: gears, bearings, screws, springs.
  • Vehicle technology: seat belts, truck tarpaulins
  • Medicine: implants such as vascular prostheses

Amorphous PET shows less rigidity and hardness than C-PET, but better impact strength.Because it is transparent but lighter than glass, it is used as a material for beverage bottles and packaging for food and cosmetics. In electrical engineering, PET films are used as a carrier material for magnetic tapes.

High performance polymers

In addition to the plastics that are produced in large quantities for mass-produced articles, polymers for special applications have also been developed in the past. Some examples are:

Liquid crystalline polymers

As Liquid crystalline polymers (engl. Liquid Crystalline Polymers (LCP)) are polymers whose chains form so-called liquid crystalline phases in the melt. In crystals there is generally a fixed order, while in liquids / melts the distribution of the molecules or atoms is usually largely random. In this respect, the expression "liquid crystalline" actually represents a contradiction. In LCPs, however, the polymer chains orient themselves parallel to bundles due to intramolecular interactions. For example, aromatic polyamides in sulfuric acid in combination with calcium or lithium chloride form such phases.[23] Such a solution is pressed from a spinneret through an intermediate space with air into a precipitation bath(Dry-jet-wet spinning process), fibers are obtained in which the chains are oriented in the direction of the longitudinal axis.[24] Such fibers are able to withstand an unusually high tensile load for plastics, which is comparable to metals or carbon fibers. Due to their low density, they are embedded in synthetic resins (Composites) in aircraft and vehicle construction. Further applications are bulletproof vests, protective helmets, protective suits, surfboards, sailboat construction, etc. Important brands are: Kevlar, Nomex and fiber B.

Electrically conductive polymers

Plastics are generally considered to be excellent insulators. This is due to the fact that polymers completely lack the basic requirement for electrical conductivity, quasi free electrons. By adding substances (Doping)that either add electrons to the chain (Reduction) or by distance (Oxidation) Create free spaces for the movement of electrons, it is possible to produce electrically conductive polymers. For example, polyacetylene and poly (p-phenylene) electrically conductive when doped with bromine, iodine or perchloric acid. Other important electrically conductive polymers are polyaniline, doped with hydrochloric acid and polypyrrole from anodic oxidation. Applications are materials for electrodes and battery elements, as well as antistatic coatings. The polymers mentioned above can also be given semiconducting properties by suitable doping. Polymer light-emitting diodes, for example, are made from such materials. The scientists Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded the Nobel Prize in Chemistry in 2000 for the development of conductive polymers.

Plastics in medicine

Plastics fulfill a variety of tasks in medicine: They serve as containers for infusion solutions, components of medical devices, disposable items (e.g. syringes, plasters, catheters, tubes, etc.) and implants (heart valves, bone substitutes, joint sockets, resorbable bone screws, etc.). For materials that are in direct or indirect contact with living tissue, special requirements naturally apply: On the one hand, the plastic must not damage the organism, and on the other hand, the biological environment must not impair the material properties of the plastic. If these conditions are met, one speaks of biocompatibility. The most important argument for the use of plastics in medicine was and is hygiene, so medical instruments made of glass or metal could be replaced by disposable items made of plastic.[25] A notable example is polylactic acid (also: Polylactide), a polyester of naturally occurring lactic acid. It is spun into fibers that are used as absorbable surgical sutures.[22] After the threads have been used, they are broken down enzymatically.[22] The duration of the degradation can be adjusted via the stereochemistry (choice of chains from dextrorotatory or levorotatory lactic acid) of the polymer.

Environmental issues

Since disposable items are also made from plastics, the problem of disposal inevitably arises. On the one hand, the polymeric components of the plastics are not water-soluble and, on the other hand, they are not able to pass through the cell membranes of microorganisms; that is, an interaction with living organisms is largely excluded, with the exception of biodegradable plastics. This has the advantage that polymers can be classified as absolutely harmless to health, but a transformation in living nature can also be excluded. So plastics only rot very slowly. Basically, microorganisms can only process plastics using extracellular enzymes, which break the material down into smaller components that can then be absorbed by the cell.[26] However, the enzymes are too bulky to effectively penetrate the rotting material, so that this process can only take place as surface erosion.[26] Toxic intermediate stages of biochemical processes can also accumulate in nature if they cannot be further implemented.[26] The additives added to the plastics, such as plasticizers, dyes or flame retardants, also pose a risk. Various strategies are therefore being pursued to master the mountains of rubbish.

Plastic recycling

Main article: Recycling plastic waste

Basically, there are three ways of recycling:

Material recycling

Thermoplastics, once formed into a workpiece, can be melted down again and shaped into a new product. However, the sequence of heat treatments leads to a progressive loss of quality of the material (Downcycling).[27] The biggest problem with renewed material recycling, however, is the separation of the individual plastics. If you mix different polymers in one material, this usually leads to a severe loss of quality and significantly poorer mechanical properties. In 1988, the recycling code was introduced to facilitate separation. The recycling of non-sorted waste, such as household waste, is nevertheless very difficult. The current separation processes are very labor-intensive and require a large amount of water and energy, so that both a cost-benefit calculation and the ecological balance are negative.

Material recycling is therefore currently used almost exclusively where large quantities of a single-origin material are available. For example, foam polystyrene packaging is collected in Germany that can be reused as a soil improver in agriculture or in the production of foam polystyrene concrete or bricks. The recycling rate for expanded polystyrene in 2000 was around 70 percent.[27] There is also a take-back system for PVC, mainly floor coverings, roofing membranes, window profiles and PVC pipes. Further areas of application for material recycling are, for example, in the recycling of vehicles or beverage bottles, or in countries of the second or third world, where collecting single-type plastic waste contributes to income. The secondary raw materials are used to create new packaging or products such as window profiles, pipes, flower and beverage boxes, new foils, window frames or watering cans.

Recycling of raw materials

By means of pyrolysis, plastics can be split back into their respective monomers or other petrochemically usable substances such as methanol or synthesis gas. For the production of the monomers, however, the availability of pure material is also a prerequisite. Examples are the Hamburg procedure,[28] which is currently operated by BP and is used both for the extraction of monomers and petrochemical raw materials and the degradative extrusion process developed by Walter Michaeli and others,[29] that is able to convert mixed plastic waste into gases, waxes and oils that can be used as raw materials. Naturally, these processes are mainly used for the recycling of mixed plastics, which can only be separated with great effort.

Energy recovery