It's a paradox, but epoxy resin, obtained for the first time almost a century ago, remains a very popular material.
Epoxy resins (the word "epoxy" is derived from two Greek roots: epi - over and oxy - sour) are used in adhesives for various purposes, sealants, for the manufacture of household plastics, plastics, in the aviation, construction, automotive, radio-electronic industries, in the ship and mechanical engineering and even in the jewelry industry - for the production of artificial amber. They are used in the textile, leather, paint and varnish industries. It would seem that it is time to find something new that would send epoxy to the archives of history, as happened with many organopolymers - ebonite, gum arabic, tar, tree resins, rosin, polymethyl methacrylate, ordinary polystyrene, alkyds. After all, one-component curable formulations and more convenient water-acrylic resins have appeared long ago. Nevertheless, the world market for epoxy resins is estimated at 15–20 billion US dollars, of which about a third "spin" in the North American continent, another third - in China, where dozens of resin manufacturers operate. In the post-Soviet space, epoxy resin (ED-20) is produced only in Russia (Dzerzhinsky NIIP, plus factories in Tatarstan and Bashkiria), although almost all chemical enterprises are packed (bottled).
The "progenitor" of epoxy resin can be considered the Russian chemist Alexander Pavlovich Dianin, a contemporary of Mendeleev. He was the first who, in 1891, received one of the key components for the production of epoxy resins - bisphenol A. Another name for this monomer is diphenylolpropane, and also dian. On behalf of A.P. Dianin, the name "epoxy-dian resins" (in English "epoxy-dian resins") * has gone.
The synthesis of diphenylolpropane, which was later used to obtain not only epoxy resin, but also polycarbonates, adhesives, varnishes, did not become a noticeable event for contemporaries (as well as the discoveries of other polymers - then they were scornfully considered by-products). They were really interested only in pre-war Europe. The first industrial epoxy resins were obtained in the mid-1930s by chemist P. Shlak from the German concern IG Farbenindustri. In 1936, the Swiss P. Castan from the Ciba concern patented the most famous epoxy, bisphenol, and then the American S. Greenlee obtained and patented its modern form. The first epoxy adhesive - "Araldite 1", based on the works of Castan, was released by Ciba in the early 1940s.
The USSR was slightly late with the introduction of epoxy, but already in the 1960s it launched such a production that even the West envied it. In those years, industrial production was organized in Dzerzhinsk, Sumgait, Kotovsk, Ufa and Leningrad. Later, production was launched in the Ukrainian SSR (Donbass) and the Tatar Autonomous Soviet Socialist Republic. The most popular in the USSR glue "Epoxy-Dian with a hardener polyethylene polyamine" (hence - "Glue EDP", or epoxy resin ED-20) was born in Dzerzhinsk. Since then, it has been sold under this name, and with the same packaging design - in plastic and glass jars, placed in simple red and white cardboard boxes. Some adjustments to the design were made at one time only by the Baltic manufacturers, packing it in tubes.
Now there are six large classes of epoxy resins - bisphenol (A and F), novolac (phenolic and cresol), aliphatic (mono- and highly functional), glycidyl, acrylic epoxy. Dozens of their subspecies are known, including light-cured and water-borne ones (their widespread introduction is held back by their high cost). But one class of epoxides is especially popular - the same oligomer based on bisphenol A.
Epoxies are cured, as a rule, with amines - yellow-brown viscous liquids (with an unpleasant odor and poisonous). Among the most popular amine hardeners are DEET, TETA, TEA, AEP, HMDA. But the most popular is still polyethylene-polyamine PEPA - a technical mixture of almost 25 amines, among which all are named (except for HMDA).
By combining resins and hardeners and changing the curing conditions, materials can be obtained from hard (stone-like) to semi-liquid and elastic. They can be used for critical products, such as high-strength or heat-resistant, and for work in humid environments and water. But this is still "exotic". The most widely used in everyday life and in small industries for non-food and non-heat-resistant products is cold curing epoxy using PEPA.
Epoxy glue is valued for its high adhesion (that is, good adhesion) to inorganic (metal, glass / ceramics, stone) and polymer (polystyrene, wood, PVC) materials, as well as the strength of the adhesive bond. Often, the disadvantages of glue become its advantages. For example, a “non-self-drying” and long-hardening resinous composition is often needed.
Epoxy polymers remain unsurpassed adhesives-compounds, that is, adhesives that do not change their volume after drying. Such a durable, moreover, transparent material for pouring is needed everywhere - from electronics to construction.
and design work. Epoxy resins are used in the manufacture of impregnating material for glass fabrics or fiberglass, fiberglass used in the construction industry, in shipbuilding, in the manufacture of machine parts, etc. They are also used to prepare coatings for waterproofing and chemically resistant coatings, paints for outdoor and indoor use, strengthening and waterproofing impregnations for concrete, wood and other porous materials.
The initial components for the production of epoxy resins are rather specific and poisonous reagents - epichlorohydrin, alkalis, acids and other "harsh chemistry". The mass ratio of resin - hardener is very different: 1: 1, 5: 1, 10: 1. Sometimes, for correct dosage, the amount of hardener is slightly increased, while the polymerization time is reduced, but at the same time the strength of the polymer decreases. Resin in a sealed container can be stored for decades, the hardener (also in an airtight container) without moisture and light can be stored for years and even withstands low temperatures.
Of course, epoxy has many disadvantages and limitations in its use. It is not heat-resistant, that is, it can withstand temperatures only up to 150-200 ° C. Further, the product darkens and loses its strength, and at about 300-350 ° C it becomes charred. In its pure form - without flame retardants and stabilizers - it is easily ignited by open fire and does not extinguish itself. A couple of seconds is enough to set fire to any product with the fire of a simple lighter (if fillers are not used). Epoxy resin is unstable in some important household and industrial solvents - acetone, some ethers, it quickly decomposes in concentrated nitric acid and very strong oxidants.
Finally, it is far from always convenient to work with it, moreover, you need to be able to do it - a beginner can ruin the product. The hardener is quite capricious: if the storage conditions are not observed, it can quickly change its properties. So, if moisture gets into it, curing simply will not occur properly. Cases of poisoning with hardeners are not uncommon. The epoxy itself can penetrate the skin and cause itching and redness of the skin. When working with a large amount of epoxy resin, "inhalation poisoning" is also possible, since it always contains up to 1 wt% of toxic volatile components - epichlorohydrin and toluene, which give the resin a peculiar aroma. Therefore, the use of epoxy for the manufacture of household items, toys is strictly limited to the list of acceptable resins.
Superglue used in everyday life, PVA, Desmokol or rubber adhesives are fast-setting and one-component. The result of their application is visible almost immediately, and only skills and desire are required to work with them. But with epoxy, a mistake in curing agent dosage or heating temperature can be worth the work. In my practice, there were cases when serious orders were canceled due to non-curing, and sometimes the mixture did not cure and not only the product but also the expensive form deteriorated.
Will epoxy go out of use over time under the pressure of new adhesives and bonding materials? Most likely, yes - when it will be replaced by a resin similar in properties, but water-soluble and non-toxic, especially if it will also be stored for a long time. But while epoxy remains in demand in everyday life and industry, as it was 50-70 years ago. Therefore, the Russian chemist Alexander Dianin will be remembered for a long time due to the commonly used term formed on his behalf - "epoxy-dian resins" - "epoxy-dian resins".
Epoxy resins are oligomers, partially polymerized substances (therefore, they are usually viscous like honey), containing in a molecule one or more glycine –CH2 – HCOCH2 or epoxy ... COC ... groups. In both cases, the oxygen atom forms a triangular unstable epoxy cycle in the molecule.
The C-O bond in the epoxy cycle has a pronounced polar character, that is, the bond electrons are strongly displaced in one direction. Due to the high tension of the epoxy cycle, it tends to react more quickly - to open up. The opening of the cycle occurs under the influence of substances that cause the curing of the viscous resin, as a result of which a solid cross-linked polymer is obtained.
Hardeners are divided into three large groups - polycondensation action, polymerization-catalytic and mixed. Polycondensation hardeners crosslink resin molecules, while releasing water - these are amines, anhydrides, amides, olefins. Polymer-catalytic hardeners, as the name implies, catalyze resin polymerization - these are acids (iron trichloride or boron trifluoride) and Lewis bases, phenols, alkalis. In the segment of mass consumption, they settled on two large groups of hardeners - amines and anhydrides. With amines, the polymer is generally stronger, with anhydrides, it is more chemically stable.