Epoxies and Resin Technologies

Summary

Epoxies and resins are chemical systems, as opposed to single compounds, that are used in a variety of applications. Their value derives from their polymerization into three-dimensional or cross-linked polymeric materials when the components are combined and allowed to react. Epoxies are so-named because the principal component is a reactive epoxide compound. The combination of the epoxy compound with a second material that promotes the polymerization reaction is called a resin. The term also applies generally to any polymerizing combination of materials that is not epoxy-based. Epoxies and resins are used primarily in structural composite applications, in which the combination of a reinforcement material (usually a specialized fiber) bound within a solid matrix of polymerized resin provides the advantages of high strength, low weight, and unique design capabilities. Resins are also used in injection molding and other molding operations, extrusion and pultrusion, prototype modeling, and as high-strength adhesives.

Definition and Basic Principles

In the field of polymers, resin refers to the material or blend of materials that is specifically prepared to undergo a polymerization reaction. In this type of reaction, molecules add together sequentially to form much longer and larger molecules. A polymerization reaction can proceed in a linear manner to form long-chain single molecules whose bulk strength derives from the physical entanglement of the molecules. It also can proceed with branching to form large, multiple-branch molecules that derive their bulk strength from their sheer size and complex three-dimensional interlinking bonds between the molecules.

The particular combination of materials used to prepare the resin for polymerization is chosen according to the extent and type of polymerization desired. Monomers containing only one reactive site or two functional groups can form only linear polymers. Three-dimensional polymers require the presence of three or more functional groups or reactive sites in at least one of the resin components. The polymerization reaction can proceed as a simple addition reaction, in which the single monomer molecules simply add together by forming chemical bonds between the reactive sites or functional groups on different molecules.

Polymerization reactions are generally driven to completion with heating, although the heat produced by exothermic reactions must be controlled to prevent overheating, decomposition, and dangerous runaway reactions from occurring. The polymeric product of the reaction may be thermoplastic—becoming soft or plastic with heating. Thermoplastics are characterized by this behavior change at the glass transition temperature, Tg. Below this temperature, the material is solid and fractures in the characteristic conchoidal manner of glass rather than along any regular planes denoting a regular crystal structure. At the Tg, the material begins to deform rather than to fracture. The Tg is always stated as a fairly broad temperature range. At its higher value, the material has no resistance to deformation that would result in fracture, although it may not yet be entirely liquefied.

A thermosetting resin produces a polymer that does not soften with further heating and exhibits conchoidal fracture behavior at all temperatures at which it is stable. Such polymers will undergo thermal decomposition (also called thermolysis) when heated, as their Tg is at a higher temperature than the temperature at which they break down.

Epoxies are a specific type of resin in which one of the components is an epoxide compound. A second component, typically an amine, reacts irreversibly with the epoxide functional group, causing its three-membered ring structure to open up. The intermediate form produced reacts in a chainlike manner with other epoxide molecules to form complex, three-dimensional polymeric molecules.

Various technologies, methods, and applications are encompassed by the field of epoxies and other resins. These range from molecular design and testing in the chemical and material sciences laboratory to injection molding and hand layup of fiber-reinforced plastics and the repair of structures made from resin-based materials. The production of specific resin formulations on an industrial scale is particularly exacting because of the regulations governing the certification of the materials for specific critical uses and concomitant purity requirements throughout the handling of the product. Specialized training and equipment are required to safely produce and transport the materials.

Background and History

Resins and their property of solidifying have been known and used since ancient times. During explorations of the New World and Asia, European explorers like Christopher Columbus and Hernan Cortés found Indigenous peoples playing sports with balls that could bounce and wearing waterproofed clothing and footwear. The Indigenous peoples used natural latex materials derived from plants to make these and other objects.

In the mid-nineteenth century, as the industrial sciences, especially chemistry, blossomed in Europe, natural latex materials called caoutchouc and gutta-percha were imported, and their unique properties were used in many ways. Gutta-percha, for example, was used to make the corrosion-resistant coating and insulation for the first underseas telegraph cables laid across the English Channel between England and France. Other resins produced were semisynthetic, chemical modifications of vegetable oils and latexes. The development of synthetic polymers such as Bakelite, especially after World War II, paved the way for untold applications. The unique and customizable properties of plastics and polymer resins were the foundation of a vast and growing industry that has constantly sought new materials, innovations, and applications.

How It Works

Resins and Chain Reactions. The term resin was initially used to refer to secretions of natural origin that could be used in waterproofing. It has since come to mean any organic polymer that does not have a distinct molecular weight. Typically, organic polymers form through sequential addition reactions between small molecules forming much larger molecules through a chain reaction mechanism. Once initiated, the progress of such a reaction chain becomes entirely random. Any particular reaction chain will continue to add monomer molecules to the growing polymer molecule as long as it encounters them in an orientation that permits the additional step. Typically, this happens several thousand times before a condition, such as an errant impurity, is encountered that terminates the series of reactions. The exact molecular and chemical identity of any individual polymer molecule is determined precisely by the number of monomer molecules that have been combined to produce that particular polymer molecule. However, within a bulk polymerization process, billions of individual reaction chains progress at the same time in competition for the available monomers, and there is no way to directly control the individual reactions. As a result, any polymerized resin contains a variety of homologous molecules whose molecular weights follow a standard distribution pattern. In thermoplastic resins, this composition, consisting of many chemical compounds, is the main reason that the Tg is characterized by softening and gradual melting behavior over a range of temperatures rather than as the distinct melting point typical of a pure compound.

Polymerization Reaction Processes. Polymerization reactions occur in one of two modes. In one, monomer molecules add together in a linear, head-to-tail manner in each single chain reaction. This occurs when only two atoms in the molecular structure function as reactive sites. In the other mode, there is more than one reactive site or functional group in each molecule. Polymerization reactions occur between reactive sites rather than between molecules. The presence of more than one reactive site in a molecule means that the molecule can take part in as many chain reaction sequences, with the resulting polymer molecules being cross-linked perhaps thousands of times and to as many different polymer chains. The result can, in theory, be a massive block of solid polymeric material composed of a single, large molecule.

Epoxy Resins and Cross-Linking. Polymerization and cross-linking bonds arise as the reactive site or sites of the molecules become connected by the formation of chemical bonds between them. As a bond forms from the atom at one end of a reactive site, the atom at the other end becomes able to form a bond with the reactive site of another molecule. The resulting resin is called an epoxy when the reactive site is an epoxide ring structure. Epoxy resins are two-part reaction systems, requiring the mixture and thorough blending of the epoxide compound and the catalyst. This second compound initiates the ring opening of the epoxide. This is typically an amine, and the relative amount of amine to epoxide controls the rate at which the polymerization occurs. This represents essentially all the control that can be exercised over the progress of a polymerization reaction. It is, therefore, critical to control the relative amounts of epoxide and catalyst in an epoxy resin blend.

Applications and Products

The value of epoxy and other resins is in their versatility. They are used in innumerable products across industries that have become central to modern society. When epoxy resins cure, they become a tough, resilient, and durable solid highly resistant to impact breakage, fracturing, erosion, and oxidation. They are also reasonably good thermal conductors that tolerate rapid temperature changes very well.

Aircraft. An excellent example of resin application is in aircraft technology, particularly modern fighter jets. The fuselage and wing structures of many aircraft are constructed of fiber-reinforced plastics. The materials used in aircraft production must be able to tolerate drastic changes in temperature and pressure. For example, an aircraft may be stationed on the ground in a desert with surface temperatures over 60 degrees Celsius (140 degrees Fahrenheit), and less than one minute later, the aircraft may be in the air at altitudes where the air temperature is −35 degrees Celsius (-31 degrees Fahrenheit) or colder. The structural materials of such an aircraft can repeatedly withstand abrupt changes in temperature and physical stresses, indicating the strength, toughness, and thermal properties of the epoxy resins used in its construction.

Electronic Devices. The thermal stability of epoxy resins is also evident in their use in the packaging material of integrated circuits, transistors, computer chips, semiconductors, and other electronic devices. The operation of these devices produces a great deal of heat because of the friction of electrons moving in the semiconductor material of the actual chip. Pushed to extremes, the devices can fail and burn out, but it is far more usual for the packaging material to adequately conduct and safely dissipate heat, allowing whatever process is running to continue uninterrupted. That may be something as trivial as some spare-time gaming or as crucial as an emergency response call, the flight control program of an aircraft in the air, or an advanced medical procedure.

Structural Composite Applications. There are numerous applications of and products produced from resins. The combinations of materials for the production of resins are essentially limitless, and each combination has specific qualities that make it suitable for particular applications. Thus, the varieties and possibilities of epoxies and resin technologies are virtually limitless. A significant area of application for epoxy resins and other types of resins is in structural composites, particularly in fiber-reinforced plastics and as insulating or barrier foams. The particular application of a resin is determined as much by the desired properties of the product as by the properties of the polymerized resin. Resins that produce a hard, durable polymer, such as those produced by epoxy resins, are used in products of a corresponding nature. Resins that have good shape-retaining properties coupled with high compressibility, such as those used to produce urethane foams, are used in products such as furniture cushions, pillows, mattresses, floor mats, shoe insoles, and other applications in which the material provides protection from impact forces. Resins that exhibit high levels of expansion while forming a fairly rigid polymer with good thermal resistance are used in sealing and insulating applications, such as those for which urea-formaldehyde resin combinations are so useful. Some resins are also used in construction and civil engineering. For example, a type of epoxy called polyacrylates is used for concrete patching and making traffic stripes on roads.

Resin Production and Supply. A completely different set of technologies and applications is related to the supply and material processing of the resins themselves. Chemists and chemical engineers expend a great deal of effort and time in developing and testing resins to identify new commercially valuable materials or customize existing materials' properties. When the new product leaves the laboratory for commercial applications, a system must be established to produce and safely transport the material from the supplier to the user. Systems and methods must also be established so the end user can prepare the intended products from the material. Resins for low-volume use can be packaged in cans and other small containers, while those for high-volume use may be transported by rail or in different types of large containers. Production methods must produce the resin material in a sufficiently pure state to avoid polymerizing the material en route. Methods of transport must also be such that the resin is protected against any contamination that could result in the initiation of polymerization. This requires specialized applications in transportation technology. The end user of the resins will need the means to manipulate the resin, typically in spray-on applications or molding operations. The equipment used in the molding operations also requires the creation of molds and forms appropriate to the product design. There is, accordingly, a substantial sector of skilled support workers in industries and applications for resin usage.

Careers and Course Work

The demand for resin technology products will likely increase with the expanding human population. The facility with which large quantities of objects can be produced by epoxy and resin technologies ensures the field's continued growth as it keeps pace with the population's needs. The need for new or improved qualities in the materials being used in resin products means a need for materials scientists with advanced training and knowledge in organic and physical chemistry.

An individual who chooses to make a career in resin technologies must learn the chemical principles of polymerization by taking courses in advanced organic chemistry, physical chemistry, analytical chemistry, reaction kinetics, and specialty polymer chemistry. They will also need classes in mathematics, statistics, and physics. Specialized engineering fields related to epoxy and resin technologies include chemical, mechanical, and civil engineering (regarding specific special infrastructure applications). Aircraft maintenance engineers and technicians must undergo specific training programs in using resins and epoxies as they apply to aircraft structural maintenance standards. These are hands-on training programs focused on the physical use and applications of the materials rather than courses of instruction in chemical theory. No special training is required for the layperson to use the materials sold in the automotive and marine supply sections of many retail outlets and by certain hobby and craft suppliers.

Social Context and Future Prospects

A vast quantity of plastics is produced from resins. The strongest of these are the epoxy resins. In concert with the commercial and social benefits of epoxy and resin technologies are the logistical problems inherent in the materials themselves. Using resin-based technologies carries the responsibility of disposing of the products properly. Thermoplastics are relatively easily managed because of their built-in ability to be reused. Because they can be rendered into a mobile fluid form simply by heating, used objects made from thermoplastic resins can be melted and formed into new products.

Thermosetting resins, however, cannot be reformed and must be processed for disposal in other ways. Thermoset plastics, such as epoxy resins, are resistant to facile reprocessing as they are generally impervious to solvents and all but the strongest oxidizing agents. Historically, this has meant that most goods made from thermosetting resins have been relegated to landfills, off-shore dumps, or left as litter and refuse. In the late twentieth century, efforts began to put such materials to other uses, the most common being simply grinding them up for bulk filling materials. Thermal recycling using an incinerator is possible. This method allows expensive oils, fillers, and energies to be recovered, but the quality of the properties in the recovered materials is lowered. Additionally, many countries have laws preventing such methods of recycling thermosetting resins because of the greenhouse gasses produced. Chemical and mechanical recycling methods allow the recovery of some fillers, but these processes are expensive and require long processing times. Scientists continue researching and developing new materials and technologies with built-in recyclable properties.

Epoxy and resin technologies and the plastics industry have had a considerable impact on modern life, becoming essential to the infrastructure of contemporary society. Essentially, every government and university research program, industry, business sector, and corporation dealing in material goods work with resins and plastics. The automotive industry is responsible for a considerable amount of the epoxy resin market in the twenty-first century. Because of the demand for energy-efficient and earth-friendly vehicles, demand continues to rise. The wind energy sector is also an increasingly important sector in the purchase and use of epoxy resins. New ventures are established almost daily for the production of material goods explicitly designed to be produced by epoxy or resin technologies.

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