Overview Of Epoxy Resins: Features, Manufacturing Process, And Applications

“Epoxy” is derived from the two Greek words, ‘epi’ which means upon and ‘oxy’ which means sharp or acidic. Epoxy has been around since the late 1800’s but has increasingly become popular around the mid 1900’s. Prileschajev (a Russian chemist) was the first to realise that alkene’s would react with peroxybenzoic acid. From this the reaction formed epoxides which is used to form epoxy. Epoxy can be an adhesive, plastic, or other material made from a class of synthetic thermosetting polymers containing epoxide groups. Epoxy is used to describe both the basic component itself as well as the product of epoxy resins. Epoxy is capable of forming very tightly linked polymer structures that can either be made into adhesives or cured to become high strength polymers. Epoxide falls under the cyclic ether of organic compounds, which in turn makes it highly reactive.

A thermosetting polymer is a polymer that will become irreversibly hardened once it has been cured completely. Because thermosetting polymers are network polymers, they set permanently through the curing process and will not soften due to heating. The heat treatments for thermosetting polymers the bonds fuse the chains together resulting in a material which is able to resist vibrational and rotational motions at relatively high temperatures. These bonds are fused by the means of cross linking, this is where a polymer chain bonds to another chain. These bonds can react with themselves or a range of co-reactants which are termed hardeners or curatives. The reaction with themselves or other co-reactants forms a thermosetting polymer, which often has desirable mechanical properties such as high thermal and chemical resistance.

The curing process can be activated in a few different ways such as by heat, or some sort of radiation, it can be prompted by high pressures or it can be activated by the use of a catalyst. Most epoxy resins are in liquid form or a mostly malleable prior to being cured, this has a great deal when it comes to the purpose of moulding or used as an adhesive. What makes epoxy resins unique from thermoplastic polymers is that once hardened it cannot be changed or liquefied whereas thermoplastics can be reshaped after the initial curing.

Manufacturing/Processing

Manufacturing process of epoxy resins can either be in liquid or solid states:

The most common epoxy resin today is produced from the reaction of epichlorohydrin (ECH) and bisphenol-A (BPA), where bisphenol-A can be replaced with other materials to produce specified or alternative resins for desired uses. Epichlorohydrin is the product of the reaction of propylene and chlorine where Bisphenol-A is the product of the reaction of acetone and phenol.

  1. Epichlorohydrin and bisphenol-A are charged in the reactor vessel.
  2. Then a solution of boiling caustic soda is added to the reactor vessel. The solution must consists of around 20% to 40% caustic soda.
  3. The epichlorohydrin which has not reacted will evaporate. By adding in a solvent such as methyl isobutyl ketone the two phases will separate.
  4. Once that is completed the resin is washed in water and the methyl isobutyl ketone solvent is removed by vacuum distillation.
  5. Resins which are specified or require certain property enhancements, then certain compounds are added in to change the epoxy’s properties. This process can alter the flexibility, colour, viscosity, adhesiveness and the hardness of the resin for its desired use.
  6. The resin is cured with hardener such as amines, this is to produce infusible, rigid and hard epoxy materials. Depending on the curing agent added to the solution the curing temperatures can range from 5°C to 150°C. There are epoxy resin and hardener combinations which may take place at ambient conditions, and then for specialised epoxies they require even greater temperatures. The reason for heat being present is that without a sufficient amount of heat there will be a network of incomplete polymerisation, which will mean that maximum mechanical properties desired will not be achieved. The curing process transforms the material into a plastic, or an elastomer which is rubber like, by means of cross linking the chains of the polymer. The higher the crosslink density and aromatic content of a thermoset polymer, the greater the mechanical properties are for the means of resistance to heat degradation and chemical attack.

Applications

Expo resin has become very popular now days, because of the fact that they are well known for their chemical and heat resistance, its excellent adhesion properties, its exceptional electrical insulating properties and as well its great mechanical properties. So epoxy resin can be found in numerous sectors such as Paints and coatings, adhesives, Industrial tooling and composites, electrical systems and electronics, petroleum and petrochemical, consumer and marine applications, aerospace applications, biology, and art.

Adhesive applications:

Epoxy adhesives fall in the class of structural adhesives because of its high strength bonding capabilities. They have been manufactured in such a way that the can suit almost any adhesive application. Some of the epoxy adhesives can be manufactured to be flexible or rigid depending on its demand, transparent or coloured. Epoxy adhesives can be either be slow or fast setting which can affect the finish and quality of the adhesive.

Art, paint, coating and consumer applications:

When epoxy resin is mixed with certain pigments, it may be used as a form of painting medium. Epoxy resin can be used for a wide range decorating labels, countertops, tables and decoupage applications. In wood work art epoxy can act as filler, with either a clear finish, transparent finish, coloured finish or even a fluorescent glow to it by the addition of a fluorescent pigment. Epoxy floor coating is the same as having concrete floors as it is exceptionally durable, but the coating protects the underlying concrete from moisture, stains, cracks and grease.

Two part epoxy coatings have been manufactured for heavy duty metal pieces or structures. The epoxy based paints and coatings protect against corrosion and rust if exposed to harsh environments such as chemicals, heat or constant liquid contact. Epoxy coatings improve the adhesion of paints if it is used as a primer for metal surfaces.

Aerospace applications:

The use for epoxy in the aerospace industry falls under the category of adhesives as well because they use epoxy resins as a structural glue because it can to applied to many types of surfaces, rough and smooth. The aerospace industry uses a structural matrix material which is composed of epoxy which has been reinforced by either glass, Kevlar, carbons and boron fibres.

Industry and electrical applications:

Epoxy is used for industrial tooling because it is used to produce the fabrication of short run, master moulds and for castings. The epoxy is used to produce laminates so that composite materials may achieves improved strength, insulation or appearance. For the electrical sector, epoxy resins are a valuable asset to have due to their excellent electrical insulation property. Epoxy resin serves as a protective layers which protects components from shorting, dust and moisture by encapsulation compounds and impregnating varnishes for electric and electronic equipment. It can also act as a connector for copper foil to circuit boards or it is used for sealing and preserving transformers and inductors because cured epoxy is an electrical insulator and it is a better alternative for conducting heat which increases the life span of components.

Marine applications:

For the marine industry epoxy is used predominantly as repair resins, but it is not recommended for external use as the epoxy resin deteriorates after long exposure the UV light. Marine use of epoxy falls into two main areas of use. They can either be used to manufacture components for a high strength weight ratio, or they can be used for its extensive gap filling and adhesive capabilities.

Environment and Sustainability

For epoxy to be consider as a hazardous material it has to be in its uncured state, and only when its uncured it is most dangerous if inhaled or swallowed but it can also harm skin if contact is made. Long term exposure to epoxy can be asthma, skin allergies, and irritations of the eyes, nose and throat. If the resin were inhaled or absorbed from contact, the uncured solution can affect the brain causing nausea, dizziness, headaches, slurred speech and loss of consciousness. Bisphenol-A which is one of the key chemicals in epoxy has been shown to affect individual’s hormone levels and even delays in reproductive developments. Exposure to epichlorohydrin, the other key chemical in epoxy, can cause respiratory tract illnesses and haematological effects.

Advantages:

  • Epoxy can be easily used for multiple sectors and in certain situations the process of applying the epoxy can be relatively quick and stress free.
  • The cured product of epoxy resin becomes nontoxic. Once it is cured it does not emit any toxic fumes, but may deteriorate due to the photolysis.
  • Epoxy resin has a long life span with very high performance, weather and water resistance. Due to its long life span the end product is not easily recycled but this exhibits the reliability and quality of epoxy resins.
  • The performance of the epoxy resin is not determined by the type or texture of the surface which it is applied to.

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