Bisphenol-A epoxy resins are a class of thermosetting resin polymers which consists of a number of cross-linking polymers such as amino resins, unsaturated polyester resins, and phenol-formaldehyde resins. Bisphenol-A (BPA) is an organic chemical compound predominantly used as the basic building block for the chemical production of BPA epoxy resins. In modern life, BPA epoxy resins products have had a wide range of application in the manufacturing industries. This is because of their physical and chemical properties which increase their versatility in industrial application. These polymers have gained worldwide acceptance for their effective use as protective coatings, structural and electrical applications due to their specialized properties. Examples of these properties include; thermal and chemical resistance, toughness, excellent adhesion and electrical properties. However, the most common and largest use of these polymers is in industrial products that require excellent adhesion and corrosion resistance. Due to its wide industrial application, there has been an increased demand for its production over the year. As such, extensive study of these compound has been carried out on its production, use and health effect. This paper will cover in details on the production, properties, manufacture and the application of BPA epoxy resins in the modern world.
The increased industrial application of Bisphenol A- epoxy resins for the production of a variety of industrial products has led to a rise Its demand. This has led to large quantity industrial production of Bisphenol A which is the basic building block of epoxy resins. Bisphenol-A is a preferred due to its low cost of production. These polymers are categorized into two main groups; glycidyl epoxy and non-glycidyl epoxy resins. The glycidyl epoxies classified further into glycidyl-amine, glycidyl ester, and glycidyl ether. Non-glycidyl epoxies are classified as either cyclo-aliphatic or aliphatic epoxy resins. The two main categories of epoxy resins have different preparation methods and vary in their physical and chemical properties. However, they all share a common basic building block organic compound called Bisphenol A. About 30% all commercially produced Bisphenol A compound is accounted for in the production of epoxy resin. Currently, the estimated annual production of Bisphenol A is between 2-3 million metric tons.
BPA Epoxy Resin Synthesis
Bisphenol A, an organic compound is the principal monomer used in the production of epoxy resins. The synthesis of epoxy resins begins with a reaction between bisphenol –A and epichlorohydrin (ECH) compounds which are the two main raw materials. The two main raw materials are reacted in a specific ratio of 10:1 in order to prevent the formation of high molecular weight species due to polymerization (Cowie and Arrighi, 2008).This reaction takes place inside a reactor where 20-40 % caustic soda solution is added into the chamber and the solution brought to a boiling point. High temperatures result in evaporation of the unreacted ECH followed by addition of inert solvents mainly methylisobutylketon (MIBK) to separate the two reaction phases. During the first phase, chlorohydrins form as an intermediate product while the second phase involves dehydrohalogenation of the intermediate product. Unreacted phenol and acetone compounds are dislodged during the reaction followed by addition of two glycidyl groups (epoxy group) to the bisphenol-A (Cowie and Arrighi, 2008). This process leads to the formation of diglycidyl ether of bisphenol A (DGEBA). The reaction also produces HCL as a by-product which reacts with the caustic soda solution to form sodium chloride. The end product of this reaction is DGEBA epoxy resin polymer as shown in the reaction below. The resulting molecular weight of the product is dependent on the ratio of the two main raw materials, bisphenol-A to epichlorohydrin.
A typical end product of this reaction is liquid in nature with an average molecular weight of approximately 370, a viscosity of 11,000-15,000 mPa at 250 c and a weight of 188 per epoxide (Cowie and Arrighi, 2008). Upon completion of this reaction, water is used to wash the resin and the solvent separated by means of vacuum distillation. Additives are then added in proportional ratio to produce desired properties depending on the intended use of the product. In addition, curing of epoxy resins through chemical processes is done in order to convert them into hard, rigid and infusible materials. The image below represents a diepoxy molecule with a polymerization degree of one.
Properties of Epoxy Resins Polymer
A low molecular epoxy resin with a mean molecular weight of 380 is fluid at room temperature while an epoxy resin with a mean molecular weight of 1000 is solid at room temperature (Cowie and Arrighi, 2008). Epoxy resins are characterized by varying properties depending on the curing agent used. This property gives epoxy resins a unique quality upon exposure to factors such as heat, chemicals, and pressure. They have a high resistant property to most of these factors. The curing process can be harnessed to produce products of varying properties, however, all the products possess specific basic properties. The properties are explained as follows.
Adhesion is one of the basic characteristic property that is common to all epoxy resins. It is the ability of the epoxy resins to adhere to a number of substrates. The adherence properties are due to the presence of ether bonds and the polar hydroxyl groups. In addition, the minimal shrinkage reduces tension disturbance by enhancing contact between the resin and the substrate. As a requirement, excellent adhesion is only achieved only when the epoxy resins surface tension value is under the critical surface energy value of the substrates (van Tonder, 2014).
When correctly formulated, epoxy resins displays a high mechanical strength than any other hard plastic. Its tensile strength exceeds 80 Mpa. This is attributed to negligible shrinkage which limits inbuilt tensions. It has a flexural strength of 112N/mm2 and a compressive strength of 190N/mm2 and when subjected to water for 24 hours at 230c, its absorption capacity is 5-10 mg (0.06-0.068%) (van Tonder, 2014). This property exceeds those of other hard plastics. It is therefore preferred in various application and in the manufacture of a wide range of products that require a resin of high mechanical strength.
Thermal properties of epoxy resins are categorized into thermal shock, thermal decomposition, flammability and smoke emission. These properties differ depending on the catalyst used and the ratio of the main raw materials during the synthesis and curing process. Thermal shock property described as zero effect at 200 cycles (90 sec. at 750) without being subjected to dry ice. Thermal decomposition occurs at 3500c. the polymer has a property of low smoke emission and placed at class 0 for its high resistance to flammability.
d. Electrical properties
Anhydride based epoxy resins are preferred for plotting and encapsulation purposes. This is especially so for appliances that require high current voltage and high temperatures. Electrical properties of epoxy polymers are categorized into; volume resistivity, surface resistivity, arc resistance, dielectric constant and dissipation factor (Flick, 2001). Volume resistivity property of epoxy resins is a measure of the electrical resistance of an insulating material for a current passing through a solid specimen of a specific thickness under specific conditions. A typical epoxy resin has a volume resistivity of >1010 ohm-cm at 250 C. Its surface resistivity, a measure of electrical resistance for a current along a surface of specified thickness and conditions is also >1010 ohm-cm at 25oC. its dielectric value expressed in volts/mil is about 420-480 volts/mil for a specimen with a thickness of 0.125 inches. Arc resistance of a typical epoxy resin ranges from 100-300 seconds. It expresses the resistance to high voltage to the point of insulator breakdown. The dissipation factor is a measure of the total energy lost to the dielectric component in the field of an alternating current.
e.Stability to light
Prolonged exposure to UV light deteriorates the quality of epoxy resins and their associated products. This phenomenon is known as photodegradation (Mittal, 2011). UV of a value between 315-400 nm contains enough energy that breaks the bonds of polymers including epoxy resins polymers. Free radicals are then formed which actively attack the polymer structures. This phenomenon can, however, be minimized by addition of organic and inorganic UV absorbers and other stabilizers.
Epoxy resins based products have a wide range of application due to their excellent properties and a variation in these properties. This application range from coatings, composite materials, adhesives, electrical appliances, aerospace industries and many others depending formulations and desired properties. Some of the uses include the following;
a.Paints and coatings.
Epoxy resins are highly resistant to corrosion and are therefore very useful in products where resistant to corrosion is important for example in metal surfaces. They act as primers to enhance adhesion of paints, especially in the marine industry. Another application is in the gas and oil industries where the are used to coat the steel pipes and crude oil tanks to minimize corrosion. They are also useful in food industries that produce canned food products. The metallic food containers are coated with specific epoxy polymers to prevent rusting thus prolonging the shelf life of the product. Decorative flooring also applies epoxy resins products due to the availability of the product with different colors which improves the aesthetic appearance.
b.Composite & plastic tooling
This application entails production of fibre-reinforced products useful in the production of strong materials that is used to supplement metal and wood (Flick, 2001). The reinforced fiber is used in plastic tooling to produce castings, molds and master models. This is due to the excellent thermal and mechanical properties of the polyester. The final products are less expensive, quicker and are generally more efficient. Industrial composites are manufactured from thermoset resins. The process requires conversion of the thermoset resins from liquid to solid through a process called polymerization (Mittal, 2011). An example of an epoxy resin plastic tooling product is a hammer, as shown below
Epoxy resins are used to produce a class of high-performance adhesives useful in structural and engineering industries. This type adhesive is widely used in automotive and aircraft engineering where bonds of high strengths are required. They are suitable for almost all industrial application due to their ability to resist heat and chemicals compared to other forms of adhesives. Furthermore, they are available in a variety of forms based for example on color or flexibility and thus satisfy consumer preferences.
d.Electrical & electronic application
Formulation of epoxy resin products has produced excellent products useful in the electrical industry. Epoxy resins used in the production of electrical & electronic appliances are divided into two categories based on the curing process: amine-based epoxy resins and anhydride-based epoxy resins. Anhydride-based epoxy resins are generally used to produce sealants and impregnants products while amine-based compounds are used in plotting and encapsulation purposes (Flick, 2001). They are also applied as insulators due to the ability to withstand high voltage current, heat and ability to withstand moist conditions (encapsulation). They are therefore used to prevent short circuiting, dust and moisture thus protecting electrical appliances. An example is these products is shown in the image below.
Other applications include inductors and transformer plotting and in the manufacture of circuit bonds
They are extensively used in industrial flooring because of their waterproof property and inflammability. In the industries, they form part of the roofing system, bonding, and sealing. They are also used in concrete repair due to their light weight and high strength thus offering protection to the concrete walls. Industrial flooring adapts epoxy resins in the creation of rubber carpet for playgrounds and floor due to their resistance to tear and wear. An example of an epoxy resin end product is shown in the image below and is extensively used in domestic and industrial flooring.
In addition, most of the fire escape route in industries are also coated with thermal resistant epoxy polymers which can withstand high temperatures.
Super reinforced matrix fiber are used to create structural matrix material containing the epoxy polymer. The fiber is mainly composed of carbon, boron, Kevlar and glass and has an advantage of light weight yet remaining strong. The advanced composite matrix materials exhibit extremely high tensile strength, low density and low volume fraction suitable for aerospace engineering (Mittal, 2011). They are used as an alternative to a variety of metal components due to their low weight and relatively low cost of production.
The current leading end-use industries in the consumption epoxy resin based products are the automotive, aerospace, construction, transportation, electrical & electronics and industrial and marine coatings. With a rise in demand and a constant supply of the raw materials especially from china, the global epoxy resin market is expected to reach USD 14.26 billion by 2024 (Inkwood Research, 2017). China is also the current leading producer of epoxy resin with a total of 30% of the total global output. India, Italy, and Canada have recorded a significant growth in this industry in the recent years. The epoxy resin global market is distributed on the basis of geography and application of epoxy resin products. Based on application, the segment divided further into composites, constructions, adhesives, coating and paintings, electrical & electronics, coatings & paintings and other applications (Inkwood Research, 2017). The market data indicates that the composite material application is the fastest growing segment estimated to grow at a CAGR of 7.5 % in the forecast period 2016-2022. The aviation and automotive industries continuous application of lightweight epoxy resin based products is predicted to further increase the demand and production of epoxy resin. Currently, coating & paints segment dominate the market and is expected to continue its dominance in the forecast period. The segments are represented as follows in the chart below
The report also indicates that the approximate global use of epoxy resin in 2014 was 2.9 million metric tons with an approximate sale value of USD 6.8 billion. Consumption of epoxy resins products was estimated to have an annual growth rate of 8.2% in the year 2014 with an overall 7.7 % annual increase in global demand. By the end of 2015, electrical & electronics and coating industries increased their annual consumption of epoxy resins by 6.8 & 8.3% respectively with the composite industry recording an increased rate of 8.1% during the same year. Consumption remains high in developing countries such as Asian countries while demand for the same product remains lower than average in most developed countries (Ihs.com, 2017).
Uncured epoxy resins are of low quality in term of their physical, chemical, mechanical and heat resistance properties. The epoxy resin must be converted into a rigid, hard and infusible material by cross-linking the polymers using hardening agents in a process called curing. The curing process plays a key role in determining a variety of specialized properties exhibited by epoxy resins (Chen, Bulkin and Pearce, 2001). It requires the use of curing agents (hardeners) which react with the epoxy group forming a highly cross-linked rigid material. The curing agents contain active hydrogen atoms which initiate the reaction. It is process is quick and occurs easily at a wide range of temperature of between 50c and 1500 c depending on the curing agent used. As a thumb rule, room temperature hardened epoxy resins requires about 7 days to attain maximum properties, but one can attain about 70 -80 % of the final properties in about 24 hours (Flick, 2001). Curing agents that are commonly used include; polyamides, phenolic resins, amines, anhydrides, polymercaptan and isocyanates (Cowie and Arrighi, 2008). The choice of hardener is dependent on the intended application of the processed product and desired properties. Phenolic and amine resins are predominantly used for the curing process. The stoichiometry of the epoxy-hardener system also affects the properties of the cured material (Cowie and Arrighi, 2008). Both primary and secondary multifunctional amines react actively with the epoxy group. On the other hand, tertiary amines act as catalyst or accelerators in the curing process. Amine curing agents produce a final product exhibiting high chemical resistance and durability properties than the amide-based curing products. A disadvantage of amine based cured epoxy products is the blushing effect in presence of moisture. In order to achieve faster curing, the catalyst compound is used in excess amounts even though it leads to interference in the thermal stability of the end product (Mittal, 2011). In contrast, phenolic based hardeners used in the curing process produce a product exhibiting excellent strength, adhesion, flame and chemical properties. A resins full potential is only reached when the curing process is complete. Heat realized during the curing process must be properly regulated to minimize shrinkage. In general, the catalytic activity of the catalyst (hardener) used affects the physical properties of the cured polymer.
Chen, C., Bulkin, B. and Pearce, E. (2001). New epoxy resins. II. The preparation, characterization, and curing of epoxy resins and their copolymers. Journal of Applied Polymer Science, 27(9), pp.328-331.
Cowie, J. and Arrighi, V. (2008). Polymers. 1st ed. Boca Raton: Taylor & Francis.
Flick, E. (2001). Epoxy resins, curing agents, compounds, and modifiers. 1st ed. Park Ridge, N.J: Noyes Publications.
Ihs.com. (2017). Epoxy Resins - Chemical Economics Handbook (CEH) | IHS Markit. [online] Available at: https://www.ihs.com/products/epoxy-resins-chemical-economics-handbook.html [Accessed 24 Mar. 2017].
Inkwood Research. (2017). Epoxy Resins Market Analysis , Trends, Share Report 2016-2022 | Inkwood Research. [online] Available at: http://www.inkwoodresearch.com/reports/global-epoxy-resins-market-forecast-2016-2022/ [Accessed 24 Mar. 2017].
Mittal, V. (2011). In-situ Synthesis of Polymer Nanocomposites. 1st ed. Hoboken: John Wiley & Sons, p.72.
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