A chemical species containing at least one 3 membered epoxide ring within its chemical structure. When reacted with a suitable curing agent it is capable of forming a thermoset polymer with superior chemical properties including excellent mechanical, electrical insulation, chemical resistance with strong adhesion to most surfaces. They are generally manufactured via the chemical reaction between epichlorohydrin and bisphenol A.
The chemical resistance of an epoxy coating is governed by its inert aromatic backbone fortified by the stable ether linkages. The excellent bonding characteristics of epoxy resins are the combined result of the hydrophobic polar characteristics of the polymer backbone and the pendant polar hydroxyl groups on the polymer chain forming strong interactions with the substrate surface.
The epoxy equivalent weight is the number of grams of epoxy resin required to give 1 mole of epoxy groups. It is basically a measure of the concentration of epoxy groups within the resin.
The sintering of solid epoxy resins is a function of its glass transition temperature (Tg) which in turn is governed by its molecular weight. Solid resins with low Tg's can easily soften and fuse together to form lumps if the atmospheric conditions change or they experience an excessive pressure. As a rule of thumb, the higher the resin molecular weight (higher the EEW) the higher the glass transition temperature, and the lower the tendency of the product to sinter.
In tropical warm conditions, type 1 solid epoxy resin (such as epotec YD 011) readily sinters on storage, type 2 solid epoxy resins (such as epotec YD 012) have a possibility of sintering and type 3 and higher solid epoxy resins (such as epotec YD 903, YD 904, YD 013) are resistant to sintering when properly stored.
One thing to remember is that sintering is purely a physical phenomenon, there is no change in the chemical structure of the product. Hence the sintered product can be crushed to the required particle size and used again just like normal product.
Liquid Bisphenol based epoxy resins exist as supercooled liquids at room temperature. Their true melting point is 420C but because of high viscosities and a slight distribution of molecular sizes it is difficult for the molecules to align together to a solid form, hence they remain a liquid at normal room temperature. However as the temperature decreases the tendency to align and solidify becomes more prevalent. Bisphenol F liquid resins have a greater tendency to crystallize than Bisphenol A liquid resins because of their lower viscosity and more linear molecular structure.
Epoxy crystallisation generally need 'seed' crystals present for it to initiate. The most common causes of epoxy crystallization are extreme low temperatures and temperature cycles (such as day and night temperature transitions). Once a resin is warmed, molecular motion is enhanced allowing the resin molecules to orientate themselves around 'seed' crystals. As the temperature cools crystallization becomes rapid until the resin forms a solid mass. Therefore a moderate, constant storage temperature is preferred. The introduction of seed materials should be avoided when the resin is in use. For example lids and mouths of containers should be kept clean of foreign material such as solid resin or dust / dirt etc.
A curing agent (also known as hardener) is a multi functional chemical species which contains reactive groups that can react with either the epoxide or hydroxyl groups on the structure of the epoxy resin. The majority of curing agents available for use with epoxy resins become an integral part of the final thermoset polymer structure when cured, thus the choice of curing agent used can have a significant effect on the final properties.
The most common type of epoxy curing agents are based on amines. Primary and secondary amine functional groups contain active hydrogens that will react with an epoxy group and form the final cured thermoset polymer. AHEW is the amount of curing agent in grams which contains one mole of active hydrogen.
Phr is per hundred of the resin. In simple language it is the amount of curing agent required to cure 100 parts by weight of resin. Phr is calculated using the following equation:
phr of Curing Agent = AHEW of curing agent ?X ?100
? ? ? ? ? ? ? ? ? ? ? ? ? ? EEW of resin
It ensures there is an equal number of active hydrogens and epoxy groups in the coating formulation. Therefore by calculating the phr, the coating formulator can determine the desired mixing ratio between epoxy resin and curing agent.
Curing agent selection is determined by the application and performance needs and factors such as pot-life, cure speed, appearance, chemical resistance must be considered. When just considering amine based curing agents: Polyamides offer long pot-life, low cost, high colour, good water repellency and are generally used in primer applications. Aliphatic amines offer fast cure, high strength and are generally used in adhesives and civil applications such as mortars and grouts. Cycloaliphatic amines offer good colour, low-yellowing, low blushing and are generally used in flooring applications. Aromatic amines offer good chemical and heat resistance and are generally used in coatings / flooring for harsh environments.
A coating is polymeric, substantially organic material which when applied in the form of a thin layer and cured appropriately; protects, decorates and obliterates the surface of a substrate.
Put simply a 1-pack coating means the uncured coating is supplied to the end user in 1 package and the user can apply the coating to the substrate as received. A 2-pack coating means the uncured coating is supplied to the end user in 2 separate packages and the user has to mix the 2 components together effectively before the coating can be applied. 1-pack coatings generally cure via air drying, heating or UV irradiation with powder coatings and solvent based can and coil coatings being the common examples of epoxy 1-pack coatings. 2-pack coatings generally cure upon mixing at room temperature and many epoxy based civil systems are 2-pack, including protective coatings, self-leveling flooring, grouts and mortars.
In principle the chemical resistance increases with increasing functionality of the resin used. The Epotec YDPN 600 series is specifically designed and developed for this application. Epotec YDPN 638 has highest functionality (F ~ 3.6) and hence will give the highest chemical resistance. It must also be remembered that correct selection of curing agent is equally important.
Amine blush also termed as sweating or blooming describes the tendency of amine functional curing agents to react with moisture and carbon dioxide in the atmosphere before reaction with epoxy groups. The reaction product, called carbamate, is white in colour and gets deposited onto the surface marring the appearance, reducing gloss and giving a stained, blotchy look.
Amine blush can be reduced by one or all of the below :
1.Select a curing agent based on more hydrophobic amines eg. cycloaliphatic amines
2.Use the correct stoichiometry as recommended
3.Select amines with higher AHEW
4.Use fast reacting reactive diluents eg. Cresyl glycidyl ether in place of C12-C14 glycidyl ether
5.Accelerate the system
6.Use higher reactive amine curing agents such as mannich bases
7.Improve the compatibility of the curing agent by adduct formation
8.Offer induction time to increase the compatibility
9.Avoid ketonic solvents
Surface preparation is a process where the surface of a substrate is cleaned of any residual dirt, dust, oily/greasy material, rust etc. to make it receptive to the applied coating.
Good surface preparation is essential as it profoundly affects the performance of a coating. Poor surface preparation invariably develops a poor adhesive bond between coating and substrate and causes pre-mature coating failure.
There are various ways of preparing a surface before a coating can be applied. Broadly these are divided into: solvent cleaning, abrasion cleaning, etching, conversion coating, shot blasting, flame treatment.
The selection of the cleaning process is governed by the performance needs combined with application conditions. The gauge of a substrate also plays a role in selection of cleaning method. Invariably more than one method from above are combined and used for surface preparation.
Standard liquid epoxy resin (YD 128) is a highly viscous material and can be difficult to handle and formulate into a coating. Reactive diluents are small molecules that contain epoxy groups which when mixed with liquid epoxy resin can reduce its viscosity and react into the final cured resin coating. The use of reactive diluents can reduce or even totally eliminate the need for solvent in a coating formulation thus making it more user and environmentally friendly.
The choice of reactive diluent depends entirely on the properties you want to achieve. Mono functional aliphatic reactive diluents are better at reducing viscosity and extending the system pot-life. Mono functional aromatic reactive diluents are the best at maintaining mechanical properties (compressive and flexural strength) and chemical (acid, water, solvent) resistance.
Since the high performance properties associated with epoxy coatings come from the Bisphenol based structure of the liquid resin (YD 128), replacing some of this quantity with diluents will lead to a lowering of the performance affecting properties such as hardness, compressive strength etc. and water/chemical resistance. Hence a judicious use of reactive diluent quantity is recommended. As a general practice 10-20 % of epoxy resin is replaced with reactive diluents.
We recommend Epotec YD 515 (reactive diluent modified epoxy resin) with Epotec TH 7301 (modified cycloaliphatic amine curing agent). This system gives excellent gloss, hardness and other high performance properties.
Normal epoxy systems need the ambient temperature conditions to be at least 100C in order to cure at a sufficient rate. When it comes to low temperature curing particularly sub zero temperatures there are limited options available. One possible solution is use a phenalkamine as the curing agent.
Phenalkamines are the reaction products of cardanol, a phenolic compound obtained from cashew nut shell liquid, formaldehyde and an amine. Because of the chemical structure of cardanol, phenalkamines are self-accelerating meaning they are capable of achieving a state of full cure even at temperatures below zero degrees centigrade, albeit with a slow cure rate.
There are quite a few advantages of using phenalkamine curing agents as below:
1.Excellent water / salt water resistance
2.Good adhesion even to not so well prepared surfaces
3.Based on a renewable source
4.Capable of curing at low temperatures e.g. -5?C
5.Good chemical and corrosion resistance
8.Moisture tolerant during curing reaction
FBE powder coatings are special powder coatings with specific functional needs. These products are mainly used for the protection of steel bars in the construction of high rise buildings, dams, bridges etc. and pipes used for transportation of petroleum products, water etc. We offer resin and curing agent grades specific for these needs.
Our recommended products for this application are Epotec YD 942, YD 972, YD 903HE and phenolic curing agents Epotec TH 981.