- What are the general types of polymer coatings for corrosion protection?
Polymers refer to compounds of high molecular weight derived from the addition of many smaller molecules or by the condensation of many small molecules along with the elimination of water, alcohol, or the like. The word polymer comes from Greek where “poly” means many and the word “mer” means unit, hence many units. Numerous different polymer based coatings are used for protective coatings. Coatings are based on polymers such as epoxies, polyesters, vinyl esters, polyurethanes, polyureas, and others. In industry specialty coating applications, the most common coatings systems are based on epoxy, polyurethane, vinyl ester, and acrylic polymers. Within these polymer choices exist numerous versions of each resin technology along with many varied modifications to each.
- What is solids content? What does the term “100% solids” mean? What is a VOC?
The percent solids or solids content of a coating is the ratio of the volume of its non-volatile components to its total wet volume. The solids content figure is usually expressed as a percentage.
The proportion of solids to liquid determines the thickness of the coating film after it dries. The greater the percentage of solids to liquid, the thicker the coating film is at a given spread rate. The percentage of solids contained in a gallon of paint indicates how much of the volume in the coating forms the protective paint film, and how much disappears as the paint cures. It is a general indicator of paint/coating quality.
By definition, the term “100% solids” means the coating system does not use any solvent to dissolve, carry or reduce any of the coating resins. Further, the resins normally still in a liquid state, will convert, 100%, to a solid film after application. The viscosity of a coating system is determined by the selection and design of the resin components. It is not determined by the addition of a solvent, although solvents have usually been used in the industry for the purpose of reducing viscosity. A good chemist shall therefore be able to design a 100% solids epoxy and polyurethane coating system having a relatively lower viscosity without using any solvents.
The term 100% Solids also means that the coating contains no solvents or VOC’s. VOC stands for Volatile Organic Component. Zero VOC coatings have no fire hazard and low health risk while application. They are also very environment friendly as hazardous organic solvent vapour is not generated and released into the air. VOC’s contribute to ground level ozone and smog. There are increasing environmental restriction on using VOC’s in coatings.
- How to choose epoxy coatings?
Epoxies are without question the most widely used coatings in the specialty coating industry. Epoxy coatings are generally made through the reaction of phenols with acetone or formaldehyde. Those reactants are then further reacted with epichlorohydrin. The resultant materials are diglycidyl ethers of what are called Bisphenol A epoxies, Bisphenol F epoxies, or phenolic novolac epoxies. These resins are then crosslinked via polymerization reactions with various curing agents or blends of curing agents. There are different types of curing agents used for different performing characteristics of an epoxy coating. The curing agents react with the epoxy resins and form a cross-linked network that converts the epoxy resin into a fully cured epoxy coating. Please find below some basic differences between polyamide epoxies, polyamine epoxies, phenolic epoxies and novolac epoxies
Polyamide epoxy coatings are based on a curing agent using some sort of reactive polyamide resin. These reactive polyamides are the result of long chain fatty acid dimers reacted with multifunctional amines. These terminal amines have a great molecular distance between them that greatly reduces the vapor pressure of the resin. Polyamide epoxies are normally used in primers for recoatability, flexibility, adhesion, better wetting characteristics, and improved corrosion resistance. They tend to be more resistant to moisture, weather and alkali, but have poor chalk resistance and only moderate acid resistance.
Polyamine Epoxies (Amine-epoxies)
Polyamine-cured epoxies are normally used to improve film hardness, abrasion resistance and chemical resistance. There are wide varieties of amine categories such as aliphatic amines, cycloaliphatic amines, aromatic amines, ketamine, and phenalkamines. These different types of amines offer a wide range of chemical resistant characteristics to the epoxy coatings. Polyamine-cured epoxies provide improved resistance to microbiologically induced corrosion. These epoxies are characterized by shorter pot life and recoat windows as well as chemical resistance and heat resistance.
Note the key difference here: Polyamide cured epoxies provide better flexibility, better abrasion resistance, improved corrosion resistance, and are relatively safer to use whereas polyamine-cured epoxies are tough, more chemical resistant, brittle, and provide improved abrasion resistance.
Phenolic epoxy has been typically referred to as a system where the epoxy resin is heat-cured with a phenolic resin. These type of formulations have traditionally been used for high chemically-resistant can, drum, and tank liners.Novolac Epoxies
Novolac epoxy resins are based on the epoxidation of the acid catalyzed reaction product of phenol and formaldehyde. These epoxies are typically used in applications where there is a need for higher chemical and/or higher temperature resistance. They usually require heat to fully cure but sometimes can be formulated to cure at room temperature.
As the polymer technology evolves so does the different types of epoxy resins and curing agents that perform in particular conditions. There are so many variants of each of these that it is very difficult to judge comparable values from just the product data sheet information alone. Always contact a SPC technical representative before selecting a particular type of epoxy coating for a particular application.
- How to choose polyurethane coatings?
The basic building blocks for polyurethanes are diisocyanates and polyols. Below are some different types of polyurethane coatings:
Acrylic Aliphatic Polyurethanes
Aliphatic polyurethanes are polyurethanes based on aliphatic isocyanates (e.g. HDI and IPDI) and mostly polyester and/or acrylic polyols. Aliphatic-acrylic polyurethane coating are thin film decorative coatings, typically providing only 1-4 mils (25-100 microns) dry film thickness per coat. This type of polyurethane can have exceptional color and gloss retention and is used exclusively for atmospheric (mostly U.V. light resistance) protection and color retention. Acrylic aliphatic polyurethane coatings are typically used as topcoats over epoxy base/intermediate or aromatic polyurethane coats to protect steel or other metal substrates from weathering or mildly corrosive environmental exposures.
Aromatic polyurethanes are polyurethanes based on aromatic isocyanates (e.g. MDI and TDI) and mostly polyether polyols. Aromatic polyurethane coating are less expensive than aliphatic polyurethane, but providing harder, more chemically resistant film and they are often used for interior, lining or underground applications.
Elastomeric vs. rigid polyurethanes
Polyurethanes are versatile polymers which provide products ranging from soft, thermoplastic elastomers to hard and rigid, thermosetting coatings and rigid insulating foams. The chemical bonds in the more rigid systems are highly cross-linked to each other to create hard, dense systems that have very good chemical and moisture resistance. The rigid systems usually have excellent adhesion and are the best choice for the corrosion protection of metals. On the other hand, the elastomers have a more linear structure with much less cross linking that allows them to be very stretchy and elastic. These systems normally have great impact strength and flexibility. Elastomers are generally have good elongation and recoverable elongation properties. As such, these lining films can bridge minor crack movement in concrete substrates. However, the chemical resistance properties of elastomers are often lesser than more highly crosslinked harder, rigid polyurethane coating films. One generally gives up chemical/heat/corrosion resistance and permeability resistance properties for flexibility and elongation properties. The chemical and corrosion resistance of the elastomeric coatings can be compensated if the systems are applied relatively thicker.
Moisture cure polyurethanes
Moisture-cure polyurethanes are isocyanate-terminated pre-polymers that are formulated to cure with ambient water. Moisture cure polyurethanes are most widely used as primers for steel. They use up substrate moisture, adhere well, and are fast to recoat.
- Why most good high solids/100% solids epoxy and polyurethane coatings are in the 1:1 format?
The two components of high solids and/or 100% solids epoxy or polyurethane coatings designed by Strathmore are reacted normally by around the ratio range of 1:1.
To apply high solids and/or 100% solids epoxy and polyurethane coatings with plural component equipment, the 1:1 format is the easiest way to configure the set up.
A high solids and/or 100% solids epoxy and polyurethane coating system with a mixing ratio higher than 1:1 will have better chance to cause mis-metering problems (often called as off-ratio) during its application. The higher ratio is, the higher possibility it will occur.