Epoxy and Polyurethane Coating Systems for Marine Applications

Topics Covered

Background

Chemistry

Epoxy Resins

Polyurethane Resins

Coating Requirements

Solvent Based and Solvent Free Epoxy Systems

Handling & Safety

Background

Sophisticated two part epoxy and polyurethane coating systems are used increasingly by the marine industry in preference to conventional one component products due to their superior mechanical properties and protection against long term environmental degradation.

Both epoxy and polyurethane two part coating systems are of the reaction type whereby the base resin and a hardener are combined just prior to application. This is true of both solvent-free and solvent-based types. Once applied to the surface, the coating cures either immediately (in solvent-free products) or after the solvents have evaporated from the film (in the solvent-based systems). It is a direct result of this chemical reaction that in aggressive environments these materials can offer significant advantages over one-component coatings which rely on drying or surface oxidation to cure. This article discusses the relative merits of epoxy and polyurethane systems in their various forms and provides a basic guide to the properties of the two systems.

Chemistry

In order to understand fully the different film properties and applications of each product, a basic understanding of their chemistry is worthwhile.

Epoxy Resins

Epoxy resins consist of a linear chain molecule with a reactive epoxy group at each end of the chain. Each particular type of epoxy resin varies in terms of its detailed structure and the length of the chain between the epoxy groups. For example, short chains give liquid resins whereas longer chains give solid epoxy resins. A variety of agents can be used to cure epoxies but for marine coating applications the main hardeners are amine based. When mixed with the epoxy resin, the amine ‘active site’ reacts with the epoxy ‘active site’ and a chemical bond is formed which links the resin and hardener chains together. Once all the amine sites have reacted with the epoxy sites a three dimensional network is achieved. An important feature of this curing mechanism is that there is a fixed resin-to-hardener ratio. If there is an excess of either component, a full cross-linked network will not be achieved because some of one or other component will remain unreacted in the polymer network and therefore the mechanical properties of the coating will be degraded. It is for this reason that manufacturers of epoxy systems stress the importance of accurate mixing to the specified ratio.

The actual properties of the final coatings are very much dependent on the type of resin and hardener used. Solid epoxy resins have larger molecules and thereby the distance between cross-linking points is greater, which results in more flexible and resilient films. Liquid resins with shorter chains give harder and stronger films due to more dense cross-linking.

Polyurethane Resins

Technically speaking the term epoxy refers to the uncured resin component only (the hardener is usually an amine) whereas polyurethane is the generic term for the cured product which is formed from a chemical reaction between a polyol resin and a polyisocyanate hardener. When the two components are mixed the hydroxyl groups (-OH) in the resin react with the isocyanate groups (N=C=O) in the hardener and a three dimensional molecular structure is produced. Only one isocyanate group can react with one hydroxyl group so there is an ideal ratio of hardener molecules to resin molecules which will give optimum mechanical properties. Despite this fact, it is possible to vary this ratio slightly either way in order to modify the mechanical properties of the system. This is because the isocyanate hardener is also capable of cross-linking with itself in the presence of atmospheric moisture. More hardener than the optimum ratio will produce coatings which are harder, more brittle and have greater resistance to chemical attack. Less hardener will have the opposite effect: the film will be more flexible and its resistance to weathering will decrease. Therefore it follows that a polyurethane coating can accommodate a degree of variance in the mixing ratio but not without this having some effect on the properties of the final film. Generally speaking polyurethane systems cure faster than their epoxy counterparts. They can however, be difficult to use when cured in moist conditions at low temperatures, again because of the affinity of the isocyanate for moisture. This may result in an inadequate cure and premature embrittlement.

Coating Requirements

The main role of a coating, whether it is used as a primer, intermediate coating or top coat is to protect the substrate to which it is applied and, in some cases, to enhance its appearance. To be considered effective it must give the desired protection level for as long as possible against environmental and mechanical damage. In order to achieve this level of performance good adhesion to the surface and a resistance to moisture and sunlight (ultraviolet) are essential.

Table 1 Relative Performance of Epoxy and Polyurethane Systems.

Property

Epoxy

Polyurethane

Adhesion

Excellent

Good

Water Resistance

Excellent

Good

Cure Speed

Good

Excellent

Susceptibility to moisture during curing

Little to Considerable

Considerable

Gloss Retention

Poor

Very Good

Abrasion Resistance

Good

Excellent

It is apparent that epoxy systems are particularly suitable for use as primer and intermediate coatings because of their good adhesion and water resistance whereas polyurethanes offer very good colour stability and gloss retention and therefore they tend to be used for top coats where a cosmetic finish is required. Where there is a requirement for good adhesion and water resistance as well as high gloss and colour stability (i.e. non-yellowing) epoxy and polyurethane systems can be used in combination on the same substrate.

The benefits of this combination of materials are clearly illustrated on many of the high performance wooden racing dinghies and cruiser/racers currently racing, where a top quality clear coating is required to enhance the woodwork.

Solvent Based and Solvent Free Epoxy Systems

Two methods are currently employed to form a film of epoxy on a substrate. The first involves using a liquid resin and a liquid hardener which are mixed together and applied to the substrate. The second method comprises a solution of resin and hardener in a solvent which is designed to evaporate once the material has been applied in a thin film. A solvent-free or 100% coating is produced by the first method whereas the second is referred to as a solvent-based coating. As it is not possible to manufacture polyurethane resins with low viscosities, all polyurethane resin coatings are solvent-based. Each system has its advantages and disadvantages, a summaryof which is outlined in Table 2.

Table 2. Solvent Based versus Solvent Free Systems

Property

Solvent-Free

Solvent Based

Max. Film Thickness (one application)

Unlimited

75 microns

Cure Speed

Comparable

Comparable

Moisture Susceptibility During Cure

High

Low

Pot Life

Short

Long

Mixing Accuracy

Must Be Accurate

More Tolerant

Water Resistance

Very Good

Very Good

Film Strength

Strong but can be Brittle

More Flexible

Colour Stability

Poor

Fair to Good

Film Shrinkage on Cure

Virtually None

Up to 70%

The major advantage of solvent-free systems is that a very thick coating can be applied in a single application. It is this feature which gives solvent-free coatings their good gap filling properties.

In comparison, solvent-based systems are normally limited to a wet film thickness of approximately 75 microns because a greater thickness could result in solvent entrapment which prolongs drying and can lead to a semi-cured coating.

However, the thick coating advantage can be more than offset by the disadvantage of the short pot life of solvent-free systems which can pose difficulties if large areas are to be coated. This problem can be overcome by using heated mixing and spraying equipment but the throughput of coating work has to be sufficient to justify the capital expenditure of such equipment. The pot life of solvent based systems does not present the same problems because the solvents used have a retarding effect on the in-the-pot gel time.

In addition solvent-free epoxy systems can suffer from a surface by-product caused by interaction of the chemicals of the system with water and CO2 in the air, and which should be removed before overcoating.

Another problem with solvent-free epoxy systems is their potential susceptibility to moisture which can result in a ‘blooming’ effect on the finished surface. This is caused by the active amine curing agents which are used to achieve a fast thin film cure. These tend to react with water and therefore care should be taken not to apply the system in a damp environment. Solvent-based systems do not suffer to the same degree from this problem as they employ solid resins and pre-reacted hardeners. The two types of system do not suffer to the same extent from ultraviolet degradation. Generally, solvent-based epoxy systems exhibit superior light stability and clarity.

Handling & Safety

As with almost all resin systems, it is important that both polyurethanes and epoxies are used in accordance with the manufacturer’s recommendations regarding safety and handling. All solvents are potentially hazardous if used incorrectly and care should be taken to apply solvent-based systems in a well ventilated area.

Skin irritation is often seen as a particular problem with epoxy systems. However, many manufacturers have minimised this problem by using a partially reacted hardener. Barrier cream and/or gloves should be used as a further precaution. Care should still be taken to avoid skin contact where possible and any material which does come into contact with the skin should be removed as soon as possible using a resin removing cream.

Source: SP Systems. This article was derived from ‘Epoxy & PU Marine Coatings.pdf’ which can be downloaded from the SP Systems website.

For more information on this source please visit SP Systems

 

Date Added: Dec 10, 2001 | Updated: Jun 11, 2013
Comments
  1. frian bellows frian bellows United States says:

    What are the UV exposure characteristics of the two types of coatings?  In other words, how well does epoxy stand up to UV (or sunlight) vs cross-linked polyurethane?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.
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