A coating is a barrier used to separate a substrate from its corrosive environment. A coating must be a completely continuous film in order to fulfill its function. Any imperfection becomes a focal point for corrosion and the breakdown of the structure.

 Paint, in the traditional sense of the word, primarily fulfils a decorative function. While it may have some durability and modest resistance to its environment, it cannot provide long term corrosion resistance. A coating differs in two main ways. First, unlike paint, it cures via a chemical reaction (called cross linking) as opposed to drying via evaporation of solvents and/or water. Most importantly, the chemical cross linking provides the continuous, impermeable barrier required for long term corrosion protection.

 There are three recognized ways that coatings can protect steel: barrier protection, inhibition, and sacrificial action.

A coating protects as a barrier by blocking moisture, oxygen, and other chemicals from the steel substrate. All coatings are permeable to some degree, but those coatings that protect through a barrier mechanism have relatively low moisture permeability. Polyurethanes, such as Madison’s CorroCote and CorroPipe families with highly cross linked structures have particularly low permeability. Coatings that protect by inhibition contain special pigments to inhibit or interfere with the corrosion reactions on the steel surface. As moisture passes through the coating film, the anti-corrosive pigments slowly dissolve and aid in stopping corrosion. Finally, sacrificial action is the method used by zinc- and aluminum-rich coatings. Zinc-rich polyurethane coatings with additional aluminum and other anti-corrosive pigments are excellent primers for steel surfaces. Refer to our InfoTech Bulletin No. 19, available in our Technologies section, for a discussion on our Alumizinc ‘S’ Next-Generation zinc-rich primer.

 Broadly speaking, a polyurethane coating is an organic coating made from the cross linking reaction of isocyanate-rich and polyol-rich compounds. Polymerization is made possible by using di- and poly-functional isocyanates and polyols. There is no byproduct, other than heat from the reaction between isocyanates and polyols. This is important in terms of cost-effectiveness and health risk.

R-N=C=O + R’-OH --> R-NH-CO-O-R’ + Heat
isocyanate       polyol       polyurethane

A note of caution: “Urethane", as used most often in the term "urethane coating", is a misnomer for "polyurethane coating". "Urethane" is used broadly and has two different meanings. One refers to ethyl carbamate, which is a monourethane. Although it is a member of the urethane family, it exists as a totally separate chemical that is not used in polyurethane coatings. The other meaning of "urethane" should more properly be stated as "polyurethane" and generally refers to coatings or other materials made with isocyanate compounds that contain multi-NCO groups.

Overall interest in all types of polyurethane coatings, and in particular fast setting 100% solids products, is growing rapidly because of their high chemical and abrasion resistance, excellent electrical properties, very high adhesion, excellent resistance to undercutting corrosion and very low water absorption and permeability

Plural-Component Polyurethanes
When the polyurethane is formed in the manner above, it is typically referred to as a plural-component polyurethane. These comprise the majority of Madison’s polyurethane coatings family. The basic polyurethane reaction is an exothermic (heat-generating) reaction. Therefore most polyurethane coatings, especially those which are solvent-free, will have "cold-curing" ability. The heat generated by the reaction will be enough to help the coating to cure even at -40°C. Furthermore, the rapid setting properties of these coatings allow them to be applied at very high film builds, with minimal sagging, in a single coat. This group of products provides the highest level of corrosion protection in the Madison coatings family.

Moisture-Cure Polyurethanes
Moisture-cure polyurethanes are a segment of polyurethanes that Madison employs to a lesser extent than plural products. (A note of clarification: our ‘moisture-cure’ technology is more complex than the ‘textbook’ definition for these products; however for our purposes here, we’ll discuss it in basic terms). True moisture cure polyurethanes are single-package polyurethane prepolymers. Following application, the prepolymer or the isocyanate group reacts with moisture in the atmosphere to form the final cross-linked coating. The fact that these urethane coatings are cured from moisture in the air is sometimes a disadvantage. The curing time is reduced rapidly at high humidity leading to film defects, such as bubbles and haziness, while it is lengthened to the point of no cure if the humidity is very low. Madison recognized these limitations and formulated products called ‘Pre-Catalyzed’ single component polyurethanes. Essentially they utilize a hybridized technology of moisture cure and ‘regular’ plural component cure to avoid the pitfalls of humidity variation, ensuring a uniform, defect free film. These provide the user with an easy to apply format, while retaining a significant amount of the exceptional corrosion protection offered by plural component polyurethanes, particularly for atmospheric service.

 Polyurethanes cure to a safe, inert product. It has been estimated that the average family owns approximately 25 to 100 pounds of polyurethane present in such areas as insulation for refrigerators, foams for bedding and cushions, and as dashboards and bumper covers in cars. The polyols used for polyurethane formation are derived from natural products such as sucrose; hence they are not classified as hazardous substances. The main hazard in polyurethanes is the isocyanate. However, isocyanates are not carcinogenic as many people might believe. Allergic reactions develop in a very small percentage (< 1%) of the population but these reactions desist once the over-exposure is stopped. Furthermore, the isocyanates Madison uses are pre-polymers, which means that the isocyanate monomer is reacted with a small amount of polyol to produce a larger, less volatile and less hazardous polymer.

Some of our ‘mix-and-apply’ polyurethanes contain solvents – those found typically in many industrial coatings – and do not pose a hazard greater than normally anticipated for solvented coatings (i.e., flammability and toxicity). Proper engineering controls and personal protective equipment are therefore required in those cases. This is furthered addressed below in FAQ No. 10.

 Occasionally these terms are used interchangeably. However, they are very different coatings. Both polyurea and polyurethane are made from an isocyanate. An isocyanate will react with any compound containing reactive hydrogen. An isocyanate reacts with a polyol to form polyurethane, while a polyurea is made from the reaction between an isocyanate and a polyamine. While polyureas have some apparent advantages, for example extremely rapid cure, on balance they do not match the level of performance that is the hallmark of well formulated polyurethanes. Please refer to a technical paper, 100% Solids Polyurethane and Polyurea Coatings Technology: Chemistry, Selection and Application, available in our Resource section, which provides an excellent overview of the differences between these technologies.

R-N=C=O       +       R’-NH2   ---->   R-NH-CO-NH-R’
isocyanate             polyamine             polyurea

 VOC stands for Volatile Organic Component, and it usually – though not exclusively – refers to solvents in the system. The lower the amount of VOC's in the coating, generally speaking, the safer it is to use. VOC's increase the degree of flammability of the coating and also increase the health risk associated with application of the coating. From an air quality perspective, VOC's contribute to the formation of ground level ozone and smog. Many jurisdictions have strict limits on VOC's and this may prohibit use of some products. Many of our coatings have zero VOC's, while others have low levels that are compliant with virtually all air quality standards.

 No. There are three main types of exposure to which coatings are subjected: atmospheric exposure, immersion, and underground (embedded) exposure. A coating under atmospheric exposure must endure a variety of conditions, including exposure to UV rays, cycles of heating and cooling, precipitation, and pollutants. Immersion coatings are primarily subjected to fluids including potable water, waste water, and a variety of fuels and chemicals. Coatings used in underground applications must be resistant to ground water, variations in soil conditions (alkalinity, acidity, salinity) and abrasion from rocks, backfill, and directional drill/jacking installations. Each of these situations poses unique challenges to a formulator. For atmospheric exposure, for example, resin and pigment selections are made with the effect of sunlight in mind. Immersion coatings, for example, typically have very high resistance to absorption of fluids, but are not formulated to deal with UV. Madison researchers over the years have tailored their formulations to deal with specific service life requirements.

 Polyurethane coatings are quite versatile and are used in a variety of applications, from sealing hardwood floors to lining water pipelines. Madison Chemical polyurethanes are primarily utilized to protect infrastructure. By this we mean structures and facilities that support and sustain our daily lives; that deliver or store the ‘stuff’ we depend on. This includes pipelines to deliver water, utility structures to deliver electricity, tanks to store hazardous chemicals, and towers to support wind turbines. We are always exploring new technologies and markets to broaden the scope of our company’s product offering.

 Madison’s plural and single component products are typically applied by spray. Therefore, we recommend that one wears safety glasses or goggles, an approved NIOSH respirator, gloves and protective clothing when applying the product. For brush and roller application, depending on the conditions, a respirator may not be required. In fact, these precautions apply to any spray or hand applied industrial coating and is not exclusive to polyurethane.

 100% solids refers to a lack of volatile ingredients – typically solvents. The thickness of a 100% solids coating remains the same – whether wet or dry – because there are no solvents to evaporate. There are many advantages to 100% solids coatings. First, these coatings are more environmentally friendly and generally safer to use due to their decreased levels of flammability and health risk. Second, on a cost-per-mil of thickness, they are very cost-effective when compared to solvent based products. Finally, most 100% solids coatings have the advantage of additional thickness that helps to increase their physical properties and chemical resistance.

 Plural-component coatings are those in which the isocyanate part is packaged separately from the polyol part. Plural component products use specialized airless pumps that meter very accurate volumes to ensure the best possible results. Just before the coating is applied to a surface, the two parts are delivered through individual fluid lines to a mixing device, which is located within the spray gun or directly before the spray tip. It is very important that the right mixing ratio is obtained; otherwise defects in the coating will develop. Thus, both sides must have balanced viscosities (i.e., equal ‘thicknesses’) in order to be sprayed on ratio. It is possible to apply some two component products using a brush or roller. Single-component products are applied ‘straight out of the can’, although catalysts may be added to the product to speed up the cure time.

 The main difference between these two types of coatings is the degree of cross-linking density. Soft, elastomeric coatings are made from long chain polyols that have little or no branching. Rigid coatings, on the other hand, are made from short chain polyols that generally feature higher degrees of branching, such as triols and quadrols. Water and chemicals have easier passage through an elastomeric coating due to the linear nature of the chemical bonds, whereas rigid coatings show a high degree of chemical and moisture resistance due to their high cross-linking density. Rigid coatings also demonstrate excellent adhesion to substrates; this quality, together with strong chemical resistance, position rigid coatings as the best choice for protecting metal from corrosion. Although elastomeric coatings do not perform as well in such areas, they are superior in terms of abrasion and impact resistance. Therefore, more flexible coatings are generally suited to the protection of substrates that demonstrate more movement than steel, such as concrete. We recommend you contact us to discuss your specific needs if in doubt as to the appropriate product for your application.

 Aliphatic and aromatic coatings differ in the types of polyols and isocyanates used in the formulation, hence their stability in atmospheric conditions differ significantly. Because they are very stable when exposed to ultraviolet light, weathering, and hydrolysis, aliphatic coatings are the superior choice for exterior protection. Aromatic coatings do not fair as well against atmospheric exposure since the UV light causes yellowing and chalking. Aliphatic coatings are produced as a result of the reaction between aliphatic isocyanates, such as HDI and IPDI, and polyester or acrylic polyols. In contrast, aromatic coatings are designed from aromatic polyisocyanates and polyether polyols. The raw materials used in formulating aliphatic systems are generally more expensive and have higher viscosities than their aromatic counterparts. The need for solvents to reduce viscosity has been an obstacle in trying to achieve 100% solids aliphatic polyurethane coatings. New developments such as the use of lower molecular weight resins, reactive diluents, and new polyurethane prepolymers have resulted in higher solids systems.

 There are several situations where polyurethane coatings are preferred:

1. Rapid throughput: Polyurethane coatings can be made to cure at virtually any given time by changing the amount of catalyst or the type of polyol in the formulation. Thus, fast-setting, one coat polyurethanes have a much faster turn around time than most epoxy systems, for example. This results in significantly lower applied costs. Epoxy coatings generally take several days to fully cure and to allow the solvents to evaporate. Some also require force curing.

2. VOC and safety: 100% solid polyurethane coatings are solvent free. Due to solvent content, many epoxy coatings are extremely flammable and may be subject to VOC restrictions.

3. Low temperature cure: Due to their exothermic (heat releasing) nature polyurethane coatings can cure at almost any ambient temperature. This means that polyurethane coatings can be applied even during the coldest months of the year. Waterborne and epoxy coatings, on the other hand, usually require temperatures above 50°F (10°C) to cure. High levels of humidity can also impact the finish of waterborne and epoxies.

4. Corrosion protection: Polyurethanes, and in particular those with 100% solids, have superior adhesion, water absorption and undercutting resistance compared with epoxy and waterborne technologies. These are critical parameters that provide maximum corrosion resistance.

5. Exterior exposure: Aliphatic polyurethanes have excellent resistance to UV, while it is well known that epoxies fade and chalk significantly when used in exterior applications.

 Epoxies are preferred under these circumstances:

1. Exceptionally corrosive environments: For example, storage vessels for strong acids and many aggressive solvents are well protected by heat cured epoxy hybrids called novolac.

2. Sealing and priming: When a porous substrate such as concrete requires coating, certain epoxy products are first applied in thin films to seal pores and provide an ‘anchor’ for subsequent coating layers, including polyurethanes. Madison recently discovered that one of its epoxy products provides the same function on wood and utilizes this in production of high performance prefinished wood siding and decking.

3. Ease of use: Most epoxies may be spray applied with conventional airless pumps, or even simply rolled or trowelled. They typically do not require a plural component metering pump as do most of the fast set polyurethanes. For smaller operators that cannot justify purchasing a plural pump, 100% solids epoxies offer an excellent alternative as a high build tank lining.

 Yes. In fact, this is a technology that we recognize as becoming more important as jurisdictions place tighter controls on emissions (recall our discussion on VOCs). Also, they require fewer ingredients made from petroleum feed stocks. In 2008, we introduced a family of waterborne products called FusionClad. They are designed for light to moderate service requirements (i.e., not immersion, embedment or extensive pooling water). Some examples include structural steel for building components (girders, trusses and beams), utility storage tanks and low corrosivity industrial facilities. Furthermore, in many cases they require a less aggressive surface preparation than our traditional products. We call this ‘No-Blast’, meaning rather than preparing the steel with abrasive blasting, we utilize a safe and easy to use surface conditioner called FerroGrip. This saves appreciable time and money. For a detailed discussion of our No-Blast and FusionClad technologies, refer to our InfoTech Bulletin No.024 Performance Properties of No-Blast Waterborne Protective Coating Systems for Mild to Moderate Service ver 02 2010 02 01 in the Resources section.

 What is ‘No-Blast’ technology? Where can it be used?