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049CWM0303.pdf !"#$%#&'!!()$*!+,!!+-./!0% 100% Solids Polyurethane and Polyurea Coatings Technology he need for corrosion pro- tection has provided a When it comes to corrosion protection, great challenge for today’s how do these technologies stack up? coatings industry to for- mulate and to provide By Shiwei William Guan coatings products that can Madison Chemical Industries meet various combined requirements. These standing about the differences among tems’ structure, properties and appli- include environmental the three systems as well as the cation characteristics and provides Tand safety compliance such as volatile advantages and disadvantages of each guidelines on the selection of the three organic compound (VOC) content, system’s application and performance. coating systems by highlighting the cost-effectiveness, appearance and A lot of hype also exists, especially development of several new systems of high performance. with respect to 100% solids elastomer- 100% solids polyurethane and The ideal corrosion protection coat- ic polyurea coatings, which needs to be polyurea coatings technology and their ing system must be environmentally put in perspective by providing an application in high-performance corro- friendly, worker-safe, durable and able accurate gauge to decide whether a sion protection. These new systems to expose little or no metal/substrate polyurea or polyurethane is the best include modifications by ceramic surface to the environment while choice for an application. and/or anti-microbial additives and the being resistant to environmental, This paper reviews the basic chem- world’s first 100% solids, rigid and mechanical and chemical damage istry and development of three coat- aliphatic polyurethane coatings. from the initial stage of handling and ings systems currently available: installation through its entire service 100% solids elastomeric polyurethane, Chemistry and Development life. It should also come at a reason- 100% solids elastomeric polyurea and The history of polyurethane coatings able cost in terms of materials, appli- 100% solids rigid (or structural) began in the late of 1930s after Otto cation, repair and operation mainte- polyurethane. This paper discusses Bayer and coworkers discovered the nance. One-hundred-percent solids the differences among the three sys- diisocyanate addition polymerization polyurethane and polyurea coatings technology is regarded as a coating solution that stacks up well against this long list of demands and is the fastest growing technology of choice Figure 1: Polyurethane chemistry. for a number of industries. The expan- sion of polyurethane technology into new markets is believed not to be a question of if but when1. Currently, there are three 100% solids coating systems available: elas- Figure 2: Polyurea chemistry. tomeric polyurethane, elastomeric polyurea and rigid polyurethane. Each system has unique and outstanding properties. However, there are many misperceptions and much misunder- Figure 3: Isocyanate/water reaction. www.coatingsworld.com COATINGS WORLD • March 2003 49 050CWM0303.pdf !!"#$%#&'!!1)1"!+,!!+-./!1& Type I Type II Type III Type IV Type V Type VI ASTM description One-package One-package One-package Two-package Two-package One-package pre-reacted moisture cured heat cured catalyst polyol non-reactive lacquer Characteristics Unsaturated Contains free Blocked isocyanate Isocyanate prepolymer Part A – isocyanate Fully drying oil modified; isocyanate plus catalyst rich; Part B – polyols polymerized no free isocyanate or amines polyurethane dissolved in solvents Main curing mechanism Oxidation of drying oil; Reaction with Thermal release of Reaction of Reaction between Part A Solvent Solvent evaporation atmospheric blocking agents isocyanate with and B; instant curing evaporation moisture and then reaction moisture and/or possible components in catalysts Polymer Alcoholysis products Higher molecular Prepolymer forms to Prepolymer similar to Relatively lower Thermoplastic of drying oils reacted weight diols an adduct with blocking Type II but catalysts molecular weight polymer with with isocyanate and triols agents such as phenol could contain polyol/ relatively high amine molecular weight Chemical resistance Fair Fair to good Good to excellent Fair to excellent Good to excellent Fair Corrosion resistance Poor Poor to good Fair to good Fair to excellent Fair to excellent Poor to fair Corrosion protective Exterior or interior; Exterior or interior; Not used for anti- Similar to Type I but Used for many Not normally used applications non-immersion services; non-immersion corrosive coatings in the design of catalyst substrates; Elastomer for for corrosion Wood; Concrete; Metal services; Wood; the field; Automotive may change the concrete; Rigid protection; Auto- Concrete; Metal and product finishes properties; Some concrete; Rigid motive and product suitable for immersion for metals finishes. Special considerations Better abrasion than Properties and Heat required for Similar to Type II Special equipment VOC limitation most oil paints curing affected cure but with speed may be required by humidity of curing Table 1: Six ASTM polyurethane coatings types. procedure that resulted in the prepa- dioxide gas provides as a benefit the castor oil derivatives. The amine com- ration of many polyurethanes and principal source of gas for blowing in ponents can be aliphatic or aromatic polyureas3,4. Polyurethane chemistry, the manufacturing of low-density flex- amine resins and amine-terminated based on the exothermic reaction ible polyurethane foams, the gas is chain extenders. The selection of dif- between di- or poly-isocyanates and unwanted in the application of a ferent resin types has great impact on compounds with hydroxyl end-groups polyurethane coating. The carbon the properties of the finished coating. such as polyols, is illustrated in Fig. 1. dioxide causes bubbles or foaming For example, aliphatic isocyanates are It is the exothermic nature of this within the coating during cure. If recommended to make aliphatic reaction that provides fast-setting, there is a significant number of bub- polyurethane or polyurea coatings for cold-temperature curing ability and bles in the coating and defoaming has exterior and above-ground applica- unlimited film build-up of 100% solids not been taken place, chemical and tions because of their UV and weather polyurethane coatings. Similarly, but physical properties of the coating will resistance. Aromatic coatings are often much more quickly, di- or poly- be diminished. The finished surface of often used for interior or underground isocyanates can react with compounds the coating may become dull, and applications for their chemical resist- with active hydrogen groups such as foaming, blistering and bubbling may ance and low cost. When certain types amines to form polyureas (Fig. 2). occur. of resins are used, varying the type Isocyanates can also react with The above simple polyurethane or and the degree of branching of the water, yielding a substituted urea at polyurea chemistry provides a great polyols/amine and isocyanates, as well the end of the process. This two-step deal of versatility to coatings formula- as NCO/OH ratio, a great variety of reaction is controlled by the much tors that no other coatings resin coating properties can also be slower isocyanate/water reaction, pro- chemistries can provide. There are obtained ranging from very flexible to ducing a substituted carbamic acid hundreds of different isocyanates and hard and brittle films. that breaks down into an amine and thousands of polyols and amines avail- The development of polyurethane carbon dioxide gas. The amine then able for the formulator to choose from, coatings technology matches the rate reacts with further isocyanate to yield resulting in millions of permutations of development and commercialization the substituted urea (Fig. 3). and combinations. Examples of iso- of polyurethanes science and technol- References to polyurea chemistry usu- cyanate groups are aliphatic iso- ogy. During World War II and the post- ally refer to the one-step reaction cyanates (HDI, m-TMXDI, and IPDI) war period, various polyurethane shown in Fig. 2 rather than the two- and aromatic isocyanates (TDI and products proved to be of great com- step reaction shown in Fig. 3. MDI). Examples of polyols include mercial importance, especially in the When the production of carbon polyethers, polyesters, acrylics and production of flexible and rigid foams. 50 COATINGS WORLD • March 2003 www.coatingsworld.com 051CWM0303.pdf !!"#$%#&'!!1)1"!+,!!+-./!1$ Figure 4: Elastomeric polyurethane coatings. Figure 5: Rigid polyurethane coatings. Figure 6: Structure-property relationships in 100% solids polyurethane and polyurea coatings. Since the 1960s, castable polyure- the reaction of difunctional iso- Industries, Inc. In 1975, ULC thane elastomers have also become cyanates with long chain difunctional (Underwriters Laboratories of widely used, particularly in the auto- polyols or a mixture of di- and tri- Canada) issued the first listing for motive industry. Polyurethane adhe- functional polyols, using short-chain cathodically protected steel tanks with sives then became accepted in a vari- difunctional polyols or diamines as a rigid polyurethane coating. In 1981, ety of commercial applications. chain extenders. The major advantage the same technology was approved for Finally, polyurethane coatings began of 100% solids elastomeric polyure- use in the STI-P3 tank by the Steel to
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