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PU CoatingsPolyurenthanes are the latest in the family of Polymer coatings for application in the areas of corrosion resistance, insulation, wood protection and other materials etc. The usage of PU coatings are so versatile that, it can be used on textiles, metals, silver wares, precious metals and even for paper protections. PU coatings are normally supplied in two packs, where one part is the basic polymer and the second part is the catalyst. Since PU is new generation polymer, there are many questions asked and we have tried to reply as many questions asked in the following paragraphs. The basic features of PU system are the outstanding performance, resistance to aggressive corrosion and high abrasion and impact resistance, high micron build up, fast application and curing capability at temperature below Sub Zero Temperatures. SRL under technical guidance and cooperation with an eminent German Scientist has developed, in-house, PU coatings at most economical price range and has made it affordable for all classes of people for whom, so far, applying PU coatings has been a dream. The most commonly asked questions on Pu coatings are replied here under and if you still have a question, we will be glad to communicate with you. 1.What are Polyurethane coatings? Polyurethane coating usually consists of two components, one is Isocyanate rich solution and the other is polyol rich solution. When two components are combined together in a given ratio, a rapid and exothermic chemical polymerisation reaction takes place and raise to a polyurethane film. 2.What are aliphatic polyurthanes? What are aromatic polyurethanes? Aliphatic polyurethanes are polyurthanes based on aliphatic isocyanate and mostly polyster and/or acrylic polyols. Aromatic polyurethanes are polyurthanes based on aromatic isocyanates and mostly polyether polyols. Aliphatic polyurthanes are most expensive, but provide the best UV resistance and colour stability and so, they are used for exterior applications. The design of a aliphatic polyurethane system is very difficult and few coatings manufacturers have such a technology. Aromatic polyurthanes are cheaper and often used for interior lining or underground applications. Polyurethanes will undergo a certain degree of color change i.e. yellowing after a few months of UV exposure. However, their UV resistance is generally better than that of common epoxies. Most of the polyurethanes available today are aromatic polyurethanes. 3. What are elastomeric polyurethanes? What are rigid polyurethanes? The Polyurethane vary from very soft, rubbery elastomers to hard, ceramic like systems. The chemical bonds in the more rigid systems are highly cross-linked to each other to create hard, dense systems that have a more good chemical and moisture resistance. 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, but relatively poor adhesion and chemical resistance. The chemical and corrosion resistance of the elastomers can be improved if the systems are applied relatively thicker. 4. How do polyurethane coating differ from other 100% solids coatings, such as epoxies and powder coatings? 100% solids epoxy and polyurethane are currently two main sprayable coatings systems that offer the 100% solids advantage to the corrosion industry. They both do, together with some industrial powder coatings, provide excellent corrosion resistance and unique physical and mechanical properties. Some advantages of polyurethanes over epoxies are:
Comparison with Powder coatings The sprayable 100% solids epoxies and polyurethanes will not require application set up or equipment as expensive and complicated as those for powder coatings. 5. What advantages do polyurethane coatings offer? Polyurethanes coatings offer very unique handling, performance and environmental advantages, i.e.
Accompanying these handling advantages are a number of important performance advantages, These include;
6. How to apply polyurethane coatings ? Any special equipment required? Polyurethane technology is available in a variety of setting times to allow application techniques including brush and roller, hand spray, automatic line travel and casting. Typically, the setting or cured to touch time for these polyurethane systems ranges from less than one minute to several hours. SOME DATA ON POLYURETHANE (I) Performance results of the 100% solids aliphatic polyurethane Physical and chemical properties of the 100% solids aliphatic polyurethane system, as well as its gloss and colour retention, are all high. The following typical results are given together with those of a commercial 100% solids aromatic polyurethane and a commercial high solid ( 70% solids) alipahatic polyurethane systems. Weathering was investigated by placing panels in QUV cabinet fitted with UVB-313 bulbs for 5,000 hours as per ASTM G53. Panels were exposed to cycle of eight hours UV at 50°C followed by four hours condensation at 40°C. Gloss values of the panels before and after exposure were measured as per ASTM D523. After 5,000 hours exposure, both the 100% solids and the 70% solids commercial aliphatic polyurethane systems had gloss retention of greater than 90%, compared with 65% with the 100% solids aromatic polyurethane system. 25,000 hours QUV 313 B test results (ASTM G53)
Long term outdoor weathering test with these systems is currently underway but initial results for 18 months has already been obtained. The 100% aromatic polyurethane system showed chalking and darkened in colour as was expected. Both the 100% solids aliphatic and the 70% solids commercial alipahatic polyurethane systems had gloss retention of greater than 95% and showed no colour changes by a visual examination. II. 96 hours chemical resistance test results (ASTM D714) Chemical resistance was evaluated for 06 hours as per ASTM D714 in 10% H2SO4, 5% HNO3 and 25% NaOH solutions. The 96 hours of continuous immersion exposure is far more than the average duration of a chemical spill. Yet, this has become the standard used by some standard coatings manufacturers as part of the extra care that they take for end users.
No colour changes were found with the 100% solids aliphatic and the 70% solids commercial aliphatic polyurethane systems. They were however slightly softened, which is typical for most high solids aliphatic polyurethane systems after immersion. As is to be expected from most of the advanced 100% aromatic polyurethane systems, the tested 100% solids aromatic polyurethane coating was discoloured without any other physical property changes. III. Results of abrasion test (ASTM D4060, 1 Kg. load, CS17, 1000 cycles)
IV. Results of adhesion test (ASTM D4541)
P.S. : These data have been collected from foreign journal extracts and research papers, published by Dr. Shiwei Guan, Ph.D., Ontario, Canada. |
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