Polyurethanes are versatile polymers with a broad range of applications across many industries. In part one of this series, I discussed the chemistry of isocyanates / polyisocyanates and polyols. Here, I’ll further explore the uses of polyurethanes in coatings and adhesives and sealants. I’ll also touch on polyurethane elastomers and thermoplastic polyurethanes.
Polyurethane coatings are applied to products to improve their appearance and provide protection from environmental conditions, such as sunlight UV, abrasion, rain and airborne contaminants and particulates. Combined with a quality pretreatment for metals, a formidable coating system provides many years of corrosion-free protection. On automobiles, polyurethane coatings provide the exterior a high gloss and color retention, while contributing to scratch and corrosion-resistance. In aerospace applications, the coatings also provide a high-gloss finish and low surface energy and help protect the skin of the plane from rust and pitting over extreme temperature differences—from intense summer heat to below-freezing at cruising altitudes.
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A special class of polyurethane coatings incorporates fluorine into the polyol. These resins, which are sometimes called “fluorourethanes,” generally provide even higher performance than non-fluorinated urethanes, and find use in extremely harsh environments such as pipe linings, offshore marine platforms and chemical plants. Companies that provide these polymeric materials include Asahi Glass (AGC) under the tradename Lumiflon®, Daikin Industries’ Zeffle™, and Arkema’s Kynar®.
Other types of polyurethane coatings are used in construction in diverse applications for: steel trusses and concrete supports for bridges, Floating Production Storage and Offloading (FPSO) vessels and oil platforms. In the energy sector, they find use on wind turbine towers, blades and nacelles, and for the protection of the backsheet for solar panels. Coatings are typically thought of as an application of a wet material to a substrate, but may also refer to a film. Films are finding greater uses in aerospace, solar, and transportation, as they don’t require a window of application and cure for field uses as do liquid coatings.
Polyurethane Adhesives and Sealants
Polyurethane adhesives and sealants provide strong bonding in a variety of applications. Polyurethane adhesives provide an initial bond before fully curing, which is referred to as "green strength." This reduces clamping and holding materials, which may help cut costs by increasing throughput.
Polyurethane adhesives are used in automotive interiors, windshield bonding, in the assembly of shoes, and for in-home use in glue products such as Gorilla Glue® and Excel Adhesives. The ends of conveyor belts are typically closed using polyurethane adhesives. Polyurethane sealants are used in road repair, plumbing, construction and anywhere else a high-strength, water-resistant seal is needed.
Polyurethane elastomers can be created with a wide variety of properties and molded into almost any shape. Depending on the intended use, polyurethane elastomers can provide resistance to:
- oil and solvents
- mold, mildew and fungus
- cuts and tears
Polyurethane elastomers easily return to their original shape after stressed (i.e., compressed, bent or stretched), and this “memory” is very useful in many applications. Wheels for shopping carts, skateboards, roller coasters, and heavy trash containers are all produced from polyurethane, due to its high load-bearing capacity and abrasion resistance.
Thermoplastic polyurethane (TPU)
Thermoplastic polyurethane (TPU) offers an infinite number of physical property combinations and processing applications. It is highly elastic, flexible and resistant to abrasion, impact and weather. TPUs can be colored or fabricated in a wide variety of methods and their use can increase a product's overall durability.
TPU is an elastomer that is fully thermoplastic, and as such, is elastic and melt-processable. In addition, it can be processed by extrusion, injection, blow and compression molding equipment. It can be vacuum-formed or solution-coated and is well suited for a wide variety of fabrication methodologies. TPU can provide a considerable number of physical property combinations, making it an extremely flexible material adaptable to dozens of applications such as construction, automotive and footwear.
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