CompositesWorld
Issue link: https://cw.epubxp.com/i/632301
FEBRUARY 2016 6 CompositesWorld COMPOSITES: PAST, PRESENT & FUTURE » Polyurethanes resins (PURs) are a group of polymers formed by the reaction of two molecules: an isocyanate and a polyol. Such a simple reaction suggests a simple product but, in fact, this poly- addition reaction yields one of polymer chemistry's most complex and diverse set of reaction products. PUR technology, after more than 50 years, exhibits a wide range of end-products with an even wider range of properties: foams, elastomers, coatings, adhesives, sealants, fbers and flms 1 . Te fundamental PUR building blocks allow for customization in many areas, including glass transition temperature (<25°C to >200°C), thermal properties (linear mole- cules to highly cross-linked structures) and mechanical properties (soft elastomers to hard thermosets and thermoplastics) 2 . But, to date, PUR chemistry in composites applications has been limited, accounting for only about 3% of the total composites resin market 3 . PUR chemistry in fber-reinforced composites, however, is a natural ft because the PU matrix exhibits many characteristics that are superior to matrices of epoxy, polyester and vinyl ester. PUR-based chemistry has long been known for greater tough- ness than other thermosetting resins due to the high fexibility and elongation of its cured polymer structure. PU formulations can be tailored to a wide range of processing conditions and desired fnal properties due to the tailorability of the polyol's backbones. 4 Also, in pultrusion processes involving carbon fber reinforcements, greater fber/matrix adhesion has been observed with PURs than with polyester, vinyl ester or epoxy. 5 So why then, given its perfor- mance advantages, has PUR not been adopted in place of other thermosetting resins in a host of applications? A major hurdle to widespread adoption has been the high degree of reactivity of the mixed PUR system compared to other thermosetting chemistries. Even the most optimized two-compo- nent traditional polyurethane system exhibits gel times an order of magnitude faster than its epoxy counterpart. Processing with PUR, therefore, is difcult because most conventional composite manufacturing operations have been designed around thermoset- ting resins with a much longer pot life. Additionally, the isocya- nate in the two-component system readily reacts with water (i.e., moisture/humidity) to form carbon dioxide that causes bubbling (voids) in the laminate structure. It's no wonder, then, that when PUR is used in processes designed for thermoset resins with longer pot lives, poor part performance results. Accelerating the growth of PUR chemistry into widespread composite applications will take a more collaborative approach from the entire value chain including resin suppliers, machine manufacturers and composites fabricators. An excellent example of such collaboration has already enabled polyurethane pultrusion. Traditional pultrusion operations use an open bath for coating and impregnating the fber with resin before feeding the coated fbers into a heated die to form and cure the part. Te open bath design is simple, easy to use and most importantly refects the needs (or lack thereof ) of the particular chemistries (esters or epoxies). PUR-based pultrusion, however, requires a closed method to minimize interaction with environmental moisture. It incorporates in-line mixing and injection technology (in place of the open bath) and a contraction die design that facilitates homogenous fber wet out. Te injection and impregnation technology requires addi- tional tooling from the manufacturer that is attached to the front of the curing die. To make the tooling as small as possible, very low- viscosity resins have been designed to facilitate fast and efcient wet out without sacrifcing fnal part performance. Tis combina- tion of tooling and chemistry took place at a pultruder's facility. Te pultruder provided valuable feedback based on decades of pultrusion experience to adjust the tooling and chemistry. Today's method of closed polyurethane pultrusion minimizes waste, elimi- nates VOC emissions and enables pultruders to run faster line speeds at lower pull forces compared to open-bath technology. Further, it wouldn't have been possible without contributions from collaborators with diferent backgrounds and experience. New chemistries, like PURs, should be embraced because they encourage us to rethink production processes throughout the value chain. Simply dropping a highly reactive resin into a conven- tional process may not provide a desirable result. But, tweaking process conditions to accommodate the nuances of the chemistry might result in signifcant cycle time reduction. Or, redesigning the process, as described with the pultrusion process above, can yield signifcant benefts by reducing or eliminating waste with a one-time tooling expense. Large steps forward in technology are never easy, but when all parties in the value chain add their eforts, the sum is much greater than the individual parts. ABOUT THE AUTHOR Dr. Kevin J. Meyer is an associate scientist, Dow Polyurethanes, The Dow Chemical Co. (Midland, MI, US). Meyer currently works in Dow's Polyurethanes Formulated Systems business, where he provides applications development and technical support to the flament winding and pultrusion markets. His background includes processing and characterization of both thermoset and thermoplastic composites. Meyer holds a BS from Florida State University and a Ph.D from Virginia Tech, both of which are in chemical engineering. kjmeyer@dow.com Polyurethanes: Collaboration and adaptation for optimal application REFERENCES 1 R. Herrington and K. Hock, eds., Flexible Polyurethane Foams, 2nd Ed., The Dow Chemical Co. (Midland, MI, US), 1997. 2 M. Sonnenschein and W. Koonce, "Polyurethanes" in Encyclopedia of Polymer Science and Technology, 4th Ed., John Wiley & Sons Inc. (Hoboken, NJ, US), 2012. 3 "Growth Opportunities in Global Composites Industry: 2013-2018," Lucintel LLC (Irving, TX, US), 2012. 4 M. Sonnenschein, Polyurethanes: Science, Technology, Markets and Trends, John Wiley & Sons Inc. (Hoboken, NJ, US), 2015. 5 G. Bramante, L. Bertucelli, A. Benvenuti and K. Meyer, "Polyurethane Composites: A versatile Thermo-set Polymer Matrix for a Broad Range of Applications - Mechanical Analysis on Pultruded Laminates," Proceedings of Polyurethanes 2014 Technical Conference (Dallas, TX, US), 2014.