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Pultruded parts have long been fabricated using polyester or other types of resins. When polyurethane resin is substituted, the intrinsic properties of polyurethane allow for advantageous changes to the pultrusion process and result in superior properties in the finished part.
(Published on January - February 2008 – JEC Magazine #38)
JOHN HAYES, PRINCIPAL SCIENTIST, POLYURETHANE COMPOSITES AND
CRAIG SNYDER MARKET CHANNEL REPRESENTATIVE BAYERMATERIALSCIENCE LLC
For decades, the pultrusion process has been utilized to manufacture composite parts. Resins used in the process have traditionally included polyester, vinylester, epoxy or other resins. Using polyurethane in the pultrusion process is a relatively new development. Bayer MaterialScience (BMS) LLC is making significant strides in furthering polyurethane pultrusion technology and has experienced a number of commercial successes with polyurethane pultruded products.
A wide range of benefits
This new two-component polyurethane chemistry is based on a proprietary blend of polyols and a highly reactive isocyanate that was selected to optimize the processing characteristics for pultrusion. These characteristics include: low viscosity, to ensure good fibre wetting, long relative gel time to make setup and shutdown easy and forgiving, rapid polymerization to allow for high process speeds, good surface finish, and competitive cost.
The polyurethane pultrusion process offers a wide range of benefits compared with other materials. It allows the use of a higher concentration of fibreglass, making the end product substantially stronger. When window frame components are made from fibreglass-reinforced pultruded polyurethane, for example, they become eight times stronger than PVC frames. They are also 40 times less conductive than aluminium, so they are much better insulators. And because pultruded polyurethane frames are less brittle, they won’t crack or splinter, resulting in superior fastening.
Inline Fibreglass Ltd. of Toronto, Canada, is now producing window frames using the polyurethane pultrusion process and Bayer MaterialScience’s Baydur PUL 2500 polyurethane resin. By pultruding polyurethane resin, Inline can now produce larger, stronger, more complex parts with thinner walls (Figure 1). The polyurethane pultrusion technology allows the manufacture of window lineals that are vastly superior to their aluminium, wood and vinyl counterparts. Therefore, Inline is able to provide new offerings for the window frame manufacturing industry – and is exploring a variety of new applications in other industries as well.
To fully understand the multiple benefits polyurethane offers, it is advantageous to take a closer look at the physical and mechanical properties of this material, as well as the fabrication and environmental advantages polyurethane pultrusion offers.
Polyurethane offers improved physical properties across the board, compared with other resins such as unsaturated polyester or vinylester.
As an example, Baydur PUL 2500 polyurethane resin, formulated specially for pultrusion applications, exhibits superior flexural strength, flexural modulus and short beam shear compared with other resins used for pultrusion. (See the “More Information” box). In fact, the strength and stiffness properties of polyurethane are two to five times greater than those of polyester resins.
It is also worth noting that polyurethane pultruded materials show significantly lower water absorption than other resins, owing to the fact that they have a hard surface and low void content (Figure 2).
When pultruded materials absorb water or other solvents, the penetrating liquids can cause degradation, especially for resins that are prone to hydrolyze. For added moisture and UV protection, the surfaces of PU pultruded parts can be painted.
Because polyurethane offers improved impact and transverse properties, it is often possible to use fibreglass rovings instead of a fibreglass mat. Eliminating mats offers several advantages.
Traditional resin pultrusion of some profiles can require up to four or five different mats that constantly have to be sliced and formed into shape. Eliminating the mats reduces raw material costs, as well as the cost of labour required for mat handling. Mats also tend to break apart into pieces that can plug the processing machines, negatively impacting production. By eliminating mats, it is possible in many cases to increase line speed, thereby translating into more efficient and cost-effective manufacturing operations.
There are other benefits, as well. By replacing mats with fibreglass rovings, the fibre volume can be increased to approximately 80%, compared with 60% in most nonpolyurethane pultruded products. This combination of higher glass content along with better-performing resin results in polyurethane pultruded profiles that exhibit increased strength and stiffness.
The increased strength of polyurethane pultruded components also opens the door to a number of exciting, new possibilities. Polyurethane pultruded parts are strong enough to tackle applications that would be impossible with polyester – replacing steel and aluminium in the building/construction, infrastructure and transportation markets, for example.
Pultruders interested in exploring the use of polyurethane should understand that the process can be accomplished easily and economically; no major capital investments are required. Converting existing pultrusion systems for use with polyurethane is relatively simple and existing dies, heaters and machinery can still be used.
The two specifications required for polyurethane pultrusion are a resin metering/mixing system and a resin injection box. Because polyurethane is a two-component system, a special meter-mix unit is required. Bayer MaterialScience has worked with equipment manufacturers to develop meter-mix units for this purpose that are currently commercially available (Figure 3).
The reactivity of the polyurethane resin system also requires the conventional open, wet-resin bath system be removed and substituted with a resin injection system to accommodate polyurethane’s faster gel time. Bayer MaterialScience has custom-designed an injection system specified for achieving optimum glass wetout during the pultrusion process. An injector box is mounted flush to the entrance face of the pultrusion die (Figure 4).
The injector box can be machined from high-density polyethylene to minimize cost, promote ease of cleaning and avoid fibre damage. The two components of the resin are pumped at a rate that matches the rate of resin consumption and maintains sufficient pressure in the injector box to ensure adequate wetout.
In addition to the physical properties and fabrication advantages previously described, polyurethane pultrusion also offers assembly advantages, particularly in the area of fastening. Because of the exceptional strength of polyurethane, it is not necessary to pre-drill holes prior to sinking a screw into a polyurethane pultruded part, which saves time and labour costs. Conversely, the amount of force required to pull out a screw attached to a polyurethane pultruded part is more than twice that required of a screw attached to a polyester pultruded part (Figure 5).
As companies are increasingly looking for ways to “go green,” it is important to note that Baydur PUL 2500 polyurethane resin is a more environmentally-friendly alternative to other types of resins commonly used for pultrusion such as polyester, which contains VOC-emitting styrene as well as peroxide catalysts that require refrigeration. In fact, 10% of the resin is comprised of bio-based material derived from renewable polyols. Of course, the bottom line can’t be ignored. Fortunately, polyurethanes are cost-competitive with traditionally used, non-renewable resins. Furthermore, the future holds potential for achieving an even greater percentage of bio-based content in pultruded materials.
As this paper illustrates, the many benefits of polyurethane pultrusion are sure to make it a process that continues to gain momentum in coming years. Already, Bayer MaterialScience has made advancements in polyurethane pultrusion technology that allow for the manufacturing of complex profiles that would be very difficult to pultrude using other types of resins. Certainly, there will be many more exciting developments on the horizon as this proven, economically viable technology continues to penetrate new markets and offer new opportunities for pultruders.