Fiberglass pultrusion manufacturers have always celebrated — and even pushed for — innovative new ways to improve their products. One of the most important areas in which such innovations occur concerns the resins used in the pultrusion process.
Historically, unsaturated polyesters (UPE) and vinyl esters (VE) accounted for the bulk of resin used in the pultrusion industry.
Today, however, many manufacturers have begun working with another material: two-component polyurethane (PU).
While polyurethane resin offers compelling benefits, many people outside the pultrusion industry still do not understand its appeal.
This article takes a closer look at resin versus polyurethane, examining the benefits and drawbacks of this emerging material.
Benefits of Polyurethane Pultrusions
The growth of polyurethane in the pultrusion industry has been driven by compelling research about its superior performance attributes.
Here are five of the most important benefits offered by PU systems.
Increased Strength
Polyurethane fiberglass pultrusions have been extensively tested against traditional resins with respect to tensile, compressive, and flexural strength. In terms of compression strength, PU performed on par with other resin types.
However, PU showed marked advantages in tensile and flexural strength, particularly when measured under stresses applied in the important 90-degree direction.
For instance, polyurethane fiberglass pultrusions withstood 48.7 MPa of 90-degree tensile stress. Vinyl ester pultrusions, by contrast, only withstood 40.5 MPa of stress.
Unsaturated polyesters performed even more poorly, with a stress of 30.8 MPa. The flexural strength of PU showed similar advantages.
Polyurethane also demonstrated a marked improvement over other resins in apparent shear strength.
Shear strength quantifies pultrusion's ability to resist delamination forces, whether from drilling, nailing, or blunt impact.
Delamination reduces the load-carrying capacity of a composite, which is why shear strength is critical to long-term performance.
Excellent Strength-to-Weight Ratio
Polyurethane's well-documented strength advantages stem from several combined properties, including its fiber coupling efficiency and reduced incidence of resin-phase defects.
Because PU pultrusions are so much stronger, manufacturers can safely reduce wall thickness — and hence composite weight. This puts PU well ahead of other resins in terms of strength-to-weight ratio.
Better Toughness
While strength is important, it is far from the only indicator of a material's long-term performance profile.
Just as important is the composite's toughness — that is, its ability to absorb stress without fracturing. In other words, toughness encompasses both a degree of pliability and a material's damage tolerance.
Fiberglass pultrusion composites are often subjected to intense real-world applications. The toughness of polyurethane makes it better able to withstand the types of impacts to which composites are regularly exposed.
That toughness also gives polyurethane distinct advantages in terms of:
- Reinforcement costs
- Long-term warranty costs
- Customer satisfaction
- Scrap material reduction
Finally, polyurethane's improved toughness benefits secondary operations, such as component assembly.
Unlike other forms of fiberglass pultrusion, polyurethane does not need to be pre-drilled for screws and other fasteners. Instead, self-tapping screws can be used safely, often significantly reducing assembly times.
Greater UV Resistance
Fiberglass pultrusion products are often used for outdoor applications that require long-term exposure to natural ultraviolet radiation.
Such UV light can cause problems for certain polymers, leading to marked reductions in strength over time. Polyurethane exhibits a much greater resistance to UV damage than both UPE and VE.
One study subjected various types of fiberglass pultrusion to 1000 hours of ultraviolet light and then measured changes in transverse strength.
VE experienced strength reductions between 25 and 40 percent, and UPE experienced reductions between 15 and 20 percent. PU, meanwhile, only lost 10 to 15 percent of its strength.
Challenges of Polyurethane Pultrusions
Despite their clear mechanical property advantages, polyurethane pultrusions have certain drawbacks, particularly from a manufacturer's perspective.
To a large extent, this fact simply reflects the relatively recent emergence of polyurethane pultrusions. As more manufacturers adopt PU processes, the perceived drawbacks should continue to recede.
Cost
One area in which polyurethane is disadvantaged is in material costs. Simply put, PU resin tends to be more expensive than UPE or VE resins.
To an extent, this increased cost is offset by the strength benefits of polyurethane, thereby reducing the need for secondary reinforcements.
Similarly, the inherent strength of polyurethane enables manufacturers to reduce costs by using less expensive fiberglass or by replacing costly glass mats with more affordable roving.
Nonetheless, consumers should understand that for the time being, PU pultrusions will likely carry a higher upfront cost than those made with traditional resins.
Equipment Compatibility
Polyurethane also presents certain challenges from a manufacturer's standpoint. Most standard pultrusion equipment is not suitable for PU unless modifications are made.
For instance, most traditional equipment uses open resin baths. PU pultrusions, by contrast, require proprietary injection boxes.
These enclosed boxes meter and inject the two components of the polyurethane resin at a rate that matches the consumption rate.
The injection box also maintains the required pressure to ensure adequate wet-out.
Finally, injection boxes are designed to limit the possibility of dead spots — in other words, places where the combined PU resin might accommodate and cure.
Beyond these specialized equipment requirements, however, the PU pultrusion process largely proceeds in a manner similar to that of other resins.
Line Speeds
PU pultrusion systems also exhibit minor drawbacks with respect to line speed. Whereas many commercial processes require line speeds of up to 8 feet per minute, many polyurethane systems typically operate at approximately 5 feet per minute.
Fortunately, those numbers reflect the technology used in the earliest iterations of PU rollout.
As time goes on, it is reasonable to expect that engineers will continue to refine the equipment and chemistry that drive the PU pultrusion process.
Eventually, PU systems should be able to compete — if not surpass — the speeds of traditional resin pultrusion systems.
When it comes to the best material for a particular fiberglass pultrusion, there is no one-size-fits-all formula. Instead, you should consult an experienced company regarding the most appropriate resin type to meet your project requirements.
For more information about resin vs polyurethane and other fiberglass pultrusion questions, contact the industry experts at Tencom Ltd.
