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Polyurethane Pultrusion and Its Applications

In recent years, polyurethane (PU) resins have stood out in composite materials due to their excellent toughness, rapid curing, and absence of styrene smoke. With advancing mastery in PU molding techniques and progress in controlling its reactivity to extend usability, polyurethane has entered the composite material domain traditionally dominated by unsaturated polyester and vinyl ester resins. Previously, PU composite materials were mainly used in automotive interiors and exteriors through Structural Reaction Injection Molding (SRIM), such as pickup truck boxes, underbodies, roof racks, and inner door panels (using foamed polyurethane). However, in recent years, technologies like pultrusion, filament winding, vacuum infusion, and long fiber spraying have developed, utilizing non-foamed PU composites to manufacture items like window frames, bathtubs, light poles, and large parts for trucks and off-road vehicles.

Polyurethane Pultrusion: Polyurethane pultrusion typically features low viscosity, moderate to high reactivity, excellent impact strength and toughness, and good short-beam shear properties. Compared to other materials, polyurethane pultrusion offers multiple benefits. It allows for a higher glass fiber content in products, significantly enhancing strength. For instance, window frames made by pultruding glass fibers with PU resin are eight times stronger than PVC frames and have 40 times lower conductivity than aluminum, thus offering far better insulation. Additionally, PU pultruded window frames are less brittle, making them more durable and crack-resistant.

High-Performance PU/Glass Fiber Composites are high-strength, high-modulus, lightweight polymer composites produced using continuous pultrusion with a hard PU elastomer matrix and glass fibers as reinforcement.

Products of Polyurethane Pultrusion Technology not only offer higher strength and better thermal insulation than traditional materials but are also lighter and more environmentally friendly. Their application range is broad, from luxurious bathtubs to surfboards and ski boards, to today’s window frames and container floors, integrating PU composites into various aspects of our daily lives.

In the past few years, China’s demand for composite materials has shown a steady increase. Composite materials are high-tech solutions integrating the properties of various materials to create ones with exceptional new performance. Due to their unique properties, such as lightweight, high strength, rigidity, cost-efficiency, and ecological responsibility, PU composites have gained attention across industries, particularly in construction and transportation, where innovative technologies and applications are in the spotlight.

Performance Characteristics of PU/Glass Fiber Composites: After years of development, PU pultrusion molding has been commercialized abroad. During the PU pultrusion process, more reinforcing fibers can be used, significantly increasing product strength. Moreover, the inherent excellent impact, tensile, and interlaminar shear strengths of polyurethane allow for thinner, lighter products. For instance, using fewer continuous roving mats and more untwisted rovings can produce thinner I-beams, reducing their thickness from 3.3mm to 2.6mm while maintaining longitudinal rigidity. This results in a 13% reduction in weight and a 7% cost saving. Additionally, the less brittle nature of PU pultruded products allows for conventional assembly without cracking or breaking.

Advantages of Polyurethane Pultrusion Technology include:

  1. Traditional resin pultrusion of certain profiles may require up to 4 or 5 different glass fiber mats, which need to be cut and shaped. PU pultrusion often uses glass fiber untwisted rovings instead of mats, reducing material costs and the labor involved in handling mats. This change can also increase production line speed, thereby improving cost-effectiveness.
  2. Replacing mats with untwisted rovings can increase fiber volume content to about 80%, compared to 60% in most non-PU pultruded products. The combination of higher glass fiber content and superior resin results in PU pultruded profiles with improved strength and stiffness.
  3. Transitioning from existing pultrusion systems to PU pultrusion systems is simple, convenient, and economical, requiring no significant investment. Existing molds, heaters, and units can still be used.
  4. In addition to physical properties and molding advantages, PU pultruded parts also offer assembly benefits, especially in terms of easy fastening. Due to the strength of PU, screws can be inserted into PU pultruded products without pre-drilling, saving time and labor. Conversely, the force required to pull a screw out of a PU pultruded product is more than twice that needed for a polyester pultruded product.
  5. With the same fiber structure, PU/glass fiber pultruded products outperform ordinary thermosetting resins in all aspects, with similar flexural modulus, significantly higher impact strength, higher screw pull-out strength, better crack resistance, superior wear resistance,excellent secondary processing capability, and heat resistance above 240 degrees Celsius.
  6. Pure polyurethane/glass fiber is currently the best-performing pultruded composite material, featuring complex cross-sections, smooth surfaces, fast pultrusion speed, excellent resistance to water, acids, alkalis, salts, good flame retardancy, paintability, and being aliphatic PU system, solvent-free, styrene-free, and environmentally friendly.

Main Uses of PU/Glass Fiber Composite Materials:

  1. In Building Materials: In recent years, building energy conservation has become an important part of China’s sustainable development. In buildings, doors, windows, exterior walls, roofs, and floors are the main energy-consuming parts, with doors and windows being the least insulated. It’s estimated that in China, the energy consumption of doors and windows is about four times that of walls, five times that of roofs, and more than twenty times that of floors, accounting for 40% to 50% of the total energy consumption of building envelopes. Therefore, enhancing the insulation of doors and windows to reduce their energy consumption is crucial for improving indoor thermal environment quality and building energy efficiency. The rise of composite materials in applications offers new ideas, making their use in the transformation of energy-saving doors and windows possible.

Polyurethane pultruded window frames, compared to traditional materials, offer superior dimensional stability, higher lateral mechanical performance, higher specific strength and rigidity, and better thermal insulation. Additionally, the unique pultrusion technology of PU materials, which are VOC-free, is also highly environmentally friendly.

Glass fiber reinforced PU pultruded door and window profiles are produced using advanced injection impregnation pultrusion processes with glass fibers as reinforcement and PU as the matrix. The development of GRPU door and window profiles aims to provide energy-saving window solutions with a total heat transfer coefficient K ≤ 2.0 W/(m•K) for high-rise buildings. Polyurethane (PU) has inherent thermal insulation capabilities and has been used in the energy-saving window industry for making foam sealants, sealing strips, and thermal breaks. Using PU to manufacture the entire window frame for energy-saving windows is a first in China.

The advantages of GRPU window frames are determined by the properties of the GRPU material itself. GRPU, as a new type of composite material, offers growth potential for window frames. Using this resin allows for the production of larger, thinner, and sufficiently strong profiles, suitable for large window frames and even curtain walls. Reportedly, these window frames are much better than aluminum, wood, and plastic frames. They have excellent expansion and contraction properties, can withstand various climatic conditions from the extreme cold of the Arctic to the intense heat of the desert, and the humidity of coastal areas. They can be painted or post-processed to create a wood-like appearance. Polyurethane composite window frames have the following basic characteristics:

  • High Insulation: GRPU profiles, like solid wood and PVC, have a very low thermal conductivity at room temperature (0.22 W/m•K), only about 1/700th of that of aluminum alloys, making them excellent insulating materials.
  • Low Thermal Expansion: The linear thermal expansion coefficient of GRPU is approximately 7×10-6/K, much lower than that of aluminum alloys, and similar to that of building walls. Thus, GRPU frames do not create gaps with the wall during temperature changes, ensuring good sealing and insulation in environments with significant temperature differences.
  • Corrosion Resistance: GRPU profiles are highly resistant to most acids, alkalis, salts, organic substances, seawater, and humid air. They do not rust or corrode, offering better corrosion resistance than other door and window materials, especially suitable for coastal, corrosive, and generally humid environments.
  • Excellent Electrical Properties: GRPU profiles are good insulating materials, unaffected by electromagnetic waves and do not reflect radio waves, making them particularly useful for buildings with communication systems.

Other Uses: Polyurethane pultrusion products include profiles, rods, and plates, such as ladder rods, tool handles, crossarms for electrical poles, poles, hockey sticks, dock pilings, container flooring, etc.

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