Polyurethane Pultrusion Process and Application

 

The pultrusion process refers to the process of impregnating roving and felt through a resin tank under the pulling action of a traction device, then entering a heated mold for curing, and then cutting to form the final product.

 

 

There are still some problems to be solved in continuous fiber reinforced polyurethane composites: At present, there is a phenomenon that the design and preparation process of polyurethane resin formula are isolated from the manufacturing process of polyurethane composites in China; polyurethane reacts violently, releases a large amount of heat that is difficult to control, and is sensitive to ambient temperature and humidity and easy to foam.

Therefore, appropriate process equipment (such as closed mold molding) should be used in the preparation process to control ambient temperature and humidity, reduce the reactivity of raw materials or use non-foaming polyurethane.

 

Reinforcement Materials

 

Glass Fiber

As an inorganic fiber with excellent performance, glass fiber is a brittle material with high elastic modulus, high tensile strength and low elongation at break. It has the advantage of designable mechanical properties and can flexibly design product structure according to needs to improve the overall performance of the product. It is widely used as a reinforcement material for resin-based composites. The following table shows the main properties of the two types of glass fibers.

 

Main properties of the two types of glass fibers

 

 

Glass fiber roving for pultrusion Source: Jushi Research shows that the increase in the N content on the surface of glass fiber can significantly increase the bonding strength between glass fiber and polyurethane resin. This may be because the nitrogen-containing functional groups react with the residual isocyanate in the polyurethane resin to form urea groups, which play a chemical bonding role between the two phases.

 

Carbon Fiber

As an excellent reinforcing material, carbon fiber has the advantages of high specific strength and modulus, low density, small thermal expansion coefficient, low friction coefficient, low elongation, and good low temperature resistance. Carbon fibers of different specifications and properties can be selected according to the application requirements of different fields. The specifications and properties of carbon fiber are shown in the table below.

 

Specifications and Properties of Carbon Fiber

 

Polyurethane resin has good wettability to fibers, but a higher bonding strength is required to form a good interface between the two. Therefore, in the production process of carbon fibers, carbon fibers are usually sizing. This is mainly to prevent the fibers from absorbing impurities and water in the air and to protect the active groups on the fiber surface; at the same time, it can improve the fiber’s bundling function and improve its wettability, which has a certain reinforcement effect.

 

 

Continuous carbon fiber However, most of the current research on carbon fiber reinforced polyurethane resin-based composites focuses on short fibers. In contrast, when continuous carbon fiber is used in polyurethane-based composites, the content of carbon fiber is greatly increased, the interface area with the polyurethane resin matrix is ​​large, the interface bonding force is strong, and the reinforcement strength is much higher than that of short fibers.

 

Other Fiber-Reinforced Materials

In addition to carbon fiber and glass fiber, commonly used fiber-reinforced materials include basalt fiber and aramid fiber. Basalt fiber is a high-performance fiber-reinforced material made from natural basalt ore.

 

Basalt fiber has high corrosion resistance and chemical stability, and has a wide operating temperature range and high thermal insulation and sound insulation.

 

Door and window profiles made of basalt fiber reinforced polyurethane composites have good thermal insulation performance and energy-saving effect. Compared with glass fiber windows, they have higher corrosion resistance and anti-aging performance.

 

Aramid fiber, that is, aromatic polyamide fiber, has the advantages of high strength, high modulus, low density and good wear resistance. It is known as the world’s three major high-tech fibers together with carbon fiber and high-strength and high-modulus polyethylene fiber.

 

Fillers and Auxiliary Materials

The main raw materials of fiber-reinforced resin-based composite materials are matrix and reinforcement materials. In actual production, adding fillers can not only improve the performance and molding processability of composite materials, but also reduce production costs and meet appearance requirements. In addition, there are some auxiliary materials, such as colorants and release agents. Colorants are generally used in the form of pigment paste; release agents should have extremely low surface free energy and can evenly wet the mold surface to achieve demolding effect.

 

Pultrusion Technology of Continuous Fiber Reinforced Polyurethane Composite Materials

 

Pultrusion Molding Process

Pultrusion molding began in the 1950s and is still in a stage of rapid development. It is a molding process that continuously and massively produces various linear profiles with constant cross-sectional dimensions by molding and curing impregnated continuous fibers through extrusion molds under the action of traction.

 

 

 

Flow chart of pultrusion process of continuous fiber reinforced polyurethane composites

 

Polyurethane resin system is applied to pultrusion molding process, which has the advantages of short molding cycle, high production efficiency and no styrene volatilization, making polyurethane pultrusion products a hot spot in research and development. After several years of development, polyurethane pultrusion molding has been commercialized, and polyurethane resin systems dedicated to pultrusion products have appeared at home and abroad.

 

Advantages of Pultrusion Molding Process

 

Continuous fiber reinforced polyurethane composites are formed by pultrusion process, which has the following advantages:

 

① Glass fiber roving is often used instead of glass fiber mat. It reduces costs and avoids the impact of glass fiber mat on production due to its easy breakage.

 

② There is no need to replace the original pultrusion equipment, and the assembly of the pultrusion system is simple and economical.

 

③ The fiber mass fraction can reach 60%~90%, and the dipping is carried out under tension, which can give full play to the role of the fiber and make the product stronger.

 

④ The production process is continuous and the length can be cut at will.

 

⑤ The production process can be fully automated and the production efficiency is high. KraussMaffei Group’s new iPul pultrusion system has increased the production speed from the usual 0.5~1.5m/min to about 3m/min (laboratory data), opening up a new market for pultrusion technology. 3. Performance characteristics of pultruded composite materials

 

Performance Characteristics of Pultruded Composite Materials

 

In the pultrusion process of continuous fiber-reinforced polyurethane composite materials, more reinforcing fibers can be used to greatly improve the strength of the product, and the resin and fiber form a good complementary advantage in performance.

 

Therefore, the fiber-reinforced material produced by pultrusion with polyurethane resin as the matrix has the following properties and characteristics:

 

① Lightweight and High Strength Taking the Polyurethane Auxiliary Frame Prepared by Pultrusion as an Example, it has excellent mechanical properties without supporting the skeleton, and the tensile strength and specific strength are more than 10 times that of PVC (polyvinyl chloride) plastic doors and windows, much greater than aluminum alloy and steel, and the density is only 1/4 of the density of steel and 1/2 of the density of aluminum alloy. The performance of pultruded fiber-reinforced polyurethane composite materials is shown in the table. Performance of fiber-reinforced polyurethane pultrusion composite materials.

 

 

②Good Corrosion Resistance

Glass fiber reinforced polyurethane composite materials have good resistance to strong acid, strong alkali and high concentration salt at room temperature. The loss of its mechanical properties in the wet heat aging test is less than 3%, and it has good aging resistance. Moreover, when used as a building material, it can last as long as the building, and the surface does not need to be treated. It has strong corrosion resistance to cement slurry in alkaline or acidic environments.

 

③Good Dimensional Stability

The linear shrinkage of continuous fiber reinforced polyurethane composite materials is less than 0.2%, and the deviation from the design size is very small. In addition, the linear expansion coefficient of polyurethane composite doors and windows formed by pultrusion is much smaller than that of aluminum alloy and PVC plastic, and is close to that of building walls, which effectively avoids the cracks caused by thermal expansion and contraction, and improves the sealing and thermal insulation performance.

 

④Strong Designability

The high fiber content in continuous fiber polyurethane pultrusion gives the composite material excellent mechanical properties, and the preferred fiber mass fraction is 60%~90%. The performance of polyurethane resin has strong designability, and the performance of products obtained with different raw material formulas and additives is also different.

 

Application of Pultruded Fiber Reinforced Polyurethane Composite Materials

 

Application in the Field of Power Engineering

 

With the rapid development of my country’s economy and power grid, the application trend of new composite materials has become more prominent year by year. Traditional poles are susceptible to severe damage in harsh climates, causing devastating blows to the power grid.

 

 

Polyurethane Composite Poles Source:

BASF Continuous fiber reinforced polyurethane composite poles are lightweight, high-strength, and have good electrical insulation properties. They can replace traditional poles in coastal, mountainous, and highly polluted areas. Therefore, the China Electricity Council standard “Composite Electric Poles for Distribution Networks” stipulates that polyurethane resin must be used and the poles should be formed by pultrusion.

 

 

Polyurethane Composite Electric Poles Source:

BASF Polyurethane resin insulated power poles are only 1/10 of the mass of concrete poles and can be directly molded into supporting parts, saving transportation, installation and labor costs. Due to the excellent properties of polyurethane itself, no curing agent, anti-aging agent, etc. need to be added during molding, which reduces the production cost of power poles. However, the anti-tracking performance of glass fiber reinforced polyurethane composite materials is insufficient, and it is difficult to be directly used in high electric fields.

 

Application in the Field of Building Doors and Windows

 

The application of continuous fiber reinforced polyurethane pultrusion materials in construction is mainly door and window profiles. Although aluminum alloy windows, PVC plastic steel windows, aluminum clad wood windows and other doors and windows have good thermal insulation performance and are widely welcomed by the market, such materials will cause a lot of energy loss during the production process, easily generate solid waste and harmful volatiles, and have poor fire resistance

 

 

Polyurethane Pultruded Door and Window Profiles Source:

Sitong Group During the molding of polyurethane pultruded door and window profiles, no styrene is volatilized, which avoids environmental pollution and conforms to the concept of “green environmental protection”. Because glass fiber has good flame retardancy and heat resistance, the polyurethane pultruded material itself has fireproof properties; and the heat transfer coefficient of polyurethane pultruded door and window profiles is only 0.22W/(m2·K), and the thermal resistance is 9.96mK/W, with good thermal insulation performance. The heat transfer coefficient and air tightness of several energy-saving windows are shown in the table below.

 

Heat transfer coefficient and air tightness of several energy-saving windows

 

 

Compared with doors and windows made of other materials, polyurethane pultruded doors and windows have stable dimensions, good air tightness and sound insulation. According to the national standard “Classification and Testing Methods for Air Tightness, Water Tightness and Wind Pressure Resistance of Building External Doors and Windows”, the air tightness of polyurethane composite doors and windows can reach level 8, water tightness can reach level 6, and sound insulation can reach level 5, and they have been widely used.

 

Application in the Field of Railway Sleepers

 

Wooden sleepers and reinforced concrete sleepers are currently the two main sleeper materials. The former is not corrosion-resistant, has a short service life, and is environmentally unfriendly; the latter has cheap and easy-to-obtain raw materials, stable performance, and long service life, but its elasticity is poor and the hardness is high, and it is easy to crack. In contrast, fiber-reinforced polyurethane synthetic sleepers, regardless of the performance of the raw materials themselves or the characteristics of the pultrusion process, have brought huge advantages to the construction and application of sleepers.

 

 

Composite Sleepers Source:

Deyilong Qiao Dongping et al. used pultrusion to prepare a new type of synthetic sleeper. Performance test results show that the bending load resistance of the new polyurethane composite sleeper can reach 181kN, and the pull-out strength of the screw can reach 76kN. This type of sleeper not only has excellent performance in mechanical properties such as shear strength and impact strength, but also has excellent performance in corrosion resistance and aging resistance.

 

Application in Other Fields

 

Automobile Floor:

This type of automobile floor has high strength and light weight, which improves fuel economy; there will be no small molecule volatilization during use, it is moisture-proof and corrosion-resistant, and has a certain flame retardancy, which is safe and reliable; the thermal expansion coefficient of fiber-reinforced polyurethane material is small, there is no problem of thermal deformation, and it has a long service life.

 

Car Body:

Due to the high fiber volume fraction, the composite material has excellent mechanical properties, low quality, and reduces fuel consumption and environmental damage.

 

Photovoltaic Frames and Brackets:

Continuous glass fiber reinforced polyurethane pultrusion composite materials can be used to make photovoltaic module frames and brackets. This is a rapidly developing new market. Replacing aluminum alloy frames with composite materials can reduce costs and reduce weight. In addition, the good insulation and chemical resistance of composite materials make them more advantageous in applications such as sea surfaces and saline-alkali land. At present, many companies have completed relevant certifications and are in the stage of partial mass production. Composite frames have even become a special selling point for photovoltaic modules.

 

 

Composite Material Frame Photovoltaic Module Source:

 

At present, polyurethane pultruded composites still need to solve some problems to achieve large-scale application in photovoltaic frames and brackets. On the one hand, it is necessary to fully verify the long-term weather resistance and strength performance, which currently mainly relies on third-party testing and certification endorsement; on the other hand, the current polyurethane pultrusion molding speed is generally less than 1m/min, which is difficult to meet the production needs for GW-level installations, which requires continuous improvement of pultrusion production equipment.