Why Choose Polyurethane Pultrusion Composites for Photovoltaic Frames? Polyurethane Pultrusion Molding Process and Application

 

The use of pultrusion technology to manufacture composite products has a history of decades. Traditional resins used in pultrusion technology include polyester, unsaturated resin, phenolic resin, epoxy resin, etc. Highly automated pultrusion molding technology can give full play to the mechanical properties of fibers and is suitable for continuous production of resin-based composite materials.

 

Polyurethane stands out from traditional resins with its excellent mechanical properties, low-pressure molding, fast curing, and no styrene volatilization, becoming the “new favorite” in the resin-based composite material industry.

 

Raw Materials

 

Polyurethane Resin Matrix

 

Polyurethane is a polymer with a main chain containing a carbamate group (-NHCOO-) synthesized from oligomer polyols and isocyanates as the main raw materials. Compared with traditional resins, polyurethane has better bonding with reinforcing materials, so it has excellent impact resistance, mechanical properties and weather resistance.

 

Polyurethane resin does not contain volatiles such as styrene, which fully meets the country’s requirements for “strengthening the application and research and development of green products and promoting lightweight and low-power technology and processes”.

 

Polyurethane has the process advantages of low viscosity and faster molding speed when used in pultrusion molding. The typical related properties of polyurethane resin for pultrusion are shown in the table below.

 

 

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 structures according to needs to improve the overall performance of the product. It is widely used as a reinforcing material for resin-based composite materials. The following table shows the main properties of the two types of glass fibers.

 

 

 

Studies have shown 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 phase interfaces.

 

Carbon Fiber

 

As an excellent reinforcing material, carbon fiber has the advantages of high specific strength and specific 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.

 

 

 

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 fiber, carbon fiber is usually sizing. It is mainly to prevent the fiber from absorbing impurities and water in the air and 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.

 

However, most of the current research on carbon fiber reinforced polyurethane resin-based composites focuses on short fibers. In contrast, when continuous carbon fibers are used in polyurethane-based composites, the content of carbon fibers is greatly increased, the interface area with the polyurethane resin matrix is ​​large, the interface bonding force is large, 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, aramid fiber, etc.

 

Basalt fiber is a high-performance fiber-reinforced material made from natural basalt ore. Basalt fiber has high corrosion resistance and chemical stability, a wide range of operating temperatures, and high thermal insulation and sound insulation.

 

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.

 

Continuous Fiber Reinforced Polyurethane

 

Pultrusion Molding Technology of 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.

 

 

 

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

 

Advantages of Pultrusion Process

 

Continuous fiber reinforced polyurethane composite materials 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.

 

Performance Characteristics of Pultrusion 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:

 

LightWeight and High Strength

 

Take the polyurethane auxiliary frame prepared by pultrusion as an example, which has excellent mechanical properties without the need for a supporting skeleton. 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.

 

 

 

Good 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 mechanical properties in the wet heat aging test is less than 3%, which has good aging resistance.

 

Moreover, when used as building materials, 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.

 

Good 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 occurrence of cracks caused by thermal expansion and contraction, and improves the sealing and thermal insulation performance.

 

Strong 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.

 

Pultrusion-Molded Fiber-Reinforced Polyurethane

 

Application of 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 and more prominent year by year. Traditional poles are susceptible to severe damage in harsh climates, which can cause devastating blows to the power grid.

 

Continuous fiber-reinforced polyurethane composite poles are light and high-strength, with good electrical insulation performance, and can replace traditional poles in special areas such as coastal areas, mountainous areas, and high pollution areas. Therefore, the China Electricity Council standard “Distribution Network Composite Poles” stipulates that polyurethane resin must be used, and poles should be formed by pultrusion.

 

Polyurethane resin insulated power poles have a mass of only 1/10 of that of concrete poles, and can be directly formed into support 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.

 

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.

 

In 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 performance; 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.

 

 

 

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 External Doors and Windows of Buildings”, 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 friendly; 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 great advantages to the construction and application of sleepers.

 

A new type of synthetic sleeper prepared by pultrusion molding. The 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 screws 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 certain flame retardancy, safety and reliability; the fiber-reinforced polyurethane material has a small thermal expansion coefficient, there is no thermal deformation problem, and 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 Frame and Bracket: 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 it more advantageous in applications such as sea surface and saline-alkali land.

 

At present, many component companies have completed the certification of related products and are in the stage of partial mass production. Composite frames have even become a special selling point for photovoltaic modules.