Why Use Polyurethane Pultruded Composites for Photovoltaic Frames? Polyurethane Pultrusion Molding Process And Applications

Why Use Polyurethane Pultruded Composites for Photovoltaic Frames? Polyurethane Pultrusion Molding Process And Applications

The pultrusion molding process refers to the impregnation of roving and felts in resin tanks under the pulling action of traction equipment, then into heated molds for curing, and cutting to form the final product.

The pultrusion process has been used for decades to manufacture composite products. The traditional resins used in the pultrusion process are polyester, unsaturated resins, phenolic resins, epoxy resins and so on. Highly automated pultrusion molding technology can give full play to the mechanical properties of the fibers and is suitable for the continuous production 

of resin matrix composites.

Polyurethane with its excellent mechanical properties, low-pressure molding, fast curing, no styrene evaporation and other advantages from the traditional resin, and become the resin matrix composite material industry's “new favorite”.

I. Raw materials

1. Polyurethane resin matrix

Polyurethane is an oligomer polyol and isocyanate as the main raw material synthesized a main chain containing urethane groups (-NHCOO-) polymer. Compared to conventional resins, polyurethane binds better to reinforcement materials, resulting in excellent impact resistance, mechanical properties and weatherability.

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

Polyurethane for pultrusion molding has the advantages of low viscosity and faster molding speed. Typical properties of polyurethane resins for pultrusion are shown in the table below.

Relevant properties of polyurethane resins for pultrusion

Continuous fiber-reinforced polyurethane composites still have some problems to be solved: at present, there exists the phenomenon that the design of polyurethane resin formulations and preparation process are mostly isolated from the manufacturing process of polyurethane composites; polyurethane reacts violently, releasing a large amount of heat which is not easy to be controlled, and it is easy to be foamed due to its sensitivity to the ambient temperature and humidity.

Therefore, suitable process equipment (such as closed molding) should be used in the preparation process, environmental temperature and humidity should be controlled, and the reactivity of raw materials should be reduced or non-foaming polyurethane should be used.

2. Reinforcement materials

Glass fiber

As an inorganic fiber with excellent performance, glass fiber is a brittle material with high modulus of elasticity, high tensile strength and small elongation at break, which has the advantage of designable mechanical properties, and can be flexibly designed according to the need to improve the overall performance of the product structure, and it is widely used as reinforcing material for resin matrix composites. The following table shows the main properties of the two types of glass fibers.

Comparison of Main Properties of Two Types of Glass Fiber

It is shown that the increase of N element content on the surface of glass fiber can significantly increase the bonding force between glass fiber and polyurethane resin, which may be due to the fact that the nitrogen-containing functional groups will react with the residual isocyanate in the polyurethane resin to generate urea groups, which will act as a chemical bonding agent at the interface between the two phases.

Carbon Fiber

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

Carbon fiber specifications and properties

The polyurethane resin has good wettability to the fiber, but a higher bond strength is needed if the two are to form a good interface. Therefore, in the production process of carbon fiber, carbon fiber is usually sizing treatment. Mainly to prevent the fiber adsorption of impurities in the air and water, to protect the fiber surface active groups; at the same time, it can improve the fiber cluster function, improve its infiltration properties, with a certain reinforcing effect.

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

Other fiber reinforced materials

In addition to carbon fiber and glass fiber, commonly used fiber reinforcing materials are basalt fiber, aramid fiber and so on.

Basalt fiber is a high-performance fiber reinforced material made from natural basalt ore. Basalt fiber has high corrosion resistance and chemical stability, and a wide range of use temperature, with high heat and sound insulation.

Basalt fiber reinforced polyurethane composite material made of door and window profiles, good thermal insulation performance, energy saving effect, compared with fiberglass windows have a higher corrosion resistance and anti-aging properties.

Aramid fiber, that is, aromatic polyamide fiber, with high strength, high modulus, low density and good abrasion resistance and other advantages, and carbon fiber, high strength and high modulus polyethylene fibers and known as the world's three major high-tech fibers.

3. Fillers and auxiliary materials

Fiber reinforced resin matrix composites is the main raw material of the matrix and reinforcing materials, in the actual production of fillers can not only improve the performance of composite materials and molding process, but also reduce the production cost, to meet the demand for appearance and so on.

In addition, there are some auxiliary materials, such as coloring agent, mold releasing agent and so on. The coloring agent is generally used in the form of pigment paste; the release agent should have very low surface free energy and can uniformly wet the surface of the mold to achieve the effect of mold release.

II. Continuous fiber reinforced polyurethane composites pultrusion molding technology

1. Pultrusion molding process

Pultrusion molding began in the 1950s, is still in the high-speed development stage, is an impregnated continuous fiber under the action of traction, through the extrusion die molding, curing, continuous large-scale production of a variety of cross-sectional dimensions of unchanged linear profiles of the molding process.

      Polyurethane resin system used in pultrusion molding process has the advantages of short molding cycle, high production efficiency, no styrene volatilization, which makes polyurethane pultruded products become a hot spot for research and development. After several years of development, polyurethane pultrusion molding has been commercialized, and special polyurethane resin systems for pultruded products have appeared at home and abroad.

2. Advantages of pultrusion molding process

Continuous fiber reinforced polyurethane composite materials using pultrusion molding process, has the following advantages:

1). Glass fiber untwisted roving is often used instead of glass fiber mats. Reduced costs, but also to avoid the glass fiber mat is easy to break and affect the production.

2). No need to replace the original pultrusion equipment, pultrusion system assembly is simple and economical.

3). Fiber quality fraction up to 60% - 90%, impregnation under tension, can give full play to the role of fiber, product strength is higher.

4). The production process is continuous and the length can be cut arbitrarily.

The production process can be fully automated and controlled, with high production efficiency.

5). 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 whole new market for pultrusion molding technology.

3. Performance characteristics of pultruded composites

In the continuous fiber reinforced polyurethane composites pultrusion molding process, more reinforcing fibers can be used, so that the strength of the product is greatly increased, the resin and fiber in the performance of the formation of good complementary advantages. Therefore, the fiber-reinforced materials produced by pultrusion molding process with polyurethane resin as matrix have the following properties and characteristics:

1). Light weight and high strength

Pultrusion process to prepare polyurethane frames, for example, which do not need to support the skeleton has excellent mechanical properties, tensile strength and specific strength and other properties of PVC (polyvinyl chloride) plastic doors and windows is more than 10 times, much larger than the aluminum alloy and steel, while the density of only 1/4 of the density of steel, aluminum alloy density of 1/2. Pultruded fiber-reinforced polyurethane composite material properties are shown in the table.

2). Good corrosion resistance

Glass fiber reinforced polyurethane composites have good resistance to strong acid, strong alkali and high concentration of salt at room temperature, and the loss of its mechanical properties is less than 3% in the wet and hot aging test, which has good aging resistance.

Moreover, it can be used as building materials with the same life as the building, the surface does not need to be treated, and it has strong corrosion resistance to cement paste and other alkaline or acidic environments.

3). Good dimensional stability

The linear shrinkage of continuous fiber-reinforced polyurethane composites is less than 0.2%, and the deviation from the designed size is very small. In addition, the linear expansion coefficient of the pultruded polyurethane composite doors and windows is much smaller than that of aluminum alloy and PVC plastic, and it is close to that of the building wall, which effectively avoids the appearance of cracks caused by thermal expansion and contraction and improves the sealing and heat preservation performance.

4). Strong designability

The high fiber content in continuous fiber polyurethane pultrusion molding endows the composites with excellent mechanical properties, and the preferred fiber mass fraction is 60% - 90%. The performance of polyurethane resin has a strong designability, different raw material formulations and additives obtained from the products have different properties.

III. The application of pultruded fiber reinforced polyurethane composites

1. Application in the field of electric power engineering

With the rapid development of China's economy and power grid, the application trend of new composite materials is highlighted year by year. Traditional poles are susceptible to serious damage in harsh climates, which can devastate the power grid.

Continuous fiber-reinforced polyurethane composite poles are lightweight and high-strength, with good electrical insulation performance, which can replace traditional poles in special areas such as coastal, mountainous and highly polluted areas. Therefore, the CLP standard “Composite Electricity Poles for Distribution Networks” stipulates that polyurethane resin must be used and the poles should be molded by pultrusion process.

Polyurethane resin insulated power poles and towers, the quality of which is only 1/10 of concrete poles, and can be molded directly into the support members, saving transportation, installation and labor costs. Due to the excellent characteristics of polyurethane itself, there is no need to add curing agent, anti-aging agent, etc. in the molding, which reduces the production cost of power pole tower. However, the anti-leakage starting trace performance of glass fiber reinforced polyurethane composites is insufficient, and it is difficult to be used directly at high electric fields.

2. Application in the field of building doors and windows

The application of continuous fiber reinforced polyurethane pultruded materials in construction is mainly for door and window profiles. Aluminum alloy windows, PVC plastic steel windows, aluminum clad wood windows and other doors and windows, although good thermal insulation performance, widely welcomed by the market, but such materials in the production process will cause a lot of energy loss, easy to produce solid waste and harmful volatiles, and fire resistance performance is not good.    Polyurethane pultruded window and door profiles in the molding, no styrene volatilization, to avoid environmental pollution, in line with the “green” concept. As the glass fiber has good flame retardancy and heat resistance, the polyurethane pultruded material itself has fire resistance; and polyurethane pultruded window and door profiles have a heat transfer coefficient of only 0.22W/(m2-K), with a thermal resistance of 9.96mK/W, which is a better thermal insulation performance. The heat transfer coefficient and airtightness of several types of energy-saving windows are shown in the table below.

Compared with other materials windows and doors, polyurethane pultruded window and door profiles have stable dimensions, good airtightness and sound insulation. According to the national standard “building exterior doors and windows airtight, watertight, wind pressure resistance performance grading and testing methods” test, polyurethane composite doors and windows of airtightness up to 8 levels, watertightness up to 6 levels, sound insulation performance up to 5 levels, has been widely used.

3. Application in the field of railway sleepers

Wooden sleepers and reinforced concrete sleepers are currently two major sleeper materials, the former is not corrosion-resistant, short service life, and contrary to environmental protection; the latter raw materials are cheap and easy to get, stable performance, long service life, but its poor elasticity and high hardness, easy to rupture. In contrast, fiber-reinforced polyurethane synthetic sleepers, regardless of the performance of the raw material itself or pultrusion process characteristics, have brought great advantages to the construction and application of sleepers.        

Qiao Dongping and other pultrusion molding process to prepare a new type of synthetic sleepers. Performance test results show that the new polyurethane composite sleeper bending load of up to 181kN, screw pull-out strength of up to 76kN. the kind of sleeper is not only in the shear strength, impact strength and other mechanical properties have excellent performance, in corrosion resistance, aging resistance is also excellent performance.

4. In other areas of application

Automobile floor: this kind of automobile floor has high strength and light weight, which improves fuel economy; there will be no volatilization of small molecules in the process of using, moisture-proof and corrosion-resistant, and has a certain degree of flame retardant, safe and reliable; the thermal expansion coefficient of the fiber-reinforced polyurethane material is small, and there is no problem of thermal deformation, which has a long service life.

Body: Due to higher fiber volume fraction, the composite material has excellent mechanical properties, lower mass, reduced fuel consumption and environmental damage.

Photovoltaic frame and stent: continuous glass fiber reinforced polyurethane pultruded molding composites can be used to make photovoltaic module frames and stents, which is a rapidly developing new market. Replacing aluminum alloy frames with composites can play a role in cost reduction, lightweighting and so on, and the composites' good insulating properties, chemical resistance and other properties make them have more performance advantages for application in the sea surface, saline and alkaline land and other fields.

At present, a number of companies have completed the relevant certification, in part of the mass production stage, the composite frame has even become a special selling point of photovoltaic components.

At present, polyurethane pultruded composite material in the photovoltaic frame and stent to achieve large-scale application, still need to solve some problems, on the one hand, long-term weathering and strength and other properties of the full verification, is mainly relying on third-party testing and certification endorsement; on the other hand, the current polyurethane pultruded molding speed is generally less than 1m/min for the installed demand of the GW level at any time, it is difficult to meet the production needs, which requires pultrusion molding production equipment continues to progress. This requires continuous progress of pultrusion molding production equipment.