Surface Felt: The Invisible Guardian in High-end Fields Such As Wind Power.

Fiberglass surface felt, as a key supporting material in the field of composite materials, plays an irreplaceable role in improving the surface quality of composite products, enhancing interlayer bonding strength, and optimizing protective performance due to its uniform fiber distribution, excellent air permeability, and good resin wetting properties. Its materials encompass various types, including glass fiber, polyester fiber, and carbon fiber, and its production process is becoming increasingly mature. Its applications have penetrated multiple high-growth sectors such as wind power, automotive, chemical, and construction. In terms of market size, surface felt, as a core sub-category of felt products, benefits from the rapid development of the downstream composite materials industry, showing steady growth in both the global and Chinese markets, bringing broad development opportunities to industry enterprises.

 

I. Core Application Scenarios: Composite Material Molding for Wind Turbine Blades

 

Driven by global "dual carbon" goals, the wind power industry is entering a period of rapid development. As a core component of wind turbine generators, wind turbine blades need a design life of 20-25 years or more and must withstand long-term exposure to complex environments such as strong wind loads, ultraviolet radiation, and alternating temperature and humidity changes, placing stringent requirements on the molding quality of composite materials. Surface felt, as a key reinforcing and protective material in the molding of wind turbine blade composites, is widely used in core parts such as blade shells, tips, and leading edges. Its application value is mainly reflected in the following three aspects:

 

(I) Improving Surface Smoothness and Aerodynamic Performance: The aerodynamic performance of wind turbine blades directly determines power generation efficiency, requiring a blade surface roughness Ra≤15μm. Fiberglass surface felt, with its fine fibers and uniform spreading, is laid as the outermost layer in the blade composite layup process, effectively masking the texture of the underlying reinforcing material (such as fiberglass cloth) and filling micropores. After resin impregnation and curing, it forms a smooth, flat surface, reducing wind resistance and improving the blade's aerodynamic efficiency. Simultaneously, the smooth surface reduces damage from rainwater erosion and dust abrasion, extending service life.

 

(II) Enhancing Interlaminar Bond Strength and Structural Stability: Wind turbine blades must withstand repeated bending and torsional loads during operation, and interlaminar delamination is a common failure mode. The surface felt fibers are randomly distributed, forming a "bridge" between composite layers to effectively transfer stress and improve interlaminar shear strength and impact resistance. Furthermore, the good wettability of the surface felt with the resin ensures uniform resin distribution, avoiding defects such as insufficient resin or bubbles, enhancing the overall integrity and stability of the blade structure, and ensuring safe operation under long-term loads.

 

(III) Optimizing Protective Performance and Durability Wind turbine blades are exposed to the outdoor environment for extended periods, requiring resistance to various forms of corrosion, including UV aging, damp heat corrosion, and salt spray erosion (offshore wind power). The surface felt, combined with a specialized resin, forms a dense protective layer on the blade surface, reducing the degradation effect of UV radiation on the matrix resin, minimizing moisture penetration, and improving the blade's aging and corrosion resistance. For offshore wind turbine blades, the use of glass fiber surface felt with excellent salt spray resistance can effectively delay the erosion of the blade by the marine environment, ensuring its service life under harsh conditions.

 

(IV) Key Application Processes In the wind turbine blade molding process, the laying of the surface felt is usually combined with processes such as hand lay-up molding and vacuum injection molding. First, a gel coat is evenly applied to the surface of the blade mold. After the gel coat has cured to a semi-dry state, a surface mat is laid, ensuring a tight bond between the surface mat and the gel coat, free of wrinkles and bubbles. Then, reinforcing materials such as fiberglass cloth and unidirectional cloth are laid. A vacuum injection process is then used to fully impregnate all reinforcing materials with resin. Finally, the mat is cured by heat, demolded, and trimmed to complete the molding process. In terms of material selection, surface mats for wind turbine blades primarily use E-glass fiber, with fiber diameters typically ranging from 9-13μm, lengths from 6-25mm, and a basis weight range of 30-120g/㎡, balancing molding quality and structural strength.

 

II. Mainstream Manufacturing Processes: Detailed Explanation of Wet Molding Process

 

The manufacturing processes for surface mats are mainly divided into wet molding, dry molding, and hot pressing. Among these, the wet molding process has become the mainstream production process for surface mats used in wind turbine blades and high-end fiberglass products due to its advantages such as smooth product surface, uniform fiber distribution, and good resin impregnation. In particular, the vast majority of fiberglass surface mats are produced using the wet molding process. The following details the process flow, core technologies, and key equipment for wet-process molding of fiberglass surface mats.

 

The global market for fiberglass surface mats, segmented by product type, is dominated by wet-process technology.

 

(I) Definition and Advantages of Wet-Process Molding Wet-process molding is a nonwoven fabric production method developed from papermaking technology. It uses chopped glass fibers as the main raw material. The fibers are dispersed in an aqueous solution, special chemical additives are added, and the process involves molding, dehydration, sizing, drying, and curing to produce a mat-like material. The core advantages of this process are: the fibers can be fully dispersed in water, resulting in a uniform fiber distribution and consistent thickness in the surface mat; the product has excellent air permeability, facilitating rapid resin penetration and effectively eliminating air bubble defects during composite material molding; and it can produce ultra-thin surface mats (gram weight < 50 g/m²), meeting the surface molding requirements of different products.

 

(II) Complete Process Flow Chart The complete process flow for wet forming of glass fiber surface mat is as follows: Fiberglass stubble cutting → Weighing and batching → Slurry preparation (dispersion, slurry storage) → Sizing → Dehydration and forming → Sizing → Drying and curing → Slitting → Winding → Packaging. The core functions and key operational points of each critical process are as follows:

 

1. Fiberglass Stubble Cutting Continuous glass fiber filaments (mainly E-glass or C-glass) are selected and stubbled using longitudinal rolling cutters or transverse cutting cutters, controlling the fiber length within the range of 3-25mm (6-13mm is commonly used for surface mats for wind turbine blades), with a single filament diameter of 3-16μm. The moisture content of the stubbled fiber needs to be controlled at 6%-10% to avoid affecting the subsequent dispersion effect. To improve the dispersibility of the fiber in water, some filaments are pre-treated with a water-soluble wetting agent. Stubbled fibers containing wetting agents can be stored for more than 6 months; if water-drawn stubbled fibers (without wetting agents) are used, they must be used within 72 hours, otherwise surface hydrolysis will lead to adhesion and decreased strength.

 

2. Pulping Process (Core Process) The core objective of the pulping process is to uniformly disperse chopped glass fibers in an aqueous solution to form a stable fiber suspension. This process requires strict control of parameters such as pulp concentration, pH value, and the amount of chemical additives.

 

3. Sizing and Dewatering Molding The prepared pulp is pumped to a dilution tank for further dilution to the required molding concentration (0.01%-1.0%), and then fed into a pre-mesh box. The pre-mesh box is equipped with stepped diffusers and perforated plates to achieve uniform lateral distribution of the pulp. Subsequently, the pulp is carried away by a continuously running inclined forming conveyor belt, where it undergoes gravity dewatering and vacuum dewatering in two forced dewatering tanks to form wet felt. During dewatering, the dewatering speed is controlled by adjusting the perforated plates in the dewatering tanks to ensure the lateral uniformity of the wet felt; the dewatering width is controlled by adjusting the baffles in the dewatering tanks to match subsequent production requirements.

 

4. Sizing Process After dewatering, the wet felt is sent to an impregnation machine for sizing treatment. The core purpose of sizing is to bond the dispersed fibers together using an adhesive, thereby improving the mechanical strength of the surface felt and its bonding performance with the resin. Commonly used adhesives include urea-formaldehyde resin, polyvinyl acetate emulsion, and acrylic resin, with the appropriate type selected based on the application scenario of the surface felt. For example, surface felt for wind turbine blades often uses a mixture of acrylic resin emulsion and coupling agent (such as KH560) to improve compatibility and interfacial bonding strength with epoxy resin. The sizing method uses overflow sizing; the adhesive is pumped via a diaphragm pump to the overflow applicator above the sizing belt, uniformly wetting the wet felt. Excess adhesive is recovered through a suction box, separated in a separation tank, and recycled. The adhesive content in the final product is controlled below 30%.

 

5. Drying, Curing, and Post-treatment The sizing wet felt is then sent to a drying oven, where hot air penetration convection drying is used. The drying oven is divided into a preheating zone and multiple drying and curing zones, each with independently controllable temperature. The drying temperature and airflow are automatically adjusted by a computer to ensure complete curing of the adhesive while preventing fiber oxidation and degradation. After drying, the surface felt is cut to the required width by a slitting machine, then wound into rolls by a winding machine, and finally packaged, inspected, and stored.

 

(III) Key Equipment and Quality Control Key equipment for the wet molding process includes: a chopped strand machine, a dispersion tank, a suspension mixing tank, a wire mesh box, an inclined wire mesh forming line, an impregnation machine, a drying oven, a slitting machine, and a winding machine. Among these, the wire mesh speed of the inclined wire mesh forming line, the uniformity of the slurry distribution in the wire mesh box, and the temperature control accuracy of the drying oven directly affect product quality. Key quality control points include: ensuring dispersion effect through fiber dispersion grade evaluation (divided into 5 grades, with the best grade being completely dispersed into single fibers); real-time monitoring of product thickness and weight uniformity using thickness gauges and basis weight meters; verifying product mechanical properties through tensile strength testing; and ensuring that resin wetting requirements are met through air permeability testing.

 

III. Surface Felt Market Size and Development Trends

 

The surface felt market size is deeply intertwined with the development of downstream composite material applications. In recent years, driven by global "dual carbon" goals, demand from wind power, new energy vehicles, green buildings, and other fields has surged, directly driving the continuous expansion of the surface felt market. From a market hierarchy perspective, the market can be divided into three dimensions: the global market, the Chinese regional market, and core segmented markets. Specific data and characteristics are as follows:

 

(I) Steady Growth in Global Market Size

From the overall felt product market structure, surface felt, as a core segment of industrial felt, occupies a significant share. Focusing on core application areas, growth is mainly driven by rigid demand from wind turbine blades, composite pipes, and shipbuilding.

 

For fiberglass surface felt, in terms of global market size and application segmentation, composite pipes are the largest downstream market.

 

(II) China Holds a Core Global Position

As the world's largest producer and exporter of composite materials, China is also a major producer and consumer of surface felt, contributing over 28% of global surface felt production capacity. From an industrial perspective, the Yangtze River Delta and Pearl River Delta regions of China have gathered over 600 large-scale felt product enterprises, most of which are involved in surface felt production, with an annual output exceeding 800,000 tons, forming a complete industrial chain. On the demand side, with the continuous increase in domestic wind power installed capacity, the rapid growth in the penetration rate of new energy vehicles, and the in-depth promotion of green building policies, the demand for surface felt has maintained rapid growth. In particular, fiberglass surface felt, as a key material for wind turbine blades and new energy vehicle structural components, has seen its market size growth rate significantly exceed the industry average. Furthermore, Chinese surface felt products not only meet domestic demand but are also exported in large quantities to Southeast Asia, Africa, and other regions, giving them a strong cost-performance advantage in the global market.

 

(III) Demand from Sub-sectors Drives Market Structure Upgrading

From the perspective of sub-sector demand, wind power, construction, and automotive are the core consumer markets for surface felt, contributing a combined share of over 70%. Among them, the demand growth in the wind power sector is the most prominent. With the rapid increase in global wind power installed capacity, the demand for fiberglass surface felt for wind turbine blades has continued to surge, with domestic wind power surface felt consumption increasing by over 18% year-on-year in 2024. In the construction sector, the global market size for construction felt reached US$6.8 billion in 2023. Surface felt, a key material for roof waterproofing and exterior wall insulation, saw its penetration rate in green buildings jump from 18% in 2020 to 34% in 2023. Driven by the EU's building energy efficiency directive and domestic green building policies, demand in this sector is expected to maintain an annual growth rate of 12%. In the automotive sector, benefiting from the trend towards lightweighting in new energy vehicles, demand for high-end surface felt products such as battery pack insulation felt and engine compartment sound insulation felt has surged. The global automotive felt market size is projected to reach US$3.1 billion by 2025, with surface felt accounting for over 30%. Furthermore, emerging fields such as chemical corrosion protection and medical protection are also providing new growth drivers for the surface felt market. The demand for corrosion-resistant fiberglass felt in the chemical industry is increasing at a rate of 4.5% annually, and this segment is expected to exceed US$1.2 billion by 2025.

 

IV. Industry Development Outlook

 

Considering the current market growth trend and changes in downstream demand, the demand for surface felt is expected to maintain a steady upward trend. In the future, the development direction of the surface felt industry will focus on three aspects: First, high performance, improving the aging resistance, corrosion resistance, and high strength of products through raw material modification and process optimization to adapt to high-end application scenarios such as offshore wind power and aerospace; second, green and environmentally friendly development, researching and developing environmentally friendly raw materials such as formaldehyde-free adhesives and biodegradable fibers, optimizing production processes, and reducing wastewater and exhaust emissions; and third, customized services, providing personalized product solutions based on the specific needs of different application scenarios. Member companies of the China Composites Industry Association will play a core role in technological innovation and supply chain collaboration, leveraging the huge domestic market demand to further enhance their global market competitiveness and promote the high-quality development of the surface felt industry.