High Strength and High Modulus Polyvinyl Alcohol (PVA) Fibers

Abstract

This paper introduces the production process, chemical reactions and microstructure of high strength and high modulus PVA fibers manufactured by Inner Mongolia Shuangxin Environmental Protection Materials Co., Ltd. It also details the domestic and international production processes of high strength and high modulus PVA fiber cement and elaborates on their characteristics. Based on relevant application cases at home and abroad, development suggestions for China's PVA special fiber cement are summarized and proposed.

Preface

Since the early 1980s, the harmfulness of asbestos to human health has attracted great attention, and the international community has been committed to the research, development and promotion of non-asbestos fiber cement products. At present, most EU countries, as well as the United States, Japan, Australia and other countries have banned the production and use of asbestos cement products, which have been replaced by non-asbestos fiber cement products. Developing countries such as China and Brazil are also actively engaged in the R&D and application promotion of non-asbestos fiber cement products. Especially in recent years, the development of fiber cement products in Vietnam, Thailand and other Southeast Asian countries has been more active. South Korea is also actively seeking enterprises and partners in China that can produce and export non-asbestos fiber cement products. More and more domestic enterprises are gradually shifting from asbestos cement products to non-asbestos fiber cement products, and some of them are already mass-producing and exporting non-asbestos fiber cement products.

At present, there are still different views on the safe use of asbestos in China and Russia. However, judging from the international development trend, the transition from asbestos cement products to non-asbestos fiber cement products is irreversible. Initially, high strength and high modulus polyvinyl alcohol (PVA) fibers and modified polyacrylonitrile fibers were used in the industry to replace asbestos. However, the strength (7–8 cN/dtex) and Young's modulus (140–180 cN/dtex) of modified polyacrylonitrile fibers are lower than those of special fibers (10–14 cN/dtex in strength and 240–330 cN/dtex in Young's modulus), thus limiting the application of modified polyacrylonitrile fibers. According to long-term experimental research and production practice, Eternit AG of Switzerland has proved that PVA fiber is the most ideal substitute for asbestos.

Inner Mongolia Shuangxin Environmental Protection Materials Co., Ltd. is one of the few companies in the world that can industrially produce high strength and high modulus PVA fibers. Its products have been exported to more than 30 countries and regions in North America, South America, Europe, Australia and Southeast Asia, and are deeply favored by downstream manufacturers. The company is investing a great deal of human and material resources in R&D of special high strength and high modulus PVA fibers to meet the needs of various fields.

The specific performance indicators of PVA fibers commonly used in the construction field produced by Inner Mongolia Shuangxin Environmental Protection Co., Ltd. are shown in Table 1. SX4 refers to PVA fibers with a length of 4 mm, SX6 to 6 mm, and SX8 to 8 mm.

1 High Strength and High Modulus Polyvinyl Alcohol (PVA) Fibers

Free vinyl alcohol is extremely unstable and cannot exist independently. Therefore, to obtain practically useful polyvinyl alcohol, vinyl acetate is usually polymerized as a monomer, followed by alcoholysis or hydrolysis to produce polyvinyl alcohol. Inner Mongolia Shuangxin Environmental Protection Materials Co., Ltd. uses calcium carbide to produce acetylene and acetic acid as raw materials. At about 200 °C and atmospheric pressure, the gas is passed through zinc acetate catalyst supported on activated carbon to produce vinyl acetate.

The specific process flow is shown in the following chemical equations:

• Reaction of acetylene and acetic acid
• Polymerization of vinyl acetate
• Alcoholysis of polyvinyl acetate
• Also included are:
• Schematic diagram of the unit cell structure of high strength and high modulus PVA fibers
• SEM micrograph of high strength and high modulus PVA fibers
• Micrograph of the combination of high strength and high modulus PVA fibers and matrix

2 Production Processes of High Strength and High Modulus PVA Fiber Cement Products

According to the characteristics of manufacturing processes, the production of high strength and high modulus PVA fiber cement products can be divided into the following categories:

(1) Hatschek Process: Modified from the traditional wet process for asbestos cement products to produce non-asbestos fiber cement products. The Hatschek process is still the most widely used process internationally.

(2) Flow-on Process: The main advantage of this technical route is lower investment cost. Some enterprises in Guangdong, China adopt this method to produce non-asbestos fiber cement products.

(3) Extrusion Process: Improved from the traditional extrusion process for certain asbestos cement products. This process adds polymer plasticizers to the mixture to meet the plasticity requirements for extrusion molding. Since such plasticizers account for a high proportion of raw material costs, only a few enterprises worldwide use this method to produce non-asbestos fiber cement products. In China, Beijing New Building Materials also adopts this process to produce fiber cement products.

(4) Dry & Semi-Dry Process: Originally developed by a Japanese enterprise and currently only used by this company internationally. Its characteristics are that cement, fibers and other admixtures are dry-mixed or stirred with a minimal amount of water, then formed under pressure. The water consumption is based on the theoretical water demand for cement hydration, hence the name dry or semi-dry process. Products produced by this method feature high performance and quality, holding a large market share in Japan.

3 Characteristics of High Strength and High Modulus PVA Fiber Cement Products

The elastic modulus of high strength and high modulus PVA fibers is second only to steel fibers, glass fibers, carbon fibers and ultra-high molecular weight polyethylene fibers. Their tensile strength is comparable to steel fibers, glass fibers, carbon fibers, ultra-high molecular weight polyethylene fibers and para-aramid fibers. PVA fibers have the advantages of low cost, low density, high adhesion, good dispersibility and easy mixing.

Compared with other synthetic fibers, PVA fibers have good hydrophilicity, high elastic modulus and large specific surface area, offering excellent compatibility with cement and remarkable reinforcing effects. In addition, they have good light resistance, alkali resistance and excellent chemical stability. With the continuous development of high-strength PVA fibers, their applications have received increasing attention, especially in cement-based materials, showing promising prospects.

High strength and high modulus PVA fibers can reinforce cement mainly because:

(1) High tensile strength and modulus;

(2) Good chemical compatibility with Portland cement;

(3) Excellent hydrophilicity, enabling uniform dispersion in cement matrix;

(4) Strong interfacial bonding between high-strength PVA fibers and cement matrix, due to:

(a) The non-circular and irregular cross-section enlarges the bonding area between PVA fibers and cement matrix;

(b) The -C-OH groups in the PVA fiber molecular structure form strong hydrogen bridges with -OH groups in cement hydrates.

Cement products and building materials reinforced with high strength and high modulus PVA fibers offer the following advantages:

(1) Good mechanical properties, improving toughness and impact resistance. The flexural strength of building materials can be increased by 200%, from 195 kg/cm² to 225 kg/cm², with enhanced elastic fatigue resistance and crack prevention.

(2) Excellent acid and alkali resistance, suitable for all grades of cement.

(3) Good dispersibility, maintaining a smooth surface without spalling for a long time.

(4) Improved bending temperature resistance and cold resistance of cement boards and bricks by 50%.

(5) Significantly improved operating conditions.

(6) The dosage of PVA is only one-fifth of asbestos, reducing the unit weight of products.

(7) Low permeability of concrete, preventing corrosion of steel bars and improving durability against weathering and climate effects.

4 Main Applications of High Strength and High Modulus PVA Fiber Cement Products

Cement mixed with a certain proportion of high strength and high modulus PVA fibers is defined as PVA fiber cement, which can be used for:

(1) Mass concrete construction such as high-standard airport runways, high-grade highways, aprons, long-span bridges, curved roofs, transfer beams, columns and floor slabs of high-rise buildings, river dikes, ports and wharves, mine tunnels, culverts, water storage tanks and swimming pools.

(2) Manufacturing various cement products, including roofs, colored tiles, decorative wall panels, lightweight partition boards, floors, floor tiles, indoor ceilings, large-diameter sewers, water pipes and joints, urban sculptures, greenhouse supports, fireproof boards, ventilation ducts, manhole covers and frames.

(3) Production of commercial dry-mixed mortars such as anti-cracking finishing mortars.

4.1 Exterior Decorative Cladding Panels Made of High Strength and High Modulus PVA Fiber Cement

High strength and high modulus PVA fiber cement exterior decorative cladding panels feature:

(1) Weather resistance – long-term aesthetic retention;

(2) Sound insulation – effective noise isolation;

(3) Thermal insulation – excellent thermal performance;

(4) Safety – asbestos-free;

(5) Durability – stable strength;

(6) Fire resistance – A1 fire rating;

(7) Seismic resistance – safety for dry-hung installation.

From the perspectives of environmental protection, energy conservation and safety, the demand for dry-constructed PVA fiber cement cladding panels will continue to grow. Notably, these panels possess a unique self-cleaning function. The photocatalytic nano-coating on the surface has stronger hydrophilicity than contaminants, forming a water film between contaminants and the panel surface to prevent adhesion of hydrophobic substances such as soot. Rainwater washes away contaminants along with the water film. These panels have been used in Japan for over 40 years, achieving a service life matching that of buildings. Installation is simple, requiring only single-direction steel or wooden keels with a wall thickness of 1.6–2.3 mm, reducing steel consumption by 75% and cutting costs while improving efficiency.

4.2 High Strength and High Modulus PVA Fiber Engineered Cementitious Composites (PVA-ECC)

PVA-ECC refers to cementitious composites using cement, cement with fillers, or cement plus fillers and fine aggregates as the matrix, reinforced with PVA fibers. Studies at home and abroad show that PVA-ECC achieves ultra-high toughness with a low fiber dosage, exhibiting tensile strain greater than 3%, multiple cracking with crack widths less than 3 mm, and excellent crack resistance. It also features high temperature resistance, wear resistance, fatigue resistance and environmental friendliness.

Due to increasing traffic loads, environmental changes (freeze-thaw cycles), chloride ion corrosion and other factors, roads and bridges worldwide face durability issues and huge economic losses from maintenance and reconstruction. Tests and applications demonstrate that PVA-ECC effectively mitigates the shortcomings of conventional concrete through superior wear resistance, deformability and durability.

PVA-ECC is widely used in oil and gas pipeline coatings, irrigation canal repair, shear structures of super high-rise buildings, infill walls, beam-column joints, etc.

4.3 Anti-Cracking Modified Mortar with High Strength and High Modulus PVA Fibers

Foamed concrete is widely used in building partitions due to its thermal insulation, sound absorption and light weight. However, improper construction techniques (treating foamed concrete blocks as traditional materials) often cause quality problems such as peeling, wall cracking and hollowing, mainly due to its "ink bottle" pore structure.

High strength and high modulus PVA fiber anti-cracking modified mortar effectively solves these problems. Adding PVA fibers increases the consistency of finishing mortar and restrains the sinking of heavier particles in cementitious materials. PVA fibers form a network structure in fresh mortar, restraining aggregate settlement.

As flexible synthetic fibers with excellent hydrophilicity and strong bonding with cementitious materials, PVA fibers enhance the viscosity and consistency of mortar, restrict aggregate movement and prevent flow through interlocking with the matrix, thus eliminating cracking, hollowing and delamination.

Conclusion

High strength and high modulus PVA fibers can replace asbestos in the production of asbestos-free fiber cement boards, tiles and pipes. However, domestic research on the application of high strength and high modulus PVA fibers remains at a relatively low level.

Inner Mongolia Shuangxin Environmental Protection Materials Co., Ltd., targeting world-class PVA fiber technology, has increased investment and cooperated with domestic universities and foreign research institutions to catch up rapidly.

The use of high strength and high modulus PVA fibers to replace carcinogenic asbestos is superior in performance, complies with national industrial policies and belongs to encouraged projects. At present, most domestic high strength and high modulus PVA fibers are exported, with limited domestic demand. Further market development and policy support are needed to promote the sustainable and sound development of the PVA fiber industry.

References

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