I. Core Factors Influencing the Waterproof Performance of Woven Membranes: A Comprehensive Analysis from Raw Materials to Processes

The waterproof performance of woven membranes is not determined by a single step, but rather by the combined effects of raw material characteristics, production processes, product structural design, and post-treatment. The selection and control of each step directly or indirectly affects the penetration path and blocking efficiency of water molecules. Only by precisely controlling the key nodes throughout the entire chain can a balance be achieved between waterproof performance, cost, and other functions.

(I) Raw Material Characteristics: The "Foundation" of Waterproof Performance

Raw materials are the core prerequisite for determining the waterproof performance of woven membranes. The molecular structure and density of the resin, as well as the type and ratio of functional additives, directly affect the microstructure density and water molecule blocking ability of the membrane material, forming a crucial line of defense for waterproof performance control.

1. Resin Molecular Structure: The "Source Guarantee" of Waterproof and Dense Properties

The arrangement, branching degree, and crystallinity of the resin's molecular chains determine the size and distribution of the micropores in the woven membrane, thus affecting the difficulty of water molecule penetration. This is a key intrinsic factor influencing waterproof performance:

Molecular Chain Arrangement and Density: The tighter the resin molecular chain arrangement and the higher the density, the smaller the micropores in the membrane material, and the stronger the waterproof performance. HDPE (High-Density Polyethylene) resin, due to its high molecular chain linearity and tight arrangement, has a density of 0.941-0.965 g/cm³, resulting in woven membranes with significantly better waterproof performance than PP (Polypropylene) resin (density 0.900-0.915 g/cm³) and LLDPE (Linear Low-Density Polyethylene) resin (density 0.910-0.925 g/cm³). For example, the water vapor transmission rate (PVP) of HDPE woven film of the same thickness (100μm) is typically 3-5 g/(m²·24h), while that of PP woven film can reach 8-12 g/(m²·24h), and that of LLDPE woven film is 6-9 g/(m²·24h). A chemical company used HDPE woven film to package moisture-absorbing polyethylene granules, and after 6 months of storage, the moisture content of the raw materials was still below 0.1%; however, after switching to PP woven film, the moisture content of the raw materials rose to 0.3% under the same conditions, and slight clumping occurred.

Branching degree: The lower the branching degree, the easier it is for the molecular chains to arrange themselves tightly, and the better the waterproof performance. HDPE resin has extremely low branching (almost no branches), allowing its molecular chains to pack tightly, forming a dense barrier layer. LLDPE resin, due to the presence of short branches (20-30 branches/1000C), has increased spacing between molecular chains, resulting in slightly inferior waterproofing performance compared to HDPE. While PP resin also has low branching, the presence of methyl side groups on its molecular chains leads to less compact packing than HDPE, resulting in relatively weaker waterproofing performance. For example, in building waterproofing applications, the permeability coefficient of HDPE geomembranes is typically less than 1×10⁻¹¹ cm/s, while that of LLDPE geomembranes is approximately 1×10⁻¹⁰ cm/s, and that of PP geomembranes is 5×10⁻¹⁰ cm/s. HDPE membranes are more suitable for applications with stringent waterproofing requirements, such as landfills and artificial lakes.

Crystallization: Higher crystallinity results in more regular arrangement of resin molecular chains, stronger membrane density, and better waterproofing performance. HDPE resin has a crystallinity of 70%-80%, significantly higher than PP resin (50%-60%) and LLDPE resin (30%-40%), thus exhibiting a substantial advantage in waterproof performance. Further enhancing waterproofing performance can be achieved by adjusting process parameters to increase resin crystallinity. For example, one company, in producing HDPE Waterproof woven membranes, optimized the cooling rate (from 5℃/min to 10℃/min), increasing the resin crystallinity from 75% to 82%, and reducing the membrane's water vapor permeability from 4 g/(m²・24h) to 2.5 g/(m²・24h), resulting in a 37.5% improvement in waterproofing performance.

2. Functional Additives: "Reinforcing Support" for Waterproofing Performance

By adding specific functional additives, the waterproofing performance of woven membranes can be specifically improved, or the bonding stability between the waterproofing layer and the base membrane can be enhanced. Common additive types include waterproofing agents, densifying agents, and interface compatibilizers:

Waterproofing Agents: Adding waterproofing agents (such as silanes and fluorocarbons) directly to the resin can form a hydrophobic layer on the membrane surface, reducing water molecule adsorption and penetration. The addition ratio of silane waterproofing agents is typically 0.5%-1.0%, which can increase the surface contact angle of the woven membrane from 70°-80° to 100°-110°, significantly reducing surface hydrophilicity; the addition ratio of fluorocarbon waterproofing agents is 0.3%-0.8%, which can increase the surface contact angle to over 120°, resulting in a more durable waterproofing effect, but at a higher cost. For example, an outdoor product company added 0.8% silane-based waterproofing agent to its PP woven membrane. The resulting outdoor sunshade film exhibited a water permeability of only 0.5 L/m² within 24 hours during heavy rain; in contrast, a similar product without the added waterproofing agent had a permeability of 5 L/m², failing to meet outdoor usage requirements.

Denseizing Agents: Adding denser agents (such as nano-calcium carbonate or talc, with a particle size of 10-50 nm) can fill the gaps between resin molecular chains, improving the membrane's density and reducing water molecule penetration channels. The addition ratio of nano-calcium carbonate is typically 2%-5%, which can reduce the water vapor permeability of HDPE woven membranes by 15%-20%; the addition ratio of talc is 3%-6%, which has a more significant effect on improving the waterproof performance of PP woven membranes. For example, a packaging company added 4% nano-talc powder to PP woven film, reducing the film's water vapor permeability from 10g/(m²・24h) to 8.2g/(m²・24h), improving waterproof performance by 18%, while only increasing costs by 3%, making it suitable for cost-sensitive food packaging applications.

Interfacial compatibilizers: Adding interfacial compatibilizers (such as maleic anhydride-grafted PP or PE) to composite woven films can improve the bonding tightness between the waterproof functional layer (such as PE Film or aluminum foil) and the woven base film, preventing the formation of water molecule penetration channels due to interfacial gaps. The compatibilizer addition ratio is typically 1%-3%, which can increase the interfacial peel strength of the composite film from 1.5N/15mm to over 3N/15mm, significantly enhancing the stability of waterproof performance. For example, when a company produced PP/PE composite waterproof woven membrane, without adding a compatibilizer, tiny gaps existed at the composite interface. After 3 months of use, the water vapor transmission rate increased from 5 g/(m²・24h) to 8 g/(m²・24h). After adding 2% maleic anhydride-grafted PP compatibilizer, the interface bonded tightly, and after 6 months of use, the water vapor transmission rate remained stable at 5.2 g/(m²・24h).

(II) Production Process: "Process Guarantee" of Waterproof Performance

The production process is the key step in transforming raw materials into woven membranes with specific waterproof properties. From extrusion, stretching, weaving to lamination (or coating), precise control of process parameters at each step affects the membrane's microstructure, density, and interfacial bonding, thus determining the product's waterproof performance. Deviations in process parameters are one of the main reasons for insufficient waterproof performance.

1. Extrusion Process: The "Key to Forming" for Waterproof Layer Density

For single-layer waterproof woven membranes (such as HDPE waterproof membranes), the extrusion process, by controlling temperature, screw speed, and die pressure, determines the density and surface smoothness of the flat filaments, directly affecting waterproof performance:

Extrusion Temperature: This needs to be precisely set according to the resin type to ensure the resin fully melts and does not degrade, forming a dense flat filament structure. The extrusion temperature of HDPE resin is typically 160-200℃. If the temperature is too high (e.g., exceeding 220℃), the resin will undergo thermal oxidative degradation, molecular chain breakage, and micropores will appear inside the flat filaments, potentially increasing water vapor permeability by 20%-30%. If the temperature is too low (e.g., below 150℃), the resin will not melt sufficiently, resulting in unmelted particles inside the flat filaments, forming pores and reducing waterproof performance. For example, when a company produces HDPE waterproof flat yarn, increasing the extrusion temperature from 180℃ to 230℃ increases the water vapor transmission rate of the flat yarn from 3 g/(m²・24h) to 3.8 g/(m²・24h); when the temperature drops to 140℃, the unmelted particle rate inside the flat yarn reaches 3%, and the water vapor transmission rate increases to 4.2 g/(m²・24h).

Screw speed: Affects the plasticizing effect and molecular chain arrangement of the resin, and is usually controlled between 30-60 r/min. Too high a speed (e.g., >80 r/min) shortens the resin residence time in the screw, resulting in uneven plasticizing, unevenness on the surface of the flat yarn, increased water molecule adsorption area, and decreased waterproof performance; too low a speed (e.g., <20 r/min) results in low production efficiency, and the resin may degrade due to excessive residence time, affecting density. For example, to increase production, a company increased the extrusion screw speed of HDPE flat yarn from 50 r/min to 90 r/min. This increased the surface roughness (Ra) of the flat yarn from 0.8 μm to 1.5 μm, and the water vapor transmission rate from 3.2 g/(m²・24h) to 3.9 g/(m²・24h). When the speed was reduced to 15 r/min, the flat yarn showed slight degradation, turning yellow, and the water vapor transmission rate increased to 3.7 g/(m²・24h).

Die pressure: Die pressure is typically controlled between 10-20 MPa. Higher pressure results in a more compact arrangement of resin molecular chains, stronger flat yarn density, and better waterproof performance. If the pressure is too low (e.g., <8MPa), the flat yarn structure becomes loose, with increased internal pores, potentially increasing water vapor permeability by 15%-25%. If the pressure is too high (e.g., >25MPa), the flat yarn is prone to excessive stretching, resulting in micro-cracks on the surface, which negatively impacts waterproof performance. For example, when a company was producing HDPE waterproof flat yarn, reducing the die pressure from 15MPa to 7MPa increased the water vapor permeability from 3.1g/(m²・24h) to 3.9g/(m²・24h). When the pressure was increased to 28MPa, micro-cracks appeared on the surface of the flat yarn, and the water vapor permeability increased to 3.8g/(m²・24h).