Silane coupling agents are a class of compounds that can "bridge" inorganic materials and organic materials through chemical action. Their core function is to improve the interfacial bonding strength between two materials of different polarity. They are widely used in composite materials, coatings, adhesives, textiles, and other fields. Their application effects are closely related to the hydrolysis process and addition ratio, and there are significant differences in practical methods due to industry demand variations. The following provides a detailed analysis from three dimensions: hydrolysis process, addition ratio, and industry differences.

1. Hydrolysis Process of Silane Coupling Agents: Principles and Key Points

The molecular structure of silane coupling agents is usually "Y-R-Si-(OR')₃" (Y is an organic functional group, such as amino, epoxy, etc.; R is an alkylene group; OR' is a hydrolyzable alkoxy group, such as methoxy, ethoxy). Its function premise is that the alkoxy group (OR') first hydrolyzes into silanol (Si-OH), and the silanol then condenses with hydroxyl groups (-OH) on the surface of inorganic materials (such as glass, metal, fillers) to form chemical bonds; simultaneously, the organic functional group (Y) reacts with organic substrates (such as resin, rubber), thus achieving "bridging." Therefore, hydrolysis is the core step in the application of silane coupling agents, and process details directly affect its effectiveness.

1. Basic Principle of Hydrolysis

The alkoxy group undergoes hydrolysis in the presence of water:

Si-(OR')₃ + 3H₂O → Si-(OH)₃ + 3R'OH (e.g., methoxy hydrolyzes to methanol, ethoxy to ethanol)

The hydrolyzed silanol (Si-OH) has high reactivity but is also prone to self-condensation (Si-OH + HO-Si- → Si-O-Si- + H₂O). Excessive condensation forms polymers insoluble in the substrate, causing coupling failure. Therefore, the hydrolysis process must balance "hydrolysis efficiency" and "avoiding excessive condensation."

2. Key Factors Affecting the Hydrolysis Process

The hydrolysis effect is mainly influenced by water amount, pH value, temperature, and solvent. Adjustments are needed based on silane type (e.g., amino, epoxy, mercapto):

Water amount: Theoretically, 1 mol of silane requires 3 mol of water for complete hydrolysis, but in practice, excess water is needed (usually the molar ratio of water to silane is 5:1 to 10:1). Excess water can increase the hydrolysis rate and prevent condensation of hydrolysis products due to water deficiency. Insufficient water leads to incomplete hydrolysis, leaving residual alkoxy groups unable to react with inorganic materials; too much water may dilute the system, affecting compatibility with the substrate.

pH value: It is the core factor controlling hydrolysis rate and condensation degree. Silane hydrolysis needs to be accelerated under acidic or alkaline conditions (neutral water hydrolysis is very slow), but pH must be strictly controlled:

Acidic conditions (pH=3~5): Commonly adjusted with acetic acid or hydrochloric acid, suitable for most silanes (e.g., epoxy silane KH-560, vinyl silane KH-570). In acidic environments, H⁺ acts as a catalyst to promote alkoxy bond cleavage, and hydrolysis products are more stable with slower condensation rates, suitable for scenarios requiring long-term storage of hydrolysate (e.g., coating pretreatment).

Alkaline conditions (pH=8~10): Commonly adjusted with ammonia or NaOH, suitable for amino silane (e.g., KH-550). Amino silane itself contains basic groups and hydrolyzes faster under alkaline conditions, but note: excessively strong alkalinity (pH>11) accelerates silanol condensation, causing the hydrolysate to quickly become turbid (forming Si-O-Si polymers), requiring fresh preparation and immediate use.

Temperature: Increasing temperature can accelerate hydrolysis rate (e.g., 30~60 minutes at room temperature, shortened to 15~30 minutes at 50~60℃), but too high temperature (>80℃) causes: ① rapid solvent evaporation (if used); ② intensified silanol condensation (especially under alkaline conditions). Therefore, most scenarios use "room temperature to 60℃" hydrolysis, with slight heating only for difficult-to-hydrolyze silanes (e.g., long-chain alkoxy silanes).

Solvent: If silane has poor compatibility with water (e.g., long-chain alkyl silanes), organic solvents (such as ethanol, isopropanol, acetone) are added as "media." The solvent-to-water ratio is usually 1:1 to 3:1 (volume ratio). Solvents reduce system surface tension, allowing silane to disperse evenly in water, avoiding uneven hydrolysis caused by silane "floating." Silanes with good compatibility (e.g., KH-550, KH-560) can be hydrolyzed directly with water (adjusting pH with acid or base).

3. Actual Hydrolysis Operation Steps (Using Common "KH-560" as an Example)

Taking "preparing 100g hydrolysate (for coating substrate treatment)" as an example, the steps are as follows:

Take 50g deionized water, add 0.1~0.3g acetic acid (adjust pH to 3~4), stir evenly;

Slowly add 10g KH-560 (mass ratio of silane to water about 1:5), stirring while adding (speed 200~300 r/min);

Stir at room temperature for 30 minutes, observe solution state: if transparent without layering, hydrolysis is complete (if turbid, pH may be improper or stirring insufficient, add a small amount of acetic acid and extend stirring);

The hydrolysate should be used within 24 hours (store at room temperature, avoid exposure to sunlight or high temperature to prevent condensation).

4. Precautions for the Hydrolysis Process

Avoid "excessive hydrolysis": Excessive hydrolysis time (e.g., over 24 hours) or pH loss of control (e.g., overly acidic) causes extensive silanol condensation, turning the hydrolysate from transparent to turbid or even gelled, losing activity, requiring fresh preparation and immediate use;

Water Purity: Deionized water or distilled water is required. If tap water (containing metal ions such as Ca²⁺, Mg²⁺) is used, the metal ions will complex with silanol groups, causing accelerated condensation and reduced stability of the hydrolyzed solution;

Stir evenly: When adding silane, it should be added dropwise slowly and stirred thoroughly to avoid locally high concentrations (excess local silane will rapidly condense due to lack of water, forming particles).

2. Silane Coupling Agent Addition Ratio: No fixed value, must be "scenario-matched"

There is no unified standard for the addition ratio of silane coupling agents. The core principle is "sufficient to cover the surface of inorganic materials without waste (excess leads to decreased compatibility with the organic phase)." The ratio should be determined comprehensively based on the application scenario, substrate type, coupling agent type, and desired performance, usually calculated as "mass fraction relative to the inorganic substrate" or "fraction relative to the total system mass." Common ranges are 0.1%~5%, with special scenarios reaching up to 10%.

1. Core factors affecting the addition ratio

Specific surface area of the inorganic substrate: The larger the specific surface area (such as nano-scale fillers, ultrafine powders), the more coupling agent is needed to cover the surface. For example:

Ordinary calcium carbonate (specific surface area 1~5m²/g): addition ratio is usually 0.5%~1% (relative to filler mass);

Nano silica (specific surface area 50~200m²/g): addition ratio needs to be increased to 2%~5% (to cover more surface hydroxyls).

Type of coupling agent: The longer the molecular chain (such as long-chain alkyl silanes), the smaller the coverage area per unit mass, requiring a higher ratio; conversely, short-chain silanes (such as methyltrimethoxysilane) have high coverage efficiency and the ratio can be reduced.

Application purpose: If "strong interfacial bonding" is required (such as composite reinforcement), the ratio must be sufficient; if only "basic modification" is needed (such as anti-settling in coatings), the ratio can be lower. For example:

Glass fiber reinforced plastics (FRP): to fully cover the glass fiber surface with coupling agent, the ratio is usually 1%~3% (relative to glass fiber mass);

Preventing filler settling in coatings: only a small amount of coupling agent is needed to improve compatibility between filler and matrix, ratio 0.1%~0.5% (relative to total system mass) is sufficient.

Addition method: The "pre-treatment method" (coating the coupling agent on the inorganic substrate surface first) saves more coupling agent than the "direct addition method" (directly adding to the organic matrix). For example, when pre-treating glass fiber, a coupling agent ratio of 0.5%~1% is sufficient; when directly added to resin, 1%~2% is needed (because some coupling agent may not contact the substrate and dissolve directly in the resin).

2. Practical tips for ratio adjustment

First conduct a "gradient experiment": test ratios at 0.1%, 0.5%, 1%, 2%, 5% gradients, determine the optimal value through performance indicators (such as tensile strength, adhesion, water resistance) (usually there is a "peak value" after which performance declines due to excess coupling agent forming a "weak layer" at the interface);

Combine with substrate pre-treatment: if the inorganic substrate surface has oil or impurities, it must be cleaned first (e.g., wiped with ethanol), otherwise the coupling agent will react with impurities and the ratio needs to be increased (possibly 1~2 times);

Synergy with other additives: if the system contains dispersants, plasticizers, etc., which may affect coupling agent adsorption, the ratio should be appropriately increased (e.g., originally 1%, can be increased to 1.5%).

3. Industry differences in silane coupling agent applications: different demands lead to different processes and ratios

Core demands vary greatly among industries (such as mechanical properties, weather resistance, functionality), so there are significant differences in silane coupling agent "hydrolysis process," "addition ratio," and "type selection," requiring targeted adjustments:

1. Construction industry: Focus on "substrate bonding and weather resistance," hydrolysis requires weather resistance adjustment

The construction industry mainly uses it for concrete interface treatment, stone protection, and architectural coatings. The core demands are "improving adhesion" and "resistance to rainwater/UV aging."

Hydrolysis process: mostly uses "weakly acidic hydrolysis" (pH=4~5) because construction scenarios require slightly longer storage time of the hydrolyzed solution (e.g., 1~2 days on construction sites). Under weak acidity, the hydrolyzed solution is more stable (less prone to condensation); ethanol is commonly used as solvent (low cost, easy to volatilize) to avoid residue affecting adhesion.

Addition ratio: relatively low (0.5%~2%) because construction substrates (concrete, stone) have small specific surface area and do not require excess; for example, in concrete interface agents, silane coupling agents (such as KH-550) usually account for 1%~1.5% (relative to total interface agent mass), sufficient to cover surface hydroxyls.

Type selection: prioritize amino silanes (good compatibility with cement-based materials) and alkyl silanes (stone waterproofing, such as isooctyl triethoxysilane).

2. Automotive industry: Focus on "composite strength and aging resistance," mainly pre-treatment

The automotive industry mainly uses it for glass fiber reinforced plastics (such as bumpers) and metal coatings (such as electrophoretic paint). Core demands are "improving composite mechanical properties" and "resistance to high/low temperature and oil aging."

Hydrolysis process: mostly uses "pre-treatment hydrolysis" (hydrolyze the coupling agent first, then coat glass fiber/metal surface), strictly controlling pH during hydrolysis (e.g., for glass fiber using KH-560, pH=3~4 acidic hydrolysis), and preparing the hydrolyzed solution fresh to avoid condensation affecting coating uniformity; temperature controlled at 40~50℃ (accelerates hydrolysis, ensuring completion within 30 minutes, matching production line rhythm).

Addition ratio: Medium to high (1%~3%), because automotive composite materials require high interface bonding (e.g., bumpers need impact resistance); for example, during glass fiber pretreatment, the KH-560 ratio is 2%~3% (relative to the glass fiber mass) to ensure complete coverage of the fiber surface (fiber specific surface area about 10~20m²/g).

Type selection: Epoxy silane (good compatibility with resin), mercapto silane (metal coating, improves adhesion to electrophoretic paint).

3. Electronics industry: Focus on "insulation and low volatility," hydrolysis requires low impurities.

The electronics industry mainly uses electronic packaging materials (such as epoxy encapsulants) and circuit board substrate treatment, with core requirements of "high insulation" and "low volatile substances (to avoid chip corrosion)".

Hydrolysis process: Use "high-purity water hydrolysis" (to avoid metal ion impurities affecting insulation), strictly control pH (e.g., for encapsulant using KH-560, pH=3.5±0.2), and do not add organic solvents (to avoid solvent volatilization residue); after hydrolysis, "degree of condensation" must be tested (judged by viscosity, discard if viscosity exceeds 50mPa·s).

Addition ratio: Precisely controlled (1%~2%), because excessive coupling agent may cause changes in the dielectric constant of encapsulation materials (affecting insulation); for example, during pretreatment of silicon micropowder (encapsulation filler), the coupling agent ratio is 1.2%~1.5% (relative to silicon micropowder mass), covering the surface without excess residue.

Type selection: Epoxy silane (matches encapsulation epoxy resin), methyl silane (low polarity, good insulation).

4. Textile industry: Focus on "functionality and hand feel," hydrolysis needs to consider compatibility.

The textile industry mainly uses fiber waterproofing/anti-wrinkle treatment (such as outdoor clothing, shirts), with core requirements of "functionality (waterproof, anti-wrinkle)" and "does not affect fiber hand feel (not stiff)".

Hydrolysis process: Use "water-ethanol mixed solvent hydrolysis" (ethanol content 50%~70%) because fiber surfaces are hydrophobic and pure hydrolysis solution is difficult to wet; solvent reduces surface tension; pH is neutral to slightly alkaline (pH=7~8) to avoid acidic hydrolysis solution damaging fibers (e.g., cotton fibers have poor acid resistance).

Addition ratio: Relatively high (2%~5%), because a continuous "functional film" needs to form on the fiber surface (e.g., waterproofing requires alkyl silane to form a hydrophobic layer); for example, in outdoor clothing waterproof treatment, alkyl silane (such as octadecyl triethoxysilane) accounts for 3%~4% of the treatment solution (relative to the treatment solution mass).

Type selection: Alkyl silane (waterproof), amino silane (anti-wrinkle, improves hand feel).

Summary

The application of silane coupling agents requires "hydrolysis adjusted by process, ratio determined by scenario, solution selected by industry".

The core of the hydrolysis process is "control pH, control water amount, prevent condensation"; acidic conditions suit most silanes, alkaline conditions suit amino silanes, adjustments depend on stability requirements.

There is no fixed addition ratio; it needs to be adjusted based on substrate specific surface area and application purpose; gradient experiments are a practical method to determine the optimal value.

Industry differences are reflected in "demand orientation": construction emphasizes weather resistance, automotive emphasizes strength, electronics emphasizes insulation, textiles emphasize functionality; type selection, hydrolysis, and ratio adjustments must be targeted accordingly.