Silane Coupling Agent Knowledge

Silane Coupling Agent

Ⅰ. Definition and Performance Characteristics

Silane coupling agent is a class of organosilicone compounds containing two different chemical groups in the molecule at the same time. In the formula, Y is a non-hydrolyzable group (also an organic group, which can be epoxy, methacryloyloxy, sulfhydryl, amino, alkyl, isocyanate and vinyl) that can react chemically or form hydrogen bonds with the polymer to form a strong bond; X is a hydrolyzable group (including Cl, Me-O-, Et-O-, i-Pr-O-, MeO-CH2CH2-O- Silane coupling agents can react with inorganic materials containing hydroxyl groups (e.g., glass, metals, or minerals).

Due to this special structure, silane coupling agents act at the interface between inorganic materials (such as glass, metals or minerals) and organic materials (such as organic polymers, coatings or adhesives), combining or coupling two distinctly different materials. It enhances the affinity between organic and inorganic compounds and improves the physical and chemical properties of the composite material, such as strength, toughness, electrical properties, water resistance and corrosion resistance.

Performance Features and Advantages

When using glass fibers or mineral-reinforced organic polymers, the interface or interfacial phase between the polymer and the inorganic material involves a complex cross-talk between many physical and chemical factors. These factors are related to adhesion, physical strength, coefficient of expansion, concentration gradients, and retention of product properties. An important destructive force affecting adhesion is the migration of water into the inorganically reinforced hydrophilic surface. The moisture erodes the interface and breaks the bond. “Real” coupling agents form water-resistant bonds at the interface between inorganic and organic materials. Silane coupling agents have unique chemical and physical properties that not only increase bond strength, but more importantly, prevent disintegration of bonds at the interface during aging and use of the composite. The coupling agent imparts a stable bond between two dissimilar, difficult-to-bond surfaces.

Silane coupling agent can not only be used as an elastic bridging agent between the matrix, that is, to improve the adhesion between two materials with different chemical properties, to improve the product’s mechanical, electrical insulation, anti-aging and hydrophobic and other comprehensive performance purposes; can also be used as a surface modifier, giving waterproof, anti-static, anti-mildew, anti-odor, anti-blood coagulation and physiologically inert properties; can also be used as a crosslinking curing agent for non-crosslinked polymer systems, so that it can be used to improve the performance of the product. Ambient temperature and pressure curing. In composites, the right choice of silanes can increase the flexural strength of composites by more than 40%. Silane coupling agents also enhance the bond strength between the coating and the adhesive, while increasing resistance to humidity and other harsh environmental conditions.

Other advantages that silane coupling agents can provide include.

  1. Better wetting of inorganic materials
  2. Lower viscosity when compounding
  3. Smoother composite surfaces
  4. Reduce the inhibitory effect of inorganic materials on the catalyst of thermoset composites.
  5. Clearer and more transparent reinforced plastic

Ⅱ. Silane coupling agent mechanism of action

The role and effect of silane coupling agent has been recognized and affirmed, but there is no complete set of coupling mechanism to explain why a very small amount of coupling agent at the interface can have such a significant effect on the performance of composite materials. The mechanism of the action of the coupling agent at the interface between two materials of different properties has been studied by many researchers, and explanations such as chemical bonding and physical adsorption have been proposed. Among them, the chemical bonding theory is the oldest, but so far is considered to be a more successful theory.

1. Chemical Bonding Theory

The theory is that the coupling agent contains a chemical functional group that can form covalent bonds with the silanol groups on the surface of glass fibers or other inorganic filler surface molecules; in addition, the coupling agent also contains a different functional group and polymeric molecules bonding to obtain good interfacial bonds, coupling agents play a role as a bridge between the inorganic and organic phases of the connection between each other.

The following is an example of a silane coupling agent to illustrate the chemical bonding theory. For example, when aminopropyltriethoxysilane is used to first treat inorganic fillers (such as glass fibers, etc.), the silane is first hydrolyzed into silanol, then the silanol group reacts with the surface of the inorganic filler in a dehydration reaction to carry out chemical bonding, the reaction formula is as follows.

The Y group in the coupling agent interacts with the organic polymer and ultimately bridges the gap between the inorganic filler and the organic material when the inorganic filler is filled and prepared as a composite material.

Silane coupling agents are available in a wide variety of varieties, and the different Y groups in the general formula indicate the different types of polymers for which the coupling agent is suitable. This is because the Y groups are selective in their reaction to the polymer, and silane coupling agents containing vinyl and methacryloyloxy, for example, are particularly effective in unsaturated polyester resins and acrylics. The reason is that the unsaturated double bond in the coupling agent and the unsaturated double bond in the resin are the result of a chemical reaction in the presence of an initiator and an accelerator. However, coupling agents containing these two groups are less effective when used in epoxy and phenolic resins because the double bonds in the coupling agent do not participate in the curing reaction of epoxy and phenolic resins. However, the coupling agent with epoxy group is especially effective for epoxy resins. Because the epoxy group can react with the hydroxyl group in unsaturated polyester, the coupling agent with epoxy group is also suitable for unsaturated polyester. Silane coupling agents containing -SH are widely used in the rubber industry.

Through these two reactions, silane coupling agents improve the adhesion between polymers and inorganic fillers in composite materials through chemical bonding, which greatly improves their performance. It can be characterized by extrapolation of the theoretical bonding force.

According to the interfacial chemistry of adhesion theory, the adhesion force per unit area between the adhesive and the object to be adhered to the second-valent bond mainly considering the dispersion force.

2. Wetting effect and surface energy theory

In 1963, ZISMAN, in a review of what was known about surface chemistry and surface energy in relation to adhesion, concluded that in the manufacture of composite materials, good wetting of the liquid resin to the adherent is of primary importance, and that if complete wetting can be obtained, then physical adsorption of the resin to the energetic surface will provide a higher bond strength than the cohesive strength of the organic resin.

3. The theory of deformable layer

In order to moderate the interfacial stresses caused by the difference in thermal shrinkage between the resin and the filler as the composite cools, it is desirable that the resin interface adjacent to the treated inorganic material be a pliable deformable phase so that the composite is most ductile.

The surface of the coupling agent-treated inorganic material may selectively absorb one of the coordinating agents in the resin, and uneven curing in the interphase region may result in a flexible resin layer that is much thicker than the multimolecular layer of the coupling agent between the polymer and the filler. This layer is called the deformable layer, which can relax the interfacial stress and prevent the expansion of interfacial cracks, thus improving the interfacial bond strength and improving the mechanical properties of the composite.

4. Constrained layer theory

In contrast to the deformable layer theory, the confining layer theory states that the resin in the inorganic filler region should have some modulus between the inorganic filler and the matrix resin, and that the function of the coupling agent is to “tightly bind” the polymer structure in the interphase region. The function of the coupling agent is to “tightly bind” the polymer structure in the interphase region. In terms of the performance of the reinforced composite, a confining layer at the interface is required for maximum adhesion and hydrolysis resistance.

As for the titanate coupling agent, its bonding with organic polymers in thermoplastic systems and thermoset composites containing fillers is mainly based on the solubilization and intertwining of long-chain alkyl groups and the formation of covalent bonds with inorganic fillers. The above hypotheses all reflect the coupling mechanism of coupling agents from different theoretical aspects. In the actual process, it is often the result of several mechanisms working together.

Ⅲ. Silane coupling agent selection principle

Matching the organic functional group on a silicon atom to the type of resin-polymer to be bonded can guide which silane coupling agent to use in a particular application. Organic groups on silanes can be either reactive (e.g., organofunctional) or non-reactive organic groups. These groups can be hydrophobic or hydrophilic, or have a variety of heat-stabilizing properties. Because of the different organic structures, the solubility parameters of the groups vary; to some extent, this affects the interpenetration between the polymer system and the siloxane system used for surface treatment.

The number of reactive sites per unit surface area of the treated material (substrate) and the thickness of the surface covered by the silane coupling agent are key factors in determining the amount of coupling agent required to silylate the substrate surface. In order to obtain monolayer coverage, the Si-OH content of the substrate must first be determined. It is known that the Si-OH content of most silicone substrates is 4-12 Si-OH/µm2, so that 1 mol of silane coupling agent can cover about 7500 m2 of the substrate when evenly distributed.

Silane coupling agents with multiple hydrolyzable groups affect the accuracy of the calculations somewhat due to their own condensation reaction. If Y3SiX is used to treat the substrate, a monolayer coverage consistent with the calculated value can be obtained. However, since Y3SiX is expensive and the hydrolysis resistance of the cover is poor, it has no practical value. In addition, the number of Si-OH on the substrate surface also varies with the heating conditions.

For example, the number of Si-OH is 5.3/µ㎡ silicon substrate under normal condition, after heating at 400℃ or 800℃, the value of Si-OH can be reduced to 2.6/µ㎡ or <1/µ㎡ accordingly. Conversely, high Si-OH content can be obtained by treating the substrate with hot and humid hydrochloric acid, and silanol anion can be formed by treating the substrate surface with alkaline detergent. The wettable surface area (WS) of silane coupling agent is the area (m²/g) of the substrate that can be covered by a solution of 1 g of silane coupling agent. The amount of silane coupling agent required to cover a single molecular layer can be calculated by correlating it with the surface area value (㎡/g) of the silicon-containing substrate.

The amount of silane coupling agent W (g) needed to form a single molecular layer covering the filler surface is directly proportional to the surface area of the filler (㎡/g) and its mass, and inversely proportional to the wettable area of silane WS (㎡/g). Accordingly, the formula for calculating the amount of silane coupling agent is as follows: Silane amount (g) = surface (S) value of some common fillers.

Ⅳ. Usage of silane coupling agent

Surface treatment method

This method uses a silane coupling agent to bond the inorganic material to the polymer interface in order to obtain optimum wetting values and dispersion. The surface treatment method requires that the silane coupling agent be acidified to a dilute solution to facilitate contact with the surface to be treated. The solvents used are water, alcohols or hydroalcoholic mixtures, water that does not contain fluoride ions, inexpensive and non-toxic ethanol and isopropanol.

The solutions prepared from other silanes should be hydrolyzed by adding acetic acid as a catalyst and adjusting the pH value to 3.5-5.5. Long-chain alkyl and phenyl silanes are not suitable for use as aqueous solutions because of their poor stability. The hydrolysis of chlorosilanes and acetoxysilanes will be accompanied by severe condensation reactions.

For the poorly water-soluble silane coupling agent, 0.1%-0.2% by mass of non-ionic surfactant can be added first, and then water can be added to make a water emulsion and used. In order to improve the hydrolytic stability of the product economically, a certain proportion of non-carbon functional silane can be added to the silane coupling agent. For the treatment of difficult-to-stick materials, a mixed silane coupling agent can be used or a carbon-functional siloxane can be used in combination.

Once the treatment solution has been prepared, the material can be treated by dipping, spraying or brushing. Generally speaking, lumpy materials, granular materials and glass fibers are usually treated by impregnation method; powder materials are usually treated by spraying method; and if the surface of the substrate needs to be coated as a whole, it should be treated by brush coating method.

Use silane coupling agent alcohol water solution treatment method.

This method is simple and easy, firstly, make a solution of 95% EtOH and 5% H2O, add AcOH to make the pH 4.5-5.5, then add silicon coupling agent to make the concentration reach 2%, after 5min of hydrolysis, the hydrolysate containing SiOH will be produced. When the glass plate is treated with SiOH, it can be immersed in EtOH for 1-2min with a little stirring, then taken out and immersed in EtOH and rinsed for 2 times, dried and transferred to an oven of 110 for 5-10min, or dried at room temperature and 60% relative humidity for 24h.

HOAc is not necessary if an aminalkylsilane coupling agent is used, but the aqueous alcohol treatment is not suitable for chlorosilane-type coupling agents, which undergo polymerization in aqueous alcohol solutions. When treated with a 2% concentration of a trifunctional silane coupling agent solution, a 3-8 molecule thick coating is obtained.

Treatment with an aqueous silane coupling agent solution

This method is mostly used in industry to treat glass fibers. The specific process is to dissolve the alkoxy silane coupling agent in water and make a solution of 0.5%-2.0%. For the less soluble silanes, 0.1% nonionic surfactant can be added to water to form an aqueous emulsion, and then AcOH can be added to adjust the pH to 5.5. The glass fiber is then treated by spraying or impregnation. After removal, the product is solidified under 110-120 for 20-30min.

For example, a simple aqueous solution of alkyl alkoxysilane is stable for only a few hours, while an aqueous solution of ammonium silane is stable for several weeks. For example, a simple alkyl alkoxy silane solution is stable for a few hours, whereas an aqueous aminosilane solution can be stable for several weeks. This method cannot be used due to the low solubility parameters of long-chain alkyl and se-based silanes. When preparing an aqueous silane solution, it is not necessary to use deionized water, but water containing fluoride ions cannot be used.

Treatment with a solution prepared with silane coupling agent organic solvent.

When using a silane coupling agent solution to treat a substrate, the spray method is usually used. Before treatment, it is necessary to know the amount of silane and the water content of the filler. The dosage of silane coupling agent is about 0.2%-1.5% of the filler mass. The treatment can be finished after 20 min, and then the filler can be dried by dynamic drying method.

In addition to alcohols, ketone esters and hydrocarbons can be used as solvents and formulated to a concentration of 1-5% by mass. In order to hydrolyze the silane coupling agent, or to partially hydrolyze the solvent, it is necessary to add a small amount of water, or even add a little HOAc as a hydrolysis catalyst, and then the material to be treated is added to the solution with stirring, and then filtered, dried and cured under 80-120 for a few minutes to obtain the product.

The powder filler can also be treated by spraying, using the original silane coupling agent or its hydrolysate solution. When dealing with metal, glass and ceramics, it is appropriate to use 0.5%-2.0% (mass fraction) concentration of silane coupling agent alcohol solution, and use impregnation, spraying and brush coating and other methods to deal with, according to the shape and properties of the substrate, it can either be dried and solidified immediately, or can be kept under 80-180 for 1-5min to achieve dry and solidified.

Integrated blending method

The monolithic blending method involves mixing the original silane coupling agent into the resin or polymer before the filler is added. Therefore, it is required that the resin or polymer not react prematurely with the silane coupling agent to reduce its viscosity increasing effect. In addition, before the material can be cured, the silane coupling agent must migrate from the polymer to the surface of the filler and then complete the hydrolytic condensation reaction.

For this purpose, metal carboxylates can be added as catalysts to accelerate the hydrolytic condensation reaction. This method is especially convenient and effective for fillers that are suitable for surface treatment with silane coupling agents, or for systems in which the resin and filler are to be mixed and stirred prior to molding, and can overcome some of the disadvantages of the filler surface treatment. The advantages and disadvantages of blending and surface treatment have been compared using various resins. It is believed that, in most cases, the blending method is inferior to the surface treatment. The doping process involves the migration of the silane coupling agent from the resin to the fiber or filler surface and then to the filler surface.

Therefore, after the silane coupling is doped into the resin, it must be left for a certain period of time to complete the migration process and then cured in order to obtain a better result. Theoretically, the migration of the silane coupling agent molecules to the filler surface is only equivalent to the amount of single molecular layer formed on the filler surface, so the amount of silane coupling agent required is only 0.5%-1.0% of the mass of the resin. It should also be pointed out that when using additives with good surface compatibility and low molar mass in composite formulations, special attention should be paid to the feeding order, i.e. adding the silane coupling agent first and then the additives in order to obtain better results.

Ⅴ. Application areas of silane coupling agents

Coatings, Inks

Coatings using silicone have the following advantages.

  1. Corrosion resistance
  2. Improve adhesion
  3. Improve rheology
  4. Improve the dispersion of dyes and fillers
  5. UV resistance
  6. Waterproof and chemical resistant

Synthetic elastomers and resin-based coatings often encounter adhesion problems on hydrophilic silicate and metallic surfaces, especially at high tide, in water or salt, and rely on adhesion promoters on the substrate surface for protection.

Fiberglass composites

With UMC, glass fibers will have the following benefits.

  1. Improve the test performance from hot pole to cold pole cycle.
  2. Glass fiber wettability and electrical properties are improved.
  3. fiber filament clustering, protection and processing performance.

Glass fiber greatly increases the physical strength of composites, even to the point of rivaling metal. All manufacturers of glass fiber reinforced materials use treated products to achieve good product performance, and coupling agents are still the first choice. Silane coupling agents are most commonly used in the treatment of fiberglass composites and are a key component of glass fiber reinforced polymers.

Wire and Cable

Since the 1970s, vinyl silanes have been used in cross-linked polyethylene homopolymers and their copolymers, silicone silicone cross-linked polyethylene, as wire and cable insulation and jacketing for high temperature resistance. The technology is also used in the manufacture of materials for hot water piping, which can withstand high temperatures for long periods of time.

Sealants

Co-Silicone silicones are widely used to improve the hermeticity of sealants, as well as to improve adhesion to inorganic materials such as metal, glass and stone. Sealants are based on filler-based, curable elastomers that perform the dual function of being waterproof, resistant to air and chemical penetration, and in some cases can also be used as adhesives.

Their applications in the aerospace, automotive and construction industries depend on their ability to form durable bonds with metal, glass, concrete and other surfaces that are resistant to heat, UV, moisture and water.

Rubber, and Elastomers

As the need to manufacture elastomers in a variety of colors other than black grows, and mechanical properties similar to those of formulations containing carbon black are required, there is no question that associosilicone plays an important role in enabling effective combinations of inorganic fillers and organic elastomers.

Ussilicon organosilane coupling agents and inorganic fillers offer the following advantages.

  1. Anti-wear
  2. More effective binding of complexes
  3. Improve rheological control
  4. Reduce tire rolling resistance
  5. Improve toughness
  6. Improve the electrical performance under humid conditions.

Building Material Waterproof

The use of UMC silicone modified water repellents has the following effects on concrete protection.

  1. Effectively reduce the penetration of chlorine ions through concrete pores to prevent internal steel corrosion.
  2. greatly reduce the water absorption rate of concrete, prevent concrete corrosion, weathering, breeding microorganisms.
  3. Effectively prevent freeze-thaw damage to concrete.
  4. excellent permeability and breathability – respirability to achieve the exchange of materials inside and outside the concrete, to prevent pressure difference between inside and outside.
  5. Excellent alkali resistance can be used in new concrete and high alkaline substrates.
  6. Textile chemicals (silicone finishing agent)

Ussilicon organosilane coupling agents are used in the synthesis of silicone oils and silicone emulsions to provide comfort, softness, and smoothness to fabrics. The use of modified silicone oil silicone finishing agent has the following advantages:

  1. so that the fabric has a soft and beautiful feeling or “feel” with low yellowing.
  2. improved tear strength and durability.
  3. Designed to enhance water repellency (hydrophobicity) or improve water absorption (hydrophilicity), depending on the formulation.
  4. Improved elastic recovery, wrinkle and abrasion resistance.
  5. reduce shrinkage.
  6. Improve elasticity.