1、 General principles for selecting silane coupling agents
It is known that the hydrolysis rate of silane coupling agents is determined by the silicon group Si-X, while the reactivity with organic polymers is determined by the carbon functional group C-Y. Therefore, it is crucial to choose the appropriate silane coupling agent for different substrates or processing objects. The selection method is mainly through experimentation, pre selection, and should be based on existing experience or patterns. For example, in general, unsaturated polyesters often use silane coupling agents containing CH2=CMeCOOVi and CH2-CHOCH2O; epoxy resins often use silane coupling agents containing CH2CCHH2O and H2N; phenolic resins often use silane coupling agents containing H2N and H2NCONH; polyolefins often use vinyl silane; and rubber vulcanized with sulfur often uses alkyl silane, etc. Due to a series of factors affecting the bonding strength between dissimilar materials, such as wetting, surface energy, interfacial layer and polar adsorption, acid-base interactions, interpenetrating networks, and covalent bond reactions. Therefore, relying solely on experimental pre selection is sometimes not precise enough, and it is necessary to comprehensively consider the composition of the material and its sensitivity to silane coupling agent reactions. To improve hydrolysis stability and reduce modification costs, trialkylsilane can be added to silane coupling agents for use; For difficult to stick materials, polymers crosslinked with silane coupling agents can also be shared.
When silane coupling agents are used as thickening agents, they mainly form chemical and hydrogen bonds with polymers; Wetting and surface energy effects: achieved by improving polymer crystallinity, acid-base reactions, and the generation of interpenetrating polymer networks. The thickening mainly revolves around three systems: (1) inorganic materials on organic materials; (2) Inorganic materials to inorganic materials; (3) Organic materials to organic materials. For the first type of bonding, it is usually required to bond inorganic materials to polymers, so priority should be given to the reactivity between Y in silane coupling agents and the functional groups contained in polymers. The latter two types of bonding belong to the same type of materials, so silane coupling agents are chosen for their own anti hydrophilic polymers and inorganic materials that require increased adhesion.
2、 Usage method
As mentioned earlier, one of the main application areas of silane coupling agents is the treatment of inorganic fillers used in organic polymers. The latter can be treated with silane coupling agents to transform its hydrophilic surface into an organic friendly surface, which can not only avoid particle aggregation and rapid thickening of polymers in the system, but also improve the wettability of organic polymers to reinforcing fillers. Carbon functional silane can also achieve strong bonding between reinforcing fillers and polymers. However, the effectiveness of using silane coupling agents is also related to the type and dosage of silane coupling agents, the characteristics of the substrate, the properties of the resin or polymer, as well as the application scenarios, methods, and conditions. This section focuses on two methods of using silane coupling agents, namely surface treatment method and bulk blending method. The previous method involves treating the substrate surface with a dilute solution of silane coupling agent; The latter method involves directly adding the original solution or solution of silane coupling agent into a mixture of polymer and filler, making it particularly suitable for material systems that require stirring and mixing.
1. Calculation of dosage of silane coupling agent
The number of reactive sites per unit specific surface area of the processed material (substrate) and the thickness of the silane coupling agent covering the surface are key factors determining the amount of coupling agent required for siliconization of the substrate surface. To obtain monolayer coverage, it is necessary to first determine the SiOH content of the substrate. It is known that the SiOH content in most siliceous matrices is 4-12 per square meter, so when uniformly distributed, 1 mol of silane coupling agent can cover approximately 7500m2 of the matrix. Silane coupling agents with multiple hydrolyzable functional groups may affect the accuracy of calculations to some extent due to their own condensation reactions. If Y3SiX is used to treat the substrate, a monolayer coverage consistent with the calculated value can be obtained. However, due to its high cost and poor hydrolysis resistance, Y3SiX has no practical value. In addition, the Si OH number on the surface of the substrate also varies with heating conditions. For example, under normal conditions, the SiOH number of the silicon matrix is 5.3 per square meter. After heating treatment at 400 or 800, the SiOH value can be correspondingly reduced to 2.6 per square meter or 1 per square meter. On the contrary, treating the matrix with moist heat hydrochloric acid can result in high SiOH content; Treating the substrate surface with alkaline detergent can form silanol anions.
2. Surface treatment method
This method uses silane coupling agents to connect inorganic and polymer interfaces together to achieve optimal wetting values and dispersibility. The surface treatment method requires the silane coupling agent to be acidified into a dilute solution to facilitate sufficient contact with the treated surface. The solvents used are mostly water, alcohol, or a mixture of water and alcohol, and it is advisable to use water that does not contain fluoride ions and inexpensive, non-toxic ethanol and isopropanol. Except for aminoalkylsilane, solutions prepared from other silanes require the addition of acetic acid as a hydrolysis catalyst and the adjustment of pH to 3.5-5.5. Long chain alkyl and phenylsilane are not suitable for use as aqueous solutions due to their poor stability. During the hydrolysis process of chlorosilanes and acetoxysilanes, severe condensation reactions will occur. It is also not suitable for making aqueous solution or aqueous alcohol solution. For silane coupling agent with poor water solubility, non-ionic surfactant with mass fraction of 0.1% -0.2% can be added first, and then water can be added to make water lotion for use. In order to improve the economic benefits of hydrolysis stability of the product, a certain proportion of non carbon functional silane can also be added to the silane coupling agent. When dealing with difficult to stick materials, mixed silane coupling agents or carbon functional siloxanes can be used in combination.
After the treatment solution is prepared, it can be treated by dipping, spray or brushing. Generally speaking, block materials, granular materials, and glass fibers are often treated by impregnation method; Powder materials are mostly treated by spray method; If the surface of the substrate needs to be coated as a whole, the brush coating method is used for treatment. Below are several specific processing methods.
（1） Treatment of alcohol water solution with silane coupling agent
This method has a simple process. Firstly, an alcohol water solution is prepared by mixing 95% EtOH and 5% H2O, and then adding AcOH to adjust the pH to 4.5-5.5. Stir and add silicon coupling agent to achieve a concentration of 2%. After hydrolysis for 5 minutes, a hydrolysate containing SiOH is generated. When using it to process glass plates, it can be immersed for 1-2 minutes with slight agitation, taken out and rinsed twice in EtOH, dried, then transferred to a 110 oven for 5-10 minutes, or dried at room temperature and relative humidity of 60% for 24 hours to obtain the product. If using an aminoalkylsilane coupling agent, there is no need to add HOAc. However, the alcohol water solution treatment method is not suitable for chlorosilane type coupling agents, which will undergo polymerization reactions in alcohol water solutions. When treated with a 2% concentration of trifunctional silane coupling agent solution, coatings with a thickness of 3-8 molecules are mostly obtained.
（2） Treatment with silane coupling agent aqueous solution
This method is mostly used in industrial processing of glass fibers. The specific process is to first dissolve the alkoxysilane coupling agent in water and prepare it into a solution of 0.5% -2.0%. For silane with poor solubility, 0.1% non-ionic surfactant can be added to water in advance to prepare water lotion, and then AcOH can be added to adjust the pH to 5.5. The glass fibers are then treated with spray or impregnation. After removal, cure at 110-120 for 20-30 minutes to obtain the product. Due to the significant difference in stability of silane coupling agent aqueous solutions, simple alkyl alkoxysilane aqueous solutions can only remain stable for a few hours, while ammonia hydrocarbon silane aqueous solutions can remain stable for several weeks. Due to the fact that long-chain alkyl and arylsilane aqueous solutions can only be stable for a few hours, while ammonia hydrocarbon silane aqueous solutions can be stable for several weeks. Due to the low solubility parameters of long-chain alkyl and silylsilane, this method cannot be used. When preparing silane aqueous solution, deionized water is not required, but water containing fluoride ions cannot be used.
（3） Solution treatment using silane coupling agent and organic solvent
When silane coupling agent solution is used to treat the matrix, the spray method is generally used. Before processing, it is necessary to grasp the amount of silane and the moisture content of the filler. Prepare the coupling agent into a 25% alcohol solution first, then place the filler into a high-speed mixer and pump in a fine mist of silane coupling agent solution under stirring. The amount of silane coupling agent used is about 0.2% -1.5% of the filler mass. The treatment can be completed after 20 minutes, and then dry it using dynamic drying method.
In addition to alcohols, ketone esters and hydrocarbons can also be used as solvents and prepared at a concentration of 1% -5% (mass fraction). To hydrolyze the silane coupling agent or partially hydrolyze the solvent, a small amount of water or even a small amount of HOAc can be added as a hydrolysis catalyst. Then, the material to be treated is added to the solution under stirring, filtered, and dried at 80-120 ° C for a few minutes to obtain the product.
Powder filler can be treated by spray method, and silane coupling agent stock solution or its hydrolysate solution can also be used. When processing metal, glass and ceramics, it is advisable to use silane coupling agent alcohol solution with concentration of 0.5% -2.0% (mass fraction), and use immersion, spray, brush coating and other methods for treatment. According to the shape and performance of the substrate, it can be dried and cured immediately, or it can be dried and cured at 80-180 for 1-5 minutes.
Treatment of hydrolysis products using silane coupling agents
Silane is first hydrolyzed into a hydrolysate through controlled hydrolysis and used as a surface treatment agent. This method can achieve better treatment effects than pure silane solution, and it can be dried and solidified without further hydrolysis.
3. Integral blending method
The overall blending method is to mix the original solution of silane coupling agent into the resin or polymer before adding the filler. Therefore, it is required that resins or polymers should not react with silane coupling agents too early to avoid reducing their thickening effect. In addition, before the material solidifies, the silane coupling agent must migrate from the polymer to the surface of the filler, and then complete the hydrolysis condensation reaction. For this purpose, metal carboxylic acid esters can be added as catalysts to accelerate the hydrolysis condensation reaction. This method is particularly convenient and effective for fillers that are suitable for surface treatment with silane coupling agents, or for systems where resin and fillers need to be mixed and stirred before molding. It can also overcome some of the disadvantages of filler surface treatment methods. Someone compared the advantages and disadvantages of blending and surface treatment methods using various resins. It is believed that in most cases, the mixing method is less effective than the surface treatment method. The process of blending involves the migration of silane coupling agent from the resin to the surface of fibers or fillers, and then it interacts with the surface of fillers. Therefore, after silane coupling is added to the resin, it needs to be left for a period of time to complete the migration process, and then cured to achieve better results. It is theoretically speculated that the migration of silane coupling agent molecules to the surface of the filler is only equivalent to the amount of monolayer generated on the surface of the filler, so the amount of silane coupling agent required is only 0.5% -1.0% of the resin mass. It should also be pointed out that in the formulation of composite materials, when using additives with good compatibility with the filler surface and low molar mass, special attention should be paid to the feeding sequence, that is, adding silane coupling agent first and then adding additives, in order to obtain better results.
