Use of silane coupling agents

It is known that the hydrolysis rate of the silane coupling agent is derived from the silicon energy group Si-X, and the reactivity with the organic polymer is derived from the carbon functional group C-Y. Therefore, for different substrates or processing objects, it is very important to choose a suitable silane coupling agent. The method selected is mainly through experiments, pre-selection and should be carried out on the basis of existing experience or regularity. For example, under normal circumstances, unsaturated polyesters mostly use silane coupling agents containing CH2=CMeCOOVi and CH2-CHOCH2O; epoxy resins mostly use CH2CHCH2O and H2N silane coupling agents; phenolic resins mostly use H2N and H2NCONH silanes Coupling agent: vinyl silane is mostly used for polyolefins; hydrocarbyl silane is mostly used for rubber vulcanized with sulfur. Since the bonding strength between dissimilar materials is affected by a series of factors, such as wetting, surface energy, interfacial layer and polar adsorption, acid-base interaction, interpenetrating network and covalent bond reaction, etc. Therefore, the preselection by test alone is sometimes not accurate enough, and the composition of the material and its sensitivity to the reaction of the silane coupling agent should also be comprehensively considered. In order to improve the hydrolytic stability and reduce the modification cost, the silane coupling agent can be mixed with trihydrocarbyl silane for use; for difficult-to-stick materials, the polymer cross-linked by the silane coupling agent can also be shared.

When the silane coupling agent is used as a tackifier, it is mainly achieved by forming chemical bonds and hydrogen bonds with the polymer; wetting and surface energy effects: improving the crystallinity of the polymer, the acid-base reaction, and the formation of the interpenetrating polymer network, etc. of. The viscosity increase mainly revolves around three systems: (1) inorganic material to organic material; (2) inorganic material to inorganic material; (3) organic material to organic material. For the first type of bonding, the inorganic material is usually required to be bonded to the polymer, so the reactivity of Y in the silane coupling agent and the functional group contained in the polymer should be given priority: the latter two belong to the bonding between the same type of materials , so the anti-hydrophilic polymer of the silane coupling agent itself and the silane coupling agent selected when the inorganic material requires viscosity increase.

Instructions 

As mentioned above, 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 agent to convert its hydrophilic surface into an organic hydrophilic surface, which can not only avoid particle agglomeration and sharp polymer thickening in the system, but also improve the wetting of organic polymers to reinforcing fillers. The carbon-functional silane can also make the reinforcing filler and the polymer achieve strong bonding. However, the use effect of the silane coupling agent is also related to the type and amount of the silane coupling agent, the characteristics of the substrate, the properties of the resin or polymer, and the application occasion, method and conditions. This section focuses on two methods of using silane coupling agents, namely surface treatment method and integral blending method. The former method is to treat the surface of the substrate with a dilute solution of the silane coupling agent; the latter method is to directly add the silane coupling agent stock solution or solution to the mixture made of polymer and filler, so it is especially suitable for material systems that require stirring and mixing.

1. Calculation of the dosage of silane coupling agent

The number of reactive sites occupied by the specific surface area of the object to be treated (substrate) and the thickness of the surface covered by the silane coupling agent are the key factors to determine the amount of coupling agent required for the silylization of the substrate surface. To obtain monolayer coverage, the SiOH content of the matrix needs to be determined first. It is known that the SiOH content of most siliceous substrates is 4-12 per square meter, so when evenly distributed, 1 mol of silane coupling agent can cover about 7500 m2 of the substrate. Silane coupling agents with multiple hydrolyzable groups will somewhat affect the accuracy of the calculation due to self-condensation reactions. If the substrate is treated with Y3SiX, monolayer coverage consistent with the calculated values can be obtained. However, because Y3SiX is expensive and has poor hydrolysis resistance, it has no practical value. In addition, the number of Si-OH on the surface of the substrate also varies with heating conditions. For example, under normal conditions, the number of SiOH is 5.3/m2 of the siliceous substrate. After heat treatment at 400 or 800°C, the SiOH value can be reduced to 2.6/m2 or 1/m2 accordingly. Conversely, high SiOH content can be obtained by treating the substrate with moist-heat hydrochloric acid; silanol anions can be formed by treating the substrate surface with an alkaline detergent.

2. Surface treatment method

In this method, the two interfaces of inorganic matter and polymer are connected together by silane coupling agent to obtain the best wetting value and dispersibility. In the surface treatment method, the silane coupling agent needs to be acidified into a dilute solution to facilitate full contact with the treated surface. The solvent used is mostly water, alcohol or a mixture of water and alcohol, and water without fluoride ion and cheap and non-toxic ethanol and isopropanol are suitable. In addition to aminohydrocarbyl silane, the solution prepared by other silanes should be added with acetic acid as a hydrolysis catalyst, and the pH value should be adjusted to 3.5-5.5. Long-chain alkyl and phenyl silanes are not suitable for use in aqueous solutions due to their poor stability. During the hydrolysis of chlorosilane and acetoxysilane, a severe condensation reaction will occur. It is also not suitable for use in aqueous solution or hydroalcoholic solution. For silane coupling agent with poor water solubility, 0.1%-0.2% mass fraction of non-ionic surfactant can be added first, and then water is added to process it into an aqueous emulsion. In order to improve the economic benefit of the hydrolytic stability of the product, a certain proportion of non-carbon functional silane can also be incorporated into the silane coupling agent. When dealing with difficult-to-stick materials, a mixture of silane coupling agents or a combination of carbon-functional siloxanes can be used.

After the treatment liquid is prepared, it can be treated by dipping, spraying or brushing. Generally speaking, bulk materials, granular materials and glass fibers are mostly treated by dipping method; powder materials are mostly treated by spray method; if the substrate surface needs an integral coating, brush coating method is used. Several specific processing methods are described below.

(1) Treatment method using silane coupling agent alcohol aqueous solution

The process of this method is simple. First, an alcohol aqueous solution is prepared from 95% EtOH and 5% H2O, and AcOH is added to make the pH 4.5-5.5. Add silicon coupling agent under stirring to make the concentration reach 2%, and after hydrolysis for 5min, the hydrolyzate containing SiOH is formed. When using it to treat the glass plate, it can be immersed for 1-2 minutes with a little stirring, taken out and immersed in EtOH for 2 rinses, after drying, moved to a 110 degree oven to dry for 5-10 minutes, or at room temperature and relative humidity of 60% The product was obtained by drying under conditions for 24h.

If an aminohydrocarbyl silane coupling agent is used, HOAc need not be added. However, the alcohol aqueous solution treatment method is not suitable for the chlorosilane type coupling agent, which will polymerize in the alcohol aqueous solution. When treated with a 2% concentration of trifunctional silane coupling agent solution, the resulting coating is mostly 3-8 molecules thick.

(ii) Treatment with aqueous solution of silane coupling agent

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 make it into a 0.5%-2.0% solution. For silanes with poor solubility, 0.1% non-ionic surfactant can be added to water to prepare an aqueous emulsion, and then AcOH can be added to adjust the pH to 5.5. Then, the glass fibers are treated by spraying or dipping. After taking out, solidify at 110-120 for 20-30min to obtain the product. Because the stability of the aqueous solution of silane coupling agent varies greatly, for example, the simple aqueous solution of alkylalkoxysilane can only be stable for a few hours, while the aqueous solution of ammonia hydrocarbon silane can be stable for several weeks. Since long-chain alkyl and aryl silane aqueous solutions are only stable for a few hours, ammonia hydrocarbon silicon aqueous solutions can be stable for several weeks. This method cannot be used due to the low solubility parameters of long-chain alkyl and silyl silanes. When preparing the aqueous silane solution, deionized water is not required, but water containing fluoride ions cannot be used.

(iii) Solution treatment prepared with organic solvent of silane coupling agent

When the substrate is treated with a silane coupling agent solution, the spray method is generally used. Before treatment, it is necessary to master the amount of silane and the water content of the filler. The coupling agent is first prepared into a 25% alcohol solution, and then the filler is placed in a high-speed mixer, and a fine mist of silane coupling agent solution is pumped under stirring. The amount of silane coupling agent is about the mass of the filler. 0.2%-1.5%, the treatment can be finished for 20min, and then it is dried by dynamic drying method.

In addition to alcohols, ketone esters and hydrocarbons can also be used as solvents, and the concentration is 1%-5% (mass fraction). In order to hydrolyze the silane coupling agent, it is necessary to add a small amount of water or even a small amount of HOAc as a hydrolysis catalyst to the solvent for partial hydrolysis. -120 under drying and curing for a few minutes, the product can be obtained.

The powder filler is treated by the spray method, and the silane coupling agent stock solution or its hydrolyzate solution can also be used. When dealing with metals, glass and ceramics, it is advisable to use an alcohol solution of silane coupling agent with a concentration of 0.5%-2.0% (mass fraction), and use methods such as dipping, spraying and brushing. According to the shape and performance of the substrate, It can be dried and cured immediately, or it can be kept at 80-180 for 1-5min to achieve drying and curing.

(iv) Treatment with silane coupling agent hydrolyzate

That is, the silane is first prepared as a hydrolyzate by controlled hydrolysis and used as a surface treatment agent. This method can achieve better treatment effect than pure silane solution, it can be dried and cured without further hydrolysis.

3. Overall blending method

The overall blending method is to mix the silane coupling agent stock solution into the resin or polymer before the filler is added. Therefore, it is required that the resin or polymer should not react with the silane coupling agent prematurely, so as not to reduce its tackifying effect. In addition, before the material is cured, the silane coupling agent must migrate from the polymer to the surface of the filler, and then complete the hydrolytic condensation reaction. To this end, metal carboxylate can be added as a catalyst 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 systems where resin and fillers need to be mixed and stirred before molding, and can also overcome some of the shortcomings of filler surface treatment methods. Some people use various resins to compare the advantages and disadvantages of the blending method and the surface treatment method. It is considered that in most cases, the effect of the blending method is inferior to that of the surface treatment method. The action process of the blending method is that the silane coupling agent migrates from the resin to the surface of the fiber or filler, and then acts on the surface of the filler. Therefore, after the silane coupling is incorporated into the resin, it must be placed for a period of time to complete the migration process, and then cured to obtain better results. It is also speculated theoretically that the migration of silane coupling agent molecules to the surface of the filler is only equivalent to the amount of monolayer formed on the surface of the filler, so the amount of silane coupling agent only needs to be 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, the silane coupling agent is added first, and then the additives are added. good results.