A multifunctional composite cotton fiber material and its preparation method and application

Abstract

The present invention relates to the technical field of plant-based fiber material preparation, and specifically to a multifunctional composite cotton fiber material and a preparation method and application thereof, the method is based on the principle of hydrophobic modification, takes plant-based fiber as raw material, and simultaneously adopts 3-aminopropyl triethoxysilane and other long carbon chain alkyl alkoxysilane to hydrophobically and lipophilically modify it, through the synergistic effect of the cellulose surface grafted amino group to repel nicotine and the low surface energy long chain alkyl hydrophobicity, so that the modified multifunctional composite cotton fiber material simultaneously absorbs less or no water and nicotine in cigarette smoke, and absorbs more tar in cigarette smoke. Thereby reducing the harmful components inhaled by smokers, while not affecting the smoking experience, improving the use compliance of filter tips, the whole preparation process is simple and convenient, and industrialized large-scale production can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of plant-based fiber material preparation, and in particular to a multifunctional composite cotton fiber material for filtering harmful components in cigarette smoke, and a preparation method and application thereof. Background Art

[0002] Plant-based fiber materials have a reticular porous structure, are conducive to adsorption, and have the advantages of being non-toxic, low-cost, biodegradable, chemically stable, biocompatible and wettable. After functional modification, they are widely used in the separation field.

[0003] Existing research on the modification of plant-based fiber materials mainly focuses on using hydrophobic and lipophilic plant-based fiber materials in oil-water separation applications of light oils such as organic solvents, motor oil, diesel, and vegetable oil. However, there are fewer research reports on the adsorption of heavy oils, especially the adsorption of harmful components in cigarette smoke.

[0004] In the published studies on the adsorption of harmful components in cigarette smoke, cellulose is mostly treated with 3-aminopropyltriethoxysilane alone as a hydrophobic modifier. The fiber material prepared by this method has poor hydrophobicity and cannot reduce the adsorption of water and nicotine while adsorbing tar in cigarette smoke. In addition, there are no reports on the use of plant-based fibers as raw materials, using two or more organic silane hydrophobic modifiers to modify them hydrophobically and lipophilically and then use them as cigarette filters to adsorb more tar in cigarette smoke and less or no water and nicotine in cigarette smoke. Therefore, it is of great significance to develop a multifunctional plant-based fiber composite material with the advantages of abundant sources, low price, good biodegradability, good chemical stability, good biocompatibility, simple preparation conditions, and industrial large-scale production. Summary of the invention

[0005] The purpose of the present invention is to provide a multifunctional composite cotton fiber material and a preparation method and application thereof. The multifunctional composite cotton fiber material can better adsorb polycyclic aromatic hydrocarbon components in flue gas tar, but less adsorbs nicotine, and has the advantages of being non-toxic, low cost, environmentally friendly, and having a simple preparation process, and can realize industrial large-scale production.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing a multifunctional composite cotton fiber material comprises the following steps:

[0008] Step 1, adding long carbon chain alkyl alkoxysilane and 3-aminopropyl triethoxysilane to an organic solvent, adding catalyst acetic acid dropwise under stirring, stirring for 0.5h to mix well, to obtain a reaction solution; the volume ratio of the long carbon chain alkyl alkoxysilane, 3-aminopropyl triethoxysilane, organic solvent and catalyst is 1:1:50:1;

[0009] Step 2: Add cotton fiber to the reaction solution obtained in step 1 and completely immerse it in the reaction liquid. After standing for 2-12 hours, take it out and repeatedly rinse it with ethanol to remove unreacted raw materials. Then put the modified cotton fiber into an oven and dry it until constant weight is reached to obtain functionalized cotton fiber.

[0010] A method for preparing a multifunctional composite cotton fiber material comprises the following steps:

[0011] Step 1, adding a silicon precursor, a long carbon chain alkyl alkoxysilane, and 3-aminopropyltriethoxysilane to an organic solvent, and gradually adding a catalyst acetic acid, stirring for 0.5h to mix, to obtain a reaction solution; the volume ratio of the silicon precursor, the long carbon chain alkyl alkoxysilane, the 3-aminopropyltriethoxysilane, the organic solvent, and the catalyst is 1 to 5:1:1:50:1;

[0012] Step 2: Add cotton fiber to the reaction solution obtained in step 1 and completely immerse it in the reaction solution. After standing for 2 to 12 hours, take it out, rinse it with ethanol, and dry it in an oven until constant weight is obtained to obtain a multifunctional composite cotton fiber material.

[0013] Furthermore, the cotton fiber is absorbent cotton or surface-roughened cotton fiber.

[0014] Furthermore, the method for preparing the surface roughened cotton fiber is:

[0015] Add silicon precursor, water and organic solvent into a reaction container, stir and mix, then add catalyst acetic acid or ammonia water drop by drop and stir for 0.5 to 1 hour, add cotton fiber after mixing, stir for 0.5 to 4 hours, take out, wash repeatedly with ethanol, put into an oven at a temperature range of 50 to 80°C for drying, and obtain surface roughened cotton fiber; the volume ratio of silicon precursor, water, organic solvent and catalyst is 1:1:8 to 20:1.

[0016] Furthermore, the drying temperature of step 1 and step 2 is 50-80°C.

[0017] Furthermore, the organic solvent is any one of ethanol, methanol, ethyl acetate, acetone, and toluene, or a mixture of two or more thereof.

[0018] Furthermore, the silicon precursor is any one of triethoxysilane, tetramethoxysilane and tetraethoxysilane, or a mixture of two or more thereof.

[0019] The present invention also provides a multifunctional composite cotton fiber material obtained by the above preparation method, which comprises cotton fiber, silicon precursor, long carbon chain alkyl alkoxy silane and 3-aminopropyl triethoxy silane as raw materials.

[0020] The present invention also provides the use of the multifunctional composite cotton fiber material prepared by the preparation method of the multifunctional composite cotton fiber material in cigarette smoke filtering products.

[0021] The term "hydrophobic" refers to a material surface that exhibits water-repellent properties. A useful method for determining this property is to measure the water contact angle. The "water contact angle" is the angle between the tangent to the liquid / vapor-interface shape and the solid surface (including the liquid phase side) when the liquid / vapor interface meets the solid surface. It quantifies the wettability of a solid surface via Young's equation.

[0022] Cellulose is a macromolecule composed of repeating glucoside units, such as Figure 1 As shown, the presence of a large number of hydroxyl groups in a long-chain polymer rich in hydroxyl groups makes the cellulose macromolecule have good hydrophilicity. The principle of hydrophobic modification involved in the invention is as follows: the oxygen atoms in the water molecules nucleophilically attack the silicon atoms in the silicon precursor and the long carbon chain alkyl alkoxy silane, and form a coordination intermediate in the presence of a catalyst. At this time, the silicon atom is negatively charged, weakening the Si-O bond between the alkoxy group and the silicon atom, so that the alkoxy group is replaced from the silicon atom to form an alcohol. The hydrolysis product of the silicon precursor undergoes a dehydration condensation reaction to form silica gel, and the hydroxyl groups in the hydrolysis product of the long carbon chain alkyl alkoxy silane undergo further condensation reaction with the silica gel. In this case, it is beneficial for the long hydrophobic alkyl chain to be grafted on the cellulose surface, which greatly enhances the hydrophobicity of the cellulose surface. In addition, according to the -NH2 electron-donating group contained in 3-aminopropyl triethoxy silane, it forms an amino-amino alkaline-alkaline functional group repulsion effect with nicotine, so the multifunctional composite cotton fiber material prepared by adding 3-aminopropyl triethoxy silane modification has a significantly reduced adsorption effect on nicotine.

[0023] The present invention is based on the principle of hydrophobic modification, using plant-based fibers as raw materials, and using 3-aminopropyl triethoxysilane and other long carbon chain alkyl alkoxysilanes to perform hydrophobic and lipophilic modification on them. Through the synergistic effect of the nicotine rejection of the amino groups grafted on the cellulose surface and the hydrophobicity of the low surface energy long chain alkyl, the modified multifunctional composite cotton fiber material simultaneously absorbs less or no water and nicotine in cigarette smoke, and absorbs more tar in cigarette smoke. Thereby reducing the harmful components inhaled by smokers, while not affecting the smoking experience, and improving the use compliance of the filter tip. At the same time, the preparation process is simple and convenient, and can be industrialized and mass-produced.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The raw material cotton fiber described in the present invention is simple and easy to obtain, and has the advantages of large specific surface area, high porosity, wide source, low price, light weight, and biodegradability.

[0026] 2. The method of the present invention is simple to operate, low in cost, mild in reaction conditions, short in production cycle, and does not require large equipment or special instruments.

[0027] 3. The multifunctional composite cotton fiber material prepared by the method of the present invention can not only be used in environmental protection fields such as oil-water separation, but also can selectively and effectively adsorb tar components in cigarette smoke and be applied in the field of cigarette filtration technology, with good adsorption performance and high selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the structural diagram of cellulose;

[0029] Figure 2 The surface wettability test diagram of the fiber material obtained by modifying cotton fiber with 3-aminopropyl triethoxysilane alone and in combination with long carbon chain alkyl alkoxysilane;

[0030] Figure 3 This is an electron microscope photograph of the multifunctional composite cotton fiber material obtained in Example 2;

[0031] Figure 4 This is a photograph of the contact angle of the multifunctional composite cotton fiber material obtained in Example 3 to water. DETAILED DESCRIPTION

[0032] Now the present invention is further described in conjunction with specific examples, and the following examples are only for explaining the present invention, but do not constitute a limitation of the present invention. The test samples and test procedures used in the following examples include the following contents (if the specific experimental conditions not indicated in the examples, usually according to conventional conditions, or according to the conditions recommended by the reagent company; the reagents, consumables, etc. used in the following examples, if not otherwise specified, can be obtained from commercial sources). The endpoints and any values ​​of the range disclosed in this article are not limited to the precise range or value, and these ranges or values ​​should be understood as including values ​​close to these ranges or values. For numerical ranges, between the endpoint values ​​of each range, between the endpoint values ​​of each range and a separate point value, and between separate point values, one or more new numerical ranges can be combined with each other, and these numerical ranges should be considered as specifically disclosed in this article.

[0033] Example 1

[0034] This embodiment adopts the method of in-situ growth and simultaneous introduction of electron-donating amino groups and low-surface-energy chemical groups long-chain alkyl groups to prepare a functional composite cotton fiber material, which specifically includes the following steps:

[0035] 1) At room temperature and pressure, add 2 mL of 3-aminopropyltriethoxysilane and 2 mL of octadecyltrimethoxysilane to 100 mL of anhydrous ethanol, add about 2 mL of glacial acetic acid dropwise while stirring at room temperature, and stir at room temperature for 0.5 h. Add absorbent cotton fiber and soak for 2-12 h.

[0036] 2) Drying: The functionalized cotton fiber is taken out, washed with anhydrous ethanol, and then dried in an oven at 60° C. until constant weight is obtained to obtain a multifunctional composite cotton fiber material.

[0037] The water contact angle of the multifunctional composite cotton fiber material obtained in this embodiment is 153.3±2.7°, as shown in Table 1. The surface wettability comparison experiment between the material obtained in this embodiment and the cotton modified with 3-aminopropyltriethoxysilane alone is shown in Table 1. Figure 2 As shown. According to thermogravimetric analysis, the multifunctional composite cotton fiber material obtained in this embodiment was pyrolyzed at 538°C to reach a constant weight, and its ash residue was 5.76%. The adsorption performance test results of the multifunctional composite cotton fiber filter material obtained in Example 1 are shown in Table 2. The adsorption amount of total particulate matter in cigarette smoke is 1.42 times that of cellulose acetate filter, the adsorption amount of tar in cigarette smoke is 1.46 times that of cellulose acetate filter, the adsorption amount of 16 polycyclic aromatic hydrocarbons in cigarette smoke is 1.17 times that of cellulose acetate filter, and the adsorption amount of nicotine in cigarette smoke is 55.24% lower than that of cellulose acetate filter.

[0038] Table 1 Contact angle test results of the multifunctional composite cotton fibers obtained in Example 1

[0039] Hydrophobic cotton fiber material Contact Angle (CA) Cellulose acetate filter 0 3-Aminopropyltriethoxysilane modified cotton fiber 0 Example 1 153.3±2.7°

[0040] Table 2 Adsorption performance test results of the multifunctional composite cotton fiber filter material obtained in Example 1

[0041]

[0042] Example 2

[0043] 1) Sol-gel method to construct the rough surface of cotton fiber material: At room temperature and pressure, in a mixed solution of 90 mL of ethanol, 10 mL of tetraethyl orthosilicate, and 10 mL of ultrapure water, 10 mL of ammonia water was added drop by drop as a catalyst to the mixed solution, and the absorbent cotton fiber was completely immersed in the prepared silica sol and magnetically stirred for 0.5 h.

[0044] 2) In-situ growth and simultaneous introduction of electron-donating amino groups and low-surface-energy chemical groups with long-chain alkyl groups: At room temperature and pressure, add 2 mL of 3-aminopropyltriethoxysilane and 2 mL of octadecyltrimethoxysilane to 100 mL of anhydrous ethanol, add about 2 mL of glacial acetic acid dropwise while stirring at room temperature, and stir at room temperature for 0.5 h. Add silica-coated cotton fibers and soak for 2-12 h.

[0045] 3) Drying: The functionalized cotton fiber is taken out, washed with anhydrous ethanol, and then dried in an oven at 60° C. until constant weight is obtained to obtain a multifunctional composite cotton fiber material.

[0046] The microstructure of the multifunctional composite cotton fiber material obtained in this embodiment is as follows: Figure 3 As shown in the figure, it can be seen that the diameter of the modified cotton fiber is significantly increased, and the surface is rough, with SiO2 nanoparticles attached. The water contact angle of the multifunctional composite cotton fiber material obtained in Example 2 is 153.0±2.1°, as shown in Table 3. The surface wettability comparison experiment of the material obtained in this example and the cotton modified with 3-aminopropyltriethoxysilane alone is shown in Table 3. Figure 2 As shown. According to thermogravimetric analysis, the multifunctional composite cotton fiber material obtained in this embodiment was pyrolyzed at 553°C to reach a constant weight, and its ash residue was 12.83%. The adsorption performance test results of the multifunctional composite cotton fiber filter material obtained in Example 2 are shown in Table 4. The adsorption amount of total particulate matter in cigarette smoke is 1.22 times that of cellulose acetate filter, the adsorption amount of tar in cigarette smoke is 1.26 times that of cellulose acetate filter, the adsorption amount of 16 kinds of polycyclic aromatic hydrocarbons in cigarette smoke is 1.19 times that of cellulose acetate filter, and the adsorption amount of nicotine in cigarette smoke is 53.42% lower than that of cellulose acetate filter.

[0047] Table 3 Contact angle test results of the multifunctional composite cotton fibers obtained in Example 2

[0048] Hydrophobic cotton fiber material Contact Angle (CA) Cellulose acetate filter 0 3-Aminopropyltriethoxysilane modified cotton fiber 0 Example 2 153.0±2.1°

[0049] Table 4 Adsorption performance test results of the multifunctional composite cotton fiber filter material obtained in Example 2

[0050]

[0051] Example 3

[0052] 1) Sol-gel method and composite modification with two siloxane modifiers: At room temperature and pressure, add 2 mL of ethyl orthosilicate, 0.4 mL of 3-aminopropyltriethoxysilane and 0.4 mL of octadecyltrimethoxysilane to 20 mL of anhydrous ethanol, add about 0.4 mL of glacial acetic acid dropwise under stirring at room temperature, and stir at room temperature for 0.5 h. Add hydrophobic cotton fiber and soak for 2-12 h.

[0053] 2) Drying: The functionalized cotton fiber is taken out, washed with anhydrous ethanol, and then dried in an oven at 60° C. until constant weight is obtained to obtain a multifunctional composite cotton fiber material.

[0054] The water contact angle of the multifunctional composite cotton fiber material obtained in this embodiment is 153.4±3.1°. Figure 4 As shown in Table 5, the surface wettability comparison experiment of the material obtained in this example and the cotton modified with 3-aminopropyltriethoxysilane alone is as follows Figure 2 As shown. According to thermogravimetric analysis, the multifunctional composite cotton fiber material obtained in this embodiment was pyrolyzed at 589°C to reach a constant weight, and its ash residue was 20.66%. The adsorption performance test results of the multifunctional composite cotton fiber filter material obtained in Example 3 are shown in Table 6. The adsorption amount of total particulate matter in cigarette smoke is 1.22 times that of cellulose acetate filter, the adsorption amount of tar in cigarette smoke is 1.25 times that of cellulose acetate filter, the adsorption amount of 16 polycyclic aromatic hydrocarbons in cigarette smoke is 1.35 times that of cellulose acetate filter, and the adsorption amount of nicotine in cigarette smoke is 50.21% lower than that of cellulose acetate filter.

[0055] Table 5 Contact angle test results of the multifunctional composite cotton fibers obtained in Example 3

[0056] Hydrophobic cotton fiber material Contact Angle (CA) Cellulose acetate filter 0 3-Aminopropyltriethoxysilane modified cotton fiber 0 Example 3 153.4±3.1°

[0057] Table 6 Adsorption performance test results of the multifunctional composite cotton fiber filter material obtained in Example 3

[0058]

[0059] Cigarette harmful substances adsorption performance test

[0060] The multifunctional composite cotton fiber materials prepared in Examples 1-3 were subjected to a simulated smoking experiment. An appropriate amount of the multifunctional composite cotton fiber materials prepared in Examples 1-3 of the present invention was weighed, and a cigarette was connected to a simulated commercially available acetate fiber filter, and a control experiment was conducted with commercially available acetate fiber of the same specifications without the outer layer of paper wrapping the filter.

[0061] Moisture adsorption determination method: 20 commercially available acetate filter cigarettes and 20 cigarettes of Example 3 were used for a simulated smoking experiment using a homemade smoking device. The cigarettes were puffed once every 5 seconds, and the total puffing time of one cigarette was 3 minutes. The weight was immediately weighed after the puffing was completed to obtain the total adsorption of the total particulate matter of the cigarette smoke by the filter material. The adsorbate was then extracted with a solvent, and the water content adsorbed in the filter was measured by gas chromatography.

[0062] Method for determining adsorption of polycyclic aromatic hydrocarbons: 20 commercially available acetate filter cigarettes and 20 cigarettes from group 3 of Example 3 were used to simulate smoking experiments using a homemade smoking device. The cigarettes were puffed once every 5 seconds, and the total puffing time of one cigarette was 3 minutes. The weight was immediately weighed after the puffing was completed to obtain the total adsorption of the filter material on the total particulate matter of cigarette smoke. The adsorbate was then extracted with a solvent, and the content of polycyclic aromatic hydrocarbons adsorbed in the filter was measured by gas chromatography-mass spectrometry analysis.

[0063] Nicotine adsorption determination method: 20 commercially available acetate filter cigarettes and 20 cigarettes of Example 3 were used for a simulated smoking experiment using a homemade smoking device. The cigarettes were puffed once every 5 seconds, and the total puffing time of one cigarette was 3 minutes. The weight was immediately weighed after the puffing was completed to obtain the total adsorption of the filter material on the total particulate matter of the cigarette smoke. The adsorbate was then extracted with a solvent, and the nicotine content adsorbed in the filter was measured by gas chromatography-mass spectrometry analysis.

[0064] Tar adsorption determination method: According to the national standard of the People's Republic of China GB / T 19609-2004, m 焦油 =m 总粒相物 -m 水 -m 尼古丁 .

[0065] As can be seen from Tables 2, 4 and 6, the multifunctional composite cotton fiber filter of the present invention has significantly higher adsorption capacity of tar and polycyclic aromatic hydrocarbons than commercially available cellulose acetate filter, and the tar adsorption capacity is increased by more than 25%.

[0066] As shown in Tables 2, 4 and 6, the amount of nicotine adsorbed by the multifunctional composite cotton fiber filter of the present invention is reduced by more than 50%, which is significantly lower than the commercially available cellulose acetate filter. Smokers only need to smoke half the amount of cigarettes to get the same experience as before.

[0067] It can be seen that smokers can obtain the same nicotine intake with only half the original amount of cigarettes by using the filter made of the multifunctional composite cotton fiber material provided in the above embodiment, and obtain the satisfaction of smoking, while the amount of tar inhaled is significantly reduced, and the harm to health is greatly reduced. In addition, the filter can also improve the dry feeling of smoking and has better palatability.

[0068] The cigarette filter rod made of the multifunctional composite cotton fiber material based on the above embodiment uses cotton fiber as raw material, and has the advantages of safe and healthy components, wide sources, low cost, and environmental friendliness. The cotton fiber modified by super hydrophobicity has better adsorption selectivity and adsorption capacity for tar substances including harmful components of polycyclic aromatic hydrocarbons; and in its preparation process, the introduced amino group repels the functional group formed by nicotine, thereby achieving less adsorption of nicotine. As an adsorption material for cigarette filter rods, it can provide different application scenarios.

[0069] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. Application of a multifunctional composite cotton fiber material in a cigarette smoke filtration product, characterized in that: The preparation method of the multifunctional composite cotton fiber material comprises the following steps: Step 1, adding long carbon chain alkyl alkoxysilane and 3-aminopropyl triethoxysilane to an organic solvent, adding catalyst acetic acid dropwise under stirring, stirring for 0.5 h to mix well, to obtain a reaction solution; the volume ratio of the long carbon chain alkyl alkoxysilane, 3-aminopropyl triethoxysilane, organic solvent and catalyst is 1:1:50:1; Step 2: Add cotton fiber to the reaction solution obtained in step 1 and completely immerse it in the reaction liquid. After standing for 2-12 hours, take it out and repeatedly rinse it with ethanol. Then, put the modified cotton fiber into an oven and dry it until constant weight is obtained to obtain functionalized cotton fiber with hydrophobicity; the cotton fiber is absorbent cotton or surface roughened cotton fiber.

2. Application of a multifunctional composite cotton fiber material in a cigarette smoke filtration product, characterized in that: The preparation method of the multifunctional composite cotton fiber material comprises the following steps: Step 1, adding a silicon precursor, a long carbon chain alkyl alkoxysilane, and 3-aminopropyltriethoxysilane to an organic solvent, and gradually adding a catalyst acetic acid, stirring for 0.5 h to mix, to obtain a reaction solution; the volume ratio of the silicon precursor, the long carbon chain alkyl alkoxysilane, the 3-aminopropyltriethoxysilane, the organic solvent, and the catalyst is 1 to 5:1:1:50:1; Step 2, adding cotton fiber to the reaction solution obtained in step 1 and completely immersing it in the reaction liquid, taking it out after standing for 2 to 12 hours, rinsing it with ethanol, and drying it in an oven until constant weight is obtained to obtain a multifunctional composite cotton fiber material; the cotton fiber is absorbent cotton or surface-roughened cotton fiber.

3. The use of a multifunctional composite cotton fiber material as claimed in claim 1 or 2 in a cigarette smoke filtration product, characterized in that , the preparation method of the surface roughened cotton fiber is: Add silicon precursor, water and organic solvent into a reaction container, stir and mix, then add catalyst acetic acid or ammonia water drop by drop and stir for 0.5 to 1 hour, add cotton fiber after mixing, stir for 0.5 to 4 hours, take out, wash repeatedly with ethanol, put into an oven at a temperature range of 50 to 80°C to dry, and obtain surface roughened cotton fiber; the volume ratio of silicon precursor, water, organic solvent and catalyst is 1:1:8 to 20:

1.

4. The use of a multifunctional composite cotton fiber material as claimed in claim 1 or 2 in a cigarette smoke filtration product, characterized in that: The drying temperature of step 2 is 50-80°C.

5. The use of a multifunctional composite cotton fiber material as claimed in claim 1 or 2 in a cigarette smoke filtration product, characterized in that: The organic solvent is any one of ethanol, methanol, ethyl acetate, acetone and toluene, or a mixture of two or more thereof.

6. The use of a multifunctional composite cotton fiber material in a cigarette smoke filtration product as claimed in claim 2, characterized in that: The silicon precursor is any one of triethoxysilane, tetramethoxysilane and tetraethoxysilane, or a mixture of two or more thereof.

7. The use of a multifunctional composite cotton fiber material in a cigarette smoke filtration product as claimed in claim 3, characterized in that: The silicon precursor is any one of triethoxysilane, tetramethoxysilane and tetraethoxysilane, or a mixture of two or more thereof.

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