Mildew-proof powder for high-efficiency photovoltaic glass and preparation method of mildew-proof powder
By modifying mesoporous silica to load organic and inorganic acids, and combining hydroxyethylidene diphosphate with flow aids, a synergistic antibacterial system is formed, which solves the problem of poor anti-mold effect of photovoltaic glass anti-mold products and achieves a highly efficient, long-lasting and environmentally friendly anti-mold effect.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- IRICO HEFEI PHOTOVOLTAIC CO LTD
- Filing Date
- 2026-01-26
- Publication Date
- 2026-04-21
AI Technical Summary
Existing anti-mold products for photovoltaic glass have poor anti-mold effects, short duration of action, are corrosive to the glass surface, and are not environmentally friendly. Furthermore, some products generate harmful substances during use, affecting the power generation performance of photovoltaic modules and the environment.
Modified mesoporous silica is used as a functional carrier. By grafting polymethyl methacrylate segments onto its surface, a physical barrier is formed, which loads organic and inorganic acids. The mesoporous channel structure enables the slow release of acids. It also combines with hydroxyethylidene diphosphate to form an ion complex with the glass surface, thus forming a synergistic antibacterial system. At the same time, flow aids are used to improve the flowability of the powder and ensure a uniform coating.
It achieves a highly efficient and long-lasting anti-mildew effect, avoids the loss of acidic components, inhibits mold growth, reduces shedding during storage and transportation, ensures the transparency and environmental friendliness of the glass surface, and avoids the residue of harmful substances.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of coating technology, and in particular to a high-efficiency anti-mildew powder for photovoltaic glass and its preparation method. Background Technology
[0002] As a crucial component of solar photovoltaic modules, the cleanliness and light transmittance of photovoltaic glass directly impact the power generation efficiency of these modules. During the production, storage, and transportation of photovoltaic glass, factors such as ambient temperature and humidity can easily lead to mold growth on the glass surface. Mold not only disrupts the smoothness of the glass surface but also reduces its light transmittance, thereby affecting the power generation performance of the photovoltaic modules.
[0003] Currently, most commercially available anti-mold products for photovoltaic glass use single-component or simple compound formulations, resulting in poor anti-mold effects, short duration of action, and corrosiveness to glass surfaces. Some anti-mold products also generate harmful substances during use, polluting the environment and contradicting current green and environmentally friendly development principles. Therefore, developing an anti-mold powder for photovoltaic glass that offers superior anti-mold performance, long-lasting action, is non-corrosive, and environmentally friendly is of significant practical importance. Summary of the Invention
[0004] The purpose of this invention is to provide a high-efficiency anti-mildew powder for photovoltaic glass, so as to improve the problems of poor anti-mildew powder effect, short duration and environmental unfriendliness in the prior art.
[0005] The present invention also aims to provide a method for preparing a high-efficiency anti-mildew powder for photovoltaic glass.
[0006] In a first aspect, the present invention provides a high-efficiency anti-mildew powder for photovoltaic glass, comprising the following components in parts by weight: 30-70 parts of modified mesoporous silica; Organic acids 20-50 parts; 5-20 parts of inorganic acid; Hydroxyethylidene diphosphate 0.5–5 parts; 3-8 parts of flow aid; The modified mesoporous silica is surface-grafted with polymethyl methacrylate segments.
[0007] By employing the above technical solutions, mesoporous silica possesses a high specific surface area and mesoporous channel structure, providing adsorption sites for organic and inorganic acids. This allows acidic antifungal components to be loaded onto the interior and surface of the channels. Simultaneously, by grafting polymethyl methacrylate (PMMA) segments onto the mesoporous silica, the hydrophobic properties of the PMMA segments form a physical barrier, preventing the rapid swelling and loss of acidic components due to environmental moisture. Furthermore, the strong binding force between the PMMA segments and the mesoporous silica, through a polyethylene glycol acrylate transition layer, further enhances the loading stability of the acidic components and reduces the shedding rate of acidic components during storage and transportation. Organic acids can bind to the amino groups on the mold cell membrane through their carboxyl groups, disrupting cell membrane integrity and inhibiting mold metabolism. Inorganic acids, on the other hand, can penetrate into the mold cell, interfering with enzyme active sites and blocking energy metabolism pathways. Together, they form a synergistic antibacterial system of physical destruction and biochemical inhibition. Hydroxyethylidene diphosphate can complex calcium and magnesium ions on and inside the photovoltaic glass surface through its phosphonic acid groups, inhibiting salting-out and mold growth caused by ion precipitation. Simultaneously, its polar groups can form hydrogen bonds with organic and inorganic acids, further enhancing the synergistic effect of the anti-mold components. Furthermore, hydroxyethylidene diphosphate is readily soluble in water and can be completely removed with water during subsequent cleaning, leaving no residue on the photovoltaic glass surface. Flow aids can improve powder flowability, reduce friction between powder particles, prevent agglomeration or dust during spraying, and ensure that the anti-mold powder forms a uniform thin coating on the glass surface.
[0008] Preferably, the modified mesoporous silica is prepared by the following method: A1. Disperse mesoporous silica in anhydrous ethanol, ultrasonically disperse, add aminosilane coupling agent, react at 70-80℃ for 20-24h, centrifuge, wash, and dry to obtain aminated mesoporous silica. A2. Aminated mesoporous silica was dispersed in methanol solution and ultrasonically dispersed. Polyethylene glycol acrylate was added and reacted for 20-24 hours. After washing and drying, mesoporous silica modified with polyethylene glycol acrylate was obtained. A3. Polyethylene glycol acrylate-modified mesoporous silica is added to an aqueous nitric acid solution, followed by the addition of methyl methacrylate monomer and cerium ammonium nitrate. The reaction is carried out under a nitrogen atmosphere at 30–40°C for 20–24 hours. The silica is then washed and dried to obtain the final product.
[0009] Preferably, in step A1, the ratio of the amount of mesoporous silica to aminosilane coupling agent is 1g: (1-2)mL.
[0010] Preferably, the aminosilane coupling agent includes γ-aminopropyltriethoxysilane and N-aminoethyl-γ-aminopropyltrimethoxysilane.
[0011] Preferably, in step A2, the ratio of the amount of aminated mesoporous silica to polyethylene glycol acrylate is 1g: (0.3~0.8)g.
[0012] Preferably, the ratio of polyethylene glycol acrylate-modified mesoporous silica, methyl methacrylate monomer, and cerium ammonium nitrate is 0.1 g : (0.8-1.2) mL : (0.5-0.6) g.
[0013] Preferably, the organic acid includes fumaric acid and / or adipic acid; the inorganic acid is boric acid.
[0014] Preferably, the flow aid is polyethylene glycol monomethyl ether and polyolefin wax micro powder in a mass ratio of 1:(2-4).
[0015] A method for preparing a high-efficiency anti-mildew powder for photovoltaic glass includes the following steps: S1. Mix 30-70 parts of modified mesoporous silica with 20-50 parts of organic acid and 5-20 parts of inorganic acid in a high-speed mixer to obtain mixture A; S2. Mix mixture A, 0.5-5 parts of hydroxyethylidene diphosphate and 3-8 parts of flow aid evenly and then homogenize them to obtain a high-efficiency anti-mildew powder for photovoltaic glass.
[0016] Preferably, in step S2, the homogenization process is carried out at a rotation speed of 300–800 rpm for 10–30 min.
[0017] The beneficial effects of this invention are: The high-efficiency anti-mildew powder for photovoltaic glass of this invention uses modified mesoporous silica with surface-grafted polymethyl methacrylate segments as a functional carrier. This not only fully utilizes the high specific surface area and pore structure of the mesoporous material to achieve the adsorption and slow release of anti-mildew active ingredients such as organic and inorganic acids, but also greatly improves the compatibility and dispersibility of the powder system through the introduction of surface polymer segments. Specifically, the PEGA transition layer solves the compatibility problem between PMMA and polar acids, providing hydrogen bond adsorption sites for organic and inorganic acids. The PMMA segments form a hydrophobic barrier to inhibit the loss of acid components due to moisture swelling, and the mesoporous channels provide space for acid storage and slow release. The synergistic effect of these three elements avoids the defects of traditional single carriers that are prone to detachment. Meanwhile, organic acids disrupt the cell membranes of molds, and inorganic acids interfere with the active sites of enzymes. These two mechanisms inhibit mold growth through a dual pathway of physiological structure and biochemical metabolism. Hydroxyethylidene diphosphate, on the other hand, complexes calcium and magnesium ions in photovoltaic glass, inhibiting salting-out mold growth and forming a synergistic defense against both mold growth and bacterial growth. Finally, a specific ratio of flow aids not only optimizes the physical flowability and dispersibility of the powder but also promotes uniform mixing and stable existence of the components during processing and use, avoiding clumping during storage and uneven application. This multi-layered synergy gives the final product a rapid and long-lasting anti-mold effect. Detailed Implementation
[0018] A high-efficiency anti-mildew powder for photovoltaic glass comprises the following components in parts by weight: 30-70 parts of modified mesoporous silica; Organic acids 20-50 parts; 5-20 parts of inorganic acid; Hydroxyethylidene diphosphate 0.5–5 parts; 3-8 parts of flow aid; By employing the above technical solutions, mesoporous silica possesses a high specific surface area and mesoporous channel structure, providing adsorption sites for organic and inorganic acids. This allows acidic antifungal components to be loaded onto the interior and surface of the channels. Simultaneously, by grafting polymethyl methacrylate (PMMA) segments onto the mesoporous silica, the hydrophobic properties of the PMMA segments form a physical barrier, preventing the rapid swelling and loss of acidic components due to environmental moisture. Furthermore, the strong binding force between the PMMA segments and the mesoporous silica, through a polyethylene glycol acrylate transition layer, further enhances the loading stability of the acidic components and reduces the shedding rate of acidic components during storage and transportation. Organic acids can bind to the amino groups on the mold cell membrane through their carboxyl groups, disrupting cell membrane integrity and inhibiting mold metabolism. Inorganic acids, on the other hand, can penetrate into the mold cell, interfering with enzyme active sites and blocking energy metabolism pathways. Together, they form a synergistic antibacterial system of physical destruction and biochemical inhibition. Hydroxyethylidene diphosphate can complex calcium and magnesium ions on and inside the photovoltaic glass surface through its phosphonic acid groups, inhibiting salting-out and mold growth caused by ion precipitation. Simultaneously, its polar groups can form hydrogen bonds with organic and inorganic acids, further enhancing the synergistic effect of the anti-mold components. Furthermore, hydroxyethylidene diphosphate is readily soluble in water and can be completely removed with water during subsequent cleaning, leaving no residue on the photovoltaic glass surface. Flow aids can improve powder flowability, reduce friction between powder particles, prevent agglomeration or dust during spraying, and ensure that the anti-mold powder forms a uniform thin coating on the glass surface.
[0019] In some embodiments, modified mesoporous silica is prepared by the following method: A1. Disperse mesoporous silica in anhydrous ethanol, ultrasonically disperse, add aminosilane coupling agent, react at 70-80℃ for 20-24h, centrifuge, wash, and dry to obtain aminated mesoporous silica. A2. Aminated mesoporous silica was dispersed in methanol solution and ultrasonically dispersed. Polyethylene glycol acrylate was added and reacted for 20-24 hours. After washing and drying, mesoporous silica modified with polyethylene glycol acrylate was obtained. A3. Polyethylene glycol acrylate-modified mesoporous silica is added to an aqueous nitric acid solution, followed by the addition of methyl methacrylate monomer and cerium ammonium nitrate. The reaction is carried out under a nitrogen atmosphere at 30–40°C for 20–24 hours. The silica is then washed and dried to obtain the final product.
[0020] By adopting the above technical solution, mesoporous silica is first aminated to provide reaction sites for subsequent polyethylene glycol acrylate modification using the amino groups of the aminosilane coupling agent, ensuring uniform grafting of the transition layer. Then, hydrophilic segments are introduced through polyethylene glycol acrylate modification to solve the compatibility problem between the hydrophobic segments of polymethyl methacrylate and the hydrophilic surface of mesoporous silica, while providing polar adsorption sites for acidic antifungal components. Finally, polymethyl methacrylate segments are grafted onto the surface of the hydrophilic transition layer using a cerium ammonium nitrate initiation system, resulting in modified mesoporous silica with stable structure and good interfacial compatibility.
[0021] In some embodiments, the aminosilane coupling agent includes γ-aminopropyltriethoxysilane and N-aminoethyl-γ-aminopropyltrimethoxysilane; the selected coupling agent can effectively introduce amino groups onto the silica surface, and the reaction conditions are mild and the grafting efficiency is high.
[0022] In some embodiments, in step A1, the ratio of mesoporous silica to aminosilane coupling agent is 1 g: (1-2) mL; this range of amounts ensures sufficient surface amination while avoiding excessive coupling agent leading to self-polymerization.
[0023] In some embodiments, in step A2, the ratio of the amount of aminated mesoporous silica to polyethylene glycol acrylate is 1 g: (0.3-0.8) g; this ratio ensures that the polyethylene glycol acrylate molecules are fully grafted to form a sufficient density of reaction sites, which is beneficial to the subsequent grafting growth of polymethyl methacrylate segments.
[0024] In some embodiments, the ratio of polyethylene glycol acrylate-modified mesoporous silica, methyl methacrylate monomer, and cerium ammonium nitrate is 0.1 g : (0.8–1.2) mL : (0.5–0.6) g. This ratio can achieve a high grafting rate of methyl methacrylate segments on the silica surface, which can act as a hydrophobic barrier without completely blocking the mesoporous channels, ensuring that acidic components can be released slowly.
[0025] In some embodiments, the organic acid includes fumaric acid and / or adipic acid; the inorganic acid is boric acid; fumaric acid and adipic acid have good antifungal activity and environmental friendliness; boric acid can enhance the antibacterial spectrum and weather resistance of the antifungal powder, and has good compatibility with other components.
[0026] In some embodiments, the flow aid is polyethylene glycol monomethyl ether and polyolefin wax micro powder in a mass ratio of 1:(2-4). The polyethylene glycol monomethyl ether can form a hydrophilic adsorption layer on the surface of the powder particles, improving the ease of cleaning, while the polyolefin wax micro powder can fill the gaps between the powder particles and reduce the adhesion between the particles. The two work together to balance the flowability and ease of cleaning of the powder, avoiding the problems of insufficient flowability or excessive residue caused by a single aid.
[0027] A method for preparing a high-efficiency anti-mildew powder for photovoltaic glass includes the following steps: S1. Mix 30-70 parts of modified mesoporous silica with 20-50 parts of organic acid and 5-20 parts of inorganic acid in a high-speed mixer to obtain mixture A; S2. Mix mixture A, 0.5-5 parts of hydroxyethylidene diphosphate and 3-8 parts of flow aid evenly and then homogenize them to obtain a high-efficiency anti-mildew powder for photovoltaic glass.
[0028] By employing the above technical methods, the high-speed mixing in step S1 allows organic and inorganic acids to be initially adsorbed onto the mesoporous channels and surface of the modified mesoporous silica, achieving initial fixation by utilizing the adsorption force of the mesopores and the barrier effect of the polymethyl methacrylate shell. The homogenization treatment in step S2 further breaks up local agglomeration, allowing hydroxyethylidene diphosphonic acid, flow aid, and mixture A to fully integrate, ensuring that each component is evenly distributed in the powder, and avoiding localized failure of anti-mildew properties or decrease in light transmittance due to uneven component distribution.
[0029] In some embodiments, in step S2, the homogenization process is performed at a speed of 300–800 rpm for 10–30 min. These process conditions ensure thorough and uniform mixing while avoiding overheating or particle breakage due to excessive shearing or prolonged time, thus maintaining the integrity of the powder structure.
[0030] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.
[0031] Preparation Example
[0032] Preparation Example 1: A modified mesoporous silica was prepared by the following method: A1. Disperse 1g of mesoporous silica in 50mL of anhydrous ethanol, sonicate, add γ-aminopropyltriethoxysilane, react at 80℃ for 24h, centrifuge, wash with anhydrous ethanol, and dry at 50℃ to obtain aminated mesoporous silica. A2. 1g of aminated mesoporous silica was dispersed in 50mL of methanol solution and ultrasonically dispersed. 0.6g of polyethylene glycol acrylate was added and reacted at 40℃ for 24h. The silica was washed with deionized water and dried at 50℃ to obtain polyethylene glycol acrylate modified mesoporous silica. A3. 0.1 g of polyethylene glycol acrylate-modified mesoporous silica was added to 50 mL of 0.1 M nitric acid aqueous solution, along with 1 mL of methyl methacrylate monomer and 0.5 g of cerium ammonium nitrate. The reaction was carried out under a nitrogen atmosphere at 40 °C for 24 h. The silica was then washed with deionized water and dried at 50 °C to obtain a modified mesoporous silica.
[0033] Preparation Example 2, a modified mesoporous silica, differs from Preparation Example 1 only in that, in step A3, the amount of methyl methacrylate monomer is 0.1 mL.
[0034] Example
[0035] Example 1: A high-efficiency anti-mildew powder for photovoltaic glass is prepared by the following method: S1. Mix 50 parts of the modified mesoporous silica prepared in Preparation Example 1 with 40 parts of fumaric acid and 10 parts of boric acid in a high-speed mixer to obtain mixture A. S2. Mix mixture A, 3 parts of hydroxyethylidene diphosphate and 5 parts of flow aid evenly and then homogenize them at 400 rpm for 20 min to obtain a high-efficiency anti-mildew powder for photovoltaic glass; wherein the flow aid is polyethylene glycol monomethyl ether and polyolefin wax micro powder in a mass ratio of 1:2.
[0036] Example 2: A high-efficiency anti-mildew powder for photovoltaic glass is prepared by the following method: S1. Mix 30 parts of the modified mesoporous silica prepared in Preparation Example 1 with 20 parts of fumaric acid and 5 parts of boric acid in a high-speed mixer to obtain mixture A; S2. Mix mixture A, 0.5 parts of hydroxyethylidene diphosphate and 3 parts of flow aid evenly and then homogenize them at 400 rpm for 20 min to obtain a high-efficiency anti-mildew powder for photovoltaic glass; wherein the flow aid is polyethylene glycol monomethyl ether and polyolefin wax micro powder in a mass ratio of 1:2.
[0037] Example 3: A high-efficiency anti-mildew powder for photovoltaic glass is prepared by the following method: S1. Mix 70 parts of the modified mesoporous silica prepared in Preparation Example 1 with 50 parts of fumaric acid and 20 parts of boric acid in a high-speed mixer to obtain mixture A. S2. Mix mixture A, 5 parts of hydroxyethylidene diphosphate and 8 parts of flow aid evenly and then homogenize them at 400 rpm for 20 min to obtain a high-efficiency anti-mildew powder for photovoltaic glass; wherein the flow aid is polyethylene glycol monomethyl ether and polyolefin wax micro powder in a mass ratio of 1:2.
[0038] Comparative Example
[0039] Comparative Example 1 is a high-efficiency anti-mildew powder for photovoltaic glass, which differs from Example 1 only in that the amount of modified mesoporous silica added is 10 parts.
[0040] Comparative Example 2, an anti-mildew powder for high-efficiency photovoltaic glass, differs from Example 1 only in that the modified mesoporous silica prepared in Preparation Example 2 is used instead of the modified mesoporous silica in Example 1.
[0041] Comparative Example 3 is a high-efficiency anti-mildew powder for photovoltaic glass, which differs from Example 1 only in that it does not contain hydroxyethylidene diphosphate.
[0042] Comparative Example 4: A high-efficiency anti-mildew powder for photovoltaic glass, which differs from Example 1 only in that it does not contain organic acids.
[0043] Performance testing
[0044] 1. Accelerated mold growth test: The high-efficiency photovoltaic glass prepared in Examples 1-3 and Comparative Examples 1-3 were uniformly sprayed with anti-mold powder between two pieces of photovoltaic glass, and then placed in a constant temperature and humidity chamber for accelerated mold growth test at a temperature of 60℃ and a relative humidity of 85%. The mold growth on the glass surface was observed after 20 days, and the test results are shown in Table 1; 2. On-site storage test on the production line: To more realistically evaluate the anti-mold durability of the present invention, the anti-mold powder prepared in Example 1 and commercially available conventional anti-mold powder were applied to the company's actual photovoltaic glass production line. After the glass was cut, the anti-mold powder was sprayed onto the surface of the glass sheets according to standard operating procedures, and then stacked and stored in the raw glass warehouse. The warehouse environment was under natural conditions, experiencing daily fluctuations in temperature and humidity; Table 1 Performance test results
[0045] The results of on-site storage tests on the production line involved periodic sampling of glass slides over a storage period exceeding 30 days. The results showed that the glass stacks using the anti-mold powder prepared in Example 1 consistently exhibited no mold growth, and the glass surfaces remained smooth and transparent. In stark contrast, the control group glass using commercially available conventional anti-mold powder, under the same storage conditions, showed some speckled mold spots appearing after approximately 25-30 days.
[0046] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.
Claims
1. A high-efficiency anti-mildew powder for photovoltaic glass, characterized in that, The components include the following parts by weight: 30-70 parts of modified mesoporous silica; Organic acids 20-50 parts; 5-20 parts of inorganic acid; Hydroxyethylidene diphosphate 0.5–5 parts; 3-8 parts of flow aid; The modified mesoporous silica is surface-grafted with polymethyl methacrylate segments.
2. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 1, characterized in that, The modified mesoporous silica is prepared by the following method: A1. Disperse mesoporous silica in anhydrous ethanol, ultrasonically disperse, add aminosilane coupling agent, react at 70-80℃ for 20-24h, centrifuge, wash, and dry to obtain aminated mesoporous silica. A2. Aminated mesoporous silica was dispersed in methanol solution and ultrasonically dispersed. Polyethylene glycol acrylate was added and reacted for 20-24 hours. After washing and drying, mesoporous silica modified with polyethylene glycol acrylate was obtained. A3. Polyethylene glycol acrylate-modified mesoporous silica is added to an aqueous nitric acid solution, followed by the addition of methyl methacrylate monomer and cerium ammonium nitrate. The reaction is carried out under a nitrogen atmosphere at 30–40°C for 20–24 hours. The silica is then washed and dried to obtain the final product.
3. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 2, characterized in that, In step A1, the ratio of the amount of mesoporous silica to aminosilane coupling agent is 1g: (1~2)mL.
4. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 2, characterized in that, In step A1, the aminosilane coupling agent includes γ-aminopropyltriethoxysilane and N-aminoethyl-γ-aminopropyltrimethoxysilane.
5. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 2, characterized in that, In step A2, the ratio of the amount of aminated mesoporous silica to polyethylene glycol acrylate is 1g: (0.3~0.8)g.
6. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 2, characterized in that, The ratio of the amounts of polyethylene glycol acrylate-modified mesoporous silica, methyl methacrylate monomer, and cerium ammonium nitrate is 0.1 g : (0.8–1.2) mL : (0.5–0.6) g.
7. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 1, characterized in that, The organic acid includes fumaric acid and / or adipic acid; the inorganic acid is boric acid.
8. The high-efficiency anti-mildew powder for photovoltaic glass according to claim 1, characterized in that, The flow aid is polyethylene glycol monomethyl ether and polyolefin wax micro powder in a mass ratio of 1:(2-4).
9. A method for preparing a high-efficiency anti-mildew powder for photovoltaic glass, used to prepare the high-efficiency anti-mildew powder for photovoltaic glass according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Mix 30-70 parts of modified mesoporous silica with 20-50 parts of organic acid and 5-20 parts of inorganic acid in a high-speed mixer to obtain mixture A; S2. Mix mixture A, 0.5-5 parts of hydroxyethylidene diphosphate and 3-8 parts of flow aid evenly and then homogenize them to obtain the high-efficiency photovoltaic glass anti-mildew powder.
10. The method for preparing a high-efficiency anti-mildew powder for photovoltaic glass according to claim 9, characterized in that, In step S2, the homogenization process is performed at a rotation speed of 300–800 rpm for 10–30 min.