Weatherable and yellowing resistant high-transparency silicone rubber and preparation method thereof

Abstract

The present application relates to the technical field of organic silicon polymer materials, and particularly relates to weather-resistant and yellowing-resistant high-transparency silicone rubber and a preparation method thereof.The silicone rubber comprises the following raw materials in mass fractions: 100 parts of methyl vinyl silicone rubber raw rubber, 15-25 parts of white carbon black, 3-8 parts of a structure control agent, 0.2-1.0 parts of a composite additive, 0.05-0.15 parts of a release agent, 0.3-1.5 parts of water and 0.8-1.8 parts of a vulcanizing agent.The present application solves the technical problems of easy yellowing, poor weather resistance, low light transmittance and high haze of conventional high-transparency silicone rubber by optimizing the raw rubber system, high-transparency reinforcing filler, weather-resistant and yellowing-resistant additive and high-transparency vulcanizing system, and combining precise mixing and vacuum heat treatment processes.The key performance of the obtained silicone rubber is as follows: light transmittance > 80%, haze < 10%, no obvious yellowing in ultraviolet aging, no discoloration in high-temperature thermal aging, and good mechanical properties and processability.

Description

Technical Field

[0001] This invention relates to the field of organosilicon polymer materials technology, and in particular to a weather-resistant, yellowing-resistant, high-transparency silicone rubber and its preparation method. Background Technology

[0002] Solid silicone is widely used in electronics, construction, photovoltaics and other fields due to its excellent resistance to high and low temperatures, elasticity and biocompatibility. However, traditional solid silicone has two major pain points: First, it is difficult to balance light transmittance and mechanical strength. After adding reinforcing fillers, agglomeration is easy to occur, resulting in decreased light transmittance and increased haze. Second, when used outdoors, long-term exposure to ultraviolet light can easily cause yellowing and embrittlement, resulting in loss of optical stability and mechanical properties.

[0003] In existing technologies, some transparent silicone materials maintain light transmittance by reducing the amount of filler added, but their mechanical strength is insufficient (tensile strength is usually <1.0MPa), which cannot meet the structural support requirements; another type uses MQ silicone resin or conventional nanofillers for reinforcement, which can improve strength, but the light transmittance is mostly below 75%, the haze exceeds 15%, and the weather resistance is insufficient, with the light transmittance decreasing by more than 20% after 1000 hours of outdoor use.

[0004] Existing technology discloses a high-transparency, high-hardness, and anti-aging silicone material for LED encapsulation. This material improves mechanical properties by adding functional additives such as glass fibers (30-60μm) and ceramic particles (60-100μm). However, this technology has significant drawbacks: Firstly, the particle size (30-100μm) of the functional additives is much larger than the wavelength of visible light (380-780nm), inevitably causing severe light scattering, making it difficult for the material's transmittance to exceed 80%, and its haze is generally higher than 18%. Secondly, the inorganic phase characteristics of glass fibers and ceramic particles result in poor compatibility with the silicone matrix, making them prone to interfacial delamination during long-term outdoor use. Combined with the introduction of weather-sensitive components such as epoxy resin and polycarbonate, the transmittance decreases by more than 30% after 1000 hours of UV aging, and the yellowing index exceeds 4.0, failing to meet the long-term reliability requirements of high-transparency outdoor applications.

[0005] For outdoor LED packaging, photovoltaic module light-transmitting layers and other scenarios, the core requirements of "transmittance > 80%, haze < 10%, and stable weather resistance" are difficult to achieve synergistic optimization of optical performance, mechanical strength and weather resistance due to improper filler selection, insufficient formulation compatibility and process design defects. Therefore, it is urgent to develop a new formulation and process that takes into account all three. Summary of the Invention

[0006] The purpose of this invention is to provide a weather-resistant, yellowing-resistant, high-transmittance silicone rubber and its preparation method, so as to solve the problems of yellowing, poor weather resistance, insufficient light transmittance, and high haze of existing high-transmittance silicone rubber.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: One of the technical solutions of the present invention provides a silicone rubber comprising the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 15-25 parts of silica, 3-8 parts of structure control agent, 0.2-1.0 parts of composite additive, 0.05-0.15 parts of release agent, 0.3-1.5 parts of water, and 0.8-1.8 parts of vulcanizing agent.

[0008] The second technical solution of this invention provides a method for preparing the aforementioned silicone rubber, comprising the following steps: (1) Mix raw methyl vinyl silicone rubber, structure control agent, mold release agent and water, add silica and composite additives, and heat treat to obtain rubber compound; (2) The rubber compound is back-refined. After the back-refining is completed, a vulcanizing agent is added and mixed evenly. The mixture is then vulcanized and molded to obtain silicone rubber.

[0009] Compared with the prior art, the present invention has the following beneficial effects: (1) The weather-resistant and yellowing-resistant high-transmittance silicone rubber prepared by this invention has a light transmittance of >86% and a haze of <8%, which is far higher than the requirements of "light transmittance >80% and haze <10%" for outdoor LED packaging, photovoltaic module light transmittance layer and other scenarios. The composite additives and high-transmittance vulcanizing agents work synergistically to make the silicone rubber not yellowed significantly after 300h of ultraviolet light and 48h of heat aging. It is weather-resistant and yellowing-resistant, and stable in long-term use. It has moderate hardness (Shore A≤20) and stable tensile properties, which meet the requirements of processing and use.

[0010] (2) The silicone rubber of the present invention has a high overall filling amount and no diluent is added. It has no precipitation, good dispersibility, and will not cause white carbon black clumps, oil seepage and roller sticking. It has good processing performance and is suitable for industrial mass production.

[0011] (3) This invention achieves a balance of high light transmittance, low haze, weather resistance, yellowing resistance and good mechanical properties through the synergistic optimization of raw rubber, fillers, additives and processes. Detailed Implementation

[0012] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0013] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0014] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0015] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0016] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0017] All raw materials used in this invention can be obtained commercially or prepared using existing technologies.

[0018] This invention provides a silicone rubber comprising the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 15-25 parts of silica, 3-8 parts of structure control agent, 0.2-1.0 parts of composite additive, 0.05-0.15 parts of release agent, 0.3-1.5 parts of water, and 0.8-1.8 parts of vulcanizing agent.

[0019] In this invention, the methyl vinyl silicone rubber raw material is vinyl-terminated methyl vinyl silicone rubber raw material or methyl-terminated methyl vinyl rubber raw material. The vinyl-terminated methyl vinyl silicone rubber raw material has a vinyl content of 0.05-0.22%, for example, 0.05%, 0.08%, 0.1%, 0.15%, 0.2%, or 0.22%; the molecular weight of the vinyl-terminated methyl vinyl silicone rubber raw material is 620,000-680,000, for example, 620,000, 630,000, 640,000, 650,000, 660,000, 670,000, or 680,000; and the volatile matter content is ≤0.8%. The methyl-terminated methyl vinyl silicone rubber raw material has a vinyl content of 1-3%, for example, 1%, 1.5%, 2%, 2.5% or 3%; the molecular weight of the methyl-terminated methyl vinyl silicone rubber raw material is 510,000 to 600,000, for example, 510,000, 540,000, 550,000, 580,000 or 600,000; and the volatile content is ≤1.5%.

[0020] In this invention, the methyl vinyl silicone rubber raw material is colorless and transparent, has low impurities, and excellent light transmittance.

[0021] In this invention, the silicone rubber comprises 15 to 25 parts of silica, for example, 15 parts, 18 parts, 20 parts, 22 parts or 25 parts, etc.

[0022] In this invention, the specific surface area of ​​the silica is 200~300m². 2 / g, for example, can be 200m 2 / g、220m 2 / g、240m 2 / g、250m 2 / g、280m 2 / g or 300m 2 / g etc.

[0023] In this invention, the silica is preferably high-permeability gas-phase silica, which has low metal ion content, low impurities, moderate surface silanol groups, is easy to disperse and does not agglomerate, thus ensuring high permeability and low fogging.

[0024] In this invention, the silicone rubber includes 3 to 8 parts of a structure control agent, for example, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, or 8 parts.

[0025] In this invention, the structure control agent is one or more of dimethyldimethoxysilane, hydroxyl silicone oil, and hexamethyldisilazane; the viscosity of the hydroxyl silicone oil is 10-50 μL. 2 / s, for example, can be 10m 2 / s、20m 2 / s、30m 2 / s, 40m 2 / s or 50m 2 / s etc.

[0026] In a preferred embodiment of the present invention, the structuring control agent is composed of hydroxyl silicone oil and hexamethyldisilazane; in the silicone rubber, the hydroxyl silicone oil is 5 parts and the hexamethyldisilazane is 0.3 parts by mass.

[0027] In this invention, the structuring control agent is a high-transmittance structuring control agent, which can improve the dispersion of silica, reduce haze, and increase light transmittance.

[0028] In this invention, the silicone rubber includes 0.2 to 1.0 parts of composite additives, for example, 0.2 parts, 0.3 parts, 0.4 parts, 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, or 1 part, etc.

[0029] In this invention, the composite additive is one or more of hindered phenolic antioxidants, ultraviolet absorbers, blue modifiers, and low-hydrogen silicone oil; the hydrogen content of the low-hydrogen silicone oil is 0.3~0.8%, for example, it can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7% or 0.8%, etc.

[0030] In this invention, the hindered phenolic antioxidant is preferably Irganox-1726.

[0031] In this invention, the ultraviolet absorber is preferably UV-329.

[0032] In this invention, the blue modulator is preferably a silicon-based blue pigment.

[0033] In this invention, the composite additive is preferably a weather-resistant and yellowing-resistant composite additive that is resistant to high temperatures without yellowing and resistant to ultraviolet aging.

[0034] In this invention, the silicone rubber includes 0.05 to 0.15 parts of a release agent, for example, 0.05 parts, 0.08 parts, 0.1 parts, 0.12 parts, or 0.15 parts.

[0035] In this invention, the release agent is one or both of zinc stearate and high-transparency silicone release agent.

[0036] In this invention, the high-transparency silicone release agent is phenyl silicone oil.

[0037] In this invention, the release agent is preferably a high-transparency release agent composed of zinc stearate and phenyl silicone oil, which is highly efficient in small amounts, does not affect transparency, and does not precipitate whitening.

[0038] In this invention, the silicone rubber includes 0.8 to 1.8 parts of vulcanizing agent, for example, 0.8 parts, 1 part, 1.2 parts, 1.5 parts or 1.8 parts, etc.

[0039] In this invention, the vulcanizing agent is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

[0040] In this invention, the vulcanizing agent is a highly transparent 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, which is colorless, odorless, does not yellow, and does not affect light transmission after vulcanization.

[0041] The present invention also provides a method for preparing the above-mentioned silicone rubber, comprising the following steps: (1) Mix methyl vinyl silicone rubber raw rubber, structure control agent, mold release agent and water, add silica and composite additives, and heat treat to obtain rubber compound; (2) The rubber compound is back-refined. After the back-refining is completed, a vulcanizing agent is added and mixed evenly. The mixture is then vulcanized and molded to obtain silicone rubber.

[0042] Step (1) of this invention involves adding methyl vinyl silicone rubber raw material, structuring control agent, release agent and water to a kneader and stirring for 3-6 minutes. After mixing evenly, silica is added in 3-5 batches, stirring for 10-18 minutes each time to ensure full dispersion and a smooth, particle-free rubber compound. Then, composite additives are added and stirred for 30-50 minutes. Heat treatment is then performed to remove low boiling point, improve transparency and weather resistance, and obtain the rubber compound.

[0043] In this invention, the heat treatment temperature is 150~190℃, the vacuum degree is -0.06~-0.09MPa, and the time is 1~2.5h.

[0044] Step (2) of this invention involves cooling the rubber compound, then refining it, plasticizing it for 10-18 seconds until the surface is smooth, adding a vulcanizing agent and mixing it thoroughly, then vulcanizing and molding it to obtain silicone rubber.

[0045] In this invention, the number of refining cycles is 10 to 18.

[0046] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0047] Unless otherwise specified, in the embodiments of the present invention, the methyl vinyl silicone rubber raw material is vinyl-terminated methyl vinyl silicone rubber raw material, the vinyl content of the vinyl-terminated methyl vinyl silicone rubber raw material is 0.12 mol%, the molecular weight is 650,000, and the volatile matter is ≤0.8%; the specific surface area of ​​the high-permeability gas-phase silica is 250 m² / g. 2 / g; The viscosity of hydroxyl silicone oil is 30mm. 2 / s; the weather-resistant and yellowing-resistant composite additives are Irganox-1726, UV-329, and silicone-based blue pigment, mixed in a ratio of 3:4:1; the high-transparency release agent is a compound of zinc stearate and phenyl silicone oil, with a mass ratio of zinc stearate to phenyl silicone oil of 1 / 2; the high-transparency vulcanizing agent is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

[0048] Example 1 Silicone rubber is composed of the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 20 parts of high-permeability fumed silica, 5 parts of hydroxyl silicone oil, 0.3 parts of hexamethyldisilazane, 0.6 parts of weather-resistant and yellowing-resistant composite additives, 0.1 parts of high-permeability release agent, 1.0 part of deionized water, and 1.2 parts of high-permeability vulcanizing agent.

[0049] The preparation method of silicone rubber is as follows: (1) According to the mass fraction, add methyl vinyl silicone rubber raw rubber, hydroxyl silicone oil, hexamethyldisilazane, zinc stearate and deionized water into a kneader and stir for 4 minutes; then add high permeability gas phase method silica in 3 batches; stir thoroughly for 12 minutes after each addition until the surface of the rubber compound is smooth; (2) After the powdering process is completed, add composite additives according to the mass fraction and continue stirring for 30 minutes. Heat to 170°C, vacuum, and heat treat at a vacuum of -0.08 MPa for 2 hours to obtain silicone rubber. The cooled silicone rubber compound is re-milled on a two-roll mill and plasticized for 15 seconds until the surface is smooth. Then, a high-permeability vulcanizing agent is added and mixed thoroughly. After vulcanization and molding, performance testing is performed.

[0050] Example 2 (Reduction of silica content control) Silicone rubber is composed of the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 15 parts of high-permeability fumed silica, 5 parts of hydroxyl silicone oil, 0.3 parts of hexamethyldisilazane, 0.6 parts of weather-resistant and yellowing-resistant composite additives, 0.1 parts of high-permeability release agent, 1.0 part of deionized water, and 1.2 parts of high-permeability vulcanizing agent.

[0051] The preparation method of silicone rubber is the same as in Example 1.

[0052] Example 3 (Increased silica control) Silicone rubber is composed of the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 25 parts of high-permeability fumed silica, 5 parts of hydroxyl silicone oil, 0.3 parts of hexamethyldisilazane, 0.6 parts of weather-resistant and yellowing-resistant composite additives, 0.1 parts of high-permeability release agent, 1.0 part of deionized water, and 1.2 parts of high-permeability vulcanizing agent.

[0053] The preparation method of silicone rubber is the same as in Example 1.

[0054] Example 4 (Structured control agent reduction control) Silicone rubber is composed of the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 20 parts of high-permeability fumed silica, 2 parts of hydroxyl silicone oil, 0.1 parts of hexamethyldisilazane, 0.6 parts of weather-resistant and yellowing-resistant composite additives, 0.1 parts of high-permeability release agent, 1.0 part of deionized water, and 1.2 parts of high-permeability vulcanizing agent.

[0055] The preparation method of silicone rubber is the same as in Example 1.

[0056] Example 5 (Control without weather-resistant and yellowing-resistant additives) Silicone rubber is composed of the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 20 parts of high-permeability fumed silica, 5 parts of hydroxyl silicone oil, 0.3 parts of hexamethyldisilazane, 0 parts of weather-resistant and yellowing-resistant composite additives, 0.1 parts of high-permeability release agent, 1.0 part of deionized water, and 1.2 parts of high-permeability vulcanizing agent.

[0057] The preparation method of silicone rubber is the same as in Example 1.

[0058] Example 6 (Common Vulcanizing Agent Control) Silicone rubber is composed of the following raw materials in parts by weight: 100 parts methyl vinyl silicone rubber raw rubber, 20 parts high permeability fumed silica, 5 parts hydroxyl silicone oil, 0.3 parts hexamethyldisilazane, 0.6 parts weather-resistant and yellowing-resistant composite additives, 0.1 parts high permeability release agent, 1.0 part deionized water, and 1.2 parts benzoyl peroxide vulcanizing agent.

[0059] The preparation method of silicone rubber is the same as in Example 1.

[0060] Example 7 (Low Vinyl Raw Rubber Control) Silicone rubber is composed of the following raw materials in parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 20 parts of high-permeability fumed silica, 5 parts of hydroxyl silicone oil, 0.3 parts of hexamethyldisilazane, 0.6 parts of weather-resistant and yellowing-resistant composite additives, 0.1 parts of high-permeability release agent, 1.0 part of deionized water, and 1.2 parts of high-permeability vulcanizing agent.

[0061] The vinyl molar content of the methyl vinyl silicone rubber raw material is 0.05%.

[0062] The preparation method of silicone rubber is the same as in Example 1.

[0063] The transmittance, haze, yellowing index, UV transmittance after 300 hours, heat aging at 200℃ for 48 hours, hardness, tensile strength and elongation of silicone rubber were tested according to GB / T 2410-2008. The test results are shown in Table 1.

[0064] Table 1. Performance test results of the silicone rubbers prepared in Examples 1-7

[0065] As shown in Table 1, the silicone rubber prepared in Example 1 has the best performance, with a light transmittance >86% and a haze <8%, which is far higher than the requirements of "light transmittance >80% and haze <10%" for outdoor LED packaging, photovoltaic module light transmittance layers and other scenarios. The composite additives and high-transmittance vulcanizing agents work synergistically to ensure that the silicone rubber does not yellow significantly after 300 hours of ultraviolet light and 48 hours of heat aging. It is weather-resistant, resistant to yellowing, and stable in long-term use. It has moderate hardness, excellent tensile strength and elongation, and meets the requirements for processing and use. Compared to Example 1, Example 2 reduced the amount of silica while still meeting the transmittance and haze standards, but the reinforcement decreased and the mechanical properties were reduced. Example 3, compared to Example 1, increased the amount of silica; excessive silica easily led to slight agglomeration, resulting in a slight decrease in transmittance and an increase in haze. Mechanical properties were improved, but the high transmittance advantage was weakened. Example 4, compared to Example 1, reduced the amount of structure control agent; insufficient structure control agent resulted in poor silica dispersion, substandard transmittance, excessive haze, and increased hardness. Example 5, compared to Example 1, omitted the weather-resistant and yellowing-resistant additives, and the initial transmittance met the standards. However, its weather resistance and yellowing resistance are extremely poor. The silicone rubber yellows significantly after 300 hours of UV exposure, and it yellows severely and becomes slightly sticky after heat aging, making it unsuitable for long-term use. In Example 6, compared with Example 1, the high-transparency vulcanizing agent was replaced with a common vulcanizing agent. The common vulcanizing agent is prone to yellowing and has an odor. The silicone rubber yellows significantly after 300 hours of UV exposure, and it yellows and has an odor after heat aging, affecting its transparency and weather resistance. In Example 7, compared with Example 1, the vinyl content in the methyl vinyl silicone rubber raw rubber was reduced. The low-vinyl raw rubber has better light transmission and haze, but the crosslinking density is low, and the mechanical properties are slightly reduced.

[0066] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A silicone rubber, characterized in that, The raw materials include the following parts by weight: 100 parts of methyl vinyl silicone rubber raw rubber, 15-25 parts of silica, 3-8 parts of structure control agent, 0.2-1.0 parts of composite additive, 0.05-0.15 parts of release agent, 0.3-1.5 parts of water and 0.8-1.8 parts of vulcanizing agent.

2. The silicone rubber according to claim 1, characterized in that, The methyl vinyl silicone rubber raw material is a vinyl-terminated methyl vinyl silicone rubber raw material or a methyl-terminated methyl vinyl rubber raw material. The vinyl-terminated methyl vinyl silicone rubber raw material has a vinyl content of 0.05-0.22% in the molar fraction; the molecular weight of the vinyl-terminated methyl vinyl silicone rubber raw material is 620,000-680,000, and the volatile matter is ≤0.8%. The methyl-terminated methyl vinyl silicone rubber raw material has a vinyl content of 1-3% in the molar fraction; the molecular weight of the methyl-terminated methyl vinyl silicone rubber raw material is 510,000-600,000, and the volatile matter content is ≤1.5%.

3. The silicone rubber according to claim 1, characterized in that, The silica is a high-permeability phase silica; the specific surface area of ​​the silica is 200~300 m² / g. 2 / g.

4. The silicone rubber according to claim 1, characterized in that, The structure control agent is one or more of dimethyldimethoxysilane, hydroxyl silicone oil, and hexamethyldisilazane; the viscosity of the hydroxyl silicone oil is 10-50 μL. 2 / s.

5. The silicone rubber according to claim 1, characterized in that, The composite additive is one or more of hindered phenolic antioxidants, ultraviolet absorbers, blue modifiers, and low-hydrogen silicone oil; the low-hydrogen silicone oil has a hydrogen content of 0.3-0.8%.

6. The silicone rubber according to claim 1, characterized in that, The release agent is one or both of zinc stearate and high-permeability silicone release agent.

7. The silicone rubber according to claim 1, characterized in that, The vulcanizing agent is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

8. The method for preparing silicone rubber according to any one of claims 1 to 7, characterized in that, Includes the following steps: (1) Mix raw methyl vinyl silicone rubber, structure control agent, mold release agent and water, add silica and composite additives, and heat treat to obtain rubber compound; (2) The rubber compound is back-refined. After the back-refining is completed, a vulcanizing agent is added and mixed evenly. The mixture is then vulcanized and molded to obtain silicone rubber.

9. The preparation method according to claim 8, characterized in that, The heat treatment temperature is 150~190℃, the vacuum degree is -0.06~-0.09MPa, and the time is 1~2.5h.

10. The preparation method according to claim 8, characterized in that, The number of refining cycles is 10-18.

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