Condensed type rubber adhesive as well as preparation method and application thereof

Abstract

The invention discloses a condensed rubber adhesive as well as a preparation method and application thereof, and relates to the technical field of high polymer materials. 350 to 600 parts of calcium carbonate; 80 to 160 parts of white carbon black; 460 to 900 parts of silica powder; 150 to 280 parts of cerium oxide; 12-30 parts of a coloring agent; 90 to 220 parts of a cross-linking agent; 30 to 50 parts of a silane coupling agent; the mass ratio of the white carbon black to the calcium carbonate is 1: (3.5-5); by introducing the white carbon black and the calcium carbonate in a specific mass ratio and matching with the silica powder, the cerium oxide and other components, on the basis of remarkably reducing the use amount of the white carbon black and the application cost of the adhesive, the adhesive still has excellent mechanical properties and high-temperature resistance, still has excellent mechanical properties after being aged in a high-temperature environment, and is suitable for industrial production. Meanwhile, the construction performance of the adhesive is not influenced at all, and the curing uniformity and the like of the adhesive are not influenced.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high polymer materials, and particularly relates to a condensation type rubber adhesive as well as a preparation method and application thereof. BACKGROUND

[0002] High-temperature-resistant adhesive is a kind of special adhesive that can maintain good bonding performance, mechanical strength and stability in high-temperature environment, and is widely used in the fields of aerospace, automobile manufacturing, electronic packaging, industrial equipment, etc. Traditional high-temperature-resistant silicone rubber adhesive usually takes silicone as the matrix and adds white carbon black (fumed silica) as the main filler to improve the heat resistance, mechanical properties and rheological properties of the adhesive. White carbon black becomes the key component of high-temperature-resistant adhesive due to its high specific surface area, excellent reinforcing effect and thermal stability.

[0003] However, the production process of white carbon black is complex and the cost is high, and the price is significantly affected by raw materials and market supply and demand, resulting in high overall cost of high-temperature-resistant adhesive, which limits its large-scale application in cost-sensitive industries (such as civil electronics, building materials, etc.).

[0004] At present, in order to reduce the cost of high-temperature-resistant adhesive, the industry tries to replace white carbon black with other mineral fillers (such as talc powder, mica powder or silicon powder), but these materials often have the following problems: 1. Insufficient thermal stability: the temperature resistance of some fillers is poor, which easily decomposes or causes the performance of the adhesive to deteriorate at high temperature (> 250℃); 2. Poor compatibility: the uncoated filler has weak bonding force with the resin matrix, which affects the mechanical strength and durability of the adhesive; 3. Process performance decreases: the replacement of fillers may change the rheological properties of the adhesive, resulting in unsuitable construction viscosity or uneven curing.

[0005] Therefore, it is necessary to provide a filler that can maintain high-temperature-resistant performance, significantly reduce cost, and not affect the application performance of high-temperature-resistant adhesive to meet the application requirements of high-temperature-resistant adhesive, which is a technical problem to be solved. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a condensation type rubber adhesive as well as a preparation method and application thereof.

[0007] To achieve the purpose of the present application, the following technical solutions are adopted:

[0008] In a first aspect, the present application provides a condensation type rubber adhesive, which comprises, by weight:

[0009] 1000-2500 parts of polyorganosiloxane;

[0010] 350-600 parts of calcium carbonate;

[0011] 80-160 parts of silica;

[0012] 460-900 parts of silica powder;

[0013] 150-280 parts of cerium oxide;

[0014] Colorant 12-30 parts;

[0015] 90-220 parts of crosslinking agent;

[0016] 30-50 parts of silane coupling agent;

[0017] Catalyst 0.5-1.5 parts;

[0018] The mass ratio of the silica to calcium carbonate is 1:(3.5-5), for example 1:3.5, 1:4, 1:4.5, 1:5, etc., preferably 1:(3.5-4.5).

[0019] This invention, by replacing a portion of the silica with calcium carbonate and incorporating components such as silica powder and cerium oxide, significantly reduces the amount of silica used and lowers the application cost of the adhesive while maintaining excellent mechanical properties and high-temperature resistance. The mechanical properties remain excellent even after aging at high temperatures, and the adhesive's workability and curing uniformity are completely unaffected. By limiting the mass ratio of silica to calcium carbonate, this invention enables the adhesive to maintain viscosity, thermal stability, and mechanical strength while leveraging the low cost of calcium carbonate to reduce the overall cost of the adhesive by more than 15%.

[0020] In this invention, the amounts of calcium carbonate, silica powder, and cerium oxide added must be within the limits specified in this invention in order to obtain an adhesive that meets application requirements while maintaining low cost.

[0021] In this invention, the 1000-2500 parts of the polyorganosiloxane can be 1000 parts, 1200 parts, 1500 parts, 1800 parts, 2000 parts, 2200 parts, 2500 parts, etc.; the 350-600 parts of calcium carbonate can be 350 parts, 400 parts, 450 parts, 480 parts, 500 parts, 520 parts, 550 parts, 580 parts, 600 parts, etc.; the 80-160 parts of silica can be 80 parts, 100 parts, 110 parts, 120 parts, 130 parts, 140 parts, 150 parts, 160 parts, etc.; and the 460-900 parts of silica powder can be 460 parts, 500 parts, 550 parts, 600 parts, 650 parts, 700 parts, 750 parts, 800 parts, 850 parts, 900 parts, etc. The amounts of cerium oxide (150-280 parts) can be 150, 160, 180, 200, 220, 250, 260, 280, etc.; the amounts of colorant (12-30 parts) can be 12, 15, 18, 20, 22, 25, 28, 30, etc.; the amounts of crosslinking agent (90-220 parts) can be 90, 100, 150, 180, 200, 220, etc.; the amounts of silane coupling agent (30-50 parts) can be 30, 32, 35, 38, 40, 42, 45, 48, 50, etc.; and the amounts of catalyst (0.5-1.5 parts) can be 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, etc.

[0022] Preferably, the calcium carbonate has a particle size D50 ≤ 100 nm, such as 100 nm, 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, etc.

[0023] This invention limits the particle size of calcium carbonate to D50≤100 nm. Smaller particle size calcium carbonate can be more uniformly dispersed in the matrix material, giving polyorganosiloxane a better reinforcing and toughening effect, and better dimensional stability, thus enabling condensation-type rubber adhesives to have excellent performance.

[0024] If the particle size of calcium carbonate is too large, its dispersibility will be poor, which will lead to a decrease in the mechanical properties and high-temperature resistance of condensation-type rubber adhesives.

[0025] In this invention, the test method for particle size D50 is dynamic light scattering (DLS), specifically in accordance with ISO 22412:2017 "Particle size analysis - Dynamic light scattering method".

[0026] Preferably, the calcium carbonate is calcium carbonate modified by an interface modifier.

[0027] This invention, by selecting ultrafine active calcium carbonate and treating its surface with an interface modifier, can increase the compatibility between calcium carbonate and the matrix, thus avoiding any impact on the mechanical strength and durability of the adhesive.

[0028] Preferably, the silica is a hydrophobic silica.

[0029] Preferably, the polyorganosiloxane is selected from hydroxyl-terminated polydimethylsiloxane, and the viscosity of the hydroxyl-terminated polydimethylsiloxane at 25°C is 5000-80000 mPa·s, such as 5000 mPa·s, 10000 mPa·s, 20000 mPa·s, 30000 mPa·s, 40000 mPa·s, 50000 mPa·s, 60000 mPa·s, 70000 mPa·s, 80000 mPa·s, etc.

[0030] Preferably, the crosslinking agent includes a ketoxime crosslinking agent.

[0031] Preferably, the silane coupling agent comprises any one or a combination of at least two of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, or bis(amino)silane oligomers.

[0032] Preferably, the catalyst comprises an organotin catalyst.

[0033] The adhesive provided by the present invention has excellent high temperature resistance. Preferably, after aging at 300°C for 24 hours, the mechanical strength retention rate of the condensation rubber adhesive is ≥85%, such as 85%, 86%, 88%, 90%, 91%, etc., and the hardness retention rate is ≥90%, such as 90%, 91%, 92%, 93%, 95%, etc., and there is no bubbling phenomenon.

[0034] In a second aspect, the present invention provides a method for preparing a condensation-type rubber adhesive as described in the first aspect, the method comprising:

[0035] The components in the formula are mixed to obtain the condensation-type rubber adhesive.

[0036] Preferably, the preparation method includes:

[0037] In a planetary mixer, the components of the formula are initially mixed for 5-10 minutes (e.g., 5 min, 6 min, 7 min, 8 min, 9 min, 1000 rpm, 1100 rpm, 1200 rpm) at a revolution speed of 30-60 rpm and a rotation speed of 800-1200 rpm. Then, under vacuum, the temperature is raised to 100-120℃ (e.g., 100℃, 105℃, 110℃, 115℃, 120℃) and mixed for 1-3 hours (e.g., 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours). Finally, the temperature is lowered to below 40℃ and mixed for 85-140 minutes (e.g., 85 minutes, 90 minutes, 100 minutes, 110 minutes, 120 minutes). min, 120 min, 130 min, 140 min, etc.

[0038] Preferably, the preparation method includes:

[0039] After a preliminary mixing of polyorganosiloxane, calcium carbonate, silica powder, cerium oxide and colorant, a crosslinking agent is added for a second mixing, silica is added for a third mixing, and finally a silane coupling agent and catalyst are added for a fourth mixing to obtain the condensation-type rubber adhesive.

[0040] Preferably, the preparation method is carried out in a planetary mixer.

[0041] Preferably, the preparation method includes:

[0042] S1. In a planetary mixer, the polyorganosiloxane, calcium carbonate, silica powder, cerium oxide, and colorant are initially mixed for 5-10 minutes at a stirring rate of 20-50 rpm (e.g., 20 rpm, 25 rpm, 30 rpm, 35 rpm, 40 rpm, 45 rpm, 50 rpm) and a rotation rate of 300-500 rpm (e.g., 300 rpm, 350 rpm, 400 rpm, 450 rpm, 500 rpm) for 300-500 rpm for 300-500 rpm for 300-500 rpm for 300-6 ... Mix at a stirring rate of 800-1200 rpm (e.g., 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm) for 1-3 hours, such as 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, etc., then cool to below 40°C and mix at a stirring rate of 10-30 rpm (e.g., 10 rpm, 15 rpm, 20 rpm, 25 rpm, 30 rpm) and a rotation rate of 100-300 rpm (e.g., 100 rpm, 150 rpm, 200 rpm, 250 rpm, 300 rpm) for 30-60 minutes, such as 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, etc.

[0043] S2. Add the crosslinking agent and mix under vacuum at a stirring rate of 20-50 rpm (e.g., 20 rpm, 25 rpm, 30 rpm, 35 rpm, 40 rpm, 45 rpm, 50 rpm) and 300-500 rpm (e.g., 300 rpm, 350 rpm, 400 rpm, 450 rpm, 500 rpm) for 25-35 minutes (e.g., 25 minutes, 28 minutes, 30 minutes, 32 minutes, 35 minutes).

[0044] S3. Add silica and mix under vacuum at a stirring rate of 30-60 rpm (e.g., 30 rpm, 35 rpm, 40 rpm, 45 rpm, 50 rpm, 55 rpm, 60 rpm) and 800-1200 rpm (e.g., 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm) for 10-15 minutes (e.g., 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes, 15 minutes).

[0045] S4. Add silane coupling agent and catalyst, and mix under vacuum at a stirring rate of 20-50 rpm (e.g., 20 rpm, 25 rpm, 30 rpm, 35 rpm, 40 rpm, 45 rpm, 50 rpm, etc.) and 300-500 rpm (e.g., 300 rpm, 350 rpm, 400 rpm, 450 rpm, 500 rpm, etc.) for 20-30 min (e.g., 20 min, 22 min, 25 min, 28 min, 30 min, etc.) to obtain the condensation-type rubber adhesive.

[0046] By limiting the specific order of adding materials and the stirring rate, this invention can ensure that the final condensation rubber adhesive is not only uniformly mixed, but also prevents abnormal increase in system viscosity or gel formation caused by excessive shearing and heat generation, thereby compromising the storage stability, application period, and final curing performance of the condensation rubber adhesive.

[0047] The preparation method provided by this invention enables the final adhesive to have a uniform distribution of each component and superior stability.

[0048] Thirdly, the present invention provides the application of the condensation-type rubber adhesive as described in the first aspect in sealing materials for aerospace, automotive manufacturing, electronic packaging, or industrial equipment.

[0049] Preferably, the present invention is applicable to the sealing needs of equipment and devices that need to be used in high-temperature working environments, such as automobile engines, high-temperature pipelines, or electronic appliances operating at high temperatures, and has excellent application prospects.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] (1) This invention replaces part of the silica with calcium carbonate on the basis of introducing silica, and at the same time, by combining silica powder and cerium oxide and other components, it can significantly reduce the amount of silica used and reduce the application cost of adhesives, while still having excellent mechanical properties and high temperature resistance. The mechanical properties remain excellent after aging in a high temperature environment, and at the same time, it does not affect the construction performance of the adhesive, and its curing uniformity is not affected.

[0052] (2) The condensation-type rubber adhesive provided by the present invention retains ≥85% of its mechanical strength and ≥90% of its hardness after aging at 300℃ for 24 h, and there is no bubbling phenomenon. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0054] Unless otherwise specified, the raw materials involved in the following specific embodiments of the present invention are all conventional materials in the art, and can be purchased from commercially available products. Some raw material information is as follows:

[0055] Polyorganosiloxane 1: Hydroxyl-terminated polydimethylsiloxane, with a viscosity (25℃) of about 5000 mPa·s, purchased from Xin'an Chemical, is room temperature vulcanizing methyl silicone rubber 5000;

[0056] Polyorganosiloxane 2: Hydroxyl-terminated polydimethylsiloxane, with a viscosity (25℃) of approximately 20,000 mPa·s, purchased from Xin'an Chemical, is 20,000 mPa·s of room temperature vulcanizing methyl silicone rubber;

[0057] Calcium carbonate 1: Particle size ≤100 nm, not treated with interface modifier, purchased from Guangxi Huana New Materials, model is basic nano calcium carbonate powder;

[0058] Calcium carbonate 2: Particle size ≤100 nm, purchased from Guangxi Huana New Materials, model CCS-32;

[0059] Calcium carbonate 3: Particle size 400-600 nm, purchased from Guangxi Huana New Materials, model CCR-05;

[0060] Silica: Hydrophobic silica, purchased from Evonik Chemicals, model R-974;

[0061] Silicon micro powder: Purchased from Huwei Silicon Micro Powder, model B03-1;

[0062] Cerium oxide: Hebei Yuanying New Materials, model number Hangzhuo cerium oxide;

[0063] Colorant: Purchased from Delong Carbon Black, model N630;

[0064] Crosslinking agent: purchased from Hangzhou Sibao, model HG-3104;

[0065] Silane coupling agent: KH-550, purchased from Hubei Jianghan New Materials, model JH-A110;

[0066] Catalyst: Purchased from Guizhou Mingtai Chemical Co., Ltd., model number MT 1201.

[0067] Examples 1-4 and Comparative Examples 1-3

[0068] This embodiment and comparative example provide a condensation-type rubber adhesive and its preparation method. The specific components are shown in Table 1.

[0069] Table 1

[0070]

[0071] The preparation method is as follows:

[0072] S1. In a planetary mixer, polyorganosiloxane, calcium carbonate, silica powder, cerium oxide and colorant are initially mixed for 5 min at a stirring rate of 20 rpm and 500 rpm. After scraping the edges, the cylinder head is closed and the vacuum pump is turned on to evacuate to -0.09 MPa. The temperature is raised to 120°C and mixed at a stirring rate of 30 rpm and 1200 rpm for 3 h. Then the temperature is lowered to below 40°C and mixed at a stirring rate of 10 rpm and 300 rpm for 30 min.

[0073] S2. Add crosslinking agent and mix under vacuum at a stirring rate of 20 rpm and 500 rpm for 25 min;

[0074] S3. Add silica and mix under vacuum at a stirring rate of 60 rpm and 800 rpm for 15 min.

[0075] S4. Add silane coupling agent and catalyst, and mix under vacuum at a stirring rate of 20 rpm and 500 rpm for 20 min to obtain the condensation-type rubber adhesive.

[0076] Examples 5-8 and Comparative Examples 4-5

[0077] This embodiment and comparative example provide a condensation-type rubber adhesive and its preparation method. The specific components are shown in Table 2.

[0078] Table 2

[0079]

[0080] The preparation method is as follows:

[0081] S1. In a planetary mixer, polyorganosiloxane, calcium carbonate, silica powder, cerium oxide and colorant are initially mixed for 10 min at a stirring rate of 50 rpm and 300 rpm. After scraping the edges, the cylinder head is closed and the vacuum pump is turned on to evacuate to -0.09 MPa. The temperature is raised to 100℃ and mixed at a stirring rate of 60 rpm and 800 rpm for 1 h. Then the temperature is lowered to below 40℃ and mixed at a stirring rate of 30 rpm and 100 rpm for 60 min.

[0082] S2. Add crosslinking agent and mix under vacuum at a stirring rate of 50 rpm and 300 rpm for 35 min;

[0083] S3. Add silica and mix under vacuum at a stirring rate of 30 rpm and 1200 rpm for 10 minutes.

[0084] S4. Add silane coupling agent and catalyst, and mix under vacuum at a stirring rate of 50 rpm and 300 rpm for 30 min to obtain the condensation-type rubber adhesive.

[0085] Example 9

[0086] This comparative example provides a condensation-type rubber adhesive and its preparation method.

[0087] The difference from Example 1 is that, in this comparative example, the preparation method is as follows:

[0088] In a planetary mixer, at a stirring rate of 60 rpm for revolution and 800 rpm for rotation, polyorganosiloxane, calcium carbonate, silica powder, cerium oxide and colorant are initially mixed for 5 min. After scraping the edges, the cylinder head is closed and the vacuum pump is turned on to evacuate to -0.09 MPa. The temperature is raised to 120℃ and mixed for 3 h. Then the temperature is lowered to below 40℃ and mixed for 30 min. Then a crosslinking agent is added and mixed for 25 min. Silica is added and mixed for 15 min. Silane coupling agent and catalyst are added and mixed for 20 min to obtain the condensation-type rubber adhesive.

[0089] Example 10

[0090] This comparative example provides a condensation-type rubber adhesive and its preparation method.

[0091] The difference from Example 1 is that, in this comparative example, the preparation method is as follows:

[0092] In a planetary mixer, the components of the formula are initially mixed for 5 minutes at a stirring rate of 60 rpm for revolution and 800 rpm for rotation. After scraping the edges, the cylinder head is closed and the vacuum pump is turned on to evacuate to -0.09 MPa. The temperature is raised to 120°C and mixed for 3 hours. Then the temperature is lowered to below 40°C and mixed for 90 minutes to obtain the condensation-type rubber adhesive.

[0093] Performance testing

[0094] The performance of the samples provided in Examples 1-10 and Comparative Examples 1-5 was tested using the following methods:

[0095] (1) Mechanical properties (tensile strength and elongation at break): tested according to GB / T 528.1-2019, wherein the dumbbell-shaped sample has a working section length of 25 mm and an experimental rate of 500 mm / min;

[0096] (2) Hardness: The test was conducted according to GB / T 531.1-2008 Rubber Pocket Hardness Tester Indentation Hardness Test Method;

[0097] (3) Aging performance: After the sample is placed at 300℃ for 24 h, observe whether the surface morphology is bubbling, and at the same time test its mechanical properties and hardness after aging.

[0098] (4) Curing state: After drying the condensation rubber adhesive at 300℃ for 24 h, observe the state of the adhesive and divide it into three states: good (still has elasticity and toughness), slightly hardened and obviously hardened.

[0099] The test results are as follows:

[0100] Table 3

[0101]

[0102] As can be seen from the examples and performance tests, the condensation-type rubber adhesive provided by the present invention has superior mechanical properties and high temperature resistance. Its mechanical properties remain excellent after aging in a high temperature environment, and its construction performance is not affected at all. Its curing uniformity and other properties are also unaffected.

[0103] As can be seen from the comparison of Examples 1-4 and Comparative Examples 1-3, in this invention, by limiting the mass ratio of calcium carbonate to silica to be within the range of (3.5-5):1, the condensation-type rubber adhesive provided by this invention can have better high-temperature aging resistance.

[0104] As can be seen from the comparison of Examples 5-6 and Examples 7-8, the present invention, by using calcium carbonate treated with an interface treatment agent and having a particle size D50 ≤ 100 nm, enables the condensation-type rubber adhesive to have better mechanical properties and high-temperature aging resistance.

[0105] As can be seen from the comparison between Examples 1 and Examples 9-10, the preparation method of the present invention can make the condensation rubber adhesive have better performance.

[0106] As can be seen from the comparison between Examples 5-6 and Comparative Examples 4-5, the amount of calcium carbonate introduced in this invention needs to be within the limits of this invention in order to ensure the performance of the condensation-type rubber adhesive.

[0107] The present invention has been illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A condensation-type rubber adhesive, characterized in that, By weight, it includes: 1000-2500 parts of polyorganosiloxane; Calcium carbonate 350-600 parts; 80-160 parts of silica; 460-900 parts of silica powder; 150-280 parts of cerium oxide; Colorant 12-30 parts; 90-220 parts of crosslinking agent; 30-50 parts of silane coupling agent; Catalyst 0.5-1.5 parts; The mass ratio of the silica to calcium carbonate is 1:(3.5-5).

2. The condensation-type rubber adhesive according to claim 1, characterized in that, The particle size of the calcium carbonate is D50≤100nm.

3. The condensation-type rubber adhesive according to claim 1, characterized in that, The calcium carbonate is calcium carbonate modified by an interface modifier.

4. The condensation-type rubber adhesive according to claim 1, characterized in that, The silica is a hydrophobic silica.

5. The condensation-type rubber adhesive according to claim 1, characterized in that, The polyorganosiloxane is selected from hydroxyl-terminated polydimethylsiloxane, and the viscosity of the hydroxyl-terminated polydimethylsiloxane at 25°C is 5000-80000 mPa·s.

6. The condensation-type rubber adhesive according to claim 1, characterized in that, The crosslinking agent includes ketoxime crosslinking agents; And / or, the silane coupling agent comprises any one or a combination of at least two of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, or diaminosilane oligomers; And / or, the catalyst includes organotin catalysts.

7. The condensation-type rubber adhesive according to any one of claims 1-6, characterized in that, The condensation-type rubber adhesive retains ≥85% of its mechanical strength and ≥90% of its hardness after aging at 300℃ for 24 h.

8. A method for preparing a condensation-type rubber adhesive as described in any one of claims 1-7, characterized in that, The preparation method includes: The components in the formula are mixed to obtain the condensation-type rubber adhesive.

9. The preparation method according to claim 8, characterized in that, The preparation method includes: After a preliminary mixing of polyorganosiloxane, calcium carbonate, silica powder, cerium oxide and colorant, a crosslinking agent is added for a second mixing, silica is added for a third mixing, and finally a silane coupling agent and catalyst are added for a fourth mixing to obtain the condensation-type rubber adhesive.

10. The application of a condensation-type rubber adhesive as described in any one of claims 1-7 in sealing materials for aerospace, automotive manufacturing, electronic packaging, or industrial equipment.

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