An acrylate dispersion and its use in pressure sensitive adhesives

Abstract

The application relates to the technical field of adhesives, belongs to the technical field of adhesives, and specifically relates to an acrylate dispersion liquid and application of the acrylate dispersion liquid to pressure-sensitive adhesive, wherein the acrylate dispersion liquid is prepared from the following raw materials in percentage by weight: 35-50 wt% of an acrylic compound, 5-10 wt% of a diluent, 0.5-1 wt% of a carboxyl compound, 7-12 wt% of an organic solvent, 5-7 wt% of an additive, 0.01-0.2 wt% of an initiator, 0.2-0.4 wt% of a dispersant, and water, which makes up the rest to 100%. The organic solvent and water are mixed in a reaction container, the acrylic compound, the diluent, the carboxyl compound, the initiator and the dispersant are added, and then the mixture is obtained by heating and cooling to room temperature. The additive is added to the mixture for purification and separation, and the pressure-sensitive adhesive is prepared. The pressure-sensitive adhesive has good adhesion, water resistance and acid and alkali resistance, and the prepared pressure-sensitive adhesive still has excellent local viscosity after repeated adhesion.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of adhesives, and specifically relates to an acrylate dispersion and application thereof to pressure-sensitive adhesive. BACKGROUND

[0002] The pressure-sensitive adhesive is a viscoelastic body with liquid viscosity and solid elasticity, which can withstand the influence of both bonding contact and destruction, has high viscosity, and has sufficient cohesion and elasticity when pressed by fingers.

[0003] The pressure-sensitive adhesive is mainly divided into medical pressure-sensitive adhesive, acrylate medical pressure-sensitive adhesive, hydrocolloid medical pressure-sensitive adhesive, conductive pressure-sensitive adhesive, and corrosion-resistant pressure-sensitive adhesive. With the expansion of the demand for packaging, decoration, office supplies, and various labels, the demand for pressure-sensitive adhesive is also increasing. Non-toxic, non-polluting, and excellent aging resistance, weather resistance, pressure sensitivity, and adhesion gradually become the development direction of pressure-sensitive adhesive.

[0004] Patent CN201711277879.4 discloses an acrylate pressure-sensitive adhesive, a pressure-sensitive adhesive tape, and a preparation method of the acrylate pressure-sensitive adhesive. The preparation raw materials include: isooctyl acrylate, butyl acrylate, acrylic acid, vinyl acetate, methyl methacrylate, hydroxyethyl methacrylate, trimethylolpropane triacrylate, double-bond-containing isoprene rubber, and double-bond-containing SBS elastomer. The acrylate pressure-sensitive adhesive has electrolyte resistance and high-temperature resistance, but poor acid and alkali resistance.

[0005] Patent CN201811585270.8 discloses a heat-resistant and cold-resistant outdoor pressure-sensitive adhesive and a pressure-sensitive adhesive tape thereof. The preparation raw materials include: soft monomer, hard monomer, functional monomer, epoxy resin, phenolic epoxy resin, initiator, catalyst, organic solvent, and glycidyl methacrylate. The pressure-sensitive adhesive has heat resistance and cold resistance, and is suitable for outdoor use, but has poor repeated adhesion. SUMMARY

[0006] The present application provides an acrylate dispersion. The preparation raw materials, by weight percentage, include: 35-50wt% acrylic compound, 5-10wt% diluent, 0.5-1wt% carboxyl compound, 7-12wt% organic solvent, 5-7wt% additive, 0.01-0.2wt% initiator, 0.2-0.4wt% dispersant, and water to make up the balance to 100%.

[0007] The main body of the present application is to control the glass transition temperature, cohesive energy of the synthetic acrylic polymer and improve its adhesion, so as to facilitate the spreading of the adhesive on the film, while improving the low temperature processing performance, maintaining good adhesion and storage stability. The preferred glass transition temperature of the acrylic ester polymer is less than -20℃. To reduce the glass transition temperature of the polymer, the conventional method is to select the long esterification chain structure of the acrylic ester monomer, so as to improve the flexibility of the polymer molecular chain. But with the increase of the length of the branched chain, the polarity group of the product decreases, and the adhesion decreases rapidly. Therefore, the method uses the principle of multi-random copolymerization, and a large amount of acrylic ester monomers with different long and short esterification branched chains are used for polymerization, so as to obtain a main chain structure change, irregular polyacrylic ester polymer, thereby reducing the glass transition temperature of the product. The acrylic ester monomer is a combination of a plurality of ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, n-propyl acrylate, lauryl methacrylate, isodecyl methacrylate;

[0008] Further preferred monomers are long branched lauryl methacrylate and isodecyl methacrylate monomers, preferably in a complex composition, the mass ratio of lauryl methacrylate and isodecyl methacrylate is 0.7-1.0:0.5-0.9; which is conducive to increasing the asymmetry of the molecular structure, thereby increasing the free volume between the molecules and reducing the glass transition temperature of the product.

[0009] Because the glass transition temperature of the synthesized polymer is low, and the molecular chain composition structure is changed, the cohesive energy of the polymer product is relatively weak, thereby affecting its adhesive strength. In order to further improve the adhesive performance of the polymer product, the amount of diluent needs to be increased. The preferred amount is 5-10wt%. The functions of these diluents include two aspects: adjusting the viscosity of the system and acting as a crosslinking agent in the polymerization reaction. The diluent is one or a combination of propoxylated neopentyl glycol diacrylate, ethylene glycol diacrylate, diethylene glycol dimethacrylate, and triethylene glycol dimethacrylate;

[0010] Further preferably, the diluent is a combination of diethylene glycol diacrylate and diethylene glycol dimethacrylate, and the mass ratio of diethylene glycol diacrylate to diethylene glycol dimethacrylate is 0.3-0.7:1.1-1.5;

[0011] At the same time, because the main functional monomer in the formula is an ester group, the monomer polarity is weak, which will limit the adhesive performance of the product, so a small amount of carboxyl compound is added to the formula. The preferred one or combination of several of methacrylic acid, crotonic acid, maleic acid, itaconic acid, fumaric acid, 2-acrylamide glycolic acid-hydrate, and cinnamic acid.

[0012] Further preferably, the carboxyl compound is a mixture of methacrylic acid and crotonic acid, and the mass ratio of methacrylic acid to crotonic acid is 1.3-1.7:0.4-0.8.

[0013] In order to improve the solubility of the polymerization product, the organic solvent is an organic ester compound, including one or a combination of triethyl phosphate, ethyl acetate, isoamyl acetate, n-propyl acetate, and ethyl acetate.

[0014] Further preferably, the organic solvent is a mixture of ethyl acetate and triethyl phosphate, and the mass ratio of ethyl acetate to triethyl phosphate is 1:0.5-0.8.

[0015] In addition, other organic solvents can be added as cosolvents according to different application needs, such as ethanol, n-butanol, glycerol, and ethylene glycol.

[0016] In the dispersion liquid, the initiator used is a thermal initiator, including one or a combination of benzoyl peroxide, lauryl peroxide, ammonium persulfate, potassium persulfate, dimethyl azobis isobutyrate, azobis isobutyronitrile, tert-butyl peroxybenzoate, diisopropyl peroxydicarbonate, tert-butyl hydroperoxide solution, hydrogenated p-menthane hydroperoxide, and cumene hydroperoxide. The addition amount is preferably between 0.01 and 0.2 wt%.

[0017] The dispersion liquid also contains a specific dispersant: one or a combination of vinyl bis-stearamide, glyceryl monostearate, glyceryl tri-stearate, zinc stearate, and polyethylene wax. The dispersant improves the water resistance of the product, and its proportion is 0.2-0.4%.

[0018] In addition, different additives can be added in the above-mentioned formula according to needs, without affecting the main function of the product, and details are not described.

[0019] The application also provides an application of the acrylic ester dispersion liquid, which is applied to pressure-sensitive adhesive.

[0020] Preferably, the preparation method of the pressure-sensitive adhesive comprises the following steps: mixing the organic solvent and water in a reaction container, then adding the acrylic compound, the diluent, the carboxyl compound, the initiator, and the dispersant, heating to 75-85°C for 5-6h, then cooling to 25-30°C to obtain a mixture, adding additives to the mixture for purification and separation, and preparing the pressure-sensitive adhesive.

[0021] Preferably, the glass transition temperature of the pressure-sensitive adhesive is less than -20°C.

[0022] Beneficial effects:

[0023] (1) The present application prepares the pressure sensitive adhesive by mixing organic solvent, water, acrylic compound, diluent, carboxyl compound, initiator, dispersant, additive, which has good adhesion, water resistance and acid and alkali resistance, and the prepared pressure sensitive adhesive still has excellent local viscosity after repeated bonding.

[0024] (2) The present application adds amine group-containing dispersant and multi-branched glyceride dispersant which can participate in the reaction, which can improve the water resistance, acid and alkali resistance, adhesion and repeated bonding of the prepared pressure sensitive adhesive, especially when the dispersant is a compound of vinyl bis-stearamide, glycerol monostearate and glycerol tristearate, and the mass ratio of vinyl bis-stearamide, glycerol monostearate and glycerol tristearate is 1.2-1.6:0.7-1.3:0.7-1.1, the prepared pressure sensitive adhesive has excellent water resistance, acid and alkali resistance, adhesion and repeated bonding. The reason may be that the addition of carboxyl compound causes the decrease of water resistance and alkali resistance of the pressure sensitive adhesive, the addition of amine group-containing vinyl bis-stearamide and multi-branched glycerol monostearate and glycerol tristearate promotes the interaction between the ester groups in the acrylic compound and diluent and the amine groups and ester groups in the dispersant, the acrylic compound and diluent are uniformly dispersed in the branched structure of glycerol monostearate and glycerol tristearate, forming a dense and directional arrangement of cross-linked polymer structure, preventing the interaction of water, acid and alkali with carboxyl groups in the acrylic compound and diluent. In addition, the applicant found that when the compound of carboxyl compound is methyl methacrylate and crotonic acid, and the mass ratio of methyl methacrylate and crotonic acid is 1.3-1.7:0.4-0.8, it interacts with the acrylic compound and diluent in the system under the action of initiator, and a pressure sensitive adhesive with good adhesion can be prepared. However, the addition of carboxyl compound may increase the local viscosity of the pressure sensitive adhesive and affect the repeated bonding. By controlling the amount of organic solvent, water and dispersant, the compatibility of the system components can be improved, the size of the cross-linked polymer structure can be controlled, the local viscosity of the pressure sensitive adhesive can be made more uniform, and the repeated bonding performance of the pressure sensitive adhesive can be improved.

[0025] (3) The present application mixes organic solvent and water in a reaction container, adds acrylic compound, diluent, carboxyl compound, initiator and dispersant for heating reaction, then cools to room temperature to obtain a mixture, and adds additive to purify and separate the mixture to prepare a pressure sensitive adhesive, which has good adhesion, water resistance and acid and alkali resistance, and the prepared pressure sensitive adhesive still has excellent local viscosity after repeated bonding. DETAILED DESCRIPTION

[0026] Example 1

[0027] Example 1 provides an acrylate dispersion, the raw materials for preparing the acrylate dispersion include, by weight percentage, 35 wt% acrylate compound, 5 wt% diluent, 0.5 wt% carboxyl compound, 7 wt% organic solvent, 5 wt% additive, 0.01 wt% initiator, 0.2 wt% dispersant, and water to make up the balance to 100%.

[0028] The acrylate compound is a mixture of lauryl methacrylate and isodecyl methacrylate, the mass ratio of lauryl methacrylate and isodecyl methacrylate is 0.7:0.5, the CAS number of lauryl methacrylate is 142-90-5, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd., the CAS number of isodecyl methacrylate is 29964-84-9, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd.;

[0029] The diluent is a mixture of diethylene glycol bisacrylate and diethylene glycol dimethacrylate, the mass ratio of diethylene glycol bisacrylate and diethylene glycol dimethacrylate is 0.3:1.1, the CAS number of diethylene glycol bisacrylate is 4074-88-8, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd., the CAS number of diethylene glycol dimethacrylate is 2358-84-1, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd.;

[0030] The carboxyl compound is a mixture of methacrylic acid and crotonic acid, the mass ratio of methacrylic acid and crotonic acid is 1.3:0.4, the CAS number of methacrylic acid is 79-41-4, and it is purchased from Shanghai Macklin Biochemical Technology Co., Ltd., the CAS number of crotonic acid is 107-93-7, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd.;

[0031] The organic solvent is a mixture of ethyl acetate and triethyl phosphate, the mass ratio of ethyl acetate and triethyl phosphate is 1:0.5, the CAS number of ethyl acetate is 141-78-6, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd., the CAS number of triethyl phosphate is 78-40-0, and it is purchased from Shanghai Macklin Biochemical Technology Co., Ltd.;

[0032] The additive is a mixture of ethanol and n-butanol, the mass ratio of ethanol and n-butanol is 1:0.7;

[0033] The initiator is a mixture of tert-butyl hydroperoxide solution and hydrogenated p-menthane hydroperoxide, the mass ratio of tert-butyl hydroperoxide solution and hydrogenated p-menthane hydroperoxide is 1.3:0.4, the concentration of tert-butyl hydroperoxide solution is 70% aqueous solution, the CAS number is 75-91-2, and it is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd., the CAS number of hydrogenated p-menthane hydroperoxide is 80-47-7, and it is purchased from Hubei Ximingtai Chemical Co., Ltd.;

[0034] The dispersant is a compound of vinyl bis-stearamide, glycerol monostearate and glycerol tristearate, the mass ratio of vinyl bis-stearamide, glycerol monostearate and glycerol tristearate is 1.2:0.7:0.7, the vinyl bis-stearamide has a CAS number of 110-30-5 and is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., the glycerol monostearate has a CAS number of 123-94-4 and is purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd., and the glycerol tristearate has a CAS number of 555-43-1 and is purchased from Shanghai Macklin Biochemical Technology Co., Ltd.;

[0035] The second aspect of the present application provides an application of the acrylate dispersion, and the acrylate dispersion is applied to a pressure-sensitive adhesive.

[0036] The preparation method of the pressure-sensitive adhesive comprises the following steps:

[0037] The organic solvent and water are mixed in a reaction container, and then the acrylate compound, the diluent, the carboxyl compound, the initiator and the dispersant are added, the temperature is increased to 75 DEG C and reacted for 5 hours, then the temperature is decreased to 25 DEG C, a mixture is obtained, and the additive is added to the mixture for purification and separation, so that the pressure-sensitive adhesive is prepared.

[0038] Example 2

[0039] The acrylate dispersion provided in Example 2 is prepared from raw materials including, by weight percentage, 45wt% acrylate compound, 8wt% diluent, 0.8wt% carboxyl compound, 10wt% organic solvent, 6wt% additive, 0.015wt% initiator and 0.3wt% dispersant, and water is added to make up the balance to 100%.

[0040] The acrylate compound is a compound of lauryl methacrylate and isodecyl methacrylate, the mass ratio of lauryl methacrylate and isodecyl methacrylate is 0.8:0.7, the lauryl methacrylate has a CAS number of 142-90-5 and is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., and the isodecyl methacrylate has a CAS number of 29964-84-9 and is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.;

[0041] The diluent is a compound of diethylene glycol bis-acrylate and diethylene glycol dimethacrylate, the mass ratio of diethylene glycol bis-acrylate and diethylene glycol dimethacrylate is 0.5:1.4, the diethylene glycol bis-acrylate has a CAS number of 4074-88-8 and is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., and the diethylene glycol dimethacrylate has a CAS number of 2358-84-1 and is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.;

[0042] The carboxyl compound is a compound of methacrylic acid and crotonic acid, the mass ratio of methacrylic acid and crotonic acid is 1.5:0.7, the CAS number of methacrylic acid is 79-41-4, which is purchased from Shanghai McLean Biotechnology Co., Ltd., and the CAS number of crotonic acid is 107-93-7, which is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.;

[0043] The organic solvent is a compound of ethyl acetate and triethyl phosphate, the mass ratio of ethyl acetate and triethyl phosphate is 1:0.7, the CAS number of ethyl acetate is 141-78-6, which is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., and the CAS number of triethyl phosphate is 78-40-0, which is purchased from Shanghai McLean Biotechnology Co., Ltd.;

[0044] The additive is a compound of ethanol and n-butanol, the mass ratio of ethanol and n-butanol is 1:0.8;

[0045] The initiator is a compound of tert-butyl hydroperoxide solution and hydrogenated p-menthane hydroperoxide, the mass ratio of tert-butyl hydroperoxide solution and hydrogenated p-menthane hydroperoxide is 1.4:0.5, the concentration of tert-butyl hydroperoxide solution is 70% aqueous solution, the CAS number is 75-91-2, which is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., and the CAS number of hydrogenated p-menthane hydroperoxide is 80-47-7, which is purchased from Hubei Ximingtai Chemical Co., Ltd.;

[0046] The dispersant is a compound of vinyl bis-stearamide, glycerol monostearate and glycerol tri-stearate, the mass ratio of vinyl bis-stearamide, glycerol monostearate and glycerol tri-stearate is 1.4:1.1:0.9, the CAS number of vinyl bis-stearamide is 110-30-5, which is purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., the CAS number of glycerol monostearate is 123-94-4, which is purchased from Shanghai Aldrin Biotech Co., Ltd., and the CAS number of glycerol tri-stearate is 555-43-1, which is purchased from Shanghai McLean Biotechnology Co., Ltd.;

[0047] The second aspect of the application provides an application of the acrylate dispersion liquid, and the acrylate dispersion liquid is applied to pressure-sensitive adhesive;

[0048] The preparation method of the pressure-sensitive adhesive comprises the following steps:

[0049] The organic solvent and water are mixed in a reaction container, then the acrylic compound, the diluent, the carboxyl compound, the initiator and the dispersant are added, the temperature is increased to 85 DEG C and reacted for 5.5 hours, then the temperature is decreased to 28 DEG C, a mixture is obtained, the additive is added to the mixture for purification and separation, and the pressure-sensitive adhesive is prepared.

[0050] Comparative example 1

[0051] The specific implementation is the same as that of Example 2, except that the dispersant is a compound of vinyl bis stearamide, glyceryl monostearate, glyceryl tristearate, the mass ratio of vinyl bis stearamide, glyceryl monostearate, glyceryl tristearate is 0.3:0.7:0.4, the CAS number of vinyl bis stearamide is 110-30-5, which is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd., the CAS number of glyceryl monostearate is 123-94-4, which is purchased from Shanghai Aladdin Biochem Technology Co., Ltd., and the CAS number of glyceryl tristearate is 555-43-1, which is purchased from Shanghai Macklin Biochemical Technology Co., Ltd.

[0052] Comparative Example 2

[0053] The specific implementation is the same as that of Example 2, except that the dispersant is vinyl bis stearamide, the CAS number of vinyl bis stearamide is 110-30-5, which is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd.

[0054] Comparative Example 3

[0055] The specific implementation is the same as that of Example 2, except that the carboxyl compound is a compound of methacrylic acid and crotonic acid, the mass ratio of methacrylic acid and crotonic acid is 0.3:0.4, the CAS number of methacrylic acid is 79-41-4, which is purchased from Shanghai Macklin Biochemical Technology Co., Ltd., and the CAS number of crotonic acid is 107-93-7, which is purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd.

[0056] Performance test: water absorption rate test, acid resistance, alkali resistance, and repeated adhesion

[0057] Water absorption rate test: according to the GB / T 173393 standard, two pieces of pressure-sensitive adhesive prepared in Example 1-2 and Comparative Example 1-3 were taken, respectively, (the pressure-sensitive adhesive provided in Example 1-2 and Comparative Example 1-3 was prepared by mixing raw materials of acrylate dispersion at a temperature of 75°C, and the reaction was carried out for 5h to form the pressure-sensitive adhesive) the first piece of pressure-sensitive adhesive prepared in Example 1-2 and Comparative Example 1-3 was placed in an oven for drying, and after cooling to room temperature, the weight was measured and recorded, the other piece of pressure-sensitive adhesive prepared in Example 1-2 and Comparative Example 1-3 was placed in water, taken out, and the water on the surface of the pressure-sensitive adhesive was absorbed with filter paper, and the weight of the pressure-sensitive adhesive after water absorption was measured, the weight of the pressure-sensitive adhesive after water absorption was subtracted from the weight of the pressure-sensitive adhesive after drying, and divided by the weight of the pressure-sensitive adhesive after drying, and multiplied by 100%, to calculate the water absorption rate.

[0058] Acid resistance: Prepare an acid solution with the following ratio: phosphoric acid (H3PO4: HNO3: CH3COOH: H2O = 15: 1: 1: 2). After the pressure-sensitive adhesive prepared in Example 1-2, Comparative Example 1-3 is soaked in the acid for 1 h, it is taken out, washed with water, and air-dried. The whitening, bubbling, and delamination of the surface of the pressure-sensitive adhesive are observed.

[0059] Alkali resistance: The pressure-sensitive adhesive prepared in Example 1-2, Comparative Example 1-3 is attached to a glass plate, respectively, and is compacted by a roller. After being placed at room temperature for 30 min, the initial tack of the pressure-sensitive adhesive is tested. After the initial tack of the pressure-sensitive adhesive prepared in Example 1-2, Comparative Example 1-3 is tested, the pressure-sensitive adhesive is again compacted by a roller and is placed at room temperature for 7 h. Then, the pressure-sensitive adhesive is placed in a sodium hydroxide alkaline solution with a pH of 13 at 65°C for 2 h. After being taken out and air-dried, the residual tack of the pressure-sensitive adhesive is tested.

[0060] Repeatability: According to the GB / T2792 standard, the pressure-sensitive adhesive prepared in Example 1-2, Comparative Example 1-3 is attached to a glass plate. After being pasted and peeled off 100 times, the peeling strength of the pressure-sensitive adhesive is tested.

[0061]

Claims

1. An acrylate dispersion, characterized in that, By weight percentage, including: 35~50wt% acrylic compound, 5~10wt% diluent, 0.5~1wt% carboxyl compound, 7~12wt% organic solvent, 5~7wt% additive, 0.01~0.2wt% initiator, 0.2~0.4wt% dispersant, water to make up the balance to 100%; The acrylic compound is a compound of lauryl methacrylate and isodecyl methacrylate, and the mass ratio of lauryl methacrylate to isodecyl methacrylate is (0.7-1.0):(0.5-0.9); The diluent is a compound of diethylene glycol diacrylate and diethylene glycol dimethacrylate, and the mass ratio of diethylene glycol diacrylate to diethylene glycol dimethacrylate is (0.3-0.7):(1.1-1.5); The carboxyl compound is a compound of methacrylic acid and crotonic acid, and the mass ratio of methacrylic acid to crotonic acid is (1.3-1.7):(0.4-0.8); The dispersant is a compound of vinyl bis-stearamide, glycerol monostearate and glycerol tristearate, and the mass ratio of vinyl bis-stearamide to glycerol monostearate to glycerol tristearate is (1.2-1.6):(0.7-1.3):(0.7-1.1); The additive is a compound of ethanol and n-butanol; The organic solvent is a compound of ethyl acetate and triethyl phosphate.

2. Use of an acrylate dispersion according to claim 1, characterized in that, Applied to pressure-sensitive adhesive.

3. Use of an acrylate dispersion according to claim 2, characterized in that The glass transition temperature of the pressure-sensitive adhesive is less than -20℃.

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