Low-temperature stable organic peroxide emulsion and safe preparation method thereof

Abstract

The invention discloses a low-temperature stable organic peroxide emulsion and a safe preparation method thereof, and the emulsion comprises diisobutyryl peroxide, a hydroperoxide stabilizer, an organic solvent containing 2, 2, 4-trimethyl-1, 3-pentanediol diisobutyrate, and an emulsifying / dispersing agent. Through a low-temperature high-shear dispersion coupling cyclic emulsification method and in combination with real-time combustible gas / oxygen / smoke monitoring, the problem of freezing of the emulsion under an extremely cold condition (-40 DEG C) and the decomposition risk in the preparation process are solved. The obtained emulsion has significantly improved storage stability (greater than or equal to 85 days), low viscosity and excellent fluidity, and is suitable for industrial production.

Description

Technical Field

[0001] The invention belongs to the technical field of fine chemical industry, and in particular relates to a low-temperature stable organic peroxide emulsion and a safe preparation method thereof. Background Art

[0002] Organic peroxides are free radical polymerization initiators, widely used in polyethylene, polyvinyl chloride, polystyrene, acrylic resin, ethylene-vinyl acetate copolymer and other industries, and are key additives in the polymer industry. Diisobutyryl peroxide is a low-temperature organic peroxide with a self-accelerating decomposition temperature of 0°C. It has a low operating temperature and high initiation efficiency, and has good application value in the field of low-temperature polymerization.

[0003] After a period of storage, a large amount of gas will be generated in the container storing the emulsion according to conventional known techniques, and the container will expand, and the peroxide will decompose, resulting in a decrease in content, which will reduce the product quality and hide a large safety risk. After analyzing the emulsion that produces a large amount of gas, its acidity has an increasing trend. We found that it is mainly isobutyric acid, and the content of diisobutyryl peroxide is also significantly reduced. The decomposition product also has oxygen, etc. The storage stability period of the emulsion product is generally no more than 1 month, or even no more than 2 weeks, which undoubtedly greatly reduces the service life, and its industrial production and application are obviously limited.

[0004] At the same time, when the diisobutyryl peroxide emulsion is stored at different temperatures, when the storage temperature is lower than -30°C, the emulsion undergoes a freezing phenomenon of condensing into micelles, and the viscosity also increases significantly, which undoubtedly limits its pumping, transportation and application. It is of great significance to provide a diisobutyryl peroxide emulsion that does not freeze at temperatures lower than -30°C and can be stored or transported under a wider range of temperature conditions, especially having good fluidity under extremely cold conditions (such as -35°C or even -40°C).

[0005] In addition, the diisobutyryl peroxide emulsion is prepared according to conventional known technology, that is, diisobutyryl peroxide, water, methanol, emulsifier, dispersant, etc. are added to the kettle for high-speed shearing. The high shearing action of the disperser during the preparation process causes the material to generate a large amount of heat. Even if there is a low-temperature refrigerant to refrigerate the emulsified material, the material temperature will quickly rise from -10°C to above 0°C, and the self-accelerating decomposition temperature of diisobutyryl peroxide is 0°C. The safety risk of the preparation process is greatly increased. At the same time, the organic peroxide decomposes during the preparation of the emulsion, resulting in a certain degree of reduction in the product yield. If the overtemperature range is obvious, for example, the temperature exceeds 5°C, or even reaches 10°C, the material may decompose, decompose oxygen and combustible gas, produce a large amount of smoke, and have a very high risk of explosion. When considering emulsification at a lower temperature (such as -15°C or lower), it is found that the too low emulsification temperature increases the viscosity of the material, increases its energy consumption under high shear, and the material still has a large temperature rise, resulting in the inability to complete good emulsification. It can be seen that the efficiency of preparing diisobutyryl peroxide emulsion by conventional methods is relatively low, and the risk is high.

[0006] In summary, the defects of the prior art are: Poor storage stability: Conventional diisobutyryl peroxide emulsion is easy to decompose and produce gas (isobutyric acid / oxygen) during storage, causing the container to swell and explode. The stability period is usually less than 30 days; Insufficient adaptability to low temperatures: When the temperature is less than -30°C, the emulsion freezes and solidifies, and the viscosity is relatively high, which cannot meet the transportation requirements in extremely cold areas (such as -40°C); when the transmission temperature is controlled below -25°C or even -30°C during use, the emulsion freezes and solidifies, which cannot meet the application requirements.

[0007] Production safety hazards: First, shear heat generated during the emulsification process makes the material temperature easily exceed 0°C (self-accelerating decomposition temperature of diisobutyryl peroxide), causing decomposition and explosion. Second, isobutyryl chloride, isobutyric acid and isobutyrate in the material promote chain decomposition reaction of diisobutyryl peroxide. Third, the traditional emulsification process has low heat exchange efficiency and cannot export shear heat in time. Summary of the invention

[0008] In view of the above technical problems, the present invention provides a low-temperature stable organic peroxide emulsion and a safe preparation method thereof, which solves the problems of ultra-low temperature storage of diisobutyryl peroxide emulsion, product instability, and the emulsification safety and efficiency of the emulsion product.

[0009] The low-temperature stable organic peroxide emulsion of the present invention comprises the following components in mass fractions: Diisobutyryl peroxide 23%~50%; Stabilizer hydroperoxide 0.5%~1.0%; 10% to 18% of an organic solvent, wherein the organic solvent is a solvent containing 2,2,4-trimethyl-1,3-pentanediol diisobutyrate; Water 15%~30%; methanol 20%~30%; emulsifier 0.5%~0.9%; dispersant 0.1%~0.3%.

[0010] The stabilizer hydroperoxide is one or more of tert-amyl hydroperoxide, tert-butyl hydroperoxide, tert-hexyl hydroperoxide, and 1,1,3,3-tetramethylbutyl hydroperoxide. One or two of tert-amyl hydroperoxide and 1,1,3,3-tetramethylbutyl hydroperoxide are preferred. The emulsifier is fatty alcohol polyoxyethylene ether; the dispersant is polyvinyl alcohol with an alcoholysis degree of 50% to 80%. The viscosity of the emulsion is 60 to 80 mPa·s when stored at -40°C, and no freezing or solidification occurs.

[0011] The safe preparation method of the low-temperature stable organic peroxide emulsion of the present invention is to mix the components under low temperature conditions (-15°C to -5°C) and form the emulsion by high shear dispersion coupling circulation emulsification method; during the preparation process, the combustible gas concentration, smoke and oxygen concentration on the surface of the emulsion in the kettle are monitored in real time. Specifically, the following steps are included: (1) Add methanol, water, emulsifier and dispersant into the emulsifier, start stirring, and cool to low temperature conditions of -15℃~-5℃; (2) Add diisobutyryl peroxide, organic solvent and stabilizer hydroperoxide, and control the temperature to -15℃~-5℃ for 10~20min; (3) Turn on the circulating emulsification pump and its pipeline refrigeration to cool the material in the kettle to -15~-5℃; (4) Adjust the frequency of the circulating emulsification pump to 20-40 Hz, start the high shear disperser for 20-40 minutes after 15-20 minutes, and control the disperser speed to 1000-2000 rpm; (5) After stopping the disperser, continue to circulate for 10 to 20 minutes, stop the circulating emulsification pump, and finally stop stirring to obtain the final emulsion.

[0012] The emulsion is circulated from the bottom of the kettle to the top of the kettle through a circulating emulsification pump, with a pumping rate of 1~3 L / s; the circulating emulsification pump and its pipeline are cooled with ethylene glycol cold salt, and the temperature is controlled at -25℃~-15℃.

[0013] The combustible gas concentration is less than 5% LEL, there is no smoke, and the oxygen concentration is less than 23.5%; the combustible gas, smoke and oxygen concentrations are fed back in real time through an online monitoring system. If the combustible gas concentration is ≥5% LEL or the oxygen concentration is ≥23.5%, shearing is stopped immediately and refrigeration is intensified.

[0014] In order to solve the problems of ultra-low temperature storage, transportation and application, an organic solvent accounting for 10%-18% of the mass fraction of the emulsion is added, and the proportion of methanol is appropriately increased (such as accounting for more than 27% of the mass fraction of the emulsion) when necessary. The emulsion has good anti-low temperature freezing performance, and the emulsion does not freeze when cooled to -40°C. The emulsion also has good fluidity when stored and transported below -38°C or even below -40°C. According to the method of the present invention, the viscosity of the emulsion is also significantly reduced. At -40°C, the viscosity decreases from about 120 mPa·s to less than 80 mPa·s, and the viscosity decreases significantly. At -10°C, the viscosity of the emulsion mixed with 5% mass fraction of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate decreases by more than 45% compared with the emulsion without the solvent. It should also be pointed out that 2,2,4-trimethyl-1,3-pentanediol diisobutyrate can also be used as an environmentally friendly plasticizer, which will not have an adverse effect on the polymer materials produced downstream.

[0015] The most common method of synthesizing diisobutyryl peroxide in various existing production technologies is to react isobutyryl chloride with hydrogen peroxide under alkaline conditions (Qin Rongguang et al., Synthesis of diisobutyryl peroxide, Journal of Northwest Normal University (Natural Science Edition) No. 2, 1982). Since the reaction is in an aqueous system and is alkaline in most cases (Chinese patent CN116396200A, Chinese patent CN111072542A, etc.), isobutyryl chloride is very easy to hydrolyze and alkaline hydrolyze, thereby forming isobutyric acid and (or) isobutyrate salts in the product. Due to incomplete reaction, a small amount of isobutyryl chloride is also present in the organic peroxide, and its decomposition increases the acidity. On the one hand, isobutyric acid and (or) isobutyrate salts cause the decomposition of peroxides, and on the other hand, incompletely reacted isobutyryl chloride itself is a volatile irritating gas. Therefore, the diisobutyryl peroxide synthesized in the prior art inevitably contains isobutyryl chloride, isobutyric acid and its salts, which are all the reasons for the decomposition and instability of the product. In order to solve the above problems and overcome the shortcomings of the prior art, a small amount of hydroperoxide (such as tert-amyl hydroperoxide, etc.) is innovatively added to react with isobutyryl chloride or isobutyric acid (salt), which can convert it into peroxy isobutyrate (such as tert-amyl peroxy isobutyrate, etc.). In this way, the formed peroxide is miscible with diisobutyryl peroxide on the one hand, and can greatly reduce or prevent the material from continuing to decompose and produce acid, thereby reducing or inhibiting the decomposition of the product and stabilizing the diisobutyryl peroxide product (emulsion). This type of hydroperoxide includes tert-amyl hydroperoxide, tert-hexyl hydroperoxide, tert-butyl hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, preferably one or two of tert-amyl hydroperoxide and 1,1,3,3-tetramethylbutyl hydroperoxide, and the addition amount is 0.5% to 1.0% of the mass of the emulsion, preferably 0.5% to 0.8%.

[0016] In order to solve the safety risks of the above-mentioned emulsification process (the emulsification process is prone to generate a lot of heat and thus has a higher risk), as well as the problem of low emulsification efficiency. The inventor chose to control the material temperature at -15°C~-5°C during high-speed shear dispersion in the emulsification kettle, and more preferably, controlled it at around -10°C. A circulating pump was also introduced to circulate the product, and the circulating pump has an emulsification and homogenization function. By adding ethylene glycol cold salt to the circulation line for cooling, the problem of maintaining low temperature during the circulation process is solved. The surface renewal of the material is improved by the circulation of the product material, which significantly increases the contact surface area between the material and the emulsification kettle jacket and the cooling coil, and increases the heat exchange area. At the same time, this surface renewal makes the high shear of the disperser and the homogenization mixing of the emulsification pump more efficient and effective.

[0017] During the experiment, the inventor found that when the diisobutyryl peroxide emulsion is overheated (greater than 3°C or higher), it will decompose oxygen (concentration significantly exceeds 23.5%, sometimes up to 30%) and combustible gas. When the overheating is obvious (greater than 10°C or higher), obvious smoke will be generated, and the emulsion preparation process has great risks. In order to monitor the risk of explosion and combustion caused by the decomposition of oxygen and combustible gas and the generation of a large amount of smoke, the inventor transmits the monitoring data in real time on the surface of the emulsion in the emulsifier through online monitoring methods such as combustible gas alarm instruments and smoke sensing instruments, providing early warning for taking emergency measures. Among them, the best control conditions are that the combustible gas concentration is less than 5% LEL, there is no smoke, and the oxygen concentration is less than 23.5%. When the combustible gas concentration is greater than 5% LEL, or the oxygen concentration is greater than 23.5%, the shear emulsification should be stopped, and the stirring and emulsion circulation cannot be stopped. At the same time, the cooling and refrigeration should be increased. The emulsification preparation can be carried out again after the gas concentration is lower than the control value and the temperature is observed to be lower than the control value. If visible smoke or a smoke alarm is generated during the emulsion preparation process, emergency measures should be initiated immediately, including immediately stopping the disperser, using water spray, injecting large amounts of cold water, emptying the material into the emergency pool, etc., to avoid subsequent accidents.

[0018] The beneficial effects of the present invention are as follows: by adding hydroperoxide to react with the by-products in the material, the purpose of stabilizing the product is achieved, and the product generated by the reaction is still an organic peroxide, which does not affect the downstream use; at the same time, by using 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, which can be used as both a solvent and a viscosity reducer, the viscosity of the emulsion material decreases by more than 40% at -40°C, and the content retention rate is greater than 97% after storage for 90 days, and no freezing occurs. The innovation of the present invention is that the added stabilizer and the generated product are both organic peroxides. In addition, the present invention uses a circulating emulsification method through high shear dispersion to accelerate material mixing and surface renewal, increase heat exchange area and efficiency, improve emulsification efficiency and reduce safety risks. By online monitoring of combustible gas, oxygen concentration and smoke, etc., the safety risk control of the corresponding emulsion preparation process is further enhanced. DETAILED DESCRIPTION

[0019] In order to better illustrate the technical method of the present invention, the preparation process is described in detail below and illustrated by examples. Example 1

[0020] (1) Add 602 kg of methanol, 537 kg of water, 60 kg of emulsifier solution (pre-prepared, containing 20 kg of water, 20 kg of methanol and 20 kg of fatty alcohol polyoxyethylene ether emulsifier), and 60 kg of dispersant solution (pre-prepared, containing 48 kg of water, 6 kg of methanol and 6 kg of polyvinyl alcohol dispersant) into the emulsifier, start stirring at a speed of 180 rpm, and cool to -5°C.

[0021] (2) Add 399 kg of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate and 587 kg of diisobutyryl peroxide, and 12 kg of 1,1,3,3-tetramethylbutyl hydroperoxide, control the temperature to -6°C, and maintain for 15 min.

[0022] (3) Start the circulating emulsification pump and its pipeline refrigeration, and control the temperature of the ethylene glycol cold salt used for refrigeration to -20℃.

[0023] (4) Cool the contents of the kettle to -10°C, lower the stirring speed to 150 rpm, adjust the frequency of the circulating emulsification pump to 25 Hz, and after 15 minutes, start the disperser (speed 1800 rpm) for 35 minutes.

[0024] (5) Stop the disperser, continue to circulate for 15 minutes, then stop the circulating emulsification pump, and finally stop stirring to obtain diisobutyryl peroxide emulsion.

[0025] During the preparation process, the combustible gas concentration is 0% LEL, there is no smoke, and the oxygen concentration is less than 21.5%.

[0026] The content of diisobutyryl peroxide in the emulsion is 24.4%. The viscosity is tested by DV-1 BROOKFIELD VISCOMETER (61# rotor, 100 rpm). The viscosity of the emulsion is 44 mPa·s at -10℃. The emulsion flows well at -35℃ without freezing and solidification. The viscosity is 60 mPa·s. The content is 23.9% after storage for 85 days. The content is 23.6% after storage at -25℃ for 85 days. Example 2

[0027] (1) Add 602 kg of methanol, 537 kg of water, 60 kg of emulsifier solution (pre-prepared, containing 20 kg of water, 20 kg of methanol and 20 kg of fatty alcohol polyoxyethylene ether emulsifier), and 60 kg of dispersant solution (pre-prepared, containing 48 kg of water, 6 kg of methanol and 6 kg of polyvinyl alcohol dispersant) into the kettle, start stirring at a speed of 180 rpm, and cool to -5°C.

[0028] (2) Add 399 kg of organic solvent (300 kg of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate and 99 kg of isododecane), 587 kg of diisobutyryl peroxide, and 12 kg of 1,1,3,3-tetramethylbutyl hydroperoxide. Control the temperature to less than -6 °C and maintain for 15 min.

[0029] (3) Start the circulating emulsification pump and its pipeline refrigeration, and control the temperature of the ethylene glycol cold salt used for refrigeration to -20℃.

[0030] (4) Cool the contents of the kettle to -10°C, lower the stirring speed to 150 rpm, and adjust the frequency of the circulating emulsification pump to 25 Hz. After 15 minutes, start the disperser (speed 1800 rpm) for 35 minutes.

[0031] (5) Stop the disperser, continue to circulate for 15 minutes, then stop the circulating emulsification pump, and finally stop stirring to obtain diisobutyryl peroxide emulsion.

[0032] During the preparation process, the combustible gas concentration was 0% LEL, there was no smoke, and the oxygen concentration was less than 20.9%.

[0033] The content of diisobutyryl peroxide in the emulsion is 24.2%. The viscosity is tested by DV-1 BROOKFIELD VISCOMETER (rotor 61#, 100 rpm). The viscosity of the emulsion is 46 mPa·s at -10℃; the emulsion flows well at -40℃, with a viscosity of 66 mPa·s, and no freezing and solidification occurs. When stored at different temperatures, the content is 23.7% after 85 days at -40℃; and 23.5% after 85 days at -25℃. Example 3

[0034] (1) Add 496.5 kg of methanol, 432 kg of water, 54.5 kg of emulsifier solution (pre-prepared, containing 20 kg of water, 20 kg of methanol and 14.5 kg of fatty alcohol polyoxyethylene ether emulsifier), and 36 kg of dispersant solution (pre-prepared, containing 29 kg of water, 3.5 kg of methanol, and 3.5 kg of polyvinyl alcohol dispersant) into the kettle, start stirring at a stirring rate of 180 rpm, and cool to -5°C.

[0035] (2) Add 299 kg of organic solvent (100 kg of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, 199 kg of isododecane) and 1233 kg of diisobutyryl peroxide and 16 kg of tert-amyl hydroperoxide, control the temperature at -5°C and maintain for 15 min.

[0036] (3) Turn on the refrigeration of the circulating emulsification pump and its pipelines, and control the temperature of the ethylene glycol cold salt used for refrigeration to -20°C.

[0037] (4) Cool the contents of the kettle to -11°C, lower the stirring speed to 150 rpm, and adjust the frequency of the circulating emulsification pump to 25 Hz. After 15 minutes, start the disperser (speed 1500 rpm) for 30 minutes.

[0038] (5) Stop the disperser, continue to circulate for 10 minutes, then stop the circulating emulsification pump, and finally stop stirring to obtain diisobutyryl peroxide emulsion.

[0039] During the preparation process, the combustible gas concentration is 0% LEL, there is no smoke, and the oxygen concentration is less than 21.5%.

[0040] The content of diisobutyryl peroxide in the emulsion is 47.4%. The viscosity is tested by DV-1 BROOKFIELD VISCOMETER (rotor 61#, 100 rpm). The viscosity of the emulsion is 58 mPa·s at -10 ℃; it flows well at -40 ℃, with a viscosity of 79 mPa·s, and no freezing and solidification occurs. After storage for 90 days, the content is 46.7%.

[0041] Comparative Example 1 As a comparative example, the other parts are the same as Example 2, except that the stabilizer hydroperoxide 1,1,3,3-tetramethylbutyl hydroperoxide is not added. During the preparation process, the combustible gas concentration is 2% LEL, there is no smoke, and the oxygen concentration is less than 21.5%.

[0042] The prepared emulsion has a diisobutyryl peroxide content of 24.6%. The viscosity is 47 mPa·s at -10°C using a DV-1 BROOKFIELD VISCOMETER (61# rotor, 100 rpm). It flows well at -40°C, with a viscosity of 64 mPa·s, and no freezing occurs. When stored at different temperatures, the content is 20.7% after 85 days at -40°C and 19.3% after 85 days at -25°C.

[0043] It can be seen that in Comparative Example 1, after storage at -40°C for 85 days, the content decreased from 24.6% to 20.7%, with a decrease rate of 15.8%; after storage at -25°C for 85 days, the content decreased from 24.6% to 19.3%, with a decrease rate of 21.5%.

[0044] In Example 2, after storage at -40°C for 85 days, the content decreased from 24.2% to 23.7%, a decrease rate of 2%; after storage at -25°C for 85 days, the content decreased from 24.2% to 23.5%, a decrease rate of 2.9%.

[0045] Comparative Example 2 As a comparative example, the other conditions are the same as those in Example 3, except that the organic solvent added is isododecane. During the preparation process, the combustible gas concentration is 0% LEL, there is no smoke, and the oxygen concentration is less than 21%.

[0046] The prepared emulsion has a diisobutyryl peroxide content of 47.6%. The viscosity was tested using a DV-1 BROOKFIELD VISCOMETER (rotor #61, 100 rpm), with a viscosity of 72 mPa·s at -10°C and 118 mPa·s at -40°C. Slight freezing and solidification occurred, and the content was 45.5% after storage for 90 days. It can be seen from Example 3 that if only isododecane is added as an organic solvent, the viscosity of the emulsion at -40°C increases by more than 40%.

Claims

1. A low temperature stable organic peroxide emulsion, characterized in that: The following components are included in mass fraction: Diisobutyryl peroxide 23%~50%; Stabilizer hydroperoxide 0.5%~1.0%; 10% to 18% of an organic solvent, wherein the organic solvent is a solvent containing 2,2,4-trimethyl-1,3-pentanediol diisobutyrate; Water 15%~30%; methanol 20%~30%; emulsifier 0.5%~0.9%; dispersant 0.1%~0.3%.

2. The low temperature stable organic peroxide emulsion according to claim 1, characterized in that The stabilizer hydroperoxide is one or more of tert-amyl hydroperoxide, tert-butyl hydroperoxide, tert-hexyl hydroperoxide, and 1,1,3,3-tetramethylbutyl hydroperoxide.

3. The low temperature stable organic peroxide emulsion according to claim 1, characterized in that The emulsifier is fatty alcohol polyoxyethylene ether; the dispersant is polyvinyl alcohol with an alcoholysis degree of 50% to 80%.

4. The low temperature stable organic peroxide emulsion according to claim 1, characterized in that The emulsion has a viscosity of 60-80 mPa·s when stored at -40°C, and does not freeze or solidify.

5. A safe method for preparing a low-temperature stable organic peroxide emulsion according to any one of claims 1 to 4, characterized in that: The components are mixed under low temperature conditions to form an emulsion by a high shear dispersion coupled cyclic emulsification method; During the preparation process, the concentration of combustible gas, smoke and oxygen on the surface of the emulsion is monitored in real time; specifically, the following steps are included: (1) Add methanol, water, emulsifier and dispersant into the emulsifier, start stirring, and cool to low temperature conditions of -15℃~-5℃; (2) Add diisobutyryl peroxide, organic solvent and stabilizer hydroperoxide, and control the temperature to -15℃~-5℃ for 10~20min; (3) Turn on the circulating emulsification pump and its pipeline refrigeration to cool the material in the kettle to -15~-5℃; (4) Adjust the frequency of the circulating emulsification pump to 20-40 Hz, start the high shear disperser for 20-40 minutes after 15-20 minutes, and control the disperser speed to 1000-2000 rpm; (5) After stopping the disperser, continue to circulate for 10 to 20 minutes, stop the circulating emulsification pump, and finally stop stirring to obtain the final emulsion.

6. The safe preparation method of the low-temperature stable organic peroxide emulsion according to claim 5, characterized in that: The emulsion is circulated from the bottom of the kettle to the top of the kettle through a circulating emulsification pump, with a pumping rate of 1~3 L / s; the circulating emulsification pump and its pipeline are cooled with ethylene glycol cold salt, and the temperature is controlled at -25℃~-15℃.

7. The safe preparation method of the low-temperature stable organic peroxide emulsion according to claim 5, characterized in that: The combustible gas concentration is less than 5% LEL, there is no smoke, and the oxygen concentration is less than 23.5%; the combustible gas, smoke and oxygen concentrations are fed back in real time through an online monitoring system. If the combustible gas concentration is ≥5% LEL or the oxygen concentration is ≥23.5%, shearing is stopped immediately and refrigeration is intensified.