JPH0239527B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous ethylene-vinyl acetate copolymerization method that can prevent the formation of gel-like substances when ethylene and vinyl acetate are continuously polymerized in a solvent using a radical initiator. Conventionally, various methods have been known for polymerizing vinyl monomers, but the continuous polymerization method is widely used as the polymerization method for vinyl acetate because it is industrially advantageous in terms of productivity and uniformity of quality. has been done. Generally, when the polymerization reaction is carried out continuously, polymers and gel-like substances tend to form and accumulate on the walls of the polymerization tank and in so-called dead spaces where the flow within the tank stagnates, resulting in a rapid decrease in the heat transfer coefficient. Therefore, it becomes impossible to remove the heat of polymerization, and the discharge of the gel-like material causes clogging of the line, making long-term continuous operation impossible. There is a problem in that cleaning the inside of the tank is unavoidable and the advantages of continuous polymerization are lost. Even in solution polymerization of ethylene-vinyl acetate system, which is a homogeneous system, the problems caused by the formation and accumulation of gel-like substances are significant, and from the viewpoint of enabling stable continuous operation over a long period of time, it is necessary to prevent the formation and accumulation of gel-like substances. was one of the important issues that needed to be solved. The present inventors have investigated various factors of the polymerization conditions in the ethylene-vinyl acetate copolymerization reaction, such as the copolymerization conditions, the composition of the copolymer and its characteristic values, the type of initiator, and the gelatin during continuous polymerization. As a result of a detailed study of the relationship between the formation of such substances in the polymerization tank, we found that
We have arrived at the present invention, which prevents the formation of gel-like substances and enables stable continuous polymerization over a long period of time. That is, when producing an ethylene-vinyl acetate copolymer by continuously copolymerizing ethylene and vinyl acetate, 2,2'-azobis-
Ethylene-vinyl acetate continuous copolymerization characterized by solution polymerization using (4-methoxy-2,4-dimethylvaleronitrile) at a polymerization solvent concentration of 20% by weight or less and a polymerization temperature of 35 to 80°C. It's legal. Although the cause of the formation of gel-like substances in the polymerization tank is still unclear, it is believed that cross-linking tends to occur due to chain transfer of vinyl acetate, and that cross-linked structures that are produced as by-products as the degree of cross-linking increases in the reaction solution. This is thought to be because it is insoluble in Furthermore, the site of the ethylene-vinyl acetate copolymerization reaction requires the dissolution of ethylene, and the ethylene concentration increases as the ethylene content increases, and the solubility of the crosslinked structure of the copolymer decreases. It seems that there is a connection. Normally, in order to obtain the copolymer with a relatively high degree of polymerization, it is necessary to keep the solvent concentration in the polymerization tank low.On the other hand, when the solvent concentration is kept low and continuous polymerization is carried out, the gel-like material There is a trade-off that production and accumulation become significant. For example, when saponified ethylene-vinyl acetate copolymer is used as a molding raw material, it is desirable to maintain the solvent concentration at 20% by weight or less in order to secure the necessary degree of polymerization, but in such a low solvent concentration range, In particular, the formation and accumulation of gel-like substances is remarkable. The present invention exhibits extremely significant effects at such low solvent concentrations and eliminates the above-mentioned problems. When the solvent concentration exceeds 20% by weight, the formation and accumulation of gel-like substances tends to be alleviated, and the effects of the present invention tend to be relatively reduced. Since the degree of polymerization of the copolymer decreases as the polymerization temperature increases, in order to ensure the desired degree of polymerization,
The polymerization temperature is often selected to be 80°C or lower. In such a temperature range, gel-like substances tend to form, although this seems to be related to an increase in the degree of polymerization and an increase in the viscosity of the polymerization system. The present invention exhibits remarkable effects particularly in such a relatively low temperature range, and eliminates the above-mentioned problems. On the other hand, in the temperature range higher than 80℃,
There is a tendency for the effects of the present invention to be relatively reduced as the degree of polymerization of the copolymer and the viscosity of the polymerization system are reduced. It is preferable that the polymerization temperature be as low as possible in order to maintain the degree of polymerization, but as the temperature decreases, the polymerization rate decreases, which is an economic problem. In order to compensate for this, measures such as increasing the initiator concentration can be considered, but at low polymerization temperatures of 35°C or lower, the degree of polymerization increases, the decomposition rate of the initiator decreases, and the viscosity of the polymerization system increases further. It seems to be related to the above, and there is a tendency for the gel formation preventing effect of the present invention to decrease. If the polymerization solvent is different, the solubility of the gel-like material will change, so the generation and accumulation state of the gel-like material will change depending on the solvent. From this point of view, methanol or t-butanol is suitable as the solvent. From an industrial standpoint, inexpensive methanol is advantageous, and when a high degree of polymerization is desired, t-butanol is preferred. In the copolymerization reaction of ethylene and vinyl acetate, it is essential that ethylene be dissolved in the vinyl acetate and solvent in the polymerization tank, and the necessary dissolved ethylene concentration increases as the ethylene content of the copolymer increases. . It seems that the solubility of the gel-like product produced in the copolymerization reaction solution decreases as the dissolved ethylene concentration and the ethylene content of the copolymer increase, and as the ethylene content increases, the degree of formation and accumulation of the gel-like product increases. becomes significant. Ethylene content is 60
If the amount exceeds mol%, the effect of the present invention tends to decrease. On the other hand, in regions where the ethylene content is low, the formation and accumulation of gel-like substances is thought to be alleviated, but on the other hand, in these regions, under conditions such as the same solvent concentration and the same polymerization temperature, the ethylene content is A considerable amount of polymer gel-like material was observed to be produced, probably because the degree of polymerization of the produced copolymer increased as the temperature decreased. Especially when the solvent concentration is 20
Although the problem is significant in the range of % by weight or less, the effects of the present invention can be enjoyed even in the range of low ethylene content and low solvent concentration. The present invention also exhibits its effects in a copolymerization reaction system containing ethylene and vinyl acetate as main components and further containing a third component. If the amount of the third component increases, the properties of the resulting polymer will change, and the type of third component will also affect the expression of the effect. In order to enjoy the present invention, the third component is
CH ₂ CRR' [where R is hydrogen or a methyl group, and R' is a methyl group or COOR. ], and the amount of the third component contained in the produced copolymer is such that the molar ratio (Z/Y) of the copolymer to the acetic acid component is 0 to
Preferably it is 0.1. When the content of the third component exceeds this range, the effects of the present invention tend to decrease. The effects of the present invention are related to the reaction rate of vinyl acetate in ethylene-vinyl acetate copolymerization, and when the reaction rate exceeds 85%, the effects tend to decrease. Therefore, the reaction rate is preferably 80% or less. In addition, the state of formation and accumulation of gel-like substances is closely related to the average residence time in the polymerization tank, and becomes more noticeable as the average residence time decreases. Usually, when the average residence time is 15 hours or less, the formation and accumulation of gel-like substances occurs. is remarkable. The method of the present invention is particularly effective when applied to areas where the average residence time is 15 hours or less and where the formation and accumulation of gel-like substances is significant, and more preferably 12 hours or less. When the average residence time is less than 1 hour, the formation of gel-like substances is extremely significant and the effect of the present invention appears to be reduced, but when the average residence time is 1.5 hours or more, the effect is fully recognized. The relationship between the average residence time and the gel-like material is presumed to be related to changes over time in the polymerization reaction and the decomposition reaction of the initiator, but it is not yet clear. In the present invention, suitable opening agents include 2,
2'-azobis-(4-methoxy-2,4-dimethylvaleronitrile). The ethylene-vinyl acetate copolymer may be used by itself or may be saponified in the presence of a caustic alkali, an alkali metal alcoholate, etc., and used as a saponified copolymer. The saponified copolymer has excellent oxygen barrier properties and is a suitable raw material resin for food packaging materials. Among radical polymerization initiators, it is strictly required that the saponified product be substantially odorless when heat treated with the alkaline substance, since the saponified product is used in food packaging materials. From this point of view, 2,2′-azobis-(4-
methoxy-2,4-dimethylvaleronitrile)
When the copolymer is saponified in the coexistence of the alkaline substance, the initiator itself and the initiator decomposition products do not substantially emit any odor, and the present invention can be applied to many fields and its effects can be enjoyed. It is an excellent initiator with no limitations. The present invention will be described in detail below with reference to Examples, but the present invention is not limited to these Examples. Example 1 In order to produce an ethylene-vinyl acetate copolymer using a polymerization tank with a capacity of 100 and equipped with an internal cooling coil and a four-blade paddle type stirrer,
Continuous polymerization was carried out. The polymerization conditions are as shown below. Vinyl acetate supply amount 5.0Kg/hr Methanol supply amount 0.68 〃 Propylene 〃 0.1 〃 2,2'-azobis-4 (4-methoxy-2,4
-Dimethylvaleronitrile) Supply amount 2.5g/hr Polymerization temperature 60℃ Polymerization tank ethylene pressure 45Kg/cm ² G Average residence time 7hrs 2,2'-azobis-(4-methoxy-2,4-
The half-life of dimethylvaleronitrile (dimethylvaleronitrile) at 60°C (in methanol, same hereinafter) is approximately 15 minutes. As a result, ethylene-vinyl acetate-propylene copolymer with an ethylene content of 34 mol% was produced at 3.2 kg/hr.
The vinyl acetate polymerization rate at this time was about 55%. During continuous operation for 50 days, the polymerization temperature was suppressed extremely stably by the cooling coil, and no gel-like substances were observed in the polymer solution discharged from the polymerization tank. After 50 days of continuous polymerization operation, the inside of the polymerization tank was inspected and no gel-like substances were observed on the cooling coil, stirrer shaft, four blades, tank inner wall, etc. Comparative Example 1 In Example 1, the initiator was changed to 2,2'-azobisisobutyronitrile, and continuous polymerization was carried out under the same conditions except for that. The feed rate of 2,2'-azobisisobutyronitrile was 3.5 g/hr, and the half-life at 60°C was about 22 hours. The ethylene content of the obtained ethylene-vinyl acetate-propylene copolymer was 34 mol%, and the vinyl acetate polymerization rate at this time was about 55%. From the 3rd day of continuous polymerization, gel-like substances began to be observed in the polymer solution discharged from the polymerization tank, and on the 8th day, the polymerization temperature control became unstable, so continuous polymerization was stopped. After inspecting the inside of the polymerization tank, approximately 1.5 kg was found in the cooling coil, stirrer shaft, tank inner wall, etc.
A gel-like substance was attached. Examples 2 to 4 Using the same polymerization tank as in Example 1, continuous polymerization was carried out under the following conditions to produce an ethylene-vinyl acetate copolymer. Vinyl acetate supply amount 3.5Kg/hr t-Butanol supply amount 0.5 〃 Polymerization temperature 60℃ Polymerization tank ethylene pressure 50Kg/cm ² G Average residence time 8.5hrs Initiator 2,2'-azobis-(4-methoxy-
(2,4-dimethylvaleronitrile) In this case, the amount of 2,2'-azobis-(4-methoxy-2,4-dimethylvaleronitrile) supplied is adjusted to change the polymerization rate of vinyl acetate, thereby increasing the ethylene content. An ethylene-vinyl acetate copolymer of 38 to 41 mol% was obtained, and the relationship between the change in polymerization rate, the number of days of continuous operation, and the amount of gel deposited in the polymerization tank was as shown in the table below.

[Table] Examples 5 to 7 Using the same polymerization tank as in Example 1, continuous polymerization was carried out under the following conditions to obtain an ethylene-vinyl acetate copolymer with an ethylene content of 30 to 33 mol%. Polymerization solvent methanol concentration 10% Polymerization temperature 65℃ Polymerization tank ethylene pressure 45Kg/cm ² G Initiator 2,2'-azobis-(4-methoxy-
(2,4-dimethylvaleronitrile) Vinyl acetate polymerization rate: Approximately 40% In this case, the average residence time was changed by adjusting the amounts of vinyl acetate, methanol, and initiator supplied, but the average residence time, number of days of continuous operation, and polymerization tank Regarding the amount of adhesion of the inner gelatinous substance, the results shown in the table below were obtained.

[Table] Examples 8 to 10 Using the same polymerization tank as in Example 1, continuous polymerization was carried out under the following conditions to produce an ethylene-vinyl acetate copolymer. Vinyl acetate supply rate 3.5Kg/hr t-Butanol supply rate 0.5Kg/hr Polymerization temperature 60℃ Average residence time 7-8.5hrs Initiator 2,2'-azobis-(4-methoxy-
(2,4-Dimethylvaleronitrile) Vinyl acetate polymerization rate: approximately 40% In this case, the ethylene pressure in the polymerization tank was adjusted to change the ethylene content of the ethylene-vinyl acetate copolymer. The results shown in the table below were obtained regarding the ethylene content of the polymer, the number of days of continuous operation, and the amount of gel-like substance deposited in the polymerization tank.

【table】

Claims (1)

[Claims] 1. Continuous copolymerization of ethylene and vinyl acetate,
When producing ethylene-vinyl acetate copolymer, 2,2'-azobis-
Ethylene-vinyl acetate continuous copolymerization characterized by solution polymerization using (4-methoxy-2,4-dimethylvaleronitrile) at a polymerization solvent concentration of 20% by weight or less and a polymerization temperature of 35 to 80°C. legal. 2. Ethylene according to claim 1, wherein the polymerization solvent is methanol or tertiary butanol.
Vinyl acetate continuous copolymerization method.