JPH0586403B2 - - Google Patents

Description

[Detailed description of the invention]

A Technical Field of the Invention The present invention relates to an aqueous emulsion dispersion stabilizer such as an emulsion polymerization stabilizer. In recent years, stable emulsions have been obtained by emulsion polymerization of ethylenically unsaturated monomers such as vinyl acetate alone or together with ethylene, acrylic esters, etc. in an aqueous medium in the presence of water-soluble emulsion-stabilizing protective colloids. It is widely used in applications such as agents, paints, and paper processing agents. Polyvinyl alcohol polymer (sometimes abbreviated as PVA) is used as a water-soluble protective colloid, especially when partially saponified.
A large amount of PVA is used to create an emulsion with good adhesive properties and excellent mechanical performance. However, current PVA-based emulsion polymerization stabilizers do not necessarily have sufficient emulsion polymerization stabilization performance, and a highly stable product that can provide stability with a smaller amount is desired. B. Prior art and its problems PVA, which is conventionally used as a dispersion stabilizer for emulsion polymerization, is a polyvinyl ester obtained by radical solution polymerization of vinyl ester monomers such as vinyl acetate in methanol, and residual monomers are separated and removed. After that, water-soluble water is saponified with an alkaline catalyst such as sodium hydroxide in methanol.
PVA is a typical example. Parameters that indicate the performance of dispersion stabilizers for emulsion polymerization, such as the surface tension of the PVA aqueous solution, the emulsifying power of the vinyl acetate monomer, and the protective colloidal property, are determined by the fact that partially saponified products with a lower degree of saponification are better than completely saponified ones with a higher degree of saponification. Partially saponified products are mainly used as dispersion stabilizers for emulsion polymerization because they are superior to chemical compounds. However, with lower saponification degree
PVA has various problems such as lower water resistance than PVA with a high saponification degree, becoming insoluble in water if the saponification degree becomes too low, and showing a cloud point phenomenon where it is soluble in water at low temperatures but precipitates at high temperatures. Therefore, it is difficult to extremely lower the degree of saponification. In an attempt to obtain PVA with higher dispersion stability for emulsion polymerization with the same degree of saponification, polyvinyl acetate obtained by ordinary methanol solution polymerization was mixed with methanol and a solvent such as methyl acetate or benzene as a solvent during saponification. It is known to saponify in a mixed solvent system. It has been shown that the partially saponified PVA obtained by this method has a lower surface tension in its aqueous solution than that saponified with methanol alone, making it possible to produce a dispersion stabilizer for emulsion polymerization with better surfactant ability. ing. However, the increase in surfactant ability is still not sufficient, and there is currently a need for a PVA-based dispersion stabilizer for emulsion polymerization that has higher surfactant ability and emulsion-protecting colloid performance. C Structure, purpose, and effect of the present invention As a result of intensive studies in view of the above circumstances, the present inventors surprisingly discovered that a specific highly branched polyvinyl acetate polymer (hereinafter referred to as PVAc polymer) with a degree of branching of 1 or more Polyvinyl ester such as (sometimes abbreviated as ) is saponified using a specific mixed solvent system in which the dielectric constant is adjusted to 18 to 27 (CGSesu; units are omitted below) by mixing methanol with a solvent with a small dielectric constant. Water-soluble partially saponified PVA with a saponification degree of 50 to 95 mol% obtained by saponifying a normal PVAc polymer with a degree of branching of less than 1 in the above mixed solvent system We have completed the present invention by discovering that it is an excellent dispersion stabilizer for emulsion polymerization that has significantly superior surface activity and emulsion-protecting colloid performance compared to the above. By using the dispersion stabilizer for emulsion polymerization made of water-soluble partially saponified PVA of the present invention, emulsion polymerization can be carried out stably and an excellent emulsion can be obtained. D More detailed description of the present invention The dispersion stabilizer for emulsion polymerization of the present invention is a polyvinyl ester polymer having a branching degree of 1 or more and a mixed solvent system containing methanol and having a dielectric constant of 18 to 27. It is made of water-soluble partially saponified PVA obtained by saponification and whose vinyl ester portion has a degree of saponification of 50 to 95 mol%. The degree of branching here indicates the average degree of branching of the polyvinyl ester polymer, and is expressed by the following formula. Degree of branching = (degree of polymerization of polyvinyl ester/degree of polymerization of fully saponified polyvinyl alcohol obtained by completely saponifying polyvinyl ester) -1 The degree of polymerization in the above formula means that for polyvinyl ester, the intrinsic viscosity at 30°C in acetone, completely For saponified PVA, measure the intrinsic viscosity in water at 30℃,
The so-called Sakurada method {Ichiro Sakurada et al.; Industrial Chemistry Magazine 47 ,
135-137 (1944)}. The highly branched polyvinyl ester of the present invention needs to have a degree of branching represented by the above formula of 1 or more, preferably 2 or more, and the higher the degree, the more desirable. However, those having too high a degree of branching and being insoluble in the above mixed solvent system during saponification are naturally unsuitable. Polyvinyl ester with a high degree of branching can be produced by increasing the polymerization rate to a high polymerization rate, but highly branched polyvinyl ester can be easily obtained by polymerization using suspension polymerization or emulsion polymerization. This is preferable. Particularly preferred is suspension polymerization. As the vinyl ester, various vinyl esters that can be polymerized and saponified can be used, such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate. In particular, vinyl acetate is available industrially at low cost. It is preferable because it has good polymerization and saponification properties. As a suspension polymerization method, for example, a vinyl ester such as vinyl acetate and a radical polymerization initiator that is sparingly soluble in water, such as azobisisobutyronitrile, are added to an aqueous solution containing a suspension polymerization stabilizer, such as partially saponified PVA.
This is carried out by adding an initiator such as lauryl peroxide or benzoyl peroxide, stirring, and heating and polymerizing the suspension. In order to obtain a highly branched polyvinyl ester with a degree of branching of 1 or more, the polymerization rate should be 50%.
It is desirable to increase the ratio to above, preferably 70% or more. The degree of polymerization of polyvinyl ester can be lowered by adding a chain transfer agent during polymerization.
Furthermore, a vinyl ester copolymer obtained by copolymerizing a vinyl ester monomer and a comonomer copolymerizable with the vinyl ester monomer can also be used. Emulsion polymerization using a water-soluble initiator with or without an emulsion stabilizer is also useful as a method for obtaining highly branched polyvinyl esters with a degree of branching of 1 or more, but the separation of the polymer is not easy. is disadvantageous compared to the suspension method. The obtained polyvinyl ester polymer is saponified after removing and separating unpolymerized vinyl ester monomers. At that time, a mixed solvent containing methanol and having a dielectric constant of 18 to 27 is used as the saponification solvent. It is necessary to strengthen it. By partially saponifying polyvinyl ester with a degree of branching of 1 or more in a specific mixed solvent system containing methanol with a dielectric constant of 18 to 27, the significantly superior surfactant ability and emulsion-protecting colloid performance of the present invention can be achieved. A dispersion stabilizer for emulsion polymerization is obtained, which has the following Comparative Example 3.
As is clear from the above, partially saponified PVA obtained by saponifying ordinary polyvinyl ester with a low branching degree of less than 3 in a methanol-containing mixed solvent system with a dielectric constant of 18 to 27, and partially saponified PVA with a branching degree of 1 or more. Partially saponified PVA (Comparative Example 1) obtained by saponifying a large amount of polyvinyl ester in methanol, which has a high dielectric constant of 32.6, cannot provide an excellent dispersion stabilizer for emulsion polymerization such as the one of the present invention. The dielectric constant of the mixed solvent system containing methanol in the present invention means a value measured by the Bridge method at a temperature of 25°C. (The unit is CGSesu) The dielectric constant of methanol is 32.6, and by mixing a solvent with a small dielectric constant with methanol, a mixed solvent with a dielectric constant of 18 to 27 can be prepared. Various solvents can be used as the solvent to be mixed with methanol, but a system in which the resulting mixed solvent can dissolve the polyvinyl ester is desirable. Examples include aromatic hydrocarbons such as benzene, toluene, and xylene, esters such as methyl acetate and ethyl acetate, and solvents with a low dielectric constant of 10 or less such as 1,4 dioxane and ethyl ether. Among them, aromatic hydrocarbons such as benzene, toluene, and xylene exhibit excellent emulsification and dispersion stability.
PVA can be obtained, which is preferable. The mixing ratio of methanol and the solvent to be mixed is selected so that the resulting mixed solvent system has a dielectric constant of 18 to 27. However, in order to obtain a significant mixing effect, methanol/methanol/ The solvent to be mixed is preferably 40/60 to 90/10, more preferably 40/60 to 90/10.
80/20 is preferred, and 50/50 to 70/30 is particularly preferred. Methanol is necessary for methanolysis in the saponification reaction and is consumed as methyl ester, so if the methanol content becomes too small, the saponification reaction will slow down, which is undesirable. Further, it is preferable that the amount of mixed solvent used is at least twice the amount of methanol required for saponification. The saponification catalyst is preferably an alkaline catalyst such as sodium hydroxide, potassium hydroxide, lithium hydroxide, or sodium methylate. The degree of saponification can be adjusted by changing the amount of saponification catalyst used, and the amount used is appropriately determined depending on the amount of water in the system and the like. In molar ratio to vinyl ester units
It is desirable to use 0.001 or more, preferably 0.002 or more. Saponification temperature is usually 10~70℃, preferably 20~
Selected from the range of 60℃. Not only anhydrous methanol but also a small amount of water-containing methanol can be used. The PVA of the present invention has a saponification degree of vinyl ester units of 50 to 95 mol%, preferably 60 to 90 mol%.
It must be water-soluble, partially saponified PVA. Highly saponified PVA with more than 95 mol%
It is not preferable that the dispersion stability performance for emulsion polymerization, which is a feature of the present invention, cannot be achieved, and on the other hand, if it is less than 50 mol %, sufficient water solubility cannot be achieved. Note that the water-soluble partially saponified PVA referred to in the present invention includes those that have dispersion stability performance for emulsion polymerization in water, ranging from those that completely dissolve in water to those that are dispersed and dissolved in water. The degree of polymerization of the partially saponified PVA of the present invention is not particularly limited, but is preferably 50 to 5,000, preferably 100 to 3,000. The degree of polymerization is determined by measuring the intrinsic viscosity in water at 30°C.
It is derived from the Sakurada style mentioned above. The dispersion stabilizer for emulsion polymerization made of water-soluble partially saponified PVA of the present invention is characterized by having significantly superior surfactant ability and emulsion-protecting colloid performance compared to conventional PVA-based dispersion stabilizers for emulsion polymerization. It is in the point where it is. Therefore, it is useful as a stabilizer for emulsion polymerization of vinyl monomers such as vinyl esters, and has good stability during emulsion polymerization of vinyl acetate, etc.
The resulting emulsion has a small particle size and is highly viscous and stable. Also acrylic,
It can also be used as a dispersion stabilizer for emulsion polymerization of styrene, vinyl chloride, styrene-butadiene, etc. For use as a dispersion stabilizer for emulsion polymerization, known methods are employed. PVA of the present invention as a dispersion stabilizer for emulsion polymerization
There is no particular limit to the usage ratio, and it can be determined as appropriate.
It has superior dispersion stability for emulsion polymerization compared to conventional PVA, so the amount used can be reduced. Furthermore, the PVA of the present invention may be used in combination with other known unmodified or modified PVA, surfactants, or other types of additives as long as the purpose of the present invention is not impaired. EXAMPLES The present invention will be explained in more detail below with reference to Examples, but the present invention is not limited to these in any way. Note that parts and percentages in the examples are based on weight unless otherwise specified. Example 1 (a) Synthesis of polyvinyl acetate with a degree of branching of 1 or more In a reaction tank equipped with a stirrer and a cooler, Kuraray Poval 220E (degree of polymerization = 2050, degree of saponification 88 mol%) was added.
15,000 parts of a 0.005% aqueous solution, 2,790 parts of vinyl acetate monomer (hereinafter abbreviated as VAc), 5.6 parts of azobisisobutyronitrile as an initiator, and acetaldehyde as a chain transfer agent to adjust the degree of polymerization.
Added 19.5 copies. After replacing the inside of the tank with nitrogen, lower the internal temperature to 60°C while stirring.
The temperature was raised to .degree. C., and suspension polymerization was carried out for 5 hours. The resulting bead-like polymer was deliquified and separated, and then dried under reduced pressure to remove unreacted monomers and attached water. The obtained beads are dissolved in methanol and methanol vapor is blown in to drive out and remove a small amount of unreacted VAc.
Polyvinyl acetate (hereinafter referred to as PVAc)
A methanol solution was obtained. The PVAc concentration is 40%, the solution volume is 4900 parts, and the polymerization rate of VAc is 70
It was warm at ℃. The methanol was removed from the obtained methanol solution of PVAc to take out PVAc, and the degree of polymerization (
_AC ) was 6320. On the other hand, after heating this methanol solution to 40℃, 2N
A methanol solution of sodium hydroxide was added so that the molar ratio of alkali to vinyl ester units was 0.1, and complete saponification was carried out. The degree of polymerization ( _A ) of the completely saponified PVA obtained by washing, purifying and drying the polymer powder was determined from the intrinsic viscosity of the aqueous solution at 30°C, and _A was 1650. In other words, calculated from the above formula, the degree of branching =
A highly branched PVAc of 2.83 was obtained. (b) Partial saponification of PVAc Highly branched with the degree of branching obtained above = 2.83
100 parts of 40% methanol solution of PVAc in methanol
15 parts and 80 parts of benzene were added, mixed well, and heated to 50°C. Add 0.52 parts of sodium hydroxide (molar ratio of PVAc to VAc units) onto this while stirring.
0.028) and 5 parts of methanol,
Stir well. (The saponification solvent system is methanol/
Benzene = 50/50 (weight ratio), and the dielectric constant is 20. ) After 72 seconds, the entire product gelled and solidified. After 5 minutes, the gel was ground with a grinder, washed with methanol, heated (70 to 80°C) in a mixture of methanol: methyl acetate: water = 10:3:0.5, and colored with phenolphthalein. The residual alkali was neutralized until it disappeared. The neutralized polymer was dried at 70°C. Analysis of the polymer revealed that it was partially saponified PVA with a saponification degree of 88.6 mol% and a polymerization degree of 1729. Comparative Example 1 The branching degree = 2.83 obtained in Example 1 above.
100 parts of 40% methanol solution of PVAc in methanol
Add 95 parts of sodium hydroxide at 50°C and add 0.3 parts of sodium hydroxide.
A solution containing 5 parts of methanol and 5 parts of methanol was mixed and saponified using methanol as a single solvent without benzene. (Dielectric constant of methanol sole solvent system = 32.6)
Post-treatment was carried out in the same manner as in Example 1 to obtain partially saponified PVA with a degree of saponification of 87.5 mol% and a degree of polymerization of 1720. Comparative Example 2 The branching degree = 2.83 obtained in Example 1 above.
To 100 parts of a 40% methanol solution of PVAc, 23 parts of methanol and 73 parts of acetone were added and mixed well. 50℃
Then, a solution containing 0.74 parts of methanol hydroxide and 5 parts of methanol was added and stirred thoroughly for saponification. That is, it was saponified using a mixed solvent system of methanol/acetone = 55/45 (weight ratio), which has a high dielectric constant of 29. Post-treatment was carried out in the same manner as in Example 1 to obtain partially saponified PVA with a degree of saponification of 88.8 mol% and a degree of polymerization of 1715. Comparative Example 3 Methanol in a polymerization tank equipped with a stirrer and cooler
260 parts, 740 parts of VAc, and 0.025 parts of azobisisobutyronitrile were added, and after purging with nitrogen, the temperature was raised to 60° C. and solution polymerization was carried out for 3 hours while stirring. After cooling the content solution and stopping polymerization, it was evaporated under reduced pressure while adding methanol to remove unpolymerized monomers by azeotropic distillation with methanol, and further concentrated to obtain PVAc- with a PVAc concentration of 40%.
1020 parts of methanol solution was obtained. The polymerization rate was 55%. When _AC and _A of PVAc were measured in the same manner as in Example 1, _AC = 3030, _A = 1700, and the degree of branching was 0.78, indicating that PVAc did not have many branches. Using this PVAc, partially saponified PVA with a saponification degree of 88.6 mol% and a polymerization degree of 1750 was saponified in a mixed solvent system (dielectric constant = 20) of methanol/benzene = 50/50 (weight ratio) under the same conditions as in Example 1. I got it. Next, in order to evaluate the dispersion stability performance of the partially saponified PVA obtained in Example 1 and Comparative Examples 1, 2, and 3 for emulsion polymerization, we investigated the surface tension of the PVA aqueous solution as an index of surfactant ability and the protective colloid performance. The specific viscosity of a VAc monomer and PVA aqueous solution mixed emulsion system was measured as an indicator of The results are shown in Table-1. The surface tension was measured at 20℃ for a 0.2% PVA aqueous solution, and the specific viscosity was measured at 20℃.
The specific viscosity (ηsp) of the VAc-PVA aqueous solution mixture system was determined from the following equation. ηsp=η _EM / η _PV-OH −1 However, η _EM : Viscosity of VAc and PVA aqueous solution mixed emulsion (20°C, VAc7.87% by volume,
PVA7.5wt% aqueous solution) η _PV-OH : Viscosity of PVA7.5wt% aqueous solution (20℃)

[Table] As is clear from Table 1, the PVA of the present invention has excellent dispersion stability performance for emulsion polymerization,
In other words, a specific PVAc with many branches with a branching degree of 1 or more
It can be seen that a dispersion stabilizer for emulsion polymerization with extremely excellent dispersion stability performance for emulsion polymerization can only be obtained by saponifying it with a specific mixed solvent system containing methanol and having a dielectric constant of 18 to 27. Next, the performance of the partially saponified PVA obtained in Example 1 and Comparative Examples 1, 2, and 3 as a dispersion stabilizer for emulsion polymerization was evaluated by emulsion polymerization of VAc. The results are shown in Table-2. The emulsion polymerization was carried out under the following conditions. To a solution of 40 parts of partially saponified PVA obtained in Example 1 and Comparative Examples 1, 2, and 3 dissolved in 800 parts of ion-exchanged water, 160 parts of VAc was added and mixed with stirring for 68 hours.
The temperature was raised to .degree. C., 1.6 parts of potassium persulfate was added, and emulsion polymerization was carried out by heating for 3 hours. The polymerization rate was determined by analyzing the residual VAc monomer and the solid content, and the appearance and viscosity of the particles of the emulsion were measured by microscopic observation, and the performance as a dispersion stabilizer for emulsion polymerization was evaluated.

[Table] As is clear from Table 2, the partially saponified PVA of the present invention has no coarse particles compared to the comparative example, and a stable emulsion with high viscosity can be obtained, making it an excellent dispersion stabilizer for emulsion polymerization. I understand that there is something.

Claims (1)

[Claims] 1 Degree of branching {degree of branching = (degree of polymerization of polyvinyl ester/degree of polymerization of completely saponified polyvinyl alcohol obtained by completely saponifying polyvinyl ester)
-1} is 1 or more, a highly branched polyvinyl ester containing methanol and a dielectric constant of 18 to 27 (CGS
A dispersion stabilizer for emulsion polymerization consisting of water-soluble partially saponified polyvinyl alcohol with a degree of saponification of 50 to 95 mol%, obtained by saponification in a mixed solvent system of esu).