Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-09-20
2002-09-17
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S503000, C526S202000
Reexamination Certificate
active
06451898
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aqueous emulsion having the advantages of good water resistance, high-temperature viscosity stability and low-temperature storage stability, and to a method for producing such an aqueous emulsion having the advantage of good polymerization stability.
2. Description of the Related Art
Heretofore, polyvinyl alcohol (hereinafter referred to as PVA) is widely used as protective colloid in emulsion polymerization of an ethylenic unsaturated monomer, especially a vinyl ester monomer such as typically vinyl acetate. Aqueous emulsions of vinyl ester polymers obtained through emulsion polymerization with PVA serving as protective colloid have many applications in various fields of adhesives for paper products, wood products, plastics, etc., binders for dip-coated paper, non-woven fabrics, etc., as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.
Controlling the degree of hydrolysis of PVA therein makes it possible to change the physical properties of such aqueous emulsions, some of which will have low viscosity just like Newtonian fluid, and will be relatively highly resistant to water, while some others will have high viscosity relatively independently of ambient temperatures. Depending on such their properties, aqueous emulsions containing PVA have many applications in various fields.
However, some aqueous emulsions containing PVA have drawbacks in that their fluidity (high-speed coatability) is poor, that their water resistance is poor, that their viscosity greatly varies depending on ambient temperatures, and that their viscosity significantly increases at low temperatures. It is known that the properties of aqueous emulsions containing PVA greatly depend on PVA therein used for emulsion polymerization.
PVA serving as a dispersant for emulsion polymerization is generally grouped into “completely hydrolyzed PVA” having a degree of hydrolysis of 98 mol % or so and “partially hydrolyzed PVA” having a degree of hydrolysis of 88 mol % or so. Aqueous emulsions containing the former PVA could have relatively good water resistance and fluidity (high-speed coatability), but are problematic in that their viscosity greatly increases and therefore they soon gel when left at low temperatures. On the other hand, aqueous emulsions containing the later PVA could have improved properties in some degree in that their viscosity increase at low temperatures is retarded and they are not so much gelled even at low temperatures. However, they are still problematic in that their water resistance is poor. To solve the problems with these aqueous emulsions containing PVA, using the two types of PVA as combined in aqueous emulsions, or using PVA having an intermediate degree of hydrolysis therein is tried. However, no one has heretofore succeeded in realizing aqueous, PVA-containing emulsions that satisfy the two requirements of good water resistance and good low-temperature storage stability for their viscosity. In that situation, PVA with ethylene units therein has been proposed for use in aqueous emulsions (see Japanese Patent Laid-Open Nos. 81666/1996, 80709/1994, 226774/1998, etc.). Containing it, aqueous emulsions have much improved water resistance and low-temperature storage stability. However, their high-temperature viscosity stability is not still good. In other words, their viscosity still varies, depending on ambient temperatures, and, in addition, their high-temperature storage stability is not also good.
SUMMARY OF THE INVENTION
The present invention is to solve the problems with the related art techniques, and its object is to provide an aqueous emulsion having the advantages of far improved high-temperature viscosity stability (that is, the emulsion is anywhere stable independently of ambient temperatures, and its high-temperature storage stability is good) and far improved water resistance and low-temperature storage stability, and also to provide a method for producing such an aqueous emulsion having the advantage of good polymerization stability.
Specifically, the invention provides an aqueous emulsion which comprises, as the dispersant, a polyvinyl alcohol containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, and, as the dispersoid, a vinyl ester polymer, and which is characterized in that, when its film is immersed in water at 20° C. for 24 hours, the degree of water solubility of the film is at most 1.5%, and the degree of water absorption of the film is at most 30%, and that the ratio of the emulsion viscosity at 60° C. (T
60° C.
) to the emulsion viscosity at 20° C. (T
20° C.
), T
60° C./T
20° C.
is at most 2.
In this, the degree of emulsion release and the degree of water absorption are measured according to the methods mentioned hereinunder. Films having a degree of water solubility of them of at most 1.5% and having a degree of water absorption of at most 30% are meant to have good water resistance. Preferably, the degree of water solubility of the film obtained from the emulsion of the invention is at most 1.1%; and the degree of water absorption of the film is at most 27%.
The ratio of the emulsion viscosity at 60° C. (T
60° C.
) to the emulsion viscosity at 20° C. (T
20° C.
), T
60° C.
/T
20° C.
is measured according to the method mentioned hereinunder. In general, ordinary vinyl ester polymer emulsions containing, as the dispersant, an ethylene-containing PVA have high viscosity at around 60° C. However, the aqueous emulsion of the invention is so defined that its viscosity change ratio T
60° C.
/T
20° C.
is at most 2. This means that the viscosity increase in the aqueous emulsion of the invention is retarded at around 60° C., or that is, the temperature dependence of the aqueous emulsion of the invention is small. As its viscosity increase at around 60° C. is thus retarded, the aqueous emulsion of the invention shall have the advantages of viscosity increase retardation anytime during polymerization to give the emulsion, and even during high-temperature storage, transportation and use of the emulsion. Accordingly, the workability and the handlability of the emulsion of the invention are much improved.
More preferably, the ratio of the emulsion viscosity after stored at 60° C. for 1 week (H
60° C.
) to that after stored at 20° C. for 1 week (H
20° C.
), H
60° C.
/H
20° C.
, is at most 2.5. The viscosity change ratio H
60° C.
/H
20° C.
of the emulsion is measured according to the method mentioned hereinunder. The emulsion having a viscosity change ratio, H
60° C.
/H
20° C.
of at most 2.5 is meant to have good high-temperature storage stability. Concretely, the viscosity of the emulsion having such a limited viscosity change ratio does not increase so much even when it is stored at high temperatures of around 60° C. for 1 week or so, and the emulsion can be well worked and handled anytime at such high temperatures.
The emulsion of the invention can be prepared by polymerizing a vinyl ester monomer through emulsion polymerization in the presence of (1) a modified PVA containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, which serves as a dispersant, and (2) at least one polymerization initiator selected from hydrogen peroxide, ammonium persulfate and potassium persulfate in a molar ratio to the vinyl ester monomer of from 0.001 to 0.01, in such a polymerization mode that (3) from 5 to 20% by weight of all the vinyl ester monomer is fed into the reactor in the initial stage of polymerization and the initiator is fed thereinto all at a time in a molar ratio to the. initial feed of the vinyl ester monomer of from 0.005 to 0.025.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The modified PVA containing from 1 to 15 mol % of ethylene units in the molecule and having a degree of hydrolysis of at least 95 mol %, which serves as the dispersant in the aqueous emulsion of the invention, is obtained
Inomata Naokiyo
Tanimoto Seiji
Kuraray Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Reddick Judy M.
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