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
1995-04-24
2002-05-14
Wilson, D. R. (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...
C528S50200C, C528S50200C, C528S503000
Reexamination Certificate
active
06388000
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for the production of a hydrophilic resin having an acrylate as a main component thereof. More particularly it relates to a method for the production of a hydrophilic resin which has only a small residual monomer content and shows virtually no increase in the residual monomer content under varying conditions of use.
The hydrophilic resin according to this invention can be produced easily and inexpensively and is excellent in quality and safety and, therefore, can be used as an absorbent resin and water-soluble resin in a wide range of applications.
2. Description of the Prior Art
The hydrophilic resins can be generally classified by their solubility in water into roughly two types, water-soluble resins and absorbent resins.
Water-soluble resins are hydrophilic resins of the type which dissolve in water and are used, for example, as water treatment grade flocculants, oil drilling additives, food additives, and viscosity enhancers.
The water-soluble resins which are known to the art include, for example, polysodium acrylate (JP-B-48-42,466 and JP-B-42-9,656), polyacrylic acid and polyacrylamide (JP-A-54-145,782 and JP-A-57-18,652), polymers of 2-acrylamide-2-methylpropane sulfonic acid (JP-A-2-173,108), partial hydrolyzate of polyacrylamide (JP-A-52-137,483), acrylic acid-acrylamide copolymer (JP-A-59-15,417), (meth)acrylic acid-itaconic acid copolymer (JP-A-58-91,709), and polyvinyl alcohol.
Absorbent resins are water-insoluble hydrophilic resins of the type which absorb water and consequently undergo gelation and are widely used in the fields of agriculture and forestry and in the field of civil engineering as well as in the field of hygienic materials such as disposable diapers and sanitary napkins.
The absorbent resins which have been heretofore known include, for example, partially neutralized cross-linked polyacrylic acid (JP-A-55-84,304, JP-A-55-108,407, and JP-A-55-133,413), hydrolyzate of starch-acrylonitrile graft polymer (JP-A-46-43,995), neutralized starch-acrylic acid graft polymer (JP-A-51-125,468), saponified vinyl acetate-acrylic ester copolymer (JP-A-52-14,689), hydrolyzate of acrylonitrile copolymer or acrylamide copolymer (JP-A-53-15,959) or cross-linked derivatives thereof, and cross-linked cationic monomer (JP-A-58-154,709 and JP-A-58-154, 710).
Numerous compounds have been proposed as monomers for the production of these hydrophilic resins. From the viewpoint of the quality of the product and the cost of production, a partially or completely neutralized acrylate (hereinafter referred to as “acrylate”) is predominately used today. The acrylate type polymers which have acrylates as the main component of their monomers are produced today in large amounts for both absorbent resins and water-soluble resins and have been finding extensive utility in the fields of hygienic materials and foodstuffs.
By the current technical standard, it is normal that the acrylate type polymers which are in wide use generally contain such a residual monomer as unaltered acrylic acid (or a salt thereof) in a concentration in the range of 500 to 3,000 ppm. Thus, the desirability of decreasing the residual monomer content in the polymers has been finding enthusiastic recognition.
In these hydrophilic resins, particularly the absorbent resins used in sanitary materials, a lower the residual monomer content is required. In recent years, the prevailing demand is to lower the residual monomer content to below 100 ppm. This decrease of the residual monomer content is particularly difficult to attain in the absorbent resins among the hydrophilic resins because the absorbent resins are hydrophilic resins of the type having a cross-linked structure and, therefore, more often than not have a neutral pH value.
The absorbent resins having a cross-linked structure are not easily polymerized uniformly as compared with water-soluble resins. When absorbent resins fresh from polymerization are to be mixed with an additive to decrease the residual monomer content or with an organic solvent, a uniform mixture is not easily obtained because of the cross-linked structure of the polymers. Thus, a decrease of the residual monomer content in the absorbent resins has been extremely difficult to attain. Moreover, since acrylates are such that their polymerization velocities are lowered proportionately as their pH values approach neutrality, a decrease of the residual monomer content as a consequence of polymerization has been extremely difficult to attain in neutral acrylate type absorbent resins.
Heretofore, in the field of macromolecular flocculants, for example, there has been an attempt at decreasing the residual monomer content in hydrophilic resins. Even now, numerous hydro-philic resins mentioned above as well as acrylate type polymers and absorbent resins are still the subjects of a study in a search for measures of decreasing the residual monomer content.
The techniques known to the art are broadly divided into the following six types (a) to (f):
(a) Methods for lowering the residual monomer content by increasing the polymerization ratio of the polymer itself.
The methods of this type include, for example, increase of the amount of polymerization initiator and the use of a composite initiator (JP-A-50-96,689), elevation of the reaction temperature, an increase in the polymerization concentration, lengthening the polymerization time, and the specification of aging conditions (JP-A-53-145,895), the two-stage addition of a polymerization initiator (JP-A-56-72,005), the exposure of a resin fresh from polymerization to radiation (JP-A-63-43,930), and the irradiation of a resin fresh from polymerization with ultraviolet light (JP-A-62-260,906).
(b) Methods for converting the residual monomer in the polymer into an extraneous derivative by use of an additive.
The methods of this type include, for example, the subsequent addition of a primary or secondary amine (JP-A-50-40,649), the subsequent addition of sulfur dioxide (U.S. Pat. No. 3,780,006), and the subsequent addition of an alkali metabisulfite (U.S. Pat. No. 4,306,955).
(c) Methods for extracting the residual monomer from the polymer.
The methods of this type include, for example, the extraction by the use of a hydrophilic organic solvent (U.S. Pat. No. 4,794,116) and the supercritical extraction by the use of carbon dioxide.
(d) Methods for treating the residual monomer with a micro-organism capable of decomposing the residual monomer
The methods of this type include, for example, decomposition of residual acrylamide with a microorganism (U.S. Pat. No. 4,742,114).
(e) Methods for volatilizing the residual monomer at elevated temperatures
The methods of this type include, for example, volatilization of residual acrylonitrile at an elevated temperature (JP-A-54-119,588).
The methods of (a), however, are actually such that since their effects in lowering the residual monomer content are not sufficient, the residual monomer generally persists in a concentration of at least 0.03% and the self-crosslinking occurs and basic molecular weight of the hydrophilic resin are necessarily degraded by the harsh conditions during polymerization and the aftertreatment possibly to the extent of increasing the water-soluble content of the absorbent resin, lowering the gel strength, and impairing the physical properties of the resulting hydrophilic resin.
Further, the two-stage addition of a polymerization initiator and the use of a large amount of initiator increases the possibility of the polymerization initiator persisting in the produced resin and consequently jeopardizing the safety of the produced polymer.
The methods of (b) and (c) are purportedly capable of lowering the residual monomer content to below 0.03%. However in (b), in addition to complexed process, an additive used and the adduct formed of the additive with the residual monomer and in (c) the organic solvent such as methanol used for the extraction of residual monomer, never fail to persist in the hydrophilic resi
Hatsuda Takumi
Irie Yoshio
Ishizaki Kunihiko
Kajikawa Katsuhiro
Burns Doane , Swecker, Mathis LLP
Nippon Shokubai Co. , Ltd.
Wilson D. R.
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