Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-06-04
2004-04-20
Cheung, William (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S206000, C525S209000, C525S217000, C252S182130, C252S182140, C252S182150, C252S182240, C252S182230, C252S182340, C119S166000, C119S169000, C119S171000, C600S362000
Reexamination Certificate
active
06723797
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a crosslinking agent for the production of a water absorbing agent by surface-crosslinking of particles of water absorbing resin, a water absorbing agent so obtained, and a method for the production of such a water absorbing agent.
BACKGROUND ART
Water absorbing resins have a high water absorbing capacity and because of this capacity they are in wide use as sanitary or hygienic materials as well as in the fields of food, agriculture and forest industry, or engineering works.
Partially neutralized salts of polyacrylic acid or polymethacrylic acid have been heretofore used as water absorbing resins for sanitary materials in many cases. Water absorbing resins for disposable diapers, among a variety of sanitary materials, are required to have a high water absorbing capacity (water retention) and high speed of water absorption under increased pressures (under a load) as well as under normal pressures (under no load), and besides such performance characteristics are required to be well-balanced.
One of measures to obtain a water absorbing resin having high water absorbing capacity is to increase crosslinking density of the resin. However, as is well-known, for example, a resin obtained by polymerization of acrylic acid and/or its salt (neutralized salt) by using a polyfunctional copolymerizable monomers as an inner crosslinking agent in a large quantity is crosslinked uniformly with high density, and has high water absorbing capacity under a load, but inferior in water absorbing capacity under no load.
In order to solve such problems, a measure is also known in which water absorbing resin particles are surface-crosslinked. According to the method of surface-crosslinking, particles of water absorbing resin which has carboxylic acid groups and/or carboxylic acid salt groups are used as starting materials. The particles are crosslinked mainly at the surface region by using a crosslinking agent to increase the crosslinking degree mainly at the surface region of the particles of the resin so that the particles have a large water absorbing speed while maintaining the crosslinking density low inside the particles so that the particles are not reduced in water absorbing capacity. The crosslinking agent used has at least two reactive groups (functional groups) in the molecule that are capable of reacting with the carboxylic acid groups and/or carboxylic acid salt groups of the particles of the water absorbing.
As such a surface-crosslinking agent used in the surface-crosslinking method as mentioned above, for example, epoxy compounds typified by ethylene glycol diglycidyl ether are proposed in Japanese Patent Application Laid-open No. 57-44627, while polyhydric alcohols typified by ethylene glycol, diethylene glycol or glycerin in Japanese Patent Application Laid-Open No. 58-180223. Besides, polyfunctional amine compounds, polyfunctional aziridine compounds or epoxy compounds having amino groups are proposed in Japanese Patent Application Laid-open No. 63-195205, while the reaction products of epihalohydrin with ammonia or low molecular weight primary amines such as ethylenediamine in Japanese Patent Application Laid-open No. 2-248404.
When an alcohol such as glycerin or an amine is used as a crosslinking agent in the surface-crosslinking method, the surface-crosslinking reaction of water absorbing resin particles must be carried out usually at a temperature as high as 180° C. However, the surface-crosslinking reaction at such a high temperature causes thermal crosslinking or thermal deterioration of water absorbing resin itself to make it difficult to control the crosslinking degree of resin particles, and the resulting water absorbing agent is deteriorated in absorbing capacity and speed instead of being improved in such performance.
On the other hand, crosslinking agents such as epoxy compounds (i.e., ethylene glycol diglycidyl ether), epoxy compounds having amino groups in the molecule, amine compounds, aziridine compounds or isocyanate compounds irritate the skin. Accordingly, there arises a problem of safety when such a crosslinking agent is used for the production of water absorbing agent for sanitary materials for infants as it is possible that part of the crosslinking agent remains unreacted and persists on the absorbing agent and makes contact with the skin of infants.
The reaction product of an epihalohydrin with ammonia or a low molecular weight primary amine such as ethylenediamine is a simple adduct formed by the addition of amine to the epoxy group of the epihalohydrin and has no reactive functional group in the molecule so that it cannot achieve effective crosslinking even under heating and improvement in water absorbing speed.
The invention has been accomplished to solve the above-mentioned problems involved in the production of a water absorbing agent by the conventional surface-crosslinking of water absorbing resin particles. Accordingly, it is an object of the invention to provide a crosslinking agent for use in the production of water absorbing agent by surface-crosslinking or inner-crosslinking of water absorbing resin particles. It is a further object of the invention to provide a water absorbing agent obtained by using such a crosslinking agent and a method for the production of such a water absorbing agent.
More specifically, it is an object of the invention to provide a surface-crosslinking agent that is safely used since it has no epoxy groups in the molecule and that crosslinks the surface region of water absorbing resin particles at lower temperatures more effectively than the conventional surface-crosslinking agents such as alcohols or amines, thereby providing a water absorbing agent which is superior in water absorbing capacity especially under a load, and preferably either under no load or under a load. It is still an object of the invention to provide a water absorbing agent obtained by using such a crosslinking agent and a method for the production of such a water absorbing agent.
It is a further object of the invention to provide an inner crosslinking agent for use in the production of water absorbing agent by inner-crosslinking of water absorbing resin particles.
DISCLOSURE OF THE INVENTION
The invention provides a crosslinking agent for use in the crosslinking of water absorbing resin particles which comprises at least one halohydrin compound (A) selected from the group consisting of:
(A1) a first halohydrin compound (A1) which has in the molecule at least two halohydrin groups D having the general formula (I)
wherein R
1
is a hydrogen atom or an alkyl group, and X is a chlorine atom or a bromine atom; and
(A2) a second halohydrin compound (A2) which has in the molecule at least two the said halohydrin groups D and at least one ammonium group M having the general formula (II)
wherein R
2
is independently a hydrocarbon group of 1-4 carbon atoms or a benzyl group.
In particular, it is preferred that the halohydrin compound (A) has, as the above-mentioned halohydrin group D, a halohydrin group Da having the general formula (Ia)
wherein R
1
a is hydrogen atom or an alkyl group, and X is a chlorine atom or a bromine atom.
The invention further provides a water absorbing agent obtained by adding 0.01-20 parts by weight of the above-mentioned crosslinking agent to 100 parts by weight of particles of water absorbing resin which has carboxylic acid groups and/or carboxylic acid salt groups, and heating the resulting mixture to crosslink the particles of the water absorbing resin.
The invention still further provides a method for the production of a water absorbing agent which comprises adding 0.01-20 parts by weight of the above-mentioned crosslinking agent to 100 parts by weight of particles of water absorbing resin which has carboxylic acid groups and/or carboxylic acid salt groups, and heating the resulting mixture to crosslink the particles of the water absorbing resin.
BEST MODE FOR CARRYING OUT THE INVENTION
The crosslinking agent of the invention for use in the crosslinking of particles of water absor
Aoki Nobuaki
Fujii Satoru
Hosomi Tetsuya
Morita Yoshiyuki
Cheung William
Merchant & Gould P.C.
Nagase Chemtex Corporation
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