Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-07-31
2002-12-03
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S165000, C162S158000, C162S179000
Reexamination Certificate
active
06488813
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the use of blocked urethane prepolymers as wet strength agent to be added to the stock to obtain paper with increased strength in the wet state. It also relates to a process for making paper with increased wet strength including the addition of a blocked urethane prepolymer to the stock.
BACKGROUND OF THE INVENTION
Paper comprises, in addition to cellulose fiber, a variety of additives such as fillers, strength agents and the like. Wet strength agents are an additive which gives paper having increased strength in the wet state. Examples of widely used wet strength agents include urea resins, melamine resins, dialdehyde starch, polyethyleneimine, epoxylated polyamide etc.
Recently, several attempts have been made to use polyurethanes and urethane prepolymers as wet strength agent. JP-A-06173196 discloses use of a urethane prepolymer containing tertiary amine or quaternary ammonium moieties without blocking the free isocyanate groups. The prepolymer is either added into the stock or applied to webs formed therefrom in the form of an aqueous dispersion or emulsion. GB 2068034A discloses use of a urethane prepolymer amine salt as wet strength agent. The prepolymer amine salt is produced by blocking a urethane prepolymer having free isocyanate groups with a ketoxime, and reacting the blocked urethane prepolymer with a polyfunctional amine to give amine containing-blocked urethane prepolymer followed by dissolving the reaction product in water in the form of an acid addition salt. This amine salt is either added to the stock or applied to a web formed therefrom. JP-A-05051896 discloses a wet strength agent of the type to be applied to a web or paper. The agent comprises a water-soluble urethane prepolymer blocked with sodium hydrogen sulfite or a water soluble urethane prepolymer blocked with ketoxime or other blocking agent which is solubilized by introducing a hydrophilic moiety using the reaction with dimethylolpropionic acid.
Urethane prepolymers containing free isocyanate groups are generally unstable in the presence of water. Blocked urethane prepolymers, on other hand, require heating for unblocking. For blocked urethane prepolymers to be added in the stock, it is imperative to be unblocked under conditions to be encounted in the dryer part of paper making machines (up to 130° C. in several minutes).
A need exists, therefore, a water-soluble blocked urethane prepolymer which, when added in the stock as wet strength agent, is capable of unblocking under conditions to be normally encountered in the dryer part of paper making machines.
SUMMARY OF THE INVENTION
The present invention relates to the use of a water-soluble blocked urethane prepolymer as paper wet strength agent. According to the present invention, said blocked urethane prepolymer is blocked with a phenolic compound having an electron-attracting group at the ortho- or para-position. This blocking agent is capable of unblocking at a temperature lower than the unblocking temperature of oximes and, therefore, water-soluble urethane prepolymer blocked therewith may be used for producing paper with increased wet strength by adding in the stock without need for modifying conventional or existing paper making machines.
The present invention provides, therefore, a method for making paper with increased wet strength comprising:
adding a water-soluble, blocked urethane prepolymer having a hydrophilic group and a blocked isocyanate group blocked with a phenolic compound having an electron-attracting group at the ortho- or para-position to the stock, forming a web from the stock, and drying the web at a temperature up to 130° C.
Suitable examples of said phenolic compounds include p-hydroxybenzoic acid, esters thereof, salicylic acid and esters thereof. They are used as preservative of foods and pharmaceutical preparations due to their safety to human beings. Accordingly, the presence thereof in paper is out of concern if they are produced by the unblocking reaction of the blocked prepolymer and remain in paper. Also, if they are contained in the effluent from paper mills, the ecological affects thereof are considered to be minimum.
DETAILED DISCUSSION
As is well-known in the art, urethane prepolymer are produced by reacting a polyisocyanate with a polyether or polyester polyol at an NCO/OH equivalent ratio greater than 1. Examples of the starting polyether polyols, polyester polyols and polyisocyanates are all well-known in the polyurethane industry. Any of known materials may be used in the present invention.
Examples of suitable polyether polyols include those having a functionality of at least three and a molecular weight from 1,000 to 50,000. These polyether polyols may be prepared starting from a polyhydric alcohol such as glycerine, trimethylol propane, pentaerythritol and the like as an initiator by addition reaction with an alkylene oxide such as ethylene oxide or propylene oxide in the presence of a basic catalyst.
The use of polyester polyols as polyol component of the urethane prepolymer is effective to increase the wet strength of paper. Polyester polyols are generally produced by reacting an dihydric alcohol with a dicarboxyl acid or its acid hydride. Examples of dihydric alcohols include ethylene glycohol, 1,4-butanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol and other aliphatic glycols. Also included in usable dihydric alcohols are polyether polyols produced by the addition-polymerizing reaction of glycols or bifunctional phenols such as bisphenol A with ethylene oxide and/or propylene oxide and/or butylene oxide. Examples of dicarboxylic acids and acid anhydrides include adipic, terephthalic, isophalic or sebacic acid and maleic anhydride. Ring opening polymers of a lactone such as &egr;-caprolactone or a cyclic carbonate such as trimethylene-carbonate are included in the class of polyester polyols. Polyester polyols having a functionality greater than two may be produced by using a polyhydric alcohol having a functionality greater than two as a portion of the alcoholic component.
The starting polyisocyantes are preferably non-yellowing polyisocyanates, namely polyisocyanates not containing an aromatic ring. Examples thereof include aliphatic diisocyanates such as hexamethylenediisocyanate, alicyclic diisocyanates such as isophoronediisocyanate, dimers, trimers or adducts thereof with a polyhydric alcohol such as trimethylolpropane.
A first method for introducing a hydrophilic group into the blocked urethane prepolymer comprises the steps of reacting a portion of free isocyanate groups in the urethane prepolymer produced from a polymer polyol component and a polyisocyanate component with a hydrophilic group-introducing active hydrogen compound (hydrophilic group precursor), and then reacting said phenolic blocking agent to block the remaining free isocyanate groups.
The urethane prepolymer must have a free isocyanate content of at least 2% by weight and preferably from 5 to 10% by weight. A portion of the free isocyanate content is consumed by the reaction with the hydrophilic group-introducing active hydrogen compound. A number of such active hydrogen compounds are disclosed in JP-A-06017196, GB 2068034A and JP-A-05051896 cited above. Any of disclosed active hydrogen compounds may be used in the present invention. However, preference is made to an aliphatic active hydrogen compound having a tertiary amino group and an active hydrogen-containing group such as hydroxyl or primary amino group. Typical examples thereof are N,N-dimethyldiethanolamine, N,N-dimethylethylenediamine or their homologs. Dihydric tertiary aminoalcohols are preferable. The hydrophilic precursor compound is bound to the urethane prepolymer by the reaction between the active hydrogen-containing group thereof and a portion of the free isocyanate groups of the urethane prepolymer. The remainder of free isocyanate groups is then blocked with a blocking agent disclosed herein. The water-soluble blocked urethane prepolymer is prepared by quaternization of bound tertiary amino g
Doi Takeshi
Kouchi Masahiro
Saiuchi Naofumi
Dai-Ichi Kogyo Seiyaku Co. Ltd.
Fortuna Jose
Millen White Zelano & Branigan P.C.
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