EP1902177A1

EP1902177A1 - Use of non-thermosetting polyamidoamines as dry-strength resins

Info

Publication number: EP1902177A1
Application number: EP06786959A
Authority: EP
Inventors: Mark T. Crisp; Wim Stevels
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 2005-07-11
Filing date: 2006-07-07
Publication date: 2008-03-26
Also published as: US20070056706A1; WO2007008945A8; KR20080024195A; CN101263262A; CA2615002A1; JP2009500541A; BRPI0613467A2; AU2006268229A1; ZA200801403B; WO2007008945A1; MX2008000123A

Abstract

The invention relates to a process for utilizing resins useful for imparting dry-strength to paper without substantially increasing the paper's wet-strength wherein the resins comprise non-thermosetting crosslinked polyamidoamine- epihalohydrin resins. The invention also relates to the paper produced containing the resins.

Description

USE OF NON-THERMOSETTING POLYAMIDOAMINES AS DRY-STRENGTH

RESINS

FIELD OF THE INVENTION

The invention relates to a process for manufacturing paper using resin systems useful for imparting dry-strength to paper.

BACKGROUND OF THE INVENTION

It is well known to add certain resins to paper, usually during the papermaking process, to improve dry-strength of the resultant paper. It is also well known to add certain resins to paper to improve wet-strength of the resultant paper. It is also well known that certain additives increase both a paper's wet- strength and dry-strength. However, it is not always desirable that paper with increased dry-strength also exhibit an increased wet-strength since increasing a paper's wet-strength makes paper more difficult to repulp. If it is difficult for a paper product to be repulped, the papermaker will find it difficult to reprocess the material thereby increasing the amount of unusable waste associated with the papermaking process.

Many polymers that improve a paper's dry-strength are anionic under normal papermaking conditions, e.g., sodium carboxymethylcellulose, carboxymethyl guar, and copolymers of acrylamide and acrylic acid or sodium acrylate.

Alternatively, many cationic resins are used to improve a paper's dry- strength, including glyoxalated cationic poly(acrylamide)s, high molecular weight cationic polyacrylamides, thermosetting polyamidoamine-epichlorohydrin resins and poly(vinylamines). These resin are sometimes applied with anionic co- factors such as poly(acrylamide-co-acrylic acid) or carboxymethyl cellulose. None of these anionic or cationic resins is universally applicable and suffers from one or more of the following drawbacks: low solids, significant levels of permanent wet strength, effective over limited pH range, sensitivity to specific ions, subject to hydrolysis under papermaking conditions or limited shelf-life. There is a continued need for dry strength products addressing all or most of these drawbacks.

In U.S. Pat. No. 5,338,406 to Smith, a dry-strength system for a "water- soluble, linear, high molecular weight, low charge density cationic polymer having a reduced specific viscosity greater than two deciliters per gram (>2 dl/g) and a charge density of 0.2 to 4 milliequivalents per gram" with "at least one water-soluble, anionic polymer having a charge density less than 5 meq/g" is disclosed. The polyelectrolyte complex of Smith is useful as an additive for providing dry-strength to all types of paper, particularly for those papers which are produced using unbleached pulp.

In U.S. Pat. No. 5,338,407 to Dasgupta, a process for enhancement of paper dry-strength without reducing its softness is disclosed. The process comprises adding a mixture of an anionic carboxymethyl guar, carboxymethyl bean gum or carboxymethyl hydroxyethyl guar with various cationic additives to a bleached pulp furnish. The cationic additive may be a polyamidoamine- epichlorohydrin resin. If the cationic additive is a wet-strength resin, the paper's dry-strength is enhanced without reducing its softness. Additionally, the wet- strength of the paper is increased.

In Canadian Patent No. 1 ,110,019, "a process for manufacturing paper having improved dry-strength which comprises mixing an essentially alum-free pulp slurry with a water-soluble cationic polymer and subsequently adding a water-soluble anionic polymer to the essentially alum-free pulp slurry" is disclosed. In addition to the above, polyamidoamine-epichlorohydrin resins have been used extensively as wet-strength agents for paper. Typically, these resins are prepared in a two-step process.

In a first step, a polyamidoamine prepolymer is prepared from a diacid

(e.g. adipic acid) and a polyamine (e.g. diethylenetriamine).

In a second step, the polyamidoamine prepolymer is reacted with epichlorohydrin in an amount equal to or greater than the amount of secondary amine groups in the prepolymer. A small amount of epichlorohydrin reacts to effect branching of the prepolymer, accompanied by an increase in molecular weight. However, a majority of the epichlorohydrin reacts with the prepolymer to give reactive functional groups, specifically, either aminochlorohydrin or azetidinium. It is well known to those skilled in the art of papermaking that the above-described polyamidoamine-epichlorohydrin resins may be used in combination with anionic acrylamides or anionic cellulose derivatives. However, papers containing these combinations exhibit increased wet-strength as well as increased dry-strength, thereby making papers containing these combinations difficult to repulp.

In U.S. Pat. No. 6, 294,645 to Allen, et al., the disclosure of which is incorporated herein by reference in its entirety, a dry-strength system for paper comprising: a cationic component and an anionic component is disclosed. In this dry-strength system, the cationic component may comprise a cationic polyamidoamine epihalohydrin polymer. When the cationic component comprises a cationic polyamidoamine epihalohydrin polymer, an intralinker comprises epihalohydrin. The epihalohydrin may be selected from the group consisting of epichlorohydrin, epibromohydrin, epiiodohydrin, epifluorohydrin and alkyl-substituted epihalohydrins. Preferably, the epihalohydrin comprises epichlorohydrin

Additionally, it is well known that in manufacturing various types of paper, namely creped paper products such as tissue products, to use resins as creping adhesive chemicals. Rather than adding the creping adhesive chemicals directly to paper, these creping adhesive chemicals are typically sprayed directly onto a surface of a rotating drying cylinder (creping drum) which adheres a paper web as it is passed over the drying cylinder. The paper web is removed and creped from the surface of the drying cylinder by the use of a creping blade often called a doctor blade.

Creping adhesive chemicals which are widely used include polyvinyl alcohols, poly(ethylene vinyl acetate) copolymers, polyvinyl acetate, polyacrylates and thermosetting cationic polyamides which comprise the water- soluble reaction products of an epihalohydrin and a polyamide containing secondary amino groups. These chemicals may be used alone or in combination with each other in order to achieve the desired effect.

In U.S. Pat. No. 5,234,547 to Knight, et al. discloses a method of creping a paper which comprises applying a synthetic anionic polymer to the creping drum prior to the application of the paper web to be creped. The polymers used are (meth)acrylate polymers and especially polymers of acrylic or methacrylic acid.

EP-A-O 063 301 relates to water-soluble polymers obtainable by reacting an optionally modified polyamidoamine and/or polyureaamine with a bifunctional dihaloalkylene derivative. This document further discloses the use of said polymers as creping additives in the manufacture of creped paper. The creping additives are preferably applied on the paper sheet prior to the contact with the heated surface of the creping drum.

EP-A-O 739 709 discloses a composition for creping fibrous web comprising a polyamine/epihalohydrin resin creping adhesive and a creping release agent that is a plasticizer for the polyamine/epihalohydrin resin. Most of these creping adhesive chemicals and particularly those polyamides become crosslinked by the input of thermal energy and dehydration which occur on the surface of the drying cylinder.

EP 0 856 083 B1 , the disclosure of which is incorporated herein by reference in its entirety, discloses a method of creping a paper which comprises applying directly to the surface of the creping drum a water-soluble, non- thermosetting polyamidoamine or modified polyamidoamine which is crosslinked with an epihalohydrin.

It would be desirable to provide a process for imparting dry strength to paper which uses readily available creping adhesive chemicals as a dry strength resin. It is also desirable to obtain a dry strength resin for which anionic co- factors are not a prerequisite. Furthermore, it is desirable to obtain a dry strength resin which is available at favorable solids levels with good stability and limited levels of permanent wet strength, whilst providing dry strength over a range of practical conditions.

SUMMARY OF THE INVENTION The invention relates to a process for manufacturing paper having dry strength comprising the following steps: forming an aqueous suspension of cellulose fibers; adding a non-thermosetting crosslinked polyamidoamine- epihalohydrin resin to the aqueous suspension of cellulose fibers; and sheeting and drying the aqueous suspension of cellulose fibers to form paper. The non- thermosetting crosslinked polyamidoamine-epihalohydrin resin comprises a reaction product of a polyamidoamine and an epihalohydrin and wherein the epihalohydrin to amine is in a ratio of less than 0.10:1 on a molar basis, preferably, the epihalohydrin to amine is in a ratio in the range of about 0.01 :1 to less than about 0.10:1 on a molar basis.

In producing the non-thermosetting crosslinked polyamidoamine- epihalohydrin resin, of use in manufacturing paper, the polyamidoamine has a molecular weight as measured by its reduced specific viscosity (RSV) of greater than 0.13dl_/g prior to reaction with the epihalohydrin.

The polyamidoamine of use in forming the non-thermosetting crosslinked polyamidoamine-epihalohydrin resins comprises a polyalkylene polyamine having at least two primary amine groups and also at least one secondary and/or at least one tertiary amine group. The polyamidoamine may be selected from the group consisting of diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), iminobispropylamine (IBPA), N-methyl-bis- (aminopropyl)amine (MBAPA), bis-hexamethylenetriamine (BHMT) and mixtures thereof. Preferably, the polyamidoamine is diethylenetriamine (DETA).

The epihalohydrin of use in forming the non-thermosetting crosslinked polyamidoamine-epihalohydrin resins comprises a epihalohydrin selected from the group consisting of epichlorohydrin, epibromohydrin, epiiodohydrin, epifluorohydrin and alkyl-substituted epihalohydrins. Preferably, the epihalohydirn is epichlorohydrin.

DETAILED DESCRIPTION OF THE INVENTION This invention pertains to a method for providing dry-strength to paper while not substantially increasing the paper's wet-strength and comprises a non- thermosetting crosslinked polyamidoamine-epihalohydrin resin. The non- thermosetting crosslinked polyamidoamine-epihalohydrin resin comprises a reaction product of an epihalohydrin with a water-soluble polyamidoamine comprised of a dicarboxylic acid and a polyamine containing secondary and/or tertiary amines. The epihalohydrin and amine are reacted with one another in a ratio. This ratio expressed on a molar basis of less than 0.10:1 on a molar basis of epihalohydrin to amine. The amines of the reaction product may be either secondary or tertiary amines. Preferably, the ratio of epihalohydrin to amine is in the range of about 0.01 :1 to less than about 0.10:1 on a molar basis.

One aspect of the invention pertains to dry-strength systems in which a water-soluble polyamidoamine's molecular weight, as measured by the polyamidoamine's RSV, is of greater than 0.13dl_/g prior to reaction with the epihalohydrin. Preferably, the polyamidoamine's RSV is greater than 0.13dl_/g but less than 0.19dL/g prior to reaction with the epihalohydrin. More preferably, the polyamidoamine's RSV is greater than 0.15dl_/g but less than 0.18dL/g prior to reaction with the epihalohydrin.

In one embodiment of the invention, the non-thermosetting crosslinked polyamidoamine-epihalohydrin resin may be a crosslinked polyamidoamine epihalohydrin polymer where the epihalohydrin is selected from the group consisting of epichlorohydrin, epibromohydrin, epiiodohydrin, epifluorohydrin and alkyl-substituted epihalohydrins. Preferably a crosslinked polyamidoamine comprises epichlorohydrin polymer where the epihalohydrin is epichlorohydrin.

The non-thermosetting crosslinked polyamidoamine-epihalohydrin is a reaction product of a water soluble polyamidoamine comprised of a dicarboxylic acid and a polyamine with secondary and/or tertiary amines present in the polyamidoamine, and an epihalohydrin. The RSV of the water soluble polyamidoamine is greater than 0.13dl_/g prior to reaction with the epihalohydrin.

In this embodiment of the invention, the non-thermosetting crosslinked polyamidoamine-epihalohydrin resin comprises poly(adipic acid-co- diethylenetriamine) reacted with epichlorohydrin at a mole ratio of less than 0.10 moles of epihalohydrin per mole of amine, preferably at a mole ratio of less than 0.08 moles of epihalohydrin per mole of amine, alternatively at a mole ratio of less than about 0.07 moles of epihalohydrin per mole of amine. In this embodiment of the invention, the polyamidoamine is poly(adipic acid-co- diethylenetriamine). In this embodiment, the polyamidoamine's molecular weight is controlled by regulating the amount of condensation water removed during the reaction of the dibasic acid and the polyamine.

The non-thermosetting crosslinked polyamidoamine-epihalohydrin is synthesized by first producing a polyamidoamine and subsequently alkylating and crosslinking the polyamidoamine with epihalohydrin, preferably epichlorohydrin. The polyamidoamines useful in the method of the present invention are prepared by the condensation of aliphatic, cycloaliphatic, araliphatic or heterocyclic (preferably aliphatic) polyamines containing at least two amino groups, at least one of which must be a primary amino group, with a saturated or unsaturated aliphatic or aromatic (preferably aliphatic) dicarboxylic acid having from 2 to 12 carbon atoms or their functional equivalents, preferably having from 3 to 10 carbon atoms or their functional equivalents. The dicarboxylic acids and dicarboxylic acid derivatives of use in producing the polyamidoamine comprise two amidization reactive carboxyl (i.e., --COOH) groups. Suitable dicarboxylic acids for use in producing the polyamidoamine include the C₂ -C₁₂ dicarboxylic acids. Particular dicarboxylic acids which are suitable include oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic, sebacic, maleic, fumaric, itaconic, phthalic, isophthalic, and terephthalic acids.

Suitable dicarboxylic acid derivatives for producing the polyamidoamine include dicarboxylic acid esters and dicarboxylic acid halides. Preferred derivatives are the esters.

Dicarboxylic acid esters which may be used include esters of the C₂ -Ci₂ dicarboxylic acids, and especially the C₁-C₃ diesters of these acids. Particular diesters which are suitable include dimethyl adipate, dimethyl malonate, diethyl malonate, dimethyl succinate, and dimethyl glutarate.

The preferred dicarboxylic acid is adipic acid. Examples of functional equivalents of dicarboxylic acids include dicarboxylic acid halides. Appropriate dicarboxylic acid halides include adipoyl chloride, glutaryl chloride, and sebacoyl chloride.

Alternatively, a corresponding diester may be used instead of the above mentioned dicarboxylic acids in the formation of the polyamidoamine. When a diester is used instead of dicarboxylic acid, prepolymerization can be conducted at a lower temperature, specifically, about 110⁰C and at atmospheric pressure. In this case, the byproduct is an alcohol with the type of alcohol dependent upon the identity of the diester. For instance, where a dimethyl ester is employed the alcohol byproduct will be methanol, while ethanol will be the byproduct obtained from a diethyl ester.

The polyamine comprising a polyalkylenepolyamine, may be selected from the group consisting of diethylenetriamine (DETA), triethylenetetraamine (TETA), and tetraethylenepentamine (TEPA), lminobispropylamine (IBPA), N- methyl-bis-(aminopropyl)amine (MBAPA), bis-hexamethylenetriamine (BHMT) and mixtures thereof. The polyamine is charged into a reaction vessel having sufficient mixing. While the polyamine is being mixed, the dicarboxylic acid is added to the reaction vessel over a period of time. Over this period of time, the temperature of the reactants is allowed to rise and is maintained below about 125°C during this stage of the reaction. The temperature of the reactants is then raised to about 170⁰C and an amount of water contained in the reactants is driven off. At this stage in the reaction, polymerization to polyamidoamine is essentially complete. The aqueous polyamidoamine solution is to have an RSV of greater than 0.13dL/g at this stage of the process.

An amount of water is added to the reactor, and the resultant polyamidoamine is stirred until it dissolves in the water. The amount of water added to the reactor is not critical to the process.

An amount of the aqueous polyamidoamine solution is charged into a reaction vessel and diluted with water. The total aqueous polyamidoamine solution is not critical. An amount of an epihalohydrin, preferably epichlorohydrin, is charged into the reaction vessel to provide a reaction solution having a concentration of about 30% by weight total solids (polyamidoamine + epihalohydrin). The temperature of the reactants is raised to about 45⁰C to about 70⁰C, preferably about 52°C to about 62°C, more preferably about 57 to about 58⁰C. The viscosity of the solution is monitored. When a viscosity is achieved which is indicative of the desired level of reaction of the polyamidoamine with the epihalohydrin, the reaction is stopped by diluting the polymer with cold water. Alternatively, the reaction can be stopped through the adjustment of the pH of the solution with a mineral acid to a pH of about 3.5. The final solids of the resultant crosslinked solution is from about 5% to about 30% by weight, preferably about 10% to 25% by weight, more preferably about 15% to about 18% by weight.

To increase the molecular weight of the crosslinked polyamidoamine- epihalohydrin resin, it is preferred to react the polyamidoamines or modified polyamidoamines with a substoichiometric amount of epihalohydrin. By using a substoichiometric amount, it is ensured that the epihalohydrin completely reacts with the polyamidoamine or the modified polyamidoamine so that no further crosslinking can take place under elevated temperature conditions. To produce the non-thermosetting crosslinked polyamidoamine-epihalohydrin resins of the present invention wherein a substoichiometric amount of epihalohydrin is used, the epihalohydrin to amine is in a ratio in the range of about 0.01 :1 to less than 0.10:1 on a molar basis, preferably in a ratio in the range of about 0.03:1 to about 0.08:1 on a molar basis, more preferably in a ratio in the range of about 0.05:1 to about 0.07:1 on a molar basis.

In case of incomplete reaction of the epihalohydrin or use of more than substoichiometric amounts of epihalohydim, any functional groups which remain after crosslinking and can result in further crosslinking under the elevated temperature conditions can be "neutralized" by reacting the crosslinked polyamidoamine or modified polyamidoamine with suitable agents. Any remaining free epoxy functionality of the epihalohydrin, which could lead to further crosslinking, can e.g. be removed by reacting the crosslinked polyamidoamine or modified polyamidoamine with an amine or ammonia.

As stated earlier, preferably, the polyamidoamines useful in the method of the present invention are obtained by the condensation of a dicarboxylic acid and an amine containing two primary amino groups and at least one secondary amino group, e.g., diethylenetriamine. The condensation results in polyamidoamines which contain about ten dicarboxylic acid derived units and the corresponding amount of amine derived units. In order to increase the molecular weight by crosslinking, the condensation product is reacted with an epihalohydrin, preferably epichlorohydrin. However, in contrast to the preparation of resins conventionally used as wet strength agents, the epihalohydrin is used in substoichiometric amounts to make sure that no free reactive functionality is included in the crosslinked polyamidoamine which would make it crosslinkable and thus thermosetting.

Indeed it is already known to use crosslinked polyamidoamines or modified polyamidoamines which are useful in the present invention as adhesion-improving agents in paper creping for direct application to the surface of the drying cylinder or as retention aids in the paper making process. However, such adhesion-improving agents or retention aids have never been used to provide dry strength to paper.

In the prior art, many modifications of thermosetting polyamidoamines useful as wet strength agents or non-thermossetting polyamidoamines useful as retention aids or as adhesion-improving agents for paper creping are described. All of these modified polyamidoamines are also useful in the method of the present invention as long as they are non-thermosetting, i.e. crosslinking of the polyamidoamines has been effected by the use of substoichiometric amounts of epihalohydrin or any functional crosslinkable groups remaining after crosslinking have been "neutralized" (see above). Examples for modifications of polyamidoamines are disclosed in U.S. Pat. No.4,501 ,862, incorporated herein by reference in its entirety, DE-A-33 23 732. U.S. Pat. No.4,673,729, incorporated herein by reference in its entirety, DE-C-24 34 816, DE-A-18 02 435, and EP-A-74 588. Preferred modified polyamido amines are disclosed in DE-A-34 21 557. These are polyamidoaminepolyamines formed by transamidation of polyamidoamines with polyamines which are obtainable by reacting under substantially anhydrous conditions and at elevated temperatures of at least 15O⁰C. The preparation of the polyamidoamines or modified polyamidoamines useful in the method of the present invention is well known to a person of ordinary skill and described in detail in the prior art such as the prior art documents cited above.

Gelling and thermosetting of polyamidoamine resins result from the presence of reactive epihalohydrin functionality. Both gelling and thermosetting entail the formation of intermolecular connections between discrete resin molecules. Gelling and thermosetting are caused by reaction between reactive epihalohydrin functionality and epihalohydrin reactive amine groups of different resin molecules; the reactive epihalohydrin functionality crosslinks the different molecules, and these molecules accordingly form an interconnected structure which is insoluble.

Particularly in the case of a thermosetting resin, the act of heating and/or drying the resin hardens it, as well as rendering it insoluble. In the prior art, resin solutions are acid stabilized, so that heating will not gel or thermoset the resin.

In contrast, the non-thermosetting crosslinked polyamidoamine- epihalohydrin resin of the present invention is non-gelling. With substantially all of the epihalohydrin already reacted to link polyamidoamines, the dearth of reactive epihalohydrin functionality precludes, or at least greatly limits, reaction between the discrete resin molecules. The non-thermosetting crosslinked polyamidoamine-epihalohydrin resin can accordingly be redissolved after drying and/or heating.

The process for manufacturing paper comprises three principal steps: (1) forming an aqueous suspension of cellulose fibers; (2) adding a strengthening additive; and (3) sheeting and drying the fibers to form paper.

The step of forming an aqueous suspension of cellulosic fibers is performed by conventional means, such as known mechanical, chemical and semi-chemical, etc., pulping processes. Alternatively, a suspension may be formed by repulping paper or paperboard. After the mechanical grinding and/or chemical pulping step, the pulp may be washed to remove residual pulping chemicals and solubilized wood components. These steps are well known, as described in, e.g., Casey, Pulp and Paper (New York, lnterscience Publishers, Inc. 1952).

The step of adding the strengthening additive, e.g. a non-thermosetting crosslinked polyamidoamine-epihalohydrin resin is carried out according to conventional means through direct addition to the papermaking system. Previously, resins having similar chemistries as the non-thermosetting crosslinked polyamidoamine-epihalohydrin resins of use in the present invention had been applied directly to the surface of a creping drum rather than to the wet end of the papermaking system.

The step of sheeting and drying of the fibers to form paper is carried out according to conventional means, such as those described in Casey, Pulp and Paper, cited above.

The preferable level of addition of the non-thermosetting crosslinked polyamidoamine-epihalohydrin resin is about 0.1 to 2% based on the dry-weight of the pulp.

The process for manufacturing paper having dry strength may also comprise use of additives, such as a crosslinked starch. The crosslinked starch may be added at a level of about 0.15% to about 2.0% by weight of the paper, preferably about 0.25% to about 1.5% by weight of the paper, more preferably about 0.5% to about 1.25% by weight of the paper. The crosslinked starch may be any crosslinked starch used in the paper-making process. The crosslinked starch may be selected form the group consisting of potato starch, tapioca starch, wheat starch, corn starch and other crosslinked starches derived from waxy maize. Crosslinked starches of use in the instant invention are described in U.S. Patent No. 4,643,801 incorporated herein by reference in its entirety. The process for manufacturing paper having dry strength of the invention may also comprise use of a wet-strength resin. The wet-strength resin may be added at such levels to the paper so as not to significantly increase the paper's wet-strength. The process for manufacturing paper having dry strength of this invention may also be used to enhance the dry-strength of wet-strengthened papers. A wet-strength resin can then be added to at such levels to provide only the needed amount of wet-strength, and the non-thermosetting crosslinked polyamidoamine-epihalohydrin resin used in this invention can be used to increase the dry-strength without further increasing the wet-strength. Some examples of wet-strength resins available from Hercules Incorporated are Kymene^®557H resin, Kymene^®736 resin, Kymene^®450 resin, Kymene^®557LX resin and Kymene^®Plus resin. The wet-strength resin may be added at a level of about 0.025% to about 1.5% by weight of the paper, preferably about 0.05% to about 1.0% by weight of the paper, more preferably about 0.075% to about 0.75% by weight of the paper. Polyamidoamine epichlorohydrin ("PAE") resins are the most preferred wet-strength resins. Most preferred is Kymene^® 557H resin, in which adipic acid is reacted with diethylenetriamine (DETA) to form a polyamidoamine that is alkylated and crosslinked with epichlorohydrin to form a PAE resin, namely, adipic acid-DETA polyamidoamine epichlorohydrin. Alternatively, the wet-strength resin may comprise an aldehyde-functionalized starch or a glyoxal-modified polyacrylamide resin.

The process for manufacturing paper having dry strength of the invention may also comprise use of a retention aid. The retention aid may be a high molecular weight polyacrylamide or a high molecular weight flocculent.

Alternatively, the retention aid may be poly(ethyleneoxide). Alternatively, the retention aid may be a microparticulate retention aid. The microparticulate retention aid may be selected from the group consisting of bentonite and colloidal silica. Alternatively, the microparticulate retention aid may comprise a synthetic polymeric microparticle.

The process for manufacturing paper having dry strength of the invention may also comprise use in paper which contains a highly crosslinked material for charge control or for fine particle retention. The highly crosslinked material for charge control may be selected from the group consisting of alum, polyaluminum chloride, poly(diallyldimethylammonium) chloride, poly(dialkylamine- epichlorohydrin) and polyethyleneimine.

Other additives useful in the papermaking process of this invention include sizes, defoamers, fillers, wetting agents, optical brighteners, inorganic salts, etc.

The process for manufacturing paper having dry strength of the invention is of utility in manufacturing many types of paper. The process for manufacturing paper having dry strength of the invention is of particular utility in manufacturing papers selected from the group consisting of bleached board, linerboard, corrugating medium, newsprint, printing and writing paper, tissue and towel. The process for manufacturing paper having dry strength of the invention is preferably used in the manufacture of recycled liner board and recycled corrugating medium.

The method for the determination of a material's reduced specific viscosity (RSV) is as follows:

REDUCED SPECIFIC VISCOSITY

The reduced viscosity of a 2% solution of polymer in 1 N ammonium chloride is determined at 25.0⁰C by means of a Ubbelohde viscometer and a Brinkmann Viscotimer. Flow times of a 2% polymer solution and a pure solvent are measured and the relative viscosity (Nrel) calculated. The reduced viscosity is calculated from the relative viscosity. This method is based on ASTM D446. Apparatus used to determine RSV:

(1) Ubbelohde Viscometer tubes, No. 1 , with Viscometer Constant C=O.01^» available from Visco Systems, Yonkers, N.Y., or Schott, Hofheim, Germany, or Brinkmann Instruments. (2) Brinkmann Viscotimer C-available from Brinkmann Instruments Inc., Cantiague Rd., Westbury, N.Y. 11590.

(3) Ubbelohde Viscometer Support-ibid., Cat. No. 21-00-032-9.

(4) Constant temperature water bath maintained at 25+/-0.1⁰C. Cooling capability (cold water or ice pack) may be necessary to maintain constant temperature. An ASTM 45⁰C thermometer should be used to monitor the temperature near the viscometer tube mounting location.

(5) Volumetric flask, 50 ml_, Class A.

(6) Beaker, 1O mL

(7) ASTM 45 C. thermometer, calibrated, designed for measurements at 25°C with 0.05 degree divisions-available from VWR Scientific, Cat. No. 61118-

923, or equivalent.

(8) Source of vacuum-Preferably a water aspirator for cleaning of viscometers.

(9) Filter or stainless steel screen, ca. 100 mesh.

Reagents used to Determine RSV :

(1) Ammonium chloride, granular. ACS reagent grade.

(2) Solvent (1 N ammonium chloride). Add 53.5+/-0.1 g of NH₄ Cl to a 1 -liter volumetric flask, dilute to volume with distilled water and mix.

Ammonium Chloride Flow Measurement:

The ammonium chloride flow time should be measured once per day that Polymer RV measurements are made. This value should be used in the RV calculation.

(1) The viscometer is mounted in the 25°C. constant temperature bath in a vertical position and allowed to equilibrate for at least 15 minutes. The bath must be at 25+/-0.1°C. (2) The viscometer is filled with ammonium chloride solvent, through tube "L", so that the level of liquid falls between the marks on bulb "A". The viscometer is placed in the constant temperature bath and is allowed to stand for at least 5 minutes in order to reach the correct temperature. (3) The Ubbelohde viscometer is connected to the Viscotimer with the attached tubing. The Viscotimer is turned on and is allowed to run.

(4) Measurements are recorded at least 3 flow times. The average of three measurements that agree within 0.2 seconds is calculated If after 4 measurements, agreement is not reached, the viscometer tube is cleaned and the flow is measured 3 times again.

(5) The viscometer is then cleaned and dried.

Polymer Flow Measurement:

The following procedure is used: (1) Determine the total solids content of the polymer.

(2) Calculate the amount of polymer required for 1.000+/-0.02O g of solids using Equation 1.

(3) Weigh, to the nearest 0.0001 g, the appropriate amount of sample, calculated in Step 2, into a 50 ml_ volumetric flask. Alternately, the sample can be weighed into a small beaker and quantitatively transferred to the 50 ml_ volumetric flask with 4 or 5 washings of ammonium chloride solution.

(4) Add 20-25 ml_ of 1 N ammonium chloride to the flask and gently swirl until the sample has completely dissolved. Then add ammonium chloride solution to within 1/4" of the mark. (5) Place the flask and contents in the 25°C constant temperature bath and allow the temperature to equilibrate for at least 15 minutes.

(6) (6) Mount the viscometer in the 25°C constant temperature bath in a vertical position and allow it to equilibrate for at least 15 minutes. The bath must be at 25+/-0.1⁰C. (7) Slowly make up to the volume mark with more solvent and finally mix to obtain complete homogeneity. This will give a 2.000+/-0.040% solution. Calculate the actual concentration to the Polymer solution, to the nearest 0.001 g/100 mL (8) After equilibration of the polymer solution and adjustment to volume at 25°C, filter the solution through a 100 mesh stainless steel screen or comparable pore size filter.

(9) Fill the viscometer through tube "L" so that the level of liquid falls between the marks on bulb "A". Place the viscometer in the constant temperature bath and allow to stand for at least 5 minutes in order to reach the correct temperature.

(10) Connect the Ubbelohde viscometer to the Viscotimer with the attached tubing. Turn the Viscotimer on and allow it to run. (11) Measure and record at least 3 flow times. Calculate the average of three measurements that agree within 0.2 seconds. If after 4 measurements, agreement is not reached, clean the viscometer tube and measure the flow times again. Prepare a fresh solution if agreement still cannot be obtained. (12) Clean the viscometer immediately after use. (13) Calculate the relative viscosity (Nred) of the polymer using Equation 3 and the reduced viscosity (Nred) using Equation 4.

Calculation:

100H-S=Ws Eq(1)

where:

TS=% total solids of Polymer

Ws=weight of sample required for 1.000+/-0.020 g of solids

(Ws.times.TS)/50=Cp Eq(2)

where:

Ws=actual weight of Polymer sample

TS=% total solids of Polymer

50=mL of diluted Polymer solution Cp=concentration of Polymer solution, g/100 ml_

where: t_s =average flow time of the 2% sample solution at 25°C sec. t_o =average flow time of the solvent at 25°C, sec. Nrel=relative viscosity (Nrel-1)/Cp=RSV Eq(4)

where:

Nrel=relative viscosity

Cp=concentration of the polymer solution in grams of polymer solids per 100 ml_ of solution. RSV=reduced specific viscosity

Note: Carry out this value to the nearest 0.001 unit.

The tensile tests were determined using TAPPI test method T494. The Mullen burst was determined using TAPPI test method T807. The Ring Crush was determined using TAPPI test method T818, and the Scott Bond was determined using TAPPI Method T569.

The following examples will serve to illustrate the invention, parts and percentages being by weight unless otherwise indicated.

EXAMPLES

Example 1 A non-thermosetting crosslinked polyamidoamine was prepared in two steps.

In the first step, a mixture of adipic acid, triethylenetetramine (TETA) and diethylene triamine (DETA) was condensed at elevated temperature to a low molecular weight poly(amidoamine) and diluted to a 35% solids solution in water (RSV 0.17dL/g). In a second step, this polymer was crosslinked using a substoichiometric amount of epichlorohydrin to obtain a non-thermosetting resin as a 24% solids solution in water (RSV 0.39 dL/g) (Resin A1).

Paper of 115 g/m² was made on a model papermaking machine using re- dispersed commercial neutral recycled linerboard furnish, with conductivity controlled at 2000 micro Siemens per centimeter and pH of 7. Resin A1 was added at several addition levels to the furnish. The properties of the dried paper were compared to an untreated control. Properties studied included dry tensile (MD and CD), Ring crush resistance (MD and CD), and Mullen burst strength.

In Table 1 , the results of this Example are shown. Properties determined in MD and CD directions are expressed as their geometric mean (or breaking length for dry tensile). The Table shows dry strength properties of paper prepared with several additional levels of Resin A1 to the papermachine wet end.

Table 1

Table 1 shows Resin A1 provides dry strength improvements at commercially useful addition levels.

Example 2 As in the method of Example 1 , non-thermosetting crosslinked polyamidoamine resin was prepared in two steps.

In the first step, a mixture of adipic acid, triethylenetetramine (TETA) and diethylene triamine (DETA) was condensed at elevated temperature to a low molecular weight poly(amidoamine) and diluted to a 35% solids solution in water (RSV 0.17 dL/g).

In a second step, this polymer was crosslinked using a substoichiometric amount of epichlorohydrin to obtain a non-thermosetting crosslinked polyamidoamine resin as a 25% solids solution in water (RSV 0.39 dL/g) (Resin A2).

In a similar way, resins were made based on the use of DETA and a mixture of TEPA (tetraethylenepentamine) and DETA to provide after crosslinking respectively Resin B at 15% solids and Resin C at 24.1% solids.

Paper of 115 g/m² was made on a model papermaking machine using re- dispersed commercial neutral recycled linerboard furnish, with conductivity controlled at 2000 micro Siemens per centimeter and a pH of 7. Resins A2, B and C were added at several addition levels and the properties of the dried paper were compared to the untreated control. Properties studied included dry tensile (MD and CD), Ring crush resistance (MD and CD), and Mullen burst strength, wet tensile and Scott internal bond.

In Table 2, the results of this study are shown at a dose level of 0.15%, as obtained by averaging the results at 0.10, 0.15 and 0.20%. Properties determined in MD and CD direction are expressed as their geometric mean (or breaking length for dry tensile.

Table 2

Clearly, Resins A, B and C provide dry strength improvements over the untreated control at a commercially useful addition levels. It is not intended that the examples presented here should be construed to limit the invention, but rather, they are submitted to illustrate some of the specific embodiments of the invention. Various modifications and variations of the present invention can be made without departing from the scope of the appended claims.

Claims

We claim:

1. A process for manufacturing paper having dry strength comprising the following steps, (a) forming an aqueous suspension of cellulose fibers;

(b) adding a non-thermosetting crosslinked polyamidoamine- epihalohydrin resin to the aqueous suspension of cellulose fibers; and

(c) sheeting and drying the aqueous suspension of cellulose fibers to form paper,

wherein the non-thermosetting crosslinked polyamidoamine-epihalohydrin resin comprises a reaction product of a polyamidoamine and an epihalohydrin and wherein the epihalohydrin to amine is in a ratio of less than 0.10:1 on a molar basis and, and wherein the polyamidoamine has a molecular weight as measured by its reduced specific viscosity (RSV) of greater than 0.13dL/g prior to reaction with the epihalohydrin.

2. The process for manufacturing paper of claim 1 , wherein the polyamidoamine comprises a polyalkylene polyamine having at least two primary amine groups and also at least one secondary and/or at least one tertiary amine group.

3. The process for manufacturing paper of claim 2, wherein the polyalkylene polyamine has two primary amine groups and also at least one secondary and/or at least one tertiary amine group.

4. The process for manufacturing paper of claim 2, wherein the polyamidoamine is selected from the group consisting of diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), iminobispropylamine (IBPA), N-methyl-bis-(aminopropyl)amine (MBAPA), bis- hexamethylenetriamine (BHMT) and mixtures thereof.

5. The process for manufacturing paper of claim 2, wherein the polyamidoamine is diethylenetriamine (DETA).

6. The process for manufacturing paper of claim 2, wherein the polyamidoamine comprises a mixture of diethylenetriamine (DETA) and triethylenetetramine (TETA).

7. The process for manufacturing paper of claim 2, wherein the polyamidoamine comprises a mixture of diethylenetriamine (DETA) and tetraethylenepentamine (TEPA).

8. The process for manufacturing paper of claim 1 , wherein the epihalohydrin is selected from the group consisting of epichlorohydrin, epibromohydrin, epiiodohydrin, epifluorohydrin and alkyl-substituted epihalohydrins.

9. The process for manufacturing paper of claim 1 , wherein the epihalohydrin is epichlorohydrin.

10. The process for manufacturing paper of claim 1 , wherein the polyamidoamine has a molecular weight as measured by its reduced specific viscosity (RSV) of greater than 0.13dl_/g but less than 0.19dl_/g prior to reaction with the epihalohydrin.

11. The process for manufacturing paper of claim 8, wherein the polyamidoamine has a molecular weight as measured by its reduced specific viscosity (RSV) of greater than 0.15dl_/g but less than 0.18dL/g prior to reaction with the epihalohydrin.

12. The process for manufacturing paper of claim 1 , wherein the epihalohydrin to amine is in a ratio in the range of about 0.01 :1 to less than 0.10:1 on a molar basis.

13. The process for manufacturing paper of claim 11 , wherein the epihalohydrin to amine is in a ratio in the range of about 0.03:1 to about 0.08:1 on a molar basis.

14. The process for the manufacturing paper of claim 12, wherein the epihalohydrin to amine is in a ratio in the range of about 0.05:1 to about 0.07:1 on a molar basis.

15. The process for the manufacturing paper of claim 6, wherein the epihalohydrin to amine is in a ratio in the range of about 0.05:1 to about 0.07:1 on a molar basis, and wherein the epihalohydrin is epichlorohydrin.

16. The process for manufacturing paper of any one of claim 1 wherein the non-thermosetting crosslinked polyamidoamine-epihalohydrin resin is added to the aqueous suspension of cellulose fibers in an amount based on about 0.1 to 2% dry-weight of the cellulose fibers.

17. The process for manufacturing paper of claim 16, wherein the non- thermosetting crosslinked polyamidoamine-epihalohydrin resin is added to the aqueous suspension of cellulose fibers in an amount based on about 0.15% dry- weight of the cellulose fibers.

18. Paper made by the process of any one of claims 1-18.