CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to Chinese Patent Application Serial No.

201410698600.X filed on November 27, 2014, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention, which is in the field of paper-making process, relates to a paper-making aid composition and its preparation process, to a process for increasing tensile strength, especially dry tensile strength and wet tensile strength of paper, and to a paper-making process.

BACKGROUND OF THE INVENTION

Strength (including dry strength, wet strength and temporary wet strength, etc.) is a structural property of paper, which mainly depends on the interfiber bond and the nature of fibers in the paper sheet. During the paper making process, the strength properties of paper can be improved by adding strengh agents to the paper stock by adjusting the ratio of fibers and pulping to fibrillate as well as by virtue of film-forming properties of a surface sizing agent. Strength agents have become increasingly important as they can improve the strength properties of paper and avoid the defect of pulping which accompanied with a deterioration in drainage property of paper stock and change the paper properties. Furthermore, the paper and paper board will lose alomost all mechinical strength after soaking in water and only 4%-10% of dry paper strength can be retained due to the interfiber bonding force from hydrogen bonding in celluloses. However, some types of paper are required to not only possess a certain dry strength but also retain notable mechanical strength and characteristics after being soaked in water, such as photo paper, military map paper, banknote paper, paper tableware, wallpaper base, etc. In this case, a special strength agent can be added to the paper to impart it with essential wet strength. Therefore, a strength agent is usually divided into a dry strength and a wet strength agent.

At present, popular dry strength agents include natural polymers, such as cationic starch, Carboxymethyl Cellulose (CMC) and guar gum, and synthetic polymers such as polyacrylamide (cationic, anionic and amphoteric) , glyoxalated polyacrylamides (GPAMs), polyvinylamine, etc. Di-aldehyde functionalized polyacrylamide, which was prepared from di-aldehyde and polyacrylamide, was developed first as a temporary wet strength resin (see US3556932A, US4605702A) back in the 1970's and 1980's. It was then developed as a dry strength resin used in combination with a wet strength resin. In this category of di-aldehyde functionalized polyacrylamide, glyoxalated polyacrylamide (GPAM), prepared from glyoxal and backbone polyacrylamide, is the most commonly used dry strength aid. Anionic and amphoteric (WO0011046A1), as well as cationic (US7641766B2, US7901543B2) di-aldehyde functionalized polyacrylamides (mostly GPAMs) usually could be used separately, and they were developed to impart paper with enhanced dry strength, wet strength or drainage ability.

Di-aldehyde functionalized polymers, including cationic, anionic and amphoteric acrylamide polymers, particularly the glyoxalated modified diallyldimethylammonium chloride (DADMAC)/acrylamide copolymers, are widely used as a dry strength and temporary wet strength aid in the paper and paperboard area. Such polymer strength aid is of great interest for paper and paperboard manufacturer since (1) it provides good temporary wet strength together with good dry strength, and (2) it also helps improve the drainage ability and the paper machine runnability.

Wet strength agents are commonly used in the paper-making industry, including polyamide polyamine - epichlorohydrin (PAE) resin, melamine-formaldehyde (MF) resin, urea-formaldehyde (UF) resin and other types of wet strength agents . MF resin and UF resin can't be used widely since each can only be used in acidic conditions and contains harmful and volatile formaldehyde. Another type of wet strength agent, such as polyethyleneimine (PEI), has not been commercialized in a large scale due to the unrefined technology. PAE resin is a water-soluble, cationic and thermosetting resin featured with good wet strength property, formaldehyde-free, less yellowing of paper, easy to use, and the like, so it is particularly suitable for hand sheet in neutral and alkaline condition. However, if the PAE resin dosage exceeds a certain range, the increase in wet strength of paper will be greatly reduced. To some types of paper which requires a relatively high wet strength, the one-sided excessive addition of PAE resin cannot achieve the effect of imparting high wet strength. Moreover, PAE resin has drawbacks including a long time for curing, difficult for waste paper recovery, high level of organochlorine, and not environmentally friendly.

Thus, there exists a need to develop a new solution and technology, which has both dry and wet strength performance. Research has been conducted in this area and has brought forward a series of technical solutions. For example, US5427652 disclosed that by combining a cationic dialdehyde functionalized polyacrylamide (GPAM) and PAE resin, the paper is equipped with the wet tensile strength as well as the good decomposition performance in the paper recycling process.

US6294645B1 reported a dry strength agent for paper which comprises PAE, amphoteric polyacrylamide and wet strength resin, wherein the wet strength resin could be GPAM. Furthermore, US5783041 also disclosed a dry strength agent for paper, comprising PAE resin, glyoxylated cationic polyacrylamide copolymer, and a high charge density cationic polymer resin.

WO0011046 disclosed the synthesis of amphoteric and anionic glyoxylated polyacrylamide copolymers, and suggested that the anionic or amphoteric GPAM could be used alone or in combination with a cationic promoter, wherein the cationic promoter could be starch, PAE resin, polyamines. However, this document does not pay special attention to the optimized combination of anionic GPAM with PAE resin.

In addition, CN103215853 disclosed a wet strength agent, and pointed out that the combination of a permanent wet strength agent with a temporary wet strength agent in a ratio between 0.02-0.5: 0.2-5 could impart to the paper both wet tensile strength and good water solubility, wherein the permanent wet strength agent could be PAE resin and the temporary wet strength agent could be glyoxylated polyacrylamide resin.

However, there remains a need to optimize the strength agent in the prior art, especially in terms of composition and amount, so as to further improve the utilization of the paper strength agent, reduce cost and decrease the negative impacts on the environment caused by using permanent resins such as PAE resin in large quantities.

SUMMARY OF THE INVENTION The inventors have performed intensive research and surprisingly found that the combination of a polyamide polyamine-epichlorohydrin (PAE) resin with an anionic dialdehyde-modified polyacrylamide (GPAM) strength agent in a specific PAE resin to GPAM strength- agent mass ratio of about 5 : 1 and about 1 : 1.6 can significantly increase dry strength and wet strength of paper, while maintaining other advantageous properties of the paper.

It may be advantageous when each of the anionic dialdehyde-modified polyacrylamide and the polyamide polyamine-epichlorohydrin, i.e. the active substances, is contained in the paper-making aid composition in an amount ranging from 1 to 50 mass- , preferably 10 to 30 mass-%.

Furthermore, the present inventors have unexpectedly found that, the dry/ wet strength of paper could be further improved by separate feeding of the anionic dialdehyde-modified polyacrylamide (GPAM) strength agent and the polyamide polyamine-epichlorohydrin (PAE) resin as compared with the pre-mixing addition method.

One aspect of the present invention is to provide a paper-making aid composition, comprising an anionic dialdehyde-modified polyacrylamide (GPAM) and a polyamide polyamine-epichlorohydrin (PAE) resin, wherein the mass ratio of the PAE resin to the anionic GPAM is between about 5: 1 and about 1: 1.6. Preferably, the composition does not contain an amphoteric or cationic dialdehyde-modified polyacrylamide. More preferably, the composition may only consist of these two components and water as a medium.

Further, the mass ratio of the PAE resin and the anionic dialdehyde-modified polyacrylamide is between about 3.5 : 1 and about 1 : 1.6, preferably between about 2 : 1 and about 1 : 1.23, more preferably between about 1.2 : 1 and about 1 : 1.

Aspects of the present invention provide a process to increase strength of paper, especially dry strength and wet strength, wherein the mentioned paper-making aid composition is added to the liquor comprising pulp in the paper-making process. In one preferred embodiment, the components of above strength aid composition, especially the polyamide polyamine-epichlorohydrin (PAE) and the anionic dialdehyde-modified polyacrylamide (GPAM), are dosed into the pulp in the separate addition manner (i.e., they are not pre-mixed or not added simultaneously). In the context of this application, "in the separate addition manner", which is different from pre-mixing addition method or simultaneous addition method, means that the components are added in sequence with a certain interval, and particularly means that the two main components PAE and GPAM are added separately.

Another aspect of the present invention is to provide a paper-making process, comprising the steps of:

(a) providing a pulp slurry; simutaneously or before or after

(b) providing the above paper-making aid composition;

(c) adding the paper-making aid composition into the paper slurry to obtain a paper stock;

(d) forming the paper stock obtained in the step (c) to obtain a wet paper web;

(e) pressing and draining the wet paper web obtained in the step (d) to obtain a wet paper sheet; and

(f) drying the wet paper sheet obtained in the step (e) to obtain a paper sheet.

It would be appreciated that in the above process for increasing the tensile strength of paper and the paper-making process, there is no special limitation to the addition manner of PAE resin and anionic GPAM. The two components can be added to a pulp separately or simultaneously, or the two components can be first mixed with each other to form a pre-mixed strength agent and then added into a pulp. However, it is preferred that the two components are fed in the separate addition method as described above.

1. Anionic dialdehyde-modified polyacrylamide

In the context, the dialdehyde-modified polyacrylamide is a strength agent for paper-making, which may be prepared by modifying a base polymer of polyacrylamide type with a dialdehyde. The dialdehyde modified polyacrylamide-type strength agents are usually used as dry strength agents while some of them can be used to provide the paper with wet strength and drainage properties.

The dialdehyde-modified polyacrylamide used herein is anionic. Correspondingly, the polyacrylamide-type base polymer is also anionic.

The anionic polyacrylamide-type base polymer is a copolymer of one or more acrylamide monomer(s) and one or more anionic monomer(s). For example, the anionic polyacrylamide-type base polymers disclosed in WO0011046A1 are applicable to the present invention and corresponding dialdehyde-modified polyacrylamides, and their preparation methods. The contents disclosed in this document are all incorporated herein by reference.

"Acrylamide monomer" means the monomer of formula

R 0

H ₂ C=C-CNHR ₂ wherein Ri is H or C1-C4 alkyl and R ₂ is H, C1-C4 alkyl, aryl or arylalkyl. Preferably, acrylamide monomers are, for example, acrylamide or methacrylamide.

The following definitions apply in the context:

"Alkyl" means a monovalent group derived from a straight or branched chain saturated hydrocarbon by the removal of a single hydrogen atom. Representative alkyl groups include methyl, ethyl, n- and iso-propyl, cetyl, and the like.

"Alkylene" means a divalent group derived from a straight or branched chain saturated hydrocarbon by the removal of two hydrogen atoms. Representative alkylene groups include methylene, ethylene, propylene, and the like.

"Aryl" means an aromatic monocyclic or multicyclic ring system of about 6 to about 10 carbon atoms. The aryl is optionally substituted with one or more C1-C2 ₀ alkyl, alkoxy or haloalkyl groups. Representative aryl groups include phenyl or naphthyl, or substituted phenyl or substituted naphthyl.

"Arylalkyl" means an aryl-alkylene-group where aryl and alkylene are as defined above. Representative arylalkyl groups include benzyl, phenylethyl, phenylpropyl, 1-naphthylmethyl, and the like, e.g., benzyl.

There is no special limitation to the di-aldehyde. The di-aldehyde may be selected from glyoxal, malonaldehyde, succinic aldehyde and glutaraldehyde, preferably glyoxal.

There is no special limitation to the anionic monomer. The anionic monomer can be one or more selected from a group consisting of acrylic acid, methacrylic acid, itaconic acid, maleic acid, and maleic anhydrid and the salts thereof. Preferably, the anionic monomer can be acrylic acid, itaconic acid, a salt of acrylic acid, and/or a salt of itaconic acid.

In the context, there is no special limitation to the sum of the anionic monomers, as long as a stable polymer is prepared. For example, the sum of the anionic monomers can be 0.1-50 mol , such as 5-30 mol , of the copolymer, depending on the practical application, but without being limited to those.

In the context, there is no special limitation to the ratio of dialdehyde, especially glyoxal, to acrylamide (G/A ratio) in the dialdehyde modified polyacrylamide. The G/A ratio can be 0.01 :1-1: 1 (molar ratio), e.g., 0.1 :1-0.8: 1 (molar ratio).

In the context, the abbreviation "GPAM" used herein refers to a dialdehyde modified polyacrylamide, especially glyoxal-modified polyacrylamide.

There is no special limitation to the weight average molecular weight of the dialdehyde modified polyacrylamide, as long as it can be used as a strength agent (such as a dry strength agent). The weight average molecular weight of the dialdehyde modified polyacrylamide can be 100,000-10,000,000 Dalton, or 500,000-2,000,000 Dalton, or 800,000-1,500,000 Dalton, or 1,000,000-1,200,000 Dalton.

The dialdehyde-modified polyacrylamide can be prepared according to the known technology, for example, the process disclosed in US Patent No. 7641766 B2, the contents disclosed in this document being incorporated by reference into the present application in their entirety. It shoud be noted that, in the process of producing the dialdehyde-modified polyacrylamide, a cross-linking agent and / or a chain transfer agent can be used to provide a branched / cross-linked structure of the copolymer.

2. PAE resin

PAE resin is generally formed by reacting a carboxylic acid, especially a dicarboxylic acid, with a polyalkylene polyamine under conditions which produce a water-soluble, long-chain aminopolyamide containing the recurring groups: — NH(C _n H _2n HN) _x — CORCO—

wherein n and x are more than 2 and R is the divalent, organic radical of the dicarboxylic acid,

and then reacting the polyamide with epichlorohydrin.

Dicarboxylic acids used in preparing the aminopolyamide could be saturated aliphatic dicarboxylic acids, preferably containing about 3 to 8 carbon atoms, such as malonic, succinic, glutaric, adipic, and so on, together with diglycolic acid. Of these, the saturated aliphatic dicarboxylic acids having about 4 to 6 carbon atoms in the molecule, such as adipic acid, are preferred. Blends of two or more dicarboxylic acids may be used, as well as blends which include a suitable amount of higher saturated aliphatic dicarboxylic acids, such as azelaic and sebatic acids, as long as the resulting long-chain polyamide is water soluble or at least water dispersible.

The polyalkylene polyamines useful in preparing the aminopolyamide include polyamines containing two primary amine groups and at least one secondary amine group in which the nitrogen atoms of the secondary amine group are linked together by groups of the formula -C _n H2 _n - (where n is an integer of 1 to 6, and preferably 2 to 4), and the number of such groups in the molecule ranges from up to eight, preferably four. The nitrogen atoms of the secondary amine group may be attached to adjacent carbon atoms in the -C _n H2 _n - group or to carbon atoms further apart, but not to the same carbon atom. Examples of such polyalkylene polyamines include but are not limited to diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenetriamine, and the like. These polyalkylene polyamines can be used alone or in mixtures of two or more of them.

The method for preparing the PAE resin entails reacting the obtained aminopolyamide with epichlorohydrin in a mole ratio of epichlorohydrin to free amino groups of about 0.5: 1.8, preferably 0.5:1.5 and more preferably 1 :1.25 in aqueous solution. The temperature may vary from about 45°C to about 100°C.

Illustratively, the preparation of a PAE resin starting from preferred adipic acid and diethylenetriamine as well as epichlorohydrin is performed according to the following reaction scheme: «ίΚΧΚ>·- i ^' .:n. ^: : COOH+ :ΛΗ _; ί¾)·> \Η C¾ _: ϊ¾

A person skilled in the art can prepare the PAE resin of the present invention by referring to the above contents and the contents disclosed in, for example, US5783041. As to more detailed information about the preparation of PAE resin, please refer to US5783041, the contents of which are incorporated by reference into the present application in their entirety.

PAE resin carries a strong positive charge, tends to be retained on the fiber surface and can also further attract negatively charged GPAM, which enables PAE resin to play an excellent bridging role between fiber and anionic GPAM. As described above, the mass ratio of PAE resin to anionc dialdehyde-modified polyacrylamide is between about 5 : 1 and about 1 : 1.6, advantageously between about 3.5 : 1 and about 1 : 1.6, preferably between about 2 : 1 and 1 : 1.23 , more preferably between about 1.2 : 1 and about 1 : 1. When PAE resin and anionic dialdehyde-modified polyacrylamide are used in one of the foregoing ratios, it is possible to the achieve dry tensile strength and wet tensile strength superior to those achieved by analog products comprising cationic or amphoteric dialdehyde-modified polyacrylamide.

3. Other Components

Optionally, in addition to the specific combination of PAE resin and anionic dialdehyde-modified polyacrylamide, the paper-making aid composition according to the invention may contain or may not contain other chemical aids for paper-making, especially synthetic polymer aids for paper- making, e.g., polyvinyl alcohol (PVA), urea-formaldehyde resin, melamine formaldehyde resin, polyethyleneimine (PEI), polyethylene oxide (PEO), etc. The paper-making aid composition according to the invention may contain or may not contain other dry strength agents. In the case that the paper-making aid composition contains other chemical aids for paper-making, those skilled in the art can select the suitable kinds and amounts of the other chemical aids for paper-making as required. The amount of the other chemical aids for paper-making is in the range of 0-50%, preferably 0-20%, and more preferably 0-5%.

Further, as one embodiment, the paper-making aid composition may only consist of a combination of the PAE resin and anionic dialdehyde-modified polyacrylamide and water as a medium.

In addition, in another embodiment, the paper-making aid composition may contain a cationic polyacrylamide polymer as a retention aid. The cationic polyacrylamide polymer is a copolymer formed by one or more acrylamide monomers and one or more cationic monomers. Herein, there is no special limitation to the cationic monomers. They can be one or more selected from the group consisting of diallyldimethylammonium chloride, N-(3-dimethylaminopropyl) methacrylamide, N-(3-dimethylaminopropyl) acrylamide, methacryloyloxyethyl trimethylammonium chloride, acryloyloxyethyl trimethylammonium chloride, methacryloyloxyethyl dimethylbenzylammonium chloride, acryloyloxyethyl dimethylbenzylammonium chloride,

(3 - aery lamidopropyl)trimethylammonium chloride, methacrylamidopropyl trimethylammonium chloride, 3-acrylamido-3-methylbutyl trimethylammonium chloride, 2-vinyl pyridine, 2-(dimethylamino)ethyl methacrylate, 2-(dimethylamino)ethyl acrylate, preferably one or more selected from the group consisting of diallyldimethylammonium chloride (DADMAC), N-(3-dimethylaminopropyl) methacrylamide, acryloyloxyethyl trimethylammonium chloride, 2-(dimethylamino)ethyl methacrylate.

However, in one preferred embodiment, the paper-making aid composition of the present invention may not contain a cationic polyacrylamide polymer, as the inventors have surprisingly found that the paper-making aid composition not compising a cationic polyacrylamide polymer according to the present invention might lead to a better tensile strength.

As described above, another aspect of the present invention is to provide a paper-making process, comprising the steps of:

(a) providing a pulp slurry; simutaneously or before or after

(b) providing the above paper-making aid composition;

(c) adding the paper-making aid composition into the paper slurry to obtain a paper stock;

(d) forming the paper stock obtained in the step (c) to obtain a wet paper web;

(e) pressing and draining the wet paper web obtained in the step (d) to obtain a wet paper sheet; and

(f) drying the wet paper sheet obtained in the step (e) to obtain a paper sheet.

In the context, "paper-making process" or "process for paper-making" means a method of making paper products from pulp comprising forming an aqueous cellulosic papermaking furnish, draining the furnish to form a sheet and drying the sheet.

In the context, "pulp slurry" or "pulp" is intended to mean a product obtained from a pulping process. "Pulping" involves a production process of dissociating the plant fiber raw materials by a chemical method or a mechanical method, or a combination of the both, to form a paper pulp with an inherent color (unbleached pulp) or further to form a bleached pulp. The pulp can be any known pulp, including but not limited to, mechanical pulp, chemical pulp, chemical mechanical pulp, and recycled waste paper pulp, for example, a pulp containing mechanical pulp and / or recycled fiber.

In the context, the pulp is subject to the pulping and additive adjustment, producing a fiber suspension which can be used in handsheeting. Such a fiber suspension is called as "paper stock", so as to be distinguished from the paper slurry which is not subject to a pulping and an additive adjustment.

In the context, "wet paper sheet" refers to a product obtained after the pulp stock passed the headbox, the forming section and the press section to be formed and partially drained, wherein the dryness of the wet paper sheet can be in a range of from about 35% to 50%. For the sake of clarity, the product which comes from the forming section but is not subject to drainage in the press section is called as "wet paper web", which can have a dryness in a range of from about 15% to 25%.

In the context, "paper sheet" refers to a product obtained after the wet paper sheet is dried in the dryer section. The dryness of the paper sheet can be in a range of from about 92% to 97%.

In general, the paper-making process according to the invention can be carried out by the following procedure, but not limited to this, i.e., the paper- making process according to the invention can be also carried out by other known paper-making procedures in the art.

1. The treatment before the paper stock flowing onto the wire comprises:

(1) the preparation of paper stock: the paper slurry can be made into a paper stock, and the preparation of the paper stock comprises pulping and additive adjustment (adding additives such as sizings, fillers, pigments and aids). The paper slurry is first subject to pulping wherein the fiber of the paper slurry undergoes treatments such as necessary cutting, swelling and fine fibrosis, so as to render the paper having physical properties and mechanical properties required by a certain sort of paper and meeting the requirements of a paper-making machine. In order to render the paper sheet useful for writing and resistant to liquid impregnation, improve the paper color, white and tone, increase the opaqueness of the paper and increase the printing performance of the paper, etc., the paper slurry can undergo sizing, adding filler and staining. Furthermore, various chemical aids can be added to provide the paper with some special properties (for example, enhancing the dry strength, wet strength and eliminating bubbles).

(2) supplying the paper stock to the slurry supply system: the paper stock is supplied into the slurry supply system, undergoes treatments such as storing, screening, purifying, de-slagging, de-sanding, de-gasing, and dischages the metal, nonmetal impurities, fiber bundle, lump and air, etc., so as to avoid the adverse effect on the quality of the paper and hinder the paper-making process. The slurry pass undergoes slurry proportion, dilution, concentration adjustment, dosage and pressure elimination, and then flow into the head box and onto the wire for making paper.

2. The paper-making of paper comprises:

(1) stock flow approching: the paper stock is delivered to the forming section (wire section) through the headbox. The headbox is useful in dispering the fiber homogeneouly and flowing the slurry onto the wire smoothly. The additives for paper making, such as the dry strength aids for paper, the wet strength aids for paper, can be added in the process of stock flow approching.

(2) forming: in the forming section, the paper stock delivered by headboxis formed into a wet paper web by draining on the wire. The forming section is also referred to as wire section. The dryness of the wet paper web can be in range of from about 15% to 25%. (3) pressing and draining: in the pressing section, the wet paper web from the forming section is subject to a mechanical pressing to form a wet paper sheet. The dryness of the wet paper sheet can be in a range of from about 35% to 50%.

The steps (d) and (e) can be carried out by the above 2.(2) and 2.(3).

(4) drying: in the dryer section, the wet paper sheet from the pressing section is dried with a dry cylinder to form a paper sheet. The dryness of the paper sheet can be in a range of from about 92% to 97%.

The step (f) can be carried out by the above 2.(4).

Moreover, the paper sheet can undergo, as required, finishing procedures such as calendering, winding and cutting, paper-sorting or rewinding, packaging, etc., so as to produce the paper sheet in to a finished paper in the form of flat or roller. Additionally, in order to improve the quality of the paper sheet, surface sizing, coating and online soft calender or offline supercalender can be carried out in the dryer section.

In the paper-making process, the paper slurry provied by a paper stock preparation system is generally subject to a slurry supply system (undergoing a treatment before the paper stock flows onto the wire), the headbox and the forming section, the press section, dryer section, etc.

The paper-making aid composition is added into the pulp slurry in an amount of between about 0.01 kg/ton dry fiber and 50 kg/ton dry fiber, e.g., 0.1 kg/ton dry fiber and 10 kg/ton dry fiber, based on the weight ratio of the sum of the dialdehyde-modified polyacrylamide and PAE resin to the dry fiber in the pulp slurry.

Examples

The invention is described in more detail by referring to the folowing Examples and Comparative Examples, but is not limited to these Examples.

1. Paper-making process and characterization of paper (a) Method for making hand sheet

The pulp slurry (thick stock) is obtained from a paper mill. The thick stock comprises a mixed slurry of softwood bleached kraft pulp and hardwood bleached kraft pulp, or other pulp, as main component. Sheet-making is performed after the thick stock is diluted with tap water or white water from paper-making plant to a concentration of about 0.7%.

Semi-automatic Tappi standard sheet-making machine, provided by FRANK-PTI Co., is used as the sheet-making machine. The specific test method is described in T205 Introduction sp-02. To the diluted pulp, a fixing agent, test additives and retention aids are added successively at a rotation speed of about 800 rpm.

The pulp added with the agents is poured into a forming cylinder of paper-making machine and undergoes filtering and forming. Afterwards, the forming cylinder is opened, and a bibulous paper is taken to cover the wet paper sheet which is then covered with a flat clamp to remove part of water. Then the paper sample is transferred to a new bibulous paper which is then covered with stainless steel clamp, onto which a bibulous paper is covered again, the wet paper sample is thus accumulated. When accumulating 5 to 10 paper samples, they are provided into a special press machine to perform a two-section pressing, further removing water from paper.

The pressed paper is transferred to a constant temperature and humidity lab (about 50% humidity at 23°C), and every single paper sample is placed into a special metal ring. Piling up the metal rings and placing a heavy object onto the metal ring where the paper sample lies on. After air drying for about 24 hours, the paper sample can be peeled successively from stainless steel clamp for corresponding test.

(b) Test Method for internal bonding strength

The principle of the internal bond impact tester is to measure the energy required to separate the paper sheet by a mechanical equipment so as to reflect the magnitude of the internal bonding strength. The measurement of the internal bonding strength is to express the resistant force that is required to overcome for separating the single or multiple fiber layer(s), which is frequently used to discuss the delamination problem of the paper sheet or paperboard. The test method adopted in the experiment comprises the determination of the force applied by a pendulum to splitting the paper along Z-direction. When the fibers of a hand sheet align in X-Y plane, the exhausted energy is mainly used for the bonding of the fiber, and the length of the fiber and the strength of the fiber itself have no influence on the Scott bonding.

The equipment used in the experiment was purchased from PTI company. The test method refers to Tappi T569.

For a test, a paper with a size of around 25.4 mmx200 mm is cut out previously, and then tape and paper sample are attached to a base following a sequence of tape-paper sample-tap, and the double-sided adhesive tape and the paper sample are attached to each other closely by applying a force. Afterwards, a pendulum is released to knock and separate the paper sample when the equipment automatically records the force that is required to separate the bonding of the fiber layers for each time, expressed in kg»cm/in ² , J/m ² .

(c) Determination of Viscosity

Brookfield Programmable LVDV-II+viscometer, manufactured by Brookfield Engineering Laboratories, Inc, Middleboro, Mass., is utilized in this experiment.

0-100 cps, measured by Spindle 1 at 60 rpm

100-1000 cps, measured by Spindle 2 at 30 rpm

1000-10000 cps, measured by Spindle 3 at 12 rpm 2. Preparation example

(a) Preparation of PAE resin

PAE resin used in the examples and comparative examples was polyamide polyamine epichlorohydrin, manufactured and sold by Nalco. Co., which was prepared according to the following process:

About 82 kg of diethylenetriamine, about 15 kg of distilled water and about 1 kg of p-toluenesulfonic acid were put into a reaction vessel. Then, about 110 kg of adipic acid was added portion-wise with stirring, and the mixed solution was allowed to automatically warm up to about 125°C. After fractionation of water, the system was further heated to about 150~160°C, and kept the temperature for about 3 h. When the total amount of distillated water and amine was approximately 35 kg, the reaction tended to complete. Then, the system was cooled to below 100°C, and about 160 kg of water was added and stirred until a uniform bright red, transparent viscous liquid was obtained, having a solids content of about 50% and a viscosity (25°C) of about 600~1000mPa»s. About 400 kg of water was added to the above-obtained polyamide, and about 80 kg of epichlorohydrin was added with stirring. After reacting at 70°C for about 1-2 h until the required viscosity was achieved, acetic acid was added to adjust pH to about 3-5, giving the PAE resin.

The basic properties of the PAE resin:

Active substance: polyamide polyamine epichlorohydrin

Solids content: 25%

Viscosity: 600-1000 mPa»s

pH value: 3-5

(b) Preparation of Glyoxalated polyacrylamide (GPAM copolymer) solution

GPAM copolymer used in the examples and comparative examples was prepared according to the following process:

(1) Synthesis of polyacrylamide base polymer 1 (intermediate 1)

To a 2 L three-neck flask with a heating and a cooling tube, about 90 g deionized water, about 0.1 g ethylenediamine tetraacetic acid (EDTA) and about 160 g diallyldimethylammonium chloride (DADMAC) were added. An initiator comprising about 4 g ammonium persulfate and about 16 g deionized water was added once the obtained solution was heated to about 100°C and the addition took about 137 minutes to complete. The addition of monomer phase containing about 625 g acrylamide (concentration 50%) was started after adding the initiator for about 2 minutes. The addition of monomer phase took about 120 minutes to complete. After completing the addition of the initiator, the solution was incubated at about 100°C. The reaction ended in about 1 hour, affording an intermediate 1 with a solids content of about 41 wt% and a viscosity of about 2000 cps, wherein the concentration of cationic monomelic units was about 12 mol%.

(2) Synthesis of polyacrylamide base polymer 2 (intermediate 2)

To a 2 L three-neck flask with a heating and a cooling tube, about 113.486 g deionized water, about 16.25 g 48% sodium hydroxide aqueous solution, about 26.27 g 75% phosphoric acid solution, about 7.6 g sodium formate, and about 0.1 g ethylenediamine tetraacetic acid were added. An initiator comprising about 4.4 g ammonium persulfate and about 13.2 g deionized water was added dropwise once the obtained solution was heated to about 100°C and the addition took about 130 minutes to complete. The addition of a mixed solution containing about 768.401 g 50% acrylamide and about 20.6 g 100% acrylic acid was started after adding the initiator for about 2 minutes. The addition took about 120 minutes to complete. After completing the addition of the initiator, the solution was incubated at about 100°C. The reaction ended in about 2 hours, affording an intermediate 2 with a solids content of about 41 wt%, a viscosity of about about 1380 cps, and a molecular weight of about 15,000-25,000, wherein the concentration of anionic monomeric units was about 5 mol%.

(3) Synthesis of polyacrylamide base polymer 3 (intermediate 3)

To a 2 L three-neck flask with a heating and a cooling tube, about 200.78 g deionized water, about 16.25 g 48% sodium hydroxide aqueous solution, about 26.27 g 75% phosphoric acid solution, about 7.6 g sodium formate, about 0.1 g ethylenediamine tetraacetic acid and about 109.4 g diallyldimethylammonium chloride (concentration 62%) were added. An initiator comprising about 4.4 g ammonium persulfate and about 13.2 g deionized water was added dropwise once the obtained solution was heated to about 100°C and the addition took about 130 minutes to complete. The addition of a mixed solution containing about 609.5 g 50% acrylamide and about 12.5 g 100% acrylic acid was started after adding the initiator for about 2 minutes. The addition took about 120 minutes to complete. After completing the addition of the initiator, the solution was incubated at about 100°C. The reaction ended in about 2 hours, affording an intermediate 3 with a solids content of about 39 wt%, a viscosity of about about 530 cps, and a molecular weight of about 15,000-20,000, wherein the concentrations of cationic and anionic monomeric units were respectively about 8.5 and 3.5 mol%.

(4) Synthesis of Glyoxalated cationic polyacrylamide copolymer 1 (GPAM 1)

To a 2L glass container, about 727 g deionized water, about 195 g the above intermediate 1 and about 49 g 40% glyoxal solution were separately added and mixed at about 25°Cin a mechanical stirrer for about 15 minutes. The pH value of the obtained solution was adjusted to about 8.4 with a 48% sodium hydroxide solution. During the reaction, samples were taken for the determination of the viscosity until a product with a viscosity of about 18 cps was obtained. The obtained product was adjusted with a 50% sulfuric acid until pH value is about 3, affording a modified polymer having a solids content of about 10 wt% and a molecular weight of about 1,200,000 Dalton. The final product was marked with "GPAM 1".

(5) Synthesis of Glyoxalated anionic polyacrylamide copolymer 2 (GPAM 2)

To a 2L glass container, about 783.5 g deionized water and about 155.5 g the above intermediate 2 were added, and the obtained solution was adjusted to have a pH value of about 9 with about 3 g 48% sodium hydroxide solution. Then, about 47.2 g 40% glyoxol solution was added, and the pH value was adjusted to about 8.5 with about 6.8 g 5% sodium hydroxide solution. The reaction was carried out at a normal temperature, and a viscometer was used to monitor the viscosity of the reaction solution. At the beginning, the viscosity of the reactant was about 4-5 cps. When the reactant reached a viscosity of about 14 cps, 50% sulfuric acid was added to adjust the pH value of the product to be about 3, so as to obtain a polymer having a solids content of about 8 wt% and a molecular weight of about 1,200,000 Dalton. The final product was marked with "GPAM 2".

(6) Synthesis of Glyoxalated amphoteric polyacrylamide copolymer 3 (GPAM 3)

To a 2L glass container, about 732.63 g deionized water and about 205.5 g of the above intermediate 3 were added, and the obtained solution was adjusted to have a pH value of about 9 with about 4.07 g 48% sodium hydroxide solution. Then, about 50.3 g 40% glyoxol solution was added, and the pH value was adjusted to about 8.5 with about 7.5 g 5% sodium hydroxide solution. The reaction was carried out at a normal temperature, and a viscometer was used to monitor the viscosity of the reaction solution. When the reactant reached a viscosity of about 18 cps, 50% sulfuric acid was added to adjust the pH value of the product to be about 3, so as to obtain a polymer having a solids content of about 10 wt% and a molecular weight of about 1,000,000 Dalton. The final product was marked with "GPAM 3". (c) Preparation of cationic polyacrylamide copolymer

The cationic polyacrylamide copolymer used in the examples was prepared according to the following process:

To a 2 L three-neck flask equipped with a heating and a cooling tube, about 21.1 g deionized water, about 546 g acrylamide (concentration 50%), about 10 g oxalic acid, about 15 g urea, about 105 g acryloyloxyethyl trimethylammonium chloride (DMAEA · MCQ), about 20 g crude oil and about 15 g of sorbitan monooleate were added. The solution was heated to about 45°C and rapidly stirred until fully dissolved. Subsequently, nitrogen gas was charged, and about 0.3 g azobisisobutyronitrile was added. The reaction was carried out at about 45 °C for about 3 hours until complete, to obtain a cationic polyacrylamide copolymer having a solids content of about 35 wt% and a viscosity of about 1500 cps.

3. Examples

Example 1

The PAE resin solution and GPAM 2 were respectively diluted 15 times by adding the ionized water. The diluted PAE resin solution and GPAM2 solution were added into the furnish in sequence in an active concentration mass ratio of about 1.25 : 1. The interval of adding the components was about 60 s. The hand sheet samples of the invention were prepared according to the hand sheet preparation method as described above with two different dosages (about 3 kg/ton or about 6 kg/ton). The thick stock used in this Example was a mixed slurry of softwood bleached kraft pulp and hardwood bleached kraft pulp.

It should be noted that the dosage of the tested additive herein refers to the amount of the active ingredient in the solution (agent) relative to the dry fiber in the pulp slurry. The meaning of dosage also applies to the following examples. The composition and amount ratios of different paper-making aids used in the example and the measured properties were listed in Table 1.

Table 1. Composition and ratios of different paper- making aids and measured properties Dry Wet Wet Cohesi

Dosage of Dry

Composition and tensile tensile tensile Cohesio on paper-ma tensile

ratio (mass ratio) of strength strengt strength n increas king strength

paper-making aid increase h increase J/m ² e

aids, kg/t N- m/g

% N- m/g % %

Blank 21.1 1.44 74.48

100% PAE 3 23.77 12.7 4.94 243.1 89.68 20.4

100% PAE 6 26.11 23.7 6.15 327.1 97.28 30.6

100% GPAM2 3 20.02 -5.1 1.66 15.3 71.44 -4.1

100% GPAM2 6 20.6 -2.4 1.69 17.4 69.92 -6.1

PAE :GPAM2_ 1.25

3 26.34 24.8 5.25 264.6 103.36 38.8 A

PAE :GPAM2_ 1.25

6 32.17 52.5 7.49 420.1 133.76 79.6 A

As seen from Table 1 , the use of a combination of PAE resin and anionic GPAM2 in a mass ratio of about 1.25 : 1 according to the present invention as strength agent will result in better dry tensile strength and wet tensile strength and higher tensile strength increase as compared with the use of the PAE resin or GPAM2 alone at the same dosage. Meanwhile, in the case of comparable dry tensile strength and wet tensile strength and tensile strength increase, the amount of the strength aids, especially the amount of polluting PAE resin, can be significantly reduced by using the paper-making aid according to the present invention as strength agent.

Example 2

The PAE solution and GPAM 2 were respectively diluted 15 times by adding the ionized water, and added into the furnish in sequence in different active concentration mass ratios (see Table 2 below). The interval of adding the components was about 60 s. The hand sheet samples of the invention were prepared according to the hand sheet preparation method as described above with two different dosages (about 2 kg/ton or about 4 kg/ton). The thick stock used in the Example was a mixed slurry of softwood bleached kraft pulp and hardwood bleached kraft pulp.

Table 2 Optimization of the combination of PAE resin and GPAM2

As seen from Table 2, when the mass ratio of PAE resin and anionic GPAM2 is in the range as claimed in the present invention, the use of a combination of these two components as strength agent will result in better tensile strength as compared with the use of the PAE resin or GPAM2 alone. However, when the mass ratio is below or equal to about 1 :2 (for example, 1 : 2 or 1 : 3), as the mass of GPAM2 increases, the wet tensile strength increase will significantly decrease, even inferior to the use of the PAE resin. Example 3

This example was carried out on the basis of Example 2 making a further comparison in the vicinity of the active mass ratio of PAE resin: GPAM2 = 1 : 1 so as to obtain the optimum mass ratio. In this example, the operation of Example 2 was repeated except for using further specified mass ratios as shown in Table 3 below. The data of tensile strength as measured were listed in this table.

Table 3 Further optimization of active mass ratio of PAE resin to GPAM2

As summarized from Table 1, Table 2 and Table 3, it can be seen that, when the active mass ratio of the PAE resing to GPAM2 is controlled between about 1.2: 1 and 1: 1, the optimum dry and wet tensile strength can be achieved.

Example 4

This example was carried out to compare the addition manners of the PAE resing and GPAM2. In this example, the PAE resin and GPAM2 as prepared above were added to a pulp in a active mass ratio of about 1 :1 in two different manners, i.e. separate addition (separately) or pre-mixing addition (pre-mixing). When the PAE resin and GPAM2 were added separately, PAE resing was added first and then, after about 60 s, GPAM2 was added. Table 4 Comparison of addition manners of PAE resin and GPAM2

As seen from Table 4, separately adding the PAE resin and GPAM2 performed obviously better than the pre-mixing manner. Example 5

Comparison was made between different charged GPAM when combining with 64897 as strength solutions.

PAE resing , GPAM 1, GPAM 2 and GPAM 3 were diluted 15 times respectively using ionized water first, the PAE resin combined with GPAM1, GPAM2 and GPAM3 were used as test additives with about 1.2:1 mass ratio in two dosages (about3 kg/t and about 6 kg/t) in the preparation of the handsheet samples of the invention according to the handsheet preparation method described above. The other steps of the experiment are the same as Example 1.

As seen from Table 5, the combination of PAE resin with anionic GPAM 2 performed better than the combination of PAE resin with cationic or amphoteric GPAM copolymer. Table 5

Example 6

This example was focused on a comparison between a dual component strength solution composed of the PAE resin and GPAM 2 and a ternary component strength program composed of PAE resin, GPAM 2 and cationic polyacrylamide copolymer. The PAE resin and GPAM2 was added with about 1 :1 mass ratio, followed by the cationic polyacrylamide copolymer. The addition of chemistries was done in about 60 second intervals. The dual and ternary component strength solutions were used as test additives in the preparation of the handsheet samples of the invention according to the handsheet preparation method described above. Specific dosage of the additives was presented in Table 6. The thick stock used in the Example was a mixed slurry of softwood bleached kraft pulp and hardwood bleached kraft pulp. As seen from Table 6, the ternary component strength program composed of the PAE resin, GPAM2 and the cationic polyacrylamide copolymer was less effective than the dual component strength program composed of the PAE resin and GPAM2. Moreover, the higher dose of the cationic polyacrylamide copolymer was the worst with respect to 5 strength performance.

Table 6

Example 7

Application of the invented strength solution in carton board furnish was studied in this 10 experiment, wherein the active mass ratio of PAE resing to GPAM 2 was about 1 : 1 , the addition of chemistries was done in about 60 second intervals. The new strength solutions were used as test additives in two dosages(about 3 kg/t and about 6 kg/t) in the preparation of the handsheet samples of the invention according to the handsheet preparation method described. The thick stock used in the Example was a mixed slurry of 15 bleached chemi-mechanical pulp, de-inked pulp, hardwood bleached kraft pulp and waste paper pulp.

As seen from Table 7, the new strength solution composed of the PAE resin and GPAM 2 showed comparable wet strength but higher dry strength than using the PAE resin alone. Table 7