NO891527L - POLY (VINYL ALCOHOL-VINYLAMINE) COPOLYMERS FOR IMPROVING THE PAPER PRODUCTS COMPRESSION STRENGTH IN MOISTURE CONDITION. - Google Patents

Description

The invention relates to the production of paper and cardboard by depositing pulp of wood from an aqueous slurry and, more specifically, relates to the improvement in the compressive strength in a moist state for such paper products.

In the production of paper, cardboard and corresponding wet-laid paper webs (hereinafter referred to as "paper"), a cellulose fiber suspension, which may contain rosin-resin glue and mineral constituents (fillers) depending on the desired product, is deposited and dripped onto a continuously moving wire cloth. In this method free water passes through the paper tap which undergoes further processing to give the desired paper product.

Cartons made in this way that are stored or stacked under high humidity conditions (90+$) absorb a high level of moisture of approx. 14-17$. This absorbed moisture causes the cartons, especially those near the bottom of the stack, to buckle or give way so that the stack can collapse and cause damage to the contents.

A solution to this problem would be to increase the weight of the components in the box carton, but prepackagers prefer to use lighter and stronger cartons.

There is therefore a need for a method for improving the compressive strength in a moist state for paper products.

US Patent No. 3,597,314 describes a process for removing water from aqueous cellulose fiber suspension in the manufacture of paper by adding to the suspension from 0.05 to 4% of a water-soluble polymer consisting mainly of units derived from N-vinyl-N-methylformamide having 60-100$ of the formic acid residues of the polymer split off by acid hydrolysis. H.H. ESPY, TAPPI Proceedings, 1983, Papermakers Conference, pages 191-195 describes that combinations of carboxymethyl cellulose (CMC) and poly(aminoamide) epichlorohydrin (PAE) wet strength resins improve wet and dry strength more than PAE resin alone.

The present invention provides a method for increasing the compressive strength in a moist state for a paper product produced by depositing cellulose fiber from an aqueous slurry. The improvement includes adding to the aqueous slurry of cellulose fiber an effective amount of an additive which improves the compressive strength in the wet state, consisting mainly of poly(vinyl acetate-vinylamide), preferably in combination with an anionic polymer, such as carboxymethyl cellulose in a ratio of 0.5 to 5 parts by weight cationic polymer to one part anionic polymer. The vinyl acetate units of the poly(vinyl acetate-vinylamide) are hydrolysed to vinyl alcohol units to an extent of at least 75 mol-%, preferably 90 mol-$, and the vinylamide units are hydrolyzed so that the polymer gets 1-50 mol-$ vinylamine units.

In addition to improving the wet compressive strength of the paper product, such paper products are more easily returned to pulp because the wet tensile strength is lower than that normally achieved using the PAE wet strength resins.

The process for improving the wet compressive strength of paper products is incorporated into the wet batch stage of the papermaking process. An amount of additive composition effective to increase the wet compressive strength of the paper product, for example 0.1 to 3 wt% based on cellulosic fibers, is added to the aqueous slurry of cellulosic fibers, such as wood pulp. The added composition consists mainly of a poly-(vinyl alcohol-vinylamine) [P(VOH-VAm)], preferably in combination with a carboxymethyl cellulose (CMC). The most preferred composition will contain 0.5-5 parts of P(VOH-VAm) per part anionic polymer, preferably approx. 2 parts P(VOH-VAm) per part anionic polymer.

P(VOH-VAm) is a poly(vinyl acetate-vinylamide) in which the vinyl acetate units are hydrolysed to an extent of at least 75 mol-#, preferably 90+ mol-$, to vinyl alcohol units and most preferably approx. 99+ mol-% hydrolysed, i.e. completely hydrolysed. The amount of vinylamide units in the polymer to be hydrolyzed depends on the molar ratio of poly(vinyl acetate) to poly(vinylamide), but should be sufficient to provide at least 1 mol-$ vinylamine units based on total polymer, preferably 3 to 30 mol -$, and most preferably 6 to 12 mol%. Accordingly, the P(VOH-VAm) component of the additive composition may contain amounts of unhydrolyzed vinylamide and/or vinyl acetate units. Suitable P(VOH-VAm) polymers will contain approx. 1-50 mol-%, preferably 3-30 mol-% vinylamine units, the remainder being made up of vinyl alcohol units, and in the case where the polymer is less than completely hydrolysed, amounts of vinyl amide and vinyl acetate units. The molecular weight of P(VOH-VAm) can vary from 6,000 to 300,000, preferably 10,000 to 100,000.

The process for producing P(VOH-VAm) suitable for use in the present invention is the same as the well-known process for producing polyvinyl acetate and hydrolyzing this to polyvinyl alcohol, only with the incorporation of a vinylamide monomer in the polymerization reaction. The vinylamide monomer can be a vinylacetamide or preferably a vinylformamide, especially N-vinylformamide.

Representative references for the preparation of P(VOH-VAm) are W.M. Brouwer, et al., J. Polvm. Sei.. Polvm. Chem. Ed.. 22, 2353 (1984 ) and C.J. Bloys van Treslong, et al., Eur. Polvm. J.. 19(2), 131-4 (1983).

The preferred method for producing P(VOH-VAm) is described in the simultaneously submitted application with no.

entitled "Method For Preparing Poly(vinyl alcohol)-Co-Poly(vinylamine) Via a Two-Phase Process".

In addition to P(VOH-VAm), the additive composition for improved wet strength preferably includes an anionic polymer such as anionic starch, carboxymethylguar, polyacrylic acid, partially hydrolyzed polyacrylamide or most preferably CMC. The degree of carboxymethyl substitution in CMC can vary from 0.2 to 1.2, and the molecular weight can vary from 10,000 to 300,000.

The amount of the additive composition for improving the wet strength added to the slurry of cellulose fibers is approx. 0.1-3 weight-# based on fiber, and is preferably approx. 0.2-2 weight #. P(VOH-VAm) is added to the slurry prior to the addition of the anionic polymer. If the two components are mixed together before they are added to the slurry, a rubbery mixture results. It is preferred to add P(VOH-VAm) and the anionic polymer as aqueous solutions.

It is assumed that P(VOH-VAm), which is cationic, is absorbed by the anionic cellulose fibers and, especially when the anionic polymer is added, electrostatic bonds are formed leading to polyelectrolyte complexes that increase the strength of the fiber matrix by stronger bonds between the fibers.

By forming such polyelectrolyte complexes by electrostatic bonding and reducing the susceptibility of the fiber structure to the weakening effects of moisture, the treated paper is better able to withstand compaction forces, e.g. when cartons made from components treated in this way are stacked under conditions of high humidity and high moisture content. Nevertheless, the bonds between the fibers can still be broken during typical processes for renewed pulp formation.

Example 1

For the following experiments, unbleached kraft pulp of pine (Southern pine) was ground in tap water to a freeness value (CSF value) of 400 ml. The ground mass was then classified (fines removed) resulting in a final freeness of 700 ml. (The purpose of classifying the mass was to avoid variable interactions of the polymer and the fine fraction which could affect the retention of fine particles and, consequently, strengthen the properties). The pulp was then dewatered to approx. 25$ solid and stored at 4.4°C in the presence of a small amount of preservative until used in hand paper. Immediately prior to the manufacture of hand paper, the dewatered pulp was suspended at 1.5% consistency in tap water using a "British" disintegrator. The stock was then diluted to 0.5% consistency and an amount equivalent to 30g (oven dry basis) was used in the manufacture of each batch of hand paper. Solutions of several P(VOH-VAm) were prepared at approx. 0.25$ solid by first stirring and then aging the suspensions overnight in distilled water at room temperature. The resulting solutions are filtered through a 325 mesh screen to remove undissolved polymer. CMC (D.S.=0.7) of low viscosity was dissolved with 1% solid in distilled water (by boiling at 95°C for 20 minutes) for use in hand paper. A commercial grade of PAE ("Kymene" from Hercules, Inc.) was diluted to 1% solids prior to pulp addition. In those cases where polymer combinations were involved, a measured amount of the cationic component was first stirred into the mass for 5 min. followed by CMC and another 5 min. stirring. A portion of the pulp sufficient to form a hand paper of 2.5 g (63 g/m<2>) was added to a "Noble and Wood" mold where the molding consistency was 0.04$.

The hand sheets were formed on a 100 mesh monel thread, with rent on a tracing paper stack, pressed 5 min. at 446.10 kPa, and dried for 7 min. on a steam drum at 104.4°C. Control hand papers were prepared from untreated pulp and from pulp treated with the additives shown in Tables 1 and 2. After conditioning at 50% relative humidity and 22.8°C, the hand papers were examined for basis weight, wet tensile strength, and dry and moist compressive strength state. Wet compressive strength and wet tensile strength factors were determined by dividing the value for the treated sheet by that of the untreated control sheets. Table 1 shows the amount of cationic and anionic additives in the fiber slurry, as well as performance data for the paper product produced from a commercial pulp of seed wood (southern pine) at ph 7-9. In the experiments in Table 2, a fiber slurry maintained at pH 4.5 throughout the papermaking process by the addition of dilute sulfuric acid was used. It appears from the data in Table 1 that the resulting paper product according to the present invention (experiments 5-16) provides compressive strengths in dry and moist conditions as well as wet tensile strength that are superior to the values for the untreated control paper product. The combination of P(VOH-VAm) and CMC produced paper products that were superior to the paper products made using P(VOH-VAm) alone. (CMC itself has little or no beneficial effect on paper strength properties when added to the fiber suspension.) The addition of 1$ P(VOH-VAm) coupled with 0.2 or 0.4$ CMC gave maximum or near maximum compressive strength properties.

Table 2 shows that the effect of P(VOH-VAm) in combination with CMC is somewhat weaker at acidic pH compared to corresponding values under alkaline pH.

The wet tensile strength level produced using P(VOH-VAm) was lower than that normally achieved using PAE wet strength resins in combination with CMC and suggests that P(VOH-VAm)-treated paper may be easier to re-pulp. , which is a desirable property.

Example 2

An investigation of the possibility of renewed pulp formation was carried out using different hand paper series prepared with (a) no additive (blind control), pH 4.5; (b) PAE (1$), pH 4.5; (c) PAE (1$) and CMC (0.4$), pH 4.5; (d) P(VOH-VAm) (1$), pH 4.5; (e) P(VOH-VAm) (1$), pH 7-8; and (f) P(VOH-VAm) (1$) and CMC (0.4$), pH 7-8. P(VOH-VAm) had 6$ amine functionality and a molecular weight of 130,000.

The hand papers containing P(VOH-VAm) [(d), (e) and (f)] all regenerated significantly better than the reference papers [(b) and (c)]. The hand papers prepared at pH 7-8 containing P(VOH-VAm) alone or with CMC present had more undispersed fibers than P(VOH-VAm) at pH 4.5. Results from visual inspection of the diluted fiber slurries correlated with the observations discussed above.

Consequently, the combination of high compressive strength in the dry and wet state, coupled with a better possibility of renewed mass formation, which is achieved by the use of P(VOH-VAm) represents a significant advantage compared to combinations containing PAE.

Accordingly, the use of P(VOH-VAm), preferably with CMC, as an additive in the wet batch stage of a papermaking process provides a paper product having improved wet compressive strength.

Claims (10)

1. Process for the production of a paper product by depositing cellulose fiber from an aqueous slurry thereof, characterized in that in order to increase the compressive strength of the paper product in a moist state, an effective amount of an additive consisting mainly of a cationic poly is added to the aqueous slurry of cellulose fiber (vinyl acetate-vinylamine), where the vinyl acetate units are hydrolysed to an extent of at least 90 mol-% to vinyl alcohol units and which contain 1-50 mol-% vinylamine units. 2. Method according to claim 1, characterized in that the copolymer which is added contains 3 to 30 mol-% vinylamine units. 3. Method according to claim 1, characterized in that the vinyl acetate units of the copolymer that are added are hydrolysed to an extent of approx. 99+ mol-$ to vinyl alcohol units. 4. Process for the production of a paper product by depositing cellulose fibers from an aqueous slurry thereof, characterized in that an effective amount of an additive composition is added to the aqueous slurry of cellulose fibers to increase the compressive strength in a moist state consisting mainly of 0.5-5 parts by weight of poly( vinyl acetate-vinylamide), where the vinyl acetate units are hydrolysed to an extent of at least 90 mol-$ and the vinyl amide units are hydrolysed so that the polymer consists of 1-50 mol-% vinylamine units, per part anionic polymer. 5 . Method according to claim 4, characterized in that the anionic polymer is anionic starch, carboxymethylguar, carboxymethylcellulose, polyacrylic acid or partially hydrolyzed polyacrylamide. 6. Method according to claim 4, characterized in that the anionic polymer is carboxymethyl cellulose. 7 . Method according to claim 4, characterized in that the vinyl acetate units are hydrolysed to an extent of approx. 99+ mol% to vinyl alcohol units. 8. Method according to claim 4, characterized in that the cationic polymer consists of 3-30 mol-% vinylamine units. 9. Method according to claim 4, characterized in that the additive composition essentially consists of approx. 2 parts poly(vinyl acetate-vinylamide) per part anionic polymer. 10. Process for the production of a paper product by depositing cellulose fibers from an aqueous slurry thereof, characterized in that it includes the addition to the aqueous slurry of cellulose fibers of 0.1 to 3 wt-#, based on the fibers, of an additive composition for improving compressive strength in moist state consisting mainly of 0.5-5 parts by weight of poly(vinyl acetate-vinylamide), where the vinyl acetate units are hydrolysed to an extent of at least 90 mol-$, and the vinylamide units are hydrolysed so that the polymer comprises 1-50 mol-# vinylamine units, per part carboxymethyl cellulose.