Papermaking additives and mechanical pulp with primary amino...

Details Papermaking additives and mechanical pulp with primary amino... Papermaking additives and mechanical pulp with primary amino...

2001-07-02

2002-09-24

Acquah, Samuel A. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From carbohydrate or derivative as a reactant

C527S312000, C527S600000, C524S800000, C525S054210, C162S146000, C162S176000, C162S177000

Reexamination Certificate

active

06455661

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method for the production of a novel type of retention aid and strength additive employed in the manufacture of pulp and paper. The additive is particularly useful in the production of paper blends containing at least a portion of mechanical pulp.
BACKGROUND ART
During the fabrication of paper sheet, a flat jet of a dilute fibrous suspension is injected onto the surface of a specially designed textile, called a forming fabric, or into a converging gap formed by two forming fabrics. The function of the forming fabric is to enable a rapid drainage of water and retain as much of the fibres in the suspension as possible. The bulk of the water is rapidly drained through the fabrics, and a large portion of the suspended solids, such as fibres, fines and filler, is retained on or between the fabrics. Ideally, all the solid material dispersed between the fabrics would be retained in the paper sheet however, a portion, especially very small particles and colloidal material, escapes through the interstices of the forming fabrics. Retention is always less than ideal and, for light grades of paper and tissue, it usually varies between about 30% and 80%. When the retention is poor, a large amount of material must be recirculated to form a sheet with the desired basis weight. Only a fraction of a second is usually available for water drainage on rapidly-operating, modern paper machines. Therefore, for good machine operation, it is critical that drainage occurs rapidly. However, during rapid drainage a high shear stress occurs in the forming zone which tends to separate the particles of fillers and fines from the fibres, thus impairing their retention.
The primary component of mechanical pulps is cellulosic fibres, but the pulps also contain about 30% of small wood debris usually referred to as fines. Furthermore, mineral pigments of small particle size are often used as fillers in amounts ranging from a few % to over 40% of sheet mass. These fillers are added to improve the printing quality of the paper and to reduce its cost. The fines and fillers are too small to be retained on the forming fabrics by filtration. In the absence of chemical additives, a large proportion of these materials passes through the forming fabric and recirculates in the white water loop. Poor retention causes a loss of valuable papermaking material, impairs product quality and increases the cost of both production and waste water treatment.
In common papermaking practice polymeric retention aids are added to a fibrous suspension to improve the retention of fines and fillers. The retention aids are adsorbed on to the surface of the fines and fillers causing coagulation of fine particles into larger conglomerates which are adsorbed on the surface of the pulp fibres. Such polymeric additives, either singly or in complex systems consisting of one or two components and a mineral powder, or as a polymer plus a small molecular weight co-factor, are described in the literature and are sold commercially.
As with most natural fibres, pulp fibres are negatively charged. Most retention aids are therefore positively charged polymers which are adsorbed on to the negatively charged fibres via electrostatic interactions. This mechanism of retention can be efficient for chemical pulps, which are composed of relatively pure cellulose, as most of the lignin and hemicellulose originally present in the wood is eliminated during pulping and bleaching. By contrast, mechanical pulps contain almost all of the original wood mass, including almost all the hemicellulose and lignin. Compared with cellulose, these non-cellulosic wood components usually carry a much greater negative charge. Because of the very large specific surface of mechanical pulp a large amount of negative charge is thus present for electrostatic interaction with dissolved cationic polymers. Negative charges also reside on the dissolved and colloidally dispersed wood components which are present in the mechanical pulp suspensions. Thus, the efficiency of the common retention aid is greatly diminished.
The cationic charge of many papermaking polymers is due either to the presence of quaternary amino groups, which remain cationic at all values of solution pH, or as tertiary amino groups which are cationic only in acidic solution. Polyethelenimine is one polymer, which contains a certain proportion of its amino groups in their primary form.
Recently, chitosan was reported to be an efficient retention additive and strengthening agent for mechanical pulps, [M. Laleg and I. I. Pikulik, Nordic Pulp and Paper Res. J., Vol.7, No. 4 page 174 (1992)]. Chitosan is a natural polysaccharide with a structure similar to cellulose but different from cellulose in that every glucose unit of chitosan contains one primary amino group. In acid solution these amino groups become positively charged, making chitosan, in solution, strongly cationic. Chitosan can thus be used in papermaking as a cationic, polymeric retention aid. Chitosan in its papermaking form, is produced from the shells of sea crustaceans. The procedure for the preparation of chitosan from this source is complex, requiring a large amount of chemicals, and yielding only about 20% based on weight of dry shells. Chitosan is, therefore, relatively expensive. Since the world supply of sea shells suitable for industrial production is limited, chitosan from this source cannot be relied upon to satisfy a large scale demand from the paper industry. Thus, a new class of retention additives having the properties of chitosan, but which could be produced in large quantities at low cost, would be highly desirable.
Dry-strength additives are often used to increase the strength of dry paper and board; cationic starches and water-soluble synthetic polymers such as polyacrylamides are examples. In contrast, wet-end additives increase the strength of paper which was previously dried and then rewetted. Urea-formaldehyde resins and similar materials can be used for this purpose. These additives are capable of cross linking the cellulosic network by covalent bonds, but cross linking only occurs at the elevated temperature and low moisture content encountered in the dryer section of a paper machine, and “curing” often continues for weeks after the paper has been fully dried. The wet strength caused by these additives is usually permanent and it can be difficult to disintegrate such paper or board during recycling. Wet-strength additives are well known, and have been extensively described in the literature.
Wet-web strength additives are capable of increasing the strength of a freshly-formed, never-dried wet web as it proceeds on the paper machine towards the dryer section of a papermachine. These additives are new in the industry and are not widely used. Only three wet-web strength additives have been described in the literature: chitosan, polyethelenimine and cationic aldehyde starch.
Several published reports describe the preparation of cellulose derivatives that contain primary amino groups. In each case, the product is a solid material which is insoluble in water and which is, therefore, not suitable for application as a papermaking additive. Most of these reports are academic describing preparation procedures that are completely unsuitable for industrial application.
Several investigators have reacted starch with epichlorohydrin. H. Dreyfus [German patent 550,760 (1929)] and M. Hartman [U.S. Pat. No. 1,777,970 (1930)] describe the production of starch substituted by tertiary amino groups, and which is insoluble in water. Syntheses described by C. P. L. Vaughan [U.S. Pat. Nos. 2,591,748 (1952) and 2,623,042 (1952)] yielded starches containing tertiary amines which, at high degree of substitution, were soluble in dilute acids. The preparation of tertiary amines was also described by P. Schlack in U.S. Pat. No. 2,131,120 (1938), while C. L. Hoffpauir and J. D. Guthrie [Textile Res. J., Vol. 20, page 617 (1950)] and E. F. Evans [U.S. Pat. No. 2,76

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