EP0693148B1 - Method of monitoring the deposition of "stickies" from paper-pulp suspensions - Google Patents

Abstract

The invention relates to a process for controlling the deposition of stickies from paper stock suspensions in papermaking. According to the invention, the deposition of stickies is controlled by addition of an effective quantity of a degradation product of native starch to the paper stock suspension.

Description

Field of the Invention

The invention relates to a method for controlling the settling of sticky contaminants from pulp suspensions in papermaking.

State of the art

Already when paper was invented in the 2nd century, the use of waste material, i.e. the technique of - at least partially - recycling plays a certain role. In today's world, recycling technology is becoming increasingly important as ecological awareness increases. With an ever increasing production of paper, questions of raw material supply and waste prevention are becoming increasingly important.

By using secondary pulp by recycling waste paper, savings can now be made in terms of raw materials, landfill space and the energy required for paper production. This However, technology is associated with specific difficulties.

In the field of processing waste paper, sticky contaminants, commonly referred to as stickies , can significantly disrupt the production process and negatively affect the quality of the paper produced. The stickies get into the paper manufacturing process if the used paper contains glue spots, adhesive tapes or finished products such as coated, laminated or coated papers or cardboards. In addition, however, the resin of the wood and its interaction with paper auxiliaries can form adhesive contaminations.

If the stickies are in a compact form, they can be removed relatively easily mechanically with the help of sorting machines. As a rule, the stickies are not only available in a compact form, they are also contained in dispersed form in the pulp and are very difficult to remove in this form. Recently, therefore, the increasing use of waste paper in paper production and the narrowing of the water cycles have led to an increasing increase in the proportion of stickies in the circulating water.

Stickies not only cause a number of problems or faults in paper production, but also in paper processing. Because of their stickiness, deposits form on machine parts, pipe walls, screens, wet felts, dry felts, drying cylinders, smoothing rolls, calender rolls and, moreover, also in the finished paper, which causes web breaks in the paper machine and a deterioration in paper quality due to holes, stains, markings (see HL Baumgarten, Das Papier, 1984, 38 , Issue 10A, pp. V121 - V125 ). HL-Baumgarten notes that sticky contaminants have been described in industry and institute publications as the biggest problem of waste paper recycling for years. Even minimal amounts of glue can still tear on paper and printing machines, making cleaning stoppages necessary. Baumgarten explains: "If 2 g of adhesive is applied at a well-chosen point in the paper machine, several 100 kg of paper can be rejected" (loc. Cit., Page V122, right column).

The origin of stickies is not uniform. Essentially, they come from the resin of wood, aids in paper production, binders for painting paper and cardboard, adhesives for paper processing, printing ink binders and materials for paper processing. Of particular importance in the context of the task of the present invention are those adhesive contaminants that come from the resin of the wood and the adhesives used in paper processing.

Depending on the type of wood, the resins present in the pulp and wood pulp contain around 1 - 5% by weight of so-called harmful resins. These can be in a colloidal unbound form or adhere to the paper fibers. As J.Weigl et. al. have faced the difficulties of resin deposits in the manufacture and processing of paper in recent years various reasons steadily increased (see J. Weigl et al. Das Papier, 1986 , p. V52 - V 62; there p. V53, left column).

The adhesives used in paper processing can be divided into three groups: the pressure sensitive adhesives, the dispersion adhesives and the hot melt adhesives.

The pressure sensitive adhesives are permanently adhesive and permanently adhesive products. Adhesion is achieved by pressing the surfaces of the parts to be bonded. A large number of base materials in combination with corresponding additives, e.g. tackifying resins, plasticizers or antioxidants. Typical base polymers include Natural rubber, butyl rubber, styrene-butadiene copolymers (SBR rubber), acrylonitrile copolymers, polychloroprene, polyisobutylene, polyvinyl ether, acrylates, polyesters, polyurethanes, silicones.

In the case of dispersion adhesives, the polymers which are suitable for forming the adhesive layer are present as solid particles in an aqueous dispersion medium. During manufacture, the base monomers are first emulsified in an aqueous phase and then polymerized therein, a technique known as emulsion polymerization. The polymer is then in the form of small particles with different particle sizes, which can vary in the range from molecularly disperse to coarsely disperse. As a rule, a joint storage and an associated one Sedimentation of the polymer particles counteracted by adding protective colloids or emulsifiers to the system.

The so-called hot melt adhesives, also called "hot melts", belong to the group of thermoplastics. These substances have the property of softening when heated, which makes them flowable. When they cool down, they solidify again. Examples of polymers which are used as hot-melt adhesives include polyamides, copolyamides, polyaminoamines, saturated polyesters and ethylene-vinyl acetate copolymers.

With the stickies a distinction is made between primary and secondary stickies. Primary stickies are adhesive contaminants that are not dispersed due to their high resistance to wet grinding. They are therefore in a compact form and are easy to excrete.

The existence of the secondary stickies stems from the fact that the adhesive impurities are subject to a change in their particle size in the course of the waste paper processing, which is caused by thermal, chemical and mechanical influences. This means that even impurities that are still in a rather coarse form at the beginning of the reprocessing can be reduced to a greater or lesser extent in the reprocessing of waste paper. In particular, the processes in the hot shredder of waste paper processing result in dispersion of sticky contaminants. For example, stickies with a low melting point are liquefied and then finely dispersed. Also crumbly or brittle Stickies break up into very small particles. The particle size of the dispersed stickies then ranges from coarse to colloidal to molecular.

In other words: many adhesive contaminants have good dispersibility, with the result that they are present in finely divided form after dissolution and are not detected by the sorting. These substances run the risk of forming agglomerates in the paper machine due to thermal, mechanical or chemical effects, which are referred to as secondary stickies. It is these secondary stickies that cause problems in the course of further paper processing. You will e.g. transported with the paper webs, travel through the paper machine and thus arrive at the most diverse places, where they lead to undesirable deposits, in particular on press felts, dryer fabrics, drying cylinders, smoothing rollers. In addition, they can of course also be found in the finished paper, whose quality is affected.

It is therefore clear from the relationships described that, in principle, all parameters which favor the agglomeration of particles entail the risk of secondary stickies being formed. Two very important parameters in this context are the pH value and the presence of certain aids in paper production. In detail:

Small solid particles that touch each other or between which there is a very small distance pull due to molecular interactions, the so-called Van der Waals forces, on. However, the van der Waals forces acting on an agglomeration generally do not come into play in the alkaline milieu - that is, the milieu typical for waste paper processing - since the particles are surrounded by an electrical double layer which, in the same direction, repels each other charged particles are responsible. On the other hand, the paper machine is usually used in a neutral or slightly acidic environment, which reduces the repulsive negative forces.

The drainability of the pulp suspensions that were made using waste paper is generally poor. In practice, aids are therefore often used, which are referred to as drainage or retention aids. The person skilled in the art understands retention agents as substances that bind fine fibers and fillers to the long paper fibers (long fibers). This binding of the short fibers and the fillers to the long fibers prevents a kind of fleece formation of the fine materials, which makes dewatering of the paper pulp suspension difficult. In this way, retention agents improve the drainability by binding the fine substances to the long fibers.

The retention aids can be divided into three groups. A distinction is made between inorganic products such as aluminum sulfate or sodium aluminate, synthetic products such as polyethyleneimines, polyamines or polyacrylamides and modified natural products such as cationic starch.

The mode of action of the retention agents is based on the deposition of fine substances and fillers on the paper fibers. An important mechanism is that polyelectrolytes with sufficient chain length can bridge the distance between two particles and thus cause agglomerate formation. For example, JLHemmes et al. Report that cationic polyelectrolytes, for example cationic starch, are suitable as scavengers for anionic contaminants ( Wochenblatt für Papierfabrikation 1993, pages 163-170 ).

In summary, it can be stated that, according to the general state of the art, on the one hand a neutral or acidic environment, and on the other hand the use of cationic auxiliaries for shifting drainage and retention are conditions which favor agglomeration of particles. With regard to the sticky problem mentioned, this means that the person skilled in the art logically sees these conditions as favorable for the formation of stickies.

Another key role in controlling stickies is temperature. The reason for this is that many adhesives belong to the thermoplastic (hotmelts), the stickiness of which increases with temperature.

In addition, it should be noted that the manifestation of the undesirable properties of adhesive contaminants for the process of paper production or processing depends on a large number of parameters, the details of which are not yet sufficiently known (cf. HL Baumgarten, loc. Cit. P. V122, left column ). It is even possible that normally harmless contamination from the Interaction of mechanical, chemical and thermal influences during the production process can be converted into adhesive contaminants (see B.Brattka, Wochenblatt für Papierfabrikation 1990 , pp. 310-313 ).

Various methods are now known from the prior art which attempt to counteract the manifestation of the negative properties of adhesive contaminants for the paper production process. The approach of stopping stickies by means of an additive so that the disturbances caused by the adhesive properties are reduced to a technically acceptable level is of particular importance to experts. The methods based on this approach are referred to below as SDC methods ("stickies deposition control").

A method is known from US Pat. No. 4,923,566 in which the stickies are checked with the aid of urea .

According to the teaching of the American patent US 3,081,219, the control of the stickies in the pulp of the sulfite pulping of wood is achieved with the aid of N-vinyl-2-pyrrolidone .

Attempts have also been made to control the stickies by adding bentonites, diatomaceous earth and the like . This approach, which has been known for a long time, is based on the idea of introducing fine particles which are able to bind adhesive contaminants to their surface (cf. US 3,081,219, column 1, lines 40-44). On Another approach is based on the addition of segregating agents , for example polyphosphates (cf. US 3,081,219, column 1, lines 45-50). Finally, attempts have also been made to use various dispersants , for example the sodium salts of sulfonated formaldehyde / naphthalene condensates, but this entails disadvantages in the case of neutral pH values and leads to unfavorable interactions with cationic auxiliaries (cf. US 3,081,219, column 1, lines 51-58).

The American patent application US 4,744,865 describes an SDC process in which the coagulation of adhesive contaminants by polymers containing methoxy groups is to be reduced.

The subject of the American patent application US 4,871,424 is an SDC process using polymers which contain hydroxyl groups . However, only cellulose derivatives such as hydroxypropylmethyl cellulose and polyvinyl alcohol , which can be obtained from polyvinyl acetate by hydrolysis or partial hydrolysis, are explicitly disclosed as polymers.

Finally, G. Galland and F. Julien Saint Amand describe that primary acrylate stickies can be removed by flotation in an alkaline environment and in the presence of soap (see EUR. Comm. Eur. Communities 14011, 1992 , pp. 235 - 243 ) . However, by its nature, this approach cannot help solve the problem of secondary stickies.

Description of the invention

Overall, the state of the art in the field mentioned here is very heterogeneous and a method for checking stickies that is satisfactory in every respect has not yet been implemented. HL Baumgarten's statement still applies: "A look at the problem chain" Adhesive contaminants "in waste paper" ... makes it clear that in addition to the manufacturers of waste paper processing systems, in particular the manufacturers of paper processing and paper processing aids, which usually contain plastics, and the chemical ones Industry as their raw material supplier are asked to intensively help the paper industry "(Das Papier, 1984 , Issue 10A, page V124). There is therefore a constant need for new or alternative problem solutions for controlling the stickies in paper production.

It was therefore an object of the present invention to develop a process for controlling the settling of adhesive contaminants in papermaking which avoids the disadvantages of the known prior art. This method should generally be applicable to a wide variety of types of adhesive contaminants, but especially to pressure sensitive adhesives, dispersion adhesives and hot melt adhesives. In addition, it was to be striven for that the auxiliaries to be used in the process are largely biologically compatible and therefore, from an ecological point of view, meet the requirements which are becoming increasingly important in the paper processing industry today. Finally, the task relates to present invention in particular on the problem of secondary stickies.

This object has been achieved according to the invention by a method for controlling the settling of sticky impurities (stickies) from paper pulp suspensions in papermaking, the pulp suspension being metered in with an effective amount of a degradation product of native starch.

The present invention therefore relates to a process for controlling the settling of sticky impurities (stickies) from paper pulp suspensions in papermaking, which is characterized in that an effective amount of a degradation product of native starch is metered into the pulp suspension.

In the context of the present invention, degradation products of native starch are understood to mean those products which are accessible by thermal, hydrolytic or enzymatic degradation of native starch and which have a lower average molecular weight than the underlying native starch and a higher average molecular weight than glucose, the product of one show complete degradation. However, it should be expressly stated that the products of a chemical derivatization such as the esterification, etherification, acetylation, etc. do not fall under the definition of the degradation products of native starch used here.

In the context of this invention, those degradation products of native starch are preferred which have an average molecular weight in the range from 1,200 to 600,000. The preferred type of Manufacture of these products is acidic and enzymatic hydrolysis or a combination of these methods.

The type of native starch which is used to produce the degradation products suitable according to the invention is not subject to any particular restriction. For example, Potato starch, corn starch, rice starch or canna starch can be used as starting materials. However, it is particularly preferred to use potato starch degradation products.

The method according to the invention is generally applicable to a wide variety of types of adhesive contaminants. However, it is particularly suitable for solving the problems caused by pressure-sensitive adhesives, dispersion adhesives and hotmelt adhesives.

In a preferred embodiment, the method according to the invention relates to those pulp suspensions which have been produced from waste paper or from paper products which contain waste paper components.

It has also been found that the effect of the degradation products of native starch which are suitable according to the invention can be improved by additionally carrying out the process in the presence of a cellulose derivative. Carboxymethyl cellulose, methyl hydroxypropyl cellulose and mixtures of these substances are particularly preferred as the cellulose derivative.

Another object of the present invention is the use of degradation products of native starch to control the settling of sticky contaminants (stickies) from pulp suspensions in papermaking.

The method according to the invention is suitable in principle for checking the setting and sticking of stickies of different types and thus also of different chemical and physico-chemical nature. However, the advantages of the method according to the invention are particularly evident in the case of stickies based on pressure-sensitive adhesives and hotmelt adhesives.

The degradation products of native starch according to the invention can in principle be metered in at any point in the entire papermaking process. They are metered in either in the form of solid particles or in the form of an aqueous solution or dispersion. The effective amount of native starch required depends on the extent to which the waste paper or paper types to be processed, which contain waste paper components, contain sticky contaminants. As a rule, however, the degradation products according to the invention of native starch are used in an amount in the range from 0.001 to 5.0% by weight, preferably 0.1 to 1.0% by weight, based on the fiber material.

The following examples serve to illustrate the invention and are not to be understood as restrictive.

Examples 1. Substances and materials used 1.1. Polymers

• a) MHPC: methyl hydroxypropyl cellulose (MHPC 50, Aqualon)
• b) AKS: degraded potato starch (Noredux 118, Henkel)

1.2. Pressure sensitive adhesives

• a) styrene / butadiene
• b) vinyl esters
• c) acrylate

2. Carrying out the detacking tests 2.1. Principle of the method

The detacking test used here is already known in principle to the person skilled in the art from the American patent application US 4,886,575 and the above-mentioned. Article by B.Brattka (op. Cit. P.311) known. The method consists in immersing a selected adhesive tape in an aqueous solution with the substance to be tested. The tapes are then glued together under defined conditions and then the (still existing) adhesive strength is determined in a universal testing machine.

2.2. Details of the execution of the tests

200 ml of solution were prepared from different polymers and filled into 200 ml beakers. In these Various adhesive tapes were immersed in solutions for a period of exactly 30 seconds. The tapes were then dried within 4 hours at a temperature of 23 ± 1 ° C. Two tapes treated in the same way were now glued together. The contact pressure was set constant with a press to a value of 1 N / mm ² . The adhesive strips that were open at one end by inserting two release papers were clamped in a universal testing machine and peeled from one another at a speed of 250 mm / min (analogous to DIN 53282 "angle peeling test"). The peeling forces determined as a function of the selected concentration of the polymers are summarized in the tables below. The peeling force is to be regarded as an indicator of the ability of the respective polymer to exert a controlling influence on the stickie formation: the lower the measured force, the better the polymer prevents the tapes from sticking to one another and thus preventing agglomeration of adhesive particles, which ultimately that cause sticky problems. The values given in the tables represent mean values from five measurements.

Comparative experiment 1

tested polymer: MHPC Polymer concentration (% by weight) Peel strength (N / cm) at Styrene / butadiene Vinyl ester Acrylate 0 3.6 2.9 2.5 0.5 1.3 1.1 1.2 1.0 1.0 0.9 1.0 2.0 0.8 0.8 0.9

example 1

tested polymer: AKS Polymer concentration (% by weight) Peel strength (N / cm) at Styrene / butadiene Vinyl ester Acrylate 0 3.6 2.9 2.5 0.5 1.0 1.1 0.8 1.0 0.7 0.8 0.7 2.0 0.7 0.6 0.7

2.3. discussion of the results

From the tables above it is clear that the degradation products of native starches according to the invention achieve better results than with MHPC, the structurally closest native polymer which is known from the prior art.

The extent of the advantages that can be achieved with the products according to the invention was not predictable and makes it clear that modified starches behave quite differently from modified celluloses.

3. Application example

A 3-week practical test was carried out in a paper mill that produces tissue papers from 100% waste paper. A 5% by weight solution of degraded potato starch (AKS, see Example 1) was metered in at a rate of 16 kg / hour. The paper machine produced 4,000 kg of paper per hour. The number of holes on the paper web fell from an average of 0.2 / m ² to 0.05 / m ² . At the same time, the deposits on the sieves and felts decreased, so that the machine running times between the cleaning intervals could be extended. The number of demolitions dropped from 4 per week to 2.

Claims (8)

1. A process for controlling the deposition of stickies from paper stock suspensions in papermaking, characterized in that an effective quantity of a degradation of native starch is added to the paper stock suspension.
2. A process as claimed in claim 1, in which the paper stock suspensions used are produced from wastepaper or from paper products containing wastepaper constituents.
3. A process as claimed in claim 1 or 2, in which a degradation product of potato starch is used.
4. A process as claimed in any of claims 1 to 3, in which the process is additionally carried out in the presence of a cellulose derivative.
5. A process as claimed in claim 4, in which carboxymethyl cellulose and/or methylhydroxypropyl cellulose is used as the cellulose derivative.
6. The use of degradation products of native starch for controlling the deposition of stickies from paper stock suspensions in papermaking.
7. The use claimed in claim 6, in which the paper stock suspensions used are produced from wastepaper or from paper products containing wastepaper constituents.
8. The use claimed in claim 6 or 7, in which the degradation product of native starch is produced from potato starch.