EP0038008B1 - Cleaning process for regenerated size baths - Google Patents

Abstract

A method of processing regenerated liquors of polyacrylate, polyvinyl alcohol and carboxymethylcellulose (CMC) sizes for reuse, wherein the impurities are substantially removed, without noticeably changing the properties of the size, by constantly aerating the liquor and separating off the precipitate thereby formed.

Description

The invention relates to a process for the preparation of regenerate fleets of difficult to biodegradable sizes such as polyacrylate, polyvinyl alcohol and carboxymethyl cellulose (CMC) sizes for reuse. By continuously aerating the liquor and separating the resulting precipitate, the impurities are largely removed without noticeably changing the properties of the size.

As is well known, the warp threads are sized before weaving so that they can better withstand the mechanical stress in the loom. Vegetable starch in native or chemically modified form as well as polyacrylates, carboxymethyl cellulose and polyvinyl alcohol are mainly used as sizing agents. The finished fabric usually has to be desized in order to be able to carry out subsequent finishing processes such as bleaching, dyeing, printing or finishing. The desizing liquor, which contains the sizing agent washed off the fabric along with other substances referred to below as impurities (e.g. preparations of synthetic fibers, sizing greases, scorching dust, natural cotton components such as wax, pectins, etc., desizing agents) has so far mostly been completely or partially discarded , and pollutes the wastewater considerably. For this reason and for reasons of economy, efforts are increasingly being made to recycle the washed sizing agent, that is to say to use the desizing liquor as a regenerated liquor for sizing. Firstly, there are problems with the concentration of the sizing agent in the fleet. The present invention is not concerned with this. Secondly, problems arise because the impurities washed off the fiber during desizing accumulate more with each cycle when the fleet is used again. Thirdly, the desizing and regenerated fleets are not stable in storage: after a few days, there is such a strong smell of putrefaction that it is unacceptable for the staff to use them again. The present invention addresses both of these problems.

The problem of the accumulation of impurities comes to the fore as the yield in the recovery of water-soluble sizes increases through improved techniques. As long as part of the desizing fleet is discarded and the sizing agent lost is replaced by fresh one, there is a certain level of impurities which does not increase continuously and which may still be tolerated. The problem only arises when the sizing agent washed off the fabric is used more or less completely again. The only known and practiced solution so far is ultrafiltration. However, it requires a lot of equipment and energy.

The odor formation has so far been suppressed by keeping the recovery liquor hot (energy-consuming) or by using bactericides, that is to say foreign substances which may be disruptive.

The invention was therefore based on the object of developing a simple and economical method which, at least for some of the customary sizes, namely the biologically difficult to biodegrade, solves both of the latter problems at the same time.

The solution to this problem consists in the method according to the claims.

Desizing, which can be carried out by any method, is not the subject of the invention. A prerequisite for the reuse of sizes and therefore also for the method according to the invention is that no desizing agents are used for desizing.

The fleet can be aerated in any way: All measures are suitable in which the fleet comes into intensive contact with air, e.g. vigorous stirring, pumping with free fall or trickling of the liquor over surfaces, preferably introducing air under the liquor surface. The air can be introduced in any manner. A pipe or hose that is immersed in the liquor is sufficient. However, a certain degree of mixing of the fleet is expedient - most simply by the air flow itself. The liquor temperature should be in the range from 10 to 40, preferably 15 to 30 ° C.

The aeration measures must be initiated immediately after the recovery before a putrefaction process begins (smell) and should be carried out without any longer interruption until the fleet is used again. An interruption (e.g. during the transport of the fleet) of the aeration measures by 2 to 3 d depending on the climate can be tolerated if the molecular oxygen content of the fleet does not drop below 2 mg / l.

Of course, oxygen can also be blown in instead of air, but this has no advantages.

The introduction of oxygen via chemical oxygen carriers such as peroxides, e.g. Hydrogen periodix, or oxidizing agents, e.g. Permanganate is unsuitable because it can damage the metabolic processes required for the process according to the invention and also the size (especially polyvinyl alcohol, PVA and carboxymethyl cellulose, CMC). In addition, the newly added contaminants, i.e. the reduced oxidizing agents (e.g. manganese dioxide), the quality of the regenerate.

If the oxygen content of the fleet drops below 2 mg / l before aeration is initiated or in between, there is a risk that (further) aeration will no longer be successful in the sense of the invention This means that the fleet is no longer odorless or only becomes odorless after a long period of inactivation.

It is usually not necessary to vaccinate the fleet with bacteria. A small sample of desizing bath treated according to the invention from the previous cycle (approx. 5%), however, noticeably speeds up cleaning within the first 12 hours, so that a sufficient cleaning effect can be achieved after only 24 hours.

Sizes suitable for the process according to the invention are those which are difficult to biodegrade, ie those which are organic according to the modified confirmatory test by W. Huber and KH Popp, _" Tenside Detergents" 11 (1974), pp. 195-197 bound carbon of the size (Total Organic Carbon = TOC) by no more than 20, preferably no more than 10%. These are primarily sizes based on polyacrylate, i.e. copolymers of 30 to 90% by weight of acrylic acid or methacrylic acid, 10 to 70% by weight of acrylonitrile and possibly small amounts of acrylamide and acrylic esters of lower alcohols (such polyacrylates are described, for example, in DE-B no 1594905 and 2004676 and DE-A No. 2714897), furthermore carboxymethyl cellulose (CMC) with a degree of substitution (DS) of more than 0.5, in which more than half of all primary hydroxyl groups are carboxymethylated, and sizes based on polyvinyl alcohol, which are surprisingly insensitive to oxidative degradation in the presence of fiber accompanying substances, even during long ventilation times (about 8 d).

In view of the sizing effect and the avoidance of corrosion of the apparatus, the sizing agents should be as free as possible from corrosive salts. This should be noted especially with CMC, since it often contains considerable amounts of table salt during manufacture.

The ventilation should be carried out at room temperature for at least 1 to 3 days. In the presence of aerated regenerate from the previous cycle, 1-2 d is sufficient. The ventilation time is only limited by practical considerations (essentially the storage room; the energy required to blow in air is negligible). The ventilation time can therefore be adapted to the rest of the operation within very wide limits. The process of converting the dissolved impurities into a solid, sedimentable substance is initially rapid, then gradually slower. It is a question of practical considerations how far you want to and can drive the ventilation and thus the cleaning process. As a rule, the fleet will be used again after 3 to 8 days. The longer the ventilation has been carried out, the longer it can be used without damage, i.e. without the molecular oxygen content falling below 2 mg / I, are otherwise interrupted under the same conditions. A longer transport of the regenerated liquor is therefore more practical towards the end of the ventilation period than at the beginning. But even after long ventilation, it should not be interrupted longer than necessary.

The precipitate formed during aeration can be separated off in the usual way (by filtration, centrifugation, decanting). The result is a largely odorless, practically odorless sizing bath which has been largely freed from impurities (approximately 50 to 90%, preferably 70 to 80%), and which, depending on the concentration, can be reused immediately or after strengthening (by adding fresh sizing agent or removing water by evaporation or ultrafiltration) whose properties do not differ for practical purposes from those of a corresponding completely freshly made one.

It was not foreseeable that this conceivably simple process would work at all and in particular so well, because it involves the removal of contaminants of a completely different nature, the sum of which is only a small fraction of the concentration of the sizing agents dissolved in the fleet. In addition, the loosened sizing agent must not be changed noticeably. A change had to be expected under these conditions, even in the case of sizes which are difficult to biodegrade per se, as experiments with, for example, the slowly biodegradable polyvinyl alcohol have shown (cf. Wheatley, QD, Baines, FC _" , Textile Chemistand Colorist" 8 (1976) , 28). In combination with one of the modern, simple processes for the production of relatively highly concentrated size regeneration liquors, for example according to DE-B No. 2543815 or DE-A 2937002, the present invention can replace the very complex ultrafiltration process in the simplest way.

The parts and percentages given in the examples relate to the weight.

example 1

500 g of a cotton fabric containing 6 percent by weight of a copolymer made of 70 parts by weight of acrylic acid and 30 parts by weight of acrylonitrile with a viscosity of 310 mPa-s, measured on a 10th, produced with ammonia neutralized (pH 7) analogously to Example 1d of DE-B No. 2004676 % aqueous solution at 20'C and pH 7, were used to recover the polyacrylate size. The above-mentioned fabric was cut into strips about 6 cm wide, which were sewn one behind the other.

The fabric tape thus obtained was passed through a trough which was filled with water at 50 ° C. The residence time of the fabric tape in the water was about 2 s. The fabric tape was squeezed between two rubber rollers (Shore hardness 80-85) at a pressure of 4 bar. The fabric speed was approx. 6 cm / s, the dwell time of the fabric from the moment of immersion until it reached the nip was approx. 8 s, the liquor absorption was approx. 140%. After passing through the fabric strip once, 325 g of a 5% aqueous size solution were collected. This corresponds to 16.2 g size active substance or 54% of the theoretically recoverable size.

The size solution obtained was concentrated in a water jet vacuum by heating to about 10% solids content. With this regrind concentrate, carded cotton yarn Nm 68/1 was sized on a laboratory sizing machine in such a way that a weight of 13% by weight sizing solid substance was achieved.

The yarn-technological test data of the yarns air-conditioned for 24 hours at 21 ° C and 70% relative air humidity included the determination of breaking load, elongation and the number of abrasions up to break. The mean values obtained from 20 individual determinations are summarized in Table 1.

The number of abrasions, a criterion for the sizing effect in staple fibers, was measured according to the specification by E. Keuk, _" Textile Practice", Vol. 7, 698 (1952) on a commercially available yarn-abrasion tester.

The breaking load and elongation at break were measured on a skill testing machine with a static force measuring device type Statigraph N from Textechno Herbert Stein, Mönchengladbach, Regentenstr. 37-39.

The differences between the measured values b and c are just outside the number of abrasions, the others within the range of the measurement methods. You can expect the same web result in the first recovery cycle. However, it is known from the literature (Trauter et al., _" Melliand Textile Reports" 5/1979, p. 379) that the enrichment of preparations, waxes, size greases or regrowth products leads to a drop in the weaving results beyond a certain limit. In principle, this enrichment occurs with every recovery cycle. If you look at the abrasion index values mentioned above, you can see that the regenerate drops slightly compared to the original product. This influence is apparently due to the softening effect of accompanying yarn substances, which, even when the regenerate is stored for a long time, cause an odor nuisance due to biological degradation processes, which makes it difficult or impossible to reuse the regenerates.

It was therefore the task of developing an economical, simple cleaning process which enables the troublesome accompanying substances (fibers, waxes, preparations) to be separated off cleanly and which prevents odor nuisance when storing the size liquors. The addition of sizing preservatives does not bring a satisfactory solution in terms of odor (Table 2) and does not solve the cleaning problem.

As you can see, all preservatives delay rotting, but don't solve the problem. A further increase in the concentration of the preservative is not advisable since the sizing effect can be impaired or crosslinking reactions can occur in the case of formaldehyde or its derivatives with the sizing agent. Apart from this, increased concentrations sometimes cause an unacceptable odor nuisance from the bactericide itself (e.g. formalin, phenol). In general, it is not advisable to add auxiliaries to the size regeneration, since these water-soluble components accumulate in the course of repeated recycling and, as a rule, also irrespective of this, impair or otherwise interfere with the size effect.

Surprisingly, the problem of preventing putrefaction and at the same time cleaning undesirable accompanying substances by introducing air into the regenerate was solved. For 3 d, enough air was blown into the regenerate liquor through a glass tube that the liquor was mixed thoroughly. The liquor temperature was 25 ° C. The treatment formed a well sedimentable precipitate, which was separated by separators. The dried precipitate weighed 0.8% of the liquor. If the regenerate treated in this way was used again to finish the cotton mentioned at the outset, the results shown in Table 3 were obtained.

The results show that the treatment according to the invention has raised the quality of the regrind to the level of the original size.

Example 2

• Schlichte A

Warp threads for the polyester / cotton sheathed poplin fabric defined in Table 4 were treated with the following polyacrylate sizes:

• Sizing A

The size consisted of a copolymer of 65% by weight of acrylic acid and 35% by weight of acrylonitrile, in which 45 mol% of the acrylic acid groups were present as the calcium salt and 55% by weight in the ammonium salt form. The polymer ₃ t was based on example 1 of DE-B N _I. 2004676 and had a viscosity of 250 mPa · s in 15% aqueous solution, measured in a Brookfield Viscometer RVT at 100 tr / min and a temperature of 85 ° C.

Plain B

Copolymer of 60% by weight of acrylic acid and 40% by weight of acrylonitrile, 50 mol% of the acrylic acid being present as the calcium salt and the rest as the sodium salt. The polymer was prepared analogously to size A. The viscosity was 320 mPa's under the same measurement conditions as for size A.

Plain C

Magnesium salt of a copolymer of 35% by weight of acrylic acid and 65% by weight of acrylonitrile, which was obtained according to the preparation specification of DE-A No. 2714897, Example 1. Viscosity: 350 m Pa · s.

In all three cases (based on dry Hongarn), the coating of polyacrylate size on the chain was approx. 16% by weight of rock substance. The chains were woven on Rüti C looms with a room climate of 70% RH / 21-22 ° C and a weft insertion rate of 230 / min (number of thread breaks / loom hour for A: 0.62, B: 0.54, C: 0, 38).

Approx. 5000 m each of the coated poplin fabrics made with sizes A, B and C with a size coverage of 10.5-11% were treated to recover the size according to Example 2 of DE-A No. 2543815. The recovery rate for all three sizes is between 80-85% of theory. Regenerate fleets with an average active ingredient concentration of 6% were obtained.

In order to be able to finish the warp threads for the above-mentioned poplin article again with an order of approx. 16% with these regenerates, an approx. 11% size fleet was required.

The regenerated liquors from size A, B and C were concentrated to a solids content of 12% in a commercially available forced-circulation evaporator in order to compensate for the condensate content in the size cooker. Due to the thermal stress during drying, singeing and evaporation of the regrind, there was a pH drop in the size A of 5.3 compared to the original size (pH 6.5), which was corrected by 25% ammonia solution. This measure was not necessary for sizes B and C. With the regenerates concentrated to 12% active ingredient, size and weaving tests were carried out on the one hand with 5 d old untreated, and on the other hand with the regenerates treated according to the invention.

The comparison of the number of warp thread breaks per loom hour shows that the treatment according to the invention brings increased weaving security compared to the untreated, stored size, and in fact to the same amount as the original size.
(Table on the next page)

Example 3

In order to transfer the process to other water-soluble staple fiber sizes such as polyvinyl alcohol and carboxymethyl cellulose, the influence of air treatment on the COD (chemical oxygen demand) and the pH, essential criteria for a chemical change in the size, was investigated. In order to be able to recover and reinsert a sizing several times, the smallest possible chemical change in the sizing agent is a prerequisite.

Approx. 6% solutions of the original sizes (Table 6), corresponding solutions with the addition of size aids (Table 7) and fleets containing cotton or polyester / cotton (65:35) contaminants (Table 8) were examined. These impurities were obtained by aqueous extraction of the corresponding yarns without the addition of wetting agents, a process that more or less takes place every time a water-soluble size is recovered. Sedimented impurities were not separated.

Chemical character of size greases: Fat No. 1

Meltable sizing fat (49-53 ° C) based on fatty acid triglyceride with emulsifier system based on C _13/15 oxo alcohol sulfonate and nonylphenol ethoxylate.

Fat number 2

Meltable size grease (46-48 ° C) based on polyethylene glycol with an average molecular weight of molecular weight. = 6000 and stearic acid polyglycol esters (molecular weight polyethylene glycol = 1000).

Fat number 3

Stearic acid monoglyceride.

From the data shown in Tables 6, 7 and 8, it can be seen that the influence of sizing auxiliaries differs and that there is no systematic relationship to the sizing agent. The same applies to the influence of the accompanying fibers. Polyacrylates are best suited for a recycling process because their pH value and the chemical structure change only slightly under the various influences of the accompanying substances and thus the greatest production security in the weaving mill is guaranteed. If the addition of type 2 size greases is avoided in the process according to the invention, polyvinyl alcohol and salt-free carboxymethyl cellulose are also accessible to the process with minor restrictions (e.g. CMC for polyester / cotton, Table 8). Surprisingly, the polyvinyl alcohol (Table 6), which degrades strongly when oxygen is introduced, is largely protected against oxidative degradation by some sizing agents or accompanying substances of the fiber.

Claims (2)

1. A process for purifying regenerated liquors of sizes which are difficult to biodegrade, wherein, following the recovery of the size, the content of molecular oxygen in the liquor is prevented from dropping below 2 mg/I by aeration, and the precipitate formed is separated off after not less than one day's aeration.

2. A process as claimed in claim 1, wherein the regenerated liquor employed is a liquor obtained by recovery of a polyacrylate size, a polyvinyl alcohol size or a carboxymethylcellu(ose size, the carboxymethylcellulose having a degree of substitution greater than 0.5.