Patent Publication Number: US-2004049858-A1

Title: Method for impregnating a textile material

Description:
[0001] This invention relates to a process for the continuous impregnation of textiles, more particularly wet textiles, in an impregnating compartment containing the impregnating liquor through which the textile is passed, the impregnating liquor containing as impregnating medium a lye, more particularly soda lye, or a lye, more particularly soda lye, and in addition an oxidative bleaching agent, more particularly hydrogen peroxide, part of the impregnating liquor being branched off, more particularly continuously, through a bypass, filtered and delivered to one or more sensors for determining the concentration of the impregnating medium. After filtration, that part of the impregnating liquor which has been branched off is preferably additionally subjected to working up in a particular way before being delivered to the sensors.  
       [0002] One such process is known from an article in the journal Int. Dyer, August 1997, pages 20-22. In this case, the concentration of hydrogen peroxide is continuously measured in a bleaching liquor. The samples are continuously removed from the bleaching process, i.e. from the impregnating compartment, through a bypass and are returned after their removal. The sample removed is first filtered to remove coarse impurities from the solution. For the determination of hydrogen peroxide, a buffer solution is introduced to adjust the pH to 7 because this is necessary for the measurement. The sample is then automatically delivered to the sensor. To calibrate the sensor, calibrating solutions for two-point calibration are delivered to the sensor at cyclic intervals.  
       [0003] After amplification of the sensor signal, the concentrations are calculated therefrom and displayed. The values obtained may be used to actuate a dosing unit for hydrogen peroxide and/or soda lye. The data can be stored in a personal computer and are available for printout.  
       [0004] The invention is intended in particular for continuous wet-in-wet impregnation as commonly used in the textile industry. In this process, the wet or damp textile is continuously fed to the impregnating compartment. Since water is continuously introduced into the impregnating compartment, the problem is to keep the concentration of the impregnating medium in the impregnating compartment constant. The concentration in the impregnating compartment can be periodically measured, as is standard practice, and the impregnating medium replenished accordingly. Alternatively, the necessary quantity of impregnating medium to be added can also be determined on the basis of the weight of the textile introduced into the impregnating compartment.  
       [0005] In the case of dry-in-wet impregnation where dry textile material is introduced into the impregnating compartment, the liquor in the impregnating compartment cannot be diluted so that it is merely necessary in this case to add impregnating liquor with the same concentration.  
       [0006] However, the continuous wet-in-wet impregnation already mentioned is generally practised in the textile industry. Particular problems in determining the quantity of impregnating medium to be added arise whenever the type of textile is changed. Another problem lies in the delay between measurement of the existing concentration in the impregnating compartment and the resulting addition of impregnating medium. This is because, at present, the concentration is always determined in practice by manual titration. In general, titration is carried out at 30-minute intervals. Shorter titration intervals are very rare and involve far more personnel. Since the textile material passes through the impregnating bath at high speeds and since the type of textile material is often changed, the concentration in the impregnating compartment has already significantly changed in the half hour mentioned.  
       [0007] The process mentioned in the above-cited article in Int. Dyer is in principle a distinct improvement in the existing practice of the textile industry because it enables the concentration of the impregnating medium, hydrogen peroxide, in the impregnating compartment to be quickly and continuously determined so that the necessary quantity of impregnating medium can be added at virtually the same time. Accordingly, an automatic control system can be built up on the basis of this process which continuously monitors the concentration in the impregnating compartment and adjusts it accordingly.  
       [0008] Unfortunately, considerable problems arise in the practical application of the process described in the journal article. The filters used for the filtration step of the sample preparation block up relatively quickly. If, on the other hand, no filtration is carried out, the analysis system and especially the sensors are clogged up by the fine residual particles present in the impregnating liquor which are entrained by the textile material into the impregnating compartment and which emanate from earlier treatment of the fibers and the textile material. Particular damage is caused by starch constituents which represent the principal component of the sizes used in the weaving process. It was found in the development of the process according to the invention that the starch which coagulates at low temperatures and which is soluble at high temperatures is primarily responsible for the blockage of the filters which have a pore size of the order of 0.2 μm.  
       [0009] Accordingly, the problem addressed by the present invention was to achieve continuous, uninterrupted and exact replenishment of the impregnating medium, more especially soda lye or a combination of soda lye and hydrogen peroxide, in the process mentioned at the beginning.  
       [0010] According to the invention, the solution to this problem in the process mentioned at the beginning is characterized in that the part of the impregnating liquor which is branched off is heated before filtration. Heating prevents coagulation of the starch constituents in the impregnating liquor and the already coagulated starch paticles are re-dissolved. In tests conducted during the development of the process according to the invention, it was found that the problems otherwise occurring through blockage of the filters are no longer observed when the impregnating liquor is heated to a suitable minimum temperature. Uninterrupted operation over a long period is guaranteed in this way.  
       [0011] In one particular embodiment of the invention, that part of the impregnating liquor which is branched off is heated before filtration to a temperature in the range from 30 to 60° C. and more particularly to a temperature of about 40° C. The tests according to the invention have shown this temperature range to be particularly advantageous for uninterrupted and exact determination of the concentration of the impregnating medium over long periods.  
       [0012] Another embodiment of the invention is characterized by cyclic automatic rinsing of the filtration unit, more particularly with heated process water. The effect of this rinsing is that residues of the impregnating liquor remaining in the filters which cannot be dissolved by heating like the starch constituents are regularly washed out into the wastewater pipe. Uninterrupted filtration is thus significantly extended.  
       [0013] In another embodiment of the invention, amperometric and/or potentiometric sensors are used to determine the concentration of the impregnating medium, the branched-off part of the impregnating liquor is optionally diluted for pH adjustment to about pH 7—in line with the requirements of the hydrogen peroxide sensor—before its concentration is determined and is galvanically separated from the remaining part of the impregnating liquor. The sensors mentioned are known from the prior art and are commercially obtainable. The dilution of the impregnating liquor is only necessary for the determination of hydrogen peroxide because the corresponding sensor can only be used for concentrations of hydrogen peroxide that are well below the typical concentrations in the impregnating liquor. The galvanic separation which can be effected by a drop mechanism is of advantage for obtaining reliable results with certainty. This is because, with the sensors mentioned, even a small leakage current or a small error voltage can lead to an error in the determination of concentration.  
       [0014] Other advantageous embodiments of the invention are covered by the other subsidiary claims.  
       [0015] Finally, in order to use the measured concentration values for automatically maintaining the required concentration, the signals of the sensors corresponding to the measured concentrations are delivered to a control unit for replenishing the impregnating medium.  
       [0016] The control unit advantageously operates on the basis of the following correlations discovered by the inventors. Whenever the fabric is changed, i.e. whenever the textile to be impregnated is changed, there is generally a change in the concentration of the impregnating medium in the impregnating compartment. The change in concentration detected after 5 minutes can be used to calculate the concentration in the new equilibrium because the difference in the equilibrium concentrations according to the observations and tests according to the invention is around  8  times greater than the change in concentration in the first 5 minutes. The concentration in the new equilibrium is obtained in this way and can be used to estimate the additional quantity of impregnating medium that needs to be added or the extent to which the quantity of impregnating medium to be added should be reduced. If the concentration of impregnating medium in the new equilibrium after a change of the textile material is to be determined even more exactly, it is favorable to calculate this concentration using an exponential formula which is a function of time.  
       [0017] The advantages of the invention lie in particular in the following:  
       [0018] 1. Constant textile qualities are achieved through the invention because the concentration of the impregnating liquor can be kept substantially constant, even when the textile is changed.  
       [0019] 2. For documentation purposes, reports can be prepared and printed out or the stored data can be correspondingly further processed so that, for example in the event of possible errors detected afterwards, the cause can easily be located.  
       [0020] 3. The process can even be carried out with known installations because such installations can easily be modified.  
       [0021] 4. The textiles produced have fewer defects than in the prior art because there is no longer an over-concentration of hydrogen peroxide in the impregnating liquor or hence on the textile.  
     
    
    
     [0022] One example of embodiment of the invention is described in detail in the following with reference to the single drawing. FIG. 1 schematically illustrates an installation suitable for carrying out the process according to the invention and a corresponding flow chart without the pipes and parts of no relevance to the invention.  
     [0023] The process according to the invention can be divided into three main stages. Firstly, the sample to be measured has to be removed and prepared. Then comes the measurement. Thirdly, the liquor to be introduced is dosed on the basis of the results obtained so that a constant concentration of the particular impregnating medium is always maintained in the impregnating compartment.  
     [0024] The items of equipment for carrying out the first two steps mentioned are shown in the installation illustrated in FIG. 1. The last stage of the process according to the invention can be built up and carried out in known manner using standard elements.  
     [0025] The liquor to be measured is removed from the impregnating compartment  1  through a bypass line  2  and heated in a heat exchanger  3  to a temperature of about 40° C. in order to prevent coagulation of the residual constituents of the size in the following membrane filters  4 ,  5  to which the heated sample is delivered by a flow inducer  6 . The unfiltered, excess part of the liquor is returned to the impregnating compartment  1  through a bypass return line  2   a.    
     [0026] Impurities&gt;0.2 μm are removed from the sample in the membrane filters  4 ,  5 . Impurities remaining behind in both filters are eliminated by cyclic rinsing of the filters. To this end, water is introduced through a process water connection  29  into a heater  30  where it is heated to a temperature of around 60° C. and immediately transported into the two filters by a flow inducer  6 . The impurities washed out are removed from the system through a wastewater connection  31 .  
     [0027] The filtrate from the first membrane filter  4  is used to determine the concentration of hydrogen peroxide. The filtrate flows through a level measuring tank  7  to two dilution stations between which a pressure equalizing chamber  8  is arranged. Distilled water is delivered to the sample solution from a storage tank  9   a  through the pipe  10  while a buffer solution is delivered from a storage tank  9   b  through the pipe  11  so that the sample is in the pH range and the concentration range suitable for the sensor. In addition, flow inducers  12 ,  13  are provided at those places where distilled water and buffer solution are introduced for the first and second dilutions.  
     [0028] In addition, for the two-point calibration of the sensor, calibrating solutions can be fed into the pipe from two storage tanks  14 ,  15 , one solution having a low concentration and the other solution a high concentration of hydrogen peroxide. If calibration does take place, the calibrating solutions are introduced in a particular order. Between the introduction of the two calibration points, the drop chamber  16  is quickly emptied at a particular time by means of the membrane pump  16 a in order to avoid mixing of the solutions and hence to accelerate calibration.  
     [0029] The sample now diluted twice is then delivered via a drop chamber  16  used for galvanic separation to the sensor  17  for hydrogen peroxide. The sample is then delivered by a flow inducer  18  to a collecting tank  28  for the waste solution. The sensor  17  is an amperometric molecularly selective chemosensor and is commercially obtainable from Zabs GmbH.  
     [0030] In the sensor  17 , the peroxide is continuously decomposed at a platinum electrode. The electrical current obtained represents the measured value which is compared with calibration values previously obtained in a following electronic device (not shown).  
     [0031] To determine the concentration of soda lye in the impregnating liquor, the filtrate from the second membrane filter  5  is delivered through the first channel  19   a  of the flow inducer and a drop chamber  20  used for galvanic separation to a sensor  21  for sodium ions and its reference system  22 . Two storage tanks  24 ,  25  are used for calibrating the sensor, the corresponding calibrating solutions being delivered to the sensor  21  from the tanks  24 ,  25 .  
     [0032] The sensor  21  is a potentiometric ion-selective chemosensor. The concentration of the soda lye is measured via the membrane potential. After the measurement, the sample solution is delivered through the second channel  19   b  of the flow inducer to the above-mentioned collecting tank  28  for the waste solution.  
     [0033] In addition, a reference solution of potassium chloride has to be fed via the reservoir  23  into the reference system  22  of the sodium ion sensor  21  from another storage tank  26  by means of a flow inducer  27 . This potassium chloride solution represents the reference solution for the analysis and is therefore continuously delivered.  
     [0034] If the described system is not used for analysis, standby operation is essential to prevent drying out and salting up of the sensors  17 ,  21 . To this end, a standby circulation pipe containing a standby vessel  35  is built into the analysis section of the hydrogen peroxide sensor  17 . From the vessel  35 , buffer solution is continuously delivered through the hydrogen peroxide sensor  17  and back into the vessel  35  during the standby phase.  
     [0035] The standby operation of the soda lye sensor  21  is effected by the permanent supply of sodium chloride solution from a storage tank  36  instead of the sample solution.  
     [0036] In order to guarantee the operation of the soda lye sensor  21  for long periods, the reference system  22  of the sensor  21  has to be cleaned at regular intervals. To this end, distilled water instead of the sodium chloride solution is transported through the reference system  22  from a storage tank  32 , the potassium chloride reservoir  23  being short-circuited by means of two pinch valves  33 ,  34  in order to accelerate the rinsing process.  
     [0037] In order to keep the required concentration of impregnating medium in the impregnating compartment at a constant level, a strengthened, i.e. more highly concentrated, solution of the impregnating medium has to be continuously added. Since every textile material is different in regard to the carryover of liquor and the exchange of liquor with the moisture already adhering to the textile, the concentration of the impregnating liquor—for a constant addition—approaches a certain concentration value, which differs according to the particular textile, until an equilibrium is reached. Other parameters, such as mechanics, temperature and chemicals used, can also produce changes in the equilibrium concentration value of the impregnating medium. However, since these other parameters mostly remain the same, they only play a minor role.  
     [0038] Accordingly, a different quantity of impregnating medium always has to be added in dependence upon the treated textile in order to guarantee a constant concentration in the impregnating compartment.  
     [0039] The changes in concentration can be determined as a function of time using an exponential equation so that the equilibrium concentration can be calculated. Accordingly, there is no need for the trial and error method normally applied in practice. Whenever there is a change of textile material, the necessary quantity of strengthening liquor to be added can be calculated in at most a few minutes. To this end, the weight of the textile material per meter (fabric weight passing through the bath in kg/m) has to be indicated. The liquor (Q2) carried over by the textile from the impregnating compartment is calculated. The quantity of water (Q1) carried over by the textile into the impregnating compartment is generally known or can be reliably determined. Another parameter, the exchange factor (f), can be determined (by iteration) very quickly as a function of time and the developing concentration. The following equation is used for this purpose:  
       Kt=Ko−Mc*Kc /( Q 1* f*M*v+Mc )* exp{−t *(( Q 1* f*M*v+Mc )/ Mbo )}+ Mc*Kc /( Q 1* f*M*v+Mc ),  
     [0040] where  
     [0041] Kt is the concentration after a time of t minutes,  
     [0042] Ko is the concentration on entry at time t=0 minutes  
     [0043] Kc is the concentration of the total strengthening liquor (I/I)  
     [0044] Mc is the liquor difference per unit of time (I/min)  
     [0045] Mbo is the volume of the bath  
     [0046] Q1 is the quantity of water introduced into the impregnating compartment (I/kg)  
     [0047] M is the textile weight (kg/m)  
     [0048] v is the rate of travel of the textile (m/min)  
     [0049] f is the exchange factor.  
     [0050] Once the exchange factor f has been determined, the necessary quantity of strengthening liquor is calculated on the basis of the following equation:  
     Addition ( I /min)= M*v* ( Q 2− Q 1+ Q 1 *f )/ Rf    
     [0051] where  
     [0052] Q1 is the quantity of water introduced into the impregnating compartment (I/kg)  
     [0053] Q2 is the liquor carried over (I/kg)  
     [0054] Rf is the strengthening factor (concentration in the strengthening liquor/required concentration, the total strengthening being calculated as DQ=Q1-Q2)  
     [0055] M=textile weight (kg/m)  
     [0056] v=rate of travel (m/min)  
     [0057] f=exchange factor  
     [0058] Since control can be carried out continuously, the required concentration of impregnating medium in the impregnating compartment can be kept constant, even when the textile material is changed.  
     [0059] List of Ref renc Numerals:  
     [0060] 1  impregnating compartment  
     [0061] 2  bypass pipe  
     [0062] 2   a  bypass return pipe  
     [0063] 3  heat exchanger (40° C.)  
     [0064] 4  first membrane filter  
     [0065] 5  second membrane filter  
     [0066] 6  flow inducer  
     [0067] 7  level measuring tank  
     [0068] 8  pressure equalizing chamber  
     [0069] 9   a  storage tank for distilled water  
     [0070] 9   b  storage tank for buffer solution  
     [0071] 10  pipe for distilled water  
     [0072] 11  pipe for buffer solution  
     [0073] 12  flow inducer  
     [0074] 13  flow inducer  
     [0075] 14  storage tank for calibrating solution (low H 2 O 2  concentration)  
     [0076] 15  storage tank for calibrating solution (high H 2 O 2  concentration)  
     [0077] 16  drop chamber  
     [0078] 16   a  a membrane pump  
     [0079] 17  sensor for hydrogen peroxide  
     [0080] 18  flow inducer  
     [0081] 19   a  first channel of the flow inducer  
     [0082] 19   b  second channel of the flow inducer  
     [0083] 20  drop chamber  
     [0084] 21  sensor for soda lye  
     [0085] 22  reference system of the sensor for soda lye  
     [0086] 23  reservoir for potassium chloride solution  
     [0087] 24  storage tank for calibrating solution (low NaOH concentration)  
     [0088] 25  storage tank for calibrating solution (high NaOH concentration)  
     [0089] 26  storage tank for potassium chloride solution  
     [0090] 27  flow inducer  
     [0091] 28  collecting tank  
     [0092] 29  process water pipe  
     [0093] 30  heater (60° C.)  
     [0094] 31  wastewater connection  
     [0095] 32  storage tank for distilled water  
     [0096] 33  pinch valve  
     [0097] 34  pinch valve  
     [0098] 35  standby vessel  
     [0099] 36  storage tank for sodium chloride solution