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Finishes requiring acid release catalystsIn the case of finishes requiring acid releasing catalysts for the cross linking with the cellulose and polymerization in the subsequent curing, alkalinity on the fabric would neutralize the same and thus stall the chemical reaction of the finishing agent. Variation in pH of the substrate would therefore correspondingly affect the consistency and uniformity of polymerization of the resin matrix / cross-linking on the substrate and consequently reflect on the performance parameters like CRA /DP ratings / wash fastness etc. the temperature and dwell time are the other two important factors that need to be controlled, besides pH for any given catalyst employed. The flame retardant finish, resin / silicon applications and various other reactant finishes require specific heat treatments and catalysts. It is therefore necessary to neutralize the fabric substrate fully and the bath pH brought down to the recommended acidic pH and maintained. Neutralization of the substrate needs careful planning and execution. Invariably this operation is carried out on a stenter and acetic acid is used for this purpose. The foregoing example is also applicable to neutralization operation. Application of ‘finish’ by padding Application of finish by padding is more convenient and many of the problems related to exhaust techniques can be avoided. Primarily padding technique is most suited for finishes that are non / less substantive, at least at the padding stage. (Subsequent post treatments may impart fixation / bonding). Where the finish mix is likely to sediment / separate, continuous agitation is necessary. Where substantivity is involved care need to be taken to take this factor into consideration and provide safeguards against preferential exhaustion in the pad bath. We may discuss this point with a case study.
OBA (Optical Brightening Agent) had been applied at the padding mangle of stenter. The investigation and trials revealed that bleach before OBA was good and it was at the OBA application stage that the tailing / centre – selvedge variation had occurred. The Mangle pressure across the width was found to be satisfactory. The fabric absorbency and the dryness (moisture) were satisfactory. In this case application of the substantive OBA in the pad box of a stenter gave not only tailing but also centre – selvedge variation. The OBA from the stock tank was fed to the stenter through a distributor with delivery at left, middle and right of the pad trough. Such distribution could have been got away with in yester years, but with precision stitching of garments and more discerning customers, the variation in the whiteness is not permissible now. This problem was sorted out by providing a closed circuit circulation between the pad box and a side service tank through a pump and feeding the fresh feed OBA through the side tank. The distributor was also provided with properly sized additional nozzles to evenly distribute the circulated liquor. Care was taken to point the nozzles away from the relatively a little higher than the concentration of OBA in the pad box at the start to even out substantively factor. This concentration was arrived at by carrying out initial trials. The centre selvedge problem was also solved due to the circulation and even distribution. The Stenter machine was run at a constant speed right through the OBA application. These arguments and logic shall also apply to other finishes having high substantivity and in the neutralization operation. Variation in finish is generally caused at the time of such pad applications particularly where substantive finishes are employed. Unless the finish is substantive it will not last and then either substantive or reactive finishes are preferred. It is therefore necessary to safeguard against tailing and evenness problems.
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