Quantitative Analysis Of Citric Acid/Sodium Hypophosphite Modified Cotton And Preliminary Exploration Of Non-phosphorus Catalyst

Citric acid/sodium hypophosphite (CA/SHP) is widely used as a green non-formaldehydealternative to crosslink carbohydrate polymers such as cellulose and starch for performanceenhancement. However the actual role of SHP in this ester crosslinking system remains unclear.Regarding the application as formaldehyde-free durable press finishing agent for cotton fabrics,the system is known to suffer from substantial yellowing or discoloration and inferior durability ofgained resilience. In order to gain the insight of the CA/SHP crosslinking system, quantitativeinvestigations over its reaction with cotton fabrics under high curing temperatures were carried out.The observed changes in macroscopic physical properties were correlated with molecular leveltransformations.To quantitatively examine the modification of cotton cellulose by citric acid (CA)/sodiumhypophosphite (SHP), isocratic HPLC was used in conjunction with conductometric titration (CT).The major innovation of the approach is that all identifiable secondary polycarboxylic acids (PCA)from thermal decomposition of CA were included in the HPLC analysis. Specifically, thesecondary PCA are cis-aconitic acid, trans-aconitic acid, itaconic acid, citraconic acid and methylsuccinic acid. Analytes were made from both the saponification liquid (fixed PCAs fixed) and thewash liquid (PCAs failed to react) of cotton fabrics finished under various conditions. The concentrations of CA and the above mentioned secondary PCAs were determined for each curingtemperature and curing time. The amounts of free carboxylic groups on treated and washed fabricswere determined by conductometric titration and used in combination with the HPLC results tocalculate the degree of ester-crosslinking. The results showed that CA decompositionoverwhelmed the desirable esterification reaction at higher curing temperatures so that theconcentration of ester crosslinks [XL-PCA] initially increased with rising temperature then fellafter the peak at170°C. To understand the inconsistency of calculated [XL-PLA] and measuredWRA of the treated fabric, the degree of SHP mediated crosslinking was examined by elementalanalysis of phosphorous content on fabric. It was found the improvement in crease resistance orWRA results from both ester crosslinking and SHP mediated crosslinking. The co-existence of twodistinct crosslinking mechanisms distinguishes the CA/SHP crosslinking system from systemsbased on other PCAs such as BTCA and MA.Systems with added polyols were also examined. Other important findings of the HPLC-CTinvestigation include:1) Whiteness of the treated fabric corresponded well with the sum ofidentifiable unsaturated PCAs found on fabric. The species contributing to fabric yellowing ordiscoloration was likely to be an oligomer of IA and/or CCA.2) Adding polyol was confirmed toeffectively inhibit CA decomposition. The crease resistance correlates strongly with CApreservation. The dosage of polyol should be kept below an inflexion point so that the undesirablecompetition against cellulose remained minor. For the best extender, xylitol, the inflexion pointwas at0.15equivalents to CA.Initial investigations on the possibility of using non-phosphorous catalyst as an alternative toSHP were also carried out. The change in fabric properties were measured and compared. Thecatalysts tried were sodium acetate, trisodium citrate, imidazole, cyanamide, ethylene carbonate,sodium hydroxide and potassium hydroxide. Among the above mentioned catalysts, NaOH led tothe highest WRA of up to271°. However, the whiteness of CA/NaOH finished fabrics was lessthan20and not acceptable. Direct addition of xylitol to the CA/NaOH system gave only marginalimprovement in fabric whiteness. Pre-oligomerization of CA and xylitol was carried out forvarious length of time prior to addition of NaOH. It was found that pre-oligomerized CA/xylitolgave substantial improvement in terms of fabric whiteness. However, the WRA of thus treated fabrics were not satisfiable.The combination of HPLC and conductometric titration was demonstrated to be useful instudying the CA/SHP-cellulose crosslinking system. The findings have implications for betterapplication of CA/SHP in polysaccharide modifications in general. The superior performance ofSHP lies in its weak alkaline nature and the ability to participate in crosslinking reaction byreacting with unsaturated PCAs. The finding has implications for the development ofnon-phosphorus catalyst.