Application Of Organophosphorus Acid-Urea Flame Retardant Systems To Cotton Fabric

Cotton fabric as traditional dressing-oriented and industry-oriented materials is popular due to its excellent physical and mechanical properties, high moisture absorption performance, natural handle, snugness of fit, and economy. However, cotton fabric is a flammable material, so its flame retardant treatment is concerned with for a long time. Unfortunately, up to date, durable fireproofing finishing of cotton fabric is needed badly because of inaccessibility to the requirements of durability, harmlessness to environment and human, slight hurting of original physical and mechanical properties, low cost ,and no requiring complex or expensive devices.Although the cotton fabric finished by a combination of diammonium hydrogen phosphate(DAP) and urea(U) possessed high phosphorus retention rate under soft or hard water washing conditions and excellent flame retardancy was kept after soft water laundering, the flame retardant ability was lost after hard water laundering owing to the ion exchange, occurring between the ions of hydrogen and ammonium in cellulose-phosphate acting as the main products and the ions of Ca (II) and Mg (II) in the hard water, which caused the production of insoluble and refractory phosphates by virtue of which upon combustion of cotton fabric the phosphate acid couldn't be released timely to promote the dehydration and carbonization of cellulose through a phosphorylation-dephosphorylation mechanism so as to result in the loss of flame retardant effectiveness. In order to solve the problem, several organophosphorus compounds, for example, dimethylol phosphinate(DMLP)-(HOCH₂)₂P(O)OH and ammonium dimethoxyphosphate (ADMP)-(CH₃O)₂P(O)O^-·NH₄⁺, whose structure general formula was R₂P(O)OR', were combined with urea to flame retard cotton fabric. The purpose of the design was to produce organic cellulose-phosphates and then avoid the ion exchange during hard water washing, finally imparting cotton fabric to the durability of flame retardant effectiveness to hard water laundering.In order to provide a basis of comparison and analysis with the application of R₂P(O)OR'-U systems to cotton fabric, the DAP-U flame retardant system was used to repeat exactly the flame retardant treatment of cotton fabric according to the process conditions provided by the related literatures. The LOI values of the unwashed finished article and those articles after once deioned water washing and once hard water laundering were 46.8%, 39.6% and27.0% respectively. Those LOI values showed the reproducibility of phosphorylating reaction of cellulose on or in the cottonfabric.ADMP was prepared by using dimetylphosphite as raw material, hydrogen peroxide as oxidant, ammonium hydroxide as a pH regulator of reaction system with a product yield of 86%. The cotton fabric was immersed in a solution containing the combination of 20%(w.t.) ADMP and 40%(w.t.)urea, then passed through a laboratory padder with a pick-up of 80% after two dips and two nips, dried at 105℃for 3 minutes, and finally cured at 170℃for 3 minutes in a setting machine. When the curing temperature was 170℃, the LOI values of the unwashed finished article and those articles after once deioned water washing and once hard water laundering were 47.7%, 29.5% and 20.7% respectively. The IR analysis results of the samples finished by ADMP-U system and the samples treated by DAP-U system showed that all the samples had exactly the same IR characteristic peaks and that the IR characteristic peaks of-OCH₃ was absent. Therefore, it could be concluded easily that P-O-CH₃ of ADMP was cleaved during curing and phosphate acid or ammonium salts of phosphate were produced. It meant that for both DAP-U-Cellulose and ADMP-U-Cellulose reaction systems the same reaction process occurred and the same main product-cellulose phosphate containing H⁺ and NH₄⁺, was produced. This was probably the principal reason that the finished articles by ADMP-U system were not durable to hard water laundering.Due to the poor thermal stability of ADMP shown by the cleavage of P-0 bond, DMLP was synthesized, whose excellent thermal stability resulted from P-C bond with good heat endurance DMLP was prepared by mixture of sodium hypophosphite monohydrate and paraformaldehyde at a molar ratio of 1:2.1 ,refluxing for 20 hours at 135-140℃with a product yield of 88%. The cotton fabric was immersed in a solution containing the combination of 20%(w.t.) DMLP and 40%(w.t.)urea, then passed through a laboratory padder with a pick-up of 100% after two dips and two nips, dried at 105℃for 3 minutes, and finally cured at 165℃for 5 minutes in a setting machine. The LOI values of the unwashed finished article and those articles after once deioned water washing and once hard water laundering were 49%, 22.5% and 19.8% respectively. The data showed a sharp decrease of flame retardancy of cotton fabric.The phosphorus contents of the finished articles after once deioned water washing and once home laundering were 0.466% and 0.402% respectively, which showed the weak reactivity between DMLP-U system and cellulose. Low yields of products covalently bonded to cellulose resulted from the weak reactivity. This explained why flame retardant effectiveness of finished articles by DMLP-U system was so low. The probable main reason why the reactivity of DMLP-U-cellulose system was so low was the unusual difficulty of occurrence of SN₂ reaction which resulted from the partial double bond nature of P-OH in DMLP molecular and the protonation of hydroxyl groups of cellulose caused by padding solution (pH=1.96-2.05). Therefore, the selection of phosphorylation catalysts was made in order to enhance the reactivity of DMLP-U-cellulose system. Experimental results showed that although sulfuric acid was the best catalyst upon improving the flame retardancy of finished articles after soft water washing, the flame retardant performance of those finished articles after hard water laundering was decreased and even inferior to that of the articles which were not catalyzed. This is probably because addition of sulfuric acid caused the protonation of hydroxyls of DMLP which made a certain quantity of P-C bonds cleaved to produce phosphate acid or ammonium salts of phosphate which resulted in the production of cellulose-phosphate with H⁺ and NH₄⁺. And then the endurance degree of the finished samples to hard water laundering was reduced because of their ion exchange ability. Comparatively, the finished samples without catalysis after 25 home launderings and 5 HL had the same flammability. This showed that the finished articles by DMLP-U system could keep their original flame retardant ability under hard water laundering condition. Unfortunately, those articles didn't have notable flame retardant effect owing to the low product yield of esterification by direct dehydration-condensation means. If reasonable structures of organophosphorus acids are design or effective phosphorylation catalysts are found out to enhance the reactivity between organophosphorus acids and cellulose, then it is a promising research direction to impart cotton fabric to durable flame retardant performance though phosphorylation of cellulose.