Study On The Microencapsulation And Application Of Water-soluble Flame Retardant

The paper dealt with the study of the microencapsulation of a water-soluble flame retardant dimethyl methylphosphonate （DMMP） which was used for the flame retarding finishing of cotton fabric, when it was microencapsulized and built-up with some inorganic compounds as the synergists of suppress afterflame or afterglow. Factors affecting the microencapsulation and the flame retarding efficiency of the built-up system were discussed. The mass-transfer mechanism of synthesizing microcapsules by W/O/W multiple emulsion method was also studied.The work mainly consisted of three parts. First, systems of built-up flame retardants, which included the water-soluble flame retardant DMMP and inorganic compounds as the synergists of suppress afterflame, afterglow or smoke suppression, were studied. Second, the microencapsulation of DMMP and its flame retarding application on cotton fabric were researched, and the optimal technical parameters for microencapsulation were confirmed. Third, the stability of W/O emulsion and the mass-transfer mechanism were delved.Part 1. DMMP was selected as the flame retardant of the study and the core of the microcapsules. The flame retarding efficiency had been evaluated by means of measurements of char length, afterflame time, afterglow time, and fuming amount. From the built-up experiment results, we found that inorganic compounds containing some nonmetal elements （P, N, and B, such as phosphoric acid, red phosphorus, sodium dihydrogen phosphate, ammonium chloride, borax） and metal ions (Mg²⁺, Al³⁺, Ca²⁺, Zn²⁺, Cu²⁺, and Mn⁴⁺, such as copper sulfate, magnesium chloride, aluminium oxide, calcium carbonate, aluminium hydroxide, zinc hydroxide, magnesium hydroxide, copper oxide, manganese dioxide) could suppress the afterflame and afterglow well. The production of fuming amount could also be suppressed by adding the inorganic compounds containing more active metal ions (such as Ca²⁺, Al³⁺, Cu²⁺, and Zn²⁺) or some anion ions （such as oxide and hydroxide anions）. So they could be used as additive to the system of flame retardant.Part 2. The microcapsules were synthesized by the w/o/w multiple emulsion method, with the water-soluble DMMP as the core material and the acetal product of polyvinyl alcohol （PVA） and glutaraldehyde （GA） as the shell material. Factors affecting the average particle size and distribution of microcapsules were discussed. Based on a large amount of experimental data, the optimal condition of synthetic technology was established: the mass concentration of PVA was 3%, the volume percentage of emulsifier SPAN80 in cyclohexane was 5%, the optimal volume ratio between inner phase and intermediate phase was 1 : 1 - 4 : 5, the time of emulsification was 20 minutes and the speed of agitator was 1400 r/min, the time of cure was 1 hour and the speed of agitator was 400 - 600 r/min, the volume percentage of PVA solution was 50%-60%, the volume of water in inner phase was 6mL, the volume of catalyst HCl in interior phase was 1mL.The DMMP microcapsules were characterized to realize their properties well. It was showed by DSC thermal analysis that the thermal behavior of the microcapsules shell was determined by the concentration of GA, and it was suitable for shell burst as the volume percentage of GA was 15%. The shell materials were acetal made from PVA and GA by IR spectrum analysis certified in the formation of the large amount of bond C-O-C. The solubility test showed that the acetal could not dissolve in several common solvents owing to a certain degree of stereo-reticular structure. It was verified that DMMP was capsuled in the microcapsules successfully through qualitative analysis of phosphor.The microcapsules, with flame retardant DMMP as the core material, had a good flame retarding efficiency to the fire caused by unextinguished stump.The microencapsulated DMMP was built-up with a series of inorganic compounds, and then the optimal finishing formula of suppress afterflame and afterglow was confirmed, according to measurements of char length, afterflame time and afterglow time. It was found that an excellent flame retarding efficiency was obtained when the DMMP microcapsule and inorganic compounds containing Sb₂O₃ and H3PO4 as built-up system were used in flame retarding of cotton fabric.Sb₂O₃ was selected as synergist of suppressing afterflame, which was dissolved in HCl, and H₃PO₄ as synergist of suppressing afterglow. Further more, the microcapsules of built-up system were synthesized by the w/o/w multiple emulsion method, with the water-soluble DMMP and Sb₂O₃ or H₃PO₄ respectively as the core material and the acetual product of polyvinyl alcohol （PVA） and glutaraldehyde （GA） as the shell material. The optimal condition of synthetic technology of two series built-up systems was determined:（1） The optimal parameters of synthesizing microcapsules which could suppress afterflame: 20 mL DMMP, 6 mL water, 5 mL HCl, 1.5 g Sb₂O₃ and 5 mL GA were contained in the inner phase; and in the intermediate phase the volume of cyclohexane was 50 mL, emulsifier SPAN80 6 mL; in the outer phase the volume of 3% PVA solution （mass concentration） was 110 mL; the time of emulsification was 15 minutes, and the time of cure was 1 hour.（2） The optimal parameters of synthesizing microcapsules which could suppress afterglow: 20 mL DMMP, 12 mL water, 5 mL HCl, 10 mL H₃PO₄ and 5 mL GA were contained in the inner phase; and in the intermediate phase the volume of cyclohexane was 50 mL, emulsifier SPAN80 6 mL; in the outer phase: the volume of 3% PVA solution （mass concentration） was 110 mL; the time of emulsification was 20 minutes, and the time of cure was 1 hour. The excellent flame retarding efficiency was obtained after the two kinds of microencapsulated flame retardants were used for cotton fabric. Compared with unmicroencapsulated flame retardants, the durability of flame retardants was greatly improved, and the loss of cotton fabric strength was reduced.Part 3. Based on the stability test of W/O initial emulsion, the optimal technical parameters for preparation of W/O emulsion were confirmed as follows, water content: 60 %vol; ratio of Span80 and Tween80: 4/1 （v/v）; amount of emulsifier: 3.5 %vol; pH value of water phase: 1.05; emulsification temperature: 40℃; time of emulsification: 20min; emulsification intensity: 1500r/min.Premised on the single-factor experiments of synthesizing microcapsules, factors affecting the average particle size and size distribution of the microcapsules were confirmed. Those factors included water content, amount of emulsifier Span80, amount of catalyst HCl, concentration of PVA, time of emulsification and solidification, intensity of emulsification and solidification. However, affecting regularity of GA amount was not detectable.By the comparison of the average particle size among inner phase droplets, W/O initial emulsion droplets, W/O/W multiple emulsion droplets and microcapsules, the mass-transfer mechanism of PVA and GA in the acetalization was found. The shell of microcapsules was formed at the interface between middle phase and outer water phase by the acetalization of GA with PVA to produce deposit as GA passed through middle phase to outer water phase.In summary, the study of the microencapsulated water-soluble flame retardants used for flame retarding finish of cotton fabric has rarely been reported. It was difficult for microcapsule technique in encapsulizing water-soluble materials. The fundamental research in this area had both theoretical and practical importances. Although the work described in this paper was a preliminary attempt in this field, it established a very useful foundation for further research.