Comparison of two Organophosphorus Flame Retardant for Silk FabricsGUAN Jinping CHEN GuoqiangSchool of Material Engineering, Soochow University, Suzhou (215021)E-mail：helengjp@AbstractFlame resistance finishing of silk fabrics is still challenging.N-Methylol dimethylphosphonopropionamide(MDPA),known as “Pyrovatex CP” commercially, was applied onto silk fabrics in our previous research, but it has some shortcomings ,such as causing yellowish, degradation of breaking strength etc. In this research, we applied a self-prepared novel reactive organophosphorus flame retardant named diethyl-2-(methacryloyloxyethyl) phosphate (DEMEP) onto silk fabrics. We explored the process of the two flame retardants onto silk fabrics. After flame resistance treatment, we compared the flame resistance effect of the two retardants by LOI, char length, laundry cycles. The toxicity was investigated by measuring the release of formaldehyde. We also explored the physical and chemical properties of the two flame resistance samples, such as whiteness, strength, permeability and hygroscopy.Key words: Silk; Flame resistance; Flame retardant; Pyrovatex CPINTRODUCTIONAccording to fire statistics, about 50 % of fires are caused by textiles in the world [1]. Silk fabric is widely used as pajamas, domestic decoration materials for its luster, soft handle, wearing comfort and aesthetic appearance. So it is vital to be endowed flame resistance property. Silk has low flammability with LOI value about 23 %, which can be attributed to itshigh nitrogen content (about 15~18%) [2]. But it still needs added flame resistance finish tofulfill some commercial requirements.Flame resistance finishing on silk fabrics can be traced back to 200 years ago, when silk fabrics were immersed in the mixture of borax and boric acid in solution. But this mixture iseasily removed by water [3]. In the mid 1980s W.B.Achwal et al. reported that silk fabrics treated with a urea phosphoric acid salt, U4P, by a pad-dry process had high flame resistancewith an LOI value higher than 28% and the flame resistance finish was fast to dry cleaningwhile not fast to washing [4].To this day, there is still no suitable flame retardant for silk fabrics. The well-known flame retardant N-Methylol dimethylphosphonopropionamide(MDPA),known as “Pyrovatex CP”, is effective for cotton,in our previous research , we applied it onto silk fabrics and gained good flame resistance property but at the loss of some physical properties of silk fabrics, such as whiteness ,strengthetc. Recently, we prepared a new reactive flame retardant for silk fabrics named diethyl-2-(methacryloyloxyethyl) phosphate (DEMEP). In this paper, we fully compared the process method and effect on physical, chemical properties of silk fabrics of the two flame retardants.EXPERIMENTALMaterialsDegummed and bleached silk fabric (plain weave, 36g/m2) was kindly supplied by Zhejiang Haoyunlai Group. N-Methylol dimethylphosphonopropionamide(MDPA,PyrovatexCP)and the cross-linking agent hexakis (methoxymethyl) melamine (HMM) were kindly provided by Ciba Specialty Chemicals (China) Ltd. Shanghai Branch, diethyl-2-(methacryloyloxyethyl)phosphate (DEMEP) was prepared in our laboratory and its purity was 95%. Other reagents were reagent grade commercial products.Synthesis of vinyl phosphate DEMEPThe mixture of 13.1 g (0.1 mol) of 2-hydroxyethyl methacrylate (HEMA) and 10.1 g (0.1 mol) triethylamine was added dropwise to 15.3 g (0.1 mol) phosphorus oxychloride in 150 ml dichloromethane at 0 °C, under nitrogen. After the addition, the mixture of 12.0 g (0.25 mol) absolute ethyl alcohol and 20.2 g(0.2 mol) triethylamine was added dropwise at the same condition as above. After completion of the addition, keep stirring at 0~5 °C for 1 h, then warming the mixture to room temperature and stirred overnight. The precipitated triethylamine hydrochloride was removed by vaccum filter.The combined filtrates were washed with an aqueous solution of 2 % NaOH, the solvent was evaporated and the residue was distilled under vacuum after adding a small amount of hydroquinone. The product is pale yellow viscous oil named diethyl-2-(methacryloyloxyethyl) phosphate (DEMEP), and the yield is 56 %, its boiling point is 104 °C (0.5 mmHg).The molecular scheme of DEMEP is as follows:C O1H NMR(CDCl,ppm)δ 6.17(s, 1H, =CH),δ 5.61(s, 1H, =CH),34.37-4.11(m,8H,-OCH2),1.96(s,3H,=C-CH3),1.37(t,6H,-CH3).13C NMR(1H,CDCl3,ppm)δ 170.2(-C=O),139.5(H2C=C-),128.7(=CH2),68.2-66.7(OCH2),18.8-16.9(CH3)IR(cm-1)1721(C=O),1637(C=C),1297(P=O),1032 and 988(P-O-C)Fabric treatmentMDPA treated process:Fabric samples (30×20cm) were dipped in finishing solution (containing 250g/l of MDPA,50 g/l of HMM, 7.5 g/l of phosphoric acid (85%) and 0.1 g/l of JFC)for one minute and thenpadded on a two roll padding mangle at a fixed pressure; wet pickup was about 95% after twodips, two nips. Then the fabric was dried in a heat-setting stenter at 90 °C for 3 minutes, andthen cured at 170 °C for 3 minutes. Then, the cured silk fabrics were soaked in water (liquorratio 1:50) containing neutral soap flakes 2g/L at 60 °C for 20min,then rinsed with tap waterand then dried at 105 °C for 2h. Samples were placed in a desiccator over silica gel before measurements. Weight gains were calculated with the following formula: Weight gain (%)=(w2-w1)/w1×100,Where w2 and w1 are the weight of treated silk fabric and original fabric respectively.DEMEP treated process:Silk fibers were immersed in a finish solution containing 100 % DEMEP (on the weight offibers [o.w.f.]), 0.9 % potassium persulfate (on the weight of monomer [o.w.m.]), 2% emulsifier(o.w.m., the emulsifier is the mixture of Tween 80 and Span 80 mixed at weightratio 64:36),The reaction pH value was adjusted with formic acid or sodium hydroxide at 3;The material-liquor ratio was 1︰30.The reaction system was heated from room temperatureto 85 °C and maintained in vibrational water for 45 min. At the end of the reaction, silk fiberwas washed with water (containing 0.5 g/l non-ionic surfactant OP) at 60 °C for 30 min, andthen rinsed with cold water. After that removed ungrafted flame retardant with a propersolvent and then rinsed with tap water and dried at 105 °C for 2h.Samples were placed in a desiccator over silica gel before measurements.Grafting yield was calculated as follows:Grafting yield (%) = (w2-w1)/w1×100, where w2 is the oven-dried weight of grafted silk fibers and w1 is the oven-dried weight of the original silk fibers.MeasurementsWhiteness and Yellowness Index was measured by WSD III whiteness instrument. The result was the average of 8 measurements.Char length was determined by the GB/T 5455-1997 (equivalent to ISO 6940:1984) Textiles-Burning behavior-Vertical method [5].Tensile properties were measured under standard conditions on an YG026A Electrical Fabric Strength Tester. The values presented were the average of 10 tests.The Limiting oxygen index (LOI) was assessed with the HC-2 LOI instrument, according to the GB/T5454-1997 (equivalent to ISO 4589-2 or to ASTM D 2863) Textiles-Burning behavior-Oxygen Index [5].Laundry cycles are according to the GB/T 17596-1998 (equivalent to ISO 10528-1984) Textiles-Commercial Laundering Procedure for Textile Fabrics [5].Permeability was determined by the GB/T 5453-1997 (equivalent to ISO 9237:1995) Textiles-Determination of the permeability of fabrics to air [5].The instrument is YG (B) 461D digital fabric permeability tester.Formaldehyde content on fabrics is determined by GB/T 2912.1-1998(equivalent to ISO/FDIS 14184-1:1997) Textiles-Determination of formaldehyde-Part 1: Free and hydrolyzed formaldehyde (Water extraction method).Hygroscopy of fabrics is determined by FZ/T 01071-1999 Textiles-Capillary effect test method [5].RESULTS AND DISCUSSIONComparison of MDPA and DEMEP process methodTable I Structure of MDPA and DEMEPFlame retardant Structure formulaMolecularweightPhosphoruscontent /%FunctionalgroupWatersolubilityMDPA211 14.69 YesDEMEP 266 11.65 No From Table I we can see that MDPA has higher phosphorus than DEMEP. And the functional group of MDPA is hydroxyl, which has some reactivity with hydroxyl or amino group on tyrosine or lysine of the silk fibers, but this reactivity is not enough so itmust add crosslinking agent HMM to improve its fixation amount onto silk fabrics[6].While the functional group of DEMEP is vinyl. As we all know, vinyl monomers arevery effective for silk fabrics to produce many added function by graft copolymerizationtechnique, such as wrinkle resistance, water repellency, anti bacteria etc[7-9].In most cases , MDPA is applied onto fabrics by pad-dry-cure technique accompanied by HMM,so dyeing and finishing are not in the same bath. But DEMEP can be applied either in exhausting method or in pad-dry-cure method with crosslinking agent. It can be appliedonto fabrics by plasma or radiation initiation technique [10].MDPA is water soluble, whileDEMEP is not soluble in water. So before applying DEMEP onto fabrics, it must me pre-emulsified.Flammability properties of treated silk fabricsTable II Flammability properties of grafted silk fabrics Flammability Properties Samples Weight gain or graft yield /% Char Length /cm LOI/%Control sample 0 BEL 23.0MDPA treated sample 30.41 6.2 31.1MDPA treated sample after 15 laundry cycles - 11.129.4MDPA treated sample after 50 laundry cycles - 14.1 26.7DEMEP treated sample 40.5 3.85 30.6DEMEP treated sample after 15 laundry cycles - 5.529.2DEMEP treated sample after 50 laundry cycles - BEL 23.6Note: LOI= Limiting Oxygen Index; BEL=Burns Entire LengthTable II shows that, after adding a flame resistance finish, silk fabrics have a satisfactory LOI value of about 30%, which is much more than that of the control silk fabric (23.0%). The char length can fulfill the flame resistance requirement of char length less than 200 mm, as specified by China National Standard GB 17951-1998, “Woven flame retardant fabrics” [5]. DEMEP treated silk fabrics present better flame resistance than MDPA treated silk fabrics because DEMEP treated silk fabrics have higher LOI with 30.6% and lower char length with3.85 cm than MDPA treated silk fabrics with LOI 31.1% and char length 6.2 cm. After 15 laundry cycles, DEMEP treated silk fabrics still shows better flame resistance than MDPA treated silk fabrics. But when experienced 50 laundry cycles, MEPA treated silk fabrics still have some flame resistance, but DEMEP treated silk fabrics are almost without any flame resistance. That is, MDPA treated silk fabrics have much excellent flame resistance durability than DEMEP treated silk fabrics.Comparison of phosphorus content on silk fabricsa b cFig.1 Element content distribution on silk fabrics by SEM-EDS(a is control sample; b is MDPA treated sample; c is DEMEP treated sample)Table III Element content of silk fabricsSamples Element content / Wt /%Control sample C 64.49;N 13.24;O 22.27MDPA treated sample C 56.49;N 17.44;O 25.21;P 0.86DEMEP treated sample C 58.55;N 10.09;O 27.73;P 3.35;Cl 0.27Phosphorus is an effective component in flame resistance. Only phosphorus attain a certain amount can textiles have flame resistance [11] Fig.1 shows that the element content distribution on silk fabrics and Table III listed the specific data. Table III shows that on control sample, there is no phosphorus, so it is easy to burn.The DEMEP treated samples have higher phosphorus content 3.35% than MDPA treated sample with phosphorus content 0.86%, so the former exhibits much excellent flame resistance than the latter.Physical properties of treated silk fabricsTable IV Physical properties of treated silk fabricsElongation at Break/ % Samples Whiteness index Yellowing index Tensile strength/N16.92 Control sample 88.64 10.6 352.2 MDPA treated sample 78.56 30.37 310.0 11.58DEMEP treated sample 82.63 20.68 319.5 18.40Table IV indicated that after flame resistance finish, silk fabrics become yellow which is shown by the decrease of whiteness index and the increase of yellowing index. And the treatment will also cause the loss of tensile strength, which may be the result of the hydrolysis of some sensitive peptide bonds caused by acid in wet and thermal state [12], but this loss is negligible. The elongation at break of MDPA treated silk fabrics is lower than that of control sample, which is because after treated with MDPA, silk fabrics become crisp and easily broken. While the DEMEP treated samples have higher elongation at break 18.40 % than that of control sample 16.92 %. That is , after treated with DEMEP, silk fabrics have better elasticity, so it is hard to break, which should be mainly attributed to the changes of the inner orientation of silk fibers caused by flame retardant. DEMEP treated silk fabrics had lower average orientation than that of control sample, the polymer grown during the grafting process filled the space available within the fiber matrix, which disturbs the arrangement of the fibroin chains in the amorphous regions and partially hindering their mobility when subjected to tension [13]. Effect on comfort characteristics of silk fabricsTable V Permeability, hygroscopy and toxicity of silk fabricsCapillary effect / cmSamples Permeability5 min/mm/s 10 min20 min 30 minControl sample 1027 4.6 6.3 7.3 7.4 MDPA treated sample 1599 3.3 3.4 5.4 6.2DEMEP treated sample 685 5.1 7.0 7.6 8.3 Wear comfort, is an outstanding characteristic of silk fabrics,esp permeability ,hygroscopy and health care.Table V shows that after treated with MDPA , silk fabrics have higher permeability than DEMEP treated samples,which is because MDPA samples become loose when dry and cure under heat and tension conditions. MDPA treated samples have poorer hygroscopy than DEMEP treated samples. With the time increase, the capillary effect value increased. So, DEMEP treated samples will give excellent wear comfort.CONCLUSIONSThe self-prepared organophosphorus flame retardant vinyl phosphate ester DEMEP is a reactive retardant. Only less phosphorus content can make silk fabrics gain excellent flame resistance. Silk fabrics treated with DEMEP can endure 15 laundry cycles. While MDPA can be fixed onto silk fabrics effectively only with the help of crosslinking agent HMM. And MDPA-treated silk fabrics have better laundry endurance. Both flame retardants can cause silk fabrics yellow and damage.But both strength losses are negligible. DEMEP treated silk fabrics have better elasticity and wear comfort than MDPA treated silk fabrics.ACKNOWLEDGEMENTSWe express our appreciation to Zhejiang Haoyunlai Group for providing silk fabrics.REFERENCES1. Nair G.. P., Colourage, V ol 47, No. 8, 2000, pp27-34.2. Hui-xuan LI, Jian-gang WANG, Jilin Engineering College Acta, V ol 12, No.2,1991,pp68-71.3. Hongxiang ZHOU, Guangxi Textile Science Technology, V ol 24, No. 2, 1995,pp 35-39.4. Achwal W.B.,et.al, Colourage, No.6, 1987,pp16-30.5. China Textile Standards Compile (I), (II), China Standards Press, Beijing, 2000.6. 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