Article pubs.acs.org/IECR
An Antidripping Flame Retardant Finishing for Polyethylene Terephthalate Fabric Qingli Feng,†,‡ Xiaoyu Gu,†,‡ Sheng Zhang,*,†,‡ Bin Zhao,‡ Jun Sun,† Xueyan Li,† and Mingzhe Dong†,‡ †
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ‡ College of Materials Science & Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: A flame retardant system containing 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) and ammonium sulfamate (AMS) was proven to be effective on improving the fire performance of polyethylene terephthalate (PET) fabric. The flammability tests indicate that this FR system can impart dripping resistance to PET fabric and the limiting oxygen index value can reach up to 28. The thermal behavior of PET fabric was investigated by thermo-gravimetric analysis. The results indicate that the residual char of treated samples was significantly improved by the presence of HEDP and AMS. The morphology of treated PET fabric residue was also characterized by scanning electron microscope analysis. It has been suggested that intumescent char structures have been formed.
1. INTRODUCTION PET fabric has been extensively used in both military and civilian areas for many years because of its excellent properties, such as the low-density and fairly high mechanical strength etc.1 However, its flammability and melt dripping problems cannot meet the industrial requirements in many cases. Several flame retardant systems and techniques have been developed to improve the flame retardancy of PET fabric.2−4 Halogenated flame retardants have been proven to be effective on synthetic fabric.5−9 However, halogenated flame retardants have been undergoing more and more scrutiny due to the increasing concern about the potential hazard caused to the environment. Therefore, phosphorus-based intumescent flame retardant systems have attracted more and more attention for flame retardant treatment of synthetic textiles.10−12 Several aromatic brominated and phosphorus-containing compounds,13−15 in particular, aromatic polyphosphonates16−18 were found to be efficient flame retardants for PET. Flame retardant finishing has often been used to improve the flame retardancy of PET fabric because of its convenience and inexpensiveness during processing. One of the challenges in flame retardant finishing of PET fabric is the formation of melt dripping during burning which usually leads to the spreading of fire. Our previous study has demonstrated that the intumescent flame retardant system containing ammonium polyphosphate (APP), melamine, and pentaerythritol (PER) can improve flame retardancy and reduce dripping tendency of nylon-6,6 fabrics.19 However, it is very difficult to further improve the fire performance of nylon-6,6 because of the low solubility of the above-mentioned components. In this study, 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) and ammonium sulfamate (AMS) have been introduced to the antidripping flame retardant (FR) system to improve the flame retardancy of PET fabrics. It is suggested that this could be the first time that both HEDP and AMS were used in enhancing the flame retardancy of PET fabric. The fire performance of treated samples has been evaluated and the © 2012 American Chemical Society
thermal property and the morphology of residual char have also been discussed.
2. EXPERIMENTAL SECTION 2.1. Materials. PET fabric (100% ) with a density of 200 g/ m2 was supplied by Hairong Technical Textiles Co. Ltd., Cheng Du, China. 1-Hydroxy ethylidene-1,1-diphosphonic acid (HEDP) was supplied by Shandong Taihe Water Treatment Co. Ltd., China. Pentaerythritol (PER) was supplied by Fuchen Chem, Tian Jin, China. Ammonium sulfamate (AMS) was supplied by Laizhou Sanding Chemicals Co. Ltd., China. 2.2. Sample Preparation. PET fabric samples were washed in a sodium hydroxide aqueous solution (30 g/L) at 100 °C for 20 min before being submitted to washing with water to remove the impurities before use. Pretreated PET fabric samples were immersed in a flame retardant (FR) solution, then passed through a padder with two dips and two nips under the pressure of 0.3 MPa between the two rolls, and finally dried at 110 °C for 4 min. All concentrations were based on the weight of bath solution (w/w, %). 2.3. Characterization. The limiting oxygen index (LOI) was measured according to GB/T2403-1993 by using JF-3 LOI instrument, Jiangning, China. The vertical flammability was measured according to GB/T 5455-1997 by using CZF-3 instrument, Jiangning, China. The replicates number for this test is 5. Thermogravimetric (TG) experiments were carried out by using HENVEN HCT-1 TG analyzer. Sample mass is in the range of 2−3 mg. All samples for TG were measured from 25 to 800 °C with a heating rate of 10 °C/min in static air atmosphere. The replicates number for this test is 3. Received: Revised: Accepted: Published: 14708
June 7, 2012 October 18, 2012 October 23, 2012 October 23, 2012 dx.doi.org/10.1021/ie301508q | Ind. Eng. Chem. Res. 2012, 51, 14708−14713
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Table 1. The Flammability Testing Results of PET Fabric Samples Treated with HEDP or AMS flame retardant
samples
concentration (%)
after-flame (S)
damage length (cm)
LOI (%)
dripping
HEDP
1 2 3 4 5
5 10 15 20 25
21.37 11.87 2.67 0.67 0.78
12.50 9.51 8.93 8.37 8.33
22.0 24.3 25.4 25.5 25.7
dripping dripping dripping none none
AMS
6 7 8
5 10 15
10.00 7.40 0.92
11.50 10.60 7.87
22.5 22.8 23.1
dripping dripping none
SEM analysis was carried out on HITACHI S4700 instrument in order to investigate the surface of residual char and the melting drip of the sample. Treated and untreated PET fabric samples were burnt in air and their residual char samples were used for SEM tests. The possible chemical structure treated and untreated fabric samples were characterized by Fourier transform infrared (FTIR) spectrometer (ThermoNicolet Nexus 670, USA), equipped with a variable-angle attenunated total reflection (ATR) accessory (PIKE ATRMax II) with ZnSe (n = 2.43) as internal reflection element wafer. The tensile strength of fabric samples were measured by a computerized universal testing machine (L R30 K PLUS, Lloyd, United Kingdom) at a speed of 50 mm/min according to GB/ T 3917.2-1997. All samples were tested five times, and the average values were used as the final data, with a variance of +5%. The washing durability test of the fabric samples after washing was tested according to AATCC test method 1242001.20
Table 2. The Flammability Testing Results of Samples Treated with HEDP, AMS and PER samples
HEDP/AMS/PER mass ratioa
afterflame (S)
damage length (cm)
LOI (%)
dripping
9 10 11 12 13 14 15 16
4:1:0 4:2:0 4:3:0 4:4:0 0:0:1 0:3:1 4:0:1 4:3:1
2.8 1.5 0.0 0.0 40 14.4 3.25 0
6.83 6.07 5.15 5.87 11.8 9.8 9.8 6.6
26.1 27.5 28.1 27.8 24.1 27.3 27.3 28.1
none none none none none none none none
a
The concentration of the solution is 20%
The antidripping phenomenon has been observed during the burning process of all samples treated with HEDP and AMS. Flame spreads on the top or bottom of the treated PET sample in the initial stage of burning; a char zone is formed afterward on the sample surface which finally prevents the flame from spreading and restricts the fuel transmitting from the substrate. 3.2. The Thermal Behavior. The TG curves of HEDP treated samples are presented in Figure 1. It shows HEDP itself
3. RESULTS AND DISCUSSION 3.1. The Flammability. The PET fabric was treated with HEDP and AMS separately. The LOI and vertical flammability test results of samples treated with different concentrations of HEDP and AMS are presented in Table 1. It can be seen that LOI increases with the HEDP concentration. LOI reaches its highest value of 25.7% when the concentration of HEDP is 25%. The increase of LOI value for the sample treated only with AMS is minimal, while the vertical flammability can be gradually improved when the concentration of AMS increases. This suggests that AMS acts as a gas source which can release inert gas and absorb heat during the combustion, and hence can reduce the burning rate of PET fabric and reduce the dripping tendency. Melt dripping has not been observed during the burning process for the samples treated with 20% solution, 25% HEDP solution, and 15% AMS solution respectively. The PET fabric was also treated with FR solution containing HEDP, AMS, and/or PER and the flammability result is shown in Table 2. The fire performance of samples 9−12 was significantly improved compared with samples 1 to 8. The addition of AMS can not only improve LOI value, but also reduce after-flame time and damaged length. It is suggested that there exist synergic effects between HEDP and AMS in improving char formation and hence flame retardancy, which will be discussed in section 3.5. The flame retardancy of samples containing only PER which is usually used as a char source has not been significantly improved. It is suggested that PET itself could not be used alone in FR finishing of PET fabric.
Figure 1. TG curves of the untreated PET fabric and samples treated with HEDP.
has a much earlier starting decomposition temperature of 150 °C than the PET samples. The untreated PET fabric starts to decompose at 380 °C. There are two mass-loss stages which contain a major mass-loss peak at around 415 °C and a minor mass-loss peak at around 530 °C. The PET fiber starts to decompose with the abruption of the polymer chain.21 The untreated PET fabric sample loses 96% of its original mass at 700 °C. The initial exothermic temperature of treated samples 14709
dx.doi.org/10.1021/ie301508q | Ind. Eng. Chem. Res. 2012, 51, 14708−14713
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decreases gradually with the increase of HEDP (Figure 1). The first mass-loss peaks of treated samples are bigger than that of the untreated sample, which is speculated due to the decomposition of the HEDP. The temperature range of the second step mass loss of treated sample is similar to the untreated sample, which may correspond to the decomposition of the PET chain. The residual char of the sample treated with HEDP is about 8.0% at 700 °C, which is higher than that of the untreated PET fabric (4.0%). It is suggested that HEDP can affect the initial exothermic temperature, decomposition rate, the first mass-loss stage, and the residual char formation of PET fabric. Figure 2 shows the TG curves of samples treated with AMS. It can be seen that the starting decomposition temperature of
Figure 3. TG curves of samples treated with different ratio of HEDP/ AMS.
Figure 2. TG curves of untreated PET fabric and samples treated with AMS.
the treated samples is much lower than that of the untreated sample. The AMS treated sample starts to decompose at 300 °C, while the untreated sample starts to decompose at around 380 °C. The maximal mass loss rate of treated samples is smaller than that of untreated sample. The first small mass loss peak may be caused by the lower decomposition temperature of AMS. However, the effect of AMS on improving char formation of PET fabric is much less than that of HEDP. Figure 3 shows the TGA curves of samples treated with both HEDP and AMS. It shows that the sample has an initial exothermic peak at 250 °C which is much lower than that of sample treated with HEDP or AMS, respectively (Figure 1 and 2). Meanwhile, the char residue of sample 12 is more than double compared with that of untreated sample. So the presence of both HEDP and AMS can enhance the char formation effectively. Figure 4 shows the TGA curves of samples treated with HDPE/AMS and HDPE/AMS/PER solution. When PER is added to the FR system, the initial exothermic temperature (sample 16) is lower than that of sample without PER (sample 11 in Figure 4). There is a minor weight-loss stage at 260 °C, which might be due to the dehydration of pentaerythritol.22 Table 3 gives the selected TGA data of PET and treated samples. It can be seen that all treated samples have an earlier decomposition temperature than pure PET. The char residue of samples treated with both HEDP and AMS is much higher than that of the samples treated with only one component. The theoretical char of sample 11 is 7.8% which is calculated
Figure 4. TG curves of untreated PET fabric and samples treated with different ratio of HEDP/AMS/PER.
Table 3. The Selected TGA Data of PET and Treated Samples samples
HEDP/ AMS/PER mass ratio
concentration %
PET 5 6 11 16
1:0:0 0:1:0 4:3:0 4:3:1
10 5 20 20
starting decomposition temperature, °C char residue, % 380 320 300 260 200
2.1 3.6 2.2 10.1 10.2
according to the method in our previous paper,23 and it is less than the experimental char residue of 10.1. This indicates that HEDP and AMS have synergic effects in enhancing the char formation despite that they can lower the starting decomposition temperature of PET fabric. It is suggested that the lower bond energy of P−C and P−O in HEDP than that of C− C and C−O in PET chain is responsible for the earlier decomposition of samples treated with FR solution containing HEDP. The degraded products can prevent flame from spreading both in the gaseous phase and condensed phase. The decomposition temperature ranges of treated samples have become much wider than that of untreated sample. It shows that the presence of HEDP and AMS can enhance the thermal 14710
dx.doi.org/10.1021/ie301508q | Ind. Eng. Chem. Res. 2012, 51, 14708−14713
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Figure 5. SEM photographs of residue of PET fabric samples: (A) control sample; (B) sample 4 treated with 20% HEDP; (C) sample 8 treated with 15% AMS; (D) sample 11 treated with HEDP and AMS; (E) sample 16 treated with HEDP, AMS and PER.
stability of PET fiber at a high temperature range (above 450 °C), which icontributes to formation of stable char layer. Comparing the LOI values in Table 1 and 2 with the char residue in Table 3, we can see the strong correlation between them. Samples treated with only one component (HEDP or AMS) have much lower LOI values than samples treated with a combination of HEDP and AMS, and the former samples also have less char residue than the latter samples. 3.3. Char Residue Morphology. Figure 5 is SEM photographs of the residue of treated and untreated PET fabric samples. In Figure 5A, a smooth and flat surface of pure PET can be observed, which is because the residue is composed of melted and partially decomposed PET. Figure 5B indicates that residual char of sample treated with HEDP has more microconvexities with little uneven holes on the surface, which might be caused by the HEDP promotion to char formation and gas release. The residual char of the sample treated with
AMS displayed in Figure 5C has an even frothy structure with a relatively flat and smooth surface. It is proposed that AMS decomposes and releases gas which finally can impel melt substance to form a porous residue structure. Figure 5D shows that residual char of a sample treated with both HEDP and AMS is continuous and thick with a many-layered deposit in which many big and deep holes are distributed. In Figure 5E, residual char of sample treated with HEDP, AMS, and PER is frothy and swollen. Intumescent char structures have been observed in Figure5D,E. 3.4. Flame Retardant Mechanism. Figure 6 shows the effect of flame retardant concentration on the LOI value of treated PET fabric. The increased LOI value of PET fabric treated with HEDP or AMS can be calculated by the following formula I from Figure 6. ΔLOI = LOI − LOI 0 14711
(I)
dx.doi.org/10.1021/ie301508q | Ind. Eng. Chem. Res. 2012, 51, 14708−14713
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Figure 6. Effect of concentration of HEDP or AMS on the LOI value of treated PET fabric.
Figure 7. Curves of the increased LOI value vs the FR mass ratio.
where ΔLOI is the increased LOI value, LOI is the LOI value of PET fabric treated with FR and LOI0 is the LOI value of untreated PET fabric Table 4 shows the calculated results of increased LOI value. It can be seen that the LOI value of HEDP treated sample is 1.5 Table 4. The Calculated Results of Increased LOI Value HEDP
AMS
mass ratio HEDP/ AMS
concentration (%)
increased LOI value (%)
concentration (%)
increased LOI value (%)
4:1 4:2 4:3 4:4
16 13 11 10
4.0 3.8 3.3 2.8
4 7 9 10
0.9 1.2 1.3 1.3
higher than that of AMS treated sample at the concentration of 10%. HEDP could enhance the formation of char which is more effective in preventing heat and fuel transfer than the just a single porous structure induced by AMS (see Figures 1−5). For the PET fabric treated with HEDP, the ΔLOI is gradually increased with the concentration from 10% to 16%. The significant increase trend levels off at the concentration of 16%. For the PET fabric treated with AMS, the ΔLOI is relatively low. The theoretical increased LOI values (ΔLOIT) of PET fabric treated with both HEDP and AMS should be the overlapping of the increased LOI value of HEDP treated sample and the increased LOI value of AMS treated sample, and therefore, it can be calculated by the following formula: ΔLOI T = ΔLOIHEDP + ΔLOIAMS
Figure 8. FTIR curves of the residue chars of original and modified PET fabrics.
corresponds to the stretching vibration of CC bonds. It is suggested that HEDP dephosphorylates at high temperature and catalyzes the formation of substances with CC bonds in the residue char. Our recent research has demonstrated that ammonium sulfamate decomposes into NH3, SO2, and SO3 when the temperature is above 360 °C (unpublished results), so the effect of ammonium sulfamate cannot be observed in terms of FTIR curves of residue char. It is proposed that the flame retardant system take effect through the follows process: (1) Catalytic dehydration and endothermic effect. HEDP decomposes and then releases phosphoric acid which can esterifies the polymer containing hydroxyl. Then dehydration of polyhydric alcohol phosphate ester occurs and the water vapor is released by esterification and the nonflammable gas emitted by AMS at appropriate amount. (2) Intumescent char formation in solid phase. HEDP acts as the main acid source, AMS acts as the gas source, PET and HEDP act as the char source. The treated sample can form intumescent char layer during combustion, which can improve the dripping resistance. (3) Flame retardant effects in gas phase. AMS decomposes and releases a large amount of nonignitable gas which can dilute oxygen and volatile flammable gas. Furthermore, some gases, such as ammonia and nitrogen, could trap the free radicals during combustion.
(II)
The theoretical overlapped increased LOI values (ΔLOIT) can be compared with the experimental increased LOI values (ΔLOI) (see Figure 7). ΔLOIT is 0.3 higher than ΔLOI at the ratio of 4:1; however, the ΔLOI is rapidly increased and reaches up to 6.6 which is much higher than ΔLOIT at the ratio of 4:3. This further demonstrates that a synergism between HEDP and AMS does exist. FTIR analysis on the residue chars of original and modified PET fabrics has been undertaken, and the results are shown in Figure 8. Compared with original PET fabric sample (curve A), an additional absorption peak at around 2231 cm−1 was observed in the FR treated sample (curve B), which 14712
dx.doi.org/10.1021/ie301508q | Ind. Eng. Chem. Res. 2012, 51, 14708−14713
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(4) Kim, Y. H.; Jang, J.; Song, K. G.; Lee, E. S.; Ko, S. W. Durable flame-retardant treatment of polyethylene terephthalate (PET) and PET/cotton blend using dichlorotribromophenyl phosphate as new flame retardant for polyester. J. Appl. Polym. Sci. 2001, 81, 793−799. (5) Burnell, M. R. U.S. Patent 2,953,480; Sept 20, 1960. (6) Nemes J. J., Polansky R, Herbes W. F. U.S. Patent 3,308,089; Mar 7, 1967. (7) Stoddard, J. W.; Pickett, O. A.; Cicero, C. J. Flame retarded nylon carpets. Text. Res. J. 1975, 45 (6), 474−483. (8) Horrocks, A. R.; Zhang, S. Char formation in polyamides (Nylons 6 and 6.6) and wool keratin phosphorylated by polyol phosphoryl chlorides. Text. Res. J. 2004, 74 (5), 433−441. (9) Horrocks, A. R.; Kandola, B.; Davies, P. J.; Zhang, S. Developments in flame retardant textiles: Q review. Polym. Degrad. Stab. 2005, 88 (1), 3−12. (10) McNeill, W. C.; Drews, M. J.; Barker, R. H. J. Fire Retard. Chem. 1977, 4, 222. (11) Einsele, U. Melliand Textilber Int. 1976, 57, 64. (12) Liepins, R.; Surless, J. R.; Morosoff, N.; Stannet, V. T. Localized radiation grafting of flame retardants to poly(ethylene terephthalate). I. Bromine-containing monomers. J. Appl. Polym. Sci. 1977, 21, 2529. (13) Annakutty, K. S.; Kishore, K A novel approach to structure flammability correlation in polyphosphate esters. Polymer 1988, 29 (7), 1273−1276. (14) Annakutty, K. S.; Mallick, I. M. Condensation polymers of bisphenol A with alkyl phosphorodichloridates: Synthesis,characterization and thermal studies. Polymer 1988, 29, 762−764. (15) Kannan, P.; Kishoret, G.; Kishoret, K. Novel photo-crosslinkable flame retardant polyvanillylidene arylphosphate esters. Polymer 1997, 38, 4349. (16) Wu, W. D.; Yang, C. Q. Statistical analysis of the performance of the flame retardant finishing system consisting of a hydroxyfunctional organophosphorus oligomer and the mixture of DMDHEU and melamine formaldehyde resin. Polym. Degrad. Stab. 2004, 85 (1), 623− 632. (17) Hui, Y.; Yang, C. Q. Durable flame retardant finishing of the nylon/cotton blend fabric using a hydroxyl-functional organophosphorus oligomer. Polym. Degrad. Stab. 2005, 88 (3), 363−370. (18) Ou, Y. X.; Han, T. J.; Zhao, Y. Application of intumescent flame retardants in fibers and textiles. China Synth. Fiber Ind. 2006, 29 (4), 46−58. (19) Li, L.; Chen, G.; Liu, W.; Li, J.; Zhang, S. The anti-dripping intumescent flame retardant finishing for nylon-6,6 fabric. Polym. Degrad. Stab. 2009, 94, 996−1000. (20) AATCC test method 124e2001; American Association of Textile Chemists and Colorists: Research Triangle Park, NC, 2001. (21) Levchik, S. V.; Weil, E. D. A review on thermal decomposition and combustion of thermoplastic polyesters. Polym. Adv. Technol. 2004, 15, 691−700. (22) Lv, P.; Wang, Z. Z.; Hu, K. L.; Fan, W. C. Flammability and thermal degradation of flame retarded polypropylene composites containing melamine phosphate and pentaerythritol derivatives. Polym. Degrad. Stab. 2005, 90, 523−534. (23) Yu, L.; Zhang, S.; Liu, W.; Zhu, X.; Chen. X.; Chen, X. Improving the flame retardancy of PET fabric by photo-induced grafting. Polym. Degrad. Stab. 2010, 95, 1934−1942
3.5. Tensile Strength and Washing Durability. Table 5 shows the tensile strength and washing durability results of Table 5. The Tensile Strength and Washing Durability of PET Samples samples tensile strength (N) LOI
untreated
pretreateda
FR treatedb
after washing
15.3
14.5(±0.5%)
13.9 (±0.5%)
14.0 (±0.5%)
28.1 (±0.5)
23.2 (±0.5)
a
Pretreated: PET fabric samples were washed in a sodium hydroxide aqueous solution (30 g/L) at 100 °C for 20 min. bFR treated: PET fabric samples were treated with 20% HEDP and AMS solution (HEDP/AMS = 4:3).
PET fabric sample 11 treated with 20% HDEP and AMS solution. It can be seen that the pretreatment can cause a reduction of 5.2% to the tensile strength of PET fabric and then FR treatment can cause a further reduction of 9.1% to the tensile strength. It is suggested there is no serious damage to the strength while the PET fabric is treated with the FR system containing HADS and AMS. The LOI value dropped to 23.2 from 28.1 after the sample was washed in boiled 1% detergent water solution for 30 min, indicating the bonding forces between the FR and fabric are mainly physical forces rather than chemical bonding forces.
4. CONCLUSIONS The presence of both HEDP and AMS can improve flame retardancy and reduce dripping tendency of PET fabric. The highest LOI can reach up to 28.1. The after-flame time is 0 s. Treated fabric samples can form intumescent char structure and release less heat than untreated fabric during combustion. However, the padding/curing process could cause a tensile strength reduction of around 10% for the treated samples and the durability is not ideal because of the physical bonding forces between the FR and the fabric. It is suggested there exists a synergism between HEDP and AMS in improving both char formation and burning behavior. Further investigation is being undertaken to improve the washing durability of FR PET fabric in our laboratory.
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AUTHOR INFORMATION
Corresponding Author
*Tel.:+86(10)64436820. Fax: +86(10)64436820. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science of China (Grant No. 20974009/B0402 and 21061130552/B040607).
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REFERENCES
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