Cotton Blend Fabrics Using a

The treated fabrics showed high levels of flame retardant performance ... reduces hydrolysis resistance of the HFPO on the nylon/cotton blend fabrics...
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Ind. Eng. Chem. Res. 2008, 47, 2160-2165

Flame Retardant Finishing of Nylon/Cotton Blend Fabrics Using a Hydroxy-Functional Organophosphorus Oligomer Hui Yang and Charles Q. Yang* Department of Textiles, Merchandising and Interiors, The UniVersity of Georgia, Athens, Georgia 30602

In previous research, we found that dimethyloldihydroxyethyleneurea (DMDHEU) is able to covalently bond hydroxy-functional organophosphorus oligomer (HFPO) to nylon and to cotton by formation of a crosslinked polymeric network. In this research, we applied HFPO to two nylon/cotton blend military fabrics using DMDHEU as the bonding agent. The treated fabrics showed high levels of flame retardant performance and excellent laundering durability. The treated fabrics passed the vertical flammability test after 40-50 laundering cycles. The DMDHEU concentration used in a finishing solution plays a critical role in determining the flame retardant performance and physical properties of the treated fabric. Increasing the HFPO/DMDHEU ratio increases phosphorus concentration after one laundering and reduces the fabric stiffness, but it also reduces hydrolysis resistance of the HFPO on the nylon/cotton blend fabrics. We also found that the fabric treated with HFPO and DMDHEU suffers little tensile strength loss. The pH of a finishing solution has no significant effect on the flame retardant performance and tensile strength retention of the treated nylon/cotton blend fabric in the pH range from 2 to 6. Introduction

Scheme 1. Structure of HFPO

Durable flame retardancy is a desirable property for nylon/ cotton blend fabric when it is used for protective clothing.1-3 However, durable flame retardant finished nylon/cotton blend is not available commercially, and limited research has been reported on this topic.4-7 No practical technology has been commercially successful for the durable flame retardant finishing of nylon/cotton blend fabrics. In our previous research, we developed a durable flame retardant finishing system based on a hydroxy-functional organophosphorus oligomer (HFPO) shown in Scheme 1 for cotton and cotton blend fabrics.8-18 The bonding agent includes dimethyloldihydroxyethyleneurea (DMDHEU), trimethylolmelamine (TMM), and 1,2,3,4-butanetetracarboxylic acid (BTCA). We compared the performance of DMDHEU and TMM as the bonding agents for HFPO on cotton.13,14 Since TMM contains more synergistic nitrogen than DMDHEU, the cotton treated with HFPO/TMM has higher flame retardant performance than that treated with HFPO/DMDHEU. However, the cotton treated with HFPO/DMDHEU showed higher laundering durability than that treated with HFPO/TMM.13 We treated the 50/50 nylon/cotton blend battle dress uniform (BDU) fabric using HFPO and a mixture of DMDHEU and TMM as the bonding agents. The treated fabric showed high levels of flame retardant performance and laundering durability.18 The fabric thus treated also showed significantly increased stiffness. Our study also showed that both DMDHEU and TMM are able to bond HFPO to the nylon fabric by the formation of a cross-linked polymeric network shown in Scheme 2, and DMDHEU is more effective as a bonding agent for HFPO to nylon than TMM.19 The formation of the cross-linked HFPO/ DMDHEU polymeric network improves the laundering durability of HFPO on the blend.19 In this research, we evaluated the flame retardant performance and physical properties of the nylon/cotton blend fabric treated using HFPO and DMDHEU. We also studied the effects of * To whom correspondence should be addressed. Tel.: (706) 5424912. Fax: (706) 542-4890. E-mail: [email protected].

finishing solution pH on the flame retardant performance of the treated fabric. Experimental Section Materials. Two fabrics were used in this study: (1) a 50%/ 50% nylon/cotton BDU pure finish ripstop fabric printed with three-color “day desert” camouflage weighing 216 g/m2 (military specification: MIL-C-44031 CL1); (2) a 50%/50% nylon/cotton BDU pure finish twill fabric with three-color “woodland” camouflage weighing 220 g/m2 (military specification: MILC-44436 CL3), both supplied by Bradford Dyeing Association, Bradford, RI. HFPO with the commercial name of Fyroltex HP (also known as Fyrol 51, CAS Registry No. 70715-06-9) was supplied by Akzo Nobel Phosphorus Chemical Division, Dobbs Ferry, NY. DMDHEU was a commercial product (44% solid content) under the trade name of Freerez 900 supplied by Noveon, Cleveland, OH. The catalyst was an NH4Cl-based commercial product under the trade name of Catalyst RD supplied by Eastern Color & Chemical, Greenville, SC. Fabric Treatment and Laundering Procedure. The fabric was first immersed in a solution and passed through a laboratory padder with two dips and two nips. The nylon/cotton blend fabric thus treated was then dried at 90 °C for 3 min and finally cured in a Mathis curing oven at 165 °C for 2 min. All concentrations presented here were based on weight (w/w, %), and concentrations of HFPO and DMDHEU in all solutions were based on the solid of the chemicals in those commercial products. The wet pick-up of the nylon/cotton blend fabric was 77 ( 2%. After curing, the treated fabric was subjected to a specified number of laundering cycles using the reference detergent AATCC Standard Detergent 1993. The laundering process was done according to AATCC Test Method 124-1996, and the water temperature of laundering was approximately 46 °C.

10.1021/ie071146a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008

Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2161 Scheme 2. Cross-Linked Polymeric Network of HFPO/DMDHEU

Evaluation of the Fabric Flame Retarding Performance. The vertical flammability of the fabric was measured according to ASTM Standard Method D6413-99. The limiting oxygen index (LOI) of the fabric was measured according to ASTM Standard Method D2863-97. Evaluation of the Fabric Stiffness and Strength. The fabric stiffness was measured according to ASTM Standard Method D6828-02 using a Handle-O-Meter tester (Model 211-300) manufactured by Thwing-Albert Instrument Company, Philadelphia, PA. The slot width used in this study was 5 mm, and the beam size used in this study was 1000 g. The fabric stiffness presented in this study was the mean of total stiffness obtained of five specimens. The tensile strength of the fabric was measured according to ASTM Standard Method D5035-95. The tensile strength retention is calculated by dividing the tensile strength of the treated fabric by that of the untreated control fabric. Determination of Phosphorus Concentration of the Treated Fabric. Approximately 2 g of the treated fabric sample taken from three different parts of a “30 cm × 25 cm” fabric specimen was ground in a Wiley mill into powder to improve sample uniformity. A 2 mL volume of concentrated H2SO4 was added to 0.1 g of the powder in a beaker. Then 10 mL of 30% H2O2 was added dropwise to the mixture, allowing the reaction to subside between drops. The reaction mixture was then heated at approximately 250 °C to digest the powder and to evaporate the water until dense SO3 vapor was produced. The completely digested sample as a clear solution was transferred to a 50 mL volumetric flask and then diluted with distilled/deionized water. The sample thus prepared was analyzed with a Thermo-FarrellAsh (Model 965) inductively coupled plasma atomic emission spectrometer (ICP/AES) to determine the phosphorus concentration. The percent phosphorus retention is calculated by dividing the phosphorus concentration of the fabric after laundering by that of the fabric before laundering.

Table 1. Phosphorus Concentration of the Nylon/Cotton Fabric (Woodland) Treated with HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 mina phosphorus concentration (%) HFPO (%)

DMDHEU (%)

after 1 laundering

after 20 launderings

after 40 launderings

32 32 32 32 32 32

1 2 4 6 8 10

0.56 1.34 1.81 2.47 2.59 2.69

0.30 0.52 0.72 1.79 2.00 2.18

0.25 0.42 0.64 1.01 1.13 1.84

a The phosphorus concentration of the treated fabric before wash is 3.43%.

use 32% HFPO in the formula. The phosphorus concentration of the fabric thus treated and subjected to different numbers of laundering cycles is shown in Table 1. The phosphorus retention of the treated fabric is presented against the DMDHEU concentration in Figure 1. When DMDHEU increases from 1 to 10%, the phosphorus concentration of the treated fabric after one laundering increases from 0.56% (16% phosphorus retention) to 2.69% (78% phosphorus retention). The percent phosphorus retention after one laundering is also called “percent phosphorus fixation” by the industry. A higher percent phosphorus fixation is a qualitative indicator of higher relative quantity of the flame retarding organic phosphorus agent bound to the fabric after curing. A similar trend is observed on the

Results and Discussion The nylon/cotton fabric (woodland) is treated with 32% HFPO and DMDHEU at different concentrations and cured at 165 °C for 2 min. In our previous research, we found that 40% HFPO was needed for the flame retardant finishing of the nylon/cotton BDU fabric to maintain ∼1% phosphorus on the fabric after 40 launderings when TMM was used as a bonding agent.18 Since DMDHEU is a more effective bonding agent for cotton,10 we

Figure 1. Percent phosphorus retention of the nylon/cotton fabric (woodland) treated with 32% HFPO and DMDHEU at different concentrations and cured at 165 °C for 2 min.

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Table 2. LOI (%) of the Nylon/Cotton Fabric (Woodland) Treated with HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 mina LOI (%) HFPO DMDHEU after 1 after 10 after 20 after 40 (%) (%) laundering launderings launderings launderings 32 32 32 32 32 32 a

1 2 4 6 8 10

22.9 25.0 26.5 27.7 27.9 28.0

22.2 23.2 25.1 27.1 27.5 28.0

21.6 22.5 23.0 26.8 27.2 27.4

21.2 22.2 22.7 24.8 25.8 27.0

The LOI (%) of the untreated fabric is 20.1.

Table 3. Vertical Flammability of the Nylon/Cotton Fabric (Woodland) Treated with HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 mina

Figure 2. Percent phosphorus retention of the nylon/cotton fabric (desert) treated with 32% HFPO and DMDHEU at different concentrations and cured at 165 °C for 2 min.

char length (mm) HFPO DMDHEU after 1 after 10 after 20 after 40 (%) (%) laundering launderings launderings launderings 32 32 32 32 32 32 a

1 2 4 6 8 10

>300 77 80 77 79 49

>300 >300 94 99 66 62

>300 >300 >300 88 83 68

>300 >300 >300 114 105 81

Table 5. LOI of the Nylon/Cotton Fabric (Desert) Treated with 32% HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 mina LOI (%) DMDHEU (%)

after 1 laundering

after 25 launderings

after 40 launderings

after 50 launderings

6 8 10

28.0 28.4 28.5

23.8 26.1 27.1

23.1 24.4 25.5

22.5 23.2 24.8

The char length of the untreated fabric is >300 mm.

Table 4. Phosphorus Concentration of the Nylon/Cotton Fabric (Desert) Treated with 32% HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 min phosphorus concentration (%) DMDHEU (%)

after 1 laundering

after 25 launderings

after 50 launderings

6 8 10

2.11 2.50 2.69

0.71 1.28 1.68

0.50 0.66 0.96

treated fabric after 20 and 40 launderings (Table 1 and Figure 1). The phosphorus concentration and phosphorus retention decrease as the number of laundering cycles increases due to the hydrolysis of the HFPO bound to the fabric. It is noticed that the fabric treated with 32% HFPO and 10% DMDHEU after 40 laundering cycles still retains 1.84% phosphorus (54% phosphorus retention). The LOI and vertical flammability of the fabric thus treated are shown in Tables 2 and 3, respectively. As the DMDHEU concentration is increased from 1 to 10%, the LOI of the treated fabric after one laundering cycle increases from 22.9 to 28.0%. The LOI decreases as the number of laundering cycles for the fabric increases. The fabric treated using 1% DMDHEU fails the vertical flammability test after one laundering cycle, that treated using 2% DMDHEU also fails the test after 10 launderings, and that treated using 6% or higher DMDHEU concentrations passes the test after 40 launderings (Table 3). The LOI of the fabric treated with 32% HFPO and 10% DMDHEU (after 40 laundering cycles) reaches 27.0% with a char length of 81 mm. The data presented here clearly show that DMDHEU concentration plays a critical role in determining the flame retardant performance of the nylon/cotton blend fabric treated with HFPO/DMDHEU. This is because a higher DMDHEU concentration increases the amount of HFPO bound to the treated fabric and it also improves the hydrolysis resistance of the HFPO bound onto the treated fabric by forming a crosslinked HFPO/DMDHEU polymeric network.19 This fact is understandable because the bonding of HFPO to both nylon and cotton depends on the reactions of DMDHEU as discussed

a

The LOI (%) of the untreated fabric is 20.1.

previously.10,12,19 An increase in DMDHEU concentration improves the flame retardancy and the laundering durability of the treated fabric. In addition, DMDHEU also functions as a synergistic nitrogen provider for the flame retardant finishing system. In our previous research, we have discovered that the nitrogen of DMDHEU does have a synergistic effect for the HFPO-based flame retardant system on cotton,12 but this effect becomes less predominant on the nylon/cotton blends. We applied the same flame retardant finishing system to the nylon/cotton BDU fabric (“desert”) with the same chemical composition but different structure. The fabric treated with 32% HFPO and DMDHEU at different concentrations is subjected to different numbers of laundering cycles. The phosphorus concentration of the nylon/cotton fabric thus treated is shown in Table 4. The percent phosphorus retention of the nylon/cotton fabric (desert) thus treated is presented against the DMDHEU concentration in Figure 2. When the DMDHEU concentration is increased from 6 to 10%, the phosphorus concentration of the treated fabric after one laundering cycle increases from 2.11% (61% retention) to 2.69% (78% retention). After 50 launderings, the fabric treated with 32% HFPO and 10% DMDHEU retains 0.96% phosphorus (28% retention), which is almost twice of that of the fabric treated with 32% HFPO and 6% DMDHEU (0.50% phosphorus). The data presented here demonstrate that the DMDHEU concentration notably affects the phosphorus retention and also improves the hydrolysis resistance of the HFPO bound to the fabric. One can also observe that woodland fabric has higher phosphorus retention than the desert one under the same conditions. The LOI (%) and vertical flammability of the nylon/cotton blend fabric (desert) thus treated are presented in Tables 5 and 6, respectively. After one laundering cycle, the LOI of the treated fabric increases from 28.0 to 28.5% as the DMDHEU concentration is increased from 6 to 10%, respectively, and all three fabric samples pass the vertical flammability test. As the number of laundering cycles increases, the difference among the fabric

Ind. Eng. Chem. Res., Vol. 47, No. 7, 2008 2163 Table 6. Vertical Flammability of the Nylon/Cotton Fabric (Desert) Treated with 32% HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 mina

Table 8. LOI of the Nylon/Cotton Blend Fabric (Desert) Treated with 8% DMDHEU and HFPO at Different Concentrations and Cured at 165 °C for 2 min

char Length (mm)

LOI (%)

DMDHEU (%)

after 1 laundering

after 25 launderings

after 40 launderings

after 50 launderings

HFPO (%)

after 1 laundering

after 25 launderings

after 40 launderings

6 8 10

68 74 53

>300 94 81

>300 103 92

>300 >300 92

24 32 40

27.8 28.4 28.4

26.6 26.1 26.0

24.7 24.4 24.1

a

The char length of the untreated fabric is >300 mm.

Table 7. Phosphorus Concentration of the Nylon/Cotton Blend Fabric (Desert) Treated with 8% DMDHEU and HFPO at Different Concentrations and Cured at 165 °C for 2 min phosphorus concentration (%) HFPO (%)

after 1 laundering

after 25 launderings

after 40 launderings

24 32 40

1.90 2.50 3.14

1.34 1.28 0.81

0.81 0.66 0.54

samples treated using different DMDHEU concentrations becomes more evident. After 25 launderings, the fabric treated with 6% DMDHEU has LOI of 23.8% and fails the vertical flammability test, whereas that treated with 8% DMDHEU has LOI of 26.1% and passes the flammability test. The fabric treated with 10% DMDHEU has LOI of 24.8% and passes the vertical flammability test after 50 launderings. The data presented here again demonstrate that DMDHEU plays a decisive role in determining the flame retardant performance of the nylon/cotton blend fabric treated with HFPO and DMDHEU. We also studied the effects of HFPO concentration on the performance of the treated fabric. The nylon/cotton fabric (desert) is treated with 8% DMDHEU and HFPO at different concentrations, cured at 165 °C for 2 min, and finally subjected to different numbers of laundering cycles. The phosphorus concentration and percent phosphorus retention are shown in Table 7 and Figure 3, respectively. When the HFPO concentration is increased from 24 to 40%, the phosphorus concentration of the treated fabric after one laundering cycle also increases from 1.90 to 3.14%, but the percent phosphorus retention changes little (Figure 3). The effect of HFPO concentration becomes obvious when the number of laundering cycles increases (Table 7). After 25 launderings, the fabric treated with 24% HFPO still retains 1.34% phosphorus (53% retention), which is significantly higher than that of the fabric treated with

Figure 3. Percent phosphorus retention of the nylon/cotton blend fabric (desert) treated with 8% DMDHEU and HFPO at different concentrations and cured at 165 °C for 2 min.

Table 9. Vertical Flammability Test of the Nylon/Cotton Blend Fabric (Desert) Treated with 8% DMDHEU and HFPO at Different Concentrations and Cured at 165 °C for 2 min char length (mm) HFPO (%)

after 1 laundering

after 25 launderings

after 40 launderings

24 32 40

75 74 48

96 94 203

98 103 >300

Table 10. Tensile Strength of the Nylon/Cotton Fabric (Woodland) Treated with 32% HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 min (After 1 Laundering Cycle) tensile strength (N)

tensile strength retention (%)

DMDHEU (%)

warp

filling

warp

filling

2 4 6 8 control

703 694 685 701 721

454 449 445 451 458

98 96 95 97

99 98 97 98

40% HFPO (0.81% phosphorus and 19% phosphorus retention). The data presented here demonstrate that, at a constant DMDHEU concentration, increasing HFPO concentration results in higher initial phosphorus concentration on the fabric, but lower hydrolysis resistance of the HFPO bound to the nylon/cotton blend fabric after multiple launderings. The LOI and char length of the fabric thus treated are shown in Tables 8 and 9, respectively. When HFPO concentration is increased from 24 to 40%, the LOI of the treated fabric after one laundering cycle increases from 27.8 to 28.4% due to higher phosphorus concentration on the fabric as shown in Table 7. However, the LOI of the treated fabric after 25 laundering cycles decreases from 26.6 to 26.0%. As we discussed previously, formation of the cross-linked polymeric network requires the HFPO/DMDHEU ratio in a range in which the number of hydroxyl groups of HFPO is close to that of the methylol groups of DMDHEU. The formation of the HFPO/DMDHEU polymeric network, which is discussed in detail in a more recent paper, improves the laundering durability of the HFPO bound to both cotton and nylon.19 The formation of a similar crosslinked HFPO/TMM polymeric network on cotton was discussed previously.10 At a constant DMDHEU concentration, an extremely high HFPO concentration in a formula indicates an extremely high number of hydroxyl groups relative to the quantity of DMDHEU, which favors DMDHEU functioning as a bridge between HFPO and cotton cellulose and reduces the formation of HFPO/DMDHEU polymeric network, thus reducing the laundering durability of the HFPO on the blends. The nylon/cotton fabric (woodland) is treated with 32% HFPO and DMDHEU at different concentrations and subjected to one laundering cycle. The tensile strength of the fabric thus treated after one laundering cycle is shown in Table 10. When DMDHEU concentration increases from 2 to 8%, the tensile strength in the warp direction is in the range from 703 N (98%

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Table 11. Tensile Strength of the Nylon/Cotton Fabric (Desert) Treated with 32% HFPO and DMDHEU at Different Concentrations and Cured at 165 °C for 2 min (After 1 Laundering Cycle) tensile strength (N)

tensile strength retention (%)

DMDHEU (%)

warp

filling

warp

filling

6 8 10 control

604 599 599 635

416 415 408 416

95 94 94

100 99 98

retention) to 685 N (95% retention), respectively. The tensile strength in the filling direction is in the range from 445 N (97% retention) to 454 N (99% retention). The data presented in Table 10 demonstrate that the fabric treated with HFPO and DMDHEU has negligible tensile strength loss. The nylon/cotton fabric (desert) was treated with 32% HFPO and DMDHEU at different concentrations, and cured at 165 °C for 2 min. Table 11 shows the tensile strength and tensile strength retention of the fabric thus treated after one laundering cycle. When DMDHEU concentration is increased from 6 to 10%, the tensile strength retention is 94-95% at the warp direction and 98-100% at the filling direction (Table 11). The data presented here again indicate that increasing DMDHEU concentration causes almost no fabric strength loss. The nylon/cotton fabric (woodland) treated with 32% HFPO and DMDHEU at different concentrations was cured at 165 °C for 2 min and subjected to one and 10 laundering cycles. The stiffness of the fabric thus treated after one and 10 laundering cycles is shown in Figure 4. At 32% HFPO, increasing DMDHEU concentration from 1 to 10% causes an increase of the fabric stiffness from 483 to 1208 g (after one laundering cycle). One also can observe that the stiffness shows a steep increase as the DMDHEU concentration increases from 4 to 6% (Figure 4). After 10 laundering cycles, the fabric stiffness only increases from 479 to 685 g at the same DMDHEU concentration range (Figure 4). It is clear that DMDHEU concentration plays an important role in determining the fabric stiffness. The fact that the stiffness of the treated fabric increases with the DMDHEU concentration is attributed to the formation of a cross-linked polymeric network of HFPO/DMDHEU/cotton on the cotton and on the nylon of the blend fabric. The fabric stiffness due to higher DMDHEU concentrations decreases considerably when the number of laundering cycles increases (Figure 4). Multiple laundering significantly decreases the fabric stiffness, thus making the fabric stiffening as a result of the flame retardant finishing less of a problem. The nylon/cotton blend fabric (woodland) was treated with 6% DMDHEU and HFPO (4-40%). The stiffness of the fabric thus treated is shown in Figure 5. The fabric stiffness increases from 782 g at 4% HFPO to its maximum at 1280 g at 16% HFPO, and then decreases to 833 g at 40% HFPO. Figure 6 shows the stiffness of the nylon/cotton blend fabric (desert) treated with 6% DMDHEU and HFPO (4-40%). One observes a similar phenomenon on the fabric (desert). When HFPO concentration is increased from 4 to 40%, the fabric stiffness increases from 445 g to its maximum at 729 g, and then decreases to 488 g (Figure 6). The data demonstrate that the fabric stiffness is also affected by the HFPO concentration in the formula. The HFPO/DMDHEU ratio affects the formation of the cross-linked polymeric network, thus changing the fabric stiffness. This issue was discussed in more detail in another paper.19 The stiffness of the nylon/cotton fabric (desert) treated with 32% HFPO and DMDHEU at different concentrations after one

Figure 4. Stiffness of the nylon/cotton fabric (woodland) treated with 32% HFPO and DMDHEU at different concentrations and cured at 165 °C for 2 min, and finally subjected to one and 10 laundering cycles.

Figure 5. Stiffness of the nylon/cotton fabric (woodland) treated with 6% DMDHEU and HFPO at different concentrations and cured at 165 °C for 2 min, and finally subjected to one laundering cycle.

Figure 6. Stiffness of the nylon/cotton fabric (desert) treated with 8% DMDHEU and HFPO at different concentrations and cured at 165 °C for 2 min, and finally subjected to one laundering cycle.

laundering cycle is shown in Figure 7. The fabric stiffness increases from 224 g (the control) to 504 g for the fabric treated using 6% DMDHEU. It increases to 830 g as the DMDHEU concentration is increased to 10%. The absolute increase in the fabric stiffness as a result of high DMDHEU concentrations for this fabric (desert) shown in Figure 7 is considerably smaller than that on the previous fabric (woodland) (Figure 4). The lower stiffening of the treated desert fabric can be attributed to two factors. First, there is less formation of a polymeric crosslinked network on the fabric (desert) as demonstrated by lower phosphorus retention after multiple launderings. Second, the fabric stiffness also depends on the structure of the fabric. It should also be pointed out that stiffness of the treated fabrics

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the DMDHEU to HFPO ratio at a constant HFPO concentration increases the percent fixation of HFPO on the fabric and the laundering durability of the treated fabric. Both the HFPO and DMDHEU concentrations and their ratio also affect fabric stiffness. The fabric stiffness decreases drastically after multiple home launderings. An optimum formula can be developed based on the requirements of the end-use products. Literature Cited

Figure 7. Stiffness of the nylon/cotton fabric (desert) treated with 32% HFPO and DMDHEU at different concentrations cured at 165 °C for 2 min and subjected to one laundering cycle. Table 12. LOI, Phosphorus Concentration, and Percent Phosphorus Retention of the Nylon/Cotton Blend Fabric (Desert) Treated with 32% HFPO/8% DMDHEU at Different pH Levels and Cured at 165 °C for 2 min (After 1 Laundering Cycle)a filling

pH

LOI (%)

phosphorus concentration (%)

3.0 4.2a 5.0 6.0

28.2 28.4 28.4 28.3

2.40 2.46 2.34 2.24

tensile strength (N)

tensile strength retention (%)

409 416 405 414

98 100 97 99

a The original pH of the finishing solution containing 32% HFPO and 8% DMDHEU is 4.2.

could be reduced by the use of a fabric softener or softeners in the formulation. The nylon/cotton blend fabric was treated with 32% HFPO and 8% DMDHEU with the solution pH adjusted from 3.0 to 6.0 using either a 1 M NaOH or 1 M HCl solution, and then cured at 165 °C for 2 min. Table 12 shows the LOI (%), phosphorus concentration, tensile strength, and percent tensile strength retention of fabric thus treated after one laundering cycle. The LOI of the treated fabric after one laundering cycle is ∼28.4% independent of pH. The phosphorus concentration is also in a narrow range (2.24-2.50%). The tensile strength in the filling direction is in the range of 405 N (97% retention) to 416 N (100% retention). Thus, the data presented here indicate that pH of a finishing solution has no notable effect on the flame retardant performance or the tensile strength. Conclusions The combination of HFPO and DMDHEU is effective in imparting durable flame retardant performance to two 50/50 nylon/cotton blend BDU fabrics. The treatment causes almost no fabric strength loss and increases the fabric stiffness. The concentrations of HFPO and DMDHEU and the HFPO/ DMDHEU ratio in a finishing solution play critical roles in determining the performance of the treated nylon/cotton fabrics. An increase in the HFPO/DMDHEU ratio at a constant DMDHEU concentration increases the amount of HFPO bound to the fabric, but reduces the laundering durability. Increasing

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ReceiVed for reView August 21, 2007 ReVised manuscript receiVed November 23, 2007 Accepted December 24, 2007 IE071146A