Synthesis of 2,3-dibromo-succinic anhydride and application on cotton,polyester and polyester/cotton blended fabric

Abstract

For a poly(ethylene terephthalate) (PET)/cotton blended fabric, a reactive-type flame retardant 2, 3-dibromo-succinic anhydride (DBSA) was used to endow durable flame retardancy via the pad-dry-cure method. DBSA was synthesized via the addition reaction of maleic anhydride and bromine in ethyl acetate solution and extracted by solventing-out crystallization. DBSA was used to finish a cotton fabric firstly with sodium hypophosphite as the catalyst. Thermal behaviors and amount of DBSA that esterified with the cotton was determined. Pyrovatex CP new was applied as a comparison. It was speculated from the thermogravimetric analysis result that esterification of cellulose with DBSA worked similar to phosphate-ester at the initial stage of thermal decomposition. DBSA was also applied to PET and PET/cotton blended fabrics. Flame retardancy, thermal behaviors and durability of the finished fabrics were investigated. Evidence of the condensed phase effect of DBSA was also observed on PET and PET/cotton.

Keywords

2 3-dibromo-succinic anhydride polyester/cotton blended fabric brominated flame retardant finishing

Poly(ethylene terephthalate) (PET)/cotton blends are widely used due to their excellent physical and chemical properties and high performance with low cost. However, they are highly flammable in air. The flame retardant finishing of these blended fabrics, especially PET/cotton (65/35), is difficult because of the different burning performance of cotton and polyester fibers, causing the so-called “scaffolding effect”.^1–3 There were many studies to disclose the use of phosphorus-containing flame retardant agent to treat PET/cotton blended fabric, but few of those flame retardant finishing systems had been commercialized.^4–6 There were also many studies on the application of chemicals such as tetrabromobisphenol A (TBBPA), tris(2, 3-dibromopropyl)phosphate, dimethyl N-hydroxymethyl carbamoyl ethyl phosphonate and dichlorotribromophenyl phosphate, but they were restricted in many fields due to their toxicity.^7–10 Although great efforts have been made for several decades, no good practical solution has been developed so far and the research is continuing.^11–13

Brominated flame retardants (BFRs) are usually the cheapest chemicals to improve flame retardancy of polymers with low oxygen content. However, the environmental pollution and toxic effect of BFRs, especially the safety of polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD) and TBBPA, have been attracting great concern for the last two decades in many countries.^14–16 The acute toxicity of BFRs is low (LD-50 > 5g/kg for animals mostly), but toxic substances such as dibenzo-dioxins and dibenzo-furans will be produced during the burning process.¹⁷ The World Health Organization (WHO) makes great efforts to assess the safety of BFRs and several BFRs are added to the list of Substances of Very High Concern (SVHC) by REACH or restricted by RoHS Directive.¹⁸ Oeko-Tex Association also imposes restrictions on the use of HBCD, polybrominated biphenyls (PCBS) and PBDEs on textile products.¹⁹ However not all of the BFRs are banned from use and the evaluation of BFRs is also undertaken actively. The use of alternative flame retardants for PBDEs and HBCD is likely to increase. The US Environmental Protection Agency (EPA) suggested using butadiene styrene brominated copolymer to replace HBCD and using low polyphosphate-ester polyols to replace penta-PBDE.²⁰ Although the alternatives are under development, BFRs are still widely used in many products.

The type of flame retardant could be reactive or additive. The reactive flame retardants usually have reactive groups that can react with polymer and incorporate into the polymeric materials by covalent bonding, whereas the additive types are dissolved in the polymer.²¹ TBBPA, dibromoneopentyl glycol, tetrabromophthalic anhydride and brominated styrene are the most commonly used bromine-based reactive flame retardants.^22,23 Wang et al.²⁴ reported a synthesis process of 2, 3-dibromo-succinic anhydride (DBSA), which is water soluble, and pointed out that it could be used as a reactive flame retardant for polymers. However, there were few reports about the application of that.

In 1988, Welch²⁵ reported that 1, 2, 3, 4-butanetetracarboxylic acid (BTCA) was able to provide effective cross-linking for cotton cellulose by ester linkage and proposed that polycarboxylic acids esterify the cellulose through the formation of a five-membered cyclic anhydride as a reactive intermediate. Yang²⁶ and Yang et al.²⁷ investigated the cross-linking of cotton cellulose by esterification using different multifunctional carboxylic acids. According to their results, the dicarboxylic acids can only be grafted to cellulose.

DBSA is a five-membered cyclic anhydride. This study focused on the synthesis of DBSA and its graft to cotton via esterification and application of DBSA as a flame retardant for PET/cotton blended fabric. Most of the processing parameters, such as concentration, pH, temperature and time, have been examined, but only a part of them is discussed in this article. The amount of DBSA that esterified with cotton was determined by a titration method. The flammability of the fabrics was evaluated by the limited oxygen index (LOI) and vertical burning test. The flame retardant mechanism was investigated by thermogravimetric (TG) analysis.

Experimental details

Materials

Three types of fabrics were used in this study and the characteristics of the fabrics are given in Table 1.

Table 1.

The characteristics of the fabrics

Fabrics	Weave structure	Fabric weight /g/m²
100% Cotton	Plain weave	109
100% PET	Woven	115
50/50 Polyester /cotton	Woven	102

PET: poly(ethylene terephthalate).

Maleic anhydride, bromine, ethyl acetate, dimethyl benzene, methyl salicylate, sodium hypophosphite and phosphoric acid were obtained from Sinopharm Chemical Reagent Co., Ltd, China. Pyrovatex CP new (CP for short in this article), Lyofix CHN (CHN) were obtained from Ciba Specialty Chemicals (China) Ltd.

Synthesis of DBSA

Maleic anhydride (19.6 g) and ethyl acetate (80 mL) were added into a four-neck flask equipped with a mechanical stirrer, a thermometer, a dropping funnel and a reflux condenser. The mixture was stirred until the maleic anhydride completely dissolved. Then bromine (38.4 g) was added drop by drop into the above solution under stirring at 48℃ for half an hour and the mixture was stirred for 5 h under 48℃. The above procedure was similar to that reported by Wang et al.²⁴ While the reaction mixture turned to a colorless transparent solution, a mixture of dimethyl benzene and water was added as an anti-solvent followed by evaporation of ethyl acetate using a vacuum evaporator. Through filtrating and drying, the white product DBSA was obtained and the yield was above 85%.

Characterization of DBSA

The Fourier transform infrared (FT-IR) spectrum of the synthesized DBSA was conducted by a FT-IR spectrophotometer (FT-IR 640 Varian) with the potassium bromide pellet technique. The proton nuclear magnetic resonance (¹H-NMR) spectrum was obtained on a NMR spectrophotometer (Avance-400, Bruker, Switzerland) using deuterated acetone as the solvent. The melting point was measured by a differential scanning calorimeter (DSC 204 F1, Netzsch) under N₂ at a heating rate of 10℃/min. The thermal decomposition was observed using a TG analyzer (TG 209 F1, Netzsch) under N₂ at a heating rate of 20℃/min.

Finishing of fabrics with DBSA

Cotton, PET and PET/cotton fabrics were finished through the pad-dry-cure method. Finishing baths with different concentration of DBSA were prepared and NaH₂PO₂ (SHP) was used as the catalyst for esterification with the ratio DBSA:SHP = 1:1.5 (mol). The pH value of the finishing bath was adjusted to 3 using NaOH. Fabric was immersed in the bath and passed through a laboratory padder with two dips and two nips to give an approximately 85% wet pickup. Then the fabric was dried at 80℃ for 3 min and subsequently cured at 170℃ for 3 min. The finished fabric was immersed in water with stirring for 5 min and rinsed with running water for 3 min, dried at room temperature.

The flame retardant CP was applied to cotton fabric using the method described above. The finishing bath of CP was prepared with 35% (w/w) CP, 5% (w/w) CHN as the cross-linking agent and 2% (w/w) phosphoric acid as the catalyst.

Evaluation of the flame retardancy of the finished fabrics

The limiting oxygen index (LOI) value of the fabrics was measured according to the Chinese standard GB/T 5454-1997 (equivalent to ISO 4589-2 or to ASTM D2863) “Textiles-Burning Behavior-Oxygen Index method” using a LOI-type burning tester (ATS, ATS FAAR Co.). The vertical burning test, including measurement of char length, after-flame time and after-glow time, was conducted according to GB/T5455-1997 (equivalent to ISO 6940:1984 or ASTM D6413-99) “Textiles-Burning Behavior-Vertical method” using a flammability tester (YG(B)815D-1). The thermal decomposition behavior of the fabrics were observed at the temperature range of 20–600℃ under N₂ at a heating rate of 20℃/min using the TG analyzer (TG 209 F1, Netzsch).

Determination of whiteness

Whiteness of the fabrics was determined according to GB/T 8424.2-2001 (equivalent to ISO 105-J02:1997 W04) “Textiles-Tests for color fastness-Instrumental assessment of relative whiteness” on a WSB-II digital whiteness instrument (Shanghai Xinrui, Shanghai, China).

Determination of tensile property

The tensile property of the fabrics was measured according to GB/T 3923.1-1997 (equivalent to ISO 13934.1-1994) “Textiles-Tensile properties of fabric part 1: Determination of breaking force and elongation at breaking force-Strip method” on an H10K-S type electronic textile strength machine (Tinius Olsen).

Durability to home laundering

After finishing, the fabric samples were subjected to different numbers of home laundering cycles (HLCs) according to GB/T 17595-1998 (equivalent to ISO 10528-1995) “Textiles-Domestic laundering procedure for textile fabrics prior to flammability testing” in a home washing machine with 2 g/L detergent. The water temperature for laundering was approximately 40℃, and one HLC was 45 min.

Determination of bromine content of the finished fabric

The finished samples were oxidized by combustion in a closed system (a bomb containing oxygen under pressure) following standard procedures of BSEN14582:2007. Initially, about 0.2000 g of accurately weighed sample were placed in the calorimetric bomb and the cover of the bomb was tightened securely. Then oxygen was admitted to a pressure of 2 MPa and the circuit was closed to ignite the sample. The combustion gases were collected in an absorption solution containing 0.024 mol/L Na₂CO₃ and 0.030 mol/L NaHCO₃. Ion chromatography was used to analyze the combustion product.

Determination of the number of ester linkages

Lejaren A et al.²⁸ suggested that the number of ester linkages would be determined by back-titration with Ca(CA)₂-NaOH. The carboxyl groups were supposed to react with calcium acetate as shown in Scheme 1.

Scheme 1.

Chelation reaction of carboxyl and calcium acetate.

Hu and Zhou²⁹ used the back-titration method at first in our lab. The method was adopted to determine the amount of DBSA that esterified with cellulose. The fabric finished with DBSA was cut into small pieces of about 0.5 cm × 0.5 cm, and mixed thoroughly. The sample was dried at 105℃ for 2.5 h, and then cooled to room temperature in a dryer. A total of 1.000 g of the sample was put into a 250 mL Erlenmeyer flask, and 50 mL fresh 0.1 M Ca(CA)₂ solution was added, the flask was kept in ice water for 2 h, then titrated with 0.02 M NaOH standard solution.

Amount of COOH on fabric = (V - V_{o}) M / m \times 1000 mmol / kg

where V (mL) is the standard base solution consumed in titration of the finished fabric sample, V₀ (mL) is the standard base solution consumed in titration of the unfinished fabric sample, M is the molar concentration of the standard base solution and m (g) is the weight of the sample.

Results and discussion

Characterization of DBSA

DBSA was prepared via the addition reaction of maleic anhydride and bromine without any catalyst, as shown in Scheme 2.

Scheme 2.

The addition reaction of maleic anhydride and bromine.

The structure of the product was investigated by the FT-IR spectrum, as Figure 1 shows. The bands at 1685 and 1731 cm⁻¹ correspond to the symmetric and asymmetric stretching of a five-membered cyclic anhydride, respectively. The three explicit bands at 1388, 1264 and 1184 cm⁻¹ are attributed to the C-O-C stretching modes of the five-membered cyclic anhydride, while the band at 572 cm⁻¹ is associated with the stretching mode of C-Br stretching vibration. The band at 3014 cm⁻¹ is assigned to O-H stretching vibration, which could be observed because of hydrolysis of the anhydride.

Figure 1.

FT-IR spectrum of the synthesized product.

The ¹H-NMR spectrum of the product is shown in Figure 2. There is only one proton resonated peak at δ = 4.75, which indicates that the hydrogen existing in the product has only one form.

Figure 2.

Proton nuclear magnetic resonance spectrum of the product.

Figure 3(a) shows the DSC curve of the product in an atmosphere of nitrogen. There was an endothermic peak in the temperature range below 170℃, which was the melting point. The melting point was also tested by the capillary tube method and the result was 167–170℃. These results agree with that of DBSA recorded in the book Organic chemical raw materials (169–170℃). The TGA and DTG curves of the product are presented in Figure 3(b). As shown in the TG curve, the main pyrolysis stage occurred in the temperature range from 170℃ to 225℃ and yielded no residue. The rate of weight loss reached its maximum at 209.4℃, as indicated by the peak in the DTG curve. The decomposition of the product at this temperature range corresponded to the endothermic peak at 217.3℃ in the DSC curve. The decomposition temperature of the product was slightly lower than the BFRs in common use.

Figure 3.

Differential scanning calorimetry (DSC) (a), thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) (b) curves of the product.

From these results, we affirmed that DBSA was synthesized successfully.

Finishing of cotton fabric with DBSA

Different concentrations of DBSA (10–30%, w/w) were applied to the cotton fabric by the method described in the Experimental details section. The LOI values and vertical burning test data were compared with those of unfinished fabric; the results are given in Table 2.

Table 2.

Flame retardancy of cotton with different concentrations of 2, 3-dibromo-succinic anhydride (DBSA)

DBSA (%, w/w)	Vertical burning test			LOI (%)
DBSA (%, w/w)	T_flame (s)	T_glow (s)	L_char (cm)	LOI (%)
Untreated	24.7	1.3	TD ^a	17.7
10	22.0	6.1	21.6	23.7
15	18.4	5.4	12.4	26.6
20	5.2	0	4.9	32.7
25	6.4	0	6.3	31.5
30	7.2	0	9.2	30.6

TD denotes that the samples were totally destroyed during the test, the same below.

LOI: limited oxygen index.

From Table 2, it can be seen that with increasing concentration of DBSA, the char length of the finished cotton fabric decreased while the LOI value increased to reach the top point (32.8) at the concentration of 20%. The finished cotton fabrics before washing were considered to be flame retardant when the concentration of DBSA was above 20%, because those fabrics with a LOI value greater than 27 (or 26 in some material) were generally known to be self-extinguishing.

When dissolved in water, DBSA changes to a dicarboxylic acid. According to the mechanism that polycarboxylic acids esterify cellulose through the formation of a five-membered cyclic anhydride under catalyzing of SHP,³⁰ the possible esterification of DBSA with hydroxyl groups of the cellulose would be carried out as shown in Scheme 3.

Scheme 3.

Reaction of 2, 3-dibromo-succinic anhydride and cellulose.

The changes of the chemical structure of the cotton fabric before and after finishing were investigated by the FT-IR spectrum. Figure 4 shows the FT-IR spectra of finished and unfinished cotton. It was evident that the FT-IR spectrum of the finished cotton fabric (spectrum b) was almost identical to that of the unfinished cotton fabric (spectrum a), except in the wave number of about 1732 cm⁻¹, where a new absorption peak appears, which could be attributed to the overlapped absorption of the carboxylic acid carboxyl band and the ester carboxyl band; the absorption peak at 1400 cm⁻¹ was significantly enhanced (C-O stretching vibration), which indicated that the flame retardant esterified with the hydroxyl groups of cotton cellulose.

Figure 4.

Fourier transform infrared spectra of unfinished (a) and finished (b) cotton.

The number of ester linkages could be calculated by titrating the amount of free carboxyl groups on the cotton fabrics before and after curing. Results of the titration are presented in Table 3.

Table 3.

Amount of free carboxyl groups on cotton samples

Sample	Amount of free carboxyl groups (mmol/kg)
Before curing	704
After curing	405
Cured and rinsed	297

The amount of DBSA that was available for reaction could be determined by the results of titration before curing, 704 ÷ 2 = 352 mmol/kg. The calculated amount of DBSA applied to cotton fabric could be obtained by wet pickup and the concentration of the finishing bath, 20% × 85% ÷ 257.8 = 618 mmol/kg. The measured value was lower than the calculated value. The reason might be due to the addition of the pH adjusting agent NaOH, which changed carboxyl groups to carboxylate anion. The difference value between free carboxyl on the fabrics before and after curing was the amount of flame retardant that esterified with the cellulose. By calculating (704 – 405) / 618 = 48.4%, 48.4% of the flame retardant applied esterified with cellulose of the cotton fabric. If it was true, the finished fabric contained 5.1% bromine by weight. If the data of the cured and rinsed sample is used, bromine content would be the same. The bromine content of the finished sample was also determined by the combustion method to be 4.6%, which was consistent with the calculated value according to free carboxyl group titration.

CP has been one of the most commonly used phosphonate durable flame retardant agents for cotton fabric and is usually applied together with a cross-linking agent such as CHN by a pad-dry-cure acid-catalyzed procedure. CHN is an N-hydroxymethyl cross-linking agent that is added to increase the nitrogen content for the synergistic effect with the phosphorus. The flame retardancy of cotton fabric finished with DBSA (20%, w/w) was compared with that finished by CP using the recommendation formulation and condition. The results are shown in Table 4. The flame retardancy of cotton fabrics finished with DBSA was better than that with CP. Meanwhile, the breaking strength retentions were all about 70%.

Table 4.

Flame retardancy and physical properties of cotton treated with Pyrovatex CP (CP) or 2, 3-dibromo-succinic anhydride (DBSA)

Sample	Vertical burning test			LOI (%)	Whiteness index	Tensile strength retention (%)
Sample	T_flame (s)	T_glow (s)	L_char (cm)	LOI (%)	Whiteness index	Warp	Weft
Unfinished	24.7	1.3	TD	17.7	83.6	100	100
Finished with CP	2.6	0	9.5	29.4	79.4	70.6	73.2
Finished with DBSA	5.2	0	4.9	32.7	80.8	71.5	69.8

LOI: limited oxygen index.

The TGA and DTG curves are illustrated in Figure 5 and the related data are listed in Table 5.

Figure 5.

Thermogravimetric analysis (a) and derivative thermogravimetric (DTG) (b) curves of finished and unfinished cotton.

Table 5.

Data from the thermogravimetric analysis and derivative thermogravimetric of finished and unfinished cotton

Cotton sample	Peak of weight loss of cotton^a		Residual at 600℃ (%)
Cotton sample	V_max (%/min)	T_max (℃)	Residual at 600℃ (%)
Unfinished	24.6	368	10.5
Finished with CP	9.8	297	36.2
Finished with DBSA	13.5	332	33.3

V_max denotes the maximum decomposing rate; T_max denotes the temperature of the maximum decomposing rate.

It can be seen from Figure 5(a) that the unfinished cotton fabric started to lose weight at about 300℃ and the rate of weight loss reached its maximum at 368℃, as indicated by the peak in the derivative thermogravimetric (DTG) curve (Figure 5b). After finished with CP, as could be expected, the decomposition temperature and the rate of decomposition were all reduced, while the amount of char at 600℃ increased greatly. After the cotton fabric was finished with DBSA, its thermal decomposition also shifted to a lower temperature, but higher than that of the fabric finished with CP. The T_max of DBSA finished cotton was 332℃ and the rate of decomposition was reduced apparently compared with that of the unfinished fabric. The unfinished cotton lost 89.5% of its original weight with 10.5% residual at 600℃, while the DBSA finished cotton retained 33.3% of its original weight. The data indicated that the presence of DBSA on the cotton fabric lowered the decomposition temperature and enhanced the formation of char after pyrolysis of the sample, which was an evidence of condensed phase mechanism. It is known that phosphorous-containing FRs can reduce cellulose flammability, primarily by dehydration, phosphorylation and phosphate-ester decomposition mechanisms.¹ The phosphate-ester contributed to the forming of a cross-linked network within the cotton, which can inhibit the release of volatile combustible fragments and enhance char formation.³¹ As shown in the DTG curves there is a shoulder peak on the DBSA finished curve at about the T_max of the CP finished curve, and this indicated that DBSA might have some similar flame retarding mechanism with CP, meaning that the esterification of DBSA with cellulose worked similar to phosphate-ester at the initial stage of thermal decomposition. The LOI value of the cotton finished with DBSA was higher than that with CP. The flame retardancy must be enhanced by the gas-phase effect of bromine.

Finishing of PET fabric with DBSA

The curing temperature for PET fabric was set at 170℃, which was lower than the normal thermosol dyeing temperature of 180–210℃ because DBSA started to decompose at 170℃. Dependence of the LOI value and vertical burning test data on the DBSA concentration is shown in Table 6.

Table 6.

Flame retardancy of poly(ethylene terephthalate) with different concentrations of 2, 3-dibromo-succinic anhydride (DBSA)

DBSA (%, w/w)	Vertical burning test			LOI (%)
DBSA (%, w/w)	T_flame (s)	T_glow (s)	L_char (cm)	LOI (%)
Untreated	35.6	0	TD	20.5
10	11.3	0	12.6	23.7
20	4.8	0	10.4	25.0
30	2.4	0	9.7	26.4
40	5.8	0	10.3	25.6

LOI: limited oxygen index.

As shown in Table 6, the LOI value increased with increasing DBSA content (below 30%), and the char length was decreased. DBSA could be adsorbed on the polymer chain and diffuse into polymer when the segmental movement of the adjacent chains is such that a hole is formed of a size sufficiently large to accept the DBSA molecule, which would happen above certain temperature. However, the migration of DBSA into the internal area of PET fiber might be restricted by the lower curing temperature. However, bromine content of the sample finished with 30% DBSA was determined to be 6.5% by weight. This revealed that a certain fraction of the DBSA has migrated into the internal are of PET fibers under this temperature.

Figure 6 shows that there was no changes in the IR spectroscopy after the PET finished with DBSA, which also indicates that there was no chemical reaction between DBSA and PET fibers.

Figure 6.

Fourier transform infrared spectra of unfinished (a) and finished (b) poly(ethylene terephthalate).

Finishing of PET/cotton blended fabric with DBSA

The PET/cotton blended fabric was finished with 20% DBSA via the pad- dry-curing method. The flammability and physical properties of finished and unfinished fabrics are presented in Table 7.

Table 7.

Flammability and physical properties of finished and unfinished poly(ethylene terephthalate)/cotton

Sample	Br (%)	Vertical burning test			LOI (%)	Whiteness index	Tensile strength retention (%)
Sample	Br (%)	T_flame (s)	T_glow (s)	L_char (cm)	LOI (%)	Whiteness index	Warp	Weft
Unfinished	0	9.2	5.9	TD	17.0	85.1	100	100
Finished	6.0	3.4	0	10.9	27.8	79.8	68.2	66.4

LOI: limited oxygen index.

As shown in Table 7, DBSA improved the LOI value and greatly shortened the char length to 10.9 cm, which indicated that DBSA could endow good flame retardancy to PET/cotton blended fabric. The breaking strength retentions were about 67%.

Thermal analysis

The TGA and DTG curves of the cotton, PET and PET/cotton are illustrated in Figure 7 and the related data are listed in Table 8.

Figure 7.

Thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) curves of finished and unfinished fabrics: (a) cotton; (b) poly(ethylene terephthalate) (PET); (c) PET/cotton.

Table 8.

Data from the thermogravimetric analysis and derivative thermogravimetric curves of finished and unfinished fabrics

Samples		1st weight-loss stage			2nd weight-loss stage			Residual at 600℃ (%)
Samples		V_max (%/min)	T_max (℃)	Weight loss (%)	V_max (%/min)	T_max (℃)	Weight loss (%)	Residual at 600℃ (%)
C	Unfinished	24.6	368	285–394℃:83.3				10.5
C	Finished	13.5	332	225–369℃:35.1				33.3
PET	Unfinished				19.4	432	370–472℃:81.2	18.9
PET	Finished				22.1	434	372–481℃:52.1	35.1
PET/C	Unfinished	16.9	365	295–378℃:51.5	8.9	427	378–463℃:33.9	10.2
PET/C	Finished	7.1	278	196–343℃:23.3	9.2	442	343–488℃:36.9	30.5

PET: poly(ethylene terephthalate).

For PET fabric in Figure 7(b), the rate of weight loss of the finished and unfinished reached their maximum at 434.4℃ and 432.4℃, respectively. The TGA curve revealed that the finished PET fabric lost 64.9% of its original weight at 600℃ with 35.1% residual, which was higher than that of the unfinished (18.6%). The reason for the change of the thermal behavior of the finished PET might be explained by the fact that the unsaturated residual of flame retardant after losing halogen atoms could promote the polyester to carbonize,³² thus reducing the generation of combustible gas and contributing to the decrease of flammability.

The results of Figure 6(c) indicated that the amount of residue of the unfinished blended fabric was 10.2% at 600℃, which was obviously lower than the calculated value of 14.6% (10.5 × 50% + 18.9 × 50%). The data indicated that the thermal decomposition of cotton and PET promoted each other, which increase the difficulty of flame retardant finishing. After finished with DBSA, the peak of weight loss of the cotton moiety appeared at the lower temperature of 278℃, which was lower than cotton finished with DBSA (332℃, Figure 7(a)) and the weight loss between 196℃ and 343℃ was 23.3%. The results seemed that the condensed phase flame retardant effect was improved. This might be because the melting of PET covered the cotton to bring about inhibiting the volatilization of the cellulose decomposition product. The second V_max, which corresponded mainly to the PET moiety shifted to a higher temperature of 442℃ and the weight loss between 343℃ and 488℃ was 36.9%, higher than the unfinished. The possible reason was that the charring residue of the cotton moiety formed the skeleton, which increased the volatilization of the decomposition product of PET. Simultaneously, the cotton that had been covered by melting PET obtained the chance to decompose again along with the decomposition of PET. However, after finishing the amount of residue was 30.5% for PET/cotton, which was much higher than that of unfinished.

Durability to home laundering

The durability to home laundering was investigated by determination of the LOI values. The LOI values before and after different numbers of HLCs are shown in Table 9. After one HLC the LOI value of cotton fabric was notably lower than that before laundering; however, the LOI was still up to 26.2 after 12 HLCs. For PET fabric the LOI values had few changes with increasing the times of laundering. The fastness should be because the flame retardant molecules diffused into the PET fiber during the finishing process, which were not easy to exude during laundering. The downward trend of LOI value of PET/cotton blended fabric was between the cotton and PET fabrics.

Table 9.

Effect of home laundering on the limited oxygen index (LOI) value of finished fabrics

Numbers of HLCs	Cotton				PET				PET/cotton
Numbers of HLCs	T_flame (s)	T_glow (s)	L_char (cm)	LOI(%)	T_flame (s)	T_glow (s)	L_char (cm)	LOI(%)	T_flame (s)	T_glow (s)	L_char (cm)	LOI(%)
0	5.2	0	4.9	32.7	2.4	0	9.7	26.4	3.4	0	10.9	27.8
1	11.2	1.6	9.2	28.7	4.6	0	12.4	25.0	10.4	5.4	15.3	26.5
5	18.1	5.2	11.4	26.8	5.3	0	12.6	24.8	NR^a	NR	TD	25.8
12	22.3	6.7	18.6	26.2	6.4	0	13.2	24.5	NR	NR	TD	25.2

NR denotes no record because of the complete destruction of the fabric

HLC: home laundering cycle; PET: poly(ethylene terephthalate); LOI: limited oxygen index.

During the vertical burning test, the finished cotton fabric could self-extinguish after it was washed for 12 HLCs, while the finished PET/cotton blended fabric could not pass the vertical burning test after it was washed five and 12 times. The results indicate that the fixation of DBSA with fiber may not have been sufficient.

Conclusions

A reactive-type flame retardant DBSA was synthesized via the addition reaction of maleic anhydride and bromine and was characterized by FT-IR, ¹H-NMR, DSC and TGA. The range of decomposition temperature of DBSA was 170–225℃. Durable flame retardancy was endowed to cotton fabric and PET/cotton blended fabric through the esterification of DBSA with hydroxyl of cellulose and migration into PET via the pad-dry-cure process. The LOI value of finished cotton was 32.7, while PET/cotton was 27.8 with 20% (w/w) DBSA. In TGA, the cotton, PET and PET/cotton blended fabrics finished with DBSA showed evidence of the condensed phase mechanism. For the cotton fabric, by comparison with CP finished cotton, it was speculated that esterification of cotton cellulose with DBSA worked similar to phosphate-ester at the initial stage of thermal decomposition. The breaking strength of the cotton fabric finished with DBSA retained about 70%, similar to that finished with CP. For the PET/cotton blended fabric the retention was about 67%. The LOI value of PET/cotton finished with DBSA remained at 25.2 after 12 HLCs.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

References

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