Metal compounds as catalysts in the intumescent flame retardant system for polyethylene terephthalate fabrics

Abstract

Forming a carbonaceous protective layer during combustion has been proven to be an effective way to improving the flame retardancy of polymers and it has been found that some metal compounds could promote dehydrogenation and aromatization of the degradation products. In this research, several divalent or multivalent metal compounds were added in the intumescent flame retardant (IFR) based on ammonium polyphosphate, pentaerythritol and melamine for polyethylene terephthalate (PET) fabrics. The prepared flame retardant dispersions containing 25 wt% IFR and 1 wt% metal compounds were applied on PET fabrics by the common pad-dry-cure method. None of the treated samples generated any dripping during the vertical burning test, with the highest limiting oxygen index being up to 33.4%. Thermogravimetric analysis showed that the catalytic effect of metal compounds may occur in promoting the formation of char residues. The morphology of char residues of two of the IFR/metal compound-treated PET fabrics, IFR/FeCl₃ and IFR/ZnCl₂, were smooth and compact, as shown by scanning electron microscopy. Laser Raman spectroscopy of the char residues suggested that a higher graphitization degree was obtained by adding the metal compounds. Real-time Fourier transform infrared spectroscopy showed that metal compounds made more phosphorus available for phosphorylation and char formation. The results of the present study indicate the possibility for enhancing char formation via the catalytic effect of metal compounds to improve the flame retardant and anti-dripping properties of the PET fabrics.

Keywords

intumescent flame retardant system metal compound catalytic effect polyethylene terephthalate fabric anti-dripping

Textiles made of polyethylene terephthalate (PET) fiber are widely used in upholstery, transportation, clothing and for individual protection.^1–3 However, PET is flammable, and due to the temperature difference between the melting temperature (250–260℃) and ignition temperature (480℃), shrinking and melt dripping may happen during combustion. Melt dripping is dangerous in fires, as it may stick on the skin, thus causing severe injuries, or it may act as a secondary ignition source to promote fires.^4–6 Therefore, great attention has been paid to impart both flame retardancy and anti-dripping performance in PET fabrics.

Phosphorus-containing compounds, such as 2-carboxyethyl (methyl) phosphinic acid, 9,10-dihydro-9-oxa-10-phosphaphenanthrenyl-10-oxide (DOPO) adduct of itaconic acid and cyclic phosphonate ester,^7–9 as substitutions for effective halogenated flame retardants have been applied to PET plastic or fiber, and satisfactory performance was obtained. These phosphorus-containing flame retardants increase the limiting oxygen index (LOI) values of PET to more than 28%, but the melt-dripping phenomenon still exists.^10–12 However, in lab tests, melt dripping acts as a contributor to flame retardancy due to enhancing the loss of heat and flammable matter from the burning materials. The flame retardation of PET sometimes seems to conflict with its anti-dripping behaviors. Addressing this issue has become a challenging assignment in the flame retardation of PET.

For the last 20 years, researchers have focused on the flame retardant and anti-dripping modification of PET, and many positive results have been achieved.^4,5,13–22 Among them, forming carbonaceous protective layers through the application of phosphorus-containing intumescent flame retardant (IFR) has been proven to be an effective approach.^5,15,17 In general, there are three major components in an IFR, an acid source, a carbon source and a blowing agent.²³ In order to raise the efficiency of IFR, some metal compounds have been used and proved to be effective in catalyzing carbonization.^24–27

It is well known that transition metal compounds as catalysts have been widely applied in catalyzing some organic reactions, such as oxidative dehydrogenation and esterification. As a result, the metal compounds could be applied in catalyzing the carbonization of polymer materials. The most adopted mechanism is that the metal compounds are able to accept an electron pair to create a coordinate bond,^24,25 and polymers with electronegative groups can coordinate with them leading to crosslinking and carbonization. The reported metal compounds are metal chlorides (NiCl₂, CoCl₂, ZnCl₂ and FeCl₃), metal chelates (of Cu, Co and Ni), metal oxides (La₂O₃, Ni₂O₃, Fe₂O₃, LaMnO₃, NiFeO and CoFeO), metal acetates, metal acetyl acetonates, metal borates and metal sulfates, which were primarily used in polypropylene (PP),^27–29 acrylonitrile butadiene styreneacrylonitrile (ABS),^26,30 polyamide (PA),³¹ polyethylene (PE),^32,33 polyacrylonitrile (PAN),³⁴ epoxy resin (EP),^35,36 poly(lactic acid) (PLA)³⁷ and cotton fabric.^38,39 These previous studies suggested that the incorporation of a very low concentration of metal compounds could promote the crosslinking of the non-charring polymer or enhance flame retardant efficiency, increase the char yield and improve the char quality and the thermal stability of formed char. However, to the best of our knowledge, the effect of involving metal compounds in IFR on the flammability of flame retardant PET fabrics has not been reported so far.

In the present work, several divalent or multivalent metal compounds were added to IFR based on ammonium polyphosphate (APP), pentaerythritol (PER) and melamine (MEL), and applied to PET fabric, aiming to investigate the potential use of metal compounds in IFR to enhance the charring performance of PET fabric. The effects were investigated by means of LOI, vertical burning properties, micro-scale combustion colorimeter (MCC), thermogravimetric analysis (TGA), laser Raman spectroscopy (LRS), real-time Fourier transform infrared spectroscopy (RT-FTIR) and scanning electron microscopy (SEM).

Experimental details

Materials

PET knitted fabric with an area density of 110 g/m² was purchased from Shanghai Xinfanglian automobile interior Co., Ltd (Shanghai, China). APP with an approximate degree of polymerization (DP) of above 1000 was purchased from Jinan Taixing Fine Chemicals Co. Ltd (Shandong, China). PER, MEL and the metal compounds (NH₄)₂Fe(SO₄)₂, FeCl₃, ZnSO₄, ZnCl₂, NiSO₄, NiCl₂ and Zr₂(HPO₄)₂ were purchased from Sinopharm Chemical Reagent (Shanghai, China).

Preparation of flame retardant dispersion

A mixture containing 25 wt% IFR (APP:PER:MEL = 20:4:1), 1 wt% metal compound and tap-water was ground in a planetary ball mill (QM-3SP2, Nanjing University Instrument Factory, China) for 4 h until a slightly mushy dispersion was obtained that could maintain this state for about 1 h.

Flame retardant treatment of PET fabrics

The prepared flame retardant dispersion was applied on PET fabric through the pad-dry-cure method. The fabric was immersed in the dispersion for 1 min and passed through a laboratory padder to give an approximately 100% wet pickup. Then the fabric was dried at 80℃ and cured at 180℃, both for 3 min. The add-on percentage mass of flame retardant was calculated from the following equation

add - on % = [(M_{1} - M_{0}) / M_{0}] \times 100 %

where M₁ and M₀ represent the mass of the PET fabric after and before IFR or IFR/metal compound treatment, respectively; the add-on of all the samples after flame retardant treatment was about 25%.

Characterization

The metal content on the fabric was analyzed by an inductively coupled plasma-atomic emission spectrometer (Leeman Prodigy, USA) according to JY/T 015-1996. The LOI value of the fabric was measured at room temperature (RT) on an oxygen index instrument (10095104, NOAELAB-AFS, Italy) according to ASTM D2863-97. A vertical burning test was conducted on a fabric flame retardant performance tester (YG(B)815D-I, Wenzhou Darong, China) according to ASTM D6431-99. TGA was performed on a thermal analyzer (TG 209F1, NETZSCH, Germany) in air atmosphere with a heating rate of 10℃/min between RT and 800℃. MCC measurements were made using a micro-scale combustion calorimeter (FTT 0001, FTT, UK) according to ASTM D7309-2007 (Method A), the sample (∼5 mg) was heated from RT to 750℃ in 80 cm³/min nitrogen stream followed by a linear heating rate of 1℃/min, then the gaseous pyrolysate was mixed with 20 cm³/min oxygen stream and combusted in the furnace at 900℃ for 10 s. RT-FTIR was performed on a FTIR spectroscope (Avatar 380, Thermo Electron, USA) to characterize the structure of the char residues obtained by roasting the fabric sample at specific temperatures in a muffle furnace for 10 min under air atmosphere. LRS measurement was carried out at RT using a laser Raman spectrometer (SPEX-1403, SPEX, USA) to characterize the graphitic structure of the char residues obtained by roasting the fabric sample at 600℃ and 800℃, respectively, in a muffle furnace for 10 min under air atmosphere; the excitation wavelength was 532 nm and the scanning scope was from 1000 to 2000 cm⁻¹. SEM (TM-1000, Hitachi, Japan) was used to examine the surface morphology of the treated PET fabrics and char residues obtained by roasting the fabric samples at 600℃ in a muffle furnace for 10 min under air atmosphere; the surface of the fiber or char residues was sputter-coated with a platinum layer before examination. The whiteness of the fabrics was determined according to GB/T 8424.2-2001 (equivalent to ISO 105-J02:1997 W04) on a Datacolor spectrophotometer (650, Datacolor, USA). The bursting strength of the fabrics was measured according to GB/T 19976-2005 (equivalent to ISO 3303-1994) on a textile strength machine (YG026A, Changzhou, China). Washing durability was determined by subjecting the fabric samples to five home laundering cycles (HLCs) according to the procedure of AATCC 124.

Results and discussion

Flammability

The LOI and vertical burning tests are commonly used to estimate the flame retardancy of materials. Before studying the effect of the metal compounds on the flammability of IFR-treated PET fabric, the appropriate amount of APP, PER and/or MEL to endow flame retardancy to PET fabric was investigated. When the total content of 25 wt% IFR remained constant, by changing the composition of APP, PER and/or MEL, the LOI value of PET fabrics increased from 27.8% (25 wt% APP alone) to 29.5%, and finally APP:PER:MEL = 20:4:1 was adopted for follow-on experiments.

The LOI values and vertical burning performance of IFR/metal compound-treated PET fabrics are presented in Table 1.

Table 1.

Effect of metal compounds on the flame retardant and anti-dripping performance of intumescent flame retardant (IFR)-treated polyethylene terephthalate (PET) fabrics

Sample	Metal content on PET fabric (mg/g)	LOI (%)	Flammability from vertical burning test			Dripping
Sample	Metal content on PET fabric (mg/g)	LOI (%)	Damaged length (cm)	After flame time (s)	After glow time (s)	Dripping
IFR	/	29.2 ± 0.3	8.2 ± 1.0	0	0	No
IFR/(NH₄)₂Fe(SO₄)₂	3.9	27.4 ± 0.5	9.2 ± 0.8	0	0	No
IFR/FeCl₃	5.5	33.4 ± 0.5	6.7 ± 1.0	0	0	No
IFR/ZnSO₄	6.1	28.7 ± 0.2	7.6 ± 0.5	0	0	No
IFR/ZnCl₂	6.8	32.0 ± 0.5	6.6 ± 0.5	0	0	No
IFR/NiSO₄	5.8	26.3 ± 0.5	9.6 ± 0.5	0	0	No
IFR/NiCl₂	6.6	25.5 ± 0.3	10.5 ± 0.5	0	0	No
IFR/Zr₂(HPO₄)₂	7.0	29.5 ± 0.4	8.7 ± 0.8	0	0	No
Untreated	/	21.0 ± 0.2	14.5 ± 0.3	14.7	0	Yes

LOI: limiting oxygen index.

It can be seen from Table 1 that the LOI values of IFR/FeCl₃, IFR/ZnCl₂ and IFR/Zr₂(HPO₄)₂-treated PET fabrics were higher than the sample treated with IFR alone, while samples containing (NH₄)₂Fe(SO₄)₂, ZnSO₄, NiSO₄ and NiCl₂ had lower LOI values. The highest LOI value of 33.4% was obtained from IFR/FeCl₃, which represented an increase of 4.2 LOI units, followed by ZnCl₂, having the LOI value of 32.0%.

In the vertical burning test, the damaged lengths of the treated samples were less than 10 cm, except IFR/NiCl₂, with no after flame or after glow observed; according to GB/T 17951-2006, B1 rating was achieved. Pictures of some samples after the vertical burning test are presented in Figure 1 (taking IFR-treated, IFR/ZnCl₂-treated and IFR/FeCl₃-treated PET fabrics as examples). It was noted that on contact with the ignition source, the melt shrinkage of the treated PET fabrics became very slow and a ring of black char was formed at the edge of the fabric and no dripping was observed during testing. In accordance with the LOI value, the damaged lengths of the IFR/FeCl₃ and IFR/ZnCl₂ samples in the vertical burning test were the shortest.

Figure 1.

Pictures of untreated (a), intumescent flame retardant (IFR) (b), IFR/ZnCl₂ (c), and IFR/FeCl₃ (d) treated polyethylene terephthalate fabrics after the vertical burning test.

From the above results, it could be seen that the effect of different metal compounds on the flame retardant performance varied. It was supposed that the influence of metal compounds on the LOI value was associated with the change of stability of the metal compounds, the pH of the system during pyrolysis²⁵ and the covalent and ionic radii of the metal compounds.⁴⁰ In general, the shorter the metal ionic radius, the more stable the covalent bond, and the higher the oxidation state of metal ions, the greater the catalytic ability could be obtained. Of the seven metal compounds used in this research, Fe³⁺ possesses the largest electric negative properties and the shortest atomic radius, thus endowed the treated PET fabric with the best flame retardant performance.

Thermal degradation behavior

The thermal degradation behavior of untreated, IFR-treated and IFR/metal compound-treated PET fabrics was investigated by TGA under air atmosphere. TG and differential thermogravimetry (DTG) curves of these samples are shown in Figure 2. The TGA results are summarized in Table 2.

Figure 2.

Thermogravimetry and differential thermogravimetry curves of untreated, intumescent flame retardant (IFR)- and IFR/metal compound-treated polyethylene terephthalate fabrics under air atmosphere.

Table 2.

Thermogravimetry and differential thermogravimetry data of untreated, intumescent flame retardant (IFR)- and IFR/metal compound-treated polyethylene terephthalate fabrics under air atmosphere

Sample	T_5% (℃)	T_max (℃)		Maximum mass loss rate (wt%/min)		Residue (wt%)
Sample	T_5% (℃)	Stage 1	Stage 2	Stage 1	Stage 2	600℃	800℃
IFR	328	374	548	11.7	1.2	19.9	9.6
IFR/(NH₄)₂Fe(SO₄)₂	318	364	551	11.6	1.1	22.8	13.3
IFR/FeCl₃	323	372	/	9.7	/	28.0	18.7
IFR/ZnSO₄	323	365	/	9.5	/	20.5	11.6
IFR/ZnCl₂	324	367	/	9.2	/	24.7	14.0
IFR/NiSO₄	324	360	/	10.2	/	20.9	10.9
IFR/NiCl₂	332	362	/	10.7	/	21.0	9.8
IFR/Zr₂(HPO4)₂	330	374	649	7.5	1.5	22.6	10.2
Untreated	363	429	551	17.9	1.2	0.8	0

The initial mass loss temperature T_5% of PET was 363℃, and it showed a two-stage weight loss in air; the former with the temperature of the maximum mass loss rate (MMLR) T_max = 429℃ involves the decomposition of the main chain via the formation of carboxylic acids and vinyl ends as the starting point, and the latter with T_max = 551℃ involves the further oxidation of the residue formed during the first stage.

The T_5% and T_max values of the IFR-treated fabric were lower than that of the untreated sample, and the mass loss rate decreased. In the case of IFR-treated PET fabric, polyphosphoric, phosphate ester and NH₃ formed by the degradation of IFR. Balabanovich⁴¹ investigated the effect of APP on the thermal degradation behavior of polybutylene terephthalate (PBT), the results showed that APP interacted with PBT and the main reaction occurring on pyrolysis was the ammonolysis of PBT. In our recent research,⁴² the pyrolysis products of IFR-treated PET fabric were analyzed by pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS), and benzonitrile was detected, which means that by the decomposition of APP and MEL, nitrogen-containing chemical fragments such as NH₃ formed and caused a degradation of PET through the ammonolysis mechanism. It had been known that polyphosphoric or phosphate ester would promote the degradation of PET. Both the ammonolysis and phosphorylation were related to the decline of the T_5% and T_max values. At the same time, the reaction of APP with PER during pyrolysis could lead to the formation of char,²³ hindering the passage of combustible gases to the flame and protecting the PET fabric from oxygen and heat.

It can be seen from Table 2 that the addition of metal compounds, except for Zr₂(HPO₄)₂ and NiCl₂, further lowered the T_5% value of the treated samples, which demonstrated that decomposition of PET was accelerated and occurred at lower temperatures due to the effect of metal compounds.

Coupled with metal compound and IFR, the degradation profile of PET changed, the MMLR of the first degradation stage decreased and the second stage almost disappeared, except for (NH₄)₂Fe(SO₄)₂ and Zr₂(HPO₄)₂. The following sequences were obtained: the MMLR at the first stage follows IFR/Zr₂(HPO₄)₂ < IFR/FeCl₃ ≈ IFR/ZnSO₄ ≈ IFR/ZnCl₂ < IFR/NiSO₄ ≈ IFR/NiCl₂ < IFR/(NH₄)₂Fe(SO₄)₂ < IFR; the char yield of 600℃ follows IFR/FeCl₃ > IFR/ZnCl₂ > IFR/(NH₄)₂Fe(SO₄)₂ ≈ IFR/Zr₂(HPO₄)₂ > IFR/NiCl₂ ≈ IFR/NiSO₄ ≈ IFR/ZnSO₄ > IFR. These results showed that the presence of the metal compounds was advantageous to the enhancement of the thermal stability of IFR-treated PET fabric and the char was also stabilized against further oxidation at the higher temperature range. A similar sequence was obtained in Jang et al.'s research work;⁴³ when four metal chlorides (NiCl₂, CoCl₂, ZnCl₂ and FeCl₃) were investigated on the thermal degradation of ABS, ZnCl₂ and FeCl₃ were found to be more effective than NiCl₂ or CoCl₂ in catalyzing carbonization, but this phenomenon has not been further explored.

To understand the interaction between the metal compounds and PET fabric, FeCl₃, ZnCl₂, ZnSO₄ and NiCl₂ were chosen to treat PET fabric individually. PET fabric treated with these metal compounds (1 and 5 wt%) were investigated by TGA under air atmosphere. Figures 3 and 4 show the TG and DTG curves of PET fabrics treated with metal compounds, and the TGA results are summarized in Table 3.

Figure 3.

Thermogravimetry and differential thermogravimetry curves of untreated, FeCl₃ and ZnCl₂-treated polyethylene terephthalate fabrics under air atmosphere.

Figure 4.

Thermogravimetry and differential thermogravimetry curves of untreated, ZnSO₄ and NiCl₂-treated polyethylene terephthalate fabrics under air atmosphere.

Table 3.

Thermogravimetry and differential thermogravimetry data of untreated and metal compound-treated polyethylene terephthalate fabrics under air atmosphere

Sample	T_5% (℃)	T_max (℃)			Maximum mass loss rate (wt%/min)			Residue (wt%)
Sample	T_5% (℃)	Stage 1	Stage 2	Stage 3	Stage 1	Stage 2	Stage 3	600℃	800℃
1% FeCl₃	361	421	457	/	14.2	6.2	/	2.7	2.6
5% FeCl₃	350	423	466	/	14.6	9.1	/	8.8	8.7
1% ZnCl₂	356	431	488	546	16.4	2.2	4.3	3.4	3.4
5% ZnCl₂	340	426	486	533	13.5	4.7	2.4	5.9	5.9
1% ZnSO₄	347	430	540	/	16.9	2.4	/	1.9	1.6
5% ZnSO₄	337	429	540	/	16.4	2.6	/	2.7	1.9
1% NiCl₂	350	414	504	/	19.6	1.9	/	0.7	0.2
5% NiCl₂	346	406	537	/	17.2	2.1	/	3.9	3.9
Untreated	363	429	551	/	17.9	1.2	/	0.8	0

It can be seen that all of the T_5% values of samples treated with metal compounds were below that of the untreated PET. The higher the amount of metal compounds, the more the T_5% value decreased. For the FeCl₃ and ZnCl₂-treated samples, there was two-stage thermal degradation at the temperature range of 420–490℃, which indicated that the residues formed at about 420℃ volatilized at a slightly higher temperature. However, unlike untreated PET and samples treated with ZnCl₂, there was no further degradation for FeCl₃-treated samples at the temperature range of 530–550℃. The char residues of 1% FeCl₃ and ZnCl₂-treated samples were 2.6% and 3.4% at 800℃; considering that the total mass of metal compounds was 1%, the majority of char residue should be pyrolytic products of PET itself. When the dosage of metal compounds increased, more char residue was obtained; the sample treated by 5% FeCl₃ had 8.8% residue remaining at 600℃, and the char was also stabilized against further oxidation at higher temperatures, since the amount of char residues at 800℃ was almost the same as that at 600℃. For NiCl₂ and ZnSO₄-treated samples, the degradation profiles were relatively similar to that of untreated PET, only with a slight increase of char residues, which meant that the catalytic effect of NiCl₂ or ZnSO₄ was not obvious.

Micro combustion performance

Micro-scale combustion calorimetry (MCC) is able to measure the following flammability parameters using milligram sample sizes: heat release capacity (HRC), heat release rate (HRR), peak heat release rate (PHRR), temperature at peak heat release rate (T_PHRR) and total heat release (THR) based on the oxygen consumption principle. Generally the higher the PHRR, THR and HRC values, the greater the fire hazard.^44,45

In this research, MCC was applied to evaluate the flammability of PET fabrics treated with IFR and/or different metal compounds. Figure 5 shows the HRR curves of the untreated, IFR- and IFR/metal compound-treated PET fabrics as a function of the heating temperature. Table 4 includes the corresponding MCC parameters.

Figure 5.

The heat release rate (HRR) versus temperature curves of the untreated, intumescent flame retardant (IFR)- and IFR/metal compound-treated PET fabrics.

Table 4.

Summary of the micro-scale combustion colorimeter results of the untreated, intumescent flame retardant (IFR)- and IFR/metal compound-treated polyethylene terephthalate fabrics

Sample	HRC [ J·(g·K)⁻¹]	PHRR (W·g⁻¹)	THR (kJ·g⁻¹)	T_PHRR (℃)
IFR	310 ± 5	310 ± 4	12.6 ± 0.2	382 ± 0.5
IFR/(NH₄)₂Fe(SO₄)₂	253 ± 5	252 ± 4	11.9 ± 0.2	384 ± 0.5
IFR/FeCl₃	218 ± 5	218 ± 5	11.3 ± 0.1	382 ± 1
IFR/ZnSO₄	228 ± 5	224 ± 5	12.3 ± 0.1	383 ± 0.5
IFR/ZnCl₂	237 ± 3	235 ± 5	12.4 ± 0.2	384 ± 0.2
IFR/NiSO₄	180 ± 5	180 ± 3	11.2 ± 0.2	397 ± 0.4
IFR/NiCl₂	282 ± 5	280 ± 5	13.2 ± 0.3	388 ± 0.4
IFR/Zr₂(HPO₄)₂	273 ± 4	273 ± 4	11.2 ± 0.1	389 ± 0.5
Untreated	379 ± 5	379 ± 4	16.6 ± 0.2	432 ± 0.2

HRC: heat release capacity; PHRR: peak heat release rate; THR: total heat release.

As can be seen from Figure 5, for untreated PET fabric, the thermal decomposition started at approximately 360℃ and intensified with increase of the temperature, as indicated by the HRR increasing to its peak value at approximately 430℃. When PET fabric was treated with IFR, the T_PHRR value was decreased by 50℃.

Comparing the untreated PET fabric with the IFR-treated sample, it can be seen that the flame retardant treatment of PET fabric with IFR modified the combustion behavior of PET by decreasing HRC [310 versus 379 J·(g·K)⁻¹], PHRR (310 versus 379 W·g⁻¹) and THR (12.6 versus 16.6 kJ·g⁻¹ for untreated PET); the decreasing degree reached 18.2%, 18.2% and 24.1%, respectively. It is worth noting that the addition of most of the metal compounds led to a further decrease of HRC, PHHR and THR values. PHRR and HRC were mainly affected by two factors, the MMLR and the heat of combustible gases released. The larger the MMLR, the more flammable gases released, the greater the heat generated, and the higher the corresponding PHHR or HRC.

From the above results, it was clear that the IFR was able to inhibit the release of combustible gases, protect the polymer from heat transfer and reduce the fuel by stimulating char formation of PET, and the introduction of the metal compounds to the IFR system further enhanced the flame retardancy of the treated PET fabrics.

Considering the LOI value, vertical burning performance, thermal stability and MCC results, the best flame retardant and anti-dripping effect of PET fabric was obtained by treating with IFR/FeCl₃ or IFR/ZnCl₂.

Morphology of fabric and char residue

SEM images of untreated, IFR, IFR/ZnCl₂ and IFR/FeCl₃-treated PET fabrics are shown in Figure 6.

Figure 6.

Surface morphology of untreated (a), intumescent flame retardant (IFR) (b), IFR/ZnCl₂ (c) and IFR/FeCl₃ (d) treated polyethylene terephthalate fabrics.

The surface of untreated PET fiber was smooth; after treatment with IFR or IFR/metal compounds, the accumulation of flame retardants on the surface of the fiber and the interspace between the fibers was observed, and it seems that IFR/metal compound-treated samples were relatively homogeneously covered by the flame retardant particles.

The physical structure of char residue plays a critical role in the performance of a flame retardant system. The untreated, IFR, IFR/ZnCl₂ and IFR/FeCl₃-treated PET fabrics were roasted at 600℃ in a muffle furnace for 10 min under air atmosphere. Digital photographs and SEM images of the char residues of these samples are shown in Figure 7.

Figure 7.

Digital photographs (a)–(d) and scanning electron microcopy images (e)–(h) of char residues of untreated, intumescent flame retardant (IFR), IFR/ZnCl₂ and IFR/FeCl₃-treated polyethylene terephthalate fabrics roasted in a muffle furnace at 600℃ for 10 min.

As can be observed, for the untreated PET sample the char residue was very little, with holes and no expansion, being structurally conducive to gas diffusion and heat transfer. On the contrary, an intumescent char layer was observed for the IFR-treated PET fabric; this structure could provide a much better physical barrier to the spreading of flammable gases and heat transfer.

When ZnCl₂ was combined with IFR in flame retardant finishing of PET, thicker and more compact char residue was formed. Meanwhile, compared with the IFR/ZnCl₂ sample, the char layer of the IFR/FeCl₃ sample was more continuous and smooth. The results further confirmed that ZnCl₂ and FeCl₃ improved the charring performance of IFR-treated PET and enhanced the quality of the intumescent char.

Structure analysis of char residue

The Raman spectrum is usually used to characterize the graphitic structure of materials, and to evaluate the ordered degree of carbon materials in terms of two characteristic absorbance bands, the G and D bands. The G band, at about 1600 cm⁻¹, corresponds to the vibration of sp² bonded carbon atoms, which indicates the formation of a crystalline graphite phase. The D band, at about 1360 cm⁻¹, is attributed to the presence of amorphous carbon. The relative intensity ratio of the D peak to the G peak (I_D/I_G) is inversely proportional to the in-plane microcrystalline size and/or in-plane phonon correlation length.^46–48 Generally, the higher the value of I_D/I_G, the lower the graphitization degree of chars.

The Raman spectra of the char residues of untreated, IFR-treated, IFR/ZnCl₂ and IFR/FeCl₃-treated PET fabrics roasted in a muffle furnace at 600℃ and 800℃, respectively, are shown in Figure 8. The I_D/I_G values and the amount of char residue from TGA are summarized in Table 5.

Figure 8.

Raman spectra of the char residues of untreated, intumescent flame retardant (IFR), IFR/ZnCl₂-and IFR/FeCl₃ treated polyethylene terephthalate fabrics: (a) 600℃ muffle; (b) 800℃ muffle.

Table 5.

I_D/I_G values and the amount of char residues from thermogravimetric analysis

Sample	600℃		800℃
Sample	I_D/I_G	Residue	I_D/I_G	Residue
IFR	2.87	19.9	2.00	9.6
IFR/ZnCl₂	2.62	24.7	1.90	14.0
IFR/FeCl₃	2.41	28.0	1.59	18.7
Untreated	1.52	0.8	/	0

IFR: intumescent flame retardant.

As shown in Figure 8 and Table 5, the I_D/I_G value of the char residues obtained at 600℃ increased from 1.52 of untreated PET to 2.87 of the sample treated with IFR, while the amount of char residue of the sample treated with IFR increased compared to that for the untreated sample. So, it is deduced that the IFR applied mainly promoted the formation of disordered carbon.

However, the value of I_D/I_G decreased from 2.87 for the IFR-treated PET sample to 2.62 for the IFR/ZnCl₂ sample, and further decreased to 2.41 for the IFR/FeCl₃ sample, which indicated that the two metal compounds contributed to more graphitic structure formation. Combined with the fact that samples containing ZnCl₂ or FeCl₃ had much more residue char content, it was concluded that ZnCl₂ or FeCl₃ induced more formation of char residue and turned the amorphous char into a graphitic structure, thus enhancing the flame retardant properties. Considering with SEM images of char residues, more compact charred layers were formed by adding metal compounds, which demonstrates that the size of carbonaceous microstructures⁴⁶ from the IFR/metal compound-treated samples was smaller than that from the IFR sample, implying that ZnCl₂ or FeCl₃ inhibited the increase of the carbonaceous micro-domain in size during burning. Both the promoting of graphitization and inhibiting of microcrystalline size of char residue contributed to the lower I_D/I_G value.

When samples were further heated to 800℃, no char remained for the untreated PET sample. For the IFR, IFR/ZnCl₂ and IFR/FeCl₃-treated samples, the I_D/I_G value decreased compared with the char residues of 600℃, which suggested that the unstable amorphous char further degraded at the higher temperature range and retained more graphited char. Meanwhile, the decline of the I_D/I_G value of the IFR-treated sample was much larger (from 2.87 to 2.0), getting closer to the value of the IFR/ZnCl₂ sample, but with much less residue. The I_D/I_G value of the IFR/FeCl₃ sample reached even as low as 1.59, with the highest amount of residue. These results reconfirmed that a high degree of graphitization was achieved by adding a metal compound, and the effect of FeCl₃ was more significant.

To further understand the catalytic mechanism of metal compounds on IFR-treated PET fabric, residues obtained by roasting the fabric samples at specific temperatures in a muffle furnace for 10 min under air atmosphere were analyzed by means of FTIR. FTIR spectra of the residues of untreated, IFR- and IFR/FeCl₃-treated PET fabrics are shown in Figure 9.

Figure 9.

Fourier transform infrared spectra of residues of untreated (a), intumescent flame retardant (IFR) (b) and IFR/FeCl₃ (c)-treated polyethylene terephthalate fabrics at different temperatures.

As shown in Figure 9(a), the typical absorptions of untreated PET fabric were detected at 2985 and 2893 cm⁻¹ (symmetric and antisymmetric stretching vibration of -CH₂), 1245 cm⁻¹ (stretching vibration of -C-OH) and 720 cm⁻¹ (stretching vibration of -C = O). The spectra at RT, 300℃ and 400℃ were nearly the same, confirming that PET was thermally stable below 400℃. When the temperature was above 450℃, a new absorption peak at around 1600 cm⁻¹ appeared, which belongs to the stretching vibration of C = C, suggesting the formation of char. In the case of IFR-treated PET fabric, the intensities of the characteristic absorption peaks of PET decreased before 400℃, lower than those of untreated PET, suggesting that IFR promoted the decomposition of PET at the initial stage, which was consistent with the result of TGA. At 250℃ and above, an additional peak at 975 cm⁻¹ was detected, assigned to the symmetric stretching vibration of P-O-C⁴⁹; this peak still existed at high temperature, which indicated that the carbon layer formed contained a P-O-C structure. The FTIR spectra of IFR- and IFR/FeCl₃-treated PET fabrics were different and the main differences were the additional peak at 890 cm⁻¹ when the IFR/FeCl₃ fabric was heated to 450℃ and above, which was attributed to the antisymmetric stretching vibration of P-O-P,^50,51 and the peak at 1600 cm⁻¹ for IFR/FeCl₃-treated PET fabrics was broader than that for IFR-treated fabrics. According to the literature,⁵² the broad band in the region of 1500–1600 cm⁻¹ is typical for carbonaceous char that undergoes graphitization, and hence this indicated the addition of FeCl₃-enhanced formation of char with a high graphitization degree.

Catalytic carbonization mechanism

On the basis of the above discussion, it can be concluded that during the decomposition of the IFR/metal compound-treated PET fabric, the metal compound further increased the formation of char residue and the char was stabilized against further oxidation at a higher temperature. The proposed catalytic mechanism of metal compounds in the IFR system for PET fabrics depended both on the interaction between the metal compound and IFR, and that between the metal compound and PET. The metal compound can influence the thermal decomposition process of APP and improve the thermal stability of the as-formed char.⁵³ As for the interaction of metal and PET, it is known that PET first decomposes through an intra-molecular six-membered cyclic transition state with the formation of benzoic acid and vinyl benzoate.⁵⁴ The metal compounds with the Lewis acid characteristic could interact with the electron pairs of carbonyl groups, generating coordinate bonds. Then decarboxylation occurred during the further decomposition process, facilitated the formation of a C = C double bond, thus resulting in the formation of char.^24,25,55,56 On the other hand, metal compounds could catalyze the crosslinking of PET at an elevated temperature with the formation of char residue with high aromatic content. Taking FeCl₃ as an example, the interaction is illustrated in Figures 10 and 11.

Figure 10.

Schematic diagram of decarboxylation of polyethylene terephthalate catalyzed by FeCl₃.

Figure 11.

Schematic diagram of the crosslinking of polyethylene terephthalate catalyzed by FeCl₃.

Washing durability and physical properties

The data of bursting strength and whiteness of untreated, IFR, IFR/ZnCl₂ and IFR/FeCl₃-treated PET fabrics and the LOI value of the as-treated PET fabrics after 5 HLCs are shown in Table 6.

Table 6.

Washing durability and physical properties of untreated, intumescent flame retardant (IFR), IFR/ZnCl₂ and IFR/FeCl₃-treated polyethylene terephthalate fabrics

Sample	LOI (%) after 5 HLCs	Bursting strength (N)	Whiteness index
IFR	22.0 ± 0.2	488.0 ± 15	87.6 ± 0.5
IFR/ZnCl₂	22.3 ± 0.3	460.1 ± 12	86.2 ± 0.5
IFR/FeCl₃	22.7 ± 0.2	441.9 ± 12	73.5 ± 1.0
1% ZnCl₂	/	417.8 ± 10	82.0 ± 0.5
1% FeCl₃	/	405.0 ± 13	72.4 ± 1.0
Untreated	21.0 ± 0.2	433.3 ± 10	82.3 ± 0.2

LOI: limiting oxygen index; HLC: home laundering cycle.

As shown in Table 6, the bursting strength of the IFR-treated PET fabric was higher than that of the untreated fabric; the IFR physically coated on the fiber surface and acted like “sizing”, thus improving the bursting strength of the treated PET fabric. The two metal chlorides, especially FeCl₃, caused a reduction of 9% to the bursting strength; however, the strength of IFR/FeCl₃-treated PET fabric was still slightly higher than that of the untreated PET fabric. The whiteness of PET fabric treated by IFR/FeCl₃ decreased due to the reddish brown color of FeCl₃, but since the amount of FeCl₃ was small, no serious damage was observed.

After 5 HLCs, LOI values dropped to about 22%, which indicated that due to the lack of bonding force between the flame retardants and PET fabrics, the washing durability was poor. Further investigation is being undertaken to improve the washing durability in our laboratory.

Conclusion

The function of the IFR composed of APP, PER and MEL on the flame retardant performance of PET fabric and particularly the effect of metal compounds catalyzing carbonization based on IFR-treated PET fabric has been studied. The main conclusions are as follows.

The presence of IFR and IFR/metal compounds imparted PET fabric with excellent flame retardancy and anti-dripping properties. All seven metal compounds applied in this study enhanced the char formation and decreased the HRC, HRR and PHRR of the treated samples to varying degrees, which was affected both by the metal ions and the non-metal part.

Only Zr₂(HPO₄)₂, ZnCl₂ and FeCl₃ increased the LOI value compared with the IFR alone treated sample, with the highest LOI value up to 33.4%. FeCl₃ and ZnCl₂ were proved to be effective catalysts for the carbonization of PET at an elevated temperature. Furthermore, FeCl₃ and ZnCl₂ enhanced the graphitization degree of chars of IFR flame-retarded PET fabrics.

The effect of metal compounds in the IFR system for PET fabrics was supposed to depend both on the interaction between the metal compound and IFR, and the metal compound and PET. Metal compounds interacted with carbonyl groups, then a decarboxylation occurred during the further decomposition process, resulting in the formation of char; metal compounds catalyzed crosslinking of PET to promote char formation. Of the seven metal compounds studied in this research, Fe³⁺ possesses the largest electric negative properties and the shortest atomic radius, and thus is possessed of much stronger catalytic ability.

Due to the lack of bonding forces between the flame retardant/metal compound and PET fabric, the washing durability was poor; further investigation is being undertaken to improve the washing durability in our laboratory.

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 disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Chinese National High Technology Research and Development Program 863 Project (2013AA06A307).

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