Xu Jianzhong, Zhao Xiaolong, Qu Hongqiang
(College of Chemistry and Environmental Science, Hebei University, Hebei Province, Baoding 071002, China)

1 INTRODUCTION
Nowadays more and more people pay attention to the hazards from fires. Because fires not only cause many deaths, injuries and considerable financial losses, but also pollute our environment seriously. The fibrous products' flammable nature is one of the major problems of the present time because of many fires from textiles. Wool with natural flame retardant property has provoked people's interest.
    Wool, a cutin fibrous protein, contains many kinds of cysteine, thiocarbamic acid, and cross-linking polypeptides with a helical structure. The natural flame retardant property of the wool is connected with its relatively high nitrogen content (16%), high moisture content (10-14%) ^[1], high ignition temperature (570-600ºC)^[2], low heat of combustion (20.5KJ/g), low flame temperature (677ºC) and a relatively high limiting oxygen (LOI) (25-28%) ^[3]. As a kind of fiber used extensively, it is necessary to apply flame retardant treatment to wool. Many reports have been published on the use of flame retardant reagents for wool^[3,4]. Benisek studied the effectiveness of titanium ( ) salts in the presence of a-hydrolysis carboxylic acids^[5]. THPC^[6], vinyl phosphonate^[7,8] and (NH₄)₃Cl[Ca(H₂PO₄)₄], (NH₄)₃Cl[Mg(H₂PO₄)₄]^[9] et al. were also reported having improved the flame retardant property of wool. As far as we know, however, few systematic study of of wool treated with Titanium tetrachloride, Ammonium bifluoride and one of dicarboxylic acids has been reported. In this paper, the thermal properties of wools treated with a series of such flame retardant reagents were studied by thermal analysis, LOI, char yield and SEM.

2 EXPERIMENTAL
2.1 Flame retardation of wool
A fabric of SANLI (Hebei Province Baoding Fabric Plant, P.R. China) wool was first boiled with water at 100ºC for 2h. Then it was washed with water and dried out.
    Sample (I) was pure wool. Sample (II)-(IX) were obtained by dipping the dried wool into an aqueous solution of one of the eight flame retardant reagents, namely: 0.07%TiCl₄ +1.5% NH4HF₂, 0.07% TiCl₄ +1.5% NH₄HF₂ + oxalic acid, 0.07% TiCl₄ + 1.5% NH₄HF₂ + malonic acid, 0.07% TiCl₄ + 1.5% NH₄HF₂ + succinic acid, 0.07% TiCl₄ + 1.5% NH₄HF₂ + glutaric acid, 0.07% TiCl₄ + 1.5% NH₄HF₂ + adipic acid, 0.07% TiCl₄ + 1.5% NH₄HF₂ + suberic acid, 0.07% TiCl₄ +1.5% NH₄HF₂ + azelaic acid (the consentration of all the samples is mass fraction). The weight ratio of liquor to wool was 30:1. The molar ratio of TiCl₄ to dicarbonic acid was 1:1.
    HCl (37% mass fraction) was added to each of above flame retardant reagents. Each reaction mixture was exhausted at 85ºC for 1h. After being exhausted, the treated wools were dried at 60ºC Finally, they were used for thermal analysis, LOI, char yield and SEM.
2.2 Thermal analysis
Differential thermal analysis (DTA) and thermogravimetry (TG) were carried out on a DT-40 thermal analyzer. The DTA and TG curves were run under static air at a scanning rate of 10Kmin^-1. Calcined alumina was taken as the reference material.
2.3 Calculation
The kinetic parameters for the various stages of pyrolysis of wool without and with flame retardant treatment were determined using the method described by Broido^[10]. The equation of Broido can be written as
Ln(ln1/y)=-E/RT+ ln(RZT²/Eb)
    Where y is the mass fraction of initial samples has not been decomposed, T is the temperature of maximum reaction velocity, b is the rate of heating (Kmin^-1), Z is the frequency factor, E is the activation energy and R is the gas constant equal to (8.314Jmol^-1K^-1). Using Broido's method, the activation energies can be determined from the slopes by plotting of ln(ln1/y) versus 1/T. Table 1 presents activation energies and the decomposition temperatures of the exothermic peaks for all the samples.
2.4 Limiting oxygen index
The LOI values were determined in accordance with ASTM D2863-70 by means of a General Model HC-1 LOI apparatus.
2.5 Char yield
The samples were weighed accurately and put into a weight changeless crucible. Then the crucible was put in the muffle furnace. The experiment was carried out at 400ºC for 30 min with the muffle furnace full of N₂. Finally, stop heating the muffle furnace and made the temperature drop naturally. When the temperature reached the room temperature, the weight of the char was quantified. Char yield values were calculated by the equation:
Char yield(%)= w₂/w₁
Where w₁ and w₂ were the weight of wool samples before combustion and the residuce after combustion of wool samples respectively.
2.6 Scanning electron microscopy
AMARRY-1000B scanning electron microscopy was used in this experiment in order to analyze the interior structures of pure wool and treated wools.

3 RESULTS AND DISSCUSSIONS
3.1 Thermal analysis
Figure.1 and Figure.2 are DTA and TG curves of the pure wool and treated wool. The kinetic parameters obtained from the DTA and TG curves are listed in Table 1. The results indicate that three processes take place in the wool pyrolysis progress. The first, which ends at around 120-160ºC, is an endothermic process and corresponds with the desorption of water physically bound to fiber and the dehydration of wool.
    The second important process coincides with the temperature range over which a number of defined pyrolysis reactions take place in wool. The hydrogen- bond peptide helical structure ruptures and the ordered regions of the wool undergo a solid to liquid phase change, also cleavage of the disulfide bonds occurs and a number of volatiles are released including hydrogen sulfide and sulfur dioxide^[11]. As shown in Figure.2, compared with pure wool, TG curve of sample (IV) has lower decomposition temperature, following a smaller activation energy (E₁). It is the fact that the presence of TiCl₄, NH₄HF₂ and dicarboxylic acid, which reacts on condensed phase^[8], catalyzes the thermal decomposition of the wool.
    The third process is an exothermic reaction that the char oxidation reactions dominate. In Figure.2, the exotherm in DTA curves of sample (IV) is much sharper and take place at lower temperature (T_m) than that of pure wool, and TG curve of sample (IV) in this process is sharper than that of pure wool. Because the presence of the flame retardant reagents appears to have resulted in a cross- linked complex, which can be achieved in graphite-like structure, and possibly aromatic char which has a high-than-expected resistance to oxidation^[11], the treated wools show higher activation energies (E₂).

Figure.1 DTA and TG curves of sample (I)(pure wool)

Figure.2 DTA and TG curves of sample (IV) (wool treated with 0.07%TiCl₄ +1.5% NH₄HF₂+ malonic acid)

3.2 LOI, char yield and SEM
The LOI and char yield values are listed in Table 1. Compared with the pure wool, the LOI and char yield values of the wool treated with TiCl₄ and NH₄HF₂ increase and the values of LOI and char yield of the wools treated with TiCl4, NH₄HF₂ and dicarboxylic acids increase furtherly. This indicates that TiCl₄, NH₄HF₂ and one of dicarboxylic acids consist of a flame retardant synergistic system which more efficiently prevents wool from burning. The SEM photographs also proved this effect. Figure.3-5 are SEM photographs of sample(I), sample (II) and sample(IV). In Figure.4, we can see that a kind of membrane structure begins to form in sample (II). In Figure.5, the excellent membrane structure has completely formed in sample (IV). Because such membrane structure can form a barrier to inhibit combustible gases and heat energy to go into the interior room of wools, it is beneficial to the improvement of flame retardance. From this we can infer that the flame retardant property of sample (IV) is much better than that of pure wool and sample (II). This is in accordance with LOI and char yield.

Table 1 Thermal characteristics and kinetic parameters of the pure wool and modified wools

Sample No.	LOI (%)	Char yield (%)	Tm (ºC)	E1 (KJ/mol)	E2 (KJ/mol)
(I)	25.5	23.16	535	91.1	196.9
(II)	30.5	26.29	510	81.2	267.5
(III)	34.5	33.27	478	77.1	327.6
(IV)	38.0	36.41	523	75.8	361.9
(V)	36.0	36.83	492	78.2	246.6
(VI)	35.5	35.36	515	79.8	252.0
(VII)	38.0	39.69	523	80.0	386.5
(VIII)	34.5	28.53	515	84.5	385.1
(IX)	33.5	31.52	522	86.2	324.2

sample (I): pure wool;   sample (II): wool treated with 0.07% TiCl₄ +1.5% NH₄HF₂ ; sample (III)-(IX): wool treated with 0.07% TiCl₄ +1.5% NH₄HF₂ + oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid and azelaic acid respectively.

Figure.3 SEM photograph of sample (I) (pure wool)	Figure.4 SEM photograph of sample (II) ( wool treated with 0.07% TiCl4 +1.5% NH4HF2 )

Figure.5 SEM photograph of sample (IV) ( wool treated with 0.07% TiCl4 +1.5% NH4HF2 + malonic acid)	Figure.6 Fourier transform IR spectra of sample (III) (wool treated with 0.07% TiCl4+ 1.5% NH4HF2 + malonic acid) (a) and sample (I) (b) charred at 200℃.

3.3 Flame retardant mechanism
From the IR spectra we can see that the treated wool shows the bond of TiF₆^2- at 585cm^-1. This indicates TiF₆^2- is formed by the reaction of TiCl₄ with NH₄HF₂^[12]. It is known that the TiF₆^2- is stable and largely undissociated particularly in strongly acid solutions^[13]. Therefore it is exhausted on to the positively charged wool in acid solution as the negative TiF₆^2- ion according to the equation:
TiF₆^2-+wool-NH₃⁺→wool-NH₃⁺TiF₆^2-
The TiF₆^2- in the fiber very probably hydrolyses to form TiOF₂ after several washes. Heating the treated wool also can make TiF₆^2- convert into TiOF₂^[14]_. TiOF₂ is not a combustible substance. Because TiOF₂ exists on the surface of the wool and form a barrier to inhibit air to go into the interior room of wool, it is responsible for the flame retardance. The existence of dicarboxylic acids stabilizes titanium combined with wool maybe by the formation of such compound^[15]:

(R is (CH₂)_n), so the flame retardant property of wool treated with the synergistic system is improved.

4 CONCLUSION
Compared with pure wool, the treated wools show : (a) lower decomposition temperature, (b) increases in LOI and char yield values, (c) decrease in activation energy in the second process and increase in the third process. The flame retardant propertis of treated wools are much better than that of pure wool.

REFERENCES
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[2] Benisek L. British Patent 1372694, 1974.
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[4] Wang J Q and Feng D M. Polym. Deg. Stab., 1994, 43: 93.
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[7] Friedman M and Stillin. Text. Research J., 1970, 40: 70.
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[9] Tian C M, Shi Z H and Zhang H Y et al. Thermochimica Acta, 1995, 284: 435.
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[11] Davies D J, Horrocks A R and Miraftab M et al. Polym. Int., 2000, 49: 1125.
[12] Benisek L. Text. Research J., 1975, 45: 351.
[13] Schmitt R M, Grove E L and Brown R D et al. J. Amer. Soc., 1960, 82: 5292.
[14] Mellor J W. Inorganic and Theoretical Chemistry, 1963, 7: 138.
[15] Thelen J, Knott J and Zahn H. Bull Sci Inst Text, 1980, 9 (36): 279.