Thermal Degradation of Cotton Cellulose Modified with THPC
and Its Metal Complexes
12LI Chenming & WU Weihong
(1 Department of Resources and Environmental Engineering, North China Institute of Science & Technology, Beijing 101601, China;
2 College of Science, Agriculture University of Hebei, Baoding 071000, Hebei, China)
Abstract: Cotton cellulose has been treated with tetrakis (hydroxymethyl) phosphonium chloride (THPC), urea and small amounts of ammonium dihydrogen
phosphate (ADP) to impart flame retardancy. Complexe of cell-THPC-urea-ADP with metal ion such as Cu(?) have been prepared. The thermal behavior of
samples in air has been studied by differential thermal analysis (DTA) and thermogravimetry (TG) from ambient temperature to 800?. From the resulting data,
various thermodynamic parameters for different stages of thermal degradation have been obtained by following the method of Broido. Experimental data show
that for treated samples there is a decrease in the activation energy of decomposition process and an increase in char yield compared with cotton cellulose. The
IR spectra of the thermal degraded residues indicate that dehydration takes place earlier for treated samples compared with pure cellulose.
Keywords: cotton cellulose; degradation; flame retardant; metal complexes; thermal analysis
Textile materials are used extensively to make life pleasant, comfortable and colorful. Unfortunately, with few exceptions, they
are flammable and cause a fire hazard. As for cotton cellulose, one of the major textile materials, it is extremely flammable. So the
emphasis on reducing combustibility has centered by the chemical modification of cotton cellulose to increase safety of use. Cotton
cellulose is made flame-resistant by treatment with proper flame-retardants. However, with environmental sustainability and safe
required, the effects of flame-retardants on both smoke generation and the toxicity of the combustion products have become of [1,2]special importance, as flame-retardant compositions have been reported to produce denser smoke than untreated composition. In [3,4]previous papers, compounds of transition metals have been found to be most effective smoke retarders. So the main objective of the work reported here was to investigate the effects of transition metal ions on the thermal degradation of cellulose modified with
tetrakis(hydroxymethyl) phosphonium chloride (THPC), urea and ammonium dihydrogen phosphate (ADP).
In this paper, coordinate complexe of cell-THPC-urea-ADP with transition metal ion such as Cu(?) was prepared. The kinetics
of the thermal degradation of samples were studied from ambient temperature to 800 ? by using thermogravimetry. Further, the
composition of the charred products at certain temperature was evaluated by spectroscopic techniques. 2 Experiment
Cotton cellulose of commercial grade (Beijing Sanitary Plant, China) was selected for flame-retardant treatment. The cotton cellulose was immersed in 24% NaOH solution at room temperature for 24 h (mercerization process). The alkali was then filtered off and
the sample was washed repeatedly with distilled water. The sample was dried in an oven at 60 ? and then stored in a desiccator.
2.2 Fiber Treatment
THPC(The Pesticide company of Shanghai, China) was neutralized with NaOH to give a pH value equal to 6.5 and its 45% solution was mixed with 22.5% urea solution. The pH value was adjusted to 6.5 and a small amount of ADP was added. The resulting mixture
was used as the treating solution. The mercerized cotton cellulose was immersed in the treating solution for 30 min at room temperature.
The treated cotton cellulose was dried at 60 ? in an oven for 60 min. Curing of these treated cellulose was carried out by heating at 160 ? for 5 min in the oven. After cooling, the sample was thoroughly washed with distilled water for an hour and dried in an oven at
60 ?. Cu(?) complexe of cell-THPC-urea-ADP were prepared by treating 6g of cell-THPC-urea-ADP in each instance with 5% aqueous solutions of CuSO•5HO at room temperature for 72 h under constant stiring. Each product was washed repeatedly with water 42
until the filtrate was free from metal salt and dried overnight in an oven at 60 ? then stored in a desiccator.
2.3 Thermal Analysis
Differential thermal analysis (DTA), thermogravimetry (TG) and differential thermogravimetry (DTG) were carried out using a DT-40 thermal analyzer(Shimadzu,Japan). The DTA, TG and DTG curves were obtained under a dynamic air (dried) atmosphere at a -1heating rate of 20 ?min. The DTA measurements were made relative to calcined alumina.
2.4 Infared Spectral Analysis
For the IR studies, KBr discs containing 2% of charred products of cotton cellulose and treated cellulose were prepared and analyzed using a Bio-Rad FTS-40 FTIR spectrophotometer. The charred samples were prepared by heating the compounds in a DTA cell. Heating was stopped at the desired temperature for half an hour and the residues were quickly transferred into a stoppered
3 Results and Discussion
The DTA, TG and DTG curves of (i) cotton cellulose, (ii) cell-THPC-urea-ADP, (iii) Cu(?) complexes of cell-THPC-urea-ADP
were obtained in a dynamic air atmosphere from ambient temperature to 800 ? and are shown in Figure 1-3. To support the
interpretations involved in the discussion of the various process taking place during the thermal degradation of cell-THPC-urea-ADP
and its metal complex, the samples were subjected to the thermal degradation in air in the temperature range 200-350 ? and the IR
spectra of chars and shown in Figures 4-6, respectively. The related date was determined and are given in Table 1
Table 1 Thermal and thermodynamic parameters for cellulose, cell-THPC-urea-ADP and its metal complex in air
in DTA /? TE/ / EChar Peaka1a2Samples Compound -1-1st(kJ•mol) (kJ•mol) yield Endo 1 exo 2st exo
i Cotton cellulose 75 364 460 189.47 97.61 24.5
ii Cell-THPC-urea-ADP 69 335 491 80.89 25.61 45.9
Cu(?) complex of iii 71 331 604 146.33 17.71 37.6 Cell-THPC-urea-ADP
3.1 Differential Thermal Analysis Studies
From the DTA curves of samples (i-iii), the peak temperatures for various endotherms and exotherms are shown in Table 1. An endotherm below 90 ? in all the compounds is due to the loss of absorbed moisture. The curve for cotton cellulose shows two large exotherms with their peak maxima at 364 and 460 ?. The first exotherm represents a degradation and decomposition process. The second exotherm, as the thermal degradation was carried out in a dynamic air medium, represents oxidation of the charred residues.
The degradation process dominates at low temperatures and leads to rearrange, evolution of carbon monoxide, carbon dioxide, etc.,
and formation of carbonyl and carbonyl groups and ultimately a carbonaceous residue. At higher temperatures, cleavage of glycosyl units by intramolecular transglycosylation starts, forming ultimately a tarry mixture with levoglucosan as the major constituent. Levoglucosan decomposes into volatile and flammable products, such as furans, aldehydes, ketones, aromatic hydrocarbons, etc.,
and therefore plays a key role in the flammability of cellulose. The heat liberated is partially transferred back to cellulose surfaces to
continue polymer pyrolysis, maintaining a continuous supply of gaseous fuel for further propagation. The main role of flame-retardants is to minimize the formation of levoglucosan by lowering the decomposition temperature of cellulose and enhancing char formation by catalyzing the dehydration and decomposition reaction.
T EXOTHERM?DTA?50 T EXOTHERM? DTA?50MASS LOSS, %MASS LOSS, %
DTGDTG?ENDOTHERM 100100?ENDOTHERM 01002003004005006007008000100200300400500600700800TEMPERATURE, ?TEMPERATURE, ?
Fig.1 Thermal analysis of cotton cellulose in air Fig.2 Thermal analysis of cell-THPC-urea-ADP in air
MASS LOSS, %
Fig.3 Thermal analysis of Cu(?) complex of Cell-THPC-urea-ADP in air
The DTA curve of cell-THPC-urea-ADP is quite distinct from that of cotton cellulose. It shows two exotherms with their peak maxima at 335 ? and 491 ?. The first exotherm is shifted to a lower temperature but the second is shifted higher. On heating, dehydrohalogenation and dephosphorylation take place, which can catalyze the dehydration of cellulose and some bond formation. The bond formation is probably due to a phosphorylation reaction at the C-6 hydroxyl group of the anhydroglucose unit as suggested by Hendrix. Phosphorylation at this position would inhibit the formation of levoglucosan and prevent further breakdown to flammable gases. This would account for the increased amount of char formed over that for untreated cotton cellulose. Because oxidation of charred residues becomes difficult, the second exotherm is shifted to a higher temperature. The cause of the increased
char oxidative resistance is believed to be a consequence of a combination of reduced oxygen diffusivity of the char physical structure and the presence of phosphorus in the cross-linked char chemistry.
In the DTA curves of the Cu(?) complexe of
cell-THPC-urea-ADP (sample iii), the first exotherm with their peak maxima in the range 321-345 ?, represents the decomposition process.
In the second exotherm, the peak temperatures of 604?, which are
higher than the second exothermic peak temperature of
cell-THPC-urea-ADP. These indicates that oxidation of the charred residues becomes more difficult due to the existence of metal. 3.2 IR Spectra
The above interpretations are also supported by the IR spectra of the chars of cotton cellulose, cell-THPC-urea-ADP and Cu(?) complex
From Figure 4-6, the changes in the IR spectrum are as follows. With the temperature increased, there is a decrease in the intensity of the absorption bands due to hydroxyl stretching and bending (3300 and 3400 -1) for cotton cellulose. At 350 ?, it still keeps a little absorptive cm
peak. But for phosphorylation cellulose, the absorption of this band
Fig.4 IR spectra of (a) cotton cellulose, and disappears at 300 ?. For Cu(?) complex of cell-THPC-urea-ADP,
(b-e) the chars and of cotton cellulose at 200, there is almost no absorptive peak at 250 ?. These indicate hydration
250, 300 and 350 ?, respectively process starts earlier for treated cellulose than untreated cellulose. The -1bands at 2900 cm (C-H stretching) are missing for cotton cellulose and cell-THPC-urea-ADP at 300 ?. But for Cu(?) complex this band
disappears at 250 ?, which indicates that the decomposition is taking -1place at a lower temperature. At 300 ?, new bands appear at 1720 cm -1(C=O stretching) and 1630cm (C=C stretching) for cotton cellulose,
which indicates that bond formation is taking place. However, this band appears at 250 ? for cellulose treated with flame-retardants, which also indicates that decomposition and bond formation starts earlier than -1pure cotton cellulose. At 300 ?, there is a shift of the band at 1630 cm -1to 1600 cm (due to conjugated C=C) for treated cellulose, which indicates that the extension of the conjugation of the C=C bonds in the residues from flame-retardant cellulose is taking place. At 350 ?, all
the normal bands due to cellulose and its derivatives disappear and -1-1intense bands at 1720 cm and 1600 cm remain, suggesting the
formation of compounds containing C=O and C=C groups, respectively. 3.3 Thermogravimetric studies The TG curves of samples (i-iii) (Figs1-3) show two significant Fig.5 IR spectra of (a) cell-THPC-urea-ADP, (b-e) areas of weight loss, which have been termed here the first stage and the the chars of cell-THPC-urea-ADP at 200, 250, 300 second stage. The kinetic parameters for these two stages were determined using the following equation, given by Broido: and 350 ?, respectively
Where y is the fraction of the number of initial molecules not yet decomposed, T the temperature of the maximum reaction rate, β the m
rate of heating and Z the frequency factor. Using the Broido method, from the slopes of the TG curves in Figure 1-3, plots of Ln(Ln1/y) vs.
1/T for various stage of thermal degradation were drawn and linear plots were obtained in each instance. The values of the energies of activation Ea determined from the slopes are given in Table 2.
For the first stage, the major degradation and weight-loss stage, the energies of activation for treated samples (in ranges -180.89-146.33kJ•mol) are low compared to cotton cellulose -1(189.47kJ•mol). The reason is that flame-retardants catalyze the decomposition reaction . This stage corresponds to the first exotherm in the DTA curves. After the major decomposition stage, the activation -1energies of the second stage are in the range 17.71-25.61kJ•molfor Fig.6 IR spectra of (a) cell-THPC-urea-ADP, -1treated samples. But cotton cellulose is 97.61 kJ•mol, higher than the And (b-e) the chars of Cu(?) complex of others. These indicate that the decomposition reaction is easer and Cell-THPC-urea-ADP at 200, 250, 300 and occurs at lower temperature, resulting in less levoglucosan which 350 ?, respectively produces less flammable gas. So the combustibility of cellulose was
decreased by the treatment.
3.4 Char Yield
It is generally observed that the amount of char formed during thermal degradation of cotton cellulose treated with a particular flame-retardant is related to the degree of flame resistance exhibited by the treated cellulose sample. In order to understand the flame-retardant properties of these compounds, the char yields (in mass %) were determined at 400 ? from TG curves and are given in Table 2. Samples (ii-?) show much higher char yields than cotton cellulose. Larger amount of char is formed at the expense
of combustible volatile products of thermal degradation, thus suppressing combustion and making for more effective flame-retardant property.
For the treated cellulose, the activation energy of decomposition and char yield are found to decrease, which indicates that the decomposition reaction is easer and occurs at lower temperature, resulting in less levoglucosan which produces less flammable gas. The char yield is found to increase. It is suggested that the lager amount of char is formed at the expense of combustible volatile products of thermal degradation, thus suppressing combustion and making for more effective flame-retardant property, resulting in the lower combustibility of treated cellulose which can be used more safely.
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