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Thermal characterization of rubberwood-polymer composites
Radiat. Phys. Chem. Vol. 33, No. 3, pp. 197-204, 1989 Int. J. Radiat. Appl. Itwtrtm. Part C Printed in Great Britain. All rights reserved
0146S724/89 $3.00 + 0.00 Copyright 0 1989 Pergamon Press plc
THERMAL CHARACTERIZATION OF RUBBERWOOD-POLYMER COMPOSITES K. Y. Department
CHAN,’ M. G. S. YAP? L. H. L. CHIA’ and K. G. NEOH~
of Chemistry and 2Department of Chemical Engineering, National University of Singapore, Kent Ridge 0511, Singapore (Received 10 May 1988)
Abstract-The thermal properties of five types of radiation-induced wood-polymer composites based on a tropical hardwood, rubberwood (Ifeoea bruziliensts),was studied by oxygen index measurement, differential thermal analysis (DTA) and thermogravimetry (TO). The DTA and TG curves of composites were different from those of rubberwood, which can be attributed to the presence of the incorporated polymers. Of the five composites, the one impregnated with bis(2-chloroethyl)vinyl phosphonate reduced the initial temperature of decomposition, increased the peak temperatures of exothermic reactions, and increased the char yield. Comparison with physical blends of rubberwood and the corresponding polymer provided some evidence of chemical interaction of wood and polymer in some of the composites.
INTRODUCTION The impregnation of vinyl monomers into wood followed by in-situ polymerization is known to result in composite materials with enhanced strength, di-
mensional, and thermal properties. Flame retardancy has been imparted to woods that were incorporated with phosphorus or chlorine containing polymers, or the copolymerization products of these with more flammable polymers like polymethylmethacrylate, polyacrylonitrile and polystyrene (Janazzi et al., 1964; Arni and Jones, 1964; Raff et al., 1966; Ahmed et al., 1970; Siau et al., 1972; Lebedev et al., 1972). There is however, relatively little work that relates the observed flame retardancy in composites with the chemical modification of wood thermal decomposition behaviour by the polymer impregnant. In recent years, a number of thermal analytical techniques have been used in the study of thermal degradation of wood (Tang, 1970; Nguyen et al., 1981; Shafizadeh, 1985) and wood impregnated with polymers (Lubke and Jokel, 1983; Czechowski and Zakrzewski, 1983). In the present study, the thermal properties of rubberwood (Heuea braziliensis), and five of its polymer-composites are characterized by means of oxygen index measurements, thermogravimetry (TG) and differential thermal analysis (DTA). This work is part of the ongoing research at the National University of Singapore investigating various wood-polymer composites based on Malaysian tropical hardwoods (Chia and Kong, 1981; Chia et al., 1986). MATERIALS AND METHODS Monomers
Methylmethacrylate, (1) Schuchardt).
MMA
(Merck-
Methylmethacrylate-25% vinyl(2) 75% idenechloride, MVDC (the latter from Aldrich Chem.). (3) 75% Methylmethacrylatc-25% bis(Zchloroethyl)vinyl phosphonate, MBP (the latter from Polysciences Inc.). (4) Acrylonitrile, AN (Aldrich Chem.). (5) 60% Styrenm% acrylonitrile, STAN (styrene from Merck-Schuchardt). Preparation of test specimens Test specimens for oxygen index studies were cut from bulk material to dimensions of 6.5 f 0.5 mm width, 3.0 f 0.5 mm thickness, and about 100 mm length. A sufficient number of specimens (about 20 for each type of composite) were impregnated with the appropriate monomer solution and irradiated with a dosage of 4.0 Mrad from a cobalt-60 source according to the method described by Chia and Kong (1981). All specimens were dried in a vacuum-oven at 60°C to constant weight before testing. Thermal analysis was carried out on fine shavings of rubberwood (RB) or composites (RB-polymer) less than 1 mm thick and weighing 8.0 &-0.1 mg. The amount of polymer in composites could only be estimated from the polymer loading of the bulk material from which the shavings were obtained. For purpose of comparison, samples of the various polymers and their physical blends with RB were similarly prepared for analysis.
Oxygen index measurements The oxygen index (01) is defined to be the minimum concentration of oxygen, expressed as percent volume, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature. The design of the 01 test apparatus was based on ASTM D2863-77. 197
198
K. Y.
CHAN et
The criterion for a positive burn was chosen arbitrarily to the sustenance of 100 mm of combustion. A single positive burn was accepted as indicating a concentration at or above the 01; but at least three negative burns were required for identifying a concentration that was below the 01. In each case, combustion was allowed to continue uninterrupted until glowing ceased. The flaming and glowing times and other flaming characteristics were also recorded. TG and DTA measurements
Thermal analysis by TG and DTA was carried out using Netzsch Simultaneous Thermal Analysis Apparatus STA 409. DTA measurements were referenced to burnt kaolin. The test sample was heated in a crucible from 20 to 800°C at lO”C/min and then held at 800°C for a further 20min. Air flow through the sample was maintained at 100 ml/min at room temperature and pressure. The residual weight of sample as given by TG curves was expressed as fractional weight loss (FWL): Fractional
wg- U’ weight loss = ~ I+‘0 ’
(1)
where w0and w are the residual weights at 100°C and temperature t”C (t > 100) respectively, both corrected for buoyancy. An initial residual weight at 100°C was chosen so that any moisture content in the sample was eliminated. Duplicate analysis of randomly selected samples confirmed the reproducibility of the results. DISCUSSION OF RESULTS
Oxygen indices and Jaming
characteristics
The results of the 01 measurements are presented in Table 1. The 01 test assumes that inherently less flammable materials require greater oxygen concentrations to produce the heat necessary for the con-
al.
tinued production of flammable volatiles and flame propagation. Therefore among the five composites tested, only RB-PMBP exhibited improved flame retardancy as it had an 01 which is significantly higher than that of RB. It is also significant that the burning rate of RB-PMBP (as indicated by the flamming time of 2.7-2.8 min) was comparable to that of RB even though the former had been tested at such high oxygen concentrations as 34%. Effective flame retardancy is associated with small localized flames, minimal smoke/fume generation, short glowing time, high char yields, and in the case of synthetic polymers, minimal dripping of burning polymer melt. It is now known that decomposition of the cellulose fraction is primarily responsible for the flaming combustion, and solid-phase oxidation of the lignin fraction is primarily responsible for the glowing reactions (Barker and Drews, 1985). Table 1 shows that RB-PMBP compared favourably against the other composites in terms of these characteristics. These observations concerning PMBP are consistent with the findings of other researchers (Ahmed et al., 1970). Thermal decomposition of rubberwood
The thermal behaviour of RB is indicated by its TG curve [Fig. l(a)i] and DTA curve [Fig. 2(a)i]. These curves are typical of wood in general and can be regarded as a summation of the individual behaviours of its carbohydrate and lignin components. Mixtures of cellulose, hemicellulose and lignin in the same proportions as in wood have been found to give similar TG and DTA curves (Tang, 1970). The TG curve of RB was computed based on an original mass of wood at 100°C. A small weight loss prior to 100°C was attributed to the evaporation of moisture. It has been suggested from literature (Tang 1970; Nguyen et al., 1981) that pyrolysis takes place between 150 and 200°C with the hemicellulose start-
Table I. Oxygen indices and flaming Polymer loading (%I
26-21
RB RB-PMMA
RR-PMVDC
RB-PMBP
RB-PAN
RB-PSTAN
Oxygen index (%)
2&39
&I5
42-53
1l-18
45-64
25-26
28-29
33-34
27-28
28-29
Flaming time (min) 2.80
Flame description Candlelike,
characteristics Smoke and fumes
Dripping
Char
1.37
Little
White fumes, black smoke
Liquid exudation
1.70
Little
2.12
Rigld rod, breaks under own weight
-
Whole length of char
localised
4.26
Candlelike,
2.82
Big flame turning small and localised
White fumes, black smoke
Liquid exudation
2.70
Small localised flame, splits into vertical flames
Small amounts of fumes and black smoke
Little
2.32
Strong flame splits into non-vertical flames
White fumes and some black smoke initially
Liqud exudation with spattering
I .98
Rigid rod, breaks under own weight
Strong flame engulfing the whole length of rod
Tall stream of smoke, soot flies about
LIttIe, but rod expands and splits
I .78
Little
4.07
localised
Glowing time (min)
Thermal characterization
ing to decompose at about 2OO”C,followed by lignin at about 220°C and subsequently by cellulose at 250°C. At temperatures greater than 250°C rapid weight loss was observed which is due to the volatilization of the wood components. Pyrolysis can be considered to be complete around 360°C as indicated by a small shoulder in the TG curve around this temperature and at a FWL of 0.72. Since the test was performed using air, the next phase of weight loss around 400°C can be attributed to the oxidative combustion of the non-volatile pyrolysis products. Complete decomposition was achieved at around 480°C when the TG curve asymptotically approached a FWL of 0.99. Corresponding to the decomposition behaviour described above, the DTA curve of RB shows two exothermic peaks. The broad peak at 340°C was caused by the highly exothermic ignition and combustion of the volatile products. The second exothermic peak at 450°C is due to the oxidative combustion of the carbonaceous char. Thermal &composition
of RB-polymer
composites
The TG curves and DTA curves of the RBpolymer composites are shown in Fig. l(a-e) and Fig. 2(a-e) respectively. Three particular aspects of these curves are examined: (1) the initial temperature of decomposition; (2) the temperatures of the first and second exothermic peaks; and (3) the char yield. The
1.0
of WFC
199
effectiveness of a flame retardant is often measured in terms of its ability to lower the decomposition temperature of the treated wood specimen. Jain et al. (Jain et al., 1985) reported that if the onset of thermal decomposition took place at lower temperatures, then a lower percent of flammable volatile products would be formed, leading ultimately to a high char yield. Furthermore, the exothermic reactions associated with the combustion of pyrolysis products will take place at higher temperatures. This seems to be the mode of operation of inorganic salts (Shafizadeh, 1985). The initial temperature of decomposition, Tdr of RB and the other samples in this work was chosen on the criterion of a 5% weight loss (FWL = 0.05 on the TG curves). These temperatures are listed in Table 2. It can be seen that only PMVDC and PMBP lowered the T, of RB from 255 to 210 and 216°C respectively. These two polymers have relatively low Td themselves (below 2OO”C), and therefore, their flame retardant action in this temperature range was expected. To assess the char yield, FWL values were read off from the TG curves at 340 and 450°C (the two exothermic peaks of RB), and were tabulated in Table 3. A low FWL corresponded to high char yield. At 34OC, RB-PSTAN had the lowest FWL among the five composites, followed jointly by RB-PMMA and RB-PMBP. At 450°C however, RB-PMMA was almost completely burned while RB-PSTAN and
(a)
0.9
0.6
3
0.7
E -I
0.6
2 i ._a
0.5
g 2 .P z
0.4
t e IL 0.3
0.2
0.1
0 300
500
400
Temperature
Fig. 1. (a)
(‘Cl
600
700
200
K. Y.
Cnm et al.
Temperature
(‘C )
Fig. I. (b)
‘.O - (cl 0.9 0.0
_
-
0.7-
? -1
0.6-
2 i .”
0.5
-
0.4
-
0.3
-
0.2
-
0.1
-
s d ,cj t e IL
0 100
200
300
500
400 Temperature
Fig. 1. (c)
L”C1
600
700
Thermal characterization 1.0
-
of WPC
201
(d)
0.9
0.8
0.7 3 L -ia
0.6
s E .P
0.5
g d .E
0.4
t
/
z
/
0.3
0.2
, / 0.1
*’
/
/
//
/’ I
I
0
/
500
400
I 600
I 700
Temperature (“Cl
Fig. 1 (d) 1.0
3 3 LL j
0.7
0.6
E 9 3
0.5
G .;5 0 e L
0.4
0.3
0 100
200
300
500
400
Temperature
600
700
(‘0
Fig. 1 (e) Fig. l.(a) TG curves of(i) RB (-), (ii) RB-PMMA (-.-.), (iii) RB/PMMA blend (OOOOOO), and (iv) PMMA (- -- --). (b) TG curves of(i) RB (-), (ii) RB-PMVDC (. -. -), (iii) RB/PMVDC blend (000000). and (iv) PMVDC (----- ). (c) TG curves of(i) RB (-), (ii) RB-PMBP (.--.--), (iii) RB/PMBP blend (OOOOOO), and (iv) PMBP (-----). (d) TG curves of(i) RB (-), (ii) RB-PAN (.-.--), (iii) RB/PAN blend (000000). and (iv) PAN (-----). (e) TG curves of(i) RB (-), (ii) RB-PSTAN (.-.-e), (iii) RB/PSTAN blend (000000).
K. Y. CHAN et al.
(a)
(iv)
200
300
400
Temperature
500
600
L_I
1°C)
400
Temperature
500
600
(‘Cl
(d)
I
I
I
I
I
200
300
400
500
Temperature
I 600
200
300
400
Temperature
(“Cl
500
600
(‘0
(e)
A Temperature
(1)
Fig. 2.(a) DTA thermograms of(i) RB, (ii) RB-PMMA, (iii) RMjPMMA blend and (iv) PMMA. (b) DTA thermograms of(i) RB, (ii) RB-PMVDC, (iii) RB/PMVDC blend and (iv) PMVDC. (c) DTA thermograms of(i) RB, (ii) RB-PMBP, (iii) RBjPMBP blend and (iv) PMBP. (d) DTA thermograms of(i) RB, (ii) RB-PAN, (iii) RB/PAN blend and (iv) PAN. (e) DTA thermograms of(i) RB, (ii) RB-PSTAN, (iii) RB/PSTAN blend and (iv) PSTAN.
(‘Cl
Thermal characterization of WPC Table
2. Peak
temperatures in DTA thermograms. shoulder or a plateau region Polymer loading/ composition W)
RB Composires RB-PMMA RB-PMVDC RB-PMBP RB-PAN RB-PSTAN Bl.?d RB/PMMA RB/PMVDC RB/PMBP RB/PAN RB/PSTAN Polymers PMMA PMVDC PMBP PAN PSTAN
indicate
First exotherm (“C)
Decomposition temperature (“C)
a
Second exotherm (“C)
340
450
39.2 14.5 51.3 10.6 54.4
270 210 216 250 270
410 345 400 340 405
450 450 470 515
34.6 10.0 46.2 13.3 52.5
260 236 208 250 250
410 340
460 460 -
335 405
465 495
260 200 192 324 340
2x-l -
400 480 -
290 -
600 540
weight loss at selected temperatures
At 340°C
PMMA PMVDC PMBP PAN PSTAN
Dashes
255
Table 3. Fractional
Polymer used
203
At 450°C
At 600°C
~ RB
RB-polymer
0.59 0.59 0.59 0.59 0.59
0.45 0.55 0.47 0.55 0.27
Polymer
RB
0.74 0.57 0.56 0.05 0.03
0.92 0.92 0.92 0.92 0.92
RB-PMBP still had a char yield of about 25%. Only the char of RB-PMBP was sustained through the high temperature oxidation to beyond 600°C. The impregnated polymers also affected the temperatures of the two exothermic peaks of RB (at 340 and 450°C respectively). Table 2 shows that the most drastic changes were observed in RB-PMBP and RB-PSTAN. In these composites, the temperature of the first exotherm has been increased to around 400°C and in RB-PSTAN, the second exotherm has been increased to 515°C. Of particular interest is the absence of a second exothermic peak in RB-PMBP. This observation is consistent with the absence of glowing combustion in RB-PMBP. Based on these observations, the effectiveness of PMBP as a flame retardant can be seen in its ability to lower the T,, of RB and to maintain a high char yield. The flame retardant action of organophosphorus compounds such as bis(2-chloroethyl)vinyl phosphonate is often considered to operate in the condensed phase via the generation of phosphorous acid (H,PG,). The latter alters the pyrolytic decomposition reactions of the wood components, resulting in a higher carbonaceous char yield (Barker and Drews, 1985; Jain et al., 1985). Comparison of RB-polymer physical bled
composites with their
If there is no chemical interaction
between the
RB-polymer 0.97 0.92 0.75 0.81 0.74
Polymer
RB
1.oo 0.82 0.75 0.15 0.89
0.99 0.99 0.99 0.99 0.99
RB-polymer
Polymer
1.00 0.98 0.91 I .oo 1.00
1.00 1.00 0.82 0.60 0.99
wood components and polymer in a composite, then the rule of mixture that can be applied in the calculation of the FWL values is as follows: (FWL), = (FWL);
W, + (FWL);
W,,
(2)
where W is the weight fraction and the subscripts c, p and w represent composite, polymer and gross wood respectively. Given that equation 2 holds, the TG curve of the composite would be expected to fall between the RB and polymer curves for the temperature range scanned, and would closely resemble the TG curve for the physical blend. In addition, the DTA curve of a composite, where there is no chemical interaction, would be similar to that of the blend. The blends of RB and polymer were prepared such that their compositions corresponded to the polymer loadings of the composites. Their TG and DTA curves are also shown in Fig. l(a-e) and Fig. 2(a-e) respectively. The TG curves of the composites RB-PMVDC and RB-PAN are intermediate between RB and the respective polymer. A close similarity between the TG and DTA curves of the composite and the blend was also observed in each case. As postulated, these observations indicate the absence of chemical interaction between polymer and wood. The fact that the TG curve of the composite RB-PMMA was not intermediate between RB and PMMA (from 250 to 35O’C) suggests the presence of
K. Y. CnAN et al.
204
wood-polymer interaction. However, this is contrary to reports that PMMA merely bulk the wood voids physically with little or no interaction with the cell walls (Timmons et al., 1971). In view of the small difference between the FWL values of RB and PMMA in this temperature range, the rule of mixture expressed in equation 2 might not be strictly adhered to by RB-PMMA. Since both the TG and DTA cuves of the composite and the blend matched rather well, it is possible to infer the absence of chemical interaction. Evidence of chemical wood-polymer interaction was found in the composites RB-PSTAN and RBPMBP. In both cases, the TG of the composite was non-intermediate between RB and polymer, and was also different from the blend: throughout most of the entire temperature scan, the FWL of composite was lower than that of the blend. The possibility of wood-polymer interaction is also reflected in the DTA thermograms. For PSTAN, the second exotherm of the composite was 20°C higher than that of the blend-the widest separation between any composite/blend pair. For PMBP, the sole exotherm found in the composite at 400°C was absent in the blend. These observations suggest that an appreciable difference in chemical nature exists between composite and blend. Some of the polymeric materials had probably grafted onto the wood components in these composites. CONCLUSION
By studying the thermal properties of RB and its composites, the effectiveness of different polymers as flame retardants for wood can be assessed. From TG and DTA measurements, it was found that the impregnated polymer either reduced the initial temperature of decomposition, increased the temperatures of one or both of the exothermic peaks, or increased the char yield. Only PMBP caused all the three effects. There was some evidence of chemical
interaction between PMBP and the wood constituents. Taking into consideration the oxygen index data and the various flaming characteristics, PMBP stood out as a promising fire-retarding polymeric system for impregnation into wood. Acknowledgements-We thank the Head, Physics Department, for the use of the y-radiation facilities and the Heads in the Departments of Chemistry and Chemical Engineering for the opportunity of carrying out this work in their departments and for the various facilities extended.
REFERENCES
Ahmed A. U., Takeshita N. and Gotoda M. (1970) Nippon Genshiryoku Kenkyt&o Nempo 82. Arni P. C. and Jones E. (1964) J. Appl. Chem. 14, 221. Barker R. H. and Drews M. J. (1985) In: Cellulose Chemistry and Its Applications (Edited by Nevell T. P. and Zeronian S. H.). Ellis Horwood, Chichester. Chia L. H. L. and Kong H. K. (1981) J. Macromol. Sci. Chem. A16, 803. Chia L. H. L., Chua P. H., Hon Y. S. and Lee E. (1986) Radiat. Phys. Chem. 21, 207. Czechowski Z. and Zakrzewski R. (1983) Zesz. Probl. Postepow Nauk Roln. 271, from CA 101, 74553h. Ianazzi F. D., Perri F. G., Jr., Levins P. L. and Lindstrom R. S. (1964) USAEC Report No. TID-21434. Jain R. K., La1 K. and Bhatnagar H. L. (1985) J. Appl. Polym. Sci. 30, 897. Lebedev V. T., Suminov S. I., Shiryaeva Cl. V., Karpov V. L. and Novikov V. Ya. (1972) Vysokomol. Soedin. Ser. A 14, 442. Lubke H. and Joke1 J. (1983) Zest. Probl. Postepow Nauk Roln. 260, 281; from CA 101, 56676~. Nguyen T., Zavarin E. and Barall E. M. (1981) J. Macromol. Sri. Rev. Macromol. Chem. C20, 1. RatT R. A. V., Herrick I. W. and Adams M. F. (1966) Forest Prod. J. 16, 43. Shafizadeh F. (1985) In: Fundamentals of Thermochemical Biomass Conversion (Edited by Overand R. P., Milne T. A. and Mudge L. K.), Elsevier, London. Siau J. F., Meyer J. A. and Kulik R. S. (1972) Forest Prod. J. 22, 31. Tang W. K. (1970) In: Differential Thermal Analysis, Vol. 1 (Edited by Mackenzie R. C.). Academic, London. Timmons J. K., Meyer J. A. and Cote W. A. (1971) Wood Sri. 4, 13.
0146S724/89 $3.00 + 0.00 Copyright 0 1989 Pergamon Press plc
THERMAL CHARACTERIZATION OF RUBBERWOOD-POLYMER COMPOSITES K. Y. Department
CHAN,’ M. G. S. YAP? L. H. L. CHIA’ and K. G. NEOH~
of Chemistry and 2Department of Chemical Engineering, National University of Singapore, Kent Ridge 0511, Singapore (Received 10 May 1988)
Abstract-The thermal properties of five types of radiation-induced wood-polymer composites based on a tropical hardwood, rubberwood (Ifeoea bruziliensts),was studied by oxygen index measurement, differential thermal analysis (DTA) and thermogravimetry (TO). The DTA and TG curves of composites were different from those of rubberwood, which can be attributed to the presence of the incorporated polymers. Of the five composites, the one impregnated with bis(2-chloroethyl)vinyl phosphonate reduced the initial temperature of decomposition, increased the peak temperatures of exothermic reactions, and increased the char yield. Comparison with physical blends of rubberwood and the corresponding polymer provided some evidence of chemical interaction of wood and polymer in some of the composites.
INTRODUCTION The impregnation of vinyl monomers into wood followed by in-situ polymerization is known to result in composite materials with enhanced strength, di-
mensional, and thermal properties. Flame retardancy has been imparted to woods that were incorporated with phosphorus or chlorine containing polymers, or the copolymerization products of these with more flammable polymers like polymethylmethacrylate, polyacrylonitrile and polystyrene (Janazzi et al., 1964; Arni and Jones, 1964; Raff et al., 1966; Ahmed et al., 1970; Siau et al., 1972; Lebedev et al., 1972). There is however, relatively little work that relates the observed flame retardancy in composites with the chemical modification of wood thermal decomposition behaviour by the polymer impregnant. In recent years, a number of thermal analytical techniques have been used in the study of thermal degradation of wood (Tang, 1970; Nguyen et al., 1981; Shafizadeh, 1985) and wood impregnated with polymers (Lubke and Jokel, 1983; Czechowski and Zakrzewski, 1983). In the present study, the thermal properties of rubberwood (Heuea braziliensis), and five of its polymer-composites are characterized by means of oxygen index measurements, thermogravimetry (TG) and differential thermal analysis (DTA). This work is part of the ongoing research at the National University of Singapore investigating various wood-polymer composites based on Malaysian tropical hardwoods (Chia and Kong, 1981; Chia et al., 1986). MATERIALS AND METHODS Monomers
Methylmethacrylate, (1) Schuchardt).
MMA
(Merck-
Methylmethacrylate-25% vinyl(2) 75% idenechloride, MVDC (the latter from Aldrich Chem.). (3) 75% Methylmethacrylatc-25% bis(Zchloroethyl)vinyl phosphonate, MBP (the latter from Polysciences Inc.). (4) Acrylonitrile, AN (Aldrich Chem.). (5) 60% Styrenm% acrylonitrile, STAN (styrene from Merck-Schuchardt). Preparation of test specimens Test specimens for oxygen index studies were cut from bulk material to dimensions of 6.5 f 0.5 mm width, 3.0 f 0.5 mm thickness, and about 100 mm length. A sufficient number of specimens (about 20 for each type of composite) were impregnated with the appropriate monomer solution and irradiated with a dosage of 4.0 Mrad from a cobalt-60 source according to the method described by Chia and Kong (1981). All specimens were dried in a vacuum-oven at 60°C to constant weight before testing. Thermal analysis was carried out on fine shavings of rubberwood (RB) or composites (RB-polymer) less than 1 mm thick and weighing 8.0 &-0.1 mg. The amount of polymer in composites could only be estimated from the polymer loading of the bulk material from which the shavings were obtained. For purpose of comparison, samples of the various polymers and their physical blends with RB were similarly prepared for analysis.
Oxygen index measurements The oxygen index (01) is defined to be the minimum concentration of oxygen, expressed as percent volume, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature. The design of the 01 test apparatus was based on ASTM D2863-77. 197
198
K. Y.
CHAN et
The criterion for a positive burn was chosen arbitrarily to the sustenance of 100 mm of combustion. A single positive burn was accepted as indicating a concentration at or above the 01; but at least three negative burns were required for identifying a concentration that was below the 01. In each case, combustion was allowed to continue uninterrupted until glowing ceased. The flaming and glowing times and other flaming characteristics were also recorded. TG and DTA measurements
Thermal analysis by TG and DTA was carried out using Netzsch Simultaneous Thermal Analysis Apparatus STA 409. DTA measurements were referenced to burnt kaolin. The test sample was heated in a crucible from 20 to 800°C at lO”C/min and then held at 800°C for a further 20min. Air flow through the sample was maintained at 100 ml/min at room temperature and pressure. The residual weight of sample as given by TG curves was expressed as fractional weight loss (FWL): Fractional
wg- U’ weight loss = ~ I+‘0 ’
(1)
where w0and w are the residual weights at 100°C and temperature t”C (t > 100) respectively, both corrected for buoyancy. An initial residual weight at 100°C was chosen so that any moisture content in the sample was eliminated. Duplicate analysis of randomly selected samples confirmed the reproducibility of the results. DISCUSSION OF RESULTS
Oxygen indices and Jaming
characteristics
The results of the 01 measurements are presented in Table 1. The 01 test assumes that inherently less flammable materials require greater oxygen concentrations to produce the heat necessary for the con-
al.
tinued production of flammable volatiles and flame propagation. Therefore among the five composites tested, only RB-PMBP exhibited improved flame retardancy as it had an 01 which is significantly higher than that of RB. It is also significant that the burning rate of RB-PMBP (as indicated by the flamming time of 2.7-2.8 min) was comparable to that of RB even though the former had been tested at such high oxygen concentrations as 34%. Effective flame retardancy is associated with small localized flames, minimal smoke/fume generation, short glowing time, high char yields, and in the case of synthetic polymers, minimal dripping of burning polymer melt. It is now known that decomposition of the cellulose fraction is primarily responsible for the flaming combustion, and solid-phase oxidation of the lignin fraction is primarily responsible for the glowing reactions (Barker and Drews, 1985). Table 1 shows that RB-PMBP compared favourably against the other composites in terms of these characteristics. These observations concerning PMBP are consistent with the findings of other researchers (Ahmed et al., 1970). Thermal decomposition of rubberwood
The thermal behaviour of RB is indicated by its TG curve [Fig. l(a)i] and DTA curve [Fig. 2(a)i]. These curves are typical of wood in general and can be regarded as a summation of the individual behaviours of its carbohydrate and lignin components. Mixtures of cellulose, hemicellulose and lignin in the same proportions as in wood have been found to give similar TG and DTA curves (Tang, 1970). The TG curve of RB was computed based on an original mass of wood at 100°C. A small weight loss prior to 100°C was attributed to the evaporation of moisture. It has been suggested from literature (Tang 1970; Nguyen et al., 1981) that pyrolysis takes place between 150 and 200°C with the hemicellulose start-
Table I. Oxygen indices and flaming Polymer loading (%I
26-21
RB RB-PMMA
RR-PMVDC
RB-PMBP
RB-PAN
RB-PSTAN
Oxygen index (%)
2&39
&I5
42-53
1l-18
45-64
25-26
28-29
33-34
27-28
28-29
Flaming time (min) 2.80
Flame description Candlelike,
characteristics Smoke and fumes
Dripping
Char
1.37
Little
White fumes, black smoke
Liquid exudation
1.70
Little
2.12
Rigld rod, breaks under own weight
-
Whole length of char
localised
4.26
Candlelike,
2.82
Big flame turning small and localised
White fumes, black smoke
Liquid exudation
2.70
Small localised flame, splits into vertical flames
Small amounts of fumes and black smoke
Little
2.32
Strong flame splits into non-vertical flames
White fumes and some black smoke initially
Liqud exudation with spattering
I .98
Rigid rod, breaks under own weight
Strong flame engulfing the whole length of rod
Tall stream of smoke, soot flies about
LIttIe, but rod expands and splits
I .78
Little
4.07
localised
Glowing time (min)
Thermal characterization
ing to decompose at about 2OO”C,followed by lignin at about 220°C and subsequently by cellulose at 250°C. At temperatures greater than 250°C rapid weight loss was observed which is due to the volatilization of the wood components. Pyrolysis can be considered to be complete around 360°C as indicated by a small shoulder in the TG curve around this temperature and at a FWL of 0.72. Since the test was performed using air, the next phase of weight loss around 400°C can be attributed to the oxidative combustion of the non-volatile pyrolysis products. Complete decomposition was achieved at around 480°C when the TG curve asymptotically approached a FWL of 0.99. Corresponding to the decomposition behaviour described above, the DTA curve of RB shows two exothermic peaks. The broad peak at 340°C was caused by the highly exothermic ignition and combustion of the volatile products. The second exothermic peak at 450°C is due to the oxidative combustion of the carbonaceous char. Thermal &composition
of RB-polymer
composites
The TG curves and DTA curves of the RBpolymer composites are shown in Fig. l(a-e) and Fig. 2(a-e) respectively. Three particular aspects of these curves are examined: (1) the initial temperature of decomposition; (2) the temperatures of the first and second exothermic peaks; and (3) the char yield. The
1.0
of WFC
199
effectiveness of a flame retardant is often measured in terms of its ability to lower the decomposition temperature of the treated wood specimen. Jain et al. (Jain et al., 1985) reported that if the onset of thermal decomposition took place at lower temperatures, then a lower percent of flammable volatile products would be formed, leading ultimately to a high char yield. Furthermore, the exothermic reactions associated with the combustion of pyrolysis products will take place at higher temperatures. This seems to be the mode of operation of inorganic salts (Shafizadeh, 1985). The initial temperature of decomposition, Tdr of RB and the other samples in this work was chosen on the criterion of a 5% weight loss (FWL = 0.05 on the TG curves). These temperatures are listed in Table 2. It can be seen that only PMVDC and PMBP lowered the T, of RB from 255 to 210 and 216°C respectively. These two polymers have relatively low Td themselves (below 2OO”C), and therefore, their flame retardant action in this temperature range was expected. To assess the char yield, FWL values were read off from the TG curves at 340 and 450°C (the two exothermic peaks of RB), and were tabulated in Table 3. A low FWL corresponded to high char yield. At 34OC, RB-PSTAN had the lowest FWL among the five composites, followed jointly by RB-PMMA and RB-PMBP. At 450°C however, RB-PMMA was almost completely burned while RB-PSTAN and
(a)
0.9
0.6
3
0.7
E -I
0.6
2 i ._a
0.5
g 2 .P z
0.4
t e IL 0.3
0.2
0.1
0 300
500
400
Temperature
Fig. 1. (a)
(‘Cl
600
700
200
K. Y.
Cnm et al.
Temperature
(‘C )
Fig. I. (b)
‘.O - (cl 0.9 0.0
_
-
0.7-
? -1
0.6-
2 i .”
0.5
-
0.4
-
0.3
-
0.2
-
0.1
-
s d ,cj t e IL
0 100
200
300
500
400 Temperature
Fig. 1. (c)
L”C1
600
700
Thermal characterization 1.0
-
of WPC
201
(d)
0.9
0.8
0.7 3 L -ia
0.6
s E .P
0.5
g d .E
0.4
t
/
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/
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0.2
, / 0.1
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I
0
/
500
400
I 600
I 700
Temperature (“Cl
Fig. 1 (d) 1.0
3 3 LL j
0.7
0.6
E 9 3
0.5
G .;5 0 e L
0.4
0.3
0 100
200
300
500
400
Temperature
600
700
(‘0
Fig. 1 (e) Fig. l.(a) TG curves of(i) RB (-), (ii) RB-PMMA (-.-.), (iii) RB/PMMA blend (OOOOOO), and (iv) PMMA (- -- --). (b) TG curves of(i) RB (-), (ii) RB-PMVDC (. -. -), (iii) RB/PMVDC blend (000000). and (iv) PMVDC (----- ). (c) TG curves of(i) RB (-), (ii) RB-PMBP (.--.--), (iii) RB/PMBP blend (OOOOOO), and (iv) PMBP (-----). (d) TG curves of(i) RB (-), (ii) RB-PAN (.-.--), (iii) RB/PAN blend (000000). and (iv) PAN (-----). (e) TG curves of(i) RB (-), (ii) RB-PSTAN (.-.-e), (iii) RB/PSTAN blend (000000).
K. Y. CHAN et al.
(a)
(iv)
200
300
400
Temperature
500
600
L_I
1°C)
400
Temperature
500
600
(‘Cl
(d)
I
I
I
I
I
200
300
400
500
Temperature
I 600
200
300
400
Temperature
(“Cl
500
600
(‘0
(e)
A Temperature
(1)
Fig. 2.(a) DTA thermograms of(i) RB, (ii) RB-PMMA, (iii) RMjPMMA blend and (iv) PMMA. (b) DTA thermograms of(i) RB, (ii) RB-PMVDC, (iii) RB/PMVDC blend and (iv) PMVDC. (c) DTA thermograms of(i) RB, (ii) RB-PMBP, (iii) RBjPMBP blend and (iv) PMBP. (d) DTA thermograms of(i) RB, (ii) RB-PAN, (iii) RB/PAN blend and (iv) PAN. (e) DTA thermograms of(i) RB, (ii) RB-PSTAN, (iii) RB/PSTAN blend and (iv) PSTAN.
(‘Cl
Thermal characterization of WPC Table
2. Peak
temperatures in DTA thermograms. shoulder or a plateau region Polymer loading/ composition W)
RB Composires RB-PMMA RB-PMVDC RB-PMBP RB-PAN RB-PSTAN Bl.?d RB/PMMA RB/PMVDC RB/PMBP RB/PAN RB/PSTAN Polymers PMMA PMVDC PMBP PAN PSTAN
indicate
First exotherm (“C)
Decomposition temperature (“C)
a
Second exotherm (“C)
340
450
39.2 14.5 51.3 10.6 54.4
270 210 216 250 270
410 345 400 340 405
450 450 470 515
34.6 10.0 46.2 13.3 52.5
260 236 208 250 250
410 340
460 460 -
335 405
465 495
260 200 192 324 340
2x-l -
400 480 -
290 -
600 540
weight loss at selected temperatures
At 340°C
PMMA PMVDC PMBP PAN PSTAN
Dashes
255
Table 3. Fractional
Polymer used
203
At 450°C
At 600°C
~ RB
RB-polymer
0.59 0.59 0.59 0.59 0.59
0.45 0.55 0.47 0.55 0.27
Polymer
RB
0.74 0.57 0.56 0.05 0.03
0.92 0.92 0.92 0.92 0.92
RB-PMBP still had a char yield of about 25%. Only the char of RB-PMBP was sustained through the high temperature oxidation to beyond 600°C. The impregnated polymers also affected the temperatures of the two exothermic peaks of RB (at 340 and 450°C respectively). Table 2 shows that the most drastic changes were observed in RB-PMBP and RB-PSTAN. In these composites, the temperature of the first exotherm has been increased to around 400°C and in RB-PSTAN, the second exotherm has been increased to 515°C. Of particular interest is the absence of a second exothermic peak in RB-PMBP. This observation is consistent with the absence of glowing combustion in RB-PMBP. Based on these observations, the effectiveness of PMBP as a flame retardant can be seen in its ability to lower the T,, of RB and to maintain a high char yield. The flame retardant action of organophosphorus compounds such as bis(2-chloroethyl)vinyl phosphonate is often considered to operate in the condensed phase via the generation of phosphorous acid (H,PG,). The latter alters the pyrolytic decomposition reactions of the wood components, resulting in a higher carbonaceous char yield (Barker and Drews, 1985; Jain et al., 1985). Comparison of RB-polymer physical bled
composites with their
If there is no chemical interaction
between the
RB-polymer 0.97 0.92 0.75 0.81 0.74
Polymer
RB
1.oo 0.82 0.75 0.15 0.89
0.99 0.99 0.99 0.99 0.99
RB-polymer
Polymer
1.00 0.98 0.91 I .oo 1.00
1.00 1.00 0.82 0.60 0.99
wood components and polymer in a composite, then the rule of mixture that can be applied in the calculation of the FWL values is as follows: (FWL), = (FWL);
W, + (FWL);
W,,
(2)
where W is the weight fraction and the subscripts c, p and w represent composite, polymer and gross wood respectively. Given that equation 2 holds, the TG curve of the composite would be expected to fall between the RB and polymer curves for the temperature range scanned, and would closely resemble the TG curve for the physical blend. In addition, the DTA curve of a composite, where there is no chemical interaction, would be similar to that of the blend. The blends of RB and polymer were prepared such that their compositions corresponded to the polymer loadings of the composites. Their TG and DTA curves are also shown in Fig. l(a-e) and Fig. 2(a-e) respectively. The TG curves of the composites RB-PMVDC and RB-PAN are intermediate between RB and the respective polymer. A close similarity between the TG and DTA curves of the composite and the blend was also observed in each case. As postulated, these observations indicate the absence of chemical interaction between polymer and wood. The fact that the TG curve of the composite RB-PMMA was not intermediate between RB and PMMA (from 250 to 35O’C) suggests the presence of
K. Y. CnAN et al.
204
wood-polymer interaction. However, this is contrary to reports that PMMA merely bulk the wood voids physically with little or no interaction with the cell walls (Timmons et al., 1971). In view of the small difference between the FWL values of RB and PMMA in this temperature range, the rule of mixture expressed in equation 2 might not be strictly adhered to by RB-PMMA. Since both the TG and DTA cuves of the composite and the blend matched rather well, it is possible to infer the absence of chemical interaction. Evidence of chemical wood-polymer interaction was found in the composites RB-PSTAN and RBPMBP. In both cases, the TG of the composite was non-intermediate between RB and polymer, and was also different from the blend: throughout most of the entire temperature scan, the FWL of composite was lower than that of the blend. The possibility of wood-polymer interaction is also reflected in the DTA thermograms. For PSTAN, the second exotherm of the composite was 20°C higher than that of the blend-the widest separation between any composite/blend pair. For PMBP, the sole exotherm found in the composite at 400°C was absent in the blend. These observations suggest that an appreciable difference in chemical nature exists between composite and blend. Some of the polymeric materials had probably grafted onto the wood components in these composites. CONCLUSION
By studying the thermal properties of RB and its composites, the effectiveness of different polymers as flame retardants for wood can be assessed. From TG and DTA measurements, it was found that the impregnated polymer either reduced the initial temperature of decomposition, increased the temperatures of one or both of the exothermic peaks, or increased the char yield. Only PMBP caused all the three effects. There was some evidence of chemical
interaction between PMBP and the wood constituents. Taking into consideration the oxygen index data and the various flaming characteristics, PMBP stood out as a promising fire-retarding polymeric system for impregnation into wood. Acknowledgements-We thank the Head, Physics Department, for the use of the y-radiation facilities and the Heads in the Departments of Chemistry and Chemical Engineering for the opportunity of carrying out this work in their departments and for the various facilities extended.
REFERENCES
Ahmed A. U., Takeshita N. and Gotoda M. (1970) Nippon Genshiryoku Kenkyt&o Nempo 82. Arni P. C. and Jones E. (1964) J. Appl. Chem. 14, 221. Barker R. H. and Drews M. J. (1985) In: Cellulose Chemistry and Its Applications (Edited by Nevell T. P. and Zeronian S. H.). Ellis Horwood, Chichester. Chia L. H. L. and Kong H. K. (1981) J. Macromol. Sci. Chem. A16, 803. Chia L. H. L., Chua P. H., Hon Y. S. and Lee E. (1986) Radiat. Phys. Chem. 21, 207. Czechowski Z. and Zakrzewski R. (1983) Zesz. Probl. Postepow Nauk Roln. 271, from CA 101, 74553h. Ianazzi F. D., Perri F. G., Jr., Levins P. L. and Lindstrom R. S. (1964) USAEC Report No. TID-21434. Jain R. K., La1 K. and Bhatnagar H. L. (1985) J. Appl. Polym. Sci. 30, 897. Lebedev V. T., Suminov S. I., Shiryaeva Cl. V., Karpov V. L. and Novikov V. Ya. (1972) Vysokomol. Soedin. Ser. A 14, 442. Lubke H. and Joke1 J. (1983) Zest. Probl. Postepow Nauk Roln. 260, 281; from CA 101, 56676~. Nguyen T., Zavarin E. and Barall E. M. (1981) J. Macromol. Sri. Rev. Macromol. Chem. C20, 1. RatT R. A. V., Herrick I. W. and Adams M. F. (1966) Forest Prod. J. 16, 43. Shafizadeh F. (1985) In: Fundamentals of Thermochemical Biomass Conversion (Edited by Overand R. P., Milne T. A. and Mudge L. K.), Elsevier, London. Siau J. F., Meyer J. A. and Kulik R. S. (1972) Forest Prod. J. 22, 31. Tang W. K. (1970) In: Differential Thermal Analysis, Vol. 1 (Edited by Mackenzie R. C.). Academic, London. Timmons J. K., Meyer J. A. and Cote W. A. (1971) Wood Sri. 4, 13.