Potassium aluminum sulphate, hereafter referred to as alum, was used as a flame-retardant for some African timbers. It was observed that this treatment did not drastically reduce the strength of these timbers. Reasons are adduced to explain this result

Akpabio, I. O.

Amanfo, I.

Eboatu, A. N.

University of Uyo Institutional Repository

Effect of Flame-Retardant Treatment on the Mechanical Properties of Some Tropical Timbers A. N. EBOATU,* 1. AMANFO, and 1. 0. AKPABIO Chemistry Department,  Usrnanu Danfodiyo University, P.M.6 2346, Sokoto, Nigeria SYNOPSIS Potassium aluminum sulphate, hereafter referred to as alum, was used as a flame-retardant for some African timbers. I t  was observed that this treatment did not drastically reduce the strength of these timbers. Reasons are adduced to explain this result. INTRODUCTION Since the dawn of civilization, timber has been widely used both structurally and as a decorative material in buildings. Unlike some other traditional building materials, such as concrete and steel, timber will burn if exposed to an igniting source under fa- vourable conditions. It is hardly surprising, there- fore, that some considerable energy has been ex- pended in finding the right flame-retardant (FR) formulations for wood and other cellulosics.'-4 In a recent paper, we have shown5 that alum functions as a FR for some tropical timbers. It has also been reported6 that in certain circumstances treatment with flame-suppressant formulations can reduce the strength of some species of solid timber. In this article, some tropical timbers were im- pregnated with alum and the effect of this on the mechanical properties was examined. The same  timber^,^ viz., Chlorophra exceka, Nuclea diderichii, Ceiba pentandra, and Terminulia superba, subse- quently to be called by their trade names of iroko, opepe, ceiba, and afara, respectively, were used. EXPERIMENTAL Materials The four timbers, obtained from a local sawmill, have the characteristics shown in Table I. The alum * To whom correspondence should be addressed. Journal of Applied Polymer Science. Vol. 44, 239-242 (1992) 0 1992 John Wiley & Sons, Inc. CCC 002l-S995/92/020239~4$04.00 was procured from the Federal Superphosphate Fertilizer plant a t  Kaduna, Nigeria, and assays as: A1203 (25.5% ) , Fe (0.0025% ) , insoluble matter (0.084% ) , and moisture content ( 18.47% ) . Method 1. Flame-retardant treatment: The method used is the same as outlined in ref. (5). 2. Moisture regain: Oven-dry treated and un- treated wood splints were kept in a constant humidity (71%) chamber at  25°C for 40 d. Moisture regain was calculated as: Moisture regain ( % )  Wt after exposure - Wt of oven-dry sample 100 X-. - Wt of oven-dry sample 1 3. Measurement of mechanical properties: The INSTRON Table Model 1026, at  crosshead speed of 100 mm minpl and chart speed of 200 mm min-', was used to measure the ten- sional breaking load. Gauge length was 5 cm. Sample width and thickness of about 1.5 and 0.15 cm, respectively, were maintained. In the case of bending load, the Dennison Machine Model No. T4 282, M/C No. 23313, was em- ployed. The sample was held horizontally be- tween two iron bars (gauge length still 5 cm) . By means of a metal rod projecting from the machine head, pressure was gradually applied to the centre of the sample until break. 239 240 EBOATU, AMANFO, AND AKPABIU Table I Characteristics Iroko Opepe Afara Ceiba Specific gravity 0.56 0.23 0.45 0.18 Moisture content (%) 27.80 33.70 35.70 37.30 Porosity index (% water imbibition) 36.60 49.0 29.40 68.80 RESULTS AND DISCUSSION In Figure 1 it is evident that the quantity of alum absorbed by the wood samples depends on the liquor concentration. In the type of system, i.e., polymer solution, the manner of chemisorption is well rep- resented by the Fick‘s Laws. The first law is: where J = rate of accumulation of the reagent per unit area of the reference plane oriented normal to the X-axis, D = diffusion coefficient, and C = local reagent concentration at a point distance X from the origin of coordinates. A second differential of the first law expression with respect to time is the Fick’s second law: LIQWR CONCENTRATION ( M) Figure 1 Effect of liquor concentration on add-on ( % ) iroko (M), afara (M), ceiba (-), and opepe (-). a J  - a2c - - D p  at which implies that the rate of accumulation of the reagent of the surface, and hence its penetration into the wood matrix, would essentially be linked to the bath concentration. On the basis of this argu- ment, the observations highlighted in Figure 1 are in accord with theoretical considerations. It is also observed that lighter timbers imbibe more alum than heavier ones. This is hardly surprising as more po- rous materials, as shown in Table I, are expected to accommodate more of the salt. For the four timbers, it can be seen from Figure 2 that there are some moderate decreases in the bending load as the quantity of alum in the sample increases, i.e., the sample becomes more embrittled. This effect, however, does not seem to depend di- rectly on the relative amounts of alum; for example, ceiba, which absorbs the highest quantity, is not the most deleteriously affected. It is therefore likely that bending strength losses as a result of alum treatment has to do with the timber species. All that is definite a t  present is that the greater the density of these timbers the higher the bending strength. In fact, ceiba and opepe, whose oven-dry densities are sim- ilar, have very close bending strengths at all alum concentrations. 0.025 3 0.0 OD3 0.1 0s 075 LIQUOR CONCENTRATION (M) Figure 2 Effect of FR imbibition on maximum bending load of iroko (M), afara (e--e--S), ceiba (M), and opepe (-). FLAME RETARDANTS AND TROPICAL TIMBERS 241 Figure 5 Effect of concentration on pH of liquor of iroko (w), afara (M), ceiba (M), and opepe (-). low the fibre saturation point, the strength and elas- I 0 03 0.1 0-5 075 00 LIQUOR CONCENTRATION (M) Figure 3 Effect of FR imbibition on load at break of iroko (w), afara (E&Et-€l), ceiba (M), and opepe (O--O-O). A rather complex picture is presented in Figure 3. The effectiveness of untreated wood in resisting an applied force is a function of the total cell wall and extractive materials, manifested in the sample density. This perhaps explains the trend in the ten- sile strengths of the untreated timbers (Fig. 3). Be- A 2l I A ad son3 0.1 05 075 LIQUOR CONCENTRATION (M)  Figure 4 Effect of FR imbibition on moisture regain of iroko (w), afara (Ek-El-B), ceiba (M), and opepe (-). tic characteristics of cellulosics, including wood, vary inversely as the moisture content. This is attribut- able to the well-explored plasticization phenomenon. Alum is a deliquescent double-salt and is therefore expected to increase the natural moisture absorp- tivity of wood (Fig. 4),  thereby contributing to strength reduction. Other possible contributors to strength losses as a result of innoculating wood with alum could be acid attack and the physical pulling apart (rupture of H-bonds, etc.) of the wood grains. In fact, Figure 5 shows that the pH of the padding liquor decreases as the concentration increases. Also at the cure stage, some quantity of alum decomposes according to the equation: thereby yielding the strongly acidic gas, SO3. The observations depicted in Figure 3 indicate that de- spite all these possibilities the FR treatment of the timbers with alum results in but little losses in ten- sile strength. Wood is naturally hygroscopic. Alum is deliques- cent. Incorporation of alum into wood is therefore expected to enhance moisture absorption. This is borne out by the results in Figure 4. The sorption of small molecules, including water, by polymers, has been extensiveIy discussed in the literature '-lo and is now believed to take place by a number of interdependent mechanisms, such as the formation of solid-sorbate solution, dissolution of sorbate in a series of flexible polymer chains (the Florry-Hug- gins law), attachment of the sorbate onto active 242 EBOATU, AMANFO, AND AKPABIO centres on the polymer (the Hill-Rowen postulate), and the Brunaur-Emmet-Teller mechanism, whereby there is a polynuclear adsorption on pore walls. It is felt that the trend observed in Figure 5 is due to the water molecules being attached to the cellulose macromolecules by some or all of the mechanisms above, as well as attraction by the alum within the wood interfibre or matrix spaces. CONCLUSION The following conclusions can be drawn from this work: 1. 2. The use of alum as a FR material for these trop- ical timbers does not drastically reduce the strength of the timbers. On the other hand, moisture regain is appreciably enhanced. REFERENCES 1. F. Ward, J ,  SOC. Dyers Color, 71,559 (1955). 2. J. Wyld, Br. Pat. 551 (1735). 3. Tetsuya Nakao and Takeshi Okano, J .  Polym. Sci., 4. A. R. Horrocks, Rev. Coloration, 16,62 (1986). 5. A. N. Eboatu and Bashiru Garba, J. Appl. Polym. Sci., 6. Br. Wood Preseru. Assoc. Monogr., 1 1 , 2  (1971). 7. H. Tamaru Oyama and T. Nakajiva, J. Appl. Polym. 8. A. Ye Chalykh, A. P. Belokurova, and T. R. Komarova, 9. H. T. Oyama and T. Nakajiva, J. Polym. Sci., A-I, 10. T. L. Hill and J. W. Rowen, J. Polym. Sci., 9, 93 Polym. Lett. Ed., 23 ( 12), 646 (1985). 39,109 (1990). Sci., 29,2143 (1984). Polym. Sci. (USSR), 25 (5),  1240 (1983). 21,2987 (1983). (1952). Received December 21, 1990 Accepted January 24, 1991 

Effect of flame-retardant treatment on the mechanical properties of some tropical timbers

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Effect of flame-retardant treatment on the mechanical properties of some tropical timbers

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