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Properties of Woodceramics Obtained By High Temperatures and Phenolic Resin Impregnation

Posted on: Tuesday, 4 October 2005, 03:01 CDT

By Oh, Seung Won

Abstract

This research investigated variations in density, weight loss, shrinkage, and mechanical properties of "woodceramics," as affected by resin impregnation percentage and carbonizing temperature. These woodceramics were formed by carbonizing medium density fiberboard (MDF) at 600, 800, 1000, and 1200C that had been impregnated with varying levels of phenolic resin. As the resin impregnation percentage and the carbonizing temperature increased, density increased until the carbonizing temperature reached 1000C, then began to decline. On the other hand, as resin impregnation percentage increased, weight loss and shrinkage decreased. As the carbonizing temperature increased, weight loss and linear shrinkage both increased. In addition, as resin impregnation percentage and the carbonizing temperature rose, so did both bending and compressive strength, due to an increase in the hardening of the phenolic resin located in the cell walls of the impregnated MDF.

"Woodceramics" are new, porous, carbonaceous materials that are formed by impregnating a thermosetting resin into wood or wood- based materials and carbonizing at a high temperature. In general, ceramics are generally thought of as porcelain or china made of clay and minerals. However, in a broad sense, ceramics are inorganic materials having ionic and covalent bonds. Therefore, even carbon materials, which belong in the class of inorganic materials, can be included in ceramics. Okabe and Saito ( 1995a) used the term woodceramics for porous carbon materials made by impregnating resin into wood and carbonizing. Woodceramics have many advantages, such as lightness, hardness, corrosion resistance, durability, electromagnetic shielding capability, far-infrared emission properties, etc. It is therefore expected that they may have various applications in industrial uses, and several basic studies have been done on properties and possible uses (Hokkirigawa et al. 1995, 1996a, 1996b; Okabe and Saito 1995b; Kanoetal. 1996; Kasai et al. 1996; Okabe et al. 1995a, 1995b, 1996b, Shibata et al. 1997).

The properties of woodceramics are affected by a variety of factors such as the density of the wood or wood-based materials, the carbonizing temperature, the temperature rise rate, and the cooling temperature. Moreover, resin impregnation has an important influence on the property of woodceramics, converting the material into glassy carbon during the process of carbonization.

This study was undertaken to provide basic data for the development of woodceramics, utilizing various levels of carbonizing temperatures and resin impregnation percentages, and then investigating the physical properties of the resulting materials.

Experimental procedure

Materials

This research used medium density fiberboard (MDF) density, 0.84 g/cm3; MC, 6.5%; UF resin content, 10% - made of a radiata pine for forming woodceramics. Phenolic resin (KPD-L777, Kolon Chemical Co., Ltd.; solids 51% to 53% ; SG 1.06; viscosity 45 to 65 cps; and gelation time 80 to 95 sec) was used for impregnation.

Impregnation and forming woodceramics

MDF pieces, 8 by 8 by 1.2 cm in size, were placed in a decompression impregnator that contained liquid phenolic resin, and impregnated at 1 atmospheric pressure, with the percentage of resin impregnation ranging from 50 to 82 percent. The manufacturing process consisted of an impregnation tank, an ultrasonic wave generator, a vacuum pump, and a compressor. Nineteen ultrasonic transducers (ferrite magnetic-strained oscillators of frequency 25 kHz) were installed in the bottom of the impregnation tank to produce ultrasonic vibrations in the decompressed tank. After impregnating, the specimens were put into a dryer for 10 hours at 60C, and for 8 hours at either 100C or 135C for drying and hardening, respectively. The material was then carbonized at 600, 800, 1000, or 1200C by using a vacuum sintering furnace (KOVAC KSF- 100. Ko-Ryeo Vacuum Corp., Ltd.). The carbonizing was done in the presence of air. The experiment was conducted by subjecting the sample to a temperature rise rate of 5C/min until reaching the target temperature, maintaining the sample at the target temperature for 2 hours, and then cooling the sample at a cooling rate of 5C/ min.

Figure 1. - Relationship between resin impregnation percentage and density of woodceramics.

Measuring the physical properties

To examine the physical properties of woodceramics. research was conducted with 20 resin-impregnated specimens made under various conditions, placed in a vacuum desiccator for 2 weeks, and then carbonized at varying temperatures. The weight and dimensions of the samples produced were measured, and the weight loss, shrinkage, and variation in density

were measured. In addition, bending strength and compressive strength were measured according to Korea Standards 2206 (KS 1995a) and 2208 (KS 1995b). The crosshead speed was 10 mm/min using a universal testing machine (Autograph. AGS-10KNG. Shimadzu). In this testing process, the sample sizes of the bending specimens were 70 mm long by 10 mm wide and 10 mm thick; the compressive samples were 20 mm long by 10 mm wide and 10 mm thick.

Results and discussion

Changes in density

As shown in Figure 1, the density of the woodceramics differed in relation to resin impregnation percentage. At a carbonizing temperature of 600C. the density was 0.73 g/cm^sup 3^ at a resin impregnation percentage of 50 percent; 0.76 g/cm^sup 3^ at 60 percent impregnation percentage, and 0.79 g/cm^sup 3^ at 80 percent impregnation percentage; while, at 800C, the density was 0.79 g/ cm^sup 3^ at 52 percent, 0.82 g/cm^sup 3^ at 60 percent, and 0.87 g/ cm^sup 3^ at 82 percent. These results indicate that the density of woodceramics after carbonizing increases with increasing resin impregnation percentage; the same trend appeared at carbonization temperatures of 1000 and 1200C. At a given resin impregnation percentage, the density of woodceramics varied in relation to carbonizing temperature as follows; at a resin impregnation percentage of 82 percent, density was 0.77 g/cm^sup 3^ at 600C, 0.87 g/cm^sup 3^ at 800C, 0.85 g/cm^sup 3^ at 1000C, and 0.80 g/cm^sup 3^ at 1200C. showing that density slightly decreased at carbonizing temperatures above 1000C. These results are in agreement with those of Oh and Byeon (2002). who examined the density variation of woodceramics with varying carbonizing temperatures using MDF at a resin impregnation percentage of 60 percent, and reported a similar rise in density from 500 to 800C, followed by a slight decrease at 1000C.

Figure 2. - Relationship between resin impregnation percentage and weight loss.

Weight loss and size shrinkage

Figure 2 shows sample weight loss in relation to resin impregnation percentage and carbonizing temperature. The weight loss decreased with increasing resin impregnation percentage at all temperatures, but increased slightly with rising temperature at all resin impregnation percentages. These results suggest that as the resin percentage in MDF increases, a great deal of resin that is changed into glassy carbon may be left in the MDF. Oh and Byeon (2002) reported a similar tendency for weight loss to increase with increasing carbonizing temperatures over the range 500 to 1000C, for woodceramics made of MDF with a resin impregnation percentage of 40 percent.

Figures 3 and 4 show linear shrinkage and thickness shrinkage of the woodceramics samples after carbonizing. Linear shrinkage had a tendency to decrease slightly with increasing resin impregnation percentages at all temperatures, but increased with increasing carbonizing temperatures at all impregnation levels. Thickness shrinkage also decreased with increasing resin impregnation percentages at all temperatures, but, in a reverse trend from that of linear shrinkage, also decreased with increasing carbonizing temperatures at all resin impregnation levels. Okabe et al. (1996b) reported linear shrinkage and thickness shrinkage after forming woodceramics with MDF as 14 percent in length and 24 percent in thickness at 500C, rising to 21 percent in length and 25 percent in thickness at 800C. However, our results produced slightly larger values, and thickness shrinkage decreased, not increased, with increasing temperatures in our study. It is likely that the differences between the two studies came from the conditions of forming such as the resin impregnation percentage, the use of rising firing temperatures etc., and differences in the physical properties of the materials.

Figure 3. - Relationship between resin impregnation percentage and linear shrinkage.

Figure 4. - Relationship between resin impregnation percentage and thickness shrinkage.

Figure 5. - Relationship between resin impregnation percentage and bending strength.

In addition, Okabe et al. (1996a) reported that dimensional and weight changes in woodceramics carbonized at temperatures under 1000C are attributed to structural changes due to wood and resin carbonization. Cellulose, the main component of wood, undergoes dehydration between 150 and 240C and then undergoes several reactions, for example the fission of C-C bonds. At around 400C cellulose is carbonized, forming a polyaromatic structure. Similarly, i\n phenol resin, a cross-linking structure is formed at 300C and above by dehydrogenation between and within molecules. Then, at around 500C, it is carbonized, forming a polyaromatic structure by elimination reactions of methane and hydrogen.

Mechanical properties

Bending strength and compressive strength in relation to the resin impregnation percentage and carbonizing temperature are shown in Figures 5 and 6. For both bending and compression, the strength increased with increasing resin impregnation percentages at all temperatures and also with increasing carbonizing temperatures at all resin impregnation percentages; the only exception was that bending strength at 800C was actually higher than at 1000 and 1200C. It is known that, in the thermal decomposition of wood, the hydrocarbon structure forms through the dehydration and depolymerization of cellulose between 250 and 310C, and that condensation aromatic polynuclear structures start to form above 400C and develop gradually above 500C. For phenol compounds, it is known that depolymerization occurs between 300 and 400C. and that the aromatic polynuclear structure starts to form above 400C and develops above 500C. As a result, the uniting of phenol compounds advances as the carbonization temperature rises, which results in an increase in strength. Oh (2001). investigating the bending strength of woodceramics that were from thinned Cryptomeria logs and formed at a carbonizing temperature of 650C, also reported that bending strength increased as the resin impregnation percentage rose: bending strength was 120 kgf/cm^sup 2^ at 50 percent resin impregnation, and rose to 160.0 kgf/cm^sup 2^ at 90 percent. Furthermore. Oh and Byeon (2002) reported that bending strength increased both with rising carbonizing temperatures and with rising resin impregnation percentages. At a carbonizing temperature of 500C. bending strength was 50.3 kgf/cm^sup 2^ at a resin impregnation percentage of 40 percent, and 65.2 kgf/cm^sup 2^ at 60 percent: while, at 1000C, bending strength was 142.0 kgf/cm^sup 2^ at 40 percent resin impregnation and 173.5 kgf/cm^sup 2^ at 60 percent. Oh et al. (2000), examining compressive strength as affected by the resin impregnation percentage after forming woodceramics at 650C with a board made from Aomori Hiba wood, reported that as the resin impregnation percentage increased, the compressive strength rose from 275.0 kgf/cm^sup 2^ at 30 percent resin impregnation to 367.0 kgf/cm^sup 2^ at 85 percent. The increase in strength of the woodceramics with increasing resin impregnation percentage was possibly due to transformation of a large amount of phenolic resin in the cell walls into glassy carbon during the process of carbonizing, thereby strengthening the cell wall. As for the relationship between carbonizing temperature and strength, the density of the woodceramics rose as the carbonizing temperature increased, thereby influencing the strength.

Figure 6. - Relationship between resin impregnation percentage and compressive strength.

Conclusions

Woodceramics, which are made of wood or wood-based materials impregnated by thermosctting resins and then carbonized at a high temperature, are porous carbon materials, the properties of which vary according to manufacturing conditions. This study examined the properties of woodceramics, which were made from MDF impregnated at various impregnation percentages with phenolic resin, and then carbonized at various temperatures. With increasing resin impregnation percentages, the density of the woodceramics after carbonizing increased. The density of woodceramics formed at a carbonizing temperature of 800C was the highest of all. With increasing resin impregnation percentages and carbonizing temperatures, both bending and compressive strength increased. This increase was possibly due to the fact that the phenolic resin in the cell walls of the woodceramics made from impregnated MDF was changed into glassy carbon during the process of carbonizing, thereby strengthening the cell wall, increasing the density, and thus influencing the strength.

Literature cited

Hokkirigawa, K., T. Okabe, and K. Saito. 1995. Development of porous carbon material "woodceramics" - Fundamental wear properties under unlubricated condition on air under base-oil impregnated condition and in water. J. of the Soc. of Mat. Sci. of Japan 44(501):800-804.

__________, __________, and __________. 1996a. Wear properties of new porous carbon materials: Woodceramics. J. of Porous Materials 2:229-235.

__________, __________, and __________. 1996b. Friction properties of new porous carbon materials: Woodceramics. J. of Porous Materials 2:237-243.

Kano, M., M. Momota, T. Okabe, K. Saito, and R. Yamamoto. 1996. Thermogravimctric and differential thermal analysis of woodceramics. Trans. of the Materials Res. Soc. of Japan 20:40-43.

Kasai, K., K. Shibata, K. Saito, and T. Okabe. 1996. Humidity sensor characteristics of woodceramics. Transactions of the Materials Res. Soc. of Japan 20:92-95.

Korea Standard (KS). 1995a. The method of compressive test. KSF 2206.

__________. 1995b. The method of bending test. KSF 2208.

Oh, S. W. 2001. Properties of woodceramics made from thinned logs of Cryptomeria Japonica D. Don - Effect of steam injection and its time. J. of Korean Wood Sci. and Tech. 29(2):69-75.

__________ and H.S. Byeon. 2002. Properties of woodceramics made from MDF. J. of Korean Wood Sci. and Tech. 30(2):115-120.

__________. T. Hirose, and T. Okabe. 2000. Manufacturing characteristics of woodceramics from thinned small logs (II) - Dimensional change, weight change and compressive strength. J. of Korean Wood Sci. and Tech. 28(4):56-60.

Okabe, T. and K. Saito. 1995a. Development of woodceramics. Trans. of the Materials Res. Soc. of Japan 18:681-684.

__________ and __________. 1995b. The examination of the manufacturing method of woodceramics (I) - Structural changes affected by burning temperature. Inter. Ecomaterial Conf. Proceedings. IUMRS, Xian, China. pp. 1-4.

__________, __________, and K. Hokkirigawa. 1996a. New porous carbon materials woodceramics - Development and fundamental propertics. J. of Porous Materials 2:207-213.

__________, __________, and __________. 1996b. The effect of burning temperature on the structural changes of woodceramic. J. of Porous Materials 2:215-221.

__________, __________, H. Togawa, and Y. Kumagai. 1995a. Electromagnetic shielding characteristic of porous carbon material "woodceramics." Inter. Ecomaterial Conf. Proceedings, IUMRS, Xian. China. pp. 9-12.

__________, __________, __________, and __________. 1995b. Development of porous carbon material "woodceramics" - Electromagnetic shielding characteristics. J. of the Soc. of Mat. Sci. of Japan 44(498):288-291.

Shibata, K., T. Okabe, K. Saito, T. Okayama, M. Shimada, A. Yamamura, and R. Yamamoto. 1997. Electromagnetic shielding properties of woodceramics made from wastepaper. J. of Porous Materials 4:269-275.

Seung Won Oh*

The author is an Associate Professor, Division of Forest Sci., College of Agriculture and Life Sci., Chonbuk National Univ., Chonbuk, Korea (ohsw@moak.chonbuk.ac.kr). This paper was received for publication in February 2004. Article No. 9840.

* Worest Products Society Member.

Forest Products Society 2005.

Forest Prod. J. 55(7/8):27-30.

Copyright Forest Products Society Sep 2005

Source: Forest Products Journal

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