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Selected physical and mechanical properties of resin infused porous carbon composites made from medium density fiberboard.

Converting wood into carbon has been done for centuries, and the products have been used for fuels, adsorbents, and industrial raw materials. Recent studies (Byrne and Nagle 1997a, 1997b) on the production of crack-free monolithic porous carbon from wood for use as precursors of structural ceramics and composites has demonstrated new uses for wood which has attracted interest from a number of research groups. Carbon produced through the controlled thermal decomposition of wood in an inert atmosphere has been shown to retain the ultrastructural integrity of the parent wood (Blankenhorn et al. 1972). Because of its high reactivity and excellent machinability, wood-derived carbon has been used as a template in the synthesis of high performance biomorphous carbide ceramics (Klingner et al. 2003, Zollfrank and Sieber 2004, Rambo et al. 2005). Carbonized wood also has shown promise as a precursor for the production of carbon/carbon and carbon/polymer composites (Byrne and Nagle 1997c). To date, however, no comprehensive study has been conducted to investigate the mechanical properties of such composites.

Carbonization of natural solid wood has several limitations including: long carbonization processing times for large samples, retention of structural defects (knots), marginal reproducibility, and dimensional limitation of products due to the size of trees and the solid wood cut from those trees. Therefore, to expand the breadth of products that can be produced from carbonized wood, wood-based composites, such as medium density fiberboard (MDF), have been used to make porous carbon (Kercher and Nagle 2002). When MDF is used as the raw material, the carbonization of large samples can be completed within 1 day (Kercher and Nagle 2002) without crack formation, and some of the properties of the final products can be engineered through the control of density and particle size of the MDF panels (Treusch et al. 2004).

The primary objective of this study was to conduct a preliminary investigation to determine the potential for use of carbon preforms made from wood-based materials in the production of high-value composites by evaluating some of the basic physical and mechanical properties of carbonized medium density fiberboard (CMDF) infused with epoxy and phenolic resins.

Materials and methods

Carbonization of MDF

Hardwood MDF with a nominal thickness of 19 mm, purchased from Home Depot (Catonsville, Maryland), was cut into 305 mm long and 203 mm wide pieces. The boards were carbonized in argon with a flow rate of 0.5 L [min.sup.-1] with the entire process conducted in a retort furnace (CM 1200 Rapid-Tem, CM Furnace Inc., Bloomfield, New Jersey). The thermal schedule was modified from the prior work of Nagle and coworkers (Byrne and Nagle 1997a, Kercher and Nagle 2002) and is outlined as follows:

50[degrees]C/h to 110[degrees]C: maintain for 3 hours

15[degrees]C/h to 200[degrees]C

30[degrees]C/h to 400[degrees]C

15[degrees]C/h to 600[degrees]C

50[degrees]C/h to 1000[degrees]C: maintain for 1 hour

300[degrees]C/h to 25[degrees]C

Resin infusion (specimen shaped before resin infusion)

After carbonization, all of the CMDF boards had a uniform thickness of 11 mm. The CMDF boards were machined into test coupons prior to resin infusion, with the length direction of the coupons parallel to the panel formation direction, and thickness being equal to the MDF. For apparent density, 50 by 50 mm square blocks were used. For flexural properties tests, bar-shaped coupons were used (178 mm long and 23 mm wide). For tensile property evaluation, dog-bone shaped specimens were used with an overall length, overall width, length of the narrow section, and width of the narrow section of 165 mm, 22 mm, 57 mm and 13 mm, respectively. The dimensions of the fracture toughness specimens were 102 mm long and 23 mm wide. For each property test and each type of material, five test coupons were prepared, producing a total of 60 test coupons.

Infusion of epoxy resin.--The epoxy resin system, ProSet 125 (resin) and 229 (hardener) by Gougeon Brothers Inc. (Bay City, Michigan), was used in this study. The resin and hardener were mixed at a weight ratio of 10:3 at room temperature according to the manufacturer's manual. Before infusion, the specimens were submerged and weighted down in the resin mixture. A vacuum of -25 in. Hg (gauge) was applied and maintained for 10 minutes before release; this process was repeated an additional time. Specimen surfaces were then cleaned of excess resin by wiping, and the cleaned specimens were stored at room temperature for 15 hours followed by post-curing at 83[degrees]C for 8 hours. After the resins were cured, all of the test coupons were examined using standard radiographs (x-ray imaging at 80 kV) to ensure uniform resin infiltration.

Infusion of phenolic resin.--The phenolic resin, Durite SC 1008 by Hexion Specialty Chemicals (Columbus, Ohio), was selected as it is specially designed for laminate infusion. Although this resin is cured by heating to 100[degrees]C for 30 minutes, slight heating will decrease the viscosity and facilitate infusion. Preliminary studies showed that when the resin was heated to the temperature range of 60[degrees]C to 70[degrees]C, viscosity was reduced for a period long enough to permit complete resin infusion of the samples. The same vacuum procedure described for epoxy resin infusion was used for phenolic resin infusion, except that the process was conducted at 70[degrees]C. After cooling of the submerged samples to room temperature, the samples were cleaned and then stored at 50[degrees]C for 24 hours, followed by curing under the following temperature regime:

150[degrees]C/h to 80[degrees]C

80[degrees]C/h to 94[degrees]C and remain for 20 minutes

150[degrees]C/h to 150[degrees]C and remain for 60 minutes

300[degrees]C/h to room temperature

All of the test coupons were imaged by x-ray after resin curing as specified in the section on epoxy resins.

Mercury porosimetry measurement

Total porosity and pore size distribution of the samples before and after the polymer infusions were measured using a mercury porosimeter (AutoPore IV 9500, Micromeritics Instrument Corporation, Norcross, Georgia). A contact angle of 130[degrees] and a surface tension value of 473 mN/m for mercury were used in the calculation of pore sizes (Blankenhorn et al. 1978). A total of about 2 g particulate samples were collected from different parts of five specimens of each type of composite to obtain an average evaluation of the porosity for that type of material.

Mechanical tests and calculations

A screw-driven ATS universal test machine (Series 910, Applied Test Systems Inc. Butler, Pennsylvania) was used for all of the mechanical tests. Dog-bone tensile tests, the four-point bending test, and the single-edge-notch fracture test were conducted according to ASTM D638-03, ASTM C651-91, and ASTM D5045-99, respectively. A constant crosshead speed of 1.27 mm/min was used in all of the tests. All displacements were measured using a MTS extensometer (Model: 632.118-20, MTS, Eden Prairie, Minnesota), and the data were collected using a HP data acquisition unit (Model: 34970 A, Hewlett-Packard Company, Loveland, Colorado). Sample orientation and load direction in the mechanical tests are as illustrated in Figure I.

During the mechanical tests, all failures took place at the expected locations in the test coupons, and all data from the fracture toughness tests followed the criteria specified in the test standard. Therefore, data from all of the tests were included in the calculation of mechanical properties. Statistic analysis of the data were conducted using Microsoft Office Excel 2003 at a confidence level of [alpha] = 0.05.

Results and discussion

Porosity and pore distribution

The original CMDF had a porosity of 58.9 percent, but after resin infusion, the porosity decreased to 4.8 percent for epoxy-infused samples and to 29.1 percent for the phenolic-infused samples (Fig. 2). Most of the pores in the original CMDF had a diameter smaller than 10 [micro]m. After the carbonized material was infused with epoxy resin, most of the pores were filled with resin and only a small amount of the pores, in the size range between 2.5 [micro]m and 9.0 [micro]m, remained. For the samples infused with phenolic resin, pores smaller than 1.5 [micro]m were filled with resin, but there were a relatively large number of unfilled pores in the size range between 1.5 [micro]m and 9.0 [micro]m (Fig. 2). The high porosity in the phenolic-infused samples is primarily due to high shrinkage of the resin during polymerization and resin bleeding in the curing process which is discussed in the next section.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Apparent density

Samples infused with epoxy displayed a greater apparent density than those infused with phenolic resin (Table l) with the former having a weight gain of 96.7 percent compared to 66.7 percent for the latter. The epoxy resin used in this study was designed to cure at room temperature, and during the infusion process the epoxy resin began to polymerize shortly after infusion into the porous carbon preforms. The heat released from the polymerization reaction appears to have facilitated the curing process permitting more resin to be retained in the samples. The smaller apparent density associated with the phenolic resin infused samples was primarily associated with bleeding of the resin during the curing process at an elevated temperature. During the infusion process, as the temperature dropped from 70[degrees]C to room temperature, the viscosity of the resin increased and the resin stayed temporarily within the structure of the porous carbon preform. But, because the resin was not cured to the B stage during the 50[degrees]C-24 h storage period, when the temperature increased during the curing cycle, the viscosity of the resin decreased, resulting in resin bleed from the samples. Vaporization of the resin solvent and the water produced from the condensation reaction also could have prompted resin bleeding via the formation of bubbles. It is conceivable that postcuring of the phenolic resin while maintaining air pressure on the samples would have reduced resin bleed to produce a greater density sample.

Mechanical property tests

Both the original CMDF and resin-infused samples displayed the abrupt fracture behavior typical of brittle materials as shown by their tensile stress-strain profiles (Fig. 3). The resin-infused CMDF samples, however, displayed significantly greater elongation at failure, with the epoxy-infused samples increasing by 130 percent and the phenolic-infused samples increasing by 70 percent (Table 1). The increased ultimate tensile strain of the resin-infused samples is attributed to the greater elongation of the resin phase in the system. Because the resin-infused CMDF has improved elongation properties relative to the uninfused material, the results imply that the CMDF does not constitute a continuous carbon phase with uniform properties.

The tensile properties of the original CMDF were considerably improved after resin infusion (Table 1), with tensile strength increasing 434 percent and 235 percent for the epoxy-infused CMDF and phenolic-infused CMDF, respectively, and the tensile modulus increasing 138 percent and 97 percent for the epoxy-infused and phenolic-infused samples, respectively. The flexural properties of CMDF also increased significantly after resin infusion. Bending strength (modulus of rupture) increased 219 percent and 107 percent for the epoxy-infused and phenolic-infused samples, respectively. Resin infusion into the original CMDF significantly improved its resistance to fracture as well with plane-strain fracture toughness increasing 443 percent for epoxy-infused samples and 258 percent for phenolic-infused samples.

The MDF used in this study was typical of many commercial MDF samples and possessed a sandwich structure in profile with high-density surfaces composed of short fibers and fines and a lower density core composed of longer fibers. The long fibers in the core provide superior strength compared to the short fiber and fines at the surfaces. Bending strength, however, which is greatly impacted by surface layer properties, could be altered by density changes occurring in the surface layers, which occurred in this study. Because the sandwich structure was retained after carbonization, no significant difference was found between tensile and bending strength of uninfused CMDF. After resin infusion, density differences between the surface layers and the core were reduced because more resin penetrated into the lower density material. This resin infusion disproportionately improved the properties of the CMDF core material compared to the surface layers; this resulted in a material with proportionally greater tensile strength than bending strength.

The mechanical properties of the samples produced in this work were less than those of a carbon-fiber reinforced polymer composite with longitudinal tensile strength being only about 2 percent that of a unidirectional carbon-fiber reinforced epoxy composite (fiber volume ratio: 0.63) (Daniel and Ishai 1994). Carbonized wood, however, provides an easy and economical approach to produce net-shaped carbon composites, and they can be used in specific applications in which carbon composites are required, but high mechanical properties are not. These applications would include those in which carbon composites are used for their high dimensional stability at elevated-use temperatures, for applications requiring fire safety, or when static dissipation and shielding from radio frequency is needed (Savage 1993, Inagaki 2000).

[FIGURE 3 OMITTED]

In addition to the production of the CMDF-resin-infused samples, it may also be possible to produced high-value carbon-carbon composites by repeated carbonization and resin infusion of similarly produced samples. Current products requiring similar carbon-carbon materials include rocket nozzles, rocket reentry heat shields, shuttle nose cones, and brake pads and rotors in aircraft and racing cars (Savage 1993). It is conceivable that the use of the CMDF material would significantly reduce the production cost as complex fiber handling and forming procedures can be reduced or eliminated for some applications.

Conclusions

Selected physical and mechanical properties of resin-infused CMDF were investigated. Epoxy-infused CMDF samples were less porous and had greater density and mechanical properties than phenolic-infused samples. Because of resin bleed and the production of gas bubbles during the curing process with the phenolic resin, less resin was retained in the porous carbon preform after curing, resulting in lower mechanical properties for this material. The properties of resin-infused CMDF make it attractive for use in applications where high temperature, thermally stable core materials may be required. Altering the curing temperature profile, postcuring under pressure, or adding filler into the resin will be explored as methods for increasing phenolic resin retention in future studies.

Acknowledgments

This is publication 3026 of the Maine Agricultural and Forest Experiment Station.

Literature cited

ASTM Standard D638-03 Standard Test Method for Tensile Properties of Plastics. ASTM International. West Conshohocken, PA. www. astm.org.

ASTM Standard C651-91 Standard Test Method for Flexural Strength of Manufactured Carbon and Graphite Articles Using Four-Point Loading at Room Temperature. ASTM International. West Conshohocken, PA. www.astm.org.

ASTM Standard D5045-99 Standard Test Methods for Plan-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials. ASTM International. West Conshohocken, PA. www.astm.org.

Blankenhorn, P.R., D.P. Barnes, and D.E. Kline. 1978. Porosity and pore size distribution of black cherry carbonized in an inert atmosphere. Wood Sci. 11(1):23-29.

--, G.M. Jenkins, and D.E. Kline. 1972. Dynamic mechanical properties and microstructure of some carbonized hardwoods. Wood and Fiber 4(3):212-224.

Byrne, C.E. and D.C. Nagle. 1997a. Carbonization of wood for advanced materials applications. Carbon 35:259-266.--and--. 1997b. Carbonized wood monoliths-characterization. Carbon 35:267-273.

--and--. 1997c. Cellulose derived composites--A new method for materials processing. Materials Res. Innovations 1:137-144.

Daniel, I.M. and O. Ishai. 1994. Engineering Mechanics of Composite Materials. Oxford Univ. Press, Oxford, NY.

Inagaki, M. 2000. New Carbons: Control of Structure and Functions. Elsevier, Amsterdam and New York.

Kercher, A.K. and D.C. Nagle. 2002. Evaluation of carbonized medium-density fiberboard for electrical applications. Carbon 40:1321-1330.

Klingner, R., J. Sell, and T. Zimmermann. 2003. Wood-derived porous ceramics via infiltration of Si[O.sub.2]-sol and carbothermal reduction. Holzforschung 57:440-446.

Rambo, C.R., J. Cao, O. Rusina, and H. Sieber. 2005. Manufacturing of biomorphic (Si, Ti, Zr)-carbide ceramics by sol-gel processing. Carbon 43:1174-1183.

Savage, G. 1993. Carbon-Carbon Composites. Chapman and Hall, London.

Treusch, O., A. Hofenauer, F. Troger, J. Fromm, and G. Wegener. 2004. Basic properties of specific wood-based materials carbonized in a nitrogen atmosphere. Wood Sci. Technol. 38:323-333.

Zollfrank, C. and H. Sieber. 2004. Microstructure and phase morphology of wood derived biomorphous SiSiC-ceramics. J. Eur. Ceram. Soc. 24:495-506.

Xinfeng Xie *

Barry Goodell *

Dennis Nagle

Dajie Zhang

Roberto Lopez-Anido

The authors are, respectively, Post-Doctoral Research Associate and Professor, Wood Sci. and Technology, Univ. of Maine, Orono, Maine (xinfeng.xie@umit.maine.edu, goodell@umit.maine.edu); Professor and Research Scientist, Advanced Technology Lab., Johns Hopkins Univ., Baltimore, Maryland (dnaglel@jhem.jhu. edu, dzhang9@jhu.edu); and Associate Professor, Civil Engineering, Univ. of Maine, Orono, Maine (rla@maine.edu). This paper was received for publication in April 2008. Article No. 10474.

* Forest Products Society Member.
Table 1.--Apparent density and mechanical properties of CMDF before and
after resin infusion. (a)

 Tensile

Sample Apparent density Strength Modulus

 (g/[cm.sup.3]) (MPa) (GPa)

CMDF 0.62 (1.3%) 8.75 (14.9%) 4.40 (5.2%)
Epoxy/CMDF 1.22 (2.8%) 46.72 (9.1%) 10.49 (3.8%)
Phenolic/CMDF 1.01 (3.8%) 29.30 (12.7%) 8.66 (8.8%)
Epoxy (b) 1.14 68.77
Phenolic (b) 1.28 61.36

 Tensile Bending

Sample Elongation Strength

 (%) (MPa)

CMDF 0.20 (10.1%) 9.68 (15.2%)
Epoxy/CMDF 0.46 (4.6%) 30.88 (12.3%)
Phenolic/CMDF 0.34 (4.5%) 20.06 (12.8%)
Epoxy (b) 4.0 120.76
Phenolic (b)

 Bending
 Fracture
Sample Modulus Toughness

 (GPa) (MPa [m.sup.1/2])

CMDF 2.13 (5.5%) 0.0009 (5.2%)
Epoxy/CMDF 3.76 (4.5%) 0.0048 (3.7%)
Phenolic/CMDF 3.38 (4.8%) 0.0032 (8.8%)
Epoxy (b)
Phenolic (b)

(a) Values in parenthesis are coefficient of variation.

(b) Data from the product manual provided by the manufacturer.
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Author:Xie, Xinfeng; Goodell, Barry; Nagle, Dennis; Zhang, Dajie; Lopez-Anido, Roberto
Publication:Forest Products Journal
Geographic Code:1USA
Date:May 1, 2009
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