Nanomechanical Properties of Wood Cell Walls as Affected by Wood Processing

Transcription

1 International Conference on Nanotechnology for the Forest Products Industry June 25-27, 2008 St. Louis, MO, USA Nanomechanical Properties of Wood Cell Walls as Affected by Wood Processing Cheng Xing Siqun Wang Sang-Hoon Shim Zhaobin Zhou Yang Zhang

2 1. Project Summary Nanointentation was used to study the nano-mechnical properties of wood cell wall as affected by different wood processing: A) Thermo-mechanical refining B) Thermo-consolidation C) Thermo-extraction D) Thermo-treatment

3 2. Introduction Two characteristics of wood Cellular solid: Natural polymer:cellulose, hemicellulose, lignin, extractives and ash. Wood exhibits compressible and viscoelastic behaviors.

4 Products relate to heat and pressure Forest Logs Heat, pressure Composites Heat Heat, pressure Heat, pressure Lumber Extracted strands Compression molded products

5 (1) Thermomechanical refining Thermomechanical refining plays an important role in the manufacture of MDF and HDF. Do some mechanical properties change in fiber cell wall during the refining processing? What is the effect of refining steam pressure?

6 (2) Thermal consolidation Heat and pressure play a very important role in the manufacturing either wood based composites or solid wood molded products. Wood subject to heat and pressure conditions and to be consolidated and the wood cells would be reconstructed. Temperature sensor Platen Wood sample Platen

7 After hot-pressing Does the wood cell wall reconstruction affect cell wall mechanical property, increased or decreased???

8 (3) Thermal extraction Wood strands Aqueous stream Feedstock for Bio-products Bio-energy Solid stream OSB Does the hemicellulose extraction harm the cell walls?

9 (4) Thermal treatment Heat-treated wood is a new, ecological wood product, which main advantage is the ecological method of manufacture. Improved dimensional stability, durability, rot fungi-resistant. What is the real time mechanical properties of cell walls subjected to higher temperature? Does the heat treatment affect cell walls mechanical property?

10 Summary of Introduction Thus, more information is needed to understand the effects of manufacturing processing on the properties of wood cell walls! How? Nanoindentation! The nanoindentation technique has been established as the primary tool for investigating the hardness and modulus from a small volume of materials in a nano scale!

11 3. Experimental & methodology MW Thermo-mechanical Loblolly pine Refining JW 2-18bar Thermo-consolidation Poplar CR:20-80%; T: ; t: 20-60min. Pine and Red Oak Thermo-extraction strands T: ; t: 20-60min MW fibers JW fibers Dandified wood Extracted Strands Nanoindentation Loblolly pine (ring 32th) Thermal treatment T:room-250 Real time mechanics

12 Nanoindentation E r E Hardness (H): H s = = = (1 p max A Elastic Modulus (E s ): π ( dp 2 ν 2 s dh 25 ) = c h c p h c = h max.75 dp / ) unloading.4 h 2 c 1 E r p max max dh 1 ν 2 i E i P max Load P (mn) B C D hc Displacement h (nm) dp/dh Er: reduced modulus for indenter and sample combination ν s and νi (0.07) are Poisson's ratio of the specimen and indenter; Ei is the elastic modulus of indenter (1141 GPa).

13 4. Result and Discussion 4.1 Refining pressure effect a. Physical damage on MW fibers 2 bar-fiber 12bar- fiber 18bar- fiber

14 b. Physical damage on JW fibers 2 bar-fiber 10bar- fiber 14bar- fiber

15 C. Elastic modulus of MW fiber vs pressure Elastic modulus (GPa) Refining pressure (bar)

16 d. Hardness of MW fiber vs pressure Hardness (GPa) Refining pressure (bar)

17 e. Elastic modulus of JW fiber vs pressure Elastic modullos (GPa) Refining pressure (bar)

18 d. Hardness of JW fiber vs pressure Hardness (GPa) Refining pressure (bar)

19 Summary of reining pressure effect No obvious damages were observed on 2-4 bar MW fibers; Small cracks (<1μm) on 6-12 bar MW and 2-10 bar JW fibers; Large cracks (>1-6 μm) for higher pressure fibers (14-18 bar MW, and bar JW fibers); Elastic modulus and hardness greatly decreased for higher pressure fibers (14-18 bar for MW, and 12-18bar for JW).

20 4.2 Thermal consolidation effect a. Compression ratio effect (reconstruction) 40 % 80 %

21 b. Modulus and hardness vs. compression ratio Elastic modulus (GPa) y = x R 2 = Hardness (GPa) y = x R 2 = Compression ratio (%) Compression ratio (%) Note: Platen temperature 160 o C, 40 min.

22 c. Modulus and hardness vs. compression temperature Elastic modulus (GPa) Hardness (GPa) Temperature ( o C) Temperature ( o C) Note: Compression ratio 50%, and 40 min.

23 d. Modulus and hardness vs. compression time Modulus / GPa y = x R 2 = Hardness / GPa y = x R 2 = Time / min Time / min Note: Platen temperature 160 o C, and compression ratio 50%.

24 Summary of thermo-consolidation effect Thermo-consolidation can increase cell wall mechanical properties; Compression ratio is the most important factor to the increase of cell wall mechanical properties. Can we make enhanced fiber with design lumen size?

25 4.3 Thermal extraction effects a. Southern yellow pine strands

26 24 Elastic modulus (GPa) Weight loss>10 %, elastic modulus decreased extraction Weight loss (%) Weight loss>18 %, hardness decreased. Hardness (GPa) extraction Weight loss (%)

27 b. Red Oak strands

28 30 Elastic modulus (GPa) y = x R 2 = Extraction Linear (Extraction) Weight loss (%) Hardness (GPa) Extraction Weight loss (%)

29 Summary of thermo-extraction effects For pine, modulus was not significant difference as weight loss<10 %, and then decreased; while hardness was not difference as weight loss<18%, and then decreased. For red oak, modulus increased, while hardness decreased with weight loss.

30 4.4 Real time mechanical property Temperature ( o C) E (GPa) STDEV (GPa) H (GPa) STDEV (GPa) Max depth (nm) STDEV (nm) (post)

31 Summary of real time mechanical properties Cell wall mechanical properties were not significantly changed as temperature below 100 C, and dramatically dropped as temperature increased to more than 150 C. At 250 C, the measurements were failed. Both modulus and hardness were increased after heat treatment cooling down to room temperature.

32 Conclusion Thermo-mechanical refining steam pressure has important effects on the physical damage, elastic modulus and hardness of refined fiber cell walls; Compression ratio is the most important factor to the improvement of wood cell wall elastic modulus and hardness; For southern yellow pine, modulus started to decrease as weight loss>10%, while it is true for hardness as weight loss>18%; For red oak, modulus increased with weight loss up to 32%, while hardness decreased with weight loss; Heat treatment can increase cell wall modulus and hardness; but the real time modulus and hardness of cell wall dramatically dropped as temperature >= 150 o C.

33 Acknowledgement Dr. John Dulap; Dr. Steve Kelly, Dr. Les Groom, Dr. Brian Via, and Mr. Omid Hosseinaei Oak Ridge National Laboratory USDA NRI grant number # USDA Wood Utilization Research Grant

34 I has accepted a position as a R&D Scientist of BioComposites Group at TTS Inc. in the coming July. Here is my new contact information: th Avenue Edmonton, AB CANADA T6E 5Z5 Tel: Fax: cheng.xing@ttsfpl.com

35 Tekle Technical Services (TTS) Inc. ISO accredited testing lab Pilot of continuous microwave drier Pilot of Structural Insulated Panel (SIP) Developing and testing facility for biocomposites and wood plastic composites products

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