Challenges in Wood Densification:Processing and Properties

Parviz Navi

5th International Scientific and Technical Conference, 15-16 November, 2012, University of Forestry, Sofia “INNOVATION IN FOREST INDUSTRY AND ENGINEERING DESIGN

contents

1- Introduction

2- Wood densification

2.1 Open systems (atmospheric pressure)

2.2 Closed systems (pressurized gas environment)

3. Effects of THM processing

3.1 Size effects

3.2 Shape memory or set-recovery

3.3 TH wood degradation, fixation set-recovery

3.4 Effects of THM parameters on wood properties

4. Conclusions

1- Introduction

1.1- unidirectional densification

(a) (b)

(a) Spruce before densificationd= 430 kg/m3

(b) After densification d= 1290 kg/m3

degree of densification 68 %

(c) Pine before densificationd= 490 kg/m3

(d) After densification d= 1300 kg/m3

degree of densification 68 %

(Navi and Girardet, 1997)

(c) (d)

Sample after 5% densification, early wood

Sample after  65% densification, late wood (Navi and Heger, 2005)

1.2 The nature of deformation: large strain; buckling and apparently non permanent 

1.3 Two-directional densification

• Transformation of a circular trunk to a square-section by two-dimensional densification (Ito et al., 1998a)

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1.6 Surface densification

Scanning micrograph in the vicinity of the boundary between the compressed and uncompressed portions (Imbue et al. 1990)

1.7 Why it is possible to densify or mould wood

1- Wood is made of natural polymers

2- Wood is porous material

3- Water is a natural wood plasticizer (reduces the wood

constituents Tg-glass transition temperature)

4- Wood polymers may be considered thermoplastic, lignin

at higher temperature acts like thermoset.

2. - Densification methods 2.1 Open systems (atmospheric pressure)

Schema of the process

THM densification by open-system process using an hydraulic-press with heated plates, moisture content of the panels is 15% maximum

2.1 Open systems (atmospheric pressure)

Production of a rolled densified, glued wooden tube in a mold (courtesy from P. Haller)

a) charging b) Closing c) Forming

2.1 Open systems Example of tube construction

Tube Spruce, fabricated by densified bended wood panels, Photo P. Haller (2008)

Filament winding of moulded tubes, Peer Haller (2009)

Viscoelastic Thermal Compression device (VTC):

Diagram showing the press inside to the chamber, platens and the specimen (veneers) between the platens (F. Kamk)

3.1 unidirectional (platen press), VTC, (Mixed system)

2.2 Closed systems (pressurized gas environment)

Photograph of a THM (pressure vessel or reactor) together with control panels

Schematic representation of a THM reactor showing steam generator, ingoing and outgoing steams to cylindrical chambers

2-2 Diagram of THM processing (Treatment chamber) – 4 stages Small elements

Densifying a trunk by THM closed system –large elements

Photo of a two layered THM reactor, opened state. Diameter of the cylinder is 20 cm.

3.1 Variation of the temperature and steam pressure during densification inside the wood element, the reactor and mantel.

3.1 Densified trunk of pine (one dimensional)- large element

Two cross-sections of Radial densified trunk of pine after drying. This shows cracks formed on the wood during drying (Girardet and Navi, 2007)

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3.1 Up‐scaling demonstration in THM

Photo of a two layers THM reactor, opened state. Diameter 20 cm Girardet and Navi (2007)

Production line,Plato international TH Wood, Netherlands (2009)

3.4 Effects of THM parameters on wood properties

The swelling of beech as a function of time (logarithmic scale) during humidification : (a) before densification, (b) after densification by open system (Huguenin & Navi, 1995).

3.4 Shear strength of the initial wood (black) and wood densified in a THM closed system (grey) for spruce and maritime pine (average values)(Navi & Girardet, 2000).

3.4 Brinel surface hardness of initial wood and densified wood in a THM closed system for spruce, beech and maritime pine (average values) (Navi & Girardet, 2000).

3.4 Longitudinal tensile strength and Young´s modulus of densified samples

20 mi. 3h. 1h. 20m. 4mi.

4.Conclusions

1. Research on wood densification still needs the laboratory works

2. Few attempts has been made to commercialize the technology with open system

3. Domain is with high innovation. it is clean processing only effects of T,H,M & t

4. Challenges exist on the moulding of large scale elements

5. Further study, experiments and numerical simulation of Virtual processing is needed

2.2 Closed systems

Multi-layer THM reactor

3.2 Effect of Temperature on wood, shape-memory, (recovery test)

Samples are compressed at different temperatures between110°C and 200°C in a closed system, under saturated steam (Heger, 2004).

3.3 Chemical degradationPost-treatment time to achieve complete fixation of the compression-set vs. 1/(RT)

3.3 degradation of wood constituents

Chemical degradation of wood constituents

is function of Temperature, Moisture Content

and Processing Time.

In both Heat-treatment and THM wood

densification under the same T & H the

degradation is similar.

Chemical degradation is such that there is

time humidity and temperature equivalency.

In THM densification the process needs water

or (steam).

________________Definition of hydrolysis : A chemical reaction in which water is used to break down a compound; this is achieved by breaking a covalent bond in the compound.

Chemical degradation of wood at high temperature steam is important (here only up to 200C°considered)

The most important of degradation is wood hydrolysis (specially the hemicelluloses), function of T, MC and t

Photograph of a specimen after fracture under tensile testing (Navi & Heger, 2005).

Natural wood, Wood densified at 140°C during 20 minutespost-treated at 140°C for 3 hours, post-treated at 160°C for 1 hour, post-treated at 180°C for 20 minutes, post-treated at 200°C for 4 minutesFrom each type of sample, at least 10 specimens were tested in tension.

3.4 MOR and MOE of THM wood

3.3 What happens to wood under Heat treatments

Acknowledgment

Dr. Frederic Heger,Mr. Fred Girardet