properties and processing of thm wood - tu...

Properties and processing of Wood

Densification by THM

Parviz Navi

Thematic Session “Modelling of THM treated

wood, Joined Action FP0802, FP0904

August 24, 2011, Helsinki

contents

1- Compression methods

1.1 Open systems (atmospheric pressure)

1.2 Closed systems (pressurized gas environment)

1.3 Mixed systems

1.4 unidirectional (platen press)

1.4 Compression forming (Two-directional compression)

1.5 Isobar (membrane press, 3D)

contents

2. Effects of THM processing on wood

2.1 Physical effects: Glass transition, Elasto-viscoplasticity,

and shape memory or compression set-recovery

2.2 Chemical : TH wood degradation, fixation set-recovery

2.3 Effects of THM parameters on wood properties

3. Conclusions



Schema of the process

THM densification with open-system process using an hydraulic

press with heated plates, moisture content is 15% maximum


Production of a rolled wooden tube in a mold (courtesy from P.

Haller)

a) charging b) Closing c) Forming

1.1 Open systems

Example of tube construction

Tube Spruce, fabricated by densified

bended wood panels, Photo P. Haller

(2008) Filament winding of moulded tubes

(Haller, 2009)


1.2 Closed systems (pressurized gas environment), THM treatment

Densifying Apparatus (pressure vessel) up to 150°C and saturated

steam: (a) closed condition, (b) open condition,1) chamber, 2) piston,

3) mould, 4) cover, 5) densified wood, (Navi & Girardet, 2000).

1.2 Closed systems (pressurized gas environment)

Photograph of a THM

(pressure vessel or reactor)

together with control panels

Schematic representation of a

THM reactor showing ingoing

and outgoing streams to

cylindrical chambers

1.2 Closed systems

Multi-layer THM reactor

Diagram of THM processing (Treatment chamber) – 4 stages

Forming a trunk by THM closed system

Photo of a two layered THM reactor, opened state.

Diameter of the cylinder is 20 cm.

Densified trunk of pine (one dimensional)

Two cross-sections of one directional radial densified trunk of pine

after drying. The trunk after densificatin sawn into two parts and

dried. This shows cracks formed on the wood during drying

(Girardet and Navi, 2007)

1.3 unidirectional densification

(a) (b)

(a) Spruce before densification

d= 430 kg/m3

(b) After densification

d= 1290 kg/m3

degree of densification 68 %

(c) Pine before densification

d= 490 kg/m3

(d) After densification

d= 1300 kg/m3

degree of densification 68 %

(Navi and Girardet)

(c) (d)

Viscoelastic Thermal Compression device (VTC):

Diagram showing the press inside to the chamber, platens

and the specimen between the platens (F. Kamk)

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

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

Large Viscoelastic Thermal Compression device (VTC):

Photograph of the interior of the chamber (Courtesy of F. Kamk)

Viscoelastic thermal compression process

(VTC)

Phase 1 – steaming

Phase 2 – venting

Phase 3 – compression

Cooling – specimens are cooled under compression


THM apparatus (pressure vessel or reactor) and the internal

moulding block (Ito et al. 1998 )


• Transformation of a circular trunk to a square-section by two-

dimensional densification (Ito et al., 1998a)


Compressed Lumber Processing System

Temperature in the THM chamber during the post-processing

1.4 Compression forming (multi-directional compression)

Photo of two pieces of densified wood fabricated by

“Compressed Lumber Processing system”

1.5 Isobar (membrane press - 3D)

• This is a 3D densification referring to the

membrane press method (pressure vessel + a

flexible membrane and steam) where

compression occurs under three-dimensional

uniform applied stress. Constant fluid pressure

(isobaric) on the membrane causes

compression of substrate in regions with least

resistence, (F. Kamke).

2. Effects of THM processing parameters on wood

2.2 Chemical : TH wood degradation

Samples are compressed at different temperatures between110°C

and 200°C in a closed system.

Degree of set-recovery of compressed samples measured under

water soaking-drying cycles (Heger, 2004).

2.2 Chemical : TH wood degradation

Set-Recovery (R) of samples compressed at different temperatures between

110°C and 200°C at a compression rate of 10 mm/min (Heger, 2004).

Sample of densified spruce

compressed to 35 %; detail of

a partially crushed earlywood

zone.

Sample of densified spruce

compressed at140°C under

saturated moisture conditions.

2.2 Chemical : degradation of wood constituents

High temperature steam, chemically degrades

wood constituents (here only up to 200C° considered)

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.

In THM densification the process needs water

or (steam).

In wood at high temperatures, water plays three

important roles:

it makes hydrolysis possible

it ensures the mobility of the protons

it participates in the acidification of the reactive

medium

________________

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.

The most important of degradation is wood

hydrolysis (specially the hemicelluloses), function of

T, MC and t

1- Hydrolysis of polysaccharides:

Treatments Man

Xyl Gal Ara Total

Reference (natural) 13.6 5.6 2.8 1.2 23.2

Densified 9.7 4.7 1.2 0.7 16.3

Densified and post-treated

(180°C, 30 min)

5.6 2.0 0.5 no-

traces

8.1

Densified and post-treated

(200°C, 5 min)

5.4 2.8 0.4 no-

traces

8.6

Quantitative analysis of hemicelluloses by GPC (gas

phase chromatography)

Percentages of the different neutral sugars in treated wood

after washing with water (the rest solved in water).

2- Determination of the degree of polymerization of

cellulose by capillary viscometer (according to the AFNOR (NF G06-037)

Cellulose microfibrils are chemically more resistant

to the hydrolysis.

Degree of polymerization of three spruce samples

Treatments: densification densification densification

post-treatment at

180°C, 30 min

post-treatment at

200°C, 4 min

1533 947 837

4- Index of crystallinity (CrI) of the cellulose and diameter (L) of cellulose crystallites

Treatment Temperatur

e (oC)

Time

(minutes)

CrI Diameter

, L

(Å)

No treatment - - 0.710 25.0

Densified 140 20 0.778 28.7

Densified and post-

treated under saturated

moisture conditions

140 60 0.809 31.3

-``- 140 120 0.804 31.3

-``- 140 180 0.830 32.5

Densified at 140°C and

post-treated under

saturated moisture

conditions

160

30

0.818 31.9

-``- 160 60 0.811 32.5

-``- 160 90 0.844 34.5

-``- 190 8 0.831 33.8

-``- 190 16 0.843 35.2

-``- 190 24 0.853 36.7

-``- 210 2 0.844 35.2

-``- 210 4 0.846 36.7

-``- 210 8 0.868 40.1

-``- 210 16 0.875 43.1

5- Determination of the glass transition temperature (Tg) of lignin

by DSC (Differential Sweeping Calorimetry with a single scan)

Depending to temperature & time lignin condenses or

depolymerises

What happens to wood after THM treatment

Unity or integrity of wood (cellulose-hemicelluloses-lignin)

Covalent and hydrogen bands

2.3 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).

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).

Brinel hardness of initial wood and densified wood in a THM

closed system for spruce, beech and maritime pine (average

values) (Navi & Girardet, 2000).

Photograph of a specimen after fracture under tensile

testing (Navi & Heger, 2005).

Natural wood,

Wood densified at 140°C during 20 minutes

post-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

minutes

From each type of sample, at least 10 specimens were tested in

tension.

MOR and MOE of THM wood

Longitudinal tensile strength and Young´s modulus of densified

samples

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

Influence of stress level on properties of THM wood

0

25

50

75

100

125

150

175

200

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500

Tem

pera

ture

[oC

]

Pre

ssu

re o

r S

tress [

MP

a]

Time [s]

Stress Steam Temperature

Kutnar and Kamke, 2010

Load

[MPa]

Temperature

[°C]

Saturated steam

pressure

[psi]

Compression

loading steam

environment

Venting time

prior to

compression

[s]

5.5 150 68 Saturated steam 0

Superheated steam 180

Transient conditions 10

160 98 Saturated steam 0



170 114 Saturated steam 0




Test parameters of each THM treatment

Oven dry density after THM treatment

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Transient Superheated steam Saturated steam

OD

density [g/c

m3]

150°C 160°C 170°C


0

1

2

3

4

5

6

7

8

9

10

Transient Superheated steam Saturated steam

Mois

ture

conte

nt

[%]

150°C 160°C 170°C

Moisture content of THM treated wood

after conditioning at 20°C and RH 65%


Treatment Temperature [°C] MOE

[GPa]

MOE/ρemc

[GPa cm3/g]

Untreated - 7.67 (1.06) 19.5 (1.35)

Saturated steam 150 11.6 (2.08) 20.3 (3.68)

160 22.9 (4.90) 20.3 (4.26)

170 28.7 (6.63) 24.8 (5.21)

Transient conditions 150 16.5 (4.80) 19.2 (4.34)

160 22.6 (5.91) 20.8 (3.12)

170 31.8 (3.01) 26.9 (2.92)

Superheated steam 150 11.6 (2.08) 15.8 (1.68)

160 15.8 (4.83) 18.4 (4.04)

170 19.6 (5.02) 18.2 (2.93)

MOE and MOE/ρemc of THM specimens


Treatment Temperature

[°C]

MOR

[MPa]

MOR/ρemc

[MPa cm3/g]

Untreated - 76.6 (10.9) 194 (9.60)

Saturated steam 150 222 (38.2) 193 (35.5)

160 219 (45.1) 194 (39.9)

170 249 (16.7) 215 (30.8)

Transient conditions 150 139 (42.0) 161 (36.7)

160 186 (55.4) 170 (19.6)

170 276 (16.7) 233 (16.3)

Superheated steam 150 101 (21.3) 137 (19.6)

160 140 (40.8) 164 (41.9)

170 171 (49.3) 158 (28.8)

MOR and MOR/ρemc of THM specimens


0

1

2

3

4

5

6

7

8

0 100 200 300 400 500 600

Time [s]

Ste

am

pre

ssu

re [

MP

a],

Co

mp

ressio

n s

tre

ss [

MP

a]

0

50

100

150

200

250

Te

mp

era

ture

[°C

]

Compression stress

Steam pressure

Temperature

Influence of post heat-treatment at 200°C

on properties of THM wood


MOR/ density post heat-treated THM wood at 200°C


Viscoelastic thermal compression process

(VTC)

Phase 1 – steaming

Phase 2 – venting

Phase 3 – compression

Cooling – specimens are cooled under compression Kutnar and Kamke, 2008

MOR and MOE of VTC wood

0,0

20,0

40,0

60,0

80,0

100,0

120,0

140,0

160,0

180,0

Control VTC 63% VTC 98% VTC 132%

MO

R [

MP

a]

0,0

5,0

10,0

15,0

20,0

25,0

MO

E [

GP

a]

MOR [MPa] MOE [GPa]


Effect of densification temperature on

maximum bending strength (I) and tensile strength (II)

Fang C.H., Mariotti N., Cloutier A., Koubaa A., Blanchet P. 2011. Densification of

wood veneers by compression combined with heat and steam. Eur. J. Wood Prod.

DOI 10.1007/s00107-011-0524-4

aspen (black) and hybrid

poplar (grey).

Effect of densification temperature on

MOE of bending (I) and tension (II) (?)

Fang C.H., Mariotti N., Cloutier A., Koubaa A., Blanchet P. 2011. Densification of

wood veneers by compression combined with heat and steam. Eur. J. Wood Prod.

DOI 10.1007/s00107-011-0524-4

aspen (black) and hybrid

poplar (grey).

3.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 because

of T,H,M & t effects on mechanical and

chemical degradation

4. Challenges exist on the moulding of large

scale elements

5. Need for a Numerical simulation of Virtual

processing of THM

Acknowledgment

Dr. Frederic Heger,

Mr. Fred Girardet

Dr. Andreja Kutnar

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