Rowell R.M. (ed.) Handbook of Wood Chemistry and Wood Composites
Подождите немного. Документ загружается.
TABLE 5.4
Penetration of the Heartwood of Various Softwood and Hardwood Species
a
Ease of Treatment Softwoods Hardwoods
Least difficult Bristlecone pine (Pinus aristata)
Pinyon (P. edulis)
Pondersosa pine (P. pondersosa)
Redwood (Sequoia sempervirens)
American basswood (Tilia americana)
Beech (white heartwood) (Fagus grandifolia)
Black tupelo (blackgum) (Nyssa sylvatica)
Green ash (Fraxinus pennsylvanica var.
lanceolata)
Pin cherry (Prunus pensylvanica)
River birch (Betula nigra)
Red oaks (Quercus spp.)
Slippery elm (Ulmus fulva)
Sweet birch (Betula lenia)
Water tupelo (Nyssa aquatica)
White ash (Fraxinus americana)
Moderately difficult Bald cypress (Taxodium distichum)
California red fir (Abies magnifica)
Douglas fir (coast) (Pseudotsuga
taxifolia)
Eastern white pine (Pinus strobus)
Jack pine (P. banksiana)
Loblolly pine (P. taeda)
Longleaf pine (P. palustris)
Red pine (P. r esinosa)
Shortleaf pine (P. echinata)
Sugar pine (P. lambertiana)
Western hemlock (Tsuga heterophylla)
Black willow (Salix nigra)
Chestnut oak (Quercus montana)
Cottonwood (Populus sp.)
Bigtooth aspen (P. grandidentata)
Mockernut hickory (Carya tomentosa)
Silver maple (Acer saccharinum)
Sugar maple (A. saccharum)
Yellow birch (Betula lutea)
Difficult Eastern hemlock (Tsuga canadensis)
Engelmann spruce (Picea engelmanni)
Grand fir (Abies grandis)
Lodgepole pine (Pinus contorta var.
latifolia)
Noble fir (Abies procera)
Sitka spruce (Picea sitchensis)
Western larch (Larix occidentalis)
White fir (Abies concolor)
White spruce (Picea glauca)
American sycamore (Platanus occidentalis)
Hackberry (Celtis occidentalis)
Rock elm (Ulmus thomoasi)
Yellow-poplar (Liriodendron tulipifera)
Very difficult Alpine fir (Abies lasiocarpa)
Corkbark fir (A. lasiocarpa var.
arizonica)
Douglas fir (Rocky Mountain)
(Pseudotsuga taxifolia)
Northern white-cedar (Thuja
occidentalis)
Tamarack (Larix laricina)
Western red cedar (Thaja plicata)
American beech (red heartwood) (Fagus
grandifolia)
American chestnut (Castanea dentata)
Black locust (Robinia pseudoacacia)
Blackjack oak (Quercus marilandica)
Sweetgum (redgum) (Liquidambar styraciflua)
White oaks (Quercus spp.)
a
As covered in MacLean (1952).
1588_C05.fm Page 114 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
5.2.3 TREATMENT PROCESSES
There are two general types of wood-preserving methods: pressure processes and nonpressure
processes. During pressure processes wood is impregnated in a closed vessel under pressure above
atmospheric. In commercial practice wood is put on cars or trams and run into a long steel cylinder,
which is then closed and filled with preservative. Pressure forces are then applied until the desired
amount of preservative has been absorbed into the wood.
5.2.3.1 Pressure Processes
Three pressure processes are commonly used: full-cell, modified full-cell, and empty-cell. The full-
cell process is used when the retention of a maximum quantity of preservative is desired. The steps
include the following: (1) The wood is sealed in a treating cylinder and a vacuum is applied for a
half-hour or more to remove air from the cylinder and wood, (2) the preservative (at ambient or
elevated temperature) is admitted to the cylinder without breaking the vacuum, (3) pressure is
applied until the required retention, (4) the preservative is withdrawn from the cylinder, and (5) a
short final vacuum may be applied to free the wood from dripping preservative. The modified full-
cell process is basically the same as the full-cell process except for the amount of initial vacuum
and the occasional use of an extended final vacuum.
The goal of the empty-cell process is to obtain deep penetration with relatively low net retention
of preservative. Two empty-cell processes (the Rueping and the Lowry) use the expansive force of
compressed air to drive out part of the preservative absorbed during the pressure period. The
Rueping empty-cell process is often called the empty-cell process with initial air. Air pressure is
forced into the treating cylinder, which contains the wood, and then the preservative is forced into
the cylinder. The air escapes into an equalizing or Rueping tank. The treating pressure is increased
and maintained until desired retention is attained. The preservative is drained and a final vacuum
is applied to remove surplus preservative. The Lowry process is the same as the Rueping except
that there is no initial air pressure or vacuum applied. Hence, it is often called the empty-cell
process without initial air pressure.
5.2.3.2 Nonpressure Processes
There are numerous nonpressure processes and they differ widely in their penetration and retention
of a preservative. Nonpressure methods consist of (1) surface applications of preservative by
brushing or brief dipping, (2) cold soaking in preservative oils or steeping in solutions of waterborne
preservative, (3) diffusion processes with waterborne preservatives, (4) vacuum treatment, and (5)
various other miscellaneous processes.
5.2.4 PURCHASING AND HANDLING OF TREATED WOOD
The EPA regulates pesticides, and wood preservatives are one type of pesticide. Preservatives that
are not restricted by EPA are available to the general consumer for nonpressure treatments, whereas
the sale of others is restricted only to certified pesticide applicators. These preservatives can be used
only in certain applications and are referred to as restricted-use. Restricted-use refers to the chemical
preservative and not to the treated wood product. The general consumer may buy and use wood
products treated with restricted-use pesticides; EPA does not consider treated wood a toxic substance
nor is it regulated as a pesticide.
Consumer Safety Information Sheets (EPA-approved) are available from retailers of treated
wood products. The sheets provide users with information about the preservative and the use and
disposal of treated-wood products. There are consumer information sheets for three major groups
of wood preservatives (see Table 5.5): (1) creosote pressure-treated wood, (2) pentachlorophenol
pressure-treated wood, and (3) inorganic arsenical pressure-treated wood.
1588_C05.fm Page 115 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
TABLE 5.5
EPA-Approved Consumer Information Sheets for Three Major Groups of Preservative Pressure-Treated Wood
Preservative Treatment Inorganic Arsenicals Pentachlorophenol Creosote
CONSUMER
INFORMATION
*This wood has been preserved by pressure-treatment
with an EPA-registered pesticide containing
inorganic arsenic to protect it from insect attack and
decay. Wood treated with inorganic arsenic should be
used only where such protection is important.
*Inorganic arsenic penetrates deeply into and remains
in the pressure-treated wood for a long time.
However, some chemical may migrate from treated
wood into surrounding soil over time and may also
be dislodged from the wood surface upon contact
with skin. Exposure to inorganic arsenic may present
certain hazards. Therefore, the following precautions
should be taken both when handling the treated wood
and in determining where to use or dispose of the
treated wood.
*This wood has been preserved by pressure-
treatment with an EPA-registered pesticide
containing pentachlorophenol to protect it from
insect attack and decay. Wood treated with
pentachlorophenol should be used only where
such protection is important.
*Pentachlorophenol penetrates deeply into and
remains in the pressure-treated wood for a long
time. Exposure to pentachlorophenol may present
certain hazards. Therefore, the following
precautions should be taken both when handling
the treated wood and in determining where to use
and dispose of the treated wood.
*This wood has been preserved by pressure-
treatment with an EPA-registered pesticide
containing creosote to protect it from insect attack
and decay. Wood treated with creosote should be
used only where such protection is important.
*Creosote penetrates deeply into and remains in
the pressure-treated wood for a long time.
Exposure to creosote may present certain hazards.
Therefore, the following precautions should be
taken both when handling the treated wood and
in determining where to use the treated wood.
HANDLING
PRECAUTIONS
*Dispose of treated wood by ordinary trash collection
or burial. Treated wood should not be burned in open
fires or in stoves, fireplaces, or residential boilers
because toxic chemicals may be produced as part of
the smoke and ashes. Treated wood from commercial
or industrial use (e.g., construction sites) may be
burned only in commercial or industrial incinerators
or boilers in accordance with state and Federal
regulations.
*Avoid frequent or prolonged inhalation of sawdust
from treated wood. When sawing and machining
treated wood, wear a dust mask. Whenever possible,
these operations should be performed outdoors to
avoid indoor accumulations of airborne sawdust from
treated wood.
*Dispose of treated wood by ordinary trash
collection or burial. Treated wood should not be
burned in open fires or in stoves, fireplaces, or
residential boilers because toxic chemicals may
be produced as part of the smoke and ashes.
Treated wood from commercial or industrial use
(e.g., construction sites) may be burned only in
commercial or industrial incinerators or boilers
rated at 20 million btu/hour or greater heat input
or its equivalent in accordance with state and
Federal regulations.
*Avoid frequent or prolonged inhalation of
sawdust from treated wood. When sawing and
machining treated wood, wear a dust mask.
*Dispose of treated wood by ordinary trash
collection or burial. Treated wood should not be
burned in open fires or in stoves, fireplaces, or
residential boilers, because toxic chemicals may
be produced as part of the smoke and ashes.
Treated wood from commercial or industrial use
(e.g., construction sites) may be burned only in
commercial or industrial incinerators or boilers in
accordance with state and Federal regulations.
*Avoid frequent or prolonged inhalation of sawdust
from treated wood. When sawing and machining
treated wood, wear a dust mask. Whenever
possible these operations should be performed
outdoors to avoid indoor accumulations of
airborne sawdust from treated wood.
1588_C05.fm Page 116 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
*When power-sawing and machining, wear goggles to
protect eyes from flying particles.
*Wear gloves when working with the wood. After
working with the wood, and before eating, drinking,
toileting, and use of tobacco products, wash exposed
areas thoroughly.
*Because preservatives or sawdust may accumulate on
clothes, they should be laundered before reuse. Wash
work clothes separately from other household
clothing.
Whenever possible, these operations should be
performed outdoors to avoid indoor accumulations
of airborne sawdust from treated wood.
*Avoid frequent or prolonged skin contact with
pentachlorophenol-treated wood. When handling
the treated wood, wear long-sleeved shirts and long
pants and use gloves impervious to the chemicals
(for example, gloves that are vinyl-coated).
*When power-sawing and machining, wear
goggles to protect eyes from flying particles.
*After working with the wood, and before eating,
drinking, and use of tobacco products, wash
exposed areas thoroughly.
*If oily preservatives or sawdust accumulate on
clothes, launder before reuse. Wash work clothes
separately from other household clothing.
*Avoid frequent or prolonged skin contact with
creosote-treated wood; when handling the treated
wood, wear long-sleeved shirts and long pants and
use gloves impervious to the chemicals (for
example, gloves that are vinyl-coated).
*When power-sawing and machining, wear
goggles to protect eyes from flying particles.
*After working with the wood and before eating,
drinking, and use of tobacco products, wash
exposed areas thoroughly.
*If oily preservatives or sawdust accumulate on
clothes, launder before reuse. Wash work clothes
separately from other household clothing.
USE SITE PRECAUTIONS *All sawdust and construction debris should be
cleaned up and disposed of after construction.
*Do not use treated wood under circumstances where
the preservative may become a component of food or
animal feed. Examples of such sites would be use of
mulch from recycled arsenic-treated wood, cutting
boards, counter tops, animal bedding, and structures
or containers for storing animal feed or human food.
*Only treated wood that is visibly clean and free of
surface residue should be used for patios, decks, and
walkways.
*Do not use treated wood for construction of those
portions of beehives that may come into contact with
honey.
*Treated wood should not be used where it may come
into direct or indirect contact with drinking water,
except for uses involving incidental contact such as
docks and bridges.
*Logs treated with pentachlorophenol should not
be used for log homes. Wood treated with
pentachlorophenol should not be used where it
will be in frequent or prolonged contact with bare
skin (for example, chairs and other outdoor
furniture) unless an effective sealer has been
applied.
*Pentachlorophenol-treated wood should not be
used in residential, industrial, or commercial
interiors except for laminated beams or building
components which are in ground contact and are
subject to decay or insect infestation and where
two coats of an appropriate sealer are applied.
Sealers may be applied at the installation site.
Urethane, shellac, latex epoxy enamel, and
varnish are acceptable sealers for
pentachlorophenol-treated wood.
*Wood treated with creosote should not be used
where it will be in frequent or prolonged contact
with bare skin (for example, chairs and other
outdoor furniture) unless an effective sealer has
been applied.
*Creosote-treated wood should not be used in
residential interiors. Creosote-treated wood in
interiors of industrial buildings should be used
only for industrial building components that are
in ground contact and are subject to decay or
insect infestation and wood-block flooring. For
such uses, two coats of an appropriate sealer must
be applied. Sealers may be applied at the
installation site.
*Wood treated with creosote should not be used in
the interiors of farm buildings where there may be
direct contact with domestic animals or livestock
that may crib (bite) or lick the wood.
(Continued )
1588_C05.fm Page 117 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
TABLE 5.5
EPA-Approved Consumer Information Sheets for Three Major Groups of Preservative Pressure-Treated Wood (Continued)
Preservative Treatment Inorganic Arsenicals Pentachlorophenol Creosote
*Wood treated with pentachlorophenol should not
be used in the interiors of farm buildings where
there may be direct contact with domestic
animals or livestock that may crib (bite) or lick
the wood.
*In interiors of farm buildings where domestic
animals or livestock are unlikely to crib (bite) or
lick the wood, pentachlorophenol-treated wood
may be used for building components that are in
ground contact and are subject to decay or insect
infestation and where two coats of an appropriate
sealer are applied. Sealers may be applied at the
installation site.
*Do not use pentachlorophenol-treated wood for
farrowing or brooding facilities.
*Do not use treated wood under circumstances
where the preservative may become a component
of food or animal feed. Examples of such sites
would be structures or containers for storing silage
or food.
*Do not use treated wood for cutting-boards or
countertops.
*In interiors of farm buildings where domestic
animals or livestock are unlikely to crib (bite) or
lick the wood, creosote-treated wood may be used
for building components that are in ground
contact and are subject to decay or insect
infestation if two coats of an effective sealer are
applied. Sealers may be applied at the installation
site. Coal tar pitch and coal tar pitch emulsion are
effective sealers for creosote-treated wood-block
flooring. Urethane, epoxy, and shellac are
acceptable sealers for all creosote-treated wood.
*Do not use creosote-treated wood for farrowing
or brooding facilities.
*Do not use treated wood under circumstances
where the preservative may become a component
of food or animal feed. Examples of such use
would be structures or containers for storing
silage or food.
*Do not use treated wood for cutting-boards or
countertops.
1588_C05.fm Page 118 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
*Only treated wood that is visibly clean and free
of surface residue should be used for patios,
decks, and walkways.
*Do not use treated wood for construction of those
portions of beehives that may come into contact
with the honey.
*Only treated wood that is visibly clean and free
of surface residues should be used for patios,
decks, and walkways.
*Do not use treated wood for construction of those
portions of beehives that may come into contact
with the honey.
*Pentachlorophenol-treated wood should not be
used where it may come into direct or indirect
contact with public drinking water, except for
uses involving incidental contact such as docks
and bridges.
*Do not use pentachlorophenol-treated wood
where it may come into direct or indirect contact
with drinking water for domestic animals or
livestock, except for uses involving incidental
contact such as docks and bridges.
*Creosote-treated wood should not be used where
it may come into direct or indirect contact with
public drinking water, except for uses involving
incidental contact such as docks and bridges.
*Do not use creosote-treated wood where it may
come into direct or indirect contact with drinking
water for domestic animals or livestock, except
for uses involving incidental contact such as
docks and bridges.
1588_C05.fm Page 119 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
There are two important factors to consider depending upon the intended end use of preserva-
tive- treated wood: the grade or appearance of the lumber, and the quality of the preservative
treatment in the lumber. The U.S. Department of Commerce American Lumber Standard Committee
(ALSC), an accrediting agency for treatment quality assurance, has an ink stamp or end tag for
each grade stamp and quality mark. These marks indicate that the producer of the treated-wood
product subscribes to an independent inspection agency. The stamp or end tag contains the type of
preservative or active ingredient, the retention level, and the intended exposure conditions. Retention
levels are usually provided in pounds of preservatives per cubic foot of wood and are specific to the
type of preservative, wood species, and intended exposure conditions. Be aware that suppliers often
sell the same type of treated wood by different trade names. Depending on your intended use and
location, there will be different types of treated wood available for residential use. Also, be aware
that some manufacturers add colorants (such as brown) or water repellents (clear) into some of
their preservative treatments. When purchasing treated wood, ask the suppliers for more information
to determine what preservative and additives were used, as well as any handling precautions.
Note that mention of a chemical in this article does not constitute a recommendation; only
those chemicals registered by the EPA may be recommended. Registration of preservatives is under
constant review by EPA and the U.S. Department of Agriculture. Use only preservatives that bear
an EPA registration number and carry directions for home and farm use. Preservatives, such as
creosote and pentachlorophenol, should not be applied to the interior of dwellings that are occupied
by humans. Because all preservatives are under constant review by EPA, a responsible state or
Federal agency should be consulted as to the current status of any preservative.
REFERENCES
ASTM. (2000). Annual Book of ASTM Standards. American Society for Testing and Materials, West
Conshohocken, PA.
AWPA. (2003). AWPA 2003 Book of Standards. American Wood-Preservers’ Association, Selma, AL.
Cassens, D.L., Johnson, B.R., Feist, W.C., and De Groot, R.C. (1995). Selection and Use of Preservative-
Treated Wood, Forest Products Society, Madison, WI.
Eaton, R.A. and Hale, M.D. (1993). Wood: Decay, Pests and Protection. Chapman & Hall, New York.
EPA. (2002). Whitman announces transition from consumer use of treated wood containing arsenic, U.S.
Environmental Protection Agency.
FPL. (1999). Wood Handbook: Wood as an Engineering Material. U.S. Department of Agriculture, Forest
Service, Forest Products Laboratory, Madison, WI.
Gutzmer, D.I. and Crawford, D.M. (1995). Comparison of Wood Preservatives in Stake Tests. U.S. Department
of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 124.
Johnson, B.R. and Gutzmer, D.I. (1990). Comparison of Preservative Treatments in Marine Exposure of Small
Wood Panels. U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison,
WI, 28.
Mac Lean, J.D. (1952). Preservation of Wood by Pressure Methods. U.S. Department of Agriculture, Forest
Service, Washington, DC, 160.
PMRA. (2002). Chromated Copper Arsenate (CCA). Canadian Pest Management Regulatory Agency.
Rowell, R.M. (1991). Chemical modification of wood. In: Hon, D.N.-S. and Shiraishi, N., Handbook on Wood
and Cellulosic Materials. Marcel Dekker, New York, pp. 703–756.
USFSS. (1968). Wood Preservation Treating Practices. U.S. Federal Supply Service, Washington, DC.
USFSS. (1969). Fungicide: Pentachlorophenol. U.S. Federal Supply Service, Washington, DC.
1588_C05.fm Page 120 Thursday, December 2, 2004 3:47 PM
© 2005 by CRC Press
6
Thermal Properties
Roger M. Rowell
1,2
and Susan L. LeVan-Green
1
1
USDA, Forest Service, Forest Products Laboratory, Madison, WI and
2
Department of Biological Systems Engineering, University
of Wisconsin, Madison, WI
CONTENTS
6.1 Pyrolysis and Combustion
6.2 Fire Retardancy
6.3 Testing Fire Retardants
6.3.1 Thermogravimetric Analysis (TGA)
6.3.2 Differential Thermal Analysis (DTA) and Differential
Scanning Calorimetry (DSC)
6.3.3 Tunnel Flame-Spread Tests
6.3.4 Critical Oxygen Index Test
6.3.5 Other Tests
6.4 Fire Retardants
6.4.1 Chemicals that Promote the Formation of Increased Char
at a Lower Temperature than Untreated Wood Degrades
6.4.2 Chemicals which Act as Free Radical Traps in the Flame
6.4.3 Chemicals Used to Form a Coating on the Wood Surface
6.4.4 Chemicals that Increase the Thermal Conductivity of Wood
6.4.5 Chemicals that Dilute the Combustible Gases Coming
from the Wood with Non-Combustible Gases
6.4.6 Chemicals that Reduce the Heat Content of the Volatile Gases
6.4.7 Phosphorus-Nitrogen Synergism Theories
6.4.8 Fire-Retardant Formulations
6.4.8.1 Phosphorus
6.4.8.2 Boron
6.4.9 Leach Resistant Fire-Retardants
References
The traditional question at the start of a class on thermal properties of wood is, “Does wood burn?”
The students have all been warmed in front of a wood-burning fire before, so they are sure the
answer is yes—but since the professor asked the question, there must be some hidden trick to the
obvious answer. Going with their experience, their answer is “yes, wood burns.” But, the actual
answer is no, wood does not burn. In short, wood undergoes thermal degradation as it heats up,
giving rise to volatile, flammable gases which burn when they contact a source of ignition. So it
is the flammable gases that burn, not the wood itself. This is, of course, an oversimplified explanation
of the pyrolysis and burning processes which are the subject of this chapter.
Lignocellulosic materials decompose on heating and when exposed to an ignition source by
two different mechanisms. The first, dominant at temperatures below 300˚C, degrades polymers
1588_C06.fm Page 121 Friday, December 3, 2004 10:17 AM
© 2005 by CRC Press
by the breaking of internal chemical bonds; dehydration (elimination of water); formation of free
radicals, carbonyl, carboxyl, and hydroperoxide groups; formation of carbon monoxide and carbon
dioxide; and finally, the formation of reactie carbonaceous char. Oxidation of the reactive char
results in smoldering or glowing combustion, and further oxidation of the combustible volatile
gasses gives rise to flaming combustion (Antal 1985, Bridegwater 1999, Czernik et al. 1999,
Shafizadeh 1984).
The second mechanism, which takes over at temperatures above 300˚C, involves the cleavage
of secondary bonds and formation of intermediate products such as anhydromonosaccharides, which
are converted into low molecular weight products (oliosaccharides and polysaccharides), which
lead to carbonized products (Kawamoto et al. 2003).
6.1 PYROLYSIS AND COMBUSTION
The simplest method of evaluating the thermal properties of wood is by thermogravametric analysis
(TGA). Figure 6.1 shows a schematic of the equipment used for this analysis. A sample is placed
in a metal pan in a furnace tube. Nitrogen gas is passed through the system and the furnace tube
is slowly heated at a constant rate. The percent of weight loss is measured as function of temperature
and is printed out on a strip chart. The temperature is usually raised to 500–600˚C in nitrogen at
a rate of a few degrees Celsius per minute. The temperature is then lowered to around 300˚C,
oxygen is introduced into the system, and the temperature is increased again. This second scan
shows the combustion of the char in oxygen.
A simple thermogram run in nitrogen is shown in Figure 6.2 for pine (Shafizadeh 1984). As
the wood is heated from room temperature to 100˚C, very few chemical reactions take place. At
approximately 100˚C, any moisture in the wood is vaporized out. As the temperature of the wood
increases, very little degradation occurs until about 200˚C, when chemical bonds start to break via
dehydration and, possibly, free radical mechanisms to eliminate water and produce volatile gases.
In the absence of oxygen, or in the presence of limited amounts, this thermal degradation process
FIGURE 6.1 Schematic diagram of a simple thermogravimetric analysis system.
DEMODULATOR
X-Y
RECORDER
TIME
GENERATOR
TIME
TEMPERATURE
SWITCH
FURNACE
CONTROL &
PROGRAMMER
FURNACE
CONTROLLING
THERMOCOUPLE
SAMPLE
THERMOCOUPLE
SAMPLE
DAMPER
VACUUM
PUMP
TARE WT.
WT. CALIBRATOR
TRANSDUCER
COIL
TRANSDUCER
ARMATURE
SPRING
1588_C06.fm Page 122 Friday, December 3, 2004 10:17 AM
© 2005 by CRC Press
is called pyrolysis. The volatile gases produced diffuse out of the wood into the surrounding
atmosphere. Figure 6.3 shows the first derivative of the TGA curve for pine.
Whole wood starts to thermally degrade at about 250˚C (see Figure 6.4). Between about
300–375˚C, the majority of the carbohydrate polymers have degraded and only lignin remains. The
hemicellulose components start to decompose at about 225˚C and are almost completely degraded
by 325˚C. The cellulose polymer is more stable to thermal degradation until about 370˚C, and then
decomposes almost completely over a very short temperature range. Both the acid lignin and milled
wood lignin start to decompose at about 200˚C, but are much more stable to thermal degradation
as compared to the carbohydrate polymers. The curve for whole wood represents the results of
each of the cell wall components. The pyrolysis products given off when wood is thermally degraded
are shown in Table 6.1 (Shafizadeh 1984).
Since the major cell wall polymer is cellulose, the thermal degradation of cellulose dominates
the chemistry of pyrolysis (Shafizadeh and Fu 1973). The decomposition of cellulose leads mainly
to volatile gases, while lignin decomposition leads mainly to tars and char. Figure 6.5 shows the
pyrolysis and combustion of cellulose (Shafizadeh 1984). In the early stages of cellulose degradation
(below 300˚C), the molecular weight is reduced by depolymerization caused by dehydration reac-
tions. The main products are CO, CO
2
produced by decarboxylation and decarbonylation, water,
and char residues. In the presence of oxygen, the char residues undergo glowing ignition. CO and
CO
2
form much faster in oxygen than in nitrogen, and this rate accelerates as the temperature
FIGURE 6.2 Thermogravimetric analysis of pine.
FIGURE 6.3 Derivative thermogravimetric analysis of pine.
1588_C06.fm Page 123 Friday, December 3, 2004 10:17 AM
© 2005 by CRC Press