Rowell R.M. (ed.) Handbook of Wood Chemistry and Wood Composites

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15.8 WATER SOLUBLE POLYMERS AND SYNTHETIC RESINS

Wood can be impregnated with water-soluble treatments such as PEG or resins (Impreg) that become

insoluble after curing. Keeping the wood in a partially or completely swollen state increases

dimensional stability, as well as strength and water repellency.

15.8.1 POLYETHYLENE GLYCOL (PEG)

PEG is a chemical with the following structure:

HO-CH

-O-CH

-(O-CH

)

-O-CH

-OH

PEG 1000 is most commonly used when treating wood, usually undried, green wood (Mitchell

1972). It is a waxy, white solid that has an n value (average molecular weight) of 1,000 and it can

penetrate the cell wall because of its small size. It melts at 40˚C, readily dissolves in warm water,

is noncorrosive, odorless, and colorless, and has a very high ﬁre point (305˚C) (Rowell and Konkol

1987). Molecular weights up to 6000 are soluble in water.

To treat wood with PEG 1000, the wood is placed in a container and covered with a 30-50

weight percent solution dissolved in water. The treatment is based on diffusion and therefore soak

time will vary depending upon the thickness of the specimen. PEG remains in the cell walls when

the wood is dried because of its low vapor pressure. The rate of diffusion into the cell wall increases

as water evaporates from the solution (Stamm 1964).

Treatment temperature is usually from 21–60˚C, but diffusion can be accelerated with increasing

the temperature and/or the concentration of the solution. After treatment, the wood is air-dried in

a ventilated room. The drying time also depends upon specimen size.

PEG is not chemically attached to the wood, and because it is water soluble, it will leach out

if it gets wet (see Chapter 4). Glycol attracts moisture, so if the relative humidity reaches above

70%, the wood becomes sticky.

PEG has many uses especially in prevention or reduction of cracking due to drying sound wood

for tabletops, to partially decomposed wooden artifacts, or archeological waterlogged wood.

15.8.2 IMPREG

Impreg is wood that has been treated with a thermosetting, ﬁber-penetrating resin and is cured

without compression (Stamm and Seborg 1962). Phenol-formaldehyde resin-forming systems with

low molecular weights are the most successful thermosetting agents. The resins penetrate the cell

wall (25–25% weight gain) and keep the wood in a swollen state, dried at 80–93˚C for 30 minutes,

and then are polymerized by heat (155˚C for 30 min) to form a water-insoluble resin in the cell

wall (Rowell and Konkol 1987). Treatments are usually done on thin veneers (less than 9 mm thick)

due to time.

The cured product is usually reddish brown with reduced swelling, shrinkage, grain raising, and

surface checking. It improves the compression strength, but reduces the impact bending strength.

Impreg shows resistance to decay, termite, and marine-borer attack, and it has a high resistance to

acid. It is suited for pattern and die models as well as electrical control equipment.

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Plasma Treatment of Wood

Ferencz S. Denes, L. Emilio Cruz-Barba,

and Sorin Manolache

Biological Systems Engineering and Center for Plasma-Aided

Manufacturing, University of Wisconsin, Madison, WI

CONTENTS

16.1 Wood Surface Properties

16.2 Surface Treatment of Wood

16.2.1 Wood Protection

16.2.2 Wood Coating

16.3 The Plasma State

16.3.1 Classiﬁcation of Plasmas

16.3.1.1 Natural and Man-Made Plasma

16.3.2 Plasma Parameters

16.3.3 Atmospheric Pressure Cold Plasma

16.4 Cold Plasma Modiﬁcation of Wood

References

This chapter presents a review of the various plasma-mediated treatments of wood reported in the

literature. Some of these works presented extensive detail, and others only described the process

with the plasma-parameters. This is because of the high diversity of the plasma tools involved in

the modiﬁcation of wood surfaces, the complexity of the environment in the plasma discharges,

and the main focus of many researchers in the characteristics of the plasma-generated surfaces,

which result in a black-box-like representation of the plasma processes.

16.1 WOOD SURFACE PROPERTIES

The properties of wood surfaces depend on the species, environmental conditions, age, and mechan-

ical processing (see Chapters 2, 3, and 8). Depending on its ﬁnal use, wood needs to undergo

surface improvement treatments to overcome issues resulting from surface inactivation, formation

of weak boundary layers, and material processing such as machining, drying, and aging.

Surface inactivation can be deﬁned as the detriment of wood surface properties as a result of

different factors. These factors can be summarized as the migration of hydrophobic extractives to

the surface during drying, which results in poor wettability, acidity or reactivity of extractives

affecting adhesion, oxidation of the wood surface as a result of aging, molecular reorientation of

the surface functional groups, and closure of wood cell micro-pores, which reduces adhesive

absorption into the wood (Christiansen 1990, 1991).

Weak boundary layers are the result of chemical and mechanical variations on the wood surfaces.

Chemical weak boundary layers are the result of extractive materials migrating to the surface. Resin

acids, fats/fatty acids, sterols, and waxes will render the surface hydrophobic, while sugars, phenols,

tannins, and proteins will render it hydrophilic. Mechanical weak boundary layers are the result of

degradation by light, damaged surfaces from machining processes, and trapped air at the interface.

Morphology of the wood surface reveals important properties such as surface roughness and

mechanical weak boundary layers. Surface roughness affects wettability due to capillary force, and

changes on it can lead to improved mechanical interlocking. Adhesion is affected by the presence

of damaged ﬁbers resulting from machining, as well as cracks and splits.

16.2 SURFACE TREATMENT OF WOOD

Depending on the intended application of wood products, it is generally necessary to treat the wood

to provide speciﬁc surface properties. A typical problem in softwoods is yellowing, a degradation

resulting from UV-radiation damage caused by sunlight exposure. Softwood products that have to

be used outdoors need to be treated with a UV-protecting layer.

Hardwood products can undergo surface treatments where the coating is used to cover only

the surface or to ﬁll up the pores on the structure. There is an increased use of wood-based board

materials in a wide variety of applications. These engineered composite materials have improved

properties and structural characteristics. The most common board materials are chipboards, ﬁber-

boards, and veneer.

16.2.1 WOOD PROTECTION

In order to protect wood from weather conditions and pest attack, it is necessary to treat it with

preservatives. The preservatives used can be divided into water-based (e.g., sodium phenylphenox-

ide, benzalconium chloride, copper chrome arsenate); organic solvent-based (e.g., triazoles, per-

methrin, copper and zinc naphthenates); borates; and tar oils.

16.2.2 WOOD COATING

The main purpose of a surface treatment is to protect the surface of wood and give it a good

appearance. Different materials can be used for ﬁnishing, such as ﬁllers, stains, primers and top

coatings, oils, and waxes. Fillers are used to seal cracks and small cavities on the wood surface.

Stains are usually applied under transparent coatings and have the purpose of giving color to the

wood to emphasize the natural beauty and structure of it. Lacquers and paints are the most used

coating materials and are usually the outermost layer of the treated wood. Coating materials consist

of binders, ﬁllers, pigments, ﬂatting agents, solvents, and additives.

The main properties of the materials to be used for surface treatment depend greatly on the

binder. Binder materials commonly used for the treatment of wood include: amino resins, polyure-

thane, acrylate, polyester, and nitrocellulose. Special properties can be achieved by the use of a

combination of these materials.

Amino resins, such as urea and melamine, and alkyd resins, which are often modiﬁed with

nitrocellulose, are used as binders in acid-curing treatments. These materials harden as a result of

a polycondensation process that is initiated when a catalyst is added. Such a catalyst (acid hardener)

should be added to the coating before application. The hardening process can be signiﬁcantly

accelerated by increasing the drying temperature and boosting air ﬂow. Acid-curing coatings have

good durability against chemical attack and mechanical impact.

Emulsions, colloidal dispersions, and totally water-soluble binders can be used for water-borne

treatments. Use of these products is mainly because emission of toxic organic solvents is greatly

reduced or totally avoided during the treatment. In addition, these materials have good light and

ﬁre resistance.

There are a wide variety of water-borne products with different properties that can be used,

depending on the application for which the wood is needed. Water-borne coatings can be alkyd-,

polyurethane-, acrylate-, or polyester-based. The main disadvantages of these materials are that

storage and transportation can take place only at temperatures above 0˚C, and the possibility of

swelling of the wood.

Polyurethane materials dry as a result of chemical reaction between the isocyanate and hydroxyl

groups. Most polyurethane materials are organic solvent-based, although there are some water-

based products. Compared to acid-curing materials, polyurethanes dry more slowly. Polyurethanes

are known for good resistance to chemicals and mechanical impact, as well as excellent moisture

resistance. Their ﬂexibility enables them to provide good resistance against swelling and shrinking.

Acrylate combined with a photo initiator is generally used as a UV-curing coating. Under UV

radiation, the photo initiator starts a fast curing process. These materials, which can be acrylates

or polyesters, need to be dried before the curing in order to remove all of the solvent.

Moisture content causes wood to swell or shrink, and the results can differ depending on the

kind of wood. The moisture content of wood should be kept stable during the treatment processes,

as well as during storage, transport, and usage. Cycled or even single swelling and shrinking of

the wood surface can cause severe damage to the treated surface, resulting in cracking and splitting

which appears on the surface of the coating.

The coatings are usually applied to the wood surfaces by pressurized impregnation; by soaking,

brushing; or spraying; by dipping or immersion; and by thermal diffusion (immersion in a hot bath

of preservative).

16.3 THE PLASMA STATE

Broadly the plasma state can be considered to be a gaseous mixture of oppositely charged particles

with a roughly zero net electrical charge (Chen 1965; Venugopalan 1971; Nasser 1971; Vossen and

Kern 1978; Chapman 1980; Chen and Pfender 1983; Herman 1988; Hershkowitz 1989; MacRae

1989; Smith et al. 1989; Cecchi 1990; Chang and Pfender 1990; Leveroni and Pfender 1990; Raizer

1991; Danikas 1993; Grill 1994; Hitchon 1999; Becker 2003). Sir William Crooks suggested the

concept of the “fourth state of matter” (1879) for electrically discharged matter, and Irving Langmuir

ﬁrst used the term “plasma” to denote the state of gases in discharge tubes.

Ionization processes can occur when, for instance, molecules of a gas are subjected to high-

energy radiation, electric ﬁelds, or high caloric energy. During these processes the energy levels

of particles composing the gas increase signiﬁcantly and, as a result, electrons are released, and

charged heavy particles are produced.

Increasing the energy content of solid phase matter sufﬁciently leads to phase transformation

processes, and, depending on the speciﬁc molecular structure, molecular weight and temperature;

a system that has been heated will frequently be in solid, liquid, or gaseous phase or a mixture of

these. Not all materials can undergo all of these transformation processes. Some of them will

undergo structural changes at speciﬁc absorbed-energy levels, through molecular fragmentation

processes. At atmospheric pressures and temperatures around 5000 K materials exist only in the

gaseous phase. Above 10,000 K few atoms, but mainly ions, are the constituent particles of matter.

Under these conditions and at even higher temperatures matter is considered to be in the plasma

state. More elevated temperatures induce very high degrees of ionization, and temperatures of 10

and higher lead to mixtures of bare nuclei and electrons.

Gas-phase plasmas are the most common plasmas, although the plasma state can be present in

liquid and solid phases. Man-made plasmas can be produced in the laboratory by raising the energy

content of matter regardless of the nature of the energy source. Thus plasmas can be generated by

mechanical (close to adiabatic compression), thermal (electrically heated furnaces), chemical (exo-

thermic reactions, e.g., ﬂames), radiant (high-energy electromagnetic and particle radiations, e.g.,

electron beams), nuclear (controlled nuclear reactions), and electrical (arcs, coronas, DC and RF

discharges) energies, and by the combination of them, as in the combination of mechanical and

thermal energies (e.g., explosions).

Most electrical discharge processes are initiated and sustained by electron impact ionization

phenomena. The plasma state is created when “omni-present” free electrons (e.g., free electrons

generated by cosmic radiation) in a gas environment are accelerated by an electric or an electro-

magnetic ﬁeld to energy levels where ionization and molecular fragmentation processes occur

through non-elastic collision mechanisms. The electrons involved in non-elastic collision reactions

lose part of their energy by generating more free electrons, ions of either polarity, excited species,

and free radical species through the fragmentation of molecular structures. Electrons traveling along

the free path (λ) with a speciﬁc distribution must be accelerated to energy levels by an applied

electric ﬁeld (E) to gain sufﬁcient kinetic energy (ε

kin

) for the initiation of ionization, excitation,

and fragmentation (ε

) of atoms or molecules of a speciﬁc gas environment:

kin

= e ·E·λ > ε

Accordingly, to initiate the discharge, low-pressure gas environments (large λ values) or very

intense electric ﬁelds (E) are required. Most of the ionization energies of atoms involved in organic

structures are in the range of 10–13 eV, so if an electron must gain a kinetic energy of 10 eV at

atmospheric pressure (λ=10

−5

cm) E must exceed 10

V/cm. Under low-pressure conditions like

1 mTorr (at 1 mTorr and room temperature λ

argon

is about 8 cm) E must exceed only 1.25 V/cm

(Gericke et al. 2002; Baars-Hibbe et al. 2003; Schefﬂer et al. 2004).

16.3.1 CLASSIFICATION OF PLASMAS

Plasma states can be divided into two main categories: hot plasmas (near-equilibrium plasmas) and

cold plasmas (non-equilibrium plasmas). Hot plasmas are characterized by comparable and very

high-temperature electrons and heavy particles, both charged and neutral, and they are close to

maximal degrees of ionization (100%). Cold plasmas are composed of low-temperature particles

(charged and neutral molecular and atomic species) and relatively high-temperature electrons, and

they are associated with low degrees of ionization (10

−4

–10%). Hot plasmas include electrical arcs,

plasma jets of rocket engines, thermonuclear reaction-generated plasmas, etc., while low-pressure

DC and RF discharges (silent discharges), discharges from ﬂuorescent (neon) illuminating tubes,

and corona discharges can be identiﬁed as cold plasmas.

16.3.1.1 Natural and Man-Made Plasma

It is estimated that more than 99% of the known universe is in the plasma state, the exception being

cold celestial bodies and planetary systems. The stars of the universe including our sun have

extremely high temperatures and they consist entirely of plasma. It is also suggested that the space

between the stars and galaxies are star-origin, radiation-induced, rareﬁed plasmas. Our planet is

not an exception to the presence of the plasma state. The ionosphere, “northern lights” (aurora

borealis), and lightning also represent plasma states.

Meteoritic impacts on the atmosphere of the earth also create plasma states. An incoming

meteorite traveling at extremely high speed will compress the atmospheric gas components to very

high densities and convert its tremendous kinetic energy into caloric energy. Molecular fragmen-

tation and ionization processes are initiated as a consequence and the plasma state is generated.

The plasma state can be produced in the laboratory by raising the energy content of matter

regardless of the nature of the energy source. Plasmas lose energy to their surroundings through

collision and radiation processes; consequently, energy must be supplied continuously to the system

in order to sustain the discharge. The easiest way to inject energy into a system in a continuous

manner is with electrical energy, and that is the reason why electrical discharges are the most

common man-made plasmas. Electrical discharges usually generate non-homogeneous plasma

media owing to various shapes and geometrical locations of the electrodes (antennas) and the

geometry of the plasma reactor. As a result of a more or less intense interaction (reaction) of plasma