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Hygric Properties of Natural and

Manufactured Timber

David Bailey

Advanced Study Dissertation

Supervisor: Dr. Matthew Hall

Date: 7 May 2009

Nottingham University

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ABSTRACT

Understanding the hygroscopic properties of wood is the first step in ensuring itsproper application within a structure. As each species of timber has unique properties,

no set model has yet been designed to determine the way wood will react to moisture

and the most reliable data is still obtained through physical experimentation.

This paper seeks to explore the continuous state of equilibrium flux that manifests

itself as a moisture sorption curve when wood is exposed to varying relative humidity

values under isothermal conditions. These sorption isotherms allow a comparison

between the samples of timber in a number of respects, including: natural vs.

manufactured timber, the relationship between hardwood and softwood and the effect

of density on sorption. The results show that the actual wood used in the construction

of manufactured timber has little effect on its sorption characteristics, which are

mostly controlled by the moisture resistant chemical treatments added to them after

production. The paper also expands on the isotherm data in two in two key areas,

modelling of data and time sorption time dependency. The analysis of a diffusion

coefficient model, proved its validity, as calculated data from the sorption isotherm in

conjunction with the model, was corroborated against research from other papers. The

study into sorption time dependant variables is dealt with under three headings, time

taken to reach equilibrium, the dependence of that time on the environmental relative

humidity and finally the hysteresis of the water mass absorbed and desorbed by the

timber over a period of time. Results call into question the definition of equilibrium in

relation to hardwood moisture sorption. The final area of research does lead to an

interesting theory of constant hysteresis, initiated by observed trends in the sorption

mass curves.

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Table of contents

ACKNOWLEDGEMENTS .........................................................................................4

List of figures ................................................................................................................5

List of Graphs ............................................................................................................... 6

List of Tables ................................................................................................................ 7

1. INTRODUCTION .................................................................................................... 9

1. 1 Overview of the problems .................................................................................10

1.1.1 Humidity control in buildings Comfort, Energy and Structure ................10

1.2 Aims and objectives ...........................................................................................111.2.1 Overall Aim .................................................................................................11

1.2.2 List of Objectives ........................................................................................ 11

1.3. Limitations of Study ......................................................................................... 12

1.3.1 Practical Limitations ................................................................................... 12

1.3.2 Theoretical Limitations ............................................................................... 12

2. BACKGROUND LITERATURE ......................................................................... 14

2.1 Wood structure .................................................................................................. 14

2.2 Moisture Sorption/Desorption ......................................................................... 16

.............................................................................................................................. 18

2.2.1 Hysteresis ................................................................................................... 18

2.3 Methods of measuring moisture content .........................................................19

2.4 Moisture buffering capacity of materials ........................................................20

................................................................................................................................. 21

2.5 Strength/Dimensions of structure ................................................................... 21

2.6 Phenomenological Macroscopic Models ........................................................ 23

3. METHODOLOGY ................................................................................................27

3.1 Background ....................................................................................................... 27

3.2 Materials and Preparation ...............................................................................28

3.3 Procedure .......................................................................................................... 29

3.4 Primary Testing and Modifications .................................................................30

4. RESULTS & DISCUSSION ..................................................................................33


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4.1 Characterisation ................................................................................................33

4.1.1 Isotherms ................................................................................................... 33

4.1.2 Diffusion Coefficient & Model Comparisons ............................................. 38

4.1.3 Parametric Review ...................................................................................... 41

4.1.4 Storage Function ......................................................................................... 43

4.2 Time Dependant ................................................................................................ 44

4.2.1 Time Taken To Reach Equilibrium .............................................................44

4.2.2 Dependence of on RH () .........................................................................47

4.2.3 Hysteresis of mw at duration of .............................................................48

5. CONCLUSIONS ................................................................................................... 53

6. EXPERIMENTAL RECOMENDATIONS AND FURTHER RESEARCH ....54

6.1 Experimental Recommendations ......................................................................54

6.2 Future Research ................................................................................................54

REFERENCES ...........................................................................................................55

APPENDIX A: TEST RESULTS ............................................................................. 58

APPENDIX B: DATA CONCLUSIONS ................................................................. 69

APPENDIX C: DIFFUSION COEFFICIENT CALCULATIONS .......................75

ACKNOWLEDGEMENTS

Dr. Matthew Hall, Lecturer, Nottingham University I would like to express

my gratitude for his continual support, instructions and guidelines. His

teaching and supervision have been invaluable throughout the year and have

have helped to make the project what it is today.David Bailey Page 4 of 77 Advanced StudyUniversity of Nottingham Dissertation

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David Allinson, Nottingham University For helping in various areas of the

experimentation and in the laboratory. .

List of figures

Figure no. Page no.

1 Hardwood Structure 132 Softwood Structure 14

3 Surface view and section through bordered pits in conducting cells 15

4 An example of the shape of a typical wood sorption isotherm 15


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5 Graphical representation of the stages of sorption. 17

6 Sorption Isotherms of white spruce at 25oC, demonstrating the hysteresis

effect. 18

7 Sorption Possible energy savings 20

8 The FSP of several wood species 21

9Relationship of Longitudinal compression strength to moisture content of

the wood species 21

10 Permeability experimental setup 29

11 Comparative Diffusion Coefficients for Pine 38



14 Comparative Diffusion Coefficients for Oak 39

List of Graphs

Graph no. Page no.

1 Change in mass over a set time for the wet-cup/dry-cup testing. 30

2 Moisture content against environmental RH for Pine wood 32

3 Moisture content against environmental RH for Medium densityFibreboard 32

4 Moisture content against environmental RH for Oak wood 33

5 Moisture content against environmental RH for Chipboard 33

6 Moisture content against environmental RH for Plywood 347 Hardwood vs. Softwood Absorption isotherm 36

8 Hardwood vs. Softwood Desorption isotherm 38

9 Absorption Isotherms-All 5 Samples 41

10 Desorption Isotherms-All 5 Samples 42

11 Time taken to reach absorption equilibrium for Pine and Oak at varying

RH values 45

12 Time taken to reach absorption equilibrium-All 5 samples 45

13 Change in mass over time-Pine mw hysteresis 48

14 Change in mass over time-MDF mw hysteresis 48

Graph no. Page no.

15 Change in mass over time-Oak mw hysteresis 49

16 Change in mass over time-Chipboard mw hysteresis 49

17 Change in mass over time-Plywood mw hysteresis 50

18 Change in mass over time- Chipboard; predicted mw hysteresis 51


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List of Tables

Table No. Page No.

1 Desorption isotherm data 34

2 Absorption isotherm data 34

3 Calculated density of the samples 35

4 Calculated Diffusion Coefficients of the samples at varying RH values 38

5 Model vs. Experimental Equilibrium Moisture Content Comparison 40

6 Storage Function slopes 43

7 Absorption- for all 5 samples at varying RH 46

8 Absorption- for all 5 samples at varying RH 47

9 Calculation of the constant value of hysteresis for all 5 samples 50

10 Oven Drying of Samples Appendix A

11 Absorption at different RH levels for Group A Appendix A

12 Absorption at different RH levels for Group B Appendix A

13 Absorption at different RH levels for Group C Appendix A

14 Desorption at different RH levels Appendix A

15 Permiability testing results Appendix A

16 Equilibrium Values for all three tests Appendix B

17Absorption moisture content as a percentage of dry weight for all 5samples Appendix B

18Desorption moisture content as a percentage of dry weight for all 5samples Appendix B

19 Time to reach equilibrium for all samples at varying RH levels Appendix B

20 Days to reach equilibrium vs. Mass of water at equilibrium Appendix B21 Diffusion coefficient variables and calculation data Appendix C

Nomenclature

A the area (m2)

cp the specific heat of air (J/kgK)

cv the compressibility of water vapour

D the transverse diffusion coefficient (m2/s)

Da the diffusion coefficient of vapour in airDavid Bailey Page 7 of 77 Advanced StudyUniversity of Nottingham Dissertation

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Dbt the transverse bound water diffusion coefficient

Dv the vapour diffusion coefficient in the lumens

Eb the activation energy

Gd the nominal specific gravity of wood substance (at a given bound water

content.)

Gm the specific gravity of wood (at a given moisture content)

h the relative vapour pressure (kPa)

J the diffusion flux (mol/m2s)

K1, K2, W, ms(T),n(T),i(T) temperature dependant variables

L the sample thickness (m)

M the bound water content on the surface of a sample (kg/kg)

m the moisture content (kg/kg)

mr the mass (kg)

mw the mass of water contained within the wood (kg)

Mw the molecular weight of water (kg/kmol)

the concentration in dimensions of (amount of substance/length3)

the relative humidity (%)

R the gas constant (kmol K)

Ra the ideal gas constant for water vapour (Nm/kgK)

S the surface emission coefficient (cm/s)

T the temperature in Celcius (C)

the time (s)

Tk the temperature in Kelvin (K)

u0 the initial moisture content

ue the equilibrium MC

va the porosity of wood

x the position (length)

the heat transfer coefficient of air (W/m2K)

the time it takes for a sample to reach equilibrium moisture content at a given

relative humidity (days)

the density of wood (kg/kg)

0 the oven dry density of wood (kg/kg)

a the density of air (kg/m3

at a given temperature)David Bailey Page 8 of 77 Advanced StudyUniversity of Nottingham Dissertation

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w the density of water (kg/m3)

the relative humidity (RH %)

s the saturation humidity of air

the constant value of hysteresis (g)

1. INTRODUCTION

Moisture absorption in wood is characteristic of that of most hygroscopic materials, in

that it seeks to achieve equilibrium with the surrounding environment and will

continue to do so as long as there is a variation in moisture content (MC) between the

timber and air. This attribute is both useful and destructive and this study seeks to

quantify the timber moisture relationship in terms of absorption and permeability in

order that proper design and application of wood in a structure can be carried out. As


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each species of timber has unique properties, no set model has yet been designed to

determine the way a wood will react to moisture (ASTM 2007). Through this study

we will seek to identify this moisture relationship in 5 different species of wood.

1. 1 Overview of the problems

1.1.1 Humidity control in buildings Comfort, Energy and Structure

Accurate temperature and humidity control within a building envelope is essential to

the comfort and wellbeing of its occupants as well as long term preservation of the

building fabric itself (Highley Terry L. 1999). It is necessary therefore to take into

account hygroscopic properties of wood, as one of the most widespread materialsused in construction.

Studies by C.J. Simonson et al. (2004, cited in Osanyintola Olalekan F. et al.

2006:1271) demonstrated that wood can reduce peak humidity by up to 35%,

depending on the species and ventilation rate, subsequently reducing heating and

cooling energy consumption by up to 5% and 30% respectively.

The amount of moisture in timber can have a dramatic effect on its strength, elasticity

and dimensions, all very important variables in the construction industry. Being able

to predict the response of timber to changing humidity levels can also help to prevent

mould and bugs associated with excessive moisture (generally above 30% moisture

content) in timber (Sunley John et al. 1985). These can cause scarring and sometimes

irreparable and hazardous damage, especially in older buildings where wood is less

likely to be treated with chemicals.

This study will gather data on 5 different types of wood. The data will help to

establish the equilibrium moisture content, the fibre saturation point, and various

other hygric properties of the wood. The psychometrics involved in the study include

understanding the relationship between the absorption and permeability of moisture in

wood, the movement of moisture and the physics behind it. The experimentation will

provide wood sorption isotherms for each of the samples in which the hysteresis

effect should be demonstrated (Bell Leonard N. et al. 2000). The samples will also be

tested for vapour permeability, defined as the ratio between the density of vapor flow


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rate at that point and the magnitude of the vapor pressure gradient in the direction of

the flow (Miniotaite Ruta 1998).

As each different type of wood has unique properties, no one model has yet been

designed to determine the way a wood will react to moisture (Sunley John et al.

1985). The data acquired during this study should help to validate those theoretical

models that do exist or even become the foundation to a new model.

1.2 Aims and objectives

1.2.1 Overall Aim

The main aim of this study is to obtain sorption characteristics for a number of natural

and manufactured timber species with a strong emphasis on the application of these

characteristics in existing models, and a range of time dependant tests. By analysing

the sorption data collected it is hoped that various useful trends can be identified, such

as a tendency of one species to absorb greater amounts of moisture, more rapidly, or

the affinity of one species to remain outside of the mould and insect moisture content

region.

1.2.2 List of Objectives

To review existing research in the field of wood moisture sorption, including

wood structure, hysteresis, diffusion models and structural and dimensional

changes resulting from moisture sorption and desorption.

Obtain samples of natural and manufactured timber for testing.

Prepare and test the specimens based on two well known methods of

absorption and permeability testing.

Analyse data within 2 sub-categories

1. Characterisation: Evaluation of the sorption curves themselves,

Moisture content (MC) vs. Relative Humidity (RH), Fibre saturation

point (FSP), Equilibrium moisture content (EMC) vs. RH. Then a

comparison of the different types of timber in relation to the sorption

curves. A diffusion coefficient analysis.


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2. Time Dependant: A study of the sorption curve in relation to time, MC

vs. Time to reach EMC. An in-depth analysis of the hysteresis effect;

MC vs. RH, and time vs. MC for absorption and desorption

comparison. Compare natural and manufactured samples.

Suggest possible applications of the data and trends observed along with ways

to improve the study.

1.3. Limitations of Study

1.3.1 Practical Limitations

The practical limitations can be broken down into two main categories:

Time: The 4 months or so set aside for testing was sufficient enough to obtain

accurate and useful data in the case of the absorption tests, but with the

permeability testing the recommended period of observation is over a year

(discussed further in section 3.4).

Equipment: The absorption testing focused on a weight measuring method of

MC monitoring which is simple and fairly accurate; ideally x-ray imaging

could be used as it provides a much more detailed picture. The lack of such

equipment leads to the necessary assumption that the moisture is distributed

uniformly throughout the sample and renders the FSP and diffusion coefficient

modelling imprecise.

1.3.2 Theoretical Limitations

This report is concerned primarily with the absorptive properties of wood and whilst

there is some background into the physical changes that occur, things like strength

and dimensional changes are not actually measured. Wood structure also plays an

enormous part in the absorption properties of a species, but as there is little or no

uniformity between the different types of manufactured wood, therefore the structure

of those samples is not discussed. It is also not known how the bonding materials used

in the manufactured types of wood affects the absorption.


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2. BACKGROUND LITERATURE

The literature review will focus on the mechanics and methods of moisture sorption,

wood structure, hysteresis, diffusion models and structural and dimensional changes

resulting from moisture sorption and desorption.

2.1 Wood structure

Natural timber can be divided up into two primary categories, soft wood and

hardwood. Both types are made up of long hollow cells, elongated and parallel to each

other along the trunk of the tree (Miller, 1999).

Figure 1: Hardwood Structure

In hardwood these cells vary in size and wall thickness, the largest used primarily for

the transport of sap throughout the tree and the smaller ones used for structural

rigidity and strength. In hardwood there are far more transverse cells than in

softwood, again adding to the strength and density of the wood.


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Figure 2: Softwood Structure

Softwood has basically the same structural makeup, except the cells are all similar in

size and generally orientated in the same direction. They are less compacted and have

larger pores and openings. The experimentation within this project is looking at the

absorption and desorption of water within the wood and it is within this cell structure

that the moisture transfer at the microscopic cellular level occurs. For both hardwoods

and softwoods the cell walls are made up of layers containing micro-fibrils whichgive the walls strength and rigidity (Dinwoodie J.M. 1989). The cell wall and its

constituentsalso contain water in various forms that are relevant to theexperimentation.

As Haque M. Nawshadul (2002) discusses, water exists within the wood in one of 3

forms, free water, bound water and water vapour.

Free water occurs within the cell Lumina or macroscopic capillaries.

Bound water is trapped within the actual cellulose of the cell wall of the wood.

Water vapour, like bound water, is held within the cell wall Lumina, but in

comparison with free water and bound water its effects are insignificant.

Moisture movement through the wood is attributable to either pressure difference or

moisture gradient (Skaar 1988) It moves through passageways in the structure of the

wood, interconnected by small openings or thin areas in the walls known as pits

(Dinwoodie 2000). The permeability of moisture longitudinally, within the

passageways is many times greater than via the pits, in the transverse direction.David Bailey Page 15 of 77 Advanced StudyUniversity of Nottingham Dissertation

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Figure 3: Surface view and section through bordered pits in conducting cells; right- solid view of two pits cut in half: I, pit

opening; II, torous; III, margo strands formed from the primary wall; IV, pit cavity; V, secondary wall (Desch and Dinwoodie,

1996).

It is worth noting that in softwoods the cell structure off the wood is not as dense,

allowing for larger passageways and greater ease of movement and theoretically faster

and greater absorption. This phenomenon will be further investigated in a comparison

between both the natural hardwood and softwood that are to be tested.

2.2 Moisture Sorption/Desorption

Figure 4: An example of the shape of a typical wood sorption isotherm. (Note the Equilibrium moisture content or EMC only

ever reaches 30% or the fibre saturation point. Also note the hysteresis effect between sorption and desorption.)


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Within the field of timber sorption, principally timber drying, there are three primary

driving forces that are used to predict the rate of moisture sorption and absorption

within the wood: Moisture content, the partial pressure of water vapour, and the

chemical potential (Skaar, 1988; Keey et al., 2000). These along with capillary forces

are the means by which the wood reaches equilibrium. The experimentation deals

with two types of water within the cell wall.

Free Water: Termed free water due to the fact that it is not chemically bound

to the wood, it is simply held via capillary forces (Skaar, 1988),and therefore

easily desorbed or absorbed.

Bound Water: On a molecular level the cell wall is negatively charged and

therefore attracts the water (which is a polar substance), due to van der Waals

forces (Haque M. Nawshadul 2002). The resulting hydrogen bonds are what

keep the water within the wood. The tests being carried out will deal largely

with bound water.

There are detailed sorption theories discussed by Simpson (1973, 1980) and Simpson

and Rosen (1981) (all three cited in Bastas Marcia Vidal et al. 2005:146) and Skaar

(1988). The theories are designed to provide an explanation into the sorption of water

in hygroscopic materials, including wood.

In one theory Skaar (1988) and Dinwoodie J.M. (2000) reviewed water sorption from

a molecular point of view, thereby dividing the sorption curve into 3 separate parts as

shown in figure 5. Water is hydrogen bonded to the hydroxyl groups of the cellulosic

and hemicellulosic portions of wood (Bowyer Jim L. et al. 2003). In zone 1 of figure

5, water is subject to Van der Waals forces. In moisture absorption, water molecules

gradually cover the external surface of the cell wall and are held in a rigid state via the

chemical bonds. This layer is termed the monolayer. Once the monolayer is saturated

Zone 2 begins. The sorption graph becomes linear as more molecular layers are added

to the cellular structure of the wood. Zone 3 demonstrates the appearance of liquid

water within the capillaries of the wood due to capillary condensation. This is

signified by the steep increase in MC on the sorption graph.


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Figure 5: Graphical representation of the stages of sorption. (Skaar 1988)

2.2.1 Hysteresis

There are a few hypothesis used to explain hysteresis in wood: Excess energy

produced by water sorption (Richardson Barry A. 1976), plasticity in relation to the

irreversible inelastic exchanges of hydroxyl groups (Barkas Wilfred W. 1949), and

the behaviour of hydroxyl groups within the cellulose and lignin(Dinwoodie J. M

2000). The most popular theory relates to the differences in contact angle and

pressures of the advancing and receding water through the capillaries of the wood and

into the porous structure (Mujumdar A. S. 2006).

Sorption and desorption of water within the cellular structure does not happen

instantaneously with changes in the surrounding RH. When absorption occurs due to a

high surrounding RH, capillaries in the wood begin to fill with water. The capillaries

in turn are attached to the pores in the wood, but in order for the pores to fill up the

partial pressure of the vapour in the air, needs to be greater than that of the vapour

pressure of the liquid in the pore. Only then will the moisture move into the porous

structure of the wood. For desorption the wood is already saturated, the pores are full

and the process is reversed, but as the pores are already full the MC will be

consistently higher along the drying curve (see figure 6). This theory does assume a

rigid pore structure and therefore is valid in a timber application. The explanation is

that the contraction and swelling is superimposed on the drying and wetting process,

producing states of tension in the capillaries and pores, leading to varying EMC


100

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depending on whether desorption or absorption is in progress (Mujumdar A. S. 2006,

pg. 16). A study by Stamm (1964) showed the tendency of white spruce to conform

to a middle curve when exposed to alternating RH conditions.

Figure 6: Sorption Isotherms of white spruce at 25oC, demonstrating the hysteresis effect. (Stamm 1964)

Higgins (1957, cited in Bowyer Jim L. et al. 2003:177) found that the hysteresis effect

varied considerably from one species to another as demonstrated by the fibre

saturation points in figure 8.

2.3 Methods of measuring moisture content

Bowyer Jim L. et al. (2003) discuss a number of methods that are used to measure the

moisture content of wood. Continuous examination of the moisture content can be

done via a neutron gauge in tandem with a gamma radiation gauge to measure the

total mass of the sample; the MC can be also be measured via the amount of

microwave power that the sample is absorbing. A simpler method is to use a

resistance meter which measures the electrical resistance between two pins inserted in

the wood. These are only generally reliable from 6% to around 30% or FSP moisture

content. A similar method uses the capacitance of the wood as it varies with density

and MC. Prado Pablo J. (2001) did a study into the use of magnetic resonance fields

(MRF), similar to those use in the oil industry and various moisture detection

applications. For MC under 30% the study showed that MRF results were consistentDavid Bailey Page 19 of 77 Advanced StudyUniversity of Nottingham Dissertation

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with existing MC models. A method widely used in the food and archaeological

industries is the x-ray diffraction method where a sample is bombarded with x-rays.

The diffraction data is capable of giving important structural information as well as an

accurate MC reading (Chinachoti Pavinee et al. 2006). Due to the somewhat limited

means of this study the simple interpolation method based on known EMC points was

used. The EMC points are determined using the process described in section 3.2.

2.4 Moisture buffering capacity of materials

The ability of wood to moderate variations in RH is defined as its moisture buffering

capacity (MBC) and depends on a number of factors, the outdoor climate, theventilation rate and even things like furniture. Research has shown that peak indoor

humidity levels can be reduced by up to 35% and minimum levels raised by up to

15% (ASTM 2007) given proper application of hygroscopic materials. The most

widely recognised method of calculating the MBC of hygroscopic materials is based

on a NORDTEST project. The project was initiated after previous MBC testing

proved inconclusive. The subsequent results of the NORDTEST define the moisture

buffering ability of materials in the indoor environment and present a test to measure

the MBC of the materials in question (ASTM 2007). This project is unable to perform

the calculations that establish moisture effusivity through the NORDTEST method, as

they require data from the permeability tests (refer to section 3.4).

Physical testing of MBC includes an interesting method known as the suction

technique; adapted from the analyzation of porous building materials in a high RH

environment, to measure the storage capacity of wood (Thygesen Lisbeth G. et al.

2007). The method is limited though as it assumes moisture is initially removed from

water saturated samples and therefore only produces a desorption isotherm. A simpler

and nearly as accurate method is to measure the amount of water a sample can absorb

at a given RH. Once the samples weight has stabilised within the controlled RH

environment it has reached the Equilibrium Moisture Content (EMC) of wood,

defined as: The moisture content of the timber, such that there is no inward or

outward diffusion of water vapour. This will correspond to a specific temperature and

RH of the surrounding environment (Dinwoodie J. M. 2000). The EMC is the state


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that we wish to obtain via the absorption experimentation as it will allow the RH-MC

relationship to be quantified and provides points for an accurate sorption profile for

all RH conditions.

Figure 7: Sorption Possible energy savings when the indoor temperature in the hygroscopic case is (a) decreased while

maintaining the same indoor RH and in the non-hygroscopic case, and (b) increased while maintaining the same comfort and air

quality conditions as in the non-hygroscopic case. (Chinachoti Pavinee et al. 2006)

2.5 Strength/Dimensions of structure

The rate of moisture absorption is naturally related to the species of wood and

correspondingly the rate of expansion or shrinkage, based on the absorption of the

moisture is also related. Structurally wood needs to be quite stable, in that to much

expansion or shrinkage in a supporting beam could cause irreparable and possibly

harmful damage. A 3% volume change for a 30% RH difference is deemed acceptable

in most circumstances (Sunley John et al. 1985), but depending on the application

changes of up to double that are acceptable. It is important to recognise the

significance of the fibre saturation point in terms of strength and dimensional change

due to MC variation. The Fibre Saturation Point (FSP) of wood is defined as: the

moisture content at which the cell walls are saturated but no free water remains in the

cell cavities. Moisture content of the individual cell walls at the fibre saturation point

is usually about 30%, but may be lower for some species (Simpson William T.

1991). The FSP is taken as an average over a piece of timber and thus detailed results


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might be false as not all parts of the timber are at the same MC. The FSP of wood is

important to moisture sorption as it defines the point at which the wood changes

strength and dimensions. It also helps in the comparison of hysteresis in varying

species.

Figure 8: The FSP of several wood species. Higgins (1957)

Once the amount of moisture in the cell wall falls below the FSP the moisture

contained within the cellulosic strands or microscopic capillaries, discussed in section

2.1, begins to leave. This removal of water from the hemicellulose/lignin matrix then

causes the shrinkage, primarily along the length of the cell (Sunley John et al. 1985).

Figure 9: Relationship of Longitudinal compression strength to moisture content of the wood species (Dinwoodie J.M. 1989).

The figure clearly shows the strength relationship apparent when the MC of the wood

falls below the 30% FSP. The removal of water from the hemicellulose/lignin matrix

causes microfibrils to compact, and move closer together thus causing shrinkage in

the wood, but an increase in strength as the wood dries.David Bailey Page 22 of 77 Advanced StudyUniversity of Nottingham Dissertation

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Another structural matter related to the moisture content of wood is the growth of

mould and fungi. The initial infection of a piece of timber requires a MC of at least

20% (Desch, H.E. et al. (1996) or the approximately the FSP of most species of

timber; should an infected piece of timber fall below that MC threshold the fungus

will cease its growth. Fungal attack destroys the cellular structure of the wood and

eventually wood can lose up to 80% of its air dry mass(Dinwoodie J.M. 1989).

2.6 Phenomenological Macroscopic Models

Whilst the data gathered from a wood sorption isotherm can be used directly in a

physical process, oftentimes it is used to validate mathematical models developed to

predict the properties of wood. It is well known that no it is not possible to exactly

predict the nature of a piece of timber, but many people (Skaar, Dinwoodie, Siau)

have devised equations based on existing wood sorption data. These models can be

accurate and are sometimes the only way to obtain the data required. The sorption

data gathered from this experiment will be used in one of these models to determinethe Moisture Diffusion Coefficient (MDC). Moisture is the predominant mechanism

of fluid moisture movement through wood, below the FSP(Dinwoodie J. M. 2000).

There are various different coefficients of diffusion, as moisture does not move at the

same speed through all elements of the wood Siau (1984, cited in Dinwoodie J. M.

2000:46). The MDC model that has been chosen, gives a transverse diffusion

coefficient (D) and a surface emission coefficient (S) by utilising the data from the

experiments in conjunction with the model calculations.

The model chosen incorporates within it a simpler model to define the MC based on

the variable of temperature. This in itself though is a choice as there are many such

equations that have been developed to predict the MC of a sample based on a couple

given variables. A study by BastasMarcia Vidal et al. (2005) compares 7 such

models and has determined that the most accurate results, based on the parameters of

this experiment, are given by the Malmquist model, developed in 1958. It calculates


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the MC based on temperature and relative vapour pressure and the formulation is as

follows:

The results from the model are compared with the theoretical MC results in the MDC

model in section 4.1.2.The model chosen was developed by Baronas R. et al. (2001) and is based around the

fundamentals of Ficks second law:

As derived from Ficks first law:

The MDC model calculations are as follows:

MC derivation (note the similarities to the Malmquist model)

where

K1 = 4.737 + 0.04773Tc 0.00050012 2

K2 = 0.7059 + 0.001695 0.0000056382

W = 223.4 + 0.6942 + 0 .01853

2

Saturated Vapour Pressure

ps = 3390 exp(-1.74 + 0.0759 T 0.000424 T2 + 2.44 10-6 T3)

Activation Energy

Eb = (40.195 - 71.179m + 291m2- 669.92m3) 106


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Diffusion coefficient of bound water in cell walls according to the Arrhenius Equation

Dbt = 7 10-6 exp(-Eb/RTk)

Nominal Specific GravityGd = 1.54/(1.0 + 1.54 m)

Interdiffusion Coefficient of Vapour in Air

Da = 9.2 10-9 Tk

2.5/(Tk + 245.18)

Vapour Diffusion Coefficient in the Lumens

The Transverse Diffusion Coefficient

A secondary part to the model is the evaluation of a surface emission coefficient and

is calculated as follows:

Specific Gravity of Wood

Porosity of Wood

Saturation Humidity


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The Surface Emission Coefficient

The model used to determine the MDC, uses a 2 dimensional formulation process,

best suited to applications where the sample is long and thin and therefore will lose

the majority of its water only in a 2 dimensional field (see Section 3.3). Ideally a 3

dimensional formula should be used given the square shape the samples but it was not

practical due to the amount of extra information and assumptions required for its

completion. The moisture diffusion coefficient is a step toward a complete moisture

sorption profile for a species of wood and is used as a vital part of other moisture

sorption models. Results and analysis of the MDC model are discussed in section

4.1.2.


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3. METHODOLOGY

3.1 Background

The first step in determining the hygric properties of timber is to obtain a sample of

the specific species required and perform tests on the sample itself. The tests have

been chosen to give a good indication of the sorption and desorption properties for the

selected timber.

3.1.1 Absorption

The primary reasoning behind the absorption test was simplicity and a necessity. As

mentioned in section 1.3.1 the lab does not contain many of the advanced technology

required for detailed absorption analysis. Thus an experimental procedure based

around a simple but accurate scale, and some controlled humidity jars was devised.

Salt solutions discussed later provide the humidity control needed to acquire the

necessary data. As sorption and desorption takes place, the weight of the wood is

monitored. This weight can be divided by the dry weight of the wood to get the

amount of moisture being absorbed. Once the wood has reached EMC it can be

moved up or down to another RH jar. The information is then compared on a sorption

isotherm graph.

3.1.2 Permeability

A detailed description of the cup method is available in works by Siau (1984 and

1985, cited by Olek Wieslaw 2003:16). The mechanics of the cup method are similar

to the absorption testing in that they rely on the humidity control of salt solutions and

are discussed in depth in section 3.3. The steady state of the bound water during the

test in combination with the one dimensional travel through the wood allows for a

linear regression of time vs. mass change and the flux of the bound water can be

calculated:


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The Flux of the Bound Water

3.2 Materials and Preparation

Initially 5 different species of timber were procured, 2 natural, Pine-Sample 1 and

Oak-Sample 3, and 3 manufactured, MDF-Sample 2, Plywood-Sample 5 and

Chipboard-Sample 4. The timber was then cut into blocks of 50x50x50mm (3 blocks

for each species) and 4 species also were cut into disks 105mm in diameter and 10mmthick (3 disks for each species). While the dimensions were not strictly adhered to,

this does not affect the results of the test as the final results are largely ratio based.

The identical tangential dimensions of the square blocks used in the experimentation

should help to maintain a universally similar rate of moisture transfer (see section

2.1). The short length of the samples should help to reduce the increased permeability

rate caused by the longitudinal effect of the cells.

All the samples were oven dries to remove as much excess water as possible and

gauge a base dry weight from which all the other tests will stem.

Sealable glass jars containing a salt solution and a small metal shelf provided the

moisture controlled environments for the absorption tests. The chemicals within the

salts control the humidity levels in the air and by mixing the salts with distilled water

it ensures that no extraneous minerals counteract the effect of the salt.

The controlled environment for the permeability cups is a sealed box, containing a

small fan and a tray for the high RH salt solution. Once the wood disks have been

sufficiently oven dried, they are sealed round the edges with aluminium tape to ensure

only one dimensional moisture transfer. A low RH salt solution is added to the cups

and the wood disks are sealed to the top with epoxy resin. They are then placed within

the climate controlled box.


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3.3 Procedure

The absorption test involves exposing the samples of wood to a controlled relative

humidity (RH) for a period of time and then measuring the weight change once theyhave achieved equilibrium. (BS Standard of 0.1% initial change over a 24hr period)

By subtracting the dry weight from the weight as set by the water absorbed in the

controlled RH we obtain the absorption ratio. This test is performed over 5 varying

RHs and that allows for 5 separate ratios and subsequently enough information to

draw up a relatively accurate absorption isotherm for the species of wood. A

controlled RH is achieved by placing 1 cube of each type of wood is placed on a small

rack in a sealed jar or multiple jars containing a mixture of salt solutions. The jars are

then kept in a chamber at approx. 100%humidity to provide an approximate constant

temperature of 23oC. The salts in the water have a measured dampening effect on the

evaporation of moisture into the air. This effectively controls the relative humidity in

the jar to an exacting degree. The salts being used are:

MgCl2 32.9%RH@23oC

K2CO3 43.16%RH@23oC

NaBr 58.2%RH@23oC

NaCl 75.36%RH@23oC

KNO3 94.00%RH@23oC

The process is repeated with each of the samples going through each of the stages of

humidification and then the results can be averaged to form a more accurate

absorption isotherm for the species. The drying curve is the same process as wetting,

just the samples are moved down the RH scale. Time constraints may limit this to just

one set of wood going through the process.

The principle behind the permeability tests is very simple, the liquid in the cup seeks

to escape through the wood due to the imbalance of humidity inside and outside the

cup.


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Figure 10: Permeability experimental setup: 1-wood disk sample, 2-sealant, 3-salt solution, 4-supporting rack, 5-climate

controlled environment (temp. and RH), Olek Wieslaw (2003)

The cup is periodically weighed to establish how much water has escaped through the

sample. The results provide a graph based on the equation in section 3.1.2.

3.4 Primary Testing and Modifications

During the first week the absorption samples were tested on an almost daily basis.

After the first week it was deemed necessary to increase the weighing intervals as

constant testing was causing disruption to the controlled RH environment within the

jars. A subsequent complication arises due to the need to interpolate some of the data

(see section 4.2.1).

The initial stages of the permeability test involved waiting for the system to

equilibrate. The weight of the cups was in constant flux as shown in the graph below:


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Graph 1: Change in mass over a set time for the wet-cup/dry-cup testing.

When the cups failed to reach a state of equilibrium within a few weeks, the

experiment was deemed unfeasible due to time limitations. Some possible reasons for

the constant flux are:

Saturation of the air within the sealed box caused by a lack of water in the tray

of salt solution within the box, leading to equilibrium between the RH in the

wet cup and the air. This theory of saturation within the box is supported by

the appearance of mould on two of the samples which requires a high level of

moisture.

The cups are not sealed properly, releasing so much water moisture into the air

that the salt solution within the tray is unable to counteract the effects, again

resulting in equilibrium between the RH of the wet cup and the interior of the

sealed box.

Because of the nature of the test equipment is necessary to remove the lid of

the sealed box to get at the samples to weigh them. It is possible that the

frequent testing prevented the box from attaining a steady RH, governed by

the salt solution in the tray.

There is however a visible downward trend after 30 days, and it is possible that the

experiment was not a failure and the testing phase just needs to be extended; although

similar experiments by Olek Wieslaw (2003) and Bandyopadhyay A. et al. (2002)

provided trend data after only 2 days.


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The permeability experiment will be continued, but as far as this paper is concerned

the experiment yielded little to no relevant data, and it is impossible to apply the Flux

of bound water equation.


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4. RESULTS & DISCUSSION

4.1 Characterisation

4.1.1 Isotherms

Graph 2: Moisture content against environmental RH for Pine wood

Graph 3: Moisture content against environmental RH for Medium density Fibreboard


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Graph 4: Moisture content against environmental RH for Oak wood

Graph 5: Moisture content against environmental RH for Chipboard

Graph 6: Moisture content against environmental RH for Plywood


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There are 3 specific areas that need to be evaluated:

1. Hysteresis

All of the samples seem to follow the general trend of sorption curves given

by previous research (see figure 6) with a constant curve for absorption and

slight variations in the slope at various points for the in the desorption curve.

The separation between the two curves implies the existence of hysteresis

which in turn helps to understand the response of wood, to constant wetting

and drying in a natural setting (see section 5 further experimentation). As

absorption hysteresis is still an active area of research this relationship in the

samples tested could be used to define the parameters for the theory of

hysteresis within wood.

2. Sorption Isotherms Natural vs. Manufactured Timber

DESORPTION Moisture ContentRelativeHumidity Pine MDF Oak

Chipboard

Plywood

94%

17.83

%

15.52

%

14.95

% 15.63%

15.48

%

75.26%15.56

%13.49

%13.38

% 14.10%14.39

%

58.20%11.90

%10.97

%10.74

% 11.49%11.24

%43.16% 9.72% 9.04% 8.87% 9.19% 9.27%

33%9.03

%7.92

%8.20

% 8.22% 8.15%

Table 1: Desorption isotherm data

ABSORPTION Moisture ContentRelativeHumidity Pine MDF Oak

Chipboard

Plywood

33% 4.56% 4.02% 4.40% 4.30% 5.72%

43.16% 6.42% 5.58% 5.49% 5.73% 6.22%58.20% 8.70% 7.45% 7.51% 7.67% 7.78%

75.26%11.99

%10.22

%10.46

% 10.62%10.60

%

94%17.83

%15.52

%14.95

% 15.63%15.48

%


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Table 2: Absorption isotherm data

Given the sorption data, the manufactured timber isotherms appear very

similar to those of the natural timber. Initial assumptions gave the

manufactured timber a lower EMC due to additives in their composition.

Given the data in tables 1&2 any glues or resins used in their production seem

to have had varying effects on their sorption characteristics. There also

appears to be an unusual similarity between the final moisture contents of the

three manufactured samples with percentages all within 0.15% of each other at

the end of the sorption curve. This may be a coincidence, or it may be that the

wood used to make the manufactured timber is of similar origin. It is also

possible that the glue, used to bond the various timber elements together, has a

moisture absorption factor of its own that is affecting results. There are

difficulties in specifying an exact isotherm for the various types of

manufactured timber as there is no strict regulations on their components.

Different companies may have different methods of constructing

manufactured timber and therefore it is necessary to do a separate test or

model for each sample.

Plywood can be made up of either softwood or hardwood, but in this case the

final MC value resembles that of the Oak, more so than the Pine, in both

sorption curves. The results for MDF should be similar to the pine sorption

isotherm, as MDF is predominantly of softwood origins. However, the resin

used to bind the MDF together appears to have impeded the flow of moisture

and sorption characteristic is more so of a hardwood. Another theory is the

increased density of the wood slows the moisture flow through it. Using the

data from the Diffusion Coefficient Model we can clearly see that the MDF isboth denser and the speed at which the moisture is transported through the

wood is slower (section 4.1.2).

Type

Oven dryDensity(kg/m3)

Pine 472

MDF 576

Oak 660

Chipboa 616David Bailey Page 36 of 77 Advanced StudyUniversity of Nottingham Dissertation

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rd

Plywood 888

Table 3: Calculated density of the samples

Chipboard is made up of tiny pieces of softwood bonded together to form a

sheet. The sample tested here is a low grade, fairly porous example, which

should have allowed a large amount of moisture in and the predicted MC

should be considerably higher than the other manufactured types. The reason

that it obviously isnt, is simple; Chipboard is used in a variety of applications

which cause it to come into contact with fire and water, therefore it is treated

in its raw form with fire resistant and moisture resistant chemicals. The sample

tested was not chipboard in its raw form, it was in-fact, treated chipboard. The

results of the experiment show that the treatment worked and the wood

moisture sorption is on a similar level as that other manufactured timber and

the natural oak.

3. Sorption Isotherms Hardwood vs. Softwood

Graph 7: Hardwood vs. Softwood Absorption isotherm


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Graph 8: Hardwood vs. Softwood Desorption isotherm

Theoretically softwood should have both a greater MC, and rate of sorption than

hardwood as the structural makeup of softwood is much more porous. The confirms

this and gives both a higher rate of absorption and a higher overall MC for pine in

both the absorption and desorption curves. The softwood has an FSP of nearly 18%

while the hardwood is closer to 15%. However, a study by Matiasovsky Peter and

Takacsova Zuzana (1996)which includes the sorption characteristics of Spruce

(softwood) and Walnut (hardwood) shows a different trend, with the FSP of the

softwood at approx 10% and the hardwood at approx 15%MC. This reversal of

hardwood and softwood MC boundaries may be down to the different species that

were used in the experiments by Matiasovsky and this paper. But if both sets of data

are valid then the conclusion is that neither hardwood of softwood has exclusively

higher MC and that it is down to the specific species within both categories.

4.1.2 Diffusion Coefficient & Model Comparisons

The Diffusion Coefficients in the table below are based on the equations from the

model in section 2.6, and the data collected from the samples.


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Diffusion Coefficients of Wood Samples Environment RH

32.90% 43.16% 58.20% 75.36% 94%

Wood Type D m2/s D m2/s D m2/s D m2/s D m2/s

Pine 3.16E-11 3.9E-11 5.27E-11 7.87E-11 1.89E-10

MDF 1.98E-11 2.45E-11 3.32E-11 4.98E-11 1.21E-10

Oak 1.42E-11 1.75E-11 2.38E-11 3.59E-11 8.81E-11

Chipboard 1.68E-11 2.08E-11 2.82E-11 4.25E-11 1.04E-10

Plywood 6.35E-12 7.88E-12 1.08E-11 1.63E-11 4.08E-11

Table 4: Calculated Diffusion Coefficients of the samples at varying RH values

The following are diffusion coefficients from a paper by Sehlstedt-PerssonMargot (2003):

Figure 11: Comparative Diffusion Coefficients for Pine

Unext. and Extr. relate to the location that the sample was taken from within the tree,

ie. the heartwood or the sapwood. These variations are not taken into account in this

paper.

Two other models examined by Olek Wieslaw and Weres Jerzy (2006), review the

inverse method and the analytical sorption method in relation to Scots pine:



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Fotsing Joseph Albert Mukam and Tchagang Claude Wanko (2004) used the wet cup

dry cup test in conjunction with Ficks laws to determine the diffusion coefficient for

red oak:

Temprature 30C 35C 40C

DRadial (ms-1) 1,35 x 10-11 3,70 x 10-11 5.37 x 10-11

DLongitudinal (ms-1) 3,23 x 10-11 5,38 x 10-11 6,73 x 10-11

DTangential (ms-1) 1,16 x 10-11 2,65 x 10-11 3,05 x 10-11

Figure 14: Comparative Diffusion Coefficients for Oak

Again the basis of the test was a variation in temperature with a constant RH, but the

data obtained is still relevant to.

When compared with results from similar models and other research, the diffusion

coefficient of Pine and Oak, as calculated via the demonstrated model, are validated.

There are some small discrepancies, but those can be attributed to experimental error.

The conclusion therefore is that the diffusion coefficients for the manufactured

timber, as calculated by the same model, are also valid. As research into the sorption

characteristics of manufactured timber is limited the model data could prove useful in

a suitable application.

Model vs. Experimental Equilibrium Moisture ContentComparison

Experimentally Deduced Moisture

ContentDavid Bailey Page 40 of 77 Advanced StudyUniversity of Nottingham Dissertation

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RelativeHumidity

ModelPredictedmoisturecontent Pine MDF Oak

Chipboard

Plywood

33% 6.50% 4.56% 4.02% 4.40% 4.30% 5.72%

43.16% 8.03% 6.42% 5.58% 5.49% 5.73% 6.22%58.20% 10.59% 8.70% 7.45% 7.51% 7.67% 7.78%

75.26% 14.63%11.99

%10.22

% 10.46% 10.62%10.60

%

94% 22.39%17.83

%15.52

% 14.95% 15.63%15.48

%

Table 5: Model vs. Experimental Equilibrium Moisture Content Comparison

As displayed in the table above the predicted moisture content for the various types of

timber are not species specific, but rather based on environmental temperature. The

application of those predicted moisture contents is when the species characteristics are

entered into the equation. The predicted data is considerably higher than the actual

MC values and as such the predicted diffusion coefficient may be slightly greater than

in reality.

4.1.3 Parametric Review

The sorption isotherm of a timber species provides MC data that can be used to

specify the correct environment in which it should be installed. Two areas of

importance are comfort and the conditions at which the timber is most likely to

undergo fungal attack.

The RH comfort zone for people is between 25% and 75% with an ideal RH of

approximately 60%. In terms of comfort, a building wants timber that has the greatest

variation in MC as this signifies its ability to absorb and desorb moisture at a greater

rate, thereby keeping a room within the RH comfort zone when there is a change in

room RH.

The sorption isotherms display this as slope, with the steepest slope indicating the

highest rate of sorption. For all the samples, between 80%-100% RH there is a steep

sorption trend. For desorption the curve actually flattens out somewhat over the

comfort zone and then theoretically drops suddenly again after 25%.


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Graph 9: Absorption Isotherms-All 5 Samples

Amongst the samples tested there is no clear steeper gradient for the absorption

curves. Plywood is probably the least desirable as the arc is somewhat shallower than

the rest at between 30%-55%. Pine has a higher MC level, but the gradient is still

almost exactly the same as the other samples.

Graph 10: Desorption Isotherms-All 5 Samples


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The desorption isotherm gives a steeper slope for pine, chipboard and plywood, with

pine at the highest gradient, consequently labelling it as the most appropriate timber

for relatively rapid humidity control.

The similarities between the slopes displayed may be down to the relatively

straightforward testing process. Given more exact equipment it may be possible to see

a greater differentiation between the samples.

Mould, as discussed in section 2.5, requires a minimum of 20% MC to propagate and

given that the isotherms show an equilibrium point of well below 20% for all the

samples, the existence of mould is theoretically impossible. There was however small

outbreaks on various samples. This is due to the different levels of MC within the

wood. The surface of the wood may be at 25% even 30% and the middle could be 5%

MC. This method of testing assumes a uniform distribution of moisture throughout

the wood sample and therefore we can see mould on a sample of MC under 20%.

4.1.4 Storage Function

It is possible to use a pressure plate in conjunction with sorption isotherms to form a

continuous moisture storage function covering the entire range from dry state to

capillary saturation but as we only have access to data from the sorption isotherm

testing the storage function will be calculated as the slope of the isotherm over stage

2, as defined in figure 5. At this stage the monolayer is completely saturated and more

molecular layers are being added to the cell walls, but the water vapour pressure is not

enough to overcome the pore pressure within the capillaries, resulting in a temporary

decline in moisture content over the midrange RH values.

Referring back to the sorption isotherm graphs in section 4.1.1 we see that the Oak,

Chipboard isotherms show a continuous curve with little or no change in slope. ThePine and MDF samples have a slight incline in their gradient after 75% RH indicating

a shallower slope before that point. Plywood exhibits the greatest effect, between 33-

50% the slope is dramatically reduced resulting in a lower storage function.


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Storage Functionbetween 33-75%RH

Sample Slope

Pine R = 0.997MDF R = 0.997Oak R = 0.990Chipboard R = 0.995

Plywood R = 0.958

Table 6: Storage function slopes

This storage function data is useful in validating current theoretical models

4.2 Time Dependant

4.2.1 Time Taken To Reach Equilibrium

The time taken to reach equilibrium () is based on the definition of equilibrium (see

section 3.3). The process involves taking the dry weight of the sample and calculating

its equilibrium variation, ie. 0.01% of its mass. Each set of data is then studied to

determine the point at which the mass change fell below the equilibrium variation

point. It is important to note that the equilibrium point is not necessarily the total

testing time for the sample. Due to the incapacity to test the samples every day some

linear interpolation of the data is sometimes required to estimate an approximate

equilibrium time for both absorption and desorption tests. However, the absorption

data is based on three separate tests and the calculated times are an average,

increasing the precision of the results. Therefore most conclusions will be drawn from

the absorption data unless there needs to be a direct comparison between the results

(Section 4.2.3).

Variation in according to natural or manufactured origins

Graph 12gives no general similarity between the manufactured samples and the

natural samples. The varied make-up of the manufactured samples means that it is

impossible to class them under one heading. The MDF and Chipboard are both of

softwood origins, whereas the Plywood is made from hardwood. The comparison then


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becomes MDF and Chipboard against Pine, and Plywood against Oak; manufactured

softwood and hardwoods against natural hardwood and softwoods.

The relationships now emerge as Pine, MDF and Chipboard all reach equilibrium

faster than Oak and Plywood. The initial stages of sorption place the two varieties of

wood closer together but at higher RH the differences become apparent.

For both comparisons, the natural wood has a quicker time. Oak is faster than

Plywood and Pine faster than MDF and Chipboard. This is most likely down to

bonding materials in the manufactured samples and the treatments applied to them.

The time for oak drops considerably and one point during the absorption phase, but

there is no corresponding drop in the desorption period. The conclusion is that the

anomaly is due to an experimental error at some point.

Variation in according to class

Graph 11: Time taken to reach absorption equilibrium for Pine and Oak at varying RH values

Apart from the slight variance at RH ~40% there is a very clear separation between

the two classes. The basis behind the faster for pine are based primarily in the

structure of the wood as considered in section 2.1; larger capillaries, a more porous

cellular makeup and various other compositional matters aid in the speed of

absorption in softwood, and therefore its . The structure of the wood is tied directly

into the density which is examined further by graph 12.


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There is one very noticeable similarity shared by all the samples, and displayed

clearly in graph 11, and that is the apparent increase in as RH approaches 100%.

This inclination is discussed in more detail in section 4.2.2.

Variation in according to the density :

Graph 12: Time taken to reach absorption equilibrium-All 5 samples

Denser wood takes longer to reach equilibrium at each different RH stage as shown

by the continually higher for plywood. The data shows that for a similar sized

sample the plywood was significantly heavier, and therefore denser than the other

samples, the exception being the oak. The oak sample has smaller dimensions and

therefore will reach equilibrium quicker as there is less distance for moisture to travel

for a given moisture diffusion coefficient. A larger sample of the same species will

have the same diffusion coefficient but greater volume to fill with water, thus

validating the oak and plywood results. The graph plainly shows that the denser

plywood sample takes marginally longer to equilibrate but follows the similar pattern

as the rest of the samples in that, as the RH approaches 100%, increases

dramatically. This means that in a real life application where timber is being used as

an active moisture buffer it should ideally be as porous as possible. Dense woods

(generally hardwoods) seem more suited to seasonal variations as their response time

is too slow for day to day effectiveness.


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4.2.2 Dependence of on RH ()

Graph 12 displays obvious variations in the gradient, or slope, of for different

values of RH. Within that spectrum we also have a variation in slope depending on

the species of timber.Absorption-Time to reach equilibrium

(days)

RH Pine MDF Oak Chipboard

Plywood

33% 8 6 11.5 6 9.543.16% 7 7 5.5 6 8.5

58.20% 9 7 10 8.5 10.5

75.26% 8.5 8 12 9 15

94% 18 21 26 20 37Table 7: Absorption- for all 5 samples at varying RH

Desorption-Time to reach equilibrium

(days)

RH Pine MDF Oak Chipboard

Plywood

94%

75.26% 19.5 14 12 14 13

58.20% 10 7 9 11 10

43.16% 8 4 6.5 8 4

33% 7 5 4.5 4 5.5Table 8: Absorption- for all 5 samples at varying RH (Unlike the dry weight it is almost impossible to know the fully saturated

weight so we cannot tell how long it would take to get from fully saturated to 94% RH equilibrium.)

Absorption results show a fairly constant rate of over all the samples, up to around

70% RH. Oak and Plywood, as the two hardwood representatives have a slightly

higher average for the absorption curve, but have a lower over the desorption

curve. This may be due to the density of the hardwood. The moisture absorbed during

the absorption phase penetrated less into the core of the sample. When desorption then

occurs the water within the timber is closer to the surface and equilibrium is achievedfaster. Another possibility is based on the definition of equilibrium. The transfer rate

of moisture from the hardwood samples may be lower than the stipulated equilibrium

requirement of 0.01% mass change over 24 hours. For the absorption phase this is not

the case as the porous structure of the wood absorbs moisture more quickly then it

desorbs (see section 2.2.1). The resulting outcome is a longer during the absorption

phase and a deceptively short for the desorption period. The wood has not in fact


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reached an equilibrium point during desorption, the rate of mass change is simply

incredibly slowed due to the hysteresis effect.

At RH levels above 70% the increases dramatically throughout the range of

samples. Section 4.2.1 examines the tendency of hardwoods to have a longer during

absorption, due to their structural makeup. The above 70% on the desorption curve

is also steeper, but generally the rate is less than that of the absorption equilibrium.

Pine seems to have the longest average , which is unusual as the untreated natural

softwood should have the greatest transport rate and therefore reach equilibrium the

fastest. It is likely that the results are subject to previously discussed equilibrium rate.

The fact that pine loses over 0.01% of its dry weight during a 24 hr period actually

means that it is considered not to have reached equilibrium for a longer time, whereas

the other samples will still be losing moisture long after Pine has reached equilibrium;

they are just doing so at a much slower rate.

4.2.3 Hysteresis of mw at duration of

Graph 13: Change in mass over time-Pine mw hysteresis


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Graph 14: Change in mass over time-MDF mw hysteresis

Graph 15: Change in mass over time-Oak mw hysteresis

Graph 16: Change in mass over time-Chipboard mw hysteresis


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Graph 17: Change in mass over time-Plywood mw hysteresis

The correlation between the mass variation and time is easily identifiable. The area

that this section focuses on is the variation in water mass over time at varying RH

values. Graphs 13-17 show that over time the hysteresis between absorption and

desorption mass remains fairly constant. The only reason that the two points connect

is due to the reasoning in table 6. Were this not the case, it is a valid hypothesis that

the two lines would be parallel at a constant value of hysteresis () throughout the

absorption/desorption processes.

Should this hypothesis be correct, it is possible to estimate the mass of water

contained within a drying sample of wood, given only the change in mass over time

absorption curve and the for that species of timber.

A very crude calculation of yields the following results, based on data from the

experiments:

Calculation of

Weight difference between

sorption curves (g)

RHPine

MDF

Oak

Chipboard

Plywood

94%0.0

00.0

00.0

0 0.00 0.00

75.26%2.0

52.3

01.8

9 2.61 4.20

58.20%1.8

52.4

82.0

9 2.87 3.83

43.16%1.9

12.4

32.1

9 2.59 3.3733% 2.5 2.7 2.4 2.94 2.68


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6 4 7-taken asthe averagevalue ofhysteresis

2.09

2.49

2.16 2.75 3.52

Table 9: Calculation of the constant value of hysteresis for all 5 samples

The more constant the hysteresis between absorption and desorption, in the

experimental results, the more accurately can be calculated. Graph 18 is the

application of the theory in the case of sample 4, Chipboard. At lower levels of MC or

in other words when the RH is lower the theory seems to be valid.

Graph 18: Change in mass over time- Chipboard; predicted mw hysteresis

There are two samples that vary slightly from this theory, Pine and Plywood. The

Pine sample shows a very rapid drop in mass over the first few days, and then levels

out to similar profile as the other samples. Plywood remains at an almost constant

hysteresis for about the first 25 days and then there is a jump in the absorption rate

with a corresponding decrease in . This may be due to an estimation of the

desorption time, as the mass of water is an experimental result. Further

experimentation to test this hypothesis is advised.


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5. CONCLUSIONS

The primary component of this project was the characterisation of sorption isotherms

for 5 different samples of natural and manufactured timber. The result of the

experimentation gave data which matched current research within the field of wood

sorption and allowed a certain amount of expansion on the basic sorption isotherm

curves. These areas of expansion included various time dependant variables,

hysteresis and diffusion coefficient (models). Within each section there was an

analysis of the experimental results and a comparison between the samples.

The sorption isotherms display FSP, MC and EMC at varying RH values leading to

the following conclusions. There certain types of timber such as Pine and other

softwoods that can absorb greater amounts of moisture, quicker than hardwood

varieties and are therefore more applicable to moisture control within most buildings.

Manufactured timber tends to display the same sorption characteristics as hardwood

as it is generally treated with moisture retardant chemicals. However, when used it

large amounts the effect that manufactured timber has on a sealed building envelope

can be significant.

Within the species themselves the data gathered on the amount of hysteresis between

the drying and wetting curves is comparable to known values and therefore may be

useful within that field of research. The hysteresis of of mw at duration of gave

some interesting results and the theory of constant value of hysteresis has been put

forward for further research. The definition of equilibrium has been called into

question, with the results of the desorption curves not matching the predicted

values; the speed at which hardwood desorbes water being brought forward as the

cause. The analysis of a diffusion coefficient model was successful, with calculated

data corroborated against research from other papers. Application of this data in other

models is the logical next step.

Given the experimental limitations, specifically time related, the data and conclusions

drawn from the data appear to be valid when compared against known research from

respected sources.


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6. EXPERIMENTAL RECOMENDATIONS AND FURTHER

RESEARCH

6.1 Experimental Recommendations

In order to get a more detailed picture of the actual diffusion rate it would be

advisable to use a more complex method of moisture measurement, such as an

x-ray imager. It is important not only to know how much moisture is in the

sample, but also where it is.

Smaller samples would work better within a short timeframe such as this as

they would respond faster and with more accuracy given the inability to

measure the depth of moisture ingress.

Test physical properties of the wood, such as expansion, using detailed

measuring equipment.

Allow more time for the permeability testing and use a sealed box containing a

scale so RH disruption is limited.

6.2 Future Research

Detailed sorption isotherm testing with daily MC measurements, to test the

theory of constant value of hysteresis between 30-70% RH

Test samples under a temperature variation with constant RH. Results include

graphs of EMC vs. Temperature.

Heating and cooling a sealed volume with a known MC, that contains a

sample of wood

Test physical properties at different moisture contents, including strength,

elasticity and dimensional change.

Perform a constant wetting and drying of on sample between two relative

humidity levels, and monitor the absorption and desorption characteristics.


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APPENDIX A: TEST RESULTS

Table 10: Oven Drying of Samples

Sample Original Weight of Wood and Room RH; 2:00pm Tues 18 Nov

A B C D E F1 66.64 63.78 66.54 56.46 45.99 55.92

2 78.2 77.97 77.97 7

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