Permeability in surface treated Norway spruce samples-

Effect of wood properties Fuktupptag i ytbehandlade granprover-

Virkesegenskapers påverkan

Författare: Josefin Nilsson

Handledare LNU Åsa Blom

Examinator LNU Göran Peterson

Termin: HT12 15hp

Examensarbete TS9903

III

Summary

The tradition of using Norway spruce (Picea abies) in outdoor applications goes way

back in history. In Norway spruce there’s a large natural variation in durability but

knowledge about what is causing this variation is missing.There are many factors that

affect the durability of wood. Since fungal growth demands water the woods ability

to water uptake got a direct effect on durability. In this thesis the focus were put into

investigating how the water uptake ability was affected by the influence of

heartwood versus sapwood and wood properties such as density and annual ring

width.

Material divided into the following groups was investigated: narrow grown wood,

heartwood or sapwood and wide grown wood, heartwood or sapwood. In addition to

testing untreated samples two different coating systems were used: alkyd and acrylic,

both of them frequently used in Sweden.

The following assessments were done: density, percentage of latewood, liquid water

permeability and the Mycologg method. Liquid water permeability was checked

trough a modified version of EN 927-5 where priming oil was used as the first layer

on the investigated surface and the back of the sample was left untreated. The same

samples were later used in the Mycologg method, an accelerated weathering test

where the samples are subjected to moisture fluctuations.

The highest density was found in the narrow grown wood. Sapwood samples had

higher density then heartwood samples, in both the narrow and wide grown wood.

The percentage of latewood ranged from 23-13% in the examined samples.

The wide grown wood with a coating system (priming oil and alkyd or acrylate paint)

absorbed less water than the narrow grown wood in the permeability test. In the

accelerated testing only untreated samples from wide grown wood reached a

moisture content level high enough for fungal growth.

The Mycologg test showed a clear difference between wide and narrow grown wood.

Density cannot be the sole explanation to this. Further research is necessary to

examine the impacts of wood properties when it comes to water uptake.

IV

Sammanfattning

Traditionen av att använda granvirke (Picea abies) utomhus går långt tillbaka i tiden.

Man har sett en stor naturlig variation på beständighet på granvirke utan att kunskap

funnits kring vad som orsakar denna variation. Det finns flera faktorer som påverkar

beständigheten på virke. Då mögel kräver vatten så är har virkets

vattenupptagningsförmåga en direkt påverkan på virkets beständighet. I denna

uppsats är fokusen på att undersöka vattenupptagningsförmågans påverkan från

kärnved kontra splintved samt virkesegenskaper såsom densitet och årsringsbredd.

Material indelat i följande kategorier har undersökts: kärnved och splintved från

tätvuxet virke samt kärnved och splintved från frodvuxet virke. Utöver obehandlat

virke så undersöktes två olika färgsystem, alkyd och acryl varav båda är vanligt

förekommande i Sverige.

Följande undersökningar gjordes: densitet, andel sommarved, vattenpermeabilitet

samt en accelererad åldringsmetod (Mycologg metoden). Vattenpermeabiliteten

undersöktes enligt en modifierad version av EN 927-5 där första lagret på den

undersökta ytan var en grundolja och baksidan på provet lämnades obehandlad.

Samma provbitar användes senare till Mycologg metoden, en accelererad

åldringsmetod där man utsätter proverna för flukterande fuktcykler.

Den högsta densiteten bland de undersökta proverna fanns i det tätvuxna virket.

Splintveden hade högre densitet än kärnveden i både det tätvuxna och det frodvuxna

virket. Andelen sommarved varierade från 13-23% av årsringen.

Det frodvuxna virket behandlat med grundolja och ett färgsystem absorberade

mindre vatten än det tätvuxna virket, behandlat med samma system, i

vattenpermeabilitetstestet. I det accelererande åldringstestet var det endast frodvuxen

splintved som nådde en fuktkvot tillräckligt hög för mögel påväxt.

Mycologg testet visade på en tydlig skillnad mellan frodvuxet och tätvuxet virke.

Densitet kan inte vara den enda förklaringen till detta. Mer forskning är nödvändig

för att vidare undersöka de olika virkesegenskapernas påverkan för vattenupptagning

i trävirke.

V

Abstract

In Norway spruce there’s a large natural variation in durability but knowledge about

what is causing this variation is missing. In this thesis the focus were put into

investigating how the water uptake ability was affected by the influence of

heartwood versus sapwood and wood properties such as density and annual ring

width. Liquid water permeability was checked trough a modified version of EN 927-

5 and samples were also investigated trough the Mycologg Method.

The wide grown wood with a coating system (a priming oil and alkyd or acrylate

paint) absorbed less water than the narrow grown wood in the permeability test. The

Mycologg test showed a clear difference between wide and narrow grown wood.

Further research is necessary to examine the impacts of wood properties when it

comes to water uptake.

Keyword: Norway spruce, Picea abies, durability, water uptake, wood properties

VI

Preface

The work presented in this bachelor thesis was carried out at Linnaeus University, the Department of Forest and Wood Technology. This thesis is a part of a larger research project, called Above ground durability for surface treated spruce that was started up at former Växjö University during the autumn of 2009.

I would like to thank my supervisor, Dr Åsa Rydell Blom and also the laboratory technician Mr Jonaz Nilsson, for giving me support and helping me carry out the tests. To both of you: Thanks for all the patience you showed me and all the good laughs you gave me!

VII

Table of contents

1. Introduction ________________________________________________ 1 1.1 Background and hypotheses __________________________________________ 1 1.2 Aims and objectives _________________________________________________ 1

1.3 Limitations ________________________________________________________ 2

2. Theory ____________________________________________________ 3 2.1 Wood structure _____________________________________________________ 3 2.2 Water flow in timber ________________________________________________ 4

3. Material and method _________________________________________ 6

3.1 Material __________________________________________________________ 6 3.2 Coating system and the application _____________________________________ 6 3.3 Assessment of density _______________________________________________ 7 3.4 Assessment of percentage of latewood __________________________________ 7

3.5 Assessment of the liquid water permeability ______________________________ 7 3.6 The Mycologg method _______________________________________________ 8

4. Result ____________________________________________________ 10

4.1 Density __________________________________________________________ 10 4.2 Percentage Latewood _______________________________________________ 10

4.3 Permeability- Floating test ___________________________________________ 11 4.4 Permeability – Mycologg test ________________________________________ 12

5. Discussion and conclusion ____________________________________ 16 5.1 Discussion _______________________________________________________ 16 5.2 Conclusion _______________________________________________________ 18

6. References ________________________________________________ 19

.

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1. Introduction

1.1 Background and hypotheses

Panels made out of Norway spruce (Picea abies) can show big variance in

durability even though they are covered with the same coating system

(Elowson et al. 2003). Further research needs to be done to find the factors

behind the variance.

The tradition of using Norway spruce in outdoor applications goes way back

in history. It has been common knowledge that spruce has better durability

than pine sapwood but less durability then pine heartwood. Spruce on the

other hand was seen as a homogenous material without any difference

between heartwood and sapwood. This is a myth that recent research been

showing is not true (Bergström & Blom 2005). But furthermore studies are

necessary since the different factors behind the variation in durability are not

established. Factors such as density, annual ring width, heartwood and

sapwood need to be further examined. Research needs to be done about

whether different wood properties affect the durability when there is a

coating system applied, and if that is the case, what importance do they

have?

This thesis is a part of a bigger research project called Above ground

durability for surface treated spruce that was started up at former Växjö

University during the autumn of 2009. The hypotheses were:

1. There are resources within spruce, when it comes to durability, that are not being used. By selecting the right wood, spruce could achieve status as a more durable tree species.

2. Difference between heartwood and sapwood is the most important factor for water uptake, both vapour and liquid.

3. The wood properties have an impact on the water uptake even for panels with a coating system.

1.2 Aims and objectives

The aim of this study is to obtain further knowledge about what is causing

the large natural variation seen in Norway spruce durability. The main

objective is to find measurable factors that are causing the variations that

occur in usage of Norway spruce applications above ground. The two fields

that will be investigated are different coating systems and effects of wood

properties.

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The goal is to find a way of selecting wood material that with the correct

type of paint system can show a high durability, without need of additional

treatments that can be hazardous to the environment. With the knowledge on

how to select natural high quality wood there is a possibility to get an

environmentally friendly building material, regarding production of carbon

dioxide both throughout the production as well as the service life and

recycling.

1.3 Limitations

There are many factors that affect the durability of the wood samples but the

focus were put into investigating the influence of heartwood versus sapwood

and wood properties such as density and annual ring width.

In addition to testing the untreated samples two different coating systems

were used, alkyd and acrylic, both of them frequently used in Sweden. Other

popular coating systems such as linseed oil or calcimine were not tested.

Because of time limitations the Mycologg test will only run for some weeks,

not enough time to establish any fungal attack on the test panels. Therefore

the Mycologg test will only result in data on water vapour uptake. The

moisture content in the test samples are checked via sensors by means of

electrical resistance, therefore there is an upper limit of calculating moisture

contents up to 32 % down to 7 %.

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Josefin Nilsson

2. Theory

2.1 Wood structure

Central trough the tree trunk runs the pith, surrounded first by heartwood

and then sapwood (fig 1). It is the sapwood that performs the role of support,

conduction and food storage (Thörnqvist 1991). The very young tree

consists of only sapwood but with time the tree starts to transform the

sapwood near the pit into heartwood (Dinwoodie 2000). In the process of

creating heartwood the main tasks for the wood is closed down, pores get

clogged to stop transportation of fluids and nutrians. The heartwood is left

with function as support only (Dahlgren et al. 1999). The extractives are the

same kind in the heartwood and the sapwood but the content is higher in the

sapwood. (Bergström et al. 2002). The growth of the tree takes place in the

cambium, both new wood cells and the living inner bark is produced there

(Dahlgren et al. 1999).

Figure 1. Sketch of wood anatomy 1. Sapwood 2. Cambium 3. Inner bark 4. Outer bark 5. Annual rings 6. Heartwood 7. Rays 8. Nutrients transport 9. Pith (Dahlgren et al. 1999).

Depending on the time of the year (and where in the world) the growth of

the tree varies. For Norway spruce in Sweden, the summer is the time to

produce new wood cells, known as latewood, with the main function of

giving support to the tree, the cell walls are thick (fig 2). During the spring,

earlywood cells with thinner walls and larger bordered pits are produced

since the main focus in this growth period is conduction, transporting

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Josefin Nilsson

nutrients to the tree crown (Dinwoodie 2000). Earlywood and latewood

together form an annual ring, also known as year ring. The growth of the

latewood is relatively constant while the earlywood grow depending on

growth conditions. A fast growing tree produces a higher percentage of

earlywood than a slow growing tree. Since the amounts of pits are higher

and the cell walls are thinner in the earlywood, a fast growing tree has lower

density (Dahlgren et al. 1999).

Directly effected by the percentage of earlywood is the permeability.

Permeability is a measure on how much liquid the wood can absorb. It is the

proportion of cavity, which is depending on the density that affects the

permeability. A porous material got higher permeability than a dense

material (Dahlgren et al. 1999). Wood with the density of 400kg/m3 can

absorb maximum 720l/m3 while wood with a density of 800kg/m

3 only can

absorb 400l/m3 (Bergström et al 2002). Density of Norway spruce varies

between 300-480 kg/m3, the heartwood normally has a little lower density

(Dahlgren et al. 1999).

Figure 2. Picture taken with a mikroskop showing the thick cell structure of the latewood and the big hollow structure of the earlywood (woodanatomy.ch 2012)

2.2 Water flow in timber

There are three states of moisture that is present in timber, free liquid water

in the cell cavity, lumen, giving rise to bulk flow above the fiber saturation

point. Second there is bound water within the cell wall, which moves by

diffusion below the fibre saturation point and finally water vapour that

moves by diffusion in the lumbers both above and below the fibre saturation

point (Dinwoodie 2000). The fiber saturation point is where there in theory

is no free water in the cell cavities while the walls are holding the maximum

amount of bound water. The fiber saturation point varies between species, in

spruce it is said to be around 30 % moisture content (MC). The removal of

water from these areas within the cell wall results first in increased strength

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Josefin Nilsson

and, second, in marked shrinking. Such changes are reversible or almost

reversible (Dinwoodie 2000).

In the heartwood process, not only lumen gets clogged but also the pores

(for transportation between the cells) aspirates (fig 3). When the pores

aspirates, torus that works as a ventilator, seals the pit (Dinwoodie 2000).

The displacement of the torus effectively seals the pit and markedly reduces

the level of permeability of dry earlywood. In the latewood, the degree of pit

aspiration is very much lower after drying then in the earlywood (Dinwoodie

2000). In the mechanical drying of timber the free water gets removed from

lumen and also the bound water within the cell walls gets removed to some

extent. Determining the moisture content, MC, is done trough the following

formula:

Depending on expected use of the timber, different levels of MC is

desirable, for windows, building materials an expected moisture content

should be around 12-15%. Timber expected to be used only outdoors (but

protected from direct rainfall) can have a moisture content of 14-18%

(Dahlgreen et al. 1999). Fungus grows in 25-120% MC, rot fungi is found

from 25% MC and above (Deacon 1997).

Since timber is hygroscopic not even mechanical drying is permanent. The

timber will absorb moisture from the atmosphere when if it is dry and also

yield moisture to the atmosphere when it is wet (Dinwoodie 2000). Since the

humidity of the atmosphere varies over the year, a panel will be exposed to

humidity fluctuations (Dalhgreen et al. 1999).

Figure 3. Cross-section of a borded pit in the sapwood of a softwood timber. Left picture shows the timber in the green condition with the torus in the “normal” position; the right picture shows the timber in the dried state with the torus in an aspirated position (Dinwoodie 2000).

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3. Material and method

3.1 Material

The choice of wood has been done accordingly: wide grown wood with an

width of the annual rings above 8 mm and narrow grown wood with an

annual ring width of less then 4 mm. This wood was gathered from forests in

the close surroundings of Växjö. The wood was cut and taken to a sawmill

2008-05-20. The logs were taper sawn so that pieces both with horizontal

and vertical annual rings were made. The timber was sorted and heartwood

and sapwood were marked out before drying. The timber was sawn into 25

mm thick boards and thereafter dried to a moisture content of 17%.

Wood (without knots and other defects) for the test pieces (70x18x150± 2

mm) were sawn out of the boards and put into a climate room (20ºC and

65% RH). After being conditioned to a moisture content of 12% the exact

dimensions were sawn out. Thereafter the panels were randomly split into

the three different coating groups, uncoated, alkyd and acrylic, twenty

panels of each coating system.

3.2 Coating system and the application

One untreated and two different coating systems were investigated. The

systems came from Becker Acroma and from Alcro Becker. The paint

systems used was one acrylic and one alkyd. Both systems consisted of a

total of four layers: one layer priming oil, one layer base coat and two layers

of top coat.

All the panels, also the “untreated” ones, were treated with two layers of

impermeable PUR-paint (Becker-Acroma Reafen) on the sides (fig 4). The

end-grains were sealed additionally with two layers of silicon (Bostic). For

the samples with a coating system on the investigated surface, first the top

surface was treated with a layer of priming oil and then three layers of paint

according to the manufacturers’ recommendations. The back surface was left

untreated on all the samples. The same test samples were used in both the

liquid permeability test and in the Mycologg test. Between the two tests the

samples were conditioned in a climate room (20ºC and 65%RH) so they had

an approximated moisture content of 12% when starting the Mycologg test.

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Josefin Nilsson

Figure 4. Sketch over the test pieces, measurement and coating system application areas. A, Back surface: left untreated on all samples B, end grains: sealed with two layers of PUR-paint plus two layers of silicon on all samples C, Investigated surface: untreated or treated with priming oil and investigated paint system D, Sides: two layers of PUR-paint on all the samples.

3.3 Assessment of density

3 to 5 small reference samples from each board were used for density tests.

The samples were soaked in water during the night to obtain maximum

volume, thereafter they were weighed (m1) and put in an oven (103°C) to

completely dry to achieve zero moisture content. After drying the samples

were weighed again (m2).

The basic density was assessed by:

 

densitywood (kg/m3) =m2

m1

*1000

3.4 Assessment of percentage of latewood

For all 60 test samples data was collected about number of year rings, total

width of year rings and width of latewood. After determining the average

width of the year ring (the width of the earlywood varies from year to year

but the latewood is more or less constant in width), the percentage of

latewood was calculated.

3.5 Assessment of the liquid water permeability

A modified EN 927-5 method (CEN 2000) was used. This is a test method

that is intended to assess the permeability of coating materials applied to

wood. It can also be used for testing untreated samples (Bergström et al

2005).

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Josefin Nilsson

60 test samples were made (3 for each examined coating system). Following

the standard they had the length of 150 ± 2 mm, height of 18 mm and width

of 70 mm (fig 4).

The modification implied that the backside of the test panels were left

untreated so that the moist could freely be transported trough the entire test

piece. All the test panels, except the untreated ones, were treated with a

sealer or a priming coat of oil over the entire investigated surface. None of

the samples were sanded before the coating system was applied. The

modifications were made to make the test closer to how the coating systems

are actually used in real life.

The samples were weighed before the test, then left floating (the investigated

surface facing the water) in de-ionized water and weighed after 1, 2, 4, 8, 24,

48 and 72 hours. Thereafter the samples are reconditioned in a climate room

(20ºC and 65%RH) and weighed after 24 and 96 hours. The result from the

test was put into a spread sheet; out of this a diagram was created.

3.6 The Mycologg method

This is an accelerated test method where the test panels are simultaneously

set under fungal attack and subjected to moisture fluctuations. The test

panels are put into a chamber with a moisturiser that produces the mist (fig

5). Sensors are screwed into the samples (from the back side, to the depth of

three mm from the surface) where they measure the moisture content. The

operator sets the relative humidity cycles in time increments. During those

19 days that the Mycologg test was running (fig 6) it was set on cycles of 2

days of mist and 3 days of drying. Information about relative humidity and

temperature in the chamber plus data from the sensors in the samples were

stored every half hour in the control unit (consistent of a computer with

hardware and software). This information was put into tables, out of those

diagrams were created.

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Figure 5. Scematic layout sketch of the Mycologg.

Figure 6. Photo on the Mycologg in action.

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4. Result

4.1 Density

Regarding density narrow grown sapwood was the wood with the highest

density, 394kg/m3 (fig 7). Second highest density had narrow heartwood,

376kg/m3. Second lowest density had the wide grown sapwood, 368 kg/m

3.

Lowest density was found in the wide grown heartwood, 342 kg/m3.

Density

310

320

330

340

350

360

370

380

390

400

sapwood heartwood

W ood type

De

ns

ity

kg

/m

3

wide

narrow

Figure 7. Diagram of density in wide- and narrow grown sapwood and heartwood. Highest density was found in the narrow grown sapwood and heartwood. Sapwood had higher density then the heartwood in both the narrow grown and the wide grown wood.

4.2 Percentage Latewood

It was found that the wide grown sapwood had the highest percentage

latewood, around 30% (fig 8). Second most latewood, 23 %, was found in

the narrow grown heartwood. Third most latewood, 17%, was found in

narrow grown sapwood and the least amount, 13%, of latewood was found

in the wide grown heartwood.

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Josefin Nilsson

Average part latew ood

0

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0,1

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0,25

0,3

0,35

Sapwood Heartwood

W ood type

La

tew

oo

d p

art

of

tota

l a

nn

ua

lrin

gw

idth

Wide

Narrow

Figure 8. Diagram of percentage of latewood of total annual ring width in wide- and narrow grown sapwood and heartwood. Wide grown sapwood had the higest amount of late wood (30%) then came narrow grown heartwood (23%) then narrow grownsapwood (17%) and least amount of latewood was found in the wide grown heartwood (13%).

4.3 Permeability- Floating test

The floating test showed that the untreated wide grown heartwood absorbed

the highest amount of water, 10g/m2 during 72 hours (figure 9). The

heartwood from the narrow grown spruce was the test piece taking up the

least amount of water, 7,5g/m2, among the untreated samples. Second most

water uptake 8,9g/m2, had the untreated narrow grown sapwood closely

followed, 8,5g/m2, by the untreated wide grown sapwood.

Among the acryl coating systems very little difference among the samples

was found. The narrow grown heartwood and sapwood showed almost no

difference at all between them with 2,7g/m2 water uptake by the narrow

grown sapwood and 2,65g/m2 by the narrow grown heartwood. Just a little

bit less water, 2,3g/m2 was absorbed by the wide grown heartwood closely

followed by the wide grown sapwood that absorbed 2,1g/m2.

The narrow grown sapwood and heartwood absorbed close to no water at all

treated with the alkyd coating systems, 0,2g/m2 and 0,15g/m

2. Even less

water was absorbed by the wide grown sapwood, 0,1g/m2 and almost no

water at all, 0,05g/m2, was absorbed by the wide grown heartwood.

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Josefin Nilsson

Figure 9. Diagram of water absorption (g/m2 during a period of 72 hours) in a floating test of wide- and narrow grown sapwood and heartwood with three different coating systems, untreated, alkyd and acryl.

4.4 Permeability – Mycologg test

The untreated samples are showing a clear difference between wide and

narrow grown wood (figure 10). During this moisture cycle (two days mist

and three days of drying) the wide grown heartwood had the highest

moisture content, starting at 15% MC going up to 30% and then sinking to

17% at the end of the 4th

day. The wide grown sapwood closely followed

this curve starting at 15 % MC going up to 28% MC, sinking to 16-17%

MC. The narrow grown sapwood started at 15% MC, went up to a maximum

of 22,5% MC and then sank to 16 % MC. The narrow grown heartwood

started at a lower MC then the other samples, around 13% MC, going up to a

maximum of 23% MC and then sank to just below 15% MC in the end of

the measured period.

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Moisture Content Mycologg - Untreated

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Date

Mo

istu

re

Co

nte

nt

%

W.sapwood

N.sapwood

W.heartwood

N.heartwood

Figure 10. Diagram of moisture content (MC) in wide- and narrow grown sapwood and heartwood, untreated test samples. The narrown grown reached a MC of 23% and the wide grown wood a MC of 30%.

With the alkyd coating system the same result was found, the wide grown

sapwood and heartwood was the test samples that reached the highest

moisture content (figure 11). The wide grown sapwood started at 12% MC,

went up to 21% MC, going down to 16% MC. The wide grown heartwood

started at 12 % MC, went up to 21% MC and went down to 14% MC. The

difference in MC between wide grown and narrow grown was at most

around 10 %. The narrow grown sapwood started at 12%, had a top MC at

17% already the first day, went back to a MC just below 15% MC and

planned out at a moisture content around 12,5-13 %. The narrow grown

heartwood started at a 11% MC, slowly reached a top level of 12,5% gowing

down to 12% in the last day of measure.

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Moisture Content Mycologg- Alkyd

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Date

Mo

istu

re

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nte

nt

%

W.sapwood

N.sapwood

W.heartwood

N.heartwood

Figure 11. Diagram of moisture content (MC) in wide- and narrow grown sapwood and heartwood, Alkyd coated test samples. The wide grown wood reached at most a MC level of 21% and the narrow grown wood had a top of 17 % MC.

The results from the test pieces with the acryl coating showed some

difference from the other coating system (figure 12). The wide grown

sapwood remained at the same level as with the alkyd system, at most a MC

of 21 %, starting at 16% MC gowing down to 17% MC. The narrow grown

sapwood and the wide grown heartwood on the other hand reached the same

level of MC, around 17% both starting at 13-14% MC. The difference

between them was that the wide grown heartwood reached and kept that

level one day before the narrow grown sapwood, after that they dried up at

the same rate, both ending at 15%. The narrow grown heartwood showed

almost no change in MC, the highest level reached was an MC about 14%

(going up from 12% ending at 13%).

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Moisture Content Mycologg - Akrylat

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Date

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istu

re

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nte

nt

%

W.sapwood

N.sapwood

W.heartwood

N.heartwood

Figure 12. Diagram of moisture content (MC) in wide- and narrow grown sapwood and heartwood, Acryl coated test samples. The wide grown sapwood reached a maximum of 21% MC, both the narrow grown sapwood and the wide grown heartwood reached a level of 17% MC and the narrow grown heartwood reached about 14% MC.

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5. Discussion and conclusion

5.1 Discussion

Just like theory said the narrow grown wood had a higher density then the

wide grown wood. According to the theory, sapwood in spruce contains a

higher amount of extractives (Bergström et al. 2002), this could explain the

higher density in the sapwood compared to the heartwood (in both the

narrow and wide grown wood).

Narrow grown wood showed the least increase in moisture content in the

accelerated Mycologg tests, independent on coating system. Even the

untreated narrow grown wood never reached an MC high enough for fungus

to grow. Maximum MC was 23 %, fungus grow in 25-120%, rot fungi is

found from 25% MC and above (Deacon 1997).

In this test the pieces were treated with a sealer and a priming coat of oil.

The oil is normally only used on end-grains. This could have an influence of

the results, compared to findings of the non-modified versions of EN 927-5.

Was it right to not follow the standard when it came to cover three sides

with the PUR-paint? It was argued that the water should not be closed into

the panel. What actually was created was a water vapour transmission

situation with two different RH on the surfaces that were not totally sealed.

This change did tough make it less idealised and closer to reality just like the

modifications was meant too-also all the pieces had the same treatment so

the comparison was still possible.

Since the knowledge about the amount of aspired pits in the sapwood is

unknown it is hard to analyse the impact of this in relation to different water

uptake between sapwood and heartwood.

Wide grown wood could have absorbed more oil than the narrow grown;

this could then explain why the narrow grown wood absorbed more water

out of the two.

The Mycologg test continued running for more months after this paper was

finished. No discolouration or fungus attack was noted on the test pieces

with coatings system. Discolouration was found on the untreated ones. One

possible reason for this could be that the Mycologg is a much protected

environment and the fungus needs some friction to get a hold on the wood

coating. The painted test pieces had a very smooth surface, maybe they were

simply too slippery for the fungus to get a grip?! Other research has shown

that raw sawn (and not planed) wood easier get attacked by fungus (Terziev

1996).

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Josefin Nilsson

The results are showing some different tendencies. In the permeability test

among the untreated samples there was a notable difference between the

wide and narrow grown heartwood. The narrow grown heartwood had the

lowest water uptake while as the wide grown heartwood had the highest

uptake. Earlier research is showing that sapwood from spruce normally

absorbs water easier than the heartwood. Also in the Mycologg test the wide

grown heartwood reached a higher MC and faster than the wide grown

sapwood. This could depend on the fact that priming oil was used. The oil

penetrates deep into the wood and should, in theory, penetrate deeper in a

more permeable wood structure. If sapwood, which has been shown to be

more permeable (Dahlgren et al. 1999), soaked up more oil the water uptake

should also be lower. This in combination with the fact of the low amount of

latewood in the wide grown heartwood (only 13% latewood compared to

23% latewood in the sapwood) could be the explanation why the wide

grown heartwood had a higher water uptake than the sapwood.

Since the permeability was presented in g/m2 and the accelerated testing

presented in moisture content a direct comparison between the tests cannot

be made. If repeated, the test results about water permeability should also be

presented in moisture content.

The Mycologg test showed a clear difference between wide and narrow

grown wood. Density cannot be the sole explanation to this since the

variation in MC between narrow grown heartwood and sapwood are less

than their variation in density. If density should have been the most

important factor the order of the density test should reflect in all the other

tests, which was not the case. Further research is necessary to examine the

impacts of wood properties when it comes to water uptake.

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Josefin Nilsson

5.2 Conclusion

In this test the narrow grown wood had higher density than wide grown

wood. The sapwood had higher density than the heartwood.

In the Mycologg test, none of the narrow grown samples reached a moisture

content level high enough for fungal growth.

The wide grown wood with a coating system (priming oil and alkyd or

acrylate paint) absorbed less water than the narrow grown wood. The wide

grown wood with a coating system also reached a lower MC in the

accelerated weathering test. This leads to the conclusion that the wide grown

wood (independent on sapwood or heartwood) performs better with a

coating system than the narrow grown wood when treated with priming oil.

In this test there was a bigger difference between narrow grown and wide

grown wood then the difference between heartwood and sapwood.

Since the density itself doesn’t seem to be the sole key to low water uptake

and low MC more research needs to be done about the impact of wood

properties.

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Josefin Nilsson

6. References

Bergström M and Blom Å (2005) Above ground durability of Swedish softwood. Thesis for the degree of Doctor of Technology, Växjö University Press, Sweden

Blom Å and Bergström M (2005) Mycologg – a new accelerated test method for wood durability above ground. Wood science and technology 39(8)663-673

CEN (2000) EN 927-5. Paints and varnishes – Coating materials and coating systems for exterior wood – Part 5: Assessment of the liquid water permeability. European Committee for Standardisation.

Dahhlgren T, Wistrand S and Wiström M (1999) Nordiska träd och träslag, 3 ed. Arkitektur förlag AB, Byggförlag, Stockholm. (In Swedish)

Deacon, J.W. (1997) Modern Mycology, third ed. Blackwell Science, USA, ISBN 0-632-03077-1

Dinwoodie JM (2000) Timber: Its nature and behaviour. 2 ed. E & FN Spon, New York, USA

Elowson T, Bergström M and Hämäläinen M (2003) Moisture Dynamics in Norway Spruce and Scots Pine during Outdoor Exposure in Relation to Different Surface Treatments and Handling Conditions Holzforschung / Vol.57 / 2003 / No2

Samuelsson, A (1990) Calibrations Curves for Resistance-type Moisture Meters, Trätek, Report I 9302012, Stockholm, Sweden

Terziev, N. (1996). "Low-Molegular Weight Sugars and Nitorgenous Compounds in Scots Pine". Acta Universitatis Agriculturae Suecuae, Silvestria 6. Uppsala, SLU: 7-32.

www.woodatanomy.ch Species list: Picea abies Karsten, transversal section, stemwood 2012-08-10 13.27

Institutionen för teknik 351 95 Växjö tel 0772-28 80 00, fax 0470-76 85 40