Conference PaperPDF Available

Restriction of liquid water spreading in overlaid plywood top veneer

December 2018

DOI:10.22616/rrd.24.2018.013

Conference: Research for Rural Development, 2018

Authors:

Dace Cirule

Institute of wood chemistry

Edgars Kuka

Latvian State institute of Wood chemistry

Anrijs Verovkins

Latvian State institute of Wood chemistry

Ingeborga Andersone

Latvian State institute of Wood chemistry

Content uploaded by Edgars Kuka

Content may be subject to copyright.

86 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1

FORESTRY AND WOOD PROCESSING DOI: 10.22616/rrd.24.2018.013

RESTRICTION OF LIQUID WATER SPREADING IN OVERLAID

PLYWOOD TOP VENEER

Dace Cirule1, Edgars Kuka1,2, Anrijs Verovkins1, Ingeborga Andersone1

1Lavian State Institute of Wood Chemistry, Latvia

2Riga Technical University, Latvia

xylon@edi.lv

Abstract

Plywood overlaid with resin impregnated paper lms is used in various decorative applications for which high

stability of aesthetic qualities is of great importance. The top veneer of the plywood for these materials is perfectly

protected from a direct contact with water as far as the covering lm is not damaged. However, in case of lm

damage water can cause clearly visible defects in a relatively short period of time. To reduce these types of defects,

unsophisticated and efcient way was developed. It involves lling the vessel system of the top veneer with industrial

phenol-formaldehyde resin by using a hot-press. Inuence of some phenol-formaldehyde resin properties on its

penetration in birch veneer as well as the effect of wood moisture content were tested. The proposed top veneer

pre-treatment method with phenol-formaldehyde resin reduced the average swelling zone distance from damage site

by half for oven pre-dried plywood with 3% moisture content before treatment. Moreover, it was found that using

plywood with moisture content of 9% the swelling zone distance was reduced by two thirds compared to untreated

overlaid plywood.

Key words: plywood; phenol-formaldehyde resin; water spreading.

Introduction

Plywood manufacturing from wood veneer layers

by gluing them together with resins is one of the largest

wood-based composite industries (Chang et al., 2018).

In comparison with solid wood products, the design

of plywood with veneer sheets glued together with

adjacent layers having their wood grain perpendicular

to one another results in reduced tendency of swelling

and shrinking, providing improved dimensional

stability in changing humidity environment.

Moreover, such a design makes the plywood panel

less anisotropic regarding its strength properties.

Different varieties of plywood have been designed

and are manufactured for multitude of applications

such as furniture, engineering constructions, vehicles,

sporting goods and equipment, packaging, ships and

yachts and others (Hrázský & Král, 2007). Among

others, plywood overlaid with a resin impregnated

paper lm has gained its niche for various decorative

applications. Mostly melamine and phenolic resins

are used for production of resin lms, which are

subsequently hot-pressed onto one or both of plywood

sheet surfaces. Beside changed appearance, covering

of plywood with resin-impregnated lm signicantly

improves water and abrasion/wear resistance as well

as facilitates cleaning and maintenance of the surface.

Moreover, the resin lm coating perfectly protects

the top veneer of plywood from a direct contact with

water as long as the covering lm is intact. However,

in case of the lm damage, the inherent wood swelling

characteristics can provoke noticeable surface failure

even after a relatively short contact with water.

Intensive research has been done and is still under

experimentation to overcome the high moisture

sensitiveness of wood thus improving its utility. There

are numerous modication processes proposed to

reduce wood swelling, which can be divided into two

groups: an active modication involving changes in

wood chemical structure and passive modication,

which does not alter the chemistry of wood (Hill,

2006; Epmeier, Westin, & Rapp, 2004; Sandberg,

Kutnar, & Mantanis, 2017). The active modication

of wood comprises wood treatment methods, which

are quite sophisticated and therefore costly, such

as thermal modication, acetylation, furfurilation.

Besides, wood modication through alteration of its

chemical structure often involves reduction of strength

properties (Epmeier, Westin, & Rapp, 2004). Passive

modication mainly is performed by impregnation

with an aim to bulk the wood cell wall thus preventing

dimensional changes of wood caused by varying

humidity conditions or direct contact with liquid

water. It is found that water-soluble low molecular

weight phenol-formaldehyde (PF) resins can penetrate

into a wood cell wall and improve wood dimensional

stability (Seborg, Tarkow, & Stamm, 1962; Furuno,

Imamura, & Kajita, 2004; Kielmann, Butter, & Mai,

2018). However, even a very high resin loadings

cannot totally prevent dimensional changes of wood

(Hill, 2006). Another type of passive impregnation

involves cell lumens lling. It cannot prevent swelling

but can substantially hinder liquid water spreading

through the wood porous structure. In such a way, the

region subjected to action of water is reduced.

The ow of liquid primarily moves in the path of

least resistance. In hardwoods, the least resistance

path is the capillary structure of vessel network,

which is characterized by relatively wide lumens and

end-to-end connection with perforation plates of high

permeability (Kamke & Lee, 2007). The penetration

87RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1

rate of liquid into wood varies depending on both the

wood structure and properties and the characteristics

of the penetrant solution (Meijer, Thurich, & Militz,

2001; Meijer, 2004; Kučerová, 2012). It implies

that lling of vessel lumens could hinder fast water

spreading into the top veneer under damaged covering

lm thus reducing the surface area subjected to

swelling and lessening the visual defects.

The present study was aimed at nding an

unsophisticated and effective way for the enhancement

of covered plywood resistance to rapid local swelling

in case of top veneer contact with water because of the

covering lm damage.

Materials and Methods

All of the materials used for experiments were

supplied by JSC ‘Latvijas Finieris’. Plywood overlaid

with PF resin impregnated lm was used to determine

the swelling zone expansion rate from the damaged

site. The damaged site was created by drilling in the

surface of the overlaid plywood an approximately 5

mm deep hole with diameter of 2.8 mm. Afterwards

the damaged site was exposed to liquid water and the

swelling zone distance was measured after certain

periods of time (10, 30, 60, 120, 240 and 1440 min)

with an accuracy of 1 mm.

Partly polymerised PF resin was obtained by

heating the industrial PF resin in water bath for 3 h at

50 ± 1 °C. Partly dehydrated PF resin was obtained

from the industrial PF resin by using a rotary vacuum

evaporator with process parameters: vacuum – 30 mbar,

temperature – 20 °C, rotation speed – 100 rpm. To obtain

PF resins with a different water content, dehydration

was performed for 1 h, 2 h and 4 h. Viscosity of the

PF resins was measured by using rotational viscometer

HAAKE Viscotester 6 plus. The water content of

the PF resins was determined by drying the PF resin

(~ 2 g) in an oven at 140 °C for 1 h. The mass of PF

resin was weighed before and after drying. From

these measurements the mass loss (water content) was

calculated assuming that only water evaporates.

Rotary-cut birch (Betula spp.) veneers with

thickness 1.4 mm were used for impregnation with the

industrial PF resin as well as with the resins modied

by polymerization and dehydration. The set of six

veneer specimens with dimensions 100 × 100 mm

per each resin type was used for impregnation. The

impregnation was carried out under vacuum (22 mbar)

for 10 min. The excess of resin was wiped off from

the veneer surface with a paper towel immediately

after removing the specimens from the impregnation

container. To polymerize the impregnated resin, the

specimens were kept in an oven (140 ± 2 °C) for 12

h and weight percent gain (WPG) was calculated as a

percentage increase of the dry weight of the specimens

after the impregnation.

To evaluate how the water content of the industrial

PF resin is changed after spreading it over the plywood

surface depending on the wood moisture content,

an equal amount of PF resin was paintbrush applied

on three surfaces (100 cm2 surface area): aluminium

plate (as control), conditioned plywood (RH 65 ± 5%,

temperature 20 ± 2 °C) with 9% moisture content and

oven pre-dried (102 ± 3 °C) plywood with 3% moisture

content. After ve and ten minutes the PF resin was

removed with a scraper and the water content was

determined as previously described. The control was

used to exclude from the calculations the water that

evaporates during exposure for 5 and 10 min.

To assess the effect of the top veneer pre-

treatment with PF resin on reducing of the rapid

swelling zone of the overlaid plywood, PF resin was

applied with a paintbrush onto the plywood surface

(amount: 120 – 140 g m-2) prior to the covering lm

hot-pressing process. Manufacturing of overlaid

plywood specimens was performed in a laboratory

by using hydraulic laboratory press. To manufacture

the specimens, plywood with pre-treated top veneer

was covered with a PF resin impregnated lm and

pressed under 1.9 MPa pressure for 5 min at 140 °C

temperature.

To assess resin distribution in the top layer of

the pre-treated plywood, 1 cm wide bars were sawn

from specimen central parts and, after softening

by boiling in water for 3 h, samples were sliced for

a light microscopy examination. The examination

was performed with a transmitted light microscope

“Leica DMLB” connected to the video camera „Leica

DFC490”.

Results and Discussion

The results of the experiment in which the rate of

the swelling zone expansion was evaluated show that

liquid water transport in the top veneer is relatively

rapid and the covered distance can be quite large. The

average distance from the site of damage to the end of

the swelling zone after 24 hours (1440 min) was 32 ±

7 cm. Moreover, more than 60% of the total swelling

zone distance was reached during the rst 30 minutes.

The development of the swelling zone from damage

site during the rst four hours after articial swelling

initiation is presented in Figure 1.

The results show that the damage site can cause a

fast development of swelling zone on the surface of the

overlaid plywood when in contact with liquid water.

Besides, the results show that there is a limit how

far the swelling zone can develop from the damage

site, and the distance depends on the site’s specic

wood anatomic structure. The main cause of the fast

development of the swelling defects is attributed to the

rapid liquid water transport through the birch wood

vessel system. Therefore, the restriction of water ow

Dace Cirule, Edgars Kuka,

Anrijs Verovkins, Ingeborga Andersone

RESTRICTION OF LIQUID WATER SPREADING

IN OVERLAID PLYWOOD TOP VENEER

88 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1

by lling the vessels with PF resin could be one of the

possibilities to reduce or eliminate the formation of

such aesthetic defects.

To evaluate the possibility of lling birch wood

vessels with PF resin and to estimate the effect of

resin properties on this process, vacuum impregnation

of birch veneers for 10 min was performed. The resin

penetration was evaluated by measuring the weight

percent gain (WPG) of the impregnated birch veneer

and the results are presented in Table 1.

The results show that the resin penetration into

birch veneers is signicantly affected by the resin

properties and the largest inuence is attributed

to the resin viscosity. By increasing the phenol-

formaldehyde resin temperature by 10 °C, which

resulted in a decrease of resin viscosity by 43%, the

WPG increased twice reaching the average value

of 26.4%. Also, for partly polymerised PF resin the

WPG signicantly increased by rising the resin

temperature by 20 °C which caused decrease in resin

viscosity by 47%. Moreover, partial polymerisation

of the resin reduced its ability to penetrate into the

birch veneer. However, it was still possible to achieve

high WPG (34.9%) by increasing the temperature of

the partly polymerised resin. For partly dehydrated

PF resin, the WPG values were the lowest from

all the tested resin types. As the dehydration was

performed in vacuum without increased temperature,

no substantial polymerisation of resin is supposed

to occur. Therefore, inability of partly dehydrated

resin to penetrate into veneer is mainly attributed to

the very high resin viscosity due to reduced water

content. These results suggest that viscosity is highly

important regarding the resin penetration into wood

and therefore should be well analysed. Meijer (2004)

in his review paper concluded that penetration capacity

of coating is mainly attributed to the viscosity, which

is in agreement with the results of the present study.

In addition, he concluded that wetting and surface

tension of the coating seem to play only a minor role.

As previously shown, a partial dehydration of PF

resin can cause a signicant increase in resin viscosity

that results in low WPG of the impregnated birch

veneer. Therefore, it is important to determine how

water content of phenol-formaldehyde resin inuences

the resin viscosity. The results of the experiment are

presented in Figure 2. The PF resin dehydrated for 4

h is not included in the gure because its viscosity at

20 °C was too high for measuring with the viscometer

used in the study.

Dace Cirule, Edgars Kuka,

Anrijs Verovkins, Ingeborga Andersone RESTRICTION OF LIQUID WATER SPREADING

IN OVERLAID PLYWOOD TOP VENEER

DOI: 10.22616/rrd.24.2018.013

Figure 1. Development of swelling zone from the damage site on overlaid plywood. Error bars represent

standard deviations.

The results show that the damage site can cause a fast development of swelling zone on the surface of the overlaid

plywood when in contact with liquid water. Besides, the results show that there is a limit how far the swelling zone

can develop from the damage site, and the distance depends on the site’s specific wood anatomic structure. The

main cause of the fast development of the swelling defects is attributed to the rapid liquid water transport through

the birch wood vessel system. Therefore, the restriction of water flow by filling the vessels with PF resin could be

one of the possibilities to reduce or eliminate the formation of such aesthetic defects.

To evaluate the possibility of filling birch wood vessels with PF resin and to estimate the effect of resin properties

on this process, vacuum impregnation of birch veneers for 10 min was performed. The resin penetration was

evaluated by measuring the weight percent gain (WPG) of the impregnated birch veneer and the results are

presented in Table 1.

Table 1

Effect of phenol-formaldehyde resin properties on resin penetration in birch veneer by vacuum

impregnation

Industrial resin type

Resin

temperature

(T), C

Resin

viscosity,

mPa s

Veneer average weight

percent gain after

impregnation, %

Industrial phenol-formaldehyde resin

212

13.4 (3.4)

Industrial phenol-formaldehyde resin

120

26.4 (4.7)

Partly polymerised phenol-formaldehyde resin

180

6.5 (3.8)

Partly polymerised phenol-formaldehyde resin

34.9 (9.9)

Partly dehydrated (4 h) phenol-formaldehyde resin

2600

4.7 (1.6)

Standard deviation in parentheses

The results show that the resin penetration into birch veneers is significantly affected by the resin properties and

the largest influence is attributed to the resin viscosity. By increasing the phenol-formaldehyde resin temperature

by 10 C, which resulted in a decrease of resin viscosity by 43%, the WPG increased twice reaching the average

value of 26.4%. Also, for partly polymerised PF resin the WPG significantly increased by rising the resin

temperature by 20 C which caused decrease in resin viscosity by 47%. Moreover, partial polymerisation of the

resin reduced its ability to penetrate into the birch veneer. However, it was still possible to achieve high WPG

(34.9%) by increasing the temperature of the partly polymerised resin. For partly dehydrated PF resin, the WPG

values were the lowest from all the tested resin types. As the dehydration was performed in vacuum without

increased temperature, no substantial polymerisation of resin is supposed to occur. Therefore, inability of partly

dehydrated resin to penetrate into veneer is mainly attributed to the very high resin viscosity due to reduced water

content. These results suggest that viscosity is highly important regarding the resin penetration into wood and

therefore should be well analysed. Meijer (2004) in his review paper concluded that penetration capacity of coating

is mainly attributed to the viscosity, which is in agreement with the results of the present study. In addition, he

concluded that wetting and surface tension of the coating seem to play only a minor role.

As previously shown, a partial dehydration of PF resin can cause a significant increase in resin viscosity that results

in low WPG of the impregnated birch veneer. Therefore, it is important to determine how water content of phenol-

100

030 60 90 120 150 180 210 240

Proportion of total swelling

zone distance

Time, min

Figure 1. Development of swelling zone from the damage site on overlaid plywood.

Error bars represent standard deviations.

Table 1

Effect of phenol-formaldehyde resin properties on resin penetration

in birch veneer by vacuum impregnation

Industrial resin type Resin

temperature

(T), °C

Resin

viscosity,

mPa s

Veneer average weight

percent gain after

impregnation, %

Industrial phenol-formaldehyde resin 20 212 13.4 (3.4)

Industrial phenol-formaldehyde resin 30 120 26.4 (4.7)

Partly polymerised phenol-formaldehyde resin 30 180 6.5 (3.8)

Partly polymerised phenol-formaldehyde resin 50 96 34.9 (9.9)

Partly dehydrated (4 h) phenol-formaldehyde resin 65 2600 4.7 (1.6)

Standard deviation in parentheses

89RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1

The results clearly show that water presence in PF

resin is highly important to ensure appropriate resin

viscosity for efcient resin penetration. The resin

viscosity increased more than 20 times when the water

content of the resin was reduced by only 6% (from

48% to 42%). A similar trend has been observed for

urea-formaldehyde and polyvinyl acetate adhesives

(Hass et al., 2012). The results suggest that any loss

of water content of PF resin could be crucial in the

process of lling wood vessels.

By using resin pre-treatment, it is possible that

plywood with low moisture content due to its inherent

hydrophilic nature could remove some of the water

from PF resin before the resin itself can penetrate

into the vessels, which subsequently would result in

higher resin viscosity and hindered penetration into

wood. Therefore, the wood moisture content effect on

removal of water from the PF resin was tested. The

results are presented in Figure 3.

The results show that a signicant decrease in

water content of PF resin is noticeable due to wood

ability to absorb water. After 5 min water content

of the resin decreased by 6% and 35%, but after 10

min by 10% and 67% compared to the control for

conditioned (moisture content 9%) and oven pre-

dried (moisture content 3%) plywood, respectively.

In case of oven pre-dried plywood, such changes in

water content of resin can dramatically affect further

resin penetration in wood vessels because of the

enormous increase in resin viscosity. These results

are in accordance with the studies in which similar

tendencies have been established regarding glue and

water-borne paint penetration into wood depending

on the wood moisture content (Hrázský & Král, 2007;

Meijer, Thurich, & Militz, 2001).

Several overlaid plywood specimens were

produced and the swelling zone from articially-

created damage site was measured to test how addition

of PF resin between the plywood surface and PF

impregnated lm can affect the average distance of the

swelling zone. The results are presented in Figure 4.

An addition of PF resin signicantly reduces the

average swellings zone distance from the damaged site

on the overlaid plywood surface. Moreover, the results

Dace Cirule, Edgars Kuka,

Anrijs Verovkins, Ingeborga Andersone

RESTRICTION OF LIQUID WATER SPREADING

IN OVERLAID PLYWOOD TOP VENEER

DOI: 10.22616/rrd.24.2018.013

formaldehyde resin influences the resin viscosity. The results of the experiment are presented in Figure 2. The PF

resin dehydrated for 4 h is not included in the figure because its viscosity at 20 °C was too high for measuring with

the viscometer used in the study.

Figure 2. Effect of phenol-formaldehyde resin water content on resin viscosity at 20 C.

The results clearly show that water presence in PF resin is highly important to ensure appropriate resin viscosity

for efficient resin penetration. The resin viscosity increased more than 20 times when the water content of the resin

was reduced by only 6% (from 48% to 42%). A similar trend has been observed for urea-formaldehyde and

polyvinyl acetate adhesives (Hass et al., 2012). The results suggest that any loss of water content of PF resin could

be crucial in the process of filling wood vessels.

By using resin pre-treatment, it is possible that plywood with low moisture content due to its inherent hydrophilic

nature could remove some of the water from PF resin before the resin itself can penetrate into the vessels, which

subsequently would result in higher resin viscosity and hindered penetration into wood. Therefore, the wood

moisture content effect on removal of water from the PF resin was tested. The results are presented in Figure 3.

Figure 3. Water content of phenol-formaldehyde resin after application on plywood surface. Error bars represent

standard deviations.

The results show that a significant decrease in water content of PF resin is noticeable due to wood ability to absorb

water. After 5 min water content of the resin decreased by 6% and 35%, but after 10 min by 10% and 67%

compared to the control for conditioned (moisture content 9%) and oven pre-dried (moisture content 3%) plywood,

respectively. In case of oven pre-dried plywood, such changes in water content of resin can dramatically affect

further resin penetration in wood vessels because of the enormous increase in resin viscosity. These results are in

accordance with the studies in which similar tendencies have been established regarding glue and water-borne

paint penetration into wood depending on the wood moisture content (Hrázský & Král, 2007; Meijer, Thurich, &

Militz, 2001).

2000

4000

6000

8000

10000

12000

41 42 43 44 45 46 47 48 49

Viscosity, mPs

Water content, %

Control Conditioned

plywood Oven pre-dried

plywood Control Conditioned

plywood Oven pre-dried

plywood

After 5 min After 10 min

Resin water content, %

Figure 2. Effect of phenol-formaldehyde resin water content on resin viscosity at 20 °C.

DOI: 10.22616/rrd.24.2018.013

formaldehyde resin influences the resin viscosity. The results of the experiment are presented in Figure 2. The PF

resin dehydrated for 4 h is not included in the figure because its viscosity at 20 °C was too high for measuring with

the viscometer used in the study.

Figure 2. Effect of phenol-formaldehyde resin water content on resin viscosity at 20 C.

The results clearly show that water presence in PF resin is highly important to ensure appropriate resin viscosity

for efficient resin penetration. The resin viscosity increased more than 20 times when the water content of the resin

was reduced by only 6% (from 48% to 42%). A similar trend has been observed for urea-formaldehyde and

polyvinyl acetate adhesives (Hass et al., 2012). The results suggest that any loss of water content of PF resin could

be crucial in the process of filling wood vessels.

By using resin pre-treatment, it is possible that plywood with low moisture content due to its inherent hydrophilic

nature could remove some of the water from PF resin before the resin itself can penetrate into the vessels, which

subsequently would result in higher resin viscosity and hindered penetration into wood. Therefore, the wood

moisture content effect on removal of water from the PF resin was tested. The results are presented in Figure 3.

Figure 3. Water content of phenol-formaldehyde resin after application on plywood surface. Error bars represent

standard deviations.

The results show that a significant decrease in water content of PF resin is noticeable due to wood ability to absorb

water. After 5 min water content of the resin decreased by 6% and 35%, but after 10 min by 10% and 67%

compared to the control for conditioned (moisture content 9%) and oven pre-dried (moisture content 3%) plywood,

respectively. In case of oven pre-dried plywood, such changes in water content of resin can dramatically affect

further resin penetration in wood vessels because of the enormous increase in resin viscosity. These results are in

accordance with the studies in which similar tendencies have been established regarding glue and water-borne

paint penetration into wood depending on the wood moisture content (Hrázský & Král, 2007; Meijer, Thurich, &

Militz, 2001).

2000

4000

6000

8000

10000

12000

41 42 43 44 45 46 47 48 49

Viscosity, mPs

Water content, %

Control Conditioned

plywood Oven pre-dried

plywood Control Conditioned

plywood Oven pre-dried

plywood

After 5 min After 10 min

Resin water content, %

Figure 3. Water content of phenol-formaldehyde resin after application on plywood surface.

Error bars represent standard deviations.

90 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1

for conditioned plywood with 9% moisture content

were better than for the oven pre-dried plywood with

3% moisture content, which is in accordance with

previously discussed results about the wood ability

to absorb water from PF resin depending on the

wood moisture content. The examination of the top

veneer cross-sections with microscope showed that

the reduction in the average swelling zone distance

is mainly due to PF resin penetration into wood

vessels, which restricts rapid liquid water transport

in top veneer of the overlaid plywood. These results

suggest that for more efcient lling of birch wood

vessels with PF resin and subsequent restriction of

water spreading, the plywood moisture is of a great

importance and the plywood of the highest admissible

moisture content regarding the technological process

should be used in order to ensure top veneer with

reduced water transportation ability.

However, in Figure 5 presented light microscopy

images of specimens prepared from one plywood

sample pre-treated with PF resin show that PF resin

penetration in birch wood vessels can be rather

uneven. The area with all vessel lumens empty (a),

area with only part of lumens lled (b), and the

area with completely lled vessel lumens (c) were

detected even within one specimen in some cases.

The inhomogeneous resin penetration in wood

vessels could be partly attributed to the differences in

wood anatomy (Meijer, 2004). This can account for

only reduction and not complete elimination of fast

water spreading into the top veneer of plywood with

damaged covering lm by using the proposed method.

Conclusions

1. The damage of overlaid plywood covering lm in

presence of liquid water causes a rapid formation

and spreading of well noticeable swelling zone

and during the rst 30 minutes more than 60% of

the total swelling zone is reached.

2. Resin viscosity signicantly affect its penetration

ability into veneer. Reduction of the water content

of phenol-formaldehyde resin causes an increase

in viscosity which hinders resin penetration into

wood.

3. The best penetration of the resin into veneer was

achieved for specimens with a higher moisture

content, which is in accordance with the ndings

that wood readily absorbs water from resin and the

absorption capacity is inversely related to wood

moisture content.

4. In the present study, a method for the enhancement

of covered plywood resistance to rapid local

swelling was developed comprising pre-treatment

of top veneer with phenol-formaldehyde resin

to ll wood vessels followed by hot-pressing of

covering lm.

DOI: 10.22616/rrd.24.2018.013

Several overlaid plywood specimens were produced and the swelling zone from artificially-created damage site

was measured to test how addition of PF resin between the plywood surface and PF impregnated film can affect

the average distance of the swelling zone. The results are presented in Figure 4.

Figure 4. Effect of phenol-formaldehyde resin pre-treatment and plywood moisture content on reduction of the

average swelling zone distance from the damage site. Error bars represent standard deviations.

An addition of PF resin significantly reduces the average swellings zone distance from the damaged site on the

overlaid plywood surface. Moreover, the results for conditioned plywood with 9% moisture content were better

than for the oven pre-dried plywood with 3% moisture content, which is in accordance with previously discussed

results about the wood ability to absorb water from PF resin depending on the wood moisture content. The

examination of the top veneer cross-sections with microscope showed that the reduction in the average swelling

zone distance is mainly due to PF resin penetration into wood vessels, which restricts rapid liquid water transport

in top veneer of the overlaid plywood. These results suggest that for more efficient filling of birch wood vessels

with PF resin and subsequent restriction of water spreading, the plywood moisture is of a great importance and the

plywood of the highest admissible moisture content regarding the technological process should be used in order

to ensure top veneer with reduced water transportation ability.

However, in Figure 5. presented light microscopy images of specimens prepared from one plywood sample pre-

treated with PF resin show that PF resin penetration in birch wood vessels can be rather uneven. The area with all

vessel lumens empty (a), area with only part of lumens filled (b), and the area with completely filled vessel lumens

could be partly attributed to the differences in wood anatomy (Meijer, 2004). This can account for only reduction

and not complete elimination of fast water spreading into the top veneer of plywood with damaged covering film

by using the proposed method.

Figure 5. Top veneer cross sections of overlaid birch plywood pre-treated with phenol-formaldehyde resin: a)

empty vessel lumens; b) part of vessel lumens filled; c) completely filled vessel lumens.

Conclusions

1. The damage of overlaid plywood covering film in presence of liquid water causes a rapid formation and

spreading of well noticeable swelling zone and during the first 30 minutes more than 60% of the total swelling

zone is reached.

2. Resin viscosity significantly affect its penetration ability into veneer. Reduction of the water content of phenol-

formaldehyde resin causes an increase in viscosity which hinders resin penetration into wood.

3. The best penetration of the resin into veneer was achieved for specimens with a higher moisture content, which

is in accordance with the findings that wood readily absorbs water from resin and the absorption capacity is

inversely related to wood moisture content.

100

Oven pre-dried plywood

(3 % moisture content)

Conditioned plywood

(9 % moisture content)

Reduction in the avarage

swelling zone distance, %

Figure 4. Effect of phenol-formaldehyde resin pre-treatment and plywood moisture content on reduction of

the average swelling zone distance from the damage site. Error bars represent standard deviations.