Zbornik gozdarstva m lesarstva, 42, 1993, s. 237 - 245 
GDK 813.8:862.2( =20) 
FORMALDEHYDE EMISSIONS, DETERMINED BY MODIFIED WKI -
METHOD, VS. PARTICLE BOARD THICKNESS AND THE KINO OF 
COATING 
Saša PIRKMAIER * 
Abstract 
It was found that formaldehyde em1ss10ns are decreasing with an increasing 
particleboard thickness. Formaldehyde emissions from the side areas are 
aproximately by two times higher than the respective emissions from the surface 
layer. The highest formaldehyde emissions were found in uncoated particleboards, a 
bit lower in surface coated particleboards and the lowest in side coated 
particleboards. 
Key words: particleboard, formaldebyde, layer, emissio11 
ODVISNOST EMISIJE FORMALDEHIDA, UGOTOVLJENE Z 
MODIFICIRANO WKI METODO, OD DEBELINE IVERNIH PLOŠČ 
VRSTE OPLEMENITENJA 
Izvleček 
Ugotovljeno je bilo, da em1s1Ja prostega formaldehida pada z narascaJoco debelino. 
Emisija formaldehida iz robnih površin je približno dvakrat višja kot iz površine 
zunanjega sloja. Najvišja emisija formaldehida je bila ugotovljena pri 
neoplemenitenih ploščah, nekoliko nižja pri površinsko oplemenitenih in najnižja pri 
robno oplemenitenih ivernih ploščah. 
Ključne besede: iverna plošča, formaldehid, sloj, emisija 
* dr. dipl.ing., associate professor, University of Ljubljana, Biotechnical Faculty, Department of 
Wood Science and Technology, Rožna dolina, Cesta VIJl/34, 61000 Ljubljana 
238 
Zbornik gozdarstva in lesarstva, 42 
CONTENTS 
1 INTROOUCTION ................................................................... 239 
2 FUNOAMENTALS ................................................................. 239 
3 SOME METHOOS OF FORMALOEHYDE 
OETERMINATION ................................................................ 239 
3.1 Perforator method ................................................................... 239 
3.2 WKI flask method .................................................................. 240 
3.3 Modified WKI method ........................................................... 240 
3.4 Gas analysis ............................................................................. 240 
3.5 Chamber analysis ..................................................................... 241 
4 METHOOOLOGY ANO EXPERIMENTALS .................. 241 
5 RESULTS ANO OISCUSSION ........................................... 242 
6 CONCLUSIONS ...................................................................... 244 
POVZETEK ............................................................................. 244 
REFEREN CES ........................................................................ 245 
1 INTRODUCTION 
239 
Pirkmaier, S.: Formaldehyde emissions 
The production and the use of particle boards have risen by almost 36 % 
in Europe over the past decade. 
The majority of particle boards are of type V20 (according to the DIN 68 
763 standard). They are used for the manufacture of furniture, interior 
fumishings and in civil engineering. Formaldehyde - based glue, obtained by 
the synthesis of urea and formaldehyde, has been mainly used in the 
manufacture of particle boards of type V20. The disadvantage of these 
particle boards lies in formaldehyde emissions which are hazardous to 
health. This problem has been therefore given great attention in Europe 
during the past decade. An increasing number of European countries has 
decided to make particle boards of class E1 since then. 
2 FUNDAMENTALS 
Three - layer particle boards of type V20 (12 mm, 18 mm and 22 mm 
thick) have been chosen and tested for correlation between the 
formaldehyde emissions and the varying share of core and the varying 
gluing factors and the kinds of gluing. The share of single layers depends 
on the thickness of particle board. The gluing factor depends on the 
specific particle surface respectively. Particle boards can be either coated, 
surface and / or side coated or uncoated. 
In brief, we wanted to find out how the thickness of particle board and 
the respective kind of gluing depending again on the particle board 
thickness (uncoated, surface coated, side coated) affect the formaldehyde 
emissions. 
3 SOME METHODS OF FORMALDEHYDE DETERMINATION 
3.1 Perforator method 
This is probably the most widely used method which bas been applied for 
more than 20 years and standardized as a European standard EN 120. 
240 
Zbornik gozdarstva in lesarstva, 42 
According to this method, formaldehyde is extracted with hot toluene and 
transformed into distilled water. The share of formaldehyde in the aqueous 
solution is determined by iodometry and / or spectrophotometry and given 
in mg / 100 g a. d. by particle board. The disadvantage of this method 
presented for many years the dependence of formaldehyde share on the 
moisture content in the particle board. This method bas been mainly used 
to determine the share of formaldehyde in · uncoated particle boards and 
fiber boards. 
3. 2 WKI flask method 
The WKI flask method is quite a simple method. Sample pieces with 
dimensions of 25 mm x 25 mm x thickness are suspended on a rubber 
strip over a 50 ml distilled water in 500 ml PE bottle and left for 24 
hours or longer at temperatures of 40 °c. This is the so - called diffusion 
method. The disadvantage of this method presents an unfavourable 
relationship between the side and surface area. Here also, the share of 
formaldehyde in the solution is determined by iodometry or 
spectrophotometry. 
3. 3 Modified WKI method 
The modified WKI method, as developed by Roffael, differ from the 
simple WKI method in both a larger size of test sam ples ( 40 mm x 50 
mm x thickness) and a bottle content. Furthermore, saturated sodium 
chloride solution is used instead of distilled water. 
3. 4 Gas analysis 
This method is known as a DIN standard 52 368. It is based on gas 
extraction of formaldehyde from the particle board. Under given conditions 
formaldehyde containecl in air mixture turns into distillecl water. The share 
of formaldehycle is then determinecl by spectrophotometry. 
3. 5 Chamber analysis 
241 
Pirkmaier, S.: Formaldehyde emissions 
This method allows the testing of larger samples or even of entire 
furniture pieces. The research parameters (such as, volume, temperature, 
moisture content, air velocity, air changes and chamber load) are preset. 
This method again is based on gas extraction of formaldehyde from a 
particle board. The content of formaldehyde is given in ppm. The results 
perfectly correlate with the results obtained by .other methods. 
4 METHODOLOGY ANO EXPERIMENTALS 
In the investigation under study, a modified WKI tlask method has been 
used for the determination of formaldehyde emissions. This is again a 
diffusion method. The diffusion takes place according to the first Fick's 
law: 
dn = DxSx de 
dt dx 
where means: 
dn 
dt 
D 
s 
de 
dx 
number of formaldehyde molecules diffusing from the particle 
board in a given tirne unit 
coefficient of diffusion 
a surface through which formaldehyde diffuses 
coefficient of concentration 
The advantage of this method over similar methods is in the fact that it 
remains unaffected by the moisture content of the particle board. By using 
sodium chloride it is possihle to maintain the relative humidity ahove the 
solution more exactly. Larger test pieces (40 mm x 50 mm x thickness) 
allow for a larger relationship between the surface and side area. A 400 
ml bottle, 90 mm high, bas a diameter of 85 mm. Here too, the test 
242 
Zbornik gozdarstva in lesarstva, 42 
pieces are suspended on a rubber strip over the solution. The bottle is 
airtight closed and subject to temperatures of 40 °c for 24 hours. The 
relative humidity above the bottle is 85 % RH. The above mentioned 
three - layer test pieces (12 mm, 18 mm and 22 mm thick) of type. V20 
and the emission class El have been tested. Urea - formaldehyde glue bas 
been used in the manufacture of particJe boards. The gluing facfor was 11, 
7 % for the surface layer and 7, 5 % for the core respectively. The test 
pieces to be tested for formaldehyde and moisture content have. been cut 
and shaped according to a drawing. The experiment was C.-.'lrried out in 
various alternatives. 10 test pieces have been always used in each 
alternative. 
Ali tree thicknesses of a particle board have been tested in the following 
three alternatives: 
I uncoated particle board 
II surface coated particle board 
III side coated particle board 
The surface was coated with a melamine - resin impregnated foil, with a 
basis weight of 80 / 200 g / m2. The sides were coated with a threefold 
coat of polyurethane varnish. 
5 RESULTS AND DISCUSSION 
The following formaldehyde emissions and their correlations have been 
found in the investigation under study: 
- Formaldehyde emissions vs. board thickness in uncoated particle boards 
(in mg / 100 a. d. ) . A linear correlation with a correlation coefficient 
R = O, 96 was determined. 
- Formaldehyde emissions vs. board thickness in coated particle boards (in 
mg / 100 g a. d. ) . A linear correlation with a correlation coefficient R 
= O, 95 was determined. 
- Formaldehyde emissions vs. board thickness in side coated particle boards 
(in mg / 100 g a. d. ) . A linear correlation with a correlation coefficient 
R = O, 68 was determined. 
243 
Pirkmaier, S.: Formaldehyde emissions 
- Formaldehyde flux vs. the share of side areas in uncoated particle boards 
(in mg / m2 / h) . The correlation takes a shape of a curve. The 
correlation coefficient is R = O, 96. 
- Formaldehyde flux vs. the share of side areas in coated particle boards 
(in mg / m2 / h) . The correlation takes a shape of a curve. The 
correlation coefficient is R = O, 91. 
Let us now look at the graphs of formaldehyde emissions in ali three 
thicknesses and ali three alternatives of surface coating. 
mg/100 g atr?. 
8 
6 
4 
2 
o 
12 mm 
1 variant 
18mm 
22mm 
As the graph clearly shows, formaldehyde em1ss10ns are decreasing with the 
increasing particle board thickness of uncoated, surface coated and side 
coated particle boards. The reasoh lies in the relationship between the 
number of board layers and the gluing factors. The share of side areas is 
increasing with the increasing particle board thickness which inhibits the 
formaldehyde flux. In side coated particle boards, the formaldehyde 
emissions from the surface layer are decreasing with the increasing board 
thickness owing to the share of surface layer and a higher gluing factor. 
Further, the graphs show that the formaldehyde emissions have been the 
highest in uncoated particle boards and the lowest in side coated particle 
boards. This applies to ali three board thicknesses. In uncoated particle 
244 
Zbornik gozdarstva in lesarstva, 42 
boards, the relative share of formaldehyde em1ss10ns released from the side 
area in relation to the total formaldehyde emissions does not depend on 
the board thickness. The same applies also to the formaldehyde emissions 
from the surface layer. 
The above comparison shows that formaldehyde emissions from side areas 
reach approximately twice the amount of formaldehyde emissions from the 
surface layer. 
6 CONCLUSIONS 
- Reduction of formaldehyde emissions by increasing particle board 
thickness; 
- Formaldehyde emissions are the highest in uncoated boards, and the 
lowest in side coated particle boards; 
- The relative share of formaldehyde emissions from the side areas does 
not depend on the board thickness; 
- The relative share of formaldehyde emissions from the surface layer 
shows only a light interdependence with the board thickness; 
- Formaldehyde emissions from the side areas are approximately by two 
times higher than those from the surface layer; 
POVZETEK 
Prispevek govori o rezultatih ugotavljanja emisij formaldehida iz ivernih 
plošč tipa TP 20 , kjer je bilo uporabljeno urea - formaldehidno lepilo. Pri 
ivernih ploščah je bila ugotavljana odvisnost med emisijami formaldehida, 
debelino plošč in vrsto oplemenitenja. Preiskušane so bile tri različice 
ivernih plošč (neoplemenitene, površinsko in robno oplemenitene) in treh 
različnih debelin (12 mm, 18 mm in 22 mm) . Emisije formaldehida so 
bile ugotovljene z modificirano WKI metodo. Ta metoda je zelo praktična 
in uporabna za vse tri vrste ivernih plošč:neopleme - nitene, površinsko in 
robno oplemenitene. 
Ugotovljeno je bilo, da em1s1Je formaldehida padajo z naraščajočo debelino 
ivernih plošč. Emisije formaldehida iz površine robov so približno dvakrat 
245 
Pirkmaier, S.: Formaldehyde emissions 
višje kot emisije iz površine zunanjega sloja. Najvišja emisija formaldehida 
je bila ugotovljena pri neoplemenitenih ivernih ploščah, nekoliko nižja pri 
površinsko oplemenitenih in najnižja pri robnooplemenitenih ivernih plošcah. 
REFERENCES 
PIRKMAJER, S., 1991. The manufacture of wood based materials from 
desintegrated wood in Slovenia in the period from 1984 to 1990. - LES, 11 / 
12, pp. 318 - 319, 
PIRKMAJER, S. / MIHEVC, S., 1986. Formaldehyde emissions from uncoated and 
coated particleboards and parts of furniture: actual states and goals. - LES, 7 / 
8, pp. 195 - 203, 
ROFFAEL, E., 1982. Formaldehydeemissionen aus den Spanplatten und anderen 
Holzwerkstoffen. - DRW Verlag, Stuttgart, pp. 43 
ROFFAEL, E., 1989. Formaldehydeabgabe von Holzwerkstoffen: ein Rueckblick. -
Holz als Roh und Werkstoff, 47, pp. 41 - 45, 
SUNDIN, B. / ROFFAEI, E., 1991. Bestimmung der Formaldehydeabgabe von 
Holzwerkstoffen nach der Flaschenmethode. - Holz - Zentralblatt, Stuttgart, pp. 
597 - 598, 
TIŠLER, V., 1987. Reasons far different results obtained in the detcrmination of 
formaldchyde emissions in particleboards using the Perforator method. -
Potentials of particle and fiberboards development in Yugoslavia. - 3rd Federal 
Symposium Proceedings. Nova Gorica, pp. 192 - 200.