Properties of Extruded Chipboards Laminated with Surface Layers from Flakes

Publisher’s version / Version de l'éditeur:

Technical Translation (National Research Council of Canada), 1964

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la

première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/20331670

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Properties of Extruded Chipboards Laminated with Surface Layers from

Flakes

Buro, A.; May, H-A.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=60873d8e-8d0e-4d6f-98db-61121d397963 https://publications-cnrc.canada.ca/fra/voir/objet/?id=60873d8e-8d0e-4d6f-98db-61121d397963

PROPERTIES OF EXTRUDED CHIPBOARDS LAMINATED WITH

SURFACE LAYERS FROM FLAKES

I

.)

-c.x..'

t

NATIONAL RESEARCH COUNCIL OF CANADA TECHNICAL TRANSLATION 1129

BY

ANDREAS BURO AND HANS-ALBRECHT MAY

FROM

HOL.Z AL.S ROH· UNO WERKSTOFF, 19: 479·482, 1961

TRANSLATED BY

D. A. SINCLAIR

THIS IS THE ONE HUNDRED AND EIGHTEENTH OF THE SERIES OF TRANSLATIONS PREPARED FOR THE DIVISION OF BUILDING RESEARCH

OTTAWA

PREFACE

The interest in German extrusion processes for the manufacture of "chipboards" and thicker panels in Canada has ahlays been

tem-pered by two weaknesses inherent in the board structure. These

are the lack of tensile strength in the long (machine) direction,

and the large swelling coefficient in that direction. The board

is ideal as core stock for IIsandwt ch" shapes utilizing strong

facings. This paper shows the very effective composites that can

be formed by bUilding up pressed chipboards or flakeboards on both sides of the extruded stock.

The Division is pleased to make this paper available to all who recognize the clear potential of these methods and products

for Canadian development and use. In all its research work

in-volving the use of wood and wood products, the Division works in the closest liaison with the Forest Products Research Branch of

the Federal Department of Forestry. This translation is

accord-ingly published in this series with the knowledge and agreement of Dr. J.H. Jenkins, Director of the Branch.

The paper was translated by Mr. D.A. Sinclair of the Translations Section of the National Research Council Library, to whom the Division of BUilding Research Wishes to record its thanks. Ottawa May

1964

,

Title:

NATIONAL RESEARCH COUNCIL OF CANADA Technical Translation 1129

Properties of extruded chipboards laminated with surface layers from flakes

(Eigenschaften von Strangpressplatten mit aUfgepressten

Deckschichten aus ｓ｣ｨｮ･ｩ､ｳｰｾｮ･ｮＩ

Authors: Andreas Buro and Hans-Albrecht May

Reference: Holz ala Roh- und Werkstoff, 19: 479-482, 1961

PROPERTIES OF EXTRUDED CHIPBOARDS LAMINATED WITH SURFACE LAYERS FROM FLAKES

Introduction and 3tatement of Aims

Basically, there are two different methods of manufacturing chipboards,

the pressed board and the extrusion method. The two systems result in special

properties. The pressed board is isotropic in the plane of the board. It

can be built up in several layers, making it possible to employ special chips

(surface chips, internal chips) in order to produce certain properties. The

pressed board exhibits high bending strength and bending stiffness. The

extruded chipboard, on the other hand, has high transverse tensile strength

and exhibits little swelling. It is anisotropic in the plane of the board and

its low bending strength makes it necessary, as a rule, to ｶ･ｾ･･ｲ the boards

or to coat them in some other manner.

The purpose of the present investigation was to combine the good pro-perties of the two different board types by means of a laminated structure. For this purpose extruded chipboards were finished on both sides with surface

layers from flakes. Unlike the usual veneer, these surface layers do not

only act in a single direction, but because of their surface isotropy they serve uniformly in all directions.

Principles for Combining Extruded Chipboards and Pressed Boards In order to understand how strength is produced by this combination, we must first examine the properties of the two types of board in some detail. The basic structural differences between pressed boards and extruded

chip-boards are shown in Fig. 1. Here sections of the two types of board are

shown schematically and are so arranged that the plane of dispersion against

the pressure is exerted is at the top. It is immediately evident that the

main properties of the boards are derived and can be explained essentially from the position of the plane of dispersion and the direction of pressing. Thus the strength values in the plane of dispersion, in which the chips over-lap, are always sUbstantially higher than on the axis perpendicular to the plane of dispersion, where the chips are only more or less glued together. In Fig. I the tensile strength was used as the strength criterion, since it

can be determined in all three directions. All other strength values are

closely related to it.

-4-The swelling values also depend strongly on the direction of pressing. The tendency of the cells that are deformed in the course of pressing is to return to their original shape and in the pressed board this tendency

mani-fests itself in swelling in the direction of the thickness of the board. In

the case of the extruded chipboard, however, the swelling is predominantly in the longitudinal direction parallel to the direction of pressing.

Consideration of the properties of the two types of board, therefore, shows that the weak point of both, with respect to strength and swelling, is

situated on the axis parallel to the direction of pressing. If an extruded

chipboard is veneered with a thin pressed board, as was done in the course of these tests, the pressing directions of the two types of board are per-pendicular to each other, resulting in a certain mutual cancellation of the

negative properties. i'lith such a combination, for example, less surface

swelling can be expected compared with the extruded chipboard, and at the same time the thickness swelling should be greatly reduced compared with the

pressed board. The transverse tensile strength, even for comparatively light

boards, should be increased considerably compared with pressed boards, because of the favourable transverse strength of the extruded chipboard and the greater densities of the laminated layers, while at the same time, as a consequence of the veneering, the anisotropy of the extruded chipboard should be reduced, depending on the extent of the lamination, and the bending

strength will be increased by the statically favourable position of the veneer in the surface zones.

These more or less theoretical considerations lead to the conclusion that by combining the two methods of manufacture favourable results may be

expected. The first experiments in the production and testing of such boards

were carried by G. Ohse, with positive results (unpublished). G. Ohse also

veneered and lacquered test specimens of these boards. They exhibit good

properties and to some extent they also proved effective under very severe weathering conditions.

The Tests and Their Results

16 mm extruded chipboards with a bulk density of approximately 0.58 gfcm3

were sprayed with 20 g of a 50% urea resin solution per square metre of sur-face and then sprinkled on both sides with thin, flaky spruce chips (thickness

about 0.2 mm, length about 15 - 20 mm, width about 5 mm) containing about

8%

binding agent relative to the dry weLght. of the wood. Both sides of the

unpressed board were then sprayed \'lith 50 g/m2 _{wat er , after which the board}

ｾＭ

minutes, depending on the surface layer thiclcness). The quantities of chips

applied were chosen so as to produce surface layers of 1 and

3

densities of 0.65 and 0.90 g/cm3

• Figure 2 ShOHS the structure of the

resulting board.

Bending Strength and Bending Stiffness

As expected, even with a small degree of lamination (ratio of weight of

surface layer to weight of the entire board) the boards ･ｹｾｩ｢ｩｴ･､ considerably

improved bending strength and bending stiffness. The effect of the surface

layer thickness on the overall strength is evident from Table I. A layer

1 rom thick already increases the bending stiffness by about 100 kp/cm2 _{and a}

3 mm layer brings about an increase of only 150 kp/cm2 • Thus, relating the

increase of stiffness to the thickness of the surface layer, we get only

50 kp/cm2 _{in the second case, compared with 100 kp/cm}2 _{when the ｬｾｾｩｮ｡ｴｩｯｮ}

is only 1 mm thick. From this standpoint, therefore, the surface layer

thickness should not be too great, since the effectiveness of the lamination decreases with increasing thickness.

The influence of the bulk density within the given range is shown in

Table II. It is evident that increased density in the surface layers is

advantageous and brings about a somewhat better utilization of the chips

employed. To clarify this, we divide the mean increase of strength attained

in both directions by the bulk density of the surface layers, and thereby

obtain an effectiveness of approximately 220 for a bulk density of 0.68 g/cm3

and an effectiveness of 270 for 0.90 g/cm3_{• The difference is not very great,}

to be sure, so that when technical and economic factors are taken into con-sideration the useful range of values is found to be between approximately

0.75 and 0.85 g/cm3

•

In all the test boards the original anisotropy of the extruded

chip-board still influenced the bending strength. Its influence decreased, of

course, vlith increasing degree of lamination, so that the board with a

surface layer of

3

as practically isotropic (cf. Tables I and II). At the same time it should

be mentioned here that such a board already exceeds considerably the bending strength values called for in the quality standard for pressed boards.

To give a rough idea of the bending strength that can be obtained for a

given lamination,Vlc may use the degree of lamination. In Fig. j the quality

coefficient for bendinG is plotted against the degree of lamination (ratio of surface layer weight to total weight of chips) for various surface layer

thickness and bulk densities. It will be seen that in rough approximation

-6-to the direction of pressing can in each case be joined by a straight line. The steeper broken line, which represents the values of boards with equal layer thickness but varying bulk density of the surface layer, again indicates that increasing the density of the surface layers brings about a somewhat greater increase of bending strength than increasing the layer thickness. Nevertheless, a rough estimate of the effect of lamination can be obtained by asswning that surface layer thickness and surface layer bulk density are corresponding factors which can be reciprocally varied within certain limits, depending on the requirements.

Transverse Tensile Strength

The transverse tensile strength of extruded chipboard is known to be

very high. The boards used here as interior layers have a transverse tensile

strength of 17.6 kp/cm2

• The laminated boards, on the other hand, showed an

average transverse tensile strength of 9.2 kp/cm2

• The value is thus reduced

by approximately one-half. In the laminated boards the break always occurs

in the zone of transition between the extruded chipboard and the laminated surface layer, so that this transition zone must determine the transverse

tensile strength of the whole. Surprisingly, the bond between the extruded

chipboard and the surface layer was not influenced by the bulk density of the surface layer, nor, consequently, by the pressure employed in bonding, so

that the stated average value of 9.2 kp/cm2 applies to all the test boards.

We did not investigate the question of whether thicker coating of the extruded chipboard before lamination would result in a greater transverse tensile

strength. This did not appear important from the practical standpoint, since

the strength already obtained is about twice that of ordinary pressed boards. Swelling

All swelling measurements indicated that this property is not materially

affected by the degree of lamination. The swelling values were thus

summa-rized for laminated boards and compared with those for unlaminated ones

(Table III). The swe Ll.Lng value in thickness, wnLch is particularly

favour-able in the case of the extruded chipboard because of its structure, was not

greatly influenced by lamination. The swelling in the plane of the board,

on the other hand, is considerably reduced by the lamination. This is

par-ticularly true for the severe swelling of the extruded chipboard in the

direction of pressing which is reduced by lamination from 25% to

3%

24 hours in water. The swelling of 0.)% in the plane of the board

perpendicu-lar to the direction of pressing corresponds approximately to the value for the standard pressed board.

-7-Kind of Hood Used in the Surface Layer

In the kinds of board investigated here the densities of the surface

layer are high. Now, it is well-known from investigations of the strength

properties of pressed boards that the quality coefficients for the strength properties increase with increasing density of the board, and that for heavier boards the effect of the kind of wood employed, and especially its bulk

density, decreases progreSsivcly(l). For example, in the case of a light

board of, for example, 0.55 g/cm3_{, a board made from beechwood chips shows}

only about 55? of the strength of one from spruce chiPs(2), but at a density

of 0.8 g/cm3

the beechwood board shows

87%

board. This is due to the fact that at higher board densities even the

beechwood chip can be compressed to such an extent that adequate surfaces of adhesion are able to form between the chips and transmit the wood strength to a large extent.

It would be expected, therefore, that surface layers of beechwood would give almost as good a planking effect as the laminated surfaces of spruce

chips. Figure 4 shows the results for boards with a laminated layer

thick-ness of 3 mm and a bulk density of 0.87 g/cm3 _{for the spruce chips and}

0.80 g/cm3 _{for the beechvlood. Although the latter are not qUite so}

com-pressed, they give approximately the same bending stiffness values.

Transverse tensile strength and swelling for the beechwood boards corresponded to those of the spruce boards.

This result is important from both the technological and economic

stand-point. It means that the considerably cheaper beechwood can be employed with

equal effectiveness in place of the more expensive spruce. Orientation of the Surface Chips

From the investigations of the Institut fllr HOIZforschung(3) it 1s known that materials from wood chips with directional strength values can be

pro-duced by suitable orientation of the chips. We now ｜ｾｩｳｨ to determine what

the properties of extruded chipboards with laminated surface layers would be

if the chips were orientated. The results of tests in which the chips in the

surface layers were orientated perpendicular to the direction of extrusion,

are given in Table V. As expected, the development of the strength is

determined decisively by the orientation of the chips. The effect of the

extruded chipboard, accordingly, is reduced. The resulting strength pattern

is similar to that of the veneered extruded chipboard.

The above-mentioned investigations(3) indicate that the strength pattern

is scattered over a fairly larGe angle relative to the direction of orienta-tion, the strength obtained in this direction is less than if the angle is

very small. The directed strength of the material can thus be varied by

varying the orientation angle. It ,may therefore be expected that surface

isotropic boards can be produced by'means of a suitably weal< orientation of

the chips of the surface layers for the extruded chipboard. For these the

direction of the chips in the surface layer must be adjusted so that the resulting increase in strength compensates for the weaker strength pattern of the extruded chipboard parallel to the direction of extrusion.

Summary

The aim of these ｩｮｶ･ｳｴｩｾ｡ｴｩｯｮｳ was to determine the properties of

extruded chipboards laminated with surface layers from flakes. The test

results showed that by such a lamination boards with high bending and

per-\

pendicular tensile strength and with little swelling can be manufactured which comply with the quality conditions for flat-pressed boards (bending

strength with a surface layer thickness of

3

0.68 g/cm 3 _{parallel to extrusion 160 kp/cm}2

, perpendicular to extrusion

220 kp/cm2

) . An investigation of the influence of surface layer thickness

and density showed that the effect of both factors on the bending strength is mainly determined by the portion of chips in relation to the total quantity

of chips used. Surface layers from beechwood chips proved to be equivalent

to surface layers from spruce chips. In the course of several tests it could

be proved that boards with special properties can be obtained by the orientation of the surface layer chips.

The ｴ･ｾｴ results, especially with regard to the numerous different types

of extruded chipboards, show that, in view of the economic as well as of the technical standpoint, certain advantages are to be expected from the combina-tion of extruded chipboards and flat-pressed boards which justify further research on the possibilities of manufacture and applicability of extruded chipboards laminated with surface layers from flakes.

-9-References

"

1. Klauditz, W. Untersuchungen uber die EiGnung verschiedener Holzarten,

insbesondere von Rotbuchenholfi zur Hcrstellung von Holzspanplatten. Bericht 25/52 des Instituts fur Holzforschung Braunschweig, 1952.

2. K1auditz, W. and Buro, A. Untersuchungen an Spanplatten aus Spangemischen

verschiedener Holzarten. Holz-Zentralbl. 86: 1195, 1960.

3. Klauditz, VI., Ulbricht, H.J., Kratz, W. and Buro, A. Herstellung und

Eigenschaften von H01zspanwerkstoffen mit gerichteter Festigkeit. Holz als Roh- und Werkstoff, 18: 377-385, 1960.

Table I

Influence of surface layer thiclcness on the bending strength of extruded chipboards with laminated surface layers from flakes

Surface layer thickness*

rom

Bending strength

lcp/cm2

Parallel

I

to the direction of extrusion

E-Modulus (bending)

kp/cm2

Parallel IAt right angles to the direction of extrusion

o

* Bulk density of surface layer ｲｾ

=

Table II

Influence of bulk density of surface layers on the bending strength of extruded chipboards with laminated surface layers from flakes

Bulk density of surface layer* gjcm3 Bending strength kp/cm 2

Parallel

I

to the direction of extrusion

E-Modulus (bending)

lcp/cm2

Parallel IAt right angles to the direction of extrusion

0.68 0.90 12 161 270 80 223 289 300 ＲＳｌｾＰＰ 32200 3000 32900 40300

-10-Table III

Swelling in thickness and length of extruded chipboards laminated with flakes by the flat pressure method.

Since the swelling was similar for all kinds of lamination the values were summarized

Swelling after 24 hours in

_%

Type of board _{Parallel At right angles}

Thickness to direction of pressing Ordinary extruded chipboard

5.3

25

0.7

0.3

Influence of the kind of wood in the flakes of the surface layer on the bending strength of extruded chipboards

with laminated surface layers from flakes

Kind of wood in the sur-face flakes Bulk densities of surface layers* gfcm3 Bending strength kp/cm2

Parallel IAt right angles to direction of extrusion Spruce Beech

0.87

0.80

270

289

-11-Table V

Bending strength of extruded chipboards with surface layers

from orientated flakes. Flake orientation

parallel to the direction of extrusion

...

Position of flakes

Surface layer

Thickness Bulle density

rom g/cm2

BendinG Strength

lep/cm2

Parallel

I

to direction of extrusion Non-orientated Orientated

3.0

3·0

o

o

Comparison of pr-opcr-tte s of cxtr-uceo chipboards and pressed boards

The indicated va Lue o nre ordc!'u of magnitude only

-12-Fig. :2

Cross-section of extruded chipboard with laminated surface layers of flakes 3 rom thick

Above: View of the plane perpendicular to the direction of

pressing (plane of scatter)

Below: View of the plane in the direction of preSSing

lir---.,...ｾＮＭＭＭＭＬＬＮ｟｟ＮＭＭＭＬ

killo

f!J JY JI1 C<eilt"'l!eJUmil'\djon

Fig. 3

Effect of degree of lamination (ratio of weight to surface layer

to weight of board) on the quality coefficient for the bending of extruded chipboards With laminated surface layers of

flakes. The broken line curves join points

ot

surface layer thickness, but unequal surface layer bulk densities