Note: Descriptions are shown in the official language in which they were submitted.
1~38148
This invention relates to a method of manufacturing stressed skin
panels for use as building elements.
In order to utilize the advantages of rational operation, effective
production control, constant manufacturing conditions, simplified work and
efficient assembly on a building site, building panels are being factory pro- - duced to an ever increasing extent.
In such manufacture face materials, such as plasterboard, plywood,
fiberboard or particle board are usually nailed to a wooden framework of studs
or joists. However, in this type of construction the material is not utilized
optimAlly, since only the framework is load supporting and must be dimensioned ~ -
~ ~O us~
accordingly, whereas he~se is made of the supporting ability of the face
material.
For this reason increased efforts are now being made to manufacture
"stressed skin" panel elements as building elements, by which it is understood
that the face or "skin" interacts with the framework to make the load support-
ing part of the element. In order to achieve this a good interaction between
the framework and the skin must be established so that the latter substantially
supports the load while the framework stiffens the construction and prevents
outward bending and breaking of the board.
Such so-called "stressed skin" construction has other advantages in
addition to the better utilization of required materials. The reduced demand
on the framework makes the whole panel element lighter, which simplifies
manufacture as well as assembly. As the framework can also be made from the
face material, this avoids the use of wood which is much more expensive and
difficult to get in large sizes. Today's demands on better insulation, which
usually means thicker walls, roofs and floors, accentuate the need of an al-
ternative framework material. The cheap and uniform face material will make
stressed skin panel elements of thie type very suitable for industrial prefab-
rication.
To obtain the requisite good interaction between the framework and
1. ~
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103814~ ~
the skin an adhesively bonded construction is required. An ordinary nail joint
does not have sufficient elasticity and stiffness in respect of outward bend-
ing and breaking to function in this typs of element. Relatively great
demands are made on an adhesive joint in this construction. The adhesive
must be able to take up the forces of the load on the building element and
must not be weaker than the associated construction materials to enable utiliza-
tion of the advantages of stressed skin constructions. The adhesive must not
age or creep even under a heavy load over a long time. The adhesive joint
must be achieved even when the joined materials show certain distortions and
irregularities. -
Setting adhesives can be used for a & esive joints of this type.
Setting adhesives require a certain minimum time of contact between the
faces of the joint before the joint has sufficient stability to make it pos-
sible to handle the product. Moreover, to meet the demands on the joint a
high pressure has heretofore been applied to the joint during setting of the
a & esive. As an alternative nail bonding has been used as joining method.
In nail bonding the faces of the joint are kept together and under
pressure by means of nails joining the face and framework after adhesive has
been applied. This method is simple but has other evident disadvantages.
Nailing is a relatively time-consuming procedure. Nailing also causes damage
to the surface layer which must be filled and sanded, in spite of which the
damage will appear again after some time. Nailing requires a certain minimum
thickness and quality of the framework that may exceed that which is necessary
in view of strength. Therefore wood is normally required in the framework.
Moreover, the requirement for a certain minimum thickness of the framework
elements has the effect that thick elements must be used instead of many thin
elements, which would be preferred in view of breakage. If the use of nails
requires wooden framework elements the disadvantages discussed above in
connection with nailed building elements will also arise.
3G When adhesively bonding under pressure the faces of the joint are
103814~
kept together and under a relatively high pressure during setting of the
a & esive. Many of the nailing problems are avoided when bonding, but other
problems appear instead. Certainly all possible sizes of the stressed skin
type of elements can be produced with bonding under pressure, e.g. small wall
elements in dimensions of 1 x 2.5 m, but the greatest advantages are obtained
with big panels making up whole wall, floor and roof sections in sizes of up
to 10 m or more. It is obvious that the pressing means necessary for the
manufacture of such big building elements will be enormous, expensive and give
low flexibility, especially as pressures in the order of 5-10 kp/cm2 are used
in construction bonding. In order not to be forced to apply this pressure
over the whole surface of the elements, they are usually bonded in sections,
which still means that a section of 2 x 2.5 m requires a total force of 15-20
tons. As the a &esive requires a certain reaction time, equipment presently
used is provided with a high-frequency heating line, which is obviously another
complication with this production method, i.e. in respect of energy. ~ ;
As a consequence of these disadvantages and manufacturing problems
with nail glueing and pressing, stressed skin constructions have not to any
significant extent been commercially utilized in spite of their excellent prop-
erties and great advantages.
According to the present invention there is provided a method for
the manufacture of stressed skin building elements, preferably building
panel elements, consisting of a framework with a face material attached to
at least one side thereof by an a &esive characterized in that the adhesive is
a joint filling, setting adhesive, that a substantially non-warping material
is used for the framework, t,hat the face material is semi-rigid, and that the
framewcrk and the face material are put together on a planar support and
bonded under a pressure which does not substantially exceed that correspond-
ing to the dead weight of the parts or the weight of several building elements
piled on each other during manufacture
If these characteristic features are present it is not difficult to
provide suitable precision when cutting the parts, applying the adhesive and
1038148
controlling other conditions such that substantially no joint discontinuities
will arise by a combination of the parts having a greater gap than can be
bridged with adequate strength by the joint filling adhesive, even when the
pressure exerted on the joint surfaces only corresponds to the contact pressure
of the individual parts. The pressure thus obtained does not normally exceed ~`
2.5 kp/cm preferably not 0.2 kp/cm2 at the joint surface.
Huilding elements produced according to this invention show the same
physical properties as elements made according to methods previously known,
e.g. nailing or nail bonding of board materials on a wooden framework or
high-frequency bonding under high pressure of board material on a framework
using an a &esive that does not fill the joint.
In connection with this invention it is understood by a joint filling
adhesive an adhesive that does not require a direct contact everywhere between
the faces of the joint but is capable of filling out gaps between the faces
and forming a solid joint despite the presence of such gaps.
Normally it is required that a gap in excess of 0.5 mm can be filled
by the adhesive.
The theoretical m~ximum required shear strength of the adhesive joint
in stressed skin structures is often lower than 4-5 kp/cm2. However, in
practice it is not hard to achieve strength values in excess of 15-20 kp/cm
with common adhesive compositions. As mentioned above, the joint must have ~a good creep resistance. ~-
A series of known types of adhesive can be made to satisfy the nec-
essary requirements to make stressed skin structures. Examples of such
adhesives are modified or unmodified epoxy adhesives, polyurethane adhesives
and adhesives based on condensation products of aldehydes, preferably form-
aldehyde, and melamine, urea, thio-urea, mono- or diphenols or mixtures there-
of.
Of these adhesives the condensation resins based on formaldehyde are
preferred to the resins based on epoxy and polyurethane because of their
4.
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103814~
relative cheapness, non-toxicity and non-allergenic nature, as well as suit-
able curing time and good wetting properties in respect of wood products.
Especially good results have been obtained with resorcinol or
resorcinol/phenol resins based on formaldehyde due to their execellent
wetting and filling properties as well as good long-term characteristics.
With the condensation resins based on formaldehyde an addition of
fillers known per se is preferred, e.g. cocoanut shell flour and/or colloidal
silicic acid. A suitable filler content is 5-60 percent by weight, based
on the composition especially about 10-25 percent by weight.
Moreover, with the condensation resins based on formaldehyde a high
solids content is preferred. The solids content should exceed 55% by weight
based on the adhesive composition, preferably exceeding 65% by weight. How-
ever, with adhesives that are solid in pure form handling problems and
problems with the wetting properties with solids contents in excess of 80%
by weight will arise.
From a practical point of view it is unsuitable to work with easy
flowing a & esive compositions. In order to obtain in a simple way a well-
filled a & esive joint it is therefore preferred that the a &esive has prop-
erties allowing a certain shaping thereof, e.g. so that if the a & esive is
applied in the form of a bead it will remain as such during the working mom-
ents preceding the direct abutment of the joint faces.
Such properties, e.g. a high viscosity, can be obtained by addition
of a filler, by a suitable selection of the molecular weight ratio amongst the
components included in the a & esive or in other known ways.
If the adhesive is gliven thixotropic properties it will be easy to
handle and stable in form before and during the assembly. This property
can be obtained by adding known thixotropic agents, e.g. colloidal silicic acid.
The following has been found to be a suitable range of thixotropy as measured
with Brookfield RVT rotation viscosimeter, sp. 7
at 1 rpm 20,000 - 2,000,000 cP
103t~14~ ~
at 5 rpm 55,000 - 550,000 and
~ 50 ~ 10,000 - 100,000 cP.
The size of the adhesive bead applied and consequently the amount of
adhesive per running-metre are important to obtain in a simple manner a good
filling of the adhesive joint. The diameter of the bead should exceed 2 mm
and should preferably be within the range of 3-10 mm. In the case of a
coarse framework, e.g. studs, double beads are preferably laid to obtain a
good bond.
Finally it is advantageous that the adhesive is cold-setting, i.e.
it hardens within reasonable time at a temperature of not more than 35C.
The term substantially non-warning used with reference to the frame-
work material is intended to describe a material which, with variations of
moisture and temperature will be subject to no or only little warping or
other changes in form. Variations in length width and thickness, while each
substantially uniforn, may be independent of one another.
. .
Materials suitable for use as framework in stressed skin panels of ~ :
the invention are wood-based fiber and composite materials. The materials
can be different fonms of fibre board, particle board, plywood or blockboard.
As a rule pure wood does not meet the requirements as to stability of
form as it expands non-uniformly in each direction when absorbing mositure and
thus will be warped.
It i~ an advantage but not a requirement that the material of the
framework also has a certain flexibility so that it can adopt the shape of
the support even when placed on edge.
Particle board, fibr,e board and plywood have been used as material
for the framework elements with great success.
The term semi-rigid as applied to the face material is intended to
mean a nominally rigid material having such bending properties that it is
when in sheet form, capable of assuming a planar form with small deviations,
when placed on a planar support surface.
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~03814~
For those face materials that are substantially planar by nature,
relatively high values of their modulus of bending elasticity can be allowed.
In general3however,it is desired that the modulus of elasticity is less than
100,000 kp/cm2. It is preferred that the value is below 50,000 kp/cm2.
Suitable materials are substantially the same as indicated for the
framework. Very good results have been obtained with particle board, fibre
board and certain forms of (planar and/or flexible) plywood.
The planar support should be levelled and can e.g. consist of a
framework of wood or steel sections, on which a fiberboard or a plate of
steel or aluminium has been placed. The support can also consist of a
smoothly ground concrete slab.
When building elements of the invention are manufactured such
amounts of the adhesive are preferably applied either to the board or the
edge of each framework element that the edge will be wholly covered by ad-
hesive after the face material has been placed thereon. It is suitable that
the adhesive is applied in the form of a bead, which is flattened out when the
face material is placed thereon.
The adhesive can be applied manually or automatically, e.g. by using
a gun, in which adhesive and hardener are mixed automatically in suitable
proportions, or by separate application of adhesive and hardener.
The framework can either be placed on the face member put on the
planar support, or else the framework can first be placed on the support and
the face member on top of this.
In the accompanying drawings which illustrate an exemplary embodi-
ment of the present invention:
Figure 1 is a plan view of a framework;
Figure 2 is a detail of the framework of Figure l; and
Figure 3 illustrates various stages during the manufacture of a
building element.
In Figure 1 the illustrated framework consists of a number of parallel
~.038141!~
crossbars 1 (see Figure 1). Perpendicular to these are additional crossbars
2 at their ends and crossbars 3 between their ends to prevent lateral dis-
placement of parallel crossbars 1. In this case a number of comparbments are
created.
Before bonding the framework and its crossbars can be kept in
position by means of a jig, internal coupling connection, e.g. by means of
Dowels, by means of locating pieces ~4 in Figure 2) nailed to the support or
in one or both of the boards, by means of spot curing with a high frequency
heating means or in another way.
The space between the crossbars 1 can be provided with a vapour
barrier and insulation in the forn of glass or mineral fiber mats or directly
foamed plastic material. Any necessary wiring and the like can also be -
installed in this space.
After putting together face material and framework, hardening can be
allowed to take place at room temperature, or the process can be accelerated
by heating, possibly by heating the parts before their combination.
A suitable working method will now be described more in detail in
connection with Figure 3.
After assembly of the framework 11, adhesive is applied to the
edges of the crossbars by means of the gun 15, as is apparent from Figure
3(a). Then the framework is placed on the planar support 14 and face member
12 is put thereon at (b). This procedure is now repeated on top of the
first element until a full pile 17 (Figure ~) has been obtained. One addit-
ional face member is placed on top of the last element to improve the contact.
When the adhesive has set sulfficiently for handling strength the elements are,
at (d) turned 180. After this any required insulation 16, vapour barrier
etc. are inserted at (e). Adhesive is applied to the other side of the
crossbars (f) and the other face member 13 is placed thereon ( ~ . The pro-
cedure is repeated until a full pile of finished elements has been obtained
(h)-
1o38~4l!~
If the elements are intended for useas wall panels they can bepainted and papered directly after curing. Doors, windows, and cupboards
can also be mounted which minimimizes the site work .
Ex~mple 1
A framework of particle board studs sawn from a 19 mm particle board
was held together in a jig and was placed on a 10 mm particle board placed
on a planar support. Both edges of the studs were coated with an adhesive
layer of min. 0,5 mm thickness of a thermosetting resin of resorcinol type
with joint filling properties after which another particle board was put
on the upper side of the framework so that a panel element was formed.
The a & esive was left to cure without any other applied pressure
than the dead weight of the uppermost particle board. The composition of
the adhesive used was as follows:
Resorcinol resin (dry content about 60 %) 100 parts by weight
Formalin Solution (37 %) 25 " n ~,
Cocoanut shell flour 25
In the same procedure when the face material was slightly bent or
warped so that no satisfactory contact was obtained at the joint, a light
fixing pressure was applied which did not exceed 0.2 kp/cm surface unit of
framework.
Example 2
A framework of particle board studs placed on edge were put together
in a fixture and connccted by means of a few staples in one instance and by
a hot melt adhe~ive in another. An insulating material was placed in the
framework. A joint filling resorcinol adhesive according to Example 1
having the following composition was applied to the edges of the framework:
- Resorcinol-phenol resin (dry content about 70 %) 100 parts by weight
Formalin solution (37 %) 25 " " "
Cocoanut shell Flour 25 " tt t~
A particle board was placed on the edges coated with adhesive and
103~14~
attached with a few braces. The construction was turned upside down and
adhesive beads were applied on the opposite edges of the framework and
another particle board was put thereon. A number of elements thus finished
were placed on each other and stored for 3 hours, after which the adhesive
had obtained such a strength that the elements could be handled. The shear-
ing strength was min. lS kp/cm2.
Example 3
Elements were made in the same way as in example 2 with the only
difference that the particle boards were pre-heated on a heating plate of
140~ for 2 min. before their combination with the framework. The shearing
strength was determined after different periods. The following results were
obtained:
3 min. after combination 2.5 kp/cm2
5 It tt tt 6 "
10 tt ~ 13 tt ~' -
20 " tt tt 23 tt
Due to the preheating the hardening of the adhesive was accelerated
and consequently an extra rapid handling of the glued panel elements was
obtained.
After less than 5 min. handling strength had thus been obtained and
after 10 min. the strength was on a level with what is required for the
finished construction.
When detenmining the shearing strength ruptures were always obtained
in the particle board and not in the adhesive joint, in spite of the fact
that no pressure was used during hardening of the adhesive. The contact
between the particle boards and the framework was perfect in the joint thanks
to the fact that a planar material of even thickness had been used in com-
bination with a joint filling adhesive.
Exanple 4
A series of tests was carried out on a laboratory scale with differ-
10 .
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~038~48
ent combinations of material in bonding of panel elements of the si~e
100 x 50 x 10 cm, different face materials, framework materials and dimens-
ions being used. As face material different types of plywood, fibre board
and particle board were used and as framework material the same materials
and, additionally, wood. The thickness of the materials varied from 10-20 mm.
A joint filling adhesive of the type resorcinol-phenol having the following
composition was used:
Resorcinol-phenol resin (dry content about 50 ~) 100 Parts of Weight
Fornalin solution (37 %) 25 " " ~
Cocoanut shell flour 21 " " "
p~ 7,5 - 8,0
No extra pressure in excess of the dead weight of the boards was
used. The test results are indicated in the enclosed table. The results
show unambiguously that face materials having a relatively low modulus of
bending elasticity, i.e. not more than 120,000 kp/cm and preferably below
50,000, are required to carry out the method successfully at no pressure or
low pressures ( ~ 0.2 kp/cm2). This means that the method can be carried out
with particle boards and fibre board and even with plywood having a low
modulus of bending elasticity. As framework particle boards, board and
plywood can be used.
ExamPle S
Stressed skin panels, well insulated, measuring 8 x 2.5 m and intended
for outer walls were made in the following way. As face material structural
particle board measuring 8 x 2.5 m and with thickness 12 mm was used. The
framework was made of studslof 16 cm width and of different lengths from the
same particle board. The same joint filling adhesive was used as in example
4. As insulation material glass fiber wool of the thickness 16.4 cm was used.
The construction is shown in Figure 1. The framework was 20 mm inside the
outer edge~ of the faces and the distance between the framework studs was
11.
, ' :. ~" .
1038148
50 cm.
A particle board was laid on a planar, vertically adjustable table
and the relatively soft board material lay wholly close to the table. In
order to be able to place a framework according to Figure 1 on the bottom
board without complicated mechanical equipment, pieces 4 of particle board
measuring 2 x 2 x 1.5 cm were nailed to the board according to Figure 2 in
a pattern agreeing with the framework to be built. The distance between
adjacent, pairs of pieces 4 was about 80 cm and the distance between the
pieces within a pair was fully 12 mm. (The tolerance of the width of the studs
was + 0.1 mm). Before assembly an approx. 5 mm high bead of the joint
filling adhesive was put on one edge of the studs after which the studs were
mounted in position on the lower board. In the end positions the studs were
fixed to each other by means of staples, which were applied by means of a gun
at a distance of about 2 cm from the upper and lower edges of the framework.
The support was planar, the face material was relatively soft and the cross-
bar studs had good stable dimensions, which gave a very good contact of the
framework with the bottom board. Nowhere could a joint gap of more than
1.5 mm be measured.
Strips of particle board measuring 20 x 12 x 490 mm were placed
between the transverse cross bars according to position 3 in Figure 1 in order
that cross bars 1 could not be laterally displaced. Glass fiber wool and
vapour barrier were inserted into the box structure thus created. Beads of
the same adhesive were spread on the upper edges of the framework, after
which the upper particle board was placed in position and kept there by means
of a few particle board pieces of above-mentioned type.
On top of the element thus finished an additional five elements
were made, a pile of six elements placed upon each other thus was obtained.
In order to obtain a good contact of the uppenmost board in the pile a part-
icle board of 22 mm of the same measurements was placed on top of the pile.
In spite of the practically non-existent pressure and the big fonmat a very
12
: ' ~. ' ~ '. :
,~
103l~148
good contact in all adhesive joints of the pile was obtained thanks to the
planar support, the low and uniform stiffness of the framework and its stable
dimensions and no joint gaps greater than 1.2 mm could be measured The
pile was left for about 2-3 hours to harden. After this time absolutely
fixed, non-creeping and weather-proof joints were obtained. By the coaction
of the adhesive joints with the face members a stiff and tight construction
was obtained, the length deviation of which amounted to max. + 2 mm The
face members of the elements remained quite intact and after mounting of
windows no special working in the form of puttying etc. before painting or
papering was required.
Samples were taken from the bonded panels and the shearing strength
of the adhesive layers was measured to be 60 kp/cm2 on an average. The con-
struction load on such a joint amounts to max. 15 kp/cm2. A wall element
was studied in detail and after total brnak down the joint showed more than
90 % fiber rupture. The surprisingly good bonding result must also be
ascribed to the good wetting and joint filling properties of the adhesive
and to the fact that the special combination of hard surface and board edge
is favourable to adhesive even with the very low pressures used in this case.
As adhesive for stressed skin elements a setting resin type is
necessary since the finished joint must not creep. If a joint creeps the
adhesive is as a rule too soft, and there is a risk that the strength of the
adhesive joint will be reduced, especially if the construction is loaded.
As reliable adhesives for wood usually thermosetting resins based on form-
aldehyde are used, which, however, normally require considerable pressures
to provide perfect glueing. In this case a joint filling resorcinol-phenol
adhesive was selected.
,
-` 10381~
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