CA1038148A

CA1038148A - Method of manufacturing skin stressed building elements

Info

Publication number: CA1038148A
Application number: CA245,772A
Authority: CA
Inventors: Ove H. Mattsson; Evert W. Perciwall
Original assignee: Casco AB
Current assignee: Casco AB
Priority date: 1975-02-21
Filing date: 1976-02-16
Publication date: 1978-09-12
Also published as: FI760436A; NO146628C; DE2606924A1; HU178483B; DD125192A5; PL111434B1; FI57640B; SE405268B; NO146628B; AU1088476A; NO760535L; DK73176A; DE2606924B2; FI57640C; SE7502001L; DE2606924C3; GB1536329A; CS191292B2; JPS5234909A; AU497143B2

Abstract

ABSTRACT OF THE DISCLOSURE

A method of manufacturing stressed skin panel elements for use in building construction is disclosed. A framework of wood-based fibre or composite materials is made up and held in an assembled condition while a joint-filling settable adhesive is applied to one side. A face member, also of a wood-based fibre or composite material such as particle wood, and exhibiting semi-rigid properties, is placed on the frame work. In alternative procedures either the framework or the face member is supported on a planar support during the setting of the adhesive. During setting of the adhesive the joints are subjected to a pressure that does not substantially exceed that corresponding to the dead weight of the parts or the weight of several elements piled on each other.

Description

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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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 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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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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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 .

- .:-~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.

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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 : ' ~. ' ~ '. :
,~

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.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the manufacture of stressed skin building elements, pre-ferably building panel elements, consisting of a framework with a face mater-ial attached to at least one side thereof by an adhesive, characterized in that the adhesive is a joint filling, setting adhesive, that a substantially non-warping material is used for the framework, that the face material is semi-rigid and that the framework and the face material are put together on a planar support and bonded under a pressure which does not substantially exceed that corresponding to the dead weight of the parts or the weight of several building elements piled on each other during manufacture.

2. The method of claim 1, characterized in that the pressure on the joint surface does not exceed 2,5 kp/cm2.

3. The method of claim 2, characterized in that the pressure does not exceed 0,2 kp/cm2.

4. The method of claim 1, characterized in that the framework is placed on the plane support, that adhesive is applied and that the board is then placed on the framework.

5. The method of claim 4, characterized in that a second framework is placed on top of the board thus glued and a second board is glued to the second framework after which the method is possibly repeated again until a pile has been obtained.

6. The method of claim 4 or 5, characterized in that the element or elements, after enough setting of the adhesive used to make the elements handable, is turned upside down that adhesive is applied to the joint sur-faces on the other side of the element and that a board is glued also to this side of the element or elements.