CA1123722A - Multi-layer board and process of preparing the same - Google Patents

Abstract

Schulze 6 MULTI-LAYER BOARD AND PROCESS OF PREPARING THE SAME

ABSTRACT OF THE DISCLOSURE

A multi-layer board comprises a core layer con-sisting of a cementitious binder material and a cover layer consisting of a fiber-reinforced material of plastics or the binder material containing plastics. The flexural resistance of the layers is adjusted to a desired mechanical property of the board and the layers have interfaces connected in a force-transmitting manner.

Description

~ ~z,3~7~

(1) The uresent invcntion relates to a multi-laycr board and a process of preparing the same.
Multi-layer boards comprised of laminated layers are know. U.S. Paten-t No. 2,806,811, for e~ample, describes a paper-covered gypsum board wherein the paper layers are bonded to the gypsum board with a resin adhesive.
Laminated boards taking advantage of the good qualities of inorganic materials used for the core layer and organic plastics for the cover layer or layers are very useful.
~owever, considerable difficulties have been encountered in providing a true and lasting bond between layers of such different materials since not only the adhesion but also the mechanical properties of the materials cause problems.
Glueing or bonding with adhesive does not provide strong enough lamination.
Attempts to bond the two layers to each other while their materials were still wet and the layers were, there-fore, in a plastic condition failed because when an organic plastics layer was cast on a fresh core layer of concrete or a like water-containing cementitious binder material, which was not yet hardened, a water layer formed between the layers. Similar disadvantages are found when a material in a plastic condition was applied to a hardened layer.
It is the primary object of this invention to provide a multi-layer board and a process of preparing the same, in which a cementitious material core layer and at least one cover layer of organic plastics are combined while ~T
.

.

i~.Z37~2

(2) I assuring strong adhesion therebetween so that the~e is no ¦ danger of the cover l~yer separating fro~ the core layer when I the multilayer board is in use, i.e. to connect the ¦ interfaces of the layers in a force-transmitting manner, the ! 5 board constituting an integral structure as ~ar as its j m~chanical characteristics are concerned.
The above and other objects of one aspect of the present invention are accomplished with a multi-layer board compris-~ ing a core layer consisting at least primarily of a cement-! ` lo itious binder material, and at least one cover layer consist-ing of a fiber-reinforced material selected from the group consisting of plastics and the binder material containing plastics, the flexural stiffness factor of each of the - layers being calculated to a desired mechanical on the basis 15 of the board on the basis of the following equation:
E bc3 + 2E ~ bl2t + bt (-2-) wherein SB is the flexural stiffness of the multi-layer board, E is the modulus of elasticity, index c signifies the core layer, c is the thickness of the core layer, b is the j width of-the structural part, index t signifies the cover i layer(s), and t is the thickness of the cover layer, and ¦ the layers having interfaces connected in a force-transmit-ting manner. The cementitious binder material is illustrated 25 by such materials as cement, lime, gypsum, magnesite, and/or mixtures of magnesia and magnesium chloride, and may be fiber-reinforced. The force-transmitted connection between the layers is pro~ided by the structure of the interfaces, .. . . ... . . . . . .

(3) ~'~ 2~7Z~
by fibers projecting therefrom into the adjclcent interface, by cllemical or vander Waals forces, or by an intermediate layer connecting the interfaces of the core layer and the cover layer, the intermediate layer consisting of at least one hydrophilic natural or synthetic resin and/or the cementitious binder material containing plastics.
In a preferred embodiment, the board comprises another one of the cover layers, the core layer being disposed between the two cover layers.
According to another aspect of this invention, the multi-layer board is prepared by forming a core layer con-sisting at least primarily of a cementitious binder material, adjacently forming a cover layer of a fiber-reinforced material selected from the group consisting of plastics and the binder material containing plastics, superposing the layers while the material of at least one layer is still wet, and permitting the wet material to harden at or above room temperature until the layers are connected in a force-transmitting manner. The material of one layer may be wet and the other rigid, or the materials of both layers may be wet. The wet material may be premitted to harden to an elevated temperature, such as under infrared radiation of in a furnace. The concept of contacting the layers while we-t means that the layer materials have not yet hardened. The concept of contacting a dry or rigid layer with a wet layer means that the material of a layer which has not yet hardened is applied to a layer of hardened material.
The true connection of the layers is provided by an

(4) ~l~.Z37~Z
interface between the layers in which the layers actually grow into each other or are mechanically locked to each other, which is meant by the layers having interfaces connected in a force-transmitted manner. In other words, they form an integral multi-layered structure and no bond-ing agent is used to adhere the layers to each other.
Rather, the connection between the layers is established by making use of chemical bonding forces, such as van der Waals forces, created by the materials of the layers and/or the interlocking provided by projecting fibers forming bridges between the layers. By suitably adjusting the flexural stiffness of the board according to the equation, the elongation, the modulus of elasticity and other mech-anical properties of the individual layers, a substantial-ly tension-free product is obtained which constitutes a very advantageous laminated plate combining the advantages ; of inorganic materials with those of organic plastics.
The core layer has a high rigidity or stiffness and there is no danger of aging since progressive hydration of the aementitious binder material will actually improve it.
- Subsequent shrinkage and cracks caused thereby are avoided and there is no decrease in rigidity because the evaporation of the water is impeded.
The core has a resistance to deformation which imparts to the multi-layer board the characteristics of a single-layer board, or to a pipe made therefrom, that of homo-geneous layer pipe. The cover layers are also load-carrying ,: .
X

, ' , ' , :, .

t5) ~ 7.22 and contribute to the hicJh quality of the board and enhance it. I~eretofore, tlO sheet material of this type was known which was so well adapted for the production of round bodies and imparted to them the required ri~idity. seing resistant

5 to abrasion and wear, for instance by corrosion, the cover layers operate as protective layers and simultaneously provide a desired surface configuration.
They also absorb maxima of tensile forces. Using an - intermediate layer will remove difficulties arising from 10 surface irregularities at the interfaces. An intermediate ~ layer transmits shearing forces, prevents the spreading i of cracks and imparts great stability to the multi-board.
It is a connecting layer constituting an advanlageous transition between the core an~ cover layers.
In contr~st to the cover layers in known multi-layer boards, the cover layers in the boards of the invention are load-carrying layers, whereby the load-carrying capabil-ity of the board is increased. Since the board is a sub-~^ stantially integral structure, it combines this mechanical 20 advantage with the advantages obtained by the use of ~ inorganic and organic structural materials.
; The multi-layer board of the present invention is far superior in its mechanical properties to known laminates of this type and will find particular application in the " 25 construction industry, in water pipe systems, and in build-ing tanks or like containers.
The binder material of the core layer may be cement, .,~

, (~) particularly a fast-hardenincJ type of cement, or a magnesite type material, or a mixture of magnesia and magnesium chloride. Other cementitious binder materials useful for the board of the invention are burnt lime and gypsum or liquid silicates which may be rapidly hardened by the admixture of suitable additives. If desired, the cementitious binder material may contain fillers of natural or synthetic substances, such as sand, slag, bau~ite or corundum and the like, as well as fibers of inorganic substances, including glass fibers, asbestos fibers and/or other mineral or metallic fibers, and fibers of organic substances. It is also possible to add to the cementitious binder material hydrophilic, self-hardening plastics, such as a vinyl ester resin, a phenolic resin and/or other 15 plastics with or without suitable curing agents. Such plastics additions to the cementitious binder material will improve the extensibility of the inorganic binder material matrix. Depending on the final purpose of the finished board and the desired characteristics thereof, the matrix 20 may consist of plastics and the cementitious binder material may be added thereto.
The cover layer comprises strong plastics and operates not only as a protective layer but forms, in fact, an integral part of the core layer in the multi-layer board, 25 the cover layer reinforcing the core at its weak points, in addition to being resistant to abrasion, wear and cor-rosion, as well as being capable to impart a desired surface configuration to the board.

i~a , :

' (7) ~-2372Z
The ma-terials fornling the cover layer may contain the same matrix material as thc cover layer. The matrix of the cover layer may consist of curable plastics which may contain inorganic fillers and/or carbon fibers. The fibers may be filamen-ts, staple fibers, fibrous webs or rovings arranged in parallel. Also, fibers may be arranged in the core in one or several superposed layers.
If desired, the cover layers may consist of several plies which differ from each other in their mechanical properties. If the core is covered on both surfaces with a cover layer, one of the cover layers may consist of plastics binder material and the other cover layer may consist of a cementitous binder material.
The favourable properties of the multi-layer board of the present invention are further enhanced by the use of intermediate layers between the core and cover layers, the intermediate layer being comprised of hydrophilic plastics which may be cold hardening or thermosetting resins of the type of natural or synthetic elastomers. The inter-mediate layer may have a thickness of about 0.1 to about5 mm or more. It may be reinforced with inorganic fibers, fabrics or webs.
Such an intermediate layer does not only add to the rigidity of the multi-layer board but it also transmits shearing forces, reduces such forces, and impedes tears and cracks, thus increasing the stability of the board and shaped structures made therefrom. The intermediate layers also operate as bonding layers which provide a favourable interface between the core and cover layers.

3~7~
(~) rhe stress in the intcrmediate layer clue to a ]oad on the board is preferably adjuste~d to the parameters of the core and cover Layers, the following tension value being desir-able: ~ ~ c x bt z wherein ~z is the stress of the intermediate layer, ~c is the stress of the core layer and ~t is the stress of the cover layer.
The calculated stress value ~z, which is given by the product ~z x Ez, is then to be adjusted so that ~z is high with respect to -the extensibilities of the core and cover layers, and Ez is small with respect to the other moduli of elasticity.
~z is the strain in the intermediate layers and E is lS the modulus of elasticity.
Obviously, the possibilities of varying the composition of the layers are quite manifold and may be suitably selected by those skilled in the art to match the desired requirements of the finished board. In any case, it is es-sential to keep in mind the desired bending resistance, modulus of elasticity, elongation and other mechanical properties. It will be possible to adjust the flexural stiffness of the several layers of the board in each case on the basis of the above equation.
As indicated, we have found that the multi-layer board may be prepared wet-on-wet, i.e. the core layer with a matrix of a cementitious binder material may not yet be hardened when it is laminated with a cover layer of plastics not yet polymerized. The process of the invention has over-~.23~
(~) come the well known di~Liculties of binding inoryanic and organic materials in their still workablc condition, so that it i5 now possible to particularly deposite orgallic lay~rs in tlle workable condition on inorganic layers in the workable condition, that is wet-on-wet and also wet-on-dry.
The accompanying drawing illustrates, by way of example, some embodiments of a multi-layer board according to the present invention.
FIG. 1 is a cross sectional view of a portion of a flat board comprised of a core faced by two cover layers.
FIG. 2 is a like cross section of such a board further comprising intermediate layers between the core and cover layers.
FIG. 3 is a like cross section showing a modification of the embodiment of FIG. 2.
FIG. 4 is a side view of a portion of a multi-layer board constructed according to any of the preceding embodi-ments but being arcuately curved and indented, rather than extended rectilinearly.
FIG. 5 is an end view of such a board shaped into a pipe.
FIGS. 6 to 9 are end views of variously shaped boards incorporating the structu~e of the embodiments of FIGS. 1, 2 or 3-The multi-layer board of FIG. 1 is comprised of core layer 1 and two cover layers 2 and 3.
The board of FIG. 2 is comprised of core layer 10, a cover layer 11 bonded to the core layer by intermediate layer 12, and another cover layer 13 bonded to the core .~

~.Z37ZZ
(10) layer by intermediate layer 14.
In the modlfication of tl~is board s}lown in ~IC. 3, the ~ore layer has two plies 20 and 22 wllerebetweel- there extended a fibrous layer 21, cover layer 24 being bonded to the t~70-ply core by intermediate layer 23 and cover layer 26 being bonded to the core by intermediate layer 25.
In FIG. 6, the board has a U-shaped section, in FIG. 7 it has a truncated V-shape with longitudinal flang~s, the board of FIG. 8 is corrugated, and FIG. 9 is a rectang-ular hollow cross section. Other shapes may obviously befabricated.
The following specific examples further illustrate the practice of this invention, all parts being by wei~ht unless otherwise indicated.
~xample 1 (Board according to FIG. 1) A flat one-square meter multi-layer board according to FIG. 1 was produced in the following manner. Core 1 was glass fiber-reinforced concrete having a thickness of 20mm and consisting of a very rapidly hardening modified Portland cement having a water cement value (ratio of water to cement) of 0.4 and containing 5%, by volume, of alkali-resistant glass staple fibers. Each cover layer 2 and 3 had the following composition:
One hundred grams of a styrene-containing vinyl ester resin (a polymerized adduct of an epoxy resin and acrylic acid dissolved in styrene, with a styrene content of 45-50~) were dissolved with two grams of 50~ methyl ethyl ketone Z 37~;?d2 (11) peroxide in a plasticizer, 0.125 g of cobalt octoate ~6% Co in styrene) and 1.2 g of dimethyl aniline (10% in styrene) being added as catalysts and activators. A glass fiber web weighing 450 g/m2was disposed in the vinyl ester resin and the fiber-reinforced layer was cured on the core after the latter has been aged for 48 hours.
The core layer was aged in a mold for 48 hours and the two cover layers were applied to the aged, hardened core layer before they were cured and were then cured in contact with the core layer.
The parameters of the layers calculated according to the above equation were as f~llows:
Modulus of elasticity of the cover layers, Et = 260,000 kp/cm2 Modulus of elasticity of the core layer, Ec = 200,000 kp/cm2 Thickness of the cover layer, t = 0.1 cm Thickness of the core layer, c = 2.00 cm Thickness of the structural part, b = 1.00 cm SB = 190,707 kp/cm EXample 2 (Board according to FIG. 2) ., .
The multi-layer board of FIG. 2 was prepared from the sam~e preformed core layer and cover layers as in Example 1, but bonded by interp~sed intermediate layers, each of said ;~ 25 intermediate layers 12 and 14 having the following composit-ion:
Hundred parts of "Beckopox" (T.M.) VEP 22 ("Beckopox"
' ' ' ~.237~2 (12) being liquid or solid epo~y resin of Farbwerke lloechst, Germany, whicll may be cured with the addition of commercial-ly available curing agents or special "Beckopox" curing agents and/or in conjunction with phenolic or amino resins at ambient or elevated temperatures), 80 parts of "Beckopox" special curing agent (also available from Farbwerke Hoechst, as a variety of modified polyamines and polyamide amines capable if imparting to the "Beckopox"
epoxy resins different curing conditions and properties of the cured product), and 10 parts of alkali-resistant glass fibers having a length of 5 to 10 mm.
The surfaces of the cover layers facing the intermediate layer were roughened before the cover material was cured.
The components of the intermediate layer were mixed and the mixture was deposited as intermediate layer 12 onto the not yet cured but roughened cover layer 11. Then the pre-formed core 10 was pressed onto the intermediate layer 12.
Then the following intermediate layer 14 was deposited between the core 10 and the cover layer 13.
Modifications of the compositions were made by replac-ing the rapidly hardening modified Portland cement by an ordinary cement to which an accelerator was added, the specific accelerating agent used being calcium chloride.
In either cement formulation, the vinyl ester resin was replaced by an unsaturated polyester or any epoxy resin.
As special curing agents, "Beckopox" VEH 29 or VEH 14 were used, as well as the aliphatic polyamine "H 105 B"
sold by Rutgerswerke Meiderich, Germany (curing period 20 to .

.
' Z37.~d2 (13) 40 hours at 25C). The "Beckopox" resins or curing agents were replaced by epoxy resins and curing agents therefor, sold by Ciba, of ~asle, Switzerland, with substantially the same results.
Any suitable thermosetting plastics containing suit-able curing agents may be used. Also, in addition to the mentioned glass fibers, it is possible to use resin-covered glass fibers, carbon fibers, graphite fibers, steel fibers or organic fibers.
The residual contents of water in the cementitious binders of the core layer had no perceptible influence on the bonding quality of the intermediate or cover layers to the core. No delamination occurred under mechanical stress and the mechanical quality of the multi-layer boards was excellent. Even higher qualities were obtained by adding to the cementitious binder material of the core layer about 5 to lO percent by weight of the plastics used in the cover or intermediate layers.
Example 3 (Board according to FIG. 2) The composition of the core layer was as follows:
100 parts of magnesia, 6 parts of a urea-formaldehyde conde-sation product, 142 parts of 20% aqueous solution of magne-sium chJoride, 0.6 parts of glycerol or butyl glycol as a plasticizer. All components were thoroughly mixed and the mixture was placed into a mold for hardening.
The composition of the cover layer was as follows:
100 parts epoxy resin "Ciba (T.M.) X20", 90 parts of "Ciba HT 907" epoxy resin curing agent, lO parts of "DY 040"

~a ~L~.Z37.~2 (14) (an accelerator sold by Ciba), 1 part o~ Cib~'s DY 062 epoxy resin accelerator, 50 parts of hydrocarbon resin E
"Lithoplast") (T.M.) and lOO parts of a glass fiber web.
The components were mixed, the mixture was molded into a plate and cured at a temperature 130C in 90 minutes.
"Lithoplast" is dark brown resin with a softening point of 100C and a melting temperature of about 120C to 140C, having a molecular weight of 1000 to 2000. It is a hydro-carbon resin of aromatic character which contains hydro-carbons condensed in a ring, direct C-C bonds, secondary and tertiary C-atoms, and 2 to 3 double bonds per molecule.
It is weakly polar.
The composition of the intermediate layer was as fol-lows: 100 parts of Ciba's hydantoin resin, 100 parts of Ciba's curing agent for hydantoin resin, 20 parts of glass fibers and 1 part of polyester fiber unwoven web (KT 1751"
of the firm Freudenberg, Weinheim, Germany).
The intermediate layer composition is poured in the liquid state over the shaped core and the formed cover layer was placed thereover, and the laminate was subjected to a temperature of 80C for two hours.
A water-soluble epoxy resin, with a suitable curing agent therefor, was used instead of the hydantoin resin with the same results.
Example 4 (Board according to FIG. 2) The core layer had the following composition:
100 parts of Portland cement ("PZ 550"), 20 parts of mineral aggregates having a maximum dimension of 2 mm, 50 parts of water, 0.06 parts of liquefier, 6 parts of zirconium glass 3 ~ Z~
(15) fibers, O.l parts o~ lO~ liquicl sodium silic~te.
~ \s is w~ nown, tlle nlineral ~gcJreclates use~l ill cerllcnt include such materials as aranaceous quartz, ~3ranite, diorite, quartz porphyry, basalt, quartzite, quart~itic sandstone, other sandstones, dense limestone, other lime-stones and blast-furnace slag, as well as mixtures thereof, in grain sizes of 0.1 to 30 mm, preerably 0.8 to 8 mm.
Such aggregate additions may also be used with advantage up to about lO~, by weight, in the cover and intermediate layer materials, fine cement also having been used as an advantageous additive in the intermediate layers.
The cover layer had the following composition:
lO0 parts of unsaturated, highly reactive polyester ("P 8"
of BASF) of medium viscosity, having a double bond value of 0.20, 0.3 parts of a cobalt acceleratox solution con-tain-ing 1% Co, 2 parts of a catalyst paste (methyl ethylketone peroxide), and lO0 parts of a roving fabric, the rovings consisting of short staple glass fibers.
The intermediate layer had the following composition:
lO0 parts of "Beckopox" VEP 22 epoxy resin, 50 parts of "Beckopox" VEH 14 curing agent, and l part of a polyester-cotton fabric, the denier of the polyester fibers being 5 to lO mm.
The multi-layer board of FIG. 2 was produced wet-on-wet from the core, intermediate and cover layers of Examples3 and 4, i.e. cover layer ll was the lowest layer and the subsequent layer was superimposed thereon in the illustrat-ed sequence.

(16) Instead of t~le "P3" polyester, we used mixtures of this resin w1th resin "E~ 200" of BASF, with the same result.
Also useful for this purpose were the alkali-resistant product "A 410" of BASF as well as such resins as "W 41"
or "W 45" of sayer Leverkusen or similar resins of Hoechst.
By preparing the laminates in the wet~on-wet process, i.e. by superimposing the layers in the given sequence before the individual layers are hardened, the mechanical properties and resistance to peeling of the board are con-siderably improved. In this connection, it has provenparticularly useful to place a polyester or polyethylene web or fabric in the intermediate layer, which contains wool or cotton fibers, i.e. fibers which will absorb the resin and produce a defined intermediate layer. Very good results 15 are obtained with three-dimensional fabrics.
The thicknesses of the layers may be freely chosen to suit the end use of the multi-layer board, practical ranges encompassing 3 to 300 mm for the core layer, 2 to 10 mm for the cover layer, and 0.5 to 2 mm for the intermediate layer.

In the wet-to-wet process, several plies of the cover layer may be applied to providing superposed plies of plastics on the core layer and/or the core itself may con-sist of a plurality of superpose plies. In the latter case, as shown in FIG. 3, a fibrous layer may be disposed central-25 ly in the core layer, which will prevent any propagation of cracks from ply to ply.
In providing multi-ply cover layers, the outer ply composition may be so selected as to make it resistant to chemical reactions and/or this ply may be mixed with sand to (17) ~37~z make the board useful in an abrasive environment. The surface layer of the outer cover layer in a multi-ply cover layer can be formed of a thermoplastic material, such as polyethylene, polypropylene, polyvinyl chloride, poly-vinylidene fluoride, or other thermoplastics or also poly-imides. For a good adhesion between the surface layer and the adjacent thermosetting resins, i.e. the resins of the cover layers ~Example 3), it is preferred to press a thin fibrous reinforcement - in the main glass fiber - into the thermoplastic material so that upon curing a strong bonding is obtained. With polyvinyl chloride a known binder may be used for applying the thin glass fiber fabric.
With such surface film or layer of the above thermo-plastics an excellent corrosion-resistant layer is obtained.
At the same time, said thermoplastics - which may have a thinkness of preferably 0.1 to about 10 mm - are useful in sealing shapes, as shown in Figures 4 to 9. As illustrated, the board may be shaped into any desired form, including tubes. They may be molded into the desired shapes at the time of manufacture and various methods may be used in pre-paring tubes or pipes, including a centrifugal method in which layer after layer is consecutely applied in a continu-ous process from nozzles supplying the compositions of the respective layers. The multi-layer tube is then cured by means of warm air or infrared radiation, at a maximum temperature of about 80C.
The tube may also be produced by winding the cover layers over the core layer which is produced on a mandrel 72~2 (1~) which receives a ribbon of the core layer composition wound about the mandrel.
~ fter the multi-layer board has been finished, it may be subjccted to desired sur:face treatments, for instance a plastic coating and/or polishing.

~2a

Claims (21)

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

1. A multi-layer load-bearing board comprising at least three layers of different composition, including (a) a basic layer comprised of a cementitious inorganic material, (b) a cover layer subjectable to mechanical forces and consisting of a fiber-reinforced organic plastic and (c) an intermediate layer of a hydrophilic resin dilutable or miscible with water, interconnecting the basic layer and cover layer in a force-transmitting manner.

2. The multi-layer load-bearing board of Claim 1, wherein the cementitious inorganic material of the basic layer comprises a binder selected from the group consisting of cement, lime, gypsum, magnesite and a mixture of magnesia and magnesium chloride.

3. The multi-layer load-bearing board of Claim 2, wherein the cementitious inorganic material of the basic layer contains aggregates.

4. The multi-layer load-bearing board of Claim 2 wherein the cementitious inorganic material of the basic layer contains fillers.

5.The multi-layer load-bearing board of Claim 1, wherein the basic layer is comprised of mortar or concrete.

6. The multi-layer load-bearing board of Claim 1, wherein the cemetitious inorganic material contains admixtures.

7. The multi-layer load-bearing board of Claim 1, wherein the cemetitious inorganic material is fibre-reinforced.

8. The multi-layer load-bearing board of Claim 7, wherein the reinforcing fiber is selected from filaments, staple fibers, fibrous mats and rovings.

(20)

9. The multi-layer load-bearing board of Claim 1, wherein the intermediate layer is fiber-reinforced.

10. The multi-layer load-bearing board of Claim 9, wherein the reinforcing fiber is selected from filaments, staple fibers, fibrous mats and rovings.

11. The multi-layer load-bearing board of Claim 1 or 9, wherein the intermediate layer contains fillers.

12. The multi-layer load-bearing board of Claim 1 or 9, wherein the intermediate layer contains minor amounts of water.

13. The multi-layer load-bearing board of Claim 1 or 9, wherein the intermediate layer contains minor amounts of cement.

14. The multi-layer load-bearing board of Claim 1, wherein the fiber-reinforced plastic of the cover layer comprises a synthetic resin.

15. The multi-layer load-bearing board of Claim 14, wherein the synthetic resin of the cover layer is a thermosetting resin.

16. The multi-layer load-bearing board of Claim 14, wherein the synthetic resin of the cover layer is a thermoplastic resin.

17. The multi-layer load-bearing board of Claim 15 or 16, wherein the cover layer contains fillers.

18. The multi-layer load-bearing board of Claim 1, wherein the reinforcing fiber of the cover layer is selected from filaments, staple fibers, fibrous mats and rovings.

19. The multi-layer load-bearing board of Claim 1, comprising two of said cover layers and two of said intermediate layers interconnecting the basic and respective ones of the cover layers in a force-transmitting manner, the basic layer being interposed between the intermediate layers.

20. The multi-layer load-bearing board of Claim 1, further comprising a coating over at least one of the cover layers.

(21)

21. The multi-layer load-bearing board of Claim 20, wherein a reinforcing material is embedded in the coating.