This invention concerns fire resistant construction elements according to the introductory of claim 1.

Construction elements of the plaster/fiber type, e.g. plaster and wood, will under the influence of for example fire result in the emission of crystal water from the plaster at a temperature of about 103 °C. The crystal water will naturally decrease flame development, and on occasions where the fire is localised and limited, continued flame development maybe also completely terminated. However, during stronger and persistent flame influence, e.g. in a fire which has been allowed to develop for a certain period of time, the fire resistant effect of the crystal water will only last temporarily. NO Patent Publication No. 166273, laid open to public inspection, describes fiber plates having a binder based on alkali silicate, sulphur and/or sulphurous compounds, including calcium and/or magnesium compounds, having the combined object to decrease the emission of formaldehyde, improve water resistance, strength and flame resistance. As a calcium compound, Ca(OH) ₂ , CaO and CaCl ₂ are examplified. These calcium and/or magnesia compounds are, according to paragraph two and three on page 4 of the specification present in relatively small amounts: from 2 to 24 weight percent of the plate mass. Accordingly, the fire resistance of such plate is relatively passive, i.e. the liberation of suffocating gaseous components will occur only from the optional Ca(OH) ₂ component, and, only in one step (at 580°C). The object of DE Patent No. 3.508.933 is to provide a fire protective construction plate having sufficient air and vapor permeability, the latter feature being said to be advantageous during production of the plate. This construction plate is a laminate structure having a plaster core, optionally containing fibers, and two layers of glass fiber with the core located therebetween. On the outer surface of one of the glass fiber layers, a layer of fine grained inorganic material is formed, e.g. plaster, dolomite and magnesite. The inorganic components will, among other things, serve as a fire barrier by emitting water and carbon dioxide during a fire. However,

because of this thin surface layer, the active fire resistance is limited, and the plate structure is relatively complex.

The object of the present invention is to provide a construction element having as much intrinsic and active fire resistance as possible, without loss of strength properties of the plate.

Another object is to provide fire protection in such construction elements that actively resists the influence of heat and flames in a broad temperature range, even at temperatures up to 900°C. This object is achieved with a construction element according to the characterizing part of patent claim 1. Further features appear from the dependent claims.

Construction elements of plaster and a fibrous material, e.g. wood fibers, are in accordance with the present invention provided with active fire retarding properties by adding the mineral dolomite during production of the elements. The term "active fire retarding" means the element emits gaseous non-combustible components under the effect of heat. The fire protective properties are substiantially achieved from the carbonate component of the mineral, which component is liberated in the form of carbon dioxide gas. The chemical composition of dolomite is CaMg(CO ₃ ) ₂ , i.e. calcite CaCO ₃ and magnesite MgCO ₃ , and the mineral frequently contains brucite Mg(OH) ₂ . Dolomite having from 17 to 20 weight percent of brucite is particularly preferred in connection with the present invention. Under the influence of heat, the magnesite component will evolve carbon dioxide at a temperature of about 300 °C; the vapor pressure of carbon dioxide from magnesite is 1 atmosphere at about 353 °C. The calcite component in dolomite will evolve carbon dioxide in the temperature range of 700°C to 900°C. If the dolomite also contains brucite, the brucite component will initiate the formation of free water at a temperature of from 350°C to 450°C. In this way, the dolomite mineral added will evolve gaseous carbon dioxide and optionally water vapor that prevents further supply of oxygen to the combustion. This active fire protection mechanism will, as mentioned above, occur in different temperature ranges, in which one component after another takes over as the temperature rises. In addition to the fire resistant properties, construction elements of such type having a content of dolomite, will exhibit improved mechanical properties such as improved modulus of elasticity and tensile strength.

In this connection, fibrous material is meant to comprise fiber preferably from wood, such as chips, splinter, wood fiber, paper fiber, and the like. However, other fibrous materials can be used, such as plastic fiber, nutshells, and so on. The term construction element is meant to comprise plates, different furnishing components, such as door panels, door frames, armrests on chairs, and the like.

The emission of water and carbon dioxide from the different components occur in accordance with the following reaction scheme:

CaSO ₄ -2H ₂ O > CaSO ₄ + 2H ₂ O

MgCO ₃ > MgO + CO ₂ Mg(OH) ₂ > MgO + H ₂ O

CaCO ₃ > CaO + CO ₂

If the amount of fiber increases outside the specified limit, the construction element may absorb water more easily and swell, a property that is undesirable for a construction material of this type. On the other hand, if the amount of plaster increases outside the specified limit, the solidity and strength properties of the construction element will decrease. With respect to fire protective properties, we have found that the content of dolimite must be at least about 10 weight percent with respect to the plate product. Should the dolomite content exceed about 50 weight percent, the plate strength properties will decrease. Accordingly, we have found that the dolomite content in a finished construction plate should be between 10 and 50 weight percent with respect to the amount of calcined plaster, whereas the amount of plaster constitutes the remaining 75 to 35 weight percent. In a preferred embodiment having regard to fire resistance and solidity and strength properties, dolomite constitutes 20 weight percent, whereas the plaster content constitutes about 65 weight percent, the remaining comprising substantially fibers. The construction element according to the invention is homogenous in the sense of being uniformly distributed with respect to each other.

In the production of a construction plate, crude plaster (CaSO ₄ -2H ₂ O) and crude dolomite, substantially CaMg(CO ₃ ) ₂ , are treated in a mill, in which the components are mixed and ground to form a powdery mass. The dolomite particle size should be as small as possible to provide the best possible dispersion of the mineral in the finished plate. In practice, we have found that a suitable particle size is about 20 μm

or less, and a particle size of about 10 μm is preferred, but the invention is not dependent on such particle sizes. However, should the dolomite particles become too large, there may be a risk of the construction element having a coarse grained surface on the construction element, whereupon both fire protection and strength properties are decreased. The basic rule is: the less dolomite particle size, the better strength properties. Thus, it is important to perform the mixing step described above thorougly. The mixture of plaster and dolomite is then transported to a calcination unit having a temperature of about 150 °C. In this calcination unit, the di-hydrate is converted to semi hydrate, i.e. the amount of crystal water is decreased from 20% to about 5%. However, the dolomite does not become calcined, but any humidity present is removed. This treatment is performed to put the plaster in a condition to better bind water in a later process step. Water and wood fiber in a desired amount is then added to the calcined powdery mixture comprising plaster and dolomite. The amount of water added should theoretically be 15% of the plaster mass to form plaster di-hydrate, but a certain excess of water must be added to fulfil the formation of di-hydrate. The mixture is then spread upon a steel plate, which is inserted in a press. The pressed plate is thereafter dried to a resultant water content of 1-2 weight%. The mixture is preferably provided with a retarder, e.g. amphotheric surfactants, to retard the curing of the plaster, whereupon the surfactants prevent the water from binding to the plaster, resulting in a prolonged formation of the di- Φ hydrate. Retardan , a calcium salt of an amino acid, is a preferred retarder that is added to the water in an amount of about 0.09 weight percent with respect to the - plaster mass. Corresponding methods of production can also be used for the production of other elements, as examplified above, but in which the pressing step is selected dependent on the end requirements.

If no retarder is added, the plaster will react immediately, and the curing will occur prior to the mass leaving the mixing unit. Dolomite will serve as an accellerator for the plaster. Accordingly, it is necessary to increase the amount of retarder when adding dolomite. Crude plaster can also serve as an accellerator in the process, which is used in the plaster cardboard industry. Other retarders can theoretically be used, such as tri-natrium citrate, but they will after a certain period of time precipitate as salts at the plate surface as long as the plate is not covered

with cardboard.

The following Tables 1 and 2 illustrate typical mutual mixing quantities and composition of ordinary plaster/wood fiber plates of similar type with added dolomite.

Table 1 Composition of ordinary plates and plates comprising 10% dolomite

Table 2 Composition of plates added 10% and 20% dolomite

A mixture of 1145 kg will result in about 0.7 m ³ net plate, whereas a mixture of 1106 kg will result in about 0.5 m ³ net plate.

In order to illustrate the different steps and the contributions from the plaster and dolomite components, a summary of fire progress with a 100 kg construction element having a content of 10% dolomite, of which 2 kg brucite, 3.7 kg magnesite and 4.3 kg calcite; 75 kg plaster, 13 kg dry chips and 2 kg free humidity:

- At 120°C, an evaporation of crystal water and free humidity will occur, as is also the case for ordinary construction plates without dolomite, and in this case, the amount will constitute 15 kg water from the plaster and 2 kg free humidity. - At about 300°C, the magnesite component will initiate emission of carbon dioxide, in a total quantity of 1.94 kg.

- In the temperature range from 350°C to 450°C, the brucite component starts emitting carbon dioxide, in a total quantity of 0.62 kg.

- In the temperature range from 700°C to 900°C, the calcite component will emit a total of 1.89 kg carbon dioxide gas.

According to the above mentioned, construction elements according to the present invention exhibit a total of four active fire retarding phases, of which two are water evaporation phases and two are CO^ evaporation phases, which is contrary to ordinary elements of plaster and wood fiber which exhibit only one such phase. The quantity of CO ₂ emitted will naturally correspondingly increase if the dolomite content in the construction element is increased.

Example 1

This example illustrates the improved strength properties achieved in a plate comprising plaster and wood fiber by the addition of dolomite. Six different plates having a content of 10 weight percent dolomite, 18 weight percent wood and the

® remainder plaster and Retardan , of which three had a plate thickness of 8 mm and the remaining about 7 mm, were subjected to simple tests with respect to flexure strength, modulus of elasticity and tensile strength. As can be seen from the following tables, all specimens exhibit significantly improved strength, except from flexure strength, compared with ordinary plaster/wood fiber plates without dolomite.

Table 3 Simple test of plates having 10% dolomite and 2.4% free humidity, with respect to strength properties.

Tensile

Thickness Weight strength

Plate No. (mm) _?. (N/mm ³ )

Table 4 Simple test of plates having 10% dolomite and 0.9% free humidity, with respect to strength properties.

Modulus

Flexure of Fracture Tensile

Thickness Weight Density strength elastisity stress strength

Plate No. (mm) (g) (kg/m ³ ) (N/mm ² ) (N/mm ² ) (N) (N/mm ³ )

The above results show that by adding dolomite to construction elements of the plaster/wood fiber type, active fire protection is achieved in a wide temperature range for a construction element having excellent strength properties.