CA2016321A1 - Process for making low density gypsum products - Google Patents

Abstract

ABSTRACT

A peroxide compound, such as Hydrogen Peroxide, is added to an aqueous mixture containing plaster, and a suitable catalyst which decomposes the peroxide into free oxygen is added before the mixture has been cast and is allowed to set. The oxygen produced causes immediate and substantial expansion of the mixture, the volume and control of which are maintained through the use of a suitable viscosity adjusting and solids suspending agent. The product has a foam-like structure after drying and may be made to possess a density as low as 0.25 g/cc, or 25% that of water. The invention is distinguished from prior art methods by its replacement of a mechanical method for aerating plaster by a chemical method which is more immediate and effective. The method is particularly applicable to the manufacture of laminated gypsum wall-board.

Description

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SPECIFICATIONS:

This invention relates to a ~ethod for producing a light-weight gypsum or plaster product; such as a laminated wall-board, which is suitable for construction purposes.

Gypsum is mined from ancien~ sea beds and is also known as: fully hydrated Calcium Sulphate, or CaS04.2H20. To transform gypsum into a water reactive material, gypsum is calcined to remove some of the water of hydration, in order to form plaster or Plaster of Paris, which is also known as partially hydrated Calcium Sulphate, or CaS04.1/2H20. When mixed with water, plaster forms a quick-setting paste or mixture which can readily be poured, shaped or formed, which hardens on drying. Thus plaster may be used to produce a variety of molded articles such as sculptures or figurines, and in particular, a wall-board suitable for construction purposes.

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~; When plaster is mixed with water, a chemical reaction takes place ,~
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CaS04.1/2H20 + 11/2 H20 ---> CaS04.2H20 Formula Weight: 145.14 27 172.14 - 201~32:~

Or, per 100 parts plaster: 100 parts 18.6 parts 118.6parts Hence, essentially 100 parts of plaster will react with only 18.6 parts of water, known as the water of hydration. This reaction gives rise to the foxmatio~ of a homogeneous solid which may be described as crystalline, with properties ranging between anisotrophy and amorphousness, the solid being mostly amorphous.
The formation of a crystalline solid is evidenced by setting or hardening of the plaster, calcium ions (Ca++) and sulphate ions S04=) being bonded to one another in a matrix which also includes an average of two water molecules for every calcium or sulphate ion. The lattice energy of fully hydrated calcuim sulphate is low -~
compared to that of other crystalline solids, and unlike other solids which are strongly bonded, plaster castings are easily fxactured and may be reduced to powder if handled excessively. -~

Furthermore, it is also found by experience, that a mixture containing only 18.6 parts water per every 100 parts plaster cannot ~::
be usefully cast or poured; therefore there is usually added to such mixture an excess of water in order to allow the mixture to be cast, shaped, or pouredj before setting and hardening. Thus the water content of such aqueous plaster mixtures may range from say 30 parts of water to say 200 parts of water per every 100 parts of plaster, to permit workability, depending on the consistency 2~32:~

desired. The excess water added, beyond that required for hydration, is usually removed by drying at a temperature below 100 degrees Centigrade and after such mixtures have been allowed to set and to harden, to avoid calcining the material and removing the water of hydration.

It is also known that various substances may be added to plaster or to aqueous plaster, in order to affect setting and hardening times, and to achieve desirable properties in the finished products. For example, substances, including and similar in action to calcium triphosphate, or silicon dioxide may be added to dry plaster, in order to promote free flow characteristics and ease of handling in mechanical apparatii, before the addition of water to form aqueous mixtures is made. Substances such as potassium sulphate (K2S04), ammonium sulphate ([NH4]2S04), or other metal salts (NaC1), may be added in order to ac~elerate the setting time of aqueous mixtures. Similarly, substances which retard the rate of setting are known and employed. Detergents may be added to promote wettability. Fillers may be added to extend or to promote hardness, lateral strength, and flexibility. For example, wood flour, cellulose powder, or cellulitic materials consisting primarily of cellulose or hemi-cellulose ~ay be used as extenders.
~iatomaceous earth, calcium oxide, calcium silicates, or even talc, clays or other magnesium or alumino-silicates can also be added as extenders, or to improve hardness. Fibrous or granular materials possessing reactivity with plaster such as gluten from wheat flour, ~ 2016321 casein from milk, keratin from wool, hair or silk, etc., may be added to improve hardness, strength, and lateral stability, etc.
Water-soluble starches, proteins, plant gums, or synthetic polymers may be added to improve viscosity characteristics of aqueous mixtures, to prevent the settling of solids before hardening, and to promote the adhesion of plaster to other substances such as paper, etc. Colouring agents such as iron oxides, titanium dioxide, etc., may be added, and fungicides can be used to prevent the growth of mold on articles which have been re-wetted after drying and hardening.

Thus, a great many substances are known which can be used to improve or alter the final properties of a plaster product. The application of these substances, and their range of concentration may be readily demonstrated, and are well known to anyone s~illed in the art of making a plaster product. Particularly as relates to the manufacture of gypsum based wall-board for construction purposes for example, a type of starch is used which promotes the adhesion of plaster to paper coatings.

Now, as relates to the use of gypsum or plaster for the manufacture of wall-board for construction purposes, a description of prior art methods is hereby given as follows:

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In order to form a wall-board product, calcined gypsum or plaster is admixed with various agents as aforementioned to form a dry .~ .

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powder. To the dry powder is then added in continuous fashion, an aqueous solution, consisting primarily of water, to which other ingredients have also been added in order to improve the flow characteristics of the aqueous mixture which is formed, and also to adjust the final properties of the wall-board. For example starch is added to promote adhesion of the plaster to a paper coating which is applied after casting.

The plaster and the solution are then also mixed continuously using suitable mechanical means employing an excess of water, and the a~ueous mixture which is formed is pumped or extruded, and cast onto a continuous sheet of paper. A second continuous sheet of paper is at some point then applied to the exposed surface of the plaster sheet which has been cast in this fashion, and is similarly enduced to bond onto this surface. Thus, a laminated sheet or sandwich is formed which is allowed to set into a rigid form. The continuous shaet produced is then at some suitable point in time cut into individual sheets of such dimensions as to be suitable for construction purposes, and the sheets are finally dried to remove excess moisture without removing the water bound by hydration.

When such prior art techniques are applied to wall-board manufacture however; it is found that the products, when cast and hardened and also after drying, often possess densities which range from 50 to 60 pounds per cubic foot, or from 0.8 to 1.0 grams per cubic centimeter; or, from 80 to 100% the density of water.

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20~6321 Therefore, since the addition of any of the afore-mentioned well-known additives does not result in any significant reduction in density, and in order to obtain a lower density in the finished products after drying, there is usually employed a method of aerating the aqueous mixture before casting. Such method is usually achieved by foaming the solution to which dry plaster, containing additives is added, using a mechanical means. A suitable foaming agent such as for example, a natural substance such as starch, protein, a hydrocolloid, or a synthetic polymer is added to the solution, and air is beaten in by means of suitable agitation or using other mechanical means. When a plaster powder is mixed with such foamed solution, the resulting aqueous mixture also becomes aerated. Alternatively, the aqueous plaster mixt~re itself may be aerated by mechanical means, after the plaster powder has been mixed into the aqueous solution. In either case, suitable solids suspending agents may also be added to prevent solid particles from settling before the mixture has been cast and has set.

When the preceding techniques are applied to wall-board manufacture however, it is found that the resulting products, when dried, although possessing a lower density than non-aerated plaster products, are still relatively heavy, and may possess densities of only 40 to 50 pounds per cubic foot, or 65 to 80% that of water.

Thus, the resulting products are cumbersome and difficult to handle, as anyone skilled in the art of gypsum wall-board application will testify. Furthermore, as the application of wall-632~
9board for construction purposes generates considerable waste during cutting operations in order to frame doors and windows and to prepare close fitting wall and ceiling junctions, the disposal of such cutting waste imposes difficulties. In addition, considerable amounts of fuel energy must be expended in order to transport existing wall-board products to building sites. Finally, a wall-board product possessing a low density due to trapped air has superior heat insulating properties when compared to existing products. Thus the advantages of an aerated wall-board product possessing reduced weight and density compared to wall-board products made using prior art techniques, are obvious to anyone skilled in the art of manufacture and in the use of such products.

It is an object of this invention therefore, to provide a method of producing a gypsum wall-board product which possesses a density much reduced in comparison to that of any wall-board products made using prior art methods.

It is a further object of this invention, to provide a method of producing low density gypsum or plaster which is suitable for sculpting or molding purposes.

It will be shown that the method of this invention results in products which possess densities as low as 25% that of water, which are unobtainable using prior art methods.

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While the method of this invention also employs a technique of foaming, or a means of introducing a gas such as air into a gypsum product, the invention is distinguished from prior art methods in the manner ~y which such aeration or gasification is achieved.

More specifically, the invention employs a foaming or gassing agent which is added to an aqueous plaster mixture, and is made to decompose upon addition of a suitable catalyst thereto, therefore, the invention employs a chemical method which obviates the mechanical means employed by prior art techniques in order to produce products exhibiting reduced densities., The inventive idea which the new process em~odies is that, in place of a mechanical method of aerating plaster a chemical method is ~e ~c f~7r~c employed, wherein hydrogen peroxide~or a peroxide compound of an active metal, is added to an aqueous plaster mixture, as a foaming agent, and is caused to decompose at a controlled rate into free oxygen by means of a given concentration of a suitable catalyst.
In the case of hydrogen peroxide, the products of decomposition are oxygen and water, as shown below:

H202 -----> H20 + 1/202 34 parts 18 parts 16 parts ';~

In the case of a peroxidP compound of an active metal, the products ~

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of decomposition are oxygen and an oxide of the metal:

CaO2 ~ > CaO + 1/202 The catalyst may be the salt of a metal derived from one of the Transition Elements of the Periodic Table such as iron (Fe), or Copper (Cu), but is most preferably a catalase enzyme preparation which is known to have a strong affinity for hydrogen peroxide especially.

The action of catalase on hydrogen peroxide is one of the fastest of all known reaGtions, and 1 molecule of catalase is sufficient to effect the decomposition of almost 40,000 molecules of hydrogen peroxide per second. Hence, a relatively small quantity of catalyst is required to promote reaction. Furthermore, it will be demonstrated by means of the following mathematical example, that relatively small amounts of a foaming agent such as hydrogen peroxide are required to produce a substantial reduction in density of the product.

If, for example, 1 mole or 145 grams of plaster, requiring 27 grams water of hydration were to be dispersed in 145 grams of water to give a mixture possessing a volume of say 250ml, and therefore a density of 1.16 g/cc, in order to produce a two-fold expansion of the mixture to say 500 ml in volume, it would be necessary to add 250 ml or 0.25 litre of a gas such as oxygen to the mixture. The .,. : ~ : -~;, ~ , , -.

number of moles of oxygen required may therefore be approximated using the Ideal Gas Law:

n = PV = l.0 atm. x 0.25 litre _ _ RT 0.08206 (gas constant) x 293 degrees K (room temperature) = 0.0104 moles Since 1 molecule of oxygen is evolved from 2 molecules of hydrogen peroxide, the quantity of hydrogen peroxide required to obtain the desired expansion is : 2 x 0.0104, or 0.0208 moles, or approximately 0.7 grams.
Hence, the ratio of hydrogen peroxide to plaster to be employed in the above example is approximately 1 part for every 200 parts of plaster.
Furthermore, if the initial density of the mixture were only 1.16 g/cc according to the preceding exa~ple, then after expansion, the mixture would possess a density of only half as much, or 0.58 g/cc.
If the excess water, which is given by: (145-27) = 118 grams, were to be removed by drying/ the density of the dried product would be further reduced since there would be no volume change on drying.
Hence, the density after drying is given by:

rl45 + 27! = 0.344 g/cc, or 34.4% the density of water.

Therefore, it can be shown by means of example, that a relatively small quantity of hydrogen peroxide is necessary to produce a significant change in the density of the product. The densities obtained are thus seen to be less than those obtained by prior art methods. Furthermore, since the decomposition of hydrogen peroxide . :, . . : ,. , :

2~ i ~321 by catalase is a rate dependant reaction, according to the quantity of enzyme added, the reaction may be controlled by adjusting the amount of catalase enzyme added to the mixture, as well as the quantity of peroxide employed.

Hence, it has been illustrated by means of the preceding example, that the process of this invention is practical, and is economically viable by virtue of the small quantities of foaming agent and catalyst required to achieve desirable properties in the final products, and also since the quantity of plaster required to prodllce a given volume of product, is considerably reduced.
Furthermore, hydrogen peroxide is found to ~e unreactive with many common ingredients used to make plaster products as afore-mentioned, and with plaster itself, and being available in a liquid and aqueous form, and therefore more easily employable, is therefore a preferred foaming agent for the process.

It has been found in practice, that a peroxide compound may be added;either d1rectly to the aqueous solution to which the plaster is later added, or directly to the aqueous plaster mixture itself.

It is also found that when a catalyst, particularly a catalase i~ enzyme, is added to an aqueous mixture containing plaster and hydrogen peroxide for example, that an immediate and substantial foaming takes place and that the mixture expands so as to increase considerably in volume. Furthermore, in order to maintain the 2~321 .

volume of the foamed mixture before the plaster has reacted with water of hydration and has set, it is found that a variety of su~stances which have the property of increasing the viscosity of water may be used for this purpose. For example, various starch compounds, proteins, or hydrocolloids may be used for this purpose.
In particular, it i5 found that cellulose compounds such as methyl cellulose, or hydroxy propyl methyl cellulose for example, may be used for this purpose Furthermore, it is also found that the use of many of these compounds also prevents solid particulates in the aqueous plaster mixtures from settling out before the plaster itself has set -these solids are plaster itself, or other modifying agents which have been added and are insolu~le in water. The types of additives which may ~e used to maintain the shape and volume of expanded mixtures and to keep solids in suspension are well known to anyone skilled in the art, and are not a su~ject matter of this invention, except as applies to their use.

Finally, it is found that after such aqueous plaster mixtures have been foamed, accordin~ to the process of this invention, they may ~e cast in a manner similar to that descri~ed in the prior art, and may for example be used to form a wall-board product. Foamed Plaster castings are allowed to set and are cut, dried and hardened in the ordinary manner. After drying, it is found that the products made according to the process of this invention have a foam-li~e 2~:~632~

structure, and possess densities as low as 0.25 g/cc or 25% that of water, and are therefore lower in density than existing gypsum products, and are hence considerably reduced in weight when compared to existing products of eqwivalent volume. The oxygen used to create such f~amed products is gradually replaced ~y am~ient Nitrogen from the atmosphere during casting and dryin~ operations, so that the products may be said to bei aerated. It is also found that such products are easy to cut and to handle, and do not conduct heat as well as plaster which has been cast using a prior art method. Furthermore, it can be demo:nstrated that foamed products which have been laminated with paper so as to form a wall-:board type construction, :may be fastened to a suitable backing without brea~ing or shattering.

It can be seen from the disclosure that anyone skilled in the art should be capa~le of reproducing the pr~cess of this invention, and that a variety of additives and a range of concentrations of foaming agent and catalyst as above descri~ed may ~e used to produce products which are considerably improved over those ~ade ac~ording tn prinr art ~ethnds The Lnvent-in-n ;6 fllrther illu~trateA hy mean~ of pratctical examnle6 which follnw:
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A d:ry powder mixture w~ prepared i.:; .: , i- . .

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Plaster of Paris - 94 parts Powdered cellulose - 5 parts Potassium Sulphate - 1 part 100 parts An aqueous solution of hydroxy propyl methyl-cellulose was prepared by dissolving 0.9 parts of the gum in 200 parts of water. To this solution was added 25 parts of a 3% hydrogen peroxide solution to give 226 parts in total.
The plaster mixture was then added to the solution and suspended with rapid stirring.
To the aqueous plaster mixture was then added 0.3 parts of a catalase enzyme preparation.
Upon addition of the enzyme, immediate and substantial foaming of the mixture was o~served to occur, and the mixture was noticed to expand in volume. A weight/volume measurement taken at this point, determined that the density of the expanded mixture was 0.78 g/ml.
The foamed mixture was cast in the shape of a flat sheet of approximately 1/2" thickness and allowed to set. The product was stripped from the mold and dried at an ambient temperature of 60 degrees centigrade. After drying~ the product was ~ound to possess a density of only 0.252 g/ml, but was hard and cowld be e~ily h~rlr~

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20~321 Plaster of Paris - 99 Potassium Sulphate - 1 100 parts The above was added to a solution consisting of 1.2 ~arts gum added to 150 parts water and 15 ~parts 3% hydrogen peroxide solution, totalling 166.2 parts.

0.~ parts of a catalase enzyme preparation was added to the aqueous mixture and the mixture was observed to expand as in example A, and had a density of 0.82 g/ml.

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The product was cast and dried in a manner similar to A, and was ~ ~ found to possess a density of only 0.326 g/ml.

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EXAMPLE C: - Laminated product , :~

Plaster of Paris - 99 i 3 ~ ~

Potassium Sulphate - 1 100 parts The dry mixture was added to 1.0 parts of gum dissolved in 100 parts water and 5 parts of 3% hydrogen peroxide solution, totalling 106 parts.

0.15 parts of a catalase enzyme preparation when added produced immediate expansion. The product was cast onto a sheet of paper, to form a layer approximately 1/2" thick. A second sheet of paper was applied to the upper surface of this layer, and the casting was allowed to set, and finally dried at 60 degrees Centigrade.

The final product had a density of 0.35 g/ml, or 22 lb/ft3 and was nailed onto a piece of wood without shattering or breaking.

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Claims (8)

1) A process for producing a low density gypsum product by means of adding a peroxide compound to an aqueous plaster mixture, containing plaster, plaster of Paris, or calcium sulphate partial hydrate, and effecting decomposition of the peroxide into oxygen by means of a suitable catalyst, before casting the mixture and allowing it to set.

2) A product of the process according to claim 1).

3) A process according to 1), wherein the product is a gypsum based wall-board.

4) A process according to 1), wherein the peroxide compound is hydrogen peroxide, or ureaperoxide.

5) A process according to claim 1), wherein, the peroxide compound is a peroxide of a metal element, chosen from one of the active elements, comprising columns I A, IIA, of the Periodic Table of Elements.

6) A process according to 1), wherein, the catalyst is a preparation of a catalase enzyme.

7) A process according to 1), wherein the catalyst is a salt of a heavy metal element, chosen from one of the transition elements of the Periodic Table of Elements.

8) A process according to 1), wherein a viscosity adjusting or solids suspending agent is also added, to maintain the shape and volume of the mixture before setting and hardening.