Alkylhydroxyalkylcellulose (AHAC) for gypsum-setting building material systems

The present invention relates to the preparation of alkylhydroxyalkylcellulose (AHAC) and the use thereof in gypsum- setting building material systems, preferably in gypsum-setting plasters and filling compounds, particular preferably in gypsum machine plaster.

The class of substances consisting of the cellulose ethers, including the group consisting of the binary alkylhydroxyalkylcelluloses with the commercially utilized members methylhydroxyethylcellulose (MHEC) and methylhydroxypropylcellulose (MHPC) , has been an academic and industrial field of activity for many decades and has been often described. A review of the chemical fundamentals and principles of preparation (preparation processes and process steps) and a material compilation and description of the properties and potential applications of the various derivatives are given, for example, in Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry] , Makromolekulare Stoffe [Macromolecular Substances] , 4th edition, volume E 20, page 2042 (1987) . The commercially utilized methylhydroxypropylcelluloses form viscous solutions in water at room temperature and are insoluble in hot water at temperatures above the flocculation point.

The preparation of alkylhydroxyalkylcelluloses can be summarized as follows: the activation of the cellulosic starting material, preferably with alkali solution, is effected in an upstream part-step. Thereafter, the alkali cellulose formed is subjected to a forced reaction with the corresponding alkylene oxide and methyl chloride, any alkali used in excess expediently being substantially neutralized with superstoichiometric amounts of methyl chloride. In the subsequent purification step, salt formed and other byproducts are separated off, preferably by washing with hot water.

The alkyl substitution is described in cellulose ether chemistry generally by the DS. The DS is the average number of substituted OH groups per anhydroglucose unit. The methyl substitution is stated, for example, as DS (methyl) or DS (M) .

Usually, the hydroxyalkyl substitution is described by the MS. The MS is the average number of moles of the etherification reagent which are bound by an ether bond per mole of anhydroglucose unit. The etherification with the etherification reagent propylene oxide is accordingly stated as MS (hydroxypropyl) or MS (HP) .

The still water-moist cellulose ether obtained by washing with hot water, purified to remove byproducts and having a water content of about 50% by weight, based on the total mass, is converted - optionally after further pretreatment (conditioning) - into a saleable form by drying and milling. The cellulose ethers are preferably marketed in powder or granule form having a water content of from about 1 to 10% by weight.

Cellulose ethers are used in building material systems, such as, for example, hand-applied and machine plasters, filling compounds, tile adhesives, air-placed concrete materials, self-leveling floor screeds, cement extrudates and emulsion paints, as thickeners and water retention agents.

The properties of these building material systems, in particular the consistency and the setting behavior, can be considerably influenced by the choice of the cellulose ether.

Particularly in gypsum-setting building material systems, i.e. gypsum-containing base mixes to which water has been added, lumps or nodules are often observed and, in the most unfavorable case, may lead to unevenness and grooves and may at least result in delays through intensive reworking.

Attempts have been made to eliminate some of these problems by combinations of admixtures. Thus, WO 99/64368 discloses a mixture of additive which consists mainly of cellulose ether and small amounts of a polymerized carboxylic acid and a methacrylate or acrylate homo- or interpolymer . Unfortunately, the preparation of this mixture of additives is complicated, requires additional mixing units and does not always leads to reduction in lumps. In addition, use of aqueous carboxylic acid solutions can lead to a pH induced chain degradation of the cellulose ether.

Aside from the above it is known a process from the prior art according to which a treated alkylhydroxyalkylcellulose (AHAC) is prepared, the process consists in spraying an already dried and milled cellulose ether with an aqueous solution of a crosslinking agent in a continuous high-intensity mixer and subsequently removing the water by further drying, as described, for example, in EP-B 0 868 436. This process requires an additional drying step and a special mixing member.

Likewise known is a process in which an alkylhydroxyalkylcellulose (AHAC) is suspended in a suspending medium, a crosslinking agent is added and the suspending medium is then separated off again. Examples of this procedure are to be found in US 2 879 268. According to this process, however, additional process steps are introduced which can be integrated into the preparation process only with difficulty.

It is therefore an object of the present invention to provide a composition which does not lead to agglomeration on stirring with water - the production of a gypsum- setting building material system - and in which the addition of combinations of additives which are complicated to prepare, as are described in the prior art, can be dispensed with.

Surprisingly, this object was achieved by using, in the composition according to the invention for a gypsum-setting

building material system, an alkylhydroxyalkylcellulose (AHAC) treated with a crosslinking agent, the crosslinking agent serving for temporary crosslinking of the AHAC.

Therefore, the present invention provides a composition for a gypsum- setting building material system, wherein an alkylhydroxyalkylcellulose (AHAC) treated with from 0.01 to 0.3% by weight of a crosslinking agent, calculated as anhydrous and solvent- free substance and based on the dry AHAC, is present in addition to ingredients of a gypsum mixture .

Furthermore, the present invention provides a gypsum-setting building material system formed by adding water to the composition which is described in more detail below as well as a gypsum-set building material system based on the before mentioned gypsum-setting building material system.

Additionally, the present invention provides a process for the preparation of an alkylhydroxyalkylcellulose (AHAC) treated with a crosslinking agent, wherein a) from 0.01 to 0.3% by weight of a crosslinking agent, calculated as anhydrous and solvent- free substance and based on the dry AHAC, is added to a moist AHAC, and b) the AHAC thus treated is then dried and milled, characterized in that from 3 to 100% by weight of water, based on the dry AHAC, are added together with the crosslinking agent in step a) . Also provided is an alkylhydroxyalkylcellulose obtainable by said process.

Finally, the present invention provides an AHAC which was treated with from 0.01 to 0.3% by weight of a crosslinking agent, calculated as anhydrous and solvent- free substance and based on the dry AHAC, and more than 95% by weight of which passes through a sieve having a mesh size of 315 μm. Also provided is the use of the mentioned AHACs for the preparation of compositions for gypsum-setting building material systems.

In a preferred embodiment of the invention, the alkylhydroxyalkylcellulose (AHAC) present in the composition in addition to ingredients of a gypsum mixture was treated with from 0.02 to 0.2% by weight of the crosslinking agent, calculated as anhydrous and solvent-free substance and based on the dry AHAC. The amount used of the crosslinking agents known per se is smaller than the amount used in the prior art in other applications, for example on use in emulsion paints.

The following alkylhydroxyalkylcelluloses (AHAC) are preferably used for the preparation of the composition of the present invention: methylhydroxyethylcellulose (MHEC), ethylhydroxyethylcellulose (EHEC) , methylhydroxypropylcellulose (MHPC) and the ternary mixed ethers methylethylhydroxyethylcellulose (MEHEC) and methylhydroxyethylhydroxypropylcellulose (MHEHPC) or a mixture of these cellulose ethers. Methylhydroxyethylcellulose (MHEC) is particularly preferably used.

The alkylhydroxyalkylcelluloses (AHAC) used for the inventive composition preferably have a DS (alkyl) of from 0.8 to 2.0 and particularly preferably from 1.3 to 1.8.

The MS (hydroxyalkyl) of the AHAC used for the inventive composition is preferably from 0.1 to 1.2, particularly preferably from 0.15 to 0.8 and very particularly preferably from 0.2 to 0.6. In the case of ternary mixed ethers, the "MS (hydroxyalkyl)" is understood as meaning the total MS.

The DS and MS are determined by the Zeisel method known to the person skilled in the art and described, for example, in P. W. Morgan, Ind. Eng. Chem. Anal. Ed. 18 (1946) 500 - 504 and R. U. Lemieux, CB. Purves, Can. J. Res. Sect. B 25 (1947) 485 - 489. _.

In a preferred embodiment of the invention, the above mentioned crosslinking agent is selected from the group consisting of monoaldehydes , dialdehydes, silicon compounds of

the formula Si (Rl) (R2) (R3) (R4) and silicon compounds containing aldehyde or carbonyl groups, or mixtures thereof, wherein glyoxal is preferred as the crosslinking agent. The crosslinking agent is additionally described in more detail below.

Preferably, the treated AHAC is present in the composition according to the invention in an amount of from 0.01 to 5% by weight, more preferably in an amount of from 0.1 to 0.8% by weight and particularly preferably in an amount of from 0.2 to 0.4% by weight, based on the total mass of the composition.

A particular embodiment of the invention therefore uses a process which requires no additional drying stage and can be readily integrated into the existing preparation process. This process comprises, as a substantial feature, the mixing of a moist cellulose ether purified to remove byproducts, as obtained in the preparation process after the purification stage, with the crosslinking agent and the drying and milling of the cellulose ether thus treated.

It has proven to be particularly advantageous to add the crosslinking agent together with a certain minimum amount of a solvent for the crosslinking agent, preferably water. Surprisingly, forced mixing with plasticizing in a kneader can be dispensed with, as described in DE-A 1 518 213 as being necessary if a moist cellulose ether from the purification stage of the preparation process is used.

The present invention also relates to a process for the preparation of an alkylhydroxyalkylcellulose (AHAC) treated with a crosslinking agent by a) addition of from 0.01 to 0.3 and preferably from 0.02 to 0.2% by weight of a crosslinking agent, calculated as anhydrous and solvent-free substance and based on the dry AHAC, to a moist alkylhydroxyalkylcellulose, and b) subsequent drying and milling of the AHAC thus treated,

characterized in that at least 3% by weight, preferably at least 5% by weight, particularly preferably at least 10% by weight, of water, based on the dry AHAC, are added together with the crosslinking agent in step a) .

Preferably not more than 100% by weight, preferably not more than 80% by weight, particularly preferably not more than 50% by weight, of water are added.

Together with the crosslinking agent, further additives and modifiers may be added, for example a buffer system for adjusting the pH according to EP-B 1 316 563 (in particular paragraphs [0030] to [0033]) or process auxiliaries according to EP-A 1 445 278 (in particular paragraphs [0019], [0026] to [0033] and [0043] ) .

The process according to the invention can be used without additional apparatuses or process steps, such as, for example, a drying step. Thus, the crosslinking agent and optionally the buffer mixture can be completely or partly mixed with the solvent which is added to a moist AHAC for establishing desired properties. The adjustment of the moisture content of cellulose ethers is described, for example, in EP-B 1 127 895.

An alkylhydroxyalkylcellulose isolated from the reaction mixture, neutralized, purified sufficiently to remove byproducts and still moist is preferably used as starting material. The moist alkylhydroxyalkylcelluloses (AHAC) to be used according to the invention can be obtained, for example, by reaction of cellulose with alkyl chloride and alkylene oxide and subsequent purification with hot water in the form of a usually water-moist filter cake or centrifuge residue.

Moist alkylhydroxyalkylcelluloses, as are to be used in the process according to the invention, have a total moisture content of preferably 30 - 70% by weight, particularly preferably of 40 - 60% by weight, based in each case on the

dry cellulose ether, regardless of the manner in which the moisture content is established.

In addition to the crosslinking agent and water, further additives or modifiers, e.g. the buffer system according to

EP-B 1 316 563 or the process auxiliaries according to

EP-A 1 445 278, or further water can be added to the moist alkylhydroxyalkylcellulose (AHAC) . The moist alkylhydroxyalkylcellulose (AHAC) can, for example, be mechanically compacted or cooled.

The addition of the crosslinking agent and of the solvent can be carried out, for example, in a commercially available mixer or in a conveying screw.

The crosslinking agent forms a bond with the hydroxyl groups of the cellulose, which bonding is reversible in aqueous solution. Dialdehydes and organosilicon compounds according to the prior art are preferably used, as described, for example in the following patents: US 2 879 268, EP-B 1 316 563 and EP 1 452 544 or JP08183801 and DE 32 32 467.

Thus, for example, the following can be used:

a) Dialdehydes having 2-8 C atoms, such as, for example, glyoxal. Glyoxal is preferably used in the form of commercially available aqueous solution of about 40% by weight .

b) Monoaldehydes having 1-8 C atoms on which can form full acetal bonds, for example formaldehyde.

c) Silicon compounds of the formula Si (Rl) (R2 ) (R3) (R4) , wherein the radicals Rl-4, independently of one another, are aryloxy, alkoxy, acyloxy, aryl , alkyl, alkenyl, N-silyl or O-silyl. The radicals can optionally be substituted, wherein for the 0- and N-silyl radicals the substituents Rn

(e.g. R5, R6, R7 , etc.) in turn, independently of one

another, are aryloxy, alkoxy, acyloxy, aryl, alkyl, alkenyl, N-silyl or 0-silyl. i) Preferred compounds are those in which at least 2 of the 4 substituents are optionally substituted alkoxy or acyloxy groups and 0-2 substituents are optionally substituted alkyl groups having 1-2 C atoms. Examples of preferred compounds are:

- ethyltriacetoxysilane

- methyltriacetoxysilane - methyl trimethyoxysi lane

- tetraacetoxysilane ii) Furthermore preferred are those polymeric siloxanes which have at least two optionally substituted alkoxy or acyloxy substituents in the polymer chain. Examples of such compounds are:

- H ₃ C (OCH ₃ ) ₂ Si-O-Si (OCH ₃ ) ₂ CH ₃ , (Rl= CH ₃ , R2=R3= OCH ₃ , R4= OSi (OCH ₃ ) ₂ CH ₃ )

- H ₃ C-Si(CH ₃ ) (OC ₂ Hs)-[Si(CH ₃ ) (OC ₂ H ₅ ) ] _n -CH ₃ , n= 1, 2, 3, etc . - H ₃ C-Si(CH ₃ ) (0OC ₂ H ₃ )- [Si(CH ₃ ) (0OC ₂ H ₃ ) J _n -CH ₃ , n= 1, 2, 3, etc . .

d) Mixed forms of crosslinking agents which contain both at least one aldehyde or carbonyl group and at least one group of the formula -Si (Rl) (R2 ) (R3 ) , the radicals R1-R3, independently of one another, being aryloxy, alkoxy, acyloxy, aryl, alkyl, alkenyl, N-silyl or 0-silyl. The radicals may be optionally substituted, wherein for the 0- and N-silyl radicals the substituents Rn (e.g. R5 , R6 , R7 , etc. ) in turn, independently of one another, are aryloxy, alkoxy, acyloxy, aryl, alkyl, alkenyl, N-silyl or 0-silyl. In this case, at least one of the radicals Rn is an aryloxy, alkoxy or acyloxy radical. An example of such compound is: HC (0) CH ₂ Si (OCH ₃ ) ₃ .

e) Crosslinking agents having at least one reversibly bonding unit selected from the group consisting of aldehyde, carbonyl and silanes -Si(Rl) (R2) (R3) and at least one more

strongly bonding unit from the group consisting of epoxides, alkyl esters, acid chlorides, alkyl chlorides, alkyl bromides, alkyl iodides. Here, the radicals R1-R3, independently of one another, are aryloxy, alkoxy, acyloxy, aryl, alkyl, alkenyl, N-silyl or O-silyl. The radicals may be optionally substituted, wherein for the 0- and N-silyl radicals the substituents Rn (e.g. R5 , R6 , R7 , etc.) in turn, independently of one another, are aryloxy, alkoxy, acyloxy, aryl, alkyl, alkenyl, N-silyl or O-silyl. In this case, at least one of the radicals Rn is an aryloxy, alkoxy or acyloxy radical. An example of such compound is glycidyloxypropyltrimethoxysilane (GPTMS)

CH ₂ (O)CHCH ₂ OCH ₂ CH ₂ CH ₂ Si (OCH ₃ ) ₃ .

Glyoxal is particularly preferably used.

Methods for milling and drying cellulose ethers are known, for example from the teaching of GB 2 262 527, DE 38 39 831, EP 1 127 910 and EP 1 127 895. Various mill types can be used, for example pinned-disk mills, bowl mills, hammer mills, screen basket mills, cross beater mills and impact mills. The drying effect is preferably supported by the use of gases or gas mixtures heated to temperatures of > 80 ⁰ C, preferably > 100 ⁰ C, in the comminution apparatus. The combined milling/drying can be followed by further milling steps and/or drying steps. However, the milling is preferably single-stage and the mill used is a screenless high-speed impact mill.

After the milling and drying, which is preferably carried out simultaneously in the form of a combined milling/drying, the alkylhydroxyalkylcelluloses (AHAC) have a residual moisture content of from 0.1 to 15% by weight, preferably from 1 to 10% by weight, particularly preferably from 1.5 to 7% by weight, measured as loss on drying at 105 ⁰ C after 20 h and based on the weight of the dried and comminuted cellulose ether taken.

The present invention furthermore relates to the alkyl- hydroxyalkylcelluloses (AHAC) obtainable by the process according to the invention.

The alkylhydroxyalkylcelluloses (AHAC) according to the invention preferably have the following grading curve:

Undersize Percent

< 315 μm 95 to 100

< 250 μm 90 to 100

< 200 μm 80 to 100

< 160 μm 70 to 99.9

< 125 μm 50 to 95

< 100 μm 30 to 90

< 63 μm 10 to 70

Therefore, the present invention provides an AHAC which was treated with from 0.01 to 0.3% by weight of a crosslinking agent, preferably from 0.02 to 0.2% by weight, calculated as anhydrous and solvent-free substance and based on the dry AHAC, and more than 95% by weight of which passes through a sieve having a mesh size of 315 μm. In a preferred embodiment 95 to 100% by weight of the before mentioned AHAC passes through a sieve having a mesh size of 315 μm, and 90 to 100% by weight passes through a sieve having a mesh size of 250 μm, and 80 to 100% by weight passes through a sieve having a mesh size of 200 μm, and 70 to 99.9% by weight passes through a sieve having a mesh size of 160 μm, and 50 to 95% by weight passes through a sieve having a mesh size of 125 μm, and 30 to 90% by weight passes through a sieve having a mesh size of 100 μm, and 10 to 70% by weight passes through a sieve having a mesh size of 63 μm.

The viscosity of the products obtainable according to the invention, measured as a solution in water of 2% by weight at 23°C using a Haake Rotovisko at 2.55 s ^"1 , is preferably from 10 to 200 000 mPa-s, particularly preferably from 100 to

150 000 mPa-s and very particularly preferably from about 1000 to 100 000 mPa-s, in particular from 10 000 to 80 000 mPa-s.

The bulk density of the products, measured on a loose heap, is preferably from 200 to 700 g/1, particularly preferably from 250 to 650 g/1 and very particularly preferably from 300 to 600 g/1.

The alkylhydroxyalkylcelluloses (AHAC) according to the invention, are distinguished in particular in that, compared with other alkylhydroxyalkylcelluloses (AHAC) of the prior art, they lead to a particularly low level of formation of lumps in gypsum machine plasters. For example, according to

WO99/64368, an already dried MHAC is coated with a polymeric acid and then dried. However, drying twice is very expensive in terms of apparatus and energy. Furthermore, the addition of acid can lead to a reduction in the viscosity.

In addition to the advantages which relate to the preparation process according to the invention, there are also improvements for the AHAC according to the invention itself. Thus, the process according to the present invention results in a more uniform distribution of the crosslinking agent over the AHAC, in contrast to processes of the prior art in which the crosslinking agent is added to the dried cellulose ether and then mixed in. The prevention of agglomeration on stirring of treated AHAC is improved in the case of the AHAC according to the invention.

The present invention therefore furthermore relates to the use of the alkylhydroxyalkylcelluloses (AHAC) according to the invention in gypsum- setting building material systems, preferably in gypsum-setting plasters and filling compounds, particularly preferably in gypsum machine plaster.

As mentioned already above the treated alkylhydroxyalkylcelluloses (AHAC) are typically used in amounts of from 0.01 to 5% by weight, preferably from 0.1 to

0.8% by weight and particularly preferably from 0.2 to 0.4% by weight, based in each case on the dry mass of the building material system.

In addition to the cellulose ether according to the invention and gypsum, the dry mass of the building material system (the gypsum mix) may consist, for example, of the following customary ingredients, but without being limited to these:

• slaked lime 0 - 30% by weight

• mineral aggregates 0 - 30% by weight

(e.g. quartz sand, limestone sand, limestone gravel, limestone powder, mica)

• light aggregate 0 - 20% by weight (e.g. Perlite)

• plastic dispersion powder 0 - 20% by weight

• fibers 0 - 2% by weight (e.g. cellulose fibers)

• accelerator 0 - 0.8% by weight • added thickener 0 - 0.5% by weight

(e.g. starch derivatives and guar derivatives, synthetic thickeners, polyacrylamide, polyvinyl alcohol)

• retardant 0 - 0.5% by weight • air pore former 0 - 0.1% by weight

The gypsum- setting building material system according to the presen invention is simply formed by adding water to the above described inventive composition.

The water/solids ratio in the gypsum-setting building material systems according to the invention is preferably from 0.2 to 0.9, particularly preferably from 0.4 to 0.8.

After having set the gypsum- setting building material system becomes a gypsum-set building material system. Therefore, also such a gypsum-set building material system based on the above

described gypsum- setting building material system is part of the present invention.

It is known to the person skilled in the art that alkylhydroxyalkylcellulose (AHAC) can be used usually not alone but with a number of aggregates and/or modifiers in the formulations. Thus, methylhydroxypropylcelluloses mixed with small amounts of auxiliaries and aggregates, e.g. antifoams, swelling agents, fillers, light aggregates, polyacrylates, polyacrylamides, air pore formers, dispersants, water repellants, plasticizers, superabsorbers, stabilizers and synthetic, semisynthetic and natural thickeners, can be used, for example, in building material base mixes.

Examples

Unless stated otherwise, all stated percentages are based on weight .

Example 1:

A milled and dried methylhydroxyethylcellulose (MHEC) having a DS (methyl) of 1.58 and an MS (hydroxyethyl) of 0.30 was treated with glyoxal. For this purpose, the cellulose ether was suspended in 10 times the amount of acetone and a 40% strength by weight aqueous glyoxal solution was added. The suspending medium was evaporated. Analysis of the products gave a glyoxal content of 0.03, 0.11 and 0.24% by weight, respectively, based on the dry cellulose ether. The products were obtained in the form of powders which were apparently not distinguishable from the starting material.

Example 2 :

Glyoxal-treated samples El to E3 from example 1 were tested in a gypsum filler. The comparative substance used was the starting material Cl not treated with glyoxal .

A dry gypsum filler mix containing 0.48% by weight of cellulose ether is sprinkled into water (water/solid factor

0.6) and, after a maturing time of 8 minutes, is stirred for

1 minute manually with a trowel. After a further maturing time of 10 minutes, the mix is stirred up (15 min) and applied to a gypsum plasterboard by means of a spatula. Striking differences in the agglomeration and in the thickening behavior were found. While all samples according to the invention showed very little agglomeration, these samples with increasing amount of glyoxal showed gradual thickening. This unexpected slight thickening on stirring after 8 minutes may induce the processor to use more of the dry gypsum filler mix and thus establish an excessively high water/solid factor.

Example 3 (process )

A water-moist filter cake of a methylhydroxyethylcellulose (MHEC, DS (methyl) = 1.55, MS (hydroxyethyl) = 0.28, NaCl = 3.4%, based on the dry cellulose ether), having a water content of 57% (based on the total mass of the filter cake) , is moistened with water to a water content of 62% (based on the total mass of the filter cake) . The water for moistening the filter cake contains various amounts of glyoxal and a constant amount of a buffer mixture. The buffer mixture consists of sodium dihydrogen phosphate and disodium hydrogen phosphate in the molar ratio 1:1. The moistening is carried out in a horizontal mixer equipped with plowshare mixers and having a capacity of 400 1 at a batch size of 80 kg of the water-moist filter cake. For this purpose, the glyoxal and the phosphate buffer system are dissolved in 10.53 kg of water and added to 34.4 kg of MHEC (calculated as dry mass) . The mixing was effected at room temperature.

The product thus prepared was then subjected to combined milling/drying in a screenless impact mill according to EP 1 127 910.

The MHEC from experiments E4 to E6 showed reduced agglomeration in a gypsum filler which was stirred analogously to example 2. With the use of the sample C2 not treated with glyoxal, agglomeration was observed.