DK164737B - Process for producing an aqueous silica gel - Google Patents

Description

in

DK 164737 B

The invention relates to a process for preparing an aqueous silica gel in which an acid-cured silica hydrogel is washed with an aqueous acidic solution to a pH of 2.5-5 in the silica hydrogel which is dried to a water solution. 5 content of 20 to 60% by weight and ground to an average particle size of from 1 to approx. 40 ^ u.

Silica needles are used as polishes and cleansers in dentifrices. Silica zerels are prepared by slowly drying silica hydrogels so as to induce significant 1 ° equal shrinkage of the hydrogel structure and form a dense, rigid structure. Although silica zerels are highly effective polishes and cleansers, the necessary drying increases manufacturing costs, and the gel structure absorbs some of the valuable liquid components of the agent so that the effectiveness of these components is reduced or directly wasted.

DE Publication No. 2,704,504 proposes dentifrices containing an aqueous silica gel having a higher water content (15 to 35% by weight) than that present in a zero gel used as a detergent and polish. However, even with these compositions, to a large extent overcoming the above disadvantages, it would be desirable for certain purposes to improve the abrasiveness of the aqueous silica gel. It is precisely the object of the present invention to provide a solution to this problem.

The process of the invention is then characterized in that the dried gel is contacted with an alkaline medium in sufficient quantity to raise its pH to a value in the range of 6 to 10 as measured in a 5% by weight aqueous slurry. gel.

30 As can be seen from the following, the aqueous silica gel so produced is well suited for use as a polishing and cleansing agent in dentifrices, the abrasiveness being substantially increased.

The gels can be dried quickly so that the production price is reduced and the amount of wetting agent needed to produce a given volume of dentifrice is reduced due to the lower absorption in the 2

DK 164737 B

aqueous pores. Dental care products containing a silica gel made in accordance with the invention provide effective cleaning and polishing as measured by the radioactive dentin values (RDA) determined according to the procedure prescribed by The American 5 Dental Association described in Hefferren, J. Dent. Res., Pages 563-573 (July-August 1976), with the following exceptions. The RDA values are determined using a slurry containing 6.25 g of the aqueous silica gel instead of the 10.0 g of the abrasive powder used in The American Dental Associations1 procedure. In addition, throughout the present disclosure, the RDA values are based on an RDA value of 500 for the calcium pyrophosphate reference standard instead of the value 100 normally assigned to this reference standard by The American Dental Association's procedure.

The aqueous silica gels prepared by the process of the invention preferably have an RDA value of at least approx. 400. The aqueous silica gels prepared from hydrogels washed at ca. 27 ° C to approx. 38 ° C and contacted with an alkaline medium by the process according to the invention has an RDA value of at least approx. 600th

The present aqueous silica gels are prepared from acid cured silica hydrogels. Silica hydrogel can be prepared by reacting an alkali metal silicate and mineral acid in acidic medium to form a silica hydrosol which is allowed to cure to a hydrogel. When the amount of acid reacted with the silicate is such that the final pH of the reaction mixture is acidic, the resulting product is considered an acid-cured hydrogel. Sulfuric acid is the most commonly used acid, although other mineral acids such as hydrochloric, nitric or phosphoric acid can be used. Sodium or potassium silicate may e.g. is used as the alkali metal silicate.

Sodium silicate is preferred because it is the cheapest and most readily available. The concentration of aqueous acidic solution is generally from ca. 5 to approx. 70% by weight, and the aqueous silicate solution generally has a SiO2_3

DK 164737 B

ISLAND

content from approx. 6 to approx. 25% by weight and a weight ratio of SiO2 to Na2O from approx. 1: 1 to approx. 3,4: 1st

The mineral acid solution and the alkali metal solution are mixed to give a silica hydrosol. The relative proportions and concentrations of the reactants are controlled so that the hydrosol contains from ca. 6 to c. 20 and has a pH of less than about 20% by weight. 5 and usually between approx. 1 and approx. 3. Generally, continuous processing is used and both reactants are metered separately in a fast-running mixer. The reaction may be carried out at any convenient temperature, e.g. from approx. 15 to approx. 80 ° C, and is usually carried out at ambient temperature.

The silica hydrosol will cure to a hydrogel of generally from ca. 5 to approx. 90 minutes and then washed with an aqueous acidic solution to remove residual alkali metal salts formed during the reaction. For example, when sulfuric acid and sodium silicate are used as the reactants, sodium sulfate is trapped or entrapped in the hydrogel. Prior to washing, the gel is usually cut or broken into pieces 20 having a particle size in the range of from approx. 12.5 to approx. 7.5 cm. The gel can be washed with an aqueous solution of mineral acid such as sulfuric acid, hydrochloric acid, nitric acid or phosphoric acid or other highly acidic medium, e.g. an organic acid such as formic acid, acetic acid or propionic acid.

The wash solution contains a sufficient amount of acid to provide a pH value of approx. 2.0 to approx. 5.0, but preferably from ca. 2.5 to approx. 4.5. The temperature of the wash solution affects the properties of the aqueous silica gel product.

Generally, the temperature of the solution is from ca. 27 to approx. 93 ° C. Preferably, the wash solution has a temperature of from 27 to approx. 38 ° C to further enhance or improve the abrasive performance of the aqueous silica gel product. It is believed that the lower washing temperatures decrease the bond between the micelles 35 and facilitate the shrinkage upon subsequent drying.

4

DK 164737 B

The gel is washed for a sufficient period of time to reduce the total salt content to below approx. 5% by weight. The gels can e.g. have a Na 2 O content of approx. 0.05 to approx. 3% by weight and a SO 0.05 to approx. 3 weight-5 percent, based on the dry weight of the gel. The usual amount of time needed to achieve the desired salt removal is from approx. 6 to approx. 30 hours. Although the effect is not as great as at the reduced washing temperatures, shorter wash times increase the abrasive capacity of the aqueous silica gel products because the bonding between the micelles is further reduced and shrinkage upon subsequent drying thus increases. It is preferred that the hydrogel is washed with the aqueous acidic solution at a temperature of approx. 27 to approx. 38 ° C for approx. 6 to approx. 15 hours, especially for approx. 6 to approx. 12 hours.

15 The pH of the gel increases as the salts are removed by the acid wash. To prepare aqueous silica gels suitable for use in dentifrices, the pH of the finished gel, after completion of the wash, should be in the range of 2.5 to 5, as measured in a 5% by weight aqueous slurry of the gel.

The washed silica hydrogel generally has a water content, as measured by loss after annealing at 950 ° C, of 60 to 75% by weight and a particle size in the range of about 1 /: to approx. 50 mm. The hydrogel is then dried to the desired water content from 20 to 60% by weight, preferably from ca. 35 to 60% by weight.

Oven drying, rotary drying, cascade drying or other known drying methods can be used. For example, the washed hydrogel may be dried in a rotary dryer or cascade dryer 30 at an air inlet temperature of approx. 310 to approx. 320 ° C and an air outlet temperature of approx. 49 to approx. 52 ° C with a residence time of approx. 4 hours to produce a product containing 44% by weight water.

The gel is ground to an average particle size of from 1 to approx. 40 µm, preferably from ca. 5 to approx. 20 µ, allowing it to be used in dental care. Average score 5

DK 164737 B

The cell size here refers to "aggregate" or "secondary" particles and may be that determined in a microspore particle size analyzer. An average particle size of not more than 1% by weight greater than 50 µ is preferred for use in dentifrices. The hydrogels can be ground by several different methods under a variety of conditions to provide the appropriate particle size. Since water is usually removed during grinding, the hydrogel can be dried to within about 5 hours. 10% by weight or slightly more than 10 the desired final water content and the remaining water is removed during grinding. When the water content of the hydrogel is above approx. 70% by weight, it can be pre-dried in any suitable dryer at a temperature and for a time sufficient to reduce the water content of the hydrogel to less than about 10% by weight. 70% by weight so that surface moisture is removed and the hydrogel's feed into the heated mill is facilitated.

In a preferred embodiment, the silica hydrogel is milled and dried simultaneously in a mill in the presence of heated air, thereby providing the desired water content and a suitable average particle size for use in dentifrices. Eg. For example, the hydrogels may be simultaneously ground and dried in a mechanical grinding means such as a impact mill, e.g. a rotary hammer mill into which a moving storm of heated air is introduced. In this simultaneous operation, the mill can be operated using air at substantially atmospheric pressure with inlet temperatures of approx. 93 to approx. 260 ° C and ambient air temperatures from approx. 27 to approx. 66 ° C. A particularly suitable mill is an air classification hammer mill such as the MicroPul ACM 100 mill in which the input air flow is heated. After drying and grinding, the aqueous silica gel product is contacted with an alkaline medium to increase its abrasiveness. The alkaline medium can e.g. be ammonia, an organic amine, an alkali metal hydroxide or an alkali metal carbonate. Preferably, the aqueous silica gel is contacted with an ammonia medium.

DK 164737 B

6

The aminoniacal medium may be gaseous ammonia, aqueous ammonia or other aqueous ammonia media containing e.g. aliphatic amines and especially alkylamines and alkylene diamines such as ethylamine, ethylenediamine, propylamine, propylenediamine or diethylenamine. Ammonia is strongly absorbed by the gel so that ammonia-containing solutions of widely varying concentrations and prepared from a wide variety of ammonia-containing compounds can be used. The aqueous silica gel is contacted with the alkaline medium in a sufficient amount to provide a gel having a pH of from 6 to 10 and preferably from 8 to approx. 9.5. The pH is measured in a 5% by weight aqueous slurry of the gel. The use of a gaseous alkaline medium is preferred because of the greater uniformity and rate of adsorption. The gel can be contacted with the alkaline medium by circulating the gas or spraying the solution over the gel until the gel has adsorbed the required amount of base and reached the desired pH.

In a preferred embodiment of the method 20 according to the invention, the gel is contacted with gaseous aqueous ammonia after its exit from the mill. Preferably, the gel is contacted with ammonia no more than 1 minute after grinding. The air stream containing the gel has a temperature of approx. 27 to approx. 66 ° C so that contact with 25 ammonia provides rapid adsorption into the gel particles.

The aqueous silica gels can be used in dentifrices in an effective amount for polishing and cleaning. Generally, the gel comprises approx. 5 to about 50%, but preferably from about 5% to about 50%. 30 to approx. 20%, based on the weight of the dentifrice.

The process according to the invention is illustrated in more detail in the following examples. All percentages and percentages in the Examples are by weight unless otherwise noted.

35

DK 164737 B

7

ISLAND

Example 1

An aqueous sulfuric acid solution with a strength of 36 ° Baumé and an aqueous sodium silicate solution with a strength of 36.5 ° Baumé are pumped into a fast-running 5 mixer with flow rates of 68 and 201 liters, respectively. minute. An excess of sulfuric acid is maintained to such an extent that the medium is 0.6 N. The resulting silica hydrosol has a SiC 2 content of 18% and a pH of about 1.5. The silica hydrosol is deposited on a 10-moving conveyor belt and cures to a hydrogel over 15 minutes. 453 kg of the hydrogel is placed in a washing basket and washed with an aqueous solution of sulfuric acid having a pH of 4 and a temperature of 31 to 32 ° C at a rate of 15 liters per liter. per minute for 12 hours.

After washing, the washed products are cascaded and mixed from four repeated experiments. The mixture has a total content of volatiles of 64% and Na 2 O and SO 4 contents of 0.15 and 0.28% respectively. An aqueous slurry containing 5% of the hydrogel has a pH of 4.1.

A series of samples of the mixed material is then fed into a MicroPul ACM 10 mill, in which a stream 3 of heated air is introduced at a rate of approx. 20 m per minute. The feed rates and air inlet temperatures are varied to provide the desired degree of drying. As determined in a microspore particle size analyzer, the average particle size of the product ranges from approx. 12 to approx. 15 µ. In half of the experiments, anhydrous gaseous ammonia is injected into the mill discharge line and brought into contact with the gel so as to provide the desired pH increase. The properties of the various aqueous silica gel products are shown in the following Table I. The size% TV indicates the total amount of volatiles lost after annealing at 950 ° C.

35

DK 164737 B

8

X

w g g g g g g

Qi ft ft ft ft ft Φ g ft ft ft ft ft

• P

Cn OOOO OOO

• H O r * in pH rH! —I 10

tO iH

Ό O <0> * 6 (0. £ ft 'to φ ro (N in i — i 0 * 1 θ' »* # P. g 10 pH * 3 <<J \ O ^

P ► «K Λ P \ tH

pH pH pH pH pH pH

ffl CM

> Γ0

H

<D

tr

G

n | n in ^ · η w tu s}, cn <0 g • H j2j j r · * \ ·>. Λ to OOO -P 0 <t3

(0 φ QJ

-G m H -P

Ό O pH H c

-PH O Η H G ID ^ ro -H

1) t3- CO * »^ ttJ» x »» »n to

> H -P OOO Λ O O O

Η Ό 0) G

G <A ° Λ ^

pHttf-P O ^ HfO D tO «tf m O

(DrGcn cjpHpHp- <ro cm> h «tf

Λ (D ftJ O

GrQ> £ 000 OOO II

B - aT »

s · I

ISLAND

ISLAND

pH

<000 OOO

Q tD tO CO to to <3 *

At 0 ° C at pH ro * 3 · g for 5 s pH 10

Ϊ Qi tD «tf II

ft O o ^^ PH r- CM pH

G C0 03 CO ^ dfi · Η «tf« tf «tf to (Λ G 9 g ιΟ

CM

> Γ '' tO O UD «tf O ^ r \ * \ i * \ * s n o \ 00 r-

DsgJ in «tf ό ko« tf co X

9

DK 164737 B

Example 2

A series of silica hydrogels is prepared by the procedure of Example 1. All the gels are washed with an aqueous solution of sulfuric acid having a pH of 4.0 at a rate of 15 liters per minute. minute. Type A gels are washed with the solution at 32 ° C for 8.5 hours to a gel pH of 3.2. Type C gels are washed with the solution at 82 ° C for 7.5 hours to a gel pH of 4.0. All the gels are ground and dried in Micro-Pul ACM 10 and contacted with ammonia by the procedure of Example 1. The properties of the aqueous silica gel products are listed in the following Table II.

10

DK 164737 B

<| in a o O o> n

Di r ~ r-j r ~ O O

es; o co σ \ «µ in co Φ w Ϊ> 1

rh

Π3 Λ C 3 id C

ii. o ® m.

2 X3 ** s Λ ON. On I (Tv D * EY «d * τί <d <» m m w (5, _j j_i. — I

U

M

mr \ £

® .C

ti w

^ <D

• h £

to o-oo O O O

g § oid _i

> M

Old X

Ck I _ £> raj «*? j ΦΙ co σι ni η o o • JJ! Ej co o n o o g1 ^ * N ^ * -n

y. ^ S in r-ι h O OOO

H> I CN!

Λ I

05 tn * m ['d * ro E-ι -H ni m nj no »Z! ^^ i i i

* j OOO

s-ι i o m r * cn .- «

§ · j O 'Ό r \ j CM OOO

in C / 3jff »I» \ tP «on» o. o ^

o r-i p-i OOO

di> ϋ * g1! o cn η η «tf'd'in

W o \ I C \ J vD t "OOO

* φ e-s. rs V- «e **» * n <rs

2 0 0 0 OOO

rh

W

Go - IS O tf ί ιο m m co r-

Qj t-S ^ on o * n * - O- On * ° · * co n t "co oo co n c> j o. — I co co n o

£ - ** 5N ON * N 0 ~ N yN. "N

i in n co co o r-> R; > n «d · rn m ^» n

I <D

φ £ <<<U u υ

O 4J

DK 164737 B

11

Example 3

Silica hydrogels are prepared by the method of Example 1, washed by the Type A method of Example 2 to a total Na 2 and SO 2.0% and ground 5 and dried in MicroPul ACM 10 to microspore average particle sizes in the range of 9.0 to 12 ^. The ground and dried gels are treated with varying amounts of ammonia so that the pH of the gel is varied and the untreated control samples are prepared. The properties of the gels are listed below 10 in Table III

Table III

Sample No.% TV pH RDA

15 1 24 7.5 990

Control 31 3.2 580 2 31.5 6.1 780 20 3 31 6.9 930 4 28 7.7 1070 5 35 7.0 790 6 34.5 7.5 860 7 33 8.2 950 25 -

Control 39 3.2 325 30 Example 4

In the method of Example 2, a series of gels of type A and C are prepared without ammonia treatment. The properties of the gel products are listed in Table IV: 35 12

DK 164737 B

^ I O O O O LO lH

Q Cft t "O 00 <£> ro P2; i — I <M" ςΐ * · Μ tø tn> 1

H

Π3

G

H3 5-) o.-q'HHin oo oa r-t g ^ vd n o nK oT o?

g - Η Η Η = H Η H

5 o * s <! J2 ® rag g

tL tho ft ft CM

>% a ft ft o

Htfl OOOO O o o '

_ O rH H

rH Ό _ (th eifl Λ> to E- · “(0 n ft m t o o ro n H ^ j.m ** gr ^^ o H —I oj

ti P ~ θ '. ·> «V

g * m.HOO o o o

red CM

> ro m

• H

® CM cm o O CO o ra «tfoocor ^ O o.-i -t-i O (Τ '^ CO rH O O Ooo • Θ-

-P

dP

tilooCTiCM-O '^. ^. g1! pmOco ^ j · Ooo ►? i ra · - * v * ^ * ».

zoo o Ooo> 1 o co oo o ^ j. ^ C-, * · »V <v. ^

o LO co m o O

to m tf n in ri.

<<<O U o

ISLAND

DK 164737 B

13

Example 5

In the method of Example 2, a series of type A gels without ammonia treatment are prepared to illustrate the relationship between the total amounts of * volatiles 5 and RDA as listed in Table V:

Table V

approximate

ITV RDA

10 56 190 49 170 45 270 38 395 (average of 2 values) 28 610 15 21 640 20 25 30 35

Claims (7)

A process for preparing an aqueous silica gel in which an acid-cured silica hydrogel is washed with an aqueous acidic solution to a pH of 2.5-5 in the silica hydrogel which is dried to a water content of 20 to 60% by weight and ground to an average particle size of from 1 to approx. 40 µg, characterized in that the dried gel is contacted with an alkaline medium in sufficient quantity to raise its pH to a value in the range of 6 to 10 as measured in a 5% by weight aqueous slurry of the gel . Process according to claim 1, characterized in that the gel is contacted with the alkaline medium after drying and grinding. A method according to claim 1 or 2, characterized in that the gel is contacted with an ammonia medium. Method according to claim 3, characterized in that the gel is contacted with gaseous ammonia. Process according to claim 1 or 2, characterized in that the gel is contacted with an alkali metal hydroxide. Process according to any one of claims 1 to 25, characterized in that the pH is raised to the range of 8 to 9.5. Process according to any one of claims 1 to 6, characterized in that the silica hydrogel is washed with an aqueous acid solution at a temperature of 27 to 93 ° C. 35