CA1331503C - Precipitated silicas, process for producing same and their use - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for producing silicas having a cleaning, polishing and thickening effect and being intended for use as dentifrices. This process is characterized in that the sili-cas obtained in a conventional manner by precipitation and having varying particle sizes and particle densities are homo-geneously mixed with each other in the suspension phase and that the mixtures are further treated in the usual manner by filtering, drying and grinding.

Description

1331~03 `~ This is a divisional application of copending Application 552,383, filed November 20, 1987.

The present Inventlon relates to preclplt~ted slll-cas, a process for thelr productlon and to thelr use In tooth pastes.
Synthetlcally produced preclpltated slllcas have been of great Importance as a comPonent of dentlfrlces for many years. They are very pure, toxlcologlcally unobJectlon-able and compatlble wlth the other substances contalned In tooth pastes, as for example, glycerol, sorbltol, thlckeners, detergents, dyes and aromatlc substances and, when requlred, ¦ soluble fluorlne compound%.
Synthetlc preclpltated slllcas are produced by pre-clpltatlon from alkall-slllcate solutlons wlth aclds In a stlrrlng proces~, by fllterlng the suspenslon off, washlng, drylng and grlndlng. The preclpltatlon process provldes the posslblllty of readlly predetermlnlng Important propertles, such as partlcle dlameter, partlcle shape and partlcle denslty or hardness by operatlon~ In the flocculate phase.
The present Inventlon provldes a process for the productlon of cleanlng, pollshlng and thlckenlng preclpltated slllcas for dentlfrlces In whlch the slllcas of varylng partl-cle slze and partlcle denslty obtalned In the current manner by preclpltatlon are homogeneously mlxed wlth each other In the suspension phase and the mlxtures are further treated In a conventlonal manner by fllterlng, washlng, dryln~ and grlndlng.
The present Inventlon also provldes a preclpltated slllca whlch Is characterlzed by the followlng physlcal-cheml-cal characterlstic data:

loss on drying (DIN 53198) % 3 - 7-conductlvlty at 25C
(4X suspenslon) ~ S 400 - 800 pH value (6% accordlng to DIN 53200) 5 - 7 _ 1 _ '.' .~ .~
,, "" 1331503 "

BET surface area (DIN 66131) sqm/~ 130 - 150 tamped denslty (DIN 53194) g/lltre 100 - 150 macropore volume D , 30 nm ml/g 3.2-4.0 ~`
(accordlng to the H~ pressure method) oll absorptlon ~ccordln~ to G~rdner m//100 g 180 - 200 water retentlon value % 76 - 79 Cu abraslon mg 5 - 14 RDA abraslon 35 - 100 REA abrasion 40 - 90 scratches few to very few ~' v I scos I ty mPas 6000-10000 (In a 16X glycerol-water dlsperslon 1:1, splndle D wlth hellpath) Fe con~ent p. p.m. 240-280.

The present Inventlon also provldes a process for produclng the preclpltated slllca whlch Is characterlzed by the followlng physlcal-chemlcal characterlstic data:
loss on drylng (DIN 63198) X 3 - 7 oonductlvlty at 26C ~ S 400 - 800 (4X suspenslon) pH value (6% accordlng to DIN 53200) 6 - 7 BET surface area (DIN 66131) sqm/g130 - 150 tamped don~lty (DIN 63194) g/lItre100 - 150 macropore volume D ~ 30 nm m/~3.2-4.0 (accordlng to the H~ pressure method) :~ oll absorptlon accordlng to Gardner ml/100g 180 - 200 water retentlon value % 76 - 79 Cu abraslon mg 6 - 14 RDA abr~slon 36 - 100 REA ~bra310n 40 - 90 -~
scratches few to very few v I scos I ty mPas 5000-10000 (in a 16X glycerol-water dlsperslon ! -. splndle D wlth helIpath) ~:
`.

!~ -- 2 !

` 13~1~03 Fe content p. p.m. 240 - 280 In whlch process flrstly a thlckenlng preclpltated slllca sus-penslon Is produced by react I ng an a I kall-slllcate solutlon wlth ~n acld whlle avoldlng the formatlon of a gel, the alkall-slllcate solutlon and the acld beln~ slmultaneously fed Into an a~ueous a I kall-slllcate solutlon whlle for the preclp-ltat I on of the slllca Into an a I kall-slllcate solutlon havlng ~
a concentratlon of approxlmately 5 to 25 g of Sl02 per lltre :
of solutlon the acld and the alkall-slllcate solutlon are so fed wlth speclflc solutlon concentratlons and sPeclfic feed rates whlle ma I ntalnln~ a preclpltat I on temperature of between 80 and 90C In the reactlon medlum, the vlscoslty of the reac-tlon medlum Is kept unlformly low durln~ a perlod of at least 30X of the total preclpltatlon tlme, the pH value Is kept ~-between 10 and 12 and the addltlon of the reactants Is terml-nated before the vlscoslty, on passlng through a maxlmum, has decreased to ~ value of less than 100X above the Inltlal vls-co~lty, the pH Is adJusted wlth sulPhurlc acld to a value below 7 and, ~ocondly an abraslve preclpltatod slllca suspen-slon Is producod by ~ process In whlch an orlglnal preclpl-tated slllca suspenslon whlch Is produced by preclpltatlon of the slllca from an alkall-slllcate solutlon havlng a concen-tratlon of approxlmately 5 to 25 g of Sl02 per lltre of solu-tlon wlth an acld alkall metal-slllcate solutlon havlng a spe-clflc solutlon concentratlons and spec~flc feed rates whlle malntalnlng a preclpltatlon temperature of between 80 and 90C
In the reactlon medlum, whlle, when requlred, the vessel con-tent Is Intenslvely sheared durlng the entlre preclplt~tlon ~; tIme, the vlscoslty of the reactlon medium Is kept unlformly ~: low durlng a perlod of at least 30X of the entlre preclplta-tlon tIme and the pH value Is kept between 10 and 12 and the addltlon of the reactants Is termlnated before the vlscoslty, ;l ~i ,.
,,1 .
~ _ 3 _ ,.,j on p~ssln~ through a maxlmum, has decreased to a value of Jess than 100% above the Inltlal vl~coslty, the pH Is adJusted wlth sulphurlc acld to ~ value of 7 to 9, followed by dllutlng wlth hot water to a preclpltated slllca content of 10 to 30 ~ per lltre and a sodlum sulph~te content of 8 to 20 g per lltre and by heatln~ to 80 - 90C and whlle keepln~ thls pH value con-stant by a slmultaneous feed of alkall metal-slllcate solu- ;
tlon, sulphurlc acld and, when requlred, hot water over a pre-clpltatlon tlme of 15 to 180 mlnutes a fInal concentratlon of preclpltated slllca of 40 to 80 g per lltre Is obtalned, whereupon the suspenslon Is acldlfled wlth sulphurlc acld to a pH value of below 7, the two preclpltated slllca suspenslons are mlxed wlth each other and the preclpltatlon slllca Is f I 1-tered off, wash, spray-drled and $hen 3round wlth an ~Ir-Jet mlll. In a preferred embodlment the thlckenlng slllca suspen-slon Is produced substantlally accordln3 to DE-AS 1467019.
The abraslve preclpltated slllca suspenslon Is produced sub-stantlally accordlng to DE-OS 3114493, Ex~mple 10.
The present Inventlon also relates to a preclpltated slllca characterlzed by the foliowlng physlcal-chemlcal char-acterlstlc data:
IOS8 on dryln~ (DIN 53198) % 3 - 7 conductlvlty at 25C ~ S 250 - 400 (4X suspenslon) pH value (5X accordlng to DIN 53200) 6 - 7 BET surface area (DIN 66131) sqm~g 220 - 250 tamped denslty (DIN 53194) g/ll~re 80 - 120 `

macropore volume D > 30 nm ml/g3.2-4.0 (accordlng to the Hg pressure method) oll absorptlon accordln~ to Gardner ml/100g 200 - 220 water retentlon value % 77 - 78 Cu abraslon mg 8 - 12 RDA abr~slon 60 - 80 1331~03 REA abraslon 60 - 90 scratches few v I scos I ty mPas 14000-18000 (In a 16% ~Iycerol-water dlsperslon 1:1, splndle D wlth helIpath) Fs content p. p.m. 70 - 90.
The present Invent ! on also relates to a process for produclng the preclpltated slllca whlch has the followlng physlca I -chemlcal characterlstlc data:
loss on dryln~ (DIN 53198) % 3 - 7 conductlvlty ~t 25C ~S 250 - 400 (4X suspenslon) pH value (6% accordlng to DIN 53200) 6 - 7 BET surface area (DIN 66131) sqm/g 220 - 250 tamped denslty (DIN 53194) ~/lltre 80 - 120 U

macropore volume D ~ 30 nm m I /~ 3 . 2-4 . O
(accordlng to the Hg pressure method) oll absorptlon accordlng to Gardner ml/lOOg 200 - 220 water retentlon value % 77 - 78 Cu abrasOon m~ 8 - 12 RDA abra~lon 60 - 80 REA abraslon 60 - 90 scratchos few vlscoslty mPns 14000-18000 (ln a 16X glycerol-wator suspenslon 1:1, splndle D wlth hellpath) Fo content p.p.m. 70 - 90.
In the process flr~tly an abraslve preclplt~ted, :~
slllca suspenslon Is produced In that an orlglnal preclpltated slllca suspenslon whlch Is so produced by preclpltatlng the slllca from an alkall-slllcate solutlon havlng a concentratlon of approxlmately G to 26 kg of Sl02 per lltre of solutlon wlth an acld and alkall slllcate solutlon havlng speclflc solutlon concentratlons and speclflc feed rates whlle malntalnlng a preclp!tatlon temperature of between 80 and 90C in the reac-1331~03 tlon medlum, the vlscoslty of the reactlon medlum Is kept unl-formly low durlng a perlod of at least 30X of the total pre-clplta~lon tlme and th0 pH value Is kept between 10 and 12 and the addltlon of the reactants Is termlnated be~ore the vlscos-Ity, on passlng throu~h a maxlmum, has decreased to a value less than 100% above the Inltlal vlscoslty, the pH Is adJusted wlth sulphurlc acld to a value of 7 to 9, followed by dllutlng wlth hot water to a preclpltated slllca content of 10 to 30 g per lltre and a sodlum sulphate content of 6 to 20 9 of Na2S04 per lltre and by heatln~ to 80 - 90C, adJustlng the pH wlth sulphurlc acld to a value of 7 to 9 and whlle keeplng thls pH
value constant by a slmultaneous feed of alkall metal-slllcate solutlon, sulphurlc acld ~nd, when requlred, hot water over a preclpltatlon tlme of 150 to 180 mlnutes a fInnl concentratlon of preclpltated slllca of 40 to 80 g per lltre Is ob~alned, whereupon the suspenslon Is acldlfled wlth sulphurlc acld to a pH value of below 7, and s~condly a further preclpltated sll-Ica suspenslon Is produced by a proce~s whlch tho preclplta-tlon Is carrled out In a strongly acld pH range, preferably at a pH value of between 1.6 ~nd 2.6 by a conventlonal slmultane- ~.
ous addltlon of the 30dlum slllc~te and acld solutlons to a ~:
recelver wlth qubstances dlssolved In water whlch form Iron-soluble complex compounds, the two preclpltated slllca suspen-slons are mlxed wlth each other and the preclpltated slllca Is flltered off, washed, spray-drled and then ground In an alr-Jet mlll. In a preferred embodIment the abraslve preclpltated slllca suspenslon Is produced substantlally accordlng to DE-OS
3114493, Example 10. The further preclpltated slllca suspen-slon Is preferably produced accordln~ to DE-PS 1293138, the substances deflned thereln, preferably sodlum fluorlde and i oxallc acld, belng used as complexlng agents.
It h~s now been found th~t by mlxlna, In the suspen-1331~03 .-slon phase, precipltated slllcas havln~ varylng propertlespartlculsrly valuable preclplt~ted slllcas for Use In dentl-frlce~ are obtalned. Sultable preclpltated slllcas are obtalnable directly by preclpltating or by resuspendlng the fllter dough of preclpltated slllca. Preclpltated 31 ~ Ica sus-penslons produced dlrect~y by preclpltatlon are preferably used.
The preclpl~ated sll~cas obt~lned by mlxlng preclpl-tated slllcas In accordance wlth the present Inventlon, I.e., by fllterln~, dryln~ and ~rlndlng, have more favourable pro-pertles In den~lfrlces than dry mlxtures of the same startlng preclpltated slllcas In the same ratlo.
The preclpltated slllcas accordln3 to the pre~ent Inventlon show syner~lstlc effects such that wlth ~Iven effect of vl~coslty and thlckenlng there occur an unexpectedly hlgh abraslvlty, I.e., the vlscoslty-abraslvlty relatlon surprls-Ingly does not proceed llnearly (see Flg. 1).
The preclpltated slllcas accordln~ to the present Inventlon~can be produced wlth advanta~o In a partlcularly easy manner slnce the amounts can be exac~ly adJusted contlnu-20 9U8 I y or In batches by means of volume or w~lght measurements at known solld contents. Furthermore, tho wet proc~ss does not cause any dust problems. Separatlon phenomena are also mlnlmlzed. As compared wlth dry mlxtures the components In wet mlxtures lose Part of thelr Identlty. On mlxlng and pre- -sumably on drylng In partlcular there result Interactlons, smaller ones of large partlcles, the smail partlcles belng added on to larger ones or growln~ thereon.

Varlous forms of mu~ual flocculatlon and penetratlon In the aqueous ph~se must be taken Into ~ccount.
A first clear dlstlnctlon between dry mlxture and wet mlxture Is evldent from the partlcle slze dlstrlbutlon of - 1331~03 the ground samples. Whlie the dry mlxture of two preclpltated slllcas of dlfferent pArtlcle slze and denslty shows, as expected, two maxIm~ In the measurement wlth the Coulter counter and the correspondln~ curve ha~ a poak, the curve of the same preclpltated slllcas, mlxed wlth each other In the same ratlo whlle wet, follows a completely harmonlc course (see Flg. 2).
A further appreclable dlfference lles In the effect of thlckenlng. The preclpltated slllca ~ccordln~ to the pre- ;
sent Inventlon Increases the vlscoslty of a model mlxture of ~-10 ~Iycerol and water dlstlnctly less than the dry mlxture. ~-Preclpltated slllcAs havln~ moderate thlckenln3 -~
power allow hlgher degrees of fllllng and thus the productlon ;~
of pastes of ~full n char~cter.
As compared therewlth, pastes fllled to a lower ~;
degree have an "empty" ~nd "aqueous" effect In the mouth.
The thlckenlng offect of the preclpltated slllca must fully develop on belng Incorporated Into the toothpaste m~ss, I.e., thereafter there must be no change In ~he rheolog-Ical characterlstlc of the toothpaste whlch would render the fllllng and subsequent emptyln~ of the tubes and contalners dlfflcult or make It even Impos Ible.
Fundamentally a dsntlfrlce Is suPPosed to supportand Increase the cloanslng actlon of ~ tooth brush In that there Is bulId-up between brlstle and tooth surface a fllm that mechanlcally removes layers formed on the teeth.
Thls performance can only be produced by an agent h~vlng abraslve propertles.
It Is self-evldent that an Identlcal ~braslon per-formance preclpltated slllcas produclng the fewest scratches to be preferred.

The preclpltated slllcas accordlng to the present ~ - 8 -s 1331~03 stable vlscoslty development to an avera~e extent wlth hlgh abraslve and cleanslng actlon whlle avoldln~ deep scratches.
The measurement of the abraslvlty Is carrled out on extracted human teeth rendered radloactlve, I.e., on the exposed dentlne (RDA value) or on the tooth enamel (REA
value). In each case the Increase of the radloactlvlty of the suspenslon of the dentlfrlce to be tested after deflned work-Ing on the test teeth wlth brushes Is measured.
As a substltute the decrease In wel~ht of copper plates on brushln~ wlth test suspenslon can be determlned.
However, the values thus obtalned can frequently no~ be com-pared with RDA and R~A values.
Wlth Improved, I.e., mlcroscoplc methods In speclal laboratorles Informatlon on the cleanslng and pollshlng effect of dentlfrlces Is obtalned and samples causlng many or deep -~
scratches are elImlnated. These tests are carrled out on extracted anlmal and human teeth.
An Idea of the slze and number of scratches caused by a dentlfrlce Is obtalned oven by mlcroscoPlc or stereo-mlcroscoplc examlnatlon of metal Plates treated In a deflnedmanner llke those obtalned, for example, In the measurement of copper dust.
Apart from thlckenlny propertles and ~entle cleans-lng and pollshlng act!on, for health reasons a preclpltated slllca Is today expected to have a hl3h degree of purlty wlth re~ard to heavy metal contents. Furthermore, hl~her Iron con-tents can result In dlscoloratlons of the Pastes, whlch to some extent are coloured and transparent. Thus, a paste coloured blue wlth Indl~otln can have a ~reenlsh dlscol- ;

oratlon. Furthermore, the appllcant has also found that In the selectlon of certaln slllcas by means of the wet mlxtures :

_ 9 - ~

133~03 very pure products wlth partlcularly low Iron contents are obtalned.
The present Inventlon also relates to the use of the preclpltated slllca accordlng to the presen~ Inventlon as abraslve and pollshlng agent In toothpastes.
Ex~mole ~ 9 The sollds content (preclpltated slllca content) of -the preclpltated slllca suspenslon Is determlned as follows:
250 m/ of the preclpltated slllca suspenslon, cooled to 20C, are flltered by suctlon on ~ porcelaln suctlon fllter (dlam.
120 mm) and the fllter cake Is then washed wlth 500 m/ of ;

water untll It Is low In electrolytes. The cake Is drled at 106C to a constant welght. ~ -Computation: g wel~hed out x 4 ~ of solIds per lltre.
Exam~lç 1 A preclpltated ~IIIca havlng a thlckenln~ effect Is substantlally produced accordlng to DE-AS 1467019. In a varl-atlon, for economlc roason~ concentrated sodlum ~IIIcate solu-tlon denslty ~ 1.353 g/ml) and concentrated sulphurlc acld (96%) are used for thls Purpose. Thls results In a sollds content of 86 ~ per lltre In the preclpltated slllca suspenslon.
In a rubber-coated 120-lltre preclpltatlng vessel 73 lltres of hot water ~nd 5.25 lltres of sodlum slllcate solu-tlon (denslty 6 1.353 g~m/, molar ratlo Sl02:Na20 - 3.4~) are heated to 85C whlle stlrrIng. DurIng the next 90 mlnutes whlte stlrrlng and malntalnlng the temperature there are slmultaneously added to thls alkallne preclpltatlon recelver 16.5 lltres of sodlum sllicate solutlon (denslty ~ - 1.353 g/ml, molar ratlo Sl02:Na20 3.46) wlth 11.0 lltre/hour and 1.448 lltres of sulphurlc acld (96%) wlth 0.965 per -, 1 0 --` 1331~03 lltre/hour. The addltlon of the reactants Is termlnated before the vlscoslty of the reactlon medlum, on passlng through medlum, has decreased to a value less than 100% above the Inltlal vlscosl~y. The preclpltated slllca suspenslon thus obtalned Is then adJusted wlth sulphurlc acld (96%) to a pH value of 3.5. Thls Is achleved by an Inflow of acld at 1.25 lltres/hour exten,dlng over several mlnutes. The preclpl-tated slllca suspenslon thus obtalned has a sol Id content of ;~
85.0 g per lltre.
Exam~e Thls example descrlbes the productlon of an abraslve ~,~
preclpltated slllca accordlng to DE-OS 31 ~4 493, Example 10, but wlth the dlfference that the entlre amount of water Is added to the preclpltatlon recelver so that the addltlon of water durln~ th¢ prcclpltatlon Is omltted and the preclplta-tlon tlme Is extended from 60 to 100 mlnutes.
In a rubber-coated 120-lltre preclpltatlon vessel 75 lltres of hot wator and 5.26 lltro~ of ~odlum slllcate solu-tlon (denslty 1.353 g/ml; mol~r ratlo: Sl02:Na20 ~ 3.46) are heated to 85C whlle stlrrlng. DurIng the subsequent 90 mlnutes whlle stlrrln~ and malnta I nlng the temper~ture of 85C
there are added slmult~neously to thls alkallne preclpltatlon recelver 15.5 lltros of sodlum slllcate solutlon (denslty -1.353 ~/ml; molar rat!o Sl02:Na20 - 3.46) wlth 11.0 lltres/h and 1.448 lltres of concentrated sulphurlc acld (96X) wlth 0.965 lltre per hour. The addltlon of the reActants Is termlnated before the vlscoslty of the reactlon medlum, on -~
passln~ through A maxlmum, has decreased to a value less than 100X above the Inltlal vlscoslty.
Tho preclpltated slllca suspenslon thus obtalned Is then adJusted wlth sulphurlc acld (96X) to a pH value of 8.5.
Thls Is achleved by an Inflow of acld at 1.25 lltres per hour , _ 1 1 _ ``` 1~31503 extendlng over several mlnutes. The preclpltated slllca sus-penslon thus obtalned has a solId con~ent of 85 g per lltre;
Its Na2S04 content Is 55 g per lltre.
Durlng the entlre preclpltatlon tlme of 90 mlnutes the vessel content Is Intenslvely sheared by a centrlfugal pump clrculatlng the vessel content several tImes. Detalled data on the equlpment and apparatus and on the condltlons of shearlng are evident from DE-PS 17 67 332, partlcularly from column 8, llnes 31 to 68.

Tho orlglnal preclpltated slllca suspon~lon thus produced Is adJusted wlth water to a preclpltated slllca con-tent of 13 9 per lltre and 8.5 ~ of Na~S04 per lltre.
16.06 lltres of thls suspenslon are heatod to 85CIn a rubber-coatçd 120-lltre preclpltatlon vessel whlle stlr-rlng. Whlle malntalnlng thls temperature and a pH value of 8.5 there are 8 Imultaneously added to the preclpltated slllca suspenslon durIng a perlod of 100 mlnutes 23.1 lltres of sodlum sillcate solutlon (denslty ~ ~ 1.33 g/ml; molar ratlo Sl02:Na20 - 3.46) at a rate of 231.0 m/ per mlnute and 1.94 lltres of sulphurlc acld (~6X) at a rate of 19.4 m/ per mlnute. The preclpltated slllca suspenslon Is subsequently adJusted wlth sulphurlc acld (96%) to a pH value of approxl-mately 3.5. The preclpltated slllca suspenslon has ~ solld content of 92.0 9 per lltre. The preclpltated slllc~ obtalned on fllterlng off and washlng has the physlcal-chemlcal charac-terlstlc data complled In Table 1. -~
Ex~mDI~ 3 18.6 cu m of the suspenslon of the ~braslve preclpl-tated slllca suspenslon havlng a preclpltated slllca content of 92.0 g per lltro accordlng to Example 2 are mlxed wlth 40.0 cu m of thlckonlng preclpltated slllca accordln~ to Example 1 havlng a content of preclpltated slllca of 85.0 ~ per lltre `- 1331503 correspondlng to a welght ratlo of 1:2 - (relatlve to the pre-clpltated slllca content).
For the further treatment the preclpltated slllca of thls suspenslon mlxture Is separated by means of fllter presses. The preclpltnted slllca content In the fllter press dough Is 24%. The washed and spray-drled fllter press Is ground In an alr-Jet mlll. The preclpltated slllca thus obtalned has the physlcal-chemlcal characterlstlc data com-Plled In Table 1.
ExamDI~ 4 1.0 kg of the washed, drled and ~round thlckened preclpltated slllca of Example 1 Is mlxed, whlle dry, wlth the abraslvo preclpltated slllca of Example 2 that also has been washed, drled and ground In a comparable manner, I.e., the welght ratlo of the two preclpltated slllcas Is 1:2 as In Example 3.
Tho preclpltated slllca obtalned has the physlcal-chemlcal characterlstlc data complled In T~ble 1.
Examolo 6 As defIned In DE-OS 31 14 493.4, Example 12 an abra-20 slve preclpltated slllca l~ flrst produced as follows: In a ~-~

preclpltatlon vossel fltted wlth a stlrr~r 27.0 cu m of water and 10.0 cu m of preclpltatod slllca suspenslon substantlally ~
as In DE-AS 14 67 019, whlch Is acldlfled only to pH 8.5 and ~:
has a sillca content of 8.5 g per lltre, are mlxed and heated to 85C. Dlfferln~ from DE-AS 14 67 019, concentrated sodlum : slllcate solutlon (denslty ~ 1.3B3 g/ml) and concentrated sul- ~ ~;
phurlc acld (96X) are aPPlled for economlc reasons, resultlng In a oontent of sollds of 85 g per lltre In the preclpltated slllca suspenslon. At thls temperature and whlle malntalnlng a pH vzlue of 8.~ to 9 there are slmultaneously added for a perlod of 80 mlnutes 9.6 cu m of sodlum slllcate solutlon ~:' 1331503 ~ ~:

(density ~ 1.363 g/ml; modulus StO2:Na20 ~ 3.46), 0.94 cu m of concentrated sulphurlc acld (96%) and 20.0 cu m of hot water.
On completed preclpitation the pH of the suspenslon Is adJusted to a value of 3.5, using concentrated sulphurlc acld (96%). The content of precipltated slllca Is 66.0 9 per lltre.
25.75 cu m of thls preclpltated slllca suspenslon are mlxed wlth 40.0 eu m of the suspenslon of a ~h~ckenlng slllca content of 85.0 g per lltre correspondlng to a welght ratlo of 1:2 (relatlve to the solld content).
For the fur~her treatment the procedure Is llke that In Example 1. The slllca content In the fllter press dou~h Is 21%.
The preclpltated slllca obtalned has the physlcal-chemlcal characterlstlc data complled In Table 1.
Exam~lg_Ç
A preclpltated slllca low In Iron content Is pro-duced accordln~ to Examplo 1 of D 47 633 (- DE-PS 12 93 138).
18.6 lltres of water are put Into a cyllndrlcal, 30-lltre wooden tank provlded wlth a wooden bl~de mlxer (dlameter 20 cm, helght 6 cm, 90 r.p.m.).
The w~ter 18 Indlrectly heated to 85C wlth an acld~
proof steam-coll. In thc recelver 26 ~ of sodtum fluorlde and 6.25 g of oxallc acld are dlssolved.
Whlle stlrrlng and malntalnlng a pH value of 2.0 5.45 lltres of a commerclal sodlum slllcate (molar ratlo of Na20:SI02 1:3.36, denslty ~ 1.34) and 0.55 lltre of sul-phurlc acld 96X of H2S04) were fed In wlthln 90 mlnutes at opposlte polnts.

¦ 30 Stlrrlng of the nascent suspenslon Is contlnued for approxlmately 30 mlnutes, followed by fllterIn~ on suctlon fllters In a conventlon~l manner, usln~ twlce the amount of ~5 --` 1331503 water, relatlve to the volume of the suspenslon, whlch Is then .
washed, spray-drled and ground wlth an ~Ir-Jet mlll.
ExamDle 7 18.3 cu m of suspenslon of the abraslve slllca accordlng to Example 2 havlng a preclpltated slilca content of 92.0 9 per lltre are then mlxed wlth 40.0 cu m of suspenslon of a preclpltated slllc~ low In Iron accordlng to Example 1 of D 4763~ (' DE-PS 12 93 138) havln~ a preclpltated slllca con-tent of 83.8 g per lltre correspondlng to a welght ratlo of 1:2.
For the further treatment the procedure is llke that In Example 3. The sllic~ content In the press dough Is 23X.
The preclpltated slllca obtalned has the physlcal-chemlcal characterlstlc data complled In Table 1.
ExamDle 8 For the productlon of a preclpltated slllca that Is low in Iron and has thlckenln~ and cieansln~ propertles the suspon~lon of a low-lron content slllca produced accordlng to Example 1 of DE-PS 12 93 138 (- D 4763~3) Is mlxed In speclflc -ratlos wlth the susponslon of the abraslve slllca produced accordlng to Example 2. Table 2 shows that any welght ratlo the Iron content of the mlxture Is lower than that of the com- ~
parable dry mlxture. .
T~bl 2 Mlxture Ratlos In Parts by Welaht abraslve sllica as descrIbed2 slllca low In Iron accordlng to Example 1 of D 47633 1 1 2 .
wet mlxture 157 p.p.m. ~18 p.p.m. 80 p.p.m.
Fe Fe Fe . . . _ _ .

, - 15 -1331~03 dry mlxture 1~8 p.p.m. 137 p.p.m. 101 p.p.m.
Fs F~ Fe ~ .:
~':
: :
: :

~ 30 ;
:

1331~03 P, ^ C u~ ) G`.o El P~ ~~ O O
n ~s ~ n ~ o n 3 ~ u) n ~ C~
~_" o op ~ ' O 1~ ~1_ D C
~' 'C , ~' O O 1'' Ul O ~ OP O
~- C P
~ 1_ o ~ _ OI o Z
~ n ;~

o ~ 3 ~ o~ _ 3 ~ 3 ~ 3 ~ . ~ .
u~ ~ ~ x ~-~ N ,~ oo N W CO 0~ ~ W ~n ~

~: X ', o o 'C o o o0~ o ~ O o ~0 o 3 o ` W W .~ ~ '~ u~ x ~ ~
o o ~ o o ~ o~ ~ o o o w o 3 : --~ .

N O ~ ~ o ` ~ N ~ N ~ ~ ~n X
o 'C O W ~ O~ O Ul ~ O Vl 'O ' ' , ~o . ~ ~ _, ~

W O ~ ~ ~ ~ ~ ~ ~ ~ o Vl :
O ~h a~

~o ~ ~ O O ~ O Co o ~n ~~ o --` 13315~3 The physlcal-chemlcal characterlstlc data deflned In the present applIcatlon are dctermlned as follows:
Loss on Drylng accordlng to DIN 53198.
Conductlvlty at 25~C/4% suspenslon.
A sample of 4.0 g Is heated wlth 50 m/ of completely fully desalted water In a 150 m/ beaker and bolled for one mlnute whlle stlrrlng. The suspenslon Is then transferred Into a 100 m/ flask, cooled and fllled up to the mark wlth completely full desaltsd wat~r. The electrlc conductlvlty is moasured wlth a commerclal measurlng devlce, for example, of the "Wlssenschaftllche Werkstatten (WTW), conductImeter type LF530, at 25C.
pH Value In a 5% dlsperslon accordlng to DIN 53200.
BET Surface Aren accordlng to DIN 66131.
A sample Is rlnsed wlth nltrogen at 100C. The vol~
umetrlc measurement is carrled out wlth pure nltrogen ~t the temperature of the llquld nltrogen (-195.8C).
T~mped Denslty accordlng to DIN 63194.
The test 18 carrled out wlthout pretreatln~ the 5ample.
MAcropor~ Volume D ~ 30 nm accordlng to ~he Hg pres-sure method.
The measurement Is c~rrled out wlth the Mercury Pressure Poroslmeter, 200 Serles of the flrm of Carlo Erba Strumentazlone.
011 Absorptlon accordlng to Gardner-Coleman accord-lng to ASTM:D 1483-60.
Water Retente Value.
The water retentlon value Indlcates the portlon of water contalned In the press dou~h upon squeezlng out preclpl-tated slllca suspenslon and washlng to a pauclty of elec-trolyte. For the computatlon 260 ~ of press dough are drled _ 18 -- 1331~03 at 105C untll a constant welght Is attalned.
Computatlon -~w~ah~ Q~ Q0 100 - , % water retentlon value wel~hed sample.
Partlcle Slze Dlstrlbutlon accordlng to Coulter ~-Counter/100~ caplllary. -The measurement Is carrled out wlth the Coulter Counter model TA ll of the flrm of Coulter Electronlcs GmbH.
O Dstermlnatlon of the Cu Abr~slon In a 10X Glycerol Dl~perslon: --a) Productlon of the Glyc~rol Dlsp~rslon 153 g of ~Iycerol (anhydrolcs, purest Ph Eur, BP USP, denslty 1.26 g/m/, flrm of Merck, Darmstadt) are welghed Into a polyethylene beaker (250 m/). 17 g of preclpltated slllca ar~ carefully Inter-mlxed wlth ~ spatula. The mlxture Is subsequently homogenlzed for 12 mlnutes wlth a blade agltator (dl~meter 5.2 cm) at 1500 r.p.m.
b) Abraslon Measurlng Procedure The determlnatlon of the abraslon measurement Is carrled out In an abr~slon testlng devlce, whlch Is known from followlng publlcatlons: (1) Pfungle: Fette, Selfen, Anstrlchmlttel, ~ (6) (19B1), Page 445 to 451. "Abraslon und Relmgunskraft von Putz Korpern In Zahnpasten" ~2) A. Reng, F. Danny; Parfumerle und Kosmetlk 5~ (2) (1978), Page 3~ to 45; "Anwendung-~techmlsche Prufung von Zahnpasten". For thls purpose each of the slx troughs of the testlng devlce Is coated wlth 20 m/ of the homo~eneous dlsperslon. The abraslon caused by slx surface-ground nylon brushes on slx surface-ground CU sheets (electrolyte-copper) In fIve hours wlth 50000 up and down strokes Is determlned by dlfferentlal welghlng.
When computlng the abraslvlty average values are formed from :

I , ,~

`` 1 3315~3 the values obtalned. The abraslon (abraslvlty) Is deflned In mg of Cu.
RDA Abraslon.
The method Is descrlbed In Journal of Dental Research, 55~4), 563 (197~) and Is ~Iso applled to the REA
abraslon test.
REA Abraslon.
Scratches are determlned vlsually by means of a mlcroscope.

Determln~tlon of the Vlscoslty measured In a 16%
glycerol-water dlsperslon (mlxture 1:1) wlth a Brookfleld Vls-coslmeter RTV, splndle 10, wlth helIpath at 10 r.p.m.1. Test Mlxtures 48 g of slllca 126 g of glycerol (approxlmately 87%, purest Ph Eur. BP, den-slty , 1.26 g/ml fIrm of Merck, Darmstadt) 126 g of dlstllled water 300 g of a 16X dlsperslon, relatlve to slllca.

2. Procedurc.
In a 400 m/ bcaker gla~s (broad shape) the abraslve slllcas are stlrred by hand Into the glycerol-water mlxture wlth a glass rod (1 mlnute) and allowed to stand for 24 hours. The vlscoslty Is measured thereafter.

3. Measurement.
The measuremont of the vlscoslty Is carrled out In the same beaker wlth a*Brookfleld-vlscoslmeter RVT, splndle 10 wlth .
helIpath at 10 r.p.m.

4. Computatlon.

~ Read scalo value X factor - vlsco~lty In mP~s .

:
* Trademark --:
:

~j - 20 -

Claims (6)

1. A precipitated silica having the following physical-chemical features:

*BET - Brunauer-Emmett-Teller equation, for surface area determination in study of sorption **RDA - Radioactive Dentin Assay ***REA - Radioactive Enamel Assay.

2. A process for the production of a precipitated silica as in claim 1, comprising:
(A) producing an abrasive precipitated silica suspension by providing an alkali silicate solution reaction medium with a concentration of approximately 5-25 g of SiO2 per liter solution, (Claim 2 con't.[Page 2]) adding to said reaction medium an acid and alkali metal silicate solution with selected solution concentrations and selected flow velocities, while maintaining a precipitation temperature of between 80°C. and 90°C., in the reaction medium, said reaction medium having an initial viscosity value, holding the viscosity of the reaction medium uniformly low and maintaining the pH level between 10 and 12 during a time period of at least 30% of the total precipitation time, the viscosity of the reaction medium thereafter increasing up to a maximum value, terminating the addition of the said acid and alkali metal silicate solution before the viscosity of the reaction medium following passage through said maximum value drops to a value less than 100% above the initial viscosity, adding sulfuric acid to adjust the pH to 7 to 9, the suspension thus obtained being diluted with hot water to a precipitated silica content of from 10-30 g/l and a sodium sulfate content of 6-20 g of Na2SO4/l, heating the suspension to 80°C.-90°C., and while holding this pH level constant simultaneously adding alkali metal silicate solution, sulfuric acid, for a precipitation time of from 15 to 180 minutes, to thereby obtain a precipitated silica final concentration of from 40 to 80 g/l, acidifying the suspension with sulfuric acid to a pH
below 7 to thereby obtain an abrasive precipitated silica, and (B) providing a second precipitated silica suspension in which the precipitation is carried out in the strongly acidic pH range by simultaneous feed of sodium silicate and acid solution to reaction vessel containing at least one substance dissolved in water, which form soluble complexes with iron, and thereafter mixing the two precipitated silica suspensions (A) and (B) together, (Claim 2 con't. [Page 3]) and recovering a combined precipitated silica and then thoroughly comminuting with an air jet mill.

3. The process of claim 2 wherein said second silica suspension is produced by a reaction with pH of 1.5 to 2.5.

4. The process of claim 3 wherein the two silica suspensions are mixed together in the ratio range of 1:2 to 2:1 based on the solids content.

5. The process of claim 2 wherein the acid is sulfuric acid and the alkali silicate is sodium silicate.

6. Use of a precipitated silica as defined in claim 1 as an abrasive or polishing medium in a toothpaste.