CA1190378A

CA1190378A - Dicalcium phosphate dihydrate having improved monofluorophosphate compatibility

Info

Publication number: CA1190378A
Application number: CA000417237A
Authority: CA
Inventors: Fawzy G. Sherif; Helmut W. Majewski; Francis A. Via
Original assignee: Stauffer Chemical Co
Current assignee: Solvay USA Inc
Priority date: 1981-12-31
Filing date: 1982-12-08
Publication date: 1985-07-16
Also published as: GB2112762B; DE3246884C2; JPS59121106A; GB2112762A; JP2594251B2; DE3246884A1

Abstract

DICALCIUM PHOSPHATE DIHYDRATE HAVING IMPROVED
MONOFLUOROPHOSPHATE COMPATIBILITY

Abstract Dicalcium phosphate dihydrate compositions having improved monofluorophosphate compatibility are prepared by adding pyrophosphoric acid to the di-calcium phosphate dihydrate reactor and terminating the reaction by which the dicalcium phosphate dihydrate is formed at a pH ranging from above about 2.2 to below about 4.9.

Description

~1--BACKGROUND OF T~IE INVENTION
The present in~ention relates to dicalcium phosphate compositions having improved monofluoro-phosphate compatiblility, and to a process for the preparation thereoE.
Dicalcium phosphate dihydrate has been used as a dental polishing agent in toothpastes and powders for many years.
This matexial is typically produced by first reactin~ a slaked lime slurry with phosphoric acid to form a dicalcium phosphate dihydrate precipitate, and then separating the dicalcium phosphate dihydrate precipitate from the mother li~uor, after which it is dried and milled to form the final product as a fine powder.
; One serious problem which was initially en-countered in the use of dicalcium phosphate dihydxate in toothpaste was the tendency of the dicalcium phos-phate to "set~up" and become lumpy. When this occurs in toothpaste formulations, it makes it difficult to extrude the toothpaste Erom the tube in which it is usually packaged.
A second problem was encountered with the advent of the use of rnonofluorophosphate additives in toothpaste formulations. It was found that the mono~
fluorophosphate components woul~ react with th~ dicalcium phosphate whereby the monofluorophosphate component was converted Erom a water-soluble form to an insoluble form, Since the ~eneficial effect of monofluorophos-C-61~0 '7~3 phate additives in too-thpaste are understood to be derived principall~ from the water~soluble formr it has become important to develop toothpaste form-ulations which permit an effective amount of mono-flu3rophosphate component tG remain in the water-soluble state.
~ he term "mono~luorophospha-te compatibilit~"
has ~een used as a term-of-art to describe the tendency of such formulations to permit the monofluoro-phosphate component to remain in the water solublestate.
The monofluorophosphate compatibility of a particular formulat.ion may be determined by a variety of methods. Preferably~ the monofluorophosphate com~
patibility of a formulation is determined by actual.ly preparing the formulation, placing it in storage for a predetermined period of time under controlled condi~ions, and then determining the amount of water-soluble mono~luorophosphate which remains in the formulation after having been stored under these conditions~ A1-ternatively~ a simulated ~ormulation, such as the di-calcium phosphate dihydrate to be tested, glycerine and a known amount of a monofluorophosphate component~
such as sodium monofluorophosphate can be "quick aged"
. b~ maintaining it at an elevated temperature for one or more hours r and then determining the ~mount of water-soluble monofluorophosphate remaining a~ter such conditioning D There are, of course, many other me-thods for measuring the relative monofluorophosphate compat-;bili.ty o~ various samples o~ dicalcium phosphatedihydrate.

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U. S. Patent 2~287,699 teaches that dicalcium phosphate dihydrate may be stabilized by adding a small amount of an alkali meta:L pyrophosphate to the mother liquor, at a controlled pH, during :the preparation of the dicalcium phosphate. Specifically, it is taught that after precipitation of the dicalcium phosphate in the mother liquor, a small amount of alkali metal pyrophosphate should be added and the entire slurry then heated for a short period of time, while maintain-ing the pH of the mother liquor ahove 7.
Alternatively, the precipitate may be treat-ed during the subsequent washing step.
It is also known to those skilled in the art that other forms of pyrophosphate can also be used to stabilize the dicalcium phosphate.
~ nother method for stabili2ing dicalciu~
phosphate is disclosed in U.S. Patent 2,01g,410.
This patent teaches that dicalcium phosphate can be stabilized by the addition ~hereto oE a magnesium salt such as trimagnesium phosphate, magnesium sulfate, magnesium stearate, or dimagnesium phosphate.

United States Patent 4,312,843 teaches a method for preparing dicalcium phosphate dihydrate compositions having improved monofluoro-phosphate compatibility which involves the addition of pyrophosphoric acid to the reaction mixture and termination of the reaction within a very limited pH range of from about 4.9 to about 5.5.

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Surprisingly and unexpectedly in view of the teachings of the pr.ior art, it has now been found that improved monofluorophosphate compatibility can be achieved with the addition of pyrophosphoric acid and termination of the reaction at pHs below about 4.9.
Summary_of the Invention . _ In accordance with the present invention there is now provided a process for preparing dicalcium phosphate dihydrate having improved monofluorophos-phate compatibility comprising the steps of (a) reacting a slaked lime slurry with phos-phoric acid to form a monocalcium phosphate solutiGn;
(b) adding to the solution additional slaked lime slurry and pyrophosphoric acid in amounts sufEicient to form a dicalcium phosphate dihydrate slurry having a pH rangin~ from above about 2.2 to below about 4.9; and (c) separating the dicalcium phosphate dihydrate from the slurry.
Detailed Description of the Invention In accordance with the present invention it has now been discovered that termination of the re-action at pHs ranging from above about 2.2 to belowabout 4,9 together with addition of pyrophosphoric acid to the reaction mixture resul-ts in the formation of di.calcium phospha-te dihydrate having improved mono-fluorophosphate compatibility. This is contrary to the prior art which teaches the pH range of from 4.9 ~o 5.5.

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It has now been found that the formation of dicalcium phosphate dihydrate crystals during addition of the lime slurry to the monocalcium phosphate solutio~ begins at a pH of about 2.2, and that the S crystals which are formed at that point have a very high degree o~ monofluorophosphate ccmpatibility.
Yield at this low pH however, is relatively low.
The dicalcium phosphate dihydrat,e crystals which are formed at pHs between about 2.2 and about 3 appear to have the same very high degree of mono-fluorophosphate compatibility~ but yield increases as additional lime slurry is added to increase the pH. Thus, higher pHs are accompanied by higher yields.
It appears that the crystals which are formed at p~'s above about 3.3 are less compatible with mono-fluorophosphate than those formed at lower pH's but that the overall monofluorophosphate compatibility of the total mixture of crystals formed remains quite high even at a final pH o~ about 4.9.
The lime which is used in the practice of the present invention is the same type rotary kiln lime or shaft kiln lime as is used in conventional di-calcium phosphate processes.
The slaked lime slurry is prepared by mixing lime with either water or recycled mother liquor (i.e,, that which remains after removal of dicalcium phosphate dihydrate product from the final slurry), or both, in amounts from about 100 to about 150 grams CaO/liter and at a temperature preferably ranging from about 70C. to about 74C. At higher concentrations the mixture will become a gelatinous mass which will be difficult to handle, while at concentrations below the range specified the process "payload" will be unnecessa:rily reduced.
The slaked lime slu:rry is then added to phos-phoric acid to form a monocalcium phosphate solution.
The acid which is used is preferably a foodgrade phosphoric acid, preferably at an initial conoen-tration o~ about 85~ Varying amounts of recycled mother liquor may also be added to the lime slurry and phosphoric acid, with the specific amount in each case being determined in accordance with the preferences of the individual practitioner. The compositional range of the monocalcium phosphate solution will be approximately as followsO

High(Wt.~) Low(Wt.%) CaO 4 2 pH 2 These ranges are set forth as examples o~
those which are typical, and are in no way intended to be limitations on the scope of the pxesent invention.
Those skilled in the art will understand that higher and lower amounts may also be used, provided that the reaction mixture meets the requi~ements of the prac-titioner.
When th~ lime slurry and phosphoric acid are brought together under the conditions specified above, a reaction will ensue and a monocalcium phos-phate solution will be Eormed. The essential comple-tion of the reaction will be indicated by a steady-state pH of from about 1.0 to about 2Ø

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The preparation of the monocalcium phosphate solution can be carried out as a continuous, batch or semi-batch process.
Once the monocalcium phosphate solution has been formed, the pyrophosphoric acid and additional slaked lime slurry are added to Form the dicalcium phosphate dlhydrate slurry. Th.is reaction is exo-thermic and external cooling is required to control the reaction temperature. The react.ion temperature should preferably be controlled at or below about 50C.
It is preferable to first add the additional slaked lime slurry to the monocalcium phosphate solu-lS tion until a desired pH is reached, and then the pyrophosphori.c acid is added. The minimum amount of pyrophosphoric acid which should be added is about 0.1~ by weight of di.calcium phosphate dihydrate to be prepared while the maximum added should be about 1.0%. The pH will, of course, drop slightly when the pyrophosphoric acid is added.
Although it ls preferable to add the pyro-phosphoric acid and slaked lime slurry to the mono-calcium phosphate solution in the sequence just de-scribsd, it is within the scope of the invention to add these two ingredients in other than that sequence.
It is, however, important that the final pH after both of these ingredients are added, ranges from above about 2.2 to below about 4.9 and preferably, that is be from about 3.3 up to about 4.7.

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The amount of pyrophosphoric acld added should ranye from about 0.1% to about 1.0% by weight oE dicalcium phosphate dihydrate to be produced, and preferably, from about 0.3~ to about 0.4~.
Once the dicalcium phosphate dihydrate slurry : has been formed as described above, the dicalcium phosphate dihydrate product is separated from the mother liquor. The mother liquor may then be re-cycled to the beginning of the process, or discarded.
The separation of the dicalcium phosphate d.ihydrate from the slurry can be accomplished by any of several conventional techniques. These techniques include, but are not limited to~ decantation, centri-fugation~ filtration and the like, although decant-ation is preferred because of its simplicity.
The stabilizers which are usually added to dicalcium phosphate dihydrate are intended to prevent the licaking" and "lumping" which occurs in unstabillzed dicalcium phosphate dihydrate as a result of dehydration.
There are many stabilizers known to be useful for this purpose, These include, but are not limited to di-magnesium phosphate, trimagnesium phosphate, mag-nesium stearate and magnesium sulfate. The amount of stabilizer added ranges from about 0.5~ to about 5.0~ by weight of dica].cium phosphate dihydrate.
Preferred stabilizers for use in the practice of the present invention are dimagnesium phosphate trihy-drate, trimagnesium phosphate octahydrate, and mixtures thereGf.

It is preferred to add the stabili2er to the dicalcium phosphate dihydrate by dry-blending these two components after the dicalcium phosphate dihydrate has been dried ox a~ter it has been dried and milled.
It is, however, within the scope of the invention to add the stabilizer to the product slurry before separating the dicalcium phosphate dihydrate therefrom;
or to the "wet" dicalcium phosphate dihydrate prior to drying and milling.
In order that the present invention be more fully understood, the following examples are given by way of illustration. No specific details or enumerations contained therein should be construed as a limitation on the present invention except insofar as they appear in the appended claims.

Slaked lime slurry (10,85~ CaO~ prepared by slaking lime with recycled mother liquor from a dicalcium phosphate dihydrate synthesis, was added ; 5 with stirring to 750 grams o~ a clear solution of ; monocalcium phosphate prepared using the same recycled mother liquor, until the ~pH reached 5.88. Pyro-phosphoric acid, in the amount of 1.13 grams was then added, and stirring continued for another 30 m.inutes, at which time the final pH was found to be 5,2, The temperature of the mixture was maintained at 40~C.
during the foregoing additions, through the use of external cooling, The resulting dicalcium phosphate dihydrate product was recovered from the slurry by filtration, after which it was dried and milled, A portion of the dicalcium phosphate dihydrate was then blended with 2~ trimagnesium phosphate, by weight of dicalcium phosphate dihydrate, and used to prepare a standard toothpaste formulation which also included sodium monofluorophosphate in an amount equivalent to 1000 ppm. fluori.de ion, The toothpaste formulation was then aged for . three weeks at 49C., after which the amount of water soluble monofluorophosphate remaining was determined.
The results are shown in Table I, .

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EXAMPLE II

A quantity of dicalcium phosphate dihydrate was prepared as in Example 1, except that the slaked lime slurry addition was terminated at a pH of 3.4 ("Terminal p~ A small amount of pyrophosphoric acid was added and the slurry stirred for an additional 30 minutes, after which the f.inal pH was found to be 3.3.
The dicalcium phosphate dihydrate was then used to prepare a standard toothpaste formulation, which was aged and tested for monofluorophosphate compatibility as in Example 1. The results are shown i.n Table I.

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A slaked lime slurry was prepared by mixing 310 grams CaO with 2400 ml. distilled water at a temperature which varied between 50C. and 78C~
The slurry was then passed through a 140 mesh sieve and then cooled to room temperature~ The slurry was found to contain 124 grams CaO/liter.
A monocalcium phosphate solution was prepared by mixing 358 grams of the slaked lime slurry with 401 grams of 85% H3PO4 and 608 ml. distilled water, To the monocalcium phosphate slurry was then added additional slaked lime slurry, at a temperature of 40C., until the pH of the mixture reached 5.8 (the l'Terminal pH was therefore 508).
Pyrophosphoric acid t in the amount of 2.0 grams, was then added and the mixture stirred for another 30 minutes. The final pH was then found to be 5.4. The dicalcium phosphate dihydrate product was then recovexed by filtration, dried, milled and blended with trimagnesium phosphate as in Example I.
A portion of the product was then used to prepare a standard toothpaste formulation, as in the previous examples, and aged for three weeks at 49~C. after which

2 the amount of water soluble monofluorophosphate re-maining was determined. The results are shown in Table II.

The procedure of Exc~mple 3 was repeated, except that the addition of lime slurry was terminated at a pH of 5.0 and the final pH after addition of pyrophosphoric acid was 4.7. The monofluorophosphate compatibility test results are shown in Table II.

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EXAMPLE S

The procedure of Example 4 was repeated, except that the terminal pH was 4.5 and the final pH was 4.5. The monofluorophosphate compatibility test results are shown in Tahle II.

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TABLE I
MONOFLUOROPHOSPHATE COMPATIBILITY OF DICALCIUM
PHoA~l~TE DRATE

(Product Made Using Recycled Mother Liquor) Example ~ Compatibility*1 No. Terminal Final Product Control*2 1 5.9 5.2 630 650 ~ 3.4 3~3 700 : TABLE II
MONOFLUOROPHOSPHATE COMPATIBILITY OF DICALCIUM
i ~5 PHOSPHATE DYHYDRATE
_ (Product Made Using Distilled Wa-ter) Example pH Compatibility*1 No. Term ~ al Product ~-- Control* 2 . . ~
~0 3 5.8 5.4 6~0 625

4 5~0 ~7 690 4.5 4,5 ~70 "
. *1: Monofluorophosphate compatibility~ expressed as : 25 ppm F-.
*2: The control samples for both tables I and II were taken from the same source.
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Claims

Claims l. A process for preparing dicalcium phosphate dihydxate having improved monofluorophosphate compatibil-ity comprising the steps of:
a) reacting a slaked lime slurry with phos-phoric acid to form a monocalcium phosphate solution;
(b) adding to said solution additional slaked lime slurry and pyrophosphoric acid in amounts suffic-ient to form a dicalcium phosphate dihydrate slurry having a pH ranging from above about 2.2 to below about 4,9; and (c) separating said dicalcium phosphate dihydrate from said slurry.
2. The process of Claim l wherein the amount of pyrophosphoric acid added ranges from about 0.1%
to about 1.0% by weight of dicalcium phosphate dihydxate to be produced.
3. The process of Claim 1 wherein said pH
ranges from above about 3.3 up to about 4.7.
4. A dicalcium phosphate dihydrate composition having an improved monofluorophosphate compatibility comprising a mixture of a stabilizer with a dicalcium phosphate dihydrate product prepared by the steps of:
(a) reacting a slaked lime slurry with phos-phoric acid to form a monocalcium phosphate solution;
(b) adding to said solution additional slaked lime slurry and pyrophosphoric acid in amounts sufficient to form a dicalcium phosphate dihydrate slurry having a pH ranging from above about 2.2 to below about 4.9;

(c) separating said dicalcium phosphate di-hydrate product from said slurry, drying, milling and (d) blending said dicalcium phosphate dihydrate with a stabilizer.
5. The composition of claim 4 wherein said stabilizer is dimagnesium phosphate, trimagnesium phosphate, magnesium stearate, magnesium sulfate or any combination thereof.
6. The composition of claim 5 wherein said stabilizer is present in an amount ranging from about O.5% to about 5.0% by weight of dicalcium phosphate dihydrate.