ACID RESISTANT CALCIUM CARBONATE FILLER

Abstract: An improved form of calcium carbonate which is acid resistant to enable its use as a filler material in the making of neutral to weakly acid paper, and a process for producing this acid resistant calcium carbonate are provided. This acid resistant calcium carbonate consists of at least about 0.1 percent, based on the dry weight of the calcium carbonate, of a mixture of weak acids in admixture with the calcium carbonate.

Description: ACID RESISTANT CALCIUM CARBONATE FILLER RΆΠ Γ.ROTTND OF THE INVENTION This invention relates generally to calcium carbonate for use in papermaking, and related industries, and more particularly to a calcium carbonate having acid resistant properties- Titanium dioxide and calcined clay have traditionally been utilized as filler materials in the preparation of neutral to weakly acidic paper in order to improve the optical properties, especially the brightness, of the resultant product. These materials, however, especially titanium dioxide, have the disadvantage of being very expensive, resulting in higher manufacturing costs and an uncompetitively priced paper product . Calcium carbonate, particularly precipitated calcium carbonate, has been used as a filler material in the making of alkaline paper. Such usage results in a paper with enhanced optical properties, without the expense incurred in using titanium oxide fillers, resulting in a much less expensive product. Calcium carbonate, however, cannot generally be used as a filler in acidic paper because it decomposes in an acidic environment. Consequently, there has long been a need to develop a calcium carbonate composition which is acid stabilized and resistant to decomposition at low pH, so that it can be utilized as a filler material in the manufacture of acidic paper, such as groundwood paper, where the use of an alkaline filler would have a negative impact on the final paper properties. Paper made from mechanical pulps has been traditionally produced under acidic papermaking conditions because of "fiber alkaline darkening" that occurs as pH rises. This means that there is a reduction in brightness of the paper (brightness reversion) when the pH is raised from acid to alkaline in wood-containing systems. Alkaline darkening will occur to some degree in any wood pulps with significant lignin content. The degree of darkening depends on the particular pulps, Ph, and water quality. In general, ground calcium carbonate and precipitated calcium carbonate fillers buffer wet end in the 7.5-8.2 pH range. Acid-resistant calcium carbonate compositions thus provide a means for reducing the degree of fiber alkaline darkening and brightness reversion due to their ability to maintain a stabilized pH. U. S. Patent 5,043,017 discloses and claims an acid- stable calcium carbonate resistant to degradation in a mildly acidic environment which comprises a mixture of a calcium-chelating agent or a conjugate base, and a weak acid, such that calcium carbonate is coated by, and is in equilibrium with, the calcium-chelating agent or conjugate base and the weak acid. Preferred calcium carbonate compositions contain sodium hexametaphosphate and phosphoric acid. OBJECTS OF THE INVENTION It is an object of the present invention to provide stabilized and acid resistant calcium carbonate compositions especially suitable for use in papermaking applications. It is a further object of the present invention to provide a process for the preparation of the aforesaid calcium carbonate compositions. A still further object of the present invention is to provide a paper having enhanced optical qualities prepared using the calcium carbonate compositions of the present invention. SUMMARY OF THE INVENTION The present invention relates to an improved form of calcium carbonate which is stabilized and thus, acid resistant, to enable its use as a filler material in the making of neutral to weakly acid paper, and a process for producing this acid resistant calcium carbonate. More particularly, this invention concerns an acid resistant calcium carbonate consisting essentially of at least about 0.1 percent, based on the dry weight of the calcium carbonate, of a mixture of two or more weak acids, in admixture with the calcium carbonate. It has surprisingly been found that the inclusion of the mixture of two or more weak acids confers a higher degree of stability and acid resistance for calcium carbonate in the presence of fiber slurry, and a longer term of pH stability than the prior art acid-stabilized calcium carbonate compositions. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a graph comparing the 24 hours ageing of scalenohedral precipitated calcium carbonate compositions of the present invention containing 1, 2 or 6% polyacrylate acid, and various concentrations of phosphoric acid. FIGURE 2 is a graph comparing the 24 hours ageing of rhombic precipitated calcium carbonate compositions of the present invention containing 1% or 2% polyacrylate acid, and various concentrations of phosphoric acid. FIGURE 3 is a graph comparing the 24 hours ageing of ground precipitated calcium carbonate compositions of the present invention containing 1% or 2% polyacrylate acid, and various concentrations of phosphoric acid. FIGURE 4 is a graph showing the pH of a scalenohedral precipitated calcium carbonate composition of the present invention containing 3% polyacrylate acid, and 5% phosphoric acid. FIGURE 5 is a graph showing the pH of a scalenohedral precipitated calcium carbonate composition of the present invention containing 1% polymaleic acid, and 4% phosphoric acid. FIGURE 6 is a graph showing the pH of a scalenohedral precipitated calcium carbonate composition of the present invention containing 6% polymaleic acid and 1% phosphoric acid. DF.TATT.F.D DESCRIPTION OF THE INVENTION The improved form of calcium carbonate prepared by the instant invention is stabilized, and thus, acid resistant, to enable its use as a filler material in the making of neutral to weakly acid paper. While not wishing to be bound by any particular theory as to the operability of the present invention, it is believed that the acid resistance conferred upon the calcium carbonate compositions of the present invention is a result of the inactivation of the surface of the calcium carbonate by the addition of the mixture of the two weak acids. In the practice of the present invention, the calcium carbonate compositions are rendered acid resistant by the inclusion of at least about 0.1 percent, based on the dry weight of the calcium carbonate, of a mixture of two or more weak acids. Especially preferred as one component of the mixture of weak acids is an organic, polymeric weak acid, such as polyacrylate or polymaleic acid. While not wishing to be bound by any theory, it is believed that the capability of the acid-stabilized calcium carbonate of the present invention to resist dissociation in an acidic environment is due to the polymer adsorption on the surface of the calcium carbonate, absorption of polymer functional groups on the calcium carbonate surface, and the formation of a buffering system between anionic functional groups of the polymer and a weak acid. This mechanism of polymer adsorption is distinct from absorption or reaction of the prior art sodium hexametaphosphate on the surface of calcium carbonate. Polymer adsorption can provide a barrier coating on the surface of calcium carbonate which reduces the dissolution reaction of calcium carbonate. On the other hand, the absorption or reaction of sodium hexametaphosphate is limited to the surface of the calcium carbonate. The weak acids utilized in the compositions of the present invention are preferably weak acids selected from the group consisting of phosphoric acid, metaphosphoric acid, hexa etaphosphoric acid, ethylenediaminetetraacetic acid (EDTA) , citric acid, sulfurous acid, boric acid, acetic acid, weak acids derived from organic polymeric acids, such as polyacrylate acid, polymaleic acid and polycarboxylic acid, and mixtures thereof. As noted hereinbefore, the mixture of weak acids preferably contains at least one weak acid which is derived from an organic, polymeric acid. These organic polymeric acids are typically an organic polymer having a weight average molecular weight, M„, in the range of 750-1,000,000, consisting of regularly repeating units or chemically similar units, connected by primary covalent bonds. The total amount of the weak acids utilized is at least 0.1 percent, based on the dry weight of the calcium carbonate, and is preferably about 1 to about 8 percent, based on the dry weight of the calcium carbonate. A preferred combination of weak acids for use in the present invention is polyacrylate acid with phosphoric acid. The calcium carbonate utilized is preferably finely divided and it can be either a precipitated calcium carbonate or a natural ground limestone. The process for producing this acid resistant calcium carbonate involves forming a mixture of calcium carbonate with at least about 0.1 percent, based on the dry weight of the calcium carbonate, of the mixture of weak acids. The resultant mixture is blended for a sufficiently long period of time to ensure uniform mixing of the ingredients. The calcium carbonate can be utilized in the above- described process either as a dry powder or an aqueous slurry with up to about 60 percent by weight solids content. The weak acids can be utilized in the process of preparation in either pure concentrated forms or as aqueous solutions. In a preferred embodiment of the instant process, either the polymeric acid or the phosphoric acid can be first added, followed by the addition of the second acid. Alternately, the polymeric acid and phosphoric acid can be added at the same time, or the polymeric acid and phosphoric acid can be mixed together and then added to the calcium carbonate. The composition of the present invention can be utilized to.improve the optical properties of neutral to weakly acidic paper by its addition to the paper during standard manufacturing processes. Typically, the calcium carbonate composition of the present invention is added to a first paper furnish containing components necessary for making acidic paper to thereby form a second paper furnish. The invention will be further illustrated by the following Examples, which are to be considered illustrative of the invention, and not limited to the precise embodiments shown. EXAMPLE 1 Scalenohedral Precipitated Calcium Carbonate Stabilized with Polvacrvlate Acid and Phosphoric Acid Acid stabilized scalenohedral precipitated calcium carbonate slurry can be obtained by the addition of a polymeric acid such as polyacrylate acid, and a weak acid such as phosphoric acid. Initially, 1%, 3%, and 6% polyacrylate acid, based on the dry weight of calcium carbonate, was added into a 19.7% solids slurry of scalenohedral precipitated calcium carbonate. The pH of the untreated scalenohedral precipitated calcium carbonate slurry was 9.05. After mixing, 1-6% phosphoric acid, based on the dry weight of calcium carbonate, was added. A plot of the pH was measured for each sample after 24-hours ageing as shown in Figure 1. A composition containing 1% polyacrylate acid, based on the dry weight of calcium carbonate, and 3% phosphoric acid, based on the dry weight of calcium carbonate was found to have an initial pH of 5.28, which rose to 5.44 after 24 hours ageing. EXAMPLE 2 Rhombic Precipitated Calcium Carbonate Stabilized with Polvacrvlate Acid and Phosphoric Acid The initial pH of rhombic precipitated calcium carbonate was 8.29. First, 1% or 2% polyacrylate acid, based on the dry weight of calcium carbonate, was added into a 17.3% solids slurry of rhombic precipitated calcium carbonate. After blending, 1-6% phosphoric acid, based on the dry weight of calcium carbonate, was added as shown in Figure 2. The samples containing 2% polyacrylate acid have lower pHs than the 1-6% phosphoric acid and 1% polyacrylate acid, after 24 hours ageing. EXAMPLE 3 Ground Calcium Carbonate Stabilized with Polvacrvlate Acid and Phosphoric Acid The initial pH of ground calcium carbonate was 8.01. Initially, 1% and 2% polyacrylate acid, based on the dry weight of calcium carbonate, was added into 20% solids slurry of ground calcium carbonate. After blending, 1-6% phosphoric acid, based on the dry weight of calcium carbonate, was added as shown graphically in Figure 3. The samples with the addition of 2% polyacrylate acid had a lower pH at the addition of 3-6% phosphoric acid compared to 1% polyacrylate acid after 24 hours ageing. EXAMPLE 4 Long Term pH Stability of Scalenohedral Precipitated Calcium Carbonate Slurry A long-term low pH stabilized scalenohedral precipitated calcium carbonate slurry can be obtained by the combination of polyacrylate acid and phosphoric acid. The initial pH of scalenohedral precipitated calcium carbonate slurry was 9.05. At the beginning, 3% polyacrylate acid, based on the dry weight of calcium carbonate, was added to 19.8% solids scalenohedral precipitated calcium carbonate slurry, followed by the addition of 5% phosphoric acid, based on the dry weight of calcium carbonates, as shown in Figure 4. The pH of scalenohedral precipitated calcium carbonate slurry were measured and found to be 6.32 after two weeks ageing. EXAMPLE 5 Scalenohedral Precipitated Calcium Carbonate Stabilized with Polymaleic Acid and Phosphoric Acid Acid stabilized scalenohedral precipitated calcium carbonate slurry can be obtained by the addition of a polymeric acid such as polymaleic acid, and a weak acid such as phosphoric acid. Initially, 1% polymaleic acid, based on the dry weight of calcium carbonate, was added into 19.7% solids slurry of scalenohedral precipitated calcium carbonate. The pH of untreated scalenohedral precipitated calcium carbonate slurry was 9.05. After mixing, 4% phosphoric acid, based on the dry weight of calcium carbonate, was added. A plot of the pH was measured after 47 hours ageing as shown in Figure 5. The initial pH of the slurry was measured and found to be 5.38, which rose to 6.80 after 47 hours ageing. EXAMPLE 6 Scalenohedral Precipitated Calcium Carbonate Stabilized with Polymaleic Acid and Phosphoric Acid Acid stabilized scalenohedral precipitated calcium carbonate slurry can be obtained by the addition of a polymeric acid such as polymeric acid, and a weak acid such as phosphoric acid. Initially, 6% polymaleic acid, based on the dry weight of calcium carbonate, was added into 19.7% solids slurry of scalenohedral precipitated calcium carbonate. The pH of untreated scalenohedral precipitated calcium carbonate slurry was 9.05. After mixing, 1% phosphoric acid, based on the dry weight of calcium carbonate, was added. A plot of the pH was measured after 118 hours ageing as shown in Figure 6. The initial .pH of the slurry was measured and found to be 5.70. After 118 hours ageing, the pH was again measured and found to be 6.46.

Claims: WHAT IS CLAIMED IS: 1. An acid resistant calcium carbonate consisting essentially of at least about 0.1 percent, based on the dry weight of the calcium carbonate, of a mixture of two or more weak acids, in admixture with the calcium carbonate. 2. The acid resistant calcium carbonate of claim 1 wherein the weak acids are selected from the group consisting of polyacrylate acid, polymaleic acid and phosphoric acid. 3. The acid resistant calcium carbonate of claim 1 wherein the mixture of weak acids includes at least one weak acid which is an organic, polymeric acid. 4. The acid resistant calcium carbonate of claim 3 wherein the mixture of weak acids is comprised of polyacrylate acid and phosphoric acid. 5. The acid resistant calcium carbonate of claim 3 wherein the mixture of weak acids is comprised of polymaleic acid and phosphoric acid. 6. The acid resistant calcium carbonate of claim 1 wherein the mixture of weak acids is present in an amount of about 1 to about 8 percent, based on the dry weight of the calcium.carbonate. 7. A process for the preparation of an acid resistant calcium carbonate which consists essentially of: a) forming a mixture of calcium carbonate with at least about 0.1 percent, based on the dry weight of the calcium carbonate, of a mixture of two or more weak acids to the mixture ; and b) blending the resultant mixture to ensure uniform mixing. 8. The process according to claim 7 wherein the weak acids of the mixture are selected from the group consisting of polyacrylate acid, polymaleic acid and phosphoric acid. 9. The process according to claim 7 wherein the mixture of weak acids includes at least one weak acid which is an organic, polymeric acid. 10. The process according to claim 7 wherein the mixture of weak acids is comprised of polyacrylate acid and phosphoric acid. 11. The process according to claim 7 wherein the mixture of weak acids is comprised of polymaleic acid and phosphoric acid. 12. The process according to claim 7 wherein the mixture of weak acids is present in an amount of about 1 to about 8 per cent, based on the dry weight of the calcium carbonate. 13. A method of improving the optical properties of neutral to weakly acidic paper by the addition of a calcium carbonate composition which consists essentially of a mixture of at least about 0.1 percent, based on the dry weight of the calcium carbonate, of a mixture of two or more weak acids, in admixture with the calcium carbonate.

Inventors: WU KUAN-TING, WU, KUAN-TING
Applicants: ECC INT INC, ECC INTERNATIONAL INC.
Application year: 1996
Main classification: D21H17
IPC codes: C01F11, C09C1, C09C3, D21H17