1. Field of the Invention
This invention relates to silica compositions useful as an abrasive in a dentifrice and methods for making the same. The invention also relates to dentifrices, preferably toothpastes, which effectively clean teeth without a high degree of dentin and/or enamel abrasion.
2. Relevant Art
Dentifrices, e.g., such as toothpastes, are used to clean teeth. Abrasive substances are formulated in the toothpaste as the primary cleaning agent. In particular, such dentifrices aid in the removal of food particles, the removal of discoloration caused by substances such as tobacco or tea, and the removal of firmly adhering bacterial films, referred to as plaque, from the surface of the teeth.
To achieve cleaning, the abrasive in dentifrices have to provide a certain degree of abrasiveness with respect to the surface of the teeth. It is important, however, that abrasiveness with respect to dental enamel and dentine be at an acceptably low level to prevent the surface of the teeth from being damaged by the daily use of the toothpaste. The rate of enamel removal through brushing should not exceed the rate at which it is replenished through natural remineralization processes.
The abrasive used should also be compatible with the other components of the toothpaste. It should lend itself to processing with water, humectants and consistency regulators to form a ductile paste readily dispensable from tubes or dispensers. It also should not adversely affect known caries inhibitors, for example, fluoride carriers, such as NaF or Na monofluorophosphate, even in the event of prolonged storage.
U.S. Pat. No. 3,957,968, discloses tooth pastes containing a combination of alpha-aluminum oxide (corundum) and a second abrasive having a Mohs hardness of less than about 6 which are said to have good cleaning and polishing effects. Alpha-aluminum oxide, however, has a Mohs hardness of 9, and has a relatively strong abrasive effect on dental enamel. To reduce the enamel abrasion, certain calcium, magnesium or sodium salts have been added to a dentifrice containing the aforementioned abrasives.
U.S. Pat. No. 4,303,641 discloses an alkaline treatment for increasing the abrasiveness, and as a result its cleaning performance, of dentifrice silica gel compositions. The process described in this patent does not require employing the processing and drying steps typically used to prepare prior art gels. It also discloses that treating silica gels with alkaline materials enhances the cleaning performance of the gels as evidenced by increased Radioactive Dentine Abrasion (RDA), defined later below. The Examples in the '641 patent illustrate the alkaline treatment on gels having average particle sizes greater than 10 microns, e.g., about 14-16 microns. The RDA values shown for these alkaline treated gels, however, are quite high as evidenced by “powder” RDA's which this patent reports to be over 1,000 (and over 200 if measured using RDA methods disclosed herein) for some samples. This indicates that the alkaline treated gels exhibit a high degree of abrasiveness on dentin surfaces.
Non-alkaline treated silica xerogel abrasives are also well known in the art. Such gels typically have particle sizes from 10 to 50 microns, depending on the ‘grittiness’ desired. The aforementioned '641 patent discloses that non-alkaline treated silica gels are effective polishing and cleaning agents while causing low amounts of damage to underlying tooth materials such as dentin and enamel. However, the '641 patent also discloses that more effective cleaning is achieved via alkaline treatment of such silica gels.
U.S. Pat. No. 5,651,958 discloses using a combination of specific silicas in dentifrices to balance cleaning with minimal abrasion to dentin and enamel surfaces. The '958 patent discloses combining precipitated silica having a narrow particle size range distribution of soft particles having a mean value ranging from 8 to 14 microns with a silica gel in which 70% of the gel particles have a diameter below 25 microns and a Radioactive Dentin Abrasion from 62 to about 100. The silica gel particles described in this patent have an Einlehner hardness from about 3 to about 15 as measured using a brass screen.
U.S. Pat. No. 5,589,160 discloses using a combination of two precipitated silicas as a dentifrice abrasive. One of the precipitated silicas has a mean particle size of about 5 to 11 microns and an Einlehner hardness of 0.8 to 2.5 as measured using a brass screen. The other precipitated silica has a mean particle size of from about 5 to about 11 and an Einlehner hardness from about 3 to about 8 as measured using a brass screen.
U.S. Pat. No. 3,670,076 discloses a combination of relatively small and large alumina particles as providing superior abrasives and cleaning.
U.S. Pat. No. 5,939,051 also discloses silica gel abrasives which impart good cleaning properties to dentifrice compositions, but results in relatively low abrasion. The preferred abrasive, which is identified as XWA 300 from W. R. Grace & Co.-Conn., has a relatively small particle size, e.g., 2 to 4 microns. WO 99/51196 discloses a similar abrasive. That abrasive is obtained by sustained milling which in turn results in substantial drying. As a result these particles are somewhat more expensive to make. They also typically have a volatiles content of 20% or less. The most preferred cleaning and abrasivity performance using the XWA 300 abrasive is obtained by combining it with a larger, less abrasive silica gel having particle sizes of 12 microns or greater. See WO 99/51196.
The above-mentioned measures, however, have not completely solved the problem of obtaining effective cleaning without the excessive abrasion of dental enamel. Workers in the art have continued to search for dentifrices containing abrasive systems which have reduced abrasion of dental enamel, but still effect acceptable cleaning.