Hi bdoviack-ga, and thanks for your interesting question.
I was somewhat surprised that more information on this topic was not
readily available, but often industrial information is kept out of the
public eye to protect intellectual property.
I did dredge up a patent that describes the removal of gypsum from the
skin. This technique, as described, is primarily meant to remove
residue from the skin after casts are removed. The Plaster of Paris
used for casting contains gypsum, which is the most difficult
component to remove. I assume that you are planning to remove gypsum
from dental hardware that has been molded from a negative gypsum cast
made in a patient, but the method below would work for other
applications. Also, since the method outlined below is designed for
direct application to the skin, it should be gentle enough for not
damage the underlying dental hardware, although tests would be prudent
to rule out an unforeseen chemical reaction.
US Patent #5702693, filed December 7, 1995 can be read in full at these sites:
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=5702693.WKU.&OS=PN/5702693&RS=PN/5702693
http://www.patentstorm.us/patents/5702693.html
The above document gives general parameters for a gypsum removing
solution, as well as multiple examples with test results for how well
gypsum was removed from skin.
First, it may be necessary to convert gypsum to Plaster of Paris,
which is a reversible reaction. If this is the case, the document
gives one possible method:
"When gypsum is heated at about 125.degree. C., a portion of the water
of crystallization is lost and Plaster of Paris is formed according to
the following equilibrium reaction: [ 2[CaSO4 * 2H2O {gypsum} <->
(CaSO4)2 * H20 {Plaster of Paris} + 3H20 ] This reaction is
reversible. Thus, when Plaster of Paris, impregnated in the fibrous
matrix, is mixed with enough water to form a creamy paste, the reverse
reaction occurs and the resulting crystals of gypsum interlace to form
a rigid mass."
__________
Although not directly relevant to your application, the patent above
describes a three step process for the removal of gypsum, which I will
quote below to make reading other references to this patent more
clear:
"(i) applying to the skin of a patient an aqueous liquid composition
comprising a water-miscible organic solvent, an acid and an emollient,
the aqueous liquid composition having a pH in the range of from about
2 to about 5;
(ii) dissolving the gypsum in the aqueous liquid composition; and
(iii) removing the aqueous liquid composition containing dissolved
gypsum from the skin of the patient. "
In the body of the patent, these are referred to as Steps (i) - (iii).
__________
With regard to the general parameters of the gypsum removal agent, the
patent has this to say:
"The aqueous liquid composition used in Step (i) of the present method
comprises a water-miscible organic solvent, an acid and an emollient.
The aqueous liquid composition is further characterized by having a pH
in the range of from about 2.0 to about 5.0. At a pH level below about
2.0 the aqueous liquid composition becomes relatively corrosive
resulting in an increased likelihood of the occurrence of skin
irritation as a side effect. At a pH level above about 5.0, the
aqueous liquid composition is so weakly acidic that it will be
inefficient or ineffective in dissolving, and thereby facilitating
removal of, gypsum from the skin of the patient. Preferably, the
aqueous liquid composition has a pH in the range of from about 2.0 to
about 4.0, more preferably from about 2.5 to about 3.5. The ideal pH
is about 3.0.
The aqueous liquid composition used in Step (i) of the present method
comprises an acid. Preferably, the acid is an organic acid.
Preferably, the organic acid is a weak acid having a pK.sub.a value in
the range of from about 1.0 to about 5.0, more preferably from about
2.5 to about 5.0, most preferably from about 3.0 to about 4.0.
Non-limiting examples of suitable organic acids for use in the aqueous
liquid composition in Step (i) of the present method may be selected
from the group consisting of acetic acid, ascorbic acid, benzoic acid,
citric acid, lactic acid, malic acid, malonic, acid, oxalic acid,
salicylic acid, glycolic acid, tartaric acid and mixtures thereof.
Preferably, the organic acid is selected from the group consisting of
acetic acid, ascorbic acid, citric acid, lactic acid, malic acid,
malonic acid, oxalic acid, glycolic acid and mixtures thereof. More
preferably, the organic acid is selected from the group consisting of
citric acid, lactic acid, malic acid and mixtures thereof. The most
preferred organic acid is citric acid. "
Their argument in favor of citric acid is as follows:
"As between organic acids and inorganic acids, it is preferred to
utilize organic acids as these tend to mitigate the risk of skin
irritation compared to inorganic acids. Further, inorganic acids are
relatively toxic to handle and use."
"The most preferred acid (organic or inorganic) suitable for use in
the aqueous liquid composition used in Step (i) of the present method
is citric acid. Ideally, citric acid is used in an mount in the range
of from about 2.0 to about 2.5 percent by weight of the aqueous liquid
composition."
________
Next, the patent describes the relative compositions of the gypsum
solvent, which are not strict.
"Generally, the acid is [u]sed in an [a]mount to ensure a significant
stoichiometric excess relative to the amount of particulate gypsum
being removed from the skin of the patient. In the case of organic
acids, for all practical purposes, the amount of acid used preferably
is in the range of from about 2.0 to about 5.0, more preferably in the
range of from about 2.0 to about 4.0, most preferably in the range of
from about 2.0 to about 3.0, percent by weight of the aqueous liquid
composition.
The most preferred acid (organic or inorganic) suitable for use in the
aqueous liquid composition used in Step (i) of the present method is
citric acid. Ideally, citric acid is used in an mount in the range of
from about 2.0 to about 2.5 percent by weight of the aqueous liquid
composition.
The liquid composition used in Step (i) of the present method is
aqueous (i.e. it contains water at some level) and contains a
water-miscible organic solvent. While the precise nature of the
water-miscible organic solvent is not critical, it is preferred that
the water-miscible organic solvent be selected from the group
consisting of C.sub.1 -C.sub.10 alcohols. Alternatively, the
water-miscible organic solvent may be a ketone such as acetone. More
preferably, the alcohol is selected from the group consisting of
ethanol, isopropyl alcohol and mixtures thereof. Most preferably, the
water-miscible organic solvent is isopropyl alcohol. Of course, those
of skill in the art will recognize that it is desirable to utilize a
water-miscible organic solvent which can be safely applied to the skin
of a patient without any significant side effects and which does not
have a significant drying effect when applied to skin.
While the relative [a]mounts of water and water-miscible organic
solvent used in the aqueous liquid composition are not particularly
restricted, it is preferred that amounts be chosen to ensure
dissolution of the other components of the aqueous liquid composition,
namely the acid and the emollient. Practically, it is preferred that
the aqueous liquid composition comprises a water:water-miscible
organic solvent weight ratio in the range of from about 80:20 to about
20:80, more preferably in the range of from about 70:30 to about
30:70, even more preferably in the range of from about 60:40 to about
40:60, most preferably in the range of from about 55:45 to about
45:55. "
As you are not applying this agent to skin, an emollient is obviously
not necessary.
_______
Acids other than citric acid can also be used. This may be useful
information, depending on the composition of the material from which
you are removing the gypsum and its chemical reactivity.
"The acid present in the aqueous liquid composition used in Step (i)
of the present method may also be an inorganic acid such as
hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid
and the like. Generally, these strong acids must be used in very
dilute form since they are very corrosive and could give rise to skin
irritation. For example, aqueous hydrochloric acid could be used in
the aqueous liquid composition. The amount of aqueous hydrochloric
acid required at various normalities and pH levels can be determined
according to Table 1. Of course, those of skill in the art will
readily be able to calculate the amounts of other strong, inorganic
acids required to produce a pH value in the range of from about 2.0 to
about 5.0. Of course, as will be apparent to those of skill in the
art, the amount of strong, inorganic acid may be increased to ensure
dissolution of the particulate gypsum on the skin of the patient
provided that the acid is buffered such that it has a pH in the range
of from about 2.0 to about 5.0. The choice and amount of buffering
agent used (if present) is within the purview of a person skilled in
the art."
In Table 2, they give specific examples and the outcome in terms of
gypsum removal ability:
TABLE 2
______________________________________
Exam- Isopropyl
Dioctyl Acid
ple Water Alcohol Malate.sup.1
Acid Amount
Results
______________________________________
1 40.0 56.0 2.0 Malic 2.0 Good
2 40.0 56.0 2.0 Ascorbic
2.0 Good
3 40.0 56.0 2.0 Acetic 2.0 Good
4 -- -- -- Salicylic.sup.2
2.2 Good
5 40.2 56.0 2.0 Citric 1.7 Good
6 40.0 56.0 2.0 Citric 2.0 Good
.sup. 7.sup.3
40.2 56.0 2.0 Citric 1.7 Good
8 40.5 56.0 2.0 Citric 1.5 Marginal
9 40.7 56.0 2.0 Citric 1.3 Marginal
10 41.0 56.0 2.0 Citric 1.0 Marginal
11 42.0 56.0 2.0 none -- Ineffective
12 100.0 -- -- none -- Ineffective
______________________________________
This table is somewhat hard to read. You can either download this
page from the US Patent site above (Click on Images and go to this
page), or, until it expires, you can find this page here:
http://img509.imageshack.us/img509/6572/dimg1en.png
=========================================
Although somewhat further from your application, the following journal
article may also be relevant to your application:
Removal of Gypsum from Microfossiliferous Shales
Royal H. Mapes, Gene Mapes
Micropaleontology, Vol. 28, No. 2 (1982) , pp. 218-219
You can purchase the full text article via JSTOR at this link:
http://links.jstor.org/sici?sici=0026-2803%281982%2928%3A2%3C218%3AROGFMS%3E2.0.CO%3B2-P
This article describes a process by which gypsum can be removed from
gypsum-rich micro-fossils. This group used a solvent known as
Stoddard solvent, which is a petroleum product with a variable
composition. It is usually used in dry cleaning, and is commercially
available under the trade names Varsol and Texsolve.
You can read more about it here:
http://www.atsdr.cdc.gov/HEC/CSEM/stoddard/stoddard.pdf
There are occupational risks associated with Stoddard solvent, which
you can read about in this article:
http://occmed.oxfordjournals.org/cgi/reprint/51/2/136.pdf
______________
The process described by the above article is as follows (remember,
they're talking about shale, but this may also be applicable to your
application):
"1. The shale sample must be completely dried. Drying time depends on
the volume of the sample and the type of oven used (e.g. 150-200 C, 12
hours for 25 kg of shale). When the sample is completely dry, the
sample bearing containers are allowed to cool, after which the shale
is immersed in Stoddard solvent. Other organic solvents such as
kerosene, Varsol, or gasoline can also be used, but these
hydrocarbon-based solvents often leave a strong odor on the final
washed and dried micro-fossils. Additionally, gasoline is highly
flammable and its use is inadvisable for safety reasons..."
"2. The solvent is removed and water is added. The solvent does not
appear to enter into the reaction involved in disaggregating the
shale. Once decanted and filtered to remove contaminating impurities,
the solvent can be safely re-used. Water is added to the drained
sample, and the shale is allowed to soak until slaking is complete.
If large quantities of the sample are process, it is necessary to stir
the shale thoroughly to insure complete water contact throughout the
sample. Immediate shale disaggregation, or slaking, can often be
observed; however, clods and larger lumps of shale may require soaking
time. Some samples require water-soaking for as long as 24 hours."
"3. The sample is washed and sieved through appropriate screen sizes
in order to remove the clay from the fossil bearing residue. When
examination of this residue shows a large percentage of gypsum,
additional preparation may be desirable to facilitate micro-fossil
picking."
"4. This step represent the basic modification we suggest for samples
with high gypsum content. The gypsum/fossil-bearing residue must be
totally re-dried. Drying must continue until the gypsum's water of
hydration has been completely driven off. When this has been
accomplished (usually after 24 hours at a temperature of 150-200 C),
the residue is again soaked in Stoddard solvent for about 1 hour. The
Stoddard solvent is again decanted, filtered and saved, and the
residue is soaked in water for no longer than 30 minutes. The residue
must then be immediately washed through appropriate sieves. A longer
soaking time apparently allows the dehydrated gypsum (anhydrite) to
partially recombine molecular water into a rigid framework and results
in a white, plaster-of-Paris-like cake that envelopes the entire
residue including the micro-fossils. Removal of the fossils from this
cake is difficult to impossible."
==========================
I hope this information in useful. Please feel free to request
clarification prior to rating.
Best,
-welte-ga |
,
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