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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to liquid organic solvents, and
more particularly, to such a solvent which effectively dissolves and
removes a wide range of materials, yet which is also comparatively
non-hazardous to the environment and personnel.
2. Background
Liquid solvent compositions are widely used throughout modern industry.
Just a few of the many applications for such solvent compositions include
the following processes: oil and grease removal; cleaning of paint guns
and lines; stripping paint; washing paint rollers; cleaning and
reclamation of silk screens; aluma-printing; deglazing, ink removal, and
roller washing in printing processes; lacquer washing.
As is well known to those skilled in the art, the many various materials
which must be solubilized in these applications differ greatly in how
effectively they can be dissolved by different solvent materials. For
example, the materials may be characterized as "polar" or "non-polar" (or
somewhere in between) as a result of their molecular structure; it has
been found that as a general rule, "like dissolves like", so that
materials having a generally polar character will tend to be most
effectively solubilized by solvents also having a polar character (i.e.,
those solvents which rely largely on their electrical dipole
characteristics for their solvent action), while materials of a non-polar
character are usually more effectively solubilized by solvents which also
have non-polar characteristics (i.e., those solvents which work primarily
on the basis of their dispersion forces).
While it would thus in some respects be "ideal" to mate the material which
is sought to be removed with a solvent which is matched to it in terms of
its polar/non-polar characteristics, this is frequently not possible, or
is at least impractical. For one thing, coatings and other materials may
themselves be made up of both polar and non-polar constituents, and it is
necessary for the solvent to be able to act on both of these in order to
successfully remove the material. Furthermore, it is simply desirable from
an economic and convenience standpoint to have a "general purpose" solvent
available which can be relied on to perform many different cleaning tasks
involving a wide variety of materials.
Unfortunately, relatively few solvents formulations have been found which
are capable cleaning up a broad spectrum of materials having the varied
characters discussed above. In general, the search for such general
purpose solvents has focused on compounds which exhibit both polar and
non-polar characteristics in a single molecule; for example, some
molecules are essentially "polar" at one end, and "non-polar" at the
other, in terms of their solvent characteristics. Some of these materials
(e.g., fatty acids and the like, which are used in detergent mixtures) are
characterized by a long-chained molecular structure, and are generally
unsuitable for use in many industrial applications, due to the excessive
residue which they leave behind, and the amount of rinsing or other
secondary washing which is necessary to remove this.
On the other hand, a handful of organic compounds have been identified
which have been successfully used as broad spectrum solvents in high
technology industries, such as the aerospace and electronic industries, as
well as in more commonplace applications. Unfortunately, the great
majority of these have ultimately been found to present undesirable
toxicologies and serious hazards to the environment; examples of these
compounds include methylene chloride and methyl ethyl ketone (MEK), as
well as toluene, xylene, and other aromatics, may of which include the
additional hazard of high flammability. For example, although MEK has long
been considered a satisfactory solvent from the standpoint of cleaning
effectiveness, there is a growing concern that the toxicity and
flammability of MEK exposes users to unnecessary risks. Also, because used
MEK is considered a threat to the environment, and so is classified as a
hazardous waste, the expense associated with the safe disposal of MEK is
on the order of 5-10 times greater than the amount which the user
initially pays for the solvent. Moreover, because of its relatively high
vapor pressure, the lose of MEK to the atmosphere during use is excessive,
necessitating the use of large and expensive collection systems such as
vacuum hoods.
Because of the concern for the safety, health, and environmental hazards
which these known organic solvents thus present, both the federal and
state governments are promulgating increasingly stringent criteria which
solvent users must comply with. For instance, the California State
Legislature limits the use of volatile solvents by requiring that they
have a vapor pressure below about 45 mmHg at 20.degree. C. In addition,
regulations require that solvents be disposed of in a manner that will not
adversely effect the environment; for may users of such solvents, this
disposal generally translates into increased operating costs, as noted
above.
For the above reasons, a primary consideration for many users of organic
solvents has become the toxicity of a particular solvent mixture, and also
the hazards which it presents to the environment. This has lead to a
number of attempts to find safe substitutes for the hazardous organic
solvents which have been used in the past. As an example, methylene
chloride has been widely used in industry, especially for formulating
paint strippers, lacquer removers, and paint clean-up systems, but it
suffers from high volatility which leads to excessive evaporation,
contributing to worker exposure and environmental pollution. Attempts have
consequently been made to replace methylene chloride using various, safer
organic solvents, but for the most part these efforts have not yielded
solvent compositions which are sufficiently effective or quick in action
to gain acceptance, and, furthermore, many of the proposed substitutes
have proven to costly to be economically feasible. For example,
n-methyl-pyrrolidone (NMP) has sometimes been found to be a suitable
substitute for MEK or methylene chloride in terms of its solvent
abilities, and it exhibits a very low volatility which drastically reduces
the flammability hazard and evaporative losses. However, both the cost of
NMP renders it use prohibitive in the concentrations which are necessary
to make many of the proposed formulations perform effectively as solvents.
Furthermore, NMP is excessively harsh for many applications, in that it
will cause damage to the underlying substrate; for example NMP can cause
severe deterioration of rubber and plastics, such as PVC. It also tends to
cause irritation and defatting of user's hands.
Accordingly, a need exists for a substantially non-toxic solvent
composition which exhibits superior cleaning ability when applied to a
variety of substances, and which exhibits low flammability and relatively
low vapor pressure so as to limit evaporative losses. Furthermore, there
is a need for such a solvent which safely degrades in the environment and
in biological systems, and which is also sufficiently inexpensive to be
economical for large scale use.
SUMMARY OF THE INVENTION
The present invention has solved the problems cited above and is a low
toxicity solvent composition, this comprising broadly (a) an alicyclic
carbonate for providing the composition with a polar solvent component;
(b) a terpene for providing the composition with a non-polar solvent
component; and (c) a short-chained, non-ionic surfactant for coupling the
alicyclic carbonate and the terpene in a homogeneous, single-phase
solution.
The alicyclic carbonate may be selected from the group consisting of
propylene carbonate and ethylene carbonate, and is most preferably
propylene carbonate. The propylene carbonate may be present in the
composition in an amount from about 25% to about 60% by weight.
The terpene which is used in the composition may be a monoterpene, and this
may be selected from the group consisting of d-limonene and l-limonene,
with d-limonene being preferred. The d-limonene may be present in an
amount from about 1% or less to about 20% by weight.
The short-chained non-ionic surfactant may be a glycol ether. This may be a
glycol ether which is selected from the group consisting of
propylene-based and ethylene-based glycol ethers, with the propylene-based
glycol ethers being preferred. Tripropylene glycol methyl ether is most
preferred, and this may be present in the composition in an amount from
about 40% to about 60% by weight.
DETAILED DESCRIPTION OF THE INVENTION
As an overview, it has been found that a particularly effective liquid
solvent is provided when a liquid mixture of an alicyclic carbonate and a
terpene is formed, this being stabilized in a single phase solution by a
small-chained, non-ionic surfactant, such as a glycol ether. For example,
it has been found that a preferred mixture of propylene carbonate,
d-limonene, and tripropylene glycol methyl ether provides a liquid solvent
which exhibits superior cleaning properties with respect to a wide variety
of materials, a virtual absence of toxicity, and a low vapor pressure
which greatly reduces evaporative loss and personnel exposure in use.
Furthermore, this mixture is economical to produce, which advantage is
enhanced by the fact that the stability of the mixture and the low
evaporative loss greatly extends the service life of a given amount of
this solvent.
As was noted above, a known (but extremely hazardous) organic solvent which
has the ability to solubilize a wide range of materials is methyl ethyl
ketone, and this ability is believed to stem from the fact that MEK
combines both polar and non-polar characteristics in a single molecule.
The present invention, in turn, is intended to produce a solvent material
which, on a macro scale, may mimic some of the permanent electrochemical
characteristics which are exhibited by MEK in a single molecule.
Accordingly, the cyclic carbonate provides the polar component of the
solution, and the terpene provides the non-polar component. The glycol
ether, in turn, serves as a "coupling agent" which maintains the two
solvent portions (i.e., the polar and non-polar portions) in a
homogeneous, single phase solution; this enables the solvent system to
develop a synergism which permits it to solubilize materials having
solvent-related characteristics which lie either at or between these
extremes in terms of polarity. The net result is that this composition has
been found to exhibit a solvent ability which out-performs any of the
three components used separately; this increase in performance has been
observed with respect to numerous different materials, and especially with
respect to solvent-based paints.
As was noted above, the polar portion of the solvent composition is
provided by the alicyclic carbonate compound, these compounds being of the
formula:
##STR1##
wherein R.sub.1 and R.sub.2 are selected from the group consisting of
hydrogen, methyl, or ethyl. Examples of alicyclic carbonate compounds
having this formula which are preferred for use in the composition of the
present invention include propylene carbonate and ethylene carbonate. As
is known to those skilled in the art, the polar character of those
carbonate compounds stems from the strong positive polarity which the
oxygen bonding imparts to the carbonate end of the molecule.
The reasons the alicyclic carbonate compounds have been found advantageous
for use in the composition of the present invention appear to be at least
twofold. Firstly, the cyclic structure of these compounds renders the
molecules significantly more compact than their straight-chained
equivalents, and this greatly reduces steric (mechanical) hindrances
between the molecules of the solvent composition and those of the material
which is sought to be solubilized. For example, the compact nature of the
cyclic versions of the carbonates appears to make it much easier for those
to penetrate the micro-pore structure which exists at the solvent
interface with coatings of paint. Secondly, it has been found that these
cyclic carbonates are much more readily maintained in a homogeneous,
single phase solution with the cyclic terpenes which are the preferred
non-polar component of the composition, when these are combined with the
glycol ether coupling agent, this apparently being due to "stacking" of
the cyclic structures of the carbonate and the terpene in the solution.
Of the two preferred cyclic carbonates noted above, the propylene carbonate
is most preferred in the solvent composition of the present invention, in
that, while both ethylene and propylene carbonate degrade quickly and
safely in the environment, and both exhibit desirably low vapor pressures
(as will be discussed further below), the propylene carbonate is
considerably safer from the standpoint of personnel exposure, inasmuch as
it degrades to safe intermediates and end products within the human
metabolic system, while this is not true of ethylene carbonate. In fact,
the safety of propylene carbonate is attested to by the fact that it has
been widely used is cosmetics. Propylene carbonate suitable for use in the
composition of the present invention is available from several sources;
for example, Texaco Chemical Company, Thousand Oaks, Calif., supplied
suitable propylene carbonate under the brand name TEXACAR.sup.* PC.TM..
Turning now to the non-polar constituent of the composition, this (as was
noted above) is provided by a suitable terpene. As is known to those
skilled in the art, such non-polar solvents generally rely more on their
dispersion forces for their solvent action, rather than on electrical
dipole characteristics, as is the case with the polar compounds.
Terpenes are hydrocarbons often found in essential oils, resins, and other
vegetable aromatic products, and, in general, are perceived to be polymers
of a 5-carbon moiety referred to as an isoprene unit. Related to the
terpenes are the hemiterpenes (C.sub.5 H.sub.8), sesquiterpenes (C.sub.15
H.sub.24), diterpenes (C.sub.20 H.sub.32), and the polyterpenes
(n(C.sub.10 H.sub.16)). The monoterpenes (C.sub.10 H.sub.16) which are
preferred for use in the solvent composition of the present invention are
primarily of plant origin, a very large number of these having been
isolated and characterized; many have long been used in perfumes and
medicines, and consequently many of these present significant advantages
in terms of safety for personnel exposure. Examples of suitable terpenes
include the di-pentenes d-limonene and l-limonene, and also pinene,
terpinene, and terpinolene.
As was noted above, the cyclic terpenes are generally preferred for use in
the solvent composition of the present invention, due to their compact
molecular structure and ability to "stack" with the cyclic carbonate
component of the solution, the most preferable of these being d-limonene,
a by-product of the citrus industry; it is also believed that the
double-bonded structure of d-limonene provides a flatter ring
configuration which more effectively emulates the solvent characteristics
of the aromatic hydrocarbons which are sought to be replaced. This
compound is derived in various amounts from the rinds or peels of oranges,
grapefruits, and other citrus fruits; it safely biodegrades in the
environment, and does not present a personnel hazard in terms of exposure,
this material having in fact been used in various food products. The
structure for d-limonene is given below:
##STR2##
An extensive discussion of d-limonene, and its derivation from various
sources, is presented in a book by J. W. Kesterson, R. Hendrickson, and R.
J. Braddock, entitled "Florida Citrus Oil", and published in December 1971
by Agricultural Experiment Station, Institute of Foods and Agricultural
Sciences, University of Florida, Gainesville, Fla.
The d-limonene employed in the compositions of the present invention can be
of a relatively impure grade without causing significant degradation of
the solvent capabilities of the resultant solution. However, some
researchers believe that the presence of significant amounts of impurities
in d-limonene speeds the formation of decomposition products to which the
skin of some people's hands may be sensitive, and so more refined grades
of d-limonene are preferable from this standpoint. Also, the more highly
refined grades of d-limonene lack the citrus odor which is characteristic
of the material, but the odor itself is generally not considered
offensive.
The final component of the solvent composition is the "coupler" which
permits the two, dissimilar solvent portions to exist together in a
homogeneous, single phase solution. Without the inclusion of such a
"coupler" (or "coupling agent"), it is simply not possible to get the
polar carbonate and non-polar terpene to stay in the solution together;
these compounds very strongly tend to "bead" when any attempt is made to
mix them together in the absence of a coupler, and they will quickly
separate out from one another, even after vigorous mixing and agitation.
A characteristic of effective coupling agents has been found to be that
they exhibit both polar and non-polar characteristics in the same
molecule; in other words, each molecule is part polar and part non-polar,
so as to essentially provide a link between the polar and non-polar
molecules in the solution. In this regard, it has been found that
small-chained, non-ionic surfactants are suitable for use as coupling
agents in the solvent composition of the present invention, and that
glycol ethers and their acetates are preferable for this purpose. Suitable
glycol ethers and their acetates have the structure:
(R.sub.4 --O).sub.n2 --(R.sub.3 --O).sub.n1 --R.sub.5
wherein .sub.n1 and .sub.n2 are the numerals 1-3, and wherein R.sub.3 is a
hydrocarbon radial having 2-3 carbon atoms, R.sub.4 is a hydrogen or a
hydrocarbon radical having 1-4 carbon atoms, and R.sub.5 is a hydrogen or
a hydrocarbon radial having 1-4 carbon atoms.
Both ethylene- and propylene-based glycol ethers have been found
particularly effective as coupling agents for the solvent composition of
the present invention, and suitable examples of these two families are
listed below:
Propylene-Based Glycol Ethers
Propylene Glycol
Methyl Ether
Dipropylene Glycol
Methyl Ether
Tripropylene Glycol
Methyl Ether
Propylene Glycol
Methyl Ether Acetate
Dipropylene Glycol
Methyl Ether Acetate
Propylene Glycol
n-Butyl Ether
Propylene Glycol
n-Butyl Ether
Propylene Glycol
t-Butyl Ether
Ethylene-Based Glycol Ethers
Ethylene Glycol
n-Butyl Ether
Diethylene Glycol
n-Butyl Ether
Triethylene Glycol
n-Butyl Ether
Diethylene Glycol
Methyl Ether
For reasons essentially similar to those which were discussed above with
respect to the advantages of propylene carbonate relative to ethylene
carbonate, the propylene-based glycol ethers are generally preferred over
the glycol-based glycol ethers, since the propylene-based compounds
degrade safely in a living system. In particular (as is also the case with
propylene carbonate), the main metabolite of the propylene-based glycol
ethers is propylene glycol, which is used extensively in cosmetics and as
a food additive. Most preferred for use in the composition of the present
invention is tripropylene glycol methyl ether, because of its very low
vapor pressure. The vapor pressure of tripropylene glycol methyl ether is
lower than that of either the propylene carbonates of the d-limonene, with
the result that the glycol ether is the last to evaporate off; this
ensures that the coupling agent will always be present in the composition
so as to prevent the carbonate and terpene from separating out, this
relationship being maintained regardless of any evaporative losses which
may occur over a long period of use. The ethylene-based glycol ethers, by
contrast, while they also biodegrade well in the environment, are less
desirable from the standpoint of personnel exposure, inasmuch as some are
readily absorbed through the skin and do not degrade as safely within
living systems.
EXAMPLE FORMULATIONS
The novel solvent compositions of the present invention will be more fully
understood from a consideration of the following examples, which
illustrate preferred embodiments. It is to be understood, however, that
these examples are given by way of illustration and not of limitation.
EXAMPLE I
A solvent composition was prepared by simple mixing of the following,
preferably always maintaining a sufficient amount of the glycol ether in
the solution to prevent beading of the other two components:
______________________________________
Component % (By Weight)
______________________________________
(a) Propylene Carbonate
25
(CAS #108-32-7,
1,3-Dioxolan-2-one
methyl)
(b) d-Limonene 17
(CAS #5989-27-5,
4-isopropenyl-1-
methylcyclo hexene)
(c) Tripropylene Glycol
58
Methyl Ether
(CAS #25498-49-1
______________________________________
This particular formulation (which has been designated "EP921") has
widespread applicability, and has been found particularly effective for
use in cleaning up guns and lines have been used for applying paint and
other coatings, and also for the deglazing of rollers which are used in
offset printing.
EXAMPLE II
A solvent composition was prepared containing:
______________________________________
Component % (By Weight)
______________________________________
(a) Propylene Carbonate
60
(CAS #108-32-7,
1,3-Dioxolan-2-one,
methyl)
(b) d-Limonene 1 (or less)
(CAS #5989-27-5,
4-isopropenyl-1-
methylcyclo hexene)
(c) Tripropylene Glycol
39
Methyl Ether
(CAS #25498-49-1)
______________________________________
This formulation is somewhat more specific in its applicability than that
of Example I, this having been found useful primarily for the cleaning and
reclamation of silkscreens, and in the clean-up of aluma-printing surfaces
and equipment. This is the result of reducing the amount of d-limonene to
1% or less, which tends to enhance the polar aspect of the solvent
mixture. Nevertheless, it has been found that, even at these relatively
low levels (e.g., 0.5-1%), the d-limonene enables the synergism to be
developed with the propylene carbonate such that the effective solvent
action of this mixture is significantly greater than that which would be
exhibited by the mixture if it contained only the propylene carbonate and
the glycol ether.
Other exemplary formulations in accordance with the present invention may
be prepared as follows:
EXAMPLE III
______________________________________
by weight
______________________________________
(a) Propylene Carbonate
36.2%
(b) d-limonene 7.4%
(c) Tripropylene Glycol
56.4%
Methyl Ether
______________________________________
EXAMPLE IV
______________________________________
by weight
______________________________________
(a) Propylene Carbonate
40.3%
(b) d-limonene 4%
(c) Propylene Glycol
55.7%--
Ether Acetate
______________________________________
The compositions provided by the exemplary formulations given above have
been found to be highly effective solvents which are non-c | | |
