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BACKGROUND OF THE INVENTION
A variety of correction compositions have been commercially-available over
the years. Such compositions comprise a masking pigment such as titanium
dioxide, a resinous binder material such as ethyl cellulose and a volatile
organic solvent for the binder material such as benzene, ethyl acetate,
ethanol, trichlorethylene, or the like.
Such compositions have one or more disadvantages which render them unsafe
and/or unsatisfactory for their intended function. The low boiling point
hydrocarbons, ketones, esters and alcohols are flammable and represent a
danger when used in the presence of an open flame such as a match lit by a
typist. Moreover, such volatile solvents have rapid evaporation rates so
that they evaporate from the applicator brush and from the mouth of the
bottle during use, causing caking of the composition on the brush,
narrowing of the inner neck of the bottle and the deposit of particles of
solidified resin in the composition, which particles form an uneven
masking coating over the images being covered during the subsequent use.
The use of less volatile solvents such as higher-boiling point
hydrocarbons, including toluene and xylene, and certain chlorinated
hydrocarbons, such as perchlorethylene, overcomes the problems of
premature drying, caking, etc., and provides correction compositions which
have a good levelling power, i.e., ability to form a smooth, even deposit
over the image being masked prior to the evaporation of the solvent.
However, such hydrocarbons are also flammable and their evaporation rate
is so slow, under ambient conditions, that the typist must wait an undue
length of time for the composition to dry and solidify prior to the typing
of the correct image thereover, with resultant loss of time and
efficiency.
Certain other solvents, such as trichlorethylene, have a satisfactory
evaporation rate providing good levelling properties and rapid drying time
but are dangerous to use because their vapours are dangerous to health,
i.e., carcinogenic at the 100 ppm level, and it is possible to build up
this concentration in an office having poor ventilation.
OBJECTS OF THE INVENTION
It is the object of the present invention to provide a masking composition
having good levelling properties, a relatively rapid drying rate and
evolving solvent vapors which are non-toxic or non-carcinogenic and
non-flammable under ordinary ambient conditions.
This and other objects and advantages of the present invention will be
apparent to those skilled in the art in the light of the present
disclosure.
SUMMARY OF THE INVENTION
The novel masking compositions of the present invention comprise a
film-forming binder material, an opaque pigment and a volatile solvent
mixture comprising methyl chloroform and perchlorethylene. We have found
that the combination of methyl chloroform and perchlorethylene provides a
mixture having a good evaporation rate, non-toxicity and non-flammability
under normal ambient conditions.
Methyl chloroform (1,1,1-trichlorethane), per se, has a boiling point of
about 74.degree. C. and thus provides a masking composition which dries
too rapidly, has unsatisfactory levelling properties and tends to cake on
the applicator brush and in the neck of the bottle, particularly when used
under warm or hot ambient conditions.
Perchlorethylene, per se, has a boiling point of about 121.degree. C. and
thus provides a masking composition which dries too slowly for practical
use. Also, this solvent is a health hazard in concentrations of about 100
ppm and thus is dangerous to use per se under ordinary office conditions.
We have found that the use of mixtures of methyl chloroform and
perchlorethylene containing from about 20% up to about 95% by weight of
methyl chloroform and from about 80% down to about 5% by weight of
perchlorethylene provide an excellent solvent for correction liquids based
upon resinous or film-forming binder materials which are soluble in such
mixtures. Most preferably the solvent mixture contains from about 70% to
about 90% by weight of the methyl chloroform so as to have a rapid drying
rate.
Such mixtures have an evaporation rate which can be tailored to suit the
exact preference of the user with respect to drying time of the masking
composition by adjusting the relative percentages of the more volatile
component, methyl chloroform, and the less volatile component
perchlorethylene. The presence of at least about 5% by weight of the less
volatile component reduces the evaporation rate of the mixture
sufficiently that the masking composition will not dry prematurely on the
applicator brush or in the neck of the applicator bottle, and permits the
masking composition to level or form a smooth coating over the erroneous
image during drying. The evaporation temperature of the mixture is
adjustable between about 78.degree. C. and 110.degree. C. depending upon
the ratio of the two solvents used.
Conversely, the presence of at least about 20% by weight of the more
volatile component, methyl chloroform, not only provides a mixture having
the desired drying rate but also provides a mixture having a sufficiently
high threshold limit value that it is not a health hazard under conditions
of normal office use and is therefore safe. Perchlorethylene, per se, has
a threshold value limit of only 100 ppm and therefore is dangerous to use
in poorly ventilated areas. However, we have found that the threshold
value limit of the present mixtures of perchlorethylene and methyl
chloroform (which has a threshold value limit of about 350 ppm) range from
about 150 ppm up to about 335 ppm, and thus are safe to use under normal
office use conditions, since such concentrations will not be exceeded
under such conditions.
Aside from the novel solvent mixtures of the present masking compositions,
the other ingredients of the present compositions may be the same as those
used in other conventional masking compositions based upon organic
solvents. Such ingredients comprise a film-forming binder material which
is soluble in the solvent mixture and which dries on solvent evaporation
to form a removal-resistant film, a major amount by weight of an opaque
masking pigment having a color corresponding to the color of the paper on
which corrections are to be made, and optional ingredients such as
plasticizers to render the film-forming binder material less brittle and
resistant to cracking, wetting agents to improve the dispersing of the
pigment in the solvent composition, tinting agents to modify the color of
the composition, etc.
The preferred film-forming binder materials are the cellulose ethers such
as ethyl cellulose, hydroxyethyl cellulose, ethyl hydroxyethyl cellulose,
and the like. However, other film-formers are also suitable such as
polystyrene, chlorinated rubbers, acrylic resins such as methyl
methacrylate, n-butyl methacrylate and iso-butyl methacrylate, vinyl
polymers such as vinyl chloride-vinyl acetate copolymers, and similar
film-forming materials which are soluble in the solvent mixtures of the
present invention.
Suitable opaque masking pigments are titanium dioxide, talc, china clay,
whiting, and other conventional opaque pigments having good covering or
hiding properties. The preferred material is titanium dioxide, with a
small amount of fumed silica being added to give the dried composition a
dull, matte finish corresponding closely to surface of most copy papers.
Various tints may be added to color the pigments blue, pink, green or other
color corresponding to the color of the paper on which the composition is
to be used.
Suitable plasticizers for the film-formers include dibutyl phthalate,
tricresyl phosphate, castor oil and other oily liquids which are at least
partially compatible with the particular film-forming binder materials
used.
Small amounts of conventional wetting agents may also be included to assist
dispersion of the pigments and brushability of the masking compositions
over the paper being corrected.
The following example is given as an illustration of a composition coming
within the present invention:
EXAMPLE
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Ingredients Parts by Weight
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Ethyl cellulose 3.0
Castor Oil 4.0
Wetting Agent 2.0
Titanium dioxide 30.0
Fumed silica 1.0
Perchlorethylene 10.0
Methyl chloroform 50.0
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The above ingredients are uniformly blended and mixed to form a homogeneous
white masking composition having a threshold value limit of nearly 300 ppm
and a solvent evaporation temperature of about 81.degree. C., providing a
good drying rate which permits the composition to flow or level on the
copy paper while drying fairly rapidly.
The foregoing example is given by way of illustration and should not be
considered limitative. Variations and modifications can be made within the
scope of the appended claims.
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