The shrinkage of tape joint compounds, which normally comprise
water, filler, and binding resin, is significantly reduced by minimizing
the amount of water-absorbing materials such as attapulgite and
hydroxyethyl cellulose. Comparable improvements can be made to spachtling
1. In a ready-mix tape joint or wall repair compound of the type
that hardens by drying comprising particular filler, binder, and
the improvement wherein said composition has a diluted yield value
no greater than about 15 Pa.s,
whereby said composition is readily spreadable and resists sagging
but shrinks no more than about 25% upon drying.
2. The compound of claim 1 wherein the diluted yield value is at
least 0.5 Pa.s.
3. The compound of claim 1 wherein the viscosity of the compound
is on the order of 2000-3000 Pa.s.
4. The compound of claim 3 wherein the diluted yield value is on
the order of 1-10 Pa.s.
5. The compound of claim 4 wherein shrinkage upon drying is no
more than about 20%.
6. The compound of claim 4 wherein shrinkage upon drying is no
more than about 10%.
7. The compound of claim 4 wherein hollow glass microspheres make
up a portion of the filler solids.
BACKGROUND OF THE INVENTION
This invention relates to tape joint compounds and spachtling (or
spackling) compounds and is especially concerned with compositions
having low shrinkage upon drying.
During installation, gypsum board surfaces frequently develop cracks,
pits, etc. When gypsum board is mounted, there are inevitably dimples
at the location where individual sheets are nailed or screwed to
studs or ceiling joists. A strip of perforated tape is commonly
applied over the indented adjacent edges of a space between adjoining
gypsum board panels, spreadable joint filling composition (commonly
called "compound") being applied both under and over the
tape and allowed to dry or cure.
Even if the original gypsum board or plaster installation is substantially
free of defects, shrinkage or uneven settling is likely to cause
cracks at a later time. When pictures are moved from one location
to another, nail holes remain at the first location. Flaws of these
type are typically repaired with spachtling compound, which is a
spreadable composition that is similar to, but somewhat "dryer"
than, tape joint compound, which is applied and similarly allowed
to dry or cure.
Both tape joint compounds and spachtling compounds include polymeric
binder, filler, and water, but also typically include preservatives,
water-retention agents, wetting agents, defoamers, plasticizers,
non-leveling agents, etc. The density of such compositions can be
reduced by incorporating expanded perlite or hollow glass microspheres,
the latter sometimes being referred to as glass microbubbles; see,
e.g., U.S. Pat. Nos. 3183107 3386223 4086098 4391647
Because it is necessary for tape joint compositions to include
enough water to permit them to be readily and smoothly applied with
a spatula or trowel, a substantial amount of shrinkage typically
results during drying. It is thus generally necessary to apply such
compositions in several separate thin coats, sanding at least after
the last coat has dried, in order to avoid leaving a disfiguring
depression where the joint has been filled. For example, when filling
taped joints between abutted gypsum board panels, it is usually
necessary to apply three coats of a joint filling composition having
a shrinkage of 30-40%, drying and preferably sanding between applications.
Certain premium compositions having a shrinkage of 20-30% may require
only two applications, but it is believed that no previous composition
would yield satisfactory results in a single application. Since
all joint filling operations are highly labor intensive (typically
labor costs amount to about 90% of the cost of the project), the
total cost has been extremely high when compared to the cost of
the composition employed, and the advantages of a one-pass compound
appear to be obvious. Interestingly, however, both manufacturers
and purchasers profess to be extremely concerned about the cost
of the compositions, and no such one-pass compound has been available;
indeed, it is believed that no one recognized how to make such a
Manufacturers of joint filling compositions formerly included asbestos
fibers to impart the non-sagging properties considered essential
to a successful product. When asbestos was recognized as a carcinogen,
manufacturers turned to more innocuous substitutes to impart the
same properties. In the absence of asbestos, attapulgite (a micronized
swelling or gelling clay having a fibrous crystalline structure
and a high oil absorbence; cf. U.S. Pat. Nos. 3907725 and 4454627),
thickeners (e.g., cellulose ethers such as hydroxypropyl methyl
cellulose or hydroxyethyl cellulose; cf. U.S. Pat. No. 4454627;
starch; precipitated or fumed silica), and adjuvants such as gelling
agents and flow control agents became typical ingredients in joint
filling compositions. These additives--especially the attapulgite
and cellulose ethers, which adsorb a substantial amount of water
and impart thixotropic and non-sagging properties to the finished
composition and which are present in substantial amounts--unfortunately
have also made it impossible to achieve a spreadable composition
having low water content. As a result, prior art compositions have
been subject to significant shrinkage (up to about 50%), making
it necessary to apply more than one coat (typically three coats,
as previously noted) to achieve an acceptable appearance.
SUMMARY OF THE INVENTION
The present invention provides joint filling compositions that
have extremely low shrinkage while still possessing desirable attributes
of easy application and resistance to sagging. Satisfactory filling
can be effected in two applications, and often in a single application,
dramatically reducing labor cost. Because the compositions of the
invention contain a higher volume percentage of relatively high
density solid material than prior art compositions, the weight per
unit volume of the least expensive compositions is somewhat higher;
density can, of course, be reduced by incorporating hollow fillers
such as expanded perlite or, preferably, hollow glass microspheres,
as part of the solid material, albeit at some increase in the cost
of raw materials.
The present invention is based on the deceptively simple discovery
that prior art joint filling compositions can be significantly improved
and shrinkage drastically reduced by minimizing the volume percentage
of water-adsorbing additives such as the conventionally included
attapulgite and hydroxyethyl cellulose, thereby obtaining compositions
having lower water content than has heretofore been considered possible.
Although the shrinkage is reduced, the handling and non-sagging
properties of the prior art compositions are retained. Simple and
logical though the invention may seem in retrospect, it flies in
the face of what has been "known" by those skilled in
Compositions of the invention exhibit shrinkage upon drying of
no more than about 25%, preferably no more than about 20%, and still
more preferably no more than about 10%. This decreased shrinkage
significantly reduces the need for additional coatings, even where
the area to be filled is deep or extensive, and thereby greatly
lowers labor cost.
Compositions of the present invention are significantly more nearly
Newtonian than those of the prior art. If, for example, viscosity
is measured at room temperature on a Brookfield Model RVT Viscometer
using spindle "4" at 0.5 rpm and 1 rpm, the viscosity
difference in poises (known as the "Brookfield yield value")
would be 0 for a completely Newtonian liquid, more plastic fluids
having much higher values. To demonstrate clearly the difference
between prior art compositions and those of the present invention,
250 cc of water is added to 500 cc of the composition to be tested,
mixed until smooth and the diluted composition allowed to stand
for 30 minutes before performing yield value measurements. When
evaluated in accordance with this test, conventional tape joint
compositions exhibit what will be referred to herein as "diluted
yield values" on the order of 250-500 poises (25-50 Pa.s),
while compositions of the present invention generally have diluted
yield values on the order of 10-100 poises (1-10 Pa.s) or somewhat
less (e.g., 5 poises, or 0.5 Pa.s) and in no event exceed 150 poises
The invention may thus be characterized as an improved tape joint
or wall repair composition comprising particulate filler, binder,
water and optionally adjuvant substances, typically having a viscosity
on the order of 2000-3000 Pa.s when measured on a Brookfield RVT
Viscometer with Helipath, Spindle F, at 5 rpm. The improvement resides
in reducing the amount of water-adsorbing materials to ensure that
the diluted yield value of the composition is on the order of 1-10
Pa.s (in no event greater than about 15 Pa.s) whereby the composition
is readily spreadable and resists sagging but shrinks substantially
less upon drying than do conventional compounds having a diluted
yield value on the order of 25-50 Pa.s.
To perform careful measurements of the amount of shrinkage resulting
from the use of a given wall repair composition, it was found necessary
to devise a more precise test than the highly qualitative and subjective
evaluations presently used in the tape joint compound industry.
This procedure will now be described.
A reusable mold, formed from RTV silicone, so as to provide a cavity
approximately 2 cm.times.2 cm.times.20 cm, is weighed. A Fisher
Grease Pycnometer is then used to determine the density of each
joint compound to be tested, after which the mold cavity is filled
with the compound and the sample-filled mold weighed. The volume
of the joint compound in the mold is calculated by subtracting the
weight of the empty mold from the weight of the compound-filled
mold and dividing the result by the density of the composition.
The sample is then dried at room temperature until a constant weight
is achieved, typically 48 hours, after which it is carefully removed
from the mold and weighed. All six faces of the dried sample are
then sealed with hair spray and allowed to dry, minimizing any water
absorption in the subsequent volume displacement test.
Water is added to a tared 100 ml graduated cylinder until the bottom
of the meniscus reaches the 100 ml mark. The water-filled cylinder
is reweighed and the tare weight subtracted, yielding the true volume
of the cylinder, based on an assumed 1.0 g/ml density for water.
The cylinder is then emptied and dried, the sealed and dried sample
placed in it, and the cylinder plus sample tared. Water is then
added until the meniscus is even with the 100 ml mark, tapping the
cylinder from time to time to make certain that no air bubbles cling
to the surface of the sample. For any sample having a density of
less than that of water, an eye dropper is employed to push the
sample just below the surface of the water before reading the meniscus.
The weight of the water added is then determined and subtracted
from the weight of water previously required to fill the empty cylinder,
thereby arriving at the volume of the dried sample. The percentage
shrinkage is then calculated as follows: ##EQU1##
Tape joint compounds are generally classified as either "heavy
weight" or "light weight", the filler in the former
constituting only solid particulate inorganic material and the filler
in the latter including hollow glass microspheres or other cellular
material. As a general rule, heavy weight compounds shrink about
30-50%, while light weight compounds shrink about 20-30%.
Tabulated below are a variety of tape joint compounds, Table I
listing the compositions and Table II listing certain physical properties
of the compositions. In preparing these formulations the attapulgite
and "Tamol" dispersant were vigorously mixed with approximately
half of the water, using an air motor and a high shear mixer so
as to thoroughly disperse the clay. This dispersion was combined
with the polyvinyl acetate (supplied as an aqueous emulsion) and
preservative, using a Ross double planetary mixer, Model LOM-2.
The remaining dry ingredients were blended by hand and added to
the planetary mixer. The ingredients were then mixed for 5 minutes,
adding just enough water to provide a stiff but fluid mass. After
the sides of the mixing vessel had been scraped down, the remaining
additional water was added to obtain the desired viscosity, on the
oder of 2000-3000 Pa.s.