Note: Descriptions are shown in the official language in which they were submitted.
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TAPE-ON CORNER BEAD FOR DRYWALL CONSTRUCTION
TECHNICAL FIELD
The present invention relates to tape-on corner beads for drywall
(gypsuni board) construction, and more particularly to those having some
protection
against the abrasive action of sanding the joint cement compound used to
secure the
coiner bead in position and provide a topping coat. Standard specifications
for joint
tape and joint compound for finishing gypsum board are set forth in ASTM
Designation: C475.
BACKGROUND OF THE INVENTION
Tape-on corner beads for drywall construction normally comprise a
thin corner angle of galvanized steel covered by a joint reinforcing paper
tape which
extends laterally as wings beyond the longitudinal side edges of the corner
angle.
Commonly the corner beads are classified as hard-line and soft-line to denote,
respectively, those having a central corner rib and those having a rounded
central
bullnose. These corner beads present a respective shoulder facing each wing to
serve
as a stop for joint cement applieci to the respective paper wing, the adjacent
surface of
the drywall, and the paper surface of the corner bead extending laterally
outward from
the shoulder. When the joint cernent is being applied as a retaining layer to
the colner
bead to hold it in position the joint cement commonly slops as excess material
beyond
the shoulder onto part of the paper on the central part of the corner bead.
After
drying, the retaining joint cement layer is sanded to provide a smooth surface
from
the wallboard over the corner bead. During this sanding operation the central
portion
of the paper layer of the corner bead adjoining the shoulders is commonly
subjected
to some of the sanding action t:o an extent scuffing the surface such that
when the
wallboard and corner bead are painted, a smooth painted surface is difficult
or
impossible to attain.
It is preferred to use substantially the same type of paper for the paper
layer on tape-on type corner beads as is normally used as the tape for
covering the
joints of installed gypsum board. This paper properly absorbs water from joint
cement when applied thereto and its fibers are spaced such as to be adequately
coated
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with the joint cement materials to provide a good bond between the joint
cement and the
paper. However, the characteristics of the joint tape making it excellent for
receiving the joint
cement make it more vulnerable to scuffing and loose fiber exposure when
subjected to a
sanding operation as discussed above.
In United States Patent No. 5,131,198 issued July 21, 1992 the corner scuffing
problem on tape-on type corner beads was addressed by applying a protective
coating (film)
of a suitable material such as an acrylic resin to the outside paper layer in
longitudinal zones
(bands) thereof adjoining the stop shoulders. The applied protective material
penetrated the
surface of the paper, but normally did not penetrate through the thickness of
the paper. In
commercial use of corner beads with bands of protective coatings disclosed in
the patent it
has been found that on some occasions the sanding operations to smooth the
joint cement are
so overly aggressive that objectionable scuffing still results.
In the International Application Number PCT/IB96/00236, filed 13 February
1996, which was published 22 August 1996, under International Publication
Number WO
96/25570, there is disclosed a tape-on corner bead for drywall construction in
which the paper
layer is not like that used in joint tape, but is stated to be made from a
Kraft stock paper about
0.005 inches in thickness which is impregnated with a latex containing an
acrylic resin to
improve the wet and dry strength properties. The international application
states that a
suitable such impregnated paper is produced by Thorold Specialty Papers of
Etobicoke,
Ontario, Canada, under the trademark WALLSTRIP, and is superheated to 300 F.
during
production. This paper is used for all of the paper in the corner bead. Hence,
the paper wings
as well as the portions of the paper layer adjoining the stop shoulders are
paper which has
been impregnated throughout with acrylic latex. As a result the paper wings
resist penetration
by joint cement far more than the normal joint tape type of paper. This
detracts from good
bonding of the joint cement to the corner bead and therefore the joint cement
is not always
effective in bonding the corner bead to the underlying wall board. Also, crack
lines are more
likely to develop.
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SUMMARY OF THE INVENTION
The present invention aims to provide an improved tape-on corner
bead which is superior in performance to both of the afore-discussed prior art
tape-on
corner beads. This is accomplished by impregnating joint tape type of paper
for the
paper layer of the corner bead throughout its thickness with a suitable
synthetic resin
applied only through the part of the width of the paper which is later likely
to be
subject tc, abrasion during the finishing of the joint cement on the corner
bead. A
synthetic resin film is applied as a protective coating over the resin
impregnated
portion o f the paper after the impregnating resin has set (dried). Both resin
applications are preferably performed before the paper is applied to the metal
core.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional view showing a typical tape-on corner bead
with a hard-line configuration;
Fig. 2 is a cross-sectional view showing a typical tape-on corner bead
with a so ft-line configuration;
Fig. 3 is a detail sectional view to an enlarged scale of the circled
portion indicated in Fig. 1 when the illustrated colner bead embodies the
present
invention.;
Fig. 4 is a detail sectional view to an enlarged scale of the circled
portion indicated in Fig. 2 when the illustrated corner bead embodies the
present
invention;
Fig. 5 is a fragmentary plan view of a paper tape for the corner bead of
Fig. 3;
Fig. 6 is a fragmentary plan view of a paper tape for the corner bead of
Fig. 4; ar.id
Fig. 7 is a perspective schematic view showing application of the
impregn2Lting material to the paper.
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DETAIL:~ED DESCRIPTION OF THE INVENTION
Referring to Figures 1 and 2, it is seen that typical hard-line and soft-
line tape-on corner beads 10, 11 have a core strip 12, 13 and a respective
paper cover
strip 14, 15. The core strip may be a galvanized steel strip about 0.009 to
0.015
inches thick which has been rolll-formed to the hard-line shape (Fig. 1)
having a
corner rib 12a and side flanges 121), or to the soft-line shape (Fig. 2)
having a rounded
bullnose 13a, a pair of step-down sloped shoulders 13b, and side flanges 13c.
Commonly the core strip is 0.012 to 0.013 inches thick. A typical hard-line
core strip
12 will have its side flanges 12b at ninety degrees to one another and about
one inch
wide, an(i a typical soft-line core strip 13 will have its bullnose 13a shaped
with a
radius in the range of about'/4 to 1'/z inches, and its side flanges 13c at a
right angle to
one another and about an inch wide. The corner rib 12a on the hard-line unit
will
typically be about 0.0625 inches lligh and about 0.125 inches wide. On the
soft-line
unit each of the shoulders 13b wi:ll typically be about 0.125 inches wide and
have an
offset of about 0.0625 inches from the corresponding outer edge of the rounded
nose
section 13a and the adjoining siiie flange 13c. The paper cover strips 14, 15
are
bonded to the core strips 12, 13 as by a hot melt glue and extend laterally as
wings
14a and 15a beyond the core strips, typically about one inch.
Tape-on colner beads are normally installed by applying a thin layer of
joint conipound (joint cement) about four inches wide to the outer
intersecting edge
portions of two intersecting drywall sheets and then the selected corner bead
is
pressed against this layer of joint compound. An outer layer of joint compound
is
then applied to the corner bead arid extending several inches onto the
adjoining faces
of the drywall. This outer layer is dressed by a finishing knife to the paper
on the
'25 shoulders of the colner rib 12a ir.t the case of a hard-line corner bead,
and is dressed
onto the paper on the shoulders 13b in the case of a soft-line corner bead.
During this
dressing operation normally some of the joint compound is wiped onto the paper
on
the noses of the hard-line and soft-line beads. After the joint compound has
dried, the
joint cement is smoothed and feathered by a sanding operation. The present
inventioii protects the paper at 1:he noses of the corner beads during this
sanding
operation.
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In accordance with the invention preferably two applications of
protective materials are applied to the paper layer of the hard-line corner
beads. The
first application involves a thorough impregnation of the paper for its entire
thickness
in the ceritral band (zone) of the paper strip (tape) which will cover the
central rib 12a
5 (See Fig. 3) of the core strip 12 in the case of the hard-line bead, and
which will cover
the bullnose 13a and adjoining shoulders (see Fig. 4) in the case of the soft-
line bead.
This is done with an acrylic latex formulation before the paper is applied to
the core
strip.
A preferred formulation for the impregnating material includes the
lo following ingredients, together with water, given by weight percentages:
Acrylic polymer ennulsion 35-40%
Surfactant 1-3%
Glycol Ether EB 10-15 /0
Alcohol 190 5-10%
The acrylic polynler emulsion preferably has a molecular weight
greater than 200,000, an acid number of 50, and a viscosity of 700 cps. The
formulation permits the impregnating material to be applied by spraying at
normal
room teniperature and then pressed through the entire thickness of the paper
as will
now be described.
For economy of production, the paper used for the paper layer of the
corner bead is preferably treated with the impregnating resin in a procedure
in which
a plurality of bands spaced across the width of a roll of paper are
simultaneously
impregnated throughout their wicith and thickness while the roll is being
unwound
from one mandrel and rewound on a second mandrel. A suitable impregnation
procedur.- is shown in Figure 7 in which an untreated roll of paper is
indicted at 50.
The paper feed& from the roll via rollers 53 through 56 to pass by a row of
spray
heads 58 which apply parallel bands 60 of impregnating resin to the passing
paper.
The paper travels from the spray heads to first and second sets of front and
back
squeeze rollers 62-63, and 64-65. The front rollers 62, 64 preferably comprise
conventional print cylinders. The squeezing action on the paper by the squeeze
rollers forces the impregnating resin applied by the spray heads through the
entire
thickness of the paper. The paper then advances by infrared heaters 66 and
through a
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heater box 67 while guided by rollers 68-69. Additional rollers 70-72 guide
the paper
to the ro ll 52. The described procedure may be performed with the paper
traveling at
about 600 feet per minute. It will be apparent that use of the described
apparatus
results in a roll of paper with laterally spaced bands of paper which are
fully
impregnated with the material sprayed onto the paper by the spray nozzles. The
pattern of the impregnated bands is set so that when the paper roll is slit
between the
bands resulting tapes will each have a central impregnated band 80 as shown in
Fig. 5
when the paper is to be used for the hard-line tape-on bead 10. As shown in
Fig. 6,
each tape slit from the roll will liave a wider central band 80' of
impregnated paper
1o corresponding to the locations of the shoulders 13b when the paper is to be
used for
the soft-line bead 11.
After the impregnating material has been applied as described the roll
52, is un.wound and the impregnated bands 80, 80' are given a protective
coating 82,
82' (film.) by a standard flexographic procedure while being guided to an
additional
mandrel for storage. The treated roll of paper can then be slit when required
to
provide the individual tapes. The following formulation, with water added as
needed,
is preser.itly preferred (by weight) for providing the protective coating 82:
Polyvinyl acetate (latex) 20-25%
Acrylic Styrene enlulsion (film former) 5-10%
Acrylic styrene resin (hard) 5-10%
Calcium carbonate (filler) 17-23%
Surfactants 1-3%
Solvents 1-3%
Zinc oxide solution (crosslinker) 1-3%
The acrylic styrerie emulsion may have a molecular weight greater
than 200,000, an acid number 4:5, and a viscosity of 1, 300 cps, and the
polyvinyl
acetate rnay have a viscosity of 400 cps. The roll with impregnating and
coated bands
can later be slit into individual sitrips (tapes) preparatory to applying the
paper strips
to the metal core.
The metal for the cores of the corner beads is fed as flat stock from a
reel into a roll forming machine together with a strip of the paper which has
been
previously impregnated and coated through part. of its width as described
above. Hot
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melt glue is applied to the paper strip and/or the metal strip before the
paper strip
merges with the metal strip in the roll forming machine so that as the metal
is shaped
to the desired bead cross-section, pressure rolls ensure a proper bond of the
paper and
metal together as is well known ir,L the art
The protective coating 82, 82' may be as described in Patent 5,131,198
and appli.ed during the roll forming operation rather than as described
previously.
The treated paper strips 14', 15' on the improved corner beads 10, 11
are preferably 70 to 95 pound bleached Kraft paper like that normally used for
wallboard joint tape. This paper has proven to be the most satisfactory in
practice
to when standard joint compounds for drywall construction are applied thereto.
By the
present invention the areas of the tape-on corner beads to be covered with
joint
compound are not treated with a protective material, but remain in their
natural state
to propei-ly bond to the joint compound. The impregnating treatment shown in
Fig. 7
holds the paper fiber down in the band areas 80, 80' after the corner bead has
been
sanded in case the coating 82 is violated by the sanding. This impregnation
together
with the coating 82 provide maximum scuffing protection without interfering
with
normal operation of the untreateci remaining areas of the joint tape type of
paper in
the papers layers.
Although the invention has been described applied to tape-on type
corner beads having paper wings, the invention is also applicable to nail-on
type
beads of' the same construction except that the paper wing portions are
narrower and
are doubled back and bonded to the back side of the core strip so as to cover
the
longitudinal edges of the core strip. Nails are driven through the paper layer
and the
side flanges of the core strip and then joint compound is applied and sanded
to cover
the nail heads and give a smooth transition from the wallboard to the nose of
the
corner bead. -
From the foregoinõ it will be appreciated that, although specific shapes
of tape-on corner beads have been shown for examples and various procedures
have
been described herein for purposes of illustration to apply the protective
materials,
various Tnodifications may be made without deviating from the spirit and scope
of the
invention. Furthermore, it will be apparent that the metal cores in the corner
beads
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could be plastic. Accordingly, the invention is not limited except as by the
appended
claims.
