U.S. patent application number 10/274635 was filed with the patent office on 2003-02-27 for fungus resistant gypsum-based substrate.
Invention is credited to Bruce, Robert B., Harriss, David M..
Application Number | 20030037502 10/274635 |
Document ID | / |
Family ID | 27382667 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030037502 |
Kind Code |
A1 |
Bruce, Robert B. ; et
al. |
February 27, 2003 |
Fungus resistant gypsum-based substrate
Abstract
The present invention relates to a fungus resistant gypsum
board, made of first and second polymeric fibrous sheets with a
gypsum core sandwiched there between. The gypsum core containing
less than 0.03% of formulation additives that serve as fungus
nutrients and less than 0.5% of the dry gypsum core contains of
fungus nutrients. The fibrous sheets are preferably nonwovens and
the gypsum core preferably contains a fungicide. The invention is
also directed to a process for making a fungus resistant gypsum
board.
Inventors: |
Bruce, Robert B.;
(Burlington, CA) ; Harriss, David M.;
(Chesterfield, VA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
27382667 |
Appl. No.: |
10/274635 |
Filed: |
October 21, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10274635 |
Oct 21, 2002 |
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09513097 |
Feb 25, 2000 |
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60121697 |
Feb 25, 1999 |
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60121698 |
Feb 25, 1999 |
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Current U.S.
Class: |
52/517 ;
52/783.1 |
Current CPC
Class: |
E04C 2/043 20130101;
C04B 22/0013 20130101; C04B 2103/69 20130101; B32B 13/14 20130101;
C04B 28/14 20130101; C04B 2111/00629 20130101; C04B 28/14 20130101;
C04B 2111/20 20130101; C04B 28/14 20130101; Y10T 428/249932
20150401; Y10T 442/665 20150401; B28B 19/0092 20130101 |
Class at
Publication: |
52/517 ;
52/783.1 |
International
Class: |
B27K 001/00; E04B
001/62; B27K 003/00; B27K 005/00 |
Claims
What is claimed is:
1. A fungus resistant gypsum board, comprising: a first polymeric
fibrous sheet, said first sheet having a first surface and opposite
first and second edges; a second polymeric fibrous sheet, said
second sheet having a first surface and opposite first and second
edges; a gypsum core sandwiched between said first and second
nonwoven sheets, said gypsum core containing less than 0.03% by
weight, based on the weight of the dry gypsum core, of formulation
additives that serve as fungus nutrients; and a synthetic adhesive
on the first and second edges of said second sheets, said synthetic
adhesive adhering the first edge of said first nonwoven sheet to
said first edge of said second nonwoven sheet, and adhering the
second edge of said first nonwoven sheet to the second edge of said
second nonwoven sheet.
2. The board of claim 1 wherein said gypsum core contains less than
0.5% by weight, based on the weight of the dry gypsum core, of
fungus nutrients.
3. The board of claim 1 wherein said gypsum core contains a
fungicide.
4. The board of claim 3 wherein said fungicide is a metal/inorganic
derivative.
5. The board of claim 4 wherein said fungicide is boric acid, and
the gypsum core is comprised of between 0.04 and 0.25 weight
percent, based on the weight of the dry gypsum core, of boric
acid.
6. A fungus resistant gypsum-based substrate, comprising: a first
polymeric fibrous nonwoven sheet, said first nonwoven sheet having
a first surface and opposite first and second edges; a second
polymeric fibrous nonwoven sheet, said second nonwoven sheet having
a first surface and opposite first and second edges; a wet and
hydrated gypsum core sandwiched between said first and second
nonwoven sheets, said gypsum core containing less than 0.02% by
weight, based on the weight of the wet gypsum core, of formulation
additives that serve as fungus nutrients, and less than 0.33% by
weight, based on the weight of the wet gypsum core, of fungus
nutrients; a synthetic adhesive on the first and second edges of
said second nonwoven sheet, said synthetic adhesive adhering the
first edge of said first nonwoven sheet to said first edge of said
second nonwoven sheet, and adhering the second edge of said first
nonwoven sheet to the second edge of said second nonwoven sheet;
and wherein said first surface of said first nonwoven sheet and
said first surface of said second nonwoven sheet adhere to said wet
gypsum core with an adhesive strength of at least 7.5 lb.
7. The gypsum board of claim 6 wherein said first surface of said
first nonwoven sheet and said first surface of said second nonwoven
sheet have pores containing set gypsum of said gypsum core
intertwined with the fibers in the first and second nonwoven
sheets.
8. The gypsum board of claim 7 wherein said first surface of said
first nonwoven is comprised of a web selected from the group of
needle punched staple fiber sheets, hydroentangled fibrous sheets,
and spunbond sheets.
9. The gypsum board of claim 6 wherein said first surface of said
first nonwoven sheet and said first surface of said second nonwoven
sheet are coated with a primer layer of a high density gypsum
slurry having a density that is at least 1.1 times the density of
the gypsum core.
10. The gypsum board of claim 6 wherein said first and second
sheets adhere to said wet gypsum core with an adhesive strength of
at least 10 lb.
11. A process for manufacturing a gypsum-based substrate,
comprising the steps of: adding calcined gypsum, formulation
additives and water to a mixer, said mixture containing less than
0.02% by weight, based on the weight of the total slurry mix, of
formulation additives that serve as fungus nutrients; mixing the
gypsum and water in the mixer to produce a gypsum slurry that is
comprised of 50 to 65 weight percent gypsum; providing a first
polymeric fibrous sheet, said first sheet having a first surface
and opposite first and second edges; pouring said gypsum slurry
from said mixer onto the first surface of said first sheet and
spreading the gypsum slurry over said first surface of said first
sheet; providing a second polymeric fibrous sheet, said second
sheet having a first surface and opposite first and second edges;
applying a synthetic adhesive on the first and second edges of said
second sheet; placing said first surface of said second sheet over
the gypsum slurry that has been spread over the first surface of
said first sheet; adhering the adhesive on the first edge of said
second sheet to said first edge of said first sheet, and adhering
the adhesive on the second edge of said second sheet to said second
edge of said first sheet; enclosing the gypsum slurry between said
first and second sheets to bring the slurry into intimate contact
with said first and second sheets and form an elongated strip of
gypsum slurry sandwiched between said first and second sheets;
allowing said elongated strip of gypsum slurry to set up and harden
to form a stiff elongated strip having a solid, wet gypsum core
sandwiched between said first and second sheets; cutting said stiff
elongated strip into gypsum board of desired length; and drying
said gypsum board in a dryer to remove excess water from the gypsum
boards.
12. The pcocess of claim 11 wherein in the step of adding calcined
gypsum, formulation additives and water to a mixer, said mixture
contains less than 0.33% by weight, based on the weight of the
total slurry, of fungus nutrients.
13. The process of claim 11 wherein after the elongated strip of
gypsum slurry has set up and hardened to form a stiff elongated
strip having a solid, wet gypsum core sandwiched between said first
and second sheets, said first and second sheets adhere to said wet
gypsum core with and adhesive strength of at least 7.5 lb.
14. The process of claim 11 wherein said first and second sheets
are nonwoven sheets.
15. The process of claim 14 wherein said first surface of said
first nonwoven sheet and said first surface of said second nonwoven
sheet have open pores between fibers of sufficient size for the
gypsum slurry to enter the pores and become intertwined with the
fibers in the sheets when the gypsum slurry is enclosed between
said first and second nonwoven sheets.
16. The process of claim 15 wherein said first and second sheets
each have a mean flow pore size, measured according to ASTM
F316-86, of at least 8.0 microns.
17. The process of claim 16 wherein said first and second sheet
each have a mean flow pore size, measured according to ASTM
F316-86, in the range of 8.7 to 40 microns.
18. The process of claim 15 wherein said first surface of said
first nonwoven sheet and said first surface of and second nonwoven
sheets are comprised of a web selected from the group of needle
punched staple fiber sheets, hydroentangled fibrous sheets, and
spunbond sheets.
19. The process of claim 12 wherein said first surface of said
first sheet and said first surface of said second sheet are coated
with a primer layer of a high density gypsum slurry having a
density that is at least 1.1 times the density of the gypsum
slurry.
20. The process of claim 14 wherein the dried gypsum board is
comprised of less than 0.03% by weight, based on the weight of the
gypsum core, of formulation additives that serve as fungus
nutrients, and less than 0.5% by weight, based on the weight of the
gypsum core, of fungus nutrients.
Description
[0001] This application claims benefit of priority from Provisional
Application No. 60/121,697 filed on Feb. 25, 1999, and from
Provisional Application No. 60/121,698 filed on Feb. 25, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to gypsum-based construction
materials. More particularly, the invention relates to a fungus
resistant gypsum-based substrate faced with a synthetic polymeric
sheet material that is suited for use as a construction material
such as wallboard or ceiling panels. The invention is also directed
to a process for manufacturing a fungus resistant gypsum-based
substrate faced with a synthetic polymeric sheet material.
[0004] 2. Description of Related Art
[0005] Fungi frequently can be found in the walls of buildings.
Common fungi include mold and mildew. Fungi are especially
troublesome in walls with poor ventilation where moisture can
become trapped in the wall. The walls of portable buildings, such
as temporary classrooms, have proved particularly susceptible to
fungus growth because water often seeps in around the openings and
joints of such structures. In buildings with poor ventilation or
inefficient heating and air conditioning systems, the building
walls are more likely to become breeding grounds for fungus. Some
funguses that grow in walls, such as the stachybotrys chartarum
(atra) fungus, produce toxins that have been known to render
structures uninhabitable.
[0006] Conventional gypsum-based construction materials have the
disadvantage that they support fungus growth when used in moist
environments. Fungus needs both moisture and nutrients to survive.
Naturally occurring organic matter that is a part of conventional
gypsum board products, such as cellulose, paper fibers, starch, and
contaminants, serve as nourishment for many strains of fungus.
Accordingly, when conventional gypsum board becomes chronically
moist or water damaged due to excessive humidity, water leaks,
condensation, or flooding, fungus will grow on or in the gypsum
board. Fungus growth can be exacerbated in gypsum board walls when
vinyl wall coverings are used on the interior surface of the walls.
Such vinyl wall coverings can trap moisture inside the gypsum board
where it facilitates fungus growth.
[0007] Gypsum wallboard and gypsum panels are traditionally
manufactured by a continuous process. In this process, a gypsum
slurry is first generated in a mechanical mixer by mixing calcium
sulphate hemihydrate (also known as calcined gypsum), water, and
other agents. The gypsum slurry is normally deposited on a paper
sheet. The gypsum slurry may include additives such as cellulose
fibers that help to strengthen the gypsum core once it is dry.
Starch is conventionally added to the gypsum slurry in order to
improve the adhesion between the gypsum core and the paper facing.
An upper continuously advancing paper sheet is laid over the gypsum
and the edges of the upper and lower paper sheets are pasted to
each other with a starch paste. The paper sheets and gypsum slurry
are passed between parallel upper and lower forming plates or rolls
in order to generate an integrated and continuous flat strip of
unset gypsum sandwiched between the paper sheets that are known as
facing or liners. This strip is conveyed over a series of
continuous moving belts and rollers for a period of 2 to 5 minutes
during which time the core begins to hydrate back to gypsum and
hardens. During each transfer between belts and/or rolls, the strip
is stressed in a way that can cause the paper facing to delaminate
from the gypsum core if the adhesion between the gypsum core and
the facing is not sufficient. Once the gypsum core has set
sufficiently, the continuous strip is cut into shorter lengths or
even individual boards or panels of prescribed length. Once again,
it is important for there to be good adhesion between the paper
sheets and the set, but still wet, gypsum core or the cutting
action will pull the edges of the paper facing sheet away from the
gypsum core.
[0008] After the cutting step, the gypsum boards are separated and
grouped through a series of belts and rollers and then flipped over
before being fed into drying ovens or kilns where the boards are
dried so as to evaporate excess water. The hydration from
hemihydrate to gypsum must be essentially complete at this point,
normally between 7 and 15 minutes after mixing. When the gypsum
boards are accelerated, flipped and fed into the drying ovens, the
boards are subjected to a variety of stresses that can cause the
facing to peel away from the gypsum core of the boards unless there
is good adhesion between the set (but still wet) gypsum core and
the facing material. Inside the drying ovens, the boards are blown
with hot drying air at speeds up to 4000 feet/minute which can
cause further delamination of the paper facing if there is not good
wet adhesion between the gypsum and the paper liners. When portions
of the facing sheets delaminate from the gypsum core during drying
in the oven, the liner becomes entangled in the rollers and the
gypsum crumbles as it dries which jams the oven and requires
frequent shut downs of the line while the loose gypsum is cleaned
out of the ovens. The gypsum boards are dried in the ovens for
anywhere from 30 to 75 minutes. After the dried gypsum boards are
removed from the ovens, the ends of the boards are trimmed off and
the boards are cut to desired sizes.
[0009] The fully dried gypsum adheres well to the paper facing
sheet materials as long as the gypsum board is kept dry. However,
paper facing has a number of inherent properties that can be
detrimental in a gypsum wallboard product. As discussed above,
paper facing material (sometimes called a paper liner) is made of
cellulose which serves as a nutrient for fungus growth. Paper
facing also is not as strong or abrasion resistant as needed for
certain construction applications. In addition, because the
strength of paper differs significantly depending on the direction
in which the strength is measured, paper facing must be relatively
thick in order to achieve satisfactory multidirectional strength.
Paper faced gypsum-board products also suffer from a lack of
abrasion resistance. Paper facing used on conventional gypsum board
becomes especially weak and subject to delamination from the gypsum
core when the paper becomes damp due to leaks or high humidity.
[0010] Paper-faced gypsum boards must generally be coated with
another material, such as paint or a wallcovering material, in
order to achieve sufficient abrasion resistance. For example,
paper-faced wallboard is often covered with vinyl wallcovering, a
hard plastic sheet, or a plastic film when used in high traffic
areas. Unfortunately, such coatings and coverings tend to trap
moisture inside the wall where it can precipitate fungus
growth.
[0011] Canadian Patent No. 1,189,434 discloses gypsum panels made
with a facing of a moisture vapor permeable spunbonded nonwoven
material. Canadian Patent No. 1,189,434 discloses gypsum panels
faced with Tyvek.RTM. spunbonded olefin sheet material. Tyvek.RTM.
is a registered trademark of E.I. du Pont de Nemours and Company of
Wilmington, Del. Tyvek.RTM. sheets are made by solution
flash-spinning polyethylene to form fine plexifilamentary fibril
structures that can be thermally bonded to form sheet material.
U.S. Pat. No. 5,704,179 discloses gypsum board faced with mats of
fiberglass or synthetic resin fibers. While the panels disclosed in
these patents eliminate naturally occurring organic matter from the
facing sheets of the gypsum board, these patents are not directed
to reducing or eliminating fungus growth. Accordingly, the patents
do not disclose removal of nutrients from the gypsum core or other
enhancements needed to reduce fungus growth in the gypsum
board.
[0012] In addition, while it has been possible to produce gypsum
boards faced with polymeric fibrous sheet materials on a small
laboratory scale, it has proven difficult to produce gypsum boards
faced with such sheets on a commercial scale. This is because the
adhesive strength between conventional fibrous synthetic fibrous
sheets and the wet gypsum core (known as wet adhesion) tends to be
low. Thus, the facing peels away from the gypsum core during
various points in the production process before the boards are
fully dried in the drying ovens.
[0013] There is a need for a process by which gypsum board that is
free of fungus nutrients such as cellulose, starch, and natural
fibers that can be manufactured on a commercial basis. There is
also a need for a gypsum board that does not trap mold supporting
moisture. Finally, there is a need for gypsum boards that actually
include substances that prevent the growth of fungus.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention is directed to a fungus resistant
gypsum board, comprising: a first polymeric fibrous sheet, the
first sheet having a first surface and opposite first and second
edges; a second polymeric fibrous sheet, the second sheet having a
first surface and opposite first and second edges; a gypsum core
sandwiched between the first and second nonwoven sheets, the gypsum
core containing less than 0.03% by weight, based on the weight of
the dry gypsum core, of formulation additives that serve as fungus
nutrients; and a synthetic adhesive on the first and second edges
of said second sheets, the synthetic adhesive adhering the first
edge of said first nonwoven sheet to the first edge of the second
nonwoven sheet, and adhering the second edge of the first nonwoven
sheet to the second edge of the second nonwoven sheet. Preferably,
the gypsum core contains less than 0.5% by weight, based on the
weight of the dry gypsum core, of fungus nutrients.
[0015] According to a preferred embodiment of the invention, the
gypsum core contains a fungicide such as a metal/inorganic
derivative. More preferably, the fungicide is boric acid, and the
gypsum core is comprised of between 0.04 and 0.25 weight percent,
based on the weight of the dry gypsum core, of boric acid.
[0016] According to a preferred embodiment of the invention, the
first and second polymeric fibrous sheets are nonwoven sheet. In
the preferred embodiment of the invention, the first surface of the
first nonwoven sheet and the first surface of the second nonwoven
sheet adhere to said wet gypsum core with an adhesive strength of
at least 7.5 lb. The first surface of the first nonwoven sheet and
the first surface of the second nonwoven sheet have pores
containing set gypsum of the gypsum core intertwined with the
fibers in the first and second nonwoven sheets. The sheets may be
comprised of a needle punched staple fiber sheet, a hydroentangled
fibrous sheet, or a spunbond sheet. Alternatively, the first
surface of the first and second nonwoven sheets may be coated with
a primer layer of a high density gypsum slurry having a density
that is at least 1.1 times the density of the gypsum core.
Preferably, the first and second sheets adhere to said wet gypsum
core with an adhesive strength of at least 10 lb.
[0017] The present invention is also directed to a process for
manufacturing a gypsum-based substrate. The process includes the
steps of: adding calcined gypsum, formulation additives and water
to a mixer, the mixture containing less than 0.02% by weight, based
on the weight of the total slurry mix, of formulation additives
that serve as fungus nutrients; mixing the gypsum and water in the
mixer to produce a gypsum slurry that is comprised of 50 to 65
weight percent gypsum; providing a first polymeric fibrous sheet,
the first sheet having a first surface and opposite first and
second edges; pouring the gypsum slurry from the mixer onto the
first surface of the first sheet and spreading the gypsum slurry
over the first surface of the first sheet; providing a second
polymeric fibrous sheet, the second sheet having a first surface
and opposite first and second edges; applying a synthetic adhesive
on the first and second edges of the second sheet; placing the
first surface of the second sheet over the gypsum slurry that has
been spread over the first surface of the first sheet; adhering the
adhesive on the first edge of the second sheet to the first edge of
the first sheet, and adhering the adhesive on the second edge of
the second sheet to the second edge of the first sheet; enclosing
the gypsum slurry between the first and second sheets to bring the
slurry into intimate contact with the first and second sheets and
form an elongated strip of gypsum slurry sandwiched between the
first and second sheets; allowing the elongated strip of gypsum
slurry to set up and harden to form a stiff elongated strip having
a solid, wet gypsum core sandwiched between the first and second
sheets; cutting the stiff elongated strip into gypsum board of
desired length; drying the gypsum board in a dryer to remove excess
water from the gypsum boards.
[0018] In the process of the invention, the gypsum slurry contains
less than 0.33% by weight, based on the weight of the total slurry,
of fungus nutrients. After the elongated strip of gypsum slurry has
set up and hardened to form a stiff elongated strip having a solid,
wet gypsum core sandwiched between the first and second sheets, the
first and second sheets preferably adhere to the wet gypsum core
with and adhesive strength of at least 7.5 lb. According to the
more preferred process of the invention, the first and second
sheets are nonwoven sheets. It is preferred that the first surface
of the first nonwoven sheet and the first surface of the second
nonwoven sheet have open pores between fibers of sufficient size
for the gypsum slurry to enter the pores and become intertwined
with the fibers in the sheets when the gypsum slurry is enclosed
between the first and second nonwoven sheets. It is desirable that
the first and second sheets each have a mean flow pore size,
measured according to ASTM F316-86, of at least 8.0 microns, and
more preferably in the range of 8.7 to 40 microns.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0019] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate the presently
preferred embodiments of the invention and, together with the
description, serve to explain the principles of the invention.
[0020] FIG. 1 is a schematic representation of a portion of the
process of the invention.
[0021] FIG. 2 is a cross-sectional view of a gypsum-based substrate
made according to the invention.
[0022] FIG. 3 is a schematic representation of another portion of
the process shown in FIG. 1.
DEFINITIONS
[0023] As used herein, "fungus nutrients" means carbohydrate or
cellulosic based materials, or other organic materials which are
biodegradable by fungi commonly found in building construction
materials.
[0024] As used herein, "fungicide" means a group of materials that
destroy fungi or inhibits the growth of fungi. Fungicides include
synthetic compounds that are sulfur containing, halogens
containing, metal containing, aliphatic, aromatic (phenol compounds
and deriviatives), naphthol, quinoline, and imidazole
derivatives.
[0025] As used herein, "formulation additives that serve as fungus
nutrients" means raw materials that are used to manufacture gypsum
board and become incorporated into a final gypsum board product
which materials fall within the definition of "fungus
nutrients."
Test Methods
[0026] In the description above and in the non-limiting examples
that follow, the following test methods were employed to determine
various reported characteristics and properties. ASTM refers to the
American Society for Testing and Materials.
[0027] Mold Resistance of wallboard samples was measured according
to ASTM D3273 using 3.75 inch by 4 inch (9.5 cm by 10.2 cm)
wallboard samples, prepared as described in Example 1 below. The
wallboard samples were tested in triplicate by suspending the
samples above a mold-soil inoculum in an environmental humidity
chamber equipped with a fan to circulate the mold spores,
constructed as detailed in ASTM D3273. A white pine control was
also tested, as described in ASTM D3273. The soil inoculum for the
study was prepared by seeding the incubating soil with three
strains of fungi: Aureobasidium pullulans (ATCC 9348), Aspergillus
niger (ATCC 6275), Penicillium Sp. (ATCC 9849). ATCC refers to the
American Type Culture Collection, 12301 Parklawn Drive, Rockville,
Md. 20852The inside of the chamber was maintained at between about
90% to 100% relative humidity and a temperature of 88.degree.
F.-92.degree. F. (31.degree. C.-33.degree. C.). The test samples
were continuously equilibrated and challenged with mold for 4.5
weeks, during which time the samples were rated each week on a
scale of 0 to 10 using photographic standards (ASTM D3274). A
rating of 10 indicates undetectable mold growth and a rating of 0
indicates substantial mold growth. Any sample ratings that differed
by more than an increment of 2 from the others for a particular
example were disregarded and the average of the ratings was
calculated based on the remaining data points.
[0028] At the conclusion of the study, the gypsum board samples
were removed and microscopically examined at a magnification of
50.times. to distinguish mold growth from any soil particles that
may have contaminated the board during the test. To further
differentiate mold from soil particles, chlorine bleach was applied
to the spots in question. Black spots disappearing on contact with
the bleach were considered to be mold whereas spots which were
unaffected by the bleach solution were considered to be soil
particles. After microscopic examination and treatment with bleach,
the boards were rated again on a scale of 0 to 10 using
photographic standards (ASTM D3274). Any sample ratings that
differed by more than an increment of 2 from the others for a
particular example were disregarded and the average of the ratings
was calculated based on the remaining data points and reported in
Table 1 as the final rating.
[0029] Wet Adhesion was measured using an Instron tensile tester
according to the following procedure.
[0030] Gypsum boards were prepared using a mold comprising a
laminated board having three aluminum rails of 1/2 inch height
(12.7 mm) screwed thereto to define three sides of a rectangular
mold with one open end. The aluminum rails were sized to form a
mold having a length of about 20 inches (50.8 cm) and a width of
3.75 inches (9.53 cm). With one of the longer side rails removed, a
nonwoven sheet having a length of 19 inches (48.3 cm) and a width
of 5.5 inches (14.0 cm) was placed on the bottom of the mold to act
as a liner on the first side of the gypsum board. After re-screwing
the 20 inch (50.8 cm) rail to the bottom of the mold, 1.75 inches
(4.44 cm) of the 5.5 inch (14.0 cm) liner width extended outside of
the mold, underneath the aluminum rail. The portion of the liner
extending outside the mold forms an overhanging portion of the
liner on the final gypsum board, which is inserted into the clamp
of the Instron testing machine during wet adhesion testing.
Immediately after mixing, as described in the examples below, a
gypsum slurry was poured into the mold onto the nonwoven liner and
spread evenly over the surface thereof. A second piece of nonwoven
sheet material having dimensions of about 20 inches (50.8 cm) by
3.75 inches (9.53 cm) was placed on top of the gypsum slurry to act
as a liner on the second side of the gypsum board. The board was
allowed to sit at room temperature for 20 minutes to allow the
gypsum to set. The temperature of representative mixes were
monitored to ensure that the hydration was complete within this
time frame (the temperature of the mix rises during hydration, then
holds steady, and finally drops once hydration was complete).
Hydration times of 16 to 18 minutes were recorded for the boards
produced in the examples below. The boards were removed by
unscrewing and removing the side rail which was on top of the
overhanging section of liner and sliding each board out of the
mold. The boards were flipped over so that the first side having
the overhanging liner was on the top surface.
[0031] Immediately after the gypsum had set, each board was cut,
using a utility knife, into three, four or five 3 inch (7.6 cm) by
3.75 inch (9.53 cm) sections (with the top liner having dimensions
of 3 inches (7.6 cm) by 5.5 inches (14 cm) due to the overhang).
The top nonwoven liner on each board section was cut parallel to
the 3.75 inch (9.53 cm) side into three 1 inch (2.54 cm) wide
strips and each strip was cut in the perpendicular direction such
that the length of the liner section to be pulled off the board was
2 inches (5.1 cm) (in addition to the 1.75 inch (4.45 cm)
overhang). The time taken to cut the board and the liner strips was
no more than about 10 minutes.
[0032] The board was clamped in the Instron machine and the center
strip was pulled from each board section with the Instron set at 20
or 50 pounds force (89 or 222 Newtons). During testing, the liner
strip was pulled from the board in the direction parallel to the
length of the liner. The wet adhesion was measured as the force in
pounds at which the 2 inch length of liner was completely pulled
away from the board. Testing of all of the three to five samples
was completed within 5 minutes of testing the first sample. The wet
adhesion is reported as the average (+/- standard deviation) of the
three to five samples tested for each board.
[0033] Basis weight was determined by ASTM D-3776, which is hereby
incorporated by reference, and is reported in g/m.sup.2.
[0034] Maximum, minimum, and mean flow pore sizes were measured for
the nonwoven liners on a Coulter Porometer II according to ASTM
F316-86 using Porofil wetting fluid, available from Coulter. The
max pore size is an indicator of the diameter of the largest pore
channels in the distribution of pore sizes supporting flow through
the web. The mean flow pore size is an indicator of the mean pore
channel diameter for the pores supporting the total flow. The
minimum pore size is an indicator of the minimum pore channel
diameter for the pores supporting the total flow through the web.
Pore size calculations were made using a size factor of 0.64, a
tortuosity factor of 1.00, and a sample thickness of 10
microns.
DETAILED DESCRIPTION OF THE INVENTION
[0035] Reference will now be made in detail to the presently
preferred embodiments of the invention, examples of which are
illustrated below. The present invention is directed to a gypsum
board product that is resistant to the growth of fungus. The
invention is also directed to an improved process for manufacturing
gypsum board having a gypsum core lined with a synthetic nonwoven
facing material without the addition or use of materials or
additives that may serve as fungus nutrients.
[0036] As discussed in the background section above, gypsum
wallboard is traditionally manufactured by a continuous process. A
commercial process for manufacturing the fungus resistant gypsum
board material of the invention is shown in FIG. 1. As shown in
FIG. 1, a gypsum slurry is first generated in a mechanical mixer 10
by mixing calcium sulphate hemihydrate (also known as calcined
gypsum), water, water reducing agents, foam, bonding agents, and
set control agents. The foam is a mixture of water, a foaming agent
such as alkyl sulfate/alkyl ether sulfate mixtures, and air. Other
additives, such as anti-burning agents, can be added to the slurry
as needed. Formulation additives that serve as fungus nutrients,
such as cellulose fibers or starch, preferably comprise less than
0.02% of the gypsum slurry, which corresponds to less than 0.03% by
weight of the dried gypsum core. More preferably, there are no
formulation additives that are fungus nutrients. In addition, the
gypsum used in the slurry is preferably comprised of less than
0.5%, by weight of the gypsum, of naturally occurring organic
contaminants (such as plant or animal matter) that may serve as a
fungus nutrient. According to a preferred embodiment of the
invention, the gypsum slurry includes a fungicide such as a
metal/inorganic derivative. More preferably, the gypsum slurry
includes between 0.025% and 0.17% by weight boric acid, based on
the total weight of the gypsum slurry.
[0037] The gypsum slurry 12 is deposited on the central portion of
a continuously advancing first polymeric fibrous sheet 14. The
edges of the first sheet 14 are folded upward. As can be seen in
the cross-sectional view of FIG. 2, each of the ends of the
upturned edges of the sheet 14 are folded toward each other along
folds a short distance, depending on the thickness of the board,
from each of the first folds so as to form strips 16 and 18 that
are substantially parallel to the bottom of the sheet. An upper
continuously advancing second polymeric fibrous sheet 20, with a an
adhesive applied on opposite edges of its bottom surface as
adhesive strips 22 and 23, is laid over the gypsum slurry such that
the edge paste strips 22 and 23 contact the folded over strips 16
and 18 of the first sheet 14. According to the invention the
adhesive is material, such as a synthetic pressure sensitive
polymeric adhesive that does not act as a fungus nutrient.
Preferably, the adhesive strips 22 and 23 are comprised of fast
tacking polyvinyl alcohol based adhesives, synthetic resin based
adhesives, or hot melt adhesives. The polymeric fibrous sheets 14
and 20 may be woven or nonwoven synthetic sheets. Nonwoven sheets
made of fiber forming thermoplastic polymers are preferred.
[0038] As can be seen in FIG. 1, the first and second nonwoven
sheets and gypsum slurry are passed between parallel upper and
lower forming plates 26 or rolls in order to generate an integrated
and continuous flat strip 30 of unset gypsum sandwiched between
synthetic fibrous sheets which are referred to as synthetic facing
or liners. The strip 30 is conveyed over a series of continuous
moving belts 32 and rollers (not shown) for a period of 2 to 5
minutes during which time the gypsum core 28 sets up. It is
important that a good bond be formed quickly between the wet gypsum
and the fibrous sheets 14 and 20 because the strip 30 can move at
speeds in excess of 500 ft/min over a distances of 1200-2000 feet,
during which time the strip 30 is transferred between multiple
belts and rollers. During each transfer between belts and/or rolls,
the strip 30 is stressed in a way that can cause the synthetic
facing to delaminate from the gypsum core 28 if the adhesion
between the gypsum core and the facing is not sufficient. Once the
gypsum has set, the continuous strip 30 is cut into shorter lengths
or individual boards or panels 34 of prescribed length by means of
the rotating serrated blades 38 and 39. Once again, it is important
for there to be good adhesion between the synthetic fibrous sheets
14 and 16 and the set, but still wet, gypsum core 28. Otherwise the
blades 38 and 39 pull the edges of the synthetic facing sheet 14
and 16 away from the gypsum core 28 as the blades rotate during the
cutting process.
[0039] After the cutting step, the gypsum boards 34 are accelerated
on rollers 36 to separate the boards from each other. The separated
gypsum boards are then lifted from the line and flipped over by my
means of a plurality of lifting arms. The boards are fed, with
their top sides down, into drying ovens or kilns where the boards
are dried so as to evaporate excess water. When the gypsum boards
are accelerated, flipped and fed into the drying ovens, the boards
are subjected to a variety of stresses that would cause the
synthetic facing to peel away from the gypsum core of the boards
but for the excellent wet adhesion between the set (but still wet)
gypsum core and the facing material that is obtained by means of
the process of the invention. Inside the drying ovens, the boards
are blown with hot drying air at speeds up to 4000 feet/minute. The
absence of loose edges where there is not good adhesion between the
nonwoven synthetic sheets 14 and 16 and the wet gypsum core 28
means that the facing is not pulled away from the gypsum core by
the hot drying air. The gypsum boards are dried in the ovens for
anywhere from 30 to 75 minutes. After the dried gypsum boards are
removed from the ovens, the ends of the boards are trimmed off and
the boards are cut to desired sizes.
[0040] According to the invention, steps can be taken to improve
the wet adhesion between the setting gypsum slurry and the
synthetic fibrous sheet, that do not include the addition of fungus
nutrients, such as starch, to the gypsum panels. According to one
preferred embodiment of the invention, wetting agents that do not
act as fungus nutrients are added to the gypsum slurry or applied
to the synthetic nonwoven sheet before the gypsum slurry and
nonwoven liner are brought into contact with each other. Such
agents include synthetic chemicals with hydrophilic and hydrophobic
groups which are known to reduce surface tension of aqueous
solutions and reduce contact angles with hydrophobic solids. A wide
range of wetting agents will perform this function such as soaps
and detergents, or even the foaming agents which are described
above for adding foam to the gypsum core.
[0041] A preferred wetting agent is polyvinyl alcohol (PVA). While
effective as a wetting agent, it can also be used to replace the
starch that is normally used in a conventional gypsum board
manufacturing process to improve the bonding of the liner to the
dried gypsum core. Polyvinyl alcohol is commonly used as an
adhesive and it has now been found that during the drying process,
the polyvinyl alcohol will migrate to the interface between the
liner and gypsum core and improve the bonding of the liner to the
dried gypsum core to the extent that starch is not needed. Indeed,
PVA has been found to be a more effective dry bond adhesive than
the starch for synthetic fibrous polymeric liners. The starch, a
fungus nutrient, can be replaced by PVA, a formulation additive
that does not serve as a fungus nutrient.
[0042] According to another preferred embodiment of the invention,
the first and second synthetic sheets each have a first surface
characterized by pores or spaces formed between fibers, which pores
are of sufficient size for a gypsum slurry to enter the pores and
become intertwined with the fibers in the sheets so as to form a
strong mechanical bond between the gypsum core and the fibrous
synthetic sheets when the gypsum sets up. The gypsum slurry is
deposited on this first porous surface of the first sheet and the
first porous surface of the second sheet is juxtaposed against the
gypsum such that when the gypsum slurry is enclosed between said
first and second nonwoven sheets, the slurry impregnates into the
pores or spaces between the fibers on the surfaces of the first and
second fibrous sheets. According to this embodiment of the
invention, a strong bond is formed between the wet gypsum core and
the sheets in the absence of naturally occurring additives, such as
starch, that can serve as fungus nutrients. Preferably, the first
and second nonwoven sheets each have a mean flow pore size,
measured according to ASTM F316-86, of at least 8.0 microns.
According to a more preferred embodiment of the invention, the
first and second nonwoven sheets each have a mean flow pore size,
measured according to ASTM F316-86, in the range of 8.7 to 40
microns. This range of pore sizes allows the wet, set gypsum layer
to intertwine with the fibers of the synthetic fibrous liner,
providing good wet adhesion, without the gypsum slurry penetrating
completely through the nonwoven liner.
[0043] According to the invention, the first and second sheets may
be nonwoven sheets comprised of meltspun substantially continuous
fibers, carded staple fiber webs, needle punched staple fiber webs,
hydroentangled fibrous webs, or other porous nonwoven synthetic
structures. The fibers in the first and second nonwoven sheets are
comprised of synthetic melt spinnable polymer. The preferred fibers
are comprised of one or more of any of a variety of polymers or
copolymers including polyethylene, polypropylene, polyester,
aramids, nylon, elastomer, and other melt spinnable polymers. For
example, the fibers of the first and second nonwoven sheets may be
comprised of at least 50% by weight polyester polymer, such as
poly(ethylene terephthalate), poly(propylene terephthalate), or
poly(butylene terephthalate) polymer. Alternatively, the fibers may
be comprised of at least 50% by weight of a nylon polymer, a
polyolefin polymer such as polyethylene or polypropylene, or an
elastomeric polymer such as polyurethane or co-polyether ester.
[0044] According to one especially preferred embodiment of the
invention, the first and second nonwoven sheets may be comprised of
small denier polymeric fibers that, when made into a sheet
structure, form numerous very small pores. The fibers of such sheet
can be melt spun and air drawn according to the process disclosed
in U.S. Pat. No. 5,688,468. Such nonwoven sheet may be a unitary
fibrous sheet comprised of melt spun substantially continuous
filament polymer fibers wherein the sheet has a basis weight of
from 13 g/m.sup.2 to 125 g/m.sup.2 and substantially all of the
fibers are melt spun fibers. The fibers in such nonwoven sheets
have a cross sectional area of between about 20 and about 90
.mu.m.sup.2, and more preferably, of from about 25 to about 70
.mu.m.sup.2, and most preferably from about 33 to about 60
.mu.m.sup.2. Such melt spun microfibers sheets have a tensile
strength (in both the machine and cross directions), normalized for
basis weight, of from 0.7 to 5 N/(g/m.sup.2), and more preferably
from 0.8 to 4 N/(g/m.sup.2), and most preferably from 0.9 to 3
N/(g/m.sup.2).
[0045] In another preferred embodiment of the invention, the
surface of the synthetic nonwoven liner which contacts the gypsum
slurry has a textured surface comprising depressions and/or
protrusions. Such textured surfaces can be found in embossed woven
and nonwoven fabrics (e.g. thermally point-bonded nonwoven fabrics)
or embossed woven fabrics. The gypsum slurry flows into the
depressions or around the protrusions on the textured surface and
mechanically locks the gypsum layer to the liner as the gypsum
layer expands during setting. Preferably the depressions or
protrusions have dimensions in the range of about 50 to 2000
microns in the plane of the liner and from about 30 to 500 microns
in depth with between 20 and 100 depressions/protrusions per square
centimeter. More preferably, the dimension of the protrusions in
the plane of the liner is between about 100 and 1000 microns, the
depth of the protrusions is between about 200-500 microns, and
there are between 30 and 75 depressions/protrusions per square
centimeter. The dimensions of the protrusions/depressions can be
measured by microscopic analysis using scanning electron microscopy
techniques known in the art.
[0046] According to another preferred embodiment of the invention,
the process of the invention may include the steps of coating the
first surface of each of the first and second synthetic sheets with
a thin coating of a dense gypsum slurry. Preferably, the first
surface of said first sheet and said first surface of said second
sheet are coated with a layer of a high density gypsum slurry
having a density that is 1.1 to 3 times the density of the gypsum
slurry used to form the core of the gypsum board. Preferably, the
dense gypsum layer has a thickness in the range of {fraction
(1/32)} to 1/8 inch and has a dry density of between about 0.70 and
1.72 g/cc (corresponding to a wet density of between about 1.06 to
about 1.98 g/cc). The gypsum slurry density may be calculated based
on a density of water of 1 g/cc and a gypsum density of 2.32 g/cc
or can be measured using methods known in the art. Typical
commercial gypsum board core densities are approximately 0.96 g/cc
(wet) and 0.63 g/cc (dry). Gypsum boards lined with a synthetic
polymeric fibrous liner having a high density layer adjacent the
liner can be produced using methods known in the art for paper
liners. For example, the high density gypsum slurry may be coated
onto a synthetic nonwoven liner using the roller-coating apparatus
and method described in U.S. Pat. No. 5,879,486, which is hereby
incorporated by reference. Alternately, a defoaming agent can be
applied to the surface of the synthetic polymeric nonwoven liner
which results in an increase in the gypsum density immediately
adjacent the liner, as described in U.S. Pat. No. 4,327,146, which
is hereby incorporated by reference. In paper-faced gypsum boards,
the paper has good wet adhesion to the gypsum slurry and the high
density gypsum layer is used to improve the dry bond between the
paper liner and the gypsum. In the current invention, the use of a
high density gypsum layer results in improved wet adhesion between
the synthetic fibrous liner and the gypsum slurry, allowing the
board to be manufactured using conventional gypsum board
manufacturing processes.
EXAMPLES
[0047] The following non-limiting examples are intended to
illustrate the product and process of the invention and not to
limit the invention in any manner.
Example 1
[0048] A gypsum slurry was prepared by pre-blending, in a plastic
bag, 400 grams of General Purpose White Molding Plaster (available
from USG Corporation), a beta-type hemihydrate plaster similar to
that used in a commercial gypsum board factory, and 0.67 grams of a
very finely ground gypsum accelerator having an average particle
size of less than 2 microns. The pre-blended powder was then sifted
over a period of approximately 2 minutes onto the surface of a
polyvinyl alcohol (PVA) solution contained in a Waring blender,
allowing the gypsum to wet out and fall to the bottom of the
blender. The PVA solution comprised 305 ml of a stock solution
prepared by dissolving 22.1 grams of Elvanol.RTM. 90-50 polyvinyl
alcohol (available from E.I. du Pont de Nemours and Company) in
1000 ml of water, heating to dissolve the polyvinyl alcohol, and
cooling the resulting solution at room temperature for at least 24
hours. Immediately after the addition of the pre-blended powder to
the PVA solution was completed, foam that had been prepared by
blending 65 ml of a 0.5 weight percent solids solution of an alkyl
sulphate/alkyl ether sulphate blend (Cedepal FA-406, manufactured
by Stepan Chemicals) in a separate Waring blender for approximately
2 minutes was poured on top of the water/solids mixture and the
blender turned on for 10 seconds. The resulting gypsum slurry was
used immediately for preparing the gypsum boards.
[0049] Half-inch thick gypsum boards were prepared by coating the
gypsum slurry on a 3.75 inch by 16 inch (9.5 cm by 40.6 cm) sheet
of Tyveke.RTM. 1058D flash-spun high density polyethylene
(available from E.I. du Pont de Nemours and Company) placed on the
bottom of a mold and covering the gypsum slurry with a second
Tyvek.RTM. 1058D sheet. Rubber gloves were worn throughout the
board preparation process to avoid contamination of the liner
surface with oils and dirt. After casting the board, it was removed
from the mold, and dried in a General Signal Blue M Series forced
air oven at 127.degree. C. for 90 minutes, after which the oven
temperature was ramped down to 75.degree. C. over a period of 45
minutes. The oven was then turned off and allowed to cool to room
temperature overnight. The dried gypsum board was removed from the
oven and cut into four equal 4 inch.times.3.75 inch sections for
mold testing. A {fraction (1/16)} inch hole was drilled about 1/8
inch from one of the 3.75 inch edges, centered on the edge, used
for suspending the boards in the environmental chamber during mold
testing.
[0050] The boards were tested for resistance to mold growth as
described above and the results are reported in Table 1 below.
There was no evidence of delamination of the nonwoven liner during
testing.
[0051] Boards were prepared for wet adhesion measurements and
tested as described above. An average wet adhesion of 7.1.+-.1.3
pounds-force (31.5.+-.5.8 Newtons) was obtained.
Example 2
[0052] Gypsum boards were prepared as described in Example 1,
except that 16.7 ml of Aqualite 70 wax emulsion (available from
Monsey Bakor, Inc.) was added to the Waring Blender containing the
305 ml of PVA solution prior to adding the pre-blended gypsum
powder.
[0053] The boards were tested for resistance to mold growth as
described above and the results are reported in Table 1 below.
There was no evidence of delamination of the nonwoven liner during
testing.
Example 3
[0054] Gypsum boards were prepared as described in Example 1,
except that the lining was a thermally point bonded spunbonded
polypropylene fabric having a basis weight of 2.1 oz/yd2 containing
2 weight % pigment. The fibers had an effective diameter of about
10 microns. The gypsum board was prepared with the embossed sides
of the liner facing the gypsum slurry.
[0055] The boards were tested for resistance to mold growth as
described above and the results are reported in Table 1 below.
There was no evidence of delamination of the nonwoven liner during
testing.
Example 4
[0056] Gypsum boards were prepared as described in Example 1,
except that the lining was a point bonded spunbonded polyester
fabric having a basis weight of 1.9 oz/yd.sup.2, and comprising
fibers having an effective diameter of approximately 8.6 microns.
The spunbonded liner was thermally point bonded between an engraved
oil-heated metal calender roll and a smooth oil heated metal
calender roll. The engraved roll had a chrome coated non-hardened
steel surface with a diamond pattern having a point size of 0.466
mm.sup.2, a point depth of 0.86 mm, a point spacing of 1.2 mm, and
a bond area of 14.6%. The point bonded sheet had a minimum pore
size of 14.69 microns, maximum pore size of 70.63 microns, and a
mean flow pore size of 29.01 microns. The gypsum board was prepared
with the embossed sides of the liner facing the gypsum slurry.
[0057] The boards were tested for resistance to mold growth as
described above and the results are reported in Table 1 below.
There was no evidence of delamination of the nonwoven liner during
testing.
[0058] Boards were prepared for wet adhesion measurements and
tested as described above. An average wet adhesion of 12.3.+-.2.2
pounds-force (55.+-.10 Newtons) was obtained.
Example 5
[0059] Gypsum boards were prepared as described in Example 1,
except that the lining was Sontara.RTM. E88-320 spunlaced polyester
having a basis weight of 4 oz/yd.sup.2 (available from E.I. du Pont
de Nemours and Company).
[0060] The boards were tested for resistance to mold growth as
described above and the results are reported in Table 1 below.
There was no evidence of delamination of the nonwoven liner during
testing.
[0061] Boards were prepared for wet adhesion measurements and
tested as described above. An average wet adhesion of 10.6.+-.6.6
pounds-force (47.2.+-.29 Newtons) was obtained.
Comparative Example A
[0062] In this example, a commercial paper-lined Sheetrock.RTM.
gypsum board, manufactured by USG Corporation was tested for mold
resistance.
[0063] The boards were tested for resistance to mold growth as
described above and the results are reported in Table 1 below. The
paper liner was observed to start to delaminate from the gypsum
core during testing.
1TABLE 1 Fungus Resistance Ratings for Gypsum Boards Micro- Core
ASTM ASTM scopic Final Example Liner Additives Ratings Average
Ratings Rating 1 Tyvek .RTM. PVA 8, 8, 10 9 10, 10, 10 1058D 10 2
Tyvek .RTM. PVA/Wax 10, 8, 6 8 10, 10, 10 1058D emulsion 6 3
Spunbond PVA 8, 10, 8 8 6, 10, 10 PP 10 4 Spunbond PVA 8, 8, 8 8
10, 10, 10 PE 10 5 Sontara .RTM. PVA 10, 8, 8 9 8, 10, 10 E88-320
10 Comp. A Paper Commer- 2, 10, 4 3 2, 10, 4 3 cial Board White --
-- 4 4 4 4 Pine Control
[0064] It can readily be seen that the gypsum boards made in
accordance with the invention without the introduction of materials
that serve as fungus nutrients are far more resistant to mold and
fungus growth than is the conventional gypsum board of Comparative
Example A. It will be apparent to those skilled in the art that
modifications and variations can be made in process and gypsum
board material of invention. It is intended that all matter
contained in the foregoing description, drawings and examples shall
be interpreted as illustrative and not in a limiting sense.
* * * * *