U.S. patent application number 12/339951 was filed with the patent office on 2009-07-09 for acoustically absorbent ceiling tile having barrier facing with diffuse reflectance.
Invention is credited to Natalia V. Levit, Eric W. Teather.
Application Number | 20090173570 12/339951 |
Document ID | / |
Family ID | 40456808 |
Filed Date | 2009-07-09 |
United States Patent
Application |
20090173570 |
Kind Code |
A1 |
Levit; Natalia V. ; et
al. |
July 9, 2009 |
ACOUSTICALLY ABSORBENT CEILING TILE HAVING BARRIER FACING WITH
DIFFUSE REFLECTANCE
Abstract
An acoustically absorbent ceiling tile includes a core of
acoustically absorbing material having two major surfaces, and a
facing for covering the core on at least one major surface. The
facing comprises a porous flash spun plexifilamentary film-fibril
sheet having a coherent surface and comprising a plurality of pores
having a pore diameter between about 100 nm and about 20,000 nm and
a mean pore diameter of less than about 20,000 nm. The facing has
highly diffuse reflectance of light, and a reflectance of greater
than about 86%. The use of the facing improves the acoustic
absorption of ambient sound at frequencies below about 1200 Hz. The
facing provides a barrier to moisture and particles including
microorganisms so that the ceiling tile is suitable for use in
environments in which cleanliness is critical.
Inventors: |
Levit; Natalia V.; (Glen
Allen, VA) ; Teather; Eric W.; (Elkton, MD) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
40456808 |
Appl. No.: |
12/339951 |
Filed: |
December 19, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61008881 |
Dec 20, 2007 |
|
|
|
Current U.S.
Class: |
181/286 ;
181/290 |
Current CPC
Class: |
E04B 9/04 20130101; E04B
2103/04 20130101 |
Class at
Publication: |
181/286 ;
181/290 |
International
Class: |
E04B 1/84 20060101
E04B001/84 |
Claims
1. A ceiling tile comprising: a core of acoustically absorbing
material having two major surfaces; and a facing for covering the
core on at least one major surface thereof, the facing comprising a
flash spun plexifilamentary film-fibril sheet having a coherent
surface, having a basis weight of no greater than about 140
g/m.sup.2, comprising a plurality of pores having a pore diameter
between about 100 nm and about 20,000 nm and a mean pore diameter
of less than about 20,000 nm and having a light reflectance of
greater than about 86%.
2. The ceiling tile of claim 1, wherein the acoustic absorption of
the ceiling tile at a frequency below about 1200 Hz is at least
about 5% higher than the acoustic absorption of the ceiling tile
without the facing.
3. The ceiling tile of claim 1, wherein the delamination resistance
of the flash spun sheet is at least about 0.028 N/m.
4. The ceiling tile of claim 1, wherein the facing is
perforated.
5. The ceiling tile of claim 1, wherein the flash spun
plexifilamentary film-fibril sheet comprises particulate filler
having a refractive index greater than the refractive index of the
polymer.
6. The ceiling tile of claim 1, wherein the core of acoustically
absorbing material has a noise reduction coefficient of between
about 0.3 and about 0.9 and the noise reduction coefficient of the
ceiling tile is approximately equivalent to the noise reduction
coefficient of the core.
7. The ceiling tile of claim 1, wherein the facing has a Parker
surface smoothness not less than 6 micrometers.
8. The ceiling tile of claim 1, wherein the facing has a tensile
strength of at least about 20 N/2.54 cm.
9. The ceiling tile of claim 1, wherein the facing comprises a
graphical image printed thereon.
10. The ceiling tile of claim 1, wherein the facing is free of
nutrients that support growth of microorganisms.
11. The ceiling tile of claim 1, wherein the facing has a log
reduction value of at least about 2.
12. The ceiling tile of claim 1, wherein the facing comprises a
polymer selected from the group consisting of polyethylene and
polypropylene.
13. A method of improving acoustic absorption and light reflectance
in an environment comprising: (a) providing a ceiling tile
comprising a core of acoustically absorptive material covered by a
facing of flash spun sheet having a plurality of pores having a
pore diameter between about 100 nm and about 20,000 nm and a mean
pore diameter of less than about 20,000 nm and having a light
reflectance of greater than about 86%; and (b) positioning the
ceiling tile within the environment to cause ambient sound to be
absorbed and light to be reflected by the ceiling tile.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to ceiling tiles for use in a building
interior.
[0003] 2. Description of the Related Art
[0004] Acoustically absorbent ceiling tiles are known in the art
for use reducing the amount of noise and/or reverberation within a
given area, such as a building interior. In such ceiling tiles, a
core of acoustically absorbing materials, i.e., materials having a
high absorption coefficient, reduces noise by absorbing acoustic
energy as sound waves strike and enter the acoustically absorbing
material. Many known acoustically absorbing materials are formed of
unconsolidated or partially unconsolidated, lofty fibrous materials
including compressed fibers, recycled fiber or shoddy materials,
fiberglass or mineral fiber batts and felts and require a facing to
contain the core of fibrous materials. Other known acoustically
absorbing core materials including foam, materials having a
honeycomb structure, microperforated materials and acoustically
absorbing materials utilizing air spaces also utilize a protective
and/or decorative facing for use in a building interior.
[0005] Facings for covering acoustic absorbent ceiling tiles serve
as durable coverings that protect the core during handling, use and
maintenance. It is desirable that facings for covering acoustically
absorbent materials be materials that are either acoustically
transparent or absorbent, but not acoustically reflective, in order
to enhance the absorption of sound. Facings which are acoustically
reflective undesirably contribute to the ambient noise. Known
facings for covering acoustically absorbent ceiling tiles include
fabric, nonwoven sheet, paper, film and perforated solid
materials.
[0006] U.S. Pat. Nos. 5,824,973 and 6,877,585 disclose a sound
absorption laminate useful as a ceiling tile comprising a porous
insulation substrate and a paper, fabric or perforated film facing
sheet having an air flow resistance between 200 and 1210 rayls.
U.S. Pat. Appl. Pub. 2007/0151800 discloses an acoustic insulating
sheet material comprising a primary sound absorbing sheet and a
dense porous membrane which can be a spunbond web, melt blown web,
spunlaced web, carded or airlaid staple fiber web, woven web,
wet-laid web or combination of such webs having an airflow
resistance of about 5,000 rayls or less. U.S. Pat. No. 3,858,676
discloses a thin sound-absorbing panel especially for frequencies
below 500 Hz and its use in a ceiling system wherein the panel
comprises a perforated backing, a heavy textile facing having a
basis weight of 12 to 2,140 g/m.sup.2 and a specific airflow
resistance of 300 to 1,800 rayls, and a fiberglass core. U.S. Pat.
No. 5,832,685 discloses a self-supporting sound absorbing panel and
its use in a ceiling system comprising a nonwoven fabric having a
basis weight of about 10 to 15 oz/yd.sup.2 which can be a spunbond
fabric or a fabric comprising bonded staple fibers. These known
facing materials have the disadvantage that they are open to the
penetration of water, dust, mold and microorganisms, thus limiting
their application in critical environments.
[0007] It is desirable that visible light be reflected from the
surface of ceiling tile facings in a diffuse, even distribution, as
opposed to specular (mirror-like) reflection in which light is
reflected only at an angle equal to the incident angle. Diffuse or
Lambertian reflectance is the uniform diffuse reflection of light
from a material in all directions with no directional dependence
for the viewer, according to Lambert's cosine law. Diffuse
reflectance originates from a combination of external scattering of
light from features on the surface of a material, and internal
scattering of light from features within a material. Internal light
scattering can arise, for example, from features within a material
such as pores and particles. The light scattering cross section per
unit feature volume of materials containing closely spaced
refractive index inhomogeneity is maximized when the mean diameter
of the features is slightly less than one-half the wavelength of
the incident light. The degree of light scattering is also
increased when there is a large difference between the refractive
index of the scattering feature and refractive index of the phase
in which the feature is dispersed.
[0008] It would be desirable to have acoustically absorbing ceiling
tiles having a combination of diffuse reflectance of light and
acoustic absorption which are suitable for use in a variety of
critical environments.
SUMMARY OF THE INVENTION
[0009] According to one embodiment, the invention is directed to a
ceiling tile comprising:
[0010] a core of acoustically absorbing material having two major
surfaces; and
[0011] a facing for covering the core on at least one major surface
thereof, the facing comprising a flash spun plexifilamentary
film-fibril sheet having a coherent surface, having a basis weight
of no greater than about 140 g/m.sup.2, comprising a plurality of
pores having a pore diameter between about 100 nm and about 20,000
nm and a mean pore diameter of less than about 20,000 nm and having
a light reflectance of greater than about 86%.
[0012] According to another embodiment, the invention is directed
to a method of improving acoustic absorption and light reflectance
in an environment comprising:
[0013] (a) providing a ceiling tile comprising a core of
acoustically absorptive material covered by a facing of flash spun
sheet having a plurality of pores having a pore diameter between
about 100 nm and about 20,000 nm and a mean pore diameter of less
than about 20,000 nm and having a light reflectance of greater than
about 86%; and
[0014] (b) positioning the ceiling tile within the environment to
cause ambient sound to be absorbed and light to be reflected by the
ceiling tile.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a graph depicting the acoustic absorption,
reflection and sound transmission of a flash spun sheet (block
measurement).
[0016] FIG. 2 is a graph depicting the acoustic absorption,
reflection and sound transmission of a flash spun sheet (anechoic
measurement).
[0017] FIG. 3 is a graph comparing the acoustic absorption
coefficients of an acoustic absorber without a facing and two
acoustic absorbers with facings useful in the ceiling tile
according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] According to an embodiment of the invention, a ceiling tile
is provided having an advantageous combination of acoustic
absorption, diffuse reflectance of light and barrier to the
penetration of fine particles, microorganisms and moisture. Ceiling
tiles according to the invention have facings that are durable,
waterproof, hypoallergenic, non-linting, non-off-gassing and
resistant to the penetration of moisture, dust, mold and
microorganisms without impeding the reflectance and acoustic
absorption capabilities.
[0019] The terms "acoustic absorbent" and "acoustically absorbing"
herein refer generally to the ability of a material to absorb
incident sound waves.
[0020] The term "diffuse reflectance" refers to the uniform diffuse
reflection of light from a material in all directions with no
directional dependence for the viewer, according to Lambert's
cosine law. Diffuse reflectance can be approximated as the total
reflectance minus specular reflectance.
[0021] The acoustically absorbing ceiling tile of the invention
includes an acoustically absorbing core and a nonwoven facing
covering at least one surface of the core. The facing is
acoustically transparent, in that the facing does not detract from
the acoustic absorption of the core, or the facing can enhance the
acoustical absorption of the ceiling tile core. The nonwoven facing
comprises a flash spun plexifilamentary film-fibril sheet having a
coherent surface. By "coherent surface" is meant the surface of the
sheet is consolidated and/or bonded. The bonding method can be any
known in the art, including but not limited to thermal calendering,
through-gas bonding, and point-bonding. The core and facing are
bonded to each other, by any known suitable bonding techniques such
as adhesive bonding, solvent bonding, ultrasonic bonding, thermal
bonding, point bonding, stitch bonding or the like. The bonded
material is subsequently cut into ceiling tiles.
[0022] The acoustically absorbing core includes any known
acoustically absorbing material and/or an air space. The core has a
noise reduction coefficient (NRC) between about 0.3 and about 0.9,
as measured by ASTM C423, mounting A (without air space). Suitable
acoustically absorbent materials include nonwoven fabrics, such as
spunbonded nonwovens, carded nonwovens, needlepunched nonwovens,
air-laid nonwovens, wet-laid nonwovens, spunlaced nonwovens,
spunbonded-meltblown-spunbonded composite nonwovens and meltblown
nonwovens, woven fabrics, knit fabrics, three-dimensional meshes,
including honeycomb structures and foams, combinations thereof and
the like. The term "nonwoven" means a web including a multitude of
randomly distributed fibers. The fibers can be staple fibers or
continuous fibers. The fibers can comprise a single material or a
multitude of materials, either as a combination of different fibers
or as a combination of similar fibers each comprised of different
materials. Other materials suitable for use as the core are foams,
such as open-cell melamine foam, polyimide, polyolefin, and
polyurethane foams, and perforated sheets. According to preferred
embodiments of the invention, the core is substantially free of
volatile organic compounds (VOCs). One preferred material is
formaldehyde-free fiberglass batting. In general, the greater the
thickness of the core material, the greater the acoustic absorption
of the ceiling tile will be, especially at low frequencies. An air
space covered with the facing can serve as the absorbing core.
[0023] Acoustically transparent facings for use with acoustically
absorbent articles including ceiling tiles are known in the art.
Such facings typically have between about 5% and about 50% open
area, i.e. the area of the pores on the surface with respect to the
total surface area, depending on the need for acoustic absorption.
If high frequency absorption is not required, 5-15% open area is
appropriate. (M.D. Egan Architectural Acoustics, J. Ross
Publishing, 2007, p. 74-76). The percent open area and diameter of
the holes affects the acoustic transparency by determining the
critical frequency, the frequency after which the sound absorption
decreases rapidly. The critical frequency (f.sub.c) above which
sound absorption drops rapidly can be approximated using the
following equation:
f.sub.c.about.40P/D
[0024] where:
[0025] f.sub.c represents the critical frequency, Hz
[0026] P represents open area, %
[0027] D represents pore diameter, in
[0028] Examples of known acoustically transparent facings include
woven meshes, fabrics with low density and nonwoven scrims. The
drawback of such facings is very low barrier, e.g., resistance to
penetration of water, dust, and/or microorganisms.
[0029] The facing for use in the ceiling tile of the invention is
highly resistant to the penetration of water and fine particles
including microorganisms. The void fraction of the facing, i.e., 1
minus the solids fraction, is between about 0.5 and about 0.7. The
facing has a pore diameter as measured by mercury porosimetry (H.
M. Rootare, "A Review of Mercury Porosimetry" from Advanced
Experimental Techniques in Powder Metallurgy. Plenum Press, 1970,
pp.255-252) between about 100 nm and about 20,000 nm and even
between about 100 nm and about 1.500 nm. For the purpose of this
invention, the pores include intra-fiber pores and inter-fiber
pores. Intra-fiber pores are randomly distributed throughout the
interior of a fiber and have a mean pore diameter from about 100 nm
to about 1,000 nm. Inter-fiber pores are randomly distributed
interstices between fibers in a plexifilamentary film-fibril sheet.
The porous structure of the plexifilamentary film-fibril sheet
consist of both types of pores forming torturous pore structure,
rather then through hole structure found in mechanically perforated
prior art facings. The mean pore diameter of the facing is less
than about 20,000 nm, even less than about 5,000 nm, even less than
about 2,000 nm, even less than about 1,000 nm and even between
about 10 nm and about 1,000 nm. Pore sizes between 10 nm and 1000
nm represent intra-fiber pores. Summing the volume of pores with a
diameter between 10 nm to 1000 nm gives the volume of intra-fiber
pores, called for the purpose of this invention as Vpore. Specific
pore volume SPV (in units of cm.sup.3/m.sup.2) is defined as
mathematical product of the nonwoven sheet basis weight (in units
of g/m.sup.2) and the sheet pore volume (in units of cm.sup.3/g)
for pores of a given mean diameter as disclosed in US Pub. No.
2006/0262310, also assigned to DuPont.
[0030] For some uses, such as cases in which the absorbing material
contains no dust or nutrients to support the growth of
microorganisms, it may be desirable to mechanically perforate the
facing in order to open the structure and to increase the critical
frequency value. It has been found that by perforating the facing,
the overall acoustic absorption of the ceiling tile can be
improved.
[0031] For some uses, it is desirable for the facing of the ceiling
tile to provide a barrier to microorganisms including bacteria,
viruses and mold. The facing has a log reduction value (LRV), which
is a measure of microbial filtration, at least about 2 or even at
least about 4, as measured according to ASTM F2638-07 and ASTM
F1608. It is desired for the facing to have no flow rate or
time-dependent LRV such that the facing has stable barrier
efficiency and does not build up barrier over time during use, such
as is the case for known laminated paper. The facing further does
not include nutrients that support the growth of microorganisms,
including bacteria, yeast and fungus, without any additional
antibacterial or antifungal treatment.
[0032] The nonwoven facing for use in the ceiling tile of the
invention includes a plexifilamentary film-fibril sheet formed by
flash spinning, also referred to herein interchangeably as a flash
spun plexifilamentary film-fibril sheet or a flash spun sheet. The
nonwoven facing of the invention is lightweight, thin and strong.
The basis weight of the facing is less than about 140 g/m.sup.2,
even between about 34 g/m.sup.2 and about 120 g/m.sup.2. The
thickness of the facing is not more than about 1 mm, even between
about 0.02 mm and about 0.40 mm, and even between about 0.10 mm and
about 0.25 mm. Previously used thin facing materials provided
negligible acoustic absorption and a low level of strength and
durability. The flash spun facing according to the invention
imparts a high degree of isotropic strength and durability which is
important for the assembly and handling of the ceiling tile of the
invention and stable long term performance. The preferred tensile
strength of the facing in both machine and cross directions is not
less than about 20 N/2.54 cm as measured by ASTM D5035.
[0033] It has been generally believed that for effective acoustic
absorption, the wavelength of the sound to be absorbed and the
thickness of the absorptive material should be on the same order of
magnitude. FIG. 1 shows that the acoustic reflection coefficient is
nearly 1.0 for flash spun plexifilamentary sheet for use as the
nonwoven facing when tested in a blocked configuration in an
impedance tube, and there is no acoustic absorption detected. By
contrast, as depicted in FIG. 2, the same flash spun
plexifilamentary sheet surprisingly exhibits acoustic absorption
demonstrated by absorption coefficients between 0 and 0.2 and low
acoustic reflection when tested in an anechoic configuration (with
an air space located behind the sheet in the impedance tube) at
low- and mid-range frequencies, e.g., between about 200 and about
1200 Hz. It was previously believed that only thick materials and
thick perforated facings with continuous through-holes were able to
act as acoustic absorbers near the individual hole resonant
frequencies (Helmholtz resonators) with closed air space behind the
facing. Surprisingly, the facing of the ceiling tile of the
invention, which does not have through-holes and which is very
thin, has been found to enhance acoustic absorption at low- and
mid-range frequencies.
[0034] The flash spun sheet is produced by the following general
process, also disclosed in U.S. Pat. No. 3,860,369. The flash
spinning process is conducted in a chamber which has a
vapor-removal port and an opening through which sheet material
produced in the process is removed. Polymer solution is prepared at
an elevated temperature and pressure and provided to the chamber.
The pressure of the solution is greater than the cloud-point
pressure, which is the lowest pressure at which the polymer is
fully dissolved in the spin agent forming a homogeneous single
phase mixture. The single phase polymer solution passes through a
letdown orifice into a lower pressure (or letdown) chamber where
the solution separates into a two-phase liquid-liquid dispersion.
One phase of the dispersion is a spin agent-rich phase which
comprises primarily spin agent and the other phase of the
dispersion is a polymer-rich phase which contains most of the
polymer. This two-phase liquid-liquid dispersion is forced through
a spinneret into an area of much lower pressure (preferably
atmospheric pressure) where the spin agent evaporates very rapidly
(flashes), and the polyolefin emerges from the spinneret as
plexifilaments which are laid down to form the flash spun sheet.
During the flashing process, impurities are flashed along with the
spin agent, so that the resulting flash spun sheet is free of
impurities.
[0035] The term plexifilamentary or plexifilaments as used herein
refers to a three-dimensional integral network of a multitude of
thin, ribbon-like, film-fibrils of random length and with a mean
fibril thickness of less than about 4 micrometers and a median
width of less than about 25 micrometers. In plexifilamentary
structures, the film-fibrils are generally coextensively aligned
with the longitudinal axis of the structure and they intermittently
unite and separate at irregular intervals in various places
throughout the length, width and thickness of the structure to form
a continuous three-dimensional network. Such structures are
described in further detail in U.S. Pat. Nos. 3,081,519 and
3,227,794.
[0036] The sheet is consolidated which involves compressing the
sheet between the belt and a consolidation roll into a structure
having sufficient strength to be handled outside the chamber. The
sheet is then collected outside the chamber on a windup roll. The
sheet can then be bonded using methods known in the art, such as
thermal bonding, through gas bonding and point bonding.
[0037] The diameter of the film-fibrils of the flash spun facing,
i.e. between about 4 micrometers and about 25 micrometers, is in
the range of ultrasound wavelengths. At frequencies between about
100 Hz and about 1600 Hz, the wavelength of sound is several orders
of magnitude larger than the diameter of the film-fibrils.
Nevertheless, thin plexifilamentary film-fibrils of the facing
according to the invention surprisingly enhance the acoustic
absorption of the acoustic absorber between about 100 Hz and about
1600 Hz, even between about 100 Hz and about 1200 Hz. This is the
range of frequencies most often emitted by mechanical equipment and
the human voice, and therefore most often encountered as
undesirable noise in building interiors. Without wishing to be
bound by theory, it is believed that the pore size distribution of
the plexifilamentary film-fibrils of the flash spun sheet enhances
the acoustic absorption of an acoustically absorbing core of an
acoustically absorbing material or air space when the sheet is used
as a facing on at least one surface of the core. It has furthermore
surprisingly been found that flash spun sheet exhibits extremely
high airflow resistance.
[0038] Polymers from which facings of the acoustically absorbing
ceiling tile according to the invention can be made include
polyolefin (e.g., polyethylene, polypropylene, polymethylpentene
and polybutylene), acrylonitrile-butadiene-styrene (ABS) resin,
polystyrene, styrene-acrylonitrile, styrene-butadiene,
styrene-maleic anhydride, vinyl plastic (e.g., polyvinyl chloride
(PVC)), acrylic, acrylonitrile-based resin, acetal,
perfluoropolymer, hydrofluoropolymer, polyamide, polyamide-imide,
polyaramid, polyarylate, polycarbonate, polyesters, (e.g.,
polyethylene napthalate (PEN)), polyketone, polyphenylene ether,
polyphenylene sulfide and polysulfone. Preferred amongst the
polymers are the polyolefins, e.g., polyethylene and polypropylene.
The term polyethylene as used herein includes not only homopolymers
of ethylene, but also copolymers wherein at least 85% of the
recurring units arise from ethylene. A preferred polyethylene is
linear high density polyethylene having an upper limit of melting
range of about 130.degree. to 137.degree. C., a density in the
range of 0.94 to 0.98 g/cm.sup.3 and a melt index (as defined by
ASTM D-1238-57T, Condition E) of between 0.1 to 100, preferably
between 0.1 and 4. The term polypropylene as used herein includes
not only homopolymers of propylene but also copolymers wherein at
least 85% of the recurring units arise from propylene units.
[0039] Nonwoven facings can further comprise a known UV stabilizer,
antistatic agent, pigment and/or flame retardant dispersed within
the polymer of the fibers of the nonwoven substrate.
[0040] The facing of the ceiling tile has the desirable combination
of barrier, i.e., resistance to penetration of water, dust and/or
microorganisms, and porosity resulting in high air flow or
permeability and good acoustical performance. Acoustical absorption
is a function of acoustic impedance, which is determined by a
complex combination of acoustical resistance and acoustical
reactance. The acoustical reactance is governed largely by material
thickness, while acoustical resistance is governed by air flow
through the material. Significant porosity is needed for
acoustically transparent facings. On the other hand, barrier
properties are needed for particulate and liquid resistance of the
facing.
[0041] Facings of the ceiling tile according to the present
invention can comprise single or multiple layers of flash spun
sheet provided the acoustical absorption is not compromised. The
multilayer sheet embodiment is also useful for averaging out
nonuniformities in single sheets due to nonuniform sheet thickness
or directionality of sheet fibers. Multilayer laminates can be
prepared by positioning two or more sheets face to face, and
lightly thermally bonding the sheets under applied pressure, such
as by rolling the sheets between one or more pairs of nip rollers.
Laminates of sheets are preferably prepared by adhering the sheets
together with an adhesive, such as a pressure sensitive adhesive.
Adhesives of utility are those that maintain sufficient structural
integrity of the laminate during normal handling and use. Adhesives
of utility include moisture curable polyurethane, solvated
polyurethane adhesives and water-borne acrylics.
[0042] The reflectance of the ceiling tile facing is at least about
86%, even at least about 88%, even at least about 90% and even at
least about 94%, over the visible light spectrum, i.e., over
wavelengths between about 400 nm and 700 nm. The reflectance of
flash spun facings according to the present invention decreases
with increased thermal bonding. Thermal bonding reduces the volume
of intra-fiber pores having a high scattering cross section per
unit pore volume that contribute substantially to diffuse
reflectance. Thermal bonding also reduces the volume of inter-fiber
pores that also contribute to the diffuse reflectance. Flash spun
sheet of utility in ceiling tiles according to the invention is
preferably not bonded so heavily that reflectance is less than
about 86%. Flash spun sheet of utility in ceiling tiles according
to the invention is consolidated, and is preferably bonded to a
degree necessary to maintain structural integrity of the sheet
during manufacture of ceiling tiles. In particular, the sheet
should have sufficient structural integrity so that the edges do
not fray as the sheet is laminated to the ceiling tile core and
subsequently cut into tiles. Preferably, the delamination
resistance of the flash spun sheet is at least about 0.028 N/m.
Delamination resistance is a measurement reported in units of
force/length defined by ASTM D 2724 and relates to the extent of
bonding in certain types of sheet, for example bonding in nonwoven
sheet made from plexifilamentary film-fibrils.
[0043] The scattering and diffuse reflection of light by flash spun
facings is due to reflection of light at air-polymer interfaces of
the inter-fiber and intra-fiber pores resulting from the flash
spinning process. Reflection will increase with an increase in the
difference between the refractive index of the pore phase (air,
refractive index of 1.0) and the refractive index of the fiber
polymer phase. An increase in light scattering is observed
typically when the difference in refractive index between two
phases is greater than about 0.1. Polymer comprising the flash spun
facing preferably has a high refractive index (for example
polyethylene, refractive index of 1.51) and low absorption of
visible light.
[0044] Flash spun facings according to the invention can further
comprise particulate filler dispersed in the polymer phase forming
the flash spun sheet fibers. Particulate fillers of utility will
have a refractive index larger than the polymer and thus light
scattering of the nonwoven sheet will increase with an increase in
the difference between the refractive index of the pore phase (air,
refractive index of 1.0) and the refractive index of the fiber
polymer phase. Particulate fillers of utility have a high
refractive index, high light scattering cross section and low
absorption of visible light. Particulate filler enhances light
scattering and thereby use of particulate filler can provide higher
average reflectance for a given sheet thickness. Particulate
fillers can be any shape and have a mean diameter of from about
0.01 micrometer to about 1 micrometer, preferably from about 0.2
micrometers to 0.4 micrometers. Flash spun sheets containing
particulate filler comprise at least about 50% by weight polymer,
and particulate filler comprises from about 0.05 weight % to about
50 weight %, preferably 0.05 weight % to about 15 weight %, based
on the weight of the polymer. Example particulate fillers include
silicates, alkali metal carbonates, alkali earth metal carbonates,
alkali metal titanates, alkali earth metal titanates, alkali metal
sulfates, alkali earth metal sulfates, alkali metal oxides, alkali
earth metal oxides, transition metal oxides, metal oxides, alkali
metal hydroxides and alkali earth metal hydroxides. Specific
examples including titanium dioxide, calcium carbonate, clay, mica,
talc, hydrotalcite, magnesium hydroxide, silica, silicates, hollow
silicate spheres, wollastonite, feldspar, kaolin, magnesium
carbonate, barium carbonate, magnesium sulfate, barium sulfate,
calcium sulfate, aluminum hydroxide, calcium oxide, magnesium
oxide, alumina, asbestos powder, glass powder and zeolite. Known
methods are used to make the present nonwoven sheets containing
particulate filler, such as those disclosed in U.S. Pat. No.
6,010,970 and PCT publication number WO2005/98,119.
[0045] The acoustically absorbent ceiling tile of the invention is
particularly useful in critical indoor environments in which indoor
air quality and cleanliness are critical, such as in schools,
hospitals, cleanrooms, and the like. As a result of the flashing
process during flash spinning of the facing, the resulting facing
is free of impurities and the facing does not generate off-gassing
of any volatile compounds. Furthermore, the facing is non-linting
in that it does not release particles or fibers as a result of the
high degree of consolidation of the single film-fibrils within the
sheet structure. Furthermore, the acoustically absorbing core
preferably contains substantially no VOCs. The facing can be
cleaned by wiping or washing. The facing can also be sterilized by
known methods including solution cleaning, physical energy
radiation or gas sterilization. In situations in which cleaning and
sterilizing the facing are not convenient, the flash spun facing
can be disposed of and replaced at minimal expense and effort.
[0046] The facing of the ceiling tile can be further printed with a
graphic design such as a pattern, image and/or text in order to be
aesthetically desirable for the intended use. It is convenient to
have the ability to replace the facing in order to change the image
and/or text. By changing the facing, the aesthetics of the ceiling
tile can easily and inexpensively be changed.
[0047] The present invention further includes a method of improving
acoustic absorption and light reflectance in an environment
comprising: (i) providing a ceiling tile comprising a core of
acoustically absorbent material covered by a facing of flash spun
non-woven sheet having a plurality of pores wherein the pores have
a diameter between about 100 nm and about 20,000 nm and even
between about 100 nm and about 1500 nm and wherein the pores have a
mean pore diameter of less than about 20,000 nm, even less than
about 5,000 nm, even less than about 2,000 nm, even less than about
1,000 nm and even between about 10 nm and about 1,000 nm; and (ii)
positioning the ceiling tile within the environment to cause
ambient sound to be absorbed and light to be diffusely reflected by
the ceiling tile.
EXAMPLES
Test Methods
[0048] Basis Weight was measured by the method of ASTM D 3776,
modified for specimen size, and reported in units of g/m.sup.2.
[0049] Tensile Strength was measured according to ASTM D5035 and
reported in units of N/25.4 cm.
[0050] Gurley Hill Porosity was measured according to TAPPI T460
and reported in seconds.
[0051] Frazier Air Permeability was measured according to ASTM
D737-75 in CFM/ft.sup.2 at 125 Pa differential pressure.
[0052] Hydrostatic Head was measured according to AATCC TM 127, DIN
EN 20811 with a test rate of 60 cm of H.sub.2O per minute.
[0053] Parker Surface Smoothness was measured according to TAPPI
555 at a clamping pressure of 1.0 MPa and is reported in
micrometers.
[0054] Specific Airflow Resistance is equivalent to the air
pressure difference across a sample divided by the linear velocity
of airflow measured outside the sample and is reported in
Ns/m.sup.3. The values reported herein were determined as follows.
The volumetric air flow Q was calculated by dividing the air
permeability of the sample at a differential pressure of 125 Pa by
the sample area (38 cm.sup.2), using the following equation:
Q (in m.sup.3/s)=0.000471947.times.(air permeability (in
CFM/ft.sup.2)/area (in ft.sup.2)).
Gurley Hill porosity (in seconds) is used for relatively low air
permeability materials. For flash spun sheet of less than 101
g/m.sup.2, the Frazier air permeability of 0.6 m.sup.3/min/m.sup.2
(2 ft.sup.3/min/ft.sup.2) corresponds to about 3.1 seconds;
therefore Frazier air permeability (in CFM/ft.sup.2) of the samples
herein was approximated as 3.1/Gurley Hill porosity (in
seconds).
[0055] Next, the airflow resistance in units of Pa-s/m.sup.3 was
calculated by dividing the differential pressure by the air flow Q.
Finally, the specific airflow resistance in units of Ns/m.sup.3 was
calculated by dividing the airflow resistance by the area of the
sample.
[0056] Transmission, Reflection, and Absorption Coefficients as
reported in FIGS. 1 and 2 were determined in anechoic and blocked
impedance tube configuration according to ASTM E 1050 and ISO
10534.
[0057] Sound Absorption Coefficient as reported in FIG. 3 was
measured using a laboratory setting including a reverberant room in
compliance with ASTM C 423, specimen mounting A (without air space)
according to ASTM E 795. The absorbers were placed on the floor of
the reverberant room in a 1 inch high aluminum test frame. The
edges of the frame were sealed to the floor using duct tape to
eliminate flanking noise. The sound absorption measurements were
conducted at 1/3 octave bands from 80 to 5,000 Hz. Ten decay
measurements were taken for every microphone position.
[0058] Total Reflectance Spectra of flash spun sheets were obtained
by the method of ASTM E1164-02 (Standard Practice for Obtaining
Spectrophotometric Data for Object-Color Evaluation) using a SP64
Portable Sphere Spectrometer available from X-Rite, Grand Rapids,
Mich., USA. Diffuse white light is used as the illuminant and the
reflectance was measured at 8 degrees using a spectral dispersion
system. The output is a percent reflectance at each wavelength and
the spectral range measured is 400 nm to 700 nm in 10 nm intervals.
Detection is made by blue-enhanced silicon photodiodes. The X-Rite
standard provided with the instrument is traceable to National
Institute of Standards and Technology, Gaithersburg, Md., USA.
Tristimulus values are calculated by the method of ASTM E308-01
using the CIE 10.degree. 1964 standard observer and illuminant
D65.
[0059] Noise Reduction Coefficient was calculated as an average of
the Sound Absorption Coefficients at 250, 500, 1000, 2,000 and
4,000 Hz as measured in accordance with ASTM C423.
[0060] Porosity and pore size distribution data are obtained by
known mercury porosimetry methodology as disclosed by H. M. Rootare
in "A Review of Mercury Porosimetry" from Advanced Experimental
Techniques in Powder Metallurgy, pp. 225-252, Plenum Press,
1970.
[0061] Total Porosity was estimated from basis weight, thickness
and solids density as follows:
Porosity=1-((Basis weight/density of solid.times.thickness))
[0062] Microbial Filtration Efficiency was measured according to
the ASTM F2638-07 and ASTM F1608. Log reduction value or LRV
characterizes barrier efficiency of the membrane and is determined
from the test. The test can use both, polystyrene particles and
actual spores to challenge the membrane.
Examples 1-2
[0063] Samples of acoustically absorbing material according to the
invention were formed using a layer of open cell melamine foam
(from Illbruck Acoustic Inc., Minneapolis, Minn.) having a
thickness of 13 mm, a basis weight of 9.4 kg m.sup.3 and a specific
airflow resistance of 120 rayls. A 0.1 mm thick, 17 g/m.sup.2 basis
weight nylon 6, 6 spunbond scrim was laid on both sides of the foam
and the scrims and foam were quilted together using a pattern of
approximately 11 cm.times.11 cm diamonds. The acoustically
absorbing samples were made by the lamination process described
below. A vinyl acetate water based glue (WA 2173 available from efi
Polymers, Denver, Colo.) was applied by a roller onto one surface
of the quilted foam layer at a rate of approximately 0.3
kg/m.sup.2. A melt blown polyester nonwoven layer having a
thickness of 20 mm, a basis weight of 0.33 kg/m.sup.2, and a
specific airflow resistance of 130 rayls was laminated to the
quilted foam layer to form the absorber core. A flash spun nonwoven
facing available from DuPont under the trade name DuPont.TM.
Tyvek.RTM. style 1055B was wrapped around the core to form Example
1. A flash spun nonwoven facing available from DuPont under the
trade name DuPont.TM. Tyvek.RTM. style 1443R was wrapped around the
core to form Example 2. The total absorber thickness of each of the
examples was about 25 mm. The table includes properties of the
facings used in the example absorbers. The range indicated for
Gurley Hill porosity of Example 1 is based on the typical range
within which the flash spun nonwoven varies according to
specification. The average reflectance is the average of 31
measurements at wavelengths between 400 nm and 700 nm taken at 10
nm increments. Flash spun facing of Example 1 has a hydrostatic
head of at least 180 cm of H.sub.2O, and facing of Example 2 has a
hydrostatic head of at least 24 cm of H.sub.2O according to the
product specification (tested per AATCC TM 127, DIN EN 20811 with a
test rate of 60 cm of H.sub.2O per minute). The Table includes
properties of the facings used in the example absorbers.
[0064] The Gurley Hill porosity of Example 1 and 2 was measured
experimentally and it is well in agreement with the typical range
within which the flash spun nonwoven varies for both Tyvek.RTM.
styles according to specification. Air permeability, as measured by
Gurley Hill porosity and Frazier air permeability characterizes the
general porosity or openness of the structure. The range for air
permeability for various types of nonwoven structures is very wide.
Typically, all nonwovens have much more open structure with Frazier
air permeability of about 50 cfm or higher. Solid films have very
closed, solid structure, which is why films are called impervious,
with Gurley Hill porosity well above 10,000 s. The air permeability
of the flash spun facing can be changed from Gurley Hill range of
about 4,000 s, like for Example 1 to Frazier air permeability to
about 30 cfm, giving a range of Specific Air Flow Resistance of
about 31,000,000 to 800 rayls.
[0065] Total porosity of the structure can be roughly estimated
from the facing's basis weight, thickness and density of the
polymer. Knowing polyethylene has a density of about 0.98
g/cm.sup.3, the total porosity can be estimated as being about 0.6
for facing of Example 1 and about 0.7 for facing of Example 2. This
is well in agreement with total porosity as measured by mercury
porosimetry. The pore size range was from 10 nm to about 8,000 nm
for Example 1 and from 10 nm to about 10,000 nm for Example 2, as
measured by mercury porosimetry. The mean pore size was about 2,000
nm for both, Example 1 and Example 2. Solid films have total
porosity of about 0, which means they have no voids or pores inside
the structure. This is why solid films have extremely good barrier
properties. Despite being very porous, inventive flash spun facing
exhibits the water resistance range similar to the water resistance
of solid impervious films as measured by hydrostatic head. The
typical range of hydrostatic head for the inventive facing is from
about 24 to about 230 cm H.sub.2O, as illustrated by Example 1 and
2.
[0066] As can be seen from the table, inventive flash spun facings
have various surface features as was measured by Parker surface
smoothness. Example 1 has a Parker surface smoothness of about 4.5
micrometers; therefore, it exhibits a smooth sleek surface, similar
to the printing quality paper. Contrarily, Example 2 has a Parker
surface smoothness of about 8 micrometers, representing a rough
surface with 3-dimensional features, in this case, ribbon-like
features. The wide range of Parker surface smoothness allows the
production of aesthetically pleasing surfaces to compliment design
in various architectural spaces. Inventive facings can further
comprise graphical images.
TABLE-US-00001 TABLE Gurley Parker Specific Basis Hill surface
Tensile airflow Ex. weight, Thickness porosity, smoothness,
strength, resistance, Avg. No. g/m.sup.2 micrometer second .mu.m
N/25.4 mm rayls Reflectance, % 1 61 163 3860 4.54 (face 89 (MD
30,800,000 88.9 side) and CD) 11.26 (reverse side) 2 42.3 140 77
7.93 (face 26 (MD 615,156 88.2 side) and CD) 4.22 (reverse
side)
[0067] A comparative sample was prepared similarly without the
flash spun facing. The thickness of the comparative sample was
about 25 mm.
[0068] The examples and comparative sample were conditioned at room
temperature for at least two weeks after manufacturing, and at
controlled conditions (temperature of 23.degree. C. and RH of 60%)
for 24 hours before acoustic testing. Absorption coefficient data
were obtained for each sample.
[0069] As can be seen in FIG. 3, the absorbers of Examples 1 and 2
as represented by curves 1 and 2, respectively, provide
continuously improved absorption as compared with the comparative
example as represented by curve C over the frequency range from 400
Hz to 1200 Hz.
* * * * *