Note: Descriptions are shown in the official language in which they were submitted.
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MICROSPHERES
The present invention relates to a process for the production of paper or
nonwoven and thermoplastic expandable microspheres useful therefore.
Expandable thermoplastic microspheres comprising a thermoplastic polymer
shell and a propellant entrapped therein are commercially available under the
trademark
EXPANCEL~ and are used as a foaming agent in many different applications.
In such microspheres, the propellant is normally a liquid having a boiling
temperature not higher than the softening temperature of the thermoplastic
polymer shell.
Upon heating, the propellant evaporates to increase the internal pressure at
the same
time as the shell softens, resulting in significant expansion of the
microspheres. The
temperature at which the expansion starts is called Tstarc, while the
temperature at which
maximum expansion is reached is called Tm~. Expandable microspheres are
marketed in
various forms, e.g. as dry free flowing particles, as an aqueous slurry or as
a partially
dewatered wet-cake.
Expandable microspheres can be produced by polymerising ethylenically
unsaturated monomers in the presence of a propellant. Detailed descriptions of
various
expandable microspheres and their production can be found in, for example, US
Patents
3615972, 3945956, 5536756, 6235800, 6235394 and 6509384, and in EP 486080.
It has been disclosed to use microspheres in papermaking, for example in
US Patents 3556934 and 4133688, JP Patent 2689787 and in O. Soderberg, "World
Pulp
& Paper Technology 1995/96, The International Review for the Pulp & Paper
Industry" p.
143-145.
It is an object of the invention to provide a process for the production of
paper or
nonwoven with low bulk density.
It is another object of the invention to provide expandable thermoplastic
microspheres that can be used in the production of paper or nonwoven with low
bulk
density.
It has previously been believed that expandable thermoplastic microspheres of
large size would have poor expansion properties. However, it has now been
found that
such microspheres, when also having high content of propellant, give higher
expansion
than expected when used in the production of paper or nonwoven for increasing
the bulk
thereof.
The invention thus concerns use of thermally expandable microspheres
comprising a thermoplastic polymer shell and from about 17 to about 40 wt%,
preferably
from about 18 to about 40 wt%, most preferably from about 19 to about 40 wt%,
particularly most preferably from about 20 to about 35 wt% of a propellant
entrapped in
said polymer shell, and having a volume-average diameter from about 17 to
about 35 pm,
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preferably from about 18 to about 35 pm, more preferably from about 19 to
about 35 Nm,
most preferably from about 20 to about 30 pm, particularly most preferably
from about 21
to about 30 pm, in the production of paper or non-woven for increasing the
bulk thereof.
The term expandable microspheres as used herein refers to expandable
microspheres that have not previously been expanded, i.e. unexpanded
expandable
microspheres.
All figures for volume-average diameter given herein refer to values obtained
by
measuring according to ISO 13319:2000, "Determination of particle size
distributions
Electrical sensing zone method". Detailed description of this measuring method
can be
obtained from, for example, Swedish Institute For Standards, Stockholm.
The invention further concerns a process for the production of paper or
nonwoven from fibres comprising the steps of adding thermally expandable
microspheres
comprising a thermoplastic polymer shell and a propellant entrapped therein to
a stock
comprising fibres or to a web of fibres, forming paper or nonwoven from the
stock or the
web, and applying heat to raise the temperature of the microspheres
sufficiently for them
to expand and thereby increase the bulk of the paper or the nonwoven. The
expandable
microspheres have a volume-average diameter from about 17 to about 35 pm,
preferably
from about 18 to about 35 pm, more preferably from about 19 to about 35 pm,
most
preferably from about 20 to about 30 pm, particularly most preferably from
about 21 to
about 30 pm. The amount of propellant in the expandable microspheres is from
about 17
to about 40 wt%, preferably from about 18 to about 40 wt%, most preferably
from about
19 to about 40 wt%, particularly most preferably from about 20 to about 35
wt%.
An embodiment of the invention concerns a process for the production of paper
comprising the steps of adding expandable microspheres as described above to a
stock
containing cellulosic fibres, dewatering the stock on a wire to obtain paper,
and drying the
paper by applying heat and thereby also raising the temperature of the
microspheres
sufficiently for them to expand and increase the bulk of the paper. The
expandable
microspheres may be added separately or together with one or more other
additive used
in the papermaking process.
The expandable microspheres can be added in any form, although it from a
practical point of view is most preferred to add them in the form of an
aqueous slurry,
preferably having a solids content from about 5 to about 55 wt %, most
preferably from
about 40 to about 50 wt%. The slurry preferably also comprises a thickener
compatible
with paper making, such as anionic or cationic starch, optionally in
combination with a salt
such as sodium chloride. Starch may, for example, be present in the slurry in
an amount
from about 0.1 to about 5 wt%, preferably from about 0.3 to about 1.5 wt%.
Sodium
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chloride, or another salt, may, for example, be present in the slurry in an
amount from
about 0.1 to about 20 wt%, preferably from about 1 to about 15 wt%.
The amount of expandable microspheres added to the stock is preferably from
about 0.1 to about 20 wt%, most preferably from about 0.2 to about 10 wt% dry
microspheres of the dry content in the stock. Any kind of paper machine known
in the art
can be used.
The term "paper", as used herein, is meant to include all types of cellulose-
based
products in sheet or web form, including, for example, board, cardboard and
paperboard.
The invention has been found particularly advantageous for the production of
board,
cardboard and paper board, particularly with a basis weight from about 50 to
about 1000
glm2, preferably from about 150 to about 800 g/m2.
The paper may be produced as a single layer or a multi-layer paper. If the
paper
comprises three or more layers, the expandable microspheres are preferably not
added to
the portion of the stock forming any of the two outer layers.
The stock preferably contains from about 50 to about 100 wt%, most preferably
from about 70 to about 100 wt% of cellulosic fibres, based on dry material.
Before
dewatering, the stock besides expandable microspheres, may also contain one or
more
fillers, e.g. mineral fillers like kaolin, china clay, titanium dioxide,
gypsum, talc, chalk, ground
marble or precipitated calcium carbonate, and optionally other commonly used
additives,
such as retention aids, sizing agents, aluminium compounds, dyes, wet-strength
resins,
optical brightening agents, etc. Examples of aluminium compounds include alum,
alumina-
tes and polyaluminium compounds, e.g. polyaluminium chlorides and sulphates.
Examples
of retention aids include cationic polymers, anionic inorganic materials in
combination with
organic polymers, e.g. bentonite in combination with cationic polymers or
silica-based sots in
combination with cationic polymers or cationic and anionic polymers. Examples
of sizing
agents include cellulose reactive sizes such as alkyl ketene dimers and
alkenyl succinic
anhydride, and cellulose non-reactive sizes such as rosin, starch and other
polymeric sizes
like copolymers of styrene with vinyl monomers such as malefic anhydride,
acrylic acid
and its alkyl esters, acrylamide, etc.
At drying, the paper, and thereby also the microspheres, is preferably heated
to
a temperature from about 50 to about 150°C, most preferably from about
60 to about
110°C. This results in expansion of the microspheres and thereby also a
bulk increase of
the paper. The magnitude of this bulk increase depends on various factors,
such as the
origin of cellulosic fibres and other components in the stock, but is in most
cases from
about 5 to about 50 % per weight percentage of retained microspheres in the
dried paper,
compared to the same kind of paper produced without addition of expandable
microspheres or any other expansion agent. Any conventional means of drying
involving
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transferring heat to the paper can be applied, such as contact drying (e.g. by
heated
cylinders), forced convection drying (e.g. by hot air), infrared techniques,
or combinations
thereof. In the case of contact drying, the temperature of the contact
surfaces, e.g. the
cylinders, is preferably from about 20 to about 150°C, most preferably
from about 30 to
about 130°C. The paper may pass a series of several cylinders, e.g. up
to 20 or more, of
increasing temperature.
The cellulosic fibres in the stock may, for example, come from pulp made from
any
kind of plants, preferably wood, such as hardwood and softwood. The cellulosic
fibres may
also partly or fully originate from recycled paper, in which case the
invention has been found
to give unexpectedly good results.
Another embodiment of the invention concerns a process for the production of
nonwoven comprising the steps of forming a web of fibres, adding to said web a
binder and
expandable microspheres as described above, and forming nonwoven and applying
heat to
raise the temperature of the microspheres sufficiently for them to expand and
thereby
increase the bulk nonwoven. The expandable microspheres and the binder may be
added
separately or as a mixture. The amount of expandable microspheres added is
preferably
from about 0.1 to about 30 wt% of dried product, most preferably from about
0.5 to about 15
wt% of dried product. The amount of binder added is preferably from about 10
to about 90
wt% of dried product, most preferably from about 20 to about 80 wt% of dried
product.
The term "nonwoven" as used herein is meant to include textiles made from
fibres
bonded together by means of a binder.
The web of fibres can be formed in any conventional way, for example by
mechanical or aerodynamical dry methods, hydrodynamical (wet) methods, or
spunbonded processes. The binder, preferably pre-mixed with expandable
microspheres,
can then be added to the web also in any conventional way, for example by any
kind of
impregnation method such as immersion of the web in a bath of binder or
coating the web
by kiss roll application or knife coating with a doctor blade or floating
knife.
The web comprising a binder and expandable microspheres can then be heated
to a temperature sufficient for the microspheres to expand, preferably from
about 70 to
about 200°C, most preferably from about 120 to about 160°C.
Preferably, curing of the
binder takes place at the same time. The heating can be effected by any
suitable means,
such as contact drying (e.g. by heated cylinders), forced convection drying
(e.g. by hot
air), infrared techniques, or combinations thereof.
The fibres can be any kind of commercially available fibres, natural fibres,
mineral
fibres, as well as synthetic inorganic and organic fibres. Examples of useful
fibres include
polypropylene, polyethylene, polyester, viscose, and polyamide fibres, as well
as fibres
made from two or more of the above polymers.
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The binder can be any kind of natural or synthetic adhesive resin, such as
resins of
polyacrylates and co-polymers thereof, polymethacrylates and co-polymers
thereof,
rubber latexes such as styrene/butadiene copolymers, acrylonitrile/butadiene
copolymers,
polyvinyl chloride) and copolymers, polyvinyl ester) such as polyvinyl
acetate) and co-
y polymers, e.g. with ethylene, polyvinyl alcohol), polyurethane, and
aminoplast and
phenoplast precondensates such as urea/ formaldehyde, urea/ melamine /
formaldehyde
or phenol/ formaldehyde.
Preferred expandable microspheres to be used according to the invention are
described below.
The thermoplastic polymer shell of the expandable microspheres is suitably
made of a homo- or co-polymer obtained by polymerising ethylenically
unsaturated
monomers. Those monomers can, for example, be nitrite containing monomers such
as
acrylonitrile, methacrylonitrile, a-chloroacrylonitrile, a-
ethoxyacrylonitrile, fumaronitrile or
crotonitrile; acrylic esters such as methyl acrylate o~r ethyl acrylate;
methacrylic esters
such as methyl methacrylate, isobornyl methacrylate or ethyl methacrylate;
vinyl halides
such as vinyl chloride; vinyl esters such as vinyl acetate other vinyl
monomers such as
vinyl pyridine; vinylidene halides such as vinylidene chloride; styrenes such
as styrene,
halogenated styrenes or a-methyl styrene; or dienes such as butadiene,
isoprene and
chloroprene. Any mixtures of the above mentioned monomers may also be used.
Preferably the monomers comprise at least one acrylic ester or methacrylic
ester
monomer, most preferably methacrylic ester monomer such as methyl
methacrylate. The
amount thereof in the polymer shell is preferably from about 0.1 to about 80
wt%, most
preferably from about 1 to about 25 wt% of the total amounts of monomers.
Preferably the monomers comprise at least one vinylidene halide monomer,
most preferably vinylidene chloride. The amount thereof in the polymer shell
is preferably
from about 1 to about 90 wt%, most preferably from about 20 to about 80 wt% of
the total
amounts of monomers.
Most preferably the monomers comprise both at least one acrylic ester or
methacrylic ester monomer and at least one vinylidene halide monomer.
Preferably the monomers comprise at least one nitrite containing monomer, most
preferably at least one of acrylonitrile and methacrylonitrile, particularly
most preferably at
least acrylonitrile. The amount thereof in the polymer shell is preferably
from about 1 to
about 80 wt%, most preferably from about 20 wt% to about 70 wt% of the total
amounts
of monomers.
In an advantageous embodiment the monomers comprise at least one acrylic
ester monomer, at least one vinylidene halide and at least one nitrite
containing
monomer. The polymer shell may then, for example, be a co-polymer obtained
from
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monomers comprising methyl methacrylate in a preferred amount from about 0.1
to about
80 wt%, most preferably from about 1 to about 25 wt% of the total amounts of
monomers,
vinylidene chloride in a preferred amount from about 1 to about 90 wt%, most
preferably
from about 20 to about 80 wt% of the total amounts of monomers, and
acrylonitrile in a
preferred amount from about 1 to about 80 wt%, most preferably from about 20
to about
70 wt% of the total amounts of monomers.
It may sometimes be desirable that the monomers for the polymer shell also
comprise crosslinking multifunctional monomers, such as at least one of
divinyl benzene,
ethylene glycol di(meth)acrylate, di(ethylene glycol) di(meth)acrylate,
triethylene glycol
di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol
di(meth)acrylate, 1,6-
hexanediol di(meth)acrylate, glycerol di(meth)acrylate, 1,3-butanediol
di(meth)acrylate,
neopentyl glycol di(meth)acrylate, 1,10-decanediol di(meth)acrylate,
pentaerythritol
tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, triallylformal
tri(meth)acrylate, allyl
methacrylate, trimethylol propane tri(meth)acrylate, tributanediol
di(meth)acrylate, PEG
#200 di(meth)acrylate, PEG #4.00 di(meth)acrylate, PEG #600 di(meth)acrylate,
3-
acryloyloxyglycol monoacrylate, triacryl formal or triallyl isocyanate,
triallyl isocyanurate
etc. The amount thereof in the polymer shell is preferably from about 0.1 to
about 10
wt%, most preferably from about 0.1 to about 1 wt%, particularly most
preferably from
about 0.2 to about 0.5 wt% of the total amounts of monomers.
The propellant is normally a liquid having a boiling temperature not higher
than
the softening temperature of the thermoplastic polymer shell and may comprise
hydrocarbons such as propane, n-pentane, isopentane, neopentane, butane,
isobutane,
hexane, isohexane, neohexane, heptane, isoheptane, octane or isooctane, or
mixtures
thereof. Aside from them, other hydrocarbon types can also be used, such as
petroleum
ether, or chlorinated or fluorinated hydrocarbons, such as methyl chloride,
methylene
chloride, dichloroethane, dichloroethylene, trichloroethane,
trichloroethylene,
trichlorofluoromethane, perfluorinated hydrocarbons, etc. Preferred
propellants comprise
isobutane, alone or in a mixture with one or more other hydrocarbons. The
boiling point at
atmospheric pressure is preferably within the range from about -50 to about
100 °C,
most preferably from about -20 to about 50 °C, particularly most
preferably from about
-20 to about 30 °C.
Apart from the polymer shell and the propellant the microspheres may comprise
further substances added during the production thereof, normally in an amount
from
about 1 to about 20 wt%, preferably from about 2 to about 10 wt%. Examples of
such
substances are solid suspending agents, such as one or more of silica, chalk,
bentonite,
starch, crosslinked polymers, methyl cellulose, gum agar, hydroxypropyl
methylcellulose,
carboxy methylcellulose, colloidal clays, and/or one or more salts, oxides or
hydroxides of
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metals like AI, Ca, Mg, Ba, Fe, Zn, Ni and Mn, for example one or more of
calcium
phosphate, calcium carbonate, magnesium hydroxide, barium sulphate, calcium
oxalate,
and hydroxides of aluminium, iron, zinc, nickel or manganese. If present,
these solid
suspending agents are normally mainly located to the outer surface of the
polymer shell.
However, even if a suspending agent has been added during the production of
the
microspheres, this may have been washed off at, a later stage and could thus
be
substantially absent from the final product.
Some of the microspheres described above are novel. The invention thus also
concerns thermally expandable microspheres comprising a thermoplastic polymer
shell
made of a co-polymer obtained by polymerising ethylenically unsaturated
monomers
comprising at least one acrylic ester or methacrylic ester monomer and at
least one
vinylidene halide monomer, and from about 17 to about 40 wt%, preferably from
about 18
to about 40 wt%, most preferably from about 19 to about 40 wt%, particularly
most
preferably from about 20 to about 35 wt% of a propellant entrapped in said
polymer shell,
wherein the expandable microspheres have a volume-average diameter from about
17 to
about 35 pm, preferably from about 18 to about 35 pm, more preferably from
about 19 to
about 35 pm, most preferably from about 20 to about 30 pm, particularly most
preferably
from about 21 to about 30 pm. Regarding further possible and preferred
embodiments of
the novel microspheres, applicable parts of the above description of the
process for the
production of paper or nonwoven is referred to.
The novel expandable microspheres can be prepared by polymerising the
monomers in the presence of the propellant with the same methods as described
in the
earlier mentioned US Patents 3615972, 3945956, 5536756, 6235800, 6235394 and
6509384, and in EP 486080.
In a preferred batchwise procedure for producing expandable microspheres, the
polymerisation is conducted as described below in a reaction vessel. For 100
parts of
monomer phase (suitably including monomers and propellant, the ratio of which
determines the amount of propellant in the final product), one or more
polymerisation
initiator, preferably in an amount from about 0.1 to about 5 parts, aqueous
phase,
preferably in an amount from about 100 to about 800 parts, and one or more
preferably
solid colloidal suspending agent, preferably in an amount from about 1 to
about 20 parts,
are mixed and homogenised. The size of the droplets of monomer phase obtained
determines the size of the final expandable microspheres, in accordance with
principles
described in e.g. US Patent 3615972 and can be applied for all similar
production
methods with various suspending agents. The temperature is suitably maintained
from
about 40 to about 90°C, preferably from about 50 to about 80°C,
while the suitable pH
depends on the suspending agent used. For example, a high pH, preferably from
about 6
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to about 12, most preferably from about 8 to about 10, is suitable if the
suspending agent
is selected from salts, oxides or hydroxides of metals like AI, Ca, Mg, Ba,
Fe, Zn, Ni and
Mn, for example one or more of calcium phosphate, calcium carbonate, chalk,
magnesium hydroxide, barium sulphate, calcium oxalate, and hydroxides of
aluminium,
iron, zinc, nickel or manganese. A low pH, preferably from about 1 to about 6,
most
preferably from about 3 to about 5, is suitable if the suspending agent is
selected from
silica, bentonite, starch, methyl cellulose, gum agar, hydroxypropyl
methylcellulose,
carboxy methylcellulose, colloidal clays. Each one of the above agents have
different
optimal pH, depending on, for example, solubility data.
In order to enhance the effect of the suspending agent, it is also possible to
add
small amounts of one or more promoters, for example from about 0.001 to about
1 wt%.
Usually, such promoters are organic materials and may, for example, be
selected from
one or more of water-soluble sulfonated polystyrenes, alginates,
carboxym.ethylcellulose,
tetramethyl ammonium hydroxide or chloride or water-soluble complex resinous
amine
condensation products such as the water-soluble condensation products of
diethanolamine and adipic acid, the water-soluble condensation products of
ethylene
oxide, urea and formaldehyde, polyethylenimine, polyvinylalcohol,
polyvinylpyrrolidone,
amphoteric materials such as proteinaceous, materials like gelatin, glue,
casein, albumin,
glutin and the like, non-ionic materials like methoxycellulose, ionic
materials normally
classed as emulsifiers, such as soaps, alkyl sulfates and sulfonates and long
chain
quaternary ammonium compounds.
Conventional radical polymerisation may be used and initiators are suitably
selected from one or more of organic peroxides such as dialkyl peroxides,
diacyl
peroxides, peroxy esters, peroxy dicarbonates, or azo compounds. Suitable
initiators
include dicetyl peroxy dicarbonate, tert-butyl cyclohexyl peroxy dicarbonate,
dioctanoyl
peroxide, dibenzoyl peroxide, dilauroyl peroxide, didecanoyl peroxide, tent-
butyl
peracetate, tert-butyl perlaurate, tert-butyl perbenzoate, tert-butyl
hydroperoxide, cumene
hydroperoxide, cumene ethylperoxide, diisopropyl hydroxy dicarboxylate, azo-
bis
dimethyl valeronitrile, azo-bis isobutyronitrile, azo-bis (cyclo hexyl
carbonitrile) and the
like. It is also possible to initiate the polymerisation with radiation, such
as high energy
ionising radiation.
When the polymerisation is essentially complete, microspheres are normally
obtained as an aqueous slurry or dispersion,- which can be dewatered by any
conventional means, such as bed filtering, filter pressing, leaf filtering,
rotary filtering, belt
filtering or centrifuging to obtain a so called wet cake that can be used as
such. However,
it is also possible to dry the microspheres by any conventional means, such as
spray
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drying, shelf drying, tunnel drying, rotary drying, drum drying, pneumatic
drying, turbo
shelf drying, disc drying or fluidised bed-drying.
The invention will now be further described in connections with the following
Examples, which, however, not should be interpreted as limiting the scope
thereof. If not
otherwise stated, all parts and percentages refer to parts and percent by
weight.
EXAMPLE 1: A three layer paper board with a basis weight of about 180 g/m~
was produced in a pilot paper machine with a machine speed of 7 m/min and
having
recirculated process water. The pulp was composed of 40 wt% hardwood and 60
vut%
softwood pulp and was beaten to a Schopper-Riegler value of 25°SR and
then dispersed
to give a pulp slurry/stock. An aqueous slurry of expandable microspheres was
before the
mixing pump added to the stock used for the middle layer in an amount of about
1 wt%
dry microspheres of the dry substance in the stock. As retention aid 0.1 wt%
PolyminT"'
SIC was used. In the drying section the paper web was heated by cylinders
having a
temperature profile from 30 to 130°C. Different kinds of expandable
microspheres were
tested, all having isobutane as propellant and a polymer shell from vinylidene
chloride
(VDC), acrylonitrile (ACN) and methyl methacrylate (MMA) but in various
ratios. In order
to determine the retention of the microspheres, paper samples were taken
before the
press section for determination of the amount of microspheres (using GC). The
retention
was calculated from the microspheres addition and the content of microspheres
in the
paper. Moreover, samples from the dried paper were taken for determination of
bulk and
thickness . The results are shown in Table 1.
Table 1
VDC/ACN/MMA Amount Particle Increased bulk
in of size (% per percentage
polymer shell propellant(um) of
(wt%) (wt%) retained microspheres)
56/35/9 15.8 13.2 6
56135/9 24.4 24.6 39
73/24/3 16.5 12.3 19
173/24/3 24.8 28.0 39
EXAMPLE 2: A single layer paper board with a basis weight of about 200 g/m2
was produced in a pilot paper machine with a machine speed of 4 m/min and not
having
recirculated process water. The pulp was composed of 50 wt% hardwood and 50
wt%
softwood pulp and was beaten to a Schopper-Riegler value of 25°SR and
then dispersed
to give a pulp slurrylstock. An aqueous slurry of expandable microspheres was
before the
mixing pump added to the stock in an amount of about 1.75 wt% dry microspheres
of the
dry substance in the stock. As retention aid Compozil~, 0.1 % BMA-OTM and
0.75%
RaisamylT"" 135, was used. In the drying section the paper web was heated by
cylinders
having a temperature profile from 65 to 122°C. Expandable microspheres
with the same
propellant and same monomers in the polymer shell as in Exampel 1 were tested.
The
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retention of microspheres and the bulk/thickness of the paper were determined
as in
Example 1. The results are shown in Table 1
Table 2
VDCIACNIMMA Amount of ParticleIncreased bulk
in propellant size (% per percentage
polymer shell (wt%) (Irm) of
(wt%) retained microspheres)
56/35/9 36.2 17.8 47
56/35/9 12.5 12.7 17
56/35/9 14.0 11.2 11
73/24/3 24.5 - 20.5 34
73/24/3 19.4 19.5 33
73/24/3 20.4 33.5 32
73/24/3 18.4 18.3 29
73/24/3 15.9 26.1 28
73/24/3 23.5 12.8 23
73/24/3 30.9 17.8 21
73/24/3 15.1 17.3 19
73/24/3 15.4 12.1 14
73/24/3 13.4 15.1 14-
~
5 It appears that the overall trend is that the combination of high amount of
propellant and
large particle diameter gives high increase of the bulk of the paper. However,
due to
difficulties to exactly measure the amount of retained microspheres, some
individual
results may be inconsistent with the overall trend.
