Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
ACRYLIC AND PARA-ARAMID PULP AND PROCESSES OF MAKING
SAME
BACKGROUND OF THE INVENTION
1. Field of the Invention.
This invention relates to acrylic and para-aramid pulp for use as
reinforcement material in products, such as seals and friction materials.
The invention further relates to processes for making such pulp.
2. Description of Related Art.
Fibrous and non fibrous reinforcement materials have been used
for many years in friction products, sealing products and other plastic or
rubber products. Such reinforcement materials typically must exhibit high
wear and heat resistance.
Asbestos fibers have historically been used as reinforcement
materials, but due to their health risks, replacements have been made or
proposed. However, many of these replacements do not perform as well
as asbestos in one way or another.
Research Disclosure 19037, 74-75, published February 1980, discloses
the manufacture of pulp made from fibrillated KEVLAR brand para-
aramid fibers of variable lengths and use of such pulp as a reinforcement
material in various applications. This publication discloses that pulp made
from KEVLAR brand para-aramid fibers can be used in sheet products
alone, or in combination with fibers of other materials, such as NOMEX
brand meta-aramid, wood pulp, cotton and other natural cellulosics, rayon,
polyester, polyolefin, nylon, polytetrafluoroethylene, asbestos and other
minerals, fiberglass and other, ceramics, steel and other metals, and
carbon. The publication also discloses the use of pulp from KEVLAR
brand para-aramid fiber alone, or with KEVLAR brand para-aramid short
staple, in friction materials to replace a fraction of the asbestos volume,
with the remainder of the asbestos volume being replaced by fillers or
other fibers.
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U.S. Patent 5,811,042 (to Hoiness) discloses a composite friction
or gasketing material made of a thermoset or thermoplastic matrix resin,
fiber reinforcing material, and substantially fibril free aramid particles.
Poly (p-phenylene terephthalamide) and poly(m-phenylene
isophthalamide) are preferred fiber reinforcing materials, and the fibers
can be in the form of floc or pulp.
U.S. Patent Application 2003/0022961 (to Kusaka et al.) discloses
friction materials made from a friction modifier, a binder and a fibrous
reinforcement made of a mixture of (a) a dry aramid pulp and (b) wet
aramid pulp, wood pulp or acrylic pulp. Dry aramid pulp is defined as an
aramid pulp obtained by "the dry fibrillation method". The dry fibrillation
method is dry milling the aramid fibers between a rotary cutter and a
screen to prepare the pulp. Wet aramid pulp is defined as an aramid pulp
obtained by "the wet fibrillation method". The wet fibrillation method is
milling short aramid fibers in water between two rotary discs to form
fibrillated fibers and then dehydrating the fibrillated fibers, i.e., the
pulp.
Kusaka et al further disclose a method of mix-fibrillating fibers by first
mixing plural types of organic fibers that fibrillate at a definite ratio, and
then fibrillating the mixture to produce a pulp.
There is an ongoing need to provide alternative reinforcing
materials that both perform well in products, such as seals and friction
applications, and that are low in cost. Despite the numerous disclosures
proposing lower cost alternative reinforcement materials, many of these
proposed products do not adequately perform in use, cost significantly
more than currently commercial products, or have other negative
attributes. As such, there remains a need for reinforcement materials that
exhibit high wear and heat resistance, and that are comparable or less
expensive than other commercially available reinforcement materials.
BRIEF SUMMARY OF THE INVENTION
The invention relates to a first embodiment of a process for making
an acrylic and para-aramid pulp for use as reinforcement material,
comprising:
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(a) combining pulp ingredients including:
(1) acrylic fiber comprising acrylonitrile units which are at
least 85 wt % of the total acrylic fiber, the fiber being 10 to 90 wt % of the
total solids in the ingredients, and having an average length of no more
than 10 cm;
(2) para-aramid fiber being 10 to 90 wt % of the total solids
in the ingredients, and having an average length of no more than 10 cm;
and
(3) water being 95 to 99 wt % of the total ingredients;
(b) mixing the ingredients to a substantially uniform slurry;
(c) co-refining the slurry by simultaneously:
(1) fibrillating, cutting and masticating the acrylic fiber and
the para-aramid fiber to irregularly shaped fibrillated fibrous structures
with
stalks and fibrils; and
(2) substantially uniformly dispersing all solids in the refined
slurry; and
(d) removing water from the refined slurry to no more than 60 total
wt % water,
thereby producing an acrylic and para-aramid pulp with the acrylic
and the para-aramid fibrous structures having an average maximum
dimension of no more than 5 mm, a length-weighted average of no more
than 1.3 mm, and the acrylic fibrils and/or stalks are substantially
entangled with the para-aramid fibrils and/or stalks.
The invention is further related to a second embodiment of a
process for making an acrylic and para-aramid pulp for use as
reinforcement material, comprising:
(a) combining ingredients including water and a first fiber from the
group consisting of.
(1) acrylic fiber comprising acrylonitrile units which are at
least 85 wt % of the total acrylic fiber, the fiber being 10 to 90 wt % of the
total solids in the ingredients; and
(2) para-aramid fiber being 10 to 90 wt % of the total solids
in the ingredients;
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(b) mixing the ingredients to a substantially uniform suspension;
(c) refining the suspension in a disc refiner thereby cutting the fiber
to have an average length of no more than 10 cm, and fibrillating and
masticating at least some of the fiber to irregularly shaped fibrillated
fibrous structures;
(d) combining ingredients including the refined suspension, the
second fiber of the group of (a)(1 and 2), and water, if necessary, to
increase the water concentration to 95 -99 wt % of the total ingredients;
(e) mixing the ingredients, if necessary, to form a substantially
uniform suspension;
(d) co-refining the mixed suspension by:
(1) fibrillating, cutting and masticating solids in the
suspension such that all or substantially all of the acrylic and para-aramid
fiber is converted to irregularly shaped fibrillated acrylic and para-aramid
fibrous structures with stalks and fibrils; and
(2) substantially uniformly dispersing all solids in the refined
slurry; and
(h) removing water from the refined slurry to no more than 60 total
wt % water,
thereby producing an acrylic and para-aramid pulp with the acrylic
and the para-aramid fibrous structures having an average maximum
dimension of no more than 5 mm, a length-weighted average of no more
than 1.3 mm, and the acrylic fibrils and/or stalks are substantially
entangled with the para-aramid fibrils and/or stalks.
The invention is further directed to an acrylic and para-aramid pulp
for use as reinforcement material, comprising:
(a) irregularly shaped, acrylic fibrous structures comprising
acrylonitrile units which are at least 85 wt % of the total acrylic fibrous
structures and being 10 to 90 wt % of the total solids;
(b) irregularly shaped, para-aramid fibrous structures being 10 to 90
wt % of the total solids; and
(c) water being 4 to 60 wt % of the entire pulp,
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whereby the acrylic and the para-aramid fibrous structures having
an average maximum dimension of no more than 5 mm, a length-
weighted average of no more than 1.3 mm, and stalks and fibrils where
the acrylic fibrils and/or stalks are substantially entangled with the para-
aramid fibrils and/or stalks.
The invention is further directed to a friction material, comprising a
friction modifier; optionally at least one filler; a binder; and a fibrous
reinforcement material comprising the pulp of the present invention.
Moreover, the invention is directed to a sealing material, comprising
a binder; optionally at least one filler; and a fibrous reinforcement material
comprising the pulp of the present invention.
BRIEF DESCRIPTION OF THE DRAWING(S)
The invention can be more fully understood from the following
detailed description thereof in connection with accompanying drawings
described as follows.
Figure 1 is a block diagram of apparatus for performing a wet
process for making "wet" pulp in accordance with the present invention.
Figure 2 is a block diagram of apparatus for performing a dry
process for making "dry" pulp in accordance with the present invention.
Figure 3 is an image of a photomicrograph of para-aramid particles
used as an optional ingredient to the process of the present invention.
Figure 4 is an image of a photomicrograph of pulp made according
to the process of the present invention.
GLOSSARY
Before the invention is described, it is useful to define certain terms
in the following glossary that will have the same meaning throughout this
disclosure unless otherwise indicated.
"Fiber" means a relatively flexible, unit of matter having a high ratio
of length to width across its cross-sectional area perpendicular to its
length. Herein, the term "fiber" is used interchangeably with the term
"filament" or "end". The cross section of the filaments described herein
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can be any shape, but are typically circular or bean shaped. Fiber spun
onto a bobbin in a package is referred to as continuous fiber. Fiber can
be cut into short lengths called staple fiber. Fiber can be cut into even
smaller lengths called floe. Yarns, multifilament yarns or tows comprise a
plurality of fibers. Yarn can be intertwined and/or twisted.
"Fibril" means a small fiber having a diameter as small as a fraction
of a micrometer to a few micrometers and having a length of from about
to 100 micrometers. Fibrils generally extend from the main trunk of a
larger fiber having a diameter of from 4 to 50 micrometers. Fibrils act as
hooks or fasteners to ensnare and capture adjacent material. Some fibers
fibrillate, but others do not or do not effectively fibrillate and for
purposes
of this definition such fibers do not fibrillate. Poly(para-phenylene
terephthalamide) fiber fibrillates readily upon abrasion, creating fibrils.
Acrylic fibers of this invention also fibrillate.
"Fibrillated fibrous structures" means particles of material having a
stalk and fibrils extending therefrom wherein the stalk is generally
columnar and about 10 to 50 microns in diameter and the fibrils are hair-
like members only a fraction of a micron or a few microns in diameter
attached to the stalk and about 10 to 100 microns long.
"Floc" means short lengths of fiber, shorter than staple fiber. The
length of floe is about 0.5 to about 15 mm and a diameter of 4 to 50
micrometers, preferably having a length of 1 to 12 mm and a diameter of 8
to 40 micrometers. Floe that is less than about 1 mm does not add
significantly to the strength of the material in which it is used. Floc or
fiber
that is more than about 15 mm often does not function well because the
individual fibers may become entangled and cannot be adequately and
uniformly distributed throughout the material or slurry. Aramid floe is
made by cutting aramid fibers into short lengths without significant or any
fibrillation, such as those prepared by processes described in U.S. Patent
Nos. 3,063,966, 3,133,138, 3,767,756, and 3,869,430.
"Length-weighted average" means the calculated length from the
following formula:
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[(Each Individual pulp length) 2 j
Length weighted average = ------------------- ----_----
E [Each Individual pulp length]
"Maximum dimension" of an object means the straight distance
between the two most distal points from one another in the object
"Staple fiber" can be made by cutting filaments into lengths of no
more than 15 cm, preferably 3 to 15 cm; and most preferably 3 to 8 cm.
The staple fiber can be straight (i.e., non crimped) or crimped to have a
saw tooth shaped crimp along its length, with any crimp (or repeating
bend) frequency. The fibers can be present in uncoated, or coated, or
otherwise pretreated (for example, pre-stretched or heat-treated) form.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to processes for making an acrylic and
para-aramid pulp for use as reinforcement material. The invention is also
directed to acrylic and para-aramid pulp, that can be made by the
processes of the invention, for use as reinforcement material. The
invention is further directed to products, such as sealing materials and
friction materials, incorporating the pulp of this invention, and processes
for making them.
1. First Embodiment of the Inventive Process
In a first embodiment, the process for making an acrylic and para-
aramid pulp comprises the following steps. First, pulp ingredients are
combined, added or contacted together. Second, the combined pulp
ingredients are mixed to a substantially uniform slurry. Third, the slurry is
simultaneously refined or co-refined. Fourth, water is removed from the
refined slurry.
Combining Step
In the combining step, the pulp ingredients are preferably added
together in a container. The pulp ingredients include (1) acrylic fiber, (2)
para-aramid fiber, (3) optionally substantially or completely fibril-free,
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granular, para-aramid particles, (4) optionally other minor additives, and
(5) water.
Acrylic Fiber
The acrylic fiber is added to a concentration of 10 to 90 wt % of the
total solids in the ingredients, preferably 25 to 60 wt % of the total solids
in
the ingredients, and most preferably 25 to 55 wt % of the total solids in the
ingredients.
The acrylic fiber preferably has an average length of no more than
cm, more preferably 0.5 to 5 cm, and most preferably 0.6 to 2 cm.
Prior to combining the pulp ingredients together, any acrylic fibers in the
form of continuous filaments can be cut into shorter fibers, such as staple
fibers or floc.
Acrylic Polymer
The acrylic fiber useful in this invention includes acrylonitrile units
which are at least 85 wt % of the total acrylic fiber. An acrylonitrile unit
is -
(CH2-CHCN)-. The acrylic fiber can be made from acrylic polymers made
up of 85 % by weight or more of acrylonitrile with 15 % by weight or less of
an ethylenic monomer copolymerizable with acrylonitrile and mixtures of
two or more of these acrylic polymers. Examples of the ethylenic
monomer copolymerizable with acylonitrile include acylic acid, methacrylic
acid and esters thereof (methyl acrylate, ethyl acrylate, methyl
methacylate, ethyl methacrylate, etc.), vinyl acetate, vinyl chloride,
vinylidene chloride, acrylamide, methacylamide, methacrylonitrile,
allylsulfonic acid, methanesulfonic acid and styrenesulfonic acid.
Para-Aramid Fiber
The para-aramid fiber is added to a concentration of 10 to 90 wt %
of the total solids in the ingredients, preferably 40 to 75 wt % of the total
solids in the ingredients, and most preferably 40 to 55 wt % of the total
solids in the ingredients. The para-aramid fiber preferably has a linear
density of no more than 10 dtex, more preferably 0.5 to 10 dtex, and most
preferably, 0.8 to 2.5 dtex. The para-aramid fiber also preferably has an
average length along its longitudinal axis of no more than 10 cm, more
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preferably an average length of 0.65 to 2.5 cm, and most preferably an
average length of 0.65 to 1.25 cm.
Para-Aramid Particles
Optionally, in one embodiment, the pulp ingredients further include
substantially or completely fibril-free, granular, para-aramid particles. If
these particles are added, they are added to a concentration of no more
than 50 wt % of the total solids in the ingredients, preferably 20 to 50 wt %
of the total solids in the ingredients, and most preferably 25 to 35 wt % of
the total solids in the ingredients. Being made of para-aramid, they
contribute superior wear resistance and dispersability to the pulp being
produced. Because the particles are substantially fibril-free, they also
serve as a compounding agent to assist in dispersing the other ingredients
in the mixture and slurry. Particles that perform this function are often
known as processing agents or aids. The substantially or completely fibril-
free, granular, para-aramid particles have an average maximum
dimension of 50 to 2000 microns (0.05 to 2 mm), preferably 50 to 1500
microns, and most preferably 75 to 1000 microns. Particles below about
50 microns, however, lose effectiveness in friction and sealing
applications. Particles above about 2000 microns do not adequately stay
dispersed in the water with the other ingredients when mixed. Figure 3 is
an image of a photomicrograph of para-aramid particles capable of being
used as an ingredient to the process of the present invention.
Aramid Polymer
Polymers suitable for use in making the aramid fiber and aramid
particles of this invention are synthetic aromatic polyamides. The
polymers must be of fiber-forming molecular weight in order to be shaped
into fibers. The polymers can include polyamide homopolymers,
copolymers, and mixtures thereof which are predominantly aromatic,
wherein at least 85% of the amide (-CONH-) linkages are attached directly
to two aromatic rings. The rings can be unsubstituted or substituted. The
polymers are para-aramid when the two rings are para oriented with
respect to each other along the molecular chain, Preferably copolymers
have no more than 10 percent of other diamines substituted for a primary
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diamine used in forming the polymer or no more than 10 percent of other
diacid chlorides substituted for a primary diacid chloride used in forming
the polymer. Additives can be used with the aramid; and it has been
found that up to as much as 13 percent by weight of other polymeric
material can be blended or bonded with the aramid. The preferred para-
aramids are poly(para-phenylene terephthalamide)(PPD-T) and its
copolymers.
Optional Other Additives
Other additives can optionally be added as long as they stay
suspended in solution in the mixing step and do not significantly change
the effect of the refining step on the mandatory solid ingredients listed
above. Suitable additives include pigments, dyes, anti-oxidants, flame-
retardant compounds, and other processing and dispersing aids.
Preferably, the pulp ingredients do not include asbestos. In other words,
the resulting pulp is asbestos free or without asbestos.
Water
Water is added to a concentration of 95 to 99 wt % of the total
ingredients, and preferably 97 to 99 wt % of the total ingredients. Further,
the water can be added first. Then other ingredients can be added at a
rate to optimize dispersion in the water while simultaneously mixing the
combined ingredients.
Mixing Step
In the mixing step, the ingredients are mixed to a substantially
uniform slurry. By "substantially uniform" is meant that random samples
of the slurry contain the same wt % of the concentration of each of the
starting ingredients as in the total ingredients in the combination step plus
or minus 10 wt %, preferably 5 wt % and most preferably 2 wt %. For
instance, if the concentration of the solids in the total mixture is 50 wt %
acrylic fiber plus 50 wt % para-aramid fiber, then a substantially uniform
mixture in the mixing step means each random sample of the slurry has
(1) a concentration of the acrylic fiber of 50 wt % plus or minus 10 wt %,
preferably 5 wt % and most preferably 2 wt % and (2) a concentration of
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para-aramid fiber of 50 wt % plus or minus 10 wt %, preferably 5 wt % and
most preferably 2 wt %. The mixing can be accomplished in any vessel
containing rotating blades or some other agitator. The mixing can occur
after the ingredients are added or while the ingredients are being added or
combined.
Refining Step
In the refining step the pulp ingredients are simultaneously co-
refined, converted or modified as follows. The acrylic fiber and the para-
aramid fiber are fibrillated, cut and masticated to irregularly shaped fibrous
structures having stalks and fibrils. If para-aramid particles are added with
the other ingredients, at least some of the para-aramid particles are
masticated into smaller, rounder, substantially fibril-free, particles. All
solids are dispersed such that the refined slurry is substantially uniform.
"Substantially uniform" is as defined above. The refining step preferably
comprises passing the mixed slurry through one or more disc refiner, or
recycling the slurry back through a single refiner. By the term "disc
refiner" is meant a refiner containing one or more pair of discs that rotate
with respect to each other thereby refining ingredients by the shear action
between the discs. In one suitable type of disc refiner, the slurry being
refined is pumped between closely spaced circular rotor and stator discs
rotatable with respect to one another. Each disc has a surface, facing the
other disc, with at least partially radially extending surface grooves. A
preferred disc refiner that can be used is disclosed in U.S. Patent
4,472,241. If necessary for uniform dispersion and adequate refining, the
mixed slurry can be passed through the disc refiner more than once or
through a series of at least two disc refiners. When the mixed slurry is
refined in only one refiner, there is a tendency for the resulting slurry to
be
inadequately refined and non uniformly dispersed. Conglomerates or
aggregates entirely or substantially of one solid ingredient, or the other, or
both, or all three if three are present, can form rather than being dispersed
forming a substantially uniform dispersion. Such conglomerates or
aggregates have a greater tendency to be broken apart and dispersed in
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the slurry when the mixed slurry is passed through the refiner more than
once or passed through more than one refiner.
Because a substantially uniform slurry containing multiple
ingredients is co-refined in this step of the process, any one type of non-
pulp ingredient (for example, para-aramid fiber) is refined into a pulp in the
presence of all the other types of non-pulp ingredients (for example,
aramid material pieces and optionally para-aramid particles) while those
other ingredients are also being refined. This co-refining of non-pulp
ingredients forms a pulp that is superior to a pulp blend generated by
merely mixing two pulps together. Adding two pulps and then merely
mixing them together does not form the substantially uniform, intimately
connected, fibrous components of the pulp generated by co-refining of
non-pulp ingredients into pulp in accordance with the present invention.
Removing Step
Then water is removed from the refined slurry to no more than 60
total wt % water, preferably 4 to 60 total wt % water, most preferably, 5 to
58 total wt % water. The water can be removed by collecting the pulp on
a dewatering device such as a horizontal filter, and if desired, additional
water can be removed by applying pressure or squeezing the pulp filter
cake. The dewatered pulp can optionally then be dried to a desired
moisture content, and/or can be packaged or wound up on rolls.
Figures 1 and 2
This process will now be described in reference to Figures 1 and 2.
Throughout this detailed description, similar reference characters refer to
similar elements in all figures of the drawings.
Referring to Figure 1, there is a block diagram of an embodiment of
a wet process for making "wet" pulp in accordance with the present
invention. Pulp ingredients I are added to container 2. Container 2 is
provided with an internal mixer, similar to a mixer in a washing machine.
The mixer disperses the ingredients into the water creating the
substantially uniform slurry. The mixed slurry is transferred to a first
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refiner 3 which refines the slurry. Then, optionally, the refined slurry can
be transferred to a second refiner 4, and optionally then to a third refiner
5. Three refiners are illustrated but any number of refiners can be used
depending on the degree of uniformity and refining desired. After the last
refiner in the series of refiners, the refined slurry is optionally
transferred
to a filter or sorter 6 that allows slurry with dispersed solids below a
chosen mesh or screen size to pass and recirculates dispersed solids
larger than a chosen mesh or screen size back to one or more of the
refiners such as through line 7 or to a refiner 8 dedicated to refine this
recirculated slurry from which refined slurry is again passed to the filter or
sorter 6. Suitably refined slurry passes from the filter or sorter 6 to a
horizontal water vacuum filter 9 which removes water such that the pulp
has a concentration of water of no more than 75 wt % of the total
ingredients. Slurry can be transferred from point to point by any
conventional method and apparatus such as with the assistance of one or
more pump 10. Then the pulp is conveyed to a dryer 11 that removes
more water until the pulp has a concentration of water of no more than 60
wt % of the total ingredients. Then the refined pulp is packaged in a baler
12.
Referring to Figure 2, there is a block diagram of an embodiment of
a dry process for making "dry" pulp in accordance with the present
invention. This dry process is the same as the wet process except after
the horizontal water vacuum filter 9. After that filter, the pulp goes through
a press 13 which removes more water until the pulp has a concentration
of water of no more than 20 wt % of the total ingredients. Then the pulp
goes through a fluffier 14 to fluff the pulp and then a rotor 15 to remove
more water. Then, like the wet process, the pulp is passed through a
dryer 11 and packaged in a baler 12.
11. Second Embodiment of the Inventive Process
In a second embodiment, the process for making the acrylic fiber
and para-aramid pulp is the same as the first embodiment of the process
described above with the following differences.
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Prior to combining all ingredients together, either the acrylic fiber or
the para-aramid fiber, or both the acrylic fiber and the para-aramid fiber,
may need to be shortened. This is done by combining water with the fiber
ingredient. Then the water and fiber are mixed to form a first suspension
and processed through a first disc refiner to shorten the fiber. The disc
refiner cuts the fiber to an average length of no more than 10 cm. The
disc refiner will also partially fibrillate and partially masticate the fiber.
The
other fiber, that was not previously added, can be shortened this way too
forming a second processed suspension. Then the other fiber (or the
second suspension, if processed in water) is combined with the first
suspension.
More water is added before or after, or when, other ingredients are
added, if necessary, to increase the water concentration to 95 - 99 wt % of
the total ingredients. After all ingredients are combined, they can be
mixed, if necessary, to achieve a substantially uniform slurry.
The ingredients in the slurry are then co-refined together, i.e.,
simultaneously. This refining step includes fibrillating, cutting and
masticating solids in the suspension such that all or substantially all of the
acrylic and para-aramid fiber is converted to irregularly shaped fibrillated
fibrous structures. This refining step also disperses all solids such that the
refined slurry is substantially uniform. Then water is removed as in the
first embodiment of the process. Both processes produce the same or
substantially the same acrylic and para-aramid pulp.
The Inventive Pulp
The resulting product produced by the process of this invention is
an acrylic and para-aramid pulp for use as reinforcement material in
products. The pulp comprises (a) irregularly shaped, acrylic fibrous
structures, (b) irregularly shaped, para-aramid fibrous structures, (c)
optionally substantially fibril-free, granular, para-aramid particles, (d)
optionally other minor additives, and (e) water.
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The concentration of the separate ingredient components in the
pulp correspond, of course, to the concentrations described beforehand of
the corresponding ingredients used in making the pulp.
The irregularly shaped, acrylic and para-aramid fibrillated fibrous
structures have stalks and fibrils. The acrylic fibrils and/or stalks are
substantially entangled with the para-aramid fibrils and/or stalks. The
fibrils are important and act as hooks or fasteners or tentacles which
adhere to and hold adjacent particles in the pulp and final product thereby
providing integrity to the final product.
The acrylic and para-aramid fibrillated fibrous structures preferably
have an average maximum dimension of no more than 5 mm, more
preferably 0.1 to 4 mm, and most preferably 0.1 to 3 mm. The acrylic and
para-aramid fibrillated fibrous structures preferably have a length-weighted
average of no more than 1.3 mm, more preferably 0.7 to 1.2 mm, and
most preferably 0.75 to 1.1 mm.
If para-aramid particles are included in the pulp, the acrylic and
para-aramid fibrous structures also additionally contact and are wrapped
partially around at least some of these rounder, substantially fibril-free,
para-aramid particles. These para-aramid particle also preferably have a
dimension of at least 50 microns, more preferably, 50 to 100 microns, and
most preferably 50 to 75 microns. Fibrils on and along the acrylic and
para-aramid fibrous structures can contact and form a partial cocoon
around the rounder, substantially fibril-free, para-aramid particles
The acrylic and para-aramid pulp is without substantial aggregates
or conglomerates of the same material. Further, the pulp has a Canadian
Standard Freeness (CSF) as measured per TAPPI test T 227 om-92,
which is a measure of its drainage characteristics, of 100 to 700 ml, and
preferably 250 to 450 ml.
Surface area of pulp is a measure of the degree of fibrillation and
influences the porosity of the product made from the pulp. Preferably, the
surface area of pulp of this invention is 7 to 11 square meters per gram.
Figure 4 is an image of a photomicrograph of acrylic and para-
aramid pulp made according to the process of the present invention.
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It is believed that aramid particles and fibrous structures, dispersed
substantially homogeneously throughout the reinforcement material, and
the friction and sealing materials, provide, by virtue of the high
temperature characteristics of the para-aramid polymers and the
fibrillation propensity of the para-aramid fibers, many sites of
reinforcement and increased wear resistance. When co-refined, the
blending of the acrylic and para-aramid materials is so intimate that in a
friction or sealing material there is always some para-aramid fibrous
structures close to the acrylic structures, so the stresses and abrasion of
service are always shared.
Sealing Material
The invention is further directed to sealing material and processes
for making the sealing materials. Sealing materials are used in or as a
barrier to prevent the discharge of fluids and/or gases and used to prevent
the entrance of contaminants where two items are joined together. An
illustrative use for sealing material is in gaskets. The sealing material
comprises a binder; optionally at least one filler; and a fibrous
reinforcement material comprising the acrylic and para-aramid pulp of this
invention. Suitable binders include nitrite rubber, butadiene rubber,
neoprene, styrene-butadiene rubber, nitrite-butadiene rubber, and
mixtures thereof. The binder can be added with all other starting
materials. The binder is typically added in the first step of the gasket
production process, in which the dry ingredients are mixed together.
Other ingredients optionally include uncured rubber particles and a rubber
solvent, or a solution of rubber in solvent, to cause the binder to coat
surfaces of the fillers and pulp. Suitable fillers include barium sulfate,
clays, talc, and mixtures thereof.
Suitable processes for making sealing materials are, for example, a
beater-add process or wet process where the gasket is made from a slurry
of materials, or by what is called a calendering or dry process where the
ingredients are combined in an elastomeric or rubber solution.
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Friction Material
The pulp of the present invention can be used as a reinforcement
material in friction materials. By "friction materials" is meant materials
used for their frictional characteristics such as coefficient of friction to
stop
or transfer energy of motion, stability at high temperatures, wear
resistance, noise and vibration damping properties, etc. Illustrative uses
for friction materials include brake pads, brake blocks, dry clutch facings,
clutch face segments, brake pad backing/insulating layers, automatic
transmission papers, and friction papers.
In view of this new use, the invention is further directed to friction
material and processes for making the friction material. Specifically, the
friction material comprises a friction modifier; optionally at least one
filler;
a binder; and a fibrous reinforcement material comprising the acrylic and
para-aramid pulp of this invention. Suitable friction modifiers are metal
powders such as iron, copper and zinc; abrasives such as oxides of
magnesium and aluminum; lubricants, such as synthetic and natural
graphites, and sulfides of molybdenum and zirconium; and organic friction
modifiers such as synthetic rubbers and cashew nut shell resin particles.
Suitable binders are thermosetting resins such as phenolic resins (i.e.,
straight (100%) phenolic resin and various phenolic resins modified with
rubber or epoxy), melamine resins, epoxy resins and polyimide resins, and
mixtures thereof. Suitable fillers include barite, calcium carbonate,
wollastonite, talc, various clays, and mixtures thereof.
The actual steps for making the friction material can vary,
depending on the type of friction material desired. For example, methods
for making molded friction parts generally involve combining the desired
ingredients in a mold, curing the part, and shaping, heat treating and
grinding the part if desired. Automotive transmission and friction papers
generally can be made by combining the desired ingredients in a slurry
and making a paper on a paper machine using conventional paper making
processes.
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TEST METHODS
The following test methods were used in the following Examples.
Canadian Standard Freeness (CSF) is a well-known measure of
the facility for water to drain from a slurry or dispersion of particles.
Freeness is determined by TAPPI test T227. Data obtained from conduct
of that test are expressed as Canadian Freeness Numbers, which
represent the milliliters of water which drain from an aqueous slurry under
specified conditions. A large number indicates a high freeness and a high
tendency for water to drain. A low number indicates a tendency for the
dispersion to drain slowly. The freeness is inversely related to the degree
of fibrillation of the pulp, since greater numbers of fibrils reduce the rate
at
which water drains through a forming paper mat.
Length-weighted average is measured using a "FiberExpert"
tabletop analyzer (also now known as "PulpExpertFS", available from
Metso Automation of Helsinki, Finland). This analyzer takes photographic
images of the pulp with a digital CCD camera as the pulp slurry flows
through the analyzer and then an integrated computer analyzes the fibers
in these images and calculates their length-weighted average.
Temperature: All temperatures are measured in degrees Celsius (0
C).
Denier is measured according to ASTM D 1577 and is the linear
density of a fiber as expressed as weight in grams of 9000 meters of fiber.
The denier is measured on a Vibroscope from Textechno of Munich,
Germany. Denier times (10/9) is equal to decitex (dtex).
EXAMPLES
This invention will now be illustrated by the following specific
examples. All parts and percentages are by weight unless otherwise
indicated. Examples prepared according to the process or processes of
the current invention are indicated by numerical values.
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Example 1
In this example of the invention, the pulp of this invention was
produced from a feedstock of para-aramid fiber and acrylic staple. Acrylic
staple having a cut length of 2 inches and having a filament linear density
of 3 dpf (3.3 dtex per filament) was obtained from Solutia, Inc., with offices
in St. Louis, MO. Para-aramid fiber in the form of commercially available
KEVLAR brand floc, Style 1 F178, having a'/4" cut length, was obtained
from E. 1. de Pont de Nemours and Company with offices in Wilmington,
Delaware.
Acrylic staple and water together were fed directly into a Sprout-
Waldron 12" Single Disc Refiner using a 10 mil plate gap setting and pre-
pulped to reach an acceptable processing length in the range of 13mm.
The pre-pulped acrylic fiber and the cut para-aramid fiber plus
water were then combined into a highly agitated mixing tank at a solids
concentration of 50 wt % para-aramid fiber and 50 wt% acrylic staple and
mixed to form a uniform, pumpable slurry of about 2-3 wt % of the total
ingredients concentration. The slurry was then recirculated and co-refined
through a Sprout-Waldron 12" Single Disc Refiner.
The refiner simultaneously:
(1) fibrillated, cut, and masticated both the para-aramid fiber
and the acrylic staple to irregularly shaped fibrous
structures having stalks and fibrils.
(2) dispersed all solids such that the refined slurry was
substantially uniform, substantially uniform being as
previously defined.
This refined slurry was then filtered using a filter bag and was
dewatered through pressing and placed in large ZIPLOC type storage
bags. The fibrous structures had an average maximum dimension of no
more than 5 mm and a length-weighted average of no more than 1.3 mm,
as measured by FiberExpert .
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Example 2
This example illustrates another method by which a co-refined pulp
can be made from a feedstock of para-aramid fiber and acrylic fiber.
Acrylic staple, having a cut length of 2 inches and having a filament linear
density of 3 dpf (3.3 dtex per filament) available from Solutia, Inc., is cut
with a guillotine cutter two to three times at right angles in order to
produce a random-length fiber with most fibers shorter than 3 inch (1.91
cm) and averaging about'/ inch (1.27 cm) long.
Para-aramid fiber in the form of commercially available KEVLAR
brand multifilament yarn, available from E. I. de Pont de Nemours and
Company on bobbins, is prepared by cutting the para-aramid yarn to a
nominal % inch (1.27 cm) cut length on a Lummus Cutter (available from
Lummus Industries with offices in Columbus, Georgia). Other KEVLAR
brand para-aramid fiber, which initially is not on bobbins and is of multiple
long lengths, is cut by a guillotine cutter two to three times at right angles
in order to produce a random-length fiber with most fibers shorter than %
inch (1.91 cm) and averaging about'/ inch (1.27 cm) long.
The two ingredients prepared as described above plus water are
then combined into a highly agitated mixing tank called a hydrapulper at a
solids concentration of 50 wt % para-aramid fiber and 50 wt % acrylic fiber
and mixed to form a substantially uniform, pumpable slurry having a total
solids concentration of about 2-3 wt % of the total ingredients. The slurry
is pumped through a series of three refiners, as described in U.S. Patent
4,472,241. The refiners simultaneously:
(1) fibrillate, cut, and masticate the acrylic fiber and the para-aramid
fiber into irregularly shaped fibrous structures having stalks and fibrils;
and
(2) disperse all solids such that the refined slurry was substantially
uniform with substantially uniform as previously defined.
This refined slurry is then dewatered using a horizontal filter and
dried in an oven to a desired moisture content of 50 total wt % for wet
pulp. The wet pulp is then packaged into bales by a baler. When
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measured by FiberExpert , all of the ingredients in the pulp have a length-
weighted average of no more than 1.3 mm.
Example 3
This example illustrates further process steps and another
embodiment of the pulp of this invention. The procedure of Example 2 is
followed. However, after the pulp is dewatered on the horizontal filter, the
pulp is pressed in a mechanical press to further remove water; and the
pulp is then fluffed using a Fluffer (available from Bepex Corporation with
offices at Santa Rosa, California) to better separate the pressed wet pulp.
The fluffed wet pulp is then dried in an oven to approximately 8 total wt %
moisture and is then further processed in an ultrarotor (model IIIA
available from Altenburger Machinen Jackering GmbH with offices in
Voisterhauser, Germany) such as is disclosed in U.S. Patent 5,084,136 to
further fluff and disperse the dried pulp. The dried pulp is then packaged
into bales. When measured by FiberExpert , all of the ingredients in the
pulp have a length-weighted average of no more than 1.3 mm.
Example 4
This example illustrates another embodiment of the pulp of this
invention. The process of Example 2 is followed with the exception that
one third by weight of the para-aramid fiber is replaced by para-aramid
particles. The para-aramid resin particles are prepared by reacting para-
phenylenediamine and teraphthaloyl chloride continuously in a screw
extruder as is generally disclosed in U.S. Patent 3,884,881, but using N,
methyl pyrollidone/calcium chloride as the solvent, producing a crumb-like
polymer that precipitates from the solvent. The solvent is extracted, and
the polymer crumb washed and dried to a particulate powder of mixed
particle size. The para-aramid resin particles are then treated
substantially the same as the para-aramid fiber is treated in Example 2.
However, the refiner not only refines the fibers but also cuts and/or
masticates the para-aramid particles into rounder, substantially fibril-free
particles. After dewatering, some of the resulting pulp having a moisture
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content of 50 total wt percent is then packaged into bales. The remainder
of the resulting pulp is further pressed to a moisture content of
approximately 8 total wt percent and then fluffed, dispersed, and
packaged as in Example 3. When measured by FiberExpert , all of the
ingredients in the pulp have a length-weighted average of no more than
1.3 mm.
Example 5
Disc brake pads incorporating the pulp of this invention were made
in the following manner. Approximately 20 kilograms of a non-asbestos-
containing base compound powder comprising a mixture of 7 wt %
cashew nut shell resin, 17 wt % inorganic fillers, 21 wt % graphite, coke
and lubricants, 18 wt % inorganic abrasives, and 16 wt % soft metals was
mixed together for 10 to 20 minutes in a 50-liter Littleford mixer. The
mixer had two high-speed choppers with blades of the "stars and bars"
configuration and a slower rotating plough.
kilograms of the well-blended base compound powder was then
combined with the pulp of this invention (a co-refined pulp being 50 wt %
para-aramid and 50 wt % acrylic fiber) in an amount of 3.8 wt %, based on
the combined weight of the compound powder and the pulp. The pulp
was then dispersed in the base compound powder by mixing for an
additional 5 to 10 minutes. Once mixed, the resulting brake pad
composition had a normal visual appearance with the fiber well dispersed
in and completely coated with the base compound powders, with
essentially no detectable balling up of the pulp or segregation of any
constituents.
The brake pad composition was then poured into a single-cavity
steel mold for a front disc brake pad and cold pressed to a standard
thickness of about 5/8 inch (16 mm) then removed from the mold to form a
pre-formed brake pad having an approximate weight of 200 grams. The
pre-form had no excessive spring-back or swelling, and was robust
enough to endure normal handling without damage. Twelve replicate pre-
forms were made. The pre-forms were then placed in two multi-cavity
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molds, placed in a commercial press, and press-cured (the binder
phenolic cross-linking and reacting) at 300 F (149 C) for about 15
minutes, with periodic pressure release to allow phenolic reaction gases to
escape, followed by lightly constrained oven curing at 340 F (171 C) for 4
hours to complete the phenolic binder crosslinking. The cured, molded
pad was then ground to the desired thickness of about half an inch (13
mm). When compared visually with a commercial brake pad containing
an equivalent amount of all para-aramid pulp or acrylic pulp, the test pad
was indistinguishable and had good compound flow into the backing plate
holes and no edge chipping.
A sample of the brake pad incorporating the pulp of this invention
was then tested to determine its frictional performance. Coupons,
typically one inch by one inch and about 3/16 inch (5 mm) thick, from test
pads were assessed on the Chase Machine available from Link
Engineering, Detroit, MI, using test protocol Society of Automotive
Engineers (SAE) J661 to determine the hot and cold friction coefficient
during constant pressure and controlled temperature drag tests against a
heated steel drum. The sample was periodically measured for wear
(thickness loss). This was repeated with two more test samples cut from
other replicate pads. The samples of the brake pad incorporating the pulp
of this invention exhibited hot and cold friction performance substantially
equivalent to commercially available pads containing a substantially
equivalent amount of all para-aramid pulp. The test further indicated the
pad-to-pad uniformity and an average friction rating was also substantially
equivalent.
The pad was then tested for friction and wear under various braking
conditions using a dynamometer (single piston dynamometer with a rolling
radius of 289.0 mm at Link Testing Laboratories, Inc., in Detroit, MI) using
test protocol J2681 (ISO-SWG4). This test was comprised of seventeen
scenarios of from 5 to 200 brake applications each, and measured
coefficient of friction as a function of applied brake pressure, temperature,
braking speed and deceleration rate. This test also had two high-
temperature fade sections, during which the brake pad was subjected to
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increasingly high initial temperatures during constant deceleration, and
reached temperatures exceeding 600 C. Wear was measured as the
reduction in thickness and weight of the pad at the end of the test (608
brake applications.) Results for the pads made with the compound of this
example showed very little fade and what fade there was recovered well
(where fade is defined as the loss of friction at the highest temperature
brake applications), acceptable coefficient of friction of 0.25 to 0.4 in non-
fade sections, absence of pad surface cracking, and acceptable wear
rates for both the pad and the rotor.
Example 6
This example illustrates how the pulp of this invention can be
incorporated into a beater-add gasket for sealing applications. Water,
rubber, latex, fillers, chemicals, and the pulp of this invention are
combined in desired amounts to form a slurry. On a circulating wire sieve
(such as a paper machine screen or wire), the slurry is largely drained of
its water content, is dried in a heating tunnel, and is vulcanized on heated
calender rolls to form a material having a maximum thickness of around
2.0 mm. This material is compressed in a hydraulic press or two-roll
calender, which increases the density and improves sealability.
Such beater-add gasket materials generally do not have as good
sealability as equivalent compressed-fiber materials and are best suited
for moderate-pressure high-temperature applications. Beater-add gaskets
find applicability in the making of auxiliary engine gaskets or, after further
processing, cylinder head gaskets. For this purpose, the semi-finished
product is laminated onto both sides of a spiked metal sheet and is
physically fixed in place by the spikes.
Example 7
This example illustrates how the pulp of this invention can be
incorporated into a gasket made by a calendering process. The same
ingredients as in Example 6, minus the water, are thoroughly dry mixed
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together and are then blended with a rubber solution prepared using an
appropriate solvent.
After mixing, the compound is then generally conveyed batchwise
to a roll calender. The calender consists of a small roll that is cooled and
a large roll that is heated. The compound is fed and drawn into the
calender nip by the rotary movement of the two rolls. The compound will
adhere and wrap itself around the hot lower roll in layers generally about
0.02 mm thick, depending on the pressure, to form a gasketing material
made from the built-up compound layers. In so doing, the solvent
evaporates and vulcanization of the elastomer commences.
Once the desired gasketing material thickness is reached, the rolls
are stopped and the gasketing material is cut from the hot roll and cut
and/or punched to the desired size. No additional pressing or heating is
required, and the material is ready to perform as a gasket. In this manner
gaskets up to about 7 mm thick can be manufactured. However, most
gaskets made in this manner are much thinner, normally being about 3
mm or less in thickness.
