U.S. patent application number 10/648736 was filed with the patent office on 2004-05-13 for non-woven sheet of aramid floc.
Invention is credited to Levit, Mikhail R..
Application Number | 20040089432 10/648736 |
Document ID | / |
Family ID | 25079923 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089432 |
Kind Code |
A1 |
Levit, Mikhail R. |
May 13, 2004 |
Non-woven sheet of aramid floc
Abstract
The present invention relates to a wet-laid non-woven sheet made
from a essentially binder-free aramid floc, wherein the aramid floc
includes at least 25 percent by weight of para-aramid floc based on
the total weight of aramid floc only, and a process for making the
sheet.
Inventors: |
Levit, Mikhail R.;
(Richmond, VA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
25079923 |
Appl. No.: |
10/648736 |
Filed: |
August 25, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10648736 |
Aug 25, 2003 |
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09767584 |
Jan 23, 2001 |
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Current U.S.
Class: |
162/157.3 ;
162/205; 442/408; 442/409; 442/411; 442/415 |
Current CPC
Class: |
D21H 25/005 20130101;
H05K 1/0366 20130101; D21H 25/04 20130101; Y10T 428/23943 20150401;
Y10T 442/697 20150401; D21H 13/26 20130101; Y10T 442/692 20150401;
C08J 5/04 20130101; Y10T 442/689 20150401; Y10T 442/69 20150401;
Y10T 442/659 20150401 |
Class at
Publication: |
162/157.3 ;
442/409; 442/411; 442/408; 442/415; 162/205 |
International
Class: |
D21H 013/26 |
Claims
What is claimed is:
1. A wet-laid non-woven sheet comprising essentially binder-free
aramid floc, wherein the aramid floc includes at least 25 percent
by weight of para-aramid floc based on the total weight of aramid
floc only.
2. The wet-laid non-woven sheet of claim 1, wherein the aramid floc
includes at least 50 percent by weight of para-aramid floc based on
the total weight of aramid floc only.
3. The wet-laid non-woven sheet of claim 1, wherein the aramid floc
includes at least 75 percent by weight of para-aramid floc based on
the total weight of aramid floc only.
4. The wet-laid non-woven sheet of claim 1, wherein the aramid floc
includes 100 percent by weight of para-aramid floc based on the
total weight of aramid floc only.
5. The wet-laid non-woven sheet of claim 1, wherein the aramid floc
has an average length of from 3 to 13 millimeters
6. The wet-laid non-woven sheet of claim 1, wherein the floc has
been hydroentangled.
7. The wet-laid non-woven sheet of claim 1, which has been hot
calendered.
8. The wet-laid non-woven sheet of claim 6, which has been hot
calendered.
9. The wet-laid non-woven sheet of claim 1, wherein the aramid floc
comprises para-aramid floc and meta-aramid floc.
10. A prepreg that includes the wet-laid non-woven sheet of claim
1.
11. A prepreg that includes the wet-laid non-woven sheet of claim
7.
12. A prepreg that includes the wet-laid non-woven sheet of claim
8.
13. A laminate made from the prepreg of claim 10.
14. A laminate made from the prepreg of claim 11.
15. A laminate made from the prepreg of claim 12.
16. A printed wiring board that includes the laminate of claim
13.
17. A printed wiring board that includes the laminate of claim
14.
18. A printed wiring board that includes the laminate of claim
15.
19. A process for making a sheet of essentially binder-free aramid
floc, wherein the aramid floc includes at least 25 percent by
weight of para-aramid floc based on the total weight of aramid floc
only, comprising the steps of: a) preparing an aqueous dispersion
of 0.005 to 0.08 weight percent of essentially binder-free aramid
floc that includes at least 25 percent by weight of para-aramid
floc; b) pouring the dispersion onto a moving screen in a paper
making machine; c) withdrawing water from the aqueous dispersion to
leave a wet paper felt; and d) drying the wet paper felt on the
supporting screen to make a sheet of essentially binder-free aramid
floc.
20. The process of claim 19 wherein the aramid floc has an average
length of from 3 to 13 millimeters.
21. The process of claim 19 further comprising the step of
hydroentangling the non-woven sheet before or after the non-woven
sheet is dried.
22. The process of claim 19 further comprising the step of hot
calendering the dried sheet of essentially binder-free aramid
floc.
23. The process of claim 21 further comprising the step of hot
calendering the dried sheet of essentially binder-free aramid
floc.
24. The process of claim 19, wherein the aramid floc comprises
para-aramid floc and meta-aramid floc.
25. The process of claim 19 further comprising the step of
impregnating the sheet of essentially binder-free aramid floc with
a resin to form a prepreg.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a non-woven sheet that includes
aramid floc. Such sheets may be impregnated with a resin and used
as a substrate for a printed wiring board or other composite
structures.
[0003] 2. Description of the Related Art
[0004] It is known in the art to use non-woven aramid sheets as
substrates for printed wiring boards or other composite structures.
These sheets are usually impregnated with a resin and made into
laminates that have a low coefficient of thermal expansion and that
are used in printed wiring boards.
[0005] Non-woven aramid sheets can be produced by wet papermaking
technology by using various binders with the aramid fibers. U.S.
Pat. No. 5,314,742 discloses a non-woven aramid sheet that is
prepared from 75 to 95 weight percent para-aramid floc and from 5
to 25 weight percent poly(m-phenylene isophthalamide) fibrids. U.S.
Pat. No. 5,223,094 discloses a non-woven aramid sheet made from 45
to 97 weight percent poly(p-phenylene terephthalamide) floc, from 3
to 30 weight percent poly(m-phenylene isophthalamide) fibrids, and
from 0 to 35 weight percent of quartz fiber. In both of these
patents, the disclosed fibrids act as a binder during the
paper-forming process.
[0006] U.S. Pat. No. 4,698,267 discloses high density para-aramid
papers that contain short para-aramid fibers selected from the
group consisting of para-aramid pulp, para-aramid floc and mixtures
thereof. The paper contains from 5 to 25 weight percent of a binder
of water dispersible polymeric materials or aramid fibrids. This
patent teaches that the binder may be omitted when batch methods of
paper preparation are used. However, while not identified as such,
the para-aramid pulp acts as a binder in these papers.
[0007] The binder in the non-woven sheets described in the above
prior art references is an impurity that acts mostly as a defect in
the sheet when that sheet is impregnated and used as a substrate.
The negative impact of a binder used in the papermaking process on
a composite structure depends on the type and quantity of binder
used, but in general the problems caused by such a binder include
low physical and mechanical properties compared to aramid floc,
higher moisture absorption, and poorer thermal properties. Further,
such a binder can interfere with the interaction between aramid
floc and a primary binder, such as an epoxy, polyimide or other
resin, used in a laminate in a composite structure, which decreases
the physical and mechanical properties of that composite
structure.
[0008] In the manufacture of a non-woven sheet, it is known to
hydroentangle a sheet made by a wet-lay forming process. However,
the hydroentangling is used only with low modulus fibers. For
example, U.S. Pat. No. 4,902,564 describes a method for producing
highly absorbent non-woven by wet-lay forming followed by
hydroentangling wherein the non-woven consists of polypropylene,
nylon, or polyester fibers, and wood pulp as a binder.
[0009] Heretofore, non-woven sheets made from aramid fibers that
have been hydroentangled have been made only by dry-laid processes.
Further, these sheets have had significant directionality, meaning
significant differences in strength in the machine and cross
directions of the sheet.
[0010] The problems in the prior art are overcome by the present
invention because the sheet in the wet-lay or papermaking process
is essentially free of binders used in the papermaking process.
Also, the wet-laid non-woven sheet, and products made using that
non-woven sheet, are free from the negative effects of having a
binder present in the non-woven sheet. The inventive sheet may be
hydroentangled and calendered to improve the physical properties of
the sheet.
[0011] The present invention relates to a wet-laid non-woven sheet
of essentially binder-free aramid floc, wherein the aramid floc
includes at least 25 percent by weight of para-aramid floc based on
the total weight of aramid floc only.
[0012] The invention also relates to a process for making a sheet
of essentially binder-free aramid floc, wherein the aramid floc
includes at least 25 percent by weight of para-aramid floc based on
the total weight of aramid floc only, by a) preparing an aqueous
dispersion of 0.005 to 0.08 weight percent of essentially
binder-free aramid floc that includes at least 25 percent by weight
of para-aramid floc; b) pouring the dispersion onto a moving screen
in a paper making machine; c) withdrawing water from the aqueous
dispersion to leave a wet non-woven felt; and d) drying the wet
non-woven felt on the supportive screen to make a sheet of
essentially binder-free and pulp-free aramid floc. The sheet may be
hydroentangled before or after being dried. Further, the sheet may
be hot-calendered to increase its strength, whether or not that
sheet has been hydroentangled.
DETAILED DESCRIPTION
[0013] The present invention provides an improved wet-laid
non-woven sheet that is useful, for example, as a substrate for
printed circuit boards. The non-woven sheet is made from aramid
floc and is essentially free of binders such as pulp, fibrids or
water dispersible polymeric materials. The non-woven sheet is made
in a wet-laid non-woven process. Optionally, the non-woven sheet
may be hydroentangled before or after being dried. Further, the
non-woven sheet may be hot-calendered. The sheet may be impregnated
with a resin and may be formed into a laminate for use in a final
composite structure such as a printed circuit wiring board.
[0014] It is surprising and unexpected that essentially binder-free
para-aramid floc, without being refined or subjected to other
treatment, can be used to make a wet-laid sheet having sufficient
strength to be processed on a paper-making machine and to be wound
up in a roll. Also, it surprising and unexpected that a non-woven
structure, such as a sheet, made from relatively short para-aramid
floc (3-13 mm) can be successfully hydroentangled. A further
discovery is that an essentially binder-free aramid sheet can be
dramatically strengthened by a hot calendering process.
[0015] A non-woven sheet of the invention exhibits a better
uniformity than a similar sheet made using binders in the
papermaking process. Further, such a sheet delivers essentially
pure aramid floc to the impregnation step of the process for
preparing a final composite structure that includes the impregnated
non-woven sheet.
[0016] The term "floc", as used herein, means fibers that are cut
to a short length that is customarily used in the preparation of
wet-laid sheets. Such floc has a specific surface area of less than
one square meter per gram. Aramid floc is made by cutting
continuous aramid fibers, such as those prepared by processes
described in U.S. Pat. Nos. 3,063,966, 3,133,138, 3,767,756, and
3,869,430.
[0017] The term "aramid fiber", as used herein, means aromatic
polyamide fiber, wherein at least 85% of the amide (--CONH--)
linkages are attached directly to two aromatic rings. Additives can
be used with the aramid, and it has been found that up to as much
as 10 percent by weight of other polymeric material can be blended
with the aramid or that copolymers can be used having as much as 10
percent of other diamine substituted for the diamine of the aramid
or as much as 10 percent of other diacid chloride substituted for
the diacid chloride of the aramid. Para-aramid polymers are the
primary polymers in fibers used to make the floc of this invention
and poly(p-phenylene terephthalamide)(PPD-T) is the preferred
para-aramid. Meta-aramid fibers may also be used in the floc of
this invention and poly(m-phenylene isophthalamide)(MPD-I) is the
preferred meta-aramid.
[0018] The term "fibrids", as used herein, means very small,
nongranular, fibrous or film-like particles with at least one of
their three dimensions being of minor magnitude relative to the
largest dimension. These particles are prepared by precipitation of
a solution of polymeric material using a non-solvent under high
shear. The term "aramid fibrids", as used herein, means
non-granular film-like particles of aromatic polyamide having a
melting point or decomposition point above 320C. The fibrids have
an average length of 0.2 to 1 mm with a length-to-width aspect
ratio of 5:1 to 10:1. The thickness dimension is on the order of a
fraction of a micron. Such aramid fibrids, before being dried, can
be used wet and can be deposited as a binder physically entwined
about the aramid floc component of a paper.
[0019] The term "pulp", as used herein, means cellulose fibers or
fibrillated made-made fibers, which are refined or subjected to
some other special treatment to be fibrillated. The term "aramid
pulp" means aramid material having many fibrils, mostly attached to
fiber "trunks" but also unattached. The trunk 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 trunk and about 10 to 100 microns
long.
[0020] The term "essentially binder-free", as used herein, means
that any binder from a papermaking process present in the non-woven
sheet is present in an amount less than 3 weight percent,
preferably less than 2 weight percent, more preferably less than 1
weight percent, and most preferably 0 weight percent. These
percentages are based upon the total weight of binder from a
papermaking process and floc only. The term "binder from a
paper-making process", as used herein, means fibrids, pulp,
adhesives or polymeric materials that are typically used to bind
fibers together in a paper-making process. In the making of paper
or sheets from para-aramid floc, it has long been the practice to
include binders to assure that the paper fibers adhere together to
yield a sheet of acceptable strength and integrity. As an
alternative, the para-aramid papers have been made using pulp
instead of floc. Para-aramid pulp is refined and has many fibrils
which are believed to act as hooks or fasteners to hold together
adjacent particles in the paper and provide integrity to the is
finished paper sheet. Pulp typically has a specific surface area of
from 5 to 15 square meters per gram.
[0021] With the invention, there is no need to use a binder in the
wet-lay process and is preferred to not use a binder for the
reasons mentioned above. However, while the resultant product is
not as good, very small amounts of binder may be used within the
weight ranges above to make the non-woven sheet of the
invention.
[0022] Polymeric dispersants or processing aids or other polymeric
additives which could be used in the wet-lay process to improve
sheet uniformity, reduce the use of water, or any other purpose,
even though such materials may have some binding effect, their
primary purpose is not as a binder and therefore these materials
are not included in the meaning of the term "binders from a
papermaking process".
[0023] Wet-lay non-woven sheets and papers made from aramid floc
were not, until now, made using essentially binder-free aramid floc
because there was a belief that a floc-only sheet would not hold
together and would lack strength and integrity. Surprisingly, it
has been found that there is enough cohesiveness in the para-aramid
floc in wet and dry sheets to make acceptable non-woven sheets.
Therefore, it is possible to produce a continuous wet-lay non-woven
sheet from essentially binder-free aramid floc only using known
sheet-forming apparatuses and supporting screens in the drying
section of a papermaking machine.
[0024] A further surprising result of the invention is that an
aramid non-woven sheet made from para-aramid floc, which is
inherently relatively short, can be successfully hydroentangled. A
third surprising result is a dramatic increase of the strength of a
wet-laid non-woven sheet made from essentially binder-free
para-aramid when that sheet has been subjected to a hot calendering
process, whether or not that sheet has been hydroentangled. Until
now, there was a belief that it is necessary to have a binder
present in the sheet in order to increase the strength of a
wet-laid non-woven sheet made from para-aramid because it was
believed that the binder, when subjected to the hot calendering
process, exhibited a reversible transition from a solid to a soft
stage and back to a solid that fixed the densified structure and
increased the strength of the sheet.
[0025] The present invention is directed to a essentially
binder-free wet-laid non-woven sheet comprising aramid floc,
wherein the aramid floc includes at least 25 percent by weight of
para-aramid floc based on the total weight of aramid floc only.
Preferably the aramid floc contains at least 50 percent by weight,
and more preferably at least 75 percent by weight, and most
preferably 100 percent by weight, of para-aramid floc. If less than
25 percent by weight of para-aramid floc is used, the resulting
sheet will not have sufficient strength to be processed on a
papermaking machine.
[0026] The remaining aramid floc is preferably meta-aramid floc.
However, depending on the final application of the non-woven sheet,
other floc can be used in combination with the para-aramid floc.
For example, polypropylene, polyester or another thermoplastic floc
can be added to the para-aramid floc of the invention to produce,
for example, shaped forms from the sheet in a molding process. If
the floc includes one or more different flocs in addition to
para-aramid floc, the additional flocs are blended with the
para-aramid floc preferably in an aqueous dispersion.
[0027] The aramid floc used in the invention has an average length
of from 3 to 13 millimeters, preferably from 5 to 8 millimeters.
The para-aramid floc is straight floc obtained by cutting aramid
fiber or yarn without any further refining or other fibrillating
treatment of the floc. Crimped aramid floc may be blended with the
straight floc and used to make the non-woven sheets of the
invention. It is preferred that the aramid floc include at least 25
weight percent of straight floc based on the total weight of
straight floc and crimped floc only.
[0028] There is no limitation on the tex or on the cross-sectional
shape of the fiber used in the floc of this invention.
[0029] The wet-laid sheet of the invention may be made by any known
process for wet-lay forming a non-woven sheet. However, if the
non-woven sheet to be dried is not hydroentangled before the sheet
is dried, then a supporting screen is necessary in the drying stage
of the process. An example of such a process for making the sheet
of the invention includes the steps of:
[0030] a) preparing an aqueous dispersion of 0.005 to 0.08 weight
percent of essentially binder-free aramid floc that includes at
least 25 percent by weight of para-aramid floc;
[0031] b) pouring the dispersion onto a moving screen in a paper
making machine;
[0032] c) withdrawing water from the aqueous dispersion to leave a
wet non-woven felt; and
[0033] d) drying the wet paper felt on a supporting screen to make
a sheet of essentially binder-free aramid floc.
[0034] Optionally, the non-woven sheet may be hydroentangled before
or after being dried. Hydroentangling results in a wet-laid
non-woven sheet of aramid floc that exhibits an increase in density
and strength compared to a wet-laid non-woven sheet that has not
been hydroentangled. The hydroentangling may be performed by
conventional processes, such as is disclosed in U.S. Pat. No.
4,902,564, the text of which is hereby incorporated by
reference.
[0035] As a further option, the wet-laid non-woven sheet may be hot
calendered, which leads to an unexpected and significant increase
in the strength of the non-woven sheet of aramid floc. The
calendering temperature can be from 100 to 400C., preferably from
240 to 380C. It is preferred that the sheet to be hot calendered is
hydroentangled, but this is not a requirement.
[0036] The wet-laid non-woven sheet can be impregnated with resins,
such as epoxy, polyimide, phenolic, silicone, or other, or
combinations thereof, using conventional processes to make a
prepreg. The prepreg can then be made into a laminate using
conventional processes. These laminates can be made into wiring
boards or other composite structures.
[0037] The aramid non-woven sheets of the invention, in addition to
aramid floc and less than 3 weight percent of a binder from a
papermaking process, can include 15 to 85 weight percent filler and
0 to 30 weight percent other components. The fillers can be
inorganic materials such as diatomaceous earth, talc, carbon, and
the like. The other components can include other fibers, such as
glass fibers, or ceramic fibers, and the like.
[0038] Non-woven sheets of essentially binder-free para-aramid floc
can be made by a wet-lay manufacturing process that is simpler
compared to processes for wet-lay manufacturing of non-woven sheets
that include a binder from a papermaking process. Further, by
incorporating an essentially-binder free non-woven sheet, a final
composite structure that includes a resin-impregnated sheet of the
invention has better physical and mechanical properties at the same
testing conditions compared to a composite structure having a
non-woven sheet that includes a binder from the papermaking
process, because that binder is mostly an impurity that can cause
defects in the final composite structure.
[0039] In addition to their use in printed wiring boards, non-woven
sheets of this invention can be used in the manufacture of
composite structures such as multi-chip modules and electronic
interconnect devices. Also, the inventive sheets can be used in the
aircraft industry and other end uses of composites that include
para-aramid fibers.
TEST METHODS
[0040] The following test methods were used in the Examples below.
The tensile strength of the wet-laid non-woven sheets was measured
on an Instron-type testing machine using test specimens 2.54 cm
wide and a gage length of 18 cm, in accordance with ASTM D 828.
[0041] The thickness and basis weight of the non-woven sheets were
determined by measuring the thickness and the weight of an area of
a sample of the test non-woven sheet in accordance with ASTM D 645
and D646, respectively.
[0042] The peel strength of copper clad laminates was determined in
accordance with IPC-TM-650, Number 2.4.8.
[0043] The coefficient of thermal expansion (CTE) in the z-axis of
the laminates was determined by thermal mechanical analysis (TMA)
performed in accordance with IPC-TM-650, number 2.4.24.
[0044] The CTE in-plane of the laminates was determined by TMA in
accordance with IPC-TM-650, number 2.4.41.
[0045] The water absorption of the laminates was determined in
accordance with IPC-TM-650, Number 2.6.2.1.
EXAMPLES
Examples 1-6
[0046] Aqueous dispersions of para-aramid floc of varying length
and diameter were made, and wet-laid non-woven sheets were prepared
from those dispersions by wet-forming the dispersion on a Rotonier
machine (a combination of cylinder paper making machine and the
Fourdrinier) equipped with a horizontal through-air dryer with a
supportive metal screen (Kitchen & Perry Flat-Bed Thru Dryer).
The floc was from poly (paraphenylene terephthalamide) fiber sold
by E.I. du Pont de Nemours and Company under the trademark
KEVLAR.RTM. 49. No binders were added to the floc.
[0047] The tensile strength and elongation in both the machine
direction and the cross direction of the non-woven sheets made from
this para-aramid floc were measured and are shown in Table 1. These
data demonstrate that while all of the essentially binder-free
aramid floc resulted in a usable non-woven sheet, a length of
para-aramid floc of about 6-7 mm provided a better wet-laid
non-woven sheet compared with a floc of 3 mm or a floc of about 13
mm. Note that in Examples 5 and 6 a non-woven sheet of the
invention was made, but not in a continuous sheet, and therefore it
was not possible to perform tensile strength or elongation tests on
those sheets.
1TABLE 1 Tensile Para- Basis Thick- strength Elongation Ex. aramid
wt. ness N/m % No. floc used g/m2 mm MD CD MD CD 1 KEVLAR .RTM. 49
113 1.0 115 76 1.2 1.7 0.16 tex, 6.7 mm 2 KEVLAR .RTM. 49 190 1.7
140 92 2.0 3.0 0.16 tex, 6.7 mm 3 KEVLAR .RTM. 49 127 1.2 67 45 2.2
3.3 0.25 tex, 6.7 mm 4 KEVLAR .RTM. 49 201 1.9 86 65 2.0 2.1 0.25
tex, 6.7 mm 5 KEVLAR .RTM. 49 195 1.9 * * 0.25 tex, 3.3 mm 6 KEVLAR
.RTM. 49 207 2.0 * * 0.25 tex, 13 mm *Sheets made were not
continuous and standard tensile and elongation tests could not be
conducted.
Examples 7-9 and Comparative Example 10
[0048] Aqueous dispersions were made as in Examples 1-6 except that
the dispersions included both meta-aramid floc and para-aramid floc
at different weight ratios. These aqueous dispersions were made
into wet-laid non-woven sheets as in Examples 1-6.
[0049] The meta-aramid floc was poly (metaphenylene isophthalamide)
floc of a linear density of 0.22 tex and a length of 0.64 cm and is
sold by E.I. du Pont de Nemours and Company under trade name
NOMEX.RTM..
[0050] The para-aramid floc was made using poly (para-phenylene
terephthalamide) fiber sold by E.I. du Pont de Nemours and Company
under the trademark KEVLAR.RTM. 49, and had a linear density of
0.16 tex and a length of 6.7 mm. No binders were added to the
floc.
[0051] The tensile strength and elongation in both the machine
direction and the cross direction of the non-woven sheets were
measured and are shown in Table 2. These data show that if
para-aramid floc and meta-aramid floc are blended together and used
to make an essentially binder-free non-woven sheet of the
invention, it is necessary to have at least 25% of para-aramid floc
in such a blend to permit the continuous production of a wet-laid
non-woven sheet. Note that in Example 9 a non-woven sheet of the
invention was made but not in a continuous sheet and therefore it
was not possible to perform tensile strength or elongation tests on
that sheet.
2TABLE 2 Wt. % of meta- amarid floc in blend with Tensile para-
Basis Thick- strength Elongation Ex. amarid wt. ness N/m % No. floc
g/m2 mm MD CD MD CD 7 25 205 1.9 110 72 1.8 2.6 8 50 190 1.7 57 44
2.8 3.2 9 75 197 16 * * * * C10 100 201 ** ** ** ** ** *Sheet made
was not continuous and so standard tensile and elongation tests
could not be conducted. **Sheet made was too weak to be transited
from the forming wire to the drying section
Example 11
[0052] A wet-laid non-woven sheet was made as described in Examples
1-6, and had a basis weight of 374 g/m.sup.2. The non-woven sheet
was impregnated between two polyester screens with water-based
phenolic resin diluted to 15% of solids and then was dried at 120
C. to form a prepreg. The phenolic resin used was Durez 33-304 sold
by OxyChem Co.
[0053] The floc used was the same as in Examples 7-10. No binders
were added to the floc.
[0054] The prepreg produced contained 30.3 wt.% of phenolic resin
and had a density of 0.1 g/cm.sup.3 and a tensile strength of 14.7
and 7.3 kN/m in the machine and cross directions respectively.
These data show that an acceptable prepreg can be prepared from a
wet-laid aramid non-woven sheet of this invention immediately after
forming the sheet, without any hydroentangling or calendering of
the wet-laid sheet.
Examples 12-13
[0055] A wet-laid non-woven sheet was made as described in Examples
1-6 except that the dried formed sheet was hydroentangled, with
preliminary wetting, on a pilot washing machine.
[0056] A metal screen of 100 mesh was used as a carrier belt on the
washing machine. The hydroentangling process was conducted at a
speed of 23 m/min.
[0057] The para-aramid floc used was the same as in Examples 7-10.
No binders were added to the floc.
[0058] The sheet in Example 12 was hydroentangled by consecutive
treatment from both sides. One side was treated with water jets
from two manifolds, which had a 0.18 mm orifice diameter at 8
orifices per cm, at a pressure of 2.1 and 3.4 MPa respectively, and
from three manifolds, which had a 0.13 mm orifice diameter at 16
orifices per cm, and at a pressure of 5.5 MPa. Another side was
treated with water jets from two manifolds, which had a 0.18 mm
orifice diameter at 8 orifices per cm, and at a pressure of 2.1 and
3.4 MPa respectively, from three manifolds, which had a 0.13 mm
orifice diameter at 16 orifices per cm, at a pressure of 5.5 MPa,
and from one manifold, which had a 0.10 mm orifice diameter at 32
orifices per cm, and at a pressure of 5.5 MPa.
[0059] The sheet in Example 13 was hydroentangled by treatment only
from one side. The treatment was conducted consecutively with water
jets from two manifolds, which had a 0.18 mm orifice diameter at 8
orifices per cm, at a pressure of 2.1 and 3.4 MPa respectively, and
from three manifolds, which had a 0.13 mm orifice diameter at 16
orifices per cm, and at a pressure of 5.5 MPa.
[0060] Properties of hydroentangled materials are shown in Table 3.
These data show that hydroentangling of a wet-laid aramid non-woven
sheet from essentially binder-free aramid floc provides
densification and additional strengthening of the sheet compared to
a sheet that is not hydroentangled, such as the sheets in Examples
1-6.
3 TABLE 3 Basis Thick- Tensile strength wt. ness N/m % Elongation
g/m.sup.2 (mm) MD CD MD CD Example 92 0.64 130 89 1.7 2.6 12
Example 94 0.81 109 78 1.5 2.3 13 Example 67 0.42 66 48 1.5 2.2 14
Example 34 0.23 29 21 1.4 1.9 15 Example 33 0.20 38 34 1.6 2.2
16
Example 14
[0061] An aqueous dispersion was made of para-aramid floc, and a
wet-laid non-woven sheet was formed from this dispersion on a
Deltaformer inclined wire machine.
[0062] The material was collected in wet form after leaving the
wire and was hydroentangled on a pilot washing machine as in
Examples 12-13.
[0063] The para-aramid floc used was the same as in Examples 7-10.
No binders were added to the floc.
[0064] The non-woven material was hydroentangled by consecutive
treatment from both sides. One side was treated with water jets
from eight manifolds, which had a 0.13 mm orifice diameter at 16
orifices per cm, and at a pressure of 1.4, 5.5, 8.3, 11.0, 13.8,
13.8, 13.8, and 13.8 MPa respectively. Another side was treated
with water jets from four manifolds, which had a 0.13 mm orifice
diameter at 16 orifices per cm, and at a pressure of 3.4, 6.9,
10.3, and 13.8 MPa respectively.
[0065] The properties of the hydroentangled non-woven sheets are
shown in Table 3. These data show that wet-laid aramid non-woven
sheets of the invention can be successfully hydroentangled before
being dried. Further, when compared with the data in Examples
12-13, these data show that the non-woven sheets of the invention
can be hydroentangled before or after being dried and still provide
acceptable physical properties.
Example 15
[0066] An aqueous dispersion was made of 100 weight percent
para-aramid floc, and a wet-laid non-woven sheet was formed from
this dispersion in a handsheet mold and dried on a metal screen of
100 mesh. Afterwards, the sheet was hydroentangled, with
preliminary wetting, on a pilot washing machine as in Examples
12-13.
[0067] The para-aramid floc used was from poly (paraphenylene
terephthalamide) fiber sold by E.I. du Pont de Nemours and Company
under the trademark KEVLAR.RTM. 149 and had a linear density of
0.16 tex and a length of 6.7 mm. No binders were added to the
floc.
[0068] The non-woven material was hydroentangled by consecutive
treatment from both sides. One side was treated with water jets
from four manifolds, which had a 0.13 mm orifice diameter at 16
orifices per cm, at a pressure of 1.4, 3.4, 5.5, and 6.9 MPa
respectively. Another side was treated with water jets from four
manifolds, which had a 0.13 mm orifice diameter at 16 orifices per
cm, and at a pressure of 2.1, 4.8, 6.9, and 8.3 MPa
respectively.
[0069] The properties of the hydroentangled non-woven sheet are
shown in Table 3. These data show that the non-woven sheets of this
invention can be hydroentangled successfully using a para-aramid
floc and as KEVLAR.RTM. 149, which is characterized by a very high
modulus and a low moisture regain compared with KEVLAR.RTM. 49 or
KEVLAR.RTM. 29.
Example 16
[0070] A wet-laid non-woven sheet was formed as in Example 15
except that the para-aramid floc used was made from poly
(para-phenylene terephthalamide) fiber sold by E.I. du Pont de
Nemours and Company under the trademark KEVLAR.RTM. 29 and had a
linear density of 0.096 tex and a length 6.7 mm.
[0071] The non-woven material was hydroentangled by consecutive
treatment from both sides. Each side was treated with water jets
from four manifolds, which had 0.13 mm orifice diameter at 16
orifices per cm, at a pressure of 0.7, 2.1, 3.4, and 4.8 MPa
respectively.
[0072] The properties of the final non-woven sheet are shown in
Table 3. These data show that a reduction in the diameter of the
para-aramid floc can lead to some improvement in the mechanical
properties of a hydroentangled sheet made from a wet-laid non-woven
of essentially binder-free aramid floc.
Example 17
[0073] Two wet-laid non-woven sheets of different basis weights
were formed and hydroentangled as in Example 12. In addition, the
sheets were hot calendered at 300C. between two metal rolls each
having a diameter of 20.3 cm diameter and at a linear pressure of
about 2000 N/cm. The properties of the calendered sheets are shown
in Table 4. When compared with the data in Table 3, these data show
that calendering dramatically increases the strength of an aramid
non-woven sheet of this invention. Also, the data indicate that it
is possible to produce an aramid non-woven sheet having about the
same strength in both the machine and the cross directions of the
sheet.
4 TABLE 4 Basis Tensile strength weight Thickness (N/m) %
Elongation (g/m.sup.2) (mm) MD CD MD CD 67.8 0.067 1600 360 0.53
3.0 90.5 0.104 1240 1240 0.19 0.49
Example 18
[0074] A wet-laid non-woven sheet was formed and hydroentangled as
in Example 12 and was calendered as in Example 17. The sheet was
impregnated on a vertical prepregging tower using an epoxy resin
system with Tg about 165 C. The epoxy resin system used was L-1070
sold by Fortin Industries, Inc. A 2 ply copper clad laminate was
produced using a vacuum press. The final laminate contained 46.7%
of the resin.
[0075] Properties of the laminate are shown in Table 5 and are
compared with published values for a THERMOUNT.RTM. laminate based
on a DuPont Type E-200 reinforcement from KEVLAR.RTM. 49 floc
having a binder of meta-aramid (NOMEX.RTM.) fibrids. The data show
that for the same type of KEVLAR.RTM. floc and the same type of
resin in the final laminate, the wet-laid non-woven sheet of the
invention improves three very important properties for printing
wiring boards and similar applications compared to a standard
wet-laid non-woven sheet that contains greater than 3 weight
percent of a binder from the papermaking process: namely, CTE,
cooper peel strength, and moisture absorption.
5 TABLE 5 Typical value for THERMOUNT .RTM. laminate based on
DuPont Type E- 200 reinforcement with Laminate of epoxy Property
Example 18 (48% resin content) In-plane CTE 7.0 ppm/C 8.2-8.5
ppm/C* (average between MD and CD) Copper peel 11 N/cm 7-8 N/cm*
strength Moisture 0.23% 0.4-0.5% absorption *THERMOUNT .RTM.
Laminate and Prepreg Containing Type E210, E220, E230 Non-woven
Aramid Reinforcement For Printed Wiring Boards, Multichip Modules,
and Electronic Interconnect Devices (Product Bulletin, H-52775,
Rev. 5, 2/98)
* * * * *