U.S. patent application number 11/011637 was filed with the patent office on 2005-05-19 for polytetrafluoroethylene molded articles coated with fused fluoropolymer resin.
Invention is credited to Nishio, Takao.
Application Number | 20050106325 11/011637 |
Document ID | / |
Family ID | 13439188 |
Filed Date | 2005-05-19 |
United States Patent
Application |
20050106325 |
Kind Code |
A1 |
Nishio, Takao |
May 19, 2005 |
Polytetrafluoroethylene molded articles coated with fused
fluoropolymer resin
Abstract
The present invention relates to a molded article of
polytetrafluoroethylene or modified polytetrafluoroethylene having
a fluoropolymer resin coating, the coating comprising a
heat-flowable tetrafluoroethylene copolymer wherein the surface of
the coated article has a reduced roughness compared to the molded
article prior to coating. The coating for the molded article is
preferably a fused powder, most preferably formed by
electrostatically applying a fluoropolymer powder resin to the
molded PTFE article. In a preferred embodiment, the fluoropolymer
powder resin comprises a mixture of heat-flowable
tetrafluoroethylene copolymer powder and a polytetrafluoroethylene
that has a temperature of crystallization of at least 305.degree.
C. and a heat of crystallization of at least 50 J/g. The surfaces
of the articles are smoother than the original articles so that
they resist adhesion of chemical contaminants and have
applicability for chemical containers and transport pipes in the
rigorously clean environment of the semiconductor industry.
Inventors: |
Nishio, Takao;
(Shimizu-City, JP) |
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: |
13439188 |
Appl. No.: |
11/011637 |
Filed: |
December 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11011637 |
Dec 14, 2004 |
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10658083 |
Sep 9, 2003 |
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10658083 |
Sep 9, 2003 |
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09498215 |
Feb 4, 2000 |
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6673416 |
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Current U.S.
Class: |
427/384 ;
427/374.4 |
Current CPC
Class: |
C08L 2205/02 20130101;
C09D 127/18 20130101; Y10T 428/24355 20150115; Y10T 428/31544
20150401; C08J 7/0427 20200101; C08J 2427/00 20130101; C08J 7/044
20200101; C08J 2327/18 20130101; C08L 27/18 20130101; C09D 127/18
20130101; C08L 2666/04 20130101 |
Class at
Publication: |
427/384 ;
427/374.4 |
International
Class: |
B05D 003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 1999 |
JP |
H11-70701 |
Claims
What is claimed is:
1. A process for forming a smooth surface on a molded article of
polytetrafluoroethylene or modified polytetrafluoroethylene by
coating said article with a heat-flowable fluoropolymer powder
resin and heating the coated article to form a fused fluoropolymer
resin coating, wherein the surface of the coated article has a
reduced roughness compared to the molded article prior to
coating.
2. The process of claim 1 wherein said heat-flowable fluoropolymer
resin comprises a tetrafluoroethylene copolymer.
3. The process of claim 1 wherein said fluoropolymer resin coating
comprises a copolymer of tetrafluoroethylene and perfluoro(alkyl
vinyl ether) (PFA).
4. The process of claim 1 wherein said fluoropolymer resin coating
comprises a copolymer of tetrafluoroethylene and
hexafluoropropylene (FEP).
6. The process of claim 1 wherein said fluoropolymer resin coating
comprises a mixture of heat-flowable tetrafluoroethylene copolymer
and a polytetrafluoroethylene that has a temperature of
crystallization of at least 305.degree. C. and a heat of
crystallization of at least 50 J/g.
7. The process of claim 1 wherein said coating is formed by
electrostatically applying said fluoropolymer powder resin to said
molded PTFE article.
8. The process of claim 1 wherein the surface of the coated article
has a roughness that is reduced by at least 25% as compared to the
molded article prior to coating.
9. The process of claim 1 wherein the surface of the coated article
has a roughness that is reduced by at least 50% as compared to the
molded article prior to coating.
10. The process of claim 1 wherein the surface of the coated
article has a roughness that is reduced by at least 75% compared to
the molded article prior to coating.
11. The process of claim 1 wherein the coated surface has a center
line average roughness (R.sub.a) of less than 1.5 micrometers.
12. The process of claim 1 wherein the coated surface has a center
line average roughness (R.sub.a) of less than 1.0 micrometer.
13. The process of claim 1 wherein the coated surface has a center
line average roughness (R.sub.a) of less than 0.5 micrometer.
14. The process of claim 1 wherein the thickness of the fused
fluoropolymer resin coating is 100 .mu.m or less.
Description
FIELD OF INVENTION
[0001] The present invention relates to molded PTFE articles for
use as containers for high purity chemicals, typically in the area
of semiconductor manufacturing, or as a lining material for
transport pipes.
BACKGROUND OF THE INVENTION
[0002] Because of the excellent heat and chemical resistance of
polytetrafluoroethylene (hereafter PTFE) and modified
polytetrafluoroethylene, articles formed from these polymers are
extensively used in transport pipes that handle corrosive or high
temperature fluids or as corrosion resistant lining material for
tanks, machines, equipment, and the like.
[0003] However, generally, the surface of PTFE molded articles is
not sufficiently smooth. This is a cause for concern when such
articles are used as containers for high purity chemicals or
linings for transport pipes, since contaminants tend to adhere to
the surface and are difficult to remove even after attempts are
made to clean the articles. In particular, the removal of
contaminants has become a rigorous requirement in areas requiring
sophisticated physical properties, particularly in semiconductor
fabrication or the like.
BRIEF SUMMARY OF THE INVENTION
[0004] The present invention provides for a molded article of
polytetrafluoroethylene or modified polytetrafluoroethylene having
a fluoropolymer resin coating, the coating comprising a
heat-flowable tetrafluoroethylene copolymer wherein the surface of
the coated article has a reduced roughness as compared to the
molded article prior to coating. The coating for the molded article
is preferably a fused powder, most preferably formed by
electrostatically applying a fluoropolymer powder resin to the
molded PTFE article. In a preferred embodiment the fluoropolymer
powder resin comprises a mixture of heat-flowable
tetrafluoroethylene copolymer powder and a polytetrafluoroethylene
powder that has a temperature of crystallization of at least
305.degree. C. and a heat of crystallization of at least 50
J/g.
[0005] The invention further provides for a process for forming a
smooth surface on a molded article of polytetrafluoroethylene or
modified polytetrafluoroethylene by coating the article with a
heat-flowable fluoropolymer powder resin and heating the coated
article to form a fused fluoropolymer resin coating wherein the
surface of the coated article has a reduced roughness compared to
the molded article prior to coating.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0006] FIG. 1 is a three-dimensional profile display of the surface
of a molded PTFE article (Example 3) coated with a mixture of PFA
powder and low molecular weight PTFE powder in accordance with the
present invention, measured using a laser type three-dimensional
surface roughness measuring instrument.
[0007] FIG. 2 is a three-dimensional profile display of the surface
of a molded PTFE article (Comparative Example 1) without a
heat-flowable coating measured using a laser type three-dimensional
surface roughness measuring instrument.
DETAILED DESCRIPTION OF THE INVENTION
[0008] PTFEs, because of their excellent characteristics such as
heat resistance and chemical resistance, have been known to exhibit
satisfactory performance when used as containers for high purity
chemicals. Previously, they were used with no modification. However
in recent years, in high technology industries, particularly, in
semiconductor manufacturing areas, there is a stringent requirement
for removing contaminants, so that the existing PTFE alone can no
longer satisfy the requirement. The present invention satisfies the
need for a molded article having the heat and chemical resistance
properties of PTFE and in addition having a smoother surface that
resists adhesion of contaminants to the surface. Specifically, a
molded PTFE article with a smoother surface can be prepared by
electrostatically powder coating the surface of a PTFE molded
article with a fluoropolymer powder resin. The fluoropolymer powder
resin comprises a heat-flowable tetrafluoroethylene copolymer.
Preparation of an article with this electrostatic coating permits
the entire surface of the article, regardless of configuration, to
be covered with heat-flowable fluoropolymer powder resin, thereby
considerably improving the surface smoothness of the molded
polytetrafluoroethylene article.
[0009] By smoother surface is meant that the surface of the coated
article has a reduced roughness as compared to a molded article
prior to coating.
[0010] % Reduced Roughness is calculated by the following equation
(1): 1 Surface Roughness ( uncoated article ) - Surface Roughness (
coated article ) Surface Roughness ( uncoated article ) .times. 100
( 1 )
[0011] The coated articles of the present invention preferably have
a surface roughness that is reduced by at least 25%, more
preferably by at least 50% and most preferably by at least 75% as
compared to a molded article prior to coating. The coated surface
of the molded articles of this invention have a center line average
roughness (R.sub.a) of less than 1.5 micrometers, preferably less
than 1.0 micrometer, and most preferably less than 0.5
micrometer.
[0012] Molded PTFE
[0013] PTFE molded articles, substrates in this invention, are
molded articles obtained by molding tetrafluoroethylene homopolymer
(PTFE) or modified polytetrafluoroethylene. By modified PTFE is
meant that PTFE contains a small amount of comonomer modifier which
improves film-forming capability during baking (fusing), such as
perfluoroolefin, notably hexafluoropropylene (HFP) or
perfluoro(alkyl vinyl ether), notably wherein the alkyl group
contains 1 to 5 carbon atoms, with perfluoro(propyl vinyl ether)
(PPVE) being preferred or fluoroalkyl ethylene or
chlorotrifluoroethylene. The amount of such modifier will be
insufficient to confer melt-fabricability to the PTFE, generally
being no more than 1 wt % . The PTFE, also for simplicity, can have
a single melt viscosity, usually at least I.times.10.sup.9 Pa.s,
but a mixture of PTFEs having different melt viscosities can be
used to form the molded fluoropolymer component.
[0014] Heat-Flowable Fluoropolymer
[0015] Heat-flowable fluoropolymer resins that can be used in this
invention are copolymers of tetrafluoroethylene and other
comonomers that melt and liquify to flow at temperatures above
their melting points. Examples of such copolymers include
copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl ether)
(PFA) such as perfluoro(propyl vinyl ether), and copolymers of
tetrafluoroethylene and hexafluoropropylene (FEP). "Heat-flowable"
as used herein means that the fluoropolymer resin means that
particles of the fluororesin will flow and fuse together upon
heating above their melting temperature.
[0016] Preferred among the heat-flowable fluoropolymer resins that
coat the surface of PTFE molded articles, is PFA powder that is a
crystalline copolymer powder of tetrafluoroethylene and
perfluoro(alkyl vinyl ether), the copolymer powder having a
perfluoro(alkyl vinyl ether) content in the copolymer of 1-10% by
weight. Suitable perfluoro(alkyl vinyl ether) comonomers are
perfluoro(propyl vinyl ether), perfluoro(ethyl vinyl ether), and
perfluoro(methyl vinyl ether).
[0017] The copolymer powder is a heat-flowable fluoropolymer resin
that permits melt molding such as melt extrusion molding, injection
molding and the like, and preferably has a melt flow rate (MFR) at
372.degree. C..+-.1 .degree. C. of 0.5-500 g/10 minutes, more
preferably 0.5-50 g/10 minutes.
[0018] Another useful heat-flowable fluoropolymer resin is FEP
powder, which is a copolymer powder of tetrafluoroethylene and
hexafluoropropylene where the hexafluoropropylene content in the
copolymer is 10-15% by weight of the copolymer powder. This
copolymer powder is a heat-flowable fluoropolymer resin that
permits melt molding such as melt extrusion molding, injection
molding, or the like.
[0019] Low Molecular Weight PTFE
[0020] The above copolymer powders e.g., PFA or FEP can be used
alone, but the use of a mixture with a specific low molecular
weight PTFE can further improve surface smoothness. Such a PFA
powder that is most suitable for the objectives of the invention is
a mixture of a PFA powder and a polytetrafluoroethylene powder
having a temperature of crystallization of at least 305.degree. C.
and a heat of crystallization of at least 50 J/g. The average
particle size of low molecular weight PTFE is less than 100
micrometers, and preferably 2-20 micrometers. A PFA composition
containing low molecular weight PTFE for use as a molding
composition is previously described in U.S. Pat. Nos. 5,473,018 and
5,603,999. Alternatively, the low molecular weight PTFE may be
added in the form of dispersion wherein the average particle size
is 0.05 micrometer to 1 micrometer.
[0021] Use of a conventional PTFE having physical properties other
than the low molecular weight PTFE described above as a component
will make it difficult to achieve a smooth coating onto a substrate
material by electrostatic coating, failing to give a PTFE molded
article with excellent surface smoothness. In the above
composition, the amount of PTFE to be incorporated is 0.01-50% by
weight, particularly 0.01-4% by weight with respect to the
composition. Using too little PTFE reduces the effect of improving
surface smoothness, while using a level exceeding 50% by weight
produces a poor coating onto the substrate.
[0022] Electrostatic Powder Coatinq Procedure
[0023] The present invention calls for electrostatically powder
coating the surface of a PTFE molded article substrate with a
heat-flowable fluoropolymer resin e.g., PFA powder or an FEP powder
for improved surface smoothness. PTFE molded articles, because of
their excellent water repellency and oil repellency features, are
difficult to surface-coat. It is extremely difficult to coat the
surface of a PTFE molded article with PFA or FEP by an impregnation
coating or similar techniques. Also, there are limitations on the
available configurations of coatable PTFE molded articles even when
the PTFE molded article is pretreated to make the surface of the
molded article more receptive to the coating. However, it has been
discovered that use of electrostatic coating of a heat-flowable
fluoropolymer powder resin, e.g., PFA powder or FEP powder, readily
provides an excellent coating and removes previous limitations with
respect to the configuration of the PTFE molded article to be
coated. Electrostatically powder coating using a PFA powder or a
FEP powder permits controlling the thickness of the film generated
on the surface of a PTFE molded article, making it possible to
provide a film thickness of 100 .mu.m or below. Thin films can
produce a smooth surface on large articles of molded PTFE. The use
of thin film coatings permits economic fabrication of large
articles producing surfaces that can resist adhesion of
contaminants.
[0024] There are no particular limitations as to the method of
electrostatically powder coating the surface of a PTFE molded
article with a heat-flowable fluoropolymer resin. Use of any
conventional electrostatic powder coating method/equipment is
suitable for the application of the powder to produce smooth,
coated, molded PTFE articles of this invention.
[0025] A PTFE molded article of this invention, obtained by coating
the surface with a heat-flowable fluoropolymer resin retains the
excellent physical properties of fluororesins, such as heat
resistance and chemical resistance, while exhibiting excellent
surface smoothness. The surface smoothness of the article
substantially suppresses contamination in fluid handling equipment
and therefore may be adapted to a broad range of applications in
the area of precision industry materials such as in semiconductor
processing areas.
Test Methods
[0026] Temperature of Crystallization
[0027] A Perkin Elmer differential scanning calorimeter DSC Model 7
is used to determine Temperature of Crystallization. A 5 mg sample
is weighed in a dedicated aluminum pan, crimped by means of a
dedicated crimper, mounted in a DSC instrument, and the sample is
heated from 200.degree. C. to 380.degree. C. at 10.degree. C./min.
After the sample is held one minute at 380.degree. C., it is cooled
from 380.degree. C. to 200.degree. C. at 10.degree. C./min,
producing a crystallization curve from which the crystallization
peak temperature is obtained as the temperature of crystallization
(Tc).
[0028] Heat of Crystallization
[0029] The heat of crystallization (Hc) is obtained from the
crystallization curve by connecting with a straight line the point
where the curve departs from the base line to the point at which
the curve returns to the base line before and after a given
crystallization peak and measuring the peak area enclosed.
[0030] Surface Roughness
[0031] The center line average roughness (Ra) of the surface of
PTFE molded articles is measured using a laser type
three-dimensional surface roughness measurement instrument
(manufactured by Laser Tex, Model 1LM21).
EXAMPLES
[0032] The present invention is now more specifically described
using the examples below. The compositions, physical properties,
and manufacturing processes and the like, of the fluoropolymer
resins used as substrates and as coatings in these examples and
comparative examples are given below.
[0033] Fluoropolymer Resins
[0034] (1) PTFE Powder:
[0035] Average Particle Size 301 .mu.m
[0036] "Teflon 170-J" (Registered trademark, Mitsui DuPont
[0037] Fluorochemicals KK)
[0038] (2) PFA Powder:
[0039] Average Particle Size 28 .mu.m
[0040] Manufactured by Mitsui DuPont Fluorochemicals KK
[0041] Copolymer of tetrafluoroethylene and perfluoro(propyl vinyl
ether) (PFA)
[0042] [perfluoro(propyl vinyl ether) content 3.6 wt %]
[0043] (3) FEP Powder:
[0044] Average Particle Size 32 .mu.m
[0045] Manufactured by Mitsui DuPont Fluorochemicals KK
[0046] Copolymer of tetrafluoroethylene and hexafluoropropylene
[0047] (FEP) [hexafluoropropylene content 12.0 wt %]
[0048] (4) Low Molecular Weight PTFE Powder
[0049] Average Particle Size 2- 20 .mu.m
[0050] Temperature of Crystallization 305.degree. C. or higher
[0051] Heat of Crystallization 50 J/g or higher.
[0052] PTFE Molded Article
[0053] PTFE molded articles are formed by filling a mold, 170 mm
long.times.170 mm wide.times.15 mm thick with 350 g of PTFE powder
(1) and compressing at a pressure of 150 kg/cm.sup.2 and a
compression speed of 10 mm/min until a thickness of 4 mm is
reached. After having reached the desired thickness, the pressure
is held 1 minute at 150 kg/cm.sup.2, followed by deaerating to give
a PTFE pre-molded article.
[0054] The resulting PTFE pre-form molded article is placed on a
metal plate and sintered according to the following temperature
cycle.
[0055] (i) Heating from room temperature to 370.degree. C. in 2
hours.
[0056] (ii) Holding at 370.degree. C. for 1 hour.
[0057] (iii) Cooling from 370.degree. C. to 100.degree. C. at the
rate of -30.degree. C./hr.
[0058] (iv) On reaching 100.degree. C., the sample is removed from
the oven and cooled to room temperature.
[0059] A PTFE molded article (sheet), 170 mm.times.170 mm.times.4
mm thick comprising 350 g of PTFE powder is produced.
Examples 1 and 2
[0060] A heat-flowable fluoropolymer resin of PFA (2) or FEP (3)
powder, as indicated in Table 1, is electrostatically spray coated
onto a molded PTFE sheet prepared as described above. The sheet is
grounded, 25 cm away. Coating is conducted at an electrical voltage
of 10 kV (negative) and a rate of discharge of 50 mg/min using an
electrostatic powder coater (manufactured by Onoda Cement KK;
GX-200T) and an electrostatic powder coating gun (Onoda Cement KK
GX-107). The applied powder weight is uniformly 2.2 g, thereby
generating a 100 .mu.m coated thickness after sintering. The
coating environment is at a temperature 25.degree. C. and a
moisture level of 60% RH. The coated PTFE sheet is sintered in a
forced air circulation oven at 370.degree. C. for 30 minutes and
then cooled to room temperature producing a molded PTFE article
coated with fused fluoropolymer resin of heat-flowable PFA or
FEP.
[0061] The center line average roughness (Ra) of the surface of the
resulting fused fluoropolymer coated PTFE sheet is measured using a
laser type three dimensional surface roughness measurement
instrument and the results are shown in Table 1.
Example 3
[0062] A molded PTFE sheet prepared as described above is
electrostatically powder coated in a manner similar to that of
Example 1. However, in this example the powder is a mixture of 99%
by weight of the PFA powder (2) and 1% of a low molecular weight
PTFE powder (4) having a temperature of crystallization of at least
305.degree. C. and heat of crystallization of at least 50 J/g,
thereby obtaining a fused fluoropolymer resin coated PTFE
sheet.
[0063] Center line average roughness of the surface of the
resultant PTFE sheet coated with a PFA containing a low molecular
weight PTFE is measured using a laser type three dimensional
surface roughness measurement instrument. The results are given in
Table 1. A three-dimensional profile obtained using this apparatus
is shown in FIG. 1.
Comparative Example 1
[0064] A molded PTFE sheet prepared as described above but not
having a fused fluoropolymer powder resin coating, e.g., without
any modification, is subjected to measurement of its surface center
line average roughness (Ra) using a laser type three dimensional
surface roughness measurement instrument. The results are given in
Table 1. A three-dimensional profile obtained using this apparatus
is also given in FIG. 2.
1 TABLE 1 Example 1 Example 2 Example 3 Comp. Ex. 1 Substrate
Material PTFE PTFE PTFE PTFE Molded article Molded article Molded
article Molded article Fluoropolymer PFA (wt %) 100 -- 99 -- resin
Powder FEP (wt %) -- 100 -- -- Low molecular -- -- 1 -- wt PTFE (wt
%) Method of coating Electrostatic Electrostatic Electrostatic --
powder coating powder coating powder coating Center line average
roughness (.mu.m) 0.33 0.25 0.21 2.15
[0065] The results of Table 1 (FIGS. 1 and 2) clearly show that in
contrast to the surface of the unmodified PTFE molded articles
having rough surfaces with indentations and protrusions, the fused
fluoropolymer resin coated PTFE molded articles of this invention
exhibit substantially improved surface smoothness. The coated
molded articles of Examples 1, 2 and 3 with average surface
roughness of 0.33, 0.25 and 0.21 respectively have a roughness that
is reduced by 85%, 88% and 90% respectively as compared to the
molded article with no coating of Comparative Example 1.
* * * * *