Fluorine-containing synthetic resin shaped articles having improved surface properties and a method for the preparation thereof

The invention provides a shaped article of a fluorine-containing synthetic resin having improved surface properties such as increased wettability with water, printability, susceptibility to adhesive bonding and less accumulation of static electricity. The improved shaped article is obtained by subjecting the surface of the article to exposure to low temperature plasma generated in a low pressure atmosphere of a nitrogen-containing gaseous organic compound such as amines, imides and amides.

BACKGROUND OF THE INVENTION 
The present invention relates to a shaped article of a fluorine-containing 
synthetic resin having improved surface properties and a method for the 
preparation thereof. More particularly, the invention relates to a shaped 
article of a fluorine-containing synthetic resin having increased 
wettability, printability, susceptibility to adhesive bonding, moderate 
antistatic performance and the like improved surface properties and a 
method for the preparation of such an improved shaped article of a 
fluorine-containing synthetic resin by the method of treatment of the 
article with low temperature plasma produced in specific gaseous 
atmosphere. 
As is well known, shaped articles of fluorine-containing synthetic resins, 
typically represented by polytetrafluoroethylene resins, are very 
excellent in general in their chemical stability and heat resistance as 
well as in their electric properties, very low coefficient of friction and 
weathering resistance. On the other hand, some of their unique surface 
properties cause several serious problems in the practical use of such 
shaped articles. Specifically, the surfaces of shaped articles of 
fluorine-containing synthetic resins have poor wettability with water, 
poor receptivity of printing inks, insusceptibility to adhesive bonding 
and extremely strong accumulation of static electricity to cause various 
disadvantages and inconveniences in the practical application of them so 
that their application field is narrowly limited. 
With an object to solve the above described problems in the shaped articles 
of a fluorine-containing synthetic resin, several methods have been 
proposed in the prior art in which the surface of the shaped article is 
subjected to the treatment by electric corona discharge or to the exposure 
to an atmosphere of low temperature plasma of an inorganic gas to cause 
surface oxidation resulting in the improvements of the surface properties 
to some extent. These methods are, however, far from satisfactory because 
the improvement obtained thereby is insufficiently low and, moreover, the 
effect obtained by the above mentioned treatment has very poor durability 
and permanency. 
On the other hand, several attempts have been made by the treatment with 
chemicals of graft-copolymerization on the surface by utilizing 
irradiation with actinic rays such as electron beams, ultraviolet light, 
gamma rays and the like in order to obtain improved surface properties of 
the shaped articles. These methods are, however, disadvantageous from the 
practical standpoint due to the complicated process involved in the method 
and difficulty in obtaining desired degree of improvement presumably due 
to the extreme chemical stability of the fluorine-containing synthetic 
resins. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a novel and 
improved shaped article of a fluorine-containing synthetic resin having 
improved surface properties such as increased wettability and 
printability, increased susceptibility to adhesive bonding and moderate 
antistatic performance. It is also an object of the present invention to 
provide a novel and improved method for the preparation of a shaped 
article of a fluorine-containing synthetic resin having improved surface 
properties as mentioned above. 
Thus, the shaped article of a fluorine-containing synthetic resin provided 
by the invention as a result of the extensive investigations undertaken by 
the inventors is characteristic in that the surface thereof has been 
subjected to exposure to an atmosphere of low temperature plasma under a 
pressure of 10 Torr or below of a nitrogen-containing gaseous organic 
compound represented by the general formula 
##STR1## 
in which R.sup.1, R.sup.7, and R.sup.9 are each a substituted or 
unsubstituted monovalent hydrocarbon group, R.sup.2, R.sup.3, R.sup.4, 
R.sup.5, R.sup.6, R.sup.10 and R.sup.11 are each a hydrogen atom or a 
substituted or unsubstituted monovalent hydrocarbon group and R.sup.8 is a 
substituted or unsubstituted divalent hydrocarbon group. 
Consequently, the method of the present invention comprises subjecting the 
surface of the shaped article of a fluorine-containing synthetic resin to 
exposure to an atmosphere of low temperature plasma of a 
nitrogen-containing gaseous organic compound specified above under a 
pressure of 10 Torr or below. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is described above, the inventive shaped article of a 
fluorine-containing synthetic resin has a surface having been subjected to 
a low temperature plasma treatment in a gaseous atmosphere of a specified 
nitrogen-containing organic compound and the effect of improvement in the 
surface properties obtained by this treatment is very remarkable and 
outstandingly excellent in the durability and permanency in comparison 
with the effects obtained in a low temperature plasma treatment using a 
conventional inorganic or organic gaseous compound. This unexpectedly 
remarkable improvement obtained by the plasma treatment of a specific 
nitrogen-containing organic compound is presumably due to the formation of 
an extremely thin layer of modified nature of the surface having a 
thickness of 0.1 .mu.m or smaller, of which the adhesion to the body of 
the fluorine-containing synthetic resin is extremely firm and the velocity 
of its formation is much larger than on the surface of other types of 
synthetic resins resulting in the very easy and rapid appearance of the 
effect of improvement in the surface properties of the inventive article. 
As indicated in the analytical examination by the surface-reflective 
infrared spectrophotometry and electron spectrometry such as the ESCA 
method, such a modified layer actually exists on the surface of the shaped 
article and can be identified to be a layer of a crosslinked polymer 
including structures with linkages of R-N, R'.dbd.N and R".tbd.N, where R, 
R' and R" are each an organic group, and the presence of such a layer is 
directly visible by use of a scanning-type or transmission-type electron 
microscope. 
Needless to say, the effect of the plasma treatment is limited to the very 
surface of the shaped article and the excellent properties of the bulk 
body of the article inherent to the fluorine-containing synthetic resin 
are little or not at all affected by the plasma treatment including the 
high chemical stability and heat resistance. 
The fluorine-containing synthetic resin of which the inventive shaped 
article is formed is not particularly limitative including various types 
of homopolymers and copolymers having carbon-to-fluorine linkages in the 
molecule in general. Exemplary of the fluorine-containing synthetic resins 
to which the present invention is applicable are polytetrafluoroethylenes, 
polychlorotrifluoroethylenes, polyvinylidene fluorides, polyvinyl 
fluorides, copolymers of tetrafluoroethylene and hexafluoropropene, 
copolymers of tetrafluoroethylene and a perfluoroalkyl vinyl ether, 
copolymers of tetrafluoroethylene and ethylene, copolymers of 
chlorotrifluoroethylene and ethylene, copolymers of vinylidene fluoride 
and hexafluoro isobutene and the like. 
The shaped article of the above named fluorine-containing synthetic resin 
may be prepared in any conventional molding method without particular 
limitations including compression molding, ram extrusion molding, paste 
extrusion molding, calendering and dispersion fabrication as well as 
extrusion molding, injection molding, melt shaping and the like. The form 
or configuration of the shaped article is also not particularly limitative 
provided that uniform exposure of the surface of the article to the plasma 
atmosphere is ensured. The shaped article may be fabricated with the resin 
formulated with various kinds of additives conventionally used in the 
fabrication of fluorine-containing synthetic resins including fillers such 
as glass fibers, graphite powder, molybdenum disulfide, bronze powder and 
the like, surface active agents, emulsifiers, stabilizers and plasticizers 
such as fluorocarbon oils. The effect of improvement in the surface 
properties by the plasma treatment is little affected by the formulation 
of these additives. 
The most characteristic feature in the inventive shaped article is that the 
surface thereof has been subjected to the treatment with low temperature 
plasma of a specific nitrogen-containing gaseous organic compound 
represented by either one of the above given three general formulas. 
Exemplary of such nitrogen-containing organic compounds are amines, 
imines, amides and imides as well as derivatives thereof including methyl 
amine, dimethyl amine, trimethyl amine, ethyl amine, diethyl amine, 
triethyl amine, n-propyl amine, di-n-propyl amine, tri-n-propyl amine, 
n-butyl amine, n-amyl amine, n-hexyl amine, lauryl amine, ethylene 
diamine, trimethylene diamine, hexamethylene diamine, ethanol amine, 
diethanol amine, allyl amine, aniline, N-methyl aniline, allyl dimethyl 
amine, di(2-aminoethyl) ether, 1-dimethylamino-2-chloro ethane, 
cyclopropyl amine, cyclohexyl amine, ethylene imine, 1-methyl ethylene 
imine, formamide, N,N-dimethyl formamide, capronamide, aminoacetal, benzyl 
amine, piperidine, pyrrolidine, morpholine and the like as well as 
derivatives thereof, of which the non-heterocyclic compounds are 
preferred. When the nitrogen-containing organic compound has a relatively 
high boiling point or relatively low vapor pressure at room temperature 
not suitable for introduction to the plasma atmosphere as such, sufficient 
vapor pressure can be obtained by heating the compound. 
The inventive shaped article of the fluorine-containing synthetic resin is 
obtained by subjecting the shaped article to exposure to the atmosphere of 
low temperature plasma of the above named nitrogen-containing organic 
compound so that the shaped article is imparted with remarkably improved 
surface properties such as excellent wettability, printability, 
susceptibility to adhesive bonding and adequately controlled antistatic 
performance and these improved surface properties are retained durably and 
permanently during the serviceable life of the shaped article. 
The plasma treatment according to the present invention is performed by 
generating low temperature plasma in a plasma chamber containing the 
shaped article while the pressure inside the plasma chamber is maintained 
at 10 Torr or below by continuously introducing the nitrogen-containing 
gaseous organic compound at a controlled rate into the plasma chamber with 
simultaneous pumping out so that the surface of the shaped article is 
exposed to the atmosphere of low temperature plasma of the gas for a 
desired length of time. 
The pressure inside the plasma chamber should be maintained throughout not 
to exceed 10 Torr or, preferably, in the range from 0.001 Torr to 1 Torr 
in order to obtain excellent and sufficient effect of improvement of the 
surface properties. When the pressure in the plasma atmosphere is 
increased over 10 Torr, the desired effect of improvement rapidly 
decreases in contrast to the conventional phenomena encountered in the 
plasma treatment within an atmosphere of other gases and in the plasma 
polymerization. It is of course that an extremely low pressure below 0.001 
Torr is undesirable due to the instability of the electric plasma 
discharge across such an atmosphere. 
The conditions for generating low temperature plasma in a low-pressure 
atmosphere of a gas is well known in the art. For example, a 
high-frequency electric power of 10 watts to 100 kilowatts at a frequency 
of 10 kHz to 100 MHz is supplied to the electrodes of a plasma chamber, 
the electrodes being installed either inside or outside of the plasma 
chamber, while the pressure inside the chamber is maintained at a desired 
low pressure by introducing the specified gas at a controlled rate. 
Sufficient effects of the plasma treatment can be obtained regardless of 
the type of the electric discharge which may be glow discharge or corona 
discharge. The length of time for the plasma treatment may widely differ 
depending on various factors including types of the synthetic resins, 
desired effects of improvements, types of the nitrogen-containing organic 
gaseous compound, conditions of plasma discharge and so on but sufficient 
improvements can be obtained usually by the treatment for a length of time 
in the range from a few seconds to several tens of minutes. 
It is of course optional that the above specified nitrogen-containing 
gaseous organic compounds may be used either singly or as a combination of 
two kinds or more according to need expecting a possible synergistic 
effect. It is further optional that the gaseous atmosphere of the low 
temperature plasma is constituted of a gaseous mixture of the 
nitrogen-containing organic compound and an organic gaseous compound of 
other types or an inorganic gas such as inert gases, e.g. helium and 
argon, nitrogen, oxygen, air, hydrogen, water vapor, carbon dioxide, 
carbon monoxide and the like. The combined use of such additional gases 
may be effective in some cases to give additional secondary improvements 
on the properties other than those as the primary object of the present 
invention. It should be noted, however, that the partial pressure of such 
an additional gas in the plasma chamber is desirably 0.5 Torr or below or 
smaller than one tenth of the partial pressure of the essential gaseous 
component of the specified nitrogen-containing organic compound. 
Following are the examples to illustrate the invention in further detail 
but not to limit the scope of the invention in any way. In the following 
examples, the effectiveness of the plasma treatment of the shaped articles 
of fluorine-containing synthetic resins according to the invention was 
evaluated by measuring several surface properties of the articles before 
and after the treatment including the contact angle of water on the 
surface as a measure of the wettability, receptivity of printing inks as a 
measure of the printability, adhesive bonding strength by use of two kinds 
of adhesives and charge voltage by rubbing as a measure of the antistatic 
performance. The procedures for the measurement of these items were as 
follows. 
Receptivity of printing inks: a printing ink was uniformly applied on to 
the surface of the test specimen and the completely dried film of the 
printing ink was cut with a sharp knife in a checkerboard-like manner at 1 
mm intervals of the incision lines in each direction to make 100 squares 
of each 1 mm.times.1 mm wide. Then, a commercially available adhesive tape 
of sufficient width was applied and bonded by pressing on to the area of 
the specimen surface cut in the above manner and the adhesive tape was 
peeled off at a peeling velocity of 10 cm/second to examine the number of 
the 1 mm.times.1 mm squares of the ink film left on the surface unremoved 
by the adhesive tape. 
Susceptibility to adhesive bonding: two pieces of the test specimen each 1 
inch wide and 3 inches long were prepared. One of them was coated with an 
adhesive uniformly on the 1 inch.times.1 inch area at one end and the 
other test piece was exactly laid thereon. After curing of the adhesive at 
50.degree. C. for 7 days, the test pieces were pulled apart at a pulling 
velocity of 200 mm/minute in a 90.degree. direction to determine the 
strength of adhesive bonding in kg/inch. The adhesive used was either 
Araldite (a tradename of a modified epoxy adhesive manufactured by Ciba 
Geigy Co., hereinafter referred to as adhesive A) or Bond KU15A (a 
tradename of a urethane adhesive manufactured by Konishi & Co., 
hereinafter referred to as adhesive B). 
Charge voltage by rubbing: the test specimen was rubbed in a rotary static 
tester and the charge voltage was determined after 1 minute from the 
beginning of rubbing. The rubbing body was a cotton cloth under a tension 
of 200 g and the rubbing rate on the test surface was 750 times per minute 
.

EXAMPLE 1 
(Experiments No. 1 and No. 2) 
A sheet of polytetrafluoroethylene resin (Teflon TFE, a product by E. I. 
DuPont Co.) was placed in a plasma chamber of a low temperature plasma 
generator and, after evacuation of the chamber to a pressure of 10.sup.-4 
Torr, vapor of ethylamine was introduced into the chamber at a controlled 
rate to give a constant inside pressure of 0.1 Torr. Low temperature 
plasma was generated for 2 minutes inside the plasma chamber by applying a 
high frequency electric power of 400 watts at a frequency of 13.56 MHz to 
the electrodes so as to expose the surface of the resin sheet to the low 
temperature plasma. 
Measurement of the surface properties of the test specimen undertaken 
before (Experiment No. 1) and after (Experiment No. 2) the plasma 
treatment in the above described manner gave the results shown in the 
Table given below. 
EXAMPLE 2 
(Experiment No. 3) 
The same polytetrafluoroethylene resin sheet as used in Example 1 was 
plasma-treated in substantially the same manner as in Example 1 excepting 
the replacement of ethylamine with methylamine, reduction of the pressure 
from 0.1 Torr to 0.05 Torr, decrease of the high frequency electric power 
from 400 watts to 200 watts and extension of the treatment time from 2 
minutes to 5 minutes. 
The Table gives the results of the surface property measurement of the thus 
plasma-treated resin sheet. 
EXAMPLE 3 
(Experiments No. 4 to No. 7) 
The same polytetrafluoroethylene resin sheet as used in Example 1 was 
placed in the plasma chamber and, after evacuation of the chamber to a 
pressure of 10.sup.-5 Torr, air was introduced into the chamber at a 
controlled rate to give a constant inside pressure of 0.05 Torr or 0.1 
Torr. Then, vapor of methylamine was introduced into the chamber at a 
controlled rate so as to give a constant partial pressure of from 0.1 to 
1.5 Torr of the methylamine vapor mixed with the air inside the chamber. 
Low temperature plasma was generated inside the plasma chamber for 20 
seconds by applying a high frequency electric power of 3 kilowatts at a 
frequency of 110 kHz to the electrodes so as to expose the surface of the 
resin sheet to the low temperature plasma. 
The Table below gives the results of the surface property measurement of 
the thus treated resin sheets along with the partial pressures of air and 
methylamine vapor. 
EXAMPLE 4 
(Experiments No. 8 and No. 9) 
A sheet of polyvinyl fluoride resin (Tedlar, a product by E. I. Du Pont 
Co.) was placed in the plasma chamber of the plasma generator and, after 
evacuation of the chamber to a pressure of 10.sup.-2 Torr, argon gas was 
introduced into the chamber at a controlled rate so as to give a constant 
pressure of 0.2 Torr under flow of argon gas. Then, vapor of allylamine 
was introduced into the chamber at a controlled rate and mixed with the 
argon gas so as to give partial pressures of argon and allylamine of each 
0.2 Torr. Low temperature plasma was generated in the thus controlled 
inside atmosphere of the plasma chamber for 1 minute by applying a high 
frequency electric power of 1 kilowatt at a frequency of 13.56 MHz to the 
electrodes so as to expose the surface of the resin sheet to the low 
temperature plasma. 
The surface properties of the resin sheet before (Experiment No. 8) and 
after (Experiment No. 9) the plasma treatment were measured in the above 
described manner to give the results shown in the Table given below. 
EXAMPLE 5 
(Experiment No. 10) 
The same polyvinyl fluoride resin sheet as used in the preceding example 
was placed in the plasma chamber and, after evacuation of the chamber to a 
pressure of 10.sup.-5 Torr, carbon dioxide gas was introduced into the 
chamber at a controlled rate to give a constant pressure of 0.1 Torr 
inside the chamber. Then, vapor of ethylene diamine was introduced into 
the chamber at a controlled rate and mixed with the carbon dioxide gas 
therein so as to give partial pressures of carbon dioxide and ethylene 
diamine vapor of 0.1 Torr and 0.3 Torr, respectively. Low temperature 
plasma was generated for 10 seconds in the thus controlled atmosphere of 
the plasma chamber by applying a high frequency electric power of 5 
kilowatts at a frequency of 110 kHz to the electrodes so as to expose the 
surface of the resin sheet to the low temperature plasma. 
The surface properties of the thus plasma-treated resin sheet were as shown 
in the Table as measured in the above described manner. 
EXAMPLE 6 
(Experiments No. 11 and No. 12) 
A copolymeric resin sheet of tetrafluoroethylene and ethylene (Aflon, a 
product by Asahi Glass Co.) was placed inside the plasma chamber and, 
after evacuation of the chamber to a pressure of 10.sup.-2 Torr, vapor of 
trimethylamine was introduced into the chamber at a controlled rate to 
give a constant pressure of 0.08 Torr inside the chamber. Low temperature 
plasma was generated for 2 minutes in the thus controlled atmosphere of 
the plasma chamber by applying a high frequency electric power of 500 
watts at a frequency of 13.56 MHz to the electrodes so as to expose the 
surface of the resin sheet to the low temperature plasma. 
The surface properties of the resin sheet were measured before (Experiment 
No. 11) and after (Experiment No. 12) the plasma treatment in the above 
described manner to give the results shown in the Table. 
EXAMPLE 7 
(Experiment No. 13) 
The same copolymeric resin sheet as used in the preceding example was 
placed in the plasma chamber and, after evacuation of the chamber to a 
pressure of 10.sup.-4 Torr, vapor of formamide was introduced into the 
chamber at a controlled rate to give a constant pressure of 0.05 Torr 
inside the chamber under flow of the vapor. Then, vapor of acetic acid was 
introduced into the chamber at a controlled rate and mixed with the vapor 
of formamide so as to give constant partial pressures of formamide vapor 
and acetic acid vapor of 0.05 Torr and 0.03 Torr, respectively. Low 
temperature plasma was generated for 1 minute in the thus controlled 
atmosphere of the plasma chamber by applying a high frequency electric 
power of 2 kilowatts at a frequency of 110 kHz to the electrodes so as to 
expose the resin sheet to the low temperature plasma. 
The surface properties of the thus plasma-treated resin sheet were as shown 
in the Table as measured in the above described manner. 
__________________________________________________________________________ 
Partial pressures Adhesion 
Adhesive 
in atmosphere (Torr) 
Contact 
of printing ink, 
strength 
Charge 
Exper- 
N-containing 
Inorganic 
angle 
number of 
kg/inch, 
voltage 
iment 
organic 
and other 
of water, 
remaining 
with adhesive 
by rubbing, 
No. gas gases 
degrees 
squares A B volts 
__________________________________________________________________________ 
1* -- -- 132 0 0 0 7750 
2 Ethylamine 
-- 42 93 14 9 210 
(0.1) 
3 Methyla- 
-- 51 96 15 12 380 
mine (0.05) 
4 Dimethyla- 
Air (0.05) 
47 94 17 10 230 
mine (0.1) 
5 Dimethyla- 
Air (0.05) 
40 90 13 9 310 
mine (0.5) 
6 Dimethyla- 
Air (0.01) 
49 98 17 11 170 
mine (0.2) 
7 Dimethyla- 
Air (0.1) 
38 77 8 5 850 
mine (1.5) 
8* -- -- 113 0 0 0 6300 
9 Allylamine 
Argon 
38 91 15 14 250 
(0.2) (0.2) 
10 Ethylene- 
Carbon 
38 93 17 17 390 
diamine 
dioxide 
(0.3) (0.1) 
11* -- -- 108 0 0 0 5500 
12 Trimethyl- 
-- 36 97 13 11 210 
amine 
(0.08) 
13 Formamide 
Acetic 
45 91 13 10 390 
(0.05) acid (0.03) 
__________________________________________________________________________ 
*Comparative experiment without plasma treatment