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Plasmas and Polymers, Vol. 8, No. 3, September 2003 (°

C 2003)

Deposition of Ethylene-Hexafluoropropene
Gradient Plasma-Copolymer Using Dielectric
Barrier Discharge Reactor at Atmospheric
Pressure: Application to Release Coatings
on Pressure-Sensitive Tape1
Kunihito Tanaka2,3 and Masuhiro Kogoma2

Received January 23, 2003; accepted May 14, 2003

Plasma-polymerized hexafluoropropene (PPHFP) film deposited using a dielectric bar-

rier discharge reactor at atmospheric pressure had low enough adhesive strength,
22.2 Nm−1 , for use as a release coating of pressure-sensitive adhesive tapes, but the
bond strength between PPHFP film and a poly (ethylene terephthalate) (PET) substrate
film was slightly weak: some part of the PPHFP deposits could be peeled from the
PET substrate. Since the XPS results indicated that the bond strength between plasma-
polymerized ethylene (PPE) film and PET substrate was strong enough, we tried to
deposit PPE and plasma-polymerized ethylene - hexafluoropropene gradient plasma-
copolymer between the PET substrate and the PPHFP film. This multi-layer film (MLF)
had low enough adhesive strength, 36.6 Nm−1 , for use as the release coating; this value
was near that of a control sample, Teflon sheet, 21.6 Nm−1 . Moreover, the bond strength
between MLF and PET substrate became stronger than that between PPHFP and PET
films.

KEY WORDS: Hexafluoropropene; atmospheric pressure glow plasma; adhesive;

plasma processing and deposition.

1. INTRODUCTION
Pressure-sensitive adhesive (PSA) tapes are generally kept in rolls. Conse-
quently, so that we can peel them out smoothly, PSA tapes consist of three layers:
the adhesive layer, the base film and the release coating. Some silicone polymers

1 Extended version of a paper presented at the International Symposium on High Pressure Low
Temperature Plasma Chemistry, HAKONE VIII, Pühajärve, Estonia, July 21–25, 2002.
2 Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho,

Chiyoda-ku, Tokyo 102-8554, Japan.

3 To whom correspondence should be addressed. E-mail: tanaka@ch.sophia.ac.jp

199
1084-0184/03/0900-0199/0 °
C 2003 Plenum Publishing Corporation
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200 Tanaka and Kogoma

have frequently been used as a release coating because of their quite low adhe-
siveness. When one uses PSA tape, a very small amount of the release coating
transfers to the adhesive layer side. Such areas of release coating on the adhesive
will desorb to the atmosphere and then readsorb elsewhere. This may result in
serious damage to certain electric devices such as hard disk drives (HDDs), for
example; silicone coatings adsorbed on a hard disk can crash a HDD head.
Generally, the adhesive strength between PSA tapes and solids strongly de-
pends on the surface morphology and the surface energy of the solid: a smoother
solid surface and a lower surface energy make the adhesive strength weaker.(1)
Thus, it is clear that any material with sufficiently low surface energy is suit-
able for depositing release coatings. According to research,(1,2) some fluorinated
polymers such as polytetrafluoroethylene (PTFE) and polyhexafluoropropene have
lower critical surface tensions, from which the surface energies are estimated, than
the values of some silicone polymers. However, they are rarely used as release
coatings since there is no suitable solvent for applying the fluorinated polymers
onto PSA tape films.
We have already reported that atmospheric pressure glow (APG) plasma can
polymerize some fluorinated plasma-polymers.(3,4) In our previous study, we found
that a plasma-polymerized hexafluoropropene (PPHFP) film deposited using a
dielectric barrier discharge (DBD) reactor had low enough adhesive strength for use
as the release coating; the results showed the validity of the atmospheric pressure
glow discharge technique.(4) They also showed that the adhesiveness of PPHFP
film was lower than that of plasma-polymerized trifluoroethylene (CF2 CFH) film;
this result agreed with the general propensity that a fluorinated polymer with a
trifluoromethyl functional group (CF3 ) has a lower critical surface tension (surface
energy) than one without CF3 .(1,5) However, two problems still remained, namely
that a large amount of the plasma-polymer was deposited inside the plasma reactor,
and that a small amount of PPHFP was peeled from the base film (poly (ethylene
terephthalate), PET) during the peel force measurement. The former problem can
be readily solved by improvement of the plasma reactor. If any deposited film
would make strong bonds with PET, it was supposed that the introduction of such
a plasma-polymer layer between PET and PPHFP would be a possible solution for
the second problem. Since ethylene is a hydrocarbon monomer compatible with
PET and is easy to deposit and handle, it was selected as the monomer for the
intermediate layer. Therefore, in this study, we built a new reactor and examined
plasma-polymerized ethylene (PPE), PPHFP and multi-layer films for application
in the release coating technology describe earlier.

2. EXPERIMENTAL
The DBD reactor used in this study is shown in Fig. 1, as operated in ambient
atmosphere. The upper high voltage electrode made of duralumin alloy was coated
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PPHFP Film Deposit Using Dielectric Barrier Discharge Reactor 201

Fig. 1. Schematic diagram of the reactor used for film

deposition. (1) Upper aluminum electrode assembly;
(2) lower stainless electrode (rotating drum); (3) dis-
charge zone; (4) film substrate; (5) motor; radio fre-
quency high voltage generator.

with aluminum oxide by an anodic oxidation treatment (“alumite” treatment):

an aluminum assembly is soaked in an acid solution, and some DC voltage is
applied between the aluminum anode and some metal cathode. Then the aluminum
assembly surface is coated with aluminum oxide. This electrode assembly had a
0.5 mm by 50 mm rectangular slit in its bottom surface whose size is 12 mm by
62 mm. The lower grounded electrode was a stainless steel drum, around which
was wrapped the PET substrate film during plasma deposition. Thus the discharge
area size was almost the same as the area of the bottom surface of the upper
electrode. The drum, 40 mm diameter, was rotated at a speed of 4.4 cm·s−1 during
the discharge treatment. In some experiments, we used a glass plate as substrate,
in which cases a flat aluminum plate replaced the stainless steel drum as the lower
electrode.
Hexafluoropropene (C3 F6 ) and ethylene (C2 H4 ) were used as monomers.
One or both of these monomers were diluted with helium, and the gas mixture
was injected into the discharge zone, resulting in plasma-polymer deposition on
the substrate, but not inside the upper electrode assembly. The discharge was
generated stably as well as calmly under the discharge conditions in Table I. The
calm (low discharge power) discharge was expected to allow the original monomer
structure to enter into the deposited film as far as possible. As already mentioned,
0.05 × 80 × 125 mm PET sheet or a glass plate was used as substrate.

Table I. Treatment Conditionsa

Discharge Frequency/kHz 100
Discharge Power/W 20
Monomer Flow Rate/sccm 3–20
He Flow Rate/slm 5
Distance/mm 1.5
a The distance between the slit and the
sample surface.
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202 Tanaka and Kogoma

The thickness of plasma-polymer deposited on the glass plate was measured

with a surface profile measuring system (DEKTAK 3, Sloan) equipped with a
diamond stylus, having nominal resolution and minimum thickness value of 5 Å.
The peel force was measured with a 180◦ peel test: No.31B PSA tape (Nitto Denko
Co., Ltd) of 20 mm width was pressed onto the deposited film and then peeled
off at a speed of 300 mm·min−1 . Although peel force was formerly presented in
units of grams (force) per 20mm, the SI unit, Nm−1 , is used in this article, the
relationship between these units is 1.0 gf (20 mm)−1 ≈ 4.9 × 10−1 Nm−1 . The
chemical state of the deposited films was determined by XPS (ULVAC-Phi Co.,
Ltd., ESCA-5800ci). The XPS spectrum binding energy was corrected by shifting
the hydrocarbon component (CHx , x = 0 to 3) peak in the C1s spectrum to 284.6 eV
in most cases. Then we carried out curve fitting with the XPS program of ULVAC-
Phi by reference to the interval energies between each chemical group observed.(6,7)
If CHx peak in the C1s spectrum was not identified clearly, for example PPHFP
film, we set the peak position of F1s spectrum at 689 eV, which value is the F1s
peak energy of a general Teflon sample.(8) Then, the same process as mentioned
earlier was followed.

3. RESULTS AND DISCUSSION

To obtain the optimum discharge conditions, we examined the deposition
rates, DR, of PPHFP and PPE and the peel force, PF, of PPHFP, that depended
on the monomer flow rate, FR. The glass plate substrate was used only for the
measurement of DR. The plasma-polymers on the glass substrate deposited only
in the discharge area, which was about 12 mm by 62 mm. The centerline of the
deposited film was thickest just under the slit and the thickness became thinner
toward the deposited film edge. Thus the thickness on the centerline was measured
along the slit line, and those ten values were averaged. Figure 2 shows plots of DR
of PPE and PPHFP versus monomer FR. Both DRs increased up to FR = 6 sccm
of ethylene and FR = 3 sccm of HFP, respectively, and then decreased with rising
FR. We believe that this observed decrease is due to a drop in discharge energy
per monomer molecule.(9) Moreover, we believe that excess HFP, which can act
as an etchant, accounts for the sharply decreasing DR of PPHFP.(10) To reduce
the treatment time, we used the optimal ethylene flow rate of FR = 6 sccm in this
study. However, for the reason described below, the optimal HFP flow rate was not
FR = 3 sccm.
Figure 3 shows plots of PF of PPHFP versus FR. Since we intended to use the
deposited film for release coatings, the lowest PF values were preferred. Contrary
to the trend of DR, PF decreased with rising FR values. The PPHFP film obtained
with FR = 20 sccm of HFP after 40 min deposition time had the same PF as the
Teflon sheet used as a control specimen had. Thus, the optimal HFP flow rate
was considered to be FR = 20 sccm. The long 40 min deposition time needed to
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PPHFP Film Deposit Using Dielectric Barrier Discharge Reactor 203

Fig. 2. Plot of DR of h PPE and • PPHFP as a func-

tion of FR of the monomers. The substrates were glass
plates.

achieve low enough PF was due to non-uniform, hence incomplete coverage of

the PET sheet with PPHFP for shorter discharge time. We expect that this problem
can be solved with improvements of the reactor. Next, the differences in PF were
examined with the help of XPS spectra of the different PPHFP samples.
Figure 4 shows the atomic ratios, F/C and O/C, obtained from XPS spectra
of PPHFP samples. While the dominant oxygen source was post-oxidation by
atmospheric oxygen for O/C < 0.07, PET substrate is believed to contribute for
O/C > 0.07. Although O/C was constant for FR > 8 sccm of HFP, F/C increased
slightly. Figure 5 shows the concentration of carbon bonded as CFx (x = 1 to 3) and
CF0 (carbons bond only with other neighboring carbons, never with fluorine; e.g.,
---- CF2 ---- C(CF3 ) ---- CF ---- ) in XPS C1s spectra. Figure 6a shows CF0−3 spectra.
The concentration of CFx structures is seen to have increased with increasing

Fig. 3. Plot of PF of PPHFP as a function of FR of

HFP. The symbols designate different discharge times:
h, 10 min; ◆, 20 min; 4, 30 min; •, 40 min. The dotted
line shows the value of the control sample (Teflon sheet,
22 Nm−1 ).
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204 Tanaka and Kogoma

Fig. 4. Plot of atomic ratios • F/C and h O/C ob-

tained from XPS spectra of PPHFP samples after dis-
charge time of 20 min as a function of FR of HFP.
F/C and O/C values on the ordinate axis correspond
to PET.

FR of HFP. Since surface energy generally decreases with increasing fluorine

concentration at the surface, the PPHFP film deposited at high FR values of HFP
showed lower PF values.(11)
Figures 6a–e show C1s XPS spectra of PPHFP and of the adhesive layer
on virgin PSA tapes, and of those used for 180◦ peel tests of PPHFP, PPE, and
PPE/PP(E-gradient-HFP)/PPHFP multi-layer film (MLF, described in the follow-
ing section), respectively. Although the spectrum in Fig. 6b shows that the virgin
PSA tape had only C, C ---- O and C ----
----O structures, some CFx structures appeared
on the adhesive layer used to peel PPHFP, as shown in Fig. 6c. The result indicates
that part of the PPHFP deposits was peeled from the PET sheet and stuck to the
adhesive layer, as confirmed on Fig. 7a. On the other hand, since there is no ap-
parent difference between Figs. 6b and 6d, it seems that PPE was not peeled from

Fig. 5. Plot of the ratio of • CFx (x = 1-3) and h CF0

structures in C1s spectra of the same PPHFP samples
as in Fig. 4.
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PPHFP Film Deposit Using Dielectric Barrier Discharge Reactor 205

Fig. 6. XPS C1s spectra of (a) PPHFP (FR of HFP:

20 sccm; discharge time: 40 min) and the adhesive
layer side of the PSA tapes; (b) virgin tape, and
tapes after 180◦ peel tests; (c) PPHFP; (d) PPE; and
(e) MLF, respectively.

the PET sheet; if PPE was peeled off, C ---- O and C ----
----O peaks of Fig. 6d would
have become smaller. Moreover, we do not observe any traces of peeled PPE on
the adhesive layer in Fig. 7b. Therefore, we expect that the deposition of PPE
and plasma-polymerized ethylene-hexafluoropropene gradient plasma-copolymer
(PP(E-gradient-HFP)) between the PET substrate and the PPHFP will increase the
bond strength between them; we therefore tried a new deposition process.
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206 Tanaka and Kogoma

Fig. 7. Photographs of the adhesive layer sides of PSA tapes after 180◦ peel tests of (a) PPHFP; (b)
PPE; and (c) MLF, respectively. The bi-direction arrows in the photographs indicate the PSA tape
width, 20 mm.

The preparation of PPE/PP(E-gradient-HFP)/PPHFP multi-layer film (MLF)

was carried out as follows: first, PPE was deposited for 10 minutes with FR = 6
sccm of ethylene. Then PP(E-gradient-HFP) was deposited with FR = 4.5 sccm
of ethylene and FR = 5 sccm of HFP mixture gas for the next 10 minutes. In the
same way, while PP(E-gradient-HFP) was being deposited, the ethylene flow was
decreased by 1.5 sccm and the HFP flow increased by 5 sccm every 10 minutes.
Finally, PPHFP was deposited for 10 minutes with FR = 20 sccm of HFP. Ethylene
and HFP flow rates during each deposition step are listed in Table II. According
to the XPS spectra in Figs. 6b and 6e, there was no difference between the C1s
spectra of the virgin adhesive layer and that of the tape used to peel the MLF; this
conclusion was confirmed in Fig. 7c by the fact that very little MLF is observed on
the adhesive layer. Therefore, our expectation was achieved. Moreover, though PF
of MLF did not show as a low value as that of Teflon sheet and PPHFP, as shown
in Fig. 8, we thought that this value was low enough.

4. CONCLUSIONS
The deposition did not occur inside the upper electrode. Of course, the plasma-
polymer was deposited on the bottom surface of the upper electrode assembly. We

Table II. Monomer Flow Rates of the MLF

Deposition During each Deposition Step
Ethylene/ HFP/
Step # sccm sccm
1 6.0 0
2 4.5 5
3 3.0 10
4 1.5 15
5 0 20
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PPHFP Film Deposit Using Dielectric Barrier Discharge Reactor 207

Fig. 8. Histogram of the peel forces on PET, MLF,

PPHFP, and Teflon surfaces.

thought that another reactor, for example one that has two drum-type electrodes,
would be needed so as not to waste the monomer.
The lowest adhesive strength of PPHFP, 22.2 Nm−1 , was observed at highest
FR = 20 sccm of HFP after 40 min deposition time; this value was the same PF
as that of the control specimen, Teflon sheet, 21.6 Nm−1 . While PPHFP deposits
show weak bonding to PET film, as in a previous study, the bond strength be-
tween PET and PPE deposits was strong. PPE/PP(E-gradient-HFP) layer raised
the bond strength between PET and PPHFP layers, we could thereby solve the bond
strength problem. However, the effect of the PPE/PP(E-gradient-HFP) layer have
not been clarified yet. Whether the layer has the same effect on other substrates,
for example polypropylene, polyethylene and other hydrocarbon polymers, is also
undetermined. Moreover it is obvious that the discharge time is too long to be
economical. Thus, we intend to examine those problems in future research.

ACKNOWLEDGMENT
The authors wish to express their gratitude to Professor Frank Scott Howell
S. J. for his valuable comments.

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