Download Surface activation of plastics by plasma for adhesion promotion

Surface activation of plastics by
plasma for adhesion promotion
Uwe Stöhr, Ph. D.∗
1 Introduction
In many fields a good adhesion between
two materials is necessary. The adhesion
should exist at the whole interface without the need of a mechanical connection.
To achieve this the physical effects of adhesion and cohesion can be used. For
some materials also welding and soldering can be used. For plastics high temperatures are often not possible. Large
area plasma treatments offer the possibility to connect two materials via chemical covalent bonds at temperatures below
50 °C. Plasma treatments create either reactive coatings on the surface or chemically functional groups and radicals in the
surface. This allows to connect metals
chemically with plastics as well as plastics with plastics.
to corresponding literature instead: [1, 2].
2 Adhesion of plastics on
plastics
Many plastic types are inert against
most chemicals under standard conditions. That means that one can even
dissolve plastics but the polymer chains
do not chemically react. Polyethylene
(PE) is for example inert against acids
and bases and gluing is only possible with
high effort. In comparison polyamide can
easily be glued because of the chemically
reactive groups in the polymer. These
so-called functional groups in the polymer chains can form chemical bonds to
another polymer. The result is an ideal
adhesion.
The energy in a plasma can crack chemical bonds in the polymer of plastics. The
open bonds can react with chemical substances (for example glues) or functional
groups can be attached to them in the
plasma.
This article gives an overview about the
chemistry and the practice of surface
treatments by plasma for adhesion and
activation. The focus is hereby the adhesion of plastics on plastics and plastics
on metals and alloys. The adhesion for
metallization of plastic is not described
because the effects are different and too
2.1 Activation with noble gas
complex for this overview. It is referred
∗
For contact details see the end of this document.
The most simple method to chemically
activate plastics is the usage of an argon
1
plasma. The substrate is hereby put into
a vacuum chamber that will be filled with
the noble gas argon. By applying an electric voltage to an electrode in the chamber
some of the argon atoms are ionized and
a plasma is ignited. The argon ions try
to return into an electrically neutral state
by catching an electron. The reactivity
of the ions is so strong that electrons are Figure 2: Possible reactions of PP due to
removed from the chemical bonds of the
UV radiation during the plasma treatpolymers. The result are open bonds (unment. Image from [3].
paired electrons) in the plastic surface, see
Fig. 1.
polymers will therefore be cross-linked.
The electromagnetic radiation created in This effect can intentionally be used to
the plasma is also strong enough to crack increase for example the shore hardness
bonds in the polymer. Some atoms are at the surface of elastomers.
excited in the plasma. They emit radiation in the range of infrared to ultraviolet The part itself keeps its elasticity as be(UV). The UV radiation is the part that fore the plasma treatment. The surface
has enough energy to crack bonds. The however has a lower surface energy so that
possible reactions for polypropylene (PP) parts don’t adhere anymore at each other
and most particles from the environment
are shown in Fig. 2 as example.
don’t stick anymore on the surface. For
Plasma treatments with argon have the polymers that strongly react with UV
advantage that the surface chemistry of light, like e. g. poly(methyl methacrythe surface is not changed. The disad- late) (PMMA), the plasma treatment can
vantage is that the open bonds recom- change the bulk properties permanently.
bine quite quickly. To have open bonds Depending on the UV penetration depth
also after some hours after the activa- the plastic gets cracks or staining. As that
tion the plasma process time needs be be change depends on the radiation dose,
quite long (several minutes). A side effect these materials should be treated as short
is that open bonds of different polymer as possible. The bulk material needs to be
chains can react with each other. Chained tested afterwards.
Ar*
H
C
H*
Ar
H
H
H
H
C
C
C
C
Polymer
C
2.2 Activation with reactive gas
H
H
C
C
Polymer
Figure 1: Effects of argon ions on PE.
(Also C−C bonds will be cracked. The
hydrogen ion can react in the plasma
with an electron or it can be attached
to other surfaces.)
2
By using a plasma consisting of molecules,
the created open bonds can be saturated
with functional groups. One of the most
used gas for plasma activation is oxygen because it creates quickly (within seconds) hydroxyl groups (OH groups) in the
surface. Fig. 3 shows the occurring chem-
O
O
O2
+
CH2
C
CH2
CH2
CH2
CH3
CH3
OH
O
H
O
CH2
CH2
CH3
C
O
CH2
CH2
CH2
CH3
CH3
C
CH2
CH3
Figure 3: Creation of hydroxyl groups in PP in an oxygen plasma. Image according
to [4].
ical reactions.
If the oxygen plasma treatment time is
too long the plastic will be oxidized. The
surface is then not only activated but also
etched.
The hydroxyl groups in the surface are
able to react with other chemical groups
in the surface of a second material resulting in a covalent bond between the two
materials. For example OH groups can
react with NH2 groups (amino groups) in
a condensation reaction by loosing a water molecule.
C
Polymer
C
H
C
C
H
H
C
H
H
C
H
O
O
C
H
H
C
C
Polymer
C
O
C
C
H
H
H
O
H
H
C
C
C
C
+
H
Polymer
Polymer
Figure 4: Theoretic reaction of two activated PE surfaces.
parts. After the plasma activation of the
surface there are 3 possibilities to achieve
an adhesion:
• The adhesion promoter is directly
applied by plasma and the elastomer
is molded subsequently.
One might assume that it is sufficient to
only activate both materials in a plasma
• The adhesion promoter is applied
to have later on a good connection. Startwithout plasma and the elastomer is
ing with Fig. 3, Fig. 4 shows the theomolded subsequently.
retical reaction: An O−O bond (peroxide) would be created. But this bond
• The elastomer is directly molded.
is not stable and no permanent chemical
That requires that the material alconnection between the two materials is
ready contains an adhesion promoter
formed. One therefore needs a “spacer”
that will react during the molding.
between the polymers of both materials.
Two examples for the first possibility are
This is a molecule that can connect the
shown in Fig. 5. The adhesion promoter
two polymers. It is therefore named adis hereby a molecule that contains a douhesion promoter.
ble bond or an amino group, respectively.
A typical application is for example the To attach the molecules no oxygen is necmolding of elastomers onto other plastic essary. A plasma is ignited that con-
3
CH2
Elastomer
Elastomer
CH2
CH2
CH
CH2
CH2
CH3 O Si O CH3
O
CH3
H
C
H
H
H
C
C
C
CH
Plasma
CH4
CH3 O Si O CH3
O
H
H
H
C
C
C
C
Polymer
CH2
Molding
Temperature
CH3 O Si O CH3
O
H
H
H
C
C
C
C
Polymer
Polymer
(a) Adhesion promoter for an elastomer.
Synthetic resin
NH2
Synthetic resin
O
OH
CH3 O Si O CH3
O
CH3
H
H
H
H
C
C
C
C
NH2
NH
Plasma
CH4
CH3 O Si O CH3
O
H
H
H
C
C
C
C
Polymer
Molding
Temperature CH O Si O CH
3
3
O
H
C
H
C
C
Polymer
H
C
Polymer
(b) Adhesion promoter for a synthetic resin.
Figure 5: Example for an adhesion promoter that is applied by plasma.
plastic
OH
H2N
H2N
F
F
F
F
F
F
NH2
H
OH
H
O
H
H
C
C
C
C
Polymer
F
OH
H
C
F
H 2O
F
Molding
Temperature
C
F
F
F
NH
NH
H2N
F
F
Wet chemistry
NH2
OH
F
plastic
H
C
Polymer
H
C
F
OH
H
C
F
H 2O
F
NH
C
H
C
H
C
Polymer
Figure 6: Example for an adhesion promoter that is not applied by plasma onto a
plasma-activated surface.
4
sists of a mixture of a carrier gas (usually nitrogen or argon) and a coating
gas. The coating gas consists of the adhesion promoter molecules. Depending
on the coating gas, a carrier gas is not
necessary. This coating process is the
“plasma-enhanced chemical vapor deposition” (PECVD). The adhesion promoter
molecule can bind in the plasma to an
open bond in the surface. In the examples in Fig. 5 methane is created that will
be pumped away. After the plasma treatment the functional groups of the adhesion promoter are available for chemical
reactions. During the molding of plastic
upon the treated surface the functional
groups can react with the plastic because
of the heat of this process. The result
is a chemical bond between both polymers. The advantage of the reactions of
the functional groups shown in Fig. 5 is
that there are no other reaction products
like water.
chemical structure of the polymer is crucial for the selection of the adhesion promoter and the chemistry of the plasma
treatment.
Tab. 1 lists the chemical reactions of functional groups that are often used for adhesion promotion. Due to the various
plastic additives it is necessary to test in
every individual case what functionalization and adhesion promoter can be used
in practice.
2.3 Influence of the polymer
chains on the activation
For the plasma treatment of plastics it
is important to understand that polymer
chains are movable. The chains can rotate
so that functional groups that were attached to the polymer in a plasma process
will not stick out of the surface after a certain time. They are then not available for
reactions. A direct activation of chained
polymers is therefore temporally not stable. For example, the activation with oxygen lasts for typical PE types only a few
hours up to 2 days. Fig. 7 illustrates the
effect of the rotation of activated polymer
chains.
An example for the second possibility is
illustrated in Fig. 6. Hydroxyl groups are
created at the surface of the plastic by a
plasma. An adhesion promoter is subsequently applied to the plastic outside the
plasma chamber using wet chemical processes. The promoter contains on both
“ends” amino and hydroxyl groups which
H
H H H H H
H H O H H
C C C C C
can react with the hydroxyl groups of
C C C C C
Time
H H O H H
H H H H H
the activated material. In the example
H
the molded plastic contains amino groups
that can react with the hydroxyl group of Figure 7: Principle of the rotation of acthe adhesion promoter.
tivated polymer chains.
The examples show that the chemistry of
both materials has to be known to get an Within cross-linked polymers the mobiladhesion via chemical bonds. Every plas- ity of the polymer is very limited because
tic contains of a polymer and additives the chain segments are short. In heavlike release agents, antistatic agents, dyes ily cross-linked polymers (thermosetting
etc. Since the adhesion is only the re- polymers) the cross-linking is so strong
sult of chemical bonds to the polymer the that an activation is usable up to weeks.
5
Table 1: Possible chemical reactions of commonly used functional groups.
Reaction
hydroxyl + amino
hydroxyl + epoxy
hydroxyl + carboxy
hydroxyl + vinyl
Result
R1 NH2 + OH R2
R1 OH
R1 OH
R1 OH
+
HC
O
HO
+
O
R1 NH R2
+ H 2O
OH
CH R2
R1 O CH2 CH R2
O
C R2
+ CH2 CH R2
R1 O
C R2
+ H 2O
R1 O CH2 CH2 R2
O
hydroxyl + isocyanate
amino + epoxy
amino + carboxy
amino + vinyl
R1 OH
+O
R1 NH2 +
HC
O
HO
R1 NH2 +
R1 NH2
O
C N R2
R1 O C NH R2
OH
CH R2
R1 NH CH2 CH R2
O
C R2
+ CH2 CH R2
R1 NH C R2 +H2O
R1 NH CH2 CH2 R2
O
amino + isocyanate
vinyl + thiol
6
R1 NH2 + O C N R2
R1 SH
+ CH2 CH R2
R1 NH C NH R2
R1 S CH2 CH2 R2
This property can also be used for low
cross-linked or chained polymers by applying a heavily cross-linked polymer onto
their surface. In this case the polymer
is at first activated in a plasma. Subsequently a plasma is ignited in a gas consisting of so-called precursor molecules.
Fig. 9 shows the principle of the plasma
polymerization. The precursor molecules
are fragmented and ionized in the plasma.
When the fragments and ions hit the activated surface they are chemically bound
to it. The molecule fragments form a
plasma polymer coating. Plasma polymers are heavily cross-linked and don’t
contain defined repeat units. Therefore
e. g. silicone-based plasma polymers have
different properties compared to chained
silicones.
In fact, by activating the applied plasma
polymer coating one activates a layer of
thermosetting polymer and the activation
is therefore usable for a long time.
NH2
NH2
Time
CH3 O Si O CH3
O
H
H
H
C
C
C
C
Polymer
CH3 O Si O CH3
O
H
H
H
C
C
C
C
Polymer
Figure 8: Example for a molecule attached to a plastic surface. It cannot
completely rotate into the polymer because of its size.
Plasma polymer coatings are not in every
case necessary for a temporally stable activation. If an adhesion promoter consisting of large molecules is applied directly
after the activation the molecules cannot
rotate into the polymer because of their
size, see Fig. 8
2.4 Influence of the process
pressure on the activation
The activation of plastic surfaces as well
as the application of adhesion promoters by plasma can be performed at
atmospheric pressure or at low pressure / vacuum.
The plasma treatment at atmospheric
pressure requires less equipment than at
low pressure. At atmospheric pressure
the method of dielectric barrier discharge
(DBD) is often used. The part to be activated is thereby used as dielectric in a capacitor setup. One electrode delivers an
alternating high voltage while the other
electrode is grounded. To achieve a homogeneous treatment a constant size of the
gap between the electrode and the part is
necessary. This is automatically the case
for flat and even surfaces like for example foils or the side walls of yogurt cups.
It is possible to use special gases in DBD
processes for adhesion promotion.
A disadvantage of atmospheric pressure
is the high consumption of quite expensive process gases. At atmospheric pressure gas flows of liters/min are necessary
while at low pressure (0.1 – 10 Pa) flows in
the range of cm3/min are sufficient, depending on the vacuum chamber size. If
3D molded surfaces need to be activated
the DBD is geometrically limited because
of the necessary uniform gap. The electrode must therefore have the same shape
as the surface which makes is complicated
to activate several parts at one. In contrast in a vacuum chamber it is possible to activate different parts with complex geometries at once because the complete volume of the chamber is filled with
plasma. Due to this possibility and the
much lower gas consumption, activation
7
CH3
CH3
CH3
CH3
CH3
CH3
H
CH3
H Si O Si H
CH3
H
O Si
H Si O Si H
CH3
H
H
H
H
C
C
C
C
CH2
H Si
CH3
CH2
Plasma
H
C
Polymer
H2
CH2
Si
CH3
Si O
H
CH3
CH3
H
C
H
C
H
C
Plasma
CH4
CH2 CH
CH2 Si
Si
O
CH2 CH2
Si
CH2
O
H
H
C
Polymer
C
C
C
Polymer
Figure 9: Principle of the plasma polymerization.
of non-planar surfaces at low pressure is gen plasma in advance. The plasma polyin most cases more cost effective than at mer is covalent-bonded to the substrate
atmospheric pressure.
and the above described techniques for
adhesion of plastic on plastic can be used.
It is nevertheless not possible to activate
Fig. 10 illustrates the proceeding.
all plastic types at low pressure. Shortchained hydrocarbons like e. g. waxes are
solid at atmospheric pressure and get liquid at low pressure. These substances are 4 Summary
therefore migrating in vacuum to the surface of the plastic. In effect a liquid film at The plasma treatment of plastic offers
the surface will be activated and not the a chemically stable connection between
polymer. A good adhesion to the poly- the plastic and coatings. In plasma
mer can therefore not be achieved. Prob- chemical bonds in the surface of plastics
lematic plastics in this respect are the PE are cracked. The open bonds are availtypes PE-LLD, PE-LD and copolymers of able for chemical reactions; the plastic is
PE and PP. These plastics therefore re- thereby activated. Activated plastics can
quire preliminary tests to check if they be glued, imprinted and coated by various
methods.
are suitable for vacuum processes.
Coatings or adhesion promoters can directly be applied onto the activated sur3 Adhesion of plastics on faces by plasma treatments.
For a successful plasma treatment it is
important to know in detail the surface
chemistry of the substrate and of the deAll metals (with few exceptions like gold) sired coating to perform a suitable treatand metal alloys have a native oxide layer ment.
at their surface that also contains hydroxyl groups. The oxide layer can be An adhesion to metal substrates can be
used for adhesion promotion by applying achieved by plasma polymerization.
a plasma polymer onto it. Depending on By plasma activation at atmospheric
the material of the substrate it is advan- pressure no vacuum equipment is necestageous to oxidize the surface in an oxy- sary. The possible geometries of the parts
metal and metal alloys
8
NH2
CH3
CH3
CH3 O Si O CH3
O
CH3
H Si O Si H
CH3
CH3
CH3
CH3
H Si O Si H
CH3
O
H
O
Fe
CH3
O
Steel
O
Fe
Plasma
CH2 Si CH2
O
Si CH2
O
Fe
O
O
NH2
CH2 O Si O CH3
CH2 Si CH2 O
CH
O
Plasma
CH
Si
O
Fe
O
Si
O
Fe
CH2
Si
O
O
Fe
Steel
Steel
Figure 10: Principle of the adhesion promotion for plastic on metals and metal alloys.
to be treated are hereby limited. The activation in vacuum allows also complex
geometries and reduces the costs for activation/coating gases. The longer process
time to create the vacuum is in practice
more than compensated by the ability to
treat several parts at once.
References
[1] Jörn Großmann. Einfluß von Plasmabehandlungen auf die Haftfestigkeit vakuumtechnisch hergestellter
Polymer-Metall-Verbunde.
Dissertation, University Erlangen-Nürnberg,
2009,
http://opus4.kobv.de/opus4fau/files/940/JoernGrossmann_Dissertation.pdf.
using atmospheric pressure discharges.
J. Phys. D: Appl. Phys, 36(6): 666 – 685,
2003, http://dx.doi.org/10.1088/00223727/36/6/309.
Contact
PLASMA ELECTRONIC GmbH
Otto-Lilienthal-Str. 2
79395 Neuenburg
Germany
Phone: +49 (0)7631 7017 - 0
Fax: +49 (0)7631 7017 – 20
[email protected]
[2] Richard
Suchentrunk.
KunststoffMetallisierung. Eugen G. Leuze Verlag,
Bad Saulgau, 3. Auflage, 2006.
[3] Uwe Stöhr. Development and applications of stamps for area-selective
plasma treatment and plasma-enhanced
coating. Dissertation, University of
Freiburg,
2010,
www.freidok.unifreiburg.de/volltexte/7469/.
[4] R. Dorai and M. J. Kushner. A model
for plasma modification of polypropylene
9