The surface of plastics can be activated or coated using plasma, thereby creating excellent adhesion to other surfaces. Such plasma treatment allows use of adhesives, direct printing onto the surface or spray application of other plastics without a requirement for wet processing. Plasma treatment works to break down some of the bonds in the polymer. These fractured bond sites are then chemically available, thereby forming an activated polymer surface. At these fractured bond sites, covalent bonding to the polymer is facilitated thus increasing adhesion. The duration of such an activated state depends on the type of polymer in question. Plasma treatment also allows the introduction of chemically active groupings into the polymer, which themselves enable bonding to another polymer type or a metal. Surfaces with a smooth geometry can be treated at atmospheric pressure while components with more complex geometry require vacuum plasma treatment. In the latter case, the polymer must be able to withstand vacuum conditions.
Oberflächenaktivierung von Kunststoffen zur Haftungsverbesserung mittels Plasmatechnik
Kunststoffoberflächen können mit Plasma aktiviert oder beschichtet werden, um dadurch eine sehr gute Haftung zu anderen Oberflächen zu gewährleisten. Die Plasmabehandlung ermöglicht ein Verkleben, direktes Bedrucken oder Aufspritzen von anderen Kunststoffen ohne nasschemische Behandlung. Bei der Behandlung werden in der Oberfläche des Substrats Bindungen im Polymer aufgebrochen. Diese Bruchstellen sind chemisch angreifbar, das Polymer ist somit aktiviert. Die Plasmabehandlung ermöglicht dadurch, dass chemisch kovalente Bindungen zum Polymer und dadurch eine optimale Haftung entstehen kann. Je nach Kunststofftyp bleibt die Aktivierung unterschiedlich lange bestehen. Im Plasma können auch chemisch aktive Gruppen in das Polymer eingebracht werden, die ebenfalls eine Verbindung mit einem weiteren Kunststoff oder einem Metall ermöglichen. Ebene Oberflächengeometrien können bei Atmosphärendruck behandelt werden, während komplex geformte Teile im Vakuum in Plasmakammern behandelt werden müssen. Dafür ist Voraussetzung, dass der Kunststoff den Bedingungen im Vakuum widerstehen kann.
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.
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 complex for this overview. It is referred 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.
2.1 Activation with noble gas
The most simple method to chemically activate plastics is the usage of an argon 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 removed from the chemical bonds of the polymers. The result are open bonds (unpaired electrons) in the plastic surface (Fig. 1).
The electromagnetic radiation created in the plasma is also strong enough to crack bonds in the polymer. Some atoms are excited in the plasma. They emit radiation in the range of infrared to ultraviolet (UV). The UV radiation is the part that has enough energy to crack bonds. The possible reactions for polypropylene (PP) are shown in Figure 2 as example.
Fig. 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)
Fig. 2: Possible reactions of PP due to UV radiation during the plasma treatment [3]
Plasma treatments with argon have the advantage that the surface chemistry of the surface is not changed. The disadvantage is that the open bonds recombine quite quickly. To have open bonds also after some hours after the activation the plasma process time needs be be quite long (several minutes). A side effect is that open bonds of different polymer chains can react with each other. Chained polymers will therefore be cross-linked. This effect can intentionally be used to increase for example the shore hardness at the surface of elastomers.
The part itself keeps its elasticity as before the plasma treatment. The surface however has a lower surface energy so that parts don’t adhere anymore at each other and most particles from the environment don’t stick anymore on the surface. For polymers that strongly react with UV light, like e. g. poly(methyl methacrylate) (PMMA), the plasma treatment can change the bulk properties permanently. Depending on the UV penetration depth the plastic gets cracks or staining. As that change depends on the radiation dose, these materials should be treated as short as possible. The bulk material needs to be tested afterwards.
2.2 Activation with reactive gas
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. Figure 3 shows the occurring chemical 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.
Fig. 3: Creation of hydroxyl groups in PP in an oxygen plasma [4]
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.
One might assume that it is sufficient to only activate both materials in a plasma to have later on a good connection. Starting with Figure 3, Figure 4 shows the theoretical reaction: An O – O bond (peroxide) would be created. But this bond is not stable and no permanent chemical connection between the two materials is formed. One therefore needs a spacer between the polymers of both materials. This is a molecule that can connect the two polymers. It is therefore named adhesion promoter.
Fig. 4: Theoretic reaction of two activated PE surfaces
- The adhesion promoter is directly applied by plasma and the elastomer is molded subsequently.
- The adhesion promoter is applied without plasma and the elastomer is molded subsequently.
- The elastomer is directly molded. That requires that the material already contains an adhesion promoter that will react during the molding.
Two examples for the first possibility are shown in Figure 5. The adhesion promoter is hereby a molecule that contains a double bond or an amino group, respectively. To attach the molecules no oxygen is necessary. A plasma is ignited that consists 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 Figure 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 Figure 5 is that there are no other reaction products like water.
Fig. 5: Example for an adhesion promoter that is applied by plasma; (a / above) Adhesion promoter for an elastomer; (b / below) Adhesion promoter for a synthetic resin
Fig. 6: Example for an adhesion promoter that is not applied by plasma onto a plasma-activated surface
The examples show that the chemistry of both materials has to be known to get an adhesion via chemical bonds. Every plastic contains of a polymer and additives like release agents, antistatic agents, dyes etc. Since the adhesion is only the result of chemical bonds to the polymer the chemical structure of the polymer is crucial for the selection of the adhesion promoter and the chemistry of the plasma treatment.
Table 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. Figure 7 illustrates the effect of the rotation of activated polymer chains.
Fig. 7: Principle of the rotation of activated polymer chains
Fig. 8: Principle of the plasma polymerization
Within cross-linked polymers the mobility of the polymer is very limited because the chain segments are short. In heavily cross-linked polymers (thermosetting polymers) the cross-linking is so strong that an activation is usable up to weeks. 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. Figure 8 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.
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 (Fig. 9).
Fig. 9: Example for a molecule attached to a plastic surface; it cannot completely rotate into the polymer because of its size
2.4 Influence of 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 it 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 of non-planar surfaces at low pressure is in most cases more cost effective than at atmospheric pressure.
It is nevertheless not possible to activate all plastic types at low pressure. Short-chained hydrocarbons like e. g. waxes are solid at atmospheric pressure and get liquid at low pressure. These substances are therefore migrating in vacuum to the surface of the plastic. In effect a liquid film at the surface will be activated and not the polymer. A good adhesion to the polymer can therefore not be achieved. Problematic plastics in this respect are the PE types PE-LLD, PE-LD and copolymers of PE and PP. These plastics therefore require preliminary tests to check if they are suitable for vacuum processes.
3 Adhesion of plastics on metal and metal alloys
All metals (with few exceptions like gold) and metal alloys have a native oxide layer at their surface that also contains hydroxyl groups. The oxide layer can be used for adhesion promotion by applying a plasma polymer onto it. Depending on the material of the substrate it is advantageous to oxidize the surface in an oxygen plasma in advance. The plasma polymer is covalent-bonded to the substrate and the above described techniques for adhesion of plastic on plastic can be used. Figure 10 illustrates the proceeding.
Fig. 10: Principle of the adhesion promotion for plastic on metals and metal alloys
4 Summary
The plasma treatment of plastic offers a chemically stable connection between the plastic and coatings. In plasma chemical bonds in the surface of plastics are cracked. The open bonds are available for chemical reactions; the plastic is thereby activated. Activated plastics can be glued, imprinted and coated by various methods. Coatings or adhesion promoters can directly be applied onto the activated surfaces by plasma treatments.
For a successful plasma treatment it is important to know in detail the surface chemistry of the substrate and of the desired coating to perform a suitable treatment. An adhesion to metal substrates can be achieved by plasma polymerization.
By plasma activation at atmospheric pressure no vacuum equipment is necessary. The possible geometries of the parts 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] J. Großmann: Einfluß von Plasmabehandlungen auf die Haftfestigkeit vakuumtechnisch hergestellter Polymer-Metall-Verbunde; Dissertation, Universität Erlangen-Nürnberg, 2009, http://opus4.kobv.de/opus4-fau/files/940/JoernGrossmann-_Dissertation.pdf
[2] R. Suchentrunk: Kunststoff-Metallisierung; 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.uni-freiburg.de/volltexte/7469/
[4] R. Dorai, M. J. Kushner: A model for plasma modification of polypropylene using atmospheric pressure discharges; J. Phys. D: Appl. Phys, 36(6): 666–685, 2003, http://dx.doi.org/10.1088/0022-3727/36/6/309.
DOI: 10.7395/2016/Stoehr01
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