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CHRISTIAN BUSKEALEXANDER KNOSPE

Plasma – the “fourth state of matter”– describes a material that has a highinstable energy level. Energy is con-

stantly introduced as heat via the three ag-gregate states: solid, liquid and gas (Fig.1).The plasma technology does not stop at

the gaseous state: If energy is additional-ly introduced into the material by meansof electrical discharge, the electrons ac-quire a higher kinetic energy and breakfree from the electron shell. The result isfree electrons, ions and molecule frag-ments. Under normal pressure, however,this state is virtually unusable because ofits instability. It is only with the Openair

atmospheric pressure plasma process,conceived and patented by Plasmatreat ofSteinhagen/Germany, that new possibili-ties have been created. Through the de-velopment and deployment of plasmanozzles, it has now become possible to al-

In-Line Plasma ProcessesSurface Technology. The Openair plasma process has, for some years now, been

creating a variety of new applications, especially for the cleaning, activation and

coating of moulded parts and film. Now, a modular system has been developed for

the in-line pretreatment of PP film that works with flexible widths and allows a

high level of activation.

Principles of the plasma process

Fig. 1. Plasma as the fourth state of matter

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Nozzle for filmactivation

Translated from Kunststoffe 11/2005, pp. 82–86

Kunststoffe plast europe 11/2005

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so utilize this aggregate state (which has,until now, seldom been used in industri-al applications) in production processes.

Electrically Neutral Plasma Jet

The nozzles are operated with air or withsome other process gas in combinationwith a high voltage (Fig. 2). The emerg-ing jet of plasma can, depending on thenozzle geometry, reach a treatment widthof up to 25 mm; the source of plasma ismoved at a distance of 40 mm from thesurface being treated. The emerging plas-ma is electrically neutral, which signifi-cantly extends and simplifies application.Depending on the introduced energy andthe structure of the plasma source, thetemperature of the emerging plasma isbetween 300 and 1500°C.This allows veryhigh processing speeds with optimum ef-fect. The typical heating-up of the plasticsurfaces during treatment is ∆T<20°C.

Because the plasma modifies the sur-faces of plastics, it is used for ■ the cleaning of surfaces, e.g. the re-

moval of release agents and additives,■ activation, i.e. to generate functional

groups, allowing the adhesion of ad-hesives and coatings,

■ improving the properties of the com-posite through a plasma-polymer coat-ing.

Non-polar materials such as plasticsbondings cannot generally be bondedwithout pretreatment. After activationwith Openair plasma, the tensile shearforce increases around 50-fold (Fig. 3).This is still the case eight weeks later,which means that the treatment remainsstable over the long term.

Numerous ApplicationPossibilities

There are no limits to the possible appli-cations of this efficient plasma technolo-gy. It is used for cleaning and de-coating,for improving adhesion properties andfor coating applications generally.

Through the use of Openair plasma,processes like the removal of mould re-lease agents from moulded polyurethaneparts – for example, furniture profiles,airbag covers (Fig. 4) and bicycles saddles– can be considerably streamlined com-pared with conventional processes. Thelayer-by-layer removal of organic coat-ings, the stripping or partial removal ofpaint or metal before bonding, the pro-duction of car headlamps and the treat-ment of reflectors are just some examplesof the successful inline application of the

Openair plasma system in cleaning andcoating processes.

Whether it is a question of high-techbonding, simple wet-labelling or thesticking together of folding cartons, theprecise pretreatment of the bond surfaceby the highly developed plasma nozzlesenables the use either of modern solvent-free UV adhesives or of natural, water-borne systems. For example, after pre-treatment with Openair plasma, the poly-carbonate display windows can be bond-ed into the half-shells of mobile phonehousings with solvent-free UV adhesives(Fig. 5). On the other hand, casein adhe-sives can be used equally well for the la-belling of plastic drums.

Plasma coating is a process, which, un-til now, could only be carried out in a vac-uum. If the Openair technology is com-bined with a newly developed feeding-inof precursor material, coating under at-mospheric pressure is also possible. Theprecursor is an evaporating reactive mate-rial that is left behind after the treatmentin the form of a coating on the plastic. Theadvantage of this technology can now alsobe used for coating plastics. For example,water vapour barriers are already being ap-plied in this way to CD blanks. This is anewly developed process from Plasmatreat,which is being widely used on an industri-al scale for such coating applications.

In addition to the technical benefits,the process also saves considerable mate-rial costs because, for example, polycar-bonates can be used instead of expensivecycloolefins.

Kufoplas Research Project

Thanks to a number of specific develop-ments over the last three years as part ofthe Kufoplas research project (sponsoredby the BMBF – the German Federal Min-istry of Education and Research), Ope-nair plasma is now also available for ap-plications with large treatment widths.For the first time, these processes can beapplied to the treatment of plastic film.

Kufoplas – the shortened form of theGerman for “plastic film plasma treat-ment” – is a research and developmentproject within the framework concept"Research for the production of tomor-row". There are three partners in the proj-ect: Plasmatreat GmbH in Steinhagen,

Fig. 2. Depending on the nozzle geometry, theemerging plasma is available in a treatmentrange of up to 25 mm; the distance from the sur-face is approx. 40 mm

Comparison of bonds

Fig. 3. Tensile shear forces of plastics bonds with and without pretreatment

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which is responsible for developing theplant engineering, Treofan GermanyGmbH & Co. KG in Neunkirchen, whichis responsible for selecting various filmsubstrates and integrating the pretreat-ment into the process for manufacturingthe film, and the Fraunhofer Institute forManufacturing Technology and AppliedMaterials Research (IFAM) in Bremen,which is working on the research of thescientific principles as the basis for all theprocessing engineering developments ofrelevance to this subject.

Polypropylene film is used in all kindsof different applications in industry and thehome, for example for the packaging offood and cigarettes, as labels, and as tech-nical film in the electrical industry. Toguarantee good printing properties andbonding of such films, the surface energyand thus the proportion of polar groupson the film surface must be increased.

For this purpose, the industry has, formany years now, been using flame treat-ment and the corona technology, allow-ing large film widths to be pretreated atmedium to high speeds. However, partic-ularly when using the corona technique,there is a risk of both sides becomingtreated, which can result in a blocking ofthe film on the roll. Furthermore, thismethod of pretreatment produces onlymoderate to poor activation and, becausethe resultant effects are not very stable,the effect declines quickly during storage.

Openair plasma has been used for sev-eral years in the activation of all kinds ofdifferent plastics and is also noted for its

long-lasting surface effect. As potential-free atmospheric pressure plasma, it guar-antees single-side pretreatment with con-sistently high surface effects. Surface en-ergies are created, which, in contrast tosurfaces treated with corona, even allowthe printing of plastics with water-bornecoating systems.

Project Targets

The goal of the project, which reached itsconclusion this year, was to implement anatmospheric plasma system for the acti-vation of PP-BO (also called BOPP) filmsin an industrial manufacturing facility for

Fig. 4. Through the use of Openair plasma, processes such as the removal of mould release agentsfrom polyurethane mouldings (e.g. airbag covers) are speeded up considerably

Fig. 5. Following pretreatment with Openairplasma, display windows made of polycarbon-ate can be bonded into the half-shells of mobilephone housings with solvent-free UV adhesives

Activation of the atmospheric pressure plasma process

Fig. 6. XPS analysisshows that pretreat-

ment with Openairplasma causes the

introduction of car-bonyl, carboxyl, ether

and hydroxy func-tions plus nitrogen-oxygen compounds

into the surface(source: Fraunhofer Institute

for Manufacturing Technolo-

gy and Applied Materials

Research)

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polypropylene film as an in-line process,based on laboratory and pilot plant tri-als. The objectives were:■ to obtain heat-sealing properties,■ speeds of above 200 m/min,■ film widths up to 10 m,■ single-sided pretreatment (already

guaranteed by the plasma),■ surface tension after pretreatment of

above 50 mN/m,■ long storage life of the film (at least

36 mN/m after six months).Three places were considered for in-tegrating the plasma system into the ex-trusion and stretching process. Locatingit directly after the extrusion has the prob-lem that the film width is low and the sub-strate suffers high thermal stress due tothe low pretreatment velocity. In ad-dition, there are two film-stretchingprocesses to follow. Even if it is locatedafter the longitudinal stretching, thefilm width is still low and it faces anoth-er film stretching process. In any case, ahigh pretreatment velocity is needed. Ifthe plasma system is not inserted untilafter the longitudinal and transversestretching stages, there are no morestretching processes to follow and the

pretreatment speed is high. The resultanteffect is not subjected to any subsequentstresses.

Results

According to current findings, it is notpossible to locate the plasma system di-rectly after extrusion because the acti-vation effect will not be able to with-stand two further stretching processes.Also when positioned after the longitu-dinal stretching, the effect declines be-cause there is still another stretchingprocess to come. During the course ofthe project, attempts were made to solvethis problem by applying a suitable lay-er of keying agent. If the system is lo-cated after the longitudinal and trans-verse stretching phases, high treatmentwidths are needed, making it necessaryto have a modular, flexible system struc-ture.

XPS analysis shows clearly that pre-treatment with Openair plasma leads tothe incorporation of carbonyl, carboxyl,ether and hydroxy functions as well as ni-trogen-oxygen compounds into the sur-face (Fig. 6).

Because of the necessary large pre-treatment width when positioning theplasma system after both the longitudinaland transverse stretching stages, tests werecarried out with positioning it after thelongitudinal stretching. The aim was togenerate the surface tension with the aidof a suitable coating and to largely main-tain this during the last stretching process.For this purpose, a polymerisable precur-sor gas was added to the plasma via a mod-ified nozzle head. Under the influence ofthe plasma energy, the gas then formed alow-crosslinked layer on the surface.Basedon the initial successes, a tool was devel-oped that allows the transfer of the resultsto the pilot plant scale (Fig. 7).

Conclusions

A modular system was developed for thein-line pretreatment of PP film. It allowshigh film activation and can be designedin flexible widths. At process speeds of250 m/min, the sealing properties of thefilm are not affected. In addition, furtherexperiments in a pilot plant showed thatthe coating process can be integrated in-to the production line. ■

ACKNOWLEDGEMENTThis research and development project is sponsoredwith funds from the Federal Ministry of Education andResearch (BMBF) within the framework concept "Re-search for the production of tomorrow". It is lookedafter by the project executing organisation of theBMBF for Production and Manufacturing Technolo-gies, Karlsruhe/Germany research centre.

THE AUTHORSDIPL.-ING. CHRISTIAN BUSKE, born in 1963, is the

managing shareholder of Plasmatreat GmbH and isresponsible for the development, patenting and inter-national expansion of the Openair plasma technology;christian.buske@plasmatreat.de

DR. ALEXANDER KNOSPE, born in 1969, is in amanagerial position with responsibility for applica-tions technology, layer development and the furtherdevelopment of coating technology;alexander.knospe@plasmatreat.de

Fig. 7. Flexible nozzle system with specially developed evaporation, in which the gap between thenozzles can be adjusted at will

© Carl Hanser Verlag, München Kunststoffe plast europe 11/2005

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