Corona pretreatmentto obtain wettability and adhesion

The prerequisite for the printabilityand adhesion of plastics, metalsand paper is the wettability withthe printing inks, primers and bondingagents. lf aqueous inks, primers andbonding agents are used, the wetta-bility must often be set by pretreat-ment. Pretreatment with a high-fre-quency corona is a common methodapplied in the surface modificationof plastic, metal and paper webs. Thereasons for the wide acceptance ofthis method are the good results,the excellent possibilities of controland the easy handling of the equip-ment used.This report deals with the essentialfeatures of corona discharges. Par-ticular emphasis is Iaid on the uniquepossibilities for the surface modifi-cation of polymers.

Wetting of surfaces

By wettability we mean the behaviourof a liquid on the surface of a solid. lf forexample water is applied to a hydro-philic surface, the water will spreadforming a uniform skin of water. On ahydrophobic surface the same quantityof water will form a multitude of tinydrops. The angle between the surfaceof the drop and the surface of thematerial describes the wetting beha-viour. Wetting depends on the chemicalcomposition and structure of the sur-faces in question, since the contactangle is defined by the surface energiesof the liquid and the solid surface(Young’s equation). The entire surfaceenergy and the P fraction which stemsfrom the polar atoms on the surfacemay be easily determined from the wet-ting behaviour. Typical values for thesurface energy and polarity of polymersare listed in Table 1.

The rule of thumb is that polymers arewetted by a liquid when the surfaceenergy of the polymer exceeds that ofthe wetting liquid. A comparison withTable 2 shows that polymers are usuallywetted by conventional organic sol-vents but not by water. However, a highdegree of wettability is an essential con-dition for the application of water-based paints, primers and bondingagents which are becoming increasinglypopular owing to their environmentalcompatibility. The required degree ofwettability may be obtained by surfacetreatment by structural and chemicalmodification of the polymer surfacewithout destroying the volume charac-teristics of the polymer. Figure 1 showsthe effect of the substitution of hydro-gen in PE on the surface energy. Coronatreatment in atmospheric air increases

oxygen and nitrogen concentrationthereby increasing the surface energyand wettability of polymers.

Whereas in the case of printing the sur-face energy must not be set too high, toprevent the ink spreading, in the case ofbonding complete spreading is desiredto obtain as large a bonding area aspossible. Experiments have shown thatwith complete wetting by the bondingagent a maximum bond is obtainedwhere the polarity of the substrate coin-cides with that of the coating, so thatunder certain circumstances it is also de-sirable to set the polarity as well as thesurface energy. However, upon detailedconsideration of adhesion in compositesystems, mechanical deformation in theproximity of the boundary layer mustalso be taken into account.

Repo r tN r. 10 2 E

Material surface energy (dyn/cm) polarity P

polyethylene terephthalate PET 43.0 0.02polyamide 11 PA 43.0 0.02polyvinyl chloride PVC 39.5 0.10polystyrene PS 33.0 0.05polyethylene PE 31.0 0.04polymethyl methacrylate PMMA 29.8 0.28cellulose triacetate 29.0 0.30polybutyl methacrylate PBMA 26.0 0.16polyvinyl acetate PVAC 24.9 0.32polytetrafluoroethylene PTFE 18.5 0.04polydimethyl disiloxane 14.1 0.04

Liquid surface tension (dyn/cm) polarity P

water 72.1 0.72Benzylalkohol 40.0 0.29Toluol 28.4 0.081-Oktanol 27.6 0.23Tetrachlorkohlenstoff 27.0 0.01Methanol 22.6 0.39Ethanol 22.1 0.21

Table 1 Table 2

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the entire length of the station andprevents the formation of strong sepa-rate sparks. Using ceramic electrodes,however, detracts from the pretreat-ment effect (see Figure 2) and so theseare only used in practice to treat metals.The typical electrode gap of 1.5 mm isvery small which means that directcorona treatment of this nature, wherethe material to be treated is transportedthrough the electrode gap, is limitedto the treatment of web material. The

Principle of a corona station

The typical corona treatment stationcomprises a roller on electric framepotential, guiding the web to bepretreated, and a system of electrodeson high electric potential. The voltageis so high that electric flashover occursbetween the electrode system and theroller. A dielectric coating on the rolleror the electrodes creates an even dis-tribution of the discharge sparks over

electrode gap is flushed with ambientair to cool the electrodes and to evacu-ate the ozone which is always formedin an air-operated corona. The physicaland chemical effects in the corona arevery complex (see insert page 5), but thepretreatment effects can be easily con-trolled by varying the web speed andthe power rating. The equipment is easyto handle and the pretreatment effectsare easy to reproduce.

The effects of coronaon polymers

When the corona strikes a plastic certainchemical reactions are initiated on itssurface. These chemical reactions maylead to crosslinking, i.e. new linksbetween adjacent molecule chains or tobreaks in the molecule chains. Since thestrength of atomic bonds depends onthe chemical structure, the pretreat-ment effect of the corona also dependson the chemical structure of thepolymer material to be pretreated, i.e.different polymers require differenttreatment intensities to obtain thesame surface energy. It is known, forinstance, that even the degree of cry-stallinity influences the effect of coronatreatment.

The polymer is oxidized very quickly atthe breaks in the molecule chains. The

oxidation leads to various functionalgroups on the polymer surface, e.g.hydroxyl, ketone, carboxylic acid,epoxy, ether and ester groups, all ofwhich contribute to an increase in sur-face energy. At present there is noknown process to direct the chemicalreactions released by corona treatmentin one particular direction. However, ithas been demonstrated that the bal-ance between the chain break andcrosslinking is affected by the presenceof water molecules. This means in prac-tice that the effect of corona treatmentmay depend on the relative humidity ofthe atmospheric air.

Owing to the slight degree of crosslink-ing in the modified surface the mobilityof the chemical groups modifying thesurface is also high. As a result of this areduction in the pretreatment effectoccurs in many polymers on storage

subsequent to pretreatment. Althoughthis undesirable occurrence can be mini-mized nowadays it still cannot be com-pletely eliminated.

Molten plastic films such as those usedin extrusion coating and laminating areoxidized by ozone treatment (SORBEX®)to render thern adhesive.

Figure 1The effect of substitution of hydrogen in PE on surface energy

Figure 2Effectivity of different etectrodes in the pretreatment of potypropylene

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The corona treatment in practice

The corona treatment equipmentcomprises a high-power generator andan electrode system. Modern gener-ators produce sinusoidal alternatingcurrent up to 20 kV with a frequency inthe range of 20 to 40 kHz. A high fre-quency is favourable for the effectivityof the corona discharge since it pro-duces more, albeit less intensive dis-charge sparks (see insert, page 5). It alsoensures an even corona discharge andimproves the service life of the dielec-tric.

The generator and the electrode systernare matched to each other and there-fore guarantee a high degree of utili-zation of the loaded electric energy.In modern generators the requiredimpedance matching occurs automati-cally which means that there is alwaysmaximum coupling of electric energyin the electrode gap, irrespective of thetype, thickness or width of the web tobe treated.

In order to set a particular surfaceenergy on the material a certainamount of energy must be applied toan area of the material to be treated.On the basis of numerous experimentsthe following extremely significant

equation was found to determine thecorona dosage D:

P: generator powerCB: corona widthv: web speed

Applying this formula it is possible todefine the required corona dosage in asmall pretreatment station on a labora-tory scale and then extrapolate to pro-duction conditions.

The effect of the applied corona dosagedepends on the electrode system.Although the wettability of water-soluble inks on plastics presents a cer-tain challenge, it can be achieved for allpolymers even under production condi-tions. The use of high-power electrodesdeveloped in recent years has proved tobe successful in this area. One of themost prominent of these high-powerelectrodes is the multiblade (MM) elec-trode system. In this system the coronapower is distributed over 4 to 10 parallelelectrode blades to obtain an evencorona with many small sparks. Figure 2illustrates the greater effectivity of the

MM electrode in comparison withsimple metal and dielectric electrodes.At the same time the reduction in sur-face energy during storage after treat-ment is less in cases where pretreatmentwas carried out using MM electrodes.

The required corona dosage dependson the surface condition of the polymerin each case and so the following valuesare only typical values. To obtain a sur-face energy of 45 dyn/cm the followingoutput is needed with an MM elec-trode:

PETP 5 W min/m2

LDPE 7.5 W min/m2

PP copolymer 12.5 W min/m2

PP homopolymer 25 W min/m2

The dependency of the required coronadosage means that the pretreatmentequipment must be adapted to the ma-terial to be treated to obtain optimumresults.

To meet the requirements of the print-ing and conversion industry a numberof corona treatment stations were de-signed. There are special units whichmay be integrated into printing ma-chines for sheets, cables, cups or tubes.There are also special designs for UVpainting systems, laminating machines,

extrusion coating ma-chines and folding boxmachines. The latterhave been developedto improve adhesiveproperties where bond-ing is carried out usingcoldsetting adhesives.Figure 3 shows a unitfor the pretreatment ofplastic films with multi-blade electrodes.

Today in practice coro-na widths of up to 8 mare treated at webspeeds of up to 800m/min.

Figure 3: Corona treatment station for plastic films comprising a ceramic-coated rollerwith (retracted) multiblade electrodes

D = PCB x v

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New developmentsand future prospects

The application of direct corona treat-ment where the materials are insertedinto the electrode gap is limited to filmsup to a few millimetres thick. Amongthe latest developments attempting toovercome this barrier, plasma treatmentat reduced pressure has become signifi-cant. Furthermore it seems possiblewith this process to set polarity as wellas surface energy using suitable gasesas part of the treatment. By adding hydro-carbons or silicon compounds to suchplasma, stable semi-organic layers canbe deposited on polymer materials. Ithas been discovered that the surfaceenergy of these materials may be setand that the surface energy duringstorage at ambient temperature canremain stable (see Figure 5).

Initial success has also been achievedwith indirect corona systems where thematerial to be pretreated passes by theelectrode. In this case a counter-elec-trode is not required and so this inno-vative method can be used to treat theouter surfaces of moulded parts of anythickness (e.g. foam rubber, cables,bottles).

SOFTAL has developed the IONAL sys-tem for such applications (see SOFTALReport No. 114).

Figure 5Surface energy of two

a-Si:C:N:H layers plasma polymerizedat approx. 20 mTorr

of hexamethyl disfloxaneduring storage in air

Figure 4SOFTAL test inks and test pens (38 mN/m) to measure surface energy

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The basic corona characteristics

Corona discharges occur when an elec-tric current passes through a gas-filledcapacitor with asymmetrical electrodes.Figure 6 shows a diagram of processesin a corona discharge in air for a modelarrangement (positive wire and nega-tive plate). lf the voltage applied is lowthere is no electric flashover. Howevera slight electric current still flowsbecause electrons (e–) are releasedfrom the wire owing to the highelectric field intensity in that area.The electrons are accelerated in theelectric field towards the level elec-trode and collide with atoms andmolecules of the ambient atmos-phere. In the process, besides otherelectrons, an immeasurable numberof molecule fragments, new com-binations and positive ions (M+) areproduced, which may also be elec-tronically excited (M*). An area ofhigh ionization is formed in the im-mediate proximity of the active elec-trode, which can be seen by macro-scope since the electrically excitedparticles may return to their initialenergetic state by the emission oflight. In their drift the electrons arecaptured by neutral atoms and mo-lecules forming negative ions (M–), sothat outside the ioniza-tion area the electric cur-rent is transported mainlyby positive ions. Owingto this electron attach-ment atomic oxygen andsubsequently ozone isformed in the air.

Irrespective of the polarity of theactive electrode, the density of thenegative (ne

–) and the positive (nM+)

charge carriers is approximately thesame in the ionization area. Theenergy of the ions, 1 to 2 eV, issignificantly greater than the ther-mal energy of the neutral gas par-ticles. Since the corona gas remainscold, the thermal effect of the coronadischarge is negligible. Therefore theionization area is similar to the plas-mas which occur with gas dischargesat reduced pressures. The drift regionon the other hand is characterized bythe predominant presence of only onetype of charge carrier, depending onthe polarity of the active electrode.Whereas the ionization area acts as achemical reactor, the ion energy(EM

+) in the drift region is too low toset off chemical reactions. lf the pres-sure in the capacitor gap is reduced,the ionization area spreads and thecorona discharge becomes a plasmadischarge.

lf the electric voltage in the arrange-ment as described is increased, theproduction rate of ions increasesmore quickly than the electrode wirecan absorb excess charges. The plas-ma of the ionization area expandsand forms a conductive channel, orstreamer, to the level electrode. Apowerful current may flow: a tem-porary short-circuit occurs. The peaksof the expanding discharge channelsare important for chemical processesin two respects. First, the energy ofthe positive ions (EM

+) may exceed100 eV at this point (this energy isvery high by comparison with atomicbinding energies in solids in theregion of 4 eV) and second, thelocally high electron energy (Ee

–) Of12 ... 16 eV leads to the increasedfragmentation of the surroundingmolecules so that chemical processestake place mainly at the head of theexpanding discharge channel. Forpractical application this means thatthe discharge sparks should beproduced as frequently as possible.Since the typical appearance fre-quency of the streamers is 10 kHz,it is recommended to operate co-rona equipment at a higher excita-tion frequency than 10 kHz. Due tothe electron attachments the streamer

quenches very quicklyafter approximately 20 ns.

Fig. 6: Schematic diagram of corona discharge processes,positive wire opposite negative plate

When the corona voltage isincreased the electric current alsoincreases and the electrodes areheated at the points where thedischarge sparks strike. With theresultant increase in electrode tem-perature the thermally stimulatedcharge carrier emission exceeds the

field emission. The discharge sparksthen produced are thermal arcswhich are in thermal balance, that ishot, as opposed to the plasma likestreamers. When poly mers areexposed to these hot sparks, a heattreatment of the material occurswhich may burn holes in films. In

corona treatment the aim is to haveas regular a spark pattern as possibleand so high currents are only usedwhere they can be distributed over anumber of electrodes (multibladeelectrode system).

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Electrode system CBAE for single side treatmentof conductive and isolating film.

Electrode system CRI for single side treatmentof BOPP film.

Corona discharge with Multi-blade-electrode (SOFTAL patent), BOPP-filmand silicone roller

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