Plasma treatment has an explosive increase in interest and use in industrial applications as for example in medical, biomedical, automobile, electronics, semiconductor and textile industry. A lot of intensive basic research has been performed in the last years, also in the field of textiles and technical textiles.


This has resulted in an increasing knowledge of the possibilities of this process regarding demands as wettability, shrinkage resistance of wool, dyeability, printability, coating and washability of conventional and technical textile. All day problems of wettability and adhesion, together with the environmental driven forces have increased the interest of industry today.


A fundamental problem at this moment for the implementation of this technique at a higher level is the lack of adapted machines. Continuous or semi-continuous treatment is one of the typical working methods in textile processing and thus the most important demand of this sector. In the US and Europe there was no standard manufacturing of these machines.


Europlasma, a Belgian manufacturer of plasma machines has built yet several tailored made batch machines for (semi-) continue treatment from roll to roll, which operate at this very moment especially in the field medical textiles. These industrial sized machines can already fill in the needs of various applications. The production of a full continuous machine for plasma treatment of yarns is planned for the near future. These special demands related to the different applications ask for a custom design approach. Chamber design, system selection and process optimization are part of the job.


In this presentation laboratory results obtained in close cooperation with Centexbel, the textile research institute in Belgium, regarding the treatment of wool, cotton, polypropylene and polyester will be presented. Results on a factorial design experimentation for the most important plasma parameters; treatment time, pressure, power will be discussed. Also comparative results for different plasma generators (KHz, MHz, GHz) and plasma gasses will be presented along.


Further the results of some industrial implementations will be discussed, together with the limits and strong elements of the available plasma equipment.


Market needs


Increasing interest from the textile industry and more specific the medical and technical textiles has made it necessary to produce continue and semi-continue plasma treatment machines.


Starting from the plastic and medical market where plasma is knowing a still growing market segment since more than ten years, now the textile market is starting to know plasma as an interesting, environmental and economic alternative or even unbearable tool for making a surface (textile) wettable or to prepare it for an optimal adhesion after colouring, laminating, bounding or whatever where adhesion is of importance.


A lot of basic research has been performed. Plasma treatment and its capabilities have been cleared out and so possibilities related to increasing wettability, dyeability, printability, coating and laminate adhesion or decreasing shrinking resistance of wool are well known now.


Economical problems in the textile industry stresses the sector to diversificate to for example technical textiles with a lot of new applications as result. The exchange of traditional materials by for example polyolefines as polypropylene because of recycling properties of the latter one has placed the textile industry in front of new problems and challenges. The typical low energy, thus low wettability of these materials include problems related to this wettability, but also to all adhesion based production processes. Plasma is known for years in the sector of plastics for its capability to solve this problem.


The discussed problems together with the environmental or technical driven forces has brought us to the interest of today. Taking not into account the Russian equipment and the very recently designed and manufactured laboratory machines, a lack of adapted industrial sized plasma treatment machines for the textile industry was reality in Europe. A clear demand of the sector is the need for semi- and full continue machines for treatment from roll to roll, in line or off line.


Because the technical textiles market will grow and create a lot of potential, the need to adapted plasma treatment machines is urgent. Especially in the market were products with a high technical specifications and added value are produced (medical, aerospace, automobile, ...) plasma will find its way.


 

Plasma advantages


Following advantages are reached by using the technique:


  • Environmental friendly technique: because of the low energy consumption, the fact it is a dry technique (no additional drying step), no waste disposal problem and disposal cost.
  • Qualitative and full controllable process: all parameters are controlled by the unit and quality control possible by print-out and data-logging.
  • Effective treatment: higher degree of activation, longer shelf-life than alternative methods as corona and flaming.
  • Operator friendly technique : no chemical products, gases, etc.
  • No substrate damage or bulk property changes
  • Different processes can run in the same unit
  • No limit to substrate geometries: small and large, simple or complex, parts or textiles are possible


All these advantages makes it part of the future techniques for surface preparation and modification.


Plasma applications


The different possible treatments and their are applications are presented below.


Plasma can be used and industrial systems are on the market for following substrates:


  • small parts like hubs or balloons up to very huge and complex substrates
  • fibres, non wovens, paper
  • plastic foils
  • metal and ceramic parts


Still a lot of companies are dealing with certain adhesion problems when gluing, printing, coating or bonding has to be applied. In most cases one is still working with the standard chemical or mechanical preparation techniques.


Plasma can be an interesting alternative or solution with described advantages of being environmental and operator friendly, dry and complete controllable.


Therefore, plasma knows a growing market.


Another factor is the increased possibilities to modify the surface of quite a lot of materials. Nowadays and future needs in the medical market can be solved by looking to and using the plasma technology.

Europlasma Machines


Europlasma, a Belgian manufacturer of plasma treatment machines has several production machines in the market. Also different laboratory setups are available for testing applications directly in the field. The different types will be presented and described together with their possibilities.


Europlasma is building standard and custom designed machines. Under standard machines is understood 4 types of machines with different chamber volumes, thus different production rates. Both a NON-PC and a PC controlled version are available.


Standard this equipment is delivered with a shelve system. But also barrel and roll-to-roll systems can be placed in this equipment.


The second group of machines are custom designed towards the client needs and products. This can vary from very small to very big systems with additional load/unload or other specifications.


For roll-to-roll systems there is a collaboration with a contractant which has several years of experience in the field of vacuum based roll systems.


Small roll-to-roll system


For laboratory purposes or specific applications of low amount and small scaled roll material our standard equipment can be dedicated to such purposes by using a small roll-to-roll cassette. The advantage of this system is the flexibility of the equipment because both shelves and roll system can be placed in the same equipment.


This system is also very useful for laboratory set-up and testing of the plasma treatment for your application.


Typical properties of CD600 type are:


Volume: 216 liter

roll diameter: 20 cm max roll

roll width: 30 cm

speed: 0.1-10 cm/sec


And CD1000 machine:


Volume: 490 liter

roll diameter: 30cm

roll width: 45cm

speed: 0.1-10 cm/sec


These systems can be used were no high demands on tension and production rates are posed.


Large roll-to-roll system (1m width)


More sophisticated batch systems for roll-to-roll treatment of roll material with a dimension of 1 m width, 0.6 m diameter are yet used in production environment in the field of medical filter materials. The speed for these materials is typically 0 to 5m/min. The final speed is dependant on the application and the grade of treatment.


The tension of the material is controlled, which makes it possible to treat sensitive materials.


 

Properties:


Volume: 1680 liters

Speed: 0-20 m/min

roll width: 1 m

roll diameter: 0.6m

Advantage of this system is that it can be upscaled to higher widths without problem.


The loading/unloading limits the acceptable weight of the rolls.


60" web treater (width 1.5m)


A more easy loading/unloading system to treat rolls with a diameter of 0.5 m.


Advantage of this system is the low to the ground loading/unloading facilities.


Properties of the system:


Speed: 0-20 m/min

roll width: 1.5m

roll diameter: 0.5m


Fiber treatment


The 4th type of machine is dedicated to the treatment of fibres. A continue and semicontinue system is available. Fibres of different kinds can be plasma treated, only by small adaptations to the machine.


The continue system treats fibres from air to air, in line with a speed of 0-40m/min.


The semi-continue machine is a batch process, which means that the roll material is placed in the chamber.


Properties:


Bundle diameter: 5mm

Speed: 0-100 m/min


Experience learns that in line treatment has more disadvantages than advantages because speeds are not necessary equal and some materials deliver strucking problems due to the properties of the material.


In general


Customer design leads to very specific systems, which posed up till now no problems towards the effectivity of the treatment. The dimensions and properties of the textile determine the final mounting system of the roll material. Special feature like pull tension on the material and guiding of the rolls have to be provided.


Capacity problems can be solved by scaling up the machines which increase of course the machine cost.


A very important and useful tool is the PC control installed by EUROPLASMA on its plasma equipment. This PC control increases the efficiency of working and following up of the system.


From full automatic treatments to basic manual operations is possible with this system. The clear and schematic interface improves the understanding and following up of the operation by the operator. By means of recipes different treatments can be programmed and easy being selected. All information of runs, errors and parameters is automatically stored and can be consulted any time by the registered persons.


 

Other features:


- password protection

- batch and operator identification

- parameter control

- process automation

- data-logging

- process diagnostics

- traceability

- flexibility




Future


At this moment, EUROPLASMA is thinking out the concept of continue machines for treatment of fibres and webs. The roll material, which is then mounted in atmospheric environment is guided through the system (under vacuum) and coming again in atmospheric environment. This must allow the customer to automise and speed up the plasma treatment. Nevertheless first the batch based systems will conquest the market.


Some R&D results


In close collaboration with Centexbel, the textile research Institute in Belgium, a regional research project has lead to some interesting results regarding the treatment of wool, cotton and polypropylene.


Results are off-course depending on treatment conditions: plasma generator, plasma frequency, plasma chamber, primary or secondary plasma, vacuum, plasma gas composition, treatment time, plasma power, and also on differences in material and material presentation form. Therefore it is sometimes difficult to compare results or it is dangerous to generalise too much in describing the obtained effects. During the above mentioned project the efforts were focused upon oxidative plasma conditions using in most cases oxygen as plasma gas, or in a few cases gases with a partial concentration of oxygen.


Without going into detail the effects of oxidative plasma treatment are summarised for the most important textile materials.


Treatment of wool


Plasma treatment of wool has already been examined by several research institutes.


In general similar results are reported although the intensity of the obtained effect might differ from case to case.


For our trials a knitted wool fabric, without special pre treatment was used.


Plasma treatment was performed in a plasma chamber of 1 m3, using a secondary plasma generated with a 40 kHz plasma generator with a plasma power between 2000 and 3500 watt. Treatment times were varied between 1 and 7 minutes. The most important effects observed during our evaluations are:


  • improvement in wettability with a factor 100 to 1000. The original fabric is very hydrophobic, after treatment a water drop is taken up within one to two seconds,
  • no significant change in mechanical properties of individual fibres,
  • important change in mechanical properties of the yarns
  • increase in yarn tenacity with up to 50%
  • increase in yarn elongation at break up to 250 % (depending of the original value before plasma treatment)
  • improved anti-felting character and shrink resistance during laundering.


The original surface shrinkage of 57 % (according TM 31) can be reduced below 10% after plasma treatment, offering "super-wash" quality to the treated fabric.


 

Regarding the origin of the improved anti-felting behaviour, contradictive explanations are proposed. According to us the anti-felting behaviour is due to an increase in fibre/fibre friction. This reduces the slipping of the fibres one to another that causes the shrinkage and felting of the wool material. The increased friction offers also an explanation for the increased yarn tenacity and elongation at break. It was indeed shown that fibre/fibre friction can be increased with more than 50 % after plasma treatment of wool sliver.


Treatment of cotton


Similar effects can be observed after treatment of cotton fabrics or yarns in oxygen plasma. In general the effects such as improved wettability, increased strength or fibre/fibre friction are less pronounced if compared to wool materials.


Much depends upon the level of pre treatment already applied to the material. For instance regarding wettability the improvements are not significant if the plasma treatment is applied to full bleached cotton. If intensive oxygen plasma treatments are applied to cotton fabric also negative effects can be observed namely a reduced tear and abrasion resistance. The negative impact can be minimised by selecting appropriate treatment conditions.


Treatment of PP


PP is a very interesting material for plasma treatment. PP is a very hydrophobic material with extreme low surface tension. On the other hand PP is used in a large number of technical applications were an improved wettabillity or adhesion properties are advantageous. This is also the case for PP technical textile applications such as filters, medical or hygiene applications etc.


Using an oxidative plasma important improvements in surface tension can be obtained within a very short plasma treatment. In order to obtain an optimisation of the plasma effects, a systematic approach (factorial experimental design) was followed for the evaluation of the plasma parameters upon the obtained effects.


PP non-woven filter webs were used as test samples. The treated samples were tested for wettability using liquids with a range of surface tensions and also the filtration time for fixed amounts of water was determined. Reference materials without plasma treatment can only be wetted with liquids with surface tension < 35 mN/m.


Without treatment, no water can pass trough the PP-web without applying a high pressure.


Treatments were performed with two different plasma generators: a kHz and a GHz system. In both cases a 64 I. plasma chamber is used. Parameters varied were: treatment time, vacuum level, treatment power. In addition some tests were performed with an adapted gas composition in stead of using 02.


The obtained effects were somewhat surprising. As expected we could observe for all plasma treatments an increase in surface tension. However the Increase in surface tension is not in correlation with the intensity of the plasma treatments. We expected an increase in the Improvement in the wettability properties the higher the plasma power and the treatment time is and the lower the vacuum pressure is. Once an optimum level is reached we expected the wettability would become constant and independent from the treatment intensity.


The results showed that an increase in wettability can indeed be observed but only at relatively low treatment intensities. Once the optimum is reached a sharp drop in wettability is obtained if the plasma treatment intensity is raised further. For instance a treatment in the kHz apparatus at 250 mTorr, 200 Watt and 120 sec. gives far better results than a treatment at 150 mTorr, 800 Watt and 240 sec. Especially the vacuum pressure and the applied plasma power have a large influence upon the observed over-treatment.


Also for the GHz plasma treatments similar effects are observed, although in general the treatment parameters are less critical. Using the GHz instrument it was observed that efficient plasma treatments still can be obtained at a relatively high vacuum pressure level of 1500 mTorr and higher. This can offer advantages since a high vacuum is difficult to maintain especially during continuous processes.


Most trials were performed with O2 as plasma gas, since it was thought this gas would offer the best results for an oxidative treatment in order to obtain wettability.


 

Some additional tests were performed using air or argon/O2 blends as plasma gas.


The results indicated that often air or a blend of gasses with a lowered amount of O2 offer a better wettability to the treated PP material. This effect can also be linked to the previous observations. Plasma treatment with pure O2 seems to be so aggressive that very easily an "over-treatment" is obtained reducing again the wanted hydrophilicity effects. Treatment with air as plasma gas might reduce the aggressiveness of the treatment and as a consequence the plasma parameters become less critical. Future trials have to confirm these findings for a larger range of individual plasma parameters.


As a conclusion we can state that for PP oxidative plasma treatments can be very beneficial in order to improve wettability and increase the surface tension. However the treatment conditions should be selected with utmost care especially since selecting conditions that are too aggressive will result in a lowering of the positive effects.


Often, when the feasibility of an industrial oxidative plasma treatment has to be proven, the first trials are performed with an intensive plasma. If the results are insufficient people tend to increase further the plasma intensity. As was discussed above it can be worth while to try the opposite and to lower treatment time or plasma power or to increase the vacuum pressure. This might not only simplify and reduce plasma treatment costs but can also improve the wanted properties.


Applications


The number of applications where plasma is or can be used successfully is very large. Also the type of application can be very different. A short list of application were plasma machines are used is given below:


Medical industry


  • wettability of filter material (paper and synthetic)
  • medical textiles and blood filters


Fibers


  • cord for industrial and automotive belts


Technical textile:


  • automobile textiles


Safety textiles:


  • work wear


Structural textiles


  • fibres for structural composites (building, sports wear)


Foils


  • Packing material and foils


Europlasma capabilities


Following services can be delivered:


  1. custom designed machines
  2. test facilities
  3. service contracting


Europlasma is a growing company with sales allover the world. Agents in Germany, Swiss, France, Asia, U.K and a division in the USA. Standard and custom designed machines are produced in Belgium and placed all over the world.

 

Custom design is performed on client specification and demands and in close collaboration with experienced contractants.


Europlasma has its own test facilities including the test set-up for fibres and textiles on laboratory and production size. This first evaluation is giving the opportunity to convince ourself on the capability of plasma before production integration.


For small scaled production a service is delivered to plasma treat products in reasonable time and cost.


Future will bring us to real continue machines which can be placed in-line or off-line.


The high technical demands, environmental aspects will force textile industry to pre treat their material and to get the most optimal adhesion or wettability performance. The presently available materials, read polymers, will always need surface activation or preparation before further treatment.


At this moment batch wise machines will fill in the demands of the sector, and as it is for web coaters, batch based machines still perform day in, day out their job.


Plasma technology: what is plasma?


A new alternative for better modification of polymer, ceramic and metal surfaces and textiles.


Introduction


Surface preparation and modification has gained in the last decennia an enormous interest and discovered new applications. It is a complete other approach to modify only the surface properties without changing the bulk properties. This delivers new materials with new possibilities, which opens perspectives to resolve production or design problems or even develop complete new applications.

Production problems are mainly caused by the substitution of the base material to new materials for example polymers, which have not the correct surface behaviour for further processing.


In design it asks of course another way of thinking because one has to take distance from the conventional mechanical and chemical modification of surfaces.


The low pressure plasma technology is such an alternative where on a dry, environmental friendly and cost-efficient way the surface IS modified on microscopic level. This without manual operations or the use of chemical products.


Low pressure plasma technique


A plasma is a partially ionised gas containing ions, electrons, atoms and neutral species. To be able to ionise the gas in a controlled and qualitative way the process acts under vacuum conditions. Therefore a vacuum vessel is pumped down to a pressure in the range of 10-2 to 10-3 mbar with the use of high vacuum pumps. The gas which is than introduced in the vessel IS ionised with the help of a high frequency generator. The formed environment is the so called 4th state of matter. Which means that if sufficient energy is supplied that solids can be melted to liquids, liquids can be vaporised into a gas, and gases are ionised into a plasma. A specific characteristic of a plasma is the visible glow discharge with colours ranging from blue-white to dark purple depending on the type of gas (see pictures above). The high reactive particles react with the surface of the substrate. The advantage of this plasma is that it is a well controlled and reproducible technique.







 

Table: Typical surface tension and contact angle values after plasma treatment



Surface Energy

dynes/em


Water CONTACT ANGLE

degrees


BEFORE

AFTER

BEFORE

AFTER

1. Hydrocarbons





Polypropylene

29

>73

87

22

Polyethylene

31

>73

87

42

Polystyrene

38

>73

7`2.5

15

ABS

35

>73

82

26

Polyamide. Polyethylen copolymer

<36

>73

63

17

Epoxy

<36

>73

59.0

12.5

Polyester

41

>73

71

18

Rigid PVC

39

>73

90

35

Phenolic

Note

>73

59

36.5

2. Fluorcarbons





Polytetrafluorethylene/

Polyethylene copolymer

37

>73

92

53

Fluorinated ethylene propylene

22

72

96

68

Polyvlnylidene

25

>73

78.5

36

3. Elastomers





Silicone

24

>73

96

53

Natural rubber

24

>73

Note

Note

Latex

Note

>73

Note

Note

Polyurethane

Note

>73

Note

Note

Styrene butadiene rubber

48

>73

Note

Note

4. Fluoroelastomers





Fluor carbon copolymer elastomer

<36

>73

87

51.1

5.Engineering thermoplastics





Pet

41

>73

76.5

17.5

Poly carbonate

46

>73

75

33

Polyamide

40

>73

79

30

Poly aramid

Note

>73

Note

Note

Polyaryel ether ketone

<36

>73

92.5

3.5

Poly acetal

<36

>73

Note

Note

Poly phenylene oxide

47

>73

75

38

Pbt

32

>73

Note

Note

Poly sulfone

41

>73

76.6

16.5

Poly ether sulfone

50

>73

92

9

Polyarylsulfone

41

>73

70

21

Poly phenylene sulfide

38

>73

84.5

28.5

 

Gas plasma process


Following treatments are possible in one and the same equipment:

  • Surface activation
  • Surface cleaning
  • Micro etching
  • Plasma polymerisation


The formed reactive particles react in a direct way with the surface without damaging the bulk properties of the treated part. In fact the surface modification is limited to the outermost 10 to 1000A of the substrate. Following process mechanisms are possible on polymer, fluoropolymer and other plastic surfaces:


Surface activation


Is the replacement of weak bondings by high reactive carbonyl, carboxyl and hydroxyl groups. The surface is super active after treatment which is not the case for an untreated polymer surface. Activation can also be performed with amino groups or other functional groups.


Cleaning


Is the removal of the organic contaminants at the surface. Non visible oil films, rest contamination layers, Sicontamination and even partially absorbed contaminations can be removed by plasma cleaning. Particles or anorganic contamination are not removed by plasma, because there is no mechanical interaction. Also visible oil films will not be removed economically by this technique. Therefore it is called super-fine cleaning.


Micro-Etching


Is the process to break down the weak covalent bondings at the surface. The outer most molecular layers are affected by this as is also present contaminant layers which are of the same type of weak C-H bondings. In this way the surface is cleaned, oil films and possible plastic injection moulding additives or weak bondings at the surface are removed to result in a uniform active polymer surface.


Deposition


Is the process where out of gases a thin polymer coating is formed at the surface. By choosing the gas and process parameters these thin coatings can be deposited with various properties or acting as barrier layer.


Cross linking


Is the result of an inert gas action to cross link the polymer surface. A stronger and harder micro surface is formed. In some occasions it leads to more wear or chemical resistance of the polymer.


Through gas selection and parameter setting it can be determined which of these physical processes will be the main one.


For metals and ceramics plasma treatment is mainly a matter of cleaning and deposition as can be understand from the physics described above.