Surface Tension: The maximum is not always the optimum
A discussion on possible measuring methods, by Softal electronic. (Photos, charts)
By Softal electronic GmbH -- Converting Magazine, 7/3/2009 2:00:00 AM
Softal electronic GmbH (Hamburg, Germany) hosted a workshop organized by the Fraunhofer Institute for Mechanics of Materials which dealt with the dichotomy of optimum and maximum surface tension. On a day filled with intense discussions, the speakers, representing various research institutes and manufacturing companies, presented their solutions to a highly qualified and very dedicated audience.
Initial Situation
Most plastic materials have a surface tension which is too low to make them suitable for certain applications and/or further processing. The corona treatment , dielectric barrier discharge (DBE), is an established industrial process which was developed to improve the wettability of plastic surfaces. The use of atmospheric pressure plasma for plastic surface activation and modification offers great advantages with regard to efficiency and achievable adhesion characteristics, making it one of the most popular technologies in the film manufacturing and processing industry. However, plasma parameters, film materials and gas phase conditions have an impact on surface treatment effectiveness.
Industrial Evaluation of the Corona Treatment
In practice, the effectiveness of the corona treatment is usually measured using test inks. Right: The resulting value for a film surface tension can be determined with an accuracy of ±2 mN/m using a standard test ink set (blue) for surface tension measurement of plastic materials: 34…60 mN/m (dyn/cm).
Problem Statement
Reliability of a measurement using test ink need not be a matter of debate. The values in question can be determined with an accuracy which meets general practical requirements. However, a serious disadvantage of this method is the fact cannot it be carried out in line, but only selectively outside the production process.
This may leads to users applying a high dosage treatment to assure the required minimum surface tension. However, such an 
overdose, or over treatment, runs the risk of damaging the material. And in many cases, the converting processes (printing, lamination…..) will be hampered by the negative influence that over treatment has on the adhesion properties. What's more, the correlation of surface energy and adhesion is not guaranteed and must be examined for each case.
The workshop included a lecture reflecting the Fraunhofer Institute's position on how corona treatment can be effected by climatic conditions which sparked a lively discussion among the expert audience and brought up a score of suggestions for further examination.
Several measuring methods, starting with those used in practice today, down to those theoretically possible tomorrow, were presented and evaluated in the course of the workshop. The long and short of it is that even though a lot of work is being invested in developing a new method of inline measurement, not one of those methods is currently ready for industrial use. However, it seems only a question of time until one of these new methods will experience a breakthrough. The workshop participants expected the introduction of a new measurement method on the technical market before the end of 2009.
Five practical questions to the specialist, Softal´s Dr. Frank Foerster:
What is the optimum surface tension, how do I recognize it, and how do I achieve it?
Every plastic type, i.e. PE, PET, PP, PA, shows a characteristic behavior in relation to surface tension versus corona dosage and therefore have characteristic curves. Increasing corona dosage, optimum surface tension is given at the inflection point. Surface tension at inflection point provides optimum corona dosage. See graph at left.
What are the risks of overtreatment?
Increasing corona dosage beyond inflection point of surface tension will create low molecular weight oxidized material due to polymer chain scission. This material lies like dust on the polymer plastic surface with no chemical bonding to the bulk and will create insufficient adhesion in converting processes such as coating and printing. Furthermore, an odour can be created due to decomposition of additives which may have a negative influence in applications for food packaging.
What are the risks of too small a dosage i.e undertreatment?
Low corona dosage applied to plastic, i.e. a dosage less than optimum corona dosage, will lead to low surface tension due to insufficient functionalisation of the polymer, and will risk insufficient adhesion in converting processes.
Can I adjust after the first treatment?
Plastics corona treated in extrusion processes normally require a refresh treament of surface tension during converting processes to achieve the required adhesion properties.
How does time influence the surface tension achieved?
Surface tension of corona treated plastics during extrusion process decreases normally with time. Decrease in surface tension is due to the molecular rotation of polymer chains, and migration of additives from polymer bulk to the film surface. Polar groups created by corona treatment rotate from plastic surface into the bulk due to rotation of polymer chain. Additives migrating from bulk to plastic surface mask the polar groups and lead to decrease of surface tension.
A refresh corona treatment in a converting process will -oxidize non-polar groups of plastic surface and remove additives masking polar groups.
Latest technologies such as AldyneTM tackle this stability problem by providing and even surpassing the performance of expensive surface coatings at a fraction of the running costs and capital investment, this in combination with the reliability and ease of use of the long proven corona technology.
AldyneTM is based on an atmospheric pressure plasma discharge in carefully controlled gaseous atmospheres that can achieve extremely high level, longest lasting adhesion on various polymer films such as BOPP, cast PP, BOPET, PE and many others.
MORE INFO:
Softal electronic GmbH,softal@ylichtenauer.de, www.softal.de
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