THIN FILM IWASTE MANAGEMENT
Some of the demanding requirements of industries in thin film manufacturing are outlined below: Thinner films down to 20 microns thick which are particularly difficult to handle and process especially in sheet form.
The films often have structured rather than flat surfaces. This structure can entrap contaminants within its topography resulting in reduced cleaning effectiveness.
Many of the coatings used are soft and easily damaged through pressure or scratching.
Very small particles can cause “killer defects”
To achieve satisfactory yields in these high technology applications exemplary levels of cleanliness in both coating and conversion operations is required. This must however be achieved without impairing the processing of and without damage to the substrates involved.
State of the art in surface cleanliness is currently the preserve of the semiconductor wafer industry where particles of even a few nanometres must be removed not only from the wafers themselves but also from all the process equipment surfaces. The primary cleaning techniques used in this industry are scrubbing using PVA brushes combined with Ultra and
Megasonics, lasers, plasma and blasting with CO2. While these techniques are ideal for rigid substrates with small surface areas they are not adaptable to processing wide webs or flexible products. Many coatings used are moisture sensitive and any wet technique such as scrubbing or Ultrasonics would damage the coatings. In addition “the adhesion forces of submicron sized particles is sufficiently large that non-contact methods of particle detachment will not remove these particles. The removal energy cannot be transported to the particle sue to the static fluid boundary layer on the surface.” (Kohli, R, 2002)
Clean contacts
Contact Cleaning has been successfully used in the film coating and conversion industries for many years. The currently accepted wisdom regarding the technology is that softer rollers clean more efficiently, ie that cleaning is a function of the Shore Hardness of the elastomer roller. The downside of softer rollers is that they apply a higher adhesion force to the film substrate resulting in tension issues, stretching and scratching in web processing and wrapping of thin films in sheet operation. Currently film processors are in a dilemma namely do they uses harder films to reduce processing issues at the expense of cleaning effectiveness or do they focus on maximum cleaning. Teknek has recognised the need a achieve a balance between optimising cleaning, faster processing and minimising damage
Figure 2. Surface of Teknek Nanocleen Elastomer
Figure 3. Diecut edge of a structured film with contamination
especially for sheet conversion operations. It has developed a model of its contact cleaning system based on the substantive body of research into the physics of small particles and the forces affecting their adhesion to substrates developed within the semiconductor industry. (Mittal, K, 1995, 1999, 2002, 2006 ) Small particles are considered to be those under 10 microns as there is a transition away from gravitational forces being dominant at around that particle size.
Research efforts
The aim of this research work was to maximise the removal force on the particle while reducing the adhesion or grab force on the filmic substrate. There are three elements which interact to form the adhesion forces system of the particles on the substrates. These are Particle Transport and Deposition, Particle Adhesion and Particle Detachment. Contact cleaning is concerned with particle detachment but to optimise this element the relative adhesion forces between the particle and the substrate, the elastomer roller and the adhesive roll all have to be taken into account. Initially the adhesion force of deposition onto the elastomer roller must be higher than the adhesion force of the particle to the substrate. Meanwhile the adhesion force of the elastomer to the substrate must be low to avoid processing issues and damage. Finally the adhesion force between the adhesive roll and the particle must be higher than that between the elastomer roll and the particle while the adhesion force between the adhesive roll and the elastomer must be low.
Figure 1. Surface of Teknek F3 Elastomer
As particle size reduces the surface properties of the particle become more dominant than the bulk properties of the material from which the particle is formed. There are, in addition, many
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