Surfaces and barrier performance. Charles A Bishop C.A.Bishop Consulting Ltd

Transcription

1 Surfaces and barrier performance Charles A Bishop C.A.Bishop Consulting Ltd

2 Surface contamination debris organic contamination water - O - - O - H + H + H + - O - - O - - O - - O - - O - H + H + H + H + H + H + - O - - O - - O - - O - - O - - O - - O - H + H + H + H + H + H + H + H + substrate

3 Pinholes In theory All that is needed is a single perfect barrier layer i.e. defect free layer But The reality grain boundaries metallization debris perfect barrier layer substrate After cleaning debris typically > 300nm Barrier coating thickness <30nm substrate

4 Pinholes metallization debris Pinhole Pinhole extended into a scratch substrate substrate Debris typically >0.5 microns metallized layer nm

5 An example of an aluminium metallised PET film showing pinholes and scratches. The metallised film was held over a lamp and a photograph taken of an area ~150mm x 150mm

6 Diffusion coefficients Water vapour in air D H2O 0.26cm 2 /s Oxygen in air D O2 = 0.15cm 2 /s Oxygen in Water D O2 = 2 x 10-5 cm 2 /s at 20 C Oxygen in Quartz D O2 = cm 2 /s Oxygen in Silica D O2 = cm 2 /s Oxygen in Silica Gel D O2 = 10-3 cm 2 /s Oxygen in Polypropylene D O2 = 10-7 cm 2 /s Oxygen in Polyester D O2 = 10-9 cm 2 /s

7 Defects voids Grain boundaries & holes through film are next cause of poor barrier Dominant defects are pinholes the primary limitation to good barrier substrate

8 No defects i.e. pinholes, cracks or pores results in perfect barrier Barrier Improvement Factor The first defect has a catastrophic effect on the barrier performance An increasing number of defects further reduces the barrier performance When 5% of the surface area is covered by pinholes or other defects the coating contributes virtually nothing towards the barrier performance Defect area The effect of defects in vacuum deposited coatings on barrier performance

9 1.00E E E+11 Barrier Improvement Factor (BIF) 1.00E E E E E E E E E E Pinhole radius in microns Pinhole model 1 per sq m Coverage model 1 per sq m Pinhole model 1,000 per sq m Coverage model 1,000 per sq m Pinhole model 1,000,000 per sq m Coverage model 1,000,000 per sq m

10 Barrier improvements Eliminate pinholes Until pinholes are prevented making other improvements becomes pointless Once pinholes are controlled other areas can be improved Adhesion maximises interfacial bonding Minimise voids maximise coating wetting Densify coating, reducing porosity Control grain size minimise effects of grain boundaries Protect coating from damage consider overcoating

11 Contaminants Two levels of cleaning generally required The first level removes the gross dust/debris contamination The second cleaning process works at the molecular level & can include surface modification by ablation, crosslinking &/or chain scission.

12 What is dirt? Human Hair Clothing Fiber Packaging Fiber Slitting Dust Core Debris Machinery debris, bearings, paint chips Ink from printed stock Debris from metallizing Atmospheric particluates

13 Activity generating particles microns Rubbing ordinary painted surface 90 Sliding metal surfaces (unlubricated) 50 to 150 Crumbling or folding paper 60 Rubbing epoxy painted surfaces 30 to 75 Seating and unseating screws 25 to 120 belt drive 5 to 35 Writing with ballpoint pen on paper 15 to 30 Activity Snap smock Membrane coverall (particles/min) (particles/min) Sitting or standing still 100, Light movement: head, leg, arm 500, Heavy movement: head, leg, arm, foot 1,000, Change position: sitting down, rising up 2,500, Walking 0.9 m/s 5,000, Walking 1.6 m/s 7,500, Walking 2.2 m/s 10,000,000 1,000 Bringing in the team to find the cause of the dirt: 1,000,000,000 at the most critical part of the process

14 Black line from particulates at web edges e.g. slitting debris Image courtesy of Teknek Shows high level of web edge contamination

15 1,000,000, ,000, ,000, ,000, ,000, ,000, ,000, ,000, ,000, ,000, number of particles per cu ft for different particle sizes in microns Particle size distribution of atmospheric a concentration of 69 micrograms per cu m Number of particles in sample passed round ~ 47,797,610 (range 0.01 to >22 microns)

16 Surface defects Light scattering from surface defects Light can be scattered from the surface by any of the 3 processes shown on the left. Using optical microscopy it is possible to count the number of defects & approximate the size of each. bump from protruding filler particle dent in surface debris sitting on surface This can be time consuming but using modern image analysis it is possible to automate the process.

17 Surface defects Surface defects i.e. dents or bumps lead to defects in the coatings the coating thinner over the defect (> surface area for same coating thickness) & the edges of the defect are prone to cracking Substrate

18 Surface defects The following two techniques were used to obtain the measurements shown. Incident Dark Field Optical Microscopy Differential Interference Contrast Microscopy Area used for the analysis was 170 x 170 microns with a magnification of 400x. Average of 6 different fields of view was taken The number of defects below 1 micron is huge & is not shown The smaller the defects the greater the number present. It is almost impossible to remove defects of 0.3 microns or less from the surface & hence there will always be a high number of very small defects present if only cleaning is considered as a remedy.

19 Surface defects Film Type Thickness Defect Count Comments Average of 6 fields Polycarbonate (cast) SKC Mylar D Mylar D Upilex Both sides same Melinex O knurled Melinex co-ex A Melinex co-ex B ANO NB After slitting the defect count was, on average, 2x greater Average 1-2microns Not the worst tested

20 Web contamination Areas where dust/debris can be generated or come in contact with web on a polymer web production process. casting drum forward draw sideways draw ovens edge trim rewind rolls extruder Clean air hoods Class 1 or 10 + Positive pressure Additional high vacuum extract above & below slitting area Tack roll cleaning C.A.Bishop Consulting Ltd 2005

21 Cleaning options The list below shows a number of different options that are available for cleaning surfaces. Cloth wipe Static brush Rotating brush Air blower / knife Vacuum cleaner Combined ultrasonic air jet & vacuum Transfer tack rolls Overcoating Carbon Dioxide snow jet

22 Wiping with Rags Wiping with fabric roll / belt Limitation accumulation of debris at contact point, may cause scratching Roll can be rotated to refresh contact area with clean fabric Roll/belt system can work well with hard surfaces such as foils or drums

23 Cleaning methods When winding web at atmospheric pressure there is a boundary layer of air moving with the web. This boundary layer is hard to penetrate & makes simple vacuum or blowing cleaning techniques less effective than might be expected. Surrounding dirty air pulled in Boundary layer Vacuum System Blowing System Courtesy of Teknek

24 Cleaning methods electrostatic neutralized ultrasonically pulsed air vacuum extract to remove debris released from surface Space due to wearing bristles letting some debris past fixed brush Polymer web fluttered by the ultrasonically pulsed air The fluttering shakes off the debris Space due to wearing bristles letting some debris past rotating brush cleaner Courtesy of Teknek

25 Substrate cleaning Air flow creates turbulence disrupting the boundary layer releasing particles to be removed by vacuum extract vacuum air in vacuum electrostatic neutralisation shower substrate vacuum air in vacuum

26 Tack roll cleaning High adhesive tack roll - As layer fills with dirt it is peeled off giving fresh surface Adhesive roll High adhesive tack roll - picks up debris from Blue tack roll keeping blue roll clean Clean Surface presented to the incoming Material - No repeater s Tack roll - picks up debris Idle Roller - provides nip

27 Cleaning methods Efficiency of Cleaning Methods 100% 90% 80% 70% 60% 50% Contact Clean Machine 40% Ultrasonic 30% High Velocity Vacuum 20% Brush & Vac 10% Air Knife 0% Courtesy of Teknek Particle Size, Microns

28 Tack roll cleaning This photograph shows an example of a wide tack roll assembly used on a production film line just prior to the wind-up. The blue roll is the low tack roll that is in contact with the web. The upper white roll is the high tack collection roll. Courtesy of Teknek

29 Cleaning surfaces There is no point in cleaning one side only Clean one side & surface is recontaminated on the next wind-up This applies to dirt/debris oligomers &/or other low molecular weight material Similarly stabilising the low molecular weight material on only one surface using a plasma will result in recontamination from the other side on the next wind-up

30 Tacky roll cleaning Images courtesy of Polymagtek

31 Polymer pre-layer No cleaning requires thick coating Large debris Unfilled PET Filled PET + Unfilled PET Filled PET Cleaning = fewer, smaller debris & thinner coating

32 Thank you for listening Any questions?