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For corona treated polymers water can be used as the single liquid to
determine SFE in accordance with ASTM D5946-0. The SFE result is then
presented directly in the Static window when the ASTM parameter in the
Options Menu is set to “ON”(see Section 9.11).
For determination of SFE using other liquids please see Section 11 of this
User’s Manual.
Please note SFE calculations can only be done from static contact angle test
results!
8.9
Options screen
8.9.1 Enhance Mode
When a water droplet is applied on a surface of low reflectance (e.g. dark,
rough craft liner), the droplet base will appear against a dark background.
If the light level is increased, or the parameter CameraGain /Threshold is
adjusted to distinguish the droplet base from its background, the droplet
top can become too bright against the illuminated background. This is
where the image enhancing routine will be very helpful. The enhancing
routine is available in Static Mode, Dynamic Mode and Advancing/Receding
Mode. Please note the image enhancing function remains in operation also
when a new test mode is selected (e.g. changing from Static to Dynamic
Mode).
a)
Double click on the labels Offset Gain, Offset Exposure and
Enhance to start with the default settings.
b)
Place the instrument on the test surface and apply a liquid
droplet on the surface.
c)
Double-click in the video image to switch from greyscale to a
binary image if necessary.
Adjust the Camera Gain control until the top of the droplet is completed.
Then use the Enhance control to create a solid black contour all the way
down to the base of the droplet.
8.9.2 Base Line determination
A water droplet applied on a reflecting background will appear as a dark
’saucer’ when the contact angle is below 90 degrees. A high contact angle
above 90 degrees will appear as a ’black snowman’ with a waist against
the white background. Similarly a contact angle of 90 degrees will result
in a black circular shape. All these droplet silhouettes will be analyzed
automatically and the program will assign a base line at the interface
between the droplet and the test surface. The determined base line is
displayed as a dotted line in the captured image. The base line can be
adjusted manually as described.
By definition, the base line is the lowest section of the droplet resting on top
of the test surface.
8.9.3 Camera Gain and Threshold
The default value for Camera Gain and Threshold is established during
calibration. This light level determines if a pixel is black or white in the binary
image. These values can be modified by the operator to manage unusually
bright or dark test surfaces as part of a test profile stored under a button on
the Options screen
8.9.4 Field of View limitations
The measurement of very low contact angles is limited by the horizontal field
of view.
A standard PGX+ instrument has a horizontal view of approximately 6.5
mm. Droplets of 0.5 µL will reach a width of 3.7 mm at a contact angle of
six degrees. At an angle of 2.5 degrees this droplet will have a width of
five mm. In comparison a 0.25 µL droplet will reach a width of 4.6 mm at a
contact angle of 1.5 degrees. As we cannot expect the droplet to be perfectly
centered in the image, a useful width is some 4.5 mm at the droplet base.
This means 0.5µL droplets can be measured down to 3 degrees. Similarly, a
0.25 µL droplet can be measured down to a contact angle of 1.5 degrees. For
even lower contact angles, the droplet volume has to be decreased further.
