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| Coating equipment |
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| Example of coating machine conformation |
Click to enlarge |
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Anchor coating (AC) head: Coats the adhesive (AC agent) |
| • |
Top coating head: Coats the PVDC latex |
| • |
Drier: Dries the coated AC agent and PVDC latex |
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| Materials
of coating machine |
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Contact with iron, aluminum, or copper will cause PVDC latex to coagulate; never use these materials where they will contact the PVDC latex. |
| • |
Materials which can be used in contact with PVDC latex include polyethylene, polyvinyl chloride, epoxy-lined steel, FRP, glass-lined steel, and SUS 316 stainless steel. |
| • |
Because a small amount of
hydrogen chloride is generated during drying,
anti-corrosion measures are necessary for materials
which will not contact the PVDC latex. |
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Coating methods
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| Air
knife coating: |
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Used
principally for coating film and paper. Coating
weight easily changed. Suitable for small lots
with frequent changeover. |
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| Reverse
gravure roll coating: |
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Used
principally for coating film and paper. High coating
speed possible. Suitable for mass production.
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| Spray
coating: |
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Used
principally for fiber binder and polyurethane
foam. |
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Dip coating: |
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Used
principally for polyurethane foam. |
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| Effect of storage conditions on physical properties |
Physical properties of PVDC latex are greatly affected by storage temperature. Improper storage temperature may result in diminished coating performance. Always maintain storage temperature at 3–30°C.
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| pH
and chlorine ion concentration: |
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The
pH and Cl-ion concentration of the latex will
increase with higher storage temperature. |
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| Viscosity: |
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Viscosity
of the latex will decrease with higher storage
temperature. |
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| Surface
tension: |
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Surface
tension generally tends to increase with higher
storage temperature. |
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| Coating
properties: |
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Barrier
properties of coatings deteriorate with higher
storage temperature. Transparency of coatings
also deteriorates with higher storage temperature,
with color tending to become yellow or reddish. |
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| Latex
condition: |
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Latex
which has frozen will not recover its original
properties when melted. High temperature may cause
coagulation. Even when stored in the proper temperature
range of 3–30°C, the quality of the
latex is not guaranteed beyond the specified shelf
life. |
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Coating with PVDC latex
For reference, typical coating on film is described here.
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| Selection
of substrate film: |
| •
|
Substrate
film must be able to withstand the drying temperature,
typically 100°C or higher. |
| • |
To obtain a
uniform coating, select a substrate film grade
with good thickness uniformity. |
| • |
Avoid substrate
film containing antistatic agent. Such antistatic
agent tends to bleed during coating, resulting
in poor adhesion and poor coating uniformity of
anchor coat and PVDC latex. |
|
| Corona
treatment: |
|
Corona
treatment is recommended to ensure strong adhesion
between substrate film and anchor coat. This is
especially vital when OPP or other low-adhesion
material is used as substrate film. |
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| Selection
of anchor coat (adhesive) agent: |
|
An
undercoat of adhesive, usually called anchor coat
(AC), is necessary to obtain good adhesion of
PVDC latex coating on substrate film. An unsuitable
AC agent will cause coloration when coated with
PVDC latex. Always perform trial coatings to confirm
the suitability of the AC agent. A range of AC
agents may be used, both solvent-borne and water-borne.
In our experience, excellent results have been
obtained with polyurethane AC agents. |
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| Additives: |
|
Additives
may be used to improve surface smoothness and
anti-blocking. Typical additives include inorganic
amorphous silica and organic wax. Caution is required
as an excessive additive loading will cause poor
barrier performance and hazing. Amorphous silica
will not disperse sufficiently if added directly
to PVDC latex; first add silica to water and mix
until well dispersed, then blend this mixture
with the latex. |
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Adjustment of
solids content: |
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When
diluting PVDC latex with water to adjust solids
content, never use water with hardness greater
than 300 ppm. Higher water hardness will cause
coagulation. |
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| Drying: |
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To
prevent skinning, it is generally recommendable
to set the temperature a little lower in the front
part of the dryer and a little higher in the back
part of the dryer. |
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| Wind-up: |
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Caution
is required with respect to the wind-up tension
and resulting inter-layer pressure in the wound
roll. Excessive pressure may cause blocking. |
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| Aging: |
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Heat
treatment after coating will improve barrier performance.
This is generally referred to as aging. It is
recommended that aging always be performed to
obtain the best barrier performance. |
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