Digital BW, The Print

Paul Roark wrote:
> Kodak's story (working from memory here) with respect to the RC cracking was
> that the OBAs were originally titanium dioxide.  

Paul, if I recall it correctly, titanium dioxide is a highly
opaque white pigment only but it can not be combined with OBAs
as it reacts with the OBAs causing  yellowing. I saw that
information not so long ago when I checked whether transparent
OBAs exist. TiO2 is still used in thin polyethylene foils to
give the foils a high white opacity  despite their thickness.

They put out a strong
> oxidant that attacked the polyethylene and cracked it.  In response, a
> number of companies put anti-oxidants in the coating to deal with this
> problem, but they yellowed.  Now, presumably, there is a combination of less
> reactive OBAs and non-yellowing anti-oxidants that solve the RC problems --
> according to the manufacturers.
> 
> Polyethylene is apparently a broad category of materials, within which there
> can be significant performance differences.  Whether what is used in the
> papers is up to the job might depend on more than just what the common name
> is for the material.

There are several grades and copolymeres. For foils like that
you would use high density polyethylene. Main problem of
plastics like polyethylene and polypropylene is getting a
bond between the coating and the plastic itself. The foil has
to be corona treated to change the polarity on the surface
after that there's a short time to create the better bond. In
time that may be lost again. Polyethylene will have
plasticisers added if there's a need for more flexibility,
paraffin wax is the more ordinary one. In itself not a nice
material to create a bond on. It also migrates from the
polyethylene in time. For this barrier purpose I do no think
that it is needed, the foil is thin enough to be flexible. For
outdoor use polyethylene has one bad property, it is easily
attacked by UV light that will shorten the long molecules to
shorter ones and make the material brittle in time.

> 
> One industry person I spoke with thought the new technology did involve a
> barrier, but it might not be polyethylene.

Enough other barriers possible and most likely it will be a
sandwich of more plastic foils. PVA for example often is used
in combination with other foil(s) to combine a gas and fluid
barrier in one layer and/or to create a better bond to paper
or coating.

> 
> Lots of uncertainty here ...

Right, you only have to check who supplies what in the coating
industry to know that there are a thousand combinations possible.

Ernst

                    --
           Ernst Dinkla


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