JEC COMPOSITES MAGAZINE - Issue #93 - December 2014 - 63

release liner. Depending on the stiffness
of the fibres and the radius of the rollers, this can be a challenge. Again, it is
important to have the right release liner
with the right release level to prevent
delamination.

Automatic tape layer
The prepreg then enters the final processing step to create the composite component. An automatic tape layer (ATL) is
often used to cut the prepreg into the end
product. ATLs often have small rollers,
which means that the radius the prepreg
has to follow is very small. Again, to
prevent pre-release of the prepreg from
the release liner, the release level of the
silicone coating has to be tight enough.
Most ATLs are able to slit the prepreg
into the needed shapes during the lay-up
step. In order to cut through all of the
fibres, the knife is normally adjusted very
low and therefore cuts into the paper as
well. If the paper is not strong enough,
this might lead to web breaks, which is
not acceptable for the production process. Laufenberg and its paper suppliers
have developed special paper grades with
enhanced tear resistance to prevent web
breaks during this process.
When the prepreg is finally laid to build
up a composite part, the release liner has
to fulfil its duty and "release". If the release level is too tight, it will not release
well. If the release level is too easy, then

Wide portfolio of test methods

Advanced slitting and finishing operation

there will be a pre-release, which is not
acceptable as well.
By controlling the release level of the
silicone coating with a so-called "controlled release additive", it is possible to
achieve a stable release without bonding
or pre-dispensing from the composite
part. Laufenberg has been supplying
the composite industry for more than
20 years and is highly experienced in
adjusting the release to the needs of the
customer and to the application.

The right base paper
The choice of the right base paper is
important as well. Laufenberg has developed special paper grades for prepreg
applications meeting the requirements of
the process.
An important issue in prepreg processing
is paper debris. If a slitting edge is not
well cut, paper fibres stick out. If these fibres pile up, they might be transferred as
paper debris to the prepreg. Laufenberg
is able to offer a paper grade that gives
an enhanced slitting edge and therefore
reduces the appearance of paper debris.
To get a better runability for ATL application, the company offers two more
paper grades with enhanced friction
properties. These grades are perfect for
ATL applications as they offer a higher
tear resistance as well. Because of its high
strength, this paper grade offers the best
protection against cutting into the paper
on an ATL.

with very little tack. In order to provide
release liners for these types of resins,
Laufenberg has developed a paper with a
special surface, which supports the adhesion of the prepreg. This liner combines
strength, low friction and a good adhesion of prepregs with nearly no tack.
For special applications where a high
dimensional stability is required, the
company offers a paper with a moisture
barrier on both sides. Therefore, the paper is protected from shrinking through
the loss of moisture and from expanding
through the absorption of moisture from
the environment. The end result is a high
dimensional stability. Unfortunately, this
grade is limited in temperature stability
but for applications involving lower temperatures, especially in combination with
moisture-sensitive resins like cyanate
esters, this grade could be an interesting
choice.
For some prepregs, a transparent film
substrate like silicone-coated PET could
be interesting as well. The PET film has
a very smooth and even surface. This
gives a nice surface to the prepreg as well.
Of course, this liner is also limited in
temperature.
As the release liner plays an important
role in the production and further
processing of prepregs, it is important
to select the right release liner from the
beginning in order to prevent further
problems. n

There is a tendency on the market to
use more and more resins without or
No93 December 2014 /

More information:
www.laufenberg.info

jec composites magazine 63

http://www.laufenberg.info

JEC COMPOSITES MAGAZINE - Issue #93 - December 2014

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