We had occasion to process a moderately thin (.009”) and expensive, clear PTFE film. This film was to be used as component to a part that was, in turn, to be used in a sealed environment. Among the specifications was a requirement that the materials should not emit VOC’s above a certain level. The film
could not be made, at extrusion, to meet this requirement. It had to be secondarily cured to achieve an acceptable level of emissions.
This secondary process was nothing more than subjecting the film to an elevated temperature for a certain period of time, with significant airflow to draw off the VOC’s as they were discharged from the film. This was to be accomplished by running the film, roll to roll through a large convection oven that we typically use to cure wet coatings on a variety of substrates. Because the oven heat source and air movement were isolated to the top side of the web, the film had to be fed through the oven twice, inverting the film for the second pass. The processing values – temperature, airflow cfm and dwell (translated to run speed, given oven length) – were calculated by our customer’s engineers, based upon their understanding of the test that the film was to undergo to verify compliance to the maximum allowable emissions.
At the elevated temperature, the film became quite soft and malleable. This tendency created an impossible situation vis-à-vis web tension control necessary to carry the film through the oven from unwind to rewind. It would stretch and neck down such that it could not be made to run in proper spatial relationship to edge guides, and would rope and wrinkle in the rewound roll. These tendencies were compounded during the second pass, dramatically compromising yields. Much time, energy and film were expended endeavoring to correct these problems, without success.
Our solution was simple and intuitive. We purchased an inert paper stock, in a roll somewhat wider than the film, and used it to bear the pressure of web tension. It was stable under processing conditions and would properly track, using edge guides, from unwind to rewind. The film was then fed onto the paper from a secondary unwind, with its leading edge attached to the paper (with conventional splicing tape). The angle of feed of the film onto the paper was calculated to achieve a modest static bond between the two, thus keeping the film in contact with the paper, despite the turbulence of the airflow in the oven. The trailing edge of the film was also, of course, similarly attached to the paper. The paper was allowed to interleave with the film at the rewind station.
The resulting rewound roll was acceptably flat and wrinkle-free, with a tight and uniform paper wind interleaved with a suitably relaxed film.
This composite roll of paper and film had then to be fed again through the oven, such that the film was inverted for the second pass. This was elegantly achieved by the simple expedient of back-stripping the single outside layer of the paper, attaching the end of the film to what was now the opposite side of the paper, placing the composite roll (without changing edge orientation) on the primary unwind station (utilizing the edge guide) and feeding the paper and film combination through the oven for a second time with identical processing parameters.
Results for this second pass were virtually identical to the first pass, with the condition of the final interleaved roll of paper and film so improved that the customer dramatically increased his yield with this very expensive film.