I had a follow-up question relating to my answer about “orange peel” defects in BOPET films. The question is, “Is there any effect of the required addition of antiblock agent “silica particles” to the skin layer on the formation of an orange peel appearance?”
Answer: While antiblock particles can impact film appearance, In general I have a bit of a different idea as what “orange peel” is which I described in the earlier post and in general it would not equate with silica addition (for me orange peel is a surface defect caused by chemical deposits on the chill roll). I might be inclined to think that an interfacial instability might cause an optical defect more like an orange peel appearance than the silica addition. A viscosity mismatch between skin and core, which causes a slight interfacial instability, will cause an optical variance in the film which could be mistaken for an orange peel. It is possible that some rheological change in the skin resin, associated with the addition of the silica containing resin, causes an interfacial instability and the silica (which must be added) is blamed for the defect.
However, there are many defects in a film which can cause some sort of optical distortion in the film appearance or a disruption in the film surface gloss which might be like an orange peel. So part of the question is how do we differentiate between various defects so that we give them a proper designation so as not to cause confusion? It might be that the addition of the additives cause a variation in skin layer.
As to the impact of the silica itself, I can imagine that the use of silica could cause some optical defects in a film, but I would generally call it a surface or bulk haze effect. In general the addition of particles to a film surface is absolutely necessary to insure that the films we produce give good web handling and winding performance. It is necessary to add particles and the best place to add them is in the skin of a coextrusion so that the particles are where they need to be, at the surface. The use of coextruded films let us take the particles out of the core of the BOPET and will improve clarity by decreasing the internal haze of the film. Particles can cause haze in several ways, from the refractive index of the particle, the size of the particle and the amount of particles. Because the particles are located in a skin layer in this case it could cause gloss variations if poorly dispersed and could be miss identified as orange peel
Silica particles have a refractive index of about 1.55 while the refractive index of PET is 1.65 or more depending on the film density. This refractive index difference between the polymer and particle will cause some light scattering (haze) which could be called orange peel if it impacted surface gloss. Next if the particles are a particular diameter, they will scatter light just due to the size. Light scattering increases when the particles are approximately the same size as the wave length of light. For me an optimum particle diameter for winding in a thin skin would be about 4 microns in diameter. This should not cause much haze if used at about 2300 ppm. If used at higher levels they could cause a drop in surface gloss which might be mistaken for orange peel. The third source of haze is simply a large number of particles causing a rough film surface and bulk light scattering.
In general if the silica particles are properly dispersed in the skin layer I do not think they would cause an optical distortion in the film which I would equate with an orange peel but could cause a change in surface gloss of the film.
For more background on light scattering from articles, you can download a copy of a TAPPI paper I wrote “Optics, principle sources of haze and gloss” on film optics from my Website www.emmount-technologies.com