ANSWER
Generally polyester films have some low molecular weight material on the surface. This is most frequently oligomer that is residual material from the polymerisation process. There may also be other materials added that can migrate to the surface, such as slip agents in OPP, that may also reduce the coefficient of friction (CoF).
Often the surface of the polymer is treated to change the surface energy to improve the adhesion. This treatment to increase the surface energy can crosslink the low molecular weight material into the bulk polymer or can volatilise the material and remove it. Either way the low surface energy material is removed and the CoF will get worse (increase).
Even if there is no surface treatment the metallization itself will cover up the low molecular weight low surface energy surface and again the CoF will increase.
Thus there is no fixed CoF that specifically relates to a particular Optical Density (OD) but rather the CoF relates to the history of the polymer web and any surface modification and surface treatment that has been carried out. Pure PET films will readily block and so they are usually have fillers added to provide a controlled surface roughness. The surface roughness reduces the contact area and hence the CoF. The size, shape, type and distribution of these fillers controls the precise value of the CoF. This is usually higher than is ideal for and easy to handle film. Coupled with the residuals this can make the ease of handling acceptable.
Sometimes the back surface of the web is not treated and thus there may be the transfer of some of the low molecular weight low surface energy material from the back surface to the front surface once the metallized web has been re-wound. This can cause a problem in reduced adhesion of any subsequent coatings to the metallized layer but the transferred material may also aid the ease of handling.
Where a link may exist between OD and CoF is in the metallization nucleation and growth. Any low molecular weight, low surface energy contaminant on the surface, that helps reduce the CoF, will stop the aluminium wetting the surface well. This will make the aluminium nucleate and grow as a series of hemispherical islands. This will make the coating quite thick before it completely covers the surface. Whereas it the surface has been treated to raise the surface energy and correspondingly the CoF rises the aluminium will wet the surface well and instead of the hemispherical-like growth the islands will be very flat and spread well over the surface this helps make the coating completely cover the film at a lower thickness. Thus the same OD may be achieved at a slightly lower thickness than for a non-wetting surface, lower CoF film. However there will not be a direct correlation as the CoF can be varied by the filler or surface roughness as well as by any surface contaminant.
Thus for some materials there may appear to be a link between OD and CoF but this link may disappear depending on the film processing or with a change of film supplier and may never be apparent in other materials.
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