You ask two different questions : one on transparency the second on color.

For the transparency you have two contributions for light diffusion : light diffusion from the surface, i.e. the surface roughness at nanometric scale, usually named gloss in industrial measurements, light diffusion inside the volume,coming from fluctuations in refractive index widely recognized in semi-crystalline polymers, usually named haze in industrial measurements. With these keywords you should find a large number of papers mainly devoted to the optical characterization of films in the same range of thickness.

For the color a classical way is to measure the transmission spectrum with a UV-visible spectrometer. It is possible to predict color components in a color-space through a conversion. You can use the CIELab or other space. Please refer to the Commission internationale de l'éclairage web site for further information. There is also a lot of literature in that field which can found with these keywords. Some meaurements are also possible with goniospectrocolorimeters to measure the color change as a function on incident and observation angles. You should be able to measure the yellowness of yours samples. The same obseravtions are also possible with light-reflection.

Jon, I am also working on PLA polymeric films, and my recomandation is to do the optical measurement with chromamether, but this equipment is mesuring only visual colours up to 400nm. its very easy and you can have all the paramethers L, a b...

You could measure several properties such as O2 or H2O permeability, mechanical properties, bright, colour, etc. If you need help to perform this measurement we could find a way to collaborate. In my institution we have film characterization equipments under international standards.

Best regards.

Dear Jon, I wonder why you are interested in OPTICAL properties, especially in the VIS-range, while for food packaging purposes, the permeability for reactive species such as oxygen, humidity or small carbon-containing molecules might be even more important as also the UV-light transmittance has its importance (e.g. for oxidation or catalytic processes within your food stuff. So I would recomment (as Gabriel mentioned above) permeability studies and optical transmission etc in the ultraviolet spectral range! Best regards, Dirk

Hi! thank to all of you very much!

I have already measured barrier, mechanical and thermal properties among others, i just wanted to make some extra analisis, but thank you very much for your sugestion.

The optical properties, as transparency, in my opinion are important, because the industry wants the customer to see the product that is only possible if the film is transparent. If you have a film but is yellowish which happens for example with some pla/clay composites, the might might look like rotten and that´s not good.

But that´s truth that I should had measured in a bigger range than i made (200 nm - 600 nm) to study the protection against UV light which degradates the food more than the visible light.

One known method is the ellipsometry, from such an experiment you can extract the refractive index knowing the thickness or vice versa and eventually test the thermo-optical properties with the appropriate heating source.

Dear Jon,

Here you have a list of several test you could carry out:

-Differential Scanning Calorimetry (DSC)

-overall migration into olive oil by total immersion.

-overall migration into aqueous simulant by total immersion

-Permeability Oxygen

-Permeability water

-Artificial ageing, artificial radiation exposure.

Best regards.

Hi! Thank you very much for your suggestions ;)

For thinner films you could go with ellipsometry, but in this case I would just coat one of the faces of a UV cuvette and take a plain spectral curve with a spectrophotometer. It will not get any easier than that. If you want to measure the thickness, just use a plain profilometer. In addition, if you want to measure other properties, you could try coating a microscope slide and take a FTIR.

You ask two different questions : one on transparency the second on color.

For the transparency you have two contributions for light diffusion : light diffusion from the surface, i.e. the surface roughness at nanometric scale, usually named gloss in industrial measurements, light diffusion inside the volume,coming from fluctuations in refractive index widely recognized in semi-crystalline polymers, usually named haze in industrial measurements. With these keywords you should find a large number of papers mainly devoted to the optical characterization of films in the same range of thickness.

For the color a classical way is to measure the transmission spectrum with a UV-visible spectrometer. It is possible to predict color components in a color-space through a conversion. You can use the CIELab or other space. Please refer to the Commission internationale de l'éclairage web site for further information. There is also a lot of literature in that field which can found with these keywords. Some meaurements are also possible with goniospectrocolorimeters to measure the color change as a function on incident and observation angles. You should be able to measure the yellowness of yours samples. The same obseravtions are also possible with light-reflection.

Maybe spectroscopic ellipsoemtry is a sensitive non-destructive method getting the optical porperties out of the spectra. We also use Imaging technique to observe other effects, like temperature, swelling and pH of the polymeric material.

Best regards

Chrsitian

Haze and gloss are the most common requirements for packaging films.