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Due to the UV absorption, I knew that the plastics can be easily affected, especially PP. But is it normal that the change is always into faded blue from white color? and Why?

Because I made an experiments, but the results made me surprised. One sample of opaque white PP material was put on sunlight, one sample of same material having same conditions was put in lightproof black color bags. Then measurements was conducted every hour till 8 hours by using spectrophotometry which works in CIE Lab color space.

According to the results, b-value of sample exposed to sunlight had changed into the minus value, describing blue color in L a b color space, while L- and a-values are relatively stable. The b-value of another sample in lightproof bags had changed into the plus value, describing the yellow color in L a b color space, while L- and a-values are again relatively stable. So we can see that the effect of sunlight is only on b- value, which describes the values of blueness and yellowness of the sample.

My question is that what is the phenomena behind this?

What do you mean by reflected and transmitted lights? Even looked with naked eyes, easily seen that it becomes slightly faded blue color. The object is opaque white color PP material.

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  • $\begingroup$ Think about what happens to blue light that hits something "blue", and to blue light that hits something "red". I wish I could find a photo of an old Pepsi can that's been in the sun a decade. It's a great illustration of the phenomenon. $\endgroup$
    – uhoh
    Commented Jul 24, 2017 at 14:27
  • $\begingroup$ Look more faded blue in transmittance? Or in reflection? Undefined? $\endgroup$
    – Alchimista
    Commented Jul 24, 2017 at 17:38
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    $\begingroup$ What color is the plastic to start with? Dark blue? Transparent? Any color? The question likely needs additional context to give a good answer. $\endgroup$
    – jerepierre
    Commented Jul 24, 2017 at 19:54
  • $\begingroup$ Please don't re-post your question, you can edit it to bump it $\endgroup$
    – orthocresol
    Commented Jul 31, 2017 at 8:22
  • $\begingroup$ It is very hard to generalise about things like this since plastics often contain a variety of minor additives (for example colourants or stabilisers). In addition, plastic degradation in light is complex. So the phenomenon could be scattering-related or due to some differential reaction of some other additive (e.g. red compounds tend to fade quickly in sunlight as the light they absorb has higher energy and loss of a small amount of red will lead to a bluer colour). $\endgroup$
    – matt_black
    Commented Aug 5, 2017 at 15:50

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I don't think its actually blue, I think the blue is a result of Rayleigh scattering.

Over time the dyes are destroyed by light removing any color. simultaneously the plastic is degraded and becomes rough on the surface and acquires an inhomogeneous refractive index within the structure due to the destruction of bonds and oxidation and the end of chains. These two structural changes cause scattering, and since blue is scattered more than the other colors due to the relation of scattering with wavelength to the fourth power, the material appears blue when looking at the reflected light. If you look at transmitted light going through the plastic it should appear yellow.

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  • $\begingroup$ Yes, it is why I asked the author to specify in which way he/she looks at the plastic. I really think as you do - scattering. But in principle you can have absorbers in the blue region, that once destroyed let more blue passing through your object. $\endgroup$
    – Alchimista
    Commented Jul 25, 2017 at 12:48
  • $\begingroup$ More context was added to the question $\endgroup$
    – Sercan
    Commented Jul 25, 2017 at 13:18

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