Yes, PP natural is opaque/translucent because it contains crystals of PP. The crystals have a different refractive index than the amorphous (non-crystalline) part and are of the right size to scatter light. This makes the material cloudy. You can use additives to make the crystals smaller so that they don't scatter light and this gives PP that is close to transparent. A leading manufacturer of such clarifying agents is Milliken.
PP homopolymer is normally translucent. Light can pass through it but is scattered fro the reasons mentioned in the previous post.
PP copolymers are inherently more transparent.
While it is true to reasonable degree to say that all crystalline polymers are translucent or opaque, it is not strictly true, as some materials (including PP) can have such small crystals that they do not refract light, and Polymethyl Pentene (TPX) has crystals with the same refractive index as the matrix amorphous resin, so light is nor refracted and scattered
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
You can also make PP transparent by cooling it so fast from the melt that it has no time to crystalise. For that you would need a thinn film and something very cold like liquid nitrogen.
I suspect that the metallocene PP mentioned in the previous post contains a clarifying agent. You can ask good suppliers for information about such matters. I would recommend Borealis.
Remember that PP is a macromolecule and it has two major shapes: isotatic and atatic (besides the sindiotatic). The first shape of the molecules give to PP the possibility of "packing" it. The consequence is some oppacity in material, because the light is more difficulty to pass through the macromolecules. In the second case, PP is not so "packing" and the light can pass through much better. The result is a translucend material.
I think Kutzweil is confusing refractive index and opacity. The chain packing affects density, higher density means higher refractive index (see the book Properties of Polymers by Van Krevelin). What makes something opaque is when a material has domains of different refractive index than the bulk. Examples are crystals of PP in the amorphous PP matrix or fillers in polymers.
There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell
Maybe it is a translation problem, but you really have it quite wrong.
Basic high school physics books will explain about how light is transmitted, reflected or absorbed, and how these might happen to different extents at different wavelengths for the light.
It will also explain how light is transmitted and deflected and reflected depending at the angle to the interface between 2 transparent materials.
Then consider for plastics that transmit light, the effects of inclusions of different refractive index.
Another chapter will contain some information on crystal structures. You will also find some info on super saturated solutions and growth of crystals in high school chemistry books.
Also find a book on the formation of crystals in semi crystalline plastics.
Something on thermodynamics and kinetic energy in molecules will also be illuminating.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
I agree that the chain packing affects density (isotatic PP has 0.9 to 0.91 g/mL). You can see in "Principles of Polymerization" (Odian) that "some polymers are highly crystalline primarily because their structure is conducive to packing, while others are crystalline primarily because of strong secondary attractive forces between molecules. The extent to which a polymer crystallizes depends on whether its structure is conducive to packing into the crystallize state and on the magnitude of the secondary attractive forces". The major comercial PP have "isotatic" conformation (for more information, see Odian, p. 24) and this stereoregularity gives, I insist, the aspect not so clear to PP ("opacity"). When PP has a "atatic" conformation (or randomic), the proximity between macromolecules is difficulted, because there is no 3D regularity. In consequence, this particular PP is much more "clear" ("translucent") that PP isotatic.
For more informations about "crystalinity / polymer structure"), please check Polymers: Chemistry & Physics of Modern Materials - 2nd edition (Cowie)
Sorry Kutzweil but I that's not really right. Isotacticity allows crystallisation. The crystals have different refractive index than the surrounding non crystallised (amorphous) PP and so the light is scattered. This is easy to prove.
For example, clarifiers from Milliken and others alter the crystals to be smaller, you still have crystals but they are too small to scatter visible light so the PP is still crystalline but transparent.
Or, you can cool the PP so fast that it can't crystallise, again it will be transparent.
In very rare cases you find a semi-crystalline polymer where the refractive index of the crystals is the same as the amorphous matrix and it this case the polymer is crystalline but transparent.
Another case is for example, HIPS or ABS where there are no crystals. The polymer (PS or SAN respectively) is amorphous but the rubber particles in it have different refractive index than the matrix and scatter light so the materials is opaque. Believe me, I've worked on this for some years and even have a patent coming on a new material based on this effect.
There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell
Translucent transmits light but scatters it like frosted glass.
Transparent transmits light but does not scatter it like spectacle lens glass.
Density has nothing to do with transparency other than in the same material it will change slightly with degree of crystallinity.
Some transparent plastics have an SG of as low as 0.8 while other transparent plastics have an SG of over 1.3.
Generally semi crystaline plastics are translucent (not opaque) in their pure form.
This is because the crystals have a different refractive index and so deflect light at the amorphous/crystaline interface, making the material translucent.
The size, shape and refractive index of the crystals vs the amorphous matrix all effect this. If the crystals are below about 40 nanometres in their longest dimension, they will be transparent. If the refractive index of the amorphous matrix is the same as the crystal, they will be transparent.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
I verified my dictionary... and in my language "translucent" has similarity with "transparent". But in English, it does not occur. I keep my last considerations, but I change "translucent" for "transparent".
