Why is polypropylene not transparent

This is particularly the case in injection mouldings, which elsewhere might well be made from high density polyethylene. Polypropylene is a relatively easy materials to injection mould in spite of its semicrystalline nature. The absence of any real need for high molecular weight, from the mechanical properties view point, leads to low melt viscosity easy flow. The pseudoplastic nature of polypropylene enhances this effect at high shear rates fast filling rates.

Typically melt temperatures for injection moulding are between and oC. Though they can be as high as , or even oC, for short periods of time. With Flame Retardant grades it recommended that oC is not exceeded. Mould filling rates are generally on the high side, to ensure good surface finish and strong mouldings free of weld lines and flow fronts. Adequate venting of the mould is essential to prevent burn marks. The melt flow index gives a rough guide to melt flow behaviour, but as a result of the pseudoplastic nature of polypropylene which is strongly dependant on the molecular weight distribution this should not be taken too literally.

Spiral flow mould data is sometimes provided by manufacturers as a practical means of assessing the flow behaviour, but this is no substitute for the fundamental melt rheological and thermal data which is now becoming more widely available. Important factors which affect the shrinkage of polypropylene mouldings include:. As a result of the ease of flow of polypropylene materials they are often chosen for large area mouldings where it is desirable to operate with minimum clamp forces.

However for many current polypropylene mouldings, especially where good surface finish is required, it is common practice to use melt temperatures of oC combined with high packing pressures typically about 80 MPa.

Three types of polypropylene are currently available. Each suits particular specifications and costing although there is often some overlap. Homopolymers - A General Purpose Grade that can be used in a variety of different applications. Their toughness can be further enhanced by the addition of impact modifiers, traditionally elastomers in a blending process. Random copolymers - incorporate co-monomer units arranged randomly as distinct from discrete blocks along the polypropylene long chain molecule.

Polypropylene can be processed by virtually all thermoplastic-processing methods. Expanded Polypropylene EPP may be moulded in a specialist process.

The choice of grade for any application is based on consideration of any, or all, of the following points:. Good chemical resistance. Good fatigue resistance. Better temperature resistance than HDPE. Lower density than HDPE. It is also translucent in nature so it allows less light through it than polypropylene. It has a lower static charge than polypropylene so it attracts less dust and dirt.

Tags: polyethylene , polypropylene. Global Plastic Sheeting, Inc. Mailing Address: P. Box Chula Vista, CA CEO Lee Hinsley. Plastic Sheeting Blog. Click on Home for all our product categories Polypropylene- Is it different from Polyethylene? How is Polypropylene different from Polyethylene? MORE Polypropylene can be combined with other materials, as can polyethylene.

Products made from polypropylene Polyethylene has more demand however than polypropylene. Here is a list of some of the differences between polyethylene and polypropylene: Polyethylene and Polypropylene are very similar as far as physical properties.

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. Demon3 is correct. 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 FAQ for tips on how to make the best use of Eng-Tips Fora.

Some of the latest metallocene catalyzed pp's have transparency close to PS - very stiff too. Cheers Harry. You can also make PP transparent by cooling it so fast from the melt that it has no time to crystalise. Dear, Remember that PP is a macromolecule and it has two major shapes: isotatic and atatic besides the sindiotatic. 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. Kutzweil 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. Marine microbe mediated biodegradation of low and high density polyethylenes.

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History Reviewed 13 Sept Received 06 June Carina Longo. Figures 5 Formulas 2. Google Google Scholar.