Crystallization of Polypropylene
Zitierlink des Filmsegments
Partially crystalline thermoplastics such as polypropylene are employed for the production of hard, shock resistant die-cast parts.
A pigmented granulate is used as raw material for coloured formed parts.
The process of die-casting is often regulated by processors,
which define temperature, pressure, cooling rate and production rate.
The single parts are expelled and dropped onto a conveyer belt.
The mechanical properties depend on the microscopic texture of the polymer. Crystalline modifications of polypropylene Due to birefringence under the polarising microscope, transparent 20 micrometer thick samples of pure polypropylene reveal different regions of partial crystallisation. Above, radially grown spheroliths of different dimensions. Below left, oriented fibre like dendrites. Below right, dust particles with the same effects as seeds. Generally, we observe mixed forms of dendritic crystals on the surface of
formed parts, and spheroliths in the bulk material. In homopolymerized samples free of seeds, shape and size of the crystallites mainly depend on the cooling conditions within the melt. The following sequences of crystallisation are shown in time lapse cinematography. This homogeneous melt of polypropylene in a heating stage is shown when the temperature is slowly lowered to immediately below the melting point. The frame width is about 600 micrometers. Due to the limited thickness of the sample, the frontiers of solidification of the spheroliths do not form spherical surfaces, as in thicker samples, but circles. These eventually make contact with each other. From this moment on, straight boundaries result from the uniform speed of growth. In reality, straight boundaries represent plane surfaces.
With rapid cooling, crystallisation starts at many different points simultaneously. A fine spherolithic texture has been formed. So far the whole sample has a uniform temperature. If, within our field of view, the sample is cooled from the left, and a high temperature gradient is produced between the left and right boundary, here 10 degrees Celsius per millimetre, a frontier of solidification proceeds rapidly from the left to the right.
This leads to an orientation of the crystallising dendrites.
As a rule, both types of crystallisation generally appear together within a formed part, depending on local temperature gradients. To the left, preferably
dendrites are being formed, and in the centre of the sample mainly spheroliths.
If the polymer contains seeds, the crystallisation initially starts there independent of the cooling rate. Dust particles of a few micrometers in diameter may also induce crystallisation of the melt.
This is a spliced asbestos fibre. Pure homopolymer preferable forms spheroliths starting from foreign seeds. After melting just above melting point and subsequent cooling again, the spheroliths crystallize in nearly the same places as before. It is quite different, when heated until far above the melting point. After slow cooling down, the crystallisation also starts at other points of the foreign impurity.
If a defined concentration of seeds is added to the homopolymer, small spheroliths develop almost independently of the cooling rate of the sample, here with 0.1 per cent seed added. An increased concentration of seeds, here 0.5 per cent, consequently produces even smaller spheroliths. The cooling of a polypropylene copolymer with a chemical structure different from that of the homopolymer, results in precipitation of preferably small crystallites, even under constant cooling conditions.
Still smaller spheroliths and finer dendrites are produced, even under moderate cooling conditions, if seeds are added to the copolymer.
In the case of effective temperature gradients, altogether an oriented solidification is observed, but now as a mixture of dendritic and spherolithic crystallites.
Crystalline hyperstructures The crystalline hyperstructure in polypropylene is influenced by three different factors. First, by the molecular structure. Left a solidified homopolymer, right a copolymer. Secondly, by the manner of temperature change, with rapid cooling left, smaller spheroliths develop than in slow cooling, right. Dendrites are produced with higher temperature gradients, here 20 degree Celsius per millimetre. Finally, the addition of seeds between 0.1 per cent and 0.5 per cent enables a well defined change of texture in the solid polymer. This influences the properties of the finished products.
Proof <Graphische Technik>
Proof <Graphische Technik>
|Titel||Crystallization of Polypropylene|
|Alternativer Titel||Kristallisation von Polypropylen|
|Autor||Großkurth, Klaus Peter|
CC-Namensnennung - keine kommerzielle Nutzung - keine Bearbeitung 3.0 Deutschland:
Sie dürfen das Werk bzw. den Inhalt in unveränderter Form zu jedem legalen und nicht-kommerziellen Zweck nutzen, vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen.
|IWF-Filmdaten||Film, 16 mm, LT, 199 m ; F, 11 min|
|Abstract||Formation and growth of crystalline structures in polypropylene as example for solidification of partially crystalline thermo plastics. Spheroliths and dendrites, influence of cooling conditions, molecular structure and seed formation. Micro cinematography using heat controlled stands and polarization.|
crystallization / Polypropylene