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IMPORTANT CRITERIA FOR MATERIALS AND COMPOUND SELECTION

In their daily work moulders and designers often rely on known material commercial Brands because they are synonymous of a certain polymer matrix with typical, physical, mechanical, chemical and surface characteristics.

In Technical Compound production, compounders rely more on specific polymers matrix because ideal for their purpose thanks to their specific physical and rheological, behaviors.

The final aim of their work and efforts is of course to produce a specific material with specific behavior, often to satisfy specific applications needs.
This is why we 2Mila people are used to asking our customers more info about the final applications. Sometimes we look a bit too curious to the eye of a customer but the reason is only that.

The information received guides our R&D toward a satisfactory alternative to standard solution products, with the best balance between performance and price.

Morphology and thermal properties in final applications

One of the key questions you can get from us is about the working temperature of the application; followed by a question about the type of work the part can (or should) perform at that temperature.

As a natural fact every polymer matrix has a different physical and thermal behavior according to its origins (Amorphous or Semi-crystalline), and this origin could influence the final physical and mechanical performance of the application (e.g: anisotropy = higher warpage due to the differential shrinkage typical of semi-crystallines, or isotrophy = flatness thanks to a homogeneous shrinkage typical of amorphous materials).

The elastic modulus (material rigidity), as we know, depends on the temperature.

Amorphous polymers in this aspect are more predictable in loss of modulus, within their range of applicability, because not influenced by factors other than the temperature (e.g. not by the moisture absorption) and this just up to their Tm (Melting Temperature) which usually is about 100°C – 150°C lower than their actual standard processing temperature (T).

The Semi-Crystallines instead present a higher degree complexity because they have a “Tg” (Glass Transition, related to their amorphous area, often around 50°C – 60°C) and a higher “Tm” (Melt Temperature), very close to the T processing temperature (about 50°C) offering like that a wide thermal range of use.
Fillers and reinforcements add to them higher thermal and mechanical (rigidity) properties than to their Amorphous cousins.

Environmental variables

In case of a PA6 another variable should be considered: humidity. The polymer absorbs it and is “plasticized” by it. This additional variable changes according to the environment and influences its flex modulus (see dry Vs. conditioned plot in the graph) but in the meantime provides it with a higher tenacity. This is why all tests on PA are made in “dry conditions”: to avoid this significant variable. A variable to consider when two or more pieces are snapped together.

Within the Semi-crystallines, Polypropylene is moving differently instead, as above 0°C its Tg is over; only the processing T remains. Therefore it keeps on losing modulus, degree after degree.

Its loss of modulus is continuous and ends at 162°C (Tm), the T at which it is processable with injection machines. To a certain extent this behavior is quite predictable and can be handled in final applications if the working temperature of the final part is known.
The modulus is not influenced by humidity, therefore the available fillers and reinforcements could help to obtain the required mechanical modulus for a specific working temperature.

The graph here represented is made to show some typical standard Amorphous behavior Vs. standard Semi-Crystallines and PP lines, with regards to the modulus change in temperature.

We think this can be a good way to underline and remember such differences for any development you may have to face in the future. We at 2Mila are at your service to find the best solution for you.

The Semi-Crystalline materials and compounds

We need to highlight that these types of materials reach superior mechanical behaviors thanks to the fillers. Glass is used for a higher modulus and Mineral for a compromise of modulus and planarity.

Their chemical structure allows to carry a higher amount of fillers than Amorphous.

Fillers do also improve significantly their thermal properties and this is immediately visible on HDT (Heat Distortion Temperature) at 0.45MPa or 1.80MPa.

These two data (the temperatures under load) are very important during the material selection phase in view of the expected final application functionalities.

Article by Emilio Sitta

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