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Determination of Temperature-Affected Polymer Softening

When designing plastic parts for structural applications, selecting the correct materials depends on various parameters. Since the properties of polymers are highly temperature-dependent, their thermal application limits must be identified prior to use. Beside well-known techniques such as dynamic-mechanical analysis or differential scanning calorimetry, technological methods like the Vicat Softening Temperature (VST) are generally used for estimating the utilizable temperature range or evaluating materials for quality assurance.

Introduction

The term “heat distortion resistance” describes a specimen’s capability of maintaining its shape at raised temperatures or not exceeding a certain deformation at a defined testing temperature.

Essentially, two main transitions limiting the application of a polymer can be identified. These are, on the one hand, the glass transition temperature Tfor amorphous thermoplasts, and on the other hand, the melting temperature of the crystalline structures for semi-crystalline thermoplasts Tm. The most important measurement methods for determining the heat distortion resistance are the Vicat Softening Temperature VST (described by ISO 306) and the Heat Distortion Temperature HDT (described by
ISO 75-1 to 75-3). For both methods, a specimen is heated to a defined, constant heating rate under a defined, constant load.

It is important to mention, that both methods lead to different results. This is caused by methodological reasons, as described in the next chapters.

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