The processing behavior and the achievable product properties of plastics can change enormously depending on the residual moisture content of the polymer raw material. In most cases, too high residual moisture leads to problems, but in some cases quality problems or processing problems also occur due to too low residual moisture. Consequently, it is important to know the possible property changes of the material as a function of the residual moisture content so that these can be specifically influenced.
Core moisture or surface moisture?
Plastics exhibit different behaviour towards moisture and are therefore divided into two groups: hygroscopic plastics and non-hygroscopic plastics. Hygroscopic plastics absorb water inside the material, i.e. also inside a granulate grain. In these plastics, the water molecules are deposited in the macromolecular structure of the polymer or even form a chemical bond with the macromolecules.
Non-hygroscopic plastics, on the other hand, can also absorb moisture, but with these plastics the moisture is only deposited on the surface of the material. However, if components of hygroscopic materials (additives, fillers, etc.) are present in a non-hygroscopic plastic, moisture can also be deposited inside the non-hygroscopic material.
Moisture in plastics can lead to various problems, which can be seen both in the processing process and in the subsequent product quality. Examples of such defects are:lower viscosity of the plastic, lower melt strength
- defects in the extrudate, e.g. steam bubbles
- deteriorated surface quality, e.g. streaks
- deteriorated mechanical properties
- reduced molecular weight due to hydrolytic degradation
- etc.
It is obvious that such problems are unwanted and should be avoided wherever possible. To avoid the problems caused by moisture, hygroscopic plastics are usually dried before processing. During the drying process, moisture is extracted from the plastic. For example, a volume flow of warm, dried air flows through the material in a container (e.g. dry air dryer) so that moisture can be extracted from the plastic and the residual moisture in the material sinks. The moisture of the plastic is absorbed by the warm, dried air and transported away from the material. In the subsequent drying process, the warm air releases the moisture again (e.g. adsorption) and can be returned to the granulate. However, there are also other drying methods such as warm air drying, compressed air drying, vacuum drying or microwave drying.
The decisive factor in every drying process is that the desired residual moisture content of the plastic can be achieved on the one hand, but also that it can be achieved as accurately and constantly as possible. The production of a very constant residual moisture content in particular often turns out to be very difficult in practice, and these are the most frequent causes of fluctuations:
- the initial moisture of the material is unknown (e.g. because it is not measured)
- the processes are not logistically well coordinated, dried material is stored for hours before it is processed, so that it can absorb moisture again
- the drying parameters are uniform and constant, there is no adaptation to the real conditions (e.g. drying is always carried out for 6 hours at 80°C, regardless of the residual moisture content of the starting material)
The parameterization of dryers is therefore a decisive task that is often neglected. However, the selection of optimal drying parameters can have a major influence on product quality. In the following some essential parameters will be mentioned:
- drying temperature
- Excessively high temperatures (above glass transition temperature) can cause material damage or even melting of the material (angel hair, bridging).
- Too high temperatures lead to very fast drying (diffusion rate), the material can be overdried and show problems during processing (e.g. high viscosity).
- dry air volume
- If the air volume (volume flow) is set too low, this can have a negative effect on the drying performance, as the air can absorb less moisture the more it is loaded with moisture.
- A very high air volume (volume flow) can accelerate the drying process, but becomes uneconomical at a certain point due to the increasing counterpressure.
- dew point temperature
- The dew point temperature is a measure of the residual moisture content in the dried air. It is not necessarily correct to assume that the lower the dew point temperature, the faster the drying process, since the diffusion rate is also important for this.
- Very low dew points require very high energy inputs for drying the air and are therefore often uneconomical.
- drying time
- The drying time is one of the most important setting parameters for dryers. This is material-specific and depends on the diffusion rate of the material.
- If drying times are too short, the desired residual moisture cannot be achieved.
- If drying times are set too long, unnecessary energy consumption or even material damage due to overdrying can occur.
The different drying variants will be presented and compared in a later article.
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