Measuring devices 21.03.2022
Brabender Aquatrac V determines the water content of polymers
However, for those whom it is relevant, probably well known, let us start with the importance of the moisture content of polymers. Some materials due to their molecular structure shows so-called hygroscopic properties. This means, such substances can “collect” the water molecules from the air humidity. The most typical examples are the PC, PA, PET, PBT, ABS, PMMA, PU.
During the processing of such resins, regardless if it is extrusion or injection molding or almost anything else, the too high moisture content can cause you unwanted issues through the viscosity dropping or even evaporating of the water. To prevent this, a widely used process is the drying and proper packing of these materials.
To know the moisture content is therefore essential for the proper setting of the drying process, knowing, that the material to be processed is actually the right moisture content range. Nowadays during the rocketing energy prices a remarkable aspect for you to prevent the overdrying, not only due to the classical reason, to prevent some degradation, but reducing your costs. Of course, an important requirement to check the water content before and after the drying procedure.
What kind of moisture measuring methods do exist?
A commonly known one is the gravimetric – typically special balances with a heating unit, weighing the sample before and after some heating. They are very simple, relatively cheap either, but having one limitation: you do not measure the real water content. This roots in the fact, that during this heating many other volatile components evaporate.
Another, very sophisticated, accurate and already water selective method/instrument is the so called Carl-Fischer titrator. You have the limitation on the “other end”: you require actually laboratory environment and skilled operator.
More than 30 years ago, when we first launched our instrument, the goal was having the advantages of these properties above: compact, robust, suited for industrial environment, yet water selective and accurate.
Let us have a look at the working principle and some technical details of how it is achieved.
The measurement is based on the chemical reaction of water (vapor) with solid calcium-hydride (CaH2 + 2H2O → Ca(OH)2 + 2H2), forming hydrogen gas in a closed system. As a first step your sample (depending on the estimated moisture content 0,1-100g) placed in the measuring vessel. It is closed with some reagent above on a sieve and evacuated. Afterwards, is heated up to a temperature, where all the volatile components evaporate, including the water. But only the latter shows this reaction above! Hence, we have now hydrogen (proportional to the water amount in your sample) and some other various component expelled from your material. Remember, there’s no air, as we evacuated our system before. After precipitating the other components, only the hydrogen remains, which can be detected via pressure measurement. As it comes from the water content of the polymer sample (phrased other way proportional to the water content of the sample), the moisture concentration can be calculated very easily. Of course this step you do not have to do by yourself, the built-in touchscreen simply show you the result in % or in ppm. So does with these steps – after putting your sample and reagent, and closing the lid, all this happens under the hood without your supervision. Also, for the necessary settings the instrument contains an extensive database of the different polymers, you have to select only the type you measure.
Regarding the material consistency, you can test besides the granules powder and shredded-ground samples either. The measuring time depending on the sample size and moisture content, approx. 15-40 minutes. The entire system has a size of a microwave oven, and what you see here on the pictures is the entire system – no other parts or accessories are needed. Due to the robustness and compactness it is practically portable, you can use right on the spot.