Tuesday, 2nd march 2021 | Technical information
Marco Arezio - Consulente materie plastiche - Which Systems to Use for the Calculation of Moisture in Polymers?

The drastic reduction of humidity in the plastic materials that must be used for the molding process of items intended for sale, is a very important operation, more important than is normally believed .

In fact, even those who normally apply a moisture reduction treatment must ensure that the levels themselves are low enough so as not to create defects on the final product , process variations and breakdowns of press components.

As we have discussed in other articles concerning hygroscopic and non-hygroscopic plastics, the importance of dehumidification of the material, especially for those families of polymers that easily absorb moisture , like PC, PA and PET, is to preserve the polymer chains which, in the presence of an excessive amount of water, are subject to degradation through hydrolysis.

In fact, in some plastic materials, the mere presence of humidity of 200 ppm will negatively affect the polymer chains, short or cut, with the degradation of the molecular weight of the polymer.

But to realize the measurement magnitude of a quantity of humidity of 200 ppm we consider that this value corresponds in percentage to 0.0200, therefore about 9.1 grams of water for 45.360 grams of material.

A high humidity concentration in the polymer can be easily noticed during the molding of articles following the formation of bubbles on the surfaces of the products, increased vaporization in the machine and all the negative consequences on the quality of the product from a mechanical and aesthetic point of view.

But a small amount of moisture, which still interacts with the polymer chains, creating aesthetic and structural damage, is not normally detected during the molding phases but will be assessable on the characteristics of the final product.

It is important to analyze the plastic resin before the molding operations, dry it completely and not fall into the temptation to mix parts of dry resin with wet parts, because the qualitative characteristics of the dry parts will be negatively affected by the wet parts.

But what are the methods for controlling humidity?

Weight difference method: the sample is taken from the packaging or from the hopper and placed in a container for its heating during the analysis phase. Before the granules are heated, a weigh is made and, subsequently, the material is heated to a temperature suitable for the resin family being analyzed.

Once the ideal temperature has been reached, the moisture fraction will come out of the granules and, with it, all the other volatile parts that are represented by release agents, stabilizers, antistats or others chemicals that were found in the polymer.

At the end of the process the granule is weighed and compared with the previous weight which represented the wet granule. For simplicity we would tend to consider the difference in weight as the expression of the amount of moisture present in the material to be used.

In reality this is not the case, as it is not possible to know exactly how much moisture and how many volatilized chemical agents are the expression of the weight difference.

Method of the analysis of the specific humidity: the initial analysis operation is comparable with the previous system, based on the weight difference, but the process is carried out in an atmosphere of dry nitrogen, in which humidity and volatiles will be expelled as in the previous analysis, but the machine will calculate exactly the part of humidity present in the raw material without taking care of the volatile parts as it is able to separate the different chemical compounds.

In this case we will talk about specific humidity, as we will precisely evaluate the weight of the same net of the ejected birds.

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Automatic translation. We apologize for any inaccuracies. Original article in Italian.

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