The Influence of Moisture in Plastics

No external factor is as critical to a plastic before processing as moisture. Depending on the material, it is not necessary to leave it in the rain, on the floor or near a wall that is mocking so that the absorbed moisture content interferes with the process or the quality of the finished product. Generally, even when the resin is packaged and palletized, drying is required before any kind of processing - remember that moisture is in the air.

There are polymers that are extremely sensitive to water, such as those in the polyester family (PET, PBT, PC) which must be dry; while others with high absorption levels such as polyamides and ABS the drying is optional and it is most required when the mechanical properties of the material are already at the limit of use or when the question of the surface appearance of the part is a prime factor.

What are hygroscopic plastics?

They are materials that absorb moisture due to their chemical characteristics, such as polyamides, which in the presence of moisture form hydrogen bonds: undesirable before processing but necessary afterwards, since without hydrating the pieces they may become brittle.

In some non-hygroscopic polymers such as PP and PE, moisture is not a critical factor, but there is always surface moisture in the granules that depending on the part can help to form some stains. It is recommended to give at least a "preheated" resin for at least 30 minutes to eliminate any future suspected moisture spots on the product. In addition, preheating to some extent aids in processing, since the fact that the material is hot decreases the requirement for the electric resistances of the injector/extruder.

Resin in aluminized packaging, need steaming?

Aluminized bag
Aluminized bag
The aluminum barrier bag has the function of more effectively preventing the ingress of moisture, but this does not mean that the material has been completely dry and has been vacuum packed.

This type of packaging is also very common among nylon recyclers, which does not help much, since most of the extruders of these recyclers are conventional, where the material is cooled in the water before being granulated which in itself already leaves the content of moisture in the heights. This is not to mention the time the granulate is exposed to the environment just after leaving the granulator until it is packed.

What is recommended for the transformer is to do two tests: one with dry material and the other with the same material under the conditions in which it is supplied. There are "n" factors that can influence the final result, which can be a stark difference between the two tests as it can be a minimal difference to the point that it is possible to eliminate the drying step, which generates a good cost reduction.

What can high moisture cause in plastic?

- Sneezing in the piece*
- Bubbles
- Changes in mechanical properties
- Variations of fluidity
* Do not confuse with flow marks, which can be generated by excessive / poor pressure or degradation. Moisture-sneezing has thicker lines, covers most of the part, and can be interspersed with small air bubbles.

This moisture can still be verified in the purge of the machine, evidenced by the large volume of steam that rises from the material coming out of the nozzle. In addition to the foam appearance draff.

How to measure moisture? Is there a standard for this?

There are equipment that does a very simple way to measure the moisture content with the use of a precision scale and an electric resistance, which is very useful in the manufacturing industry, because it is possible to establish ideal drying times eliminating those excessively long periods drying established only for guarantee.

In the above case, the test to be done would be the one of moisture content, different from the test of humidity absorption, normalized by ASTM D 570 and ISO 62. It turns out that the latter are widely disclosed in technical sheets of plastic resins, however it has few or no utility for transformers since in most cases the critical point is in the process and not in the final product.

Temperatures and Drying Times

The table below is the result of a comparison between several datasheets of different resin manufacturers, with the average temperature and time:

Family Plastics Temperature Time (h)
Acetalics POM 70°C / 158 °F 2
Acrylics PMMA 75°C / 167°F 3.5
Amides PA6, PA66 80°C / 176°F 3.5
PA610 80°C / 176°F 3
PA11, PA12 90°C / 194°F 5.5
Styrenics PS 75°C / 167°F 2
HIPS 80°C / 176°F 2.5
ABS, SAN 80°C / 176°F 3
ASA 95°C / 203°F 3.5
Polyesters PBT 120°C / 248°F 4.5
PC 120°C / 248°F 3.5
PET 145°C / 293°F 4.5
Polyolefins PE, PP 80°C / 176°F 2


Bibliography:
HARPER, Charles A.; PETRIE, Edward M. Plastics Materials and Process: A Concise Encyclopedia. Hoboken: John Wiley & Sons, Inc., 2003.
CANEVAROLO JR., Sebastião V. Ciência dos Polímeros: Um texto básico para tecnólogos e engenheiros. 2.ed. São Paulo: Artliber Editora, 2002.
WIEBECK, Hélio; HARADA, Júlio. Plásticos de Engenharia: Tecnologia e Aplicações. São Paulo: Artliber Editora, 2005.
Article posted in May 31, 2017
About the author: Daniel Tietz Roda is Plastics Technologist graduated from the FATEC/ZL and Mechanical Design Technician from ETEC Aprígio Gonzaga, in São Paulo, Brazil. Roda worked 5 years with technical assistance and development of plastics in industries and nowadays is the publisher of this website.
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