Why it is Important to Understand the Moisture Content of Plastics

In plastics, moisture content is regarded as a significant parameter, and therefore, accurate measurements are important for manufacturing these materials and for assuring their process quality. Moisture impacts the processability of almost all plastics, particularly hygroscopic equivalents like nylon (polyamide) and polybutylene adipate terephthalate (PBAT).

Image Credit: Aznan/shutterstock.com

However, when these plastics are molded or processed with moisture content beyond the accepted range, structural or visual defects may occur, which are considered a major problem. Another factor that is equally important is an in-depth understanding of the moisture content, because certain polymers tend to degrade when molded wet, leading to decreased material characteristics. Although several accepted methods are available for determining moisture content in plastic materials, the majority of conventional techniques come with their own disadvantages, with accuracy and speed being the major problems.

Issues with Measuring Plastic Moisture Content

Traditionally, the plastics’ moisture content has been established through Karl Fischer titration (ASTM D 6869) or through methods using vacuum and heat to remove moisture until a constant weight is achieved. Conversely, these techniques are not only time-intensive but also depend heavily on the thermal stability and the equilibrium water content of the polymer. This means it is important to make sure that all the moisture present in the raw material is detected and extracted effectively and that any other products that may emerge from the plastic are not confused with the water while the polymer is being heated.

Most often, a water content of about 1%–2% by weight can be safely determined through a vacuum oven at temperatures where the rate of evaporation is adequately high yet lower than the polymer’s decomposition temperature. This process is mostly carried out when testing for drying conditions prior to injection molding. Sometimes, broad line NMR can be used for establishing water fraction but this method has to be calibrated. The same holds true for NIR or IR results acquired from plastic films.

Moisture impacts the processability of almost all plastics. Image Credits: shutterstock/Mr_Mrs_Marcha

Nylon Moisture Problems

As an example of the issues associated with polymer molding, the ability and the tendency of nylon to absorb moisture represent a constant challenge. In the case of nylon, moisture can impact an array of polymer properties, which consecutively affect dimensional stability, processability, acoustic, mechanical, chemical, electrical, and optical properties, and eventually the performance of the product. In nylon, moisture behaves as a plasticizer in order to decrease the bonding and entanglement between molecules and thus boost their mobility and volume.

Nylon contaminated with moisture has a lower glass transition temperature (T_g), which makes it easier for extra crystallization to take place. In this manner, the increased moisture content can cause extreme variations in material behavior under load; for instance, it can increase ductility and reduce stiffness and strength. In due course, this introduces changes in the material’s properties and will potentially result in substandard products.

Plastic Manufacture

Currently, during the production of plastic and polymer parts, precision components that have to be defect-free and of high quality are molded. Prior to each manufacturing run, plastic manufacturers carefully dry pellets, which means they have to check moisture content intermittently until acceptable levels are achieved. Therefore, long moisture test times are major issues because they lead to a costly and extended time in the pellet dryer.

Infrared moisture balances are rapid techniques that can be used for determining moisture, although it can take 20 minutes or more to complete the process. It is also possible to fit dedicated optical sensors to pellet dryers, but these are highly specialized, costly, and need constant calibration. The SMART Q presents a novel option for determining moisture levels. This method can offer fast on-line testing along with a direct loss on drying, and since it is a primary technique, calibration is not needed. It is also easy to use; insensitive to density, color, or compound changes; and has the ability to withstand adverse manufacturing environments.

The SMART Q from CEM

The SMART Q

The SMART Q™ moisture analyzer has been specifically developed to enable precise measurement of low moisture levels in plastic pellets. Equipped with three-digit moisture readout and a highly accurate four-place analytical balance, the infrared-based SMART Q employs a combination of proprietary and patented technologies to offer repeatable results and controlled heating in approximately five minutes without any need to preheat the cavity. The alternative method will take 5 to 8 hours utilizing direct oven techniques.

Exceptional features include a sealed test cavity with active ventilation, direct sample temperature monitoring, and the utilization of a patented collimated infrared energy system. To expedite the drying process, active ventilation to the sealed sample cavity removes moisture vapor from the sample. The SMART Q moisture analyzer enables direct moisture analysis and hence does not require calibration, unlike other spectroscopic techniques.

Compared to other direct competitors, the SMART Q moisture analyzer is faster and offers better accuracy and precision. It can quickly examine a host of plastics for moisture with an average difference of below 0.003% when compared to the air-oven technique. A recent study on the SMART Q inspected five entirely different types of plastic such as nylon, rubber, ABS, polypropylene, and polycarbonate. The results revealed a close correlation with oven-achieved moisture content results, but in a fraction of the time, that is, 5 minutes instead of the usual 5 to 8 hours.

References

1. Charles A. Harper, Edward M. Petrie, Plastics Materials and Processes: A Concise Encyclopaedia, ISBN: 978-0-471-45603-2 August 2003, Wiley
2. Inke Paetau, Chin-Zue Chen, and Jay-lin Jane, Biodegradable Plastic Made from Soybean Products. 1. Effect of Preparation and Processing on Mechanical Properties and Water Absorption, Ind. Eng. Chem. Res., 1994, 33 (7), pp 1821–1827
3. C. O. Willits, Methods for Determination of Moisture-Oven Drying, Anal. Chem., 1951, 23 (8), pp 1058–1062
4. M. Kohan, Nylon Plastics Handbook, Hanser/Gardner Publications, Inc., New York, 631 pages, (1995)

This information has been sourced, reviewed and adapted from materials provided by CEM - Process Control.

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