Determining the moisture content of plastics is an important step in the extrusion and molding process of many industrial resins.
Excessive moisture content can lead to microscopic steam pockets that can blister or weaken the integrity of the molded product. Plastic pellets in a saturated or ‘wet’ state can also mislead buyers of bulk plastic since a substantial portion of the resin purchased may be water. These and many other factors have prompted the industry to look toward moisture analyzers for accurate and reliable measurements of moisture content.
Testing for Moisture in Plastics
There are several options available for moisture analysis in plastics. Karl Fischer Titration (KF) utilizes a chemical interaction with iodine and water to yield a measureable electric current. This electrical current can be quantified, and the moisture content can be deduced. This form of analysis is considered to be accurate but is plagued with many interferences and typically long test times ranging from 20 minutes to several hours depending on moisture content. With time being a valuable resource, quicker forms of analysis are sometimes required in order for plastic molders to purchase and mold plastics efficiently.
Saving time during testing is much more than just a convenience; shorter test times also enable more accurate results on hygroscopic plastics such as nylons, acrylonitrile butadiene styrene (ABS) and polycarbonate (PC). Because these plastics continuously absorb moisture from the air, the material you have yet to process will continue to gain moisture as it waits for your current Karl Fischer (KF) test to finish. In essence, the moisture content of your first sample will likely be lower than that of the second sample because the second sample has had more time to absorb moisture from the air.
Arizona Instrument’s rapid loss on drying (LOD) analyzer, Computrac® MAX® 4000XL combines a heating source and an analytical balance within a sealed chamber to provide significantly shorter test times (2-10 minutes). The Computrac operates on a thermogravimetric principle, radiating an even heat over a sample and precisely measuring the loss in weight. The Computrac also displays a real-time graph of the evolving moisture content and the rate of loss (%/min). Larger sample loads can also be used by this form of analysis compared to traditional KF methods; this allows for a more representative sample, especially if the sample is a blend of two or more resins.
Three plastic samples were analyzed using a Coulometric Karl Fischer titrator and as well as the rapid LOD Computrac® MAX® 4000XL. Samples included a Nylon 66, a ABS, and a PC. The specified maximum moisture limits for molding of these plastics are 0.2000% for Nylon 66 and ABS, and 0.0200% for PC. These plastics were dried by a desiccant bed drying hopper set to the manufacturers recommended drying parameters. These plastics were then placed into separate glass jars, capped and allowed to cool in a desiccator prior to analysis. The ambient humidity of the laboratory in Arizona was approximately 30%. All samples were run in quintuplicate (5 times) in sequential order for both KF and LOD analysis and were poured from the same mason jar.
Because some plastic samples have the propensity to gain moisture from the ambient humidity, locking in on a fixed moisture level is sometimes difficult. The best way to assure a statistically valid sample is to test a number of samples as quickly as possible. Figure 1 demonstrates the speed advantage of the Computrac® MAX® 4000XL, which is 2-3 times faster than the Karl Fischer titration method. Error bars represent the standard deviation of the total set of test times for each set of analysis.
Accuracy & Precision
The faster test times of the Computrac have a measureable impact on its accuracy. Figure 2 demonstrates the averaged data of all five tests for both KF titration and MAX 4000XL, with error bars showing one standard deviation above and below the mean. Nylon 66, ABS and PC were chosen to demonstrate this moisture progression since these three plastics are widely used in the industry for many different products. For this suite of tests, the MAX® 4000XL correlates with KF titrator averages but has tighter precision for each plastic owing to the faster test times. Figures 3a through 3c present the individual data points in sequential order of testing. As time goes on, all three products begin to gain moisture through exposure to ambient humidity during testing. Having prolonged test times keeps the sample out in the air longer, even if the sample is sealed within a glass jar between tests. The red line observed in Figure 3a-c represents the moisture specification allowed for plastic molders. If the plastic is reported over this line, then the whole batch of plastic must be re-dried in the hopper for several hours. It is therefore crucial that the results reflect the true nature of the whole batch and not just the drifting nature of the small sample.
*Moisture spec (red line) = 0.2000% Moisture
*Moisture spec (red line) = 0.2000% Moisture
*Moisture spec (red line) = 0.0200% Moisture
The Computrac® MAX® 4000XL Moisture Analyzer is an effective tool for determining moisture content in various plastics. For all three plastics showcased, the MAX® 4000XL demonstrated quicker test times and showed a tighter standard deviation and coefficient of variation than Karl Fischer titration. Having quick and efficient analysis allows the operator to capture a true representation of the moisture content within their bulk lot of plastics. If the analysis is biased high due to the hygroscopic nature of the plastic, than unwarranted drying may delay shipment, receiving or molding. Therefore, time is not only useful in preserving the accuracy of the moisture content, but important for developing a streamlined business practice.
Table 1 below demonstrates all of the statistics generated from this paper. The ‘Delta’ and the ‘% Gained’ refer to the overall change in moisture content from the first run to the last. The ‘per cent gained’ is the per cent of the delta divided by the initial moisture of each data set.
Although moisture gain is observed through both sets of analysis, the KF titration method yields drastically higher moisture gains than the MAX® 4000XL. Although Nylon 66 and ABS have a higher water absorption capacity, the PC shows a higher rate of water absorption while having the lowest moisture specifications when compared to other resins. This makes the Computrac® MAX® 4000XL useful in isolating the true moisture for a dried plastic so the analyst is not wasting their time and money running and re-drying samples. The MAX® 4000XL does not consume hazardous chemicals and is robust enough to be used right on the production floor. Graphical data can be downloaded by the USB port or by the webserver and the instrument can store up to 1000 test results on the internal memory. The MAX® 4000XL can measure moisture content in plastics accurately, reliably and in a timely fashion.