The Need for Water Concentration Monitoring in Plastics Extrusion

For plastics extrusion, the presence of moisture in the raw material can be a major problem. Many raw materials are hygroscopic in nature so it is a common practice to dry them before extruding, to avoid polymer chain breaking or bubbling in the final product, which is caused by the presence of moisture. The problem with the non-hygroscopic pellets is the possibility of environmental moisture build-up during storage.

To address these problems, it is necessary to measure the moisture content of all pellets before extruding to ascertain that the quality of finished products meet the specifications set by the manufacturers.

Karl Fisher titration is the conventional method used for measuring water concentration, but this requires harmful chemical titrants and expensive glassware. Also, although traditional methods are accurate and reliable, the area in which the devices can be mounted is limited and they can be significantly costly due to operating and waste concerns.

Arizona Instrument’s Solution

To address these problems, Arizona Instrument has designed the Computrac® Vapor Pro® 3100L, which detects moisture content without using solvents. The Computrac® Vapor Pro® 3100L is a moisture specific, sensor based device capable of determining water concentrations using a polymer capacitor that shows a measurable change in capacitance in the presence of water.

Experimental Procedure

Sample Preparation

This analysis involved the testing of polycarbonate, acetal, polypropylene, and nylon 6/6 to compare Computrac® Vapor Pro® 3100L against a Karl Fisher Titrator. Before testing, the Dri-Air RH5 resin drier was used to dry each resin under different prescribed conditions, followed by storing the dried materials in tightly sealed Mason jars in upside down condition. The jar was opened only to feed enough sample for each test and resealed between tests.

Karl Fisher

Unique testing conditions were used for each material to determine the moisture content, but the instrument preparation was the same, with a chamber purge lasting 5min, an initial background below 0.3μg•sec-1, and an ending criteria of 0.1μg•sec-1. The Coulomat titrants were changed every week and the glassware was cleaned and dried. The NIST traceable 1μl capillary tubes were used to perform daily calibration checks.

Computrac® Vapor Pro® 3100L

For each test, the testing parameters were established individually. All analyses conducted with a 30mL capacity vial fitted with a metal cap and new septum for every test. The NIST traceable 1μl capillary tubes were used to perform daily calibration checks and a chain of three tests were performed on the first day of the work-week. Dryness and leak checks were also performed daily.

Experimental Results

The average test results obtained from the two methods are summarized in Tables 1 and 2, showing a strong correlation. Although there is difference in analysis times between the two methods, the value varies with the sample size used.

Table 1. Test Results for Materials Analyzed with the Computrac® Vapor Pro® 3100L.

Material Average Result Standard deviation Test time Test temp Ending Criteria Sample size
Nylon 6/6 0.1437% 0.0061 18:00 225°C Rate; 0.01 0.5g ± 0.05g
Polypropylene 0.0283% 0.0020 7:36 180°C Rate; 0.10 0.5g ± 0.05g
Aceta I 0.0202% 0.0022 6:48 130°C Rate; 0.05 1.0g ± 0.1g
Polycarbonate 0.0174% 0.0004 5:59 220°C Rate; 0.10 1.0g ± 0.1g

Table 2. Test Results for Materials Analyzed with the Karl Fisher Titrator.

Material Average Result Standard deviation Test time Test temp Sample size
Nylon 6/6 0.1457% 0.0092 40:23 200°C 0.5g ± 0.05g
Polypropylene 0.0298% 0.0008 8:00 200°C 0.35g ± 0.05g
Aceta I 0.0209% 0.0031 5:25 190°C 0.5g ± 0.05g
Polycarbonate 0.0168% 0.0004 3:30 250°C 1.0g ± 0.1g

Figures 1 and 2 show the graphs generated by the Vapor Pro® 3100L. The total moisture graph shows the ending of the test as the slope of the line reaches zero. The rate graph delineates the reason for the slope to approach zero.

A significant drop can be observed at 540s, revealing that the ending criteria is adequate to measure the total moisture content of the material. Vapor Pro graphs created for the other materials revealed that they had same profiles. Graphing feature is not available for the Karl Fisher titrator.

Total moisture profile for polypropylene resin.

Figure 1. Total moisture profile for polypropylene resin.

Rate loss profile for polypropylene resin.

Figure 2. Rate loss profile for polypropylene resin.

Conclusion

The Computrac® Vapor Pro® 3100L has several benefits over Karl Fisher Titrators to provide moisture content measurements for extrusion resins. Both instrument methods have established standard testing procedures through the ASTM to yield corollary results.

The Vapor Pro® 3100L avoids the use of harmful aqueous chemical titrants as in the case of KF analysis. Moreover, the key features of the Vapor Pro® 3100L interface enable users to optimize results and throughput options by determining suitable testing criteria.

The Vapor Pro® 3100L can be staged anywhere on production floors and quality control labs due to its robust case. These features are helpful in monitoring extrusion materials and drying techniques, as they provide a more comprehensive analysis with rapid results devoid of adverse chemical waste, which will shorten throughput times and save money associated with waste disposal.

About Arizona Instrument

Initially known as the Quintel Corporation, Arizona Instrument LLC was founded in 1981 by a group of engineers breaking away from The Motorola Corporation who were dedicated to the idea of providing precision moisture analysis instruments that were accurate, reliable, and easy to use.

The first instrument released was the MA Moisture Analyzer, but the company quickly expanded its Computrac® moisture analysis line and became an accepted leader in moisture analysis, setting a standard that has been adopted by many Fortune 500 companies. Today the Computrac® line is comprised of three technologies: rapid loss-on-drying, high temperature loss-on-ignition, and moisture specific analysis using polymer capacitance sensor, GREEN alternative to Karl Fischer.

Arizona Instrument

This information has been sourced, reviewed and adapted from materials provided by Arizona Instrument.

For more information on this source, please visit Arizona Instrument.

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