Chloe Zhang
Moisture content in plastics is an important factor in the manufacturing process, as it can impact the quality of the final product. An excess of moisture can cause various issues, including mechanical problems, reduced shelf life, and surface defects such as dents, bubbles, and discolouration. Therefore, it is crucial to monitor and control moisture levels during production to ensure the desired product characteristics are achieved. Several methods are available to measure moisture content in plastics, including the widely used Karl Fischer titration test, which provides an accurate estimation of moisture levels in a range of materials, including plastics. Other methods include Differential Scanning Calorimetry (DSC), Gravimetric Vapour Sorption (GVS), and ASTM's Standard Test Method for Determination of Moisture in Plastics by Loss in Weight, which utilises moisture analysers.
| Characteristics | Values |
|---|---|
| Why measure moisture content in plastic? | Moisture content affects the quality, performance, and mechanical properties of plastics. It can cause defects in the final product such as dents, bubbles, discolouration, and brittleness. |
| Sample preparation | Samples can be prepared from raw resins or finished articles in powder or granular form. |
| Weighing | The initial weight of the sample is measured using a precise scientific technique. |
| Drying | Samples are dried in a ventilated oven at a controlled temperature. This step may not be suitable for heat-sensitive plastics. |
| Final weighing | Samples are cooled to room temperature and weighed again to determine the final weight. |
| Moisture content estimation | Moisture content is calculated by measuring the difference between the initial and final weights and dividing it by the initial weight. |
| Interpretation of results | Low moisture content is desirable as it indicates good material quality and potentially better layer strength and mechanical properties. |
| Test methods | ASTM D6869, ISO 15512, Karl Fischer titration, halogen moisture analyzer, differential scanning calorimetry (DSC), gravimetric vapor sorption (GVS) |
| Test equipment | HX204 moisture analyzer, METTLER TOLEDO instruments, moisture balance, coulometric Karl Fischer apparatus |
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What You'll Learn
Using the Karl Fischer titration method
The Karl Fischer titration method is a scientific method that provides a highly precise estimation of moisture content in a broader range of materials, including plastics. It is especially useful for materials with low moisture levels. The method involves using a reagent and solvent in the titration cell. When a sample is introduced to the cell and dissolved, an electrical current is induced, releasing the reagent. The amount of current required to convert the water is used to determine the moisture content. This method can accurately measure small amounts of moisture, with a sensitivity of 0.1 micrograms of water.
There are two variations of the Karl Fischer titration method: the volumetric method and the coulometric procedure. In the volumetric method, a Karl Fischer solution containing iodine is added until the first trace of excess iodine is present. The amount of iodine converted is determined by the volume of the iodine-containing solution. This method is ideal for samples with higher levels of moisture, generally over 1% or 2%.
The coulometric procedure, on the other hand, involves generating iodine directly in the titration cell by electrochemical oxidation of iodide until a trace of unreacted iodine is detected. Faraday's law is used to calculate the amount of iodine produced from the quantity of electricity used. This method is suitable for samples with a wide range of water content, from high levels to as low as the ppm range.
The Karl Fischer titration method is a reference method used for many substances and is a chemical analysis based on the oxidation of sulfur dioxide by iodine in a methanolic hydroxide solution. It is a highly accurate method for determining water content, from a few ppm to near saturation. The ASTM has recognized this method as the standard test method for determining the residual moisture content in plastics.
The HX204 moisture analyzer is an alternative to the Karl Fischer titration method for routine testing. It offers high resolution of 0.001% MC and a weighing capacity of 200g, making it ideal for applications with low moisture content such as plastics.
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ASTM D6869 test
ASTM D6869 is a standard test method for the coulometric and volumetric determination of moisture in plastics using the Karl Fischer reaction. This reaction involves the reaction of iodine with water. The Karl Fischer titration method is a scientific method that provides a precise estimation of moisture content in a wide range of materials, including plastics. It is especially useful for materials with low moisture levels.
The ASTM D6869 test is an accurate way to determine the water content in plastics, which is vital for quality assessment, quality control, refining, handling, transportation, and sales. The test uses reagent-grade chemicals, and the purity of the water used is crucial. The anode solution is a mixture of commercial Karl Fischer anode solution and reagent-grade xylene. It is important to note that reagent performance deteriorates with use and should be regularly monitored by injecting a known quantity of water.
The ASTM D6869 test is a valuable tool for industries that rely on plastics with specific moisture content requirements. By controlling the moisture content, manufacturers can ensure the production of high-quality plastic parts with smooth surfaces and ideal mechanical properties. The test provides a standardized and consistent method for measuring moisture in plastics, enabling comparisons and quality control across different batches and suppliers.
Additionally, the ASTM D6869 test can help identify potential issues related to moisture content. For example, high moisture content can cause surface imperfections or degradation by hydrolysis, while low moisture content coupled with high temperatures can lead to polymerization. Therefore, the test plays a crucial role in maintaining the desired characteristics and performance of plastic products.
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HX204 moisture analyzer
Measuring moisture content in plastics is essential for maintaining product quality and consistency. The HX204 Moisture Analyzer from Mettler Toledo is a reliable solution for this purpose. Here's how it works and why it's beneficial for measuring moisture content in plastics:
The HX204 Moisture Analyzer is a halogen moisture analyzer that offers exceptional measurement performance for various sample types, including plastics. It boasts a high-resolution weigh cell and an innovative hanging weighing pan design. This instrument is designed with compliance features in mind, adhering to ALCOA+ principles and offering automated export and printing functions.
Key Features:
- Readability: The HX204 provides highly accurate readings with a readability of 0.001 %MC (0.1 mg), making it ideal for detecting low moisture content in plastics.
- Weighing Capacity: With a weighing capacity of 200g, it is well-suited for analyzing plastic pellets and resins.
- User Guidance: This analyzer offers user guidance and results prediction, making it user-friendly.
- Data Management: It has network connectivity and versatile reporting options, allowing easy connection to PCs, servers, and printers for data management and reporting.
- Drying Programs: The HX204 features four drying programs and a motorized lid for convenient sample handling.
- Compliance: It includes smart compliance features, such as user management and the option to have "no results on the instrument," ensuring data integrity.
Measuring Moisture Content in Plastics:
The HX204 Moisture Analyzer is particularly useful for measuring moisture content in plastics due to its high resolution and accuracy. Here's how it can be applied in the context of plastics:
- Alternative to Karl Fischer Titration: The HX204 offers an accurate and easy alternative to the traditional Karl Fischer titration method for determining residual moisture content in plastics.
- ISO 15512 Compliance: This analyzer can be used in accordance with ISO 15512, which specifies methods for measuring moisture content in plastics, including test planning, weight estimation, and moisture content estimation.
- Quality Control: By controlling the moisture content in plastic resins, manufacturers can ensure trouble-free compounding, injection molding, smooth surfaces, and ideal mechanical properties of the final plastic parts.
- Surface and Chemically Bonded Moisture: The HX204 can help identify surface moisture and chemically bonded moisture, which can impact the quality and performance of plastics.
In conclusion, the HX204 Moisture Analyzer from Mettler Toledo is a reliable and accurate instrument for measuring moisture content in plastics. Its high resolution, user-friendly features, and compliance options make it a valuable tool for quality control and research applications in the plastics industry.
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Differential Scanning Calorimetry (DSC)
DSC is commonly used for the study of biochemical reactions and can be applied to nearly all plastic and rubber materials, including thermoplastics, thermosets, and elastomers. It can also be used to study liquid crystals, observing the small energy changes that occur as matter transitions from a solid to a liquid crystal and from a liquid crystal to an isotropic liquid.
The result of a DSC experiment is a curve of heat flux versus temperature or time. This curve can be used to calculate enthalpies of transitions, such as crystallization and melting, and to determine transition temperatures. The ability to measure these values makes DSC valuable for producing phase diagrams for various chemical systems.
DSC testing provides insights that support product development, quality control, and material selection. It is a fast and reliable method that requires minimal sample preparation, making it ideal for routine screening or large sample sets. The typical weight of a polymer sample is about 10 milligrams, and metal samples can be up to about 40 milligrams due to their higher densities.
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Gravimetric Fume Sorption (GVS)
GVS provides significant insights into the moisture sorption isotherm of a substance, which describes how moisture content varies with relative humidity. This information is crucial for understanding the storage, stability, processing, and performance of materials. For example, in the case of plastics, an understanding of moisture content is essential for maintaining material quality and consistency.
The GVS technique is often used to study the behaviour of pharmaceutical drugs, personal care materials, and food products. In pharmaceuticals, GVS can be used to determine the hygroscopicity of a drug substance, which is useful for early-stage development. Personal care materials such as hair and skin samples can be analysed using GVS to understand the effects of different chemical and mechanical treatments on moisturisation and hydration behaviour. Similarly, GVS can be applied to food products to determine moisture content, which is critical for establishing shelf life and freshness.
GVS is a relatively straightforward method of analysis. Modern GVS instruments are flow-through automated devices that perfuse a carrier gas, commonly nitrogen, over a sample suspended from a microbalance. The carrier gas can also be argon, oxygen, or air. The change in mass of the sample is then measured as a function of relative humidity, or through the sorption of organic vapours. It is important to ensure that the mass of the sample reaches equilibrium with the environmental relative humidity before progressing to the next relative humidity level.
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Frequently asked questions
Moisture content in plastics is important to measure because it can affect the performance of plastics and the molding process. It can also cause defects such as bubbles, jet marks, and silver threads on the surface of the product.
There are several standard methods to determine moisture content in plastics. The ASTM D6869 method, also known as the Karl Fischer Titration method, is a widely recognised test method. This method uses the reaction of iodine (I2) with water to determine the moisture content. Other methods include Differential Scanning Calorimetry (DSC) and Gravimetric Vapour Sorption (GVS).
A moisture content analyser is a common piece of equipment used to measure moisture content in plastics. The HX204 moisture analyser by METTLER TOLEDO is specifically designed for plastics and offers high resolution and data handling capabilities.
When choosing a test method, it is important to consider the type of plastic being tested and its unique characteristics. For example, some plastics may be heat-sensitive, while others may contain volatile components that can interfere with certain reactions. It is also important to ensure that the test method is suitable for the range of moisture content expected in the plastic.
Preparing plastic samples for testing typically involves the following steps: Test planning, where plastic test samples are prepared according to specific shapes, sizes, and pretreatment requirements. This is followed by an initial weight estimation, where the weight of each test sample is carefully measured to establish a baseline for determining moisture content.
