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This document specifies a general method for the rapid determination of water content of polyamide (PA) using Near-Infrared (NIR) spectroscopy, mainly involving relevant terms and definitions, the principle of the test, instrument requirements, sample selection, procedures for establishing, validating and maintaining calibration models, as well as the steps for sample measurement. This document is applicable to polyamides including but not limited to PA46, PA6, PA66, PA69, PA610, PA612, PA11, PA12, PA6T/66, PA6I/6T, PA6T/6I/66, PA6T/6I, PA6I/6T/66 and PA MXD6, as well as copolyamides and polyamide compounds. This document is suitable for use in research, quality control, specification testing and process control of PA.
Polyamide, commonly known as nylon, is a core category within the special engineering plastics. It has become a class of key fundamental materials supporting the advancement of strategic emerging industries such as new energy vehicles, aerospace, and high-end electronics, owing to its excellent mechanical strength, chemical resistance, and processing compatibility. Given the water content is the critical parameter of PA, this proposal aims to establish a standard specifying a method for the rapid determination of water content using NIR spectroscopy. The current ISO standard for the determination of water content of the polyamides industry is ISO 15512: 2019, Plastic — Determination of water content, which specifies six methods: extraction with anhydrous methanol (Method A), water vaporization using a heating tube oven (Method B1), water vaporization using a heated sample vial (Method B2), manometric method (Method C), water detection by using a diphosphorus pentoxide cell (Method D), and determination of water content by calcium hydride method (Method E). Among these, Method A involves heating and extraction with anhydrous methanol, followed by Karl Fischer titration. This method features complex pretreatment procedures, a long testing cycle (several hours per sample), and low efficiency. Method B is a heating evaporation method, using either a tube oven or heated sample vial, where water in the sample is evaporated at high temperature and transferred as vapor to a titration cell for determination by coulometric Karl Fischer titration. In this method, excessively low heating temperatures may lead to incomplete water evaporation, resulting in underestimated results; while excessively high temperatures may cause polymer decomposition, leading to overestimated results. Method C is a physical method that calculates water content by measuring the gas pressure generated after water evaporation. This method is susceptible to interference from volatile components, which can cause significantly higher results. Furthermore, the accuracy of results heavily depends on system sealing integrity and temperature control stability, involving various influencing factors and resulting in low data credibility. Method D, based on electrolytic chemistry, offers high precision and sensitivity but requires expensive equipment, and its electrolytic cell is high sensitive to ambient humidity. For materials from which water is difficult to release, an additional heating furnace accessory is needed, increasing testing costs. Method E involves mixing the sample with calcium hydride (CaH2) powder in a sealed container, where a vigorous reaction with water produces hydrogen gas. The water content is then calculated by measuring the pressure or volume of the generated hydrogen gas. The advantage of this method is that CaH2 reacts specifically with water and is not interfered with by other volatiles. However, the reaction is vigorous, generates hydrogen gas (posing safety risks), requires pressure-resistant equipment, limiting its application scope. equipment, limiting its application scope. In polyamide production, a widespread issue exists: the poor solubility of polyamide materials leads to a severe lag in intermediate control data. Because the performance parameters such as water content cannot be timely fed back, the duration of material reactions often relies heavily on process control experience, making it difficult to remedy quality issues once they arise. In continuous production lines, multiple product grades are often manufactured on shared equipment. During grade transition, the generated transition material (off-grade, substandard, or waste) accounts for a non-negligible proportion of product costs. The amount of transition material produced is closely related to the detection time of performance parameters (such as water content). A shorter product inspection time enables earlier confirmation of reaction completion for qualified products (reducing energy consumption). Improved inspection efficiency helps enhance product yield, ensure stable product quality, thereby significantly saving production costs, improving production efficiency, and achieving the goals of cost reduction, quality improvement, and efficiency gain. Therefore, it is highly necessary to develop a rapid testing method for performance parameters of polyamide product, such as water content. content. NIR spectroscopic analysis utilizes the characteristic spectra generated by the vibration or rotation of chemical bonds containing hydrogen groups, X—H (where X = C, N, O, etc.), within the wavelength range of approximately 780 nm to 2500 nm. The O—H bonds in water molecules exhibit strong absorption bands at approximately 1450 nm and 1940 nm, which can be used for quantitative analysis. Based on the fundamental principles of chemometrics, the calibration model shall be established by collecting the spectral data of polyamide and applying multivariate correction algorithms such as multiple linear regression (MLR), principal component analysis (PCA), or partial least squares regression (PLS), in conjunction with standard polyamide samples of known water content. The NIR spectral data of polyamide samples with unknown water content shall then be processed by the calibration model to achieve rapid determination of its water content value. Compared to traditional methods (such as Karl Fischer titration), NIR spectroscopy offers advantages including speed, non destructiveness, and environmental friendliness. The NIR method requires no chemical reagents, aligning with the principles of green manufacturing and sustainable development. However, there is currently a lack of unified technical standards and operational specifications for the rapid determination of the critical parameter—water content—in polyamide using NIR spectroscopy around the world. This situation leads to significant discrepancies in quality assessment and improvement processes for polyamide products across different regions and enterprises, hindering the comprehensive optimization and sustainable development of the polyamide industry. The core objective of this proposal is to provide the industry with a recognized and reliable analytical benchmark for polyamide water content, ensuring the comparability and traceability of results. It aims to address the shortcomings of traditional testing methods, such as long duration, chemical reagent pollution, and inability to perform online detection, thereby filling a key gap in the existing standard system. This initiative will promote the intelligent and green upgrading of polyamide production quality control technologies globally, lead the high-quality development of the polyamide industry, and enhance the international competitiveness of polyamide products in various countries
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