Standards Development

Scope

This document specifies a general method for the rapid determination of performance parameters of polyamides (PA) using Near-Infrared (NIR) spectroscopy. It provides the relevant terms and definitions, test principles, instrument requirements, sample selection, procedures for establishing and validating and maintaining the calibration model, and sample determination. It is applicable to determine the water content, relative viscosity, terminal amino group content and relative molecular mass of PA. The method is applicable to the polyamides such as PA46, PA6, PA66, PA69, PA610, PA612, PA11, PA12, PA6T/66, PA6I/6T, PA6T/6I/66, PA T/6I, PA6I/6T/66 and PA MXD6, as well as to copolyamides, compounds of polyamides. This document is suitable for use in quality control, specification testing and process control of PA

Purpose

Polyamide (PA, commonly known as nylon) is one of the most widely used synthetic materials in the world. With its excellent mechanical strength, chemical resistance, and processing adaptability, it is extensively applied in the textile, automotive, and electrical industries. It has become a key foundational material supporting the upgrading of strategic emerging industries such as new energy vehicles, aerospace, and high-end electronics. This proposal aims to establish a standard to regulate the procedure for determining the performance parameters of polyamides using near-infrared (NIR) spectroscopy, such as water content, relative viscosity, relative molecular mass, and terminal amino group content.

Currently, the polyamides industry relies on traditional testing methods such as polyamides water content (ISO 15512:2019), polyamides relative viscosity (ISO 307:2019), polymer relative molecular weight (ISO 16014-1:2019), and polyamides terminal amino groups (ISO 6353-1:1982). These methods all have disadvantages such as high solvent toxicity, reliance on chemical dissolution and complex manual operation, and long testing periods.

ISO 15512:2019 "Plastics - Determination of water content" specifies six methods including 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). Method A is an extraction method using anhydrous methanol followed by a Karl Fischer titration of the extracted water. This method has complex pretreatment steps, a long testing time (several hours per sample), and low efficiency. Method B is a vaporization method (using a tube furnace or a heated sample bottle). The water contained in the test sample is vaporized and carried to the titration cell by carrier gas, followed by a coulometric Karl Fischer titration. Low heating temperatures may result in incomplete water evaporation, leading to underestimated results, while high temperatures may cause polymer decomposition, resulting in overestimated results. Method C is a manometric method which water content is calculated from the increasing gas pressure when the water is evaporated under a vacuum. This method is susceptible to interference from volatile components, resulting in significantly higher water content testing results; and the accuracy of the results depends heavily on the sealing of the system and the stability of temperature control. This method has many influencing factors and low data credibility. Based on the electrolysis principle, method D has high accuracy and sensitivity, while the electrolytic cell is sensitive to environmental humidity. For some materials that are difficult to release water content, heating furnace accessories need to be added, and the testing cost is high. By mixing water content of the sample with calcium hydride(CaH) in a closed container to generate hydrogen gas in a violent reaction, the water content is calculated by measuring the pressure or volume of the hydrogen in Method E. This method has the advantage that CaH only reacts with water and is not interfered by other volatile substances. However, the reaction process is vigorous for hydrogen gas generated is highly dangerous and the equipments are also required high pressure resistance, limiting application range of method E.

ISO 307:2019 " Plastic-Polyamides-Determination of viscosity number " involves dissolving polyamides in specified solvents (sulfuric acid, formic acid, m-cresol, phenol, 1,1,2,2-tetrachloroethane, or a mixture of phenol and 1,1,2,2-tetrachloroethane) and testing the viscosity of the dilute solution. This method relies on chemical dissolution and manual operation with complex operation and long waiting time due to its poor solubility, cannot meet the requirements of rapid detection. ISO 16014-1:2019 " Plastic — Determination of average molecular weight and molecular weight distribution of polymers using size-exclusion chromatography — Part 1: General principles " specifies the general method for determining the average and dispersity of molecular weight of polymers using size-exclusion chromatography (SEC). Currently, there is no specific application standard for the relative molecular weight of polyamides. Since polyamides is a highly polar polymer with extremely strong intermolecular and intramolecular hydrogen bonding, it cannot be dissolved and completely dissociated in common solvents such as tetrahydrofuran (THF) or chloroform. Special solvents such as hexafluoroisopropanol (HFIP), which are highly acidic, toxic, and expensive, must be used. Moreover, the instruments are expensive along with high costs, needing complex maintenance and high requirements for the skills of operators.

Currently, there is no specific application standard for the content of terminal amino groups in polyamides. ISO 6353-1:1982 stipulates the basic operation requirements for potentiometric titration. During the test, due to the poor solubility of polyamides, trifluoroethanol or a mixture of m-cresol and isopropanol is generally used as the solvent, and the testing is conducted using the hydrochloric acid potentiometric titration method. The disadvantage of this method is that the solvent trifluoroethanol is highly toxic, and the dissolution time of polyamides is long (1-2 days) resulting in a long testing cycle. In the manufacturing enterprises of polyamide products, there is a widespread problem that the long testing cycle of polyamides (due to its poor solubility) leads to a serious lag in the process control data. As the product indicators cannot be promptly fed back, the length of the material reaction time often heavily depends on the experience of process control, and the arising quality issues are difficult to remedy. In the production line of the continuous process, there is also the co-production of multiple grade products. During the grade conversion process, the transitional materials (auxiliary grades, defective products, waste materials) account for a significant proportion that cannot be ignored in the product cost. The production amount of transitional materials is closely related to the performance index testing time. The shorter the product quality inspection time, the earlier the qualified product reaction completion can be determined (reducing energy consumption); the improvement of quality inspection efficiency helps to increase the product's yield rate and ensure product quality stability, thereby significantly saving production costs, improving production efficiency, and achieving cost reduction, quality improvement and efficiency enhancement. Therefore, it is very necessary to develop a rapid detection method for polyamides product indicators.

Near-infrared spectroscopy (NIR) analysis technology utilizes the characteristic spectra produced by the vibration or rotation of chemical bonds containing hydrogen groups XH (X = C, N, O, etc.) at wavelengths of approximately 780 nm to 2500 nm. The performance parameters of polyamides samples, such as water content, relative viscosity, relative molecular mass and terminal amino content, cause changes in their NIR spectra. There is a correlation between the changes in performance parameters and the corresponding molecular spectral changes. A correlation is established between the NIR spectra of polyamides and their properties through a multivariate mathematics. The properties of an unknown polyamides sample shall then be rapidly estimated by applying its NIR spectrum to this calibration model. NIR technology has advantages such as rapidity, non-destructiveness and environmental friendliness compared to traditional methods. The NIR method does not require chemical reagents and is in line with the concepts of green manufacturing and sustainable development.

However, currently there is a lack of unified technical standards and operational norms for the determination of polyamides performance parameters using near-infrared methods on an international scale. This situation leads to significant differences in the quality assessment and improvement of polyamides products among different regions and enterprises, affecting the comprehensive optimization and sustainable development of the polyamides industry. The purpose of this proposal is to provide a recognized and reliable analytical benchmark for PA industry, ensuring the comparability and traceability of results, solving the problems of long testing time, chemical reagent contamination, and inability to achieve online detection that exist in traditional testing methods. It can fill the key gaps in the existing standard system. This proposal can promote the intelligent and green upgrading of quality control technology for PA production in various countries, and lead to the high-quality development of PA industry