1. Background of Standard Development
With the increasing global call for environmental protection and the strengthening of environmental regulations in various countries, the production and variety of powder coatings, as "green coatings" that meet environmental requirements, are constantly increasing and expanding. Fluororesin possesses excellent weather resistance, durability, and chemical resistance, as well as special surface properties and good electrical characteristics. Applying fluororesin to the powder coating field offers the advantages of both high performance and environmental friendliness, and has broad development prospects.
The superior performance of fluororesin is attributed to the high bond energy and low polarizability of the C-F bonds in the resin. Therefore, the C-F bond content in the coating base has a significant impact on product performance and also affects product cost. Therefore, it is necessary to establish a method for determining the fluorine content of powder coatings and formulate corresponding standards to meet the needs of R&D, production, and supervision in the powder coating industry.
Based on the above objectives, the Secretariat of the Coatings and Pigments Technical Committee of the CSTM/FC05/TC05 Chemical Materials Committee submitted a standard development plan in 2019. which was approved. The standard development process lasted one year and was submitted for approval in December 2019. The standard was released in April 2020.
2. Purpose and Significance
Prior to the implementation of this standard, there were no corresponding domestic or international standards guiding the determination of fluorine content in the powder coating industry. This standard establishes a scientific and accurate testing method for the first time, providing a basis for the determination of fluorine content in powder coatings, meeting the needs of industry development, filling a gap, and reaching the international advanced level. The formulation and release of this standard provides manufacturers with a convenient and quick method for product quality control; provides research institutions with accurate data support for studying the relationship between elements and product performance and developing new products; and provides regulatory authorities with an accurate and powerful tool for accurately understanding the industry situation. It is conducive to standardizing the powder fluoropolymer coating industry, improving product quality, promoting technological and economic development, and enhancing industry competitiveness.
The chemical industry standard "Fluoropolymer Powder Coatings," formulated in 2020. pioneered the use of this standard as a standard method for determining fluorine and PVDF content, setting a precedent for the use of group standards in the coating industry. The formulation of this standard will further promote technological progress in the powder fluoropolymer coating industry and guide the healthy development of the industry.
3. Standard Introduction
This standard is a group standard for test methods, applicable to manufacturers, research institutions, testing organizations, and industry regulatory departments for determining the fluorine content in solvent-soluble substances of powder coatings. It can also be used as a reference for determining the organic fluorine content in related materials. This standard specifies the principles, reagents or materials, instruments and equipment, samples, test procedures, data processing, and test reports for determining the fluorine content in powder coatings.
The core content of this standard: For powder coatings containing polyvinylidene fluoride (PVDF) or fluoroolefin/vinyl ether (ester) copolymer resin (FEVE), the oxygen bomb combustion-ion selective electrode method is used to determine the fluorine content in the solvent-soluble substances; for powder coatings containing polyvinylidene fluoride (PVDF), the centrifugation method is used to determine the PVDF resin content in the solvent-soluble substances.
4. Standard Features
This standard provides two methods for testing organic fluorine-containing substances in powder coatings with different resin systems:
4.1 Determination of Fluorine Content in Solvent-Soluble Matter
This method is applicable to powder coatings containing polyvinylidene fluoride (PVDF) or fluoroolefin/vinyl ether (ester) copolymer resin (FEVE).
The principle of the method is as follows: For powder samples containing pigments and fillers, N,N-dimethylformamide (DMF) is added as a solvent. After heating and ultrasonication, the solvent-soluble matter is separated. The solvent-soluble matter is then decomposed by combustion in an oxygen bomb, and the decomposition products are absorbed with sodium hydroxide solution. Using a fluoride ion selective electrode as the indicator electrode and a saturated calomel electrode as the reference electrode, the fluoride ion concentration in the absorbent is determined by the standard addition method, and the fluorine content in the solvent-soluble matter is calculated (the fluorine content in the sample can be calculated based on the pigment-to-binder ratio). For powder samples without pigments and fillers, centrifugation is not required for direct testing.
The key technical points of this method are explained below:
4.1.1 Pretreatment Method
Fluorine in fluoropolymers is organic fluorine, with a high C-F bond energy, requiring significant energy to release fluorine. To ensure the method's recovery rate, the decomposition of the fluoropolymer and the release of fluorine should be carried out in a closed environment. Furthermore, the standard method should be as simple and easy to implement as possible. The oxygen cylinder/bomb combustion method is relatively simple to operate and has been applied in relevant industry standards for fluoropolymer coatings, HG/T 3792-2014 and HG/T 4104-2019. accumulating rich experience. Compared to oxygen cylinders, the oxygen bomb combustion device provides a high-pressure, oxygen-rich combustion environment, allowing for complete combustion and decomposition of the sample, further improving the fluorine conversion rate. Therefore, the oxygen bomb combustion method is used as the method for converting organic fluorine to fluoride ions.
4.1.2 Fluorine Content Determination Method
Common methods for determining fluorides include lanthanum-alizarin complex spectrophotometry, thorium nitrate volumetric method, mercuric nitrate volumetric method, ion-selective electrode method, and ion chromatography. Considering accuracy, interference resistance, and ease of operation, the determination of fluorine content in fluoropolymers can be performed using ion-selective electrode method or ion chromatography. Ion-selective electrode method has been used in the coatings industry, accumulating rich testing experience, and the method is relatively mature; while ion chromatography has a feasible principle, its practical application has not yet been verified. Therefore, this standard adopts the ion-selective electrode method as the determination method.
4.2 Determination of PVDF Resin Content in Solvent-Soluble Matter (Centrifugation Method)
This method is applicable to powder coatings containing polyvinylidene fluoride (PVDF).
The principle of the method is as follows: PVDF resin is insoluble in solvents such as toluene and ethyl acetate, but has good solubility in N,N-dimethylformamide (DMF), while acrylate resins, FEVE resins, and polyester resins have good solubility in solvents such as toluene and ethyl acetate. Therefore, PVDF resin, acrylate resin, FEVE resin, polyester resin, etc., in powder coatings can be completely separated from solvent-insoluble matter through multiple centrifugation methods. The content of PVDF resin in solvent-soluble matter is then determined by gravimetric method.
The key technical points of this method are explained as follows: The methods for determining PVDF content currently used in published standards mainly include: 1) centrifugation; 2) melt temperature decrease method; 3) first measuring the fluorine content, then calculating the PVDF resin content based on the theoretical fluorine content of the PVDF resin. The melt temperature decrease method requires plotting a curve using pure PVDF resin used to prepare the coating, which is difficult to implement in actual testing. The method of calculating PVDF resin content from the measured fluorine content suffers from discrepancies between the actual and theoretical fluorine content due to differences in PVDF resin synthesis processes, leading to deviations in the test results. The centrifugation method is simple, convenient, and highly accurate; therefore, considering all factors, the centrifugation method was chosen for determining the PVDF resin content.
5. Standard Application
This standard provides an accurate, convenient, and reliable method for determining the fluorine content in powder coatings for the first time. After its publication and implementation, this standard has attracted industry attention, and the National Coatings Quality Inspection and Testing Center, a leading testing institution in the coatings field, was the first to obtain testing qualifications in the CNAS expansion review. This standard meets the urgent needs of the powder coating industry in research and development, production, and supervision, effectively controlling product quality, combating counterfeiting and substandard products, guiding the healthy development of the industry, and contributing to the transformation and upgrading of my country's coating industry and environmental protection.
Following its release and implementation, this standard was referenced in the chemical industry standard HG/T 6002-2021 "Fluororesin Powder Coatings" formulated by the National Technical Committee on Standardization of Coatings and Pigments in 2020. setting a precedent for national and industry standards in the coating industry to reference group standards. With the release and implementation of the standard "Fluororesin Powder Coatings," this standard will be more widely used within the industry, powerfully promoting industry progress and economic development, and achieving significant economic and social benefits.
