Advances in VOC assays in water-based coatings

0 Introduction:
with the development of the national economy and the expansion of investment scale, China has gradually become the world's largest consumer of paint, paint safety and environmental protection is closely related to people. However, coatings circulating on the market often contain a large number of toxic and harmful substances, such as formaldehyde, benzene, toluene, xylene, heavy metals, toluene diocyanate (TDI) and volatile organic compounds (VOCs). Broadly speaking, VOC is any liquid or solid organic compound that can be volatile in the atmosphere at room temperature and pressure, its main components are hydrocarbons, oxygen hydrocarbons, halogenated hydrocarbons, nitrogen hydrocarbons and sulfur hydrocarbons, showing toxicity, irritation, carcinogenicity, and special odors can lead to various unwell reactions of the human body, and have a greater impact on human health
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. To this end, a series of regulations have been formulated at home and abroad, such as the United States Environmental Protection Agency (EPA) in 1977 developed atmospheric purification regulations, 189 kinds of harmful solvents (including methanol, toluene, xylene, butyl ketones, methyl isoclode and other paints commonly used solvents) emission standards; China has formulated a mandatory limit standard (GB 18582-2001) for the VOC content of interior wall coatings, and in 2008 promulgated a newly revised mandatory limit standard GB 18582-2008, which puts forward higher requirements for VOC content in water-based coatings. The author summarizes the definition, detection method and principle of VOC in various countries or departments, and puts forward the research direction of VOC measurement in water-based coatings in the future.
1 Volatile Organic Compounds
the definition of VOC is different from country to country, and several countries are listed below. The U.S. Environmental Protection Agency (EPA) defines VOC as all carbon compounds involved in atmospheric photochemical reactions, excluding CO, CO2, carbonate, metal carbide or carbonate and ammonium carbonate. The EU definition of VOC refers to organic compounds with an initial boiling point below or equal to 250 degrees C at standard atmospheric pressure (101.3 kPa). The World Health Organization (WHO) defines VOC as a volatile organic compound with saturated steam pressure of more than 133.322 Pa at room temperature and a boiling point between 50 and 260 degrees C.
voC definition of china's two mandatory standards (GB 18582-2001, HBC12-2002) refers to the total volatiles in the coating after water. GB 18582-2008 Defines VOC the same as the EU definition of VOC. These VOC definitions have similarities and differences, such as the U.S. Environmental Protection Agency(EPA) VOC definition does not limit boiling point, primary distillation point, emphasizing participation in atmospheric photochemical reactions, and the European Union and World Health Organization (WHO) VOC definitions limit boiling point or primary distillation point (organic compounds that provide for a certain atmospheric pressure and temperature below 250 degrees C), regardless of whether they participate in atmospheric photochemical reactions. As the definition of VOC is not uniform in various countries, which brings some difficulties to the determination, how to give VOC a precise uniform definition, there is still to be further efforts.
2 standard for determining VOC content
in order to facilitate comparative research, we summarize the national VOC measurement standards.
2.1 Foreign Standard
(1) ISO 11890-1 Determination of Color and Varnish - Determination of Volatile Organic Compounds (VOCs) Content - Part 1: Difference Method. This method can be used when the expected VOC content in coating products is greater than 15% (mass fraction). This method is mainly used for the determination of conventional solvent-based coating products with high VOC content. The principle is that the various parts of the coating product are mixed according to the regulations, with the correct mass ratio or volume ratio, and if dilution is required, diluted with a suitable thinner, as a backup sample for determination. The non-volatile content, water content and exempt compounds content in the spare sample were determined separately, and the VOC content was calculated using the appropriate formula.
(2) ISO 11890-2 Color and Varnish - Determination of Volatile Organic Compounds (VOC) Content - Part 2: Gas Chromatography. This method can be used when the expected VOC content in coating products is between 0.1% and 15% (mass fraction). This method is mainly used for coating products with low VOC content. The principle is to separate the organic volatiles and exempt compounds from the spare sample by gas chromatography technology according to the regulations of each part of the coating product, with the correct mass ratio or volume ratio. The volatiles (including organic volatiles and exempt compounds) in the spare sample are qualitatively analyzed, and then the content of each organic volatile compound and exempt compound in the spare sample is quantitatively measured by the internal standard method at the peak area value, the water content in the sample is determined by the appropriate method, and the VOC content in the coating product is calculated by the appropriate formula.
(3) ISO 17895 Color paint and varnish - determination of volatile organic compound content in low VOC latex paint, i.e. top-empty sample method to determine VOC. When the expected VOC content in coating products is between 0.01% and 0.1% (mass fraction), it can be determined by this method. This method is mainly used to determine the VOC content in water-based latex paint samples with very low VOC content. The principle is to use a top-empty sampler with a diaphragm-closed vial, and preferably with an automatic sample converter gas chromatography to analyze. When the sample is heated to 150 degrees C, the volatile organic compounds are fully vaporized and transferred to the non-polar capillary separation column, with the retention time of fourteenane (boiling point 252.6 degrees C) as the integral end point, and the peak area integral of all the parts before the integral end point.
The diluted samples without the storage mixture and the four diluted samples containing the stored mixtures with different mass fractions were measured, all calibrated to the peak area of the 1 mg sample, and the average was calculated, linear regression was made with the amount of the added storage mixture, and the VOC content in the sample was calculated by the slope of the regression line and the distance of the straight line in ordinates.
(4) ASTM D 6886-03 for the detection of coatings with a VOC content of less than 5%. The principle is that the sample after weighing is diluted with tyfuran, using GC/FID, GC/MS or SPME/GC (SPME-solid-phase micro-extraction) to qualitatively the volatile organic compounds in the sample and distinguish between volatile organic compounds and exempt compounds. The content of these compounds is determined using the internal markers and based on the peak area, and the VOC content in the coating is calculated.
(5) ASTM D 3960.02 for determining volatile organic compound (VOC) content in color paints and related coatings, in much the same way as ISO 11890-1.
(6) DIN 55649, which is suitable for coatings with VOC content of 0.01% to 0.1%. The principle is that the sample diluted by the buffer solution is injected into the top empty bottle, and heated to 150 degrees C, with the top air sampler injected into the non-polar capillary, the integral retention time is less than the retention time of the fourteanes of all the parts, with 4 different concentrations of mixed standard storage fluid, with the standard overlay method extrapolated quantitative determination of VOC content. No matter what standard has a certain range of application, different VOC content of the coating should choose the corresponding measurement standards.
2.2 Domestic Standard
In 2001, China formulated a mandatory limit standard, that is, "GB18582-2001 interior decoration material interior wall paint in the limit of harmful substances", in which VOC measurement principle for the total volatile content of the paint minus water content. The total volatile content in water-based coatings is determined according to GB/T6751-1986, the conditions (105±2) are maintained at 3 h, while the water content determination method can be measured by Carl-Fisher or gas chromatography, in which gas chromatography uses the gas chromatography system of filled columns and thermal cell detectors to determine the water content of water-based coatings. The injection method is a direct sample using a trace syringe, and the quantitative method is the internal standard quantification method using isopropyl alcohol as the internal target.
GB 18582-2002 using co-boiling distillation method to determine the water content of water-based wood paint, fluorocarbon latex paint and other samples, and then calculate the VOC content by differential method. A new environmental protection industry standard (HJ/T 201-2005) was introduced in 2005. In 2008, a newly revised mandatory limit standard GB 18582-2008 was promulgated, replacing GB 18582-2001, which put forward higher requirements for VOC content in water-based coatings. It measures the content of 24 compounds commonly contained in water-based coatings by taking hexate as a temperature marker, so as to find the total VOC content of the coating. The implementation of these standards has promoted the development of china's domestic wall coatings in the direction of harmless.
3 VOC measurement method
VOC determination method in water-based coatings is one of the hot spots in current research, commonly used for VOC dating methods are: gas chromatography (GC), high-efficiency liquid chromatography (HPLC), gas chromatography - mass spectrometrety (GC-MS), fluorescent spectrophotometer and membrane import mass spectrometrometromety, in addition, there are reflective interference spectroscopy, offline supercritical fluid extraction-GC-MS method and pulse discharge detector method, which are more used GC and GC-MS.
In recent years, many scholars at home and abroad have studied the measurement methods of volatile organic compounds, such as: Li Zhenguo used sampling tanks to collect volatile organic matter in ambient air, gas chromatography - mass spectrometry to determine; Chromatography - Mass spectrometry technology to determine volatile organic compounds in surface water, Jiang Zhihong et al. to use thermal de-attachment - Gas chromatography technology to determine the total volatile organic compounds in the air; Ma Congxin used gas chromatography to determine water characteristics VOC in coatings; Wang Yuesi et al. used self-developed cumulative atmospheric sampling device, atmospheric trace organic matter two-step frozen concentrated sample system and GC/MS joint technology to analyze trace volatile organic compounds in the atmosphere; Zhang Wenxuan et al. Chromatography - Mass spectrometrography to determine volatile organic compounds in drinking water, Dai Tianye et alto and others used HPLC to analyze 10 aldehyde contaminants in air and exhaust gases, Yin Yuanfen et al. used GC-MS to determine volatile organic compounds in soil Lin Xuanyi studied the determination and control of the content of volatile organic compounds in architectural coatings.
VOC measurement methods vary depending on the reference criteria, such as GB18581-2001 simply keep the solvent-based coating at (105±2) degrees C at a constant temperature of 3 h, and then determine the reduction of its mass as the VOC content. GB 18582-2008 provides for the use of gas chromatography to determine volatile organic compounds in coatings. According to foreign standards, different ranges of VOC content of coating products need to use different methods to determine its VOC content. Gas chromatography to determine the low-mass fraction VOC, can use known pure substance control qualitative, or the use of relative retention values, retention index and other literature data qualitative, but also can be used with other instruments to use the method of qualitative.
GC-MS method in addition to high-efficiency separation and accurate qualitative identification capabilities, but also to detect the unsealed chromatography peak, and higher sensitivity, more reliable data, in the face of increasingly stringent measurement standard limit requirements, GC-MS method will gradually replace gas chromatography.
4 The current difficulties encountered in VOC assays in water-based coatings
Although there are many literature reports on VOC determination in water-based coatings, there are still many problems that need to be solved in actual testing:
(1) there are differences of opinion on the definition of VOC, resulting in different measurement methods. China's mandatory limit standard GB 18582-2008 VOC definition refers to the standard pressure (101.3 kPa), any initial boiling point below or equal to 250 degrees C of organic compounds. In the daily water-based coating determination, qualitative analysis found that many organic compounds boiling point is less than 250 degrees C, but not the standard 24 volatile organic compounds, such as 1,3-ethylene, 2,2,3,3-tetranyl-butane, styrene, benzoyl alcohol, ethylene diol monoethyl ether, isopropyl benzene and so on.
(2) GB 18582-2008 of the 24 volatile organic compounds in the alcohol is more, because alcohols are soluble in water, its recovery rate is particularly low. In order to solve this problem, the author uses waterless sodium sulfate purification, dichloromethane ultrasound and membrane filtration extraction system in daily testing, which improves the recovery rate of various compounds, especially the problem of low recovery rate of binary alcohol compounds, and is applied to the detection of actual samples.
5 Conclusion
Because solvent-based coatings pose a serious threat to human beings themselves and the ecological environment, environmentally friendly coatings instead of solvent-based coatings become inevitable. Water-based coatings with economic, energy-saving, environmental protection and safety characteristics, in line with the requirements of the development of the times, its development and promotion will be the main direction of coatings in the 21st century. Therefore, to continue basic research, to avoid short and long and vigorously expand its application areas is still the current development of water-based coatings technology imperative. Compared with the industrial developed countries, there is a big gap between the level of hydration of industrial coatings in China, and the main problem facing water-based coatings is how to improve the performance of products under the premise of acceptable cost, so that they reach the same or close level as solvent-based coatings, and further reduce VOC emissions. With the continuous development of China's water-based coating technology and continuous improvement of coating formulations, the content of VOC in water-based coating products will be lower and lower, new, more scientific synthetic water-based coating technology will continue to appear, water-based coating performance will continue to improve.