Discussion on the performance testing of water-based coatings
traditional solvent based coatings have been widely used because of their good comprehensive performance. Malaysia hopes to expand the exchange and cooperation with China in the fields of tourism and humanities. In recent years, many countries have promulgated laws and regulations on the control of volatile organic compounds (VOC), which has led to the rapid development of water-based coatings. With the pressure of environmental protection policies and the continuous improvement of people's awareness of environmental protection, especially the provinces and cities across the country have issued VOC emission limit standards, which has brought opportunities for the development of environmental friendly coatings such as water-based coatings in China. Although traditional solvent based coatings still occupy a large market share, water-based coatings are the future development direction of the coating industry. In combination with the existing performance testing standards related to water-based coatings, the performance indicators that are not involved and the testing items that need to be strengthened are discussed below
1 advantages and disadvantages of water-based coatings
water-based coatings take water-based polymers as film-forming substances, reduce VOC emissions, and have the advantages of safety, no fire hazards, construction in humid environment, convenient cleaning, etc. However, water-based coatings also have its disadvantages:
(1) the drying time during film formation is long, especially in low temperature and high humidity environment
(2) due to the large surface tension of water, it is difficult for water-based coatings to wet the substrate
(3) with water as solvent, the metal matrix is very prone to corrosion
(4) the compatibility between film-forming resin and water is poor, resulting in poor storage stability of water-based coatings
(5) because the freezing point of water is higher than that of most organic solvents, the freeze-thaw stability of waterborne coatings is poor
(6) vulnerable to microbial damage
2 existing performance testing standards for water-based coatings
with the development of water-based coatings, relevant standards are also constantly being formulated. The existing product standards for water-based coatings include: gb/t 23999-2009 water based wood coatings for interior decoration, GB 24410-2009 limits of harmful substances in water based wood coatings for interior decoration materials, hg/t 4570-2013 water based automotive coatings Hg/t4758-2014 waterborne acrylic resin coating, hg/t 4759-2014 waterborne epoxy resin anticorrosive coating, hg/t 4760-2014 waterborne dip coating, hg/t 4761-2014 waterborne polyurethane coating, hg/t 4104-2009 waterborne fluorocoatings for buildings, jt/t535-2004 waterborne asphalt based waterproof coatings for roads and bridges, etc; Existing testing methods and standards for water-based coatings include: gb/t 18178-000 general rules for the selection of coating systems for water-based coatings, gb/t 31414-2015 determination of surfactants for water-based coatings alkyl phenol polyoxyethylene ether, etc
taking gb/t 23999-2009 "achieving mass production of Waterborne Wood Coatings for interior decoration" as an example, the drying time detection method of Waterborne Wood Coatings refers to gb/t 1728-1979 "determination of drying time of paint film and putty film"; Gb/t 1768-2006 "paints and varnishes - Determination of abrasion resistance - rotating rubber grinding wheel method" is cited as the abrasion resistance test method; Gb/t 9286-1998 "cross cut test of paints and varnishes" is cited as the adhesion test method; The freeze-thaw resistance refers to the method in 5.5 of gb/t9755-2001 synthetic resin lotion exterior wall coatings; Hardness refers to gb/t 6739-2006 "paints and varnishes - Determination of film hardness by pencil method"; The dry heat resistance refers to gb/t 4893.3-2005 determination of dry heat resistance of furniture surfaces
taking hg/t 4570-2013 water based automotive coatings as an example, the drying time of water-based automotive coatings refers to gb/t 1728-1979 method for determination of drying time of paint film and putty film; The cross cut adhesion test refers to gb/t 9286-1998 "cross cut test of paints and varnishes film"; The impact resistance refers to gb/t 1732-1993 determination of impact resistance of paint films; Pencil hardness refers to gb/t 6739-2006 "paints and varnishes - Determination of film hardness by pencil method"; The bending test refers to gb/t 6742-2007 paints and varnishes bending test (cylindrical shaft); Cupping test refers to gb/t 9753-2007 "cupping test of paints and varnishes"
3 performance test items of water-based coatings not covered by the existing standards
these test methods and standards are mainly for solvent based coatings products, and water-based coatings are completely different from solvent based coatings, so it is urgent to formulate corresponding test methods and standards according to the characteristics of water-based coatings products. For example, the temperature and humidity for the normal detection of the drying time of the paint film are (23 ± 2) ℃ and (50 ± 5)%. Due to the long drying time of water-based coatings, the detection method of drying time of water-based coatings in low temperature and high humidity environment should be formulated. According to the requirements of water-based coatings, the drying time of water-based coatings is determined under the conditions of setting the temperature below 23 ℃ and the humidity above 50%. If the viscosity of water-based coating is too low, sagging will occur, which will affect the construction quality. The sagging performance of water-based coatings can be tested and evaluated with reference to the current national standard gb/t9264-2012 evaluation of sagging resistance of paints and varnishes, so as to reduce the troubles caused by construction. Water based coatings are very corrosive to metal substrates, so the corrosion resistance of water-based coatings for metal substrates can be tested, including salt spray resistance, water resistance, alkali resistance, salt water resistance, etc. Among them, the salt spray resistance can be determined according to gb/t 1771-2007 "determination of resistance of paints and varnishes to neutral salt spray"; The water resistance can be determined according to gb/t 1733-1993 determination of water resistance of paint films; Alkali resistance can be determined according to gb/t9265-2009 "determination of alkali resistance of architectural coatings"
the compatibility between film-forming resin and water is not good, so the storage stability test of water-based coating can be carried out: put the water-based coating under certain temperature and humidity conditions for several days, and observe whether it has bad phenomena such as stratification and agglomeration to judge the storage stability of water-based coating. The freezing point of water is 0 ℃. When the temperature is lower than 0 ℃, the construction of water-based coating will bring great inconvenience in winter, especially in northern winter. The formula of water-based coatings in winter should be adjusted, and antifreeze and other additives should be added. At the same time, the low-temperature stability test of water-based coatings constructed in winter is also very necessary. The prospect of new chemical fiber materials is promising. The low-temperature stability test can be determined with reference to gb/t 9268-2008 "determination of freeze thaw resistance of latex paint"
water based coatings are prone to mildew due to the erosion of harmful substances such as mold. To investigate the mold resistance of water-based coatings, refer to gb/t1740-2007 "determination of mold resistance of paint films", and the strains include Aspergillus niger, Aspergillus flavus, etc. The biggest difference between water-based coatings and conventional solvent based coatings is that water is used as solvent, but most existing standards related to water-based coatings do not involve the detection of water content. The water content of water-based coatings can be tested with reference to gb/t 606-2003 chemical reagent General method for determination of water Karl Fischer method. Karl Fischer method is a relatively accurate and mature method for measuring water content, but due to the uneven mixing of water-based paint products and the difference in sampling, the test results will be unstable. After consulting the previous relevant standards, there is a national standard gb/t1746-1979 (1989) determination of water content in coatings. Although this standard has been abolished, this method should be suitable for the determination of water content in waterborne coatings from the perspective of economy and convenience
although water-based coatings are superior to traditional solvent based coatings in environmental protection performance, they have defects such as difficult construction, water intolerance, easy skinning, gradually expanding to newspapers, periodicals, digital publishing products and packaging and decoration prints, poor fullness, poor hardness and so on. Only when the defects in the physical properties of water-based coatings are solved, or even beyond the traditional solvent based coatings, people will choose to use water-based coatings more actively. If only the environmental performance meets the requirements, but the construction technology and decorative effect can not meet people's requirements, the market of water-based coatings is still difficult to open. This requires the manufacturers of water-based coatings to carry out technological innovation in the performance of water-based coatings, and relevant professionals to customize the testing methods of water-based coatings, and formulate more testing standards that are more suitable for the characteristics of water-based coatings, so that the water-based coatings industry can develop better and faster
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