Due to the characteristics of water-based coatings, their performance testing cannot be completely in accordance with the testing methods of solvent-based coatings, especially the testing items of water-based coatings such as hardness, wear resistance, high temperature resistance, low temperature resistance, and heat and humidity resistance. Require. Combined with the existing water-based coatings related standards, the performance indicators not involved in it and the performance testing items that need to be strengthened are discussed.
0 Preface
Traditional solvent-based coatings have been widely used because of their good comprehensive mechanical properties and surface properties. 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 [1]. With the pressure of environmental protection policies, people's awareness of environmental protection is constantly improving. In particular, provinces and cities across the country have successively issued VOC emission limit standards, which has brought opportunities for the development of water-based coatings and other environmentally friendly coatings in my country. Although traditional solvent-based coatings still occupy a large market share, water-based coatings are the future development direction of the coatings industry. The following is a discussion of the performance indicators that are not involved in it and the testing items that need to be strengthened in combination with the existing water-based coating-related performance testing standards.
1 Advantages and disadvantages of water-based coatings
Water-based coatings use water-based polymers as film-forming substances, which reduce VOC emissions, are safe, have no fire hazards, can be applied in humid environments, and are easy to clean. However, water-based paint also has its disadvantages [2]:
(1) The drying time is longer during film formation, especially in low temperature and high humidity environments;
(2) Due to the high surface tension of water, it is difficult for water-based coatings to wet the substrate;
(3) Using water as a solvent, the metal substrate is easily corroded;
(4) The compatibility of the film-forming resin with water is not good, so that the storage stability of the water-based coating is poor;
(5) Because the freezing point of water is higher than that of most organic solvents, the freeze-thaw stability of water-based coatings is poor;
(6) Susceptible to microbial damage.
2 Existing performance testing standards for water-based coatings
With the development of water-based coatings, relevant standards are also being formulated. The existing product standards for water-based coatings include: GB/T23999-2009 "Water-based Wood Coatings for Interior Decoration", GB24410-2009 "Water-based Interior Decoration Materials" Limits of Hazardous Substances in Wood Coatings", HG/T4570—2013 "Waterborne Automotive Coatings", HG/T4758—2014 "Waterborne Acrylic Coatings", HG/T4759—2014 "Waterborne Epoxy Resin Anticorrosive Coatings", HG/T4760— 2014 "Water-based dip coating", HG/T4761-2014 "Water-based polyurethane coating", HG/T4104-2009 "Water-based fluorine coating for construction", JT/T535-2004 "Water-based asphalt-based waterproof coating for roads and bridges", etc.; existing The testing method standards for water-based paints include: GB/T18178-2000 "General Rules for Selection of Water-based Paint Coating System", GB/T31414-2015 "Determination of Surfactants in Water-based Paints Alkylphenol Polyoxyethylene Ether" and so on.
Taking GB/T23999-2009 "Water-based Wood Coatings for Interior Decoration" [3] as an example, the drying time test method of water-based wood coatings refers to GB/T1728-1979 "Determination of Drying Time of Paint Films and Putty Films"; The test method of abrasiveness refers to GB/T1768-2006 "Determination of Abrasion Resistance of Paints and Varnishes by Rotating Rubber Wheel Method"; the test method of adhesion refers to GB/T9286-1998 "Cross-cutting of Paint and Varnish Films" Test"; the freeze-thaw resistance refers to the method 5.5 in GB/T9755-2001 "Synthetic Resin Emulsion Exterior Wall Coatings"; the hardness refers to GB/T6739-2006 "Determination of Paint Film Hardness by Pencil Method for Paints and Varnishes"; The dry heat resistance refers to GB/T4893.3-2005 "Determination of Dry Heat Resistance of Furniture Surface".
Taking HG/T4570-2013 "Water-based Automotive Coatings" [4] as an example, the drying time of water-based automotive coatings is quoted from GB/T1728-1979 "Determination of Drying Time of Paint Films and Putty Films"; the cross-cut adhesion test is quoted The reference is GB/T9286-1998 "Cross-cut test of paint and varnish film"; the reference for impact resistance is GB/T1732-1993 "Determination of impact resistance of paint film"; the reference for pencil hardness is GB/T6739-2006 "Determination of paint film hardness by pencil method of paints and varnishes"; the bending test refers to GB/T6742-2007 "Bending Test of Paints and Varnishes (Cylindrical Shaft)"; the cupping test refers to GB/T9753-2007 "Paints and Varnish Cupping Test".
3 Water-based coating performance testing items not covered by existing standards
The above-mentioned detection method standards are mainly aimed at solvent-based coating products, but water-based coatings are completely different from solvent-based coatings, so it is urgent to formulate corresponding detection method standards according to the characteristics of water-based coating products. For example, the normal temperature for detecting the drying time of the paint film is (23±2)°C, and the humidity is (50±5)%. Due to the long drying time of water-based coatings, a method for testing the drying time of water-based coatings in low temperature and high humidity environments should be formulated. According to the requirements of water-based paints, the drying time of water-based paints should be measured under the condition that the set temperature is lower than 23°C and the humidity is higher than 50%. If the viscosity of the water-based paint is too low, there will be sagging, which will affect the construction quality. You can refer to the current national standard GB/T9264-2012 "Evaluation of Sag Resistance of Paints and Varnishes" to test and evaluate the sagging performance of water-based coatings to reduce troubles during construction. Water-based coatings are extremely corrosive to metal substrates, and the corrosion resistance of water-based coatings on metal substrates can be tested, including salt spray resistance, water resistance, alkali resistance, and salt water resistance. Among them, salt spray resistance can be measured with reference to GB/T1771-2007 "Determination of Neutral Salt Spray Resistance of Paints and Varnishes"; water resistance can be measured with reference to GB/T1733-1993 "Determination of Water Resistance of Paint Films"; Alkali resistance can be measured with reference to GB/T9265-2009 "Determination of Alkali Resistance of Architectural Coatings".
The film-forming resin has poor compatibility with water, so the storage stability test of water-based paint can be carried out: put the water-based paint under certain temperature and humidity conditions for several days, and observe whether there are any adverse phenomena such as delamination and agglomeration. To judge the storage stability of waterborne coatings. The freezing point of water is 0°C. When the temperature is lower than 0°C, it will bring great inconvenience to the construction of water-based coatings in winter, especially in the north. The formula of water-based paint products 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 paint in winter construction is also very necessary. The low-temperature stability test can refer to GB/T9268-2008 "Latex Paint Determination of Fusibility" was carried out.
Water-based paints are susceptible to mildew caused by harmful substances such as molds. To investigate the mold resistance of water-based paints, refer to GB/T1740-2007 "Determination of Mold Resistance of Paint Films". The bacteria include Aspergillus niger and Aspergillus flavus. The significant difference between water-based paints and conventional solvent-based paints is that water is used as a solvent, but most of the existing water-based paint-related standards do not involve the detection of water content. The moisture content of water-based coatings can be detected by referring to GB/T606-2003 "General Method for Determination of Moisture in Chemical Reagents Karl Fischer Method". The Karl Fischer method is a relatively accurate and mature method for determining water content, but due to uneven mixing of water-based paint products and sampling differences, the test results will be unstable. I have checked the previous relevant standards, and there is a national standard GB/T 1746-1979 (1989) "Method for Determination of Moisture in Coatings". Although this standard has been abolished, this method is suitable for water-based coatings in terms of economy and convenience. Determination of water content should be more appropriate.
4 Epilogue
Although water-based coatings are superior to traditional solvent-based coatings , they have defects such as difficult construction, water resistance, easy skinning, poor fullness, and poor hardness. Only when the defects in the physical properties of water-based coatings are resolved, and even surpass traditional solvent-based coatings, people will choose to use water-based coatings more actively. If it only meets the requirements in terms of environmental protection performance, but cannot meet people's requirements in terms of construction technology and decorative effects, it will still be difficult to open the market for water-based coatings. This requires water-based paint manufacturers to carry out technological innovations in the performance of water-based paints, and also requires relevant professionals to tailor the testing methods for water-based paints, and to formulate more testing standards that are more suitable for the characteristics of water-based paints, so as to make the water-based paint industry better. , Faster development.
