Study on Waterborne Self-cleaning Low-fluorine Fluorocarbon Coatings

MA Hai-xu, WANG Lian-sheng, JIANG Guang-ming, MA Ning
(China Academy of Building Research, Beijing 100013, China)

Abstract: A type of waterborne fluorine acrylic resin coatings with low fluorine content of 6% in base material is synthesized by molecular structure design. The contact angle of pure water on the film is measured. The self-cleaning mechanism of fluorinecontaining acrylic resin with low fluorine content is discussed. The pure water contact angle, adhesion and aging resistance of the coatings are studied under the low fluorine content. The relationship between the aging time and contact angle is analyzed, and the structure of the polymer before and after aging is characterized by IR. The results show that the waterborne fluorine acrylic resin coatings of low fluorine content still have excellent physical properties, but the contact angle decreases obviously and the hydrophobicity is harmed after the aging. The durability of the coatings needs to be further improved.

0 Introduction
Organic fluororesins are the best weathering and durability polymers found so far. However, as people ’s awareness of environmental protection and energy saving gradually increases, the restrictions on VOC (volatile organic compound) content in various countries are becoming stricter. Solvent-based fluoropolymer coatings and high-temperature curing fluoropolymers using organic solvents as film-forming media The application of coatings is limited. Water-based fluorine coatings are safe, harmless, clean, advanced in technology, low in energy consumption, and low in emissions. They are very popular. At present, water-based fluorine coatings are divided into PVDF, PEVE and fluorine-containing acrylics. In addition, the acrylic resin has a simple synthesis method, low cost, and has the characteristics of light color, high transparency, and good wettability to pigments, and is widely used as a film-forming agent for coatings. Synthesized by emulsion polymerization a fluorinated acrylate polymer has the advantages of both acrylate polymers and fluorocarbon polymers, to use it as the only retained acrylate polymerization coating film former was good characteristics, but also both the Fluorocarbon polymers have excellent weather resistance, stain resistance, hydrophobic and oleophobic resistance, acid and alkali resistance, and other advantages. Therefore, water-based fluorinated acrylic resin coatings are increasingly attracting great attention from all countries in the world, and it is an important direction for the development of the coatings industry in the future. The latest progress and development trend of water-based fluoroacrylic resin coatings is the use of molecular design theory, incorporating many new technologies and methods, such as nanotechnology, self-layering technology, self-assembly technology, supercritical fluid technology, plasma chemical vapor deposition and physical vapor Deposition, sputtering technology, ablation technology, spin coating, transfer coating, molding decoration, pre-processed coating, etc., surface modification technology has also made progress, such as the so-called “lotus effect”, that is, the coating film The surface state is modified.
In this study, a low-fluorine content water-based self-cleaning fluorocarbon coating was prepared by mixing, that is, the fluorine self-cleaning performance of the latex paint at a low fluorine content was evaluated through the technical evaluation of the hydrophobic self-cleaning performance of the latex paint at a low fluorine content. The hydrophobic self-cleaning performance and excellent physical properties, through the analysis of the durability test process, lay a good foundation for the development of more durable coating self-cleaning function.

1 Experimental part
1.1 Sample materials
1.1.1 Low-fluorine content water-based self-cleaning fluorocarbon coating material fluoroacrylic resin, Changxing Material Industry Co., Ltd .; fillers and additives: film-forming aids, Texanol, Eastman; propylene glycol (PG), Titanium dioxide (A / B), talc, defoamer (A / B / C), wetting agent, dispersant (A / B / C), domestically produced


1.1.2 Experimental substrate
Asbestos-free cement slab conforming to NAF HV grade in JC / T 412.1—2006 , its size is 150 mm × 70 mm × 4 mm.


1.2 Coating preparation
Mix the fluorine-containing acrylic resin with the above filler and auxiliary solution into a ball mill and mix well. After repeated ball milling to a suitable fineness. That is, a low fluorine content water-based self-cleaning fluorocarbon coating is prepared.


1.3 Experimental equipment and equipment
Wire rod applicator (120 μm, 80 μm), 3M tape, contact angle measuring instrument (Data Physics), xenon lamp aging instrument (ATLAS), micro-infrared (Thermo Fisher iN10-iZ10).


1.4 Structural Characterization and Performance Test
1.4.1 Contact Angle
The contact angle tester was used to measure the static contact angle between the sample surface and distilled water. The experimental temperature was (23 ± 2) ℃, the relative humidity was (50% ± 5%), and the injection volume 5 μL. In order to reduce the error, three samples were selected, and five points were measured on each sample surface, and the arithmetic average was taken as the test result.


1.4.2 General physical properties of coatings
Test experiments were performed on the fluorine content of the aqueous fluorine-containing acrylic resin coatings as well as coating adhesion, water resistance, alkali resistance, and acid rain resistance. HG / T 4104—2009 fluoride ion electrode method (water-based fluorine coating) was used to measure the fluorine content, GB / T 9286—1998 [7] grid method was used to test the coating adhesion, and GB / T 1733—1993 was used to test the coating adhesion. Water resistance, using GB / T 9265-2009 to test the coating’s alkali resistance, and GB / T 9274-1988 to test the coating’s resistance to acid rain.


1.4.3
Anti -aging performance According to GB / T 1865—2009, the artificial climatic accelerated aging test of xenon lamp is performed, the aging time is 250 h, 500 h, 1 000 h, 3 000 h, 5 000 h, and the contact angle before and after the test is tested .


1.4.4 Structural Characterization of Infrared Spectrum

The structure analysis of low fluorine content fluorinated acrylic resin before and after aging was performed by infrared spectrum.

2 Results and discussion
2.1 Contact angles of water-based fluoroacrylic resin coatings

The contact angles of three samples were tested, and the results are shown in Table 1.

Contact angle of water-based fluorine-containing acrylic resin coating

Self-cleaning coatings can be divided into hydrophilic and hydrophobic based on their affinity for water. The hydrophilic type has very low surface tension on water, with a contact angle close to 0 °, while the hydrophobic type has a large surface tension on water, with a contact angle greater than 90 °. It can be seen from Table 1 that the aqueous fluorine-containing acrylic resin coating has a large contact angle. This is due to the movement of the molecular chain and the orientation movement, that is, the fluorine-containing group extends to the air side due to the low surface energy. Occupies the polymer-air interface, reducing the polymer surface energy. The lower the surface energy of a material, the more hydrophobic it is. This property gives the coating better hydrophobic properties, which makes it difficult for pollutants to adhere, thereby achieving excellent self-cleaning performance against pollution.


2.2 General physical properties of coatings
Based on the physical properties of water-based fluorinated acrylic resin coatings, compared with the technical indicators of HG / T 4104—2009, it can be seen from Table 2 that based on non-asbestos cement slabs, water-based fluorinated acrylic resin coatings have Good adhesion and good resistance to media can be applied to the exterior wall surface of the building, thereby ensuring the durability of exterior wall decoration.

General physical properties of coatings

2.3 Analysis of the durability of low-fluorine fluorinated acrylic resins

The accelerated weathering tests of xenon lamps were performed for 250 h, 500 h, 1 000 h, 3 000 h, and 5 000 h. The results showed that the coatings of the samples all reached the aging results of no foaming, no peeling, no cracking, grade 0 and discoloration 0, reflecting the excellent weather resistance of the fluorine-containing acrylic resin with low fluorine content. This is due to the molecular structure. The structure of the side chain covering the main chain forms a good “shielding protection” for the internal molecules of the polymer, making the polymer durable.
In order to study the self-cleaning durability of low-fluorine fluorinated acrylic resins, the laboratory conducted contact angle tests after aging for 250 h, 500 h, 1 000 h, 3 000 h, and 5 000 h. The test results are shown in Figure 1. As the aging time prolongs, the contact angle of the low-fluorine content fluorine-containing acrylic resin coating shows a significant decrease, the hydrophobicity is destroyed, and the self-cleaning performance is lost.

Relationship between contact angle and aging time

2.4 Infrared spectrum analysis before and after aging
As the hydrophobicity of the fluorine-containing acrylic resin coating with low fluorine content is lost as the aging time prolongs, the laboratory conducted a comparative analysis of the infrared spectrum before and after aging, as shown in Figure 2.

Infrared spectrum analysis before and after aging

It can be seen from Figure 2 that the C = O double bond and C—O—C stretching vibration peaks at 1 743 cm-1 and 1 053 cm-1, respectively, prove the existence of ester groups in acrylic resin; 1 180 cm- One is the C-F stretching vibration peak (this peak disappears obviously at 1 180 cm-1 after aging). It is inferred that the loss of hydrophobic properties after 5,000 h aging may be significantly changed with C-F at 1 180 cm-1. related. This is because the fluorine-containing acrylic resin can spontaneously generate an orientation effect and form a low surface energy during the film formation process. As the aging time prolongs, the fluorine-containing polymer segment is destroyed, and the content of When the fluoropolymer segment decreases, the surface energy of the coating film increases, and the contact angle decreases significantly.
Through extensive literature review and experimental exploration, in order to improve the hydrophobic and oleophobic properties of polymer coating films, a fluorine monomer with a higher content and fluorine atoms on the molecular chain must be used instead of more hydrogen atoms.

3 Conclusions

The relevant tests were carried out by preparing samples, and the results showed that when the total fluorine content is not high, the aqueous fluorine-containing acrylic resin coating has a low surface energy and a large contact angle, and has a good self-cleaning ability. The coating has good adhesion and good resistance to media. In addition, low fluorine content water-based fluorine-containing acrylic resin coatings have excellent weather resistance, but compared with other surface properties, the durability of self-cleaning ability needs to be further improved. At present, the controllability of molecular chain structure design is studied internationally. , Can make the low fluorine content water-based fluorine-containing acrylic resin coating in self-cleaning durability can be further improved.

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