Research on the development trend and direction of green and environment-friendly coatings

Wang yuanyuan, Chen jingjing, wang pengzhu, wang jihu, wen shaoguo

Abstract: with the improvement of people's living standard and the enhancement of environmental protection consciousness, the environmental protection paint has received great attention. This paper reviews the research progress of several new green environmental protection coatings, such as water-borne coatings, powder coatings, dry powder coatings and water-borne UV coatings.

Keywords: green environmental protection; Water-based paint; Powder coatings; Dry powder coating; UV coating; VOC

Chinese library classification number :TQ630 document identification code :A article number: 1008-021x (2015)01-0058-04

Coating is playing an increasingly important role in people's life and has become an indispensable material for the development of modern society. However, the coating industry is one of the main sources of air pollution [1]. Paint containing organic volatile (volatileorganiccompounds, VOC) and various heavy metal ions are will cause significant damage to the health of people. VOC affects the central nervous system and immune function, causing headaches, fatigue, and inattention. VOC content in the air is less than 0. 2mg/m3 has no effect on human body; 0. 2 ~ 3mg/m3 may cause mild irritation or discomfort to human body. 3 ~ 25mg/m3 can stimulate human body or cause headache. When greater than 25mg/m3, the nerve is affected [2]. Common VOC mainly includes benzene, toluene, xylene, ethylbenzene, styrene, formaldehyde, ethanol, tetradecane, etc. When the VOC concentration in the bedroom reaches a certain concentration, people will feel headache, nausea, vomiting, fatigue and so on in a short time. In severe cases, people will have convulsions, coma, and damage to the brain, internal organs and nervous system, causing serious consequences such as memory loss [3]. Heavy metal ions mainly include mercury, lead and chromium, which are highly toxic or carcinogenic. The pigments used in the paint mainly include red lead, zinc chromium-yellow, and high-cupric acid. Copper inhibits the synthesis of heme, leading to anemia and damages to the brain, cerebellum and nerve. Lead can lead to renal dysfunction, dysfunction in the absorption of small molecules, and can also lead to vasospasm [4].

Green and environment-friendly coating is a kind of healthy energy-saving coating that saves resources and energy, is pollution-free and beneficial to human living environment [5]. The research and development of environment-friendly coating is the basic requirement of realizing the greening, high performance and high function of coating.

At present, the most studied environmentally friendly coatings mainly include water-based coatings [6-8], powder coatings [9-11], dry powder coatings [12-14] and water-based photocurable coatings [15-17].

1• water-based paint

Traditional solvent-based coatings have excellent performance, but contain a lot of VOC, which seriously damages the environment and people's physical and mental health. With the improvement of laws and regulations in various countries and the urgent needs of people, traditional solvent coatings are gradually replaced by waterborne coatings. Compared with solvent-based coatings, water-based coatings use water as a solvent to eliminate potential safety hazards caused by volatile solvents. At the same time, the VOC content of water-based coatings is very low, with very little smell. It is safe to use and meets the requirements of environmental protection and energy conservation.

The kind of waterborne coating is very much, according to the resin cent has the waterborne polyurethane coating, the waterborne epoxy resin coating, the waterborne alkyd resin coating and the waterborne acrylic ester coating and so on. Although the water-based coating is an environment-friendly coating, it still contains VOC. How to reduce the VOC in the water-based coating and improve the performance of the water-based coating is the focus of current research [18-19].

1. Waterborne polyurethane coating

In addition to the excellent properties of solvent polyurethane coating, the waterborne polyurethane coating also has the advantages of high hardness, strong adhesion, corrosion resistance, solvent resistance and low volatile organic matter content. However, there are still some defects in the properties of waterborne polyurethane coating such as adhesiveness, water resistance and weather resistance, which need to be further improved through modification [20].

1. Waterborne epoxy coating

Water-based epoxy resin coating has excellent physical and chemical properties such as good adhesion, excellent chemical resistance and solvent resistance, high hardness, corrosion resistance and thermal stability, so it has been attracting people's attention [21]. Water-based epoxy resin can be divided into two types: water-based epoxy resin and water-soluble epoxy resin.

1. Water-based alkyd resin coating

Alkyd resin is an important coating resin, its monomer source rich, low price, variety, formula change, chemical modification convenient, and good performance. Although the water-borne alkyd resin coating has advantages such as good brushability and lubricity, it also has disadvantages such as slow drying, low hardness, poor water and corrosion resistance, and poor outdoor weatherability, which need to be modified to meet these performance requirements [22].

1. 4 waterborne acrylic paint

Acrylate emulsion has promoted the rapid development of acrylate emulsion coating due to its excellent weather resistance, light resistance and small smell, etc. Moreover, the application of core shell structure can well resolve the contradiction between the vitrification transition temperature (Tg) of emulsion and the minimum film forming temperature (MFFT) [23].

2• powder coating

Powder coating is a kind of powder coating without solvent, with 100% solid content. It is usually sprayed to the surface of the coating by powder spraying. After baking, melting, leveling and curing, the coating is completed. As a solvent-free solid coating, the VOC content can truly reach 0, posing no threat to the environment and people's health, and belongs to a true environmental protection coating [24].

Powder coatings include thermosetting powder coatings and thermoplastic powder coatings. Thermoplastic powder coatings mainly include polyethylene, polypropylene, polyvinyl chloride, etc. Thermosetting powder coatings mainly include epoxy powder coatings, polyester powder coatings and acrylic powder coatings [25]. At present, thermosetting powder coatings are the most studied.

2. 1 epoxy powder coating

The pure epoxy powder coating has a long history. Its polymer resin mainly consists of epoxy resin. Epoxy resin is a polymer prepolymer containing two or more epoxies of aliphatic, aliphatic or aromatic main chains, which has excellent adhesion and corrosion resistance [26], and is currently mainly used for corrosion prevention in pipelines, ships and electrical insulation, etc. [27]. Zhang jianfei et al. [28] prepared a new type of epoxy resin powder coating. The production process of the developed powder coating was stable, meeting the requirements of the design scheme for anti-corrosion coating performance, and the product performance was similar to that of foreign products.

Curing reaction is indispensable step in the process of powder coating, powder coating of the curing temperature at 180 ℃ or so commonly, and can only be applied to the metal substrate [29]. With the requirement of energy saving and emission reduction, the low-temperature curing temperature of powder coatings is an inevitable trend. Zhou Xiaotao [30] the TC - 125 type curing agent mixed with other additives, which can realize low curing temperature is 120 ℃, the coating of various physical properties is good. The development of new curing agent is of great significance for reducing curing temperature of epoxy powder coatings. 2 - methyl imidazole as a kind of new curing agent, the curing temperature of epoxy powder coating can be adjusted for 15 min or 180 ℃ / 130 ℃ / 8 min, at the same time can solve the problem of yellow coating curing process [31]. Through modification of curing agent, compounding technology and development of new curing agent, the wet and heat resistance, flexibility, insulation, flame retardancy and corrosion resistance of epoxy powder coatings can be improved, and the application range of epoxy powder coatings can be expanded.

2. 2 polyester powder coating

Polyester powder coating is the most important weather resistant powder coating in China [32].