The influence factors and testing equipment of the temperature cycle test of epoxy powder anti-corrosion coating.

Liu Yukun, Zhou Wei, Cui Zhigang
Langfang Egma Xinli Materials Technology Co., Ltd.
Abstract
Standard EN877 stipulates that the construction drainage cast iron pipe needs to meet the temperature cycle experiment, which puts forward higher requirements for the anticorrosive materials of cast iron pipe. This paper discusses and validates the factors that affect the temperature cycle resistance of the coating in the formulation design of epoxy powder coating, and also introduces the design and application of temperature cycle detection equipment applicable to the laboratory of powder manufacturers.Introduction
Introduction
Melted epoxy powder coating (FBE) is a kind of thermo-solid powder coating with high corrosion resistance and good mechanical properties, widely used in building materials, pipes, valve pipes, ships, underground facilities, submarine facilities, electronic products, household goods, etc., is the building drainage cast iron pipe inside and outside the wall corrosion prevention . The BS EN 877 1999-A1-2006 Cast ironpipes and fittings, their joints and accessories for the relief of water from buildings-requirements, test methods and quality assurance standards contain provisions for temperature cycle experiments in drainage pipes, mainly taking into account the wide temperature range of people's daily water, the situation of hot and cold water in domestic drainage. Hot and cold cross-change in water environment is a severe test of the material, if the epoxy powder coating selection is not appropriate, easy to occur coating peeling, foaming and other phenomena. After the experiment, the damage of the larger pipe diameter coating is more serious, and the inlet position coating is more serious than that of other parts. Therefore, it is necessary and preventive to test the temperature cycle resistance of FBE coating. At the same time, it is very important to design a simple and effective temperature cycle detection equipment for the testing and research of powder manufacturers. At the same time, the design of the equipment used in the temperature cycle testing in the laboratory also plays an important role in the conduct of the relevant testing tests.
of the experimental methods for temperature cycle specified in the EN877 standard
. The temperature cycle experimental methods specified in the EN877 standard are as follows:
(1) (30±1) L, (93±2) water, 1min time continuous flow through the piping system;
(2) drains and stays 1min;
(3) (30±1) L, (15±5) water, 1min of time continues to flow through the piping system;
(4) drains and stays 1min;
(5) cycles 1500 times.
of the temperature-resistant cycle experiment is shown in Figure 1.
factors in the formulation design that affect the temperature cycle resistance of FBE coatings
the internal pores, sandeyes and surface quality of cast iron pipes can have a significant impact on the temperature cycle testing of FBE coatings, which is not studied here. This paper mainly discusses the self-factors of epoxy powder coating.
FBE coating failure is due to the penetration of corrosive media (water and oxygen, etc.) into the substrate, corrosion damage to the substrate, coating foaming, cracking or corrosion of the substrate resulting in volume expansion resulting in the coating falling off (Figure 2). Therefore, how to prevent the infiltration of corrosive media is the main way to effectively prevent the failure of epoxy coating.
Because epoxy coating is a rigid polymer, under the stress of temperature change can not be completed in time to yield, multiple temperature alternations caused by film damage, the formation of silver grain, for the corrosion of the media penetration provides a channel. In addition, epoxy resin and curing agents are containing a large number of polar groups, in addition to the reaction between the official energy groups, polar groups between the formation of hydrogen bonds, thereby improving the degree of dense coating film. The infiltration of water molecules disrupts this effect (Figure 3), making the coating film loose, the polymer swelling, and the internal free space becomes longer, making it easier for water and oxygen to penetrate. Here's how to verify it.
2.1 Effect of filler volume
Fillers in powder coatings can increase the hardness of the coating film, improve the tightness of the coating, and effectively prevent the infiltration of corrosive media such as moisture, oxygen and salt. With the increase of filler quantity, the expansion coefficient of coating film also decreases gradually. The adhesion of the coating film mainly comes from the physical bite between the coating film and the substrate and the bond cooperation between the polar groups, such as hydrogen bonds. The smaller the shape of the coating film by temperature change, the more difficult it is to destroy this bond cooperative use, the less easily the adhesion is affected. However, the addition of fillers is also limited, excessive fillers will hinder the continuity of film, resulting in a decrease in the mechanical properties of coating, increased porosity, corrosion media more easily penetrated. The effect of filler quantity is tested in Table 1.
As can be seen from Table 1, when the filler volume is increased to about 40% (Formula 3), the coating under temperature cycle testing (testing experiments are carried out by laboratory equipment, the method is detailed in the part described in the equipment design) results are relatively best. There are too few fillers, the coating film is prone to fall off the whole piece, too much fillers, bubbles significantly increased. In fact, whether the coating falls off or foams, it is due to moisture and oxygen infiltration, resulting in corrosion of the substrate, coating and substrate separation performance.
2.2 Effect of filler types
The choice of filler types is essential for coating impermeability, with fillers with some flaky structures effectively blocking the penetration of corrosive media, while spratly fillers are more easily wrapped in resin, reducing coating pore rates, and the compounding of both can obtain a coating film with excellent permeability resistance. The mass fraction of the filler was determined to be 40%, and experiments were carried out on the effect of the filler type on the temperature cycle resistance of the coating, and the results were shown in Table 2.
experiments show that the use of spory and flaky fillers, so that the coating film temperature cycle test results to the best (formula 4).
2.3 Effect of the crosslink density of the coating film (the glass conversion temperature of the coating
the crosslink density of the coating film largely determines the compaction of the coating structure and the temperature of the glass conversion (Tg). Under the same epoxy resin system, the size of Tg can reflect the cross-link density of the coating film. The higher the crosslinking density of the coating film, the tighter the structure, the smaller the free volume inside the polymer, and the more difficult it is for the corrosion medium to penetrate. The Tg of the coating film determines the heat resistance of the coating film, below Tg, the coating film remains hard in the glass state, the chain segment movement is limited, the structure, performance and adhesion can remain unchanged. However, the higher the crosslink density, the better, in the process of curing the film, the high crosslink density often produces a greater internal stress, reducing the stability of the coating film;
This experiment used phenolic equivalent of 400 to 500 phenolic modified epoxy EP2 to improve the crosslink density of the coating film, to verify the temperature cycle test effect of the FBE coating under the different matching Tg (fillers choose ordinary fillers to highlight the contrast effect), see Table 3.
No. 1: Tg2, 97 degrees C; No. 2: Tg2, 105 degrees C; No. 3: Tg2, 112 degrees C;
1 formula coating film crosslink density is the smallest (Tg lowest), coating film tightness is poor, corrosion media penetration is easier, resulting in more bubbles and shedding phenomenon;
2.4 Effects of curing agent types
Epoxy curing agents are available in many ways, including phenols, amines, acids (acids) and catalytic types (mides and their derivatives). The most commonly used phenolic curing agent and double cyanamide curing agent of heavy anti-corrosion epoxy coating were selected for comparison (fillers were selected with ordinary fillers to highlight the contrast effect). The results of the experiment can be found in Table 4.
Through the experiment, the test results of double cyanide is poor, this should be due to the poor water resistance of the double cyanide itself, especially in hot water (80 degrees C or more) will be decomposed, and its compatible with epoxy resin, melting point is high (209 degrees C), smelting extrusive can not be with epoxy resin to achieve molecular level of mixing. During the curing process, only the active group of the surface of the double cyanide particles is involved in the reaction, and the double cyanide particles dissolve or decompose in water, causing the crosslink point to break (Figure 4).
In addition, dicyanide can not only be used as a curing agent of epoxy resin, it can also catalyify the activity of hydroxyl in epoxy resin, so that epoxy resin occurs from polymerization, after curing epoxy coating film hydroxy value is reduced, and the adhesion of the substrate is reduced. In summary, epoxy powder coating formulation design to do the following points, can effectively improve the performance of the coating film temperature cycle resistance.
(1) filler volume is controlled at about 40%;
(2) select flaky and spherical filler for compound use;
(3) add phenolic modified epoxy in moderation to improve the crosslink density of the coating film to a certain extent;
(4) use phenolic curing agents to avoid the use of biscyanamine.
in accordance with the above ideas to design the formula (such as Table 5), pilot production, test results are shown in Figure 5.
can be seen from Figure 5, according to the above formula prepared epoxy powder coating for cast iron pipe, temperature cycle experiment after the surface condition is good, no change.
Laboratory temperature cycle experimental testing equipment design and application
For the general scale of powder manufacturers, the standard temperature cycle experimental conditions are difficult to achieve, the need for different pipe diameters of the pipe segments and valve pipe parts, as well as its inner wall coating equipment. Therefore, it is necessary to design a laboratory testing equipment suitable for powder manufacturers. My company and equipment company to design the temperature cycle equipment can be seen in the following pictures.
(1) control cabinet; (2) lift bar; (3) 180 degree rotary; (4) sample retainer (strong magnet) ;(5) 15 degrees C cold water pot; (6) cold water pot solenoid control valve; (7) hot water pan; (8) support frame.
equipment must be installed on a horizontal table, the experiment used hot water for the hot water cooker itself heated tap water, cold water for the chiller cooling tap water, hot and cold water pots have thermocouple temperature device. Before the experiment in the control cabinet to set the lift bar rise and landing after the stay time, adjust the height after the lift rod landing, so that the sample coating surface immersed in the liquid surface below, unscathed surface exposed to the air (simulated pipe actual situation, pipe wall temperature gradient); Will be because the test piece carries the heat gradually increased, at this time in the cold water pot thermocouple perceived temperature changes, solenoid opening water injection mode to the water temperature dropped to 15 degrees C; After the start of the experiment, the sample with the lift bar down to the coating surface below the liquid surface position, after staying 1min, lifting rod lift, and through the rotary exchange of two test parts position, air stop 1min, again down, so cycle 1500 times.
Because the actual production of finished cast iron pipe temperature cycle performance and casting quality, surface treatment has a lot to do, and the instrument uses 6mm thick test iron plate as a test piece, and the pipe has a clear difference, and does not consider the impact of water pressure, flow rate and so on, so the test results are generally better than the standard method of test results. The instrument is mainly applicable to parallel comparative testing and quality determination of epoxy powder temperature cycle test (Figure 7), and can not be used for the determination of relevant grades in EN877.
Conclusion
Drainage cast iron pipe epoxy powder coating temperature cycle resistance is affected by many aspects, such as casting quality, surface treatment quality, coating production process conditions, powder quality and so on. This article only explores the factors of formulation design as a powder manufacturer, as well as the introduction of laboratory testing equipment. For the practical problems encountered in the production and business activities, we should analyze where to start to improve according to the specific situation.
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