JPH0119700B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a baking paint that can be coated in a thick film and is used for anticorrosion coating on the inner surface of steel pipes. Paints used for corrosion prevention are classified into air-dry paints and baking paints depending on the drying process. Of these, the former is a type of paint that dries and hardens at room temperature to form a coating, while the latter is a type of paint that undergoes a curing reaction and forms a cured coating by heating it in an oven, etc. after application. It can be said that baking paints have better coating film performance than room temperature paints for the following reasons. (1) It is based on the premise of providing sufficient heat for the reaction, and it is possible to design the crosslinking density to be higher than that of air-drying paints. (2) Low molecular weight substances such as unreacted curing agent or solvent remaining in the coating film should be small. (3) The functional groups used in the curing reaction are sufficiently active near the baking temperature, and even if they do remain, they have low reactivity at room temperature, making the coating film less susceptible to infiltration of environmental substances. To become. By the way, the inner surfaces of steel pipes are usually coated with various anti-corrosion coatings depending on the type and temperature of the fluid flowing inside them. teeth,
A baking paint such as baking phenol paint or baking phenol epoxy paint is applied in 4 to 6 coats to a film thickness of about 200μ. This type of baking paint exhibits better corrosion resistance when applied thickly (150 to 300μ), and is relatively high in temperature (over 50°C) and moisture free.
It is known that long-term corrosion resistance can be obtained even in environments where oil and corrosive gases (H ₂ S, CO ₂ , etc.) are present. It is applied to steel pipes under relatively harsh environments such as lines. However, even baking paints with such excellent corrosion resistance have the following drawbacks, mainly in terms of workability. (1) From the standpoint of work efficiency, it is ideal to charge the baking paint into a baking oven immediately after painting and bake it to harden it, but in this case, solvents and reaction by-products (mainly condensed water) ) tends to cause swelling due to rapid volatilization. The degree of blistering depends on the thickness of the coating film; the thicker the film, the more blistering occurs. The critical thickness of the current baking paint for blistering is about 100μ. (2) During baking, the polymerization reaction progresses little by little as the temperature rises, but the viscosity of the applied paint is rather strongly influenced by the temperature itself and drops significantly. For this reason, sagging is likely to occur, but sagging depends on the film thickness, and the thicker the film, the more likely it is to sag.
The sagging limit of current baking paints is about 150μ wet film thickness (50-70μ dry film thickness). For these reasons, if it is assumed that the coating is baked immediately after coating, there is a problem in that with conventional baking paints, a dry film thickness of about 70 μm can be applied at most each time. For this reason, the 200μ
If a coating film thickness higher than that is required, a complicated process of 4 coats and 4 bakes must be used.
There is a problem that the painting workability is very poor. The present invention solves the drawbacks of conventional baking paints, and provides a baking paint that can obtain a thick film of 200μ or more in one coating, thereby enabling efficient painting and baking treatments. Its purpose is to provide. For this reason, the present invention provides a bake-curable resin in which an epoxy resin is blended with an acid anhydride, an aromatic amine, or a novolac phenol as a curing agent;
A mixture of multiple components whose boiling point (bp) satisfies the following formula (3), 90% by weight or more of the component whose boiling point (bp) satisfies the following formula (1), and a boiling point (bp) that satisfies the following formula (2). ) BP (℃) Baking temperature (℃) -50 (℃) ……(1) BP (℃) Baking temperature (℃) ……(2) bp (℃) Baking temperature (℃) + 40 (℃) ... (3) Its basic feature is that it contains a low molecular weight polyethylene additive. As mentioned above, conventional baking paints have problems of blistering and sagging when applied thickly, and the present invention provides a new paint that solves these two problems at the same time. be. The coating material of the present invention contains a resin, a solvent, and an additive, to which a pigment is added if necessary. It is necessary to use a resin that is cured by a crosslinking reaction rather than condensation, and epoxy-acid anhydride, epoxy-aromatic amine, or epoxy-novolac phenol type resins are used. With other resins, low-boiling reaction products such as water and aldehyde are produced as by-products during curing (reaction), and these products gasify and cause blistering. The solvent is composed of multiple components, and must satisfy the following conditions from the viewpoint of preventing blistering. That is,
For 90% by weight or more of the solvent used, the boiling point must satisfy the following formula (1), and for 40% by weight or more, the boiling point must satisfy formula (2). Boiling point baking temperature -50°C...(1) Boiling point baking temperature...(2) By using the above resin and a solvent that satisfies these conditions, the prevention of blistering, which is the first main objective of the present invention, can be achieved. can do. Regarding this solvent, conventionally, based on the basic idea of volatilizing as much of the solvent as possible before the paint film gels, it is common practice to use a considerable proportion of low-to-medium boiling point solvents with a boiling point of 150℃ or less in baking paints. It was considered customary. However, according to research conducted by the present inventors, the above-mentioned conventional thinking is not necessarily valid when applying a thick film of paint (for example, wet 400μ).
Rather, it was discovered that the unevenness of the coating film caused by early volatilization of the solvent actually promoted the occurrence of blisters.Based on this knowledge, repeated experiments revealed that It has been found that by using a large amount of a relatively high boiling point solvent that satisfies the conditions of formulas (1) and (2) above, a coating film that does not cause blistering even when applied thickly can be obtained. As mentioned above, the boiling point of the solvent is regulated in relation to the baking temperature. The baking temperature required here differs depending on the resin system, and also differs in relation to the baking time. For example, to produce cast products, bisphenol A type epoxy resin (molecular weight approx.
360) with metaphenylenediamine, it is usually left at room temperature for 24 hours and then heated at 150°C for a long period of about 6 hours. Furthermore, when curing with phthalic anhydride, heating at 150°C for about 8 hours is required. On the other hand, when using these resin systems as baking paints, long-term heating is practically impossible in terms of work efficiency, and it is necessary to increase the baking temperature and shorten the baking time. Bisphenol A mentioned above
When curing mold epoxy resin with metaphenylene diamine, it will harden in about 1 hour if the baking temperature is set to 190°C. Therefore, in the present invention, for example, when baking is performed at 190°C for 1 hour using such a resin, 90% by weight or more of the solvent used has a boiling point of 140°C or higher, and 40% by weight or more has a boiling point of 190°C or higher. Something is necessary. However, if the amount of a solvent whose boiling point is higher than the baking temperature is increased, it naturally causes a problem of residual solvent, which deteriorates the durability of the coating film. For this reason, it is practically preferable to limit the blending amount of the solvent that satisfies the above formula (2) to about 80% by weight. FIG. 1 shows the relationship between the boiling point of the solvent and the baking temperature in equations (1) and (2) above. Here, for example, the baking temperature is 180℃, and each temperature is 200℃.
a, b, with boiling points of ℃, 170℃, 150℃, 110℃,
When using a solvent consisting of components c and d, the weight percentages of each of the above components are [a],
If expressed as [b], [c], and [d], it is necessary to adjust the proportions of the components so as to satisfy the following conditions: [a] 40% [a] + [b] + [c] 90%. Such solvents include, for example, 50 parts of Solbest #150 (boiling point: 190-204°C), 20 parts of methoxybutyl acetate (boiling point: 173°C), 20 parts of cyclohexanone (boiling point: 156°C), and xylene (boiling point: 156°C).
138℃) or 40 parts of isophorone (boiling point 210℃) or butyl cellosolve (boiling point 170℃).
℃) 30 parts, cyclohexanone (boiling point 156℃) 20 parts,
and 10 parts of xylene (boiling point 138°C). The upper limit of the boiling point of the solvent is the baking temperature plus 40℃.
(See Example C) In particular, it is practically preferable to set the baking temperature plus 30°C. If a solvent has a boiling point higher than this, the solvent will remain in the coating film after baking and will gradually evaporate, causing distortion within the film, resulting in a decrease in the adhesion of the coating film. As an additive, it is essential to use a low molecular weight polyethylene additive. This additive is commonly used as a pigment settling prevention agent in air-drying paints such as tar-epoxy or epoxy paints. This takes advantage of the property of low-molecular-weight polyethylene, which swells with solvents and imparts thixotropic properties to the paint. Thixotropic property is a property in which when the viscosity of a paint is measured at low shear rates (for example, 5 to 10 rpm with a B-type viscometer) and high shear rates (60 to 100 rpm), the viscosity at the latter is lower than at the former. This property is thought to be caused by the loose internal structure of the paint liquid. If this property were to be imparted to the paint, when painting with air spray, etc., the viscosity would be reduced due to the high shear rate at the time of discharge and the atomization property would be improved. Since no shearing force is applied afterwards, the viscosity increases and sag is less likely, which is a favorable result for thick film coatings. Based on these facts, the present inventors investigated and found that when a low molecular weight polyethylene additive is used in combination with the above-mentioned relatively high boiling point solvent, thixotropic properties increase as the paint temperature increases due to baking. The inventors have discovered that this strengthens the viscosity, thereby canceling out the decrease in viscosity caused by temperature rise, and as a result, the critical film thickness at which sagging occurs can be significantly improved. This enhanced thixotropic property is probably due to the fact that the degree of swelling of the low molecular weight polyethylene additive increases as the coating temperature increases. Although the pigment is not particularly limited, it is preferable to avoid using pigments that are flat in shape, such as talc. As described above, in the present invention, the problems of blistering and sagging when thickly applied can be appropriately avoided by the combination of resin, solvent, and low molecular weight polyethylene resin, and
It is possible to paint a thick film with a dry coating of 200μ or more per coat and to load it into a baking oven immediately after painting. Next, embodiments of the present invention will be described. [Example] The compositions and baking conditions of paints (A) to (E) according to the present invention are shown in Table 1, and the compositions and baking conditions of paints (F) to (J) as comparative examples are shown in Table 2. Indicated. The paint was either a one-component type or a two-component type, and in the case of a two-component type, appropriate amounts of solvent were distributed into the base and curing agent. The non-volatile content was about 60%. Paints (A) to (E) and comparative paint (F) to test pieces (50 x 150 x 3.2 mm hot rolled steel plate, surface shot blasted)
(J) were coated with different film thicknesses, and then immediately placed in a hot air drying oven under the conditions shown in Tables 1 and 2, respectively. The coating was performed using air spray after diluting with a solvent having the composition shown in the recipe table so that the 100 revolution viscosity measured by a B-type viscometer was 15 poise. Figure 2 shows an example of the temperature rise curve (baking temperature: 190°C) when the above test piece was placed in a hot air drying oven. Although the temperature rise curve varies depending on the shape of the object to be coated and the furnace capacity, the temperature of the object to be coated will reach the furnace temperature approximately 10 to 60 minutes after charging. After taking it out, the external appearance was observed and the critical film thickness for blistering and the critical film thickness for sagging were determined.
Table 3 shows the results of this determination, and it can be seen that the films of the present invention exhibit higher values for all critical film thicknesses than those of the comparative examples.

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〔Example 〕

The inner surface of an 80A x 5.5m steel pipe (JIS G3452) was shot blasted and painted with paint (A) in Table 1 to a wet thickness of 450μ. This painting was done by inserting an airless ball gun into the steel pipe while rotating it (60 rpm). Immediately after painting, place the test tube into a propane gas furnace (hot air circulation method) set at 180℃.
After heating for 1 hour, it was taken out. The resulting coating film had a good appearance and a dry film thickness of 200 to 250 μm. According to the present invention described above, it is possible to obtain a thick film of dry 200μ or more in one coating without causing sagging or blistering when thickly coated, and this eliminates the complicated process of several coats and several bakes. This has the effect of allowing efficient painting work to be carried out without having to remove any paint.

[Brief explanation of drawings]

FIG. 1 shows the relationship between the boiling point of the solvent and the baking temperature in formulas (1) and (2) of the present invention. FIG. 2 shows an example of the temperature rise curve of the object to be coated in the hot air drying oven.

Claims (1)

[Scope of Claims] 1. A bake-hardenable resin in which an epoxy resin is blended with an acid anhydride, an aromatic amine, or a novolac phenol as a hardening agent, and a plurality of resins whose boiling points (bp) satisfy the following formula (3). 90% by weight of the components whose boiling point (bp) satisfies the following formula (1)
Above, the components whose boiling point (bp) satisfies the following formula (2) are
Solvent containing 40 to 80% by weight, bp (℃) Baking temperature (℃) -50 (℃) ... (1) BP (℃) Baking temperature (℃) ... (2) BP (℃) Baking temperature (℃) ) +40 (℃) ...(3) Thick coating type baking paint containing low molecular weight polyethylene additive.