JPH0312113B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention combines a bisphenol type epoxy resin composition that has a high molecular weight and simultaneously contains an epoxy group and one or more secondary hydroxyl groups in one molecule with a curing agent resin that reacts with the bisphenol type epoxy resin composition. By doing so, the curing properties are improved, and as a result, a can interior paint with excellent hygiene, flavor retention, corrosion resistance, adhesion, etc. is provided. (Prior Art) Paints made of bisphenol-type epoxy resins and curing resins such as phenolic resins and amino resins are widely used for the inner surfaces of metal cans. This paint system has adhesion to metal substrates, film flexibility that can withstand can processing, and physical or chemical stability that does not change even when exposed to retort processing or can contents such as food and chemicals. It has various properties desirable as a paint for the inside of cans. These bisphenol type epoxy resins used for industrial purposes are produced by a method in which epihalohydrin and bis(4-hydroxyphenyl)alkane are reacted in one step in the presence of a strong alkali (one-step method), or by this one-step method. Furthermore, bis(4) is added to the produced low molecular weight epoxy resin.
-Hydroxyphenyl) alkane is added and polymerized (two-step method). (Problems to be Solved by the Invention) However, the epoxy resin obtained by any of the above methods is obtained as a composition consisting of a mixture of components with different molecular weights, and when producing one with a certain average degree of condensation. Even in this case, it is inevitable that low molecular weight components with a low degree of condensation will be mixed. When an epoxy resin composition containing this low molecular weight component is added to a paint, a small amount of the low molecular weight component that does not participate in the curing reaction of the paint remains in the paint film, and over time, the paint film forms on the food contents of the can. and may impair the flavor of food.
Or there was a risk of food hygiene problems occurring. Furthermore, if a low molecular weight component that does not participate in the curing reaction is present in the coating film, it is expected that the adhesion between the coating film and the metal substrate and corrosion resistance will be adversely affected over a long period of time. Of course, even if care is taken to uniformly perform the polycondensation reaction between bisphenols and epihalohydrin and the polyaddition reaction between bisepoxides and bisphenols, the number average of epoxy resins for paints Since the molecular weight is generally on the order of 500 to 10,000, it is inevitable that the low molecular weight components will be mixed in with a certain probability. In the present invention, substantially all the components in the epoxy resin effectively contribute to the curing reaction, and the transfer of low molecular weight substances from the cured coating film to the inner diameter of food products is suppressed, thereby providing excellent curability, hygiene, and The purpose of the present invention is to provide an epoxy-based can interior paint that achieves flavor retention, corrosion resistance, and adhesion. (Means for Solving the Problems) That is, the present invention has a number average molecular weight (Mn) of 2000
~8000, the dispersion ratio (Mw/Mn) of number average molecular weight and weight average molecular weight (Mw) is within the range of 1.8 to 2.6, and the molecular weight is 600 or more and 1 per molecule.
Contains an epoxy resin molecule component containing 1 or more secondary hydroxyl groups in an amount of 99.4% by weight or more based on the entire epoxy resin, and the average number of epoxy groups per molecule is
a bisphenol type epoxy resin having a molecular weight of 1.2 or more;
The present invention relates to a can inner surface paint characterized by containing a curing agent resin for the epoxy resin. (Function) The present invention provides an epoxy resin molecular component having a molecular weight of 600 or more and containing one or more secondary hydroxyl groups per molecule (hereinafter referred to as a specific epoxy resin molecular component).
This is based on the knowledge that it shows high reactivity with the methylol groups of curing resins such as phenolic resins and amino resins, and provides a coating film with a high degree of curing that is suitable as a coating for the inside of a can. Further, in the present invention, when the above-mentioned specific epoxy resin molecular component accounts for 99.4% by weight or more of the entire epoxy resin, the transfer of the epoxy resin from the coating film to the contents can be almost completely suppressed. This is also based on the knowledge that. The chemical structure of epoxy resin can be explained using the example of bis-epoxide as follows: It is expressed as In the above formula, when n is zero (hereinafter referred to as n=0 component), there are epoxy groups at both ends, but there is no secondary hydroxyl group in the molecular chain, whereas n is 1 or more, it is understood that one or more secondary hydroxyl groups are necessarily present in the molecular chain. In actual epoxy resins, there are those in which one or both molecular chain ends are glycol or bisphenol, and for example, between n=0 component and n=1 component, these and those with a solvent component such as butyl cellosolve added to the molecular chain end with a molecular weight between
components) are present. In this specification, an epoxy resin molecular component having a molecular weight of 600 or more refers to an n=0 component or a n=0.5 component.
Excluding components, it means molecular components where n=1 or more. In addition, the ratio of specific epoxy resin molecular components to the entire epoxy resin can be determined by gel permeation chromatography of epoxy resin,
(GPC). Figure 1 of the attached drawings is a GPC chromatogram of a typical epoxy resin, and from the low molecular weight side, n = 0 component, n = 0.5 component,
Peaks appear in the order of n=1 component... The molecular weight of the n=0 component and the n=0.5 component is smaller than 600,
The ratio of the peak area of components with n = 1 or more to the total peak area is the weight% of the specific epoxy resin molecule component
corresponds to Conventionally, the ratio of specific epoxy resin molecular components in epoxy resins used in can paints has been 98.5% by weight or less in most cases, and the highest ratio has been about 99.0% by weight. On the other hand, according to the present invention, when the content of the specific epoxy resin molecular component is increased to 99.4% by weight or more, the gel fraction, which indicates the degree of crosslinking of the coating film, increases considerably, as shown in the example below. Extraction or migration of the epoxy resin component into the contents is almost completely suppressed. Since the difference in the ratio of specific epoxy resin molecular components between conventional can epoxy resins and the epoxy resin used in the present invention is generally small, the remarkable difference in action based on this small difference is due to the difference in the effect of the present invention. This tells us that the effect is unexpected. The exact reason for this is unknown, but due to the presence of secondary hydroxyl groups in certain epoxy resin molecular components,
Shows a high degree of reactivity with the curing resin and produces a coating film with a high degree of curing, and low molecular weight components (n = 0 component, n = 0.5 component) that are associated with an improvement in the degree of curing.
It is presumed that this is due to the fact that it is involved in the curing reaction or is embedded within the crosslinked network and has a structure that makes it difficult to extract. The number average molecular weight of the bisphenol type epoxy resin in the present invention is preferably in the range of 2000 to 8000.
If it is less than 2000, workability decreases, which is not preferable. On the other hand, if the molecular weight is greater than 8,000, the viscosity of the resin solution will be high and only a paint with a low solids content will be obtained, making it difficult to form a stable coating film thickness during coating. In addition, the epoxy resin in the present invention has an average of
Having 1.2 or more epoxy groups is important in terms of curability. Although the method for obtaining the epoxy resin used in the present invention is not particularly limited, the method of removing the above-mentioned low molecular weight components from existing epoxy resins can be easily adopted. The method will be described below. That is, the epoxy resin in the present invention can be obtained by reducing or removing low molecular weight components that do not contribute to curing from an epoxy resin having a molecular weight distribution obtained by a one-step method or a two-step method. As the means, any of the following methods can be used. First, low molecular weight components can be separated and removed using chemical methods such as recrystallization, solvent extraction, and preparative GPC. Among these, the solvent extraction method is an industrially advantageous method if the extraction solvent is appropriately selected. On the other hand, physical methods include molecular distillation (hereinafter referred to as distillation), treatment using a separation membrane, and the like. Among these methods, the method of incorporating separation and removal of low-volume components by distillation into the epoxy resin manufacturing process has the following characteristics. Namely, the epoxy resin finally obtained has high purity, can be handled in a closed system, and since resin intermediates are purified, an epoxy resin with a desired molecular weight can be obtained.
The usual distillation method uses bisphenol A type epoxy resin.
It can be carried out by heating and melting at 200°C to 280°C and distilling under reduced pressure at 0.1 to 1.0 mmHg. In this case, the heating melt viscosity of the epoxy resin is 1000 cps at 200℃.
It is preferably 300 cps or less. Therefore, the molecular weight of the epoxy resin to be subjected to distillation is preferably in the range of 600 to 1,500. Furthermore, in order to facilitate separation of low molecular weight components during distillation, 10% or less of a high boiling point epoxy resin affinity solvent may be added. If low molecular weight substances are not removed to the desired extent in one distillation, the degree of separation of low molecular weight components is adjusted by performing multiple distillations or by changing conditions such as temperature, degree of vacuum, distillation rate, etc. The epoxy resin intermediate purified product thus obtained (hereinafter referred to as epoxy resin intermediate) can be further heated with bisphenol A and a catalyst to carry out an addition reaction to obtain an epoxy resin of a desired molecular weight. On the other hand, the liquid epoxy resin (n=0) contained in the low molecular weight components separated by distillation can be used again in the reaction by a two-stage method, and therefore this method is an industrially advantageous epoxy resin manufacturing technology. be. Although the above explanation is an example of a method for producing an epoxy resin using an epoxy resin intermediate obtained by distillation, the present invention is of course not limited to the method for removing low molecular weight components of an epoxy resin using this method. In the present invention, the curing agent resin for the bisphenol type epoxy resin contains a relatively high concentration of methylol groups that react with secondary hydroxyl groups, such as phenolic hydroxyl groups, methylol groups, carboxyl groups, etc.
The resin contains at least one of these functional groups that are reactive with epoxy groups, and more specifically, resol type or novolak type phenolic resin, melamine resin, urea resin, benzoguanamine resin, etc. be. Phenol resins are obtained by reacting phenols such as carbolic acid, cresols, ethylphenols, other alkylphenols, or bisphenols with aldehydes such as formaldehyde and acetaldehyde in the presence of a basic catalyst. Alternatively, alkyl etherified phenolic resins obtained by reacting them with alcohols can be used. The amino resin may be one obtained by reacting urea, melamine, or a triazine compound with formaldehyde, or one obtained by reacting this with a monohydric alcohol having 1 to 4 carbon atoms to etherify it. The blending ratio of bisphenol type epoxy resin and the above curable resin is epoxy resin/curable resin.
A range of 95/5 to 50/50 is preferred. The bisphenol type epoxy resin and the curable resin may be simply mixed, or both resins may be precondensed in advance. A part of the bisphenol type epoxy resin may be replaced with a phenoxy resin which is a high molecular weight resin having the same repeating unit as the resin. In this case, the method of the invention can also be used for phenoxy resins. That is, it is also possible to produce a phenoxy resin by using an epoxy resin intermediate purified by distillation and increasing its molecular weight. The can interior paint of the present invention is prepared by dissolving a bisphenol type epoxy resin and a curing agent resin in a mixed solvent containing a parent solvent for the epoxy resin or a lower alcohol component at a solid content of 25 to 40%. Paint the metal material for cans with a coating thickness of 3 to 10μ. Also,
The bisphenol type epoxy resin of the present invention can also be used in the form of powder coatings and water-based coatings obtained by known methods. Examples of metal materials include untreated steel sheets, tin-plated steel sheets, galvanized steel sheets, phosphate-treated steel sheets, and chromic acid-treated steel sheets. As the coating method, conventionally known methods such as dip coating, roll coating, and spray coating can be employed. The curing conditions for the paint can be selected from a baking temperature of 180 to 230°C and a baking time of 3 to 30 minutes. In addition to the above-mentioned resin components and solvent components, the can interior paint of the present invention may contain silicone resins and waxes as paint leveling modifiers and lubricants. (Examples) Hereinafter, the present invention will be specifically explained according to Examples. In addition, parts below refer to parts by weight.
Molecular weight means number average molecular weight. Example 1 Production of epoxy resin (a) Epicote 828 (trade name of Yuka Ciel Epoxy Co., Ltd.)
A mixture of 301.1 g, bisphenol A 88.9 g, and ethyl triphenylphosphonium chloride 0.4 g
Epoxy resin 390 with a molecular weight of 900 after reacting for 2 hours at ℃
I got g. Next, 300 g of the above epoxy resin was heated to 250 to 260°C, and vacuum distillation was repeated 7 times at 0.2 mmHg for purification. The molecular weight of the epoxy resin intermediate thus obtained was 962. Additionally, 307.6 g of the above epoxy resin intermediate, 624 g of bisphenol A, 0.3 g of ethyltriphenylphosphonium chloride, and 41 g of cyclohexanone.
The mixture was reacted at 170℃ for 2 hours to obtain a molecular weight of 4650.
370 g of epoxy resin (a) was obtained. Production of epoxy-phenol paint and creation of baked painted plates A mixture of 171 g (0.75 mol) of bisphenol A, 27 g (0.25 mol) of p-cresol, 81 g (1 mol) of formalin (37% aqueous solution), and 12 g of 25% aqueous ammonia is refluxed. The reaction was allowed to proceed for 30 minutes. After the reaction was completed, 50 g of methyl isobutyl ketone and 50 g of n-butanol were added and azeotropically dehydrated to remove a small amount of water. The solid content was adjusted to 30%. A mixture consisting of 140 g of epoxy resin (a) obtained in the above treatment, 300 g of phenol resin solution (weight ratio of epoxy resin/phenol resin 70/30), 20 g of methyl isobutyl ketone, 20 g of xylene, and 20 g of n-butanol was heated at 100°C. The mixture was allowed to react while being mixed and stirred for a period of time. After cooling the reaction, the solid content was adjusted to obtain an epoxy-phenol paint. Next, this epoxy-phenol paint was applied to an aluminum plate in a thickness of about 5 μm using a #18 percoater, and baked at 200°C for 10 minutes to prepare a coated plate. Epoxy resin measurement method and paint evaluation method (i) Epoxy resin measurement by GPC Measurement conditions Equipment: JASCO Corporation TRI ROTAR Column: Showa Denko Corporation Shodex KF801 x 2 + KF803 + KF804 Temperature/flow rate 40℃, 1ml/ Minutes Detector UV-254nm (ii) Measurement of gel fraction by MEK extraction The obtained treated plate was immersed in MEK in an amount 1500 times the weight of the coating film, and refluxed at 80°C for 60 minutes to determine the gel fraction. It was measured. Gel fraction (%) by MEK extraction = {1 - (W ₁ - W ₂ / W ₁ - W ₀ )} x 100 W ₀ : Weight of uncoated aluminum plate W ₁ : Weight of aluminum plate after painting baking W ₂ : Weight of aluminum coated plate after extracting MEK and drying at 80℃ for 30 minutes (iii) Extraction test with 10% methanol/water Per 1 cm ² of coating film using 20 baked coated plates obtained in this example Extraction was performed using 1 c.c. of 10% methanol/water (1/1) extract at 60°C for 30 minutes. 10 of the obtained extract was extracted in this order using 5 each of ethyl ethyl chloroform and methanol extraction solvents and concentrated. Note that the total weight of the extract represents the total amount of the extract with each solvent of ethyl ether, chloroform, and methanol. Extraction rate from paint film (%) = Total weight of extract/weight of paint film x 100 (iv) Elution test of epoxy components by retort treatment 60 in the 10% methanol/water (1/1) extraction test mentioned above. Instead of performing extraction at 115°C for 30 minutes, distilled water was used to perform retort treatment at 115°C for 90 minutes. Thereafter, the extract was extracted using the same solvent and concentrated. This concentrate was measured using a electrolytic desorption mass spectrometer (FD-MS) to check for the presence of epoxy components. (v) The baked painted board obtained in this example is 10 x 2.5 cm.
The material was cut into strips and three cross cuts were made at 2.5 cm intervals to serve as samples for corrosion resistance tests. On the other hand, after filling an empty can with an aqueous solution of 1% citric acid, 0.6% salt, and 0.001% surfactant, five of the samples were added to each can, and the cans were double-sealed.
I made 10 cans. Next, these cans were subjected to retort treatment at 125°C for 90 minutes, and then stored at 50°C for 3 months to conduct an accelerated corrosion resistance test. After the storage test over time, open the can, take out the test piece, and observe the degree of corrosion.
Graded evaluation. 5: No abnormality 3: Partial corrosion 1: Quite severely corroded Examples 2 to 3 and Comparative Examples 1 to 2 In place of the epoxy resin (a) obtained in Example 1,
The molecular weight can be adjusted by changing the number of vacuum distillations and the degree of vacuum.
600 or more and containing one or more secondary hydroxyl groups per molecule, paints are manufactured by varying the weight percent (hereinafter abbreviated as A%) of the epoxy resin molecular component based on the entire epoxy resin, and the performance as a can inner surface paint is achieved. It is shown in Table 1 together with examples and comparative examples.

【table】

[Table] Examples 4 to 6 and Comparative Example 3 Production of epoxy resin (b) Except for using 0.05 g of tetramethylammonium chloride and 103.6 g of bisphenol A in place of ethyl triphenylphosphonium chloride in epoxy resin (a). was carried out in the same manner as in Example 1. The molecular weight of the resulting epoxy resin intermediate is
The molecular weight of the epoxy resin finally obtained was 3430. Production of epoxy resin (c) 318.2 g of the epoxy resin intermediate (molecular weight 962) obtained in Example 1, 71.8 g of bisphenol A, 0.2 g of tetramethylammonium chloride, and 97 g of cyclohexanone were mixed and then reacted at 160°C for 7 hours. An epoxy resin (c) with a molecular weight of 7840 was obtained. Manufacture of epoxy-urea paint and creation of baked painted board 253 g of (V ₁ ) mixture of epoxy resins (b) and (c) obtained in the above treatment, 87 g of cyclohexanone, n
- A mixture consisting of 43 g of butanol and 380 g of butyl cellosolve was heated and dissolved at 85° C. for 1 hour. Then add 140g of ethyl cellosolve and cool to 60℃.
Next, 74.4 g of urea resin Melan 11E (trade name, manufactured by Hitachi Chemical Co., Ltd.) was added and mixed, and the solid content was adjusted to 30% to obtain an epoxy-urea resin. Next, a baking-coated board was prepared in the same manner as in Example 1, and its suitability as a paint for the inside of a can was evaluated, and the results are shown in Table 2.

[Table] * The epoxy resins used in Example 5 and Comparative Example 3 were obtained by changing the number of distillations of epoxy resin (b).
The results of the Examples and Comparative Examples in Table 2 above show that, similar to epoxy phenol paints, even when epoxy-urea paints are used for can paints, the A% of the epoxy resin has a large effect on the hardenability of the paint film. It shows that

[Brief explanation of drawings]

FIG. 1 shows a GPC chart of a high molecular weight epoxy resin obtained under the measurement conditions shown in Examples.

Claims (1)

[Claims] 1. The number average molecular weight (Mn) is 2000 to 8000, the dispersion ratio of number average molecular weight to weight average molecular weight (Mw) (Mw/Mn) is within the range of 1.8 to 2.6, and the molecular weight is 600. Bis containing the above-mentioned epoxy resin molecular components containing one or more secondary hydroxyl groups per molecule in an amount of 99.4% by weight or more based on the entire epoxy resin, and the average number of epoxy groups per molecule is 1.2 or more. A can interior paint characterized by containing a phenolic epoxy resin and a curing agent resin for the epoxy resin. 2. The can inner surface paint according to claim 1, wherein the curing agent resin of the epoxy resin is at least one resin selected from the group consisting of phenolic resins and amino resins.