Method for observing cured state of coating material

Abstract

Provided is a method which makes it possible to evaluate, with respect to a curing process for a coating material, the cured state of the coating material in terms of temperature and time, and therefor alleviates the burden on a user by using a method for observing the cured state of the coating material in-situ, the method including: a step for bringing a sample obtained by mixing a curing agent and a coating material into contact with the surface of a diamond prism; a step for measuring the infrared absorption spectrum of the sample while controlling the temperature of the sample, and acquiring, from the obtained infrared absorption spectrum, time-series data of the amount of functional groups of the curing agent in the sample; and a step for visualizing the relationship between the time or temperature and the amount of the functional groups of the curing agent in the sample.

Description

How to observe the curing state of paint

[0001] The present invention relates to a method for observing the curing state of a paint.

[0002] Polyisocyanates derived from aliphatic diisocyanates or alicyclic diisocyanates are known to be non-yellowing curing agents with excellent weather resistance, chemical resistance, and flexibility. Blocked polyisocyanates, in which the isocyanate groups of polyisocyanates are blocked with a thermally dissociable blocking agent, do not react at room temperature when mixed with the active hydrogen compound, which is the main component. However, when heated, the blocking agent dissociates, regenerating the isocyanate groups and allowing curing to proceed.

[0003] In order for this thermosetting coating material to exhibit the desired performance, it is necessary to find appropriate curing conditions. As a method for determining these curing conditions, for example, Mori Kanji, "Method for tracking crosslink density during the curing process of a coating film - proportionality between storage modulus and crosslink density", J. Jpn. Soc. Colour Mater., 86(4), 123-127(2013) (Non-Patent Document 1), discloses a pendulum-type viscoelasticity measuring device that measures viscoelasticity that reflects changes in crosslink density that are closely related to curing, or a method that analyzes changes in the amount and reaction rate of isocyanate by infrared spectroscopy.

[0004] In addition, various countries and regions are aiming to achieve carbon neutrality by 2050, and are promoting the introduction of renewable energy and CO ２ Various efforts are being made to reduce emissions (the Chinese government plans to achieve zero by 2060). ２ When we look at the automobile industry, which has a large amount of emissions, CO ２ Approximately 25% of emissions are thought to occur during the coating process of paint, and automakers and paint manufacturers are concerned about CO emissions during this process. ２ In the coating process, after the paint is applied, it is baked and dried at high temperature to harden the paint, but shortening the hardening time and hardening at low temperature reduces CO ２ It is said to be effective in reducing emissions.

[0005] Kanji Mori, "Tracking Method for Crosslink Density in the Curing Process of Coating Films - Proportionality between Storage Modulus and Crosslink Density -", J. Jpn. Soc. Colour Mater., 86(4), 123-127(2013) Morio Tsuge, "Application of Fourier Transform Infrared Spectroscopy to the Analysis of Thermosetting Resins", Thermosetting Resins, Vol.12 No.4(1991), pp.209-234

[0006] In the infrared spectroscopy analysis method described in Non-Patent Document 1, a mixture of a hydroxyl-containing acrylic resin and an isocyanate curing agent is used as a model paint, and the curing process is evaluated at a predetermined temperature (140°C). However, while this method can evaluate changes in curing over time, it cannot evaluate the curing process due to temperature changes. In paint development, the method described in Non-Patent Document 1 requires the user to set the temperature each time and obtain analytical data, and also manually compile the data obtained for each temperature, requiring excessive effort to find the optimal curing conditions.

[0007] The present invention has been made to solve the above-mentioned problems, and its purpose is to provide a method that can evaluate the curing state of paint from the perspectives of temperature and time during the curing process of paint, thereby reducing the burden on the user.

[0008] A first aspect of the present invention relates to a method for observing the cured state of a paint in situ, comprising the steps of: bringing a sample of a mixture of a curing agent and a paint into contact with the surface of a diamond prism; measuring the infrared absorption spectrum of the sample while controlling the temperature of the sample, and obtaining time-series data on the amount of functional groups possessed by the curing agent in the sample from the obtained infrared absorption spectrum; and visualizing the relationship between time or temperature and the amount of functional groups possessed by the curing agent in the sample.

[0009] According to the method of the present invention, data on the curing state of a paint can be obtained from the viewpoints of temperature and time during the curing process of the paint, thereby reducing the burden on the user required to find the curing conditions for the paint to be evaluated.

[0010] 2 is a diagram showing the reaction formula of the acrylic urethane resin used in Experimental Example 1. FIG. 3 is a diagram showing the spectrum of the acrylic urethane resin before and after curing of the coating material in Experimental Example 1. －１ 1 is a graph showing the relationship between the reaction rate and the heat curing time at each constant temperature.

[0011] A first aspect of the present invention is a method for observing the cured state of a paint in situ, comprising the steps of: bringing a sample of a mixture of a curing agent and a paint into contact with the surface of a diamond prism; measuring the infrared absorption spectrum (IR spectrum: InfraRed absorption spectrometry) of the sample while controlling the temperature of the sample, and obtaining time-series data on the amount of functional groups that the curing agent in the sample has from the obtained infrared absorption spectrum; and visualizing the relationship between time or temperature and the amount of functional groups that the curing agent in the sample has.

[0012] The first aspect of the present invention is premised on the use of an apparatus equipped with a diamond prism and capable of measuring the infrared absorption spectrum while changing the temperature of a sample. A specific example of such an apparatus is a Fourier Transform Infrared Spectrophotometer (FTIR: Fourier Transform Infrared Spectroscopy) IRTracer-100. （商標） A heated single-reflection total reflection measurement device, Microm ATR (Czitech), 4000 series, combined with a 1000-kJ / s single-reflection total reflection measurement device (Shimadzu Corporation). （商標） Golden Gate combined with temperature controller (Specac) （商標） An example of such an apparatus is a diamond ATR (attenuated total reflection) apparatus (manufactured by Specac), but the present invention is not limited to this as long as the apparatus satisfies the above-mentioned prerequisites.

[0013] In the first aspect of the present invention, a sample of a mixture of a curing agent and a coating material is first brought into contact with the surface of a diamond prism. By directly applying the sample to the surface of the diamond prism in this way, data on impurities is not introduced into the infrared absorption spectrum.

[0014] In the first aspect of the present invention, the infrared absorption spectrum of a sample is measured while controlling the temperature of the sample, and time-series data on the amount of functional groups possessed by the curing agent in the sample is obtained from the obtained infrared absorption spectrum. In this way, in the first aspect of the present invention, the curing process of the paint is observed in situ, and data on the curing state of the paint in terms of temperature and time is obtained.

[0015] In the first aspect of the present invention, the measurement of the infrared absorption spectrum of a sample while controlling the sample temperature is preferably performed both while maintaining the sample temperature constant and while changing the sample temperature at a predetermined rate. By measuring the infrared absorption spectrum of a sample while maintaining the sample temperature constant, time-series data showing the relationship between time and the amount of functional groups contained in the curing agent in the sample can be obtained. On the other hand, by measuring the infrared absorption spectrum of a sample while changing the sample temperature at a predetermined rate, time-series data showing the relationship between temperature and the amount of functional groups contained in the curing agent in the sample can be obtained. The temperature when maintained constant is not particularly limited and can be appropriately selected depending on the temperature at which the curing agent contained in the sample cures. Furthermore, the predetermined rate of change when changing the temperature is not particularly limited, and the temperature change may be continuous or intermittent.

[0016] In the first aspect of the present invention, the relationship between time or temperature and the amount of functional groups in the curing agent in the sample is then visualized. Specifically, the obtained time-series data is graphed, with time or temperature on the horizontal axis and the amount of functional groups in the curing agent on the vertical axis, to visualize the relationship between time or temperature and the amount of functional groups in the curing agent. In this way, in the first aspect of the present invention, not only is the change in curing over time evaluated, but the curing process due to temperature changes is also evaluated. This reduces the burden on the user required to determine the curing conditions for the paint being evaluated.

[0017] Here, the functional group possessed by the curing agent is not particularly limited as long as the relationship between the infrared spectrum and the curing temperature is known, such as an isocyanate group in the case of an isocyanate curing agent. Examples of the functional group possessed by such a curing agent include an epoxy group, a primary amino group, a secondary amino group, a primary amide group, a tertiary amide group, an ether group, a hydroxyl group, and a carbonyl group, as described in Morio Tsuge, "Application of Fourier Transform Infrared Spectroscopy to Thermosetting Resin Analysis," Thermosetting Resins, Vol. 12, No. 4 (1991), pp. 209-234 (Non-Patent Document 2).

[0018] In the first aspect of the present invention, the curing agent may be a known curing agent commonly used for curing thermosetting resins, but it is preferable that the curing agent is an isocyanate curing agent and that the functional group of the curing agent in the sample is an isocyanate group. When an isocyanate curing agent is used, polyfunctional isocyanate curing agents such as aliphatic, alicyclic, aromatic group-containing aliphatic, or aromatic diisocyanates, diisocyanate dimers, and diisocyanate trimers (preferably isocyanurate-type isocyanates (so-called isocyanurates)) can be used.

[0019] Representative isocyanate curing agents include, for example, aromatic diisocyanates such as tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), xylylene diisocyanate (XDI), and metaxylylene diisocyanate (MXDI); hexamethylene diisocyanate (HDI), tetramethylene diisocyanate, 2-methyl-pentane-1,5-diisocyanate, 3-methyl-pentane-1,5-diisocyanate, lysine diisocyanate, and trioxyethylene diisocyanate. Aliphatic diisocyanates such as isocyanate; alicyclic diisocyanates such as isophorone diisocyanate (IPDI), cyclohexyl diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, norbornane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylene diisocyanate, and hydrogenated tetramethylxylene diisocyanate; and biuret, nurate, trimethylolpropane (TMP) adduct, and uretdione derivatives thereof. These may be used alone or in combination of two or more.

[0020] Blocked isocyanates can also be used as the isocyanate curing agent. Blocked isocyanates can be prepared by blocking the isocyanate groups of the polyfunctional isocyanate curing agent with a blocking agent. The blocking agent is added to the polyisocyanate group and is stable at room temperature, but can regenerate free isocyanate groups when heated to a temperature above the dissociation temperature. Commonly used blocking agents such as ε-caprolactam and butyl cellosolve can be used.

[0021] A specific example of a suitable isocyanate curing agent is Duranate, which is disclosed in Non-Patent Document 1 and contains hexamethylene diisocyanate trimer (isocyanurate) as the main component. （商標） An example is TPA-100 (manufactured by Asahi Kasei Corporation).

[0022] The paint (main component) contained in the sample used in the first aspect of the present invention is not particularly limited and may be any suitable conventional paint used in combination with an isocyanate curing agent, but a preferred example is a hydroxyl group-containing acrylic resin.

[0023] The hydroxyl group-containing acrylic resin can be suitably prepared by copolymerizing a monomer mixture containing a hydroxyl group-containing ethylenically unsaturated monomer. Examples of hydroxyl group-containing ethylenically unsaturated monomers include 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 2,3-dihydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, reaction products of these hydroxyl group-containing (meth)acrylates with ε-caprolactone, and esters of polyhydric alcohols such as polyethylene glycol mono(meth)acrylate with acrylic acid or methacrylic acid. Furthermore, reaction products obtained by ring-opening polymerization of ε-caprolactone with monoesters of the above polyhydric alcohols with acrylic acid or methacrylic acid can also be used.

[0024] The monomer mixture may contain an ethylenically unsaturated monomer other than the hydroxyl group-containing ethylenically unsaturated monomer. Examples of such ethylenically unsaturated monomers include (meth)acrylic acid alkyl ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-, i-, or t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate; carboxyl group-containing ethylenically unsaturated monomers such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, and fumaric acid; and dicarboxylic acid monoester monomers such as ethyl maleate, butyl maleate, ethyl itaconate, and butyl itaconate; amino group-containing ethylenically unsaturated monomers such as allylamine and p-vinylaniline; cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, tricyclodecanyl (meth)acrylate, and adamantyl (meth)acrylate. (meth)acrylic acid aminoalkyl ester monomers such as aminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, and butylaminoethyl (meth)acrylate; (meth)acrylic acid aminoalkylamide monomers such as aminoethyl(meth)acrylamide, dimethylaminomethyl(meth)acrylamide, and methylaminopropyl(meth)acrylamide; other amide group-containing ethylenically unsaturated monomers such as acrylamide, methacrylamide, N-methylolacrylamide, methoxybutylacrylamide, and diacetone acrylamide; vinyl cyanide-based monomers such as (meth)acrylonitrile and α-chloroacrylonitrile; saturated aliphatic carboxylic acid vinyl ester monomers such as vinyl acetate and vinyl propionate; and styrene-based monomers such as styrene, α-methylstyrene, and vinyltoluene.

[0025] These monomers may be used alone or in combination of two or more. In this specification, "(meth)acrylate" means "acrylate or methacrylate".

[0026] The monomer mixture can be polymerized by any method commonly used by those skilled in the art, such as bulk polymerization using a radical polymerization initiator, solution polymerization, or a two-stage bulk-suspension polymerization method in which bulk polymerization is followed by suspension polymerization.

[0027] A specific example of a suitable coating material is a hydroxyl group-containing acrylic resin (weight average molecular weight: 12,500, number average molecular weight: 6,430) obtained by adding 4 PHR of azobisisobutyronitrile as an initiator to a monomer composition (hydroxyl value: 150) of methyl methacrylate (weight ratio: 33), n-butyl acrylate (weight ratio: 15.9), 2-hydroxyethyl acrylate (weight ratio: 31.1), and styrene (weight ratio: 20) and polymerizing the mixture at 80°C using methyl ethyl ketone as a solvent, as disclosed in Non-Patent Document 1.

[0028] In the sample used in the first aspect of the present invention, the isocyanate curing agent and the coating material are preferably mixed so that the hydroxyl groups and the isocyanate groups are equimolar, based on the monomer compounding ratio of the hydroxyl group-containing acrylic resin and the manufacturer's analytical value of the isocyanate curing agent, as described in Non-Patent Document 1.

[0029] The first aspect of the present invention preferably further comprises the step of removing the hardened material of the sample from the diamond prism after measuring the infrared absorption spectrum.In the first aspect of the present invention, in order to avoid the data of impurities from being included in the infrared absorption spectrum, the hardening process is observed by bringing the sample into contact with the diamond prism.Therefore, after measuring the infrared absorption spectrum, the hardened material of the sample will be fixed to the surface of the diamond prism.

[0030] The hardened material of the sample can be removed by wiping the diamond prism with a cotton swab soaked in a small amount of solvent, followed by wiping it dry. The solvent is not particularly limited as long as it is a solvent that has been conventionally used to clean diamond prisms, and examples of the solvent include methanol, ethanol, 2-propanol, N-hexane, toluene, tetrahydrofuran, and ethyl acetate.

[0031] The present invention will be explained in more detail below by way of experimental examples, but the present invention is not limited to these examples.

[0032] Experimental Example 1: Fourier transform infrared spectrophotometer (FTIR) IRTracer-100 （商標） The thermal curing reaction of a paint was measured in real time using a heated single-reflection total reflection spectrometer (Shimadzu Corporation) and a heated single-reflection total reflection spectrometer (Microm ATR) equipped with a diamond prism (Czitech). During the thermal curing reaction, the paint undergoes partial structural changes. By focusing on the peaks of functional groups resulting from this structural change, optimal curing conditions can be determined. In this experimental example, an acrylic urethane paint used in automobiles was measured. This paint is based on an acrylic polyol and uses an aliphatic polyisocyanate as a curing agent. Mixing these two liquids initiates a polymerization reaction, resulting in the formation of urethane bonds (the reaction scheme is shown in Figure 1). Acrylic urethane paint is a tough paint with excellent weather and gasoline resistance. It takes longer to dry than other lacquer paints, but the drying time can be shortened by applying heat.

[0033] Measurements were carried out under the following conditions: Resolution: 4 cm －１ ・Number of measurements: 20 ・Wave number range: 4000 to 400 cm －１ Apodization function: Happ-Genzel Detector: DLATGS (Deuterated L-Alanine Triglycine Sulfate) A heating disk was used in the Microm ATR, and measurements were taken while heating (the heating disk of the Microm ATR can measure in the temperature range from room temperature to 130° C.).

[0034] For the measurement, a commercially available acrylic urethane paint (88 Panalock (manufactured by Rock Paint Co., Ltd.)) was used, and the base agent and curing agent were mixed in a ratio of 10:1. After thorough stirring, a drop of the paint was dropped onto the diamond prism of the ATR and measured. Figure 2 shows an overlay of the spectrum before curing, measured immediately after mixing the base agent and curing agent, and the spectrum after mixing, leaving it at room temperature for 3 days, and then completely curing. In the spectrum before curing, the isocyanate group contained in the curing agent is detected at 2270 cm－１ However, since no isocyanate groups are visible after curing, it is clear that by focusing on the isocyanate groups, information about curing can be obtained.

[0035] After mixing the main agent and hardener of the paint, a time course measurement is carried out at each constant temperature for 2 hours, and the reaction rate D is calculated from the obtained spectrum. Ｒ Here, Figure 3 shows the 3100-2150 cm －１ As shown in Figure 3, the spectrum is an enlarged view of the reaction rate D Ｒ is 2940 cm, which does not change before and after heat curing. －１ The peak intensity (A ＣＨ３ ) based on 2270 cm －１ The peak intensity derived from the isocyanate group in the vicinity was calculated, and the value was calculated using the following formula.

[0036]

[0037] t: Heat curing time A ＮＣＯ (t): 2270 cm at heat curing time t －１ Peak intensity around A ＣＨ３ (t): 2940 cm at heat curing time t －１ R(t): Peak intensity ratio of isocyanate group and methyl group at heat curing time t Ｒ : Polymerization reaction rate Calculated reaction rate D Ｒ The reaction rate D is plotted against the heat curing time t in Figure 4. Ｒ As the thermal curing progresses, the effective conditions for the thermal curing reaction of the paint can be estimated from Figure 4.

[0038] From Figure 4, it can be seen that at set temperatures of 80°C and 100°C, the reaction rate reached approximately 0.8 30 minutes after the start of curing, and thermal curing proceeded rapidly. ２ From the viewpoint of cost reduction, a heat curing temperature of 80°C is more suitable than 100°C. For reference, at set temperatures of 60°C and 40°C, the reaction rate was below 0.7 even after 2 hours, and it was found that more time was required for complete curing.

[0039] As described above, when the thermal curing reaction of the paint was measured in real time using FTIR and a heating ATR accessory, the thermal curing of the acrylic urethane paint progressed as the temperature rose, and no change in the thermal curing reaction rate was observed above 80°C, so it was inferred that 80°C is the optimum thermal curing temperature. By tracking the thermal curing reaction in real time, it became possible to estimate the optimal thermal curing temperature and time.

[0040] Aspects It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0041] (Item 1) One embodiment of a method for observing the cured state of a paint in situ includes the steps of bringing a sample of a mixture of a curing agent and a paint into contact with the surface of a diamond prism, measuring the infrared absorption spectrum of the sample while controlling the temperature of the sample, and obtaining time-series data on the amount of functional groups possessed by the curing agent in the sample from the obtained infrared absorption spectrum, and visualizing the relationship between time or temperature and the amount of functional groups possessed by the curing agent in the sample.

[0042] According to the method described in paragraph 1, data on the curing state of the paint can be obtained from the viewpoints of temperature and time during the curing process of the paint, thereby reducing the burden on the user required to find the curing conditions of the paint to be evaluated.

[0043] (Item 2) In the method described in item 1, the step of measuring the infrared absorption spectrum of the sample while controlling the temperature of the sample and obtaining time-series data on the amount of functional groups possessed by the curing agent in the sample from the obtained infrared absorption spectrum includes both measuring the infrared absorption spectrum of the sample while maintaining the temperature of the sample constant and measuring the infrared absorption spectrum of the sample while changing the temperature of the sample at a predetermined rate of change, and obtaining time-series data on the amount of functional groups possessed by the curing agent in the sample from the obtained infrared absorption spectrum.

[0044] According to the method described in paragraph 2, by including a case where the infrared absorption spectrum of the sample is measured while maintaining the temperature of the sample constant, and a case where the infrared absorption spectrum of the sample is measured while changing the temperature of the sample at a predetermined rate of change, it becomes easier to visualize the relationship between time or temperature and the amount of functional groups that the curing agent in the sample has.

[0045] (Item 3) The method according to item 1, further comprising the step of removing the cured material of the sample from the diamond prism after measuring the infrared absorption spectrum.

[0046] According to the method described in Section 3, by measuring the infrared absorption spectrum by bringing the sample into direct contact with the diamond prism, data on impurities is prevented from being included in the infrared absorption spectrum, as would be the case, for example, if a transparent sheet were placed between the diamond prism and the sample.

[0047] (Item 4) In the method according to any one of items 1 to 3, the curing agent is an isocyanate curing agent, and the functional group of the curing agent in the sample is an isocyanate group.

[0048] According to the method described in item 4, the relationship between the infrared absorption spectrum of an isocyanate group and its curing temperature is known, and analysis is easy.

Claims

1. A method for observing the cured state of a paint in situ, comprising the steps of: bringing a sample of a mixture of a hardener and paint into contact with the surface of a diamond prism; measuring the infrared absorption spectrum of the sample while controlling the temperature of the sample, and obtaining time-series data on the amount of functional groups contained in the hardener in the sample from the obtained infrared absorption spectrum; and visualizing the relationship between time or temperature and the amount of functional groups contained in the hardener in the sample.

2. The method according to claim 1, wherein the step of measuring the infrared absorption spectrum of the sample while controlling the temperature of the sample and obtaining time series data on the amount of functional groups contained in the curing agent in the sample from the obtained infrared absorption spectrum includes measuring the infrared absorption spectrum of the sample while maintaining the temperature of the sample constant, and measuring the infrared absorption spectrum of the sample while changing the temperature of the sample at a predetermined rate of change, and obtaining time series data on the amount of functional groups contained in the curing agent in the sample from the obtained infrared absorption spectrum.

3. The method according to claim 1, further comprising the step of removing the cured material from the diamond prism after the infrared absorption spectrum measurement.

4. The method according to any one of claims 1 to 3, wherein the curing agent is an isocyanate curing agent, and the functional group of the curing agent in the sample is an isocyanate group.