DE102013002896B4 - Multi-layer coating film and method for its production - Google Patents

Abstract

A method for producing a multilayer coating film comprising: irradiating an adhesion promoter layer made of a first UV-curable coating material and a top layer made of a second UV-curable coating material with ultraviolet light such that the following conditions are satisfied so that the curing of the adhesive layer is completed after the curing of the top layer is completed, the conditions being represented by the following formulas (1) and (2): ΔT + TT '<TP' '0 ΔT where ΔT represents time , from which the ultraviolet irradiation of the adhesion promoter layer begins until the ultraviolet irradiation of the top layer begins, TT 'represents the time from which the ultraviolet irradiation of the top layer is started until a storage module GT' of the top layer has a 95% value GT ' 95a maximum value GT'max thereof is reached, and TP '' represents the time from which the Ultr Aviolet irradiation of the adhesion promoter layer is started until the loss modulus GP ″ of the adhesion promoter layer reaches a local maximum value GP ″ max thereof.

Description

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to a method for producing a multilayer coating film, which comprises an adhesion promoter layer (primer layer) and a top layer, using UV-curable coating materials.

State of the art

When a hard coat layer is formed on the surface of a substrate or the like, the uppermost surface is often coated with a radical UV curable coating material to form a top layer so as to impart scratch resistance. For example, the JP 00 2010 066 484 A a polarizer comprising protective layers with improved hardness. In the polarizer, a first coating layer containing a cured material made of a cationic photo-curable resin composition and a second coating layer containing a cured material made of a radical photo-curable resin composition are arranged in this order on at least one surface of a polarizing film. It is also disclosed that this polarizer by first applying a cationic photo-curable resin composition to at least one surface of a polarizing film, curing the resulting applied layer by means of cationic polymerization by an ultraviolet irradiation to form a first coating layer, then applying a radical photo-curable resin composition to the first coating layer and Curing the resulting applied layer by radical polymerization by ultraviolet irradiation to form a second coating layer.

However, when the lower first coating layer is cured and then the upper second coating layer is cured, internal stress is quickly generated in the second layer because the cured material of the radical photo-curable resin composition has a high volume shrinkage ratio. As a result, a crack or breakage occurs in some cases during coating film formation. Even if the occurrence of a crack or breakage is prevented during the production of the coating film, there is a case where the strength of the coating film is lowered due to a deterioration in weather resistance or the like and becomes smaller than an internal stress. In this case, the coating film tends to crack, break or peel off from the substrate, etc.

JP 00 2001 104 868 A relates to a hard coat film provided with a layer consisting essentially of a cationic polymerization based alicyclic epoxy compound on a film, and a hard coat layer consisting essentially of a radical polymerization curing resin based on ionizing radiation .

DE 197 51 481 A1 relates to a method for producing a multi-layer coating, in which a primer coating agent is applied to a substrate that may have been precoated with a primer and / or further coating agent and then a top coat consisting of a color and / or effect basecoat layer and a transparent clearcoat layer or a top coat of a pigmented single-layer topcoat are applied, wherein as filler, as basecoat and clearcoat as well as single-layer topcoat coating agents are used which contain binders curable by radical and / or cationic polymerization and these binders are cured by means of high-energy radiation.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-described problems in the conventional art. An object of the present invention is to provide a method for producing a coating film by coating with UV curable coating materials, which method enables an internal stress generated by the volume shrinkage of the coating film to be reduced.

The present inventors made intensive studies in order to achieve the above object. As a result, the inventors found the following fact. Particularly, in a case where a top layer and a bonding layer both made of a UV curable coating material are irradiated with ultraviolet light, when the ultraviolet irradiation is started on the top layer and the bonding layer in a manner that satisfies predetermined conditions are possible to cure the top layer and then to cure the adhesion promoter layer and consequently reduce the internal stress in the coating film. This finding has led to the completion of the present invention.

• Bestrahlen einer Haftvermittlerschicht, die aus einem ersten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, und einer Deckschicht, die aus einem zweiten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, mit Ultraviolettlicht, derart, dass die folgenden Bedingungen erfüllt sind, so dass das Aushärten der Haftvermittlerschicht nach dem Abschluss des Aushärtens der Deckschicht abgeschlossen wird, wobei die Bedingungen durch die folgenden Formeln (1) und (2) dargestellt sind:

$$\Delta {\text{T+T}}_{\text{T}}\text{'}<{\text{T}}_{\text{P}}\text{''}$$

$$0\le \Delta \text{T}$$

wobei ΔT die Zeit darstellt, ab der mit der Ultraviolettbestrahlung der Haftvermittlerschicht begonnen wird, bis mit der Ultraviolettbestrahlung der Deckschicht begonnen wird, T_T' die Zeit darstellt, ab der mit der Ultraviolettbestrahlung der Deckschicht begonnen wird, bis ein Speichermodul G_T' der Deckschicht einen 95 %-Wert G_T'₉₅ eines Maximalwerts G_T'_max davon erreicht, und T_P'' die Zeit darstellt, ab der mit der Ultraviolettbestrahlung der Haftvermittlerschicht begonnen wird, bis der Verlustmodul Gp'' der Haftvermittlerschicht einen lokalen Maximalwert G_P''_max davon erreicht.In particular, a method for producing a multilayer coating film of the present invention comprises:

• Irradiating an adhesion promoter layer, which is made from a first UV-curable coating material, and a cover layer, which is made from a second UV-curable coating material, with ultraviolet light, in such a way that the following conditions are met, so that the curing of the adhesion promoter layer after Completion of curing of the top layer is completed, the conditions being represented by the following formulas (1) and (2):

$$\Delta {\text{T + T}}_{\text{T}}\text{'}<{\text{T}}_{\text{P}}\text{''}$$

$$0\le \Delta \text{T}$$

where ΔT represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the ultraviolet irradiation of the top layer is started, T _T 'represents the time from which the ultraviolet irradiation of the top layer is started until a storage module G _T ' of the top layer reaches a 95% value G _{T '95 of} a maximum value G _T ' _max thereof, and T _P ″ represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the loss modulus Gp ″ of the adhesion promoter layer reaches a local maximum value G _P '' _max reached of it.

The first UV-curable coating material is preferably a cationic UV-curable coating material and the second UV-curable coating material is preferably a radical UV-curable coating material.

• Bilden der Haftvermittlerschicht, die aus dem ersten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, auf einem Substrat,
• dann Beginnen mit der Ultraviolettbestrahlung der Haftvermittlerschicht,
• anschließend Bilden der Deckschicht, die aus dem zweiten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, auf der Haftvermittlerschicht vor dem Abschluss ihres Aushärtens und
• Beginnen mit der Ultraviolettbestrahlung der Deckschicht, derart, dass die Bedingungen, die durch die vorstehenden Formeln (1) und (2) dargestellt sind, erfüllt sind.

The method for producing a multilayer coating film of the present invention preferably comprises:

• Forming the adhesion promoter layer, which is made from the first UV-curable coating material, on a substrate,
• then start with the ultraviolet irradiation of the adhesion promoter layer,
• then forming the top layer, which is made of the second UV-curable coating material, on the adhesion promoter layer before the completion of its curing and
• Start to irradiate the top layer with ultraviolet rays so that the conditions represented by the above formulas (1) and (2) are satisfied.

• Bilden der Haftvermittlerschicht, die aus dem ersten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, auf einem Substrat,
• anschließend Bilden der Deckschicht, die aus dem zweiten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, auf der Haftvermittlerschicht, und
• dann gleichzeitig Beginnen mit der Ultraviolettbestrahlung der Deckschicht und der Haftvermittlerschicht, derart, dass die Bedingungen, die durch die vorstehenden Formeln (1) und (2) dargestellt sind, erfüllt sind.

Further, the method for producing a multilayer coating film of the present invention preferably comprises:

• Forming the adhesion promoter layer, which is made from the first UV-curable coating material, on a substrate,
• then forming the top layer, which is made from the second UV-curable coating material, on the adhesion promoter layer, and
• then simultaneously starting the ultraviolet irradiation of the top layer and the adhesion promoter layer in such a way that the conditions represented by the above formulas (1) and (2) are satisfied.

Although it is not known exactly why an internal stress in the coating film is reduced by the method for producing a multilayer coating film of the present invention, the present inventors believe the following. In particular, as it is in the 1 is shown in the method for producing a multilayer coating film of the present invention when the The top layer and the adhesion promoter layer are irradiated with ultraviolet light, the adhesion promoter layer is not fully cured and is itself in a partially cured state at a point in time when the top coat is fully cured, since the curing speed of the first UV-curable coating material that forms the adhesion promoter layer is lower than that of the second UV-curable coating material which forms the top layer. For this reason, even if shrinkage occurs in the cover layer, the adhesion promoter layer moves like a fluid and deforms. This deformation reduces internal stress (residual stress) generated by the curing shrinkage of the top layer and makes it smaller. As a result, it is believed that curvature of the substrate, crack or breakage in the coating film, and peeling of the coating film are prevented. It should be noted that in the embodiment of the present invention disclosed in FIG 1 is shown, is started with the ultraviolet irradiation of the top layer and the adhesion promoter layer at the same time. However, even in a case where the ultraviolet irradiation of the cover layer and the adhesive layer is started at different times, an internal stress in the coating film is similarly reduced, as described above, if the ultraviolet irradiation is started while the adhesive layer is being started is at a point in time in a partially cured state at which the top layer is completely cured.

In contrast, as it is in the 2 is shown, in a case where a cover layer is formed on a substrate alone, when curing shrinkage occurs in the cover layer by ultraviolet irradiation, a shrinkage stress is generated in a direction of a surface of the cover layer (an interface between the cover layer and the substrate), which is inhibited by the substrate. Such internal stress (shrinkage stress) causes curvature of the substrate. In addition, if the internal stress is larger than the strength of the coating film, a crack or breakage occurs in the coating film; moreover, the coating film peeled off when the internal stress is greater than the interfacial strength between the cover layer and the substrate.

Furthermore, as stated in the 3 is shown, in a case where curing of a top layer and an adhesive layer is started simultaneously by ultraviolet irradiation and they are terminated at the same time, an internal stress (residual stress) is generated in the adhesive layer due to the difference in the coefficient of thermal expansion of a substrate and an internal stress (Residual stress) is generated in the cover layer due to the difference in a curing shrinkage ratio with respect to the adhesion promoter layer. Such internal stress (residual stress) causes curvature of the substrate, crack or breakage in the coating film, and peeling of the coating film.

It also deforms, as in the 4th is shown, in a case where curing of an adhesion promoter layer is completed and thereafter a cover layer is cured by ultraviolet irradiation, even if curing shrinkage occurs in the cover layer, the adhesion promoter layer does not. Thus, internal stress (residual stress) caused by the curing shrinkage of the top layer is not reduced. This leads to curvature of a substrate, a crack or break in the coating film, and peeling of the coating film.

The present invention makes it possible to obtain a multilayer coating film which has reduced internal stress even when the coating film is applied with UV curable coating materials.

Figure list

• 1 Fig. 13 is a schematic drawing showing a curing process for a multi-layer coating film in a method for producing a multi-layer coating film of the present invention.
• 2 Fig. 13 is a schematic drawing showing a curing process for a single layer coating film.
• 3 Fig. 13 is a schematic drawing showing a curing process in which all layers of a multi-layer coating film are cured at the same time.
• 4th Fig. 13 is a schematic drawing showing a curing process in which an adhesion promoter layer of a multilayer coating film is cured and thereafter a top layer thereof is cured.
• 5 Fig. 13 is a graph showing the curing behavior of a UV curable coating material by ultraviolet radiation.
• 6th FIG. 13 is a graph showing the curing behavior of an adhesion promoter layer and a top layer which have been irradiated with ultraviolet light.
• 7th Fig. 13 is a graph showing the curing behavior of an oxetane-based coating material (OXT-121) used in Examples and Comparative Examples.
• 8th Fig. 13 is a graph showing the curing behavior of an acrylate-based coating material (PETIA) used in Examples and Comparative Examples.
• 9 is a photograph showing a thin film (a PET film on which a multilayer coating film has been placed) obtained in Example 1 and a thin film (a PET film on which a single-layer coating film has been placed on it ) obtained in Comparative Example 1 shows.
• 10 Fig. 13 is a schematic drawing showing a curved state of a substrate in which a coating film is placed on a surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the present invention will be described in detail according to preferred embodiments thereof.

• Bestrahlen einer Haftvermittlerschicht, die aus einem ersten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, und einer Deckschicht, die aus einem zweiten UV-aushärtbaren Beschichtungsmaterial hergestellt ist, mit Ultraviolettlicht, derart, dass die folgenden Bedingungen erfüllt sind, so dass die Haftvermittlerschicht nach dem Aushärten der Deckschicht ausgehärtet werden kann, wobei die Bedingungen durch die folgenden Formeln (1) und (2) dargestellt sind:

$$\Delta {\text{T+T}}_{\text{T}}\text{'}<{\text{T}}_{\text{P}}\text{''}$$

$$0\le \Delta \text{T}$$

wobei ΔT die Zeit darstellt, ab der mit der Ultraviolettbestrahlung der Haftvermittlerschicht begonnen wird, bis mit der Ultraviolettbestrahlung der Deckschicht begonnen wird, T_T' die Zeit darstellt, ab der mit der Ultraviolettbestrahlung der Deckschicht begonnen wird, bis ein Speichermodul G_T' der Deckschicht einen 95 %-Wert G_T'95 eines Maximalwerts G_T'_max davon erreicht, und T_P'' die Zeit darstellt, ab der mit der Ultraviolettbestrahlung der Haftvermittlerschicht begonnen wird, bis der Verlustmodul G_P'' der Haftvermittlerschicht einen lokalen Maximalwert G_P''_max davon erreicht.A method for producing a multilayer coating film of the present invention is a method for obtaining a multilayer coating film of the present invention having reduced internal stress, the method comprising:

• Irradiating an adhesion promoter layer, which is made of a first UV-curable coating material, and a cover layer, which is made of a second UV-curable coating material, with ultraviolet light, in such a way that the following conditions are met, so that the adhesion promoter layer after curing Top layer can be cured, the conditions being represented by the following formulas (1) and (2):

$$\Delta {\text{T + T}}_{\text{T}}\text{'}<{\text{T}}_{\text{P}}\text{''}$$

$$0\le \Delta \text{T}$$

where ΔT represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the ultraviolet irradiation of the top layer is started, T _T 'represents the time from which the ultraviolet irradiation of the top layer is started until a storage module G _T ' of the top layer reaches a 95% value G _{T'95 of} a maximum value G _T ' _max thereof, and T _P ″ represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the loss modulus G _P ″ of the adhesion promoter layer reaches a local maximum value G _P '' _max reached.

First, a curing behavior of a UV-curable coating material by an ultraviolet radiation is described. The 5 Fig. 13 is a graph showing changes in the storage modulus G 'and the loss modulus G''of a UV curable coating material irradiated with ultraviolet light, that is, a curing behavior of the UV curable coating material by the ultraviolet radiation. Like it in the 5 is shown, when the curing of the UV-curable coating material begins by ultraviolet radiation, the storage modulus G 'increases. Finally, the storage modulus G 'becomes constant and curing is complete. On the basis of such a change in the storage modulus G ', it is possible to determine a state (a liquid state, a solid state or a mixed state thereof) of the UV-curable coating material and to determine a point in time when curing starts or a point in time when when curing is complete. It should be noted that usually when the curing is completed, curing shrinkage occurs and heat is generated by the ultraviolet irradiation or the curing; as a result, the storage modulus G 'may not become exactly constant. For this reason, in the present invention, a point in time when a measured storage _{modulus G 'reaches a 95% value G T'95 of} the maximum value G _T ' _max thereof is regarded as a point in time at which curing is completed; and a time from the Ultraviolet irradiation is started until the storage modulus G 'G _T ' _{reaches 95} , set as the curing completion time T '.

Furthermore, the loss modulus G ″ is the product of the angular velocity ω of a sinusoidal vibration exerted on a sample and the dynamic viscosity coefficient η ′ of the sample and represents the viscous state of the sample. As shown in FIG 5 As shown, when the curing reaction of the UV curable coating material is started by the ultraviolet radiation, monomers in the coating material bind to each other. As a result, the molecular weight is increased and the viscosity of the coating material is also increased. The loss modulus G ″ is thus increased. When a certain amount or more of a crosslinking structure is formed by the progress of the curing reaction, the coating material loses fluidity and the loss modulus G "shows a local maximum value G" _max . The coating material then deforms primarily elastically and the loss modulus G ″ increases. In other words, the UV-curable coating material behaves like a viscous material from the beginning of the ultraviolet irradiation until the loss modulus G ″ reaches the local maximum value G ″ _max. This makes it possible to reduce internal stress due to the deformation. In the present invention, the time from which the ultraviolet irradiation is started until the loss modulus G ″ reaches the local maximum value G ″ _{max is} referred to as the maximum time of deformability T ″.

In a UV-curable coating material, the crosslinking reaction takes place from the beginning of the ultraviolet radiation and even after the end of the maximum deformability time T '' and thus the viscous deformation contributes less and less to the deformation of the coating material (the elastic deformation becomes even more pronounced) . At a point in time at which the contribution of the viscous deformation becomes almost constant, the storage modulus G 'becomes constant and the curing is completed. For this reason, in the case of UV-curable coating materials of the same type, the curing completion time T 'is always longer than the maximum time of deformability T' '.

It should be noted that the maximum time of deformability T "and the curing completion time T 'of a UV-curable coating material from changes in the storage modulus G' and the loss modulus G" obtained by a viscoelasticity measurement compared with the ultraviolet light in the same wavelength range and irradiation intensity (amount of irradiation per unit time and unit area) as the coating material irradiated at the time of forming the multilayer coating films can be calculated.

Next, the UV curable coating materials used in the present invention will be described. In the method for producing a multilayer coating film of the present invention, the maximum time of deformability T _P ″ of the first UV-curable coating material forming the adhesive layer and the curing completion time T _T 'of the second UV-curable coating material forming the top layer satisfy a condition represented by the following formula (3): $${\text{T}}_{\text{T}}\text{''}<{\text{T}}_{\text{P}}\text{''}$$

If T _T 'T _P ″, the coating material in the adhesion promoter layer loses its fluidity before the curing reaction in the top layer is complete. This makes it difficult to use the deformation of the adhesion promoter layer to relieve internal stress generated by the curing shrinkage of the cover layer.

Typical examples of the UV curable coating materials include cationic UV curable coating materials, radical UV curable coating materials and the like. In general, the curing speed of radical UV curable coating materials is higher than that of cationic UV curable coating materials and the curing completion time T 'of radical UV curable coating materials is shorter than the maximum time of deformability T ″ of cationic UV curable coating materials. Accordingly, in the method for producing a multilayer coating film of the present invention, it is preferable to use a cationic UV curable coating material as the first UV curable coating material forming the adhesion promoter layer, and a radical UV curable coating material as the second UV -to use the curable coating material that forms the top layer.

It is possible to use, as the cationic UV-curable coating material, a known cationic UV-curable coating material such as an UV-curable epoxy-based coating material, a UV-curable oxetane-based coating material, or a UV-curable vinyl-ether-based coating material. Further, the radical UV curable coating material is not particularly limited, and a known radical UV curable coating material such as an UV curable acrylate coating material or an UV curable coating material based on an unsaturated polyester can be used.

Next, the method for producing a multilayer coating film of the present invention will be described using a graph showing the curing behavior of the adhesive layer and the cover layer. The 6th is a graph showing the changes in the storage modulus G 'and the loss modulus G "of the adhesion promoter _{layer, which is made from the first UV-curable coating material with the maximum time of deformability T P} ", and the cover layer, which is made from the second UV- curable coating material with the curing completion time T _T ', which are irradiated with ultraviolet light, shows.

In the method for producing a multi-layer coating film of the present invention, first, the adhesion promoting layer made of the first UV curable coating material having the maximum time of deformability T _P ″ is formed on a substrate or the like, and optionally dried. Then it will be like it in the 6th is shown, started with an ultraviolet irradiation at time t _P0 in order to initiate a curing reaction in the adhesion promoter layer. Next, the cover layer, which is produced from the second UV-curable coating material which has the curing completion time T _T ', is formed on the adhesion promoter layer before its curing is complete, and optionally dried. After that, as it is in the 6th is shown, started with an ultraviolet irradiation at time t _T0 in order to initiate a curing reaction in the top layer. Thereafter, by continuing the ultraviolet irradiation, the curing reaction proceeds. The curing reaction of the second UV-curable coating material is completed at a point in time when the time t _T0 + T _T 'has passed and the curing shrinkage of the top layer has also ended.

Here, a method of setting the time t _T0 when the ultraviolet irradiation of the top layer is started will be described. In the method for producing a multilayer coating film of the present invention, in order to utilize the deformation of the adhesive layer so that an internal stress generated by the curing shrinkage of the cover layer is reduced, the adhesive layer must be subjected to a viscous deformation so that it can Dimensional change follows due to the curing shrinkage of the top layer. This can be achieved by maintaining the fluidity of the coating material forming the adhesion promoter layer at time t _T0 + T _T 'when the curing reaction in the top layer is complete. For this reason, it is necessary that the period of ultraviolet irradiation of the adhesion promoter layer at the time t _T0 + T _T 'does not exceed the maximum deformability time T _P ″ of the adhesion promoter layer. In other words, the time t _T0 + T _T 'is required to be before the time t _P0 + T _P ″. Thus the relation t _T0 + T _T '<t _P0 + T _P ''is fulfilled. This formula can be changed to t _T0 - t _P0 + T _T '<T _P ''. Since t _T0 -t _{P0 is} the time ΔT from the beginning of the ultraviolet irradiation of the adhesion promoter layer to the beginning of the ultraviolet irradiation of the top layer, inserting ΔT into the formula leads to ΔT + T _T '<T _P ", ie to the above formula (1). Further, in the method for producing a multilayer coating film of the present invention, the formula (2): ΔT (= t _T0 - t _P0 ) ≥ 0 is always satisfied when the ultraviolet irradiation of the cover layer is carried out simultaneously with the ultraviolet irradiation of the adhesive layer or after the start the ultraviolet irradiation of the adhesion promoter layer is started. Accordingly, in the method for producing a multilayer coating film of the present invention for reducing an internal stress caused by curing shrinkage of the top layer, the time t _T0 when the ultraviolet irradiation of the top layer is started must be set so that the conditions which are represented by formulas (1) and (2) are satisfied.

As described above, by further continuing the ultraviolet irradiation after the time t _T0 + T _T 'has passed, the curing reaction proceeds in the adhesive layer, and the adhesive layer loses its fluidity at a point of time when the time t _PO + T _P '' has passed. The curing reaction in the adhesion promoter layer is then also completed.

In this way, in the method for producing a multilayer coating film of the present invention, the adhesive layer is formed on a substrate or the like, and then ultraviolet irradiation is started at time t _P0 . The top layer is then formed on the adhesion promoter layer before it has cured. The time t _T0 when with the ultraviolet irradiation The top layer is started is determined so that the conditions represented by formulas (1) and (2) are satisfied, and the ultraviolet irradiation of the top layer is initiated. This makes it possible to obtain the multi-layer coating film of the present invention which has reduced internal stress generated by the curing shrinkage of the cover layer.

Here, the method for producing a multilayer coating film of the present invention was described with reference to FIG 6th described, taking as an example the case in which the ultraviolet irradiation of the adhesion promoter layer is started and then the ultraviolet irradiation of the top layer is started. However, the method for producing a multi-layer coating film of the present invention is not limited to that disclosed in US Pat 6th Embodiment shown limited. For example, the ultraviolet irradiation of the adhesion promoter layer and the top layer can also be started at the same time.

Specifically, in the method for producing a multilayer coating film of the present invention, first, the adhesion promoting layer made of the first UV curable coating material having the maximum time of deformability T _P ″ is formed on a substrate or the like and dried if necessary. The top layer, which is produced from the second UV-curable coating material with the maximum curing completion time T _T ', is then formed on the resulting adhesion-promoting layer and optionally dried. Thereafter, the ultraviolet irradiation of the adhesion promoter layer and the top layer is started at the same time. In this case, since the time from the start of ultraviolet irradiation of the adhesion promoter layer to the start of ultraviolet irradiation of the cover layer is ΔT = 0, the condition represented by (1) is through the use of the first and second UV curable coating materials satisfying the condition represented by the formula (3). This makes it possible to obtain the multilayer coating film of the present invention which has reduced internal stress.

In the method for producing a multilayer coating film of the present invention, an ultraviolet irradiation device used for curing the UV curable coating materials is not particularly limited as long as the device can emit ultraviolet light having a wavelength of 250 to 400 nm. It is possible to use a known ultraviolet irradiation device such as an ultra-high pressure mercury lamp, a high pressure mercury lamp, a low pressure mercury lamp, an ultraviolet laser, a xenon lamp or an alkali metal lamp. The ultraviolet irradiation intensity (irradiation amount per unit time and area) and the ultraviolet irradiation period are not particularly limited, but are adjusted depending on the kinds of UV curable coating materials to be used so that the coating film can be completely cured.

The substrate used in the present invention is not particularly limited. Examples thereof include polycarbonate substrates (sheets, films, or the like), polyethylene terephthalate substrates (sheets, films, or the like), and the like. Furthermore, the method for forming the adhesion promoter layer and the cover layer on such a substrate is not particularly limited. It is possible to use a known coating method such as a solution casting method, an application method by means of a knife coater, a roll coater, a gravure coater or the like, or a method such as dip coating, spin coating or spray coating.

[Examples]

In the following, the present invention will be described more specifically based on examples and comparative examples. However, the present invention is not limited to the following examples. It should be noted that the methods for preparing cationic UV curable coating materials and radical UV curable coating materials used in Examples and Comparative Examples and the method for measuring viscoelasticity are described below.

(Production of a cationic UV-curable coating material)

An oxetane-based coating material (OXT-121) or a cyclic epoxy-based coating material (2021P) was prepared by mixing 100 parts by mass of a cationic UV-curable monomer of xylylene bisoxetane ("OXT-121 (XDO)" manufactured by Toagosei Co., Ltd.), represented by the following formula (i) (n = 1 to 3) and 3,4-epoxycyclohexenylmethyl-3 ', 4'-epoxycyclohexene carboxylate (“CELLOXIDE 2021P” manufactured by DAICEL-CYTEC Company, Ltd.) represented by the following Formula (ii) with 5 PHR of a cationic photopolymerization initiator based on iodonium salt (a 3: 1 mixture of (4-methylphenyl) [4- (2-methylpropyl) phenyl] iodonium hexafluorophosphate and propylene carbonate as solvent, "IRGACURE 250", manufactured by BASF SE ) represented by the following formula (iii).

(Production of a radical UV-curable coating material)

An acrylate-based coating material (DPHA), an acrylate-based coating material (TMPTA) or an acrylate-based coating material (PETIA) was prepared by mixing 50 parts by mass of one of the radical UV-curable monomers of dipentaerythritol hexaacrylate ("ARONIX M-402 (DPHA)", manufactured by Toagosei Co., Ltd.) represented by the following formula (iv), trimethylolpropane triacrylate (“TMPTA” manufactured by DAICEL-CYTEC Company, Ltd.) represented by the following formula (v), and pentaerythritol triacrylate (“PETIA “, Manufactured by DAICEL-CYTEC Company, Ltd.) represented by the following formula (vi) with 50 parts by mass of 1-methoxy-2-propanol (Wako Special Grade, manufactured by Wako Pure Chemical Industries, Ltd.) as a solvent 5 PHR of a radical UV polymerization initiator (1-hydroxycyclohexylphenyl ketone, “IRGACURE 184” manufactured by BASF SE) represented by the following formula (vii).

(Measurement of viscoelasticity)

A coating material was applied to a parallel plate (diameter: 8.0 mm) of a viscoelasticity measuring device (“ARES-G2” (strain control, shear strain type) manufactured by TA Instruments) so that the thickness after curing was 50 µm. The distance between measuring points was set to 50 µm. Using a point UV / visible light curing system ("OmniCure ^™ 2000 series", manufactured by Lumen Dynamics Group Inc., light source: 200 W high pressure mercury vapor lamp), the coating film was exposed to ultraviolet light (wavelength range: 250 to 600 nm, primary peak wavelength : 365 nm) at an irradiation amount per unit time and area of 55 mW / cm ² for 40 seconds until the amount of light irradiation reached 2200 mJ / cm ² . Thereafter, the coating film was left to stand. During this time, the storage modulus G ′ and the loss modulus G ″ of the coating material (coating film) were measured under the conditions of an oscillation mode, temperature: 25 ° C., measurement frequency: 10 Hz and measurement elongation: 1%.

7th shows the storage modulus G 'and the loss modulus G''of the oxetane-based coating material (OXT-121) (the time at which the ultraviolet irradiation was started was set to 0 seconds). As from the in the 7th As can be seen, in the case of the oxetane-based coating material (OXT-121), which is a cationic UV-curable coating material, both the storage modulus G 'and the loss modulus G''did not change immediately after the ultraviolet irradiation. The storage modulus G 'began to increase 200 seconds after the start of the ultraviolet irradiation. After 370 seconds, the loss module G '' showed the local maximum value G '' _max . In addition, the other cationic UV-curable coating material also showed a similar behavior of the storage modulus G ′ and the loss modulus G ″. The time T ″ from which the ultraviolet irradiation was started until the loss modulus G ″ reached the local maximum value G ″ _max was determined for each of the cationic UV-curable coating materials. Table 1 shows the results. [Table 1] Cationic UV-curable coating material T '' (s) OXT-121 370 2021P 2 × 10 ⁴

Furthermore, the 8th the storage modulus G 'and the loss modulus G''of the acrylate-based coating material (PETIA) (the time at which the ultraviolet irradiation was started was set to 0 seconds). As from the in the 8th As can be seen, in the case of the acrylate-based coating material (PETIA), which is a radical UV-curable coating material, the loss modulus G ″ showed the local maximum value G ″ _max almost simultaneously (0.2 seconds later) the beginning of ultraviolet radiation. The storage modulus G 'became constant 15 seconds after the start of the ultraviolet irradiation. Furthermore, the other free-radical UV-curable coating materials also showed a similar behavior of the storage modulus G ′ and the loss modulus G ″. The time T 'until the storage modulus G' reached a 95% value G _{'95 of} a maximum value G' _max thereof was determined for each of the radical UV-curable coating materials. Table 2 shows the results. [Table 2] Radical UV-curable coating material T '(s) DPHA 1.87 TMPTA 3.45 PETIA 14.6

(Example 1)

A surface of a polyethylene terephthalate film (PET film) (“LUMIRROR S10 # 50”, manufactured by Toray Industries, Inc., width: 3 cm, thickness: 50 µm) was washed with 2-propanol (Wako 1st Grade, manufactured by Wako Pure Chemical Industries, Ltd.). Then, using a doctor blade coater (“Select-Roller A-bar”, manufactured by OSG SYSTEM PRODUCTS Corporation, doctor blade: OSP-15, wire doctor model (equivalent to: # 6,6)), the oxetane-based coating material (OXT-121) was applied to the PET film applied in such a way that the thickness after curing was 6 µm. An adhesion promoter layer was thereby formed. Furthermore, using the knife coating device, the acrylate-based coating material (DPHA) was applied to the adhesion promoter layer in such a way that the thickness after curing was 6 μm. A cover layer was thereby formed. Thereafter, using a UV curing unit (1 kW high pressure mercury lamp ("Handy 1000", manufactured by SEN Lights Corporation)), the uncured multilayer coating film thus obtained was subjected to ultraviolet light (wavelength range: 250 to 600 nm, primary peak wavelength: 365 nm ) irradiated at an irradiation amount per unit time and area of 55 mW / cm ² for 40 seconds until the amount of light irradiation reached 2200 mJ / cm ² . As a result, the cover layer and the adhesive layer were completely cured, and consequently a multi-layer transparent coating film was obtained.

(Examples 2 and 3)

Multilayer transparent coating films were obtained in the same manner as in Example 1 except that the acrylate-based coating material (TMPTA) or the acrylate-based coating material (PETIA) was used in place of the acrylate-based coating material (DPHA).

(Example 4)

One surface of a PET film (“LUMIRROR S10 # 50”, manufactured by Toray Industries, Inc., width: 3 cm, thickness: 50 µm) was washed with 2-propanol (Wako 1st Grade, manufactured by Wako Pure Chemical Industries, Ltd.). Then, using the knife coater, the oxetane-based coating material (OXT-121) was applied to the PET film such that the thickness after curing was 6 μm. An adhesion promoter layer was thereby formed. The adhesion promoter layer was with Ultraviolet light (wavelength range: 250 to 600 nm, primary peak wavelength: 365 nm) is irradiated at an irradiation ^{amount per unit time and area of 56.4 mW / cm 2} using the UV curing unit for 40 seconds until the amount of light irradiation is 2256 mJ / cm ² reached. As a result, the adhesion promoter layer was partially cured.

Next, using the knife coating device, the acrylate-based coating material (DPHA) was applied to the partially cured adhesion promoter layer in such a way that the thickness after curing was 6 μm. A cover layer was thereby formed. The top layer was irradiated with ultraviolet light (wavelength range: 250 to 600 nm, primary peak wavelength: 365 nm) at an irradiation ^{amount per unit time and area of 56.4 mW / cm 2} using the UV curing unit for 40 seconds until the amount of light irradiation was 2256 mJ / cm ² reached. As a result, the cover layer and the adhesion promoter layer were completely cured, and thus a multi-layer transparent coating film was obtained. It should be noted that the time ΔT from the start of ultraviolet irradiation of the adhesion promoter layer to the start of ultraviolet irradiation of the top layer was 300 seconds.

(Examples 5 and 6)

Transparent multilayer coating films were obtained in the same manner as in Example 4 except that the acrylate-based coating material (TMPTA) or the acrylate-based coating material (PETIA) was used in place of the acrylate-based coating material (DPHA).

(Examples 7 to 9)

Transparent multilayer coating films were each obtained in the same manner as in Examples 4 to 6 except that the cyclic epoxy-based coating material (2021P) was used in place of the oxetane-based coating material (OXT-121).

(Comparative example 1)

One surface of a PET film (“LUMIRROR S10 # 50”, manufactured by Toray Industries, Inc., width: 3 cm, thickness: 50 µm) was washed with 2-propanol (Wako 1st Grade, manufactured by Wako Pure Chemical Industries, Ltd.). Then, using the knife coater, the acrylate-based coating material (DPHA) was applied to the PET film in such a way that the thickness after curing was 6 μm. A cover layer was thereby formed. The top layer was irradiated with ultraviolet light (wavelength range: 250 to 600 nm, primary peak wavelength: 365 nm) at an irradiation ^{amount per unit time and area of 55 mW / cm 2} using the UV curing unit for 40 seconds until the amount of light irradiation was 2200 mJ / cm ² reached. As a result, the cover layer was completely cured, and consequently a single-layer transparent coating film was obtained.

(Comparative Examples 2 and 3)

Transparent single-layer coating films were obtained in the same manner as in Comparative Example 1 except that the acrylate-based coating material (TMPTA) or the acrylate-based coating material (PETIA) was used in place of the acrylate-based coating material (DPHA).

(Comparative Examples 4 to 9)

Transparent multilayer coating films were each obtained in the same manner as in Examples 4 to 9, except that the adhesive layer formed by applying in such a way that the thickness after curing was 6 µm with Ultraviolet light was irradiated at an irradiation amount per unit time and area of 55 mW / cm ² for 40 seconds until the amount of light irradiation reached 2200 mJ / cm ² , and then the coating layer was formed thereon after the resulting adhesive layer was allowed to stand for 1 month , whereupon the ultraviolet irradiation was carried out. It should be noted that the time ΔT from the start of ultraviolet irradiation of the adhesion promoter layer to the start of ultraviolet irradiation of the top layer was 2.59 × 10 ⁶ seconds.

<Measurement of the film thickness>

The thickness of the coating films thus obtained was measured using a digital micrometer (“M-30”, manufactured by Sony Magnescale Co., Ltd.). Each of the multilayer coating films obtained in Examples 1 to 9 and Comparative Examples 4 to 9 had a thickness of 12 ± 2 µm. In addition, each of the single-layer coating films obtained in Comparative Examples 1 to 3 had a thickness of 6 ± 1 µm.

<Evaluation of Internal Stress in Coating Films>

The 9 FIG. 13 shows the PET sheet on which the multilayer coating film obtained in Example 1 was arranged and the PET sheet on which the single layer coating film obtained in Comparative Example 1 was arranged. Like it in the 9 As shown, in the PET sheet on which the multilayer coating film obtained in Example 1 was placed, the substrate on the coating film side was slightly curved but substantially flat. This was also the case with the PET sheets on which the multilayer coating films obtained in Examples 2 to 9 were placed, respectively. In contrast, in the PET sheet on which the single-layer coating film was arranged, which was obtained in Comparative Example 1, the substrate on the coating film side was largely curved. This was also the case with the PET sheets each having the single-layer coating films obtained in Comparative Examples 2 and 3 and the PET sheets each having the multi-layer coating films shown in FIG Comparative Examples 4 to 9 have been obtained. Such a curvature of the PET sheet, which was the substrate, was attributed to an internal stress in the coating film formed thereon.

(vgl. „Tokou Seimaku ni okeru Micchaku Secchakusei no Seigyo to sono Hyouka“ (Einstellung und Bewertung der Klebe- und Anhafteigenschaften bei der Beschichtungs- und Filmbildung), Technical Information Institute Co., Ltd., 2005, Seite 331, Kapitel 3, Abschnitt 6, 2-2-2 „Tawami Sokutei Hou“ (Biegemessverfahren)). Dabei stellt E₁ den Young'schen Modul des Substrats dar, W stellt die Breite des Substrats dar, p stellt den Krümmungsradius des gekrümmten Substrats dar, H stellt die Gesamtdicke des Substrats und des Beschichtungsfilms dar und h₁ stellt die Dicke des Substrats dar. Wie es aus dieser Formel ersichtlich ist, ist die innere Spannung P des Beschichtungsfilms umgekehrt proportional zu dem Krümmungsradius p. Mit anderen Worten bedeutet dies, dass die innere Spannung P des Beschichtungsfilms umso geringer ist, je größer der Krümmungsradius p einer PET-Folie ist, auf welcher der Beschichtungsfilm angeordnet ist.The internal stress of the coating film can be evaluated by the radius of curvature of the PET sheet on which the coating film is placed. In particular, the PET film serving as the substrate does not plastically deform but has elasticity. Accordingly, when a uniform coating film is formed on such a substrate, residual stress of the coating film 1 causes curvature of the substrate 2 on the side of the coating film, as shown in FIG 10 is shown. The internal stress P of the coating film can be determined according to the following formula: $$P=\left(\frac{{\mathrm{E.}}_1\mathrm{<figure-callout\; id="1"\; label="W.\; H"\; filenames="DE102013002896B4\_0025.png"\; state="\{\{state\}\}">W.</figure-callout>}{H}_{}{}_1{}^3}{\mathrm{6th}\rho H}\right)\left(1+\frac{{\mathrm{<figure-callout\; id="1"\; label="H"\; filenames="DE102013002896B4\_0025.png"\; state="\{\{state\}\}">H</figure-callout>}}_1{}^2}{3H_2}\right)$$

(cf. "Tokou Seimaku ni okeru Micchaku Secchakusei no Seigyo to sono Hyouka" (Adjustment and evaluation of the adhesive and adhesive properties in the coating and film formation), Technical Information Institute Co., Ltd., 2005, page 331, chapter 3, Section 6, 2-2-2 “Tawami Sokutei Hou” (bending measurement method)). Here, E _{1 represents} the Young's modulus of the substrate, W represents the width of the substrate, p represents the radius of curvature of the curved substrate, H represents the total thickness of the substrate and the coating film, and h ₁ represents the thickness of the substrate. As can be seen from this formula, the internal stress P of the coating film is inversely proportional to the radius of curvature p. In other words, the greater the radius of curvature p of a PET film on which the coating film is arranged, the lower the internal stress P of the coating film.

A circle was directly fitted to the obtained coating film, and the radius of curvature p was determined by extrapolation. Table 3 shows the results. Table 3 Adhesion promoter layer Top layer UV irradiation ΔT (s) ΔT + T _T '(s) T _P '' (s) Radius of curvature (cm) Ex. 1 OXT-121 DPHA once 0 1.87 370 3.0 Ex. 2 OXT-121 TMPTA once 0 3.45 370 4.3 Ex. 3 OXT-121 PETIA once 0 14.6 370 6.0 Ex. 4 OXT-121 DPHA twice 300 302 370 1.0 Ex. 5 OXT-121 TMPTA twice 300 303 370 1.5 Ex. 6 OXT-121 PETIA twice 300 315 370 3.0 Ex. 7 2021 P. DPHA twice 300 302 2 × 10 ⁴ 1.0 Ex. 8 2021P TMPTA twice 300 303 2 × 10 ⁴ 3.0 Ex. 9 2021P PETIA twice 300 315 2 × 10 ⁴ 3.0 Comp. Ex. 1 - DPHA once 0 1.87 - 0.7 Comp. Ex. 2 - TMPTA once 0 3.45 - 1.0 Comp. Ex. 3 - PETIA once 0 14.6 - 1.5 Comp. Ex. 4th OXT-121 DPHA twice 2.59 × 10 ⁶ 2.59 × 10 ⁶ 370 0.7 Comp. Ex. 5 OXT-121 TMPTA twice 2.59 × 10 ⁶ 2.59 × 10 ⁶ 370 1.0 Comp. Ex. 6th OXT-121 PETIA twice 2.59 × 10 ⁶ 2.59 × 10 ⁶ 370 1.0 Comp. Ex. 7th 2021 P. DPHA twice 2.59 × 10 ⁶ 2.59 × 10 ⁶ 2 × 10 ⁴ 0.7 Comp. Ex. 8th 2021 P. TMPTA twice 2.59 × 10 ⁶ 2.59 × 10 ⁶ 2 × 10 ⁴ 1.0 Comp. Ex. 9 2021 P. PETIA twice 2.59 × 10 ⁶ 2.59 × 10 ⁶ 2 × 10 ⁴ 1.0

As can be seen from the result shown in Table 3, compared with the coating films in which the same type of cover layer was used, it was found that when the adhesive layer and the cover layer were irradiated with ultraviolet light under the condition that ΔT + T _T '<T _P ''satisfies (Examples 1 to 9), the radius of curvature of the multi-layer coating film was larger than that of the single-layer coating film (Comparative Examples 1 to 3), and the internal stress of the multi-layer coating film was smaller. In other words, it was verified that the method for producing a multi-layer coating film of the present invention ensured the fluidity of the adhesive layer even at a time when the curing of the coating layer was completed and that the internal stress in the coating film was compared to the single layer coating film was decreased.

Further, when the adhesive layer and the cover layer were irradiated with ultraviolet light under the condition of ΔT + T _T '> T _P "(Comparative Examples 4 to 9), the radius of curvature of the multilayer coating film was equal to or smaller than that of the single-layer coating film (Comparative Examples 1 to 3) and the internal stress in the coating film was not decreased. This is presumably due to the fact that the adhesion promoter layer has already lost its fluidity at a point in time at which the curing of the cover layer was completed under this condition.

As described above, the present invention makes it possible to obtain a multilayer coating film with reduced internal stress.

Therefore, the multilayer coating film of the present invention has less internal stress and is unlikely to have a curvature of a substrate, a crack or break in the coating film, and peeling of the coating film. Accordingly, coated articles comprising such a multi-layer coating film are useful as parts or components for automobiles such as automobiles, trucks, buses and motorcycles, or for other purposes.

Claims (6)

A method for producing a multilayer coating film, comprising: irradiating an adhesion promoter layer made of a first UV-curable coating material and a top layer made of a second UV-curable coating material with ultraviolet light such that the following conditions are met so that the curing of the adhesive layer is completed after the curing of the top layer is completed, the conditions being represented by the following formulas (1) and (2): $$\Delta {\text{T + T}}_{\text{T}}\text{'}<{\text{T}}_{\text{P}}\text{''}$$

$$0\le \Delta \text{T}$$

where ΔT represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the ultraviolet irradiation of the top layer is started, T _T 'represents the time from which the ultraviolet irradiation of the top layer is started until a storage module G _T ' of the top layer reaches a 95% value G _{T '95 of} a maximum value G _T ' _max thereof, and T _P ″ represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the loss modulus G _P ″ of the adhesion promoter layer reaches a local maximum value G _P '' _max reached. A method for producing a multilayer coating film according to Claim 1 wherein the first UV curable coating material is a cationic UV curable coating material and the second UV curable coating material is a free radical UV curable coating material. A method for producing a multilayer coating film according to any one of Claims 1 and 2 comprising: forming the adhesion promoter layer made from the first UV curable coating material on a substrate, then starting the ultraviolet irradiation of the adhesion promoter layer, then forming the top layer made from the second UV curable coating material on the adhesion promoter layer completing their curing and starting ultraviolet irradiation of the top layer so that the conditions represented by the above formulas (1) and (2) are satisfied. A method for producing a multilayer coating film according to any one of Claims 1 and 2 comprising: forming the adhesion promoter layer made of the first UV curable coating material on a substrate, then forming the top layer made of the second UV curable coating material on the adhesion promoter layer, and then simultaneously starting the ultraviolet irradiation of the Cover layer and the adhesion promoter layer such that the conditions represented by the above formulas (1) and (2) are met. A multilayer coating film obtained by: irradiating an adhesion promoter layer made of a first UV-curable coating material and a top layer made of a second UV-curable coating material with ultraviolet light such that the following conditions are met, so that the curing of the adhesive layer is completed after the curing of the top layer is completed, the conditions being represented by the following formulas (1) and (2): $$\Delta {\text{T + T}}_{\text{T}}\text{'}<{\text{T}}_{\text{P}}\text{''}$$

$$0\le \Delta \text{T}$$

where ΔT represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the ultraviolet irradiation of the top layer is started, T _T 'represents the time from which the ultraviolet irradiation of the top layer is started until a storage module G _T ' of the top layer reaches a 95% value G _{T'95 of} a maximum value G _T ' _max thereof, and T _P ″ represents the time from which the ultraviolet irradiation of the adhesion promoter layer is started until the loss modulus G _P ″ of the adhesion promoter layer reaches a local maximum value G _P '' _max reached. Multi-layer coating film after Claim 5 wherein the first UV-curable coating material is a cationic UV-curable coating material and the second UV curable coating material is a radical UV curable coating material.

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