UV-curable basecoat

Abstract

The present invention relates to a process which makes possible to obtain a UV-curable coating with a high TiO2 content. The strong broad absorption of TiO2 in the UV region which prevent the photons to reach into deeper layers of the film is circumvented by the introduction of fibers material in the formulation without the requirement of photosensitizer. Potassium titanate fibers affects the translucency of the composition and allows a decrease in absorbance in correlation with the (potassium titanate fibers / TiO2) x100 ratio.

Description

[0001] UV-CURABLE BASECOAT

[0002] Technical Field of the Invention

[0003] The present invention relates to a high TiCh content ultraviolet-curable coating composition to be able to used as a basecoat. The present invention relates to the possibility to decrease the strong broad absorption of TiCh in the UV region by introducing fibers materials, in UV- curing formulation.

[0004] Known State of the At (Prior Art)

[0005] Ultraviolet energy cured coatings have been used for decades in industries as diverse as automotive, electronics, printing, packaging, medical converting, flooring, furniture, and metal pipe and tube. The commonality of all these applications is that UV curing technology delivers superior economic performance.

[0006] Cationic and free radical are the two most popular mechanisms for UV curing. During a UV- curing process, radical or cationic species are generated by the interaction of UV light with a suitable photoinitiator.

[0007] Simultaneous polymerization of radical and cationic photoinitiators can yield hybrid polymer. The monomer mixtures used in the hybrid photopolymerization usually contain two main types of monomers, e.g., acrylates and epoxides. The acrylates, which undergo free-radical polymerization, exhibit high reaction rates and offer a large selection of monomers and initiators. However, the inhibition of free-radical polymerization by atmospheric oxygen is a significant challenge, leading to a decrease in mechanical performance and tacky surfaces. On the other end, the epoxides which undergo cationic polymerization, exhibit less shrinkage and also undergo post-polymerization in the dark leading to higher conversion, robust mechanical performance and good adhesion to various substrates. Especially, epoxides do not suffer from oxygen inhibition and exhibit low toxicity and shrinkage but epoxides are less reactive than acrylate, moreover water or moisture may affect their cationic process. Such hybrid systems are attractive as distinct advantages associated with each constituent can be engineered to the final material.

[0008] Major limitation in UV curing technology is that UV energy needs to penetrate the coating down to the substrate to achieve full cure. This is easily achieved with clearcoats, but is a significant challenge with pigmented systems relating to inefficiency in light transmission. The pigments, and other constituents in coating, absorb some of the UV energy making through-cure a challenge to overcome. Consequently, many applications were limited to clearcoats or thin pigmented coatings.

[0009] Especially, TiCh the most important white pigment for paint and coating exhibit intrinsically strong broad absorption in UV region. Consequently, in high content TiCh pigmented formulation, the radiation absorbance of the photoinitiator is decreased due to the absorbance and scattering of the radiation by TiCh. In other words, there is a competition between the photoinitiator and the pigment in absorbing the light. Thus, not enough photons are able to reach into deeper layers of the film. The result is a film that is polymerized at the surface but not at the bottom.

[0010] Work has been done on development of TiCh-based uv-cured coatings. These coatings are cure by free radical, cationic or hybrid mechanism, but in all case the presence of photosensitizers such as isopropyl thioxanthone (ITX) which absorbs strongly at 386 mm is required for full-curing. When activated, the photosensitizer transmits energy to the photoinitiator and photopolymerization can start. Especially, photosensitizers are prerequist with cationic photoinitiators which absorb in a spectral window where almost all the radiation is absorbed or scattered by TiCh.

[0011] For example in JP4036007B2, a cationic white formulation is described containing 10% by weight or more of TiCh to achieved a coating with a film thichness of 5p after curing. If TiCh is less than 10% by weight, the coloring power is poor. The coating composition of the present invention further contain a photosensitizer having absorption in a long wavelength region of 300 nm or more. For example, ITX, QTX, CPTX, D from KAYACURE series (manufactured by Nippon Kayaku Co., Ltd.) ETX-S, ADEKA OPTOMER SP-100 (manufactured by Asahi Denka Co., Ltd.) and the like can be mentioned.

[0012] For example in CN108395765A, 20-30 parts of titanium dioxide hybride white formulation is described with a thickness of 5 to 10 / z m. The photoinitiator system comprises 45-50 parts of a radical photoinitiator, 5-8 parts of a cationic photoinitiator and 5-8 parts of a photosensitizer which is mentioned to ITX.

[0013] Also in US5747115A patent, a radical white formulation containing 10% or less of rutile TiCh was fully cured by a xenon lamp in a very short time, such as a minute or less. Photosensitizer from the group consisting of isopropyl thioxanthone, chlorothioxanthone, thioxanthone, benzanthrone, l,3-diphenyl-2-propanone, triphenyl acetophenone, fluorenone, biphenyl, flavone, 4-acetyl biphenyl, diacetyl, fluorene, 9, 10 phenanthrenequinone, benzophenone, and benzil is needed.

[0014] ITX and similar photosensitizers give a yellow color to coating what is not wanted from an aesthetic point of view. In addition ITX is classified as toxic by ECHA (https : / / echa.europa. eu / substance-information / - / substanceinfo / 100.024.389).

[0015] Considering a different technical field, composite for dental application, the effect of filler particule size and shape as well as filler content, has been shown to affect the translucency of material such as absorbance, scattering and transmittance of light. Various filler (silica, barium glass, glass, etc..) with different shape (irregular, spheric glass or fiber) were incorporated to play with the ligh transmittance and color of resin. Chirdon et al. [1] investigated the UV curing behavior of a composite material consisting of urethane acrylate polymer and glass fiber fillers and demonstrate that the depth of curing varies according to the orientation of the fillers within the matrix. Le Bell et al. [2] have shown that fiber-reinforced composites conduct and scatter the light better than conventional resin composite and concluded that the light scattering and absorption coefficients of resin composites, which affect the light distribution, should also be taken into consideration.

[0016] However, prior art does not disclose utilization of fibers material(s), such as titanate fibers, as a part of UV-curing formulation to decrease the absorbance of TiCh-based formulation.

[0017] Brief Description of the Invention and its Aims

[0018] This novel technology presents a solution to increase the limited cure depth which is a major drawback of light activated TiCh based composites.

[0019] One purpose of the invention, is to provide a high content of TiCh UV-curing basecoat which can be easily tailored to meet the varying hiding power and thickness required for different applications.

[0020] The basecoat is formed from rutile TiCh and with a fibrous additive which is particularly suitable for the purpose and which provides effective depth curing of the formulation without the requirement of photosensitizer. Fibers matters provide also good physical properties such as flexibility and hardness.

[0021] Furthermore, specific photoinitiators mixture of the invention enables to precisely tune the photoinitiator system to absorb in a wavelength region where the TiCh is weak. The present invention is also based on the selection of fibers matters that achieved multiple internal reflection to tailor the translucency of the formulation which decrease the total absorbance and allow the depth curing of the formulation.

[0022] Definition of the Figures Describing the Invention

[0023] Figure 1: Influence of the absorbance of the ratio of potassium titanate fibers / TiCh kronos 2066.

[0024] Detailed Description of Invention

[0025] The present technology provides novel solution to UV-cure TiCh high pigmented formulation by utilizing fiber materials to play with the light distribution through the coating.

[0026] In this invention, the incorporation of UV reflective additive which is potassium titanate fibers, decrease the total absorbance of the formulation in the wavelength range of 250-400 nm where the light source is working. In other words, as the absorbance is decreased more energy will be available to the lower layer of the coating and depth curing will be achieved.

[0027] Potassium titanate fibers preferably used in this invention shown formula KzTisOn and has the very thin form of a fiber length 10-20pm, a fiber diameter 0.3-0.6pm and a true density of 3.3 to 3.5 g / cm3. The potassium titanate fiber content in the composition is comprised between 1.25% and 50% by weight.

[0028] Rutile TiCh pigment is preferred due to it high hiding power. Depending on the application, such pigment may be present in ranges of 25-35% by weight.

[0029] The TiCh pigmented coating may contain any of a variety of UV curable material having epoxy and / or acrylate reactive groups. Hybrid cationic-radical photoinitiator system is priviledged to benefit from the advantages associated with the advantages of both systems while minimizing the disadvantages of both. The O?. inhibition in free radical polymerization and the H2O inhibition in cationic polymerization is overcome by combining these systems.

[0030] The epoxy monomer contains preferentially at least two alicyclic function wherein alicyclic epoxy is selected from any one of 3,4-epoxycyclohexylmethyl-3,4- epoxy cyclohexylcarboxy late [manufactured by Synasia., trade name: Syna epoxy 21, Syna Epo 06Exy, etc.] and bis- ( 3, 4-epoxy cyclohexylmethyl) adipate (manufactured by Synasia, trade name; Syna epoxy 28).

[0031] Glycidyl ether derivative with two or more oxirane functionalities are used as reactive diluent to reduce the viscosity of epoxy resin systems. The reactive diluent comprises any one of glycerol glycidyl ether or cyclohexane glycidyl ether derivatives. Glycidyl ether derivatives are usually present in a percentage comprised between 5 to 12% by weight.

[0032] Acrylate monomer may be mono-functional , di-functional or poly- functional. The large variety of acrylic systems allow for a great deal of variation in the type of properties achievable in cured systems that contain them. Preferred acrylic monomers are those from the trimethylolpropane triacrylate series. Acrylic derivatives are usually present in a percentage comprised between 2 to 10% by weight.

[0033] Radical and cationic photoinitiators were chosen to do not undertake competition with TiCh in absorbing the light of the mercury lamp. Radical and cationic photoinitiators are relatively stable to ambient white light and present a maximum absorption wavelength greater than about 300 nm. Cationic photoinitiator can includes diary liodonium cation and triarylsulfonium cation. The salt group of the photoinitiator is selected from the class consisting of hexafluorophosphates, fluoroborates, fluoroantimonates, fluoroarsenates, and mixtures thereof. Free radical photoinitiator is from the phenyl phosphine oxide derivative family .the cationic photoinitiator comprises any one of CPI-6996, CPI-6992, Sylanto 7M-S, Sylanto 7M-P, Esacure 1187. The radical photoinitiator comprises any one of Omnirad 819, Lucirin TPO, Lucirin TPO-L, Omnirad 2022, Omnirad 2100, Omirad 4265. Cationic photoinitiator is usually present in a percentage comprised between 0,5 to 15% by weight and free-radical photoinitiator is usually present in a percentage comprised between 0,25 to 2% by weight.

[0034] Flexural strength, flexibility, and impact strength of the cured formulation are obtained by the addition of polypropylene glycol oligomers, with at least two functional group with a molecular weight between 350 and 2000 and hydroxyl number from 150 to 30 mg of KOH / g. polypropylene glycol usually present in a percentage comprised between 5 to 15% by weight.

[0035] The compositions of the invention may include additional components such as leveling agent and adhesion promoters, and mixtures thereof, to regulate the properties of the film obtained after photopolymerization. Suitable additional agents for formulation can be selected from those that are compatible with the monomers to which it is added and with the UV-curing method. The composition may further include an adhesion promoter for glass, ceramics, porcelain, plastics and / or metal. Curing was done by an mercury lamp obtained from Ultralight AG company with a measured emission power of 160 W / cm and an accurately timed exposure. The main wavelength was 365 nm.

[0036] Compositions of the present invention show advantages in the case of preparing TiCh coatings. In that case, no thermal post treatment is necessary to obtained a fully cured coating. This is one of the advantages of cationic formulations, which exhibits dark curing and therefore continues to cure after passing under light.

[0037] One film of TiCh coating of a maximum 30 micrometer thickness is formed by applying the formulation to any surface. Curing time is between 5-30 s. The systems, compositions and methods result in films with a substantially tack-free surface upon photocuring.

[0038] Basecoat preparation method comprises following steps:

[0039] • Mixing of cycloaliphatic epoxy, reactive diluent, acrylate and photoinitiators at 1500- 3000 rpm / min until the solid photoinitiators were solubilized in the monomer,

[0040] • Addition of TiCh pigment and mixing in between 30-50 min at 3000-5000 rpm / min until a homogeneous white formulation is obtained,

[0041] • Addition of polypropylene glycol oligomers, and mixing 5-15 min at 500-1500 rpm / min and if necessary additional components such as leveling agent and adhesion promoters

[0042] • Addition of potassium titanate fibers and mixing 5 min at 500-1500 rpm / min.

[0043] • Exposing the composition to mercury lamp for at least 5s (160 W / cm).

[0044] Figure 1 is a UV spectra demonstrating the decrease of the absorbance UVabs vs (titanate fiber / TiCh)xl00 ratio.

[0045] Example:

[0046] An ultraviolet-curable composition comprising relative to the total weight of the composition:

[0047] 25-35 wt% of cycloaliphatic epoxy monomer,

[0048] 5-12 wt% of reactive diluent,

[0049] 2-10 wt% of acrylate, 0,5-5 wt% of cationic photoinitiator,

[0050] 0,25-2 wt% of free-radical photoinitiator,

[0051] 25-35 wt% of titanium dioxide (TiCh )pigment,

[0052] 5-15 wt% of polypropylene glycol

[0053] 1-14 wt% of potassium titanate fiber

[0054] Performance Testing

[0055] Decrease of absorbance:

[0056] Figure 1 show the decrease of absorbance in between 250-450 nm due to the addition of potassium titanate fibers in a TiCh-based formulation. The decrease is depending on the ratio of Titanate fiber / TiCh used. When the (potassium titanate fiber / TiCh)xl 00 ratio by weight is less than 25, the absorbance of the formulation decrease. When the (potassium titanate fiber / TiCh)xl00 ratio by weight is 2.5, the absorbance is optimally reduced.

[0057] Aceton test: Aceton double rubs: Evaluation according to ASTM D4752.

[0058] To determine if a coating is under cured, a solvent rub test is conduct. The solvent rub test is performed using aceton as the solvent. When curing is complete, the coating will be resistant to 50 strokes of cotton soaked in aceton solvent, and will only partially lose its gloss. But if the curing is not complete, the coating will turn soft and will be easily peeled off.

[0059] For the above-mentioned formulations no change was observed on the sample surface after 50 repetitions.

[0060] Industrial applicability

[0061] The present invention is applicable to for example new or old surfaces such as as aluminium, steel, Polypropylene (PP), Acrylonitrile Butadiene Styrene (ABS), PVC, etc...5prior to the application of undercoats or topcoats. References

[0062] [1] William M. Chirdon, William J. O'Brien, Richard E. Robertson, Diffuse reflectance of short-fiber-reinforced composites aligned by an electric field, Dental Materials, Volume 22, Issue 1, 2006.

[0063] [2] Le Bell AM, Tanner J, Lassila LV, Kangasniemi I, Vallittu PK. Depth of light-initiated polymerization of glass fiber-reinforced composite in a simulated root canal. Int J Prosthodont. 2003 Jul-Aug;16(4):403-8. PMID: 12956496.

Claims

CLAIMS1. An ultraviolet-curable basecoat composition, comprising; titanium dioxide (TiCh) pigment in combination with potassium titanate fibers.

2. The composition according to claim 1, wherein the titanium dioxide is rutile form.

3. The composition according to any of preceding claims, weight of TiCh pigment relative to total weight of the composition is between 25-35%.

4. The composition according to claim 1, wherein the potassium titanate fibers have chemical formula of KzTisOi?, a fiber diameter of 0.3 to 0.6 pm, a fiber length of 10 to 20 pm, a true density of 3.3 to 3.5 g / cm3.

5. The composition according to any of preceding claims , weight of potassium titanate fibers relative to total weight of the composition is between 1-14%.

6. The composition according to any of preceding claims, further comprising; at least one epoxy compound.

7. The composition according to claim 6, wherein the epoxy compound selected from a group of 3, 4-epoxycyclohexylmethyl-3,4-epoxy cyclohexylcarboxy late and bis- ( 3,4- epoxycyclohexylmethyl) adipate.

8. The composition according to claim 6, weight of epoxy compound relative to total weight of the composition is between 25-35%.

9. The composition according to any of preceding claims, further comprising; at least one acrylic derivatives.

10. The composition according to claim 9, wherein the acrylic derivative selected from the trimethylolpropane triacrylate series.

11. The composition according to claim 9, weight of acrylic derivatives relative to total weight of the composition is between 2-10%.

12. The composition according to any of preceding claims, further comprising; at least one reactive diluent.

13. The composition according to claim 12, wherein the reactive diluent selected from a group of glycerol glycidyl ether or cyclohexane glycidyl ether derivatives.

14. The composition according to claim 12, wherein the reactive diluent is glycidyl ether.

15. The composition according to claim 12, weight of reactive diluent relative to total weight of the composition is between 5-12 wt%.

16. The composition according to any of preceding claims, further comprising; combination of cationic and radical photoinitiators which present a maximum absorption wavelength greater than about 300 nm.

17. The composition according to claim 16, weight of cationic photoinitiator relative to total weight of the composition is between 0,5-15%.

18. The composition according to claim 16 or 17, weight of radical photoinitiator relative to total weight of the composition is between 0,25-2%.

19. The composition according to any of preceding claims, further comprising; polypropylene glycol oligomers with molecular weight between 350 and 2000 and hydroxyl number from 150 to 30 mg of KOH / g.

20. The composition according to claim 19, weight of polypropylene glycol oligomers relative to total weight of the composition is between 5-15%.

21. The composition according to any of preceding claims, further comprising; leveling agent and / or adhesion promoters.

22. Use of the composition according to any of preceding claims for preparing film and / or coating by UV-curing.

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