ING COMPOSITIONS

Field of the invention

The present invention relates to heat curable coating compositions for use in coil coating ap plications, and more particularly to such coatings containing epoxidized fatty acid methyl es ters, wherein the fatty acids are obtained from a vegetable oil, such as linseed oil, as well as the use of such epoxidized fatty acid methyl esters as a reactive diluent in heat curable coat ing compositions, such as coil coating applications.

Background art

In the past few decades, there has been an increasing interest of utilizing renewable and sus tainable resources due to concerns regarding environment, waste disposal and fossil fuel de pletion. Another sustainability aspect is not only to use renewable resources but also to em ploy production processes that are efficient with a minimal negative fossil carbon footprint as well as a minimum of emissions from the process.

One process that addresses some of these aspects in the field of organic coatings is the pro cess for preparing pre-coated steel sheets i.e. coil coatings. The coil-coating process is a con tinuous industrial coating process used to efficiently coat metal sheets with an organic coat ing to form coated steel coils. The coated metal sheets can subsequently be used for numer ous applications such as roofing, drainage systems, etc. Compared to traditional on-site painting of exterior built steel constructions this is advantageous both from a cost perspec tive and from an environmental aspect since direct emissions of volatile organic compounds (VOCs) may thereby be reduced, and even avoided. Traditionally the coating formulation is a solvent-borne liquid coating that dries through evaporation and chemical crosslinking in a convection oven. A typical coil coating process utilizes high temperature convection ovens to reach a peak metal temperature (PMT) of 230-240°C to allow for a full cure to be obtained in less than a minute. However, during curing a vast amount of VOCs are generated which are recovered and incinerated for energy recovery to aid the energy balance of the oven. The main emission is thus CO2 rather than VOC, but it is still desirable to find other alterna tives or ways to reduce these C0 ₂ emissions.

Several technologies have been considered as alternative to solvent-borne thermally curing of sheet metals e.g. UV-curable systems, water-borne coating formulations, powder coat ings. However, most of these methods have some drawbacks e.g. clean-up difficulties due to color change and high cost of modifying the production facilities.

Another way to exclude VOCs in coating formulations, and subsequent C0 ₂ formation, is to increase the solid content by e.g. adding a reactive diluent. A reactive diluent is a molecule that can both act as a diluting solvent as well as chemically react into the final coating during curing, thus becoming a part of the final coating. In order to fulfill these demands the reac tive diluent should have low viscosity, be compatible with the other components and have reactive functional groups suitable for the specific crosslinking chemistry used.

Accordingly, WO 2005/052070 teaches the use of fatty acid methyl esters (FAMEs), and es pecially rapeseed oil methyl ester (RME), as reactive diluents in polyester/melamine based coil coatings. FAMEs provide a decrease in viscosity when mixed into the formulation thus reducing the need for conventional organic solvents. The acyl group of the methyl ester within the fatty acid furthermore allows for a transesterification between the FAME and the polyester to occur leading to an incorporation of the FAME into the resulting cured coating. An important conclusion for this system was also that the FAME under typical coil coating curing conditions either reacted into the cured coating or evaporated, but it did not remain as a non-reacted plasticizer in the cured coating, hence a good long term performance could be obtained.

The use of fatty acid methyl esters as reactive diluents in coil-coating has been further de scribed by K. Johansson in Thermally cured coil-coatings utilizing novel resins and fatty acid methyl esters as reactive diluents, Doctoral Thesis, Stockholm, Sweden, 2008. As stated therein, FAMEs must contain at least one unsaturation in order to be usable as reactive dilu ents as they otherwise will crystallize at ambient conditions. It is also stated therein that both evaporation and alkene reactions of FAME are competing factors with the transesterifi cation reaction making it important to have oxidation-stable reactants, and that the degree of alkene reactions increases with higher number of unsaturations in the fatty acid structure. It is suggested therein that the amount of incorporated reactive diluent could be increased if the reactivity and functionality of the resin or the reactive diluent are increased. FAMEs with other functionalities such as epoxy or hydroxyl groups could for example be utilized.

The FAME used in the examples of WO 2005/052070 contains at least one unsaturation, such as rapeseed oil methyl ester (RME).

It would be desirable to be able to render coil coating systems even more sustainable, in crease the overall non-fossil content of the system, and to reduce the energy consumption and increase the life time of the final coating.

It is an object of the present invention to provide a coating composition, which can be used in a coil coating system for providing one or more of the above advantages.

Summary of invention

According to the present invention, the above object has been accomplished by means of using epoxidized fatty acid methyl ester (eFAME) in a coil coating composition as a reactive diluent.

Accordingly, in a first aspect, the present invention relates to a heat-curable coating compo sition for use in coil coating which composition comprises: a binder resin having hydroxyl groups, said binder resin being hydroxyl functional; a curing agent capable of reacting with the hydroxyl groups of the hydroxyl functional binder resin; and, a reactive diluent, wherein the reactive diluent is at least one eFAME obtained from a vegetable oil.

The inventive heat-curable coating composition for use in coil coating is binder resin based. That is to say, the main component by weight of the coating composition is the binder, based on the total weight of the overall composition as calculated not taking into account the weight of pigment when present in the overall coating formulation, and not taking into account the weight of solvent when present in the overall coating formulation.

The total amount of the inventive group of components consisting of hydroxyl functional binder, curing agent, and eFAME, typically altogether constitutes at least 80 % by weight, more preferably at least 85 % by weight, and typically at least 90 % by weight of the overall inventive coating composition, not taking into account the weight of pigment when present in the overall coating formulation, and not taking into account the weight of solvent when present in the overall coating formulation.

In a preferred embodiment the hydroxyl functional binder resin is selected from a polyester resin, an acrylate resin, and a methacrylate resin, and is preferably a polyester resin.

In another aspect the present invention relates to a method of producing a cured coating on a substrate comprising the steps of: providing a substrate; applying the inventive heat-cura ble coating composition to the substrate so as to form coating layer on the substrate; and, exposing the coating layer to heat so as to form a cured coating layer on the substrate.

In a further aspect, the present invention relates to the use of at least one eFAME, which has been obtained from a vegetable oil, as a reactive diluent in a heat-curable coating composi tion.

By means of the inventive coating compositions, cured coatings having a higher degree of incorporation of reactive diluent, and an enhanced curing performance can be obtained. It has surprisingly been found that eFAME is a more effective diluent than FAME, such as RME. Accordingly, the use of eFAME as reactive diluent in a coil coating composition allows for a lower amount of reactive diluent to be used as compared to when FAME is being used. At the same time, a lower percentage by weight of eFAME in the coating composition, as com pared to the FAME used according to WO 2005/052070, can be used to obtain same per centage by weight of reactive diluent raw material incorporated into the resulting cured coating. Further advantages and embodiment are set forth in the following detailed descrip tion.

Brief description of the appended drawings

FIG. 1 shows real-time FTIR results from model reactions at different temperatures, viz. at 130°C, 150°C, and 170°C, respectively. A shows EMO:LOH reaction, B shows EMLO:LOH reac tion, and C shows EMLEN:LOH reaction.

FIG. 2 shows FTIR analysis before and after curing of inventive coating composition eFAMElO at a curing temperature of 170°C. FIG. 3 shows DSC thermograms of samples from lab cured coatings 170°C, and coil coating simulation cured coatings at a PMT of 220-240°C.

Detailed description of the invention

Epoxidized vegetable oils are readily available on the market and easy to transform into me thyl esters in the same way as vegetable-oil based biodiesel is made. Vegetable oils are tri glycerides, i.e. triesters of glycerol and three fatty acids. Methods of producing eFAMEs from vegetable oils are known in the art and are described in e.g. US 2019/0010526 Al. Epox idized vegetable oils, such as epoxidized linseed oil, and soy bean oil, are also commercially available from e.g. Arkema Inc. under the brand name Vikoflex. For example, linseed oil is sold under the tradename Vikoflex 7190 Epoxidized Linseed Oil.

The present inventors have found eFAME to be a very efficient reactive diluent in polyester binder based coating compositions. Accordingly, from tests carried out it seems that a smaller amount of eFAME, such as even about half the weight of eFAME, as compared to that of FAME, may be used to replace a given portion of organic conventional diluents in a coil coating composition, and for providing same application characteristics, such as viscosity etc., to the coating composition. Thus, according to the present invention, the amount of re newable reactants used to replace such conventional organic diluent can be reduced. The present invention allows for a more efficient use of renewable raw material, reduced use of conventional organic solvents, and a higher degree of incorporation of renewable raw mate rial into the resulting cured coating. The use of eFAME thus results in a higher portion of bio based carbon in the resulting product in terms of weight of the cured coating.

The present inventors believe eFAME to efficient as a reactive diluent in any hydroxyl func tional binder resin, such as also in acrylate, and methacrylate binder resins.

The inventive use of eFAME has been found to avoid the problem of crystallization of the re active diluent, while at the same time avoiding the problem of oxidation of the reactive dilu ent due the presence of unsaturations in the reactive diluent.

The reduced oxidation and fragmentarization of the eFAME also leads to a reduced loss of hydrocarbon during the curing of the coating composition. The epoxidized FAME used in the present invention should contain no unsaturations, i.e. the naturally occurring unsaturations in the fatty acid residues of the vegetable oil used should be fully epoxidized.

In principle, the eFAME used according to the present invention could be obtained from any epoxidized vegetable oil.

The eFAMEs used in the present invention typically exhibits 1, 2, or 3 epoxy groups.

Examples of eFAMEs which can be used according to the present invention are epoxy methyl oleate (EMO) epoxy methyl linoleate (EMLO) and epoxy methyl linolenate (EMLEN).

While not wishing to be bound by any theory, the present inventors believe that the higher the number of epoxy groups in the eFAME, the higher chance that the eFAME will react with the binder, i.e. with the hydroxy groups of the polyester binder. It is also believed that the absence of unsaturations according to the present invention prevents oxidation and frag- mentarization. Fragmentarization could otherwise lead to loss of a fraction of the coating by e.g. evaporation during curing. By virtue of the increased degree of cross-linking of the reac tive diluent in the resulting cured coating, the inventive resulting cured coating is expected to exhibit a reduced loss of weight on ageing, as compared to a coating produced from a coating composition using FAME as reactive diluent.

Epoxidized FAME has been found to be less hydrophobic than FAME, such as e.g. RME. The present inventors therefore believe eFAME to exhibit an improved miscibility with a wider range of polyester resins, which resins may not need to be required to be modified so as to make them more hydrophobic. A wider range of polyester resins could therefore be used ac cording to the present invention, than when FAME is used as a reactive diluent. Suitable res ins are commercially available.

The preferred vegetable oil is linseed oil, and consequently the preferred eFAME mixture is one derived from linseed oil, i.e. one containing epoxy methyl oleate (EMO), epoxy methyl linoleate (EMLO), and epoxy methyl linolenate (EMLEN). Soy bean oil, and eFAME mixtures derived therefrom are also believed to useful in the present invention. Epoxidized FAMEs de rived from other naturally occurring vegetable oils are also believed to be useful, provided that such eFAMEs are fully epoxidized and free from unsaturations. In eFAME obtained from an epoxidized vegetable oil there may also be saturated, non-epox- idized FAME present, resulting from the presence of saturated fatty acid residues in the veg etable oil, from which oil the epoxidized vegetable oil has been obtained. The presence of such saturated, non-epoxidized FAME is tolerable according to the invention, but should not be too high, so as to thereby cause the reactive diluent to crystallize, or so as to thereby im pair the miscibility of the reactive diluent due to an increased hydrophobicity.

The amount of eFAME or eFAMEs used according to the invention is typically within the range of 1-20 % by weight of the coating composition, preferably 1-10 %, and more prefera bly 3-7 %, and especially up to about 5 % by weight based on the total weight of the coating composition.

The ratio of hydroxyl functional binder resin, and curing agent, respectively, used in the in ventive coating composition is not critical to the invention, and can be selected by skilled person as known in the art.

By way of example, when a polyester/melamine system is used, a typical ratio of polyester resin to melamine curing agent is usually within the range of from 90:10 to 80:20 by weight, and, when a polyester/isocyanate system is used, a typical ratio of polyester resin to isocya nate curing agent is usually within the range of from 85:15 to 70:30 by weight.

Any conventional polyester resin conventionally used in coil coating applications could be used in the invention.

The curing agent used according to the present invention is a compound capable of reacting with the hydroxyl groups of the polyester resin. The curing agent is preferably an amino compound, or an isocyanate functional curing agent, such as isophorone diisocyanate (IPDI), more preferably an amino compound, and more preferably a melamine compound, such as e.g. hexamethoxymethyl melamine (HMMM).

An acid generating catalyst is required for the desired reaction of the expoxide groups of eFAME to occur during curing by heat activation. For this purpose, according to the present invention, a conventionally used blocked acid catalyst can used, such as a blocked para tolu ene sulphonic acid (PTSA), especially p-dodecylbenzenesulfonic acid (DDBSA). In the exam ples herein p-dodecylbenzenesulfonic acid (DDBSA) has been used. The coil coating compositions of the present invention are preferably polyester/melamine based.

The inventive compositions are not prone to yellowing during curing, and seem to exhibit a reduced degree of yellowing as compared to known compositions containing FAME as reactive 5 diluent. It is believed that the reduced degree of yellowing may be due to the absence of un saturations in the reactive diluent used according to the present invention.

From the experimentation carried out, it seems that coating compositions containing 5 wt% eFAME show most promising results, both as coating composition, and as coating, i.e. re sembling the flow properties and thermal properties of a conventional coil coating composi te) tion containing 10 % by weight of rape seed methyl ester (such as the coating composition designated RME10 used in the examples).

The inventive coating composition is primarily intended for use on metal substrates, which can withstand the curing conditions. Suitable substrates are steel, such as galvanized steel, and aluminum substrates, preferably in the form of sheet metal. Preferred metal substrates 15 are various varieties of steel, such as hot dip galvanized (HDG) steel.

Examples

The invention will now be described in more detail by way of the following examples. Materials used

Epoxidized linseed oil (ELO), rape seed methyl ester (RME), and two model coil-coating poly- 20 ester/melamine (hexamethoxymethyl melamine (HMMM)) formulations were provided by PTE Coatings AB. The ratio by weight of polyester binder to melamine curing agent in the two model formulations was about 85:15. One formulation containing 10 wt% RME, and the second did not contain RME. The polyester in the coil-coating formulations was hydroxyl functional with a hydroxyl value of 120.9 mg KOH/g resin, and acid value of 8 mg KOH/g 25 resin, and a molecular weight (Mw) of 4000±500 g/mol. Lauryl alcohol (LOH), and p-do- decylbenzenesulfonic acid (DDBSA) were purchased from Sigma-Aldrich. Epoxy methyl ole- ate (EMO), epoxy methyl linoleate (EMLO), and epoxy methyl linolenate (EMLEN) were pro duced from the ELO. Hot dip galvanized (HDG) steel substrates were provided by SSAB. All materials were used as received from the suppliers. In the following coil-coating formulation containing 10 wt% RME will be referred to as "RME10", while coil-coating formulations with no RME will be referred to as "non-RME".

Analytical methods used

Fourier transform infrared spectroscopy (FTIR) and real-time FTIR

For FTIR and real-time FTIR analysis a Perkin-Elmer spectrum 100 instrument was used, equipped with an ATR accessory unit (Golden Gate) from G rase by Specac LTD (Kent, Eng land). Data was processed in Spectrum 10 software from Perkin-Elmer. For real-time FTIR measurements data were collected using TimeBase ^® software from Perkin-Elmer.

Nuclear magnetic resonance (NMR)

In order to obtain ¹ H NMR and ¹³ C NMR spectra of the different compounds a Bruker spec trometer (400 MHz) was used. The obtained spectra were analyzed with MestReNova v9.0.0- 12821 (Mestrelab Research S.L. 2013).

Differential scanning calorimetry (DSC)

DSC analysis was performed in order to obtain thermal properties of the coatings using a Mettler Toledo DSC-1 equipped with Gas Controller GC100. To analyze the cured coatings approximately 5-10 mg of each sample was measured into 100 pi aluminum crucibles. The data were collected using a heating/cooling rate of 10°C min ¹ from -50 to 200°C with 5 min isotherms. All analyses were carried out in nitrogen gas of 20 ml min ¹ . Stare Excellence Soft ware was used to evaluate the collected data. The glass transition (T _g ) was acquired from the second heating scan and reported as the midpoint of the heat capacity change.

Pendulum test (ASTM standard D4366-16)

Pendulum damping test (ASTM D4366-16) was followed in order to evaluate the hardness of the coatings. Glass substrates were coated and used for the evaluation. The thicknesses of the coatings were approximately 150 pm.

Rheometry Viscosity measurements were obtained from rheology measurements on a TA Instrument (New Castle, DE, USA) DHR-2 (Discovery Hybrid 2) equipped with a Peltier plate for tempera ture control. The temperature for the analysis was set to 30°C.

In the Examples, for the ease of testing and evaluation of properties of the heat-curable coating compositions, and resulting cured coatings, no pigment was used in the formula tions.

Example 1

Preparation of eFAME - transesterification of epoxidized linseed oil

The epoxy fatty methyl esters (eFAME)s were obtained from epoxidized linseed oil (ELO) by methanolysis of the triglycerides, as known in the art. ELO (20 g) was dissolved in 250 ml of 0.02 M NaOH in methanol and set at reflux condition for 1 h. The obtained eFAMEs were ex tracted in 4 x 100 ml n-heptane. The n-heptane phase was then dried over MgS04 (s) and fil tered. Solvent was then removed by rotary evaporator to give a yield of 75% (15 g). For the model reactions the eFAME mixture was purified, and each different eFAME was evaluated. Automated column chromatography with gradient elution of n-heptane/ethyl acetate as mo bile phase was used in order to purify the pure eFAMEs. The products in the crude eFAME mixture were methyl stearate, epoxy methyl oleate (EMO), epoxy methyl linoleate (EMLO), and epoxy methyl linolenate (EMLEN).

Example 2

Model reactions

In order to determine the occurrence and rate of the different reactions between e-FAME and non-RME, model reactions were performed by mixing LOH in a 1:1 molar ratio with the respective pure epoxy fatty methyl ester (EMO, EMLO, and EMLEN, respectively). DDBSA was weighed to approximately 3 wt% of the total reactants amount, and then added as a catalyst. The reactions of the respective mixture were then monitored by in situ real-time FTIR measurements at 130°C, 150°C, and 170°C, respectively.

A schematic overview of reactions occurring during model experiments followed by RT-FTIR is set out forth below.

"Ring-opening" Transesterification

The different reactions were followed by observing the alcohol region (approximately 3500 cm ⁴ ), the carbonyl region (1750-1720 cm ⁴ ), the linear ether region (1100-1050 cm ⁴ ), and the epoxide ring-vibration (860-790 cm ⁴ ). To better visualize how the reactions occurred time profiles were generated from the area under the curves over time for the respective re gion as can be seen in FIG. 1. Real-time FTIR data obtained showed that a higher tempera ture resulted in a faster reaction rate in all systems. In addition, during the reactions in 150°C and 170°C it was observed that a transesterification reaction occurred first, followed by ring opening of an epoxide in all cases under set conditions. Reactions occurring in 130°C re- suited only in a transesterification reaction. In all reactions, transesterification was con firmed by the increase and shift in carbonyl region while a decrease in OH region occurred simultaneously indicating a reaction between the methyl ester of EMO/EMLO/EMLEN with the hydroxyl group on LOH. A ring-opening reaction of an epoxide in the reaction mixture could occur by either another epoxide or OH group acting as a nucleophile. Both these reac- tions result in formation of linear ether and secondary alcohol. This was observed by an in crease in linear ether with a simultaneous decrease in the epoxide region. As mentioned pre viously, the OH profiles decrease at first, followed by an increase. This behavior further con firms that at first the transesterification due to reaction with methyl ester occurs, followed by epoxide ring-opening reaction, i.e. formation of secondary alcohol groups. In addition, it was possible to qualitatively assess that an increase in the number of epoxides, i.e. EMO < EMLO < EMLEN, resulted in a decrease in transesterification rate. Furthermore, the reaction rates were increased by increasing the temperature.

Example 3 Inventive formulations were prepared by mixing the eFAME-mixture obtained in Example 1 with Non-RME. The inventive formulations contain 5 % (eFAME5), 10 % (eFAMElO), 15 % (eFAME15), and 20 % (eFAME20) by weight, respectively, of the eFAME mixture of Example 1. The four different inventive compositions are set forth in Table 1 below, along with two comparative compositions, i.e. Non-RME and RME10. Viscosity was measured on the coating formulations and the measurements are set forth in Table 1.

Table 1: Coil coating formulations and their respective physical properties. Viscosity is measured on the coating compositions. The T _g , Pendulum test, and Contact angle are obtained from cured coatings in laboratory scale at 170°C.

Designation Amount eFAME [wt.%] Viscosity T _g [°C] ^C Pendulum test [min] ^f Contact Angle [°] ^d resin [Pa s]

a Coil-coating formulation without rapeseed methyl esters (RME), ^b Coil-coating formulation with 10 wt% rapeseed methyl esters (RME), ^c Thermal properties of cured coatings, ^d Contact angle of cured coatings on steel substrate; steel substrate without coating had contact angle 54±2, ^e Specimens from simulated coil-coating curing. These specimens were cured on steel substrate at 220-240°C peak metal temperature, ^f Pendulum test on cured coatings, performed on glass substrates with coating thickness of approx. 150 pm

One of the properties desired for a good coating composition is the ability to flow easily and form a uniform film when applied on a substrate. In addition, a reactive diluent should act both as a solvent and a reactant. In order to establish the flowing properties of the different coating compositions, the viscosity of the coating formulations was characterized by rheom- etry. Table 1 shows that with increasing amount of eFAME in the coating formulation a de crease in viscosity is obtained. The result shows that approximately 5 wt% of eFAME is needed on order to obtain similar flow properties to those of RME10.

Example 4

Curing of formulated clear coats

The different formulations prepared in Example 3 were spread on both glass and steel sub strates using 60 pm or 150 pm applicators and then put in oven at 130°C, and 170°C, respec tively, for 30 min. The curing of the resin formulations was also followed spectroscopically by in situ real-time FTIR measurements at 170°C in the same way as for the model studies. The glass substrate samples with coating thickness of 150 pm and cured at 170°C were used for pendulum hardness evaluation. The results are presented in Table 1 above.

All coating compositions had well spreading on both glass and steel, and uniform films were obtained. However, tack-free clear coatings were only obtained from curing at 170°C. For 130°C the coating compositions were left to cure for about 1 h, however tack-free coatings were not obtained.

Samples for FTIR were taken before and after curing of eFAMElO at 170°C as shown in FIG.

2. As the previously mentioned model reactions predicted, the reaction was confirmed by the decrease in alcohols 3500 cm ¹ , while an increase and shift in carbonyl region (1750- 1720 cm ¹ ) indicated that a transesterification has occurred. In addition, the epoxides (860— 790 cm ⁴ ) disappeared after the reaction, while an increase in linear ether region (1100- 1050 cm ¹ ) was observed.

Example 5

Simulated coil-coating curing

The coating compositions eFAME5, eFAMElO, and RME10 were cured on steel substrates for 30 seconds in a simulated coil-coating curing environment at peak metal temperature (PMT) of 220-240°C, and the curing performance was evaluated. The curing was carried out at PTE Coatings AB to resemble pilot scale coil-coating curing. Clear coatings were obtained. FTIR analysis revealed that the coating compositions were fully cured resembling lab oven cured samples. Values obtained for T _g , Pendulum test, and Contact angle were obtained from the cured coatings, and are set forth in Table 1 above.

Example 6

Thermal properties of fully cured coatings FIG. 3 shows the thermal properties of fully cured coatings at 170°C, and from coil-coating simulation, respectively. DSC analysis of lab cured coatings demonstrated that by increasing the amount of eFAME, a decrease in T _g was obtained. The decrease in T _g is believed to be due to that an increased amount of eFAME provides more aliphatic chains in the final struc ture of the cured coating. Interestingly, eFAME5 and RME10 had similar T _g values, both from lab cured samples and from coil-coating simulations.