Toughening of Epoxy Coating Systems with Novel Biobased Materials. Erwin Honcoop, Croda, Netherlands William McNamee,Croda US

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1 Toughening of Epoxy Coating Systems with Novel Biobased Materials Erwin Honcoop, Croda, Netherlands William McNamee,Croda US Introduction The market is looking for possibilities to reduce weight by reduction of the thickness of metal sheets and maintain the protective properties. To meet these needs, there is a demand for more flexible, but tough coatings without compromising the other properties. Epoxy systems have found widespread use in coatings, sealants and adhesives applications. These systems provide hard, highly crosslinked coatings with excellent protective properties. Pripol dimerised fatty acids have traditionally been used in epoxy resins, in polyamide curatives and in modified epoxies. The benefits that they add to these resins are flexibility, vibration resistance and stress absorption, hydrophobicity, water resistance, thermo-oxidative resistance, and compatibility with for example hydrocarbon resins. For some years, Croda has a range of polyester diol derivatives of dimerised fatty acids under the product name of Priplast. Recently, Croda has developed new biobased polyester diacids based on renewable dimerised fatty acids. The carboxylic acid functionality allows these polyesters to be grafted on the epoxy, as building blocks for epoxy systems. Depending on the molecular weight and polarity of the polyester diacid in relation to the epoxy, microscale phase separation occurs. This reaction induced phase separation leads to a morphology with low Tg micro-segments that are homogeneously distributed inside a hard matrix, resulting in hard and at the same time tough systems. Next to bringing good toughening performance, the polyester diacids cause a reduction in moisture diffusion and enhance thermo-oxidative stability. The relatively low viscosity allows ease of formulation, handling and formulation flexibility that is combined with the enhanced environmental profile of a renewable impact modifier. As such, polyester diacids based on dimerised fatty acids can be used as relatively low-viscous, built-in impact modifier. This paper describes incorporation of the polyester diacids as building block in epoxy systems, to bring the combination of flexibility and toughness to coating systems. Initially, the influence of polyester diacids on the epoxy morphology was tested. The influence of polyester composition and molecular weight were examined. Subsequently, the properties of modified epoxies were studied, in comparison with a conventional rubber toughener and a traditional flexible epoxy system. Dimer fatty acids Natural oils and fats have for years provided the polymer chemist with a variety of building blocks, such as glycerin and castor oil. Less well known is the use of a fatty acid derivative, the so-called dimerised fatty acids. These are obtained by the conversion of unsaturated fatty acids (from sources like soybean oil or tall oil) by a combination of pressure, temperature and catalysis. This process generates a mixture of products, the most important being dimerised fatty acid. Others are trimerised fatty acid, and isostearic acid. Figure 1 gives an overview of the dimerisation process.

2 C=C C=C-C-C=C heat, pressure, catalyst O=C HO trimerised fatty acid O=C HO dimerised fatty acid Pripol product range mono fatty acid Figure 1: Reactions in oligomerisation of fatty acids Starting from the C18 acids that nature typically provides, the dimer acid is a molecule with 36 carbon atoms, which makes it by far the longest dioic acid available. This hydrocarbon nature makes dimer acid and polymers in which it is included extremely hydrophobic. Besides, the combination of hydrocarbon character and non-crystallinity provides lubricity and flexibility, even at very low temperatures. Dimerised fatty acids have found their application in such areas as polyamide epoxy curatives, polyamide resins, polyester and polyurethane coatings, adhesives, elastomers and foam. In all these applications, the value of the dimerised fatty acid is related to the features mentioned: flexibility and impact strength, lubricity and flowability, hydrophobicity and hydrolytic resistance. Dimer fatty acids for toughness modification of epoxies form yet a relatively unexplored area. Epoxy Resins Epoxy resins have a wide use in applications like coatings flooring, sealants, grouts and adhesives. Epoxy systems are versatile because of the large amount of potential epoxy resins and the curing agent combinations, each giving a different structure to the final material. The broad spectrum of the potential epoxy/amine systems generally provides excellent adhesion to a wide range of substrates, resistance to a spectrum of chemicals and good mechanical properties. These properties and easily varied formulations allow the formulator to tune a resin system s cure, handling and characteristics. However with the high solids or solventless formulations, the challenge for the formulators to get the potlife and cure temperate at the desired level is one, but another difficult challenge is to obtain highly flexible, yet tough materials for coatings, floors, adhesives, grouts and sealants. The general experience has been that improving one property normally means weakening several other properties. Toughness and flexibility are important to distribute the stress that occurs during the duration of the epoxy systems service life, especially in thick film applications such as tank linings, floor coatings and adhesives. A number of approaches are available for the formulator to create flexibility and toughness, but rarely both to any significant extent. One of the traditional ways to incorporate a flexible backbone is via the curing agent which are available in many types: polyamides from dimer acids, adducts from ethylene-or propylene-oxide and polymeric anhydrides.

3 However these often mostly provide flexibility. Additionally modified epoxy resins which have dimer acid incorporated in the backbone are used. These are normally used as modifying agent at levels of approximately 1% on the epoxy component. They increase the flexibility and toughness modestly at these levels. At higher degrees of addition the flexibility increases but the toughness is lost. Another way is the use of thermoplastic polymer which can be separated into products that exist as a separate phase like acrylonitrile-butadiene rubbers; those that separate into a second phase during cure e.g. carboxyl terminated butadiene-acrylonitrile (CTBN) rubbers and soluble polymers that exist as separate phase in the cured system. These polymers work through phase separation in the cured system and provide an increasing toughness but have a small effect on the flexibility. Dimer Acid for Epoxies The dimer acids provide epoxy resins with enhanced flexibility, but the reduced glass transition temperature is not always favourable. Building of dimerised fatty acid into carboxylic acid-terminated polyesters, makes it available for incorporation in epoxy resins as a larger molecular weight species (see figure 2 below). Due to the apolar character of the dimer based polyesters, it can be expected that they will form a separate phase in the more polar epoxy matrix. As such, they can be introduced as a second-phase toughness modifier. By reaction induced phase separation, the modifier forms soft domains within the hard epoxy matrix. It can be expected that these dimer based polyesters will have little impact on the glass transition temperature, bring better fracture toughness, enhance peel strength, and provide a moisture barrier to the epoxy. A range of dimer based polyesters with carboxylic acid functionality has been developed and tested in epoxy adhesives, in order to substantiate the above mentioned assumptions. Dimer acid PRIPOL HOOC CO Acid functional Polyester made with dimer acid PRIPLAST HOOC---- E-E CO Figure 2: Dimer technology for epoxy resins Initially, dimer based polyesters with carboxylic acid functionality have been developed with a range of polarities and molecular weights. They were introduced in epoxy coatings, and the influence of dimer based polyester composition on the morphology was examined. This study was followed by testing the properties of these modified epoxies, in comparison with a conventional rubber modified epoxy. Effect of Biobased Polyester Diacides Polarity on Morphology To investigate the morphology effect of the biobased polyester diacids, a structural epoxy formulation has been used. The formulation used was a heat curable formulations contained 66% of pre-reacted material, 25% high molecular weight Bisphenol A epoxy (Epikote 11), 7% phenolic Novolac resin and a catalyst. These formulations were cured at 175 C. [1] Biobased polyester diacids were produced from a combination of dimerised fatty acid and a low molecular weight dioic acid with glycol. The polarity was varied by varying the ratio of dimer acid to short dioic acid, in which the short dioic acid increases the polarity. The molecular weight was kept constant at about Mw 15 (schematically overview see Table 1). The influence of these polyesters on epoxy morphology was examined using electron microscopy.

4 Product code Polarity indication Polyester diacid low high P1 Ratio dimer/short dioic acid > 5% dimer P2 5/5 P3 < 5% dimer Table 1: Schematic overview of the polarity effect of biobased polyester diacids. Initially, polyester from dimerised fatty acid without short dioc acid was used as epoxy modifier. Figure 3a shows the microscope picture of the material; the dimer based polyester can be seen as large droplets of about 4 µm diameter. Due to the highly hydrophobic character, the material was too incompatible with the epoxy resin. No grafting reaction had occurred between the dimer based polyester and the epoxy. The final resin was brittle, like unmodified epoxy, and sticky because of the separated polyester. 3a 3b Figure 3: Electron microscope picture of epoxy modified with polyester. Figure 3a: Ratio dimer acid : dioic 1:, no grafting; Figure 3b: Ratio dimer acid : dioic 75:25, only some grafting. To enhance compatibility, the dimer based polyester was made more polar by introducing a low molecular weight dioic acid. Polyester was made, combining 75 parts dimerised fatty acid and 25 parts short dioic acid. Figure 3b shows the microscope picture of the epoxy modified with this polyester. The dimer based polyester droplets were considerably reduced in size. Some grafting reaction had occurred, but still the components are not compatible enough. Again, the resin was brittle and sticky. Another polyester was made, combining 5 parts dimerised fatty acid and 5 parts short dioic acid. Figure 4 shows two microscope pictures of the epoxy modified with this polyester. Full grafting took place, resulting in excellent phase separation on micro-scale. The dimer based polyester droplets were about 5 µm in size. The final resin was hard and tough. By reaction induced phase separation, the modifier formed soft domains within the hard epoxy matrix, thereby toughening the epoxy.

5 Figure 4: Electron microscope pictures of epoxy modified with polyester. Ratio dimer acid : dioic 5:5. Effect of Biobased Polyester Diacid Molecular Weight Polyesters diacids were produced from a combination of dimerised fatty acid and a low molecular weight dioic acid at 5:5 ratio. As was discusses previously, polyesters of this polarity result in proper reaction induced phase separation in the epoxy matrix. The molecular weight of the polyester was varied, and the influence on epoxy morphology was examined using electron microscopy. Increasing the molecular weight resulted in a reduction in size of the soft dimer polyester domains. Results are given in Table 2. Due to the larger size of the hydrophobic blocks that are built into the more polar epoxy, phase separation results in more finely distributed soft domains. Table 2: effect of dimer ester molecular weight on size of the soft domains within the epoxy matrix. Mw 125 Mw 25 Mw 4 Soft domain size 5 µm 2 5 µm 1 µm For all samples, the apolar dimer polyester particles were homogeneously distributed through the more polar epoxy matrix. The hydrophobic dimer polyester particles make the epoxy moisture resistant. Moisture diffusion was examined of these modified epoxies, to explore the influence of morphology on moisture resistance. It was found that a more finely distributed morphology, with smaller soft domains, resulted in lower water diffusivity. This is shown schematically in figure 6 below.

6 Low Mw Medium Mw Hydrophobic particles Continuous Higher Diffusivity,8x1-4 Medium High Mw Epoxy Phase Moisture Diffusivity,5x1-4 Low Diffusivity,1x1-4 cm 2 /s Figure 6: Effect of biobased polyester molecular weight and morphology on moisture diffusion Preparation of Modified Epoxy Resins Acid functional dimer based polyesters were grafted onto the epoxy matrix. They were pre-reacted with low molecular weight Bisphenol A epoxy (EPIKOTE 828), to obtain epoxy functional material with 67% of the dimer based polyester. In the case of the comparative reference formulation, a low molecular weight carboxylic acid terminated copolymer of acrylonitrile and butadiene, CTBN 13x13, was pre-reacted in similar fashion. Additionally a flexible epoxy resins was prepared by reacting PRIPOL 117 with low molecular weight Bisphenol A epoxy (EPIKOTE 828). The evaluated formulations were consisting of EPIKOTE 828, modified epoxy resin and amine curative Epikure 372. Effect of Epoxy Modifyers on Coating Properties A number of basic properties of coatings like flexibility and toughness depend on the viscoelastic behaviour, physical transitions and relaxations. Coatings, as the polymers from which they are prepared, are viscoelastic in nature, that is, they behave both as viscous liquids and as elastic solids. The coatings have elastic recovery and yet will flow with time when placed under a stress. In general, viscoelastic behaviour and mechanical properties are markedly affected when a coating enters the glass transition or relaxation. The effect on glass transition can be evaluated by e.g. differential scanning calorimetric methods. The ability to withstand stress can be validated by using bending- and impact tests. This will be an indication towards flexibility, the ability of the coating to undergo a bend without failure and towards toughness, the ability to withstand great strain in a short time without breaking or rupture. The premise of the evaluation was to study the effect of the new polyester diacids in comparison to a dimer modified and a non modified epoxy resin. As reference a CTBN (Hypro13X13) adduct, commonly used as toughening agent in epoxy systems was taken along. For the evaluation of the epoxy system, consisting of the Epikote 828, modified epoxy resin and curing agent, were mixed at room temperature and diluted to 6 mpa.s with xylene /butanol (ratio 1/1). The impact of the addition of the modified epoxy resin to the Epikote 828 was measured. Figure 7 shows the impact on the epoxy resin viscosity when incorporating the 5% rubber component in the epoxy system. The formulated epoxy resin with polyester diacids, give only a limited increase to the viscosity when compared to the CTBN modified resin and are also lower then the dimer modified resin. This will allow the formulator to keep the solvent level low.

7 Dyn. 25 C (Pa.s) ref ( %) P1 P2 P3 CTBN PRIPOL 117 Figure 7: Effect on viscosity of the epoxy when incorporating 5% of the rubber component in the epoxy system. A range of formulations was made containing,5,1 or 15% rubber phase on the system. The resulting clear varnish was applied 15µ wet by drawdown bar on the various test substrates. After initial screenings a selection was made with a focus on the use of 5% of the rubber component in the epoxy system. Figure 8 shows the effect of the use of 5% of the rubber component in the epoxy system on hardness and Tg. On the hardness, measure according DIN EN ISO1522 one can observe that the hardness of the modified epoxy based on polyester diacid P2 and the PRIPOL 117 don t loose the surface hardness where as the CTBN modified, polyester diacid P1 and P3 have a reduced hardness against the unmodified epoxy system. Konig Hardness (s) Tg ( C) ref ( %) P1 P2 P3 CTBN PRIPOL117 Modified epoxy resin Hardness Tg Figure 8: Effect on hardness and Tg when incorporating of 5% modified epoxy on the system

8 When the glass transition is evaluated in relation with the hardness we see that the Tg of the PRIPOL 117 based coating only decreases a few degrees which would normally reduce the hardness of the coating. However one has to keep in mind that the crosslink degree in this system is higher than from the CTBN and polyester diacids P1/2/3 which will increase the hardness. The lower Tg of the polyester diacids would suggest that the coatings will demonstrate a more elastic behaviour. To check the elastic behaviour the coatings were tested by using the cylindrical mandrel test (DIN EN ISO 1519). The incorporation of the rubber component in the epoxy system shows increase of flexibility, as can be seen in figure 9 Cylindrical mandrel (mm) ref ( %) P1 P2 P3 CTBN PRIPOL117 Modified epoxy resin Figure 9: Effect on flexibility when incorporating of 5% of the modified epoxy on the system. Combining the flexibility data with the hardness data of figure 7, the polyester diacid P2 shows good flexibility but at the same time gives a surface hardness comparable to the reference epoxy system based on Epikote 828 / Epikure 372. This indicates that the polyester diacid P2 has the ability to resist crack formation during a deformation. Comparing to the CTBN modified resin we see a good flexibility but the hardness drops. This is caused by incompatibility of the CTBN-modified epoxy in the hard matrix giving a bad phase separation. Although to a lesser extent, the polyester diacids P1 / 3 have lost some hardness but improved on deformation resistance over the reference epoxy system. Most likely the polarity difference between the epoxy resins based on polyester diacids P1 and P3 didn t give the optimum phase separation. The PRIPOL 117 modified epoxy resin shows a retaining hardness and an improved flexibility but doesn t reach the level of the polyester diacid P2 modified epoxy. It lacks the ability to absorb the strain imposed on the coating. Impact resistance testing is an additional means of predicting energy storage and loss as a function of temperature. For good impact resistance a coating must consist of a polymer with strong intermolecular entanglements, flow and combined with energy dissipation. Impact tests were done according DIN EN ISO 6272 at 25 C and after 24h exp osure to -25 C. Figure 1 shows the test results of this evaluation.

9 2 Direct impact (cm.kg) ref ( %) P1 P2 P3 CTBN C Figure 1: Impact resistant at 25 and -25 C when in corporating of 5% modified epoxy on the system We can see that the coatings consisting of the modified epoxy based on polyester diacid P1 and P2 have the ability to absorb the inflicted deformation at room and lower temperature. The good phase separation of the rubber phase in the rigid epoxy phase enables the coating to dissipate the energy of the impact. The CTBN- and PRIPOL 117 modified epoxy resins have comparable to lower impact resistance than the reference due to compatibility issues with the epoxy matrix giving a bad phase separation. Conclusion The newly developed, biobased polyester diacids based on dimer fatty acid can well be used for toughness modification of epoxy systems. Flexibility of the chemistry allows tailoring of the polyester molecular weight and polarity, in order to fit the base epoxy resin. It allows the desired morphology to be exactly achieved. Modifying epoxies with polyester diacids enhances the flexibility at equal surface hardness when incorporated at a 5% rubber component level on the epoxy system. Especially in the Epikote 828 / Epikure 372 system the polyester diacid P2 gives a good performance due to the desired microphase separation. Additionally the lower viscosity of the polyester diacid modified epoxies, allows formulation flexibility. The lower viscosity is useful for the ease of handling and allows the use of more fillers. Based on the results found for epoxy toughened adhesives [1], it is expected that the biobased polyester diacids based on its renewable dimer fatty acid exhibit moisture resistance due to the hydrophobic nature, and increases hydrolytic, thermal and oxidative resistance. [1] Dr. Angela L.M. Smits, Dr Paul Cameron, Hans Ridderikhoff, Dimerised fatty acids technology for Epoxy toughening, presented on European coating conference 27 The information in this publication is believed to be accurate and is given in good faith, but no representation or warranty as to its completeness or accuracy is made. Suggestions for uses or applications are only opinions. Users are responsible for determining the suitability of these products for their own particular purpose. No representation or warranty, expressed or implied, is made with respect to information or products including, without limitation, warranties of merchantability, fitness for a particular purpose, non-infringement of any third party patent or other intellectual property rights including, without limit, copyright, trademark and designs. Any trademarks identified herein are trademarks of the Croda group of companies.