strength epoxy coatings

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1 30 Softness gives strength Advanced toughening technology for epoxy systems. By William L. Heaner IV, Fabio Aguirre-Vargas, William Heath, Amber Stephenson, Adam Colson and Nathan Wilmot, The Dow Chemical Company Fotolia.com There is a growing trend for higher solids contents in epoxy systems. However, the low molecular weight epoxies required often lead to brittle coatings which are easily damaged. Flexibilisers can improve some properties but compromise others. Traditional toughening agents increase viscosity but can improve a wider range of properties. Data on a newly developed toughener is provided. Epoxy coatings are commonly used as primers and mid-coats for corrosion protection in marine, military, infrastructure and oil and gas sectors. Volatile organic compound regulations and asset owners demands for higher film thickness per coat have driven demand for high-solids (HS) epoxy formulations [1, 2]. The solvent reduction requirements have forced formulators to use lower molecular weight (MW) epoxy resins to maintain formulation viscosity at manageable levels. However, HS are typically more brittle than conventional low solids formulations, which have lower crosslink densities due to the use of higher MW epoxy resins and also to the residual solvent that helps to plasticise the coatings. Hence, HS epoxy systems are more susceptible to damage by impact, abrasion and gouging in the demanding environments in which they serve. For example, a typical commercially available HS epoxy coating will often fail a ¼-inch (6.5 mm) mandrel bend and will only provide about 20 in-lb (2.25 N-m) of direct, rapid impact resistance at 25 C; at freezing or sub-zero temperatures most HS become even more brittle. Coating damage can have serious consequences The inability of HS coatings to adequately dissipate impact energy leads to holidays, cracking, gouging and loss of adhesion to the substrate. Once the barrier properties of the coating have been compromised, water, chemicals, and electrolytes can reach the metal surface leading to further crack propagation, delamination, and blistering of the coating as well as corrosion of the substrate. Replacing and/or repairing coatings in the field can significantly increase the maintenance downtime cost of the asset. To reduce the rate of failure and extend a coating s service life, flexibilisers and toughening agents can be added to the formulation. Toughening agents are the preferred method of protecting against most forms of mechanical failure, as they allow impact energy to be dissipated without compromising the coating s glass transition temperature (T g ) [3, 4]. Several toughening agents based upon a wide range of chemistries are commercially available for. Figure 2: DMA curves of a coating formulation with Toughener # Peak tan delta (67.35 C) Figure 1: Phase morphology in epoxy resins with and without toughening agent. G' ( ) [Pal] Peak tan delta ( C) tan_delta ( ) [ ] a. Control, no toughener b. 20 % Benchmark A c. 25 % Benchmark A Temp. in C

2 31 Results at a glance There is growing pressure to move solventbased epoxy coatings towards higher solids content. However, the low molecular weight epoxies required to achieve workable viscosities generally lead to high crosslink densities and brittle coatings. While epoxy flexibilisers can reduce resin viscosity and improve elongation and impact resistance, they compromise T g and resistance to chemicals, abrasion and corrosion. Epoxy tougheners increase system viscosity but can improve a wider range of properties without significantly affecting T g, chemical resistance or corrosion resistance. The nature of the phase separation between toughener particles and the resin system is critical. Data is presented on the use of a newly developed toughener which provides an improved balance of properties compared with existing standard products. In this article, the differences between flexibilising and toughening agents will be briefly discussed. The paper provides data on new recently developed tougheners as well as a data set from a commercial application of one of these products. Turning visions into reality Bühler engineering services. Key differences between flexibilisers and tougheners The brittleness of can be reduced by either toughening or flexibilisation, depending upon the nature of the corrosive environment to which the coating would be exposed. While there may be some overlap in the properties that are enhanced, toughening and flexibilisation require different approaches. Epoxy flexibilisation can be achieved with additives that plasticise the matrix or with oligomeric hardeners or reactive diluents which allow for greater free rotation of polymer chains between crosslinks. The main function of epoxy flexibilisers is to increase the elongation of a coating - a stress response, which would be required, for example, in a bending deformation. Examples of flexibilisers that plasticise an epoxy system include solvents and non-reactive diluents; those that increase degrees of rotation include polyaminoamides, polysulfides, aliphatic polyamines and dimerised fatty acids [5]. While epoxy flexibilisers can reduce the viscosity of the resin and improve elongation and impact resistance, they compromise T g, chemical, abrasion and corrosion resistance and, hence, are not recommended for coatings that need to perform in atmospheres of corrosion class greater than C3 as per ISO Epoxy tougheners, on the other hand, can improve elongation, impact resistance, gouge resistance and abrasion resistance without significantly affecting T g, chemical resistance or corrosion resistance [5]. Some examples of toughening agents include core-shell rubbers (CSR), carboxy-terminated butadiene-acrylonitrile, urethane acrylates and copolymers. Toughening agents will have varied impacts on viscosity depending upon their structure; however, compared to epoxy flexibilisers they are higher in viscosity. The higher viscosities of some tougheners, such as CSRs, may result in limited loading in HS systems as well as reducing the ability to add fillers because of the impact they have on processing. Considering extending or building a new plant? Then you are in the right hands at Bühler. Benefit from our long-term experience and vast capabilities in handling complex projects all over the world! We provide single-sourced, complete solutions from start to finish. No matter whether for small or large-scale production our experts will support you in turning your vision into reality. Bühler AG, Grinding & Dispersion, CH-9240 Uzwil, T , F , grinding.dispersion@buhlergroup.com, Innovations for a better world.

3 32 The significance of phase separation in toughening A key distinction between flexibilising and toughening is that in toughened epoxy systems a distinct phase separated from the bulk of a epoxy polymer is present. The phase separation provides domains for energy dissipation without reducing the T g of the bulk epoxy matrix. This phase morphology can be created either by dispersing preformed rubber particles (CSR) or through kinetic phase separation (urethane acrylates, reactive and unreactive block-copolymers, etc.). Depending on the amount of toughener employed as well as its structure, micro- or macro-phase separation may be observed. Scanning electron microscopy (SEM) is an excellent method for analysing the phase morphology of a system. In the SEM images depicted in Figure 1, three different phase morphologies are shown in a coating formulation containing a recently developed toughener. In the absence of the toughener, as in image a, only the epoxy matrix is observed. At 20 weight percent toughener loading, discrete micro-phases are formed as shown in image b. When the loading is increased to 25 weight percent as shown in image c, a co-continuous macro-phase separated morphology develops. The most beneficial morphology for toughened is micro-phase separation, as the toughener is inherently better distributed throughout the epoxy matrix. If the morphology is characterised by macro-phase separation, as in image c, then the full benefits of the toughening agent might not be obtained and T g can decrease [5]. Thus, optimising phase morphology in toughened epoxy systems is critical to maximising the desired coating performance. Performance of different tougheners compared A new liquid toughener with acrylic functionality for and adhesives was recently released. This product offers enhanced elongation and impact resistance without affecting T g, and has a significantly lower viscosity relative to dispersed rubber toughener technologies. This toughener, plus a second acrylic functional toughener and a nonreactive toughener, were evaluated in a standard damage-tolerant coating formulation. The formulations were based on D.E.R. 331, a standard bisphenol-a epoxy resin, and employed a toughener at 10 weight percent loading. The coatings were cured with D.E.H. 530, a modified cycloaliphatic amine. Samples were prepared in a Speedmixer and cured at ambient temperature for seven days. Impact resistance samples were prepared on Q-panel Bonderite 1000, phosphated and P60 chromium treated panels with a ground finish. The dry film thickness was 6-8 mils ( µm). Samples for mechanical analysis (micro-tensile, ASTM D1708) were prepared in open moulds of 3 mm thickness. A summary of the mechanical analysis and impact resistance results can be found in Table 1. The non-reactive toughener had the lowest viscosity, 3.5 Pa s, relative to the reactive tougheners which had much higher viscosities of 25.0 and 18.8 Pa s. Thus, the ease of processing and formulation latitude was best with Benchmark B. When comparing the impact performance of the coat- Table 1: Mechanical properties of a toughened epoxy resin at 10% toughener loading. Control Benchmark A (Reactive ) Toughener #1 (Reactive) Benchmark B (Non-reactive) Toughener viscosity (25 C, Pa.s) 12.0* Wt% additive Tensile (psi) Elongation (%) Modulus (psi x 1000) Direct impact (in lb, 25 C) Indirect impact (in lb, 25 C) Direct impact (in lb, -30 C) fail Not tested Indirect impact (in lb, -30 C) fail Not tested *Viscosity of D.E.R. 331 resin Table 2: Four-point bend flexibility test NACERP Section H4.2 Procedure B. Temp. Formula Bend /PD %Elongation Result 25 C 0 C Control 1/2" % Fail 5% Toughener #1 1/2" % Pass 5% Benchmark #1 1/2" % Pass 5% Benchmark #2 1/2" % Pass 5% Benchmark #3 1/2" % Pass 5% Benchmark #4 1/2" % Pass Control 1/4" % Fail 5% Toughener #1 1/4" % Pass 5% Benchmark #1 1/4" % Fail 5% Benchmark #2 1/4" % Fail 5% Benchmark #3 1/4" % Fail 5% Benchmark #4 1/4" % Fail

4 33 ings, the reactive tougheners provided greater improvement in both the direct and indirect test even though all tougheners produced phase-separated morphologies. Nonetheless, reactive Toughener #1 provided the best performance, passing direct impacts at 160 in lb (18 N-m) and indirect impacts up to 80 in lb (9 N-m), which is believed to be due to its better phase separation. Benchmark A and Toughener #1 were also evaluated at -30 C. At this extreme temperature, well below the T g of an epoxy, Toughener #1 performed best, passing direct impacts at 40 in lb (4.5 N-m) and indirect at 20 in lb. (2.25 N-m). Although the viscosity of Toughener #1 was not the lowest, it is still significantly lower than some rubber emulsion tougheners and provided similar toughness. Differences in toughening mechanisms revealed The tensile strength, elongation and modulus of the coatings were also measured. The control, with no toughener, had the highest tensile strength and modulus - 8,708 and 284,000 psi respectively (approximately 60 and 1960 MPa). The non-reactive toughener provided the next highest modulus and tensile strength but the lowest elongation at 7%. This result was somewhat surprising, but may indicate that the phase separation in the system was very poor compared to Benchmark A and Toughener #1. The highest elongation, 10.6%, was observed when Toughener #1 was used. The tensile strength and modulus decreased the most when this was used. In fact, the modulus was reduced by more than half to 123,000 psi (850 MPa). The formulation employing Toughener #1 produced the least brittle coating as was previously discussed with respect to impact resistance. In Figure 2, the dynamic mechanical spectrum of the coating formulation containing Toughener #1 is shown to provide some insight into the toughening mechanism. In addition to the T g at 67 C, there is a second T g at -69 C. The presence of the T g at -69 C originates from the toughener, and this is one explanation for the improved impact performance at -30 C. The T g of the control system was 62 C. Toughener #1 did not reduce the glass transition temperature of the system; in fact it slightly improved it. Thus, Toughener #1 afforded the greatest improvement in properties relative to the control and the other tougheners evaluated. A case study will next be discussed in which Toughener #1 was employed in a formulated system for an industrial coating application. Flexibility issues examined in case study In a customer trial in which flexibility, pull-off adhesion (ASTM Type 5) and Taber abrasion (ASTM ) were critical performance metrics, five tougheners were evaluated. The coatings contained 5% of a toughener and were cured under ambient conditions for seven and 14 days. Only coatings that passed four-point bend flexibility test NACE RP Section H4.2 Procedure B were subsequently evaluated for adhesion and abrasion performance. In the flexibility screening at 25 C, all trials except the control with no toughener passed (Table 2). At 0 C, only the formulation containing Toughener #1 passed the flexibility test. The control formulation and two formulations containing Toughener #1 were evaluated for adhesion and abrasion resistance. Toughener #1 was used at 5% and 10% loadings to optimise its performance (Table 3). Adhesion and abrasion were evaluated after 7 and 30 days of cure. After 7 days cure, formulations containing Toughener #1 had superior adhesion compared to the control. The better performance of these coatings in this test was due to the ability of the toughener to absorb energy as force was applied. After 30 days of cure, superior performance was still observed for formulations containing Toughener #1; however, the performance gap had shrunk

5 34 Research and development of toughening agents are active areas. 3 questions to William Heaner Are there further application fields for high solids, apart from the already mentioned ones? Transportation applications, such as railcar linings for grain transportation, do not require significant chemical resistance but better flexibility at subzero temperatures. Toughening agents that offer excellent cold temperature performance and can impart flexibility are ideal for addressing such challenges. Is research on toughening agents for HS going on, what additional properties are feasible? Research and development of toughening agents are active areas, and ones in which Dow Chemical is invested. An on-going challenge with high-performance toughening agents is to reduce their viscosity to minimise their impact on processing and enhance their formulation latitude. Additionally, solutions that offer improved performance at lower temperatures are needed. I believe that continued viscosity improvements are feasible without compromising the other properties of toughening agents. Our solutions can be used as high as 50% of the total mass of the resin to provide excellent flexibility. We are utilising our High Throughput Research facilities to assist in the development of toughening agents to address these challenges. Will Heaner Technical Service and Development The Dow Chemical Company WHeaner@dow.com How elaborate is it to incorporate the newly developed tougheners in existing formulations? It can be quite easy to incorporate new toughening agents into existing formulations. It may be necessary to make some adjustments in ratio to account for slight changes in equivalent weight; however, new toughening agents, such as, Voraspec 58 can allow for much easier processing in both formulation and application. relative to the performance at 7 days of cure. The increase in strength of the was expected, but it appears that coatings with Toughener #1 approached their maximum strength more quickly than the control. The order of performance in Taber abrasion at seven and 30 days of cure are the same. The 5% loading of Toughener #1 performed best followed by the control and then the 10% loaded coating. The increase in loss in the formulation with 10% toughener is probably due to decreasing the modulus of the coating too much. From these experiments and with respect to this formulation, it was determined that Toughener #1 provided the best performance and a 5% loading was optimal. Demand for tougheners is expected to increase The market trend in the use of HS is upwards. The ability to improve the toughness of HS coatings will facilitate their growth in field applied, industrial applications where low solids are currently used. Toughening agents can enhance the durability of coatings without compromising the T g, although a compromise in viscosity will be necessary compared to flexibilising agents. Data from recently developed tougheners shows they can provide superior impact resistance, adhesion and abrasion resistance without compromising T g or significantly increasing viscosity. Continued innovations in toughening agents will only serve to increase their relevance in the market and extend their service into ever harsher environments. REFERENCES [1] Linak E., Yoneyama M., Epoxy Surface Coatings, CEH Marketing Research Report, March 2011, A. [2] IRFAB Global Industrial Coatings Markets, [3] Sue H.-J., Craze-like damage in a core-shell rubber-modified epoxy system, Jnl. Mater. Sci., 1992, Vol. 27, pp [4] Spontaka R.J. et al, Model acrylate-terminated urethane blends in toughened epoxies: a morphology and stress relaxation study, Polymer, 2000, Vol. 41, pp [5] Pham H.Q., Marks M.J., Encyclopedia of Polymer Science and Technology, Vol. 9, Epoxy Resins, 2002 (2004 online), pp Table 3: Pull-off adhesion (ASTM 4551-Type 5) and Taber abrasion (ASTM ) results. 7 days cure 30 days cure Formula Adhesion (psi) Taber Abrasion 1,000 cycles, C7 wheel, 1 kg load (mg loss) Adhesion (psi) Taber Abrasion1,000 cycles, C7 wheel, 1 kg load (mg loss) Control % Toughener # % Toughener #