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2023 CoatingsTech Conference Highlights

Conference Highlights June 26-28 | Cleveland, Ohio Coatings Technologies: Adaptation in a Complex World ACA’s 2023 CoatingsTech Conference was a remarkable success that brought together members of the coatings industry […]

ACA Industry Awards Dinner

[…] world. ACA and related local coatings organizations provided the opportunity to present new products and technologies, while building relationships in the industry.” Equi holds a BS in Chemical Engineering from […]

Members Only Industry Pulse: Industrial Coatings

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Members Only Industry Pulse: Architectural Coatings

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Polyamine Curing Agents Meeting the Industry Need for Enhanced Productivity

[…] challenging world of  industrial, protective, and marine coatings, two-component (2K) epoxy systems are established as the benchmark technologies due to the combined offerings of excellent corrosion protection and compliance with regional volatile organic compound standards. Today, productivity has emerged as a major driver, and innovation focuses on developing epoxy coatings that have greater application versatility while providing enhanced performance properties, such as dry speed, rapid recoat, and through-cure. For marine and protective coatings, the need is for faster cure and blush-resistant coatings when applied under adverse, low temperatures conditions. In the OEM sector, the wet-on-wet application process means there is a requirement for epoxy systems to provide rapid overcoatability with polyurethane and/or polycarbamide topcoats within minutes after initial application of the primer. In this sector, the drivers are to increase overall productivity with faster application of multiple layers coupled with lower bake temperatures which can provide energy savings. This article will focus on the performance attributes of a novel polycyclic-aliphatic amine and its use in the development of new epoxy curing agents designed to provide benefit in the above markets. The supporting data includes the functional properties thermal analysis, glass transition (Tg ), and infrared (IR)-cure profile, confirming the rapid through-cure and crosslinking capabilities in epoxy systems. In addition, a review of model coating formulations and their key performance attributes, including the rapid recoat times, improved intercoat adhesion, and excellent corrosion protection properties will be discussed. Introduction Global megatrends are reshaping the world we live in, driving common requirements of improved productivity and reducing costs, while addressing emerging environmental concerns. Epoxy coatings used in marine and protective coatings are based on either solid or liquid epoxy resins, derived from bisphenol A digylcidylether and cured in combination with polyamides or modified aliphatic or cycloaliphatic amine hardeners. Typical modifications include amine adducts, Mannich bases, phenalkamines, and specialty ketimine curatives,1 designed to ensure an optimum balance of handling and end performance properties. With the introduction of maximum volatile organic compound (VOC) limits in coating applications, development work in epoxy coatings has moved away from using traditional solid epoxy resins (SER) to systems based on the lower viscosity liquid epoxy resin (LER). A standard solvent-free LER has a viscosity of 10,000 mPa.s and is characterized by an epoxy equivalent weight (EEW) ±190 (functionality ±2). The use of LER enables the formulator to achieve higher coating solids (≥80%) vs the traditional solventborne SER systems where solids are in the 40%–60% range. This approach influences the handling and performance characteristics of the formulated coating. Reaction kinetic studies demonstrate that there is a negative impact on the workable pot life coupled with an extension of the dry time. The latter effect is due to the polymeric network having to react and build up sufficient molecular weight to reach the gel point or dry-to-touch state, whereas with the SER systems, these are already high molecular weight polymers and dry-to-touch or lacquer dry is observed as soon as the solvents evaporate from the coating film.2 To overcome the slower dry speed, formulators typically incorporate tertiary (3°) amine accelerators (e.g., tris -2,4,6-dimethylaminomethyl phenol) at ±5% by weight based on active curing agent into the formulation. The acceleration mechanism of the 3° amine is that it polarizes the C–O bond in the epoxy group and makes it more susceptible to nucleophilic attack with the primary (1°) and secondary (2°) amines present in the additional curing agent.3 Formulators can only use small quantities of this type of accelerator because a high concentration can result in driving the homopolymerization of the epoxy resin in favor of the preferred crosslinking reaction between the epoxy resin and amine curing agent. Excessive homopolymerization often results in brittle coatings and free unreacted amines, the presence of which may result in a decrease in the corrosion resistance properties of the cured film. Our approach to amine design has resulted in the development of a new polycyclic amine, Poly , that has a balance of 3°, 2°, and 1° amines built into the polymeric backbone.4 Poly delivers a unique balance of properties by acting as a reactive accelerator and co-curative all in one. The amine is capable of enhancing the molecular weight build up in applied epoxy so coatings can reach their dry-to-touch state faster, while driving the crosslinking reaction so that coatings achieve rapid through-cure at both ambient and low-temperature cure conditions. This is evident when monitoring the degree of reaction via infrared (IR) spectroscopy. The Poly shows a higher level of hydroxyl formation compared to the standard tertiary amine accelerator, which in the presence of liquid epoxy resin is shown to form ether linkages (Figure 1). High ether linkage formation is a clear indicator that the epoxy system undergoes homopolymerization rather than amine-epoxy crosslinking during the curing process. The novel polycyclic amine technology enables rapid property development when formulated with conventional polyamides, modified polyamides, aliphatic, and other formulated epoxy curing agents. Examples of curing agents developed based on Poly amine include two new polycyclic amine polyamides, RDPA-1 and RDPA-2. The new polyamides undergo rapid cure, which enables application of topcoats based on similar epoxy or polyisocyanate technology within 15–30 min while maintaining excellent intercoat adhesion and corrosion protection. It provides excellent film appearance without loss of gloss, distinction of image and surface wrinkling, and dive-back of topcoats into the primer. This performance attribute allows applicators to spray-apply multiple layers within quick succession, to increase the overall productivity of the coating application at the job site. Key Features and Benefits of the Poly Amine Building Block Common amines used in the design of amine curing agents are classified as aliphatic or cycloaliphatic amines, examples of which include diethylenetriamine (DETA), triethylenetetramine (TETA), diaminocyclohexane (DACH), and isophoronediamine (IPD). Although aliphatic amines such as DETA and TETA have high functionality and reactivity, they have a strong tendency to blush and form amine carbamate due to poor compatibility of the amine with the epoxy resin. On the other hand, cycloaliphatic amines have excellent compatibility with epoxy resin because of the cyclocaliphatic backbones but have slower reactivity than aliphatics, especially at low temperature. Good compatibility between curing agent and epoxy resin is essential to provide coatings with good surface appearance, excellent overcoatability, and corrosion resistance. It has been a challenge in epoxy coatings to design a new amine building block that possesses the benefits of both the aliphatic and cycloaliphatic amines: the reactivity of an aliphatic amine and the resin compatibility of a cycloaliphatic amine. As highlighted, Poly is a novel polyheterocyclic amine that crosslinks with an epoxy resin delivering fast through-cure while maintaining good resin compatibility over a range of application temperatures. Poly is a polymeric amine with moderate viscosity, has a low color (water white), and can be used as a sole curing agent or as a co-curing agent with other amines. The general handling properties are outlined in Table 1, and the following sections detail the amine’s unique performance properties.  Fast Property Development of Poly Amine Poly amine was evaluated against aliphatic amines, cycloaliphatic amines, and Mannich base curing agents to exemplify its fast development of coating properties. For this, Poly amine was plasticized with ±30 wt% of benzyl alcohol, comparable to the concentration contained in many commercial curing agents, and cured with standard bisphenol A liquid epoxy resin (EEW=190) at 1:1 stoichiometry. Clear coating formulations for thin film set time (TFST), Persoz hardness, and gloss were deposited on glass substrates at 150 mm wet film thickness using a bird applicator. The TFST was determined using a Beck-Koller recorder, in accordance with ASTM D5895. Persoz hardness was conducted in accordance with ASTM D4366 after coatings were cured at 23°C, 10°C, and 5°C, and 50% RH for designated cure duration of one day, two days, and seven days. Gloss was determined at an angle of 20° and 60° using a Gardner gloss meter according to ASTM D523. Measurements were made with the glass panel placed on a black cardboard background to minimize reflection. The test summary in Table 2 clearly shows that clear coatings based on Poly delivered fast dry speed and Persoz hardness development at ambient and low temperatures. The Poly amine demonstrated faster dry speed than a Mannich base curing agent, which is typically used as fast curative especially for low-temperature cure. Poly coatings also exhibit good coating appearance similar to cycloaliphatic amine, and better than the Mannich base curing agent. High gloss coating is an indication of good compatibility between resin and curing agents. By comparison, the aliphatic amine exhibited poor compatibility with the epoxy resin and, as such, the resultant coatings were greasy and the Persoz hardness could not be obtained. In addition, Figure 2 shows the cure and property development of clear coatings across a range of application temperatures, from ambient temperature to low temperature down to 5°C. The fast cure property is demonstrated by the early hardness development and ultimate hardness both at ambient and low application temperatures, thus making the Poly amine an ideal amine building block for a variety of coating systems where low temperature cure is critical. Poly amine not only delivers fast cure speed, but also shows good compatibility with epoxy resin.  Fundamental Study of Fast Cure Mechanism of Poly Amine To understand the cure mechanism of Poly amine, dynamic mechanical analysis (DMA) was utilized to monitor the cure process. DMA provides the mechanical property information and the crosslinking density of the cured samples. The crosslinking density is expressed as the average molecular weight between crosslinking points, Mc, and is calculated from the analysis data, while mechanical property such as storage modulus was measured during the analysis.5 In this experiment, Poly amine was compared with a cycloaliphatic amine and an aliphatic amine. The amines were cured with standard bisphenol A liquid epoxy resin at 1:1 stoichiometry, and the samples were prepared by making the plaques in silicone rubber molds and allowed to cure at ambient temperature and humidity for a week. Figure 3 compares the average molecular weight between crosslinking points, Mc, of the samples of the Poly amine, an aliphatic amine, and a cycloaliphatic amine after one-day, three-day, and seven-day cure. Mc of Poly amine is low and remained unchanged after one-day cure, while Mc of aliphatic amine and cycloaliphatic amine are higher after day 1 and then decrease over time, indicating lower initial degree of crosslinking. However, further crosslinking develops over time, thus a longer reaction time is necessary to achieve full through-cure. Mc of cycloaliphatic amine was the highest, suggesting the slowest reaction rate and lowest crosslinking density. Furthermore, Figure 4 shows the comparison of storage modulus, G’, after one-day cure. G’ of Poly remained relatively flat and was the highest among the samples, indicative of highest degree of cure, and no post cure in the rheometer. However, G’ of aliphatic amine and cycloaliphatic amine shows a greater increase, indicating significant post cure in the rheometer. Cycloaliphatic sample exhibited the greatest increase in G’ in the rubbery region, suggesting that it had the lowest degree of cure at ambient temperature. The DMA data of Mc and G’ demonstrate that Poly is a much faster curing agent than aliphatic and cycloaliphatic amines, and can reach a high degree of through-cure and crosslinking within a shorter time period. New Polycyclic Polyamide Curing Agents The Poly amine is further derivatized to prepare a range of amine curing agents. Of specific interest is the use of the amine is the synthesis of polycyclic-polyamide curing agents that deliver enhanced rapid dry characteristics. Two examples, RDPA-1 and RDPA-2, have been developed and will be reviewed in this article. Both new curing agents provide rapid through-cure at both ambient and low application temperatures, and are capable of being rapidly overcoated either self-on-self or with polyisocyanate-based technologies within a 15 min window after initial application. The technology also delivers the high levels of corrosion resistance properties demanded by the marine, protective, and industrial maintenance coating markets. The typical handling and performance properties of the new curing agents are summarized in Table 3 and subsequent sections below. Near FTIR spectroscopy (NIR) is a powerful and versatile technique for monitoring transient chemical change during the cure process.6 It offers a unique possibility to obtain detailed information about molecular orientation and relaxation behavior and is an effective tool to monitor the extent of the epoxy-amine reaction. Using an NIR spectrometer, Model 6500, the conversion of oxirane (epoxy) and primary amine during the cure was monitored by the C–O stretch of oxiran ring at 1646 cm-1, and the N–H stretch of the primary amine at 2026 cm-1. Figures 5 and 6 show the conversion of primary amine and epoxy during the cure process, comparing the RDPA-1 with a standard high solid polyamide (HSPA-1) and the same polyamide blended with a tertiary amine (HSPA-1a). In Figure 5, the primary amine conversions were similar among the three samples due to the fast reactivity of available primary amines with the epoxy. However, in Figure 6, RDPA-1 exhibits the fastest epoxy conversion—faster vs the HSPA-1a and significantly faster vs the unmodified polyamide HSPA-1. Although HSPA-1a showed a fast epoxy consumption vs HSPA-1, there was a lower level of hydroxyl formation in the matrix. The data indicates that a high percentage of the epoxy conversion is a result of the epoxy homopolymerization reaction instead of the amine crosslinking with the epoxy resin. The conclusion is further collaborated by dynamic mechanical analysis. Table 4 shows the Tg and Mc of the three polyamide systems. The coating system HSPA-1a containing the tertiary amine accelerator exhibited the highest Tg and lowest Mc, followed by RDPA-1, whereas the standard polyamide HSPA-1 exhibited the lowest Tg and highest Mc values. Both modified polyamides had lower Mc vs HSPA-1, indicating a higher degree of crosslinking. However, in the case of HSPA-1a, the presence of the tertiary amine also drives the competing homopolymerization reaction, which in turn, leads to lower Mc, a higher Tg, and potentially an increase in coating brittleness. Further evidence supporting the excellent low-temperature cure characteristics of RDPA-1 is shown by the faster dry speed development obtained in clearcoat formulations (Figure 7). When used with liquid epoxy resin, the thin film set times as measured using a Beck Koller (BK) instrument offer a significant improvement over both HSPA-1 and a special modified high solid polyamide adduct (HSPA-2), commonly promoted for low-temperature cure applications. At room temperature, the phase III, thin film set time is 4 h compared with 10 h and 7 h, respectively for HSPA-1 and HSPA-2. At lower application temperatures, the performance benefits of RDPA-1 are clearly demonstrated where a phase III dry time of 14 h is achieved, compared with 48 h for HSPA-1 and 30 h for HSPA-2.   Additional analysis via DSC highlights the differences in the degree of cure development of the polyamides at low application temperatures. Analysis was conducted via measurement of the residual exotherm, during the curing process. Samples were prepared at ambient temperature and then the sealed DSC cells were immediately stored at 5°C in a climate chamber for one to seven days. After the allotted cure time, samples were removed and scanned by DSC (TA Instruments–model Q200) at a ramp rate of 10°C/min. The percentage cure for each sample was calculated by using the following equation.7 When cured with the standard liquid epoxy resin, DSC analysis shows that the new curing agent RDPA-1 undergoes excellent cure development at 5°C (Figure 8). When compared with HSPA-1 and HSPA-2, RDPA-1 achieves a degree of cure after one day of 64%, which is two times faster than HSPA-1 which only achieved 30% conversion. HSPA-2 achieved 40% degree of cure, however, still significantly slower vs the new curing agent technology. After seven days’ cure at 5°C, the extent of cure for RDPA-1 was >95%, compared with 55% and 83% for HSPA-1 and HSPA-2 respectively. The RDPA-1 curing agent […]