CYCLALIPHATIC EPXY RESINS Robert Kultzow* and Stephanie Foxhill Huntsman Advanced Materials Advanced Technology Center 8600 Gosling Road The Woodlands, Texas Presented at a meeting of the Thermoset Resin Formulators Association at the Hyatt Regency Savannah in Savannah, Georgia, September 10 through 11, 2007 This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA
Abstract: Cycloaliphatic epoxy resins are a class of materials of choice for a wide variety of electrical, electronic and structural applications. Having fully saturated molecular structures, they are ideally suited for use in applications requiring resistance to ultraviolet degradation and electrical arc-tracking. This paper describes the commercially available types of cycloaliphatic epoxy resins, presenting their features and applications. Physical property data for a number of systems containing these resins is provided. This paper also explores alternative epoxy resins in formulations where cycloaliphatic epoxies are typically used. INTRDUCTIN Cycloaliphatic epoxy resins are a unique class of materials that are characterized by non-aromatic saturated rings in their molecular structures. There are two commercial types of cycloaliphatic epoxy resins in widespread use. Among their most notable features are inherently low viscosity, coupled with excellent weathering and electrical performance. Both commercial cycloaliphatic resins are ideally suited for applications in severe environments such as near seacoasts and in areas of high industrial pollution. APPLICATINS An important application for cycloaliphatic epoxy resins is for the fabrication of nonceramic outdoor electrical insulating components. This would include electrical insulators, bushings and switchgear parts used for the transmission and distribution of electrical power. Compared to porcelain, components made with epoxy formulations are lighter in weight, show greater impact resistance, excellent electrical properties, reduced maintenance and can be fabricated economically into complex shapes with embedded inserts 2,4. The outstanding resistance to both ultraviolet degradation and carbon tracking provided by these resins are noteworthy 1. They are among the primary requirements for electrical components used in severe outdoor environments. The inherent low viscosity of these resins enables them to be formulated with higher levels of inorganic fillers. This enhances mechanical and electrical track resistance for such components. Encapsulation of transformers, coils and various electronic devices is another important area of application for cycloaliphatic epoxy resins. Their characteristics of low dielectric loss and high electrical resistivity, up to or above their glass transition temperatures provide high performance and reliability in both AC and DC circuitry. In such applications, low viscosity enables facile penetration into tightly-wound electrical coils. Void-free encapsulation is a pre-requisite in high voltage electrical coils. It prevents failure due to internal arcing and partial discharge. The inherent low color characteristics coupled with high resistance to yellowing from ultra-violet light absorption makes the cycloaliphatic epoxies useful in formulations for such applications as light emitting diodes. Cycloaliphatic epoxy resin formulations are used to fabricate many fiber-reinforced structural components. Their low viscosity allows for rapid fiber wet-out in commonly used processes that include filament winding, pultrusion and resin transfer molding. Formulations incorporating these resins can exhibit high glass transition temperatures in the range of 200 C. These
are used to fabricate structural components requiring high temperature service. Cycloaliphatic epoxies show distinct advantages over the glycidyl ether epoxy materials in their ability to produce high bond strengths on poorly cleaned and oily surfaces. A number of commercially available adhesive formulations include these materials for this benefit. CMMERCIAL CYCLALIPHATIC EPXIDES An important and widely used cycloaliphatic epoxy resin is the diglycidyl ester of hexahydrophthalic acid. This resin is referred to Epoxy A throughout this paper. Its structure is shown in Figure 1. recent supply issues has the chemical name of 3,4 epoxycyclohexylmethyl -3,4 epoxy-cyclohexane carboxylate. This resin is referred to as Epoxy B throughout this paper. Its chemical structure is illustrated in Figure 2. Figure 2. Epoxy B structure Typical properties of this material are shown in Table 2. Table 2 C C CH 2 CH CH 2 CH 2 CH CH 2 Typical Properties Color Gardner 1 EEW 131-143 Specific Gravity 1.14 1.18 Viscosity, 25 C 350 450 cps. Flash Point > 200 F Vapor Pressure, 20 C <0.1 mbar Figure 1 Epoxy A structure Typical properties of this resin are given in Table 1. Table 1 Typical Properties Color Gardner 3 max. EEW 164-172 Specific Gravity 1.20 1.25 Viscosity@ 25 C, cps. 700-900 Flash Point 374 F Vapor Pressure, 20 C <0.1 mbar Another widely used cycloaliphatic epoxy resin that has been readily available until Some cured physical properties of these cycloaliphatic epoxy resins, as neat systems (without filler) are given in Table 3. Table 3 [Neat systems cured with HHPA + BDMA] Property @ 25 C Epoxy A Epoxy B Tensile strength (psi) 8,000 9,900 Tensile modulus (psi) 472,000 480,000 Elongation (%) 1.9 1.2 Flexural strength (psi) 19,300 12,900 Flexural modulus (psi) 490,000 440,000 Comp. strength (psi) 22,300 21,200 Tg ( C) 110-125 150-160 Dielec. constant, 60 3.3 3.3 Hz, Diss. Factor, 60 Hz 0.005 0.005 Volume resist. (Ω/cm) 5 E16 5 E16 Dielectric Strength
@ 1/8 (V/mil) 476 433 The most common curing agents for the diglycidyl ester cycloaliphatic epoxy resin include the carboxylic acid anhydrides and acid catalytic types. For outdoor applications it is recommended to use fully saturated curing agents in order to retain both optimum uv light resistance and electrical arc-tracking resistance. The most common acid anhydrides of this type include both hexahydrophthalic anhydride and methylhexahydrophthalic anhydride. At room ambient temperature, HHPA is a solid but melts to a low very viscosity liquid at 35-37 C. The MHHPA is a very low viscosity liquid at room temperature having a melting point of less than -15 C. Lewis acid catalysts including both BF 3 and BCl 3 amine complexes are used to formulate single component systems with this epoxy resin. Such formulations exhibit excellent latency at room temperatures with rapid cure at elevated temperatures. Two-component casting formulations utilizing this epoxy resin that employ saturated acid anhydride curatives commonly contain inorganic mineral fillers. These fillers are used to increase thermal conductivity and modulus while reducing thermal expansion, reaction exotherm during gelation and cost. The recommended fillers for outdoor electrical applications are silica and hydrated alumina. Silane pre-treatment of these fillers has been shown to significantly improve retention of mechanical and electrical properties on long term exposure in outdoor conditions and in hostile environments. Such silane coupling agents provide a stronger bond between the resin matrix and filler surfaces 3,5. Some typical properties of a pre-filled cycloaliphatic epoxy system based on the Epoxy A cycloaliphatic type containing 62% by weight silanized silica filler are given in Table 4. Table 4 Epoxy A / HHPA System Containing 62% by weight silanized silica filler Property @ 25 C Typical Values Tensile strength, psi 12,000 Tensile modulus, psi 1,500,000 Flexural strength, psi 18,000 Flexural modulus, psi 1,450,000 Tg, C 110-120 Dielectric constant, @ 60 Hz 3.5 Dissipation factor @ 60 Hz 0.005 Dielectric strength, 1/8, v/mil 450 Time-to-track @ 2.5 kv (ASTM-D 2303), minutes >2000 Thermal endurance testing is used to predict the long term stability of an insulating material. Property changes such as tensile or flexural strength are measured after exposure to heat. The test times may vary between 5,000 and 20,000 hours. A thermal endurance profile, in the form of an Arrhenius plot, is shown in Figure 3 for anon-filled formulation utilizing the Epoxy A type cycloaliphatic resin 6. These tests were performed in accordance with the IEC 216 Standard. Time at 50% Initial Strength (hours) 100000 10000 1000 100 280 260 240 220 200 180 160 140 120 Temperature ( C) Figure 3 Thermal Endurance Profile: Epoxy A / HHPA System Non-Filled
The 3,4 epoxycyclohexylmethyl-3,4 epoxycyclohexane carboxylate resin has exceptionally low viscosity as well as very low color. Similar to the diglycidyl ester cycloaliphatic type, the most common curing agents used with this epoxy are the acid anhydrides and certain acid catalytic types. This epoxy type is manufactured by epoxidizing a diolefin with peracetic acid. Since chlorine is not present in these raw materials they are free of hydrolysable chlorine. This characteristic is of great importance in formulating high purity electronic encapsulants. Nadic-methyl anhydride is frequently used in systems demanding the highest glass transition temperatures that can exceed 180 C, and with certain high temperature cure conditions, they can exceed 200 C. However, NMA is generally less reactive that other anhydrides and usually requires longer cure cycles to develop full properties. For such high Tg applications this resin can either be used alone or in combination with other epoxy resins for the best balance of physical properties. Certain Lewis acid type catalysts such as BF 3 - amine complex types can be used with this epoxy yielding systems with moderate latency at room temperature coupled with a very rapid elevated temperature cure. In applications that require superior electrical properties in outdoor environments, either of the cycloaliphatic epoxy resins discussed in this paper can be used if cured with fully saturated acid anhydrides. In most of these applications, glass transition temperatures in excess of 125 C are not required and generally not desired. For such cases the Epoxy A resin can be used without modification. However if the type B resin is used for such applications, it would commonly be modified with flexibilizing additives, such as polyether or polyester polyols. This modification would effectively reduce the Tg to the desired range. Where outdoor electrical performance is not a primary requirement but low viscosity coupled with high Tg are of paramount importance, the Epoxy B type resin would be a material of choice. As an alternative, this resin can be substituted for blends of the Epoxy A with low viscosity multi-functional epoxy resins such as 4- glycidyloxy-n,n-diglycidyl aniline. This particular tri-functional epoxy is referred to Epoxy C in this paper. Its chemical structure is illustrated in Figure 3. The Tg values of several blends of Epoxy A and Epoxy C, cured with two different carboxcylic acid anhydrides are shown in Table 5. Figure 4 Epoxy C structure Table 5 Cure: 2 h / 90 C + 1 h / 150 C + 1 h / 200 C System Component Formulations (pbw) Epoxy B 100 100 - - - - Epoxy A - - 75 50 75 50 Epoxy C - - 25 50 25 50 MTHPA 110-105 120 - - NMA - 115 - - 110 130 1-m. imidazole 0.5 0.5 0.5 0.5 0.5 0.5 Tg by DSC, C 183 206 135 158 156 181
CNCLUSINS Cycloaliphatic epoxy resins continue to be used in a wide variety of electrical, electronic and structural applications. Their fully saturated molecular structures make them ideal for use in applications where protection in outdoor and severe environments is needed In such cases they are an attractive alternative to porcelain 4. There are two structurally different commercially available cycloaliphatic epoxy resins today. For many systems intended for use in outdoor high voltage electrical insulating components, either of these epoxy resins can be used if properly formulated. Each of these cycloaliphatic epoxy resins has a number of unique characteristics favoring them for use in certain specialized applications. Formulations that utilize these versatile epoxy resins can be tailored to meet the requirements of many of these specialized applications. ver a Decade, Georgia Power Company, 1996. 5.. Margreve, B. Mistiaen, Silica-Filled Cycloaliphatic Epoxy Resin Insulation, Laboratory Research and Field Experience, CIGRE 12 WG Instrument Transformers, Chapter 2, Manufacture and Quality Tests, GEC AlthomT&D Balteau S.A. 6. E. Hubler, B. Hanisch, Long Term Experience with Cycloaliphatic Epoxy Systems, SEE Conference, Paris, 1994 REFERENCES 1. U. Massen, C. Beisele, Cycloaliphatic Epoxy Insulators Experiences oner 30 Years, EFG-Fachtagung, Bad Nauheim, Germany, (1999). 2. P. Mahonen, utdoor Endurance of Cycloaliphatic Epoxy Insulation, International Symposium on Modern Insulator Technologies, Coral Gables, Florida, (1997). 3. U. Massen, Worldwide utdoor Experiences with Cycloaliphatic Epoxy Insulators over 30 Years, Symposium on Non-Ceramic Insulator Technology, Singapore, (June 1996). 4. P.M. Vagle, Cycloaliphatic Epoxy utdoor Bushings / Insulators In a Heavily Polluted Industrial Area A Ultility s View