What is epoxy resin

Epoxy resin is a compound that contains two or more highly reactive epoxy groups in its molecules. Various types of epoxy resin are commercially available, including the most typical bisphenol A epoxy resin, and they are used according to their characteristics [Table 1]. Epoxy resin is a low-molecular-weight compound that is either liquid or solid, so it cannot be used as a resin for paints or adhesives as it In other words, epoxy resin is usually made into a solid resin with properties suited to its intended use by reacting it with a compound called an epoxy resin hardener.

Table 1: Examples of Epoxy Resins

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Type	Specific examples	Characteristics and main applications
Bisphenol A		・A general-purpose epoxy resin that comes in liquid and solid forms ・Used in paints, adhesives, laminates, and the encapsulation of electrical and electronic components, etc.
Cresol novolac type		・Excellent in heat resistance, chemical resistance, and water resistance ・Semiconductor encapsulation material, heat-resistant laminate ・Powder coating
Biphenyl type		・Low melting viscosity, low hygroscopicity ・Semiconductor encapsulation material
Brominated epoxy resin		・Adds flame retardancy ・Semiconductor encapsulation material, laminate
Alicyclic epoxy resin		・LED encapsulation material ・Low viscosity, long pot life, good color stability and thermal stability ・Encapsulation material for transformers and coils

Epoxy resin hardeners

Epoxy resin hardeners are classified into two types: those that cure at low temperatures or room temperature, such as aliphatic polyamines, polyaminoamides, and polymercaptans, and those that require heating to cure, such as aromatic polyamines, acid anhydrides, phenolic novolac resins, and dicyandiamide (DICY) [Table 2]. In general, epoxy resins cured with low-temperature to room-temperature curing agents have a low glass transition point and become soft cured materials, so they are often used in paint applications. On the other hand, epoxy resins cured with heat-curing agents have a high glass transition point and are excellent in terms of heat resistance and mechanical strength, so they are often used in electrical and electronic components.

Table 2: Examples of epoxy resin hardeners Hardening type

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Hardening type	Type	Specific examples	Characteristics of the compound
Low temperature to room temperature hardening type	Aliphatic polyamine	triethylenetetetramine	・Excellent adhesion ・Small-scale casting, adhesive
	Polyaminoamides		・Long pot life ・Paints
	Polymer captan		・Excellent low-temperature curing properties ・Adhesive
Heat-curing type	Aromatic polyamine	Diaminodiphenylmethane, etc.	・Excellent heat and chemical resistance ・For use in glass fiber laminates
	Acid anhydride	3-Methyl tetrahydrophthalic anhydride	・Compound has low viscosity and cured product has excellent electrical properties ・LEDs, sealing materials, laminates, powder coatings
	Phenol novolac resin	Phenol novolac resin	・Excellent chemical resistance and electrical properties ・Semiconductor encapsulation material, laminate
	Dicyandiamide（DICY）	Dicyandiamide（DICY）	・Excellent in terms of latency (long pot life) ・Laminates, powder coatings, adhesives

※Potential: Curing reaction occurs only when a certain temperature is reached.

Epoxy resin curing accelerator

In the case of epoxy resins that use a heat-curing type curing agent, if the curing agent is an aromatic polyamine, the curing reaction progresses relatively easily with heating. However, in the case of curing agents that are (1) acid anhydride-based, (2) phenol novolac resin-based, or (3) DICY-based, the reactivity between the functional groups they possess and the epoxy groups is low, so the curing reaction of the epoxy resin does not progress easily even when heated. Therefore, in order to speed up the curing reaction and increase the hardness and strength of the resin, it is necessary to use an epoxy resin curing accelerator in combination. For example, liquid bisphenol A epoxy resin and an acid anhydride curing agent will not cure even if they are heated at 120°C for one hour, but if they are used in combination with San-Apro's curing accelerator “U-CAT SA 102”, they will cure in just over ten minutes. In epoxy resins that use heat-curing hardeners, the role of epoxy resin curing accelerators is extremely important, and there are currently many different types on the market [Table 3]. San-Apro also has a wide range of products, including DBU (1,8-diazabicyclo (5,4,0)-undec-7-ene) and DBN (1,5-diazabicyclo (4,3,0)-non-5-ene), which are organic super strong base systems, and the “U-CAT SA®” series, which are organic acid salts of these compounds, as well as the “U-CAT®” series, which includes special amine, urea, and phosphorus systems. , and the U-CAT® series, which includes special amine, urea, and phosphorus-based products, are used in a wide range of applications [Table 4].

Table 3: Examples of epoxy resin curing accelerators

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Type	Specific examples
Tertiary amine Tertiary amine salt		Tris(dimethylaminomethyl)phenol
Imidazole	1-cyanoethyl-2-ethyl-4-methylimidazole	2-ethyl-4-methylimidazole
Phosphine Phosphonium salt	Triphenylphosphine	Tetraphenylphosphonium Tetraphenylborate
Specialty products	3-phenyl-1,1'-dimethylurea	sulfonium salt

Table 4: San-Apro epoxy resin curing accelerators

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Product name	Composition	Main curing agent	Main applications
DBU	1,8-diazabicyclo(5,4,0)-undecene-7	phenol novolak resin	adhesives, paints, semiconductor encapsulants
DBN	1,5-diazabicyclo(4,3,0)-non-5-ene-	phenol novolak resin	Adhesives, paints, semiconductor encapsulants
U-CAT SA 1	DBUのphenolsalt	Anhydride	LED, artificial marble, liquid sealant, powder coating
U-CAT SA 102	Octylate of DBU
U-CAT SA 506	DBU p-toluenesulfonate
U-CAT SA 603	DBU formate
U-CAT SA 810	DBU o-Phthalate
U-CAT 5002	Tetraphenylborate of DBU Derivative
U-CAT 5003	Phosphonium salt, grade 4
U-CAT 18X	Special amine
U-CAT SA 831・841・851	Phenolic novolak resin salt of DBU	phenol novolak resin	semiconductor encapsulants
U-CAT 881	DBN phenolic novolak resin salt
U-CAT 891	DBN special phenol resin salt
U-CAT 3512T	Aromatic dimethylurea	Dicyandiamide (DICY)	Laminated boards, powder coatings
U-CAT 3513N	Aliphatic dimethylurea

(1) Curing accelerator for acid anhydride-based curing agents

The coils of motors, transformers, and electrical and electronic components such as LEDs are impregnated or sealed with resin to protect them from moisture and vibration. Epoxy resins that use acid anhydride curing agents are generally used for this purpose. When used as a sealing material for LEDs, epoxy resin curing agents must not only not impair electrical characteristics, but also not cause rapid heat generation during the curing reaction, and must also not cause the resin to discolor after curing. San-Apro's epoxy resin curing accelerator “U-CAT SA 102” (octylate of DBU) is a typical grade for liquid acid anhydride curing systems, and has a low exothermic effect during curing and produces little discoloration of the cured resin. Figure 1 shows the curing acceleration data (gel time) when using U-CAT SA 102, and Figure 2 shows the curing exothermic data. Rapid curing exothermicity causes the cured resin to discolor and the generated internal strain worsens the adhesiveness. U-CAT SA 102 can be said to be ideal for LED encapsulation applications. A product with even more moderate curing exothermic behavior is U-CAT SA 506. This product is a salt made from DBU and the strong acid p-toluenesulfonic acid, and because the temperature at which its catalytic activity is expressed is high, when used in combination with U-CAT SA 102, the curing exothermic behavior becomes very moderate. Furthermore, in recent years, white LEDs have been used in fields where high-level color reproduction is required, such as backlighting for TV and PC displays and car instrument panels, because they have low energy consumption and a long lifespan. In this application, where LEDs are more difficult to replace than lighting, it is necessary to maintain high-level color reproduction for longer periods of time, in other words, to maintain the transparency of the LED encapsulating material for longer. San-Apro has developed a new epoxy resin curing accelerator that can maintain a high level of color reproducibility by changing the substituents of the cationic components in the structure and other composition modifications. Epoxy resins cured using this product have improved heat and light resistance. In our own durability tests, the durability of LED encapsulants using this new product was improved by around 30% compared to conventional products [Figures 3 and 4]. In addition, as the performance required of LED encapsulants is becoming more diverse, we have added two types of epoxy curing accelerator to our lineup (phosphonium-based 'U-CAT 5003' and ammonium-based 'U-CAT 18X').

Figure 1: Curing Acceleration Characteristics when Using U-CAT SA 102　

Figure 2: Curing exothermic curve when using U-CAT SA 102

Figure 3: Heat resistance of epoxy resins using phosphonium and ammonium systems

Figure 4: Heat resistance of epoxy resins using phosphonium and ammonium systems

(2) Curing accelerator for phenolic novolac resin-based hardeners

Semiconductor chips are usually covered in black resin. This resin is a composite of epoxy resin and filler, known as semiconductor encapsulation material, and protects the silicon chip cut from the silicon wafer from dust, moisture, impact, etc. [Figure 5]. The most common types of epoxy resin used in this sealing material are cresol novolac and biphenyl, while the most common type of hardener is phenol novolac. In addition, more than 70% by weight of fine powdered silica is used as a filler to improve heat resistance and mechanical properties. The following are the requirements for curing accelerators used in this application Firstly, it must have a hardening accelerator performance that matches the molding cycle. In semiconductor encapsulation, the transfer molding method is mainly used, and after the encapsulation material is injected, the mold is removed after 1 to 2 minutes at 175℃ (after that, an after-cure is performed if necessary). Therefore, it is necessary to make the gel time, when the fluidity of the epoxy resin disappears, about 25 to 35 seconds.
Secondly, it must be in powder form. Encapsulating materials are made by mixing solid epoxy resins, hardeners, fine powdered silica and other powdered ingredients together and kneading them with a roller or similar device. Therefore, it is preferable for the curing accelerator to be in powder form so that the ingredients can be well mixed together.
Thirdly, the cured material must exhibit good mechanical properties and electrical insulation. San-Apro's typical curing accelerators suitable for this application are the “U-CAT SA831, SA841, SA851” series, which are salts composed of DBU and phenolic novolac resin. The resulting epoxy resin hardener has excellent electrical insulation, heat and moisture resistance, and adhesion properties. Figure 6 shows data on the curing behavior of epoxy resin when using products from this series.

Figure 6: Cross-section of a semiconductor package (for discrete devices)

Figure 7: Curing behavior when using U-CAT SA 841

③DICY-type curing agent accelerator

DICY-type curing agents are used in epoxy resins used in carbon fiber composite materials such as wind turbine propellers. At room temperature, DICY-type curing agents do not dissolve in epoxy resins, but are dispersed in them, so the mixture of DICY-type curing agents and epoxy resins is stable at room temperature (at high temperatures, they dissolve and react with the epoxy resin). However, DICY alone has a high curing temperature of around 200℃, and because DICY does not dissolve completely, there is a drawback that some unreacted material tends to remain in a particulate form. To address this problem, San-Apro's urea-based curing accelerators 'U-CAT 3512T' or 'U-CAT 3513N' are effective. These curing accelerators do not impair the room temperature reactivity of epoxy resin/DICY, and the curing reaction begins at temperatures of 120℃ or higher, and they have a structure that allows DICY to be highly soluble in epoxy resin. Table 5 shows the curing characteristics (gel time) and storage stability (pot life) data for “U-CAT 3512T” and “U-CAT 3513N”.

Table 5: Curing characteristics and storage stability

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Product name	Amount added (parts)	Gel time (min)	Pot life (days)
U-CAT 3512T	0.5	18	13
U-CAT 3513N	3.0	21	＞60

The above is an overview of epoxy resin curing accelerators, focusing on San Apro's existing products.