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PART 2 – Exploring the Different Types of Epoxy Curing Agents: Advantages, Disadvantages, and Applications in Speciality Polymer Coatings

In our previous discussion, we delved into epoxy curing agents such as polyamines, polyamides, and phenalkamines. Continuing our exploration, this article sheds light on three more pivotal curing agents: Anhydrides, Phenalkamides, and Mercaptans.

4. Anhydrides:

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Anhydrides are derived from organic acids that typically contain two acid functional groups – which are dehydrated to form the anhydride group. Epoxy systems cured with anhydrides are renowned for their heat resistance, electrical properties, and swift cure at moderate temperatures.

Advantages and Disadvantages:

Curing Characteristics:
Anhydrides provide relatively fast curing times compared to other curing agents such as amines. They can achieve a high degree of cure within a short period, making them suitable for applications that require quick turnaround times. Additionally, anhydrides cure at moderate temperatures, typically in the range of 100-180°C. This moderate curing temperature allows for easier processing and minimizes thermal stress on substrates.

Performance Properties:
Anhydrides offer excellent heat resistance, making them suitable for applications that require high-temperature resistance. Anhydride-cured epoxy systems exhibit good electrical insulation properties, making them suitable for electrical and electronic applications such as laminates, encapsulation, and potting compounds. Additionally, anhydride-cured epoxy systems often demonstrate good dimensional stability, maintaining their shape and properties over a wide temperature range. This characteristic is important in applications that require precise tolerances or minimal thermal expansion.

Considerations and Limitations:
Since anhydride are dehydrated carboxylic acids, they can be very sensitive to moisture. This may cause issues such as blushing or reduced performance if moisture is present during the curing process. It is crucial to handle anhydride-cured epoxy systems in a dry environment and avoid exposure to moisture. Also, anhydrides typically have a shorter pot life (working time) compared to some other curing agents. Therefore, careful consideration and efficient handling are required to prevent premature curing and ensure proper application. Finally, some anhydrides, such as phthalic anhydride, can pose health hazards due to their potential for irritations or sensitizations.

5. Phenalkamides

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Phenalkamides are a relatively new class of epoxy curing agents that are essentially a hybrid between phenalkamines and polyamides. Phenalkamide curing agents have been developed for all-weather applications. They are also well suited for tropical climates, where currently polyamide hardeners are preferred over the short pot life phenalkamine resins. At NSPC, our phenalkamides have been precisely engineered at a molecular level, whereby strong chemical bonds link amide and phenalkamine groups to ensure long lasting performance coatings. Here is a link to our Phenalkamide offerings.

Advantages and Disadvantages:

Curing Characteristics:
Phenalkamides provide a workable pot life that is comparable to polyamide resins in tropical conditions. They generally have a viscosity lower than polyamides but higher than phenalkamines. They exhibit faster reactivity with epoxy resins compared with polyamide systems. They also effectively cure in cold weathers. Furthermore, like phenalkamines, phenalkamides offer good moisture tolerance during the curing process, making them suitable for applications in damp or humid conditions. Their viscosity is typically lower than polyamides but higher than phenalkamines.

Performance Properties:
Phenalkamides, similar to phenalkamines, provide excellent corrosion resistance and chemical resistance, offering protection against a wide range of chemicals, solvents, and corrosive substances. Since phenalkamides are not sensitive to moisture as polyamides are, they do not show blushing when cured in humid or damp conditions. Also, our phenalkamides CURE-O-POXY 8115, 8125 and 8140 are resistant to UV and heat, thereby they can be used for top coat applications.  They also adhere well to various substrates, including metals, concrete, and other materials and exhibit good flexibility and toughness. Consequently, they are best suited for high-performance protective coatings in all-weather climates where they provide the best of both worlds from polyamide and phenalkamine systems.

Considerations and Limitations:
Phenalkamides generally have a slightly shorter pot life compared with polyamides but longer than phenalkamines. They also cure much slower than phenalkamines in extremely cold climates, although faster than polyamides. Similar to phenalkamines, they can be sensitive to moisture during the storage and handling of the uncured material. It is therefore important to store and handle phenalkamide resins in dry conditions to avoid moisture absorption or degradation.

6. Mercaptan Curing Agents:

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Mercaptans, also known as thiol curing agents, are a type of epoxy curing agent that contain sulfur-based mercaptan or thiol functional groups. They react with epoxy resins to form a thioether cross-linking network. They offer good flexibility, low-temperature curing capabilities, and excellent adhesion to various substrates. They are often used in applications where low-temperature or low-viscosity curing is required.

Advantages and Disadvantages:

Curing Characteristics:
Mercaptans offer curing at low temperatures, including room temperature or below. This characteristic makes them suitable for applications where low-temperature curing is required or desired. Mercaptans also provide relatively fast curing rates compared to some other curing agents, such as anhydrides or polyamides. They allow for rapid development of mechanical and adhesive properties in the cured epoxy system. Additionally, Mercaptan curing agents typically have low viscosities, which allows for improved flow and wetting properties, making them suitable for applications that require low-viscosity systems.

Performance Properties:
Mercaptan-cured epoxy systems exhibit good flexibility and resilience. They can withstand mechanical stresses, impacts, and vibrations, making them suitable for applications that require excellent toughness and flexibility. They also promote strong adhesion to various substrates, including metals, concrete, and plastics. Furthermore, mercaptan curing agents tend to exhibit low shrinkage during the curing process, resulting in reduced internal stress within the cured epoxy system. This characteristic can be beneficial for applications where dimensional stability is critical.

Considerations and Limitations:
The major limitation of mercaptans is their strong and distinct odor, which can be perceived as unpleasant. Adequate ventilation and proper handling procedures should be followed to minimize exposure to the odor. Also, some mercaptan curing agents may undergo color changes or yellowing over time, especially when exposed to UV radiation or outdoor conditions. This may affect the aesthetic appearance of the cured epoxy system in certain applications. Finally, mercaptan curing agents can be sensitive to moisture, and precautions should be taken to avoid moisture contamination during storage and handling. Exposure to moisture can cause undesirable side reactions or impact the curing process.

Further readings:

  1. Nadkarni SPC – Technology for more on our technology and specialization.
  2. Polymer Science Learning Center for epoxy resin basics.
  3. Polymer Innovation Blog for more on anhydride curing agents.
  4. Polymer Innovation Blog for more on mercaptan curing agents.
  5. Future Market Insight for epoxy curing market.

Author:
Shikhin Nadkarni

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sikhin

Shikhin is currently a PhD Student in Coatings and Polymeric Materials Department at North Dakota State University. He is a member of Dr. Dean Webster’s Research Group and his research focuses on Non-Isocyanate Polyurethanes as well as novel Epoxy systems. He is passionate about incorporating bio-based materials in polymers so as to reduce our dependance on petrochemicals.

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