They are a class of thermosetting polymers that are finding increasing applications in demanding engineering applications. They were first developed in the 1970s as an alternative to epoxy resins and have been commercially available since the early 1980s. The monomers used to synthesize cyanate ester resins have one or more cyano groups (—OCN) attached to an aromatic ring. When cured, the cyano groups react to form triazine rings providing the crosslinked thermoset network.
Monomer Chemistry and Polymerization Process
The most common monomer used is bisphenol-E cyanate ester, which is synthesized by reacting bisphenol-E with cyanogen bromide. Other derivatives of bisphenol-A and other aromatic compounds are also used. The monomers are low viscosity liquids that undergo an addition polymerization reaction when heated to form a highly crosslinked thermoset polymer network. The polytriazine network formed has high temperature and chemical resistance. The curing reaction can be carried out with or without a catalyst. Catalysts like tertiary amines are typically used to lower the curing temperature and shorten the cure time.
Unique Properties
The unique properties of Cyanate Ester Resins stem from the rigid and thermally stable triazine rings in the crosslinked polymer structure. Some key properties include:
- Excellent high temperature resistance - Glass transition temperatures of 200°C or higher allow use up to 250°C continuously.
- Superior chemical resistance - Good resistance to acids, bases, and solvents better than many other thermosets.
- Very low moisture absorption - Absorption is typically less than 1% even after prolonged exposure.
- Low dielectric constant and dissipation factor - Makes it suitable for electronic applications requiring insulation.
- Good electrical properties - Used as matrix resins for electrical laminates and composites.
- Low shrinkage upon curing - Dimensionally very stable throughout curing and subsequent heating.
- Easy processing - Can be readily fabricated using sheet molding, resin transfer molding etc.
Typical Applications of Cyanate Ester Resins
Due to their unique set of properties, cyanate ester resins have found use in many applications that require high performance at elevated temperatures. Some of their major application areas are:
Printed Circuit Boards
The excellent dimensional stability and electrical properties have made cyanate esters a popular choice for high frequency circuit boards experiencing temperatures up to 200°C. They provide boards with lower losses at microwave frequencies compared to conventional epoxy circuits.
Aircraft and Space Applications
Cyanate ester composites are used to manufacture structural components in aircraft engines experiencing high temperatures over long durations. They are also used to develop radomes, spacecraft components and payloads requiring thermal and structural stability.
Electronics Enclosure and Housings
Components inside electronic devices experience higher operating temperatures. Cyanate ester composites provide housings and enclosures that can withstand over 200°C continuously without degradation.
Optoelectronic Applications
Their low shrinkage and dimensional stability along with high glass transition temperature and transparency in the infrared region make cyanate esters suitable for optical applications like infrared domes.
Alternative to Metal Components
Cyanate ester composites are increasingly replacing metallic parts in applications where weight savings, corrosion resistance and insulation are needed while still being able to operate continuously above 100°C.
Advancements in Cyanate Ester Resin Technology
Research is ongoing to further broaden the use of cyanate esters and address some of the issues associated with their cure reactions:
- Development of flame retardant variants without compromising properties.
- Synthesis of cyanate ester monomers and polymers with tailored Tg and flow properties based on end-use needs.
- Use of latent catalysts that allow for out-of-autoclave molding of large complex parts.
- Blending cyanate esters with thermoplastics to combine processing advantages while maintaining high temperature capability.
- Optimizing processing techniques like resin transfer molding for reduced void formation and enhanced mechanical performance.
- Modelling cure kinetics to better control exothermic cure temperatures and reduce cycle times.
Cyanate ester resins have emerged as an important class of high performance thermosets with a unique combination of thermal, chemical and electrical properties. Their continuing development and newer application areas hold promise to displace conventional materials in niche segments that demand sustained performance at elevated temperatures. Future advancements in monomer design and processing techniques will further stimulate their use in diverse engineering applications.
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