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.

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