Bonding Substrates with Different Thermal Expansion Coefficients

Interview conducted by G.P. ThomasApr 4 2013

In this interview, Venkat Nandivada Manager of Technical Sales at Master Bond Inc talks to AZoM about the tricky task of bonding two dissimilar substrates together.

In your opinion, what is one of the toughest challenges regarding the bonding of dissimilar substrates?

Over the wide range of applications related to adhesives, one of the toughest challenges is bonding together substrates with different coefficients of thermal expansion (CTE).

Some real-world examples are listed below:

• rubbers to metals
• ceramic to plastics
• aluminium to glass
• glass to plastics

Furthermore, thermal cycling and service temperature extremes can cause extra problems and sometimes complicate an already difficult situation.

What qualities must be taken into account when deciding how to bond two dissimilar substrates?

There are many factors that need to be taken into account with regards to bonding these difficult substrates together.

Cure schedule, service conditions and joint design all need to be considered when selecting a possible product for bonding two materials with differing CTE’s.

Could you elaborate on the types of compound that have been developed to meet this need?

These cases require specially formulated compositions that basically come in two major categories.

The first are “toughened” two and one component systems. They comprise a major amount of a traditional high strength epoxy and a smaller amount of an elastomer modified system designed to accommodate thermal and/or mechanical shock and vibration induced stresses.

A second category includes much more flexible compounds via the incorporation of flexible epoxies, urethanes, polysulfides or silicone type polymers, however the bond strength is slightly compromised.

Has Master Bond formulated any unique products for where low thermal expansion is desired?

The two part epoxy system EP30LTE has been developed specifically for applications which require excellent dimensional stability as well as low shrinkage and low CTE. Further benefits include its excellent properties of electrical insulation and the fact that it is easily dispensed. The CTE of EP30LTE is 15-18 x 10^-6 in/in °C.

EP30LTE-LO, a variation approved by NASA, has the same exceptional properties of EP30LTE and the low outgassing requirements of ASTM E595 required for critical optical and aerospace applications.

What are the key properties that an adhesive system must possess to be beneficial under extreme thermal stress?

To compensate for the CTE mismatches, it is necessary to consider an adhesive system that would be compliant under cyclic loads caused by temperature swings.

For bonding dissimilar substrates, low modulus stress absorbers are preferred as they take up the deflections of the adjoining materials, allowing the bonded entities to move more freely with little constraint.

How do these systems improve the bonding of substrates with different CTEs?

These systems enhance resistance to thermal cycling as well as shock and vibration. They also minimize the potential of failure from fatigue or hysteresis due to the forces generated in the joint.

Does Master Bond produce a “toughened” two or one component system?

A no-mix one component toughened system produced by Master Bond is Supreme 10HT.

This offers brilliant peel and shear strength and can withstand thermal cycling, mechanical shock and vibration. Supreme 10HT has a CTE of 45-50 ppm/°C.

Silicone elastomers are widely employed for bonding dissimilar materials. Do you offer any rapid cure systems?

Master Bond’s MasterSil 711 is a silicone elastomer compound with many useful specifications.

With a CTE ranging from 250 to 300 ppm/°C MasterSil 711 has a fast and non-corrosive cure.

Further properties of MasterSil 711 include:

• Serviceable up to +400ºF
• high thermal shock resistance
• extremely soft
• outstanding flexibility and elasticity
• tack free time of 2 to 5 minutes at room temperature

In conclusion, how should people approach the challenge of bonding two substrates with different coefficients of thermal expansion?

To make sure that the right product is chosen to meet the most critical specifications of this tricky application, careful consideration of the situation is needed.

Service conditions, CTE mismatches and processing requirements are just some of the requirements that need to be taken into consideration.

About Venkat Nandivada

Venkat Nandivada has been the Manager of Technical Sales at Master Bond Inc since 2010. He has a Masters in Chemical Engineering from Carnegie Mellon University. His hobbies include table tennis, cricket and soccer.