Inside Story: Low Outgassing Adhesives from Master Bond

Master Bond (Hackensack, NJ) focuses on developing the best in epoxies, silicones, UV cures, and other specialty adhesive systems including compounds that have passed NASA low-outgassing specifications and exceed the ASTM E595 test requirements. These products are successfully employed in applications ranging from disk drives and flat panel displays, to advanced avionics and aerospace components. Tech Briefs spoke with Rohit Ramnath, Senior Product Engineer, and Venkat Nandivada, Manager Technical Support, to learn more about these low-outgassing products.

Tech Briefs: How important are low-outgassing adhesives for applications in the aerospace, electronics, and optical industries?

Rohit Ramnath: Low-outgassing adhesives and potting compounds are widely used in various applications within these industries, mainly because they ensure there is little to no volatile organic content, or VOCs. Some examples of devices where low outgassing typically is critical are photonic sensors or optical lenses. They can also be used in manufacturing components for satellites, where the adhesive could be subjected to very high levels of vacuum. Considering it is outer space, outgassing is quite important.

Tech Briefs: How is outgassing measured and what is the ASTM E595 specification?

Venkat Nandivada: The industry standard for measuring outgassing in adhesives and other materials is ASTM E595. This test was developed by NASA to screen outgassing materials for use in space and determines the volatile content of the material that is placed in a heated vacuum chamber. The test chamber is at 125 C with a vacuum level around 10-6 Torr. During this time in the test chamber, volatiles that outgas from the sample escape to a port and condense on a collector plate. The particles that condense on the collector plate are then weighed and this determines the total mass loss, which is known as the TML. Also, the amount of collected volatile condensable materials is known as the CVCM. The materials will pass or fail the test based on these TML and CVCM measurements. To pass the test, the TML should be a maximum of 1% and the CVCM needs to be a maximum of 0.1%.

Tech Briefs: What new developments have been made in the various curing mechanisms for adhesives that offer low-outgassing properties?

Ramnath: Traditionally, curing mechanisms for low-outgassing rated adhesives can range from two-component, room-temperature-curable epoxies to one-component, heat-curable chemistries. Recent innovations in adhesive technology have enabled not only the use of ultraviolet (UV) light-curable adhesives to meet the ASTM E595 specification, but also the development of low outgassing dual-curing systems. What we mean by dual-curing is that you can cure them either by UV light and/or moderate heat. This means that even shadowed-out portions will not remain uncured. This is very useful, especially in cases where ultrafast cures are needed while at the same time, excessive heat cannot be added because of sensitive electronics. Additionally, this class of dual curing adhesives works even when none of the substrates are optically clear.

Tech Briefs: What are some of the technical advancements that low-outgassing adhesives have made for electronic applications?

Nandivada: There are special grades of low-outgassing adhesives that are suitable for use in various electronic devices. Some uses for low-outgassing adhesives in the electronic industry include conformal coatings, potting, underfills, and die attach adhesives. Advancements have been made in order to achieve more convenient handling and processing along with rapid curing. This becomes important especially in applications with high-volume production. Some specialized one-component epoxies are available for use that would fully eliminate the need for measuring and mixing, and also provide unlimited working life or unlimited pot life at room temperature. Some of these products also offer rapid curing, especially at higher temperature around 125 C.

Tech Briefs: Can you give us a real-world example of where a low outgassing adhesive has been successfully used?

Ramnath: There are published research articles that reference specific Master Bond part numbers or products and many include applications where low-outgassing adhesives performance was critical. Among them, none has been as stringent as the requirements where one of our epoxies was used for the Laser Interferometer Gravitational-Wave Observatory (LIGO), which was a large-scale physics experiment to detect gravitational waves. Master Bond EP30-2 was qualified for several bonding and sealing applications in the LIGO project. Considering the sensitivity involved in these applications, meeting the ASTM E-595 standard was not sufficient. EP30-2 surpassed more stringent requirements of having a very low outgassing rate for various gases, such as hydrogen, helium, and total hydrocarbons. Most of the critical testing was done at MIT Lincoln Lab.

Tech Briefs: How do silicones compare among the other adhesive chemistries for their outgassing properties, and what are the advantages as well as tradeoffs of using a silicone?

Nandivada: Let’s take an example of epoxies versus silicones. Epoxies are definitely more widely used in applications where low outgassing is critical. This is because general-purpose silicones are not known for providing low outgassing properties. There are some special high-purity grades of silicones that actually meet the ASTM E595 specification. I mentioned previously that a material passes the standard if it has a maximum TML of 1%, but some of these special silicone grades have a TML of just 0.1% or less. These silicones are utilized in many sensitive optical applications or electro-optical, electronics, aerospace, or similar industries and also in devices where low stress is extremely important. Silicones have a very high degree of flexibility and low modulus. That’s why they ensure that in these devices, stress is minimum. They are also known for their ability to withstand rigorous thermal shock. These would be among the advantages of a low-outgassing silicone as compared to a highly stiff or very rigid low-outgassing epoxy. The main tradeoff is that silicones are not known to provide structural strength. Many epoxies are suitable for applications where you need high shear or tensile or compressive strength – that is where you’d go to an epoxy, and it would perform much better.

Tech Briefs: What’s on the horizon for Master Bond low-outgassing adhesives?

Ramnath: Some areas that Master Bond is currently focusing on revolve around speed of cure or speed of the process where the adhesive is being applied. Another topic of relevance would be reducing the weight of components in various assemblies because that also reduces the overall carbon footprint, which is critical in this day and age. Some of the other aspects that Master Bond is innovating and keeping an eye on would be novelty fillers ranging from micron-sized fillers for electronics applications and delving into nanoparticle fillers. We’re also paying close attention to sustainability of new raw materials, while not compromising on meeting low outgassing requirements. Maintaining that balancing act is very important.

Nandivada: We’re working on a graphene-based epoxy, which will be launched soon and provides very thin bond lines. It can be used in applications where high heat dissipation is required with low-outgassing properties. We’re also working on an LED curing product that cures at 405 nm and provides low-outgassing properties, especially in optical applications where UV adhesives cannot be used.

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