Low Outgassing Adhesives: Characteristics, Tests and ApplicationsFebruary 01, 2018
How does the world’s premier supplier of adhesives, sealants and coatings stay on top in a market where competitors are quick to imitate good ideas?
By getting better, of course.
Take, for instance, adhesives and polymers used in aerospace applications. Spacecraft use these compounds for a variety of mechanical, structural and electronic purposes that include bonding, potting, encapsulating, coating and sealing. When subjected to high vacuum and extreme cold, these products must function properly without degrading other components that may be sensitive to outgassed chemical compounds.
The term “outgassing” refers to the release of gas trapped within a solid, such as an adhesive or PCB material. This released gas can condense on optical or electro-optical components such as lenses, fiber optic assemblies or solar cells, which can become clouded or fogged by the outgassed chemicals. Semiconductor manufacturing processes also can be very sensitive to outgassing materials.
The adhesives that outgassing most severely affect are those that cure through the action of solvents or moisture. These can include a variety of pressure-sensitive and contact adhesives and related encapsulation and sealing compounds.
Adhesives directly exposed to space for long periods of time also must resist radiation and atomic oxygen, which are formed in Earth’s thermosphere as a result of exposure to the sun's ultraviolet radiation. Atomic oxygen is a highly reactive form of oxygen that interacts readily with many materials, including all hydrocarbon-based polymers and graphite, primarily on spacecraft flying in low Earth orbit (LEO). Lengthy exposure to atomic oxygen can erode adhesives.
The challenge then is two-fold: achieving sufficiently low-outgassing levels while offering physical properties that suit the given application.
Fortunately, Master Bond can render assistance, and engineers can find a variety of adhesives, encapsulants and sealants that have been specially formulated for low outgassing performance, as well as meet NASA low outgassing requirements by passing an industry-standard outgassing test known as ASTM E595-07.
Among the exacting procedures for evaluating materials prior to their use in space is a standard test for measuring outgassing in adhesives and other materials known as ASTM E595, developed by NASA. Its full title is, “A Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment.” ASTM International is a well-respected materials standards organization and ASTM E595 gauges key material parameters such as total mass loss (TML) and collected volatile condensable materials (CVCM) to evaluate the mass changes of different materials in a vacuum environment as would occur due to outgassing. Simply put, the test determines the volatile content of material samples placed in a heated vacuum chamber.
Samples to be tested are first preconditioned at 50 percent relative humidity for 24 hours and then weighed. Following this, they go into the test chamber for another 24 hours with the temperature set at 125° C and the vacuum at a minimum of 5x10-5 torr (by way of comparison, so-called deep space is 10-14 torr). During the time in the test chamber, volatiles that outgas from the sample escape through a port and condense on a cooled (25° C) collector plate. The sample and condensate on the collector plate are then weighed to determine the TML by the sample and the amount of CVCM on the collector plate. Materials pass or fail the test based on these TML and CVCM measurements.
If CVCM does not exceed 0.10 percent and TML does not exceed 1.0 percent, the material passes the test. If CVCM exceeds 0.10 percent—regardless of TML—the material fails. However, if CVCM is less than 0.10 percent, but TML exceeds 1.0 percent, a further measurement is made to see how much water vapor the specimen can reabsorb after reconditioning exposure, which takes place in a chamber with 50 percent relative humidity at 25° C for an additional 24 hours. The water vapor regained (WVR), expressed as a percentage, represents the mass of water vapor recovered compared to the mass of the original specimen. If the difference between the TML and WVR does not exceed 1.0 percent (and CVCM does not exceed 0.10 percent), the material passes the test.
The following is a pass/fail summary of the measurements taken:
· If CVCM <0.1 percent and TM <1 percent, the material passes.
· If CVCM <0.1 percent and TM >1 percent, the material can pass if the TML-WV <1 percent.
· If CVCM >0.1 percent or TML-WV >1 percent, the material fails.
UV-Cured Low Outgassing Products
It is well-known that UV curing can substantially decrease drying times compared to room temperature or heat-curing chemistries. Shorter lead times, in turn, can improve a customer’s competitiveness and profitability in the marketplace by reducing labor and assembly costs. What’s more, UV curing is obviously better for the environment than using solvents. Until recently, UV-curable adhesives were not able to pass ASTM E595, which kept these fast-curing adhesives out of some otherwise suitable applications; however, that’s no longer true.
Master Bond’s UV-curable adhesive systems offer low outgassing properties that pass NASA low outgassing specifications and exceed the ASTM E595 test requirements.
As a result, these UV formulations can be used in a wide array of optical, electro-optical and vacuum applications—from disk drives to semiconductor manufacturing to automotive electronics to displays to printed circuit boards. These products can be conveniently dispensed manually or automatically and require little manufacturing space. Select systems exhibit excellent thermal stability; low shrinkage; optical clarity; resistance to thermal cycling, chemicals, moisture and solvents; and meet safety, health and ecological concerns.
Let’s look at one example, Master Bond’s UV22DC80-1. A one-component, low viscosity, nanosilica-filled, epoxy-based system for bonding, sealing and coating, UV22DC80-1 is formulated to cure readily upon exposure to UV light. Curing at thicknesses of 0.01-0.02 in. can be achieved in seconds with exposure to UV at 365 nm with 20-40 mW/cm2 of energy. Faster cures can be achieved at higher energy outputs. Shadowed areas where the material is not exposed to UV light are curable with the addition of heat (80° C for 40-60 minutes). This dual-cure low temperature mechanism is advantageous when working with plastics or other heat-sensitive substrates. In particular, dual cure systems are useful in high-tech applications such as aerospace, optical and opto-electronics, among others, to rapidly fixture parts via UV curing followed by heating..
As a lower-viscosity system, UV22DC80-1 features a cationic cure mechanism for the UV portion of curing. This, combined with it being a nano-filled system, results in exceptionally low shrinkage and outstanding dimensional stability. UV22DC80-1 is not oxygen-inhibited and bonds well to a wide variety of substrates including metals, glass, ceramics and most plastics.
Not content to sit on its laurels, Master Bond continues to make other significant technical advancements. Recently, the engineering troops at Master Bond developed a high temperature-resistant, NASA-low-outgassing-approved epoxy for die attach adhesives, which are increasingly accepted for microelectronic semiconductor packaging as well as for chip on board (COB) assemblies. Die attach offers outstanding bond strength for adhering components directly onto printed circuitry. Formulated specifically for die attach applications, Master Bond EP17HTDA-1 is a one-component epoxy that can also be used for bonding and sealing. EP17HTDA-1 can be used in vacuum situations as it passes NASA’s low outgassing testing. EP17HTDA-1 has the consistency and flow needed for die attach applications and features a high glass transition temperature (Tg) of 195-205° C. It also offers excellent electrical insulation properties—even at elevated temperatures—and low exotherm upon curing. This system features excellent die shear strength (force sufficient to shear the die from its mounting) and can be used in typical die sizes ranging from 4 to 400 mm2.
As a one-component, heat-cured formulation, EP17HTDA-1 does not require any mixing for use and is curable in the temperature range of 300-350° F. A typical curing schedule is two to three hours at 300° F or one to two hours at 350° F. It bonds well to a wide variety of substrates, such as metals, ceramics, plastics and composites. It also has minimal shrinkage upon curing. This epoxy is available for use in 10- and 30-cc syringes and has a shelf life of three to six months when stored at 40-50° F.
For More Information
For a list of Master Bond’s line of ASTM-E595 compliant epoxy, silicone and UV-curable adhesives, visit https://www.masterbond.com/certifications/nasa-low-outgassing.