Tough Polymer Solves Aerospace Optics Assembly Problem

Master Bond a worldwide adhesive manufacturer solved a major aerospace manufacturer’s problem with the special development of its EP21TDC-2LO formulation. The demand for more sophisticated and reliable optical devices is presenting design and manufacturing engineers with increasingly more difficult bonding challenges. Recently, a major aerospace company approached Master Bond with a difficult, design problem but only after first spending months of unsuccessful engineering effort. As described by Don Dodge the project engineering manager: “We realized the design team needed urgent help with the selection of a suitable adhesive when design schedules began slipping, due dates were passing and project costs started escalating. The crux of problem was in specifying the right adhesive to adhere the coated optical glass lens into its aluminum supporting frame. Adhesives were being recommended and tested but one by one they failed on key parameters. The completed assembly had to be capable of withstanding severe mechanical shocks and vibration as well as large temperature cycling from as low as -50°C to as much as 120°C. In addition, use in a high vacuum environment required very low outgassing. We spent months purchasing and testing compounds for the long term effects that stress can play on the performance of bonded optics. The effect of stress can be delayed for days, weeks or even months as some epoxies and UV adhesives complete their cure cycles or age.” We were running out of solutions by the time we finally called on the Master Bond engineers to analyze all the requirements an offer a solution. Instead of suggesting an off the shelf product, Master Bond developed a specially designed elastomer modified epoxy resin system which successfully not only met but significantly surpassed all of our adhesive performance and manufacturing requirements.” Robert Michaels, V.P. Technical Sales at Master Bond describes the principles used to develop an adhesive that can meet the myriad of concurrent performance criteria required in high performance optical systems. "The application engineers at Master Bond are used to dealing with unusual problems. Our engineering knowledge base, developed over 30 years and thousands of applications, has led us to develop a systematic approach to formulating application-oriented adhesives, sealants and coatings. In this case, where precision optical systems are involved, only an experience-driven systematic approach can be used to select the exact additives used to solve quality problems stemming from thermal-expansion mismatches and build a reliable optical compound. We choose a basic formulation having more than the necessary strength and environmental performance characteristics. We then look at which modifiers we need to add to bring in the additional necessary properties, in this case mainly stress elimination. In our approach, stresses induced by an adhesive are minimized by using base adhesives and modifiers that: Minimize the shrinkage on(upon) cure; Lower the modulus of the cured adhesive polymer; Utilize polymers with a small glass transition, Tg effect; Use polymers with a small overall Coefficient of Thermal Expansion (CTE), independent of the Tg glass transmission temperature. In the end, our chemical engineers came up with optical adhesive EP21TDC-2LO. It is an epoxy adhesive with outstanding physical strength properties, remarkably high mechanical shock resistance - up to 400 g’s and it easily accommodates the temperature, as well as the humidity and low outgassing protocol. Maintaining performance while putting in flexibility and thereby eliminating almost all stress causing factors is not a trivial accomplishment." There are three major phenomenon surrounding stress-factors in optical systems that need to be considered. The first is stress from shrinkage during cure. All adhesives experience some shrinkage during cure (polymerization). Shrinkage on cure for most industrial epoxies and UV adhesives typically range from 2 to 5%. This shrinkage may move some optics out of alignment. Glass can break when adhesive-induced stress exceeds the tensile strength of glass. Materials are available with shrinkage levels of less than 0.2% and an induced stress of less than 100 psi, thus facilitating fine optics assembly with tight tolerances. Robert Michaels explains: “When choosing an adhesive one attempts to match the coefficient of thermal expansion of an adhesive to the CTEs of the different substrates. If not closely matched, the resulting differential expansions create both stress and relative movement between bonded parts. The EP21TDC-2LO has a low thermal expansion coefficient of 90 X10-6 in/in°C.” Another factor in optical systems is stress induced birefringence which can lead to optical failure. Birefringence, sometimes called double refraction, is a property of an anisotropic material where two differing indices of refraction exist for orthogonal planes of incident polarization. While most optical elements are isotropic and have no natural birefringence, the application of mechanical stress will cause many optical materials to become anisotropic. Thus, stress from shrinkage or thermal excursions can make an isotropic material birefringent, creating a pattern from which double refraction occurs. Robert Michaels of Master Bond adds; “The birefringence may not exist immediately with initial bonding, rather it arises as non-fully cured adhesives develop their properties over time or the optics are subjected to thermal cycling. For that reason it is important to have an easy mixing ratio and convenient cure schedule. You have to take the repeatability of manufacture into account or quality issues arise incessantly. In this case the EP21TDC-2LO was formulated to be an easily applied paste-like elastomeric epoxy adhesive with a convenient non critical three-to-one mix ratio, weight or volume. Also it is relatively insensitive to mixing ratio or substrate cleaning procedures. It has a convenient one-hour working-life after mixing and can be readily cured at ambient or more quickly at elevated temperatures. Additionally, the EP21TDC-2LO which is 100% reactive, containing no volatile ingredients, could be conveniently processed with their existing commercial dispensing equipment. The successful use of the Master Bond EP21TDC-2LO system has led the aerospace manufacturer to utilize the same compound for bonding plastics including polycarbonates and acrylics to metals.” Don Dodge concludes: "Attaching the glass assembly to the supporting ring was a major challenge for us. The technical team at Master Bond has been most supportive of our development program. We have been able to use their skill, experience and laboratory resources to engineer a product that has significant functionality and productivity gains over the prior-art. Ease of application and quick curing were also factors as EP21TDC-2LO only requires 2 hours to cure. Developing an adhesive that is both optimal performance-wise as well as easy to manufacture was a major achievement." For more information on EP21TDC-2LO low outgassing adhesives, call +44-(0) 207-039-0034 or visit www.masterbond.com. For more than 30 years, Master Bond has been an industry leader offering a comprehensive line of engineered adhesives, coatings, sealants, potting compounds, encapsulation compounds, impregnating resins & encapsulants. We pride ourselves on the personalized application engineering & full technical support that we provide. As a manufacturer of industrial & custom UV curing adhesives, structural epoxy adhesives, polyurethane, silicon, rubber adhesives, acrylic, hot melt adhesives, instant cyanoacrylate, contact adhesives, films & performs & pressure sensitive adhesives we arte able to offer optimal solutions. These include special purpose adhesives & coatings for industries including OEM aerospace, electronic, microelectronic, automotive, telecommunications, medical, optical, photonics, underfill, chip bonding, PCB & surface mount adhesives. New & custom, environmentally responsible formulations include advanced epoxy heat resistant adhesives, electrically conductive, thermally conductive adhesives, permanent & removable adhesives, rework, eco-safe, low V.O.C. & medically approved adhesives. These adhesives and coatings meet ASTM, USP Class VI, NASA, FDA & UL94 V-0 specifications.