The demand for more sophisticated and reliable optical devices is presenting design and manufacturing engineers with increasingly more difficult bonding challenges. As the performance-envelope of optical systems keeps advancing, problems relating to thermal and stress management have become major concerns. Optical devices/assemblies are required to perform with increased accuracy in ever more confined spaces.
To complicate matters, these devices/assemblies are hybrid in nature, often involve special coatings, and require the use of highly different materials that must interface with each other during their operational life. The generation of heat from both internal and external sources causes these varied materials to expand at widely different rates. Add to that the higher altitudes and accelerations modern aerospace devices are subjected to and you realize why thermal shock, thermal cycling, vibration, mechanical shock, and a myriad of other factors all become major design hurdles. It is the attempt to solve these concurrent, often contradictory performance requirements that creates the challenge for the specialty adhesive manufacturer.
Difficult Aerospace Design Problem
Leading adhesive manufacturers have come up with systematic approaches to formulating high performance compounds for use in multi-parameter applications. Recently an aerospace company approached one such specialty formulator, Master Bond of Hackensack, NJ, with a difficult design problem. However, this came only after spending months of unsuccessful engineering effort on their own.
"We realized the design team needed urgent help with the selection of a suitable adhesive when design schedules began slipping 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," explained project engineering manager Don Dodge. "The completed assembly had to be capable of withstanding severe mechanical shocks and vibration as well as large temperature cycling from as low as -60°F to as much as 250°F. We spent months purchasing and testing compounds for the long term effects that stress can play on the performance of our 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."
In the end, quite a number of different adhesive systems were experimentally investigated by the aerospace manufacturer''s engineering group as recommended by various adhesive suppliers. The engineering staff analyzed the problem resulting from the large mismatch of the thermal expansion coefficients of the glass and metal components -- a mismatch which was further aggravated by the specified mechanical shock, humidity, and temperature requirements. These compounds included acrylic, polyurethane, silicone, and certain epoxy resin formulations.
All the products investigated failed to meet the required performance specifications for the glass/metal assembly. In part this was due to additional performance specifications which included extensive humidity cycling over a wide range of temperatures without loss of adhesive strength.
"Historically engineers designing precision optical apparatus have relied primarily on silicone-based systems to resolve stress-related problems," Don Dodge explains. "In this instance, however, while they did pass thermal and mechanical shock tests, the silicone adhesives lacked adequate physical strength properties. In addition, although silicones have demonstrated success in relieving stress, their inherently poor adhesion and release agent characteristics have created additional costly handling steps for production operations such as: 1. application of a primer required to improve adhesion, and 2. extensive masking procedures to avoid silicone ''contamination'' of areas where the bonding of other adhesives or coatings is critical.
"Polyurethane had marginal physical strength, shock, and vibration performance while failing humidity testing protocol. Acrylic and some specific epoxy resin formulations were not capable of meeting the shock and vibration requirements, especially at the low service temperatures. Our design problems manifested themselves as compromises between seal reliability and image quality.
"We were running out of solutions by the time we finally called on the Master Bond engineers to analyze all the requirements and offer a solution. Instead of suggesting an off-the-shelf product, Master Bond developed a specially designed elastomer modified epoxy resin system which not only met but significantly surpassed all of our performance and manufacturing requirements."
Custom Solution for Tough Bonding Problem
Robert Michaels, VP of 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 compound.
Master Bond chose a basic formulation having more than the necessary strength and environmental performance characteristics. The engineers then looked at which modifiers were needed to bring in the additional necessary properties, in this case mainly stress elimination. In their approach, stresses induced by an adhesive are minimized by using base adhesives and modifiers that:
- Minimize the shrinkage on 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.
"Our polymer engineers developed a product -- EP21TDC -- specifically designed for them," Michaels continues. "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 protocol. Maintaining performance while putting in flexibility and thereby eliminating almost all stress-causing factors is not a trivial accomplishment."
Stress Factors in Optical Systems
Three major phenomena surrounding stress factors in optical systems need to be considered. The first is stress from shrinkage during cure. All adhesives experience some shrinkage during cure (polymerization). Shrinkage on cure from most epoxies and UV adhesives typically range from 2-5%. 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 on cure levels of less than 0.2% and an induced stress of less than 100 psi, thus facilitating fine optics assembly with tight tolerances.
Stress from shrinkage is an inherent property of the chemicals making up the adhesive compounds. Chemical bond changes and molecular distance contribute to shrinkage. Bonds from the relatively distant molecules in a liquid adhesive polymerize to form the shorter bonds of the cured adhesive (polymer). In addition, on the molecular level, the molecular bonds of a polymer are shorter than those in a monomer. Molecular bond length changes are independent of either fast or slow cure curing processes. However, the degree of cure gives the effect of changing apparent shrinkage and may change with the aging of the optical assembly.
In addition to shrinkage, bonded optics can fail due to a second factor: thermal stresses induced during thermal excursions. Thermal stress can be quite large even during mild thermal excursions. Assembly failures can happen when thermal stress is excessive. This can occur when the stress exceeds the bond strength the adhesive. In this case the adhesive delaminates. When stress exceeds the yield strength of the bonded optic or the adhesive, then the optical alignment suffers, and when the stress exceeds the ultimate strength of the optic or adhesive, then the optic shatters or the adhesive shatters or tears and the device falls apart. 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. Designers can use models to determine and minimize stress. Such calculations can be very accurate, predictive, and after providing them with the data, most engineers are capable of handling very complex bonding configurations."
The third factor 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.
If polarization is not critical in the optical system, the birefringence will not cause failure. Nevertheless, the lens will suffer transmission loss if adhered with a stress-causing adhesive. Examples of devices sensitive to polarization include polarization-coupled lasers or lasers having polarization dependent gain. Induced birefringence in these devices can lead to operational dysfunction.
"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," adds Robert Michaels of Master Bond. "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 incessantly arise. We formulated EP21TDC to be an easily applied paste-like elastomeric epoxy adhesive with a convenient noncritical one-to-one mix ratio, weight or volume, and it is relatively insensitive to mixing ratio or substrate cleaning procedures. Also 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, which is 100% reactive, containing no volatile ingredients, could be conveniently processed with their existing commercial dispensing equipment."
Bonding Plastics to Metals
The successful use of the Master Bond EP21TDC system has led the aerospace manufacturer to utilize the same compound for bonding plastics, including polycarbonates and acrylics, to metals.
"Attaching the glass assembly to the supporting ring was a major challenge for us and the technical team at Master Bond has been most supportive of our development program," concludes Don Dodge. "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. The bond shear strength (aluminum/aluminum) is 2,970 psi while peel strength T-peel is 32 pli. Ease of application and quick curing were also factors, as EP21TDC only requires 30 minutes to cure. Their being able to develop an adhesive that is both optimal performance-wise as well as easy to manufacture was a major achievement with significant payoff for our company."