Selection of Optical Adhesives Andrew Clements
Abstract
This paper presents an overview of the issues involved with selection of an optomechanical adhesive. It proposes a very general approach by which selection of optomechanical adhesives may be considered. It gives a brief historical overview of the major developments in the field of optical adhesives. It discusses many of the important material properties that are used to characterize and distinguish optomechanical adhesives. It presents common ways in which adhesives are categorized. It mentions some applications that place more stringent requirements than usual upon adhesives and the challenges they present. It discusses the failure mechanisms of adhesives and some of the things that can go wrong in the processing of adhesives. Finally, it addresses some issues that may be pertinent to applications that demand adhesives with good chemical and solvent resistance.
1. Introduction
Adhesives are very useful materials with widespread application in optomechanics. At the most basic level, the purpose of an adhesive is to join two parts together. There is a host of reasons for using adhesives rather than mechanical fasteners in different applications. For example, one or both of the materials may be brittle and prone to fracture if threaded. The application may require fixing the pieces together without obscuring the transmitted optical path, thus demanding an optically transparent adhesive. The application may take advantage of the ability of adhesives to more broadly distribute stresses than mechanical fasteners. Adhesives also have their drawbacks, prime among which are the host of thermal issues that they introduce. When an application has been identified which could benefit from use of an adhesive, the task of the engineer is to determine which adhesive to choose from among the plethora which are now available. Unfortunately, there is no magic formula that one can use to quantitatively determine the best adhesive for the particular application. Likewise, there is no one adhesive that is suitable to all applications. Selection of adhesives becomes a balancing act of trading off several material properties according to the priorities of the particular application. This can be further complicated by the fact that the properties of adhesives are dependent upon their processing and curing parameters as well as their use environment. While making no pretense of presenting an all-purpose formula or procedure for selecting optomechanical adhesives, this paper will discuss many of the issues pertinent to identifying candidate adhesives that are likely to be well suited to an intended application.
If any approach at all could be suggested, I would recommend first becoming familiar with the material properties that characterize and distinguish optomechanical adhesives, to understand which of those properties are the most important for the intended application, and to try to narrow the search down to one or two classes of adhesives that would best address the main requirements of the bonding application. From there, one would seek out suppliers of this type of adhesive, survey their products, and choose some formulations as prime candidates based on a review of online selection tools and telephone technical support. After a second look to determine if any details about the material would disqualify it for the application, one should request samples and test a handful of the most likely materials, trading off performance, cost, and secondary material properties. With the large array of suppliers and the wide variety of adhesives that each supplier offers, finding the optimal product for a particular application may require a bit of work and a fair amount of luck, but finding an acceptable adhesive in fairly short order.
2. Short History of Optical Adhesives
When selecting optical adhesives, it is useful to have at least a cursory understanding of the historical development of optical adhesives so that the extensive offerings available today can be put in some perspective. The following is a brief summary of the history of optical adhesives, as set forth in a very informative paper by James T. Magyar. The reader is directed there for additional detail. The original optical adhesive was Canada Balsam, the distilled and filtered sap from the balsam tree. It had limitations at high and low temperatures, which became apparent during its use in optical instruments in World War II. A replacement, dubbed M1-10-2A, was developed in 1945-1946 that withstood a temperature range of –50 to +100 degrees C. However, it required a cure temperature of 100 degrees C. Since the bond was achieved at such an elevated temperature, the stresses that resulted from returning to room temperature often cracked the optical elements. In 1959, it was reformulated to include a cobalt catalyst, which allowed curing in 4 days at room temperature or in 2 hours at 70 degrees C. This became the industry standard MIL-A-3920 and is sold today by Eastman Kodak and Summers Laboratories under the designations HE79 and HE80 (Kodak) and Lens Bond Type C-59 and Type M-62 (Summers). High production volume and demanding tolerances began to demand an optical cement that could pre-cure in seconds rather than minutes and full-cure in minutes rather than hours. Summers introduced the first UV-curing adhesive in 1966 to meet this need.
Since then, the number of available optical adhesive formulations has skyrocketed. Paralleling this has been further development in structural adhesives suitable for optomechanical applications. The selection available today is considerable.
3. Material Properties for Characterizing Adhesives
3.1. Pre-Cure Properties
3.1.1 Shelf Life
The shelf life of an adhesive (how long it can be stored without a significant change to its properties) may or may not be an important factor in the selection of an adhesive for a particular application, but it is important to know for the proper use of adhesives. A typical shelf life for two-part epoxies is around 6 to 12 months. One-part epoxies are typically much shorter than this. Manufacturers may recommend ways to extend shelf life such as refrigeration or freezing.
3.1.2 Pot Life
This applies to 2-part systems and is the time during which an adhesive can be used after mixing without a significant change in viscosity. It is usually defined as the time from mixing to a 10% increase in viscosity. This can vary widely, from 30 seconds to 5 days, and can be an important consideration in the selection of an adhesive, depending on the requirements of the fabrication process. Adhesives with longer pot life are easier to work with, but may result in longer full cure duration than might be desired for quick fabrication of test samples or high volume production.
3.1.3 Viscosity
Viscosity is defined as resistance to flow or shear stress. It is an important property in determining the ease of handling of an adhesive in its uncured state. It is easy to control the bead size and position of high viscosity adhesives. Low viscosity adhesives tend to have improved wetting. However, very low viscosity adhesives will run and are consequently hard to control. Viscosity is directly proportional to temperature. It is important to consider this, since an adhesive that has a good viscosity for room temperature application may flow under a high temperature cure. Viscosity is measured in centipoise. Optical adhesives can vary widely in viscosity from about 100 cps for a low viscosity UV-cured acrylic to about 90,000 cps for a two-part silicone.
3.1.4 Wetting
Wetting is the ability of an uncured adhesive to make intimate contact with the substrate in order to facilitate a bond. The important physical parameter related to this is surface tension. An adhesive must have a lower surface tension than the substrate to make good contact and avoid beading. As a rule of thumb, the surface tension should be about 10 dynes/cm below the surface energy of the substrate. Most optical adhesives have a surface tension of around 30 to 35 dynes/cm.3
3.2 Cure Properties
3.2.1 Cure Process/Temperature/Time
The details of the cure process for the adhesive can be important factors in its selection. Adhesives can cure in as little as 30 seconds or may require several days to achieve a full cure. Adhesives may be curable at room temperature, may require an elevated temperature for curing, or may have a range of cure schedules that offer some compromise of cure time and temperature. Some curing processes are photosensitive and require a certain type of UV light source. Others are moisture sensitive and may require a particular level of humidity for curing. Note that the final properties of the cured adhesive often depend upon the details of the cure temperature and schedule. Further details are discussed below in the section on processing methods.
3.2.2 Out-gassing
Out-gassing is the liberation of volatile constituents by the adhesive. This can occur in the uncured state, during cure, and throughout its lifetime after cure. It can occur at room temperature, but is particularly likely upon exposure to vacuum or elevated temperature. Out-gassing is important for two main reasons. First, it can result in shrinkage as the adhesive volume decreases due to the lost gases, resulting in stress. Also, the liberated volatiles can thereafter condense on nearby cooler surfaces and potentially degrade optical surfaces or attack structural or electrical components. In addition, out-gassing can be critical in high-power laser applications because it can result in laser-induced optical damage. Out-gassing is quantified by two measurements: TML (Total Mass Lost, in %) and CVCM (Collected Volatile Condensable Material, in %).3 Out-gassing is most critical in space applications. NASA maintains an excellent guide for selecting low-outgassing materials compatible with spaceflight. In general, the higher the cure temperature, the less outgassing the material will experience after cure.2 Using solvent-free (100 percent solids) adhesives will prevent voids due to solvent evaporation upon cure. However, some small bubbles can still result from dissolved air or water in the adhesive. To prevent this, the adhesive should be de-gassed before use. If the adhesive is 2-part, it is mixed and then placed in a vacuum to remove any trapped air [2].
3.2.3 Shrinkage Upon Cure
All adhesives shrink in volume at least somewhat during cure. Older solvent-loss adhesives were particularly notorious for this. Modern epoxies typically have about 3-5% shrinkage upon cure. UV-cured adhesives can be as low as 0.2% [3]. Modern solvent-free (100 percent solids) formulations minimize shrinkage because no solvent is released during the curing process. Low shrinkage is necessary both to minimize stress and to provide long term stability. 3.3 Optical Properties
3.3.1 Optical Transmission
Spectral transmittance is an important consideration for optical adhesives. Figure 1 shows the spectral transmittance of a representative optical cement, Norland Products’ NOA 61. The two primary issues of concern are usually coloration and attenuation. Ideally, an optical cement should have a very flat spectral transmittance curve throughout the spectrum of interest, thus imparting no coloration to the scene. Likewise, the transmittance should be high in the spectral region of interest. Spectral transmittance plots are usually given for some specified bond thickness. Transmittance (or attenuation) at some other bond thickness can be calculated according to Beer’s law:
Where t is the transmittance at some wavelength, x is the bond thickness, and a is the attenuation constant.
Figure 1. Spectral transmittance of Norland NOA 61 adhesive.
3.3.2 Refractive Index
The refractive index of an adhesive is also important for optical cements. The closer the refractive index can be matched to the substrates, the lower the Fresnel reflection losses will be. Properties describing the elastic behavior of adhesives include Young’s modulus (E), the shear modulus (G), Poisson’s ratio (v), and the bulk modulus (B). Given any two of these properties, the other two can be derived from the relations:
G = E / [2.(1+n)]
and
B = E / [3.(1-2n)]
3.4 Mechanical Properties
3.4.1 Modulus of Elasticity (Young’s Modulus)
Normal, or longitudinal, stress (s) is defined as the applied force divided by the cross sectional area. (See fig. 2.) Longitudinal strain (e) is defined as the elongation from an applied force divided by the original length. (See fig. 3.) For small deflections of homogeneous material, stress is linearly proportional to strain. The constant of proportionality relating the two is Young’s modulus (E), according to the relation:
Figure 2. Longitudinal stress [8].
Figure 3. Longitudinal strain [8].
Materials with a high Young’s modulus are rigid, whereas materials with a low Young’s modulus are compliant. Adhesives usually have a much lower Young’s modulus than the materials that they are bonding. Thus, the strains experienced by the adhesives are typically much larger than those experienced by the substrates [7]. Conversely, if an adhesive has a high Young’s modulus, it will experience high stresses as a result of shrinkage upon cure or differential expansion due to mismatch in thermal expansion coefficients, which may approach the strength of the adhesive [6]. An adhesive with a low Young’s modulus can provide stress relief as the material stretches. The challenge with adhesives is to strike a balance between being compliant enough to accommodate stresses due to differential expansion, vibration, etc., yet being rigid enough to resist sag or shifting [5].
3.4.2 Poisson’s Ratio
When a material is in axial tension, there will generally be a strain in the transverse direction as well, opposite in sign to the longitudinal strain (see fig4):
Poisson’s ratio is the ratio of transverse strain to longitudinal strain:
Figure 4. Transverse and longitudinal strain [8].
Poisson’s ratio is by definition limited to values between 0 and 0.5. Some substances approach these limits. Cork has a Poisson ratio of nearly zero and rubber has a Poisson ratio of nearly 0.5. Most materials have a Poisson ratio between 0.25 and 0.35. A Poisson ratio of 0.5 means that the material maintains constant volume when it is deformed.
Adhesives with high Poisson ratios behave much differently than their Young’s moduli would predict when they are constrained into thin layers. When a thin bond is put into compression, the thickness of the adhesive layer is slightly reduced. If it has a high Poisson ratio, the adhesive will try to maintain constant volume, but the only place for the material to go is to bulge out at the free edges. In thin bonds, not much free surface area is exposed so not much material can be displaced, and the material exhibits a stiffening that exceeds the Young’s modulus. This new “apparent modulus” behaves exactly like the Young’s modulus (i.e. the ratio of stress/strain) but is a function of the bond thickness. More precisely, it is a function of the ratio of the thickness of the bond over the bond diameter. For thick bonds (i.e. thickness larger than diameter), the apparent modulus approaches Young’s modulus. For very thin bonds (i.e. thickness less than 1/100th of the bond diameter), the apparent modulus can approach the bulk modulus. (See fig. 5.)
Figure 5. The apparent stiffening of an elastomer in thin bonds [7].
3.4.3 Shear Modulus
Shear stress (t) is defined as shear force (V) distributed over area (A). (See fig 6.) Shear strain (g) is the angular deformation (in radians) caused by the shear stress. (See fig 7.) The constant of proportionality that relates these two quantities for homogeneous materials and small deflections is the shear modulus (G), according to:
Figure 6. Shear stress [8].
Figure 7. Shear strain [8].
According to the above relations, the shear modulus is necessarily limited between values of:
According to the above relations, the shear modulus is necessarily limited between values of:
Note that the shear properties of an adhesive are unchanged by compressing it into a thin layer [7].
3.4.4 Bulk Modulus
The bulk modulus (B) is a measure of the compressibility of a material. If a material is subjected to a hydrostatic pressure, P, it will have a corresponding change in volume:
The bulk modulus (B) is defined as:
3.4.5 Hardness
Hardness is defined as the resistance to indentation (after full cure). Hardness is usually characterized by a “Shore D” value. A wide range of hardness values is possible among the various adhesive types, from around Shore D 25 for a soft silicone elastomer to a Shore D 87 for a polishable fiber optic epoxy (e.g. EPO-TEK 353ND).
3.4.6 Strength
The strength of an adhesive indicates the maximum amount of stress that can be applied in a particular geometry (i.e. tensile, compressive, shear, peel, or cleavage) without failure of the adhesive. For permanent bonds, this should be high; for temporary bonds, it should be low. Structural adhesives typically have a tensile or shear strength in excess of 1,000 psi. Lapshear strengths are most commonly reported [3].
3.4.7 Elongation at Failure
This indicates how far the adhesive can be stretched before it fails. Note that much of this deformation is plastic, so this cannot be easily calculated using the Young’s modulus of the adhesive. This is an indicator of how much differential expansion an adhesive can withstand.
3.4.8 Adhesion
This is the name of the game with adhesives and is obviously a very important property. Good adhesion is absolutely necessary to withstand shock, temperature cycling, and differential expansion. But, it can be tricky to quantify. Adhesive suppliers usually describe the general types of materials (e.g. glass, metal, plastic, etc.) to which an adhesive formulation will adhere well. However, the degree of adhesion is highly dependent upon the details of the surface, the surface preparation, and the details of the curing process. This is discussed in more detail below under failure modes of adhesives.
3.5 Thermal Properties
Temperature can strongly affect the properties of adhesives. In fact, this is one of the major tradeoffs when considering the use of adhesives vs. mechanical fasteners. The following are several important temperature related properties of adhesives.
3.5.1 Maximum Continuous Operating Temperature
This is the highest temperature that the cured adhesive can endure for an extended time without deteriorating. For epoxies, this is usually 125 degrees C or higher [3]. Adhesive suppliers normally report both the maximum and minimum recommended extended exposure temperatures for their formulations and it is important to consider both extremes in the context of the proposed application.
3.5.2 Glass Transition Temperature (Tg)
Adhesives behave like rubbery materials at high temperature and brittle materials at low temperature due to the change in mobility of the polymer chains in the adhesive with temperature [3]. The glass transition temperature (Tg) is the temperature at the mid-point of the transition region between these two types of behavior. The glass transition temperature is an indicator of the maximum usable temperature range because of the drastic change in properties that result when it is crossed. Rubbery materials can become brittle below their Tg.3 Likewise, rigid materials can lose their structural integrity above their Tg.5 While in many cases, the glass transition temperature falls somewhat below the maximum continuous operating temperature, in applications where the bond will be subjected to large stresses, the Tg should be above the maximum projected use temperature [2]. Alternatively, if rubbery behavior is desired, the Tg should be below the minimum projected use temperature. Note that the glass transition temperature is a function of the adhesive formulation as well as the cure temperature and schedule. To maximize Tg, the adhesive must be cured at the highest recommended temperature for a sufficient length of time.
3.5.3 Coefficient of Thermal Expansion (CTE)
Materials expand linearly with an increase in temperature. The coefficient of thermal expansion (denoted CTE or a) is the constant of proportionality between the temperature change and the induced thermal strain:
The CTEs of adhesives are typically much higher than the structural materials that they are joining (e.g. 30-300 ppm/deg C for adhesives compared to 23 ppm/deg C for aluminum and 7ppm/deg C for BK7). Typically, the designer attempts to match the CTEs of the two substrates as closely as possible and then find an adhesive with a CTE near that value.3 If there is a large mismatch between the CTE of the adhesive and substrate(s), large stresses can result upon temperature variations. If the adhesive has a low Young’s modulus, the stresses resulting from thermal expansion can be lessened. But, in general, low CTE adhesives are desired. 3.5.4 Creep rate This could perhaps be best described as a long term shape change and is critical for applications where dimensional stability is important. Keep in mind that creep rate is exponentially proportional to temperature and is also influenced by stress and humidity.
The CTEs of adhesives are typically much higher than the structural materials that they are joining (e.g. 30-300 ppm/deg C for adhesives compared to 23 ppm/deg C for aluminum and 7ppm/deg C for BK7). Typically, the designer attempts to match the CTEs of the two substrates as closely as possible and then find an adhesive with a CTE near that value [3]. If there is a large mismatch between the CTE of the adhesive and substrate(s), large stresses can result upon temperature variations. If the adhesive has a low Young’s modulus, the stresses resulting from thermal expansion can be lessened. But, in general, low CTE adhesives are desired.
3.5.4 Creep rate
This could perhaps be best described as a long term shape change and is critical for applications where dimensional stability is important. Keep in mind that creep rate is exponentially proportional to temperature and is also influenced by stress and humidity[9].
3.6 Chemical Properties
3.6.1 Chemical and Solvent Resistance
This is the resistance of a cured adhesive to degradation by chemical attack. Once again, this is a property that is hard to quantify. Adhesive suppliers will typically include comments such as “excellent chemical and solvent resistance” in their adhesive selection tables and may post some limited data reporting testing against certain solvents. However, a particular formulation may be resistant to a handful of chemicals, but much less resistant to others. Also, the chemical resistance of a particular adhesive is strongly dependent upon the details of its curing and aging. This is discussed in more detail below.
4. Classification of Adhesives
There is no uniformly accepted method for classifying adhesives. Some authors categorize them according to their chemical family, others according to their cure method, others according to their usage. In all cases, there is some overlap among these different classification methods.
4.1 Chemical Families of Adhesives
4.1.1 Epoxies
Epoxies are available in one- or two-part varieties. Two-part epoxies generally demonstrate good adhesion, high strength, and some degree of resiliency. Their chief limitations are critical mix ratio dependency, long room temperature cure times or high temperature cure requirements, high CTE, and high Young’s modulus. The latter two result in rigid bonds and high stress levels [3]. When used in structural applications, they sometimes have filler materials, such as silver powder to increase electrical conduction, aluminum oxide to enhance thermal conduction, or silica powder to act as a thickening agent [6].
4.1.2 Urethanes
Available in one- or two-part forms, urethanes (or polyurethanes) form strong, durable bonds to many types of substrates. They are flexible and consequently good for bonding components with differing CTEs. Their environmental resistance is worse than that of epoxies. The upper temperature limit at which they can be used is about 100 degrees C [6].
4.1.3 Silicones
Silicones are elastomers, tend to be somewhat flexible upon cure, and are often used to bond materials with dissimilar thermal expansion coefficients. They are chemically inert and generally function well over a temperature range of about –80 to 204 degrees C. A common type of silicone adhesive is RTV, which stands for “room temperature vulcanizing.” These have become very popular because they are very easy to apply. However, as with all adhesives, cleaning and priming metal surfaces before applying RTV is very important. Silicones are usually used as sealants. They are generally not considered to be structural adhesives, although they are sometimes used to bond lightweight components. Some silicones have significant outgassing both during and post cure. Some have been formulated for low outgassing, such as Dow Corning DC93-500. About 1% shrinkage is typical for silicones. A typical CTE is around 200-300 ppm/degree C. Silicones usually have a tensile strength is of around 30-90 psi (which is why they are seldom used for structural mounting). Cure times depend strongly upon temperature. The following are typical cure schedules: 2 days at 25 deg C, 4 hours at 65 deg C, 1 hour at 100 deg C, 15 minutes at 150 deg C. Some varieties rely on atmospheric humidity to cure and require about 50% relative humidity for thorough curing. Cure is from the surface inward, so thick bonds may require longer times. Silicones are usually available in colors as well as clear and translucent varieties. The thermal conductivity of silicones is usually about 8.4 W/m2-K, but some varieties have additives to increase this value by up to 5 times. This increases their usefulness as a potting compound for electronic components that need temperature stabilization [6].
4.1.4 UV-Cured Acrylics
4.1.4 UV-Cured Acrylics These materials currently dominate the market for optical cements due to their ease of use and fast cure times, which makes them very attractive for high-volume production of optical components. Most UV-cured optical cements are members of the acrylic family and are doped with UV photo-initiators to allow curing upon exposure to a UV light source. The bond strength of UV-cured acrylics is typically considerably lower than those achieved by thermosetting epoxies, but are often sufficient for the intended applications. Suppliers claim that new formulations are beginning to rival the strengths achieved by epoxies.
4.1.5 Cyanoacrylates
Cyanoacrylates (e.g. “superglue”) are single-component adhesives that cure by exposure to the alkaline moisture and ionic substances found on most surfaces. For best adhesion to inert or acidic surfaces, a primer may be applied. Cyanoacrylates characteristically have low viscosity and a short cure time to fixate (typically less than 30 seconds). Some have elastomeric additives to increase the adhesive’s flexibility and gap-filling properties. These materials will quickly bond skin, so working with them requires care in application and full eye protection. Cyanoacrylates have several limitations. They have a potential for failure above 71 degrees C, especially in high humidity. Their bond strength on glass or ceramics may decrease with time. In addition, vapor emitted by uncured adhesive may leave a white residue on nearby surfaces [6].
4.2 Processing Methods
4.2.1 Two-part and one-part Thermosets
In thermosetting adhesives, cementing and alignment is done at room temperature, which is followed by curing at elevated temperature. Often, these adhesives can be cured in few hours at elevated temperature or in several days at room temperature. The manufacturers usually provide recommended cure temperatures and schedules and their recommendations should be followed.
Two-part systems begin cross-linking upon mixing and thus have a short useful life after mixing, but also offer extended shelf lives. Typical room temperature shelf lives are around 6 months to a year at room temperature. The pot life of a 2-part epoxy indicates the length of time between mixing and significant change in viscosity. This can be anywhere from a few minutes to several days. Two-part epoxies cure uniformly throughout the bulk and thus are preferred for thick bonds [9].
Single-component systems are easier to work with because they do not require precise mixing of the two components, but their viscosity gradually increases as they are stored. Freezing or refrigerating it can slow this process down [2].
In most cases, the initiator for single-component systems is the water vapor in the air and curing relies upon diffusion of the initiator. As such, it can potentially take years for a very thick bond to fully cure [9].
In general, both one- and two-part thermosets usually emit some reaction byproducts, which can sometimes be corrosive or flammable, and result in some degree of shrinkage [9].
4.2.2 UV Cures
UV-curing adhesives are one-component systems that are cured upon exposure to UV light. The manufacturers instructions should be followed in terms of the wavelength of the UV source (i.e. UV sunlamp, 365 nm source, etc.), the distance from the part to be cured (e.g. 1” or 12” are common), and the duration of exposure. Most UV-curing adhesives recommend a short pre-cure on the order of tens of minutes followed by a long full cure exposure on the order of an hour or two. Uniform irradiation is important to prevent distortions and stress gradients due to uneven curing [6]. Note that generally only small, low mass optics (e.g. up to about 3” diameter) should be mounted with UV adhesives, due to the limitations in bondline thicknesses which can be easily cured without creating too much stress or shrinkage. Acceptable bondline thicknesses are on the order of 0.010” to 0.250” [5].
4.2.3 Thermoplastic
These materials liquefy upon heating to about 120 degrees C. Re-heating can separate bonded elements.
4.2.4 Solvent Loss
This type of curing process relies upon evaporation of a solvent at an elevated temperature. Consequently, considerable shrinkage occurs. Canada Balsam is an example of such an adhesive [6].
4.3 Uses for Adhesives Adhesives
are also often classified according to their usage. Typical uses include optical bonding (glass to glass), which would call for optically transparent adhesives, optical mounting (metal to glass), which would call for adhesives with low elastic modulus and low CTE in order to accommodate differential expansion due to the differing CTEs of the two substrates, structural bonding, which would call for high strengths (e.g. tensile or shear strengths in excess of 1000 psi),3 locking adjustment threads, or making temporary bonds, etc.
5. Applications with Special Requirements for Adhesives
5.1 Military Environments
Military systems have extreme reliability requirements and consequently more stringent requirements than most commercial optical devices. In some cases, military systems have very unique requirements based on the field conditions they may encounter. The reader is directed to an excellent paper by D. H. Krevor, et al. Some key points from this paper are highlighted here.
Optomechanical adhesives used in military systems must be able to resist moisture, rain, and extreme temperatures (i.e. -50 to +80 degrees C working temperatures for avionics; more extreme for storage), as well as thermal shock (fast temperature change) and prolonged sunlight/UV radiation exposure. Free radical can be generated by UV exposure and often this requires free-radical scavengers and anti-oxidants to be included in the adhesive formula.
Military adhesives must also be fungus resistant. Fungus-nutrient materials include epoxy resin and polyurethane, so these would be poor choices for military adhesives. Silicone resin is a fungus-inert material.
Military adhesives must also have good chemical and solvent resistance. In particular, they must be resistant to degradation upon exposure to jet fuels, hydraulic fluids, coolants, detergents, soaps, fire-fighting foams, Halon and Halon-replacements, and de-icing solvents. Furthermore, they must resist degradation upon exposure to chemical warfare agents and their decontaminants. They must also resist absorption of these materials, to prevent a prolonged exposure hazard to crew members.
Military adhesives must also withstand extreme environments such as volcanic ash, salt atmospheres, and explosive atmospheres.
Military adhesives must also withstand extreme mechanical stresses – vibration, shock, gunfire, and sudden acceleration. Laminating adhesives are often required to provide viscoelastic damping for vibration abatement.
Edge-seal adhesives used in military liquid crystal displays must be non-porous and impervious to liquid crystal materials (which are typically linear aromatic and aliphatic organics). They must also cure without residual volatiles that could leach into the LCD. Both thermal and UV-cure materials are used.
A low ambient pressure (high altitude) environment requires low-outgassing adhesives. Long burn-in times or bake treatments are generally part of the manufacturing processes for optical systems used in aerospace applications. Differential Scanning Calorimetry (DSC) is a technique that is often used to verify that curing conditions have removed all volatiles.
Because of the extreme temperature range, finding adhesives with low CTEs becomes very important. It is also important to find adhesives that do not have glass transition temperatures within this extreme temperature range because of the radical change in properties such as modulus of elasticity and CTE on either side of this temperature. Temperature cycling above and below the glass transition temperature can be particularly detrimental, causing adhesives to become hazy because of phase separation as a function of molecular weight [10].
5.2 Space Environments
Space is a rigorous environment in which materials are exposed to extreme vacuum and extreme solar radiation without the benefit of atmospheric filtering. Consequently, resistance to out-gassing as well as resistance to UV exposure is crucial for space-based applications [10]. According to NASA, space-qualified materials should have TML < 1.0% and CVCM < 0.1% [4].
5.3 Medical Adhesives
Medical adhesives are required by law to be solventless (100 percent solids before and after curing) due to concerns about the short-term toxic effects of solvents as well as indications that many of them are carcinogenic. Other requirements for medical adhesives include good adhesion to organic and inorganic surfaces, good wetting properties, compatibility with radiation, chemical, and autoclave sterilization, and fungus- and bacteria-resistance. Many optical adhesive suppliers have a line of adhesives that are qualified for medical use.
6. Adhesive Suppliers
While not exhaustive, the following table lists several optomechanical adhesive suppliers and contact information. Many of them specialize in a particular category of adhesives, but several of them have a broad selection of products across different adhesive categories. Most or all of them provide significant amounts of technical information, recommendations, and guidelines regarding their materials on their websites.
| Supplier | Contact Information |
|---|---|
| 3M Adhesives | www.3m.com |
| Ablestik Electronic Materials & Adhesives | www.ablestik.com |
| Armstrong Products | http://www.ellsworth.com/ArmstrongEpoxy.html |
| Dow Corning | www.dowcorning.com |
| Dymax Corporation | www.dymax.com |
| Eastman | www.eastman.com |
| Emerson and Cuming | www.emersoncuming.com |
| Epoxy Technology | www.epotek.com |
| Extreme Adhesives | www.extremeadhesives.com |
| Fiber Optic Center (distributor of AngstromBond) | www.focenter.com |
| GE | www.ge.com |
| Loctite Corp. | www.loctite.com |
| Master Bond, Inc. | www.masterbond.com |
| Norland Products | www.norlandprod.com |
| Summers Optical | Http://www.emsdiasum.com/summers/optical/cements/default.htm |
7. Failure Modes of Adhesives
There are three primary failure modes for adhesives. First, and most common, is adhesive failure. This occurs when the stresses exceed the strength of the adhesive’s grip upon the substrate. Second, is cohesive failure. This happens when the grip of the polymer molecules to each other in the bulk of the adhesive is exceeded, usually by a shear stress. Finally, and least likely, is fracture of the substrate. This can occur when the bond is so strong that the element that fails is actually the substrate. This is unlikely, since the strengths of adhesives are usually significantly lower than the strengths of typical substrates, but this can occasionally happen. The first two are explained in further detail below. For more detail in failure analysis of adhesives, the reader is directed to a very informative paper by H. F. Woods.
7.1 Adhesion
Adhesion is the attraction between the adhesive and the substrate. There are three primary processes involved in adhesion: wetting, acid-base reaction,
and van der Waals forces.
7.1.1 Wetting
In order for good contact between the adhesive and the substrate surface, the surface tension of the adhesive must be much less than the surface energy of the substrate. An accepted rule of thumb is that there needs to be at least 10 dynes/cm difference. Most optical adhesives have surface tensions of 30 to 35 dynes/cm and most glasses and metals have surface energies in excess of 100 dynes/cm, so wetting to these surfaces is not often problematic. However, optical plastics usually have surface energies between 30 and 40 dynes/cm, so it can be challenging to wet many optical adhesives to optical plastics, so one must select adhesives which are specially formulated for this purpose if plastic bonding is required [1]. Even when there is good wetting of the adhesive to the optical surface, the grip of the adhesive can be rather weak due to the high polish of many optical surfaces, so other mechanisms are required for a strong bond.
7.1.2 Acid-Base Reaction
Optical glasses are usually alkaline and optical adhesives are usually acidic. During cure, the substrate and adhesive will exchange ions, creating a strong ionic bond between them. Very strong bonds can be formed if the pH of the adhesive is between 3 and 4. The acid-base reaction can be left incomplete in UV-cured bonds due to the speed with which the liquid converts to a solid, resulting in weaker adhesion.
7.1.3 Van der Waals Attractions
The third component of adhesion is the attraction between the adhesive and substrate due to van der Waals forces.
7.2 Cohesion
Cohesion is the attraction of the adhesive molecules to each other in the bulk of the adhesive. The two primary processes involved in cohesion are van der Waals forces and crosslinking.
7.2.1 Van der Waals Attractions
Although they play a role in adhesion, van der Waals forces are predominantly involved in cohesion. These forces impart some chemical resistance to the adhesive.
7.2.2 Crosslinking
Thermoset adhesives form chemical bonds between the polymer chains called crosslinks, which are much stronger than van der Waals forces. These also impart chemical resistance to the adhesive bond.
8. Chemical and Solvent Resistance
Chemical resistance is generally lower in UV-cured adhesives than in thermoset epoxies. Magyar indicates that this may be a result of the UV initiator causing the long chain molecules to link, but leaving the crosslinks to occur by themselves over a longer period of time. Tests have shown that UV initiated thermoset cements showed much higher chemical resistance if heat-treated at 40 degrees C for 1 hour following their full UV cure than if they did not undergo heat treatment [1].
Yoder indicates that the solvent resistance of typical UV-cured adhesives is high, especially after aging for about 3 weeks [6]. This may be a result of delayed cross-linking.
If high chemical resistance is desired in an application that demands UV-cure, one might consider the UV-curing epoxy, EPO-TEK UVO 114. Unlike most UV-curing adhesives, which are acrylics, this is an epoxy and exhibits superior chemical and moisture resistance as well as tolerance of high temperature conditions. It also exhibits no significant shrinkage upon cure.
Medical adhesives are specifically formulated to be solvent-free (100 percent solids) and thus are likely to be more chemical resistant than generic adhesives. One example is EPO-TEK 377, which is noted as the most moisture-resistant and chemical-resistant adhesive in Epoxy Technology’s line of medical adhesives [11].
Silicones are considered chemically inert and are likely to have very good chemical and solvent resistance [6].
9. Conclusion
There is no simple and clear-cut formula for selecting the best adhesive for any particular application. However, an understanding of the material properties that characterize and distinguish optomechanical adhesives will help the optical engineer make informed choices. Understanding the historical context of optical adhesives and the general classes of optomechanical adhesives, including their strengths and weaknesses will aid the engineer in focusing his or her search for an appropriate adhesive for their application. Some applications have special requirements that limit the available adhesive choices. For such applications, including military optical systems, space based systems, and medical devices, it is crucial not to overlook the details of these sometimes stringent requirements. Familiarization with optomechanical adhesive suppliers and their product offerings, as well as making use of the online tools and telephone technical support that they provide, is helpful in selecting some likely candidates for the desired application. For applications that require chemical and solvent resistance, it is important to understand the failure processes of bonds and the things that can contribute to making an adhesive weak to chemical attack. Finally, it is important to make selection decisions with cost always in mind, so that the most economical and effective solutions are found.
10. REFERENCES
2. A.L. Delmarsh and R.H. Estes, “Selecting Epoxies for Optical and Fiber Optic Applications,”, Technical Paper GB-56, http://www.epotek.com/ssc-white-papers.asp.
3. http://outgassing.nasa.gov/
6. P.R. Yoder, Jr., Opto-mechanical Systems Design, 3rd ed., CRC Press, Boca Raton, FL, 2006, pp. 128-140.
8. J. Burge, Course notes, OPTI-521 Introductory Optomechanical Engineering, University of Arizona, 2006.
9. B. Olbert, Presentation: “Adhesive selection & characterization – What you don’t know can kill you,” UASO Engineering Seminar, Aug 2004.
11. R.H. Estes, “The suitability of epoxy-based adhesives for use in medical devices,” Technical Paper GB-63, http://www.epotek.com/ssc-white-papers.asp.