Choosing Optomechanical Adhesives Selecting Suitable “Sticky Stuff” for Making Your Stuff Stick or Presented by: Andy Clements College of Optical Sciences University of Arizona December 12, 2006

The Big Picture Material A Adhesive Material B What properties are used to characterize this stuff? What properties are most important to my application? What general classes of this stuff are out there? Who supplies this stuff? What is the best trade-off?

Outline n Proposed Methodology n Historical Perspective n Properties of Adhesives n Classes of Adhesives n Applications with Special Requirements n Adhesive Suppliers n How Adhesives Fail n Chemical and Solvent Resistance n Cost

General Methodology for Optomechanical Adhesive Selection or How to Catch a Fish n What are the properties that characterize and distinguish optomechanical adhesives? n Which of these properties are the most important to my application? n If possible, narrow down to one or more general classes of adhesive. n Who supplies these things? (Anybody know any good fishing holes?) n Survey what’s available and choose some prime candidates. (Go fishing.) n Scour the details and make sure there are no disqualifying properties. (Is it a keeper?) n Get a sample. (Reel it in.) n Test it! (Don’t believe the big fish story.)

Putting Things in Perspective: A Brief Historical Overview

Material Properties for Characterizing Adhesives n Pre-cure properties n Cure process properties n Optical properties n Mechanical properties n Thermal properties n Chemical properties

Pre-Cure Properties n Shelf Life n Pot Life n Viscosity n Wetting

Shelf Life n How long an adhesive can be stored without degradation to its properties n Ex: Two-part epoxies = 6-12 months n One-part epoxies shorter than this n Extend by refrigeration or freezing

Pot Life n What in the world is a “pot” and why do we care how long it lives? n “Potting” refers to the process of fixing a lens in its cell by injecting and curing an adhesive. n “Pot life” is the length of time during which an adhesive can be used after mixing. n Pot life can range from 30 seconds to 5 days.

Viscosity n Resistance to flow or shear stress n Measured in Centipoise (100-90,000 cps) n High Viscosity –Easy to Control Bead Size & Position n Low Viscosity –Improved Wetting –Hard to Control n Linearly Proportional to Temperature!

Wetting n Ability to make contact with substrate surface n Surface tension should be ~10 dynes/cm less then substrate surface energy n Typical adhesive surface tension = dynes/cm

Cure Process Properties n Cure Process n Cure Temperature n Cure Time n Out-gassing n Shrinkage Upon Cure

Cure Process n Thermoset (one-part or two-part) n UV-cure n Thermoplastic n Solvent Loss

Cure Time and Temperature n These are inter-related n 30 seconds to several days n Room temperature to > 100 degrees C

Out-gassing n Driven by vacuum or high temperatures n Negative consequences: –Shrinkage –Volatile materials condense on cooler optics, electronics or structures n Two measures: –TML (Total Mass Lost, in %) –CVCM (Collected Volatile Condensable Material, in %)

Shrinkage Upon Cure n All adhesives have some shrinkage n Examples: –0.2% for a UV-cure acrylic –3-5% for epoxies n Negative consequence: Stress build-up

Optical Properties n Spectral Transmittance –Coloration –Attenuation n Refractive Index –Fresnel Reflection

Mechanical Properties n Modulus of Elasticity (Young’s Modulus) n Poisson’s Ratio n Shear Modulus n Bulk Modulus n Hardness n Strength n Elongation at Failure n Adhesion

Modulus of Elasticity (Young’s Modulus) n High modulus = rigid n Low modulus = compliant n Adhesives have much lower Young’s modulus than the materials they are bonding n Low modulus is desirable to provide stress relief n A balance must be struck; too low of a modulus can result in sag or shifting   E

Poisson’s Ratio n Ratio of transverse strain to longitudinal strain n Ranges between 0 (cork) and 0.5 (rubber)

High Poisson Ratio Materials Confined in Thin Layers n Exhibit “stiffening” because they try to maintain constant volume, but there is no room for the material to escape to n The result is an “apparent modulus” which is much higher (stiffer) than the material’s Young’s modulus

Hardness n Resistance to Indentation n Wide range available: –“Shore D” 22 (good for damping) to 87 (polishable fiber optic connector)

Strength n Tensile, Compressive, Lapshear, Peel, or Cleavage n Ultimate Strength (Strength at Failure) n Should be high for permanent bonds, low for temporary bonds n Structural Adhesives > 1,000 psi

Elongation at Failure n How far the adhesive can be stretched before failure n Much of this is plastic deformation n Indicates how much differential expansion an adhesive can withstand

Adhesion n The name of the game n Tricky to quantify n Highly dependent upon surface properties, cleaning and preparation n Highly dependent upon the details of the curing procedure

Thermal Properties n Maximum Continuous Operating Temperature n Glass Transition Temperature n Coefficient of Thermal Expansion (CTE) n Creep Rate

Maximum Continuous Operating Temperature n Highest temp for long-term exposure without degrading n Usually 125 deg C or higher for epoxies n Max and Min are important

Glass Transition Temperature n Transition between “rubbery” and “brittle/glassy” behavior n Tg should not fall between Min and Max projected use temperatures for high stress applications n A function of adhesive formulation and cure temperature

Coefficient of Thermal Expansion n CTE of adhesive is usually much higher than substrate(s) n Typical values are ppm/deg C n Usual approach is to match CTEs of two substrates and try to find an adhesive with a similar CTE n Low CTEs are desired

Creep Rate n Long term shape change n Exponentially related to temperature n Also influenced by humidity and stress

Chemical Properties n Chemical and Solvent Resistance –Hard to quantify –Some data available, but not a lot –Strongly dependent upon the details of curing –Dependent on material aging

Classification of Adhesives n Chemical Families n Processing Techniques n Usage

Chemical Families n Epoxies –Thermosetting (or UV) n Urethanes –Limited to use < 100 deg C n Silicones –Elastomers n Acrylics –Most UV-curing cements are Acrylic n Cyanoacrylates (super-glue)

Processing Techniques n Thermosets –One- and two-parts n UV-cure –10 minute pre-cure followed by 1 hour cure n Thermoplastic –Heat above 100 deg C n Solvent Loss

Optomechanical Uses for Adhesives n Optical bonding (glass to glass) n Optical mounting(glass to metal) n Structural bonding n Locking threads n Temporary bonds

Applications with Special Requirements for Adhesives n Military –Wide temperature range (-50 to +80 deg C) –Extreme environments n Space –Vacuum –Intense UV exposure n Medical –Biocompatibility - requires solvent-free

Optomechanical Adhesive Suppliers (Good Fishing Holes)

How Do Adhesive Bonds Fail? n Adhesive Failure n Cohesive Failure n Substrate Fracture

Adhesion n Wetting –Surface tension at least 10 dynes/cm less than substrate surface energy –High polish can still result in weaker grip n Acid-Base Reaction –Adhesive pH between 3 and 4 for strong ionic bonds with alkaline glass surface –Speed of UV-curing can leave the acid-base reaction incomplete n van der Waals Forces

Cohesion n Van der Waals Forces –Important for cohesion –Impart some chemical resistance n Crosslinking –Much stronger than van der Waals forces –Impart chemical resistance –Can be left incomplete by the speed of UV- curing or by insufficient thermal curing

Chemical and Solvent Resistance n Class of Material n Formulation n Curing Details n Aging

Adhesive Selection for Chemical Resistance n Silicones are chemically inert n Epoxies are highly resistant n UV-cured acrylics are not as good as epoxies, but are very resistant n Urethanes are worse n Choose low out-gassing materials n Medical grade are required to be solvent-free (100 percent solids)

Best Guesses for Maximizing Solvent Resistance n General –Meticulous surface preparation –Age for 3 weeks n UV-cured Adhesives –Use UV-cured epoxies over acrylics –Less intense, slower UV-cure (hours) –Post-cure heat treat at max recommended temp n Thermosets –Cure at the max recommended temp for a sufficient time

Cost n Ultimately, we want a cost-effective solution that will meet our technical requirements n Pay attention to the price!

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