Presentation Transcript

1. Adhesive Selection, Development and Testing

   For the Average Engineer Dustin Urone University of Arizona Opti 521, Fall 2010
2. Introduction Problem: You are tasked with developing a reliable adhesive bond for an engineering effort, but you don’t know where to start. This Tutorial: Guides you through the general process of how to define adhesive requirements, select adhesives for testing, and develop the process.
3. Comments on Adhesive Bonds An adhesive bond comprises two primary elements; the adhesive and the substrate(s). The quality, or reliability, of an adhesive bond is the combined effect of many factors including: Adhesive chemistry Substrate material and surface chemistry The process used to create the adhesive bond The environment and loads the bond is subjected to This tutorial outlines the consideration of these factors and gives a process to define and test them. For information on adhesive types and applications, see the references [6, 11, 12]
4. Getting Started Initial Considerations Before Opting for an Adhesive Bond Is an adhesive bond is the appropriate course of action? Start with the lifetime requirement of the bond Consider the operational environment; if harsh, adhesives will suffer. All adhesive bonds will eventually fail [1]. Adhesives should be avoided if lifetimes of 10 years or more are required [1]. Consider any alternative mechanical restraint methods. Weigh the risk, complexity and cost of the potential methods and select one.
5. Environmental Conditions Adhesive Method The Basic Process Physical Loads Steps to Selecting and Testing Adhesives Define Requirements The entire process of selection and development revolves around proper definition of the requirements on the adhesive bond. Details of each step are given in the following sections. Failures Lifetime Requirements Compare to “Known” Solution Assembly Requirements Budget, Schedule & Politics Adhesive Research, Selection and Testing Use “Known” Solution
6. Environmental Conditions Adhesive Method Define Application Requirements Physical Loads Define Requirements What are the requirements for the adhesive bond? Five primary categories: Environmental Conditions Physical Loads Failures Lifetime Requirements Assembly Requirements Additional possible requirements: Toxicity, cost, appearance, etc. A complete list of requirements must be defined to select and test the adhesive properly. Failures Lifetime Requirements Compare to “Known” Solution Assembly Requirements Budget, Schedule & Politics Adhesive Research, Selection and Testing Use “Known” Solution
7. Environmental Conditions What are the expected environmental conditions? Typically broken into two categories: Operational and Storage Conditions Basic environmental conditions will include: Temperature, humidity, pressure and any chemicals present Special attention should be paid to corrosive or vacuum environments. Rate of change of environmental conditions Fast temperature changes will cause additional stress on the bond Industry standards often define the requirements NASA (www.outgassing.nasa.gov) Telcordia (www.telcordia.com) MIL Standards (www.mil-standards.com) If specifying your own environmental conditions do not make the problem more difficult than necessary. Limit temperature extremes to what makes sense Avoid phase change of water (if possible) Make a complete, well defined list of the environmental conditions
8. Example List of Environmental Conditions The environmental conditions will be used to define the test methods and conditions. Special adhesives can be chosen with resistances to the specified environmental conditions [11, 12].
9. Physical Loads Example Shear stress from CTE mismatched substrates over a temperature change ∆t. ∆L What are the physical loads that the bond will endure? Can be separated into short and long term loads. Consider stresses from environmental conditions: Thermal expansion Vibration Shock Draft a list of the physical loading requirements for comparison to adhesive specifications. Ensure a factor of safety > 4. [2] See the appendix for a list of first order stress equations. γ Glass, αG Adhesive H Aluminum, αAL Resulting Shear Stress, τ L
10. Failures Example Two lenses bonded together with optical adhesive. What constitutes a failure of the adhesive bond? Can be separated into two categories: Reversible and irreversible changes Obvious failure is catastrophic Other failures may include inelastic strain due to stress or a change in optical properties. Define failures in terms of both: Change in adhesive Change in device performance Evaluate failure criteria – is it unreasonably difficult to achieve? Use failure criteria to help select adhesives and to generate test methods. Nominal Specifications Optical Loss < 0.2 dB Wavefront Error < ¼ λrms Design residual assumed to be zero. Example Failure Criteria
11. Types of Adhesive Failures There are two primary types of catastrophic adhesive failure: Cohesive Adhesive In general, a cohesive failure indicates a very good adhesive bond. The chemical bond between surfaces exceeded the adhesive’s material strength. http://www.lpdlabservices.co.uk/problem_solving/typical_technical_expertise/interfaces_adhesion_adhesives_and_coatings.php
12. Lifetime Requirements What is the lifetime requirement of the adhesive bond? How critical is a failure? Is the device serviceable? Adhesive lifetime prediction is complex and prone to extreme error [2] Lifetime must be accelerated for testing purposes To make a lifetime prediction, must know rate at which lifetime was accelerated. For more information, look up Arrhenius equation (also see appendix) [8] Check for industry standards for lifetime requirements. Never specify a lifetime unless required to do so. Instead, satisfy industry standard tests (i.e. Telcordia or MIL standards) Use extreme caution with adhesives if a long lifetime is required (5-10 years+) [2]
13. Assembly Requirements What are the assembly requirements when making the adhesive bond? Heat sensitive components? Eliminates option of high temperature cure adhesives. Sensitive alignment? May want UV cure adhesives for active alignment operations. Stress sensitive? With large CTE differences, high temp. cure process may build in stresses. Define the assembly process based on these requirements and select an adhesive type.
14. Requirements Defined The most important part of the process of selection, is defining the requirements of your application. The list of requirements will be used for selection, testing and development of the adhesive process. Get it right the first time! Application Requirements Environmental Conditions Physical Loads Failures Lifetime Requirements Assembly Requirements
15. Environmental Conditions Adhesive Method Compare to “Known” Solution Physical Loads Define Requirements Are there any “Known” adhesive solutions for similar applications at your facility/company? Have they been tested explicitly? Ask to see data; often reliability is implied by no recent memory of failure. Discuss with those who have worked with them, there may be a wealth of knowledge. Would they satisfy your defined requirements? This may not be clear without testing, but review of the datasheets is a perfect first step. It is worthwhile to create a spreadsheet and compare several other adhesives to any “Known” adhesive solutions. Failures Lifetime Requirements Compare to “Known” Solution Assembly Requirements Budget, Schedule & Politics Adhesive Research, Selection and Testing Use “Known” Solution
16. Environmental Conditions Adhesive Method Budget and Schedule Physical Loads Define Requirements What is the allotted timeline and budget for developing the adhesive bond? Often very little budget or timeline are allotted for said development. For critical applications, adhesive bonds should be thoroughly tested for reliability. Regardless of whether or not the adhesive is a “known” solution. Remember, adhesive bond reliability is defined by the process used to create it, any small change can result in drastically different results. Compile a list of failure criteria, development tasks and tests. Compare with budget and schedule, reduce scope as necessary. Make risks and consequences known to management. Do not allow the consequences of budget limited test & development to appear as a lack of diligence on your part. Failures Lifetime Requirements Compare to “Known” Solution Assembly Requirements Budget, Schedule & Politics Adhesive Research, Selection and Testing Use “Known” Solution
17. Environmental Conditions Adhesive Method Research and Selection Physical Loads Define Requirements Based on the compiled lists of requirements, research adhesives and compare specifications to make a selection. Often there will be many adhesives with similar specifications. Ultimate selection may require arbitrarily limiting list to a few for physical trials. Note that if the “known” solution has acceptable, even if not the best specifications, it is often a good choice. Failures Lifetime Requirements Compare to “Known” Solution Assembly Requirements Budget, Schedule & Politics Adhesive Research, Selection and Testing Use “Known” Solution
18. Testing and Development Define the testing and development that will be done. At minimum, develop the adhesive process thoroughly, defining and documenting every step explicitly. Some form of testing should be done to confirm the adhesive bond meets the reliability / lifetime requirements. Topics requiring development: Cleaning and surface preparation of substrates Bond cure schedule Bond geometry Bond reliability
19. Develop Adhesive Bond Process An adhesive bond is only as reliable as the process used to create it. The basic steps of creating an adhesive bond are outlined above. Each step should be developed, tested, proven and documented. An example test method is outlined here, but each application is different and may not apply. Substrate Cleaning and Surface Preparation Adhesive Preparation Adhesive Application Adhesive Curing
20. Adhesive Test Methodology Substrate A thin “Coupon” made of the desired material Shear Test Bonded coupons are pulled until the adhesive fails Test samples can be prepared with the proposed process and shear tested to find the ultimate shear strength (τ). Use ultimate shear strength as figure of merit for development. See ASTM D 1002 for standards regarding this test. F F 4” 1” Adhesive Bond Covering an Area, A 1” 1/16”
21. Surface Preparation Development Use Adhesive Process to Bond Samples Together Obtain Substrate Samples (see ASTM D 1002) Apply Various Surface Preparation Techniquesto the Samples This test methodology will develop a surface preparation technique to maximize the shear strength of the adhesive bond. See references for a source on surface preparation techniques [5, 9]. Lap Shear Test Samples According to ASTM D 1002 Compare Results, Look for Cohesive Failures, Iterate if Necessary
22. Environmental Test Method This test method compares the degradation of shear strength to both duration and type of environmental conditioning. The adhesive with the least degradation of performance could be the chosen solution. See the references for an example implementation of this process [9]. Apply the Chosen Cleaning and Surface Preparation Technique to All of the Samples Obtain Substrate Samples (see ASTM D 1002) Use Adhesive Process to Bond Samples Together 25% of the Samples to the Environmental Test Group (2) 25% of the Samples to the Environmental Test Group (3) 25% of the Samples to the Control Group 25% of the Samples to the Environmental Test Group (1) Lap Shear Test (see ASTM D 1002) Evaluate Results
23. Summary Adhesive selection and testing requires a solid and complete definition of the requirements. Following the steps outlined here allows the reader to: Define the adhesive chemistry based on the list of requirements and adhesive specifications, Develop the adhesive process with tests that evaluate relevant figures of merit. Test the adhesive bond reliability as a whole with relevant figures of merit.
24. References B. Olbert, Presentation: “Adhesive selection & characterization – What you don’t know can kill you,” UASO Engineering Seminar, Aug 2004. Vukobratovich, Daniel, and Suzanne M. Vukobratovich. Introduction to Opto-Mechanical Design. Tucson: Raytheon Systems Co. PDF. J. Burge, Course notes, OPTI-521 Introductory Optomechanical Engineering, University of Arizona, 2010. Telcordia. "Telcordia GR 1221." Generic Reliability Assurance Requirements for Passive Optical Components. Web. http://telecom-info.telcordia.com/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-1221& Wegman, Raymond F. Surface Preparation Techniques for Adhesive Bonding. Park Ridge, N.J., U.S.A.: Noyes Publications, 1989. Print. Epoxy Technology. The Adhesive Primer for Advanced Assembly. Billerica: Epoxy Technology, 2006. Print. P.R. Yoder, Jr., Opto-mechanical Systems Design, 3rd ed., CRC Press, Boca Raton, FL, 2006. Clark, Jim. "Rate Constants and the Arrhenius Equation." Helping You to Understand Chemistry. 2002. Web. Nov. 2010. <http://www.chemguide.co.uk/physical/basicrates/arrhenius.html>. Lacy, H. De, and M. Tavakoli. "The Effects of Surface Pre-Treatment of Stainless Steel on Joint Strength and Durability." Euradh, Adhesion. United Kingdom, Cambridge. 3 Sept. 1996. Lecture. ASTM D 1002 Clements, Andrew, “Selection of Optical Adhesives,” University of Arizona. 2006. http://www.optics.arizona.edu/optomech/student reports/tutorials/ClementsTutorial1.doc Tianquan, Su, “Choice of Structural Adhesives,” University of Arizona. http://www.optics.arizona.edu/optomech/student%20reports/tutorials/521%20tutorial%20Tianquan%20Su.pdf
25. appendix
26. Equations
27. Stress Figures Tensile Stress Shear Stress H ∆X