Transcript Adhesive Selection & Pitfalls

Adhesive Selection,
Development and Testing
For the Average Engineer
Dustin Urone
University of Arizona
Opti 521, Fall 2010
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.
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]
Getting Started
• 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.
Initial Considerations Before
Opting for an Adhesive Bond
The Basic Process
• 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.
Steps to Selecting and
Testing Adhesives
Environmental
Conditions
Adhesive
Method
Physical Loads
Define
Requirements
Failures
Lifetime
Requirements
Compare to
“Known” Solution
Adhesive Research,
Selection and
Testing
Assembly
Requirements
Budget,
Schedule &
Politics
Use “Known”
Solution
Define Application Requirements
• What are the requirements for the
adhesive bond?
• Five primary categories:
–
–
–
–
–
Environmental Conditions
Physical Loads
Failures
Lifetime Requirements
Assembly Requirements
Physical Loads
Define
Requirements
Failures
Lifetime
Requirements
Compare to
“Known” Solution
• Additional possible requirements:
Assembly
Requirements
Budget,
Schedule &
Politics
– Toxicity, cost, appearance, etc.
• A complete list of requirements
must be defined to select and test
the adhesive properly.
Environmental
Conditions
Adhesive
Method
Adhesive Research,
Selection and
Testing
Use “Known”
Solution
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
Example List of Environmental
Conditions
Environmental Condition
Temperature
Temperature Rate of Change
Relative Humidity
Atmospheric Pressure
Chemicals Present
Operational
Range
Storage Range
Special Attention
Required
-5 oC to +60 oC
-20 oC to +80 oC
Yes, water freezes
5 oC/min
10 oC/min
0% to 55%
0% to 95%
15 psi
15 psi
Salt Water
Yes, water absorption
Yes, corrosion
• 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].
Physical Loads
• 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.
Example
Shear stress from CTE mismatched substrates
over a temperature change ∆t.
∆L
Glass, αG
H
Adhesive
Aluminum, αAL
L
Resulting Shear Stress, τ
γ
Failures
• 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.
Example
Two lenses bonded together with optical
adhesive.
Nominal Specifications
• Optical Loss < 0.2 dB
• Wavefront Error < ¼ λrms
• Design residual assumed
to be zero.
Example Failure Criteria
Failure
Mode
Adhesive
Change
Device Performance
Change
Adhesive
Creep
1% Strain
Wavefront Error = ¼
λrms
Adhesive
Darkening
5% absorption
increase
Optical Loss = 0.2 dB
Catastrophic
Delamination
∞
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/interf
aces_adhesion_adhesives_and_coatings.php
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]
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.
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
Compare to “Known” Solution
• 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.
Physical Loads
Define
Requirements
• It is worthwhile to create a spreadsheet
and compare several other adhesives to
any “Known” adhesive solutions.
Failures
Lifetime
Requirements
Compare to
“Known” Solution
• Would they satisfy your defined
requirements?
– This may not be clear without testing,
but review of the datasheets is a
perfect first step.
Environmental
Conditions
Adhesive
Method
Assembly
Requirements
Budget,
Schedule &
Politics
Adhesive Research,
Selection and
Testing
Use “Known”
Solution
Budget and Schedule
• What is the allotted timeline and budget
for developing the adhesive bond?
Physical Loads
• 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.
Environmental
Conditions
Adhesive
Method
– Often very little budget or timeline are
allotted for said development.
Define
Requirements
Failures
Lifetime
Requirements
Compare to
“Known” Solution
Assembly
Requirements
Budget,
Schedule &
Politics
Adhesive Research,
Selection and
Testing
Use “Known”
Solution
Research and Selection
• 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.
Environmental
Conditions
Adhesive
Method
Physical Loads
Define
Requirements
Failures
Lifetime
Requirements
Compare to
“Known” Solution
Assembly
Requirements
Budget,
Schedule &
Politics
Adhesive Research,
Selection and
Testing
Use “Known”
Solution
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
Develop Adhesive Bond Process
Substrate Cleaning
and Surface
Preparation
Adhesive
Preparation
Adhesive
Application
Adhesive
Curing
• 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.
Adhesive Test Methodology
Substrate
Shear Test
A thin “Coupon”
made of the desired
material
Bonded coupons
are pulled until the
adhesive fails
F
F
4”
F

A
•
1”
1/16”
•
•
1”
Adhesive Bond
Covering an Area, A
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.
Surface Preparation Development
Obtain Substrate
Samples
(see ASTM D 1002)
Apply Various Surface
Preparation Techniques
to the Samples
Use Adhesive
Process to Bond
Samples Together
Lap Shear Test
Samples According
to ASTM D 1002
Compare Results, Look
for Cohesive Failures,
Iterate if Necessary
• 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].
Environmental Test Method
Obtain Substrate
Samples
(see ASTM D 1002)
25% of the Samples
to the Control
Group
Apply the Chosen Cleaning
and Surface Preparation
Technique to All of the
Samples
25% of the Samples
to the Environmental
Test Group (1)
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)
Lap Shear Test
(see ASTM D 1002)
Evaluate Results
•
•
•
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].
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.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
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/sitecgi/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
12.
Tianquan, Su, “Choice of Structural Adhesives,” University of Arizona.
http://www.optics.arizona.edu/optomech/student%20reports/tutorials/521%20tutorial%20Tianquan%20Su.pdf
APPENDIX
Equations
Tensile Stress, σ
Tensile Deflection, ∆L
Tensile Strain, ε
Young’s Modulus, E
Compressive Deflection, ∆T
Bulk Modulus, EB
Shear Stress, τ
Shear Deflection, ∆X
Shear Strain, γ
Shear Modulus, G
𝐹
= 𝜀𝐸
𝐴
𝐹𝐿
∆𝐿 =
𝐸𝐴
𝜎=
𝜀=
∆𝐿
𝐿
E = Material Constant
∆𝑇 =
𝐸𝐵 =
𝜏=
𝐹𝑇
𝐸𝐵 𝐴
(1)
(2)
(3)
(4)
(5)
𝐸
3 1 − 2𝜐
(6)
𝐹
= 𝛾𝐺
𝐴
(7)
∆𝑋 =
𝐹𝐻
𝐺𝐴
∆𝑋
𝐻
𝐸
𝐺=
2 1+𝜐
𝛾=
(8)
(9)
(10)
Poisson’s Ratio, ν
ν = Material Constant
(11)
Thermal Expansion, ∆L
∆𝐿 = 𝛼𝐿∆𝑇
(12)
Coefficient of Thermal
Expansion, α
α = Material Constant
(13)
Stress Figures
Tensile Stress
Shear Stress
H
∆X