1. Adhesion Victor X. Lu, Ph.D.
Pressure Sensitive Adhesive Tapes for Sales and Marketing Professionals
Tape University Basics May 12, 2009
Adhesion
Victor X. Lu, Ph.D.
Cytec Industries, Inc.

2. Disciplines of Adhesion
Reference: Petrie, E.M., Handbook of Adhesives and Sealants, p. 9, McGraw-Hill, 2000.
Physics
Science of Adhesion
Surface
Science
Joint
Design
Polymeric Materials
Chemistry
Mechanics

3. PSA Fundamentals Tg Viscoelastic Behavior Surface Energy
-35 oC to +10 oC
Viscoelastic Behavior
Solid component
Liquid component
Surface Energy
Interfacial Forces

4. Theories of Adhesion Mechanical Interlocking Diffusion Electronic
Adsorption
No one theory explains adhesion but Adsorption Theory is the most popular.

5. A List To Stick By ... Things to know about PSA functionality
SUBSTRATE
The material that the adhesive/tape
needs to stick to?
Things to know about PSA functionality
ADHESION
How the adhesive is supposed to behave?
FUNCTIONALITY
What is the tape supposed to do?
You cannot always assume that the customer knows what he/she needs or what will or will not work!
ENVIRONMENTAL
CONDITIONS
What are the conditions the tape will function in?
APPLICATION
How is the tape applied?

6. Knowing The Answers To These Parameters Will Help To Select ...
Adhesive Type
Backing Type
Product Construction
Fitness For Use Requirements
Quality Control Tests
Cost Implications

7. PARAMETER CONDITION IMPLICATIONS SUBSTRATE ADHESION FUNCTIONALITY
SURFACE TEXTURE
SURFACE CONTOUR
SURFACE ENERGY
ADHESIVE THICKNESS
ADHESIVE TYPE
ADHESION
PERMANENCE
REMOVABLE
TACK
SHEAR RESISTANCE
ADHESIVE PROPERTIES
FUNCTIONALITY
DURABILITY
FLAMMABILITY RESISTANCE
BARRIER PROPERTIES
BACKING TYPE
ADHESIVE TYPE
CONSTRUCTION
ENVIRONMENTAL
CONDITIONS
WATER/MOISTURE
CHEMICAL EXPOSURE
U.V. EXPOSURE
HOT/COLD
BACKING TYPE
ADHESIVE TYPE
APPLICATION
HAND APPLIED
MACHINE APPLIED
LINER REMOVAL
TEMPERATURE
TACK VALUES
ADHESION VALUES

8. The focus of this presentation: To discuss ...
What conditions must exist for maximum adhesion to occur
How to maximize adhesion when these conditions are compromised

9. Critical Parameters For Bond Formation
Bulk adhesive properties
Viscous and elastic behavior
Surface properties
Surface tension in high energy surfaces
Interfacial properties
Ionic, Van der Waals and hydrogen bonds

10. Bond Formation
Adhesive must “wet” substrate* surface. Surface energetics control wetting.
Surface energy properties of both the adhesive and of the substrate
Adhesive must “flow” over substrate. Rheology controls flow.
Viscous flow properties of the adhesive (affects wet-out)
* Substrate refers to the material/surface
that the adhesive is attempting to adhere to
(E.G. METAL, CONCRETE, PVC , etc.)

11. Bond Energies Bond Energy, Kj/mol Covalent 60-700 Ionic 600-1000
Van der Waals
0.1-40
Polar
4-20
Hydrogen
Up to 40
Adhesion is governed by Van der Waals, Polar and Hydrogen Bonds which require intimate contact between the substrate and adhesive

12. Surface Energy

13. What Is Surface Energy? Surface Energy
The ability of an adhesive to wet out over the surface of a material is related to its surface energy. Low surface energy materials do not allow the surface to wet out - high surface energy surfaces do
LIQUID
WAX
LIQUID
PAINT
PAINT
METAL
METAL
LOW SURFACE ENERGY
HIGH SURFACE ENERGY
Poor To Average
Adhesive Wet-out
Good
Adhesive Wet-out
The difference is similar to the behavior of water on the finish of a car. On a newly waxed car, water beads up (low surface energy) and on an older finish it wets out (high surface energy).

14. Surface Energetic
Surface Energy is expressed as dyne/cm. It can be altered by corona and flame treatment or primer application of surface.
FOR WETTING TO OCCUR, THE SURFACE ENERGY OF THE ADHESIVE MUST BE LOWER THAN THE SURFACE ENERGY OF THE SUBSTRATE.

15. Surface Energy J O y Wets out easily on high surface energy substrates
Wets out poorly on low surface energy substrates

16. The study of the flow and deformation of matter.
Rheology
The study of the flow and deformation of matter.

17. Viscoelasticity
A solid will not continuously change its shape when subjected to a given stress (elastic).
A liquid will continuously change its shape (ie, flow) when subjected to a given stress, irrespective of how small that stress is (viscous).
A viscoelastic material has mechanical properties that have both elastic and viscous components, properties that show a marked time and temperature dependence.

18. Log Modulus versus Temperature
From: Polymer Structure, Properties, and Applications, Deanin, Cahners Books, © 1972, p. 89.

19. Peel Adhesion
Wetting
Surface Tension (Intermolecular forces and Polarity)
Rate of Wetting
Surface roughness and viscosity of adhesive
Bonding
Chemistry across interfaces to form interphase
Displacement of weak boundary layer
Viscoelasticity of Adhesive
Viscoelasticity
Molecular Weight and MWD
Glass Transition Temperature
Degree of Cross-linking and Cross-link Density
Polymer architecture
De-Bonding

20. Other Issues To Consider For Adhesive Bonding

21. Substrate or Surface Contour
The contour of the substrate will influence product performance.
ADHESIVE
STRESS ON ADHESIVE
SUBSTRATE
CONVEX CURVES 1 3
IDEAL SURFACE - FLAT
CORNERS
CONCAVE ANGLES 2 4

22. Substrate Contour Substrate Contour IMPLICATIONS/RECOMMENDATIONS
Regardless of the strength of the adhesive formulation, it is very difficult to overcome a continuous stress placed on it by a rigid backing trying to return to its original form.
Choose a conformable product
Consider adding stress relief to the converted part via scoring or perforation

23. Surface Contamination
The presence of surface contaminant can prevent contact of the adhesive to the substrate and results in weak bonding
Plastisizers (PVC)
Dirt/Dust
Oils
Chemicals
Water
EXAMPLE: DIRT PARTICLES
ADHESIVE
SUBSTRATE
The areas highlighted in red are areas where there is no adhesive contact

24. Surface Contamination
IMPLICATIONS/RECOMMENDATIONS
Most contamination is not visible to the eye but can be identified analytically
It may not be possible to obtain an acceptable bond without cleaning a contaminated surface
Surface contamination may be present if one can detect loose material on the surface of the substrate or if the material feels slippery, greasy etc.
Contamination may also be suspect if testing indicates poor bond strength and the adhesive feels “dead” after removal from the substrate

25. Surface Contamination
IMPLICATIONS/RECOMMENDATIONS - CONTINUED
The amount of surface preparation depends on the required bond strength, desired environmental aging resistance and economics. For maximum strength oxide films, oils, dust, release agents and all other surface contaminants must be completely removed. There are four principal ways for preparing surfaces:
Solvent degreasing;
Abrasion, including emery paper, sand, shot or grit blasting;
Chemical etching and anodizing;
Use of surface primers.
Wiping.

26. Surface Contamination
IMPLICATIONS/RECOMMENDATIONS - CONTINUED
Fortunately, most applications do not require preparation at this level, and usually an acceptable compromise can be found at some point in the sequence:
No treatment at all;
Solvent wash;
Solvent wash, abrade, solvent wash;
Solvent wash, abrade, solvent wash, chemically etch
Any of the above plus an appropriate primer.

27. Substrate texture has an impact on the strength of the adhesive bond
Surface Roughness
Surface Roughness
It’s a common misconception that more adhesive means better adhesion. On smooth surfaces, good surface contact can be achieved with relatively thin layers of adhesive, typically 2 mils. On rough or textured surfaces, more adhesive is required to fill in the hills and valleys, typically 5 mils.
ADHESIVE
ADHESIVE
ADHESIVE
Smooth
Slight Texture
Heavy Texture
Substrate texture has an impact on the strength of the adhesive bond

28. Surface Roughness
Textured substrates do not allow for complete contact (wet-out) of the adhesive with the substrate
The less contact, the smaller the bonding area and lower the adhesion
IMPLICATIONS/RECOMMENDATIONS

29. Substrate or Tape Temperature
IMPLICATIONS / RECOMMENDATIONS
Most tapes are good heat insulators and take several hours to reach room temperatures
A tape that is too warm can become soft & gummy
A tape that is too cold can become hard and lose tack and adhesion
Control the temperature of the tape and/or substrate
If temperature is difficult to regulate, consider the use of the following adhesives:
20 oF to 150 oF - RUBBER
0 oF to 400 oF - ACRYLIC
-20 oF to 500 oF - SILICONE

30. PSA Performance Depends Upon
Surface Energy
Rheology
Application Environment
Speed of Application
Surface Contour and Roughness
Surface Cleanliness
Chemicals and Moisture
Time for Bond Formation
Durability
Tape Chemistry and Construction
Understanding the functional performance attributes of the application is critical to the selection and design of PSA tape