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.

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

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

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

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?

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

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

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

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

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.)

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

Surface Energy

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).

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.

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

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

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.

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

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

Other Issues To Consider For Adhesive Bonding

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

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

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

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

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.

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.

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

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

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

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