Document describes lab testing on bonding double sided tapes to difficult to bond surfaces, or low energy substrates. litclipVHB Labr
- Customer complain 20mil clear High Bond
–Not sticking sufficiently to plastic surface
- Customer sent 1-5/16” plastic pieces
- Pieces too small for reliable Instron peel test
- A subjective value testing performed
- The 20mil clear VHB easily peeled away from plastic
- Also tested black and grey VHB from Guardia (40mil),3M and Duraco, and exhibited slightly tighter bond than the clear.
- Determined that the plastic has low surface energy
–Dynes range of 34/cm
- Used 3M Tape Primer 94 to increase surface tension of plastic.
- Let Primer 94 dry for 5 minutes. Then applied 20mil clear VHB tape.
–Significant improvement was immediately evident in bonding strength to plastic. After 24 hours the VHB was fully cured
- After 24 hours, when VHB is at 100% strength, it was not possible to remove the clear VHB from plastic surface by hand without breaking adhesive into small pieces
- If a VHB is used in the application, a tape adhesion promotor is recommended for permanent bonding.
- 3M Tape Primer 94 should be used in this application
–Primer 94 is specifically formulated for bonding VHB tapes to plastics
–Apply a single thin layer of Primer 94 to plastic surface (an acid brush or automated system) at room temperature
–Let dry for 5 minutes before applying the VHB
Other alternatives to VHB that does not require an adhesive promoter is a polyethylene low surface energy foam tape 1/32”
WHAT MAKES A PSA TAPE WORK?
Surface Contact: This is fundamental to adhesive performance. To maximize the contact to a substrate:
1)It must be unified (no particulate), dry, and free of contaminants
The next slides go into what factors impact the bond/adhesion of the tape to the substrate
2)Firm pressure must be applied to increase cold flow and contact of adhesive with the substrate
3)Typically, time and temperature will increase contact and adhesion values.
- Adhesion- The molecular force of attraction between unlike materials (similar to a magnetic force).
- Surface Energy- A measure of the molecular attraction of the facial contact of a material.
- Wet Out- The ability of an adhesive to flow and/or reflow over a surface to maximize bond strength based on higher contact area.
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
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 Contour
The contour of the substrate will influence product performance.
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
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
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
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 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.
- 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
- The adhesive must “wet” the substrate
- The surface energy of the adhesive must be lower than the substrate
- Surface roughness and contaminants affect adhesion
- After wetting, the adhesive must flow over the surface of the substrate
- Strength of the bond depends upon the “chemistry” at the interface and proximity for bond formation (5-10Ao)
- Formation of Dipole-Dipole, Van der Waals, ionic and hydrogen bonds and mechanical “locks” increases bond strength