Adhesion is a surface phenomenon. A surface's texture, porosity, and flexibility affect the adhesive strength of a plastic joint. Ditto for any contaminants such as mold release agents, plasticizers, or other resin formulations that migrate to the surface.
Surface energy also influences adhesion. It defines the ability of adhesives and pressure-sensitive adhesive (PSA) tapes to "wet out" plastic surfaces to allow adhesion. Surface wet out refers to how well a liquid or viscoelastic solid flows and intimately covers a surface. Maximum adhesion develops when the adhesive or viscoelastic PSA tape thoroughly wets out the surface to be bonded. The greater the wet out, the better the surface coverage and the greater the attractive forces between adhesives and plastic surfaces. Surfaces with high surface energy bond more readily because they are easier to wet with conventional adhesives and tapes than are low-energy surfaces.
Surface energy is a relative phenomenon. To gauge the effects of surface energy on adhesion, one must compare the surface energy of a liquid or viscoelastic solid to that of a solid surface. A liquid or viscoelastic solid possessing a lower surface energy than a solid surface will spontaneously wet out the solid surface.
High surface energy plastics such as ABS and polycarbonate bond well with standard conventional adhesives and tapes possessing lower surface energy (LSE). But wet out becomes a challenge when using these same adhesives and tapes to bond LSE plastics such as polypropylene, TPOs, and polyethylene. Conventional adhesives and tapes can't wet them out properly resulting in minimal contact with the plastic surface and unsatisfactory bonds.
Traditionally, these LSE plastics have been primed, flame treated, or corona treated to raise surface energy and make them more suitable for bonding with conventional tapes and adhesives. As design and production engineers shift to LSE plastics, they need better and more efficient ways of attaching LSE plastics to themselves, metals, or other materials.
NEW ADHESIVES AND TAPES FOR LSE PLASTICS
The most recent advance in adhesive technology allows structural bonding (in excess of 1,000 psi in overlap shear) of LSE plastics without priming or other pretreatment steps. It is based on a two-part solvent-free acrylic adhesive technology. This room-temper-ature-curing adhesive saves cost, time, oven-curing space, UV lamps, and heaters. It also resists many chemicals, water, humidity, and corrosion.
Another adhesive technology that bonds LSE plastics without pretreatment involves sprayable hot melt adhesives. They combine high heat resistance with high strength and low creep and are pressure sensitive when first applied. These adhesives handle light to medium- weight bonding and can be used in making furniture or transportation interiors manufacturing to bond fabric with polypropylene or polyethylene foams or foam to foam.
Acrylic pressure-sensitive adhesives often provide the best balance of adhesion and performance properties for many applications, but generally do not bond to LSE plastics. Historically, some acrylic PSAs have been formulated to bond to LSE plastics. But they've often had to compromise performance at high heat and with chemicals to better wet out the LSE surface. A relatively new acrylic PSA technology now bonds to a wide variety of LSE plastics while maintaining excellent high-temperature and chemical resistance and high-peel strength. This technology is available as an adhesive transfer tape and as a double-coated tape. It works in light to medium-weight bonding applications, as for bonding nameplates to LSE plastic parts or bonding carpet onto polypropylene door panels.
New tape and adhesive technologies that bond to LSE plastics without a pretreatment step offer increased efficiency, reduced costs, and improved design flexibility to manufacturing operations.
|Relative surface energy of plastics|
SURFACE ENERGY (Dynes/cm)
250 to 500
HIGH SURFACE ENERGY
|Kapton (DuPont)|| |
LOW SURFACE ENERGY
|Teflon (DuPont)|| |
Information for this article was contributed by Dr. Barry W. Kostyk, 3M Bonding Solutions, St. Paul.