Surface energy is a measure of how readily something spreads out over a surface, also known as wet-out. Materials with high surface energy wet out very well and form bonds easily while those with low surface energy resist wetting out. Surface energy is the reason it’s simple to bond metals and difficult to bond plastics.
In between those extremes are a whole range of substrates with medium surface energies – not very difficult to bond, but each with its own specific considerations. This group of traditional materials includes old technologies such as glass, ceramics and concrete as well as more natural materials like wood, leather and fabric (both natural and synthetic). In many cases designers choose a specific material for its appearance, which can also affect the choice of adhesive.
Glass is most often chosen for transparency, but it’s also strong, scratch-resistant, impervious to liquids and easy to clean, all of which make it popular with consumers. Glass has a wide range of applications from television and smartphone screens to signage and architecture panels. 3M™ adhesives for bonding glass include VHB™ tapes and structural epoxies.
Wood is natural, can bear heavy loads, takes paint or stain well and is aesthetically pleasing. This renewable resource is used in many applications from construction to furniture to trim. The most basic factor in bonding wood is its absorptivity: to use tape you want to smooth the surface with paint or varnish and thus bond the finish, whereas a liquid adhesive will best wet out unfinished wood.
Fabrics range from natural materials like cotton or wool to synthetics made of LSE plastics such as polyester. Common applications where bonding is preferred to stitching include floor coverings and seat cushions as well as some clothing and accessories. Bonding concerns with fabric are usually about the base material rather than the fact that it’s woven into fabric.
Leather is tanned animal skin and is prized for its durable yet flexible nature. It is most commonly used in clothing, sporting goods and furniture, often in applications where a bond is preferable to a visible stitch. The back side of leather is generally porous and easy to bond, but the finished side may have oil or finishing pigments that the adhesive has to bond rather than the leather itself.
Products for Bonding Fabric and Leather
While the material can be critical in choosing an adhesive for fabric or leather, hold-out versus absorption and the adhesive delivery system are also important factors. Hold-out is how much adhesive remains at the surface to bond the other substrate while absorption is the amount soaking into the fabric. Spray adhesives are often a good choice because the finer particles rest on the surface for a time; contact adhesives that are rolled or otherwise spread can be another good option.
Made from fired clay, ceramics are prized for strength, hardness and resistance to wear and chemicals. They also exhibit excellent temperature resistance and low thermal expansion, which is why they are often used as heat shielding. Ceramics have many applications in the consumer goods, electronics and aerospace industries. Industrial and technical ceramics are nearly always glazed, so the bond is really to this vitreous or glasslike glaze rather than the porous ceramic beneath.
Concrete is very malleable when mixed but hardens to a strong, durable form used in all kinds of construction from roadways to housing to commercial buildings. Even smooth-finished concrete is comparatively rough and absorptive, so an adhesive that flows can fill in the rough surface. And because we expect concrete to last indefinitely, any adhesive bond should also have a long life expectancy.
Products for Bonding Ceramic and Concrete
Both ceramics and concrete are good candidates for one-part urethane adhesives and two-part structural adhesives. These liquid adhesives can flow into any gaps and thus provide as much bonding surface as possible.
In determining which adhesive will perform best, it is very often helpful to consider the assembly type. The six assembly types shown below have different design characteristics that often determine the best adhesive or tape.