Signs are not only important for wayfinding but are a critical part of any business’s branding message. As such, signs must be creatively designed and reliably constructed. Structural adhesives can replace welding, riveting and other mechanical fasteners. They offer improved appearance, lower costs and shorter assembly time. Adhesives also insulate against galvanic corrosion and are environmentally and chemically resistant, which translates into longer life for the sign. Many adhesives are available to choose from, so it’s important to select the right one for the job.
Type of Substrate
Many materials are available for use as substrates in signage. Common substrates include: aluminum, stainless steel, aluminum composite panels (ACMs), polycarbonate, acrylic, foams, glass and natural materials such stone, concrete and wood. An important consideration is how to effectively bond these materials.
- Metal-to-metal bonding is one common scenario in sign construction, and acrylic adhesives such as the LORD Signlok™ series are an excellent choice. Minimal surface preparation is required – saving time and resources.
- Plastic-to-plastic bonding is also common and typically urethane adhesives are the best choice, although for thermoplastics such as acrylic panels, LORD Signlok 400 Series acrylic adhesives may be an option.
When bonding dissimilar materials, a key consideration is whether the materials have different coefficients of thermal expansion (CTE), which would cause the substrates to expand and contract at different rates when the temperature changes. Metals and plastics have different CTEs. Our urethane adhesives can offer a solution, as urethanes provide the flexibility to tolerate the stresses produced by thermal expansion, preventing stress fractures and/or bond failure. LORD 7542 an equal-mix, two-component urethane adhesive system used to bond PC, acrylic, prepared metals and other composite plastics with little surface preparation. LORD 7610DTM is a one-part, general purpose sealer/adhesive for non-structural assemblies.
There is a minimum level of surface prep that is critical to any adhesive performance. For acrylic adhesives, minimal surface preparation is required on bare metal. For urethane adhesives, metal surfaces should be painted or primed for best long-term results. Bonding surfaces on plastic should be scuffed, free of loose debris and greases.
Our sign bonding adhesives provide options to the sign designer to evaluate exposure, size and substrate to find the best adhesive solution.
Signs installed outdoors may be larger than indoor signs, exaggerating the effects of different rates of thermal expansion. Outdoor installations may also be subjected to loading that causes the sign to flex. LORD 7542 urethane adhesive is a good choice for accommodating these conditions.
Other factors can influence adhesive choice. For example, coatings used on outdoor signs can present special requirements for adhesive chemistries. Ease of dispensing the product in the field may also be a consideration. Large installations with tight tolerances may require adhesives with indefinite open times and our No-Mix Series (LORD 201 and LORD 204 used with Accelerator 4) provides this. LORD 7650 single component urethane may also be an option. No-Mix adhesives can be applied on opposite mating surfaces of the substrates to be bonded. Curing does not start until the parts are brought together. Porous, natural materials such as stone or concrete may require other adhesives, like epoxies (LORD 320/322), to be considered.
During sign design, adhesive options should be tested for performance; if sign materials or installed conditions are not straightforward, both acrylics and urethanes should be tested. Tradeoffs may also need to be made. For example, adhesive stiffness should be chosen not only for ease of use but to prevent failure (or “pop-off") on a cross bonded assembly.
Finally, it should be remembered that sometimes an adhesive is not appropriate for sign bonding. The rule of thumb is to use adhesives where the joint design is loaded in shear or compression, instead of peel or tension. Also, areas like butt joints or “T” joints do not always allow sufficient overlap for the adhesive to bond properly. In these circumstances, mechanical fasteners may be required or used in combination with adhesives.