FOR IMMEDIATE RELEASE
April 14, 2009
Top 10 Questions for Using Structural Adhesives in the Sign Industry
The Sign Industry has used adhesives for many years: cyanoacrylates bond a variety of substrates; two-part epoxies are used for wood and foam bonding, as well as methylene chloride and contact cements for trimcap. More recently, the Sign Industry, along with the Automotive and Aerospace Industries, recognize there are alternatives to welding, taping and other conventional fasteners such as screw and rivets. Adhesives can structurally bond metals and plastics with unique advantages in design, production methods and costs.
1. What are the advantages of structural adhesives?
The first advantage is the shear strength performance that structural adhesives provide. Adhesives are very high in shear strength ranging from 2000 to 4000 psi. This allows them to be as strong as welding and most other conventional fastening methods. Adhesives also maintain their strength over a wide temperature range (40C to 170C.) Unlike screw, bolts or rivets that concentrate the stress at the point of attachment, adhesives allow the stress to spread over the entire bond area.
The second advantage of structural adhesives is the ease of use. The benefits include the following: Available in convenient cartridges with an applicator that permits instant application without measuring and hand-mixing; Require minimum surface preparation and eliminate the need to pre-drill holes and fill in over screw heads; Eliminate rework caused by oil canning or burning from welding operations; Compensate for dimensional irregularities often filling in gaps on poor fitting parts; Act as a sealant keeping out moisture that cause problems with the electrical components in the sign, thereby reducing maintenance. From a design standpoint, adhesives allow for complicated shapes and design faces to be bonded without protruding screws and bolts.
Overall, adhesives can reduce the cost to manufacture a sign, as well as improve the aesthetics and the durability.
2. Can you use an adhesive on every substrate?
Adhesives can be used to join aluminum, stainless steel, cold rolled steel, brass, copper and most other metals. Adhesives can bond Sintra®, acrylic, polycarbonate, foam, trimcap and fiberglass and can be used with most factory painted metals such as Dibond®. Some adhesive manufacturers have Selector Guides (Figure 3) that list the appropriate adhesive to use with various substrates.
When substrates are used for cross bonding, such as bonding metal to plastic, it is very important that the adhesive supplier is contacted before starting the application. There are extreme differences in expansion coefficients of metal and plastics. When using conventional fastening methods, a sign manufacturer will cut oversize holes and loosely fasten a plastic backer to a metal face to prevent the plastic from breaking. If the holes are cut for its exact size or screws are fastened too tight, the sign could come apart. Choosing the proper adhesive when cross-bonding substrates is critical for a successful bond.
3. Is surface preparation important?
Surface preparation is important in any bonding application. Dust, dirt, grease, oil, adhesive residue and anything that can get between the adhesive and the substrate can be a possible source of failure. The surface preparation can be as simple as wiping with a clean rag. A wipe with isopropyl alcohol will often remove contaminates. Sometimes scuffing or grinding is required. The best source proper surface preparation is from the adhesive supplier.
4. Is there one adhesive that bonds all substrates?
The chemistry of an adhesive is a key indicator in determining which substrates it can bond. These adhesives are based on polymers made from acrylics, urethanes or epoxies each having a specific chemistry.
Acrylic-based adhesives are primarily used to bond metals. They are very aggressive and require minimum surface preparation. Acrylic adhesives work well on plastics, but because they are so aggressive, they may craze or stress crack some plastics.
Urethanes are a good choice for bonding plastics, wood and foam. Urethanes do not adhere well to metal without a primer, so they are not used as often for metal bonding.
Epoxies can be used on metal, plastic, concrete, wood and foam. Because epoxy-based adhesives tend to have a slow cure rate and become brittle at low temperatures, acrylics and urethanes are better choices for bonding metals and plastics.
Sign adhesives are available to provide the necessary work time, fixture time and handling strength best suited for a particular application.
5. What does the terms “work time,” ”fixture time” and “handing strength” mean?
Work time is also referred to as “pot life” — the time allowed to use an adhesive after it is mixed. Most adhesives are two components. When dispensed from cartridges, they go through a mixing tube. When two-part adhesives are continuously dispensed through the mixing tube cartridge, work time problems are greatly reduced. However, if an adhesive is not dispensed during its stated work time, the adhesive will solidify in the mixer.
Fixture time is also referred to as “assembly time” — the time allowed to mate parts and position the panel before the adhesive begins to cure. For example, an adhesive that has a 5-minute fixture time may not be appropriate for bonding a large panel, as the panel’s size may require an adhesive with a longer fixture time. The adhesive could begin to cure before the entire panel is bonded together and sections of the panel with weak bonds may come part.
Handling strength is the amount of time needed before the bonded part can be moved without coming apart. Generally, this is when an adhesive reaches strength of 50 psi.
Full cure is the amount of time needed for the adhesive to become strong enough to ensure that the panel can be successfully installed or processed further. The adhesive is considered to be fully cured when it has reached >90% of its strength.
Adhesive manufacturers list values at a specific temperature, such as 25C. As the temperature increases, the time decreases, often significantly. For example, in the summer when temperatures reach 40C, the work time may be 50% faster than work times during winter temperatures. The temperature of the substrate can also influence the full cure properties; by increasing the temperature of the substrate, the adhesive will cure faster.
6. Are adhesives hazardous?
Since adhesives contain chemicals, users need to wear protective equipment and clothing. Safety glasses or goggles are required in most plants to prevent injury and should be worn when applying adhesives. Gloves are used to protect the hands. There should be sufficient ventilation in the area to protect workers from fumes and vapors.
In general, users should use good hygiene practices. Read and understand the application procedure and safety information provided by the adhesive manufacturer.
7. When is it not appropriate to use adhesives?
The rule of thumb is to use adhesives only where the joint design is in shear or compression mode, not peel or tension. Areas like butt joints or T-joints do not always allow a sufficient overlap for the adhesive to bond properly. Sometimes it is faster and easier to assemble some designs with conventional methods. Your adhesive supplier can provide the information on which method works best.
8. Are adhesives expensive?
Adhesives provide a total cost reduction. A customer may spend more on adhesives than on a rivet or a screw, but adhesives save time in labor, application, etc.
9. Where are signs that have been bonded using adhesives?
Across Canada and throughout the United States there are signs bonded with adhesives, some as many as 10 years ago. A sign for Two Time Electronics on Dundas Street in Toronto was designed and installed by Sheriff Signs. Signage Systems made an entire storefront for Moore Clothing For Men in Brampton. The May 2002 Signs Canada issue featured World of Lights using adhesives for Farmbro Van and Truck Equipment.
10. How can end users get more information?
Information can be accessed on the Web, as most adhesive suppliers have websites (figure 3) and almost all suppliers provide toll-free phone numbers for assistance. Local distributors such as Acme or EM Plastics carry adhesive supplies and are able to provide assistance with their products.
About LORD Corporation:
With headquarters in Cary, N.C., and sales in excess of $700-MM, LORD Corporation is a privately-held company that designs, manufactures and markets devices and systems to manage mechanical motion and control noise and vibration; formulates, produces and sells general purpose and specialty adhesives and coatings; and develops products and systems utilizing magnetically responsive technologies. With manufacturing in eight countries and offices in more than 15 major business centers, LORD Corporation employs more than 2,400 worldwide. Visit www.lord.com for more information.
LORD Corporation
111 Lord Drive
Cary , NC 27512
Phone: 1-877-ASK-LORD
E-Mail: kkayler@constructivecommunication.com
Website URL: www.lord.com
