Instant Adhesives

Blooming or frosting is a phenomenon that occurs when cyanoacrylate monomer volatilizes off a part, reacts with moisture in the surrounding environment, and falls back down on the part leaving a white powdery residue behind. (Cool fact: It is actually used by detectives to find fingerprints at crime scenes.) There are several ways to prevent blooming such as sufficient ventilation over the assembly area, applying less adhesive and/or curing the adhesive faster. The other option is to simply use a LOCTITE^® Low Odor/Low Bloom instant adhesive which is less volatile and thus less likely to bloom.

Instant adhesives are acid stabilized and require a weak base (i.e. moisture) to react. Thus acidic substrates (e.g. paper and zinc dichromated steel) or residue (e.g. vinegar) on a part can slow or even inhibit instant adhesives from curing. Surface insensitive instant adhesives are formulated to react much faster than traditional instant adhesives on inactive and active surfaces, as well as dry climates, for a more consistent, reliable bond.

There are several methods to toughen instant adhesives, but the most popular ways are using rubber or anhydride toughening agents. The toughening agents are designed to absorb and distribute impact and stress applied to the adhesive bondline, resulting in a tougher bond.

A thermoplastic plastic, as opposed to a thermoset plastic, is a solid polymer that can be reshaped or molded with pressure or temperature. Hot melts and instant adhesives are the most common thermoplastic adhesive.

Instant adhesives bond skin very rapidly because of their high moisture content. An easy way to separate bonded fingers and skin is to inflict a peel force on the bond line by simply rolling a pen or pencil through the bond joint. You could also try warm water and soap.

In addition to a unique shape, LOCTITE^® patented 20 g (1 oz) instant adhesive bottles possess a thinner and more uniform wall thickness that reduces the required squeeze force by over 30%, and improves the moisture barrier resulting in a longer shelf life. The nozzle is strategically designed to accept most standard dispense needle tips and is protected by an easy twist on/off cap that snaps in place.

For the best shelf life results it is recommended to store unopened bottles of instant adhesive in a refrigerator. Once the bottle is opened it is best to store the bottle at room temperature as storing in the refrigerator will cause any moisture contamination to condense and prematurely cure the adhesive.

An adhesive has “fixtured” when the bond is strong enough that the assembly can be handled without compromising the long-term strength of the adhesive. On a lab scale, fixture time is equivalent to the time required for 1” wide lap shears bonded with a ½” overlap to support a 3 kg weight in shear mode for at least 5 seconds. For instant adhesives, this time is usually seconds. Full cure is achieved when the adhesive has reached its highest bond strength. For instant adhesives this is typically 24 hours.

In general, instant adhesives resist non-polar solvents (ex. motor oil, ether, and toluene) very well but demonstrate poor resistance to strong polar solvents (ex. caustic, acetic acid, acetone, etc.). Consult the products TDS for more specific chemical resistance data.

Instant adhesive primers are solvent-based products that are pre-applied to promote adhesion (not cure speed) to "inactive" surfaces so that they can be bonded with cyanoacrylate adhesives. Time must be allowed for the solvent carrier to flash off the substrate (typically < 1 min) prior to applying the adhesive. Primers may not be recommended for use on all substrates so it’s suggested to consult the products TDS or the LOCTITE^® Plastic Bonding Guide for more detailed data.

Instant adhesive accelerators are solvent-based products that are pre-applied or post-applied to increase the cure speed and/or increase the gap-filling capabilities of instant adhesives. They can also be post-applied to cure exposed instant adhesive resin that squeezed out of a bond joint or to tack something into place such as a wire. They are especially useful in the winter months when the environment is very cold and dry.

Leaving uncured liquid instant adhesive remaining on some plastic substrate (such as polycarbonate, acrylic, and polysulfone) in a stressed condition may cause the formation of cracks in the plastic. Some remedies for avoiding/minimizing stress cracking are closing the bond joint quickly, using reinforced grades of plastic, anneal parts, or speed curing the instant adhesive with accelerator or light (for flashcure products only).

When using an instant adhesive to bond glass, the strength is initially strong but degrades dramatically over time. The cure speed is extremely fast due to the basic nature of glass which likely generates high stresses in the bond line immediately adjacent to the glass at a molecular level. This high stress makes the cured polymer around the bondline susceptible to chemical and physical degradation as a response overtime to contraction and expansion of the joint with changes in temperature, or hydrolytic attack by moisture, etc. A permanent bonding in joints cannot be achieved when bonding substrates involving glass.

Traditional instant adhesives are acid-stabilized, single-component adhesives that react rapidly at room temperature when contacted with moisture (a weak base) between tight bond lines. Henkel also offers light cure instant adhesives (flashcure) which have the traditional CA adhesive cure mechanism but also contain photoinitiators that polymerize rapidly upon exposure to the correct wavelength of light. Two component static-mix instant adhesives are also available for applications with large gaps or tolerances.