Cyanoacrylates Adhesives, dispensing

Two of the most difficult types of adhesives to dispense are Cyanoacrylate adhesives and Anaerobic adhesives. Cyanoacrylates cure by interaction with the surface moisture on a component part. Anaerobics remain liquid in the presence of oxygen but automatically cure once enclosed in a joint. 

Anaerobic reactive adhesives and cyanoacrylate adhesives are applied by means of special metering units that dispense small predetermined quantities at the required time intervals. 

Cyanoacrylates and Light-Curing Acrylic Adhesives - These types of adhesives have achieved widespread acceptance in high-speed manufacturing because they cure rapidly, offer high bond strength to many substrates and are easy to dispense 9. (See Reference 189 for the structure of these chemicals.)... 

The adhesive is put under pressure and the dispense valve is opened for a certain time to provide the correct quantity of adhesive (see Figure 10.15). Since the cyanoacrylate contacts the pressurised air, the dryness of the air is important and an air filter on the mains air line is recommended. A pressure/time system is best suited for cyanoacrylates up to a viscosity of 5,000 mPa-s, i.e., self levelling products. 

When excess adhesive has been applied, the surface to volume-of-adhesive ratio is too low and the moisture on the surface will be insufficient to neutralise the stabiliser in the adhesive. The cyanoacrylate vapour will escape and fuming will occur. Reduce the quantity of the adhesive by using fine bore application nozzles and/or dispensing equipment. 

Cyanoacrylates are quite sensitive to acidic or basic impurities. Their stability and reactivity should be checked after storage in dispensers. Clean-up can often be a problem with cyanoacrylates where the adhesive often cures in, for example, drip trays or on the outside of joints. Precise application is the best solution, otherwise disposable drip trays or trays coated with poly tetrafluoroethylene (PTFE) should be used. 

In the electronics industry, the list of commonly dispensed fluids includes adhesives (anaerobic, cyanoacrylates, epoxy, and UV) conformal coatings flux (liquid and paste) RTV silicone (room-temperature vulcanization — curing without heat) solder mask, and solder paste. Given the vast differences in the properties of these fluids, in particular their viscosities and typical dispensed volume, dispensing... 

As with the anaerobic adhesives, moisture-curing adhesives, such as cyanoacrylates, are typically applied using bottles, cartridges etc., made for specific applications. Since moisture exclusion is imperative for premature polymerization, metering and dispensing equipment is designed specially to prevent contact of moisture-curing adhesives with air, prior to use. 

Most industrial adhesives (e.g., cyanoacrylate, epoxy, poljmrethane, room-temperature-vulcanising silicone and most acrylic adhesives) do not adhere to PP and PE. Indeed these adhesives are often packaged in PP or PE bottles so that the adhesive itself can be dispensed without sticking to the bottle. 

The cure method of the adhesive very often dictates the type of dispensing equipment to be used. Cyanoacrylates are extremely sensitive to moisture and so a clean dry air supply to the pressure pot is essential to ensure the adhesive does not thicken in the pot. 

This guide provides a full explanation of the cure mechanisms and discusses the performance benefits for four types of engineering adhesives (cyanoacrylates, epoxies, two-part acrylics and UV curing adhesives). There are also chapters on joint design, dispensing systems, the surface preparation for difficult plastics and information on several Other adhesive technologies. 

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