Cyanoacrylate adhesives often referred to as “super glues,” exhibit the high bond strength and fast cure times that manufacturers need for product quality and manufacturing productivity. But cyanoacrylates are not without their drawbacks. Materials are costly, improper dispensing can lead to product rejects, and care must be taken to limit worker exposure to the adhesives.
The intent of this blog is to discuss the basics of dispensing these adhesives to help you minimize material waste and worker exposure, while maximizing the productivity and performance of cyanoacrylate adhesive bonding operations.
These adhesives provide an extremely high-bond-strength and are moisture-curable. The use of cyanoacrylates has grown rapidly as the preferred method to bond a wide variety of substrate materials used to manufacture automotive components, appliances, electrical components, disposable medical devices, musical instruments and other consumer products. A major advantage of these adhesives is they do not require heat, light, precise mixing of multiple components or catalysts to cure. Humidity in the atmosphere and on the substrate surfaces is sufficient to cure the material which triggers the chemical reaction for the resulting polymerization of the cyanoacrylate monomer creating its high bond strength.
It is very important that care is taken to minimize air exposure of the material prior to dispensing. The materials can also absorb moisture from other sources, such as skin perspiration and other moisture present in the dispensing operation. The fastest curing bonds use the least amount of cyanoacrylate material.
Typical methods of manually dispensing these adhesives in many manufacturing operations is with using either small tubes or larger squeeze bottles along with disposable polyethylene fine dispensing tips. The inherent problem is often the pressure a worker physically applies to a tube, the bottle nozzle orifice diameter and adhesive viscosity determines the amount of material dispensed. Operator fatigue, shift changes and the guessing game of “just the right amount” can impact how well the parts have been fixtured. This often means either too little material for an adequate bond, or too much material, which not only affects bond strength and bond line appearance (too much glue = ugly bond line/glue squeeze out), but wastes costly material. The key to dispensing a cyanoacrylate is that superior bonds are achieved with thinner, not thicker, layers of adhesive.
So to debunk an old myth, more is not better. Dispensing more adhesive does not usually produce better results than less, unless insufficient adhesive is being dispensed to cover bonding surface. Even more, excess adhesive is a waste of material and time. More importantly, excess adhesive may not cure correctly or might not cure at all or may cause white blooming (also known as chlorosis) on the material surface which creates an aesthetic anomaly. Consistently dispensing the thin and uniform coating that is required for a good bond is arguably the major challenge in using cyanoacrylates in production.
A metal or plastic pick is often used as another manual application method to spread a thin film of cyanoacrylate across a larger surface area such as on a corner molds for an automotive weather strip application. The pick is held like a pen or pencil in the operators hand and a cup of cyanoacrylate is used to hold a reservoir of the adhesive in the assembly cell. The operator dips the pick into the pool of cyanoacrylate and then quickly wipes the thin coating of cyano onto the rubber surface. This is an effective means to apply cyanoacrylate, however it leads to several negative consequences including cross contamination of the pick mixing rubber particulate or other accelerant chemicals from the rubber compound back into the cyanoacrylate reservoir leading to thickening (fouling) of the cyanoacrylate. Also, the metal pick will eventually have a solid residue of adhesive build up that will need to be removed from time to time. Process engineers usually live with the negative trade off due to the high efficiency and accuracy of adhesive application.
Cyanoacrylate adhesives have been dispensed from tubes or squeeze bottles or by picks for years, and they still are. However, over time various types of dispensing systems have been adapted for dispensing cyanoacrylates in manufacturing applications, including pinch tubes, diaphragm valves and pneumatic-actuated syringe dispensers.
Pinch tube dispensers are ideal for manually dispensing continuous beads or applying small amounts of low viscosity liquids including solvent dispensing and cyanoacrylate dispensing.
This equipment minimizes maintenance and are designed for low viscosity fluids such as glues, solvents, and cyanoacrylates.
With this method, the amount of adhesive being dispensed depends on “pinching,” the flow of adhesive. Pinch tubes are ideal for applying thin amounts of material. In addition, this method requires a certain range of viscosity. Too low a viscosity and lack of control can occur, while for the higher viscosity gels, resistance to flow of the material can be an issue. Then, too, the pinch tube assembly must be discarded after use.
Pneumatic fluid dispensers or “time/pressure” dispensers are simple in design and consist primarily of a plunger, or piston, and a syringe. Pulsed air from a compressor “slaps” the top of the plunger in a rhythmic pattern moving the plunger forward in the syringe to push the material out through the needle tip at the end of the syringe. Pneumatic fluid dispensers are relatively inexpensive and are ideal in automated or bench top semi-automated bonding operations. One major downside to pneumatic dispensers for cyanoacrylates is the supplied air be must be filtered, cleaned and free of oil and moisture (-40 F dew point) and the syringe itself cannot be more than half full.
Ideal applications where the diaphragm valve can best be deployed include dispensing dots or beads for precise control of the cyanoacrylate. The diaphragm valve is excellent for many applications where automation is preferred. Diaphragm valves are often the system of choice when dispensing cyanoacrylates because the CA will not come in contact with the metal in the valve’s moving part.
The viscosity of the CA formulation required, and the level of performance that can be accepted, are determining factors in choosing one of these systems for a particular application. So take great care in understanding the viscosity of the formulation and the performance level required.
It is also very important to exercise great care at all times when working with cyanoacrylate adhesives. That said, plastic gloves and safety glasses with eye shields are strongly recommended for personnel protection. The assembly area should also be well ventilated to minimize exposure to the pungent vapor given off by CAs. A downdraft system that moves cyano vapor away from assembly associates face towards the floor is best practice.
In summary, CA’s have historically been dispensed through a manual, hand operator process. But over time, more efficient and time saving methods have been made available to the industry. No one method is right for every application, so it is important to understand the formulation and the performance level required prior to selecting the right dispensing method. You can contact Toagosei America, directly for more information and guidance on the best dispensing methodology for your application.