Cyanoacrylates
Cyanoacrylates (CAs) are single-component, quick fixturing, room temperature curing adhesives, which provide excellent adhesion to most substrates. Due to the single-component nature of CAs, these adhesives easily can be automated and integrated into production lines. CAs come in a wide range of viscosities from water-thin to gels and have good resistance to moisture and non-polar chemicals.
Cyanoacrylates anionicly polymerize in the presence of a weak base, such as water (which can be present in the form of humidity from the air on parts), and the neutralization of acid stabilizers present in the adhesive. As the acid stabilizers in the cyanoacrylate are neutralized, rapid polymerization occurs.
CAs do have some limitations: CAs bond to skin rapidly, have limited gap filling and curing capabilities, have poor polar solvent resistance (isopropanol, acetone, methylene chloride) and have poor long-term durability on glass substrates.
Other limitations of CAs include slow cure speed on dry or acidic surfaces, poor impact-resistance and peel strength (due to the ridged nature of the cured adhesive), poor high-temperature performance and low bond strengths to polyolefins.
Accelerators are solvent-based products designed to increase the cure speed of cyanoacrylates, reduce fixture time and increase the gap cure capabilities of the adhesive system. Primers are solvent-based products which are applied to a substrate and used to assist in promoting adhesion on difficult-to-bond substrates with low surface energy, such as polyolefins. Some CAs contain rubber toughening agents which enhance peel and impact strengths of CAs on bonded assemblies.
Surface-insensitive adhesives provide enhanced bonding performance on acidic surfaces and enhanced adhesive performance in low-humidity curing environments. Surface-insensitive CAs typically provide the highest adhesive bond strengths on most plastics. In destructive assembly testing, often the plastic material fails before the adhesive3.