Cyanoacrylates surface preparation

Surface preparation. Cyanoacrylate adhesives are very sensitive to surface preparation methods and surface contamination, owing to the nature of the cure and to the high polarity of cyanoacrylate monomers and polymers. 

Alkyd parts are generally very rigid, and the surfaces are hard and stiff. Surface preparation for alkyd parts consists of simple solvent cleaning and mechanical abrasion. Epoxies, urethanes, cyanoacrylates, and thermosetting acrylics are commonly used as structural adhesives. 

Typical surface preparation calls for cleaning with acetone, MEK, or other common solvent. Once clean, the substrate is then mechanically abraded with sand, grit or vapor blast, or steel wool. The surface is again wiped clean with fresh solvent. Typical adhesives that are employed include epoxies, urethanes, and cyanoacrylates. Polysulfides, furanes, and polyester adhesives have also been suggested. 

The common surface preparation treatment for epoxy resins is to wipe with solvent, mechanical abrasion, and final solvent cleaning. Epoxy parts can be most easily bonded with an epoxy adhesive similar to the material being bonded. Urethanes, cyanoacrylates, and thermosetting acrylics have also been used when certain properties or processing parameters are required. 

The most important step in the successful application of a cyanoacrylate adhesive is the application of a thin adhesive film between two well-mated surfaces. The thiimer the film is, the faster the rate of bond formation, and the higher the bond strength. Bond strength is dependent on proper surface preparation. 

Disadvantages of cyanoacrylate adhesives include poor thermal and moisture resistance on metals and glass, brittleness, sensitivity to surface preparation and poor cure through... 

Modified cyanoacrylates are now available which overcome the surface acidity and cure well on these surfaces (see Section 10.4.1). Acidic deposits may also be present as a result of an incorrect cleaning or surface preparation process. 

Bond stresses, materials, type of cyanoacrylate, surface preparation, method of application and production requirements should all be considered in relation to each other at the... 

Silicone rubber is difficult to bond with cyanoacrylates. The adhesive does not wet the surface properly without special surface treatment, due to the very low surface energy, and therefore will not bond. Polypropylene (PP), polyethylene (PE), polytetrafluoro ethylene (PTFE) and acetal plastics and Santoprene rubber also fall into this category and cannot be bonded without prior surface preparation. 

Surface finish and surface preparation are both key factors in the success of an adhesively bonded joint and, in many applications, roughening the plastic surface can be beneficial to the overall bond strength and the durability. If the adhesive is injection moulded, it is often possible to spark erode the mould tool to give a slightly rougher surface finish at the bond line thus improving the mechanical keying of the adhesive to the surface. A surface finish of between 1 and 2 Ra will invariably improve the adhesion performance of cyanoacrylates to thermoplastics. 

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. 

Aboubakar et al. [47] studied the physico-chemical characterization of insulin-loaded poly(isobutyl cyanoacrylate) nanocapsules obtained by interfacial polymerization. They claimed that the large amount of ethanol used in the preparation of the nanocapsules initiated the polymerization of isobutyl cyanoacrylate and preserved the peptide from a reaction with monomer, resulting in a high encapsulation rate of insulin. From their investigations, it appears that insulin was located inside the core of the nanocapsules and not simply adsorbed onto their surface. 

The industrial manufacturing process for cyanoacrylate monomers is designed to generate pure organic compounds free of metals. The prepared monomer is functionally very reactive and is polymerised by several mechanisms of which the most common is by anionic methods. In most applications the initiation is usually carried out by the nucleophilic contaminant (water or moisture) found on most surfaces. These adhesives differ from other adhesives in that they are monofunctional and can homopolymerise rapidly at room temperature. A number of modifiers have been added to impart a range of desired properties and these include stabilisers, inhibitors, thickeners, plasticisers, tracers, colorants and preservatives. 

Quite contrary to popular belief - a belief founded on the use of traditional adhesives - reliable joints can be obtained from unprepared surfaces. Of course, there is no denying that the better the preparation the better the overall performance. But, providing contamination is not gross, perfectly adequate levels of performance can normally be obtained from Anaerobic Cyanoacrylate Plastisol Toughened acrylic and Toughened, heat-cured, epoxide-based adhesives. 

---