Cyanoacrylates thermoplastics

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

An example of this improvement in toughness can be demonstrated by the addition of Vamac B-124, an ethylene/methyl acrylate copolymer from DuPont, to ethyl cyanoacrylate [24-26]. Three model instant adhesive formulations, a control without any polymeric additive (A), a formulation with poly(methyl methacrylate) (PMMA) (B), and a formulation with Vamac B-124 (C), are shown in Table 4. The formulation with PMMA, a thermoplastic which is added to modify viscosity, was included to determine if the addition of any polymer, not only rubbers, could improve the toughness properties of an alkyl cyanoacrylate instant adhesive. To demonstrate an improvement in toughness, the three formulations were tested for impact strength, 180° peel strength, and lapshear adhesive strength on steel specimens, before and after thermal exposure at 121°C. 

The data also demonstrate that the addition of the thermoplastic, PMMA, does not have the significant effect on the toughness or adhesion properties as does the addition of the rubber, Vamac B-124. Clearly, the physical properties of the polymeric additive determine the magnitude of the adhesive physical property modifications, which result from their addition to an alkyl cyanoacrylate monomer. 

Cyanoacrylates can be used to bond many materials, including most thermoplastics and even the more difficult ones like polyethylene, polypropylene, and ethylene-propylene-diene ( EPDM ) rubber. The best results are obtained with close contact and narrow bonds (some formulations have limited ability to bridge large or irregular gaps between the surfaces). 

Since they are thermoplastic the cyanoacrylate adhesives have limited resistance to heat they do not resist moisture, and can be softened by highly polar solvents like ketones. They are expensive but since only a small quantity is necessary to form a bond, their overall economy in use is good. 

Parts molded from polyetherimide can be assembled with all common thermoplastic assembly methods. Adhesives that are recommended include epoxy, urethane, and cyanoacrylate. However, service temperature must be taken into consideration in choosing an adhesive because PEI parts are generally used for high-temperature applications. Good adhesion can be effected by simple solvent wipe, but surface treatment by corona discharge, flame treatment, or chromic acid etch will provide the highest bond strengths. 

The acrylic plastics use the term acryl such as polymethyl methacrylate (PMMA), polyacrylic acid, polymethacrytic acid, poly-R acrylate, poly-R methacrylate, polymethylacrylate, polyethylmethacrylate, and cyanoacrylate plastics. PMMA is the major and most important homopolymer in the series of acrylics with a sufficient high glass transition temperature to form useful products. Repeat units of the other types are used. Ethylacrylate repeat units form the major component in acrylate rubbers. PMMAs have high optical clarity, excellent weatherability, very broad color range, and hardest surface of any untreated thermoplastic. Chemical, thermal and impact properties are good to fair. Acrylics will fail in a brittle manner, independent of the temperature. They will suffer crazing when loaded at stress about halfway to the failure level. This effect is enhanced by the presence of solvents. 

The new technical developments have made possible quick bonding to woods, papers, and porous surfaces. Polyolefins, which comprise approximately 50% of the U.S. thermoplastic production, are now bondable with the cyanoacrylic ester adhesives. These new capabilities are sure to provide for continued market growth in the years ahead. 

Thermoplastics Cyanoacrylates, methacrylates, radiation-curing adhesives, polyurethanes (depending on the degree of crosslinkage). 

Elastomeric 1, Natural rubber. 2, Neoprene. 3, Nitrile. 4, Urethane. 5, Styrene-butadiene. Thermoplastic 6, Poly(vinyl acetate). 7, Polyamide. Thermosetting 8, Phenol-formaldehyde. 9, Resorcinol, Phenol-resorcinol/formaldehyde. 10, Epoxy. 11, urea-formaldehyde. Resin 12, Phenolic-poly(vinyl butyral). 13, Polyeser. Other 14, Cyanoacrylate. 15, Solvent. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

Cyanoacrylates polymerize by anionic mechanism initiated by moisture or basic ions. The polymers formed tend to be more brittle than those formed from other acrylics, and the bond-lines are usually not resistant to degradation by moisture. Adhesive formulations are singlecomponent, fast-curing products, however, and suited to many hard-to-bond surfaces such as rubber and many thermoplastics. 

The most common thermoset adhesives are epoxies, phenoUcs and thermoset polyurethanes. The most widely used thermoplastic adhesives are acrylics (including anaerobics, hot melts, cyanoacrylates) and thermoplastic polyurethanes. A brief description of some adhesives is given in the EUROCOMP Handbook, 5.3.4. 

Most of the adhesive families have either a thermoset or thermoplastic base. This is also the primary and the most traditional way of categorising adhesives, although within some adhesive families, such as polyurethanes, both thermoset and thermoplastic adhesives may be found. Thermoset adhesives form bonds that are essentially infusible and insoluble after curing and they typically have a much higher load-bearing capability than thermoplastic adhesives. Thermoplastic adhesives are fusible, soluble, soften when heated and their creep resistance is lower than that of the thermoset adhesives. The most common thermoset adhesives are epoxies, phenolics and polyurethanes, while the most widely used thermoplastic adhesives include acrylics (including anaerobics, hot melts and cyanoacrylates) and thermoplastic polyurethanes. A brief description of these adhesives (both thermoset and thermoplastic) is given below from reference 5.20 and 5.28. 

Thermoplastic slmctural adhesives are less significant for the more-demanding applications however. Cyanoacrylate adhesives have gained considerable importance for applications at ambient temperatures and less severe environments, particularly on plastic substrates, mainly as a result of their very rapid cure speeds (seconds) and ease of application. 

Acrylic resins are a thermoplastic type of resins formed by derivatives of acrylic acid (CH2=CH-C00H). The acrylic group is a vinyl group (CH2=CH-) (Fig. i). The monomers in acrylic resins are acrylic acids and methacrylic acids and their esters, cyanoacrylic acid and its esters, and acrylamides and acrylonitrile. Numerous different acrylic monomers therefore exist and, as a result, a multitude of different polymers and resins are produced. 

Confidence in the ability of certain synthetic resin-based adhesives to provide satisfactory structural bonds has developed during the past 40 years. The requirement that the adhesive be able to withstand loads over long periods of time means that most structural adhesives are based on thermosetting resins, whose crosslinked structure provides good creep-resistance. However, certain thefhioplastic resins (notably linear thermoplastic polyimides and cyanoacrylates) do appear to be useful for some structural bonding applications. 

Polyalkyl cyanoacrylates are clear, colorless thermoplastics whose physical properties are roughly equivalent to those of other common thermoplastics such as PMMA or polystyrene. Because the majority of adhesives sold today are based on ethyl cyanoacrylate, the properties of polyethyl cyanoacrylate (PECA) will be discussed in detail, and existing data for other esters will be tabulated. 

The discussion of heat durability in this section should make clear that this is a complex phenomenon. Embrittlement, retropolymerization, thermoplasticity, and the loss of adhesion are all factors affecting the adhesive s performance on metal surfaces. Based on the published state of the art, optimum cyanoacrylate heat durability could be achieved using a combination of a heat-resistant adhesion promoter, a crosslinking agent, and a plasticizer. The heat durability promoters discussed in this section are summarized in Table XI. 

Thermoplastics like polymethyl methacrylate (PMMA) or polystyrene should be thoroughly cleaned with a non-solvent such as isopropanol to remove surface contaminants. Abrasion is sometimes necessary to remove all traces of mold release agents. Cyanoacrylates form very strong bonds with many thermoplastics because the monomer swells into the adherend, and when cured, forms an intimate mixture of the plastic and the polycyanoacrylate. Thermosets such as filled phenolics should be treated in the same way as thermoplastics. Rubbers usually require abrasion as well as solvent cleaning to prepare them for bonding. 

Solvent resistance. Although polyalkyl cyanoacrylates are thermoplastics, they are resistant to frequently encountered low- and medium-polarity organic liquids. 

Durability. Cyanoacrylates suffer from poor heat and moisture durability. This failing is pronounced on metal adherends, but minimal on most plastic or rubber adherends. Poor heat resistance is due to several causes the thermoplastic nature of the polycyanoacrylate, the tendency to retropoly-merize, and the loss of adhesion experienced on heat aging of cyanoacrylate bonds. The poor moisture resistance is due in part to the hydrolytic degradation of the polymer and in part to the loss of adhesion caused by exposure to moisture. 

Another way to classify polymers results from the consideration of their typical applications. Typical classes are Compression molding compounds, injection molding compounds, semi-finished products, films, fibers, foams (urethane foam, styrofoam), adhesives (synthetic adhesives are based on elastomers, thermoplastics, emulsions, and thermosets. Examples of thermosetting adhesives are Epoxy, polyurethane, cyanoacrylate, acrylic polymers), coatings, membranes, ion exchangers, resins (polyester resin, epoxy resin, vinylether resin), thermosets (polymer material that irreversibly cures), elastomers (BR, silicon rubber). 

Polychloroprene, nitrile, natural rubber (polyisoprene), styrene butadiene rubber (SBR) and butyl are amongst the types of rubber that can be readily bonded with cyanoacrylates. Ethylene propylene diene monomer (EPDM) and fluroelastomers (Viton, registered trade mark of DuPont) can also be bonded, although only with specific grades of cyanoacrylate. Silicone rubber and thermoplastic rubber (Santoprene, registered trade mark of Advanced Elastomer Systems) can be bonded with the aid of a primer. Typical applications and techniques for bonding different grades of rubber are discussed in Section 10.11. 

---