Curing mechanisms

Polyimides (PI) were among the eadiest candidates in the field of thermally stable polymers. In addition to high temperature property retention, these materials also exhibit chemical resistance and relative ease of synthesis and use. This has led to numerous innovations in the chemistry of synthesis and cure mechanisms, stmcture variations, and ultimately products and appHcations. Polyimides (qv) are available as films, fibers, enamels or varnishes, adhesives, matrix resins for composites, and mol ding powders. They are used in numerous commercial and military aircraft as stmctural composites, eg, over a ton of polyimide film is presently used on the NASA shuttle orbiter. Work continues on these materials, including the more recent electronic apphcations. 

V,/V dipheny1ethy1enediamine. The cure mechanism probably involves an amine-catalyzed decomposition of the sulfonyl chloride group or a path of radical anions. The cross-link probably involves the HVA-2. Calcium hydroxide or other SO2 absorbers must be included for development of good mechanical properties. 

J. F. Hagman, Curing Mechanisms ofVMMMC Bulletin EA-030.0684, Du Pont Polymers, Stow, Ohio, 1980. 

The chemistry of this cure system has been the subject of several studies (44—47). It is now generally accepted that the cure mechanism involves dehydrofluorination adjacent to hexafluoropropylene monomer units. The subsequent fluoroolefin is highly reactive toward nucleophilic attack by a variety of curatives (eg, diamines, diphenols). 

Fig. 4. Addition-cure mechanism, employing hydrosilyation involving a vinyl group and hydride m > n.

Whilst the curing mechanisms may be quite complex and the cured resins too intractable for conventional analysis some indication of the mechanisms involved has been achieved using model systems. 

Forms of BR and polyisobutylene. The properties of butyl rubber and polyisobutylene depend on their moleeular weight, degree of unsaturation, nature of the stabilizer incorporated during manufacture and, in some cases, chemical modification. It is common to produce halogenated forms of butyl rubber to increase polarity and to provide a reactive site for alternate cure mechanisms [6],... 

Each of these product types will be discussed, including the method of application, the type of substrates that are bonded by each approach, and one or more typical adhesive formulations. In order to understand the adhesive cure mechanism, a brief review of the common urethane reactions is needed. 

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

Blocked isocyanates are particularly helpful in dual cure mechanisms. In one instance, UV light first polymerizes an acrylate polymer containing hydroxyl groups. The system also contains a malonate ester-blocked isocyanate. The one-component system is heated, which starts the polymerization of the acrylate. Higher temperatures unblock the isocyanate, permitting the cure of the urethane to proceed [15]. 

These adhesives differ from normal hot-melt adhesives, such as the standard ethylene vinyl acetate hot melts. Standard hot-melt adhesives like EVA have no curing mechanism. They are heated above the crystalline melting point and applied as a low-viscosity liquid in the same manner as is the curing hot melt. The bond is closed in the same manner and strength is developed upon crystallization. 

Fig. 5. Curing mechanism of a maleimide resin via Michael addition.

Of the commercially available EB-curable adhesives [9-12], the resins fall within one of two categories based on their curing mechanisms. The majority of EB-curable resins are based on (meth)acrylate-functionalized oligomers involving a free-radical curing mechanism. The second category is the epoxy resins that cure by a cationic mechanism. 

An analogous mechanism should also produce polymers on irradiation of epoxies. Crivello s recent mechanistic suggestions [29] are consistent with the mechanisms given above. One can conclude that radiation-induced polymerization of epoxies can proceed via several mechanisms. However, further work is needed to determine the relative contributions of the different mechanisms, which might vary from one epoxy to another. As part of the Interfacial Properties of Electron Beam Cured Composites CRADA [37], an in-depth study of the curing mechanism for the cationic-initiated epoxy polymerization is being undertaken. 

Materials and additives that are chemically basic in nature have a detrimental effect on the curing of cationic-initiated epoxy systems. These substances can either stop the curing mechanism completely or produce under-cured polymers. Therefore such additives as amines or imides that are known to be adhesion promoters cannot be used in the EB-curable epoxy adhesive formulations. 

Epoxy ester Epoxy esters are a type of alkyd where a high molecular weight resin is reacted with alkyd resin. The curing mechanism remains primarily through the oil-oxidation reaction and their properties are in no way similar to the chemically reacted epoxies. They have similar properties to alkyds although with improved chemical resistance but inferior appearance. They form a reasonably hard, oil-resistant coating, which can sometimes be suitable for machinery enamels, but are primarily for interior use, since they tend to chalk rapidly on exteriors. Their best use is for chemical or water resistance where circumstances dictate that finishes that are more superior cannot be used. 

The curing mechanism shown below demonstrates the behaviour of one small polyamine molecule with four epoxy resin molecules. Similar reactions will occur at the other end of the epoxy resin molecules. 

Kamon, T., Furukawa, H. Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins. Vol. 80, pp. 173 — 202. 

Free-radical cure mechanism, silicone network preparation via, 22 568-569 Free-radical cyclization process, 27 147 Free-radical-initiated polymerization, 20 211... 

The synthesis and proposed cure mechanisms of this resin are described in reference 2. While the cure mechanism of the BCB terminated resin is not yet known, it is speculated that it reacts via one of two different routes. Initially the strained four member ring of the benzocyclobutene undergoes a thermally Induced ring opening. The opened rings then react with one another by a linear type addition to form a network type of structure or by cycloaddition to form linear polymer chains. An Illustration of the proposed polymerization mechanism of benzocyclobutene (BCB) terminated resins is shown below. 

Proposed Benzocyclobutene (BCB) Terminated Resin Cure Mechanism... 

The cure mechanism of the BCB/BMI blend is believed to differ from that of the BCB oli omer. Following the thermally induced ring opening of the strained four member ring of the benzocyclobutene it can then react with the dienophile introduced to the system by the BMI resin via a Diels-Alder reaction An illustration of that reaction is shown below. 

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