The Cross-linking Reactions

Diglycidyl ethers of Bisphenol A cannot be cross-linked through heating alone. Chemical cross-linking agents must be added. Most commonly used compounds are tertiary amines, polyfunctional amines, and acid anhydrides. Lewis acid, phenols, and compounds like dicyandiamide, however, are also used. The reactions between tertiary amines and epoxy groups result in formations of quaternary bases:

The product reacts with hydroxyl compounds to form anions:

The anions in turn initiate polymerizations of the epoxy groups:

Because the monomer is a diepoxide, a three-dimensional lattice results. A similar three-dimensional product forms from reactions with other cross-linking materials, like boron trifluoride-etherate, boron trichloride-amine complexes, and imidazole derivatives. All these compounds initiate polymerizations of the epirane ring. Cross-linking of the epoxy resins with primary and secondary amines is somewhat different because they react by nucleophilic addition to the epirane ring:

A considerable amount of evidence suggests that this reaction is accelerated by proton donors. Such donors may be water, phenols, or alcohols [136]:

Epoxy resins can also be cured with the aid of ionic liquids [136]. The cross-linking reaction takes place at elevated temperatures Highly idealized pictures of reaction products from Bisphenol A diglycidyl ethers with diamines can be found in the literature [137]. In actuality, the products are probably more complex. They are certainly complex when reactions involve higher molecular weight epoxy resins. Many different aliphatic and aromatic polyamines are available for cross-linking epoxy resins. Some of these are ethylenediamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, and many others. Among the aromatic polyamine are p-phenylene diamine, m-phenylene diamine, 4,40-diaminodiphenyl-methane, and diamino diphenyl sulfone. Aspecial cross-linking agent for epoxy resins is dicyanodiamide, also referred to by its trade name as cyanoguanidine. It is used for high temperature cures and it is believed that the compound condenses with the epirane structures to form 2-aminooxazoline derivatives [137]:

Earlier, however, it was speculated that guanidyl urea forms [140]. The curing mechanism of epoxy resins with imidazole was investigated. The following mechanism was proposed [137]:

Many cyclic acid anhydrides are used industrially for cross-linking epoxy resins. These are mono and polyanhydrides. Following are some typical commercially used anhydrides:

The anhydrides react with traces of moisture or with pendant hydroxyl groups first. This opens the anhydride groups and frees the carboxylic acids for reactions with the epoxy rings:

When only epoxy and hydroxyl groups are present, very little reaction takes place even at elevated temperatures, as high as 200C. Proton donors, however, catalyze the reaction [139]. When anhydrides are present in the reaction mixtures, diesters form as a result of termolecular transition states. Kinetic data support that [136]:

Tertiary amines are very effective in catalyzing reactions of anhydrides with epoxies:

Reactions in the presence of alcohols take a somewhat different path [142, 143]:

where, ROH represents both epoxide molecules with hydroxyl function and other alcohols in the system.

Another similar group of epoxy resins, called epoxy novolacs, forms from reactions of epichlorohydrin with low molecular weight phenolic novolacs (phenolic novolacs resins are discussed in the next section):

where, n is typically 2.2–3.8 for liquid epoxy novolacs and 3–7 for solid resins. Epoxy resins are also prepared commercially from Bisphenol F that is a blend of ortho and para diphenol methanes:

Another material is based on a condensation product of glyoxal with phenol:

Epoxy resins formed by condensations of epichlorohydrin with resorcinol-based phenolic resins are also formed commercially:

Several nitrogen-containing aromatic epoxy resins were also commercialized. These are condensation products of aromatic amines with epichlorohydrin. Following are some examples [140]:

The cross-linking reactions of tetrafunctional epoxy resins with aromatic primary diamines were investigated by spectoscopy [141]. UV-Visible and fluorescence spectroscopies of the materials, after gelation, show significant amounts of amines in the finished products. The infrared spectra also show that ether formation becomes significant only late in the cure. In addition, during the cure, especially in air, some oxidations and degradations occur [141]. This results in color formation.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يباشر أعمال إدامة شباك مقام الإمام المهدي (عجّل الله فرجه)

المجمَع العلمي ينظم ندوته القرآنية الأسبوعية في جامعة الكوفة

جامعة العميد تقيم الملتقى العلمي الثاني لخريجي كلية طب الأسنان

ضمن دورتها العلمية (في رحاب العقيدة) الهيأة العليا لإحياء التراث تقدم محاضرة عن مفهوم الأسبقية في الإسلام

المزيد

الآخبار الصحية

اللحظات الأخيرة قبل الموت.. هذا هو تسلسل "فقدان الحواس"

وخز الأطراف وضعف الذاكرة.. علامات على نقص فيتامين هام

عادة صباحية بسيطة.. تأثيرها قد يحسن ضغط الدم

عادات يومية تدمر قلبك بصمت.. خبراء يطلقون التحذير

المزيد

الآخبار التكنلوجية

الذكاء الاصطناعي يفك لغز آثار أقدام الديناصورات

القمر أولا.."سبيس إكس" تؤجل خططها الخاصة بالرحلات إلى المريخ

الذكور أم الإناث؟ دراسة "تنسف الشائع" بشأن التوحد

خبير يكشف عن اقتراب العلماء من فك شفرات "لغات الحيوانات" !

المزيد

مواضيع ذات صلة

Resols

Cycloaliphatic Epoxides

Preparation of Commercial Epoxy Resins

Polyurethane Foams

Polyurethane Elastomers

Polyurethane Fibers

The Effect of Catalysts

Chemical Reactions of the Isocyanates

Commercial Polyisocyanates

Polyurethanes

Polysulfones

Poly(p-xylylene) s