Resols

These thermosetting resins form in reactions of phenols with formaldehyde in water in the presence of catalytic amounts of bases. Under these conditions, phenol exists as a resonance-stabilized anion:

The addition of the phenol anion to formaldehyde is a typical nucleophilic reaction:

Both ortho and para methylolphenols form in the above reaction. Phenol is very reactive and monosubstituted phenols are hard to isolate from the reaction mixture, because di- and trisubstitution occurs rapidly. No substitutions were ever shown to take place in the meta position. The overall reaction is as follows:

When an aqueous reaction of phenol and form aldehyde, catalyzed by sodium hydroxide, is carried out at 30C for 5 h, the products are [147]:

The remaining 3% is unreacted phenol. As the reaction continues, methylolphenols condense with each other to form methylene bridges:

The para substituted methylolphenols, of course, react in the same manner:

Formation of methylene bridges take place by one of two mechanisms. One is a direct SN2 displacement:

The other one is addition of methylolated phenols to molecules of quinone methides that form at typical reaction conditions, particularly when the temperatures are elevated [144–146]:

Two methylol-substituted phenols react with each other by the same mechanism:

The samecanbeshownforparasubstituted methylolphenols. As the reaction continues, it leads to formation of trinuclear and tetra nuclear phenolic resins. Atypical liquid resole is quite low in molecular weight. It may contain no more than two or three benzene rings. Carried a little further, the condensation yields a solid resole. The pH is usually adjusted to neutral before the resoles are heated further for cross-linking. Under neutral or slightly acidic conditions, the methylol groups tend to form dibenzyl ethers:

These dibenzyl ethers are unstable at higher temperatures, such as 150C, and decompose to yield methylene bridges and formaldehyde [147]:

The structure of a typical resole contains both dimethylene ether and methylene bridges as well as methylol groups. Fusible and soluble resols are called A-stage resins. Further reactions cause these resins to pass through a rubbery stage where they can still be swollen by solvents. This is called B-stage. The finally cross-linked material is called C-stage resin. The cross-linking process involves complex and competing reactions. Each may be influenced by reaction conditions. When cross-linking of resoles take place at neutral or slightly acidic conditions, both methylene and ether linkages form. Upon heating, the ethers in turn split out formaldehyde, as shown above. The dibenzyl ethers [144–146, 148] also break down at elevated temperatures and form quinone methides:

The quinone methides can undergo a variety of reactions including cycloadditions with other methides to form chroman groups:

The para quinone methides can couple:

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

Cycloaliphatic Epoxides

The Cross-linking Reactions

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