Polyurethane Elastomers

Solid elastomers can be divided into three categories, namely cast, millable, and thermoplastic ones. The cast elastomers are formed by casting liquid reaction mixtures of low molecular weight prepolymers into heated molds where they cross-link and convert to high molecular weight materials. Slightly branched polyesters are combined first with diisocyanates, like toluene diisocyanate, to form the prepolymers and degassed in a vacuum at elevated temperature (about 70C). The reaction mixtures are then poured into molds and heated for several hours at about 110C to form solid elastomers. These elastomers are soft and resilient, but they lack good mechanical strength. Elastomers with better mechanical strength form from linear, hydroxyl-terminated polyesters or polyethers. These macro glycols are also prereacted first with the diisocyanates, similarly to the procedure used for expandable fibers. The products, however, are mixed with low molecular weight glycols or diamines and then heated in molds at 110C for 24 h. Slightly less than stoichiometric amounts of glycols or diamines are used so that the polymers are terminated with isocyanate groups. These terminal isocyanate groups react in the mold with urethane hydrogens to form allophanate cross-links. Trimerization of the isocyanate groups might also take place, though this usually requires a catalyst to form at temperatures below 130C:

The concentrations of the allophanate links vary with the time of cure [129]. Also, if the cross linking reactions are conducted in inert nitrogen atmospheres, very little scission of cross-links takes place and a network structure forms during the cure. In open air, however, the scissions of cross-links are extensive [129] and the products have poorer physical properties. A drawback to the cast elastomers is limited shelf life and a need to store them in the absence of moisture. As a result, millable elastomers we redeveloped. These are produced by first forming hydroxy terminated linear polyurethanes through reactions of linear aliphatic polyesters or polyethers with diisocyanates. The prepolymers are rubbers or gums that can be compounded on rubber mills with other ingredients and cross-linked. Cross-linking is accomplished by adding either more diisocyanates, or sulfur, or peroxides. Diisocyanates dimers that dissociate at about 150C are often used:

Unsaturated prepolymers cross-link with peroxides or sulfur. This unsaturation can be present in the backbone or in the pendant groups. Vulcanization or cross-linking of elastomers with sulfur or peroxides is discussed in Chap. 8. Thermoplastic elastomers exhibit physical properties that are similar to those of cast and millable elastomers at ambient temperatures. These materials, however, are not cross-linked and flow at elevated temperatures. They are fabricated like other thermoplastic polymers, are high in molecular weight, and are hydroxyl-terminated. Such polymers form from linear hydroxyl-terminated polyester or polyethers that are condensed with diisocyanates and glycols. Strict stoichiometry must be maintained to achieve high molecular weights. A structure study was carried out on a model compound of one elastomer [130] prepared with 4,40 diphenylmethane diisocyanate and butanediol hard segments. It was shown that the chains are probably linked together in stacks through C¼O H–N hydrogen bonds between the urethane groups. This bonding stabilizes the overall structure in both directions, perpendicular to the chain axis. Such an arrangement of the molecules was also proposed earlier [131].

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

The Cross-linking Reactions

Preparation of Commercial Epoxy Resins

Polyurethane Foams

Polyurethane Fibers

The Effect of Catalysts

Chemical Reactions of the Isocyanates

Commercial Polyisocyanates

Polyurethanes

Polysulfones

Poly(p-xylylene) s