Aromaticity rigid foams

Polyester Polyols. Initially polyester polyols were the preferred raw materials for polyurethanes, but in the 1990s the less expensive polyether polyols dominate the polyurethane market. Inexpensive aromatic polyester polyols have been introduced for rigid foam appHcations. These are obtained from residues of terephthaHc acid production or by transesterification of dimethyl terephthalate (DMT) or poly(ethylene terephthalate) (PET) scrap with glycols. 

Isocyanates that are produced fi om aliphatic amines are utilized in a limited range of polyurethane products, mainly in weatherable coatings and specialty applications where the yellowing and photodegradation of the aromatic polyurethanes are undesirable [5]. The aliphatic isocyanates are not used more widely in the industry due to the remarkably slow reaction kinetics of aliphatic isocyanates compared to their aromatic counterparts [6]. Due to the slow reactivity of aliphatic isocyanates, it is not practical to use them in the preparation of flexible or rigid foams, which are the main commercial applications for polyurethane chemistry. 

Thesecompounds, widely employed at the outset of the development of polyurethanes have been displaced from their market by polyether-polyols. For the production of rigid foams however, use is still made of aromatic polyols produced by the transesterification of dimethyl terephthalate by glycoL... 

The polyols for rigid foams (referred to as rigid polyols) discussed before (Chapters 13 and 14), are based on the alkoxylation of different polyols or polyamines, commercialised in a relatively high purity form. Some important starters for rigid polyols are obtained by the synthesis of the starter in situ, before the alkoxylation reaction, by the condensation reaction of some aromatic compounds (phenols, melamine and so on) with aldehydes (mainly formaldehyde), followed by the reaction of the resulting condensate with alkylene oxides. Some important rigid polyols based on the condensates mentioned are ... 

By solubilisation of propoxylated bisphenol A with the structure 15.28, in a sucrose-based polyether polyol for rigid foams, an homogeneous mixture is obtained [29]. The viscosities of these mixtures increase with the content of propoxylated bisphenol A. From these mixtures rigid PU foams were obtained. Due to the aromaticity introduced by the propoxylated bisphenol A, the physico-mechanical properties of the resulting rigid PU foams were superior to the rigid PU foams made with the sucrose-based poly ether polyol alone [29]. 

Due to the low cost, the excellent physico-mechanical properties of the resulting urethane - isocyanuric foams, thermal and fire resistance and low level of smoke generation, the most important applications of aromatic polyester polyols are for rigid PU/PIR foams in the boardstock market (continuous rigid foam lamination) and for building insulation. 

The rigid PU/PIR foams [1,4-32] enjoyed an enormous success in recent years, making the aromatic polyester polyols dominate the USA polyol for rigid foam business, bypassing the polyether polyol business [4, 6. In Europe, the penetration of PU/PIR rigid foams based on aromatic polyesters has been limited, but the tendency is for a slow growth of aromatic polyester production. 

The most important structures of rigid polyester polyols presented in this chapter (Chapter 4.4) are the low functionality aromatic polyester polyols with terephthalic or phthalic structures, used for PU/PIR rigid foams. 

Tetrabromobisphenol A is a very interesting bromine containing raw material produced industrially. By the ethoxylation of tetrabromobisphenol A with 8-9 mols of ethylene oxide (EO), an interesting aromatic bromine diol is obtained which is useful as a reactive flame retardant for urethane - isocyanuric rigid foams (18.7). 

In the second step, the aromatic amines formed react with ethylene oxide or with propylene oxide. Two layers are formed. The top layer is the same polyol used in the original foam formulation (around 30% from the total volume). The bottom layer is a high functionality polyol, which it is possible to use successfully in various rigid foams formulations [34, 41]. 

The MDI is of functionality around 2.2. This functionality together with aromatic nature of the MDI will tend to give rigid foams. 

Conventional polyurethane foams are prepared from aromatic diisocyanates and aliphatic polyols. For manufacturing rigid foams, a high excess of isocyanate may be used for creating cyclic isocyanurate structures in the presence of the suitable... 

Polyether foam n. A type of polyurethane foam that has been made by reacting isocyanate with a polyether rather than a polyester or other resin component. For rigid foams, polyethers often used as the propylene oxide adducts of materials such as sorbitol, sucrose, aromatics, diamines, pentaerythritol, and methyl glucoside. These range in hydroxyl numbers from 350 to 600. For flexible foams, polyethers with hydroxyl numbers ranging from 40 to 160 are used. Examples are condensates of polyhydric alcohols such as glycerine, sometimes containing small amounts of ethylene oxide to increase reactivity. 

Diaminodiphenylmethane is an aromatic diamine used as a curing agent in epoxy resins of the bisphenol A type, as in the production of plastics, isocyanates, adhesives, elastomers, polyurethane (elastic and rigid foams, paints, lacquers, adhesives, binding agents, synthetics rubbers, and elastomeric fibers) and butyl rubber. Diaminodiphenylmethane is also a by-product of azo dyes. It is also possibly formed by hydrolysis of diphenylmethane-4,4 -diisocyanate. 

These materials are often used in a partially polymerized form, known as prepolymers. These are reacted with glycols to form high-molecular-weight polymers, or with mixtures containing controlled amounts of water to form rigid foams (note that water reacts with isocyanates to form CO2, which is generated in situ to form the foam). R and R may be aromatic or aliphatic, to form aromatic, aliphatic, or mixed polyurethanes. Also, different polyurethane prepolymers are produced from diisocyanates and diamines, giving rise initially to a urea bond (NH —CO—NH—) instead of urethane ... 

Aromatic Isocyanates. In North America, aromatic isocyanates ate heavily used as monomers for addition and condensation polymers. The principal appflcafions include both flexible and rigid polyurethane foam and nonceUulat appflcations, such as coatings, adhesives, elastomers, and fibers. 

With amine initiators the so-called self-catalysed polyols are obtained, which are used in the formulation of rigid spray foam systems. The rigidity or stiffness of a foam is increased by aromatic initiators, such as Mannich bases derived from phenol, phenoHc resins, toluenediamine, or methylenedianiline... 

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