Polyether Foam Production

Most flexible foams produced are based on polyether polyols ca 8—10% (15—20% in Europe) of the total production is based on polyester polyols. Elexible polyether foams have excellent cushioning properties, are flexible over a wide range of temperatures, and can resist fatigue, aging, chemicals, and mold growth. Polyester-based foams are superior in resistance to dry cleaning and can be flame-bonded to textiles. 

In more recent years, molded flexible foam products are becoming more popular. The bulk of the molded flexible urethane foam is employed in the transportation industry, where it is highly suitable for the manufacture of seat cushions, back cushions, and bucket-seat padding. TDI prepolymers were used in flexible foam mol ding ia conjunction with polyether polyols. The introduction of organotin catalysts and efficient siHcone surfactants faciHtates one-shot foam mol ding, which is the most economical production method. 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

Polymerization of the oxiranes is typically propagated from a starter molecule that is chosen to define the functionality if) of the final polyol. The functionality and the molecular weight of polyols are the main design features that define the polyurethane properties in the end-use applications. Additionally, the balance of EO and PO in the polyether polyols, mainly for flexible foam polyols, is tailored to enhance the compatibility of formulations and the processability of the foam products. The exact composition of the polyols defines the crucial performance features of the final polyurethane product. Even seemingly small differences in polyol composition can result in changes to polyol processabihty and polyurethane performance. This becomes a crucial issue when replacing conventional petrochemical polyols with polyols from different feedstocks. To demonstrate the sensitivity of commercial formulations to changes in feedstocks, a simple example is offered below. 

Polyethers are typically products of base-catalyzed reactions of the oxides of simple alkenes. More often than not, ethylene oxides or propylene oxides and block copolymers of the oxides are used. A polypropylene oxide-based polymer is built and then capped with polyethylene oxides. An interesting aspect of this chemistry is the use of initiators. For instance, if a small amount of a trifunctional alcohol is added to the reactor, the alkylene oxide chains grow from the three alcohol end groups of the initiator. Suitable initiators are trimethylol propane, glycerol or 1,2,6 hexanetriol. The initiator is critical if one is to make a polyether foam for reasons that we will discuss shortly. 

For the reaction of TDI with a polyether triol, bismuth or lead compounds can also be used. However, tin catalysts are preferred mainly because of their slight odor and the low amounts required to achieve high reaction rates. Carboxylic acid salts of calcium, cobalt, lead, manganese, zinc, and zirconium are employed as cocatalysts with tertiary amines, tin compounds, and tin—amine combinations. Carboxylic acid salts reduce cure time of rigid foam products. Organic mercury compounds are used in cast elastomers and in RIM systems to extend cream time, ie, the time between mixing of all ingredients and the onset of creamy appearance. 

One-shot polyether foams were studied, using a variety of catalysts. The formula contained 100 parts by weight of poly(oxypropylene)triol of 3000 M.W., 38 parts of 80 20-TDI, 2.9 of water, 0.3 of 4-dimethyl-aminopyridine, 0.5 of lV,iV-dimethylbenzylamine, varying amounts of metal catalysts, and 0.1 part of X-520 siloxaneoxyalkylene copolymer. All of the gas was evolved from these systems within 60 sec after mixing. Viscosity measurements were not satisfactory due to fracture of the polymeric phase. Analysis of the reaction mixture at the end of 55 sec reaction time indicated the relative rate of formation of various products, as indicated in Table 22. The importance of selecting the proper catalyst to avoid undesirable side reactions is readily apparent. The results shown in Table 22 indicate that both tin catalysts promote the isocyanate/water reaction more than the isocyanate/hydroxyl reaction in the system studied. This is unusual, since other reports, though often of dilute solution studies, have shown the tin catalysts to promote the isocyanate/ hydroxyl reaction more [145,147,196]. 

Classification. Flexibie urethane foams have the largest market of all polyurethane products. The production properties and applications of various flexible urethane foams are described in the following sections. Flexible urethane foams are defined as open-cell urethane foams having the property of complete recovery immediately after compression. They can be classified into two kinds, i.e., polyether foams and polyester foams. Polyether foams are further classified as follows conventional flexible foams, high-resilience flexible foams (HR foams), cold-molded foams, super-soft foams, and viscoelastic foams. 

Over 90% of all flexible PU foam production is now based on polyether polyols and 80 20 TDI. Some specialty foams have been developed recently using MDI, either alone or in combination with TDI. These new materials have been produced primarily for furniture and vehicle seating. ... 

It can be seen that moulded flexible PU foams using EO capped polyether polyols (block copolymers PO-EO with terminal poly[EO] block) represent only 22% of total worldwide consumption and that the majority of foams are flexible slabstock PU foams which use random copolyethers of PO-EO. It can therefore be concluded that the most important polyols for flexible PU foams production are in fact the random copolyethers PO-EO. 

For example, in practice, polyethers with an alkaline ion content of 50-400 ppm are used successfully. This is possible because in rigid PU foam production the one shot technique is used predominantly. The prepolymer technique is used to a small extent for one component rigid PU foams, used as sealants or in coatings. In this case the polyol needs less than 2 ppm potassium ion (for example propoxylated glycerol), in order to avoid the gellification of the prepolymers, due to the trimerisation of -NCO groups catalysed by K+ ions. 

Not taking cyclic molecules into account, the general structures of industrial silicone surfactants for flexible slabstock foam production can be seen in Figure 2.13. The main building blocks of these materials are a PDMS backbone and attached polyethers based on ethylene oxide and propylene oxide addition products. The siloxane backbones can either be linear or branched and can have their polyether substituents attached in an either pendant or terminal location. These four general structures are outlined in Figure 2.13). 

Polyether polyol types used in PU foam production are found to be safe (they are low in oral toxicity with no irritation caused to the eyes and skin). 

The formation of cellular products also requires surfactants to facilitate the formation of small bubbles necessary for a fine-cell structure. The most effective surfactants are polyoxyalkylene-polysiloxane copolymers. The length and ethylene oxide/PO (EO/PO) ratio of the pendant polyether chains determine the emulsification and stabilizing properties. In view of the complexity of the interaction of surfactant molecules with the growing polymer chains in foam production, it is essential to design optimal surfactants for each application. Flexible polyurethane foams require surfactants that promote improved cell-wall drainage. This allows the cell walls to become more open during the foaming reaction. Also, the shift away from TDI to MDI in molded HR foams adds new demands on foam surfactants (97). 

Bayer acquired the other primary polyether polyol producer, when it purchased the polyol husiness of Arco (now Lyondell) in 1999. Also, in 1999 Dow further strengthened its position in polyols when it acquired Union Carbide. Polyether polyols, mainly used for flexible foam production, accoimt for 65 wt% in a flexible foam formulation, 35% in rigid polyurethane foams, and even less in PUIR foams. 

Tolylene diisocyanate is invariably the preferred isocyanate for the production of flexible polyurethane foams. As previously mentioned, flexible foams are based either on polyesters or polyethers polyether foams account for about 80% of current flexible foam output. The two types of foams are considered separately in this section. 

The Price Award patent covers elastomeric polyurethanes made from the reaction of diisocyanates with the propylene oxide adducts of polyols. These polyether urethanes have proved to be of great commercial value as foamed rubber products, which have contributed greatly to the comfort and well-being of mankind. Approximately 1 billion lbs of these superior foamed products are used each year in the United States, particularly in cushioning for furniture and cars. 

The volatile products detected during degradation of the three foam types are summarised in Table 5.2. In agreement with TGA data, no products were formed upon heating Foams 1 or 3 much below about 200 °C, whereas Foam 2 evolved carbon dioxide and a volatile alkene in small concentrations at about 100 °C. Between 200 °C and 300 °C, the three foam systems degraded to similar products. The polyether Foams 1 and 2 formed mixtures of products apparently arising from the oxidative... 

Polyol. A wide range of polyols is available for the production of polyurethane foams. Polyether foams normally employ triols of molecular weight 3000-5000. 

Polyether foams. "Flexible polyether foams are most commonly produced by a one-shot process. In this procedure, diisocyanate, polyol, water, catalysts and surfactant are all mixed simultaneously. About 80% of such foam is produced in slabstock form in large blocks (with cross-sections up to about 12 m wide and 1.25 m high) and 20% is moulded. Commonly, in the production of slabstock the mixture is fed continuously into a moving trough... 

Major uses for propylene oxide are in the production of urethane polyether foams and propylene glycols. It also is used to make glycol ethers, polyglycols, glycerine, surfactants, and amino propanols. 

The catalytic activity of products I, II and HI, as well as of some comparison amines, was assessed by viscometric measurements. A polyether triol, Laprol 5003-2 B-lOl was combined with commercial toluene diisocyanate T 80 (Bayer AG)2 plus some water and acetone, for simulation of foam production conditions. The molar ratio of hydroxyl to isocyanate groups was 3/1. The amines tested for catalytic activity were also present they were 1,4-dimethylpiperazine (DMP), bis(2-morpholino-ethyl) ether (BMDEE) and l,4-diazabicyclo[2.2.2]octane (DABCO). The concentrations of amines were in the range 0.006 to 0.4 M. (DABCO is used in industrial production of PU foams, while DMP and BMDEE are used as co-catalysts.)... 

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