Rigid PU foam

A split phase glycolysis process for the recovery of polyols from PU foam waste is described. Applications of the polyols in the manufacture of flexible and rigid PU foams are examined, and the economics of the process are analysed. 2 refs. 

Arcus gasification combuster is described and the principles upon which it works are explained. This combuster combines solid fuel gasification with the burning of the lean gases produced on a small capacity scale. The types of solid fuels which can be used are listed and these include segregated municipal waste and industrial waste such as rigid PU foam and plastics mixed with other materials. Uses of the gas produced are included. 

The dimensional stability of low density, water blown rigid PU foams for pour-in-place thermal insulation applications was improved by the use of a phthalic anhydride based polyester polyol containing a dispersed cell opening agent. The foam systems obtained allowed some of the carbon dioxide to be released through the cell windows immediately after filling of the cavity, and to be rapidly replaced by air. Studies were made of the flowability, density, open cell content, dimensional stability, mechanical properties, thermal conductivity and adhesion (particularly to flame treated PE) of these foams. These properties were examined in comparison with those of HCFC-141b blown foams. 21 refs. 

A stndy was made of the effects of foam formulation and process conditions and liner composition on the adhesion of HCFC-141b blown rigid PU foam thermal insulation to refrigerator liner protective layers made of ABS, high-impact PS (HIPS), PE and blends of HIPS and PE containing a compatibiliser and adhesion promoter. A tensile test was used to quantify the level of adhesion before and after thermal cycling, and the Brett mould was used for laboratory simulations of foam adhesion within... 

Also phosphorus- and nitrogen-containing polyols are shown to be effective in flame retardancy of PU foams24 such as polyols based on phosphonic acid ester or obtained by partial or full substitution of methylol groups of tetrakis(hydroxymethyl)phosphonium chloride with amine several examples of such polyols were reported by Levchik and Weil.15 Rigid PU foam modified with these polyols showed improved oxygen index values moreover better results were achieved with higher functionality polyols. 

For rigid PU foam Standards of performance to BS 476 parts 6 and 7, French Epiradiateur NFP92-501, German DIN 410238... 

In order to simplify the procedure of using too many components, a masterbatch , that is a mixture of the components that do not react with each other, (e.g., oligo-polyol, water, chain extender, catalysts, etc.), is made before foaming. Then it is possible to use only two components one is the polyolic component (called component A or formulated polyol, containing a mixture of all raw materials except for the isocyanate, in the proportions needed) and the second component is the isocyanate (called component B or isocyanate component). The polyurethane that results is a consequence of the very efficient contact between the isocyanate component and the polyolic component. Usually, in rigid PU foams only two components are used. In flexible foams, the polyolic component is divided into two components, especially in order to avoid the contact of some hydrolysable component with water, (e.g., stannous octoate). The gravimetric ratio between the components is verified before the foaming process and if necessary, it is corrected. 

The same reaction (11.4) is currently used to obtain silicon emulsifier for flexible and rigid PU foams, by the reaction of polydimethylsiloxane of relatively high MW (3000-5000 daltons or more) having several -Si-H groups in the main polysiloxanic chain and a propylene oxide (PO) - ethylene oxide (EO) copolymer, block or preferably random copolymers, having minimum 50% EO units (reaction 11.6). 

By chemical recovery of polyester [poly(ethylene terephthalate) (PET)] (Chapter 16) and PU wastes, by alcoholysis or by aminolysis (Chapter 20), new polyols are obtained that can be used in rigid PU foam fabrication. The vegetable oil polyols, obtained by chemical transformation of the double bonds in vegetable oils in various hydroxyl groups are a very attractive route to obtain polyols from renewable resources (Chapter 17). 

A special group of polyols for rigid PU foams is the group of reactive flame retardant polyols containing phosphorus, chlorine or bromine, which confer fire resistance to the resulting PU (Chapter 18). 

The general synthesis reaction of polyether polyols for rigid PU foams by polymerisation of alkylene oxides (PO, EO) initiated by polyolic starters is presented in reaction 13.1. 

The most important low molecular weight polyols used as starters for polyether polyols destined for rigid PU foams synthesis are glycerol, trimethylolpropane (TMP), triethanolamine, pentaerythritol, dipentaerythritol, a-methyl glucoside, xylitol, sorbitol and sucrose [1-27]. The main properties of these starter polyols, which are of interest for polyurethane chemistry, are presented in Table 13.1. 

Table 13.1 Some characteristics of polyols used frequently as starters for polyether polyols for rigid PU foams ...

A second important group of starters used in the synthesis of polyether polyols for rigid PU foams is the group of polyamines, aliphatic or aromatic, having 2-3 amino groups/mol (primary or secondary amino groups) such as ethylenediamine (EDA), diethylenetriamine (DETA), ortho-toluene diamine (o-TDA) and diphenylmethanediamine (MDA) [1,2] (see Chapter 4.2). The main properties of these polyamines which are of interest in polyurethane chemistry are presented in Table 13.2. 

Both structures (13.2 and 13.3) are used successfully in rigid PU foams. Structure 13.2, having a high hydroxyl number, is used more frequently as crosslinker in many polyurethane applications (rigid PU foams, coatings). 

A very interesting catalyst used in the synthesis of polyether polyols for rigid PU foams is urea [41]. Sucrose poly ether polyols obtained in the presence of urea as catalyst have a very light colour [41]. Unfortunately with urea it is possible to obtain lower molecular weight polyether polyols, with an hydroxyl number (OH ) higher than 500 mg KOH/g. 

Polyether polyols for rigid PU foams are obtained in the same type of polymerisation reactors as those used for high molecular weight polyether polyols, i.e., in stainless steel loop reactors, with an external heat exchanger, preferably with the possibility of generating a large surface of the liquid reaction mass, by a spray technique or by an ejector technique... 

Continuous processes for the synthesis of rigid polyether polyols are discussed [42], Generally a synthesis of a polyether polyol for rigid PU foams has the following steps ... 

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. 

The most important polyether polyols from this first group of low melting point starters are sorbitol-based polyether polyols, which are considered to be the universal polyols for rigid PU foams. They can be used in all applications of rigid polyurethane foams, such as thermoinsulation, wood imitations, packaging, flotation materials and so on. 

The polyether polyols for rigid PU foams based on polyols which are liquid under the conditions of alkylene oxides polymerisation are glycerol and TMP polyether triols, of various molecular weights, sorbitol-based polyols (based on a mixture of sorbitol - glycerol, sorbitol - dipropyleneglycol, sorbitol - dithylene glycol) and xylitol-based polyether pentaols. 

The mixtures of sucrose - triethanolamine, usually of 1-1.5/1 (sucrose/triethanolamine) [9] are very stirrable mixtures, at the propoxylation temperature, and are frequently used in practice. Triethanolamine can be replaced by diethanolamine, monoethanolamine and even by ammonia [59]. The triol is formed in situ by the reaction of ammonia or primary or secondary ethanolamines with PO. The polyols based on sucrose - triethanolamine (Table 13.6) are frequently used to make rigid PU foams for thermoinsulation of freezers. The mixtures of sucrose - sorbitol lead easily to high functionality polyols, sorbitol having an excellent solvating capability for solid sucrose. 

Table 13.6 The characteristics of a representative sucrose-triethanolamine-based rigid polyether polyol for rigid PU foams (structure I) ...

Polyether polyols for rigid PU foams or intermediate polyether polyols (with a higher hydroxyl number than the final polyether) proved to be excellent reaction media for the propoxylation of solid polyols, especially to obtain very high functionality polyols. These polyether polyols, used as a liquid reaction medium, are called heel . For example, at the PO polymerisation temperature (110-120 °C) a mixture of 60% sucrose and 40% sucrose-based polyether polyol is a perfectly stirrable mixture. 

The resulting mixtures of aminic polyols (4.10) are excellent crosslinkers for rigid PU foams and other PU products. The aminic polyols, due to their intrinsic high reactivity are used especially in rigid spray PU foams. 

Mannich polyols are aromatic polyols, which confer excellent physico-mechanical, thermal and fire proofing properties to rigid PU foams. Mannich polyols, especially those based on p-nonyl phenol, have a very good compatibility with pentanes used as blowing agents (for example sucrose polyether polyols have a poor compatibility with pentanes, giving emulsions at normal concentrations for foaming, but not real solutions). 

A very interesting Mannich polyol, of low functionality (f = 3.5 OH groups/mol) and low hydroxyl number (OH = 325 mg KOH/g), derived from nonyl phenol was developed successfully for all water blown rigid PU foams [19]. 

The resulting novolak polyols, in spite of their low functionalities and low hydroxyl numbers, give rigid PU foams with a very uniform cellular structure, with excellent physico-mechanical, thermal and fire proofing properties and good dimensional stability, characteristics which are associated with the high aromatic structure of novolak polyols. 

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