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Method of molding rigid polyurethane foams
Updated: 2013-10-22 14:40 Source: PUWORLD share:

In one preferred embodiment chain extenders and/or crosslinking agents are included. Unlike the polyols, these are not polymers in their own right. Chain extenders are used to join together lower molecular weight polyurethane chains in order to form higher molecular weight polyurethane chains, and are generally grouped as having a functionality equal to 2. They are usually represented by relatively short chain or low molecular weight molecules such as hydroquinone di (β-hydroxyethyl)ether, ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol (BDO), neopentyl glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, methyldiethanolamine, phenyldiethanolamine, combinations thereof, and the like. Particularly frequently used are 1,4-butanediol (BDO), diethylene glycol (DEG) and combinations thereof.

Crosslinking agents serve to promote or regulate intermolecular covalent bonding between polymer chains, linking them together to create a more rigid structure. The crosslinking agents are generally grouped as having a functionality equal to 3 or more. They also are usually represented by relatively short chain or low molecular weight molecules such as glycerine, ethanolamine, diethanolamine, trimethylolpropane (TMP), 1,2,6-hexanetriol, triethanol-amine,pentaerythritol, N,N,N',N'-tetrakis(2-hydroxypropyl)-ethylenediamine, diethyl-toluenediamine, dimethylthiotoluenediamine, combinations thereof, and the like. Particularly frequently used are glycerine, 1,4-trimethylolpropane (TMP), and combinations thereof.

Some molecules may contribute to both chain extension and crosslinking. Those skilled in the art will be familiar with a wide range of suitable chain extenders and/or crosslinking agents. When used, the crosslinker and/or chain extender may be used in amount up to 8 wt% of the polyol formulation.

Suitable surface-active substances are, for example, compounds which serve to aid the homogenization of the starting materials and may also be suitable for regulating the cell structure of the foam. Those are supplied under the trademarks NIAX, DABCO and TEGOSTABby Momentive, Air Products and Degussa, respectively. Other examples which may be mentioned are emulsifiers such as the sodium salts of castor oil sulfates or of fatty acids and also amine salts of fatty acids, eg. diethylamine oleate, diethanolamine stearate, diethanolamine ricinoleate, salts of sulfonic acids, eg. alkali metal or ammonium salts of dodecylbenzene- or dinaphthylmethanedisulfonic acid and ricinoleic acid. Foam stabilizers include for example, siloxane-oxalkylene copolymers and other orgariopolysiloxanes, ethoxylated alkylphenols, ethoxylated fatty alcohols, paraffin oils, castor oil or ricinoleate esters, Turkey red oil and peanut oil and cell regulators such as paraffins, fatty alcohols and dimethylpolysiloxanes. The above-described oligomeric acrylates having polyoxyalkylene and fluoroalkane radicals as side groups are also suitable for improving the emulsifying action, the cell structure and/or stabilizing the foam. The surface-active substances are usually employed in amounts of from 0.01 to 5 parts by weight, preferably 0.5 to 4 parts per 100 parts of polyol formulation. Agents, such as perfluoroalkanes are important additives in the field of rigid foams since they help regulate foam cell structure, hence they can be used with the present invention. Any known liquid or solid flame retardant can be used in the present invention. Generally such flame retardant agents are halogen-substituted phosphates, phosphate esters, phosphonate esters and inorganic flame proofing agents. Generally such flame retardant agents are halogen-substituted phosphates, inorganic flame proofing agents or organo-phosphous compounds. Common halogen-substituted phosphates are tricresyl phosphate, tris(1,3-dichloropropyl phosphate, tris(2,3- dibromopropyl) phosphate, tris(2-chloropropyl)-phosphate, chloropropyl bis(bromopropyl) phosphate and tetrakis (2-chloroethyl)ethylene diphosphate. Inorganic flame retardants include red phosphorous, aluminum oxide hydrate, antimony trioxide, ammonium sulfate, expandable graphite, urea or melamine cyanurate or mixtures of at least two flame retardants. In general, when present, flame retardants are added at a level of from 5 to 50 parts by weight, preferable from 5 to 25 parts by weight of the flame retardant per 100 parts per weight of the polyol formulation.

Examples of fillers include talcs, clays, silicas, calcium carbonates, graphites, glass, carbon black, plastic powders such as ABS; glass fibers or other ceramics, or polymers such as polyamide, propylene or recycled polyurethane foam. Fillers can be used in an amount of up to 20 % by weight of the polyol formulation.

Suitable polyisocyanates used in the present invention are aliphatic, cycloaliphatic, alicyclic, arylaliphatic, aromatic polyisocyanates and derivatives thereof. Such derivatives include allophonate, biuret and NCO terminated prepolymer. Aromatic isocyanates, especially aromatic polyisocyanates are preferred. It is preferred to use aromatic diisocyanates such as isomers of toluene diisocyanate (TDI), crude TDI, isomers of diphenyl methane diisocyanate, m- and p-phenyldiisocyanate, and higher functional polymethylene polyphenyl polyisocyanate; aromatic triisocyanates such as 4,4',4"-triphenyl methane triisocyanate and 2,4,6-toluene triisocyanate; aromatic tetraisocyanates; aliphatic isocyanates such as hexametliylene-1,6-diisocyanate; and alicyclic isocyanates such as hydromethylene diphenyldiisocyanate.

In one embodiment, it is preferred to use polymethylene polyphenylene polyisocyanates (MDI). As used herein MDI refers to polyisocyanates selected from diphenylmethane diisocyanate isomers, polyphenyl polymethylene polyisocyanates and derivatives thereof bearing at least two isocyanate groups. The crude, polymeric or pure MDI can be reacted with po

 

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