Graft polyols

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

In automotive applications, e.g., automotive seats, molded HR foam is the major process used. HR foam is produced by using four kinds of technologies (a) crosslinker technology, b) graft-polyol technology, (c) PHD polyol technology, and (d) specialty isocyanate technology. 

Graft Polyol Technology. Graft polyols (or polymer polyols) are prepared by grafting both acrylonitrile and styrene monomer or acrylonitrile alone to conventional polyether polyols. Graft polyols provide increased load-bearing ability as well as cell-opening, which prevent or minimize the formation of closed-cell foams, because closed-cell flexible foams readily shrink. 

Normally a 70/30 to 50/50 blend of a 4500-6500 EO-capped polyether triol with a polymer polyol is used, together with an 80 20 blend of TDI (80/20 isomer ratio) and polymeric MDI. Recently, higher-solids-content graft polyols, e.g., 30-50% solid polyols, have become available in the market. 

As the latter has more importance regarding the performance of the material, it is necessary in real systems to turn to other alternatives such as the use of triols or graft polyols to increase molecular weight without seriously affecting the ability of the system to reach a certain degree of phase separation. 

Graft polyols made from grafting polyacrylonitrile onto the basic polyether (also ethylene oxide -"capped"). 

Many of the same basic raw materials shown in Table II for RIM fascia systems are also used in high modulus systems. Additionally, however, polyether polyols "filled" with dispersions of polyureas are used( 2) These are the so-called PHD polyols developed by Bayer AG, the PHD being an abbreviation for Polyharnstoff-Dispersion. These polyols provide the same "filler" effect as the graft polyols (Table II) for increasing the modulus of the polymer without increasing the amount of extender. 

For example what is the estimated hydroxyl number of a graft polyol obtained from 800 kg of polyether polyol of hydroxyl number 36 mg KOH/g, after a grafting reaction with 200 g of a mixture ACN - styrene (a 100% yield of radical reaction is assumed) By using equation 6.17 one obtains ... 

Critchfield et al. (109) have also reported the use of acrylonitrile-styrene graft polyols in urethane polymers. The presence of styrene prevents the tendency toward yellowing of the acrylonitrile graft copolymers, which is due to cyclization of the polyacrylonitrile chains at elevated temperatures. Similar types of graft polyols have been reported elsewhere (110). 

BASF Wyandotte Corp. Urethane Applications Research Bulletin, "Pluracol Polyol 637. A Graft Polyol for Slab Stock Foam," Jan 8, 1974. 

Graft-Polyols Polyurethane foams are synthesized from the polycondensation reaction between a polyol and a diisocyanate. Polyols can be of two types conventional ones (polyethers) and graft-polyols [50]. The latter are obtained by the copolymerization of SAN in the presence of a polyol polyether using a free-radical initiator. Some of the SAN chains are grafted to the polyol backbone, while others remain free. The grafting is promoted... 

The most important compounds are polyetherols, polyesterols, and graft polyols (Figure 1). Polyetherols are the most common polyols and most commercial products are made chiefly from ethylene oxide and propylene oxide. Polyesterols are widely used in elastomeric polyurethanes. Typical compounds are aliphatic polyesters, such as poly(ethyl-ene adipate), made so that there are only hydroxyl end groups, with no residual acid groups. Graft polyols (also called polymer polyols) consist of a matrix of a conventional polyol, which also contains rubbery (such as styrene/acrylonitrile copolymer) particles chemically bound to polyol molecules. These grafted particles reinforce the final polyurethane, giving improved physical properties. 

Catalysts. To make the final polyurethane, common catalysts are tertiary amines and organotin compounds, often used in mixtures. Amine catalysts favor the reaction of isocyanate with water, producing urea linkages and CO2, which acts as a blowing agent. On the other hand, organotin catalysts favor the isocyanate/hydroxyl reaction. Additional catalysts and catalyst decomposition products may be present if graft polyols are part of the mixture. The most common of these catalysts are free radical initiators. Phosphorus compounds may be present in the case of carbodiimide-modi-fied MDI. 

There is an increasing market for higher resilience foams using the so-called polymer polyols. Amongst the earliest to become established were suspensions of styrene-aerylonitrile copolymer in the polyol. A variation involved some grafting of SAN, either instead of or in addition to the use of a suspension. 

Multihydroxyl containing monomeric or oligomeric p-cyclodextrins (PCD) such as those attained by grafting with glycidyl ethers of protected polyols (glycerol and pentitols) appeared rather promising components for their amphiphilic character, connected to the presence of an hydrophobic pocket and an external hydrophilic shell with an amplified number of hydroxyl groups. 

The big question is whether the seemingly ideal properties of a reticulated foam will be maintained when we start to change the chemistry (for cell adhesion, extraction, etc.). Changes in the polyol or isocyanate will inevitably affect its physical properties. A balance of chemical activity would have to be established. In many cases, this balance will degrade the desirable properties. An answer is the recent development of a composite of a chemistry designed according to desirable chemical features grafted to a reticulated scaffold. " Such a composite was developed and patented and it will be cited as an example for several applications. 

With amine initiators the so-called self-catalyzed 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, phenolic resins, toluenediamine, or methylenedianiline (MDA). In the manufacture of highly resilient flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. 

An early application of starch graft copolymers claims that incorporation of graft copolymers of starch or other polyols with ethyl or butyl acrylate into polystyrene improves the impact resistance if the polyol content is less than 20% of the graft.102... 

The IPNs prepared were composed of a rubbery polyurethane and a glassy epoxy component. For the polyurethane portion, a carbodiimide-modified diphenyl-methane diisocyanate (Isonate 143L) was used with a polycaprolactone glycol (TONE polyol 0230) and a dibutyltin dilaurate catalyst (T-12). For the epoxy, a bisphenol-A epichlorohydrin (DER 330) was used with a Lewis acid catalyst system (BF -etherate). The catalysts crosslink via a ring-opening mechanism and were intentionally selected to provide minimum grafting with any of the polyurethane components. The urethane/epoxy ratio was maintained constant at 50/50. A number of fillers were included in the IPN formulations. The materials used are shown in Table I. 

Modified polyether polyols have appeared in recent years, i.e., graft polyether polyols (polymer polyols, copolymer polyols) which were first developed by Union Carbide Corp. in the mid-1960 s. 

TECHNIQUE HOT-CURE COLD-CURE GRAFT PHD POLYOL SPECIALTY... 

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