One shot process

The one-shot polyethers now form the bulk of the flexible polyurethane foam now being manufactured. This is a result of the favourable economics of polyethers, particularly when reacted in a one-shot process, and because the polyethers generally produce foams of better cushioning characteristics. A typical formulation for producing a one-shot polyether foam will comprise... 

The secondary hydroxyl groups of these poly(oxypropylene) glycol diols are less reactive than the primary hydroxyl groups of the earlier polyesters. At the time of the introduction of these polyethers, the catalysts then available were insufficiently powerful for one-shot processes to be practical and so these polymers have been used primarily in prepolymer processes. 

Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

In addition to one-shot processes, quasi-prepolymer systems are used commercially with rigid polyether foams. The quasi-prepolymer is commonly produced using excess TDI rather than diphenylmethane di-isocyanate. Sinee the former isocyanate is light in colour and the latter dark, quasi-prepolymer foams... 

Figure 4.9 One-shot process for polyurethane elastomer preparation.

A number of issues related to this reaction should be discussed. First, a polymer is rarely isolated in this form. In the early 1950s a technology was developed that has since come to be known as the one-shot process. While the technique certainly produces a capped polyol, it immediately reacts further to achieve its ultimate form (Figure 2.6, bottom). You will notice that the capped polyol still has isocyanate functionalities as end groups. Regardless of the process, these end groups must continue to react (by the addition of water and/or a catalyst) to complete the process. While this reaction produces one of the most commonly constructed polyurethanes, it is rarely isolated as an end product. 

The reaction shown in Figure 2.6 produces what is referred to as a prepolymer via the production method of choice before the one-shot process was developed. Prepolymers are still commonly used. Small molding operations, elastomers, and hydrophilic polyurethanes involve production of prepolymers. 

The one-shot process was developed in the 1950s. It is a technique by which the polyol and the isocyanate are mixed together. Without help, however, the reaction would be too slow to be useful. For this reason, the key to the process is the development of a formulation that accelerates tlie isocyanate reaction. We will discuss the design of a formulation to meet a specific end use in the next chapter. To complete our discussion of the processes by which polyurethane foams are made, consider the formulation for a conventional foam used for furniture cushions (Table 2.3). 

FIGURE 2.13 One-shot process for production of polyurethane foam. 

Whether the one-shot process or prepolymer technique is used, the development of a foam involves the juxtaposition of gas generation and the development of tensile strength within the developing foam. The evolution of gas can be via the use of blowing agents or the in situ generation of CO2 from the reaction of water with an isocyanate to produce a water-blown foam. In any case, the gas evolution creates an internal pressure that must be resisted by the development of a gel structure via polymerization reactions as shown in Figure 3.13. 

The last chemical characteristic is the attachment of ligands. Unlike the other properties (reservoir capacity and biocompatibility) that could be incorporated into a one-shot process, this aspect is most conveniently practiced at the prepolymer level. The philosophy is to use some of the isocyanate functionality of the prepolymer to attach active side chains. This is illustrated in two examples that we will discuss again. 

To obtain elastomers, one or two diols can be reacted with the isocyanate. When two diols are used, the first one is a macrodiol with a molar mass in the range 500-10000 g mol-1, and the second one is a short diol, typically 1,4-butanediol. The PU may be prepared by either the one-shot process (three components reacting together), or the prepolymer approach a prepolymer is prepared first (Eq. 2.36) and then reacted with the short diol (chain extender) ... 

A typical formulation for the reaction-injection molding (RIM) process is shown below (one-shot process) ... 

Processes of Urethane Foam Preparation. Urethane foams can be prepared by the one-shot process, semi-prepolymer (quasi-prepolymer) process or prepolymer process. The one-shot fnrocess is most commonly used. The semi-prepolymer process is sometimes preferred because of the advantages of easy processing, stabilized-foam rise and lower exotherm. 

The one-shot process is used for flexible and rigid foams. In the case of slabstock foams, the ingredients are separately supplied to the mixing head. In order to adjust viscosity and mixing accuracy, some of the ingredients, such as polyol and tin catalyst, water and amine catalyst, are pre-mixed. 

In the case of machine foaming, a one-shot process is widely used. All ingredients are pumped into the mixing head through separate streams. However, from the standpoint of viscosity, mixing ratio and ease... 

Manufacturing Process. Both prepolymer and one-shot processes are available, but the polymeric isocyanate-based one-shot process is used in preference because of the easy processing due to the low viscosity of the system, relatively low toxicity of polymeric isocyanates, and fewer environmental problems. However, a disadvantage of the one-shot process is a possible risk of shrunken-foam formation due to its higher closed-cell content. In contrast, the TDI-based prepolymer process has advantages including better in-mold flowability and higher open-cell content. 

One-Shot Process. Monomeric TDI (80/20 and 65/35 isomer ratios) was used as the polyisocyanate component in the early stages of the urethane-foam industry. But due to the toxicity problems of TDI at the present time, most of the isocyanates used for producing semi-rigid foams have been replaced by polymeric isocyanates, and the use of TDI is somewhat limited. 

In principle, both the one-shot process and semi-prepolymer processes have been used for rigid-urethane-foam manufacturing. However, the monomeric TDI-based one-shot process was used only in the initial stage of the rigid-urethane-foam industry because of the toxicity problems of TDI and difficulties in controlling reactivity due to the high NCO percent. For these reasons TDI-prepolymers, blends of TDI prepolymers and polymeric isocyanates, and 100% polymeric isocyanate are most widely used. 

In the household-refrigerator industry, however, the one-shot process employing crude TDI (which has a lower percent NCO and contains oligomeric compounds, such as isocyanurate- and carbodiimide-types of oligomers) is still widely employed because of its low cost. 

Crude TDI—One-Shot Process. In place of the TDI-semi-prepolymer process, a one-shot process using crude TDI is widely used for refrigerator insulation because of its lower cost, although the flowability of this system is inferior to the TDI-prepolymer process. 

Polymeric MDI—One-Shot Process. Polymeric isocyanate-based fluorocarbon-blown rigid urethane foams are the most widely used rigid urethane foams at the present time because the foams have a high thermal... 

Preparation. Urethane-modified isocyanurate foams are mostly fH epared by the one-shot process based on the jH inciple discussed previously in the urethane modification section. The semi-prepolymer process is used only in limited cases because of viscosity problems. This section describes several examples of formulations for producing block foams, slabstock foams, laminate foams, and spray foams. 

Recently, oxazolidone-urethane-modified isocyanurate foams made by the one-shot process have been reported by Fuzesi et al (128, 209, 210). This process consists of a three-component system. Unmodified oxazolidone foams will be described in the following section. 

Imide-Modified Isocyanurate Foams. The imide linkage is a thermally stable linkage, and therefore, imide-modified isocyanurate foams have higher thermal stability and flame retardance than urethane-modified isocyanurate foams. R. Grieve (114) prepared such foams in a one-shot process by reacting a polycarboxylic acid anhydride with an organic polyisocyanate in the presence of a catalytic amount of a monomeric homocyclic polyepoxide and a tertiary amine. 

Methods for making isocyanate-based polyimide foams include a one-shot process by admixing carboxylic dianhydrides with an organic polyisocyanate at room temperature in the presence of a dipolar aprotic organic solvent (103, 106, 142, 251). The resulting foams from this method exhibited outstanding thermal resistance, as shown in Figure 47. 

In the prepolymer method for one-shot polyether flexible urethane foams the primary role of the silicone surfactant is to lower surface tension and to provide film (cell-wall) resilience. Resilient films prevent the collapse of the foam during foam rise and continue to stabilize it until the foam is self-supporting. A secondary, but nevertheless important role of the silicone surfactant is cell-size control. The silicones can be added to the formulation in any of the 2 to 6 streams usually fed to the mixing head in the one-shot process. Usually, however, the silicone is metered separately, in combination with the polyol, or added as a wa-ter/amine/silicone mixture. It can also be added in the fluorocarbon blowing agent (52). 

Polyurethanes can be synthesized using either a one-shot process or a two-stage, prepolymerization technique. For the one-shot process, all chemicals are combined together at once and allowed to react. Although this method is faster and less labor-intensive, the final product tends to have a less organized structure than that achieved using the two-stage method. In the... 

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