One shot technique

One of the most used techniques to obtain polyurethanes is the one-shot technique, which consists of the very efficient mixing, in one step only, in a short time, of all the raw 

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. 

All the previous information regarding the general chemistry of polyurethanes and the structure of isocyanates have a role in the better understanding of how the oligo-polyols get chemically inserted in the high MW polyurethane structure and to understand the role played by the polyol structure in the properties of the resulting polyurethanes. 

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The full-prepolymer, quasi-prepolymer, and one-shot techniques may appear to be quite similar, but they each have important bearing on equipment requirements and physical properties. The full-prepolymer method, for example, is die process of choice for making high-performance cast elastomers with superb dynamic properties, such as industrial bumpers and bushings. Quasi-prepolymers are used in less demanding applications like shoe soles, adhesives, and spray elastomers. One-shots make up the rest, including most foams and elastomers. 

Analytical techniques. See also Light scattering technique One-shot technique TEM technique ... 

One-shot techniques, 216, 217, 236-237 Optical fiber waveguide, 271-272 Optically active poly(arylene), synthesis of, 518-519... 

The high exotherm and the number of feeds have led to the popularity of the quasiprepolymer variation of the one-shot technique. Quasiprepolymers are a version of the standard prepolymer method, where some of the polyol is reacted with an excess of diisocyanate. This is later reacted with the remainder of the polyols, including the chain extender and some catalysts. The advantage of this scheme is that the exotherm is spread between the preparation of the quasiprepolymer and the final cured part. In MD1 systems, BDO is often used as the curative. As the molecular weight of BDOs is low, the amount added is low. By dissolving it in the polyol, a more even mix ratio can be obtained. 

There are several reasons to work through the prepolymer route as opposed to the one-shot technique. These include ... 

One of the most significant advances in commercial practice occurred through the development of highly active catalysts about 1958. Prior to that time, it has been the custom to react the isocyanate with all or part of the polyol to form a prepolymer. Subsequently, this prepolymer was reacted with water and the balance of polyol. The advent of the very active catalysts—l,4-diaza-(2,2,2)-bicyclooctane (69, 70) and tin compounds (SS, 68)—provided a means to practice the so-called one-shot technique whereby the prepol3uner step is avoided (71). Thus, all the reactants and catalyst can be mixed and formed into desired polyurethane in one step with substantial economies. 

The above-mentioned manner of contacting the catalytic species is based on the analysis of the heat released during the one-shot technique, Le., the lifetime of the process is a little more than 30 min. In addition, it is well known that the catalyst (lactamate anion), in the absence of any impurities, is stable up to 48 h at the reaction temperature. Figure 6 illustrates the heat released at 90 °C when the same amount of Cl, as that used in the one-shot method, was introduced by 12, 21 and 42 successive injections, respectively. 

In polyurethane fabrication, some special techniques are used, such as prepolymer technique, quasiprepolymer technique and one shot technique. 

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. 

Diol linseed fatty amide is treated with TDI at room temperature in a one shot technique to obtain polyurethane, using a minimal amount of xylene as the solvent and without any chain extender or catalyst. The reaction is stopped when it has reached the desired hydroxyl value and the solvent is removed through a rotary vacuum evaporator to obtain poly(urethane amide) resin. 

Depending on the material structure and on the conformation of diisocyanate, there were observed large differences between the modulus and tan (5 loss factors respectively. The material 3, obtained with MDI via the one shot technique in melt (Table 3.11), showed an amorphous structure. The modulus above Tg was the lowest of all the materials, and correspondingly the loss factor tan in the glass transition region was the highest. For the samples PU 1, 2, 4 and 5 (DBDI, one shot), the Tg values ranged between -28°C (PU 2) and -14°C (PU 3), while tan varied between 0.127 (PU 2) and 0.394 (PU 3). 

Not applicable for PUs, but what about us I would gladly wave back when it is cold outside. Numerous studies had been made on the influence of temperature on the PUs deformation. A series of model MDI based PUs were characterized by Crawford et al [220] before, and after varying degrees of strain aging to study the effect of strain on the polymer morphology and mechanical properties. The segmented materials were synthesized by using the one-shot technique. The molar ratios of the model polymers were varied. 

Soft polyurethane rollers (from about 15 IRHD to about 55 IRHD) have been in use for many years, mainly in the printing industry. These polyurethanes are generally made by a one-shot technique and are usually based on polyesters chain-extended with TDI. They are one-phase amorphous elastomers with very low compression set (>10%) and good ink and solvent resistance. Harder polyurethane rollers are also used from about 55 IRHD to about 95 IRHD for more demanding mechanicalhandling applications, for example, in the steel, textile and paper industries. 

Most of these systems use the one-shot technique and are based on MDI with either a polyether or a polyester. Crude or polymeric MDI or other liquid variants of MDI predominate. Formulations usually contain high catalyst concentrations of either tin or mercury salts to give a very rapid cure at room temperature within a few minutes. It is usual to use additives to provide specific properties of density and lower the cost. Applications are commercial vehicle floor mats, cable jointing and potting compounds, sealants, moulds and the like. 

One-shot techniques have become feasible with the availability of suitable catalysts to control the rate of the various competing reactions. Thus polyol, isocyanate and chain extender can be simultaneously blended and by the use of such catalysts as stannous octoate it is possible to promote the NCO/OH reaction to a rate similar to that of the NCO/NH2 reaction. The one-shot process has the disadvantage of having free isocyanate at the casting stage which presents a vapour hazard. 

A series of novel polyurethane (PU) thermoplastic blends based on diisocyanates of variable geometry was recently obtained. On employing the one shot technique, either in solution or in melt and using DBDI as a diisocyanate of variable geometry, by selective dissolution... 

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