Pre-polymer process

Before the discovery of the catalytic activity of tin compounds, polyether foams were prepared by the pre-polymer technique. In this two-stage process, the low reactivity of the polyether secondary hydroxy groups is compensated for by carrying out the reaction between isocyanate and diol before the addition of water. The polyether is treated with excess of isocyanate to give an isocyanate-terminated pre-polymer which is stable in the absence of moisture. Addition of water, tertiary amine and surfactant to the pre-polymer results in a flexible foam of good quality. The pre-polymer process has been largely displaced by the more economical one-shot process but is still occasionally used since it offers more scope in the design of compounds. 

A related technique is the quasi pre-polymer (or semi pre-polymer ) process in which the polyol is treated with a large excess of isocyanate to give a polymer of low molecular weight. This product is then treated with additional polyol, water and catalyst to produce a foam. The advantage of this procedure is that the initial polymer, being of low molecular weight, is less viscous than the product obtained in the pre-polymer process described previously and is easier to handle. 

Elastomer and Polymer Processing Systems, Bulletin MPR 76,1976, Pre-Press, Bulletin FO 175 and French Dual Cage Screw Press, Bulletin OP8130,1981, The French Oil Mill Machinery Co., Piqua, Ohio. 

Many polymers have been processed by casting, e.g., acrylics, polystyrene, polyamide (nylon 6), phenolics, PVC/plasticiser. Many of these are used in a pre-polymer form, which polymerise on the casting belt, or the polymerisation can be completed later by application of heat. 

This process, commonly used with thermosets, is sometimes used for specific thermoplastics such as monomers or pre-polymers of PMMA or polyamide (Nyrim). For the latter, a mix is made just before casting with ... 

Aciylamide is required in very large quantities as the pre-polymer of the polyacrylamide that is very widely used in polymer and flocculant apphcations. The chemical manufacture of acrylamide has been estabhshed for a long time. The original process involved treatment of acrylonitrile with sulphuric acid at 90°C. More recently processes have been introduced that require the use of copper catalysts and high temperatures (80-140°C), but result in the formation of large quantities of toxic waste, including HCN. The expensive copper catalyst used is difficult to regenerate. In addition the chemical process produces aciylamide that requires considerable purification, for instance because the... 

In RIM processes, two or more reactive components are mixed together, starting the reaction between the components before the mixture is dispensed into the mold. This tends to increase the viscosity of the liquid that is dispensed due to an increase in molecular weight of the polymers or pre-polymers formed in the initial reaction. An increased viscosity can prohibit complete filling of the mold and permeation of the preform. This tends to decrease the adhesion between the matrix and the fibers. Poor interfacial adhesion between the reinforcement and matrix phase can cause a material to have less than desirable stiffness and strength. 

In the multiple zone process an olefin mixture is polymerized in a first reaction zone to produce a pre-polymer that has a lower molecular weight up to 35-65% the final conversion. After this step, volatile materials, such as hydrogen, are removed. The polymerization continues in a second reaction zone by adding more of the olefin mixture to produce the final polymer with a higher molecular weight (6). 

Processing of polyurethanes is different from other elastomers in that most polyurethanes are cast into a mold. The mixing of the pre-polymer and curative can be done with specially designed machines in large shops or by hand in small shops or for small runs. 

PBDA and BDA with some transition metal ions are shown in Table 1. For the polymer ligand, the complexation process appears to proceed in two steps a) accumulation of M2 + ions into the PBDA domain (pre-equilibrium process) due to electrostatic interaction of the anionic polymer backbone of PBDA with M2+,... 

Brearley etal.9 at DuPont (Wilmington, DE, USA) used in-line transmission NIR to monitor carboxyl end groups and DP in PET oligomer and pre-polymer melt streams in a new polyester process. The business value was derived from several sources. 

First and most important, real-time NIR monitoring enabled real-time control of the process. For a given product, the molecular weight and end-group balance in the prepolymer exiting the front end or melt part of the process must be controlled at specified levels in order for the back end or solid-phase part of the process to successfully produce the intended polymer composition. In addition, the variability in pre-polymer composition must be controlled with very tight tolerances to keep the variation in final product composition within specification limits. Since the process dynamics in the front end were more rapid than those in conventional PET processes, the conventional analytical approach involving off-line analysis of samples obtained every 2-4 hours was not sufficient to achieve the desired product quality. 

Figure 11.9 Predictions of the NIR model for pre-polymer carboxyl ends over an eight-day period. Three states of a designed experiment, as well as a period of process upset and the return to lined-out operation, are indicated. Reprinted with permission from Brearley and Hernandez (2000).9...

The yields of radiation-induced polymerizations can be very high. No additives are required, which makes it possible to synthesize very pure polymers. The initiation step is temperature independent giving rise to an easily controlled process at any desired temperature. These features account for the commercial interest in radiation polymerization. The very high speeds attainable within the layers of monomers subjected to powerful electron beams explain the wide use of this technique in radiation curing of adhesives, inks and coatings. The corresponding formulations are "solvent-free" and involve pre-polymers and monomers as reactive diluents. 

The concepts of positive or negative systems are by no means a strict classification since modifications of the original pre-polymer with additives, with suitable changes In the development processes can be utilized to obtain positive resists whilst the unmodified material Is a negative resist. For example, tetra-chloro dlazo cyclopentadienes in general are polymerisable by UV light to obtain negative resist systems. 

The pre-polymer has a very low DP (x < 3). The norbornene-ended polyimide (norbornene-2,3-dicarboxylic imide also known as nadic imide) is further polymerized by heating it at about 300 C. The thermosetting process of norbornene-ended polyimide can be investigated by Py-GC/MS. For example, the crosslinking of norbornene-ended polyimide PMR-II, was investigated using Py-GC/MS [20] (the pre-polymer PAR-11 structure is shown below) ... 

In PP manufacture, modern bulk (liquid monomer) and gas-phase processes have largely replaced the earlier slurry processes in which polymerization was carried out in hydrocarbon diluent. The most widely adopted process for PP is Basell s Spheripol process.317 Homopolymer production involves a pre-polymerization step at relatively low temperature, followed by polymerization in a loop reactor using liquid propylene random co-polymers are produced by introducing small quantities of ethylene into the feed. The pre-polymerization step gives a pre-polymer particle with the capacity to withstand the reaction peak, which occurs on entering the main loop reactor. The addition of one or two gas-phase reactors for EP co-polymerization makes it possible to produce heterophasic co-polymers containing up to 40% of E/P rubber within the homopolymer matrix. 

Source The Japanese Ministry of International Trade Industry, cellulosic fibres is expected to decline marginally at the rate of 0.3%. The interplay between fibre structure, morphology and chemical composition is an essential part of all pre-treatment processes and thus, it is necessary to know the differences in the structures of different polymers and their effects on the properties of the fibres. There are many good books on this subject and hence only general fibre chemistry and manufacturing processes are presented in reference form and then proceeded to discuss how preparatory processes are chosen for use as fibre processing. 

Several aspeets of this discussion are common to all polymers. These include the major applications of polymers, the most common methods of polymer processing, the most frequently used fillers and their typical concentrations, additives used to incorporate fillers, special methods of filler incorporation, fillers pre-treatment, and special considerations affecting the selection of a filler. These data found in the most recent literature are recorded in a tabular form for clarity and ease of comparison. This is followed by some examples of current use of fillers in particular polymers. The examples can be used to develop numerous new applications for fillers in material improvement. They elaborate on the most recent developments in filler and filled material improvements. 

Production of acrylamide (Fig. 13) by hydration of acrylonitrile under the action of the intracelluar nitrile hydratase in Rhodococcus rhodochrous (Nitto Chemical Industry Co., Ltd., fed-batch process). The annual production amounts to >30000 tons (see also Table 6). Acrylamide is one of the most important commodity chemicals and is required in large quantities as the pre-polymer of polyacrylamide that is widely used in polymer and floccu-lent applications. The advantages of this hydratase approach in comparison with the classical chemical nitrile hydration are higher product end concentration, quantitative yields, no formation of acrylic acid, no need for copper catalyst, and only five chemical/technical operations instead of seven [73,112,113,171]. An analogous process for nicotinamide is being commercialized by Lonza (see also section 6). 

By far the acrylates are the monomers of choice in UV curable systems. Not only do they cure at extremely rapid rates compared to other monomer systems (acrylic > methacrylic > vinyl > allylic), but they are also available in a wide range of structures which are monofimctional, difunctional, trifunctional, and tetrafunctional. Additionally, as shown in the oligomer section, acrylates can be used to derivatize oligomers or pre-polymers. Commonly in UV curable formulations it is necessary to use a number of monomers in order to achieve a balance between speed of cure and properties of the final film. It is not unheard of to use four or five monomers in a single UV curable formulation. For instance, tri- and tetra-functional acrylates result in highly crosslinked films when incorporated into UV curable resins however, they severely limit the extent and rate of the curing process. Thus, one often combines a tetrafunctional acrylate to increase crosslink density with a mono and/or difunctional acrylate to increase the cure rate. 

---