Latex properties

Monomers. A wide variety of monomers can be used, and they are chosen on the basis of cost and abiUty to impart specific properties to the final product. Water solubiUties of iadustriaHy important monomers are shown ia Table 1 (38). The solubiUty of the monomer ia water affects the physical chemistry of the polymerization. Functional monomers like methacrylic and acryUc acid, infinitely soluble ia water, are also used. These monomers impart long-term shelf stabiUty to latices by acting as emulsifiers. The polymerization behavior of some monomers, such as methacrylic acid, as well as the final latex properties are iafiuenced by pH. For optimum results with these acids, polymerization is best performed at a pH of ca 2. After polymerization, the latex is neutralized to give adequate shelf stabiUty at tractable viscosities. 

Continuous Polymerizations As previously mentioned, fifteen continuous polymerizations in the tubular reactor were performed at different flow rates (i.e. (Nj g) ) with twelve runs using identical formulations and three runs having different emulsifier and initiator concentrations. A summary of the experimental runs is presented in Table IV and the styrene conversion vs reaction time data are presented graphically in Figures 7 to 9. It is important to note that the measurements of pressure and temperature profiles, flow rate and the latex properties indicated that steady state operation was reached after a period corresponding to twice the residence time in the tubular reactor. This agrees with Ghosh s results ). 

Polymer production technology involves a diversity of products produced from even a single monomer. Polymerizations are carried out in a variety of reactor types batch, semi-batch and continuous flow stirred tank or tubular reactors. However, very few commercial or fundamental polymer or latex properties can be measured on-line. Therefore, if one aims to develop and apply control strategies to achieve desired polymer (or latex) property trajectories under such a variety of conditions, it is important to have a valid mechanistic model capable of predicting at least the major effects of the process variables. 

On-line State Estimation. Optimal Sensor Selection and Control. Realistically, it is very difficult to have on-line measurements of all the major polymer or latex properties of interest, but perhaps one could rely upon one or two of the sensors available for the on-line measurement of a few states (e.g. conversion). In order to estimate some of the other states (e.g. particle diameter averages), Kalman filters or Observers should be used. A number of papers have investigated these state estimation schemes (58,6 , 67). 

The development of on-line sensors is a very costly and time-consuming process. Therefore, if one has available a dynamic model of the reactor which predicts the various polymer (or latex) properties of interest, then this can be used to guide one in the selection and development of sensors. Ideas from the optimal statistical design of experiments together with the present model expressed in the form of a Kalman filter have been successfully used (58) to select those combinations of existing or hypothetical sensors which would maximize the information that could be obtained on the states of the polymerization system. Both the type of sensors and the precisions necessary for them are easily investigated in this way. By changing the choice of the measurement matrix and... 

Typical emulsion polymerization recipes involve a large variety of ingredients. Therefore, the possibilities of variations are many. Among the variables to be considered are the nature of the monomer or monomers, the nature and concentration of surfactants, the nature of the initiating system, protective colloids and other stabilizing systems, cosolvents, chain-tranfer agents, buffer systems, short stops, and other additives for the modification of latex properties to achieve the desired end properties of the product. 

Grafting and Stabilizers. The degree of grafting of poly(vinyl acetate) (PVAc.) on poly(vinyl alcohol) (PVA) and other stabilizers during emulsion polymerization strongly affects latex properties such as viscosity, rheology, and polymer solubility. 

In the polymerization of latexes such as polystyrene the free radical initiators contribute covalently bonded hydrophilic end groups at each end of every polymer molecule and these hydrophilic groups tend to accumulate in the surface of latex particles where they influence latex properties. The techniques developed for titration of these hydrophilic sites on latex particles may be of general application however. 

Despite these generalizations, the reduction or elimination of coagulum is usually best accomplished by a "systems approach", i.e., a consideration of latex properties to be achieved in the emulsion polymerization, the economics of the polymerization process, and the deliberate design of the reactor system for that particular polymerization system. Each polymerization system must be considered as a separate system and treated as such. The most effective approach to reduce or eliminate the formation of coagulum is to determine the mechanism by which it is formed and... 

The objective of this study was to investigate the feasibility of using a tubular reactor for the seeded emulsion polymerization of vinyl acetate, and to study the effect of process variables on conversion rate and latex properties. 

The mass versus particle size distribution of several polymer latices with diameters in the range of 30 nm to 1500 nm was determined in less than 20 minutes using an integrated hydrodynamic chromatograph. Distributions obtained were compared with those found by other particle sizing techniques such as electron microscopy to verify validity of the technique. The instrument employed was able to analyze latices re-producibly with different optical properties, even though some of the injected particles may have been trapped within the column. Latex properties were correlated with particle size distribution data to illustrate the benefit of this particle sizing technique. 

Since emulsifiers determine the particle size of the latex, we studied the influence of the distribution ratio of the emulsifiers between monomer feed emulsion and initial reactor charge on the latex properties. We found that small amounts of the nonionic emulsifier in the initial reactor charge caused coagulation during latex preparation. Hence, the total... 

The most useful on-line instruments would be those capable of measuring the latex properties given in Eqs, (56-60), the free soap concentration and some molecular weight characteristics. In this section we will not attempt on exhanstive survey, but will only concentrate on a few of the more rugged and more promising on-line measurements that have been used. 

In some situations where one or more of the latex properties are measured either directly or indirectly through their correlation with surrogate variables and where extreme nonlinearities such as the periodic generation of polymer particles does not occur, one can use much simpler modehng and control techniques. Linear transfer function-type models can he identified directly from the plant reactor data. Conventional control devices such as PID controllers or PID controllers with dead-time compensation can then be designed. If process data is also used to identify... 

The properties of the latices and the paints produced from them were evaluated by several commercial firms. Ihe latex properties varied somewhat from sample to sample but, in general, the paints produced from the latexes were comparable to those produced from high-grade commercial latexes made by conventional means. One property of the paint produced from the radiation-catalyzed latex, enamel holdout, which is one measure of the capability of a paint to serve as a primer for enamel on unpainted wood, was superior to that of paint made from conventionally produced latex. 

TABLE I Comparison of Polymer Latex Properties of the Product... 

It can be noted that latexes of low-density polyethylene (LDPE) are prepared via free-radical emulsion polymerization as a specialty product [56]. However, fhe low variability wifh respect to fhe polymer microstructure is disadvantageous for tailoring latex properties (e.g. film-forming properties), resulting in a narrow property profile. In addition, working at pressures in excess of 1000 bar in fhe presence of water is challenging for the equipment used. 

A number of issues and challenges will be of particular interest in the future. Up to now, little data is available regarding latex properties, such as stability and film formation, and on the properties of the resulting films (such as adhesion to substrates of varying polarity). In aqueous olefin polymerizations, these properties... 

Some examples of monomeric surface-active azo initiators as well as their synthesis, application in emulsion polymerization, and resulting latex properties are described in Refs. [43, 44]. These inisurfs are symmetrical and exhibit a methylene chain as hydrophobic part of the molecule and an ionic hydrophilic group (Inisurf 4, Inisurf 5). The main residts of these investi tions may be summarized as follows ... 

The limited control that can be exerted over polymer and latex properties greatly restricts the commercial utility of batch processes. Nevertheless, batch processes have an important role, particularly in more fundamental studies. The principal uses batch processes may be divided into the duee categories described in the following sections. 

Process models are also important components of reactor control schemes. Kiparissides et al. [17] and Penlidis et al. [16] have used reactor models for control simulation studies. Particle number and size characteristics are the most difficult latex properties to control. Particle nucleation can be very rapid and a strong function of the concentration of free emulsifier, electrolytes and various possible reagent impurities. Hence the control of particle number and the related particle surface areas can be a difficult problem. Even with on-line light scattering, chromatographic [18], surface tension and/or conversion measurements [19], control of nucleation in a CSTR system can be difficult. The use of a pre-made seed or an upstream tubular reactor can be utilized to avoid nucleation in the CSTR and thereby imjHOve particle number control as well as increase the number of particles formed [20-22]. Figures 8.6 and 8.7 illustrate open-loop CTSR systems for the emulsion polymerization of methyl methacrylate with and... 

Further advances in initiators can give more degrees of fieedom for the poly(vinyl acetate) (PVAc) producer. Low residual monomer is a primary coiKem for many latex produces, and new initiator systems can aid in reducing final monomer levels. Initiato type also affects latex properties by altering molar mass. Thus a variety of initiators allows for the productitHi of differoit products. 

The present review will mainly focus on inverse emulsion polymerization, the most commonly employed water-in-oil synthesis method and on inverse microemulsion polymerization which is more recent and offers some new prospects. The formulation components and their actions, the various structures of the colloidal dispersions prior to polymerization and some latex properties will be discussed. The kinetics and the mechanisms occurring in these water-in-oil systems will also be analysed and compared to the more conventional emulsion polymerization process. 

Chem. Descrip. Poiyamide in propyiene giycoi monomethyi ether Uses Thixotrope, antisagging agent, antisettiing agent for water-soi. resin systems and water-reducible latexes Properties Gel 20% solids Use Level 0.5-2.0%... 

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