Chemicals Added During Polymerisation

In this category is included the addition of up to 10% mineral oil (to impart flexibility to the product during the manufacture of high impact polystyrene by the copolymerisation of styrene and synthetic rubbers). 

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The formulation, or the chemical composition, is one major factor affecting the form and properties of the final product. Some components are added to the recipe before or during polymerisation. Some are added after polymerisation (post-additives) to modify the latex properties. There are two phases in an emulsion the dispersed (oil) phase containing the monomers and other monomer-soluble components, and the continuous (aqueous) phase containing water-soluble components. The individual components that comprise the oil and aqueous phases in an emulsion, and their functions, are described below. 

A counterion (A ) is incorporated during synthesis to balance the charge on the polymer backbone. Common chemical oxidants are FeClj and (NH4)2S20g, which provide Cl and HS04 /S04, respectively, as the dopant anions. Electrochemical oxidation provides greater flexibility in terms of the anion that can be incorporated from the electrolyte (MA salt or HA acid) added to the polymerisation medium. 

Plastics and rubbers are not used alone in their pure states, but usually they contain various chemicals which may have potential health hazards for humans. These chemicals can be either those that exist initially in the system and may be left after polymerisation (such as, monomers, catalysts), or those that are added afterwards during processing (i.e., plasticisers, stabilisers). 

At times, it is necessary to add a quantity of material all at once to a reactor in which an emulsion is already polymerising. For example, a relatively small amount of functional monomer could be added to a latex near the end of the polymerisation to produce a functionalised particle surface. Such a process is known at the shot-growth method (283). This method is often used in emulsifier-free polymerisations (69, 360) that contain no surfactant (but do use highly water-soluble, surface-active, functional monomers instead). During the early stages of emulsifier-free polymerisation, the particle size is determined primarily by the amount and type of initiator (such as potassium persulfate) and functional monomer (such as sodium styrene sulfonate). Latexes made in this manner tend to be narrow in size distribution. At high conversion (approximately 90%) an additional quantity or shot of functional monomer is added, and it chemically binds at the surface of the... 

The layman s term for the time/lemperature related molecular cross-linking process - known more correctly as polymerisation - which occurs when a thermoset resin changes from the typically viscous liquid to the solid state. This cure process is normally also promoted following chemical activation by a catalyst - or often called, hardener -addition. Other chemical accelerators may be added by the resin manufacturer, or alternatively added to the resin immediately prior to the catalyst addition. Cure can also be activated in other ways, such as by the influence of ultra-violet radiation. No by-products are formed during the formation of these long molecular cross-linked chain links. 

Polymerisations may be categorised by both the polymerisation mechanism (e.g. radical polymerisation, anionic polymerisation etc.) and by the polymerisation technique (e.g. solution polymerisation, emulsion polymerisation etc.). A third factor is how the reactor is operated in batch mode, or by adding monomers during the process (semi-continuous) or by continuous operation. Mechanism, technique and process strategies (mode of operation) all have an influence on rates of polymerisation and characteristics of the formed polymer. In this chapter, we will focus on the special characteristics that can be distinguished in an emulsion polymerisation related to rate, development of molar mass and chemical composition. In Chapter 4 the effects of the process strategy will be discussed. 

For instance, this procedure is followed in manypapers dealing with the semi-continuous emulsion copolymerisation of vinyl acetate and butyl acrylate (e.g. El-Aasser et al, 1983). Two main situations can be distinguished with respect to the monomer addition rate, (a) Flooded conditions the addition rate is higher than the polymerisation rate, (b) Starved conditions the monomers are added at a rate lower than the maximum attainable polymerisation rate (if more monomers were to be present). The latter process (starved conditions) is often applied in the preparation of homogeneous copolymers/latex particles. In this case after some time during the reaction, because of the low addition rates, a steady state is attained in which the polymerisation rate of each monomer is equal to its addition rate and a copolymer is made with a chemical composition identical to that of the monomer... 

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