Post-Polymerisation Additives

After polymerisation is completed, some additives may be required to enhance the performance or stabihty properties of the final latex product (62). It may be necessary to modify the viscosity of the latex, improve film formation ability, or protect the latex from biological or oxidative degradation. 

Latexes should be stable under zero-shear conditions at room temperature. However, when latex is pumped from the reactor to a tank, or from tank to tank, it is subjected to shear forces that can cause shear-induced coagulation of the latex particles. Additional surfactant, called a post-stabiliser (296), is often added to latexes to improve shear stability. Also, in cold climates, latexes may be exposed to extreme temperatures that cause the aqueous phase to begin to freeze, thereby compressing the latex particles together, and causing coagulation (358). Poststabilisers are also added to improve freeze-thaw stability and electrolyte tolerance. 

Latexes are typically low in viscosity since they are water-based dispersions. However, in some cases, it may be necessary to increase the viscosity of the final latex product. For example, the viscosity of latex paints should be such that they flow evenly on a substrate, but do not run off. Latex viscosities may be controlled by the addition of a viscosity modifier (thickener) to the aqueous phase (371). Polyethylene oxide (PEO), hydroxyethyl cellulose (HEC), and various associative thickeners (such as HEUR (91, 281)) or HASE (130, 282) thickeners) are often used as viscosity modifiers. 

Latex coatings such as paints are often exposed to the outdoor environment, during which time they are vulnerable to attack by microbial and fungal growth. High moisture environments that promote the growth 

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A variation on PVC that is important for some pipe applications is Chlorinated PVC (CPVC). This is manufactured by chlorination at a post-polymerisation stage. The modified polymer which carries additional chlorine atoms has a stiffer polymer chain resulting in better high temperature performance than PVC. CPVC has therefore been used for piping hot water and industrial chemicals where it continues to be used in competition with PP and crosslinked PE. The mechanical properties and chemical resistance characteristics of CPVC are similar to PVC. 

Another significant application of the ADMET polymerisation relates to the preparation of star-shaped polymers, which are branched macromolecules in which several linear polymer chains are attached to a unique branching point or core [48]. Montero de Espinosa, Winkler and Meier [49] described an ADMET approach to obtain those architectures (three- and four-arm) using small tri-acrylates and tetra-acrylates. More recently, Unverferth and Meier [50] reported the synthesis of well-defined star-shaped polymers via a head-to-tail ADMET polymerisation whereby di(trimethylolpropane)tetra-acrylate (four-arm) and dipentaerythritol hexa-acrylate (six-arm) served as core units, and fatty acid-derived 10-undecenyl acrylate as asymmetric a,(0-diene monomers. In this case, star-shaped polymers containing arms of 10 or 20 monomer units with an a,(0-unsaturated ester backbone and their subsequent post-polymerisation via a base-catalysed Thia-Michael addition were prepared. 

Azide Derivatives. Azidoformates are prepared from a hydroiC yl - Containing monomer via the chloroformate and sodium azide. Cenerally hydrolytically stable, they can be used either in cdreot copolymerisation or as post - additives, functioning by a secondC ary reaction. In the latter case the reaction is not restricted to azoformates of polymerisable acids. The crosslink at 70 prodC uoes a urethance type linkage. 51)... 

Rate Theory Pre-gel and Post-gel Loops. In order to account for the effects of pre-gel and post-gel intramolecular reaction on modulus, it is necessary to use a theory which describes the continuous growth of intramolecular reaction throughout an irreversible polymerisation. The rate theory(21-24) is being further developed to this end. The theory already allows prediction of Nf and Oq given only the initial dilution of reactive groups, and reactant molar mass, chain structure and functionality, and has been applied to the interpretation of experimental values of N. in linear(21) and non-linear reaction systems(22) and to the correlation of experimental values of 00(22.2377 In addition, correlations between Op and M /MpO have been achieved for an RAa self-polymerisation( ) and a resume of the results obtained are presented here. 

Step-growth condensation copolymerisations give rise to additional experimental difficulties, with respect to the former reactions studied, due to the continuous release of e.g. water. Indeed, the evaporation of water produced by the reaction may obscure the detection of the cure process and prohibit a reliable quantification of the reaction heat and the reaction conversion. To illustrate how condensation polymerisations can be studied by MTDSC, the post-cure condensation reactions of melamine-formaldehyde (MF) resins will be discussed [91]. 

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. 

Resin-water emulsions can be produced by the post-reaction emulsification of a condensation or addition polymer, by the use of surfactants or emulsifiers to form an oil-in-water emulsion of the polymer in the aqueous phase. This is simply a method of obtaining a polymer in an aqueous medium. This type of emulsion is different from a tme emulsion polymer made by the process of emulsion polymerisation. 

During the deposition experiments, the top electrode block moved back and forth over the sample at a constant speed (4 m/min) with a 380 mm displacement range. A treatment with 12 passes took 68.4 s. It has been demonstrated that the coating properties can be improved when an additional plasma post-treatment is performed on pre-existing plasma polymerised coatings [22]. This post-treatment, which has an effect on both the chemical composition and structure of the ppHMDSO layers. 

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