Polymeric latex stabilisation

Polymeric Suifactantsfor Stabilisation of Preformed Latex Dispersions 357... 

Other types of surfactants are the polymeric (steric) stabilisers, such as partially hydrolysed polyvinyl acetate. Also oligomeric species formed in situ, when SO radicals react with some monomer units in the aqueous phase, will have surface active properties, and can even form a colloidally stable latex Electrosteric stabilisers combine steric and electrostatic functionalities for example, inclusion of acrylic acid in a recipe results in chains with blocks comprised largely of poly(acrylic acid) which for in the aqueous phase, then pick up enough hydrophobic monomer to enter the particle and continue polymerisation in the particle interior. The hydrophilic component remains in the aqueous phase and provides colloidal stability both sterically and, imder the appropriate conditions of pH, electrostatically. This mode of stabilisation is very common in surface coatings, because it gives excellent freeze-thaw stability. 

Most reports on emulsion polymerisation have been limited to commercially available surfactants which, in many cases, are relatively simple molecules such as sodium dodecyl sulphate and simple nonionic surfactants. However, studies on the effects of surfactant structure on latex formation have revealed the importance of the structure of the molecule. Block and graft copolymers (polymeric surfactants) are expected to be better stabilisers when compared to simple surfactants. The use of these polymeric surfactants in emulsion polymerisation and the stabilisation of the resulting polymer particles is discussed below. 

Polymeric Surfactants for Stabilisation of Performed Latex Dispersions I 359... 

The polymerisation and the properties of the latexes depended sensitively on the emulsifier and on the charge of the initiator. There was no visible correlation between the properties of the final latexes and the properties of the emulsifiers such as surface activity, solubilisation capacity, or the ability to stabilise the initial monomer emulsion. When a cationic (2,2 -dimethyl-2,2 -azo-N-benzylpropionamidine hydrochloride, VA-552 from Wako Chemicals) initiator was employed, all emulsifiers lead to stable monodisperse latexes, except for polysoaps with low hydrophobe content. The polymerisable, as well as the polymeric emulsifiers, yielded latex solutions with very high surface tensions, different from the use of the standard surfactant. In contrast, the use of an anionic initiator (potassium persulphate) can pose difficulties. The... 

The principal consideration in selecting an appropriate soluble polymeric component for the steric barrier is that it should be freely soluble in the dispersion medium to be usedo This had been recognised empirically for some years before Napper (4) undertook quantitative studies which showed that sterically stabilised latexes flocculated at or around the theta point for the soliible polymer in the dispersion medium. Later studies have shown that both upper and lower critical flocculation temperatures may occur in organic solventso... 

Anti-foaming agents, preservatives and stabilisers can all be added to latex paints. Polymeric de-foamers of the Bevaloid type are popular, as they combine long life and do not cause wetting or livering troubles. Silicone de-foamers can cause cissing and other surface faults, whilst the nonanol type de-foamers tend to dissolve in the polymer particles and are then ineffective. 

---