Composite latex particles

Synthesis of Composite Latex Particles and Their Role in Sound and Vibration Damping... 

A short review of the synthesis of composite latex particles is presented and their utility as damping... 

A prolific variety of composite latex particles appears in both the open and patent literatures. The subject has been reviewed (1,2) by several authors. Composite implies the presence of at least two dissimilar components either of which could, in principle, constitute the major component by volume. Some features of composite particles, which retain colloidal stability during preparation and subsequent storage, that is where the product is a dispersion in which flocculation, aggregative, and coalescence processes are largely absent so long as the continuous phase remains, will be described here. There are alternative and important processes for preparing composite particles which give flocculated particles readily separated from the liquid diluent phase and dried for use as powder. 

Preparation by Sequential Polymerisation. Two-polymer composite latex particles may be prepared using either emulsion or dispersion polymerisation techniques. A dispersion (latex) of particles of a first polymer may be prepared in the usual manner after complete conversion of monomer to polymer, a different monomer or monomer mixture is added and polymerised to provide the second polymer. 

Three factors, listed below, which might influence the morphology of composite latex particles will now be discussed. 

In conclusion, it has been shown that the predicted order of miscibility in composite latex particle systems is not necessarily bourne out when the extent of miscibility is guaged by dynamic mechanical analysis, and, very recently, by the same authors using solid-state NMR spectroscopy. Control over particle morphology, and, hence, over damping behaviour can be exercised by the differences in hydrophilicity between the polymer pair in question, by the degree of crosslinking in the first network and by whether or not the first-formed polymer is above or below its Tg when the second monomer is polymerised. 

Young JLS, Richard J, DeSimone Joseph M. Synthesis of two-stage composite latex particles by dispersion polymerization in carbon dioxide. Pol5mr Prepn (ACS Div Polym Chem) 1999 40 829-830. 

The morphology of latex particles is controlled by the thermodynamic and kinetic factors. The thermodynamic factors determine the ultimate stability of the multiphase system, inherent in the production of a composite latex particle, while the kinetic factors determine the ease with which such a thermodynamically favored state can be achieved. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition (monomer starved or batch) and the use of crosslinking agents. Of course, crosslinked core/shell latexes constitute IPNs, see Section 6.4.1. 

Luo YD, Dai C A, Chiu WY (2008) Synthesis of P(AA-S A)/ZnO composite latex particles via inverse miniemulsion polymerization and its application in pH regulation and UV shielding. J Polym Sci Part A Polym Chem 46 8081-8090... 

Al-Ghamdi GH, Sudol ED, Dimonie VL, et al. (2006) High PVC film-forming composite latex particles via miniemulsification, part 3 optical properties. J Appl Polym Sd 101 4526-4537... 

In addition, allyl methacrylate is often added to a latex formulation as a crosslinking/grafting monomer [52] especially in the case of core-shell latexes. This monomer is often used to promote adhesion between various polymeric layers, especially in multilayered composite latex particles, which are often used as impact modifiers [53]. 

Composite latex particles of poly(n-butyl acrylate) ly(benzyl methacrylate) (PBA/PBM) [67] prepared by semi-continuous seeded onulsion polymerization in the presence of a chain transfer agent (isooctyl mercrptoproprionate) (lOMP) exhibited a hemispherical morphology or else laiger domains of PBM were... 

Microscopy is a key approach which is frequently used for the characterizatitm of composite latex particles. There are a wide range of microscopes which can be used to analyze latexes, such as the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM) and the atomic force microscope (AFM). The choice of the microscope technique depends on the resolution and size range needed (i.e. nanometres to microns). The most important factor in microscopy is contrast. If the contrast is low, it becomes very difficult to distinguish between... 

The evolution of composite latex particle morphology during onulsion polymerization, and the various parameters which influence ffiis morphology have been described in this chapter. Bodi diermodynamic and kinetic factors strongly... 

Besides the topological aspects of polymer formation in seeded emulsion polymerization, the problems of phase separation during polymerization create a more complex dependence of the particle morphology on polymerization parameters and conditions in the case where the composite latex particles comprise two or more immiscible polymers. During the past several years there have been a significant number of studies which have been dedicated to the elucidation of the... 

The above computations for the fiee energy changes for composite latex particles have shown that only core-shell, inverted core-shell and hemispherical particles are stable in a thermodynamic sense. All of the other reported morphologies (e.g. sandwich-like , raspberry or confetti-like particles or occluded domains) are non-equilibrium, kinetically controlled structures, prepared... 

Figure 9.6 Transmission electron micrographs of (a) PS-300/PEMA-AIBN and (b) PS-190/PEMA-AIBN composite latex particles with a 40/60 weight ratio of seed to shell. The dark regions are polystyrene stained with RuOa and the lighter regions are acrylate domains outlined with phosphotungstic acid stain. (Reprinted from ref. 36. Copyright 1991 John Wley Sons, Inc.)...

The interfacial tension between the various polymer phases in the composite latex particles is of paramount importance in determining their final morphology. Other factors, such as the mode of monomer addition, the surface polarity of the polymer particles, initiator type, surfactant type, the presence or absence of crosslinking of one or more of the polymer phases, and the presence of chain transfer agents will also strongly influence the final particle structure. In addition, a survey of methods used to characterize the composite latex particles has been given. 

Yan et al. [52] explored the use of IPN techniques to produce a composite vinyl-acrylic latex. The first-formed polymer was produced using VAc and divinyl benzene (DVB), while the second formed polymer constituted a BA/DVB copolymer. In both cases the DVB was added at 0.4 wt%. They compared this product with another product, a bidirectional interpenetrating netwodc (BIPN) in which VAc was again polymerized over the first IPN. They noted that the compatibility between the phases was more pronounced in the BIPN than in the IPN as determined using dynamic mechanical measurements and C nuclear magnetic resonance spectroscopy. The concept of polymer miscibility has also been used to produce composite latex particles and thus modify the pafamance properties of VAc latexes. Bott et al. [53] describe a process whereby they bloid VAc/ethylene (VAc/E) copolymers with copolymers of acrylic acid or maleic anhydride and determine windows of miscibility. Apparently an ethyl acrylate or BA copolymer with 10-25 wt% AA is compatible with a VAc/E copolymer of 5-30 wt% ethylene. The information obtained from this woik was then used to form blends of latex polymers by polymerizing suitable mixtures of monomers into preformed VAc/E copolymers. The products are said to be useful for coating adhesives and caulks. 

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