Waterborne dispersions

In the gas-phase processes, the polymer is discharged into a cyclone separator, from which residual monomers are recovered and recompressed. 

Probably the main step to eliminate VOC in the polymer industry has been the substitution of solvent-based systems by waterborne products. Nowadays, about 5% of the polymers are produced by aqueous dispersion processes, mainly by suspension and emulsion polymerization [8]. The water-based polymers, however, are not totally free of monomer and VOCs, since the reaction does not reach completion and VOCs arising from impurities in the raw materials or from by-side reactions can be present. There are two main ways to reduce the residual monomer content in waterborne polymers post-polymerization or/and devolatilization. 

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The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

Acrylic emulsions, or better referred to as waterborne dispersion acrylic paints, were introduced in the 1950s and soon became the most used artists media because of their... 

EPI-REZ 3510-W-60 185-215 500-5,000 Waterborne dispersion of bisphenol A resin similar Resolution... 

EPI-REZ 3519-W-50 560-660 5,000-15,000 Waterborne dispersion of a CTBN modified epoxy resin Resolution... 

To summarise the comparison between alkyd trim paints and their waterborne counterparts, a lot of properties are provided by solventbome alkyd technology for which specific additives are required in waterborne dispersion paints. Frequently, the polar alkyd resins are capable of wetting and stabilising the pigment for which dispersing agents are needed in waterborne. 

Waterborne dispersions containing poly(vinyl acetate) are widely used as adhesives for wood or wood-based materials. Their success can be attributed to several factors such as easy application, non-flammability, low toxicity and relatively low cost. In fact, high temperatures are generally not required to dry these adhesives, their shelf-life is long and water can be used to remove the residual product from tools. Furthermore, these adhesives show an extremely low toxicity profile due to the presence of water as dispersing medium and to the very low content of volatile organic compounds (VOCs). 

Core-shell particles have been produced both chemically [134] and electrochemically [135]. For example, a dispersion of electrically conductive core-shell particles was obtained by polymerizing pyrrole or aniline in the presence of a dispersion of polyurethane or alkyd resin particles [134]. Coatings from these dispersions were reported to have conductivities in the range of 10 to 10 S/cm [134]. This coreshell approach, though yet to be fully exploited in the area of corrosion protection, enables formation of CP-containing films from waterborne dispersions. 

Waterborne dispersed polymers include both synthetic polymer dispersions and natural rubber. Synthetic polymer dispersions are produced by emulsion polymerization. A substantial part of the synthetic polymer dispersions is commercialized as dry products these include SBR for tires, nitrile rubbers, about 10% of the total PVC production, 75% of the total ABS and redispersable powders for construction materials. Carboxylated styrene-butadiene copolymers, acrylic and styrene-acrylic latexes and vinyl acetate homopolymer and copolymers are the main polymer classes commercialized as dispersions. The main markets for these dispersions are paints and coatings, paper coating, adhesives and carpet backing. 

Waterborne dispersions Carboxylated styrene-butadiene polymers Vi nyI acetate polymers Acrylic and styrene acrylic polymers... 

ABS, an elastomer modified thermoplastic used in electrical and electronic equipment, house and office appliances and in the automotive industry) and redispersable powders for construction materials. Natural rubber accounted for 6.5 million tonnes per year, including about 1 million tonnes that is commercialized as a waterborne dispersion. 

Colloid Polymer Science 278, No. 11, Nov.2000, p. 1103-8 PREPARATION OF WATERBORNE DISPERSIONS OF EPOXY RESIN BY THE PHASE-INVERSION EMULSIFICATION TECHNIQUE. II. THEORETICAL CONSIDERATION OF THE PHASE-INVERSION PROCESS... 

For waterborne dispersions the ability to deliver polymer particles to the hair near the critical 60-80% solids level will minimize the effects of water plasticization of the hair. As can be seen in Figure 34, however, waterborne dispersions rapidly gain viscosity as their solids level increases. In this regard, the more water evaporation that occurs during... 

There are two main ways to reduce the VOC content in waterborne dispersed polymers post-polymerization and devolatilization. Post-polymerization can only be applied to emulsion polymers. Water-soluble redox initiators yielding hydrophobic radicals have been found to be advantageous for monomer removal by postpolymerization. In suspension polymers the devolatilization is limited by both the diffusion through the particle (because of the usually high Tg of the polymer) and the mass transfer from the particle surface to the aqueous phase (because of the large particle size that results in a relatively small interfadal area). Therefore, the temperature and the polymer particle size play an important role in the devolatilization efficiency. 

Waterborne dispersions form films through a fascinating process. In order for crosslinking to occur and a coherent film to be built, the solid particles in dispersion must spread out as the water evaporates. They will do so because coalescence is thermodynamically favored over individual polymer spheres the minimization of total surface allows for a decrease in free energy [5]. 

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