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The aqueous emulsion characteristics of furniture paint
Time:2014-07-26 Click:
 

Emulsion is a spherical droplet (dispersed phase) dispersed in water (the continuous phase) in aqueous two-phase system composed of. In a certain volume, the diameter of ball of the most compact, there are two ways: accumulation of dense row of six party and face centered cubic (Figure 2.1). The two most closely packed spheres occupy the same volume fraction, is 0.7405. The close packing of two is large diameter ball, volume fraction will be higher. If the dispersed phase in the emulsion similar words, latex solid content can do 74%. But in fact, waterborne wood lacquer latex solid content is very low, high and no more than 50%, low was only about 28%.

If the excessive increase of the solid content, the stability of the emulsion will become poor, it will be easy to produce in the preparation and storage phase inversion, flocculation, coalescence and demulsification phenomenon, it is difficult to guarantee the waterborne lacquer furniture quality may cause difficulties in construction. According to the research, there are two reasons. One is the polymer emulsion for waterborne wood paint dispersed phase particle surface ion polymer chain end groups, and adsorption in the emulsifier emulsion particle on the surface of the latex particles with charge, thus forming a double electric layer. Electric double layer increases the apparent particle size of the emulsion particles, so that the particles appear than big, charge will cause particles to mutual exclusion, mutual penetration, the emulsion viscosity increases, on the other hand, the electric double layer reduces the concentration of emulsion particles, reduce the solid content of effective. Too close packing can make the electric double layer structure damage, eventually making emulsion breaking. Another reason is that the solid content is too high, the viscosity of emulsion is large, is not conducive to the paint and coating construction. Effects of sub particle size and particle size distribution. In addition, the higher the concentration of emulsion, the rheological properties tend to exhibit non Newtonian viscosity, at this time also depends on the shear rate.

Solvent furniture paint does not exist the thermodynamic stability problem, and the stability is a special performance of emulsion. The stability of the emulsion is destroyed, will occur latex particle aggregation, flocculation, stratification, water shrinkage, condensation phenomena even gel. Stability of emulsion stability finally decided to paint.

Factors influencing the stability of emulsion particles are:
① charged particle surface charges repel each other so that a stable emulsion particles, a charge opposite to the cause condensation;
② so that van der Waals forces between the emulsion particles tend to clump;
③ interfacial tension, interfacial tension between the particles and the dispersed phase, the more unstable emulsion;
④ steric hindrance, the spatial configuration of the surface of the macromolecular chains of the emulsion particles adsorbed or grafted hinder coalescence of the particles;
⑤ adsorbed solvent or co-solvent solvent, emulsion particles formed on the surface layer, thereby blocking the particles approach each other, increasing the stability of the emulsion.

Specific performance and emulsion stability factors are as follows:

(1) In the course of long-term storage stability of the emulsion will be placed  movement; collisions between the resulting emulsion particles aggregated; emulsion particles larger difference between the density of water, under the action of gravity tends to float or sink to produce , and produce aggregation; hydrolysis of the polymer constituting the particles or hydrophilic changed, and the effects of changes in pH value of the electrolyte, so that the system is unstable; due to microbial surface active agent, protective colloid. Deterioration or polymer emulsion damage.


(2) by mechanical shear stability, especially high shear forces, the emulsion breaking. This is likely to occur in the following cases: the interface potential drop, the protective layer of particles adsorbed. ! Thin, temperature, latex particles finer emulsion or higher concentrations.


(3) When the freeze-thaw stability of the emulsion will generate excessive cooling ice, ice expansion oppression caused by latex particle coalescence, and then heating the particles have not re-dispersed emulsion breaking. Emulsion particles larger, lower concentrations of surfactant adsorption at low temperatures and can reduce the temperature of the material came from a generation of ice will cause freeze-thaw stability deteriorates.


(4) mixing the pigment stability of the pigment away quickly charged latex particles or pigment interfacial bonding of the pigment particle surface charge and the charge of the ion dissolving out of the latex particles do not match, the emulsion loses stability. The size of the emulsion particles is generally in the micron level. When the emulsion particle size decreased from hundreds of microns, emulsion appearance will experience a significant change process: by turns milky milky white with blue, white slightly transparent, translucent milky liquid to completely transparent. This is because the wavelength of visible light of 400 ~ 750nm, the wavelength of visible light when the particle size is reduced to a much smaller (nano-scale) when the ratio of the emulsion into a transparent microemulsion (microemulsion). And the true solution is different, light scattering and X-ray diffraction showed that there is still a second phase. Is actually a microemulsion micelle emulsion (micellar emulsion), is thermodynamically stable system, a particle size in the range of IO ~ lOOnm. True solution is a single-phase system at the molecular level, excellent stability. Seen in this light, according to the size of the observed particle dispersion, microemulsion can be viewed as a transitional state from the emulsion to a true solution. The smaller particle dispersion stability becomes better. In this process, the system will exhibit a different state of instability, sub-stability and stability. In fact some have slightly transparent when dispersed system of long-term storage will have particle coarsening or phase separation occurs, this indicates that the dispersion of particles in the system was still not small enough, only in a metastable state. Table 2.3 shows the comparison of the emulsion and microemulsion. Today Waterborne Wood with polyurethane dispersion comprising acrylic modified polyurethane dispersion mostly microemulsion system, the dispersion of small particle size, permeability, high gloss paint, but the main thing is to have good storage stability, to obtain a wide range of applications.

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