Latex concentration

In order to obtain a homogenous and stable latex compound, it is necessary that insoluble additives be reduced in particle size to an optimum of ca 5 )Tm and dispersed or emulsified in water. Larger-size chemical particles form a nucleus for agglomeration of smaller particles and cause localized dispersion instabiHty particles <3 fim tend to cluster with similar effect, and over-milled zinc oxide dispersions are particularly prone to this. Water-soluble ingredients, including some accelerators, can be added directly to the latex but should be made at dilute strength and at similar pH value to that of the latex concentrate. 

Unvulcanized Latex and Latex Compounds. A prime consideration has to be the fluid-state stabihty of the raw latex concentrate and hquid compound made from it. For many years, the mechanical stabihty of latex has been the fundamental test of this aspect. In testing, the raw latex mbber content is adjusted to 55% and an 80 g sample placed in the test vessel. The sample is then mechanically stirred at ultrahigh speed (ca 14,000 rpm) by a rotating disk, causing shear and particle cohision. The time taken to cause creation of mbber particle agglomerates is measured, and expressed as the mechanical stabihty time (MST). 

More recently, a number of tests of chemical stabihty of the latex concentrate have been developed. Chemical stabihty variance in the raw concentrate has considerable effect on the dipping characteristics of latex compounds, and can also affect mechanical stabihty of the compound. A broad rule is that, while latex MST can be increased or decreased without necessarily affecting its chemical stabihty, any change in the latter always is reflected in the MST. A new test, in which chemical stabihty is deterrnined by measurement of the effect of weak 2inc acetate solution added to a second mechanical stabihty sample and the result contrasted with the original MST, is available to numerically quantify chemical stabihty (56). 

Table 11. ISO Specifications for Natural Rubber Latex Concentrates...

Consumption of NR Latex. The total world consumption of natural mbber latex was 585,000 t in 1993, more than double that of 10 years earlier. The proportion of total natural mbber used as latex concentrate also increased from 7% in 1983 to 11% in 1993. Malaysia remains the dominant latex concentrate producing country in the world (Table 12). Its decline in exports during the 1980s and early 1990s has been partly compensated by an increase in consumption within the country (Table 13). Since the 1970s there has been a gradual shift in latex consumption from the traditional consumers in Europe and the United States to Asian countries. In 1994 Malaysia consumed 175,000 t of latex, more than the combined consumption of Western Europe and the United States (see Table 13). 

Membrane-retained components are collectively called concentrate or retentate. Materials permeating the membrane are called filtrate, ultrafiltrate, or permeate. It is the objective of ultrafiltration to recover or concentrate particular species in the retentate (eg, latex concentration, pigment recovery, protein recovery from cheese and casein wheys, and concentration of proteins for biopharmaceuticals) or to produce a purified permeate (eg, sewage treatment, production of sterile water or antibiotics, etc). Diafiltration is a specific ultrafiltration process in which the retentate is further purified or the permeable sohds are extracted further by the addition of water or, in the case of proteins, buffer to the retentate. 

FIG. 18 SEM pictures of self-assembled layers of particles Ic prepared from latex dispersions of different pH value (substrate glass support modified with 3-AMDS, dipping time 1 h, latex concentration 3 mg/mL T = 23.5°C). (From Ref. 98, with permission from Elsevier, Amsterdam.)... 

Natural latex is concentrated by three main methods centrifuging, evaporation and creaming electrodecantation once showed promise, but the output of latex concentrated by this method is now negligible. 

Latex evaporators Water removed during latex concentration Dissolved organics, suspended and dissolved solids. Relatively high wastewater flow rates... 

Figure 6.9 Ultrafiltration flux with a latex solution at an applied pressure of 60 psi (in the limiting flux region) as a function of feed solution latex concentration. These results were obtained in a high-turbulence, thin-channel cell. The solution recirculation rate is shown in the figure [13]...

Figure 4 shows that higher concentrations of seed latex decrease the reaction rate, at constant mixed emulsifier concentrations, while Figure 5 shows that the smallest latex particle gave the lowest reaction rate. This effect is explained by the decreased availability of emulsifier to create micelles, since the small seed latex particle or high seed latex concentrations adsorb more of the surfactant, thus removing it from the water phase. 

Ficin The purified proteolytic substance derived from the latex of Ficus sp. (Fam. Moraceae), which include a variety of tropical fig trees. Produced as a white to off white powder completely soluble in water. (Liquid fig latex concentrates are light to dark brown.) Major active principle ficin. Typical applications used in the chillproofing of beer, in the tenderizing of meat, and in the conditioning of dough in baking. 

Figure 4.19. Streaming potentials obtained with the cell of fig. 4.18. Negatively charged polystyrene latex. Concentration of electrolyte (KCl) indicated.

Spiral wound elements have been used by a number of companies in the water purification area since 1968. Osmonics has been unique in that we have used spirals on a very large number of non-water purification applications, including oil concentration, latex concentration and of course, the concentration and fractionation of cheese whey. In fact, our installed capacity on cheese whey is approximately 324,000 sq. ft. (29,160 m2) of membrane or 20 miles (32 km) of membrane as it comes off of our machinery 3 ft. (1 meter) wide. Or, if you prefer the installed capacity is capable of handling 800,000 lbs (360,000 kg) of cheese whey per hour. 

Results of the passage of three sizes of the microspheres (0.23, 0.91, and 1.35 fim in diameter) are presented in Figure 1. Actual particle number concentrations observed at a sampling well are presented as a function of time after injection in the main part of the figure. The inset contains these data as dimensionless concentrations normalized to the latex concentration in the input. Results for the chloride tracer (not shown) indicate substantial passage after 18 days and a peak concentration at 21 days. These data indicate that removal decreased with increasing particle size in the order 0.23 >0.91 > 1.35 m. The... 

Figure 3- Increase in hardness of nitrile containing rubber antioxidants in a cyclical hot oil (150 C)/hot air (150 C) test. LML, latex concentrate (20%) diluted to 2% LMCE, latex coagulum (207 ), unextracted before incorporation into polymer LMCU, latex coagulum (20%,), unextracted before incorporation into polymer at 2%.

Figure 7. Loss of impact strength (I ) of ABS during UV exposure. L, latex concentrate (masterbatch) U, unextracted P, mechanochemical concentrate. Numbers on curves are the concentrations of photoantioxidants in the concentrates.

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