Latex mechanical stability

Latex mechanical stability The abihty of latex to resist coagulation under influence of mechanical agitation. 

In dipping generally, but particularly with the anode process, it is desirable to use tanks that circulate the coagulant and latex compound, particulady the latter. Use of circulation keeps the liquid surface dean and free from lumps, scum, or bubbles. Mechanical circulation can cause rubber particle instability, however, and eventually coagulate the compound. Therefore, tanks should be designed to minimize friction or shear action, and the compound stabilized to maintain mechanical stability. 

Unvulcanized Latex and Latex Compounds. A prime consideration has to be the fluid-state stability of the raw latex concentrate and liquid compound made from it. For many years, the mechanical stability 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 collision. The time taken to cause creation of mbber particle agglomerates is measured, and expressed as the mechanical stability time (MSI). 

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

The quality of the water used in emulsion polymerization affects the manufacture of ESBR. Water hardness and other ionic content can directly affect the chemical and mechanical stability of the polymer emulsion (latex). Solution polymerization can use various solvents, primarily aliphatic and aromatic hydrocarbons, SSBR polymerization depends on recovery and reuse of the solvent for economical operation as well as operation under the air-quality permitting of the local, state, and federal mandates involved,... 

The ability of a soap or surfactant to enhance the chemical stability of natural rubber latex was assessed by ascertaining its effect upon the mechanical stability of natural rubber latices whose stabilities had been reduced by various chemical modifications. Natural rubber latices of reduced stability were produced in three different ways as follows ... 

Wherever possible, the soaps and surfactants were added to the natural rubber latex as dilute aqueous solutions. The cases where this was not possible were (a) ethylene oxide-fatty alcohol condensates of low ethylene oxide fatty alcohol mole ratio, and (b) sparingly-soluble fatty-acid soaps such as lithium laurate and calcium soaps. The former were added as pastes with water, the latter as dry powders. In all cases, the latex samples were allowed to mature for about three days at room temperature before their mechanical stabilities were determined. This allowed some opportunity for the attainment of adsorption equilibrium. 

Saturated straight-chain fatty-acid soaps (1). Figure 1 shows the effects of increasing levels of various potassium saturated straight-chain fatty-acid soaps upon the mechanical stability of natural rubber latex. For convenience of making comparisons between the various soaps, the levels of added soap are expressed as moles per 100 g. of latex solids. 

The results summarised in Table I show the effect of equal parts by weight of each of the potassium fatty-acid soaps upon the mechanical stability of each of the three chemically-destabilised latices. For convenience in making comparisons, estimates of the corresponding results for unmodified natural rubber latex are also included. It is clear from these results that the ability of added potassium fatty-acid soaps to enhance the stability of chemically-destabilised natural rubber latex roughly parallels their abilities to enhance the mechanical stability of unmodified natural rubber latex. 

Figure 1. Effect of added straight-chain potassium fatty-acid soaps upon mechanical stability of natural rubber latex (1). Numbers appended to curves are number of carbon atoms in alkyl chain of soap.

Figure 2. Effect of alkyl chain length of added soap upon mechanical stability of natural rubber latex at four molal levels of addition (A) 0.84 X 10 4 (B) 2 X 10 4 (C) 3.36 X 10 4 (D) 4.20 X 10 4 mol/100 g of latex solids (1)...

Insofar as some of the added soaps may be very efficient in "activating" the indigenous soaps, in the sense that a small number of molecules of added soap is able to activate a large number of molecules of indigenous soap, then we have a ready explanation for the ability of some added soaps to effect large enhancements of mechanical stability even although they are present in an amount which is small compared to the concentration of indigenous soaps in the latex. 

We also note that carboxylate ions which are chemically combined at the polymer-water Interface are known to be considerably more effective in conferring mechanical stability upon a latex than are carboxylate ions which are held at the interface by adsorption. Presumably this is because the latter are able to move laterally in the particle surface, whereas the former are not.) We propose that a given number of adsorbed soap anions is more effective in conferring mechanical stability if able to move independently of... 

Figure 3. Effect of various straight-chain potassium C18 carboxylate soaps upon mechanical stability of natural rubber latex (2) (KCt8) potassium stearate (KC18) potassium oleate (KC18") potassium elaidate (KC18Z) potassium linoleate (KC=ZZ) potassium linolenate (KC1H12(oli)) potassium 12-hydroxy stearate ...

The abilities of the five laurates to protect natural rubber latex against chemical destabilisation appear to be broadly parallel to their effects upon mechanical stability. 

We have also recently discovered that added calcium laurate is able markedly to enhance the mechanical stability of natural rubber latex (2.). This observation is surprising, partly because of the low solubility of calcium laurate in water, and partly because calcium ions are known to be powerful destabilisers of natural rubber latex (j ). It indicates that the stabilising effect of the laurate anion is much greater than the destabilising effect of the calcium cation. 

The effects of a range of sodium n-alkyl sulphates and sodium n-alkyl sulphonates upon the mechanical stability of natural rubber latex are summarised in Figures 4 and 5 respectively. As in the case of added potassium fatty-acid soaps, small additions of... 

Table Ii Effect of 0.1 part by weight per 100 parts latex solids of various potassium fatty-acid soaps upon mechanical stability of unmodified and chemically-destabilised natural rubber 1atices (1 )...

Some data are also available for the effect of the counterion of a dodecyl sulphate upon its ability to enhance the mechanical stability of natural rubber latex. As in the case of the laurates the lithium, sodium, potassium and ammonium salts are similar in behaviour, but the morpholinium salt is slightly less effective. Again, the latter effect is attributed to specific adsorption of the morpholinium cation. Calcium and magnesium dodecyl sulphates are also effective in enhancing mechanical stability, their abilities being similar to that of morpholinium dodecyl sulphate. 

Some interesting results have recently become available for the effects of a range of n-alkyl triethyl ammonium bromides upon the mechanical stability of natural rubber latex. The number of carbon atoms in the alkyl group varied from 6 to 18. Figure 6 summarises the results. It is usually believed that the addition of cationic surfactants to an anionic latex such as natural rubber latex invariably leads to a reduction in colloid stability, the effect being attributed to adsorption of the cations with consequent partial neutralisation of the particle charge and reduction of the counterion cloud surrounding the particles. 

The results summarised in Figure 7 show that small additions of ethylene oxide-fatty alcohol condensates to natural rubber latex generally cause the mechanical stability of the latex to fall. This phenomenon is attributed to the displacement of adsorbed proteinaceous molecules by the condensate molecules. Although the latter are more surface active than the former, they are presumably less effective in conferring mechanical stability upon the rubber particles, perhaps because, unlike the proteinaceous molecules, they are not ionised. 

We interpret this observation as implying that, for these condensates, the effect upon mechanical stability is determined primarily by the binding of water to the ethylene oxide units which are anchored to the rubber-water interface by the fatty-alcohol moiety of the condensate. In the case of condensates for which the overall mole ratio of ethylene oxide to fatty alcohol exceeds ca. 30, the effect upon mechanical stability is much greater than would be expected on the basis of the total amount of ethylene oxide which has been added to the latex, as evidenced by the... 

The small amount of acrylic acid is required to ensure freeze/thaw stability of the latex at a pH of 7 or higher and contributes to its mechanical stability. The anionic emulsifier (Fenopon SF 78) regulates the particle size and contributes to the stability of the system during latex preparation. The nonionic emulsifier (Tergitol NP 40) is required to obtain sufficient electrolyte stability and contributes to the mechanical and the freeze/thaw stability of the latex. Borax is used as a neutralizing agent. 

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