Third, common concentration measurement methods
Currently, methods for measuring the concentration of a plant using a PAG quenching agent include a refractometer method, a viscosity method, and a true concentration method. According to the above-mentioned variation rule of the PAG quenching liquid, the following three methods are evaluated one by one.
1. Refractometer concentration method
All substances dissolved in the water change the refractive index of the aqueous solution. The contribution of the solute to the refractive index of the solution is proportional to its concentration in the solution, and the total refractive index of the solution is a simple superposition of the contribution of each solute to the refractive index of the solution. According to this rule, the total refractive index of all solutes in the solution will be measured with a refractometer. Therefore, the refractive index of the newly formulated PAG quenching liquid is the sum of the refractive indices of the PAG polymer component and the additive component therein. As shown in A of Figure 1. As mentioned earlier, after the quenching liquid is used, the relative concentration of the PAG polymer is lowered, and the relative concentration of the additive component is increased. At the same time, the concentration of soluble substances in tap water also increases due to water volatilization. In addition, the quenching liquid is inevitably contaminated, and the soluble contaminants also increase the refractive index of the solution. Thus, the refractive index of the quenching liquid after use is obtained by adding the contribution of the four components of the polymer, the additive, the self-contamination of water and the foreign pollutant to the refractive index. As shown in B of Figure 1. It can be seen that in the production of a quenching fluid concentration measured by a refractometer, in order to maintain a certain PAG concentration, a higher refractometer reading concentration must be used. The longer the quenching liquid is used, the more quenching the amount is, and the more serious the pollution, the higher the concentration of the required refractometer. If you don't understand this, continue to control the concentration of the quenching liquid according to the refractometer reading when the quenching liquid is newly prepared. The concentration of the PAG polymer is low and the cooling rate is too fast, causing quenching and cracking.
2. Viscosity measurement method
The viscosity of the PAG quenching liquid increases as the concentration of the quenching agent increases. In PAG quenching agents, the PAG polymer component plays a decisive role in the viscosity increase of the quenching liquid, while the influence of other components (including contaminants) is rather small. Therefore, by measuring the viscosity, the influence of other components can be better excluded, and the concentration of the PAG component in the solution is measured. However, for longer-lasting PAG quenching fluids, the viscosity measurement method overestimates the effective concentration in the solution. The danger that may still occur is that the solution is cooling too fast.
3. Real concentration measurement method
The true concentration measurement method was first proposed by Beijing Huali Fine Chemical Company. This method is a refractometer test method in which the concentration of the quenching liquid is determined by separating the reverse-soluble polymer in the solution from the other components without the reverse solubility by utilizing the reverse solubility of the PAG polymer. Since the concentration of the polymer having reverse solubility in the solution can be determined, the concentration of the quenching liquid can be measured and controlled relatively well over a slightly longer period of use. Since it is not affected by the content of other components, and only needs a common refractometer, it should be said that it is a better method to determine the total amount of PAG in the solution. However, as with the viscosity measurement method, the concentration is controlled only by this method, and the actual cooling rate will be high for a long time. Therefore, if you do not pay attention, there is also the danger of quenching.
4. About the effective concentration of PAG quenching liquid
In quenching and cooling, not only can it be desolvated from the water around the workpiece beyond the cloud point, but it may also adhere to the surface of the workpiece together with other de-dissolved PAG polymer. The concentration of this part of PAG, we call it the effective concentration. . Taking the quenching liquid in the beaker for heating test, it is found that the newly prepared quenching liquid, when heated to the boiling temperature, the depolymerized polymer can all be flocculated together and float first due to the wrap of water vapor. On the liquid surface. After the long-term use of the PAG quenching liquid is heated to the boiling temperature, it is possible that a part of the de-dissolved polymer does not form a flocculent shape with other polymers, but sinks to the bottom of the cup like a snowflake. Such a quenching liquid has a higher concentration measured by a viscosity method or a true concentration method, but its cooling characteristics are comparable to those of a lower concentration of a new quenching liquid. Obviously, the part of the polymer that sinks in the form of a snowflake is an ineffective part. Neither the viscosity method nor the true concentration method can separate the ineffective polymer, and thus the cooling characteristics of the quenching liquid that has been used for a long period of time cannot be accurately controlled.
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