Factors affecting the quality of finished polyvinyl alcohol products

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This post was last edited by QXZ-1966 on 2009-7-27 at 18:35. There are many factors that affect the hydrolysis reaction, including the concentrations of PVAc and base, the molar ratio of base to polyvinyl acetate, the moisture content of the system, the temperature of the hydrolysis reaction, the duration of the hydrolysis, and the conditions following the hydrolysis. Concentration of polyvinyl acetate methanol solution: The concentration of polyvinyl acetate must be maintained at a stable specified value. Too high a concentration results in high viscosity, poor flowability, and poor homogeneity when mixed with alkalis; furthermore, the residual acetate ions in the PVA product increase, and the degree of alcoholysis decreases ； If the concentration is too low, the reaction rate slows down, the reaction residence time increases, the production capacity of the equipment decreases, and it becomes more difficult to operate subsequent systems such as presses. According to the production process using the calcium carbide-acetylene method, it is appropriate to maintain the concentration of polyvinyl acetate at (31±0.5)%, as this ensures that the blowing rate of TQ～301 remains within the acceptable range, allows for stable control at this specified value, and also takes into account the manufacturing capacity of the equipment. Of course, it would be better if it could be stably controlled at (33.5±0.5)% just like in the ethylene production route. Alcoholysis reaction temperature: According to available information, for alkaline alcoholysis, every increase of 10°C in temperature results in the reaction rate roughly doubling. However, as the alcoholysis temperature increases, the final degree of alcoholysis decreases. This is because at higher initial temperatures for the alcoholysis reaction, the transesterification reaction proceeds rapidly, resulting in a large amount of methyl acetate being produced; this accelerates the occurrence of side reactions (endothermic reactions), leading to rapid consumption of the base. By the later stages of the reaction, the concentration of sodium hydroxide decreases, and the rates of transesterification and saponification **slow down**, resulting in an increase in resin at the initial stage. Furthermore, the hydrolysis temperature also has a significant impact on the material properties of the hydrolyzed polyvinyl alcohol in bulk. Practice has shown that too high an alcoholysis temperature not only increases the hardness of the polyvinyl alcohol after alcoholysis, but also accelerates the vaporization of methanol and methyl acetate within the PVA mass, increases the number of bubbles in the polyvinyl alcohol mass, and reduces its apparent specific gravity. In severe cases, the blocky polyvinyl alcohol resulting from alcoholysis expands into a foamy structure, increasing in volume; it cannot pass through filters, which can easily cause damage to the belt-type alcoholysis machine. Conversely, if the reaction temperature is too low, the reaction rate is slow, the residence time is long, curing is poor, and the material may even turn into a thin paste-like substance. In severe cases, there is no response at all, which also makes process accidents more likely. Based on pilot production experience, the alcoholysis temperature is generally maintained between (40–50)°C, with adjustments made accordingly for producing products with different degrees of alcoholysis. Generally, when producing polyvinyl alcohol with a low degree of hydrolysis, such as 88 mol%, the hydrolysis temperature is set at (48±2)℃ ； When producing products with an alcoholysis degree of 99 mol%, the alcoholysis temperature is set at (42±2)℃ ； When producing intermediate products with alcoholysis degrees of (92, 95, 97) mol%, etc., it is appropriate to control the alcoholysis temperature at (45±2)°C. By selecting this temperature for the alcoholysis reaction, not only is the internal quality of polyvinyl alcohol ensured, but the system also operates smoothly, ensuring stable production. Moisture content: The moisture content in the alcoholysis system has a significant impact on the alcoholysis reaction. Since water acts as a catalyst for the direct saponification reaction and reaction (2), as the moisture content in the system increases, the saponification reaction and side reactions accelerate, leading to increased consumption of alkali. The fundamental difference between low-alkali alcoholysis and high-alkali alcoholysis lies in the water content of the system, and this water content significantly affects process parameters such as the alkali mole ratio and reaction time. Relatively speaking, in the calcium carbide-acetylene route, process parameters such as the concentration of the concentrated alkali solution of polyvinyl acetate, the alkali molar ratio, reaction temperature, and reaction time are easier to maintain at stable specified values, whereas controlling the moisture content of the system is much more difficult. Alkali molar ratio (MR): The molar ratio of alkali (NaOH) to the monomer units in polyvinyl acetate (alkali molar ratio) needs to be adjusted accordingly based on other process conditions. The alkali molar ratio is the most important fundamental factor affecting the hydrolysis degree of polyvinyl alcohol. Bases act as catalysts for the alcoholysis reaction, and they are also involved in saponification reactions and side reactions. Without the presence of a base, the alcoholysis reaction cannot occur. The molar ratio of the base is too high, resulting in a rapid alcoholysis reaction and an excessive amount of sodium acetate as a by-product ； Polyvinyl alcohol is hard and compact; the blades of the crusher can easily get damaged ； Moreover, a large amount of PVA powder comes out of the second grinder, which easily leads to bridging between the two powders. However, if the molar ratio of the base is too low, the rate of the alcoholysis reaction slows down, requiring an extended reaction time; moreover, the alcoholysis is incomplete, resulting in a low degree of alcoholysis. In severe cases, the material on the alcoholysis machine either does not form lumps or forms lumps that are too soft to pass through subsequent processing steps, leading to accidents. Alcoholysis reaction time: The alcoholysis reaction time, which is also the residence time in the belt-type alcoholysis machine, is determined by the type of product being produced and the amount of material fed in. For the production of products with a lower degree of hydrolysis, such as PVA1788 and PVA1792, the amount of material used is certainly less, and the hydrolysis reaction takes longer, around 30 minutes. This allows for more thorough pre-alcoholysis, results in a moderate thickness of the material on the belt, and prevents the material from becoming soft. When producing PVA with a high degree of hydrolysis, due to the fast reaction rate and large amount of material used, the residence time in the hydrolysis machine can be reduced to around 15 minutes, or even less. Generally speaking, an excessive residence time and an overly thick layer of material accumulated on the belt are detrimental to increasing production volume ； The residence time is too short, the reaction time cannot be guaranteed, the degree of alcoholysis is too low, and the material is too soft to pass through the subsequent equipment. Waste liquid treatment: When producing PVA with a low degree of hydrolysis, the hydrolysis waste liquid contains partially hydrolyzed PVA particles, as well as unreacted polyvinyl acetate. If the waste liquid is not treated or is not treated properly, it can easily cause blockages in the reboilers of tower 1 and tower 4 in the recovery section; in severe cases, the tray plates can also be blocked by resin. Content of impurities such as VAc in the resin: When impurities like VAc in the resin undergo alcoholysis, acetaldehyde is generated, forming acetaldehyde condensates that reduce the degree of alcoholysis and cause PVA to discolor. Pressing rate: The level of the pressing rate also has a certain impact on the sodium acetate content in the final product. The pressing ratio is too high, resulting in a large amount of mother liquor entering the dryer; consequently, the sodium acetate content in the final product is relatively high, and the color intensity decreases.