Study On The Improvement Of Test Methods For Product Quality Of Polyvinyl Alcohol

The polyvinyl alcohol (PVA) is a kind of typical water-soluble and crystalline polymer. Because of its excellent mechanical properties and surface activity, it has been used in many industries. Especially, it is of fine water solubility and excellent membrane-formation, which has the advantage of any other polymers. The total production at home has achieved 500 kt/a, but its test method remains 1989 standards, including 12 quality characteristic examinations. It is necessary to study on the improvement of the test methods for PVA product quality, for the shortcomings that exist in the test procedures seem to be complex and long, part of the methods unsuitable, and the performance indexes for the users incomplete and so on.First, this article makes an improvement in the test method for the viscosity of PVA, and studies a fast determination for the concentration in the viscosity test, the combination of microwave and drying oven is applied instead of the traditional oven drying, and the time for the concentration test is shortened from 5 to 3 hours. In the meanwhile, the time for the whole viscosity test is reduced from 7 to 4 hours, thus the viscosity test speeds up significantly and large quantity of energy is saved. This article also improves the way of the viscosity determination. In the national standard, the viscosity is required to be determined simultaneously under the concentrations of 3.8%,4.0% and 4.2%, then charts out the viscosity at the accurate concentration of 4.00%, and this is called the three-point method. The article points out that in theory, the usual linear charting method is incorrect, while the nonlinear charting method is not practical. Therefore, the method of fixed point takes the place of the three-point method, that is, to prepare a PVA solution at the concentration of 4.0% approximately, then determine out the accurate concentration and adjust the concentration to 4.00% exactly, at last the viscosity can be determined immediately. The method of fixed points has proved to be compliant in theory and controllable in practice.Next, the article studies the determination of the intrinsic viscosity in the determination of the polymerization degree. It is used in the national standard that determines one point of the Logarithm viscosity number lnηr /c under the concentration of 10 g·L-1 as the intrinsic viscosity [η]. This article points out the determination of the intrinsic viscosity should measure the value of Huggins intermolecular interaction parameter kH first. The value of kH will decide which way of the determination and the formula of the intrinsic viscosity [η]is to be used for calculation. When the kH equals1/3, we can use one point method, which is simple; when kH≠1/3, we should make relative viscosity measurements on a series of polyvinyl alcohol aqueous solutions and the reduced viscosityηspcshould be extrapolated linearly to zero concentration so as to obtain the intrinsic viscosity [η].Then, the article researches the determination of the content of methyl alcohol (CH3OH) in polyvinyl alcohol. Water is used as the absorbent agent and the air as the carrier gas, polyvinyl alcohol is heated to 120℃and CH3OH is separated from PVA so as to collect the methyl alcohol, then alkaline potassium permanganate is used to inspect the content of the methyl alcohol. The research indicates that CH3OH can be entirely separated at 120℃when the sample is heated for 2h by bubbling continuously and then 15 mL of 4 mol·L-1 NaOH is added to make the methyl alcohol react for 30 min with the excessive potassium permanganate standard volumetric solution at (80±2)℃, the methyl alcohol is oxidized completely. With the characteristics of simple instruments and equipment and higher accuracy, this method is really suitable to be promoted in factories.Finally, this article researches the quantitative analysis about the dissolution of polyvinyl alcohol. And it suggests that polyvinyl alcohol should be heated and warmed up at a certain speed and the temperature when the sample is completely dissolved should be used to express the dissolution of polyvinyl alcohol, and so called is"the dissolving temperature". The determination of dissolving temperature for medium and high polymerization of complete alcoholysis is as follows: after the PVA sample is soaked in water for 20 min at (60±1)℃, the sample solution is heated up at 1℃/min to make the particles of polyvinyl alcohol completely dissolved, and the solution temperature is to be recorded at the same time. This method is quite adaptable at present for the domestic PVA products.