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One of the most commonly used plastics is polyurethane. They are produced by the reaction of crude polyols with isocyanates. A variety of plastics can be obtained depending on the starting material. Determining the hydroxyl value, acid value and isocyanate content plays a key role in the analysis of plastic raw materials.

To ensure consistency between batches, the acid value of polyol raw materials is usually used for quality control. In addition, it can also be used as a correction factor to determine the true hydroxyl value. In this article, the determination of acid value according to ASTM D4662 and ASTM D7253 is outlined.

Polyol is a raw material for polyurethane and consists of multiple hydroxyl groups. Therefore, the hydroxyl number of the raw material directly corresponds to the amount of polyol present, so it is a key parameter for quality control. In this article, the method for calculating the number of hydroxyl groups according to ASTM E1899 and DIN 53240-3 is outlined.

Understanding the isocyanate content is an important quality parameter for the production of polyurethane, because polyols react stoichiometrically with isocyanates. The measurement according to EN ISO 14896 Method A, ASTM D5155 Method A and ASTM D2572 is described below.

The acid value corresponds to the amount of carboxylic acid groups in the alkyl resin, polyester acrylate resin or mixture, and is expressed in milligrams of KOH per gram of sample. It is also used to evaluate plasticizers, where the acid value should be as low as possible.

No sample preparation is required.

Weigh 100 – 120 mg of benzoic acid into the titration vessel and dissolve it in 50 mL of ethanol. Next, use c(KOH) = 0.1 mol/L to titrate the solution until the first equivalence point is passed.

Add 50 mL of solvent mixture to a 150 mL beaker. After a 30-second pause, use c(KOH) = 0.1 mol/L alcohol to titrate the solution until the first equivalence point.

See the table below. Weigh an appropriate amount of sample into a 150 mL beaker. Then add 50 mL of solvent mixture and dissolve the sample. After a 30-second pause, use c(KOH) = 0.1 mol/L alcohol to titrate the solution until the first equivalence point.

The sample size depends on the expected acid value

Figure 1: Determination of acid value (blue = titration curve, pink = ERC).

The hydroxyl value provides information about the degree of lactonization of the sample in mg KOH/g sample.

Tetrabutylammonium hydroxide, c(TBAOH) = 0.1 mol/L isopropanol/methanol solution, Φ(MeOH) = 50% (v/v). If possible, the solution should be purchased from the supplier.

Put approximately 250 mL acetonitrile into a 500 mL volumetric flask and add 20 mL TSI. Next, fill the flask to the mark with acetonitrile and mix. The solution reacts violently with water, so it is recommended to work under protective gas in a fume hood. The reaction solution is stable for about 1 month.

KHP is dried in an oven at 120 °C for two hours and then cooled in a desiccator for at least one hour.

Add 60 mL deionized water to approximately 180 mg KHP. The suspension was then stirred for approximately one minute to dissolve the KHP. Next, use c(TBAOH) = 0.1 mol/L to titrate the solution until the first equivalence point.

Weigh an appropriate amount of sample (see the following formula) into the titration cup, dissolve it in 10 mL of acetonitrile, and then stir the solution at a stirring rate of 8 for 30 seconds. Add 10.0 mL of TSI solution, cover the sample, and stir the mixture at a stirring rate of 4.

After 5 minutes, add 0.5 mL of deionized water and close the lid again, then stir the solution at a stirring rate of 4 for another 60 seconds. Add 40 mL of acetonitrile and titrate the solution with c(TBAOH) until after the second end point = 0.1 mol/L. The container and burette are first rinsed with acetonitrile, and then rinsed with deionized water after each titration. The electrode is conditioned in deionized water for 1 minute.

Figure 2: Determination of the hydroxyl value (blue = titration curve, pink = ERC).

Diisocyanates are used in the manufacture of polyurethane. Since this value shows the concentration of reactive NCO groups in the sample, the isocyanate (NCO) content is a key quality parameter. The NCO content is expressed in grams of isocyanate per 100 grams of sample. In this case, calculations according to EN ISO 14896 are shown, because fewer chemicals are used compared to ASTM D5155 and ASTM D2572, making it a more environmentally friendly technology.

c(HCl) = 1 mol/L aqueous solution. If possible, this solution should be purchased from the supplier.

c(Dibutylamine) = 1 mol/L toluene solution (molecular sieve drying)

TRIS is dried overnight at 105 °C in a drying oven, and then cooled in a desiccator for at least one hour.

Weigh approximately 420 mg of TRIS into the titration vessel. Add 20 mL deionized water and 50 mL acetone. After a pause of 20 seconds, the solution was titrated with c(HCl) = 1.0 mol/L until the first equivalence point. Between two measurements, the electrode membrane was rehydrated in deionized water for one minute.

The processing and titration of the blank sample is exactly the same as the actual sample without sample.

Weigh an appropriate amount of sample (about 2 g) and add 30 mL of toluene to dissolve it. Add 18.0 mL of reaction solution, cover the container, and react on a magnetic stirrer for 10 minutes. Next, 30 mL of acetone should be added, and the excess dibutylamine should be back titrated with c(HCl) = 1 mol/L.

Figure 3: Determination of the isocyanate value (blue = titration curve, pink = ERC).

Metrohm is one of the most trusted manufacturers of high-precision chemical analysis instruments in the world. Metrohm was founded in Herisau, Switzerland in 1943 by engineer Bertold Suhner. Today, Metrohm has subsidiaries and exclusive distributors in 120 countries. The Global Metrohm Group also includes Dutch companies Metrohm Applikon and Metrohm Autolab, which are manufacturers of online analyzers and electrochemical research instruments, respectively. Recently, Vantone Raman, a leading manufacturer of handheld Raman spectrometers, joined the Vantone Group.

Metrohm is the global market leader in titration analysis instruments. Instruments for ion chromatography, voltammetry, conductivity and stability measurement complete Metrohm's ion analysis product portfolio. Near-infrared and Raman spectroscopy instruments are another strong growth segment in Metrohm's product portfolio.

Wantong is a problem solver in laboratories and industrial processes. To this end, the company provides its customers with complete solutions, including dedicated analytical instruments and comprehensive application knowledge. At the Metrohm International headquarters in Herisau, more than 30% of the company's employees are engaged in research and development.

Since 1982, the Vantone Foundation has been 100% owned by the non-profit Vantone Foundation. The Vantone Foundation has no influence on the company's business operations. It sponsors talented students in the field of natural sciences, supports charity and charitable purposes, and most importantly, ensures the independence of the company.

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Last update: May 15, 2020 at 11:29 AM

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