Polyurethanes, formed through the reaction of raw polyols with isocyanates, are one of the most widely used types of plastics today. By varying the starting materials, a diverse range of plastics can be produced.
Analyzing the acid value, hydroxyl value, and isocyanate content is essential for evaluating the quality of raw materials used in plastic production. The acid number of polyol raw material is commonly employed in quality control to ensure uniformity between batches. It is also used as a correction factor for calculating the true hydroxyl number.
Polyols are a common raw material for polyurethanes and contain multiple hydroxyl groups. As a result, the hydroxyl number of a raw material directly correlates to the number of polyols present, making it an important quality control parameter.
Isocyanate content is another key quality parameter for the production of polyurethanes, as polyols react stoichiometrically with isocyanates.
In the application bulletin discussed in this article, the determination of acid number (according to ASTM D4662 and ASTM D7253), hydroxyl number (according to ASTM E1899 and DIN 53240-3) and isocyanate content (according to EN ISO 14896 method A, ASTM D5155 method A and ASTM D2572) is described.
Acid Value
The acid value corresponds to the number of carboxylic acid groups in alkyl resins, polyester acrylate resins or mixtures and is presented in units of mg KOH per g sample. The acid value is also used for evaluating plasticizers, where acid values must be minimized.
Instruments and Chemicals
The instruments used to determine acid value were a sample changer, titrator with DET mode, buret (20 mL) and a stirrer.
A Solvotrode easyClean (6.0229.020) electrode was also employed.
The reagents used were ethanol, toluene (dry) and phenolphthalein.
Solutions
Source: Metrohm Middle East FZC
| . |
. |
| Titrant |
c(KOH) = 0.1 mol/L in ethanol or methanol If possible this solution should be bought from a supplier. |
| Solvent mixture |
Ethanol / toluene, Φ(EtOH) = 50% (v/v) Neutralized, just before use, with KOH in presence of 0.3 mL phenolphthalein solution per 100 mL solvent mixture. |
| Phenolphthalein solution |
Phenolphthalein in ethanol, β(Phenolphtalein) = 1 g / 100 mL. |
Standard
- Benzoic acid: Benzoic acid is dried in a desiccator overnight.
Sample Preparation
- No sample preparation was required.
Analysis
Titer
100 – 120 mg benzoic acid was weighed into a titration vessel and dissolved in 50 mL ethanol. The solution was then titrated using c(KOH) = 0.1 mol/L until after the first equivalence point.
Blank
A 50 mL solvent mixture was added to a 150 mL beaker. After 30 seconds, the solution was titrated until the first equivalence point using alcoholic c(KOH) = 0.1 mol/L.
Sample
An appropriate sample amount was weighed into a 150 mL beaker (as shown in the table below). A 50 mL solvent mixture was then added, and the sample dissolved. After 30 seconds, the solution was titrated until the first equivalence point using alcoholic c(KOH) = 0.1 mol/L
The amount of sample depends on the expected acid value. Source: Metrohm Middle East FZC
| Expected AV / mg KOH / g |
Sample amount / g |
Accuracy / g |
| < 7 |
6 – 8 |
0.001 |
| 7 – 15 |
2.5 |
0.001 |
| 15 – 75 |
0.5 |
0.0001 |
| > 75 |
0.2 |
0.0001 |
Parameters
Titer
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Signal drift |
50 mV/min |
| Max. waiting time |
26 s |
| Meas. point density |
4 |
| Min. increment |
10 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
all |
Source: Metrohm Middle East FZC
| . |
. |
| Application Bulletin 200/3 e |
| Acid value, hydroxyl value, and isocyanates in raw materials for the fabrication of plastics by automatic potentiometric titration |
Sample
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Signal drift |
20 mV/min |
| Max. waiting time |
38 s |
| Meas. point density |
4 |
| Min. increment |
10 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
all |
Calculation
Titer
Image Credit: Metrohm Middle East FZC
f: Titer of the selected titrant
ms: Mass of standard in mg
VEP1: Titrant consumption until the first equivalence point in mL
c(KOH): Concentration of the selected titrant in mol/L; here c(KOH) = 0.1 mol/L
MS: Molecular weight of the standard; here 122.12 g/mol
Acid value
Image Credit: Metrohm Middle East FZC
AV: Acid value of the sample in mg KOH / g
VEP1: Titrant consumption until the first equivalence point in mL
c(KOH): Concentration of the selected titrant in mol/L; here c(KOH) = 0.1 mol/L
f: Correction factor («titer») without unit
MA: Molecular weight of KOH; 56.11 g/mol
ms: Sample size in g
Example Determination
Fig 1. Determination of the acid value (blue = titration curve, pink = ERC). Image Credit: Metrohm Middle East FZC
Comments
For less soluble materials, it is recommended that the sample size is decreased or a solvent mixture of one volume of ethanol and three volumes of tert-butyl methyl ether or toluene is used. This mixture should also be neutralized.
ASTM D7253 is similar to ASTM D4662; however, the differences are the use of 100 mL isopropanol instead of the solvent mixture, c(KOH in methanol) = 0.02 mol/L, and a 50 – 60 g sample.
Hydroxyl Value
The hydroxyl value reveals the degree of esterification within the sample and is presented in units of mg KOH per g sample.
Instruments
Instruments and Chemicals
The instruments used to determine the hydroxyl value are listed below.
- Titrator with DET mode
- 1x Buret 50 mL (acetonitrile)
- 2x Buret 20 mL (reaction solution, titrant)
- 1x Buret 2 mL (deionized water)
- Magnetic stirrer for sample changer
- DIS-Cover Electrodes Solvotrode easyClean 6.0229.010 Reagents
- Acetonitrile, HPLC quality
- Toluene-4-sulfonyl-isocyanate (TSI), purum
- Potassium hydrogen phthalate, KHP, pa
Solutions
Source: Metrohm Middle East FZC
| . |
. |
| Titrant |
Tetrabutyl ammonium hydroxide, c(TBAOH) = 0.1 mol/L in isopropanol/methanol, Φ(MeOH) = 50% (v/v) If possible, this solution should be bought from a supplier. |
| TSI solution |
Approximately 250 mL acetonitrile is given into a 500 mL volumetric flask and 20 mL TSI is added. The flask is filled up to the mark with acetonitrile and mixed. The solution reacts vigorously with water, it is therefore recommended to work in a fume hood and under protective gas. The reaction solution is stable for approximately 1 month. |
Standards
- KHP: KHP is dried in a drying oven for two hours at 120 °C and allowed to cool down in a desiccator for a minimum of one hour.
Sample preparation
- No sample preparation was required.
Analysis
Titer
Deionized water (60 mL) was added to approximately 180 mg KHP, and the suspension was stirred for about one minute to dissolve the KHP. The solution was then titrated until the first equivalence point using c(TBAOH) = 0.1 mol/L.
Sample
An appropriate amount of sample, determined using the calculations listed below, was weighed into a titration vessel and dissolved in 10 mL acetonitrile. The solution was then stirred for 30 seconds at a stirring rate of 8.
The TSI solution (10.0 mL) was then added, the sample covered, and the mixture was then stirred at stirring rate 4. After 5 minutes, 0.5 mL deionized water was added, the lid closed, and the solution was stirred for another 60 seconds at stirring rate 4.
Acetonitrile (40 mL) was then added, and the solution was titrated until after the second endpoint with c(TBAOH) = 0.1 mol/L. After each titration, the buret and vessel were rinsed with acetonitrile, followed by deionized water, and the electrode was then conditioned for 1 minute in deionized water.
Image Credit: Metrohm Middle East FZC
ms: Sample size in g
OHVexpected: Expected hydroxyl value
Source: Metrohm Middle East FZC
| . |
. |
| Application Bulletin 200/3 e |
| Acid value, hydroxyl value, and isocyanates in raw materials for the fabrication of plastics by automatic potentiometric titration |
Parameters
Titer
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Pause |
30 s |
| Signal drift |
50 mV/min |
| Max. waiting time |
26 s |
| Meas. point density |
4 |
| Min. increment |
10 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
greatest |
Sample
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Pause |
30 s |
| Signal drift |
50 mV/min |
| Max. waiting time |
26 s |
| Meas. point density |
4 |
| Min. increment |
10 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
all |
Calculation
Titer
Image Credit: Metrohm Middle East FZC
f: Titer of the selected titrant
ms: Mass of standard in mg
VEP1: Titrant consumption until the first equivalence point in mL
c(TBAOH): Concentration of the selected titrant in mol/L; here c(TBAOH) = 0.1 mol/L
MS: Molecular weight of the standard; here 204.22 g/mol
Sample
Image Credit: Metrohm Middle East FZC
OHV: Hydroxyl value of the sample in mg KOH / g sample
VEP1: Titrant consumption until the first equivalence point in mL
VEP2: Titrant consumption until the second equivalence point in mL
c(TBAOH): Concentration of the selected titrant in mol/L; here c(TBAOH) = 0.1 mol/L
f: Correction factor («titer») without unit
MA: Molecular weight of KOH; here 56.11 g/mol
ms: Sample size in g
Example Determination
Fig 2. Determination of the hydroxyl value (blue = titration curve, pink = ERC). Image Credit: Metrohm Middle East FZC
Comments
The ASTM method presented here is faster than the DIN method, the former taking 12 minutes and the latter 40 minutes.
As alcohols react with the TSI, excessively high results are obtained when using solvents such as ethanol for cleaning. Therefore, acetonitrile is recommended as a solvent, with acetone also acceptable.
The tip of an electrode should never be dabbed with a tissue, as this will damage the electrode.
For samples with an expected hydroxyl value of 2 or less, a 15 to 20 g sample should be used.
Acetonitrile is added at 40 mL rather than 30 mL after the preparation to ensure the electrode is completely immersed.
Isocyanate Content
Diisocyantes are employed for the manufacture of polyurethanes, and as a result, isocyanate (NCO) content is a crucial quality parameter indicating the concentration of active NCO groups within the sample. The isocyanate content is presented in units of g of isocyanate per 100 g of sample.
In the application bulletin presented here, isocyanate content determination according to EN ISO 14896 is described and compared with ASTM D5155 and ASTM D2572. Fewer chemicals are used in this method, making it more environmentally friendly than alternatives.
Instruments and Chemicals
The instruments used to determine the isocyanate content were a sample changer, a titrator with DET mode, 2x Buret 50 mL (toluene, acetone), 2x Buret 20 mL (reaction solution, titrant), a magnetic stirrer for sample changer and a DIS-Cover.
A Solvotrode easyClean (6.0229.010) electrode was also employed.
The reagents used were toluene (dried over a molecular sieve), acetone, and dibutylamine.
Solutions
- Titrant c(HCl) = 1 mol/L aqueous. If possible, this solution should be bought from a supplier.
- Reaction solution c(dibutylamine) = 1 mol/L in toluene (dried over molecular sieve)
Standard
- TRIS: TRIS is dried overnight in a drying oven at 105 °C and allowed to cool down in a desiccator for a minimum of one hour.
Sample Preparation
- No sample preparation was required.
Analysis
Titer
Approximately 420 mg TRIS was weighed into a titration vessel, and deionized water (20 mL) and acetone (50 mL) were added. After 20 seconds, the solution was titrated with c(HCl) = 1.0 mol/L until the first equivalence point. Between measurements, the electrode membrane was rehydrated for one minute in deionized water.
Blank
A blank sample was treated and titrated following the same method as the actual sample without sample.
Sample
An appropriate amount of the sample (~2 g) was weighed out, and toluene (30 mL) was then added to dissolve it. The reaction solution (18.0 mL) was then added, the vessel was covered, and it was allowed to react for 10 minutes using a magnetic stirrer. After this, acetone (30 mL) was added, and the excess of dibutylamine was back titrated with c(HCl) = 1 mol/L.
Parameters
Titer
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Pause |
20 s |
| Signal drift |
20 mV/min |
| Max. waiting time |
38 s |
| Meas. point density |
4 |
| Min. increment |
50 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
greatest |
Blank
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Signal drift |
20 mV/min |
| Max. waiting time |
38 s |
| Meas. point density |
4 |
| Min. increment |
10 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
all |
Sample
Source: Metrohm Middle East FZC
| . |
. |
| Mode |
DET U |
| Signal drift |
20 mV/min |
| Max. waiting time |
38 s |
| Meas. point density |
4 |
| Min. increment |
10 μL |
| Max. increment |
off |
| EP criterion |
5 |
| EP recognition |
all |
Example determination
Fig 3. Determination of the Isocyanate value (blue = titration curve, pink = ERC). Image Credit: Metrohm Middle East FZC
Comments
As organic isocyanates react with atmospheric moisture, special precautions must be taken during sampling. Conventional sampling methods, even when conducted rapidly, can cause contamination of the sample with insoluble ureas. Therefore, the sample should be covered at all times with a dry inert gas, such as nitrogen, argon or dried air.
Organic isocyanates are hazardous when absorbed through the skin or when the vapors are breathed in. It is essential to provide adequate ventilation and wear protective gloves and eyeglasses.
Turbidity will be encountered in the titrations. If the mixtures are agitated vigorously, inhomogeneity can be tolerated without adversely affecting the results. Alternatively, methanolic HCl can be utilized as a titrant.
ASTM D5155 is similar to EN ISO 14896; however, the differences are the concentration of the reaction solution, the 15-minute reaction time, the quantities of toluene and reaction solution, the sample size, and the use of isopropanol rather than acetone.
References and Further Reading
- ASTM D4662 Standard Test Methods for Polyurethane Raw Materials. Determination of Acid and Alkalinity Numbers of Polyols.
- ASTM D7253 Standard Test Method for Polyurethane Raw Materials: Determination of Acidity as Acid Number for Polyether Polyols.
- ASTM E1899 Standard test method for hydroxyl groups using reaction with p-toluene sulfonyl isocyante (TSI) and potentiometric titration with tetrabutyl ammonium hydroxide
- DIN 53240-3 Binders for paints and varnishes - Determination of hydroxyl value - Part 3: Rapid test
- EN ISO 14896: Plastics — Polyurethane raw materials - Determination of isocyanate content
- ASTM D5155: Standard Test Methods for Polyurethane Raw Materials: Determination of the Isocyanate Content of Aromatic Isocyanates
- ASTM D2572: Standard Test Method for Isocyanate Groups in Urethane Materials or Prepolymers
This information has been sourced, reviewed and adapted from materials provided by Metrohm Middle East FZC.
For more information on this source, please visit Metrohm Middle East FZC.