Study on The Deblocking Reaction of Blocked pMDI

• Many analytical techniques have been applied to studying the reactions of blocked isocyanates.

• One needs to remember that reported deblocking temperature frequently depend on the method of analysis, heating rate, chemical structure of blocking agent, chemical structure of isocyanate monomer, and other variables.

• Different analytical techniques can give different deblocking temperature for the same sample.

• (Wicks and Wicks, 1999)

Thermogravimetry Analysis1. The thermal stability was investigated using a TGA 209 F3 thermal analysis

system (NETZSCH Co. Germany). Approximately 5 mg of sample was scanned from 30 to 700°C at a heating rate of 10°C/ minute in an argon atmosphere at a flow rate of 30 ml/minute. The TGA and derivative thermogravimetric (DTG) curves were tested by TGA209F3 instrument (Zhang, et al., 2014).

2. Thermogravimetric analyses were performed on a Perkin-Elmer TGA7 series. The measurements were performed in a nitrogen atmosphere with a heating rate of 20°C/min (Mohammed and Sankar, 2011).

3. TGA was performed from 30 to 600°C at a heating rate of 5°C/min under nitrogen atmosphere protection with the gas flowing rate of 90 ml/min. The samples had a mass of 5-10 mg (Zhang, et al., 2011).

Differential Scanning Calorimetry1. Differential scanning calorimetry DSC analysis of the blocked polyurethane

was performed on a NETZSCH D204 DSC, and argon at a flow rate of 30 ml/ minute was used as the purge gas. The scanning temperature ranged from 30 to 200°C at a heating rate of 5°C/minute (Zhang, et al., 2014).

2. Differential scanning calorimetric (DSC) analyses were performed on a Perkin-Elmer DSC7 series in a nitrogen atmosphere. A heating rate of 10°C/min was applied (Mohammed and Sankar, 2011).

3. The samples had of a mass of 3-8 mg were heated from 20 to 250°C, the heating rate was 5°C/min, under nitrogen atmosphere (Zhang, et al., 2011).

Deblocking temperature from aromatic isocyanate

Monomer of Isocyanate Blocking Agent Deblocking T(°C)

MDI MEKO 90-180

MDI Caprolactam 110-180

MDI DPMA 75-125

MDI NaHSO3 90-105

TDI MEKO 70-150

TDI Caprolactam 110-180

TDI DPMA -Subramani et al., 2003)

Important Note

• Sample Preparation• It is very important to ensure that the sample is in intimate contact with the

bottom of the crucible.• It is important to have an accurate measure of the sample weight,

because the accuracy of the heat of reaction measurement is only as good as the weight measurement. Recommended sample weights are between 5 and 20 mg.

(Hale, 2002)

• Initial Temperature– The selection of the initial temperature obviously depends on the

temperature at which the thermoset starts reacting. If the system reacts at room temperature, then some information will be lost.

– It is preferable to start the run at least 20°C until 40°C below the expected onset of the reaction.

(Hale, 2002)

Important Note

• Final temperature• Final temperature should be high enough to allow the reaction to go to

completion.• Ideally, final temperature will allow full development of the exotherm curve

and a flat baseline at the end of the run.Example:Epoxies can typically be scanned up to 250-275°C, but Urethanes and isocyanate group scanned lower than that.(Hale, 2002)

Important Note

• Heating Rate– It is common to employ heating rates between 5 and 20°C/min.– If the heating rate is too fast, there may not be enough time for the

reaction to proceed to completion.– If the heating rate is too slow, the signal may be too low for the instrument

to detect the heat.– The most accurate way to select the appropriate heating rate is to run

several samples at different heating rates.(Hale, 2002)

Important Note

SUMMARY• Differential scanning calorimetry (DSC) analysis:

• 5 mg of blocked pMDI were heated from 20 to 200°C, with heating rate 10°C/min under nitrogen atmosphere.

• Differential scanning calorimetry (DSC) analysis:• 5-10 mg of blocked pMDI were heated from 20 to 600°C, with heating rate

10°C/min under nitrogen atmosphere at a flow rate of 30 ml/minute.

References• Hale, A. (2002). Thermosets. Handbook of Thermal Analysis and Calorimetry Vol. 3, 3, 295–

354.• Mohammed, I. A., & Sankar, G. (2011). Synthesis, deblocking and cure reaction studies of

secondary alcohol-blocked isocyanates. High Performance Polymers, 23, 535–541. • Subramani, S., Park, Y. J., Lee, Y. S., & Kim, J. H. (2003). New development of polyurethane

dispersion derived from blocked aromatic diisocyanate. Progress in Organic Coatings, 48, 71–79.

• Wicks, D. A., & Wicks, Z. W. (1999). Blocked isocyanates III: Part A. Mechanisms and chemistry. Progress in Organic Coatings, 36, 148–172.

• Zhang, Y., Gu, J., Jiang, X., Zhu, L., & Tan, H. (2011). Investigation on blocking and deblocking isocyanates by sodium bisulphite. Pigment & Resin Technology, 40, 379–385.

• Zhang, Y., Cao, J., Tan, H., & Gu, J. (2014). New thermal deblocking characterisation method of aqueous blocked polyurethane. Pigment & Resin Technology, 43, 194–200.