BLOCKED AND DEBLOCKED ISOCYANATE WITH NaHSO3

PREVIEW

What is an isocyanate?• Name : Methylene diphenyl diisocyanate (MDI)• Synonims : 4,4’-Diphenylmethane diisocyanate, 1,1’-Methylenebis,

Isonate, Hylene M50, Methylbisphenyl isocyanate.• Molecular formula : C15H10N2O2 (mw : 130 g/mole)• Chemical structure

• One of wood adhesive with excellent bonding properties, water resistance, aging resistance, no formaldehyde emission, and other pollution problems (Tan et al., 2011).

The primary commercial form of MDI is actually polymeric MDI (pMDI) which is a mixture that contains 20-80% of 4,4’-MDI (Kapp, 2014).

• Unfortunately such systems are too reactive at room temperature, because of their sensitivity before cure to moisture either from solvents, resins or pigments, or from high humidity under application condition (Wicks and Wicks 1975).

• Due to high reactivity of isocyanate group at room temperature, this monomer is difficult to deposit for a long period (Lou and Di, 2013).

What is the solution????

ISOCYANATE BLOCKING TECHNOLOGY• Blocked isocyanate is a solution to overcome these drawbacks.

• A blocked isocyanate is an adduct/product containing comparatively weak bond formed by reaction between an isocyanate and a compound contains an active hydrogen atom, and shows the advantage of quite long shelf life because the active N=C=O groups are masked and protected (Lou and Di, 2013).

• Blocked isocyanates have wide spread use in many coatings areas: automotive, maintenance, and industrial finishing (Wicks and Wicks, 2001).

• The blocked isocyanate adhesive applied as wood adhesive should be unblocked under less than 100°C (Zhang et al., 2011)

• Furthermore, blocked isocyanates are stable at room temperature in the presence of polyols and water.

• The overall reaction in the case of formation and hydrolysis of blocked isocyanate can be seen at scheme 1, where NaHSO3 was the blocking agent (Wicks and Wicks 1999)

Scheme 1. Formation and hydrolysis reaction of bisulphite blocked isocyanate

ISOCYANATE BLOCKING TECHNOLOGY

Blocking Agent• As an abundant blocking agent, NaHSO3 has special properties such

as low unblocking temperature, low price, and no pollution. • Bisulphites have been widely used due to their low deblocking

temperature compared to alcohols, phenols, oximes, and caprolactom.

• They have been the most favoured of blocking groups for isocyanates in waterborne solution.

• Bisulphites blocked isocyanates are prepared by reacting isocyanates in aqueous solutions of sodium bisulphite to give water-soluble blocked isocyanates (Zhang et al., 2011).

• During the blocking procedure, low molecular weight sodium bisulfite must be dissolved in water, and due to the greater reactivity of isocyanates and hydroxyl compound, especially water, the reactions are devoid of undesirable side reactions (Iyer et al., 2002).

• Hydrolysis of bisulphite blocked isocyanate is pH dependent, being slowest at low pH, blocked aromatic isocyanates undergo more rapid hydrolysis than aliphatic isocyanates (Wicks and Wicks, 1999).

• The most common methods of monitoring deblocking reactions and determining deblocking temperature are Fourier transform infrared spectrometry (FTIR) (Zhang et al., 2011), differential scanning calorimetry (DSC), thermo gravimetry analysis (TGA) and solid state NMR (Wicks and Wicks, 1999).

• The use of FTIR in combination with DSC is particularly effective for studying curing mechanisms. TGA has been used to determine the kinetic parameters of deblocking reactions (Kordomenos et al., 1982).

• Solid state 13C-NMR can be used to track the intensity of carbon atoms within the blocking group, which become more intense after deblocking reaction (Cholli et al., 1983; Zuo et al., 2012).

Experimental

• pMDI : Polymeric MDI• PEG : Poly ethylene glycol• NaHSO3 : Sodium bisulphite as blocking agent• C2H5OH: Ethanol as cosolvent combine with water• 1,4-dioxanel as solvent

Material

Instrument• DSC to analyze deblocking temperature• FTIR to characterise unblocked and blocked isocyanate• TGA to investigate deblocking reaction• 13C-NMR to track the intensity of C and H atoms within 1H-NMR blocking group, which more intense after deblocking

Depend on blocking agent

Method

• 1000 mL flask was refrigated at the temperature of 0-25°C with a pressure under Nitrogen atmosphere protection

• 15-40% of NaHSO3 and cosolvent (20-50% of NaHSO3 aqueous solution) were introduced into a four-necked flask (reaction kettle)

stirred for 15 min• pMDI and solvent (1,4-dioxane) were introduced dropwise while

stirring• The temperature was maintained at 0-2°C, time for reaction was 1.5-

6 hour, stirring speed was 150-500 rpm, NaHSO3/NCO mole ratio was 1.05-1.5

Preparation of blocked isocyanate (Zhang et al., 2011).

• The deblocking reaction was carried out using oven attached to hermetic container at different temperature of 70°C, 80°C, 100°C for 30 min

Deblocking reaction of sodium bisulphite-blocked polyisocyanate (Zhang et al., 2011).

Characterisation (Zhang et al., 2011).• Deblocking temperature of blocked isocyanate was studied using

DSC with a high pressure under nitrogen atmosphere protection. The samples had of mass of 3-8 mg were heated from 20° to 250°C, the heating rate was 5°C/min.

• FTIR spectra of blocked and unblocked isocyanate adducts were recorded in the range 400-4000 cm-1 at room temperature, in KBr pellets.

• 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.

• FTIR spectroscopy: FTIR was used to examine chemical structure of blocked isocyanate. The FTIR spectra were recorded with a spectral range of 4000–400 cm-1.

• DSC measurement: To examine the thermal behavior of blocked isocyanate, was scanned at a heating rate of 10°C/min, with a temperature range of 20–300°C.

• NMR: 1H NMR and 13C NMR spectra were recorded in dimethyl sulfoxide (DMSO) on a Bruker AV400 spectrometer (Bruker Daltonic Inc., USA), working at 400.13 and 100.61 MHz, respectively.

• Particle size: Particle size was measured, using a LS-800 laser particle sizer (Omec-instruments, China). SEM: The SEM measurements were analyzed using Hitachi S-3400 scanning electron microscope (SEM) (Hitachi Limited, Japan). The cryogenically fractured surfaces were etched with chloroform for 24 h to remove the thermoplastic phase. The specimens were dried in vacuum overnight to remove the solvent. All the specimens were sputter coated with gold before taking the micrographs.

(Lou and Di, 2013)

Result• In this experiment, blocked isocyanate and unblocked isocyanate

were characterised by FTIR as shown in Figure 1.

• Deblocking temperatures of the blocked isocyanate by sodium bisulphite

• DSC analysis was used to study the phase behaviour and the deblocking temperature because the energy associated with deblocking of blocked isocyanate was quite different with structure variations

Result

ResultThe deblocking temperatures of the blocked isocyanates were determined with DSC and FTIR analysis.

BPI7 : deblocked at 70°CBPI8 : deblocked at 90°CBPI10 : deblocked at 100°C

The results of TGA and DTG curves showed the thermal stability of the blocked isocyanate

Result

• Sodium bisulphite-blocked isocyanates with good solubility were successfully synthesised with high yield during the reaction of sodium bisulphite and isocyanate by the special procedure. The optimum blocking conditions determined by blocking yield measurements was as follows: reaction time was 1.5 - 6 hour, reaction temperature was 0-25°C, stirring speed was 150-500 rpm, cosolvent was ethanol and water, solvent was 1,4-dioxane, NaHSO3/NCO mole ratio was 1.05-1.5.

Conclusions

Other method (EP0929518 A1)• Blocked TMXDI was prepared in the followin: • Sodium metabisulphite (190 g) was dissolved in 530 g of water. The

clear solution obtained was placed in a flask provided with a stirrer and cooling means.

• l-Methyl-2-pyrroIidinone (200g) was added to the solution along with 120g of m-tetramethylxylene di-isocyanate. 3.5g of triethanolamine and 0.1 g of dibutyl tin dichloride. The mixture was maintained at 5 degrees Celsius with continued stirring for seven days at which time the reaction mixture had become one aqueous suspension. After filtering this mixture, the residue contained the desired blocked isocyanate and the filtrate, after evaporating and adding excess acetone yielded a further crop of blocked isocyanate. The combined yield was 180g of the sodium salt of the blocked isocyanate.

What we are going to do?• Trying to prepare of bisulphite blocked isocyanate with mole ratio of

NaHSO3/NCO = 1.2, 1.4, 1.6, 1.8, 2.0 (max 2.0)• 15-40% of NaHSO3 and cosolvent (20-50% of NaHSO3 aqueous solution) were

introduced into a four-necked flask (reaction kettle) stirred for 15 min• The temperature was maintained at 5-15°C, time for reaction was 1.5-6 hour,

stirring speed was 150-500 rpm, NaHSO3/NCO mole ratio was 1.2-2.0• Deblocking temperature of blocked isocyanate was studied using DSC with a high

pressure under nitrogen atmosphere protection. The samples had of mass of 3-8 mg were heated from 20° to 250°C, the heating rate was 5°C/min.

• FTIR spectra of blocked and unblocked isocyanate adducts were recorded in the range 400-4000 cm-1 at room temperature, in KBr pellets.

• 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.