Deblocking Temperatures Research Articles

Dual-curable isocyanate crosslinking agents blocked by methacrylate-functionalized pyrazoles with lower curing temperature

To develop a polyurethane-based automotive clearcoat with a lower curing temperature, methacrylate-functionalized pyrazole derivatives were newly designed and synthesized as isocyanate blocking agents and their chemical structures were identified by 1H NMR. These novel blocking agents were used to block the functional groups of hexamethylene diisocyanate (HDI) trimer (commercially known as Desmodur N3300), and density functional theory simulations were performed to predict the activation energy of the deblocking process and to determine the HN distance in the pyrazole molecules. The curing behavior of the blocked isocyanate with polyol resin at 110, 120, and 130 °C was investigated using an oscillatory rheometer and a rigid-body pendulum tester. In addition, their results were compared with those for a commercially available 3,5-dimethyl pyrazole-blocked HDI trimer (Desmodur PL350). The thermal properties of the clearcoat films cured at 130 °C were investigated using thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis, and the surface mechanical properties of the cured films were evaluated using a nano-indentation tester. The novel pyrazole blocking agents contributed to lowering the deblocking temperature and enhancing the mechanical properties via supplementary radical polymerization of deblocked methacrylate-functionalized pyrazoles, resulting in interpenetrating crosslinked polymer networks. This study provides a new concept of isocyanate blocking agents with lower deblocking temperatures and higher crosslinking densities, resulting in blocked isocyanates that can be used in various coating materials.

Synthesis and properties of blocked Di- and polyisocyanates

Tricresol-blocked di- and polyisocyanates (BDPs) of variable structure are synthesized, and their main physicochemical and technological properties are defined. They have lower melting points and thermal dissociation (deblocking) temperatures than do the known BDPs based on e-caprolactam, oximes, and secondary amines.. The new blocked isocyanates are tested as latent curing agents for polyurethane varnish compositions of electroinsulating assignment.

Synthesis and Deblocking Studies of Low Temperature Heat-Curable Blocked Polymeric Methylene Diphenyl Diisocyanates

One package heat curable polyurethane system typically requires blocked isocyanates with a relatively low deblocking temperature. In this study, a series of new aromatic secondary amines blocked polymeric methylene diphenyl diisocyanate (pMDI) was synthesized and characterized. Hot-stage FTIR, DSC and TGA spectroscopic methods were used to determine the deblocking temperatures of the blocked isocyanates. Cure reaction between the blocked isocyanates and a commercial polyl (PPG-2000) were carried out to establish structure and property relationship. Deblocking temperature and gel time of the blocked isocyanates based on N-methylaniline and N-methyl-o-toludine were lower than the blocked isocyanates with imidazole and methyl 4-methylaminobenzoate. Unexpected high deblocking temperature of imidazole and methyl 4-methylaminobenzoate was due to intra molecular hydrogen bonding. Solubility behavior of the blocked isocyanates was also studied using a series of polyols (PEG-400, Terathane-2000, PPG-2000, and castor oil).

Synthesis and characterization of water-borne diisocyanate crosslinkers from methyl ethyl ketoxime/2-methylimidazole-blocked aromatic isocyanates

Water-borne blocked isocyanates (WBIs) were synthesized using toluene 2,4-diisocyanate (TDI), trimethylolpropane, dimethylolpropionic acid, 2-methylimidazole (2-MI), and methyl ethyl ketoxime (MEKO). The particle size, viscosity, pH, and storage stability of the WBI dispersions were studied and compared. De-blocking temperatures of the WBIs were characterized by Fourier transform infrared (FT-IR), differential scanning calorimetry (DSC), and thermo-gravimetric analysis (TGA) techniques. The thermal analysis revealed that MEKO-blocked TDI adducts and 2-MI-blocked TDI adducts started to de-block at about 70.7 and 125.2 °C, respectively. The storage stability results showed that all dispersions prepared were stable for more than 6 months. The dispersions prepared were all in liquid form and miscible (compatible) with acrylic dispersion. Gelation test results confirmed that MEKO-blocked isocyanate adducts dispersions de-block at higher temperatures and cure at faster rates than 2-MI-blocked isocyanate adducts dispersions. These anionically modified blocked isocyanate adducts can be used as potential crosslinkers.

Synthesis and properties of biodegradable polyurethane crosslinkers from methyl ethyl ketoxime-blocked diisocyanate

A series of novel biodegradable blocked polyurethane crosslinkers (BPUCs) were synthesized from the reaction of toluene 2,4-diisocyanate (TDI), isophorone diisocyanate (IPDI), polycaprolactone (PCL), 1,1,1-trimethylolpropane (TMP), and methyl ethyl ketoxime (MEKO). Synthesis of the accurate kind of BPUCs was confirmed by Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (1H NMR), and gel permeation chromatography (GPC). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to determine the deblocking temperatures of the BPUCs. The thermal analysis revealed that both TDI- and IPDI-based BPUC had the proper initial deblocking temperature (<160 °C) and the maximum deblocking temperature (<200 °C) for practical applications. Compared to IPDI-based BPUC, the TDI-based BPUC had a lower thermal dissociation temperature and a faster deblocking rate. Hydroxyl polyurethane (HPU) was introduced to study the crosslinking effect of prepared BPUCs. The reaction proceeded at various deblocking temperatures in different curing times. It was noticed that the elastic modulus and tensile strength of the HPU sample increased whereas elongation at break decreased with the addition of BPUC in comparison with pure HPU, which suggested better interfacial adhesion due to the strong chemical reaction between the released NCO groups from BPUC and hydroxyl groups from HPU. In addition, improvement on water resistance of the BPUC modified HPU samples compared to pure HPU samples also demonstrated the good crosslinking effect of prepared BPUCs. Open image in new window

Synthesis and characterization of reactive blocked‐isocyanate coupling agents from methyl ethyl ketoxime, ethyl cellosolve/ε‐caprolactam blocked aromatic and aliphatic diisocyanates

AbstractA series of water‐dispersible blocked polyisocyanates were synthesized from toluene 2,4‐diisocyanate (TDI), isophorone diisocyanate (IPDI), dimethylol propionic acid, methyl ethyl ketoxime (MEKO), ethyl cellosolve (EC), and ϵ‐caprolactam (CL). The physical properties, such as the viscosity, pH, and storage stability, of the blocked‐polyisocyanate adducts were measured. All aqueous dispersions of the blocked polyisocyanates showed good storage stability. The prepared blocked polyisocyanates were characterized by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry, and thermogravimetric analysis techniques. The FTIR confirmed that the NCO groups of the original TDI and IPDI molecules were completely blocked by the blocking agents. The thermal analysis measurements revealed that both the blocked TDI‐ and IPDI‐based polyisocyanates started to deblock at about 55–85°C. Compared to the CL‐blocked polyisocyanates, the MEKO‐ and EC‐blocked polyisocyanates had lower thermal dissociation temperatures and faster deblocking rates. We also found that the initial deblocking temperatures of the TDI‐based adducts were lower than those of the IPDI‐based adducts. The water resistance and tensile properties of the composite films from the blocked‐polyisocyanate crosslinkers and hydroxyl–polyurethane emulsion (HPUE) matrix were studied. The tensile strength increased and the elongation at break were lower in the composites compared to the pure HPUE film. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Amine‐blocked polyisocyanates. I. Synthesis of novel N‐methylaniline‐blocked polyisocyanates and deblocking studies using hot‐stage fourier transform infrared spectroscopy

AbstractA series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, 1H NMR, and 13C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature 1H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007

Thermal decomposition behavior of blocked diisocyanates derived from mixture of blocking agents

To improve the performance and reduce raw material costs, blocked isocyanates were prepared with mixture of blocking agents in many industries. Three blocked isocyanates (adducts) namely e-caprolactam/benzotriazole-blocked 4,4′-diphenylmethane diisocyanate (MDI), toluene-2,4-diisocyanate (TDI) and 4,4′-dicyclohexylmethane diisocyanate (H12MDI) were synthesized. Six reference adducts were also prepared by blocking MDI, TDI, and H12MDI with e-caprolactam (e-CL) or benzotriazole. The reactions were carried out in acetone medium and dibutyltin dilaurate (DBTDL) was used as a catalyst. The progress of the blocking reaction was monitored by IR spectroscopy. De-blocking temperatures (dissociation temperatures) of these adducts were studied using DSC and TGA and the results were correlated. As expected, the thermal analysis data showed that de-blocking temperature of blocked aromatic isocyanates was lower than that of the blocked aliphatic isocyanates. The low de-blocking temperature of blocked aromatic isocyanate could be due to electron withdrawing benzene ring present in the blocked isocyanates. It was also found that benzotriazole-blocked adducts de-blocked at higher temperature compared with e-CL-blocked adducts.

Chain extension studies of water-borne polyurethanes from methyl ethyl ketoxime/ɛ-caprolactam-blocked aromatic isocyanates

A series of water-based blocked aromatic polyurethane dispersions (BPUDs) were prepared by prepolymer mixing process, using toluene 2,4-diisocyanate (TDI), 4,4′-di- p-phenylmethane diisocyanate (MDI), poly (oxytetramethylene) glycol (PTMG), dimethylol propionic acid (DMPA), methyl ethyl ketoxime (MEKO) and ɛ-caprolactam (CL). Particle size, viscosity, pH, molecular weight, storage stability, and de-blocking temperature of the BPUDs were measured and compared. Chain extension of BPUD was carried out with three types of chain extenders, hexamethylene diamine (HMDA), isophorone diamine (IPDA) and tetraethylene pentamine (TEPA), at various de-blocking temperatures. Tensile and thermal properties of the chain extended BPUD films were investigated and results confirmed that MEKO-BPUDs de-blocked at lower temperature than ɛ-caprolactam-BPUDs. The T g values of the chain extended BPUDs were higher than those of the BPUDs and pure PTMG. Improvement in the thermal stability confirmed the de-blocking and chain extension reaction of blocked PU prepolymers.

Synthesis and characterizations of silylated polyurethane from methyl ethyl ketoxime-blocked polyurethane dispersion

Water-dispersible blocked polyurethane dispersions (BPUD) were synthesized by prepolymer mixing process using toluene 2,4-diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), poly(tetramethylene glycol) (PTMG), dimethylol propionic acid (DMPA), and methyl ethyl ketoxime (MEKO). The particle size, viscosity, pH and storage stability of the BPUDs were studied and compared. The aqueous dispersions were characterized by FT-IR, GPC, DSC and TGA techniques. De-blocking temperatures of the BPUDs were measured and end-capped with phenylamino propyl trimethoxysilane (PAPTMS) at different de-blocking temperatures. The thermal analysis revealed that both MDI- and TDI-based BPUDs started to de-block at about 60–85 °C. The average molecular weights of the MDI-BPUDs were higher than that of the TDI-BPUDs due to the high reactivity of MDI. It was noticed that the tensile strength increased and elongation at break decreased in the silylated BPUD compared to pure BPUDs, which confirmed that the BPUDs were de-blocked and end-capped with PAPTMS. The T g values of the silylated BPUD were higher than the BPUD and pure PTMG as well as thermal stability. Storage stability results showed that all BPUDs containing PAPTMS were stable. Water and xylene resistance tests and gel content studies confirmed that silylated PU cross-linked well after silylation of blocked PUDs.

Synthesis and deblocking of cardanol‐ and anacardate‐blocked toluene diisocyanates

AbstractMethyl anacardate and secondary butyl anacardate were prepared from anacardic acid and corresponding alcohols and were used, in addition to cardanol, as blocking agents for 2,4‐toluene diisocyanate (TDI). Blocked diisocyanate adducts were characterized via nitrogen estimation, Fourier transform infrared spectroscopy, and proton nuclear magnetic resonance spectroscopy. The deblocking temperatures of the adducts were determined using an FTIR spectrophotometer in conjunction with the carbon dioxide evolution method. The gel times of hydroxyl‐terminated polybutadiene–TDI adducts also were determined. Deblocking temperature and gel time analyses revealed that cardanol‐blocked 2,4‐TDI deblocks at a lower temperature and at a higher rate compared with anacardate‐blocked adducts. In addition, it was found that the electron‐withdrawing ester group reduces the deblocking temperature of the adduct only when it is in solvated form. All adducts were waxy solids that were found to be soluble in polyether polyol, polyester polyol, and polyhydrocarbon polyols. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4047–4055, 2004

Synthesis and dissociation of amine‐blocked diisocyanates and polyurethane prepolymers

Substituted N-methylanilines are shown to act as blocking agents for toluenediisocyanate. N-methylaniline-, N-methyl-p-anisidine- and N-methyl-p-nitroaniline-blocked toluene diisocyanates have been prepared and characterized by FTIR, 1H NMR and 13C NMR spectroscopies, and nitrogen content analysis. A new method for determining the minimum deblocking temperature of the blocked isocyanate is described. The method has advantages in that it can be used to find the minimum deblocking temperature of even non-volatile blocking agents. The minimum deblocking temperature of the adducts is found to be in the following order: N-methyl-p-anisidine–TDI adduct < N-methyaniline–TDI adduct < N-methyl-p-nitroaniline–TDI adduct. The anilines exhibit the same trend when they block a polyurethane prepolymer prepared using polypropylene glycol of molecular weight 2000 g mol−1 and tolylene-2,6-diisocyanate. The deblocking temperatures are lower in the case of blocked prepolymers than in the blocked adducts. The blocked adducts and prepolymers are reacted with pyromellitic dianhydride (PMDA) in dimethylpropylene urea (DMPU) and the evolution of carbon dioxide is monitored to study the completion of imidization. The reaction time is in accordance with the deblocking ability of the adducts. The regeneration of the blocking agent is confirmed by gas chromatography. © 2002 Society of Chemical Industry

Synthesis and dissociation of amine‐blocked diisocyanates and polyurethane prepolymers

AbstractSubstituted N‐methylanilines are shown to act as blocking agents for toluenediisocyanate. N‐methylaniline‐, N‐methyl‐p‐anisidine‐ and N‐methyl‐p‐nitroaniline‐blocked toluene diisocyanates have been prepared and characterized by FTIR, 1H NMR and 13C NMR spectroscopies, and nitrogen content analysis. A new method for determining the minimum deblocking temperature of the blocked isocyanate is described. The method has advantages in that it can be used to find the minimum deblocking temperature of even non‐volatile blocking agents. The minimum deblocking temperature of the adducts is found to be in the following order: N‐methyl‐p‐anisidine–TDI adduct &lt; N‐methyaniline–TDI adduct &lt; N‐methyl‐p‐nitroaniline–TDI adduct. The anilines exhibit the same trend when they block a polyurethane prepolymer prepared using polypropylene glycol of molecular weight 2000 g mol−1 and tolylene‐2,6‐diisocyanate. The deblocking temperatures are lower in the case of blocked prepolymers than in the blocked adducts. The blocked adducts and prepolymers are reacted with pyromellitic dianhydride (PMDA) in dimethylpropylene urea (DMPU) and the evolution of carbon dioxide is monitored to study the completion of imidization. The reaction time is in accordance with the deblocking ability of the adducts. The regeneration of the blocking agent is confirmed by gas chromatography.© 2002 Society of Chemical Industry