NCO Content Research Articles

Influence of the soft segment length and content on the synthesis and properties of isocyanate-terminated urethane prepolymers

The synthesis and structure–property relationships of one-component urethane adhesives were studied. NCO-terminated urethane prepolymers were prepared from monomeric diphenylmethane-4,4′-diisocyanate, difunctional polypropyleneglycol polyols and their mixtures. The syntheses were monitored by in-line FTIR-ATR spectroscopy. The rate of the urethane group formation increased with increasing free isocyanate (NCO) concentration in the bulk. The average molecular weight and Brookfield viscosity of the urethane prepolymers increased with decreasing NCO/OH ratio, at fixed polyol composition. When prepolymers contained longer polyol segments, the number average molecular weight increased, while Brookfield viscosity decreased due to higher content of soft segments, which made urethane chains more flexible. The prepolymers’ average molecular weights affected their glass transition temperature values, particularly when prepolymers had shorter soft segments. With increasing content of polyol with higher molecular weight and decreasing content of polyol with lower molecular weight, the glass transition temperature decreased. At lower NCO/OH ratios the rigidity and stiffness of the adhesive bond in wood-to-wood specimens increased with increasing free NCO content, whereas beyond a critical free NCO content, high stiffness was responsible for decrease in adhesion strength. The cure time of adhesives increased with increasing polyol molecular weight and decreasing free NCO content.

Thermal and dynamic mechanical characterization of polyurethane–urea–imide coatings

AbstractA series of NCO terminated polyurethane (PU)–imide copolymers were synthesized by a systematic three‐step process and were chain extended with different diol/diamine chain extenders. In the first step, isocyanate terminated PU prepolymers were prepared by reacting soft segments such as polyester (PE) polyols and polyether polyols such as polypropylene glycol (PPG‐1000) with hard segments like 2,4‐tolylene‐diisocyanate (TDI) or isophorone‐diisocyanate (IPDI) with NCO/OH ratio 2:1. In the second step, thermally stable heterocyclic imide ring was incorporated using isocyanate terminated PU prepolymers by reacting with pyromellitic dianhydride (PMDA) in a excess‐NCO:anhydride ratio of 1:0.5. The surplus NCO content after imidization was both moisture cured or partially reacted with chain extender and moisture cured. The films were characterized by thermogravimetric (TG), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) instruments. The adhesion strength of these coatings on mild steel (MS), copper (Cu), and aluminum (Al) is dependent on the nature of the substrate. The TGA analysis show good thermal stability. The DMTA results show the microphase separation between the different hard and soft segments. Finally, a structure to property correlation was drawn based on the structure of the soft, hard, and chain extender and the observed properties are useful for understanding and design of intelligent coatings. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3158–3167, 2006

Synthesis, Characterization of a Novel Light Stable Cross-Linker—Isophorone Diisocyanate Terpolymer Composite

Isophorone diisocyanate (IPDI) is an alicyclic isocyanate, in which the outcomes always possess good light and heat stability. A novel IPDI terpolymer composite were synthesized and characterized using Fourier transform infrared (FTIR), Nuclear magnetic resonance (NMR), gas permeation chromatography (GPC), and the chemical analysis method. The result showed that IPDI terpolymer composite could be prepared in the presence of Cat-5 catalyst and under the reaction temperature between 70 ∼ 90°C. The colorless or pale yellow liquid products without solvent could be obtained in a short time under optimized reaction conditions. In addition, the IPDI trimer/monomer mixture having an NCO content ranging from 25.0% to 28.0% and average nominal functionality of from 2.5 to 2.8 could be compatible with sufficient reactivity and liquidity.

Thermal and viscoelastic properties of polyurethane-imide/clay hybrid coatings

This paper reports the synthesis and characterization of polyurethane (PU)-imide/clay hybrid coatings based on two types of polyester (PE) polyols (PE-1 and PE-2). PE-1 was prepared from neopentyl glycol (NPG), adipic acid (AA) and isophathalic acid (IPA), whereas PE-2 contains NPG, AA, IPA and TMP (trimethylol propane) with the same hydroxyl value 280 as PE-1. Cetyl trimethyl ammonium bromide (CTAB) modified montmorillonite (K10) was used as the organoclay for the synthesis of the hybrid coatings. The organoclay particles (3 wt%) were well-dispersed into the PE matrix by ultrasonication method. Then the isocyanate terminated PU prepolymers were synthesized by the reaction of polyester polyols with hard segments such as 2,4-toluene diisocyanate (TDI) or isophorone diisocyanate (IPDI) in different NCO/OH ratios e.g., 1.6:1, 2:1 and 3:1, respectively. Finally the thermally stable imide rings were incorporated into the PU backbone by complete reaction of excess NCO content present in the PU prepolymer with pyromellitic dianhydride (PMDA). The thermogravimetric analysis (TGA) shows a higher thermal stability for the PU-imide hybrid coatings with respect to the corresponding PU-imide films. A higher NCO/OH ratio has resulted in higher thermal stability. The activation energies of degradation were calculated by the Broido and Coats–Redfern methods, respectively. The dynamic mechanical thermal analysis (DMTA) results show an enhancement in the glass transition temperature value ( T g) for the clay containing hybrid coatings. The surface analysis by angle resolved X-ray photoelectron spectroscopy (AR-XPS) showed an enrichment of the soft segment towards the surface, and an enhancement in the hard segment composition in the hybrid coatings, resulted in phase mixing.

FT-IR and XPS studies of polyurethane-urea-imide coatings

A series of polyurethane (PU)-urea-imide coatings were synthesized by a systematic three-step reaction process. Initially isocyanate terminated PU prepolymers were prepared by reacting soft segments such as polyester polyols (prepared from neopentyl glycol, adipic acid, isophathalic acid and trimethylol propane) or polyether polyols (polypropylene glycol-1000) with hard segments such as 2,4-toluene diisocyanate or isophorone diisocyanate with NCO/OH ratio of 2:1. Heterocyclic imide ring into the PU backbone was incorporated by co-polymerization with pyromellitic dianhydride (PMDA) from the excess NCO groups in the PU prepolymer with an NCO/anhydride ratio of 1:0.5 and the surplus NCO content after imidization was moisture cured. PU-urea-imide coatings were also obtained by partial chain extension of the excess NCO groups in the NCO terminated PU-imide copolymers, and the remaining excess NCO groups were completely reacted with atmospheric moisture. The obtained polymers were analyzed with Fourier transform-infrared (FT-IR) and angle resolved X-ray photoelectron spectroscopy (AR-XPS). The type and change in intermolecular H-bonding interaction in the PU-urea-imide films with structural variables was identified by deconvolution of the FT-IR spectra using Origin 6.0 software through Gaussian curve-fitting method. The FT-IR analyses of the PU-urea-imide coating films show dependence of phase separation on the nature of chain extender. Surface characterization data from AR-XPS suggests the dependence of phase segregation behaviour on the nature of the chain extender, which also supports the FT-IR observations.

Influence of the free isocyanate content in the adhesive properties of reactive trifunctional polyether urethane quasi-prepolymers

The effect of the free isocyanate (NCO) content on the adhesive properties of urethane quasi-prepolymers was studied. Polyether-based isocyanate ended quasi-prepolymers containing between 13 and 21 wt% NCO were prepared by reacting a trifunctional polypropyleneglycol with polymeric MDI. The quasi-prepolymers were characterized by infrared spectroscopy (IR), proton nuclear magnetic resonance ( 1H-NMR) spectroscopy, Brookfield viscosity, and differential scanning calorimetry (DSC). Immediate adhesion of quasi-prepolymers was studied by means of a probe test performed with a texture analyzer. Adhesion properties of the moisture-cured quasi-prepolymers have been obtained from single lap shear tests of vulcanized styrene–butadiene (R1) rubber/cured quasi-prepolymer joints. The increase in the free isocyanate content produced quasi-prepolymers with lower average chain length and less intermolecular interactions between polymer chains. Due to the presence of free isocyanate molecules, lower Brookfield viscosity values and lower glass transition temperatures were obtained by increasing the free isocyanate content in the quasi-prepolymers. The lower the free NCO content, the higher the cohesion and the immediate adhesion of the quasi-prepolymers because of the increase in the average molecular weight. During debonding, fibrillation was observed in the quasi-prepolymers with free NCO content lower than 20 wt%. Single lap shear strength values of R1 rubber/moisture cured quasi-prepolymers joints showed a particular trend as a function of the increase in the NCO content, which can be ascribed to the opposite trends in the cohesion and adhesion properties of the cured quasi-prepolymers.

Determination of technical grade isocyanates used in the production of polyurethane plastics.

A method for determination of technical grade isocyanates used in the production of polyurethane (PUR) is presented. The isocyanates in technical grade products were characterised as di-n-butylamine (DBA) derivatives using LC-MS and LC-chemiluminescent nitrogen detection (CLND) and the total isocyanate content was compared to a titration assay. For collection of isocyanates in air, an impinger-filter sampling technique with DBA as derivatisation reagent was used. Characterised DBA and nonadeuterium labelled DBA derivatives of isocyanates in technical products were used as calibration standards and internal standards, respectively, in the analysis of air samples. Three workplaces were studied where PUR products were produced either by spraying or by moulding. In both technical products and in air samples, a number of monomeric, oligomeric and prepolymeric isocyanates of e.g. methylenebisphenyl diisocyanate (MDI) and hexamethylene diisocyanate (HDI) were characterised. Several of these have not previously been described in workplace atmospheres. In the technical isocyanate products, between 69 and 102% of the NCO content determined by titration was accounted for by LC-CLND. Quantifications of a wide range of isocyanates in air samples were performed with correlation coefficients in the range 0.988-0.999 (n= 8) and the instrumental detection limits were 0.7-25 pg. At the two workplaces where MDI- and HDI isocyanurate-based products were sprayed, the isocyanate composition in the air reflected the composition in the technical product. At the workplace where a MDI-based product was used in a moulding process, only the monomeric isocyanates were found in the air. The advantage of using characterised technical grade isocyanates as analytical standards was clearly demonstrated and the possibility of using index compounds when monitoring isocyanate exposure is discussed.

Effect of crosslinking on mechanical and viscoelastic properties of semiinterpenetrating polymer networks composed of poly(vinyl chloride) and isocyanate crosslinked networks

Semiinterpenetrating polymer networks (SIPNs) of PVC/isocyanate/poly-triol were prepared by premixing small (150 μm dia.) porous (30% voids) unplasticized PVC particles, 10% by weight of isocyanate, and a triol at different OH/NCO mol ratios. Three types of isocyanates (methylene bis-phenyl diisocyanate (MDI), oligomeric MDI isocyanates (PAPI), and toluene diisocyanate (TDI) prepolymer/polytriol) were used. Two-roll milling was followed by hot-press curing. The tensile, flexural, and impact strengths increased when small amounts of crosslinked isocyanate networks were created in PVC. The isocyanate/polyol hydroxyl stoichiometry was varied, and the effects of crosslinking on the tensile, impact, and flexural strengths of PVC/isocyanate/triol SIPNs were examined. The strength increments were greater when the OH/NCO mole ratio went from 0 to 0.25, than when it went from 0.25 to 1.0. In many cases, increasing OH/NCO mol ratio from 0.5 to 1.0 decreased tensile, impact, and flexural strengths. Both PAPI and MDI (30% NCO content) gave bigger improvements in the these mechanical strengths than the TDI (only 9.7% NCO). These SIPN blends exhibited lower tan δ peak temperatures and a single distinct loss modulus, E″, peak values at lower temperatures than those of PVC that had been exposed to the same processing temperatures. Substantial amounts of isocyanate networks exist in SIPN phases according to DMTA studies. The OH/NCO ratio did not generally correlate with the decreases in the glass transition temperatures in these three sets of blends. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1402–1411, 2000

TSC/RMA study on the depolarization transitions of TDI-based polyurethane elastomers with the variation in NCO/OH content

Thermal behaviors of TDI-based polyurethanes were studied viz. depolarization relaxation transitions by TSC/RMA technique. Two relaxation transitions in T g region were found. One is the T g transition. The other, T global, is the relaxation transition of global nature and dominated by the hard TDI-MOCA segment in an amorphous phase. The observed T g is shifted from the cooperative motion of urethanic matrix dominated by the soft segment to the relaxation transition of the entire global molecular structure. As a result, a merged and widen T g transition range was observed in TSC with supportive observations of a broadened DSC curve for the same specimens. This widened T g transition was further analyzed by thermal windowing technique of RMA. Increases in the following T g temperature ranges of 25°C (−50°C to −25°C), 50°C (−50°C to 0°C), 55°C (−50°C to 5°C) and 60°C (−50°C to 10°C) for 1.6, 2.4, 2.8 and 3.2 NCO/OH ratio specimens were detected in the RMA spectra. The compensation parameters ( T c, log τ c) of polyurethane specimens in order of NCO content are (10.1, −2.85); (39.6, −3.08), (51.8, −3.71) and (85.8, −4.49) for NCO/OH ratios 1.6, 2.4, 2.8 and 3.2, respectively. The T global transition was observed and in association with tangent δ in the DMA measurement.

Study on thermal transitions of toluene diisocyanate-based polyurethane elastomers with poly (tetramethylene oxide) as the soft segment by TSC/RMA, DSC and DMA thermal analyzers

Thermal transitions of TDI-based polyurethane elastomers with PTMO as the soft segment were characterized by the depolarization technique in TSC and by using with the thermal windowing technique on selected specimens in the RMA measurements. Results indicate that the broadened thermal transition in the glass transition region as observed in the DSC thermogram is related to the combined Tg transition and the Tglobal transition in the TSC spectrum. This Tglobal transition is associated with the macromolecular property as detected by tan δ in DMA measurement. The increase in the Tg with a high NCO content may be explained by the structural modification found on the urethanic chain with the additional linkage of the hard segment that affects the cooperative motion of the molecular chain. Data measured from DSC, TSC/RMA and DMA with simulated DEA and wide angle X-ray data are presented for the characterization of the polyurethanes. The RMA measurement leads to a compensation search on Tg transition and provides pertinent thermokinetic data that correlates the NCO content with changes in enthalpy and entropy on the relaxation behaviors in the Tg transition of polyurethane elastomers.

Effect of HTPB structure on prepolymer characteristics and on mechanical properties of polybutadiene-based polyurethanes

Oligobutadiene-based prepolymers and polyurethanes (PU) were prepared from hydroxyl terminated polybutadiene (HTPB), three diisocyanates (aromatic, alicyclic and aliphatic) and two chain extenders (containing hydroxyl or amino groups). The free NCO content in the prepolymers varied systematically and the characteristic of these products were interpreted in terms of NCO group reactivity, propagation reactions and HTPB functionality. The mechanical properties of the polyurethane elastomers derived from the prepolymers were discussed in terms of hard and soft segment structures.

Properties of hydroxyl-terminated polybutadiene-base elastomers under the influence of γ-radiation

Following radiation treatment, a number of polyurethane samples were synthesized from hydroxyl-terminated polybutadiene and toluene diisocyanate: the equivalence ratio of NCO/OH was varied over 0–2.0 and the γ-dose over 0–720 kGy. Understanding of the chemical behavior is attempted through the observation of mechanical properties of the solid cross-linked prepolymer. With lower NCO content the ultimate stress increased with dose and the ultimate strain decreased; cross-linking and urethane formation occurred. With higher NCO, allophanate formation predominated. The stress-strain properties were markedly influenced by the presence of a small fraction of additives. A suitable mechanism is proposed.

Heparin immobilization by surface amplification.

A method to increase the amount and improve the bioactivity of heparin (HEP) immobilized on a polymer surface was developed. The surface of polyurethane-urea (PU) coated glass beads was first modified with diisocyanates, followed by surface grafting of polyfunctional polymers (PFP), including: poly(vinyl alcohol), poly(ethyleneimine), and poly(allylamine). The functional groups of the surface grafted PFP (-OH, -NH, or -NH2) were modified with diisocyanates (TDI) to amplify the surface concentration of isocyanate groups, alpha, omega-diamino-terminated polyethylene oxide (PEO; molecular weight, 4,000 daltons) was then coupled to the surface grafted PFP, and the free amino groups were derivatized with TDI. Finally, HEP was coupled to the amplified surface through free -NCO groups of PU-PFP-PEO. The surfaces were quantified during each step of the procedures for -NCO groups and HEP. All grafted surfaces showed a four to eightfold increase in -NCO content and a twofold increase in immobilized HEP content compared with HEP immobilized directly onto the PU surface. The HEP bioactivity tests (including activated partial thromboplastin time, thrombin times, and factor Xa) demonstrated an increased bioactivity of HEP when immobilized through PFP-PEO compared with PFP and PU alone.

Pultruded fibre reinforced polyurethane composites I. process feasibility and morphology

A feasibility study on the pultrusion of a fibre reinforced polyurethane (PU) composite has been conducted by using a proprietary method. The blocked NCO-terminated PU prepolymer synthesized in this study was prepared from ε-caprolactam blocked prepolymer blended with toluene di-isocyanate (TDI-80) and branched polyester. From the investigations of gel permeation chromatography, IR and 1H nuclear magnetic resonance spectra and viscosity, the molecular weight, degree of blocked reaction and viscosity of the blocked PU prepolymer were obtained. From the mono- n-butylamine back-titration (wet) method, the equivalent weight and NCO content of unblocked NCO-terminated PU prepolymer were determined. A long pot life of blocked NCO-terminated PU prepolymer with various chain extenders, such as cycloaliphatic diamine, aromatic diamine and 1,4-butane diol, was observed in the impregnation temperature range (55–70°C). A high reactivity of prepolymer was found from Brookfield viscometer measurements at elevated temperature. From a morphological study of scanning electron micrographs, it was confirmed that the wetting of fibres by blocked PU resin was complete, and the fibre bundles were evenly distributed in the PU matrices.

Determination of NCO Content of Aromatic Isocyanates

Determination of NCO Content of Aromatic Isocyanates

Study on Isocyanurate-Modified MDI. 1 — Preparation

Isocyanurate-modification of MDI was investigated. Certain modified MDIs which were stable liquids with lower viscosity were obtained. Direct trimerization of pure MDI resulted in crystallization at room temperature. After modification by diol with lower molecular weight, for instance tripropyleneglycol or tetrapropyleneglycol, at a molar ratio to isocyanate of more than 7·5/100, trimerization then produced a stable isocyanurate-modified isocyanate having an NCO content of about 25%. The trimerization of polymeric MDI by a tertiary amine catalyst, such as 2, 4, 6–tris(dimethylaminoethyl) phenol or N, N’, N”-tris(dimethylaminopropyl) hexahydro-s-triazine, followed by adding reaction-stopper and heating, also resulted in stable modified isocyanate with a 25 % NCO content.