Free Isocyanates Research Articles

Improving the mechanical, thermal and surface properties of polyaspartic ester bio-based polyurea coatings by incorporating silica and titania

Polyaspartic ester polyurea coatings are known for their eco-friendly properties due to the absence of VOCs and free isocyanates. While these coatings offer many advantages to the industry, achieving durability often requires the use of trimers with high NCO content. In such cases, applicators face challenges due to their notably short pot life. Furthermore, the second component used is a petroleum-based polyisocyanate, which limits its classification as a sustainable material. In this study, bio-based polyisocyanates were employed in polyaspartic coatings for the first time, thereby positioning them as sustainable alternatives without compromising their conventional properties. Fourier Transform Infrared Spectroscopy (FT-IR) and Nuclear Magnetic Resonance Spectroscopy (13C and 1H) were utilized to characterize the structure of polyaspartic ester coatings derived from biobased polyisocyanates along with solvent-free and catalyst-free PAEs. It was determined that the coating formulation had a convenient working time at room temperature. Pendulum hardness and contact angle measurements indicated that hard coatings with hydrophobicity were obtained with the addition of SiO2 and TiO2 particles without reducing the working time. The results revealed a notable 37% increase in tensile strength with the addition of 3 wt% silica compared to the base coating, and a remarkable 43 wt% increase when the coating obtained an equal proportion of titanium and silica at the same concentration. Additionally, Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) studies revealed that the composite coatings comprising titanium and silica particles exhibited higher glass transition temperatures and residual char yields compared to the base coating. The morphology and surface roughness of the coatings were characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM).

Chemical characteristics and sources of nitrogen-containing organic compounds at a regional site in the North China Plain during the transition period of autumn and winter

Organic nitrogen constitutes a significant fraction of the nitrogen budget in particulate matter (PM). However, the composition and sources of nitrogen-containing organic compounds (NOCs) in PM remain unclear currently in North China Plain (NCP), China. Rare local or regional studies on NOCs were conducted. In this study, ambient fine particles (PM2.5) were collected in Xianghe, a regional background site in NCP, from 26 October to 26 December 2017. The insights from this study include NOC molecule identification, concentration level, and NOC sources and origins. Specifically, we have identified and quantified >90 NOC species, with urea being the most abundant, accounting for 39.7 ± 4.7% of the total NOC followed by free amino acids (FAAs; 21.9 ± 1.5%), cyclic NOCs (15.3 ± 4.5%), amines (14.8 ± 1.5%), alkyl amides (5.8 ± 0.5%), isocyanates (1.7 ± 0.2%), and nitriles (1.1 ± 0.2%). The time series of FAAs was well correlated (r = 0.51–0.68, p < 0.01) with the organic marker of levoglucosan and was moderately correlated with Ox (r = 0.29–0.41, p < 0.01), suggesting biomass burning and secondary formation were important FAAs sources. We also show that amines can be oxidized and/or reacted by aqueous-phase processing to form secondary aerosols, which are further enhanced by the involvement of iron in the catalytic process. Using the receptor model of positive matrix factorization (PMF), six factors were identified including coal combustion, crustal sources, biomass burning, industry-related sources, traffic emissions, and secondary aerosols. Source apportionment of NOC shows biomass burning was the dominant factor, accounting for 31.8% of the total NOCs. This study provides a unique dataset of NOCs at this regional background site in the NCP, with the insights of NOC chemical composition and sources gained in this study being important for future NOC modeling as well as NOC health effects studies.

Poly(hydroxyurethane) Adhesives and Coatings: State-of-the-Art and Future Directions.

Polyurethane (PU) adhesives and coatings are widely used to fabricate high-quality materials due to their excellent properties and their versatile nature, which stems from the wide range of commercially available polyisocyanate and polyol precursors. This polymer family has traditionally been used in a wide range of adhesive applications including the bonding of footwear soles, bonding of wood (flooring) to concrete (subflooring), in the automotive industry for adhering different car parts, and in rotor blades, in which large surfaces are required to be adhered. Moreover, PUs are also frequently applied as coatings/paints for automotive finishes and can be applied over a wide range of substrates such as wood, metal, plastic, and textiles. One of the major drawbacks of this polymer family lies in the use of toxic isocyanate-based starting materials. In the context of the REACH regulation, which places restrictions on the use of substances containing free isocyanates, it is now urgent to find greener routes to PUs. While non-isocyanate polyurethanes (NIPUs) based on the polyaddition of poly(cyclic carbonate)s to polyamines have emerged in the past decade as greener alternatives to conventional PUs, their industrial implementation is at an early stage of development. In this review article, recent advances in the application of NIPUs in the field of adhesives and coatings are summarized. The article also draws attention to the opportunities and challenges of implementing NIPUs at the industrial scale.

Use of functionalized boehmite nanoparticles to improve the hardness and tribological properties of polyurethane films

Plate-like boehmite nanoparticles (BH) produced from aluminum hydroxide by hydrothermal process were functionalized in one step with two different diisocyanates. The amount of free isocyanates that were available for polymerization reaction was determined to be higher in functionalization with the aromatic diisocyanate (diphenylmethane-4,4′-di-isocyanate – MDI). In composite film production MDI functionalized BH (MDI-BH) was used. Polyurethane based nanocomposite films were produced through polymerization of non-functionalized and MDI-BH with two different polyester-polyols that were synthesized by the esterification of 1,4 butanediol with either adipic acid or phthalic anhydride. It was impossible to form films suitable for hardness and tribological tests with non-functionalized BH. Up to 1 wt% MDI-BH additions were effective in increasing the hardness and scratch resistance of films. The increases in abrasion resistance were more significant and followed the increasing trend for MDI-BH additions even up to 5 wt%. The highest increase, which was 400% with respect to the unmodified resin was observed with adipic acid based polyols and this result was obtained at MDI-BH content of 3 wt%.

Statistical experimental design screening strategies for free monomeric isocyanates determination by UPLC in materials used in cork stoppers manufacturing

&lt;p&gt;A statistical experimental design was used to screen variables of the analytical procedure to quantify free monomeric isocyanates presented in polyurethane based pre-polymers in trace amounts.&lt;/p&gt;&lt;p&gt;For this purpose, diphenylmethane-4,4’-diisocyanate (4,4’-MDI), 2,4-toluene diisocyanate (2,4-TDI) and 2,6-toluene diisocyanate (2,6-TDI) were analysed by Ultra Performance Liquid Chromatography with a Photo Diode Array detector (UPLC-PDA). A preliminary study was performed with three derivatization agents, being 1-(2-piridyl) piperazine (1,2-PP) the most suitable one. Column temperature, flow and percentage of ammonium acetate (% NH4Ac.) were the factors studied at two levels each. A sequence of experiments was planned according to a 23 full factorial design with three replicates and two repetitions. Analysis of variance (ANOVA) was applied for the identification of significant factors and interactions.&lt;/p&gt;&lt;p&gt;Higher responses were achieved when the column temperature was 30 °C, a flow of 0.3 mL/min and a solvent with a percentage of ammonium acetate of 0.1 %. Figures of merit were assessed within-laboratory as a preliminary step for method validation. Similar values were obtained for TDI and MDI. Recoveries are approximately 100 %. In addition, the values of detection limits (LODs) for MDI and TDI were 0.08 and 0.11 microgram/mL, respectively, and quantification limits (LOQs) were 0.25 and 0.33 microgram/mL for MDI and TDI, respectively. The working range was between 0.01 and 10.00 microgram/mL for MDI and 0.01 – 4.95 microgram/mL for TDI. These figures of merit seemed adequate to detect low amounts of free monomeric isocyanates presented in agglomerates and foams for agglomerated cork stoppers production. This data is suitable to address the optimization of an analytical method by a response surface methodology.&lt;/p&gt;

Degradation of polyester polyurethane by an indigenously developed consortium of Pseudomonas and Bacillus species isolated from soil

This work investigated the degradation efficiency of bacterial co-culture against polyester polyurethane in comparison to the individual strains. For this purpose, a consortium of two already reported polyester polyurethane (PU) degrading bacterial strains Bacillus subtilis MZA-75 and Pseudomonas aeruginosa MZA-85 was developed. The degradation was analyzed through measurement of weight loss, scanning electron microscopy (SEM), fourier transformed infra-red (FT-IR) spectroscopy and Sturm test (CO2 evolution test). Polyester PU degradation was augmented in the presence of co-culture, as indicated by maximum weight reduction in Polyester PU film pieces within 20 days. SEM micrographs showed surface changes in the polyester PU films in the form of pits or holes that resides bacterial cells. Besides degradation of soft segment, a new peak at 2250 cm−1 appeared in FTIR spectrum, which corresponds to free isocyanates due to breakdown of urethane linkages. The quantity of carbon dioxide evolved in Sturm test was much higher due to high degradation rate in co-culture vessel as compared to control as well as the separate cultures. A sharp increase in extracellular esterase activity of co-culture against polyester PU was observed. Butanediol (BD) and Adipic Acid (AA) were detected as degradation products in relatively higher intensity than those observed in pure culture, through gas chromatography-mass spectrometry (GC-MS). Hence, it is concluded that consortium might be better than pure cultures in polyester PU waste management and used as a tool for recycling purified monomers (biochemical depolymerisation/monomerization).

Perfume‐containing polyurea microcapsules with undetectable levels of free isocyanates

AbstractCore‐shell polyurea microcapsules with a 40% fragrance load were prepared by interfacial polymerization of guanidine and a technical polyisocyanate prepolymer containing mainly the biuret trimer derived from hexamethylene di‐isocyanate (HDI). Residual free polyisocyanates were still present at a level slightly above 100 mg NCO functional group per kg as determined by liquid chromatography hyphenated with tandem mass spectrometry of HDI and of its biuret trimer. This level was decreased by a factor of about 10 when the polymerization process was allowed to proceed for a longer time and by a factor of about 500 when guanidine or NaOH were added to the microcapsule suspension to act as scavengers. In these cases, polyisocyanate conversion was observed to proceed for about one month when the microcapsules were stored at room temperature before reaching a plateau at a level below 1 mg NCO/kg. Overall, ammonia was the most efficient polyisocyanate scavenger as no residual HDI biuret trimer and only less than 2 μg NCO/kg as HDI were detected at the end of the process, a level which had dropped below the limit of detection of 0.25 μg NCO/kg after about 40 days of aging at room temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Nanoencapsulation of blocked isocyanates through aqueous emulsion polymerization

Blocked isocyanates were successfully encapsulated into polystyrene and hydroxyl and amine functionalized polymeric nanospheres via emulsion polymerization. The nanocapsules were characterized via Fourier transform infrared spectroscopy, differential scanning calorimetry and transmission electronic microscopy. The blocked isocyanates generated free isocyanate functionality upon thermal annealing of nanocapsules. This research establishes a novel encapsulating method for release and retention of free isocyanates in aqueous media. These nanocapsules can provide active isocyanates in coatings and adhesive applications, and represent a novel application of nanoencapsulated materials for controlled or delayed active material utilization.

Compatibilized blends of thermoplastic polyurethane (TPU) and polypropylene

AbstractCompatibilized blends of thermoplastic polyurethane (TPU) and polypropylene (PP) were developed using amine (primary or secondary) functionalized PP's (PP‐g‐NH2 or PP‐g‐NHR). The strategy of reactive compatibilization is based on fast reactions between amine functional groups and urethane linkages or traces of free isocyanates released by thermal degradation of TPU. Excellent compatibilization between TPU and PP was confirmed by rheological, morphological, and mechanical properties. Much finer domain size, higher interfacial adhesion, and more stable morphologies were clearly observed by scanning electron microscopy. Significant improvements in the overall mechanical properties (tensile, tear, abrasion) imply significantly more reaction between TPU and PP phases in the two TPU/PP blends containing PP‐g‐NH2 or PP‐g‐NHR than a TPU/PP blend using PP‐g‐MA as a compatibilizing agent.

Kinetic study on the reaction of 2,4‐ and 2,6‐tolylene diisocyanate with 1‐butanol in the presence of styrene, as a model reaction for the process that yields interpenetrating polyurethane–polyester networks

AbstractA classical method was utilized for determination of free isocyanates to study the kinetics of a model reaction of 2,4‐ and 2,6‐tolylene diisocyanate (TDI) with 1‐butanol, which was run in the reaction medium of liquid aliphatic hydrocarbons. Such conditions made it possible to find rate constants and activation energy values for the second‐order reactions under isothermal conditions. Those reactions can be employed to produce isocyanate prepolymers, which make intermediates for downstream polyurethanes. The findings are commented upon against the background of earlier kinetic data that can widely be found in the research reports published. Further, part of our work utilized the findings to provide interpretation of kinetic parameters for analogous reactions, the kinetic data being found in the presence of styrene—the monomer employed when producing polyurethane–polyester interpenetrating polymer network (IPN) systems. The hypothesis was put forward that styrene and isocyanate component(s) could react at the final stages of the process when the system becomes short in the hydroxyl substrate. That hypothesis was based on disturbances observed in the reactivity of NCO groups (in 2,4‐TDI) in the reaction system studied. The rate constants and activation energy values were determined and presented for the urethanization reaction of 1‐butanol. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 327–336, 2003

Fabrication and characterization of polyurethane-coated fabrics for collapsible diesel tanks

Polyurethanes synthesized from diols of low, medium and high molecular weight ( Mw = 1000, 2000 and 3000 respectively) with low free isocyanates were used to fabricate coated fabrics. The physical, chemical and mechanical properties of these coated fabrics and polymer films were studied to establish their suitability for fabricating collapsible fuel tanks to store diesel for long periods under adverse conditions. Fabrics coated with low molecular weight polyurethanes are found to be more suitable for storing diesel fuel compared with those coated with medium and high molecular weight polyurethanes.

Blocked isocyanate. Reaction and thermal behaviour of the toluene 2,4‐diisocyanate dimer

AbstractDue to the difference in reactivity between free isocyanates and uretdione rings, diisocyanate dimers can be used as blocked isocyanates. They offer the potential advantage that no volatile by‐products are formed. The toluene 2,4‐diisocyanate dimer contains one uretdione ring, formed by dimerization of two isocyanate groups, and two free isocyanate groups. The latter react rapidly with the hydroxy groups of a polycaprolactone diol at temperatures above 80°C. The pure dimer thermally dissociates above 150°C; dissociation is reversible. In the presence of OH groups, the regenerated isocyanate groups can then react to form urethanes. Our experiments have also revealed the formation of allophanate groups as reaction products of an uretdione ring with a hydroxy group. The influence of initial molar ratio of reactants and of temperature on the mechanisms have been discussed. Allophanates are not stable at high temperature in presence of remaining hydroxy groups, their dissociation leads to urethanes. Application of branching theory has shown that the system can gel due to allophanate formation in a wide range of initial molar ratios of NCO to OH groups.

Late transition metal .mu.-oxo and .mu.-imido complexes. Part 3. Sequential insertion of carbon monoxide into rhodium-nitrogen .mu.-imido/amido bonds. Formation of isocyanate and dimetallocycloimide complexes

Catalytic carbonylations of organoazides and nitroaryls are frequently postulated to proceed via metal imido intermediates. In support of this proposal imido complexes have been isolated from reactions of metal complexes with organoazides, nitroaryls, and nitrosoaryls. In the catalytic processes the formation of nitrogen-carbon bonds is a key step. In most cases, isocyanate (formed by coupling of the imido ligand and CO) is the final product or is a probable precursor to the final product. However, until recently the isolated imido complexes have given little, if any, isocyanates or products derived from isocyanates when treated with CO under mild conditions. This communication reports the facile formation of isocyanate and dimetallocycloimide complexes and free isocyanates and ureas by the formal single and double insertion of CO into the metal-nitrogen bonds of rhodium {mu}-imido/amido A-frame complexes.

Thermal degradation of urethanes based on 4,4′-diphenylmethane diisocyanate and 1,4-butanediol (MDI/BDO)

Differential scanning calorimetry (d.s.c.) and gel permeation chromatography (g.p.c.) were used to study the thermal degradation of 4,4′-diphenylmethane/1,4-butanediol based monodisperse urethane model compounds. It was found that the urethane bond was unstable above 170°C in the crystalline state as well as in the molten liquid state. Significant degradation occurred above 200°C. The annealing time was found to be as important as the annealing temperature. The degradation species present were identified by g.p.c. using an absolute molecular weight-retention volume calibration curve. Polymerization as well as depolymerization occurred during the degradation process, so that the molecular weight distribution tended to broaden. The degradation mechanism was found to be thermal dissociation of the urethanes into free isocyanates and alcohols as confirmed by infra-red spectroscopy and gas liquid chromatography. The degradation rate fits well with first order kinetics with respect to the urethane. The triblocks and oligomeric multiblocks of MDI/BDO/poly(propylene oxide) polyol/polyurethanes were also found to be unstable at elevated temperature.

An Improved Adhesive System for Textile-Reinforced Rubber Products

Abstract Resorcinol Formaldehyde Latex (RFL) adhesive systems have been used for over forty years to bond synthetic fibers to rubber compounds. The original RFL formulation was developed by Charch and Maney. The adhesive was applied primarily to rayon fibers and later extended to nylon and glass fibers. At the same time, the formulation evolved with the introduction of synthetic latexes and preformed resorcinol formaldehyde resin solutions. The RFL treated fibers are used to reinforce rubber products (e.g., tires, conveyor belts, hoses, V-belts). With the commercialization of polyester fibers, it was found that the RFL system did not satisfactorily adhere polyester to rubber compounds. The difficulty in bonding to polyester fibers has been attributed to the lack of reactive sites in the polymer structure, as well as its hydrophobic nature. New adhesive systems were developed which consisted of free isocyanates in solvent solutions. These systems were undesirable due to the associated health and fire risks. As a result, the use of aqueous dispersions of blocked isocyanates was investigated. Blocked isocyanates consist of isocyanate molecules which are chemically reacted with a blocking agent to prevent reactions with other materials at room temperature conditions. At higher temperatures, the blocking materials dissociate to free the reactive isocyanate groups. The blocked isocyanates are generally available in solid form and are chemically stable in water. Common blocking materials include phenols, oximes, and lactams. Shoaf developed a two-step, water based, adhesive system that effectively bonded polyester to rubber. This system consists of a first-step dip application which contains a polyepoxide and blocked aromatic polyisocyanate. In the second step, the treated fiber is overcoated with an RFL mixture. The reaction product of the polyepoxide and polyisocyanate has been identified as a polyurethane structure which physically adheres to polyester. Iyengar attributed this physical interaction to the similarity between the cohesive energy density values for the polyester and polyurethane structures.

Immunologic studies of hollow-fiber dialyzer extracts.

Testing failed to detect free isocyanates in dialyzer rinses or significant levels of immunoglobulin E antibodies to isocyanate-protein conjugates in the serum of individuals reacting during hemodialysis with dialyzers containing Cuprophan membranes. Extended immunization of rabbits with Limulus lysate-reactive material from dialyzer rinses demonstrated antigenic activity of the rinse that appears to originate from cotton linters used in hollow-fiber membranes. The possibility that an immunologic response to these antigens may be a mechanism in adverse reactions to hemodialysis is discussed.

Summary

Summary

Application of electrochemical detection to the measurement of free monomeric aromatic and aliphatic isocyanates in air by high-performance liquid chromatography

A method based on high-performance liquid chromatography (HPLC) with electrochemical detection is described for the determination of aromatic and aliphatic isocyanates in air. 1-(2-Methoxyphenyl)piperazine is used as the electrogenic reagent. It forms electrochemically active derivatives with all of the isocyanates of occupational hygiene interest that are readily oxidised at carbon electrodes. In addition, these derivatives are strongly absorbing in the ultraviolet region, enabling a combination of electrochemical and ultraviolet detectors to be employed to provide additional qualitative information if required.Comparison is made between the electrochemical detection method and HPLC-ultraviolet detection methods for isocyanate analysis. The electro-chemical detection method was found to be significantly more sensitive than the ultraviolet detection methods for the determination of phenyl isocyanate, toluene diisocyanate, hexamethylene diisocyanate and (4,4′-diisocyanatodiphenyl)methane in air.

Decomposition of polyurethane in a garbage landfill leakage water and by soil microorganisms

The decomposition of polyurethane, measured gravimetrially or using infrared spectrophotometry, was found to be more complete in polyurethane based on polyester and only very small in polyurethane based on polyether. In the presence of clay minerals the decomposition was inhibited. If positive, the decomposition of polyurethane followed the sequence: remaining free isocyanates→urea and amide groups→urethane groups→isocyanuric acid rings.

Behaviour of Urethane Adhesives on Rubber Surfaces

Abstract The paper presents the results of chemical investigations into some of the bonding problems of the footwear industry and pays particular attention to the importance of the substrate surfaces. The importance of the presence of metal soaps on rubber surfaces and their detrimental effect on adhesion is pointed out together with the concomitant beneficial effects of solvent soap dispersing treatments on subsequent adhesion. The special needs of the footwear industry for a single adhesive system capable of adhering strongly to a wide range of substrates led to the requirement of improving the bond of urethane adhesives to rubber surfaces. The role of free isocyanates in promoting this bond is outlined. The practical advantages of halogenating rubber surfaces in conferring excellent adhesion properties on moulded rubber surfaces when used with solvent urethane adhesives are pointed out. The observed phenomena associated with halogenation are discussed qualitatively in relation to proposed theories of a...