Interfacial studies of crosslinked urethanes: Part III. Structure-property relationships in polyester waterborne polyurethanes

These studies examine crosslinking reactions of polyurethanes (PURs) using attenuated total reflectance Fourier-transform (ATR FTIR) spectroscopy and show that higher relative humidity (RH) accelerates the crosslinking reactions leading to the formation of polyurethane and polyurea. Concentration levels of unreacted isocyanate (NCO) are greater at the film-air (F-A) interface than the film-substrate (F-S) interface. In contrast to the previous studies on polyacrylate emulsion urethanes, no stratification was detected between 0.65 to 1.14 μm near the F-A and F-S interfaces. This behavior is attributed to equivalent weight differences, 3100 g/eq for polyacrylate and 1140 g/eq for polyester. Solvent evaporation experiments show that approximately 10% of the initial water concentration remains in the film for extended periods of time, resulting in reactions leading to the formation of urea near the F-S interface. PUR film formation occurs in two stages, a solvent vapor pressure controlled stage, followed by a diffusion controlled stage. The duration of each stage depends on several factors, including the amount of shear induced on the shear thinning waterborne urethanes, which subsequently affects the exposure of isocyanate aggregates to water. Increased RH significantly affects structure-property relationships of waterborne PURs due to urea formation, which alters the glass transition temperature, storage modulus, crosslink density, and film hardness. Dept. of Polymers and Coatings, Fargo, ND 58105.     

Interfacial studies of crosslinked urethanes: Part IV. Substrate effect on film formation in polyester waterborne polyurethanes

These studies examine the effect of substrate surface tension on crosslinking reactions in water-borne polyurethanes (PUR) applied to tin-plated steel, steel, polypropylene (PP), thermoplastic olefin (TPO), and glass. The results show that, relative to tin-plated steel, removal of tin-plating from a steel substrate increases isocyanate (NCO) consumption by 42% near the film-substrate (F-S) interface. When PUR is allowed to crosslink on steel, PP, TPO, and glass, the NCO concentration is greater at the film-air (F-A) interface. Furthermore, crosslinking reactions result in a greater amount of urea-hydrogen bonded species near the F-S interface for all substrates. While an increase of substrate surface tension decreases the amount of urea-hydrogen bonded carbonyl groups near the F-S interface, TPO was found to exhibit different behavior due to talc stratification near the surface. In this case, the presence of talc in WB-PUR coatings or thermoplastic substrates decreases the amount of hydrogen-bonded species and increases NCO consumption.     

Interfacial studies of crosslinked urethanes: Part II. The effect of humidity on waterborne polyurethanes; a spectroscopic study

In this study we examined isocyanate crosslinking reactions at the film-air (F-A) and film-substrate (F-S) interfaces of waterborne two-component (2K) urethanes as a function of depth and relative humidity (RH) using attenuated total reflectance (ATR) FTIR spectroscopy. Analysis of water/isocyanate reactions revealed urea formation which further interacts with water via H-bonding. Quantitative analysis of isocyanate consumption showed that the rates of film formation for waterborne polyurethanes are faster than equivalent solvent-borne polyurethanes. These studies showed that isocyanate consumption increases at high relative humidities. Examination of isocyanate consumption as a function of depth from the F-A and F-S interfaces showed that the isocyanate concentration increases when going from 0.65 to 1.14 μm. Part I of this paper was published in the September 1997 issue of theJournal of Coatings Technology. Department of Polymers and Coatings, Fargo, ND 58105.     

Heterogeneous crosslinking of waterborne two-component polyurethanes (WB 2K-PUR); stratification processes and the role of water

Stoichiometric imbalance and crosslinking conditions during film formation of waterborne two-component polyurethanes (WB 2K-PUR) play a significant role in the development of material properties. Changing isocyanate-to-hydroxyl (NCO:OH) ratios from 1.0 to 2.2 over a range of humidities significantly affects film morphology, and these studies show that while films with higher NCO:OH exhibit increased T_g and surface roughness when crosslinked at high RH, a T_g decrease is observed at elevated RH. Higher RH conditions not only result in increased urea and decreased urethane content, but also facilitate enrichment of poly(ethylene glycol) (PEG) functionality near the film-air (F-A) interface due to stratification of PEG-modified polyisocyanate crosslinkers. Reaction-induced stratification also occurs during film formation resulting in the T_g differences between F-A and F-S interfaces: namely T_g,F-A>T_g,F-S at 32 and 52%RH, whereas T_g,F-A≤T_g,F-S at 75%RH, as determined by interfacial micro thermal analysis (μTA). This behavior is attributed to concentration gradients of water during film formation and their corresponding effects on isocyanate hydrolysis reactions in conjunction with PEG stratification near the F-A interface. Furthermore, excessive stoichiometric NCO:OH imbalance results in the formation of microscopic ‘hills’ and ‘valleys’ on the F-A surface having T_g differences of 6 °C. These phenomenological processes are incorporated into a model describing WB 2K-PUR film formation as a function of crosslinking conditions and resulting morphological features.     

Effect of carbon black on adhesion to plastics in solventborne 2K polyurethanes

These studies show that when the pigment volume content (PVC) of carbon black particles increases in polyurethanes (PUR), NCO consumption increases, and the extent of H-bonded C=O species near the film-substrate (F-S) interface is enhanced. During crosslinking, polyurea (PUA) is produced, and its concentration levels near the F-S interface are diminished for unpigmented coatings, whereas for the same system containing carbon black, the PUA formation is enhanced. Although the presence of carbon black particles at the F-S interface results in diminished adhesion of 2K PUR to acrylonitrile-butadiene-styrene (ABS), adhesion is also affected by the presence of OH and C=O functionalities on ABS, which are potential sites for H—bonding to PUR. Stronger hydrogen donor affinity of N-H functionalities in PUA as compared to the N-H groups on PUR leads to increased H-bonding in the presence of carbon black. Enhanced intermolecular H-bonding in PUR due to the presence of carbon black particles competes with the F-S interfacial H-bonding that promotes adhesion. Presented at the 77th Annual Meeting of the Federation of Societies for Coatings Technology, on October 18–22, 1999, in Dallas, TX. School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, Hattiesburg, MS 39406.     

Kinetics and mechanisms of catalyzed and noncatalyzed reactions of OH and NCO in acrylic polyol–1,6‐hexamethylene diisocyanate (HDI) polyurethanes. VI

Fourier transform infrared spectroscopy was used to study the kinetics of noncatalyzed and catalyzed polyurethanes. These studies show that for noncatalyzed acrylic polyol–hexamethylene diisocyanate (HDI) trimer reactions, the reactions between OH and NCO of HDI exhibit second‐order kinetics, with first‐order kinetics with respect to NCO and OH. On the other hand, when dibutyltin dilaurate (DBTDL) is used as a catalyst in acrylic polyol–HDI trimer reactions, the reaction rate is first order with respect to NCO and 0.5 order in OH and DBTDL concentrations. A mechanism for the catalyzed acrylic polyol–HDI trimer crosslinking reactions is proposed and it appears that an equilibrium involving associations between OH and DBTDL exists, resulting in the formation of an active anion, which interacts with NCO to generate polyurethanes. To further verify this mechanism, the influence of acidity on the reaction rate constant was investigated. When the acidity of the system is increased, retardation of urethane formation occurs. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2322–2329, 2002     

Semi-interpenetrating polymer networks from castor oil-based polyurethanes and chlorinated rubber

Semi-interpenetrating polymer networks (semi-IPNs) were prepared by crosslinking castor oil in the presence of chlorinated rubber. Crosslinking in toluene solution was effected with 2,4-toluene diisocyanate or hexamethylene diisocyanate at various NCO/OH ratios. The semi-IPNs were obtained as tough films and were characterized by their mechanical, thermal, morphological and electrical properties.     

Synthesis and characterization of rubber‐seed‐oil‐based polyurethanes

Novel biobased polyurethanes were synthesized from rubber seed oil (RSO), a renewable resource. The RSO monoglyceride, together with xylene and hexamethylene diisocyanate (HMDI), was employed to synthesize the desired urethane‐based prepolymer with isocyanate (NCO)‐terminated end groups followed by curing. The degrees of crosslinking of the polyurethane after curing were assessed with their swelling behavior. The properties of the resulting polyurethanes were found to be dependent on the type of diisocyanate and their molar ratios to the RSO monoglyceride. The network structures, which were assessed through swelling studies, showed that networks based on HMDI with an NCO/OH ratio of 1.50 were better crosslinked than with those toluene diisocyanate. The thermal properties of the samples analyzed by thermogravimetric analysis showed two and three decomposition stages in aliphatic‐ and aromatic‐based RSO polyurethanes, respectively. The highest stability with initial decomposition temperature (253°C) and percentage residual at 500°C (11.4%) was achieved with an aliphatic‐based RSO polyurethane. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Three‐group type mechanism in the curing behavior of polyacrylate and blocked toluene diisocyanate

Abstract: The curing behavior of two resins, five blocked toluene diisocyanate (TDI) crosslinkers and polyacrylate, was studied using Torsional Braid Analysis (TBA) to examine their dynamic mechanical properties. The curing process was clearly divided into two stages. Associated with data from in situ FTIR at the molecular level, the first stage was the reaction of deblocked NCO and hydroxyl in polyacrylate. In the second stage, the deblocked NCO attacked the hydrogen in NH group of the urethane bond to conduct deep curing. Computer modeling was used to investigate the lowest‐energy configuration of the crosslinker molecule. The space hindrance effect in the molecule causes this type of three‐group crosslinking curing mechanism in the blocked TDI crosslinker. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 112–120, 2002     

Surface and interfacial fourier transform infrared spectroscopic studies of latexes. XV. Orientation of the sodium dioctyl sulfosuccinate surfactant molecules in core/shell-type styrene/butyl acrylate latex films at the film–air and film–substrate interfaces

This study addresses the effect of particle structure and composition on the mobility and orientation of sodium dioctyl sulfosuccinate (SDOSS) surfactant molecules in polybutyl acrylate/polystyrene core/shell-type latex films near the film-air (F-A) and film-substrate (F-S) interfaces. In an effort to determine how the surface tension of the substrate influences the migration and orientation of the SDOSS surfactant molecules, polytetrafluoroethylene (PTFE) and liquid Hg substrates were used. It appears that, as the concentration of styrene monomer in the latex increases, SDOSS migrates toward the F-A and F-S interfaces. As the surface tension of the substrate changes from 18 mN/m in PTFE to 400 mN/m for liquid Hg, the surfactant also migrates toward the two interfaces. For the latex particles composed of 50%/50% styrene/n-butyl acrylate (Sty/n-BA) latex copolymer, the hydrophilic SO⁻₃Na⁺ groups of SDOSS surfactant are present primarily near the F-A interface, and they appear to be mostly parallel to the surface for the films cast on the PTFE surface. For the latex films cast on the liquid Hg surface, the SDOSS hydrophilic surfactant groups are found to be preferentially parallel near the F-A interface and perpendicular near the F-S interface. These studies indicate that the surfactant concentration and its orientation throughout the latex film change as a function of the initial monomer composition and the surface tension of the substrate. Furthermore, the SDOSS concentration appears to vary with depth into the latex interfaces. © 1996 John Wiley & Sons, Inc.     

Interpenetrating polymer networks from castor oil based polyurethanes and poly(ethyl acrylate)

Polyurethanes were obtained by the reaction of hydroxyl groups of castor oil with hexamethylene diisocyanate, isophorone diisocyanate or diphenylmethane diisocyanate using an NCO/OH ratio of 1.6. These polyurethanes were swollen in ethyl acrylate monomer and subsequently polymerized by radical polymerization initiated with benzoyl peroxide in the presence of the crosslinking agent ethylene glycol dimethacrylate. A series of interpenetrating polymer networks (PU/PEA IPNs) were obtained as tough films by casting in glass moulds. The characteristics of these films were determined: resistance to chemical reagents, thermal behaviour (DSC, TGA), tensile strength, Young's modulus, elongation at break (%) and Shore A hardness. The morphology was determined by scanning electron microscopy, and the dielectric properties such as electrical conductivity, dielectric constant (ε′), dielectric loss (ε″) and loss tangent (tan δ) were studied.     

Synthesis and characterization of some polyurethanes

Polymers containing urethane units were synthesized by solution polymerization of bisphenols A, C, and F with diisocyanates viz. toluene-2,4-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and diphenylmethane diisocyanate, respectively. Thus obtained polymers were pale yellow amorphous powders. These polymers were characterized by infrared spectral studies and nitrogen content. The number-average molecular weights (M?_n) were estimated from conductometric titration and limiting viscosity number [η] in dimethyl formamide. From thermogravimetric analyses, the thermal behaviour was studied. Kinetic parameters for their degradation were computed.     

Synthesis and properties of polybenzoxazine modified polyurethanes as a new type of electrical insulators with improved thermal stability

A new class of electrical insulating materials with improved thermal stability was prepared from combination of polybenzoxazine with epoxy terminated polyurethane prepolymer (EPU). EPU was prepared by the reaction of glycidol with NCO‐terminated urethane prepolymer. The prepolymer was prepared from polycaprolactone polyol (molecular weight 1000), and hexamethylene diisocyanate. Bisphenol‐A and aniline were used for preparation of benzoxazine monomer. Mode of reactions and optimum curing condition were determined by DSC and FTIR analysis. Upon these analysis as well as DMTA findings, formation of interpenetrating polymer networks was suggested for blends consisted different weight ratios of two components. Viscoelastic behavior, mechanical, thermal, and electrical properties of the prepared samples was studied and their relation to the chemical structure and composition of blends were elucidated. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Study of deblocking and crosslinking reactions of a blocked isocyanurate cationomer

Summary A cationic adduct of a partly blocked hexamethylene isocyanurate and N-methyldiethanolamine was prepared and applied as a crosslinker in one-pack water-borne polyurethane system. The deblocking and crosslinking reactions were followed by FTIR, TGA and DSC techniques. Deblocking started at about 100°C, while at higher temperatures urethane, allophanate, and urea linkages were formed. The efficiency of the cationic crosslinker was evaluated by determining the insoluble part of the crosslinked polyurethane ionomer films. Films of a good solvent resistance were obtained using 15 wt% of the cationic crosslinker and 0.05 wt% of a catalyst at 130°C.     

Stratification of polysiloxanes at the film-air and film-substrate interfaces in silicone-modified acrylic coatings; an ATR FI-IR spectroscopic study

Film-air (F-A) and film-substrate (F-S) interfaces of silicone-modified acrylic coatings were characterized using attenualed total reflectance Fourier transform infrared (ATR FT-IR)spectroscopy. Previous studies suggested that when silicone linkages are part of a polymer backbone-containing polymers, they tend to accumulate near the F-A interface. These studies show that when polysiloxanes are dispersed in a silicone-modified acrylic/polysiloxane solution, they appear to accumulate near F-S interfaces. When polysiloxanes were added to a dispersion of silicone-modified acrylic resin, the distribution of polysiloxanes between F-A and F-S interfaces became less pronounced. These differences result from the mobility of polysiloxanes, and are attributed to the molecular weight dependence and compatibility differences between the systems.     

Characterization of polyurethane foam prepared by using starch as polyol

Polyurethane foams were prepared using starch as a main component of polyols and their structural, mechanical, and absorbing properties for organic solvents were investigated. Fourier transform infrared spectra showed that urethane linkage was formed by the reactions between ? NCO of diisocyanates and ? OH of polyols. When polyurethane foams were prepared at high molar ratio of ? NCO/? OH, the unreacted ? NCO groups were detected. Also, urea linkage was formed by the reaction between diisocyanate and water, which was used as the foaming agent. The micrographs showed that the polyurethane foams had closed‐cell structure, of which the cell size increased with ? NCO/? OH molar ratio. The density of polyurethane foams increased with molecular weight of polyethylene glycol. The compressive moduli of polyurethane foams increased with ? NCO/? OH molar ratio. Polyurethane foams prepared using toluene‐2,4‐diisocyanate as diisocyanate had the highest modulus, while those prepared using hexamethylene diisocyanate had the lowest modulus. In case of the absorbency for the organic solvents, the polyurethane foams prepared at ? NCO/? OH molar ratio of 0.8 had the maximum absorbency. Among several organic solvents, the absorbency for dimethyl sulfoxide was the highest, while the absorbency for tetrahydrofuran was the lowest. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1544–1553, 2007     

Vibrational spectroscopic study of distribution of sodium dodecyl sulfate in latex films

The migration of the surfactant sodium dodecyl sulfate (SDS) to the film-air (F-A) and film-substrate (F-S) interfaces during coalescence of a latex of methyl methacrylate and ethyl acrylate functionalized with methacrylic acid was monitored by IR and ATR-FTIR spectroscopic techniques. Both F-A and F-S interfaces showed surfactant enrichment, revealing a nearly parabolic distribution of surfactant throughout the film.     

Mobility and distribution of silicone additives in coatings; a spectroscopic study

The concentration of four prepared silicone/poly(ether) additives at the film-air (F-A) and film-substrate (F-S) interfaces of films prepared on glass and poly(ethylene) substrates is characterized via quantitative attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy. Three different coating systems, an acrylic/melamine solvent based system, an acrylic/isocyanate solvent based system, and an aqueous poly(urethane) suspension, were examined. It is found that the nature of the substrate, the mechanism of film formation, and the inherent coating/additive compatibility are all factors which may significantly influence additive behavior. Also the acrylic/melamine system generally reveals a slight degree of enrichment of both additive components to the F-A interface, but the acrylic/isocyanate system exhibits a more pronounced concentration gradient due to a lesser degree of compatibility. In contrast, the water-based poly(urethane) exhibits a more complex behavior, with the poly(ether) component revealing significant enrichment to the F-A interface, and the siloxane component exhibiting a decreased concentration as the interface is approached. Here, the mechanism of film formation involves a water flux passing out of the film which may serve to transport the water soluble poly(ether) components to the F-A interface. At the F-S interface, it is demonstrated that the nature of the substrate may influence additive behavior, and it is found that the concentration of the more polar poly(ether) components generally exhibits a lesser degree of enrichment to a poly(ethylene) substrate, relative to glass.     

Elastic recovery and creep properties of waterborne two-component polyurethanes investigated by micro-indentation

Polyurethane (PUR) films were obtained by casting waterborne formulation of polyhydroxylated polyacrylate resin and hexamethylene 1,6-diisocyanate trimer hardener. Conversion of the polyaddition reaction was followed by FTIR spectroscopy and appears independent of the NCO/OH stoichiometric ratio between HDI trimer NCO groups and polyhydroxylated polyacrylate OH groups (resin), but thermally activated. The crosslinked networks were investigated by swelling experiments and dynamic mechanical analysis. Calculated values of the average mass between crosslinks allowed explaining the evolutions of the conservation modulus and loss factor with NCO/OH stoichiometric ratio. Elastic recovery and creep behavior of the PUR films were investigated by micro-indentation. A four-element viscoelastic model was used to fit the indentation depth evolution during micro-indentation creep experiments. Results show that creep instantaneous elasticity is fully controlled by the network elasticity and thus by the NCO/OH stoichiometric ratio and that the ability of the network to dissipate energy remains high for non-continuous (NCO/OH ratio < 1) networks. On the other hand, for high NCO/OH ratios, it was shown that hard PUR networks exhibit creep properties and significant retardation times. Finally the evolution of indentation springback factor vs. indentation creep factor was determined, showing that instantaneous elastic recovery behavior increases as creep behavior decreases.     

Synthesis and characterization of castor oil urethane triamine networks

Castor oil was polymerized with diisocyanate and crosslinked with primary triamine (Jeffamine T-403) to form networks. The effect of triamine as a crosslinking agent on rubbery castor oil urethane elastomer was determined by measuring network parameters such as average molecular weight between crosslinks (MC) number of polymer chains per unit volume (N), tensile strength, and modulus of the networks. The crosslinking density was varied by varying the ratio of NCO : NH₂ from 0.60 to 0.95. The results indicated the formation of highly crosslinked elastomers at all NCO : NH₂ ratios employed. The tensile stregth and modulus increased with increasing crosslink density up to a value of NCO : NH₂ 0.85 and after this there was no significant change, indicating the maximum limit of improvement attainble in terms of network characteristics.