Linking of oligoesters hydrolysis to polyurethane coatings

The hydrolytic stability of a series of oligoesters comprised of three and four different monomers was evaluated. The hydroxyl terminal oligoesters were prepared from adipic acid (AA) and isophthalic acid (IPA), with six different diols and one triol, which included: 1,4‐butanediol, 1,5‐pentanediol, 1,6‐hexanediol, neopentyl glycol, 2‐methyl‐1,3‐propanediol, trimethylolpropane, and 2‐butene‐1,4‐diol. The hydroxyl terminated oligoesters were reacted with phenyl isocyanate to form telechelic urethane groups. Hydrolysis rate constants were obtained from plots of acid number vs. time. It was observed that ternary oligoester systems had lower hydrolysis rates than quaternary systems. In addition to investigating the hydrolytic stability of the synthesized oligoesters, polyurethane coatings were produced by reacting the hydroxyl‐terminated oligoesters with an aliphatic polyisocyanate (1,6‐hexanediisocyanate trimer). Model oliogester hydrolysis was then correlated to the weatherability of a crosslinked polyurethane film. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40198.     

水性聚氨酯涂料的制备与性能研究

以甲苯-2,4-二异氰酸酯(TD I)、聚醚Ng 210和二羟甲基丙酸(DMPA)为主要原料,制得不含或含少量有机溶剂的水性聚氨酯乳液,研究了NCO/OH的摩尔比、DMPA含量等对水性聚氨酯乳液性能的影响。结果表明,当NCO∶OH(摩尔比)=2.5∶1,DMPA=5%时,乳液性能最佳。     

Polyaniline‐grafted polyurethane coatings for corrosion protection of mild steel surfaces

We report the superior corrosion‐resistant properties of conducting polyurethane networks of polyaniline (PANI), poly‐m‐aminophenol (PmAP), and poly‐o‐anisidine (PoA) coated on mild steel panels. These networks were prepared by blending conducting polyanilines with isocyanate‐containing prepolyurethanes. Free‐standing polyurethane films were obtained after a moisture cure for several days to ensure complete reaction of the excess isocyanate. The films were electrochemically active with conductivity in the range of 10^?2 to 10^?3 S/cm. The solution blends and formed films were characterized by infrared, ultraviolet, thermogravimetric analysis, and differential scanning calorimetry. Electrochemical corrosion studies of the coated films on mild steel panels showed excellent corrosion protection in the following order: PU‐PANI > PU‐PmAP > PU‐PoA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45806.     

The performance of epoxy coatings containing polyaniline (PANI) nanowires in neutral salt,alkaline, and acidic aqueous media

PANI/epoxy coatings have great promise applications in the industry as the metal corrosion protection coating, and their performance directly determines the life span of the coating and equipment durability. In this study, the performance of epoxy coatings with and without PANI nanowires immersed in 12 wt% NaCl, 5 wt% HCl, and 5 wt% NaOH solutions at different temperature were investigated for the first time. The performance and the degradation reactions of the coating cooperated with PANI nanowires were characterized by the variety of techniques and methods, including ultraviolet–visible spectrophotometry (UV–vis), field emission scanning electron microscopy (SEM), Attenuated Total Reflectance-Fourier transform Infrared spectroscopy (ATR-FTIR), and thermogravimetric analysis (TGA). The experiment results indicated that the failure mechanism of the different coatings varied with the different temperatures and solutions. Electrochemical impedance spectra (EIS) results showed that an appropriate content of PANI nanowires improve the protection performance of epoxy coatings in 12 wt% NaCl, 5 wt% HCl, and 5 wt% NaOH solutions, which is attributed to the passivation ability and shielding effect of PANI nanowires.     

Improving the helium gas barrier properties of epoxy coatings through the incorporation of graphene nanoplatelets and the influence of preparation techniques

Graphene/epoxy nanocomposite coatings were formulated by applying different dispersion and preparation methods to determine whether the coatings might be a potential gas barrier material for irradiated graphite waste which is known to release radioactive gases like tritium (³H₂). Helium was used as a substitute gas for tritium and the gas permeability was measured with the use of a helium leak detector. The dispersion and fabrication techniques influenced the ability of the coatings to reduce the helium gas permeability. Characterization of the graphene nanoplatelets and the composite morphology showed that the graphene nanoplatelet geometry and aspect ratio were altered by the applied dispersion techniques. The results showed that incorporating 2 wt % graphene into the epoxy matrix, combined with a multilayer fabrication method, reduced the helium gas permeability by 83% when compared to the reference epoxy samples. Modeling the gas permeability according to the tortuous path theory confirmed the aspect ratios which were estimated by the microscopic methods and particle size analysis. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42584.     

Failure process of acrylic polyurethane coating under alternate wetting and drying condition

The failure process of an acrylic polyurethane coating in simulated sea water under a wet‐dry cyclic condition and immersion condition was studied with the methods of electrochemical impedance spectroscopy (EIS), Fourier transform infrared spectroscopy (FT‐IR), and scanning electron microscope (SEM). The results show that the failure rate of acrylic polyurethane coating under the condition of wet‐dry alternation is obviously greater than that under complete immersion. Under wet‐dry condition, both the porosity and the water absorptivity of the coating are also greater than those under complete immersion condition. Owing to the physical effect of wet‐dry alternation, the fillers in the coating surface layer may fall off and result in micro‐pores, which could multiply the defects in the coating and accelerate the coating degradation. FT‐IR analysis shows that the isocyanate group in the coating not only participates in the curing of coating, but also hydrolyzes with water molecule. The C‐O bonds fracture partly due to hydrolysis of the main molecule chains, which is one reason of the coating failure. The failure process for acrylic polyurethane coating under a wet‐dry cyclic condition is discussed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41989.     

Multiscale investigation on molecular structure and mechanical properties of thermal-treated rigid polyurethane foam under high temperature

In order to study the relationship between the molecular structure and mechanical properties of rigid polyurethane foam (RPUF) during the mechanical and chemical failure process, the variation of the molecular structure and mechanical properties of RPUF treated in temperature range of 323–473 K were characterized by both theoretical and experimental methods. The molecular structure stability of RPUF varied with thermal treatment temperature was characterized by density functional theory method. The mechanical properties of base material of RPUF were simulated by means of molecular dynamics (MD) simulation. Then the related parameters obtained from the MD simulation were assigned into a representative volume element model of RPUF for the finite element analysis. The results indicated that the vibrational frequencies of isocyanate groups and amino acid ester groups in RPUF molecule increased while the molecular orbital energy gap of RPUF decreased with the increase of treatment temperature. It indicated that the RPUF molecule had high chemical reactivity at high temperature. The results of the multiscale simulation of mechanical properties showed that the defects and voids in RPUF generated under high temperature would grow with the increase of thermal treatment temperature, which intensified the stress concentration in RPUF and decreased the tensile properties of RPUF.     

Development of layer-by-layer assembled thin coatings on aluminium alloy AA2024-T3 for high resolution studies of local corrosion processes

The aim of this study is to develop nanometer-thin epoxy-based films on aluminium alloy AA2024-T3 as a model coating system for high resolution corrosion studies. Spin coating was used for the layer-by-layer (LbL) deposition of poly-(ethylenimine) (PEI) and poly([o-cresyl glycidyl ether]-co-formaldehyde) (CNER) bilayers. The film chemistry and the cross-linking process were characterized by means of Fourier-transform infrared spectroscopy (FTIR). Ellipsometric data confirmed the linear increase of film thickness. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) results indicate the improvement of the film barrier properties with increasing film thickness. Mapping of the topography and the volta potential was performed by means of scanning Kelvin probe force microscopy (SKPFM). The results indicate the presence of a homogeneous film structure, while the intermetallic phases can still be identified below the coating. The SKPFM analysis confirmed that the model films are suitable for investigation of corrosion processes at the coating/metal interface.     

Gas barrier properties of butyl rubber/vermiculite nanocomposite coatings

Gas barrier properties of butyl rubber/vermiculite nanocomposites coatings are described here. The coating formulations consisting of a butyl rubber latex (the rubber particles are about 1 μm in diameter) to which exfoliated vermiculite was added were applied to a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) coated Anapore ceramic disc; composites containing 0, 20 and 30 wt% of vermiculite were evaluated. The permeability of the nanocomposite coatings to various gases was measured and compared to permeation models for composites with flake-like fillers proposed by Cussler, Nielsen, Fredrickson and Bicerano, and Gusev and Lusti. The gas permeability of the nanocomposite materials was decreased remarkably by the presence of the high loadings of vermiculite. Diffusion coefficients computed from time lag data also decreased remarkably with the vermiculite content. TEM and SEM were used to observe the internal structure of the nanocomposite membranes in an effort to gain further understanding of the permeability properties. Aspect ratios ranging from 100 to 480 were predicted from the gas permeation results and the above models. TEM experiments could not provide a precise estimate of the aspect ratio but the range predicted from the models are of this order. Apparent solubility coefficients, computed by dividing the experimental permeability by the diffusivity obtained from the time lag observation, increased significantly with vermiculite content in contradiction to the theoretical prediction. Independent gas sorption isotherm experiments for CO₂ were measured and found to be larger than that in butyl rubber possibly due to adsorption on the vermiculite. While this turned out to be a very complicated system, it seems clear from these results that the transient tortuosity factor defined by the time lag is significantly larger than the steady-state tortuosity factor defined by permeability.     

Epoxy loaded poly(urea-formaldehyde) microcapsules via in situ polymerization designated for self-healing coatings

Self-healing systems are a next-generation technology that can offer autonomous crack repair and increase the service lifetime of a protective coating. Polymeric microcapsules containing healing agents can be used in that perspective and exhibit significant potential. In the current study, poly(urea-formaldehyde) microcapsules containing an epoxy healing agent as core material were successfully prepared using in situ polymerization. The effect of different process parameters was studied in respect to microcapsules' characteristics, that is, morphology, particle size, encapsulation efficiency and thermal properties. Spherical microcapsules with either smooth or rough surface were obtained with a size ranging from 90 to 165 μm, controlled by the stirring rate during the emulsification stage. The encapsulation efficiency varied between 65–77% with no significant dependence on the process conditions. Finally, the stability of the microcapsules during storage was investigated at different conditions.     

Influence of coalescing aids on the latex properties and film formation of waterborne coatings

Coalescing aids are organic solvents that are widely used to accommodate the film formation of waterborne coatings. The influence of the volatility, hydrophilicity, and amount of coalescing aids on the properties of waterborne dispersions and the dynamics of film formation was investigated. The results show that coalescing aids increased the viscosity through particle swelling and solvation, with the effect being stronger with more hydrophilic solvents. The results obtained with electrochemical impedance spectroscopy (EIS) and indentation tests show that hydrophilic coalescing aids were more efficient because of the partitioning on the particle interfaces but they could temporarily decrease the water resistance. The ease of coalescence was countered by solvent retention, which complicated the dynamics of film formation. EIS was used to optimize a real‐world coating formulation and identify the pitfalls arising from the complex dynamics of film formation. The best resistance to humidity was obtained with a combination of hydrophobic and hydrophilic coalescing aids. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45142.     

Preparation and properties of polyurethane coatings based on acrylic polyols and trimer of isophorone diisocyanate

This paper presents a study on the effect of NCO/OH ratio and an increase in hydroxyl content of acrylic polyols on the properties of polyurethane (PU) coatings. Coating properties studied are gloss, scratch resistance, flexibility and adhesion, mechanical properties include tensile strength, modulus, percent elongation and Shore hardness, while physicochemical properties include chemical resistance and solvent absorption of coated PU samples. A series of acrylic polyols (copolymers) based on butyl acrylate (BA), methyl methacrylate (MMA), styrene and 2-hydroxy ethylacrylate (HEA) were prepared by selecting different percentage of hydroxyl content. Trimer of isophorone diisocyanate (IPDI) was also synthesized in the laboratory. This trimer has trifunctionality. Isocyanurate ring of trimer increases thermal properties of PU. Polyurethanes from these acrylic polyols (containing different percent hydroxyl) and trimer of IPDI were prepared with two different NCO/OH ratios viz, 1.1:1 and 1.2:1. Polyurethanes were coated on substrates for measuring coating properties. Mechanical properties were measured on cast films of the PUs. The experimental results revealed that all polyurethane coatings based on acrylic polyols and IPDI trimer showed good gloss, scratch resistance and excellent adhesion. Thermal stability of these PU samples was found to be better. Physicochemical properties reflected that these PU have excellent chemical and solvent resistance.     

Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings

Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The microstructure of the nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.     

Development of self-healing coatings based on urea-formaldehyde/polyurethane microcapsules containing epoxy resin

In this study, the synthesis of urea-formaldehyde/polyurethane (UF/PU) microcapsules containing epoxy resin for self-healing and anti-corrosion coatings with good stability has been reported. Spherical microcapsules were prepared with a diameter of about 50–720 μm and a shell thickness of 0.6–0.7 μm via in situ polymerization in an oil-in-water emulsion using 2,4-toluene diisocyanate-based pre-polymer along with the urea-formaldehyde. Scanning electron microscopy (SEM) and optical microscopy (OM) were employed to evaluate the shape and morphology of the microcapsules. Fourier transform infrared (FTIR) spectroscopy showed the absence of free isocyanate groups within the microcapsule shell confirming the completion of shell formation reactions. OM illustrated that the microcapsules were stable over a period of 30-days in toluene and xylene. Increasing microcapsule loading improved crack repairing and anti-corrosion performance of the coating layer. Low-carbon steel coupons coated with an epoxy resin containing 10 wt% microcapsules and scribed using a scalpel blade showed no visible sign of corrosion after up to 5 weeks of exposure in a standard salt spray test chamber.     

Synthesis,characterization, and properties of PCDL aliphatic hyperbranched polyurethane coatings

The isocyanate‐terminated polyurethane pre‐polymer (PPU) was synthesized via the step‐growth polymerization approach by using polycarbonate diol (PCDL, Mn = 2000) and isophorone disocyanate (IPDI) as monomers, dibutyltin dilaurate (DBTDL) as the catalyst. Subsequently, the hyperbranched polyurethane (HBPU) was synthesized by graft copolymerization using PPU, hyperbranched poly(amide–ester) polyol (HPAE) and 1,4‐butanediol (BDO). The molecular structure of HBPU was characterized by means of FTIR, ¹H‐NMR, and ¹³C‐NMR. It was observed that HBPU was synthesized as anticipated. The thermal and mechanical properties, the microstructure, and morphologies of the filmed HBPU and LPU (linear polyurethane) were tested, respectively. The filmed HBPU, revealed better thermal stability, and higher T_g accompanied with lower viscosity than those of filmed LPU. Additionally, the mechanical experiment showed that the filmed HBPU exhibited enhanced mechanical properties because it contained certain amounts of HPAE. Compared with its linear analog (LPU) specimen, the tensile strength of the filmed HBPU containing 10 wt % HPAE increased by 1.9 times (up to 28.15 MPa), and its elongation at break increased by 1.5 times (up to 543.8%), resulting from the dual effects of the hydrogen bonding and the crosslinking density in the HBPU system. The morphologies of filmed HBPU were characterized by means of WAXD and SEM, which indicated that increasing the content of HPAE lowers the crystallinity of HBPU. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2671–2679, 2013     

Preparation and physical properties of two-component polyurethane flame-retardant coatings using trichloro modified polyesters

Trichloro modified polyester polyols were synthesized by two-step condensation reactions. An intermediate compound was synthesized by the esterification of trichloroacetic acid with trimethylolpropane in the first step. In the next step, polycondensation of the intermediate, 1,4-butanediol and trimethylolpropane with adipic acid was carried out. Two-component flame-retardant polyurethane coatings were prepared by blending these modified polyester polyols and polyisocyanate. The physical properties of these new coatings were comparable to non-flame-retardant coatings. They showed a drying time of 3–4 h and a pot life time of 10–13 h, which could belong to the flame-retardant coatings with long pot life times. Coating with 20 wt % trichloroacetic acid showed a self-extinguishing property in the vertical burning test. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 913–920, 1998     

The influence of ageing of epoxy coatings on adhesion of polyurethane topcoats and protective properties of coating systems

The dependence of adhesion and protective properties of coating systems on surface properties of epoxy intermediate coatings, aged and non-aged before an application of polyurethane topcoats, were examined. The intermediate coatings were aged 500 h in UV chamber. The surface free energy and polar groups were estimated after ageing. After applying polyurethane topcoats on aged and non-aged epoxy coatings, resistance to salt spray and thermal shocks were tested as well as internal stresses were measured before and after corrosion tests.The results showed that adhesion in coating systems with polyurethane topcoats applied on aged epoxy coatings depends strongly on the degradation degree of epoxy intermediate coatings and the value of generated internal stresses. Coatings with good adhesion retention in corrosion environments have good protective properties even when temporary blistering has occurred.     

Synthesis and properties of UV-curable waterborne urethane modified acrylic coatings with varying vinyl content

It has been an effective method to improve the metal protective performance of UV-curable waterborne coatings by increasing the crosslinking degree. Hence, a series of UV-curable waterborne urethane modified acrylic (UV-WUA) coatings with different vinyl content were prepared. Firstly, a functional prepolymer containing carboxyl and hydroxyl groups was synthesized by free radical copolymerization (FRCP) using acrylic acid (AA), 2-hydroxyethyl acrylate (HEA), methyl methacrylate (MMA) and butyl acrylate (BA). Next, it was grafted with different amounts of semi-adduct of isophorone diisocyanate (IPDI) and HEA (IPDI-HEA), and finally neutralized and hydrated, obtaining UV-WUA dispersions which were then cured by UV to acquire cured films and coatings. Meanwhile, molecular structures, molecular weights, particle sizes, and Zeta potentials were characterized. Then, the heat resistance, mechanical performance, adhesion and the pencil hardness of the coatings were also investigated. Moreover, their protective performance was tested by electrochemical methods, and the surface morphology was analyzed by atomic force microscopy. The results showed that the coating had desirable comprehensive performance when vinyl content reached 0.86 mmol·g⁻¹.     

Layer-by-layer modification of thermoplastic coatings to improve adhesion

One of the causes leading to low bond strength between a coating and a substrate (adhesion strength) - if coatings are formed at elevated temperatures in air - is assumed to be a weak boundary layer generated in the region of adhesional contact: the boundary layer consisting mostly of low-molecular-weight products resulting from thermal oxidative degradation of the polymer. It has been verified experimentally that products of oxidation diffuse from the coating surface layer to the contact area. The oxidation process is supposed to be localized within that surface layer. A method has been devised to determine the thickness of the layer, and model experiments have been conducted to show that low-molecular-weight products of oxidation deteriorate the adhesion strength. Ways have been found to increase the adhesion strength of coatings by means of modification of the coating applied in a layer-by-layer manner. The idea is to introduce separately such modifiers as antioxidants, inorganic fillers possessing high adsorption capacities, and crosslinking agents into the coating surface layer. This method of coating modification allows one to eliminate the negative effects of the low-molecular-weight products generated in the surface layers during the formation.     

Study on effect of duration of the ultrasonication process on solvent‐free polyurethane/organoclay nanocomposite coatings: Structural characteristics and barrier performance analysis

In this study, effect of duration of ultrasonication process on structural characteristics and barrier properties of solvent‐free castor oil‐based polyurethane (PU)/organically modified montmorillonite (OMMT) nanocomposites was investigated. A series of PU/OMMT composites were synthesized by in situ polymerization technique through an ultrasonication‐assisted process at various processing durations. Effect of ultrasonication duration on de‐agglomeration of clay stacks in castor oil dispersions was evaluated by optical microscopy, sedimentation test, and viscosity measurement. Wide angle X‐ray diffraction and Fourier‐transform infrared spectroscopy were employed to investigate the effect of processing time on degree of delamination of clay platelets and interfacial strength between clay layers and PU matrix. Also, surface morphology of the nanocomposites was analyzed by atomic force microscopy. The results showed that by increasing the ultrasonication time up to 60 min, the size of clay agglomerates decreased and the interlayer spacing of clay platelets increased. To evaluate the effect of ultrasonication duration on transport properties of the PU/OMMT composites, diffusion coefficient and permeability were determined through water uptake test. Electrochemical impedance spectroscopy was carried out to analyze the barrier properties and to evaluate the corrosion performance of these composite coatings on carbon steel panels. It was found that by increasing sonication time, the barrier property of nanocomposites against diffusion of water molecules improved, which is due to further separation of clay platelets, enhancement of the traveling pathways for water molecules and improvement of interactions between the two components. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012