Curing of silylated polyurethane with BF3‐monoethylamine complex as moisture‐curable adhesives and their properties

Boron trifluoride monoethylamine complex (BF₃‐MEA) was evaluated as one of the curing catalyst, for the moisture‐curable adhesive based on the polyurethane end‐capped with trimethoxysilane (silylated polyurethane). BF₃‐MEA worked as the catalyst of condensation reaction of trimethoxysilyl groups in silylated polyurethane to give the corresponding networked polymer. The curing speed, the bond strength, and the heat resistance of the crosslinked adhesives with BF₃‐MEA were compared with dibutyltin dimethoxide. It was found that BF₃‐MEA was a good curing catalyst, which showed the same performance in the bond strength and remarkable advantages in the curing speed and the heat resistance, compared with organotin compound. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Structure–property relationships in one‐component polyurethane adhesives for wood: Sensitivity to low moisture content

The causes of strength loss of wood joints and their consequent delamination from one‐component polyurethane adhesives used for bonding structural wood when used at a low moisture content was investigated by testing wood joint strength and elongation at rupture at different wood moisture contents and by ¹³C‐NMR spectroscopy and scanning electron microscopy of the hardened bond line. The combination of the relative proportion of the still‐reactive free ? NCO groups on the polyurethane, of the wise choice of degree of polymerization of the resin, and of a slower rate of reaction were the three parameters found to be important in overcoming the problem of poor or no bonding of wood at low to very low moisture contents from one‐component polyurethane adhesives. The results obtained indicated that one‐component polyurethane adhesives that had a combination of a higher proportion of still‐unreacted ? NCO groups, a lower degree of polymerization, and a slower reaction rate were capable of overcoming the problem of the high sensitivity of polyurethane gluing at low to very low wood moisture contents. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4181–4192, 2006     

Thermal stability of chemically crosslinked moisture‐cured polyurethane coatings

Polyurethane prepolymers are widely used in reactive hot melt adhesives and moisture‐cured coatings. The segmented moisture‐cured formulations, based on polytetramethylene glycol (PTMG‐1000)/trimethylol propane (TMP)/isophorone diisocyanate (IPDI) and PTMG/TMP/toluene diisocyanate (TDI), were prepared with NCO/OH ratio of 1.6 : 1.0. The excess isocyanate groups of the prepolymers were chain extended in the ratio of 2 : 1 (NCO/OH) with different aliphatic diols and 4 : 1 with different aromatic diamines. The surplus isocyanate groups of the formulations were completely reacted with atmospheric moisture, and the thermal stability of the postcured materials obtained as cast films were evaluated by thermogravimetric (TG) analysis. It was observed that initial degradation temperatures were above 270°C, with two‐ or three‐step degradation profiles. The degradation parameters were evaluated using the Broido and Coats–Redfern methods. The thermal resistance of moisture‐cured formulations using diisocyanates with the cycloaliphatic structures (IPDI) and the aromatic TDI, at the same NCO/OH ratio (1.6), and TMP content were compared from the isothermal TG experiments at different temperatures and dynamic TG experiments at different heating rates in nitrogen and oxygen environments. The observation suggests that polyurethane‐containing sulfone groups and straight‐chain diol chain extenders were more stable. It was also observed that at lower temperature polyurethane, prepared from aliphatic diisocyanates (IPDI), was more stable than the aromatic diisocyanate (TDI) containing polyurethanes. At high temperature, the stability order follows the reverse trend. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1509–1518, 2005     

One‐component polyurethane adhesives for green wood gluing: Structure and temperature‐dependent creep

Creep and temperature‐dependent creep, characteristic of one‐component polyurethane adhesives for wet and green structural wood gluing are two different parameters. Testing of standards‐approved commercial polyurethane adhesives for this purpose shows that while ambient temperature creep can be avoided or at least greatly limited according to the formulation used, this is not the case for temperature‐dependent creep. The commercial adhesive formulation characteristics that minimize or enhance ambient temperature and temperature‐dependent creep are identified. Basic principles on the structure of simple polyurethane adhesives influencing temperature‐dependent creep are also identified and discussed. The higher the proportion of all hardened network nodes, the lower creep is likely to be at ambient temperature. Instead, due to segment movement and easier disentanglement with increasing temperature, the lower the proportion of covalently linked nodes and the greater the proportion of just physical entanglement nodes, the greater the temperature‐dependent creep will be. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1231–1243, 2005     

Survey of polyurethane adhesives

Polyurethane chemistry provides a means to ‘tailor’ adhesives to specific needs. This is being achieved by polyurethane technology at a cost of more sophisticated application/bonding equipment and a need for better control of raw materials and adhesive processing conditions.     

Synthesis and stability research of reproducible aqueous polyurethane micelles with low de‐blocking temperature

Aqueous polyurethane micelles from 2,4,6‐trichlorphenol blocking terminal‐isocynate pre‐polymer were prepared which could be de‐blocked at low temperature. The prepared aqueous blocked polyurethane micelles (BPUMs) were uniform and spherical micro‐particles. The particle size was decreased with increase of the DMPA content, while it was enlarged at first and then gradually decreased by increasing R‐value. The stability of the micelles was affected by the R value, blocked efficiency and DMPA content. HDI‐BPUMs were more stable than HDI‐BPUMs at the same condition. The stability was further researched by the zeta potential combining with technique and viscosity tracking method, and the initial zeta potential be used as the stability criterion. In addition, a stabilization mechanism of the micelles was proposed. TCP‐blocked TDI‐BPUMs could be initially de‐blocked at about 75°C and TCP‐blocked HDI‐BPUMs was de‐blocked at about 80°C. The de‐blocking temperature could be further reduced in the presence of catalyst, but it was scarcely affected by the change of R‐value and DMPA content. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41908     

Preparation and properties of waterborne polyurethane adhesives modified by polystyrene

Waterborne polyurethane (WPU) adhesives modified by polystyrene (PS) were prepared through a prepolymer mixing process from diisocyanates, an anionic polyester, internal emulsifiers, a neutralizer, a chain extender, and PS dispersions. The latter was preformed via the in situ polymerization of styrene in poly(1,4‐butanediol adipate) diol. Transmission electron microscopy, Fourier transform infrared spectroscopy, and ¹H‐NMR techniques were used to characterize the PS dispersions and polyurethane (PU)–PS prepolymer. Experimental results with respect to the performance of the PU–PS adhesives indicate that suitable PS/polyester diol weight ratios improved the mechanical properties, thermal stability, water resistance, and initial adhesive strength of the pristine WPU adhesives. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and characterization of soybean‐oil‐based polyurethane composites containing industrial and agricultural residual wastes as fillers

Thermosetting composites were prepared from soybean‐oil‐based polyols (hydroxyl number = 190 mg of KOH/g, [OH]/[NCO] for 2,4‐toluene diisocyanate = 0.9) and fillers (10 wt %) from industrial and agricultural residual wastes. Different types of inexpensive residual wastes were used: black rice husk ash, coconut husk ash, calcined retorted oil shale, and retorted oil shale. The fillers were characterized by thermogravimetric analysis and measurements of particle size distribution, specific surface area, and pore size distribution. The fillers were microporous materials with different chemical compositions, with average particle diameters varying from 5.6 to 76.6 μm, specific surface areas varying between 6 and 165 m²/g, and thermal stability at the polyurethane cure temperature (65°C). All composites were characterized by dynamic mechanical analysis, flexural tests, Shore A hardness tests, thermogravimetric analysis, and scanning electron microscopy analysis. Coconut husk ash, rice husk ash, and retorted oil shale presented better mechanical properties; nevertheless, coconut husk ash and rice husk ash had higher particle sizes, which caused bad dispersion of the filler in the matrix and resulted in nonhomogeneous composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Mechanical and thermal properties of polyurethane elastomers based on hydroxyl‐terminated polybutadienes and biopitch

Biopitch is a renewable source of polyol obtained from Eucalyptus tar distillation, which was studied as an active component of polyurethane (PU). The polymerization occurred in one step, with a mixture of biopitch and hydroxyl‐terminated polybutadiene polyols reacted with 4‐4′‐diphenyl methane diisocyanate in the presence of dibutyltin dilaurate. Solid‐state ¹³C‐NMR, IR spectroscopy, elemental analysis, and thermal analysis [thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)] were used to characterize the biopitch. The biopitch sample showed an aromatic and oxygenated structure with great thermal stability at high temperatures. Multiphasic PUs were synthesized and characterized by IR spectroscopy (attenuated total reflectance), elemental analysis, thermal analysis (TGA and DSC), mechanical assays (tensile strength, elongation at break, toughness, hardness, and resilience), and water absorption resistance (ASTM D 570‐81). In a comparative study of the synthesized elastomers, biopitch content increased tensile strength and hardness and decreased thermal stability, elongation at break, and water absorption. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 759–766, 2003     

Preparation and properties of one‐pack polybenzoxazine‐modified polyurethanes with improved thermal stability and electrical insulating properties

A novel method has been developed for the combination of polyurethanes and polybenzoxazines. For this purpose, firstly, p‐nitrophenol blocked polyurethanes (BPUs) were prepared via the reaction of poly(tetramethylene ether) glycol of various molecular weights, 2,4‐tolylene diisocyanate and p‐nitrophenol. The BPUs were then mixed with 2,2‐bis(3,4‐dihydro‐3‐phenyl‐2H‐1,3‐benzoxazine)propane (Ba) at various weight ratios. To prepare poly(urethane‐co‐benzoxazine) networks, the BPU/Ba mixtures were subjected to a heating programme derived from detailed differential scanning calorimetry and gel content measurements. Results showed about 30 to 40 °C reduction of polymerization temperature for complete curing of BPU/Ba mixtures in comparison to neat Ba. This phenomenon was related to catalytic action of librated p‐nitrophenol molecules. The thermal, mechanical, viscoelastic and electrical properties of prepared thermoset polymers were measured and correlated with their chemical structures. A significant improvement of thermal stability and dielectric strength in comparison to neat polyurethanes was found. Also, enhancement of tensile properties, ease of curing and ability to be transformed into thin films are fascinating features of these newly developed materials in comparison to neat polybenzoxazines. Therefore, these polymers have potential applicability as high‐performance materials in modern electrical industries. Copyright © 2010 Society of Chemical Industry     

Long‐term development of thermophysical and mechanical properties of cold‐curing structural adhesives due to post‐curing

The long‐term changes in the thermophysical and mechanical properties of a cold‐curing structural epoxy adhesive were investigated by accelerating the curing reaction by post‐curing at elevated temperatures. Experimental data concerning the glass transition temperature for periods of up to 7 years and tensile strength and stiffness measurements could be extrapolated for a period of up to 17 years. An existing model for the long‐term development of concrete properties was modified for the prediction of the long‐term mechanical properties of adhesives. The applicability of the acceleration procedure and the new model was confirmed by several verification procedures. Structural adhesives exhibit significant increases in glass transition temperature, strength and stiffness over the long term provided that joints are adequately sealed and protected from humidity and UV radiation. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effects of the aluminum filler content on moisture diffusion into epoxy adhesives in distilled water and sea water

Epoxies are the most common of high‐performance structural adhesives, especially in automotive and aircraft manufacturing. In a variety of industrial applications, epoxy adhesives are required to have enhanced thermal conductivity. The normal method of changing this property is to add to the epoxy a filler of higher conductivity than the continuous phase. Although the improvement in the thermal properties of adhesives by the addition of metal fillers is obvious, their influence on water sorption characteristics of adhesives is not clear. It was the objective of this study to shed light on these aspects, which are lacking in the literature. The emphasis was placed on determining the moisture sorption behavior of aluminum‐powder‐filled epoxy adhesives under complete immersion in distilled water and sea water. Moisture diffusion tests show that the addition of aluminum filler into epoxy decreases the total amount of water intake at saturation in both fluids. However, there appears to be no significant effect of the aluminum filler content on the moisture diffusivity in epoxy adhesive specimens in either distilled water or seawater. It has also been determined that the adhesives adsorb a larger amount of water upon exposure to distilled water than when exposed to seawater, whereas the moisture diffusion rate in the adhesive immersed in seawater is higher than that in distilled water. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1165–1171, 2005     

Thermal stability of structural one‐component polyurethane adhesives for wood—structure‐property relationship

The relationship between the chemical structure of commercial polyurethanes and temperature‐dependent creep properties was determined in full scale tests and the results were compared with thermomechanical analysis. Comparison of mechanical performance with ¹³C‐NMR spectroscopy studies elucidated important structure‐property relationships, which either allow the reduction or elimination of temperature‐dependent creep in one‐component polyurethanes (1C‐PUR) adhesives for wood. The combination of the relative content of still reactive, free ? NCO groups on the polyurethane, careful selection of the degree of resin polymerization and a slower rate of reaction are the three most significant parameters that have to be controlled to overcome the problem of temperature‐dependent creep found in 1C‐PUR adhesives. The results obtained indicate that adhesives presenting a combination of a higher content of still unreacted ? NCO groups, a lower degree of polymerization and slower reaction rate are capable to counteract problems of high sensitivity of polyurethane to temperature‐dependent creep. Two commercial polyurethanes that fulfil the latter requirements and exhibit almost no creep were identified and characterized. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5698–5707, 2006     

Properties of reactive hot melt polyurethane adhesives with acrylic polymer or macromonomer modifications

The properties of reactive hot melt polyurethane adhesives (RHAs) modified by dihydroxyl‐terminated reactive acrylic macromonomers were examined and compared with those of RHAs modified with a nonreactive commercial acrylic copolymer. The melt viscosity and initial bond strength was increased when a portion of the polyols and tackifier was replaced by macromonomers or acrylic copolymers, which have higher glass transition temperatures. When RHA was modified by macromonomer, the storage modulus at low shear rate was reduced and set time was increased, which suggests that the chain entanglement was diminished by short‐chain branches created by the macromonomers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mechanical properties of crosslinked polyurethane elastomers based on well‐defined prepolymers

This article presents research findings for selected mechanical properties of polyurethane elastomers. The studied elastomers were synthesized with the prepolymer‐based method with the use of controlled molecular weight distribution (MWD) urethane oligomers and with the classical single‐stage method. Prepolymers with defined MWDs were obtained with the use of a multistage method, that is, step‐by‐step polyaddition. To produce elastomers, isocyanate oligomers were then crosslinked with triethanolamine, whereas hydroxyl oligomers were crosslinked with 4,4′,4′′‐triphenylmethane triisocyanate (Desmodur RE). The tensile strength of the obtained elastomers ranged from 1.0 to 7.0 MPa, the ultimate elongation approached 1700%, the Shore A hardness varied from 40 to 93°, and the abrasion resistance index fell within 15–140. The effects of the types of raw materials used, the chemical structures, the production methods, and the supermolecular structures on the mechanical properties of the obtained polyurethane elastomers were examined. When the obtained findings were generalized, it was concluded that the structural changes in the polyurethanes, which were favorable for intermolecular interactions, improved the tensile strength, hardness, and abrasion resistance of the materials and impaired their ultimate elongation at the same time. More orderly supermolecular structures and, therefore, superior mechanical properties were found for polyurethane elastomers produced with the prepolymer method. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Improved mechanical properties of shape‐memory polyurethane block copolymers through the control of the soft‐segment arrangement

High‐performance shape‐memory polyurethane block copolymers, prepared with two types of poly(tetramethylene glycol) (PTMG) used as soft segments, were investigated for their mechanical properties. Copolymers with a random or block soft‐segment arrangement had higher stresses at break and elongations at break than those with only one kind of PTMG. Random copolymers with fewer interchain interactions showed higher elongation than block copolymers. All the copolymers had shape‐recovery ratios higher than 80%. In dynamic mechanical testing, the glass‐transition behavior clearly depended on the soft‐segment arrangement: random copolymers had only one glass‐transition peak, whereas block copolymers showed two separate glass‐transition peaks. Overall, the control of the soft‐segment arrangement plays a vital role in the development of high‐performance shape‐memory polyurethane. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2410–2415, 2004     

Effect of difunctional acids on the physicochemical,thermal, and mechanical properties of polyester polyol‐based polyurethane coatings

Polyester polyol (PP)‐based polyurethanes (PUs) consisting of two difunctional acids [1,4‐cyclohexanedicarboxylic acid (CHDA) and 1,6‐adipic acid (AA)] and also two diols [1,4‐cyclohexanedimethanol (CHDM) and 1,6‐hexanediol (HDO)] were synthesized by a two‐step procedure with a variable feed ratio of CHDA to AA but fixed ratio of CHDM and HDO. The prepared PPs and/or PUs were characterized by Fourier transform infrared spectroscopy, X‐ray diffraction spectroscopy, and atomic force microscopy. The effects of difunctional acids on the thermal, mechanical, and dynamic mechanical thermal properties of PPs or PU films were investigated by thermogravimetry analysis, differential thermogravimetry and dynamic mechanical thermal analysis. The results show that PP exhibits a lowest viscosity with the mole fraction of CHDA and AA at 3 : 7 whereas it delivers a lowest melting point with the mole fraction at 9 : 1. After PPs being cross‐linked by isocyanate trimers, the impact resistance, shear strength and glass transition temperature increase the mixed‐acid formulations with increasing the content of CHDA. In detail, the resultant PU almost simultaneously exhibits the best mechanical and thermal properties when the mole fraction of CHDA and AA is kept constant at 9 : 1, thus giving rise to a high glass transition temperature of 56.4°C and a onset decomposition temperature of 350°C, and also delivering a balanced toughness and hardness with an impact resistance of 100 J/g and storage modulus as high as 10⁹ Pa. This path for synthesis of PP‐based PU provides a design tool for high performance polymer coatings. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41246.     

Use of pyrolysed oil shale as filler in poly(ethylene‐co‐vinyl alcohol)

The pyrolysed oil shale (POS) obtained from the pyrolysis of bituminous rock was used as filler in poly(ethylene‐co‐vinyl alcohol) (EVAL). The effects of vinyl alcohol content in the EVAL and the particle size of pyrolysed oil shale in the mechanical properties were investigated. The EVAL was prepared by hydrolysis of poly(ethylene‐co‐vinyl acetate) (EVA) with 8 and 18 wt % of vinyl alcohol content. The composites were prepared in a rotor mixer at 180°C with concentration of pyrolysed oil shale up to 5 wt %. Stress–strain plots of compression‐molded composites showed a synergic behavior in the mechanical properties for low concentrations (1–5 wt %) of POS in all particle sizes and EVAL used. Such behavior indicates a close packing and strong interactions between the inorganic filler and the polymer. Increasing of the vinyl alcohol content of EVAL improved the compatibility between the polymer and filler, but decreasing the POS particle size had no effect on the properties. The modulus and the ultimate tensile strength also increased in all concentrations of POS for both EVAL. Mechanical properties and dynamic mechanical analysis also demonstrated the compatibility between EVAL and POS. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1658–1665, 2004     

Characteristics of polyurethane adhesives with various uretonimine contents in isocyanate and average alcohol functionalities

Polyurethane adhesives are found in a variety of applications. To ensure durability and functionality, a polyurethane adhesive is required to possess good adhesive properties and thermal stability. In this study, polyurethane adhesives were fabricated using isocyanate and polyols of various type and functionality (f_av). It was found that peel strength of an adhesive is strongly related to the f_av of a polyol and uretonimine content. At high uretonimine content, the probability of forming hydrogen bonded component increases and as a result the adhesive's peel strength increases. At high f_av and molecular weight of a polyol, an adhesive's wettability decreases so does its adhesion to a substrate. Therefore, designing for a functional adhesive, high uretonimine content and low f_av of polyol are desired. Thermal stability of a polyurethane adhesive also improves with uretonimine content. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43737.     

Strengthening the mechanical properties of polysulfide adhesives by introducing polyurethane

Polysulfide adhesives are often subjected to breakage or even fracture caused by highly loaded vibrations and large deformations from the wing, limiting their practical use in aircraft fuel tanks. Inserting polyurethanes into liquid polysulfide systems is a viable approach to strengthening their mechanical properties and avoiding nonuniform dispersion of the curing agent. In this contribution, a series of sulfur-containing curing agents were prepared by liquid polysulfide and trimethylolpropane tris(3-mercapto propionate) (TMPMP) to minimize the effect of the polysulfide-urea (-NHCOS-) groups on water resistance. Subsequently, the two-component polysulfide adhesives were successfully synthesized via the prepolymerization method, and their chemical structure, mechanical properties, and solvent resistance were systematically evaluated. As expected, the large introduction of sulfur linkages and -NHCOS- groups provided excellent oil resistance and strong mechanical properties for polysulfide adhesives. Notably, Samples 2-4 exhibited the highest tensile strength of 1.11 ± 0.02 MPa, the greatest shear strength of 1.87 ± 0.04 MPa, and the best hardness of 81 ± 2 Shore A, with 122.0%, 523.3%, and 170.0% improvement over the control, respectively. Furthermore, the oil absorption rate of all samples was less than 3.0%, and their tensile strength remained almost unchanged after 30 days of immersion than before. We believe our paradigm can provide a valuable guideline for designing high-performance polysulfide adhesives.