Dynamic colloidal processes in waterborne two-component polyurethanes and their effects on solution and film morphology

While waterborne two-component polyurethanes (WB 2K-PURs) offer an attractive approach toward the reduction of volatile organic compounds (VOC), a number of fundamental processes related to the colloidal stability and solution morphology of WB 2K-PUR reactive dispersions are not understood. These studies focus on mechanisms of inter-particle reactant aggregation in heterogeneous aqueous dispersions resulting from solution morphological changes of poly(ethylene glycol) modified water-dispersible polyisocyanates (WDPI) and water-reducible polyester polyol coreactants, as revealed by particle size measurements and in situ NMR T₂ studies. Based on the results of these experiments, a model of film formation is proposed which relates solution morphological features and chemical reactions. WDPI reactant droplets become hydrated over time, leading to formation of carbon dioxide, which dissolves in the aqueous continuum as carbonic acid and subsequently destabilizes anionically stabilized polyol droplets, resulting in their adsorption to WDPI droplet surfaces. The implications of polyol collapse, as they pertain to film formation, are discussed in the context of solution and film morphologies.     

Preparation and properties of waterborne polyurethanes with natural dimer fatty acids based polyester polyol as soft segment

Waterborne polyurethanes (WPUs) with natural dimer fatty acids (DA) based polyester polyols as soft segments were prepared via the acetone process, and characterized by Fourier transform infrared (FTIR) spectroscopy and gel permeation chromatography (GPC). The thermal and mechanical properties were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and tensile tester. Water contact angle on the surface of the WPU films increased from 80° to 93° with increasing DA weight content in dicarboxylic acid of polyester polyol from 0 to 100%. The results indicated that among the synthesized WPUs, those containing DA exhibited excellent water resistance (water uptake decreased from 13.2% to 2.42%), outstanding hydrolytic resistance (weight loss decreased from 14.2% to 5.85%) and superior thermal stability (decomposition temperature of 50% weight loss and maximum weight loss, increased from 384 to 443 °C and from 409 to 464 °C, respectively). Compared to toluene resistance and mechanical properties of WPUs containing no DA, those of WPUs containing DA were low. For the WPU films based on polyester polyol derived from 100% DA as dicarboxylic acid, the interesting delamination occurred in special solvents, probably due to dissolution of low molecular weight molecules, and the presence of long hydrophobic branched chains and the high degree of phase separation.     

A new method to synthesize high solid content waterborne polyurethanes by strict control of bimodal particle size distribution

A new method named two-step emulsification process was developed to synthesize high solid content waterborne polyurethanes by strict control of the bimodal particle size distribution. In the first step, a series of 40% solid content polyester-based (WPU-1) with low content of hydrophilic group and large particle size were firstly synthesized. In the second step, polyether-based prepolymers (WPU-2 prepolymers) with high content of hydrophilic group were firstly prepared and WPU-1 emulsions were used to emulsify WPU-2 prepolymers to obtain the final emulsions with high solid content (WPU-3). The particle size of WPU-3 present bimodal distribution and the diameter ratio and volume percentage of large particles to small particles in WPU-3 were able to be strictly controlled by this method. The viscosity of WPU-3 with 55% solid content was only 489.1 mPa s⁻¹ when the diameter ratio of large particles to small particles was 9.2 and the volume percentage of large particles was 74%.     

Effect of the hard segment on the properties of UV curable waterborne blocked polyurethanes

Two types of UV curable waterborne blocked polyurethanes (PUs) were obtained from 2,4-tolylene diisocyanate(TDI) and isophorone diisocyanate (IPDI), which had the same soft segments and different hard segments. The PUs contained both the UV curable C=C bond and the blocked NCO groups which could be de-blocked when heated. Effect of the hard segment on the properties of the two types of PUs was investigated. The FTIR spectra method was employed to confirm the structure of the PUs. The photo-DSC was used to study the photo-polymerization rate of the PUs under UV irradiation in the presence of a photo-initiator and the C=C conversion behaviors. Heating-up IR analysis was used to track the de-blocking process and the heat curing process. The thermal degradation analysis (TGA) was employed to investigate the thermal stability of the UV cured films before and after heat curing process. Dynamic mechanical properties of the UV and heat cured films were investigated by the dynamic mechanical thermal analysis (DMTA).     

Effect of the diisocyanate structure and the molecular weight of diols on bio‐based polyurethanes

Bio‐based polyurethanes (PU) containing poly(ε‐caprolactone) diol (PCL) and hydroxyl telechelic natural rubber (HTNR) were synthesized. The effect of the diisocyanate structure and the molecular weights of diols on the mechanical properties of PU were investigated. Three different molecular structures of diisocyanate were employed: an aliphatic diisocyanate (hexamethylene diisocyanate, HDI), an aromatic diisocyanate (toluene‐2,4‐diisocyanate, TDI) and a cycloalkane diisocyanate (isophorone diisocyanate, IPDI). Two molecular weights of each diol were selected. When HDI was employed, a crystalline PU was generated while asymmetrical structures of TDI and IPDI provided an amorphous PU. The presence of crystalline domains was responsible of a change in tensile behavior and physical properties. PU containing TDI and IPDI showed a rubber‐like behavior: low Young's modulus and high elongation at break. The crystalline domains in PU containing HDI acted as physical crosslinks, enhancing the Young's modulus and reducing the elongation at break, and they are responsible of the plastic yielding. The crystallinity increased the tear strength, the hardness and the thermal stability of PU. There was no significant difference between the TDI and IPDI on the mechanical properties and the physical characteristics. Higher molecular weight of PCL diol changed tensile behavior from the rubber‐like materials to the plastic yielding. Thermal and dynamic mechanical properties were determined by using DSC, TGA and DMTA. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis of isophorone diisocyanate (IPDI) based waterborne polyurethanes: Comparison between zirconium and tin catalysts in the polymerization process

In this work, a comparative study of the catalytic activity of tin and zirconium compounds in the polymerization of isophorone diisocyanate (IPDI) based waterborne polyurethanes is presented. Zirconium acetyl acetonate is much less toxic than the commonly used dibutyltin diacetate but it is deactivated in the presence of acid groups. Nevertheless, acid groups are commonly used in the synthesis of waterborne polyurethanes and therefore these groups have to be neutralized in advance for carrying out the synthesis using the zirconium catalyst. Moreover, FTIR and ¹³C NMR results have shown that in the presence of triethyl amine and tin catalyst, the secondary isocyanate group is more reactive than the primary group, while using the zirconium catalyst, the two isocyanate groups show the same reactivity.     

Network structure and properties of polyurethanes from soybean oil

Vegetable oils are very heterogeneous materials with a wide distribution of triacylglycerol structures and double‐bond contents. The hydrogenation of epoxidized soybean oil (ESO) produces polyols having a functionality distribution related to that of soybean oil. Therefore, these polyols are convenient substances for studying the impact of structural heterogeneity on network formation and properties. Polyols of hydroxyl numbers ranging from 225 to 82 mg KOH/g and weight‐average functionalities ranging from 4.4 to 2.7 were obtained by the variation of the time of hydrogenation of ESO. An analysis of the functionality distribution in polyols shows that gel points with diisocyanates vary from 54 to 76% conversion. The molecular weights of the network chains of polyurethanes prepared from these polyols and diphenyl methane diisocyanate varied from 688 to 1993. Polyols with hydroxyl numbers above 200 mg KOH/g gave glassy polymers, whereas those below that value gave rubbers. The heterogeneity of polyols had a negative effect on the elastic properties only at low crosslinking densities. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of novel aliphatic fluoro-diol content on synthesis and properties of waterborne polyurethanes

This study synthesized a waterborne polyurethane (WPU) prepolymer by using polytetramethylene glycol, isophorone diisocyanate, and 2,2,3,3,4,4,4-Heptafluoro-butynic acid 2,2-bis-hydroxymethyl-butyl ester (HFBA). Subsequently, a series of novel HFBA/WPUs were synthesized by adopting ethylenediamine as the chain extender. The results from Fourier transform infrared spectroscopy demonstrated that the CO groups of HFBA/WPUs shifted to lower wavenumbers as the HFBA content increased. Moreover, when the HFBA content of the synthesized HFBA/WPUs increased, the WPU initial decomposition temperature, glass transition temperature, tensile strength, and Young's modulus also increased. Furthermore, the results of atomic force microscopy identified a relatively high number of humpy protrusions and roughness levels in the HFBA/WPUs with a relatively high HFBA content, causing higher hydrophobicity of these HFBA/WPUs. Furthermore, in vitro platelet and erythrocyte adhesion experiments revealed that increasing the HFBA content lowered platelet and erythrocyte adhesion on the surface of the WPUs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47356.     

Effects on the structure and properties of membranes formed by blending polydimethylsiloxane polyurethane into different soft‐segment waterborne polyurethanes

Polydimethylsiloxane polyurethane (PDMS‐PU), which was synthesized from PDMS as the soft segment, was blended into a variety of ester‐ or ether‐based soft‐segment waterborne polyurethanes with different concentrations to investigate the crystallization, thermal, and physical properties of the membrane formations. According to X‐ray analysis, the ether‐based PUs, synthesized from soft segments of poly(propylene glycol) (PPG1000) or poly(ethylene glycol) (PEG2000), were found to have maximum crystallinity at a 5% blending ratio of PDMS‐PU, but the ester‐based PU, synthesized from soft segments of polycaprolactone (PCL1250), had decreased crystallinity at a 5% blending ratio. Differential scanning calorimetric analysis revealed that the T_g,s values of PUs were highest when the blending ratio of PDMS‐PU was 5%–10%, except for PU from PCL1250. Moreover, ether‐based PUs showed maximum T_m,h values, but the T_m,h of the ester‐based PU was greatly reduced when PU with PCL1250 was blended with PDMS‐PU. In addition, the PU from PEG2000 had the highest melting entropy. Mechanical property analysis showed that the stress of ether‐based PUs would be increased when PUs were blended with a small amount of PDMS‐PU and that the stress of PU from poly(tetramethylene glycol) (PTMG1000) increased to its greatest value (20–30 MPa). On the other hand, the ester‐based PU, from PCL1250 blended with PDMS‐PU, would have reduced stress. On the whole, the stress and strain of PU from PEG1000 had excellent balance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 210–221, 2006     

Influence of molecular weight on the fracture properties of aliphatic polyketone terpolymers

The influence of polymer molecular weight on the mechanical properties of aliphatic polyketones was investigated. The molecular weight varied from 100,000 to 300,000 g mol⁻¹. The crystallinity was found to be independent of polymer molecular weight, as was the glass transition temperature. The yield strength and stiffness of the aliphatic polyketone terpolymers were also found to be independent of molecular weight. The post yield behaviour showed strong dependency on polymer chain length. The draw stress was increased significantly with higher molecular weight material. The impact resistance was increased with molecular weight, resulting in ductile fractures with large energy consumption upon fracture. The brittle-to-ductile transition temperature was lowered with increasing chain length. The difference in material deformation was linked to the higher mechanical connectivity and more stable post yield behaviour of the polymers with an increased molecular weight.     

Enhancement of hydrolytic stability and adhesion of waterborne polyurethanes

A new type of polyester polyol, with alkyl side groups, viz. poly(2,4‐diethyl‐1,5‐pentamethylene adipate) glycol (PDPAd) was synthesized and used to improve the hydrolytic stability of waterborne polyurethanes (PU). The results compared favorably with poly(tetramethylene adipate) glycol (PTAd)–based PU and blends of the two types of PU in terms of particle size, thermal, XRD, mechanical, dynamic mechanical, and swell behavior of the dispersion cast films in addition to hydrolytic stability and adhesion properties. Blends of PTAd‐based PU and PDPAd‐based PU gave significantly improved green (immediate) adhesion and hydrolytic stability due to the synergistic effects of crystallinity (heat of crystallization, high density) and amorphous regions (tack, high thermal stability). © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1961–1969, 2005     

Influence of molecular weight of polymer matrix on the structure and rheological properties of graphene oxide/polydimethylsiloxane composites

The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (C_cr). Moreover, at GO concentration just above C_cr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (M_c). However, positive ΔN is found in samples with PDMS molecular weight above M_c. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.     

Effect of different isocyanates on the properties of soy‐based polyurethanes

New types of polyurethanes were prepared by reacting soybean oil‐based polyol and different isocyanates. The polyurethanes can be used as foams, elastomers, coatings, adhesives, etc. Their properties strongly depend on crosslinking density and the structure of isocyanates. Aromatic triisocyanates impart the highest density, glass transition, modulus, and tensile strength, but have the lowest elongation at break, swelling in toluene, and impact resistance. Aliphatic triisocyanates and diisocyanates give rubbery materials with the highest elongation at break, highest swelling, and the lowest tensile strength. Polyurethanes with aromatic and cycloaliphatic diisocyanates were similar in properties, with values between those of the two groups. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2912–2916, 2003     

Effects of aggregation structure on rheological properties of thermoplastic polyurethanes

The effects of the molecular aggregation structure on the rheological properties of thermoplastic polyurethane (TPU) were investigated. The TPU was composed of poly{(tetramethylene adipate)-co-(hexamethylene adipate)} glycol as the soft segments, 4,4′-diphenylmethane diisocyanate and 1,4-butanediol as the hard segments. The TPU sheets prepared by injection molding were annealed at various temperatures from 23 to 120 °C to vary the molecular aggregation structure. Glass transition temperature of the soft segment and melting points of the hard segment domains of the TPUs decreased and increased, respectively, with increasing annealing temperature. The results of DSC, solid-state NMR spectroscopy and dynamic viscoelastic measurements revealed that the degree of micro-phase separation of the TPUs becomes stronger with increasing annealing temperature due to the progress of formation of well-organized hard segment domains. The dynamic temperature sweep experiments for molten TPUs revealed that the temperature at critical gel point, which is defined as the temperature at which the dynamic storage modulus coincides with the loss storage modulus, in the cooling process increased with the progress of aggregation of the hard segments in the TPUs observed in the solid state. The uniaxial elongational viscosity measurements showed that TPUs exhibited an obvious strain hardening behavior with strain rate owing to residual hard segment domains at an operating temperature. It was revealed that the formation of well-organized hard segment domains had a profound effect on the rheological properties of TPUs, in particular on their elongational viscosity.     

The chain extender content and NCO/OH ratio flexibly tune the properties of natural rubber‐based waterborne polyurethanes

The effects of chain extender content (ethylene diamine, EDA) and NCO/OH ratio on the properties of natural rubber‐based waterborne polyurethanes (WPUs) were investigated experimentally. The particle size of WPU increased significantly with the NCO/OH ratio, in the presence of the EDA chain extender, while it was unaffected by the EDA content. The water uptake of WPU film increased with the EDA content, while the swelling in various solvents decreased. In a thermal analysis, the second decomposition stage of a WPU film increased with the EDA content and with the NCO/OH ratio that also positively affected the dynamic mechanical and mechanical properties. These factors in WPU films had no the effect on the T_g. The stress–strain curves clearly showed the change in WPU films from soft elastomeric materials to ductile and hard plastics. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42505.     

H-bond and conformations of donors and acceptors in model polyether based polyurethanes

The molecular mechanics (MM) method with COMPASS force field was used to study the H-bonds in the polyether based polyurethane model molecules. Availability of the calculation was firstly verified in comparison of some H-bonded model molecules, which were studied by using ab initio calculation, and those calculated by MM. Based on a urethane model molecule 1,3-dimethylcarbamate, which can be donor or acceptor and behaves in various conformations, it is reasonable to have a large number of H-bond interactions between various conformational donors and acceptors. For examining all the possible interaction patterns, we studied 57H-bond complexes. This systematic modeling covers well-known four types of interaction patterns, such as NH?OC (Type I), NH?O-CO (Type II), NH?NH (Type III), NH?COC (Type IV) in the system. Obtained H-bond energies were used to analyze the probabilities of the complexes. For the interaction within the hard segments, or Type I, Type II and Type III, a predominant H-bond complex has been found in the present study, which belongs to Type I. For the interaction between the hard segment and the soft segment, two conformations of Type IV were calculated to be existed.     

Effect of macroglycol structure and its molecular weight on physicomechanical properties of polyurethanes

The molecular weight of polyester polyol plays an important role in controlling mechanical and dynamic mechanical properties of polyurethanes. Two systems are studied for evaluation of these properties. Since the properties of polyurethanes are derived from the properties of various constituents involved in their synthesis, the magnitude of properties of polyurethanes are also studied. Dynamic mechanical properties are analyzed using a dynamic mechanical analyzer that employs the principle of compound resonance. © 1995 John Wiley & Sons, Inc.     

Thermoplastic polyurethanes from renewable resources: effect of soft segment chemical structure and molecular weight on morphology and final properties

The effect of soft segment molecular weight and chemical structure on the morphology and final properties of segmented thermoplastic polyurethanes containing various hard segment contents has been investigated from the viewpoint of the degree of microphase separation. Vegetable oil‐based polyesters and corn sugar‐based chain extender have been used as renewable resources. The synthesis has been carried out in bulk without catalyst using a two‐step polymerization process. Physicochemical, thermal and mechanical properties, and also morphology, have been studied using Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, atomic force microscopy, X‐ray diffraction and mechanical testing. Chemical structure and molecular weight of polyols strongly affect the properties of the synthesized segmented thermoplastic polyurethanes. An increase in soft segment molecular weight leads to an increase of the degree of soft segment crystallinity and microphase separation, thus giving enhanced mechanical properties and higher thermal stability. Copyright © 2012 Society of Chemical Industry     

Preparation and properties of polydimethylsiloxane (PDMS)/polytetramethyleneadipate glycol (PTAd)-based waterborne polyurethane adhesives: Effect of PDMS molecular weight and content

Waterborne polyurethane (WBPU) dispersions were prepared by pre-polymer process using siloxane polyol, namely polydimethylsiloxane (PDMS), and polyester polyol, namely poly(tetramethyleneadipate glycol) (PTAd), as a soft segment. Three different molecular weights (M_n = 550, 6000, 110,000) of PDMS and one fixed molecular weight of PTAd (M_n = 2000) was used during preparation of WBPU dispersions. This research aims to explore the potential use of PDMS in complementing WBPU by boosting flexibility, water resistance, and adhesive strength. The water swelling (%), tensile strength, and adhesive strength of WBPUs were investigated with respect to PDMS molecular weight and PDMS content (PDMS mol %). The water swelling (%) and tensile strength decreased with increasing PDMS molecular weight at a fixed PDMS content (mol %) in mixed polyol of WBPU films. By contrast, the peel adhesive strength peaked at 6.64 mol % and 4.43 mol % with molecular weight of PDMS at 550 and 6000, respectively, while it only decreased when the molecular weight of PDMS stood at 110,000. The adhesive strength was almost unaffected with optimum content (6.64 mol %) of lower PDMS molecular weight (M_n = 550) in mixed polyol-based WBPU after immersing the adhesive bonded nylon fabrics in water for 48 h among all of the samples. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012