Effect of polyols and diisocyanates on thermo-mechanical and morphological properties of polyurethanes

Understanding the Interactions between Soft Segments in Polyurethanes: Structural Synergies in Blends of Polyester and Polycarbonate Diol Polyols

There are no previous studies on the interactions between polyols of different nature as a model for understanding the interactions between soft segments in PUs. In this study, different blends of two polyols of different natures (polyester-PE, and polycarbonate diol-CD) and similar molecular weights were prepared and their structural, thermal, surface, viscoelastic, and self-adhesion properties were assessed. Different experimental techniques were used: infrared spectroscopy (ATR-IR), differential scanning calorimetry (DSC), X-ray diffraction, thermal gravimetric analysis (TGA), and plate-plate rheology. PE showed a larger number of structural repeating units and a higher number of polar groups than CD, but the carbonate-carbonate interactions in CD were stronger than the ester-ester interactions in PE. The blending of CD and PE imparted synergic structural properties, particularly in the blends containing less than 50 wt.% PE, they were associated with the disrupt of the carbonate-carbonate interactions in CD and the formation of new ester-carbonate and hydroxyl-carbonate interactions. CD + PE blends with less than 50 wt.% PE exhibited higher glass transition temperatures, a new diffraction peak at 2θ = 24°, one additional thermal degradation at 426-436 °C, and a less-steep decline of the storage moduli. Furthermore, the different interactions between the polyol chains in the blends were also evidenced on their surface properties, and all CD + PE blends showed self-adhesion properties which seemed related to the existence of ester-carbonate and carbonate-carbonate interactions.

Influence of Long-Term Storage and UV Light Exposure on Characteristics of Polyurethane Foams for Cryogenic Insulation

Rigid polyurethane (PUR) foams have been the most effective insulation material used in space launchers since the beginning of cryogenic fuel use, due to their outstanding thermal and mechanical properties. In this study, spray-applied PUR foams using different ratios of amine-based catalysts were produced. Due to climate change, several restrictions have been made regarding the usage of blowing agents used for PUR foam production. Lately, hydrofluoroolefins (HFOs) have been suggested as an alternative for PUR foam production due to their low global warming potential (GWP) and ozone depletion potential (ODP), replacing the hydrofluorocarbons (HFCs) so far used. This change in blowing agents naturally altered the usage of catalysts. Reactive amine-based catalysts are less hazardous because of their low volatility and ability to react successfully with isocyanate or polyols. Spray-applied PUR foams with a potential application for cryogenic insulation were produced and tested for long-term storage, analyzing parameters such as the pH value of polyol composition, foaming kinetics (trise, tcream), etc. Athermal analysis (TG, DSC) was also applied to developed materials, as well as artificial ageing by exposing samples to UV light. It was discovered that PUR foams obtained using reactive amine-based catalysts, such as Polycat 203 and 218, have a higher integral heat capacity, but polyol mixtures containing these catalysts cannot exceed a storage time of more than 4 months. It was also observed from artificial ageing tests of PUR cryogenic insulation by exposure to UV light that the thickness of the degraded layer reached 0.8 mm (after 1000 h), but no significant destruction of cellular structure deeper in the material was observed.

Synthesis and Characterisation of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-b-poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Multi-Block Copolymers Produced Using Diisocyanate Chemistry

Bacterially derived polyhydroxyalkanoates (PHAs) are attractive alternatives to commodity petroleum-derived plastics. The most common forms of the short chain length (scl-) PHAs, including poly(3-hydroxybutyrate) (P3HB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), are currently limited in application because they are relatively stiff and brittle. The synthesis of PHA-b-PHA block copolymers could enhance the physical properties of PHAs. Therefore, this work explores the synthesis of PHBV-b-PHBV using relatively high molecular weight hydroxy-functionalised PHBV starting materials, coupled using facile diisocyanate chemistry, delivering industrially relevant high-molecular-weight block copolymeric products. A two-step synthesis approach was compared with a one-step approach, both of which resulted in successful block copolymer production. However, the two-step synthesis was shown to be less effective in building molecular weight. Both synthetic approaches were affected by additional isocyanate reactions resulting in the formation of by-products such as allophanate and likely biuret groups, which delivered partial cross-linking and higher molecular weights in the resulting multi-block products, identified for the first time as likely and significant by-products in such reactions, affecting the product performance.

A thermodynamic study on relationship between gas separation properties and microstructure of polyurethane membranes

The lattice fluid (LF) thermodynamic model and extended Vrentas' free-volume (E-VSD) theory were coupled to study the gas separation properties of the linear thermoplastic polyurethane (TPU) membranes with different chemical structures by analyzing their microstructures. A set of characteristic parameters were extracted using the repeating unit of the TPU samples and led to prediction of reliable polymer densities (AARD < 6%) and gas solubilities. The viscoelastic parameters, which were obtained from the DMTA analysis, were also estimated the gas diffusion vs. temperature, precisely. The degree of microphase mixing based on the DSC analysis was in order: TPU-1 (4.84 wt%) < TPU-2 (14.16 wt%) < TPU-3 (19.92 wt%). It was found that the TPU-1 membrane had the highest degree of crystallinity, but showed higher gas solubilities and permeabilities because this membrane has the least degree of microphase mixing. These values, in combination with the gas permeation results, showed that the content of the hard segment along with the degree of microphase mixing and other microstructural parameters like crystallinity were the determinative parameters.

Influence of Reactive Amine-Based Catalysts on Cryogenic Properties of Rigid Polyurethane Foams for Space and On-Ground Applications

Rigid polyurethane (PUR) foams have outstanding properties, and some of them are successfully used even today as cryogenic insulation. The fourth-generation blowing agent Solstice^® LBA and commercial polyols were used for the production of a low-density cryogenic PUR foam composition. A lab-scale pouring method for PUR foam preparation and up-scaling of the processes using an industrial spraying machine are described in this article. For the determination of the foam properties at cryogenic temperature, original methods, devices, and appliances were used. The properties at room and cryogenic temperatures of the developed PUR foams using a low-toxicity, bismuth-based, and low-emission amine catalyst were compared with a reference foam with a conventional tin-based additive amine catalyst. It was found that the values of important cryogenic characteristics such as adhesion strength after cryoshock and the safety coefficient of the PUR foams formed with new reactive-type amine-based catalysts and with the blowing agent Solstice^® LBA were higher than those of the foam with conventional catalysts.

Hydrolytically Stable and Thermo-Mechanically Tunable Poly(Urethane) Thermoset Networks that Selectively Degrade and Generate Reusable Molecules

Cross-linked polymeric networks that possess tunable properties and degrade on-demand have broad applications in today's society. Herein, we report on silyl-containing poly(urethane) (silyl-PU) thermoset networks, which are highly cross-linked stimuli-responsive materials with hydrolytic stability at 37.7 °C and 95% relative humidity, thermal stability of 280-311.2 °C, tensile properties of 0.38-51.7 MPa strength and 73.7-256.4% elongation, including storage modulus of 2268-3499 MPa (in the glassy state). However, unlike traditional (i.e., nondegradable) PU thermosets, these silyl-PUs selectively activate with fluoride ion under mild and static conditions to completely degrade, via cascading bond cleavages, and generate recoverable and reusable molecules. Silyl-PUs, as thin films, also demonstrated complete removal (within 30 min) from a strongly adhered epoxy thermoset network without altering the structure of the latter. Silyl-PU thermosets have potential applications in composite parts, vehicle and industrial coatings, and rigid plastics for personal devices, and may reduce environmental waste compared to nondegradable, single-use materials.

Polyurethane elastomers with amphiphilic ABA tri-block co-polymers as the soft segments showing record-high tensile strength and simultaneously increased ductility

Polyurethane elastomers with amphiphilic ABA tri-block co-polymers as the soft segments robustly show record-high tensile strength and simultaneously increased ductility via producing small and uniform hard domains.

Influence of Silica-Aerogel on Mechanical Characteristics of Polyurethane-Based Composites: Thermal Conductivity and Strength

Polyurethane foam (PUF) has generally been used in liquefied natural gas (LNG) carrier cargo containment systems (CCSs) owing to its excellent mechanical and thermal properties over a wide range of temperatures. An LNG CCS must be designed to withstand extreme environmental conditions. However, as the insulation material for LNGC CCSs, PUF has two major limitations: its strength and thermal conductivity. In the present study, PUFs were synthesized with various weight percentages of porous silica aerogel to reinforce the characteristics of PUF used in LNG carrier insulation systems. To evaluate the mechanical strength of the PUF-silica aerogel composites considering LNG loading/unloading environmental conditions, compressive tests were conducted at room temperature (20 °C) and a cryogenic temperature (−163 °C). In addition, the thermal insulation performance and cellular structure were identified to analyze the effects of silica aerogels on cell morphology. The cell morphology of PUF-silica aerogel composites was relatively homogeneous, and the cell shape remained closed at 1 wt.% in comparison to the other concentrations. As a result, the mechanical and thermal properties were significantly improved by the addition of 1 wt.% silica aerogel to the PUF. The mechanical properties were reduced by increasing the silica aerogel content to 3 wt.% and 5 wt.%, mainly because of the pores generated on the surface of the composites.

Environmental impact assessment of air-permeable plastic runway production in China

With the rapid development of plastic runways in China, incidents of toxic runways that are detrimental to human health frequently occurred. This phenomenon has resulted in public concern on the safety and cleanliness of plastic runways. To improve the sustainability of these runways, the environmental performance of the produced plastic runways should be evaluated. The critical hotspots for plastic runway studies should be determined, and a cleaner optimization path of critical materials should be explored. In this study, a cradle-to-gate life cycle assessment (LCA) on the air-permeable plastic runway was conducted. The green factory formula was identified, and the environmental impacts of the production process were quantitatively analyzed. Detailed life cycle inventory data were obtained from the on-site survey of typical plastic runway manufacturer enterprises in China. Environmental impacts were calculated using the CML 2001 method built into the GaBi 8.0 software. Results indicated that the 1: 7 ratio of polyurethane adhesive to ethylene-propylene-diene monomer rubber particles was the greenest formula with the least environmental impact. The environmental hotspots were from the front-end of raw material production during the mixing phase and the biomass steam input during the curing phase. The characteristic pollutants generated from mixing phase were CO₂, methane, NO_x, and VOCs, whereas those from the curing phase were CO₂, NO_x, SO₂, freon, HCl, and NH₃. Moreover, methylene diisocyanate (MDI) was the cleaner raw material for air-permeable plastic runway production, because the environmental impact of producing an equal mass of MDI was 39%-89% of that by toluene diisocyanate. Thus, this LCA study presents a strategy for the sustainable improvement of air-permeable plastic runway production and also proposes policy recommendations for decision makers.

Synthesis and characterization of polyethylene glycol-phenol-formaldehyde based polyurethane composite

A series of phenol-formaldehyde-polyethylene glycol polyether polyols (PF-PEGs) were synthesized through the condensation polymerization and etherification of phenol, formaldehyde, and poly(ethylene glycol) (PEG) under alkaline conditions and subsequently reacted with 1,6-hexamethylene diisocyanate to obtain polyurethane (PU) films using acetone as solvents. The influence of phenol and formaldehyde to PEG mass ratio ((P + F)/PEG) on the hydroxyl number of PF-PEGs and mechanical properties, thermal stabilities, crystallization behaviors, as well as microstructure of polyurethane composite films were studied using chemical analysis, mechanical tests, thermogravimetric analyses (TGA), dynamic mechanical analyses (DMA), X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM and TEM), respectively. Results demonstrated that PF-PEGs with (P + F)/PEG of 50/50 had the highest hydroxyl number of 323 mg K(OH)/g. The incorporation of phenol and formaldehyde into PEG improved the mechanical properties of polyurethane films, glass transition temperature (T_g), and thermal properties but resulted in the brittleness characteristic of the composite films and low crystallization properties. Moreover, the synthesis mechanism of PF-PEGs polyurethane composite films was revealed, which would provide a theoretical base for the preparation of the rigid polyurethane foams based on phenolic resins.

Recent developments in the field of barrier and permeability properties of segmented polyurethane elastomers

Polyurethane (PU) elastomers represent an important class of segmented copolymers. Thanks to many available chemical compositions, a rather broad range of chemical, physical, and biocompatible properties of PU can be obtained. These polymers are often characterized by high tensile and tear strength, elongation, fatigue life, and wear resistance. However, their relatively high permeability towards gases and water as well as their biocompatibility still limits the PU’s practical application, especially for biomedical use, for example, in implants and medical devices. In this review, the barrier and permeability properties of segmented PUs related to their chemical structure and physical and chemical properties have been discussed, including the latest developments and different approaches to improve the PU barrier properties.

Effect of chemical structure on properties of polyurethanes: Temperature responsiveness and biocompatibility

Polyurethanes are known as one of the most biocompatible and inherently blood-compatible materials and have a wide range of applications in the medical field due to their controllable structure and properties. Durability, elasticity, elastomeric structure, fatigue resistance, versatility, and easy acceptance by the biological media after the application makes these polymers preferable in medical area. In this study, polyurethane films were prepared using poly(propylene-ethylene glycol) and either toluene-2,4-diisocyanate or 4,4′-methylenediphenyl diisocyanate without adding any other ingredients such as solvent, catalyst, or chain extender to prevent negative effects of leachable molecules. Mechanical tests were performed at room temperature while swelling tests were conducted in water and phosphate-buffered saline at 4°C, 25°C, and 37°C. Temperature responsiveness was observed for the samples synthesized using toluene-2,4-diisocyanate and poly(propylene-ethylene glycol). These samples had more than 100% swelling at 4°C and about 4% swelling at 25°C and 37°C. Cytocompatibility tests were performed by culturing the samples and their extracts with mouse fibroblast cells (L929). Viability of human umbilical vein endothelial cells was studied to examine the compatibility of the films for blood contacting devices. Both toluene-2,4-diisocyanate and 4,4-methylenediphenyl diisocyanate–based polyurethane films showed no cytotoxic effect and good biocompatibility. Oxygen plasma treatment enhanced hydrophilicity of the films. After plasma treatment, human umbilical vein endothelial cell attachment on toluene-2,4-diisocyanate–based polyurethane films improved and 4,4-methylenediphenyl diisocyanate–based polyurethane films maintained their high cell affinity. Polyurethanes presenting temperature responsiveness, high biocompatibility, and high affinity for human umbilical vein endothelial cells were synthesized in medical purity and in a reaction media involving only diisocyanate and diol components without addition of any solvent, chain extender, or catalyst. Polyurethanes with these properties and as produced in this study are reported for the first time in the literature.

Investigation on the Influence of Chain Extenders on the Performance of One-Component Moisture-Curable Polyurethane Adhesives

In this work, a number of chain extended moisture-curable urethane prepolymers were synthesized in order to develop isocyanate terminated urethane prepolymer formulations that would simultaneously display both high adhesive strength and low viscosity. Proton nuclear magnetic resonance spectroscopy (¹H-NMR), size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and Brookfield viscometry were utilized for characterizing the prepared urethane prepolymers. In addition, the adhesion strength of the cured prepolymers was determined by tensile shear strength test according to the DIN EN (Deutsches Institut für Normung, the German Institute for Standardization) 1465 standard. Especially, the role of different types of linear (butanediol, pentanediol) and branched chain extenders (dipropyleneglycol (di-PPG), tripropyleneglycol (tri-PPG) and the influence of their dosage on the degree of microphase separation between hard segments (HS) and soft segments (SS) in urethane prepolymers were studied. Furthermore, the benefits of utilizing either a one-step versus a two-step polymerization process were investigated. The results revealed that the extent of phase separation of different urethane prepolymers was dependent on the extent of hydrogen bonding interactions which was extensively studied by attenuated total reflectance infrared spectroscopy (ATR-FTIR). The incorporation of branched chain extenders (di-PPG and tri-PPG) did not result in notable phase separation between hard segments and soft segments, while linear chain extenders (pentanediol and butanediol) readily promoted phase separation. The degree of phase separation was particularly pronounced for butanediol, and when the linear chain extender ratio was higher than or equal to 0.74. Compared with a two-stage process, one-stage process produced more randomly distributed polymer chains with highly dispersed hard segments. Thus, urethane prepolymers exhibiting strong adhesive strength with simultaneously low viscosity were successfully developed by systematic adjustment of structural parameters.

Morphology of Elastic and Plastic Poly(Urethane-Imide) Films Observed with TEM and FE-SEM

Direct observation of the morphology of poly(urethane-imide) films, prepared by the reaction of a polyurethane prepolymer and poly(amide acid) as a precursor of polyimide, was performed using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). The poly(urethane-imide) films were plastic or elastic depending on the ratio of the two components. In the plastic films, TEM and FE-SEM analyses clearly showed the presence of urethane-rich domains in the imide-rich continuous phase. Under the higher magnification of TEM, a lamella structure was clearly observed inside the urethane-rich domains. The size and the number of the urethane-rich domains varied with the chemical structure and the ratio of the two components. It was confirmed that the size of the urethane-rich domains in the poly(urethane-imide) films observed with TEM and FE-SEM was almost the same as that of the pores in the porous polyimide films obtained by pyrolyzing the poly(urethane-imide) films at 300–400 °C. In the elastic films, imide-rich domains were clearly seen in the urethane-rich continuous phase by the FESEM observation.

A novel ionomeric polyurethane elastomer based on ionic liquid as crosslinker

A novel flexible polyurethane was prepared using ionic liquid as crosslinker. It showed lower T_g, superelastomeric behavior with very high elongation, better tensile and oil resistance properties than a non-ionic crosslinked PU or a linear PU.

Tuned red NIR phosphorescence of polyurethane hybrid composites embedding metallic nanoclusters for oxygen sensing

Phosphorescent metallic clusters are functionalized and integrated in polyurethane by copolymerization. Solid state quantum yield measurements show a drastic and reversible enhancement of nanocomposite luminescence depending on its surrounding atmosphere.

Physical and thermal properties of thermoplastic poly(azo-urethane)s

New thermoplastic segmented poly(azo-urethane)s (PAUs) have been synthesized by one-step polycondensation from aromatic diazo-diol, ( E)-1-(4-(4-(3-hydroxypyridyl-azo)thiocarbamoylaminobenzyl)-phenyl)-3-(3-hydroxypyridylazo)thiourea, as chain extenders, phenylene diisocyanate and 20–60 mol % polyethylene glycol (PEG) with the molecular weight of 2000 g/mol. The effect of diazo-diol used in the structure and some physicochemical, thermal and mechanical properties of the segmented polyurethanes were studied. The structure was examined by proton nuclear magnetic resonance (1H NMR) and Fourier-transform infrared spectroscopy (FTIR) analyses. Polyazos were the products with good processability and high molar mass in the range 120 × 103–135 × 103 g/mol. Thermal properties, investigated by thermogravimetric analysis and differential scanning calorimetry indicated that PAUs up to 40 mol % polyol were fairly stable above 530°C, having initial weight loss of around 528–539°C, 10% gravimetric loss in the range 553–567°C and own high glass transition temperature 259–264°C. The polymers also demonstrated higher tensile strength (56.53–63.83 MPa) and elongation at break (1.58–1.63). However, compared with typical polyurethanes, PAUs with lower polyol content displayed higher Tgs, heat and mechanical stability owing to warily designed rigid hard segment structure.