Synthesis, foaming kinetics and physical properties of cellular nanocomposites based on rigid polyurethane

Durante los últimos años, la inclusión de partículas a la matriz polimérica de espumas de poliuretano (PU) termoestable ha permitido excelentes mejoras de una amplia variedad de características físicas. Sin embargo, en muchos casos esta estrategia puede modificar el proceso de espumado, lo que da lugar a consecuencias inesperadas en la morfología final de la matriz de PU y en la estructura celular. Es decir, algunas propiedades pueden mejorar, pero otras pueden empeorar. Por esta razón, es muy importante comprender cómo se modifica el proceso de espumado cuando se incorporan partículas en las espumas de PU, ya que de este modo se podría ayudar a corregir un posible desequilibrio en las reacciones, mediante la optimización de la formulación. El trabajo de investigación descrito en la presente Tesis Doctoral se centra principalmente en desarrollar una metodología para permitir el estudio del proceso de espumado de PU termoestable. Esta metodología se ha basado en el uso de las siguientes técnicas complementarias: espectroscopia de FTIR, expandometría infrarroja, medidas de temperatura interna y radioscopia de rayos X. La apropiada aplicación de estas técnicas ha permitido entender el efecto que genera la adicción de diferentes partículas (nanosilicas, nanoclays, óxido de grafeno, etc) sobre el proceso de espumado, tanto desde un punto de vista químico (cinética de reacción y morfología de la matriz polimérica seguidas por espectroscopia de FTIR y evolución de la temperatura interna medida con termopares), como desde el punto de vista físico (cinética de expansión seguida por expandometría infrarroja y evolución de la estructura celular interna monitoreada mediante radioscopia de rayos X). Además, la información obtenida con la metodología desarrollada ha permitido optimizar las formulaciones reforzadas con partículas para obtener efectos positivos en todas las propiedades de las espumas finales. La presente Tesis Doctoral se ha desarrollado en el Laboratorio CellMat del Departamento de Física de la Materia Condensada de la Universidad de Valladolid y ha sido supervisada por el Prof. Dr. Miguel Ángel Rodríguez Pérez y el Prof. Dr. Fernando Villafañe González. El conocimiento desarrollado durante esta tesis ha sido aplicado tanto en proyectos públicos como en proyectos en colaboración con empresas del sector de la construcción, refrigeración, automóvil, etc. La tesis consta de ocho publicaciones en revistas internacionales y cumple con los requerimientos para ser acreditada con Mención Internacional.Departamento de Física de la Materia Condensada, Cristalografía y MineralogíaDoctorado en Físic

Identification and quantification of cell gas evolution in rigid polyurethane foams by novel GCMS methodology

Producción CientíficaThis paper presents a new methodology based on gas chromatography-mass spectrometry (GCMS) in order to separate and quantify the gases presented inside the cells of rigid polyurethane (RPU) foams. To demonstrate this novel methodology, the gas composition along more than three years of aging is herein determined for two samples: a reference foam and foam with 1.5 wt% of talc. The GCMS method was applied, on one hand, for the accurate determination of C5H10 and CO2 cell gases used as blowing agents and, on the other hand, for N2 and O2 air gases that diffuse rapidly from the surrounding environment into foam cells. GCMS results showed that CO2 leaves foam after 2.5 month (from 21% to 0.03% for reference foam and from 17% to 0.03% for foam with 1.5% talc). C5H10 deviates during 3.5 months (from 28% up to 39% for reference foam and from 29% up to 36% for foam with talc), then it starts to leave the foam and after 3.5 year its content is 13% for reference and 10% for foam with talc. Air diffuses inside the cells faster for one year (from 51% up to 79% for reference and from 54% up to 81% for foam with talc) and then more slowly for 3.5 years (reaching 86% for reference and 90% for foam with talc). Thus, the fast and simple presented methodology provides valuable information to understand the long-term thermal conductivity of the RPU foams.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (grants MAT2015-69234-R and RTC-2016-5285-5)Junta de Castilla y Leon (grant VA275P18)Agencia austriaca para la promoción de la investigación (grant 850697

Effect of the molecular structure of TPU on the cellular structure of nanocellular polymers based on PMMA/TPU blends

Producción CientíficaIn this work, the effects of thermoplastic polyurethane (TPU) chemistry and concentration on the cellular structure of nanocellular polymers based on poly(methyl-methacrylate) (PMMA) are presented. Three grades of TPU with different fractions of hard segments (HS) (60%, 70%, and 80%) have been synthesized by the prepolymer method. Nanocellular polymers based on PMMA have been produced by gas dissolution foaming using TPU as a nucleating agent in different contents (0.5 wt%, 2 wt%, and 5 wt%). TPU characterization shows that as the content of HS increases, the density, hardness, and molecular weight of the TPU are higher. PMMA/TPU cellular materials show a gradient cell size distribution from the edge of the sample towards the nanocellular core. In the core region, the addition of TPU has a strong nucleating effect in PMMA. Core structure depends on the HS content and the TPU content. As the HS or TPU content increases, the cell nucleation density increases, and the cell size is reduced. Then, the use of TPUs with different characteristics allows controlling the cellular structure. Nanocellular polymers have been obtained with a core relative density between 0.15 and 0.20 and cell sizes between 220 and 640 nm.Ministerio de Ciencia, Innovación y Universidades (Proyects RTI2018-098749-B-I00 and PTQ2019-010560)Ente Regional de la Energía de Castilla y León (Proyecto EREN_2019_L4_UVA

Rigid polyurethane foams with infused nanoclays: Relationship between cellular structure and thermal conductivity

Producción CientíficaA water blown rigid polyurethane (PU) formulation has been used to manufacture cellular nanocomposites containing different concentrations of montmorillonite nanoclays. The PU foams have been produced using a low shear mixing technique for dispersing the nanoclays and by reactive foaming to generate the cellular structure. A detailed characterization of the cellular structure has been performed. The effect of the nanoparticles on the reaction kinetics and the state of intercalation of the nanoparticles in the foams has been analyzed. The thermal conductivity and extinction coefficient of the different materials has been measured and the results obtained have been correlated with the materials structure. A strong reduction of cell size and modifications on cell size distribution, anisotropy ratio and fraction of material in the struts has been detected when the clays are added. In addition, a reduction of the thermal conductivity has been observed. Different theoretical models have been employed to explain thermal conductivity changes in terms of structural features. It has been found that, in addition to the modifications in the cellular structure, changes in the extinction coefficient and thermal conductivity of the matrix polymer play an important role on the final values of the thermal conductivity for these materials.We would like to acknowledge to Mr. Vela and Mr. Ferrer, from BASF Española S.L., for supplying the PU formulation employed in this research. The authors are grateful to the Spanish Ministry of Science and Education which supported this work with a FPU Grant Ref-AP-2008-03602 given to Mr. Estravís. In addition, Financial assistance from the Spanish Ministry of Science and Innovation, FEDER program (MAT 2012 – 34901 and MAT2015-69234-R), the Junta de Castile and Leon (VA035U13) and the EU Commission (FP7 program, EC project NanCore number 214148) is acknowledged

Dynamic Mechanical Analysis during polyurethane foaming: Relationship between modulus build-up and reaction kinetics

Producción CientíficaThe modulus build-up and relative density evolution during the reactive foaming of four standard polyurethane formulations was monitored in-situ by Dynamic Mechanical Analysis (DMA) with a customised set-up in parallel plate geometry. The modulus increased from 0.01 MPa in the first minutes to over 1.2 MPa within 20 min. The set-up also enabled the recording of vitrification followed by curing times. These typically occur within 3 min of each other. The results of DMA are corroborated by measurements of the reaction kinetics with Infrared Spectroscopy. This goes to show that the modulus remains nearly unchanged during the stage of swiftest isocyanate conversion, while the point of gel conversion is accompanied by their increase.Junta de Castilla y León (grant VA202P20)Ministerio de Ciencia, Innovación y Universidades (grant RTI2018-098749-B-I00

A new synthesis route to produce isocyanate-free polyurethane foams

Producción CientíficaIsocyanate is a toxic substance that is one of the main reactants used in the conventional fabrication route for polyurethane foams. This study presents the synthesis of isocyanate-free polyurethane foams from cyclic carbonates and diamines using sodium bicarbonate as the foaming agent. Three different series of foams are synthesised using three types of sodium bicarbonates having different average particle sizes (i.e. 3, 13, and 22 µm) at four content levels (i.e. 5, 10, 15, and 20 wt% with respect to cyclic carbonate). The density, open-cell content, average cell size, normalised standard deviation of the cell size distribution, anisotropy, cell density and cell nucleation density are characterized for all the materials synthesized. In addition, a theoretical study of the expected densities is conducted to compare the theoretical values with the experimental densities obtained for the foams. Finally, the foams previously manufactured with high sodium-bicarbonate content are optimised by modifying the catalyst content from 0.5 to 2.0 wt% with respect to cyclic carbonate, thereby producing foams with lower densities. Nonisocyanate polyurethane foams having densities as low as 142 kg/m3 are fabricated.Ministerio de Ciencia e Innovación y Ministerio de Universidades (RTI2018 - 098749-B-I00, PID2021-127108OB-I00, TED2021-130965B-I00 y PDC2022-133391-I00)Junta de Castilla y León - UE-FEDER (VA202P20

Thermal conductivity aging and mechanical properties of polyisocyanurate (PIR) foams produced with different contents of HFO

Producción CientíficaPolyisocyanurate (PIR) foam is a thermal insulating material widely used in many industries such as construction, refrigeration, piping/tubing among others. In this research the production, characterization and modeling of the thermal conductivity of PIR foams synthesized with a hydrofluorolefin (HFO 1233zd (E)) as physical foaming agent have been studied. The results have shown that increasing the amount of HFO reduces the density, but the cellular structure is not modified. The relative mechanical properties are the same for the concentrations of HFO considered. In addition, the aging of thermal conductivity as a function of time has been studied in detail. The experimental results have been deeply analyzed using a theoretical model to predict the thermal conductivity The results show that the thermal conductivity and the rate of aging at early stages are reduced for the higher concentrations of HFO. This result has been related to the lower temperature reached during the foaming reaction for higher contents of the physical blowing agent.Ministerio de Economía, Industria y Competitividad (grant DIN2019-010840)Ministerio de Ciencia, Innovación y Universidades (MCIU) - (RTI2018 – 098749-B-I00, PID2021-127108OB-I00, TED2021-130965B-I00 y PDC2022-133391-I00)Junta de Castilla y León y programa EU-FEDER (VA202P20

Influence of silica aerogel particles on the foaming process and cellular structure of rigid polyurethane foams

Producción CientíficaWater blown rigid polyurethane (RPU) composite foams were produced using different concentrations of nanoporous silica aerogel micrometric powder (0.5, 1 and 3 wt%). The effect of these particles on the foaming kinetics was analysed from a physical and chemical viewpoint. On the physical side, the foaming process was studied by in-situ X-ray radioscopy. The inclusion of aerogel particles in the system delays the foam expansion and enhances the nucleation of cells. However, high amounts of these particles (3 wt%) lead to intense cell coalesce during foam evolution. On the chemical side, the reaction kinetics was investigated by in-situ FTIR spectroscopy and reaction temperature measurements. The addition of low contents of aerogel (below 3 wt%) reduces the conversion of isocyanate while favouring the generation of urethane groups, which explains the higher density of the foams with low aerogel contents. However, the foam with high contents of aerogel (3 wt%) does not change the reaction balance in comparison to the reference. Therefore, this foam presents similar expansion and density to those of the unfilled foam (Reference). Furthermore, higher reaction temperatures were reached by the reference foam during the foaming process, and higher dissipation speeds of these temperatures were detected for the foams containing aerogel with respect to those of the reference foam.Junta de Castilla y León (project VA275P18)Ministerio de Ciencia, Innovación y Universidades (project RTI2018-098749-B-I00

Cooperative effect of chemical and physical processes for flame retardant additives in recycled ABS

Producción CientíficaIn the present work, the effectiveness of four non-halogenated flame retardants (FR) (aluminium trihydroxide (ATH), magnesium hydroxide (MDH), Sepiolite (SEP) and a mix of metallic oxides and hydroxides (PAVAL)) in blends with recycled acrylonitrile-butadiene-styrene (rABS) was studied in order to develop a more environmentally friendly flame-retardant composite alternative. The mechanical and thermo-mechanical properties of the obtained composites as well as their flame-retardant mechanism were evaluated by UL-94 and cone calorimetric tests. As expected, these particles modified the mechanical performance of the rABS, increasing its stiffness at the expense of reducing its toughness and impact behavior. Regarding the fire behavior, the experimentation showed that there is an important synergy between the chemical mechanism provided by MDH (decomposition into oxides and water) and the physical mechanism provided by SEP (oxygen barrier), which means that mixed composites (rABS/MDH/SEP) can be obtained with a flame behavior superior to that of the composites studied with only one type of FR. In order to find a balance between mechanical properties, composites with different amounts of SEP and MDH were evaluated. The results showed that composites with the composition rABS/MDH/SEP: 70/15/15 wt.% increase the time to ignition (TTI) by 75% and the resulting mass after ignition by more than 600%. Furthermore, they decrease the heat release rate (HRR) by 62.9%, the total smoke production (TSP) by 19.04% and the total heat release rate (THHR) by 13.77% compared to unadditivated rABS; without compromising the mechanical behavior of the original material. These results are promising and potentially represent a greener alternative for the manufacture of flame-retardant composites.European Union’s Horizon 2020 - (project Creator 820477)Ministerio de Ciencia, Innovación y Universidades, Centro para el Desarrollo Tecnológico y la Innovación (CDTI) - (grant CER-20211009)Ministerio de Economía y Competitividad - (project PTQ2020-010968)Ministerio de Economía y Competitividad - (project PTQ2021-011628

X-ray radioscopy validation of a polyol functionalized with graphene oxide for producing rigid polyurethane foams with improved cellular structures

Producción CientíficaFillers can be dispersed in polyurethane (PU) components to produce PU foams with improved properties by different methods. Herein, graphene oxide (GO) is chemically linked to polyol chains in order to suppress fillers agglomeration. Then the foaming behaviour of rigid polyurethane (RPU) foams from polyols functionalized with GO (GO-f) is compared with the one of RPU foams containing GO dispersed in the polyol (GO-d) by high shear mixing. Relative density, cell size, and cell nucleation density of the RPU foam nanocomposites are monitored during the foaming process by using X-ray radioscopy. The results obtained demonstrate that the use of polyol functionalized with GO-f offers a high improvement of the cellular structure and also makes the results more reproducible.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (project MAT2015-69234-R)Junta de Castilla y León (project VA275P18)Universidad de Valladolid - Banco Santander (contract E-47-2015-0094701