High performance flame retardant rigid polyurethane foam with high thermal insulation
- Autores: Pablo Alberto Acuña Domínguez
- Directores de la Tesis: De-Yi Wang (dir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Juan Pedro Fernández Blázquez (presid.), Silvia González Prolongo (secret.), José Luis Díaz Palencia (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
Polyurethane (PU), is one of the most used and important materials in our daily life´s, being integral part of freezers, cars, buildings, footwear, food packaging and many others. PU is created by the reaction of two different components: Isocyanates and polyols. Indeed, PU is a very versatile polymer, being found on different materials, such as elastomers, adhesives, thermoplastics and on top of that, PU foams. PU foams are the most important PU-based products on the market, being around 70 % of the total market share for PU production. PU foams possess interesting properties, such as structural stability, low density, low thermal conductivity, high porosity and sound absorption. It can be divided on rigid (RPUF) and flexible (FPUF) foams and covers almost all the demands of the market. As an example, FPUF is integral part of almost every seat for cars and offices whereas RPUF can be inside every domestic and industrial building thanks to its energy conservation ability, thermal insulation performance and mechanical properties. However, due to its porosity and the nature of its components, PU foam is very flammable with strong smoke release during combustion.
In order to reduce the flammability of polymeric products, fire retardants (FR) are commonly used solutions. In this PhD thesis, the thermal and fire behavior of different FR systems into RPUFs matrixes has been systematically investigated. Also, the mechanical and morphological studies were analyzed to study the impact of the FRs into the cellular structure of RPUFs, since the addition of FRs could have direct influence in both thermal conductivity and mechanical performance. The materials used as FR on this thesis include carbon-based materials such as expandable graphite (EG) and graphene oxide (GO) as well as novel synthesized phosphorous-nitrogen-based FRs (P-N FRs). Besides, the use of vegetable-oil-based RPUFs was explored, suppressing the use of petroleum-based polyol for the PU production. The studies based on these solutions to obtain higher thermal and fire stability as well as good mechanical performance were studied separately along with the FR mechanism investigation. In detail, the strategies based on novel concepts were adopted to obtain high flame retardancy (Chapter 4), synergistic FR effect to achieve both fire resistance and higher compressive strength (Chapter 5) and the use of a FR vegetable-oil based polyol combined with high thermal insulation by nanomaterials (Chapter 6). Finally, to obtain multifunctional properties; including thermal insulation, flame resistance and good compressive strength, into one sole system was demonstrated in the Chapter 7.
In Chapter 4, the use of EG as FR additive offered a feasible solution to the flammability problem of RPUFs. The use of three different EGs (EG1, EG2 and EG3) with different particle size (EG1 and EG2 with 300 µm and EG3 with 500 µm) and volume expansion (EG1 with 250 cm3/g and EG2 EG3 with 350 cm3/g) was deeply investigated in a commercial PU rigid foam. The big size of EG affects the structure of the foam by reducing its cell size and increasing the density. Although it is commonly known that a lower size implies a lower thermal conductivity, the increase of density as well as the intrinsic high thermal conductivity of graphite, raised the thermal conductivity of the foams, weakening its thermal insulation. The internal cellular structure of the RPUFs gets dramatically affected by EG, thus the compressive performance of the foams was diminished due to slippage between the PU matrix; with a damaged cellular structure, and the EG. The thermogravimetric analysis (TGA) showed that EG does not directly interact with the PU matrix but their acidic species between the graphite layers, accelerates the degradation of the material. Nonetheless, the residue at the end of the degradation process was increased with the addition of any EG, reaching up to a maximum of 22.6 % for EG1. For the fire properties, three different tests were performed, including cone calorimeter test (CCT), limited oxygen index (LOI) and vertical burning test (UL94). As the loading up to 8 wt. % of EG, it was enough to achieve V-0 rating in the UL94 test. The highest LOI was obtained with EG2 and EG3 at 10 wt. % loading with a value of 31.8 vol %. Thus, the volume expansion is important to achieve a high LOI value as the char and gases produced can dilute the oxygen surrounding the material. On the other hand, the particles size will increment their performance on the UL94 test, as they can form a more interconnected char barrier that blocks the fire propagation. The particle size maximizes the blocking effect of EG and the rate of expansion favors lower smoke production; i.e. mass transfer. For this reason, peak heat released rate (PHRR), total heat released (THR), total some production (TSP) and mass loss measured in the CCT were incredibly improved by EG with a high particle size and high rate of expansion (EG3). Indeed, PHRR was incredibly reduced up to a 51 %, THR was reduced up to a 47 %, TSP was reduced up to an impressive 82 % and mass loss were reduced by a 39 % less compared to the commercial reference PU when EG3 was used at 8 wt. %. On the other hand, it was seen that the high particle size of EG3, affects negatively on the cellular structure, leading to a dramatic decrease of the compressive strength compared to the neat RPUF or a lower particle size EG-filled foam (EG1/EG2). 10 wt. % of EG3 decreased the compressive strength of neat PU foam by a 15 %. The different particle size (EG1/EG2 and EG3) and rate of expansion (EG1 and EG2/EG3) were systematically evaluated inside this research. It was concluded that to have an outstanding flame retardancy, it is necessary to use high particle size and high rate of expansion as key characteristics for EG, with the objective of maximizing their performance into RPUFs.
In the Chapter 5, a deeper study was performed. P-N FRs has been used in the literature due to their potential benefits for RPUFs, such as improved flame retardancy and better mechanical properties. The investigation of this chapter consisted of the achievement of a synergistic effect in both flame retardancy and compressive strength, solving the problem with EG, in a system combining P-N FRs and EG. Phenylphosphonic-aniline salt (FR1); a new P-N FR, was synthetized by our group and used as a synergistic with EG at 8 wt. % loading; with different ratios between the components, into a commercial RPUF. A ratio of 12/1 (EG/FR1) was demonstrated to obtain the highest results with respect to flame retardancy and improved compressive strength. The cellular structure of the foams was deeply affected by FR1. First, FR1 increased the density of the foams as it can stablish bonds with the isocyanate from the PU matrix. Besides, FR1 increased the cell size of the foams due to a more efficient nucleation and the production of more bubbles. All in all, FR1 increased the thermal conductivity of the foams. The TGA analysis showed that FR1 accelerated the degradation of PU matrix by a phosphoric acid depolymerization, which decreased the degradation temperature but improved the char residue at the end of the process as the resulted char species are more thermally stable. During the study of the fire properties, the optimal sample with 12/1 EG/FR1 ratio (RPU3), increased the LOI value from 19.2 % to 29.8 % and reached V-0 rating UL94 test. The PHRR was reduced up to a 45 %, THR decreased to a 24 % and the TSP 58 %. It is highlighted that the optimal sample obtained better FR results than the sample with only EG at 8 wt. % loading, confirming a synergistic improved effect between FR1 and EG. A combined synergistic FR mechanism by the char barrier effect of EG in condense phase and radical capture mechanism of FR1 in the gas phase was proposed to explain this improvement. Finally, as showed in the Chapter 4, the low performance of EG-based RPUFs in the compression strength is a problem for the RPUF applications. It was discovered that the compressive strength of the new foams increased a 9 % compared with the reference commercial foam and 11 % higher than the EG-filled foam (RPU1). FR1 has as a good interfacial adhesion between with the PU matrix along with an increase on the density of the foam provoked by the stiff phenyl groups in the FR1 structure and the proposed reaction of isocyanate groups at the PU end chains and FR1 to produce urea groups. However, the increase in thermal conductivity was still a main problem to solve in order to obtain a foam with high thermal insulation, high fire resistance and stronger compressive strength.
As mentioned before, vegetable oil is an excellent source to obtain polyols for the synthesis of RPUFs. In Chapter 6, castor oil, a vegetable oil with secondary hydroxyl groups and a fatty acid structure, was selected to synthetize a new FR polyol. This new polyol was used a biobased source for the synthesis of partly biobased RPUFs in this study. Moreover, some carbon material such as GO is an interesting nanomaterial. The hydroxyl functionalities under the GO surface, allow GO capabilities as thermal insulator agent, FR and mechanical reinforcer, which would potentially impart RPUF with the desired properties for the market. First, castor oil was modified by transmidization to increase the number of primary -OH groups (CODEOA), as they reacted faster with isocyanate groups and produced more crosslinked structures. This polyol was used to create BIO1 as the first reference foam of this study. Secondly, the remaining double bonds of the castor oil unsaturated fatty acid structure were also modified via epoxidation (ECODEOA). Finally, to obtain a polyol with intrinsic flame retardancy, phenyl phosphonic acid (PPA), a phosphorous-containing acid with phosphonate structure, was used as modifier to be introduced into the ECODEOA structure via ring opening reaction. After these modifications, a polyol with intrinsic flame retardancy, higher viscosity and increased hydroxyl number was synthetized (CPPA). CPPA was used as a substitute for the commercial polyol from the previous reference foam and produced a novel partly biobased FR RPUF (BIO2). This novel idea was able to reduce the thermal conductivity by 14 % (33.8 mW/mK) with respect to the commercial foam as well as maintained the density at levels closer to the ones used as the commercial insulators. Also, the LOI increased up to 24.0 % and the PHRR was reduced 15 % with respect to BIO1. The smoke production was also lower than BIO1 by 27 %. Following these incredible results, EG and GO were used as additives due to its previously studied flame retardancy and thermal insulation effect, respectively. With the aim of reducing the loading of FRs with respect to the previous chapters, 6 wt. % was selected as the loading for this study. GO affected positively the cellular structure, reducing the cell size and the density of the foam due to a better and homogeneous dispersion of the EG particles. The covalent bonds between GO and the PU matrix increased the crosslinking density of the foam and the gas barrier effect of GO; towards the lower cell size, improved the insulation capacity of the foam. Besides the intercalated water inside the GO surface serves as an additional blowing agent to decrease the density. The thermal conductivity of BIO2/EG/GO was 34.2 mW/mK, meaning a 10 % lower than that of BIO2/EG. In terms of flame retardancy, BIO2/EG/GO; with 6 wt.% of additives, including only 0.5 wt. % of GO, was able to obtain a V-0 rating in the UL94 test whereas BIO2/EG only achieved a V-2 rating due to the porosity of the structure, which led to higher internal temperature and melt of the polymeric matrix. Moreover, The LOI was increased to 27.2 %. In the CCT, BIO2/EG/GO reduced the PHRR a 46 % and the THR by a 5.6 %. The reason behind is the combination of a condensed phase barrier insulation effect against heat and mass transfer from EG and GO towards an intumescent FR effect of CPPA. However, the TSP increased compared to EG. The reason is the pyrolysis of the oxygen functionalities under the GO surface and the less efficient char barrier effect of GO. For the compressive strength, the use of EG as previously stated, reduced the compressive strength. On this chapter, we have seen that GO; although the density and cell size were lower than EG, was able to maintain the compressive strength closer to the initial value of BIO2. The good interfacial adhesion by the interactions of GO with the PU matrix was the responsible for this effect.
With the aim to increase the flame retardancy and compressive strength of GO into a biobased RPUF (BIO2), a modification of GO was performed in Chapter 7. PGO was synthesized by using a P-N FR such as N, N'-Diallyl-P-phenylphosphonicdiamide (FP1) and grafting it into the GO surface. The N-H groups of FP1 can make a nucleophilic attack to the epoxy and carboxylic acid groups under the GO and produced amides and/or 1,2 amino alcohols via amidation and ring-opening amination respectively. Thus, attaching FP1 under the GO surface. In terms of the multifunctional properties, the combination of thermal insulation, flame retardancy and high compressive strength into one unique system was proposed and demonstrated. PGO as modifier was able to improve the characteristics of GO. The thermal insulation was maintained at similar levels with only a slight increase of density. The cellular structure of PGO-filled BIO2 showed reduced cell size due to the fact that the amine groups in PGO, disperses EG and PGO itself more homogeneously and reduced the effect of EG into the structure. Besides, the higher interactions of PGO with the PU matrix and the stiff phenyl groups of PGO, led to an increase of viscosity towards a lower foaming pressure, which consequently helped to reduce the cell size and to increase the density. Although the slight increase of density increased the thermal conductivity a negligible 2 % in comparison to GO. The FR properties were highly improved. The LOI of BIO2/EG/PGO was as a high as 27.6 % and reduced the individual average after-flame time to only 1.6 s in 5 samples for the UL94 test, being the clearest V-0 rating among all the samples tested. Following this, BIO2/EG/PGO was able to decrease the PHRR and THR at low levels. PHRR was decreased a 50 % and THR a 6.6 % compared to BIO2, meaning an improved FR effect even compared to GO as unmodified additive. Unfortunately compared to BIO2, the TSP increased due to an intense gas-phase FR effect of PGO and CPPA. Moreover, the TSP of PGO was lower than that of GO. Finally, the compressive performance was incredible improved to about a 25 % compared to BIO2. The chemical grafting of P-N FRs into GO surface, principally by the chemical bond of -NH- with epoxy groups, lead to more free -OH groups, which subsequently can be ready to form bonds with the -NCO groups at the end of the PU chains. Also, hydrogen bonds are present, thus increasing the density and so, the compressive strength of PGO with respect to unmodified GO. After that, by the strong covalent interface interaction, it allows effective load stress transfer from PU to PGO and dramatically increased the mechanical strength of the RPUF.
All in all, we have herein demonstrated different approaches to obtain high flame retardancy, thermal insulation and good compressive strength by different systems into PU foam composites. Also, we have showed that vegetable oil is an excellent green source to obtain FR polyols and RPUFs with a density close to that of the typical ones used for the insulating industry. Besides, GO as an interesting filler for polymers, has shown that it can impart flame retardancy, low thermal conductivity, low density ang good compressive strength with the appropriate functionalization.
