Please use this identifier to cite or link to this item: http://doi.org/10.25358/openscience-5539
Authors: Markwart, Jens Christian
Title: Systematically Controlled Decomposition Mechanism in Phosphorus Containing Polymers by Precise Molecular Architecture
Online publication date: 22-Mar-2021
Year of first publication: 2021
Language: english
Abstract: Polymers are omnipresent in our daily life and with their broad use comes with an increased fire risk, and sustained research into efficient flame retardants is key to ensuring the safety of the populace and material goods from accidental fires. Halogenated flame retardants are still widely used today, but are increasing under review due to concerns regarding health and environment. A promising alternative is phosphorus, a versatile and effective element for use in flame retardants: to solve the task of developing flame-retarding polymeric materials, current formulations employ a variety of modes of action and methods of implementation, as additives or as reactants. During a fire phosphorus‐based flame retardants can act in both the gas and condensed phase. The aim of this thesis is to investigate how current phosphorus chemistry can help reducing the flammability of polymers by understanding the decomposition pathways under pyrolytic conditions, and apply this knowledge for the design of various flame-retardant polymers. Chapter 1: In this chapter, an introduction into phosphorus based flame retardants is given and the challenges of a good flame retardant are discussed. These include the retention of material properties while keeping the price low and being sustainable. Most compounds that contain phosphorus are manufactured from phosphorite, commonly known as “phosphate rock”. At current extraction rates, estimates point to phosphate rock reserves being depleted in the next 370 years, with the exception of the reserves in Morocco. In addition, recent developments in phosphorus-containing reactive and additive flame retardants are discussed while summarizing modern trends and the future of phosphorus flame retardants. Chapter 2: Flame retardants are inevitable additives to many plastics. Halogenated organics are effective flame retardants but are controversially discussed due to the release of toxic gases during a fire or their persistence if landfilled. Phosphorus-containing compounds are effective alternatives to halogenated flame retardants and have potential lower toxicity and degradability. In addition, nitrogen-containing additives were reported to induce synergistic effects with phosphorus-based flame retardants. However, no systematic study of the gradual variation on a single phosphorus flame retardant containing both P–O and P–N moieties and their comparison to the respective blends of phosphates and phosphoramides was reported. In this study general design principles for P–O- and P–N-based flame retardants were developed and will help to design effective flame retardants for various polymers. A synthesized library of phosphorus flame retardants, which only differ in their P-binding pattern from each other, were studied regarding their decomposition mechanism in epoxy resins. Systematic control over the decomposition pathways of phosphate (P═O(OR)3), phosphoramidate (P═O(OR)2(NHR)), phosphorodiamidate (P═O(OR)(NHR)2), phosphoramide (P═O(NHR)3), and their blends was identified, for example, by reducing cis-elimination and the formation of P–N-rich char with increasing nitrogen content in the P-binding sphere. The flame-retardant epoxy resins can compete with commercial flame retardants in most cases, but it was proven that the blending of esters and amides outperformed the single-molecule amidates/diamidates due to distinctively different decomposition mechanisms acting synergistically when blended. Chapter 3: Multifunctional P-based hyperbranched polymeric flame retardants were successfully synthesized with varying oxygen-to-nitrogen content and characterized via 1H and 31P NMR and GPC. Their miscibility in epoxy resins and impact on glass-transition temperatures were determined via differential scanning calorimetry. Using thermogravimetric and evolved gas analysis, pyrolysis gas chromatography/mass spectrometry, hot stage FTIR, flammability tests UL-94 and LOI, fire testing via cone calorimetry, residue analysis via scanning electron microscopy and elemental analysis, detailed decomposition mechanisms and modes of action are proposed. Hyperbranched polymeric flame retardants have improved miscibility and thermal stability, leading to high flame retardant performance even at low loadings. Polymeric, complex flame retardants increase flame retardancy, mitigate negative effects of low molecular weight variants, and can compete with commercial aromatic flame retardants. The results illustrate the role played by the chemical structure in flame retardancy and highlight the potential of hyperbranched flame retardants as multifunctional additives. Chapter 4: The current trend for future flame retardants (FRs) goes to novel efficient halogen-free materials, due to the ban of several halogenated flame retardants. Among the most promising alternatives are phosphorus-based flame retardants, and of those, polymeric materials with complex shape has been recently reported. Herein, we present novel aromatic and aliphatic, hyperbranched, halogen-free polyphosphoesters (hbPPEs), which were synthesized by olefin metathesis polymerization and investigated them as a flame retardant in epoxy resins. We compare their efficiency aliphatic vs. aromatic and further assess the differences between the monomeric compounds and the hbPPEs. The decomposition and vaporizing behavior of a compound is an important factor in its flame-retardant behavior, but also the interaction with the pyrolysing matrix has a significant influence on the performance. Therefore, the challenge in designing a FR is to optimize the chemical structure and its decomposition pathway to the matrix, with regards to time and temperature. This behavior becomes obvious in this study and explains the superior gas phase activity of the aliphatic FRs. Chapter 5: We synthesized a library of phosphorus-based flame retardants (phosphates and phosphoramides of low and high molar mass) and investigated their behavior in two epoxy resins (one aliphatic and one aromatic). The pyrolytic and burning behavior of the two resins (via TGA, TG-FTIR, Hot stage FTIR, Py-GC/MS, PCFC, DSC, LOI, UL-94, Cone calorimeter) are analyzed and compared to the results of flame retardant (FR)-containing composites. A decomposition pathway incorporating the identified modes of action and known chemical mechanisms is proposed. The overlap of decomposition temperature (Tdec) ranges of matrix and FR determines the efficacy of the system. Low molar mass FRs strongly impact material properties like Tg but are very reactive, and high molar mass variants are more thermally stable. Varying P-O and P-N content of the FR affects decomposition, but the chemical structure of the matrix also guides FR behavior. Thus, phosphates afford lower fire load and heat release in aliphatic epoxy resins, and phosphoramides can act as additives in an aromatic matrix or a reactive FRs in aliphatic ones. The chemical structure and the structure-property relationship of both FR and matrix are central to FR performance and must be viewed not as two separate but as one codependent system. Chapter 6: Hyperbranched polyphosphoesters are promising multifunctional flame retardants for epoxy resins. These polymers were prepared via thiol-ene polyaddition reactions. While key chemical mechanisms and modes of actions were elucidated, the role of sulfur in the chemical composition remains an open question. In this study, we compare the FR-performance of a series of phosphorus-based flame retardant additives with and without sulfur (thio-ethers or sulfones) in their structure. The successful synthesis of the thio-ether or sulfone-containing variants is described and verified by 1H and 31P NMR, also FTIR and MALDI-TOF. A decomposition mechanism is proposed from pyrolytic evolved gas analysis (TG-FTIR, Py-GC/MS), and flame retardancy effect on epoxy resins is investigated in pyrolytic conditions and via fire testing in the cone calorimeter. The presence of sulfur increased thermal stability of the flame retardants and introduced added condensed phase mechanisms. Likely, sulfur radical generation plays a key role in the flame-retardant mode of action, and sulfones released incombustible SO2. The results highlight the multifunctionality of the hyperbranched polymer, which displays better fire performance than its low molar mass thio-ether analogue due to the presence of vinyl groups and higher stability than its monomer due to the presence of thio-ether groups. Chapter 7: Branched polymers are an important class of polymers with a high number of terminal groups, lower viscosity compared to their linear analogs and higher miscibility, which makes them especially interesting for flame retardant applications, where the flame retardants (FR) are blended with another polymer matrix. Hyperbranched polyphosphoesters (hbPPEs) are gaining more and more interest in the field of flame retardancy, as low molar mass FRs often have the disadvantage of blooming out or leaching, which is not desired in consumer products. Here, we present the first phosphorus-based AB2 monomer for the synthesis of hbPPEs and assess its flame-retardant performance in an epoxy resin compared to a hbPPE synthesized by an A2+B3 approach. The hbPPE synthesized from an AB2 monomer exhibited a slightly higher performance compared to a similar hbPPE, which was prepared by A2 + B3 polyaddition, probably due to its higher phosphorus content. Chapter 8: Crosslinked-polymer composites currently on the market cannot be recycled. A promising alternative to reach recyclability is the development of dynamic covalent polymer networks. To date, however, additives such as flame retardants are still required for these materials if they are to follow safety regulations. Therefore, herein the first intrinsic flame-retardant dynamic polymer network based on vinylogous polyurethanes is presented and its flame retardant properties, as well as the application in composites, are assessed. In composites, vitrimers open the possibility of recycling, including reprocessing, repairing and separation of the fibers from the matrix. This is almost impossible for conventional fiber-reinforced polymer composites. In addition, the herein presented vitrimer has a similar glass transition temperature to commercially available epoxy resins and the determined values for the bending strength and bending modulus for the phosphorous-containing vitrimer lie within the range of permanently cross-linked epoxy resins reinforced with the same glass fibers. Chapter 9: Phosphodiesters are bridging elements in nucleic acids. In nature and synthesis, their negative charge protects them from hydrolysis and controls their solubility profile. RNA is a promising material for gene technology but cellular uptake is low due to negative charges. Synthetic oligonucleotides were delivered into cells by a prodrug approach relying on the enzymatic release of the polyphosphodiester oligonucleotides. In synthetic chemistry, a protective group for the P-OH functionality is often necessary, e.g. due to solubility or chemical incompatibility. Several methods for P-OH protection were proposed, but often with low selectivity or harsh conditions. Here, we present the 2-acetylthioethyl group as a versatile protective group for low molecular weight or polymeric phosphodiesters, which can be cleaved under acidic conditions in water or by hydrazine in THF to release the P-OH-functionality, but olefins remain intact. This straightforward use allows designing various synthetic polyphosphodiesters, e.g. for flame-retardant or dispersants. Chapter 10: Anisotropic materials with very high aspect ratios (“2D materials”), such as platelets are an interesting material class due to their unique properties based on their unilamellar geometry. We utilized phosphorus chemistry and simple polycondensation to introduce precisely spaced defects in polyethylene like polymers. The relatively large size and flexibility of the phosphate groups allows the control of the chain-folding during crystallization. We investigated this behavior by solid state NMR and TEM imaging. Furthermore, we showed that we are able to do chemistry “on surface”. The pendant ester group at the phosphate gives the possibility for the introduction of functional groups which are accessible for further chemical modification on the crystal surface, which was proven by the introduction of the 2-acetylthioethyl ester group and a later cleavage of the 2-acetylthioethyl ester group to P-OH. In addition we were able to control the lateral crystal size by different temperature profiles.
DDC: 540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
ROR: https://ror.org/023b0x485
DOI: http://doi.org/10.25358/openscience-5539
URN: urn:nbn:de:hebis:77-openscience-4c2d5af8-1707-4f42-a34a-16eac6bdee992
Version: Original work
Publication type: Dissertation
License: In Copyright
Information on rights of use: http://rightsstatements.org/vocab/InC/1.0/
Extent: IV, 436 Seiten, Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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