Why is DNA flame retardant and suppressive

Phosphorylation and sol / gel finishing of cellulose textiles to be made flame retardant


1 Phosphorylation and sol / gel finishing of cellulose textiles to be made flame-retardant. Paper approved by the Chemistry Faculty of the University of Stuttgart to obtain the title of Doctor of Natural Sciences (Dr. rer. Nat.) Presented by Sarah Deh from Reutlingen Main reporter: Co-reporter: Prof. Dr . Michael R. Buchmeiser Prof. Dr. Dietrich Gudat Oral Examination Day: Institute for Textile Chemistry and Man-Made Fibers Denkendorf Institute for Polymer Chemistry at the University of Stuttgart 2016


3 For my family You never notice what has already been done, you only see what remains to be done. (Marie Curie)


5 Acknowledgments At this point I would like to thank my doctoral supervisor, Prof. Dr. Michael R. Buchmeiser, thank you. On the one hand for the opportunity to provide me with a research topic for my doctorate at the Institute for Textile Chemistry and Man-Made Fibers in Denkendorf and on the other hand for the constant support of my work through constructive criticism or by conveying new solutions. Many thanks also to Dr. Frank Gähr for looking after my work. Due to his many years of expertise in the field of surface modification and flame protection, interesting conversations with the exchange of knowledge and the development of new ideas arose with regard to the research topic. Thanks to Prof. Dr. Dietrich Gudat for his willingness to be available as co-reporter and to Prof. Dr. Rainer Niewa, for taking over the chairmanship of the examination. I would also like to thank my colleagues at the ITCF Denkendorf. Advice was always available for questions or problems and a solution was sought together. Thanks also for the nice working atmosphere that made working a pleasure every day. My thanks also go to the working group of the IPOC for the cohesion and the exchange of knowledge. I would particularly like to thank Dr. Antje Ota, Dr. Stephanie Zinn, Dr. Lisa Steudle, Dr. Johanna Spörl, Manuel Clauss, Erna Muks, Dianne Weldin, Bernhardt Sandig, Laura Widmann and Iris Elser, who always had an open ear and advice for me. Finally, I would like to thank my family and friends, especially my parents and my brother, who made my studies possible at all and who supported me at all times. A very big thank you also goes to my friend, who encouraged me in good phases of work and always built me ​​up in bad phases and gave me new ideas. I would never have achieved so much without you, thank you for that.



8 Table of Contents RESULTS AND DISCUSSION 37 5 PHOSPHORUS-BASED SYSTEMS DEVELOPMENT OF A SUITABLE PHOSPHORUS-BASED SYSTEM Phosphorylation according to Yurkshtovich et al. [139] Phosphorylation according to Hawkes et al.



11 Table of contents Reaction of cellulose with FR-0 (with base) Reaction of cellulose with PSi or NSi DEVELOPMENT OF NEW SOL / GEL SYSTEMS WITH ADDITIVES Finishing of cotton fabric with guanidinium phosphate (sol / gel technique) 167 APPENDIX CHARACTERIZATIONS PHOSPHORYLATION OF COTTON CELLULOSE- TOSYLATE FOR PY-GC / MS MEASUREMENTS C 176 REFERENCES 179 V

12 Abbreviations Abbreviations AIP ATH Char, charring DEPETEOS DHG DP Dynasylan Damo EDX EGA-FTIR EtOH FF FSM FTIR GC / MS 5-HMF IL LG LGO LOI Aluminum triisopropoxide Aluminum trihydrate Carbon / carbon residue Diethylphosphatoethyltriethoxysilane 1,4; 3,6-Dianhydro-α-D -glucopyranose degree of polymerization N- (2-aminoethyl-3-aminopropyl) trimethoxysilane energy-dispersive X-ray spectroscopy Evolved gas analysis FTIR ethanol furfural flame retardant Fourier-transformed infrared spectroscopy gas chromatography coupled with mass spectrometer 5- (hydroxymethyl) -2-furancarboxaldehyde PA 6 levoglucosone index Levoglucosanic liquid Polyamide 6 PEG Py REM STA TEOS TG; TGA polyethylene glycol pyrolysis scanning electron microscopy standard deviation simultaneous thermal analysis tetraethyl orthosilicate thermogravimetry; Thermogravimetric Analysis VI

13 Introduction and summary Introduction and summary Materials made of cellulose are subject to rapid thermal decomposition as soon as they come into contact with a source of fire or heat. During the pyrolysis of cellulose, a large number of highly reactive radical species are created, which maintain the combustion cycle due to the exothermic nature of the reactions to be taken into account during pyrolysis. From a chemical point of view, there are several ways to intervene in order to reduce the flammability of a polymer such as cellulose which is flammable per se in a standard atmosphere [1]. Halogen-containing flame retardants (FSM) are of great importance in flame protection. In particular, compounds containing bromine were and are still frequently used [2]. Polybrominated diphenyl ethers (PBDE) are mostly used in consumer goods, mainly in plastics such as polyurethane foams, such as those required for mattresses, furniture and car seats. But they are also used for flame-retardant textiles. At higher temperatures, PBDE can easily split off bromine atoms, which react with free radicals (such as oxygen) in the gas phase and thus prevent the fire from continuing. Since the bromine-containing FR are only added as an additive and are not reactively bound, they can easily get into the environment and be absorbed by humans. These FSM have been clearly identified as substances active in the thyroid gland. In addition, there is suspicion of an inhibition of brain development and an influence on the reproductive system [3]. Many of the halogen-free, flame-retardant finishes used nowadays have only a low permanence and usually withstand little or no washes. These are therefore only suitable for objects that are rarely washed or for which the FSM can be reapplied after each wash. Ammonium phosphates have been used for some time as effective FSM, with which phosphorus values ​​between 1-2% can be generated on the cotton. The addition of urea is often used to swell and increase the accessibility of the cellulose and to increase the flame-retardant effect [4]. The methods practiced according to the current state of the art for permanent flame retardant finishing of textiles are limited to a few chemical methods

14 Introduction and summary the. Resin finishes (Proban finish) or formaldehyde-based finishes (Pyrovatex) have been applied to cellulose textiles such as cotton for more than 40 years. As a result of constantly tightening flame retardant standards and, above all, the EU REACH regulation [5], manufacturers of clothing, contract and technical textiles are increasingly forced to tackle new chemical developments. In addition, the requirement to develop better FSM at acceptable prices is a limiting factor, which puts the industry before the decision to further develop existing FSM and replace questionable FSM or to use well-known systems in a new way [6]. The two most important target requirements for modern flame retardant systems are firstly the avoidance of organohalogen chemicals and secondly the avoidance of formaldehyde-based, organophosphorus reactive flame retardants. The aim of the present work was therefore to create a reactive connection of this element to cellulose on the basis of phosphorus-containing compounds and to bring about a permanence beyond this covalent connection through a sol / gel finish carried out in the second step (so-called thin coating). In this second stage, elements that are classified as flame-retardant (nitrogen, silicon, aluminum) should also be incorporated so that on the basis of modern analytical methods, e.g. Py-GC / MS, STA-FTIR / MS, ATR-FTIR etc. - basic statements about possible synergisms of the elements were made possible. Particular attention was paid to the interaction of different combinations of elements, so that their analysis was a mandatory requirement. It was also of interest to investigate the effects of sulfur, especially in combination with phosphorus. Based on the element correlations, the results of this work should serve as a guide for more targeted developments of future flame retardant systems. In the present work, cotton fabrics were modified with a phosphoric acid-based system (Cell-PN) and then derivatized with compounds containing silicon, nitrogen or aluminum using the sol / gel technique. The different sol / gel precursors were compared with one another. Tetraethyl orthosilicate proved to be the most suitable for the Si-containing compounds. Both the received 2

15 Introduction and summary The surface structures of the fibers as well as the flame-retardant effect were convincing. The combination system of phosphorylation with subsequent sol / gel coating based on TEOS (Cell-PNSi) turns out to be very promising. The aim was to precisely examine and understand the relationships between the modes of action between the elements. The influence of certain elements was therefore assessed as part of the work. Nitrogen, which was used for phosphorylation in the form of urea, did not lead to any synergism in combination with phosphorus. Only the accessibility and reactivity of the cellulose was increased and an influence on the CO2 / CO ratio during pyrolysis was observed. Interestingly, this influence was only determined by looking at the individual elements and not during the analysis of the actual FSM systems (Cell-P, Cell-PN and Cell-PNSi). The necessity of investigating the individual element combinations was thus made clear. Silicon, on the other hand, was shown to be an excellent synergist to phosphorus, which also improved the permanence of the phosphorylation. In addition, the relationship between phosphorus and nitrogen showed that the P / N ratio should be as large as possible in order to achieve an optimal flame-retardant effect. It was proven that the sol / gel coating reduced the nitrogen value and that the resulting synergism with phosphorus resulted in a significant increase in the LOI values ​​as well as the residual masses. The two-stage system thus represents an optimal combination of elements. Studies of the pyrolysis behavior based on the determination of the Limiting Oxygen Index (LOI), Py-GC / MS, STA-MS / FTIR or ATR-FTIR investigations revealed clear differences between the Systems Cell-P, Cell-PN and Cell-PNSi worked out. It was found that, due to the limitation of the phosphorylation (max. 1.0% by weight P) without urea, cellulose decomposition took place significantly faster with Cell-P than with Cell-PNSi, but also with Cell-PN there was a more rapid decomposition Decomposition. When comparing the systems, Cell-PNSi clearly prevailed, as the silicate created a protective protective layer and thus improved the flame-retardant effect. It should be emphasized that all three systems showed very good char. By means of the EDX mapping of the coal residues, it was possible to demonstrate the formation of a protective layer in the form of pyrophosphate in Cell-PNSi. At the 3

16 Introduction and summary other systems this was not found. Incidentally, in all three systems, crystals formed on the fiber surfaces, which were identified as potassium phosphate compounds. In the course of the Py-GC / MS measurements, there was a noticeable reduction in levoglucosan (LG) in the systems, in return the amount of levoglucosenone (LGO) was increased. The ratio LGO / LG increased with increasing phosphorus content, which in turn underpinned the catalytic effect of phosphoric acid and its intervention in the pyrolysis mechanism. An increased release of furans and phenol derivatives also confirmed the changed pyrolysis, which resulted in the formation of stable coal frameworks and thus higher residual masses. Cell-PNSi turned out to be the most effective FSM system as a result of the investigations. Another highlight was the investigation of the specific optical smoke density. A particular difficulty, however, was to obtain meaningful results due to the very thin textiles. The Cell-PN and Cell-PNSi systems made it possible to reduce smoke density by 75%. Here, too, it turned out that the phosphorus concentration in particular is decisive for this. Another important point of the work was the structure elucidation. In the course of the investigations, a clear formation of cellulose carbamate was demonstrated. There is much to suggest that other species are formed in addition to cellulose phosphate, but clear evidence of this has not yet been provided. In addition to the tests on textile fabrics, flame retardants were used on cellulose fibers and tosyl cellulose powder in order to investigate the influence of the tosylate group and sulfur. With the FSM FR-0 high residual masses were obtained. However, it was found that excessive amounts of sulfur catalyze the dehydration of the cellulose too strongly and there is a risk of pyrolysis being too rapid, as a result of which the phosphorus component cannot fully show its effect. For further comparisons and in order to be able to refer to the experiments with phosphorylation, the cotton fabric was modified again. Sulphation with amidosulphonic acid was carried out as a second derivatization step after the phosphorylation. The combination of System Cell-PN and sulfation produced promising results. Very high LOI values ​​(> 50) were achieved, which also showed good permanence. Correlations between phosphorus, sulfur and 4

17 Introduction and short version Nitrogen content was found to be contrary to the LOI and the residual masses. The best LOI values ​​were obtained with high sulfur and nitrogen and low phosphorus values. High phosphorus levels were particularly responsible for high residual masses. Accordingly, sulfur has a very effective effect in the lower temperature ranges (up to ~ 400 C), which play a role in the LOI tests. In relation to the residual mass, sulfur has a smaller effect, since at higher temperatures most of it changes into the gas phase (SO2, SO3). This also creates a somewhat brittle coal framework. The loss of mass would be too high at very high sulfur levels. A combination of sulfur and phosphorus is particularly useful with regard to flame protection. In the event of a fire, temperatures of up to 1000 C can be reached quickly, as not every object in the vicinity usually meets the flame retardant requirements. Sulfur values ​​in the range of 1.0% by weight are, as far as the residual masses are concerned, in a tolerable range. In combination with phosphorus values ​​of 2.0% by weight, there is also a good flame-retardant effect. The P / S ratio should be above 1.0. Depending on the question, it is therefore necessary to adjust the concentration of sulfur and phosphorus. With regard to flame protection, components such as the phosphoric acid, which contributes to charring, must be included in the system. Only compounds that catalyze dehydration, such as tosylates or sulfonic acids, are not effective. In addition, well-known modes of action, such as P / N synergism, do not occur in every phosphorus and nitrogen-based system. Rather, it is necessary to examine each FSM system in detail for the individual connections, only in this way is a meaningful assessment of the synergisms possible. 5

18 Abstract Abstract Cellulose materials are highly flammable in the presence of fire resulting in the release of a huge number of highly reactive radical species. These species lead to a further increase of the fire scenario because of the exothermal reactions during the pyrolysis. Fortunately, there are chemical opportunities for enhancing the inflammability of these flammable polymers [1]. Halogen-containing flame retardants (FRs) are of great importance in the field of flame retardancy. Mainly, brominated compounds were and are still frequently applied [2]. Polybrominated FRs such as polybrominated diphenylether (PBDE) are used in customer products, e.g., polymers like polyurethane, needed for mattresses, furniture and car seats, but they are also used for textiles.With increasing temperature, PBDEs tend to release bromine radicals, which can act as radical scavengers in the gas phase and therefore inhibit the fire propagation. The brominated FRs serve as additives for the polymers but are not bound covalently to them, so they can be easily released to the environment and can be absorbed by human beings. These FRs are clearly identified as thyroid hormone disruptors and potential developmental neurotoxicants and strongly influence the reproductive system [3]. A wide range of currently applied halogen-free FRs are insufficient regarding permanence and are nonresistant against one or more washing cycles. These compounds are useful for objects which are barely washed or in case there is the possibility to repeat the application of FRs after washing. For some time, ammonium phosphates are used as effective FRs, achieving phosphorus amounts between wt .-%. The increase in the accessibility and swelling of cellulose by the addition of urea leads to an improved flame retardant effect [4]. Nowadays, the applied procedures to achieve permanent flame retardant effects are limited to a few chemical methods. For more than 40 years, cellulosic textiles like cotton are treated with resin-based finishings (Proban -Finish) or formaldehyde-based systems (Pyrovatex). Continuously intensified regulations, especially the EU-REACH regulation [5], force companies to invest into new chemical developments. Moreover, costs reduction is of great importance. The objective is to improve existing flame retardant systems and replace the forbidden ones or to use old-known systems in a new 6

19 Abstract way [6]. The most important reason for the development of new modern FRs is the avoidance of halogen containing FRs and of formaldehyde-based and organophosphorus reactive FRs. The objective of this doctoral thesis was to implement a reactive binding of the element phosphorus to cellulose as well as to achieve a permanence going beyond this covalent linkage. This was performed via a second modification by the sol / gel technique (socalled ultrathin coating). In this second step, the incorporation of other flame retardant elements such as nitrogen, silicon or aluminum was accomplished. The characterization of these systems was carried out by using modern analytical tools, like e.g. Py-GC / MS, STA-FTIR / MS or ATR-FTIR, in order to evaluate synergistic effects. The study focused on the interactions between single elements. Therefore, analysis of the diverse systems was necessary. Besides, the influence of sulfur in combination with phosphorus on the flame retardancy was considered. The results of this work in terms of the element correlations should function as a guide for the further development of new flame retardant systems. Cotton fabrics were modified with phosphoric acid-based systems and subsequently coated by sol / gel with silicon, nitrogen or aluminum-based compounds. Thereby, the comparison of different sol / gel precursors showed that tetraethylorthosilicate was the most suitable silicon-based compound. The fiber surface structures and the flame retardant effect were convincible. The combination between phosphorylation and sol / gelcoating (based on TEOS) was pormissing. Correlations between the single elements were analyzed in order to understand the underlying mechanisms. Within this work, the influence of specific elements was assessed. The nitrogen, which was introduced in the form of urea for the phosphorylation process does not affect the flame retardant effect, also no synergism could be determined between nitrogen and phosphorus. However, an increased accessibility and reactivity of cellulose as well as a change of the CO2 / CO ratio by pyrolysis was observed. Interestingly, this influence was solely indicated by regarding the single element combinations and not by analyzing the main systems, like Cell-P, Cell-PN or Cell-PNSi in total. This illustrated the importance of analyzing the single systems in detail. 7th

20 Abstract On the contrary, silicon was found to be an excellent synergist for phosphorus, also increasing the permanence of the phosphorylation. Moreover, the P / N ratio should be sufficiently high for an optimal flame retardant effect. By the additional sol / gel coating, a decrease in the nitrogen amount can be achieved and the synergism between silicon and phosphorus lead to higher LOI values ​​and residual masses. The two-step system (Cell-PNSi) is the perfect element combination. Studies of the pyrolysis behavior in terms of the Limiting Oxygen Index (LOI), Py-GC / MS, STA-MS / FTIR or ATR-FTIR showed significant differences between the systems Cell-P, Cell-PN and Cell-PNSi. The synthetically limitation of the phosphorylation concentration (1.0 wt .-%) without urea (Cell-P) resulted in a fast pyrolysis compared to Cell-PNSi, but also Cell-PN showed a faster combustion. Comparing the three systems, Cell-PNSi was identified as the best system. The silicate, which was formed during combustion, allows the formation of an additional protective layer on the fiber surface, enhancing the flame retardant effect. Nevertheless, it is mentionable that all three systems showed a great charring effect. By EDX mapping of the char of Cell-PNSi, an observation of a glassy pyrophosphate-based coating was possible. The other systems did not show such a coating. Apart from that, all char surfaces showed crystals on their surface, which could be identified as potassium phosphates. Py-GC / MS revealed the formation of decreased amount of levoglucosan (LG) for all systems. In contrast, the amount of levoglucosenone (LGO) was increased. With increasing P-amount the LGO / LG ratio also increased. This confirmed the catalytic effect of the phosphoric acid and the influence on the pyrolytic mechanism of cellulose. Higher amounts of furans and phenol derivatives also enhanced the changed combustion mechanism, resulting in stable char and increased residual masses. Because of these results, Cell-PNSi was found to be the most effective FR. The investigation of the specific optical density was of special interest. The major difficulty was to get convincing results out of the very thin cotton fabrics. The systems Cell-PN and Cell-PNSi reduced the specific optical density about 75%, which is significant. Again, the phosphorus amount was crucial. Investigations were also made concerning the structure elucidation of the phosphorylated samples. The analyzes showed the formation of cellulose carbamate. Besides cellulose phosphates, the formation of other species are possible, but there is no evidence for that until now. Next to the experiments on cotton fabrics, 8

21 Abstract also modified cellulose and cellulose tosylate powders, were analyzed regarding the influence of tosylate and sulfur, respectively. High residual masses could be achieved with the use of the flame retardant FR-0, but it could also be determined that too high sulfur amounts significantly catalyze the dehydration and therefore lead to a very fast pyrolysis, whereby the phosphorus compound is not able to develop its effectiveness completely. For sulfur-containing modifications, sulfamic acid was applied on the cotton fabrics in a second step after phosphorylation. This system was very promising, leading to very high LOI values ​​(> 50) and to a good permanence. Correlations between phosphorus, sulfur and nitrogen were contrary to the LOI values ​​and residual masses. Best LOI values ​​were received at high sulfur and nitrogen amounts, but at low phosphorus amounts. Increased phosphorus amounts resulted in high residual masses. This means that sulfur is very effective in the lower temperature area (until 400 C), which is important for the LOI test. In terms of residual mass, sulfur is of little importance, because most of it transforms into gaseous SO2 or SO3 at higher temperatures, resulting in a brittle char. A combination of sulfur and phosphorus is meaningful for a good flame retardancy. In case of a fire, temperatures over 1000 C can be easily reached, because not every object meets the flame retardant requirements. A sulfur amount around 1.0 wt% is ideal for sufficient residual masses. Combined with phosphorus amounts of 2.0 wt.%, A great flame retardant effect is observed as well. The perfect ratio of P / S should be around 1.0 for good flame retardancy. The sulfur and phosphorus amounts should be adapted according to the issue. Compounds like phosphoric acid are crucial for the charring effect and necessary for flame retardant systems. Compounds like tosylates or sulfonic acids, with their strong catalytic effect on the dehydration of cellulose, are not conducive to reach good flame retardancy. Furthermore, not every phosphorus and nitrogen based system shows a P / N synergism. In reality, every single FR systems has to be analyzed in order to understand the relations between the single elements for evaluating the synergisms. 9


23 Theoretical basics Theoretical basics 1 Development of a fire The development of a fire can be divided into three stages. These would be the beginning, the fully developed and the decreasing fire. In the case of polymeric materials, a fully developed fire typically always occurs. The three stages are shown schematically in Fig. 1. Fig. 1: Phases of a fire. Temperature development over time [7]. To prevent or inhibit a fire, it is important to consider the phases up to the flashover [7]. Since a fire makes it difficult to escape from the affected area and thus endangers health, it is of great importance to minimize the risk of fire so that a fire cannot even start or at least the flashover is not achieved. Preventing a fully developed fire allows those affected a longer period of time to escape [7]. The whole thing can be implemented with so-called flame retardants (FSM). You can specifically intervene in the decomposition of the polymeric materials and change the pyrolysis behavior of these, resulting in a flame-retardant effect. 11

24 Theoretical Basics 2 Use of Flame Retardants (FSM) The use of polymers is becoming more and more important these days. It is precisely these materials that are highly flammable [8]. A US statistic of the National Fire Protection Association (NFPA) [9] showed that in the period of 500 fires 44% textile-based materials were involved, fires through worn clothing were not considered here [10-11]. The fire is usually accompanied by the release of corrosive and toxic gases and the development of smoke [8]. In almost 80% of cases, the cause of death is inhalation of the toxic smoke [12]. Inhaling released hydrogen cyanide or CO in particular poses a great danger, as just a few breaths can lead to death [13-15]. The challenge is therefore to develop the flame-retardant behavior of polymers in such a way that they pose little or no danger in the individual applications [8]. 2.1 Decomposition of Polymers As already mentioned, polymers are highly flammable. The reason for this is their structure, which consists mainly of carbon and hydrogen [8, 16]. The fire behavior of polymers depends on many parameters, such as: the oxygen content, the ignition temperature or the amount of heat released by the combustion products [17-19]. Two factors are of particular interest in the decomposition. On the one hand, the combustible material (reducing agent) and, on the other hand, the ignition source (oxidizing agent). The latter is oxygen from the air. If the thermal decomposition begins with the release of gases, it is called pyrolysis. As the process progresses, the temperature rises, which in turn leads to bond cleavage in the polymer. The released gases are enriched in the gas phase to form a flammable mixture (fuel / combustion gases). This mixture ignites as soon as the auto-ignition temperature (activation energy for the decomposition) has been reached, releasing heat. The whole thing can be initialized at lower temperatures in the presence of additional ignition sources such as fire or sparks [8, 20]. In the course of pyrolysis, organic compounds are broken down

25 Theoretical Basics Bonds in radical fragments, whereupon further radical chain reactions follow. The radicals react with the air (oxygen), which further ignites the combustion, which leads to reactions as shown in Fig. 2. The highest energy is provided by reaction (3) [20-21]. Fig. 2: H 2-O 2 scheme. Radicals generated during pyrolysis [21]. Depending on the type of fuel, non-flammable and carbon-like substances are also formed [20]. The duration of the decomposition depends on how much heat is released and how high the thermal feedback on the substrate is. A diagram of the so-called firing cycle is shown in Fig. 3. The energy necessary to start the decomposition of polymeric materials depends on the physical properties of these compounds. In the case of partially crystalline thermoplastics, decomposition leads to softening, melting and dripping of the polymer. The ability to store heat depends in turn on the heat storage capacity, the melting enthalpy and the crystallinity index of the polymer. Therefore, the resulting temperature increase in the polymer depends on the heating rate, the temperature differences in relation to the exothermicity of the reactions involved and the specific heat and thermal conductivity of the mostly partially crystalline fiber polymers [8]. Fig. 3: The firing cycle. Schematic representation of the important parameters for the decomposition of polymers [8]. 13th

26 Theoretical principles Amorphous thermoplastics and most thermosets show, due to the lack of melting point, a direct decomposition when the temperature increases [8]. The thermal decomposition of a polymer is subject to endothermic reactions, i.e., as already explained, energy must be supplied for this process. The energy supplied must be higher than the binding energies of the weakest bonds (mostly kj / mol for C-C polymers). Accordingly, the decomposition is linked to the weakest bonds in the polymer and the presence or absence of oxygen in the solid or gas phase. The decomposition can generally be viewed as a result of the combination of heat and oxygen, with a subdivision into oxidizing and non-oxidizing decomposition [8, 16]. Thermal decomposition under non-oxidizing conditions begins with the cleavage of the chains in the polymer (pyrolysis). This cleavage varies depending on whether there is oxygen or catalyst residues or residues of previous oxidation and weak bonds along the chain. During oxidative cleavage, the polymer reacts with the oxygen in the air, with a large number of low molecular weight compounds such as ketones, alcohols, carboxylic acids, aldehydes, etc. being formed. In the process, very reactive radical species such as H and OH are formed. Recombinations of these radicals lead to cross-linking in the polymer. Nevertheless, the bond cleavage represents the dominant part of the reactions. This is driven by the cleavage of H atoms, because the oxidation stability depends on the C-H bond energy [8]. The pyrolysis gases are oxidized under oxygen to H2O, carbon oxides and heat through complex radical chain mechanisms. The energy released by this exothermic process leads to further pyrolysis of unconsumed solids, whereby more and more flammable gases are generated. This is why the combustion cycle is referred to as a self-sustaining combustion process [18, 20, 22]. The exact mechanisms will be discussed later. 2.2 Effect of flame retardants The primary task of FSM is to prevent, minimize, contain or prevent the decomposition of polymers [8, 23]. They act by interrupting the self-sustaining decomposition cycle (Fig. 4). As a result, the combustion 14

27 Theoretical principles Reduced speed or extinguished the flames and there is less heat generation [24-25]. These can work in two different ways. If there are cooling effects, the formation of protective layers or thinning effects, this is referred to as a physical effect. The FSM work chemically by being active in the gas or condensation phase [8, 26]. Mostly, the flame resistance and the increased formation of carbon residues correlate with one another. This means that if more coal is formed, the flammability of a polymer decreases [27-28]. The charring occurs through linking reactions, whereby the release of flammable pyrolysis gases is reduced. In addition, the carbon residue isolates the polymer from the heat of the flame and the barrier layer prevents the release of formed gases and the penetration of oxygen to the polymer [28-30]. Fig. 4: Intervention of flame retardants in the decomposition mechanism (graphic based on [17, 31]) Physical effect During endothermic decomposition, some FSM lead to a decrease in temperature due to heat consumption. This cooling effect in turn lowers the temperature values ​​below the polymer decomposition temperature. If the heat falls below the limit 15

28 Theoretical fundamentals for maintaining the firing cycle, the flame goes out. The dilution of the fuel results from the release of gases such as H2O, CO2 or NH3 by the FSM. This leads to a dilution of the combustion gases in the atmosphere and the concentration of reagents and the possibility of ignition are reduced [7-8, 20, 23]. Examples of such additives with this mode of action would be magnesium or aluminum hydroxide (ATH). It is possible for them to release water vapor between C [7-8, 23, 32]. Furthermore, FSM can form a solid protective layer (e.g. phosphorus or boron compounds) or a gaseous film on the substrate surface, where thermal decomposition takes place. In this way, combustible gases and, above all, the continuous diffusion of oxygen to the substrate surface are prevented.As a consequence, the amount of decomposition gases is significantly reduced, the fuel gases are physically separated from the oxygen and persistent decomposition of the polymer is prevented [7-8, 23, 33-35] Chemical effect The chemical effect occurs both in the gas phase and in the condensation phase. In the gas phase, the formation of radicals (e.g. Cl, Br in the case of FSM containing halogens) counteracts the radical mechanism of polymer decomposition. The radicals formed by FSM react with highly reactive species such as H or OH, producing less reactive compounds or inert molecules. This leads to a standstill of the exothermic reactions as well as a cooling of the system and a reduction of the combustible gases. In the condensation phase, FSM on the one hand lead to the fact that the polymer chains can be split more easily and thus the polymer drips. On the other hand, FSM lead to the formation of carbon-containing or glass-like layers on the polymer surface through chemical transformations of the decomposed polymer chains. These protective layers serve as a physical insulating layer between the gas and condensation phases, creating a barrier for the diffusion of gases, especially oxygen [7-8, 20, 23, 32-34]. 16

29 Theoretical Basics 2.3 Types of Flame Retardants There are a number of different types of FSM. In this work the focus is on phosphorus-, nitrogen-, sulfur-, silicon- and aluminum-based FSM. In principle, a distinction is made between additives and reactive FSM. Additives are integrated into the polymer and not chemically bound; a reaction only occurs when the temperature rises. The reactive FRs are anchored in the polymer chain through the polymer synthesis or modification reactions [8] Phosphorus-based FRs Both organic and inorganic phosphorus compounds serve as FRs. They are used in a wide variety of oxidation states such as 0, + III or + V. The spectrum ranges from phosphates, phosphonates, phosphinates, phosphine oxides to red phosphorus. They can be added as additives or bound directly to the polymer, being active in the gas and / or condensation phase. The efficiency in the condensation phase occurs mainly with oxygen-containing polymers such as polyester, polyamides and cellulose [8, 23, 36]. Most phosphorus-containing FRs lead to the formation of phosphoric acid, which in turn condenses to pyrophosphates and releases water. This dilutes the oxidizing gas phase. In addition, phosphoric acid and polyphosphoric acid support dehydration by reducing the pyrolysis temperature. The dehydration is catalyzed by the acids [37-38]. As a result, the pyrolysis path is preferred over dehydration, which leads to charring and the formation of C = C bonds. At high temperatures it comes to cross-linked or carbonized structures. A scheme for the decomposition mechanism of cellulose phosphate is shown in Fig. 5 [8, 23, 39-40]. At temperatures of 200 C, the area of ​​the first loss of mass, phosphate is split off from the cellulose in the form of an elimination. If the temperature rises further, the cellulose is re-esterified with the phosphoric acid formed [41-42]. At high temperatures, metaphosphoric acid (OPOOH) and the resulting polymers (PO3H) n are formed. Phosphate anions are involved in the charring, with the carbonized layer protecting and isolating the polymer from the flames. Furthermore, the release of other fuel gases and the formation of free radicals are prevented

30 Theoretical foundations bound. The continued diffusion of oxygen to the polymer is made more difficult, which slows down the decomposition. In addition, the polymer is shielded from the heat [8, 23]. Fig. 5: Scheme of the decomposition mechanism of cellulose phosphate. The formation of phosphoric acid and polyphosphoric acid leads to the formation of C = C bonds and ultimately to carbon residue [43]. Another possibility of phosphorus-based FSM is to be released into the gas phase and to form radicals there (PO2, PO and HPO) in order to act as radical scavengers of e.g. H or OH to be able to act. This ability makes the FMS very effective; the phosphorus-based FRs are five times more effective than bromine and ten times more effective than chlorine-based FRs [8, 23, 44]. The most concentrated phosphorus compound is red phosphorus. It can act both in the gas phase and in the condensation phase [45] and is also very effective even in very small amounts

31 Theoretical basics of effective FSM (e.g. <10%). The prevailing opinion is that red phosphorus reacts through oxygen and / or nitrogen in the polymers primarily through thermal oxidation to form phosphoric acid or phosphoric anhydride and consequently polyphosphoric acid is formed when the temperature is increased. This is followed by catalysis of the dehydration of the polymer and the end chains, resulting in the formation of carbon [8, 23, 46]. According to this, red phosphorus is more effective in oxygen- and / or nitrogen-containing polymers at high temperatures [2]. The effect of red phosphorus also exists with non-oxidic polymers such as e.g. PE. The red phosphorus depolymerizes to white phosphorus (P4). At higher temperatures, this evaporates in order to act as a radical scavenger in the gas phase or it reaches the polymer surface from the inside of the polymer to be oxidized there. When P2O5 comes into contact with H2O, phosphoric acid is formed. This in turn leads to charring of the polymer surface, which results in lower amounts of fuel gases and a shielding of the oxygen from the polymer [8, 23]. The disadvantage of red phosphorus is the release of toxic phosphine (PH3). This happens in reactions with moisture due to its low thermal stability. This can be counteracted by encapsulating red phosphorus or the reaction of phosphine with metal salts (AgNO3, CuO, MoS2) at high temperatures [8, 23, 47] Nitrogen-based FSM Nitrogen-based FSM are an environmentally friendly and non-toxic alternative to halogenated compounds [ 48]. A compound often used for flame retardancy is melamine. It is thermally very stable and only melts at temperatures> 345 C. The high proportion of nitrogen (67%) is also of interest for flame protection [8, 49]. Melamine is used in a wide variety of forms, e.g. as melamine cyanurate in nylon [50], melamine phosphate in polyolefins, melamine and melamine phosphate or dicyandiamide (cyanguanidine) in intumescent colors, guanidinium phosphate for textiles or wood [51-52] and guanidinium sulfamate for wallpapers (Fig. 6) [53 ]. The greatest advantages of the compounds are that they are solids with low toxicity, no dioxin or halogen acids are released in the event of fire and little smoke is released 19

32 Theoretical foundations. Their effectiveness is somewhere between that of halogens and ATH or magnesium hydroxide [48, 53]. Metal salts of dialkylphosphonic acid or calcium hypophosphite show synergistic effects with phosphorus- and nitrogen-containing compounds, such as e.g. Melamine salts [2]. Since the flame-retardant effect of nitrogen-based flame retardants is limited, a combination with phosphorus-based FSM is often used in order to increase the effectiveness through the synergism. Because the sole use of nitrogen-based FSM to develop adequate surface protection is still under discussion [54]. As it turned out in this work, a synergistic effect between nitrogen and phosphorus cannot be assumed in principle. Fig. 6: N-based FR: (A) melamine, (B) melamine phosphate, (C) melamine cyanurate, (D) guanidinium phosphate, (E) guanidinium sulfamate and (F) dicyanodiamide P / N-based FR Important representatives of this FSM class are the so-called phosphazenes. They arise from the aminolysis of phosphorus (V) halides and have the characteristic repeating unit R2P = N [55]. Phosphazenes are represented in various structures, which consist of monomeric structures such as e.g. Phosphinimines or phos- 20

33 Theoretical bases phoranimines can be obtained [56]. The spectrum ranges from high molecular weight polymers to low molecular weight oligomers. Some examples are shown in Fig. 7 [57-58]. They are used as FSM in the form of additives or are inherently flame retardant [59]. Fig. 7: Basic structures of (A) linear poly (organophosphazenes), (B) cyclic phosphazene oligomers and (C) linear phosphazene oligomers [57-58]. Cyclophosphazenes can undergo a wide variety of reactions, which in turn enables the production of numerous flame-retardant materials. They also have a thermostabilizing effect. Thermogravimetric studies have shown that their mode of action is primarily in the gas phase [56]. Linear polyphosphazenes are inorganic-organic hybrid polymers in which the inorganic component is located in the polymer backbone or in the organic side chain [58, 60]. The repeating unit P = N- results in high molecular weight molecules with linear chains, branched, cross-linked or dendritic structures. The reaction is induced by a ring-opening polymerization of hexachlorocyclotriphosphazene and a subsequent substitution reaction with various nucleophiles. In addition, homopolymers or copolymers with other monomers or polymer blends are formed. The phosphorus present in the polymer chain makes the polymer flame-retardant, shows high thermal stability, leads to high LOI values ​​and low smoke development. Functional groups such as alkyls, aryls, hydroxides, amines, sulfonic acids, etc. increase the application possibilities of the phosphazenes enormously [58]. 21

34 Theoretical Basics Silicon-based FSM silicones have a low heat release rate (HRR), release little CO and are insensitive to external heat flow. In addition, they have a low burn rate without dripping when burned, and no toxic smoke is produced without further additives. Because of these properties, silicones turned out to be suitable FSM. Polydimethylsiloxane (PDMS) is primarily used in cables and electrical lines in the higher temperature range. In contrast to organic polymers, silicones decompose at high temperatures through oxidation of e.g. Methyl groups with the release of CO2 and CO to inorganic silicates [61-62]. This shielding effect represents the basic framework for the development of silicon-based FSM. This insulating protective layer stops the decomposition products released. As a result, fewer gases get into the gas phase to further ignite the fire, there is less thermal feedback to the polymer surface, and less of the heat flow reaches the surface through the silicate layer [62]. It has been shown that the silicate ash layer controls the efficiency of the diffusion barrier of the combustion gases in the combustion zone and the access of oxygen to the unburned substrate [62-63]. The polyhedral oligomeric silsesquioxanes (POSS) are another way of binding silicon to the polymer and providing a flame-retardant effect. These are three-dimensional oligomeric organosilicon compounds with the general structure (RSiO1.5) n (Fig. 8) with n as an even number and R as a hydrogen atom or a variable organic functional group (often alkyl, alkenyl, halogen, vinyl, phenyl, epoxide) [17, 58, 64-66]. POSS consist of an inorganic core (silicon and oxygen) surrounded by organic polar or non-polar components. They are produced by the hydrolysis of organotrichlorosilanes [67-69]. The POSS molecules can be modified with both reactive and unreactive functional groups, which enables easy incorporation into the polymer. This can improve the flammability or the thermal and mechanical properties of the polymers [70]. POSS is a good precursor which forms thermally stable ceramic materials at high temperatures [8, 71]. 22nd

35 Theoretical principles Fig. 8: Structure of a polyhedral oligomeric silsesquioxane (POSS) [65]. Another current and frequently used method for generating environmentally friendly, silicon-based layers on polymers for flame protection is the sol / gel technique [54, 72-89]. The process is usually used to generate polymeric networks of hybrids from organic and inorganic compounds [54, 90]. The sol / gel technique is also of interest for other applications. In this way, properties such as hydrophobicity, antistatic, antimicrobial behavior or UV protection are obtained [72, 74, 91-94]. Detailed information on the sol / gel process will follow at a later date. Commercial FR In the last few decades, a large number of FRs have been brought onto the market [4, 95-96]. Many are based on P / N synergism. The most effective FR are those which are reactively bound to the cellulose and act in the condensation phase [97-98]. Among other things, with Pyrovatex [99-100] and derivatives, Huntsman offers a corresponding range of systems for the modification of cotton, the effectiveness of which has already been investigated and confirmed in many studies []. The system is based on dialkylphosphonocarboxamides (see Fig. 9). However, it is not a formaldehyde-free system (<75 ppm) [99]. With the stricter requirements of the REACH regulation, such values ​​will no longer be permitted in the future. With the Proban finish [4, 107], Rhodia offers an effective system that is primarily active in the condensation phase but also partially in the gas phase [108]. However, the FSM is made from the poisonous tetrakis (hydroxymethyl) phosphonium chloride. The 2-stage system 23

36 Theoretical basics tem Pekoflam ECO and SYN was brought onto the market by Clariant (Archroma) [109]. Pekoflam ECO contains the phosphorus component (phosphorous acid) and Pekoflam SYN the nitrogen component. The second stage, however, is not yet fully developed. There are still deficits with regard to the feel of the modified samples and the flame-retardant properties. It becomes clear that the need for new FSM, in spite of many systems available on the market, has by far still been exhausted. Fig. 9: General structure of PYROVATEX CP [99-100] as well as the output connections of the Proban finish [4, 107] Further applications of FSM FSM are used in other areas in addition to fire protection. The catalyzing properties of sulfur compounds (e.g. in sulfur-containing ILs) can intervene in the pyrolysis of cellulose and lead to larger amounts of levoglucosenone [110]. In addition to flame retardancy, this property is also useful for other applications. Levoglucosenone is an important chiral synthon which is used for the synthesis of various natural and synthetic compounds. A notable example is the synthesis of optically active - (-) tetrodotoxin, the nerve poison of a puffer fish. The substance is of interest not because of its toxicity, but because of its novel and complex molecular structure (Fig. 10) []. FSM are also used in carbonation. They cause the pyrolysis to start at lower temperatures and higher residual masses with a higher carbon yield are obtained []. Usually phosphorus- and sulfur-based 24 are used

37 Theoretical Foundations Connections Used. FSM lead to a reduction of the pyrolysis gases and the pyrolysis speed, but the temperature is continuously increased during the carbonization process and thus the pyrolysis cycle is kept running. For high carbon yields (carbon residue) the CO2 / CO ratio must be reduced [39] and a barrier layer must be formed on the fiber to protect against further oxidation and release of the pyrolysis gases (see also 2.2). Systems that cannot form such layers, e.g. Systems based exclusively on sulfuric acid are less suitable for carbonization. In addition, it should be taken into account that e.g. Phosphoric acid esters or phosphonates require sufficient time to quantitatively release phosphoric acid when pyrolysis occurs, in order to be able to contribute to a sufficient protective layer of pyrophosphates even at higher temperatures (see 3.2). The modes of action and efficiency of the FSM must therefore not be transferred 1: 1 to carbonization. Fig. 10: Structure of - (-) tetrodotoxin [111]. 25th

38 Theoretical Basics 3 Pyrolysis Mechanisms of Cellulose 3.1 Pathways of Pyrolysis The pyrolysis of cellulose or cotton represents a complex series of mechanisms, which in turn also run parallel to one another. An overview of the reactions taking place is simplified in Fig. 11 and shown in more detail in Fig. 12. First of all, it should be made clear that it is not the cotton itself that burns, but that the decomposition mechanism takes place in the gas phase. The breakdown and release of volatile gases from the solid phase results in a combustible mixture over time, which in turn can be ignited. The released energy leads to a further decomposition, the pyrolysis of the tissue [41]. The pyrolysis is divided into two temperature ranges. At temperatures <300 C, the active cellulose initially leads to a reduction in molecular weight, the release of H2O, CO and CO2 and the formation of carbon (Char). If temperatures between C are reached, rapid decomposition or depolymerization to anhydroglucose units and levoglucosan (1,6-anhydro-β-glucopyranose) occurs, which through further reactions lead to a tarry residue.If the decomposition proceeds at even higher temperatures, cleavage, dehydration, disproportionation and decarboxylation reactions of the anhydroglucose units occur, which in turn results in the formation of low molecular weight volatile compounds [40, 97, 102,]. The individual pyrolysis paths are discussed in more detail below. Fig. 11: Competing pyrolysis pathways in the decomposition of cellulose [115,]. 26th

39 Theoretical principles Fig. 12: Scheme of the Broido-Shafizadeh mechanism for the pyrolysis of cellulose [116, 121]. The complexity of pyrolysis is illustrated here. Reactions at low temperatures (<300 C) Cellulose is very stable over a short period of time at moderate temperatures. Nevertheless, thermal decomposition and deterioration of the physical properties occur as soon as the temperature is increased and the cellulose has to be exposed to these conditions over a longer period of time [120]. As already mentioned, in the range of T <300 C there is a loss of mass or a reduction in the degree of polymerisation of the cellulose, due to cleavage reactions, release of H2O, occurrence of free radicals, formation of carboxyl, carbonyl and hydroperoxide groups, release of CO and CO2 as well as the final formation of coal [115, 122]. The formation of hydroperoxide groups occurs primarily during pyrolysis in an air atmosphere [122]. It is assumed that the decomposition of cellulose is subject to a mechanism via the formation of free radicals, but Shafizadeh and his research group were unable to observe these radicals, but they were able to observe the formation of hydroperoxides when cellulose is heated in air analyze [115, 122]. The hydroperoxides are formed and decomposed in a similar way to the free radical mechanism. Kinetic measurements at 170 C 27

40 Theoretical principles showed a constant increase in the hydroperoxide concentration to a constant level. The disintegration, in turn, takes place with first-order kinetics under a nitrogen atmosphere. When comparing the reaction rates for the formation and the decomposition, a clear connection between the hydroperoxide formation and the bond cleavage was recognized. On the basis of these statements and findings from the irradiation of carbohydrates, a 3-stage pyrolysis mechanism at low temperatures was assumed (see Fig. 13). The initiation leads to the formation of radicals, supported by the presence of oxygen or inorganic impurities. As a result, radical chain reactions occur and the decomposition mechanism of the cellulose continues. This leads to bond cleavage, oxidation and decomposition of the molecules, which result in the formation of H2O, CO and CO2 as well as carbon residues [115]. Fig. 13: Thermally induced autoxidation of cellulose in an air atmosphere [115]. 28

41 Theoretical basics Reactions at high temperatures (> 300 C) If temperatures above 300 C are reached, the decomposition of the cellulose follows a second pyrolysis path. This includes the cleavage of glycosidic bonds. Levoglucosan, other anhydroglucose compounds, arbitrarily linked oligosaccharides and glucose decomposition products are formed here [123]. The pyrolysis of cellulose begins with the cleavage of the glycosidic compound and the condensation of a sugar unit, which is subject to further decomposition through further heating [115]. It has also been shown that the reaction is a heterolytic mechanism, as it is influenced by the electron density of the substituents on the aglycon [119, 124]. Glucosides with better leaving groups decompose at lower temperatures [124]. This finding rules out a homolytic process, although the further decomposition and carbonization of the sugar units involve radical processes. It is also an indication that thermolysis is sensitive to acids and alkaline reagents [115]. Further experiments by Shafizadeh showed that glucosides with different substituents, e.g. Amino groups, in addition to the inductive effect of the glycosidic bond, change the accessibility and reactivity of the transglycosylation side. The acetylated compounds are more stable than those with free hydroxyl groups [115, 124]. The decisive exothermic process in the decomposition of cellulose is the oxidation of CO to CO2. During the decomposition, levoglucosan is completely oxidized in the gas phase, which results in increased heat production and thus actively maintains the combustion process through the release of gases as well as the release of heat [39]. 3.2 Influence of the cellulose structure on thermal decomposition The structure of cellulose has an immense influence on the mode of action of flame retardants during pyrolysis. The linear homopolymer consists of D-glucopyranose units (anhydroglucose units) which are linked to one another via -1,4-glucosidic bonds [125]. The parallel, linear chain structures of cellulose are stabilized by inter- and intramolecular hydrogen bonds. Cellulose naturally has crystalline regions with inter- and intramolecular water 29

42 Theoretical principles of material bridges, on. It also has amorphous structures of higher energy []. The crystalline and amorphous areas of cellulose differ in pyrolysis. Studies on pyrolysis showed that the DP steadily decreases up to a temperature below 250 C and ultimately remains constant at a value of 200. The microcrystallites of cellulose show corresponding values, from which it could be concluded that thermal decomposition begins first in the amorphous, less ordered area of ​​cellulose [41, 128]. Basch et al. came to the conclusion that the proportion of amorphous cellulose structures is decisive for pyrolysis, especially since the activation energy for pyrolysis of these structures is 30 kcal / mol, only half as high as for pyrolysis of crystalline areas [129]. The orientation and cross-links influence the pyrolysis as well as the crystallinity. Basch et al. Showed that chain cleavage by breaking the glycosidic bonds (1,4 linkages) is the rate-determining step in pyrolysis. The DP therefore determines the pyrolysis rate [41, 129]. Rodrig et al. found in a cellulose crosslinked by formaldehyde that less crosslinked cellulose leads to a much easier cleavage of the hydrogen bonds than with more highly crosslinked cellulose. Due to the many connection points, stabilization occurs and the loss of hydrogen bonds is suppressed. As a result, the pyrolysis rate is immensely reduced compared to the less linked cellulose [41, 130]. In this context, Rodrig and co-workers also showed that regenerated cellulose decomposes at a higher pyrolysis rate than native cellulose. With regard to the effect of flame retardants, these previously shown influences of the cellulose structure also have an effect. Studies by Basch and Lewin on the differences between sulfur and phosphorus-containing FSM depending on the structure of the cellulose made this clear [131]. The sulfur-based FSM showed comparable results for both more highly crystalline cotton and less crystalline regenerated cellulose. The phosphorus-based FMS proved to be more effective in cotton compared to the regenerated fibers. They attributed this to the differences in the stability of the acid esters formed as intermediates. Phosphoric acid esters have a higher thermal stability, which means that the active phosphoric acid is only released at higher temperatures

43 theoretical fundamentals. The less crystalline and therefore thermally more sensitive regenerated cellulose decomposes even at lower temperatures. The proportion of phosphoric acid that has already been released is therefore still very low, and consequently only a small proportion of phosphoric acid is available for the formation of polyphosphoric acids. As a result, there is also a lower flame-retardant effect compared to native, more highly crystalline and thermally stable cotton. Due to their stability, the phosphoric acid esters can be split quantitatively into phosphoric acid and fully utilize the flame-retardant effect of the phosphoric acid formed. In contrast to this, sulfuric acid esters are less temperature-stable and are therefore split off at lower temperatures. In this case, therefore, no noticeable difference was found between the two structure types [41, 131]. 4 The sol / gel process In order to understand the sol / gel process in detail, it is first necessary to deal with some terminology. A colloid is a suspension in which the dispersed particles are so small (nm) that gravitational forces are negligible. Interactions between the particles result from their short distance, from van der Waals forces or surface charges. Their inertia is great enough that a Brownian molecular motion occurs when the particles collide. The sol is a colloidal suspension consisting of solid particles in a liquid [132]. Their interactions are very small, so that the particles stabilize. If there were no stabilization, the large surface areas of the particles would lead to agglomeration of them. The particle sizes are in a range of nm, in which the Rayleigh scattering does not yet play a role, which means that nanoparticulate dispersions are present in brines. Gels can be found in many areas such as cosmetics, food, medicine and nature. They have a very low mechanical stability and are at least 2-phase. The solid phase forms the wide-meshed network, which in turn is filled with a liquid or a gas. Depending on the pore filling, we use hydrogels (aqueous phase), alcogels (alcoholic 31

44 Theoretical basics cal phase) and xerogels (gaseous phase) differentiated. The network can consist of an organic polymer (organogel) or an inorganic network [133]. The precursors used primarily for the sol / gel process are metal alkoxides. They belong to the family of organometallic compounds in which the organic ligand is bonded to the metal or semi-metal atom. Although metal alkoxides with their metal-oxygen-carbon linkage should not actually be counted among the organometallic compounds (metal-carbon compounds), this classification is often made in the literature. An important representative of the metal alkoxides is tetraethyl orthosilicate (TEOS). Metal alkoxides are subject to rapid hydrolysis in the presence of water (Fig. 14, Eqs. (1) and (2)). This can lead to partial as well as complete hydrolysis. If complete hydrolysis does not occur, condensation reactions as in Eq. (3) and (4) (Fig. 14) can be seen [132]. Fig. 14: Hydrolysis and condensation reactions of the sol / gel process. R = alkyl radical, OR = alkoxy, ROH = alcohol [132]. The condensation reactions in turn lead to dimers, polymer chains or cross-links (ring formation) (Fig. 15). Silicon alkoxides [Si (OH) 4], with their tetrafunctionality, make it possible to form complex cross-links [132]. The production of brine is quite simple. Liquid alkoxide precursors (Si (OR) 4) are hydrolyzed in water. The resulting silicate tetrahedra can form Si-O-Si networks through condensation reactions. The particle size and the cross-linking between the particles depends on the ph value and the remainder of R [134]. With the help of a dispersing agent and whipping of the dispersion using Ultra Turrax (see diagram in Fig. 16), the 32

45 Theoretical foundations Formation of the sol supported. By e.g. HCl as a catalyst accelerates the sol / gel formation. The watery consistency of the sol enables it to be applied easily and homogeneously to the textile. The condensation reaction occurs with an increase in temperature and evaporation of the aqueous and alcoholic phases. Fig. 15: Condensation reactions to form dimers, chains and rings [132]. Fig. 16: Scheme of sol / gel formation with TEOS as a precursor. 33

46 Theoretical Basics The drying step is a very critical part of the sol / gel process, because the drying process is determined by the capillary pressure. This leads to a shrinkage of the gel with a volume decrease of the factor 10. A gradient of the capillary pressure within the pores is the reason for this mechanical damage. The capillary negative pressure that develops during drying reaches values ​​between MPa []. A prediction of the extent of the shrinkage is very difficult due to the many factors involved. (1) The pore size changes during drying, (2) the modulus of the gel changes drastically during shrinkage, (3) gels have a particle size distribution, (4) the pore shape is unknown, (5) the pore liquid is in the sol / Gel chemistry is involved and (6) chemical reactions proceed during drying, which can strengthen or weaken the gel network [137]. One way of determining the extent of the shrinkage can be based on the ratio of capillary pressure Pc and the modulus of the solid matrix: P c 2 cos / r and Pc 2 (SV SL) / rh (LV) These are (LV), ( SV), (SL) about the surface tensions between the liquid and the gas, the solid and the gas and the solid and the liquid in the pore. The negative sign indicates that the liquid is under tension. Where is the contact angle of the liquid and rh is the pore radius: r h 2VP / S P VP and SP stand for the pore volume and the surface [, 138]. There are three phases in the sol / gel process: liquid (L), solid (S) and gas (G). If, when the liquid evaporates from the pores between the liquid and the network, a contact angle of <90 occurs, concave menisci form on the outer surface, causing the liquid to come under tension and compress the gel network. If (SG)> (SL) the liquid penetrates through capillary forces from the pores, thereby the solid / gas surface is displaced from the solid / liquid surface. A concave meniscus is formed. At a contact angle = 0 or 0, the liquid in the capillary spaces is stressed, so that the solid body network is compressed. If (SL)> (SG), a convex meniscus is formed. Is the contact angle = or the gel is not compressed. 34 h

47 Theoretical principles The small pore size and the enormous capillary forces lead to cracks in the gel network during drying. In order to control the shrinkage or to reduce the inhomogeneity and the formation of cracks, the drying process is slow. In addition, the slow drying leads to a minimized bulk density, which results in an increase in pore size and volume [136, 138]. Fig. 17: Formation of the menisci during the drying of a sol / gel. (LV): surface tension (liquid / gas); (SG): surface tension (solid / gas); (SL): surface tension (solid / liquid); : Wetting angle [136]. 35


49 Results and discussion 5 Phosphorus-based systems 5.1 Development of a suitable phosphorus-based system Phosphorylation according to Yurkshtovich et al. [139] At the beginning of the work, the phosphorylation according to Yurkshtovich et al. [139] applied. A phosphoric acid / urea mixture is used to generate a flame-retardant effect on cellulose. Comparable experiments quickly showed that various parameters are important for successful phosphorylation. The drying or condensation temperature turned out to be a very important point. Initial tests showed that the postulated drying temperature of 80 C for 30 minutes and the subsequent treatment at 145 C for 1 hour cannot be used for the selected system in the long term. Rather, a significant reduction in the dwell time had to be achieved in order not to damage the cellulose too much and to achieve process engineering advantages. With regard to the condensation temperatures, it was found that with increasing temperature, more phosphoric acid could be covalently bound to the cellulose, since the LOI values ​​decreased less sharply after washing out than at lower temperatures. Furthermore, it must be taken into account that the cellulose can break down if the temperature is too high or under strongly acidic conditions (here ph ~ 1.3). This happens even with treatment at 160 C over a longer period of time. A condensation temperature of 150 C (see Fig. 18) and an average dwell time of 3-4 minutes were therefore found to be sufficient for the further tests. In these tests, the sol / gel finish was already used as a second modification. During the temperature screening, the finishing was always carried out with the same concentration (1.0% by weight) in order to exclude any influence from the sol / gel. In the first tests, a cotton fabric with a basis weight of 187 g / m 2 was used. At a later point in time, they switched to a heavier material. This must be taken into account for later comparisons, as the flame-retardant effect is influenced by the weave density of the substrate. In the case of lighter fabrics, the stitches are 37

50 is usually larger, so more air can be trapped. Accordingly, the burning of lighter tissue is usually faster. Fig.18: Dependence of the LOI value on the condensation temperature. The permanence of the phosphorylation and the sol / gel finish was tested using different washing methods (washing using a Labomat or jet). In general, there was a significant decrease in the flame-retardant effect after washing the samples. As already mentioned, a trend towards higher LOI values ​​in relation to the increased condensation temperature could also be determined after the washes. After five washes, however, the LOI sank to 20. This means that this system did not have sufficient permanence over several wash cycles. Further washes in this work were carried out with a LAB Jumbo Jet. This made it possible to set a defined volume of wash liquor and a temperature program. 38