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Tailored Metal-Organic Framework: From Hybrid to Multifunctional Flame Retardant Polymer Nanocomposites

Periodic Reporting for period 1 - MOFMAP (Tailored Metal-Organic Framework: From Hybrid to Multifunctional Flame Retardant Polymer Nanocomposites)

Reporting period: 2017-01-09 to 2019-01-08

In comparison to the traditional materials, such as metal, stone, wood, etc, synthetic polymeric materials are playing more and more important roles in social daily life due to its advantages like light-weight, corrosion resistance and easily processed. They are widely used in household appliances, cables, wires, electric devices, transportation, construction materials, etc. However, in air condition most polymeric materials are flammable and easy to burn under a relatively high temperature or in the presence of open fire. In many cases, improving flame retardancy of polymer materials is very important to reduce potential fire risks and life and property loss. Strictly speaking, the objective of flame retarding polymers is to increase the resistance of ignition and to reduce the flame spread with minimal degradation of their properties. Flame retardant materials are extremely important additives in a wide variety of materials and industries, with the increasing world market usage. In the past decade, due to the environment concern caused by the use of halogenated flame retardant, much more attention focused on studying new flame retardants for polymers. Incorporation of additives is the most common way to tune the chemical and physical properties of polymers. In particular, additives that improve the flame retardancy and mechanical properties of polymers without increasing health and environmental risks are in high demand. In recent years, phosphorus-containing molecular flame retardants have gained importance while replacing their halogenated predecessors. Nevertheless, the development of fire-safe polymers by incorporation of FR additives still poses a significant challenge. Firstly, high dosages of the FR additives are often required to achieve the desired fire-safety levels, at which the deterioration of thermal stability and mechanical properties of the corresponding composites are commonly observed. Secondly, the migration of FR molecules onto the polymer surface leads to the release of volatile organic compounds, which is concerned as a serious health-threatening issue.
Metal-Organic Frameworks (MOFs) have been prototyped as polymer additives. The resulting composite materials have been tested in a number of applications exploiting the material porosity, e.g. gas separation, nanofiltration, capture of VOC, and have been shown to exhibit better performance than those of their individual components. It has been also recognized that MOFs themselves exhibit some flame retardant properties as the metal centers within their structure can promote char formation during the combustion of the composite. However, the fire resistant effect of MOFs is limited with respect to molecular FRs which can also offer more fire suppression mechanism by quenching the combustion reactions or forming a non-flammable layer on the burning material. Conveniently, due to their intrinsic porosity, such molecular flame retardants can be incorporated into MOFs as functional guest molecules.
In this project, we aim to develop multifunctional intelligently flame retardant polymeric materials which have significantly impact on new generation polymer-based products in varied application fields. Combing the research background of the researcher, we are developing MOF and its functionalized hybrids as a new generation of polymer composites (figure 1) which have been proved by varies characterizations that this type of additive functions with good efficiency in the improvement of fire retardancy and mechanical properties of polymeric composites. At the same time, the working mechanism of each component are investigated from the perspective of molecular structure.
During the carrying out of the project, we compared the synthetic condition and the geometry property of several types of MOFs and an environment friendly structure HKUST-1 was finally assigned as the MOF platform to dose into the polymer matrix (figure 2). Different functionalization ways are implemented into the host MOF, including fulfilling non-combustible gases, organic flame retardant with different organic side groups and oxidation states (figure 3). Through the fire tests (limiting oxygen index (LOI), cone calorimetry) and mechanical tests (tensile test and charpy test), it can be found that MOF functionalized with organic flame retardants increase the LOI, reduce the heat release rate, total heat release and total smoke production compared with the neat polymer or the polymer combined with traditional organic flame retardants. At the same time, the tensile strength also increased to varies extents. By comparing the fire tests results in detail and the single crystal structure of the functionalized MOF, we concluded that the open metal site and porous system in side HKUST-1 play important roles in the fire resistance behaviours of the polymeric composites at different combustion stages. Moreover, the composites overcome the volatile organic compound release and deterioration of thermal stability, lower end-use temperature brought by many traditional organic flame retardants (figure 4). These results shed light on a new perspective of developing fire retardant and mechanical reinforced polymeric composites. Considering the environmental friendliness of this type of additive, this strategy can be widely applied in industrial areas and daily life.
we have presented a strategy to simultaneously improve mechanical and fire safety properties of polymer materials by incorporation of hierarchically functionalized MOF additives. In our investigation, the initial structure and open metal site in MOF have been made use, giving a hierarchical strategy to functionalize the MOF structure to the improve of polymeric composites properties. With the incorporation of the resulting additive, polymer composites exhibit a remarkably improved processibility without VOC release observed. This hierarchical strategy and the selection of components allowed that not only the flame retardant mechanism of the composite is regulated, but also the affinity of the additive to the polymer matrix is strengthened, resulted in more effective flame retardancy and mechanical reinforcement of the composite. The presented hierarchical strategy for developing high performance composite materials by incorporating MOF particles loaded with property enhancing molecules can be adopted to other applications by selection of polymer-compatible MOFs as well as property barring MOF-guest composite.
Furthermore, as our investigation went more deeply, by loading four types of phosphorous flame retardant into the pore system of MOF, four types polymer additives have been prepared and embedded into the life indispensable unsaturated polyester, getting a series of polymer composites 1·PFRx/UP. Systematic investigation of their fire behaviors and mechanical properties prove that the lone-pair electrons on phosphorous atom play an important role in flame retardancy and mechanical reinforcement. At the same time, their effectivity is also influenced by the chemical environment of the phosphorous atom. The preparation of polymer additive with loading PFR in MOF not only presents a liable platform to shed light on the function of high reactive PFR in polymer, but also extends the application area of these compounds by avoiding the unwanted reactions during the process procedure.
illustration of different interactions between MOF and flame retardant guest
Illustration of the systematic investigation of efficiency of phosphorous flame retardant
Illustration of composite materials with enhanced performances
Preparing polymeric composite with hierarchically functionalized MOF