Nitrogen/Phosphorus/Sulfur-Containing Monomer-Functionalized Magnesium-Aluminum Hydrotalcite And Its Synergistic Flame Retardancy In Polypropylene

Polypropylene（PP）,as a general-purpose thermoplastic synthetic resin,has the characteristics of low density,chemical corrosion resistance,good electrical insulation and excellent mechanical properties.It has been widely used in automobiles,building materials,medical equipment and food processing.However,PP is highly flammable,and produces a large amount of smoke and toxic gases during the combustion process.It has a very high fire hazard and brings a huge threat to the safety of people’s lives and property.Adding flame retardants to PP matrix is considered to be one of the most efficient ways to improve its flame retardancy.Among them,layered double hydroxide（LDH）,as a halogen-free flame retardant,has the characteristics of environmental friendliness,non-toxicity,smoke suppression,etc.,and is often added to PP matrix to enhance its flame retardant properties.However,the compatibility of LDH with the PP matrix is poor,and the hydrogen bonding between the particles easily leads to agglomeration in the PP matrix,resulting in serious damage to the mechanical properties of PP.Therefore,in order to develop hydrotalcite-based flame retardants with good compatibility with PP and excellent flame retardancy,three new hydrotalcite-based flame retardants were designed and synthesized in this paper to improve the flame retardant and smoke suppression of PP,and in-depth exploration of the structure-activity relationship between the char layer structure and flame retardancy.The main research work is as follows:（1）In order to improve the interfacial interaction between LDH and PP,a triazine-based organic covalent framework/LDH hybrid flame retardant（LDH@TOF）was prepared by in-situ polymerization between melamine and cyanuric chloride using LDH as a template.Compared with pure LDH,the hydrophobic properties of LDH@TOF are enhanced and the compatibility between LDH@TOF and PP matrix is also improved.With the addition of 20 wt%LDH@TOF,the flame retardancy of the PP/LDH@TOF composite was enhanced,and its LOI value reached 29.2%and passed the UL-94 V0 rating.Compared with pure PP,the peak heat release rate（PHRR）,total heat release（THR）and total smoke generation（TSP）of PP/LDH@TOF composites decreased by 29.8%,30.4%and 26.2%,respectively.Besides,compared with the PP/LDH composite,the tensile strength and elongation at break of the PP/LDH@TOF composite were increased by 1.9 MPa and 18.9%,respectively.The significant improvement of the flame retardancy of PP/LDH@TOF composites is mainly due to the physical barrier effect between LDH and TOF nanosheets.（2）In order to further enhance the compatibility between LDH and PP matrix and the dispersion of LDH in PP matrix,the microencapsulated hydrotalcite was microencapsulated by the in-situ polymerization of carbon-rich biomaterial tannic acid and cyanuric chloride by using LDH as a template,and then a bio-based hydrotalcite-based flame retardant（LDH@TA-CC-Fe）was constructed by utilizing the characteristics of tannin chelating Fe³⁺.Compared with pure LDH,both the hydrophobicity and thermal stability of the microencapsulated LDH@TA-CC-Fe were improved.With the addition of 20 wt%LDH@TA-CC-Fe,the PP/LDH@TA-CC-Fe composite exhibited excellent flame retardancy.Its LOI value reached 30.2%and passed the UL-94 V0 level.Compared with pure PP,the PHRR,THR,and TSP of PP/LDH@TA-CC-Fe composites decreased by 32.9%,50.3%,and 36.5%,respectively.In addition,under the same addition amount,the tensile strength and elongation at break of PP/LDH@TA-CC-Fe composites were 3.6 MPa and 26.6%higher than those of PP/LDH composites,respectively.The result is mainly related to improvement of the hydrophobicity of LDH@TA-CC-Fe.The significant reduction in fire risk of PP/LDH@TA-CC-Fe composites is mainly due to the synergistic effect of the catalytic char formation of the tannic acid-chelated Fe³⁺system and the physical barrier effect of LDH.（3）Taking advantage of the characteristics of bio-based materials,which are renewable,easy to degrade and have excellent compatibility with polymers,a green and simple core-shell structure bio-based flame retardant（LDH-LS@CS@PAMn）was prepared.This method uses water as the assembly medium,and sequentially assembles sodium lignosulfonate,chitosan and manganese phytate on the surface of LDH through electrostatic interaction.Compared with LDH,the morphology of LDH-LS@CS@PAMn is greatly changed,and its thermal stability and hydrophobicity are improved.With the addition of 20 wt%LDH-LS@CS@PAMn,the LOI value of the PP/LDH-LS@CS@PAMn composite reaches 31.6%and passes the UL-94 V0 rating.The PHRR,THR and TSP of PP/LDH-LS@CS@PAMn composites decreased by59.14%,37.62%and 47.49%,respectively,compared with pure PP.In addition,the dispersibility of LDH-LS@CS@PAMn in PP matrix and its compatibility with PP matrix have been greatly improved,which is mainly due to the better compatibility between LDH and PP matrix given by bio-based materials.Compared with PP/LDH,the tensile strength and elongation at break of PP/LDH-LS@CS@PAMn composites are increased by 3.9 MPa and 43.7%.The improvement of the flame retardancy of PP/LDH-LS@CS@PAMn composites is mainly due to the synergistic effect of the catalytic carbonization of various bio-based materials and the physical barrier effect of LDH.Compared with pure PP,the char residue after combustion of the PP/LDH-LS@CS@PAMn composite is denser and more continuous,which can effectively block the heat transfer between the condensed phase and the gas phase.