At present, it is often said that environmentally friendly flame retardants refer to products with low smoke and low toxicity during use. Including brominated flame retardants represented by decabromodiphenylethane, brominated epoxy resin and brominated polystyrene; phosphorus flame retardants represented by red phosphorus and phosphate esters; nitrogen flame retardants represented by melamine and melamine cyanuric acid; inorganic flame retardants represented by aluminum hydroxide and magnesium hydroxide.
Many scholars believe that the difference between green flame retardants and ordinary environmental flame retardants is that green flame retardants refer to green products that are environmentally friendly in raw materials, production processes and use, and recycling. With the development of bio-based materials science, more and more scholars have selected the raw materials for flame retardants in the field of bio-based materials.
The reason why bio-based materials can become raw materials for flame retardants is because many bio-based materials have high carbon content and polyhydroxy structure, resulting in excellent carbon-forming properties, such as lignin (Lig), starch (ST), cellulose, chitosan (CS), cyclodextrins and the like.
Carbon-forming performance is the most important mechanism of action of the intumescent flame retardant, the flame retardant effect of the intumescent flame retardant is mainly based on the formation of a porous foam coke layer on the surface of the material, it is a multiphase system, containing solid and liquid and gaseous products.
The flame retardant properties of the carbon layer are mainly reflected in: making it difficult for heat to penetrate the condensed phase, preventing oxygen from entering the combustion area, and preventing gaseous or liquid products generated by degradation from spilling out of the surface of the material.
The coke carbon layer formation process is: at about 150 ° C, the acid source produces esterified polyols and acids that can be used as dehydrating agents; at a slightly higher temperature, the acid and the carbon source are esterified, and the amine group in the system is used as a catalyst for the esterification reaction, accelerating the reaction; the system is melted before the esterification reaction and the esterification process, and the non-combustible gas produced during the reaction makes the system that has been in a molten state expand and foam, at the same time, the polyols and esters are dehydrated and carbonized, forming inorganic substances and carbon residues, and the system is further foamed When the reaction is nearly complete, the system gels and solidifies, and finally forms a porous layer of foam charcoal.
1 lignin
Lignin is a widely present amorphous aromatic polymer in plants, and the aromatic structure of lignin has a high residual carbon rate after decomposition. It exists in the cell wall of the plant, together with cellulose, hemicellulose to form the basic skeleton of the plant, is a polyhydroxy aromatic compound, to meet the requirements of the carbon source as an intumescent flame retardant.
During the combustion process, lignin breaks old bonds and new bonds form, and its pyrolysis process can be roughly divided into the following three stages.
The first stage is free water volatilization;
The second stage starts at about 120 °C, where the weak valence bond around the benzene ring breaks and the volatile components are recombined;
In the third stage, when the temperature reaches 800 °C, benzene ring cracking, volatilization and polymerization into polynuclear aromatic hydrocarbon compounds occur, and as the temperature further increases, the new aromatic compound undergoes a polycondensation and carbonization process. It shows that lignin has a high carbon formation capacity at high temperatures, and it is almost impossible to become charcoal during ordinary combustion.
However, its structure contains a variety of functional groups, such as methoxy, alcohol hydroxyl, phenolic hydroxyl, benzene, aldehyde, carbonyl, etc., for further chemical modification to provide a wealth of active sites, is conducive to improving its flame retardant properties. Lignin is combined with other flame retardants such as metal hydroxides and phosphorus-based compounds to further improve the flame retardant effect.
2 starches
Starch is formed by polymerization of glucose molecules and stored in cells in the form of starch granules, which is a polyhydroxy substance that can be crosslinked into charcoal when burned. It has the advantages of being degradable, renewable, and low cost, and is considered a promising sustainable material.
Its thermal degradation can be roughly divided into the following 3 stages:
(1) Physical dehydration occurs mainly, when the temperature reaches about 150 °C, the crystalline water in the starch completely disappears;
(2) Thermal decomposition and chemical dehydration of starch occur at about 300 °C, on the one hand, condensation reaction occurs between hydroxyl groups to form ether bonds and dehydration, on the other hand, adjacent hydroxyl groups in the glucose ring will also be chemically dehydrated, forming carbon-carbon double bonds or ring rupture, continuous heating, molecular chain rupture, the formation of a variety of aromatic structures;
(3) Carbonization occurs at 500 °C and forms a large aromatic conjugate ring. In the flame retardant PLA system, it can be used as a carbon source, and when it is burned, it will release carbon dioxide and carbon monoxide, and when it is compounded with the acid source, the acid source can promote the dehydration and carbonization of starch, and the carbon layer formed can inhibit the escape of combustible gases and thermal oxygen exchange.
The effective bio-based flame retardant prepared by using renewable and inexpensive potato starch as a bio-based carbonizer not only promotes the development of green flame retardants, but also greatly reduces the cost of flame retardants, which has a very valuable practical use value.
3 Cellulose
Cellulose is a cell wall component, simply put, cellulose is a straight-chain macromolecule made of glucose molecules connected by β-1,4-glycoside bonds, the molecular formula is (C6H10O5) n (where n is the degree of polymerization), widely distributed in nature, mainly in the cell walls of higher plants and bacteria, algae and fungi.
The thermal degradation process of cellulose can be roughly divided into the following 4 stages:
The first stage occurs under low temperature conditions, removing crystalline water from cellulose.
The second stage occurs chemical dehydration at about 150 °C, generating water and dehydrated cellulose, and the formation of water is conducive to accelerating the hydrolysis of glycoside bonds and promoting the degradation of cellulose; with the increase of temperature;
The third stage starts from 240 °C to produce thermal decomposition and carbonization reactions, generating liquid product tar and carbon-containing intermediate products, while dehydrated cellulose further reacts to generate carbon monoxide, carbon dioxide and water vapor;
In the fourth stage, aromatization and crosslinking of carbon occurs above 400 °C, forming coke slag.
It is worth noting that under high temperature conditions, the reaction tends to produce tar and inhibits coke production. However, rich modification technology is conducive to improving the flame retardant performance of cellulose at high temperatures. The polyhydroxy structure of cellulose makes chemical modification an effective way to improve flame retardant properties, cellulose phosphorylation is the most widely studied modification method, and esterification is the most commonly used and simplest reaction.
4 chitosan
Chitosan (CS) is prepared from chitin deacetyl and has the advantages of renewable and biocompatible. CS is a positively charged natural amino polysaccharide, which can be directly used as a carbonifier for flame retardant PLA composites, and the ring-opening reaction will occur at high temperatures, and the aromatic ring crosslinking structure will be formed by self-agglomeration in the matrix, that is, the carbon layer is generated in the condensed phase, which is conducive to inhibiting heat exchange in the matrix.
At the same time, the amino groups in CS are released into the gas phase in the form of NH3 during thermal decomposition, which on the one hand can dilute the concentration of combustible gases, and on the other hand, promote the formation of an expanded carbon layer, and the expanded carbon layer has a better protective matrix effect than the ordinary carbon layer.
Usually, CS and acid sources (such as APP) are used to form an expanded flame retardant system, and the products generated by the acid sources during thermal decomposition can promote chitosan dehydration and carbonization. In addition, due to the large number of active groups in its structure, it can be modified to optimize its flame retardant properties.
5-ring dextrin
Cyclodextrin (CD) is a cyclic oligosaccharide formed by the action of amylase, containing a large number of hydroxyl structures, and its carbon formation process includes ring opening, followed by a chemical evolution similar to cellulose, losing the glucose structure and hydroxyl groups, forming carbonyl, aromatic and other structures.
Common CDs are mainly divided into 3 categories: α-CD, β-CD, γ-CD. Among them β-CD is widely used in flame retardant PLA, polypropylene (PP) and other polymers because of its excellent carbonization, thermal stability and low cost.
The first stage occurs physical dehydration at about 40 °C, removing crystalline water from β-CD;
The second stage begins thermal decomposition and carbonization at 260 °C to generate carbon dioxide gas and residual carbon;
In the third stage, when the temperature reaches 400 °C, the residual carbon undergoes slow thermal degradation.
In addition to the polyhydroxy structure that can be used for carbonization, β-CD also contains more active primary hydroxyl and secondary hydroxyl groups, which can be modified by esterification, crosslinking and chemical modification to improve their flame retardant properties.
CD has a cavity structure with an outer hydrophilic and endolumen hydrophobic cavity, which enables it to form a clathrate with a variety of molecules, providing more space for its modification. Thermal degradation of CD can form a large amount of carbonaceous residue, which can be used as a carbon source in an expanded flame retardant system or as a embedding agent for phosphorus compounds to improve their low binding effect.
Bio-based flame retardants belong to a new variety, and many bio-based raw materials were originally used in medicine, food and other fields, and there is no special chemical grade product.
Many bio-based materials or chemicals can be properly treated to possess the properties of being used as flame retardants. Although bio-based flame retardants are still in the research and development stage of the laboratory, with the research and development of bio-based raw materials in the field of auxiliaries, bio-based flame retardants raw materials that can be applied to industrial production will be developed in the future.