With the intensification of environmental pollution and the shortage of petroleum-based raw materials, renewable resources have attracted people's active attention. PLA is a degradable thermoplastic polymer obtained from renewable resources such as corn starch. It has the advantages of low toxicity, environmental friendliness, and excellent mechanical properties. It is expected to replace non-degradable traditional petroleum-based raw materials and is widely used in medical treatment and textiles. And food packaging and other fields. However, PLA is flammable and accompanied by severe dripping, which limits its application in electronic appliances, automobiles and other fields that require high flame retardant properties. Therefore, the development of flame-retardant PLA composite materials has become an inevitable choice. In recent years, intumescent flame retardants (IFR) have been widely used in flame-retardant PLA due to their halogen-free, high-efficiency, and low-toxicity characteristics. IFR is generally composed of three parts: carbon source, acid source and gas source. It mainly achieves high efficiency flame retardancy by protecting the polymer matrix and reducing smoke emission. However, the traditional carbon source pentaerythritol (PER) has weak char-forming ability and is not renewable. Researchers have found that cyclodextrin can improve the flame retardancy of PLA while maintaining its environmental protection, and can be used as a green carbon source to replace PER.
Cyclodextrin (CD) is a cyclic oligosaccharide formed by the action of amylase. It contains a large number of hydroxyl structures. The carbon formation process includes ring opening, followed by chemical evolution similar to cellulose, losing the glucose structure and hydroxyl groups. The formation of carbonyl, aromatic and other structures. Common CDs are mainly divided into three categories: α-CD, β-CD, and γ-CD. Among them, β-CD is widely used in flame-retardant PLA, polypropylene (PP) and other polymers because of its excellent char-forming properties, thermal stability and low cost. In the first stage, physical dehydration occurs at about 40 ℃ to remove the crystal water in β-CD; in the second stage, thermal decomposition and carbonization reactions begin to occur at 260 ℃, generating carbon dioxide gas and carbon residue; in the third stage, when the temperature reaches 400 ℃ At this time, the carbon residue undergoes slow thermal degradation. In addition to the polyhydroxy structure that can be used for carbonization, β-CD also contains more active primary and secondary hydroxyl groups, which can be modified by esterification, cross-linking and chemical modification to improve its flame retardant properties.
The cyclodextrin derivative β-CD has a wide range of applications in medical, food, and environmental fields due to its more active hydroxyl groups and a special ring-shaped cavity structure. However, when it is used as a flame retardant, its application in the field of flame retardant PLA is still in its infancy due to its large amount of addition and poor compatibility with the matrix.