With the increasing concern about the global ecological environment, bio-based materials have received widespread attention [[1], [2], [3]]. Polylactic acid (PLA) is a relatively mature variety derived from renewable plant resources, such as corn, cassava, sugar beets, or other waste crops as raw materials. In the past few decades, PLA has been increasingly applied in the biomedicine, textile，and packaging industries [4] due to its distinct advantages, including biocompatibility, easy processing, and low cost [5]. However, the disadvantages of high flammability, low crystallinity, and long recycling period limit its further application, especially in the high-end areas, such as automotive, electronics, and construction industries [6]. Hence, it has great theoretical significance and application value in improving the comprehensive performances of PLA in flame retardancy, degradability, and mechanical strength.

Currently, many strategies, including copolymerization [7], melt blending [8,9], composite spinning [10], and surface modification strategies [11,12], have been fully investigated for fire resistant polyester synthesis, among which the halogen and phosphorus flame retardants (FRs) are commonly used to overcome the high flammability of PLA [13], but these FRs still have some shortcomings. Although halogen FRs have the advantages of high flame retardant efficiency and low cost, a lot of toxic gases and smoke are generated during combustion, which causes serious harm to people's health [14]. Therefore, the application of halogen FRs is gradually limited or prohibited worldwide. Phosphorous FRs are mostly applied in many industries, especially for textiles and electronics because of their excellent flame retardancy [15,16]. However, organophosphorus flame retardants are carcinogenic and biotoxic, and they can be released into the environment and pose potential risks to human health [[17], [18], [19], [20]]. To cope with the increasingly severe environmental problems of traditional FRs, the development of eco-friendly and high-efficiency FRs is quite urgent.

It is well known that biological-origin FRs have the advantages of being renewable and degradable [21]. It can also help to maintain the inherited excellent biomass characteristics of PLA. Traditionally, intumescent flame retardant (IFR) composed of carbon, acid, and gas sources, are mostly employed with good flame-resistant capability. A variety of biological origin materials can be used as carbon sources due to their multi‑carbon structure, such as starch [22], lignin [23], chitosan [24], and sodium alginate [25]. The combination of phytic acid, which is rich in phosphorus and carbon source is a normal effective way to improve flame retardant efficiency [26]. In addition, amino acids [27], proteins [28], and pyrimidines [29] can be used as bio-based gas sources to prepare bio-based IFR, among which arginine (Arg) is one of the ideal amino acids derived from any protein-containing food [30]. Kong et al. [31] added PaArg and DK2 to PBS, which improved the flame retardant and mechanical properties of PBS at the same time. However, it does not take into account the biodegradability of PBS composite. Yang et al. [32] synthesized a type of bio-based flame retardant nanosheet with a microporous structure composed of phosphazene and phytic acid (PA), which made PLA reach a V-0 level and an LOI of 24.2 % with 5 wt% addition. However, when the addition amount increased to 10 wt%, the mechanical properties were reduced due to interface defects and stress concentration. Xu et al. [33] reported the synthesis of a novel multifunctional bio-derived flame retardant composed of chitosan and methyl phosphonic acid, which could be used in PLA to fabricate multi-functional materials with excellent flame retardancy, crystallinity, and mechanical properties. Herein, chitosan is a bio-based flame retardant for its excellent charring ability and synergism with phosphorus flame retardants [34].

It is suggested that an ideal flame retardant bio-material should possess a long service life and good mechanical properties. Moreover, it should also have a high degradation efficiency after service to reduce the negative impact on the environment. Studies reported that it might take five to six years for PLA with high molecular weight to be fully absorbed and degraded by the human body [35]. In addition, the degradation rate of PLA at room temperature is pretty low because its hydrolysis mainly occurs above the glass transition temperature (Tg ≈ 55 °C) [36]. Therefore, PLA requires hydrolysis before other degradation treatments [[37], [38], [39], [40], [41]].

To tackle this dilemma, intensive research efforts toward the enhanced degradation rate of PLA were put forward by blending new types of filler. Lu et al. [42] prepared a PLA/wood flour/polymethyl methacrylate composite material and conducted hydrolysis tests on it. The results showed that the hydrolysis rate was 8.56 times faster than that of PLA under the same conditions. Tao et al. [43] blended cultured diatom particles (DFs) with PLA and found that the degradation time was reduced from 24 months to 3 months or even less, which was reduced by >8 times compared with pure PLA. In addition, the temperature of hydrolysis is also an important factor in degradation. Qiu et al. [44] prepared a new bio-based flame retardant LC-PA/TA from phytic acid (PA), L-citrulline (LC), and tannic acid (TA) and filled LC-PA/TA into PLA. The presence of LC-PA/TA accelerated the degradation rate of PLA in soil, which was of great significance for biodegradable materials. Chen et al. [45] used bio-derived core-shell flame retardant with tannins microspheres and phytic acid to achieve fire-retarding polylactide with enhanced degradation and UV protection. The above literature suggested the potential of bio-based filler in PLA degradation promotion. However, the degradation of PLA is still a challenge in the condition that no other characteristics are sacrificed.

In this work, a new type of biomass IFR system, i.e., APS, synthesized in an aqueous phase from PA, Arg, and starch was reported to prepare fully bio-based PLA composite with improved comprehensive properties, including flame retardancy, mechanical strength as well as degradation performance. Only 3.0 wt% APS made PLA composites a LOI of 24 % and passed the UL-94 grading of V-0 level. At the same time, APS could significantly promote the degradation rate of PLA. This work provides a new strategy for designing multifunctional additives for degradable bioplastic.