Polybutylene terephthalate (PBT, 1100-211S) was supplied by Changchun Plastics Co., Ltd. Aluminum diethylhypophosphate (ADP, OP 1230) was purchased from Clariant (Shanghai, China). Melamine pyrophosphate (MPP) was provided by Sichuan Institute of fine Chemical Industry Research & Desigen Co., Ltd. 3-Glycidoxypropyltrimethoxysilane (KH560, ≥97.0%), anhydrous ethanol and acetic acid (≥99.5%) were purchased from aladdin Co., Ltd. All reagents were used without further purification.

The synthesis route of KH560 oligomer was adapted from previous reports with certain modifications (Peng et al., 2023; Zhang et al., 2024). As illustrated in Figure 1, 45.0 g KH560 was dissolved in a mixture solution consisting of 105.0 mL of deionized water and 6.0 mL anhydrous ethanol at ambient temperature. The mixture was stirred at a rate of 500 rpm for 30 min in a beaker. Then, acetic acid was added into the mixture dropwise within 10 min to adjust the PH to 2–4. The solution was stirred continuously for 5 h at 40°C and the transparent KH560 oligomer solution was obtained with a solid content of approximately 28.0 g/mL.

The flame retardants consisted of ADP and MPP in a mass ratio of 2.5:1, which has been identified as the most effective ratio for the flame-retardant PBT. As shown in Figure 1, 200 g of flame retardants (FR) granules were put into a mechanical mixer with a stirring rate of 100 rpm. KH560 oligomer solution was diluted with deionized water to 100.0 mL at varying concentration of 0.08 g/mL, 0.12 g/mL and 0.16 g/mL. Then, the diluted KH560 oligomer solution was sprayed into the mechanical mixer within 10 min and the stirring rate was increased to 500 rpm for 30 min. Subsequently, the mixture of FR and KH560 oligomer solution was transferred to an oven at 120°C to completely evaporate the solvent. Finally, white powders were obtained and named as EP@FR-x, where EP@FR-1, EP@FR-2 and EP@FR-3 corresponded to FR modified with of KH560 oligomer solution at concentration of 0.08 g/mL, 0.12 g/mL and 0.16 g/mL, respectively.

PBT granules, EP@FR-x and other additives were first dried in an oven at 100°C. The dried EP@FR-x was then blended with PBT using a twin-screw extruder (CTE 35, Coperion, Nanjing Machinery Co., ltd, China) with the extrusion temperature at 220, 240, 240, 240, 240, 230, 230, 230, 230, 240°C, respectively. The detailed formulations of the flame-retardant PBT were listed in Table 1. Then the samples for flame-retardant and mechanical performance test were prepared by an injecting molding machine (HTF90W1, Haitian group, China) at temperature ranging from 230 to 250°C.

Detailed information of material characterization is provided in the Supplementary Information.

As shown in Figure 2A, the characteristic peaks of C-O and Si-O were presented in the FTIR spectrums of both EP@FR-2 and KH560 oligomer. The C-O bond was attributed to the epoxy groups from KH560 oligomer and the Si-O structure was also derived from the KH560 oligomers (Feng et al., 2024; Li et al., 2024c). XPS was carried out to investigate the chemical composition of EP@FR-2. As shown in Figure 2B, FR contained C, N, O, P and Al with elemental mass ratio of 60.4 wt%, 5.3 wt%, 23.7 wt%, 7.9 wt% and 2.7 wt%, respectively. In terms of EP@FR-2, compared to the FR, the mass ratio of C, N, P and Al all decreased, while the mass ratio of O increased. Meanwhile, Si appeared with a mass ratio of 2.3 wt%, which was attributed to the KH560 oligomer. As shown in Figure 2D, the XPS Si spectrums revealed that there was only one type of chemical bonding existing for Si. Figure 2C exhibited the thermal stability and char residue of FR and EP@FR. The results indicated that the introduction of KH560 oligomer did not affect the initial decomposition temperature (T_5%) or the max decomposition temperature (T_max) of the FR. However, the char residue was increased from 11.2% to 16.1%, which was attributed to the high char residue of the KH560 oligomer (Yang et al., 2021; Wang et al., 2023; Zhang et al., 2023). Based on the above characterization, it was concluded that KH560 oligomer was successfully coated on the surface of the FR via the condensation reaction between the Si-OH and the -OH on the surface of the FR as well as physical adhesion of the silicon couple agent itself.

The flame retardancy of the PBT composites was evaluated by the LOI and UL-94 vertical burning test. As shown in Table 2, the LOI value of the control PBT was 22.0%. After the addition of FR, the LOI value was increased to 28.3%, while the introduction of KH560 oligomer did not apparently affect the LOI value. The LOI value of PBT/EP@FR-2 was slightly increased to 28.5%. In the UL-94 vertical burning test, the control PBT completely burned out in the first ignition and did not achieve any UL-94 rating. After the introduction of FR, PBT/FR composites self-extinguished immediately after the first ignition and within 13.0 s after the second ignition, achieving a UL-94 V-1 rating. After being coated with KH560 oligomer, all the PBT/EP@FR composites self-extinguished within 10 s, achieving a UL-94 V-0 rating. There was an optimal core-shell ratio, where the solid mass ratio of KH560 oligomer to FR was 3:50. At this ratio, the self-extinguished time of PBT/EP@FR-2 in the first and second ignition was 0.0 s and 1.3 s respectively, which was the shortest. Therefore, PBT/EP@FR-2 was selected for further testing.

The cone calorimetry test (CCT) was used to investigate the heat and somke release behavior of the flame-retardant PBT composites when exposed to consistant heat flux. The key data of the CCT was listed in Table 3, including time to ignition (TTI), peak heat release rate (PHHR), total heat release (THR), total smoke prodcution (TSP), average CO and CO₂ yeild and char residue. As shown in Figures 3A, B. It was found that PBT was very flammable with PHRR and THR of 1772.9 kW/m² and 92.3 MJ/m², respectively. The addition of FR slowed down the combustion process, reducing the PHRR and THR of PBT/FR composites to 457.5 kW/m² and 77.1 MJ/m², respectively. The introduction of the KH560 oligomer on the surface of FR further decreased the PHRR and THR of PBT. The PHRR and THR of PBT/EP@EP-2 were decreased to 401.6 kW/m² and 75.7 MJ/m², respectively.

As for the smoke production shown in Figure 3C, the presence of P/N elements in the FR increased the TSP of the FR-contained PBT composites. Compared to the control PBT, the Av-COY of the FR-contained PBT was increased, while the Av-CO₂Y was decreased. This suggested that the addition of the FR resulted in the incomplete combustion of the substrate, which accounted for the increased TSP (Li et al., 2024a; Tang et al., 2023b).

It was found that KH560 oligomer was able to further improve the flame-retardant effeciency of the FR and the contribution of the KH560 was furthre investigated. As it shown in Figures 3D–F, the char residue of PBT/FR composites was porous and consisted of C (25.5 wt%), O (34.4 wt%), Al (16.6 wt%) and P (23.5 wt%) elements. While that of the PBT/EP@FR-2 composites shown in Figures 3G–I was dense with few voids observed. The proportion of Si element in the char residue was 4.3 wt%, which indicated that Si participanted in the char forming and enhanced the density of the char residue. Therefore, the transfer of the oxygen and heat was slowed down (Feng et al., 2022; Li et al., 2022; Li et al., 2024b).

The thermal stability of the flame-retardant PBT composites was presented in Figure 4; Table 4. The T_5% and T_max of the control PBT was 389.3°C and 418.8°C, respectively, with a char residue of 5.4%. Due to the catalytic effects of the phosphorus flame retardants, the T_5% and T_max of the FR-contained PBT composites were both lower and the char residue increased. The introduction of the KH560 oligomer did not apparently affect the thermal decomposition behavior of the PBT. The T_5%, T_max and char residue did not show any change, compared to the PBT/FR composites. However, notable changes were observed in the maximum decomposition rate (DTGmax). The DTG_max of PBT composites was 2.6%/^oC, while that of the PBT/FR composites was 2.4%/^oC. With the introduction of KH560 oligomer, the DTG_max further decreased to 2.3%/^oC, indicating that FR was able to reduce the thermal decomposition rate of PBT, and the KH560 oligomer further slowed down the decomposition of the substrate (Jin et al., 2017b; Tang et al., 2022).

The mechanical properties of the flame-retardant PBT were comprehensively evaluated and the results were shown in Figure 5. As shown in Figure 5A, The tensile and bending strength of the control PBT were 50.0 MPa and 76.0 MPa, respectively. It was noticed that the addition of both the FR and EP@FR slightly reduced the tensile strength while maintaining the bending strength. Specifically, the tensile and bending strength of PBT/EP@FR-2 were 46.8 MPa and 76.4 MPa, respectively. As shown in Figure 5B, the elongation at break and notched impact strength of PBT were 143.6% and 5.1 kJ/m² respectively. The addition of FR reduced the toughness of PBT with both of the elongation at break and notched impact strength decreasing. Compared with PBT/FR composites, the elongation at break of PBT/EP@FR was decreased but the notched impact strength was increased. This phenomenon was attributed to the reaction between epoxy groups and -OH and -COOH groups during reactive extrusion. As a result, the compatibility of EP@FR with PBT was better than that of FR, leading to fewer interface defects and gaps between EP@FR and PBT substrate, which resulted in a higher notched impact strength. SEM images of the fracture surface further proved it. As shown in Figure 5C, on the fracture surface of PBT/FR, FR was exposed on the outside of the substrate, while that of the PBT/EP@FR was different. As shown in Figure 5D, EP@FR was embedded within the substrate, demonstrating good compatibility with PBT. However, due to the formation of covalent bonding at the interface of PBT and EP@FR, the mobility of the molecular chains was restricted, which led to a decreased in the elongation at break.

PBT/FR and PBT/EP@FR-2 were soaked in water at 70°C for 14 days to investigate their water resistance. Figure 6A illustrated the appearance of the PBT and PBT/EP@FR composites before and after soaking in water. It was observed that the water was clear prior to soaking. However, after soaking, white precipitate appeared in water of the PBT/FR composites, which was attributed to the FR leaching out of the PBT matrix. In contrast, the water of the PBT/EP@FR composites remained clear, indicating better water resistance. The white precipitate was filtered out and dried in an oven. The FTIR spectrums of the white precipitate was shown in Figure 6B. It was found that the spectrums of the white precipitate were similar to that of FR, indicating that both ADP and MPP separated out from the PBT matrix.

The flame retardancy of the PBT and PBT/EP@FR-2 composites after soaked was evaluated by LOI and UL-94 vertical burning test. As shown in Table 5, after soaked in water, the flame retardancy of the PBT/FR composites decreased. The LOI value decreased from 28.3% to 26.2% and the UL-94 rating shifted from V-1 to V-2. However, the PBT/EP@FR composites maintained its flame retardancy. The LOI value slightly decreased from 28.5% to 27.9% and the UL-94 vertical burning rating remained at V-0.

The mechanical properties of the flame-retardant PBT composites after soaked was also characterized and the results were shown in Table 6. After soaking, the mechanical properties of PBT/FR and PBT/EP@FR-2 composites both decreased. The tensile strength, elongation at break and notched impact strength of PBT/FR composites decreased by 13.0%, 21.4% and 16.7%, respectively. These of PBT/EP@FR-2 decreased by 12.8%, 0% and 6.0%, respectively. Meanwhile, the tensile strength and notched impact strength of PBT/EP@FR-2 were both higher than those of PBT/FR after soaking.

Totally, both the flame retardancy and mechanical properties of PBT/EP@FR-2 were superior to those of the PBT/FR composites after soaking. Under humid and heat conditions, the hydrophilic nature of the FR caused it to migrate to the surface of the substrate or even separate from the substrate inducing by water. This led to mass loss and uneven distribution of the FR, which in turn resulted in a deterioration of mechanical properties and reduced flame retardancy. The loss of FR further promoted the deterioration of the flame retardancy. However, the introduction of the KH560 oligomer alleviated this issue, helping to maintain both the mechanical properties and flame retardancy of the composites.

It was hypothesized that during reactive extruding, under heat and shear, a reaction took place between the EP@FR and PBT. The epoxy groups on the surface of EP@FR reacted with the terminal -OH and -COOH of PBT, forming a C-O-C structure at the interface between PBT and FR (Figure 7) (Fu et al., 2017; Jubinville et al., 2019; Jiang et al., 2020). In this manner, the FR was covalently bonded to the substrate. As a result, the FR was effectively fixed within the PBT matrix, making it difficult for water to induce its separation. Consequently, the FR remained stably distributed in the substrate.

The torque-time curve was used to characterize the viscosity change during the melting blend to reflect the reactive extrusion and the results was shown in Figure 8A. The torques of PBT and PBT/FR showed a trend of increasing, then decreasing, and finally stabilizing, corresponding to the melting and mixing process, respectively. The max torque of PBT was 15.9 Nm at around 30 s. The addition of FR increased the max torque to 19.9 Nm at 48 s, which was attributed to the friction between PBT molecular chains and FR. It was worthy noticed that EP@FR-2 altered the viscosity during processing. Two peaks showed up in the torque curve of the PBT/EP@FR-2: the first peak, similar to PBT/FR, corresponding to the melting process. However, instead of gradually decreasing, the torque increased again, reaching a max value of 23.2 Nm at 66 s. This phenomenon indicated that the reaction between epoxy groups and terminal -OH and -COOH of PBT occurred at this moment. After most of the reactive epoxy groups were consumed, the torque started to decease and eventually equalized (Himmelsbach et al., 2023).

To further investigate this reactive extruding process, the terminal carboxyl values of the flame-retardant PBT was carried out and the results were shown in Figure 8B. For both the raw PBT materials and PBT after extrusion, the terminal -COOH value was around 20.00 mol/t. After the addition of the FR, the acid catalysis effect of the FR broke the ester bonds of PBT, leading to an increase in terminal -COOH groups. As a results, the terminal -COOH of PBT/FR increased to 32.33 mol/t. In contrast, the terminal -COOH value of PBT/EP@FR-2 decreased with the introduction of reactive epoxy groups, which was attributed to the consumption of terminal -COOH reacted with epoxy groups.