Research on modification of polylactic acid multifunctional composite materials for 3d printing

Polylactic acid is an important 3D printing material, but its further development and application in the field of 3D printing are limited due to its flammability, poor toughness, and lack of high temperature resistance. This article modifies polylactic acid wire for 3D printing by adding flame retardants aluminum hypophosphate, polycarbonate, and chain extender to prepare multifunctional composite flame retardant materials. It was found that with the increase of aluminum hypophosphate addition, the flame retardancy of polylactic acid composite materials gradually improved, while the tensile strength, impact strength, and hardness showed a decreasing trend, and the flowability decreased, but the Vicat softening temperature increased. Through 3D printing testing, when the amount of aluminum hypophosphate added is 15-20 parts, the printing quality is good and the comprehensive performance is the best, which can meet the basic usage requirements.


Introduction
The intelligent manufacturing model deeply integrates information technology and manufacturing technology, bringing revolutionary changes to the modern manufacturing industry [1] .3D printing additive manufacturing is an important representative of strategic emerging technologies.The application level of the additive manufacturing market in China is constantly deepening, and it has been increasingly widely used in fields such as aerospace, automotive, shipbuilding, nuclear industry, etc [2] .With the increasing awareness of environmental protection among people, the market demand for biodegradable and environmentally friendly plastics has increased.Poly (lactic acid) has become one of the most commonly used printing materials in FDM additive manufacturing due to its excellent processability and biodegradability.However, due to its flammability and brittleness, its application in various fields is limited [3] .
In recent years, there have been many studies on the toughening, strengthening, flame retardancy, crystallization and other modifications of 3D printed polylactic acid materials.For example, Xie et al. [4] used short cut carbon fibers to fill PLA/PBAT to prepare 3D printed wires.Through research, it was found that the thermal conductivity, crystallinity, and dynamic mechanical properties of the wires were improved.Xue et al. [5] achieved V-0 flame retardancy of 3D printed polylactic acid by adding 2% ammonium polyphosphate and 0.12% resorcinol bis (diphenyl phosphate).However, it has also been found that the improvement in a certain property (such as flame retardancy) usually leads to a decrease in other properties (such as mechanical properties).Therefore, it is of great significance to develop highly efficient flame retardant and multifunctional polylactic acid composite materials for 3D printing additive manufacturing.This article studies the preparation of multifunctional polylactic acid composite flame retardant materials suitable for 3D printing by adding aluminum hypophosphate, polycarbonate, and chain extenders.

Sample preparation
Weigh the raw and auxiliary materials PLA, PC, AHP, and ADR according to their respective proportions, and place them in a plastic high-speed mixer for uniform mixing.The mixing time is 8 minutes and the temperature is 50℃.Next, a twin screw extruder is used for melt blending extrusion, water cooling, and granulation.The screw speed control range is 30 r/min, and the temperature control ranges of the extruder are 180℃, 190℃, 200℃, 210℃, and 220℃, respectively.The nozzle head temperature is 215℃.Then place the extruded pellets in a hopper dryer at 90°C and fully dry for 4 hours.Finally, an injection molding machine was used for standard spline injection, with the barrel temperature set at 190℃, 200℃, 210℃, 220℃, 230℃, the injection mold pressure set at 100 mpa, and the molding mold temperature set at 75℃.

Performance testing and characterization
Oxygen Index (LOI): According to the relevant specifications in GB/T 2406.2-2009 [6], the size of the spline is 125 mm × 10 mm × 4 mm.
Horizontal and vertical combustion: The test standard is in accordance with the relevant specifications of GB/T 2048-2000.The sample size is 125 mm × 13 mm × 3 mm, and the ignition time is 30 seconds.
Tensile strength: According to GB/T 1040.2-2022, the sample size is 125 mm × 10 mm × 4 mm, and the stretching speed is 5 mm/min.
Melt flow rate: According to GB/T 3682.1-2018, the added load is 2.16 kg and the test temperature is 190℃.

Effect of AHP on the Flame Retardancy of PLA Wire for 3D Printing
Table 1 shows the combustion test results of PLA wires for 3D printing with the addition of AHP.
Figure 1 shows the effect of AHP addition on the oxygen index and combustion rate of PLA wires.
From the chart, it can be seen that when the amount of AHP added is less than 5 parts, the flame retardant effect is not achieved due to the low carbonization rate of AHP and PC.As the amount of AHP added increases, the flame retardant effect gradually improves.When the amount of AHP added is 20 phr, the LOI value is 29.8, with dripping, and UL94 is V-2 level.When the addition amount is 25 parts, the LOI value is 30.5, with no dripping, which is V-1 level; when the addition amount is 30 parts, the LOI value is 32, with no dripping, which is V-0 level, and the flame retardant effect is good.This is because AHP decomposes phosphorus oxygen radicals under high temperature conditions, which combine with the combustible gas of PLA to generate phosphoric acid non-combustible substances to prevent further combustion of the polymer.At the same time, AHP absorbs heat and decomposes into partially focused phosphate glass-like substances, promoting the dehydration of plastics into carbon.The glassy phosphate substances generated by chemical reactions cover the surface of the substrate, isolating combustible gases and oxygen, while preventing heat exchange [7][8] , thus achieving good flame retardancy for composite materials.
Effect of AHP addition on the combustion performance of PLA standard samples.

The Effect of AHP on the Tensile Properties of 3D Printed PLA Wire
Figure 2 shows the effect of AHP addition on the tensile properties of PLA wire.From Figure 2, it can be seen that the tensile strength and elongation at the break of PLA-modified materials decrease with the increase of AHP addition.However, when the addition amount is 20 parts, the tensile strength is still 17.0 MPa, and the elongation at break is 6.62%.Afterwards, the tensile performance decreases rapidly.The reason is that as the amount of AHP increases, more and more agglomeration phenomena gradually appear.AHP with larger agglomeration size leads to poor compatibility, leading to an increase in internal defects in PLA, resulting in poor tensile performance [9] .Whether it meets the requirements of 3D printing should be comprehensively evaluated based on factors such as printing speed and accuracy.

The Effect of AHP on the Impact Strength and Hardness of 3D Printed PLA Wire
Figure 3 shows the effect of AHP addition on the impact performance and hardness of PLA wire.
From the figure, it can be seen that the addition of AHP will reduce the impact strength and hardness of the material.When adding 15 portions of AHP, the impact strength of the PLA wire sample still reached 3.74 KJ/m 2 , but thereafter decreased rapidly with the increase of AHP dosage.The hardness decreases with the increase of AHP, but the data decrease is relatively limited, even at 30 parts added, there is still 24.4 (HRD).The reason for the decrease in impact strength and hardness is the increase in the amount of flame retardant AHP added, which hinders the crystallization of PLA and causes a decrease in crystallinity.At the same time, the aggregation of AHP in large amounts leads to an increase in internal defects in PLA, which also affects crystallinity [10] .But when the additional amount of AHP is 15 parts, its performance still meets the quality requirements of 3D printing.

Effect of AHP on Vicat Softening Temperature and Melt Flowability of 3D Printed PLA Wire
Figure 4 shows the effect of AHP addition on the Vicat softening temperature and melt flow rate of PLA standard samples.From the graph, it can be seen that as the amount of AHP increases, the Vicat softening temperature of PLA gradually increases, while the melt flow rate shows a downward trend.The reason is that the addition of inorganic flame retardant AHP increases the rigidity of the material, resulting in an increase in the Vicat softening temperature.Partial insertion of AHP into molecular chains leads to an increase in the spacing between PLA molecular chains, and at the same time, AHP also increases friction with the molecular chains, affecting the movement of PLA molecular chains, resulting in poor fluidity, increased viscosity, and reduced melt flow rate [11] .

Quality analysis of 3D printed PLA composite material samples
To test the printing quality of modified PLA composite flame retardant materials, the DP-201 desktop printer from Haoyuyuan (Beijing) Technology Co., Ltd. was used for product printing.Figure 5 shows a product printed using modified materials.Through observation of the printing process and printing quality, it was found that there were no wires such as plugs or interruptions during the printing process, and the entire printing process was smooth.The surface of the printed product is relatively smooth, without cracking, wire drawing, and other phenomena.The water ripple is small, and its surface roughness and accuracy meet general usage requirements.

Conclusions
(1) When adding AHP to prepare 3D printed PLA composite flame retardant materials, the increase of AHP can have a significant flame retardant effect.When the amount of AHP added is 20 parts, the LOI is 29.8, which meets the flame retardant standard.When the addition amount is 30 parts, the LOI is 32, with no dripping, which is V-0 grade.However, the flame retardant effect cannot be examined in one direction, and needs to be combined with the 3D printing quality of the material.(2) As the amount of flame retardant AHP increases, the mechanical properties such as tensile properties, impact properties, and hardness of the material all deteriorate.But when adding 15 portions of AHP, the tensile strength is 18 MPa, the elongation at break is 7.11%, the impact strength is 3.74 MPa, and the hardness is 24.8 (HRD), which basically meets the usage requirements of 3D printing materials.
(3) An increase in the amount of AHP will cause a decrease in the melt flow rate, which directly affects the increase in viscosity and the decrease in melt fluidity, affecting the smoothness of the 3D printing process.However, the addition of AHP at around 15 parts can still ensure processability.Although adding 30 portions of AHP can achieve a Vicat hot deformation softening temperature of 70℃, which is 9.2℃ higher than the initial temperature, it will result in significant loss of mechanical, flow, and other characteristics.
(4) Through performance testing of composite materials and multiple evaluations of 3D printing quality, it was found that when 15-20 parts of AHP were added, the comprehensive performance of the material was relatively excellent, and the 3D printing quality met the basic requirements, providing a certain reference for practical application.

Figure 3 .
Figure 3.The Effect of AHP Addition on the Impact Properties and Hardness of PLA Standard Samples.

Figure 4 .
Figure 4.The Effect of Composite Flame Retardant Addition on the Vicat Softening Temperature and Melt Flowability of PLA Standard Samples.

Table 1 .
Combustion test results of PLA composite flame retardant wire special material standard sample.