High-strength lightweight plastic cover plate material and preparation method thereof

Abstract

The invention relates to the technical field of resin, in particular to a high-strength lightweight plastic cover plate material and a preparation method thereof. According to the method, through the synergistic effect of segmented chain extension reaction extrusion and interface activated nano calcium carbonate, the problems that an existing degradable polyester thin-wall cover plate is narrow in injection molding flow window, fragile in welding line, large in warping and poor in falling resistance are solved. The preparation method comprises the following specific steps: drying polylactic acid and flexible degradable copolyester; carrying out interfacial activation on the nano calcium carbonate by adopting an epoxy group-containing silane coupling agent; in a twin-screw extruder, polylactic acid and a first-section multifunctional epoxy chain extender are subjected to melt mixing, then interface activated nano calcium carbonate is added, and finally flexible copolyester and a second-section chain extender are added for mixing, extrusion and granulation. The obtained material has the advantages of high strength, high toughness, good welding line reliability, low warping amount and excellent anti-falling performance, and is suitable for thin-wall injection molding.

Description

Technical Field

[0001] This invention relates to the field of resin technology, and in particular to a high-strength, lightweight plastic cover material and its preparation method. Background Technology

[0002] As mobile terminals and wearable devices become increasingly thinner and lighter, components such as back covers and decorative covers are increasingly employing thin-walled injection molding to reduce weight and achieve a seamless appearance. Simultaneously, the growing demand for green, low-carbon, and compostable materials has brought attention to the use of biodegradable polyesters such as polylactic acid (PLA) in cover components. However, covers are subjected to drop impacts, repeated stress, and temperature changes during assembly, transportation, and daily use, placing higher demands on the strength, toughness, and dimensional stability of thin-walled components.

[0003] Among existing biodegradable polyesters, polylactic acid (PLA) exhibits high rigidity and a good appearance, but its molecular chain movement is restricted, making thin-walled parts prone to brittle fracture at stress concentration points. When weld lines formed by gate merging exist in the product, insufficient refusion at the melt front makes the weld lines more likely to become the starting point of failure. To improve toughness, it is often blended with flexible copolyesters to enhance impact energy absorption. However, insufficient bonding at the two-phase interface can easily lead to phase separation and defects, resulting in insufficient weld line strength, accelerated crack propagation, and molding problems such as flow marks and appearance ripples.

[0004] To meet the strength and flatness requirements of thin-walled parts, inorganic fillers such as calcium carbonate are often introduced in the industry to improve rigidity and suppress shrinkage differences. However, due to the large specific surface area and high surface energy of nano-calcium carbonate, it is prone to agglomeration during processing. Insufficient wetting between the particles and the polyester matrix can create weak interfacial bonding zones, which are prone to detachment under stress and induce crack propagation. Uneven dispersion can also lead to unstable melt flow, amplifying thickness fluctuations and warpage risks during molding. Although surface treatment using silane coupling agents exists, the process is significantly affected by the medium, pH, and drying conditions. If the interfacial reaction is insufficient or deactivated during extrusion or injection molding thermal processes, unstable reinforcement effects, fluctuations in impact performance, and uncontrollable warpage may still occur.

[0005] Furthermore, to improve the melt strength of polyester for high-speed thin-walled molding, some solutions introduce epoxy chain extenders or reactive additives for thickening and molecular weight control. Chain extension reactions are affected by end-group content, residual moisture, and temperature shear history, easily leading to viscosity fluctuations, increased gel point, or narrowed flow windows. This results in increased molding pressure, insufficient refusion of the weld zone, and internal stress accumulation. If the raw material moisture content is not well controlled, the polyester may undergo hydrolytic degradation during processing, further weakening chain extension efficiency and causing inconsistent performance. In summary, existing biodegradable polyester thin-walled cover materials struggle to balance strength, weld line reliability, drop crack resistance, and low warpage. Their processing window is sensitive, making stable mass production difficult under existing injection molding line conditions. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a high-strength lightweight plastic cover material and its preparation method, so as to solve the problems of narrow injection flow window, weak and brittle weld lines, uneven shrinkage leading to warping, and difficulty in balancing strength and drop resistance reliability of existing biodegradable polyester thin-walled covers.

[0007] To achieve the above objectives, the present invention provides a method for preparing a high-strength, lightweight plastic cover material, comprising the following steps:

[0008] (1) Dry polylactic acid and flexible biodegradable copolyester separately;

[0009] (2) Nano-calcium carbonate was activated by using an epoxy-containing silane coupling agent to obtain interface-activated nano-calcium carbonate.

[0010] (3) Perform segmented chain extension reaction extrusion granulation: Add the first segment amount of dried polylactic acid and multifunctional epoxy chain extender to the extruder for melt mixing; after the first segment amount is added and before the second segment amount of multifunctional epoxy chain extender is added, add interface-activated nano-calcium carbonate; then add the second segment amount of dried flexible biodegradable copolyester and multifunctional epoxy chain extender for melt mixing, extrusion and granulation to obtain high-strength lightweight plastic cover material.

[0011] Preferably, the polylactic acid accounts for 65% to 80% of the total mass of the polylactic acid and the flexible biodegradable copolyester.

[0012] Preferably, the total amount of the multifunctional epoxy chain extender is 0.4 to 0.9 parts by mass, based on 100 parts by mass of the total mass of the polylactic acid and the flexible biodegradable copolyester.

[0013] Preferably, the multifunctional epoxy chain extender is added in two parts: a first part and a second part, wherein the first part accounts for 30% to 50% of the total amount, and the second part is the remainder.

[0014] Preferably, the amount of interface-activated nano-calcium carbonate is 2 to 8 parts by mass, based on 100 parts by mass of the total mass of the polylactic acid and the flexible biodegradable copolyester.

[0015] Preferably, based on 100 parts by weight of the nano-calcium carbonate, the amount of the epoxy-containing silane coupling agent is 0.2 to 5 parts by weight.

[0016] Preferably, the average particle size of the nano-calcium carbonate is 20–200 nm.

[0017] Preferably, in step (2), the dispersion medium for interface activation is an alcohol / water mixture system, and the mass ratio of alcohol to water is 2:1 to 10:1; and the pH is controlled at 4.5 to 6.5 by adding a buffer solution.

[0018] Preferably, in step (2), the ultrasonic dispersion time is 10-60 min, the interface activation reaction temperature is 40-80 °C, and the reaction time is 0.5-4 h.

[0019] Preferably, step (3) is carried out in a parallel co-rotating twin-screw extruder with a main feed port and at least two side feed ports, with an extrusion temperature range of 160-205°C, a screw speed of 150-450 rpm, and vacuum exhaust is opened in the exhaust section near the die head, with a vacuum degree of 60-95 kPa.

[0020] Furthermore, the present invention also provides a high-strength lightweight plastic cover material, which is obtained by the above-mentioned method for preparing high-strength lightweight plastic cover material.

[0021] The beneficial effects of this invention are:

[0022] This invention uses polylactic acid and flexible copolyester to construct a biodegradable polyester matrix. By setting the ratio range, the cover plate can have a deformable energy-absorbing phase structure while maintaining the basic appearance, and provides a material basis for lightweight thin-walled structures, thus supporting the load-bearing stability of the product under stress and impact scenarios from the material level.

[0023] Segmented reactive extrusion granulation involving multifunctional epoxy chain extenders enables molecular chain growth and branching control in the melt state, resulting in more stable melt strength and elastic response. It maintains a flow window suitable for thin-wall injection molding, reduces the sensitivity of the mold filling process to temperature and shear fluctuations, and facilitates the continuous advancement of the flow front and the re-fusion of the weld line area, thereby improving molding consistency and product reliability.

[0024] After being activated by epoxy silane interface, nano-calcium carbonate introduces reactive groups on its surface and improves the wetting and dispersion of polyester. It can chemically anchor and physically entangle with the matrix during the extrusion and molding thermal process, improve the stress transfer efficiency between particles and matrix, inhibit the initiation of cracks induced by interface debonding and agglomeration, and at the same time play a role in slowing down the shrinkage difference, reducing the warping trend, and promoting the maintenance of cover plate flatness and dimensional stability.

[0025] Under the combined effect of chain extension reaction and interface activation of nano-calcium carbonate, the polylactic acid phase and the flexible copolyester phase are more likely to form a continuous and stable interfacial bond, reducing the concentration effect of phase interface defects at the weld line and thin-wall corner, making the crack propagation path more easily passivated and dispersed, thereby improving the crack resistance and damage resistance under drop impact scenarios. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0027] This invention provides a high-strength, lightweight plastic cover material and its preparation method. The method uses biodegradable polyester as a matrix and achieves high strength, drop resistance, and low warpage of thin-walled covers through a synergistic pathway of chain extender segmented reactive extrusion, epoxy silane interface activation of nano-calcium carbonate, and reactive compatibility, without altering existing injection molding lines. The method framework includes the following steps: raw material drying (S1), nano-calcium carbonate interface activation (S2), segmented chain extension reactive extrusion granulation (S3), injection molding and annealing (S4). The segmented reactive extrusion of the chain extender (adding the first segment first, then the second segment) and the interface-activated nano-calcium carbonate, located between the two chain extension segments, are the key process organization of this invention. This stabilizes the melt rheological window and improves the stability of thin-walled mold filling, while also improving the strength, toughness, and dimensional stability of the weld line area.

[0028] A. Raw material system and formulation parameters

[0029] (1) In the high-strength lightweight plastic cover material provided by the present invention, the matrix resin is preferably a binary or multi-component system of polylactic acid (PLA) + compostable / degradable flexible copolyester to balance rigidity and toughness; wherein the flexible copolyester is preferably PBAT type, PBAT blend / copolymer system or its compostable grade (such as the ecoflex system used in the example).

[0030] In the matrix formulation provided by the present invention, the amount of PLA is preferably 60-85 wt% of the total amount of matrix resin, more preferably 65-80 wt%; the amount of flexible copolyester is preferably 15-40 wt%, more preferably 20-35 wt%; specifically, PLA:flexible copolyester = 75-80:20-25 (mass ratio).

[0031] PLA provides a high modulus and good molded appearance, but it is brittle and has insufficient weld line strength when used alone. Introducing flexible copolyester can improve impact toughness, but without reactive compatibility and melt strength control, phase separation, weld line embrittlement, and warpage fluctuations are likely to occur. This invention, through chain extension and interfacial reaction described later, achieves the goals of toughening without reducing strength and low warpage without sacrificing drop resistance simultaneously.

[0032] (2) Chain extenders added in stages (reactive thickening / branching regulation)

[0033] In the system provided by this invention, the chain extender is preferably a multifunctional epoxy chain extender (e.g., Joncryl ADR series), which achieves chain growth, branching or micro-crosslinking through the reaction of epoxy groups with polyester end groups (-COOH / -OH), thereby improving melt strength and thin-wall mold filling stability.

[0034] In the chain extender dosage provided by the present invention, based on a matrix resin (PLA + flexible copolyester) = 100 parts by mass, the total amount of chain extender is preferably 0.2 to 1.2 parts by mass, more preferably 0.4 to 0.9 parts by mass.

[0035] In the segmented addition method provided by the present invention, the chain extender is preferably added in two stages: a first stage and a second stage. The amount of the first stage is preferably 20-60% of the total amount of the chain extender, more preferably 30-50%. The second stage is the remainder (40-80%).

[0036] The segmented reactive extrusion of chain extenders helps to first build a long-chain branched / thickening platform on the PLA side, locking the melt rheological window, thereby improving the stability of thin-walled mold filling; then the reaction continues in the stage of introducing flexible copolyester, which can promote the formation of reactive compatibility structure between the two phases, reduce phase interface defects, and simultaneously improve the strength and toughness of the weld line area, avoiding the common contradiction of increased strength but more brittle weld lines.

[0037] If the chain extender content is too low, the melt strength will be insufficient and the fluctuation in thin-walled molding will increase; if the chain extender content is too high, it will easily lead to excessive melt viscosity, shear heating, or an increase in gel points, which will negatively affect the appearance and dimensional stability. Therefore, windowed control is required through the above-mentioned range and segmentation ratio.

[0038] Without departing from the technical effect of the present invention, the chain extender can also be replaced with other reactive additives that can react with polyester end groups, such as: multifunctional epoxy oligomers, epoxidized copolymers, acid anhydride chain extenders / branching agents, carbodiimide anti-hydrolysis / chain extenders, isocyanate end group coupling agents, etc.; but more preferably, epoxy multifunctional chain extenders are preferred to balance reaction efficiency and process safety window.

[0039] (3) Interface-activated nano-calcium carbonate

[0040] In the materials provided by this invention, the filler is preferably nano-calcium carbonate, and preferably activated at the interface by epoxy silane to obtain a reactive chemically anchored interface; achieving both low warpage and drop resistance with low filler content.

[0041] In the amount of nano-calcium carbonate provided by the present invention, based on 100 parts by weight of matrix resin (PLA + flexible copolyester), the nano-calcium carbonate is preferably 1 to 10 parts by weight, more preferably 2 to 8 parts by weight, and even more preferably 3 to 6 parts by weight.

[0042] In the particle size selection provided by the present invention, the average particle size of nano-calcium carbonate is preferably 20-200 nm, and more preferably 30-100 nm.

[0043] Nano-calcium carbonate can provide reinforcement and dimensional stability even with low addition levels, but its tendency to agglomerate and weak interface can lead to stress concentration and brittleness. Therefore, this invention activates the interface of epoxy silane to introduce reactive groups on the particle surface, which can chemically anchor and physically entangle with the polyester / chain extension reaction network during the extrusion reaction, thereby improving stress transfer efficiency and inhibiting interface debonding, thus reducing warpage and improving drop crack resistance.

[0044] (4) Epoxysilane coupling agent and interfacial activation conditions

[0045] In the interface activation provided by the present invention, the coupling agent is preferably a silane coupling agent containing an epoxy group, such as 3-glycidoxypropyltrimethoxysilane (GPTMS). In the amount of coupling agent provided by the present invention, based on 100 parts by weight of nano-calcium carbonate, the amount of epoxy silane is preferably 0.2 to 5 parts by weight, more preferably 0.5 to 3 parts by weight, and even more preferably 0.8 to 1.8 parts by weight;

[0046] In the interfacial activation medium provided by this invention, the dispersion medium is preferably an alcohol / water mixture; the alcohol may be ethanol, isopropanol, or a mixture thereof; the alcohol:water (mass ratio) is preferably 2:1 to 10:1, more preferably 3:1 to 8:1. In the pH control provided by this invention, a buffer solution (e.g., an acetate-sodium acetate buffer system) is preferably added to control the pH at 4.5 to 6.5, more preferably 5.2 to 6.2, for example, pH=5.8; the buffer concentration is preferably 0.05 to 0.5 mol / L, more preferably 0.1 to 0.3 mol / L.

[0047] In the dispersion and reaction conditions provided by the present invention: the mechanical stirring speed is preferably 300-1000 rpm, more preferably 500-800 rpm; the ultrasonic dispersion time is preferably 10-60 min, more preferably 20-40 min; the reaction temperature is preferably 40-80℃, more preferably 55-70℃; and the reaction time is preferably 0.5-4 h, more preferably 1-3 h.

[0048] The coupling agent can be replaced by other epoxy-functionalized silanes (such as 3-glycidoxypropyltriethoxysilane, etc.), as long as a reactive interface can be constructed on the surface of nano-calcium carbonate and the technical effect of the present invention can be achieved.

[0049] To further improve slurry stability and inhibit metal ion-induced flocculation, a dispersant / chelating agent can be added without affecting silane coupling. This dispersant is preferably one or more of citrate, tartrate, gluconate, or EDTA salt. The amount added is preferably 0.01–0.5 wt% of the nano-calcium carbonate, more preferably 0.05–0.2 wt%. The introduction of the chelating agent / dispersant can reduce the probability of particle bridging and agglomeration, thereby improving the uniformity of interfacial activation and the stability of subsequent extrusion dispersion.

[0050] B. Process parameters (drying-reactive extrusion-injection molding annealing)

[0051] (1) Drying (S1)

[0052] In the process provided by this invention, PLA is preferably dried under vacuum to reduce its moisture content; the flexible copolyester is preferably dried using hot air circulation or dehumidification. This is to avoid moisture-induced polyester hydrolysis, which would lead to a decrease in molecular weight and weaken the chain extension reaction efficiency.

[0053] The preferred drying temperature for PLA is 60–100°C, more preferably 80–95°C; the preferred drying time is 1–6 hours, more preferably 1.5–3 hours.

[0054] The preferred drying temperature for the flexible copolyester is 50–90°C, more preferably 60–80°C; the preferred drying time is 2–10 h, more preferably 4–8 h.

[0055] After drying, it is best to store it in a sealed container and proceed to the extrusion process as soon as possible to reduce moisture regain.

[0056] (2) Segmented chain extension reaction extrusion (S3)

[0057] In the extrusion process provided by the present invention, a parallel co-rotating twin-screw extruder is preferably used, and a main feed port and at least two side feed ports are provided.

[0058] Extrusion temperature range: preferably 160-205°C from the feeding section to the die head, more preferably 165-195°C.

[0059] Screw speed: preferably 150-450 rpm, more preferably 220-350 rpm.

[0060] Vacuum exhaust: It is preferable to open the vacuum in the exhaust section near the head of the machine to remove residual moisture, low molecular weight and solvent, and reduce the risk of bubbles and silver streaks; the vacuum degree (vacuum gauge pressure) is preferably 60-95 kPa, more preferably 70-90 kPa.

[0061] Filter mesh size: preferably 40-120 mesh, more preferably 50-80 mesh; to stabilize extrusion and inhibit gel / aggregates from entering the product.

[0062] Cooling and pelletizing: The water tank temperature is preferably 15-35℃, more preferably 20-30℃.

[0063] The "chain expansion before adding filler" method allows the filler to enter a higher viscosity, more dispersible platform, reducing secondary agglomeration. The "filler located between the two chain expansion stages" allows the epoxy / silane interface layer on the filler surface to participate more fully in the interface anchoring and compatibility reaction in the subsequent second stage reaction, thus achieving both low warpage and drop resistance even with low filler dosage.

[0064] (3) Injection molding and annealing (S4)

[0065] In the molding process provided by the present invention, medium-high speed injection and short mold filling time are preferred to ensure the stability of the flow front of the 0.7mm thin-walled part and reduce the risk of weld line defects and flow marks; after demolding, internal stress is released and the dimensions are stabilized by annealing.

[0066] The barrel temperature is preferably 175–220℃, more preferably 185–205℃;

[0067] The mold temperature is preferably 50–90°C, more preferably 70–85°C;

[0068] The filling time is preferably 0.5 to 1.5 seconds, and more preferably ≤1 second;

[0069] The holding pressure is preferably 40-120 MPa, more preferably 60-95 MPa; the holding time is preferably 4-15 s, more preferably 6-10 s;

[0070] The cooling time is preferably 10-35 seconds, more preferably 15-25 seconds;

[0071] The annealing temperature is preferably 60–100℃, more preferably 70–90℃; the annealing time is preferably 10–60 min, more preferably 20–40 min.

[0072] The thickness of the cover plate is preferably 0.4 to 1.2 mm, and more preferably 0.6 to 0.9 mm.

[0073] The present invention will be further described below with reference to embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0074] Example 1:

[0075] Step S1: Place 9125g of polylactic acid Ingeo Biopolymer 4032D in a vacuum drying oven, set to 90℃ and dry for 2 hours. After drying, seal and store for later use. Place 2750g of biodegradable copolyester ecoflex F Blend C1200 in a hot air circulating drying oven, set to 70℃ and dry for 6 hours. After drying, seal and store for later use.

[0076] Step S2: In a mechanically stirred reactor, 4000g of ethanol, 800g of deionized water, and 200g of acetate-sodium acetate buffer solution (0.2mol / L, pH=5.8) were added sequentially. After stirring at 600rpm, 500g of nano-calcium carbonate (average particle size 50nm) was added and ultrasonically dispersed for 30min to obtain a homogeneous slurry. Then, 6g of 3-glycidoxypropyltrimethoxysilane was added dropwise, and the reaction was continued at 60℃ and 600rpm for 2h. After the reaction was completed, the solid was collected by vacuum filtration and washed once with ethanol and once with deionized water. The solid was then vacuum dried at 80℃ for 6h and passed through a 100-mesh sieve to obtain interface-activated nano-calcium carbonate.

[0077] Step S3: Take 9125g of polylactic acid Ingeo Biopolymer 4032D (dried in Step S1) and 30g of multifunctional epoxy chain extender Joncryl ADR-4468 and dry mix them in a high-speed mixer for 5 minutes. Then, feed them into a parallel co-rotating twin-screw extruder through the main feed port. Set the extruder temperature zones from the feeding section to the die head to 170℃, 175℃, 180℃, 185℃, 185℃, and 180℃ respectively. Set the screw speed to 280 rpm. After the melt pressure and torque of the extruder stabilize, add 500g of interface-activated nano-calcium carbonate through the second feed port while keeping the screw speed constant at 280 rpm. Simultaneously add 2750g of biodegradable copolyester ecoflex F Blend C1200 (dried in Step S1) and 45g of multifunctional epoxy chain extender Joncryl through the third feed port. ADR-4468, then vacuum exhaust is turned on in the exhaust section near the die head, the vacuum degree is set to 80kPa, and the die head filter screen is kept at 60 mesh to ensure stable extrusion. The extruded strip is cooled in a 25℃ water bath and then pelletized to obtain high-strength lightweight plastic cover material.

[0078] Step S4: Injection molding and annealing

[0079] High-strength, lightweight plastic cover material was selected and molded using an injection molding machine and a mobile phone back cover mold. The barrel temperature was set sequentially from the feeding section to the nozzle to 185℃, 190℃, 195℃, and 200℃. The mold temperature was set to 80℃. The injection speed was set to medium-high speed and the mold filling time was controlled within 1 second. The holding pressure was set to 80MPa and held for 8 seconds. The cooling time was set to 20 seconds. The cover was demolded to obtain a thickness of 0.7mm. After demolding, the cover was placed in an 80℃ hot air box for annealing for 30 minutes and then naturally cooled to room temperature to obtain a high-strength, lightweight plastic cover.

[0080] Example 2:

[0081] The amount of interface-activated nano-calcium carbonate in steps S2 and S3 was adjusted from 500g to 300g, while the other conditions remained the same as in Example 1.

[0082] Example 3:

[0083] The amount of interfacial activated nano-calcium carbonate in steps S2 and S3 was adjusted from 500g to 800g, while the other conditions remained the same as in Example 1.

[0084] Example 4:

[0085] Keeping the total amount of chain extender unchanged at 75g, the amount of chain extender added in step S3 was adjusted from 30g+45g to 25g+50g, and the other conditions were the same as in Example 1.

[0086] Example 5:

[0087] Keeping the total amount of chain extender unchanged at 75g, the amount of chain extender added in step S3 was adjusted from 30g+45g to 40+35g, and the other conditions were the same as in Example 1.

[0088] Example 6:

[0089] In step S2, the amount of 3-glycidoxypropyltrimethoxysilane was adjusted from 6g to 3g, while the other conditions remained the same as in Example 1.

[0090] Comparative Example 1:

[0091] The difference between Comparative Example 1 and Example 1 is that in step S3, the addition method of the multifunctional epoxy chain extender Joncryl ADR-4468 is changed from segmented addition to a one-time addition of 75g. That is, all 75g of the multifunctional epoxy chain extender Joncryl ADR-4468 is dry-mixed with 9125g of polylactic acid Ingeo Biopolymer 4032D dried in step S1 in a high-speed mixer for 5 minutes, and then fed into a parallel co-rotating twin-screw extruder through the main feed port. At the third side feed port, only 2750g of compostable copolyester ecoflex F Blend C1200 dried in step S1 is added simultaneously, and the multifunctional epoxy chain extender Joncryl ADR-4468 is no longer added. The other conditions are the same as in Example 1.

[0092] Comparative Example 2:

[0093] The difference between Comparative Example 2 and Example 1 is that the filler added through the second side feed port in step S3 is replaced by nano-calcium carbonate instead of interface-activated nano-calcium carbonate; the other conditions are the same as in Example 1.

[0094] Comparative Example 3:

[0095] The difference between Comparative Example 3 and Example 1 is that in step S3, the 500g interface-activated nano-calcium carbonate was added from the second side feed port to the main feed port, along with polylactic acid Ingeo Biopolymer 4032D and the first-stage multifunctional epoxy chain extender Joncryl ADR-4468; the other conditions were the same as in Example 1.

[0096] Comparative Example 4:

[0097] The difference between Comparative Example 4 and Example 1 is that 2750g of biodegradable copolyester ecoflex FBlend C1200 is not added in step S3, and 9125g of polylactic acid Ingeo Biopolymer 4032D is adjusted to 11875g of polylactic acid IngeoBiopolymer 4032D; the other conditions are the same as in Example 1.

[0098] Performance testing:

[0099] Melt mass flow rate: The melt mass flow rate was determined according to GB / T 3682.1-2018. Each sample of granules was tested immediately after being vacuum dried at 80℃ for 2 hours. The test temperature was set to 190℃, the load was set to 2.16 kg, the preheating time was set to 300 s, the cutting time interval was set to 30 s, and 6 segments were cut continuously. The average mass flow rate was calculated by converting the average mass flow rate. Each sample was tested 3 times and the arithmetic mean was taken.

[0100] Tensile properties: Tensile properties were determined according to GB / T 1040.2-2022. The multi-purpose specimen (4 mm thick) prepared according to GB / T 17037.1-2019 was used as the standard specimen. The clamping distance of the testing machine was set to 115 mm, and the extensometer gauge length was set to 50 mm. During the tensile modulus test, the beam speed was set to 1 mm / min, and the modulus section was ended at 0.25% strain. Then the beam speed was switched to 50 mm / min until fracture to obtain tensile strength and elongation at break. Five specimens were tested for each sample and the average value was taken.

[0101] Bending performance: Bending performance was determined according to GB / T 9341-2008: Three-point bending tests were conducted using long strip specimens (80mm×10mm×4mm) prepared according to GB / T 17037.1-2019, with a span set to 64mm and a loading speed set to 2mm / min. The bending strength was recorded. Five specimens were tested for each sample and the average value was taken.

[0102] Weld line tensile strength: The tensile strength of the weld line was tested according to the tensile loading method of GB / T 1040.2-2022. However, the test specimens were thin-walled specimens with artificially constructed weld lines to correspond to the real defect scenario of thin-walled mold filling of mobile phone back cover. A special thin-walled flat plate with a thickness of 0.7mm was injection molded using a dual-gate opposing injection mold to form a stable weld line at the center of the plate. The injection temperature, mold temperature, holding pressure and cooling conditions were kept consistent with step S4. Then, with the weld line perpendicular to the tensile direction, dumbbell-shaped tensile specimens were cut from the center area of ​​the plate, so that the weld line was located at the center of the gauge length. The clamping distance and speed were set according to GB / T 1040.2-2022 (1mm / min for modulus section and 50mm / min for strength section) to test the tensile strength and elongation at break of the weld line. Eight specimens were tested for each sample and the average value was taken.

[0103] Flatness error of cover plate (warping): Flatness error was determined according to GB / T 11337-2004. 0.7 mm thick cover plates were used as the test objects. After annealing at 80℃ for 30 min and naturally cooling to room temperature, the cover plates were placed at (23±2)℃ for 24 h before testing. A three-coordinate measuring machine was used to evenly distribute points on the outer surface of the cover plate (6×6 grid, 36 points in total). The flatness error was evaluated using the minimum area method and used as the warping value. Five cover plates were measured for each sample, and the average value was taken.

[0104] Drop / rough handling impact performance: Impact tests caused by rough handling were conducted according to GB / T 2423.7-2018 to simulate the drop impact of a mobile phone back cover. A 0.7mm cover plate was used as the specimen for each sample. Before the test, the specimens were conditioned for 48 hours at (23±2)℃ and (50±10)% relative humidity. The cover plate and an equivalent mass of simulated internal parts were fixed as an assembly, ensuring assembly consistency. The drop height was set to 1.0m, and the drop surface was a rigid plane consisting of a 20mm thick steel plate covered with a 3mm hard rubber pad. The drop directions were front, back, long side, and short side, with one drop each, for a total of four drops. After the drops, cracks and notches were inspected under standard illumination. The maximum crack length was measured using a reading microscope, and the number of cracked pieces was counted to obtain the maximum crack length and cracking rate. Ten assemblies of each sample were tested, and the results were statistically analyzed. The test results are shown in Table 1.

[0105] Table 1 Performance Test Results

[0106] sample Melt mass flow rate (g / 10min) Tensile strength (MPa) Elongation at break (%) Bending strength (MPa) Tensile strength of weld wire (MPa) Elongation at break of weld line (%) Warpage (mm) Maximum crack length (mm) after the drop Cracking rate (%) Example 1 3.32 59.1 72.4 92.8 53.6 41.7 0.18 3.45 8 Example 2 3.58 58.0 78.6 90.9 52.4 44.9 0.21 4.20 10 Example 3 3.06 60.3 60.8 95.6 51.8 34.5 0.17 5.10 12 Example 4 3.74 57.4 83.2 89.8 51.9 48.6 0.20 3.85 9 Example 5 3.18 59.8 66.5 94.0 52.8 38.2 0.19 4.55 11 Example 6 3.49 56.8 70.1 88.7 49.6 40.5 0.24 6.32 16 Comparative Example 1 2.65 61.0 54.0 96.2 49.0 26.8 0.27 9.51 26 Comparative Example 2 3.61 54.9 58.7 86.5 46.8 24.5 0.32 12.4 34 Comparative Example 3 3.22 56.2 64.0 89.2 48.0 31.2 0.29 10.2 30 Comparative Example 4 3.08 62.0 18.6 101.5 42.7 8.5 0.25 18.6 52

[0107] Data Analysis:

[0108] As can be seen from the data in Table 1, the high-strength lightweight plastic cover material prepared by the present invention maintains a stable melt flow rate within a window suitable for thin-wall injection molding, while maintaining a high level of tensile and flexural properties. The tensile strength and elongation at break at the weld line are also relatively balanced, and the overall warpage and drop cracks of the cover are low. The possible reasons are as follows: Polylactic acid Ingeo Biopolymer 4032D forms a more stable melt structure under the action of the first-stage multifunctional epoxy chain extender Joncryl ADR-4468, which improves the melt elasticity and the self-healing ability of the weld interface; the interface-activated nano-calcium carbonate is introduced through the interfacial reactivity of 3-glycidoxypropyltrimethoxysilane, which realizes the construction of the melt support skeleton between the two chain extension stages and then completes the interface locking, thereby reducing the orientation and shrinkage differences during thin-wall molding; subsequently, it works together with the biodegradable copolyester ecoflex F Blend C1200, so that the rigid support, interfacial bonding and energy dissipation mechanism can work together, and finally, without significantly sacrificing strength, it takes into account the weld line reliability and drop crack resistance requirements of thin-wall parts.

[0109] As can be seen from the data in Example 1 and Comparative Example 1 in Table 1, when the multifunctional epoxy chain extender Joncryl ADR-4468 was no longer added in stages but was instead added all at once during the polylactic acid Ingeo Biopolymer 4032D stage, the melt mass flow rate of the material exhibited a high viscosity state, and the elongation at break and drop cracking at the weld line deteriorated accordingly. The main reason for this may be that concentrated chain extension is more likely to form excessive branching or local gelation tendency in the early stage, which narrows the interface reaction window with the biodegradable copolyester ecoflex F Blend C1200, making it difficult for the weld line area to complete sufficient interface rearrangement and molecular chain interpenetration within a short thermal history; at the same time, the high viscosity melt is more likely to generate residual internal stress and weld defects when filling thin walls.

[0110] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 2 and 3, when the nano-calcium carbonate is not activated at the interface by 3-glycidoxypropyltrimethoxysilane (Comparative Example 2), or when the nano-calcium carbonate is activated at the interface but added earlier and not within the interface locking interval between the two chain expansions (Comparative Example 3), the weld line performance, warpage, and drop crack characterization all deteriorate. The possible reasons are: lack of activation or improper addition timing leads to insufficient wetting and dispersion of the nano-calcium carbonate in the melt, resulting in weak interfacial bonding and local agglomeration. Stress during thin-walled part forming is more easily initiated and propagated at the particle-matrix interface. Furthermore, when the interface is not effectively chemically locked, the melt structure advantages brought by chain expansion are difficult to translate into weld line strength and crack resistance.

[0111] As can be seen from the data in Example 1 and Comparative Example 4 in Table 1, when the biodegradable copolyester ecoflexF Blend C1200 is missing from the system, and the material is constructed solely using polylactic acid Ingeo Biopolymer 4032D, multifunctional epoxy chain extender Joncryl ADR-4468, and interface-activated nano-calcium carbonate, the elongation at break, weld line ductility, and drop crack resistance are significantly reduced. The main reason for this may be that the polylactic acid matrix is ​​more sensitive to defects and weld lines under thin-walled conditions. Although chain extension can improve melt stability to some extent, the lack of deformation energy dissipation and crack passivation pathways provided by the biodegradable copolyester phase makes cracks more likely to propagate rapidly at stress concentration points. Simultaneously, the insufficient interdiffusion of molecular chains and energy dissipation mechanisms in the weld line region leads to a decrease in impact resistance reliability.

[0112] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a high-strength, lightweight plastic cover material, characterized in that, Includes the following steps: (1) Dry polylactic acid and flexible biodegradable copolyester separately; (2) Nano-calcium carbonate was activated by using an epoxy-containing silane coupling agent to obtain interface-activated nano-calcium carbonate. (3) Perform segmented chain extension reaction extrusion granulation: Add the first segment amount of dried polylactic acid and multifunctional epoxy chain extender to the extruder for melt mixing; after the first segment amount is added and before the second segment amount of multifunctional epoxy chain extender is added, add interface-activated nano-calcium carbonate; then add the second segment amount of dried flexible biodegradable copolyester and multifunctional epoxy chain extender for melt mixing, extrusion and granulation to obtain high-strength lightweight plastic cover material; Based on a total mass of 100 parts by mass of polylactic acid and flexible biodegradable copolyester, the total amount of the multifunctional epoxy chain extender is 0.4 to 0.9 parts by mass. The multifunctional epoxy chain extender is added in two stages: a first stage and a second stage. The first stage accounts for 30% to 50% of the total amount, and the second stage is the remainder.

2. The method for preparing the high-strength lightweight plastic cover material according to claim 1, characterized in that, The polylactic acid accounts for 65% to 80% of the total mass of the polylactic acid and the flexible biodegradable copolyester.

3. The method for preparing the high-strength, lightweight plastic cover material according to claim 1, characterized in that, Based on a total mass of 100 parts by mass of the polylactic acid and the flexible biodegradable copolyester, the amount of the interface-activated nano-calcium carbonate is 2 to 8 parts by mass.

4. The method for preparing the high-strength lightweight plastic cover material according to claim 1, characterized in that, Based on 100 parts by weight of the nano-calcium carbonate, the amount of the epoxy-containing silane coupling agent is 0.2 to 5 parts by weight.

5. The method for preparing the high-strength lightweight plastic cover material according to claim 1, characterized in that, In step (2), the dispersion medium for interface activation is an alcohol / water mixture system, and the mass ratio of alcohol to water is 2:1 to 10:1; and the pH is controlled at 4.5 to 6.5 by adding a buffer solution.

6. The method for preparing the high-strength lightweight plastic cover material according to claim 1, characterized in that, In step (2), the ultrasonic dispersion time is 10-60 min, the interface activation reaction temperature is 40-80℃, and the reaction time is 0.5-4 h.

7. The method for preparing the high-strength lightweight plastic cover material according to claim 1, characterized in that, The average particle size of the nano-calcium carbonate is 20–200 nm.

8. The method for preparing the high-strength lightweight plastic cover material according to claim 1, characterized in that, Step (3) is carried out in a parallel co-rotating twin-screw extruder with a main feed port and at least two side feed ports. The extrusion temperature range is 160-205°C, the screw speed is 150-450 rpm, and vacuum exhaust is turned on in the exhaust section near the die head. The vacuum degree is 60-95 kPa.

9. A high-strength, lightweight plastic cover material, characterized in that, It is obtained by the preparation method of the high-strength lightweight plastic cover material according to any one of claims 1-8.