Ultrathin micro-texture mobile phone rear cover plate and preparation method thereof

Abstract

The invention relates to the technical field of plastics, in particular to an ultrathin micro-texture mobile phone rear cover plate and a preparation method thereof. The method comprises the following steps: carrying out end group regulation on polyhexamethylene adipamide resin through hexamethylene diamine, and carrying out melt blending on the polyhexamethylene adipamide resin, tetraethyl orthosilicate and hydroxyl-terminated polydimethylsiloxane through a sol-gel reaction to obtain hybrid inorganic powder; hydroxyl-terminated polydimethylsiloxane and hexamethylene diisocyanate are sequentially added by adopting a step-by-step feeding strategy for modification. And then molding in a mold with micro textures through an injection molding-compression molding process, and carrying out limited annealing treatment. According to the invention, the problems of incomplete micro-texture copying, easy scratching and poor thermal dimensional stability of the ultrathin plate are effectively solved, and high-precision texture transfer, excellent surface scratch resistance and long-term flatness of the plate with the thickness of 320-380 microns are realized.

Description

Technical Field

[0001] This invention relates to the field of plastics technology, and in particular to an ultra-thin micro-textured mobile phone back cover material and its preparation method. Background Technology

[0002] As smartphones evolve towards thinner and lighter designs with a more premium feel, ultra-thin sheets have become a hot research topic in the industry for back covers. Polyamide resins, due to their excellent mechanical strength and processability, are widely used in electronic device structural components. However, when the thickness drops below 400μm, conventional injection molding processes are prone to problems such as insufficient flow and filling, significant molecular orientation, and residual stress concentration, making it difficult to meet the flatness requirements of high-end models. Furthermore, to enhance the appearance, micron-level textures are often designed on the mold surface. However, at the ultra-thin scale, the replication behavior of the melt within the microstructure is easily affected by fluctuations in local rheological properties, resulting in incomplete texture transfer or blurred outlines, affecting visual consistency and tactile quality.

[0003] Existing technologies attempt to improve resin rigidity by adding inorganic fillers; however, simple blending easily leads to powder agglomeration, exacerbating surface fiber floating or development defects during ultrathin flow. Simultaneously, the presence of hard particles reduces surface ductility, making the board more prone to visible scratches during subsequent assembly or use. Other solutions use siloxane-based additives to improve surface smoothness, but small-molecule additives are prone to migration and precipitation during high-temperature processing or long-term use, causing not only surface stickiness or gloss reduction but also potentially affecting the adhesion of subsequent coatings. Regarding molding processes, although injection-compression technology theoretically helps improve microstructure replication, if the material system itself fails to achieve synergistic control of rheological properties and interface states, simple process optimization alone cannot stably achieve a balance between high-precision texture transfer and low-stress morphology under ultrathin conditions. Summary of the Invention

[0004] In view of this, the purpose of this invention is to propose an ultra-thin micro-textured mobile phone back cover material and its preparation method, so as to solve the problem that existing ultra-thin material technology is difficult to maintain sub-millimeter thickness while taking into account high-precision micro-texture transfer, excellent scratch resistance and long-term thermal dimensional stability, and the lack of synergy between its material system and molding process.

[0005] To achieve the above objectives, the present invention provides a method for preparing an ultra-thin micro-textured mobile phone back cover material, comprising the following steps:

[0006] (1) After drying the polyhexamethylene adipamide resin, it is mixed with hexamethylenediamine, melt extruded and degassed to obtain end-group regulated polyhexamethylene adipamide particles;

[0007] (2) Tetraethyl orthosilicate is mixed with anhydrous ethanol, and hydroxyl-terminated polydimethylsiloxane ethanol solution is added. Sol-gel reaction is carried out in the presence of acidified aqueous phase. Ammonia is then added to promote polycondensation and aging. The mixture is then dried and pulverized to obtain hybrid inorganic powder.

[0008] (3) The end-group regulated polyhexamethylene adipamide particles are melt-blended with the hybrid inorganic powder and the hybrid inorganic powder is uniformly dispersed. Then, hydroxyl-terminated polydimethylsiloxane is added and kneaded, and hexamethylene diisocyanate is added and kneaded. The mixture is then extruded and pelletized to obtain modified polyhexamethylene adipamide particles.

[0009] (4) The modified polyhexamethylene adipamide particles are injection-compression molded in a mold with micro-textured structure. First, injection is performed at the initial mold closing gap, and the injection volume is 88%-92% of the cavity volume corresponding to the final mold closing thickness. Then, the compression mechanism is started to compress the mold closing gap to the final thickness of 320-380μm. After cooling and demolding, the mobile phone back cover plate blank is obtained.

[0010] (5) Anneal the mobile phone back cover blank and restrict free warping to obtain the ultra-thin micro-textured mobile phone back cover.

[0011] Preferably, in step (1), the drying temperature is 75-85℃ and the drying time is 5-7h.

[0012] Preferably, in step (1), the polyhexamethylene adipamide resin has a melt volume flow rate of 120 cm⁻¹ at 275°C / 5 kg. 3 / 10min.

[0013] Preferably, in step (1), the mass ratio of hexamethylenediamine to polyhexamethylenediamine resin is 4-8:10000.

[0014] Preferably, in step (1), the cylinder temperature during melt extrusion is 240-250℃, 250-260℃, 260-270℃, 270-280℃, and 270-280℃ respectively from the feeding section to the die head, the screw speed is 260-300rpm, and the gauge pressure of the vacuum exhaust is -0.07 to -0.09MPa.

[0015] Preferably, in step (2), the number average molecular weight of the hydroxyl-terminated polydimethylsiloxane is 500.

[0016] Preferably, in step (2), the mass ratio of tetraethyl orthosilicate to anhydrous ethanol is 80-95:280-320; and the mass ratio of hydroxyl-terminated polydimethylsiloxane to anhydrous ethanol in the hydroxyl-terminated polydimethylsiloxane ethanol solution is 15-25:90-110.

[0017] Preferably, in step (2), the acidified aqueous phase is prepared by mixing 1 wt% oxalic acid aqueous solution with deionized water; the ammonia concentration is 28 wt%.

[0018] Preferably, in step (2), the drying includes: first pre-evaporation at 50°C under a forced-air environment for 1.5-2.5 hours, and then vacuum drying at 105-115°C for 7-9 hours.

[0019] Preferably, in step (2), the median particle size D50 of the hybrid inorganic powder is 2.8-3.6 μm.

[0020] Preferably, in step (3), the mass ratio of end-group-regulated polyhexamethylene adipamide particles, hybrid inorganic powder, hydroxyl-terminated polydimethylsiloxane and hexamethylene diisocyanate is 10000:20-30:35-45:10-14.

[0021] Preferably, in step (3), the cylinder temperature during mixing is 245-255℃, 255-265℃, 265-275℃, 270-280℃, and 270-280℃ respectively from the feeding section to the die head, the screw speed is 280-320rpm, and the gauge pressure of the vacuum exhaust is -0.07 to -0.09MPa.

[0022] Preferably, in step (4), the mold preheating temperature is 165-175℃, the initial mold closing gap is 800-900μm, the injection filling speed is 260-300mm / s, and the compression mechanism is started after a delay of 60-100ms, and the mold closing gap is compressed to the final thickness of 320-380μm within 150-250ms with a compression force of 180-220kN and pressure is maintained simultaneously, wherein the pressure is 55-65MPa, the pressure holding time is 5-7s, and the cooling time is 16-20s.

[0023] Preferably, in step (5), the annealing temperature is 85-95℃ and the annealing time is 3-5h; and during the annealing process, the mobile phone back cover blank is clamped between two flat aluminum plates and 800-1200g of uniformly distributed pressure is applied to restrict free warping.

[0024] Furthermore, the present invention also provides an ultra-thin micro-textured mobile phone back cover material, which is obtained by the above-mentioned preparation method of the ultra-thin micro-textured mobile phone back cover material.

[0025] The 88%-92% injection filling volume to the final cavity volume mentioned in this invention refers to the cavity volume V when the mold is compressed to the final sheet thickness. final Based on this, the injection volume V is calculated by converting the injection mass m into the melt density ρ. inj =m / ρ, so that V inj =(0.88-0.92)×V final;where V final Calculated from the projected area of ​​the cavity and the final thickness.

[0026] The beneficial effects of this invention are:

[0027] This invention significantly improves the rheological uniformity and interfacial stability of polyhexamethylene adipamide resin during ultrathin molding through the synergistic effect of end-group regulation and hybrid inorganic powders. End-group regulation makes the distribution of reaction sites at the ends of molecular chains more regular, which helps the orderly anchoring of subsequent interfacial modifiers, thereby forming a more stable shear response behavior during the melt flow stage and effectively suppressing the interference of local viscosity fluctuations on microtexture replication. The hybrid inorganic powder, with its unique core-shell structure and surface activity, achieves nanoscale dispersion in the resin matrix. It not only acts as a hard phase support point to improve the surface anti-ploughing ability, but also constructs a gradient modulus interfacial layer through the interaction between its surface silanol groups and polydimethylsiloxane segments, reducing stress concentration during friction.

[0028] By introducing hydroxyl-terminated polydimethylsiloxane and hexamethylene diisocyanate sequentially through a stepwise feeding strategy, in-situ interfacial assembly and bridging grafting are achieved during melt blending. This sequential control avoids local aggregation of the reactants, resulting in a more uniform distribution of polydimethylsiloxane segments at the resin-powder interface. The subsequent addition of isocyanate, with its bifunctional properties, bridges the resin end groups and siloxane segments, forming a networked interfacial structure with greater migration resistance. This structure imparts durable lubricity and scratch resistance to the surface of the molded sheet, while preventing gloss degradation or decreased coating adhesion caused by small molecule precipitation.

[0029] The synergistic effect of injection-compression molding and material design further ensures the morphological accuracy and intrinsic quality of the ultra-thin sheet material. High-speed injection at the initial gap facilitates rapid filling of the micro-textured cavities by the melt, while delayed compression promotes full adhesion of the resin to the ends of the micro-nano structures through controllable rheological orientation relaxation and pressure penetration, significantly reducing springback deformation after demolding. Combined with restricted warpage control during annealing, the sheet material maintains a highly flat surface morphology and stable optical properties even after thermal history, meeting the stringent requirements of high-end mobile phones for consistent appearance over long-term use. Detailed Implementation

[0030] 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.

[0031] Example 1:

[0032] Step S1: Weigh 10,000 g of polyhexamethylene adipamide resin granules (BASF AG, Ultramid A3K, melt volume flow rate of 120 cm3 / 10 min at 275℃ / 5 kg) and place them in a dehumidifying dryer, set to 75℃ and dry for 5 h; then weigh 4 g of hexamethylene diamine and mix it together with the dried polyhexamethylene adipamide resin in a sealed container for 10 min to obtain a premix. Feed the premix into a co-rotating parallel twin-screw extruder for end-group controlled extrusion. The barrel temperature is set sequentially from the feeding section to the die head to 240℃, 250℃, 260℃, 270℃, and 270℃. The screw speed is set to 260 rpm. Vacuum exhaust (gauge pressure -0.07 MPa) is turned on in the middle and rear sections to continuously remove end-group condensation byproducts and volatiles. Extrude water-cooled strands and pellets to obtain end-group controlled polyhexamethylene adipamide granules.

[0033] Step S2: Weigh 80g of tetraethyl orthosilicate and 280g of anhydrous ethanol and add them to a reaction vessel equipped with a mechanical stirrer, stirring at 550rpm; separately weigh 15g of hydroxyl-terminated polydimethylsiloxane (Sigma-Aldrich, catalog number P433351, number average molecular weight 500) and mix it with 90g of anhydrous ethanol, stir in a 35℃ water bath for 10min to form a homogeneous siloxane-ethanol solution, and then slowly add it to the reaction vessel; subsequently, prepare the acid-catalyzed aqueous phase: weigh 0.8g of oxalic acid dihydrate and dissolve it in 99g of deionized water to obtain a 1wt% oxalic acid aqueous solution, and weigh 3g of it to add 2 An acidified aqueous phase was obtained from 8g of deionized water. This acidified aqueous phase was added dropwise to the reactor over 20 minutes and stirred at 35°C for 30 minutes. Then, 1.5g of ammonia (28wt%) was weighed and added in three portions, stirring at 35°C for 60 minutes. The mixture was then allowed to stand for 12 hours to age, resulting in a hybrid sol. The sol was then transferred to a polytetrafluoroethylene shallow dish and pre-evaporated at 50°C for 1.5 hours. It was then dried in a vacuum drying oven at 105°C for 7 hours. The sol was ball-milled to obtain powder with a median particle size D50 of 2.8μm. The powder was then vacuum-dried again at 105°C for 2 hours to obtain the hybrid inorganic powder.

[0034] Step S3: Weigh 10,000g of end-group-controlled polyhexamethylene adipamide granules and 20g of hybrid inorganic powder, mix them in a sealed container for 15 minutes to obtain a dry mixture, and then dry it again at 80℃ for 2 hours. Add the dry mixture to the main feed port of a conventional co-rotating parallel twin-screw extruder. Set the barrel temperature from the feeding section to the die head to 245℃, 255℃, 265℃, 270℃, and 270℃ respectively. Set the screw speed to 280rpm and turn on vacuum exhaust (gauge pressure -0.07MPa) in the middle and rear sections. After the material is completely plasticized and the hybrid inorganic powder is uniformly wetted and dispersed, add 35g of hydroxyl-terminated polydimethylsiloxane through the downstream liquid injection port and mix for 35s. Then add 10g of hexamethylene diisocyanate through the downstream liquid injection port closer to the die head and continue mixing for 70s. After extrusion, water cooling and pelletizing are performed to obtain modified polyhexamethylene adipamide granules.

[0035] Step S4: Add 1000g of modified polyhexamethylene adipamide granules to the hopper of an injection molding machine with injection-compression function. Set the barrel temperature to 265℃ / 275℃ / 280℃ / 280℃ (from the feeding section to the metering section), the nozzle temperature to 280℃, the screw back pressure to 6MPa, and the screw speed to 120rpm. Preheat the mirror-finish micro-textured mold to 165℃ and maintain the temperature. The surface roughness of the cavity mirror area is Ra20nm, and the micro-textured area is a composite microstructure: linear microgrooves with a width of 18μm, a depth of 5μm, and a pitch of 36μm, and random micro-dimples with an equivalent diameter of 1μm-2μm. m, depth 1μm; set the initial mold closing gap to 900μm and inject at 260mm / s to fill the mold within this gap, with a filling time of 280ms and an upper limit of injection pressure of 110MPa. After injecting to 88% of the cavity volume fraction, delay for 100ms to start the compression mechanism, compress the mold closing gap to the final thickness of 380μm within 250ms with a compression force of 180kN and hold the pressure simultaneously. Hold the pressure at 55MPa and hold the pressure for 5s. After cooling for 20s, open the mold and eject to obtain the mobile phone back cover blank, and control the external dimensions of the single blank to 167mm×75mm×380μm;

[0036] Step S5: Place the mobile phone back cover blank in a hot air circulating oven, anneal at 85°C for 3 hours, and then cool it to 35°C before removing it from the oven. During the annealing process, the sheet is clamped between two flat aluminum plates and 800g of uniformly distributed pressure is applied to restrict free warping, thus obtaining an ultra-thin micro-textured mobile phone back cover sheet.

[0037] Example 2:

[0038] Step S1: Weigh 10000g of polyhexamethylene adipamide resin granules (BASF AG, Ultramid A3K, melt flow rate of 120cm³ at 275℃ / 5kg). 3Place the premixed material in a dehumidifying dryer (10 min) and dry it at 80℃ for 6 h. Then weigh 6 g of hexamethylenediamine and mix it with the dried polyhexamethylene adipamide resin in a sealed container for 10 min to obtain a premix. Feed the premixed material into a co-rotating parallel twin-screw extruder for end-group controlled extrusion. Set the barrel temperature from the feeding section to the die head to 245℃, 255℃, 265℃, 275℃, and 275℃ respectively. Set the screw speed to 280 rpm. In the middle and rear sections, turn on the vacuum exhaust (gauge pressure -0.08 MPa) to continuously remove end-group condensation byproducts and volatiles. Extrude water-cooled strands and pelletize to obtain end-group controlled polyhexamethylene adipamide granules.

[0039] Step S2: Weigh 87g of tetraethyl orthosilicate and 300g of anhydrous ethanol and add them to a reaction vessel equipped with a mechanical stirrer, stirring at 600rpm; separately weigh 20g of hydroxyl-terminated polydimethylsiloxane (Sigma-Aldrich, catalog number P433351, number average molecular weight 500) and mix it with 100g of anhydrous ethanol, stir in a 40℃ water bath for 15min to form a homogeneous siloxane-ethanol solution, and then slowly add it to the reaction vessel; subsequently, prepare the acid-catalyzed aqueous phase: weigh 1g of oxalic acid dihydrate and dissolve it in 99g of deionized water to obtain a 1wt% oxalic acid aqueous solution, and weigh 4g of it from the solution... An acidified aqueous phase was obtained by adding 30g of deionized water. This acidified aqueous phase was added dropwise to the reactor over 20 minutes and stirred at 40°C for 30 minutes. Then, 2g of ammonia (28wt%) was weighed and added in three portions, and stirred at 40°C for 60 minutes. The mixture was then allowed to stand for 12 hours to age, resulting in a hybrid sol. The sol was then transferred to a polytetrafluoroethylene shallow dish and pre-evaporated at 50°C for 2 hours. It was then dried in a vacuum drying oven at 110°C for 8 hours. The sol was ball-milled to obtain powder with a median particle size D50 of 3.2μm. The powder was then vacuum-dried again at 110°C for 2 hours to obtain the hybrid inorganic powder.

[0040] Step S3: Weigh 10000g of end-group-controlled polyhexamethylene adipamide granules and 25g of hybrid inorganic powder, mix them in a sealed container in a drum for 15min to obtain a dry mixture, and then dehumidify and dry it again at 80℃ for 2h; add the dry mixture to the main feed port of a conventional co-rotating parallel twin-screw extruder, set the barrel temperature from the feeding section to the die head to 250℃, 260℃, 270℃, 275℃, and 275℃ respectively, set the screw speed to 300rpm, and turn on vacuum exhaust (gauge pressure -0.08MPa) in the middle and rear sections. After the material is completely plasticized and the hybrid inorganic powder is uniformly wetted and dispersed, add 40g of hydroxyl-terminated polydimethylsiloxane through the downstream liquid injection port and mix for 30s. Then add 12g of hexamethylene diisocyanate through the downstream liquid injection port closer to the die head and continue mixing for 60s before extruding and water-cooling pelletizing to obtain modified polyhexamethylene adipamide granules;

[0041] Step S4: Add 1000g of modified polyhexamethylene adipamide granules to the hopper of an injection molding machine with injection-compression function. Set the barrel temperature to 265℃ / 275℃ / 280℃ / 280℃ (from the feeding section to the metering section), the nozzle temperature to 280℃, the screw back pressure to 6MPa, and the screw speed to 120rpm. Preheat the mirror-finish micro-textured mold to 170℃ and maintain the temperature. The surface roughness Ra of the cavity mirror area is 20nm, and the micro-textured area is a composite microstructure: linear microgrooves are 20μm wide, 6μm deep, and have a pitch of 40μm; random micro-dimples have an equivalent diameter of 1μm-3μm. m, depth 1μm; set the initial mold closing gap to 850μm and inject at 280mm / s to fill the mold within this gap, with a filling time of 250ms and an upper limit of injection pressure of 120MPa. After injecting to 90% of the cavity volume fraction, delay for 80ms to start the compression mechanism, and compress the mold closing gap to the final thickness of 350μm within 200ms with a compression force of 200kN and hold the pressure simultaneously. Hold the pressure at 60MPa and hold the pressure for 6s. After cooling for 18s, open the mold and eject to obtain the mobile phone back cover blank, and control the external dimensions of the single blank to 167mm×75mm×350μm;

[0042] Step S5: Place the mobile phone back cover blank in a hot air circulating oven, anneal at 90℃ for 4 hours, and then cool it to 40℃ in the furnace before taking it out. During the annealing process, the sheet is clamped between two flat aluminum plates and a 1000g uniformly distributed load is applied to restrict free warping, thus obtaining an ultra-thin micro-textured mobile phone back cover sheet.

[0043] Example 3:

[0044] Step S1: Weigh 10,000g of polyhexamethylene adipamide resin granules (BASF AG, Ultramid A3K, melt volume flow rate of 120cm3 / 10min at 275℃ / 5kg) and place them in a dehumidifying dryer, set to 85℃ and dry for 7h; then weigh 8g of hexamethylene diamine and mix it together with the dried polyhexamethylene adipamide resin in a sealed container for 10min to obtain a premix. Feed the premix into a co-rotating parallel twin-screw extruder for end-group controlled extrusion. The barrel temperature is set sequentially from the feeding section to the die head to 250℃, 260℃, 270℃, 280℃, and 280℃. The screw speed is set to 300rpm. Vacuum exhaust (gauge pressure -0.09MPa) is turned on in the middle and rear sections to continuously remove end-group condensation byproducts and volatiles. Extrude water-cooled strands and pellets to obtain end-group controlled polyhexamethylene adipamide granules.

[0045] Step S2: Weigh 95g of tetraethyl orthosilicate and 320g of anhydrous ethanol and add them to a reaction vessel equipped with a mechanical stirrer, stirring at 650 rpm; separately weigh 25g of hydroxyl-terminated polydimethylsiloxane (Sigma-Aldrich, catalog number P433351, number average molecular weight 500) and mix it with 110g of anhydrous ethanol, stir in a 45℃ water bath for 20 min to form a homogeneous siloxane-ethanol solution, and then slowly add it to the reaction vessel; subsequently, prepare the acid-catalyzed aqueous phase: weigh 1.2g of oxalic acid dihydrate and dissolve it in 99g of deionized water to obtain a 1wt% oxalic acid aqueous solution, and weigh 5g of it to add... An acidified aqueous phase was obtained from 32g of deionized water. This acidified aqueous phase was added dropwise to the reactor over 20 minutes and stirred at 45°C for 30 minutes. Then, 2.5g of ammonia (28wt%) was weighed and added in three portions, and stirred at 45°C for 60 minutes. The mixture was then allowed to stand for 12 hours to age, resulting in a hybrid sol. The sol was then transferred to a polytetrafluoroethylene shallow dish and pre-evaporated at 50°C for 2.5 hours. It was then dried in a vacuum drying oven at 115°C for 9 hours. The sol was ball-milled to obtain powder with a median particle size D50 of 3.6μm. The powder was then vacuum-dried again at 115°C for 2 hours to obtain the hybrid inorganic powder.

[0046] Step S3: Weigh 10000g of end-group-controlled polyhexamethylene adipamide granules and 30g of hybrid inorganic powder, mix them in a sealed container in a drum for 15min to obtain a dry mixture, and then dehumidify and dry it again at 80℃ for 2h; add the dry mixture to the main feed port of a conventional co-rotating parallel twin-screw extruder, set the barrel temperature from the feeding section to the die head to 255℃, 265℃, 275℃, 280℃, and 280℃ respectively, set the screw speed to 320rpm, and turn on vacuum exhaust (gauge pressure -0.09MPa) in the middle and rear sections. After the material is completely plasticized and the hybrid inorganic powder is uniformly wetted and dispersed, add 45g of hydroxyl-terminated polydimethylsiloxane through the downstream liquid injection port and mix for 25s. Then add 14g of hexamethylene diisocyanate through the downstream liquid injection port closer to the die head and continue mixing for 50s before extruding and water-cooling pelletizing to obtain modified polyhexamethylene adipamide granules;

[0047] Step S4: Add 1000g of modified polyhexamethylene adipamide granules to the hopper of an injection molding machine with injection-compression function. Set the barrel temperature to 265℃ / 275℃ / 280℃ / 280℃ (from the feeding section to the metering section), the nozzle temperature to 280℃, the screw back pressure to 6MPa, and the screw speed to 120rpm. Preheat the mirror-finish micro-textured mold to 175℃ and maintain the temperature. The surface roughness Ra of the cavity mirror area is 20nm, and the micro-textured area is a composite microstructure: linear microgrooves with a width of 22μm, a depth of 7μm, and a pitch of 44μm, and random micro-dimples with an equivalent diameter of 2μm-4μm. m, depth 2μm; set the initial mold closing gap 800μm and inject at 300mm / s to fill the mold under this gap, with a filling time of 220ms, an upper limit of injection pressure of 130MPa, and after injecting to 92% of the cavity volume fraction, delay for 60ms to start the compression mechanism, and compress the mold closing gap to the final thickness of 320μm within 150ms with a compression force of 220kN and hold the pressure simultaneously, with a holding pressure of 65MPa and a holding time of 7s, and after cooling for 16s, open the mold and eject to obtain the mobile phone back cover plate blank, and control the external dimensions of the single blank to be 167mm×75mm×320μm;

[0048] Step S5: Place the mobile phone back cover blank in a hot air circulating oven, anneal at 95℃ for 5 hours, and then cool it to 45℃ in the furnace before taking it out. During the annealing process, the sheet is clamped between two flat aluminum plates and a uniformly distributed load of 1200g is applied to restrict free warping, thus obtaining an ultra-thin micro-textured mobile phone back cover sheet.

[0049] Comparative Example 1:

[0050] The difference between Comparative Example 1 and Example 2 is that: in step S1, 6g of hexamethylenediamine and polyhexamethylene adipamide resin particles are not weighed and added for drum premixing, and the dehumidified and dried polyhexamethylene adipamide resin particles are directly fed into a co-rotating parallel twin-screw extruder to obtain polyhexamethylene adipamide particles that have not undergone hexamethylenediamine end-group orientation regulation; the other conditions are the same as in Example 2.

[0051] Comparative Example 2:

[0052] The difference between Comparative Example 2 and Example 2 is that: after the acidified aqueous phase was added dropwise in step S2 and stirring was continued at 40°C for 30 min, 2 g of ammonia water (concentration 28 wt%) was not weighed and added for subsequent polycondensation promotion. Instead, the mixture was stirred at 40°C for 60 min and then allowed to stand for aging for 12 h to obtain the hybrid sol; the other conditions were the same as in Example 2.

[0053] Comparative Example 3:

[0054] The difference between Comparative Example 3 and Example 2 is that in step S3, after the material is completely plasticized and the hybrid inorganic powder is uniformly wetted and dispersed, instead of the stepwise feeding sequence of first adding hydroxyl-terminated polydimethylsiloxane through the downstream liquid inlet and mixing for 30 seconds, and then adding hexamethylene diisocyanate through the downstream liquid inlet closer to the die head and continuing to mix for 60 seconds, the hydroxyl-terminated polydimethylsiloxane and hexamethylene diisocyanate are added at the same time and in the same amount as in Example 2 through the same downstream liquid inlet and continuously mixed for 90 seconds before being extruded, water-cooled, and pelletized to obtain modified polyhexamethylene adipamide granules; the other conditions are the same as in Example 2.

[0055] Comparative Example 4:

[0056] The difference between Comparative Example 4 and Example 2 is that: in step S3, after the hybrid inorganic powder is uniformly wetted and dispersed, 40g of hydroxyl-terminated polydimethylsiloxane is added through the downstream liquid injection port in the same amount as in Example 2 and mixed for 30s, but 12g of hexamethylene diisocyanate is not weighed and added. After mixing for another 60s under the same mixing intensity, the mixture is extruded and water-cooled into pellets to obtain modified polyhexamethylene adipamide particles without hexamethylene diisocyanate bridging; the other conditions are the same as in Example 2.

[0057] Comparative Example 5:

[0058] The difference between Comparative Example 5 and Example 2 is that: in step S4, an injection molding machine with injection-compression function and a mirror micro-textured mold are still used and the mold is preheated to 170°C. However, the compression molding process of setting an initial mold gap of 850 micrometers, injecting into the cavity volume fraction of 90% first and then compressing to 350 micrometers is not performed. Instead, 100% mold filling is completed directly under the condition of a mold gap of 350 micrometers, and the mold gap is kept unchanged before holding pressure and cooling. Finally, a mobile phone back cover blank with an outer dimension of 167mm×75mm×350μm is obtained. The other conditions are the same as in Example 2.

[0059] Performance testing:

[0060] Melt volume flow rate: The melt volume flow rate of the modified polyhexamethylene adipamide particles obtained in step S3 was determined according to GB / T 3682.1-2018: the test temperature was set at 275℃, the load was 5.0kg, and the preheating time was 300s; the MVR was recorded by displacement method, each sample was tested 3 times and the average value was taken, and the relative deviation of the 3 results was recorded as the process fluctuation index.

[0061] Sheet thickness and thickness uniformity: 25 fixed measuring points (5 points along the length direction × 5 points along the width direction, with the edge recessed by 10mm) are selected on each 167mm×75mm×350μm sheet obtained in step S5. The thickness is measured using a sheet thickness gauge with a resolution of 1μm under a constant measuring force of 1.0N. Considering the structural undulations of the micro-textured surface, the mirror area of ​​the sheet is used as the measuring point area during measurement, and the probe is ensured to contact the same surface reference. The average thickness, maximum and minimum thickness, and thickness uniformity are output (expressed as (maximum value - minimum value) / 2). At least 5 sheets of each type are tested and the average value is taken.

[0062] Microtexture replication rate and surface roughness parameters: Surface roughness parameters are defined according to GB / T 3505-2009 and GB / T1031-2009, and the nominal characteristics of the measuring device are defined according to GB / T 6062-2009. A stylus-type surface profilometer (stylus tip radius 2μm, measuring force 0.75mN) is used to measure five 4.0mm long contour lines along the vertical direction of the microtexture area on the board. A cutoff length of 0.8mm is taken, and five sampling lengths are used to calculate Ra and Rz. Simultaneously, Ra is measured on the mirror area using the same method to distinguish the contribution of the material surface and the texture structure. The microtexture replication rate is based on the contour of the corresponding area of ​​the mirror microtexture mold (the mold contour is measured using the same equipment, the same stylus, and the same scanning parameters). The texture groove depth h and period p are extracted from both the board and the mold within the same sampling window, and the replication rate is calculated as (board h / mold h) × 100%. The average value of the five contour lines is taken as the replication rate for that piece, and at least five pieces are used to calculate the average.

[0063] Surface scratch performance and scratch visibility: The surface scratch performance of the sheet material was determined according to GB / T 41878-2022, and the scratch visibility was quantitatively evaluated according to GB / T 44303-2024. The sheet material obtained in step S5 was fixed on a rigid platform, and a diamond ball-tipped scriber (ball diameter 1.0 mm) was used with a scratching speed of 10 mm / s and a scratch length of 60 mm. Five fixed loads with normal loads of 1 N, 3 N, 5 N, 7 N, and 10 N were set to scratch the surface sequentially. Each load was used to repeat 3 scratches with a scratch spacing of 5 mm. The residual depth d and residual width w of the scratch were measured using a three-dimensional optical profilometer, and the color difference ΔE* between the scratched area and the unscratched area was measured using a colorimeter under D65 light source and 10° field of view as the scratch visibility index.

[0064] Flatness (warping) and appearance retention after thermal aging: Flatness was tested according to the geometric tolerance testing and verification principles of GB / T 1958-2017: The slab obtained in step S5 was placed on a granite platform with three-point support, and the height values ​​of 25 grid points (5×5) on the upper surface of the slab were measured using a coordinate measuring device. The flatness error F was calculated using the minimum area method and recorded. Subsequently, thermal aging was carried out according to GB / T 7141-2008: The same batch of slabs was aged for 168 hours under hot air circulation at 70℃. After cooling to 23℃, the flatness error F2 was measured again, and the gloss (GU) of 5 points in the specular area was measured using a 60° gloss meter according to GB / T 9754-2025 and the gloss change rate ((after aging - before aging) / before aging × 100%) was calculated to comprehensively evaluate the dimensional stability and appearance drift under thermal action. The results are shown in Table 1.

[0065] Table 1 Performance Test Results

[0066] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 <![CDATA[MVR(cm 3 / 10min)]]> 104.6 98.3 90.1 86.8 101.2 92.7 118.7 98.4 Average thickness (μm) 380.4 350.2 320.1 350.6 350.3 350.7 350.1 350.5 Thickness uniformity (μm) 6.5 5.0 4.8 8.6 7.8 10.2 7.0 14.5 Microtexture replication rate (%) 94.7 97.3 96.1 91.8 93.2 90.4 95.8 92.6 Mirror region Ra (nm) 23.6 21.4 22.8 29.7 20.6 33.2 19.8 25.1 Microtexture region Ra (nm) 1670.5 1978.4 2365.7 1815.6 1850.2 1762.9 1915.3 1837.4 Scratch residual depth d (μm) 6.42 5.18 5.76 8.35 6.98 9.12 5.05 7.85 Scratch residual width w (μm) 245.5 215.0 228.8 295.2 260.4 320.7 205.6 305.5 Scratch visibility ΔE* 1.1 0.8 1.0 1.6 1.4 1.9 1.5 1.7 Flatness error F (mm) 0.36 0.28 0.31 0.55 0.48 0.65 0.43 0.75 <![CDATA[Flatness F2 (mm) after thermal aging]]> 0.46 0.33 0.39 0.72 0.63 0.88 0.40 1.05 Gloss change rate (%) -1.6 -0.8 -1.2 -5.2 -3.9 -6.5 1.3 -4.8

[0067] Data Analysis:

[0068] As can be seen from the data in Examples 1-3 in Table 1, the ultra-thin micro-textured mobile phone back cover sheet prepared by this invention exhibits a relatively consistent high level in terms of melt flowability, sheet thickness stability, micro-texture replication, and surface condition. Furthermore, the roughness of the mirror area can approach the mold mirror reference without obvious particle protrusions or flow mark amplification. The roughness of the micro-textured area is mainly contributed by the geometric structure of linear microgrooves and random micro-pits, indicating that the surface layer is fully filled during injection molding and the demolding process is controlled. The residual morphology and visibility of scratches remain at a low level, indicating that the hybrid inorganic powder provides surface bearing capacity and anti-ploughing support. The hydroxyl-terminated polydimethylsiloxane, under the action of hexamethylene diisocyanate, participates in interface regulation in a more stable manner, weakening stress concentration and material tearing tendency during friction. Simultaneously, restricted annealing promotes residual stress relaxation and restricts free warping, thus maintaining flatness and suppressing gloss drift under thermal action, thereby balancing appearance texture and dimensional stability.

[0069] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, after removing the end-group orientation control, the replication consistency, scratch morphology, and appearance retention after thermal aging all showed adverse changes. The main reason is that when hexamethylene diamine is not pre-integrated into the reactive end-group environment of polyhexamethylene adipamide, the subsequent hexamethylene diisocyanate is more likely to undergo a non-directional dispersion reaction in the bulk phase, resulting in a decrease in the controllability of grafting and chain end connection, and more prominent viscoelasticity and local structural inhomogeneity of the system. This inhomogeneity is amplified in ultra-thin injection molding as surface filling fluctuations and demolding friction fluctuations, and further transforms into a deterioration in scratch visibility and flatness retention, indicating a significant synergistic effect between end-group control and bridging grafting.

[0070] As can be seen from the data in Example 2 and Comparative Example 2 in Table 1, when the formation process of hybrid inorganic powder lacks a subsequent polycondensation promoting step, some apparent indicators of surface roughness may not necessarily deteriorate synchronously, but scratch resistance and appearance stability under thermal action still show an overall downward trend. The reason for this is that the powder obtained under these conditions is more likely to exhibit an insufficiently structured state and uneven distribution of surface active sites. Although it can maintain a relatively smooth mirror appearance in the short term after entering polyhexamethylene adipamide, its contribution to surface bearing capacity and anti-ploughing is insufficient, and the interfacial coupling efficiency with the hydroxyl-terminated polydimethylsiloxane / hexamethylene diisocyanate system decreases, making it difficult to achieve the combined effect of hard phase support and stable low friction.

[0071] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, adding hydroxyl-terminated polydimethylsiloxane and hexamethylene diisocyanate simultaneously at the same location in a single batch significantly deteriorated microtexture replication, mirror appearance, scratch morphology, and stability after thermal aging. The main reason is that the one-pot method creates short-term high-concentration reaction zones in the melt, making the reaction of hexamethylene diisocyanate more likely to be accompanied by localized gelation and phase separation, while simultaneously weakening the redispersion and uniform wetting of the hybrid inorganic powders. The resulting microscale inhomogeneities are directly replicated as optical scattering and texture defects on the formed surface, and induce more severe ploughing and material rolling during the scratching process. This result demonstrates the combined effect of the step-by-step feeding strategy and bridging grafts on uniformity control.

[0072] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, without the addition of hexamethylene diisocyanate, some indicators related to flow / short-term friction may not deteriorate significantly, and even show abnormalities such as smaller scratch geometric residue. However, scratch visibility and gloss drift after thermal aging are significantly aggravated. This is because, without bridging branches, hydroxyl-terminated polydimethylsiloxane tends to exist physically and accumulate on the surface under molding or thermal action. In the short term, this can reduce the coefficient of friction and thus alleviate the plowing depth, but at the same time, it introduces changes in surface composition and refractive index gradient, leading to enhanced scattering and a blooming / brightening appearance drift. Therefore, end-group regulation and hexamethylene diisocyanate bridging branches do not only reduce friction, but also provide stable interface regulation with restricted migration, thereby achieving a synergistic gain of low visibility and high durability.

[0073] As can be seen from the data in Example 2 and Comparative Example 5 in Table 1, after maintaining the same material system and eliminating the injection-compression molding process, the thickness uniformity, microtexture replication stability, and flatness retention all showed a systematic decline, further manifested as increased scratch visibility and appearance fluctuations after thermal aging. The presumed reason is that conventional direct molding makes ultra-thin parts more prone to forming significant orientation gradients and residual stress accumulation. Simultaneously, the absence of the compression bonding stage reduces the surface layer's sufficiency in replicating mold microgrooves and random micro-pits. When subsequently heated, stress redistribution tends to be released in a warping manner, resulting in a combined shift in flatness and gloss. These results indicate that the combination of injection-compression molding and a built-in lubrication chain can form a synergistic closed loop of process and material in ultra-thin microtexture scenarios.

[0074] 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 an ultra-thin micro-textured mobile phone back cover material, characterized in that, Includes the following steps: (1) After drying the polyhexamethylene adipamide resin, it is mixed with hexamethylene diamine, melt extruded and degassed to obtain end-group regulated polyhexamethylene adipamide particles; (2) Tetraethyl orthosilicate is mixed with anhydrous ethanol, and hydroxyl-terminated polydimethylsiloxane ethanol solution is added. Sol-gel reaction is carried out in the presence of acidified aqueous phase. Ammonia is then added to promote polycondensation and aging. The mixture is then dried and pulverized to obtain hybrid inorganic powder. (3) The end-group regulated polyhexamethylene adipamide particles are melt-blended with the hybrid inorganic powder and the hybrid inorganic powder is uniformly dispersed. Then, hydroxyl-terminated polydimethylsiloxane is added and kneaded, and hexamethylene diisocyanate is added and kneaded. The mixture is then extruded and pelletized to obtain modified polyhexamethylene adipamide particles. (4) The modified polyhexamethylene adipamide particles are injection-compression molded in a mold with micro-textured structure. First, injection is performed at the initial mold closing gap, and the injection volume is 88%-92% of the cavity volume corresponding to the final mold closing thickness. Then, the compression mechanism is started to compress the mold closing gap to the final thickness of 320-380μm. After cooling and demolding, the mobile phone back cover plate blank is obtained. (5) Anneal the mobile phone back cover blank and restrict free warping to obtain the ultra-thin micro-textured mobile phone back cover blank; In step (3), the mass ratio of end-group-regulated polyhexamethylene adipamide particles, hybrid inorganic powder, hydroxyl-terminated polydimethylsiloxane and hexamethylene diisocyanate is 10000:20-30:35-45:10-14.

2. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, wherein in step (1), the polyhexamethylene adipamide resin has a melt volume flow rate of 120 cm⁻¹ at 275℃ / 5kg. 3 / 10min; In step (2), the number average molecular weight of hydroxyl-terminated polydimethylsiloxane is 500.

3. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (1), the mass ratio of hexamethylenediamine to polyhexamethylenediamine resin is 4-8:10000.

4. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (1), the cylinder temperature during melt extrusion is 240-250℃, 250-260℃, 260-270℃, 270-280℃, and 270-280℃ respectively from the feeding section to the die head. The screw speed is 260-300 rpm, and the gauge pressure of the vacuum exhaust is -0.07 to -0.09 MPa.

5. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (2), the mass ratio of tetraethyl orthosilicate to anhydrous ethanol is 80-95:280-320; the mass ratio of hydroxyl-terminated polydimethylsiloxane to anhydrous ethanol in the hydroxyl-terminated polydimethylsiloxane ethanol solution is 15-25:90-110.

6. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (2), the acidified aqueous phase is prepared by mixing 1 wt% oxalic acid aqueous solution with deionized water; the ammonia concentration is 28 wt%.

7. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (3), the cylinder temperature during mixing is 245-255℃, 255-265℃, 265-275℃, 270-280℃, and 270-280℃ respectively from the feeding section to the die head. The screw speed is 280-320rpm, and the gauge pressure of the vacuum exhaust is -0.07 to -0.09MPa.

8. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (4), the mold preheating temperature is 165-175℃, the initial mold closing gap is 800-900μm, the injection filling speed is 260-300mm / s, and the compression mechanism is started after a delay of 60-100ms. The compression force of 180-220kN is used to press the mold closing gap to the final thickness of 320-380μm within 150-250ms and hold the pressure simultaneously. The holding pressure is 55-65MPa, the holding time is 5-7s, and the cooling time is 16-20s.

9. The method for preparing the ultra-thin micro-textured mobile phone back cover material according to claim 1, characterized in that, In step (5), the annealing temperature is 85-95℃ and the annealing time is 3-5h; and during the annealing process, the mobile phone back cover plate blank is clamped between two flat aluminum plates and 800-1200g of uniformly distributed pressure is applied to restrict free warping.

10. An ultra-thin micro-textured mobile phone back cover material, characterized in that, It is obtained by the preparation method of the ultra-thin micro-textured mobile phone back cover material according to any one of claims 1-9.