1. A physically foamed lightweight sheet characterized in that: including intermediate level (11) and limit side layer (12), limit side layer (12) are located intermediate level (11) both sides, be provided with a plurality of microbubble (2) in intermediate level (11).

2. The lightweight sheet according to claim 1, wherein: the pore size of the micro-bubbles (2) is 0.05-400 microns.

3. The physically foamed lightweight sheet according to claim 1, wherein: the foaming magnification of the sheet is 1.05-5.

4. The physically foamed lightweight sheet according to claim 1, wherein: the sheet had a cell density of 10⁵-10¹⁶Per cm³。

5. The physically foamed lightweight sheet according to claim 1, wherein: the ratio of the thickness of the middle layer (11) to the total thickness of the two side layers (12) is 1-100.

6. A method of making a physically foamed lightweight sheet according to any of claims 1 to 5, characterized in that: the method comprises the following steps:

s1: melting raw materials:

melting the material of the intermediate layer (11): heating and melting the material of the middle layer (11), and adding a pore nucleating agent into the melted material of the middle layer (11) to obtain a middle melt;

melting the material of the side layer (12): heating and melting the material of the side layer (12) to obtain a side melt;

s2: physical blowing agent injection

After the intermediate melt is plasticized, injecting a physical foaming agent into the intermediate melt to obtain a melt containing the foaming agent;

s3: shaping of

And (3) introducing the side melt and the melt containing the foaming agent into a forming die, so that the side melt is positioned on two sides of the melt containing the foaming agent, and then demolding to obtain the lightweight sheet.

7. The method for preparing a physically foamed lightweight sheet according to claim 6, wherein: in the step S2, the pressure of the physical foaming agent is 2-20 MPa.

8. The method for preparing a physically foamed lightweight sheet according to claim 6, wherein: the physical foaming agent is one or more of nitrogen, carbon dioxide and C3-C6 alkane.

9. The method for preparing a physically foamed lightweight sheet according to claim 6, wherein: the raw material of the sheet is one or more of PET, PLA, PS, ABS and PP.

10. The method for preparing a physically foamed lightweight sheet according to claim 6, wherein: in the step S2, the physical foaming agent is pumped into the intermediate melt through an air pumping device (3), and the air outlet end of the air pumping device (3) is connected with an extruder for melting raw materials; the inflating device (3) comprises a gas storage tank (31), a gas supply pipe (32), a gas collecting hood (33), a gas outlet pipe (34) and a synchronous opening and closing piece (35), the gas supply pipe (32) is connected with the gas storage tank (31), a gas pump (321) is arranged on the gas supply pipe (32), the gas collecting hood (33) is arranged on the peripheral wall of the extruder charging barrel (4), a separating ring plate (331) is arranged in the gas collecting hood (33), the inner wall of the separating ring plate (331) is opposite to the peripheral wall of the extruder charging barrel (4), the separating ring plate (331) separates the gas collecting hood (33) into an accommodating ring cavity (332) close to the extruder charging barrel (4) and a gas collecting ring cavity (333) located at one side, far away from the extruder charging barrel (4), of the accommodating ring cavity (332), and the gas supply pipe (32) is communicated with the gas collecting ring cavity (333); the air inlet ends of the air outlet pipes (34) are uniformly distributed on the separating ring plate (331), the air outlet ends of the air outlet pipes (34) extend into a charging barrel (4) of the extruder, the air outlet pipes (34) are communicated with the air collecting ring cavity (333), and the inner side wall of the air inlet ends of the air outlet pipes (34) is rotatably provided with a sealing plate (344) through a rotating shaft (343);

the synchronous opening and closing piece (35) comprises a positioning torsion spring (351), a positioning abutting block (352), a linkage gear (353) and a synchronous gear ring (354), the positioning torsion spring (351) is arranged between the rotating shaft (343) and the side wall of the air outlet pipe (34), the positioning resisting block (352) is arranged on one side of the inner side wall of the air outlet pipe (34) far away from the air outlet end of the air outlet pipe (34), the positioning abutting block (352) abuts against the closing plate (344), the closing plate (344) closes the air outlet pipe (34), the rotating shaft (343) extends out of the gas collecting hood (33), the linkage gears (353) are arranged corresponding to the rotating shaft (343) one by one, the linkage gear (353) is arranged at one end of the rotating shaft (343) extending out of the gas-collecting hood (33), the synchronous gear ring (354) is rotationally arranged on the outer peripheral wall of the charging barrel (4) of the extruder, and the linkage gears (353) are all meshed with the synchronous gear ring (354).

Background

Blister is a common processing technology of plastics and is widely applied to the manufacture of packaging materials of electronics, electrical appliance industry, food industry, cosmetic industry, stationery and the like. The main principle of plastic uptake is that after a flat plastic hard sheet is heated to be soft, the plastic hard sheet is adsorbed on the surface of a mould in vacuum, and after cooling, the plastic hard sheet is molded to obtain a corresponding plastic product.

The blister packaging product mainly comprises a blister shell, a tray, a blister box and the like, and the blister packaging not only has the advantages of saving raw and auxiliary materials, light weight, convenient transportation and sealing performance, but also meets the requirement of environment-friendly green packaging.

With respect to the related art in the above, the inventors consider that: the processing of the sheet in the blister process is usually integrated, which results in a heavy sheet, which is not favorable for the subsequent processing and use of the sheet.

Disclosure of Invention

In order to reduce the weight of the sheet, the application provides a physically foamed lightweight sheet and a preparation method thereof.

In a first aspect, the present application provides a physically foamed lightweight sheet, which adopts the following technical scheme:

the utility model provides a physics foaming lightweight sheet, includes intermediate level and avris layer, the avris layer is located the intermediate level both sides, be provided with a plurality of microbubbles in the intermediate level.

Through adopting above-mentioned technical scheme, the intermediate level is because the existence of microbubble to can reduce the density in intermediate level, thereby can reduce the holistic density of sheet, alleviate the weight of sheet, the setting on limit side layer simultaneously for the avris of sheet belongs to the entity, thereby can reduce the influence to sheet mechanical properties that arouses because the setting of microbubble, thereby this application can obtain a lightweight sheet.

Optionally, the pore size of the microbubbles is from 0.05 to 400 microns.

By adopting the technical scheme, the aperture of the micro-bubbles is controlled in a proper range, the requirement of reducing the weight of the sheet is met, and meanwhile, the influence on the mechanical property of the sheet is further reduced.

Optionally, the foaming magnification of the sheet is 1.05-5.

By adopting the technical scheme, the foaming ratio refers to the ratio of the volume of the micro bubbles to the volume of the sheet material without the micro bubbles, and the foaming ratio is controlled in a proper range, so that the influence on the mechanical property of the sheet material caused by the existence of the micro pores is reduced.

Optionally, the sheet has a cell density of 10⁵-10¹⁶Pieces/cm 3.

By adopting the technical scheme, the tear strength of the sheet material can be influenced by the density of the cells, and the density of the cells is controlled within a proper range, so that the mechanical property of the sheet material can be relatively improved.

Optionally, the ratio of the thickness of the middle layer to the total thickness of the two side layers is 1-100.

By adopting the technical scheme and the proper thickness ratio, the middle layer meets the requirement on the weight reduction effect of the sheet, and the side layer meets the requirement on maintaining the mechanical property of the sheet.

In a second aspect, the present application provides a method for preparing a physically foamed lightweight sheet, which adopts the following technical scheme: a preparation method of a physical foaming lightweight sheet comprises the following steps:

s1: melting raw materials:

melting of the material of the intermediate layer: heating and melting the intermediate layer material, and adding a pore nucleating agent into the melted intermediate layer material to obtain an intermediate melt;

melting of the side edge layer material: heating and melting the material of the side edge layer to obtain a side edge melt;

s2: physical blowing agent injection

After the intermediate melt is plasticized, injecting a physical foaming agent into the intermediate melt to obtain a melt containing the foaming agent;

s3: shaping of

And (3) introducing the side melt and the melt containing the foaming agent into a forming die to enable the side melt to be positioned at two sides of the melt containing the foaming agent, then demolding, and expanding and foaming the melt containing the foaming agent to obtain the lightweight sheet.

By adopting the technical scheme, the microbubbles in the middle layer can be refined by adding the pore nucleating agent; after the middle melt is plasticized, injecting a physical foaming agent into the middle melt, so that the physical foaming agent is uniformly distributed in the middle melt, finally introducing the side melt and the melt containing the foaming agent into a forming die, so that the side melt and the melt containing the foaming agent are fused, meanwhile, the side melt seals two sides of the melt containing the foaming agent, and when the side melt is discharged from the die, the melt containing the foaming agent is recovered to normal pressure, and the physical foaming agent foams under the action of high temperature.

Optionally, in step S2, the pressure of the physical foaming agent is 2 to 20 MPa.

By adopting the technical scheme, proper pressure is selected, and the physical foaming agent is uniformly distributed in the intermediate melt while the conveying of the intermediate melt is not influenced.

Optionally, the physical foaming agent is one or more of nitrogen, carbon dioxide and C3-C6 alkane.

By adopting the technical scheme, the proper physical foaming agent is selected, so that the interlayer material can be well foamed.

Optionally, the raw material of the sheet is one or more of PET, PLA, PS, ABS, and PP.

By adopting the technical scheme, the PET (polyethylene terephthalate) has good physical and mechanical properties, and has good creep resistance, fatigue resistance, friction resistance and dimensional stability; PLA is biodegradable plastic polylactic acid, has good processing performance and good thermal stability; PS is polystyrene, and has the properties of good electrical insulation property, easy coloring, good processing fluidity, good rigidity and the like; ABS refers to acrylonitrile-butadiene-styrene copolymer, and is a thermoplastic high polymer material structure with high strength, good toughness and easy processing and molding; PP (polypropylene) as a thermoplastic material also has excellent mechanical properties; the sheet prepared by the material has good processing performance and service performance.

Optionally, in step S2, the physical foaming agent is pumped into the intermediate melt through an air pumping device, and an air outlet end of the air pumping device is connected to an extruder for melting the raw material; the inflating device comprises an air storage tank, an air supply pipe, an air collecting hood, an air outlet pipe and a synchronous opening and closing piece, wherein the air supply pipe is connected with the air storage tank, an air pump is arranged on the air supply pipe, the air collecting hood is arranged on the outer peripheral wall of the extruder charging barrel, a separating ring plate is arranged in the air collecting hood, the ring inner wall of the separating ring plate is opposite to the outer peripheral wall of the extruder charging barrel, the separating ring plate divides the air collecting hood into a containing ring cavity close to the extruder charging barrel and an air collecting ring cavity positioned at one side of the containing ring cavity far away from the extruder charging barrel, and the air supply pipe is communicated with the air collecting ring cavity; the air inlet end of the air outlet pipe is uniformly distributed on the separating ring plate, the air outlet end of the air outlet pipe extends into the material cylinder of the extruder, the air outlet pipe is communicated with the air collecting ring cavity, and a sealing plate is rotatably arranged on the inner side wall of the air inlet end of the air outlet pipe through a rotating shaft;

the synchronous opening and closing piece comprises a positioning torsion spring, a positioning support block, a linkage gear and a synchronous gear ring, the positioning torsion spring is arranged between a rotating shaft and an outlet pipe side wall, the positioning support block is arranged on one side, away from an outlet pipe air outlet end, of the outlet pipe inner side wall, the positioning support block is tightly supported with a sealing plate, the sealing plate seals the outlet pipe, the rotating shaft extends out of a gas collecting hood, the linkage gear is arranged in one-to-one correspondence with the rotating shaft, the linkage gear is arranged at one end, extending out of the gas collecting hood, of the rotating shaft, the synchronous gear ring is arranged on the outer peripheral wall of a material cylinder of the extruder in a rotating mode, and the linkage gear is meshed with the synchronous gear ring.

By adopting the technical scheme, the physical foaming agent enters the gas-collecting hood in advance, so that the gas-collecting hood is filled with the physical foaming agent, and after the pressure in the gas-collecting hood reaches the specified value, the positioning torsion spring is overcome to promote the rotating shaft to drive the closing plate to rotate; and due to the meshing action of the synchronous gear ring and the linkage gear, all the rotating shafts drive the corresponding closing plates to synchronously rotate, all the air outlet pipes are synchronously opened, so that the physical foaming agent is simultaneously and uniformly injected into different positions of the material barrel of the extruder, and the physical foaming agent is uniformly distributed in the intermediate melt.

In summary, the present application includes at least one of the following beneficial technical effects:

1. the arrangement of the micro bubbles can reduce the density of the middle layer, so that the overall density of the sheet can be reduced, the weight of the sheet is reduced, the cost is reduced, and the sheet has the characteristics of environmental protection;

2. the side layers of the solid body are arranged on the two sides of the middle layer at the same time when the micro bubbles are arranged on the middle layer, so that the foamed middle layer is enhanced, the influence of the micro bubbles on the mechanical property of the sheet material is further reduced, and the application range of the product can be expanded;

3. the sheet material has good buffering and heat-insulating properties due to the arrangement of the middle layer with the micro bubbles, and can be used in the fields of automobile sound insulation, building refrigeration house heat insulation and the like;

4. the application provides a preparation method of a lightweight sheet material, the lightweight sheet material can be conveniently obtained, and the diameter of microbubbles is 0.05-400 microns, preferably 0.05-300 microns, more preferably 0.05-20 microns, and most preferably 0.05-0.07 microns;

drawings

Fig. 1 is a schematic view of a sheet structure in example 1 of the present application.

Fig. 2 is a schematic view of the structure of the pump-up device in embodiment 1 of the present application.

Fig. 3 is a plan view of the structure of the air pump in embodiment 1 of the present application.

Fig. 4 is a sectional view taken along line a-a of fig. 3.

Fig. 5 is an enlarged view B in fig. 4.

Fig. 6 is a sectional view taken along line C-C of fig. 3.

Fig. 7 is an enlarged view at D in fig. 6.

Description of reference numerals: 11. an intermediate layer; 12. an edge side layer; 2. microbubbles; 3. an air pump device; 31. a gas storage tank; 32. a gas supply pipe; 321. an air pump; 33. a gas-collecting hood; 331. a separating ring plate; 332. an accommodating ring cavity; 333. a gas collecting ring cavity; 34. an air outlet pipe; 341. a fixed tube; 3411. a guide chute; 3412. a connecting rod; 342. a movable tube; 3421. a guide slider; 3422. a limiting spring; 343. a rotating shaft; 344. a closing plate; 35. synchronously opening and closing the parts; 351. positioning a torsion spring; 352. positioning a resisting block; 353. a linkage gear; 354. a synchronizing ring gear; 4. a charging barrel; 5. a stressed block; 6. a bearing ring.

Detailed Description

The present application is described in further detail below with reference to figures 1-7.

The embodiment of the application discloses light sheet of physics foaming.

Example 1

As shown in fig. 1, a physically foamed lightweight sheet, the material of the sheet is polyethylene terephthalate, the sheet includes a middle layer 11 and side layers 12, the side layers 12 are located on two sides of the middle layer 11, in this embodiment, the thickness of the middle layer 11 is 1 mm, the thickness of each of the two side layers is 0.143 mm, and the ratio of the thickness of the middle layer 11 to the total thickness of the two side layers 12 is 3.5. Meanwhile, a plurality of closed micro bubbles 2 are arranged in the middle layer 11, the aperture of each micro bubble 2 is 0.05-0.07 micron, the foaming ratio of the sheet is 1.05, and the cell density is 10¹⁶Pieces/cm 3.

The embodiment 1 of the application also discloses an inflating device.

As shown in fig. 2 and 3, the inflation device 3 in the present embodiment is used in combination with an extruder, the inflation device 3 includes an air tank 31, an air supply pipe 32 and an air collecting hood 33, the air supply pipe of the air supply pipe 32 is communicated with the air outlet end of the air tank 31, and an air pump 321 is installed on the air supply pipe 32; the gas collecting hood 33 is a ring-shaped hood, and the gas collecting hood 33 is covered on the outer peripheral wall of the barrel 4 of the extruder, and the ring inner side end wall of the gas collecting hood 33 is fixedly connected with the outer peripheral wall of the barrel 4 of the extruder.

As shown in fig. 4 and 5, a separating ring plate 331 is further integrally connected between the axially opposite inner side walls of the gas collecting hood 33, so that the inner wall of the separating ring plate 331 is opposite to the outer peripheral wall of the extruder barrel 4, meanwhile, the separating ring plate 331 separates the gas collecting hood 33 into two chambers, namely, an accommodating ring cavity 332 and a gas collecting ring cavity 333, the accommodating ring cavity 332 is close to the extruder barrel 4, the gas collecting ring cavity 333 is located at the side of the accommodating ring cavity 332 away from the extruder barrel 4, and the gas outlet end of the gas supply pipe 32 is communicated with the gas collecting ring cavity 333; the inflating device 3 further comprises six air outlet pipes 34 and synchronous opening and closing members 35, air inlet ends of the six air outlet pipes 34 are uniformly distributed on the separating annular plate 331 along the circumferential direction of the separating annular plate 331, and the air inlet end of the air outlet pipe 34 is communicated with the air collecting annular cavity 333.

As shown in fig. 5 and 7, the air outlet end of the air outlet pipe 34 extends into the barrel 4 of the extruder, a rotating shaft 343 is rotatably disposed on the inner side wall of the air inlet end of the air outlet pipe 34, and a closing plate 344 is fixedly connected to the rotating shaft 343. Initially, the closing plate 344 closes the gas outlet pipe 34, and only when the physical foaming agent is gathered in the gas collecting ring cavity 333, the pressure in the gas collecting ring cavity 333 reaches a specified magnitude, the synchronous opening and closing member 35 drives all the closing plates 344 to synchronously rotate, so that the gas outlet pipe 34 is synchronously opened, and thus the physical foaming agent can be uniformly pumped into each position in the basic material cylinder 4 under the action of the air pump 321.

As shown in fig. 5 and 7, the synchronous opening and closing member 35 includes a positioning torsion spring 351, a positioning abutting block 352, a linkage gear 353 and a synchronous gear ring 354, the positioning torsion spring 351 is fixedly connected between the rotating shaft 343 and the side wall of the air outlet pipe 34, the positioning abutting block 352 is fixedly connected to the side of the inner side wall of the air outlet pipe 34 far away from the air outlet end of the air outlet pipe 34, when the positioning torsion spring 351 is in a natural state, the positioning abutting block 352 abuts against the closing plate 344, and the closing plate 344 closes the air outlet pipe 34; in this embodiment, the same side ends of the six rotating shafts 343 extend out of the gas collecting hood 33, the linkage gears 353 are arranged in one-to-one correspondence with the rotating shafts 343, the linkage gears 353 are fixedly connected to the end of the rotating shafts 343 extending out of the gas collecting hood 33, the synchronizing gear ring 354 is rotatably arranged on the outer peripheral wall of the barrel 4 of the extruder, and the linkage gears 353 are all meshed with the outer peripheral wall of the synchronizing gear ring 354. Therefore, when the pressure in the gas collecting ring cavity 333 reaches a specified value, the elasticity of the positioning torsion spring 351 can be overcome, and all the sealing plates 344 rotate synchronously due to the meshing of the synchronous gear ring 354 and the linkage gear 353, so that the six gas outlet pipes 34 can be opened synchronously.

As shown in fig. 5 and 6, in the present embodiment, each air outlet pipe 34 includes a fixed pipe 341 and a movable pipe 342, the fixed pipe 341 communicates with the partition ring plate 331, a guide sliding groove 3411 is axially disposed on an inner peripheral wall of the fixed pipe 341, the movable pipe 342 penetrates through the fixed pipe 341, an outer peripheral wall of the movable pipe 342 is sealed by fitting with the inner peripheral wall of the fixed pipe 341, a guide slider 3421 is integrally connected to the outer peripheral wall of the movable pipe 342, and the guide slider 3421 is slidably connected to a groove wall of the guide sliding groove 3411 along the axial direction of the fixed pipe 341; and a limit spring 3422 is fixedly connected between the side walls of the guide sliding block 3421 and the guide sliding groove 3411 close to the partition ring plate 331.

As shown in fig. 5 and 7, the inner wall of the fixed tube 341 is provided with a connecting rod 3412, the connecting rod 3412 is integrally connected with a force-bearing blocking block 5, the inner peripheral wall of the movable tube 342 is provided with a bearing ring 6, the inner diameter of the bearing ring 6 is gradually reduced along the direction away from the separating ring plate 331, the force-bearing blocking block 5 is a conical block, and when the limiting spring 3422 is in a natural state, the force-bearing blocking block 5 is inserted on the inner wall of the bearing ring 6 and is attached to the inner wall of the bearing ring 6; while the side of the movable tube 342 that protrudes beyond the fixed tube 341 protrudes into the barrel 4 of the extruder.

As shown in fig. 5 and 7, after the physical foaming agent enters the fixed pipe 341, as the pressure in the fixed pipe 341 increases, the pressure of the limiting spring 3422 is overcome, the movable pipe 342 moves away from the partition ring plate 331, the force-bearing block 5 is separated from the support ring 6, the physical foaming agent enters the movable pipe 342 through the gap between the force-bearing block 5 and the support ring 6, and simultaneously, as the pressure in the fixed pipe 341 is released, the movable pipe 342 is pulled back to the original position by the limiting spring 3422, so that the movable pipe 342 slides back and forth in the fixed pipe 341, and the air outlet end of the movable pipe 342 moves in the barrel 4 of the extruder along the axial direction, so that the physical foaming agent can be injected into the barrel 4 of the extruder more uniformly.

The operating principle of the inflating device in embodiment 1 of the application is as follows: when the air pump 321 is started, the physical foaming agent in the air storage tank 31 enters the air collecting hood 33 through the air supply pipe 32, after the pressure in the air collecting hood 33 reaches a certain value, the closing plate 344 synchronously rotates under the meshing action of the synchronous gear ring 354 and the linkage gear 353, the physical foaming agent enters the fixed pipe 341, the movable pipe 342 is promoted to slide, and the physical foaming agent is pumped into the material cylinder 4 of the extruder through the movable pipe 342.

The application embodiment 1 also discloses a preparation method of the physical foaming lightweight sheet.

A preparation method of a physical foaming lightweight sheet comprises the following steps:

the sheet material in the examples of the present application was polyethylene terephthalate, wherein the polyethylene terephthalate was obtained from Kadaler plastics materials Co., Ltd, Dongguan, having a product number of 530.

S1: melting raw materials:

melting the material of the intermediate layer 11: adding the material of the middle layer 11 into a double-screw extruder, heating and melting at 260-280 ℃, and then adding a pore nucleating agent into the screw extruder, wherein the pore nucleating agent in the embodiment is graphite powder to obtain a middle melt;

melting the material of the side layer 12: adding the material of the side layer 12 into a single-screw extruder, and heating and melting at 260-280 ℃ to obtain a side melt;

s2: physical blowing agent injection

After the intermediate melt is plasticized, starting an inflating device 3, controlling the inflating pressure of a physical foaming agent to be 20MPa, injecting the physical foaming agent into the intermediate melt, wherein the physical foaming agent is carbon dioxide in the embodiment, and finally obtaining the melt containing the foaming agent, wherein the mass ratio of the physical foaming agent to the total amount of the sheet raw materials is 5%;

s3: shaping of

And (2) introducing the side melt and the melt containing the foaming agent into a forming die by using a melt distributor, so that the side melt is positioned on two sides of the melt containing the foaming agent, then demolding, recovering the normal pressure of the melt containing the foaming agent during demolding, and foaming the physical foaming agent under the action of high temperature to obtain the lightweight sheet.

Examples 2 to 11

Examples 2 to 11 differ from example 1 in the expansion ratio of the sheet, the diameter of cells 2 and the cell density of the sheet, as shown in the following table.

TABLE 1

Examples 12 to 15

Examples 12-15 differ from example 1 in that: the ratio of the thickness of the middle layer 11 to the total thickness of the two side layers 12 is different, as shown in the following table.

Comparative examples 1 to 2

Comparative examples 1-2 differ from example 1 in the foaming ratio of the sheet and the diameter of cells 2, as specified in the following table:

TABLE 3

Comparative example 3 differs from example 1 in that comparative example 3 does not contain microbubbles and therefore does not contain a step of injecting a material blowing agent.

And (3) performance detection:

the tear strengths of examples 1-10 and comparative examples 1-7 were tested and the results are shown in the following table.

TABLE 4

According to the detection results of the examples 1-3 and the comparative example 3, the sheet material provided by the invention can reduce the weight of the sheet material and simultaneously enable the sheet material to still have good mechanical properties by arranging proper micro bubbles, wherein the sheet material of the example 1 has the best performance.

As can be seen from examples 4-8, when the cell density is changed, the corresponding diameter of the micro-bubbles is also changed, and the magnitude of the cell density affects the tear strength of the sheet material, and the tear strength of the sheet material is better as the cell density is larger. Meanwhile, as can be seen from the test results of comparative examples 1 and 2, if the cell density and cell diameter are not well controlled, even if the weight of the sheet is reduced, the mechanical properties of the sheet are deteriorated, which affects the use of the sheet.

From the results of the tests of examples 9 to 11, it is understood that the influence of the expansion ratio on the sheet properties is not large, and the combination of examples 4 to 8 shows that the size of the cell density and the size of the cell diameter influence each other.

From the results of the tests of example 1 and examples 12 to 15, it is understood that the ratio of the thickness of the intermediate layer 11 to the total thickness of the both side layers 12 has a certain influence on the tear strength of the sheet, wherein the ratio of the thickness of the intermediate layer 11 to the total thickness of the both side layers 12 of example 1 is the optimum value.

The present application thus reduces the weight of the sheet material by providing suitable microbubbles while maintaining the mechanical properties of the sheet material constant or substantially constant. Moreover, the sheet material is light in weight, so that the transportation cost and the use cost of the sheet material are reduced, and the economic and environmental benefits are greater.

The above embodiments are preferred embodiments of the present application, and the protection scope of the present application is not limited by the above embodiments, so: all equivalent changes made according to the structure, shape and principle of the present application shall be covered by the protection scope of the present application.