Foam sheet and method for producing the same

Abstract

The application discloses a kind of foam sheet and preparation method thereof, relate to foam manufacturing field, and the foam sheet includes crosslinking resin layer, independent pore layer and open cell layer;Crosslinking resin layer, independent pore layer are respectively formed by unfoamed, foamed crosslinking polyolefin resin, independent pore layer has closed and not intercommunicating pore formed by foaming in it;Open cell layer is formed by foaming or foaming and unfoamed crosslinking polyolefin resin, and open cell layer has cavity communicating with outside in it;Open cell layer is located at at least one of upper surface and lower surface of foam sheet along the thickness direction distribution;The starting temperature of the first weight loss stage of thermal gravimetric analysis of foam sheet is 150 DEG C to 400 DEG C;The average pore number T / R of foam sheet in parallel to the thickness direction satisfies: 1.5<=T / R<=10;The average pore diameter of independent pore layer in parallel to the thickness direction is 100 μm to 500 μm.The foam sheet of the application embodiment has excellent shock absorbing performance and buffering performance.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of foam manufacturing technology, and in particular to a foam sheet and a preparation method thereof. Background Art

[0002] With the rapid development of mobile communication technology and the widespread use of smartphones, mobile phones are no longer limited to making calls, but have become an indispensable multimedia playback tool in people's daily lives. In particular, users have become increasingly demanding in terms of the sound quality of their mobile phones, expecting to enjoy high-quality listening experience similar to that of professional audio equipment. However, when playing music or voice at high volume, existing smartphones often experience unnecessary resonance and vibration on the back of the phone due to the vibration of the speakers. This not only interferes with the purity of the sound quality and reduces listening comfort, but may also accelerate the aging of internal components and shorten the overall lifespan of the phone. In particular, the dual-speaker stereo speaker system that has been widely adopted in recent years, while improving the three-dimensional level of the sound effect, also increases the risk of resonance and may even cause sealing issues, such as sound leakage, which is considered a key factor in excellent sound cavity design.

[0003] Foam has traditionally been used as a shock-absorbing material due to its excellent cushioning properties and long-lasting durability, which can mitigate shock and vibration to a certain extent on mobile phone components. The pore structure of foam relies on its internal cellular structure to absorb and dissipate sound waves, providing a certain degree of sound insulation and shock absorption through vibration and friction loss mechanisms. However, in practical applications, simple foam materials face numerous challenges: while the shock absorption effect of single- or multi-layer independent pore structures theoretically increases with the number of layers, this also leads to increased thickness and increased compressive stress. Under continuous compression, the pores are prone to excessive deformation, resulting in a decrease in shock absorption performance. Furthermore, the permanent deformation after compression is also significant, affecting its long-term effectiveness. Furthermore, while open-cell foam exhibits excellent cushioning properties, it is unable to effectively control the air cavity vibrations generated by the speaker. This cannot effectively prevent airflow vibrations from being transmitted to the phone's back cover and may even exacerbate the vibration of the outer shell.

[0004] Therefore, the industry urgently needs to develop a new solution that can effectively suppress the transmission of speaker vibrations to the back cover of the mobile phone. Summary of the Invention

[0005] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of protection of this application.

[0006] The embodiment of the present application provides a foam sheet and a preparation method thereof. The foam sheet has excellent shock-absorbing and cushioning properties. It can not only effectively reduce the vibration of the gas cavity, but also has a small permanent deformation after compression, and the shock-absorbing performance has long-term effectiveness.

[0007] The embodiment of the present application provides a foam sheet, which is formed of a cross-linked polyolefin resin and includes: a cross-linked resin layer, an independent pore layer, and an open-pore layer; the cross-linked resin layer is formed of unfoamed cross-linked polyolefin resin; the independent pore layer is formed of foamed cross-linked polyolefin resin, and has closed and mutually unconnected pores formed by foaming; the open-pore layer is formed of foamed or foamed and unfoamed cross-linked polyolefin resin, and has cavities in communication with the outside of the foam sheet; the open-pore layer is located on at least one of the upper surface and the lower surface of the foam sheet distributed in the thickness direction;

[0008] The starting temperature of the first weight loss stage of the thermogravimetric analysis of the foam sheet is between 150° C. and 400° C.;

[0009] The average number of pores in the foam sheet parallel to the thickness direction is T / R, and T / R satisfies:

[0010] 1.5≤T / R≤10;

[0011] Wherein, T is the thickness of the foam sheet, R is the average pore diameter of the foam sheet in the direction parallel to the thickness, and the units of T and R are the same;

[0012] The average pore diameter of the independent pore layer in a direction parallel to the thickness is 100 μm to 500 μm.

[0013] In some embodiments of the present application, the average density of the foam sheet is 0.025 g / cm 3 to 0.1g / cm 3 .

[0014] In some embodiments of the present application, the pore diameter range of the independent pore layer in the direction parallel to the thickness is ≤650 μm;

[0015] The thickness of the foam sheet is 0.15 mm to 4.5 mm, and the thickness range of the foam sheet is ≤0.1 mm.

[0016] In some embodiments of the present application, the thickness of the foam sheet is 0.15 mm to 3 mm.

[0017] In some embodiments of the present application, the thickness of the foam sheet is 0.15 mm to 1.2 mm.

[0018] In some embodiments of the present application, the cross-linking degree of the foam sheet is 10% to 60%.

[0019] In some embodiments of the present application, the open hole ratio on the surface of the open hole layer is ≥90%.

[0020] In some embodiments of the present application, the open hole ratio on the surface of the open hole layer is ≥95%.

[0021] In some embodiments of the present application, the through-porosity between the independent pore layer and the open-pore layer is ≤20%.

[0022] In some embodiments of the present application, the compressive stress of the foam sheet under 15% compression deformation is 1 KPa to 50 KPa; the compressive stress under 70% compression deformation is 25 KPa to 700 KPa.

[0023] In some embodiments of the present application, when the compression deformation varies in the range of 15% to 75%, the maximum increment of the compression stress is 10 KPa to 60 KPa for every 1% increase in the compression deformation.

[0024] In some embodiments of the present application, the cross-linked resin layer is in contact with the independent pore layer, and the open pore layer is in contact with the independent pore layer; the open pore layer is obtained by opening a partial area of ​​the independent pore layer or a partial area of ​​the independent pore layer and the cross-linked resin layer.

[0025] In some embodiments of the present application, the foam sheet comprises:

[0026] a cross-linked resin layer, an independent pore layer, and an open-pore layer, which are arranged in the order of the cross-linked resin layer, the independent pore layer, and the open-pore layer; or

[0027] Two cross-linked resin layers, two independent pore layers and one open pore layer, which are arranged in the order of cross-linked resin layer, independent pore layer, cross-linked resin layer, independent pore layer and open pore layer; or

[0028] A cross-linked resin layer, two independent pore layers and two open pore layers are arranged in the order of open pore layer, independent pore layer, cross-linked resin layer, independent pore layer and open pore layer.

[0029] In some embodiments of the present application, the cross-linked polyolefin resin is a cross-linked product of one or more polyolefin resin raw materials;

[0030] The polyolefin resin raw material includes any one or more of polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-butene copolymer, rubber and thermoplastic elastomer.

[0031] In some embodiments of the present application, the cross-linked polyolefin resin is a cross-linked product of at least two polyolefin resin raw materials;

[0032] The mass fraction of the n-hexane extract of the polyolefin resin raw material is 1% to 8.5%, the tensile strength is 15MPa to 35MPa, the elongation is 500% to 900%, and the softening temperature is 70°C to 100°C.

[0033] The present invention also provides a method for preparing the foam sheet as described above, the method comprising:

[0034] Mixing a polyolefin resin raw material with a foaming agent and an optional functional additive, and extruding to obtain a shaped sheet;

[0035] The shaped sheet is cross-linked, modified, and foamed to form a cross-linked resin layer and an independent pore layer having pores, thereby obtaining a foamed sheet comprising the cross-linked resin layer, the independent pore layer, and the cross-linked resin layer in sequence;

[0036] At least one side of the foam sheet is opened to form the open-hole layer, thereby obtaining the foam sheet.

[0037] In some embodiments of the present application, the step of opening holes on at least one side of the foam sheet to form the open-hole layer includes:

[0038] The cross-linked resin layer and part of the independent pore layer on one side of the foam sheet are removed to expose part of the pores in the independent pore layer, so that part of the pores in the independent pore layer are converted into cavities connected to the outside, and part of the area of ​​the independent pore layer having the cavities is converted into the open-pore layer.

[0039] In some embodiments of the present application, the step of opening holes on at least one side of the foam sheet to form the open-hole layer includes:

[0040] A cavity communicating with the outside is formed on at least one side of the foam sheet, and the cavity extends into the independent pore layer, so that a partial area of ​​the independent pore layer having the cavity and the cross-linked resin layer penetrated by the cavity are converted into the open-pore layer.

[0041] In some embodiments of the present application, the preparation method further comprises: after obtaining the foam sheet and before opening holes on at least one side of the foam sheet,

[0042] combining multiple foam sheets together to obtain a composite sheet;

[0043] The step of opening holes on at least one side of the foam sheet to form the open-hole layer includes: opening holes on at least one side of the composite sheet to form the open-hole layer.

[0044] The foam sheet of the present embodiment comprises an independent pore layer and an open-pore layer. The open-pore layer provides excellent cushioning performance and adaptively deforms when compressed, allowing the independent pore layer to maintain a stable shape and exhibit good shock absorption performance. The independent pore layer effectively absorbs sound waves and attenuates vibrations while avoiding the enhanced sound wave reflection caused by continuous pores. This allows the foam sheet of the present embodiment, when filled into a mobile phone back cover, to effectively suppress resonance within the back cover across different frequency ranges.

[0045] Other features and advantages of the present application will be described in the following description, and in part will become more apparent from the description, or understood by practicing the present application. Other advantages of the present application can be realized and obtained by the solutions described in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The accompanying drawings are used to provide an understanding of the technical solution of the present application and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the technical solution of the present application and do not constitute a limitation on the technical solution of the present application.

[0047] Figure 1 This is a schematic structural diagram of a foam sheet according to an exemplary embodiment of the present application;

[0048] Figure 2 for Figure 1 Optical microscope image of the actual foam sheet shown;

[0049] Figure 3 This is a schematic structural diagram of another foam sheet according to an exemplary embodiment of the present application;

[0050] Figure 4 for Figure 3 Optical microscope image of the actual foam sheet shown;

[0051] Figure 5 This is a schematic structural diagram of another foam sheet according to an exemplary embodiment of the present application;

[0052] Figure 6 Schematic diagram of the structure of closed hole and through hole.

[0053] The meanings of the various symbols in the accompanying drawings are:

[0054] 10 - cross-linked resin layer; 20 - independent pore layer; 21 - pores; 30 - open pore layer; 31 - cavity. DETAILED DESCRIPTION

[0055] This application describes multiple embodiments, but this description is exemplary rather than restrictive, and it is obvious to those skilled in the art that there may be more embodiments and implementations within the scope of the embodiments described in this application. Although many possible feature combinations are shown in the drawings and discussed in the detailed description, many other combinations of the disclosed features are also possible. Unless specifically limited, any feature or element of any embodiment may be used in combination with any other feature or element in any other embodiment, or may replace any other feature or element in any other embodiment.

[0056] The present application includes and contemplates combinations of features and elements known to those of ordinary skill in the art. The embodiments, features, and elements disclosed in this application may also be combined with any conventional features or elements to form a unique inventive solution. Any features or elements of any embodiment may also be combined with features or elements from other inventive solutions to form another unique inventive solution. Therefore, it should be understood that any feature shown and / or discussed in this application may be implemented individually or in any appropriate combination. Therefore, except for the limitations made according to the appended claims and their equivalents, the embodiments are not subject to other limitations. In addition, various modifications and changes may be made within the scope of protection of the appended claims.

[0057] In addition, when describing representative embodiments, the specification may have presented the method and / or process as a specific sequence of steps. However, to the extent that the method or process does not rely on the specific order of the steps described herein, the method or process should not be limited to the steps in the specific order described. As will be understood by those skilled in the art, other orders of steps are also possible. Therefore, the specific order of the steps set forth in the specification should not be interpreted as a limitation to the claims. In addition, the claims for the method and / or process should not be limited to performing their steps in the order written, and those skilled in the art can readily understand that these orders can be changed and still remain within the spirit and scope of the embodiments of the present application.

[0058] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0059] An embodiment of the present application provides a foam sheet. Figure 1 This is a schematic structural diagram of a foam sheet according to an exemplary embodiment of the present application; Figure 2 for Figure 1 Optical microscope image of the actual foam sheet shown; Figure 3 This is a schematic structural diagram of another foam sheet according to an exemplary embodiment of the present application; Figure 4 for Figure 3 Optical microscope image of the actual foam sheet shown; Figure 5 This is a schematic structural diagram of another foam sheet according to an exemplary embodiment of the present application.

[0060] The foam sheet is formed of a cross-linked polyolefin resin, such as Figures 1 to 5 As shown, the foam sheet includes: a cross-linked resin layer 10, an independent pore layer 20 and an open-pore layer 30; the cross-linked resin layer 10 is formed of the unfoamed cross-linked polyolefin resin; the independent pore layer 20 is formed of the foamed cross-linked polyolefin resin, and has closed and mutually unconnected pores 21 formed by foaming; the open-pore layer 30 is formed of the foamed cross-linked polyolefin resin, or of the foamed and unfoamed cross-linked polyolefin resin, and has cavities 31 in communication with the outside of the foam sheet; the open-pore layer 30 is located on at least one of the upper surface and the lower surface of the foam sheet distributed in the thickness direction;

[0061] The starting temperature of the first weight loss stage of the thermogravimetric (TG) analysis of the foam sheet is between 150° C. and 400° C.;

[0062] The average number of pores in the foam sheet parallel to the thickness direction (i.e., ZD direction) is T / R, and T / R satisfies:

[0063] 1.5≤T / R≤10;

[0064] Wherein, T is the thickness of the foam sheet, R is the average pore diameter of the foam sheet in the ZD direction, and the units of T and R are the same;

[0065] The average pore diameter of the independent pore layer in the ZD direction is 100 μm to 500 μm.

[0066] The foam sheet of the present embodiment comprises an independent pore layer and an open-pore layer. The open-pore layer provides excellent cushioning performance and adaptively deforms when compressed, allowing the independent pore layer to maintain a stable shape and exhibit good shock absorption performance. The independent pore layer effectively absorbs sound waves and attenuates vibrations while avoiding the enhanced sound wave reflection caused by continuous pores. This allows the foam sheet of the present embodiment, when filled into a mobile phone back cover, to effectively suppress resonance within the back cover across different frequency ranges.

[0067] In some embodiments of the present application, the pores in the independent pore layer are evenly distributed and independent of each other, which is conducive to achieving effective sound wave absorption and vibration attenuation.

[0068] Open / closed cell structure

[0069] Closed-cell foam absorbs and dissipates sound waves through its internal closed-cell structure, providing a certain degree of sound insulation and shock absorption through vibration and friction loss mechanisms. However, with continued compression, the closed-cell foam is prone to excessive deformation, resulting in a decrease in shock absorption performance. Furthermore, the permanent deformation after compression is significant, affecting its long-term effectiveness.

[0070] Although the open-pore foam has excellent cushioning properties, it is unable to control the gas cavity vibration generated when the speaker is working. It cannot effectively prevent the airflow vibration from being transmitted to the back cover of the mobile phone, but may even aggravate the vibration of the shell.

[0071] The foam sheet of the present invention combines closed-cell and open-cell structures. The pores in the independent pore layer are closed and interconnected, known as closed-cell; the cavities in the open-cell layer communicate with the exterior of the foam sheet, known as open-cell. When compressed, this foam sheet first undergoes adaptive deformation in the open-cell layer, allowing the subsequent independent pore layer to maintain a relatively stable shape, significantly reducing the occurrence of compression set.

[0072] The starting temperature of the first weight loss stage of TG analysis

[0073] This temperature generally corresponds to the point at which the material begins to experience significant mass loss.

[0074] The starting temperature of the first weight loss stage is too low, which means that the resin begins to lose mass at a lower processing temperature, which will affect the pore structure strength and pore size distribution of the foaming material and affect the shock absorption performance.

[0075] First, if the starting temperature of the first weight loss stage in the TG analysis of the foam sheet is too low, the structural strength of the pores will be reduced during the high-temperature foaming process, making it easy for "bubbles" to form. This will increase the range of pores, make the overall pores uneven, and reduce the cushioning and shock absorption performance of the foam material. Secondly, if the starting temperature of the first weight loss stage is too low, the material stability will be poor, and performance degradation will be likely during use. The gas in the independent pores will be more likely to escape during compression, resulting in poor permanent compression set performance of the foam sheet, which is not conducive to maintaining stable shock absorption performance during long-term use.

[0076] The starting temperature of the first weight loss stage is relatively high, which indirectly reflects that the material structure is more stable and the pore structure is not easily affected by changes in ambient temperature. This is beneficial to the long-term stability and durability of the shock-absorbing material. However, an excessively high thermal weight loss starting temperature indicates that it may have a higher degree of cross-linking or a more regular molecular structure, which will greatly affect the flexibility of the material's molecular chain, making it impossible or difficult to absorb the energy generated by vibration through the movement or rotation of chain segments, resulting in a weakening of the sound insulation and shock absorption performance of the foam material.

[0077] The starting temperature of the first weight loss stage of the thermal gravimetric analysis of the foam sheet of the embodiment of the present application is in the range of 150°C to 400°C. It has good pore strength, uniform and stable pore structure, low compression permanent deformation, and can provide long-term and stable sound insulation and shock absorption effects.

[0078] In the embodiment of the present application, the average pore number of the foam sheet in the ZD direction is T / R, and T / R can satisfy:

[0079] 1.5≤T / R≤10;

[0080] Wherein, T is the thickness of the foam sheet, and R is the pore diameter of the foam sheet in the ZD direction.

[0081] For example, T / R can be 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10.

[0082] If T / R is too small, during the compression process, the increment of compressive stress increases rapidly with the increase of compressive stress, and the compression curve is not flat; if T / R is too large, under the condition of continuous compression use, the pores are prone to excessive deformation, resulting in a decrease in shock absorption performance, and the permanent deformation after compression is also large, affecting its long-term effectiveness.

[0083] In some embodiments of the present application, the thickness of the foam sheet may be 0.15 mm to 4.5 mm. For example, the thickness of the foam sheet may be 0.15 mm to 3 mm. For another example, the thickness of the foam sheet may be 0.15 mm to 1.2 mm. For another example, the thickness of the foam sheet may be 0.15 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, or 4.5 mm.

[0084] In some embodiments of the present application, the average density of the foam sheet can be 0.025 g / cm 3 to 0.1g / cm 3 When the average density of the foam sheet is 0.025 g / cm 3 to 0.1g / cm 3 It is helpful to control the size and uniformity of pores.

[0085] In some embodiments of the present application, the average pore diameter of the independent pore layer in the ZD direction is 100μm to 500μm, the pore diameter range is ≤650μm, and the thickness range of the foam sheet is ≤0.1mm. Meeting these conditions is conducive to controlling the uniformity of the pores and enhancing the cushioning and shock-absorbing effect of the foam sheet.

[0086] In order to further improve the softness of the foam sheet and control the chip falling problem in subsequent processing, the cross-linking degree of the foam sheet is preferably 10% to 60%.

[0087] In some embodiments of the present application, the open-pore ratio of the surface of the open-pore layer of the foam sheet is ≥90%, preferably ≥95%. When the open-pore ratio of the surface of the open-pore layer is ≥90%, it is conducive to a gentle compression curve and can provide excellent cushioning performance. When compressed, the open-pore layer can undergo adaptive deformation, allowing the independent pore layer to maintain a stable shape and have good shock absorption performance.

[0088] In some embodiments of the present application, the porosity between the independent pore layer and the open-pore layer of the foam sheet is ≤20%. When the porosity is greater than 20%, the open-pore layer deforms excessively when compressed, and the independent pore layer cannot maintain a stable shape, resulting in reduced shock absorption performance.

[0089] In the description of this application, the through pores in the term "through porosity" are distinguished from the concept of independent pores (also called closed pores) in an independent pore layer. Independent pores refer to the pores inside the material that are independent, each pore is separated by a closed wall membrane and is not interconnected with other pores, such as Figure 6 As shown in the pore B in the figure; through pores refer to pores inside the material that are not completely separated by a closed wall membrane, and there is at least one channel connected to other pores, such as Figure 6 As shown in the pore A.

[0090] In some embodiments of the present application, the compressive stress of the foam sheet under 15% compression deformation may be 1 KPa to 50 KPa; the compressive stress under 70% compression deformation may be 25 KPa to 700 KPa.

[0091] In some embodiments of the present application, when the compression deformation of the foam sheet varies within the range of 15% to 70%, the maximum increment of the compression stress is 10 KPa to 60 KPa for every 1% increase in the compression deformation.

[0092] Here, every 1% increase in compression deformation means that the compression deformation increases according to the rule of 15%, 16%, 17%, 18%, ... 70%; the "increment in compression stress" refers to the compression stress corresponding to the previous 1% compression deformation.

[0093] When the compression deformation of the foam sheet increases by 1%, the maximum increment of the compression stress is in the range of 10KPa to 60KPa, which is conducive to the foam sheet maintaining a close and low compression strength under different compression amounts. When the foam sheet is filled in the back shell of the mobile phone, it can protect the internal components of the mobile phone from being damaged.

[0094] In some embodiments of the present application, Figures 1 to 5 As shown, the cross-linked resin layer 10 is in contact with the independent porous layer 20, and the open-pored layer 30 is in contact with the independent porous layer 20; the open-pored layer 30 can be obtained by opening a portion of the independent porous layer 20, or the open-pored layer 30 can be obtained by opening a portion of the independent porous layer 20 and the entire cross-linked resin layer, but the opening does not destroy the integrity of the pores in the independent porous layer 20.

[0095] In some embodiments of the present application, Figure 1 and Figure 2 As shown, the foam sheet may include:

[0096] A cross-linked resin layer 10 , an independent pore layer 20 and an open-pore layer 30 are arranged in the order of the cross-linked resin layer 10 , the independent pore layer 20 and the open-pore layer 30 .

[0097] In some embodiments of the present application, Figure 3 and Figure 4 As shown, the foam sheet may include:

[0098] Two cross-linked resin layers 10 , two independent pore layers 20 and one open pore layer 30 are arranged in the order of cross-linked resin layer 10 , independent pore layer 20 , cross-linked resin layer 10 , independent pore layer 20 and open pore layer 30 .

[0099] In some embodiments of the present application, Figure 5 As shown, the foam sheet may include:

[0100] A cross-linked resin layer 10 , two independent pore layers 20 and two open pore layers 30 are arranged in the order of open pore layer 30 , independent pore layer 20 , cross-linked resin layer 10 , independent pore layer 20 and open pore layer 30 .

[0101] In some embodiments of the present application, the cross-linked polyolefin resin may be a cross-linked product of one or more polyolefin resin raw materials. The foam sheet of the embodiment of the present application is formed by a polyolefin resin that is easily cross-linked, which has excellent heat resistance, weather resistance and mechanical strength, and can maintain a stable structural morphology and good cushioning and shock absorption performance under long-term pressure and high and low temperature environments. The above-mentioned polyolefin resin can achieve a cross-linking degree of 10% to 60% under any one or more cross-linking methods of chemical cross-linking, covalent cross-linking, ionic cross-linking, interpenetrating network, thermoplastic dynamic cross-linking, electron beam radiation cross-linking, gamma-ray radiation cross-linking, ultraviolet light cross-linking, enzyme-catalyzed cross-linking and wet heat cross-linking.

[0102] Polyolefin resin

[0103] In some embodiments of the present application, the polyolefin resin raw material may include polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, ethylene-propylene copolymer and ethylene-butene copolymer, etc., preferably polyethylene resin.

[0104] The selection of polyethylene resin is not particularly limited, including but not limited to low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, etc. Optionally, ethylene-α-olefin copolymers with ethylene as the main component can also be selected, wherein the α-olefin is selected from α-olefins such as propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-ethyl-1-pentene, 1-octene, 1-decene, and 1-undecene, each having 2 to 12 carbon atoms. The number of species of such α-olefins can be only one, or can be two or more. The selection of the above polyethylene resins can be used alone or in combination of two or more. Low-density polyethylene is preferred.

[0105] Examples of the polypropylene resin include propylene homopolymers, propylene-ethylene copolymers, propylene-ethylene-α-olefin copolymers, and propylene-α-olefin copolymers. These may be used alone or in combination of two or more. Specifically, the α-olefins constituting the propylene-α-olefin copolymers include 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Among these, α-olefins having 6 to 12 carbon atoms are preferred. For the propylene copolymer, the propylene content in the copolymer is 50 wt% or more.

[0106] The polyolefin resin may further contain a rubber and / or thermoplastic elastomer having a glass transition temperature of 20°C or less. Specific examples include natural or synthetic rubbers such as natural rubber, polyisobutylene, isoprene rubber, butyl rubber, chloroprene rubber, and nitrile rubber; olefin elastomers such as ethylene-vinyl acetate copolymer, polybutylene, polyisobutylene, and chlorinated polyethylene; styrene elastomers such as styrene-isoprene-styrene copolymer (SIS), styrene-butadiene-styrene copolymer (SBS), styrene-isoprene-butadiene-styrene copolymer (SIBS), and hydrogenated polymers thereof; thermoplastic polyester elastomers; thermoplastic polyurethane elastomers; and thermoplastic acrylic elastomers. The number of types of these rubbers and / or thermoplastic elastomers may be one, or two or more.

[0107] In some embodiments of the present application, the cross-linked polyolefin resin may be a cross-linked product of at least two polyolefin resin raw materials.

[0108] It should be noted that regardless of the type of polyolefin resin selected, whether an elastomeric material is added, and whether other functional additives are added, after the polyolefin-based base resin is blended, the mass fraction of n-hexane extract is preferably 1% to 8.5%, the tensile strength can be 15 MPa to 35 MPa, the elongation can be 500% to 900%, and the softening temperature (Vicat softening point) is 70°C to 100°C.

[0109] The properties of a resin are influenced by its chemical composition and microstructure, which determine how the resin performs in different applications.

[0110] Hexane extract

[0111] Hexane extracts refer to the components of the resin that are soluble in hexane. These components are generally low-molecular-weight compounds. These substances can affect the resin's radiation cross-linking and melt foaming processes, thereby affecting the structural strength of the pores. They can also affect the starting temperature of the foamed material's thermal weight loss.

[0112] In polyolefin resins, n-hexane extract refers to the soluble substances obtained by extracting the polyolefin resin sample using n-hexane as a solvent under certain conditions (see Chinese National Standard GB / T5009.58-2003).

[0113] The components of the soluble matter may include: 1. Unreacted monomers: residues of ethylene or other monomers that may not be completely polymerized during the production of polyolefin resins; 2. Low molecular weight polymers: polyethylene fragments with relatively low molecular weight formed in the polymerization reaction; 3. Catalyst residues: residues of the catalyst used in the polymerization reaction and its by-products, etc.

[0114] The influence of low molecular weight polymers on the cross-linking of polyolefin resins: The cross-linking of polyolefin resins mainly occurs between the main chain molecules of the polyolefin resin, rather than on the oligomer molecules. Too high a low molecular weight polymer content may interfere with the main chain cross-linking, resulting in reduced melt strength and large pores; too low a low molecular weight polymer content will make the melt fluidity of the resin poor, which will have a certain impact on the process flow; when the low molecular weight polymer content is appropriate, the molecular weight distribution of the cross-linked polyolefin resin is uniform and the main chain cross-linking is more sufficient, which will have better tensile strength, compressive strength, resilience, etc.

[0115] Tensile strength and elongation of polyolefin resin raw materials

[0116] Tensile strength refers to the maximum stress that a material can withstand under a tensile load. When the strength of the resin is too low, the possibility of open holes appearing in the cross-linked resin layer during foaming increases, and a complete cross-linked resin layer structure cannot be formed. The pore wall structure formed by foaming a high-strength resin is usually also strong. However, when the strength of the pore wall structure is too high, the restraining force on the bubble expansion process will also increase, and the average pore number T / R in the ZD direction may be too small.

[0117] In some embodiments of the present application, the polyolefin resin raw material may have a tensile strength of 15 MPa to 35 MPa, and an elongation of 500% to 900%.

[0118] Strength of polyolefin resin raw materials: including tensile strength / shear strength.

[0119] The high strength of polyolefin resin raw materials means that the foamed material is more resistant to damage under cutting forces. Stronger foam materials are less likely to break during cutting, resulting in cleaner cuts. However, this may also increase cutting resistance, requiring greater cutting force or sharper tools.

[0120] Hardness of polyolefin resin raw materials: High-strength foam materials tend to have relatively high hardness. The tool wears faster during cutting and chip loss may be more difficult to control.

[0121] Elongation / Ductility: Polyolefin resins have high elongation, indicating they can deform significantly when stretched without immediately breaking. Such foams can better adapt to tool pressure distribution during cutting, resulting in smoother chip removal and less chip loss. High elongation is often associated with good toughness. Tough foams are less likely to fracture brittlely during cutting, instead forming continuous, thick chips that help reduce debris.

[0122] Foam materials with high strength but low elongation tend to form a large amount of fine debris when cut. These debris are difficult to collect and may affect their use in electronic devices.

[0123] The foam material has moderate strength and high elongation. The chips produced during cutting are relatively complete, which is easy to clean and recycle, and can also reduce the risk of damage to equipment and molds.

[0124] Vicat softening point of polyolefin resin raw materials

[0125] The Vicat softening point of a resin is a measure of the temperature at which the resin transitions from a solid state to a plastic state when heated. It is also known as the softening temperature. The Vicat softening point reflects the thermal stability and operating temperature range of the resin. Generally speaking, the higher the molecular weight and the greater the degree of crosslinking, the higher the Vicat softening point. Resins with lower softening points are more likely to form a dense crosslinked network, but their ability to bind gas generated in pores during foaming is less. Selecting a resin with an appropriate Vicat softening point makes it easier to control the pore structure of the foamed material within the designed range.

[0126] In some embodiments of the present application, the Vicat softening point of the mixed resin may be 70°C to 100°C.

[0127] Influence on softness: Polyolefin resins with low Vicat softening points may exhibit softer properties at conditions close to or slightly above room temperature. If the Vicat softening point is higher, the foam material will remain harder at room temperature and can provide more support.

[0128] Influencing the compression curve: The Vicat softening point affects the compression increment and resilience of the foam material. Materials with lower Vicat softening points are more easily deformed during compression and have a flatter compression curve, while materials with higher Vicat softening points exhibit greater resistance to compression and slower deformation.

[0129] Impact on shock absorption, sound insulation, and cushioning performance: A foam's shock absorption, sound insulation, and cushioning capabilities are closely related to its hardness and elasticity. Resins with low Vicat softening points, when foamed, are softer and more easily deformed, typically absorbing and dispersing external forces more effectively, providing excellent shock absorption. For sound insulation applications, softer materials can better seal gaps and prevent sound transmission.

[0130] During the processing, if the Vicat softening point is too low, the heat generated during cutting will easily destroy the open-cell structure, resulting in unstable thickness of the foamed sheet. In addition, the friction during cutting may cause more material to fall off, making it more likely to produce debris. It is not easy to open holes during needle punching. If the Vicat softening point is too high, the foaming ratio will be low, the softness will be reduced, and the rebound performance will be poor, but the cutting may be cleaner and less chipping.

[0131] Influence on thermal stability: Resins with higher Vicat softening points usually have better thermal stability. The foamed products can maintain good morphological stability and mechanical properties at high temperatures. Resin foam materials with high Vicat softening points have relatively good dimensional stability and durability under long-term heat or stress environments, and are not easily deformed or subject to performance degradation due to temperature changes.

[0132] Pore ​​uniformity: A Vicat softening point that is too high may result in low resin fluidity during foaming, affecting gas diffusion and the formation of pore structure, and may lead to problems such as uneven pores and density distribution deviation.

[0133] The present invention also provides a method for preparing the foam sheet as described above, the method comprising:

[0134] Mixing a polyolefin resin raw material with a foaming agent and an optional functional additive, and extruding to obtain a shaped sheet;

[0135] The shaped sheet is cross-linked, modified, and foamed to form a cross-linked resin layer and an independent pore layer having pores, thereby obtaining a foamed sheet comprising the cross-linked resin layer, the independent pore layer, and the cross-linked resin layer in sequence;

[0136] At least one side of the foam sheet is opened to form the open-hole layer, thereby obtaining the foam sheet.

[0137] In some embodiments of the present application, the opening of holes on at least one side of the foam sheet to form the open-pore layer includes: removing the cross-linked resin layer and part of the independent pore layer on one side of the foam sheet to expose part of the pores in the independent pore layer, so that part of the pores in the independent pore layer are converted into cavities connected to the outside, and part of the area of ​​the independent pore layer having the cavities is converted into the open-pore layer.

[0138] In some embodiments of the present application, the step of opening holes on at least one side of the foam sheet to form the open-hole layer includes:

[0139] A cavity communicating with the outside is formed on at least one side of the foam sheet, and the cavity extends into the independent pore layer, so that a partial area of ​​the independent pore layer having the cavity and the cross-linked resin layer penetrated by the cavity are converted into the open-pore layer.

[0140] In some embodiments of the present application, the preparation method further comprises: after obtaining the foam sheet and before opening holes on at least one side of the foam sheet,

[0141] combining multiple foam sheets together to obtain a composite sheet;

[0142] The step of opening holes on at least one side of the foam sheet to form the open-hole layer includes: opening holes on at least one side of the composite sheet to form the open-hole layer.

[0143] Furthermore, two foam sheets may be thermally laminated together, and then holes may be opened on the upper and lower surfaces or a single surface of the laminated foam sheet.

[0144] In some embodiments of the present application, the mixing of the polyolefin resin raw material with the foaming agent and the optional functional additive comprises:

[0145] blending and modifying at least two polyolefin resin raw materials by extrusion to obtain a mixed resin;

[0146] The mixed resin is mixed with a blowing agent and optionally a functional auxiliary agent.

[0147] In some embodiments of the present application, the preparation method may include:

[0148] blending and modifying at least two polyolefin resin raw materials by extrusion to obtain a mixed resin;

[0149] The mixed resin is mixed with a foaming agent and optionally a functional auxiliary agent, and extruded to obtain a shaped sheet;

[0150] The shaped sheet is cross-linked, modified, and foamed to form a cross-linked resin layer and an independent pore layer having pores, thereby obtaining a foamed sheet comprising the cross-linked resin layer, the independent pore layer, and the cross-linked resin layer in sequence;

[0151] At least one side of the foam sheet is opened to form the open-hole layer, thereby obtaining the foam sheet.

[0152] In some embodiments of the present application, the preparation method may include:

[0153] Mixing a polyolefin resin raw material with a foaming agent and an optional functional additive, and extruding to obtain a shaped sheet;

[0154] The shaped sheet is cross-linked, modified, and foamed to form a cross-linked resin layer and an independent pore layer having pores, thereby obtaining a foamed sheet comprising the cross-linked resin layer, the independent pore layer, and the cross-linked resin layer in sequence;

[0155] combining multiple foam sheets together to obtain a composite sheet;

[0156] At least one side of the composite sheet is opened to form the open-hole layer, thereby obtaining the foam sheet.

[0157] In some embodiments of the present application, before the polyolefin resin raw material is extruded and molded, other functional additives can be added according to actual needs to further improve the various properties of the polyolefin foam sheet. For example, antioxidants, antibacterial agents, colorants, antistatic agents, fillers and other functional additives can be added.

[0158] Examples of antioxidants include 2,2,4-trimethyl-1,2-dihydroquinoline polymer, 4,4'-dioctyldiphenylamine, pentaerythritol tetrakis-β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, Antioxidant B225, Antioxidant 445, and Antioxidant MBA. Among these, 2,2,4-trimethyl-1,2-dihydroquinoline polymer and 4,4'-dioctyldiphenylamine are more preferred.

[0159] Resins experience high temperatures during the compounding, modification, and foaming processes. High-temperature oxidation can cause molecular chain breakage, affecting the thermal weight loss onset temperature and pore structure stability of the foam. Antioxidants are added to the resin to prevent or delay oxidation. Antioxidants capture free radicals generated during polymer chain breakage, preventing the continued free radical chain reaction and further degradation. Studies have shown that their free radical-capturing ability eliminates free radicals during electron beam crosslinking, weakening the crosslinking structure and reducing gel content. Therefore, antioxidants such as 2,2,4-trimethyl-1,2-dihydroquinoline polymer and 4,4'-dioctyldiphenylamine are preferred in the formulation, as they are less susceptible to electron beam crosslinking.

[0160] The steps of each process are as follows:

[0161] Blending modification

[0162] In the blending and modification step, the blending and modification can be performed in an extruder to obtain a mixed resin.

[0163] Extrusion

[0164] In the extrusion molding step, the polyolefin resin is mixed with the resin, foaming agent, and functional additives and passed through an extruder head to form a continuous sheet with a specific thickness.

[0165] Cross-linking

[0166] In the embodiment of the present application, after the extrusion molding of the polyolefin resin is completed, a gelation reaction process, i.e., a cross-linking modification reaction, is carried out. Cross-linking can be carried out by currently known technologies, such as any one or more of chemical cross-linking, covalent cross-linking, ionic cross-linking, interpenetrating network, thermoplastic dynamic cross-linking, electron beam radiation cross-linking, γ-ray radiation cross-linking, ultraviolet light cross-linking, enzyme-catalyzed cross-linking and wet heat cross-linking; preferably, the cross-linking modification method is selected from any one or both of chemical cross-linking and electron beam radiation cross-linking.

[0167] Further preferably, the cross-linking method of the cross-linking modification is electron beam radiation cross-linking, and radiation cross-linking is performed by irradiating the resin sheet with ionizing radiation such as electron beams, α-rays, β-rays, and γ-rays to cross-link. The irradiation amount of the above-mentioned ionizing radiation is adjusted in such a way that the cross-linking degree of the obtained foam sheet becomes the above-mentioned desired range, for example, 10 Mrad to 30 Mrad, preferably 12 Mrad to 28 Mrad, and more preferably 16 Mrad to 21 Mrad. The energy of the radiation cross-linking affects the rate of cross-linking and is usually selected to be 1.0 Mev to 3.0 Mev, preferably 1.2 Mev to 2.8 Mev, and more preferably 1.5 Mev to 2.5 Mev.

[0168] Ionizing radiation exposure affects the cross-linking structure of the resin. When the exposure is too low, the cross-linking of the resin is too low, resulting in a lower starting temperature for the first weight loss stage in the thermogravimetric analysis (TG) test of the foamed resin. This can also lead to overly large and uneven pores during the foaming process, an increase in the through-porosity of the independent pore layer, and a thinning of the cross-linked resin layer, even with holes forming. Excessive radiation exposure increases the likelihood of chain scission and degradation of the resin molecular weight.

[0169] The chemical crosslinking agent is not particularly limited as long as it is a substance conventionally used in the production of foams. Specific examples include dicumyl peroxide, 2,4-dichlorobenzoyl peroxide, benzoyl peroxide, tert-butyl perbenzoate, cumyl hydroperoxide, tert-butyl hydroperoxide, 1,1-di(tert-butylperoxy)-3,3,5-trimethylhexane, 4,4-di(tert-butylperoxy)butyl valerate, α,α'-bis(tert-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3, tert-butylperoxide, and the like. The content of the crosslinking aid is preferably 0.1 to 2 wt% based on the total weight of the polyolefin resin raw material.

[0170] Normal pressure high temperature foaming

[0171] The crosslinked sheet-like polyolefin resin composition is heated to foam a thermally decomposable foaming agent. The thermally decomposable foaming agent is, for example, one having a decomposition temperature higher than the melting temperature of the resin. For example, an organic or inorganic chemical foaming agent having a decomposition temperature of 160°C to 270°C is used.

[0172] Examples of the organic foaming agent include azodicarbonamide, azodicarboxylic acid metal salts (barium azodicarboxylate, etc.), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, hydrazonodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide), hydrazine derivatives such as toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonylsemicarbazide.

[0173] Examples of the inorganic foaming agent include ammonium carbonate, sodium carbonate, ammonium bicarbonate, sodium bicarbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate.

[0174] Among these blowing agents, azo compounds and nitroso compounds are preferred from the perspectives of obtaining fine bubbles, economic efficiency, and safety. Azodicarbonamide, azobisisobutyronitrile, and N,N'-dinitrosopentamethylenetetramine are more preferred, and azodicarbonamide is even more preferred. Azodicarbonamide is preferred. Thermally decomposable blowing agents may be used alone or in combination of two or more.

[0175] As for the addition amount of the thermal decomposition type foaming agent, it is preferably 6 to 15 parts by weight relative to 100 parts by weight of the polyolefin resin raw material, and the ash content is ≤1%.

[0176] The content of foaming agent will affect the pore size and distribution of the foamed sheet. If the content of foaming agent is too low, the average pore number T / R in the ZD direction will be small, the compressive stress of the foamed sheet will be large, and the cushioning and shock absorption performance will be poor. If the content of foaming agent is too high, the heat generated during foaming will be large, and the formation of pores will be difficult to control, which will lead to an open-pore structure in the cross-linked resin layer and even fracture of the foamed sheet, making it impossible to produce foamed sheets with a thickness of less than 1.2mm.

[0177] The ash in the foaming agent will remain in the pores after the foaming agent decomposes. Excessive ash content will lead to an increase in the amount of foam vibration debris. The ash content in the foaming agent needs to be controlled within 1%.

[0178] The temperature for heating and foaming varies depending on the decomposition temperature of the thermally decomposable foaming agent. When an azo compound foaming agent is used, the foaming temperature is usually 140°C to 300°C, preferably 160°C to 260°C.

[0179] In addition, a foaming aid can be added to help adjust the decomposition temperature and decomposition rate of the foaming agent. The foaming aids compatible with the azo foaming agent include urea, phosphate, organic acid, and metal salt foaming aids, preferably metal salt foaming aids, and more preferably metal zinc salt foaming aids such as zinc oxide, stearic acid and zinc stearate.

[0180] In some embodiments of the present application, the pore-forming method may include any one or more of cutting, needling, abrasion, chemical etching, laser drilling, high-pressure water jet cutting, hot-pressing punching, and mechanical stretching perforation. These pore-forming methods can form a uniformly distributed pore layer with a certain degree of air permeability without compromising the overall strength and acoustic performance of the foam sheet.

[0181] During the cutting step, in order to ensure that the surface of the foamed sheet is flat, free of impurities and has no uneven appearance, the foamed material can be fed into the counter-pressing roller, and the roller pressure is adjusted to 0.2MPa to 0.8MPa to keep the sheet flat before cutting. The cutting machine equipment sets the linear cutting speed to 100 mm / s to 5000 mm / s to ensure that the cut surface is flat and the pore structure is not destroyed. Set the cooling water flow rate to 5L / min to 10L / min and the temperature to 5°C to 30°C to reduce heat accumulation during the cutting process and prevent the material from melting or deforming. Control the traction force to 10N to 50N and the traction speed to 0.5m / min to 5m / min, and obtain a foamed sheet having a cross-linked resin layer, an independent pore layer and an open-pore layer by cutting.

[0182] During the needling step, in order to ensure that the surface of the foamed sheet is smooth, free of impurities and has no uneven appearance, the gap between the needling rollers can be adjusted so that the puncture depth is 50μm to 100μm, the diameter of the needle is 0.08mm to 0.12mm, and the center distance between the needles is 0.4mm to 0.6mm. The foamed material passes through the needling machine, and the numerous fine needles on the needle plate quickly penetrate the material to form a continuous pore structure.

[0183] In the wearing step, the foaming material can be brought into contact with a surface grinding roller with a mesh size of 80 at a linear speed of 20m / min to 50m / min, the speed of the grinding roller is 1m / min to 5m / min, the pressure between the foaming material and the grinding roller is 0.1MPa to 0.3MPa, and the surface grinding roller destroys the cross-linked resin layer of the foaming material to form an open-pore structure.

[0184] The embodiments of the present application also provide the use of the foam sheet as described above as a shock-absorbing material.

[0185] In some embodiments of the present application, the use may include: filling the foam sheet in a cavity to reduce airflow vibration in the cavity.

[0186] The present application also provides a display assembly, comprising: a display panel, a middle frame, and a back cover, wherein the display panel, the middle frame, and the back cover enclose an inner cavity, the inner cavity being in communication with the outside of the display assembly; the inner cavity being filled with a foam sheet;

[0187] The foam sheet is formed of a cross-linked polyolefin resin, and includes: a cross-linked resin layer, an independent pore layer, and an open-pore layer; the cross-linked resin layer is formed of unfoamed cross-linked polyolefin resin; the independent pore layer is formed of foamed cross-linked polyolefin resin, and has closed and mutually unconnected pores formed by foaming; the open-pore layer is formed of foamed cross-linked polyolefin resin, and has cavities communicating with the outside of the foam sheet; the open-pore layer is located on at least one of the upper and lower surfaces of the foam sheet distributed in the thickness direction;

[0188] The starting temperature of the first weight loss stage of the thermogravimetric analysis of the foam sheet is between 150° C. and 400° C.;

[0189] The average number of pores in the ZD direction of the foam sheet is T / R, and T / R satisfies:

[0190] 1.5≤T / R≤10;

[0191] Wherein, T is the thickness of the foam sheet, R is the average pore diameter of the foam sheet in the ZD direction, and the units of T and R are the same;

[0192] The average pore diameter of the independent pore layer in the ZD direction is 100 μm to 500 μm.

[0193] In some embodiments of the present application, the display assembly further comprises at least one of a speaker, a vibration motor, and a circuit board;

[0194] The speaker is located in the inner cavity; the space between the speaker and the middle frame and / or the space between the speaker and the back cover is filled with the foam sheet;

[0195] The vibration motor is located in the inner cavity; the foam sheet is filled between the vibration motor and the middle frame and / or between the vibration motor and the back cover;

[0196] The circuit board is located in the inner cavity; the foam sheet is filled between the circuit board and the middle frame and / or between the circuit board and the back cover.

[0197] In some embodiments of the present application, the display component may include a shell having a cavity, a speaker located in the cavity, and a foam sheet filled in the cavity, wherein the speaker is fixed in a frame, the cavity in front of the frame transmits sound to the outside of the display component through a sound outlet, and the cavity in the rear is connected to the air inside the display component through a ventilation hole; the foam sheet is die-cut into a certain shape and fixed in part or all of the above-mentioned cavity by compression or polymer adhesive.

[0198] For example, the foam sheet of the embodiment of the present application can be used in the following parts of a mobile phone:

[0199] 1. Used between the speaker and the phone case, it can help fix the position of the speaker, prevent the speaker from shifting or loosening due to long-term vibration, maintain stable sound quality, absorb and disperse the vibration energy of the speaker, and prevent this energy from being directly transmitted to the phone case, thereby reducing resonance;

[0200] 2. Circular shock-absorbing foam around the base of the vibration motor: Located between the vibration motor and the phone case, it absorbs vibrations generated by the motor when it is working, reduces noise, prevents resonance of the housing, and ensures a secure installation of the motor, extending its service life.

[0201] 3. Used as a fully laminated shock-absorbing pad on the back of the circuit board: located between the circuit board (for example, the PCB motherboard) and the mobile phone housing, on the back or edge of the circuit board, using shock-absorbing foam sheets to isolate it to prevent the circuit board from being directly transmitted to the housing due to vibration, thereby reducing the potential risk of damage to components on the circuit board;

[0202] 4. Used as a cushioning foam embedded around the camera module to prevent motor vibration from affecting the delicate optical components and maintain shooting stability;

[0203] 5. Used between the battery and the back cover or middle frame of the mobile phone: between the battery and the back cover or middle frame of the mobile phone to absorb vibration and prevent the battery from moving inside the mobile phone or being damaged by vibration;

[0204] 6. Used at the contact point between the mobile phone screen and the frame: In order to avoid abnormal screen display or noise caused by vibration, the foam sheet of the embodiment of the present application can be set at the edge of the screen as a shock-absorbing material.

[0205] An embodiment of the present application further provides an electronic product, which includes the display device described above.

[0206] In some embodiments of the present application, the electronic products may include: smart mobile communication devices, laptops, liquid crystal displays, OLED displays, e-books, tablet terminals, gaming devices, cameras, wearable electronic devices, etc.

[0207] The technical solution of the embodiment of the present application is further explained below through the foam sheet of an exemplary embodiment and its preparation process.

[0208] The foam sheets of the following examples and comparative examples have the following characteristics: Figure 1 The structure shown in FIG. 1 is obtained by preparing the foam sheet by the following method.

[0209] Example 1

[0210] (1) Blending modification of resin: 40 parts by weight of low-density polyethylene (Sinopec LD100-AC), 40 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 20 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 3.52%, the tensile strength was 25.7 MPa, the elongation was 650%, and the softening temperature was 93.5°C;

[0211] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 10.5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent, 0.5 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, and 0.5 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and then extruded into a sheet through a twin-screw extruder;

[0212] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 17.3 Mrad to cross-link the resin sheet;

[0213] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0214] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.6 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 500 mm / s, a cooling water flow rate of 5 L / min and a temperature of 20°C, a traction force of 40 N, and a traction speed of 3 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0215] Example 2

[0216] In this embodiment, the shaped sheet is cross-linked and modified by chemical cross-linking.

[0217] (1) Blending modification of resin: 40 parts by weight of low-density polyethylene (Sinopec LD100-AC), 40 parts by weight of polyolefin elastomer A (DOW AFFINITY™ PF 1140G) and 20 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 3.52%, the tensile strength was 25.7 MPa, the elongation was 650%, and the softening temperature was 93.5°C;

[0218] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) are mixed with 10.5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent, 0.5 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, 0.5 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and 2 parts by weight of diisopropylbenzene peroxide, and the mixture is uniformly mixed and extruded into a sheet through a twin-screw extruder;

[0219] (3) Foaming: heating the cross-linked modified sheet obtained in step (2) to decompose and foam the foaming agent at a heating temperature of 220° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0220] (4) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.2 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 1000 mm / s, a cooling water flow rate of 5 L / min and a temperature of 5°C, a traction force of 10 N, and a traction speed of 5 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0221] Example 3

[0222] (1) Blending modification of resin: 20 parts by weight of low-density polyethylene (Sinopec LD100-AC), 20 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 60 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 8.35%, the tensile strength was 19.3 MPa, the elongation was 860%, and the softening temperature was 89.3°C;

[0223] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 10.5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent, 0.5 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, and 0.5 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and then extruded into a sheet through a twin-screw extruder;

[0224] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 18.5 Mrad to cross-link the resin sheet;

[0225] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0226] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.8 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 100 mm / s, a cooling water flow rate of 5 L / min and a temperature of 28°C, a traction force of 30 N, and a traction speed of 0.5 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0227] Example 4

[0228] (1) Blending modification of resin: 40 parts by weight of low-density polyethylene (Sinopec LD100-AC), 40 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 20 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 3.52%, the tensile strength was 25.7 MPa, the elongation was 650%, and the softening temperature was 93.5°C;

[0229] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent, 0.2 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, and 0.2 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and then extruded into a sheet through a twin-screw extruder;

[0230] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 19.7 Mrad to cross-link the resin sheet;

[0231] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0232] (5) Needle punching of foamed sheet: The foamed sheet is fed into the needling machine, and the gap between the needling rollers is adjusted so that the puncture depth is 80 μm, the diameter of the needle is 0.1 mm, and the center distance between the needles is 0.2 to 0.5 mm. The foamed material passes through the needling machine, and the numerous fine needles on the needle plate quickly penetrate the material to form a continuous pore structure.

[0233] Example 5

[0234] (1) Blending modification of resin: 80 parts by weight of low-density polyethylene (Sinopec LD100-AC), 10 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 10 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 1.25%, the tensile strength was 16.7 MPa, the elongation was 580%, and the softening temperature was 95.7°C;

[0235] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 15.6 parts by weight of azodicarbonamide (gas generation volume of 216 ml / g, ash content of 0.5%) as a foaming agent, 0.8 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, and 0.8 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and then extruded into a sheet through a twin-screw extruder;

[0236] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 20.3 Mrad to cross-link the resin sheet;

[0237] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0238] (5) Wear of foam sheet: The foam material contacts the surface grinding roller with a mesh size of 80 at a linear speed of 40 m / min. The speed of the grinding roller is 3 m / min. The pressure between the foam material and the grinding roller is 0.2 MPa. There is relative displacement between the surface grinding roller and the foam sheet, which destroys the cross-linked resin layer of the foam material to form an open-pore structure.

[0239] Example 6

[0240] (1) Blending modification of resin: 40 parts by weight of low-density polyethylene (Sinopec LD100-AC), 40 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 20 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 3.52%, the tensile strength was 25.7 MPa, the elongation was 650%, and the softening temperature was 93.5°C;

[0241] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 10.5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent, 0.5 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, and 0.5 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and then extruded into a sheet through a twin-screw extruder;

[0242] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 17.3 Mrad to cross-link the resin sheet;

[0243] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent, the heating temperature being 265° C., to obtain a foamed sheet; the two sides of the foamed sheet are unfoamed cross-linked resin layers, and the middle is an independent pore layer having pores formed by foaming; pressing the surfaces of the two foamed sheets together in a molten state by a hot air laminating machine to form a foamed sheet having a five-layer structure of cross-linked resin layer / independent pore layer / cross-linked resin layer / independent pore layer / cross-linked resin layer;

[0244] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.6 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 500 mm / s, a cooling water flow rate of 5 L / min and a temperature of 20°C, a traction force of 40 N, and a traction speed of 3 m / min. The foam sheet A is obtained by cutting 0.5 mm from the upper surface of the foam sheet. The other surface of the foam sheet A is cut in the same manner by cutting 0.5 mm from the upper surface of the foam sheet to obtain the foam sheet.

[0245] Comparative Example 1

[0246] (1) Blending modification of resin: 40 parts by weight of low-density polyethylene (Sinopec LD100-AC), 40 parts by weight of polyolefin elastomer A (DOW AFFINITY™ PF 1140G) and 20 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 3.52%, the tensile strength was 25.7 MPa, the elongation was 650%, and the softening temperature was 93.5°C;

[0247] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) and 10.5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent were mixed uniformly and extruded into a sheet through a twin-screw extruder;

[0248] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 17.3 Mrad to cross-link the resin sheet;

[0249] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0250] (5) Needle punching of foamed sheets: The foamed sheets are fed into the needling machine, and the gap between the needling rollers is adjusted so that the puncture depth is 80 μm, the diameter of the needle is 0.1 mm, and the center distance between the needles is 0.5 mm. The foamed material passes through the needling machine, and the numerous fine needles on the needle plate quickly penetrate the material to form a continuous pore structure.

[0251] Comparative Example 2

[0252] (1) Blending modification of resin: 40 parts by weight of low-density polyethylene (Sinopec LD100-AC), 40 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 20 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 3.52%, the tensile strength was 25.7 MPa, the elongation was 650%, and the softening temperature was 93.5°C;

[0253] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) and 10.5 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent were mixed uniformly and extruded into a sheet through a twin-screw extruder;

[0254] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 10.3 Mrad to cross-link the resin sheet;

[0255] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0256] (5) Wear and opening of foam sheet: The foam material contacts the surface grinding roller with a mesh size of 80 at a linear speed of 40 m / min. The speed of the grinding roller is 3 m / min. The pressure between the foam material and the grinding roller is 0.2 MPa. There is a relative displacement between the surface grinding roller and the foam sheet, which destroys the cross-linked resin layer of the foam material to form an open-pore structure.

[0257] Comparative Example 3

[0258] (1) Blending modification of resin: 10 parts by weight of low-density polyethylene (Sinopec LD100-AC), 10 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 80 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 10.05%, the tensile strength was 13.7 MPa, the elongation was 1000%, and the softening temperature was 63°C;

[0259] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) and 15.6 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent were mixed uniformly and extruded into a sheet through a twin-screw extruder;

[0260] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 15.6 Mrad to cross-link the resin sheet;

[0261] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 265° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0262] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.6 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 500 mm / s, a cooling water flow rate of 5 L / min and a temperature of 20°C, a traction force of 40 N, and a traction speed of 3 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0263] Comparative Example 4

[0264] (1) Blending modification of resin: 90 parts by weight of low-density polyethylene (Sinopec LD100-AC) and 10 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) was fed into a twin-screw extruder to obtain a mixed resin; the mixed resin had a mass fraction of n-hexane extract of 0.95%, a tensile strength of 16.5 MPa, an elongation of 550%, and a softening temperature of 95.5°C;

[0265] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 18.7 parts by weight of a foaming agent, sodium bicarbonate (gas generation volume of 115 ml / g, ash content of 4.3%), 1.5 parts by weight of an antioxidant, 2,2,4-trimethyl-1,2-dihydroquinoline polymer, and 1.5 parts by weight of an antioxidant, 4,4'-dioctyldiphenylamine, and the mixture was extruded into a sheet through a twin-screw extruder;

[0266] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 23.7 Mrad to cross-link the resin sheet;

[0267] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 290° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0268] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.6 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 500 mm / s, a cooling water flow rate of 5 L / min and a temperature of 20°C, a traction force of 40 N, and a traction speed of 3 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0269] Comparative Example 5

[0270] (1) Blending modification of resin: 90 parts by weight of low-density polyethylene (Sinopec LD100-AC) and 10 parts by weight of polyolefin elastomer A (DOW AFFINITY ™PF 1140G) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 0.95%, the tensile strength was 16.5 MPa, the elongation was 550%, and the softening temperature was 95.5°C; (2) sheet forming: 100 parts by weight of the mixed resin obtained in step (1) was mixed with 8.5 parts by weight of azodicarbonamide (gas generation volume of 216 ml / g, ash content of 0.5%) as a foaming agent, and then extruded into a sheet through a twin-screw extruder;

[0271] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 23.7 Mrad to cross-link the resin sheet;

[0272] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 290° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0273] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.6 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 500 mm / s, a cooling water flow rate of 5 L / min and a temperature of 20°C, a traction force of 40 N, and a traction speed of 3 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0274] Comparative Example 6

[0275] (1) Blending modification of resin: 10 parts by weight of low-density polyethylene (Sinopec LD100-AC), 10 parts by weight of polyolefin elastomer A (DOW AFFINITY ™ PF 1140G) and 80 parts by weight of polyolefin elastomer B (SK Solumer ™ 875L) was put into a twin-screw extruder to obtain a mixed resin; the mass fraction of the n-hexane extract of the mixed resin was 10.05%, the tensile strength was 13.7 MPa, the elongation was 1000%, and the softening temperature was 63°C;

[0276] (2) Sheet forming: 100 parts by weight of the mixed resin prepared in step (1) was mixed with 15.6 parts by weight of azodicarbonamide (gas generation volume 216 ml / g, ash content 0.5%) as a foaming agent, 1.5 parts by weight of 2,2,4-trimethyl-1,2-dihydroquinoline polymer as an antioxidant, and 1.5 parts by weight of 4,4'-dioctyldiphenylamine as an antioxidant, and then extruded into a sheet through a twin-screw extruder;

[0277] (3) Cross-linking modification: The sheet prepared in step (2) was subjected to irradiation cross-linking at an irradiation dose of 16.1 Mrad to cross-link the resin sheet;

[0278] (4) Foaming: heating the cross-linked modified sheet obtained in step (3) to decompose and foam the foaming agent at a heating temperature of 205° C. to obtain a foamed sheet; the foamed sheet has unfoamed cross-linked resin layers on both sides and an independent pore layer with pores formed by foaming in the middle;

[0279] (5) Foam sheet cutting: The foam sheet is fed into the counter-pressing rollers, and the roller pressure is adjusted to 0.6 MPa to keep the sheet flat before cutting. The cutting machine equipment is set to a linear cutting speed of 500 mm / s, a cooling water flow rate of 5 L / min and a temperature of 20°C, a traction force of 40 N, and a traction speed of 3 m / min. The foam sheet is obtained by cutting 0.2 mm from the upper surface of the foam sheet and discarding it.

[0280] The properties of the foamed sheets prepared in the above examples and comparative examples were tested according to the following methods.

[0281] 1. Thickness and thickness difference

[0282] The thickness of the foam sheet was tested according to the Chinese national standard GB / T 40872-2021. The thickness was measured at 50 random points. The difference between the maximum and minimum readings was the thickness range. Only the center of the sample (≥5 mm from the edge) was measured. The sample was conditioned at a temperature of (23 ± 2)°C and a relative humidity of (50 ± 10)% for at least 24 hours before testing.

[0283] 2. Density

[0284] This is carried out in accordance with the provisions of the Chinese national standard GB / T 40872-2021.

[0285] 3. Average pore diameter and pore diameter range in ZD direction

[0286] A scanning electron microscope was used to observe the interfacial pores of the foamed material after liquid nitrogen brittle fracture treatment. At least 5 observation areas were randomly selected, and the diameters of the complete pores in the area parallel to the thickness direction (i.e., ZD direction) were counted. The average pore diameter in the ZD direction was calculated. The number of complete pores in the selected observation area ranged from 20 to 100.

[0287] The pore diameter range is the average value of the difference between the maximum pore diameter and the minimum pore diameter in the five observation areas.

[0288] 4. Cross-linking degree

[0289] a. Take a 100 mg sample from the foam sheet and accurately weigh the sample weight A (mg);

[0290] b. Wrap the sample with a 200-mesh metal mesh. Immerse the sample in 120°C xylene and allow to stand for 24 hours. The insoluble matter is collected within the metal mesh through filtration. After vacuum drying, accurately weigh the insoluble matter (B (mg)).

[0291] c. Calculate the degree of crosslinking (mass %):

[0292] Crosslinking degree (mass %) = 100% × (B / A).

[0293] 5. Through-hole rate

[0294] The surface of the open-pore layer of the foam sheet is observed using a scanning electron microscope. At least five observation areas are randomly selected, and the total number of pores and through-pores in all areas are counted to calculate the through-pore ratio. The number of pores in the selected observation area ranges from 20 to 100.

[0295] For the needle-punched samples, since the resin cross-linking layer is relatively complete, the independent pore layer connected to the open hole layer cannot be observed. It is necessary to cut off the resin cross-linking layer before observing the through hole situation.

[0296] 6. The starting temperature of the first weight loss stage of thermogravimetric (TG) analysis

[0297] (1) Prepare the sample to be tested. Place the sample in an oven at 105°C for 30 minutes to remove moisture or other volatile components in the sample. Transfer the sample to a desiccator and cool it to room temperature.

[0298] (2) The sample mass is 5 ± 0.5 mg and is evenly distributed on the bottom of the crucible; the sample crucible is placed in the sample chamber of the TG instrument.

[0299] (3) During the test, air was introduced at a rate of 150 ml / min to ensure that the sample was tested in a controlled atmosphere; the temperature was programmed to increase from 30°C to 800°C at a rate of 10 K / min;

[0300] (4) During the entire heating process, the instrument continuously records the mass change of the sample and generates a mass-temperature curve (TG curve). The curve shows the mass change of the sample at different temperatures, reflecting the thermal stability, decomposition behavior and composition of the material.

[0301] (5) On the TG curve, the temperature at which the curve begins to deviate from the baseline is the starting temperature of the first weight loss stage.

[0302] 7. Opening ratio

[0303] Use a scanning electron microscope to observe the surface of the open-pore layer of the foam sheet. Randomly select at least 5 observation areas, count the number of intact pores and the number of damaged pores in all areas, and calculate the open-pore ratio = number of damaged pores / (number of intact pores + number of damaged pores)*100%; the number of pores in the selected observation area ranges from 20 to 100.

[0304] 8. Compression stress at compression deformation 15% and 75%

[0305] Conduct the test in accordance with the Chinese national standard GB / T 18942.1. The specimen thickness should be at least 10 mm. For thinner materials, the specimen thickness should be increased to at least 10 mm. Before the test begins, apply a prestress of (100 ± 10) Pa to the specimen. After the preload is complete, reset the compression measurement system to zero. The specimen should be compressed at a rate of (50 ± 10)% of its initial thickness per minute. Measure the compressive stress at the first compression of 15% and 75% deformation. From the compression stress curve, read the maximum increase in compressive stress for a 1% increase in compression ratio.

[0306] 9. Vibration chip quantity test

[0307] If the pore structure is damaged during the opening process, debris in the pores may fall off during use, which may affect the normal function of the electronic product. The test uses vibration to count the amount of debris that falls out of the pores.

[0308] The test procedure involves placing a 50cm*50cm sample on a vibration platform at a frequency of 2000Hz for 30 minutes. After the test is complete, the sample is flipped over and vibrated for another 30 minutes. An electronic magnifying glass is used to observe and count the number of debris larger than 10μm that falls from the sample after vibration. A value of ≤10 debris is considered excellent, 10 < ≤ 30 debris is considered good, and >30 debris is considered poor.

[0309] 10. Shock absorption performance

[0310] The shock absorption performance of the material is evaluated using the minimum damping coefficient in the temperature range of 0°C to 35°C. When the damping coefficient is greater than 0.5, the shock absorption effect is evaluated as excellent; when the damping coefficient is less than 0.5, the shock absorption effect is evaluated as good; when the damping coefficient is less than 0.08, the shock absorption effect is evaluated as poor.

[0311] Minimum damping coefficient test method at 0℃ to 35℃:

[0312] Dynamic mechanical analysis (DMA) was used to test the damping coefficient of the foam material under the following test conditions: temperature range: -10°C to 50°C; heating rate: 5°C / min; frequency: 1 Hz; amplitude: 0.1 mm. The DMA temperature sweep mode was selected, and a compression fixture was used. The foam material was cut to a size and shape suitable for the fixture test and secured in the DMA equipment fixture. The DMA equipment parameters were set according to the test conditions. The temperature sweep test was initiated within the predetermined temperature range and heating rate. During the test, data on the storage modulus, loss modulus, and damping coefficient were collected. The damping coefficient versus temperature curve obtained from the test was smoothed using a Savitzky-Golay filter. The mean of three consecutive data points was taken as the effective value to determine the minimum damping coefficient within the range of 0°C to 35°C.

[0313] 11. Compression set rate

[0314] The compression set of the samples was tested according to the method specified in the Chinese national standard GB / T 6669-2008. The temperature was (25±2)°C, the compression specimen thickness was 50%±4%, and the compression time was (22±0.2) h.

[0315] The measured compression set is excellent if it is less than 30%, good if it is greater than or equal to 30% and less than 40%, and poor if it is greater than 40%.

[0316] Table 1 Properties of foamed sheets

[0317]

[0318] It can be seen that the foam sheet of the embodiment of the present application has good cushioning and shock absorbing properties. It can undergo adaptive deformation when compressed and will not deform excessively, resulting in a decrease in shock absorbing performance. Moreover, the permanent deformation after compression is appropriate, which can maintain long-term effective cushioning and shock absorbing performance.

[0319] Among them, the starting temperature of the first weight loss stage of the thermal gravimetric analysis of the foam sheet of the embodiment of the present application is between 150°C and 400°C, which can not only make the pores uniform and the structural strength of the pores high, so that the foam sheet has a certain softness, improve the cushioning and shock absorption performance of the foam sheet, but also ensure the performance stability of the foam sheet, maintain the permanent compression deformation performance, and thus maintain long-term effective cushioning and shock absorption performance.

[0320] The average number of pores in the ZD direction of the foam sheet of the embodiment of the present application satisfies T / R: 1.5≤T / R≤10; a relatively gentle compression curve can be obtained, thereby providing excellent cushioning and shock absorption performance, and the permanent deformation after compression will not be too large, which can maintain long-term effective cushioning and shock absorption performance.

[0321] In addition, the average density of the foam sheet is 0.025g / cm 3 to 0.1g / cm 3 The pore diameter of the independent porous layer in the ZD direction is 100μm to 500μm, the open hole ratio on the surface of the open hole layer is ≥90%, the through-porosity between the independent porous layer and the open hole layer is ≤20%, the compressive stress under 15% compression deformation is 1Kpa to 50Kpa, and the compressive stress under 70% compression deformation is 25Kpa to 700Kpa. When the compression deformation varies in the range of 15% to 75%, the maximum increment of the compression stress is 10KPa to 60KPa for every 1% increase in the compression deformation, which is beneficial to improving the cushioning and shock absorption performance of the foam sheet and the permanent deformation after compression.

[0322] When the compression deformation of the foam sheet of the embodiment of the present application varies in the range of 15% to 75%, the maximum increase in compression stress is 10KPa to 60KPa for every 1% increase in compression deformation, which is beneficial for the foam sheet to maintain good cushioning performance when vibration occurs.

[0323] The foam sheet of the comparative example has poor cushioning performance, shock absorption performance or permanent deformation after compression due to inappropriate starting temperature of the first weight loss stage of thermal gravimetric analysis, average pore number T / R in the ZD direction, average density, average pore diameter in the ZD direction, degree of crosslinking, open pore ratio on the surface of the open-pore layer, through-porosity, compressive stress under 15% or 75% compression deformation or maximum increment of compressive stress when the compression deformation varies from 15% to 75%.

[0324] Although the embodiments disclosed in this application are as described above, the contents described are merely embodiments adopted to facilitate understanding of this application and are not intended to limit this application. Any person skilled in the art to which this application belongs may make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed in this application, but the scope of protection of this application shall still be based on the scope defined by the attached claims.

Claims

1. A foam sheet, characterized in that: The invention is formed of a cross-linked polyolefin resin, and includes: a cross-linked resin layer, an independent pore layer, and an open-pore layer; the cross-linked resin layer is formed of unfoamed cross-linked polyolefin resin; the independent pore layer is formed of foamed cross-linked polyolefin resin, and has closed and mutually unconnected pores formed by foaming; the open-pore layer is formed of foamed or foamed and unfoamed cross-linked polyolefin resin, and has cavities in communication with the outside of the foam sheet; the open-pore layer is located on at least one of the upper surface and the lower surface of the foam sheet distributed in the thickness direction; The starting temperature of the first weight loss stage of the thermogravimetric analysis of the foam sheet is between 150° C. and 400° C.; The average number of pores in the foam sheet parallel to the thickness direction is T / R, and T / R satisfies: 1.5≤T / R≤10; Wherein, T is the thickness of the foam sheet, R is the average pore diameter of the foam sheet in the direction parallel to the thickness, and the units of T and R are the same; The average pore diameter of the independent pore layer in a direction parallel to the thickness direction is 100 μm to 500 μm; and the pore ratio on the surface of the open-pore layer is ≥90%.

2. The foam sheet according to claim 1, characterized in that The average density of the foam sheet is 0.025 g / cm 3 to 0.1g / cm 3 .

3. The foam sheet according to claim 1, characterized in that The pore diameter range of the independent pore layer parallel to the thickness direction is ≤650 μm; The thickness of the foam sheet is 0.15 mm to 4.5 mm, and the thickness range of the foam sheet is ≤0.1 mm.

4. The foam sheet according to claim 3, characterized in that The thickness of the foam sheet is 0.15 mm to 3 mm.

5. The foam sheet according to claim 4, characterized in that The thickness of the foam sheet is 0.15 mm to 1.2 mm.

6. The foam sheet according to claim 1, characterized in that The crosslinking degree of the foam sheet is 10% to 60%.

7. The foam sheet according to claim 1, characterized in that The open hole ratio on the surface of the open hole layer is ≥95%.

8. The foam sheet according to claim 1, characterized in that The through-porosity between the independent pore layer and the open-pore layer is ≤20%.

9. The foam sheet according to claim 1, characterized in that The compressive stress of the foam sheet under 15% compression deformation is 1KPa to 50KPa; the compressive stress under 70% compression deformation is 25KPa to 700KPa.

10. The foam sheet according to claim 1, characterized in that When the compression set varies from 15% to 75%, the maximum increase in compression stress is 10KPa to 60KPa for every 1% increase in compression set.

11. The foam sheet according to any one of claims 1 to 10, characterized in that The cross-linked resin layer contacts the independent pore layer, and the open-pore layer contacts the independent pore layer; the open-pore layer is obtained by opening a portion of the independent pore layer or a portion of the independent pore layer and the cross-linked resin layer.

12. The foam sheet according to claim 11, characterized in that The foam sheet comprises: a cross-linked resin layer, an independent pore layer, and an open-pore layer, which are arranged in the order of the cross-linked resin layer, the independent pore layer, and the open-pore layer; or Two cross-linked resin layers, two independent pore layers and one open pore layer, which are arranged in the order of cross-linked resin layer, independent pore layer, cross-linked resin layer, independent pore layer and open pore layer; or A cross-linked resin layer, two independent pore layers and two open pore layers are arranged in the order of open pore layer, independent pore layer, cross-linked resin layer, independent pore layer and open pore layer.

13. The foam sheet according to any one of claims 1 to 10, characterized in that The cross-linked polyolefin resin is a cross-linked product of one or more polyolefin resin raw materials; The polyolefin resin raw material includes any one or more of polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-butene copolymer, rubber and thermoplastic elastomer.

14. The foam sheet according to claim 13, characterized in that The cross-linked polyolefin resin is a cross-linked product of at least two polyolefin resin raw materials; The mass fraction of the n-hexane extract of the polyolefin resin raw material is 1% to 8.5%, the tensile strength is 15MPa to 35MPa, the elongation is 500% to 900%, and the softening temperature is 70°C to 100°C.

15. A method for preparing a foam sheet according to any one of claims 1 to 14, characterized in that: include: Mixing a polyolefin resin raw material with a foaming agent and an optional functional additive, and extruding to obtain a shaped sheet; The shaped sheet is cross-linked, modified, and foamed to form a cross-linked resin layer and an independent pore layer having pores, thereby obtaining a foamed sheet comprising the cross-linked resin layer, the independent pore layer, and the cross-linked resin layer in sequence; At least one side of the foam sheet is opened to form the open-hole layer, thereby obtaining the foam sheet.

16. The preparation method according to claim 15, characterized in that The step of opening holes on at least one side of the foam sheet to form the open-hole layer comprises: The cross-linked resin layer and part of the independent pore layer on one side of the foam sheet are removed to expose part of the pores in the independent pore layer, so that part of the pores in the independent pore layer are converted into cavities connected to the outside, and part of the area of ​​the independent pore layer having the cavities is converted into the open-pore layer.

17. The preparation method according to claim 15, characterized in that The step of opening holes on at least one side of the foam sheet to form the open-hole layer comprises: A cavity communicating with the outside is formed on at least one side of the foam sheet, and the cavity extends into the independent pore layer, so that a partial area of ​​the independent pore layer having the cavity and the cross-linked resin layer penetrated by the cavity are converted into the open-pore layer.

18. The preparation method according to any one of claims 15 to 17, characterized in that Also includes: After obtaining the foamed sheet, and before opening holes on at least one side of the foamed sheet, combining multiple foam sheets together to obtain a composite sheet; The step of opening holes on at least one side of the foam sheet to form the open-hole layer includes: opening holes on at least one side of the composite sheet to form the open-hole layer.

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