CN112795080B - EVA/LDPE supercritical solid foaming material and preparation method thereof - Google Patents

EVA/LDPE supercritical solid foaming material and preparation method thereof Download PDF

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CN112795080B
CN112795080B CN202110146340.5A CN202110146340A CN112795080B CN 112795080 B CN112795080 B CN 112795080B CN 202110146340 A CN202110146340 A CN 202110146340A CN 112795080 B CN112795080 B CN 112795080B
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ldpe
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CN112795080A (en
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龚鹏剑
李光宪
李艳婷
牛艳华
蒋根杰
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Long Chain Light Material Nanjing Technology Co ltd
Jiangsu Jitri Advanced Polymer Materials Research Institute Co Ltd
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Long Chain Light Material Nanjing Technology Co ltd
Jiangsu Jitri Advanced Polymer Materials Research Institute Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
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    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/122—Hydrogen, oxygen, CO2, nitrogen or noble gases
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/0061—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof characterized by the use of several polymeric components
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
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    • C08J2203/00—Foams characterized by the expanding agent
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    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C08J2323/08—Copolymers of ethene
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    • C08J2423/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2423/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2423/04—Homopolymers or copolymers of ethene
    • C08J2423/06—Polyethene
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Abstract

The invention belongs to the technical field of EVA (ethylene vinyl acetate) foam material products, and particularly relates to an EVA/LDPE (ethylene vinyl acetate/low-density polyethylene) supercritical solid foam material and a preparation method thereof. The invention provides an EVA/LDPE foamed material, which comprises the following raw materials in proportion: 60-90 parts of EVA, 10-40 parts of LDPE and 0.07-10 parts of cross-linking agent. According to the invention, the EVA/LDPE foamed material with excellent performance is prepared by adopting a supercritical solid foaming method under the condition that a small amount of cross-linking agent is added without other redundant additives except resin, namely, the environment-friendly foamed material with large foaming multiplying power (more than 8 times), small pore diameter (less than 50 um) and uniform foam pores is prepared at low temperature.

Description

EVA/LDPE supercritical solid foaming material and preparation method thereof
Technical Field
The invention belongs to the technical field of EVA (ethylene vinyl acetate) foam material products, and particularly relates to an EVA/LDPE (ethylene vinyl acetate/low-density polyethylene) supercritical solid foam material and a preparation method thereof.
Background
The ethylene-vinyl acetate copolymer (EVA) foaming material is a multifunctional novel polyolefin environment-friendly material consisting of a solid phase and a gas phase, has excellent physical and chemical properties, and is widely applied to shoe materials, luggage linings, toys, sports goods, building interiors and various emerging applications such as electronic accessories, automobile interiors and the like.
The current EVA foaming material adopts an AC series chemical foaming agent, such as azodicarbonamide (H) 2 NCON). Although AC is widely used because of its large gas evolution, its decomposition temperature is high (180-210 ℃). Thus, only at sufficiently high temperatures, ACCan be decomposed to generate a large amount of gas, such as N 2 、CO、CO 2 . At the same time, decomposition will also produce a large amount of NH 3 (Ammonia), COHNH 2 (formamide) and other harmful CN compounds. Ammonia gas as an irritant malodorous odour significantly affects the health of the user if the amount of ammonia produced during decomposition is too high, and formamide produced during decomposition is a toxic substance and more seriously affects the health of the user (western countries such as the european union stipulate that the amount of formamide in the product should be less than 10 ppm). Although the AC foaming agent generates a large amount of gas which is toxic and harmful to human bodies, the EVA foaming material prepared by the AC foaming agent is mainly used in products which are directly contacted with the skin of human bodies, such as yoga mats, infant crawling mats and the like. From the decomposition products of the AC foaming agent, the EVA foaming material manufactured by the currently adopted chemical foaming method (using the AC chemical foaming agent) is obviously harmful to the human body, and particularly has more profound influence on the infants at the growth and development stage.
Since the decomposition temperature of the AC series foaming agent is high, an activator such as zinc oxide (ZnO) is added to lower the decomposition temperature of the AC foaming agent. The Chinese invention patent CN105037782A uses ZnO modified by organic acid to accelerate the decomposition of the AC foaming agent at 155-160 ℃, and the Chinese invention patent CN111909439A uses a silane coupling agent to further improve the catalytic efficiency of ZnO on the AC foaming agent, and the EVA foaming material with uniform pore size distribution is prepared by foaming at 155 ℃.
At present, a small number of patents for preparing EVA foaming materials by a supercritical foaming method exist. For example, CN111793268A is supercritical nitrogen (N) 2 ) Saturating for 2.5-3.5h in supercritical state, and then decompressing for 20-40s to prepare the shoe material with EVA/POE foaming matrix, wherein the aperture is 85um, and the density is 0.2g/cm 3 The tensile strength of the corresponding foaming material is 2.6MPa; the invention patent CN111763370A also selects supercritical N 2 Swelling for 3.5-4.5h under the conditions of 14-18MPa of pressure and 115-125 ℃ of temperature, and then quickly relieving pressure within 15-25s to prepare the shoe material with the EVA/TPU foamed matrix, wherein the pore diameter is 60-100 mu m, and the density is 0.13g/cm 3 The tensile strength of the corresponding foam was 2MPa.
Disclosure of Invention
The object of the present invention is to prepare a porous material having a pore size of 50um or less and a lower density (less than 0.1 g/cm) 3 ) But at the same time keeps higher mechanical property (tensile strength is more than 2 MPa). Based on the above, the invention provides an EVA blend foaming material, namely, an environment-friendly EVA foaming material is prepared by selecting an EVA blend as a raw material and adopting a supercritical solid foaming mode of swelling supercritical fluid at low temperature and releasing pressure for foaming. The EVA foaming material obtained by the supercritical solid foaming method has the aperture smaller than 50um, and can still realize high strength and better buffer performance at lower density. Therefore, the EVA foaming material prepared by the supercritical solid foaming method has low cost, is environment-friendly, has no harm to human body, meets the environmental protection requirement of non-Volatile Organic Compounds (VOC), and has no toxic volatile organic compounds such as CO, COHNH2 (formamide) and other harmful CN compounds.
The technical scheme of the invention is as follows:
the invention aims to solve the first technical problem of providing an EVA/LDPE foamed material, which comprises the following raw materials in proportion: 60-90 parts of EVA, 10-40 parts of LDPE and 0.07-10 parts of cross-linking agent.
Further, the crosslinking agent is selected from: di-tert-butylperoxydiisopropylbenzene (BIBP) or dicumyl peroxide (DCP).
Further, the mass fraction of Vinyl Acetate (VA) in the EVA is 5-40%; preferably 18%.
In the invention, different functional additives can be added according to actual needs to meet different use occasions, for example, a certain amount of nucleating agent such as calcium carbonate (CaCO) can be added for improving the nucleation rate 3 ) Etc.; an antistatic agent may be added in order to prevent the generation of static electricity; conductive carbon black may be added if desired.
The second technical problem to be solved by the invention is to provide a preparation method of the EVA/LDPE foamed material, which comprises the following steps: firstly, melting and blending EVA, LDPE and a crosslinking agent to obtain a blend; then forming a micro-crosslinking system by using the obtained mixture; and finally, placing the micro-crosslinking system in a high-pressure chamber, saturating the micro-crosslinking system in supercritical gas at the temperature of between 80 and 100 ℃, and then carrying out pressure maintaining and rapid pressure relief foaming to obtain a foaming sample, namely the EVA/LDPE foaming material.
Further, in the above method, the pressure in the high-pressure chamber is 10 to 30MPa (preferably 15 MPa), the pressure holding time is 30 to 240min (preferably 30 min), and the pressure releasing time is 2 to 30s (preferably 2 s).
Further, in the above method, in the foaming method, the supercritical gas is supercritical CO 2 Or N 2 At least one of (a).
Further, in the above method, the method for forming the micro-crosslinking system from the obtained blend material comprises: keeping the temperature of the obtained mixture at 140-210 ℃ (preferably 190 ℃) for 5-10 min; if the EVA copolymer can be placed between hot plates, the EVA copolymer can be placed for 5-10min at the temperature of 140-210 ℃, so that the EVA copolymer can form a crosslinking system through full decomposition of the crosslinking agent.
Further, in the above method, the EVA, the LDPE and the crosslinking agent are melt blended to obtain a blend, that is, the melt blending is performed at a temperature equal to or higher than the melting temperature of each raw material but lower than the thermal decomposition temperature. The conventional extrusion or banburying method can be adopted, and for example, the method adopting the extrusion method comprises the following steps: weighing EVA, LDPE and a crosslinking agent according to a ratio, uniformly mixing the weighed raw materials, pouring the mixture into a double-screw extruder for extrusion granulation, wherein the double-screw extruder is divided into 10 heating zones in total according to the extrusion direction during operation, and the preferred temperature from a feed opening to a machine head is 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the preferred screw rotation speed is 100r/min.
The third technical problem to be solved by the invention is to point out that the EVA/LDPE foamed material is used as an environment-friendly foamed sneaker sole, a yoga mat or a crawling mat.
The invention has the beneficial effects that:
the EVA blending foaming material prepared by adopting the supercritical solid foaming method at low temperature without adding other additives except the cross-linking agent has the advantages of high strength, good buffering performance and the like:
(1) The invention only adds a small amount of cross-linking agent except resin, has no other redundant additives, and has low cost and simple process.
(2) The invention makes the blend cross-linked at high temperature before foaming, and achieves the expected cross-linking effect by effectively regulating the cross-linking degree to meet the performance required by the product, and is easy to regulate.
(3) The invention can prepare the environment-friendly foaming material with large foaming multiplying power (more than 8 times), small aperture (less than 50 um) and uniform foam pores at low temperature (95 ℃).
(4) The invention adopts supercritical CO 2 Or N 2 The foaming process is environment-friendly, the produced product has no toxic foaming agent residue, and the prepared EVA foaming material has more excellent mechanical property than a chemical foaming material.
(5) The invention has the advantages of simple and effective preparation process, rapidness, convenience and environmental protection, and the prepared foaming material has good elasticity and outstanding mechanical property, and can be widely used in the field of EVA foaming materials.
(6) Compared with EVA/TPU foam materials and EVA/POE foam materials prepared by a supercritical melting method, the EVA/PE foam materials prepared by the supercritical solid method can obtain higher tensile strength at lower density.
Description of the drawings:
FIG. 1a is an SEM image of the foamed material prepared in example 1; FIG. 1b is an SEM image of the foamed material prepared in example 2; FIG. 1c is an SEM image of the foamed material prepared in example 3; FIG. 1d is an SEM image of the foamed material prepared in comparative example 1; FIG. 1e is an SEM image of the foamed material prepared in comparative example 2.
Detailed Description
The invention aims to solve the first technical problem of providing an EVA/LDPE foamed material, which comprises the following raw materials in proportion: 60-90 parts of EVA, 10-40 parts of LDPE and 0.07-10 parts of cross-linking agent.
In the invention, the LDPE is selectively added into the EVA, and the EVA can cause the molecular chain regularity of the LDPE to be reduced due to the polar group contained in the EVA, so that partial incomplete small crystal grains are formed, and the small crystal grains are similar to an added nucleating agent in the foaming and nucleating process, so that the foaming and nucleating are enhanced, and the cell size is smaller and the cell density is higher. And the EVA has higher crosslinking capability and lower thermal conductivity than the LDPE, the crosslinking degree in the blend is increased along with the increase of the EVA content, and the tensile strength and the elongation at break of the EVA/LDPE blend material tend to increase along with the increase of the EVA content, so that the invention selects higher EVA content. In the invention, the cross-linking degree of the foaming material is strictly controlled, and the proper cross-linking degree can improve the viscosity of the blend and is beneficial to controlling the pore diameter of the cells; however, too high a degree of crosslinking may result in excessive crosslinking of amorphous regions, and the difference between crystalline and amorphous regions during the dissolution of foaming gas, bubble nucleation and cell growth may be increased, resulting in widening of cell size distribution intervals. In addition, too high a melt viscosity of the over-crosslinked polyolefin can limit cell growth; and the degree of crosslinking is insufficient, the melt strength is too low, cells are easy to break, and the obtained foaming material has higher shrinkage.
The second technical problem to be solved by the invention is to provide a preparation method of the EVA/LDPE foamed material, which comprises the following steps: firstly, melting and blending EVA, LDPE and a crosslinking agent to obtain a blend; then forming a crosslinking system by using the obtained mixture; and finally, placing the crosslinking system in a foaming kettle for foaming in supercritical gas at the temperature of 80-100 ℃, keeping the pressure and quickly relieving the pressure after the gas is saturated, and taking out a foaming sample to obtain the EVA blend foaming material.
The following examples are given to further illustrate the embodiments of the present invention and are not intended to limit the scope of the present invention.
The resins used in the embodiment of the invention are EVA and LDPE, the compatibility of the two resins is good, and the elasticity and the ageing resistance of the material can be improved after blending. The cross-linking agent is bis (tert-butylperoxydiisopropylbenzene) BIBP.
Example 1
The method comprises the following steps: weighing 88.45% of EVA, 11.06% of LDPE and 0.50% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the double-screw extruder into 10 heating zones in total according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: pressing the plate using a press vulcanizer: and when the temperature of the vulcanizing press reaches the set temperature of 190 ℃ and is stabilized for about 10min, putting the sample which is prepared in advance into a plate pressing machine, closing the mold and starting timing, when the pressure of the mold cavity reaches 2MPa, pressing the mold until the pressure of the mold cavity reaches 9.5MPa, timing for 30s, then opening the mold, taking out the material and cooling to obtain the cross-linked EVA/PE blend material.
Step three: and (3) placing the sample into a high-pressure cavity for foaming, wherein the foaming temperature is 95 ℃, the pressure is 15MPa, the swelling time is 30min, and the pressure relief time is 2s, and taking out the foamed sample to obtain the environment-friendly EVA blend foaming material.
Example 2
The method comprises the following steps: weighing 88.3% of EVA, 11.04% of LDPE and 0.66% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the twin-screw extruder into 10 heating zones in total according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) placing the sample into a high-pressure cavity for foaming, wherein the foaming temperature is 95 ℃, the pressure is 15MPa, the swelling time is 30min, and the pressure relief time is 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Example 3
The method comprises the following steps: weighing 88% of EVA, 11% of LDPE and 0.99% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the twin-screw extruder into 10 heating zones according to the extrusion direction during operation, and respectively controlling the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) placing the sample into a high-pressure cavity for foaming, wherein the foaming temperature is 95 ℃, the pressure is 15MPa, the swelling time is 30min, and the pressure relief time is 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Comparative example 1
The method comprises the following steps: weighing 88.9% of EVA, 11.1% of LDPE and 0% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the twin-screw extruder into 10 heating zones according to the extrusion direction during operation, and respectively controlling the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) foaming the sample in a high-pressure cavity at the foaming temperature of 95 ℃, under the pressure of 15MPa, for 30min, and for 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Comparative example 2
The method comprises the following steps: weighing 88.45% of EVA, 11.06% of LDPE and 0.50% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the double-screw extruder into 10 heating zones in total according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) placing the sample into a high-pressure cavity for foaming, wherein the foaming temperature is 110 ℃, the pressure is 15MPa, the swelling time is 30min, and the pressure relief time is 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Comparative example 3
The method comprises the following steps: weighing 88.45% of EVA, 11.06% of LDPE and 0.50% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the double-screw extruder into 10 heating zones in total according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) placing the sample into a high-pressure cavity for foaming, wherein the foaming temperature is 110 ℃, the pressure is 15MPa, the swelling time is 30min, and the pressure relief time is 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Comparative example 4
The method comprises the following steps: weighing 88.45% of EVA, 11.06% of LDPE and 0.50% of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the double-screw extruder into 10 heating zones in total according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) placing the sample into a high-pressure cavity for foaming, wherein the foaming temperature is 130 ℃, the pressure is 15MPa, the swelling time is 30min, and the pressure relief time is 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Comparative example 5
The method comprises the following steps: weighing 87.72 percent of EVA, 10.96 percent of LDPE and 1.32 percent of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the double-screw extruder into 10 heating zones in total according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head at 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) foaming the sample in a high-pressure cavity at the foaming temperature of 130 ℃, under the pressure of 15MPa, for 30min, and for 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
Comparative example 6
The method comprises the following steps: weighing 88.74 percent of EVA, 11.09 percent of LDPE and 0.17 percent of BIBP according to mass percentage, then uniformly mixing the weighed raw materials, pouring the mixture into an extruder to extrude and granulate or plate, dividing the double-screw extruder into 10 heating zones according to the extrusion direction during operation, and respectively setting the temperatures from a feed opening to a machine head to be 90 ℃, 120 ℃, 125 ℃, 130 ℃ and 130 ℃, wherein the rotating speed of a screw is 100r/min.
Step two: and pressing the plate by using a press vulcanizer, wherein the temperature of the press vulcanizer is 190 ℃, and the pressure maintaining time is 5-10min.
Step three: and (3) foaming the sample in a high-pressure cavity at the foaming temperature of 95 ℃, under the pressure of 15MPa, for 30min, and for 2s, and taking out the foamed sample to obtain the environment-friendly EVA blending foaming material.
The foaming ratios, the pore diameters and the pore densities of the foaming materials of the examples and the comparative examples obtained by EVA blends at different temperatures and different crosslinking agent contents are shown in Table 1.
TABLE 1
Figure BDA0002930536850000071
As can be seen from table 1, the foaming ratio of the environmentally friendly EVA blend foam material is gradually reduced and the pore size is gradually increased with the increase of the content of the crosslinking agent at the same foaming temperature, foaming pressure and swelling time, but the increase trend is smaller and the regularity of the pore density is poorer, because EVA and LDPE are of a phase-separated structure and the LDPE foaming is inhibited at low temperature foaming with the increase of the content of the crosslinking agent, so the foaming pore size of the LDPE phase at low temperature is smaller, and we can see the existence of large and small pores in the SEM picture, therefore, from the point of view of the foaming ratio and the pore size uniformity, it can be considered that the EVA blend foam material with the content of the crosslinking agent of 0.045 parts meets the requirements.
It can also be seen from table 1 that, after the EVA blend foam material with the same composition is crosslinked at 190 ℃, the foaming ratio of the material tends to decrease with the increase of the foaming temperature (the swelling pressure and the swelling time are the same), and the material does not substantially foam up when the foaming temperature reaches 130 ℃, because the chemical crosslinking manner adopted by the system is that the unreacted residual crosslinking agent is decomposed faster with the increase of the temperature, so that the crosslinking degree obtained during the foaming process of the material is greater than that measured before foaming, and the higher the temperature is, the greater the crosslinking degree is, the nucleation and growth of cells are inhibited, so the foaming ratio decreases, and even does not foam up. In the invention, the foaming temperature is an important parameter for controlling the appearance of the micropores, which obviously influences the dissolution and diffusion of foaming gas, and the influence on the appearance of the micropores is dual: on one hand, the temperature is raised to accelerate the molecular chain segment of the polymer matrix, so that the gas molecules are easy to diffuse from the matrix to micropores; on the other hand, when the foaming temperature exceeds a certain value, the foaming pressure inside the cells may exceed the limit that the cells can withstand, thereby causing cell breakage and cell coalescence; too low foaming temperature, CO 2 The swelling is difficult, the foaming ratio is low, the material is hard, the foaming temperature is slightly higher than the melting point of LDPE, the crystal region in the foaming is completely dissolved, the pore diameter of the obtained foaming material is larger, the pore density is low, the contractibility is large, when the foaming temperature is higher than 120 ℃, the crosslinking agent can be continuously decomposed in the foaming process, the crosslinking degree can be increased along with the increase of the swelling time, when the swelling time is longer, the crosslinking degree of the material is higher, the foaming ratio of the obtained foaming material is reduced, and the material density is increased.
The environmental-friendly EVA blending foaming materials in examples 1-3 and comparative examples 1-4 are subjected to mechanical property test, wherein the mechanical propertyThe test of (2) comprises: tensile strength and compressive strength were measured, and the results are shown in table 2. And (3) testing tensile strength: the tensile rate of the test piece is 100mm/min by adopting an American INSTRON universal material testing machine. And (3) testing the compression strength: the American INSTRON universal material testing machine performs the test, and the compressive strain is 50 percent. In addition, the sample density test was performed using a Mei Tuole density test balance, in g/cm 3 。
TABLE 2
Figure BDA0002930536850000081
In addition, SEM images of the foamed materials obtained in examples 1-3 and comparative examples 1-2 are shown in FIGS. 1 a-1 e; as can be seen from fig. 1: the cells in the microscopic morphology of the sample section processed under different crosslinking degrees are all closed cell structures, and meanwhile, the cell size is obviously increased along with the increase of the foaming temperature, the cell walls are thinned, and the cells are transited from a spherical shape to a polygonal shape. When the temperature is higher than the temperature of the EVA blend for complete melting, the crystalline region in the matrix is completely dissolved, the melt strength and the viscosity of the EVA blend are reduced, the growth resistance of the cells is reduced, the interaction force between the cells is enhanced, and the cells are extruded with each other to change the appearance (from spherical to polyhedral). As the content of the crosslinking agent increases, cell regularity increases; however, when the content of the crosslinking agent exceeds 0.66%, the cell diameter exhibits a double distribution although the cells are also regularly spherical.

Claims (9)

1. The EVA/LDPE foamed material is characterized by comprising the following raw materials in proportion: 60-90 parts of EVA (ethylene vinyl acetate), 10-40 parts of LDPE (low-density polyethylene), and the addition amount of the crosslinking agent is 1/201-1/101 of the total mass of the EVA, the LDPE and the crosslinking agent; the cross-linking agent is selected from: di-tert-butylperoxydiisopropylbenzene or dicumyl peroxide;
and the EVA/LDPE foamed material is prepared by the following preparation method: firstly, melting and blending EVA, LDPE and a crosslinking agent to obtain a blend; then forming a micro-crosslinking system by using the obtained mixture; and finally, placing the micro-crosslinking system in a high-pressure chamber, carrying out gas saturation in supercritical gas at 95-100 ℃, and then carrying out pressure maintaining and rapid pressure relief foaming to obtain a foaming sample, namely the EVA/LDPE foaming material.
2. The EVA/LDPE foamed material according to claim 1, wherein the mass fraction of vinyl acetate in the EVA is 5-40%.
3. The EVA/LDPE foamed material according to claim 2, wherein the mass fraction of vinyl acetate in the EVA is 18%.
4. The method for preparing EVA/LDPE foamed material according to any of claims 1 to 3, characterized in that it comprises: firstly, melting and blending EVA, LDPE and a crosslinking agent to obtain a blend; then forming a micro-crosslinking system by using the obtained mixture; and finally, placing the micro-crosslinking system in a high-pressure chamber, saturating the micro-crosslinking system in supercritical gas at 95-100 ℃, and then, maintaining the pressure, quickly decompressing and foaming to obtain a foaming sample, namely the EVA/LDPE foaming material.
5. The method for preparing EVA/LDPE foamed material according to claim 4, wherein the pressure in the high pressure chamber is 10-30 MPa, the pressure holding time is 30-240 min, and the pressure releasing time is 2-30 s.
6. The method for preparing EVA/LDPE foamed material according to claim 4, wherein the supercritical gas is supercritical CO 2 Or N 2 At least one of (1).
7. The method for preparing EVA/LDPE foamed material according to claim 4, characterized in that the method for forming the resulting blend into a micro-crosslinking system is: and (3) keeping the temperature of the obtained mixture at 140-210 ℃ for 5-10min, and fully decomposing the crosslinking agent to form a crosslinking system.
8. The method for preparing the EVA/LDPE foamed material according to claim 4, wherein the EVA, the LDPE and the crosslinking agent are melt-blended to obtain a blend, namely the melt-blending is carried out above the melting temperature of each raw material and below the thermal decomposition temperature.
The EVA/LDPE foamed material is used as an environmentally-friendly foamed sneaker sole, a yoga mat or a crawling mat, and is the EVA/LDPE foamed material in any one of claims 1 to 3 or prepared by the method in any one of claims 4 to 8.
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