CN113183393B

CN113183393B - Supercritical foaming condition prediction method based on continuous self-nucleation technology

Info

Publication number: CN113183393B
Application number: CN202110553128.0A
Authority: CN
Inventors: 龚鹏剑; 李艳婷; 李光宪; 黄亚江; 洪江
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2021-05-20
Filing date: 2021-05-20
Publication date: 2022-03-04
Anticipated expiration: 2041-05-20
Also published as: CN113183393A

Abstract

The invention relates to a supercritical foaming condition prediction method based on a continuous self-nucleation technology, and belongs to the technical field of supercritical foaming. The method for predicting the optimal crosslinking degree and the foaming temperature in foaming by the thermoplastic elastomer is suitable for solving the problem that the optimal crosslinking degree and the foaming temperature in foaming can be determined only by continuous experimental trials, and meanwhile, more accurate information such as a molecular chain structure, crystallization behavior and the like can be obtained.

Description

Supercritical foaming condition prediction method based on continuous self-nucleation technology

Technical Field

The invention relates to a supercritical foaming condition prediction method based on a continuous self-nucleation technology, and belongs to the technical field of supercritical foaming.

Background

The thermoplastic elastomer has the dual characteristics of rubber and plastic, and the thermoplastic elastomer foam product has the characteristics of light weight, high elasticity, shock absorption, heat insulation, toughness and the like, and can be widely applied to the fields of sports clothes, sports equipment, plastic runways, packaging, buildings, transportation, safety protection and the like. However, the thermoplastic elastomer has low melt strength, so that the foamed product has low dimensional stability, serious retraction and poor surface quality. It is therefore generally necessary to modify it by crosslinking to increase the melt strength of the material. The strength of the matrix melt is low when the matrix melt is not crosslinked, the melt cannot wrap bubbles in the foaming forming process, so that the phenomena of bubble combination, collapse and the like are generated, and gas easily escapes from the matrix, so that the obtained product has the characteristics of low foaming multiplying power, small bubble density, unstable bubble size and the like; when the crosslinking degree is lower, the viscosity of the matrix is increased, but the melt strength is still lower, and the pore wall of the cell is easy to break during foaming; after the crosslinking degree is further improved, the viscosity of the matrix is higher, the melt strength is higher, the phenomenon of pore wall breakage is obviously improved, and foams with good structures, such as uniform pore diameter, higher cell density and the like, are generated in the matrixWhen the crosslinking degree is continuously improved, the viscosity of the matrix is higher, gas diffusion is difficult, and meanwhile, the chain segment is bound by the crosslinking network to form a closed-cell structure and the cell size is smaller; if the matrix is completely crosslinked, the viscosity of the matrix is too high, and the melt strength is too high, so that bubbles cannot grow, and the foaming material cannot be prepared. Control of the degree of crosslinking during foaming is therefore of particular importance. At present, organic solvents are mostly adopted to swell at high temperature to judge the gel content of the polymer, so that the organic solvent is toxic and consumes long time; the cross-linking density is calculated by adopting a rheological method, but the rheological process is complicated, the stability is not strong, and meanwhile, a series of foaming experiments are required to obtain the optimal cross-linking degree of foaming, so that the process is extremely time-consuming. Therefore, the above two methods cannot directly determine the optimum crosslinking degree required for foaming from the crosslinking degree. In addition, the foaming temperature is an important parameter for controlling the appearance of the micropores, and the foaming temperature obviously influences the dissolving and diffusing processes of foaming gas. The influence of the micropore appearance is dual, on one hand, the temperature is raised to accelerate the movement of the molecular chain segment of the polymer matrix, and gas molecules are easy to diffuse from the matrix into micropores; on the other hand, when the foaming temperature exceeds a certain value, the foaming pressure inside the cells may exceed the limit borne by the cell walls, so that the phenomena of cell combination and collapse occur; too low foaming temperature, CO₂The dispersion into the matrix is difficult, and the molecular chain motion activity is reduced, so that the foam growth process is hindered, the foaming ratio of the product is low, and the material is hard; when the foaming temperature exceeds the melting point of the polymer, the crystal region in the matrix is completely dissolved, the obtained foaming material has larger pore diameter, low pore density and small density, but larger contractibility, if a chemical cross-linking agent is adopted for modification, the cross-linking agent is continuously decomposed in the foaming process when the foaming temperature is higher, the cross-linking degree is increased along with the increase of the swelling time, the cross-linking degree of the material is higher when the swelling time is longer, the foaming multiplying power of the obtained foaming material is reduced, and the density is increased, so the control on the cross-linking degree and the foaming temperature in the semi-crystalline polymer foaming is particularly important.

The introduction of the cross-linked network has a great influence on the crystallization and melting behaviors of the semi-crystalline polymer. At present, the influence of crosslinking on the melting point and the crystallinity of the polymer can only be judged by DSC, so that an approximate foaming temperature interval can only be determined by the melting point during foaming, and an optimal foaming temperature interval cannot be directly obtained. Whether the optimal crosslinking degree required by foaming or the optimal foaming temperature range is required to be obtained, the foaming ratio, the cell morphology, the pore diameter and the cell density can be determined by continuously foaming and then observing. Therefore, there is an urgent need for a technique capable of predicting both the optimum crosslinking degree and the optimum foaming temperature.

Disclosure of Invention

The invention provides a method for predicting the optimal crosslinking degree and the optimal foaming temperature in foaming by using a thermoplastic elastomer, which aims to solve the problem that the optimal crosslinking degree and the optimal foaming temperature can be determined only by continuous experimental attempts during foaming, can obtain more accurate information such as a molecular chain structure, crystallization behavior and the like, and provides powerful support for deeply researching the crosslinking behavior and the crystallization behavior of the low-temperature foaming of the thermoplastic elastomer.

A supercritical foaming condition prediction method based on a continuous self-nucleation technology comprises the following steps:

step 1, obtaining polymer materials prepared under the condition of different cross-linking agent dosage;

step 2, obtaining a DSC curve of the polymer material by a continuous self-nucleation/annealing (SSA) method;

step 3, on DSC curves under different cross-linking agent content conditions, taking the cross-linking agent dosage corresponding to the critical position at the area disappearance position of the main melting peak as an optimal value;

and 4, regarding the next melting peak of which the position is deviated to the low-temperature direction as a secondary melting peak, and regarding the temperature corresponding to the temperature approaching position of each secondary melting peak on each DSC curve as the optimal foaming temperature.

In one embodiment, the polymer is a semi-crystalline polymer.

In one embodiment, in step 1, the polymer material is prepared by blending a polymer raw material and a crosslinking agent.

In one embodiment, the polymeric material is composed of one polymer or is a blend of two or more polymeric materials.

In one embodiment, when the polymer material is obtained by blending two polymer raw materials, the main melting peaks of the two polymer raw materials are respectively subjected to the prediction of the amount of the crosslinking agent according to step 3, and the range between the two amounts is taken as an optimum value.

In one embodiment, when the polymer material is obtained by blending two polymer raw materials, the sub-melting peaks of the two polymer raw materials are respectively subjected to the prediction of the foaming temperature according to step 4, and the range between the two temperatures is taken as an optimal value.

In one embodiment, when the polymer is derived from a blend of two or more polymers, the position of the major and minor melting peaks for each polymer is determined by comparison to the peak positions of the polymer obtained in a single DSC test.

In one embodiment, the optimal value is a value having a range of deviations.

In one embodiment, the deviation is 0.01-10% of the optimal value.

In one embodiment, the foaming is supercritical foaming, wherein the foaming temperature is 50-300 ℃ and the pressure is 5-60 MPa.

In one embodiment, the SSA method comprises the steps of:

step a, heating to eliminate the thermal history of the polymer;

step b, determining the Ts temperature, and sequentially carrying out temperature rise continuous self-nucleation and annealing treatment; the maximum temperature during the continuous self-nucleation process is sequentially lowered in each cycle until the full width of the melting range.

In one embodiment, the Ts temperature is 10 to 25 ℃ above the melting point of the polymeric material.

In one embodiment, the gradient of the temperature decrease is 0.1 to 10 ℃ in each cycle.

In one embodiment, the rate of temperature change during warming and/or annealing is 1-40 deg.C/min.

Advantageous effects

Compared with the prior art, the invention has the following beneficial technical effects:

1. the invention provides a method for predicting the optimal crosslinking degree and foaming temperature in foaming by using a thermoplastic elastomer, which can predict the optimal preparation parameters by a simple method; meanwhile, more accurate information such as molecular chain structure, crystallization behavior and the like can be obtained, and powerful support is provided for the deep research of the crosslinking behavior and the crystallization behavior of the low-temperature foaming of the thermoplastic elastomer.

2. The method provided by the invention can greatly shorten the experimental time and ensure a good grading effect. Therefore, the foaming efficiency can be improved by reducing the experimental process.

Drawings

FIG. 1 is a schematic view of a continuous self-nucleation/annealing (SSA) heat treatment

FIG. 2 is a schematic diagram of a continuous self-nucleation/annealing (SSA) heat treatment. In this case, the Ts temperature was 6 ℃ apart from 124 ℃ to 40 ℃ for a total of 15 self-nucleation/annealing steps.

FIG. 3 is a graph of pure EVA versus LDPE and EVA/LDPE blends after continuous self nucleation/annealing (SSA) heat treatment.

FIG. 4 is an SEM image of the foaming of an EVA/LDPE blend at 95 ℃. (a)0 wt%, (b)0.17 wt%, (c)0.33 wt%, (d)0.5 wt%, (e)0.66 wt%, (f)0.99 wt%, since the expansion ratio of the 1.32 wt% sample is extremely low, the SEM picture hardly sees an effective cell structure.

Detailed Description

The method for predicting the optimum degree of crosslinking and the foaming temperature for foaming the thermoplastic elastomer provided by the present invention is further illustrated by the following examples.

In the present invention, the used foaming material polymer may be a semi-crystalline polymer, and the cross-linking and crystallization thereof can be characterized, for example, the method can predict parameters in the cross-linking process of EVA, POE, OBC, LDPE, PBAT, PBS, etc., and can also be used for the molecular structure characterization of ethylene/olefin copolymer, branched polymer and random copolymer. In the examples which follow, illustrated by the EVA and LDPE materials employed, EVA is a typical semi-crystalline copolymer whose crystallizable fraction can be divided into a series of continuous ethylene sequences of unequal length, and the structural heterogeneity of the polyethylene chain can be observed by applying SSA techniques, which are very sensitive to any segment of the methylene sequence, due to the presence of random branches in LDPE.

In the present invention, the crosslinking agent is not particularly limited, and may be a compound that generates chemical bonds between linear molecules, so that the linear molecules are connected to each other to form a network structure, and the function of the network structure can realize crosslinking modification between polymer molecules to improve melt strength; for example, one or a combination of triallyl isocyanurate, dicumyl peroxide, di-t-butylperoxyisopropyl benzene, 2, 5-dimethyl-2, 5-bis (t-butylperoxy) hexane, and t-amyl peroxy (2-ethylhexyl) carbonate can be used.

In the present invention, the foaming method used may be a supercritical foaming process, and the supercritical gas used may be CO₂、N₂And the like.

The characterization method of the polymer crystallization behavior provided by the invention utilizes DSC to perform characterization, and firstly, a series of foaming materials prepared under different cross-linking agent dosage and different foaming temperature conditions are required to be prepared and used as initial research data. The amount of the crosslinking agent to be used may be adjusted within the range of 0.01 to 5 wt%, etc., and for example, 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, etc. may be selected. The foaming temperature can be adjusted according to the characteristics of the polymer material, and can be adjusted within the range of 60-300 deg.C, for example, 70 deg.C, 90 deg.C, 110 deg.C, 130 deg.C, 150 deg.C, 170 deg.C, 190 deg.C, 210 deg.C, 230 deg.C, 250 deg.C, 270 deg.C, 290 deg.C, etc.

The method comprises the following steps:

(1) preparing samples with different cross-linking agent contents;

(2) crosslinking the sample at a temperature;

(3) placing 5-10mg of the crosslinked sample in a DSC crucible for testing;

next, for the data analysis process, DSC curves of different samples were determined by a continuous self-nucleation annealing (SSA) thermal fractionation technique, in which a DSC test was used to characterize a semi-crystalline polymer capable of molecular separation during crystallization. The SSA method is based on the superposition of self-nucleation and annealing cycles, each of which is similar. SSA is essentially a thermal fractionation process based on the sequential application of polymer nucleation and annealing steps. After elimination of the thermal history, the melting point distribution resulting from the heterogeneity of the polymer network structure was revealed in the DSC heating program run in the last step. The polymer network structure must be heterogeneous in order to have a wide distribution of crystallizable segments, such as randomly distributed ethylene/double-stranded-olefin copolymers, crosslinked polyethylene, or crystallizable polymers to confine the chains in the microphase-separated block copolymer. Many copolymers have a highly heterogeneous distribution, i.e., the distribution of Short Chain Branches (SCBs) is heterogeneous along a particular chain, and each chain or group of chains may have a different distribution of chain branches.

After cross-linking, a three-dimensional network structure is formed among molecular chains, the movement of the molecular chains is limited, small grains and incomplete crystals are formed in the crystallization process of the polymer, the thickness of a wafer is reduced, and therefore the melting point of the polymer is reduced. Through crystallization classification, molecular chains with different wafer thicknesses can be separated, therefore, the influence of crosslinking on the molecular structure can be seen through the change of each melting peak after crystallization classification, and after the curves are arranged according to the dosage of the crosslinking agent from less to most, the position and the area of the melting peak on each curve can be seen to change.

More specifically, the test procedure is as follows:

(a) heating to a higher temperature limit eliminates the previous thermal history.

(b) An initial "standard" semi-crystalline state is created by cooling the sample from the melt at 1-40 deg.C/min.

(c) The sample is heated at 1-40 deg.c/min from a lower temperature limit to an ideal Self-nucleation temperature as determined in previous Self-nucleation (SN) experiments. Determining the Ts temperature is more critical, and determining the proper Ts temperature in the SSA test, wherein the determined temperature is usually 10-25 ℃ higher than the melting point, the maximum number of self-nuclei is ensured when the sample is saturated at the ideal Ts temperature, and the lowest temperature is cooled to the lower temperature limit to ensure that the shortest crystallization sequence is classified. The present invention is not particularly limited.

(d) The sample was kept at the desired Ts for 5 minutes.

(e) Cool from the ideal Ts.

(f) The sample was heated from the lower temperature limit to Ts,2 at 1-40 deg.C/min.

(g) The sample was cooled from Ts,2 to the lower temperature limit at 1-40 deg.C/min.

(h) The sample is heated from the lower temperature limit at 1-40 ℃/min to Ts, 3. In the above steps, the gradient temperature difference may be adjusted based on the Ts temperature determination so that a sufficient number of DSC curves are obtained. For example, the temperature difference may be 2 ℃, 4 ℃, 6 ℃, 8 ℃, 10 ℃ or the like.

(i) The steps similar to (g) and (h) are repeated at a decreasing Ts temperature as shown in fig. 1 until the full width of the melting range is included.

After the DSC curve is determined, characteristic peaks of a main melting peak and a series of small melting peaks usually exist on each DSC curve under different operating parameters, so that the change of one to two small melting peaks behind the main melting peak and the main melting peak can be observed by arranging the curves according to the sequence of increasing/decreasing the operating conditions, and the change of the characteristic peaks can be summarized to the optimal foaming condition. Both the main melting peak and the subsequent small melting peak will have different peak positions and areas under different operating conditions.

The curves are sorted according to the operating conditions (change of the amount of the cross-linking agent or change of the foaming temperature), the area of the main melting peak is reduced on each curve in turn, and the more the melting peak is reduced, the more the area of the melting peak is reduced, which indicates that the cross-linking degree is higher. The cross-linking is firstly destroyed by the most regular part of the molecular chain, so the cross-linking degree can be estimated mainly by the change of the highest melting peak, and the cross-linking agent content corresponding to the critical position of peak area disappearance is the optimal content.

In the actual calculation, there is usually no definite value for the critical value at which the peak area disappears, but a range value can be defined. For example: after the curve with the minimum melting peak area is obtained, the deviation of the content of the corresponding cross-linking agent within 10 percent can be regarded as the optimal dosage range; for example, when a curve with the smallest main melting peak area can be detected using a crosslinker amount of 0.5 wt.%, a deviation of. + -. 10% can be given over this amount range, i.e., 0.45-0.55 wt.% is taken as the optimum amount range; the deviation may be set to 0.5 wt%, 1 wt%, 2 wt%, 5 wt%, or the like depending on the accuracy of the experimental conditions. If the binary blend polymer exists, two main melting peaks exist on a DSC curve, when the optimal cross-linking agent dosage is determined, the critical cross-linking agent dosage when each peak area of the two main melting peaks disappears is still determined by referring to the process, and an interval formed by the two cross-linking agent dosages is the optimal cross-linking agent addition amount in the foaming process of the binary blend polymer. This threshold value can also be obtained according to the usual numerical determination methods in the industry, for example: on two DSC curves arranged in sequence, one curve has the minimum peak area, and the other curve does not have the peak, so that the average value of the cross-linking agent dosage corresponding to the two curves can be used as a critical value; the data can also be obtained by interpolation in a numerical calculation mode; the determination method of these critical values can be selected appropriately according to the actual situation, and the present invention is not particularly limited.

For the part located at the rear of the main melting peak (or in the low temperature region), a plurality of small melting peaks exist, the small melting peaks are shown to shift on the DSC curve and are stabilized to a specific temperature range, and the temperature corresponding to the position of the continuously approaching peak is determined as the optimal foaming temperature. The reasons for the above-mentioned melting point depression include: (1) EVA is a random copolymer of VA and PE, and the addition of LDPE destroys the regularity of a PE chain segment and lowers the melting point; (2) the cross-linking forms a three-dimensional network structure, which restricts the movement of molecular chains, and thus the melting point is lowered. The first damaged cross-linking is the most regular sequence of molecular chains, namely the largest melting peak, so that the influence of two aspects is combined, the melting peak shifts to low temperature along with the increase of the cross-linking degree, the peak area is reduced, when the temperature is determined, the first small peak (or called the secondary melting peak) which is inferior to the main melting peak can be usually taken, the small peak on each DSC curve can continuously shift the peak position to a lower temperature along with the change of condition parameters, the stability is started near a certain temperature, and the temperature corresponding to the continuously close peak can be taken as the optimal foaming temperature; if the mixture is a binary mixture, two secondary melting peaks exist, the position deviation of the two secondary melting peaks is observed by adopting the method, two approximate temperature values can be obtained, and the interval of the two temperature values is used as the optimal foaming temperature; in addition, if the deviation of the secondary melting peak is close to a specific temperature, a certain deviation of the temperature value can be taken as the range of the optimal foaming temperature, for example: when the minor melting peak approaches 90 ℃, 88 ℃, 87 ℃, 86.5 ℃ and 86.4 ℃, a certain deviation amount of 86.4 ℃ can be taken as an optimum parameter, the deviation can be 3%, the range of 83.8-89.0 ℃ can be obtained, and the deviation values can be set to be 0.5%, 1%, 2%, 5% and the like.

After the predicted value is obtained, the correctness of the optimized temperature can be checked through a verification experiment.

Example 1

The resins used in the embodiment 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 wt% of EVA, 11 wt% of LDPE and 0 wt% -1.32 wt% of crosslinking agent BIBP according to mass percentage, and respectively marking samples with different crosslinking agent contents as # 1: BIBP is 0 wt%; 2 #: BIBP is 0.17 wt%; BIBP is 0.33 wt%; 3 #: BIBP is 0.5 wt%; 4 #: BIBP is 0.66 wt%; 5 #: BIBP is 0.99 wt%; 6 #: BIBP ═ 1.32 wt%. Then, the weighed raw materials are uniformly mixed and poured into an extruder for extrusion granulation, the twin-screw extruder is totally divided into 10 heating zones according to the extrusion direction in operation, the temperature of a first zone is 90 ℃, the temperature of a second zone is 120 ℃, the temperature of a third zone is 120 ℃, the temperature of a fourth zone is 125 ℃, the temperature of a fifth zone is 125 ℃, the temperature of a sixth zone is 125 ℃, the temperature of a seventh zone is 125 ℃, the temperature of an eighth zone is 125 ℃, the temperature of a ninth zone is 130 ℃, the temperature of a machine head is 130 ℃, and the rotating speed of a screw is 100 r/min.

Step two: pressing a 10 multiplied by 2 multiplied by 1.5mm plate by using a flat vulcanizing machine, when the temperature of the flat vulcanizing machine reaches the set temperature of 190 ℃ and is stabilized for about 10min, putting a sample which is well prepared in advance into a plate pressing machine, closing a mold, vacuumizing and starting to time recording, pressing the mold after the pressure of a mold cavity reaches 2MPa until the pressure of the mold cavity reaches 9.5MPa (the limit pressure of the flat vulcanizing machine is 9.5MPa), recording for 30s, closing the vacuum, opening the mold and taking out a mold frame, controlling the process between 5 and 10min, cooling for 3 to 5min, and demolding to obtain the cross-linked EVA/LDPE blend material.

Step three: weighing 5-10mg of sample, placing the sample in a DSC crucible, and adding the sample in N₂Temperature cycling experiments were performed under an atmosphere. SSA fractionation was performed according to the following procedure:

(a) heating all samples from 25 ℃ to 160 ℃ at a ramp rate of 10 ℃/min and maintaining for 3min to eliminate thermal history;

(b) cooling the sample from 160 ℃ to 0 ℃ to and maintaining the sample for 5min at a cooling rate of 20 ℃/min;

(c) the sample is rapidly heated (the temperature rise rate is 40 ℃/min) to the set self-nucleation annealing temperature (T)_sPhase comparison of PE_mKeeping the temperature at 10 ℃ higher for 5 min;

(d) cooling the sample to 0 ℃ at a cooling rate of 20 ℃/min and keeping the temperature constant for 5 min; (e) repeating steps (b) and (d) to gradually reduce T_sEach step T_sThe gap is 6 gaps (delta T)_s＝6℃)。T_sControlling the temperature in a sample range of 124-40 ℃;

(f) finally the sample was heated from 0 ℃ to 160 ℃ at a ramp rate of 10 ℃/min, ultimately revealing a continuous self-nucleation and annealing behavior over a series of Ts.

As shown in fig. 3, the determination process for predicting the optimum amount of the crosslinking agent is reflected in the figure. Comparing the SSA heating scan results of the EVA/LDPE blend system with the EVA/LDPE blend system, a new distribution of melting point, lamellar thickness and branching can be observed. All traces clearly showed one major melting peak and a series of small melting peaks. Higher peak temperatures represent thicker average lamellae, ordered by crystallizable sequences in longer chains. At higher temperatures, the longest ethylene chain sequence is consumed first, forming the most dense crystal lattice.

According to the test results of fig. 3, pure EVA and LDPE have major melting peaks around 112.15 ℃ and 92.12 ℃ respectively, and both pure EVA and LDPE after SSA treatment show one major melting peak and a series of small melting peaks. Quantitative analysis of T at each peak_mAnd the corresponding peak areas, the changes in the molecular chain caused by the crosslinking reaction can be quantitatively understood, as shown in tables 1 and 2, respectively. The peak areas and peak positions corresponding to those in FIG. 3 are shown in the following tables, respectively:

TABLE 1

TABLE 2

As can be seen from tables 1, 2 and FIG. 3, pure LDPE has a large corresponding melting peak area at 112.15 ℃ and is the main melting peak, with high crystallinity. When foaming is carried out near the melting temperature, because the crystal regions are completely melted, the melt strength of the matrix is too low, cells are easy to collapse and merge, and the foam appearance is poor. Whereas for EVA, it has a major melting peak at 92.12 ℃; for both pure LDPE and EVA, there is a series of small melting peaks at lower temperatures (with the second melting peak occurring at 104.19 deg.C, 86.5 deg.C, respectively). For the blending material, after a large amount of EVA is added into LDPE, the EVA/LDPE blend does not have a relatively obvious main melting peak because the regularity of the LDPE is greatly damaged, and meanwhile, the molecular chain movement is greatly limited due to the generation of a cross-linking network, so that the main melting peaks corresponding to 112.15 ℃ and 92.12 ℃ shift to low temperature, and the melting peak area is reduced. When the main melting peak is at a critical position where it is about to disappear, it can be determined that the amount of the crosslinking agent is 0.66% by weight.

In EVA/LDPE blend systems, the predominant melting point of LDPE (i.e., T)_m112.15 ℃) gradually disappeared due to the crosslinking reaction. Small peaks correspond to a series of T's during SSA processing, respectively_sTemperature dependent, all below the corresponding T_sAnd (4) horizontal. After the crosslinking agent is added, the melting peaks corresponding to 112.15 ℃ and 92.12 ℃ continue to shift to low temperature, the melting peak area continues to decrease, the main melting peak areas of the EVA phase and the LDPE phase both decrease, and disappear on the curves with the crosslinking agent content of 0.5 wt% and 0.66 wt%, respectively, so that the optimal dosage of the crosslinking agent is determined to be 0.5-0.66 wt%.

Similarly, in FIG. 3 and tables 1 and 2, it can be seen that there is a small melting peak on the left side of 112.15 ℃ and 92.12 ℃ respectively, which tends to be lower toward the low temperature as the amount curve of the crosslinking agent occurs, approaching 87 ℃ and 104.19 ℃ respectively, due to CO₂Has a plasticizing effect on the polymer, so the maximum temperature of the foaming temperature can be slightly lower. Therefore, the predicted foaming temperature is optimized to 87-105 ℃.

The accuracy of the foam temperature and crosslinker content prediction by SSA fractionation was verified by the following procedure.

Cutting a sample with a proper size, placing the sample into a foaming kettle for foaming, wherein the foaming temperature is 80-110 ℃, the pressure is 15MPa, the swelling time is 90min, and the pressure relief time is 2-5s, and taking out the foaming sample to obtain the EVA/LDPE foaming material. The foaming ratios of the foamed materials obtained from the EVA/LDPE blend at different temperatures and with different crosslinker contents are shown in Table 3.

TABLE 3

As can be seen from Table 3, the foaming ratio of the EVA/LDPE blend foamed material increases and then decreases with the increase of the foaming temperature under the same pressure and swelling time after the addition of the crosslinking agent. When the temperature is low, the cross-linking agent of the blend material can not be decomposed when swelling, and the cross-linking degree is not changed; when the temperature rises, the strength of the matrix melt is reduced, which is beneficial to the nucleation and growth of foam cells, so that the foaming ratio is increased along with the rise of the temperature. In addition, due to CO₂Has plasticizing effect on polymer matrix, facilitates molecular chain movement, and can promote CO₂The dissolution and the diffusion of the foam are beneficial to the nucleation and the growth of the foam cells; however, the foaming ratio of the material tends to decrease as the foaming temperature continues to increase. Because the system adopts a chemical crosslinking mode, the decomposition speed of the crosslinking agent is accelerated along with the increase of the temperature, so that the crosslinking degree of the material in the foaming process is higher than that measured before the foaming. The higher the temperature, the larger the degree of crosslinking, and the high degree of crosslinking inhibits cell nucleation and growth, so that the expansion ratio decreases, and even too high a degree of crosslinking results in failure to expand. From the foaming ratio of the EVA/LDPE sample, it can be known that the optimum foaming temperature cannot exceed 100 ℃, which is consistent with the results we predict by SSA classification.

Fig. 4 is an SEM photograph of the foamed material. As can be seen from the SEM image, the cell morphology of the foam sample gradually became better with the increase in the content of the crosslinking agent at the same temperature when the content of the crosslinking agent was lower. When the content of the cross-linking agent reaches 0.33-0.5 wt%, the appearance of the cells is better; the pore morphology gradually worsens as the crosslinker content continues to increase. In general, the uncrosslinked samples have a low matrix melt strength, and the cells are easily broken during foaming to cause gas to escape. In the system, because EVA/LDPE has a branched chain structure and a certain entanglement structure can be formed among molecular chains, the melt strength of the system can also maintain cell growth even if no cross-linking agent is added, but the substrate melt strength is lower, so that the cell morphology is poorer. When the addition of the crosslinking agent is insufficient in improving the viscosity of the matrix, the cell wall of the cell is easy to break due to low melt strength. After the viscosity of the matrix is increased and the melt strength is increased after the crosslinking is further improved, the hole wall is strong and is not easy to break, and a hole with a good structure can be generated. When the degree of crosslinking continues to increase until the matrix viscosity is very high, gas diffusion is difficult, a closed cell structure is formed and the cells are smaller, being bound by the crosslinked network. If the matrix is fully crosslinked resulting in too high a matrix viscosity, the system will not foam. Therefore from the SEM image results of the EVA/LDPE foam we can know that the optimum crosslinker content cannot exceed 0.66 wt%, which is consistent with the results we predict by SSA classification. The prediction effect of SSA grading can be well verified by the EVA/LDPE blending foaming result. Therefore, the SSA grading method has very important significance to the foaming temperature and the content of the cross-linking agent of the semi-crystalline polymer.

Claims

1. A supercritical foaming condition prediction method based on a continuous self-nucleation technology is characterized by comprising the following steps:

step 2, obtaining a DSC curve of the polymer material by an SSA method;

step 3, on DSC curves under different foaming conditions, taking the dosage of the cross-linking agent corresponding to the critical position of the area disappearance position of the main melting peak as an optimal value;

step 4, regarding the next melting peak of which the position is deviated to the low temperature direction as a secondary melting peak, and regarding the temperature corresponding to the temperature approaching position of each secondary melting peak on each DSC curve as the optimal foaming temperature;

the polymer is a semi-crystalline polymer; in the step 1, the polymer material is prepared by blending a polymer raw material and a cross-linking agent;

the polymer material is obtained by blending more than two polymer raw materials;

the SSA method comprises the following steps: step a, heating the polymer to eliminate heat history; step b, determining the Ts temperature, and sequentially carrying out temperature rise continuous self-nucleation and annealing treatment; the maximum temperature during the continuous self-nucleation process is sequentially lowered in each cycle until the full width of the melting range.

2. The method for predicting supercritical foaming conditions based on the continuous self-nucleation technology according to claim 1, wherein when the polymer material is obtained by blending two polymer raw materials, the main melting peaks of the two polymer raw materials are respectively predicted according to step 3 for the amount of the cross-linking agent, and the range between the two amounts is taken as an optimal value;

the foaming temperatures were predicted in accordance with step 4 for the minor melting peaks of the two polymer materials, respectively, and the range between the two temperatures was set as the optimum value.

3. The method of claim 1, wherein when the polymer material is obtained by blending two or more polymer raw materials, the positions of the major melting peak and the minor melting peak of each polymer raw material are determined by comparing the peak positions of the polymer raw materials obtained in a single DSC test.

4. The method for predicting supercritical foaming conditions based on the sequential self-nucleation technique according to claim 1, wherein the optimal value is a value having a deviation range.

5. The method for predicting supercritical foaming conditions based on the sequential self-nucleation technology of claim 4, wherein the deviation range is 0.01-10% of the optimal value;

the foaming is supercritical foaming, the foaming temperature is 50-300 ℃, and the pressure is 5-60 MPa.

6. The method for predicting supercritical foaming conditions based on the continuous self-nucleation technology according to claim 1, wherein the Ts temperature is 10-25 ℃ higher than the melting point of the polymer material.

7. The method for predicting supercritical foaming conditions based on the continuous self-nucleation technology according to claim 1, wherein the gradient of temperature decrease is 0.1-10 ℃ in each cycle; the temperature change rate is 1-40 deg.C/min during the heating and/or annealing process.