CN115073798A - Method for optimizing foaming behavior of polypropylene in supercritical fluid through double-crosslinking modification - Google Patents
Method for optimizing foaming behavior of polypropylene in supercritical fluid through double-crosslinking modification Download PDFInfo
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Abstract
The invention provides a method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double cross-linking modification, which comprises the following steps: melting and blending polypropylene and an auxiliary crosslinking agent, performing hot press molding, placing the obtained green body in an electron beam under the condition of isolating air, and irradiating to enable polypropylene molecular chains to generate chemical crosslinking to obtain a crosslinked green body; swelling the cross-linked green body by using a supercritical fluid and releasing pressure for foaming, wherein in the swelling process, the polypropylene chemical cross-linked network in the cross-linked green body can reduce the crystallinity of the cross-linked green body and widen the melting limit, and meanwhile, the crystallization behavior of the polypropylene is regulated and controlled as an out-of-phase nucleation point to form a polypropylene microcrystal physical cross-linked network in the cross-linked green body; under the combined action of the polypropylene microcrystal physical crosslinking network and the polypropylene chemical crosslinking network, the melt strength and the crystallization degree of the crosslinking green body are regulated and controlled, so that the foaming temperature interval of polypropylene in a supercritical fluid is widened, the foaming temperature is reduced, and the cell structure of the polypropylene foaming material is improved.
Description
Technical Field
The invention belongs to the technical field of polypropylene foaming, and relates to a method for optimizing foaming behavior of polypropylene in a supercritical fluid through double-crosslinking modification.
Background
Polypropylene (PP), one of the five general-purpose plastics, is inexpensive, has high flexural strength, toughness, chemical resistance, fatigue resistance, high insulation, recyclability, and the like, and is widely developed and applied in various fields such as machinery, automobiles, electronic and electronic appliances, construction, textile, packaging, agriculture, forestry, fisheries, and food industry. Global polypropylene demand is expected to approach 9000 ten thousand metric tons. The PP foaming material not only has the characteristics of the PP material and ultralow dielectric constant and dielectric loss, but also has the advantages of light weight, heat preservation, heat insulation, vibration reduction, noise reduction and the like of the traditional foaming material, and has wide application in a plurality of fields of automobiles, aerospace, 5G communication and the like.
At present, the preparation method of the PP foaming material is mainly a chemical foaming method, but the chemical foaming method is difficult to prepare the PP foaming material with smaller cell size, higher foaming multiplying power and uniform cell size, which brings certain limit to the application of the PP foaming material. Although a foaming material with smaller and more uniform cell size can be obtained by supercritical fluid foaming, the problem of cell collapse is very easy to occur in the foaming process due to the excessively low melt strength and crystallization property of PP, and the suitable foaming temperature range is very narrow and usually does not exceed 1 ℃. Slight temperature fluctuation in the foaming process can cause very adverse influence on the appearance of the PP foaming material, so that the requirement of the PP on the control precision of the temperature in the foaming process is very high, and the stable quality of the PP foaming material can be ensured only by stabilizing the temperature of equipment in a very narrow foaming temperature range. The high difficulty of temperature control during foaming and the high requirement on the temperature control fault tolerance of equipment make the supercritical fluid foaming technology difficult to realize the large-scale production of PP foaming materials.
How to improve the foaming performance of PP and effectively widen the foaming temperature range of PP is the primary problem to be solved for realizing the large-scale production of PP foaming materials by adopting a supercritical fluid foaming technology, and is also the key point for improving the performance of PP foaming materials and ensuring the stable quality of the PP foaming materials.
Disclosure of Invention
Aiming at the defects that the foaming temperature range is very narrow, the requirement on temperature control precision in the foaming process is high and industrial scale-up production is difficult to realize in the production of PP foaming materials in the prior art, the invention provides a method for optimizing the foaming behavior of polypropylene in a supercritical fluid through double cross-linking modification, so as to improve the foaming performance of PP, widen the foaming temperature range of PP and provide technical support for the industrial application of preparing PP foaming materials by a supercritical fluid foaming method.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double cross-linking modification, comprising the steps of:
(1) melting and blending polypropylene and an auxiliary crosslinking agent, and carrying out hot press molding on the obtained blend to obtain a blank body;
(2) placing the green body in an electron beam under the condition of air isolation to perform irradiation so as to enable polypropylene molecular chains to generate chemical crosslinking, thereby obtaining a crosslinked green body;
(3) swelling the cross-linked green body by using a supercritical fluid, releasing pressure and foaming, wherein in the swelling process, the polypropylene chemical cross-linked network in the cross-linked green body can limit the movement capacity of a polypropylene molecular chain, partially destroy the crystalline network of polypropylene, reduce the crystallinity of the cross-linked green body and widen the melting limit; meanwhile, a polypropylene chemical cross-linked network in the cross-linked blank is used as an out-of-phase nucleation point to regulate the crystallization behavior of polypropylene, and a polypropylene microcrystal physical cross-linked network is formed in the cross-linked blank;
under the combined action of the polypropylene microcrystal physical crosslinking network and the polypropylene chemical crosslinking network, the melt strength and the crystallization degree of the crosslinking green body are regulated and controlled, so that the foaming temperature range of polypropylene in a supercritical fluid is widened, the foaming temperature is reduced, and the foam pore structure of the polypropylene foaming material prepared by foaming is improved.
In the technical scheme of the method for optimizing the foaming behavior of the polypropylene in the supercritical fluid through double-crosslinking modification, the crosslinking degree of the crosslinking green body is controlled by controlling the addition amount and the irradiation dose of the auxiliary crosslinking agent, so that the gel content of the crosslinking green body is 25-40 wt%. The main purpose is to control the crosslinking green body in the step (3) to have a proper chemical crosslinking degree and form a proper physical crosslinking network of the polypropylene microcrystals in the swelling process so as to maintain the cell structure without limiting the growth of cells, improve the cell structure of the polypropylene foaming material while widening the foaming temperature interval and enable the foaming ratio of the foaming material to be easily regulated and controlled.
The method for measuring the gel content of the cross-linked green body comprises the following steps: taking the cross-linked green body in the step (2) as a sample, wrapping the sample by using a stainless steel filter screen, placing the sample in a three-neck flask filled with a xylene solvent, heating and refluxing for 72 hours at 200 ℃, taking the sample and the filter screen out, washing the sample and the filter screen twice by using absolute ethyl alcohol, placing the sample and the filter screen in a vacuum oven, drying the sample for 2 hours at 80 ℃, and weighing the sample; this was repeated several times until the mass difference of the dried samples was negligible, and the gel content was calculated using the following formula:
in the above formula, W 1 Mass of the sample before being wrapped by the screen, W 2 Is the mass of the screen, W 3 The total mass of the filter screen and the sample after final drying.
In the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through double-crosslinking modification, the content of the auxiliary crosslinking agent in the blank body in the step (1) is preferably 1.2 to 2 weight percent; regarding the form of the auxiliary crosslinking agent, the auxiliary crosslinking agent can be directly added to the polypropylene for blending, or the auxiliary crosslinking agent can be added to the polypropylene while being supported on a carrier for blending, and when the auxiliary crosslinking agent is introduced in the form of being supported on a carrier, the content of the auxiliary crosslinking agent herein refers to the content of the auxiliary crosslinking agent alone without including the content of the carrier.
In the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through double cross-linking modification, the irradiation dose in the step (2) is preferably 20-100 kGy.
In the technical solution of the above method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double crosslinking modification, the auxiliary crosslinking agent comprises at least one of styrene, styrene analogs (such as divinylbenzene, etc.), acrylic acid derivatives (such as pentaerythritol tetramethacrylate, etc.), maleic anhydride, salts of maleic anhydride (such as diallyl maleate, etc.), compounds containing acrylic acid structures and isocyanate (such as triallyl isocyanurate).
In the step (3) of the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through double cross-linking modification, one feasible operation of swelling and pressure-relief foaming the polypropylene blank body by using the supercritical fluid is as follows: placing the crosslinked green body in a high-pressure cavity, introducing gas serving as a physical foaming agent into the high-pressure cavity, controlling the temperature of the high-pressure cavity to be 151-165 ℃, controlling the pressure of the high-pressure cavity to convert the gas serving as the physical foaming agent into a supercritical fluid state, keeping the temperature and the pressure until the physical foaming agent achieves swelling balance in the crosslinked green body, and then releasing pressure for foaming to obtain the polypropylene foaming material.
Further, in the step (3) of the technical scheme of the method for optimizing the foaming behavior of the polypropylene in the supercritical fluid through molecular structure regulation, the pressure of the high-pressure cavity is controlled to be 7.32-30 MPa.
Further, in the technical solution of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation, the gas as the physical foaming agent can be N 2 、CO 2 Or an inert gas.
In the technical scheme of the method for optimizing the foaming behavior of polypropylene in the supercritical fluid through double cross-linking modification, the polypropylene adopted in the step (1) refers to polypropylene which is not subjected to structural modification, and comprises isotactic polypropylene, syndiotactic polypropylene and atactic polypropylene.
Based on the method for optimizing the foaming behavior of the polypropylene in the supercritical fluid through double cross-linking modification, the invention also provides a preparation method of the polypropylene foaming material, which comprises the following steps:
(1) melting and blending polypropylene and an auxiliary crosslinking agent, and carrying out hot press molding on the obtained blend to obtain a blank body;
(2) placing the green body in an electron beam under the condition of air isolation to perform irradiation so as to enable polypropylene molecular chains to generate chemical crosslinking, thereby obtaining a crosslinked green body;
(3) and swelling the crosslinked polypropylene green body by using a supercritical fluid, and releasing pressure for foaming to obtain a cellular structure of the polypropylene foam material.
Furthermore, the crosslinking degree of the crosslinked green body is controlled by controlling the addition amount of the auxiliary crosslinking agent and the irradiation dose, so that the gel content of the crosslinked polypropylene green body is 25-40 wt%. The content of the co-crosslinking agent in the blank body in the step (1) is preferably 1.2-2 wt%; the co-crosslinking agent includes at least one of styrene, styrene analogs (e.g., divinylbenzene, etc.), acrylic acid derivatives (e.g., pentaerythritol tetramethacrylate, etc.), maleic anhydride, salts of maleic anhydride (e.g., diallyl maleate, etc.), compounds containing an acrylic acid structure and a cyclic isocyanate (e.g., triallyl isocyanurate). The irradiation dose in the step (2) is preferably 20-100 kGy.
In the preparation method of the polypropylene foaming material, the operation of swelling and pressure-relief foaming of the cross-linked green body by adopting the supercritical fluid in the step (3) is as follows: placing the crosslinked olefin blank body in a high-pressure cavity, introducing gas serving as a physical foaming agent into the high-pressure cavity, controlling the temperature of the high-pressure cavity to be 151-165 ℃, controlling the pressure of the high-pressure cavity to convert the gas serving as the physical foaming agent into a supercritical fluid state, keeping the temperature and the pressure until the physical foaming agent achieves swelling balance in the crosslinked blank body, and then releasing pressure for foaming to obtain the polypropylene foaming material. In the step (3), the pressure of the high-pressure cavity is preferably controlled to be 7.32-30 MPa. The gas as a physical blowing agent may be N 2 、CO 2 Or an inert gas.
After the method disclosed by the invention is adopted to optimize the foaming behavior of polypropylene in the supercritical fluid, the foaming temperature interval of the polypropylene foaming material can be widened from about 1 ℃ to about 15 ℃, the prepared polypropylene foaming material has uniform closed-cell structure foam cells, the size and distribution of the foam cell structure are uniform, and the foaming multiplying power can reach about 32 times, and generally can be between 2 and 32 times. The supercritical fluid foaming is carried out in a temperature interval suitable for foaming, the size and the foaming ratio of the foam pores of the polypropylene foaming material can be changed along with the change of the chemical crosslinking degree of the polypropylene and the change of the physical crosslinking network of the polypropylene microcrystals, and the chemical crosslinking degree of the polypropylene can be controlled by selecting the appropriate dosage of the auxiliary crosslinking agent and the irradiation dosage according to the requirement of the polypropylene foaming material in actual application, so that the physical crosslinking network of the polypropylene microcrystals is regulated and controlled, and the foam pore structure and the foaming ratio are regulated and controlled by matching with the foaming temperature.
The mechanism for optimizing the foaming behavior of the polypropylene in the supercritical fluid is as follows:
according to the invention, a chemical crosslinking structure is introduced into the polypropylene raw material by adding the auxiliary crosslinking agent and radiation crosslinking, and the crosslinked green body is controlled to have a proper crosslinking degree, so that on one hand, the chemical crosslinking network of the polypropylene in the crosslinked green body can improve the melt strength of the polypropylene raw material in the supercritical fluid swelling process; on the other hand, the polypropylene chemical cross-linked network in the cross-linked green body can limit the movement capability of a polypropylene molecular chain, partially destroy the crystalline network of polypropylene, reduce the crystallinity of the cross-linked green body and widen the melting limit; meanwhile, a polypropylene chemical cross-linked network in the cross-linked blank is used as a heterogeneous nucleation point to promote the polypropylene raw material to form a crystal region with proper grain density and small grain size, namely a polypropylene microcrystal physical cross-linked network is formed in the cross-linked blank, and the polypropylene microcrystal physical cross-linked network can improve the mechanical strength of a matrix and can also provide certain toughness. Finally, under the combined action of a proper polypropylene microcrystal physical crosslinking network and a proper polypropylene chemical crosslinking network, the melt strength and the crystallization degree of the polypropylene are regulated and controlled to be suitable for the foaming degree of the supercritical fluid, so that the foaming temperature range of the polypropylene in the supercritical fluid is widened, the foaming temperature is reduced, the foam structure of the polypropylene foaming material prepared by foaming is improved, and the performance of the polypropylene foaming material is improved.
Compared with the prior art, the technical scheme provided by the invention can produce the following beneficial technical effects:
1. the invention provides a method for optimizing polypropylene foaming behavior in supercritical fluid through double cross-linking modification, which introduces a proper amount of chemical cross-linking structures into a high-crystallinity polypropylene raw material by adding an auxiliary cross-linking agent and radiation cross-linking to the polypropylene, forms a proper polypropylene microcrystal physical cross-linking network, and improves the melt strength, the crystallization degree and the distribution of the polypropylene raw material in the supercritical fluid swelling process through the combined action of the chemical cross-linking network and the physical cross-linking network, thereby widening the foaming temperature range of the polypropylene, reducing the foaming temperature and improving the cell structure of the polypropylene foaming material. The expansion of the foaming temperature interval can improve the problem that obvious differences appear in the foam morphology and the foaming multiplying power of the foaming material caused by unstable equipment temperature control in the production process, reduce the temperature control difficulty, improve the fault tolerance of the equipment, and reduce the energy consumption by moving the foaming temperature to a low temperature, thereby having positive significance for the product quality control in the actual production. The invention can provide technical support for the large-scale production of the polypropylene foaming material by utilizing the supercritical fluid foaming technology, and promote the industrial process of the supercritical fluid foaming of the polypropylene foaming material.
2. Experiments prove that the foaming temperature range of the polypropylene foaming material can be widened from about 1 ℃ to about 15 ℃ by the method, the prepared polypropylene foaming material has uniform closed cell structure cells, the cell structure size and distribution are uniform, the foaming ratio can reach about 32 times, usually 2-32, the proper amount of the auxiliary crosslinking agent, the irradiation dose and the foaming temperature can be selected according to the requirements of the polypropylene foaming material in actual application, and the cell structure and the foaming ratio of the polypropylene foaming material are good in controllability and wide in adjustable range. The invention can solve the problems of difficult control of foam holes, wide distribution range of the size of the foam holes and lower foaming ratio of the polypropylene foaming material prepared by the existing chemical foaming method, can improve the mechanical property and the dielectric property of the existing polypropylene foaming material and improve the quality of the polypropylene foaming material.
3. The method has simple process and good process controllability, particularly widens the foaming temperature range of polypropylene, reduces the temperature control difficulty, and is favorable for popularization and application in industrial practice.
Drawings
FIG. 1 is a graph showing the gel contents of the respective crosslinked green bodies prepared in example 1, wherein 0 wt% TAIC, 1 wt% TAIC, 2 wt% TAIC, and 3 wt% TAIC represent SiO of the green body supporting the co-crosslinking agent TAIC, respectively 2 The gel content of the resulting crosslinked base material after irradiation was 0 wt%, 1 wt%, 2 wt%, 3 wt%, respectively.
FIG. 2 is a rheological curve of each crosslinked green body prepared in example 1, in which the graphs (a) to (d) are respectively SiO supporting a co-crosslinking agent TAIC 2 The contents of (a) and (b) are respectively 0 wt%, 1 wt%, 2 wt% and 3 wt%, and the rheological curve test result of the cross-linked body obtained after irradiation.
FIG. 3 is an infrared spectrum of a crosslinked green body obtained by irradiation with different irradiation doses, in which the curves correspond, from bottom to top, to irradiation doses of 0kGy, 10kGy, 20kGy, 30kGy, 50kGy, 100kGy, 200kGy, and 400 kGy.
FIG. 4 is a graph showing the change of expansion ratio of the polypropylene foam material prepared in example 2 with the addition amount of the auxiliary crosslinking agent, irradiation dose and foaming temperature, wherein (a) to (d) are respectively SiO in the form of loading TAIC 2 The foaming multiplying power of the polypropylene foaming material obtained after foaming the cross-linked green body prepared on the basis of the green body with the content of 0 wt%, 1 wt%, 2 wt% and 3 wt% changes along with the change curve of the irradiation dose and the foaming temperature.
FIG. 5 is an isothermal crystal morphology at 128 ℃ of each crosslinked green body sample observed by a polarization microscope, wherein irradiation doses of the samples in the graphs (a) to (d) are 0kGy, 20kGy, 30kGy and 200kGy, respectively.
FIG. 6 is an isothermal crystallization diagram of a cross-linked green body obtained after irradiation of the green body with different irradiation doses.
FIG. 7 is a graph showing the tendency of the crystallinity (Xc) and the melting temperature (Tm) of a crosslinked green body according to the irradiation dose.
FIG. 8 is an SEM photograph of a cross section of a polypropylene foam material prepared by foaming at 155 ℃ wherein the irradiation doses in the (a) to (g) charts are 0kGy, 10kGy, 20kGy, 30kGy, 50kGy, 100kGy and 200kGy, respectively.
FIG. 9 is an SEM photograph of a cross section of a polypropylene foam material prepared by foaming at 165 ℃ wherein the irradiation doses in the graphs (a) to (f) are 10kGy, 20kGy, 30kGy, 50kGy, 100kGy and 200kGy, respectively.
FIG. 10 shows statistics of cell size and cell density of polypropylene foams prepared by foaming at 155 ℃ and 165 ℃ and the results are shown in FIG. 10, wherein the foaming temperatures of (a) and (b) are 155 ℃ and 165 ℃, respectively.
Detailed Description
The method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double cross-linking modification according to the present invention is further illustrated by the following examples, which are only a part of the embodiments of the present invention, but not all of them. Other embodiments, which can be derived by those skilled in the art from the summary and examples of the invention without creative efforts, are within the protection scope of the present invention.
In each of the following examples, the polypropylene used was isotactic polypropylene (iPP), model T30S, and the density was 0.91g/cm 3 A melt flow index of 3.0g/10min at 230 ℃ and under a load of 2.16 kg; the auxiliary crosslinking agent is triallyl isocyanurate (TAIC), specifically SiO 2 TAIC was supported on SiO as a carrier 2 The powder formed on the surface is SiO loaded with an auxiliary cross-linking agent 2 The TAIC content is 70 wt%, and the molecular weight of TAIC is 249.27 g/mol.
Example 1
In the embodiment, the auxiliary crosslinking agents with different proportions and the iPP are subjected to melt blending, and the influence of the auxiliary crosslinking agents on the iPP irradiation crosslinking is examined.
(1) iPP granules and SiO loaded with auxiliary cross-linking agent TAIC 2 (TAIC-Supported SiO 2 Wherein the content of TAIC is 70 wt%) into a double-screw extruder, melting and blending at 180 ℃, extruding and granulating to obtain blend granules, and carrying out hot press molding on the blend granules on a vacuum molding press at 190 ℃ to obtain a sheet blank. SiO by adjusting iPP granules and TAIC (supported cross-linking agent) 2 The SiO loaded with the assistant cross-linking agent TAIC is prepared 2 The content of (a) is 0 wt%, 1 wt%, 2 wt% and 3 wt% of the green body respectively.
(2) Placing the green bodies with different contents of the co-crosslinking agents prepared in the step (1) in an electron beam (energy is 1.6MeV, and absorption dose rate is 3.6 multiplied by 10) under the condition of isolating air 4 Carrying out irradiation in kGy/h) to enable polypropylene molecular chains to carry out chemical crosslinking, and respectively controlling the irradiation dose to be 10kGy, 20kGy, 30kGy, 50kGy, 100kGy, 200kGy and 400kGy to obtain a series of crosslinking blanks.
(3) Gel content determination
Respectively taking the cross-linked green bodies prepared under the conditions in the step (2) as samples, wrapping the samples by using a 200-mesh stainless steel filter screen, placing the samples in a three-neck flask filled with a xylene solvent, heating and refluxing for 72 hours at 200 ℃, taking out the samples and the filter screen, washing the samples and the filter screen twice by using absolute ethyl alcohol, placing the samples and the filter screen in a vacuum oven, drying the samples for 2 hours at 80 ℃, and weighing the samples; this was repeated several times until the mass difference of the dried samples was negligible, and the gel content was calculated using the following formula:
in the above formula, W 1 Mass of the sample before being wrapped by the screen, W 2 Is the mass of the screen, W 3 The total mass of the filter screen and the sample after final drying.
FIG. 1 shows the gel contents of the respective crosslinked green bodies prepared in this example, wherein 0 wt% TAIC, 1 wt% TAIC, 2 wt% TAIC, and 3 wt% TAIC represent SiO of the green body supporting the co-crosslinking agent TAIC 2 The gel content of the resulting crosslinked base material after irradiation was 0 wt%, 1 wt%, 2 wt%, 3 wt%, respectively. As can be seen from FIG. 1, when the green body does not contain the co-crosslinking agent, the iPP does not generate a crosslinked structure after irradiation, and the gel content of 20 wt% is only generated when the irradiation dose is as high as 400 kGy; when the body is loaded with SiO of the assistant crosslinking agent TAIC 2 When the content of (A) is 1 wt%, the irradiation crosslinking reaction is promoted, and a crosslinking structure begins to appear when the irradiation dose is low; when the auxiliary cross-linking agent TAIC SiO is loaded in the green body 2 When the content of the cross-linked green body is increased to 2-3 wt%, an obvious cross-linked structure appears under the irradiation dose of 10-30 kGy, the gel content is greatly increased under the lower irradiation dose, and when the irradiation dose reaches 30-100 kGy, the gel content of the cross-linked green body reaches 30-40 wt%.
(4) Viscoelasticity test
The viscoelasticity of the molecular chain of the cross-linked billet can be characterized by a rheological curve, the cross-linked billet prepared under each condition in the step (2) is taken as a sample to be subjected to strain scanning, and the specific experimental conditions are as follows: the strain range is selected to be 0.01-100%, the testing frequency is 1Hz, and the testing temperature is 200 ℃. On the basis of strain scanning, selecting proper amplitude to carry out small amplitude oscillatory shear test (SAOS) to characterize the rheological property of each sample, wherein the specific experimental conditions are as follows: the frequency range is 0.01-500 rad/s, the test amplitude is 5%, and the test temperature is 200 ℃. The rheological curves are shown in FIG. 2, and the graphs (a) to (d) in FIG. 2 are respectively load-assisted curvesSiO of cross-linking agent TAIC 2 The contents of (a) and (b) are respectively 0 wt%, 1 wt%, 2 wt% and 3 wt%, and the rheological curve test result of the cross-linked green body is obtained after the green body is irradiated under different irradiation doses.
As can be seen from the graph (a) in FIG. 2, for the iPP without the addition of the co-crosslinking agent, the irradiation is mainly degraded, and the crosslinked structure appears only when the irradiation dose exceeds 200kGy, which is consistent with the previous gel content test result. As can be seen from the graphs (b) to (d) in FIG. 2, as the content of the co-crosslinking agent in the green body increases, the tail end of the rheological curve of the crosslinked green body prepared under a lower irradiation dose begins to have a plateau phenomenon, which indicates that the rheological curve begins to form a crosslinked network under a low irradiation dose, so that the elastic modulus of the crosslinked green body is improved, and the irradiation is mainly crosslinking. Comparing the viscoelastic behavior of the cross-linked green bodies after irradiation of green bodies with different contents of the auxiliary cross-linking agent, finding that SiO of the TAIC is loaded in the green bodies 2 When the content of (A) is 2-3 wt%, the effect of irradiation crosslinking is relatively better.
(5) Infrared Spectrum testing
The irradiation crosslinking reaction of iPP is influenced by the molecular structure of iPP and irradiation conditions, especially by oxygen, which is easy to cause oxidative degradation of iPP in the irradiation crosslinking process. In order to investigate whether the iPP generates oxidative degradation reaction in the irradiation crosslinking process in the process of preparing the crosslinked green body, the invention carries out oxidation degradation reaction on SiO loaded with the assistant crosslinking agent TAIC 2 A series of cross-linked green bodies prepared after irradiation of the green body having the content of 2 wt% under the conditions of irradiation doses of 0kGy, 10kGy, 20kGy, 30kGy, 50kGy, 100kGy, 200kGy, and 400kGy were subjected to infrared spectrum testing, and the results are shown in fig. 3. 1040cm in infrared spectrum -1 And 940cm -1 The peak is respectively the stretching vibration peak of C-O-C and terminal double bond (-C ═ C), the peak intensity of each cross-linking green body at the two positions is basically not changed along with the increase of the irradiation dose, which indicates that the iPP is basically not subjected to oxidative degradation reaction under the irradiation of electron beams.
Example 2
Supercritical CO was carried out based on a series of cross-linked green bodies prepared in example 1 2 Foaming and examinationThe influence of the addition amount of the auxiliary crosslinking agent and the irradiation condition on the foaming behavior of the iPP.
A series of cross-linked blanks obtained by irradiating blanks with different contents of the CO-crosslinking agents in example 1 are respectively placed in a high-pressure reaction kettle, and CO serving as a physical foaming agent is introduced into the high-pressure reaction kettle 2 Discharging air from the high-pressure reaction kettle for several times, setting the temperature of the high-pressure reaction kettle, and introducing CO into the high-pressure reaction kettle when the temperature in the high-pressure reaction kettle is close to the set temperature 2 And controlling the temperature of the high-pressure cavity to be a set temperature and the pressure to be 16MPa to swell until the foaming agent reaches swelling balance in the cross-linked blank (the swelling time is about 2 hours), and then quickly releasing pressure to foam to obtain a series of polypropylene foaming materials.
In this embodiment, different foaming temperatures are set for the cross-linked green bodies formed after irradiation of green bodies with different contents of the co-crosslinking agent, specifically as follows:
for SiO supported by auxiliary crosslinking agent TAIC 2 Setting the temperature of a high-pressure reaction kettle at 150 ℃, 153 ℃, 157 ℃, 162 ℃ and 165 ℃ respectively for a cross-linked blank prepared from the blank with the content of 0 wt.%;
for SiO supported by auxiliary crosslinking agent TAIC 2 Setting the temperature of a high-pressure reaction kettle to be 150 ℃, 153 ℃, 157 ℃, 162 ℃ and 165 ℃ respectively for a cross-linked blank prepared from the blank with the content of 1 wt.%;
for SiO supported by auxiliary crosslinking agent TAIC 2 Setting the temperature of a high-pressure reaction kettle to be 149 ℃, 153 ℃, 157 ℃, 162 ℃ and 165 ℃ respectively for a cross-linked blank prepared from a blank with the content of 2 wt.%;
for SiO supported by auxiliary crosslinking agent TAIC 2 The temperature of the high-pressure reaction kettle is respectively set to be 150 ℃, 153 ℃, 157 ℃, 162 ℃ and 165 ℃.
FIG. 4 is a graph showing the variation of the expansion ratio of the polypropylene foam material prepared in this example with the addition amount of the auxiliary crosslinking agent, the irradiation dose and the foaming temperature, wherein (a) to (d) are respectively SiO in the form of loading TAIC 2 The content is 0 wt%And the change curve of the foaming multiplying power of the polypropylene foaming material obtained after foaming the cross-linked blank prepared on the basis of the blanks of 1 wt%, 2 wt% and 3 wt% along with the irradiation dose and the foaming temperature.
As can be seen from the two graphs in fig. 4 (a), for pure iPP, the foaming effect at high temperature is poor, and during the swelling and foaming process of the supercritical fluid, the melt strength of the blank is provided by the physical cross-linking network constructed by the partially melted crystal region, and the foaming temperature range is extremely narrow, so that the melting degree of the crystal region is extremely difficult to control, the cells are difficult to grow at low temperature, and the cells collapse at high temperature; when the irradiation dose is increased to 400kGy, the foaming ratio is improved due to the construction of a chemical crosslinking network in the blank. As can be seen from the graph (b) in FIG. 3, when the SiO of the assistant crosslinking agent TAIC is loaded in the green body 2 When the content of (A) is 1 wt%, the foaming effect of the crosslinked green body at high temperature is still poor, and the foaming temperature range is still very narrow; when the irradiation dose is increased to be more than 100kGy, the foaming multiplying power of the composite material is increased, and meanwhile, the foaming temperature range is widened to a certain extent, but when the irradiation dose is 100-200 kGy, the foaming multiplying power is still low, and only when the irradiation dose is increased to be 400kGy, the foaming multiplying power is obviously improved. As can be seen from the two graphs (c) and (d) in FIG. 3, when the co-crosslinking agent TAIC SiO is loaded in the olefinic substrate 2 When the content of the iPP is 2 wt%, the chemical crosslinking network constructed in the iPP after irradiation is moderate, the existence of the polypropylene chemical crosslinking network not only provides the melt strength required by the foaming of the supercritical fluid, but also partially destroys the crystal network of the iPP, reduces the crystallinity of the iPP, and promotes the construction of the microcrystalline physical crosslinking network of the iPP in the swelling process. A 'double cross-linked network' structure with a chemical cross-linked network and an iPP microcrystal physical cross-linked network is constructed in the swelling process, so that the foaming temperature interval is widened to about 15 ℃, the foaming multiplying power can reach 32 times at most, and the pore diameter of the foaming material is uniform. When the body is loaded with SiO of the assistant crosslinking agent TAIC 2 When the content of (3) is 3 wt%, the chemical crosslinking degree of the iPP after irradiation is increased, particularly after the irradiation dose reaches 100kGy, the chemical crosslinking degree is too high, so that the movement of an iPP molecular chain is more limited,this may cause a certain degree of inhibition of cell growth.
As can be seen from fig. 1 and 4, when the gel content of the cross-linked green body is 25 wt% to 40 wt%, a proper amount of the constructed polypropylene chemical cross-linked network and a proper amount of the polypropylene microcrystalline physical cross-linked network constructed on the basis of one time are provided, and under the combined action of the polypropylene microcrystalline physical cross-linked network and the polypropylene chemical cross-linked network, the method effectively improves the foaming behavior of polypropylene in the supercritical fluid, for example, widens the foaming temperature range of polypropylene, reduces the foaming temperature of polypropylene, and enables the foaming ratio of the polypropylene foaming material to be well controllable, and to obtain a high foaming ratio.
Example 3
In this example, the influence of the irradiation dose on the iPP crystallization behavior was examined.
(1) iPP granules and SiO loaded with auxiliary cross-linking agent TAIC 2 (TAIC-Supported SiO 2 Wherein the content of TAIC is 70 wt%) into a double-screw extruder, melting and blending at 180 ℃, extruding and granulating to obtain blend granules, then carrying out hot press molding on the blend granules on a vacuum molding press at 190 ℃ to obtain a sheet-shaped blank body, wherein the blank body is loaded with SiO of the assistant crosslinking agent TAIC 2 The content of (B) is 2 wt%.
(2) Placing the blank prepared in the step (1) in an electron beam (energy is 1.6MeV, absorption dose rate is 3.6 multiplied by 10) under the condition of air isolation 4 Carrying out irradiation in kGy/h) to enable polypropylene molecular chains to be subjected to chemical crosslinking, and respectively controlling irradiation doses to be 0kGy, 10kGy, 20kGy, 30kGy, 50kGy, 100kGy, 200kGy and 400kGy to obtain a series of crosslinking blanks.
(3) Taking each cross-linked green body prepared in the step (2) as a sample, respectively carrying out isothermal crystallization test on each sample by using a polarization microscope (POM), observing the crystal appearance of isothermal crystallization in a molten state, and setting the program under the isothermal crystallization as follows: raising the temperature from 40 ℃ to 200 ℃ at a heating rate of 10 ℃/min, carrying out isothermal treatment for 5min to eliminate thermal history, then lowering the temperature to a specified temperature at a speed of 50 ℃/min, taking pictures at 1s as a time interval for observation to obtain the crystallization rate and the crystallization starting temperature of each sample, and finally carrying out more detailed research on the melting-crystallization behavior of each sample by using DSC in combination with POM, wherein the program setting of isothermal treatment is consistent with that of POM.
FIG. 5 is a diagram showing isothermal crystal morphology at 128 ℃ of each crosslinked green body sample observed by a polarization microscope, and irradiation doses of the samples corresponding to diagrams (a) to (d) in FIG. 5 are 0kGy, 20kGy, 30kGy, and 200kGy, respectively. As can be seen from FIG. 5, when the temperature is 128 ℃ for isothermal crystallization, a large amount of spherulites appear on the iPP sample which is not subjected to irradiation after 5min of isothermal crystallization, and then the crystal morphology does not change any more; when the irradiation dose is 20kGy, the sample is crystallized after 10 min; when the irradiation dose is 30kGy, the sample is crystallized after 6 min; when the irradiation dose was 200kGy, the sample completed crystallization after 11 min. This qualitatively demonstrates that the cross-linked network constructed by irradiation chemical cross-linking inhibits the regular discharge of iPP molecular chains into the crystalline region.
Fig. 6 is an isothermal crystallization plot consistent with the POM test analysis program, and fig. 7 is a plot of the crystallinity (Xc) and melting temperature (Tm) of the samples as a function of irradiation dose. When the isothermal crystallization temperature is 128 ℃, the crystallization speed of the sample which is not subjected to irradiation is higher, and the crystallization perfection degree is higher; the samples with the irradiation doses of 10kGy and 20kGy are longer in time for completing crystallization, and the difference between the crystallinity and the samples for irradiation is not large; the crystallization completion speed of the samples with the irradiation doses of 30kGy and 50kGy is improved, and the crystallinity is reduced; the completion rate of crystallization was again lowered and the crystallinity was also lower for the samples irradiated at the doses of 100kGy, 200kGy and 400 kGy. Consistent with the results observed in fig. 5.
The experimental results show that the addition of the auxiliary crosslinking agent and the irradiation can promote the construction of a chemical crosslinking network in an iPP system, and the formed polypropylene chemical crosslinking network can be used as a heterogeneous nucleation point to promote crystallization. When the irradiation dose is lower than 20kGy, the formed polypropylene chemical crosslinking network has a lower heterogeneous nucleation promoting effect than a molecular chain movement crystallization growth inhibiting effect due to the low crosslinking degree of iPP, so that the crystallization rate is reduced; when the irradiation dose is lower than 200kGy, heterogeneous nucleation of the crosslinking points of the formed polypropylene chemical crosslinking network is dominant, the crystallization rate is increased, but the crystallinity is reduced as a whole due to the obstruction of molecular chain movement; when the irradiation dose is more than 200kGy, the degree of crosslinking of iPP after irradiation is too high, and the movement of the segment has been severely suppressed, thus showing a phenomenon of slow crystallization rate and low crystallinity.
The content of this embodiment shows that the invention can regulate the structure of the formed chemical cross-linked network by regulating the chemical cross-linking degree of iPP, and further regulate the crystallization behavior of iPP and change the structure of the iPP microcrystalline physical cross-linked network formed by the cross-linked body in the process of swelling with supercritical fluid, and these factors can affect the foaming behavior of polypropylene together.
Example 4
In this example, the effect of the crosslinked network on the iPP foaming behavior was examined.
(1) iPP granules and SiO loaded with auxiliary cross-linking agent TAIC 2 (TAIC-Supported SiO 2 Wherein the content of TAIC is 70 wt%) into a double-screw extruder, melting and blending at 180 ℃, extruding and granulating to obtain blend granules, then carrying out hot press molding on the blend granules on a vacuum molding press at 190 ℃ to obtain a sheet-shaped blank body, wherein the blank body is loaded with SiO of the assistant crosslinking agent TAIC 2 The content of (B) is 2 wt%.
(2) Placing the blank prepared in the step (1) in an electron beam (energy is 1.6MeV, absorption dose rate is 3.6 multiplied by 10) under the condition of air isolation 4 Carrying out irradiation in kGy/h) to enable polypropylene molecular chains to be crosslinked, and respectively controlling irradiation doses to be 0kGy, 10kGy, 20kGy, 30kGy, 50kGy, 100kGy and 200kGy to obtain a series of crosslinked blanks.
(3) Respectively placing a series of cross-linked blanks prepared in the step (2) into a high-pressure reaction kettle, and introducing CO serving as a physical foaming agent into the high-pressure reaction kettle 2 Exhausting air from the high-pressure reactor several times, setting the temperature of the high-pressure reactor at 155 deg.C and 165 deg.C, and introducing CO into the high-pressure reactor when the temperature of the high-pressure reactor is increased to approach the set temperature 2 The pressure of the high-pressure reaction kettle is 16MPa, the temperature of the high-pressure cavity is controlled to be set temperature, the pressure is controlled to be 16MPa, and swelling is carried out until the foaming agent reaches swelling balance in the cross-linked blank(swelling time is about 2h), and then quickly decompressing and foaming to obtain a series of polypropylene foaming materials.
The cross section of the polypropylene foam material prepared in this example was observed by SEM, and the results are shown in fig. 7 to 8. The foaming temperature of fig. 8 was 155 ℃, and the irradiation doses in the graphs (a) to (g) of fig. 8 were 0kGy, 10kGy, 20kGy, 30kGy, 50kGy, 100kGy, and 200kGy, respectively. The foaming temperature in FIG. 9 was 165 ℃ and the irradiation doses in the graphs (a) to (f) in FIG. 9 were 10kGy, 20kGy, 30kGy, 50kGy, 100kGy, and 200kGy, respectively. Statistics of cell size and cell density were made based on the SEM pictures, and the results are shown in FIG. 10, where the foaming temperatures in the two graphs (a) (b) of FIG. 10 were 155 ℃ and 165 ℃, respectively.
As can be seen from fig. 8 and fig. 10 (a), when the irradiation dose is different, the polypropylene foam exhibits different cell structures. Supercritical fluid foaming was carried out at 155 ℃ and when the irradiation dose was 0kGy, since a large number of crystal domains were present in iPP, CO was present 2 Soluble only in the amorphous region, which makes CO 2 The dissolution amount is small, the molecular chain movement of an amorphous area is obviously inhibited by the existence of a crystalline area, so that the foaming is difficult, and the pore diameter of a cell is extremely small; when the irradiation dose is 10kGy, the pore diameter of the cells is increased, but the crystallinity of the cross-linked green body is still larger due to lower chemical cross-linking degree of polypropylene, so the pore diameter of the cells formed by foaming is still smaller; when the irradiation dose is increased to 20kGy, the pore diameter of the cell is increased to about 200 mu m, and the pore wall becomes thin; when the irradiation dose is increased to 30kGy, the pore diameter of the pores is stabilized at about 250 mu m; then increasing the irradiation dose to 100kGy, and still maintaining the pore diameter at a larger level, because the chemical crosslinking density of the polypropylene is increased and the crystallinity is reduced along with the continuous increase of the irradiation dose, and the physical crosslinking network and the chemical crosslinking network of the polypropylene microcrystal play a role in regulating and controlling the melt strength and the crystallinity simultaneously; when the irradiation dose was increased to 200kGy, the cell pore size was reduced because the chemical crosslinking structure existing in a large amount in the crosslinked green body suppressed cell growth.
As can be seen from fig. 9 and fig. 10 (b): when the irradiation dose is 0kGy, the iPP is completely melted because the crystal region in the iPP is completely melted and cannot maintain the cell structure, so that the foaming and forming cannot be carried out; when the irradiation dose is 10kGy, although the foaming temperature of 165 ℃ is far higher than that of pure iPP, the chemical crosslinking structure in the crosslinking green body provides strength for supporting cell growth, and at the moment, supercritical fluid foaming can still be realized; then, with the increase of the irradiation dose, the pore diameter of the cells can still be maintained at a higher level within the range of 20-100 kGy; when the irradiation dose was increased to 200kGy, the cell pore size decreased due to an excessively high degree of chemical crosslinking of the crosslinked green body.
As can be seen by combining the graphs (a) and (b) in FIG. 10, when the irradiation dose is also 50kGy or 100kGy, the cell size obtained by foaming at 155 ℃ is larger than that obtained by foaming at 165 ℃, because the chemical crosslinking network is arranged in the crosslinked green body reaching the swelling balance when swelling is carried out at 155 ℃, and the matrix has better melt strength to support the cell structure due to the physical crosslinking network of polypropylene microcrystals in a proper amount; when the foaming is carried out at 165 ℃, the swelling temperature is higher, so that in the crosslinked green body reaching the swelling balance, the physical crosslinked network of the polypropylene microcrystal is relatively less, the improvement on the strength of the matrix melt is limited, a small amount of foam cells collapse during the foaming, and the cell aperture and the cell density of the foaming material are lower than those of the foaming material prepared by foaming at 155 ℃.
Claims (9)
1. Method for optimizing the foaming behaviour of polypropylene in supercritical fluids by double cross-linking modification, characterised in that it comprises the following steps:
(1) melting and blending polypropylene and an auxiliary crosslinking agent, and carrying out hot press molding on the obtained blend to obtain a blank body;
(2) placing the blank in an electron beam under the condition of air isolation to perform irradiation so as to enable polypropylene molecular chains to generate chemical crosslinking, thereby obtaining a crosslinked blank;
(3) swelling the cross-linked green body by using a supercritical fluid, releasing pressure and foaming, wherein in the swelling process, the polypropylene chemical cross-linked network in the cross-linked green body can limit the movement capacity of a polypropylene molecular chain, partially destroy the crystalline network of polypropylene, reduce the crystallinity of the cross-linked green body and widen the melting limit; meanwhile, a polypropylene chemical cross-linked network in the cross-linked blank is used as an out-of-phase nucleation point to regulate the crystallization behavior of polypropylene, and a polypropylene microcrystal physical cross-linked network is formed in the cross-linked blank;
under the combined action of the polypropylene microcrystal physical crosslinking network and the polypropylene chemical crosslinking network, the melt strength and the crystallization degree of the crosslinking green body are regulated and controlled, so that the foaming temperature interval of polypropylene in a supercritical fluid is widened, the foaming temperature is reduced, and the cell structure of the polypropylene foaming material prepared by foaming is improved.
2. The method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double crosslinking modification according to claim 1, wherein the crosslinking degree of the crosslinked green body is controlled by controlling the addition amount of the co-crosslinking agent and the irradiation dose, so that the gel content of the crosslinked green body is 25 wt% to 40 wt%.
3. The method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double cross-linking modification according to claim 2, wherein the content of the co-crosslinking agent in the green body in the step (1) is 1.2 wt% to 2 wt%.
4. The method for optimizing the foaming behavior of polypropylene in the supercritical fluid through double cross-linking modification according to claim 2, wherein the irradiation dose in the step (2) is 20-100 kGy.
5. The method for optimizing foaming behavior of polypropylene in supercritical fluid by double crosslinking modification according to any one of claims 1 to 4, wherein the auxiliary crosslinking agent comprises at least one of styrene, styrene analog, acrylic acid derivative, maleic anhydride, salt compound of maleic anhydride, compound containing acrylic acid structure and isocyanate.
6. The method for optimizing the foaming behavior of polypropylene in a supercritical fluid by double cross-linking modification according to claim 5, wherein the styrene analog comprises divinylbenzene, the acrylic acid derivative comprises pentaerythritol tetramethacrylate, maleic anhydride, the salt compound of maleic anhydride comprises diacrylene maleate, and the compound containing acrylic structures and isocyanate cyclic esters comprises triallyl isocyanurate.
7. The method for optimizing polypropylene foaming behavior in supercritical fluid by double crosslinking modification according to any one of claims 1 to 4, wherein the operation of swelling and pressure-relief foaming of the crosslinked green body by using the supercritical fluid in the step (3) is as follows: placing the crosslinked green body in a high-pressure cavity, introducing gas serving as a physical foaming agent into the high-pressure cavity, controlling the temperature of the high-pressure cavity to be 151-165 ℃, controlling the pressure of the high-pressure cavity to convert the gas serving as the physical foaming agent into a supercritical fluid state, keeping the temperature and the pressure until the physical foaming agent achieves swelling balance in the crosslinked green body, and then releasing pressure for foaming to obtain the polypropylene foaming material.
8. The method for optimizing the foaming behavior of polypropylene in the supercritical fluid through molecular structure regulation and control as claimed in claim 7, wherein the pressure of the high pressure cavity is controlled to be 7.32-30 MPa in the step (3).
9. Method for optimizing the foaming behavior of polypropylene in supercritical fluid through molecular structure manipulation according to claim 7 or 8, wherein the gas as physical blowing agent is N 2 、CO 2 Or an inert gas.
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