CN110927208A - Method for measuring PP content in HDPE by differential scanning calorimetry - Google Patents
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Abstract
A method for measuring the PP content in HDPE by differential scanning calorimetry is characterized in that: the main component of the measured object aimed at by the method is HDPE/PP blend; the method comprises the following steps: the method comprises the following steps of firstly, predetermining a standard curve of the relation between the enthalpy variable and the PP content of the HDPE/PP blend as Y = (55-56) X, wherein Y is the enthalpy variable of polypropylene and has the unit of J/g; x is the content of polypropylene, and the unit is weight percent; secondly, carrying out enthalpy change detection on the object to be detected; determining a tested object, sampling the tested object, crushing the sample, extracting and purifying the sample, and detecting enthalpy change by using a differential scanning calorimeter to obtain an enthalpy variable; thirdly, calculating a result; and comparing the enthalpy change result obtained in the second step with the standard curve obtained in the first step to determine the PP content value in the sample.
Description
Technical Field
The invention relates to the field of waste plastic treatment, in particular to a method for measuring the content of polypropylene (PP) in high-density polyethylene (HDPE).
Background
China is the second major plastic product producing country in the world, and a large number of plastic products are widely applied to daily production and life of people due to the characteristics of corrosion resistance, cold resistance, heat resistance, low price, convenience, rapidness and the like. At present, the market value of the application of the waste plastics in China is more than 1000 million yuan. In recent years, price rise of plastic raw materials plays a great promoting role in the market, and the price of waste plastics also rises greatly, wherein the highest price of purer reclaimed materials of polyethylene and polypropylene reaches 6000 yuan/ton (11 months in 2005), and is close to the price of the raw materials before rise. Except disposable plastic bags and snack boxes, the shadows of waste plastic products in the large-city garbage can not be found at present, which shows that the recovery degree of the waste plastic is obviously improved. However, in the aspect of recycling technology, China has a great gap with the foreign advanced level. Therefore, the sustainable development of the plastic industry in China should be developed around the center of plastics and the environment.
PE and PP account for up to 65% of waste plastic cracking liquefaction raw materials in the waste plastic composition, and are also concentrated in PE and PP. Zhang Jianping et al studied the cracking of PP using molten zinc chloride as a carrier and examined the influence of the thermal cracking temperature on the yield of each component and the cracking time. The experimental result shows that the mass ratio of the heat carrier consumption to the polypropylene is 4:1, and the optimal thermal cracking temperature range is 420-. Murata et al explored the effect of reaction pressure on thermal cracking of PE under continuous operating conditions. It was found that by controlling the discharge pressure differently, the composition distribution of the product oil can be varied. Mastral et al found that different cracking temperatures and residence times had a significant effect on HDPE cleavage product composition in a fluidized bed reactor: when the temperature is 750-780 ℃, the main cracking product is gas; the longer the residence time at this temperature, the lower the gas flow viscosity. The research on the cracking liquefaction of Polyethylene (PE) and polypropylene (PP) has been reported in a large quantity. But few studies have been reported on the thermal cracking properties of HDPE/PP blends. Angyal et al studied the thermal cracking of PE/PP mixtures in tubular furnaces, and (4060)% (wt) of the polyolefin cracked at 530 ℃ for 15min was converted to light and middle distillates, and the cracked product oil contained (2550)% (wt) olefin, which was used as a steam cracking feedstock for ethylene and propylene.
Polypropylene, a thermoplastic resin made by polymerizing propylene. The polypropylene is divided into isotactic polypropylene, atactic polypropylene and syndiotactic polypropylene according to the arrangement position of methyl.
The methyl groups arranged on the same side of the main molecular chain are called isotactic polypropylene, if the methyl groups are arranged on both sides of the main molecular chain disorderly, the isotactic polypropylene is called, and when the methyl groups are arranged on both sides of the main molecular chain alternately, the syndiotactic polypropylene is called. In general, polypropylene resins produced industrially have an isotactic structure content of about 95% and the balance of atactic or syndiotactic polypropylene. The industrial product takes an isotactic substance as a main component. Polypropylene also includes copolymers of propylene with a small amount of ethylene. Usually a translucent colorless solid, odorless and non-toxic. The melting point can be up to 167 ℃ due to regular structure and high crystallization. The product is heat-resistant and corrosion-resistant, and the steam sterilization of the product is a remarkable advantage. The density is low, and the plastic is the lightest general plastic. The defects are that the low-temperature impact resistance is poor, the aging is easy, but the defects can be overcome by respectively modifying.
Polypropylene is a non-toxic, odorless and tasteless milky-white high-crystalline polymer, has a density of only 0.90 to 0.91 g/cm3, and is one of the lightest varieties of all plastics at present. It is particularly stable to water, has a water absorption of only 0.01% in water and a molecular weight of about 8 to 15 ten thousand. The formability is good, but the shrinkage rate is large (1-2.5%), so that thick-wall products are easy to dent, the requirements on parts with high dimensional precision are difficult to meet, and the surface gloss of the products is good.
The PP material of the copolymer type has a lower heat distortion temperature (100 ℃), low transparency, low gloss, low rigidity, but has stronger impact strength, and the impact strength of PP increases with the increase of the ethylene content. The Vicat softening temperature of PP is 150 ℃. Due to the high degree of crystallinity, the surface stiffness and scratch resistance properties of this material are good. PP has no environmental stress cracking problem.
The melt Mass Flow Rate (MFR) of PP is usually 1 to 100. Low MFR PP materials have better impact properties but lower ductile strength. For materials of the same MFR, the impact strength is higher for the copolymeric form than for the homopolymeric form. Due to crystallization, the shrinkage of PP is quite high, typically 1.6-2.0%.
HDPE is a thermoplastic polyolefin produced by the copolymerization of ethylene. Although HDPE was introduced in 1956, this plastic has not yet reached its mature level. This versatile material is also constantly developing its new uses and markets.
The domestic producers of high density polyethylene HDPE (the high density polyethylene does not include the high density polyethylene produced by full density polyethylene devices) in China have three enterprises of medium petroleum, medium petrochemical and medium sea oil, and 4 sets of high density polyethylene devices belonging to the medium petroleum are provided as late as 2006, namely Lanzhou petrochemical high density polyethylene devices, Daqing petrochemical high density polyethylene devices, Liaoyang petrochemical high density polyethylene devices and Jilin petrochemical high density polyethylene devices.
High density polyethylene HDPE is usually produced by the Ziegler-Natta polymerization process and is characterized by the absence of branched chains on the molecular chain, which results in a regular molecular chain arrangement and higher density. In the process, ethylene is used as a raw material in a tubular or kettle type low-pressure reactor, and oxygen or organic peroxide is used as an initiator to initiate polymerization reaction.
The most common production of PE is by slurry or gas phase processing, with few solution phase processes. All of these processes are exothermic reactions involving ethylene monomer, a-olefin monomer, a catalyst system (which may be more than one compound), and various types of hydrocarbon diluents. Hydrogen and some catalysts are used to control molecular weight. Slurry reactors are typically stirred tanks or, more commonly, large loop reactors in which the slurry is circulated under agitation. Polyethylene particles are formed as soon as ethylene and comonomer (if desired) are contacted with the catalyst. After removal of the diluent, the polyethylene particles or granules are dried and dosed with additives to produce pellets. A modern production line with a large reactor with a twin screw extruder can produce PE of 40000 pounds or more per hour. The development of new catalysts contributes to improving the performance of new grades of HDPE. The two most common catalyst types are Phillips chromium oxide based catalysts and titanium compound monoalkylaluminum catalysts. HDPE produced with Phillips-type catalysts has a medium-width molecular weight distribution; titanium-alkyl aluminum catalysts produce narrow molecular weight distributions. Catalysts for the production of polymers with narrow MDW using multiple reactors can also be used to produce wide MDW grades. For example, two reactors in series producing significantly different molecular weight products can produce bimodal molecular weight polymers having a broad range of molecular weight distributions.
The main weight loss temperature ranges of PP and HDPE are 391-446 ℃ and 431-470 ℃, respectively, and the main weight loss range of blending cracking is 393-460 ℃; when blending cracking is carried out, the temperature is 400-445 ℃, and PP can promote the cracking of HDPE.
The main factor influencing the oil yield of HDPE/PP cracking is the content of PP in the polyolefin blend, and when the proportion of PP reaches 50%, the product oil yield is improved most favorably. The factors influencing the content of the light components of the oil of the cracking product are as follows from the beginning to the end: residence time, cracking temperature, PP ratio, initial pressure. The longer the residence time, the lower the final boiling point of the product oil, the narrower the boiling range and the greater the content of light components. It is therefore essential to determine the PP content in HDPE/PP blends.
The existing method for determining the PP content in the HDPE/PP blend is infrared spectroscopy, and content determination can be carried out according to the area of a characteristic peak through FTIR, but the accuracy is greatly deviated.
Disclosure of Invention
The invention aims to provide a method for determining the content of PP in HDPE by differential scanning calorimetry, which can quickly and accurately determine the content of polypropylene in high-density polyethylene so as to overcome the problem of large deviation of determination accuracy in the prior art.
In order to achieve the purpose, the invention adopts the technical scheme that: a method for measuring PP content in HDPE by differential scanning calorimetry is provided, the main component of the measured object is HDPE/PP blend, and the PP content in the HDPE/PP blend is 0.25% -30%;
the method comprises the following steps:
the method comprises the following steps of firstly, predetermining a standard curve of the relation between the enthalpy variable and the PP content of the HDPE/PP blend as Y = (55-56) X, wherein Y is the enthalpy variable of polypropylene and has the unit of J/g; x is the content of polypropylene, and the unit is weight percent;
secondly, carrying out enthalpy change detection on the object to be detected;
determining a tested object, sampling the tested object, crushing the sample, extracting and purifying the sample, and detecting enthalpy change by using a differential scanning calorimeter to obtain an enthalpy variable;
thirdly, calculating a result;
and comparing the enthalpy change result obtained in the second step with the standard curve obtained in the first step to determine the PP content value in the sample.
In the above scheme, the method for predetermining the standard curve of the relationship between the enthalpy variable and the PP content of the PP in the HDPE/PP blend in the first step is as follows: adding multi-grade quantitative PP into quantitative HDPE in a laboratory for blending to obtain a plurality of HDPE/PP blends with different PP contents, carrying out enthalpy change continuous detection analysis on the HDPE/PP blends by using a differential scanning calorimeter, and carrying out linear fitting to obtain a standard curve of the relation between the enthalpy change amount of the PP and the PP content.
In the above scheme, the optimal standard curve of the relationship between the enthalpy variable of PP and the content of PP in the HDPE/PP blend is Y = 55.59741X.
In the scheme, the sample is crushed to 20-100 meshes in the second step.
In the scheme, the extraction and purification in the second step is to put the sample into ethanol for extraction for 4-8 hours, take out and dry the sample.
In the scheme, the enthalpy change detection is carried out by using the differential scanning calorimeter in the second step, specifically, a sample is put into a crucible of the differential scanning calorimeter, the test is carried out in a nitrogen atmosphere, the temperature is raised twice, and the test result in the second temperature raising process is taken.
The design principle of the invention is as follows:
the inventor of the invention discovers that for a measured object of which the main component is an HDPE/PP blend and the PP content in the HDPE/PP blend is 0.25% -30%, the polypropylene (PP) content and the enthalpy change of the polypropylene (PP) are in a linear relation, the principle is that the melting points of the high-density polyethylene and the polypropylene are different, the enthalpy change is fixed, and the melting points of the high-density polyethylene and the polypropylene have larger difference, so the enthalpy change can not be influenced by calculation, the enthalpy change of each substance can be respectively tested without interference, and the content of the two substances can be calculated according to the enthalpy change, so the invention can adopt a differential scanning calorimeter DSC method to carry out quantitative analysis on the polypropylene (PP) content in the high-density polyethylene (HDPE).
The invention has the advantages that: the method can quickly and accurately judge the content of the polypropylene in the high-density polyethylene. The method can quickly detect the content of each component in the waste plastic, and is favorable for classified recovery and decomposition of the waste plastic. The method is more accurate and rapid than the infrared spectroscopy for measuring the content of each component of the plastic, and the content of each component can be conveniently and rapidly quantitatively analyzed by measuring the enthalpy change value of each component through DSC and referring to a comparison chart.
Drawings
FIG. 1 is a graph showing a second temperature rise curve in the case of detection by a differential scanning calorimeter according to the present invention;
FIG. 2 is a table of data obtained from FIG. 1;
fig. 3 is a standard graph in the first step of the present invention.
Detailed Description
The invention is further described with reference to the following figures and examples:
the first embodiment is as follows: referring to figures 1-3:
a method for measuring the PP content in HDPE by differential scanning calorimetry is provided, the measured object is exemplified by a plastic barrel, the main component of the plastic barrel is HDPE/PP blend, and the PP content in the HDPE/PP blend is 0.25% -30%.
The method of the embodiment comprises the following steps:
the method comprises the following steps of firstly, predetermining a standard curve of the relation between the enthalpy variable and the PP content of the HDPE/PP blend as Y = (55-56) X, wherein Y is the enthalpy variable of polypropylene and has the unit of J/g; x is the content of polypropylene, and the unit is wt%.
The method for obtaining the specific standard curve comprises the following steps: adding multi-grade quantitative PP into quantitative HDPE in a laboratory for blending to obtain a plurality of HDPE/PP blends with different PP contents, carrying out enthalpy change continuous detection analysis on the HDPE/PP blends by using a differential scanning calorimeter, and carrying out linear fitting to obtain a standard curve of the relation between the enthalpy change amount of the PP and the PP content. Further operational examples are: taking 100 g of HDPE particles and 5 g, 10 g, 20 g and 30 g of PP particles, which are respectively recorded as 5phr, 10phr, 20phr and 30phr, and blending for 10min on an internal mixer at 170 ℃ to obtain a blend; then, the blend is detected by adopting an American TAQ2000 differential scanning calorimeter, and the specific detection mode is as follows: placing 1-3mg of the blend into a DSC crucible, testing in a nitrogen atmosphere, wherein the temperature rising procedure is a first temperature rise from 20 ℃ to 180 ℃, then reducing to 20 ℃, then increasing to 180 ℃ for a second temperature rise, the temperature rising and reducing rate is 20 ℃/min, taking the test result in the second temperature rising process, the second temperature rising curve is shown in figure 1, the data obtained from the second temperature rising curve graph 1 is shown in figure 2, different enthalpy change values are calculated by self-contained software, and a standard curve of the relation between the PP enthalpy variable and the PP content is obtained after linear fitting is carried out by using software origin8, which is shown in figure 3, and Y = 55.59741X.
Secondly, carrying out enthalpy change detection on the object to be detected;
and determining a tested object, sampling the tested object, crushing the sample, extracting and purifying, and detecting enthalpy change by using a differential scanning calorimeter to obtain an enthalpy variable. Specific examples of operations for this second step are: crushing a sample to 20-100 meshes, extracting the sample in ethanol for 4 hours, taking out and drying, and detecting the blend by adopting an American TA Q2000 differential scanning calorimeter, wherein the specific detection mode is as follows: putting 1-3mg of a sample into a DSC crucible, testing in a nitrogen atmosphere, wherein the temperature rise procedure is a first temperature rise from 20 ℃ to 180 ℃, then a second temperature rise from 20 ℃ to 180 ℃, the temperature rise rate is 20 ℃/min, taking the test result in the second temperature rise process, the second temperature rise curve is shown in figure 1, obtaining data from a second temperature rise curve chart 1 as shown in figure 2, and calculating different enthalpy change values. The temperature is increased twice, the result of the second temperature increasing process is taken because the result of the first temperature increasing process is inaccurate because of the winding and adhesion of molecules, and the accuracy of the result of the second temperature increasing process is high.
Thirdly, calculating a result;
comparing the enthalpy change result obtained in the second step with the standard curve obtained in the first step to directly determine the PP content value in the sample, wherein if the enthalpy change value is 7.1J/g in the determination result in the second step, the PP content can be calculated to be 0.13% by comparing the standard curve.
The embodiment can quickly and accurately judge the content of polypropylene in high-density polyethylene, can quickly detect the content of each component in waste plastic, is beneficial to classified recovery and decomposition of the waste plastic, is more accurate and quick than the infrared spectroscopy for determining the content of each component of plastic, and can conveniently and quickly quantitatively analyze the content of each component by determining the enthalpy change value of each component through DSC and referring to a comparison chart.
The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.
Claims (6)
1. A method for measuring the PP content in HDPE by differential scanning calorimetry is characterized in that: the main component of the measured object aimed at by the method is HDPE/PP blend, and the PP content in the HDPE/PP blend is 0.25% -30%;
the method comprises the following steps:
the method comprises the following steps of firstly, predetermining a standard curve of the relation between the enthalpy variable and the PP content of the HDPE/PP blend as Y = (55-56) X, wherein Y is the enthalpy variable of polypropylene and has the unit of J/g; x is the content of polypropylene, and the unit is weight percent;
secondly, carrying out enthalpy change detection on the object to be detected;
determining a tested object, sampling the tested object, crushing the sample, extracting and purifying the sample, and detecting enthalpy change by using a differential scanning calorimeter to obtain an enthalpy variable;
thirdly, calculating a result;
and comparing the enthalpy change result obtained in the second step with the standard curve obtained in the first step to determine the PP content value in the sample.
2. The method for differential scanning calorimetry measurement of the PP content in HDPE according to claim 1, characterised in that: the method for predetermining the standard curve of the relationship between the enthalpy variable of PP and the content of PP in the HDPE/PP blend in the first step is: adding multi-grade quantitative PP into quantitative HDPE in a laboratory for blending to obtain a plurality of HDPE/PP blends with different PP contents, carrying out enthalpy change continuous detection analysis on the HDPE/PP blends by using a differential scanning calorimeter, and carrying out linear fitting to obtain a standard curve of the relation between the enthalpy change amount of the PP and the PP content.
3. The method for differential scanning calorimetry measurement of the PP content in HDPE according to claim 1, characterised in that: the standard curve of the relationship between the enthalpy variable of PP and the content of PP in the first-step HDPE/PP blend is Y = 55.59741X.
4. The method for differential scanning calorimetry measurement of the PP content in HDPE according to claim 1, characterised in that: and in the second step, the sample is crushed to 20-100 meshes.
5. The method for differential scanning calorimetry measurement of the PP content in HDPE according to claim 1, characterised in that: and in the second step, the extraction and purification are to put the sample into ethanol for extraction for 4 to 8 hours, take out and dry the sample.
6. The method for differential scanning calorimetry measurement of the PP content in HDPE according to claim 1, characterised in that: and in the second step, a differential scanning calorimeter is used for enthalpy change detection, specifically, a sample is put into a crucible of the differential scanning calorimeter and is tested in a nitrogen atmosphere, the temperature is raised twice, and the test result in the second temperature raising process is obtained.
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| PCT/CN2020/133491 WO2021110075A1 (en) | 2019-12-03 | 2020-12-03 | Method for detecting pp content in hdpe by means of differential scanning calorimetry |
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