At present, polymer materials have been used in a wide range of applications in various industries. Depending on the application, the performance requirements for polymer materials are also different. As far as the material itself is concerned, different production or modification processes determine the difference in its properties, and the length or entanglement of the molecular chain is usually the decisive parameter that affects its performance. Therefore, optimizing the process and quality control in plastics production is particularly important.
In general, many relevant properties can be characterized by rheological tests. The main purpose of using a rheometer to determine the rheological properties of a material (such as flowability, elasticity, and fracture properties) is:
1. Characterization of the material structure, including qualitative and quantitative analysis of the molecular weight and molecular weight distribution of the polymer, and analysis of the branching properties, filling properties, tensile properties, and glass transition temperature of the polymer.
2. Simulate the processing conditions of the polymer and evaluate the processability of the polymer. Through the analysis of the processing process, the correct selection of processing conditions and guidance for formula design.
3. Evaluate the performance of raw materials, semi-finished products and final products.
Test Techniques To ensure the accuracy of rheological testing of the polymer melt, a suitable rheometer, temperature control unit, and suitable measurement clamp need to be selected.
1. The rheometer Physica MCR rheometer is equipped with a unique electronic rectification synchronous motor motor, using a permanent magnet driver. At the same time, high-precision air bearings, optical encoder disks, and normal stress sensors make it extremely sensitive and instantaneously responsive. The torque range of the rheometer is up to 7 orders of magnitude, and the speed range is up to 10 orders of magnitude. The absolute inertia correction feature makes it have excellent measurement performance even at high frequency oscillations. Therefore, true strain control and stress control can be realized on the same rheometer.
To reduce measurement errors, the latest Physica MCR rheometer is also equipped with the Toolmaster automatic identification system. When the rotor is installed, the instrument automatically recognizes the size and serial number. Replace the temperature control unit and the host will automatically update the information. In this way, there will be no case of mixing 25mm and 50mm diameter rotors, so that the instrument has the advantages of error-free and intelligent.
2, temperature control unit In general, the typical temperature range of polymer rheological testing is 150 ~ 300 °C, which is the more sensitive polymer temperature range. For many samples, even if the temperature changes by 1°C, the change in viscosity will reach 5%. Therefore, the temperature needs to be strictly controlled in the rheological test. In general, only a few temperature control units can adapt to the requirements of polymer rheology testing, and the use of open temperature control methods and passive thermal insulation temperature control methods can not replace the closed temperature control unit that controls the temperature up and down (as shown in Figure 1) ).
Fig. 1 Comparison of open temperature control, passive insulation and closed temperature control units with active temperature control up and down The following two measuring units are more suitable for rheological analysis of polymers:
(1) For polymer melts, one option is to use a measuring plate heating method with an upper electric heating shield and lower electric heating. AntonPaar offers two temperature range electric heating temperature control units: P-ETD350 (maximum temperature 350 °C) and P-ETD400 (maximum temperature 400 °C). This heating method is efficient, fast, and easy to use, and this heating method is used to fill the gas (such as nitrogen), help to quickly reach the control temperature, to avoid oxidation. In addition, temperature gradients inside the sample can also be avoided. In general, the polymer pellets can be directly placed on the lower heating plate. After the temperature equilibrium is reached, the measuring rotor is immediately lowered to the sample-squeezing position, and then a spilled sample is scraped off by a spatula to measure, and then the copper brush is used after measurement. Or the scraper cleans the upper and lower plates.
(2) CTD high-temperature furnaces that use radiation plus convection, such as the Anton Paar CTD450 temperature control unit. Due to the design features of the CTD high-temperature furnace, the measurement rotor and the sample are directly heated by gas heating instead of being placed on the lower plate, so this heating method has a longer closed-cycle temperature control than the electric heating, and can directly measure the sample temperature. In addition, the fully symmetrical design features of the CTD high temperature furnace minimize temperature gradients. This temperature control unit is not only suitable for measuring polymer melts, but also for solid DMTA testing, tensile testing and UV curing testing.
3. Measuring fixture Compared with the board measuring system, the advantage of the cone-plate measuring system is that the entire measuring gap has the same shear rate. However, since the gap between the cone plates is usually maintained at about 50 μm (cone plate system with a cone angle of 1°), experiments conducted at temperatures above or below room temperature pose a problem: if the temperature is increased or The cooling experiment results in thermal expansion and contraction, which inevitably causes millimeter-scale length changes in the axis of the rheometer support and measurement system, resulting in measurement errors. Therefore, the vast majority of scientific experiments use a board measurement system (it has a gap of 1000 μm).
However, a recent new measurement method (TruGap) can directly test and adjust the upper/lower cone/plate or plate gap in the range of -150 to 280°C. This temperature range is of great interest to polymer rheologists. The TruGap cone and plate system has a maximum gap error of no more than 1 μm over the entire temperature range.
Applications 1. Complete Flow Curve: Measurement of Zero-Shear Viscosity and Flowability of Thermoplastic Materials Flow and viscosity curves reflect the flow properties of thermoplastic materials under different shear and processing conditions. Most polymers are processed using plastic molding, which covers a wide range of shear rates (see table). In order to simulate the fluidity of different processing processes, it is necessary to measure the viscosity at the shear rate during the processing.
As shown in Figure 2, at low shear or low angular frequencies, the viscosity of the polymer is independent of the shear rate or angular frequency, ie there is zero shear viscosity. The zero shear viscosity is an important material parameter and is directly proportional to the 3.4th power of the average molecular weight Mw.
Figure 2 Polymer flow curve Using the time-temperature equivalence principle and the Cox-Merz rule, a viscosity curve can be obtained over a wider range of shear rates, which reflects the fluidity of the polymer in different processing processes. If data processing software is used, the infinite shear viscosity can be calculated, meaning that all molecules are completely unwound and oriented.
2. Polymer Weight Average Molecular Weight and Qualitative Determination of Molecular Weight Distribution In frequency sweep analysis, the qualitative analysis of weight average molecular weight and molecular weight distribution can be judged from the intersection of storage modulus and loss modulus. In general, this experiment takes approximately 5 to 10 minutes. The horizontal position of the Gx at the modulus intersection can be used to qualitatively analyze the average molecular weight. The vertical position of Gx indicates the MMD of the molecular weight. In addition, the degree of branching is also related to the horizontal shift of Gx compared to similar polymers (as shown in Figure 3).
Figure 3: Qualitative analysis of molecular weight using the intersection of energy storage and loss modulus 3. Quantitative Characterization of Weight Average Molecular Weight and Molecular Weight Distribution Through the frequency sweep, stress relaxation, and creep experiments at different temperatures, the master curve can be calculated to calculate the relaxation time spectrum. For polymers with known material parameters (such as PS, PE, PP, PC, PMMA, and PTFE, etc.), the distribution of weight average molecular weight and molecular weight can be easily calculated quantitatively using a polymer analysis software package (see Figure 4 and Figure). 5 is shown). 
Figure 4 Polymer Analysis Module
Fig. 5 Molecular weight and distribution of polymers calculated by rheology This method does not require the use of any solvent compared to molecular weight analysis by gel chromatography (GPC). Whether the polymer is in the form of granules, powder or flakes, it can be placed directly on the measuring cell. Therefore, the analysis of the molecular weight or the molecular weight distribution is not limited by too many conditions like the gel chromatography (GPC).
4. Branched polymers Generally, the number, length, and mobility of side chains all affect rheological properties. If the side chain is not long, it will result in an increase in viscosity at low shear rates. The shear thinning effect is more pronounced than the corresponding linear polymer. For long-branched polymers, low viscosity will be exhibited at low shear rates. Therefore, the product performance can be controlled by controlling the degree of branching.
The degree of branching of the polymer can usually be analyzed using tensile tests. Rotary rheometers with excellent control rate performance, such as the Physica MCR301 from Anton Paar GmbH, Austria, can be configured with melt stretching dies to directly measure the tensile properties of the polymer, reflecting the difference in the degree of branching of the sample. For this difference, it is difficult to tell which flow curve or oscillation frequency sweep curve is usually used.
Figure 6 shows the difference between branched polypropylene (B-PP) and high regularity linear polypropylene (H-PP). Both polypropylenes have a melt index MI of 3 and the viscosity curves are basically the same. Under certain tensile stress, the molecular structure of the two polypropylenes showed a significant difference. Among them, the branching polypropylene (B-PP) showed a pronounced branching effect and tensile hardening (as shown in Fig. 6a), while the elongation of the high-linearity polypropylene (H-PP) did not increase significantly. Large (as shown in Figure 6b).
Figure 6 Tensile rheological tests of branched polypropylene and linear polypropylene at different draw rates 5, the impact of filler The filler will also affect the performance of the final product, where the filler size, shape, filling amount and particle interactions are important factors. Fillers often result in increased melt viscosity or reduced extrudate expansion. From a rheological point of view, the polymer's linear viscoelastic range becomes smaller as the filler content increases. The linear viscoelastic region can generally be determined by amplitude or strain scanning.
6. Solids test: The glass transition temperature and crystallisation temperature are analyzed by means of a temperature sweep. The rheometer can perform a torsion test on solids (or “dynamicsâ€) with a suitable solid fixture (solid sample bar fixture STBF or fiber film fixture FFF). Thermomechanical analysis DMTA"). In general, solid properties are related to temperature, and testing them helps to gain insight into the morphology and performance of the sample. Measurements of the glass transition temperature (Tg) and the storage modulus (G') below the transition temperature can yield information on maximum operating temperature, impact strength, brittleness, and rigidity. For crystalline or partially crystalline polymers, the melting temperature (Tm) is another important material parameter. The DMTA test can simultaneously obtain the melting temperature data (as shown in Figure 7).
Figure 7 DMTA testing of polypropylene and glass fiber reinforced polypropylene In this experiment, the sample is usually placed in a solid sample holder and then placed in a high-temperature oven CTD450, and the temperature of the glass transition, the melting point, and the crystallization temperature can be accurately measured by performing an oscillating temperature-sweeping scan at an appropriate heating rate and frequency. For example, for a sample size of 40 mm x 10 mm x 10 mm, an oscillatory temperature-raising scan may be performed at a temperature increase rate of 1 K/min and a frequency of 1 Hz. The crystallinity of the semi-crystalline polymer can be studied by the curve between the glass transition temperature and the melting point. Of course, other accessories can also be matched with the rheometer to complete more experiments.
7. Transient testing: testing the time-response stress of the material Step changes (creep and recovery), strain step changes (stress relaxation) and velocity step changes (stress increase/start flow) can characterize the material for a given shear stress, shear The time (instantaneous) response of the shear strain or shear rate. The analysis method involves calculating some important material parameters such as: zero shear viscosity, plateau modulus, creep compliance, and converting the transient material function into the dynamic material function G'(ω)G" (ω). Shown as an example of a stress increase test.
Fig. 8 Transient test of polymer solution 8. Tracking Curing Performance of Thermosetting Resins Figure 9 shows the performance of thermosetting epoxy resins as a function of temperature. The melting temperature, gelation temperature and curing process of the epoxy resin can be conveniently judged by the curve of the rheological parameters (such as modulus or viscosity) versus temperature. If the modulus of the epoxy resin or the viscosity versus time curve is tracked at a constant melting temperature, information on the curing time and curing kinetics can be obtained. 
Figure 9 Curves of Thermosetting Epoxy Resin with Temperature Summary In summary, advanced rotary rheometers can be used to conveniently measure the rheological properties of thermoplastic and thermoset polymers and to obtain internal information about the molecular structure. Among them, information such as molecular weight, phase transition, and extensional rheological analysis can be used to determine many important parameters of a material that are important for understanding the properties of a polymer material.
In addition, the rheometer can also measure the curing reaction or chemical reaction, for example, epoxy curing or UV curing, etc., and can determine the complete curing reaction dynamic state through the isothermal curve or setting the heating rate. The parameters involved include: The minimum softening viscosity, gel point, curing time and curing temperature.
In general, many relevant properties can be characterized by rheological tests. The main purpose of using a rheometer to determine the rheological properties of a material (such as flowability, elasticity, and fracture properties) is:
1. Characterization of the material structure, including qualitative and quantitative analysis of the molecular weight and molecular weight distribution of the polymer, and analysis of the branching properties, filling properties, tensile properties, and glass transition temperature of the polymer.
2. Simulate the processing conditions of the polymer and evaluate the processability of the polymer. Through the analysis of the processing process, the correct selection of processing conditions and guidance for formula design.
3. Evaluate the performance of raw materials, semi-finished products and final products.
Test Techniques To ensure the accuracy of rheological testing of the polymer melt, a suitable rheometer, temperature control unit, and suitable measurement clamp need to be selected.
1. The rheometer Physica MCR rheometer is equipped with a unique electronic rectification synchronous motor motor, using a permanent magnet driver. At the same time, high-precision air bearings, optical encoder disks, and normal stress sensors make it extremely sensitive and instantaneously responsive. The torque range of the rheometer is up to 7 orders of magnitude, and the speed range is up to 10 orders of magnitude. The absolute inertia correction feature makes it have excellent measurement performance even at high frequency oscillations. Therefore, true strain control and stress control can be realized on the same rheometer.
To reduce measurement errors, the latest Physica MCR rheometer is also equipped with the Toolmaster automatic identification system. When the rotor is installed, the instrument automatically recognizes the size and serial number. Replace the temperature control unit and the host will automatically update the information. In this way, there will be no case of mixing 25mm and 50mm diameter rotors, so that the instrument has the advantages of error-free and intelligent.
2, temperature control unit In general, the typical temperature range of polymer rheological testing is 150 ~ 300 °C, which is the more sensitive polymer temperature range. For many samples, even if the temperature changes by 1°C, the change in viscosity will reach 5%. Therefore, the temperature needs to be strictly controlled in the rheological test. In general, only a few temperature control units can adapt to the requirements of polymer rheology testing, and the use of open temperature control methods and passive thermal insulation temperature control methods can not replace the closed temperature control unit that controls the temperature up and down (as shown in Figure 1) ).
Fig. 1 Comparison of open temperature control, passive insulation and closed temperature control units with active temperature control up and down
(1) For polymer melts, one option is to use a measuring plate heating method with an upper electric heating shield and lower electric heating. AntonPaar offers two temperature range electric heating temperature control units: P-ETD350 (maximum temperature 350 °C) and P-ETD400 (maximum temperature 400 °C). This heating method is efficient, fast, and easy to use, and this heating method is used to fill the gas (such as nitrogen), help to quickly reach the control temperature, to avoid oxidation. In addition, temperature gradients inside the sample can also be avoided. In general, the polymer pellets can be directly placed on the lower heating plate. After the temperature equilibrium is reached, the measuring rotor is immediately lowered to the sample-squeezing position, and then a spilled sample is scraped off by a spatula to measure, and then the copper brush is used after measurement. Or the scraper cleans the upper and lower plates.
(2) CTD high-temperature furnaces that use radiation plus convection, such as the Anton Paar CTD450 temperature control unit. Due to the design features of the CTD high-temperature furnace, the measurement rotor and the sample are directly heated by gas heating instead of being placed on the lower plate, so this heating method has a longer closed-cycle temperature control than the electric heating, and can directly measure the sample temperature. In addition, the fully symmetrical design features of the CTD high temperature furnace minimize temperature gradients. This temperature control unit is not only suitable for measuring polymer melts, but also for solid DMTA testing, tensile testing and UV curing testing.
3. Measuring fixture Compared with the board measuring system, the advantage of the cone-plate measuring system is that the entire measuring gap has the same shear rate. However, since the gap between the cone plates is usually maintained at about 50 μm (cone plate system with a cone angle of 1°), experiments conducted at temperatures above or below room temperature pose a problem: if the temperature is increased or The cooling experiment results in thermal expansion and contraction, which inevitably causes millimeter-scale length changes in the axis of the rheometer support and measurement system, resulting in measurement errors. Therefore, the vast majority of scientific experiments use a board measurement system (it has a gap of 1000 μm).
However, a recent new measurement method (TruGap) can directly test and adjust the upper/lower cone/plate or plate gap in the range of -150 to 280°C. This temperature range is of great interest to polymer rheologists. The TruGap cone and plate system has a maximum gap error of no more than 1 μm over the entire temperature range.
Applications 1. Complete Flow Curve: Measurement of Zero-Shear Viscosity and Flowability of Thermoplastic Materials Flow and viscosity curves reflect the flow properties of thermoplastic materials under different shear and processing conditions. Most polymers are processed using plastic molding, which covers a wide range of shear rates (see table). In order to simulate the fluidity of different processing processes, it is necessary to measure the viscosity at the shear rate during the processing.
As shown in Figure 2, at low shear or low angular frequencies, the viscosity of the polymer is independent of the shear rate or angular frequency, ie there is zero shear viscosity. The zero shear viscosity is an important material parameter and is directly proportional to the 3.4th power of the average molecular weight Mw.
Figure 2 Polymer flow curve
2. Polymer Weight Average Molecular Weight and Qualitative Determination of Molecular Weight Distribution In frequency sweep analysis, the qualitative analysis of weight average molecular weight and molecular weight distribution can be judged from the intersection of storage modulus and loss modulus. In general, this experiment takes approximately 5 to 10 minutes. The horizontal position of the Gx at the modulus intersection can be used to qualitatively analyze the average molecular weight. The vertical position of Gx indicates the MMD of the molecular weight. In addition, the degree of branching is also related to the horizontal shift of Gx compared to similar polymers (as shown in Figure 3).
Figure 3: Qualitative analysis of molecular weight using the intersection of energy storage and loss modulus
Figure 4 Polymer Analysis Module
Fig. 5 Molecular weight and distribution of polymers calculated by rheology
4. Branched polymers Generally, the number, length, and mobility of side chains all affect rheological properties. If the side chain is not long, it will result in an increase in viscosity at low shear rates. The shear thinning effect is more pronounced than the corresponding linear polymer. For long-branched polymers, low viscosity will be exhibited at low shear rates. Therefore, the product performance can be controlled by controlling the degree of branching.
The degree of branching of the polymer can usually be analyzed using tensile tests. Rotary rheometers with excellent control rate performance, such as the Physica MCR301 from Anton Paar GmbH, Austria, can be configured with melt stretching dies to directly measure the tensile properties of the polymer, reflecting the difference in the degree of branching of the sample. For this difference, it is difficult to tell which flow curve or oscillation frequency sweep curve is usually used.
Figure 6 shows the difference between branched polypropylene (B-PP) and high regularity linear polypropylene (H-PP). Both polypropylenes have a melt index MI of 3 and the viscosity curves are basically the same. Under certain tensile stress, the molecular structure of the two polypropylenes showed a significant difference. Among them, the branching polypropylene (B-PP) showed a pronounced branching effect and tensile hardening (as shown in Fig. 6a), while the elongation of the high-linearity polypropylene (H-PP) did not increase significantly. Large (as shown in Figure 6b).
Figure 6 Tensile rheological tests of branched polypropylene and linear polypropylene at different draw rates
6. Solids test: The glass transition temperature and crystallisation temperature are analyzed by means of a temperature sweep. The rheometer can perform a torsion test on solids (or “dynamicsâ€) with a suitable solid fixture (solid sample bar fixture STBF or fiber film fixture FFF). Thermomechanical analysis DMTA"). In general, solid properties are related to temperature, and testing them helps to gain insight into the morphology and performance of the sample. Measurements of the glass transition temperature (Tg) and the storage modulus (G') below the transition temperature can yield information on maximum operating temperature, impact strength, brittleness, and rigidity. For crystalline or partially crystalline polymers, the melting temperature (Tm) is another important material parameter. The DMTA test can simultaneously obtain the melting temperature data (as shown in Figure 7).
Figure 7 DMTA testing of polypropylene and glass fiber reinforced polypropylene
7. Transient testing: testing the time-response stress of the material Step changes (creep and recovery), strain step changes (stress relaxation) and velocity step changes (stress increase/start flow) can characterize the material for a given shear stress, shear The time (instantaneous) response of the shear strain or shear rate. The analysis method involves calculating some important material parameters such as: zero shear viscosity, plateau modulus, creep compliance, and converting the transient material function into the dynamic material function G'(ω)G" (ω). Shown as an example of a stress increase test.
Fig. 8 Transient test of polymer solution
Figure 9 Curves of Thermosetting Epoxy Resin with Temperature
In addition, the rheometer can also measure the curing reaction or chemical reaction, for example, epoxy curing or UV curing, etc., and can determine the complete curing reaction dynamic state through the isothermal curve or setting the heating rate. The parameters involved include: The minimum softening viscosity, gel point, curing time and curing temperature.
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