Rheological viewpoint of thermoplastic melt (capillary rheometer)

introduction
Rheology is a discipline that studies the flow and deformation of matter
Discipline, established in Hooke in the late 17th century
The principles of elasticity and viscosity introduced by Newton
On top of that. Because it is easy to shape and has a low price,
Thermoplastic polymer melts are widely used in modern engineering
Industrial manufacturing is used to prepare various products. so
We need to understand how polymer melt is formed
What a flowing one.
Polymers are complex systems with many factors involved
Affects its flow properties. Factors affecting rheological properties
Elements may include: molding temperature; Flow velocity;
Stay time, etc.
In addition, the rheological characteristics of polymers are between those of liquids
Between body and solid. This leads to the flow of polymer
Dynamic features and other important characteristics are discussed below
Some of the relationships involved.
We have made it clear that the viscosity of the melt depends on temperature.
Reduce the mold temperature until the part being formed is produced
Raw rough flour, the temperature at this time is the surface temperature of the product
The lowest molding temperature at which defects may occur (corresponding to zu)
High resin viscosity).
Lowering the temperature of the mold can save energy and shorten
Production time, therefore understanding the relationship between viscosity and temperature
The system is very important.
*During the extrusion of polymer melt, there will be a discharge
The phenomenon of mouth swelling is manifested by the diameter of the extruded material
Getting bigger. The degree of expansion at the outlet and when entering the mouth mold
The magnitude of elastic deformation of the melt is related. Additionally, it is important to note
The meaning is that, under a constant flow rate, the melt
The degree of export swelling (more precisely, extrusion swelling)
The degree depends on the length of the mouth mold. in other words
Polymer melt has time dependence, polymer melt
Gradually eliminate the elastic deformation generated during molding,
The longer the molten material stays in the mold, the larger the outlet will swell
The smaller the degree.
Melt elasticity affects the processing and shaping of many other polymers
It has a profound impact, such as:
Blow molding. The wall thickness of the product depends on the mold closure
The degree of expansion of the outlet during pre extrusion.
Vacuum forming or thermoforming. Polymers must be maintained
A certain degree of elasticity to prevent being sucked into the cold by vacuum
Sagging occurs before forming the mold. If the polymer is not
With sufficient elasticity, it has the potential to be used in vacuum applications
Contact with cold and hard molds before applying pressure.
Meanwhile, the processing performance of polymers also depends on their mixing properties
Lubricants, plasticizers, and fillers during the formation of composites
The concentration of other components. From this introduction, we
It can be understood that it is necessary to have a correct understanding of polymer melt flow
Dynamic performance requires various complex instruments.
Rheologists believe that the flowability of polymers can
Simply divided into three parts: shear characterized by viscosity
Characterized by tensile flow and modulus or swelling ratio
Elastic behavior.
In order to comprehensively characterize the properties of polymers, it is required to
The instrument used can withstand a certain temperature and shear/pull
Extract these parameters within the range of elongation rate. modern
Laboratory rheological testing instruments can be divided into two categories:
Rotational rheometer andCapillary rheometer.
This type of instrument typically requires the detection of small amounts of polymer samples,
The sample is in the form of a circular sheet, usually with a diameter of 25 millimeters
1 millimeter thick. Place the sample on a pair of parallel plates
Between them, or with a conical plate on top and a flat plate on the bottom,
The temperature can be maintained through external heating devices, such as
Hair dryer or electric heating of the board.
Modern rotational rheometers can perform various tests, including
Fully characterize the material at a certain temperature and flow rate
Flow behavior within the scope. Possible testing items
For example:
Rheological curve, measuring shear viscosity and shear rate or
The relationship between shear stress. If the shear rate is sufficient
Low, can obtain a constant viscosity, that is, the speed is sufficient
Low, can achieve a constant viscosity, also known as zero shear
Cut viscosity.
Rotational Rheometer
Application articles in rheology
2 Application articles in rheology
Figure 1: Flow curve of LDPE at 190 ° C, with viscosity at
Present a platform at low shear rates. The magnitude of zero shear viscosity is determined by
The average molecular weight of the polymer determines.
Figure 2: Creep (blue) and recovery (red) of 190 º C polypropylene
Curve, capable of measuring zero shear viscosity and equilibrium recoverable compliance.
Figure 3: Stress relaxation data of LDPE at 190 ° C. When relaxed
The inter distribution curve contains information about the distribution of polymer molecular weight.
Figure 4: Polypropylene at 190 ° C? Frequency scanning at time C. intersection point
Determined by the average molecular weight and molecular weight distribution.
Experiments have shown that zero shear viscosity depends on the polymer
The average molecular weight of, the length of the platform (i.e. viscosity)
The shear rate before reduction reflects the molecular weight fraction
The width of the fabric. Software packages can be used to retrieve this data
Determine the average molecular weight and molecular weight distribution.
Creep test (continuously applied over a certain period of time)
Constant stress is another method for measuring zero shear viscosity
A method. Combined with the response test (stress removal),
They can measure the magnitude of the elastic strain of the sample,
Because elastic materials will rebound and try to recover as much as possible
Its original shape.
The stress relaxation test is to induce instantaneous deformation of the sample
Add strain and record the variation of stress over time.
The speed of stress relaxation depends on the polymerization at the test temperature
The viscoelasticity of the compound. This data is usually obtained using a relaxation mode
Display of quantity and time coordinate graph. Modulus and Time
Relationships are often overlooked, but they can quickly determine zero
Shear viscosity and average molecular weight.
Modulus/time differential function can generate continuous relaxation
Relaxation time distribution curve. This is essentially an abnormal recurrence
The mixed function includes the distribution of polymer molecular weight
Related information.
Small amplitude oscillation test as a convenient frequency test
Function, often used to simultaneously measure the elasticity of polymers
Sexual and adhesive characteristics. The two commonly used parameters by Zui
It is the "storage" (elastic) modulus and "loss" (viscous) modulus
The modulus of elasticity (G '') characterizes the recovery of the material, respectively
(i.e. elastic response) and flow (viscous response)
Corresponding to changes in deformation rate (test frequency)
The degree of change.
The typical response of polymer melt is dominant at high frequencies
To demonstrate elasticity, it is mainly manifested at low frequencies as
viscosity. This means that there are two types of response phases
The critical frequency of waiting. It's obvious that this is a meaning
The point with clear meaning, and the intersection of this frequency and modulus
The point depends on the molecular weight of certain linear polymers and
Molecular weight distribution. Utilize this point as a quality control measure
The potential advantage of tool making lies in its ability to generate shear adhesion
Compared to the constant point of degree, the storage modulus and loss modulus
The intersection of quantities is at a higher frequency. Therefore, with the flow
Compared to dynamic curve measurement or creep test, this test
The time may be much less.
Regarding the Relationship between Molecular Structure Information and Processing Performance
A comprehensive discussion is beyond the scope of this article,
The following example can illustrate the viscosity of polymers
How does elastic characterization solve practical processing problems
The problem is:
A) Diversity of tube thickness gauges in extrusion molding
Vibration test at low frequencies (below 0.1 Hz)
Revealing the difference in elastic modulus between different batches of materials
Different. It is obvious that the thickness gauge of the pipe depends on the polymer
The degree of recovery after extrusion, therefore the larger the pipeline size
It is not surprising that the larger the elastic modulus, the greater it is.
B) Reduce the inconsistency of fiber spinning characteristics
Low frequency oscillation test can display different batches of materials
Differences in elastic characteristics between them.
Figure 5: Frequency scanning curves of two types of HDPE pipes. compare
Samples with high elastic modulus produce larger sized tubes.
Figure 7: Advantages and disadvantages of polypropylene short fiber (PP fiber) samples
The function of storage modulus and frequency. Poor quality samples have high elasticity,
Resulting in inconsistent fiber diameters.
Figure 6: Advantages and disadvantages of polypropylene short fiber (PP fiber) samples
The function of complex viscosity and frequency. Attention, there is no obvious indication
Identify differences.
Just looking at the viscosity, there is no significant difference, indicating the flatness of the material
The average molecular weight is consistent. The difference in elasticity at low frequencies is due to
Due to differences in molecular weight distribution (MWD), the MWD range
Wide circumference causes more molecular chains to entangle together, hindering
The stretching process in fiber spinning technology. This will lead to
The products produced throughout their lifetime are inconsistent.
Capillary rheometer
The capillary rheometer includes a temperature controlled
Material bucket, equipped with one or more precise holes, holes
The exit position is equipped with a capillary tube mouth mold. Melt pressure transmission
The sensor is installed directly above the mouth mold to detect when
Polymer melt flows from the die at a set flow rate
Pressure drop during extrusion. Using capillary mouth molds and "holes"
Or "zero length" mouth molds can be determined by measuring the polymer's
Shear rate and stretching rate are related to shear viscosity and stretching
The relationship between viscosity.
With the help of different attachments, laser scanning can be used
Measure the expansion of the outlet using a controllable roller set
Measurement of Melt Strength of Fiber like Polymers (Melt)
The relationship between tension and traction speed.
Generally speaking, capillary rheometer is superior to rotational rheometer
More commonly used to determine melt characteristics at high shear rates,
And measure the rheological properties under typical processing conditions.
Of particular importance is its determination of tensile strength at high stretching rates
Stretching characteristics, especially for measuring production lines
The ability to stretch at a much higher rate than other technologies (such as...)
Reverse rotation roller equipment).
Figures 8 and 9 show the shear and tensile data, stating that
Understanding is important but often overlooked: two types
Polymers may exhibit almost identical shear flow behavior,
But it is possible to exhibit extremely different stretching behaviors.
As mentioned earlier, many polymer processing techniques (fiber)
Both spinning and blow molding are actually stretching techniques
Art, therefore measuring tensile viscosity is more effective than measuring shear viscosity
More important.
Figure 8: The relationship between shear viscosity and shear rate. Two types of rubber
The data cannot be clearly distinguished.
Figure 9: Tensile viscosity and tensile rate curves, as shown in Figure 8
For the same material. The stretching characteristics are significantly different.
Figure 10: Prove that the rupture of the melt is manifested by oscillatory pressure. that
Is the material 190? Polypropylene under C.
Capillary rheometer is usually used to study processing performance, rather than just
Only used for measuring rheological parameters. Two examples are the unstable flow zone
Measurement of domain and measurement of wall slip and critical stress.
rheology