The last period is period of explosion of blown materials from melt. This is done by those who know and don't understand, so those who need to be tested, those who can and those who can't, are also at top. reaching a new height - madness. Many people called me to ask me some questions about melt blown materials testing. I found that some people pretended to understand and were deceived, and some people did not know how to read data, so they were guided by current material technology, blown from melt. The recommended standard for indicators, GB/T30923-2014, also has problems. While I was busy with this, I thought it would be better to revise this document, which was originally presented at annual meeting of China Plastics Association Expert Committee, and publish it in advance to meet needs of relevant people.

The production of meltblown fabric and meltblown materials has a long history, but recent epidemic of novel coronary pneumonia has repeatedly drawn attention to meltblown fabric and its raw materials. It is main material for production of medical masks and supply is in short supply, and world is in a hurry. From mask, price has skyrocketed.

The material blown from melt has an important technical indicator of melt flow rate, which is commonly called melt finger. Due to special melt blown process, melt index of melt blown mask material should reach 1500 g/10 min. This is important because specialty meltblown material code name is usually determined by first two significant digits of meltblown material's melt index followed by "00". For example, PPH, Y1500, which means mass flow rate of material melt.

STR=(1500±100)g/10min

Unless otherwise noted, test conditions should be 230°C, 2.16 kg, and matrix used is standard 2.095 mm. The basic design of melting finger is shown in figure.

The experimental process of blowing material from melt is as follows:

Remove weight;

Heat barrel to experimental temperature and keep temperature constant;

Remove piston rod;

Use a plug to cover outlet at bottom of matrix;

Blow through bore with nitrogen for 5~10 seconds (nitrogen pressure is about 0.05MPa);

Add approximately 14.5 g of sample from inlet port (considering flow rate is too high, add more appropriately);

Compact with pressure bar;

Insert piston rod;

Wait about 4 minutes;

Start automatic test program;

Load weight;

Removing plug;

The piston rod moves down freely and quickly, automatically registers time it takes to move down maximum effective distance (approx. converted to churn per 10 minutes calculated using following formula:

In formula: m is quality of cut spline, g;

MFI – melt index (melt consumption), g/10 min.

If melt index of experimental material is about 1500, then effective test stroke of downward moving piston rod is about 2.54 cm, then the flow time t:

Among them: L - stroke of piston rod moving down, set at 2.54 cm;

ρ is density of experimental material melt, according to our experiments it is about 0.70;

Then t=0.506 s.

There is a clear standard when it comes to "4 minutes" in an experiment. It is used to ensure uniform sample temperature. Likewise, operator cannot use it as basis for changing test results.

It is obviously difficult to precisely cut such a piece of material in such a short amount of time and then weigh it.

The above is basic method for determining melt flow rate, also known as "A" method.

(Image taken from Internet)

So, what is best and most convenient way for us to pass this test? The following three experimental methods are available.

1 bulk method

The volume method is called "method B" in standard, and its principle is to check time of outflow of a certain volume of melt.

This method is simple if piston stroke is set on instrument, because accuracy of relevant parts of instrument is quite high if tolerance of some parts is only ±0.005. mm, set stroke will be same as internal outflow volume. According to exact match, we can draw following according to time automatically recorded by instrument:

Note that target symbol has changed from MFR to MVR, which is a volumetric flow symbol indicating how much (cm3) of sample is expelled every 10 minutes:

MVR——cm3/10min

Obviously, this is much more convenient than testing MFR, why not use it? As far as I know, this method was used internationally as early as 40 years ago. Is it because there is no precedent in China for such use, so we can only use MFR uniformly?

2 Half Die and Small Die Method

Half die was proposed in new version of international standard ISO 1133-11 in 2011 to facilitate measurement of high melt index. The author conducted an experiment comparing meltblown polypropylene material with a melt density of 0.6780 g/cm3 (obtained by actual measurement) under experimental conditions of 230°C and 2.16 kg:

Use standard die, MFR=1287g/10min;

Half punch used, MFRh = 176 g/10 min.

Note that an "h" is added in lower right corner of MFR character, which means half, half stamp, to show difference, and expiration time is increased accordingly. There is no linear relationship between them, so this is not comparability).

The new version of melt flow rate test standard added a half head, original intention was to facilitate high melt index testing, and it is recommended to be used "when MFR = 75 or higher". However, if it is not used, it is assumed that not everyone uses it, and family uses it, and data cannot be transferred and compared.

The melt flow rate test standard 20 years ago also had a small die d = (1.180 ± 0.005) mm, and height was same as standard die, which was canceled in a later standard. popular melt blowing time. In experimental process of material, some also use this kind of stamps, but there is same recognition problem as half stamp, and now small stamp does not meet standard, so author disagrees.

3 complex methods

The complex method is currently applicable method, except that it is a little cumbersome, but in end, it can get a "nationwide" MFR experimental result. First, MVR is automatically obtained by volumetric method and then multiplied by melt density to convert to MFR.

MFR= ρ MVR

ρ is density of melt

In GB/T30923-2014 "Polypropylene Melt-blown Special Material", melt density of melt-blown polypropylene material is described as follows: melt density value is 0.7386 g/cm3, and melt volume is measured first. Flow rate and then use melt density value to calculate massesth flow rate of melt.

Many do it. But wrong! What's matter? The error lies in value of density of melt. The source is wrong, result is, of course, wrong. Here value of melt density becomes a constant, and those who have been involved in melt blowing know that there are many obvious bubbles on spline, which at least shows that density of melt will not be constant. So what is value of 0.7386?

We will see this constant in relevant ASTM, ISO and even GB literature and it is indeed melt density of polypropylene, but this polypropylene is pure and without any modification. After modification, this will no longer be constant, so this 0.7386 should never be used again.

Readers and friends should not bow too much to some of our standards, but should understand them scientifically. Of course, those who formulate standards must be careful and cite. I discovered a number of national standards and industry standards, some of which I participated in review and was able to revise in time; I can "make mistakes and make mistakes." The problem with this recommended standard for melt blown polypropylene materials was discovered by me during this epidemic, and many people use it.

(Image taken from Internet)

We have tested melt density of many units of meltblown polypropylene material and under test conditions it is in range of 0.66 to 0.73 g/cm3. For each material, melt density must be obtained experimentally.

3.1 Melt Density Calculation

The experimental method for determining melt density is described in detail in my 1999 paper.

Currently, many meltblown materials are modified from polypropylene. The density of melt of polypropylene is known and is indicated in relevant literature. However, after modification, melt density changes and original melt density data can no longer be used. .

If you calculate melt density under normal conditions, you will also run into problem that flow rate is too high and difficult to understand and verify. Therefore, according to characteristics of melt density, author researched and developed a special head with a special capillary for measuring melt density of refractory finger materials. At a certain temperature and load, flow rate slows down to facilitate measurement of melt density ρ at experimental temperature and standard load (T, min) .

This die should not be used as an experimental melt index die.

Since a thermoplastic melt has characteristics of an elastic body, care must still be taken when changing applied load arbitrarily in order to conveniently obtain density of melt. When using modified polypropylene material as raw material for melt blowing, there is currently a large range:

Experiment temperature: T=230℃;

Applied load: mnom=2.16 kg.

The experimental method is similar to mass melt index method (method A in standard). Insert piston rod after feeding, wait 4 minutes at constant temperature, load weight and cut off entire stroke. section while melt slowly flows down. (e.g. 1" slot), weigh it and calculate melt density:

m - average spline quality (g)

L is effective displacement of piston rod (for example, 2.54 cm).

If you use instrument with melt density test program, when you run program, required spline section will be automatically cut off and then weighted, and then interface will be entered when checking melt index.

3.2 Melting index determination

After obtaining density of melt, you can use automatic test function of device.

In interface of automatic test program, enter measured melt density and select largest test stroke.

Feeding, constant temperature melting.

Push down on die plug (also known as die plug) to prevent melted material from dripping. The upward direction of plunger must not be too large, and it must not be pressed too quickly, otherwise die will be pushed up.

When constant temperature time is reached (usually 4 minutes), you can run test program, load weight, quickly remove plug, liquid flows down instantly, equipment automatically records flow time and gives two results MVR and MFR.

Stub requirements:

a The top force of plunger head should be moderate, load should be slow, and it is best to use a damping operation;

b The head is made of heat-insulating material so as not to affect temperature of die and internal melting material;

c The plug removal speed must be fast. For example, in a test program, stub head can be automatically ejected, which is best.

3.3 Test Accuracy Analysis

In a melt flow experiment, many factors affect accuracy, so 5% is commonly used as measurement standard. If repeatability is within 3% and test of national standard is within uncertainty of sample, equipment has already been tested. It is very good. However, following factors will directly affect test results in high melting point test:

a Check effects of time

This is affected by a combination of temporal resolution, clock accuracy, and stroke accuracy. In terms of temporal resolution, if resolution is 0.01 s, then inherent numerical error of high melting point (take 1500 as an example) caused by this "one word" is already ±30.

Bubble effect b

The author has found that material with a high melting point is more or less accompanied by air bubbles when it is extruded into tester. According to melt flow rate experiment standard, air bubbles are not allowed, which will lead to undefined changes in flow rate. , which leads to numerical fluctuations, and refractory materials seem inevitable, and evenThat is, author feels that it is precisely bubbles formed that are involved in acceleration of outflow of melt. One day, author removed air bubbles in melt through sample pretreatment and other methods, causing test value to drop from four digits to three digits. The existence of bubbles has a great influence on uncertainty of experimental results.

c Melt density must be measured correctly

Different batch numbers must be tested separately and should not be confused. Correct melt density is directly proportional to test results.

Feeling of a large absolute value of d

The accuracy and repeatability of experimental results are calculated as a percentage, but absolute value of refractoriness index is relatively large, for example, MTR=1500, you can feel a big gap between 1400 and 1500, but in fact difference is only 6.7%. For experiments with high melting point materials this is still acceptable.

4 Author's definition of melt index value of a meltblown material

How does author determine melt index of a meltblown material?

First, it is necessary to confirm that melt flow meter used in experiment meets requirements. Confirm that all three elements (dimensional accuracy, roughness, Vickers hardness) of three main mechanical parts of instrument (die, cylinder and piston rod) meet standards. Confirm that instrument's four key temperature control measures meet requirements (temperature accuracy, fluctuation, settling time, and vertical temperature gradient). Make sure accuracy of stroke detection device is within 0.2%.

Use at least three of these compatible tools at same time. According to program requirements, add test material, if measured values ​​are within 5% of average value, take average value. Melt density and melt flow rate are determined according to this requirement.

Many factors, especially effect of modified air bubbles, fluctuation of melt index value of same batch of melt blown materials will be relatively large, so there is no national standard sample for melt blown materials and concepts such as it accuracy. can't confirm. It cannot be based on accuracy of low melting temperature testing (such as national standard sample), which is my experience in researching melt flow rate testing technology for nearly 30 years.

Source: Shanghai Silda Scientific Instrument Co., Ltd. Yao Hanliang, Synthetic Fiber Chief Engineer