How Long Should Glass Fibre Be? Breaking a Pellet Tells You Nothing

Reinforced material arrives, the customer breaks a pellet open by hand, looks at the glass fibres on the fracture surface, and asks you: So what is the glass fibre length in this batch?

This is not a hard question to answer because we lack the data.

It is hard because breaking open a pellet and reading fibre length off the fracture surface carries enough error that it cannot be used to answer a customer.

Two people reading the same pellet get two different numbers

The glass fibre is buried in the matrix. What you see on the fracture surface of a pellet is only the cross-sectional projection of a fibre that was cut at random, not the full length of the fibre itself.

Count a different spot on the same fracture surface, or count at a different magnification, and the number changes. Where the fibres on the fracture surface are not aligned, the projection shortens the length as well.

Two people each open a bag, each report a number, and neither convinces the other.

So a visually estimated length cannot go into a specification, and cannot be used as the basis for discussing performance with a customer.

We measure it with a polarizing microscope

On our list of testing equipment, glass fibre length and microstructure is assigned to a polarizing microscope, model DYP702, from Shanghai Dianying Chemical Instrument Co., Ltd., with 5 specimens tested and 3 times per batch, checked in house.

There is also a work instruction for this instrument, number YS/WI-10-QA-08, which turns sample preparation through image capture into a fixed sequence of steps.

The first step is sample preparation. Take a suitable amount of the modified pellets, burn them to ash and put the ash in a beaker, dilute with alcohol, and pour it onto a microscope slide.

This step is easy to overlook. Why burn to ash first? Because what has to be measured is the size of the glass fibre itself, so the resin has to be burnt away and the remaining glass fibres have to be spread out on the slide before they can be measured one by one.

The second step is illumination. Switch on the microscope lamp power supply. There is one detail worth noting: for measuring fibre and mineral powder dimensions, only the upper light source is needed, and no polarizer is required.

Polarized-light calibration is for identifying birefringent materials, where the vibration direction of the polarizer is determined using the cleavage of black mica. Measuring fibre length does not call for that.

The third step is image capture and measurement. By adjusting the aperture, the height of the stage and the objective magnification, a clear image of suitable size is obtained in the software interface, the image is captured, and then, with the objective magnification selected, the measurement toolbar is used to measure length and diameter on the image.

Beyond length, content has its own arithmetic

When a customer asks about length, what they usually really want confirmed is content. For content we have two independent lines of evidence.

The first is ash content. In the specification sheet it is called ignition residue, test standard ASTM D2584, test condition 650 °C. This line gives the proportion left after the glass fibre has burnt, so it corresponds to the glass fibre content.

Take three of our grades as examples. Glass fibre reinforced PA6 RF-1331GF20 has an ignition residue of 20±2 %, glass fibre reinforced black PA6 RF-1331GF45 is 45±2 %, and glass fibre reinforced PA66 RF-9031GF30 is 30±1 %. The GF20, GF45 and GF30 in the grade names match this line.

The control on the production side is tighter than the specification. On the extrusion process sheet, the glass fibre and ash line gives the control range: glass fibre reinforced PA6 RF-1331GF30 is held at 30 % to 33 %, glass fibre reinforced PA66 RF-1331GF50 at 50 % to 53 %, and glass fibre reinforced PA6 RF-1331GF15 at 15 % to 17 % finished glass fibre.

The second is density. Density works backwards to the glass fibre share, through weighted mixing density.

Take the PA66 system as an example, with a nylon density of 1.13 and glass fibre of 2.2. On this relation, every 10 percentage points added to the glass fibre content moves the theoretical density up by 0.107.

The most telling part of this table is that the measured density column is lower than the theoretical density in every row. GF20 is 0.07 lower, GF45 is 0.0925 lower.

The difference comes from the actual state of the interface and of fibre packing. The theoretical density is calculated, the measured density is weighed, and the gap between the two columns is the trace left by the process and by the dispersion.

So density is more useful during grade selection and grade verification than many people assume. It is not only a pass or fail number. It also tells you together whether the glass fibre actually arrived and whether the dispersion is uniform.

One step of content, how much of a difference in performance

If glass fibre length and content cannot be pinned down, performance is only an estimate. In the other direction, once content is pinned down, performance becomes verifiable.

For glass fibre reinforced PA6, RF-1331GF20 has an ignition residue of 20±2 % and RF-1331GF45 has 45±2 %. The mechanical properties written on the specification sheet for these two grades are:

Going from 20 % to 45 % glass fibre takes the flexural modulus from 4500 MPa to 8500 MPa. That span is the part of the difference a customer actually feels on the injection moulding machine, and behind it sit length and content together.

So when a customer asks about glass fibre, they are really asking about three numbers. Asking in this order saves the most time.

First, what is the content. Look at the ignition residue line, ASTM D2584, 650 °C. The GF suffix in the grade name matches this line.

Second, which band does the density fall in. Compare the density against the theoretical value. A large gap points to a dispersion problem.

Third, by what method was the length measured. This one question separates two kinds of supplier: one that gives a visually estimated number, and one that gives a microscope number.

After those three questions, what the customer wants usually becomes one concrete action: give me the grade, and send me the ash and density figures.

Need the ash and density figures for a grade, send us the grade and we will produce them by batch.

To see how glass fibre length is actually measured, reply "microscope" and we will arrange a visit to the laboratory to watch the sample preparation and the image capture.

Qingdao Yunsu Polymer Material Technology Co., Ltd.