Measurement of Polyethylene Density

Application Note 28

Summary•		Fast, accurate and repeatable•		No sample preparation•		Simple linear calibration•		Easiest, most reliable technique available; suitable 		for unskilled personnel

Application
Various grades of polyethylene are defined by their density(LLDPE, LDPE and HDPE) which relates to the crystallinity ofthe polymer. As density and crystallinity increase, stiffness
increases and impact resistance decreases. Measurement ofdensity is therefore a critical Quality Control parameter.

Why use NMR?
Traditional methods including displacement, titration andliquid pycnometry (ASTM D792, ASTM D1505, ISO 1183) canbe time consuming, require skilled operators and, in someinstances, the use of hazardous solvents.

For optimal precision, it is essential to stabilise the temperature

The MQC+ benchtop Nuclear Magnetic Resonance (NMR)of the polymer prior to measurement; usually at 40°C, whichanalyser provides an alternative method which is quick andis the same temperature as the magnet.

easy to perform, simple to calibrate, and requires minimal
sample preparation. As such it is ideal for routine operationin either a laboratory or production environment without any
requirement for additional chemicals or specialist operator
training.

Calibration
Ultimately, only two well-known standards are required tocalibrate the instrument as the calibration is linear within
the typical range of industrial materials. However initially it
is recommended that the instrument is calibrated by 3-6,

MethodThe NMR technique monitors and distinguishes the rates(spin-spin relaxation times) characterising a decay of the NMR
signal acquired from polymer samples. In a polymer sample
with a higher density, the atoms (spins) are positioned closer
to each other and, thus, the interaction processes in sucha sample are more intense compared to those in polymer
materials with a lower density. As a result, in samples witha higher density, the NMR signal decays faster than that fora sample with a lower density allowing them both to bequantified; density is proportional to the relative amount of
the tightly packed component in the sample.

preferably more, standards with known polyethylene densities
evenly spread over the range of interest. NMR is a comparativetechnique therefore cannot be more accurate than thereference technique against which it is being compared; error
is reduced by analysing more reference samples.

Measurements

Figure 1. NMR calibration for polyethylene density. The correlation coefficient andstandard deviation are 0.9930 and 0.0015 g/cc respectively

Polyethylene samples are poured into 18 mm glass
NMR tubes up to a predefined mark. The sample
tubes are placed in a temperature controlled
aluminium block at 40ºC for 30 minutes prior toanalysis. Measurement time is less than 20 seconds
per sample.

Results
Figure 1 shows a calibration for polyethylene
density. The reference values for the calibration
samples need to be very accurate given thatthe range of densities is narrow (0.92-0.96g/cm-3). However, the NMR measurement is also
very precise; Table 1 shows the repeatability ofmeasuring the same sample seven times.

Table 1. Measurement repeatability test of polymer density (g/cc)

Reference
value
0.9276

Repeat Measurements
0.9276

0.92810.92840.92820.92850.92800.9282MeanStandard
deviation

0.92820.0003Conclusions•	NMR is very stable over the long term and rarely needs calibration adjustment.
•	NMR is insensitive to the air voids between grains of powder or pellets.
•	Measurement precision is good compared to wet chemical and other manualmethods.

• Sample measurement time is rapid•	The NMR technique is non-destructive so the same sample may be measured
several times before being analysed by other techniques

visit www.oxinst.com/mqc for more information or email: magres@oxinst.com

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Do not reproduce without permission. Part no: MR/212/0420