Polyethylene (PE) is the most common plastic in use today. In 2017 alone, over 100 million tons of PE resin were produced, mainly for applications in the packaging market, and PE makes up approximately 34 percent of all plastic produced globally. There is an array of PE variants produced today, but this article looks into films composed of three of the more common PE variants. These include:
- High-density polyethylene (HDPE)
- Low-density polyethylene (LDPE)
- Linear low-density polyethylene (LLDPE)
Additionally, although HDPE is crystalline enough to characterize further, this article is chiefly concerned with the capabilities of benchtop X-ray diffraction (XRD) and its application in analyzing the percentage of crystallinity and crystallite size of the films.
For a long time, XRD has remained the gold standard method for material analysis. XRD is able to perform various analyses, ranging from phase identification and quantification for medium and highly crystalline plastics, to the determination of crystallinity percentage and crystallite size materials with lower crystallinity. This is all due to the bonding nature of plastics. The latter of these types of analyses is currently the industry standard use of XRD in plastics and polymers.
About the Thermo Scientific ARL EQUINOX 100 X-Ray Diffractometer
The Thermo Scientific™ ARL™ EQUINOX 100 X-ray Diffractometer employs a custom-designed Cu (50 W) or Co (15 W) micro-focus tube with mirror optics. Additional peripheral infrastructures and external water chillers are no longer necessary due to the instrument’s low wattage system, which allows users to easily transport it between labs and from the laboratory to the field.
Figure 1. ARL EQUINOX 100 X-Ray Diffractometer.
The ARL EQUINOX 100 X-Ray Diffractometer (c.f. Figure 1) offers rapid data collection times in comparison to alternative conventional diffractometers because of its unique curved position sensitive detector (CPS), that measures all diffraction peaks in real-time simultaneously. As a result, it is suitable for both reflection and transmission measurements.
Two samples of each film were prepared by encasing in a windowed cardboard frame, therefore allowing sufficient access to the film while also keeping it taught and free of surface variations. The sample containers were placed in the standard stationary holder (Figure 2), in reflection orientation, with the center of each sample centered in the beam path.
The samples were analyzed from 0-115° 2θ under Co Kα (1.78897 Å) radiation for 1 minute. Raw data evaluation was performed with I_MAD. Data processing, consisting of whole pattern fitting Rietveld refinement (WPF) was performed using MDI JADE 2010 equipped with the ICDD PDF – 4+. The raw XRD data for each sample was imported into MDI JADE 2010 where search/ match confirmed that the material was polyethylene.
Figure 2. Standard stationary sample holder.
The raw XRD data for each sample was imported into MDI JADE 2010. This confirmed, through search and match, that the sample material was polyethylene. In order to provide the data shown in Figure 3 and Figure 4, the data for each series B and C was refined with the WPF Rietveld analysis.
Figure 5 shows a representative report with the parameters of interest, listed below:
- Percent amorphous
- Percent crystalline
- Crystallite size
- Goodness of fit (R/E) (Table 1).
Table 1. Percent crystallinity analysis results.
HDPE is defined by a density of greater than or equal to 0.941 g/cm3. With a low degree of branching, the mostly linear molecules show well-ordered packing that maximizes intermolecular forces when compared to highly branched polymers.
Due to its high tensile strength, HDPE is often used in products such as:
- Milk jugs
- Detergent bottles
- Butter tubs
- Garbage containers
- Water pipes.1
WPF analysis generated results showing a material density of 0.95 g/cm3.
LDPE is defined by a density range of 0.910–0.940 g/cm3, and is characterized by a high degree of short- and long-chain branching. As a result, the chains do not exhibit well-defined packing within the crystal structure, which leads to weaker intermolecular forces when compared to HDPE. This is due to the lesser dipole attraction.
Therefore, the material has a lower tensile strength and increased ductility, and is used in both rigid containers and plastic film products, such as plastic bags and film wrap.2 WPF analysis performed indicated a material density of 0.93 g/cm3.
LLDPE is has a density range of 0.915–0.925 g/cm3, and possesses higher tensile strength than LDPE. This is because of its significantly linear polymeric structure characterized by extensive short branches, meaning that it exhibits higher impact and puncture resistance when compared to LDPE.
It is widely used in packaging, in a particular film for bags and sheets. Its applications also extend to the following products:
- Cable coverings
The WPF analysis performed showed a material density of 0.92 g/cm3.
Percent crystallinity analyses were carried out for all PE samples, producing the values and R-factors, with the results summarized in Table 1.
As expected, the HDPE samples are mainly in their crystalline form. Contrasting this is the LDPE and LLDPE films, which show a reversed trend in percent crystallinity when compared to their pellet forms.
LLDPE typically has a slightly higher percent crystallinity than LDPE when in pelletized form. It is hypothesized that this film is oriented along with the long polymeric structure with the orthogonal short chains in the horizontal plane to the beam, meaning that coherent diffraction was not seen from the side chains.
Figure 3. A) HDPE B) LDPE and C) LLDPE B series films refined and fit data.
Figure 4. A) HDPE B) LDPE and C) LLDPE C series films refined and fit data.
Figure 5. Representative report generated for HDPE B Film WPF refinement.
The resolution and speed of the ARL EQUINOX 100 X-ray Diffractometer facilitate the complete analysis of phase and percent crystallinity properties of plastics and polymers. To showcase its capabilities, measurement times were set for one minute, and, as a result, the ARL EQUINOX 100 benchtop X-ray Diffractometer has been shown to be an optimum choice for analyzing the crystalline nature of plastics and polymers.
References and Further Reading
- "Market Study: Polyethylene – HDPE". Ceresana Research. May 2012.
- "Market Study: Polyethylene – LDPE (2nd edition)". Ceresana. October 2014.
- "Market Study: Polyethylene – LLDPE 2nd. edition". Ceresana. November 2014.
Produced from materials originally authored by Gregory Schmidt from Thermo Fisher Scientific.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.
For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.