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Cited by 29Terahertz technology and applicationsMicrowave and Dielectric Measurement TechniquesSpectroscopy and Laser Applications

Polymer Pellet Fabrication for Accurate THz-TDS Measurements

Open access
Murphy, Keir N., Naftaly, Mira, Nordon, Alison, Markl, Daniel·2022·Applied Sciences
DOI 10.3390/app12073475
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Abstract

We investigate fabrication of compacts using polytetrafluoroethylene (PTFE) and polyethylene (PE), and the effect of compaction conditions on their terahertz transmission properties. The conditions used to fabricate compressed powder samples for terahertz time-domain spectroscopy (THz-TDS) can impact the accuracy of the measurements and hence the interpretation of results. This study investigated the effect of compaction conditions on the accuracy of the THz-TDS analysis. Two polymers that are commonly used as matrix materials in terahertz spectroscopy studies were explored using a compaction simulator and a hydraulic press for sample preparation. THz-TDS was used to determine the refractive index and loss coefficient to compare the powder compacts (pellets) to the values of solid material. Sample porosity, axial relaxation and tensile strength were measured to assess the material’s suitability for terahertz spectroscopy. It was found that PTFE is the preferable material for creating THz-TDS samples due to its low porosity and high tensile strength. PE was found to show significant porosity at all compaction pressures, making it an unsuitable material for the accurate determination of optical parameters from THz-TDS spectroscopy measurements. The larger particle sizes of PE resulted in compacts that exhibited significantly lower tensile strength than those made from PTFE making handling and storage difficult.

AI snapshot

Written from the paper’s full text.

TL;DR

This study compares PTFE and two HDPE grades as matrix materials for THz-TDS pellets, showing that compaction conditions strongly affect porosity, axial relaxation, tensile strength, and extracted optical parameters. PTFE is preferred because it forms low-porosity, robust pellets, whereas HDPE retains significant porosity at all tested pressures and is unsuitable for accurate refractive-index and loss-coefficient extraction.

Key findings

  • PTFE pellets compacted above 350 MPa had low porosity and loss coefficients within reference ranges; a compaction pressure >350 MPa was required to prevent lamination/capping in compaction-simulator samples.
  • HDPE pellets showed significant porosity at all compaction pressures, giving refractive indices significantly lower than literature values and making them unsuitable for accurate THz-TDS optical-parameter extraction.
  • At 392 MPa, tensile strength was 3.162 ± 0.050 MPa for PTFE and 0.846 ± 0.058 MPa for HDPE2; the text states PTFE and HDPE1 exceeded 2 MPa, while HDPE2 did not.
  • PTFE axial relaxation was negligible one week after compaction for compaction-simulator and hydraulic-press-without-vacuum pellets; HDPE continued relaxing up to four weeks, so THz-TDS analysis should be performed four weeks after compaction for HDPE.

Methods

  • Pellets were made from PTFE and two HDPE grades with average particle sizes of 1 µm, 42 µm, and 125 µm, respectively; batches of ten 9 mm diameter, approximately 1 mm thick pellets were prepared.
  • Compaction used a compaction simulator at 157, 235, 319, 352, 392, 436, and 478 MPa, or a hydraulic press with and without vacuum at 139 and 159 MPa; the hydraulic press used a 1 min dwell, while the simulator used a 15 s profile with 5 s dwell.
  • THz-TDS was performed on a TeraPulse Lx system with 0.04 THz resolution under nitrogen purge; refractive index and loss coefficient were extracted at 2.45–2.55 THz. Porosity was calculated using the zero porosity approximation, axial relaxation was measured by micrometer at 0, 1, and 4 weeks, and tensile strength was calculated from breaking-force measurements.

Limitations

  • The paper does not state a formal limitations section.
  • Evident from the text: the comparison between compaction methods is confounded by dwell-time differences, which the authors note must be considered; the study is also limited to PTFE and two specific HDPE grades, and HDPE1 handling was inferior because pellet surfaces crumbled and left residue on gloves.
  • Evident from the text: HDPE porosity remained too large for accurate optical-parameter extraction across all tested compaction pressures, and inter-batch differences in HDPE reinforce the need for reference samples for each polymer batch.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.3390/app12073475

Cites

Statistics

Nothing to recompute.

Claims the paper makes

0 of 15 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    PTFE pellets fabricated in the hydraulic press with a vacuum exhibit more significant axial relaxation between zero and one week after compaction.

    “Pellets fabricated in the hydraulic press with a vacuum (HPV) exhibit more significant axial relaxation between zero and one week after compaction.”
    Quote in the paperSubstantial claim
  • Findingnone given

    There is no distinguishable difference in refractive index between samples of the two HDPE grades.

    “There is no distinguishable difference in RI between samples of the two grades.”
    Quote in the paperNot a substantial claim
  • Findingown data

    Larger particle sizes of PE resulted in compacts with significantly lower tensile strength than those made from PTFE.

    “The larger particle sizes of PE resulted in compacts that exhibited significantly lower tensile strength than those made from PTFE making handling and storage difficult.”
    Quote in the paperSubstantial claim
  • Findingown data

    Raman spectra showed no change in amorphous or crystalline character of PTFE pellets compared with solid reference samples.

    “Raman spectra of the PTFE pellets and solid reference samples were acquired to determine whether a change in amorphous or crystalline character had occurred (Figure A1); no such change was observed.”
    Quote in the paperSubstantial claim
  • Findingown data

    Loss coefficient values for PTFE at compaction pressures >350 MPa are within reference ranges.

    “The loss coefficient values for compaction pressures >350 MPa are within the reference ranges.”
    Quote in the paperSubstantial claim
  • May combine two claims. 30
  • The quote only partly states this claim. 1
  • Classified as probably not a substantial claim. 4

  • May combine two claims.

    Findingown data

    Axial relaxation in HDPE samples continues to 4 weeks after compaction, with samples made using the hydraulic press (longer dwell times) showing more significant relaxation.

    “Axial relaxation in HDPE samples continues to 4 weeks after compaction, with samples made using the hydraulic press (longer dwell times) showing more significant relaxation.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    At low frequencies, PTFE pellets are in good agreement with the loss coefficient of solid material, whereas their refractive index is slightly higher.

    “It is evident at low frequencies that the PTFE pellets are in good agreement with the loss coefficient of solid material whereas the refractive index is slightly higher.”
    Quote in the paperNot a substantial claim

  • May combine two claims. The quote only partly states this claim.

    Findingown data

    PTFE samples produced at 159 MPa using the hydraulic press without a vacuum have a significantly lower refractive index, suggesting either increased sample porosity or structural defects.

    “However, the samples produced using a compaction pressure of 159 MPa have a significantly lower refractive index, suggesting either an increase in sample porosity or the inclusion of structural defects in the sample.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The variation is significantly higher for HDPE pellets produced at compaction pressures >350 MPa, suggesting that compaction pressures >350 MPa are unsuitable for sample preparation of this material.

    “The variation is significantly higher for pellets produced at compaction pressures >350 MPa, suggesting that compaction pressures >350 MPa are unsuitable for sample preparation of this material.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The loss coefficient for HDPE1 pellets is in good agreement with solid reference material below 2 THz; higher frequencies are more affected by scattering from particles, leading to significant excess loss above 1 THz.

    “The loss coefficient for the HDPE1 pellets are in good agreement with that of the solid reference material below 2 THz; however, higher frequencies are more affected by the scattering from particles, leading to significant excess loss above 1 THz.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Pycnometric density values for all polymer materials matched manufacturer values, except PTFE, which was lower.

    “The measured values were the same as those provided by the manufacturers, with the exception of PTFE, which was lower.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    HDPE1 samples increase in porosity with increasing compaction pressure, whereas HDPE2 samples have more consistent porosity values.

    “The HDPE1 samples increase in porosity with increasing compaction pressure, whereas the HDPE2 samples have more consistent porosity values.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingreasoning

    The refractive index of PTFE at 159 MPa is significantly lower than expected and exhibits high variability across ten pellets.

    “The refractive index value at a compaction pressure of 159 MPa is significantly lower than expected and exhibits high variability across ten pellets.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    PTFE refractive index values are either in good agreement or slightly above literature values, except for pellets compacted at 159 MPa.

    “2a) are either in good agreement or slightly above the values in the literature with the exception of the pellets that were compacted at a pressure of 159 MPa.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingnone given

    Samples made with the hydraulic press show slightly higher refractive indices than the compaction simulator; this increase is independent of the application of a vacuum.

    “Samples made with the hydraulic press show slightly higher refractive indices than the compaction simulator; this increase is independent of the application of a vacuum.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingreasoning

    Pellets made from HDPE2 exhibit significantly higher loss, attributed to increased scattering from the larger particle size.

    “However, pellets made from HDPE2 exhibit significantly higher loss, which is attributed to increased scattering from the larger particle size.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    Cracks in PTFE samples also affect the loss coefficient, which is higher in samples compacted at lower pressures.

    “The cracks in these samples also affect the loss coefficient, which is higher in samples compacted at lower pressures.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    HDPE pellets contain significant porosity, as their refractive index is significantly lower than that of solid reference material.

    “It is evident that the HDPE pellets contain significant porosity as the refractive index is significantly lower than that for the solid reference material.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The inclusion of pores reduces the calculated effective refractive index and the pores' scattering leads to an increased loss coefficient value.

  • May combine two claims.

    Findingown data

    The loss coefficient for HDPE1 grade is in good agreement with reference loss coefficient values, indicating low scattering.

    “The loss coefficient for HDPE1 grade is in good agreement with the reference loss coefficient values, indicating low scattering.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The hydraulic press without a vacuum performed well at 139 MPa, with both refractive index and loss coefficient values either matching or slightly above literature values.

    “The hydraulic press without a vacuum performed well at 139 MPa, with both refractive index and loss coefficient values either matching or slightly above literature values.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingnone given

    During handling of HDPE1 pellets, the surface visibly crumbled and left residue on gloves.

    “It was observed that, during handling of HDPE1 pellets, the surface visibly crumbled and left residue on gloves”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingnone given

    The particle size in HDPE primarily affected the resulting tensile strength and scattering loss, with smaller particles producing pellets with higher tensile strength and lower loss coefficients.

    “The particle size in HDPE primarily affected the resulting tensile strength and scattering loss of the material, with smaller particles producing pellets with higher tensile strength and lower loss coefficients.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The particle size values of the polymer materials provided by the material supplier are accurate.

  • May combine two claims.

    Findingreasoning

    Compaction using the hydraulic press resulted in noticeably superior samples, with a vacuum providing no noticeable benefits except reduced porosity in PTFE pellets immediately after compaction.

    “Compaction using the hydraulic press resulted in noticeably superior samples, with a vacuum providing no noticeable benefits with the exception of reduced porosity in PTFE pellets immediately after compaction.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    The refractive index value at a compaction pressure of 159 MPa is significantly lower than expected and exhibits high variability across ten pellets.

    “The refractive index value at a compaction pressure of 159 MPa is significantly lower than expected and exhibits high variability across ten pellets.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Pellets experience material- and pressure-dependent axial relaxation, leading to a change in sample thickness and, in some cases, to porosity.

    “In the case of pellets, however, it is well known that pellets experience material-and pressure-dependent axial relaxation, leading to a change in sample thickness and, in some cases, to porosity [10].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [10] Bawuah, 2020, A Fast and Non-destructive Terahertz Dissolution Assay for Immediate Release Tablets, J. Pharm. Sci. source

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Larger particle sizes will relax to a greater extent and show more significant differences between one and four weeks after compaction.

    “Larger particle sizes will relax to a greater extent and show more significant differences between one and four weeks after compaction.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The hydraulic press had a larger dwell time than the compaction simulator, explaining the better performance at lower pressures.

    “The dwell time difference between the compaction methods must also be considered where the hydraulic press had a larger dwell time compared with the compaction simulator, thus explaining the better performance at lower pressures.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Sample thickness is utilised in the extraction of the refractive index and is a potential source of error.

    “Sample thickness is utilised in the extraction of the RI and thus is a potential source of error [20].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [20] Withayachumnankul, 2014, Fundamentals of measurement in terahertz time-domain spectroscopy, J. Infrared Millim. Terahertz Waves source

      Not checked in the free run.

  • May combine two claims.

    Interpretationown data

    The loss coefficient remains constant because porosity is constant over the compaction pressure range.

    “The loss coefficient remains constant because porosity is constant over the compaction pressure range (Figures 7 and 8).”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    The lower refractive index of HDPE suggests significant porosity in all samples, meaning the polymer is unsuitable for accurate determination of refractive index.

    “The lower refractive index suggests significant porosity in all samples, meaning that the polymer is unsuitable for accurate determination of RI.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The inclusion of pores reduces the calculated effective refractive index and the pores' scattering leads to an increased loss coefficient value.

  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationreasoning

    THz-TDS analysis of HDPE pellets should be carried out four weeks after compaction, allowing repeated analysis.

    “Therefore, due to the axial relaxation of HDPE being significant for pellets produced using all compaction methods, it is clear that THz-TDS analysis should be carried out four weeks after compaction allowing for repeated analysis.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    PTFE is preferred over HDPE for use as a matrix material due to its lower porosity and high tensile strength.

    “The results demonstrate that PTFE is preferred over HDPE for use as a matrix material due to the lower porosity of PTFE pellets.”
    • Also stated in: § 5. Conclusions · plain
      “PTFE was the preferred material for use as a matrix due to its low porosity and high tensile strength.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Both PTFE and HDPE1, having tensile strengths higher than 2 MPa, are suitable for experimental handling provided they are compacted at >392 MPa.

    “As both PTFE and HDPE1 have tensile strengths higher than 2 MPa, they would both be suitable for experimental handling, provided that they are compacted at >392 MPa.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    The lack of vacuum advantage is attributed to axial relaxation after compaction, as a higher compaction pressure causes more significant axial relaxation and greater changes in porosity.

    “This is attributed to the axial relaxation after compaction, as a higher compaction pressure causes more significant axial relaxation [32] and greater changes in porosity.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [32] Bawuah, 2018, Investigating elastic relaxation effects on the optical properties of functionalised calcium carbonate compacts using optics-based Heckel analysis, Int. J. Pharm. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationown data

    Reference samples of pure matrix polymer should be created for each batch of material.

    “It is therefore advisable to create reference samples of pure matrix polymer for each batch of material.”
    • Also stated in: § 5. Conclusions · hedged
      “suggesting that inter-batch material variations reinforce the need for reference samples to be created for each batch of polymer.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    A lower compaction pressure with a sufficiently long dwell time will achieve the same performance as a higher pressure with a shorter dwell time.

    “a lower compaction pressure with a sufficiently long dwell time will achieve the same performance as a higher pressure with shorter a dwell time.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationreasoning

    Analysis immediately after compaction is not recommended, as changes in porosity will lead to reduced reproducibility.

    “Analysis immediately after compaction is not recommended as changes in porosity will lead to a reduced reproducibility.”
    Quote in the paperSubstantial claim
32 of 32

  • The paper relies on this without support.

    Comprehensive studies on the polymers employed and the optimal conditions to make these samples are scarce.

    “Even though sample preparation methods using polymers and hydraulic presses have been used extensively to analyse various materials, comprehensive studies on the polymers employed and the optimal conditions to make these samples are scarce.”

  • The paper relies on this without support.

    There is a lack of understanding of the inherent differences between polymer batches/grades and the optimal compaction pressures to create samples with the desired properties.

    “Moreover, there is a lack of understanding of the inherent differences between polymer batches/grades and the optimal compaction pressures to create samples with the desired properties.”

  • The authors state this as an assumption.

    The porosity of PTFE samples is close to zero when the refractive index is maximised.

    “It is assumed that the porosity of the PTFE samples is close to zero when the refractive index is maximised.”

  • The paper relies on this without support.

    Paraffin oil will fill surface connected pores and increase the refractive index due to a decrease in sample porosity.

    “The paraffin oil will fill surface connected pores and increase the refractive index due to a decrease in sample porosity.”

  • The paper relies on this without support.

    The inclusion of pores reduces the calculated effective refractive index and the pores' scattering leads to an increased loss coefficient value.

    “The inclusion of pores reduces the calculated effective refractive index and the pores' scattering leads to an increased loss coefficient value.”

  • The paper relies on this without support.

    The particle size values of the polymer materials provided by the material supplier are accurate.

    “Three polymer materials used for pellet fabrication, with particle size and solid true density at 25 °C provided by the material supplier.”
6 of 6

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • The sample preparation process for PTFE produces samples of negligible porosity and high tensile strength, allowing them to be treated as binary mixtures.

    “This sample preparation process for PTFE was chosen at it has been verified to produce samples of negligible porosity and high tensile strength, producing samples which can be treated as binary mixtures [21].”

    From Unveiling the Origin of Spurious Features in THz-TDS of Powder Compacts (2025) · cites this paper as [21]

    Section 3.2 reports close-to-zero porosity for PTFE pellets compacted above 350 MPa, and the Discussion reports high tensile strength when compacted above 392 MPa. The paper does not say that these properties allow the resulting samples to be treated as binary mixtures. (LLM, conf=0.86)

    Experimental — accuracy not yet measured

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Figure 7 is described as covering all pressures but captioned at one pressure

    Minor

    The results use Figure 7 to describe HDPE porosity across compaction pressures, but its caption says the plotted pellets were compacted at 159 MPa.

  • HDPE1 porosity is called both constant and increasing with pressure

    Minor

    The results attribute constant loss to constant porosity over the pressure range, then state that HDPE1 porosity increases with pressure.

  • Compaction simulator's lowest pressure is listed as both 157 and 159 MPa

    Minor

    The methods list 157 MPa among the seven simulator pressures, while the Figure 2 description lists 159 MPa for that series.

3 of 3

The citation lists haven’t been fetched yet

References 32 · Cited by 29

Research works printed but not linked: 1

  1. Zeitler, J.A., and Shen, Y.C. (2012). Terahertz Spectrscopy and Imaging, Springer. [1st ed.].

Authors
Murphy, K. N., Naftaly, M., Nordon, A., & Markl, D.
Affiliations
  • University of Strathclyde (GB)
  • National Physical Laboratory (GB)
Corresponding author
  • Murphy, Keir N.
  • Nordon, Alison
  • Markl, Daniel
ORCID
Venue
Applied Sciences
Publisher
MDPI AG
Published
2022-03-29
Volume
12
Issue
7
Pages
3475
ISSN
2076-3417 (ISSN-L)
DOI
10.3390/app12073475
OpenAlex
W4221108505
Semantic Scholar
fc3ab74fc4a4ea2f777088c00b7c6871db08bcac
Cited by
29
References
32
Citations by year
2026: 10 · 2025: 7 · 2024: 5 · 2023: 5 · 2022: 2
FWCI
1.30
Type
journal article
Language
en
Open access
Licence
Locations
  • Applied Sciences (publishedVersion, cc-by)
  • Strathprints: The University of Strathclyde institutional repository (University of Strathclyde) (acceptedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion)
Funders
  • Engineering and Physical Sciences Research Council (HH13054, EP/S02168X/1, EP/P006965/1)
  • National Physical Laboratory (EP/S02168X/1)
Topics
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Terahertz technology and applications
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Microwave and Dielectric Measurement Techniques
  • Physical Sciences › Chemistry › Spectroscopy › Spectroscopy and Laser Applications
Keywords
polymer pellets, powder compaction, terahertz time-domain spectroscopy, polytetrafluoroethylene, polyethylene, compaction conditions, porosity, tensile strength, refractive index, loss coefficient, sample preparation, particle size

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