Green TPU

NotePart 1: Synthesis of Green TPU

1. Green TPU Synthesis and Preparation

1.1 Synthetic Route & Raw Material Selection

  • Monomer System:
    • Diisocyanate: Isophorone diisocyanate (IPDI) was selected to synthesize the prepolymer. Although bio-based diisocyanates with five-membered rings are available, IPDI was chosen for its optimal balance of reactivity and performance.
    • Polyol: Pure bio-based polyol (Bio Polyol) was reacted with IPDI to form the soft-segment prepolymer.
    • Chain Extender: Purified BHET (obtained from prior PET depolymerization) was utilized as the green chain extender.
  • Hardness Tuning (S20 & H40):
    • By adjusting the stoichiometric ratio of hard and soft segments, Green TPU products with tailored mechanical properties and varied hardness can be achieved.
    • S20 and H40 correspond to Shore D hardness values of 20 and 40, respectively.
Synthesis Reaction Scheme of Green TPU
Figure 1: Reaction scheme showing IPDI, Bio Polyol, and BHET.

1.2 Structural Verification via FTIR Spectroscopy

  • Prepolymer Formation (IPDI + Bio Polyol):
    • As shown in the FTIR spectra, the characteristic absorption peak of the free \(-\text{N=C=O}\) group appears around \(2243\,\text{cm}^{-1}\) and \(2252\,\text{cm}^{-1}\), confirming the successful formation of the isocyanate-terminated prepolymer.
    • The appearance of peaks at \(3328\,\text{cm}^{-1}\) (\(\text{N-H}\) stretching of amide A), \(1698\,\text{cm}^{-1}\) (\(\text{C=O}\) stretching), \(1517\,\text{cm}^{-1}\) (\(\text{N-H}\) bending of amide II), and \(1237\,\text{cm}^{-1}\) (\(\text{N-H}\) bending of amide III) indicates urethane linkage formation.

FTIR Spectra of IPDI, Bio Polyol, and Prepolymer

FTIR Spectra of BHET, Prepolymer, and Final TPU

Figure 2 & 3: Comparative FTIR spectra monitoring the prepolymerization and chain-extension stages.

  • Final TPU Product (Prepolymer + BHET):
    • The comparison between the prepolymer and the final TPU product demonstrates the consumption and reaction of the \(-\text{N=C=O}\) peak (drastically reduced or vanished at \(2240\,\text{cm}^{-1}\)).
    • Characteristic ester carbonyl stretching (\(\sim 1712\,\text{cm}^{-1}\)) and ether/ester linkages from the BHET chain extender (\(1248\,\text{cm}^{-1}\), \(1065\,\text{cm}^{-1}\)) are successfully incorporated into the final Green TPU backbone.
TipPart 2: Dynamic Mechanical Properties (DMA)

2. Dynamic Mechanical Analysis (DMA)

2.1 Viscoelastic Behavior and Loss Factor (\(\tan\delta\)) Comparison

  • Commercial PPF-TPU Benchmark:
    • High-performance commercial paint protection film TPU (PPF-TPU) exhibits excellent toughness and mechanical strength, with a typical maximum loss factor (\(\tan\delta\)) around \(0.27\) (the standard baseline for most conventional TPUs).
  • Green TPU S20 Performance:
    • Our synthesized S20 sample shows a significantly elevated peak \(\tan\delta\) of \(0.55\) (at \(-0.23\,^\circ\text{C}\)), indicating enhanced energy dissipation and damping capabilities.
    • At room temperature (\(25.00\,^\circ\text{C}\)), S20 maintains a favorable storage modulus of \(26.07\,\text{MPa}\) compared to PPF-TPU (\(12.76\,\text{MPa}\)).
DMA Comparison: S20 vs. Commercial PPF-TPU
Figure 4: DMA curves comparing storage modulus and \(\tan\delta\) between S20 and commercial PPF-TPU.

2.2 Formulation Tuning: Ultra-High Damping in H40

  • Formula Optimization for H40:
    • By further tuning the hard/soft segment ratio and cross-linking density, the H40 formulation achieves an extraordinary damping performance.
  • Massive Leap in Loss Factor:
    • As illustrated in the second DMA profile, the maximum \(\tan\delta\) of H40 reaches an astonishing \(1.67\) (at \(30.54\,^\circ\text{C}\)), which is more than 6 times that of conventional commercial TPUs.
    • This exceptional loss factor highlights outstanding vibration damping, impact energy absorption, and unique viscoelastic profiles tailored for advanced functional applications.
DMA Comparison: S20 vs. H40 High-Performance TPU
Figure 5: DMA curves highlighting the ultra-high \(\tan\delta\) peak of H40 versus S20.
TipPart 3: Mechanical Properties

3. Mechanical Performance and Stress-Strain Behavior

3.1 Tensile Strength and Modulus Comparison

  • Raw Material Influence & Tuning Potential:
    • Due to the specific nature of the IPDI and bio-polyol system compared to commercial PPF-TPUs, the tensile strength differs from conventional high-performance aliphatic/aromatic automotive film TPUs. However, the initial modulus can remain quite comparable.
    • Adjusting the raw material types or modifying the structural formulation can easily broaden the tensile strength window to meet various industrial standards.
  • Special Functional Requirements:
    • The selection of IPDI is primarily driven by superior optical performance, weatherability, and specific cross-linking demands rather than chasing raw high tensile strength alone.
Stress-Strain Curves of PPF-TPU, S20, and H40
Figure 6: Tensile stress-strain curves comparing commercial PPF-TPU with S20 and H40 Green TPU (with inset showing initial strain behavior).

3.2 Extraordinary Elongation at Break in S20

  • Record-Breaking Stretchability:
    • The most striking feature of the S20 formulation is its extraordinary elongation at break, which exceeds 4000%.
    • This level of ultra-high deformability is virtually unprecedented in standard engineering TPU materials—even high-toughness commercial TPU grades rarely exceed \(1000\%\) elongation.
  • Structural Implication:
    • The combination of soft bio-polyol segments and network architecture allows for extreme chain extension and segment alignment without premature rupture, offering unique potential for applications requiring extreme stretch, strain absorption, or soft-elastic behavior.
TipPart 4: Optical Properties

4. Optical Performance and Transmittance Analysis

4.1 High Transparency and Full-Spectrum Transmittance

  • Optical Clarity:
    • The synthesized Green TPU exhibits exceptional optical transparency across the visible light spectrum (\(400 - 700\,\text{nm}\)).
  • Transmittance Evaluation:
    • As measured by the UV-Vis transmission spectra, the composite structure maintains high luminous transmittance, making it highly suitable for optical-grade applications, display shields, and clear protective layers.
Transmittance Spectra of PET and Green TPU Composite Film
Figure 7: Light transmittance spectra comparing \(25\,\mu\text{m}\) bare PET with the \(25\,\mu\text{m}\) PET + \(80\,\mu\text{m}\) Green TPU composite film.

4.2 Remarkable Anti-Reflective and Enhancing Effect

  • Unexpected Optical Enhancement:
    • Intriguingly, when the Green TPU is laminated or composited with the PET film, the resulting system exhibits a pronounced anti-reflective and optical-enhancing effect.
  • Performance Superiority:
    • The transmittance curve of the composite film (red line) consistently surpasses that of the pristine \(25\,\mu\text{m}\) PET substrate (black line) across almost the entire visible spectrum. This unique synergy highlights the excellent refractive index matching and potential optical anti-glare/enhancement functions of this bio-based material system.
TipPart 5: Impact Resistance

5. Impact Resistance and Energy Dissipation Behavior

5.1 Drop-Weight Impact Testing & 3D Morphological Analysis

  • Experimental Design & Benchmark Selection:
    • Driven by the exceptionally high loss factor (\(\tan\delta\)) observed in DMA, drop-weight steel ball impact tests were conducted to evaluate impact resilience.
    • PET Film (representing hard materials) and Silicone PSA (low-adhesion silicone protective film, representing soft materials) were selected as comparative benchmarks.
  • Key Observations via Keyence VK3000:
    • Using the Keyence VK3000 3D laser scanning microscope, the front/back 3D topographies and depth-length curves of the impact craters were mapped.
    • While the impact depth remains relatively consistent across samples (largely constrained by overall film thickness), the impact span/deformation area exhibits dramatic differences.
PET Film Impact Morphology & Profile

Figure 8: 3D surface topographies (front and back) and depth-length profiles of PET film after steel ball impact.

Silicone PSA Impact Morphology & Profile

Figure 9: 3D surface topographies (front and back) and depth-length profiles of Silicone PSA after steel ball impact.

Green TPU Impact Morphology & Profile
Figure 10: 3D topography and height profiles of the Green TPU film showing an expanded deformation zone.

5.2 Cross-Sectional Evidence and Energy Dissipation Mechanism

  • Wider Deformation Zone:
    • As clearly demonstrated in the cross-sectional optical micrographs (Figure 11), our Green TPU exhibits a significantly larger deformation area compared to both PET and silicone films.
  • Superior Energy Dissipation:
    • A broader deformation region means that a much larger volume of material is actively involved in absorbing and dissipating the kinetic energy of the impact, rather than concentrating stress to cause brittle failure or localized puncture.
  • Structural Origin:
    • We attribute this unique anti-impact mechanism to a well-tailored micro-phase separated structure, which synergizes elastomeric softness with robust network restriction—endowing the Green TPU with impact-resistant capabilities that surpass conventional hard and soft polymer benchmarks.
Cross-Sectional Comparison of Impact Craters
Figure 11: Cross-sectional optical micrographs comparing impact deformation behaviors among PET film, Silicone PSA, and Green TPU.
ImportantPart 6: Comprehensive Summary

6. Comprehensive Summary of Green TPU Architecture

6.1 Core Technological Breakthroughs & Performance Highlights

  • Sustainable & Tailored Synthesis:
    • Successfully synthesized a novel Green TPU using bio-based polyols, IPDI, and PET-depolymerized BHET as a green chain extender, enabling precise hardness tuning (from Shore D 20 in S20 to Shore D 40 in H40).
  • Extraordinary Viscoelastic & Damping Performance:
    • Demonstrated a massive leap in the loss factor (\(\tan\delta\)), reaching \(0.55\) for S20 and an exceptional \(1.67\) for H40 (over 6 times that of conventional commercial TPUs), indicating outstanding energy dissipation and damping capabilities.
  • Record-Breaking Stretchability:
    • The S20 formulation achieves an unprecedented elongation at break exceeding 4000%, far surpassing standard high-toughness engineering elastomers.
  • Optical Excellence & Anti-Reflective Synergy:
    • Exhibited high optical transparency across the full visible spectrum, combined with a unique anti-reflective and optical-enhancing effect when composited with PET substrates.
  • Superior Impact Resilience:
    • Through drop-weight impact tests and 3D profile analysis, the Green TPU demonstrated a significantly expanded deformation zone and superior kinetic energy dissipation compared to traditional hard (PET film) and soft (Silicone PSA) benchmarks, driven by its unique micro-phase separated architecture.

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