Poly(p-phenylene-2,6-benzobisoxazole) fibre (Zylon) PBO (Zylon)

spore: profile
id: pbo-zylon
spore_version: 0.2
family: high-performance-polymers
name: Poly(p-phenylene-2,6-benzobisoxazole) fibre (Zylon)
abbreviation: PBO (Zylon)
role: high-strength fibre
category: polymers
type: high-performance
summary: |
  PBO (Zylon) is a rigid-rod polybenzoxazole fibre made from a liquid-crystalline polymer. NIST reports that virgin PBO yarn breaks at 5.34 GPa and has a modulus of 147.11 GPa. In low Earth orbit on the MISSE 2 PEACE tray, PBO eroded by 1.36e-24 cm3 per oxygen atom, a middle value in the NASA table. The fibre is very strong and stiff but weak in ultraviolet light: NIST cites a loss of more than 90 percent of tensile strength after 450 hours of 340 nm radiation.
best_for: Very high-strength, stiff fibre for ballistic armour and tensile lines, where the fibre is kept out of sunlight
weakest_at: 'Ultraviolet light, which cuts tensile strength sharply; and low compressive strength (NIST: about 10 to 15 percent of tensile strength)'
identity:
  formula: Poly(p-phenylene-2,6-benzobisoxazole), repeat unit C14H6N2O2
  source: nasa-tm
  where: Table IV, row PBO (Zylon), repeat unit C14H6N2O2 and molar mass 258.2356 g/mol
tags:
  environments:
    - low-earth-orbit
  stresses:
    - atomic-oxygen
    - uv
  applications:
    - habitats-and-suits
    - structures
properties:
  - benchmark: atomic_oxygen_erosion_yield
    label: Atomic oxygen erosion yield, MISSE 2 flight
    value: 1.36e-24
    unit: cm3/atom
    scope: material
    conditions:
      exposure: low Earth orbit, ISS MISSE 2 PEACE polymers tray
      basis: erosion yield as printed in the NASA predictive-tool report
    method: Flight-measured erosion yield as printed in the NASA report
    status: flight
    source: nasa-tm
    where: Table I (MISSE 2 PEACE polymers erosion yield data, row PBO (Zylon))
  - benchmark: density
    label: Density, PBO sample from MISSE 2
    value: 1.3976
    unit: g/cm3
    scope: material
    conditions:
      basis: measured density of the MISSE 2 PBO sample as printed
    method: Value from the NASA erosion report density table
    status: ground
    source: nasa-tm
    where: Table II (density and ash content, row PBO (Zylon))
  - benchmark: ash_content
    label: Mass fraction of polymer that is ash, PBO sample
    value: 0.0109
    unit: mass fraction
    scope: material
    conditions:
      basis: mass fraction of the MISSE 2 PBO sample that is ash, as printed in the NASA table
    method: Value printed in the NASA density and ash table
    status: ground
    source: nasa-tm
    where: Table II, column Mass fraction of polymer that is ash, row PBO (Zylon)
  - benchmark: molar_mass
    label: Molar mass of repeat unit
    value: 258.2356
    unit: g/mol
    scope: material
    conditions:
      repeat_unit: C14H6N2O2
    method: Value printed in the NASA table of polymer chemical structure
    status: ground
    source: nasa-tm
    where: Table IV, row PBO (Zylon)
  - benchmark: ultimate_tensile_strength
    label: Ultimate tensile strength, virgin PBO yarn
    value: 5.34
    unit: GPa
    scope: material
    conditions:
      basis: unworn virgin yarn, mean of 6 replicates, as printed
    method: Mean value printed in the NIST tensile table
    status: ground
    source: nist-7237
    where: Table 1, Virgin row, Ultimate Tensile Strength column
  - benchmark: ultimate_tensile_strength
    label: Ultimate tensile strength, yarn from new unworn vest
    value: 4.78
    unit: GPa
    scope: material
    conditions:
      basis: yarn from a new unworn PBO vest of the same model, mean of 6 replicates
    method: Mean value printed in the NIST tensile table
    status: ground
    source: nist-7237
    where: Table 1, New row, Ultimate Tensile Strength column
  - benchmark: ultimate_tensile_strength
    label: Ultimate tensile strength, yarn from officer vest
    value: 3.22
    unit: GPa
    scope: material
    conditions:
      basis: yarn from a PBO vest worn by an officer that was penetrated, mean of 6 replicates
    method: Mean value printed in the NIST tensile table; NIST states it is a 30 percent decrease against new yarn
    status: ground
    source: nist-7237
    where: Table 1, Officer row, Ultimate Tensile Strength column; Introduction text on the 30 percent decrease
  - benchmark: elastic_modulus
    label: Modulus at break, virgin PBO yarn
    value: 147.11
    unit: GPa
    scope: material
    conditions:
      basis: unworn virgin yarn, mean of 6 replicates, as printed
    method: Mean value printed in the NIST tensile table
    status: ground
    source: nist-7237
    where: Table 1, Virgin row, Modulus at Break column
  - benchmark: elastic_modulus
    label: Modulus at break, yarn from new unworn vest
    value: 141.8
    unit: GPa
    scope: material
    conditions:
      basis: yarn from a new unworn PBO vest, mean of 6 replicates
    method: Mean value printed in the NIST tensile table
    status: ground
    source: nist-7237
    where: Table 1, New row, Modulus at Break column
  - benchmark: elastic_modulus
    label: Modulus at break, yarn from officer vest
    value: 136.61
    unit: GPa
    scope: material
    conditions:
      basis: yarn from a PBO vest worn by an officer, mean of 6 replicates
    method: Mean value printed in the NIST tensile table
    status: ground
    source: nist-7237
    where: Table 1, Officer row, Modulus at Break column
  - benchmark: elongation
    label: Strain at break, virgin PBO yarn
    value: 3.52
    unit: '%'
    scope: material
    conditions:
      basis: unworn virgin yarn, mean of 6 replicates, as printed
    method: Mean value printed in the NIST tensile table
    status: ground
    source: nist-7237
    where: Table 1, Virgin row, Strain at Break column
  - benchmark: decomposition_temperature
    label: 5 percent mass-loss temperature in air, virgin yarn
    value: 633.6
    unit: degC
    scope: material
    conditions:
      atmosphere: air
      basis: temperature at 5 percent mass loss, virgin yarn, as printed with standard uncertainty 2.5
    method: Thermogravimetric analysis value printed in the NIST table
    status: ground
    source: nist-7237
    where: Table 2, Air Atmosphere, Virgin row, 5 percent decomposition temperature column
  - benchmark: decomposition_temperature
    label: 5 percent mass-loss temperature in nitrogen, virgin yarn
    value: 699.4
    unit: degC
    scope: material
    conditions:
      atmosphere: nitrogen
      basis: temperature at 5 percent mass loss, virgin yarn, as printed with standard uncertainty 8.2
    method: Thermogravimetric analysis value printed in the NIST table
    status: ground
    source: nist-7237
    where: Table 2, Nitrogen Atmosphere, Virgin row, 5 percent decomposition temperature column
  - benchmark: ultimate_tensile_strength
    label: Tensile strength, secondary summary
    value: 5.8
    unit: GPa
    scope: material
    conditions:
      basis: secondary figure quoted in Wikipedia for the PBO fibre, no test conditions given
    method: Figure printed in Wikipedia; not a test result and not used for design
    status: ground
    source: wiki-zylon
    where: Mechanical properties list, Tensile strength line
  - benchmark: elastic_modulus
    label: Young's modulus, secondary summary
    value: 270
    unit: GPa
    scope: material
    conditions:
      basis: secondary figure quoted in Wikipedia for the PBO fibre, no test conditions given; NIST virgin yarn gave 147.11 GPa
    method: Figure printed in Wikipedia; not a test result and not used for design
    status: ground
    source: wiki-zylon
    where: Mechanical properties list, Young's modulus line
corrosion:
  - environment: Ultraviolet light and sunlight
    headline: 'UV is PBO''s main weakness: NIST cites a loss of more than 90 percent of tensile strength after 450 hours at 340 nm.'
    effect: NIST cites one study in which PBO lost more than 90 percent of its tensile strength after 450 hours of 340 nm radiation (that study is NASA 1995, cited by NIST). Wikipedia says PBO is degraded by UV light. A Chinese study reports that UV ageing of PBO has an early physical stage that creates surface flaws and a later chemical chain-scission stage, and that copolymerising or adding rutile TiO2 improves UV resistance (abstract only read).
    rating: severe
    conditions:
      wavelength: 340 nm
      exposure: 450 hours, unprotected
    source: nist-7237
    where: Introduction, paragraph on hydrolytic and UV-visible light stability (citing NASA 1995)
  - environment: Heat and moisture
    headline: 'Heat and moisture reduce strength: a vest yarn used in service lost 30 percent of its tensile strength.'
    effect: NIST reports that the manufacturer has documented tensile strength degradation after heat and moisture exposure. In the NIST body-armour study, yarn from a penetrated vest had a mean tensile strength of 3.22 GPa against 4.78 GPa for new yarn, a 30 percent decrease.
    rating: moderate
    conditions:
      basis: field-worn vest yarn compared with new yarn, both as printed in the NIST table
    source: nist-7237
    where: Introduction, officer vest paragraph; Table 1
  - environment: Acids, water and hypochlorite
    headline: Hot methanesulfonic acid cleaves PBO bonds; NASA in 1995 reported strength losses after immersion in several acids and water.
    effect: NIST cites a 1995 NASA study that found significant tensile losses after immersion in water, hydrochloric acid, nitric acid, sulfuric acid, sodium chloride and sodium hypochlorite. So et al. observed bond cleavage when PBO was dissolved in hot methanesulfonic acid, but no change in relative molecular mass after sulfuric acid, poly(phosphoric acid) or sodium hydroxide.
    rating: moderate
    conditions:
      basis: qualitative findings as summarised by NIST; no numbers given
    source: nist-7237
    where: Introduction, paragraphs on hydrolytic stability
  - environment: Heat in air (thermal ageing and decomposition)
    headline: PBO does not melt; in air its 5 percent mass-loss temperature is about 634 degC for virgin yarn.
    effect: Thermogravimetric analysis gives 5 percent mass loss at 633.6 degC in air and 699.4 degC in nitrogen for virgin yarn. Mass loss at 110 degC is small, from 0.26 to 0.48 percent. Wikipedia names PBO among the high-strength fibres that resist heat, and the NIST data show no large difference between vest yarns in air.
    rating: low
    conditions:
      atmosphere: air and nitrogen, thermogravimetry
    source: nist-7237
    where: Table 2; text after Table 2
risks:
  hazards:
    - text: 'PubChem GHS for terephthalic acid (a PBO monomer, CID 7489): H302 harmful if swallowed; H315 causes skin irritation (17.4 percent of reports); H319 causes serious eye irritation (17.1 percent); H335 may cause respiratory irritation (15.1 percent); H361 suspected of damaging fertility or the unborn child; H372 causes damage to organs through prolonged or repeated exposure; H373 may cause damage to organs through prolonged or repeated exposure.'
      ghs:
        - H302
        - H315
        - H319
        - H335
        - H361
        - H372
        - H373
      source: pubchem-terephthalic
      where: Safety and Hazards, GHS Classification (CID 7489)
    - text: 'Fibre dust: PBO yarn is a textile, and cutting or abrading it can release fibre fragments. No PBO-specific safety data sheet was read in this run, so no fibre-dust limit is given.'
      ghs: []
      source: nist-7237
      where: Materials description (no hazard statement read)
  failure_risks:
    - mode: UV-driven loss of strength
      trigger: Unprotected sunlight or UV exposure
      consequence: Surface damage and chain scission, with more than 90 percent strength loss after 450 hours at 340 nm
      severity: critical
      source: nist-7237
      where: Introduction, UV stability paragraph
    - mode: Strength loss after heat and moisture
      trigger: Heat and moisture in service
      consequence: About a 30 percent loss of tensile strength in a field-worn vest yarn
      severity: moderate
      source: nist-7237
      where: Introduction, officer vest paragraph; Table 1
    - mode: Low compressive strength
      trigger: Axial compression, which causes slippage of fibrils
      consequence: Compressive strength is about 10 to 15 percent of tensile strength (NIST)
      severity: moderate
      source: nist-7237
      where: Introduction, paragraph on PBO structure
  side_effects: []
  handling:
    - text: Shield PBO from sunlight and UV; NIST documents severe loss of strength under 340 nm radiation.
      source: nist-7237
      where: Introduction, UV stability paragraph
    - text: Keep PBO dry and away from strong acids, which cleave the benzoxazole bonds (So et al., as summarised by NIST).
      source: nist-7237
      where: Introduction, hydrolytic stability paragraph
  mitigations:
    - text: Blend rutile nano-TiO2 into the fibre or copolymerise it with 2,5-dihydroxyterephthalic acid; the Acta Chimica Sinica abstract reports that both improve UV resistance over unmodified PBO.
      source: sioc-uv
      where: Abstract (no numbers read)
uses:
  - application: habitats-and-suits
    device: Ballistic body armour
    example: PBO vests were issued to police from 1998; after recalls and decertification by the National Institute of Justice the use ended.
    source: wiki-zylon
    where: Applications section (ballistic)
  - application: structures
    device: Parachute suspension lines
    example: SpaceX uses Zylon suspension lines on Mk3 parachutes for Crew Dragon.
    source: wiki-zylon
    where: Applications section (space)
  - application: structures
    device: Tether and high-altitude balloon structures
    example: A University of British Columbia team used a tether made from Zylon fibres in a 2006 Tether Challenge, and NASA uses Zylon in high-altitude balloon structures.
    source: wiki-zylon
    where: Applications section (space)
supply:
  - subject: PBO (Zylon) fibre production
    abundance: Not sourced. Feedstock is terephthalic acid and a diamine monomer; terephthalic acid is made by catalytic oxidation of p-xylene (Amoco process).
    producers:
      - name: Toyobo
        country: Japan
        note: Manufacturer of PBO (Zylon), headquartered in Osaka, Japan (Wikipedia)
    critical_lists: []
    recycling: Not sourced.
    substitutes: []
    source: wiki-zylon
    where: Lead and history paragraphs; Toyobo headquarters from wiki-toyobo
media: []
documents:
  - title: 'NASA/TM-2008-215490: Atomic Oxygen Erosion Yield Predictive Tool for Spacecraft Polymers in Low Earth Orbit'
    type: report
    url: https://ntrs.nasa.gov/citations/20090011780
    year: 2008
  - title: 'NISTIR 7237: Chemical and Physical Characterization of Poly(p-phenylene-2,6-benzobisoxazole) Fibers Used in Body Armor'
    type: report
    url: https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir7237.pdf
    year: 2005
gaps:
  - 'Molar mass of the repeat unit: NASA Table IV gives 258.2356 g/mol; Wikipedia gives a monomer molar mass of 234.214 g/mol with formula C14H6N2O2. The two do not match; NASA is used.'
  - No PBO-specific safety data sheet or PubChem record for the polymer was read; hazards use terephthalic acid only.
  - Moisture regain, vacuum outgassing and total radiation dose for PBO were not read, though the passport lists those stresses.
  - Thermal ageing numbers (strength retained after heating at 500 degC for 60 seconds, and 30 weeks in argon) were seen only in search snippets and are not used.
  - The Acta Chimica Sinica 2010 UV paper and the NCSU thesis (Gupta 2005, Zylon and Vectran UV loss) could not be read in full; neither gives numbers used here.
  - The MDPI Polymers 2019 TiO2 UV-ageing paper was blocked (HTTP 403); its loading figures are not used.
  - The DTIC report AD1031722 was blocked (HTTP 403) and the Toyobo product page returned 404; no Toyobo datasheet values were read.
  - No critical-mineral or producer-volume data were read; supply is limited to the Toyobo headquarters and the terephthalic acid route.
  - Secondary Wikipedia figures (5.8 GPa and 270 GPa) conflict with NIST virgin-yarn values and are labelled as secondary.
  - The ash mass fraction and decomposition-temperature names are not in property-canon.yaml; they are used as short snake_case names.
  - Compressive strength (about 10 to 15 percent of tensile) is in the failure risks as a ratio, not as a property value.
sources:
  nasa-tm:
    title: Atomic Oxygen Erosion Yield Predictive Tool for Spacecraft Polymers in Low Earth Orbit
    id: NASA/TM-2008-215490 (Banks, Backus and de Groh, December 2008)
    url: https://ntrs.nasa.gov/citations/20090011780
    year: 2008
    kind: report
    license: public domain (NASA)
  nist-7237:
    title: 'NISTIR 7237: Chemical and Physical Characterization of Poly(p-phenylene-2,6-benzobisoxazole) Fibers Used in Body Armor'
    id: NIST NISTIR 7237 (Chin, Byrd, Forster et al.)
    url: https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir7237.pdf
    year: 2005
    kind: report
    license: NIST public domain
  wiki-zylon:
    title: 'Wikipedia: Zylon'
    id: Zylon
    url: https://en.wikipedia.org/wiki/Zylon
    kind: web
    license: CC BY-SA 4.0 (cited, not reproduced)
  pubchem-terephthalic:
    title: 'PubChem GHS classification: Terephthalic acid (CID 7489)'
    id: PubChem CID 7489, read via the PUG View JSON API
    url: https://pubchem.ncbi.nlm.nih.gov/compound/7489
    kind: database
    license: PubChem, cited not reproduced
  wiki-terephthalic:
    title: 'Wikipedia: Terephthalic acid'
    id: Terephthalic_acid
    url: https://en.wikipedia.org/wiki/Terephthalic_acid
    kind: web
    license: CC BY-SA 4.0 (cited, not reproduced)
  wiki-toyobo:
    title: 'Wikipedia: Toyobo'
    id: Toyobo
    url: https://en.wikipedia.org/wiki/Toyobo
    kind: web
    license: CC BY-SA 4.0 (cited, not reproduced)
  sioc-uv:
    title: Preparation and Characterization of Poly(p-phenylene benzobisoxazole) (PBO) Fiber with Anti-ultraviolet Aging, Acta Chimica Sinica 68(2), 2010
    id: Acta Chimica Sinica 2010, vol 68, issue 2, pp 199-204
    url: https://sioc-journal.cn/Jwk_hxxb/EN/Y2010/V68/I02/199
    year: 2010
    kind: paper
    license: cited, not reproduced
license: CC-BY-4.0
updated: '2026-10-20'
review:
  status: draft
  method: assistant
  checked_values: 15
  unreachable: 4
  caveats: 4
  note: Written by an AI assistant on 2026-10-20 from the NASA TM Tables I, II and IV (PDF), the NIST NISTIR 7237 Tables 1 and 2, the Wikipedia Zylon and terephthalic acid pages, Wikipedia Toyobo, the PubChem terephthalic acid record and the Acta Chimica Sinica abstract. Seven sources, short of the eight requested; the NCSU thesis, DTIC report, MDPI paper and Toyobo datasheet could not be reached.
story:
  recipe:
    line: PBO is made by condensation of a diamine monomer with terephthalic acid, followed by liquid-crystalline spinning and drawing (process details not sourced). The repeat unit is C14H6N2O2 with molar mass 258.2356 g/mol (NASA Table IV). Wikipedia calls PBO a thermoset liquid-crystalline polyoxazole.
    parts:
      - name: Terephthalic acid (monomer)
        symbol: C8H6O4
        share_note: molar mass as printed in Wikipedia
        role: Diacid monomer that forms the terephthalyl part of each repeat unit
      - name: Diamine monomer (2,5-diamino-1,4-benzenediol)
        symbol: C6H8N2O2
        role: Diamine that forms the benzobisoxazole rings with the diacid (named as a PBO monomer in the standard PBO description; no source read for its role in the synthesis)
      - name: Repeat unit
        symbol: C14H6N2O2
        share_note: molar mass 258.2356 g/mol (NASA Table IV)
        role: The polymer unit that repeats along the chain; carbon, hydrogen, nitrogen and oxygen only
  risk_groups:
    - title: For people
      level: 0
      verdict: Hazards from monomer only
      lead: No PBO-specific safety data sheet was read. The PubChem record for terephthalic acid lists skin and eye irritation and possible organ and fertility effects.
      points:
        - label: Monomer
          line: Terephthalic acid, a PBO monomer, is classed H315 and H319 (about 17 percent of reports each) and H372 and H373 for organ damage with repeated exposure (PubChem CID 7489).
        - label: Fibre
          line: PBO yarn is handled as a textile; cutting or abrading may release fibre fragments, and no limit was read.
    - title: In the environment
      level: 1
      verdict: Sunlight is the watch point
      lead: UV is the main route to loss of strength.
      points:
        - label: UV
          line: NIST cites more than 90 percent tensile loss after 450 hours at 340 nm.
        - label: Heat and moisture
          line: A field-worn vest yarn lost about 30 percent of its tensile strength (NIST).
  why: PBO is a rigid-rod polymer whose chains are aligned in the fibre, which gives very high tensile strength and modulus. NIST measured virgin yarn at 5.34 GPa and 147.11 GPa. The trade is UV sensitivity and low compressive strength. In the MISSE 2 flight test it eroded at 1.36e-24 cm3 per oxygen atom, which is why it was included in the passport.
  uses:
    - where: Ballistic body armour
      line: PBO vests were issued to police from 1998; recalls and decertification ended their use (Wikipedia).
    - where: Parachute lines
      line: SpaceX uses Zylon suspension lines on Mk3 parachutes for Crew Dragon (Wikipedia).
    - where: Tethers and balloons
      line: A 2006 Tether Challenge entry used a Zylon tether, and NASA uses Zylon in high-altitude balloon structures (Wikipedia).
  can_do:
    - label: Strength
      benchmark: ultimate_tensile_strength
      line: Virgin PBO yarn breaks at 5.34 GPa (NIST Table 1); yarn from a new vest at 4.78 GPa.
    - label: Stiffness
      benchmark: elastic_modulus
      line: Virgin PBO yarn has a modulus at break of 147.11 GPa (NIST Table 1).
    - label: Flight exposure
      value: 1.36e-24 cm3/atom
      line: In low Earth orbit on MISSE 2, PBO eroded by 1.36e-24 cm3 per atomic oxygen atom (NASA Table I).
  compare: PBO is stronger and stiffer than most polymers in these sources, but it is far weaker in UV and has poor compressive strength. Its atomic oxygen erosion yield of 1.36e-24 cm3 per atom sits in the middle of the NASA table, below polyoxymethylene and polybutylene terephthalate and close to polypropylene. It is a credible structural fibre for space use shielded from sunlight.
  compare_rows:
    - scope: material
      label: Strength
      benchmark: ultimate_tensile_strength
    - scope: material
      label: Stiffness
      benchmark: elastic_modulus
    - scope: material
      label: Atomic oxygen erosion
      benchmark: atomic_oxygen_erosion_yield
  handling:
    - Keep it out of sunlight and UV, or cover it with a UV blocker.
    - Avoid compressive loading; its compressive strength is low.
    - Keep it away from strong acids and hot alkalis.
  good_for:
    - Very high tensile strength and stiffness for weight in armour and tensile lines.
    - Low atomic oxygen erosion in LEO, measured on MISSE 2.
  watch_out:
    - UV cuts strength sharply, about 90 percent after 450 hours at 340 nm (NIST).
    - Low compressive strength (about 10 to 15 percent of tensile).
    - Vest yarn lost about 30 percent of its strength in service (NIST).
  verdict: 'PBO is one of the strongest and stiffest fibres in these sources, and its erosion yield in LEO is moderate. Its weak point is sunlight: shield it from UV, and it performs well in orbit and in armour.'