Doc #107 — Rubber Recycling and Vulcanization

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RECOMMENDED ACTIONS

Phase 1 — First 6 months (Months 0–6)

1. Inventory NZ’s tire recycling and rubber processing capability as part of the national skills and asset census (Doc #8). Identify all shredding, grinding, crumb rubber, and retreading operations. Record equipment type, capacity, condition, and workforce.

2. Classify end-of-life tires as a strategic feedstock — not waste. Tires removed from service due to tread wear, sidewall damage, or age degradation retain most of their rubber content. Establish collection points at regional depots. Do not burn, landfill, or export end-of-life tires.

3. Inventory all non-tire rubber goods nationally — conveyor belts (mining, agricultural, manufacturing), industrial hoses, dock fenders, rubber matting, vehicle suspension bushings, engine mounts, seals and gaskets in warehouses. These represent significant additional rubber tonnage.

4. Identify NZ-based rubber compounding and moulding capability — any facility with mixing (Banbury mixers, two-roll mills), moulding (compression, transfer, injection), and vulcanization (autoclaves, heated platens) equipment. These facilities are the foundation for producing new rubber products from recycled material.

5. Secure rubber chemistry and compounding references — locate and preserve copies of standard texts (Rubber Technology and Manufacture, Blow & Hepburn; The Science and Technology of Rubber, Mark, Erman & Roland; ASM Handbook references). These are essential for compounding and process control.

Phase 2 — First year (Months 6–12)

6. Establish systematic crumb rubber production at existing NZ facilities. Standardise grading: coarse (10–20 mesh), medium (20–40 mesh), fine (40–80 mesh), and ultra-fine (80+ mesh). Finer particles are more valuable for re-compounding.

7. Begin experimental devulcanization trials — start with thermo-mechanical devulcanization using a two-roll mill (available at any rubber processing facility) with controlled temperature and shear. Test multiple rubber types: natural rubber (NR) from truck tires, styrene-butadiene rubber (SBR) from passenger tires, and EPDM from industrial hoses and seals.

8. Develop and test basic moulded products from crumb rubber — solid wheels for handcarts and barrows, dock bumpers, anti-vibration mounts, matting, and gaskets. Use available binders: sulfur re-vulcanization of devulcanized rubber, polyurethane binder (from existing stocks — finite), or natural binders (pine resin, tallow-based compounds).

9. Begin knowledge capture from NZ rubber industry workers — document mixing formulations, process parameters, and practical compounding knowledge. The NZ rubber products industry is small but real; its institutional knowledge is irreplaceable.

10. Assess NZ sulfur supply for vulcanization — NZ has geothermal sulfur deposits in the Taupo Volcanic Zone (Rotorua, Wairakei, White Island/Whakaari). Sulfur is essential for vulcanization; NZ’s geothermal sources provide a potentially indefinite domestic supply. Assess extraction feasibility and logistics.²

Phase 2–3 — Years 1–3

11. Scale up production of solid rubber wheels and tires from recycled material for low-speed applications: handcarts, wheelbarrows, farm trailers, bicycles (solid inserts), and slow electric vehicles. Accept the performance limitations — these are heavy, hard-riding, and speed-limited, but functional.

12. Develop rubber-to-carbon-black pyrolysis capability at a pilot scale. Carbon black recovered from tire pyrolysis can be blended back into new rubber compounds as reinforcing filler, partially closing the material loop.

13. Establish regional rubber recycling hubs — at least one fully equipped facility in each main NZ region (Auckland, Waikato, Bay of Plenty, Wellington, Canterbury) with grinding, mixing, moulding, and vulcanization capability.

14. Develop seal and gasket production from reclaimed rubber — these are small, high-value products where even moderate-quality recycled rubber can provide adequate performance for many applications (water seals, low-pressure pipe gaskets, flanged connections).

15. If trade with Australia develops, prioritise import of vulcanization accelerators (thiurams, sulfenamides), antioxidants, and carbon black — these compounding chemicals are low-volume, high-value, and dramatically improve recycled rubber product quality.

Phase 3–5 — Years 3–15

16. Develop higher-performance reclaimed rubber products as devulcanization process knowledge matures — conveyor belt covers, hose linings, vibration dampers, and (eventually) retread rubber compound from reclaimed material.

17. Integrate imported natural rubber (from Pacific sail trade, Doc #142) with domestic reclaimed rubber in blended compounds. Even small additions of virgin natural rubber significantly improve recycled compound performance.

18. Develop rubber-from-plants capability as guayule or Russian dandelion cultivation matures (Doc #33, Section 5.4). Blend locally grown natural rubber with recycled rubber for the highest-quality domestic compounds achievable.

19. Establish a national rubber compounding laboratory — capable of formulation development, quality testing (tensile strength, elongation, hardness, compression set, abrasion resistance), and process control. Without systematic testing, product quality is uncontrolled and potentially dangerous for safety-critical applications.

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ECONOMIC JUSTIFICATION

The value of what is being recovered

NZ’s tire stock alone contains an estimated 150,000–200,000 tonnes of recoverable rubber polymer, based on approximately 22–25 million tires at an average rubber content of 7–9 kg per passenger tire and 50–70 kg per truck tire.³ Additional rubber in non-tire products (conveyor belts, hoses, industrial goods, footwear) might add another 20,000–50,000 tonnes, though this estimate is highly uncertain.

At pre-war commodity prices, virgin natural rubber traded at approximately NZ$2,500–4,000 per tonne and synthetic rubber at NZ$2,000–3,500 per tonne.⁴ The material value of NZ’s rubber stock is therefore in the range of NZ$400–800 million at pre-war prices — though this comparison is misleading, because under recovery conditions, the material is effectively priceless (there is no alternative source).

Person-years and infrastructure cost

Crumb rubber production requires modest labour and energy. A single shredding and grinding line staffed by 5–8 workers can process 5,000–10,000 tonnes per year.⁵ NZ’s existing facilities, expanded and maintained, could process the national tire stock over 15–30 years — well matched to the rate at which tires come out of service.

Devulcanization and re-compounding is more labour-intensive and skill-dependent. A small rubber products facility (mixing, moulding, vulcanization) requires an estimated 15–25 workers and an investment of 2–5 person-years to bring to operational readiness, using existing buildings and equipment where available. These figures are indicative and depend heavily on the starting state of available equipment and the depth of compounding knowledge in the workforce; actual requirements could be higher if NZ’s rubber manufacturing base has atrophied.

Total estimated labour for a national rubber recycling capability: 80–150 person-years over the first 5 years to establish crumb rubber production, devulcanization, and basic product manufacturing across NZ’s main regions; ongoing operation approximately 100–200 workers nationally. These are bottom-up estimates based on facility staffing norms (see footnote 5) and a target of one equipped hub per main region; they carry high uncertainty until the national skills and asset census (Doc #8) provides actual baseline data.⁶

Comparison with alternatives

Without rubber recycling, NZ loses the material value of every tire and rubber product that reaches end of life. Transport alternatives (steel wheels, wooden wheels, rail) are available (Doc #33) but represent significant performance regressions. Seals and gaskets become unavailable, threatening the integrity of water systems, hydraulic equipment, and industrial plant.

With rubber recycling, the same material serves two or more lives — first as a tire or industrial product, then as a solid wheel, gasket, mat, conveyor belt cover, or vibration mount. The quality is lower each cycle, but the service is extended by years to decades.

Breakeven: The investment in rubber recycling pays for itself immediately in the sense that there is no alternative source of rubber. The real question is not whether to recycle rubber but how aggressively to invest in quality improvement (devulcanization, compounding chemistry) versus accepting lower-quality products made from mechanically ground crumb rubber with basic binders.

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1. NZ’S RUBBER STOCK AND SOURCES

1.1 Tires — the dominant source

Doc #33 estimates NZ’s total tire stock at approximately 22–25 million tires. Not all of this stock will be available for recycling at the same time — most tires are in service or in managed storage. Tires enter the recycling stream as they are retired from service due to:

• Tread wear below minimum depth
• Sidewall damage or structural failure
• Age-related degradation (ozone cracking, UV degradation, thermal deterioration)
• Failure during retreading inspection (casings not suitable for retreading)

Tire composition varies by type, but a typical passenger tire contains approximately:⁷

Component	% by weight	Recovery potential
Rubber compound (NR, SBR, BR blends)	45–47%	Crumb rubber, devulcanized rubber
Carbon black and silica fillers	20–22%	Retained in crumb rubber; some recovery via pyrolysis
Steel (belts and bead wire)	12–16%	Magnetic separation; recycled as steel (Doc #90)
Textile (nylon, polyester cord)	3–5%	Difficult to separate; often retained in crumb or combusted
Zinc oxide, sulfur, accelerators, oils	5–8%	Partially retained in recycled compound
Other (antioxidants, wax, etc.)	3–5%	Partially retained

Truck tires have a higher proportion of natural rubber (NR) compared to passenger tires (which are predominantly SBR — styrene-butadiene rubber). Natural rubber is generally easier to devulcanize and produces higher-quality reclaimed rubber than SBR.⁸ This means truck tires, despite being fewer in number, are a premium recycling feedstock.

1.2 Non-tire rubber sources

These are a secondary but significant source:

Conveyor belts: NZ’s mining, quarrying, agricultural, and manufacturing sectors use substantial quantities of conveyor belting. A large conveyor belt contains 20–50 kg of rubber per linear metre. NZ probably has hundreds of kilometres of conveyor belting in service and in storage.⁹ Conveyor belt rubber is typically high-quality natural rubber or NR/SBR blends, making it a good devulcanization feedstock.

Industrial hoses: Hydraulic hoses, water hoses, fire hoses, suction and delivery hoses. The rubber is reinforced with textile or wire braid. Separating the rubber from reinforcement is labour-intensive but feasible.

Dock fenders and marine rubber: NZ’s ports use large rubber fenders. These represent tonnes of rubber per port facility, mostly natural rubber or NR/SBR blends.

Vehicle components: Engine mounts, suspension bushings, weatherstrips, floor mats, pedal covers. Individually small, collectively significant across 4.4 million vehicles.

Footwear: Rubber soles from boots and shoes. Modest volumes but widely distributed.

Electrical insulation: Some cable insulation is rubber-based (particularly older installations). Recovery requires stripping from the conductor.

1.3 Total estimated recoverable rubber

Source	Estimated rubber content (tonnes)	Quality for recycling
Passenger car tires (~20 million)	100,000–140,000	Moderate (mostly SBR)
Truck and bus tires (~2 million)	40,000–70,000	Good (high NR content)
Commercial tire stock (~1 million)	5,000–10,000	Variable
Conveyor belts	10,000–30,000	Good
Industrial hoses and components	5,000–15,000	Variable
Marine and port rubber	2,000–5,000	Good
Vehicle components	5,000–15,000	Variable
Other (footwear, matting, etc.)	3,000–8,000	Low to moderate
Total	170,000–290,000	—

Important caveat: These are rough estimates. The actual recoverable quantity depends on collection effectiveness, contamination, and the fraction that is too degraded to recycle. Realistic recovery rates are probably 50–70% of the theoretical total, yielding 85,000–200,000 tonnes of usable recycled rubber material over the full depletion period.

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2. MECHANICAL RECYCLING: CRUMB RUBBER

2.1 The process

Mechanical recycling — shredding and grinding tires into crumb rubber — is the most established rubber recycling technology and the one with the lowest chemical input requirements. NZ had operating crumb rubber facilities as of the pre-event period, though current operational status requires verification (see Doc #8).¹⁰

The process sequence:

1. Debeading: Removing the steel bead wires from the tire rim area. Requires a bead removal machine (hydraulic, powered by grid electricity).

2. Primary shredding: Tire is cut into large pieces (50–200 mm) by a dual-shaft shredder. Requires a heavy industrial shredder with hardened steel blades — these machines are robust but blade wear is significant, and replacement blades require hardened steel (Doc #89, Doc #91).

3. Granulation: Large pieces are reduced to granules (5–20 mm) by a granulator with screens. Further blade wear.

4. Magnetic separation: Steel belt fragments are removed by magnetic separators. Steel is recycled via the scrap metal stream (Doc #90).

5. Textile separation: Nylon and polyester cord is removed by air classification (blowing — lighter textile separates from heavier rubber) or screening. Separation is imperfect — some textile contamination remains in the crumb rubber.

6. Fine grinding: Granules are ground to the desired mesh size. Ambient grinding (at room temperature) produces irregularly shaped particles with a rough surface — good for bonding. Cryogenic grinding (using liquid nitrogen to embrittle the rubber before grinding) produces smoother, more uniform particles but requires liquid nitrogen, which NZ can produce from its air separation capabilities but at significant energy cost.

7. Screening and classification: Ground rubber is screened to produce standardised size fractions.

Energy requirements: Mechanical grinding is energy-intensive. Processing one tonne of tires to fine crumb rubber (40 mesh) requires approximately 200–400 kWh of electricity, depending on equipment efficiency and target particle size.¹¹ At NZ grid electricity, this is feasible and affordable. Annual processing of 10,000 tonnes of tires would require approximately 2–4 GWh — a negligible fraction of NZ’s grid capacity.

Blade and equipment wear: This is the key consumable constraint. Shredder and granulator blades are made from high-carbon or alloy tool steel, hardened to resist abrasion. Steel wire in tires accelerates blade wear. Blade replacement or resharpening is required at regular intervals — perhaps every 500–2,000 tonnes of throughput, depending on blade material and tire composition.¹² NZ can produce replacement blades from Glenbrook steel (Doc #33) with appropriate heat treatment (Doc #33), but the quality of domestically produced tool steel for this application is uncertain and requires development.

2.2 Crumb rubber grades and applications

Grade	Mesh size	Particle size	Primary applications
Coarse chips	< 10 mesh	> 2 mm	Fill material, drainage, mulch, energy recovery
Coarse crumb	10–20 mesh	0.85–2 mm	Playground surfacing, sports surfaces, asphalt modifier
Medium crumb	20–40 mesh	0.4–0.85 mm	Moulded products, bound rubber, asphalt
Fine crumb	40–80 mesh	0.18–0.4 mm	Re-compounding, devulcanization feedstock
Ultra-fine powder	80+ mesh	< 0.18 mm	Highest-value re-compounding, direct additive to new compounds

For recovery purposes, the most valuable grades are fine and ultra-fine — these can be incorporated into new rubber compounds with minimal reduction in mechanical properties if the particle size is small enough. Coarser grades are useful for lower-performance applications where rubber particles are bound together rather than homogeneously blended.

2.3 What crumb rubber alone can make

Without devulcanization, crumb rubber particles are still vulcanized — their cross-linked structure is intact. They cannot flow and fuse like raw rubber. Products made from crumb rubber therefore rely on one of three approaches:

Physical binding: Crumb rubber is mixed with a liquid binder that cures around the particles, holding them together. Common binders include:

• Polyurethane: The standard industrial binder for crumb rubber products. NZ has polyurethane stocks but these are imported and finite. The MDI (methylene diphenyl diisocyanate) and polyol feedstocks for polyurethane are petroleum-derived chemicals that NZ cannot produce.
• Sulfur re-vulcanization: Fine crumb rubber, when mixed with additional sulfur, accelerators, and heat, can be partially re-bonded at particle surfaces. The result is significantly weaker than a homogeneous vulcanizate but adequate for some applications. Sulfur is domestically available from NZ geothermal sources.
• Natural binders: Pine resin (from NZ’s radiata pine forests — Doc #99), tallow, and combinations thereof. Performance data for these binders with crumb rubber is limited; experimental development is required. Feasibility is uncertain but worth pursuing as a NZ-indigenous solution. Kauri gum (kapia) — fossilised and semi-fossilised kauri resin, historically collected as a major NZ export for varnish and linoleum¹³ — has adhesive and binding properties that may make it a viable tackifier or binder for crumb rubber. NZ still has kauri gum deposits, particularly in Northland, and the material’s bonding characteristics warrant experimental trials alongside pine resin and tallow.

Compression moulding at high temperature and pressure: Fine crumb rubber (40+ mesh) can be compression-moulded at 150–180 degrees C and 10–20 MPa pressure without additional binder. The surface of the vulcanized particles partially softens and bonds under these conditions. The resulting product has roughly 30–50% of virgin rubber tensile strength but adequate hardness and compression resistance for static applications (mats, bumpers, solid wheels at very low speeds).¹⁴

Asphalt modification: Crumb rubber added to hot asphalt (rubberised asphalt) improves road surface flexibility, noise reduction, and crack resistance. This is established technology used in NZ road construction and an effective way to consume crumb rubber while maintaining road surfaces.¹⁵

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3. DEVULCANIZATION: THE KEY TECHNOLOGY

3.1 What devulcanization means

Vulcanization is the chemical process that converts raw rubber from a soft, tacky, thermoplastic material into a tough, elastic, thermoset material. Charles Goodyear’s 1839 discovery that heating rubber with sulfur produces this transformation is the foundation of the modern rubber industry. The chemistry: sulfur atoms form cross-links (bridges) between adjacent polymer chains, creating a three-dimensional network that prevents the chains from sliding past each other.¹⁶

Devulcanization is the partial reversal of this process — selectively breaking sulfur-sulfur (S-S) and carbon-sulfur (C-S) cross-link bonds while minimising the breaking of carbon-carbon (C-C) bonds in the polymer backbone. If the backbone chains remain largely intact while the cross-links are broken, the resulting material regains some of the plasticity and processability of raw rubber and can be mixed with other ingredients, shaped, and re-vulcanized into new products.

The fundamental challenge: The bond energies of S-S cross-links (270 kJ/mol), C-S bonds (310 kJ/mol), and C-C backbone bonds (370 kJ/mol) are not sufficiently different to allow perfectly selective cleavage.¹⁷ Any process energetic enough to break cross-links will also break some backbone chains, reducing the molecular weight of the polymer and therefore the mechanical properties of the resulting material. This is why devulcanized rubber is always inferior to virgin rubber — the polymer has been partially degraded. The goal is to minimise backbone damage while maximising cross-link breakage, but perfection is not achievable.

3.2 Devulcanization methods

Several methods have been developed and practiced industrially. Their applicability to NZ conditions varies:

Thermo-mechanical (pan process): The oldest devulcanization method, practiced since the 1880s.¹⁸ Ground rubber is heated under pressure (typically 150–200 degrees C, 1–2 MPa) in the presence of reclaiming agents (typically mineral oils and sometimes chemical peptisers like diallyl disulfide or thiophenols) for several hours in an autoclave or “digester.” The combination of heat, pressure, and chemical agents breaks cross-links and softens the rubber. The product is then milled on a two-roll mill to produce a smooth, workable sheet of reclaimed rubber.

NZ feasibility: Good. Autoclaves (pressure vessels) are available in NZ from food processing, medical sterilisation, and composite manufacturing sectors. Two-roll mills exist at NZ rubber processing facilities. The reclaiming agents are the constraint — mineral oils are petroleum products (finite stocks), and chemical peptisers are imported specialty chemicals. However, the process can function with heat and mechanical shear alone, without chemical agents, producing devulcanized rubber with reduced plasticity and higher residual cross-link density compared to chemically assisted reclamation — a further performance reduction on top of the 40–70% property loss already inherent to devulcanization. Tallow or pine-derived oils could partially substitute for mineral oils as softening agents; their lower aromatic content makes them less compatible with SBR (which predominates in passenger tires) than with NR (which predominates in truck tires), so the substitution gap is more severe for the majority of the feedstock. No published performance data for tallow-assisted rubber reclamation has been located; experimental development in Phase 2 is required before this can be confirmed as viable.¹⁹

Mechanical shearing (high-shear milling): Ground rubber is passed through a two-roll mill with a tight nip (small gap between the rolls) at controlled temperature. The intense shear stress breaks cross-links, particularly at particle surfaces, producing a partially devulcanized material. This can be repeated through multiple passes. No chemical agents are required.

NZ feasibility: Good. Requires only a two-roll rubber mill (existing equipment at NZ rubber processing facilities) and skill. Energy input is from grid electricity driving the mill rolls. This is the most accessible devulcanization method for NZ and should be the first developed. The limitation is that devulcanization is concentrated at particle surfaces, leaving the core of larger particles still cross-linked. Using fine crumb rubber (40+ mesh) as feedstock improves the result.

Microwave devulcanization: Microwave energy selectively heats the sulfur cross-links (which absorb microwaves more readily than the hydrocarbon backbone), enabling more selective cross-link cleavage. This is a more recent development and has shown promising laboratory results.²⁰

NZ feasibility: Uncertain. Industrial-scale microwave processing equipment is specialised. NZ may have some microwave process equipment in food processing or materials research (e.g., at University of Auckland or University of Canterbury), but scaling to rubber recycling would require engineering development. Lower priority than thermo-mechanical methods.

Ultrasonic devulcanization: High-intensity ultrasound (typically 20 kHz) applied to rubber under pressure causes localised heating and cavitation that breaks cross-links. Developed principally by Isayev and colleagues.²¹

NZ feasibility: Uncertain. Requires specialised ultrasonic processing equipment. Not a near-term option for NZ but potentially valuable if equipment can be fabricated or imported via trade.

Chemical devulcanization: Specific chemicals (disulfides, thiols, amines) can selectively attack sulfur cross-links. Examples include diphenyl disulfide, 2-mercaptobenzothiazole, and various organic solvents. These processes can produce high-quality reclaimed rubber but require chemicals that NZ does not produce.²²

NZ feasibility: Poor in the near term. The required chemicals are specialised organic sulfur compounds that NZ has no domestic production pathway for. If trade develops, these chemicals would be high-value, low-volume imports worth prioritising.

Biological devulcanization: Certain sulfur-oxidising bacteria (e.g., Thiobacillus species and Acidithiobacillus ferrooxidans) can metabolise the sulfur in rubber cross-links, gradually breaking them down. This is a slow process (days to weeks) but requires no imported chemicals — only the bacteria (which are naturally occurring in NZ soils and geothermal areas) and controlled incubation conditions.²³

NZ feasibility: Moderate. NZ’s geothermal areas host sulfur-oxidising bacteria naturally (indeed, these organisms are common in the Taupo Volcanic Zone hot springs and sulfur deposits). The microbiology is within the capability of NZ’s university and research institutes. However, biological devulcanization is slow and produces variable results. It is a research priority rather than a near-term production method. Could become a viable NZ-indigenous devulcanization pathway in Phase 3–5.

3.3 Recommended NZ devulcanization pathway

Given NZ’s materials and capabilities, the recommended development sequence is:

1. Immediate (Phase 2): Mechanical shearing on existing two-roll mills. No chemical inputs required. Produces surface-devulcanized crumb rubber suitable for blending into new compounds at modest loading levels (up to 30–40% replacement of virgin rubber, if virgin rubber is available from trade).

2. Near-term (Phase 2–3): Thermo-mechanical devulcanization using autoclaves with NZ-available softening agents (tallow, pine oil). Produces bulk devulcanized rubber suitable for compression moulding and lower-performance applications.

3. Medium-term (Phase 3–4): Biological devulcanization development using NZ’s indigenous sulfur-oxidising bacteria. Potentially a low-energy, chemical-free pathway, but requires research and scaling.

4. Long-term (Phase 4–5): If chemical accelerators and devulcanization agents become available through trade, integrate them into the process to improve reclaimed rubber quality.

3.4 Honest assessment of devulcanized rubber quality

Reclaimed (devulcanized) rubber is not equivalent to virgin rubber. Depending on the devulcanization method, feedstock quality, and re-compounding skill, reclaimed rubber typically achieves:²⁴

Property	Reclaimed rubber (% of virgin)	Implications
Tensile strength	40–70%	Cannot replace virgin rubber in high-stress applications
Elongation at break	50–80%	Reduced flexibility and fatigue life
Tear resistance	30–60%	Prone to crack propagation; not suitable for dynamic sealing
Abrasion resistance	40–65%	Higher wear rate; shorter service life for tires and belting
Compression set	60–80% of virgin (i.e., worse)	Poorer sealing performance over time
Hardness	Can be formulated to match virgin	Controllable through compounding

Blending: The standard industry practice is to blend reclaimed rubber with virgin rubber to balance cost and performance. Typical blend ratios of 20–40% reclaimed rubber in a compound produce acceptable products for many applications, with modest property reductions.²⁵ In NZ’s situation, the proportion of reclaimed rubber will be much higher (potentially 80–100% for many products), and the performance degradation must be accepted and planned for.

What reclaimed rubber CAN do adequately:

• Solid tires for low-speed vehicles (< 30 km/h)
• Vibration dampers and mounts (static or low-frequency)
• Floor mats, matting, and anti-fatigue surfaces
• Dock and wharf fenders
• Water pipe gaskets and low-pressure seals
• Conveyor belt cover rubber (lower-grade applications)
• Footwear soles (durable but less flexible)
• Hose coverings (non-pressure applications)
• Road surfacing and asphalt modification

What reclaimed rubber CANNOT do adequately without significant virgin rubber blending:

• Pneumatic tire treads (inadequate abrasion resistance and flex fatigue life)
• High-pressure hydraulic seals (inadequate compression set and tear resistance)
• Conveyor belt carcass rubber (inadequate tensile and tear strength)
• Dynamic rotary shaft seals (inadequate compression set)
• Spring and suspension components (inadequate fatigue life)
• Electrical insulation above low voltages (inadequate dielectric properties)

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4. VULCANIZATION: RE-CURING RECLAIMED RUBBER

4.1 What vulcanization requires

Once rubber has been devulcanized (or when ground crumb rubber is being re-bonded), it must be re-vulcanized to create useful products. Vulcanization requires:

Sulfur: The primary cross-linking agent. NZ has a domestic supply from geothermal sources in the Taupo Volcanic Zone. Elemental sulfur deposits at Rotokawa, Wairakei, and historically from White Island (Whakaari) total thousands of tonnes.²⁶ Sulfur can also be recovered from geothermal gas (hydrogen sulfide, H2S) at NZ’s geothermal power stations. This is one of NZ’s genuine material advantages for rubber processing.

Accelerators: Chemicals that speed up the vulcanization reaction and improve the efficiency of sulfur use. Without accelerators, vulcanization requires higher temperatures, longer times, and more sulfur — producing an inferior cure (bloomy, overcured surface, undercured interior). Common accelerators include:²⁷

• Thiurams (e.g., TMTD — tetramethylthiuram disulfide)
• Sulfenamides (e.g., CBS — N-cyclohexyl-2-benzothiazole sulfenamide)
• Thiazoles (e.g., MBT — 2-mercaptobenzothiazole)
• Dithiocarbamates (e.g., ZDEC — zinc diethyldithiocarbamate)

NZ does not produce any of these. They are products of organic sulfur chemistry that requires a chemical industry NZ does not have. Existing NZ stocks of accelerators (held by rubber product manufacturers) are finite. No public data exists on total NZ accelerator inventories; the national asset census (Doc #8) should enumerate these stocks specifically. Until that census is complete, no reliable duration estimate is possible — industry usage rates and warehouse stocks are both unknown. Prioritising this inventory in Phase 1 is essential before any estimate of operational runway can be made.²⁸

Without accelerators, vulcanization is still possible but slower (hours instead of minutes), less efficient (more sulfur consumed), and produces inferior properties. The historical rubber industry operated without accelerators before their discovery in the early twentieth century — the products were serviceable but inferior to modern vulcanizates. NZ would revert to this baseline if accelerator stocks are depleted before trade provides replacements.

Activators: Zinc oxide and stearic acid activate the accelerator system. NZ has zinc oxide from imported stocks (finite). Stearic acid can be produced domestically from tallow (a fraction of the fatty acids in rendered animal fat is stearic acid). Zinc oxide could potentially be produced from NZ zinc stocks (from galvanised coatings and zinc die-castings) by oxidation, but quantities would be limited.²⁹

Fillers: Carbon black is the primary reinforcing filler in rubber. NZ has no carbon black production, but carbon black can be recovered from tire pyrolysis (Section 5). The recovered carbon black is lower quality than virgin furnace-process carbon black but adequate for many applications. Ground silica (from NZ quartz sand) and whiting (ground chalk, from NZ limestone) can serve as non-reinforcing fillers, reducing cost and providing dimensional stability at the expense of tensile strength.

Process oils and plasticisers: Naphthenic and aromatic oils are used in rubber compounding to improve processability and flexibility. These are petroleum products — finite. NZ alternatives include tallow, lanolin, pine oil, and canola oil, all of which are domestically available. Their performance as rubber process oils differs from petroleum-based products (generally less compatible with SBR, more compatible with NR), and formulations will need adjustment.³⁰

Heat and pressure: Vulcanization is typically conducted at 140–180 degrees C for 5–60 minutes under pressure (to prevent porosity from gases evolved during curing). Compression moulding presses with heated platens, autoclaves, and heated platen presses are all suitable. NZ has these in rubber processing facilities, composite manufacturing, and other industries. All are powered by grid electricity.

4.2 NZ vulcanization chemical supply chain

Chemical	Role	NZ source	Depletion risk
Sulfur	Cross-linker	Geothermal (indefinite)	Low
Zinc oxide	Activator	Imported stock; limited recycling	Medium
Stearic acid	Activator	From tallow (indefinite)	Low
Accelerators (TMTD, CBS, MBT)	Speed cure, improve properties	Imported stock only	High — duration unknown; requires Phase 1 inventory
Carbon black	Reinforcing filler	Pyrolysis recovery; imported stock	Medium
Process oils	Plasticiser	Petroleum stocks; tallow/lanolin/canola	Medium
Antioxidants	Prevent aging	Imported stock only	High
Antiozonants	Prevent ozone cracking	Imported stock only	High

The critical gap is accelerators and antidegradants. Without accelerators, vulcanization is slow and produces inferior products. Without antioxidants and antiozonants, rubber products degrade faster in service. Ozone cracking is a surface degradation mechanism driven by tropospheric (ground-level) ozone reacting with strained rubber. Tropospheric ozone concentrations in NZ’s post-event environment are uncertain; under scenarios with reduced industrial activity, tropospheric ozone may decrease in some areas while increasing in others depending on NOx and VOC source profiles. The practical implication is that outdoor rubber storage and service life under post-event conditions should be monitored, and antiozonant stocks inventoried and rationed to the highest-priority applications.³¹

4.3 Vulcanization without modern chemicals

If accelerator stocks are depleted and trade does not provide replacements, NZ must revert to sulfur-only vulcanization — the method used before the discovery of accelerators in the early 1900s.

Sulfur-only cure characteristics:³²

• Cure temperature: 140–150 degrees C (similar to accelerated systems)
• Cure time: 2–6 hours (versus 5–30 minutes for accelerated systems)
• Sulfur loading: 8–12 phr (parts per hundred rubber, versus 1–3 phr for efficient accelerated systems)
• Properties: Higher proportion of polysulfidic cross-links; lower tensile strength; poorer aging resistance; more sulfur bloom on surface
• Energy cost: Significantly higher per unit of product due to long cure times

This is a workable process — it produced functional rubber products for six decades (1839–1906) — but the products are inferior to modern vulcanizates. The combination of reclaimed (devulcanized) rubber feedstock with sulfur-only vulcanization yields products at perhaps 25–50% of virgin rubber performance. This is the realistic worst-case baseline for NZ rubber production.

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5. PYROLYSIS: RECOVERING CARBON BLACK AND OIL

5.1 The pyrolysis process

Tire pyrolysis is thermal decomposition in the absence of oxygen at 400–700 degrees C. The rubber polymer chains break down into:³³

• Pyrolysis oil (40–50% of tire weight): A complex mixture of hydrocarbons, usable as fuel (similar to heavy fuel oil) or potentially as a chemical feedstock. Quality is variable and typically requires processing before use as anything other than boiler fuel.
• Carbon black and char (30–40% of tire weight): A mixture of the original carbon black filler (partially recovered) and carbonaceous residue from polymer decomposition. This “recovered carbon black” (rCB) has different surface properties from virgin carbon black but can serve as a reinforcing filler in rubber compounds, albeit with some performance reduction.³⁴
• Steel (10–15% of tire weight): Cleanly separated from the organic material. Recycled as steel (Doc #33).
• Non-condensable gases (5–15% of tire weight): Methane, ethane, hydrogen, and other light hydrocarbons. Can be burned to provide process heat (making the pyrolysis process partially or fully energy-self-sufficient) or potentially collected as chemical feedstocks.

5.2 NZ pyrolysis feasibility

Tire pyrolysis requires:

• A reactor vessel: A retort (sealed vessel capable of withstanding operating temperature without oxygen ingress). The vessel body can be fabricated from Glenbrook plate steel (Doc #89); at operating temperatures of 400–600 degrees C, the outer shell requires external mineral wool or ceramic fibre insulation to reduce heat loss, and high-temperature cement-based castable refractory may be needed around combustion burners if combustion heating is used. Ceramic fibre blanket is available in NZ through industrial insulation suppliers; domestic stockpiles should be inventoried.
• Heat source: Electric heating (grid-powered) or combustion of the pyrolysis gases themselves. External electric heating simplifies the design.
• Condensation system: To capture the pyrolysis oil. Standard heat exchanger / condenser technology, fabricable from NZ steel and copper.
• Emission control: Pyrolysis off-gases include harmful compounds. Flaring or combustion of uncondensed gases is the minimum control measure.

Scale: A small batch pyrolysis retort processing 1–5 tonnes of tires per day is the appropriate starting scale for NZ. The key components — a sealed steel pressure vessel, a heating system, and a condensation train — can be fabricated from Glenbrook plate steel (Doc #89) and copper tubing by NZ boilermakers and engineers, drawing on the same skills used in pressure vessel work for geothermal, food processing, and chemical industries. This assessment assumes available boilermaking capacity and competent engineering design; both should be confirmed via the Doc #8 census before committing to this pathway.³⁵ Larger continuous-feed rotary kiln designs are more efficient but require more specialised fabrication and control engineering.

Energy balance: At 500–600 degrees C operating temperature, a well-designed tire pyrolysis system can be energy-self-sufficient by burning its own non-condensable gas output to heat the reactor.³⁶ If external energy is needed during startup or for scale-up, grid electricity can supply it. A 5 tonne/day unit operating at 500 degrees C requires approximately 200–400 kWh/day for heating (if not self-sufficient), which is a negligible load relative to NZ’s grid capacity — comparable to a small industrial workshop.

Assessment: Pyrolysis is feasible for NZ from Phase 2–3 onward. Its primary value for rubber recycling is the recovery of carbon black, which can be blended back into new rubber compounds as a reinforcing filler. The pyrolysis oil is a bonus — usable as fuel or potentially as a rubber processing oil substitute.

5.3 Recovered carbon black quality

Recovered carbon black (rCB) from tire pyrolysis is not identical to virgin carbon black. Key differences:³⁷

• Higher ash content (10–20% versus < 1% for virgin carbon black) due to contamination with mineral fillers (zinc oxide, silica) from the original tire compound
• Different surface chemistry (partially graphitised, lower surface area)
• Lower reinforcing efficiency in rubber compounds (typically 60–80% of virgin N330-grade carbon black)

Upgrading rCB: Washing with dilute acid removes some mineral contaminants. Fine grinding improves dispersion in rubber compounds. These steps improve rCB quality but do not achieve parity with virgin material.

Practical use: rCB is adequate as a partial replacement for virgin carbon black in rubber compounds. A compound using 50–100% rCB (with no virgin carbon black) will have lower tensile strength and abrasion resistance but adequate hardness and compression properties for many of the applications listed in Section 3.4 as suitable for reclaimed rubber.

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6. PRODUCTS NZ CAN MAKE FROM RECYCLED RUBBER

6.1 Solid wheels and tires

The highest-priority recycled rubber product for NZ’s recovery is solid rubber tires for low-speed vehicles. Doc #33 identifies this as a critical need.

Design: Solid rubber tires moulded from devulcanized or crumb rubber compound, bonded to steel rims (fabricated from Glenbrook steel). No air chamber, no inner tube, no risk of puncture or blowout. Heavier than pneumatic tires and providing a harder ride, but maintenance-free and indefinitely repairable by remoulding.

Performance limitations: Maximum practical speed approximately 25–40 km/h depending on compound quality, road surface, and load. Higher speeds cause excessive heat buildup in solid rubber, leading to internal degradation. Ride comfort is significantly worse than pneumatic tires — passengers and fragile cargo need suspension (springs, dampers) to compensate.

Applications: - Farm trailers and implements - Handcarts and wheelbarrows - Urban delivery vehicles (electric, low-speed) - Material handling equipment (forklifts already commonly use solid rubber tires) - Bicycles (solid rubber inserts replacing pneumatic inner tubes — heavier and harder-riding but puncture-proof)

Manufacturing: Compression moulding in heated steel moulds. Moulds fabricated by NZ machine shops (Doc #91). Typical cure: 160–180 degrees C, 30–60 minutes at 10–15 MPa for accelerated systems; several hours for sulfur-only systems.

6.2 Seals, gaskets, and O-rings

Critical for infrastructure maintenance. Rubber seals and gaskets are used throughout NZ’s water infrastructure, hydraulic systems, industrial equipment, and plumbing. As existing seals age and fail, replacements are essential.

Feasibility: Compression-moulded seals and gaskets have relatively modest tooling requirements compared to complex rubber goods. The moulds for flat gaskets and pipe flanges are machined steel plates with bolt-circle patterns and cut-out profiles; any NZ machine shop with a milling machine and lathe can produce them (Doc #91). O-rings require more precise mould geometry (groove radii within 0.05 mm tolerance) and consistent compound hardness; achieving this with recycled rubber compounds requires more careful process control but is within the capability of a competent moulding operation that can verify Shore A hardness with a durometer.

Limitations: Dynamic seals (rotary shaft seals, hydraulic piston seals) require consistent quality, precise dimensions, and good compression set properties. Reclaimed rubber may be inadequate for high-performance dynamic sealing. For these applications, the best available NZ-recycled rubber should be reserved, and acceptance testing is essential.

6.3 Conveyor belt repair and replacement

NZ’s mining, agricultural, and industrial sectors depend on conveyor belts. As existing belts wear out, replacement cover rubber from recycled material can extend their service life.

Cover rubber replacement: The wear surface of a conveyor belt (the “cover”) can be replaced without replacing the entire belt if the carcass (internal reinforcing layers) is intact. Cover rubber from recycled NR compounds is adequate for many applications, particularly where abrasion rates are moderate (grain handling, general freight).

Full belt manufacturing: Producing complete conveyor belts (rubber-impregnated textile or steel-cord carcass with rubber covers) is a more complex operation requiring calendering equipment (to coat fabric with rubber) and a vulcanising press long enough for belt sections. NZ may have some of this equipment in industrial belt service operations.

6.4 Hose manufacture

Rubber hoses (water, air, low-pressure hydraulic) can be produced from recycled rubber compounds using mandrel-built construction:

1. Inner tube rubber is extruded or wrapped onto a steel mandrel
2. Textile reinforcement (NZ-produced harakeke muka cord, Doc #100, or synthetic textile from existing stocks) is wound or braided over the tube under controlled tension — a braiding machine or purpose-built winding fixture is required; the tension must be consistent to achieve uniform burst pressure
3. Outer cover rubber is applied
4. The assembly is vulcanised in an autoclave or wrapping process
5. The mandrel is removed

This is labour-intensive but established technology. The resulting hoses are adequate for water delivery, low-pressure air, and general-purpose applications. High-pressure hydraulic hose manufacture requires wire-braided reinforcement and high-quality rubber compounds — more challenging but potentially feasible using steel wire from tire beads or NZ-drawn wire (Doc #33).

6.5 Vibration dampers and engine mounts

Recycled rubber is well-suited for vibration-damping applications where the primary requirement is energy absorption rather than high tensile strength. Engine mounts, equipment isolation pads, and machinery bases can be moulded from recycled compounds. These products extend the service life of vehicles and industrial equipment by replacing failed original mounts.

6.6 Matting and flooring

Floor mats, anti-fatigue matting, stable mats (for dairy sheds), and industrial flooring from recycled crumb rubber bonded with sulfur or polyurethane. These are high-volume products that consume significant quantities of crumb rubber and provide practical value. NZ dairy farms in particular use large quantities of rubber matting in milking sheds — replacements from recycled rubber are adequate for this application.

6.7 Rubberised asphalt

Crumb rubber incorporated into road asphalt at 15–20% by weight of binder improves flexibility, reduces cracking, and extends road life. This is a proven technology already used in NZ.³⁸ Under recovery conditions, rubberised asphalt serves double duty: improving road quality (Doc #60) while providing a productive use for crumb rubber.

6.8 Harakeke-based substitutes for low-stress rubber applications

For applications where rubber serves primarily as a flexible, resilient material rather than as an elastic seal, harakeke (Phormium tenax) fibre and muka (processed fibre) can provide partial substitution (Doc #100). Harakeke cannot replicate rubber’s elasticity, but it can replace rubber’s structural role in several product categories: woven or braided cordage for shock absorption and lashing; matting for anti-fatigue and livestock flooring (replacing rubber stable mats in dairy sheds); and weatherproofing seals where compressed fibre gaskets can substitute for rubber gaskets in low-pressure, static-seal applications. These are lower-performance substitutes that conserve the finite recycled rubber supply for applications where elasticity is essential.

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7. DEPENDENCY CHAINS AND INDUSTRIAL PREREQUISITES

7.1 What must exist before rubber recycling can function

Rubber recycling does not operate in isolation. The full dependency chain includes:

Steel (Doc #89, Doc #106): For shredder blades, moulds, press platens, reactor vessels, mandrels, and structural components. Glenbrook plate steel is adequate for most applications. Tool steel for blades requires appropriate alloying and heat treatment.

Machine shops (Doc #91): For fabricating moulds, machining press components, and maintaining equipment. Mould-making is the critical skill — it requires precision machining of steel to create the negative shape of the desired rubber product.

Electricity (Doc #67, Doc #65): All processing equipment (shredders, mills, presses, heating systems) runs on grid electricity. Energy consumption is modest relative to NZ’s grid capacity.

Sulfur: From NZ geothermal sources. Extraction and refining is within NZ’s chemical processing capability but must be established.

Heat-resistant materials: Moulds and press equipment must withstand 150–180 degrees C continuously. Standard carbon steel is adequate. Refractory materials are needed for pyrolysis reactors.

Testing equipment: Durometers (hardness), tensile testers, and compression set apparatus are needed for quality control. NZ engineering laboratories (e.g., at universities and Callaghan Innovation) likely have this equipment, which should be preserved and maintained.

7.2 Skills required

• Rubber compounding: The art and science of formulating rubber compounds — selecting base polymers, fillers, curatives, and process aids to achieve desired properties. This is a specialised skill held by a small number of NZ professionals. Immediate knowledge capture is essential.
• Mould design: Designing moulds for rubber products requires understanding rubber shrinkage (typically 1.5–3% during vulcanization), flash management, parting line placement, and venting. This is a skilled trade.
• Mixing and milling: Operating two-roll mills and internal mixers (Banbury type) requires experience to achieve uniform dispersion of ingredients and consistent compound quality.
• Vulcanization process control: Temperature, time, and pressure must be controlled to achieve proper cure. Undercure produces weak products; overcure produces brittle, degraded products.
• Quality testing: Interpreting test results and adjusting formulations accordingly.

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8. CRITICAL UNCERTAINTIES

Uncertainty	Impact if wrong	Resolution method
Exact NZ rubber processing and moulding capability	Determines starting point for production; if less capability exists than assumed, development timeline extends	National skills and asset census (Doc #8)
Existing stocks of rubber compounding chemicals (accelerators, antioxidants, carbon black)	Determines how long high-quality production can continue before reverting to lower-grade sulfur-only systems	Industry inventory as part of national census
NZ geothermal sulfur extraction feasibility at needed scale	Sulfur is essential for vulcanization; if extraction proves difficult, recycled rubber products cannot be vulcanized	Geological and engineering assessment, Months 3–6
Performance of tallow and pine oil as rubber process oil substitutes	Determines whether NZ can maintain rubber processability after petroleum oil stocks deplete	Experimental testing, Phase 2
Viability of biological devulcanization using NZ thermophilic bacteria	Could provide a low-energy, chemical-free devulcanization pathway	Research program, Phase 2–3
Achievable quality of NZ-recycled rubber products	Determines which applications can be served; if quality is worse than estimated, fewer products are viable	Ongoing testing as production develops
Rate of tire exit from service into recycling stream	Too fast depletes the tire stock for transport; too slow starves the recycling stream	Coordinated with tire management strategy (Doc #33)
Recovered carbon black quality from NZ-built pyrolysis	Determines whether rCB can meaningfully substitute for virgin carbon black	Pilot pyrolysis trials, Phase 2–3
Ozone depletion effects on rubber degradation rates	If ozone cracking of stored and in-service rubber is worse than expected, the usable stock shrinks faster	Monitoring program, correlation with atmospheric data

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9. CROSS-REFERENCES

• Doc #1 — National Emergency Stockpile Strategy (rubber goods and chemical stocks as strategic assets)
• Doc #8 — National Asset and Skills Census (identifying rubber processing capability, workforce, and material stocks)
• Doc #33 — Tires: Management, Retreading, and Alternatives (the primary rubber source document; this document picks up where Doc #33’s Section 5.1 leaves off)
• Doc #34 — Lubricant Production (tallow and lanolin as rubber process oil substitutes; shared bio-oil supply chain)
• Doc #56 — Wood Gasification (potential syngas source for chemical feedstocks)
• Doc #59 — Bicycle Fleet (solid rubber inserts as pneumatic tire alternative)
• Doc #65 — Hydroelectric Maintenance (rubber seals and gaskets for hydro equipment)
• Doc #89 — NZ Steel Glenbrook (steel plate for moulds, press equipment, pyrolysis reactors)
• Doc #90 — Scrap Metal (steel recovery from tire recycling; wire from tire beads)
• Doc #91 — Machine Shop Operations (mould fabrication, equipment maintenance)
• Doc #92 — Blacksmithing and Forge Work (mould-making for simpler products)
• Doc #93 — Foundry Work (cast moulds for rubber products)
• Doc #97 — Cement and Concrete (crumb rubber as concrete additive for flexibility)
• Doc #100 — Harakeke Fiber Processing (fibre reinforcement for rubber composites; partial rubber substitution)
• Doc #102 — Charcoal Production (potential carbon source; pine resin recovery)
• Doc #103 — Salt Production (salt as feedstock for chlorine, which is used in some rubber processing)
• Doc #105 — Wire Drawing (wire from tire bead steel; wire reinforcement for hoses)
• Doc #113 — Sulfuric Acid (chemical processing prerequisite for some devulcanization agents)
• Doc #138 — Sailing Vessel Design (natural rubber as a trade priority; dock fenders from recycled rubber)
• Doc #142 — Trans-Tasman and Pacific Trade Routes (natural rubber import from tropical regions)
• Doc #157 — Trade Training (rubber compounding and moulding as a trade skill)
• Doc #162 — University and Research (devulcanization research; biological devulcanization development)

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APPENDIX A: SIMPLIFIED RUBBER COMPOUND FORMULATION FOR NZ CONDITIONS

The following are indicative formulations for basic rubber products using NZ-available materials. These are starting points, not validated recipes — each requires testing and adjustment.

A.1 General-purpose solid tire compound (from devulcanized rubber)

Ingredient	phr (parts per hundred rubber)	NZ source
Devulcanized tire rubber	100	NZ recycling
Recovered carbon black (rCB)	30–50	NZ pyrolysis
Sulfur	3–5 (accelerated) or 8–12 (unaccelerated)	Geothermal
Zinc oxide	3–5	Imported stock; recycled
Stearic acid	1–2	From tallow
Accelerator (CBS or TMTD)	0.5–1.5 (if available)	Imported stock
Process oil (tallow or pine oil)	5–10	Domestic
Whiting (ground limestone)	20–40	NZ limestone

Cure: 155 degrees C, 15–30 minutes (accelerated) or 150 degrees C, 2–4 hours (unaccelerated). Compression mould at 10–15 MPa.

Expected properties: Shore A hardness 65–75; tensile strength 5–8 MPa (versus 15–25 MPa for virgin compound); elongation 100–200% (versus 300–500% for virgin). Adequate for solid tires at speeds below 30 km/h.

A.2 Gasket compound (from fine crumb rubber)

Ingredient	phr	NZ source
Fine crumb rubber (60+ mesh)	100	NZ grinding
Sulfur	5–8	Geothermal
Zinc oxide	3	Imported stock
Stearic acid	1	From tallow
Tallow (process oil)	10–15	Domestic
Ground silica	10–20	NZ quartz sand

Cure: 160 degrees C, 30–60 minutes. Compression mould at 15 MPa.

Expected properties: Shore A hardness 55–65; adequate compression for pipe flanges and low-pressure sealing. Not suitable for dynamic seals.

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APPENDIX B: NZ GEOTHERMAL SULFUR — EXTRACTION AND REFINING

NZ’s Taupo Volcanic Zone contains significant elemental sulfur deposits and ongoing sulfur deposition from geothermal activity. Key locations include:

• Rotokawa geothermal field: Active sulfur deposition; accessible from existing geothermal infrastructure
• Wairakei-Tauhara system: Sulfur present in geothermal fluid and gas
• White Island (Whakaari): Historically mined for sulfur (last commercial mining in the 1930s); substantial deposits remain but access is hazardous and logistically challenging³⁹
• Tikitere (Hell’s Gate): Sulfur deposits accessible near Rotorua
• Various solfataras and fumaroles: Smaller deposits throughout the volcanic zone

Extraction: Elemental sulfur can be collected from surface deposits near fumaroles and solfataras, or precipitated from geothermal fluids by oxidation of dissolved hydrogen sulfide (H2S). The Claus process (converting H2S to elemental sulfur) is the industrial standard but requires catalyst and process equipment. Simpler methods — bubbling geothermal gas through water and collecting precipitated sulfur — are lower-yielding but technologically accessible.⁴⁰

Refining: Crude sulfur is refined by melting (112.8 degrees C) and filtering or distilling to remove mineral and organic contaminants. Refined sulfur for rubber vulcanization should be at least 99.5% pure. This purity is achievable through simple distillation using NZ-built equipment.

Estimated NZ sulfur demand for rubber vulcanization: If NZ processes 5,000–10,000 tonnes of recycled rubber per year at an average sulfur loading of 5–10 phr, annual sulfur demand is approximately 250–1,000 tonnes. This is a modest quantity relative to the geological sulfur resources in the Taupo Volcanic Zone, which are estimated at tens of thousands to hundreds of thousands of tonnes.⁴¹

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1. Myhre, M. and MacKillop, D.A., “Rubber Recycling,” Rubber Chemistry and Technology, vol. 75, no. 3, 2002, pp. 429–474. This review summarises devulcanization technologies and reclaimed rubber properties. The 40–70% figure for mechanical properties is a generalisation across methods; specific results depend on feedstock, process parameters, and testing conditions.↩︎

2. NZ geothermal sulfur: Geological and Nuclear Sciences (GNS Science), “Geothermal Features of the Taupo Volcanic Zone.” https://www.gns.cri.nz/ — Sulfur deposition is an ongoing process at active geothermal sites. Historical sulfur mining at White Island produced several thousand tonnes before cessation. Total sulfur resources in the Taupo Volcanic Zone are not precisely quantified but are substantial. Specific resource estimates require verification from GNS Science geological assessments.↩︎

3. Tire rubber content: A typical passenger car tire weighs 8–12 kg, of which approximately 45–47% is rubber compound (including fillers); net rubber polymer content is approximately 30–35% of total weight, or roughly 3–4 kg per tire. A truck tire weighs 40–80 kg with proportionally more rubber. The total rubber content figures are estimates based on fleet composition from Doc #33 and average tire weights from industry sources. See: Rubber Manufacturers Association, “Scrap Tire Characteristics.”↩︎

4. Natural rubber price data: Historical prices for RSS3 natural rubber and SBR 1502 synthetic rubber from commodity exchanges (Singapore, Shanghai). NZ dollar prices converted at approximate pre-war exchange rates. Actual prices fluctuate significantly with market conditions.↩︎

5. Crumb rubber production capacity: Based on specifications of typical tire shredding and grinding equipment from manufacturers (e.g., BSGH, Eldan, Granutech-Saturn). A medium-capacity ambient grinding line processes approximately 2–4 tonnes per hour. Annual capacity depends on operating hours. Staffing estimates based on typical facility configurations.↩︎

6. Labour and worker estimates for rubber recycling operations: Derived from facility staffing norms in footnote 5 (crumb rubber lines at 5–8 workers per line) plus published accounts of small rubber products facilities (typically 10–30 workers for a mixing, moulding, and vulcanization operation). The total of 80–150 person-years assumes four to five regional hubs across NZ’s main regions, each requiring 15–30 person-years of establishment work. This estimate carries high uncertainty and should be revisited once the Doc #8 census establishes how many NZ rubber processing operations are actually intact and staffed. The ongoing workforce figure of 100–200 workers assumes reduced but continuous production; actual requirements depend on product mix and throughput targets.↩︎

7. Tire composition data: Typical composition of a radial passenger tire. Actual compositions vary by manufacturer, tire type, and size. Based on: Mark, J.E., Erman, B., and Roland, C.M. (eds.), The Science and Technology of Rubber, 4th edition, Academic Press, 2013; and Rubber Manufacturers Association published data.↩︎

8. Natural rubber vs. SBR devulcanization: Natural rubber (cis-1,4-polyisoprene) generally devulcanizes more readily than SBR because its polymer backbone is more regular and the molecular weight distribution of the reclaimed product is more uniform. This is well-documented in the reclaimed rubber literature. See: Adhikari, B., De, D., and Maiti, S., “Reclamation and recycling of waste rubber,” Progress in Polymer Science, vol. 25, 2000, pp. 909–948.↩︎

9. NZ conveyor belt stocks: No precise public data available. Estimate based on the scale of NZ’s mining (coal, aggregate, ironsand), dairy processing, and manufacturing sectors. NZ’s largest conveyor belt users include coal mines (Waikato, West Coast), NZ Steel (ironsand transport), and port facilities. Verification through the national asset census is required.↩︎

10. NZ tire recycling: Several NZ companies have operated tire recycling facilities, including Tyre Recycling Ltd, Green Rubber NZ, and others. The exact current operational status and capacity of these facilities should be verified. NZ’s Waste Minimisation Act 2008 and the Tyrewise programme (developed by the Tyre Industry Federation NZ) have addressed end-of-life tire management. See: Ministry for the Environment, “Waste Tyres.” https://environment.govt.nz/↩︎

11. Energy requirements for crumb rubber production: Energy consumption data from equipment manufacturer specifications and published process studies. Actual energy varies with equipment efficiency, maintenance condition, and target particle size (finer grinding requires more energy). See: Sienkiewicz, M. et al., “Progress in used tyres management in the European Union: A review,” Waste Management, vol. 32, 2012, pp. 1742–1751.↩︎

12. Shredder blade wear: Blade life varies significantly with blade material (D2 tool steel, high-speed steel, tungsten carbide inserts), tire composition (wire content), and maintenance. The 500–2,000 tonne range is indicative; actual experience varies. NZ’s ability to produce replacement blades depends on tool steel availability and heat treatment capability (Doc #89, Doc #91).↩︎

13. Kauri gum industry: Kauri gum (kapia) was a major NZ export industry from the 1840s through the early twentieth century, with annual exports reaching 10,000+ tonnes at peak. The gum was used primarily in varnish and linoleum manufacture. Extensive deposits of sub-fossil gum remain in Northland soils. Maori were the primary collectors, and Maori knowledge of gum deposits and quality is an important cultural and practical resource. See: Ministry for Culture and Heritage, “Kauri gum digging,” NZ History. https://nzhistory.govt.nz/↩︎

14. Compression-moulded crumb rubber properties: Mechanical properties of crumb rubber products bonded by heat and pressure alone (without virgin rubber or chemical binders) depend strongly on particle size, moulding temperature, pressure, and time. The 30–50% of virgin tensile strength is a rough guideline from laboratory studies. See: Karger-Kocsis, J., Mészáros, L., and Bárány, T., “Ground tyre rubber (GTR) in thermoplastics, thermosets, and rubbers,” Journal of Materials Science, vol. 48, 2013, pp. 1–38.↩︎

15. Rubberised asphalt in NZ: Crumb rubber modified (CRM) asphalt has been trialled and used in NZ road construction. NZ Transport Agency (Waka Kotahi) research reports address the performance of rubberised asphalt in NZ conditions. See: Waka Kotahi, Research Programme publications. https://www.nzta.govt.nz/resources/research/↩︎

16. Vulcanization chemistry: Standard rubber technology textbook material. See: Blow, C.M. and Hepburn, C. (eds.), Rubber Technology and Manufacture, Butterworth-Heinemann; Mark, Erman & Roland (note 6). Charles Goodyear’s discovery of sulfur vulcanization (1839) and Thomas Hancock’s independent development are well-documented in the history of polymer science.↩︎

17. Bond energies in vulcanized rubber: S-S bond energy approximately 270 kJ/mol; C-S approximately 310 kJ/mol; C-C approximately 370 kJ/mol. These values are approximate and vary with molecular environment. The selectivity challenge is a fundamental thermodynamic limitation. See: Mark, Erman & Roland (note 6), Chapter 9; Adhikari, De & Maiti (note 7).↩︎

18. History of rubber reclamation: The pan or digester process dates to the 1880s. The US rubber reclamation industry was substantial through the mid-twentieth century, driven by rubber scarcity during both World Wars. See: Rouse, M.W., “Rubber Reclamation,” in Rubber Technology, Morton, M. (ed.), Van Nostrand Reinhold, 1987.↩︎

19. Bio-based process oils for rubber: Tallow and plant oils have been investigated as replacements for petroleum process oils in rubber compounding. Compatibility varies: plant oils (epoxidised soybean oil, castor oil) show good compatibility with NR; compatibility with SBR is lower. Tallow (primarily saturated fatty acids) has different plasticising behaviour from naphthenic petroleum oils. See: Dasgupta, S. et al., “Characterization of eco-friendly processing aids for rubber compound,” Polymer Testing, vol. 26, 2007, pp. 489–500.↩︎

20. Microwave devulcanization: Novotny, D.S. et al., “Microwave Devulcanization of Rubber,” US Patent 4,104,205, 1978. More recent work by various groups has demonstrated improved selectivity. The technology remains largely at the pilot stage for tire rubber.↩︎

21. Ultrasonic devulcanization: Isayev, A.I. and Chen, J., “Continuous ultrasonic devulcanization of vulcanized elastomers,” Journal of Applied Polymer Science, vol. 59, 1996, pp. 969–975. Isayev’s group at the University of Akron has published extensively on ultrasonic devulcanization of various rubber types.↩︎

22. Chemical devulcanization: Various chemical agents have been investigated, including organic disulfides (diphenyl disulfide, bis(3-triethoxysilylpropyl)tetrasulfide), and reducing agents. See: Verbruggen, M.A.L. et al., “Devulcanization of sulfur-cured rubber with bis(3-triethoxysilylpropyl)tetrasulfide,” Macromolecular Materials and Engineering, vol. 284/285, 2000.↩︎

23. Biological devulcanization: Bredberg, K., Persson, J., Christiansson, M., et al., “Anaerobic desulfurization of ground rubber with the thermophilic archaeon Pyrococcus furiosus — a new method for rubber recycling,” Applied Microbiology and Biotechnology, vol. 55, 2001, pp. 43–48. Also: Li, Y., Zhao, S., and Wang, Y., “Microbial desulfurization of ground tire rubber by Sphingomonas sp.,” Polymer Degradation and Stability, vol. 96, 2011, pp. 1662–1668. NZ’s geothermal environments host diverse sulfur-metabolising microorganisms that may be adapted for this purpose; assessment by NZ microbiologists (e.g., at GNS Science Wairakei Research Centre or University of Waikato) is recommended.↩︎

24. Reclaimed rubber properties: Property ranges are generalised from multiple sources. Actual properties depend on feedstock rubber type, devulcanization method, re-compounding formulation, and curing conditions. The ranges given represent typical results from thermo-mechanical and mechanical devulcanization of tire rubber. See: Myhre & MacKillop (note 1); Adhikari, De & Maiti (note 7); and Formela, K. et al., “Reactive processing of ground tire rubber,” Polymer Degradation and Stability, vol. 127, 2016, pp. 32–39.↩︎

25. Blending ratios for reclaimed rubber: Industry practice typically limits reclaimed rubber content to 20–40% of total rubber in a compound to maintain acceptable properties. Higher loadings are used in lower-performance applications. See: Rubber World Magazine, various technical articles on reclaimed rubber compounding.↩︎

26. NZ geothermal sulfur: Geological and Nuclear Sciences (GNS Science), “Geothermal Features of the Taupo Volcanic Zone.” https://www.gns.cri.nz/ — Sulfur deposition is an ongoing process at active geothermal sites. Historical sulfur mining at White Island produced several thousand tonnes before cessation. Total sulfur resources in the Taupo Volcanic Zone are not precisely quantified but are substantial. Specific resource estimates require verification from GNS Science geological assessments.↩︎

27. Vulcanization accelerator chemistry: Standard rubber technology textbook material. Accelerators are classified by chemical type and speed of action. See: Blow & Hepburn (note 14); Coran, A.Y., “Vulcanization,” in Mark, Erman & Roland (note 6), Chapter 7.↩︎

28. NZ accelerator stocks: No public data identified on total NZ inventories of vulcanization accelerators. Quantities held by individual rubber product manufacturers would be visible in the national asset census (Doc #8). Priority should be given to enumerating these stocks in Phase 1 before any drawdown occurs.↩︎

29. Zinc oxide for rubber: Zinc oxide serves as an activator in the sulfur vulcanization system, forming an intermediate complex with stearic acid and accelerator that controls the cross-linking reaction. Typical loading is 3–5 phr. NZ zinc sources include galvanised steel (Doc #35, Section 6), zinc die-cast components (automotive and hardware), and zinc-containing batteries. Recovery requires oxidation of metallic zinc at relatively low temperatures (oxidation begins around 225 degrees C). Quantities recoverable from NZ sources are uncertain.↩︎

30. Bio-based process oils for rubber: Tallow and plant oils have been investigated as replacements for petroleum process oils in rubber compounding. Compatibility varies: plant oils (epoxidised soybean oil, castor oil) show good compatibility with NR; compatibility with SBR is lower. Tallow (primarily saturated fatty acids) has different plasticising behaviour from naphthenic petroleum oils. See: Dasgupta, S. et al., “Characterization of eco-friendly processing aids for rubber compound,” Polymer Testing, vol. 26, 2007, pp. 489–500.↩︎

31. Tropospheric ozone and rubber degradation: Ozone cracking in rubber is a well-characterised phenomenon caused by ground-level (tropospheric) ozone, which attacks double bonds in rubber polymer chains at points of mechanical strain. This is distinct from stratospheric ozone depletion, which affects UV flux at the surface. Tropospheric ozone concentrations in post-event NZ are uncertain and would depend on NOx emissions from vehicle traffic, industrial sources, and biomass burning. For storage planning purposes, antiozonant-treated compounds and protective covers are the practical mitigations regardless of post-event ozone levels. See: Brydson, J.A., Rubber Materials and Their Compounds, Elsevier Applied Science, 1988, Chapter 5 (Ozone and Oxidative Degradation).↩︎

32. Sulfur-only vulcanization: Before the discovery of accelerators (Oenslager’s work with aniline, 1906; subsequent development of thiazoles and other accelerator classes), all rubber vulcanization used sulfur alone at high loadings. The products were functional but cured slowly and had inferior aging properties. See: Morton, M. (ed.), Rubber Technology, Van Nostrand Reinhold, 1987, Chapter 1 (History).↩︎

33. Tire pyrolysis: Williams, P.T., “Pyrolysis of waste tyres: A review,” Waste Management, vol. 33, 2013, pp. 1714–1728. This review covers process conditions, product yields, and product quality from tire pyrolysis. Typical conditions: 400–700 degrees C, atmospheric or reduced pressure, batch or continuous feed.↩︎

34. Recovered carbon black (rCB): Norris, C.J. et al., “Recovered carbon black from waste tyre pyrolysis: A review of its composition, properties, and potential applications,” Journal of Analytical and Applied Pyrolysis, vol. 153, 2021. rCB properties differ from virgin carbon black due to mineral contamination and surface chemistry changes during pyrolysis. Reinforcing efficiency is typically 60–80% of equivalent-grade virgin carbon black.↩︎

35. Small-scale tire pyrolysis unit fabrication: The components of a batch pyrolysis retort (sealed vessel, heating jacket or electric elements, vapour outlet, condenser coil) are within the scope of NZ pressure vessel fabrication. Analogous equipment in NZ food processing (retort sterilisers) and geothermal (separator vessels) sectors uses similar materials and welding standards. NZ’s boilermaking and pressure vessel certification (to WorkSafe NZ standards under the Health and Safety at Work Act 2015) establishes the workforce competency base. Specific pyrolysis design references: Williams (note 27); Czajczynska, D. et al., “Potentials of pyrolysis processes in the waste management sector,” Thermal Science and Engineering Progress, vol. 3, 2017.↩︎

36. Pyrolysis energy self-sufficiency: Non-condensable gas (NCG) from tire pyrolysis typically has a calorific value of 35–45 MJ/kg and constitutes 5–15% of tire weight. For a system processing 5 tonnes/day, NCG yield is approximately 250–750 kg/day with energy content of 9–34 GJ/day. Heat required to bring 5 tonnes of rubber from ambient to 550 degrees C (specific heat approximately 1.9 kJ/kg/K, plus endothermic cracking energy) is approximately 1.5–3 GJ/day, well within NCG combustion capability at the higher end. Exact energy balance depends on reactor insulation, batch vs. continuous operation, and char handling. See: Williams (note 27).↩︎

37. rCB quality and upgrading: Washing with dilute HCl removes zinc oxide and other acid-soluble contaminants, reducing ash content from 15–20% to 5–10%. Further reduction requires more aggressive treatment. See: Norris et al. (note 28).↩︎

38. Rubberised asphalt in NZ: Crumb rubber modified (CRM) asphalt has been trialled and used in NZ road construction. NZ Transport Agency (Waka Kotahi) research reports address the performance of rubberised asphalt in NZ conditions. See: Waka Kotahi, Research Programme publications. https://www.nzta.govt.nz/resources/research/↩︎

39. White Island (Whakaari) sulfur mining: Commercial sulfur mining on Whakaari occurred intermittently from the 1880s to the 1930s. Operations were hazardous (a lahar killed 10 miners in 1914) and ultimately uneconomic. Following the 2019 eruption, the island’s hazard status makes renewed mining extremely challenging. Sulfur deposits remain substantial but access is a serious safety concern. See: GNS Science, “Whakaari/White Island.” https://www.gns.cri.nz/↩︎

40. Sulfur recovery from geothermal H2S: The Claus process (2H2S + SO2 -> 3S + 2H2O) is the industrial standard for converting hydrogen sulfide to elemental sulfur. It requires a catalyst (typically alumina or bauxite) and careful temperature control. Simpler oxidation methods using air or iron oxide as oxidant are lower-yielding but more accessible for initial NZ production. See: Kohl, A.L. and Nielsen, R.B., Gas Purification, Gulf Professional Publishing, 5th edition, 1997.↩︎

41. NZ geothermal sulfur: Geological and Nuclear Sciences (GNS Science), “Geothermal Features of the Taupo Volcanic Zone.” https://www.gns.cri.nz/ — Sulfur deposition is an ongoing process at active geothermal sites. Historical sulfur mining at White Island produced several thousand tonnes before cessation. Total sulfur resources in the Taupo Volcanic Zone are not precisely quantified but are substantial. Specific resource estimates require verification from GNS Science geological assessments.↩︎