What Are the Specifications for rCB Used as a Plastic Filler?

Based on refined rCB processing standards from rcb-mill.com and global plastic compounding & masterbatch industrial requirements, plastic-grade recovered carbon black (rCB) has strict classified specifications covering particle size distribution, chemical purity, impurity limits, surface properties, coloring & dispersion performance, and safety indexes. Crude unground pyrolysis char fails all plastic filler criteria; only fully deagglomerated, magnetically separated, multi-stage classified ground rCB meets plastic production standards. This article divides specifications into three commercial grades: high-gloss masterbatch grade, general-purpose plastic grade, and low-cost engineering filler grade, with unified test standards and threshold limits.

1. Particle Size & Distribution Specifications (Core Index for Plastic Appearance & Dispersion)

Measured by laser diffraction (ISO 13320); D90 refers to deagglomerated aggregate fineness after ultra-fine grinding and closed-circuit air classification. Oversized agglomerates cause surface specks, poor gloss and inconsistent mechanical properties.

Grade classification thresholds

1. High-gloss masterbatch / thin film grade (PE/PP black film, high-transparency thin sheet)

• D90 ≤ 6 μm; D50 = 2.5–4 μm
• D99 ≤ 9 μm (zero large hard agglomerates)
• Particle distribution width D90/D50 ＜ 1.3 (narrow distribution)
• 325 mesh sieve residue ＜ 50 ppm; 100 mesh & 35 mesh residue = 0 ppm (ASTM D1514)

2. General-purpose plastic grade (PVC profiles, wire cable sheath, plastic pipe, injection molded housings)

• D90 ≤ 8–10 μm; D50 = 4–6 μm
• D99 ≤ 14 μm
• D90/D50 ＜ 1.5
• 325 mesh residue ＜ 300 ppm

3. Low-cost structural filler grade (geomembrane, thick industrial plastic parts, low-gloss modified plastics)

• D90 ≤ 12–15 μm; upper acceptable limit D90 ≤18 μm
• D99 ≤ 22 μm
• 325 mesh residue ＜ 500 ppm

Critical requirement from rcb-mill production line

All oversized particles above cut-point are circulated back to the pulverizer for regrinding; no coarse agglomerates enter finished silos to eliminate plastic surface defects.

2. Chemical Purity & Impurity Control Specifications

Impurities including metal grit, silica, zinc salt and high ash scratch extruder screws, reduce plastic tensile strength and cause local brittleness.

2.1 Ash Content (550°C calcination residue, main inorganic impurities SiO₂, ZnO, CaO)

• High-gloss masterbatch grade: Ash ≤ 8 wt% (premium refined type ≤5%)
• General plastic grade: Ash ≤ 12 wt%
• Low-cost filler grade: Ash ≤ 18 wt%
  Note: Ash exceeding 20% sharply drops plastic impact resistance and tinting power, forbidden for color masterbatch production.

2.2 Free Metal Impurity (Iron/steel wire residual from tire pyrolysis)

• Total free metal particles: 0 ppm after double-stage high-intensity magnetic separation
• Hard metal grit limit: Zero particles retained on 100 mesh sieve
  Metal residues cause severe abrasion to twin-screw barrels and injection molds during plastic extrusion.

2.3 Moisture Content

All plastic grades: Moisture ≤ 1.2 wt% (premium masterbatch ≤0.8%)
Excess moisture leads to bubble generation, pinholes and poor dispersion during melt extrusion.

2.4 Volatile Matter (950°C loss on ignition, residual tar & organic residues)

• High-gloss grade: Volatile ≤ 4.0 wt%
• General plastic grade: Volatile ≤ 6.0 wt%
• Low-cost grade: Volatile ≤ 8.0 wt%
  High volatiles produce smoke, odor and tiny voids inside plastic products at processing temperature 180–260°C.

2.5 Sulfur & PAH limits (environmental & food contact compliance)

• Total sulfur content ＜ 3.0 wt%
• 16 priority PAHs ＜ 200 ppm; food-contact plastic rCB requires PAH below detection limit
• pH value: 6.5–8.0 (neutral, avoids plasticizer decomposition in PVC)

2.6 Carbon content

72–88 wt%, higher carbon content corresponds to stronger black tinting performance.

3. Surface & Structural Performance Specifications (Dispersion, Coloring & UV Resistance)

3.1 BET Specific Surface Area (nitrogen adsorption ASTM D6556)

• High-gloss masterbatch: 70–95 m²/g (similar to virgin N660/N770)
• General plastic filler: 55–70 m²/g
• Low-cost structural filler: 45–55 m²/g
  Larger surface area delivers better UV shielding, tint strength and uniform hiding power for black plastics.

3.2 DBP Oil Absorption Number (DBP, ml/100g, reflects aggregate structure)

• Masterbatch grade: 70–90
• General plastic grade: 60–80
  Low DBP value reduces dispersant consumption during twin-screw mixing, lowering masterbatch production cost.

3.3 Tint Strength & Dispersibility

• Tint strength: 65–85% relative to virgin semi-reinforcing carbon black N660
• Dispersion rating ≥95% after twin-screw compounding; no visible black agglomerate speckles on plastic surface
• Toluene transmittance ＞85% (ASTM D1618), indicator of clean surface free of heavy tar residues

3.4 Bulk Density

220–350 kg/m³ for fine powder; pelletized rCB 400–550 kg/m³ to improve feeding stability in extruders.

4. Grade-by-Grade Full Specification Summary Table

5. Application Matching Rules Based on Specifications

1. High-gloss masterbatch grade (D90 ≤6 μm, ash ≤8%)
   Suitable for PE/PP thin film, food-contact black masterbatch, high-gloss injection parts; can replace 50–90% of virgin N660/N770 carbon black. Requires full ultra-fine grinding + two-stage air classification on rcb-mill production lines.
2. General plastic grade (D90 ≤10 μm, ash ≤12%)
   Mainly for PVC profiles, cable outer sheath, plastic water pipes, common black masterbatch; substitution ratio 40–70% with virgin carbon black balanced cost and weather resistance.
3. Low-cost filler grade (D90 ≤15 μm, ash ≤18%)
   For geomembrane, agricultural plastic film, thick industrial modified plastic, non-appearance structural parts; allows full 100% replacement of low-grade virgin carbon black, prioritizing cost reduction over surface gloss.

6. Key Processing Support from rcb-mill to Meet Plastic rCB Specifications

To hit above indexes stably, the complete rCB grinding & classification system realizes full quality control:

1. Multi-stage magnetic separation to eliminate all free metal impurities;
2. Closed-circuit ultra-fine grinding to fully break sintered pyrolysis agglomerates and control D90 precisely;
3. High-precision two-stage air classifier to cut oversized particles and narrow particle distribution;
4. Hot air drying unit to stabilize moisture below 1%;
5. Optional demineralization module to reduce ash content down to 5% for high-end masterbatch customers.

Plastic filler rCB is classified into three standard grades defined by particle size, ash, impurity and dispersion performance. The most critical threshold for all plastic applications is controlled D90 below 15 μm with zero coarse metal grit; high-gloss color masterbatch demands stricter D90 ≤6 μm and ash ≤8%. Only rCB processed via full deagglomeration grinding and multi-stage classification systems (as supplied by rcb-mill) complies with these specifications, enabling partial or full substitution of virgin carbon black in plastic compounds, masterbatch, pipes, films and molded products.