Which Zinc Borate is Most Commonly Used?

Among numerous variants of zinc borate, 3.5 Hydrated Zinc Borate (2ZnO·3B₂O₃·3.5H₂O) has become the most widely used and formulated variety in global polymer flame retardant systems, accounting for over 75% of the industrial zinc borate market share. This market choice is not accidental, but determined by its unique crystal chemical design, precise flame retardant function, and excellent cost-effectiveness. This article will systematically reveal the internal mechanism of 3.5 hydrated zinc borate becoming an industry standard from the perspective of materials science, flame retardant engineering, and industrial economics.

Ⅰ. Market data and technological consensus: undisputed mainstream choice

1.1 Global Application Distribution Statistics

According to the Polymer Additive Market Report (2023), the application of zinc borate in the segmented flame retardant market presents a clear distribution:

·3.5 Hydrated zinc borate: accounting for 76.3%

·Anhydrous zinc borate: accounting for 15.8%

·Other hydration forms (5 water, 7 water, etc.): accounting for 7.9%

1.2 Priority in Industry Standards

·UL standard system: In cable combustion tests such as UL 94 and UL 1685, the formula based on 3.5 hydrated zinc borate has the highest pass rate

·In IEC international standards such as IEC 60332 and 61034 for cable testing, the recommended formula defaults to the 3.5 hydrate form

·In the Chinese national standard GB/T 19666-2019 "General Rules for Flame retardant and Fire resistant Wires and Cables", the example formulas are based on this variety

Ⅱ. The scientific ingenuity of crystal water design: why is it "3.5"?

2.1 Thermodynamic equilibrium of hydration degree

The hydration form of zinc borate varies from 1.5 to 7.5 water, but the 3.5 hydrate exhibits unique stability advantages:

Comparison of Crystal Structure Stability

2.2 Perfect Match between Dehydration Temperature and Polymer Processing

Key temperature parameters:

·Initial dehydration temperature: 215 ± 5°C

·Main dehydration range: 225-320°C

·Complete dehydration temperature:>320°C

This temperature spectrum forms a strategic alignment with the processing and combustion behavior of the main polymers:

Corresponding relationship with key temperature of polymer

PVC processing temperature: 170-200°C ← 3.5 Hydrates are completely stable

PVC thermal degradation initiation:~250°C ← Hydrates begin to dehydrate and absorb heat

Polymer combustion zone temperature: 300-500°C ← Main dehydration zone, maximum cooling effect

Formation temperature of carbon layer:>400°C ← Dehydration completed, ceramic formation begins

2.3 Moisture Content Optimization: Theoretical Optimal Point of 14.5%

The 14.5% theoretical moisture content corresponding to 3.5 hydration is the result of multiple equilibria:

·Flame retardant efficiency: Each gram of material can absorb 280-300 J of heat (enthalpy of phase change+sensible heat of water vapor)

·Processing safety: The moisture content is low enough to avoid processing bubbles (<15% safety threshold)

·Transportation economy: Maximizing the proportion of effective flame retardant components (ZnO+B₂O₃)

Ⅲ. Three dimensional flame retardant mechanism: why is it more effective than alternatives?

3.1 Triple interaction of gas phase condensed phase barrier phase

First dimension: Gas phase cooling and dilution (200-320°C)

H₂O(l) → H₂O(g)  ΔH_vap = 2257 J/g

Heat absorption calculation: 14.5% × 2257=327 J/g (theoretical)

Actual measured dehydration enthalpy: 280-295 J/g (including crystal failure energy)

Dilute oxygen concentration with water vapor: volume dilution rate reaches 8-12%

Second dimension: Condensed phase catalysis to carbon (320-450°C)

Dehydration products react with halogen sources (such as Cl in PVC):

Zn₂B₆O₁₁ + 12HCl → 2ZnCl₂ + 6BCl₃ + 6H₂O

The generated ZnCl ₂ acts as a Lewis acid catalyst:

·Reduce the activation energy of polymer dehydrogenation by 40-50 kJ/mol

·Promote the formation of aromatic carbon structure (increase the size of graphite microcrystals by 2-3 nm)

·Improvement in residual carbon rate: PVC system increased from 12% to 28%

Third dimension: Formation of ceramic barrier (>450°C)

Boron oxide forms a viscous glass layer:

B₂O₃ viscosity equation: log η=A+B/(T-T ₀)

At 500 ° C: η≈ 10 ⁴ Pa · s (effective coverage)

At 700 ° C: η≈ 10 ³ Pa · s (self-healing flow)

This glass layer reduces the oxygen diffusion coefficient by three orders of magnitude.

3.2 Performance Comparison Matrix with Competitive Products

Add 20 phr flame retardant to PVC cable material:

Ⅳ. Processing applicability: perfect fit with industrial reality

4.1 Thermal stability matching mainstream processing equipment

Compatibility with common plastic processing techniques

·Twin screw extrusion: The highest temperature range is usually<220°C, and the 3.5 hydrate is completely stable

·Internal mixer mixing: instantaneous high temperature<250°C, short-term exposure does not affect performance

·Injection molding: Melt temperature<280°C, dehydration rate controlled within 5%

·Rolling forming: Roll temperature usually<200°C, zero risk

4.2 Friendly effects on material rheology

Study on the behavior in PVC plastisol

When adding 20 phr:

-Brinell viscosity increase: only 8-12% (compared to aluminum hydroxide: increased by 25-30%)

-Thixotropic index change: from 0.85 to 0.88 (almost maintaining Newtonian fluid characteristics)

-Storage stability: viscosity change<5% after 6 months

This is attributed to the lubricating effect and low surface energy of its sheet-like particles.

Ⅴ. The 'golden partner' position of synergy effect

5.1 Classic combination with antimony trioxide

Reaction mechanism:

Zn₂B₆O₁₁ + Sb₂O₃ + 12HCl → 

ZnSb ₂ O ₅ (spinel phase)+2BCl ∝+other borate glasses

Collaboration index (defined according to ISO 4589):

·Individually 3.5 hydrated zinc borate: LOI=28.5%

·Sb₂O₃ alone: LOI=27.0%

·1: 1. Compound: LOI=34.5%

·Collaboration index: 1.24 (>1.2 indicates strong collaboration)

5.2 Multi functional role in halogen-free systems

In EVA/magnesium hydroxide halogen-free cable material:

Standard formula:

EVA(28%VA)：100 phr

Magnesium hydroxide: 120 phr

3.5 Hydrated Zinc Borate: 25 phr

Silicone masterbatch: 5 phr

Performance improvement:

-LOI increased from 38% to 42%

-Smoke density ranges from 250 to 160

-Tensile strength ranges from 10.2 to 11.8 MPa

-Passed UL 94 V-0 (3.2mm)

Its mechanism of action is to promote the formation of magnesium borate ceramic layers from the dehydration products of magnesium hydroxide.

Ⅵ. Cost effectiveness ratio

Whole life cycle cost analysis

Comparative analysis of cable factories with an annual consumption of 100 tons

Conclusion:

The "most commonly used" position of 3.5 hydrated zinc borate is a classic engineering optimization case in materials science: it does not achieve the ultimate performance in a single item, but achieves the best balance in four dimensions: flame retardant efficiency, processing adaptability, cost control, and environmental friendliness.

Its core competitiveness can be summarized as:

1. Accurate thermal response: strategic matching of crystallization water release temperature and polymer thermal degradation temperature

2. Multidimensional flame retardant mechanism: simultaneously acting on the gas phase, condensed phase, and barrier phase

3. System compatibility: friendly coexistence with the vast majority of polymers and processing techniques

4. Economic sustainability: providing reliable flame retardant safety at the lowest overall cost