How to Choose Between 3.5 Hydrated Zinc Borate and Anhydrous Zinc Borate?

In polymer flame retardant formulation design, zinc borate is widely used as an efficient, low-toxicity, and multifunctional inorganic flame retardant. The scientific selection between hydrated forms (especially the 3.5-hydrate) and anhydrous forms constitutes a critical technical decision for engineers. This article will provide theoretical support from multiple dimensions.

Ⅰ. Chemical Nature and Structural Differences: The Decisive Role of Crystal Water

1.1 Crystal Structure and Thermal Stability Diagram

3.5 Hydrated zinc borate (2ZnO·3B₂O₃·3.5H₂O)

Crystal structure: Monoclinic system, layered structure with interlayer-embedded crystalline water molecules

Theoretical moisture content: 14.5 wt%

· Critical temperature points:

  · Crystalline water begins to be removed: 210±5°C

  · Main dehydration range: 225-320°C

  · Complete dehydration to form an anhydrous phase: >320°C

  · Glass transition temperature: 400-450°C

Zinc borate (2ZnO·3B₂O₃)

Crystal structure: High-temperature stable phase, three-dimensional network structure

· Density: 3.64 g/cm³ (approximately 12% higher than hydrate type)

Thermal stability: No phase transition up to 800°C, melting point approximately 980°C

1.2 Comparison of Physical and Chemical Properties

II. Differential Path Analysis of Flame Retardant Mechanisms

2.1 Hydrated "Three-Stage" Synergistic Flame Retardant Model

Phase I: Cooling and Dilution Effect (200-320°C)

Hydrated water absorbs heat and evaporates:

ΔH_dehydration≈ 280 J/g

Dilute the oxygen concentration with water vapor in the gas phase to reduce the temperature in the combustion zone

Phase II: Formation of carbon under Lewis acid catalysis (320-500°C)

The dehydration product reacts with the HCl released by PVC:

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

The generated ZnCl₂ acts as a strong Lewis acid, catalyzing the dehydrogenation and cross-linking of polymers

[Polymer]-H+ ZnCl₂ → [Polymer] + ZnHCl₂

The carbon layer growth rate increased by 40-60%

Stage 3: Formation of the glass barrier (>500°C)

Formation of B₂O₃ glassy body, dynamic viscosity-temperature relationship:

η(T)= A·exp(E_a/RT)

At 700°C, η≈10³ Pa·s, forming an effective oxygen barrier

2.2 "High-Temperature-Specific" Flame Retardant Mechanism of Anhydrous Type

Single-stage glass barrier mechanism

Zinc borate without water directly participates in the formation of the B₂O₃ glass phase:

2ZnO·3B₂O₃+ 6HCl → 2ZnCl₂ + 3B₂O₃ + 3H₂O↑

Key features:

There is no low-temperature heat absorption process, and all the heat is used for polymer degradation

The glass phase forms at a higher temperature (>450°C)

The carbonization catalytic activity depends on external acid sources (such as HCl from PVC decomposition)

Flame Retardant Efficiency Comparison Data

In a typical PVC cable compound formulation (25 phr flame retardant):

Ⅲ. Processing adaptability: the influence of thermal history on material behavior

3.1 Thermal stability and matching degree of processing window

PVC processing scenario (typical processing temperature 170-200°C)

·3.5 Hydrated type: Stable at processing temperature, without premature dehydration

·Anhydrous type: No risk of thermal decomposition, but attention should be paid to high-temperature shear heating

High temperature processing scenarios for engineering plastics

·PBT/PA6 processing (240-280°C):

·3.5 Hydrated type: partially dehydrated (15-20%), may produce microbubbles

·Anhydrous type: completely stable, suitable for thin-walled products

·PPS/PEEK processing (320-380°C):

·3.5 Hydrated type: completely dehydrated, actually converted into anhydrous phase

·Anhydrous type: direct use, predictable performance

3.2 Differences in the Influence on the Rheological Properties of Materials

Performance in PVC plastisol (plasticizer content 50 phr):

3.5 Hydrated Zinc Borate:

-Initial viscosity: increased by 8-12%

-Thermal aging viscosity stability: excellent (without catalytic degradation)

-Storage period:>6 months

Anhydrous zinc borate:

-Initial viscosity: increase by 4-6%

-Potential risk: The generation of trace amounts of ZnCl ₂ may catalyze the hydrolysis of plasticizers

Ⅳ. Material Performance Balance: Electrical, Mechanical, and Environmental Characteristics

4.1 Comparison of Electrical Insulation Performance

Mechanism analysis: The ordered structure of crystal water reduces ion migration rate, and the micropores generated during dehydration are filled with B₂O₃ glass phase, forming a denser insulation structure.

4.2 Mechanical performance retention rate

Comparison in EPDM cable sheath (with an addition of 30 phr):

3.5 Hydrated Zinc Borate:

-Tensile strength retention rate: 92%

-Fracture elongation retention rate: 88%

-Compression permanent deformation: improved by 15%

Anhydrous zinc borate:

-Tensile strength retention rate: 96%

-Fracture elongation retention rate: 82%

-Hardness increase:+5 Shore A

4.3 Environmental and Toxicity Characteristics

Ecotoxicity:

·Both meet RoHS/REACH requirements

·3.5 Hydrated type: Bioavailable zinc ion release rate<0.1 ppm (pH=7)

·Anhydrous type: Zinc ion release rate<0.08 ppm

Burning toxicity:

3.5 Hydrated type releases water vapor in the early stages of combustion, diluting CO concentration by 30-40% and significantly reducing acute inhalation toxicity.

Ⅴ. Formula synergy and cost-benefit analysis

5.1 Synergistic efficiency with different flame retardant systems

Synergistic with aluminum hydroxide/magnesium

·3.5 Hydration type: dehydration temperature matched with Al(OH)₃ (~220°C), synergy index 1.25

·Water free type: synergy index 1.08

Synergistic effect with phosphorus nitrogen based flame retardants

·3.5 Hydration type: promotes graphitization of the carbon layer, increasing residual carbon content by 35%

·Anhydrous type: mainly used as a ceramic precursor, with better high-temperature stability

5.2 Whole life cycle cost model

Ⅵ. Typical application scenarios and formula recommendations

6.1 It is necessary to choose the scenario of 3.5 hydrated zinc borate

Formula for low smoke halogen-free cable material

EVA (VA content 28%): 100 phr

Magnesium hydroxide: 80 phr

3.5 Hydrated Zinc Borate: 25 phr

Silicone masterbatch: 5 phr

Lubricant: 2 phr

Expected performance: Smoke density<150, LOI>38%， According to IEC 61034

6.2 Recommended scenarios for using anhydrous zinc borate

High temperature engineering plastic connector

PBT (containing 30% fiberglass): 100 phr

Anhydrous zinc borate: 15 phr

Melamine cyanurate: 8 phr

Anti drip agent: 0.3 phr

Expected performance: CTI>600V, GWIT 850°C, maintaining liquidity

6.3 Alternating Balance Scenarios

PVC wire insulation material (UL standard)

3.5 Hydration type scheme:

-Advantage: Burning through UL 1685 vertical cable bundle

-Addition amount: 18 phr+ Sb₂O₃ 6 phr

Water free solution:

-Advantage: Elongation rate remains>70% after long-term thermal aging (136°C)

-Addition amount: 15 phr+10 phr of phosphorus based flame retardant

Conclusion:

The selection of 3.5 hydrated and anhydrous zinc borate is not simply a performance evaluation, but a systematic optimization of flame retardant requirements, processing constraints, and cost structure. The key decision-making factors should follow the following priorities:

1. Temperature matching principle: The relative position between processing temperature and dehydration temperature determines the basic selection

2. Principle of complementary mechanism: The hydrated type provides multi-stage protection, while the anhydrous type focuses on high-temperature barriers

3. Performance cost ratio: Hydrated polymers provide better overall cost-effectiveness in most polymers

4. Regulatory driven: increasingly strict requirements for low smoke and low toxicity tend to favor hydration based solutions