What Is the Difference Between 2335 and Other Grades of Zinc Borate?

In halogen-free flame retardant systems, zinc borate, as an efficient multifunctional synergist, has become an indispensable component for improving overall performance. Among them, the "2335" brand (usually corresponding to the chemical formula 2ZnO·3B₂O₃·3.5H₂O) is widely recommended due to its unique performance balance. This article will systematically analyze the essential differences between 2335 grade and other mainstream grades of zinc borate from core academic dimensions such as crystalline chemistry, thermal decomposition behavior, surface morphology, and synergistic mechanisms, providing scientific basis for precise selection.

Ⅰ. The fundamental difference between crystal chemistry and composition is not just the number of water molecules

Zinc borate is a family of compounds with variable composition, and its core difference lies in the molar ratio of zinc oxide, boron trioxide, and crystal water, which directly determines its crystal structure, thermal stability, and reactivity.

Core difference analysis:

1. Strategic significance of crystal water content:

·ZB-237 (high hydration): With a crystal water content of up to 22%, it undergoes significant dehydration at lower temperatures (approximately 130 ° C). This makes it suitable as the main flame retardant filler in polymers with lower processing temperatures such as elastomers and PVC, exerting a significant effect by absorbing heat and releasing water vapor. However, at the processing temperature of engineering plastics such as nylon and PBT (>250 ° C), premature dehydration may occur, which may lead to product foaming and performance degradation.

·ZB-2335 (intermediate hydration): The key to its "performance equilibrium point" is its crystal water content of approximately 14.5%. Its dehydration starting temperature (~290 ° C) accurately matches the processing window and initial decomposition stage of most engineering plastics. The dehydration process is smooth, which can provide heat absorption cooling and gas-phase dilution in the early stage of combustion, while avoiding the risk of foaming during the processing.

·Low hydration/anhydrous varieties (such as ZB-112): have extremely high thermal stability (>350 ° C), mainly used for ultra-high temperature engineering plastics (such as PEEK, PPS) or precision electronic applications that require avoiding any water release.

2. Functional orientation of zinc/boron ratio:

·High zinc/boron ratio (such as 4:1 variety): focuses on providing a zinc source, which has a stronger ability to promote carbonization and catalyze cross-linking reactions in the condensed phase.

·ZB-2335 (2:3 ratio): The ratio of zinc to boron is more balanced, allowing it to have both strong ability to promote coalescing and moderate ability to capture gas-phase free radicals (boric acid can be converted into a glassy coating and release free radicals to quench species), achieving maximum synergistic effect.

Ⅱ. Key differences between thermal behavior and decomposition kinetics

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves intuitively reveal the behavioral differences of different grades.

·The thermal decomposition characteristics of ZB-2335: Its TGA curve shows a wide and gentle weight loss plateau in the range of 290-450 ° C, and the weight loss is consistent with the theoretical crystal water content. This "controlled, staged dehydration" behavior means that it can continue to function over a wide temperature range of polymer decomposition, providing "full range" protection.

·Compared to other grades:

·ZB-237: There is a severe and rapid weight loss peak at 130-200 ° C, which acts "early and quickly" and is misaligned with the decomposition window of engineering plastics.

·Low hydration grade: Delayed weight loss to higher temperatures may miss the critical flame retardant opportunity for early polymer decomposition.

Ⅲ.  Surface morphology

1. Particle size and distribution:

·General grades may use coarser particle sizes (D50>5 μ m) to reduce costs.

·High performance 2335 grades (such as optimized products) typically use micro refinement (D50: 1-3 μ m) and narrow particle size distribution processing. This brings two major advantages:

·Better dispersibility: reduces agglomeration in the polymer matrix, avoids becoming a stress concentration point, and causes less damage to mechanical properties (especially impact strength).

·Higher specific surface area: increases the contact area with flames and decomposition products, improves reaction efficiency, and has a more significant flame retardant synergistic effect at the same dosage.

2. Surface modification:

To improve compatibility with organic polymers (especially non-polar polyolefins) and prevent moisture absorption, high-end 2335 grades often undergo surface coating modification (such as using silane, titanate coupling agents, or fatty acids). This makes it:

·The moisture absorption rate is significantly reduced (from>2% to<0.5%), and the storage stability is better.

·The infiltration and dispersion in the matrix are better, the processing flowability is improved, and a higher filling amount can be allowed.

Ⅳ. Differences in synergistic mechanism and application performance

Due to the fundamental differences mentioned above, different grades play completely different roles and effects in actual flame retardant formulations.

1. Synergistic effect with ATH/MDH

·ZB-2335 is its' golden partner '. It can not only melt with ATH/MDH decomposition products (Al₂O₃, MgO) at high temperatures to form a strong and dense ceramic glass barrier, covering the surface of the carbon layer, greatly enhancing the insulation and oxygen barrier effect; Its moderate dehydration temperature can also fill the "temperature gap" between the ATH and MDH dehydration peaks, achieving seamless heat absorption coverage.

·Other grades: ZB-237 has poor synergistic effect due to premature dehydration; Low hydration grades result in delayed reaction initiation.

2. Synergistic effect with halogen/phosphorus flame retardants

·ZB-2335: It can effectively capture the hydrogen halides released by halogenated flame retardants, generating zinc/boron halides. These substances have stronger free radical quenching ability in the gas phase, achieving "solid halogen enhancement" while reducing corrosive smoke. For phosphorus based flame retardants, it can promote the formation of more stable pyrophosphate or borophosphate glass layers.

·The strength of the effect is directly related to the accessibility of zinc and boron sites on its active surface, so the 2335 grade with fine refinement performs better.

3. Impact on the comprehensive performance of the final product

By using optimized 2335 grade, it is usually possible to achieve the same flame retardant level (such as UL94 V-0):

·Reduce the total amount of main flame retardants (such as ATH) added to better maintain the mechanical properties and processing flowability of the matrix.

·Significantly improve CTI (compared to leakage trace index), as the ceramic insulation layer formed can effectively suppress the generation of carbide conductive paths, which is crucial for electronic and electrical components.

·Reduce smoke generation and burning droplets.

Ⅴ. Conclusion and Selection Guidelines

The core advantage of "2335" grade zinc borate (2ZnO·3B₂O₃·3.5H₂O) lies in its precise stoichiometric ratio, which achieves the optimal balance of thermal stability, reactivity, and multifunctionality.

It is neither as "impatient" as high hydration grades, nor as "slow" as low hydration grades, but has become a "precise response and versatile synergistic partner" throughout the entire life cycle of polymers from processing to combustion.

Selection decision suggestion:

1. For general engineering plastics (PA, PBT, PET) and high-performance halogen-free formulations (synergistic ATH/MDH): the preferred choice is the high-performance 2335 grade that has been surface modified and micronized. This is the stage where it maximizes its effectiveness.

2. For low processing temperature materials (such as PVC, rubber, EVA): ZB-237 can be considered to utilize its low-temperature dehydration characteristics.

3. For ultra-high temperature plastics (LCP, PPS, PEEK) or extreme moisture and heat resistant applications: low hydration/anhydrous zinc borate (such as ZB-112) should be turned to.

4. When pursuing the ultimate cost-effectiveness and having relaxed performance requirements, one can consider the general 2335 product with ordinary particle size or lower grade grades.

Therefore, when the industry generally recommends "2335", it is essentially recommending a zinc borate benchmark model that has been validated through engineering and achieves the optimal solution between performance, universality, and cost. Choosing it means choosing a reliable, efficient, and lowest risk synergistic solution.