The role of zinc borate in flame-retardant ABS formulations

Zinc borate has flame-retardant and synergistic effects in various plastics and resins, and it also increases the oxygen index and promotes char formation for flame retardancy. Its effects on the impact and tensile properties of materials vary depending on the formulation. What are the roles of zinc borate in flame-retardant ABS formulations? We will analyze this below using three flame-retardant ABS formulations.

I. Raw Materials and Formulations

The following three formulations are listed in order: Formulation 1 / Formulation 2 / Formulation 3

ABS: 100/100/100

ABS high-adhesion powder: --/--/10

Phenolic epoxy resin (NE): 8/8/8

NE/Organo-montmorillonite (OMMT): 0.5/0.5/0.5

Nanocomposite bisphenol A di(phenyl) phosphate (BOP): 12/12/12

K5609 coupling agent: ---0.5/0.5

Zinc borate: ---3/3

Other additives: Appropriate amount/Appropriate amount/Appropriate amount

II. Preparation Method

Preparation of organo-montmorillonite: Add appropriate amounts of Na-MMT and water to a three-necked flask, stir and heat to 80℃. After Na-MMT is fully dispersed, add a certain amount of organic intercalating agent OTAC, and stir at a constant temperature for 15 hours. The material was discharged, filtered, and washed until no white precipitate was detected in the washing water using AgNO3 solution. It was then vacuum dried at 80℃ for 24 hours, ground into powder, and sieved to obtain organomontmorillonite (OMMT). Preparation of NF/OMMT nanocomposite: Appropriate amounts of phenolic epoxy resin and organomontmorillonite were added to a flask, heated to 70℃, and stirred. Then, an appropriate amount of BDP was added and stirred to obtain a composite flame retardant, denoted as NE/OMMT-BDP. Preparation of flame-retardant ABS: ABS was plasticized on a two-roll mill at 170~190℃, and then pre-mixed flame-retardant ingredients were added. After uniform mixing, the mixture was discharged onto a flatbed press for tableting and sample preparation.

III. Performance and Effects

1. Formulation 1

①The synergistic flame-retardant effect of BD and phenolic epoxy resin within the studied range was significantly better than that of triphenyl phosphate (TPP) and phenolic epoxy resin: the oxygen index increased, and the heat loss temperature increased by approximately 50~100℃ at the same mass retention rate. When the total amount of the flame-retardant composite is 15%, the optimal mass ratio of NE to BDP is 2:3.

② An appropriate amount of organomontmorillonite has a certain synergistic flame-retardant effect with the BDP/NE flame-retardant system. When the mass fractions of NE, OMMT, and BDP are 6%, 0.5%, and 9%, respectively, the LOI, peak heat release rate (pHRR), and average heat release rate (mHRR) of the flame-retardant ABS are 39.0, 508 kW/m², and 220.4 kW/m², respectively, which is 10% higher than that of the flame-retardant ARS without OMMT, while PHRR and mHRR are reduced by 11% and 8.7%, respectively.

③ Within the studied range, the tensile strength increases slightly with increasing NE content; with increasing OMMT content, the impact strength of the material decreases slowly, while the tensile strength remains relatively unchanged.

2. Formulation 2

① Coupling agent KH560 improves the flame-retardant and impact properties of the material better than KH550. When 0.5% KH560 is added, the coupling agent exhibits a certain synergistic effect with the NE/OMMT-BDP flame-retardant ABS system, resulting in a maximum oxygen index. The two coupling agents show consistent trends in their impact strength, both reaching a maximum at a 0.5% addition level, with KH560 showing a significantly higher modification value than KH550.

② Zinc borate also exhibits a synergistic effect with the NE/OMMT-BDP flame-retardant ABS system within a certain range. As the zinc borate (ZB) dosage increases, the oxygen index first increases and then decreases. At a dosage of 4%, the LOI, peak pHRR, and average heat release rate (mHRR) of the flame-retardant ABS are 43.5, 350 kW/m², and 215.0 kW/m², respectively. Compared to the absence of zinc borate (ZB), the LOI increases by 12%, while pHRR and mHRR decrease by 36% and 17.0%, respectively.

③ Increasing the amount of zinc borate significantly decreases the impact strength, while the tensile strength initially increases and then decreases. The highest tensile strength, reaching 37.4 MPa, is achieved when the zinc borate content is 3%.

3. Formulation 3

The toughening effect of the ABS high-polymer powder-modified phenolic epoxy resin (NE)/organo-montmorillonite (OMMT) nanocomposite and bisphenol A di(diphenyl) phosphate (BDP) flame-retardant ABS system is superior to that of SBS. The high-polymer powder has little effect on the oxygen index and tensile strength. When the high-polymer powder content is 10%, an oxygen index of 42.5%, an impact strength of 9.60 kJ/m², and a tensile strength of 35.62 MPa can be obtained, resulting in a halogen-free flame-retardant ABS with good overall performance.

If the formulation is appropriate, zinc borate has a synergistic effect with phosphorus-nitrogen, bromine, and phosphate ester flame retardants. Appropriately increasing the amount of zinc borate improves the oxygen index and flame retardant rating of the material, but has little effect on impact, tensile, and flexural properties.