3D printed sandwich materials filled with hydrogels for extremely low heat release rate

Kaolin-embedded cellulose hydrogel with tunable properties as a green fire retardant

This study reports the synthesis of methylene bisacrylamide (MBA) crosslinked cellulose-kaolin (CMK) composite hydrogels. The internal structures of hydrogels were deduced using network parameters, viz. elastic modulus, average molecular weight, mesh size, and effective crosslink. Meanwhile, combustion behavior was investigated using the cone calorimeter test (CCT), limiting oxygen index (LOI) test, vertical flammability test (VFT), and open fire test (OFT). Our results revealed that kaolin addition improves the fire retardancy of hydrogels but reduces their swelling ability. Hydrogel having cellulose to MBA ratio of 1:2 and 2 % w/v kaolin (CM2K2) produced 63 % wt. char residue and the hydrogel-coated cotton fabric exhibited the lowest heat release rate (HRR) of 26.60 kJ/m² and total heat release (THR) of 0.9 MJ/m². The LOI of the cotton fabric surged from 20 % to 34.37 % after hydrogel coating. Kinetic analysis using the isoconversional model yielded the highest activation energy (216 kJ/mol) for the CM2K2 hydrogel, corroborating the increased LOI after kaolin addition. VFT and OFT validated the delay in the burning process and the formation of a char layer, which protected the underlying layer of cotton from burning. Overall, cellulose-kaolin hydrogels developed in this study are effective green fire retardant coatings for flammable materials.

Co-curing preparation of flame retardant and smoke-suppressive epoxy resin with a novel phosphorus-containing ionic liquid

Phosphorus-containing ionic liquid derivatives have been proven to be effective flame retardants for epoxy resin (EP). Flame retardants can accelerate the curing process and improve flame retardancy and smoke suppression of EP composites, which is challenging. In this paper, a novel phosphorus-containing ionic liquid (TPP-PF₆) was synthesized and used both as a co-curing agent with 4,4'-diaminodiphenylmethane (DDM) and as a highly effective flame retardant for EP. It has been found that TPP-PF₆ was conducive to improve the char formation of EP to inhibit the smoke release at high temperatures. For EP/TPP-PF₆ composites, the flame-retardant performance was enhanced rapidly with the increase of TPP-PF₆. With only 2 wt% of TPP-PF₆, EP/2.0TPP-PF₆ reached a UL-94 V-0 rating and a limiting oxygen index of 30.3%. The peak heat release rate, total heat release, and total smoke production values of EP/2.0TPP-PF₆ were reduced by 36.32%, 45.81%, and 15.1% compared with those of pure EP, respectively. The thermal degradation products and flame retardant mechanism in gas and condensed phases were studied. It was found that TPP-PF₆ had flame retardant effect in the barrier effect of the condensed phase and the quenching effect of the gas phase. This work explores the high-efficiency flame retardant and smoke-suppressive structures with co-curing properties for EP, thus promoting the wide application of EP materials.

Combining Low-Emissivity Thin Coating and 3D-Printed Original Designs for Superior Fire-Protective Performance

Three-dimensional (3D) printing is a very flexible process to design various objects of original shapes. Previous works highlighted the preparation of new multimaterials composed of an original sandwich structure made of the ethylene vinyl acetate copolymer containing 30 wt % of aluminum trihydroxide in which a hydrogel phase made of agar and vermiculite was incorporated. This original material revealed an extremely low heat release rate (HRR) (with a reduction of 86 and 64% with regard to the peak of the HRR and total heat release, respectively, when compared to the same sample without hydrogel filling) during its heat exposure at 50 kW/m² according to the mass loss cone calorimetry test. However, the time to ignition (TTI) of this material was not improved. This work consequently focuses on delaying the time to ignition of this hydrogel sandwich 3D-printed multimaterial. Solution consists in depositing by pulsed DC magnetron sputtering a low-emissivity thin coating on the exposed skin surface. This coating reflects most of the infrared rays responsible for heat absorption and thus delays the ignition of the underlying material. The thermal resistance performances of this coated sandwich 3D-printed multimaterial were evaluated, and a mechanism of action was proposed to explain the dramatic enhancement of the properties.