Abstract:

Objective Firefighters fighting fire disasters and accidents are facing severe environmental and operational challenges, and the performance of firefighters' clothing is vital. In parallel to the high level performance against fire threat, the comfort of firefighters' clothing is also an important requirement. The purpose of this study is to develop knitted underwear fabrics with comfort for firefighters.

Method Four meta-aramid blended yarns with different components were used as raw materials. Firstly, the basic properties such as linear density, twist, hairiness, evenness and breaking strength of the yarn were tested. Subsequently, the afterburning performance of the yarns was evaluated and the scanning electron microscopy (SEM) images before and after the combustion were analyzed. The yarn was blended with spandex of different thicknesses to prepare nine aramid blended knitted fire-fighting underwear fabrics. The flame retardant performance was the first important index, and then the thickness, surface density, vertical and horizontal density, air permeability, moisture permeability, breaking strength and elongation at break of the nine fabrics were tested. Based on the test results, the influences of yarn composition and spandex content on the flame retardancy, comfort and mechanical properties of the fabric were discussed, and the influence of spandex content on the porosity of the fabric was analyzed.

Results Z-twisted yarns were produced from blends of meta-aramid, flame-retardant (FR) viscose, FR modacrylic, and conductive fiber, yielding four yarn types with distinct blend ratios. Yarn A was 100% meta-aramid; yarn C was a 60%/38%/2% meta-aramid/FR viscose/conductive blend; yarn D was a 43%/30%/25%/2% meta-aramid/FR viscose/FR modacrylic/conductive blend. Among them, yarn D had the highest fineness (19.7 tex), while yarn C exhibited the highest twist (964 twists/m) and the lowest hairiness, indicating a compact yarn structure. Tensile tests revealed that specific tenacity and modulus increased with meta-aramid content, with yarn A achieving the highest values, consistent with the high strength of aramid fibers. However, absolute breaking force was governed primarily by yarn thickness, since thicker yarns contain more fibers to share the load; therefore, yarn D displayed the highest breaking force. Elongation at break showed no clear dependence on aramid content. As expected from its composition, yarn A demonstrated the best flame retardancy. These yarns were woven into nine fabrics using plain, rib 1+1, twill, and fish-scale loop weaves. Fabric 5^# recorded the shortest warp and weft damage lengths (22 mm and 27 mm), indicating the best flame retardancy. Fabric 6^# showed the highest air permeability (1 236 mm/s), and fabric 1^# showed the highest moisture permeability (7 088 g/(m²·d); both advantages can be attributed to their lower spandex content, which likely created a more open pore network for air and moisture transport. Regarding mechanical properties, the highest warp breaking strength was found in fabric 4^#, the highest weft breaking strength in fabric 9^#, and the highest elongation at break in fabric 5^#. Inorder to examine the influence of spandex, fabrics 1^# (1.67 tex, 4.5% spandex), 2^# (3.33 tex, 8.8%), and 3^# (3.33 tex, 10%) were compared. Fabric 2^# exhibited the best flame retardancy, suggesting an optimal combination of spandex fineness and content exists; both insufficient and excessive spandex contents compromised flame retardancy. These results highlight the importance of carefully tailoring spandex parameters to achieve a balance between flame retardancy and wear comfort in protective fabrics.

Conclusion Among the nine fabrics developed, the rib 1+1 knitted fabric (fabric 5^#) produced from 60% meta-aramid / 38% flame-retardant viscose / 2% conductive fiber blended yarn (yarn C) with 6.9% 4.44 tex spandex exhibited the best-balanced overall performance. The warp- and weft-direction damage lengths were 22 mm and 27 mm, respectively. Air permeability reached 929 mm/s and moisture permeability 5 688 g/(m²·d). Mechanically, the warp and weft breaking strengths were 347 N and 123 N, with elongation at break of 378% (warp) and 1 386% (weft), respectively, satisfying the dual demands of thermal protection and body mobility for firefighter inner layers. Compared with conventional 50% meta-aramid/50% flame-retardant viscose inner fabrics, fabric 5# has a reduced viscose content, aligning better with green manufacturing. Analysis of plain plated knitted fabrics (fabrics 1^#-3^#) further indicated that spandex incorporation has an optimal range: spandex content below 8.8% enhanced flame retardancy by densifying the structure, whereas higher content diminished it, revealing a non-monotonic relationship between spandex ratio and combustion behavior.

Key words: aramid, knitted underwear fabric, flame retardant property, air permeability, moisture permeability