| Personal protective equipment usually comprises of impermeable polyurethane or EPS foam as cushioning material,of which the comfort property is poor.Spacer fabrics which have a sandwich structure are formed in a single knitting process without any additional joining treatment.It consists of two separate multifilament outer fabric layers connecting by arrays of monofilaments,which has excellent compression resistance,energy absorption,permeability and structural variations.The use of spacer fabrics as cushioning material has attracted increasingly attention.This thesis presents an experimental study on the relationships between the cushioning properties and structural parameters of weft-knitted spacer fabrics.The results from this study provide an experimental foundation to engineer the cushioning performance of spacer fabrics.This study also realizes the integration of spacer structure and shaped knitting,which has practical significance for the development of seamless helmet.Sixteen weft-knitted spacer fabrics with different structural parameters,including four spacer yarn patterns,three spacer monofilament diameters,and three spacer yarn densities,were knitted on a computerized flat knitting machine.Since the gap between the front and back needle beds of the machine is fixed and short,outer layers of all spacer fabrics were knitted with two nylon/spandex(70D/30D)covered yarns to obtain thicker fabrics by heat treatment.It was found that spacer fabrics of different thicknesses can be produced on one computerized flat-knitting machine with high-shrinkage covered yarns by adjusting spacer yarn span distance,diameter and density.Spacer fabrics made with higher loop densities and larger areal density result in poor air permeability.The cushioning properties of the spacer fabrics were tested on an INSTRON 5969 device and analyzed in terms of compression stress–strain curves,energy absorption,and compression resilience.It was found that multifilament fineness,spacer monofilament diameter and knit pattern should be matching in order to form effective binding between outer layers and spacer monofilaments.The results also showed that spacer fabrics knitted with shorter spacer yarn span distance,coarser monofilaments,and higher monofilament density have better compression resistance and absorption energy but inferior compression resilience if their binding structures are effective.The cushioning properties of laminated spacer fabrics were also investigated in this study.The results showed that the compression process of multilayered fabric is similar to that of single-layer fabric,which can be divided into four different stages,i.e.,initial,linear elasticity,plateau,and densification.When spacer fabrics are laminated,the stiffness is much smaller than that of single-layer fabric.In plateau stage,the compression curve of double-layer fabric has 1–2 fluctuations,while the triple-layered one has 2–3 fluctuations.At the strain of 65%,energy absorption increases with number of layers,but the energy absorption per unit mass and per unit volume decreases with the increase of layers.Under the same load,the energy absorption of triple-layered spacer fabric is the maximum,and the energy absorption of single-layer one is the minimum.Twelve hemispherical spacer fabrics were designed and knitted with different structural parameters,including spacer yarn patterns,total needle numbers and remaining needle numbers.This study provides solutions to the problems encountered during the knitting process,including raveling,holes and loop dropping.The relationship between the knitting parameters and the spatial geometry of the hemispherical spacer fabrics has been identified.It was found that the number of remaining needles affects the spatial shape of spherical fabric,while the total needle numbers affect the size.The increase of span needle distance leads to large volume and long arc length of spacer fabric,but small angle. |
