Morphology and Structure of Constituent Fibres in Thermally Bonded Nonwovens

Abstract

Summary: A new process has been developed to produce three‐dimensional nonwovens directly from staple fibres. In order to establish suitable windows of the process parameters to achieve high‐quality nonwoven products, the effects of thermal bonding temperature, dwell time and mould material on the morphology and structure of the fibre have been investigated using PP/PET bi‐component fibres. It was evident from both scanning electron microscope images and Raman spectra that thermal‐induced shrinkage of the PP sheath fibre occurred in the thermal bonding process, leading to deformation and cracking of the PP sheath and exposure of the PET core. X‐ray diffraction results revealed crystal imperfection and/or less ordered polymer chains, more γ‐form and thermal contraction of the crystal lattice for the PP sheath fibre, while birefringence measurements indicated that both the birefringence and the orientation factor for the PP fibre decreased after the thermal bonding process. The degrees of the thermal‐induced shrinkage increased, and the crystallinity, birefringence and orientation factor of the PP sheath fibre decreased with increasing thermal bonding temperature, dwell time and thermal conductivity of the mould material. All these can be attributed to the different levels of modification of chemical composition caused by thermal oxidative degradation and thermal‐induced relaxation of the orientation during the thermal bonding process.Changes of morphology and crystalline features of PP/PET fibre after thermal bonding process.imageChanges of morphology and crystalline features of PP/PET fibre after thermal bonding process.

Relationships Between Birefringence of Constituent Fibers and Properties of Thermally Bonded Nonwovens

“…higher degree of chain orientation, have substantially higher tensile strength but lower elongation 10. As aforementioned, in the previous study,6 it was found that the birefringence of the PP sheath fiber was closely correlated with the degree of the thermal‐induced shrinkage, crystallinity and orientation factor after the thermal‐bonding process. Therefore, the birefringence values of the fibers in the different samples are also listed in Table 2 as a measure to explore the relationship between the properties of the constituent fiber and those of the fabrics.…”

“…It is difficult to quantify the bonding features between the fibers, but it can be qualitatively seen that the bond between the fibers in the samples produced using a lower bonding temperature and/or shorter dwell time is weaker than that in the samples produced using a higher bonding temperature and/or longer dwell time, as shown in Figure 8. Therefore, although the higher birefringence shows that the fibers in the nonwoven samples produced using the shorter dwell time and lower bonding temperature have higher breaking force,6,10 the weak bonds between the fibers lead to the lower breaking force of the corresponding fabrics as shown in Figure 1, 2 and 5.…”

“…The properties of nonwoven fabrics, including their breaking strength and abrasion resistance, are significantly dependent on not only the choice of fiber, but also the construction of the fabric 8. Therefore, following the previous paper,6 this paper further presents the breaking strength and abrasion resistance of the 3D nonwoven samples produced using the new process. The objective of this paper is to explore and provide insight into the relationships between the mechanical properties of the nonwoven fabrics and the morphology, structure of the constituent fiber and the construction of the fabrics, and, hence establish suitable windows for the process parameters to achieve high‐quality 3D nonwoven fabrics.…”

“…Note that here the birefringence applies to the constituent fibers, and not to the thermally bonded fabrics. Details about the measurement of birefringence of the constituent fibers have been given in the previous paper,6 and are thus not repeated here.…”

“…Extensive studies have been carried out to understand the process‐structure‐property relationships for the thermally bonded nonwoven fabrics produced using the new process. In a previous paper,6 scanning electron microscope (SEM), Raman spectrum, X‐ray diffraction (XRD) and birefringence measurement with a polarizing microscope were used to characterize the morphologies, crystal structures, and chain orientation states of the as‐received fiber and the fibers in the thermally bonded nonwoven fabrics produced under different process conditions. Results obtained indicated that thermal‐induced shrinkage of the PP sheath fiber occurred during the thermal‐bonding process, leading to deformation and cracking of the PP sheath and exposure of the PET core.…”