Why does PVC stretch fabric eventually break up and flake?

Main topic: Science

Other topics: Physics

Short answer:

• PVC fabrics also known as Synthetic leather are not as durable and elastic as genuine leather, and when stretched excessively, the PU coating starts to tear off.
• Due to excessive use or inappropriate storage circumstances, such as unusually cold temperature changes over the winter, they deteriorate early.

PVC Stretch Fabric

Structure and composition of PVC stretch fabric

Synthetic leather consists of a topcoat, a dense layer, a foamed middle layer, and a textile backing. Frequently, polyester fabrics covered with PVC or polyurethane films are used, rendering them entirely fossil-based. The appearance of the surface may be made to resemble leather by embossing a grain structure. 

Theories of PVC stretch fabric deterioration

Researchers from different disciplines have collaborated to develop a workable theoretical model that can accurately predict the tearing strength of coated fabrics. This is done in an effort to circumvent the need for an infinite number of experiments that must be conducted in order to replicate every conceivable scenario that may arise in the widely known fracture theory models that are utilized for coated textiles, including the stress field consideration technique, Thiele's empirical formula, the stress intensity factor theory, and the Hedgepeth stress concentration factor theory.

• The fibers are responsible for axial loads, and it was supposed that the matrix was solely accountable for shear loads. This led to the formation of a stress concentration factor at the crack tip, which may be used to make an accurate prediction about the tearing strength of the material.

• The impact of tearing speed, yarn characteristics, and weaving structure have on the tearing of a single tongue, and developed a spring model to estimate the tearing strength of a single tongue.

• The yarns in the drawing direction get started to be stretched progressively from crimp to straight when the external stress is applied, and the form of the crack can be changed from an initial closed condition to an elliptical during this first loading stage.

• A del-zone occurs when an increase in stress causes the warp yarns that are close to the crack tip to begin sliding across the transverse yarns and stripping off the coating. In this approach, a region with the rough shape of a triangle that is referred to as the del-zone is generated. An even greater increase in the load would result in an even greater number of yarns being involved in the del-zone.

• The crack begins to propagate when the strain on the warp yarn that is closest to the fracture in the del-zone increases to its elongation at the breaking. This is when the crack enters the failure stage. The specimens displayed two primary types of failure mechanisms. When fewer yarns are removed, the fracture spreads more quickly when the first one breaks, which results in an immediate breakdown of the whole specimen. The associated stress-displacement curve has very little evidence of a declining portion. Tear propagation happens alternatively at both tips of the crack as additional yarns are cut off, and new del-zones are created continually as a result of this process. As a result, the specimen is able to continue bearing the external load, and the stress-displacement curve shows a lengthy declining portion with a zigzag fluctuation; this form of failure is referred to as the progressive failure mode.

Load Ratio theory for PVC stretch fabric crack properties

As long as the starting fracture orientation is less than 45 degrees, the crack will continue to propagate along the warp direction regardless of the load ratio that is being applied. When the orientation of the fracture is larger than 45 degrees, the crack begins to grow along the weft direction. Nevertheless, there are two exceptional circumstances known as 2:1-0 and 1:2-90, in which the force that is perpendicular to the progression of a fracture is always only half the magnitude of the load that is applied in another direction during the whole ripping process. Therefore, as the external load increases, the yarns that are parallel to the crack propagation will reach their tensile strength first. This will cause a fracture to occur at the root of the extended arm, while the load in the opposite direction is still below the critical tearing load that is necessary to stimulate crack propagation.

Effect of number of cutting-off yarns

The number of cutting-off yarns that are perpendicular to the crack propagation axis is the primary factor that determines the tearing strength of biaxial specimens; however, the yarns that are loaded in the opposite direction have very little impact on the tearing resistance of the material. As a result, this demonstrates that the notions of crack orientation and crack length may also be substituted by n_c in biaxial central tearing specimens, to a lesser degree.

Distribution of the strain field all the way throughout the fracture section

The DIC equipment is used to acquire the strain distribution of the yarns that is present over the fracture section at the time when the crack is propagating. During the tearing process, the strain that is placed on the yarns when various external loads are applied to specimens with a ratio of 1:1-0-13 and 1:1-90-12. The strain in the del-zone is visibly greater than the distance from the commencement of loading to the end of ripping failure. This is the case because the del-zone is closer to the point of failure. No matter how many yarns are removed, it is possible to see that when the crack is about to extend, the strain of the first yarn at the crack tip of the specimens in the warp and weft directions is close to the break strain of the weft and warp yarns when they are subjected to uniaxial tension. This is something that can be observed.