Innovative elastomeric yarns obtained from poly(ether-ester) staple fiber and its blends with other fibers by ring and compact spinning: Fabrication and mechanical properties

2.1 Raw materials

The precise chemical nomenclature of PEE utilized in this investigation is block-copolymer poly (butylene terephthalate) (PBT)-block-(tetramethylene oxide), with the chips sourced from First Fiber Co. Limited. In the chemical structure of this thermoplastic elastomer (PEE1), the hard segment is PBT, while the soft segment is poly (oxytetramethylene), maintaining a weight ratio of 80/20. This material is designated as PEE1 in this analysis. The intrinsic viscosity of PEE1 is 1.3 dL/g at 25 ± 0.01°C in a 1:1 mixture of phenol and tetrachloroethane, with a melting point of 214.7°C and a density of 1.23 g/cm³. The other raw material for the polymer chips is PBT, obtained as an industrial product from Jiangsu Yi Zhen Co., with an intrinsic viscosity of 0.98 dL/g at 25 ± 0.010°C in a 1:1 mixture of phenol and tetrachloroethane. The combination of PEE1 and PBT also demonstrates rubber-like elasticity and is referred to as PEE2 in this research. The staple fibers of cotton and PET were purchased from the commodity market, and all materials are considered as acceptable products.

2.2 Staple fibers: PEE1 and PEE2

The staple fibers PEE1 and PEE2 were produced utilizing the aforementioned chips of PEE1 and PEE2 through the melt spinning process at the Fiber Materials Innovation Center located at Chengdu Textile College. The fabrication conditions and technological parameters were consistent with those outlined in the study by Haile et al. [7].

2.3 Production of elastomeric yarns by ring and compact spinning

The elastomeric yarns are manufactured using staple fibers from PEE1 and PEE2, along with their blends with PET staple fiber and cotton, following the specified technological process and parameters.

2.4 Testing equipment and methods

3.1 Basic properties of raw fibers

The fundamental characteristics of raw fibers utilized in the manufacturing of both homogeneous and blended elastomeric yarns are presented in Table 1.

3.2 Basic properties of obtained elastomeric yarns

Figure 1 illustrates the electron microscope observations of typical elastomeric yarns, showcasing the true twist helical and parallel arrangement of staple fibers, similar to other staple spun yarns [10]. This arrangement is a natural consequence of the ring and compact spinning processes. Ring spinning, being the predominant technology for producing staple yarns, has been modified in compact spinning. In the ring spinning process, staple fibers are intertwined to create a robust yarn. The operation of ring spinning machines involves three key steps: (1) Attenuation (or drafting) of roving or sliver to achieve the desired linear density; (2) insertion of twist to form a cohesive yarn; (3) winding the yarn onto a suitable package. This study successfully produced elastomeric yarns from elastomeric staple fibers and their blends with cotton and PET staples using ring and compact spinning techniques.

Figure 1

Electron microscope photographs of typical representatives of elastomeric yarns: (a) PEE1 and (b) PEE1/COT (50/50) by ring spinning; (c) PEE2/PET (50/50) and (d) PEE2 compact spinning.

The yarns examined in this study were subjected to a “Z” twist, as indicated in Figure 1 and confirmed through twist testing. Twist is typically quantified as the number of turns per unit length, expressed in terms of turns per meter or turns per inch. Table 2 displays the averaged results pertaining to the twist of elastic yarns.

The elastomeric yarns obtained, similar to other types of yarn, exhibited significant irregularities during the experimental tensile testing, as illustrated in Figures 2 and 3, which demonstrate considerable variation. To derive the “average load-elongation curves” or standard load-elongation curves [8], these data must be averaged through appropriate calculations. The curves generated from this analysis are also indicated in the two figures, confirming their representativeness. All tensile test data are included in this calculation, and the resulting average load-elongation curves are summarized in Figures 4 and 5. As depicted in Figure 4, the stress-strain characteristics of PEE1 yarn are quite similar to those of PEE2 yarn, particularly during the initial and primary stages of the curve, which aligns with the fact that both were made from homogeneous elastomeric staple fibers with fundamental properties.

Figure 2

Experimental load–elongation curves for PEE1 elastomeric yarn and its “average load-elongation curve” (the red curve).

Figure 3

Experimental load–elongation curves for PEE1/COT 50/50 elastomeric yarn and its “average load-elongation curve” (the red curve).

Figure 4

Average load–elongation curves of pure PEE and PEE/PET yarns.

Figure 5

Average load–elongation curves of PEE/COT yarns.

The analysis of the load–elongation curves presented in Figure 4 indicates that the homogeneous elastomeric yarn exhibits the lowest modulus and tenacity, while demonstrating the highest breaking elongation. In contrast, PET yarn displays the highest modulus and tenacity, accompanied by the lowest breaking elongation. As the proportion of PET in the blend increases, the curve progressively shifts from the characteristics of PET to those of the homogeneous elastomeric yarn, reflecting a clear pattern that aligns with the conventional theoretical framework of blend spinning, which will be elaborated upon subsequently. However, this regularity is not observed in the elastomeric staple fiber-cotton system. Figure 5 illustrates that within the load elongation curve group of various elastomeric yarns, the homogeneous elastomeric yarn PEE1 occupies a median position. Upon incorporating cotton into the blend, the curve does not exhibit a straightforward upward or downward trend; it is only when the cotton weight percentage exceeds 50% that a distinct inclination towards the curve of elastomeric yarn characterized by high tenacity, high modulus, and low breaking elongation becomes evident. For the two types of blended elastomeric yarns with lower cotton content, their modulus and tenacity are reduced compared to homogeneous elastomeric yarns, while their breaking elongation is increased. This observation poses a challenge to traditional blended spinning theory and warrants further investigation. We will explore potential explanations for this phenomenon through an analysis of the mechanics involved in yarn formation.

The fundamental mechanical characteristics are detailed in Table 3. Alongside Figures 4 and 5, this table provides a comprehensive overview of the essential mechanical performance of the elastomeric yarns produced in this study.

3.3 Effect of blending ratio on mechanical properties of blended elastomeric yarns

The blending of various fibers enables us to achieve optimal yarn properties for their intended applications. Additionally, the blending technique can enhance the processability of staple fibers during spinning. Given the significant elasticity of elastomeric staple fibers, they can pose considerable challenges in the spinning process, necessitating greater focus on the blending system. The impact of blending is reflected in the changes in properties of elastomeric yarns corresponding to the blending ratio.

In this study, PET staple fibers or cotton were blended with the elastomeric staple fibers PEE1 or PEE2 to create blended elastomeric yarns. The variation in mechanical properties between the staple fibers PEE1 and PEE2 is significantly less than that observed between the traditional staple fibers (PET or cotton) and the elastomeric staple fibers. Consequently, in the subsequent graphs (Figures 6–11) that illustrate the changes in properties with varying blend compositions, the elastomeric fibers PEE1 and PEE2 are not differentiated, and the term PEE is used collectively for both.

Figure 6

Tenacity vs weight percentage for blended yarns PEE/COT.

Figure 7

Tenacity vs weight percentage for blended yarns PEE-PET.

Figure 8

Modulus vs weight percentage for blended yarns PEE/COT.

Figure 9

Modulus vs weight percentage for blended yarns PEE-PET.

Figure 10

Breaking elongation vs weight percentage for blended yarns PEE/COT.

Figure 11

Breaking elongation vs weight percentage for blended yarns PEE-PET.

The mechanical characteristics of yarns present a multifaceted challenge, influenced by the properties of the constituent fibers and the yarn structure itself. The latter is determined by the type of spinning machine and its operational conditions, as the force and thermal histories that the fiber bundle undergoes during yarn formation are critical factors in shaping the yarn structure. The inherent complexity associated with high elastic deformation in elastomeric fibers and yarns complicates the establishment of straightforward and definitive guidelines. Consequently, this study offers only a phenomenological overview of the spinning process for elastomeric staple fibers utilizing ring and compact spinning machines.

The process of spinning multi-component staple fibers is notably more intricate and challenging. One of the earliest instances of semi-quantitative research is the relationship between the tenacity and composition of nylon and cotton staple fibers in bicomponent ring spinning. Coplan [12] demonstrated that the tenacity-composition curve of the blended yarn can be derived from that of the mixed fibers by taking into account three diminishing factors: bundle strength, Peirce’s “Weak Link,” and a twisted angle of 20 degrees. His groundbreaking research significantly enhances our comprehension of the tensile properties of blended yarns (refer also [13]).

An analogous analysis can be performed and comprehended for the two systems examined in this study, namely, PEE-cotton and PEE-PET staple, as illustrated in Figures 6 and 7, where the dashed line at the top signifies the reference standard connecting the tenacity of the two staples in accordance with Coplan’s treatment.

The variations in elastic modulus with respect to composition for these two systems are illustrated in Figures 8 and 9.

As per a simplified theory regarding staple fiber yarn [10], if the stress–strain curve of the fiber adheres to Hooke’s law, the tenacity and modulus of yarn produced through ring spinning are related to the tenacity and modulus of individual fibers in the following manner:

where σ and E are the tenacity and modulus, subscript y and f represent yarn and fiber, respectively, α is the twist angle, and k is expressed by the following equation:

where Lf is the fiber length, a is the fiber radius, Q is the migration period, and μ is the coefficient of friction.

The analysis of equation (1) indicates that the ratio of tenacity or modulus is typically less than 1. However, the friction coefficient within the equation poses measurement challenges and lacks consistency, complicating the calculation of an accurate value. Nevertheless, a comparison of the data presented in Tables 1 and 3 demonstrates that for the yarns composed of homogeneous fibers examined in this study, specifically PEE1, PEE2, PET, and cotton yarns, the tenacity and modulus of the yarns are indeed lower than those of the corresponding fibers. The sole exception to this observation is that the modulus of homogeneous PEE yarns exceeds that of the corresponding PEE fibers. This indicates that the experimentally established modulus relationship between homogeneous elastomeric yarn and its corresponding elastomeric fiber does not align with the prevailing theoretical framework.

The relationship between breaking elongation and composition for the two systems of elastomeric yarns is illustrated in Figures 10 and 11. In comparison to the homogeneous elastomeric yarn of the PEE-cotton system, the introduction of the second component (cotton), which has a lower breaking elongation, did not result in a decrease in the breaking elongation of the blended elastomeric yarns (Figure 10). Conversely, when the cotton content exceeded 50%, the breaking elongation of the blended yarns experienced a significant decline, aligning with the lower values characteristic of cotton yarn. This finding is noteworthy from a practical perspective, as it indicates that a cotton percentage below 50% is necessary to maintain adequate breaking elongation for elastomeric yarns. Additionally, the trend observed in the breaking elongation vs composition for the PEE-PET system markedly differed from that of the PEE-cotton system. These observations present complexities that are not easily explicable and warrant further investigation in future research.

3.4 Mechanical principle of elastomeric yarn formation

The compact spinning machine represents an enhanced version of the traditional ring spinning machine. Its primary innovation lies in the incorporation of a gathering device at the output end of the front roller, which allows for the yarn to be collected and twisted prior to output. This process effectively minimizes or even eliminates the twisting triangle area, resulting in a denser finished yarn. Nevertheless, the underlying principles governing both types of yarn spinning remain consistent and can be examined in conjunction. In the theoretical exploration of the ring spinning machine’s operational principles, the mechanics of the balloon within the spinning line are critically analyzed, focusing on the force and energy balance of a yarn element. A comprehensive mechanical analysis typically considers the centrifugal force acting on the yarn, the air friction affecting its movement, and the equilibrium of yarn tension, while often overlooking potential energy and the energy fluctuations resulting from balloon instability.

It has been observed that for cotton yarns ranging from 8 to 38 tex, a spindle speed of 12,000 rpm results in a tension range of 20–40 cN [15]. This study examines the variations in tension of elastomeric yarn as the spindle speed of the spinning machine is altered, as illustrated in Figure 12.

Figure 12

Experimental dependence of surface speed (the top curve) and tension (the bottom curve) on theoretical spindle speed for elastomeric yarn.

In accordance with Lord’s concepts and the data presented in Figure 12, we can create a schematic representation of the tension variations in the elastomeric yarn during the ring spinning process over the balloon line, which will aid in comprehending the spinning dynamics of elastomeric yarns. Observing from the perspective of yarn flow, beginning at point 1 and progressing through points 2 and 3 to point 4, which corresponds to the upper, middle, and lower sections of the spinning balloon illustrated in Figure 13. Upon reaching point 4, the winding point of the yarn cop, the tension in the yarn attains its peak value.

Figure 13

Schematic diagram of a typical ring spinning machine (based on Ref. [14]).

In the tension distribution of the spinning balloon [16], there are two leaps at positions 2 (guide) and 3 (traveler) in Figure 14. Both of them are related to the solid friction of the yarn with spinning machine parts, which are expressed mathematically by Amontons’ law [4] as

where F is the frictional force acting parallel to the surface in a direction opposing relative movement, µ is the coefficient of friction, and N is the force normal to the surfaces in contact. When a yarn passes round a guide, as passing positions 2 and 3, its tension must be increased by an amount necessary to overcome the frictional resistance, following an approximate equation derived from (3)

where T ₂ is the leaving tension, T ₁ is the incoming tension, and θ is the angle of contact. For example, the position of the balloon at Tensions 2 or 3 has very different contact angles around the guide in Figure 13, which leads to a great difference in friction force and can be regarded as a periodic fluctuation.

Figure 14

Schematic distribution of mean tension of yarn in yarn flow direction: 1. drafting roll, 2. guide, 3. traveler, and 4. winding point at the cop.

The mechanical examination of the aforementioned spinning balloon serves merely as a simplified representation, whereas the actual tension is dynamic and subject to fluctuations. The tension experienced by the yarn at various positions is in a constant state of change, characterized by both periodic and random variations [16]. Additionally, the yarn on the spinning machine is influenced by a specific torque due to the rotation of the balloon, leading to genuine twisting. Consequently, the staple fiber bundles within the yarn create a parallel spiral arrangement that is twisted. During the formation of the yarn, the tension is maintained through the friction among the fibers, ensuring that each fiber remains in a state of straight force, secured by the twisting process. It is important to note that in this state of stress, significant differences in modulus result in substantial tensile deformation of the elastomeric staple fibers, while the other components experience only minor tensile deformation. Upon the removal of external tension, the internal dimensions of the yarn also experience stress relaxation, reverting to their original length. As the parallel mixed fiber bundle contracts, the standard staple fibers initially return to their original length, whereas the elastomeric staple fibers retain some elongation and require further relaxation. As the elastomeric staple fibers continue to contract, the other components must bend until the elastomeric fibers return to their original length, thereby preserving the configuration of the yarn.

The tension in blended elastic yarn operates in a manner opposite to the previously described process: initially, ordinary fibers are aligned solely through winding, requiring minimal force. The primary resistance to external forces in fiber bundles is provided by elastomeric fibers. As stretching continues and ordinary fibers become straightened, their high modulus enables them to contribute to the resistance of the fiber bundles against external forces. This approach effectively elucidates why blended elastic yarns retain significant elasticity. In summary, based on the mechanical principles of yarn formation, the structure closely resembles that of elastane-covered yarn when elastomeric fibers are predominant. Conversely, a higher weight percentage of cotton or PET fibers results in a traditional blended yarn’s hybrid structure. Therefore, the structural model’s predictions align with the mechanical behavior observed in elastomeric yarns, although further experimental validation is required.

3.5 Elastic recovery of elastomeric yarns

Table 4 presents the experimental results of elastic recovery for certain samples characterized by relatively high breaking elongation, which serve as typical representatives.