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Wrinkle recovery test method

Wrinkle recovery test method

Wrinkles are ubiquitously common phenomena in natural world and are defined as ‘‘small furrows, ridges, or creases on a normally smooth surface, caused by crumpling, folding, or shrinking’’[1], and occur over a range of length scales and different types of substrates[1-3]. Although long being a research topic in textiles[4-8], it was not until more recent years, that an increasing interest has emerged on the physics of wrinkling [2, 3, 9, 10] due to mechanical instabilities such as wrinkles, creases, and folds observed in such instances as aging of human skin [1], the texturing of many citrus fruits[11], and the formation of mountains[12] are just a few everyday examples.

Wrinkling refers to the loss of the original state, morphologies, shape and appearance of a surface due to mechanical instability of the surface and “wrinkling is sensitive to exceedingly small initial imperfections that significantly reduce the compressive strain at which the instability occurs” [13], the cause for the elusiveness, ubiquity or even mysteries nature of wrinkling. Harnessing these instabilities to provide critical insight into the properties of soft materials and even as a tool in manufacturing novel soft materials have been the focus of many scientific applications [12]. Genzer and Groenewold [1] reviewed the scientific aspects of wrinkling and the related issues. Specifically, they discussed how and why wrinkles/buckles form in various materials, with several examples from everyday life, demonstrating that wrinkling or buckling is indeed a commonplace phenomenon that spans a multitude of length scales. They emphasized interestingly that wrinkling is not always a frustrating (destructive) feature, as it can help to assemble new structures, understand important physical phenomena, and even assist in characterizing chief material properties [1].

Focusing on textiles where the initial research on wrinkle and wrinkle recovery were conducted as early as in 1920s[7], fabric wrinkling is considered one of the most important surface appearance properties, and a reflection of the fabric quality and the aesthetic and visual appealing[14]. As wrinkling and its recovery is a perceived attribute, i.e., it is a result of interaction between physical, human physiological and psychological factors of both the fabric and the observer.

Visual attributes of textile fabrics represent a sub-group of sensory properties perceived by the senses of consumers and include fabric drape, surface state and luster, and, by logical extension, the fabric wrinkle recovery. Fabric wrinkle recovery on the other hand is the ability of fabric recovery from its wrinkled state – again a very important esthetic feature, and for instance the critical improvement in cotton fabric is in its wrinkle resistance or recovery ability [4, 15, 16] [17]. It is interesting to note that drape and wrinkle recovery, along with fabric hand, are interconnected and determined by the same group of mechanical properties [15] [18], laying down a useful foundation for instrumental measurement of them.

Although there are many occasions where such terms as folding (bending) and wrinkling (creasing, crumping) have been used interchangeably, there are nonetheless significant differences between them when dealing with textile wrinkling. In a typical textile fabric “f(F)riction forces serve as the only adhesive agent in our clothing to assure, apparently contradictorily, both system coherence and inter-component mobility (whether through twisting, entangling, interlacing or braiding). Sufficient integrity is thus maintained in the fibrous network structures where the seemingly opposing properties including strength, pliability, stretch, bodyshape conformity are combined harmonically”[19]. This is not true in, for instance, paper products.

As a result, when a piece of fabric is hanging on a rod tip, it drapes or turns into a wrinkled state under its own weight (Fig. 1(a)), also reflecting the anisotropy in properties of the fabric due to various directional differences [7]. We know this is essential for clothing purpose, as opposed to a normal sheet of paper or plastic film that can only bend over on a rod tip (Fig. 1(b)). It is therefore critical to differentiate the two phenomena: wrinkle is a multi-curvature deformation occurred simultaneously in more than one axial direction, whereas folding describes more appropriately the bending deformation of a uniaxial curvature. That is, wrinkling captures the inherent anisotropy in fabric performance where as bending or folding are unable to do that.

For such an important quality indicator, accurate measurement of wrinkle behavior of fabrics is of both scientific and practical significance. Various new attempts in fabric wrinkle measurement have been reviewed and compared by Liu in [8]. This is mainly due to the fact that even the physical fabric hand is basically a reflection of the overall fabric quality, attributed to many individual fabric properties [20]. However, to separately measure individual fabric properties (such as bending, compression, tensile and surface properties) deemed to be associated with fabric sensory attributes will destroy the intrinsic connections.

Testing methods:
Logically a typical wrinkle test procedure involves 3 key steps: wrinkle generation, wrinkle recovery, determination/quantification of the residual deformation in the sample. Also for any functional measurement instrument, there are 2 essential requirements: reasonable repeatability and sensitivity. For practical industrial uses, easy operation and acceptable cost will be important as well.

There exist two AATCC standard test methods as shown in Fig. 2, i.e., AATCC 66 Wrinkle Recovery Angle [21] and AATCC 128 Wrinkle Recovery of Fabrics: Appearance Method [22], and, and the corresponding testing apparatuses. During the use of the two AATCC methods, frequent reports have been published in revealing the problems with them, including:
1. Poor reproducibility, i.e., multiple tests of the same sample often do not yield reasonably close results [6];
2. Low sensitivity - only significant difference in fabric drape can be detected [20];
3. Slow and cumbersome test process [20, 23];
4. Many fabrics tend to curl and twist when cut into small specimens, and this further affects the reproducibility, and even alters the physical meaning of the test results [16];
5. In both tests, the final reading is taken by the operator through visual judgment, thus is prone to subjectivity and bias. Inconsistencies between wrinkle judgments of observers have been widely reported [4, 17], and often the results are severely influenced by the color and the fabric texture [14]. Even though imaging analysis has been [24-28] applied to the test, but it can only improve the analysis of the results, but do little in dealing the problems intrinsic in the test principles. In addition, fabric anisotropy is difficult to characterize by AATCC 66 method because multiple tests have to be run over all directions[29].

On the other hand and as a result of research by Pan and his coworkers since 1983 [20, 30, 31], a new instrument called PhabrOmeter fabric test system [32] has been developed, as shown in Fig. 3(a). Unlike the Japanese KES system, no attempt is made to separately measure individual fabric properties deemed to be associated with fabric sensory attributes. Instead, this instrument is based on the previously proposed fabric extraction method [20] with some key patent-protected improvements. During test, a force-displacement curve, Fig. 3(a), is generated through the fabric extraction process, which has been shown to contain implicitly the same fabric properties related to the fabric sensory attributes [20, 30, 31]. Then a computer algorithm was developed based on the pattern recognition technique to derive a series of parameters defining fabric hand, fabric drape and wrinkle recovery. With the key improvements over the original testing extraction action, the instrument possesses very high test repeatability, sensitivity and fast test speed. The instrument has been adopted by various companies in major countries, and some successful applications have been reported [33, 34]. In addition, an AATCC standard test method for the PhabrOmeter, AATCC TM202, has been officially established to guide the users [35]. Also it is clearly shown in Fig. 3(b) and (c) that this test method indeed creates genuine wrinkles on fabric samples tested.

In the following sections, we will compare the 3 methods by testing and analyzing the test results to establish a case that PhabrOmeter is a much better replacement of AATCC 66 and 128 methods. There are other testing methods often used along with AATCC 66 to measure fabric bending stiffness, e.g., Cantilever Stiffness Tester (ASTM D1388), but they have little relevance with our topic and hence are excluded.