Preliminary study of relating cotton fiber tenacity and elongation with crystallinity

Abstract

A fundamental understanding of the relationship between cotton fiber strength (or tenacity)/elongation and structure is important to help cotton breeders modify varieties for enhanced end-use qualities. In this study, the Stelometer instrument was used to measure the bundle fiber tenacity and elongation properties of different cotton fibers. This instrument is the traditional fiber strength reference method and could be still preferred as a screening tool owing to its significant low cost and portability. Fiber crystallinity (CI_IR) and maturity (M_IR) were characterized by the previously proposed attenuated total reflection (ATR)-based Fourier transform infrared protocol that has microsampling capability and is suitable for the tiny Stelometer breakage specimens (2 ∼ 5 mg), which cannot be readily analyzed by a conventional X-ray diffraction pattern. Relative to the distinctive increase in fiber tenacity with either CI_IR or M_IR for Pima fibers (Gossypium barbadense), there was an unclear trend between the two for Upland fibers (G. hirsutum). Although fiber elongation increases with elevated CI_IR and M_IR for Pima fibers, it generally decreases as CI_IR and M_IR increase for Upland fibers. Furthermore, small sets of Upland fibers with known varieties and growth areas were examined, and their responses to both CI_IR and M_IR are discussed briefly.

Keywords

fiber tenacity fiber elongation Stelometer fiber crystallinity index Pima cottons Upland cottons FTIR spectroscopy ATR

One of the essential attributes of cotton fibers is strength, which is required to manufacture quality goods for consumers. Over the years, a number of techniques, such as the Mantis single-fiber tester,^1,2 the Instron tensile tester,³ the Favimat single-fiber tester,^4,5 Fibrotest,⁶ the Stelometer bundle tester,^2,5–9 and the high volume instrument (HVI),^2,5,6,9,10 have been developed to measure cotton strength by testing either a single fiber or a bundle of fibers. Comparisons between two or more independent strength tests have been attempted by different investigators.^2,5,6,9 Thibodeaux et al.² determined the relationship between the single-fiber (Mantis) strength and two-bundle fiber (Stelometer and HVI) strength and found that both the Stelometer strength and HVI bundle strength are linearly proportional to the ratio of the average Mantis breaking load to the square of the average fiber ribbon width. By analyzing the results of either single-fiber testing from the Favimat or bulky fiber measurements from the HVI and Stelometer, Delhom et al.⁵ reported that single-fiber testing results in higher mean values than bundle testing. In a more recent investigation, Cui et al.⁶ observed that the Fibrotest absolute strength is lower than the Stelometer strength. There was also a report on the general conversion between the two bundle strength readings from the Stelometer and HVI measurements.⁹

A fundamental understanding of the relationship between cotton fiber strength and structure is of great interest because this knowledge could be of value to cotton breeders for cotton enhancement. In the 1990s, Benedict et al.¹¹ isolated crystalline microfibrillar fragments from different cotton fibers and reported a correlation of 0.94 between the average length of the cellulose chains in the crystalline cellulose (measured from ¹³C-nuclear magnetic resonance (NMR) spectra) and the HVI bundle fiber strength. Hsieh’s group applied the “XRAY” X-ray diffraction (XRD) analysis program to characterize the crystallite size and crystallinity parameter from the wide-angle XRD pattern of dried cotton fibers at various developmental stages.¹² They further correlated the fiber breaking force and tenacity (or strength) from the Instron tensile tester with the crystallite size and crystallinity for the Acala variety of two Upland cottons(SJ-2 and Maxxa), which are stronger and longer than typical Upland cottons.³ Within each variety, single-fiber breaking forces were positively related to both the crystallite size and crystallinity, and increasing the breaking forces and tenacities appeared to be related more to crystallite size than to crystallinity.³ In addition, the single-fiber breaking tenacities of the SJ-2 cotton fibers did not vary with fiber crystallinity, whereas those of Maxxa cotton fibers responded positively to an increase in crystallinity.³ They emphasized that, besides the crystallinity and crystallite sizes, other structural parameters, such as fibril orientation and residual stress, may also play key roles in affecting the single-fiber strength of cotton fibers.

The term crystallinity index (CI) has been used to describe the relative portion of crystalline cellulose in a simple two-phase model (crystalline vs. amorphous areas) within a cellulose sample.¹³ It is determined predominantly by a curve-fitting process that extracts individual crystalline peaks from the XRD intensity profile. In general, XRD determination of the cellulose CI provides a qualitative or semi-quantitative evaluation of the amounts of either crystalline or amorphous components in a sample.¹³ Hence, appropriate cellulose standards are desired to calibrate or validate the XRD measurement; however, these absolute standards are not easily prepared or obtained.

As a different approach, Liu et al. used attenuated total reflection (ATR)-based Fourier transform infrared (ATR-FTIR) spectroscopy to identify the spectral intensity differences between immature and mature fibers.¹⁴ They determined the key wavelengths first and then developed two simple algorithms (R₁ and R₂) for their respective discrimination, noting that the R₁ values increase with R₂ values in a dataset consisting of 402 seed bolls. Setting an R₁ threshold value at 0.40, 197 of 201 (98.0%) immature fibers and 190 of 201 (94.5%) mature samples were correctly classified. With an R₂ threshold value at 2.24, only six immature fibers and 10 mature samples were misidentified, yielding an overall 96% accuracy in their correct differentiation. Next, they proposed a formula to estimate the degree of cotton cellulose maturity (M_IR) by representing the R₁ values.¹⁴ In this concept, M_IR values of 0.0 and 1.0 were assigned to the most immature and mature fibers, respectively. Thus, the immature fibers for which R₁ < 0.40 correspond to a M_IR < 0.58 in the maturity range of 0 to 1.0, and vice versa. To validate the efficiency of accessing the M_IR directly from the ATR-FTIR measurement, cotton fibers with various maturity readings as determined from traditional image analysis (IA) and advanced fiber information system (AFIS) were examined, and strong correlations between M_IR and the referenced IA and AFIS maturity readings were reported among small sets of samples.

Similar to the concept of representing the R₁ values, the R₂ values were converted to represent cotton cellulose crystallinity (CI_IR).¹⁵ The most immature and mature fibers were set to have CI_IR values of 0 and 100%, respectively. In turn, immature fibers with R₂ < 2.24 might have CI_IR < 42%, and mature fibers with R₂ > 2.24 have CI_IR > 42%. Considering the challenge of determining the fiber CI from an XRD pattern, Liu et al.¹⁵ proposed a four-band ratio (R₃) linking four major XRD intensities with the crystalline information and then observed a high correlation of determination (R²> 0.90) between the CI_IR from the ATR-FTIR spectra and R₃ from the XRD patterns. Next, the R₃ values were converted into their respective CI_XRD readings in the range of 0 to 100%. The observed CI_XRD readings from a small set of fibers at various growth periods were in good agreement with previously reported values for two cotton varieties at different developmental stages.^1,3 When making a comparison with cottons from commercial bales, the observed CI_XRD values were between the reported CI range 73–80% (on Turkmenistan and Uzbekistan native cottons)¹⁶ and those of either 53–69% (on Egyptian cottons)¹⁷ or 57–64% (on Saudi Arabian cottons).¹⁸ Hence, they concluded that using ATR-FTIR to assess the cotton fiber CI was appropriate and reasonable. Moreover, a simple ATR-FTIR protocol avoided the need to perform any pretreatment of the cotton fibers (such as cutting), could also analyze small amounts of fiber (as little as 0.5 mg), and required only a short time (less than two minutes) for sample loading, spectral acquisition, and subsequent result reporting.

The main objectives of this study were to determine the cotton CI_IR and M_IR from Stelometer fiber breakage specimens using the ATR-FTIR technique and also to correlate the Stelometer parameters with CI_IR and M_IR. A flowchart of the experiments and projected outcomes is briefly depicted in Figure 1. In this study, the Stelometer instrument was utilized to determine the fiber bundle strength and elongation with the following considerations. First, the Stelometer protocol is a traditional laboratory-based test and is still preferred by cotton researchers and breeders as a simple screening tool because of its significant low cost and portability; however, it undoubtedly has a number of drawbacks compared to other strength-measurement methods. For example, it is a tedious and labor-intensive procedure that requires experienced operators and generates only a few fiber-quality indices. Second, Stelometer-tested fiber bundles (2 ∼ 5 mg) are too small for XRD measurements that usually require a large amount of sample (∼150 mg on an XRD aluminum holder of 25 mm diameter × 2 mm deep), but they are sufficient for ATR-FTIR, which has microsampling capability, making it possible to examine the structural differences among various portions within a sample (or a cotton boll). The assumption in this paper is that there is no spectral bias between Stelometer broken and unbroken fibers.

Figure 1.

A flow chart of experiments and projected outcomes.

Materials and methods

Cotton samples

Four sets consisting of a total of 70 lint cottons (i.e., cotton fibers acquired after the ginning process) were used. Set 1 included 32 Upland fibers from the 2009 crop year grown in several US states, but their specific varieties were not provided by the collaborator; set 2 consisted of 14 Upland fibers from the 2009 crop year obtained from international growers, namely, three from each of three countries (two African and one Asian) and five from two growing areas in one Asian country; set 3 represented 11 US Upland cottons from the 2009 crop year; and set 4 included 13 US Pima fibers from the 2009 crop year. The cotton fibers in sets 3 and 4 were collected at the Agricultural Research Service (ARS)’s Clemson facility in 2010. These samples were well conditioned at a constant relative humidity of 65 ± 2% and temperature of 21 ± 2℃ for at least 48 hours prior to Stelometer testing and the ATR-FTIR spectral measurement.

Stelometer tenacity and elongation determination

A Stelometer flat bundle tester (Spinlab, Knoxville, TN) with 1/8-inch (3.2-mm) clamp spacing was employed to determine the cotton fiber Stelometer tenacity and elongation properties according to ASTM D-1445-12.⁸ Both the tenacity and elongation of individual cotton samples were obtained by two experienced operators as an average of six bundle breaks following the established protocol. All broken bundles were retained for subsequent ATR-FTIR spectral acquisition. As cotton fiber consists of almost entirely cellulose that may be subject to conformational variation, it is of great interest to investigate the structural differences between unbroken and broken fibers in the future.

CI_IR and M_IR calculations

The ATR-FTIR spectra were collected in absorbance units with an FTS 3000MX Fourier transform IR spectrometer (Varian Instruments, Randolph, MA) equipped with a ceramic source, a KBr beam splitter, and a deuterated triglycine sulfate detector. The ATR sampling device utilized a DuraSamplIR single-pass diamond-coated internal reflection accessory (Smiths Detection, Danbury, CT), and a consistent contact pressure was applied by a stainless-steel rod and an electronic load display. Two spectra were collected for each bundle over the range 4000–600 cm⁻¹ at 4 cm⁻¹ with 32 co-added scans. This resulted in12 spectra for each sample, and their average was used for the analysis.

Following the import to Grams/AI software (V.7, Thermo Galactic, Salem, NH), the spectra were smoothed with a Savitzky–Golay function (polynomial = 2 and points = 11).

The dataset was then loaded into Microsoft Excel 2000 to calculate the CI_IR and M_IR parameters using the same algorithms previously reported.^14,15 Representative ATR-FTIR spectra in the 3600 to 600 cm⁻¹ region with different CI_IR values are shown in Figure 2.

Figure 2.

Representative ATR-FTIR spectra of cotton fibers with CI_IR readings of 54% (dotted line), 70% (solid line), and 92% (dashed line).

Results and discussion

Relationship between fiber tenacity and CI_IR

Cotton fiber is a type of natural cellulose I (β 1→4 linked glucose residues), and the quantity of cellulose varies greatly with the stages of fiber growth. Commonly, mature fibers are composed of mostly cellulose (88.0–96.5%), followed by such noncellulosic constituents as proteins (1.0–1.9%), waxes (0.4–1.2%), pectins (0.4–1.2%), inorganics (0.7–1.6%), and other substances (0.5–8%).¹⁹ In contrast to mature fibers, immature fibers contain less cellulose and more noncellulosic components. The coexistence of mature and immature fibers, along with the heterogeneous distribution in chemical structure even within one single fiber, as observed by traditional microscopy, present a challenge for accurately and consistently determining several fiber physical characteristics. For instance, the US Department of Agriculture (USDA)’s Agricultural Marketing Service (AMS) has introduced HVI readings to be used as universal standard classification indices.²⁰ One of the requirements for the strength property is that the standard deviation (SD) calculated from 60 tests of one standard cotton to calibrate the HVI system should be below 1.2 g/tex,²¹ and if the SDs of the 60 tests exceed the suggested limit, either the cotton is not sufficiently uniform or the HVI requires servicing because of high variation in the measurements. Thus, it should be expected that there is some degree of variation in the Stelometer tenacity determination among different replicates within one sample.

The Stelometer measures the force that is required to break a small, flat bundle of fibers. The broken bundle sample is then weighed, and the Stelometer tenacity (B) in the units of g/tex (1 tex = 1 g/km) is simply calculated using the breaking force (Kp) and the known weight of the sample (Wt):

B = (K_{p} \times 15.0) / Wt

(1)

Theoretically, B should be consistent or have less variation within one unique sample. Equation (1) can then be rewritten as equation (2):

K_{p} = (B / 15.0) \times Wt

(2)

Kp is proportional to Wt, and a plot of Kp against Wt should generate a linear relationship. We thoroughly examined the Kp–Wt plot consisting of six replicates from one sample measured by two operators in our Stelometer dataset and found that other factors, in addition to fiber uniformity in HVI strength mentioned earlier, might influence the Stelometer tenacity determination. These factors likely include the range of sampling weight and the interval among individual sampling weights as well as the likelihood of data input errors during the calculation.

Under a desired scenario, a plot linking Kp with Wt should show a high determination of correlation (R²), and an example is given in Figure 3(a). As a comparison, examples of samples with lower R² values are presented in Figure 3(b) to (d). The corresponding relative standard deviations (RSDs) of the breaking tenacity (defined as quotient of the SD to average the Stelometer tenacity in a sample) were 0.038, 0.048, 0.060, and 0.042 for the fibers used in Figure 3(a) to (d), respectively. Both higher R² and lower RSDs are anticipated because they reflect a more uniform distribution of tenacity property within one sample. Figure 3(b) to (d) might suggest an effect of fiber uniformity, weight range, and intervals between sampling weight on the tenacity determination. Undoubtedly, some samples with similar sampling weight ranges or intervals to those in Figure 3(b) to (d) could produce much higher correlations.

Figure 3.

Relationship between fiber breaking force and fiber mass between two independent Stelometer operators.

In the present study, only those samples with high correlations (R²> 0.90) were considered. This led to 32, 14, 11, and 13 fibers in sets 1, 2, 3, and 4 that were actually analyzed from the respective sets, including 55, 14, 11, and 18 samples. Each selected fiber was different from all the others. These chosen samples were expected to be more uniform in the fiber tenacity distribution than the excluded ones, based on six replicates within one sample. It is unclear what factors caused the exclusion of more samples from set 1 than from the other sets. The sample selection was subjective, and so the samples that were not chosen could be considered as not meeting the required (or expected) quality. In practice, caution should be taken to these samples because it is sometimes hard to understand the phenomenon.

The relationship between the Stelometer breaking tenacity and CI_IR for all sets is depicted in Figure 4(a). As anticipated, the Pima fibers in set 4 (○) show higher tenacity than Upland varieties. In general, Pima varieties have a tendency to increase in tenacity with CI_IR, whereas the tenacities of Upland fibers are nearly independent of CI_IR. Figure 4(a) also reveals that set 1 fibers have a greater range (53.5–98.5%) in CI_IR than the other sample sets (i.e., set 2 fibers are between 58.3 and 97.4%, set 3 fibers from 58.8 to 88.4%, and set 4 fibers between 62.6 and 84.8%).

Figure 4.

(a) Plot of Stelometer tenacity vs. CI_IR; Pima (○, set 4), international (▴, set 2), and U.S. (•, set 1; ▪, set 3) cottons. (b). Plot of averaged Stelometer tenacity vs. CI_IR for Pima (○) and Upland (•) cottons. (c). Plot of Stelometer tenacity against CI_IR for fibers from two U.S. cotton varieties (○ and •) in set 3, two Asian countries (Δ and ▴) and two African locations (□ and ▪) in set 2.

If the tenacities were averaged for the samples having a close CI_IR, the resultant plot in Figure 4(b) indicates a clearer relationship between tenacity and CI_IR for Pima (○) than for Upland (•) fibers. Notably, the averaged tenacities from various Upland varieties seem to be insignificantly influenced by fiber crystallinity, and this result is consistent with an earlier report on two Upland varieties (SJ-2 and Maxxa) by Hsieh et al.³ The difference in genotype between Pima and Upland varieties is probably responsible for the observations in Figure 4(a) and (b). In addition, relative ups and downs in Figure 4(b) for the Upland cottons might originate from unevenly distributed sample numbers for each set in Figure 4(a).

In order to understand the effect of a specific Upland variety on the relationship between tenacity and CI_IR, cotton fibers with known varieties and origins from sets 2 and 3 are compared in Figure 4(c). Set 3 included the fibers from two US cotton varieties. Despite a limited number of samples for each individual variety, Figure 4(c) reveals a few interesting points. For example, two US Upland varieties have a differing CI_IR range of either 55–72% (filled red circles) or 70–88% (open red circles), fibers picked in two Asian countries have CI_IR readings between 55 and 69% (filled blue triangles) or between 88 and 96% (open blue triangles), and cottons from two African locations have a CI_IR ranging from 68 to 78% or 90 to 95% (shown by ▪ and □, respectively). As far as the fibers investigated in this study, the tenacities for the US and African cottons are greater than those for Asian fibers. Nevertheless, the correlation between fiber tenacity and CI_IR based on these small sample sets is not straightforward. This resembles a previous report on two Upland varieties.³ Also, this work was not intended to differentiate US cottons from non-US cottons using Stelometer readings or ATR-FTIR. Instead, the purpose was to compare the strength and structural differences among the available diversified cottons, given the fact that the major chemical composition in mature fibers is cellulose, the representative varieties are very limited, and also the growth environment and farming practices are quite different.

Relationship between fiber elongation and CI_IR

Fiber elongation is an important physical property in raw cottons. It characterizes how far the fibers will stretch before they break. Similar to breaking tenacity, the relationship between the Stelometer elongation and CI_IR was examined for these fibers. Unlike the tenacity in Figure 4(a), Figure 5(a) indicates no distinct difference in elongation between the two categories of fibers (Pima vs. Upland). Despite a much scattered pattern in Figure 5(a), it shows an overall decrease in elongation as CI_IR increases, at least for Upland fibers.

Figure 5.

(a) Plot of Stelometer elongation vs. CI_IR; Pima (○, set 4), international (▴, set 2), and US (•, set 1; ▪, set 3) cottons. (b). Plot of averaged Stelometer elongation vs. CI_IR for Pima (○) and Upland (•) fibers. (c). Plot of Stelometer elongation against CI_IR for fibers from two U.S. cotton varieties (○ and •) in set 3, two Asian countries (Δ and ▴), and two African locations (□ and ▪) in set 2.

When the elongation values were averaged for fibers having close CI_IR values (Figure 5(b)), there is a clear decrease in elongation with elevating CI_IR for Upland fibers and also a slight gain in elongation when CI_IR is in the range of 65–84% for Pima samples. In other words, the average fiber elongation increases with amorphous amount (estimated by 100% − CI_IR) for Upland cottons. Likely, a small jump in elongation from 65 to 75% CI_IR among the Upland fibers might be related to either an unclear transition in this CI_IR range or fewer data points in Figure 5(a).

A careful analysis of the pattern in Figure 5(c) suggests, in general, that elongation for these fibers from US and African locations are greater than those from Asian countries. Clearly, more diverse samples are necessary to unravel the correlation between fiber elongation and CI_IR for each variety.

Relationship between fiber tenacity/elongation and M_IR

M_IR was also assessed from the same ATR-FTIR spectra as for CI_IR. The previous results indicated a synchronous response between M_IR and CI_IR for cotton fibers at various developmental periods.²² As expected, the relationship between fiber tenacity and M_IR in Figure 6(a) and (b) resembles that between the tenacity and CI_IR in Figure 4(a) and (b), as do the relationships between elongation and M_IR in Figure 7(a) and in Figure 5(a).

Figure 6.

(a) Plot of Stelometer tenacity vs. M_IR; Pima (○, set 4), international (▴, set 2), and U.S. (•, set 1; ▪, set 3) cottons. (b). Plot of averaged Stelometer tenacity vs. M_IR for Pima (○) and Upland (•) cottons.

Figure 7(a).

Plot of Stelometer elongation vs. M_IR; Pima (○, set 4), international (▴, set 2), and U.S. (•, set 1; ▪, set 3) cottons. (b). Plot of averaged Stelometer elongation vs. M_IR for Pima (○) and Upland (•) cottons.

Compared to Figure 5(b), a notable difference in Figure 7(b) is that the elongation stays nearly unchanged before rapid reduction around the M_IR of 0.81 for Upland fibers, whereas it increases in the M_IR range 0.70–0.80 for Pima varieties.

Relationship between fiber tenacity and elongation

The plot of averaged Stelometer breaking tenacity against averaged elongation in Figure 8(a) reveals an insignificant correlation between the fiber tenacity and elongation for the Upland fibers (•) and a relatively strong relationship for the Pima varieties (○).

Figure 8.

(a) Plot of averaged Stelometer breaking tenacity vs. averaged elongation for Pima (○) and Upland (•) cottons. (b) Plot of averaged Stelometer breaking tenacity vs. elongation for cotton fibers from two U.S. cotton varieties (○ and •) in set 3, two Asian countries (Δ and ▴), and two African locations (□ and ▪) in set 2.

Figure 8(b) highlights the correlation between fiber tenacity and elongation for the individual cotton varieties. No common trend among these small subsets was observed, but it might be of interest to examine more fibers in one variety by fiber testing methods other than the Stelometer technique.

Conclusions

Relating fiber tenacity and elongation to crystallinity and maturity has been attempted by acquiring ATR-FTIR spectra on the tiny breakage specimens used in Stelometer testing. The fibers consisted of diversified US-grown Upland and Pima varieties as well as non-US Upland cottons. Compared to an obvious increase in fiber tenacity with both CI_IR and M_IR for Pima fibers, there was an insignificant response between the two for Upland fibers. When CI_IR and M_IR increase, fiber elongation increases for Pima fibers but decreases for Upland fibers in general. When examining unique Upland varieties, the correlations between fiber tenacity/elongation and CI_IR/M_IR are inconclusive based on these small fiber sets. A probable rationale for this observation is that the fiber tenacity/elongation could be affected by multiple factors, such as crystallite size, fibril orientation, and residual stress.³ No additional work on the models was possible due to the facility closure in November 2011 and subsequent movement in equipment and personnel.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgments

The experimental work was completed at the ARS Clemson facility (officially closed in November 2011). We sincerely thank Ms. J. Linda and N. Carroll (ARS, Clemson, SC) for technical assistance in collecting the Stelometer reference data and Mr. James Knowlton (USDA, AMS, Memphis, TN) for providing the diversified cotton samples. Mention of a product or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

Appendix

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Preliminary study of relating cotton fiber tenacity and elongation with crystallinity

Abstract

Keywords

Materials and methods

Cotton samples

Stelometer tenacity and elongation determination

CIIR and MIR calculations

Results and discussion

Relationship between fiber tenacity and CIIR

Relationship between fiber elongation and CIIR

Relationship between fiber tenacity/elongation and MIR

Relationship between fiber tenacity and elongation

Conclusions

Footnotes

Funding

Acknowledgments

Appendix

References

CI_IR and M_IR calculations

Relationship between fiber tenacity and CI_IR

Relationship between fiber elongation and CI_IR

Relationship between fiber tenacity/elongation and M_IR