Poster · Pages 286–286
Abstract
Research summary supplied by the authors
When exposed to high temperatures, leather undergoes a phenomenon of dimensional contraction (shrinkage) closely related to the protein and fibrous nature of the materi- al. The studies of thermal shrinkage on leather have been focused on automotive applications, but no statistical analysis of the effect of anisotropy and inhomogeneity in the different areas have been carried out. In the present study a test method to assess the shrinkage of leather at 120°C in term s of forces by means of DMA has been proposed. The method has been applied for mapping the shrinkage stresses in the 3 segments of ISO 2418 (butt, shoulder and belly) in th e two directions parallel and perpendicular to the backbone. Different leather articles, tanned with different chemicals and different intended use (automotive, furniture, footwear, clothing and gloves) have been ana- lyzed. The aim is to increase the knowledge of the phenomenon with a correlation with the fibrous structure of the material by means of an innovative test method. Introduction Dimensional change due to environmental variations is one of the typical phenome- non related to leather. In particular, when exposed to high temperature, leather shows a negative thermal expansion (NTE) with a subsequential partial permanent loss of surface that could determine problems in different applications as for foot- wear production (lasting process), automotive applications or, in general, during dry- ing processes of leather [1]. From the quantification point of view, shrinkage has been studied in terms of stresses developed using a capacitive stress measurement trans- ducer method [1] or by placing force -measuring systems in climate chamber in a cy- clic environmental ageing (heat and relative humidity) [2]. Furthermore, in the studies available on leather [1 -3] all the assessments have been carried out without any ref- erence to the anisotropy of the materials due to its typical structure. Finally, with ref- erence to quality control of incoming materials, most of the technical specification (i.e for automotive industry) take into account large test pieces both in terms of force and deformation [4]. Finally, test methods of official standardization bodies are availa- ble only from a dimensional point of view [5]. In this study a new test method (SSIP 1701 -21) for the quantification of the forces in- volved in the shrinkage at high temperature is proposed using DMA techniques on small samples. To verify the effects of the inhomogeneous fibrous structure of leath- ers on the results, shrinkage stresses have been measured in the three segment area of ISO 2418 (shoulder, butt and belly) [6] and in two reference directions: parallel and perpendicular to leather backbone. Different leather articles, with different animal origin, tanning and destination of use have been analyzed to verify an eventual corre- lation with leather structure and characteristics. Material and Methods The measurements were performed using a DMA 850 (TA Instruments, USA), in ten- sile mode from room temperature till the target isothermal temperature of 120°C at a rate of 20°C/min. The forces have been assessed in iso -strain mode fixing a defor- mation of 0,1% and a preload of 0,05 newtons. Rectangular test pieces (30,0 ± 0,5) mm x (5,0 ± 0,5) mm have been analyzed sam- pling in accordance with ISO 2419 [7]; the iso -strain mode have been carried out at the reference distances of the clamps of (20,0 ± 0,5) mm. The isothermal has been applied for 240 minutes, as required in some automotive specifications [4], and the results have been expressed in kPa dividing the force with the transverse section of the specimen. Values after 10, 60, 120 and 240 minutes have been reported. For the validation of the test methods, two different leathers have been used: bovine for automotive tanned using glutaraldehyde and ovine chrome tanned leather for gloves. For each, 10 measurements were carried out in both sampling directions and only in the butt (ISO 2418 [6]). Afterwards, the mapping of the shrinkage in the 3 are- as of ISO 2418 (shoulder, butt and belly) [6] were carried out on several leather sam- ples, for which have been determined the elastic modulus in stress -strain tests (up to 15 newtons) and dimensional shrinkage (ISO 17130) too. Acknowledgements The authors thank Pasubio, Mario Levi and Gargiulo tannery for the support. References [1] Tuckermann M., Mertig M. , Pompe W., Reich G. — Stress measurements on chrome-tanned leather, (Journal of Materials Science, 2001). [2] Meyer M., Schröpfer M., Trommer A.— Effects of temperature and humidity on different crosslinked colla- gen structures (IULTCS Congress, 2006). [3] Meyer M., Schröpfer M., Trommer A. — Dimensional and structural stability of leather under alternating climate conditions (IULTCS Congress 2011). [4] Maserati I.DSPM 020 Delibera cuoi per rivestimenti inter- ni (2018). [5] ISO 17130:2021 Leather — Determination of dimensional change. [6] ISO 2418:2017 Leather — Sampling location. [7] ISO 2419:2012 Leather — Sample preparation and conditioning Results and Discussion In the following tables the characteristics o the sample analyzed are reported. Conclusion From the results reported in the tables and with the observations limited to the sam- ples analyzed, we can conclude that: • DMA Method shows good repeability performances • Leather with the same dimensional behavior could have very different values in terms of force or shrinkage stress • In the different sampling area there is a certain correspondence between the trend of the force values with the modulus • In many cases the shrinkage stresses have been resulted: higher parallel to the backbone for butt higher perpendicular to the backbone for shoulder • Shrinkage behavior can be affected by drying or toggling processes • Shrinkage is directly related to the fibrous structure of leather • The stress trend suggests that the time of testing can be reduced to 120 minutes; the linear forecast error to 240 minutes is lower than 2% than the measured value. TIME (MIN) AUTOMOTIVE LEATHER LEATHER FOR GLOVES PARALLEL PERPENDICULAR PARALLEL PERPENDICULAR MEAN ST.DEV MEAN ST.DEV MEAN ST.DEV MEAN ST.DEV 10 51 10,8 85 21,6 213 69,2 32 11,5 60 89 14,8 154 51,5 532 133,7 93 25,3 120 96 15,8 166 56,2 622 148,6 115 30,5 240 103 16,9 178 60,9 720 161,0 141 36,9 SAMPLE DESCRIPTION DIR. SHRINKAGE (kPa) E(5-15N) BUTT SHOUL. BELLY BUTT SHOUL. BELLY LINEAR SHR (%) AREA SHR (%) MIS_1 WASHED BOVINE 0° 37,3 25,7 42,5 45,1 32,1 31,3 0,0 5,5 MIS_1 (VEGETAL) UPPER FOOTWEAR 90° 16,8 29,1 15,2 31,2 32,3 15,0 MIS_2 WRINKLED BOVINE 0° 884,3 432,5 188,5 49,6 30,4 15,0 3,0 8,3 MIS_2 (CHROME) UPPER FOOTWEAR 90° 317,2 419,9 116,7 23,8 27,9 15,0 MIS_4 CALF BOVINE 0° 1401,4 597,4 314,1 74,5 47,3 28,0 4,0 7,5 MIS_4 (VEGETAL) LEATHER GOODS 90° 1897,6 2216,5 1251,2 102,8 107,8 70,4 MIS_6 LAMB OVINE 0° 60,8 101,7 79,5 45,9 45,2 49,6 4,0 7,5 MIS_6 (CHROME) GLOVES 90° 347,9 322,6 599,2 48,9 47,4 48,9 MIS_7 SUEDE GOAT 0° 24,8 14,4 23,1 22,5 18,9 16,8 2,0 5,6 MIS_7 (CHROME) UPPER FOOTWEAR 90° 64,4 123,9 63,4 27,1 32,7 21,7 MIS_10 CALF BOVINE 0° 53,1 14,5 35,9 19,4 28,3 12,4 4,0 7,5 MIS_10 (CHROME) APPAREL 90° 36,7 60,4 7,0 16,9 24,6 9,7 MIS_11 CALF BOVINE 0° 900,3 504,8 38,6 46,0 38,1 5,2 4,0 9,2 MIS_11 (CHROME) UPPER FOOTWEAR 90° 182,3 997,7 262,7 23,4 48,0 26,5 MIS_14 SPLIT OVICAPRINE 0° 448,0 510,5 160,3 71,4 83,1 53,8 4,7 6,4 MIS_14 (CHROME) APPAREL 90° 260,7 636,6 400,1 38,4 63,2 57,6 BCD_O OVINE 0° 744,9 226,5 617,2 8,9 7,4 21,2 3,0 6,5 BCD_O ACETONE DRIED 90° 190,5 131,6 352,1 5,9 5,5 8,1 BCD_B BOVINE 0° 1144,1 777,7 242,5 58,9 34,5 5,4 4,7 9,9 BCD_B ACETONE DRIED 90° 319,1 220,2 849,1 16,9 13,2 33,5 CP294_2 BOVINE 0° 240,7 138,9 607,5 23,9 37,9 39,8 2,1 6,8 CP294_2 (GLUTARDEHYDE) AUTOMOTIVE 90° 182,0 388,6 30,7 32,7 17,4 7,4 ML1 BOVINE 0° 458,3 600,3 208,3 32,5 30,9 33,2 4,2 7,1 ML1 (GLUTARDEHYDE) AUTOMOTIVE 90° 203,2 216,7 127,5 34,1 21,1 12,6 N245 OVINE 0° 81,1 55,0 77,3 43,3 34,9 41,1 2,7 4,4 N245 (VEGETAL) APPAREL 90° 55,2 35,0 49,5 24,7 27,9 36,2 PF23 BOVINE 0° 73,2 61,2 40,4 22,2 20,5 18,0 3,7 8,9 PF23 (VEGETAL) UPHOLSTERY 90° 69,1 105,2 38,0 22,4 23,9 13,3 PF25 BOVINE 0° 203,2 257,5 333,7 21,8 21,5 29,1 3,0 6,5 PF25 (CHROME) UPPER FOOTWEAR 90° 251,8 264,9 151,2 20,8 22,3 18,2 PF29 BOVINE 0° 244,0 234,5 654,8 30,8 24,3 46,4 3,3 8,6 PF29 (METAL FREE) LEATHER GOODS 90° 222,2 463,6 62,8 38,8 34,0 18,4 PF31 BOVINE 0° 253,7 162,5 560,4 35,7 21,9 44,1 3,7 10,7 PF31 (CHROME ) FURNITURE 90° 328,9 331,9 121,3 25,8 28,1 16,6