RPN20080225P014.qxp 2/20/2008 2:25 PM Page 1 14 Rubber & Plastics News ● February 25, 2008 www.rubbernews.com Technical Aramid yarn as a tensile member in products By Derya Gulsen Onbilger affect the tensile properties of Kevlar. and Florencio Gopez The increase in modulus and the DuPont Advanced Fibers Systems Executive summary small decrease in elongation at this low Kevlar is the registered trademark for The key properties of DuPont Kevlar product technology and product offer- temperature can be attributed to a DuPont’s family of high-temperature-re- ings are discussed. Alternative ways of making cords are discussed, e.g., core slight increase in molecular rigidity. sistant aramid fibers having a unique insertion technology and hybrid cord technology. Table IV shows thermal properties of combination of toughness, extra high Core insertion was developed to improve the resistance of aramid cords to Kevlar. As shown in Table IV, Kevlar tenacity and modulus, and exceptional compressive fatigue, while maintaining its high cord tensile properties. does not suffer significant, irreversible thermal stability. Hybrid cords, made by combining different yarns during the cabling process, shrinkage like most organic fibers when These properties offer the means to in- make it possible to engineer cords with specific properties. exposed to hot air or hot water. crease the strength and reduce weights Hybrid cords with aramid can improve fatigue resistance, higher elongation, low- Kevlar has a very small, negative co- of reinforcement, as well as extend use- er modulus, controlled shrinkage and better or equivalent strength-to-cost ratio. efficient of thermal expansion in the lon- ful wear life, in a variety of applications. gitudinal direction. Kevlar fiber consists of linear, rigid Fig. 4 shows the effect of temperature polymer chains of paraphenylene tereph- strong amide bonds confers durability high modulus compared to other fibers. on the shrinkage of Kevlar. As shown, thalamide units. These polymer mole- and heat resistance. DuPont selected Table II compares the growth and Kevlar shrinkage is not significantly af- cules are sufficiently linear and regular poly-para-phenylene terephthalamide creep values of Kevlar with those of fected by higher temperatures. as the substrate to produce and commer- glass, wire, polyester, nylon and rayon. Fig. 5 shows the effect of aging on the TECHNICAL NOTEBOOK cialize Kevlar fibers in 1972. By definition, creep is the slow, con- tensile strength of Kevlar, nylon and Edited by Harold Herzlichh Fig. 2 shows the chemical structures tinued growth or lengthening of a mate- polyester in dry air at 180°C. The yarns of two aramid fibers commercialized by rial under constant load. were tested at room temperature after in that they pack into a highly ordered DuPont: Nomex a meta-aramid fiber With organic fibers, this lengthening exposure. As shown, Kevlar retained its structure of hydrogen-bonded sheets. and Kevlar a para-aramid fiber. process is split into two regions. The strength much better than either indus- These, in turn, are efficiently aligned Table I summarizes the key differ- first is called growth and refers to the trial nylon or polyester. along the fiber axis and stack together ences between these two aramid prod- length change between 0 to 30 seconds. Fig. 6 shows the breaking tenacity to form the fiber (Fig. 1). ucts. This article basically focuses on The second region, a much more grad- and initial modulus of Kevlar, industrial The regularity of this semi-crystalline the Kevlar aramid yarn properties and ual change is called creep and occurs be- nylon, polyester and steel wire in air at structure is responsible for the extra or- applications. tween 30 seconds until the end of 30 elevated temperatures. dinary physical properties of Kevlar minutes. The entire 30 minutes is called The yarns and steel wire were tested while the outstanding chemical stability Kevlar properties creep growth. at temperature after five minutes of ex- of aromatic rings directly linked by Fig. 3 shows the stress strain behav- Table III shows the tensile properties posure in air. The breaking tenacity and ior of Kevlar compared to other industri- of Kevlar at room temperature and un- modulus of Kevlar at 177°C well exceeds Fig. 1. Kevlar fiber structure. al filament yarns, E-Glass and wire. As der arctic conditions. Exposure to arctic that of nylon, polyester and steel wire. shown, Kevlar has very high strength, conditions of -46°C does not adversely Fig. 7 shows the tenacity of Kevlar versus time at various temperatures. As Fig. 4. Effect of temperature on the shrinkage of Kevlar. shown, Kevlar has excellent strength re- Fig. 6. Breaking tenacity of industrial yarns and steel wire in air at elevated temperature. Fig. 2. DuPont meta- and para-aramids chemical structures. Fig. 7. Tenacity of Kevlar versus expo- sure time at various temperatures. Fig. 5. Effect of heat aging on tensile strength of industrial yarns in dry air. Fig. 3. Stress-strain behavior of Kevlar compared to other industrial filament yarns, E-Glass and wire. Fig. 8. Effect of twist on properties of 1,500 denier Kevlar yarn. RPN20080225P015.qxp 2/20/2008 2:27 PM Page 1 www.rubbernews.com Rubber & Plastics News ● February 25, 2008 15 Technical tention up to 180°C to 200°C. Cords twist level needs to be designed products and is useful in applications belt strength of T-956C. T-956E has bet- The level of twist in a yarn or cord will to meet end use applications. Where end where strength is the primary concern. ter strength retention. affect Kevlar physical properties (tenac- use requires resistance to compressive Kevlar T-956E (Kevlar 119): pro- Table V shows the fiber structure ity, modulus and elongation), as well as fatigue, higher twist multiplier is need- vides exceptional long-term service in and tensile properties of T-956 vs. T- its fatigue performance. ed to meet long-term durability and reli- high-fatigue applications such as power 956E. The crystallinity index of T-956E Fig. 8 shows the influence of twist on ability requirements. transmission belts and tires. It is de- is lower than T-956; crystallite size are the yarn tensile properties of 1,500 de- The resistance to compressive fatigue signed to meet demanding applications the same; the orientation angle of T- nier (1,670 dtex) Kevlar yarn. can be improved by increasing the cord where resistance to compressive fatigue 956E is higher than T-956. Yarn tenacity is maximized at a twist twist level that mitigates the effects of is of primary concern. A combination of these differences in multiplier of about 1.1. Modulus de- compressive stresses by reducing buck- Fig. 14 illustrates the belt perform- crystal structure resulted in a higher clines with increasing twist levels and ling and compressive strain. ance of T-956 T-956C and T-956E. As tenacity, higher elongation and lower more rapidly at higher twist levels. shown, our standard Kevlar product, T- modulus of the T-956E vs. T-956. These Elongation increases slightly with in- Commercial products 956 has a belt performance vs. time combination of properties resulted in a creasing twist. Twist multiplier is relat- There are three types of Kevlar prod- curve. fiber structure that is more resistant to ed to the added twist (in turns per unit ucts now used in the rubber industry. T-956C, the higher tenacity version of compressive fatigue. of yarn length) and the yarn denier. These are: Kevlar will start at a higher initial belt Fig. 15 shows the yarn stress-strain Figs. 9, 10 and 11 show the effect of Kevlar T-956 (Kevlar 29): specifically strength. Because the resistance to com- curves of T-956 vs. T-956E. As shown, T- twist on tenacity, modulus and elonga- engineered for rubber products, combin- pressive fatigue of T-956C is the same 956E has a higher tenacity, lower modu- tion for greige and water dipped 1,500 ing outstanding strength and modulus as T-956, its belt performance curve is lus and higher elongation versus T-956. denier/1/2 cords of Kevlar. with lightweight, toughness and durabili- the same as the T-956 curve, but at a Fig. 16 shows the stress-strain curve As can be seen, dipping has little ef- ty. This unique balance of properties higher strength level. of 1,500/1/2 cords of T-956 and T-956E. fect on cord tenacity. Cord modulus, on make Kevlar T-956 an excellent reinforce- The initial belt strength of T-956E is As shown, yarn properties translate di- the other hand, is “set” during dipping ment for the Mechanical Rubber Goods between T-956 and T-956C. rectly to cord properties. (which also involves stretching under applications (hose, power transmission However, because it was designed to One of the laboratory methods of test- tension) resulting in higher modulus belts and conveyor belts) and tires. give better resistance to compressive fa- ing resistance to compressive fatigue is and lower elongation. Kevlar T-956C (Kevlar 129): pos- tigue, after belt testing, the final belt the disc fatigue tester. Fig. 17 shows Figs. 12 and 13 show the effect of sesses the highest tenacity of the Kevlar strength will be higher than the final the disc fatigue test data of 1,500 de- twist multiplier and compression on fa- nier/1/2 cords of T-956 versus T-956E at tigue performance of a 1,500 denier/1/2 different twist levels. Fig. 17. Typical in-rubber cord disc fa- cord of Kevlar. Fig. 13. Effect of compression and twist This was done at 15 percent compres- tigue of 1,500/1/2 Kevlar T-956 and T- on fatigue strength of Kevlar cords (0 sion; no extension; for six hours.