TPVs in elastomeric fiber applications.
Elastomeric materials possess robber-like elasticity and can roughly be divided into two different classes: thermoset rubbers and thermoplastic elastomers.
Thermoset rubbers are materials that usually are vulcanized after being molded into the desired shape. The final product has a three-dimensional network structure which results in excellent dimensional stability, mechanical properties and elastic recovery. However, the processing of thermoset rubber articles is laborious and slow and restricted in attainable product geometries and dimensions. Furthermore, they cannot be reprocessed by heating, since the crosslinks are not thermoreversible.
Thermoplastic elastomers (TPEs) are a class of materials that have the processing characteristics of thermoplastics and the elastomeric performance of conventional thermoset rubbers. In general, TPEs mostly consist of two or more polymer phases. One of these phases is essentially thermoplastic and hard; the other is essentially elastomeric and soft. The unique performance of a TPE is directly dependent on the melting temperature of the hard thermoplastic phase ([T.sub.m, hard]) and the glass transition temperature of the soft elastomeric phase ([T.sub.g, soft]). The performance of a TPE thus depends on the properties of each of the two phases and their mutual interaction.
The useful temperature range for TPE applications is that between [T.sub.m], hard and [T.sub.g, soft]. Above [T.sub.m, hard], the hard domains become molten and the TPE is a viscoelastic or Newtonian fluid, permitting molding, extrusion or other melt processing operations. Cooling the fluid TPE below [T.sub.m, hard] causes it to resolidify and again become elastomeric. This heating-cooling cycle is reversible and enables TPEs to be recycled repeatedly with retention of their rubber properties. Below [T.sub.g, soft], the material is hard and rigid and lacks the properties of an elastomer.
TPEs can be subdivided into two different categories, i.e., segmented block copolymers, in which the rigid and soft phases are present in the same polymer backbone, and thermoplastic-elastomer blends in which the elastomer phase is either non-vulcanized (TPOs) or vulcanized (TPVs).
Segmented block copolymers
Copolyetheresters or TPE-Es are multiblock copolymers in which hard crystalline segments composed of alkylene terephthalate, and soft elastomeric segments of poly (alkylene-oxide) alternate along the polymer backbone. Crystallization of the hard segments in combination with rather strong intermolecular interactions results in the formation of a hard phase, with [T.sub.m] as melting point, which acts as thermoreversible physical crosslinks. These hard domains thus anchor or restrict the movement of the soft elastomeric chains in much the same way as crosslinks restrict the same movement in a thermoset rubber (figure 1). Above [T.sub.m] these microdomains are molten. The anchoring of the elastomer chains is no longer present and the polymer behaves as a normal polymeric fluid.
[Figure 1 ILLUSTRATION OMITTED]
Thermoplastic vulcanizates (TPVs)
Thermoplastic vulcanizates or TPVs are a class of thermoplastic elastomers which typically consist of a finely dispersed chemically crosslinked elastomer phase in a melt-processable thermoplastic matrix. This typical morphology is obtained by the so-called dynamic vulcanization process that is the vulcanization of the elastomer phase during melt-mixing with a molten thermoplastic phase using suitable vulcanization additives. Key parameters in the preparation of TPVs, and important for the final properties of the material, are the size of the dispersed elastomer particles, the degree of crosslinking of the rubber phase and the total amount of rubber that is present. By far the most common thermoplastic and rubber phases applied in TPVs are, respectively, isotactic polypropylene (PP) and ethylene-propylene-diene monomer rubber (EPDM). Because PP forms the continuous thermoplastic matrix phase, the TPV can be melt-processed above the melting point of PP. The presence of a relatively high amount of both crosslinked EPDM particles and plasticizer in the TPV results in a material with a rubber-like elasticity (figure 2). The total EPDM content (including additives such as mineral oil) can make up as much as 90% of the total TPV composition.
[Figure 2 ILLUSTRATION OMITTED]
Elastomeric textile fibers (refs. 5-8)
Elastomeric fibers are used for imparting elasticity to fabrics in a wide variety of applications, such as hosiery, lingerie and sportswear. They generally will be found hidden within a woven or knit garment fabric when clothing conforms to, extends with and physically supports the human body. The amount of elastomeric fiber depends on the type of control and comfort levels required. In constraint-type garments, such as girdles, foundations and swimming suits, 15-40% of the fabric may be elastomeric fiber. Sportswear knits may contain as much as 10% or as little as 3% elastomeric fiber.
Many materials meet the ASTM definition of an elastomer, but fail as elastomeric textile fibers. Chemically crosslinked rubber and Spandex, the generic group names of two elastomeric fibers, are both commercially important textile fibers based on, respectively, natural rubber and synthetic polyurethane.
In the production of chemically crosslinked rubber fibers, the rubber is crosslinked after extrusion which results in comparatively low spinning speeds and limitations with respect to fiber diameter.
Spandex fibers are block copolymers which consist of polyether or polyester soft segments and polyurethane-urea hard segments. This combination results in outstanding elastomeric properties. However, the molecular structure also results in poor thermal stability, and for this reason poor meltspinnability. In general, Spandex fibers are spun to mono- or multi-filament yams using either dry or wet solution spinning operations. These routes have obvious disadvantages with respect to attainable spinning speeds, fiber diameter and fiber geometry. Furthermore, there are some environmental issues related to the use of large quantities of highly toxic solvents.
In recent years, for economic and environmental reasons, a lot of research has been put in the development of meltspun elastomeric yams based on both polyurethane-esters (refs. 9 and 10) and polyether-esters (refs. 11 and 12). Both yams are produced on a commercial scale now. However, the meltspun polyurethane yarns still have limited thermal stability (important in dyeing and heat-set processes) and the meltspun polyether-ester yams show limited elastic recovery after being stretched repeatedly to higher elongations.
In principle, all thermoplastic elastomers (TPEs), including thermoplastic vulcanizates (TPVs), can be processed into continuous multi-filament fibers in conventional meltspinning operations. However, their performance is often rather poor in comparison to both elastomeric fibers based on chemically crosslinked rubbers and to synthetic polymeric yams based on segmented polyurethanes which are manufactured using dry- and wet-spinning operations. For instance, commercial TPVs based on isotactic PP, EPDM and mineral oil show limited elastic recovery when repeatedly stretched to higher elongations ([is greater than]200%). This limited elastic recovery is mainly due to yielding of the PP phase under large mechanical deformation and, therefore, this type of material is not used in the commercial production of elastomeric textile fibers.
Elastomeric fibers based on copolyetherester-EPDM TPVs
In this particular study, a new class of TPV materials is presented, and their processing behavior and properties are discussed with a special emphasis on fiber applications. In comparison to classical TPVs, the use of non-elastomeric materials such as isotactic PP is avoided and plasticizers, such as mineral oil, are omitted in order to enhance the elastomeric properties of the TPV.
Copolyetheresters were used as thermoplastic matrix material instead of PP. The TPV material was produced by combining a copolyetherester and a chemically crosslinked EPDM rubber in a dynamic vulcanization process. This new TPV combines the thermal processability of copolyetheresters with the elastomeric properties of chemically crosslinked EPDM rubber. A balance was found in the amount of copolyetherester (thermoplastic processability) and the amount of EPDM (elastomeric properties). So far, optimal results were obtained with a TPV material consisting of 40 wt.% copolyetherester and 60 wt.% dynamically vulcanized EPDM. Because of the relatively high EPDM contents, the EPDM would be present as a (co)continuous phase if no dynamic vulcanization step would be performed. So, the dynamic vulcanization does not only improve the elastomeric properties of the final TPV, but also inverts the morphology from a co-continuous system to a matrix particle system in which EPDM forms the dispersed phase.
The use of copolyetheresters as the thermoplastic matrix material results in TPV fibers with superior elastic recovery to isotactic PP-based TPVs, and the fibers are potentially heat-resistant and dyeable in polyester dyeing processes (ref. 13).
The polymers used are EM400 (polyetherester block copolymer), and K714 (ethylene-propylene-diene robber). Dynamic vulcanization of the copolyetherester-EPDM compounds, in different ratios, was performed on a co-rotating twin screw extruder (ZSK40) by using a suitable crosslinking system (phenolic resin).
Transmission electron microscopy (TEM) was performed using a Philips CM200 at an acceleration voltage of 120 kV. Contrast was obtained by staining with an Os[O.sub.4]/formaldehyde mixture.
Mechanical testing was performed on a Zwick 1455 with [L.sub.0] = 25 nun at 23 [degrees] C and 50 mm/min.
The tension set was analyzed on tapes cut from compression molded plaques. The permanent deformation was calculated after stretching the tape to a certain deformation, keeping it deformed for 10 seconds, and allowing it to relax for one hour. The tension set is defined as the permanent deformation relative to its original length.
Capillary flow measurements were performed on a Gottfert 2002 at a melt temperature of 230 [degrees] C. Melt strength and strain measurements were performed on a Gottfert 2000 in combination with a Gottfert Rheotens at a melt temperature of 230 [degrees] C.
Meltspinning was performed on an 18 mm Spintester of Fourne Polymertechnik with a spinneret diameter of 0.5 mm, L/d ratio of 2 and spinning temperatures in the order of 230 [degrees] C.
Fiber testing was performed on a Zwick 1455 in a five-cycle hysteresis (0% to 300%). Stretch resistance is the tensile force necessary to elongate the fiber to a specific elongation. Stress decay is the decrease in tensile force when the fiber is held in extended state (300%) for a specific amount of time (10 seconds). Tension set, after 5th 300% cycle, is the relative permanent increase in fiber length after cyclic stretching and relaxation, measured 30 seconds after the 5th cycle.
Transmission electron microscopy (TEM)
Figure 3 shows the morphology of a copolyetherester-EPDM (40/60 wt./wt.) blend after dynamic vulcanization. The copolyetherester phase has been stained in order to obtain contrast between both phases. Obviously, the EPDM-phase is present as small spherical particles and the copolyetherester forms the continuous matrix. The size of the EPDM-particles is in the order of three microns. By using a higher magnification, two separate phases can be distinguished in the copolyetherester part. These two phases are the hard polyester and soft polyether segments, present in the same polymer backbone and phase separated in solid state.
[Figure 3 ILLUSTRATION OMITTED]
The mechanical properties of the developed copolyetherester-EPDM TPVs, as a function of the EPDM content, are shown in figure 4. Both the tensile strength and the elongation at break decrease with increasing EPDM content. Most probably this can be ascribed to the lower tensile strength and elongation at break of the crosslinked EPDM part compared to the copolyetherester part. However, the elongation at break still exceeds 600% for all TPV combinations.
[Figure 4 ILLUSTRATION OMITTED]
The total amount of EPDM has a significant effect on the elastomeric properties of the Arnitel-EPDM TPVs, as shown in figure 5. The tension set is defined as the permanent elongation, relative to its original length, after stretching the material to a specific strain for a specific amount of time. The effect of adding a certain amount of crosslinked EPDM to copolyetheresters on the elastic properties of the final TPV is best observed at higher deformation ratios. Dynamically vulcanizing up to 60 wt.% EPDM into Arnitel decreases the tension set from 160% to 50% after deforming the material to 400%. At this deformation, the tension set of the TPVs shows an almost linear relation with the EPDM content.
[Figure 5 ILLUSTRATION OMITTED]
Dynamic mechanical spectroscopy
Introducing a vulcanized rubber into a thermoplastic has a dramatic effect on the viscosity of the latter. Figure 6 shows the viscosity as a function of the shear rate at various copolyetherester-EPDM rat-ios. Apparently, with increasing EPDM content, the rheological behavior changes from more or less Newtonian to ex-tremely shear-thinning. Typ-ical shear rates in meltspinning processes are in the range of [10.sup.4] [s.sup.-1] and higher. It is obvious that the viscosity of the copolyetherester-EPDM TPVs is almost equal at those shear rates. From figure 6 it can be concluded that the spinnability of copolyetherester-EPDM TPVs is highly de-pendent on the shear rate at which it is processed.
[Figure 6 ILLUSTRATION OMITTED]
Melt strength and melt strain
Another important property in the meltspinning of fibers is draw down. It can best be described as the difference between the spinning speed and the winding speed of the first roll. Thus, it might also be defined as the draw ratio applied to the fiber as it leaves the spinneret. At that point, the fiber is still in the molten state. At a particular spinning speed, the winding speed, diameter and properties of the fiber can be varied by varying the draw down. Figure 7 shows the maximum melt strain (draw down) and the melt strength at 230oC of the developed copolyetherester-EPDM TPVs as a function of the EPDM content. Apparently, the melt strength strongly increases with increasing EPDM content. On the other hand, the draw down shows a large decrease with increasing EPDM content. Important to note is that the melt strength and melt strain strongly depend on the temperature of the melt.
Meltspinning copolyetherester-EPDM TPVs
Meltspinning experiments have been performed with copolyetherester-EPDM TPVs containing 60 wt.% EPDM. The mechanical and elastic properties of the copolyetherester-EPDM TPV fibers largely depend on the level of orientation resulting from either the spinning conditions (shear rate and draw down) or solid-state drawing (draw ratio). As shown in table 1, solid-state drawing of the copolyetherester-EPDM TPV fiber increases the tenacity, decreases the maximum elongation, slightly decreases the stress-decay and significantly improves the elastic recovery.
Table 1 - properties of as-spun and drawn copolyetherester-EPDM TPV fiber
PP/EPDM TPV Copolyetherester as-spun as-spun Number of filaments 1 1 Fineness (tex=g/1,000m) 178 4.4 Tenacity (cN/tex) 1.3 8.3 Maximum elongation [%] 650 600 Five-cycle hysteresis (0-300%) Stretch resistance 5th cycle at 300% (cN/tex) 1.8 2.9 Stress decay 10s at 300% in 5th cycle (%) 30 20 Tension set after 5th 300% cycle (%) 103 194 Copolyether- Copolyether- ester/EPDM ester/EPDM TPV- as-spun TPV- drawn Number of filaments 1 1 Fineness (tex=g/1,000m) 60 43 Tenacity (cN/tex) 2.6 3.3 Maximum elongation [%] 610 460 Five-cycle hysteresis (0-300%) Stretch resistance 5th cycle at 300% (cN/tex) 2.9 4.5 Stress decay 10s at 300% in 5th cycle (%) 19 17 Tension set after 5th 300% cycle (%) 55 32
Both copolyetheresters and TPVs based on isotactic polypropylene, EPDM and mineral oil, show a limited elastic recovery when repeatedly stretched to high elongations (300%).
Compared to the fiber based on pure copolyetherester, adding up to 60 wt.% of a dynamically vulcanized EPDM results in melt-spun elastomeric fibers with an improved elastic recovery (permanent elongation decreases from 190% to 55% after a five-cycle/300% hysteresis). Solid-state drawing of the copolyetherester-EPDM fiber improves the elastic recovery even further (permanent elongation 32%) and also increases the tenacity.
Dynamic vulcanization of EPDM rubber in the presence of a thermoplastic copolyetherester results in raw TPV materials suitable for the meltspinning of elastomeric fibers. The resulting fibers have improved elastic properties compared to 100% copolyetheresters and traditional TPV materials. The copolyetherester-EPDM TPV fibers may well be heat resistant and dyeable in polyester dyeing processes. The mechanical and elastic properties of the copolyetherester-EPDM fibers can readily be adjusted by varying the EPDM content of the TPV. The maximum amount of crosslinked EPDM used in the dynamic vulcanization process is limited due to the limited thermal processability of the final TPV at high EPDM contents.
Initial experiments have shown that the concept described in this article can be translated to other TPE/rubber combinations, and the choice is not limited to copolyetheresters and EPDM.
[Figure 7 ILLUSTRATION OMITTED]
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|Date:||Sep 1, 1999|
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