Polyester fiber and microfiber structure by X-rays and viscoelasticimetry
We have studied three polyethylene terephthalate filaments from the same manufacturer. The results of the dyeing process depend on the filament characteristics. Dyeing modifies the macromolecular organization according to the manufacturing process. To understand the dyeing influence, each filament undergoes heat treatments that reproduce the effect of dyeing. Organizational molecular modifications are determined by x-ray diffraction measurements and by a viscoelasticimeter. The study shows that drawing and shearing align macromolecules that act on the crystallinity. Thus, the crystallinity of a trilobal filament and a microfilament are almost identical, whatever the heat treatment. The less significant drawing of a circular filament provides more amorphous and more sensitive filaments for heat treatment. Super-drawing induces the existence of macromolecular segments of great mobility. Moreover, organizational heterogeneity is detected at the time of the glass transition.
The emergence of synthetic fibers has constituted a revolution in the textile industry. Thus, poly(ethylene terephtalate) (PET) became a polymer largely used by the industry. Polyester fibers have interesting organoleptic properties, but some aspects of the behavior of these fibers still differ from those of certain natural fibers like silk, mainly due to their fineness. A reduction in the section of a fiber allows a quick increase in flexibility and brightness. Textiles created with very fine fibers are more pleasant to the touch and provide better thermal comfort [1, 12, 21]. Thus, microfibers have extended the range of uses for PET, but these new fibers require the modification of certain processes. Thus, the tinctorial behavior of these fibers is characterized by different dye absorption [16, 17], and color uniformity is also more difficult to obtain , so manufacturers have to adapt some aspects of the dyeing process. Mechanical behavior is also affected by the reduction in fiber diameter. For example, the creep phenomenon is facilitated by the microfiber structure. Our study attempts to contribute to the understanding of these modifications.
If the polymer is identical for the various textile fibers being studied, noted differences can have two origins. The first is the scale factor, but taking account of this parameter does not explain all the differences. The second origin of differences stems from modifications of the macromolecular organization, and it is this latter point that we examine in this work. For that, we consider a microfiber and two other fibers from the same manufacturer. This microfiber is obtained by drawing to a higher draw ratio than traditional fibers. We choose polyester fibers because of their properties. The macromolecular organization in crystal line fields is very different with drawing, and can support the detection of modifications that affect fiber properties. To study the structure quantitatively and qualitatively, we select two methods of analysis, the combined use of x-rays diffraction and viscoelasticity, which allows better characterization of fibers. The first technique gives information about macromolecular three-dimensional space organization in the crystalline regions. Such information is global for the entire fiber, but this technique does not allow us to detect the interactions of the macromolecular chains. For that, we use viscoelasticity, which provides, according to the temperature and the frequency, the mechanical behavior of the molecular structure. The results of the two techniques used for each fiber type will contribute to a better knowledge of polyester microfibers obtained by superdrawing.
Materials and Methods
To study microfiber structure, we have tested in parallel three kinds of polyester filaments. The first two, which we will call “classical polyester,” are manufactured by Rhone-Poulenc. These yams come directly from the manufacturer and are not subjected any treatments other than those necessary to their production (extrusion, drawing, spinning, and winding). So that the comparison is more significant, we also work with microfilaments from the same manufacturer. Table I shows the characteristics of the three products.
An examination of x-ray diffraction intensity in the plane (105) reveals the longitudinal extension of crystallites. For virgin fibers, we note that the circular fiber has a specific behavior. The diffraction intensity curve according to the azimuth presents only one peak. For the other two fibers, there are two peaks centered on the azimuth at -7.5 and 7.5 deg. This shows that there is a preferred direction of crystallites, and this direction forms an angle of 7.5 deg with the longitudinal axis of the fiber.
Considering the sit of the most intense peak, we have studied the influence of heat treatment. For the circular fiber, it is located at the 0 azimuth, whereas for the other two fibers, it is at the 7.5 deg azimuth.
Table II shows that the fall of the middle height width of the peak is globally larger for a high treatment temperature. By examining the individual behavior of each fiber, we can see that the microfiber is sensitive to heat treatment only beyond 80 deg C. On the other hand, crystallites of the circular fiber evolve quickly, confirming the results of the equatorial interference. Note that the preferred orientation of the microfibers and the trilobal fibers is retained with heat treatment.
The viscoelasticimeter provides for each temperature the value of the dynamic modulus of the fibers, which has two components. The first is E’ and characterizes the elastic field of the material. The second is the loss modulus E”, which corresponds to viscous dissipations. In the range of temperatures used, the evolution of the modulus E’ shows clearly that there is an a transition (or glass transition) for temperatures close to 100 deg C. During this transition, the macromolecular chains become more mobile, resulting in a drop in the elastic modulus. Apart from this transition, other changes do not appear so visible. The /3 transition is difficult to highlight with the E’ values, and corresponds to rearrangements of low amplitudes of some molecular segments. Its presence will be more easily detected by studying the two moduli E’ and E” in parallel.
Several parameters must be studied to analyze the data. The modulus evolution is split into three zones. In particular, we define the mechanical behavior before, during, and after the glass transition for all fibers for the various heat treatments.
This heat treatment involves a drop in the E’ modulus for the three kinds of fibers. This decrease is rather significant after a treatment temperature of 80 deg C. An increase in the heat treatment temperature will thereafter have less effect on the modulus before the transition a. The glass transition best highlights the influence of heat treatment on fibers. Thus, the conservation modulus more strongly varies according to the annealing temperature. In general, the E’ values are all the more low since the fiber has been modified by the combined action of heat and water. In this kind of treatment, the presence of water facilitates displacements of the molecular segments. In a gaseous and dry environment, temperatures must be higher to obtain a comparable modification. Table III shows the E’ values for test temperatures of 20 and 175 deg C.
Circular and trilobal virgin fibers have some points in common. Indeed, the E’ values (respectively 17.4 X 10^sup 9^ and 16.7 X 10^sup 9^ for the test temperature of 20 deg C) are very close. The difference is of the same order of magnitude as the uncertainty that exists for the measurements. The microfiber modulus is distinctly weaker. This property remains true until the glass transition, then at a given temperature, an inversion occurs as shown by the values at 175 deg C. So the mechanical properties of the microfiber are less sensitive to heat treatment without tension. After the glass transition, the behavior of circular and trilobal fibers depends on the heat treatment.
The heat treatment also influences the value of the elastic modulus at each measurement temperature. Thus, an increase in the annealing temperature involves a drop in E’ for all the fibers, but this evolution is reversed for treatments ranging between 100 and 110 deg C. Then the modulus again becomes weak for a treatment at 135 deg C. It appears to be possible to establish a correlation with the results obtained from x-ray diffraction.
Let us examine in more detail the influence of thermofixation at 100 deg C. The mechanical behavior of fibers clearly depends on the temperature of the mechanical test. Before the glass transition temperature, behavior is identical to that of virgin fibers, except for the values of the modulus, which are weaker. Beyond this temperature, the elastic modulus of the circular fiber quickly becomes weak, but the trilobal fiber retains a modulus rather close to that of the microfiber.
The a transition is significant for better understanding of the textile fiber structure. Indeed, this phenomenon corresponds to a progressive release of the macromolecules according to the temperature. This is a function of the immediate environment of each molecular section. The transition is made for a temperature range that is more significant than the environment is heterogeneous. The extended transition measurement is based on the tangent method, and consists of tracing the two base lines before and after the transition and determining the intersection of these lines with the line describing the evolution during transition. This graphic method gives values for which the maximum error is close to 5%.
Table IV shows that the extent of the transition is very significant for virgin microfibers (near 100 deg C). For other fibers, the transition is more brutal (about 60 deg C). The heat treatment at 80 degC has a dramatic effect on fibers. Thus, the trilobal fiber has a transition whose dimension increases strongly (90 deg C versus 60 deg C). On the other hand, the a transition duration becomes shorter for the microfiber. The same phenomenon occurs for circular fibers, even if the decrease in the transition duration is smaller (13 versus 45 deg C for the microfiber). An increase in the heat temperature then causes an evolution characterizing each kind of fiber. Thus, for the microfiber and the circular fiber, the increased temperature increases the dimension of the a transition. This increase is significant for the circular fiber, but for the microfiber, the dimension passes from 63 to 70 deg C. For the trilobal fiber, the extent becomes close to 90 deg C.
The tinctorial properties of fibers depend on the organization of their macromolecules, defining the zones where dye accumulates and the diffusion effectiveness of the dyebath molecules. But macromolecular organization is a result of a fiber’s history, ie., the manufacturing process (spinning, drawing, and finishing). Structural changes depend on the initial macromolecular organization. At each temperature, only certain structures will be able to change, and fiber properties will change according to the severity of the heat treatment. Our physicochemical characterization of the filaments is based on this guiding principle.
The influence of the spinning process is highlighted by x-ray diffraction. The results indicate that the circular fiber has a weaker crystallinity index and a more significant dispersion of crystallite size than the trilobal or microfiber. In spite of the heterogeneity of crystallites, there is a total orientation of the crystalline zones along the fiber axis, which is induced by the manufacturing process. During spinning, macromolecular alignment and individualization are modified by shearing at the spinneret orifices and drawing. Shearing is, at first approximation, inversely proportional to the orifice diameter and proportional to the polymer velocity. This phenomenon can induce macromolecular orientation in the filament even if the swelling after the spinneret reduces this effect. Drawing multiplies by four or five the classical filament length, mainly providing orientation to the macromolecules, then cooling fixes the internal organization of the fiber. However, microfilament drawing is four times more significant, so its macromolecules have more chances for a strong orientation.
Circular fiber macromolecules will not be completely oriented and individualized by drawing. Indeed, the crystal needs two conditions in order to form: the intermolecular distance must be sufficiently small and the chains must be correctly oriented. These conditions are met with difficulty by most macromolecules, which explains why there are crystalline fields of small size. But under high spinning and drawing action, the macromolecules might be deployed enough to decrease the distance between chains, so the size of oriented zones will be more significant. This is the case for the microfilament with super-drawing. The results show better alignment and individualization of macromolecules. Thus, the crystallinity index of the microfilament is higher and crystallite growth is different. A comparison of the results in the directions perpendicular to the plan (010) and (100) allows us to estimate the preferred orientation of crystallites. It then appears that growth is slower in the direction (100), due to interactions of the molecules of polyethylene terephthalate glycol. Indeed, dipole-dipole interactions of the adjacent ester groups along the fiber axis are more significant than interactions of aromatic electrons 7r in the a-direction. Many authors present schematic representations of these interactions [13, 14], and the polymer is (re)crystallized in the direction of the strongest interactions, thus minimizing the energy of the system.
This crystallite orientation in the microfilaments is also visible in the trilobal filaments. Thus, for the microfiber and the trilobal fiber, the maximum is detected with a 7.5 azimuth, so the direction of the crystallites forms an angle of 7.5 with the longitudinal axis of the fiber. For the trilobal filament, the particular shape of the spinneret orifice can explain its orientation. It divides the polymer melt flow into three parts, so for the same flow, shearing of the macromolecular chains is definitely more significant. Consequently, the crystallinity indexes of the trilobal filament and the microfilament are close.
Macromoleculare organization will be modified by heat treatment. In some fibers, this modification shows a drop in diffraction peak width. Many authors observed this phenomenon , which is a consequence of the growth of the crystal and the global increase in crystallite perfection. This structural evolution is stronger with a higher bath temperature.
In the case of the circular fiber, the macromolecules remain tangled up after drawing. Thus, the crystallinity index is more significant after an 80 deg C treatment, whereas the different peak widths peak do not change. This shows that crystalline fields are not wider but more numerous. For heat treatment temperatures between 100 and 120 deg C, the traditional circular fiber has a constant crystallinity index, which does not mean that the polymer structure is fixed. The middle height widths of the diffraction peaks continue to decrease, the crystallite quality increases, and certain crystalline fields are incorporated to promote wider crystallites.
The increase in the microfilament crystallinity index remains weak for an 80 deg C treatment. Indeed, superdrawing already induces significant crystallinity. After heat setting at 110 deg C, the crystallinity index is definitely larger, and between 110 and 135 deg it evolves more slightly. The same phenomenon occurs with the trilobal filament, the alignment of the macromolecular chains being the origin of this common behavior.
X-ray diffraction provides an instantaneous picture of the polymeric structure, but for mechanical applications, dynamic behavior is important. For that, a mechanical study is necessary. Mechanical experiments show that the elastic modulus is degraded after heat treatment, and its influence is more visible after the glass transition. However, an analysis of the x-ray diffraction results show an increase in fiber crystallinity, and this behavior confirms Takayanagi’s model [10, 15]. It is the oriented amorphous zone that confers part of the fiber resistance. However, during spinning, the stresses of shear and drawing involve an alignment and an untangling, more or less significant, of the macromolecular chains. Thus, amorphous zones mainly consist of macromolecules whose orientation is close to the fiber axis. In this oriented amorphous zone, the molecules of the polymer will also be in an unfolded shape. During the test, these macromolecules will offer significant resistance because of their weak possibility of expansion. However, heat setting without external tension easily modifies the amorphous zone. The heat may possibly allow the macromolecular chains to acquire sufficient mobility to adopt a more stable conformation. Internal forces within each macromolecule tend to fold the chains in the shape of ball, so global crystallization can grow by following the molecular closeness of the segments. With this more compact shape, the macromolecules of the amorphous zone are less oriented, so the fiber has greater deformation under external stress. Before this change can occur, it is necessary that the fiber be free to retract. This is not the case if an external constraint is applied during heat treatment. The importance of external stress during heat setting is obvious by looking at the mechanical behavior after the glass transition. Practically, the distribution of the modulus values according to heat setting does not change before and after the glass transition. Despite the high temperatures experienced by the filaments preceding change, the internal organization remains.
Our analysis of the modulus values must be supplemented by the glass transition study. It is during this transformation that the interactions of the molecular segments occur . The shape of the loss angle tangent curve gives an idea of the distribution of the interaction energy of the molecular segments. A significant value of the loss angle tangent indicates a strong modification of the molecular organization. The loss angle tangents at 75 deg C (a test temperature) show that the microfilament presents a significant percentage of molecular segments with great mobility. However, the end of the loss angle peak tangent for various fibers occurs at rather close temperatures. The extent of the transition is, as a consequence, greatest for the microfilament. This filament has a great heterogeneity of macromolecular segmental length that can be released. Whatever the severity of the heat treatment, the mobility of the molecular segments is increasingly more significant for the microfiber. This behavior can be explained by supposing that the superdrawing of the microfiber allows more strongly individualized macromolecules. Thus, the molecular segments will be tangled up and so more mobile. After heat treatment, folding of the macromolecular chains will not increase the tangling of the macromolecules.
The glass transition temperature is normally determined by the mean energy of the interaction of macromolecular segments. In a crystallite, the molecules are ordered and have a more compact arrangement. This favors high interaction energies of the macromolecules closed to this domain, so an increased crystallinity index must involve an increased glass transition temperature. Table V traces the change in glass transition temperature according to the crystallinity rate. The increased glass transition temperature is regular for the microfiber, and the circular and trilobal fibers behave this way overall. But this evolution is characterized by a significant fluctuation at heat setting temperatures ranging between 110 and 120 deg C.
This study emphasizes the importance of spinning conditions on filament properties. The significant parameters are drawing and shearing of the melt polymer in the spinneret orifice. Drawing and shearing allow the deployment of the macromolecules, and also their untangling more or less.
The three fibers we have examined each present an organization resulting from a particular alignment of the chains. The circular fiber is less organized, and its amorphous zone is most significant. It has crystallites of small size oriented in the fiber direction. With a more significant drawing rate, the amorphous zone decreases and crystallites are more perfect and wide. The microfilament’s macromolecules are strongly oriented. Untangling makes them more mobile as the loss angle tangent shows. Other studies with different techniques confirm this interpretation [5, 6], but this greater mobility is not generally what induces a greater heterogeneity of interaction energies. The macromolecular orientation of the trilobal fiber overall is similar to the microfilament, but it appears that the macromolecules of this filament remain more intermingled, as do the circular filaments.
Manufacturing conditions have a very significant influence on the macromolecular organization, which is reflected in their dyeing behavior. Indeed, a filament with mobile molecular segments will more easily accept dye molecules, and dye diffusion in the filaments will also be faster. Our study shows that dyeing greatly influences the mechanical properties of the filaments, modifying the elastic modulus. To limit its evolution, it is necessary to maintain the orientation of the macromolecules by the action of an external constraint during annealing. Accordingly, a study complementary to the structural characterizations due to stress and treatment temperature is necessary.
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F. DIEVAL, D. MATHIEU, AND B. DURAND
Laboratoire de Physique et de Mccanique Textiles, Ecole Nationale Superieure des Industries Textiles de Mulhouse, 68093 Mulhouse Cedex, France
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