Fibre Chemistry, Vol. 43, No. 1, July, 2011 (Russian Original No. 1, January-February, 2011)

EFFECT OF ELASTOMERIC ADDITIVES ON FABRICATION AND PROPERTIES OF FILLED FIBER MATERIALS FORMED FROM POLYMER SOLUTIONS

R. K. Idiatulov, Yu. P. Nekrasov, A. V. Genis, T. I. Samsonova, L. V. Polyakov, G. S. Kabanova, and I. F. Babich

UDC 678.074:678.027.39

We have studied the effect of elastomer additives on the properties of spin dopes for obtaining filled fiber materials by aerodynamic spinning from polymer solutions. We have obtained laboratory samples of materials with different polyurethane content. We have analyzed the results of physicomechanical, sorption, and physical tests of these materials. We show that modification of filled fiber materials by introducing elastomeric additives has only an insignificant negative impact on the sorption properties and markedly improves their physicomechanical characteristics.

Design of new efficient systems for life support and protection of the population from exposure to harmful environmental factors is based on modern technologies and materials. The latter can include sorption-active fiber materials obtained by aerodynamic spinning from polymer solutions containing solid finely dispersed fillers. A characteristic feature of these materials is high filler content (up to 70% of the total weight of the material), which in combination with extended surface area and porosity of the fibers provides high sorption capacities and rates of exchange processes. In this case, finely dispersed filler particles are firmly anchored in the structure of the polymer fibers: they do not flake off, they do not wash off, and they do not disintegrate during use. The diversity of fillers used (different grades of activated carbon, ion-exchange resins, silica gels, zeolites, active metal oxides, etc.) make it possible to obtain a wide variety of sorption-active fiber materials. At the same time, many performance properties of these materials (mechanical properties, chemical resistance, heat resistance, dimensional stability, etc.) are determined by the properties of the polymer system used. Modification of these systems makes it possible to considerably expand the range of application for fiber materials. In the technology for production of fibers and fiber materials, they are widely modified by introducing modifying additives or blending different polymers. We know of two methods for blending polymers that can be used to create a product consisting of one or more polymer components and having either a combination of their characteristics or new properties inherent only to the blend [1]. One method involves blending conventional synthetic and natural fibers (obtaining a fiber blend). In the other case, the fibers are obtained from a mixture of polymers (blending is done in the fiber forming step). In making filled fiber materials, this approach is preferred since these materials are obtained directly during aerodynamic spinning, as finished nonwoven fabrics. The aim of this study was to search for polymeric elastomers compatible in solution with a polymer that is quite well utilized in the process of aerodynamic spinning: polyacrylonitrile (PAN), and also to study their effect on the properties of the spin dopes and the filled fiber materials obtained from them. We chose elastomers as the modifying additive because of their unique properties, in particular the high flexibility of the polymer chain, which is why they can be deformed under small loads and recover their shape after significant deformation. Scientific Research Institute of Synthetic Fibers with Experimental Plant, Tver. Translated from Khimicheskie Volokna, Vol. 43, No. 1, pp. 63-66, January-February, 2011. 0015-0541/11/4301-0075 © 2011 Springer Science+Business Media, Inc.

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TABLE 1. Results of Preliminary Tests on Different Types of Elastomers Solubility in DMF

Spinnability of spin dope

Limited, solution is inhomogeneous Same Complete, transparent homogeneous solution Same Same Same Same

Unsuitable for spinning Same Unstable spinning, high breakage То же Same Stable spinning, stable Same

Viscosity, Pa·s

Viscosity, Pa·s

Elastomer

Butadiene rubber SKD Natural rubber Desmopan-385 Elastolan S70A Elastolan S80A TPU T-1413-85 TPU T-1533-90

PU Content in PAN-PU mixture, wt.%

Fig. 1 Viscosity of spin dope vs. polyurethane content: 1) PAN+PU1; 2) PAN+PU2.

Time, days

Fig. 2 Time variation of viscosity for spin dopes: 1) PAN; 2) PAN+PU1; 3) PAN+PU2.

Consequently, introducing an elastic component may lead to an increase in the relative elongation of the fibers and an overall improvement of the physicomechanical characteristics of filled fiber materials. The objects of investigation were the following elastomers: natural rubber, SKD butadiene rubber, polyurethane based on Desmopan-385 polyester, polyurethanes based on Elastolan S70A, Elastolan S80A polyesters, thermoplastic polyurethanes (TPU) based on T-1413-85 polyester and T-1533-90 polyether. As the solvent, we used dimethylformamide (DMF), which is used in production of PAN fibers. In the initial stage, we evaluated the suitability of the indicated elastomers for obtaining filled fiber materials. The investigations included study of the solubility of these elastomers in DMF, preparation of spin dopes based on a mixture of PAN and elastomer solutions, an evaluation of their spinnability. The results for this stage of the investigations are shown in Table 1. In selecting the elastomers, we started from the following requirements: complete solubility in DMF with formation of a homogeneous solution; compatibility with other components of the spin dope; good spinnability under aerodynamic spinning conditions. As follows from the data in Table 1, only the thermoplastic polyurethanes T-1413-85 (PU1 in the following) and T-1533-90 (PU2) meet the listed requirements to a large extent, so further studies were conducted using these elastomers. In order to prepare the spin dopes, we used 11%-14% solutions of PAN in DMF and 15%-25% solutions of PU1 and PU2 in DMF, which were mixed in the specified proportions. In this case, the polyurethane content in the polymer blend was varied from 0% to 90%. Finely dispersed filler (activated carbon) was introduced into the mixture of polymer solutions during their blending. The amount of filler was 100% of the total weight of the polymers. The effect of many factors such as molecular weight, concentration of the polymer, content and dispersity of the filler etc. on the viscosity of the PAN-based spin dopes was considered previously in [2, 3]. In this work, we focused special attention on studying the effect of polyurethane additives on the viscosity of the spin dope. The viscosity of the dopes was measured using a Polimer RPÉ-1M.2 viscometer. Fig. 1 shows the measurement results for the viscosity of blended dopes with equal concentrations of the components and with different polyurethane contents. As the amount of polyurethane introduced increases, we see a nonlinear decrease in the viscosity of the spin dopes both with polyester-based polyurethane (PU1) and polyether-based polyurethane (PU2). This 76

Nitrogen

Spin dope

Compressed air

Coagulator

Coagulation bath for regeneration

Fig. 3 Schematic diagram of the apparatus for spinning nonwoven material from polymer solutions: 1) spinning unit; 2) nozzle device; 3) coagulation bath feed circuit; 4) take-up drum; 5) reducer; 6,10) electric motor; 7) spin dope tank; 8) spin dope pump; 9) reducer; 11) bottom plate of the coagulation bath; 12) air flowmeter; 13) needle valve. type of variation in the viscosity is obviously connected with the lower molecular weight of the elastomer compared with the fiber-forming polymer (PAN). On the other hand, for incompatible polymer systems, including a biphasic mixture of PAN and PU solutions, a “polymer emulsion” of the “oil in oil” type can be formed. A characteristic difference between such emulsions and classical “oil in water” emulsions is the relatively low surface tension at the phase interface [4]. This leads to the fact that the concentration of entanglements at the interfacial surface is relatively low, or such entanglements are completely absent. As a result, a very thin layer of solvent may appear around a droplet. Such a low-viscosity layer may be the reason for the anomalously low viscosity of such polymer blends. This is possibly also why, in dopes based on PAN and PU solutions, structure formation processes occur more slowly and to a lesser extent, evidence for which comes from the time variation of their viscosity. From the data presented in Fig. 2, we see that the viscosity of spin dopes not containing polyurethane increases over time significantly faster than for dopes based on a PAN+PU mixture. The effect of polyurethane additives on the viscosity has to affect the behavior of spin dopes in aerodynamic spinning. This method essentially involves uniaxial stretching of a jet of polymer solution, flowing out from an annular nozzle, by a stream of compressed air [5]. Spinning was done on a laboratory apparatus to obtain filled fiber materials from polymer solutions. A schematic drawing of the apparatus is shown in Fig. 3. Assessment of the spinnability of the spin dopes involved determining the initial velocities of the compressed air at the nozzle outlet, for which we observed a change in the stability of stretching of the polymer jet. This method allows us to experimentally define the stability ranges for aerodynamic spinning for any spin dope. Previously [5, 6], we used a similar method to establish the stable spinning ranges for solutions of PAN and its copolymers (CPAN). The presence of such stability regions is also observed during production of fiber materials from polymer solutions. Within this investigation, in order to evaluate the effect of PU additives on the spinnability, we prepared spin dopes of equal viscosity, with initial Newtonian viscosity equal to 15 Pa·s. Fig. 4 shows the experimentally determined boundaries of the spinnability regions for spin dopes based on solutions of PAN and PU1: I is the primary spinnability region for the spin dope, II is the unstable spinning region, III is the secondary spinnability region. Analogous dependences were also obtained for the PAN+PU2 dopes. Analysis of the curves shows expansion of the spinnability regions as the PU content increases in the polymer blend and, as is especially important, a shift of the upper boundary of the secondary spinnability region toward higher air flow velocities. 77

TABLE 2 Basic Properties of Fibrous Carbonaceous Materials Made from Spin Dopes with Different PU Contents Composition of material

PAN + PU1 + carbon*

PAN + PU2 + carbon*

PU content in polymer blend, wt.%

Breaking load, N

Elongation at break, %

Sorption capacity relative to benzene, mg/g

Air permeability, dm3/(m2·s)

0 20 40 60 80 90 0 20 40 60 80 90

18.6 19.5 20.4 30.2 37.1 46.5 18.6 18.8 19.7 28.7 33.5 39.8

4 6 15 42 75 98 4 5 12 35 62 85

181 175 170 165 163.8 162.5 181 170 167 165 161 159.5

318 305 292 267 246 237 318 298 285 257 248 230

Air velocity at the nozzle outlet, m/sec

______________ *Carbon content is 100% of the polymer weight.

PU content, %

Fig. 4 Effect of PU content on spinnability of spin dope. Spinnability regions I-III are explained in the text. The increase in the spinning stability may be due to the increase in the fraction of highly elastic deformation in the total deformation of the jet with an increase in its draw ratio [7]. With an increase in the rate of deformation of the polymer jet by the compressed air stream in aerodynamic spinning, a dramatic decrease occurs in the time over which the forces act on the jet, down to values comparable with the relaxation time. In this case, the liquid jet begins to behave like an elastomer, having higher values of the breaking stresses and breaking forces. With an increase in the polyurethane content in the spin dope, highly elastic properties appear to an even greater degree. As a result, the spinning process takes on a more stable character, which has to affect the properties and quality of the material obtained. This is especially important if we consider the fact that the aerodynamic spinning process is a one-step process: the material is obtained and its structure is formed simultaneously while spinning. In order to evaluate the effect of polyurethane on the properties of the finished material, on a laboratory apparatus under the same conditions we obtained samples of carbon-filled fiber material with different contents of elastomers PU1 and PU2. The surface density of all the samples was 150 g/m2. The test results for these samples (Table 2) show that introducing a flexible-chain polymer (polyurethane) markedly affects all the basic properties of the material, where this effect is complex. Thus when polyurethane is introduced, we observe an increase in strength and elongation at break, especially marked for a polyurethane content greater than 40%. This is due to the fact that the physicomechanical properties are affected not only by the properties of the polymer matrix, but also by the structure of the nonwoven material itself. In nonwoven fiber material, layers of fibers are laid down in such a way that during its uniaxial stretching, we observe an effect of straightening of individual fibers. And as the deformation of the material increases, the number of straightened fibers increases. Thus here two processes occur simultaneously: straightening of the fibers and stretching of the fibers [8]. Obviously with an increase in the amount of flexible-chain polymer, the elasticity of the individual fibers increases, which leads to an increase in the breaking load for the material. Furthermore, introducing polyurethane leads to formation of 78

strong bonds between fibers at points where they intersect, and a special kind of network is formed. For a polyurethane content greater than 40%-50%, such a network takes on a stable reinforcing structure. With a further increase in the polyurethane content in the material, the density and strength of the structure increases. As a result, such materials are distinguished by higher strength and elasticity than PAN-based materials. In this case, the uniformity of the materials increases, evidence for which comes from the values of the coefficients of variation for the physicomechanical parameters. Thus the coefficient of variation with respect to the breaking load is reduced from 31.2% for PAN-based materials down to 7.9% for materials with polyurethane; with respect to breaking elongation, it is reduced from 38.9% down to 10.7% respectively. An increase in the number of bonds and compaction of the structural network in the material leads to a decrease in the pore sizes in the nonwoven fabric: the maximum pore size is reduced from 140 µm down to 80 µm, while the main pore size is reduced from 100 µm to 75 µm. Such a change in porosity on the one hand leads to some decrease in the air permeability of polyurethane-containing materials (see Table 2), but on the other hand improves their filtering capacity. At the same time, with an increase in the polyurethane fraction, we observe some decrease (up to 10%) in sorption capacity of carbon-filled fiber materials. This is due to formation of a denser shell of individual fibers, leading to a decrease in the porosity of their surface. Thus introducing an elastomer (polyurethane) into the polymer matrix of filled fiber materials, obtained from polymer solutions by aerodynamic spinning, markedly improves their fabrication process, improving the stability and spinnability of blended spin dopes, and the physicomechanical characteristics of the finished materials. Such modification of filled materials appreciably expands their variety and consequently also their range of application. The slight reduction in sorption capacity of the modified materials can be compensated by introducing an additional amount of active filler into the spin dope, which makes it possible to create reduced viscosity in blended polymer systems. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

D. R. Paul and S. Newman, eds., Polymer Blends (in 2 volumes) [Russian translation], Mir, Moscow (1981), Vol. 2, pp. 179-232. Yu. P. Nekrasov, R. K. Idiatulov et al., Khim. Volokna, No. 1, 4043 (2010). A. V. Smirnov and A. V. Genis, Khim. Volokna, No. 1, 41-45 (2002). R. F. Gold, ed., Multicomponent Polymer Systems [in Russian], Khimiya, Moscow (1974), pp. 61-71. A. V. Smirnov and A. V. Genis, Khim. Volokna, No. 3, 26-33 (2002). M. S. Mezhirov, R. K. Idiatulov et al., in: Preprints, Second International Symposium on Chemical Fibers [in Russian], Kalinin (1986), Vol. 3, pp. 304-308. A. V. Smirnov and A. V. Genis, Khim. Volokna, No. 4, 57-62 (2001). Yu. N. Filatov, Electroforming of Fiber Materials [in Russian], Neft’ i Gaz, Moscow (1997). 297 pp.

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