Micro- and Macrorheological Properties of Polypropylene-Polyoxymethylene-Copolyamide Mixture Melts.
Processing of polymer mixtures allows not only an opportunity of combining the properties of two or more components in a single product but also the possibility of obtaining unique effects. The phenomenon of the specific fiber formation  is one such effect. Under the action of rheological forces, a great number of ultrathin fibers of one of the polymer mixture components are formed in the bulk of another polymer component, and these fibers are strictly oriented along the direction of extrusion. This phenomenon has been studied in detail in our works [2, 3]. Besides the indicated fibrous structure, other various structure types may exist in the polymer mixture extrudates. According to Van Oene , structures of two main types are formed in the flow of polymer mixture melts. In layered flow (stratified), both components form continuous phases with the continuous interface. In dispersed flow, one component is in the dispersed state and distributed as droplets in the other component, forming the continuous ph ase. With certain mixture compositions, the flow of stratified morphology may go over into the flow, creating an interlacing network [the matrix structure ] for which it is impossible to define which polymer forms the dispersed phase and which polymer forms the continuous medium.
The effect of structure formation upon processing of polymer mixture melts may be controlled by addition of a third component [plasticizers, surfactants  and other substances of polymeric and non-polymeric nature [3, 6]]. Addition of a third component with ability for specific interactions with one or both components of a binary polymer mixture is of particular interest [7, 8]. There are the following types of specific interactions: hydrogen bonds, donor-acceptor, dipole-dipole, or ion-dipole intermolecular interactions. It has been shown  that such types of interactions are driving forces for the miscibility of polymers. The additives change the ratios of viscosity and elasticity of polymers, and the properties and thickness of the interphase layer, and this allows control over structure-formation processes in polymer mixtures.
This work is a study of the possibility of controlling the structure formation in polymer mixtures with the use of a third component, i.e., additives that are able to interact specifically with one of the mixture components.
Polypropylene-copolyamide (PP/CPA) mixtures with component ratios of 20/80 and 40/60 wt% were used. The additive polyoxymethylene (POM) was introduced into the indicated mixtures in the following quantities: 5, 10, 20, and 30 wt% of the PP weight. Thus, PP/CPA/POM mixtures with component ratios of 20/80/5, 20/80/10, 20/80/20, 20/80/30 and 40/60/5, 40/60/10, 40/60/20, 40/60/30 were obtained. The characteristics of the polymers used are listed in Table 1. The ternary polymer mixtures were prepared by using a screw-disc extruder of combined type. The principal mixing part of the machine is a pair of discs, moving and fixed. Mixing of the polymers takes place in the slit between the discs owing to the action of shear and tensile stresses. The mixtures were prepared at 190[degrees]C.
2.2. Methods of Investigation
The viscosity of the starting polymers and the binary and ternary polymer mixtures was determined with the use of a constant-pressure capillary viscometer. The capillary was 780 [micro]m in diameter and 7800 [micro]m in length. Values of melt elasticity of the starting polymers and mixtures were estimated by the extent of swelling of extrudates after annealing in accordance with the special procedure . Swelling was determined as a ratio of the annealed extrudate's diameter to the forming die's diameter. Annealing consists of maturing the extrudate in a liquid of necessary density at a temperature close to the polymer's melting temperature for a short time. Annealing leads to relaxation of the elastic deformations accumulated in the process of mixture melt flowing and attaining an equilibrium value of extrudate swelling. The following conditions of extrudate annealing were used in our experiments: T = 170[degrees]C, a time interval of 300s, the polymethylsilicone liquid. The processes of structure-formati on in the mixture extrudates were analyzed qualitatively with the use of micrographs of sections prepared in the transverse and longitudinal directions. Quantitative analysis was carried out by microscopic study of the dispersed polymer residue after extraction of the matrix polymer . The microscopic data were treated by methods of mathematical statistics. Average sizes, distribution variance, number and mass percentage were determined for all structure types studied. The percentage in weight of continuous fibers, short fibers, etc., was calculated on the basis of the mass of a given type of structure and the total mass of all structure types. For example, the mass of continuous fibers is calculated with use of the following data: average diameter of microfibers, number of microfibers, length of the specimen taken for investigation, additive density of PP/POM mixture. Ethyl alcohol was used as a solvent for extraction of the matrix polymer (CPA). POM was dissolved by concentrated sulfuric acid. The processability (spinnability) of the studied molten mixtures as well as of the starting polymers was estimated by the value of maximum jet stretch ([JS.sub.max]), that is, the ratio of maximum velocity of reeling on to the velocity of the melt outflow from the die. As the matter of fact, this is a process of non-isothermal longitudinal deformation, which always occurs upon the formation of fibers.
3 RESULTS AND DISCUSSION
3.1. Rheological Properties of PP/CPA/POM Mixture Melts
Lipatov  has shown that the hydrogen bonds are formed between POM and CPA macromolecules in the melted state. That is why it is to be expected that POM additives will affect the rheological properties of binary mixture melts as well as the process of PP fiber formation in a CPA matrix. The data obtained have confirmed these assumptions.
The dependence of the mixture melt viscosity ([eta]) upon POM content in the PP/CPA mixture of 40/60 composition is shown in Fig. 1. One can see that this dependence is ascending at all shear stresses studied. However, the curves of [eta] versus POM content dependence feature a minimum at POM content of 5 wt%. As will be shown further, this minimum may be explained by improvement of the PP fiber formation in the CPA matrix. The process of fiber formation involves the generation of streams of liquid PP in the bulk of CPA. It is known  that the deformation of droplets in the fluid state results in a significant decrease in the pressure gradient as against the pressure gradient when the droplets are in the non-deformed state. The flow resistance and, consequently, the viscosity are decreased since the droplets become more deformed with increasing [tau]. That is why the fiber formation process is always accompanied by a decrease in the mixture melt viscosity. On the other hand, the specific interactions betw een the macromolecules of the mixed polymers will increase the mixture melt viscosity. Thus, the data of Fig. 1 may be explained as a result of simultaneous actions of the two indicated factors. The increase in melt viscosity due to specific interactions prevails over its reduction due to the process of fiber formation. As a result, when POM content is 20 or 30 wt%, the ternary mixture melt viscosity increases by a factor of 1.7 to 3.0 as against the binary mixture.
The generated hydrogen bonds change the structure of the matrix polymer (CPA) melt and augment the flow resistance that leads to an increase in viscosity. The number of generated hydrogen bonds depends upon POM content and shear stress values. The larger the POM content and the lower the [tau], the sharper the increase in the mixture melt viscosity and the larger the ratio of the ternary mixture viscosity ([[eta].sub.t]) to the viscosity ([[eta].sub.b]) of the binary mixture (Table 2). Under the effect of large shear stresses, a part of the hydrogen bonds is broken, and as a result the ratio [[eta].sub.t]/[[eta].sub.b] decreases. The same dependencies in the variance of viscous properties upon addition of POM have been observed for PP/CPA mixtures of 20/80 in composition (Fig. 2). However, in this case, the minimum on the curves of [eta] vs. POM content is absent, and the degree of increase in viscosity depends upon shear stresses only for a PP/CPA/POM mixture of 20/80/30 in composition (Table 3).
One of the important rheological characteristics of polymer mixture melts is elasticity caused by stretching and orientation of macromolecules along the direction of flow. For polymer mixture melts, the elasticity also depends upon the process of dispersed polymer structure-formation. The extrudate swelling ("B") is an indirect characteristic of the melt elasticity. Its value is significantly larger for polymer mixtures than for each individual polymer. As a rule, the dependence of "B" versus mixture composition is represented by curves with a maximum . It has been shown  that "B" may be used as an indirect characteristic of the specific fiber-formation: the thinner the produced microfibers and the greater their number, the larger is "B." The presence of other structure types suppresses "B." The data on the effect of POM additives on the elasticity of PP/CPA mixture of 40/60 and 20/80 in composition are shown in Fig. 3. One can see that dependence of B" versus POM content is represented by curves with an extremum. Upon addition of POM, the magnitude of swelling increases, passes via the maximum, and drops lower than the value of "B" for the binary mixture at POM content of 20 to 30 wt% (Fig. 3). On the basis of this dependence, one can assume that addition of 5 to 10 wt% of POM has a positive effect on the specific fiber formation. Upon further increase in POM content, the process of PP fiber formation in the CPA matrix must be worsened. As will be shown below, the above-mentioned assumptions are validated by direct structural investigations.
3.2. Results of the Microscopic Investigations
The results of microscopic investigations are shown in Tables 4 and 5 and Fig. 4. Analysis of the data obtained reveals that the process of specific fiber formation is realized for all investigated mixtures. However, the degree of its realization varies. With addition of 5 to 10 wt% of POM both in PP/CPA mixtures of 40/60 and 20/80 in composition, the process of fiber formation is improved, that is, the number and mass of micro-fibers increase; the film content and distribution variance decrease abruptly (Table 4). An exterior cover is formed for all mixtures. Its structure and thickness are governed by the PP/CPA ratio and POM content. A PP/CPA mixture of 40/60 in composition is very close to the region of phase inversion. That is why the specific fiber formation process is not realized under real conditions of this mixture flow: 41 wt% of PP is spent on film formation, 47.7 wt% of PP forms the exterior (film-like) cover, and only 9.4 wt% of PP forms the continuous microfibers (Table 4). With addition of 5 wt% of POM, the film content is decreased abruptly, the thickness of the exterior cover decreases, and the cover becomes fibrous. Upon a further increase of POM content, the mass of polymer spent on the formation of the exterior cover increases. This increase is due to migration of the less viscous POM towards the walls of the forming die under the action of normal stresses in the mixture melt flow. To verify this assumption, special experiments have been carried out. After extraction of CPA from the extrudate of a PP/CPA/POM mixture of 40/60/30 in composition, the residue cover was placed into concentrated sulfuric acid and maintained for 24 hours with periodic agitation. Sulfuric acid dissolved POM and did not react with PP. Then the specimen was taken out of the acid, washed in water and alcohol, and dried to mass constancy. For the given specimen, the mass losses were about 50 wt%. Thus, in the polymer flow, a significant part of the POM migrates, and together with PP, forms the exterior cover.
The curves of numerical distribution of continuous fiber diameters for various POM contents are shown in Fig. 4. For the initial binary PP/CPA mixture of 40/60 in composition, the curve of numerical distribution is bimodal, with two maximums corresponding to fiber diameters of 5.3 and 10.6 [micro]m. This is an evidence of nonuniformity of the distribution of continuous fiber diameters. With addition of POM, the bimodal nature of the distribution is preserved for all POM concentrations, but the structural uniformity is improved abruptly. Thus, at 5 wt% of POM. the share of microfibers of 5.3 [micro]m in diameter amounts to 69%, and at 10 wt%--76%. At 20 or 30 wt% of POM, the distribution of the first type remains narrow, but the area of the second peak increases, and this generally leads to aggravation of the structural uniformity (Fig. 4). Thus, addition of POM gives an opportunity to realize the process of fiber formation even for a PP/CPA mixture of 40/60 in composition. For this purpose, an addition of 5 to 10 wt% of POM is recommended.
At the ratio PP/CPA of 20/80, the addition of 5 wt% of POM leads to disappearance of the second peak on the distribution curve. The maximum corresponds to the microfiber diameter of 4.8 [micro]m. At POM content of 20%, the second weak peak becomes observable. At POM content of 30%, this peak is distinctly displayed. In general, the latter is an evidence of deterioration of the fiber formation process.
Kyleznev [5, 14] has shown that the presence of anisotropic structures greatly facilitates the drawing of fibers and films from two-phase polymer mixtures. This has been confirmed in our work [2, 15], and it is valid for polymer mixture melts. Thus, our work  has shown that [JS.sub.max] of PP/CPA and PE/CPA mixture melts increases by a factor of 1.5 to 2.6 at the repeated extrusions, when the degree of dispersion increases, more liquid streams are formed, and they have smaller diameters. It is also known  that POM and CPA possess quite different abilities to be deformed longitudinally. Under the action of stretching forces, CPA is deformed 5 times as much. However, the POM/CPA mixture melts are deformed at the same degree as the initial CPA . This is explained by the formation of the hydrogen bonds between POM and CPA macromolecules. As a result, POM appears to be forcefully stretched in the flow of polymer mixture (perhaps to the same extent as CPA). Thus, POM forms liquid streams in CPA during extrusion of the temary mixture melts. This increases the PP deformability, which improves the PP fiber formation in the bulk of CPA.
Taking into consideration the distinctions in structure-formation processes of the binary and ternary mixtures, different abilities of the mixture melts for longitudinal deforming can be expected. This has been confirmed experimentally. With addition of POM, the value of [JS.sub.max] increases in comparison with the values of [JS.sub.max] of the binary mixtures, reaches its maximum (at 5 wt% of POM), and drops at POM contents of 20 to 30 wt%. The latter is explained by a sharp increase in the melt viscosity at POM contents of 20 to 30 wt%. Thus, all the data obtained confirm the recommendations of the necessity to add 5 to 10 wt% of POM into the PP/CPA mixture for the purpose of realization (or improving) of the specific fiber formation upon processing of the indicated mixtures.
Addition of POM into PP/CPA binary mixtures with PP/CPA ratios of 40/60 and 20/80 wt% causes an increase in the mixture melt viscosity. This is explained by the specific interactions of hydrogen bond type between macromolecules of POM and CPA at the interface. The degree of viscosity increase is determined by the composition of the binary mixture, value of shear stress, and POM content.
Under the influence of POM additives the structure-formation processes in PP/CPA mixtures are changed. Improvement of the specific fiber formation has taken place. The latter is realized even for compositions close to the phase inversion. There is POM migration toward the walls of the forming die during extrusion of the ternary polymer mixture melts.
The structure and thickness of the exterior cover are determined by PP/CPA ratio and POM content. The film-like exterior cover for PP/CPA mixture with a ratio of 40/60 is transformed into a fibrous cover upon addition of POM.
Addition of 5 to 10 wt% of POM into a PP/CPA mixture with a PP/CPA ratio of 40/60 or 20/80 is recommended for realization of the specific fiber-formation during processing of the indicated mixtures.
The correlation between specific fiber-formation and the mixture swelling after extrusion has been confirmed: the larger "B" is, the more distinct is the specific fiber-formation phenomenon.
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Table 1. Polymer Characteristics. Polymer Chemical structure Melting temperature, [degrees]C PP Homopolymer 170 CPA Copolymer * 180 POM Copolymer ** 169 Polymer Melt viscosity, Pa . s *** Extrudate swelling B PP 430 1.7 CPA 600 1.4 POM 300 1.3 Notes: (*)-CPA is a copolymer of caprolactam and hexamethylena-adipinate in a ratio of 50/50. (**)-POM is a copolymar of formaldehyde and 1.3-dioxolana (2 wt%) (***.)-at T = 190[degrees]C and shear stress [tau] = 5.69 * [10.sup.4] Pa. Table 2. Values of the Ratio [[eta].sub.t]/[[eta].sub.b] for PP/CPA Mixture of 40:60 in Composition. [[eta].sub.t]/[[eta].sub.b] at POM content of, wt% [tau] X [10.sup.-4], Pa 5 10 20 30 5.69 0.8 0.9 1.7 2.3 4.20 0.8 1.0 1.9 2.6 3.47 0.8 1.1 2.0 2.9 2.72 0.8 1.1 2.0 2.9 1.98 0.8 1.1 2.0 3.0 Table 3. Values of the Ratio [[eta].sub.t] /[[eta].sub.b] for PP/CPA Mixture of 20:80 in Composition. [[eta].sub.t]/ [[eta].sub.b] at POM content of, wt% [tau] X [10.sup.-4] 5 10 20 30 5.69 1.1 1.4 1.7 2.4 4.20 1.2 1.2 1.6 2.3 3.47 1.2 1.2 1.7 2.3 2.72 1.1 1.2 1.7 2.5 1.98 1.2 1.3 1.7 2.7 Table 4. Quantitative Characteristics of PP/CPA/POM Extrudate Microstructure With PP/CPA Ratio of 40/60. Continuous fibers Short fibers POM, average average content diameter, numerical diameter, numerical wt% [micro]m % wt% [micro]m % 0 6.3 40.0 9.4 5.2 8.1 5 6.2 53.0 50.0 5.0 5.4 10 6.1 70.0 40.0 5.2 5.3 20 6.3 42.0 18.0 5.3 4.2 30 6.7 57.0 21.0 5.5 5.4 Particles Films POM, average content diameter, numerical numerical wt% wt% [micro]m % wt% % wt% 0 1.6 6.3 38.0 0.1 8.6 41.0 5 2.3 6.2 38.5 0.5 1.3 16.2 10 1.5 6.7 24.0 0.2 0.5 11.5 20 0.8 5.3 44.0 0.1 1.5 16.0 30 0.9 5.4 31.0 0.5 0.4 1.9 Exterior cover POM, content numerical wt% % wt% 0 5.1 47.0 5 1.4 31.0 10 1.3 47.0 20 4.4 70.0 30 6.0 75.0 Table 5. Quantitative Characteristics of PP/CPA/POM Extrudates Microstructure With PP/CPA Ratio of 20/80. Continuous fibers Short fibers POM, average average content diameter, numerical diameter, numerical wt% [micro]m % wt% [micro]m % 0 5.2 40.6 57.6 5.0 13.0 10 4.9 46.3 69.0 4.8 4.5 20 5.1 40.0 42.7 4.9 6.7 Particles Films POM, average content diameter, numerical numerical wt% wt% [micro]m % wt% % wt% 0 18.3 5.3 45.7 1.2 0.8 2.7 10 6.9 5.4 46.8 1.5 2.0 11.0 20 7.2 5.2 51.2 1.0 1.8 7.6 Exterior shell POM, content numerical wt% % wt% 0 0.3 20.2 10 0.3 11.6 20 0.2 41.6
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|Author:||TSEBRENKO, M. V.; REZANOVA, N. M.; TSEBRENKO, I. A.|
|Publication:||Polymer Engineering and Science|
|Date:||Jun 1, 2001|
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