Polymorphic Composition of Colored Polypropylene Fibers - Crystal

CRYSTAL GROWTH & DESIGN

Polymorphic Composition of Colored Polypropylene Fibers Jan Broda* Institute of Textile Engineering and Polymer Materials, University of Bielsko-Biala, Willowa 2, 43-309 Bielsko-Biala, Poland Received July 9, 2004;

2004 VOL. 4, NO. 6 1277-1282

Revised Manuscript Received September 23, 2004

ABSTRACT: The investigations of the structure of polypropylene fibers colored with quinacridone and phthalocyanine pigments were carried out. The fibers were extruded from the melt at three different temperatures and were taken at three different velocities. In fibers two polypropylene modifications, the monoclinic R and the trigonal β, were observed. The content of particular modifications depends on the kind of added pigment as well on the spinning parameters. The highest β form content was observed in fibers colored with the quinacridone pigment extruded at the lowest temperature and taken at the lowest velocity. For fibers colored with the phthalocyanine pigment spun at such conditions the β form content is much lower. With the increase in the take-up velocity and the increase in the extrusion temperature in fibers colored with both pigments the β form content desirably decreases. Introduction The crystal polymorphism of isotactic polypropylene has been known for several decades. For years, numerous investigations were carried out and big progress in the understanding of the polymorphism was observed. As a result of performed studies three modifications of polypropylene, R, β, and γ, were discovered.1,2 The structure of the most stable and commonly encountered R form was early established. Natta and Corradini3 proposed a model based on the monoclinic unit cell. According to this model, polypropylene helices of the same hand are arranged in layers parallel to the ac plane. Layers of isochiral helices alternate with layers formed from helices of the opposite hand. The above-mentioned model was commonly accepted and confirmed by subsequent studies of Mencik,4 Hikosaka,5 and Corradini.6 Due to the complex structure the arrangement of rarer encountered β and γ forms were discovered several decades after their initial observation. The intensive investigations showed that both forms consist of the characteristic left-handed or right-handed helices arranged in the original packing schemes. During the years different models characterizing the structure of β and γ forms were proposed.7-10 Finally for the β form the frustrated structure of Lotz11 and Meile12 based on a trigonal cell containing three isochiral helices was accepted. In the case of the γ form the proposition of Bru¨ckner and Meile13,14 for the unique packing arrangement was approved. According to this model, the orthorhombic cell of the γ modification is formed by bilayers composed of two parallel helices. The direction of the chain-axis in adjacent bilayers is tilted with an angle of 80°. The different polymorphic forms of polypropylene arise during crystallization proceeded at different conditions. The R form can be obtained easily by crystallization of polymer melts or solutions. The formation of the * Institute of Textile Engineering and Polymer Materials, University of Bielsko-Biala, Willowa 2, 43-309 Bielsko-Biala, Poland. Tel.: +48 33 8279120; fax: +48 33 8279100; e-mail: [email protected].

β and γ forms requires special crystallization conditions. The β form arises during crystallization of a sheared melt,15 crystallization in a temperature gradient,16 or during crystallization in the presence of special additives.17-20 The γ form is formed by crystallization under high pressure,21,22 crystallization of polypropylene copolymers,23 or by slow cooling of low molecular fractions of polypropylene.24 Among three polymorphic modifications in polypropylene products, usually the most stable R form is observed. In polypropylene fibers as well, the R form is usually encountered.25-27 The β form can be obtained in fibers containing efficient β nucleating agents extruded at appropriately selected spinning parameters.28-29 Among several additives possessing the nucleating ability toward the β form of polypropylene some organic pigments are known. Pigments applied commonly for coloring of polypropylene fibers possess a high heat stability and form finely dispersed crystals insoluble in the polypropylene melt. During crystallization, pigments provide a foreign surface reducing the free energy of the formation of a new polymer nucleus. Many pigments crystallize forming different crystal modifications. Some of them reveal surfaces enabling the epitaxial growth of polypropylene crystals. The various geometries of the contact surface of the pigment crystals can lead to the formation of the different polypropylene modifications. This paper presents investigations of the structure of the polypropylene fibers colored with quinacridone and phthalocyanine pigments. Both pigments reveal the high nucleating ability toward polypropylene crystallization.30 In fibers colored with those pigments, the formation of the two polymorphic forms of polypropylene is observed. Experimental Section Samples. The investigations were carried out for polypropylene fibers spun in laboratory conditions using a Brabender laboratory extruder coupled with a five-hole die with a hole diameter of 0.2 mm. Three series of fibers extruded from the

10.1021/cg0497703 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/07/2004

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melt at three different temperatures 210, 225, and 250 °C were produced. The extruded filament was cooled in the air at 20 °C. The fibers were spun at the constant mass throughput 3 g/min and taken at a winding roller at three different takeup velocities 100, 200, and 300 m/min. The commercial isotactic polypropylene Mosten 52.945 characterized by a melt flow index 25 g/10 min and supplied by Chemopetrol (Czech Republic) was used. Fibers were colored with two organic pigments: the quinacridone Pigment Violet 19, C. I.73900 (Echtrot E3B - Hoechst, Germany) and the phthalocyanine - Pigment Blue 15, C. I.74160 B (Wola Kszysztoporska - Poland). Pigments were blended with polypropylene at the concentrations 0.1, 0.3, and 0.5 wt % and homogenized with the melt in a barrel of the screw extruder. Methods. The structure of fibers was evaluated by the wide angle X-ray scattering (WAXS) method. The investigations were carried out using the X-ray diffractometer HZG 4. The radiation scattered on powdered samples was measured in the angular range from 5 to 35°. The fibers were powdered on a Hardy microtome to segments of 10-15 µm. The analysis of obtained diffraction patterns was performed constructing a theoretical curve best fitted to the experimental data. The curve was composed from crystalline peaks, an amorphous halo, and polynomial describing a background scattering. The crystalline peaks were described by the linear combination of Gauss and Cauchy functions, while the amorphous halo by a logarithm-normal function. The parameters of component functions were found by minimization of the sum of squared deviations of the theoretical curve from the experimental one. The minimization was carried out by means of Rosenbrock’s method using a computer program OptiFit.31 The cristallinity index and the K value characterizing the β form content were calculated. The cristallinity index was calculated as a ratio of area under the crystalline peaks to the total area under the pattern. The K value was determined according to the proposition of Turner-Jones32 as a ratio of the intensity of (300)β peak to the sum of intensities of (110)R, (040)R, (130)R, and (300)β peaks.

Results The Quinacridone Pigment. As a result of a crystallization process the structure of polypropylene fibers is formed. The crystallization process occurring during spinning of fibers is affected by many factors, mainly by the molecular orientation and the cooling rate. In the case of low take-up velocities, the molecular orientation inside the extruded stream is minimized and the crystallization process is dominated by the cooling rate.33 The crystallization occurs at a relatively high temperature and proceeds on heterogeneous nuclei produced with a participation of pigments. For fibers taken at 100 m/min and extruded from the melt at 210 °C the cooling rate is the lowest. The low cooling rate favors the formation of the β form of polypropylene.34 Then the crystallization process starts at the relatively high temperature, above the lower critical temperature for the formation of the β form of polypropylene.35 At this temperature, the growth rate of β crystals exceeds the growth rate of the R crystals.36 The β nuclei formed on the surface of pigment crystals quickly grow forming the high amount of β crystals. For fibers colored with the quinacridone pigment the relatively high amount of the β crystals is formed already at the low pigment concentration. On WAXS patterns measured for fibers containing 0.1% of pigment the relatively strong β peak is observed

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Figure 1. Series of WAXS patterns obtained for fibers colored with the quinacridone pigment by different pigment concentrations: 1/0.1; 2/0.3; 3/0.5% (melt temperature 210 °C, takeup velocity 100 m/min).

(Figure 1). Besides this peak, the distinctly formed R peaks are visible. On the pattern obtained for fibers containing higher amounts of the pigment the intensity of the β peak increases. Simultaneously, the R peaks become weaker and for fibers containing 0.5% of pigment R peaks are only just visible. The K value achieves the relatively high value of 0.76. With the increase of the pigment concentration until 0.5% the β form content successively increases. For the pigment concentration of 0.3% the K value equals to 0.89. Then for the pigment concentration of 0.5% the K value increases to the maximal value of 0.95. The red quinacridone pigment belongs to deeply colored organic pigments characterized by a relatively small molecular size. The molecule of dimensions 1.406 × 0.52 nm is formed from five heterocyclic rings. In quinacridone crystals planar quinacridone molecules are arranged in parallel stacks, with the molecule tilted to the stacking direction. The neighboring stacks adopt a herringbone arrangement. In crystals each molecule is bonded through hydrogen bonds to four adjacent molecules. Very strong intermolecular hydrogen bonds combined with strong van der Waals’ forces ensure quinacridone pigments the high heat and chemical resistance. Seven different crystalline forms of quinacridone are known.37-39 Most synthetic methods lead to the formation of an unstable R form. Subsequent treatments afford the more stable β and γ forms, most commonly used as commercial pigments. In the investigations the γ form of the quinacridone pigment was used. For years this modification is known as a very efficient nucleating agent for the β form of polypropylene.40-41 Lotz and co-workers42 showed that the β form of polypropylene grows epitaxially on the surface of the γ crystals of the quinacridone pigment. The epitaxy involves the (110) plane of the trigonal unit cell of the polypropylene, which contacts the bc surface of the γ crystals of the quinacridone. The hydrogen atoms of stacked benzene rings form on the surface bc of the γ quinacridone crystals a

Polymorphic Composition of Polypropylene Fibers

Figure 2. Series of WAXS patterns for fibers colored with the quinacridone pigment extruded at 225 °C taken at 1/100 m/min; 2/200 m/min; 3/300 m/min (pigment concentration 0.5%).

parallel array of bulges and grooves. The spacing between grooves 0.65 nm is close to the axis repeat distance of the polypropylene helix. The arrangement of polypropylene chains on the pigment surface perpendicularly to the parallel grooves ensures nearly perfect matching of the above-mentioned dimensions and in this way enables the epitaxial growth of β crystals. The content of the β form in polypropylene fibers depends strongly on the formation parameters. Figure 2 presents series of WAXS patterns for fibers extruded at 225 °C taken at different velocities. For fibers spun at the lowest velocity two strong peaks characteristic for the β form are observed. Besides the β peaks on the pattern very weak peaks from the R form are visible. For fibers taken at higher velocities the intensity of the β peaks drastically decreases. Simultaneously the high increment of the R peaks intensity is observed. For fibers taken at 200 m/min the intensity of the β peak is comparable to the intensity of R peaks. At the take-up velocity 300 m/min the intensity of β peaks is minimal and is repeatedly lower in comparison to R peaks. The change of the peaks’ intensity does not cause a significant change of the cristallinity index. For the whole series of fibers the cristallinity index achieves values from the range 0.54-0.56. The observed changes of the patterns results from the change of the R and β forms content. For fibers taken at the lowest velocity 100 m/min the K value equals 0.95. With the increase in the take-up velocity to 200 m/min and then to 300 m/min the K value decreases to 0.33 and 0.09, respectively. Figure 3 presents series of patterns for fibers taken at 100 m/min extruded by different melt temperatures. On patterns, independently of the melt temperature strong β peaks are observed. For all those fibers the calculated K value equals to 0.95 and does not change with the increase of the melt temperature. The influence of the melt temperature on the β form content can be observed for fibers taken at the higher velocity 200 m/min (Figure 4). For those fibers with the increase of the melt temperature the intensity of the β peak considerably decreases. For fibers extruded at 210

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Figure 3. Series of WAXS patterns obtained for fibers colored with the quinacridone pigment taken at 100 m/min and extruded at the temperature: 1/210 °C; 2/225 °C; 3/250 °C (pigment concentration 0.5%).

Figure 4. Series of WAXS patterns obtained for fibers colored with the quinacridone pigment taken at 200 m/min and extruded at the temperature: 1/210 °C; 2/225 °C; 3/250 °C (pigment concentration 0.5%).

°C the intensity of the β peak is significantly greater than the intensity of the R peaks. Then at 225 °C the β peak is only a bit higher in a comparison to R peaks. For fibers extruded at 250 °C the weak β peak can be barely distinguished from the strong R peaks. As a consequence of the increase of the melt temperature the K value decreases from 0.47 for fibers extruded at 210 °C, to 0.33 and 0.15 for fibers spun at 225 and 250 °C, respectively. In the region of low velocities, the increase of the takeup velocity as well the increase of the extrusion temperature results in the increase of the cooling rate. Then the crystallization temperature moves toward lower values. The crystallization starts at lower temperature and then quickly moves below the critical temperature for the formation of the β crystals. The smaller part of the material crystallizes at conditions favorable for the formation of the β form. In fibers the higher number of R nuclei is formed. In the temperature below the low critical temperature for the formation of the β form of polypropylene the growth rate of the R crystals is higher

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Figure 5. WAXS pattern for fibers colored with the phthalocyanine pigment spun from the melt at the temperature 210 °C taken at 1/100 m/min; 2/200 m/min; 3/300 m/min (pigment concentration 0.5%).

Figure 6. WAXS pattern for fibers colored with the phthalocyanine pigment spun from the melt at the temperature 250 °C taken at 1/100 m/min; 2/200 m/min; 3/300 m/min (pigment concentration 0.5%).

than the growth rate of the β crystals.43 Then the R nuclei quickly grow, while the growth of the β nuclei is strongly constrained. As a consequence the content of the β phase in fibers successively decreases. The structure of fibers colored with quinacridone pigment spun at higher velocities and higher extrusion temperatures contains the high content of well-developed R crystals. This fact suggests that at the higher cooling rates the quinacridone pigment induces the formation of the R nuclei. The investigations of Rybnikar44 and Mathieu45 showed that the γ quinacridone reveals a versatile nucleating ability and may induce either the R or the β modification of polypropylene. The versatile nucleating ability of the quinacridone pigment results from the fact that the spacing between grooves on the bc surface of γ crystal is similar as well to the interchain distance of the isochiral helices in the (010) plane of the R form of polypropylene. By the arrangement of polypropylene helices parallel to the grooves the nearly perfect match between above-mentioned dimensions may be achieved and the epitaxial growth of R crystals may be initiated. The formation of the different polypropylene modifications on the same crystal surface bc of quinacridone crystal depends on crystallization conditions. Mathieu45 stated that the transition of the nucleating ability of quinacridone occurs at 140 °C, near the temperature at which the R and β growth rate curves versus temperature pass a crossover point. In investigated fibers, the transition of the quinacridone ability for the nucleation of the R phase occurs rather at a much lower temperature, probably near the lower critical temperature for the formation of the β form of polypropylene. The Phthalocyanine Pigment. For fibers colored with the phthalocyanine pigment in comparison to fibers colored with the quinacridone the content of the β form is much lower. In Figure 5 the series of patterns for fibers colored with the phthalocyanine pigment taken at different velocities is presented. For fibers taken at 100 m/min the crystalline peaks of R and β forms are observed. The

intensity of the β peak is comparable with the intensities of other peaks. For fibers spun at higher velocity 200 m/min the intensity of the β peak desirably decreases. For fibers taken at 300 m/min the β peak is not visible. On the pattern only R peaks are observed. The cristallinity index for all fibers equals to 0.57. With the increase of the take-up velocity the K value decreases from 0.31 for fibers taken at 100 m/min to 0.14 for fibers taken at 200 m/min. The intensity of the β peak observed on the pattern measured for fibers extruded at 210 °C with the increase of the extrusion temperature desirably decreases. For fibers taken at 100 m/min by the higher extrusion temperature 250 °C the β peak is only just visible (Figure 6). In the case of fibers taken at 200 and 300 m/min only R peaks are observed. The blue phthalocyanine pigment consists of a tetrabenzoporphyrazine nucleus with a central copper atom. The molecule assumes a planar conformation and possesses a square shape with a side length of 1.3 nm. In crystals the fairly rigid molecules of the copper phthalocyanine can be packed in different arrangements giving rise to different polymorphs. As a result of intensive research 10 different polymorphs of copper phthalocyanine were discovered.46 The first recognized R form and the most thermodynamically stable β form exhibit particularly a commercial interest. In all crystals of the copper phthalocyanine molecules are arranged in uniform stacks with the rings tilted with respect to the stacking direction. In stacks the molecules are tilted with respect to the stacking direction by 46° for the β form and 25° for the R form. The interactions between molecules within stacks are mainly defined by π-π interactions. The interplanar distance between adjacent molecules is consistent with a van der Waals bond and equals 0.34 nm. In the β modification neighboring stacks are arranged in a herringbone style, while in the R form no herringbone interactions are present. The phthalocyanine crystals tend to form needles or rods parallel to the stacking direction. The side faces of crystals are mainly covered by aromatic hydrogen atoms, while the basal faces expose the π system and

Polymorphic Composition of Polypropylene Fibers

the copper atom. The lateral surfaces exhibit nonpolar character, while the basal surfaces have relatively polar character. The aromatic hydrogen atoms occurring on the lateral surfaces of the pigments crystals are arranged in parallel rows. The shallow nonpolar grooves formed between such rows force polypropylene molecules to assume a stretched conformation over some distance, making the nucleation much easier.47 The phthalocyanine pigment applied in the investigations reveals the packing scheme of the R modification. According to the model proposed for this modification the distance between grooves on the lateral surfaces equals 1.19 nm.48 This dimension is comparable with the spacing of 1.1 nm between helices of the same hand in the trigonal cell of the β form of polypropylene. Li49 suggested that the spacing of nonpolar grooves from the range 1.1-1.3 nm is responsible for a good nucleating ability toward the β form of polypropylene of several calcium dicarboxylates. Similarly the compatibility of those dimensions for the phthalocyanine pigment may explain the formation of a certain amount of the β crystals in fibers colored with phthalocyanine pigment. In the case of fibers colored with the phthalocyanine pigment with the increase of the take-up velocity and the increase of the extrusion temperature the content of the β form drastically drops and for fibers spun at higher cooling rates the structure consisting only of the R form is obtained. One may conclude that at such conditions on the surface of the phthalocyanine pigment only R nuclei are formed. Conclusions In fibers colored with quinacridone and phthalocyanine pigments the semicrystalline structure consisting of two polymorphic modifications of polypropylene R and β is formed. The content of particular modifications desirably changes and depends on the kind of added pigment and its concentration as well on the formation parameters: the take-up velocity and the extrusion temperature. The high content of the β form can be obtained especially in fibers colored with the quinacridone pigment. The geometry of the contact plane of the γ modification of quinacridone enables the epitaxial growth of the β form of polypropylene. The formation of the β form requires the appropriate selection of spinning parameters. The highest content of the β form can be obtained in fibers taken at the lowest velocity and extruded from the melt at the lowest temperature. Then the influence of the molecular orientation on the crystallization is minimal and the formation of the row nucleated R phase is avoided. The crystallization process proceeds at a relatively low cooling rate, which enables crystallization in the suitable temperature range promoting the formation of the β modification. The increase of the cooling rate by the increase of the take-up velocity or by the increase of the extrusion temperature causes the shift of the crystallization temperature toward lower values. Then the crystallization conditions for the formation of the β form are less favorable. On the surface of pigment crystals a number

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