Elsevier

European Polymer Journal

Volume 46, Issue 7, July 2010, Pages 1436-1445
European Polymer Journal

Macromolecular Nanotechnology
Nucleation of crystallization in isotactic polypropylene and polyoxymethylene with poly(tetrafluoroethylene) particles

https://doi.org/10.1016/j.eurpolymj.2010.04.021Get rights and content

Abstract

Nucleation of crystallization of isotactic polypropylene (iPP) and polyoxymethylene (POM) with dispersed submicron particles of another polymer – poly(tetrafluoroethylene) (PTFE) was studied. The polymers were mixed with various contents of PTFE particles, in the range from 0.005 to 0.5 wt.%. iPP and POM with PTFE particles are all-polymer systems with enhanced nucleation of crystallization. PTFE particles with sizes below 300 nm added to POM and iPP efficiently decreased sizes of polycrystalline aggregates. Moreover, nonisothermal crystallization temperature of iPP by increased by up to 14 °C. iPP and POM with PTFE exhibited the elastic modulus slightly higher, by up to 10–13%, than that of the neat polymers. Other mechanical properties remained unchanged, with the exception of reduced elongation at break of POM with PTFE.

Introduction

Artificial nucleation of polymers is frequently used as it allows to shorten the solidification and to diminish sizes of spherulites. In polyolefins it can also elevate markedly the crystallization temperature (Tc) during cooling. Many nucleating agents are known for isotactic polypropylene (iPP), including talc, sorbitol derivatives, organophosphate salts and bicyclic dicarboxylate salts. Talc exhibits nucleating activity also in polyoxymethylene (POM). Benefits include shortening of processing cycle, modification of mechanical properties and, in the case of iPP, an improvement of transparency when a polycrystalline aggregate size is strongly reduced.

Efficient nucleation of bulk crystallization requires nucleating particles well dispersed in a polymer matrix. Recently, the nucleation of iPP with different nanoparticles was studied [1], [2], [3], [4], [5], [6], [7]. During nonisothermal crystallization of highly dispersed iPP/carbon nanotube (CNT) composites, containing up to 2 wt.% of CNTs, the considerable heterogeneous nucleation was observed resulting in Tc higher by up to 15 °C [3]. In iPP with 0.5–1 wt.% of SiO2 particles having sizes in the range of 30–80 nm Tc increased by 7 °C [6]. About 0.1–1 wt.% of silver nanoparticles dispersed in iPP caused an elevation of its Tc by 5 °C [7]. Other nanofillers studied, including exfoliated organo-modified monmorillonite (o-MMT) [1], [2], 16 nm SiO2 particles [5] and also nanoparticles of ZnO and Al2O3 having sizes of several tens nanometers [4], exhibited only weak nucleating activity in iPP.

Poly(tetrafluoroethylene) (PTFE) fibers and films are known to nucleate iPP crystallization [8], [9], [10], [11] and induce transcrystalline iPP morphology. Wittmann and Smith [12] reported oriented growth of several polymers, including poly(ethylene terephatalate), polyethylene, poliamide 6 and poliamide 11, on PTFE substrates prepared by the friction–deposition method which produced a thin layer of highly oriented chains with virtually single crystal orientation and organization. Using such PTFE substrate Yan et al. [13] demonstrated that iPP in its α modification crystallized epitaxially on PTFE via (110) plane. Van der Meer et al. [14] blended PTFE particles of two different sizes in various concentrations, ranging from 1 to 6 wt.%, with iPP. The high molecular weight PTFE particles, with sizes ranging up to 600 μm, were deformed into fibrillar scaffolds under shear during processing. The scaffolds consisted of bundles of individual fibrils with diameters of 20–30 nm and with an aspect ratio exceeding 25. The fibrils were found to nucleate crystallization of iPP. The second type of particles, with a smaller size and lower molecular weight, did not undergo the fibrillation. In our earlier work [15] we demonstrated that blending of small amounts of PTFE particles with a number of thermoplastic polymers resulted in enhanced nucleation of crystallization in those polymers. Recently, Bernland and Smith [16] used suspensions of fine particles of virgin ultra-high molecular weight PTFE. The particles were blended with iPP, POM, high-density polyethylene, poliamide 12 and poly(ethylene terephtalate), and subjected to controlled shear during mixing below the melting temperature of PTFE. Proper compounding conditions allowed to deform a significant fraction of PTFE particles into individual fibers which nucleated crystallization of polymer matrices. The crystallization temperature of iPP with 0.001 wt.% of PTFE during cooling at 10 °C min−1 was higher by 7 °C than that of neat iPP. It has to be noted that efficient fibrillation of solid PTFE particles of high molecular weight, during mixing with polymers, requires high shear rates and sufficiently long mixing times [16], [17].

Submicron PTFE dispersion particles, known also as emulsion particles, prepared by an emulsion polymerization are widely applied for coating and impregnations. Nascent particles are known to be composed of chain extended crystals exhibiting high melting temperature [18]. Our study was focused on the nucleation activity of those particles in iPP and POM. All-polymer systems of iPP and POM with 0.005–0.5 wt.% of PTFE particles were prepared. The crystallization, structure and mechanical properties of the materials depending on PTFE content were investigated. Unlike in Ref. [16], under the mixing conditions employed in our study only a minor fraction of particles was deformed into fibers.

Section snippets

Experimental

iPP used in the study was Malen-P F 401, having MFI of 3.0–3.5 g/10 min (230 °C, 2.16 kg), density 0.91 g/cm3, molar mass Mw of 300 kg/mol, Mw/Mn = 5.3, produced by Basell-Orlen SA, Poland. POM type polymer obtained by copolymerization of trioxane with 3 wt% of dioxolane, Tarnoform 300, having MFI of 9.6 g/10 min (2.16 kg, 190 °C) and density 1.41 g/cm3, produced by Azoty Tarnow (Poland) was also utilized.

Four grades of PTFE particles were used, three of them dispersed in water and one in the form of dry

Characterization of PTFE particles

Fig. 1 shows DSC thermograms of dried Tarflen Dispersion with melting and crystallization peaks recorded during heating and cooling, respectively. This behaviour was characteristic of all types of the PTFE particles used. The calorimetric data are collected in Table 3.

Tm1s of PTFE-T, PTFE-W1 and PTFE-W2 exceeded their Tm2s and were above PTFE equilibrium melting temperature, being in the range 332–336 °C according to Refs. [19], [20], [21]. Such high Tms of PTFE particles were measured and

Conclusions

PTFE-T in the form of submicron particles, dispersed in iPP and POM, strongly enhanced the nucleation of crystallization. Tc of iPP during cooling increased by up to 14 °C whereas a radius of polycrystalline aggregates decreased from 36 to about 2–3 μm. It can be noticed that the significant changes were observed even at low PTFE-T contents, up to 0.05 wt.%. The increase in Tc during cooling of POM was only by up to 2–3 °C, however a radius of an average spherulite was noticeably decreased, from 70

Acknowledgements

This research was supported by the Ministry of Education and Science, Poland, Grant 3T08E 059 29, 2005–2008 and Grant 3T08E 042 30 2006–2007.

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