Tough polyamide 6/core-shell blends by reactive extrusion

21 October 2013
Dongguang Yan, Liang Dong, Jiao Li, and Faliang Luo
Continuous in situ anionic polymerization of ε-caprolactam in a twin screw extruder produces tough polyamide 6 blends incorporating dispersed domains with a core-shell structure.

Polyamide 6 (PA6) is an important thermoplastic with a wide range of engineering applications. Monomer casting via anionic polymerization and hydrolytic polymerization are the two main methods widely used to prepare PA6. However, they only allow PA6 to be prepared discontinuously (e.g., in a stirred tank in the case of hydrolytic polymerization, or inside a mold in the case of monomer casting). Reactive extrusion—an entirely different technique in which polymerization and other chemical reactions take place inside the extruder during processing—would instead allow for continuous production of high molecular weight PA6. In recent years, reactive extrusion has also been examined as a possible approach for preparing both PA6 nanocomposites1–4 and PA6 blends.5–7 Novel polypropylene/PA6 blends prepared by reactive extrusion, in which the scale of PA6 dispersed domains was below 100nm, are reported in Hu.5, 6 Low-density polyethylene/PA6 blends have also been prepared using a similar technique by Fang.7 However, there are few reports on the PA6-based blends prepared in this way.

Due to its low crack propagation resistance, PA6 is brittle under severe conditions such as high strain rates, low temperatures, or in the presence of a notch. To overcome these problems, PA6 is generally toughened with modifiers having a core-shell morphology, i.e., dispersed domains consisting of a rigid core encased in a soft shell. This method can impart substantial toughening to the matrix while at the same time keeping rigidity (which otherwise tends to decrease with increasing toughness) reasonably high.8,9 Given the desirability of being able to continuously produce toughened PA6, in our work we investigated a novel approach for preparing PA6 core-shell blends by reactive extrusion that is both easy and practicable to implement industrially.10


Transmission electron microscopy images of 85/10/5 blends of polyamide 6 (PA6), styrene-ethylene/butylene-styrene block copolymer (SEBS), and polystyrene (PS) prepared by reactive extrusion show a core-shell morphology of PS encased inside poly(ethylene-butylene) (PEB) dispersed in the PA6 matrix. Magnification (a) 10,000×and (b) 20,000×.

The modifiers we chose for preparing the tough PA6 core-shell blend were styrene-ethylene/butylene-styrene block copolymer functionalized with maleic anhydride (SEBS-g-MA) and polystyrene (PS). We began the process by dissolving the SEBS-g-MA and then the PS in molten ε-caprolactam (CL), the monomer that forms the basis for preparing PA6. The resultant molten mixture was then divided into equal parts placed in two separate tanks. We added an activator of anionic polymerization to one tank, and an initiator of anionic polymerization to the other. After dissolution of the activator and initiator, the two melt solutions were fed from their respective tanks, both at the same flow rate, into a twin screw extruder. After pelletizing, the products were extracted with boiling water to eliminate the residual CL monomer (the equilibrium conversion is ~90–92%). Then, the materials were dried and injection-molded for testing.

We found that, when 10wt% SEBS-g-MA and 5wt% PS were used to compound with CL—resulting in a PA6/SEBS/PS (85/10/5) blend—a sevenfold increase in elongation at break and a 2.3-fold increase in notched Izod impact strength were achieved, at the cost of only a 4.4% loss in tensile strength and a 14.7% loss in flexural strength compared to pure PA6. Transmission electron microscopy (TEM) was used to determine the reasons for this distinct improvement in the mechanical properties. From the TEM images of this blend (see Figure 1) we clearly observed that the SEBS and PS phases in the PA6 matrix were segregated into spherical domains, with the poly(ethylene-butylene) (PEB) block of SEBS located at the interface between the PA6 and PS phases. Thus, particles with a core-shell structure, consisting of rigid PS cores encased in soft PEB shells, had formed and homogeneously dispersed in the PA6 matrix. Both the particle diameter and interparticle distance were below 1μm, which augmented matrix shear yielding and so toughened the PA6 matrix.


Fourier transform IR spectra of maleated SEBS block copolymer (SEBS-g-MA), of the graft copolymer of SEBS and PA6 (SEBS-g-PA6), and of PA6.


Schematic diagrams of the formation of PA6/core-shell blends. Anionic polymerization by reactive extrusion of ε-caprolactam, PS, and SEBS-g-MA forms a PA6 matrix containing dispersed PS domains encased in PEB, with a SEBS-g-PA6 copolymer growing from the PEB backbone.

To explore the formation mechanisms of the core-shell structure in the PA6 matrix, we used Fourier transform IR (FTIR) spectroscopy to determine the chemical structure of the PA6/SEBS/PS (85/10/5) blends. The blends were washed thoroughly with formic acid to remove the PA6 matrix and physically absorbed PA6 chains. The resulting solid was dried under a vacuum overnight for FTIR characterization. The spectra (see Figure 2) indicate that a SEBS-g-PA6 graft copolymer formed in situ during the reactive extrusion, owing to the activator reactivity of MA for anionic polymerization of CL.11 As the MA grafted only onto the PEB backbone of SEBS,12 the SEBS-g-PA6 copolymer grew from the PEB backbone, which improved interfacial bonding between PA6 and the PEB block of SEBS. On the other hand, the poor compatibility between PS and PA6 resulted in the PS phase transferring far away from PA6 matrix, leading to the PEB being located at the interface between the PA6 and PS, as indicated in Figure 1(b). On the basis of these findings, we derived the schematic diagram for the formation of the core-shell structure outlined in Figure 3.

In summary, our work demonstrated a continuous approach for preparing tough PA6/core-shell blends by reactive extrusion. The process consists first in dissolution of SEBS-g-MA and PS in molten CL, followed by in situ anionic polymerization of CL in the presence of a catalyst and activator in a twin screw extruder. This method owes its success to the different compatibility of functionalized SEBS and PS with the PA6 matrix, which results from the in situ formation of SEBS-g-PA6 copolymer during reactive extrusion. The technique worked out in this study is one that could be easily implemented on an industrial scale, with extremely wide application in the preparation of PA6 blends. The next stage of our research will be to optimize the process and explore the possibility of preparing toughened PA6 blends by reactive extrusion in a processing plant.


Authors

Dongguang Yan
Jiangsu University of Science and Technology (JUST)

Liang Dong
Jiangsu University of Science and Technology (JUST)

Jiao Li
Jiangsu University of Science and Technology (JUST)

Dongguang Yan is an instructor at JUST and has been researching the anionic polymerization of PA6 for nearly eight years. His main interests are reactive extrusion of PA6, monomer casting of PA6, and continuous-fiber-reinforced PA6 composites prepared by resin transfer molding.

Faliang Luo
Ningxia University


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DOI:  10.2417/spepro.005142