Polyamide-6 structure modification by combined solid-phase extrusion.

INTRODUCTION

It is known that when polymers are processed by solid-phase extrusion, their deformation ability and achievable properties are much influenced by the original morphology. In crystallizing polymers, the morphology is controlled by such methods as the formation of structures containing crystals with stretched chains (1), treatment by temperature and pressure (2), radiation, chemical or photochemical cross linking (3), preliminary deformation in conditions resulting in changes of the crystallographic modification of the polymer (4-8).

An alternative approach to increasing the efficiency of solid-phase extrusion is in multi-cycle (9) as well as combined (10-13) methods. They promote the creation of a high level of strain--strength characteristics unachievable under a one-stage process. The methods of processing which combine severe plastic deformation by simple shear and conventional solid-phase extrusion seem to be promising. We have recently shown that the equal-channel multiple angle extrusion (ECMAE) is an effective method of severe plastic deformation resulting in the improvement of physical and mechanical properties of crystallizing polymers (14), (15). The ECMAE implies that in one unit, there are several zones of shearing strain. In such a way, high plastic deformations are accumulated per one cycle and, at the same time, the distribution of deformation over polymer billet sections is highly uniform. Potentialities of the combined schemes of solid-phase extrusion involving the extrusion through conical die (ED) and the ECMAE in different sequence have been considered, as well (16). It is shown that for the crystallizing polymers the ED-ECMAE scheme gives a better result as compared with one-stage processes.

This paper is a continuation of studies (14-16), it deals with structural and phase rearrangements in polyamide-6 (PA-6) during ED-ECMAE as well as with the search of rational conditions for PA-6 processing by this method.

EXPERIMENTAL

The solid-phase extrusion was implemented by different schemes: ED (Fig. la), ECMAE (Fig. lb), and ED-ECMAE (Fig. lc). The ECMAE was done with deformation intensities AC, [DELTA][[GAMMA].sub.1] = 0.54 and 0.83 with the accumulated strain values [[epsilon].sub.ECMAE] = 1.3, 2.1, 4.0, 6.7

[DELTA][[GAMMA].sub.i] = 2ctg[[THETA].sub.I] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [[theta].sub.i] is the angle of channel intersection, it is the number of channel intersection angles (14). For the ED. the accumulated strain value [[epsilon].sub.ED] = 0.7, 1.1, 1.4

[epsilon] = ln [D.sup.2]/[d.sup.2] (3)

where D and d are diameters of container channel and die calibrating hole, respectively. The extrusion rate was equal to 0.6 x [10.sub.3] m/s, the extrusion temperature was 423 K. These were the optimum conditions of the process (14), (15).

[ERTANOL.sup.R] 6SA, QUADRANT was the object of our research. We chose the method of measuring microhard-ness, H, as the main investigation method. This allowed us not only to simplify the mechanical testing but also to obtain information on the uniformity of the strain over a section of the extrudates. Since the microhardness of polymers is proportional to yield strength [[sigma].sub.y] (l7), the microhardness distribution suggests the strain uniformity. Microhardness H was determined using a microhardness tester of the PMT-3 type. The indenter was a tetrahedral diamond pyramid with the vertex angle of 136 The pyramid was fluently pressed unto the sample at the loading of 0.5 N. The value of microhardness H was estimated by the formula H = 0.1854 F/[d.sup.2], where F is loading, N; d is the diagonal of the indentation; [d.sup.2]/1.854 is the area of the lateral surface of produced pyramidal indentation. For H, the relative error was not higher than 5%. The uniformity of H distribution over the sections of extrudates was estimated by value of dispersion [D.sub.H] determined by the formula:

[FIGURE 1 OMITTED]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

where n is the number of measurements, Hi is the result of an individual measurement of microhardness value, and If is the average microhardness value. The value of micro-hardness anisotropy AH characterizing the difference in the strength properties in longitudinal and cross-sections of extrudates was estimated by the formula (18):

[DELTA]H = 1 - [[bar.H].sup.1]/[[bar.H].sup.II] (5)

where [[bar.H].sup.1] and [[bar.H] .sup.II] are the average values of microhardness in cross and longitudinal sections of extrudates, respectively.

The value of die swelling (fr was calculated by the formula:

[psi] = [([d.sub.e]).sup.2] - [([d.sub.d]).sup.2]/[([d.sub.b]).sup.2] - [([d.sub.d]).sup.2] x 100%

where [d.sub.e], [d.sub.d],, and [d.sub.b], are the extrudate, die (calibrating outlet), and billet diameters, respectively.

The density of the specimens [rho] was determined by hydrostatic weighing (a balance of the AX200 type, Shi-madzu). The volume degree of crystallinity ([X.sub.c.sup.[rho]]) was calculated using the following relationship:

[x.sub.c.sup.[rho]] = ([rho] - [[rho].sub.a])/([[rho].sub.c] - [[rho].sub.a]) (7)

where [[rho].sub.a] and [[rho].sub.c] are the densities of polymer amorphous and crystalline phases, respectively.

Moisture content W was determined by the formula

W = m - [m.sub.1]/[m.sub.2] x 100% (8)

where in and [m.sub.1] are the mass of weighing bottle with material sample before and after drying, respectively, and no is the mass of the investigated material sample.

The X-ray diffraction studies were done by the method of wide-angle X-ray scattering using a X-ray diffractometer. Mn-filtrated FeK[alpha]-radiation was used in this case. The patterns were taken in the diffraction mode (the Bragg-Bretano focusing). To evaluate the degree of crys-tallinity correctly, the method of artificial randomization was used. The method implied cutting of the tested material by a sharp blade into pieces 0.5 X 0.5 X 0.5 me in size that were placed into a cavity 4.0-mm thick. As compared with other methods of reduction, the selected randomization minimized impact of shear stresses and local heating on the structure of the material.

Using Matthews method, we evaluated the relative crystallization degree [x.sub.c.sup.WAXS] by the formula:

[X.sub.c.sup.WAXS] = [V.sub.c]/([V.sub.c] + [[V.sub.a]) (9)

where [V.sub.e] is the area below crystal reflexes and ([V.sub.c] + [V.sub.a]) is the total area below the coherent scattering curve.

Transmission electron microscopy (TEM) was implemented by using a JEOL JEM-200A instrument at an acceleration voltage of 200 kV. Replicas of the initial samples and extrudates exposed in the form of longitudinal chips prepared at the temperature of liquid nitrogen were obtained as follows. Fractured surface replicas were obtained in the following way. X-ray film (matrix) welted with acetone was pasted to the fractured surface. The dried film was detached and the replica was coated by a thin carbon layer in vacuum. After removing the matrix. the carbon replica was ready for studies.

TABLE 1.    Influence of solid-phase extrusion on microhardness of PA-6.

Methods of   [DELTA][[GAMMA].sub.1]  [[epsilon].sub.ECMAE]
deformation

ED

ECMAE                          0.54                    1.3
                               0.54                    6.7
                               0.83                    2.1
                               0.83                    6.7

ED-ECMAE                       0.54                    4.0
                               0.54                    1.3
                               0.54                    1.3
                               0.83                    2.1
                               0.83                    2.1
                               0.83                    2.1

                                                   MPa
Methods of   [[epsilon].sub.ED]     P  [[bar.H].sup.I]  [[bar.H].sup.II]
deformation

ED                          0.7    90               82               128
                            1.1   285               84               156
                            1.4   390               92               190

ECMAE                             410              109               140
                                 1000              148               172
                                  475              147               170
                                  830              155               184

ED-ECMAE                    0.7   624              150               178
                            1.1   590              177               208
                            1.4   620              194               226
                            0.7   575              160               190
                            1.1   650              220               250
                            1.4   710              206               240

Methods of   [D.sub.H]  [DELTA]H
deformation

ED                1.94      0.36
                  1.74      0.46
                  1.34      0.52

ECMAE             3.70      0.22
                  1.02      0.14
                  2.86      0,16
                  1.06      0.14

ED-ECMAE          0.84      0.16
                  2.27      0.15
                  1.20      0.14
                  1.22      0.16
                  1.16      0.12
                  1.14      0.14

Uniaxial compression tests were done by using a universal testing machine and specimens of 10 mm diameter and 15 mm height. The dumbbell-shaped specimens (head size, diameter 10 mm; length, 10 mm; working-part size, diameter 5 mm; length, 32 mm) were subjected to tensile tests. The specimens were cut along the direction of extrusion. The supporting platforms approached each other at a rate of 10 mm [min.sup.-1]. The average values of yield strength [[sigma].sub.y], tensile strength [[sigma].sub.T], modulus of elasticity E, yield strain [[epsilon].sub.y], strain at break [[epsilon].sub.b], and standard deviations were determined from testing five specimens of each sample.

RESULTS AND DISCUSSION

Table 1 lists the average values of microhardness in longitudinal Hal and cross Hi sections of extrudates, the value of microhardness anisotropy AN, as well as the dispersion of microhardness nil in cross-sections for specimens produced by the ED-ECMAE scheme and for comparison for those deformed by ED and ECMAE. For the original PA-6 sample, the average microhardness value was equal to 80 MPa in both the longitudinal and cross sections.

It is seen that deformation by the ED-ECMAE scheme provides a noticeable increase in the average values of microhardness H when compared with the original polymer. The absolute values of [bar.H] are also higher than those after a one-stage ECMAE process with a higher [epsilon]. And a higher hardening effect is attained with lower extrusion pressures P. This scheme provides lower [DELTA]H values as compared to ECMAE, as well as the minimal [D.sub.H] values. Changes in technological parameters of the ED-ECMAE process, namely the increase of deformation degree [[epsilon].sub.ED] under ED, [[epsilon].sub.ECMAE] and [DELTA][GAMMA] under ECMAE favor the [[bar.H].sup.I] and [[bar.H].sup.II] increase, the decrease of microhardness anisotropy and microhardness dispersion in cross section of the extrudates.

It is known that with the conventional schemes of solid-phase extrusion, cross-section of the product undergoes the so-called die swelling at die outlet (19). It has been determined that in the case of ED, when the accumutated strain [[epsilon].sub.ED] increases from 0.7 to 1.4, the value of [psi] decreases from 22.2 to 14%. In the case of ECMAE and ED-ECMAE the die swelling is absent. This is one of the features of solid-phase extrusion processes involving the deformation by simple shear without macroscopic deformation of the material as it realizes at the microlevel.

Table 2 shows how physical and mechanical properties of PA-6 are influenced by different schemes of solid-phase extrusion. It is seen that under the combined ED-ECMAE scheme, the elastic and strength characteristics are improved the most. However, the plasticity (the yield strain 8y and breaking strain [[epsilon].sub.b]) becomes less. But the character of dependences of plasticity characteristics on accumulated strain value is, to a considerable extent, specified by scheme of the process. In the case of ECMAE and starting from certain values of [[epsilon].sub.ECMAE], the magnitudes of [[epsilon].sub.y] and [[epsilon].sub.b], come to a constant level. Such a behavior differs from that with the conventional schemes of solid-phase extrusion, ED in particular, which are typical of a continuous plasticity decrease with the accumulated strain growth. Under the combined deformation scheme when [[epsilon].sub.y] and [[epsilon].sub.b], values are larger than those in corresponding one-stage ED and ECMAE processes, the decrease in plasticity is the lowest. With the ED-ECMAE scheme, the increase of [[epsilon].sub.ED], [[epsilon].sub.ECMAE], and [DELTA][GAMMA] results in [[epsilon].sub.y] and [[epsilon].sub.b], increase.

The results of density measurements and data on water absorption are indicative of the increase of crystallinity degree. The highest effect is with the ED-ECMAE scheme; it increases with [[epsilon].sub.ED], [[epsilon].sub.ECMAE], and [DELTA][GAMMA] growth (Table 2). Wide-angle X-ray analysis (WAXS) results of crystallinity degree tests correlate with density and moisture adsorption data. In particular, the initial PA-6 has [X.sub.c.sup.WAXS] = 0.30 whereas after ECMAE ([DELTA][[GAMMA].sub.1] = 0.83, [[epsilon].sub.ECMAE] = 6.7) and ED-ECMAE ([[epsilon].sub.ED] = 1.1, [DELTA][[GAMMA].sub.1] = 0.83, [[epsilon].sub.ECMAE] = 2.1), this value was equal to 0.38 and 0.40, correspondingly.

The deviation from the general regularity has been determined only for PA-6 deformed by the ED-ECMAE scheme: when [[epsilon].sub.ED] = 1.4 and [[epsilon].sub.ECMAE] = 2.1; the density and the strain-strength characteristics become worse. The problem is that at a high degree of deformation under the ED, the subsequent ECMAE with a high enough Al' and tEcmiNE results in a partial mechanodestruction of polymer chains. The latter is also confirmed by the data on water absorption of PA-6. It is known that during sorption the hydrogen bonds are formed between water molecules and PA-6 amide groups (20). As the crystalline regions are inaccessible for moisture because of the increased density and of practically complete saturation of hydrogen bonds, then the increase of W, with the crystalline-phase volume fraction preserved, is connected with the increasing amount of free radicals in the amorphous phase which can interact with the associates of water molecules.

Thus, the use of combined deformation schemes including ED and ECMAE allows increment of hardness and strength of PA-6, conservation of high plastic characteristics, increase in density and moisture resistance, and extension of the operation temperature range of articles made of the processed polymer. Issuing from the obtained results, the following rational regime of PA-6 processing by solid-phase extrusion, that is the combined extrusion by the ED-ECMAE scheme, is proposed: [[epsilon].sub.ED] = 1.4, [DELTA][[GAMMA].sub.1] = 0.83, [[epsilon].sub.ECMAE] = 2.1. The deformation need be performed with the extrusion rate of 10.6 X [10.sup.-3] m/s and extrusion temperature of 423 K.

TABLE 2. Influence of solid-phase extrusion on physical and
mechanical properties of PA-6.

Methods of   [DELTA][[GAMMA].sub.1]  [[epsilon].sub.ED]
deformation

No
deformation

ED                                                  0.7
                                                    1.1
                                                    1.4

ECMAE                          0.54
                               0.54
                               0.83
                               0.83

ED-ECMAE                       0.54                 0.7
                               0.54                 1.1
                               0.54                 1.4
                               0.83                 0.7
                               0.83                 1.1
                               0.83                 1.4

Methods of   [[epsilon].sub.ECMAE]           [rho]  [z.sub.c.sup.[rho]]
deformation                         (g/[cm.sup.3])

No                                           1.135                 0.32
deformation

ED                                           1.139                 0.36
                                             1.139                 0.36
                                             1.139                 0.36

ECMAE                          1.3           1.142                 0.41
                               6.7           1.143                 0.42
                               2.1           1.143                 0.42
                               6.7           1.144                 0.43

ED-ECMAE                       4.0           1.143                 0.42
                               1.3           1.144                 0.43
                               1.3           1.144                 0.43
                               2.1           1.143                 0.42
                               2.1           1.144                 0.43
                               2.1           1.143                 0.42

                                    MPa
Methods of     W     E  [[sigma].sub.y]  [[sigma].sub.T]
deformation

No           8.5   900               67               69
deformation

ED           7.1  1095               91               92
             7.1  1276              108              111
             7.0  1355              115              118

ECMAE        6.7  1078               86               90
             3.8  1370              110              112
             3.8  1370              108              110
             3.6  1468              126              130

ED-ECMAE     3.8  1412              118              120
             3.6  1618              140              141
             3.4  1770              152              155
             3.7  1593              134              137
             3.4  2170              174              178
             3.9  2006              156              160

                            %
Methods of   [[epsilon].sub.y]  [[epsilon].sub.b]
deformation

No                        14.6                148
deformation

ED                        10.4                116
                           6.8                 75
                           5.0                 51

ECMAE                     10.7                139
                           9.8                120
                           9.7                120
                           9.8                125

ED-ECMAE                  10.3                132
                          10.2                134
                          10.2                135
                          10.1                132
                          10.4                138
                          10.0                130

The main reason for the increase of microhardness and strength characteristics of polymers subjected to solid-phase extrusion is in the formation of orientation order (9). From scanning electron microscopy studies (16), it is concluded that in the case of ED scheme, a clearly defined oriented structure is formed. With ECMAE and ED-ECMAE the orientation of structure is less defined.

Such a behavior can be explained as follows. The deformation induced the transformation of the original spherolytic structure (Fig. 2a) to the oriented lamellar or fibrillar structure. In the case of ED, the monotonous deformation behavior results in orientation of lamellae or fibrils along the direction of extrusion. As a result, a clearly defined macroscopic orientation is formed (Fig. 2b). Formation of such a structure stipulates, in particular, a high anisotropy of the strength properties (with the strengthening mainly aligned with the direction of orientation) and a low plasticity; the crack easily propagates along the oriented polymer chains. In the case of ECMAE, and when the ECMAE is at the final stage of deformation, that is, ED-ECMAE, the sign-alternating character of deformation under the ECMAE makes conditions for the formation of microscopic molecular orientation (Fig. 2c and d). The latter may be also responsible for conservation of high plasticity values of the deformed polymers (21).

The suggested mechanism of structural transformations in crystallizing polymers in the course of solid-phase extrusion is also confirmed by the results of TEM experiments. Figure 2e-h presents the images of replicas of the fracture surfaces of the studied PA-6 samples after different types of treatment. The fracture surface of the source PA-6 demonstrates globular formations of the size between 20 and 50 nm (Fig. 2e). The last objects may be sets of arbitrary oriented packs of crystallites.

Due to mainly tensile nature of the deformation, the extrusion through a die results in partial reconstruction of the structure into fiber-like entities with the cross size up to 10 nm and the length of 150-200 nm oriented long the extrusion direction (Fig. 21).

Globule fragmentation is observed in the case of ECMAE, resulting from realized simple shear deformation. Their size is reduced down to 20-30 nm (Fig. 2g). According to dilatometry data (16) and microhardness measurements, the structure of PA-6 exposed to ECMAE must contain areas with fiber-like entities. But TEM results do not confirm that it can be probably caused by their small size.

The first stage of ED-ECMAE process (ED, i.e. tensile deformation mainly) forms a fibrillar structure that is fragmented later and followed by partial turn of fibrils (in the course of ECMAE and simple shear deformation). Because the nature of the macroscopic strain is both shear and extensional, the result is the formation of a duplex structure consisting of elongated disoriented fiber-like entities with the diameter of 10 nm and 100-150 nm long and a number of globules of 10-20 nm in size (Fig. 2h). This structure is similar to composite containing disoriented fibers within a polymeric matrix providing combination of high strength and plasticity. Fiber-like entities oriented in several directions are also responsible for low values of anisotropy of the properties.

In the X-ray diffraction pattern of the original PA-6 (Fig. 3, curve 1) there are two well-defined reflexes for 20 = 20[degrees] and 2[theta] = 24[degrees] characterizing the a-phase and related to the inter-chain distances in plane (200) and distances between the planes of associated molecules (002 + 202) (22). Deformation according to the ECMAE scheme results in increase of [alpha] (200) and [alpha] (002 + 202) reflexes intensity by a factor of 1.4 and 1.04, respectively, and in the origination of a very weak reflex for 20 = 21.5 [degrees] characterizing [gamma]-phase of' PA-6 (Fig. 3, curve 2). In the X-ray diffraction pattern (Fig. 3, curve 3) for PA-6 processed by the ED-ECMAE scheme, the reflex [alpha] (200) is less intensive and well-defined reflex for 2[theta] = 21.5[degrees] is observed.

[FIGURE 2 OMITTED]

In view of X-ray diffraction data, high plasticity values of PA-6 samples deformed by ECMAE and ED-ECMAE may be also due to transformation of [alpha]-form crystals to [gamma]-Corm ones which possess high plasticity reserve (23).

[FIGURE 3 OMITTED]

CONCLUSION

We have established that combined solid-phase extrusion including extrusion through a conical die and the following equal-channel multiple-angular extrusion allowed us to enhance the hardness and the strength of PA-6 with the plasticity kept at a high level. It was shown that the achievement of this complex of properties is determined by the formation of a special structure containing fragmented globular formations and elongated disoriented fiber-like entities. An additional factor contributing to the plasticity retention in the extruded PA-6 is the formation of crystals of [gamma]-form with higher plasticity reserve in comparison to [alpha]-form. We have revealed rational techno-logical parameters of ED-ECMAE process with the best level of the achieved properties: [[epsilon].sub.ED] = 1.1, [DELTA][[GAMMA].sub.1] = 0.83, and [[epsilon].sub.ECMAE] = 2.1. It was shown that some values of the degree of extrusion extension and the following ECMAE with high deformation intensity and the value of the accumulated deformation demonstrate the reduction of the complex of physical and mechanical characteristics because of the material loosening due to mechanical destruction of polymer chains.

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Correvondence to: V.A. Beloshenko: [e-mail]. ua

DOI 10.1002/pen.23123

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

V.A. Beloshenko, A.V. Voznyak, Yu.V. Voznyak, V.A. Glasunova, T.E. Konstantinova

Donetsk Institute for Physics and Engineering named after A.A. Galkin, National Academy of Sciences of Ukraine, 83114 Donetsk, Ukraine