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Solid-phase extrusion of polyamide-6 by using combined deformation schemes.

INTRODUCTION

The solid-phase extrusion is known to be one of the methods for the attaining high-oriented state of polymers [1, 2]. It is traditionally implemented by pressing the polymer through a conical or slot die to produce items of various configurations possessing high strain-stress characteristics. In this case, the process of plastic deformation accumulation is accompanied by the decrease in crosssectional dimensions of extrudates.

Recently, there has been a considerable interest in the methods of solid-phase extrusion based on simple shear [2, 3]. Among them, the equal-channel angular extrusion (ECAE) is the most widely used method. Under ECAE, a polymer billet is pressed through two mutually intersecting channels of equal cross-section. The influence of ECAE on structure of crystallizing polymers and polymer-based composites has been studied previously [4-17], In contrast to conventional methods of solid-phase extrusion, the accumulation of plastic deformations is not, in this study, accompanied by changes in geometry of the polymer billet; it is attained by increasing the number of passes through the intersecting channels. We have recently shown the efficiency of the equal-channel multiple angle extrusion (ECMAE), which is a modified version of ECAE, in improving the set of physical and mechanical properties of crystallizing polymers [18, 19]. In the ECMAE plant, there are several zones of shear deformation providing the accumulation of high plastic deformations in one cycle of the process.

It is known that a combination of different solid-phase orientation methods, e.g., coextrusion and uniaxial drawing [20-23], promotes the improvement of the strainstress characteristics, in contrast to a one-stage process. A similar result may, evidently, be expected from the combining of simple shear-based solid-phase extrusion and the traditional one. In this article, polyamide (PA)-6 is taken to show potentialities of such processing by the extrusion through conical die (ED) and ECMAE of variable sequence to modify structure and properties of semicrystalline polymers.

EXPERIMENTAL

The solid-phase extrusion was implemented by different schemes: ED (Fig. 1a), ECMAE (Fig. 1b), ECMAEED (Fig. 1c), and ED-ECMAE (Fig. 1d). For ECMAE, the deformation intensity was [DELTA][[GAMMA].sub.1] = 0.54 and the accumulated strain value was [epsilon] = 1.3 - 6.7,

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

epsilon = 2 [n.summation over (i=1)] ctg[[THETA].sub.i]/[square root of (3)], (2)

where [[THETA].sub.i] is the angle of channel intersection and n is the number of channel intersection angles [18]. For ED,

[epsilon] = ln [D.sup.2]/[d.sup.2] = 0.7, (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.sup.3] m/s, and the extrusion temperature was 423 K. These were the optimum conditions of the process [18, 19].

PA-6 (ERTALON [R] 6SA; Quadrant) was the object of our research. The billets of a necessary size (15 mm diameter and 50 mm long for ECMAE and ECMAE-ED; 21 mm diameter and 25 mm long for ED and ED-ECMAE) were prepared from cylindrical rods of 16 and 30 mm diameter, respectively.

The microhardness, H, was determined by a micro-hardness meter. The indenter was a tetrahedral diamond pyramid with the vertex angle of 136[degrees]. The pyramid was smoothly forced into the sample at a load of 0.5 N. Microhardness value H was calculated by the formula

H = 0.1854 F/[d.sup.2],

where F was the load, N; d was the indentation diagonal; and [d.sup.2]/1.854 was the area of the lateral surface of the resulted pyramidal indentation. For H, the relative error did not exceed 5%.

The value of die swelling [psi] 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%, (4)

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

The density [rho] of specimens was determined by the methods of hydrostatic weighing using scales of AX200 series, Shimadzu Co. The volume degree of crystallinity ([[chi].sub.c.sup.[rho]]) was calculated using the following relationship:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

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%, (6)

where m and [m.sub.1] were the mass of weighing bottle with material sample before and after drying, respectively, and [m.sub.2] was the mass of investigated material sample.

The uniaxial compression tests for specimens of 10 mm diameter and 15 mm height were done at the room temperature using a universal testing machine. 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 traveled at a velocity 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.

Changes in linear dimensions of specimens cut in longitudinal and transverse directions with respect to the extrudate axis were determined using a dilatometer DIL 402 PC/4 Netzsch, and the heating rate was equal to 1 K/min.

Scanning electron microscopy (SEM) was implemented by using a JEOL JSM-6490 instrument at an accelerating voltage of 15 kV. A conducting layer (25-30-[micro]m-thick aurum layer) was applied to surface under investigation by cathode sputtering method. The photographs were taken of the surfaces of longitudinal spallings of original samples and extrudates. The spalling was made at the liquid nitrogen temperature.

RESULTS AND DISCUSSION

The extrusion pressure P is one of the basic parameters of solid-phase extrusion [2]. The parameter not only specifies technological potentialities of the process but affects properties of the product as well [2]. For all the investigated methods of deformation, the time dependence of P is of the same character. The material start extruding when the pressure reaches a maximum value [P.sub.m], then the billet is moving at a lower pressure. In the case of ECMAE, the maximum extrusion pressure increases with [epsilon] (Table 1). For the combined deformation schemes, [P.sub.m] value depends on a sequences of processes. It can be seen that, for the ECMAE-ED scheme realization, the extrusion pressure is almost twice as much as for the ED-ECMAE scheme.
TABLE 1. Influence of solid-phase extrusion on microhardness of PA-6.

Methods of    [P.sub.m]  [[bar.H].sup.[perpendicular  [DELTA]H
deformation   (MPa)      to]]([[bar.H].sup.||])

ECMAE               410                     109(140)      0.22
([epsilon] =
1.3)

ECMAE               505                     132(164)      0.20
([epsilon] =
4.0)

ECMAE              1000                     148(172)      0.14
([epsilon] =
6.7)

ED                   90                      82(128)      0.36

ED-ECMAE            625                     150(178)      0.16
([epsilon] =
4.0)

ECMAE              1280                      92(166)      0.45
([epsilon] =
4.0)-ED

Methods of    [D.sub.H]
deformation

ECMAE              3.70
([epsilon] =
1.3)

ECMAE              1.96
([epsilon] =
4.0)

ECMAE              1.02
([epsilon] =
6.7)

ED                 1.94

ED-ECMAE           0.84
([epsilon] =
4.0)

ECMAE              1.22
([epsilon] =
4.0)-ED


A high [P.sub.m] value attained under the ECMAE-ED implementation may, possibly, be due to the high deformation accumulated at the first stage of the process. As a result, the hardened polymer requires a higher force for the extrusion through conical die.

Because quantity H is proportional to yield strength [[sigma].sub.y] of the polymer [24], it has become possible to obtain information on the degree of strengthening and deformation uniformity in the cross-section of extrudates by the method of microhardness measurement.

Table 1 lists the average values of microhardness in longitudinal [[bar.H].sup.||] and cross [[bar.H].sup.[perpendicular to]] sections of extrudates, the value of microhardness anisotropy [DELTA]H, and the dispersion of microhardness [D.sub.H] in the cross-section for specimens produced by various schemes of solid-state extrusion. For the original PA-6 sample, the average value of microhardness makes 80 MPa both in longitudinal and cross-sections.

The ED results in [[bar.H].sup.||] growth, [[bar.H].sup.[perpendicular to]] being at the level of microhardness of the original PA-6. Such a behavior is typical of the uniaxially oriented crystallizing polymers where the anisotropy of strength properties is essential.

The ECMAE increases the microhardness in both the sections. An increment in H is the larger the higher [epsilon] is. The value of microhardness anisotropy defined as [24, 25]

[DELTA]H = 1-[[bar.H].sup.[perpendicular to]]/[[bar.H].sup.||] (7)

makes 0.22, 0.20, and 0.14 for [epsilon] = 1.3, 4.0, and 6.7, respectively. It is lower than [DELTA]H = 0.36 attained under ED with the lower value of accumulated strain. This regularity differs from the observed in the traditional case of solid-phase extrusion, when the anisotropy of strength properties increases with the accumulated plastic deformation.

The deformation by the ED-ECMAE scheme provides a noticeable increase of the average values of microhardness compared with the original polymer. The absolute values of [bar.H] are higher than those after ECMAE with [epsilon] = 6.7. Here, we state that the higher strengthening effect is attained at lower [P.sub.m]. The ED-ECMAE scheme gives lower values AH = 0.16, compared with ECMAE, [epsilon] = 4.0. For the ECMAE-ED scheme, there was a noticeable increase in [[bar.H].sup.||] and negligible growth of [[bar.H].sup.[perpendicular to]] with [DELTA]H attaining the maximum value.

[FIGURE 1 OMITTED]

Figure 2 illustrates the distribution of H in the cross-section of extrudates as a function of solid-phase extrusion method. It was seen that after ECMAE and deformation by the ED-ECMAE scheme, the values of [bar.H] at peripheral zones are higher than those in the centre. Nonuniform distribution of H in the cross-section of the extruded polymer is due to differences in structure rearrangement in peripheral and central zones of the extrudate. The differences result from polymer friction on the deforming channel [26]. The arising shearing stresses promote the strengthening of surface layers in comparison with the central part of the billet. The friction-induced warming-up of polymer billet surface can give similar result and create favorable conditions for the processes of polymer chain orientation.

[FIGURE 2 OMITTED]

The situation is opposite for ED and deformation by the ECMAE-ED scheme: the H values in central zones are higher than those at the periphery. The reason is in the die swelling of extrudates. There is no swelling after ECMAE and ED-ECMAE, whereas under ED and deformation by the scheme ECMAE-ED, it makes 22% and 16%, respectively. The developing relaxation processes are responsible for microhardness fall in peripheral zones first of all. The application of combined methods decreases the value of microhardness dispersion [D.sub.H] essentially (Table 1). The minimal [D.sub.H] values are reached with the ED-ECMAE scheme.

The use of the above methods of extrusion provides the improvement of elastic and strength characteristics of PA-6, such as the modulus of elasticity E, the yield strength [[sigma].sub.y], and the tensile strength [[sigma].sub.T] measured on compression and tension of specimens cut along the direction of extrusion (Table 2). However, the plasticity (yield strain [[epsilon].sub.y] and strain at break [[epsilon].sub.b]) decreases. Value of the effect is, to a considerable extent, specified by the scheme of the process.
TABLE 2. Influence of solid-phase extrusion on physical and mechanical
properties of PA-6.

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

No deformation                    1.135                     0.32

ECMAE ([epsilon]                  1.141                     0.36
= 1.3)

ECMAE ([epsilon]                  1.142                     0.37
= 4.0)

ECMAE ([epsilon]                  1.143                     0.38
= 6.7)

ED                                1.139                     0.35

ED-ECMAE                          1.143                     0.38
([epsilon] =
4.0)

ECMAE ([epsilon]                  1.142                     0.37
= 4.0)-ED

                      Compression

Methods of        [[sigma].sub.y] (MPa)  E (MPa)
deformation

No deformation                       68                      870

ECMAE ([epsilon]                     88                     1050
= 1.3)

ECMAE ([epsilon]                    104                     1300
= 4.0)

ECMAE ([epsilon]                    112                     1380
= 6.7)

ED                                   90                     1090

ED-ECMAE                            115                     1395
([epsilon] =
4.0)

ECMAE ([epsilon]                    108                     1345
= 4.0)-ED

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

No deformation                     14.0

ECMAE ([epsilon]                   10.6
= 1.3)

ECMAE ([epsilon]                    9.5
= 4.0)

ECMAE ([epsilon]                    9.5
= 6.7)

ED                                 10.5

ED-ECMAE                           10.3
([epsilon] =
4.0)

ECMAE ([epsilon]                   10.1
= 4.0)-ED

                            Tension

Methods of        [[sigma].sub.y] (MPa)  [[sigma].sub.T] (MPa)
deformation

No deformation                       67                       69

ECMAE ([epsilon]                     86                       90
= 1.3)

ECMAE ([epsilon]                    108                      111
= 4.0)

ECMAE ([epsilon]                    110                      112
= 6.7)

ED                                   91                       92

ED-ECMAE                            118                      120
([epsilon] =
4.0)

ECMAE ([epsilon]                    112                      115
= 4.0)-ED

Methods of        E (MPa)                [[epsilon].sub.y] (%)
deformation

No deformation                      900                     14.3

ECMAE ([epsilon]                   1078                     10.7
= 1.3)

ECMAE ([epsilon]                   1345                      9.6
= 4.0)

ECMAE ([epsilon]                   1370                      9.6
= 6.7)

ED                                 1095                     10.4

ED-ECMAE                           1412                     10.3
([epsilon] =
4.0)

ECMAE ([epsilon]                   1370                     10.2
= 4.0)-ED

Methods of        [[epsilon].sub.b] (%)
deformation

No deformation                      148

ECMAE ([epsilon]                    139
= 1.3)

ECMAE ([epsilon]                    126
= 4.0)

ECMAE ([epsilon]                    120
= 6.7)

ED                                  116

ED-ECMAE                            132
([epsilon] =
4.0)

ECMAE ([epsilon]                    128
= 4.0)-ED


The ED results in a considerable decrease in [[epsilon].sub.y] and [[epsilon].sub.b] values already at low [epsilon] compared with the original material. In the case of ECMAE and starting from certain values of [epsilon], the magnitudes of [[epsilon].sub.y] and [[epsilon].sub.b] come to a constant level. Such a behavior differs from the observed in traditional schemes of solid-phase extrusion, when the plasticity continuously decreases with the growth of accumulated strain. Under the combined deformation schemes, 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. The best combination of elastic, strength, and plasticity characteristics is attained for the ED-ECMAE scheme.

The growth of microhardness and strength characteristics of semicrystalline polymers can be conditioned by the increase in crystallmity degree of the samples [27]. This is confirmed by the data on water absorption and the results of density measurements (Fig. 3, Table 2). As known, the hydrogen bonds are formed between water molecules and amide groups of PA-6 during the sorption [28]. As the whole of polymer volume does not change, the formed associates of water molecules in polymer matrix fill only the free volume between supermolecular fonnations, whereas the crystalline regions are inaccessible for moisture because of increased density and, practically, total saturation of hydrogen bonds. Thus, the increase in the share of crystalline phase should result in the decrease in moisture content W. It can also be related with the decrease in the free volume of microdefects accessible for filling with moisture. In Fig. 3 it is seen that for the original PA-6 the water absorption reaches 8.5%. In the case of ED, this value decreases to 7.1%, whereas with the ECMAE- ED, ECMAE, and ED-ECMAE schemes it decreases to 4.1, 4.0, and 3.8%, respectively.

[FIGURE 3 OMITTED]

Figure 4 illustrates changes in elongation [DELTA]l/[l.sub.0] of original and extruded PA-6 samples under heating. In the whole of the temperature range, the original sample behaves as usual. Its length increases with temperature because of thermal expansion. For the specimens cut from extrudates in direction normal to the direction of polymer extrusion, the dependence [DELTA]l/[l.sub.0] (T) is similar to that of the original material. The value of [DELTA]l/[l.sub.0] increment with the heating temperature growth increases in the series: ECMAE, ED, ECMAE-ED, and ED-ECMAE (Fig. 4a). The increase in [DELTA]l/[l.sub.0], compared with the original material, is due to both the increase of extrudates density and the presence of compressed amorphous phase located between crystalline lamellae.

[FIGURE 4 OMITTED]

For the specimens cut along the direction of extrusion and processed by ECMAE, an extreme change in [DELTA]l/[l.sub.0] is observed (Fig. 4b). The decrease in [DELTA]l/[l.sub.0] is due to relaxation of the deformed amorphous phase confined between crystalline lamellae (the length being preserved), and the successive growth of [DELTA]l/[l.sub.0] is due to thermal expansion of the extrudates [7, 19]. It is easy to imagine that the contraction of the amorphous component stretched along the extrudate's axis would not affect changes in the length of specimen cut in direction normal to that of extrusion. At the same time, for the specimen oriented along extrudate, the folding of tie-molecules prevents, to a moment, its elongation. This process starts near the glass-transition temperature, activates with the growth of segmental mobility, and terminates with the process of thermal shrinkage [29].

In the case of deformation by the ED, ED-ECMAE, and ECMAE-ED schemes, the [DELTA]l/[l.sub.0] decreases in the whole f the temperature range studied (Fig. 4b). The character of dependence [DELTA]l/[l.sub.0] (T) points to a considerable stretching of the amorphous phase as well as to a higher degree of crystallinity, which obstructs the processes of thermal expansion up to the temperatures of extrudate melting. For the original PA-6, the temperature with which [DELTA]l/[l.sub.o] does not change makes 344 K. In the case of ED, it increases to 347 K. With the ECMAE, ECMAE-ED, and ED-ECMAE schemes, the temperatures make 353, 369, and 372 K, respectively. The increase in this temperature, with which the processes of thermal shrinkage become active, can be related to the effect of molecular orientation and to the growth of crystallinity degree of the extrudates.

In such a way, the combined deformation schemes involving ED and ECMAE make it possible to improve rigidity and strength of PA-6, to retain its plastic characteristics at a high level, to increase density and moisture resistance, and to widen the temperature range for operation of products made of the processed polymer.

In Fig. 5, there are photographs of brittle-fracture surface for the original PA-6 and that subjected to various scheme of solid-phase extrusion. For the original polymer, a preferential direction of crack propagation is absent because of the presence of isotropic spherolitic structure (Fig. 5a). For the PA-6 extrudates, the direction of crack propagation coincides with that of extrusion of the material (Fig. 5b-e). In the case of ED and ECMAE-ED schemes, the character of fracture points to the formation of clearly defined oriented structure. For ECMAE and ED-ECMAE, the orientation of structure is defined not so clearly. The results of SEM studies agree with the character of [DELTA]H change (Table 1): the highest values are attained for ECMAE-ED and ED, whereas the lowest for ED-ECMAE and ECMAE schemes. In compliance with the Brown-Windel model [30], it can be assumed that, in the case of ED and ECMAE-ED schemes, in the extrudate the macroscopic molecular orientation prevails, and for ECMAE and ED-ECMAE schemes the molecular orientation is microscopic. Similar behavior was observed, for example, under the solid-phase orientation of polycarbonate by ECAE using the routes A1 and C2 [31, 32].

The analysis of SEM results, as well as of the measurements of density, moisture content, dilatometry, and mechanical tests gives us grounds to assume that for the used [epsilon] values, the structure rearrangements occur in the amorphous phase under ED and in the crystalline phase under ECMAE. Indeed, for the PA-6 specimens subjected to ED, a considerable [DELTA]l/[l.sub.o] decrease is observed and, compared with the undeformed PA-6, the density increases negligibly and value of water absorption W negligibly decreases. On the contrary, for PA-6 samples subjected to ECMAE, we observe a negligible decrease in [DELTA]l/[l.sub.o] with a considerable gain in density and decrease in water absorption.

With the implemented combined schemes of deformation, the noticeable rearrangements of structure occur in both the amorphous and crystalline regions. It should be added that the deformation by ED-ECMAE scheme results in a more uniform oriented structure of semi-crystalline polymers.

CONCLUSION

The obtained results give us grounds to believe that the application of combined deformation schemes, including ED and ECMAE, is a promising method of solid-phase modification of semicrystalline polymers' structure and improvement of physical and mechanical properties in complex. The process is the most efficient with the ED-ECMAE scheme with which the strength and plastic characteristics are the highest, whereas the microhardness anisotropy and dispersion are the lowest. A higher strengthening effect is attained with a lower extrusion pressure.

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Correspondence to: V.A. Beloshenko; e-mail: bel@hpress.rti.ac. donetsk.ua

DOI 10.1002/pen.21835

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

[C] 2011 Society of Plasties Engineers

V.A. Beloshenko, V.N. Varyukhin, A.V. Voznyak, Yu.V. Voznyak

Department for Technological Studies of Hydropressing Processes, Donetsk Institute for Physics and Engineering Named after A.A. Galkin, National Academy of Sciences of Ukraine, Donetsk, Ukraine
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Author:Beloshenko, V.A.; Varyukhin, V.N.; Voznyak, A.V.; Voznyak, Yu. V.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXUR
Date:Jun 1, 2011
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