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Processing and mechanical properties of thermoplastic composites based on cellulose fibers and ethylene-acrylic acid copolymer.

INTRODUCTION

The use of natural fibers as a reinforcement in thermoplastic polymers has increased considerably in recent years (1). The possibility of replacing synthetic fibers such as glass or carbon fibers, with natural fibers is still considered doubtful since the mechanical performance of natural fibers in composites is poorer than that of synthetic fibers (2). Natural fibers have, however, some properties that make them attractive, such as renewability, biodegradability, unlimited availability, low cost, low density, and high specific stiffness. Other considerable advantages are their low abrasive nature and physiological harmlessness (l), (3-5). Some industries, like the automotive and packaging industries, are increasing the use of natural fibers as a reinforcement material. Furthermore, other sectors--from construction to the computer industry--are also replacing parts made with synthetic fibers with natural-fiber-reinforced composites (1), (6-10).

Some drawbacks when using cellulosic fibers as a reinforcement of thermoplastic polymers are poor adhesion between the fiber and the matrix, high moisture absorption by the fibers, poor dispersion of the fibers, and degradation of the fibers at nonnal plastic processing temperatures. Most of these drawbacks probably need to be reduced or eliminated in order to improve the performance of the final cellulose composite. The adhesion between the fiber and the matrix is very important since the load is transferred from the matrix to the stiff fiber through shear stresses at the interface. The use of compatibilizers is well-known in cellulose-fiber-reinforced composites and these agents generally improve the mechanical performance of the composite (11-17). In the present study no compatibilizer has been added, as the compatibility between the ethylene-acrylic acid copolymer (EAA) and cellulose seems to be sufficient (11), (14), (18). EAA is well-known by its application in coating, but in this case, the EAA was injection-molded since it was considered to be favorable due to its processing temperature, considerably lower than that for common thermoplastics such as for polyethylene (PE) or polypropylene (PP). The lower processing temperature reduces the risk of possible degradation of the cellulose fibers and a better mechanical performance of the composite may be expected.

Recent studies have shown that pelletizing the cellulose allows continuous feeding into the extruder, but the pelletization process results in a significant shortening of the fibers (19-23). Fiber breakage has been reported to reduce the reinforcement effect of the cellulose fibers (19), (20). Nevertheless, it has also been reported that good dispel sion of the fibers is an important parameter to improve the mechanical properties of the composite, and this means that short fibers may result in better mechanical properties than long fibers because they promote a better dispersion and consequently more cellulose fibers are used as reinforcement of the thermoplastic matrix (24). However, not only pelletizing but also the processing affects the fiber length. The processing also influences the dispersion of the fibers and can cause thermal degradation of both the cellulose fibers and the matrix. For example, it has been reported that the dispersion was improved by increasing the screw rotation in extrusion mixing without significantly affecting the fiber length and also that the dispersion was improved by two consecutive extrusions, but that this considerably reduced the fiber length and also led to greater thermal degradation of the fibers and the matrix (22). Even though cellulose-fiber-reinforced composites have been studied for several decades, there is still a need to understand how their mechanical performance can be improved by the processing.

The aim of this work has been to assess the performance of cellulose-fiber-reinforced composites using ethylene-acrylic acid copolymer as a matrix, a polymer that has been generally used as compatibilizer, and to clarify how the processing affects the fiber length, the dispersion of cellulose within the matrix, the thermal degradation and the mechanical performance of the final composite.

EXPERIMENTAL

Materials

The starting materials in this study were pelletized cellulose fiber agglomerates mixed with EAA, obtained by the Palltrusion[TM]-process method at Pallmann Maschinenfabrik GmbH & Co. KG, Zweibriicken, Germany. The process had several steps. The cellulose fibers used were softwood kraft pulp from Sodra with an estimated density of 1.5 g/[cm.sup.3]. The cellulose fibers were delivered as commercial sheets from Sodra. Since the cellulose fibers had to be disentangled in order to be fed into the agglomerator, the cellulose sheets were ground in a grinding machine with a rotor and a stator. Both rotor and stator had knives that cut the cellulose fibers when the rotor was rotating. Beneath the stator there was a sieve which defined the particle size of the ground material. The dimension of the holes in the sieve was 0.5 mm.

The ground cellulose fibers and the EAA pellets were gravimetrically fed into the agglomerator, where the fibers and the EAA became attached to each other due to the friction heat generated by the screw rotation. The EAA-cellulose material was squeezed through multiple die orifices at the perimeter of the die head and cut by rotating knives. Pellets/agglomerates are thus formed in the agglomerator machine. The moisture content of the agglomerates reported after the agglomeration process was less than 1%.

The EAA used was Primacor 3540 from Dow Chemical Company. The EAA had a 7% acrylic acid content, average molecular weight of 16100, melt flow rate (MFR) (190[degrees]02.16 kg, ISO 1133) of 8 g/10 min and a density of 0.932 g/[cm.sup.3].

The agglomerates contained 30 and 70 wt% cellulose and, in this work, are called reference A30 and masterbatch A70, respectively. The samples made from the reference A30 are called 30. Samples were also prepared by mixing the masterbatch A70 with EAA, aiming for 30 wt% cellulose content in the blend, in similarity with industrial masterbatch compounding processes. Here, A70 and EAA were hand-mixed in an open container before the processing, and the blend is denoted 30(70).

Melt Processing

The different processing techniques used were injection molding (IM), extrusion mixing (EM) and elongation dispersive mixing (EDM).

The extrusion mixing was performed with a Brabender compact extruder, Brabender OHG, Duisburg, Germany, with a screw diameter D = 19 mm and a screw length of 25D, three individually controlled temperatures zones and a temperature-controlled circular die with a diameter of 3 mm. The temperature profile along the barrel from hopper to die was 90, 130, 140, 140[degrees]C. The screw rotation speeds used were 50, 85, 100, and 150 rpm where 150 rpm was the maximum possible. The 30(70) blends consisting of masterbatch A70 and EAA was extruded at several screw rotation speeds whereas material reference A30 was extruded only at the maximum screw rotation speed of 150 rpm.

The elongation dispersion mixing technique used was intended for making use of both elongational flow and dispersion mixing, aiming for breaking up particle or fiber aggregates. In this study, the elongation dispersive mixing was performed with an injection molding machine Arburg Allrounder, 221M-250-55, Austria. One or two successive melt ejections of the agglomerates A30 and A70 were performed at the maximum pressure and flow rate in order to have very high velocity gradient but also high shear forces. The temperature profile from the hopper to the die was 90, 130, 140, 150, 150[degrees]C, the pressure was 125 MPa and the flow rate 5.6 X [10.sup.-5] [m.sup.3]/s. The screw diameter was 25 mm and the nozzle die diameter 2 mm. The elongational strain ([epsilon]) and the elongational rate ([epsilon]) applied to the material were calculated assuming constant volume flow from the equations:

[epsilon] = ln([D.sup.2]/[d.sup.2]) (1)

where [r.sub.0] is the radius of the cylinder and r the radius of the nozzle die.

[epsilon] = [v.sub.1] - [v.sub.0]/L (2)

where [v.sub.0] is the speed in the cylinder, [v.sub.1] is the speed at the nozzle and L is the total length of the nozzle and the nozzle tip.

The values were 5.05 for the elongational strain and 140 [s.sup.-1] for the elongational rate.

Injection molding of all materials was performed with the same injection-molding machine as that used for the elongation dispersive mixing. The temperature profile used was 90, 130, 140, 150, and 150[degrees]C, the injection pressure was 100 MPa, the cycle time 50 s and the mold was at room temperature. All blends were injection-molded in a final melt processing step.

The processing performed on the reference A30 and the masterbatch A70 are described in Figs. 1 and 2, respectively. In these figures, the processing techniques are indicated in a square box with dash lines while the final samples are indicated in ovals.

The three different processing series performed with the agglomerates reference A30 are shown in Fig. I. The upper line indicates that the reference material A30 was directly injection-molded obtaining the sample identified as 30 IM. Along the middle line in Fig. 1, the second processing series is described. The extrusion mixing was performed in a first step with the reference material A30 at the maximum screw speed (150 rpm) and then injection-molded in a second step to give the sample denoted 30; 150 rpm. The third and last melt processing series performed to the reference material A30, corresponding to the bottom processing line in Fig. 1, was the elongation dispersion mixing. As shown in Fig. 1, after one pass of EDM, part of the material was injection-molded to obtain the sample denoted 30; 1 pass, and the rest of the material was subjected to a second pass of EDM (indicated as EDM 2 passes). This material was, as with previous series, injection-molded to give the sample denoted 30; 2 passes.

The three different processing series performed with the agglomerates masterbatch A70 are shown in Fig. 2. The upper line in Fig. 2 indicates that first of all the masterbatch A70 was hand-mixed in solid state with EAA pellets (indicated with square box with dash line) before any further processing. In this case, the materials were hand--mixed with EAA pellets, both in solid state, to give a cellulose content of 30 wt%. From this solid blend, two different processing series were followed. First, part of this material was directly injection-molded obtaining the sample denoted 30(70) IM and the rest of the material was subjected to extrusion mixing at screw speeds of 50, 85, 100, and 150 rpm, and then injection-molded obtaining the samples denoted 30(70); 50-150 rpm, respectively. The second main processing series indicates that the material masterbatch A70 was subjected to elongation dispersive mixing without any prior blending. After one pass, part of the material was separated and subjected to a second pass. These materials, EDM 1 pass and EDM 2 passes, were separately mixed with EAA and injectionmolded to give the samples denoted 30(70); 1 pass and 30(70); 2 passes, respectively.

Measurements

Tensile Testing. The tensile mechanical properties, i.e., tensile modulus, tensile strength and strain at fracture were measured according to IS0527-1 with a Zwick 1455 tensile testing machine, Germany, equipped with a free-standing clip-on extensometer with adjustable gauge length. A cross-head speed of 6 mm/min corresponding to a strain rate of about 1.4 X [10.sup.-3] [s.sup.-1] was used and the gauge length of the extensometer was 60 mm. The measurements were performed at 23[degrees]C [+ or -] 0.5[degrees]C.

Fiber Length Measurements. The fiber length measurements were made with an optical analyzer Kajaani FS300, Metso Automation, Finland, based on light polarization measurements of fibers in water. The polymer was extracted directly from the composite with xylene and the cellulose fibers were then washed and kept in water before performing the measurements. With this method the fiber length distribution and the width distribution obtained are calculated according to the standardized methods TAPPI T271 0-7.60 mm and ISO 16065 0.20-7.00 mm. The values given are the result of five measurements for the A30 agglomerates and 16 for the A70.

Fiber Content. The actual fiber content was measured for some of the samples. A small piece of each sample was cut and weighed before extraction (Soxhlet) of the polymer with xylene during, Na72 h. After this time, the thimble containing the cellulose fibers was dried at room temperature, weighed and the fiber content was determined.

Microscopy Image Analysis. The middle region of each injection-molded sample was milled from 4 mm to a thickness of 1 mm in order to enable the size and number of cellulose aggregates of the middle layer to be determined by microscope analysis. The samples were viewed with a stereo-microscope type SteREO discovery.V20 from Carl Zeiss, Germany. The photomicrographs are shown in Fig. 3. The total characterized area was 10 x 20 [mm.sup.2]. In the images obtained, the aggregates were measured by hand and counted.

RESULTS AND DISCUSSION

Visual Characterization

Injection-molded specimens made from the blend of masterbatch A70 and EAA, denoted 30(70), are shown in Fig. 4. In the picture it can be observed that, as expected, the part with more fiber aggregation is the upper part which is furthest from the cavity gate (25-27). It was noted that the specimen directly injected molded (30(70) IM) exhibited more fiber agglomeration. It was further observed that the amount of fiber aggregates decreased as a result of extrusion mixing and decreased even more as a result of elongation dispersive mixing. After the ejection of the melt at high pressure, the fiber aggregates seemed to decrease in number and size and the dispersion of the cellulose fibers was thus improved, as can be seen in the two samples to the right in Fig. 4. The sample with the fewest aggregates was 30(70); 2 passes. A change in color was also observed. The elongation dispersive mixing samples were the darkest, presumably because the high pressure drop is associated with an excess temperature dissipation that can lead to thermal degradation of, in this case, the cellulose fibers (1), (28). It is known that thermal degradation of cellulose fibers takes place at a processing temperature of around 200[degrees]C (22). Apart of temperature, the degradation rate is significantly influenced by oxygen and water (29) but also degradation reaction products can accelerate the degradation process, so called autodegradation (30). Oxidation of EAA copolymer may occur in presence of oxygen but at higher temperatures than the one used in this work. Furthermore EAA copolymers are not sensitive to hydrolysis (31)

Microscopy Image Characterization

With microscopy image characterization it was possible to corroborate what was observed by simple visual inspection. The micrographs shown in Fig. 3 confirmed that elongation dispersive mixing was a successful method to break up the fiber aggregates and disperse the cellulose fibers. The number and size of the cellulose aggregates in sample 30(70); 2 passes was considerably lower than in the other samples. The shape of the aggregates was more elongated, indicating that some orientation was achieved. Extrusion mixing also affected the breakage of the cellulose aggregates since there were more small aggregates in the sample (30(70); 85 rpm) but the efficiency of the method was lower than that of the elongation dispersive mixing. It was also observed that the directly injection-molded sample contained the largest fiber aggregates.

Figure 5 shows the proportions of the fiber aggregates classified in size ranges. It can be observed that the maximum size of the aggregates in sample 30(70); 2 passes was in the range of 1.25-1.50 mm2 and also that the smallest size range of aggregates, concretely size range of 0-0.25 [mm.sup.2], were more frequent. The extrusion mixing sample [30(70); 85 rpm] contained the largest number of aggregates, mainly in the small size range (0-0.75 [mm.sup.2]), whereas the directly injected-molded samples [30 IM and 30(70) IMJ had more agglomerates in the larger size ranges (not shown in Fig. 5).

FIGURE 5 OMITTED

Fiber Content

The fiber contents of some of the specimens are shown in Table 1. The fiber content of the pellets of reference A30 was 34 wt% and the pellets of masterbatch A70 was 67 wt%. The cellulose content of the injection-molded compound masterbatch A70 with EAA, sample denoted 30 (70) IM, was slightly lower than for the compound of reference material A30 (sample 30 IM), concretely 28 and 30 wt%, respectively. After extrusion mixing, the cellulose content increased to 36 wt%, which was the highest value of all the samples. One pass of elongation dispersive mixing had no effect on the fiber content, but after two passes the content of cellulose in the final composite was considerably lower, only 25 wt%. In this case, the value is the result of four independent measurements with a standard deviation of 4.5. A possible reason for such differences in fiber content is the hand-mixing before the extrusion mixing but in this particular case the feeding of the materials was more difficult because it was impossible to pelletize the material obtained from the second pass of elongation dispersive mixing, possibly also causing sample-to-sample variations. This material broke very easily probably due to the degradation of the cellulose fibers. The nonuniformity in size of the pellets made it difficult to feed the solid blend of this material and EAA into the injection-molding machine. The fiber content of the samples in this case could not be expected to be homogeneous.

TABLE 1. Fiber content, tensile modulus (E), tensile strength
([delta])and elongation at break ([epsilon]) and fiber length of
all the specimens.

      Samples   Measured    E     (MPa)   [epsilon]   Fiber
               fiber      (MPa)          (%)         length
               content                               (Trim)
               (%)

IM        EAA          -    118    12.4      76 (2)       -
                           (18)   (0.1)
        30 IM         30    630    13.4      10 (1)    0.49
                           (37)   (0.3)               (0.00)
      30 (70)         28    652    16.3      10 (2)    0.45
     IM                    (64)   (0.47)              (0.01)

EM
          30;          -    588    14.1      10 (2)       -
     15Orpm                (45)   (1.0)
      30(70);          -    694    13.7      12 (2)    0.41
     50rpm                 (76)   (0.1)               (0.01)
      30(70);         36    780    15.2       9 (1)    0.44
     85rpm                 (64)   (0.5)               (0.00)
      30(70);         31    585    13.4      11 (1)    0.46
     100rpm                (44)   (0.2)               (0.00)
      30(70);          -    577    13.0      12 (1)       -
     150rpm                (21)   (0.2)

EDM
        30; 1          -    701      14      10 (1)       -
     pass                  (25)   (0.2)
        30: 2          -    750      15       8 (1)       -
     passes                (51)   (0.3)
      30(70);         31    639    15.6      13 (3)    0.43
     1 pass                (111)  (1.0)               (0.00)
      30(70);   25 (4.5)    785    18.2      12 (2)    0.38
     2 passes              (90)   (0.7)               (0.04)

The standard deviations are given in parentheses. In the sample name,
30 indicates the final cellulose content in the sample, and in case of
samples from the blend of masterbatch A70 and EAA is indicated with a
(70).


Fiber Length

The fiber length of the original cellulose fibers before the agglomeration process yielding the reference A30 and the masterbatch A70, was 2.45 mm with a standard deviation of 0.15 mm based on three measurements. The diameter of all the measured fibers was about 30 [micro]m. During the agglomeration process, the fiber length was reduced to 0.55 mm for the reference A30 and to 0.48 mm for the masterbatch A70. The standard deviations were 0.02 and 0.03 mm, respectively. This reduction in fiber length corresponded well to the grinding screen used during the agglomeration process. The fiber lengths shown in Table 1, in contrast to many other studies (21-23) indicate that the further melt processing had a small or insignificant effect on the fiber length, since all the measurements were in the same range after the first melt mixing, i.e., the agglomeration process. If the small differences in fiber length between the samples are taken into account, it is evident that the sample 30(70); 2 passes had the shortest fibers but the best mechanical properties of all the samples.

Tensile Properties

The measured mechanical properties are summarized in Table 1. All the values are averages and the standard deviation (in parentheses) of seven measurements is included. The properties of the EAA without any fibers are given as reference values. In general, when cellulose fibers were added to the matrix, the tensile modulus (E) and the tensile strength (a) increased but the elongation at break ([epsilon]) decreased considerably. The melt processing had a significant effect on the mechanical properties. Among the injection-molded samples, the blend of masterbatch A70 and EAA 30 (70) IM had a slightly higher E, a higher a and the same c as the 30 IM sample, but the standard deviations were also higher, due perhaps to the feeding of the hand-mixed materials into the injectionmolding machine.

In the case of the extrusion mixing specimens, it was noted that the screw rotation rate had an influence on the mechanical properties. When the screw rotation was increased from 50 to 85 rpm, E and a increased but when it was increased from 85 to 100 rpm both decreased. The elongation was, however, not significantly affected by the screw rotation rate. The highest screw speed resulted in the lowest E. The optimal screw rotation speed in this study was apparently 85 rpm.

The elongation dispersive mixing resulted in improved tensile properties for both materials. For the reference A30, E and a were improved by one free ejection compared with the injection-molded sample (30 1M). The same A30 material subjected to two free ejections (sample 30; 2 passes) had the highest E and a but the lowest [epsilon] of all the samples made with the reference material A30. When the elongation dispersive mixing was performed on the masterbatch A70, only r was improved by a single free ejection. With two passes, all the tensile properties were better than those of direct injection-molded sample (30 (70) 1M), E increasing from 652 MPa to 785 MPa, a from 16.3 MPa to 18.2 MPa, and e from 10 to 12%. Furthermore, the tensile properties of sample 30(70); 2 passes were the best of all the specimens, even though the content and the length of the cellulose fibers were the lowest of all the samples. These results confirmed that elongational dispersive mixing is a very effective method to break up the cellulose aggregates but also to disperse the cellulose fibers in the matrix. Hence, the smaller amount of cellulose aggregates but also smaller size indicates that more cellulose fibers are used as reinforcement of the thermoplastic matrix. Figure 6 shows the effect of the processing on the mechanical properties of the samples resulting from the blend of material masterbatch A70 and EAA which are denoted 30(70). It can be observed the ductile behavior and the high strain at break of EAA and also, as expected, that the addition of cellulose fibers to the matrix improves considerably the stiffness and increases the brittleness of the final composites independently of the compounding conditions. It can also be observed that strength can be improved by the processing since the two passes of elongational flow shows the highest strength value.

CONCLUSIONS

The processing had a great influence on the number and size of the aggregates and it had a corresponding effect on the tensile mechanical properties. The presence of fiber aggregates decreased during extrusion mixing and even more during elongation dispersive mixing. The sample with the fewest amount of cellulose aggregates was the sample 30(70); 2 passes. The elongation dispersive mixing led to a darker color, probably due to the high pressure drop with the corresponding excessive temperature dissipation that can lead to thermal degradation of the cellulose. The measured fiber content was in general about 30 wt% except for the sample denoted 30(70); 2 passes for which it was considerably lower. The manual solid mixing of cellulose masterbatch A70 and EAA probably caused some nonuniformity in fiber content and in the mechanical properties. In extrusion mixing, the screw rotation speed affected the subsequent mechanical properties; the highest screw rotation speed gave the lowest tensile modulus and tensile strength for the blend EAA and masterbatch A70. In the extrusion mixed materials, the best mechanical properties were obtained with a screw rotation speed of 85 rpm.

Elongation dispersion mixing had a very good effect on the mechanical properties. The highest tensile modulus, tensile strength were obtained with sample 30(70); 2 passes, even though its fiber content was the lowest. The processing techniques performed had a small or insignificant effect on the fiber length, and it was concluded that the most significant shortening of the fibers from 2.5 mm to about 0.5 mm was due to the grinding during the agglomeration process performed prior to our study. Photomicrographs studies confirmed that the samples subjected to elongation dispersion mixing contained the fewest fiber aggregates, also that the aggregates were smaller and more elongated. The maximum size of the fiber aggregates found in sample denoted 30(70); 2 passes was 1.5 [mm.sup.2], which was the lowest of all the studied samples. This could contribute to the mechanical performance since fewer fiber aggregates mean better fiber dispersion and consequently a more effective fiber--matrix interaction.

ACKNOWLEDGMENTS

The authors wish to acknowledge everyone involved in Formulosa for good cooperation. Furthermore, the Forest products and chemical engineering group at Chalmers University of Technology, and especially Fredrik Wernersson, are acknowledged for the use of the fiber length analyzer and for their support. Dr J.A. Bristow is thanked for the linguistic revision of the manuscript.

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This article was published online on 27 March 2012. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected on 23 April 2012. Correspondence to: Ruth Arino; e-mail: arino@chalmers.se Contract grant sponsor: VINNOVA, Sodra, Tetra Pak, Korsniis.

Ruth Arino, Antal Boldizar

Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE-41296 Goteborg, Sweden

DOI 10.1002/pen.23134
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