Microcellular of Glass Fibre Reinforced PC/ABS: Effect of ... · of the Processing Condition on the...

27Cellular Polymers, Vol. 29, No. 1, 2010

Microcellular of Glass Fibre Reinforced PC/ABS: Effect of the Processing Condition on the Morphology and Mechanical Properties

©Smithers Rapra Technology, 2010


Andrzej K. Bledzki and Joanna Kühn-Gajdzik

Institut für Werkstofftechnik, University of Kassel, Mönchebergstraße 3, 34109 Kassel, Germany

Received: 15 October 2009, Accepted: 17 December 2009

INTRODUCTION

Engineering polymers such as fi bre-reinforced thermoplastics play a major role in the automobile industry because of the very good strength and stiffness of the fi nal product. The other challenge is to create new lightweight parts with properties comparable to solid materials. That is why the injection moulding of microcellular polymer is expected to be an increasable promise for engineering applications.

Microcellular foam of thermoplastics provides many benefi ts compared to other conventional materials. One of the most important advantages of the foam is its higher specifi c fl exural modulus(1). The microscopic cell size and a large number of cells in the microcellular material can reduce material consumption as well as improve thermodynamics(2). The experiments with 45 wt.% glass fi lled PP produced with MuCell technology showed that with increased injection speed the cell nucleation density increases and with increased injection time, the fl exural and tensile properties could be improved(3).

It is possible to achieve the desired cell size and weight reduction of the foamed part by combining the classical injection moulding process with the so called precision mould opening. During this process, the cavity of the mould is volumetrically fi lled and, directly after injection, enlarged to the desired part thickness by reducing or even removing the clamping pressure. This process can also be performed with chemical blowing agents. Wall thicknesses achieved with this process can be three or four times the thickness of the initial

28 Cellular Polymers, Vol. 29, No. 1, 2010


wall. Polypropylene foam parts with a wall thickness from 10 to 12 mm have already been produced(4,5).

It was demonstrated that the uniform microcellular structures with a cell diameter less then 10 μm could be reached using a combination of precision mould opening and the gas counter pressure process(4,6).

PC/ABS blends are well-known commercial products. Their success on the market is due to the complementary properties of the components. Polycarbonate (PC) contributes to good mechanical and thermal properties whereas acrylonitrile-butadiene-styrene (ABS) provides ease of processability and a reliable notched impact resistance(7).

The original purpose of adding glass fi bres to polymers was an improvement in stiffness, strength and heat distortion temperature compared to unfi lled thermoplastics. In particular, the reinforcement of thermoplastic compounds by short fi bres has received special attention because of their use in a variety of engineering applications in both the chemical and automotive industries. The addition of glass fi bres enhanced the ultimate tensile strength and modulus and reduced elongation (both to yield and to break)(8).

Cell size distribution and cell density of foamed unfi lled and glass fi ber reinforced ABS samples compared at various pressure drops and rates in the batch process were presented by Mahmoodi and Behravesh in his studies. It could be proved that the cell size of GF-ABS samples is much smaller than that of the foamed unfi lled-ABS ones. The results indicated that the pressure drop rate does not have any noticeable effect on the cell structure of the microcellular foams in the batch process (produced at room temperature)(9).

Many research works have been conducted to study the effect of processing parameters on the microstructure of the foamed materials in various processes(9). However, very few investigations have been performed into the injection moulding process to study the infl uence of glass fi bres on the microstructure of microcellular foams in the solid state of PC/ABS.

All mechanisms of cell formation take place mostly in the amorphous regions of the polymer which is an after-effect of diffusion and absorption processes(10). Microcellular foam bubbles may be nucleated homogeneously or heterogeneously(11). It is well known that the cell density is strongly affected by using fi llers during the foaming process. The fi ller size affects nucleation during the foaming process, largely due to the fact that the heterogeneous nucleation itself has not been well studied(12).



The number of density and cell size distribution in foam is the result of coupled nucleation and growth, and possibly of coalescence. Ideal nucleants have a uniform size, shape, and surface properties(13).

Nucleation in heterogeneous polymers may occur both heterogeneously at the interface between the polymer and the second phase and homogeneously in the free volume of the single matrix phase. Heterogeneous nucleation occurs when a bubble forms at an interface between two phases such as a polymer and an additive(11). The gas absorption behaviour of fi lled and unfi lled polymers was investigated to explain heterogeneous nucleation in fi lled polymers. It was found that fi lled polymers absorb more gas compared to unfi lled ones. The gas accumulates between polymer and fi ller and helps to create nucleation during the foaming process(14).

Lin has found an important relationship between cell distribution and mechanical properties of the high impact polystyrene. Smaller cells are able to distribute the tensile force better than larger ones because smaller cells are usually associated with lower gas volume fractions(15).

Impact strength is closely related to cell morphology, in addition to weight reduction percentage and skin thickness. Fine cells and uniform distribution are critical factors for a high impact strength, and very helpful for increasing the tensile and fl exural strength as well(16).

The investigation with natural fi bre reinforced polyurethane foam indicated that the specifi c data was only slightly dependent on the cell (microvoid) content(17).

The mechanical properties of rigid high density foams can be signifi cantly modifi ed by fi bre reinforcement. Foam reinforcement will change properties such as stiffness, strength, impact, coeffi cient of thermal expansion, and property-temperature sensitivity(18).

Desai et al. concluded that the compressive properties of the composite foams do not depend on the fi bre length but instead on other parameters, such as the amount of the blowing agent and the fi bre weight fraction(19,20).

Bledzki et al. found out that the dynamic mechanical properties of natural fi bre reinforced epoxy foams were also affected by the fi bre type and orientation, as well as the fi bre and cell content of the laminates tested(21). It was also observed that the loss energy increased with increasing cell content in impact tests of the reinforced epoxy foams(22).



In this paper, the infl uence of microcellular foaming with chemical blowing agents (CBA) on the structure and mechanical properties of compact and glass fi bre reinforced polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) is investigated. The main purpose of this study was to defi ne the infl uence of processing parameters on microcellular foam morphology and also the relationship between cell structures, parameter and mechanical behaviour of a given PC/ABS microcellular foam combination produced in injection moulding process using precision mould opening. The other major objective of this research work was to study the effect of glass fi bres on the structure of compact and glass-fi lled PC/ABS microcellular foams with a desired density reduction and homogeneity distribution of cells.

MATERIALS AND PROCESSING

All processing conditions were kept constant except the enlargement of the mould cavity (defi ned density).

The analyzed material was supplied by Bayer’s Bayblend PC/ABS: one unfi lled, T85 (density 1.15 g/cm³), and T884N with 20 wt.% of glass fi bres (density 1.25 g/cm³). The material was preheated at 110 °C for 4-6 h to remove any moisture prior to injection moulding. To get the microcellular foam, Hydrocerol HP40P was used as chemical blowing agent in this study. It was obtained from Clariant Masterbacht and used as a weight percentage of the polymer.

The test bars (185×20×4 mm³) were prepared from dried granulates (at 80 °C for 24 h before processing) with an injection moulding machine (Arburg) equipped with precision mould opening (PMO) according to DIN EN ISO 294 under the following process condition (Table 1):

Table 1. Parameters of the microcellular foam injection moulding

Melt temperature TME

260 [°C]

Mould temperature TMO

80 [°C]

Injection velocity VI

40 [cm³/s]

Opening delay time TO

2 [s]

Amount of CBA 3 wt. [%]

In this study, the standard injection moulding process was combined with precision mould opening with the following enlargement of the mould cavity:



5% PMO (initial thickness of the part 3.8 mm fi nal thickness of the part 4 mm),

10% PMO (3.6 mm 4 mm),

20% PMO (3.2 mm 4 mm).

The fracture mechanism and morphology of PC/ABS microcellular foam was determined by using a scanning electron microscope type Cam Scan MV 2300 LV (SEM). The surface of the specimens was fractured in liquid nitrogen and sputter coated with a thin layer of gold to avoid electrostatic charging during examination. The structure performance was accomplished with the optical microscope at the cross section of the sanded surfaces. This investigation determines the average cell diameter, the thickness of the compact skin layer cell distribution and the cell anisotropy factor. The anisotropy factor describes the uniformity and roundness of the cell size and is defi ned as the ratio of the relationship between maximal and minimal cell diameter. If the anisotropy factor is determined with 1, this means that all cells possess geometries of an ideal circle.

The tensile and fl exural tests were realized according to EN ISO 527 and EN ISO 178 on the Zwick testing machine, UPM 1446. The Charpy notched impact test was performed with an instrumented impact hammer according to DIN EN ISO 179. The notch angle was always 45° ± 1° and the radius of notch was varied: type A: 0.25 mm ± 0.05 mm. All test specimens were prepared from the injection moulding test bars. A sketch of the bars is shown in Figure 1.

Figure 1. Sketch of the sample and the bars for tensile, fl exural and Charpy measurement



RESULTS AND DISCUSSION

Morphology

The morphology of microcellular foam can be reasonably characterized as a three-layer sandwich structure: compact skin layer – foam core centre – compact skin layer. The enlargement of the cavity leads to higher nucleation and signifi cantly increases the cell size and the thickness of the skin region (Figure 2). The reason for the uniform cell distribution of PC/ABS microcellular foam is the pressure relief of the melt during mould opening.

Figure 3 represents the progress of the cell structure in the center area of PC/ABS microcellular foam produced with 3 wt.% of blowing agents with various enlargements of the mould cavity (PMO). In all cases, a microcellular foam structure with a uniform distribution of cells in the center area could be created. At fi rst glance, the cell sizes of unfi lled PC/ABS microcellular foam as well as glass fi ber reinforced PC/ABS microcellular foam produced with higher density reduction ( ) with 20% PMO are evidently greater than those of other samples.

Comparing the microcellular process conditions of both materials produced with 5% PMO, there are no signifi cant differences between the cell sizes; and the trend is similar also with regard to other enlargements of the mould cavity, irrespective of the material. However, it can be noted that with the reinforced PC/ABS microcellular foam, a higher density reduction could be reached. This phenomenon could be explained in the way that the fi bre might act as nucleation centres in the matrix promoting a higher cell population during injection moulding.

Figure 2. Light micrographs of T85 microcellular foam a) 5% PMO, b) 20% PMO



One of the disadvantages of glass fi bres is their insuffi cient adhesion to the polymer matrix. This phenomenon can be seen in Figures 3d-e where the glass fi bre appear to be intercellular (between cells) and demonstrate a weakened interfacial character indicating a failure at the interface.

Figure 4 indicates the effect of density reduction on the thickness of the compact skin layer, the average cell size in the center area and the anisotropy factor with the standard deviation of the microcellular foam samples.

The results of the morphology analysis of the microcellular foams are shown in Table 2. The cells of the sample produced with higher enlargement of the cavity (18% ), independent of the polymer matrix, tend toward larger microcells with an average size around 85 μm. By smaller enlargement of the cavity (7% ), the cell size can be reduced up to 50%. At the same time, the thickness of the skin layer decreased from 1023 μm to 785 μm in the PC/ABS microcellular foam and from 863 μm to 763 μm in the reinforced PC/ABS microcellular foam with increasing density reduction.

It is also important to note that the higher cell anisotropy factor occurred in the microcellular foams with glass fi bres (Figure 4). These microcellular foams

Figure 3. SEM micrograph of the centre area of, T85 microcellular foam a) 5% PMO, b) 10% PMO, c) 20% PMO; microcellular foam glass fi bre reinforced T884N d) 5% PMO, e) 10% PMO, f) 20% PMO; a-c) without glass fi bre, d-e) with glass fi bre; (density reduction)



Figure 4. Effect of the density reduction of the compact skin layer and cell diameter – left PC/ABS microcellular foam; right reinforced PC/ABS microcellular foam

Table 2. Results of the morphology analysis of all microcellular foams

Materials Average cell diameter

centre area [μm]

SD Compact skin region

[μm]

SD Anisotropy factor

Density[g/cm³]

T85+5%PMO 37 11 1023 83 1,07 1,05

T85+10%PMO 55 25 903 38 1,24 1,03

T85+20%PMO 84 35 785 6 1,38 0,92

T884N+5%PMO 46 10 863 39 1,23 1,11

T884N+10%PMO 66 12 845 13 1,25 1,07

T884N+20%PMO 88 20 763 44 1,50 0,94

SD- Standard deviation

Com

pact

ski

n la

yer

(μm

)A

verage cell diameter (μ

m)

Density reduction (%)

7% 8% 18%

1200

1000

800

600

400

200

0

120

100

80

60

40

20

0

Anisotropyfactor 1.07 1.24 1.38

T85

Com

pact

ski

n la

yer

(μm

)A

verage cell diameter (μ

m)


11% 14% 25%

1200

1000

800

600

400

200

0

120

100

80

60

40

20

0

Anisotropyfactor 1.23 1.25 1.50

T884N with 20 wt.% GF



exhibit an irregular cell structure. The cell shape seems to digress from the unfi lled microcellular foams of PC/ABS which show an accurate spherical cell geometry.

Due to the foaming of unfi lled PC/ABS and glass fi bre reinforced PC/ABS, a density level in the range of PP could be reached (Table 3). Owing to the increased mechanical characteristics and signifi cantly reduced weight, it offers the possibility to replace some materials for special application.

Table 3. Results of the density of the microcellular foams

Materials Density [g/cm³]

T85+5%PMO 1,05

T85+10%PMO 1,03

T85+20%PMO 0,92

T884N+5%PMO 1,11

T884N+10%PMO 1,07

T884N+20%PMO 0,94

The volume fraction and fi bre length of glass fi bres in the moulded specimens was also determined using ahs test. The length of the glass fi bres was not signifi cantly changed during the injection moulding and foaming process and varied between 400-500 μm.

Mechanical Properties

The mechanical properties of microcellular foams are always related to their morphology, at all levels of detail, and to the properties of the polymer. Figure 5 presents the effect of the compact skin layer on the specifi c tensile modulus and specifi c tensile strength of PC/ABS microcellular foam and glass fi bre reinforced PC/ABS microcellular foam. Specifi c properties of foams are the properties divided by density of foams. Due to the lower tensile properties of compact PC/ABS in comparison to reinforced PC/ABS, the tensile modulus of the microcellular foam PC/ABS was observed to be signifi cantly (up to 50%) lower than that of the reinforced PC/ABS microcellular foam (Figure 5). However, the density reduction of both PC/ABS microcellular foams had a considerable effect on the tensile modulus. The specifi c tensile modulus as well as the specifi c tensile strength of the PC/ABS (T85) microcellular foam produced with 7% and 8% density reduction was observed to indicate the same value of the tensile modulus irrespective of the different average cell diameter



and thickness of the compact skin region (Figure 4). The tensile properties of PC/ABS microcellular foam with a thinner compact skin layer tend to drop more rapidly. In case of reinforced PC/ABS (T884N) microcellular foam, a signifi cant linear decrease of the tensile properties with decreasing thickness of the compact skin layer was observed. The level of decrease of the properties of microcellular foam is higher in samples of higher apparent density.

The values of the deformation behaviour of microcellular foam of unfi lled und reinforced PC/ABS are plotted in Figure 6 as a function of density reduction. The tensile stress at break of both materials had the same decreasing tendency with increasing density reduction. All compact T85 samples broke at an elongation

Figure 5. Effect of the compact skin layer on the specifi c tensile modulus and specifi c tensile strength of (left) PC/ABS microcellular foam and (right) glass fi bre reinforced PC/ABS microcellular foam

T85Sp

ecifi

c te

nsile

mod

ulus

(M

Pa/(

g/cm

3 ))

Specifi c tensile strength (xxx)

Thickness of compact skin layer (μm)T884N 860 840 760

4000

3500

3000

2500

2000

1500

1000

500

0

60

50

40

30

20

10

0Specifi c tensile modulusSpecifi c tensile strength


Spec

ifi c

tens

ile m

odul

us (

MPa

/(g/

cm3 )

)Specifi c tensile strength (xxx)

Thickness of compact skin layer (μm)

T85 1050 900 780

4000

3500

3000

2500

2000

1500

1000

500

0

60

50

40

30

20

10

0Specifi c tensile modulusSpecifi c tensile strength



higher than 50% under testing conditions (Figure 6 top). In the microcellular T85 foam, a small increase the percentage of elongation at break was observed with increasing density reduction as well as with decreasing skin thickness. The major role for this behaviour plays the thickness of the compact skin region by defi ned cell diameter. The improvement in tensile strain at break of T85 microcellular foam from 11% to 18% was associated with a decrease in thickness of the compact skin layer from 1023 μm to 785 μm respectively as well as with increase in average cell diameter from 37 μm to 85 μm respectively. The results of this investigation showed that there is a special morphology

Figure 6. Tensile stress and tensile strain of microcellular foam PC/ABS and glass fi bre reinforced PC/ABS at break vs. density reduction

T85

T884N 11% 14% 25%


Tens

ile s

tres

s at

bre

ak (

MPa

) Tensile strain at break (%)

Density reduction (%)T85 7% 8% 18%

70

60

50

40

30

20

10

0

25

20

15

10

5

0

Tensile stress at break (MPa)Tensile strain at break (%)50%

Tensile stress at break (MPa)Tensile strain at break (%)


Tens

ile s

tres

s at

bre

ak (

MPa

) Tensile strain at break (%)

70

60

50

40

30

20

10

0

25

20

15

10

5

0



structure with defi ned cell sizes and a thin compact surface layer to increase the tensile strain at break of microcellular T85 foam. As expected, the introduction of glass fi bres into PC/ABS lowers elongation at yielding and at breakage. Evidently, reinforced T884N microcellular foam broke at a strain value which is lower than the strain of the compact polymer (2.06%) and remained quite affected with increasing density reduction (Figure 6 bottom). All of those microcellular foam broke in the rage of 1,5% elongation. Big changes in the density of the samples did not alter the main breaking behaviour.

Figure 7 shows the specifi c fl exural modulus of the compact and microcellular foam in bending modes. The specifi c fl exural modulus of compact PC/ABS and PC/ABS with glass fi bres is 1.84 [GPa/(g/cm³)] and 3.84 [GPa/(g/cm³)] respectively. As the density reduction of the microcellular foam glass fi bre reinforced PC/ABS increased, the specifi c fl exural modulus increased linearly. There is an improvement of up to 10% in the case of higher density reduction combined with greater cell size. Result shows that sandwich structure reinforced with glass fi ber have a higher specifi c fl exural stiffness comparing with compact material as thickness of the microcellular foam increase.

Due to the increase of compact skin layer thickness in the sandwich structure the higher stress can be concentrated. In the core of the microcellular foam, the stress at the horizontal plane of the neutral is zero. The specifi c fl exural

Figure 7. Specifi c fl exural modulus of microcellular foam PC/ABS and glass fi bre reinforced PC/ABS vs. various rate of enlargement of mould cavity

Compact 5% PMO 10% PMO 20% PMO

T88N with 20wt.% GFT85

Spec

ifi c

Flex

ural

E

Mod

ul (

MPa

/(g/

cm3 ) 4500

4000

3500

3000

2500

2000

1500

1000

500

0



modulus of microcellular foam PC/ABS has been noted as not being dependent on the changed compact skin layer, the average cell diameter as well as the processing conditions and more closely resembling compact material.

The PC/ABS compact and the microcellular foam without glass fi bres did not break under the given fl exural testing conditions. In contrast, all of the compact and microcellular foam reinforced PC/ABS samples appeared to break at the same elongation between 2.5% - 2.9%.

Figure 8 shows the effect of the compact skin layer on the Charpy impact strength of PC/ABS microcellular foam and reinforced PC/ABS microcellular foam.

Figure 8. Effect of the compact skin layer on Charpy impact strength vs. (top) PC/ABS microcellular foam; (bottom) reinforced PC/ABS microcellular foam

T85


40

35

30

25

20

15

10

5

0

RT-30°C

18%8%7%Density reduction

Compact T85 1023 μm 900 μm 780 μm

T884N with 20wt.% GF

Cha

rpy

notc

hed

impa

ct s

tren

gth

(kJ/

m2 )


40

35

30

25

20

15

10

5

0

RT-30°C

25%14%11%Density reduction

Compact T88N 860 μm 840 μm 760 μm

Cha

rpy

notc

hed

impa

ct s

tren

gth

(kJ/

m2 )



When the reduced skin thickness of glass fi bre reinforced PC/ABS microcellular foam remains nearly constant (Figure 4), the values of Charpy notched impact strength observed in Figure 8 also remain constant at room temperature (RT) and also at -30 °C irrespective of density reduction.

Compared to compact PC/ABS samples, PC/ABS microcellular foam as well as reinforced PC/ABS microcellular foam samples showed lower Charpy impact testing values. A higher density reduction of reinforced PC/ABS did not cause any property changes at both testing conditions with respect to the samples at higher density which is similar to what was observed for the PC/ABS microcellular foam samples. Despite the uniform distribution of cells and the thicker compact skin region of all these systems, the reduction of the notched impact strength did not signifi cantly improve with higher density.

Figure 9 shows the fracture surface of compact and microcellular foam T85 and T884N at room temperature. Compact T85 (Figure 9a) exhibits a ductile behaviour with a clearly deformed zone and distinct fl ow lines on the fracture surface at room temperature as well as at lower temperature. After foaming, the PC/ABS appeared to be brittle which the fracture surface of the microcellular foam PC/ABS indicates (Figure 9b). The present of cell induced low stress concentration in the surrounding PC/ABS matrix, giving signifi cantly drop to a local shear yielding mechanism with a decrease of up to 20% in Charpy notched impact strength. The brittle behaviour, including the fracture surface, of reinforced PC/ABS can be observed in Figures 9c and d.

Figure 9. Effect of the cell structure on the fracture behaviour of compact and microcellular foam T85 and T884N



CONCLUSION

Using several process techniques of foaming as well as different rates of enlargement of the mould cavity, a suitable uniform morphology with desired cell diameter and compact skin region can be reached. The cell diameters of PC/ABS microcellular foam as well as reinforced PC/ABS microcellular foam were signifi cantly reduced from 85 μm to 50 μm by using a small rate of enlargement of the mould cavity (small density reduction). This caused additional pressure relief and improved the uniform cell nucleation over the complete cross section of the sample. The addition of glass fi bres to PC/ABS microcellular foam promoted a higher number of cell nucleation and produced a uniform microcellular structure. The higher density reduction of the reinforced PC/ABS microcellular foam could be reached due to the heterogeneous nucleation of the glass fi bres with a large number of cells. The glass fi bres served as an effective nucleating agent, which tended to activate the instantaneous nucleation and uniform growth and cell distribution.

The relationship of the processing parameters with the mechanical properties of PC/ABS microcellular as well as reinforced PC/ABS microcellular foam depends on the cell diameter, the cell distribution and the thickness of the compact skin region. It should be considered that the microcellular foam density and skin thickness presented a major factor in determining the mechanical properties. It has been proved that the most important advantage of reinforced PC/ABS microcellular foam is its higher specifi c fl exural modulus. The breaking mechanism in the Charpy test of PC/ABS microcellular foam and reinforced PC/ABS microcellular foam was not affected by the density reduction and thickness of the compact skin region.

One of the main aims of foaming is reducing the weight of the product. Due to a continuous requirement of improvement of the specifi c mechanical behaviour of foamed materials, it is possible to replace compact materials with foamed ones which at the same time possess better mechanical properties. By foaming unfi lled PC/ABS and glass fi bre reinforced PC/ABS, a density level in the range of PP could be reached (0.92 g/cm³). It is also necessary to take into account the cost factor and the material application aimed at.

It has been proved that the most important advantages of glass fi bre reinforced PC/ABS microcellular foam are its constant properties like Charpy notched impact strength at RT and -30 °C, tensile strain at breakage, fl exural strain at breakage during higher density reduction with a larger cell diameter and thin compact skin layer. In case of PC/ABS microcellular foam without glass fi bres, the optimum for tensile strain at breakage appears to be larger microcells and a thinner compact skin layer as well.



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