Cellulose ester-polyolefine binary blend: Morphological, rheological and mechanical properties

European Polymer Journal 48 (2012) 981–989

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Cellulose ester-polyolefine binary blend: Morphological, rheological and mechanical properties François Besson, Tatiana Budtova ⇑ MINES ParisTech, Centre de Mise en Forme des Matériaux – CEMEF, UMR CNRS 7635, BP 207, 06904 Sophia Antipolis, France

a r t i c l e

i n f o

Article history: Received 11 October 2011 Received in revised form 10 February 2012 Accepted 22 February 2012 Available online 3 March 2012 Keywords: Cellulose acetate butyrate Polyethylene Blends Rheology Morphology

a b s t r a c t Binary blends of thermoplastic polymers, one being a polyolefin (high density polyethylene) and the other a bio-based polymer (cellulose acetate butyrate) were prepared with various components proportions. No compatibiliser was used. Depending on blend composition, different morphologies were obtained, from fine nodular to co-continuous. Blends viscoelastic and mechanical properties were studied in details in all range of compositions. The results obtained were interpreted using a careful analysis of the viscoelastic properties of the initial components and classical approaches developed for immiscible blends. Except the blends containing low amount of cellulose acetate butyrate finely dispersed in polyethylene, all other blends viscoelastic and mechanical properties follow the additive mixing rule. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to depletion of fossil resources and global environmental respect awareness, interest in bio-based plastic materials is tremendously growing. Several ways are investigated to produce biopolymers [1]. Direct extraction of vegetal polymers like cellulose or starch followed by their chemical modification to bring new properties is one of these ways. Cellulose esters make a large class of thermoplastics with various properties [2]. Contrary to cellulose, they can melt and thus formed using classical polymer processing methods. The number of esterified hydroxyl function per anhydroglucose units and length of ester moieties determine their thermal and mechanical properties. A general trend is that an increase of number and length of ester moieties acts as an internal plasticization and so decreases melting temperature and tensile strength of the cellulose ester [3]. Progressively replaced by oil-based polymers in the sixties [4], cellulose esters are now reconsidered as bulk materials due to their bio-based origin.

⇑ Corresponding author. Tel.: +33 4 93 95 74 70; fax: +33 4 92 38 97 52. E-mail address: [email protected] (T. Budtova). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2012.02.010

Blending polymers is an easy way to obtain new tailored materials compared to the expensive synthesis of specific polymers. Depending on the thermodynamics of the mixed compounds and their composition, miscible and immiscible blends can be prepared. In the case of partial miscibility or immiscibility, properties are highly influenced by blend morphology [5]. Literature on miscible blends of cellulose esters with biodegradable and/or biomass-based polyesters is abundant [6–9]. In most of the cases blends are prepared in a common solvent and films are obtained by solventcasting; the expected property of theses materials is their biodegradability. Immiscible blends of cellulose esters with commodity polymers have been, however, rarely reported. Kosaka et al. [10,11] studied the morphology and mechanical properties of cellulose acetate butyrate (CAB) mixed with low density polyethylene (LDPE) blends. Maleic-anhydride grafted polyethylene was used as a compatibiliser and polyethylene was always considered as the matrix. It was found that low amount of CAB is partly miscible with LDPE. In this particular case, blends showed superior tensile properties than neat polyethylene. Wang et al. [12] investigated the formation of polyolefin or polyester microfibrils in blends with cellulose acetate butyrate. In these systems, cellulose ester was the matrix, no compatibilizer was used

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and 20 wt% of polypropylene, high density polyethylene or PET was dispersed in the molten CAB. Immiscible blends with the largest difference between components viscosity had the biggest particle size evidencing the influence of the viscosity ratio on blends morphology. In this work we perform a comprehensive study of morphological, viscoelastic and mechanical properties of binary blends based on cellulose acetate butyrate and high density polyethylene (PE), prepared in the molten state with a batch mixer. Our goal is to understand how final properties of these blends are controlled by mixture morphology. First, the rheological properties of the neat components are carefully investigated in order to predict blend morphology. Then blends properties in the whole composition range are studied. No compatibilizer is used. Classical approaches developed for immiscible blends are used to understand CAB/PE morphology, rheology in the molten state and mechanical properties in the solid state.

Torque (Nm)

Tmelt , °C

200

30

CAB added

25 1

PE added

20

180

15 2

160

10 5 0

140 0

2

4 6 time, min

8

10

Fig. 1. Evolution of melt temperature (1) and torque (2) as a function of time for the blend CAB:PE = 1:1. Arrows are showing the moments when PE and CAB are introduced into the mixing chamber.

2. Materials and methods 2.1. Materials High density polyethylene (Dow™ HDPE 53050E), ‘‘PE’’ in the following, was in pellets with melt-flow index of 8.2 g 10 min1 (190 °C, 21.6 kg). Cellulose Acetate Butyrate (Eastman CAB 531-1Ó), ‘‘CAB’’ in the following, was in powder. CAB has the degrees of substitution for Acetate, Butyrate and Hydroxyl of 0.2, 2.4 and 0.4, respectively, as given by the manufacturer. Other useful properties are given in Table 1.

10 min and CAB for 5 min. All samples were then cooled to ambient temperature and cut into pellets. Because CAB moisture absorption can be more than 2 wt%, each blend was vacuum-dried at 90 °C and kept in a desiccator prior to measurements. Samples for dynamic mechanical analysis (DMA) and dynamic oscillatory rheometry were then compression-moulded using a Daragon press at 180 °C and 200 bars for 10 min.

2.2. Blends preparation 2.3. Dynamic oscillatory rheometry Blends were prepared in the internal mixer Haake Rheomix 600 with counter-rotating roller rotors at 180 °C (set temperature) and various rotor speeds, from 25 to 150 rpm. In all preparations PE was first melted and mixed for 5 min. Then, without stopping mixing, CAB was added and blended with PE for five additional minutes. The evolution of the temperature of blends as well as torque were measured in time (see example for a CAB:PE = 1:1 in Fig. 1). Due to polymers high viscosity, viscous dissipation occurs during mixing and blend temperature may exceed the set value by 10–30°, depending on rotor speed and viscosity of the major phase. It was not possible to fix final blend temperature. As far as polymer viscosity is temperature sensitive, the real temperature will be specified for each preparation. The single components were also processed in the same way as in blends in order to have them with the same thermo-mechanical history for the further studies: PE for

Dynamic oscillatory rheometry measurements were performed on a strain controlled ARES rheometer (Advanced Rheometric Expansion System, TA Instruments). For each blend, the parallel plate geometry was used with disks of 25 mm diameter and a gap of 1.5 mm. The linear viscoelastic domain was determined at 100 rad s1 and 180 °C for blends and both initial polymers. For example, the strain limit of the linear behaviour was found to be 22% and 14% for PE and CAB, respectively. All measurements were performed in dynamic mode in the linear viscoelastic regime at strains below 5%. Samples thermal stability was evaluated at 180 °C by measuring the evolution of the elastic modulus G0 , the viscous modulus G00 and the complex viscosity g⁄ at 1 rad s1 and 5% strain for 9 h: both polymers were stable within the experimental error deviations. Frequency sweeps from 10+2 to 102 rad s1 at temperatures from 160 to 190 °C

Table 1 Properties of PE and CAB as given by the providers.

PE CAB *

Glass transition temperature (°C)

Melting temperature (°C)

Density

Molecular weight (g mol1)

110 115

120–135 135–150

0.952 1.17

241,881* 40,000

This value for HDPE 53050E was reported in Ref.[13].

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(with a 10° step) were performed for initial components and at 180 °C for blends.

1000000

G', G'' , Pa η*, Pa.s

100000

2.4. Dynamic mechanical analysis The evolution of the storage modulus E0 , loss modulus E00 and the tangent of the loss angle tan (d) with temperature was measured with the Triton Technology Tritec 2000 DMA (Malvern Instruments, UK). Compression moulded samples were 5 mm long, 4 mm wide and 1 mm thick. Strain amplitude (5 lm) was such that the material response was in the linear viscoelastic domain at the chosen frequency (1 Hz). The heating rate was 2°min1.

10000

1000

G'' η*

G'

100

ω, rad/s

10 0.01

2.5. Scanning electron microscopy (SEM) SEM images were obtained with Philips ESEM XL30. Samples were cooled in liquid nitrogen and fractured; cryofracture was used to avoid the artefacts due to plastic deformation during the breaking at room temperature. The microscope was set in environmental mode which allows observations in gaseous environment. Surfaces are not coated. Back-scattered electrons are detected and the contrast is obtained because of differences in atomic number in each phase.

1 0

100

10000

1000000

⁄

00

Fig. 2. Master plot of G , G and g for CAB at a reference temperature of 180 °C. Solid line is complex viscosity calculated according to Carreau– Yasuda model (Eq. (1)). Error bars are smaller than or equal to the point size.

1000000

G', G'' , Pa η*, Pa.s

100000

3. Results and discussion The results obtained are presented as follows. First, the rheological properties of the initial components are given making a background for predicting mixture properties and morphology. Then morphological, viscoelastic and mechanical properties of blends are presented, correlated with the properties of the initial polymers and discussed.

Dynamic oscillatory frequency sweep tests were performed for each initial polymer at various temperatures. Master plots for G0 , G00 and g⁄ were obtained at a reference temperature of 180 °C by using the time–temperature superposition principle. Fig. 2 represents the master curves of G0 , G00 and g⁄ for CAB; Fig. 3 is the same for PE. Both polymers have a shear-thinning behaviour; the Newtonian plateau of PE has not been reached. The experimental complex viscosities were approximated with the Carreau–Yasuda model: m1 a

1000

G''

η*

G'

ω, rad/s

3.1. Properties of the initial components

g ðxÞ ¼ g0  aT ½1 þ ðk  x  aT Þa 

10000

ð1Þ

where g0 is zero-shear rate viscosity (Pa s), aT the shift factor, k the characteristic time (s), m the pseudo-plasticity index and a a fitting parameter. The constants g0, aT, m, k and a were calculated with the least squares approximation by minimising the error between the model and experimental data. The parameters found for each polymer are listed in Table 2. Similar viscosity master plots fitted with Eq. (1) were made for each polymer for other reference temperatures also, from 160 to 190 °C, and will be used for the calculation of polymers viscosity ratios.

100 0.01

0.1 0

1 00

10

100

1000

⁄

Fig. 3. Master plots of G , G and g for PE at a reference temperature of 180 °C. Solid line is complex viscosity calculated according to Carreau– Yasuda model (Eq. (1)). Error bars are smaller than or equal to the point size.

Table 2 Carreau–Yasuda parameters for PE and CAB at a reference temperature of 180 °C. Polymer

g0 (Pa s)

k (s)

m

a

PE CAB

4.2  105 1.0  104

10.3 0.24

0.29 0.32

0.30 0.81

Terminal slopes of G0 and G00 in the low frequencies region has been determined for PE and CAB and are given in Table 3. Following the Maxwell model, a monodisperse flexible polymer should have the slopes of 2 and 1 for G0 and G00 , respectively. The values obtained show that high density PE is probably highly polydispersed while CAB behaviour is close to the classical one.

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Table 3 Slopes of G0 and G00 for CAB and PE in the low frequencies region. Polymer

Slope of G0

Slope of G00

PE CAB

0.82 1.72

0.72 0.97

The activation energies Ea of PE and CAB were determined from shift factors aT using Arrhenius law:

aT ¼ exp

   Ea 1 1  R T T ref

ð2Þ

where R the ideal gas constant, T is temperature in K and Tref the reference temperature. The slopes of ln(aT) versus 1/T shown in Fig. 4 give an activation energy of 30 kJ mol1 for PE and 254 kJ mol1 for CAB. Because the activation energy of CAB is almost ten times higher than the one of PE, CAB viscosity is much more temperature-sensitive as compared with PE. The rheological information on pure PE and pure CAB will be used in the study of their blends. It is known that when two immiscible fluids are mixed, the size of the droplets of the minor phase depends on the interfacial tension between the components, the ratio of their viscosity and elasticity and on the mechanical stress applied [14,15]. The droplet breaks up when the forces deforming the droplet prevail the interfacial tension forces that are trying to keep the droplet spherical and intact. In a simple shear and at a fixed interfacial tension the easiest (the lowest stress) droplet break-up occurs when the viscosity ratio of mixed Newtonian fluids gdroplet/gmatrix is from 0.1 to 1. The increase of the viscosity ratio above 1 makes droplet rupture more difficult and finally impossible at gdroplet/gmatrix > 4. When both phases are made of shearthinning fluids, the break-up behaviour is described by the theory developed for Newtonian droplets if gdroplet and gmatrix in the calculation of the viscosity ratio is taken at the shear stress applied to the matrix fluid [16]. For viscoelastic droplet fluids, higher shear stresses are required to

5

induce droplet deformation and break-up compared to a Newtonian droplet under identical conditions [16–18]. However, the viscosity ratio close to 1 is also requested for the easiest (lowest stresses) break-up of a viscoelastic droplet [19]. It is thus important to analyse the viscosity ratio of the studied mixed molten polymers as far as droplet rupture, and thus the final size of the droplet, will depend on the applied stress and gdroplet/gmatrix. Using Carreau–Yasuda approximation (Eq. (1)) for the dependence of complex viscosity of each component on frequency, the viscosity ratio g⁄CAB/g⁄PE was calculated as a function of frequency for the real temperatures to which the blends were exposed during mixing (Fig. 5). At low frequencies, below 0.1 rad s1, the viscosity ratio g⁄CAB/g⁄PE is rather low, which means that it is possible to break cellulose ester droplets in polyethylene matrix but very difficult to do the contrary. Viscosity ratio gradually increases with frequency increase to plateau values and it also increases with temperature decrease. The influence of temperature on viscosity ratio is driven by different viscosity-temperature dependences of each component. Indeed, cellulose acetate butyrate viscosity is much more temperature dependent, with much higher activation energy as compared with the polyethylene used in this study (see Fig. 4). Fig. 5 shows that by adjusting processing conditions it is possible to obtain viscosity ratio close to one which should facilitate droplets rupture. In order to predict blend morphology taking viscosity ratio as one of the background parameters, it is necessary to know the shear rates developed in the mixer to be able to use results shown in Fig. 5. We used Bousmina approach to estimate the mean shear rate produced in the internal mixer during blend preparation [20]. In this approach, the internal mixer is represented by a dual-Couette geometry exerting the same torque on the melt as the roller rotors in the mixer. A calibration was performed with the polyethylene used in the study in order to determine the intrinsic parameters of the system according to [20]. The estimations show that the mean shear rate 0.5

ln aT

ηCAB/ηPE

180°C

y = 30598x - 67.6 R2 = 0.99

0.4 187°C

1 0.3

0

193°C

2 0.2

y = 3593.8x - 7.9 R2 = 0.99

203°C

0.1

-5 0.0020

207°C

1/T , K-1 0.0021

0.0022

0.0023

0.0024

Fig. 4. Shift factor as a function of inverse temperature for PE and CAB at Tref = 180 °C.

0 0.001

ω , rad.s 0.01

0.1

1

10

100

-1

1000

Fig. 5. Viscosity ratio gCAB/gPE calculated according to Eq. (1) at various temperatures.

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between the equivalent rotors and the mixing chamber _ , is correlated with the rotor speed N (in rpm) walls, c 1=2 as follows:

c_1=2 ¼ 0:86N

ð3Þ

Table 4 correlates the experimental conditions and calculated parameters: rotor speed and the corresponding shear rate; real temperature in the mixer and the corresponding viscosity of each component (g⁄CAB, g⁄PE) calculated with Carreau–Yasuda approximation (Eq. (1)) for the given temperature and shear rate, and their viscosity ratio. Table 4 consists of two parts: one concerns the blend with 20 wt% CAB-80 wt% PE (noted as 20CAB-80PE) prepared at different rotors speeds. The second concerns CAB-PE blends of various compositions prepared with the same rotor speed of 50 rpm. In all cases the set temperature was the same, 180 °C, but real temperature in the mixer, Tmelt, varied because of energy dissipation which depends on major phase viscosity in the given conditions and thus varies from one preparation to another. Shear stress is calculated for emulsion-like blends as a product of continuous phase viscosity and applied shear rate in order to estimate the mechanical stress applied to the droplets. Let us first consider case 1: 20CAB-80PE blend prepared at various rotor speeds (Table 4). Assuming Cox-Merz rule is valid for our system, the ratio of CAB to PE viscosity is between 0.1 and 0.3. The break-up of CAB droplets in a PE matrix should be thus easy. The increase of rotor speed leads to the increase of the shear stress which continuous phase (PE) is exerting on the droplets (CAB); the size of the droplets should thus decrease. These predictions will be checked in the next section where blends morphology will be presented. In the second case mixtures of various compositions are considered (Table 4), they all were prepared at the same rotor speed. The viscosity ratio g⁄CAB/g⁄PE varies from 0.3 for CAB in the major phase to 0.15 with CAB in minor phase. This means that it should be easy to break CAB droplets in PE matrix (as shown for 20CAB-80PE blend) and difficult to do the opposite as far g⁄PE/g⁄CAB with CAB in major phase is around 4–6. Dynamic Mechanical Analysis has been performed on neat CAB and PE samples. Cellulose acetate butyrate

demonstrates a classical behaviour: a glassy plateau up to 110 °C, then a glass transition at about 125 °C characterised by a maximum of tan (d) and the beginning of a rubbery plateau at 150 °C. Polyethylene glass transition temperature that is supposed to be around 110 °C was not easy to measure: as the sample is clamped at ambient temperature, a shrink occurs when temperature is strongly reduced. Thus the clamping is less efficient and signals are noisy. Recording a maximum of tan (d) is difficult, but a tendency of a peak at about 110 °C was possible to detect. At ambient temperature, the signals are less disrupted and melting of PE occurs at about +130 °C. Complex moduli at ambient temperature of each polymer are very similar: 1.2 and 1.1 GPa for CAB and PE, respectively. 3.2. Properties of CAB-PE blends 3.2.1. Morphology Let us first consider case 1 of constant blend composition, 20CAB-80PE, prepared at various rotor speeds. The SEM images of cryo-fractured blends are presented in Fig. 6. As expected, higher rotors speed implies smaller CAB droplets. Mixing rate increase could be an easy way to obtain small CAB droplets; however, the change of blend colour from white to brown was observed indicating polymer thermo-mechanical degradation. The cryo-fractures of CAB-PE blends of various compositions (case 2) prepared with the same mixing conditions are presented in Fig. 7. Two main morphologies as a function of the composition are observed: co-continuous and emulsion-like. For mixtures containing low volume fraction of one polymer (for example, 15CAB-85PE and 85CAB-15PE, Fig. 7a and e, respectively), a nodular morphology is obtained. Droplet size depends on viscosity ratio between dispersed-to-continuous phases. In the case of CAB dispersed in polyethylene (15CAB-85PE, Fig. 7a), a fine morphology with CAB small droplets homogeneously distributed in PE bulk was obtained. Droplet diameter is about 1–2 microns. In the reverse case (85CAB-15PE, Fig. 7e), polyethylene droplets form a coarse nodular morphology with a large variation of droplets diameters from a few to 20–30 lm. The reason of different morphologies of the ‘‘symmetrical’’

Table 4 Main parameters of the blends prepared. Measured: rotor speed and melt temperature; calculated: mean shear rate, shear stress, viscosity of each component and viscosity ratio.

Case 1

20CAB-80PE

Case 2

100CAB 85CAB-15PE 75CAB-25PE 60CAB-40PE 50CAB-50PE 40CAB-60PE 25CAB-75PE 15CAB-85PE 100PE

Rotor speed (rpm)

Mean shear rate (s1)

Tmelt (°C)

25 50 100 150 50 50 50 50 50 50 50 50 50

21 43 86 129 43 43 43 43 43 43 43 43 43

187 193 203 207 186 185 187 187 188 189 191 196 202

Shear stress (Pa) 1.2  105 1.6  105 2.0  105 2.2  105 5.7  104

15.7  104 15.1  104

g⁄CAB (Pa s)

g⁄PE (Pa s)

g⁄CAB/g⁄PE

1632 715 222 142 1240 1329 1155 1155 1073 996 851 553 –

5947 3713 2276 1732 – 3917 3865 3865 3839 3814 3764 3644 3507

0.27 0.19 0.10 0.08 – 0.34 0.30 0.30 0.28 0.26 0.23 0.15 –

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Fig. 6. SEM images of 20CAB-80PE blend morphology for various preparation conditions (a) 25 rpm, (b) 50 rpm, (c) 100 rpm and (d) 150 rpm.

in composition blends is that for 15CAB-85PE blend g⁄CAB/ g⁄PE = 0.15 and for 85CAB-15PE g⁄PE/g⁄CAB = 2.9 (see Table 4). The applied stress to the minor phase is almost three times higher for the blend 15CAB-85PE than for the 85CAB-15PE and the interfacial tension is the same in both cases. As predicted in the section above, it is difficult to break polyethylene into small droplets immersed in CAB continuous phase. It should also be kept in mind that the viscosity of the continuous phase in 85CAB-15PE is low enough allowing polyethylene droplets coalescence before the melt is cooled. When the volume fraction of the dispersed phase is increased, a mixed morphology is obtained, as seen in Fig. 7b for 25CAB-75PE blend. Here both small nodular and continuous domains are formed. The latter might appear due to the coalescence because the probability of two droplets coming into contact increases with the increase of the dispersed phase fraction. A co-continuous morphology is observed for the blend with equal polymer weight fractions as shown in Fig. 7c. The morphology of the 75CAB-25PE blend (Fig. 7d) is not the same as its ‘‘symmetrical’’ one, 25CAB-75PE (Fig. 7b). The morphology of 75CAB-25PE is close to cocontinuous with very large polyethylene inclusions and small droplets of cellulose acetate butyrate embedded into polyethylene phase, despite the fact that polyethylene is in the minor phase with the volume fraction of about 30%. More elastic PE phase tends to encapsulate a less elastic one, CAB, the latter forming tiny droplets because of the favourable viscosity ratio [21]. All images demonstrate

the absence of adhesion between CAB and PE phases, which involves immiscibility between both components as far as no compatibilizer was used. Different morphologies obtained will be now correlated with blends rheological and mechanical properties. 3.2.2. Viscoelastic properties of blends The viscoelastic properties of molten blends of various compositions (case 2) were examined at 180 °C in the linear viscoelastic region. The evolution of the elastic modulus G0 as a function of frequency is presented in Fig. 8. From the first glance G0 of blends is in-between the moduli of the pure components, no anomalous behaviour is observed. A closer look shows that G0 of blends with CAB in minor phase, 15CAB-85PE and 25CAB-75PE, coincide with the one of pure polyethylene. This can be interpreted as an elastic input coming from the interfacial tension between two phases, which leads to a higher elasticity as compared with a simple additive approach. However, no noticeable shoulder on G0 (x) was observed for these blends, as it could be expected for blends with nodular morphology. The reason might be in the particular viscoelastic response of the continuous phase (here, polyethylene): even at low frequencies polyethylene studied is far from the classical terminal behaviour (see G0 and G00 slopes at low frequencies in Table 3). Complex viscosity g⁄ of blends versus frequency was recorded for all blends studied. The values at various frequencies were taken and plotted as a function of mixture composition (Fig. 9). The experimental values were

F. Besson, T. Budtova / European Polymer Journal 48 (2012) 981–989

987

Fig. 7. SEM images of CAB/PE blends made with rotor speed of 50 rpm with different weight proportions of CAB (a) 15%, (b) 25%, (c) 50%, (d) 75% and (e) 85%.

1000000

compared with mixture viscosity g⁄add calculated according to the log-additive rule (Eq. (4)):

G' , Pa

lnðgadd Þ ¼ / lnðgCAB Þ þ ð1  /Þ lnðgPE Þ

100000

10000 PE

100PE 15CAB-85PE 25CAB-75PE 40CAB-60PE 50CAB-50PE 60CAB-40PE 75CAB-25PE 85CAB-15PE 100CAB

1000

100

10

1 0.01

CAB

0.1

1

10

ω , rad.s

-1

100

Fig. 8. Elastic modulus of blends of various compositions as a function of frequency at 180 °C.

ð4Þ

where / is the volume fraction of CAB in the blend, g⁄CAB and g⁄PE are complex viscosities of the initial components. The experimentally measured complex viscosity of blends follows the log-additive rule for all compositions at high frequencies except for lower frequencies with CAB in minor phase: blends with 15CAB-85PE and CAB25-75PE are of higher viscosity than the corresponding calculated values g⁄add. This deviation is directly correlated to blends specific elasticity at low cellulose acetate butyrate concentrations that correspond to the nodular morphology with dispersed CAB fine droplets (Fig. 7a and b). 3.2.3. Dynamic mechanical properties of CAB-PE blends Dynamic mechanical analysis was performed on the blends in the solid state. The storage modulus E0 of the blends decreases with temperature increase: the

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F. Besson, T. Budtova / European Polymer Journal 48 (2012) 981–989

1.E+06

0.01rad/s

η*, Pa.s

1.E+09

E', Pa

0.1rad/s 1rad/s 10rad/s 100rad/s

1.E+05

1.E+04

φCAB 1.E+08

1.E+03 0%

25%

50%

75% φ 100% CAB

Fig. 9. Complex viscosity at various pulsations and 180 °C as a function of volume fraction of CAB in the blend: points are experimental values; solid line corresponds to the additive viscosity calculated according to the mixing rule (Eq. (4)).

0%

20%

40%

60%

80%

100%

Fig. 11. Storage modulus of blends at 80 °C as a function of volume fraction of CAB: points are experimental data; lines are E0 add calculated according to Eq. (5).

lnðE0add Þ ¼ / lnðE0CAB Þ þ ð1  /Þ lnðE0PE Þ decrease is smooth up to 110–120 °C and then is sharp due to cellulose acetate butyrate glass transition and beginning of polyethylene melting. The evolution of the loss angle tan (d) for the pure components and the blends is shown in Fig. 10. Cellulose acetate butyrate undergoes a transition to a rubbery state at about 110 °C and polyethylene is melting at 130 °C. Depending on the blend composition, the peaks corresponding to the phase transition of each polymer can be roughly identified. Despite the fact that the resolution is not good because both temperatures are close to each other, it can be concluded that the phase transition temperature of each component is not modified when polymers are mixed. Storage modulus was analysed as a function of mixture composition at 80 °C. The experimental data are compared with the additive values E0 add calculated according to a logadditive mixing rule (Eq. (5)):

tan δ

1.4

100PE 25CAB-75PE 50CAB-50PE 75CAB-25PE 100CAB

1.2 1 0.8 0.6 0.4 0.2

T °C

0 20

70

120

170

Fig. 10. Evolution of tan (d) with temperature for five blends with different proportions of CAB. Lines are given to guide the eye.

ð5Þ 0

where / is the volume fraction of CAB in the blend, E CAB and E0 PE are the storage moduli of the neat components. The results are shown in Fig. 11. In the case of non-compatibilised binary blends, the mechanical properties are usually worse that the ones of the initial components due to the lack of adhesion between two phases, i.e. the general trend is a negative deviation from the log-additive mixing rule [22,23]. For the CAB-PE blend the experimental values of the storage moduli of the blends follow log-additive behaviour. It seems that the interface does not play an important role on blend storage modulus values which is an interesting result for making multiphase materials combining the properties of each component. 4. Conclusions Cellulose Acetate Butyrate and High Density Polyethylene were melt blended in an internal mixer in various proportions without any compatibiliser. The influence of mixture composition on blend morphology, viscoelastic properties of molten mixtures and mechanical properties of solid blends was studied. It was demonstrated that different morphologies can be obtained from fine nodular (CAB dispersed in PE) to coarse (PE dispersed in CAB) and co-continuous, depending on mixture composition. These differences were explained by viscosity ratio between the neat polymers that control the size of the droplets of the dispersed phase and vary as a function of the applied stress and temperature. The fact that polyethylene is more viscous at all shear stresses used explains why it is possible to obtain small CAB droplets of 1–2 lm dispersed in PE and only large droplets of PE (10–20 lm) dispersed in CAB. The contribution of the interface between two polymer phases is reflected by viscoelastic and dynamic mechanical properties of blends at compositions corresponding to the lowest size of the droplets (CAB dispersed in PE): dynamic viscosity was

F. Besson, T. Budtova / European Polymer Journal 48 (2012) 981–989

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