MASTER'S THESIS
Development of Alternative Material for Profiles in Structural Applications
Simon Francou
Master of Science in Engineering Technology Materials Technology (EEIGM)
Luleå University of Technology
Department of Engineering Sciences and Mathematics
Feb-Sept 2011 Part 2
Simon FRANCOU
Supervisors : Roberts JOFFE, Birgitha NYSTRÖM and Guan GONG
Development of alternative
material for profiles in
structural applications
Content
Acknowledgments ... 4
Introduction ... 5
1 Literature review ... 6
1.1 Polyethylene based wood plastic composites... 6
1.2 Clay reinforced composites and the application of clay in WPCs ... 6
2 Experimental ... 9
2.1 Materials ... 9
2.2 Nanocomposite processing and sample preparation ... 9
2.3 Characterization ... 10
2.3.1 Three-point bending tests ... 10
2.3.2 Tensile tests ... 11
2.3.3 XRD measurements ... 11
2.3.4 Rheology measurements ... 12
2.3.5 Water uptake measurements ... 12
2.3.6 TGA tests... 12
3 Results and discussion ... 13
3.1 Mechanical properties ... 13
3.1.1 Influence of processing parameters on flexural properties on neat HDPE ... 13
3.1.2 Influence of processing parameters on flexural properties of HDPE/clay composites without MAPE ... 14
3.1.3 Influence of MAPE type and amount on HDPE/clay nanocomposites ... 15
3.1.4 Influence of clay loading on flexural properties of nanocomposites with E265/clay ratio of 1:1 ... 18
3.1.5 Influence of clay loading on tensile properties of nanocomposites with E265/clay ratio of 1:1 ... 20
3.2 XRD Results ... 21
3.2.1 XRD results for nanoclay composites with different MAPE types and MAPE/clay weight ratios ... 21
3.2.2 XRD results for nanocomposites with different clay loadings at E265/clay ratio of 1:1 .. ... 22
3.3 Rheological properties... 23
3.4 Water absorption ... 25
3.5 Thermal stability ... 27
4 Conclusions and future work ... 28
5 References ... 29
6 Table of Figures ... 31
7 Appendix ... 33
7.1 Sample code name ... 33
Acknowledgments
This work has been carried out at Swerea SICOMP, Swedish Institute of Composites, Piteå, and in the Division of Polymer Engineering at Luleå University of Technology (LTU), in collaboration with OFK company, (Karlskoga), between February and September 2011.
I would like to warmly thank my 3 supervisors for this master thesis, Dr. Roberts Joffe from LTU, senior scientist Birgitha Nyström and Dr. Guan Gong from Swerea SICOMP for their leadership in this project, their guidance and advises. Further thanks go to Dr. Guan Gong for the water absorption and rheology tests, and all her help for the manufacturing steps. Special thanks also for Olof Frisk, CEO, from OFK Plast AB for supporting Polywall project.
I also would like to thank Prof. Kristiina Oksman for allowing us to use the hot press and TGA device in her division, and I want to express my gratitude to the persons who helped me during this project: PhD student Konstantinos Giannadakis, research engineer Johnny Grahn and master student Latifa Melk.
Introduction
With the two-year Polywall project, the research institute Swerea SICOMP (Piteå, Sweden) and OFK Plast (Karlskoga, Sweden) attempt to develop new polyethylene (PE) based wood composites.
The global aim of the project is to fabricate materials with better barrier properties and fire retardancy compared to WPC profiles currently used in decking, wall, roof and door applications, taking advantage of nano-clay. PE/clay nanocomposites will be prepared and used as a masterbatch for the target WPCs. The opportunity of using recycled PE will also be investigated. Other additives, such as mineral filler (talc) and lubricant, etc., will be used according to the formula for current WPC profiles.
The objective of this master thesis is to optimize the compatibilizers and processing conditions to achieve full exfoliation and good dispersion of nanoclay inside of the non-polar matrix, high density polyethylene (HDPE), therefore, to enhance the thermal stability and at the same time to improve the mechanical properties of the matrix.
1 Literature review
In this part, a brief review on nanoclays and their application in WPCs will be given, along with common techniques used for clay dispersion in polymeric matrices.
1.1 Polyethylene based wood plastic composites
The use of wood plastic composites (WPCs) has grown in recent years, especially in decking products. Other applications are found in transport and building area. WPCs have several advantages compared to natural wood: lower water absorption, better aging resistance (insects, rotting), reduced maintenance (no need of wood stain for example) and greater dimensional stability (no buckle). Thermoplastic resins which are used as the matrices of WPCs also give access to fast, cost effective and continuous production.
Polyethylene (PE) is widely used as the matrix of WPCs due to its good processibility and high chemical resistance. However, the inherent incompatibility between hydrophobic PE and hydrophilic wood causes weak interface; therefore, the mechanical properties of PE based WPCs are often lower than that of neat PE [1]. To improve the compatibility of PE/wood systems, different compounds can be used. Maleic anhydride functionalized PE (MAPE) or chemicals with similar bonding properties such as siloxanes [2], titanates and polyaminoamide–epichlorohydrin [3,4] are often employed as compatibilizers.
The other big problem in WPCs is their low fire resistance, which limits their applications.
Brominated or chlorinated compounds have been widely used as fire retardants in polymeric composites. But environmental concerns in Europe (and later in North America) led to a change in legislation and those products are now banned [5]. Aluminum trihydroxide(ATH) or magnesium hydroxide (Mg(OH)2) have also been used as flame retardants, however, high amount of them, usually up to 40-60 wt%, are needed to achieve good fire retardancy, which negatively affect the mechanical properties of the resultant composites, and also introduce some processing problems [6].
1.2 Clay reinforced composites and the application of clay in WPCs
Nanoclays are made of stacks of sheet like silica platelets, with thickness in the range of nanometer. Their high aspect ratio and specific surface area make them promising reinforcements for polymer composites. After the pioneer work of Toyota [7-9], numerous studies have described the processing of nanoclay reinforced composites and their properties [10-14]. In recent years, the application of nanoclay in bio- or bio-degradable polymers also attracts more and more interests, which aims to improve the stiffness, strength and barrier properties of bioresins [15,16].When blended with polymers, three different types of structures of nanoclay can be identified depending on the interactions between polymer and clay [17]:
Phase separated: where the polymer and clay are mutually immiscible and nanoclay form clusters without good dispersion and distribution within the polymer.
Intercalated: where the polymer chains intercalate between the layers of clay.
Exfoliated: where the periodicity of clay is destroyed and the clay platelets are dispersed within the polymer matrix.
Figure 1 shows these three dispersion states of clay in polymer matrices. Therefore, different morphologies, agglomeration, intercalated, mixed intercalated and exfoliated, ordered exfoliated and disordered exfoliated, have been observed in clay reinforced composites. The superior properties of the nanocomposites are believed to achieve even at a small amount of clays when clays are fully exfoliated [18].
Suspension mixing is one of the techniques that has been used to achieve desired dispersion of nanoclay [19]. However, significant amounts of solvent (water or other diluents) must be evaporated during the matrix consolidation process. Melt compounding methods, which can be combined with conventional polymer manufacturing processes like extrusion, compression and injection molding, are now increasingly used to prepare exfoliated nanocomposites, especially from an industrial standpoint. They have mainly concentrated on twin-screw extrusion, but laboratory scale batch mixers and microextruders are also used, which are conventionally utilized to prepare masterbatches. The use of masterbatches is often favored because it supposedly leads to more effective dispersive mixing [20]. Longer residence time and exposure to shear forces may both contribute to better exfoliation of clay platelets. However, the physical properties can also be deteriorated if the materials start to degrade when kept for a long time in the mixer and exposed to strong shearing. Therefore, the processing conditions should be optimized. Strong interfacial adhesion between polymer matrices and clay are readily achieved with highly polar polymers like polyamides. However, for many commercially important, nonpolar or weakly polar polymers such as PE, polypropylene (PP), and polystyrene (PS), natural incompatibility with the polar clay surface
Figure 1. Schematic representation of different dispersion states of nanoclay inside of polymers [17]
poses a significant challenge to achieve good exfoliation and dispersion of clay. Most of the studies reported exfoliation of clay when MAPE was used either as a compatibilizer or as the polymer matrix especially at high ratio to clay [10, 21-26]. However, the application of MA functionalized polyolefins (MA-POs) in promoting dispersion of clay is still under intense research. It is because that there are various types for MA-POs, which differ from each other in terms of main chain, molecular weight, and MA content [10]. These variations directly determine the function of MA-POs. Therefore, it is also very important to optimize the type (LDPE or HDPE based MAPE) and weight ratio of MAPE to clay (depending on the MA content and the molecular weight compared to the PE matrix).
In recent years, efforts have been taken to modify WPCs by taking advantage of nanoclay. It was concluded that achieving a higher degree of dispersion for nanoclay was critical to enhance the mechanical properties and the flame retardancy of WPCs when small amounts of clay were used [27]. Lee et al., for example, added small amount (1-5wt.%) of organo-modified clay in a traditional HDPE-based WPC and observed an increase up to 30 and 20% in tensile and flexural modulus, respectively, as well as slightly improved tensile and flexural strength. The burning rate was reduced by 30% [2]. However, the systematic studies of nanoclay modified PE/wood composites, especially recycled PE/wood composites, are still needed in a great amount, because they are very complicated hybrid systems where many interactional factors operate. Polywall project aims to fabricate composites with better barrier properties and fire retardancy compared to PE based WPC profiles currently used in decking, wall, roof and door applications, by taking advantage of nano-clay.
Incorporation of PE/clay or MAPE/clay nanocomposites prepared through melt compounding as a masterbatch appears to be the best approach to incorporate nanoclay in WPCs [2,28]. It is also expected that MAPE can be used as a compatibilizer for both clay/PE systems and wood/PE composites, which simplifies the formulation and the fabrication of the composites. In the present study in order to simplify the system, PE/clay masterbatches are prepared in which neat HDPE is used. The methodology to optimize the system constitution and the processing conditions is optimized. The structures and properties of resultant nanocomposites depending on different types of MAPE and MAPE/clay weight ratios as well as clay loadings are studied.
2 Experimental
2.1 Materials
High Density Polyethylene (HDPE), MG9621 (Borealis), with a MFI of 12g/10min (190°/2,16 kg), was used as the matrix.
The organoclay, Bentone® 34, (Elementis)® served as reinforcement.
3 different MAPEs all from DuPontTM Fusabond series were used as compatibilizers: M603, a random ethylene copolymer with a MFI of 25g/10min (190°/2,16 kg); E100, a MA-grafted HDPE with a MFI of 2g/10min (190°/2,16 kg); and E265, a MA-grafted HDPE with a MFIof 12g/10min (190°/2,16 kg). The weight ratios of MAPE and clay were adopted as 1, 3, 6 and 9.
2.2 Nanocomposite processing and sample preparation
Melt compounding was performed in a twin-screw mixer Brabender (Duisburg, Germany). All components were weighed before processing according to the formula, and the total weight introduced in the Brabender was kept constant as 25g/batch .
The operating temperature was controlled by a thermometer indicating the temperature of heating liquid, and a thermocouple plugged into the metallic block displayed a temperature close to the one in the mixing chamber. The heating liquid’s temperature was around 30°K above the one given by the thermocouple. In this report, all operating temperatures for the mixer will be equal to the heating liquid’s minus 30°K, i.e. for one batch prepared at “160°C”, the thermometer indicated 190°C for the liquid.
HDPE was added first into the chamber of mixer, and MAPE wasn’t added until HDPE was totally melted. Clay was added little by little, when HDPE and MAPE were melted. The rotation speed was kept around 10 rpm until all components were blended, then was increased to 30 or 60 rpm as planned.
When melt compounding was ended, composite was manually removed from the Brabender, cooled down and crushed manually into pieces.
All the composites were then hot pressed at 150°C and 100 kN into sheets with different thickness, using Fontijne Presses LP 300. Testing samples with different shapes and dimensions according to different measurements were then cut or directly obtained from these sheets.
2.3 Characterization
2.3.1 Three-point bending tests
3-point bending tests were carried out in an Instron 4411 with a 500N load cell, following the ASTM 790M-93 standard settings. 4 specimens were tested for each material. Figure 2 shows the experimental setup for this test.
Figure 2. Experimental setup for 3-point-bending test
The support span L was set to 25 mm, compared to 50 mm for the length of the samples (meaning a 1:2 ratio). The crosshead speed was set to 8.54 mm/min according to the following equation:
R = ZL²/6d
where:
R = crosshead speed, mm/min L = support span, mm
d = thickness of the sample, mm.
The thickness was set to 1.2mm, even if small variations were observed among the different samples.
Z = rate of strain of the outer fiber, min-1. Here, Z was equal to 0.1 min-1
Flexural strain and midspan deflection are linked by the relation below :
D = rL²/6d
where:
D = midspan deflection, mm
r = strain, mm/mm L = support span, mm
d = thickness of the sample, mm
The maximal flexural stress measured at midspan is given by the equation :
S = 3PL/2bd²
where :
S = stress at midspan, MPa P = applied load at midspan, N L = support span, mm
b = width of the sample, mm d = thickness of the sample, mm
2.3.2 Tensile tests
Tensile tests were carried out in an Instron 3366 with a 10kN load cell, using the following parameters:
-Gage length: 80 mm
-Crosshead speed: 8mm/min -Strain rate : 0.1 min-1
Sandpaper was placed between clamps and samples to avoid slip.
The extensometer is limited to 10% strain. However, neat HDPE can undergo very high strain before rupture, which is out of the limitation of the extensometer. Therefore, tests were first performed with the extensometer and stopped just before 10% strain, and strain (%) – displacement (mm) was fitted by a simple linear function. This relation was then used to calculate the strain (%) from the beam displacement (mm) for subsequent tests carried out without extensometer and stopped before failure at 50% strain.
2.3.3 XRD measurements
This measurement was performed in an X-ray diffractometer (Siemens D5000 X-Ray Diffractometer). The nanoclay was measured in a powder form and all the nanocomposites were measured in a solid sheet form. The voltage and current used to excite the Cu source (wavelength of 1,54060 Å) were 40kV and 40mA, respectively. A step scan method was chosen, with 2θ ranging from 0.5° to 8° and a 0.02° increment. The counting time was 2.0 sec per step. Two samples were tested for each material.
To determine the interlayer distance, Bragg’s law is used as follow :
d = λ/(2 ⋅ sinθ)
where :
d = interlayer distance
λ = wavelength of the incident X-ray θ = half of the incidence angle
2.3.4 Rheology measurements
A stress controlled rheomoter C-VOR (Bohlin) was used for the melt rheology measurements, with a 25-mm parallel plate geometry, at temperature of 180oC. The gap distance was set to 700 μm.
At least two samples for each material were tested and representative results were shown.
Amplitude sweep tests at 6.28 rad/s were performed first to determine the stress to be applied in subsequent frequency sweep tests which were carried out in the linear viscoelastic region.
2.3.5 Water uptake measurements
Samples of 50x50x0.5 mm3 were first weighed and then immersed in water at room temperature for about a month. Samples were removed at appropriate time intervals, gently blotted with tissue paper to remove excess water on the surface and weighed. This process was continued until saturation was reached. Two films for each material were tested.
2.3.6 TGA tests
A TGA Q500 (TA Instruments) was used for the thermal degradation characterization. Samples were tested in air atmosphere from room temperature up to 500°C, with a heating rate of 10°C/min.
Two samples were tested for each material.
3 Results and discussion
3.1 Mechanical properties
3.1.1 Influence of processing parameters on flexural properties on neat HDPE
The influence of processing parameters on flexural properties of neat HDPE are shown in Figure 3. The processing parameters used are summed up in Table 1.
The results show that mechanical properties of neat HDPE are hardly dependent on the processing parameters as expected. At 130°C, flexural modulus seems to increase with rotation speed, but the standard deviation is high and therefore the high values are not valid. For maximal flexural stress results are quite similar for all setups, and all the values are close to 37.5 MPa, regardless of temperature, rotation speed and time. Flexural modulus of samples prepared at 160°C is not influenced by time or rotation speed, and are slightly higher than those prepared at 130°C. The standard deviation is also much smaller at 160oC. This could be explained by the fact that melting of HDPE was visibly not perfect at 130°C, indicating that a homogeneous material was not obtained.
The results also show that, when properly homogenized through melting at sufficient temperature, flexural properties of pure HDPE will not be influenced by rotation speed and time of mixing. Flexural properties of pure HDPE can be roughly rounded up to 1.7 GPa and 38 MPa for flexural modulus and strength, respectively.
Rotation speed (RPM)
30 60 90
Temperature in the chamber (°C)
130 4
Melting time (min)
12
160 4
12
Table 1. Different parameters for processing neat HDPE
Figure 3 Flexural properties of neat HDPE processed with different conditions: (A) flexural modulus of samples prepared at 130°C; (B) flexural strength of samples prepared at 130°C; (C) flexural modulus of samples prepared at 160°C; (D)
flexural strength of samples prepared at 160°C.
3.1.2 Influence of processing parameters on flexural properties of HDPE/clay composites without MAPE
Nanocomposites with 2 wt% clay without any MAPE were prepared with the same parameters as neat HDPE. The processing parameters are listed in Table 2. Mechanical properties of these composites given by 3-point-bending tests are shown in Figure. 4.
Rotation speed (RPM)
30 60 90
Temperature in the chamber (°C)
130 4
Melting time (min)
12
160 4
12
Table 2. Different parameters for processing HDPE/clay (2 wt%) composites
C D
A B
Figure 4. Flexural properties of HDPE/2 wt% clay composites: (A) flexural modulus of samples prepared at 130°C; (B) flexural strength of samples prepared at 130°C; (C) flexural modulus of samples prepared at 160°C; (D) flexural strength
of samples prepared at 160°C.
The flexural properties of none of these nanocomposites are better than pure HDPE. However, results show that high rotation speed is detrimental to modulus and strength, and the effect is magnified when combined with a higher temperature. Time of mixing has hardly any influence at higher temperature. Samples prepared with 30 RPM and a low temperature, are once again the ones with higher standard deviation.
Generally, scatter is higher for the lower temperature, which can be attributed to inhomogeneous melting of the samples. Increasing the rotation speed will give more homogenous material (reduced scatter). However in combination with high temperature the rotation speed seems to reduce the mechanical properties. The rotation speed 60 rpm is still selected for the subsequent trials because of the low scatter and the expected influence of MAPE.
3.1.3 Influence of MAPE type and amount on HDPE/clay nanocomposites
The study of HDPE/Clay composites indicates that the interface between HDPE and Bentone® 34 is weak, therefore, a compatibilizer, MAPE, is added. The weight ratio of MAPE/clay and processing parameters are summarized in Table 3.
A B
C D
MAPE/Clay ratio
1:1 3:1 6:1 9:1
MAPE type M603 60 Rotation speed (RPM)
E100 60
E265 30
60
Table 3. MAPE/clay ratios and rotation speeds for processing HDPE/MAPE/clay (2 wt%) nanocomposites, the processing temperature and time are fixed at 160°C and 12 min.
Mechanical results from 3-point bending test for HDPE/MAPE/clay nanocomposites are reported in Figure 5. The results of HDPE/clay (2 wt%) composites prepared with the same conditions are also listed in Figure 5 as a reference.
Figure 5. Flexural properties of HDPE/MAPE/clay (2 wt%) nanocomposites: (A) flexural modulus; (B) flexural strength 1277,6
1628,0
1531,6
1422,3
1602,1 1590,0 1652,51575,0 1648,31576,9
0 200 400 600 800 1000 1200 1400 1600 1800
PE/Clay 1:1 ratio 1:3 ratio 1:6 ratio
Flexural Modulus (MPa)
M603 E100 E265
31,76
36,54 35,11
33,31
39,0138,22 39,1237,80 39,2337,77
0 5 10 15 20 25 30 35 40 45
PE/Clay 1:1 ratio 1:3 ratio 1:6 ratio
Maximal Stress (MPa)
M603 E100 E265
A
B
The bending stiffness of nano composites is not improved compared to neat HDPE while the flexural strength is slightly improved in some cases when MAPE is added. However, adding MAPE results in a boost of mechanical properties compared to nano clay composites. The case of M603 is different from the other two MAPEs. When its concentration increases, both flexural modulus and strength decrease. This can be linked to its much lower molecular weight compared to the HDPE matrix, which makes it act as a plasticizer besides a compatibilizer, thus deteriorates the mechanical properties. But for E100 and E265, the mechanical performances are hardly influenced by the MAPE concentrations after the first improvement in stiffness and strength at MAPE/clay ratio of 1:1, adding more MAPE will not improve them any further. Since E100 has a much higher molecular weight than the neat HDPE, the improvements observed could come from either a better dispersion of clay or the reinforcing effect of E100 itself. For E265 however, its MFI is comparable to the matrix, so the exfoliation of clay can be credited to the flexural performances improvement.
Nanocomposite with E265/clay ratio of 9:1 is also prepared to see whether further increasing content of MAPE has positive effect on clay exfoliation. The effect of rotation speed on the flexural properties of the nanocomposites with E265/clay ratio of 1 and 9 is tested. As shown in Figure 6, both flexural modulus and strength are higher for the nanocomposites processed at 30 rpm than that for the nanocomposites processed at 60 rpm when the E265/clay ratio is only 1. While at E265/clay ratio of 9, the nanocomposite processed at 60 rpm show higher flexural properties than its counterpart processed at 30 rpm. Furthermore, the composites processed at 30 rpm show larger standard deviation especially at low E265/clay ratio. These results indicate that E265/clay ratio of 1 is not enough to achieve good dispersion of clay; and rotation speed of 60 rpm is more efficient than 30 rpm to achieve homogeneous materials.
Figure 6. Flexural properties of HDPE/E265/clay (2 wt%) nanocomposites with increasing E265/clay ratios: (A) flexural modulus; (B) flexural strength.
3.1.4 Influence of clay loading on flexural properties of nanocomposites with E265/clay ratio of 1:1
The flexural properties of nanoclay composites with different MAPE/clay ratio does not reveal whether a higher MAPE content is positive for the mechanical properties of the nanocomposites. As low MAPE content as possible is preferred from an economical point of view. Thus in the following trials with increasing clay load the MAPE/clay ratio 1:1 was selected. Figure 7 shows the dependence of flexural properties of the nanocomposites on clay loadings. The flexural strength and stiffness of the nanocomposites increases significantly only at the highest clay loading, 10 wt% of clay. The flexural property values of nanocomposites processed with the rotation speed of 30 rpm show bigger
1586,6
1660,2
1570,5
1277,6
1590,0 1575,0 1576,9
1583,3
1200 1300 1400 1500 1600 1700 1800
0 1 2 3 4 5 6 7 8 9 10
Flexural Modulus (MPa)
Mass of MAPE/Mass of clay ( )
30 RPM 60 RPM
38,17 39,80
38,89
31,76
38,22 37,80 37,77
39,11
30 32 34 36 38 40 42
0 1 2 3 4 5 6 7 8 9 10
Maximal stress (MPa)
Mass of MAPE/Mass of clay ( )
30 RPM 60 RPM B
A
standard deviation than that of nanocomposites processed with the rotation speed of 60 rpm. It confirms that the rotation speed of 60 rpm is more effective to achieve homogeneous materials than 30 rpm.
Figure 7. Flexural properties of nanocomposites processed at the rotation speed of 30 rpm with different clay contents, flexural properties of nanocomposites reinforced with 2 wt% clay processed at the rotation speed of 60 rpm is listed for
comparison
B A
3.1.5 Influence of clay loading on tensile properties of nanocomposites with E265/clay ratio of 1:1
Figure 8 shows the dependence of tensile properties of the nanocomposites on clay loadings.
Figure 8. Dependence of tensile properties of nanocomposites processed at the rotation speed of 30 rpm with different clay contents
The reinforcing effect of clay is hardly shown, the elastic modulus of HDPE is slightly improved by 10 wt% clay, while the tensile strength only increases slightly by 2 wt% clay then decreases with increasing clay contents. The elongation at break of HDPE decreases with increasing clay contents.
These results confirm the fact that the HDPE-clay interface isn’t effectively strengthened since insufficient MAPE is incorporated at MAPE/clay ratio of only 1:1.
3.2 XRD Results
3.2.1 XRD results for nanoclay composites with different MAPE types and MAPE/clay weight ratios
Figure 9 shows the XRD results of HDPE/MAPE/clay nanocomposites with different types of MAPE and different MAPE/clay ratios.
Figure 9. XRD patterns of HDPE/MAPE/clay (2 wt%) with different MAPE/clay ratios, processed with the same conditions (60 rpm, 160°C and 12 min): (A) M603 used; (B) E100 used; (C) E265 used; (D) E265 used and with different rotation
speeds
The first remark on the three first XRD patterns is that the relative intensity of the characteristic diffraction peak of clay (3,11°, corresponding to interlayer distance of 2.834 nm) decreases when the MAPE concentration increases, and at the same time the peak shifts to a lower angle. This indicates an increase in interlayer distance of clay platelets. Furthermore, with an MAPE/clay ratio above 3, the diffraction peak is hardly discernible in E100 and E265 modified nanocomposites, while it is still visible for M603 modified nanocomposites. When the E265/clay ratio is up to 9, the characteristic peak almost disappeared, indicating that a good exfoliation of clay has been achieved.
0 1 2 3 4 5 6 7 8
Pure organoclay
HDPE_(0:1) E100/o-clay (2wt%) HDPE_(1:1) E100/o-clay (2wt%) HDPE_(3:1) E100/o-clay (2wt%) HDPE_(6:1) E100/o-clay (2wt%)
Intensity (a.u.)
2θ (o)
B
0 1 2 3 4 5 6 7 8
Intensity (a.u.)
Pure organoclay
HDPE_(0:1) M603/o-clay(2wt%) HDPE_(1:1) M603/o-clay(2wt%) HDPE_(3:1) M603/o-clay(2wt%) HDPE_(6:1) M603/o-clay(2wt%)
2 θ (o)
A
0 1 2 3 4 5 6 7 8
Pure organoclay
HDPE_(0:1) E265/o-clay (2wt%) HDPE_(1:1) E265/o-clay (2wt%) HDPE_(3:1) E265/o-clay (2wt%) HDPE_(6:1) E265/o-clay (2wt%) HDPE_(9:1) E265/o-clay (2wt%)
Intemsity (a.u.)
2θ (o)
C
0 1 2 3 4 5 6 7 8
Pure organoclay
HDPE_(0:1) E265/o-clay (2wt%) HDPE_(1:1)E265/o-clay (2wt%)_30 rpm HDPE_(1:1)E265/o-clay (2wt%)_60 rpm HDPE_(9:1)E265/o-clay (2wt%)_30 rpm HDPE_(9:1)E265/o-clay (2wt%)_60 rpm
Intensity (a.u.)
2θ (o)
D
When comparing the effect of rotation speed on the clay dispersion in HDPE/E265/clay nanocomposites, it is found that the peak of composite processed with 30 RPM is more apparent and showing higher relative intensity compared to the composites processed with 60 RPM. It confirms that the rotation speed of 60 rpm has more positive effect to improve the exfoliation and dispersion of clay than 30 rpm.
The fact that at 1:9 ratio, 30 and 60 RPM have almost the same XRD pattern can explain the similarities in flexural properties. But surprisingly, when the E265 MAPE concentration increases, the dispersion is getting better, but flexural properties will show an improvement only at 1:1, then stabilize and increase again at 1:9 ratio. The sample at 30 RPM with 1:1 ratio gives the best flexural performances for those series, but the dispersion was not optimal.
3.2.2 XRD results for nanocomposites with different clay loadings at E265/clay ratio of 1:1
The XRD patterns of the nanocomposites with 3 different clay contents are shown in Figure 10.
The characteristic diffraction peak is detectable in 2 wt% clay reinforced nanocomposites; and the relative intensity of the characteristic diffraction peak increases as clay content increases. It reveals that E265/clay ratio of 1 is not enough to achieve exfoliation of clay. The peak position of 10 wt% clay reinforced nanocomposites appears at lower 2θ angle compared to its 6 wt% counterpart, possibly indicating the positive effect of shear force on the dispersion of clay. When more clay is added, the melt viscosity of the composite during compounding increases, leading to higher shear force, which is propitious to enlarge the interlayer distance.
Figure 10. XRD patterns of HDPE/E265/clay nanocomposites at E265/clay ratio of 1:1 and processed with rotation speed of 30 rpm, with clay concentration of 2, 6 and 10wt%, respectively
0 1 2 3 4 5 6 7 8 9
0 10 20 30 40 50 60 70 80 90
2θ (o )
Intemsity (a.u.)
2wt% clay reinforced composit 6wt% clay reinforced composit 10 wt% clay reinforced composit
3.3 Rheological properties
Rheological behavior of complex materials is very sensitive to the structure of materials. Small- amplitude oscillatory shear test is especially widely used to indicate the microstructure of hybrid materials, due to the maintenance of the original structure of the materials during the test. Figure 11 shows combination effects of MAPE type, its molecular weight (compared with HDPE matrix) and the MA group content, on the dispersion/exfoliation of nanoclay and the microstructure of the resultant nanocomposites. When no MAPE is added, the interface between nanoclay and HDPE is poor, resulting in very similar values of elastic modulous (G’) and complex viscosity (η*) of the nanocomposite and neat HDPE. G’ increases with increasing MAPE/clay ratio and when the ratio is up to 6. G’ of the nanocomposites modified with E100 (C series) and E265 (D series) show clearly platform and η* shows infinitely increase with decreasing frequency in the low frequency region, which reflect the quasi-solid behavior of the nanocomposite melts and strongly indicate that the nanoclay has been well exfoliated and dispersed, and the interfacial adhesion between nanoclay and HDPE has been greatly improved. Therefore, a 3-D percolation network, as illustrated in Figure 12, is considered to form within the nanocomposites modified by E100 or E265 at its ratio to clay of 6.
Figure 12. Elastic modulus and complex viscosity of neat HDPE (P8), 2 wt% clay reinforced HDEP (B8) and M603 modified nanocomposites with 2 wt% clay (A series), E100 modified nanocomposites with 2 wt% clay (C series) and E265 modified nanocomposites with 2 wt% clay (D series), where 1, 2, 3 indicates the compatibilizer/clay weight ratio of 1:1, 3:1 and 6:1
Figure 11. Schematic illustration of the physical jamming of clay particles and the restricted motion of polymer chains within the 3D percolated clay network
10-1 100 101 102
100 101 102 103 104 105
G' (Pa)
ω (rad/s)
P8 B8C A1 A2) A3 C1 C2 C3 D1 D2 D3
10-1 100 101 102
102 103 104 105
η* (Pas)
ω (rad/s)
P8 B8 A1 A2 A3 C1 C2 C3 D1 D2 D3
B A
M603 (A series) has the lowest molecular weight; therefore, even at weight ratio of 6, the quasi- solid behavior is not as strong as observed in the other two MAPE modified nanocomposites. Even E100 has higher molecular weight compared with E265, but the G’ and η* (at weight ratio of 6) are lower than those of E265, which may indicate that E265 has higher MA group content. Therefore, E265 shows better effect to improve the dispersion/exfoliation of nanoclay and the interfacial adhesion.
Figure 13 show the effect of E265/clay weight ratio on the rheological behavior of the nanocomposites, which indicate better exfoliated/dispersed nanoclay and stronger interface at higher MAPE content.
Figure 13. Elastic modulus and complex viscosity of neat HDPE (P8), 2 wt% clay reinforced composites (B8) and E265 modified 2 wt% clay reinforced nanocomposites (D series), which were processed at 60 rpm, and 1, 2, 3, 4 indicates the
E265/clay ratio of 1:1, 3:1, 6:1and 9:1
Figure 14 shows that when the HDPE-clay interface is strengthened by MAPE, the dispersion of clay can be further improved by higher rotation speed for composites processing.
Figure 14. Elastic modulus and complex viscosity of 2 wt% clay reinforced nanocomposites modified by different contents of E265, D3 with E265/clay ratio of 6:1 and processed at rotation speed of 60 rpm, D4 with E265/clay ratio of 9:1 and processed at rotation speed of 60 rpm and D5 with E265/clay ratio of 9:1 and processed at rotation speed of 30
rpm.
10-1 100 101 102
10-1 100 101 102 103 104 105
G' (Pa)
ω (rad/s)
P8 B8 D1 D2 D3 D4
10-1 100 101 102
102 103 104 105 106
η* (Pas)
ω (rad/s)
P8 B8 D1 D2 D3 D4
A B
10-1 100 101 102
103 104 105
G' (Pa)
ω (rad/s)
D3 D4 D5
10-1 100 101 102
102 103 104 105 106
η* (Pas)
ω (rad/s)
D3 D4 D5
A B
3.4 Water absorption
Water absorption is not expected to decrease when clay is added to HDPE since this is a water resistant matrix. However, it is important to know how the clay &/or MAPE influence the water absorption of neat HDPE, because the final product is a hybrid composite containing wood fibers and clay as well as compatibilizers and many other additives. The combined effects of the constituents are to be well understood when the individual effect on the matrix is fully understood.
Figure 15 shows the effect of clay and MAPE on the water absorption of neat HDPE, respectively. Although both MAPE (E265) and clay are polar compared to HDPE, the addition of a small amount of clay (2 wt%) increases the water absorption more significantly than a larger amount of E265 (around 18 wt%), when no MAPE is used to improve the HDPE-clay interface. However, it is worth noting that water absorption values for the HDPE/clay (2 wt%) and HDPE/E265 (18 wt%) are still very low, which are within 0.2%.
0 200 400 600 800 1000
0.0 0.1 0.2 0.3
Water uptake (%)
Time (h) HDPE
HDPE/E265 (18 wt%) HDPE/clay (2wt%)
Figure 15. Influence of clay and MAPE (E265) on the water absorption of HDPE, respectively
0 200 400 600 800 1000
0.0 0.1 0.2 0.3 0.4 0.5
Water uptake (%)
Time (h) HDPE HDPE/2wt% clay
HDPE/(1:1)MAPE/2wt% clay HDPE/(3:1)MAPE/2wt% clay HDPE/(6:1)MAPE/2wt% clay HDPE/(9:1)MAPE/2wt% clay
Figure 16. Water uptake of 2 wt% clay reinforced HDPE nanocomposites with different E265/clay ratio
Figure 16 shows the effect of MAPE (E265)/clay ratio on water uptake of the nanocomposites reinforced with 2 wt% clay. The water absorption values for the nanocomposites at the E265/clay ratio of 3 and 9 are higher than that for the nanocomposites without E265, while the water absorptions for their counterpart at the ratio of 1 and 6 are close to or slightly lower than the one without E265. It is suspected that low content of MAPE is not enough to achieve good HDPE-clay interface while excessive MAPE (high content) disperses in bulk HDPE resulting in additional polar points to absorb water. Both cases lead to higher amount of absorbed water in the resultant nanocomposites.
Figure 17. Water absorption of 2 wt% and 6 wt% clay reinforced HDPE nanocomposites with E265/clay ratio of 1 and 9, respectively, processed with the rotation speed of 30 rpm (left) and 60 rpm (right)
As shown in Figure 17, irrelevant to rotation speed and E265/clay ratio, 2 wt% clay reinforced HDPE nanocomposites posses lowest water absorption among prepared nanocomposites. It is attributed to combined negative effect of clay and MAPE on the water resistance of HDPE, since the amount of MAPE increases as clay content increases. The negative effect of either insufficient or excessive MAPE is also confirmed. Again it is noted that although the water absorption values in all the nanocomposites are higher than that of neat HDPE, they are still low, which are lower than 0.5%.
The positive effect of clay together with appropriate amount of MAPE is expected to show in the final composites containing at least 50 wt% of wood.
0 200 400 600 800 1000
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Water uptake (%)
Time (h)
HDPE HDPE/2wt% clay
HDPE/(1:1)MAPE/2wt% clay (30rpm) HDPE/(1:1)MAPE/6wt% clay (30rpm) HDPE/(9:1)MAPE/2wt% clay (30rpm) HDPE/(9:1)MAPE/6wt% clay (30rpm)
0 200 400 600 800 1000
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Water uptake (%)
Time (h)
HDPE HDPE/2wt% clay HDPE(1:1)MAPE/2wt% clay(60rpm) HDPE(1:1)MAPE/6wt% clay(60rpm) HDPE(9:1)MAPE/2wt% clay(60rpm) HDPE(9:1)MAPE/6wt% clay(60rpm)
A B
3.5 Thermal stability
Figure 18 (1) and (2) show the TGA and DTG curves of selected materials in the present study. It is clear that MAPE alone cannot increase the thermal stability of HDPE, which is proved by the fact that no improvement in the initial thermal decomposition temperature and the temperature corresponding to the highest thermal decomposition speed is shown in HDPE/E265 (18 wt%) blend compared to neat HDPE. However, clay can positively improve the thermal stability of HDPE only when appropriate amount of MAPE is used, which guarantees good exfoliation of clay and strong clay-matrix interface. It is proved by the fact that the degradation temperatures of the composite with 2 wt% clay and without MAPE are just slightly higher than those of HDPE. The initial thermal decomposition of HDPE apparently shift to higher temperature by 2 wt% clay when the E265/clay ratio is 9:1, which is increased significantly with increasing clay loadings (at E265/clay ratio of 9:1).
Furthermore, the DTG curve of 6 wt% clay reinforced nanocomposites at E265/clay ratio of 9:1 is much smoother than the others. The leftover samples after burning are visually inspected; the nanocomposite with the highest clay loading and highest E265 content (see C in Figure 19) remains as a whole piece of char while the other become very powdery (see A and B in Figure 19). These results indicate that the thermal stability of HDPE can be significantly improved by a small amount of clay provided that the clay is well dispersed/exfoliated and strong interface between clay and HDPE is formed.
Figure 18. TGA (1) and DTG (2) of different materials prepared in this study
Figure 19. The leftover of samples after TGA tests: (A) neat HDPE; (B) HDPE/2wt% clay; (C) HDPE/E265/6wt.%clay with E265/clay ratio of 9:1
A B C
0 100 200 300 400 500
0 10 20 30 40 50 60 70 80 90 100
Weight (%)
Temperature (oC) HDPE_P8
HDPE/2wt% clay_B8
HDPE(9:1)MAPE/2wt% clay(60rpm)_D4 HDPE(9:1)MAPE/6wt% clay(60rpm) HDPE/(9:1)MAPE/0wt% clay_F4
(1)
0 100 200 300 400 500
0.0 0.5 1.0 1.5 2.0 2.5 3.0
HDPE_P8 HDPE/2wt% clay_B8 HDPE(9:1)MAPE/2wt% clay(60rpm)_D4 HDPE(9:1)MAPE/6wt% clay(60rpm) HDPE/(9:1)MAPE/0wt% clay_F4
Deriv. weight (%)
Temperature (oC)
(2)
4 Conclusions and future work
The type of compatibilizer for HDPE-clay, MAPE, and its weight ratio to clay, as well as the processing conditions in melt compounding, have been optimized to improve the mechanical and barrier properties of HDPE reinforced by nanoclay.
Increased clay loading at low MAPE/nanoclay ratio did not lead to improved mechanical properties. The materials prepared with high clay loading and high MAPE content have not been mechanically tested. However the complementary characterizations i.e. XRD and melt rheology indicate that the properties have been improved.
MAPE show strongly positive effect to improve the exfoliation &/or dispersion of clay and to strengthen the HDPE-clay interface. The appropriate processing conditions, especially rotation speed, also have positive effect on clay exfoliation &/or dispersion, which is magnified when MAPE is functioning. The spatial percolation network of clay and strengthened HDPE-clay interface lead to better mechanical properties and significantly improved thermal stability of neat HDPE reinforced even by a small amount of clay. Insufficient amount of MAPE results in weak HDPE-clay interface and incomplete exfoliation of clay. Therefore, the mechanical properties of the resultant nanocomposites are slightly improved or even decreased compared to neat HDPE; and the thermal properties are appreciably increased. Excessive MAPE disperses in the bulk HDPE besides acting at the HDPE-clay interface, which also have negative effect on the mechanical and barrier properties of HDPE. The addition of MAPE &/or clay decrease the water resistance of HDPE because they are both polar compared to HDPE. Moreover, both insufficient and excessive MAPE are likely to further decrease the water resistance of HDPE.
In the future, more mechanical property measurements will be carried out to analyze the interface evolution with MAPE and processing conditions. Real fire test with ignition by a flame will be performed, and the effect of clay together with MAPE on the water resistance of WPCs will be systematically studied.
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6 Table of Figures
Figures:
Figure 1. Schematic representation of different dispersion states of nanoclay inside of polymers [17] ... 7 Figure 2. Experimental setup for 3-point-bending test ... 10 Figure 3 Flexural properties of neat HDPE processed with different conditions: (A) flexural modulus of samples prepared at 130°C; (B) flexural strength of samples prepared at 130°C; (C) flexural modulus of samples
prepared at 160°C; (D) flexural strength of samples prepared at 160°C. ... 14 Figure 4. Flexural properties of HDPE/2 wt% clay composites: (A) flexural modulus of samples prepared at 130°C; (B) flexural strength of samples prepared at 130°C; (C) flexural modulus of samples prepared at 160°C;
(D) flexural strength of samples prepared at 160°C. ... 15 Figure 5. Flexural properties of HDPE/MAPE/clay (2 wt%) nanocomposites: (A) flexural modulus; (B) flexural strength ... 16 Figure 6. Flexural properties of HDPE/E265/clay (2 wt%) nanocomposites with increasing E265/clay ratios: (A) flexural modulus; (B) flexural strength. ... 18 Figure 7. Flexural properties of nanocomposites processed at the rotation speed of 30 rpm with different clay contents, flexural properties of nanocomposites reinforced with 2 wt% clay processed at the rotation speed of 60 rpm is listed for comparison ... 19 Figure 8. Dependence of tensile properties of nanocomposites processed at the rotation speed of 30 rpm with different clay contents ... 20 Figure 9. XRD patterns of HDPE/MAPE/clay (2 wt%) with different MAPE/clay ratios, processed with the same conditions (60 rpm, 160°C and 12 min): (A) M603 used; (B) E100 used; (C) E265 used; (D) E265 used and with different rotation speeds ... 21 Figure 10. XRD patterns of HDPE/E265/clay nanocomposites at E265/clay ratio of 1:1 and processed with rotation speed of 30 rpm, with clay concentration of 2, 6 and 10wt%, respectively ... 22 Figure 11. Elastic modulus and complex viscosity of neat HDPE (P8), 2 wt% clay reinforced HDEP (B8) and M603 modified nanocomposites with 2 wt% clay (A series), E100 modified nanocomposites with 2 wt% clay (C series) and E265 modified nanocomposites with 2 wt% clay (D series), where 1, 2, 3 indicates the compatibilizer/clay weight ratio of 1:1, 3:1 and 6:1 ... 23 Figure 12. Schematic illustration of the physical jamming of clay particles and the restricted motion of polymer chains within the 3D percolated clay network ... 23 Figure 13. Elastic modulus and complex viscosity of neat HDPE (P8), 2 wt% clay reinforced composites (B8) and E265 modified 2 wt% clay reinforced nanocomposites (D series), which were processed at 60 rpm, and 1, 2, 3, 4 indicates the E265/clay ratio of 1:1, 3:1, 6:1and 9:1 ... 24 Figure 14. Elastic modulus and complex viscosity of 2 wt% clay reinforced nanocomposites modified by
different contents of E265, D3 with E265/clay ratio of 6:1 and processed at rotation speed of 60 rpm, D4 with E265/clay ratio of 9:1 and processed at rotation speed of 60 rpm and D5 with E265/clay ratio of 9:1 and processed at rotation speed of 30 rpm. ... 24 Figure 15. Influence of clay and MAPE (E265) on the water absorption of HDPE, respectively ... 25 Figure 16. Water uptake of 2 wt% clay reinforced HDPE nanocomposites with different E265/clay ratio ... 25 Figure 17. Water absorption of 2 wt% and 6 wt% clay reinforced HDPE nanocomposites with E265/clay ratio of 1 and 9, respectively, processed with the rotation speed of 30 rpm (left) and 60 rpm (right) ... 26 Figure 18. TGA (1) and DTG (2) of different materials prepared in this study ... 27 Figure 19. The leftover of samples after TGA tests: (A) neat HDPE; (B) HDPE/2wt% clay; (C)
HDPE/E265/6wt.%clay with E265/clay ratio of 9:1 ... 27
Tables :
Table 1. Different parameters for processing neat HDPE ... 13 Table 2. Different parameters for processing HDPE/clay (2 wt%) composites ... 14 Table 3. MAPE/clay ratios and rotation speeds for processing HDPE/MAPE/clay (2 wt%) nanocomposites, the processing temperature and time are fixed at 160°C and 12 min. ... 16
7 Appendix
7.1 Sample code name
Composition Processing parameters MAPE
type
MAPE/Clay ratio
Clay
content HDPE MAPE Clay Rotation
speed Temperature Time
P8 - - 0 25 0 0 60 160 12
B8 - 0 2 24,5 0 0,5 60 160 12
A1 M603 1 2 24 0,5 0,5 60 160 12
A2 M603 3 2 23 1,5 0,5 60 160 12
A3 M603 6 2 21,5 3 0,5 60 160 12
C1 E100 1 2 24 0,5 0,5 60 160 12
C2 E100 3 2 23 1,5 0,5 60 160 12
C3 E100 6 2 21,5 3 0,5 60 160 12
D1 E265 1 2 24 0,5 0,5 60 160 12
D2 E265 3 2 23 1,5 0,5 60 160 12
D3 E265 6 2 21,5 3 0,5 60 160 12
D4 E265 9 2 20 4,5 0,5 60 160 12
D5 E265 9 2 20 4,5 0,5 30 160 12
F1 E265 1 6 22 1,5 1,5 60 160 12
F2 E265 9 6 10 13,5 1,5 60 160 12
F3 E265 9 6 10 13,5 1,5 30 160 12
F4 E265 - 0 20 4,5 0 60 160 12
