Linearity and Non Linearity of Mechanical Properties in

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Linearity and Non Linearity of Mechanical Properties in Powered By Docstoc
                 OF VIRGIN AND RECYCLED HDPE’s

             Paul Miller, Industrial Research Institute, Swinburne University of Technology
                               Adjunct Prof. Edward Kosior, Visy Plastics
      Assoc. Prof. Syed Masood, Industrial Research Institute, Swinburne University of Technology
          Dr. Pio Iovenitti, Industrial Research Institute, Swinburne University of Technology

                                                                A selection of blends of virgin and recyclate HDPE have
This paper investigates the behaviour of blends of              been investigated to satisfy the moulding and mechanical
Recycled Milk Bottle Resin (R-MBR) with Injection               property performance requirements for household items
Moulding or Film Blowing grade HDPEs (IM-HDPE or                [8]. The criteria for such a blend is that the mixture must
FB-HDPE). This was done by measuring changes in                 not show a significant reduction in impact or tensile
mechanical and rheological properties as a function of          strength as compared to the virgin material currently in
blend composition and compounding intensity. There              use and should contain the maximum possible amount of
were three categories of compounding: bag mixing (BM),          recyclate [8].
single pass single screw extrusion (EBx1) and double pass
single screw extrusion (EBx2).                                  Some HDPE detergent bottles incorporate recycled HDPE
                                                                as an intermediate layer encapsulated between two layers
The results were examined for linear and non-linear             of virgin PE [8]. The outer and inner layers are virgin
trends, and relationships between morphology,                   polymers that are lightly pigmented and have good
mechanical properties and molecular weights were                environmental stress crack resistance (ESCR).
                                                                The introduction of new high performance PE resins
                     Introduction                               recently can compensate for the property deficiencies in
                                                                recycled resins [8]. This involves blending the recycled
Visy Plastics is a recycling company principally dealing        HDPE with a super-ESCR HDPE to boost the properties
with post consumer bottle recycling. The recycled milk          of the HDPE homopolymer recyclate to those of a HDPE
bottle resin (R-MBR) that results from the recycling of         copolymer, thus allowing the production of monolayer
these bottles is ideal for blow moulding applications, but      detergent bottles. Such blends of recycled HDPE together
is less appropriate for use in injection moulding and some      with recyclate tolerant virgin resins (such as Marlex™
extrusion applications. This is because the R-MBR is too        C590) have ESCR, dynamic vertical load and drop impact
viscous for injection moulding, and has a too low               test performance better or equal to the performance of the
viscosity for some extrusion applications. This project         Household and Industrial Chemical (HIC) recyclate
will try to correct these limitations by blending with a pair   grades.
of virgin HDPE’s of high and low melt viscosity. The
blending of these materials should improve the flow             Recycled HDPE copolymer (from post-consumer
problems associated with the use of R-MBR.                      detergent bottles) has been used in corrugated drainage
                                                                pipe at levels of up to 75wt% and found to pass ESCR
                                                                performance criteria [8].
                  Literature Review
The body of literature in blends of recycled and virgin                             Methodology
HDPE’s is limited. However there is a large body of
research available in the field of PE & polyolefin blends,      The R-MBR used was a Visy Plastics grade called H1
such as,                                                        recyclate. The virgin materials used were Kemcor’s
- virgin binary blends of HDPE, LDPE & LLDPE [1]                GA7260H (IM-HDPE) and GM4755F (FB-HDPE).
- recycled HDPE/LDPE [2,3]
- rubber modified recycled HDPE [4]                             Different levels of compounding intensity were used.
                                                                Bag-mixed means that the granular components were
There is even more work available in PE/PP blends:              manually mixed and then injection moulded into test
- virgin HDPE/PP [5]                                            specimens. Extrusion blending was done at on a Telford
- virgin PP/LLDPE [6]                                           Smith 28mm diameter screw, with a compression ratio of
- ternary blends of r-PP/r-HDPE/v-LLDPE [7]                     4:1, an L/D ratio of about 20:1, and a Union Carbide
                                                                mixing head in the metering section.
                                                               composition range. The increase is probably due to
Injection moulding was done with a 30 tonne clamp force,       crosslinking.      The EBx2 series displays non-linear
60g shot weight Battenfeld machine. Tensile testing was        behaviour. In this non-linear region the error bars overlap,
done on the Zwick/Z010 tensile test machine in                 casting doubt on the trend in the results, but subsequent
accordance with AS 1145. MFI’s were tested on the              tests appear to have confirmed the behaviour.
Ceast P/N6941.000 MFI Tester, and impact testing was
done on the Ceast Resil 25 Impact Tester.          SEM         The size of the error bars in the modulus tests is probably
micrographs were obtained using the Jeol JSM-35                related to the poor mixing resulting from the large
Scanning Electron Microscope.                                  disparity in MFI’s, producing inhomogeneous material
                                                               and unpredictable results.
               Results and Discussion
                                                               The yield stress tests (Figure 4) follow a similar trend
Blend Rheology                                                 display to the modulus tests.
Figures 1 & 2 represent the behaviour of unstabilized          The impact test results for the IM blends shown in Figure
blends of R-MBR with FB-HDPE/IM-HDPE. Most                     5 strongly reflect the influence of the more brittle IM
obvious is the dramatic decrease in post-extrusion MFI.        grade HDPE. Since the IM blends are less influenced by
This can be attributed to crosslinking, which usually          crosslinking, the closeness of the impact test curves seems
affects PE’s during extrusion. Also noticeable is that the     reasonable. Also, the effect of crosslinking becomes
MFI reduction principally occurs during the first              more pronounced as the fraction of R-MBR is increased,
extrusion. Another point is that the FB material is more       and this is represented by the increased dispersion
prone to crosslinking than the IM material. Figure 1           between the curves at these R-MBR levels.
shows that changes in MFI (ie, levels of crosslinking)
only become significant in the IM blends when substantial       It should be noted that the data for each compounding
amounts of the R-MBR material are present. Figure 2            type show a large degree of overlap, suggesting that
shows that the blend with FB homopolymer is more               composition is much more important than compounding
susceptible to crosslinking and the R-MBR present              in determining impact strength.
doesn’t strongly influence this. There are three possible
explanations for this behaviour in FB blends. The first is
                                                               FB-HDPE & R-MBR HDPE BLENDS
the FB material contains less stabilizer, the second is that
                                                               Modulus results for FB blends are shown in Figure 6. The
they have a higher crosslink density, or three, they are no
                                                               BM blend displays poorly mixed behaviour because the
more susceptible than the IM blends, but the crosslinks
                                                               values of the modulus are below linear additivity. As
are more effective in decreasing the MFI. For instance,
                                                               would be expected, the crosslinking has a significant
because the IM molecules are smaller than the FB
                                                               positive effect on the modulus of the blend. However, a
molecules, a crosslink between two IM molecules will
                                                               feature that would not be expected is that the EBx2 series
increase the average molecular weight by less than a
                                                               has a modulus significantly higher than the EBx1 series.
crosslink between two FB molecules. Thus, the FB
                                                               This does not correlate with changes in MFI, which were
crosslinks are more effective in decreasing the MFI.
                                                               minimal (Figure 2). This increase may be attributed to
                                                               phase and morphological changes, perhaps due to changes
Mechanical Properties                                          in crystallinity. The moduli at either end of the
IM-HDPE & R-MBR HDPE BLENDS                                    composition scale reinforce this suggestion because they
To a certain extent, the elastic modulus of the IM blend,      decrease back towards the EBx1 moduli values.
(Figure 3) reflects the crosslinking behaviour that was
noticed in the MFI plots. The BM series shows a negative       The yield stress (Figure 7) results have more in common
deviation from linear additivity, indicating that under the    with the MFI results. The BM blend displays a slightly
minimal blending conducted the BM series is not                negative change with composition while the EBx1 and
completely mixed. The extent of this negative behaviour        EBx2 series’ display similar trends, with significant
is significantly greater than that of the FB blends. This is   increases of about 15%, probably attributable to
because the ratio of MFI’s for the FB blends (≅ 17.5) are      crosslinking.
about half of that for the IM blends (≅ 35). A large
mismatch of viscosities increases the difficulty of            The impact strength data displays a wide spread of values.
compounding these materials [9]. The further away from         Each one of these data points represents the average taken
unity, the greater the difficulty in obtaining a fine          over about twelve separate tests, but data spread is still
dispersion.[10].                                               large. The trend for all three curves is that impact
                                                               strength decreases with R-MBR
The EBx1 series displays fairly linear behaviour with
modulus values increased by around 10% for most of the
In general the re-extruded blends display a slightly lower      particles. The particles act as nucleants for heterogeneous
impact performance, attributable to crosslinking                nucleation, and the finer distribution will produce a finer
embrittlement.                                                  crystallite size, resulting in a higher modulus.

Microstructure                                                                        Conclusion
Figure 9 displays surface features that are quite common        Rheological properties of these blends appear to be
for most of the IM-HDPE blends. It can be seen from the         linearly predictable, but they are severely influenced by
impact test data that the IM-HDPE blends are                    the compounding intensity involved. This compounding
significantly more brittle than the FB-HDPE blends, and         produces a decrease in MFI attributable to crosslinking
this is reinforced by the differences in microstructures of     degradation, which has very strong consequences for
the blends. Toughness is dictated by a materials ability to     modulus, yield stress, MFI and impact strength. With
deform and absorb energy. The IM blends do not display          crosslinking, all the materials become stiffer and more
this behaviour. There is a band of drawn material from          brittle.   Large scale compositional trends were also
the bottom left to the top right of the image, whilst on        observed, with the IM blends becoming stiffer and
either side of this the material is less drawn, and more        tougher with the inclusion of R-MBR, while the FB
brittle. The difference is that in the former region, crazing   blends become brittle and more flexible with the inclusion
progressed sufficiently to generate substantial void            of R-MBR.
coalescence, whilst in the latter region, the material was
simply not plastic enough for large scale fibrillation, and     Despite the presence of large scale trends, incremental
the material fractured before extensive void coalescence        changes in composition produces relatively unpredictable
could occur.                                                    changes for modulus and yield stress in compounded IM
                                                                blends. The modulus results suggest that both IM & FB
FB-HDPE                                                         blends are poorly mixed under the bag-mixing regime,
Figure 10 displays surface features common to the FB            which could be a significant factor in the unpredictability.
blends. Most prominent is the drawn nature of the               A higher MFI ratio is probably responsible for poorer
fracture surface, which is responsible for the high             mixing in the IM blends, and thus increased non-linearity
toughness of the blends.                                        and data spread.

The small anomalies in the pits at the bottom of the            Unlike modulus and yield stress for IM blends, impact
fracture structure were analyzed with Energy Dispersive         strength follows a smooth predictable curve. FB blends,
X-Ray Analysis (EDXA) to obtain information on the              while being much tougher, are much less predictable
elemental makeup of these anomalies. According to the           because of the large data scatter involved. MFI changes
results, they are mostly Al, Si and Ca compounds, with          show that the FB blends are more affected by
small amounts of Ti, K and Na. They are likely to all be        crosslinking. Anti-oxidant stabilization may reduce the
additives used in the manufacture of various bottles            data scatter and produce more predictable impact results
including milk bottles. In particular, the Ti is probably       for FB blends.
from titanium dioxide, which is a pigment. This is
usually coated with aluminium silica clay to aid                There are significant quantities of additives present in the
dispersion, with another proprietary coating on top. The        recycled resins, which play a significant role in the
Ca is probably there as a result of CaCO3, which is used        fracture behaviour of FB blends by acting as stress
as filler to reduce cost, but also as a dispersion agent.       concentration sites for crack propagation and fracture.

The structure of the fracture surface indicates that these                       Acknowledgments
particles play a critical role in the fracture behaviour. It
appears that these points are stress concentrations and act     Thanks to:
as points that fracture easily after stress is applied,                 - IRIS and Visy Plastics, for their scholarship,
providing an easier path for crack propagation. These                   supervision, materials and facilities.
particles are not readily observable in the IM blends,                  - Kangan Batman College of TAFE, for allowing
probably because the matrix is already so brittle that the              use of their extrusion compounder.
particles do not further embrittle the blend.                           - Kemcor, for the supply of virgin HDPE’s.
                                                                        - Materials Engineering Department at Monash
These particles may also be significant in the modulus                  University for allowing the use of their impact
tests. Since MFI data show that increased modulus for                   test notcher.
the EBx2 series largely cannot be attributed to
crosslinking.   A possible mechanism is that the
compounding results in a finer distribution of the impurity
  Cho, Lee, Hwang, Lee & Choe; Rheological and                                                                                             ln(MFI) vs wt% R-MBR
Mechanical properties in Polyethylene Blends, Polymer                                                                               for blends of R-MBR and FB-HDPE
Engineering and Science, December 1998, Vol.38, No.                                                           3.5
12, pp 1969-1975
2                                                                                                             2.5
  Rueda, Balta-Calleja, Viksne, Malers; Study of Blends                                                                                Bag-Mixed

based on Recycled Polyethylene Wastes; Part I Variation                                                                                Extruded x 1

                                                                                    ln(MFI) 190oC/5kg
of Mechanical Properties with Composition, Journal of                                                                                  Extruded x 2
Materials Science, Volume 29, 1994, pp 1109-1114                                                              0.5
  Rueda, Malers, Viksne, Balta-Calleja; Study of Blends
based on Recycled Polyethylene Wastes: Part II, Thermal
Behaviour and Degree of Molecular Mixing, Journal of                                                    -1.5
Materials Science, Volume 31, 1994, pp 3915-3920.
4                                                                                                       -2.5
  Tall, Karlsson, Albertsson; Improvements in the
properties of Mechanically Recycled Thermoplastics,                                                                     0               20            40       60    80     100
                                                                                                                                                       wt% R-MBR
Polymers and Polymer Composites, Vol. 6, No. 5, 1998,
pp 261-267
5                                                                                 Figure 2 MFI for RMBR/FBHDPE blends
  Niebergall, Bohse, Seidler, Grellman, Schurmannn;
Relationship of Fracture behaviour and Morphology in
Polyolefin Blends, Polymer Engineering and Science,                                                                                Effect of Processing on Modulus for
June 1999, Vol. 39, No. 6, pp1109-1118
6                                                                                                                                     blends of R-MBR and IM-HDPE
  Dumoulin, Carreau, Utracki; Rheological properties of
Linear low Density PE/PP blends: Part 2 Solid State
Behaviour, Polymer Engineering and Science, November                                                                  1300            Bag-Mixed
1987, Vol 27, No 20                                                                                                   1200
                                                                                                                                      Extruded x 1
                                                                                              Modulus (MPa)

  Kosior, Forrest, Masood, Iovenitti; The Influence of PP                                                                             Extruded x 2
content on the Properties of post consumer recycled                                                                   1100
HDPE, ANTEC 99                                                                                                        1000
  Scheirs, Polymer Recycling; Science Technology and
Applications, 1998
  Scott & Joung, Viscosity ratio Effects in the                                                                       800
Compounding of Low Viscosity, Immiscible Fluids into
Polymeric Matrices, Polymer Engineering and Science,                                                                           0          20          40      60    80    100
June 1996, Vol. 36, No. 12.                                                                                                                            wt%R-MBR
   Kukaleva, General Studies on thermodynamics of
Polymer Melts and Rheology, 1998                                                  Figure 3 Modulus for RMBR/IMHDPE blends

                                                                                                                                         Effect of Processing on
                                                                                                                                         Yield Stress for blends
                                           ln(MFI) vs wt% R-MBR
                                                                                                                                         of R-MBR and IM-HDPE
                                     for blends of R-MBR and IM-HDPE
                                                             Bag-Mixed                                                32
                          2.5                                                                                                        Extruded x 1
                                                                                                 Yield Stress (MPa)

                                                             Extruded x 1
                                                                                                                                     Extruded x 2
                          1.5                                Extruded x 2                                             28

                          0.5                                                                                         24

                          -0.5                                                                                        20

                          -1.5                                                                                        16
                                                                                                                           0            20            40      60    80    100
                          -2.5                                                                                                                        wt% R-MBR
                                 0        20     40     60      80          100
                                                 wt% R-MBR
                                                                                  Figure 4 Yield Stress for RMBR/IMHDPE blends
Figure 1 MFI for RMBR/IMHDPE blends
                                                              Effect of Processing on                                                                     Effect of Processing on
                                                            Impact Strength for blends                                                                  Impact strength for blends
                                                                                                                                                         of R-MBR and FB-HDPE
                                                              of R-MBR and IM-HDPE
                                                                                                                                              900                               Bag-Mixed
                                                            Bag-Mixed                                                                         800                               Extruded x 1
                   Impact Strength (J/m)

                                                                                                                     Impact Strength (J/m)
                                                            Extruded x 1                                                                      700                               Extruded x 2

                                                            Extruded x 2                                                                      600
                                                50                                                                                            200
                                                 0                                                                                              0
                                                     0         20          40      60       80           100                                        0      20     40     60      80            100
                                                                            wt%R-MBR                                                                               wt%R-MBR

Figure 5 Impact Strength for RMBR/IMHDPE blends                                                                    Figure 8 Impact Strength for RMBR/FBHDPE blends

                                                            Effect of processing on Modulus
                                                           for blends of R-MBR and FB-HDPE


                      1500                                                                       Bag-Mixed

                                                                                                 Extruded x 1
                                                                                                 Extruded x 2
  Modulus (MPa)






                                                0             20           40          60    80              100
                                                                                                                   Scale 9mm=50 m
                                                                                wt% R-MBR

Figure 6 Modulus for RMBR/FBHDPE blends                                                                            Figure 9 EBx1, 60%RMBR:40%IMHD, 16kV, x400

                                                         Effect of Processing on Yield Stress
                                                          for blends of R-MBR and FB-HDPE


                                                                                            Extruded x 1
        Yield Stress (MPa)

                                           45                                               Extruded x 2





                                                                                                                   Scale: 9mm=50 m
                                                0             20           40          60   80               100
                                                                                wt% R-MBR
                                                                                                                   Figure 10 EBx1, 60%RMBR:40%FBHD, 16kV, x400
Figure 7 Yield Stress for RMBR/FBHDPE blends