3.1.1. Unfilled HDPE, PP, and their blends
The mechanical properties of the unfilled blends and compos-ites are summarized in Tables 1–3 . The initial HDPE and PP plastics
exhibited very different mechanical performance, with the PP hav-ing much larger moduli and strengths but also lower impact per-formance and strains at yielding and failure (Table 1). The tensile
moduli of the blends without EPDM are clearly proportional to
the relative amounts of HDPE and PP in the blends. Not surpris-ingly, adding 10% of the low modulus EPDM reduces the moduli
of the blends. The reductions in moduli were about 25% in tensile
tests and about 20% in flexural tests. This is a common limitation of
using elastomers as compatibilizers in plastic blends. However, all
moduli for the blends were at least that of the unblended HDPE.
Tensile yield stresses and strains also appear to follow a simple
rule of mixtures. Adding 10% EPDM allows the blends to yield more
easily, at about 16% lower stress, but at about 35–40% higher
strain. The largest yield strains found were for the blends with
25:75 and 50:50 HDPE:PP blend ratios containing EPDM, and ex-ceeded those of the unblended plastics.
Representative tensile curves for the unfilled blends are shown
in Fig. 2 . The large strains exceeded the range of our strain gauge
and the strains shown are nominal strains based on the separation
of the specimen grips. Tests on unfilled HDPE were stopped after
1200% nominal strain. Unlike moduli and yield properties, addi-tions of even 25% PP to HDPE greatly reduced nominal strains at
failure. This negative deviation from a rule of mixtures demon-strates the embrittlement common in incompatible plastic blends.Adding EPDM to the blends greatly improved the nominal strains
at failure, especially those with the low PP content.
Improving impact performance is one of the major reasons for
adding elastomers to polymers. For reversed notch impact tests,
only the unfilled PP and the 25:75 HDPE:PP without EPDM were
broken during testing. As with the nominal strain at failure in
the tensile tests, notched impact energy was greatly reduced with
addition of as little as 25% PP to HDPE if no EPDM was added. How-ever, the addition of EPDM greatly improved the situation. While
adding 25% PP reduced the notched impact energy of HDPE by over
70%, adding EPDM to the 75:25 HDPE:PP blend increased its
notched impact energy over 15-fold (Fig. 3). This blend also exhib-ited considerable stress whitening due to microvoids. This
improvement fell off quickly as the PP content was increased, but
still resulted in notched impact energies well above the blends
without EPDM.
For high speed puncture tests, addition of EPDM improved im-pact performance only in the brittle, PP-rich blends (Table 2). Add-ing EPDM to the PP-rich blends led to a change from brittle to
ductile behavior (Fig. 4 ) resulting in large improvements in the
measured parameters ( Table 2). However, the PE-rich blends al-ready exhibited ductile behavior and adding EPDM did little to im-prove the high speed puncture test results.
Selected HDPE:PP blends were investigated as matrices in
wood–plastic composites. The formulations investigated are sum-marized in Table 1. The HDPE, PP and elastomeric compatibilizers
(when used) were fed into the main feed throat and the blend
formed in the first part of the extruder ( Fig. 1 ). The wood flour
was added further down the extruder and compounded with the
polymer blend along with any coupling agents used to improve
adhesion between the wood flour and polymer blend. The coupling
agent used was either a maleated PE (MAPE) for HDPE-rich blends
or maleated PP (MAPP) for PP-rich blends. These maleated polyole-fins are commonly used to improve the adhesion between wood
and polyolefins. The anhydride moiety can react with the hydrox-yls on the wood surface and form ester bonds and the polyolefin
backbone can incorporate itself into the bulk polymer. In addition
to the EPDM, a maleated EPDM was also investigated since it has
the potential to interact with both the plastics and the wood flour.
3.1.1. Unfilled HDPE, PP, and their blends
The mechanical properties of the unfilled blends and compos-ites are summarized in Tables 1–3 . The initial HDPE and PP plastics
exhibited very different mechanical performance, with the PP hav-ing much larger moduli and strengths but also lower impact per-formance and strains at yielding and failure (Table 1). The tensile
moduli of the blends without EPDM are clearly proportional to
the relative amounts of HDPE and PP in the blends. Not surpris-ingly, adding 10% of the low modulus EPDM reduces the moduli
of the blends. The reductions in moduli were about 25% in tensile
tests and about 20% in flexural tests. This is a common limitation of
using elastomers as compatibilizers in plastic blends. However, all
moduli for the blends were at least that of the unblended HDPE.
Tensile yield stresses and strains also appear to follow a simple
rule of mixtures. Adding 10% EPDM allows the blends to yield more
easily, at about 16% lower stress, but at about 35–40% higher
strain. The largest yield strains found were for the blends with
25:75 and 50:50 HDPE:PP blend ratios containing EPDM, and ex-ceeded those of the unblended plastics.
Representative tensile curves for the unfilled blends are shown
in Fig. 2 . The large strains exceeded the range of our strain gauge
and the strains shown are nominal strains based on the separation
of the specimen grips. Tests on unfilled HDPE were stopped after
1200% nominal strain. Unlike moduli and yield properties, addi-tions of even 25% PP to HDPE greatly reduced nominal strains at
failure. This negative deviation from a rule of mixtures demon-strates the embrittlement common in incompatible plastic blends.Adding EPDM to the blends greatly improved the nominal strains
at failure, especially those with the low PP content.
Improving impact performance is one of the major reasons for
adding elastomers to polymers. For reversed notch impact tests,
only the unfilled PP and the 25:75 HDPE:PP without EPDM were
broken during testing. As with the nominal strain at failure in
the tensile tests, notched impact energy was greatly reduced with
addition of as little as 25% PP to HDPE if no EPDM was added. How-ever, the addition of EPDM greatly improved the situation. While
adding 25% PP reduced the notched impact energy of HDPE by over
70%, adding EPDM to the 75:25 HDPE:PP blend increased its
notched impact energy over 15-fold (Fig. 3). This blend also exhib-ited considerable stress whitening due to microvoids. This
improvement fell off quickly as the PP content was increased, but
still resulted in notched impact energies well above the blends
without EPDM.
For high speed puncture tests, addition of EPDM improved im-pact performance only in the brittle, PP-rich blends (Table 2). Add-ing EPDM to the PP-rich blends led to a change from brittle to
ductile behavior (Fig. 4 ) resulting in large improvements in the
measured parameters ( Table 2). However, the PE-rich blends al-ready exhibited ductile behavior and adding EPDM did little to im-prove the high speed puncture test results.
Selected HDPE:PP blends were investigated as matrices in
wood–plastic composites. The formulations investigated are sum-marized in Table 1. The HDPE, PP and elastomeric compatibilizers
(when used) were fed into the main feed throat and the blend
formed in the first part of the extruder ( Fig. 1 ). The wood flour
was added further down the extruder and compounded with the
polymer blend along with any coupling agents used to improve
adhesion between the wood flour and polymer blend. The coupling
agent used was either a maleated PE (MAPE) for HDPE-rich blends
or maleated PP (MAPP) for PP-rich blends. These maleated polyole-fins are commonly used to improve the adhesion between wood
and polyolefins. The anhydride moiety can react with the hydrox-yls on the wood surface and form ester bonds and the polyolefin
backbone can incorporate itself into the bulk polymer. In addition
to the EPDM, a maleated EPDM was also investigated since it has
the potential to interact with both the plastics and the wood flour.
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