This invention is related to hollow packagings having improved optical
properties and in particular to the production of high gloss bottles, jars,
etc. formed of polyethylene by injection blow moulding.
Several methods have been sought to produce high gloss bottles presenting
good processability and good mechanical properties but all the blends and techniques
used so far present various disadvantages.
High gloss high density polyethylene (HDPE) has been used: it is characterised
by a very narrow molecular weight distribution that is typically inferior to 8.
The molecular weight distribution can be completely defined by means of a curve
obtained by gel permeation chromatography. Generally, the molecular weight distribution
(MWD) is more simply defined by a parameter, known as the dispersion index D, which
is the ratio between the average molecular weight by weight (Mw) and the average
molecular weight by number (Mn). The dispersion index constitutes a measure of
the width of the molecular weight distribution. It is known that a resin of narrow
molecular weight distribution will produce plastic containers of very high gloss
but simultaneously, that such resin will be very difficult to process and will
be characterised by very poor mechanical properties. It has also been observed
that said resins have poor mechanical properties, particularly, a very low environmental
stress crack resistance (Modern Plastic International, August 1993, p. 45).
The coextrusion of high density polyethylene (HDPE) with a thin external
layer of polyamide has been used to produce bottles of very high gloss but that
method suffers the major drawback of necessitating an adhesive layer between the
HDPE and the polyamide layers.
The coextrusion of high density polyethylene and an external layer
of low density polyethylene leads to bottles with a fair gloss. These bottles however
have an unpleasant greasy touch and offer a very poor resistance to scratching.
In another method, disclosed in co-pending European Patent Application
n° 00201155.9, high gloss plastic containers comprise an internal layer including
a polyolefin and an external layer including a styrenic component containing from
40 to 85 wt% of styrene, based on the weight of the external.
There is thus a need for a method for efficiently producing hollow
packagings of very high gloss as well as good processability and mechanical properties
by injection moulding.
An aim of the present invention is to produce hollow packagings that
offer simultaneously the desired glossy appearance and a good resistance to scratching.
It is also an aim of the present invention to obtain glossy hollow
packagings with good processability and good mechanical properties.
The present invention provides single layer hollow packagings, which
consist essentially of metallocene-produced polyethylene having a density of from
0.915 g/cm3, preferably from 0.925 g/cm3 up to 0.966 g/cm3,
or up to homopolymer densities, and a melt index MI2 of from 0.2 to 5 g/10 min
and preferably from 0.5 to 2.5 g/10min, characterised in that said hollow packagings
are produced by injection blow moulding and have an external and internal gloss
of at least 30.
In this specification, the density of the polyethylene is measured
at 23 °C using the procedures of ASTM D 1505.
The melt index MI2 is measured using the procedures of ASTM D 1238
at 190°C using a load of 2.16 kg. The high load melt index HLMI is measured using
the procedures of ASTM D 1238 at 190 °C using a load of 21.6 kg.
A number of different metallocene catalyst systems have been disclosed
for the manufacture of polyethylene, in particular medium-density polyethylene
(MDPE) and high-density polyethylene (HDPE) suitable for injection blow moulding.
It is known in the art that the physical properties, in particular the mechanical
properties, of a polyethylene product vary depending on what catalytic system
was employed to make the polyethylene
The HDPE can be polymerised with a metallocene catalyst system capable
of producing a mono- or bi- or multimodal distribution, either in a two step process
such as described for example in EP-A-0,881,237, or as a dual or multiple site
catalyst in a single reactor such as described for example in EP-A-0,619,325.
Any metallocene catalyst known in the art can be used in the present invention.
It is represented by the general formula:
wherein Cp is a cyclopentadienyl ring, M is a group 4b, 5b or 6b transition metal,
R is a hydrocarbyl group or hydrocarboxy having from 1 to 20 carbon atoms, X is
a halogen, and m-1-3, n=0-3, q=0-3 and the sum m+n+q is equal to the oxidation
state of the metal.
II. (C5R'k)g R"s(C5R'k)MQ3-gIII. R"s(C5R'k)2MQ'
wherein (C5R'k) is a cyclopentadienyl or substituted cyclopentadienyl,
each R' is the same or different and is hydrogen or a hydrocarbyl radical such
as alkyl, alkenyl, aryl, alkylaryl, or arylalkyl radical containing from 1 to 20
carbon atoms or two carbon atoms are joined together to form a C4-C6
ring, R" is a C1-C4
alkylene radical, a dialkyl germanium or
silicon or siloxane, or a alkyl phosphine or amine radical bridging two (C5R'k)
rings, Q is a hydrocarbyl radical such as aryl, alkyl, alkenyl, alkylaryl, or aryl
alkyl radical having from 1-20 carbon atoms, hydrocarboxy radical having 1-20 carbon
atoms or halogen and can be the same or different from each other, Q' is an alkylidene
radical having from 1 to about 20 carbon atoms, s is 0 or 1, g is 0, 1 or 2, s
is 0 when g is 0, k is 4 when s is 1 and k is 5 when s is 0, and M is as defined
Among the preferred metallocenes used in the present invention, one
can cite among others ethylene bis-(tetrahydroindenyl) zirconium dichloride, ethylene
bis-(indenyl) zirconium dichloride or bis-(n-butylcyclopentadienyl) zirconium
dichloride mono-, di- or tri-substituted as disclosed for example in EP-A-870,048.
The metallocene may be supported according to any method known in
the art. In the event it is supported, the support used in the present invention
can be any organic or inorganic solids, particularly porous supports such as talc,
inorganic oxides, and resinous support material such as polyolefin. Preferably,
the support material is an inorganic oxide in its finely divided form.
An active site must be created by adding a cocatalyst having an ionising
Preferably, alumoxane is used as cocatalyst during the polymerization
procedure, and any alumoxane known in the art is suitable.
The preferred alumoxanes comprise oligomeric linear and/or cyclic
alkyl alumoxanes represented by the formula :
for oligomeric, linear alumoxanes,
for oligomeric, cyclic alumoxanes,
wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C1-C8
alkyl group and preferably methyl.
Methylalumoxane is preferably used.
When alumoxane is not used as a cocatalyst, one or more aluminiumalkyl
represented by the formula AIRx are used wherein each R is the same
or different and is selected from halides or from alkoxy or alkyl groups having
from 1 to 12 carbon atoms and x is from 1 to 3. Especially suitable aluminiumalkyl
are trialkylaluminium, the most preferred being triisobutylaluminium (TIBAL).
The metallocene catalyst utilised to produce a polyethylene, as required
for preparing the high gloss hollow packagings of the present invention, can be
used in gas, solution or slurry polymerisation. Preferably, the polymerization
process is conducted under slurry phase polymerization conditions. The polymerisation
temperature ranges from 20 to 125°C, preferably from 60 to 95°C and the pressure
ranges from 0.1 to 5.6 Mpa, preferably from 2 to 4 Mpa, for a time ranging from
10 minutes to 4 hours, preferably from 1 and 2.5 hours).
It is preferred that the polymerization reaction be run in a diluent
at a temperature at which the polymer remains as a suspended solid in the diluent.
A continuous loop reactor is preferably used for conducting the polymerisation.
Multiple loop reactors can also be used.
The average molecular weight is controlled by adding hydrogen during
polymerisation. The relative amounts of hydrogen and olefin introduced into the
polymerisation reactor are from 0.001 to 15 mole percent hydrogen and from 99.999
to 85 mole percent olefin based on total hydrogen and olefin present, preferably
from 0.2 to 3 mole percent hydrogen and from 99.8 to 97 mole percent olefin.
The density of the polyethylene is regulated by the amount of comonomer
injected into the reactor; examples of comonomer which can be used include 1-olefins
butene, hexene, octene, 4-methyl-pentene, and the like, the most preferred being
The densities of the polyethylenes required for preparing the hollow
packagings of the present invention range from 0.915 g/cm3, preferably
from 0.925 g/cm3
up to 0.966 g/cm3, or up to homopolymer densities.
The melt index of polyethylene is regulated by the amount of hydrogen
injected into the reactor. The melt indexes useful in the present invention range
from 0.2 to 5 g/10 min and preferably from 0.5 g/10 min to 2.5 g/10' min.
The polyethylene resin used in the present invention can be prepared
with either a single site metallocene catalyst or with a multiple site metallocene
catalyst and it has therefore either a monomodal or a bimodal molecular weight
distribution. The molecular weight distribution is of from 2 to 20, preferably,
of from 2 to 7 and more preferably of from 2 to 5.
The polyethylene resins produced in accordance with the above-described
processes have physical properties making them particularly suitable for use as
injection blow moulding grade polyethylenes. In addition, it has surprisingly been
observed that they have excellent processability even when their molecular weight
distribution is narrow.
The polyethylene resins of the present invention are used preferably
for producing containers of a capacity ranging from 0.0005 to 2 I. They are more
preferably used for producing food packaging, such as for example milk bottles
or juice bottles, cosmetic or pharmaceutical packaging and household packaging.
The injection moulding machine, can be any one of the machines generally
used for injection-blow-moulding, such as for example the JOMAR and UNILOY machines.
They are continuous injection-blowing-ejection machines with up to 16 injection-blowing
dies that can be used for the production of polyethylene containers of up to 0.8
The hollow packagings of the present invention are characterised by
a very high gloss for both inner and outer surfaces, as measured using the ASTM
D 2457-90 test, a low haze as measured by ASTM D 1003-92, and an outstanding resistance
to drop. In addition, because of the very smooth inner surfaces, it is possible
to increase the pouring speed and to decrease the amount of residue left in the
The impact strength was measured on moulded plates at -30 °C following
the method of standard test ISO 8256.
Additionally and quite surprisingly, the production rate is very high
even though the melt index is low. The process is very stable and the packagings
are produced with an excellent success rate.
Several polyethylene resins were prepared and tested for gloss, haze,
impact strength and drop.
The polyethylene resin R1 was obtained by continuous polymerisation
in a loop slurry reactor with a supported and ionised metallocene catalyst prepared
in two steps by first reacting SiO2 with MAO to produce SiO2.MAO
and then reacting 94 wt% of the SiO2.MAO produced in the first step
with 6 wt% of ethylene bis-(tetrahydroindenyl) zirconium dichloride. The dry catalyst
was slurried in isobutane and pre-contacted with triisobutylaluminium (TIBAI, 10
wt% in hexane) before injection in the reactor. The reaction was conducted in a
slurry loop reactor with the polymerisation temperature being maintained at 90
°C. The operating conditions were as follows:
TIBAI conc (ppm) : 100-200
iC4 feed (kg/h) : 1940
C2 feed (kg/h): 3900
C6 feed (g/kg C2): 22
H2 feed (g/t): 42
Wherein, C2 is ethylene, C6 is 1-hexene, iC4 is isobutane and TIBAI is triisobutylaluminium.
The polyethylene resin R2 was prepared following the same procedure
as that used for polymerising resin R1 except that the metallocene catalyst was
bis-(butylcyclopentadienyl) zirconium dichloride. The cocatalyst was also TIBAI
(10 wt% in hexane) and the polymerisation temperature was 90 °C. The operating
conditions were as follows:
IC4 feed (kg/h) : 24
C2 feed (cc/h) : 9
C6 feed (cc/h) : 27
H2 feed (Nl/h) : 1.9
TIBAI conc (ppm) : 292
Resin R3 is a monomodal polyethylene resin produced with a chromium
catalyst, commercialised under the name ®Finathène 5502: it was prepared with
a supported chromium catalyst
Resin R4 is a low density polyethylene resin, produced by Dupont under
the name ®DuPont 20-6064 for applications in injection blow moulding.
The resins R1 to R3 were prepared with hexene as comonomer.
The properties of these resins are summarised in Table I.
The impact strength was measured on moulded plates, at a temperature
of -30 °C and following the method of standard test ISO 8256.
Resin Density g/cm3HLMI g/10' MI2 g/10' Mn Mw Mz Imp. Str. kj/m2MWD R10.93425.10.9634083881341678881702.6 R20.950271.634624927292016161302.7 R30.95317.650.19196201535581333100807.8 R40.92n.a.1.9n.a.n.a.n.a.n.a.n.a.
n.a. : not available
Resins R1 and R3 were injection-blow-moulded with the UNILOY injection-blowing
machine under the processing conditions summarised in Table II.
Resins R2 and R4 were processed with the injection-blow-moulding (IBM)
machine, 15 model available from Jomar. The injection blow moulding process is
divided into three steps:
1. the injection step, wherein the molten polymer is injected through nozzles
into heated preform moulds forming the external shape, said moulds being clamped
around core rods forming the internal shape;
2. the blowing step wherein the core rods allow compressed air into the preforms
that inflate to the shape of the chilled blow moulds; and
3. the ejection step wherein after a cooling period, the finished article is
stripped off the core rod.
The machine and mould characteristics are summarised in Table III.
The general purpose mixing screw has a diameter of 25.4 mm and a length to diameter
ratio L/D of 30:1.
The extruded articles all exhibit a very high gloss and an excellent
Resin Mass Temp. Cycle time Int. gloss 60° Ext. gloss 60° °C S % % R1200-21016.497238 R2210-22015.07n.a.62 R3230-24019.302022 R4180-20015.13n.a.n.a.
n.a.: not available
During processing, resins R1 and R2 showed a very high process-stability,
a very high percentage of well-formed bottles, a good weight consistency. The
bottles obtained were very glossy as compared to those obtained with resins R3
and R4. This can be clearly seen in Figure 1 representing bottles prepared by injection
blow moulding respectively with a low-density polyethylene produced with the metallocene
catalyst ethylene bis(tetrahydroindenyl)zirconium dichloride and a low-density
polyethylene produced by Dupont and in Figure 2 representing botlles prepared by
injection blow moulding respectively with a medium-density polyethylene produced
with the metallocene catalyst ethylene bis(tetrahydroindenyl)zirconium dichloride
and a medium-density polyethylene produced with a chromium catalyst.
Preform clamp @ 141 kg/cm2 (tons)11.4 Casting area @ 246 kg/cm2 (cm2)46 @ 387 kg/cm2 (cm2)29 Blow mould clamp @ 141 kg/cm2 (tons)2.9 Shut height (mm)203.2 Press stroke (mm)101.6 Max. die set size (mm)254 x 286 Tigger Bar Irength mm)166 Max. swing length (mm)356 Shot capacity (g)a50 Motor size (kw)15
These results show unambiguously the improved qualities of gloss and
impact strength of the hollow packagings obtained with metallocene-produced polyethylene.
A single layer hollow packaging, comprising essentially a metallocene-produced
polyethylene and produced by injection blow moulding, characterised in that
said hollow packaging has an external and internal gloss of at least 30.
A hollow packaging according to claim 1, wherein the metallocene-produced polyethylene
has a density of from 0.910 up to 0.966 g/cm3 or up to homopolymer densities
and a melt index MI2 of from 0.5 to 2.5 g/10min.
A hollow packaging according to claim 1 or claim 2 wherein the metallocene
-produced polyethylene has a molecular weight distribution of from 2 to 7.
A hollow packaging according to any one of the preceding claims wherein the
metallocene catalyst is ethylene bis-(tetrahydroindenyl) zirconium dichloride,
ethylene bis-(indenyl) zirconium dichloride or bis-(n-butylcyclopentadienyl) zirconium
Food packaging produced according to any one of the preceding claims.
Cosmetic or pharmaceutical packaging produced according to any one of claims
1 to 3.
Household packaging produced according to any one of claims 1 to 3.
Use of a metallocene-produced polyethylene to prepare hollow packaging having
an internal and external gloss of at least 30.