This invention relates to the field of bi-oriented bottles
having high transparency and prepared from compositions comprising polypropylene
(PP) and inorganic nucleating agents.
Presently, the most common material used in injection-stretch-blow-moulding
(ISBM) is polyethylene terephthalate (PET). It has the advantage of achieving very
short cycle time of the order of1500 bottles per cavity per hour and it offers the
further advantage of having excellent optical properties. It suffers from the considerable
disadvantage of being very costly.
Alternatively other thermoplastic materials such as for
example polyvinylchloride (PVC), polystyrene, acrylonitrile, polypropylene and acetals
could be used. Among those, polypropylene prepared with a Ziegler-Natta catalyst
system has been used. In order to obtain the required transparency, these resins
are clarified with sorbitols and the injection melt temperature for the production
of the pre-form must be very high. That impacts negatively the cycle time and the
There is thus a need to replace the costly PET resins with
other resins that will not suffer from the disadvantages of the sorbitol-clarified
polypropylene prepared with a Ziegler-Natta catalyst system.
It is an aim of the present invention to provide resins
that are processed at a reduced temperature for the production of the pre-form while
keeping good transparency of the final bottles.
It is also an aim of the present invention to provide resins
that can be processed for pre-form injection with a very short cycle time.
It is another aim of the present invention to provide resins
that can be used to prepare pre-forms having excellent optical properties after
It is further aim of the present invention to provide resins
that have a broad processing window in injection-stretch-blow-moulding.
Accordingly, the present invention provides pre-forms and
bottles produced by injection-stretch-blow-moulding (ISBM) prepared from a composition
comprising polypropylene and an inorganic non-sorbitol nucleating or clarifying
The pre-forms are prepared by injecting the resin at a
temperature that is lower than typically used with polypropylene resins. It is at
least 10 °C, preferably 20 °C, lower than in the prior art production.
Typical pre-form injection temperatures are of from 210 to 235 °C, preferably
of from 210 to 220 °C, more preferably of about 215 °C.
The polypropylene used in the present invention may be
prepared with a Ziegler-Natta (ZN) or a metallocene catalyst system and it may be
a homopolymer or a random copolymer of propylene. Preferably, it has a melt index
of from 2 to 100 g/10 min and preferably of from 10 to 20 g/10 min. When it is a
copolymer of propylene, the preferred comonomer is ethylene and the amount of ethylene
present in the resin is up to 10 wt%, preferably of to 4 wt%.
The melt flow index MFI is measured using the procedures
of standard test ISO 1133 at 230 °C for polypropylene resins and under a load
of 2.16 kg.
The nucleating agents that can be used at the reduced pre-form
injection temperature of the present invention without reducing the optical properties
of the finished articles, are selected from sodium salts, lithium benzoate, sodium
benzoate, talc, aluminium salts or combinations thereof.
The amount of nucleating agent varies with the nature of
the agent: it is of up to 10000 ppm, preferably of up to 2000 ppm.
List of figures.
Figure 1 represents a typical pre-form design.
Figure 2 represents a typical pre-form temperature profile
after re-heating for the stretching step.
The injection-stretch-blow-moulding can be carried out
either in a two-machine process or in a single machine process. The two-machine
process is generally preferred and it is carried out in two separate locations.
It comprises the steps of:
- providing a pre-form by injection moulding on a multi-cavity mould;
- cooling the pre-form to room temperature;
- transporting the pre-form to the blow moulding machine;
- re-heating the pre-form in the blow moulding machine in a reflective radiant
heat oven following a pre-determined temperature profile for the pre-form;
- passing the heated pre-form through an equilibrium zone to allow the heat to
disperse evenly through the pre-form wall;
- stretching the pre-form axially by a centre rod;
- orienting the stretched pre-form radially by high pressure air.
In this process, the stretching step is the critical step
as it requires re-heating of the pre-form: optimisation of the pre-form is thus
required. A typical example of pre-form design is displayed in figure 1.
The pre-forms are re-heated in an infra-red oven following
a heating profile such as displayed for example in figure 2. Typical re-heating
temperatures are of from 90 to 140 °C as shown in figure 2.
In the single-machine process, all steps are carried out
in the same machine. The cooling step, the transporting step and the reheating step
are thus replaced by a single conditioning step that consists in slightly re-heating
the pre-form following the temperature profile required for the stretching step.
The stretching is then carried out under a blowing pressure
is of from 10 to 40 bars, preferably of from 12 to 18 bars and most preferably of
about 15 bars and with a stretch rod speed of from 1000 to 2000 mm/s, preferably
of from 1400 to 1800 mm/s and most preferably of about 1600 mm/s.
The articles prepared according to the present invention
are hollow containers and bottles that can be used in various food and non-food
applications. The food applications comprise the storage of water, juices, oil,
flavoured still and carbonated beverages, isotonic drinks, dry products, fresh milk
and solid food. The non-food applications comprise the storage of cosmetic and pharmaceutical
products, dishwashing or washing detergent and dry products.
Several propylene copolymers were tested. They were all
prepared with a Ziegler-Natta catalyst system and their properties and comonomer
content are summarised in Table I. They were additivated with a commercial antioxydants
and antiacid package and additionally with various inorganic nucleating agents:
their nature and amount is also specified in Table I. Resins R1 to R3 were comparative
resins, and resin R4 was prepared according to the present invention.
amount NA (%)
The pre-form injection step was carried out on a Arburg
press using respectively injection temperatures of 215 °C and 235 °C.
The blowing step was carried out on a separate injection-stretch-blow-moulding machine.
The re-heating step was performed in an infra-red oven according to the profile
dispalyed in figure 2 and the number of bottles produced per hour and per cavity
was of 1200. The temperature profile was the same for all the tested pre-forms.
The haze results are displayed in Table II for different
pre-form injection temperatures.
As can be seen from these results, all nucleating agents
performed well and similarly at the pre-form injection temperature of 235 °C
and gave excellent haze values. When the pre-form injection temperature was reduced
to 215 °C, the haze values became prohibitively high for all the polypropylene
resins that were not additivated or that were additivated with sorbitol nucleating
agents. The resins according to the present invention kept excellent haze values
when the pre-forms were injected at a temperature of 215 °C, thus 20 °C
lower than typical temperatures. This resulted in considerable savings in time and