Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PLASTIC GRANULATE
The invention relates to a plastic granulate on the basis of a
thermoplastic polymer and natural fibres. The invention also relates to a
process
for the preparation of such a granulate, as well as to a moulding made
thereof.
Such a granulate is known from EP-A-865,891, which describes
how the granulate is obtained by mixing chopped natural fibres with a
thermoplastic polymer, followed by production of a granulate. The granulate
thus
obtained is for instance used as a starting material for mouldings, produced
with
application of techniques such as injection moulding.
During the preparation of the granulate and in the moulding
process the material is brought to a high temperature, i.e. the processing
temperature of the thermoplastic polymer, which temperature is mostly 40-100
°C
above the melting temperature of the polymer. During such treatments the
natural
fibres are also exposed to these high temperatures. At those temperatures the
natural fibres are exposed to a high thermal and mechanical burden, which
results
in degradation and decomposition of the fibres and thus ultimately loss of the
properties of the moulding.
The aim of the invention is a plastic granulate on the basis of a
thermoplastic polymer and natural fibres, obtained with much less thermal and
mechanical burdening of the natural fibres, due to which the formation of a
moulding from such a granulate gives an object having improved properties.
The plastic granulate according to the invention is characterized
in that the granulate comprises a bundle of natural fibres, predominantly
oriented
in the longitudinal direction of the granulate, the bundle being provided with
a
sheath of the thermoplastic polymer and the fibre bundle being as long or
virtually
as long as the granulate, and wherein the decomposition temperature of the
natural fibres is 20 to 80 °C higher than the melting point of the
thermoplastic
polymer.
The granulate according to the invention thus contains a bundle
of natural fibres (in the following also referred to as 'rope') provided with
a sheath
made of a thermoplastic polymer. The type of rope as such is not really
relevant to
the invention, much less than the fact that the rope is provided with a
sheath, i.e.
the outer surface of the rope is sheathed with the thermoplastic polymer. The
thermoplastic polymer can at most be partially present between the fibres of
the
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rope, i.e. there is no full impregnation of the rope. The fact is that the
characteristic feature of the invention is that during the preparation of the
granulate the fibre bundle as a whole, i.e. the rope, is only subjected
virtually
exclusively to a thermal treatment for a relatively short time, while in the
case of
full impregnation (like in the prior art) this thermal effect affects every
fibre of the
bundle, resulting in much more fibre degradation. The fibres in the granulate
according to the invention thus preserve their original properties to a high
extent
or even completely.
The fibre material used for the rope present in the granulate
according to the invention can be any natural material, of vegetable as well
as
animal origin. The fibre material is preferably selected from the group
comprising
jute, flax, kenaf, sisal and hemp fibres, but cotton and silk fibres are also
quite
suitable. Mixtures of two or more of these fibres can also be present, either
in one
mixed rope, or in the form of separate ropes.
The granulate can also comprise a mixture of natural and other
fibres. The latter can be for instance plastic fibres (such as polyolefin,
aramid or
polyester fibres), inorganic fibres (such as glass or metal fibres) or carbon
fibres.
Preferably, as 'other fibres', plastic fibres and/or glass fibres are used.
Such other
fibres can be present in one mixed rope together with the natural fibres, but
also
be present separately in the plastic granulate.
The fibres can optionally be provided with an oil or with a sizing,
in combination with an adhesion improvement agent if required. These agents,
which are known per se to the person skilled in the art, promote the
dispersion in
and/or the adhesion of the fibres to the thermoplast of the moulding to be
produced ultimately.
For a plastic granulate according to the invention it is important
that the difference between on the one hand the temperature at which the
natural
fibres begin to degrade or decompose significantly and on the other hand the
melting point of the thermoplastic polymer is sufficiently large. The
decomposition
temperature of the natural fibres must therefore be 20 to 80 °C,
preferably
35-70 °C, higher than the melting point of the thermoplastic polymer.
A suitable thermoplastic material for the plastic granulate of the
invention is in principle any thermoplastic polymer which satisfies the above-
mentioned condition concerning the melting point in relation with the
decomposition temperature of the natural fibre. Particularly suitable are
polyolefins
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and polyolefin-based polymer systems (such as blends of a polyolefin and a non-
cross-linked or cross-linked rubber, as in a thermoplastic elastomer
vulcanizate).
The polyolefin is preferably chosen from the group of polyethylene and
polypropylene, both the homopolymers and the copolymers being suitable. A
person skilled in the art is familiar with such materials. Thermosetting
materials
which are thermoplastic during the preparation of the plastic granulate and
are
only cured in the final processing stage are also suitable as material for the
granulate according to the invention.
The length and the diameter of the granulate are not critical as
such. Generally, the length of the granulate is <_ 50 mm, the diameter 5 15
mm.
For practical purposes it is preferred for the granulate to have a length of 5-
40 mm
and a diameter of 1-10 mm.
The fibre content of the plastic granulate according to the
invention can in principle be chosen freely, depending on the further use of
the
granulate. If the granulate is supplied in the form of a masterbatch, the
fibre
content will generally be high, while if the granulate is used as such a lower
fibre
content will mostly be used. On the whole the granulate preferably contains
20-85 wt.% fibres.
Since the fibres are not well dispersed in the granulate and it is
mostly a requirement that mouldings made of the granulate do have a good
dispersion, it is of advantage if the granulate already has features which
will
promote said dispersion in the moulding. One such feature is that the rope is
a
bundle of the natural fibres and readily flowing plastic fibres (i.e. fibres
which melt
in the moulding process and have a low viscosity then) so that the
thermoplastic
material spreads well through the fibres.
Another, preferred option is that the sheath of the plastic
granulate, viewed from the core of the granulate, consists of a first layer of
a low-
viscous thermoplastic polymer and a second layer of a high-viscous
thermoplastic
polymer, the ratio of the melt indices of the two polymers lying between 10
and
100. The melt index of the first layer preferably lies between 50 and 250 and
the
melt index of the second layer preferably between 0.1 and 25, both values
being
determined in accordance with ISO 1133. In order to ensure good properties of
the moulding ultimately obtained, the thermoplastic polymers are preferably of
the
same type or compatible with each other.
It should also be ensured, for instance by means of an adhesion
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promoter, that in the moulding there is good contact between the original
sheath
material and the fibres.
The plastic granulate according to the invention can further
contain the ingredients that are required for plastic objects, such as
antioxidants,
UV stabilizers, fillers, colorants, etc. Such ingredients are known to the
person
skilled in the art.
The invention also relates to a process for the preparation of a
plastic granulate as described above. In such a preparation process it is
important
that the fibres are supplied in such a form that in the sheathing procedure a
granulate is obtained in which the fibres are predominantly oriented in the
longitudinal direction of the granulate. To that end the fibres should be
combined
to form one or more continuous bundles or ropes. The manufacture of such a
bundle or rope as such is known to the person skilled in the art. The ropes
can be
of a single natural or of another fibre, they can also be a mixture of natural
fibres,
a mixture of one or more natural fibres with other fibres, etc.
The process according to the invention is characterized in that
one or more continuous bundles of the natural fibres, optionally mixed or
jointed
with other fibres, is sheathed with a melt of the thermoplastic polymer, after
which
the product obtained (the extrudate) is cooled and cut up to the desired
length. In
this process use can be made of sheathing techniques known per se, such as for
the sheathing of glass or metal fibres. For this purpose the thermoplastic
polymer
is heated, using an extruder for instance, to a temperature above the melting
point
and as such supplied to the sheathing apparatus.
In the process according to the invention the continuous
bundles) of (natural) fibres is preferably sheathed in a first step with a low-
viscous
thermoplastic polymer and subsequently or simultaneously with a high-viscous
polymer, with the ratio of the melt indices of the two polymers lying between
10
and 100.
The melt index of the low-viscous thermoplastic polymer
preferably lies between 50 and 250, while the melt index of the high-viscous
thermoplastic polymer lies between 0.1 and 25, both measured according to
ISO 1133.
The residence time, in particular that of the natural fibres, during
the sheathing process is relatively short, without the occurrence of
mechanical
degradation of these fibres and only minor, if any, thermal degradation of the
natural fibres. The contact times in the sheathing process are in general from
0.5
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and 1.0 second; then the product obtained is cooled down already. According as
the ratio of the mass of the sheath to the mass of the fibres decreases, the
effect
of the invention, i.e. diminishing/eliminating the degradation of the natural
fibre,
increases. The proportion of fibres is therefore preferably between 20 and
85 wt.%.
The process of the present invention has the further advantage
that, compared to prior art processes, the moisture content of the fibres is
less
critical; pre-drying of the fibres is now optional, instead of necessary.
The continuous bundle of natural fibres can also contain fibres
of a thermoplastic, preferably a plastic which will melt in a subsequent
shaping
process and have a low viscosity, which promotes the dispersion of the natural
fibres in the moulding.
With a view to promoting the later dispersion of the fibres in the
moulded object it can be of advantage to spread the natural fibre-bundle
slightly
already before or while they are fed into the sheathing apparatus. An
alternative
for obtaining better dispersion is to heat the fibres before they are fed into
the
sheating apparatus; this improves the wetting of the fibre by the
thermoplastic
polymer. These steps should be done with care, however, because this enhanced
spreading and/or wetting entails an enhanced risk of thermal degradation of
the
natural fibres in the sheathing process.
The invention also relates to a moulding made out of a plastic
granulate according to the invention, for instance by injection moulding, by
in-
mould decorating, by extrusion (optionally followed by deep drawing) or by
extrusion followed by compression. In such a moulding process the granulate is
fed to a moulding machine, is heated to the moulding temperature, and the
moulding is obtained.
An alternative to this moulding process can be to feed the
extrudate from the sheating process directly (i.e. without the extrudate being
cooled and cut up to granulate) to the moulding machine, a machine as
described
before. In such a process a second heating step is avoided.
The invention is illustrated below; the Examples and
comparative experiments are however not intended to restrict the scope of the
invention.
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Examples I and II and comparative experiments A and B
Example I
A bundle of jute fibres was sheated in an extruder with a polypropyle polymer.
The
bundle comprised 3 strands of jute, having a total tex of 2000 grams/1000 mtr.
The polypropylene polymer was a homopolymer with a melt-index (at
230 °C/2.16 kg) of 47 dg/min and a density of 905 kg/m3. The polymer
also
comprised Polybond~, a malefic acid anhydride modified polypropylene from
Uniroyal (GB) as adhesion improver.
The extruder was a single screw extruder from Schwabenthan
(DE) having a screw diameter of 30 mm. The extruder was provided with a fibre
bundle guide (U4SCC) from Unitek (AT), together with pinoles having a diameter
of 2.9 mm, mounted at a right angle to the extruder.
The jute bundle was passed through the extruder head at a
speed of 50 m/min and sheated with the polypropylene having a melt temperature
of 249 °C.
The so obtained strand was fed through a waterbath, cooled to
50 °C and cut into granulate having a length of 12.5 mm. The granulate
had a
fibre content of 35 wt.%.
This granulate was injection moulded to test bars with a
mculding machine from Netstal (DE), and a force of 130 ton. The temperature
applied was 230 °C, the injection time was 4 sec.
Comparative experiment A
The granulate of Example I was fed to a ZSK 30 twin-screw
extruder from Werner and Pfleiderer (DE); the extruder had a mild screw
design,
appropriate for dispersing the fibres in the polymer melt. The screw speed was
250 RPM, the melt temperature 230 °C, the through-put 12 kg/hour. The
obtained
strand was cut into granulate and used for injection moulding as per Example
I.
Example II
Example I was repeated, but now with polyethylene (Vestolen~ 1640L0 from
DSM), said polymer also comprising Yparex~, a malefic acid anhydride modified
polyethylene from DSM, as adhesion improver. The temperature of the melt
during the sheating process was 239 °C, and during the moulding process
225 °C.
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Comparative experiment B
Comparative experiment A was repeated, but now with the granulate obtained in
Example II. The temperature of the melt in the ZSK was 211 °C; the
throughput
was 5 kg/hour.
The test bars were tested for several properties:
- Tensile strength, according to ISO 527 1 B
- Flexural strength, according to ASTM D790
- Impact strength, Izod, according to ISO 180i4A
- Linear coefficient of expansion (LCE), according to ASTM D696 (23-80
°C)
- falling dart impact strength (FDI; VEM), according to ISO 66030-2V
The results of the Examples and comparative experiments are given in Table I.
Table I
Example/ comparative I A II B
experiment
Properties
Izod (kJ/m2) 4.3 2.9 11.4 4.2
FDI
E-total (Joules) 7.0 4.8 14.3 7.0
F-max (Newton) 1345 1364 1823 1206
Tensile test
Max. strength 42.3 42.9 38.2 26.0
Elongation at break 1.2 2.6 2.4 16.5
(%)
E-modulus (Mpa) 5391 4285 4464 3617
Flexural test
Max. strength (Mpa) 64 65 58 41
E-modulus (Mpa) 3511 3380 2630 1785
LCE (1/K) 3.87*10~55.89*10~56.39*10~510.9*10~5
Remarks:
The test bars from comparative experiments A and B showed black discoloration.
