Method of producing pelletized polyolefin

Details Method of producing pelletized polyolefin Method of producing pelletized polyolefin

2001-05-07

2003-06-03

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S570000, C528S50200C

Reexamination Certificate

active

06573314

ABSTRACT:

This invention relates to a method for the production of pelletized polyolefin for rotational moulding.
Rotational moulding is a moulding process in which a particulate polymer, the moulding powder, is filled into a mould which is placed in an oven and rotated so that the polymer melts and coats the inside surface of the mould. In order to ensure that the moulded product is defect free, the moulding powder must have a relatively small particle size and should preferably be uniform in particle size and composition. Generally the particle size is about 300 &mgr;m. Where, as is normal, the moulding powder has to contain colouring agents or other additives, e.g. stabilizers, the moulding powder is conventionally produced by grinding polymer pellets extruded from stabilized reactor grade powder, ie. pellets normally of size 3 to 6 mm, to the correct particle size for rotational moulding, usually with the colours or other additives being added in with the polymer pellets or mixed into the ground moulding powder. In addition to being costly, the grinding process is noisy and dusty and generally the grinding operation presents a worker unfriendly environment.
It is possible to mix polymer and additives using an extruder and to extrude pellets containing polymer and additives. The pellets however have not been found acceptable for rotational moulding as the surface of the resulting moulded product is covered with small holes, “pin holes”.
We have now found that extruded micropellets may be used in rotational moulding if their water content is reduced to less than 0.1% wt (10
3
ppm), or more advantageously less than 200 ppm. In this way the costly and worker-unfriendly grinding step of the conventional process for production of moulding powder for rotational moulding can be avoided. Furthermore the micropellets are easier to handle than the ground powder and easier to transport using conventional conveying systems, e.g. due to dry flow capability. Moreover, having a higher bulk density, the micropellets have advantage both in terms of storage and transportation volume and in terms of the rotomoulding process itself.
Thus viewed from one aspect the invention provides a rotational moulding composition comprising a pelletized olefin polymer having a particle size distribution D(v, 0.5) of 0.1 to 1 mm, preferably 0.2 to 0.9 mm, especially 0.3 to 0.8 mm, a value of the ratio of D(v, 0.9)−D(v, 0.1) to D(v, 0.5) of no more than 1, preferably no more than 0.8, e.g. 0.45 to 0.70, and a water content of less than 0.1% wt, preferably less than 200 ppm, more preferably less than 150 ppm, especially less than 100 ppm, e.g. 10-200 ppm, especially 30 to 140 ppm.
These very dry micropellets may be produced by a mixing, pelletization and drying procedure and this forms a further aspect of the invention.
Viewed from this aspect the present invention provides a method of producing a rotational moulding micropellet composition, said method comprising:
producing a mixture of a polyolefin and optionally but preferably at least one additive, e.g. one or more selected from colouring agents, stabilizers (e.g. heat or radiation stabilizers), antioxidants, UV absorbers, antistatic agents, lubricants and fillers (e.g. organic fillers;
extruding said mixture in melt form through orifices in a die;
pelletizing the mixture extruded through said orifices to form micropellets having a particle size distribution D(v, 0.5) of 0.1 to 1 mm preferably 0.2 to 0.9 mm, etc. and a value of the ratio of D(v, 0.9)−D(v, 0.1) to D(v, 0.5) of no more than 1, preferably no more than 0.8;
drying said micropellets to a residual water content of no more than 0.1% wt. preferably less than 200 ppm, etc.;
and if desired packaging said micropellets, e.g. in water-tight containers or in microperforated containers which subsequently may be coated with a shrink wrap coating.
By virtue of the pelletization process, any additives (e.g. stabilizers (for example heat or radiation stabilizers such as UV-stabilizers, in particular HALS (Hindered amine light stabilizers)), coloring agents, antistatics, antioxidants (e.g. phenolic and phosphitic antioxidants), lubricants, etc) in the mixture being pelletized are distributed very uniformly in the resulting rotomoulding pellets. This results in a high degree of homogeneity within and between the pellets. This is very important for rotomoulding since the rotomoulding process itself does not involve an extrusion step and so cannot itself cause additive distribution to become uniform.
Typically additives such as antioxidants, lubricants and UV-stabilizers will be used in quantities of about 100 to 5000 ppm, e.g. 500 to 2500 ppm, relative to the overall polymer weight.
The method of the invention can advantageously be operated on a continuous basis although the drying procedure may operate on a batchwise basis, by operating on batches of micropellets from a continuously operating pelletizer. In this way the operability (the percentage of the time that the method is in operation) of the method may be at least 95%. Operability of at least 95% is desirable for any industrial full scale polymer moulding composition production process.
Viewed from a still further aspect the invention provides the use of the micropellet compositions of or produced by the method of the invention in rotational moulding. Viewed from an alternative aspect, the invention also provides a rotational moulding process in which a particulate polymer composition is transformed to produce a moulded product, characterised in that as said composition is used a micropellet composition of or produced by the method of the invention.
Rotational moulding is a well established technique (see for example SE-A-9203167) and the micropellets of the invention can be used in conventional rotational moulding equipment.
Generally for the method of the invention the initial feedstock will be a dry polyolefin (e.g. homo or copolymers of C
2-10
l-olefins, more particularly homo or copolymers of ethylene or propylene, especially of ethylene) in pelletized or unpelletized form, optionally reactor grade polymer or molten polymer. The polyolefin may typically be produced by a polymerization process catalysed by Ziegler-Natta, or chromium based or metallocene or other single site catalysts. The polyolefin may have a narrow or broad molecular weight distribution; however a narrow molecular weight distribution, e.g. less than 4, is preferred. The polyolefin conveniently has a MFR
2.16
in the range 2 to 10, especially 3 to 6. PE of density 950 to 920 kg/m
3
is especially preferred.
Typically the initial feedstock will be at a temperature between ambient and 30° C.
Any colouring agent is preferably used in the form of a master batch, ie. already mixed with a polymer, generally the same or similar polymer as the initial feedstock. LDPE is convenient to use in this regard. The colouring agent may be an inorganic or organic material such as are conventionally used in moulded polyolefin products. Carbon black is particularly preferred.
Initial feedstock, colouring agent and any other desired additives, e.g. radiation stabilizers, antioxidants, antistatic agents, etc., can be fed to an extruder, a mixer or a melt pump by a control system that ensures the components are homogeneously mixed in the desired ratio. Generally the initial feedstock will make up at least 60% wt, more preferably at least 80% wt of the resulting mixture and that mixture will be at least 85% wt. preferably at least 90% wt. polyolefin. Thus for example 99-80% wt. particulate HDPE and 1-20% wt. of a carbon black master batch containing 40% wt. carbon black in LDPE may be fed to an extruder using two loss-in-weight feeders.
Depending on the needs of mixing and whether the polymer is already molten, a mixer, extruder or melt pump may be used to mix the components and build up the pressure necessary to ensure proper flow through the orifices of the die. Generally the mixing will involve feeding in additives from one or more storage tanks under controlled flow conditions (e.g. using appropr

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