GB2380448A

GB2380448A - Thermoforming of co-extruded laminates

Info

Publication number: GB2380448A
Application number: GB0123774A
Authority: GB
Inventors: Colin Bellis
Original assignee: HFW PLASTICS Ltd
Current assignee: HFW PLASTICS Ltd
Priority date: 2001-10-03
Filing date: 2001-10-03
Publication date: 2003-04-09
Also published as: GB0123774D0

Abstract

In a co-extrusion process, a first polypropylene is fed through a first line 4 to a first mixer and subsequently to a laminar distributor. A mixture of a second polypropylene and inorganic filler, together having the same melt flow viscosity as the first polypropylene is fed through a second line 2 to the same laminar distributor via a second mixer. A common masterbatch composition eg of highly concentrated pigments may be fed into each line prior to the mixing step. The filler may comprise talc or calcium carbonate. The co-extruded laminate is used in a thermoforming process to produce eg. containers. Scrap or rework from the thermoforming process may be incorporated into line 2 for mixing with virgin polypropylene, the filler and optionally the masterbatch.

Description

METHOD OF MANUFACTURING PLASTIC PRODUCTS This invention relates to a method of manufacturing plastic products, and more particularly to a method of thermoforming plastic containers including mineral fillers.

Thermoforming is a common method of manufacturing thin walled plastic containers such as trays, and most manufacturers of such products use a variety of commercially available plastics in sheet form or on reels to produce the trays.

It is financially beneficial for the manufacturers to be able to purchase virgin pellets of the plastic, and convert these pellets in-house into sheets in reel form from which the products can subsequently be thermoformed.

With this facility, the skeletal trim-ie. the residual sheet left over after the formed containers have been cut out-can be reground into'chips'or rework, and this rework can be fed back into the extrusion line to contribute towards the formation of further sheet. Thus the rework is a substitute for the prime material when next used, and so its value is that of the prime virgin polymer it replaces.

This can be compared with buying in sheet material, in which case the trim is either unusable or is sold back to reprocessors at a significant loss.

Another advantage of in-house production of sheet is that coloured master batch (high concentration pigments)

can be added to clear polymers to produce selectively coloured sheet on demand.

Polypropylene is a conventional material from which thin walled containers are thermoformed, although such containers can be relatively weak, while shrinkage after forming is relatively high.

It would therefore be desirable to be able to provide a stronger polypropylene tray less prone to shrinkage, and this can be achieved by using filled polypropylene-ie. polypropylene containing a mineral filler concentrate, such as calcium carbonate or talc.

However such fillers themselves have certain disadvantages. For example, whilst the filler compound is a poor pigment, on the addition of compound at levels of up to 75% to the polypropylene, together with the addition of master batch at a conventional level of about 2%, the resultant products are much lighter in colour than is actually desired.

Additionally, the filler compound contains a carrier for the calcium carbonate or talc-ie. a wetting agent for the dry powder to allow easier dispersion on extrusionand this makes the filler relatively fluid compared to conventional grades of polypropylene used in extrusion.

According to the present invention a method of thermoforming plastic containers comprises the steps of providing a first extrusion line along which is fed a first controlled quantity of polypropylene and a controlled quantity of filler, said quantities being blended, extruded

and fed as a first material to a laminar distributor, providing a second extrusion line along which is fed a second controlled quantity of polypropylene which is extruded and fed as a second material to the laminar distributor, the polypropylene of the first and second quantities being chosen such that the melt flow viscosities of the first and second materials fed to the laminar distributor are substantially the same, laminating the two materials into sheet form, and forming containers from the laminated sheet.

In a preferred method according to the invention, a controlled quantity of master batch is fed together with the quantities of polypropylene and filler along the first extrusion line to be blended therewith and to comprise part of the first material, while a further controlled quantity of master batch is also fed together with the quantity of polypropylene along the second extrusion line to be blended therewith and comprise part of the second material.

With such an arrangement, the colour of the laminate comprising the first material can be accurately determined by appropriate selection of the quantity of master batch incorporated therein.

The laminar distributor may be such as to form a two layer sheet, or, more preferably, a three layer sheet comprising an inner layer of the second material (filler) sandwiched between outer layers of the first material, the outer layers each being typically up to 30% by volume and

the central layer being at least 40% by volume of the sheet.

The outer layers are such that the colour thereof determines the colour of the final product.

Conveniently the method utilises the trim produced on formation of a container by grinding this trim into rework and feeding a controlled quantity of rework along the first extrusion line to be blended into the first material.

By way of example only, the invention will now be described in greater detail with reference to the accompanying drawing which is a schematic illustration of the component parts of equipment for carrying out the method of the invention.

In the drawing, the following abbreviations apply: V1 - Virgin polypropylene pellets for main extruder; VL-Vacuum loader; WS-Weigh station; RW-Rework ; F-Filler ; D-Drier ; MB-Master batch; V2-Virgin polypropylene pellets for co-extruder; B-Blender ; MEX-Main extruder; COEX-Co-extruder ; Ft-Filter ; P-Pump ;

LD-Laminar distributor; D-Die.

The equipment for carrying out the method of the invention is illustrated schematically in the drawing and effectively comprises a first or main extrusion line indicated generally at 2 above the dotted line I-I, and a second or co-extrusion line indicated generally at 4 below the dotted line I-I.

In the main extrusion line 2, batches of pelleted polypropylene V1, rework RW (as hereinbefore defined), filler F in the form of pelleted calcium carbonate or talc in a carrier, and master batch MB are fed to respective vacuum loaders VL, the filler by way of a drier D, and thence to respective weigh section WS, whereafter they are fed in predetermined chosen quantities to a common blender

B1.

The blended material then passes through a main extruder MEX where it is melted and pumped to and through a screen filter Ft1. A gear pump PI feeds the filtered material, which at this stage has a particular melt flow viscosity, to a laminar distributor LD.

In the mean time, in the co-extrusion line 4, batches of pelleted polypropylene V2 and master batch MB are fed to respective vacuum loaders VL and weigh stations WS, and thereafter to a blender B2. The blended material is fed to a co-extruder COEX where it is melted and pumped to and through a screen filter Ft2, through a gear pump P2 and

thence to the laminar distributor where the material has a particular melt flow viscosity.

The nature of the polypropylene pellets V1 and V2 are chosen such that the melt flow viscosities of the materials fed to the laminar distributor LD from the extrusion lines 2 and 4 are substantially the same as each other.

The laminar distributor is configured to form a three layer sheet, the inner layer of which comprises pigmented filled polypropylene from extrusion line 2 sandwiched between outer layers of pigmented polypropylene each from the extrusion line 4. Conveniently and typically the inner layer comprises about 40% by volume of the sheet and each of the outer layers comprises about 30% by volume of the sheet.

The laminate is then fed to a die D where the extruded sheet is then cooled and coiled for subsequent thermoforming. The colour of the final products is that of the relatively thick outer layers, the colour of which layers can be accurately determined by the controlled addition of master batch to the virgin polypropylene V2.

The skeletal trim from the formed products can be reused by granulating it back into chip form and adding it as rework RW to the virgin polypropylene VI without detriment to the colour of the final product, the filled polypropylene component of the rework having had master batch MB added thereto at the outset of the main extrusion line 2.

The following table indicates the % content of the constituents of the first and second materials fed to the laminar distributor LD for different proportions of rework RW included with the virgin polypropylene VI, filler F and master batch MB at the outset of the main extrusion line 2, and in particular the % master batch needed to maintain the colour of the final product for different % rework. In this table, calcium carbonate represent the filler, and PP with MF1 of 2 and PP with MF1 of 13 represent different virgin polypropylenes, while sub-table i) utilises no rework in the main extrusion line 2, sub-table iv) relates to the maximum permissible amount of rework (65% of material to be extruded in the main extrusion line 2), and sub-table ii) and iii) detail the % constituents using 50% and 75% respectively of the maximum amount of rework.

Formulations i) virgin main extruder MEX Co-extruder COEX

l%mb 2%mb PP with MF1 of 2 V1 29. 8 29.6 caC03 fíller MB F 69. 6 69.2 rework RW 0 0 masterbatch MB 0. 6 1.2 PP with MF1 of 13 V2 98. 1 96.2 masterbatch MB 1.9 3.8 ii) rework maximum main extruder MEX

l%mb 2%mb PP with MF1 of 2 V1 14. 7 14.6 CaCO) filler MB F 52. 4 52.3 rework RW 32. 6 32.5 masterbatch MB 0.3 0.6

Co-extruder COEX

pp with MF1 of 13 V2 98. 1 96. 2 masterbatch MB 1. 9 3. 8 iii) rework 3/4 maximum main extruder MEX Co-extruder COEX

limb 2tomb PP with MF1 of 2 Vl 7. 4 7. 3 CaCO3 filler MB F 43. 7 43.7 rework RW 48.8 48. 8 masterbatch MB 0. 1 0.2 PP with MF1 of 13 V2 98. 1 96.2 masterbatch MB 1. 9 3. 8 iv) rework at maximum main extruder MEX Co-extruder COEX

limb 2tomb PP with MF1 of 2 VI 0 0 CaCO3 filler MB F 35 35 Rework RW 65 65 masterbatch ME 0 0 PP with MF1 of 13 V2 98. 1 96.2 masterbatch ME 1. 9 3. 8

Clearly the method as described above may be modified or varied without departing from the scope of the invention. In particular, the filler may be other than calcium carbonate or talc, the laminar distributor may form sheet of two or more than three layers, and the % values of the individual constituents of the first and second materials, and of the layers comprising the final product, may differ from those quoted.

Other modifications and variations will be apparent to those skilled in the art.

Claims

CLAIMS 1. A method of thermoforming plastic containers comprising the steps of providing a first extrusion line along which is fed a first controlled quantity of polypropylene and a controlled quantity of filler, said quantities being blended, extruded and fed as a first material to a laminar distributor, providing a second extrusion line along which is fed a second controlled quantity of polypropylene which is extruded and fed as a second material to the laminar distributor, the polypropylene of the first and second quantities being chosen such that the melt flow viscosities of the first and second materials fed to the laminar distributor are substantially the same, laminating the two materials into sheet form, and forming containers from the laminated sheet.
2. A method as claimed in claim 1 in which a controlled quantity of master batch is fed together with the quantities of polypropylene and filler along the first extrusion line to be blended therewith and to comprise part of the first material, a further controlled quantity of master batch being fed together with the quantity of polypropylene along the second extrusion line to be blended therewith and comprise part of the second material.

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3. A method as claimed in claim 1 or claim 2 in which the laminar distributor forms a three layer sheet comprising an inner layer of the second material sandwiched between outer layers of the first material.
4. A method as claimed in claim 3 in which the outer layers are each up to 30% by volume and the central layer is at least 40% by volume of the sheet.
5. A method as claimed in any one of claims 1 to 4 in which trim produced on formation of a container is ground into rework, a controlled quantity of rework being fed along the first extrusion line to be blended into the first material.
6. A method as claimed in any one of claims 1 to 5 in which the filler is calcium carbonate or talc.
7. A method of thermoforming plastic containers substantially as described with reference to the accompanying drawings.