GB2360241A

GB2360241A - A method of and a plant for producing products from a plastics composition

Info

Publication number: GB2360241A
Application number: GB0006178A
Authority: GB
Inventors: Raj Chandrakant Mehta; Hendrik Johannes Christia Smit
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-03-14
Filing date: 2000-03-14
Publication date: 2001-09-19
Also published as: GB0006178D0

Abstract

A method of forming a plastics sheet comprises a) filling a mould 10A with a plastics material composition b) heating the mould from below to cure at least some of the basal plastics material in the mould c) heating the mould from above to cure the surface plastics material in the mould. The method preferably also comprises an intermediate step of removing at least some of any excess, uncured surface plastics material by means of suction column 18. The method is performed in a plant which includes a hopper 14 for discharging the material into the mould 10A, which is heated by bottom heater (e.g. burners of electrical elements 16) and by an overhead heater, and a leveller, which spreads out the material. Plural moulds (10a to 10d) may be placed on a conveyor.

Description

2360241 A METHOD OF PRODUCING PRODUCTS FROM A PLASTICS COMPOSITION THIS

invention relates to a method of producing products from a plastics composition.

Plastics products are made from a variety of plastics materials and compositions. For example, plastic roof sheeting is usually made from polyvinyl chloride (PVC) or glass fibre. There are, however. disadvantages with PVC and glass fibre sheets in that they tend not weather well and tend to be brittle or to become too brittle causin. them to crack. Also, plastics compositions tend to be highly flammable which decreases their safety.

The plastics composition comprises a mixture of polyethylene and an inert bulking or filler material.

C> The inert bulking or filler material may be cellulosic material, such as sawdust, a silica-containing material, such as sand, vermiculite or glass fibre.

The ratio of polyethylene to bulking or filler material is preferably in the region from about 8: 1 to about 25: 1.

The plastics composition may comprise a mixture of powdered polyethylene and sawdust or sand in a ratio of about 8.3:1.

The plastics composition may comprise a mixture of powdered polyethylene and vermiculite in a ratio of about 25:1 The plastics composition may comprise a mixture of powdered polyethylene and glass fibre in a ratio of about 6.6:1.

A flexible plastics sheet is provided which is formed from a plastics composition.

The plastics sheet can range in thickness from 0.5mm to 10.Omm, depending on the profile and quality required.

An aspect of the invention is the method of forming a plastics product comprising the steps of R= filling a mould with a plastics material composition heating the mould from below to cure at least some of the basal plastics material in the mould; and 2 heating the mould from above to cure the surface plastics material in the mould.

The method preferably also comprises an intermediate step of removing at least some of any excess, uncured surface plastics material.

According to another aspect of the invention a plant for forming a plastics product comprises a mould, a mould support, a plastics material discharge and storace facility for discharging plastics material into a mould, a leveller, an overhead beater and a bottom heater for curing the plastics material in the mould.

The plant preferably also includes an overhead suction facility for removing Z:1 any excess uncured surface plastics material from the mould.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing:

Figure 1 is a schematic representation of a plant of the invention for producing plastics sheeting, which may have a variety of profiles, depending on the mould.

A variety of plastics compositions for moulding plastics products are provided. The compositions all comprise an amount of powdered polyethylene and an inert bulkin 'c, or filler material selected from sawdust. fine sand, vermiculite and powdered glass fibre.

Polvethylene is an expensive material and thus the inclusion of cheaper buling or filling agents in a composition containing polyethylene will reduce the cost of any plastics product produced from the composition. The bulk and filling agents do, however, also have additional properties and functions. For example, the sand serves to increase the weight of a plastics product, such as a sheet, made from the composition. The sand is also nonreactive and does not affect the workability of the plastics composition. The vermiculite is a fire resistant material and it reduces the flammability of the plastics product and also provides a certain amount of insulation. The powdered glass fibre reflects or disperses heat and therefore also acts as an insulating material. Various colourants may also be added to the composition. It is important that the plastics product produced from the composition must be flexible to allow for easy attachment thereof to various surfaces. In order to ensure this, carefully selected amounts of polyethylene and the bulking and filling agents must be mixed. Typical mixtures are the following c c 3 polyethylene and sawdust 25:3 polyethylene and sand 25:3 polyethylene and vermiculite 25:1 polyethylene and powdered glass fibre 2 5:3.75 The vermiculite that is used is obtained from Mandoval Vermiculite and is a pulverized grade of vermiculite exfoliated under carefully controlled conditions from Phalaborwa quality vermiculite ore. It has the following characteristics 1. Loose Fill Bulk Densitv - 200 g/1 2. Water Absorption 220 - 280% 3. Sieve Aperture Range (microns) (%) on 25 1 -4 on 45 0 4. Packaging kg in 3 ply paper bags and it has the following., typical physical and chemical characteristics 1. Tvvical Phvsical Characteristics Melting Point 1,3 150C Sintering Temperature 1,2600C t- Specific Heat 0.2 Specific Gravity 2.6 Thermal Conductivit-% 0.062 - 0.065 W/m0C Incombustibility 1,1000c Ph 8 Cation Exchange 90 - 100 milli-equivalents 100.

Insolubility water, organic solvents Water Absorption 20% of air dry mass 4 2. Tvoical Chemical Anaivsis Weight % Silicon oxide (SiO,) 3 9.4 Magnesium oxide (Mú,0) 23.4 Aluminium Oxide (A1203) 12.1 Iron oxide (Fe203) 5.5 Potassium oxide (K20) 2.5 Calcium oxide (CaO) 1.5 Titanium oxide (Ti02) 1.3 Phosphorous oxide (P20,) 0.2 Water + 1059C 5.2 Water 1059C 6.0 Less than 0.1% - Li20, BaO, Cl, S03 There are no F, Zr02, Cr2031 V203, NiO, CoO Asbestos free Free silica content <1.0% It has been found that sheets made using these plastic compositions weather well, provide insulation and are not brittle.

Plastic sheets are made from the plastics compositions using the unique method of the invention. This method will be described with reference to Figure 1.

A plurality of empty moulds (10a to 10d) are placed one behind the other on a conveyor (12). They, are first cleaned and coated with a mould release agent. The conveyor (12) moves at a constant.. pre-set speed, which can be increased or decreased depending on the process parameters. Each empty mould (10a) is passed underneath a feed hopper (14), which holds a particulate dry plastics composition of the invention. The feed hopper (14) is opened to discharge an amount of the plastics material into the mould (10a) as it passes underneath it. There is no need for sophisticated control mechanisms on the feed hopper (14) to control the amount of plastics material that is being discharged. The plastics material within the mould (10a), is then levelled by a leveller (not shown) which spreads it out to a depth of between 1 and 50 mm in the mould.

The filled mould (10a) is then passed over a bottom heater, typically burners or heating elements or electrical elements (16) which spans a distance of about 1.0m, at a controlled speed, which heats the filled mould (10a) from below. Heat at a temperature from about 1500C to 4500C is applied to cause a basal layer of the plastics material in the mould (10a) to melt or cure to a depth ranging from 0.5mm to 1O.Omm, depending on the profile and quality required.

The mould (10a) is then passed underneath an overhead suction column (18) which removes any excess uncured powdered plastics material from the surface of the mould ( 1 Oa) and transfers it to a storage facility from where it can be re-used. The mould (10a) is then passed, at a carefully controlled speed underneath an overhead heater or overhead and bottom heater (20) which spans a distance of about 1.0m and which also applies heat at a temperature of between 150 0 C to 450 mO C to melt or cure the surface of the plastics material in the mould (10a). The removal of excess plastics material and the subsequent bottom and/or overhead curing of the remaining material ensures that a sheet with a smooth surface and a consistent thickness is obtained. The mould (10a) is then moved to a level cooling surface (21) where it is allowed to cool for between 1 to 20 minutes, typically 10 minutes, before the formed sheet (22) is removed from the mould (10a).

It is also possible for the moulds (10a) to remain stationary while the overhead discharge hopper (14), suction column (18), overhead heater or overhead and bottom heater (20) and bottom heater (16) move relative to the moulds to perform the functions set out above.

Although the formed sheets are typically 6.Om in length, the thickness, length and width of the sheets is dependent on the mould used. They take approximately three minutes to form and approximately ten minutes to cool. They may have a variety of profiles, depending, of course, on the mould, and they may be corrugated, flat or have a box-type cross sectional profile. They are relatively cheap to make and are flexible to allow for easy attachment, cutting and working thereof.

It is also possible for the method to be used for the production of sheets using other plastics materials, for example PVC, HDPE, LDPE, Polypropylene, and an inert bulking or filler material, as well as for the production of sheets using rubber with a hardener and an inert bulking or filler material.

Claims

6 CLAIMS

1 A method of forming a plastics product comprises the steps of filling a mould with a plastics composition Z> heating the mould from below to cure at least some of the basal plastics composition in the mould; and heating the mould from above to cure the surface plastics composition in the mould.

2. A method accordine, to Claim 1 which also comprises the step of removing at least some of any excess, uncured surface plastics composition intermediate the step of heating the mould from below and heating it from above.

3. A method according to Claim 2 wherein the mould is heated to a temperature between 1500C to 4500C from below.

4. A method according to Claim 2 and Claim 3 wherein the mould is heated to a temperature between 1500C to 4500C from above.

5. A method according to any one of Claims 1 to 4 which also comprises c the steps of cooling the heated mould and thereafter removing the moulded plastics product from the mould.

6. A method according to any one of Claims 1 to 5 wherein a plurality of moulds are filled, heated and cooled sequentially.

7. A method accordin. to any one of Claims 1 to 6 wherein the plastics composition comprises a mixture of polyethylene and an inert bulking or filler material.

8. A method according to any one of Claims 1 to 6 wherein the plastics composition is a mixture of plastics powder or plastics resin or rubber with hardener and. an inert bulking or filler material.

9. A plant for forming a plastics product comprising t a mould, a mould support. a plastics composition storage and discharge facility for discharging plastics composition into the mould, a leveller, and an overhead beater and a bottom heater for heating the plastics composition in the mould.

7

10. A plant according to Claim 9 which also comprises an overhead suction facility positioned intermediate the overhead heater and the bottom heater.

1 1. A plant according to Claim 9 or Claim 10 which also comprises a movable conveyor on which the moulds are placed.

12. A method according to Claim 1 substantially as herein described with reference to Figure 1.

13. A plant according to Claim 9 substantially as herein described with reference to Figure 1.