GB2177024A

GB2177024A - Process for the manufacture of expandable polystyrene products

Info

Publication number: GB2177024A
Application number: GB08614600A
Authority: GB
Inventors: Nigel Graham Rowley
Original assignee: COLUMBUS PLASTICS Ltd
Current assignee: COLUMBUS PLASTICS Ltd
Priority date: 1985-06-25
Filing date: 1986-06-16
Publication date: 1987-01-14
Also published as: GB8614600D0; GB2177024B; GB8516021D0

Abstract

A process for the production of polystyrene items 9 comprising coating unexpanded or partially expanded beads of polystyrene with an electrical conducting material and then forming the beads, in a mould, into the shape of the mould by subjection to heat and water to form a coherent mass in which the beads adhere together. <IMAGE>

Description

SPECIFICATION Process for the manufacture of expandable polystyrene products This invention relates to a process for the manufacture of products, of many kinds, from expandable polystyrene. Items such as storage or packaging containers for many types of goods, decorative structures for display or utility, storage reels on which filaments, or other products can be wound and many other items, can be produced.

The advantages of polystyrene are well known, namely it is very light weight, easily moulded, clean and relatively soft and it also has other specific attributes, but the usual expanded, low density polystyrene structures are also very weak and crushable on impact. Although the material is very suitable as a packaging material, it is usually necessary to protect it against knocks, by means of some outer casing, such as cardboard. Containers for storage or transportation of goods can be moulded in low density polystyrene to the required shape, but it is so soft and breakable that the reuse of the container is frequently very limited or impractical.

A higher density polystyrene has therefore been developed, having much improved strength and rigidity as well as being capable of accurate moulding. The mouldings made in such high density polystyrene can have edges and other shapes much better defined than equivalent ones in normal low density polystyrene, as well as having a greatly improved surface finish. High density polystyrene in this context means, generally, that having a weight of 6 to 8 Ibs/ft.3 or 100 to 120 kg/m3.

The differences in physical characteristics of the low and high density polystyrene are due to the processes used for production. In the production of items made from low density polystyrene, the initially granular material is subjected to steam at high temperature, that is at least in excess of 140 C. This causes each bead or granule to expand to many times its original size, through expansion of gases trapped within the beads. Expansion takes place in the confines of a mould of the appropriate shape and a finished moulding is produced with the required shape.

It has been found that if the beads are subjected to heat and water, at lower temperatures, namely by immersion in boiling water, they will be expanded to a lesser extent and remain harder. If they are then used to fill a mould and again subjected to heat and water, a high density structure can be produced, having the greater strength and better finish characteristics.

The high density is given, in part, by the pressure which builds up within the confines of the mould as the material attempts to expand within the mould, as heat and water come into contact with it. The strength of the material is, however, a measure of the resistance to shearing of individual beads and also of the bond between adjacent beads.

There is, however, a tendency for an electrical charge to build up on the surfaces of expandable polystyrene of any density.

In packaging of certain products, such as electronic circuit boards or other electrical or electronic devices, this electrical discharge from the expanded polystyrene can damage or prematurely age such devices.

It is the object of this invention to provide a process for the manufacture of expanded polystyrene products, in which any static electrical charge inherent in the material can be effectively neutralised.

In accordance with the invention there is provided a process for the production of expanded poly-styrene items comprising coating unexpanded or partially expanded beads of polystyrene with an electrical conducting material and then forming the beads, using a mould, into the shape of the mould by subjection to heat and water, to form a coherent mass in which the beads adhere to one another.

Preferably the process comprises the steps of subjecting raw or virgin unexpanded beads of polystyrene to heat and water for a controlled period, to create partial expansion of the beads, the beads being then coated with an electrical conducting material, then transferred into a mould of a shape to correspond with that required of the finished item, and again subjecting them to heat and water, resulting in further expansion of the beads to form a coherent mass conforming in shape to the mould, in which individual beads adhere to each other.

Preferably the electrical conducting material is a carbon based material, such as carbon black. This is conveniently applied to the beads by immersing them in a liquid in which the carbon based material is suspended.

In coating the beads, the electrical conducting material may be in a vat, into which the beads are placed, agitation being needed to ensure adequate and consistant coating of each bead. The beads may be at least partially dried after coating.

This not only assists in easy handling of the beads, but also serves to polarise adhesion of the electrical conducting material on the beads.

Preferably the coating on the beads is of such thickness or distribution that the adhesion between the beads allows formation of a strong cohesive mass.

It is preferable that, in the finished structure, the adhesion between the beads is approximately equal in strength to the breaking strength of individual beads. If this strength characteristic is obtained, the resulting structure has approximately constant breaking strength throughout.

When the final expansion takes place within the mould, the beads against the mould surfaces tend to be deformed to follow the shape of these surfaces and a generally even coating of the electrical conducting material is formed over these generally aligned surfaces, as well as throughout the moulding.

Conveniently, a drying process follows initial expansion and prior to coating of the beads with the electrical conducting material, such drying process being, however, sufficient to remove only a proportion of the water content.

The consistency and concentration of the electrical conducting material is critical. It is controlled to ensure against excess application to the beads but sufficient to build up an adequate coating. If excess material is deposited there is risk that some will be transferred to the item being packaged. In the case, for example, of electrical or electronic devices, the presence of such material could interfere with conducting or insulating properties of the devices and thus affect their performance.

Conveniently, subjection to water, when in the mould, is by immersion of the mould in boiling water for a predetermined period. After this stage, immersion in cold water arrests further tendency for expansion. After removal from the mould, a further drying process preferably follows. It is to be understood, however, that the use of steam is an laternative and references herein to heat and water may include the use of steam.

In accordance with a further aspect of the invention, there is provided an item made from expanded polystyrene, in which the individual beads, from whiwh it is formed, are coated with an electrical conducting material.

Preferably the polystyrene structure is of high density.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-section of a container constructed in accordance with the present invention, Figure 2 shows unexpanded beads of polystyrene used as the raw material in the process according to the invention, Figure 3 shows an alternative form of bead, Figure 4 shows the beads after initial, partial expansion, Figure 5 shows the beads after coating, Figure 6 is a scrap view of a surface of an expanded polystyrene item, and Figure 7 is a broken section of the item.

The process for the manufacture of products from expanded polystyrene starts with raw material in the form of beads or granules of the unexpanded material. Round beads are shown in Figure 2 at 10 and an alternative form of cylindrical shape bead 11 is shown in Figure 3. These are usually referred to as raw or virgin beads.

The beads may be treated with a substance rendering them resistant to combustion, this being introduced in the process of manufacture of the raw beads. Alternatively or additionally, the beads may contain pigment of various colours. The pigmentation is also introduced during the production of the raw beads.

Figure 4 shows, at 12, the beads which have been subjected to an initial partial expansion proces. To achieve this, the beads are loaded into a vat containing boiling water and remain there for a predetermined time sufficient to partially expand them. The timing of the immersion is critical to produce beads of the required hardness and agitation should be carried out in order to ensure even treatment of all beads.

After this treatment, the beads are removed from the boiling water and are chilled in a cold water tank.

They are then conveniently partially dried so that they can be more easily handled. Though the beads will contain some moisture, they are dry enough to be handleable in loose form and will not easily stick together.

There next follows a process for coating each bead with an electrically conducting material. A suitable material for this purpose is a carbon based material such as carbon black which contains additives to cause it to adhere to the bead surfaces.

In one practical example the following proportions, for the solution in which the beads are immersed, are used: Aqueous carbon solution 15% Polyvinyl acetate solution 15% Water 70% Conveniently the aqueous carbon solution has 35% carbon to 65% water. The polyvinyl acetate solution acts as an adhesive serving to cause the beads to stick together, as will be described.

This stage in the process comprises placing the beads into a vat containing a solution in which the carbon black and adhesive solutions are in suspension. The beads remain in contact with this material for long enough for each to be coated with the electrically conducting material. Agitation of the beads during immersion is important to ensure adequate coverage of all beads. The concentration is, however, critical in order to prevent excessive amounts being applied.

The beads 13 are next removed and excess liquid drained off and they are then at least partially dried.

The coating material is such that only the surface of each bead is coated and penetration into the interior of each bead is insignificant. The surface coating, furthermore, is not of necessity, continuous. Since the beads are generally cellular in construction, some external cells and cracks will fill with the material, whereas intervening surface portions will not. However, there is adequate covering for each bead such that a path exists for the dissipation of electrical current which may otherwise build up on the bead surfaces.

To form any item, such as the container 9 shown in Figure 1, from expanded polystyrene, moulds are used having internal cavities defining the required shapes. A mould is filled with the beads through an opening in the mould wall and this is then covered by a lid. Conveniently, a substantiall wall section is defined by the lid and when this is placed into contact with the beads in the filled mould, pressure can be applied by mechanical means such as external cams on the mould parts.

The moulds are now lowered into a container of boiling water and remain there for a period which is critical to the accurate formation of the item. The length of time required for each item, however, depends on such factors as the precise composition of the raw material and the size of the item being produced, including the relative thicknesses of various part of the item. A typical example such as the container shown in Figure 1, would require a time scale of approximately twenty minutes.

Such immersion in boiling water causes further expansion of the beads and these come together to form a coherent mass with minimal spaces between the individual beads. Great pressure is built up as a result of this process so that the external surface of the mass of beads takes up the precise and accurate form of the mould. Furthermore, a relatively smooth high gloss finish to the item is produced.

If the item is exposed to boiling water for too long a time it will tend to soften and the exges will be less well defined. The reason for this is thought to be that the gases contained within each bead escape during the expansion process and if they become completely exhausted, the bead will tend to shrink under the application of further heating.

Items thus formed, however, if correctly exposed, will have a clean and accurate external finish and will be hard and thus substantially more durable than items made from expanded polystyrene in the conventional soft low density form.

They also have a far greater adhesion between individual beads.

The electrically conducting material will not adversely affect the adhesion between individual beads and preferably the strength of the bond between adjacent beads will approximate to the breaking strength of individual beads. This is illustrated by Figure 6 in which a broken section of a moulded item is shown. There are beads shown at 14 which have been broken and thus show the internal white or other colour of the polystyrene and but are other beads 15 which have not been broken and remain complete but which have separated from their adjacent beads by breaking of the adhesive joint.

Adhesion between the beads is by chemical as well as physical bonding. Figure 6 shows the surface characteristic of the finished item where the beads have been deformed but provide a substantially flat or otherwise shaped surface.

It is, however, seen from Figure 7 that each bead, having a portion on the exterior of the item, is coated with the electrically conducting material, so that a generally continuous film is formed over the whole of the exterior surface of the item, as well as through its structure.

After immersion in the boiling water the mould, still containing the polystyrene item, is chilled in cold water for a predetermined time. After removal from the mould, the item is air dried.

Items of many kinds can be produced by the method described and will have adequate coating on their exterior surfaces to enable any build up of electrical charge to be dissipated or neutralised through any convenient earthing route. This renders the item suitable for the production, for example, of containers for electronic equipment, which might be damaged by the presence of electrical charge on the container.

Varying grades of raw material may vary the processing times required. It is also possible to use the coating technique on beads used to produce low density expanded polystyrene products.

Claims

1. A process for the production of expanded polystyrene items comprising coating unexpanded or partially expanded beads of polystyrene with an electrical conducting material and then forming the beads, using a mould, into the shape of the mould be subjection to heat and water, to form a coherent mass in which the beads adhere to one another.

2. A process as claimed in claim 1 including the steps of subjecting raw or virgin unexpanded beads of polystyrene to heat and water for a controlled period, to create partial expansion of the beads, the beads being then coated with an electrical conducting material, then transferred into a mould of a shape to correspond with that required of the finished item, and again subjecting them to heat and water, resulting in further expansion of the beads to form a coherent mass conforming in shape to the mould, in which individual beads adhere to each other.

3. A process as claimed in claim 1 or claim 2 in which the conducting material is a carbon based material applied by immersing the beads in a liquid in which the carbon based material is suspended.

4. A process as claimed in claim 3 in which the electrical conducting material is in a vat, into which the beads are placed, agitation taking place to ensure coating of the beads.

5. A process as claimed in claim 4 in which the beads are at least partially dried after coating.

6. A process as claimed in any one of the preceding claims in which a drying process follows initial expansion and prior to coating of the beads with the electrical conducting material, such drying process being, however, sufficient to remove only a proportion of the water content.

7. A process as claimed in any one of the preceding claims wherein the beads, when in the mould, are immersed in boiling water for a predetermined period.

8. A process as claimed in claim 7 in which the mould is afterwards immersed in cold water to arrest further tendency for expansion.

9. A process as claimed in claim 8 in which the item is removed from the mould and subjected to a further drying process.

10. An item made from expanded polystyrene in which individual beads, from which it is formed, are coated with an electrical conducting material.

11. An item as claimed in claim 10 wherein the electrically conducting coating material is a carbon based material.

12. An item as claimed in claim 11 wherein the electrically conducting coating material includes an adhesive material.

13. A process for the production of polystyrene items substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

14. An item made from high density expanded polystyrene substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

14. An item made from expanded polystyrene substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1 to 4, 6, and 10 to 14 above have been deleted or textually amended. (b) New or textually amended claims have been filed as follows:

1. A process for the production of high density expanded polystyrene items comprising coating partially expanded beads of polystyrene with an electrical conducting material and an adhesive material and then forming the beads, using a mould, into the shape of the mould by subjection to heat and water, to form a coherent mass in which the beads adhere to one another.

2. A process as claimed in Claim 1 including the steps of subjecting raw or virgin unexpanded beads of polystyrene to heat and water for a controlled period to create the partially expanded beads, the beads being then coated with an electrical conducting material and an adhesive material, then transferred into a mould of a shape to correspond with that required of the finished item, and again subjecting them to heat and water, resulting in further expansion of the beads to form a coherent mass conforming in shape to the mould, in which individual beads adhere to each other.

3. A process as claimed in Claim 1 or Claim 2 in which the electrical conducting material is a carbon based material applied by immersing the beads in a liquid in which the carbon based material and the adhesive material are suspended.

4. A process as claimed in Claim 3 in which the electrical conducting material and the adhesive material are in a vat, into which the beads are placed, agitation taking place to ensure coating of the beads.

6. A process as claimed in any one of the preceding claims in which a drying process follows initial expansion and prior to coating of the beads with the electrical conducting material and the adhesive material, such drying process being however, sufficient to remove only a proportion of the water content.

10. An item made from high density expanded polystyrene in which individual beads, from which it is formed, are coated with an electrical conducting material and an adhesive material.

11. An item as claimed in Claim 10 wherein the electrically conducting coating material is a carbon based material and includes an adhesive material.

12. An item as claimed in Claim 10 in which the adhesion between the beads is approximately equal in strength to the breaking strength of individual beads.

13. A process for the production of high density polystyrene items substantially as hereinbefore described with reference to and as shown in the accompanying drawings.