GB2161812A

GB2161812A - Particulate cross-linked polymer mixtures

Info

Publication number: GB2161812A
Application number: GB8516830A
Authority: GB
Inventors: John Christopher Parkins; Walter Grunwell
Original assignee: Wardle Storeys PLC
Current assignee: Wardle Storeys PLC
Priority date: 1984-07-07
Filing date: 1985-07-03
Publication date: 1986-01-22
Also published as: GB8516830D0; GB8417412D0

Abstract

A particulate, cross-linked polymeric material, e.g. polyethylene, is mixed with a compatible particulate, uncross-linked polymeric material, preferably in a weight ratio from 1:2 to 1:5, by means of a thermoplastic process e.g. extrusion, moulding or calendering process. A cross-linking agent may be added and the resulting mixture may be expanded by means of a blowing agent at a temperature in the range 190 DEG C to 210 DEG C. The starting cross-linked polymeric material may be derived from waste material.

Description

SPECIFICATION Method of manufacture of polymeric materials This invention relates to a method of manufacture of polymeric materials.

Cross-linked polymeric foams are generally much stronger than uncross-linked foams, and are suitable in applications as diverse as building materials, packaging materials, heat insulators and shock and sound absorbing materials. They are particularly useful as linings in automobile applications, for example, as covers for wheel arches. A known process for the production of a foamed, cross-linked polymer sheet, consists of the thermoplastic extrusion and chemical cross-linking of a previously uncross-linked polymer, to produce a cross-linked polymer sheet, followed by the expansion of the sheet by means of a blowing agent at a high temperature. The process can include the use of additives such as anti-oxidants and lubricants in the extrusion process.One example of a blowing agent is an azo compound which, at a temperature of around 200"C and in the presence of air, causes the polymeric sheet to expand into a foam.

Where shaped products are produced by cutting parts from a sheet material of a cross-linked polymeric foam, there is generally a large amount of waste material, typically 30% or more. This not only represents uneconomical use of the sheet material, but creates the problem of waste disposal.

Both burning the waste and burying it are expensive methods, and unacceptable for environmental reasons.

According to the present invention, there is provided a method of producing a polymeric material comprising: mixing a particulate, cross-linked, polymeric-material with a compatible particulate, uncross-linked, polymeric material by means of a thermoplastic production process.

This method allows for the recycling of waste cross- linked polymeric material such as foam.

It has been found that the resulting product has a higher degree of cross-linking, and thus greater strength, than has been obtained with conventionally produced cross-linked polymeric foams. This is because the cross-linking agent is able not only to cross-link the previously uncross-linked polymer, but also to form links between the previously uncross- linked polymer and the cross-linked polymer.

In order that the invention may be better understood, a preferred embodiment will now be described by way of example.

A low density polyethylene polymer compound (formulation A) containing a peroxide cross-linking agent and an azodicarbonamide blowing agent is extruded into sheet form using a conventional thermoplastic extrusion process at a temperature below 1300C. This sheet is then chemically crosslinked and expanded at a temperature of between 190 and 210 C in an air oven, using a conventional process. The resulting sheet is then cut into the required shapes for a production process.

Any wastage from the cutting of this sheet is then either granulated and pelletized using suitable extrusion compounding equipment or else ground to a powder, for example by cryogenic milling. The result is a particulate, cross-linked, polymeric material (formulation B).

One part of formulation B is then mixed with 2 to 5, preferably 2, parts of a second batch of formulation A. This mixture, together with a chemical cross-linking agent, is extruded into sheet form at a temperature of less than 1300C.

The resulting sheet contains cross-links not only between the molecules of formulation A, but also between the molecules of formulation B and formulation A. The sheet is then expanded into a foam using a blowing agent such as azodicarbonide, at a temperature of between 190 and 210"C, in the presence of air. The resulting expanded material may have at least equal physical characteristics to sheets produced exclusively from uncross-linked materials, due to the higher degree of chemical cross-linking. The presence in the sheet, before blowing, of a pre-cross-linked material may have the benefits of reducing the time taken to achieve the required blowing and of increasing the toughness of the final product.

It will be appreciated that particulate, crosslinked, polymeric material of any of the kinds referred to above can be incorporated in any suitable uncross-linked and/or uncross-linkable polymeric material without subsequent cross-linking. It has been found that the presence, in such composite materials, of the particulate, cross-linked, polymeric material provides satisfactory properties in comparison with the uncross-linked/uncross-linkable polymeric material. The incorporation may be by any of the methods referred to above.

1. A method of producing a polymeric material comprising: mixing a particulate, cross-linked, polymeric material with a compatible particulate, uncross-linked, polymeric material by means of a thermoplastic production process.

2. A method according to claim 1, wherein the uncross-linked polymeric material is cross-linkable, the method further comprising the inclusion of a cross-linking agent in the mix, performing the thermoplastic production process at a temperature below 1300C and then cross-linking the resulting mixture by means of a blowing agent at a temperature in the range of 1900C to 210 C.

3. A method according to claim 1 or claim 1, wherein the thermoplastic production process is an extrusion or calendering process, or a moulding process such as injection-, compression-, blow-, and press-moulding.

4. A method according to any one of claims 1 to 3, wherein the ratio of cross-linked to uncrosslinked material is between 1:2 and 1:5.

5. A method according to claim 4, wherein the ratio is substantially 1 to 2.

6. A method according to any one of claims 1 to 5, wherein the particulate, cross-linked polymeric starting material is prepared by the granula

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Method of manufacture of polymeric materials This invention relates to a method of manufacture of polymeric materials. Cross-linked polymeric foams are generally much stronger than uncross-linked foams, and are suitable in applications as diverse as building materials, packaging materials, heat insulators and shock and sound absorbing materials. They are particularly useful as linings in automobile applications, for example, as covers for wheel arches. A known process for the production of a foamed, cross-linked polymer sheet, consists of the thermoplastic extrusion and chemical cross-linking of a previously uncross-linked polymer, to produce a cross-linked polymer sheet, followed by the expansion of the sheet by means of a blowing agent at a high temperature. The process can include the use of additives such as anti-oxidants and lubricants in the extrusion process.One example of a blowing agent is an azo compound which, at a temperature of around 200"C and in the presence of air, causes the polymeric sheet to expand into a foam. Where shaped products are produced by cutting parts from a sheet material of a cross-linked polymeric foam, there is generally a large amount of waste material, typically 30% or more. This not only represents uneconomical use of the sheet material, but creates the problem of waste disposal. Both burning the waste and burying it are expensive methods, and unacceptable for environmental reasons. According to the present invention, there is provided a method of producing a polymeric material comprising: mixing a particulate, cross-linked, polymeric-material with a compatible particulate, uncross-linked, polymeric material by means of a thermoplastic production process. This method allows for the recycling of waste cross- linked polymeric material such as foam. It has been found that the resulting product has a higher degree of cross-linking, and thus greater strength, than has been obtained with conventionally produced cross-linked polymeric foams. This is because the cross-linking agent is able not only to cross-link the previously uncross-linked polymer, but also to form links between the previously uncross- linked polymer and the cross-linked polymer. In order that the invention may be better understood, a preferred embodiment will now be described by way of example. A low density polyethylene polymer compound (formulation A) containing a peroxide cross-linking agent and an azodicarbonamide blowing agent is extruded into sheet form using a conventional thermoplastic extrusion process at a temperature below 1300C. This sheet is then chemically crosslinked and expanded at a temperature of between 190 and 210 C in an air oven, using a conventional process. The resulting sheet is then cut into the required shapes for a production process. Any wastage from the cutting of this sheet is then either granulated and pelletized using suitable extrusion compounding equipment or else ground to a powder, for example by cryogenic milling. The result is a particulate, cross-linked, polymeric material (formulation B). One part of formulation B is then mixed with 2 to 5, preferably 2, parts of a second batch of formulation A. This mixture, together with a chemical cross-linking agent, is extruded into sheet form at a temperature of less than 1300C. The resulting sheet contains cross-links not only between the molecules of formulation A, but also between the molecules of formulation B and formulation A. The sheet is then expanded into a foam using a blowing agent such as azodicarbonide, at a temperature of between 190 and 210"C, in the presence of air. The resulting expanded material may have at least equal physical characteristics to sheets produced exclusively from uncross-linked materials, due to the higher degree of chemical cross-linking. The presence in the sheet, before blowing, of a pre-cross-linked material may have the benefits of reducing the time taken to achieve the required blowing and of increasing the toughness of the final product. It will be appreciated that particulate, crosslinked, polymeric material of any of the kinds referred to above can be incorporated in any suitable uncross-linked and/or uncross-linkable polymeric material without subsequent cross-linking. It has been found that the presence, in such composite materials, of the particulate, cross-linked, polymeric material provides satisfactory properties in comparison with the uncross-linked/uncross-linkable polymeric material. The incorporation may be by any of the methods referred to above. CLAIMS

5. A method according to claim 4, wherein the ratio is substantially 1 to 2.

6. A method according to any one of claims 1 to 5, wherein the particulate, cross-linked polymeric starting material is prepared by the granula tion and subsequent pelletisation of an extruded cross-linked polymeric foam in sheet form.

7. A method according to any one of claims 1 to 6, wherein the polymeric material is polyethylene or a polyolefin copolymer.

8. A method of producing a polymeric material substantially as herein described.

9. A polymeric material when made by the method of any one of claims 1 to 8.