GB2314841A

GB2314841A - A blowing agent and a process for extruding plastics material

Info

Publication number: GB2314841A
Application number: GB9708863A
Authority: GB
Inventors: George Gerard Cronin; William Anthony Walsh; Daniel Aiden Harte
Original assignee: FOAMING TECHNOLOGY Ltd
Current assignee: FOAMING TECHNOLOGY Ltd
Priority date: 1996-07-04
Filing date: 1997-04-30
Publication date: 1998-01-14
Anticipated expiration: 2017-04-30
Also published as: GB2314841B; IE960489A1; GB9708863D0

Abstract

An inward foaming Celuka process for forming a foamed extruded article, typically, a soffit, fascia board, architrave or the like comprises extruding a uPVC mixture through a twin screw extruder having a plurality of band heaters arranged around the barrel for forming a plurality of heating zones, in this case, five zones. The uPVC mixture comprises uPVC resin in powder form, a heat stabilising agent, for example tin, and a blowing agent which comprises in combination sodium bicarbonate and azodicarbonamide, and silicate. The extruded article is hauled off from the extrusion die at a haul-off rate which provides an extruded article of density of less than 0.5 gms per cm., and in general, of density in the range of 0.45 to 0.475 gms per cm. without loss of strength, and without surface blemishes, defects, surface hollows and dips, and with an increased haul-off rate.

Description

"A blowing agent and a process for extruding plastics material" The present invention relates to a blowing agent for use in the foaming of a plastics material, and to a plastics material comprising the blowing agent. The invention also relates to an article formed from a plastics material which comprises the blowing agent.

Further, the invention relates to a process for extruding uPVC material, and in particular, to a process for extruding uPVC material using an inward foaming Celuka process, and the invention also relates to an article extruded using the process, typically, an elongated article, such as, for example, a soffit, a fascia board, architrave or other building product.

The production of plastics articles using various foaming forming processes is known, for example, an inward foaming Celuka process is commonly used in forming extruded articles, for example, soffits, fascia boards, architraves and other relatively elongated articles. Such articles may be formed from any of a number of plastics materials, however, for reasons of colour stability, uPVC plastics material is commonly used in the formation of such articles. In general, a uPVC raw material mixture is prepared, which comprises a powdered mixture of uPVC resin, a heat stabilising agent and a blowing agent. The uPVC mixture is supplied at an upstream end to a barrel of a screw extruder, which in general, is a twin screw extruder.

The uPVC mixture is heated, melted and urged through the barrel by the twin screws to a downstream end of the barrel where the raw material forms an extrudate, which is in turn urged through an extrusion die for forming the article. A mandrel is placed in the centre of the extrusion die for facilitating inward foaming into the core of the extrudate. Foaming in the surface layer of the extrudate is prevented by rapid cooling of the outer surface of the extrudate as it leaves the extrusion die. Where a particularly high quality surface finish is desired on the article, a melted nonfoaming uPVC material is extruded around the surface of the extrudate adjacent the extrusion die for forming the outer surface of the article.

In such known processes the temperature of the raw material is heated from two sources, firstly, heat is supplied to the raw material through the barrel from an external heat source, typically, electrically powered band heaters which extend partly or completely around the barrel of the screw extruder, and secondly, from work done on the raw material by the screws of the extruder.

While these known processes are satisfactory for producing a foamed extruded article of density greater than 0.5 gms per cm3, in general, they are unsuitable for satisfactorily producing articles of lower density.

The main problem that arises when an extruded article is extruded with a density below 0.5 gms per cm3 is that the extruded article suffers from distortion and surface hollows and dips which result from irregular inward shrinkage of the article. In general, this problem arises as a result of the fact that the gas pressure in the cells of the foamed article drops to a level at which it is no longer able to counter-balance the cell wall surface tension prior to the temperature of the cell structure being brought below the glass transition temperature of the material of the extruded article. In foamed extruded articles of density above 0.5 gms per cm3 this problem in general, does not arise due to the higher density, and thus, the smaller cell structure and more rapid heat transfer from the extruded article. In foamed extruded articles of density below 0.5 gms per cm3 this problem could be partly overcome by more rapidly freezing the cell structure. However, in practice, this, in general, is not a practical proposition due to the quantity of heat which has to be removed from the interior of the foamed extruded article.

Similar problems arise in the extrusion of other foamed articles of other plastics materials irrespective of the foaming process, and indeed, such problems also exist in the formation of foamed articles irrespective of the forming.

It will be readily appreciated that, if it were possible to produce foamed articles of density below 0.5 gms per cm3 without loss in quality, strength and other relevant parameters, significant savings in raw material usage could be achieved, as well as production rates from an extruder could be increased.

There is therefore a need for a blowing agent for use in the foaming of a plastics material in the formation of a foamed plastics article of density below 0.5g per cm3, and indeed at or above 0.5g per cm3. There is also a need for a plastics material which comprises the blowing agent which is suitable for use in the formation of a foamed plastics article. Additionally, there is a need for a process for extruding uPVC material which overcomes the problems of known processes for the forming of a foamed extruded uPVC article, and there is a need for a more efficient process than known heretofore for extruding uPVC material to form a foamed article. There is also a need for an article of foamed plastics material, and for an article of foamed uPVC material of improved quality and strength.

The present invention is directed towards providing such a blowing agent, a plastics material, a process for forming a foamed plastics article, and foamed plastics article as discussed.

According to the invention there is provided a blowing agent for use in the foaming of a plastics material, the blowing agent comprising in combination sodium bicarbonate and azodicarbonamide.

Preferably, the ratio of sodium bicarbonate to azodicarbonamide is in the range of 5 to 1 by weight and 19 to 1 by weight. Advantageously, the ratio of sodium bicarbonate to azodicarbonamide is in the range of 13 to 2 by weight and 11 to 1 by weight. Ideally, the ratio of sodium bicarbonate to azodicarbonamide is approximately 15 to 2 by weight.

In one aspect of the invention the blowing agent comprises a modifying agent.

In another aspect of the invention the modifying agent comprises silicate. Preferably, the modifying agent constitutes in the range of 5% to 45% by weight of the blowing agent. Advantageously, the modifying agent constitutes in the range of 10% to 20% by weight of the blowing agent. Ideally, the modifying agent constitutes approximately 16% by weight of the blowing agent.

In a further aspect of the invention, the blowing agent is in powder form.

In one aspect of the invention the blowing agent is for use in forming a plastics material.

In another aspect of the invention the blowing agent is for use in extruding a plastics material using an inward foaming Celuka process.

In a further aspect of the invention the blowing is for use in extruding uPVC material.

Additionally, the invention provides use of the blowing agent according to the invention in the formation of a plastics material.

Additionally, the invention provides use of the blowing agent according to the invention in the extrusion of a plastics material.

Further the invention provides use of the blowing agent according to the invention in the extrusion of uPVC material using an inward foaming Celuka process.

Additionally, the invention provides a plastics material for use in the formation of a foamed plastics article, the plastics material comprising the blowing agent according to the invention.

Further the invention provides a uPVC material for use in forming a foamed uPVC plastics article, the uPVC material comprising the blowing agent according td the invention. In one aspect of the invention the uPVC material is for use in an inward foaming Celuka process. In another aspect -of the invention the uPVC material is in powder form.

Further the invention provides a process for extruding uPVC material using an inward foaming Celuka process to form a foamed uPVC article, the process comprising the steps of: supplying a uPVC raw material mixture to a barrel of a screw extruder at an upstream end thereof, the uPVC mixture-comprising uPVC material, a heat stabilising agent and a blowing agent, melting and urging the uPVC mixture through the barrel in a downstream direction to form an extrudate for urging through an extrusion die located at a downstream end of the barrel, supplying heat to the uPVC mixture through the barrel from an external heat source for raising the temperature of the uPVC mixture as it is urged through the barrel, wherein the blowing agent comprises the blowing agent according to the invention.

In another aspect of the invention the blowing agent constitutes in the range of 1.6% to 2% by weight of the uPVC mixture. Preferably, the blowing agent constitutes approximately 1.75% by weight of the uPVC mixture.

In another aspect of the invention the heat stabilizing agent comprises tin. Preferably, the tin constitutes in the range of 1% to 4% by weight of the uPVC mixture.

Advantageously, the tin constitutes in the range of 1.5% to 2% by weight of the uPVC mixture, and ideally, the tin constitutes approximately 1.75% by weight of the uPVC.

Preferably, the uPVC mixture is supplied to the screw extruder barrel in powder form. Advantageously, the bulk density of the uPVC mixture is in the range of 628 Kg per M3 to 632 Kg per M3.

In one aspect of the invention the temperature of the external heat source increases along the barrel in a downstream direction so that heat is supplied to the uPVC mixture from the external heat source and from work done by a screw of the screw extruder for raising the temperature of the extrudate to a foaming temperature. Preferably, the external heat source extends substantially continuously along the barrel from the upstream end to the downstream end thereof.

Advantageously, the external heat source extends substantially completely around the barrel. Ideally, the external heat source is divided into at least two zones, namely, an upstream zone and a downstream zone.

In one aspect of the invention the temperature of the upstream zone of the external heat source is maintained at a temperature in the range of 1430C to 1500C.

Preferably, the temperature of the upstream zone of the external heat source is maintained at a temperature of approximately 145 0C.

In another aspect of the invention the temperature of the downstream zone of the external heat source is maintained at a temperature in the range of 1620C to 1700C. Preferably, the temperature of the downstream zone of the external heat source is maintained at a temperature of approximately 1650C.

In a further aspect of the invention the external heat source is divided into five zones, three intermediate zones being located between the upstream and downstream zones, an upstream intermediate zone being located adjacent the upstream zone and a downstream intermediate zone being located adjacent the downstream zone, and a middle intermediate zone being located between the upstream and downstream intermediate zone.

Advantageously, the temperature of the upstream intermediate zone is maintained in the range of 1500C to 1570C. Preferably, the temperature of the upstream intermediate zone is maintained at approximately 1520C.

In another aspect of the invention the temperature of the downstream intermediate zone is maintained in the range of 1570C to 1650C. Preferably, the temperature of the downstream intermediate zone is maintained at approximately 1600C.

In another aspect of the invention the temperature of the middle intermediate zone is maintained in the range of 1520C to 1600C. Preferably, the temperature of the middle intermediate zone is maintained at approximately 1550C.

In a further aspect of the invention the upstream zone extends along the barrel for a distance in the range of 28% to 338 of the overall heated length of the barrel, and preferably, the upstream zone extends along the barrel for a distance of approximately 31% of the overall heated length of the barrel.

In another aspect of the invention the downstream zone extends along the barrel for a distance in the range of 45% to 52% of the overall heated length of the barrel.

Preferably, the downstream zone extends along the barrel for a distance of approximately 49% of the overall heated length of the barrel.

In one aspect of the invention the upstream intermediate zone extends along the barrel for a distance in the range of 14% to 18% of the overall heated length of the barrel. Preferably, the upstream intermediate zone extends along the barrel for a distance of approximately 16% of the overall heated length of the barrel.

In another aspect of the invention the downstream intermediate zone extends along the barrel for a distance in the range of 24% to 28% of the overall heated length of the barrel. Preferably, the downstream intermediate zone extends along the barrel for a distance of approximately 26% of the overall heated length of the barrel.

In a further aspect of the invention the middle intermediate zone extends along the barrel for a distance in the range of 18% to 21% of the overall heated length of the barrel. Preferably, the middle intermediate zone extends along the barrel for a distance of approximately 19% of the overall heated length of the barrel.

Preferably, the temperature of the extrudate at the downstream end just as the extrudate is entering the extrusion die is at a foaming temperature of the uPVC mixture.

In one aspect of the invention the screw of the screw extruder is rotated at a feed speed and the extruded material is hauled off at a haul-off pressure for providing foaming conditions in the extrudate at the extrusion die. Preferably, the feed speed of the screw extruder and the haul-off pressure are such as to form the extruded material with a specific gravity not exceeding 0.475. Advantageously, the feed speed of the screw extruder and the haul-off pressure are such as to form the extruded material with a specific gravity of the order of 0.45.

Additionally the invention provides an article of uPVC material extruded using the process according to the invention.

Further the invention provides an article of foamed plastics material formed from a plastics material and a blowing agent according to the invention.

Further the invention provides an article of foamed uPVC material formed from a plastics material and a blowing agent according to the invention.

The invention will be more clearly understood from the following description of some preferred embodiments thereof which are given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of an article according to the invention extruded using a process according to the invention, Fig. 2 is a cross-sectional end elevational view of the article of Fig. 1, and Fig. 3 is a diagrammatic representation of apparatus for carrying out the process according to the invention for producing the extruded article of Fig. 1.

Referring to the drawings and initially to Figs. 1 and 2 there is illustrated an extruded article, in this case, a soffit board according to the invention indicated generally by the reference numeral 1. The soffit board 1 is of uPVC material and is formed by an inward foaming Celuka process, according to the invention. The soffit board 1 comprises an outer skin 2 of dense foamed uPVC material which surrounds a central core 3 of lower density foamed uPVC material which has been formed using the process according to the invention from a uPVC mixture which comprises a blowing agent also according to the invention, and which will be described below. The boundary between the outer skin 2 and the foamed central core 3 is illustrated in Fig. 2 by broken lines. It will be appreciated by those skilled in the art that there will not be a clear cut off line between the outer skin 2 and the central core 3, rather, they will merge into each other.

Referring now to Fig. 3, apparatus for carrying out the inward foaming Celuka process according to the invention will now be described. The apparatus is similar to known apparatus for carrying out an inward foaming Celuka process and will only be briefly described, it being assumed that the reader will have sufficient knowledge of the Celuka process to understand the apparatus. The apparatus comprises a twin screw extruder 9 having an extruder barrel 10 extending from an upstream end 11 to a downstream end 12. Twin counter rotating screws 14, diagrammatically illustrated in Fig. 3, are rotatably mounted in the barrel 10 for urging raw material, in this case, the foaming uPVC mixture through the extruder barrel 10 from the upstream end 11 to the downstream end 12. A hopper 15 delivers the uPVC mixture in powder form, which is described below, into the extruder barrel 10 at the upstream end 11. The uPVC mixture is melted and urged through the extruder barrel 10 by the screws 14 to form an extrudate. The extrudate is urged from the downstream end 12 of the extruder barrel 10 through an adaptor 17 to an extrusion head 19 where the extrudate is formed into the article. The extrusion head 19 comprises an extrusion die (not shown) which will be well known to those skilled in the art, of the appropriate shape for forming the soffit. A mandrel (also not shown) located centrally in the extrusion die orifice forms a central core for facilitating inward foaming of the extrudate immediately after formation in the extrusion head 19.

A secondary screw extruder 20 melts and extrudes uPVC material which is injected through sprus (not shown) located around the periphery of the die (not shown) in the extrusion head 19 for forming the skin 2 of the soffit 1.

A cooling calibrator 21 downstream of the extrusion head 19 cools and calibrates the extrusion to the correct outer transverse dimensions. A haul-off device 22 downstream of the cooling calibrator 21 hauls off the extruded material from the calibrator 21. A saw 24 downstream of the haul-off device 22 cuts the extrusion into soffits 1 of the desired length.

An external heat source for supplying heat to the uPVC mixture as it is urged through the barrel 10 comprises a plurality of electrically powered band heaters (not shown) which extend around the extruder barrel 10, and which in this case form five zones, namely, an upstream zone, which for convenience is referred to as zone 1, a downstream zone which is referred to as zone 5, and three intermediate zones, namely, an upstream intermediate zone 2, a middle intermediate zone 3 and a downstream intermediate zone 4. The arrangement of band heaters into zones will be well known to those skilled in the art. All the band heaters extend completely around the extruder barrel 10. The band heaters are arranged so that adjacent zones are adjacent each other. In this embodiment of the invention zone 1 extends along the length of the barrel 10 for a distance of approximately 16% of the heated length of the barrel 10. Zone 2 extends for a distance of approximately 16% of the heated length of the barrel 10. Zone 3 extends for a distance of approximately 19% of the heated length of the barrel 10. Zone 4 extends for a distance of approximately 26% of the heated length of the barrel 10, and zone 5 extends for a distance of approximately 23% of the heated length of the barrel 10.

The temperatures of the band heaters forming the respective zones 1 to 5 are controlled so that the temperature of the zones from the upstream zone 1 to the downstream zone 5 increase in incremental steps in a downstream direction. In this particular embodiment of the invention the temperature of the band heaters of zone 1 are maintained at a temperature of approximately 1450C. The temperature of the band heaters of zone 2 are maintained at a temperature of approximately 1520C.

The temperature of the band heaters of zone 3 are maintained at a temperature of approximately 1550C.

The temperature of the band heaters of zone 4 are maintained at a temperature of approximately 1600C.

The temperature of the band heaters of zone 5 are maintained at a temperature of approximately 1650C.

Heat, as well as being supplied from the band heaters of the zones 1 to 5, is also supplied to the uPVC mixture in the barrel 10 as a result of work done on the uPVC mixture in the barrel 10 by the twin screws of the barrel 10. The twin screws are operated at an appropriate speed so that the temperature and pressure of the extrudate in the extrusion head 19 is at a foaming temperature and a foaming pressure for causing inward foaming into the core formed by the mandrel of the extrusion die (not shown). The foaming pressure is also controlled by the haul-off tension in the extrusion which is induced by the haul-off device 22.

In this embodiment of the invention the foaming temperature is 1750C, approximately, and the foaming pressure is 155 bar, approximately.

The uPVC mixture supplied to the hopper 15 comprises uPVC resin with a filler, both in powder form, a heat stabilising agent, which in this embodiment of the invention is tin, and the blowing agent according to the invention, all of which are in powder form. The blowing agent comprises in combination sodium bicarbonate and azodicarbonamide, which typically, are in a ratio within the range five parts sodium bicarbonate to one part azodicarbonamide to nineteen parts sodium bicarbonate to one part azodicarbonamide.

The blowing agent may also comprise a modifying agent, such as silicate, and typically, the modifying agent may constitute between 5% and 45% by weight of the blowing agent, and more typically, 10% to 20% by weight of the blowing agent. The modifying agent is provided for stabilising the extrudate as it is being extruded through the extrusion die (not shown) in the extrusion head 19 at relatively high speeds. The tin of the heat stabilising agent may be provided in the form of Butyl tin Mercaptide/Carboxalates.

The process according to the invention will now be described with reference to the following examples: Example 1 In this example a foamed extruded soffit board 1 of the type illustrated in Figs. 1 and 2 of uPVC material is produced with a density of 0.45 gms per cm3. The uPVC mixture from which the soffit board 1 is manufactured, and which is supplied to the hopper 15 comprise the following constituents in the percentages by weight set out below of the uPVC mixture: uPVC Mixture uPVC resin including a filler, both in powder form of particle size such that there is 0.5% retention on a 0.5 mm sieve 96.5% Tin (in powder form) 1.5% Blowing Agent (in powder form) 2.0% Blowing Agent Sodium Bicarbonate 75.0% Azodicarbonamide 10.0% Silicate 15.0% The proportions of sodium bicarbonate, azodicarbonamide and silicate set out above are given by weight of the blowing agent.

The filler may be any suitable filler, but in this case is calcium carbonate. The uPVC mixture may also include other minor additives which will be well known to those skilled in the art.

The outer skin 2 of the soffit 1 is formed by unfoamed uPVC material which is fed from the secondary extrusion barrel 20.

The temperature of zones 1 to 5 is as set out above, and the feed rate of the twin screws of the extruder barrel 10 and the haul-off tension induced by the hauloff device 22 are set so that the extrusion is hauled off at a stable speed of approximately 1.9 M per minute which represented a haul-off rate of the extrusion of 240 Kg per hour with a density of 0.45 gms per cm3.

Thus, the soffit 1 formed according to this example has a density of 0.45 gms per cm3.

The ratio of sodium bicarbonate to azodicarbonamide which constitutes the blowing agent of this example is 15:2.

It has been found that the soffit board 1 extruded according to this example suffers no loss of strength over and above a soffit board with a density of 0.5 gms per cm3 which is produced according to Example 4 below using a conventional process. The surface finish of the soffit board 1 of Example 1 is free from any blemishes, surface hollows and dips.

Example 2 In this example the uPVC mixture is prepared using identical constituents in identical proportions as those set out in Example 1, with the exception that the silicate is omitted from the blowing agent. In this Example 2 the extrusion from the extrusion die (not shown) exhibits slight surging or instability when exiting from the extrusion die, but rapidly becomes stable and exits from the cooling calibrator 21 without surface blemishes, surface hollows and dips.

When the soffit board 1 of this Example 2 is compared with the soffit board 1 of Example 1, it is found that the extrusion when exiting from the extrusion die in Example 1 displays no instability. It is therefore believed that the addition of silicate in the blowing agent contributes to the strength and surface slip of the extrudate.

Further experiments were carried out using a uPVC mixture with the same constituents and proportions set out in Example 1, with the exception that the quantity of silicate in the blowing agent was increased. It was found that little further improvement occurred by increasing the quantity of silicate above 15% by weight of the blowing agent, which equates to approximately 0.3% by weight of the uPVC mixture. Indeed, it was found that once the silicate exceeds approximately 37% by weight of the blowing agent, which is approximately 1% by weight of the uPVC mixture, there is a tendency for surface defects to occur.

Example 3 In this example a soffit board similar to the soffit board 1 of Figs. 1 and 2 is extruded using a uPVC mixture which comprised the following constituents in the percentage by weight of the uPVC mixture set out below: uPVC Mixture uPVC resin including a filler, both in powder form of particle size such that there is 0.5% retention on a 0.5 mm sieve 96.65% Tin (in powder form) 1.5 % Blowing Agent (in powder form) 1.85% Blowing Agent Sodium Bicarbonate 91.89% Azodicarbonamide 8.11% The proportions of sodium bicarbonate and azodicarbonamide set out above are given by weight of the blowing agent.

In this example silicate is not included in the blowing agent. The extrusion is hauled off from the extrusion die (not shown) at a rate of 1.7 M per minute which represented a haul-off rate of the extrusion of 228 Kg per minute at a density of 0.475 gms per M3. The density of the extruded soffit board 1 is 0.475 gms per cm3, which is free from surface blemishes, surface hollows and dips.

It is believed that by maintaining the density of the extruded article at or above 0.475% an acceptable article can be extruded without the need for silicate.

However, below a density of 0.475 gms per cm3, and particularly, as the density approaches 0.45 gms per cm3 the addition of silicate is desirable.

Example 4 In this example a soffit board 1 similar to that illustrated in Figs. 1 and 2 is extruded from a uPVC mixture which included the following constituents in the following percentages by weight of the uPVC mixture: uPVC resin including a filler, both in powder form of particle size such that there is 0.5% retention on a 0.5 mm sieve 96.5 % Tin (in powder form) 1.5 % Blowing Agent (Sodium Bicarbonate only in powder form) 2.0 % In this example the extrusion is hauled off from the extrusion die (not shown) at a haul-off speed of approximately 1.4 M per minute which represented a haul-off rate of 197 Kg per hour of extrusion at a density of 0.5 gms per cm3. The soffit board prepared according to this example is free from surface defects, blemishes, hollows and dips. However, the density of the soffit board is 0.5 gms per cm3.

In this example haul-off rates above 1.4 M per minute were attempted in order to reduce the density of the extrusion below 0.5 gms per cm3, however, at haul-off rates above 1.4 M per minute, while the soffit board was provided with a density of below 0.5 gms per cm3, all the soffit boards below density of 0.5 gms per cm3 suffered from surface hollows and dips.

Accordingly, it will be apparent to those skilled in the art that by providing a blowing agent which comprises in combination sodium bicarbonate and azodicarbonamide significant improvements in the production of extruded uPVC articles can be achieved.

The articles can be extruded at a significantly lower density than is known heretofore, while at the same time maintaining adequate strength and visual surface characteristics. Why a blowing agent comprising in combination sodium bicarbonate and azodicarbonamide should produce such new and surprising results is not fully understood, however, it is believed that the slower and longer expansion, in other words the time rate of decomposition of the azodicarbonamide in combination with the sodium bicarbonate, tends to counteract the tendency of the core to shrink during cooling and calibration.

As mentioned above the addition of silicate further adds to the stability of the extrusion as it exits from the extrusion die (not shown).

The following table sets out the proportions of sodium bicarbonate and azodicarbonamide by weight of the uPVC mixture, which have been used in the preparation of other soffit boards, all of which have adequate strength and are free from surface blemishes, defects, hollows and dips.

Sodium Azodi- Density of Haul-off Bicarbonate carbonamide Extruded Article rate (%) (%) (gms/cm3) (M per minute) 1.4 0.225 0.453 1.85 m/min 1.5 0.2 0.45 1.9 m/min 1.7 0.15 0.475 1.7 m/min 1.9 0.1 0.49 1.5 m/min 2.0 0.0 0.5 1.4 m/min In all the above examples, silicate was included in the blowing agent of the uPVC mixture in order to facilitate the haul-off rates set out. Without the silicate difficulty was experienced in attempting to maintain stable extrusion for prolonged periods without reducing the haul-off rate by up to 20% of the values set out above.

While the soffit board has been described as comprising a skin of unfoamed uPVC material, the skin may be omitted, and in many cases, articles may be produced according to the process of the invention without a skin of unfoamed uPVC material. Indeed, in certain cases, where a skin of unfoamed uPVC material is provided, the skin may not extend completely around the extruded article, but may be provided on only three sides, for example. It will of course be appreciated that a skin of unfoamed uPVC material may be provided around the soffit board without the need for providing unfoamed uPVC material from the secondary extruder 20.

By rapidly chilling the outer surface of the soffit a skin of dense foamed uPVC material is formed which effectively forms a smooth outer surface.

While the extruder barrel has been described as comprising five heat zones, the extruder barrel may be provided with any number of heat zones, and in certain cases, the extruder barrel may be provided with only one single heat zone.

While the process according to the invention for extruding an article has been described for extruding a soffit board, of rectangular cross-section, the process according to the invention may be used for extruding any article of any other cross-section. Indeed, it is envisaged that in certain cases, the cross-section may be L-shaped cross-section, angle cross-section, irregular cross-section, or any other desired cross section.

Claims

1. A blowing agent for use in the foaming of a plastics material, the blowing agent comprising in combination sodium bicarbonate and azodicarbonamide.

2. A blowing agent as claimed in Claim 1 in which the ratio of sodium bicarbonate to azodicarbonamide is in the range of 5 to 1 by weight and 19 to 1 by weight.

3. A blowing agent as claimed in Claim 2 in which the ratio of sodium bicarbonate to azodicarbonamide is in the range of 13 to 2 by weight and 11 to 1 by weight.

4. A blowing agent as claimed in Claim 3 in which the ratio of sodium bicarbonate to azodicarbonamide is approximately 15 to 2 by weight.

5. A blowing agent as claimed in any preceding claim in which the blowing agent comprises a modifying agent.

6. A blowing agent as claimed in Claim 5 in which the modifying agent comprises silicate.

7. A blowing agent as claimed in Claim 5 or 6 in which the modifying agent constitutes in the range of 5% to 45% by weight of the blowing agent.

8. A blowing agent as claimed in Claim 7 in which the modifying agent constitutes in the range of 10% to 20% by weight of the blowing agent.

9. A blowing agent as claimed in Claim 8 in which the modifying agent constitutes approximately 16% by weight of the blowing agent.

10. A blowing agent as claimed in any preceding claim in which the blowing agent is in powder form.

11. A blowing agent as claimed in any preceding claim for use in forming a plastics material.

12. A blowing agent as claimed in any preceding claim for use in extruding a plastics material using an inward foaming Celuka process.

13. A blowing agent as claimed in any preceding claim for use in extruding uPVC material.

14. A blowing agent for use in the foaming of a plastics material, the blowing agent being substantially as described herein with reference to the examples.

15. Use of the blowing agent of any preceding claim in the formation of a plastics material.

16. Use of the blowing agent of any of Claims 1 to 14 in the extrusion of a plastics material.

17. Use of the blowing agent of any of Claims 1 to 14 in the extrusion of uPVC material using an inward foaming Celuka process.

18. A plastics material for use in the formation of a foamed plastics article, the plastics material comprising the blowing agent of any of Claims 1 to 14.

19. A uPVC material for use in forming a foamed uPVC plastics article, the uPVC material comprising the blowing agent as claimed in any of Claims 1 to 14.

20. A uPVC material as claimed in Claim 19 in which the uPVC material is for use in an inward foaming Celuka process.

21. uPVC material as claimed in Claim 19 or 20 in which the uPVC material is in powder form.

22. uPVC material for use in forming a uPVC plastics article, the uPVC material being substantially as described herein with reference to the examples.

23. A process for extruding uPVC material using an inward foaming Celuka process to form a foamed uPVC article, the process comprising the steps of: supplying a uPVC raw material mixture to a barrel of a screw extruder at an upstream end thereof, the uPVC mixture comprising uPVC material, a heat stabilising agent and a blowing agent, melting and urging the uPVC mixture through the barrel in a downstream direction to form an extrudate for urging through an extrusion die located at a downstream end of the barrel, supplying heat to the uPVC mixture through the barrel from an external heat source for raising the temperature of the uPVC mixture as it is urged through the barrel, wherein the blowing agent comprises the blowing agent as claimed in any of Claims 1 to 14.

24. A process as claimed in Claim 23 in which the blowing agent constitutes in the range of 1.6% to 2% by weight of the uPVC mixture.

25. A process as claimed in Claim 23 or 24 in which the blowing agent constitutes approximately 1.75% by weight of the uPVC mixture.

26. A process as claimed in any of Claims 23 to 25 in which the heat stabilising agent comprises tin.

27. A process as claimed in Claim 26 in which the tin constitutes in the range of 1% to 4% by weight of the uPVC mixture.

28. A process as claimed in Claim 27 in which the tin constitutes in the range of 1.5% to 2% by weight of the uPVC mixture.

29. A process as claimed in Claim 28 in which the tin constitutes approximately 1.75% by weight of the uPVC mixture.

30. A process as claimed in any of Claims 23 to 29 in which the uPVC mixture is supplied to the screw extruder barrel in powder form.

31. A process as claimed in any of Claims 23 to 30 in which the bulk density of the uPVC mixture is in the range of 628 Kg per M3 to 632 Kg per M3.

32. A process as claimed in any of Claims 23 to 31 in which the temperature of the external heat source increases along the barrel in a downstream direction so that heat is supplied to the uPVC mixture from the external heat source and from work done by a screw of the screw extruder for raising the temperature of the extrudate to a foaming temperature.

33. A process as claimed in any of Claims 23 to 32 in which the external heat source extends substantially continuously along the barrel from the upstream end to the downstream end thereof.

34. A process as claimed in any of Claims 23 to 33 in which the external heat source extends substantially completely around the barrel.

35. A process as claimed in any of Claims 23 to 34 in which the external heat source is divided into at least two zones, namely, an upstream zone and a downstream zone.

36. A process as claimed in Claim 35 in which the temperature of the upstream zone of the external heat source is maintained at a temperature in the range of 1430C to 1500C.

37. A process as claimed in Claims 35 or 36 in which the temperature of the upstream zone of the external heat source is maintained at a temperature of approximately 145 0C.

38. A process as claimed in any of Claims 35 to 37 in which the temperature of the downstream zone of the external heat source is maintained at a temperature in the range of 1620C to 1700C.

39. A process as claimed in any of Claims 35 to 38 in which the temperature of the downstream zone of the external heat source is maintained at a temperature of approximately 1650C.

40. A process as claimed in any of Claims 35 to 39 in which the external heat source is divided into five zones, three intermediate zones being located between the upstream and downstream zones, an upstream intermediate zone being located adjacent the upstream zone and a downstream intermediate zone being located adjacent the downstream zone, and a middle intermediate zone being located between the upstream and downstream intermediate zone.

41. A process as claimed in Claim 40 in which the temperature of the upstream intermediate zone is maintained in the range of 1500C to 1570C.

42. A process as claimed in Claim 40 or 41 in which the temperature of the upstream intermediate zone is maintained at approximately 1520C.

43. A process as claimed in any of Claims 40 to 42 in which the temperature of the downstream intermediate zone is maintained in the range of 1570C to 1650C.

44. A process as claimed in any of Claims 40 to 43 in which the temperature of the downstream intermediate zone is maintained at approximately 1600C.

45. A process as claimed in any of Claims 40 to 44 in which the temperature of the middle intermediate zone is maintained in the range of 1520C to 1600C.

46. A process as claimed in any of Claims 40 to 45 in which the temperature of the middle intermediate zone is maintained at approximately 1550C.

47. A process as claimed in any of Claims 40 to 46 in which the upstream zone extends along the barrel for a distance in the range of 28% to 33% of the overall heated length of the barrel.

48. A process as claimed in Claim 47 in which the upstream zone extends along the barrel for a distance of approximately 31% of the overall heated length of the barrel.

49. A process as claimed in any of Claims 40 to 48 in which the downstream zone extends along the barrel for a distance in the range of 45% to 52% of the overall heated length of the barrel.

50. A process as claimed in Claim 49 in which the downstream zone extends along the barrel for a distance of approximately 49% of the overall heated length of the barrel.

51. A process as claimed in any of Claims 40 to 50 in which the upstream intermediate zone extends along the barrel for a distance in the range of 14% to 18% of the overall heated length of the barrel.

52. A process as claimed in Claim 51 in which the upstream intermediate zone extends along the barrel for a distance of approximately 16% of the overall heated length of the barrel.

53. A process as claimed in any of Claims 40 to 52 in which the downstream intermediate zone extends along the barrel for a distance in the range of 24% to 28% of the overall heated length of the barrel.

54. A process as claimed in Claim 53 in which the downstream intermediate zone extends along the barrel for a distance of approximately 26% of the overall heated length of the barrel.

55. A process as claimed in any of Claims 40 to 54 in which the middle intermediate zone extends along the barrel for a distance in the range of 18% to 21% of the overall heated length of the barrel.

56. A process as claimed in Claim 55 in which the middle intermediate zone extends along the barrel for a distance of approximately 19% of the overall heated length of the barrel.

57. A process as claimed in any of Claims 23 to 56 in which the temperature of the extrudate at the downstream end just as the extrudate is entering the extrusion die is at a foaming temperature of the uPVC mixture.

58. A process as claimed in any of Claims 23 to 57 in which the screw of the screw extruder is rotated at a feed speed and the extruded material is hauled off at a haul-off pressure for providing foaming conditions in the extrudate at the extrusion die.

59. A process as claimed in Claim 58 in which the feed speed of the screw extruder and the haul-off pressure are such as to form the extruded material with a specific gravity not exceeding 0.475.

60. A process as claimed in Claim 58 or 59 in which the feed speed of the screw extruder and the haul-off pressure are such as to form the extruded material with a specific gravity of the order of 0.45.

61. A process for extruding uPVC using an inward foaming Celuka process, the process being substantially as described herein with reference to and as illustrated in the accompanying drawings.

62. An article of foamed uPVC material extruded using the process as claimed in any of Claims 23 to 61.

63. An article of foamed plastics material formed from a plastics material and a blowing agent as claimed in any of Claims 1 to 14.

64. An article of foamed uPVC material formed from a plastics material and a blowing agent as claimed in any of Claims 1 to 14.