GB2449625A - One shot foaming process for rotomoulding - Google Patents

Abstract

A method of forming polymer/blowing agent units for use in a rotational moulding process, requires that the unit is not formed by an extrusion method. The unit can be a pellet formed by compression/ compaction. Using a more gentle formation method than extrusion allows a blowing agent having a lower kick temperature to be used. Consequently, the mould does not need to be heated to such high temperatures in order for the kick temperature to be reached and for foaming to commence. This results in a better quality, thicker skin layer. The edges of the pellet can be pure polymer to prevent foam breakthrough in the event of a pellet sticking to the mould surface. The blowing agent is preferably p,p'-oxobis benzene sulfonyl hydrazide.

Description

* 2449625

ONE SHOT FOAMING PROCESS FOR ROTOMOULDING

This invention relates to a one shot foaming rotomoulding process and polymer pellets for use in such a process.

Rotational moulding, or rotomoulding, is a moulding process in which polymer powder is placed in a mould which is then transferred to an oven and rotated, preferably about two axes, to distribute the polymer powder over the hot surfaces of the mould. The heating cycle is continued until all of the powder has melted and formed a continuous layer within the mould. The mould is then removed from the oven and cooled until the polymer has solidified. The moulded article is then removed.

The length of time for which the mould must be heated depends on the nature of the article being moulded, the amount and nature of polymer composition present and the temperature of the oven. Typical rotomoulding oven temperatures are 230 to 400 c, more particularly 260 to 320 C (e.g. about 290 C). The oven may optionally be pressurised to reduce the amount of time in the mould.

Typical pressures that may be used are less than 4 bar, more preferably less than 1 bar (e.g. about 0.5 bar).

Cooling may be carried out under a stream of air, water spray or mist or simply in ambient air at room temperature. A combination of these methods may also be employed. Preferably cooling is achieved using a combination of blown air followed by ambient air or just blown air. Cooling times are normally of similar magnitude to heating times or slightly longer. Slow cooling further reduces the amount of warpage in an article. The moulded article may be removed from the mould at any convenient time after solidification has occurred.

The skilled man is able to manipulate the temperature, time and rotation speed/ratio within a rotomoulding apparatus to ensure that well-formed moulded articles are produced.

Often, in order to improve the mechanical properties or reduce the weight of a product, it is desirable to create a product with a solid outer skin and an inner foam layer, or core. Traditionally this foam layer has either been added to the rotomoulded product in a separate process or via a "two shot' rotomoulding process. In a two shot process the skin layer is first fully formed in accordance with the above method, after which foa.xnable polymer pellets and a blowing agent are introduced into the mould. The mould is then heated to a certain temperature, the kick' temperature, whereupon the blowing agent undergoes a reaction which releases gases that cause the melted polymer pellets to foam.

The two shot system is lengthy as it requires two heating and cooling periods.

"One shot" foaming processes have also been developed.

In such systems the polymer which forms the skin (the skin polymer), blowing agent and foamable polymer are all introduced into the mould together.

When a one shot process is used the characteristics of the polymers must be chosen such that the skin polymer melts first and forms a surface layer on the mould wall.

This is achieved by using polymer particles of a smaller size than the foamable polymer so that upon rotation the skin polymer particles disperse to the sides of the mould and melt first. In order to prevent blowing agent from embedding itself in this outer layer, the blowing agent is mixed with the foarnable polymer to form pellets. This also creates an even distribution of foarnable polymer and blowing agent. After the skin layer has formed, and as the temperature continues to increase, the foamable polymer begins to melt and is foamed when the kick temperature of the blowing agent is reached. A one shot foaming system of this type is described in CA 0983226.

However, this type of process often results in a thin uneven skin layer. To address this problem, US 6,261,490 describes a one shot system in which the mould undergoes two distinct heating stages. During a first stage the mould is heated to a temperature of 130-140 C (just above the melting point of the skin polymer) to form the skin. The mould is then heated further to 200- 350 C to create the foam layer. it is claimed that this provides a more satisfactory skin layer. W095/19877 describes a similar process in which the pellets are coated with ceramic dust to prevent them from sticking to the mould wall while the skin layer is being formed.

A problem encountered in the one shot processes of the prior art is the limitation to the thickness of the skin layer. The thicker this layer, the greater the insulating effect, and therefore the greater the temperature needs to be to raise the temperature inside the mould to the kicking point of the blowing agent.

This can be a significant problem because the pellets of foamable polymer and blowing agent are created using an extrusion process. This involves high stresses and temperatures. The blowing agent used must therefore have a high enough kick temperature that this is not reached during the extrusion process. As a result, the kick temperature is usually significantly above the melting point of the skin polymer, which can lead to this layer re-melting and/or degrading during the foaming step, Particularly when attempting to create a thick skin layer.

In addition, the extra time taken to heat the mould to the kick temperature and then to cool the mould adds to the time and cost of the process.

According to a first aspect, the present invention provides a method of forming polymer/blowing agent units for use in a rotomoulding process, wherein the unit is not formed by an extrusion method.

The term polymer/blowing agent unit (or "unit") refers to a discrete member which provides a mixture of polymer and blowing agent. This may be in pellet form, or the polymer and blowing agent may be unified in some other way, as will be discussed more fully below.

The inventors have realised that the prior art process of forming pellets by extrusion require the use of a blowing agent with an unnecessarily high kick temperature. This in turn leads to the problems discussed above. Thus, the inventors have recognised that using a method other than extrusion allows the units to be formed at a much lower temperature and so a blowing agent with a comparatively low kick temperature can be used. The use of the units of the invention therefore has a beneficial effect on the speed and efficiency of the one step rotomoulding process.

Although the invention allows the use of virtually any blowing agent it will be appreciated that the advantages of the invention are best obtained with pellets or other units which contain blowing agent having a kick temperature only slightly higher (e.g. within 40 degrees) than the melting point of the skin polymer.

This ensures that the skin layer is properly formed whilst also reducing the amount of degradation suffered by the skin layer. Furthermore, as the mould does not need to be heated to such high temperatures, it greatly reduces the length of the process.

Pellet forming processes falling within the scope of the invention include using a gluing agent to bond the polymer and blowing agent together. The gluing agent is chosen to have a melting point above that of the skin polymer so that the pellets do not break down until after the skin layer has formed. Alternatively, other types of unit may be provided. For example, the polymer and blowing agent can be contained in sachets, bags, pouches or the like, which melt at a higher temperature than the skin polymer but below the kick temperature of the blowing agent.

However, both of the above processes involve adding additional materials (i.e. potential impurities) into the process. Therefore, the method preferably comprises the steps of mixing polymer powder with a blowing agent to form a mixture and compacting the mixture to create pellets.

The production of such pellets results from a further recognition by the inventors that suitable mechanical bonding of the polymer and blowing agent can be obtained through compression-forming the pellets. This is a much gentler method of forming pellets than extrusion and does not require the polymer to be melted. The lower forming temperature allows blowing agents with a lower kick temperature to be used. The polymer powder used in the pellet can be extruded and ground to a powder prior to mixing with the blowing agent. Alternatively, the polymer powder may be made using polymer powder directly from the polymerisatjon process (i.e. without any extrusion).

It is important to compact the pellets to such a degree that break up under torsion or applied pressure is prevented, as this may result in the pellets breaking up during handling or during the formation of the skin layer. However, the pressure used should not over-compact the pellets such that they cannot break up easily once the blowing agent begins to react. The correct impaction pressure can be selected for each polymer/blowing agent combination based on these requirements.

Preferably, the mixture should be compacted at a pressure of between 200 -1000 MPa, more preferably at a pressure of between 400-900 MPa and most preferably at a pressure of between 700-800 MPa, e.g. around 770 MPa.

It is also preferable, during formation of the pellets, to limit the shear stress at the interface between the powder and the compacting press. This reduces the damage to the pellet on removal from the press and reduces the possibility of early break up. This can be achieved by using a compaction press having reamed and honed inner surfaces.

As discussed above, the method of the invention does not require the polymer and the blowing agent to undergo high temperatures during pellet formation and therefore the blowing agent can have a low kick temperature.

Preferably, the blowing agent has a kick temperature of below 200 c and more preferably the kick temperature is below 170 C. It is important to ensure however that the kick temperature remains above the melting temperature of the skin polymer so that the foaming process is only initiated after the formation of the skin layer.

The lower kick temperature reduces the time and cost of the rotomoulding process. In addition, as the mould does not need to be heated to such high temperatures in order to activate the blowing agent, the skin layer is not degraded by the process.

According to a further aspect the invention provides a method of one shot foaming rotational moulding comprising the steps of; introducing skin polymer and polymer units formed in accordance with the first aspect of the invention into a mould; and heating the mould to a temperature not exceeding 190 C.

Because the kick temperature is lower, the insulating effect of the skin is less sig-nificant. This means that a thicker skin layer can be built up and therefore opens up the rotomoulding process to the manufacture of a greater range of products. Thus, the method of the present invention may be used to create products with a skin layer thickness of between 2 -40mm. Other preferred ranges are 5 -40mm, 5 -20mm, 5 -15mm and 2 -10mm. Most preferably the skin thickness is between 3 and 30 mm.

According to a further aspect of the invention there is provided a polymer unit for use in rotomoulding comprising polymer powder and blowing agent wherein the unit is not formed by an extrusion method.

Preferably the unit is a polymer pellet comprising compressed polymer powder and blowing agent.

Preferably, the blowing agent used is p,p'-Oxobis benzene sulfonyl hydrazide (Di-phenoxy-disulfohydrazide), which is available as, e.g. Celogen OP from Chemtura. This agent has a kick temperature of 153-163 c. Or a blend of sodium carbonate and citric acid which are available as e.g. Hydrocerol, Tracel or Supercell from Clariant, Tramaco or Americhem Preferred polymers for use in the units are thermoplastic polymers, more preferably polyolef ins, in particular polyethylene or polypropylene. In many processes polyethylene is most preferred.

The pellets may be of any practicable shape, although cylinders are likely to be preferred in most cases because of the lack of corners which are vulnerable to damage. (Spherical pellets would be ideal in most cases, but are less easy to make.) They are preferably 3-20mm, more preferably 5-15mm (say around 10mm), in length.

This is large enough to allow easy separation of the pellets from the skin polymer powder during rotation of the mould whilst allowing reasonably rapid melting.

Preferably, the pellets have equal length and width.

This reduces the possibility of the pellet sticking to the mould surface during the skin forming stage.

As previously discussed, using pellets in accordance with this aspect of the present invention allows a blowing agent with a lower kicking temperature to be used and therefore improves the one-shot rotomoulding process.

The pellets are preferably formed by blending the polymer and blowing agent and then compressing the resulting mixture within a compaction press.

In order to create an aesthetically pleasing product, separation of the layers must be as well defined as possible. This prevents any breakthrough foaming in the skin layer.

It has been found that occasionally during rotation of the mould, the mechanical forces can damage the pellet and cause powder to break of f. This powder may then be incorporated in the skin layer. Additionally, as mentioned above, it is possible for a pellet to stick to the wall of the mould during the skin forming stage. To overcome this problem, the edges of the pellet are preferably comprised purely of polymer.

It has been observed that any material breaking away from the pellet in the early stages of rotomoulding, e.g. during skin formation, originates from the outer edges of the pellet. By making these edges from pure polymer, any break up from the pellet will not result in blowing agent entering the skin layer and therefore foam breakthrough in the skin layer is prevented. The same is true if the pellet becomes stuck to the mould surface.

It has been recognised that this feature can usefully be

applied to both prior art pellets and those made

according to the invention. Thus, according to another aspect of the present invention, there is provided a polymer pellet comprising a mixture of blowing agent and polymer wherein the edges of the pellet are comprised purely of polymer.

It is to be noted that in these embodiments it is only the edge portions of the pellet which need comprise pure polymer, any smooth external sides of the pellet not forming an edge may still comprise the blowing agent/polymer blend. So, for example, a cylindrical pellet could have opposing ends of pure polymer while the circular outer wall still contained blowing agent.

Alternatively it may be decided to encase the blended polymer/blowing agent entirely within a pure polymer shell, in which case the entire outer surface of the pellet could be formed from pure polymer.

These preferred forms of pellet also help to define the skin/foam boundary by firstly allowing the skin polymer to form a layer, then the initial compressed layer of pure polymer powder and finally the compressed foam powder and blowing agent.

The invention also extends to the use of the above pellets in rotomoulding, and so, viewed from a still further aspect, the invention provides a method of rotomoulding comprising the use of a unit as discussed above. Preferably, the method uses the preferred forms of pellet herein described. Th invention further extends to products made by rotornoulding using the method of the invention.

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a pellet according to the invention; Figure 2 is a schematic cross sectional view of a press for forming a pellet; and Figures 3a and 3b are schematic cross-sections of part of a rotomoulding apparatus showing the use of such pellets.

Figure 1 shows a cylindrical foaming rotomoulding pellet 1. It is in the form of a cylinder approximately 15mm in length and 10mm in diameter. The central portion 2 comprises a mixture of blowing (foaming) agent and polymer powder. The blowing agent used is Celogen OT, which has a kick temperature of 153-163 c. The polymer is powdered polyethylene which has been produced in the conventional fashion by extruding, pelletising and then grinding into polymer. The end portions 3 do not, however, contain any blowing agent.

The pellet is made by means of a press 10 of the sort shown schematically in Figure 2. The press comprises a male portion 11. and a female portion 12. The former has a plurality of cylindrical projections that correspond to a plurality of similarly shaped recesses in the latter. Means (not shown) are provided for moving the male portion vertically relative to the female portion and, when moving it downwards, for applying significant pressure.

To produce pellets, the recesses are filled with blowing agent and polymer. At the bottom of each recess there is first provided a layer of only polymer which is to form end portion 3. This is followed by the polymer/blowing agent mixture that is to form the central portion 2 of the pellet and then polymer to form the top end portion 3.

The male portion is then lowered and pressed downwardly, as shown by the arrow until a pressure of 750 MPa is achieved. The pellets are then removed from the press.

In use, the pellets described above are put into a conventional rotomoulding apparatus along with the polymer powder that is to form the skin of the eventual product. The mould is then heated and rotated in the conventional manner.

Figure 3a shows a section through a portion of the mould. It has a wall 20 on the outside of which heat 23 is provided (typically the mould is in an oven). The wall temperature has reached the melting point of the polymer that is forming the skin 21 and so this has liquefied and is coating the inside of the wall 20. The foaming pellets 1 are, however, still solid.

In Figure 3b, the temperature within the mould has reached about 160 c; the foaming pellets have also melted and the blowing agent has kicked, causing the polymer to foam and create layer 22.

The internal temperature can be monitored using a Rotolog or similar equipment.

.After a suitable time interval to ensure even melting and foaming, the heat source 23 is removed and the mould allowed to cool. Finally, the product is removed from the mould.

Claims (16)

1. -13 -Claims: 1. A method of forming polymer/blowing agent units for use

   in a rotomoulding process, wherein the unit is not formed by an extrusion method.
2. 2. A method as claimed in claim 1, wherein the method comprises the steps of; mixing polymer powder with a blowing agent to form a mixture; and compacting the mixture to create pellets.
3. 3. A method as claimed in claim 2 wherein the mixture is compacted at a pressure of between 200 -1000 MPa.
4. 4. A method as claimed in claim 3 wherein the mixture us compacted at a pressure of between 400 -900 MPa.
5. 5. A method as claimed in claim 4 wherein the mixture is compacted at a pressure of between 700-800 NPa.
6. 6. A method as claimed in any of claims 2 to 5, further comprising the step of limiting the shear stress applied to the mixture during pellet formation.
7. 7. A method of one shot foaming rotational moulding comprising the steps of; introducing skin polymer and units formed in accordance with any preceding claim into a mould; and creating a rotomoulded product.
8. 8. A method of one shot rotational moulding comprising the steps of; introducing skin polymer and units formed in accordance with any of claims 1 to 6 into a mould; and heating the mould to a temperature not exceeding 190 C. -14-
9. 9. A method as claimed in claim 7 or 8, wherein a product is created with a skin layer thickness of between 3 -30mm.
10. 10. A polymer unit for use in rotomoulding comprising polymer powder and blowing agent, wherein the unit is not formed by an extrusion method.
11. 11. A pellet as claimed in claim 10 wherein the pellet comprises a mixture of compressed polymer powder and blowing agent.
12. 12. A pellet as claimed in claim 10 or 11 wherein the blowing agent has a kick temperature of below 200 c.
13. 13. A pellet as claimed in claim 12 wherein the blowing agent has a kick temperature of below 170 C.
14. 14. A pellet as claimed in claim 13 wherein the blowing agent is p,p'-Oxobis benzene sulfonyl hydrazide (Di-phenoxy-disulfohydrazide)
15. 15. A pellet as claimed in any of claims 10 to 14 wherein the pellets have equal length and width.
16. 16. A polymer pellet comprising a mixture of blowing agent and polymer wherein the edges of the pellet are comprised purely of polymer.