GB2508929A

GB2508929A - Cross-linkable polyethylene and conductive agent compositions for rotomoulding

Info

Publication number: GB2508929A
Application number: GB1222736.9A
Authority: GB
Inventors: Aldo Quaratino
Original assignee: MATRIX POLYMERS Ltd
Current assignee: MATRIX POLYMERS Ltd
Priority date: 2012-12-17
Filing date: 2012-12-17
Publication date: 2014-06-18
Anticipated expiration: 2032-12-17
Also published as: GB201222736D0; WO2014096777A1; GB2508929B

Abstract

A conductive composition for rotomoulding comprises a cross-linkable polyethylene and a conductive agent can be produced by blending. Preferably the conductive agent is conductive carbon black. A process for producing a rotomoulded object comprises the steps of providing a mould; providing a charge of a first polymer composition in the mould; heating and rotating the mould to form a layer of the first polymer composition; cooling the mould; and removing the rotomoulded object from the mould; wherein the first polymer composition comprises a cross-linkable polyethylene and a conductive agent (preferably conductive carbon black). Thus produced articles can include fuel tanks. The rotomoulding process can also include additional layers of different polymeric materials.

Description

COMPOSITION AND PROCESS

The present invention relates to a composition for rotomoulding, to a process of producing the composition and to a process of producing a rotomoulded object using the composition.

Rotational moulding is a moulding process for producing articles from plastics materials.

Rotational moulding is particularly suitable for large articles, because the moulds are generally cheaper than the moulds used for other sorts of plastic moulding such as io injection moulding and blow moulding. Large hollow articles such as canoes and waste bins are often made by rotational moulding.

Rotational moulding is generally based on four steps -loading (or charging), heating, cooling and unloading (or demoulding). Typically, a pre-weighed amount of material, generally in powder form, is placed in a mould located on an arm of a rotomoulding machine. The mould is then closed and the arm is moved into an oven for the heating step. During the heating phase the mould rotates biaxially on perpendicular axes. The powder (or liquid) starts tumbling inside the mould while the temperature rises. Once the melting temperature is achieved the material sticks to the wall of the mould in a layer to form the moulded part. When the material is molten, generally after a determined period of time, the arm holding the mould is moved to a cooling station where forced air, water mist or a combination of both is used to cool the mould to a temperature below the solidification point of the material. During the heating process the temperature and time must be controlled so as to sufficiently melt and distribute the polymer material around the inside of the mould without degrading or deteriorating the properties of the polymer material. Finally the arm is moved to an unloading station where the part can be removed from the mould. Loading and unloading operations are often called the service operation. A schematic view of the rotomoulding process is illustrated in Figure. 1.

Rotational moulding is a unique plastic process and is different from other processes like injection moulding or blow moulding because of the absence of pressure. This lack of pressure means that moulds can be relatively light weight and inexpensive in construction, which lends the rotomoulding process to being economic to produce relatively small runs of products. For these reasons rotomoulding is used to manufacture a very wide and diverse range of hollow, very complex and intricate shapes ranging in size from hockey balls to industrial tanks of over 50,000 litres capacity. Additionally, the absence of pressure and shear means that rotomoulded pads have low levels of moulded-in stress. The rotomoulding process also allows the production of multiple layer parts (composite structures) made using the same mould, at low cost.

A variety of materials can be used in the rotomoulding process. By far the most commonly used material in rotomoulding is polyolefins, especially polyethylene (PE), followed by polypropylene. Other materials used in rotomoulding include polyvinyl chloride (PVC), polycarbonate and crosslinkable PEs. Polyethylene is a relatively inexpensive polymer which may be successfully used in rotomoulding processes under a wide range of conditions. However, polyethylene does not provide the required properties or characteristics necessary for all products or applications. For example, where a rotomoulded object is required to be stiffer, or to be compatible with, or resistant to, specific substances, it may be preferred to use alternative polymers such as crosslinkable polyethylene, polycarbonate or crosslinkable PE. However, these polymers are more expensive than polyethylene or polypropylene, in general. Thus is there exists a need to provide polymer compositions for use in rotomoulding which can provide products having required physical properties at relatively low cost.

The present invention seeks to provide an improved polymer composition for rotomoulding, and an improved method of making such a polymer composition.

According to one aspect of the present invention there is provided a process for producing a conductive composition for rotomoulding, comprising mixing a crosslinkable polyethylene with a carbon black conductive agent.

Preferably, the crosslinkable polyethylene is Revolve XL400.

Conveniently, the conductive agent is selected from conductive carbon black CAS number 1333-86-4.

Advantageously, the conductive agent is made from Cabot and is called VULCAN® XC72 R. Preferably, the mixing of the crosslinkable PE, Revolve XL400, with the conductive agent is performed by dry blending.

Conveniently, the mixing of the crosslinkable PE with the conductive agent is performed by spraying the conductive agent on to the crosslinkable PE during a grinding step.

Preferably, the mixing of the crosslinkable PE with the conductive agent is performed by melt compounding.

Advantageously, the conductive agent is mixed with the crosslinkable PE in an amount of from about 0.1 to about 10% by weight, preferably from about 0.5 to about 5% by weight, more preferably from about 1 to about 3% by weight and most preferably from about 1.5 to about 2% by weight of the conductive composition.

io According to an aspect of the invention, there is provided a conductive composition for rotomoulding obtainable by the process of the invention.

According to an aspect of the invention, there is provided a conductive composition for rotomoutding comprising a crosslinkable FE and a conductive agent.

Conveniently, the conductive agent is selected from conductive carbon black compounds.

Preferably, the conductive agent is present in the composition in an amount of from about 0.1 to about 10% by weight, more preferably from about 0.5 to about 5% by weight, even more preferably from about 1 to about 3% by weight and most preferably from about 1.5 to about 2% by weight of the conductive composition.

Advantageously, the process further comprises incorporating the rotomoulded object into another object.

According to an aspect of the invention, there is provided an object formed by the process of the invention.

According to an aspect of the invention, there is provided a rotomoulded object comprising conductive crosslinkable FE.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic view of the rotomoulding process; Figure 2 is a cross-sectional view of a single layer rotomoulded product; Figure 3 is a cross-sectional view of a two-layer rotomoulded container; and Figure 4 is a cross-sectional view of another two-layered rotomoulded object.

As mentioned above, rotomoulding is a widely applicable method of forming physical objects from polymer compositions. It lends itself to applications which are generally complimentary to injection moulding and blow moulding techniques. One popular choice of polymer composition for use in rotomoulding is polyethylene. One of the reasons why polyethylene is a popular choice is its easy use in the rotomoulding process.

Polyethylene polymer compositions can be used in a wide variety of rotomoulding processors to produce physical objects. Close control of all of the parameters of the rotomoulding process is generally not required using polyethylene, unlike some other polymer compositions, where close control of the rotomoulding parameters is required to produce a satisfactory product.

However, for many applications a rotomoulded product produced from polymers such as polyethylene is unsuitable. This can be, for example, because the object does not have sufficient physical strength or rigidity, or does not possess other desirable physical properties.

The invention relates in general to the generation of conductive crosslinkable PE in a rotomoulding process. This involves the use of a polymer composition comprising a crosslinkable PE along with an additive that causes conductivity to occur during the rotomoulding process.

In this specification the term "crosslinkable PE" means polyethylene that can form a polymer that contains linked residues joined by covalent bonds. There are a number of different ways to form crosslinked polyethylene. These generally involve the use of a crosslinking agent which can be activated, for example, by heat to form covalent bonds between polyethylene polymer chains. Preferably, the crosslinkable PE5 comprise straight chain linked generated by the action of peroxide to form covalent bonds.

Crosslinkable PEs are commercially available from a variety of chemical companies including Matrix Polymers.

Crosslinkable PE is a polyethylene resin where a network of Carbon bond is generated by the peroxide: 2,5-dimethyl-2,5-di(tertbutylperoxy) hexyne-3 1068-27-5. Some typical properties of the crosslinkable PE XL400 (from Matrix Polymers) are listed in Table 1.

Property Method Unit XL400 Density ISO 1183 glcm3 0.945-0.950 Melting Temperature ASTM D789 -DSC °C 135 Tensile Strength (at Yield) ISO 527 (50 mmlmin) MPa 22 Flexural Modulus ISO 178 (1.3 mm/mm) MPa 950 Heat Deflection Temperature ISO 75-2 (1.8 MPa) °C 55 Water Absorption ASTM 0-570 (50% RH at 23°C) % 0.05 Impact Strength ARM (23°C 3mm) J > 170 Table 1-Typical properties of Revolve XL400 If the starting polymer is in the form of pellets, for example, the material is preferably ground at ambient conditions into a powdered form for use in rotational moulding. Table 2 summariseS some characteristics of the powder specification.

Analysis Method Specification Units

Sieve Analysis ATSM 00078 600pm 0 500pm <5 % 425 pm 5cxc20 % 300pm lOcx<40 % 212pm 10cx<40 % 150pm <10 % PAN <15 %

Table 2-Grinding specification for Revolve XL.400

Preferred conductive agents for creating a conductive crosslinkable PE io composition suitable for use in rotomoulding include carbon black compounds. Suitable conductive agents are compatible with the crosslinkable RE material and with the rotomoulding process. The conductive agent should be sufficiently stable and compatible to allow it to be mixed with the crosslmnkable RE to form a conductive composition for rotomoulding. In use, the composition should be able to be distributed around the mould on heating and rotation. During the heating process, the crosslinking reaction will occur and the crosslinkable bonds will be created. Therefore the conductive agent will be part of the polymer and a conductive path through the polymer chain will be generated. After cooling and removal from the mould, the crosslinkable PE material will retain the formed structure and be conductive.

During the rotomoulding process, the mould contains a charge of polymer powder, and is heated in an oven. The temperature of the oven increases from ambient to approximately 300°C. When the temperature inside the mould reaches around 125-o 128°C, the polymer starts melting. The mould and the material are kept in the oven until the internal temperature reaches 160-165°C where the crosslinking reaction typically takes place. The polyethylene will become crosslinked polyethylene. The process continues in the oven where the temperature inside the mould continues to rise to about 195-205°C. At this stage the material is transferred to a cooling chamber where the io cooling process begins. Due to thermal inertia, the temperature inside the mould continues to rise up to about 210°C, before cooling. This process results in a crosslinked electrically-conductive layer being formed on the inner surface of the mould.

The conductive agent preferably generates gas at a temperature of greater than about 130°C, more preferably above about 140°C, more preferably above about 150°C, more preferably above about 160°C, and most preferably above about 170°C.

Preferred conductive agents include Vulcan XC72R available from Cabot Corporation.

Another suitable conductive agent is a product called ENSACO, available from TIMCAL Ltd. Other similar conductive materials could be used in the rotomoulding process.

Through extensive research and testing, it has been found that the preferred conductive agents work well to produce conductive crosslinkable PE in rotomoulding processes.

Preferably, the conductive agent and the crosslinkable PE powder are dry-blended. Dry blending is conveniently performed by high intensity mixing of powdered crosslinkable PE and the conductive agent. High intensity mixing is used to obtain consistent dispersion of the conductive agent within the crosslinkable PE. The blending process develops shear (friction) during a relatively short period of time (such as 15 minutes).

The shear during the mixing raises the temperature of the mix. It has been found that 50 to 55 °C is an ideal mixing temperature. An additional benefit of mixing at temperatures above room temperature is that the powder is polished. The polished powder has more rounded particles (i.e. less tails and hair) improving the flow and increases the bulk density which helps to fill mould cavities.

Other methods are available for incorporating conductive into plastic materials.

Compounding and spraying additives onto powder during the grinding process are suitable methods. A preferred mixing process involves melt compounding. In this process, a polymer composition and a conductive agent are mixed by being melted together. This can be conveniently performed by extruding the polymer composition and the conductive agent together, followed by grinding of the resultant pellets. However we have found that dry mixing of the correct formulation can produce a good quality and consistent conductive product. The dry mixing method is simple to perform, cost effective and does not produce an excessive additional heat history and degradation that might be io created by compounding.

The conductive agent causes the evolution of gas during the conditions within the mould during the rotomoulding process. In the situation where a mould is charged with a polymer composition that comprises crosslinkable PE and a suitable conductive agent, the rotomoulding process will produce a mono-layered product as shown in Figure. 2.

The product 10 comprises a single layer 12 of conductive crosslinkable PE. The exterior surface 14 of the layer 12 corresponds to the internal surface of the mould. The layer 12 also has an interior surface 16 defining an internal space 18 within the object 10. The mould could be of any variety of shape suitable for use in the rotomoulding process.

The invention allowed the formation of objects having specific shapes by the rotomoulding process, which are electrically-conductive. In general, plastics are electrical insulators, and can also suffer problems from accumulation of static charges.

One particular benefit of the application is the formation of containers that can be connected to an electrical system in an easy manner. For example, fuel containers, such as vehicle fuel tanks, benefit from being connected to an electrical connection, such as an earth connection. This prevents the potentially dangerous occurrence of electrical discharges, which could cause a fire. A container formed in accordance with the invention is electrically conductive, and so can be readily connected to an electrical connection such as an earth in a variety of ways. This is particularly useful and advantageous over the prior art in that an electrical connection can be made anywhere on the electrically-conductive container, and does not need to be made in a specific location.

as The invention is particularly useful for the formation of fuel containers, including (but not limited to) vehicle fuel tanks, and fuel tanks that are not in vehicles. These include fuel containers in a fixed position, portable fuel containers and portable fuel pumps. In addition to fuel tanks for cars, containers made in accordance with the invention are suitable for use for motorbikes, commercial vehicles and industrial vehicles such as earth moves and tractors. The invention is also applicable to rotomoulded objects that are not fuel tanks.

The use of the conductive crosslinkable PE composition can be used in conjunction with other polymer compositions to produce multi-layered rotomoulded objects. For example, Figure 3 shows a rotomoulded object 20 having two layers. The first layer 22 comprises a polymer whilst the inner layer 24 comprises conductive crosslinkable PE. The object 20 may be formed by providing a mould with a charge of a polymer composition. When heated and rotated in the normal manner, this will produce the layer 22 of the polymer within the mould. After the formation of the layer 22 a second charge of a conductive crosslinkable PE composition may be added to the interior of the mould. The addition of a second charge of a polymer composition may be achieved by the use of a traditional drop-box, or by manual addition of the composition into the mould.

A drop box is typically an insulated steel container that allows powder to be dropped inside the mould at a determined moment of the heating phase of the process. A drop box may be connected to the mould by a collar. A pneumatic system opens a port between the drop box and the mould and allows polymer powder to be dropped inside the mould. Use of a drop box gives consistency of the manufactured product during the process. Alternative methods exist but they do not always give repeatable results.

Due to the rotation of the mould, the second crosslinkable PE composition will be distributed over the internal surface of the layer 22 within the mould. The conductive additive within the crosslinkable PE composition will produce the layer 24. The layer 24 is in intimate and direct contact with the layer 22. After cooling, the two layered rotomoulded object 20 may be removed from the mould. The object 20 has an outer layer 22 and an inner layer 24. The direct connection between the two layers results in a strong band.

The benefits of such a two-layered structure include the ability to produce objects having required physical properties such as strength whilst reducing the amount (weight) of polymers used in construction and cost per material used. The conductive crosslinkable PE layer can provide required conductivity, rigidity and strength to the overall structure whilst minimising the amount of the material used. In the case of crosslinkable PE5, this has cost benefits as crosslinkable PEs tend to be much more costly than polyolefins such as polyethylene.

The conductive crosslinkable PE layer may also be used in conjunction with other polymer materials, such as homo-and co-polymers of olefins, modified (adhesive) polyolefins, homo-and co-polymers of ethylene, homo-and co-polymers of propylene, high density polyethylene, linear medium density polyethylene, linear low density polyethylene, low density polyethylene, ethylene vinyl acetate copolymer, ethylene butyl acrylate co-polymer, polyolefin plastomer, styrene butadiene rubber, polyvinyl chloride and plasticised polyvinyl chloride.

Figure 4 shows a rotomoulded object 30 having two layers. In contrast to the object 20 shown in Figure 3, the object 30 in Figure 4 has an outer conductive layer 32 and an inner non-conductive layer 34. In a similar way to that described above, a mould may be provided with a first charge of a conductive crosslinkable PE composition.

Heating and rotating the mould will cause the conductive crosslinkable PE composition to be distributed around the internal surfaces of the mould. The evolution of gas by the conductive agent during the heating process will cause the production of a conductive crosslinkable FE layer. After the formation of the conductive layer 32, a second charge of a non-conductive polymer composition may be added to the mould, using a drop-box or other suitable means. Further heating and rotating will cause the non-conductive polymer composition to be distributed over the internal surfaces of the conductive layer 32, resulting in the formation of the inner non-conductivelayer 34.

Again, the object 30 is a composite structure comprising two layers of an outer conductive crosslinkable PE layer 32 and an inner non-conductive polymer layer 34. The use of these two different polymer layers produces benefits in terms of improved physical properties reduced polymer usage at reduced costs.

Other arrangements of conductive crosslinkable PE layers and non-conductive polymer layers are contemplated by the present invention, such as three or more layers of polymers, with single or multiple conductive layers being present.

After being removed from the mould, the rotomoulded object of the present invention may undergo further steps, as known to a person skilled in the art. This may include painting the object, the incorporation of the object into another product, or the incorporation of other objects within the rotomoulded product itself.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Claims

CLAIMS1. A process for producing a conductive composition for rotomoulding, comprising mixing a crosslinkable polyethylene with a conductive agent.
2. A process according to Claim 1 wherein the conductive agent is selected from conductive carbon black compounds.
3. A process according to any preceding claim wherein the mixing of the crosslinkable polyethylene with the conductive agent is performed by dry blending.
4. A process according to any of Claims 1 to 3 wherein the mixing of the crosslinkable polyethylene with the conductive agent is performed by spraying the conductive agent on to the crosslinkable polyethylene during a grinding step.
5. A process according to any of Claims 1 to 3 wherein the mixing of the crosslinkable polyethylene with the conductive agent is performed by melt compounding.
6. A process according to any preceding claim wherein the conductive agent is mixed with the crosslinkable polyethylene in an amount of from about 0.1 to about 10% by weight, preferably from about 0.5 to about 5% by weight, more preferably from about ito about 3% by weight and most preferably from about 1.5 to about 2% by weight of the conductive composition.
7. A conductive composition for rotomoulding obtainable by the process of any preceding claim.
8, A conductive composition for rotomoulding comprising a crosslinkable polyethylene and a conductive agent.
9. A composition according to Claim 8 wherein the conductive agent is selected from conductive carbon black compounds.
10. A composition according to Claim 8 or Claim 9 wherein the conductive agent is present in the composition in an amount of from about 0.1 to about 10% by weight, more preferably from about 0.5 to about 5% by weight, even more preferably from about 1 to about 3% by weight and most preferably from about 1.5 to about 2% by weight of the conductive composition.
II. A process for producing a rotomoulded object comprising the steps of: providing a mould; providing a charge of a first polymer composition in the mould; heating and rotating the mould to form a layer of the first polymer composition; cooling the mould; and removing the rotomoulded object from the mould; ic wherein the first polymer composition is as defined in any of Claims 7 to 10.
12. A process according to Claim 11 further comprising the additional steps of: providing a charge of a second polymer composition in the mould; and heating and rotating the mould to form a layer of the second polymer composition, wherein the additional steps are performed before the object is removed from the mould.
13. A process according to Claim 12 wherein the layer of the first polymer is formed before the layer of the second polymer.
14. A process according to Claim 12 wherein the layer of the second polymer is formed before the layer of the first polymer.
15. A process according to any of Claims 12 to 14 wherein the second polymer composition comprises a crosslinkable polyethylene, polyethylene, polypropylene or polycarbonate.
16. A process according to Claim 15 wherein the second polymer composition comprises polyamide 11 or polyamide 12.
17. A process according to any of Claims 11 to 16 wherein the object is a fuel container.
18. A process according to any of Claims 11 to 17 further comprising incorporating the rotomoulded object into another object.
19. An object formed by the process of any of Claims 11 to 17.
20. A rotomoulded object comprising conductive crosslinked polyethylene.
21. A process for producing a conductive composition for rotomoulding, a conductive composition for rotomoulding, or a rotomoulded object substantially as hereinbefore described with reference to and as shown in the accompanying drawings.