This invention concerns the use of plasma treated polyethylene during a rotational moulding process, in particular for bonding to Polybutylene Terephthalate (PBT) .

Rotational moulding is a process that is mainly used for making hollow plastic products. It can be used to produce a wide range of products with highly desirable characteristics, and is relatively inexpensive when compared to other thermoplastic moulding processes . The process was developed in the 1940s, and since then it has been shown that a wide range of thermoplastics can be moulded in this way. Nevertheless, polyethylene is by far the main polymer used in rotational moulding, accounting for about 90% of products made by the process. Polyethylene is used extensively in the rotational moulding industry due to its relatively low cost, good mechanical properties, and ability to withstand the prolonged oven cycles . While polyethylene can be used to make products which can successfully store liquids such as oil and diesel, it cannot be used for the storage of more volatile liquids such as petrol, as these liquids can simply permeate through the material and escape into the atmosphere.

Polybutylene Terephthalate (PBT) on the other hand is a polymer that is extremely expensive if used in rotational moulding, as it needs to be cryogenically ground into a powder in the presence of liquid nitrogen. However, its excellent resistance to the permeation of fuel makes it an ideal choice in the production of fuel tanks, especially in light of changes to environmental regulations, which may not be able to be met by current plastics used for fuel tanks .

Bonding these two materials together would enable a moulding to be produced that retains the excellent mechanical properties and low cost of polyethylene, while providing an excellent barrier to the permeation of fuel. With standard polyethylene however, this is not possible, as it is difficult to bond satisfactorily to other materials.

The one shot rotational moulding process has four main stages, which are: (1) Charging: a predetermined weight of cold powder is placed into a metal mould, and the mould is sealed. (2) Heating: the mould is indexed to an oven where it rotates biaxially. Initially all the polymer powder is free to tumble about inside the mould. Heat passes directly through the mould wall, causing the temperature of the polymer powder to rise at a constant rate. Eventually, the temperature of the mould surface and polymer are sufficiently hot to allow a first layer of polymer to adhere to the mould walls. Successive layers of polymer then build up on the mould wall until all the polymer is molten. The mould is removed from the oven once the melt has consolidated sufficiently. (3) Cooling: on removal from the oven the mould is indexed to a cooling chamber where it continues to rotate. Cooling can be by means of air, water or both. (4) Demoulding: once the mould is cooled sufficiently, the moulding is removed.

The above process will produce a single layer product, but in many cases multi-layer parts are desirable. Using multiple layers allows different materials to be incorporated into the part, for example to provide a barrier layer or to improve the structural integrity of the product. In this case, an initial layer is produced as detailed above, but when the mould is removed from the oven, a second shot of material is added to the mould. This can be achieved by simply pouring the powder through a vent tube or other opening in the mould, or by use of a specially designed λdrop-box' . The mould is then returned to the oven and the second shot is processed in the same way as the first. This can be repeated for as many layers as required.

A problem that is often encountered in multiple shot moulding is that there can be poor adhesion between different polymer layers. This is particularly true when trying to create multi-layer parts using polyethylene, as this material is extremely inert and can be difficult to bond to other materials. Whilst tie-layers such as maleic anhydride are known, they also interact with reactive materials and stop polymerisation from occurring.

Traditionally, fuel tanks were made from coated steel, but since the 1980s the demand for lighter, cheaper vehicles has led to an increase in the use of polymer tanks, typically HDPE or Nylon. However, in these tanks, fuel can escape through the tank walls over time through the process of permeation. The amount of fuel permeation through a tank will increase as the temperature increases, because this increases the driving force for diffusion. A major source of volatile organic compound (VOC) emissions occurs due to this permeation of fuel followed by evaporation into the atmosphere. VOCs can be detrimental to the environment, causing depletion of the ozone layer, contributing to global warming, creating ozone at ground level, and posing a risk to health.

The use of multilayer systems and/or barrier polymers has enabled the production of plastic fuel tanks which can meet increasingly stricter emissions standards. There are concerns however that PE, Nylon and other polymers currently used in fuel tanks will no longer meet changing regulatory requirements.

Macrocyclic polyester oligomers, mixed with a suitable polymerization catalyst system, have a number of processing advantages that make them attractive to the rotational moulding industry. When heated during processing, the cyclic rings, with the aid of a catalyst, open and quickly connect or polymerise to form the high molecular weight polymer material. What is interesting about this transformation process, and this material, is that it starts as a solid, and when heated to temperatures above 300 aF (149aC) , becomes fully molten with a viscosity range of 150 mPa.s. This step is then followed by a reduction in viscosity (polymerisation) to ranges below 20 mPa.s. This water-like viscosity allows the rotomoulding of parts with excellent surface definition. In addition, the combined benefit of low processing viscosity and rapid polymerisation allows for faster processing and reduced cycle times due to increased heating rates. However, cyclic materials are expensive, and hitherto have not bonded with for instance polyethylene.

One object of the present invention is to provide multilayer products with these materials.

Thus, according to one aspect of the present invention, there is provided use of plasma treated polyethylene (hereinafter "ptPE") in a multi-layer polymerisation rotational moulding process or system.

ptPE has been found to act as a tie-layer between otherwise incompatable layers, because it cannot adhere to other materials. It is believed that it increases the surface tension of the polyolefin, to provide the integral bonding between the layers. In particular, ptPE can be used as a tie-layer between polyolefins, such as polyethylene (PE) and polypropylene, etc, and Polybutylene Terephthalate (PBT) .

The term "tie-layer" as used herein extends to the inclusion of ptPE within one or both of the other material layers, which may or may not form a physical distinctive layer in the moulding process or moulded products. That is, ptPE could be admixed with one or more of the main layer materials, such as the polyolefin or PBT or PBT-precursor, prior to the moulding process. Plasma treated polyethylene can be provided by placing standard polyethylene in a reaction chamber and treating it with oxygen plasma. The chemical structure of the powder surface changes from a non- polar to a polar structure, increasing its surface tension. Carrying out plasma treating in this manner allows the material to be used in applications requiring adhesion to foams, paint, coating or other adhesive fixtures. It is not subject to the variations in product quality or shape which can result from manually controlled hot flame methods used to prepare parts for painting. The material provides far better long-term adhesion properties and higher quality adhesion.

The following main advantages of plasma treated polyethylene can be realised when using optimal processing conditions:

• Inherent adhesion properties in the powder means that no pre-treatment with chemicals is needed. • Direct adhesion to wide range of paints systems,, inks and similar. • Direct adhesion to polyurethane (PU) foam and similar. • Treated powder can be stored for a long time before moulding. • Moulded products can be stored before any adhesion process like painting. • Overall improved logistics by a much faster mould-to-paint process. • Excellent mechanical properties

One utility of the plasma treated PE is that it is relatively easy to paint. Standard PE cannot be painted due to its highly non-polar molecular structure, which results in very low surface tension. As the plasma treatment changes the structure to a more polar one, and thus increases the surface tension, paint will adhere to the resulting material. Whilst bonding between layers can be achieved at a ptPE/PE mixture ratio as low as 3/97, to paint the moulded article, a minimum ratio of about 25/75 is needed.

Meanwhile, it is to be noted that plasma treated PE is more than double the price of normal PE, such that it should still be used in a cost-effective manner.

One possible PBT-precursor is a cyclic (poly)butylene terephthalate resin, derived from a blend of macrocyclics polybutylene terephthalate oligomers and a suitable catalyst system, such as the Cyclics PBT resin system. This range of resins is known in the art as "CBT® Resins". These are available from Cyclics Corporation, USA. CBT® Resins are a line of Polybutylene Terephthalate (PBT) resin systems that polymerise reactively like thermosets but have the material properties of thermoplastics . Particularly suitable category of CBT® Resins include 'XB3-CA5' materials.

CBT® Resins start as a solid at room temperature, melt to low viscosity at elevated temperature, and then polymerise in the presence of a catalyst to form high molecular weight PBT thermoplastic. A distinct feature of CBT Resins is their initial low "water-like" viscosity, which promotes easy processing in a variety of applications. CBT Resins are solid (powder, pellet, flake) at room temperature and when heated are fully molten above 1600C (3200F), with a viscosity in the range of 150 mPa.s (15OcP), and drops in viscosity to below 20 mPa.s (2OcP) at 1800C (355°F) . By comparison, normal thermoplastic PBT melts to the consistency of chewing gum, making it difficult to combine with fibre and fillers, or make thin wall parts, for example.

CBT Resins require a catalyst to polymerise and process into the engineering thermoplastic, PBT. There are a large variety of known catalysts that can be used with CBT Resins, and the choice depends on many factors. Some are slow and some are fast, some are liquid at room temperature and some are solid, some are sensitive to moisture, some are not. The fabrication process employed determines the type of catalyst and how the catalyst is introduced. Importantly, the cyclic resin and catalyst combination allow polymerisation in the time frame of a rotational moulding process .

Liquid catalysts can be used in meter/mix equipment, for example, in various reaction injection moulding (RIM) applications (including RTM, SRIM, RRIM and so on) . Some catalysts can be pre-combined with Cyclic Resin, making a "one-part" system that does not require metering or mixing of the catalyst. These "one-part" systems come as a solid (e.g., powder, pellet or flake) , and begin to polymerize when the material is melted. Such "one-part" systems are applicable, for example, to making prepreg or sheet moulding compound (SMC) .

Since CBT Resins process reactively like a thermoset, but produce a thermoplastic, a variety of both traditional thermoset and thermoplastic processing technologies can be used. The advantages of CBT® Resins include:

• Low viscosity to fill intricate detail • Good barrier properties (e.g. gasoline) compared to polyethylene and Nylon • Possible to use as inner coating in polyethylene • Process above the melting point of PBT to drive the reaction quickly • Shrinkage is similar to standard PBT: • 3% linear • <1% with filler • Variety of formulations possible: • Neat PBT • PBT-based elastomers and copolymers

During polymerization, CBT® Resins are converted into PBT thermoplastic polyester resin. Commercially available PBT grades exhibit a wide range of mechanical, electrical and thermal properties when combined with typical polymer additives and fillers, making PBT thermoplastic a very versatile material. Some of these material advantages include:

• Stiffness and toughness • High thermal insulation • Chemical resistance • Dimensional stability / Low water absorption • Electrical insulation and high arc resistance • Flame retardant • Thermoformable • Post-mould operations (e.g., welding, gluing, painting) • Recyclable

Importantly, the plasma treated PE material does not adversely affect the polymerisation reaction of materials such as CBTs.

The multilayer products of the present invention are useable in a number of different areas such as chemical or hazardous liquid storage tanks, vessels, receptacles, conduits, etc, including horticultural spray tanks, solvent tanks. One particular area is fuel tanks,

One particular benefit of the products formed by the present invention is low permeability. The permeability of a polymer is driven by three main properties - the crystallinity level, the molecular weight, and its inherent incompatibility with the liquid (eg solvent) being contained. The higher the level of crystalline structure the better the impermeability (as permeation occurs mainly through the amorphous region) . A rotomoulded CBT product from the present invention can have the same level of crystallinity as injection moulded PBT, and hence its permeability performance is equal in this respect.

Also, the higher the molecular weight of the polymer the better its impermeablity. Injection moulded PBT generally has a molecular weight of approx. 60,000 g/mol, whereas a rotomoulded CBT product from the present invention can have a molecular weight in the range 120,000 to 150,000 g/mol. Thus rotomoulded CBT products have a lower permeability performance than injection moulded PBT products. Generally, it is noted that rotational moulding is a relatively slower process, compared to e.g. injection moulding, which gives the material more time and correct conditions to polymerise, hence building up greater molecular weight. The higher the molecular weight, the greater the expected impermeability. However, high crystalUnity has an adverse affect on impact performance - a major requirement of fuel tanks and the like. Hence the benefit of the present invention using a polyolefin as the λstrength' and impact absorbing part of the rotomoulded products.

The term "polyolefin" is well known in the art, and includes polyethylene, polypropylene (PP) ,polybutylene, etc. PP can be painted without any additional use of ptPE, and has better ESCR (environmental stress cracking resistance) properties than PE, with approximately the same flexural properties.

According to a second aspect of the present invention, there is provided a process for forming by rotational moulding a polyolefin and Polybutylene Terephthalate (PBT) multi-layer product comprising the steps of:

obtaining a polyolefin and PBT precursor; adding the polyolefin and PBT precursor separately or jointly into the mould; adding plasma treated Polyethylene (ptPE) either separately or jointly with the polyolefin and/or the PBT precursor into the mould; rotationally moulding each material, either simultaneously or separately; and cooling the mould and removing the moulded product from the mould. The process may be a one shot or multiple shot process depending upon the possible premixing of one or more of the precursor materials and the ptPE.

The process can provide a product having at least two different layers. The process allows the multi- layering to be any number of layers as desired or necessary.

The depth of each layer depends upon the desired properties of the final product. 'Standard' PE is an inexpensive raw material, such that the PE would generally form the majority of the thickness of rotomoulded products. It is known that the stiffness of a layer is proportional to the thickness of material cubed, so doubling the wall thickness increases the stiffness by a factor of 8. Thus the present invention provides the easy ability to increase the mechanical properties of the product by using any thickness of PE, whilst keeping the cost low.

However, this only works if there is a good bond between the layers. With no bond, or only a weak bond, the product will fail by shear at the interface, and the addition of more PE will not solve this problem. The present invention allows increased strength due to using a thicker PE layer, as there is a very good bond between the layers.

According to a third aspect of the present invention, there is provided a polyolefin and Polybutylene Terephthalate (PBT) bonded multi-layer rotationally moulded product which includes plasma treated Polyethylene (ptPE) .

The ptPE is generally in the form of a tie-layer which bonds the PE and PBT layers together.

In one embodiment, the PBT precursor is a CBT® resin, particularly a XB3-CA5 material.

The present invention extends to such bonded multi- layer moulded products being provided by a process as hereinbefore described, and/or through use of ptPE as hereinbefore described.

Embodiments of the present invention will now be described by way of the following examples and tests.

Introduction Before moulding, the materials were prepared. The CBT material was dried in an oven at 95aC for at least seven hours in order to remove any moisture. If a mixture of plasma treated PE and standard PE is to be used, then this was well mixed using the desired proportions.

There is a reasonably narrow window within which the CBT can be processed successfully, to give optimum mechanical properties. For rotational moulding applications, optimum mechanical properties are achieved when the material is held between 180sC and 215aC for up to 30 minutes. The actual length of time required will depend upon the temperature used. This temperature range is below the melting point of the final product. The benefits of this include reduced risk of degradation, shorter cycle times, and the ability to demould the product while still hot.

The properties of the CBT material, and in particular its impact properties, are heavily dependent on the level of crystallisation and crystal type. This phenomenon has been well understood for many years, and it is also well understood that the crystal type achieved in PBT is a function of the rate of cooling. With injection moulding very high cooling rates can be achieved, but rotomoulding cooling rates are many orders of magnitude slower. This means that rotomoulded CBT parts have a different crystal structure and type to injection moulded parts when they are cooled below the optimum cooling rate. Thus rapid cooling of the part is desirable in order to obtain the best morphology. Rapid cooling is also desirable to achieve a product with low void content, by minimising the effect of different crystallisation temperatures.

Moulding Processes There are many rotational moulding processes. 'One shot' involves adding all starting materials together. λ2 shot' involves moulding first one layer, then another. To exemplify the present invention, a single shot, four separate variations in the 2 shot moulding process, and two variations in the 3 shot moulding process are described. Each of these has its own utility and they are described in more detail below, starting with the 2 shot processes.

PE & ptPE / CBT In this variation standard PE alone is the first shot, and the second shot is a mixture of ptPE and CBT. The ratio of ptPE to PE can be in the range 3% upwards, and the second shot can be added manually through the breather or via a drop box.

The utility of this process is that it uses the minimal amount of ptPE to achieve a good bond between the layers. Since the ptPE has a much higher viscosity than the ultra low viscosity CBT, the two materials separate completely leaving the CBT to polymerise as the inside layer in a container, where its impermeability properties can be exploited. The ptPE material locates itself at the PE / CBT interface where it is most required.

ptPE & PE / CBT In this case ptPE is used in the first shot, with a mixture of PE and CBT in the second shot. The amount of ptPE used depends on the mould surface area as the aim is to produce a relatively thin layer over the entire surface of the mould. The amounts of PE and CBT used depend on the necessary mechanical properties and the required permeability performance respectively. A drop box can be used for the second shot or it can be added manually through the breather. As in the above process the PE and CBT will separate into two distinct layers due to their widely different melting points, viscosities, and specific gravities before the polymerisation of the CBT occurs.

The utility of this process is to produce an outer surface which is 100% ptPE, and thus has a very good surface energy. This means that it is easier to paint the part. Thus this process is used where a high quality painted finish is desired.

ptPE / PE & CBT Here a mixture of ptPE and PE are added in the first shot, with CBT alone in the second shot. The CBT can be added via a drop box or manually. The ratio of ptPE to PE would depend on the required surface energy of the part, ie whether or not it had to be painted.

The utility of this variation is to produce a part with a 100% CBT inner layer, having no contamination from either form of PE. This produces a part with maximum permeability performance, for applications where this is the key requirement.

CBT & ptPE / PE In this case CBT is added as the first shot, and a mixture of ptPE and PE as the second shot. A ratio of 3% ptPE would be used in this case to achieve an adequate bond between the layers. No more is necessary since it would be on the inner layer, which has no need to be painted. The amount of CBT used would depend on the requirements of the specific part.

The utility of this process is to place the CBT on the outside of the part, where its low viscosity allows it to form very complex surface features such as small sized text. The CBT outer layer would also provide better scratch resistance and more toughness than PE, which may be desirable in certain applications.

In this process the second shot of' the PE mixture should be added by a drop box. This is because the CBT has a melting point of about 230aC, and were the mould to be removed from the oven the CBT layer would solidify and would be very difficult to re- melt.

The two variations of the 3 shot process are:

PE / ptPE / CBT In this case PE is added first, followed by the tie layer of ptPE, and finally the CBT layer. The amount of ptPE is a factor of the part geometry as all that is required is a thin coating over the entire surface of the PE. The second and third shots can be added either by drop box or manually through the breather.

The first advantage of this process is that it uses the minimal amount of ptPE to gain a good bond, as the ptPE is 100% concentrated where it is needed at the interface, and not ^wasted' in the PE or CBT layers. Secondly this process produces a part with a 100% pure CBT layer, with the associated advantages of better permeability performance due to the absence of contamination.

CBT / ptPE / PE In this case the CBT is used as the first shot, followed by the ptPE tie layer and finally a PE layer. Again the amount of ptPE used is just enough to coat the CBT layer, while the amount of PE is dependant on the required mechanical properties . The amount of CBT used depends on the specific application.

The utility of this process is to place the CBT on the outside of the part, where it can be used to form complex surface features such as small text due to its low viscosity. It also provides a more scratch resistant, tougher surface than PE, which might be desirable in certain applications.

In this case the second and third shots must be added by way of a drop box, due to the relative melting points as described above. Single Shot Process

In the single shot process of the present invention, there is separation of the layers. This is a surprising result. Even though the materials are all introduced at the same time, the final part has distinct PE and CBT layers, with minimal contamination. This is due to greatly different viscosities, melting points and specific gravities of the two materials causing them to separate prior to the polymerisation of the CBT material.

Testing

Permeability Tests -Immersion Procedure

New standards will require an 85% reduction in plastics fuel tank permeation and a 95% reduction in fuel system hose permeation from new motorcycles beginning in 2008.

The tests regulated by the Environmental Protection Agency (EPA) are carried out with fuel which contains alcohol. Polyamide 6, 11 or 12 have generally shown good resistance to fuel which contains no ethanol, with impermeability increasing with polyamide number. However the presence of ethanol in a fuel will cause polyamides to swell, and this in turn dramatically reduces the permeation resistance. In the past, it has been shown that the resistance to permeation of ethanol, methanol and toluene is much higher in polybutylene terephthalate (PBT) than in polyamide 12. The use of PBT in fuel tanks will therefore allow the new regulations to be met.

In order to assess the permeability, immersion tests were carried out. Prior to the analysis, all of the test specimens were placed in a drying oven at 80aC. When the specimens reached equilibrium weight, individual samples of each polymer were then immersed in sealed vessels containing: methanol, ethanol and toluene at 82C, 20aC, 40aC and 60aC. The vessels containing the specimens were maintained at these temperatures throughout the duration of the tests. Specimens were removed at regular time intervals and the change in weight with immersion time was recorded using a balance, accurate to 4 decimal places. An average of 10 readings were recorded for each specimen tested.

The results from the permeability tests (based on polymer weight gain from fuel infusion) on 3 samples of rotationally moulded PBT samples are shown below. These are also compared with standard injection moulded PBT, and also with injection moulded polyamide 12. It can be seen that the rotationally moulded PBT performs much better than the Nylon in both methanol and toluene, and also performs better than the injection moulded PBT in toluene. Percentage Weight Gain of Various Polymers at 20 degrees C for 900 hours

Short Beam Strength

All samples made where tested using ASTM D 2344/D2344M-00 Standard Test Method for Apparent Interlaminar Shear Strength of Parallel Fiber Composites by Short Beam Method. This method determines short-beam strength. The sample is loaded in three-point bending. The ends of the specimen rest on two supports that allow lateral motion, the load being applied by means of a loading nose directly centred on the midpoint of the test specimen.

The dimensions of the samples were designed to cause maximum shear in the middle of the sample thickness. For a multi-layer sample with two layers of the same thickness, maximum shear will be in-between the layers. Therefore short beam strength can be correlated to measuring adhesion between the layers. The results are shown in Table 1 hereinafter. There were no interlaminar failure modes except for sample PE17 with 0% plasma treated PE, showing a good bond strength in all samples. All samples with plasma treated PE failed at a point remote from the bond. The relatively high variability in the quoted values results from some inherent variation in the properties of the CBT at the experimental grade stage, i.e. where the polymer matrix has a varying ability to with stand cracking, and not from any differences in bond strength.

The present invention provides a simple but effective process for bonding PBT and polyolefins like PE. The low viscosity prior to polymersiation allows separation of the layers, and subsequent bonding even when intimately mixed. Moreover, the polymerization is not adversely affected by the bonding, which is the case when chemical coupling agents are used.

Short Beam Shear Test Results

Table 1