WO2024036357A1

WO2024036357A1 - Rotomoulding mixture

Info

Publication number: WO2024036357A1
Application number: PCT/AU2022/050894
Authority: WO
Inventors: Grant PALLING; Lei Wang
Original assignee: Matrix Polymers Australia Pty Ltd
Priority date: 2022-08-15
Filing date: 2022-08-15
Publication date: 2024-02-22

Abstract

The present invention relates to a rotomoulding mixture, to a method of preparing the mixture and to process of preparing a rotomoulded product which comprises rotomoulding the mixture. The rotomoulding mixture comprises a post-consumer recyclate (PCR) polyethylene and a virgin polyethylene. The PCR polyethylene has a maximum particle size of 2000 µm and a melt flow index (MFI) of from 0.1 to 20. The virgin polyethylene has a maximum particle size of 800 µm and a MFI of from greater than 7 to 20.

Description

ROTOMOULDING MIXTURE

TECHNICAL FIELD

[0001] The present invention relates to rotomoulding mixtures, to methods of preparing rotomoulding mixtures, and to rotomoulded products formed from the mixtures.

BACKGROUND ART

[0002] Rotational moulding (rotomoulding) is a versatile manufacturing process with a range of possible uses in agricultural, maritime, industrial and consumer applications, amongst others. Large products including liquid storage tanks, grain storage hoppers, troughs, storage bins, weather protection units and fresh seafood bins can be produced by rotomoulding. The process is also used for an assorted range of smaller products including, for example, onboard car storage units, toys, furniture, small shipping capsules, kayaks, road and pedestrian barriers and domestic animal feeders. The initial machinery and mould investment into rotomoulding is traditionally far cheaper than other forms of plastic moulding. The process has been refined to become very economical and to produce very little waste. The rotomoulding process is designed to make long lasting and durable products which cannot necessarily be provided by other forms of plastic moulding.

[0003] The first modern rotomoulding application was used for creating children’s toys in 1940. This process used nickel-copper moulds to form polyvinyl chloride into the desired shapes, far exceeding productivity of the previously used papier-mache moulds. The rapid expansion of polyethylene around the world enabled the rapid expansion of rotational moulding. Polyethylene can be ground into a fine powder at ambient temperatures, thus eliminating the expense and complication of cryogenic techniques required for grinding many other polymers. The availability of cheap, versatile material (polyethylene) meant increased development so that products such as liquid storage tanks in the range of 50 L to 50,000 L capacity could be manufactured. Through several years of research, rotational moulders are now able to create products from several different materials such as linear low-density polyethylene (LLDPE), low- density polyethylene (LDPE), high density polyethylene (HDPE), polyurethane, polypropylene and nylon. Traditionally more than 95% of products moulded are produced from polyethylene as it remains the easiest and most cost-effective material to rotomould with.

[0004] Rotational moulding uses varying sized, hollow moulds filled with powder, or very occasionally liquid polymer, most commonly a single dose (shot) of polyethylene powder. Shot weights (otherwise known as part weights) are determined by assessing the needs of the final product, most commonly by calculating the required polymer wall thickness of the product and translating this into the amount of material required to produce such a product. The material is softened or melted using heat in an oven until the desired flow rate of the material is achieved. This allows for moulding of a product to take place under low or atmospheric pressure. The mould is biaxially rotated while being subjected to high temperatures, usually in an oven or on an open flame machine. When sufficiently heated, the molten polymer flows around the inside of the mould. As the molten plastic sticks to the wall of the hot mould, the wall thickness of the product starts to build up as more and more molten plastic sticks to the wall, creating an increasingly thick moulded product. Once the polymer has been distributed to the desired areas around the mould, the heating element can be removed.

[0005] The mould is then moved to the cooling phase. Typically, a fan is used, sometimes with the assistance of misted water, to cool the outer surface of the mould. The polymer attached to the walls of the mould will start to crystallise as the polymer decreases in temperature, the polymer shrinks and becomes separated from the mould, a crucial factor for efficient removal of the product. The mould is then opened, and the newly moulded product can be removed, ready for the process to be repeated for the next part.

[0006] The material used in rotomoulding must be carefully selected as the high temperature, low pressure environment requires several chemical and physical properties of the material for a successful product to be produced. Firstly, as the pressure inside the mould is relatively low, the melt flow index (MFI) of the material must be sufficient to evenly distribute the polymer throughout the mould. Secondly, the compound must have a high thermal stability to avoid degradation when subjected to the high temperatures inside the mould.

[0007] Currently, there are large quantities of polyethylene waste materials, predominantly from single use polymer packaging, that are not being utilised and often end up in landfill or incineration. Many post-consumer recyclates (PCR) have properties which are not generally considered compatible with the rotomoulding process. This means that many PCR materials are currently unable to be efficiently or effectively rotomoulded in a conventional manner.

SUMMARY OF INVENTION

[0008] In one aspect, the present invention is directed to a rotomoulding mixture, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

[0009] With the foregoing in view, the present invention in one form, resides broadly in a rotomoulding mixture comprising a PCR polyethylene and virgin polyethylene.

[0010] In a first aspect, the present invention provides a rotomoulding mixture comprising a PCR polyethylene and a virgin polyethylene, wherein:

(a) the PCR polyethylene has:

(i) a maximum particle size of 2000 pm, and

(ii) a melt flow index (MFI) of from 0.1 to 20; and

(b) the virgin polyethylene has:

(i) a maximum particle size of 800 pm, and

(ii) a melt flow index (MFI) of from greater than 7 to 20.

[0011] Advantageously, the rotomoulding mixtures of the present invention allow for a wider variety of PCR polyethylenes to be used in rotational moulding. For example, many polyethylenes produced from recyclates would have an MFI of less than 2, and to the inventors’ knowledge no rotomoulding mixtures have been disclosed to date which would allow the incorporation of such PCR polyethylenes into the rotomoulding process. In contrast, PCR polyethylenes with an MFI of less than 2 can be successfully used in rotomoulding mixtures of the present invention. This also provides new opportunity to reduce the amount of waste polyethylene going to landfill.

[0012] Furthermore, the present invention advantageously allows the rotomoulding mixtures of the present invention to form a rotomouldable product in one, single shot.

[0013] The inventors also believe that the rotomoulding mixtures of the present invention can form rotomoulded products using conventional processes used by rotomoulders, so that no extra equipment or knowledge is needed to produce a useable product.

[0014] As used herein “PCR polyethylene” refers to a polyethylene-based polymer derived from post-consumer recyclates, such as post-consumer and post-industrial packaging. The PCR polyethylene may contain trace contaminants normally found in consumer plastic goods, such as inks, antioxidants, and metals. The PCR polyethylene may be of at least 95%, 96%, 97%, 98% or 99% purity.

[0015] The PCR polyethylene may comprise HDPE, LDPE, or LLDPE, or a combination thereof. The PCR polyethylene may consist of HDPE, LDPE, or LLDPE, or a combination thereof. The PCR polyethylene may comprise or consist of HDPE. The PCR polyethylene may comprise or consist of LDPE. The PCR polyethylene may comprise or consist of LLDPE. In an embodiment, the PCR polyethylene is recycled milk bottle polyethylene. In an embodiment, the PCR polyethylene is recycled film grade polyethylene.

[0016] As used herein “virgin polyethylene” refers to a polyethylene polymer which is not derived from a prior polyethylene product. Virgin polyethylene may be obtained from laboratory or commercial settings. Virgin polyethylene may be at least 95%, 96%, 97%, 98% or 99% purity.

[0017] The virgin polyethylene may comprise HDPE, or LLDPE, or a combination thereof. The virgin polyethylene may consist of HDPE, or LLDPE, or a combination thereof. The virgin polyethylene may comprise or consist of HDPE. The virgin polyethylene may comprise or consist of LLDPE.

[0018] High-density Polyethylene (HDPE) is defined as any thermoplastic with a density equal to or higher than 940 kg/m³. Physical characteristics of HDPE are increased stiffness, improved gas permeability, higher tensile yield strength and improved chemical resistance. Products moulded from HDPE include water tanks ranging from 500 L to 30,000 L, troughs, weathering covers, and chemical storage tanks.

[0019] Low density Polyethylene (LDPE) is defined as a thermoplastic polymer with a density lower than 940 kg/m³, which is formed from ethylene monomers. Linear Low-density Polyethylene (LLDPE) is defined as a thermoplastic polymer with a density lower than 940 kg/m³, which is a copolymer of ethylene and another olefin. Physical characteristics of LLDPE are improved impact strength, less warpage and distortion and better environmental stress crack resistance. Products moulded from LLDPE include toys, furniture, water tanks, kayaks, pedestrian and road barriers. Therefore, the term “polyethylene” in “PCR polyethylene” and “virgin polyethylene” may include copolymers in which ethylene is one of the monomers.

[0020] The ratio of PCR polyethylene to virgin polyethylene in the rotomoulding mixtures of the present invention may be selected depending on the desired properties of the rotomoulded product, and the materials at hand.

[0021] The ratio of PCR polyethylene to virgin polyethylene may be in the range of 1:9 to 9:1. For example, the rotomoulding mixture may comprise 10 wt% PCR polyethylene and 90 wt% virgin polyethylene, by wt% of polyethylene; or 90 wt% PCR polyethylene and 10 wt% virgin polyethylene, by wt% of polyethylene. [0022] In one embodiment, the ratio of PCR polyethylene to virgin polyethylene may be from 1:9 to 9:1, or from 2:8 to 8:2, or from 3:7 to 7:3, or from 4:6 to 6:4, or from 1:9 to 1:1, or from 2:8 to 1:1, or from 3:7 to 1:1 or from 4:6 to 1:1, or from 1:1 to 9:1, or from 1:1 to 8:2, or from 1:1 to 7:3, or from 1:1 to 6:4, or from 2:8 to 9:1, or from 3:7 to 9:1, or from 4:6 to 9:1.

[0023] The PCR polyethylene may comprise (by wt% of polyethylene in the roto moulding mixture) greater than 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt% or 90 wt%. The PCR polyethylene may comprise (by wt% of polyethylene in the rotomoulding mixture) less than 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt% or 90 wt%.

[0024] The virgin polyethylene may comprise (by wt% of polyethylene in the rotomoulding mixture) greater than 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt% or 90 wt%. The virgin polyethylene may comprise (by wt% of polyethylene in the rotomoulding mixture) less than 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt% or 90 wt%.

[0025] In an embodiment, the rotomoulding mixture is a powder. The PCR polyethylene may be a powder. The virgin polyethylene may be a powder.

[0026] The PCR polyethylene may have a maximum particle size of 2000 pm 1900 pm, 1800 pm, 1700 pm, 1600 pm, 1500 pm, 1400 pm, 1300 pm, 1200 pm, 1100 pm or 1000 pm. The PCR polyethylene may have a particle size (or particle size distribution) of from 1 pm to 1700 pm, or 1 pm to 1500 pm, or from 10 pm to 1500 pm, or from 1 pm to 1300 pm, or from 10 pm to 1300 pm, or from 1 pm to 1100 pm, or from 10 pm to 1100 pm, or from 1 pm to 1000 pm, or from 10 pm to 1000 pm, or from 1 pm to 900 pm, or from 10 pm to 900 pm, or from 1 pm to 800 pm, or from 10 pm to 800 pm or from 300 pm to 1200 pm. The PCR polyethylene may have an average or median particle size of from 200 pm to 1700 pm, or from 300 pm to 1200 pm, or from 400 pm to 1000 pm. The PCR polyethylene may have an average or median particle size of from 400 pm to 900 pm, or from 450 pm to 800 pm, or from 500 pm to 700 pm, or from 400 pm to 700 pm, or about 600 pm. The PCR polyethylene may have an average or median particle size of from 300 pm to 600 pm, or from 350 pm to 550 pm, or from 400 pm to 500 pm, or about 450 pm.

[0027] The virgin polyethylene may have a maximum particle size of 750 pm, 730 pm, 720 pm, 710 pm, 700 pm, 650 pm, 600 pm, 550 pm or 500 pm. The virgin polyethylene may have a particle size (or particle size distribution) of from 1 pm to 800 pm, or from 1 pm to 700 pm, or from 10 pm to 700 pm or from 100 pm to 500 pm. The virgin polyethylene may have an average or median particle size of from 50 pm to 600 pm, or from 100 pm to 600 pm, or from 100 pm to 550 pm, or from 100 pm to 500 pm, or from 100 pm to 450 pm, or from 100 pm to 400 pm, or from 200 pm to 400 pm, or from 150 pm to 350 pm, or about 300 pm. The virgin polyethylene may have an average or median particle size of from 150 pm to 450 pm, or from 200 pm to 400 pm, or from 250 pm to 350 pm, or about 300 pm.

[0028] The PCR polyethylene may have a higher average or median particle size than the virgin polyethylene. The PCR polyethylene may have a higher maximum particle size than the virgin polyethylene. The PCR polyethylene may have a greater distribution of particles sizes than the virgin polyethylene.

[0029] Without being bound by theory, it is believed that the use of a virgin polyethylene with a smaller average particle size compared to the PCR polyethylene may allow the virgin polyethylene to achieve the required melting temperature in the rotomoulding process before the PCR polyethylene reaches its melting temperature. This means that the outer wall of the final product would be predominately formed from the virgin polyethylene, if the virgin polyethylene powder is finer than the PCR polyethylene. Conversely, if the average particle size of the PCR polyethylene is larger then it is believed that the PCR polyethylene takes more time and energy to come to melting temperature and therefore more PCR polyethylene would be present on the inner surface of the final product. This provides for a finished part with structural integrity while maintaining an acceptable surface finish.

[0030] The term “Melt Flow Index” would be well understood by a skilled person. The MFI of a powder is defined as a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed gravimetric weights (such as 2.16 kg) for alternative prescribed temperatures (such as 190 °C). Melt flow index may also be referred to as melt flow rate or melt index. The MFI of a polymer may be measured by industry standards, including ASTM D1238 (for example updated on 26 August 2013).

[0031] The MFI of a polymer may be determined as per the following: The polyethylene sample for testing is loaded into a test machine with a heating chamber. The chamber is heated to 190 °C to melt the polyethylene. Once molten, the polyethylene is compressed by use of a small plunger to ensure minimal air remains in the heating chamber. A piston is then loaded into the barrel, ready to press down onto the molten polyethylene. The polyethylene is left under the weight of the piston for 5 mins to ensure the material is completely molten and that no air remains in the chamber. After 5 minutes, a 2.16kg weight is placed on top of the piston. Once the test machine is started, the material is forced through a die (with an orifice diameter of 9.5504 +/-0.0076mm). The machine cuts a sample from the die every 60 seconds until all of the material has been forced through the die. The first and last samples are discarded and the remaining samples are allowed to cool. Each of these samples is then weighed and an average weight is determined. The formula used to calculate the Melt Flow Index (MFI) is as follows (for samples cut each 60 seconds): Average Weight (g/min) x 10 = g/10 minutes.

[0032] The PCR polyethylene may have an MFI of from 0.1 to 18, 0.1 to 16, 0.1 to 14, 0.1 to 2, 0.1 to 10, 0.1 to 8, 0.1 to 6, 0.1 to 5, 0.1 to 4, 0.1 to 3, 0.1 to 2, or 0.1 to 1. The PCR polyethylene may have an MFI of from 0.1 to 2.

[0033] The virgin polyethylene may have an MFI of from 7.5 to 20, or from greater than 7 to 18, or from 7.5 to 18, or from greater than 7 to 16, or from 7.5 to 16, or from greater than 7 to 14, or from 7.5 to 14, or from greater than 7 to 13, or from 7.5 to 13, or from 8 to 12, or from 9 to 11, or about 10.

[0034] In an embodiment, the PCR polyethylene has a lower MFI than the virgin polyethylene.

[0035] Prior to the present application, the MFI of polyethylene was selected to be between about 2 to about 20 for conventional rotational moulding. An MFI reading lower than 2 was thought to cause an uneven distribution of material throughout the product’s wall, which could cause product failure. An MFI reading lower than 2 was also thought to cause an uneven or unacceptable, porous surface finish with many holes or gaps, which would likely lead to a product with inconsistent surface finish which could cause product failure. In contrast, an MFI reading higher than 20 was thought to force the machine operator to run biaxial rotation at a far faster speed than generally required and with large moulds this could lead to machine failures and damage. However, in the present invention PCR polyethylene with an MFI of less than 2 may be successfully used.

[0036] Without being bound by theory, it is believed that the use of a high MFI virgin polyethylene allows the PCR polyethylene and the virgin polyethylene to mix more homogeneously. This may also allow the outside wall of the product to develop a smooth finish with minimal surface porosity. [0037] The rotomoulding mixture (or the virgin polyethylene or the PCR polyethylene) may include an additive selected from the group consisting of a carrier, a UV stabiliser or UV resistant compound, a slip agent, an antioxidant, a pigment, a plasticiser, an optical brightener, a flame retardant, a filler, a heat stabiliser, a release agent and combinations thereof.

[0038] In an embodiment, the virgin polyethylene contains a release agent to facilitate removal of the rotomoulded product from the mould during rotational moulding.

[0039] Advantageously, the use of additives may increase the adaptability of the rotomoulded product.

[0040] In a second aspect, the present invention provides a method for preparing the rotomoulding mixture of the first aspect, wherein the method comprises: mixing PCR polyethylene with virgin polyethylene to form the rotomoulding mixture of the first aspect.

[0041] The method of the second aspect may comprise producing PCR polyethylene (as defined in the first aspect). The step of producing PCR polyethylene (as defined in the first aspect) from PCR may comprise one or more of washing PCR polyethylene, extruding PCR polyethylene, pelletising PCR polyethylene and grinding/pulverising PCR polyethylene. PCR polyethylene may be washed before and/or after pelletising PCR polyethylene. PCR polyethylene may be pelletised after washing and extruding PCR polyethylene. PCR polyethylene may be ground/pulverised after pelletising PCR polyethylene. The step of grinding/pulverising PCR polyethylene may produce a powder. The PCR polyethylene may be combined with one or more additives (as discussed for the first aspect) prior to extruding.

[0042] The step of grinding PCR polyethylene may comprise a blade gap setting of greater than 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm or 0.8 mm. The step of grinding the PCR polyethylene may comprise a blade gap setting of less than 2.0 mm, 1.8 mm, 1.6 mm, 1.4 mm or 1.2 mm. The step of grinding PCR polyethylene may comprise a blade gap setting of from 0.6 mm to 1.6 mm, or from 0.7 mm to 1.4 mm, or from 0.8 mm to 1.2 mm, or from 0.85 mm to 1.2 mm. The step of grinding PCR polyethylene may be at a temperature of from 0 to 110 °C, or from 10 to 100 °C, or from 20 to 90 °C, or from 30 to 90 °C, or from 40 to 90 °C. The step of grinding PCR polyethylene may comprise pulverising PCR polyethylene. In one embodiment, PCR polyethylene is pulverised using a blade gap setting of between 0.85 mm to 1.2 mm at a temperature of 40 to 90°C. A blade gap setting for PCR polyethylene may be much larger than conventional settings for the manufacture of rotational moulding powders. PCR polyethylene may be subjected to only one grinding step to produce the PCR polyethylene defined in the first aspect. PCR polyethylene may not pass through a sieve to produce the PCR polyethylene defined in the first aspect.

[0043] The method of the second aspect may include the step of producing the virgin polyethylene (as defined in the first aspect). The step of producing the virgin polyethylene (as defined in the first aspect) may comprise one or more of washing virgin polyethylene, extruding virgin polyethylene, pelletising virgin polyethylene and grinding/pulverising virgin polyethylene. Virgin polyethylene may be washed before and/or after pelletising virgin polyethylene. Virgin polyethylene may be extruded before pelletising virgin polyethylene. Virgin polyethylene may be pelletised after washing and extruding virgin polyethylene. Virgin polyethylene may be ground/pulverised after pelletising virgin polyethylene. The step of grinding/pulverising virgin polyethylene may produce a powder. Virgin polyethylene may be combined with one or more additives (as discussed for the first aspect) and then extruded.

[0044] The step of grinding virgin polyethylene may comprise a blade gap setting of greater than 0.1mm, 0.2mm, 0.3 mm, 0.4 mm, 0.5 mm, or 0.6 mm. The step of grinding/pulverising virgin polyethylene may comprise a blade gap setting of less than 1.2 mm, 1.1 mm, 1.0 mm, or 0.9 mm. The step of grinding/pulverising virgin polyethylene may comprise a blade gap setting of from 0.4 mm to 1.1 mm, or from 0.5 mm to 1.0 mm, or from 0.6 mm to 0.9 mm, or from 0.65 mm to 0.85 mm. The step of grinding/pulverising virgin polyethylene may be at a temperature of from 50 to 110 °C, or from 60 to 100 °C, or from 70 to 90 °C. The step of grinding/pulverising virgin polyethylene may comprise pulverising virgin polyethylene. The ground/pulverised virgin polyethylene may be passed through a sieve screen. The mesh size of the sieve screen may be from 200 to 900 pm, or from 300 to 800 pm, or from 400 to 700 pm. Virgin polyethylene that does not pass through the sieve may be subjected to a second grinding step (for example as defined above) to produce virgin polyethylene as defined in the first aspect.

[0045] The step of mixing the PCR polyethylene and the virgin polyethylene may comprise use of a ribbon blender, dry-blender or other such blending device. The mixture is then run for a period of time sufficient to mix the materials. The mixing time may be more than 10 minutes. The mixing time may be less than 2 hours. The mixing time may be from about 10 to 60 minutes.

[0046] Products made by the above process may have several advantages. Firstly, the virgin polyethylene and PCR polyethylene can be separately made in bulk thus allowing for efficient manufacturing of them. This also allows for specific orders to be met with only the need for ratio and additive adjustments before post mixing. Secondly, it is possible to achieve different particle sizes which allows rotomoulders to keep a single shot load while keeping product quality.

Finally, pigments and other additives to be blended in with either the PCR polyethylene or virgin polyethylene which achieves a vibrant colour and various chemical properties as required.

[0047] In one embodiment, an additive may be mixed directly with the rotomoulding mixture or added to the PCR polyethylene or virgin polyethylene prior to mixing.

[0048] Any conventional equipment may be used to mix the additive, such as an extruder or an internal batch mixer. The mixture may also be prepared without extrusion. Features of the second aspect of the present invention may be as described for the first aspect of the present invention.

[0049] In a third aspect, the present invention provides a process for preparing a rotomoulded product, wherein the process comprises: rotomoulding the rotomoulding mixture of the first aspect to prepare the rotomoulded product.

[0050] In an embodiment, the process for preparing a rotomoulded product is carried out in a single- shot. In one embodiment, the rotomoulded product is a barrier or wall, for example an acoustic barrier or wall. The acoustic barrier or wall may especially used along transport corridors, for example along railways and automobile highways.

[0051] When the rotomoulding mixture of the first aspect is used to form a product, separate layers of virgin and PCR polyethylene do not form. Instead, the virgin and PCR polyethylene are interspersed, but the relative proportion of virgin and PCR polyethylene closest to the outside of the product may be different to the relative proportion of virgin and PCR polyethylene closest to the inside of the product.

[0052] In a fourth aspect, the present invention provides a rotomoulded product formed by the process of the third aspect.

[0053] In one embodiment, a rotomoulded product formed from a mixture of 9: 1 virgin polyethylene: PCR polyethylene may have one or more of a Flexural modulus of 288.5 MPa, an impact strength of 37.3 J at ambient temperature, a tensile strength of 11.678 MPa and a density modulus of 0.9019 g/cm³.

[0054] In one embodiment, a rotomoulded product formed from a mixture of 1:9 virgin polyethylene: PCR polyethylene may have one or more of a Flexural modulus of 505.14 MPa, an impact strength of 57.6 J at ambient temperature, a tensile strength of 16.79 MPa and a density modulus of 0.9379 g/cm³.

[0055] Features of the third and fourth aspects of the present invention may be as described for the first and second aspects of the present invention.

[0056] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0057] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

BRIEF DESCRIPTION OF DRAWINGS

[0058] Preferred features, embodiments and variations of the invention may be discerned from the following Description of Embodiments which provides sufficient information for those skilled in the art to perform the invention. The Description of Embodiments is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Description of Embodiments will make reference to a number of drawings as follows:

[0059] Figure 1 illustrates an overlay of the particle size distribution of PCR polyethylene (LDPE, HDPE or a combination of both as per above) and virgin polyethylene powders prepared in accordance with the present invention (left / dotted line = virgin polyethylene, fine grind; right / solid line = PCR polyethylene, coarse grind);

[0060] Figure 2 illustrates the particle size distribution of a rotomoulding mixture comprising 10 wt% PCR polyethylene and 90 wt% virgin polyethylene;

[0061] Figure 3 illustrates the particle size distribution of a rotomoulding mixture comprising 90 wt% PCR polyethylene and 10 wt% virgin polyethylene;

[0062] Figure 4 illustrates the surface of a rotomoulded product prepared from a rotomoulding mixture comprising 80 wt% PCR polyethylene and 20 wt% virgin polyethylene; and

[0063] Figure 5 illustrates the surface of a rotomoulded product prepared from a rotomoulding mixture comprising 20 wt% PCR polyethylene and 80 wt% virgin polyethylene. In this example, a red pigment colouring is included.

DESCRIPTION OF EMBODIMENTS

Rotomoulding Mixtures

[0064] The inventors have been able to manufacture rotomoulded products using rotomoulding mixtures as follows (below, a “fine grind” produces particles with a maximum size of 500 pm, and an average particle size of approximately 300 pm. A “coarse grind” produces particles with a maximum size of 2000 pm and an average particle size of approximately 450 pm):

[0065] Mixture 1 (a REblend™ formula):

20 wt% Fine Grind 10MFI 0.935 density virgin polyethylene (LLDPE);

40 wt% Coarse Grind <1MFI 0.95 density PCR polyethylene (HDPE) (from a recycled blow moulding grade polyethylene such as a milk bottle (a similar HDPE packaging product may be used instead); and

40 wt% Coarse Grind <1MFI 0.930 density PCR polyethylene (LDPE) (from a recycled film grade polyethylene).

This mixture provided a product that has good surface finish with some surface porosity and good impact properties.

[0066] Mixture 2 (a REblend™ formula):

40 wt% Fine Grind 10MFI 0.935 density virgin polyethylene (LLDPE); and

60 wt% Coarse Grind <1MFI 0.930 density PCR polyethylene (LDPE) (from a recycled film grade polyethylene).

This mixture provided a product that has an excellent surface finish and impact strength equivalent to virgin material.

[0067] Mixture 3 (a REblend™ formula):

40 wt% Fine Grind 10MFI 0.935 density virgin polyethylene (LLDPE); and 60 wt% Coarse Grind <1MFI 0.95 density PCR polyethylene (HDPE) (from a recycled blow moulding grade polyethylene such as a milk bottle (a similar HDPE packaging product may be used instead).

This mixture provided a product that has good surface finish and good impact strength (inferior to virgin polyethylene at ambient temperature) but with stiffness higher than virgin polyethylene.

[0068] Mixture 4 (a REblend™ formula):

10 wt% Fine Grind 10MFI 0.935 density virgin polyethylene (LLDPE); and

90 wt% Coarse Grind <1MFI 0.930 density PCR polyethylene (LDPE).

[0069] Mixture 5 (a REblend™ formula):

90 wt% Fine Grind 10MFI 0.935 density virgin polyethylene (LLDPE); and

10 wt% Coarse Grind <1MFI 0.930 density PCR polyethylene (LDPE).

[0070] Mixture 6 (a REblend™ formula):

10 wt% Coarse Grind <1MFI 0.95 density PCR polyethylene (HDPE);

10 wt% Coarse Grind <1MFI 0.930 density PCR polyethylene (LDPE); and

80 wt% Fine Grind 10MFI 0.935 density virgin polyethylene (LLDPE).

Particle size distributions

[0071] Figure 1 shows an overlay of the particle size distribution of LDPE/HDPE PCR polyethylene (coarse grind) and virgin polyethylene (fine grind). The virgin polyethylene has a peak at around 250 pm while the PCR polyethylene has a peak at around 570 pm. This illustrates the larger average particle size difference between the virgin and PCR polyethylene.

[0072] Figure 2 demonstrates the particle size distribution of a mixture of two powders, which comprises 10 wt% PCR polyethylene and 90 wt% virgin polyethylene, by wt% of polyethylene (i.e. Mixture 5 above). In this rotomoulding mixture which is mostly comprised of virgin polyethylene, the distribution of particle size is skewed to the smaller size particles with a maximum peak at 250 pm. However, there is still evidence of a peak in the PCR polyethylene particles around the 500 pm mark. [0073] Figure 3 demonstrates the particle size distribution of a rotomoulding mixture comprising 90 wt% PCR polyethylene and 10 wt% virgin polyethylene, by wt% of polyethylene (i.e. Mixture 4 above). Figure 3 illustrates that the particle size distribution in this rotomoulding mixture is skewed towards the PCR polyethylene with a maximum peak around 580 pm. However, the finer virgin particles can still be noticed with a small peak at 200 pm.

[0074] It is believed that the particle size distributions as illustrated in Figures 1-3 are an approximation for how the rotomoulding mixture will form to the mould when used in a rotomoulding process. The smaller the particle the sooner it will lay (i.e., melt or mould) to the mould wall and the larger the particle (as represented to the right-hand side of the distribution), the later it will lay (i.e., melt or mould) to the mould wall. Thus, by the time the larger PCR polyethylene particles lay-up on the mould there is already a layer of smaller virgin polyethylene particles on the mould wall.

[0075] It is foreseeable that different particle size distributions can be achieved when using rotomoulding mixtures with higher ratios of larger particle size PCR polyethylene compared to smaller particle size virgin polyethylene, and vice versa.

Physical Properties of Rotomoulding Mixtures

[0076] The dry flow, bulk density, and average particle size of rotomoulding mixtures of the present invention were tested in accordance with the Australian & New Zealand Standard ASNZ4766 2020. It would be appreciated that other local and international standards may also be used to measure the properties of the mixtures. The results are illustrated in Table 1.

Table 1: Physical properties of rotomoulding mixtures

[0077] As shown in Table 1, the dry flow was higher for PCR polyethylene than the virgin polyethylene indicating that the PCR flows much faster than the virgin materials. However, when combinations of 90 wt% PCR polyethylene and 10 wt% virgin polyethylene, and 10 wt% PCR polyethylene and 90 wt% virgin polyethylene were prepared they resulted in similar dry flow results despite the proportion of PCR polyethylene varying between the combinations. This indicates if there is any addition of virgin powder the dry flow will be affected. However, all of the results for dry flow testing of the combined/mixed powders fall within normal, acceptable limits required for the rotomoulding process.

[0078] The bulk density was then tested and observed for each of the material variants. The Low-density PE PCR by itself had the highest bulk density, and the high-density PE PCR by itself had the lowest bulk density. The virgin material bulk density placed between them. The two mixtures of PCR and Virgin material produced very similar bulk densities as the high and low bulk densities of the PCR material counteract each other in the mix.

[0079] The average particle size results were to be expected with the virgin powder fine grind having the lowest average particle size and the PCR coarse grind having the highest average particle size. The 10% PCR / 90% virgin mix had a slightly higher average particle size when compared to the straight virgin powder, and the 90% PCR / 10% virgin mix had a slightly lower average particle size then the straight PCR.

Producing the Rotomoulding mixture

[0080] In an exemplary process, the method for producing the rotomouldable polyethylene materials begins with the washing, extrusion and pelletisation of post-Consumer / post-industrial HDPE and post-consumer / post-industrial LDPE. This process is undertaken in what is considered a somewhat standard industry practice. After this process has been completed, the pellets need to be ground into a powder for moulding. This is achieved through a set of blades on a pulverising or grinding mill. The gap between the blades on the aforementioned mill might typically be known as the ‘bladed gap setting’. A typical blade gap setting for the pulverisation of these recycled pellets is at between 0.85mm to 1.2mm at a grinding/pulverising temperature of 40 to 90 °C. This gap setting is much larger than conventional settings for the manufacture of rotational moulding powders. Once the PCR recyclate has been ground it does not pass through a sieve and is left at that particle size which it is after the first and only pass through the grinder. The finished particle size is much larger than the conventional size, ranging from approximately 1 to 1200 pm with an average of around 450-600 pm. This product would normally be considered as too coarse for effective roto-moulding at this particle size and with the MFI characteristics associated with single use packaging PCR and post-industrial packaging waste. This recycled Polyethylene has now finished being processed and is ready for mixing. [0081] The virgin HDPE and/or LLDPE is extruded with any additives required for enhanced performance (such as UV stabilisers or anti-oxidants). It may also be processed without extrusion as there may be no need for any additives or additives may be added in during subsequent mixing/blending process steps. The resin is ground using blade gap settings of between 0.65mm to 0.85mm and a grinding/pulverising temperature of 40 to 90 °C. Once the virgin resin is ground it is passed through sieve screens with a mesh spec of between 400- 700pm, meaning the largest possible particle size for the virgin powder is < 700pm. If the powder has not been ground fine enough to pass through the sieve, it is put through a secondary grinder set with a blade gap of 0.65mm to 0.85mm then put back through the sieve screens again. This process is repeated until all the powder has reached the required particle size, achieving an average particle size of approx. 300-400 pm. It should be noted that the pulverising of resin or polymer pellets is not an exact science. It involves the ‘smashing’ of polymer pellets between two discs covered in cutting blades or edges creating a dust or powder. These numbers shown above represent a typical or working range of particle sizes and averages that arise from this process. Some variation in these numbers, be it their minimum, maximum, averages or distribution may still constitute a working range.

[0082] Once the virgin and recycled polymers have both been ground to the required particle sizes as described above, they are ready to be mixed in a blender. The two polymers may be mixed in what is commonly known as a ‘ribbon blender’ or ‘dry -blender’. A mix ratio ranging from between 10wt% of virgin to 90wt% of virgin is possible, depending on the properties desired for the finished product. Additives may be combined with this mix at this dry-blending time such as colour pigment and UV and heat stabilizers. The powders are then run for a period of whatever time is deemed necessary to obtain sufficient mixing of the many ingredients (such as between 10 and 60mins depending on the type of ribbon blender in use). The rotomoulding mixture is now finished and can be used in the rotational moulding processes.

Rotomoulded Products

[0083] The rotomoulding mixture of the present invention was shown to be suitable for preparing rotomoulded products.

[0084] Figure 4 illustrates the surface of a rotomoulded product prepared using a rotomoulding mixture comprising 80 wt% coarse ground PCR polyethylene and 20 wt% fine ground, high MFI virgin polyethylene, by wt.% of polyethylene (Mixture 1 above). Although not a perfectly smooth external product finish, this surface finish is to a high standard, and useable in certain sectors of the industry. Some surface ’’pinholes” are present but this is to be expected when using a high percentage of inconsistent PCR material.

[0085] Figure 5 illustrates the surface of a rotomoulded product prepared using a rotomoulding mixture comprising 60 wt.% coarse ground PCR polyethylene and 40 wt.% fine grind, high MFI virgin polyethylene (Mixture 2 above), with added red pigment colouring. This surface finish is to a very high industry standard and would be suitable for many industry applications.

[0086] Further rotomoulded products were formed from Mixtures 4 and 5 described above, and the physical properties of those products are provided in Table 2. For Mixtures 4 & 1, these mixtures underwent a heating cycle of seventeen minutes reaching a PIAT (Peak Internal Air Temperature) of 213°C, creating a test product with an average wall thickness of 5.5 mm. For Mixtures 5 & 6, these mixtures underwent a heating cycle of fifteen minutes reaching a PIAT of 211°C, creating a test product with an average wall thickness of 4.8 mm. When the products from said mixtures were tested, they achieved the results detailed in Table 2.

Table 2 - Physical Properties of Rotomoulded Products from Mixtures 1, 4, 5, & 6

[0087] It would be appreciated that any mixture of materials in other ratios can also lead to a combination of these properties. For example, increasing stiffness as HDPE PCR is increased and/or increasing impact properties as LLDPE PCR ingredient is increased.

[0088] In the present specification and claims, the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0089] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

Claims

1. A rotomoulding mixture comprising a post-consumer recyclate (PCR) polyethylene and a virgin polyethylene, wherein:

(a) the PCR polyethylene has:

(i) a maximum particle size of 2000 pm, and

(ii) a melt flow index (MFI) of from 0.1 to 20; and

(b) the virgin polyethylene has:

(i) a maximum particle size of 800 pm, and

(ii) a melt flow index (MFI) of from greater than 7 to 20.

2. The rotomoulding mixture of claim 1, wherein the PCR polyethylene has a higher average particle size than the virgin polyethylene.

3. The rotomoulding mixture of claim 1 or 2, wherein the PCR polyethylene has a lower MFI than the virgin polyethylene.

4. The rotomoulding mixture of any one of claims 1-3, wherein the PCR polyethylene has a MFI of from 0.1 to 2.

5. The rotomoulding mixture of any one of claims 1-4, wherein the virgin polyethylene has an MFI of from 7.5 to 20.

6. The rotomoulding mixture of any one of claims 1-5, wherein the PCR polyethylene has an average particle size of between 400 pm to 900 pm.

7. The rotomoulding mixture of any one of claims 1-6, wherein the virgin polyethylene has an average particle size of between 100 pm to 400 pm.

8. The rotomoulding mixture of any one of claims 1-7, wherein the PCR polyethylene comprises HDPE, LDPE, or LLDPE, or a combination thereof.

9. The rotomoulding mixture of any one of claims 1-8, wherein the virgin polyethylene comprises HDPE, or LLDPE, or a combination thereof.

10. The rotomoulding mixture of any one of claims 1-9, wherein the PCR polyethylene comprises greater than 10 wt% of the rotomoulding mixture, by wt% of polyethylene.

11. The rotomoulding mixture of any one of claims 1-9, wherein the ratio of PCR polyethylene to virgin polyethylene is in the range of 1:9 to 9:1

12. The rotomoulding mixture of any one of claims 1-9, wherein the ratio of PCR polyethylene to virgin polyethylene is in the range of 3:7 to 9:1.

13. The rotomoulding mixture of any one of claims 1-12, wherein the rotomoulding mixture further comprises an additive selected from the group consisting of a carrier, a UV stabiliser or UV resistant compound, a slip agent, an antioxidant, a pigment, a plasticiser, an optical brightener, a flame retardant, a filler, a heat stabiliser, a release agent and combinations thereof.

14. The rotomoulding mixture of any one of claims 1-13, wherein the rotomoulding mixture is a powder.

15. A method for preparing the rotomoulding mixture of any one of claims 1-14, wherein the method comprises: mixing PCR polyethylene with virgin polyethylene to form the rotomoulding mixture of any one of claims 1-14.

16. The method of claim 15, wherein the method further comprises the step of producing PCR polyethylene as defined in any one of claims 1-14, which comprises one or more of washing PCR polyethylene, extruding PCR polyethylene, pelletising PCR polyethylene and grinding/pulverising PCR polyethylene.

17. The method of claim 15 or 16, further comprising the step of producing virgin polyethylene as defined in any one of claims 1-14, wherein the step of producing the virgin polyethylene comprises one or more of washing virgin polyethylene, extruding virgin polyethylene, pelletising virgin polyethylene and grinding/pulverising virgin polyethylene.

18. A process for preparing a rotomoulded product, wherein the process comprises: rotomoulding the rotomoulding mixture of any one of claims 1-14 to prepare the rotomoulded product.

19. The process according to claim 18, wherein the rotomoulding is carried out in a singleshot.

20. A rotomoulded product formed by the process of claim 18 or 19.