GB2476029A

GB2476029A - Polyethylene terephthalate copolymer container manufactured using nucleating agent

Info

Publication number: GB2476029A
Application number: GB0919487A
Authority: GB
Inventors: Marco Brons; Eddie Taylor
Original assignee: PREGIS RIGID PACKAGING Ltd
Current assignee: PREGIS RIGID PACKAGING LIMITED
Priority date: 2009-11-06
Filing date: 2009-11-06
Publication date: 2011-06-15
Also published as: GB0919487D0

Abstract

A container is manufactured from a material which comprises a polyethylene terephthalate (PET) copolymer and a nucleating agent, wherein the copolymer comprises at least 90 mol% polyethylene terephthalate monomer units. The container may be a foodstuff container, such as a dual ovenable tray which is stable at temperatures found in conventional ovens. Moreover, the container may be made from at least 20 weight%, and more preferably at least 80 weight%, recycled PET based on the total PET in the container. The nucleating agent can be an organic nucleating agent, preferably a carboxylic acid. The nucleating agent may be a salt, preferably a salt of an acid. In addition to PET, the copolymer contains at least one other monomer unit, wherein the other monomer unit may be derived from isophthalic acid, cyclohexane dimethanol, neopentyl glycol and diethylene glycol. In another aspect a method of manufacturing the container is disclosed.

Description

PET Composition The invention relates to a PET material composition suitable for use as a food container.

The use of plastic containers for microwaveable food is well known. For some applications crystalline polyethylene terephthalate (PET) may be used since it can withstand high temperatures in conventional ovens as well as be used in microwave ovens, that is it is a dual ovenable tray'.

The PET is made from a homopolymer which crystallises and so provides the stability in the temperatures encountered in conventional ovens. The speed of crystallisation is also important -it needs to be as fast as possible, otherwise the overall process of making the container would be uneconomically slow.

Another common use for PET is for plastic bottles for drinks. Such bottles for soft drinks are well known and their collection for recycling is becoming increasingly popular. However there are limited markets for the recycled materials. Also, in contrast to trays, a co-polymer is normally used made predominantly from PET derived from terephthalic acid, but also including a second polymer, normally a polymer derived from isophthalic acid. The co-polymer slows and largely inhibits crystallisation thus leaving the bottles transparent, which is a particular design preference of soft drink producers. Temperature resistance afforded by crystallised PET is not a requirement for such applications.

According to the present invention there is provided a container manufactured from a material comprising a co-polymer and a nucleating agent, the co-polymer comprising at least 9Omol% polyethylene terephthalate monomer units.

Thus the inventors of the present invention have gone against the normal practice in the area of containers by utilising a co-polymer which will, as detailed below, provide a crystallised material which is stable at higher temperatures. The skilled person will be aware that the crystallisation rate of such a co-polymer is much slower than the conventional homopolymers used, and that the speed of the manufacturing process for these containers is a critical factor in the economic viability of the overall process. Therefore the skilled person would be discouraged from such a choice for a container requiring temperature stability of up to about 220°C.

For drinks bottles, crystallisation and the associated temperature stability is not required. Indeed crystallisation discolours the bottles, making them opaque, which is not aesthetically acceptable to the soft drink producers.

Thus whilst a co-polymer is chosen for such an application, a nucleating agent would not be used since this would cause crystallisation, discolour the bottles and defeat the purpose of utilising the co-polymer for the bottle in the first place (which is to prevent crystallisation).

Thus the combination of these two elements, a co-polymer and nucleating agent does not at all follow from the conventional knowledge in the art.

The inventions of the present invention have found that the addition of the nucleating agent increases the speed of the rate of crystallisation in order to offset the slower nucleation which occurs with co-polymers.

Preferably the container is a foodstuff container.

Preferably the container is manufactured from at least 2Owt% recycled PET, preferably at least SOwt%, recycled PET, more preferably at least 8Owt% based on the total PET in the container.

"Recycled PET" as used herein normally refers to materials recycled after being used or sold to consumers so called "post-consumer waste". For certain alternative embodiments, recycled PET may be material recovered from in-house manufacturing waste.

The nucleating agent is an agent suitable to initiate crystallisation and/or increase the rate of crystallisation of the co-polymer.

Preferably the nucleating agent comprises an organic nucleating agent.

Preferably the nucleating agent is a salt, more preferably a salt of an acid.

Preferably the acid is an organic acid such as a carboxylic acid.

Preferably the acid has long carbon chains, such as at least SOmol% preferably at least 8Omol% of the organic constituents are chains in the range of C20 to 040, more preferably in the range of 028 to 032.

Thus preferably the nucleating agent is a modified wax.

Preferably the salt comprises a metal cation such as a Group I or Group II metal ion. Preferably a Group I metal cation is used, such as sodium.

A preferred nucleating agent may be obtained from Clariant (www.clariant.com) under the Trade Mark LicomontTM NaV 101. This additive has a high molecular weight, low volatility and favourable migration characteristics. An alternative is sodium stearate.

Preferably the material also comprises additives.

Optionally the material comprises an impact modifier. One suitable impact modifier is sold under the name Elavoy 1224 AC by Du Pont (www.dupont.com).

For certain embodiments, there is at least lwt% of an impact modifier, preferably at least 3.5wt%, more preferably about 5wt%.

Preferably the trays have an intrinsic viscosity of around 0.85 -0.91 dllg.

For embodiments where no impact modifier is present the intrinsic viscosity tends to be less, sometimes even less than 0.85, but normally more than 0.7.

Preferably the co-polymer is crystallised by at least 10%, preferably at least 15% more preferably more than 20%. Preferably the material is crystallised by less than 40%, more preferably less than 30%. In particularly preferred embodiments the material crystallises to between 22 -30% during thermoforming. The degree of crystallisation is measured by conventional techniques known by the person skilled in the art. For example, a density column may be used comprising a fluid provided in a tall container. The material is added to the container and floats at a particular height in the density column, depending on the material's density. Thus the density of the material may be determined by the height and since this is proportional to the crystallinity, the degree of crystallisation may be deduced from the results of this experiment to determine density.

Preferably the material is opaque.

The co-polymer contains polyethylene terephthalate (PET) monomer units and at least one other monomer unit referred to as the second monomer unit. The second monomer unit may be a polymer derived from isophthalic acid, cyclohexane dimethanol, neo pentyl glycol or di-ethylene glycol. Isophthalic acid is preferred.

Normally at least 95mol% of the co-polymer is PET monomer units, preferably around 98mol%, based on the total of the co-polymer.

In addition to the co-polymer, the material may comprise a secondary polymer which may be present in order to make up the master batch during processing. The secondary polymer may be one or more of: polyethylene, polypropylene and polybutylene. Other polymers may also be used.

The material may comprise at least 8Owt% co-polymer, preferably at least 9Owt%, more preferably at least 95wt%.

Typically the material comprises at least lwt% of the secondary polymer, preferably at least 2.5wt%, preferably at least 5wt%.

Preferably the container is a foodstuff container and so is shaped to contain food. Preferably therefore the container is a tray, that is the container has an open face for access to contents of the container such as food. Normally the open face is at least 5cm wide, preferably at least 10cm wide. Normally the container has a base and walls extending upwards from the base. Preferably the open face is opposite the base.

Normally the open face has an area of at least 50%, typically at least 75% of the area of the base. Normally the open face is sealed with a lid.

Preferably the container is suitable for use in a microwave oven.

Preferably the container is suitable for use in a conventional oven. In particular preferably the container is suitable for use in both a microwave and a conventional oven, that is it is a "dual ovenable tray".

An advantage of certain embodiments of the invention is that the recycled co-polymer is normally a less expensive material.

Another advantage of certain embodiments is that the material can comprise recycled PET, and so be of benefit to the environment.

According to a second aspect of the invention, there is provided a method of manufacturing a PET container, the method comprising: combining a co-polymer and a nucleating agent, the co-polymer having at least 9Omol% polyethylene terephthalate monomer units; forming a container from the co-polymer and the nucleating agent.

Preferably the container according to the second aspect of the invention is the container according to the first aspect of the invention.

Preferably the nucleating agent is mixed with the secondary polymer before being combined with the co-polymer.

Typically the precursors to each polymer in the co-polymer are mixed before polymerisation. For example terephthalic acid and isophthalic acid are normally mixed as raw materials and then polymerized.

Typically the material is first formed into a flat sheet.

Preferably the container is formed by thermoforming. Typically hot nioulds are used in thermoforming, that is moulds with a temperature above 150 °C, preferably above 160 00, preferably below 18000 and ideally about 00 The process normally comprises moving the flat sheet into said hot mould and then into a relatively cold mould. The cold mould is typically at ambient temperatures but may be up to 40 00 or more. Typically the heat in the hot mould causes the crystallisation to initiate, whilst the cold mould forms the shape of the container.

Preferably the container is formed at a high speed, that is having a cycle time of less than 7s, preferably less than 6s, ideally less than 5s. A cycle time is herein defined as the time to make the tray, for the tray to move into the hot mould, then the cold mould and then leave the cold mould.

Preferably around 0.1 -0.7wt%, more preferably 0.3wt% -0.Swt%, of the nucleating agent is mixed with the co-polymer based on total polymer composition.

Preferably the resulting container is a food grade container, that is, a material permitted by the relevant authorities to be allowed direct contact with food. In particular preferably the resulting container abides by FDA and EFSA standards.

Embodiments of the present invention will now be described by way of

example only.

The recycled PET flake is typically bought from the open market.

Alternatively the invention may include recycling the PET material from bottles or other waste materials. To recover the material from recycled bottles, the bottles are typically first separated from other plastics, especially bottles tops etc. They are then granulated and washed and centrifuged in hot liquid, such as water. PET flakes result which, at this point in the process, are not of a food grade standard. Various treatments may be used to make the PET flakes food grade standard -one suitable treatment is to heat them to around 195 00 for about 30 minutes under high vacuum conditions.

8% of a nucleating agent, sold under the brand LicomantTM NaV 101, was compounded with polyethylene and a dye called Carbon Black.

5% of that mixture was combined with 95% of co-polymer PET flake and pellet and then fed into an extruder to produce a sheet or tray of the combined composition. The tray was then transferred to a hot mould where crystallisation was initiated. The mould was at a temperature of about 165 °C. The tray was then transferred to a cold mould and shaped accordingly into a tray. The cycle time, from being fed into the extruder to being removed from the cold mould was under 5 seconds.

The tray was then tested in an oven up to 210 00 for 25 minutes and thereafter visually assessed for distortions but was found to be resilient to such conditions.

In comparison a cycle time without a nucleant will be approximately 10 -seconds.

Improvements and modifications may be made without departing from the scope of the invention.

Claims

Claims 1. A container manufactured from a material comprising a co-polymer and a nucleating agent, the co-polymer comprising at least 9Omol% polyethylene terephthalate monomer units.
2. A container as claimed in claim 1, which is a foodstuff container and so is in the form of a tray having an open face at least 5cm wide, preferably at least 10cm wide.
3. A container as claimed in either preceding claim, which has a base and walls extending upwards from the base and an open face opposite the base, wherein the open face has an area of at least 75% of the area of the base.
4. A container as claimed in any preceding claim, which is manufactured from at least 2Owt% recycled PET based on the total PET in the container.
5. A container as claimed in any preceding clairri, which is manufactured from at least 5Owt% recycled PET based on the total PET in the container.
6. A container as claimed in any preceding claim, which is manufactured from at least 8Owt% recycled PET based on the total PET in the container.
7. A container as claimed in any preceding claim, wherein the nucleating agent comprises an organic nucleating agent.
8. A container as claimed in any preceding claim, wherein the nucleating agent is a salt, especially a salt of an acid.
9. A container as claimed in claim 8, wherein the salt comprises a metal cation such as a Group I or Group II metal ion, especially a Group I metal cation such as sodium.
10. A container as claimed in any preceding claim, wherein the nucleating agent is an organic acid such as a carboxylic acid.
11. A container as claimed in any preceding claim, wherein the acid has long carbon chains, such as at least 5Omol% of the organic constituents are chains in the range of 020 to C40, preferably in the range of 028 to 032.
12. A container as claimed in any preceding claim, wherein the nucleating agent is a modified wax.
13. A container as claimed in any preceding claim, wherein the material comprises 0.1 -0.7wt% of a nucleating agent, preferably 0.3wt% -0. Swt%.
14. A container as claimed in any preceding claim, wherein the material has an intrinsic viscosity of from 0.68-0.95 preferably 0.85 to 0.91 dl/g.
15. A container the as claimed in any preceding claim, wherein co-polymer is crystallised by at least 10%.
16. A container as claimed in claim 15, wherein the co-polymer is crystallised by at least 20%.
17. A container as claimed in any preceding claim, wherein the co-polymer is crystallised by at most 40%.
18. A container as claimed in claim 17, wherein the co-polymer is crystallised by at most 30%.
19. A container as claimed in any preceding claim, which is opaque.
20. A container as claimed in any preceding claim, wherein the co-polymer contains the polyethylene terephthalate (PET) and at least one other monomer unit derived from a precursor, the precursor selected from the list consisting of isophthalic acid, cyclohexane dimethanol, neo pentyl glycol and di-ethylene glycol.
21. A container as claimed in claim 20, wherein at least 75mo1% of the co-polymer is PET monomer units based on the total of the co-polymer.
22. A container as claimed in claim 21, wherein at least 95mol% of the co-polymer is PET monomer units based on the total of the co-polymer.
23. A container as claimed in any preceding claim, which is suitable for use in both a microwave and a conventional oven, that is it is a dual ovenable container.
24. A method of manufacturing a PET container, the method comprising: combining a co-polymer and a nucleating agent, the co-polymer comprising at least 9Omol% polyethylene terephthalate monomer units; forming a container from the co-polymer and the nucleating agent.
25. A method as claimed in claim 24, wherein the nucleating agent is mixed with a secondary polymer, such as polyethylene, before being combined with the co-polymer.
26. A method as claimed in claim 24 or claim 25, comprising forming a flat sheet then moving the flat sheet into a hot mould and then into a relatively cold mould.
27. A method as claimed in any one of claims 24 to 26, wherein the hot mould has a temperature from 150 °C to 170 CC
28. A method as claimed in any one of claims 24 to 27, wherein the cold mould has a temperatures of less than 40 C.
29. A method as claimed in any one of claims 24 to 28, wherein the time to make the tray, for the tray to move into the hot mould, then the cold mould and then leave the cold mould is less than 7 seconds.