Reusable pet bottle

Abstract

A reusable polyethylene terephthalate (PET) bottle 10 having a capacity of 500 ml and capable of withstanding at least eight pasteurisation cycles at 60°C comprises: a body portion 14 and a base porti

Description

TECHNICAL FIELD The invention relates to a reusable polyethylene terephthalate bottle. TECHNICAL BACKGROUND Polyethlyene terephthalate (PET) is widely used worldwide as a bottle material for various liquids including mineral water, soft drinks, wine and beer. It is possible to recycle PET bottles by washing, shredding, melting and subsequently extruding used bottles, or other objects made from PET, into new plastic pellets. However, while this process helps to reduce the demand on virgin plastic, it remains relatively energy intensive. The energy demands on recycling PET bottles makes the development of a viable reusable alternative desirable, since a reusable bottle simply needs to be cleaned and, for some liquids such as beer, pasteurised. Pasteurisation processes involve heating a filled bottle, typically to a temperature of approximately 60°C, to remove bacteria. However, current designs for reusable PET bottles are limited in terms of the number of pasteurisation cycles that the bottle can withstand, which limits the utility of the bottle and acts as a barrier to greater commercial implementation into products where pasteurisation in the packaging is required. It is therefore desirable to provide reusable PET bottles that can withstand a greater number of pasteurisation cycles. SUMMARY OF THE INVENTION According to a first aspect, the invention provides a reusable polyethylene terephthalate (PET) bottle having a capacity of 500 mL and capable of withstanding at least eight pasteurisation cycles at 60°C. The bottle comprises a body portion and a base portion. The base portion defines a champagne base and extends 24 mm in a direction from a bottom of the bottle towards a top of the bottle. The base portion weighs at least 15 g and a thickness of the bottle at the bottom of the base portion is at least 2.3 mm. The bottle may weigh between 50 g and 60 g. The champagne base may comprise a cavity extending upwardly into the bottle having a height of between 8mm and 14 mm, preferably between 10 mm and 12 mm. The champagne base may comprise a circular standing surface having a diameter of between 42 mm and 50 mm, preferably between 45 mm and 47 mm. The bottle may be capable of withstanding at least ten pasteurisation cycles at 60°C. The bottle may be capable of withstanding a burst pressure of at least 20 bar. In a second aspect, the invention provides a preform for making the reusable PET bottle discussed above. The preform is dimensioned such that an axial stretch ratio between the preform and the bottle is at least 2.0 and a radial stretch ratio between the preform and the bottle is less than 2.6. In a third aspect, the invention provides a method of making the reusable PET bottle discussed above. The method comprises: blow moulding the preform discussed above into a mould; and heating the mould to a temperature of between 80°C and 135°C. The method may comprise heating the mould to a temperature of between 100°C and 115°C. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the following drawings, in which: Figure 1 shows the temperature and pressure inside a PET bottle during a pasteurisation process; Figure 2 shows a reusable polyethylene terephthalate (PET) bottle; Figures 3a to 3d show different designs for a base portion of the reusable PET bottle of Figure 2; and Figure 4 shows a preform for the reusable PET bottle of Figure 2. DETAILED DESCRIPTION In general, the invention aims to provide a polyethylene terephthalate (PET) bottles that can be reused a number of times. Current PET bottles in the state of the art can only be reused a limited number of times. The invention is particularly concerned with reusable PET bottles for beer and other liquids that require pasteurisation. The pasteurisation process for PET beer bottles involves heating a sealed, full bottle to 60°C and holding that temperature for a given time period. Figure 1 shows the temperature inside the bottle over the hour-long pasteurisation cycle. Since the temperature is measured inside the bottle, the outside temperature or the temperature of water that comes into contact with the bottle may be up to 65°C. The increase in temperature inside the bottle during the pasteurisation process causes a corresponding increase in pressure. At a carbonation level of 5 g / L of CO2, the pressure at room temperature is approximately 2 bar (200 kPa). This increases to approximately 7.9 bar (790 kPa) at 60°C. The increase in temperature and pressure during the pasteurisation process causes several issues with the viability of a bottle. Firstly, the increase in temperature can lead to the relaxation of any residual stresses created when moulding the bottle, as explained in more detailed below. This can lead to the bottle deforming when subsequently heated as any straight polymer chains in residual tension coil up, causing shrinkage. The increase in pressure can also simply cause deformations in the bottle by overcoming the yield strength of the bottle at different points. For example, the base of the bottle may bulge outwards to relieve the pressure, rendering the bottle unusable. One other failure mode that can be seen for reusable PET bottles is stress cracking. This typically occurs in the base and shoulder of the bottle during washing due to the increased amounts of amorphous material there compared to the body and the neck of the bottle. Stress cracking also renders the bottle eventually unusable. The inventors have determined that these issues can be mitigated against in order to increase the number of pasteurisation cycles a PET bottle can withstand by controlling the distribution of material throughout the bottle, and in particular by increasing the amount of material in the base of the bottle. To this end, Figure 2 shows a reusable bottle 10 according to an embodiment of the invention. The bottle 10 has a base portion 12 and a body portion 14. The body portion 14 is typically shaped for PET bottles and includes a main body 14a, which narrows through a neck section, or simply neck, 14b to a finish 16. The main body 14a is shown in Figure 2 to have a substantially constant diameter, though this may not necessarily be the case. The shape of the finish 16 is determined by the type of closure the bottle 10 will utilise: this may be a crown cap closure or a screw cap closure, for example. The base portion 12 of the bottle extends 24 mm above the bottom of the bottle 10. This is determined partly by the mould used to form the bottle 10, as explained in greater detail below, but the exact extent of the base portion 12 is largely irrelevant, except when considering the distribution of material in the bottle 10. The base portion 12 incorporates a champagne base at the bottom of the bottle 10. The champagne base comprises an inwardly extending generally part-spherical cavity, or punt, 18. The punt 18 provides the bottle 10 with increased strength in order to cope with the pressure created by carbonated beverages, and during any pasteurisation process. The punt 18 is separated from the rest of the base portion 12 by a generally circular standing surface 20 on which the bottle 10 rests when upright. As mentioned above, an important feature of the bottle 10 is the increased amount of material in the base portion 12. The base portion 12 weighs at least 15 g and the thickness of the bottle at the bottom of the base portion 12 is at least 2.3 mm, preferably at least 2.5 mm. The base portion 12 is therefore both heavier and thicker than current designs for reusable PET bottles. This has been found by the inventors to improve the performance in terms of the number of pasteurisation cycles that the bottle 10 can withstand. As a result, the bottle 10 may be able to withstand eight pasteurisation cycles at 60°C, or even ten or more such cycles. The heavier base portion 12 also increases the stability of the bottle 10 when empty by lowering the centre of mass of the bottle. This may be useful for handling the bottles in a factory setting. The depth, or height, of the punt 18 may be between 8 mm and 14 mm, but is preferably between 10 mm and 12 mm. This range provides better resistance against stress cracking and against the base portion 12 being pushed out during pasteurisation. The standing surface 20 may have a diameter of between 45 mm and 47 mm. The bottle 10 itself may weigh between 50 g and 60 g. In one example, the bottle 10 weighs 54 g. The inventors have successfully tested several designs for the base portion 12, some of which are shown in Figures 3a to 3d. The base portion 12 in Figure 3a has a punt 18 having a height of 12 mm, while the standing surface 20 has a diameter of 47 mm. The base portion 12 shown in Figure 3b is largely the same as that shown in Figure 3a, except the diameter of the standing surface 20 is slightly smaller, at 45 mm. The punt 18 of the base portion of Figure 3c has a height of 10 mm, and also includes a step in the cavity where the radius of curvature increases. The standing surface 20 of the base portion of Figure 3c is also slightly smaller than that in Figure 3a, at 45.5 mm. Finally, the base portion shown in Figure 3d has a standing surface diameter of 46 mm, but also includes a sharper point at the standing surface 20 due to a change in the shape of the base portion 12 radially outside of the standing surface 20. The dimensions and weights given above refer specifically to a bottle configured to hold 500 mL of fluid but it can be assumed that, for bottles of different volumes, the various dimensions and weights can be scaled appropriately. The bottle 10, like typical PET bottles configured for a capacity of 500 mL, has a height of between 210 mm and 250 mm and a diameter of between 60 mm and 80 mm. The bottle 10 is manufactured by blow moulding, as is typical for PET bottles. The process of blow moulding PET bottles is well known and so will not be described in detail here, except to say that a preform is mounted on a pin and is inflated with a pressurised fluid, typically air. The inflating fluid forces the preform to expand to fill the space defined by a mould surrounding the preform and the pin. The mould can be comprised of multiple parts Manufacture of reusable PET bottles, such as the bottle 10 of the invention, requires certain steps to enable the bottle 10 to be successfully pasteurised. This is because additional care needs to be taken to relax any residual stresses formed by the blow moulding process in the bottle 10. As mentioned above, the presence of residual stresses can lead to deformation of a plastic product when heated (for example during washing or pasteurisation) as the polymer chains coil up. To avoid this issue, the residual stresses can be, in effect, annealed out by the application of heat to the bottle 10 while being forced against the mould. This allows the polymer chains to revert to a relaxed configuration while the bottle is still being moulded. Typically this is done by heating the mould itself, which will be in intimate contact with the bottle 10. The inventors have determined that heating the mould to a temperature of between 80°C and 135°C displays suitable results and allows the bottle 10 to undergo multiple pasteurisation cycles. Preferably, the mould is heated to a temperature of 100°C to 115°C. While the bottle 10 is typically only held in the mould for a very short period of time (around 0.5 seconds), this is sufficient to allow the relaxation of residual stresses and the reversion of the polymer chains to a relaxed configuration. Once out of the mould, the blown bottle 10 is allowed to cool to room temperature Naturally, given the importance of the distribution of material in the bottle 10, the shape and configuration of the preform used to make the bottle is also important. Figure 4 shows a preform 22 to be used in creating the bottle 10. The preform 22 extends from a first end 24, corresponding to the finish 16 of the bottle 10, to a second end 26, corresponding to the base portion 12 of the bottle 10. The preform 22 takes the general shape of a tube with a blind bore 28 extending along the length of the preform 22 from the first 24, at which the bore 28 is open, to the second end 26, at which the bore 28 is closed. The shape of the finish 16 of the bottle 10 is generally pre-moulded into the preform 22, since this region of the preform 22 is typically clamped during the blow moulding process, leaving it unable to expand. The preform 22 is specifically designed to enable the desired distribution of material in the finished bottle 10 described above and also to tailor properties of the polymer chains within certain regions of the bottle 10. Of course, one way in which this is achieved is by generally increasing the thickness of the preform 22 from the first end 24 to the second end 26. Since the outer diameter of the preform 22 remains substantially consistent along its length, this manifests itself in a gradual decrease in the width of the bore 28. As can be seen in Figure 4, the preform contains regions 30 where the width of the bore 28 decreases more quickly and regions 32 in which the width of the bore decreases very slowly, or remains substantially constant. The dimensions of the preform 22 are also configured to control the stretch ratios between the preform 22 and the bottle 10. The inventors have determined that this is important in controlling the orientation and stretch of the polymer chains. Specifically the stretch ratio in an axial direction (i.e., along the longitudinal axis of the preform 22 / bottle 10 from the first end 24 to the second end 26, or from the finish 16 to the base portion 12) is greater than 2.0, while the stretch ratio in a radial direction (perpendicular to the longitudinal axis of the preform 22 / bottle 10) is less than 2.6. In one embodiment, the stretch ratio in the axial direction is 2.1, while in the radial direction it is 2.4. St is thought that increasing the stretch ratio in the axial direction leads to more stretched polymer chains in the finished bottle 10. This helps to guard against stress cracking, which is linked to the presence of amorphous material. However, it has also been found by the inventors to prevent pushing out of the punt 18 in the base portion 12 of the bottle 5 10 during repeated pasteurisation cycles. The above description relates specifically to bottles having a capacity of 500 mL, but the principles espoused are equally applicable to both smaller and larger bottle sizes, once the necessary adjustments are made for dimensions, mass, etc, as would be appreciated 10 by the skilled person. The skilled person will also appreciate the difference between the intended capacity of a bottle, in use, and the total volume of that bottle. 15

Claims

1. A reusable polyethylene terephthalate (PET) bottle (10) having a capacity of 500 mL and capable of withstanding at least eight pasteurisation cycles at 60°C, the bottle comprising:a body portion (14) and a base portion (12) defining a champagne base, the base portion (12) extending 24 mm in a direction from a bottom of the bottle (10) towards a top of the bottle (10), wherein:the base portion (12) weighs at least 15 g and a thickness of the bottle (10) at the bottom of the base portion (12) is at least 2.3 mm.

2. The reusable PET bottle (10) of Claim 1, wherein the bottle (10) weighs between 50 g and 60 g.

3. The reusable PET bottle (10) of Claim 1 or Claim 2, wherein the champagne base comprises a cavity (18) extending upwardly into the bottle (10) having a height of between 8mm and 14 mm.

4. The reusable PET bottle (10) of Claim 3, wherein the cavity (18) has a height of between 10 mm and 12 mm.

5. The reusable PET bottle (10) of any preceding claim, wherein the champagne base comprises a circular standing surface (20) having a diameter of between 42 mm and 50 mm.

6. The reusable PET bottle (10) of Claim 5, wherein the circular standing surface (20) has a diameter of between 45 mm and 47 mm.

7. The reusable PET bottle (10) of any preceding claim, wherein the bottle (10) is capable of withstanding at least ten pasteurisation cycles at 60°C.

8. The reusable PET bottle (10) of any preceding claim, wherein the bottle (10) is capable of withstanding a burst pressure of at least 20 bar.

9. A preform (22) for making the reusable PET bottle (10) of any preceding claim, wherein the preform (22) is dimensioned such that an axial stretch ratio between the preform (22) and the bottle (10) is at least 2.0 and a radial stretch ratio between the preform (22) and the bottle (10) is less than 2.6.

10. A method of making the reusable PET bottle (10) of any of Claims 1 to 9, the method comprising:blow moulding the preform (22) of Claim 9 into a mould; andheating the mould to a temperature of between 80°C and 135°C.

11. The method of Claim 10, comprising heating the mould to a temperature of between 100°C and 115°C.

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