CH720309A1 - Stretch blow molded reusable plastic container - Google Patents

Abstract

The invention relates to a stretch blow-molded reusable container (11) made of plastic, comprising a container body (15) with a filling or pouring opening (17). The container base (21) is provided on its outside with a coating (25) at least in those areas in which cracking occurs, whereby the coating acts as a protective layer (25), in particular against alkalis.

Description

Field of the invention

[0001] The invention relates to a stretch blow-molded reusable container made of plastic according to the preamble of claim 1 and a method for applying a coating to a stretch blow-molded container made of plastic according to the preamble of claim 12.

State of the art

[0002] Reusable PET bottles are blown from a preform. When the bottle cools down, thermal stress is created in the bottom of the bottle, which can be explained by the fact that the outside of the bottle is longer at the base radius and therefore contracts more strongly when cooled than the shorter inside. This creates tensile stress on the outside of the bottle base, which leads to small, growing cracks, precisely on the base ring with a base diameter. The cracks grow larger with each washing cycle, since the base ring inevitably comes into contact with the aggressive cleaning medium during a washing cycle and also remains in it for a long time.

[0003] In addition, a reusable PET bottle is subjected to memory stresses in the base area, as the preform is originally curved inwards and the bottle blown from it is curved outwards. The bottle base wants to fold over again due to these memory stresses and take on the shape of a preform again. The base ring is the main point of attack for the so-called "folding over stresses". The stresses lead to the base rolling out slightly. This causes further small microscopic cracks to form in the base ring, which grow with each washing cycle.

[0004] In addition, stresses are caused by products that have been subjected to internal pressure. According to the boiler formula, the stresses add up particularly critically on the base ring. Due to the different stresses that add up especially on the base ring, small cracks appear directly on the base, which become more pronounced and visible as the bottle ages or with each washing cycle and, in the worst case, lead to the bottle failing. A typical example is a visual sorting out of the base, where massive cracks are already visible before the base breaks or becomes leaky.

[0005] Chemicals that penetrate the crack, especially alkalis, significantly accelerate the growth of the crack. Reusable bottles are always washed with alkali. The time and temperature of the bottle in the alkali bath are decisive for how much damage the bottle is caused by the widening of the cracks. The bottom of the bottle in the area of the base ring in particular is usually in contact with alkali residues long after the alkali bath if the bottle is standing in alkali residues. The ester bond of the polyester is attacked by the alkali. This effect is called saponification.

[0006] All of these effects mean that a refillable PET reusable bottle develops increasingly cracks, especially on the base ring, as the usage cycles progress, or becomes increasingly cloudy and unsightly due to saponification. Cracks, cloudiness and of course scratches and abrasion marks, which are caused by handling on the washing and filling line or by the consumer themselves, worsen the external appearance. The bottles must therefore be taken out of circulation for optical reasons. They are typically rejected or recycled after 12 to 25 cycles.

Object of the invention

[0007] The disadvantages of the described prior art result in the object of creating a stretch-blown reusable container made of plastic which has a significantly higher number of usage cycles than known stretch-blown reusable containers, in particular known reusable PET bottles.

Description

[0008] The solution to the stated problem is achieved in a stretch blow-molded reusable container made of plastic by the features set out in the characterizing section of patent claim 1. Further developments and/or advantageous embodiments are the subject of the dependent patent claims.

[0009] The invention is preferably characterized in that the container base is provided with a coating on its outside at least in the areas in which cracks form, whereby the coating acts as a protective layer, in particular against alkalis. Because cracks that form on the bottle base are protected, the washing liquor has no contact with the existing cracks. Accordingly, the service life of such a reusable plastic container is significantly increased. The life cycle of the reusable container can be increased to up to 100 washing cycles before the bottle has to be recycled.

[0010] In a particularly preferred embodiment, the container is a bottle with an inwardly curved base with a standing ring, the coating extending over the standing ring. This protects the standing ring, on which cracks tend to form and which is mostly in the washing solution during the washing cycles, from attack by the lye. The entire base can also be coated if cracks can also form in other places or if coating the entire base requires less effort than coating the standing ring individually.

[0011] In a further embodiment of the invention, the coating is a polar polymer. Tests have shown that polar polymers adhere well to the soil surface because they can form polar bonds with the plastic of the container.

[0012] It has proven to be useful if the coating is a polyester, an epoxy or a polyurethane. These types of polymer can form a particularly good bond to the container surface, especially if they have polar groups and ends.

[0013] The coating is preferably a cross-linked polymer. Cross-linking can also further improve the adhesion of the coating.

[0014] It has proven to be advantageous if the chain structure of the cross-linked polymer contains a dicarboxylic acid structure as a monomer. It has proven to be particularly advantageous if this dicarboxylic acid structure is formed from a terephthalic acid or an isophthalic acid. In this chemical composition, the coating has a particular affinity to the reusable plastic container, particularly if it is a reusable PET bottle. This chemical composition has also proven to be effective in recycling a reusable PET bottle when the useful life of the reusable bottle has expired. Since the coating is transesterified in the extruder during the recycling process and the coating components are broken down and integrated into the polyester raw material of the bottle, the coating does not hinder the recycling of the bottle or container.

[0015] The invention is also preferably characterized in that the coating has a thickness of between 0.00001 mm and 0.1 mm. This thin coating thickness prevents thick-walled coatings which take too long to solidify, remain sticky for a very long time and become too hard and brittle after curing.

[0016] In another particularly preferred embodiment, the coating is crosslinked and cured thermally, chemically and/or by UV radiation. This makes it possible to produce a well-adhering, dense and durable coating.

[0017] The coating is expediently applied or printed using a spray, dip or sputter process, or using sponge rollers, or with a brush. Using these application methods, the coating can be applied with high precision to exactly those parts of the floor that are prone to cracking and need to be protected accordingly. The base ring in particular can be coated precisely.

[0018] In a further embodiment of the invention, the container is made of PET (polyethylene terephthalate), PET-G (glycol modified PET), polyspiroalcohol, PEN (polyethylene naphthalate), copolymers of the above-mentioned plastics, bioplastics, in particular PLA (polylactide) or PEF (polyethylene furanoate), filled plastics and mixtures of the above-mentioned plastics. This makes the coating suitable for most plastics used for reusable containers.

[0019] A further aspect of the invention relates to a method for application, wherein the coating is applied to the outside of the container base immediately after demolding the container, after the container has been stretch blow molded at a blow molding temperature between 60°C and 260°C. As a result, the coating is ideally applied to the still very hot container after the blow molding process and cured there. This creates very good adhesion between the coating and the container surface.

[0020] Preferably, the coating is applied as a protective layer to the areas of the container base in which cracking occurs or in which cracking can be prevented by the protective layer. The coating can therefore be used very economically precisely where protection, in particular against a washing solution, is necessary.

[0021] The coating is expediently applied or printed by a spraying, dipping or sputtering process or by sponge rolling or with a brush. As already explained above, these coating processes are suitable for applying a thin and precisely positioned coating to the container surface.

[0022] Advantageously, the coating is crosslinked and cured thermally, chemically and/or by UV radiation in order to obtain a resistant coating which can withstand up to 100 washing cycles with washing solution.

[0023] Further advantages and features emerge from the following description of an embodiment of the invention with reference to the schematic representations. They show, in a representation not to scale: Figure 1: a cross section through the lower part of a preform and a bottle which is stretch blow-molded from the preform.

[0024] Figure 1 shows a reusable plastic bottle, which is designated as a whole with the reference number 11. The bottle 11 is stretch blow molded from a preform or a preform 13. The lower part of the preform 13 is shown inside the bottle 11 for illustration purposes. The container or bottle 11 can be made from PET (polyethylene terephthalate), PET-G (glycol modified PET), PEN (polyethylene naphthalate), copolymers of the plastics mentioned, bioplastics, in particular PLA (polylactide) or PEF (polyethylene furanoate), filled plastics and mixtures of the plastics mentioned.

[0025] The bottle 11 comprises a bottle body 15 with a filling or pouring opening 17, which is delimited by a bottle neck 19, and a bottle base 21. The bottle base 21 can be curved inwards, whereby a standing ring 23 is formed. Such a base is also referred to as a champagne base and is suitable for absorbing increased internal pressure, for example from carbonated drinks. However, such a base is subject to "folding stresses" since the base has a tendency to fold back into the original shape of the preform base. Such memory effect stresses, thermal stresses and stresses caused by the internal pressure cause cracks in the base 21. During each cleaning cycle of the reusable bottle 11, it is cleaned with alkalis. The standing ring 23 in particular, which is particularly susceptible to alkali attacks due to the unavoidable formation of cracks, is exposed to increased amounts of alkali during cleaning. Therefore, the area with the greatest tendency to crack is particularly exposed to the washing solution.

[0026] For this reason, at least the base ring 23 is provided on its outside with a coating 25 which serves as a protective layer, in particular against alkalis. The protective layer 25 seals and covers the cracks, whereby the alkali cannot reach the cracks and attack them. The minimum coating area which covers the base ring 23 is shown in Figure 1 by the circle. The dashed area represents the optional bottom coating. The coating 25 is provided precisely at the points on the reusable bottle 11 which are susceptible to cracks. The coating 25 can have a thickness of between 0.00001 mm and 0.1 mm.

[0027] The coating 25 is a polar polymer, preferably a polyester, an epoxy or a polyurethane. An example of a polar polyester is shown in the structural formula below.

[0028] The monomer from which the polymer of the coating is made contains a dicarboxylic acid structure, wherein the dicarboxylic acid structure is preferably formed from a terephthalic acid or an isophthalic acid.

[0029] The bottle 11 is manufactured from the injection-molded preform 13 by preheating it and stretching and blowing it in a blow mold. The coating 25 is applied directly to the bottle 11 after it has been removed from the blow mold. A blow mold temperature of between 60°C and 260°C is selected during stretch blow molding in order to relax tensions and crystallize the material more strongly and so that the bottle has as much residual heat as possible when it is coated. Due to the highest possible residual heat, the coating 25 forms a stable bond with the bottle 11 and does not accidentally detach from the bottle during the washing cycles.

[0030] The coating is applied or printed using a spray, dip or sputter process or by sponge rollers or with a brush. The coating 25 is given its high stability by being cross-linked and cured thermally, chemically and/or by UV radiation. As a result, the bottle can withstand up to 100 cleaning washes or washing cycles with 0.5% to 3% caustic soda at temperatures between 40 and 75°C and washing times between 5 and 50 minutes without the coating 25 coming off the bottle 11 or becoming damaged.

[0031] The coating 25 is dosed and applied sparingly to the areas of the bottle bottom where cracks are forming or could form. By protecting the cracks, washing liquor cannot attack them, which means that the life cycle of the reusable plastic bottle 11 can be increased to up to 100 washing cycles before the bottle 11 is recycled.

Legend:

[0032] 11 Reusable container or reusable bottle made of plastic 13 Preform 15 Bottle body, container body 17 Filling or pouring opening 19 Bottle neck 21 Bottle base, container base 23 Stand ring 25 Coating, protective layer

Claims (15)

1. Stretch blow-molded reusable container (11) made of plastic, comprising - a container body (15) with a filling or pouring opening (17) and - a container base (21), characterized in that the container base (21) is provided on its outside with a coating (25) at least in the areas in which cracking occurs, whereby the coating acts as a protective layer (25), in particular against alkalis. 2. Reusable container according to claim 1, characterized in that the container is a bottle (11) with an inwardly curved bottom (21) having a base ring (23), wherein the coating (25) extends over the base ring (23). 3. Reusable container according to claim 1 or 2, characterized in that the coating (25) is a polar polymer. 4. Reusable container according to one of the preceding claims, characterized in that the coating (25) is a polyester, an epoxy or a polyurethane 5. Reusable container according to one of the preceding claims, characterized in that the coating (25) is a cross-linked polymer. 6. Reusable container according to claim 5, characterized in that the chain structure of the crosslinked polymer contains a dicarboxylic acid structure as monomer. 7. Reusable container according to claim 6, characterized in that the dicarboxylic acid structure is formed from a terephthalic acid or an isophthalic acid. 8. Reusable container according to one of the preceding claims, characterized in that the coating (25) has a thickness between 0.00001 mm and 0.1 mm. 9. Reusable container according to one of the preceding claims, characterized in that the coating (25) is crosslinked and cured thermally, chemically and/or by UV radiation. 10. Reusable container according to one of the preceding claims, characterized in that the coating (25) is applied or printed by a spraying, dipping or sputtering process or by sponge rolling or with a brush. 11. Reusable container according to one of the preceding claims, characterized in that the container (11) is made of PET, PET-G, PEN, copolymers of the stated plastics, bioplastics, in particular PLA or PEF, filled plastics and mixtures of the stated plastics. 12. Method for applying a coating (25) to a stretch blow molded container (11) made of plastic with a container body (15) and a container base (21), wherein a preheated preform (13) is stretched and blown into the container (11) in a blow mold, characterized in that a coating (25) is applied to the outside of the container base (21) immediately after demolding the container (11), after the container (11) has been stretch blow molded at a blow molding temperature between 60°C and 260°C. 13. Method according to claim 12, characterized in that the coating (25) is applied as a protective layer to the regions of the container bottom (21) in which regions cracking occurs or in which regions cracking can be prevented by the protective layer. 14. Method according to claim 12 or 13, characterized in that the coating (25) is applied by a spraying, dipping or sputtering process or by sponge rolling, or with a brush or is printed on. 15. Method according to one of claims 12 to 14, characterized in that the coating (25) is crosslinked and cured thermally, chemically and/or by UV radiation.