WO2023025901A1

WO2023025901A1 - Process for the production of high barrier and crystallinity pet and rpet containers with standard injection-stretch-blow molding technology

Info

Publication number: WO2023025901A1
Application number: PCT/EP2022/073707
Authority: WO
Inventors: Giovanni SONZOGNI
Original assignee: Gs4Plastic S.R.L.
Priority date: 2021-08-25
Filing date: 2022-08-25
Publication date: 2023-03-02
Also published as: IT202100022292A1

Abstract

The present invention is directed to a single-stage injection-stretch-blow moulding process for the production of PET containers and bottles, which process combines the dual and simultaneous effect of the nucleant and biaxial orientation of the macromolecules of the preform to achieve barrier properties of PET and rPET containers high that would be impossible without the use of hot molds and special technologies. Moreover, thanks to this high crystallization it is possible to produce containers in rPET and PET even with high thermal and mechanical properties.

Description

Process for the production of high barrier and crystallinity PET and rPET containers with standard injection-stretch-blow molding technology

Field of the invention

[0001], The present invention is directed to the production of PET and rPET stretch-blown containers and bottles. In particular, the present invention is directed to a process that combines the use of a nucleating agent with injection stretch-blow molding technology (ISBM) for the production of containers in rPET and PET made of monomaterial, completely sustainable and at the same time have high barrier properties that would be impossible to achieve with standard technologies, such as being suitable for O2 sensitive products such as beer, ketchup or pickles.

Background of the invention

[0002], The production of stretch-blown PET containers and bottles represents an important segment of the use of PET in the world. Worldwide production of PET in 2015 was 27.8 million tons. About 70% of PET is used for the production of bottles. Plastic recycling is a huge environmental problem nowadays. Recycled PET, also known as rPET, is widely the most recycled plastic in the world. According to PETCORE the percentage of recycling in the USA was about 31% in 2012, while it was 52% in the European Union. In 2015, around 9 million tonnes of PET were recycled to produce a wide variety of products.

[0003], The EU and many companies in the sector have set important sustainability targets to safeguard global warming and limit pollution and marine littering, where by 2025 plastic recycling levels of at least 50% of virgin plastics placed on the market the previous year should be achieved. It is therefore clear that new processes need to be developed to enable rPET to be used alongside PET. At the same time, it is desirable to develop processes that allow the use of PET instead of more performing but at the same time more expensive materials.

[0004]. PET (both virgin and recycled), is the most used material in the field of stretchblowing for the production of containers in contact with food; however, for some foods particularly sensitive to oxygen (beer, juices, tomato puree, ketchup, mayonnaise), different materials must be used or, alternatively, PET should be treated to improve its O2 barrier properties. These treatments can be both chemical and physical. Among the physical treatments, we can mention the plasma treatment by which it is possible to make a surface nanometric coating of SiOx or amorphous C on the inner or outer surface of the bottle, while among the chemical treatments it is possible to mention the use of dispersed additives or inner barrier layers in the preform (and therefore in the bottle), with other polymers that reduce the permeability to oxygen. The process of this invention allows to obtain important barrier properties of the order of magnitude of multilayer materials as well as acquire excellent thermal and mechanical properties. Increasing the barrier properties of single-layer packaging would expand this sector by significantly reducing the use of low permeability packaging that is currently made from multilayer containers or films.

[0005]. Stretch blowing technology consists of the following steps: the polymer (PET, rPET or a mixture of both) is fed to an extruder in which the polymer is melted and homogenized with possible additives at a temperature of about 280°C. The molten polymer is injected into a preform. Optionally, the process can include a conditioning phase, wherein the preform is kept at the chosen temperature (normally between 140 and 160°C) before moving on to the stretch blowing stage. At this stage, the preform is inserted into the stretch blowing mould, an ironing rod pushes the bottom of the preform to the bottom of the mould and almost simultaneously high pressure (around 30 bar) air is introduced causing the bio-orientation of the material until the PET ( or rPET) completely adheres to the surface of the mould. The current technologies of stretch blowing can be roughly represented by two types of moulds: cold moulds and hot moulds.

[0006]. By the use of cold moulds, it is possible to achieve quite limited barrier properties, with crystallinity around 15% and shelf life for O2 sensitive products of the order of no more than 3 months. To increase the shelflife of these containers there are mainly 3 solutions, but all economically very expensive and some of these feasible are with dedicated and special technologies:

• Use of additives, such as oxygen scavenger (O2 absorbers), which however have a limited duration in time.

• Blow moulding of multilayer preforms (PET -MXD6- PET or other solutions) requiring injection moulding machines with more complicated and expensive double plastification cylinder.

• SiOx-based or carbon-based surface coatings to be made on the blown bottle but which also require special and expensive plasma technologies. [0007]. With some of these solutions it is possible to reach a shelflife of 9 months depending on the technology used but with an important, and often not accepted by the market, economic contribution.

[0008]. The oxygen permeability of PET is known to be inversely proportional to the crystallinity of the PET itself: a high crystallinity PET has a lower permeability than a low crystallinity PET. In order to obtain reduced oxygen permeability, the technology using hot moulds, also called hot fill or heat set, is currently used. This technology not only uses hot moulds (140-160°C), but it is only feasible with special blowers that have a specific blow molding system that can blow through perforated stretch rods in titanium and a special valve system that allows, after about 1 s of high-pressure blow molding, cold air recirculation to lower the temperature of the containers below 100°C. This very complex technology, if well engineered, allows to reach shelf life around 9 months with O2 sensitive products without addition of any additives. Thus today, to obtain containers with high barrier properties to O2 by stretch blow molding, there is only the possibility of stretch-blowing the preform in a hot mould around 140 - 160°C and with the help of special machines.

[0009]. This technology is much more expensive than the technology that uses cold moulds, because the cycle times of a hot mould process is generally 50% longer than a cold mold process.

[0010]. There is therefore a need for a process for the preparation of a hot fill packaging that has a high barrier effect against O2 and that can be obtained in a simple way and with low cycle times.

Summary of the invention

[0011]. The present invention is directed to a process for the production of single-stage stretch-blow moulding PET containers, wherein a nucleating agent is added to the PET during the granular plasticisation phase, the melted and well homogenized mixture is injected into the preform mould which is then transformed into a bottle/container inside cooled moulds.

[0012]. It has been surprisingly found that by this process it is possible to obtain PET or rPET containers having good barrier properties. Without being limited by theory, it is believed that the results of the tests done on the experimental flasks indicates that the elevated barrier properties are linked to the formation of highly oriented variable crystallinity layers in the blown bottle. In particular, it has been observed that through the process of the present invention, a container is obtained that has several layers: the outer layer of the container has a higher degree of crystallinity than the innermost layer which has a lower crystallinity. This stratification is probably responsible for the increased gas barrier of the container compared to a container having a similar degree of crystallinity. The containers obtained by the process of this invention have good barrier properties with a shelf life of more than 6 months, excellent thermal stability, high rigidity and do not require special moulds.

[0013]. In addition, the process of the present invention represents a significant improvement in terms of costs compared to state-of-the-art processes, as in addition to being feasible with standard machines, allows a shorter cycle time of about 50% compared to hot fill technology with hot moulds, a factor that allows a higher productivity of the machine.

Brief description of the figures

[0014]. Figure 1 shows an image of a 150 ml bottle obtained through the process of this invention.

[0015]. Figure 2 shows the multiplicity of layers present in a bottle according to the invention.

Detailed description of the invention

[0016]. The present invention is directed at a process for the production of containers made of PET, rPET, or mixtures thereof, wherein the process includes the following steps: addition of a nucleant to PET; homogenisation of PET and the nucleating agent in the lamination screw; injection of the homogenized mixture into a preform mould, conditioning of the preform at a temperature of between 140°C and 160°C for a period of between 0 and 30 seconds, stretchblow molding in a mould cooled to a temperature of not more than 30°C, preferably not more than 25 °C.

[0017]. The nucleating agent used in this invention may be any compound known in the art as an organic nucleating agent for plastics. The term nucleating agent refers in this description to any compound which is capable of bringing the crystallization temperature as measured by DSC (Tc, first crystallization temperature) above 200°C. This is because the effect of a nucleating agent is to favour crystallization and thus reduce the under-cooling value required for crystallization to begin. Preferred examples of nucleating agents are the alkaline salts of benzoic acid, and stearic acid. The term alkaline salts refers to the salts of lithium, potassium and sodium. The preferred alkaline metals are potassium and sodium.

[0018]. Dispersion of the nucleating agent in PET can occur in various ways. 1) Direct addition of the nucleating agent in any form (powder, granules or liquid) to PET (rPET); this method is not particularly suitable because it is difficult to control the dispersion of the agent in PET. 2) Production of the additive in PET or PETG pellets in extrusion by creating a liquid or solid compound with the nucleating agent present in suitable quantities. 3) Dispersion of the nucleating agent in the masterbatch or additive which is made to be added to the virgin raw material. Seguendo la stessa logica di produzione di compound descritta al punto 2, oltre alle percentuali di pigmento (in forma di polvere, granule, liquido, pasta colorante eccetera) e di eventual! additivi (scivolanti, cariche varie eccetera), si aggiungera una opportuna percentuale di nucleante (in polvere, granule, o liquido).

[0019]. The quantity of nucleating agent in the final composition is preferably between 0.10 % by weight and 1.50 % by weight, more preferably between 0.20 % by weight and 1.00 % by weight, more preferably between 0.30 % by weight and 0.80 % by weight. In stretch-blowing, there are two factors that push in different directions. On the one hand, too low a quantity of nucleating agent produces a final product that is not significantly different from the product obtained by the conventional cold mold process (without addition of nucleating agent). On the other hand, too high a quantity of nucleating agent produces excessive crystallization of the PET in the preform, which makes the next stretch-blowing stage unstable. As a result, it is necessary to dose the amount of nucleating agent according to the type of process.

[0020]. As mentioned above, it is possible to operate by a process with a conditioning phase, or the preform, once obtained, can be directly stretch-blown. In both cases, one of the factors determining the correct amount of nucleating agent is the total time that passes between the start of the preform injection and the start of stretch -blow moulding. In fact, a process without conditioning phase that uses an injection time of 15-20 seconds can use a greater amount of nucleating agent than a 4-station process with conditioning. For processes without conditioning, the quantity of nucleating agent should preferably vary between 0.30 % by weight and 1.50 % by weight, preferably between 0.40 % by weight and 1.20 % by weight, while for processes with conditioning or with cycle times greater than 25 seconds, the quantity of nucleating agent should preferably vary between 0.10 % by weight and 1.00 % by weight, preferably between 0.20 % by weight and 0.80 % by weight.

[0021]. Another factor that affects the amount of nucleating agent used is the blowing temperature of the preform. Normally the injection temperature is about 280 °C, but then the preform is cooled and blown at a temperature between 120°C and 160°C. As the blowing temperature of the preform increases, a higher quantity of nucleating agent can be used than a more cooled preform, bearing in mind that when the quantity of nucleating agent is too low, the benefits are not obtained but when it gets too high, the process can become unstable.

[0022]. Without being bound to theory, it is believed that the process of the present invention allows stretch-blowing of preforms containing significant quantities of crystalline phases thanks to the "lubricating" effect carried out by the amorphous phases towards them. During the phase of stretch-blowing, the spherical crystals of PET in part already present are transformed into lamellae crystals and it is precisely in this phase that it is believed that the various layers of different crystallinity and high barrier properties are formed with an increase in the crystallinity itself.

[0023]. It has been noted, as shown in Figure 2, that the bottle obtained by the process of the invention seems to be monolayer but that in reality it is composed of a large number of layers that seem to come off only after having engraved the bottle with a blade. We can therefore say that, thanks to the process of the invention, a multilayer packaging or film can be obtained, and the presence of multiple layers inside the container wall may be responsible for improving the barrier properties of the bottle.

[0024]. The process of the present invention is also directed to a process for the production of containers made of PET and a filler agent such as talc, calcium carbonate, etc. Therefore, the term PET containers also includes containers in which the PET contains fillers. In these cases, all reasoning made for pure PET (virgin and/or recycled) shall be understood to be applicable to PET mixed with fillers, and the percentages indicated (eg % by weight of nucleant, % of crystallinity) shall refer to the amount of PET present, without counting the charge.

[0025]. The process of the present invention is therefore able to provide a stretch-blown product of PET, rPET and mixtures thereof, made of monomaterial with a percentage of nucleating agent between 0.10 % by weight and 1.50 % by weight, with medium- high barrier properties and containing preferably more than 35% crystalline fraction, even more preferably more than 40%.

Experimental part

[0026]. Bottles have been tested and stretch-blow-molded both on machines with 3 stations type Aoki, and on machines with 4 stations type ASB Nissei, specifically, these 150 ml containers were made with a 4-station conditioning machine and according to the process of this invention using 0.30 % and 0.60 % nucleating agent, and for comparison a bottle was prepared with the same process but in the absence of nucleating agent. Figure 1 shows a photograph of this bottle containing 0.30 % by weight of nucleating agent.

[0027]. A sample of each bottle, taken from the centre of the bottle, was subjected to SDC analysis. To calculate crystallinity, it was used the extrapolated theoretical melting enthalpy value of 140.1 J/g of the 100% crystallised PET. In addition, with a SDC it is possible to detect the presence of a nucleating agent. In fact, during a DSC of a PET sample, while for a PET without nucleating agent the peak of crystallization during the cooling phase is very large and presents the maximum at a Tc temperature of 170-190°C, for a PET sample containing a nucleating agent, crystallization takes place earlier and the Tc increases more or less strongly depending on the effectiveness of the nucleating agent and once the type of nucleating agent has been fixed, temperature Tc increases as the amount of nucleating agent present increases. In the case of our nucleating agent, the Tc normally varies in the range between 200°C and 210°C.

[0028]. Table 1 shows the values of Tc, and the melting heat (Hm) expressed in J/g of the samples described above with an amount of 0.30 and 0.60 % by weight of nucleating agent and in the absence thereof. The melting heat represents the amount of crystalline fraction in the sample. To obtain the percentage of crystallinity it is necessary to divide the value by 140.1 J/g which is considered as the extrapolated melting enthalpy of 100% crystalline PET. The table shows the melting enthalpies and the crystallization temperatures. Interestingly, the cooling DSC shows a significant increase in Tc as the amount of nucleating agent present increases.

Table 1

[0029]. Oxygen permeability tests according to ASTM F 1307-20 using MOCON OX-TRAN 2/61 automatic equipment (No. 0698BN025 series) were carried out on the bottle obtained with 0.3 % nucleating agent and on the bottle obtained in the absence of nucleating agent.

[0030]. The samples were glued on metal plates, using two-component epoxy glue, and connected, by means of two copper tubes with swagelok fittings to the inner half-cell of the instrument where they were conditioned for at least 10 hours at 23±1 °C at 50±5% relative humidity and at a barometric pressure of 750±5 mmHg.

[0031]. The following Table 2 shows the permeability values obtained.

Tabella 2

[0032]. Since it is clear from Table 1 that the increase in crystallinity due to the presence of a nucleating agent is interesting, it can be concluded that the increase in barrier properties is probably due to a multi-layer effect occurring in the containers.

[0033]. The process was then used on a machine operating at a higher stretching ratio, testing the validity of the process according to the invention under more severe conditions. The amount of nucleating agent used was 0.25 % by weight. At higher stretching ratios, the presence of a high crystalline fraction in the preform could have prevented a correct stretch-blow phase. The results confirmed the validity of the process. A bottle obtained by this process is shown in figure 2. The bottle consists of at least 4 layers that have been manually separated. A DSC analysis was carried out on the outer layer, an intermediate layer and the inner layer and for comparison a DSC analysis was carried out on a sample obtained with hot fill technology. The results (Table 3) clearly show that the outer layer has a higher nucleating agent concentration than the intermediate layer and the intermediate layer has a higher nucleating agent concentration than the inner layer. Although it is not possible to precisely determine the concentration of nucleating agent in each layer, we can say that the increase of Tc from the inner layer to the outer layer clearly indicates how the concentration of nucleating agent increases. As for the crystallinity of the various layers, the data show that the intermediate layer has a slightly lower crystallinity than both the inner and outer layers. However, the three layers have a crystallinity value similar to that achieved with the conventional hot fill process, which requires much higher investment costs and requires a much longer cycle time and therefore much lower productivity. Thus, the process of the present invention produces a monomaterial and multilayer bottle, in which the concentration of nucleating agent increases from the innermost layer towards the outer layer.

Table 3

[0034]. The crystallinity data of Table 4 show that it is possible to obtain crystallinity values greater than 40% by the process of the present invention when operating at high stretching ratios, thus obtaining even lower values of oxygen permeability than those obtained with the samples of Tables 1 and 2.

Claims

Claims A process for the production of PET containers for hot-fill application by blow molding technology, which process comprises the following steps: a. adding a nucleating agent to a PET; b. homogenizing the PET and the nucleating agent at a temperature comprised between 260°C and 300°C; c. injecting the homogenized mixture into a preforms mold for blow molding, wherein the preforms are thermostated at a temperature comprised between 140°C and 160°C; d. blow-molding the preforms in a blow-molding mold; characterized in that the blow-molding mold is cooled at a temperature equal to or lower than 30°C. The process according to claim 1, further comprising a step cl, wherein the preforms are conditioned for a time equal to or lower than 30 seconds at a temperature comprised between 140°C and 160°C. The process according to claims 1-2, wherein the nucleating agent is a compound or composition that results in a crystallization temperature Tc of PET, as measured by DSC during first cooling, higher than 200°C. The process of claims 1-3, wherein the nucleating agent is selected from an alkali salt, preferably a sodium or potassium salt, of an acid selected from benzoic acid and stearic acid, more preferably the nucleating agent is selected from sodium benzoate and potassium benzoate. The process according to claims 1-4, wherein the nucleating agent is present in an amount comprised between 0.10 wt % and 1.50 wt %, preferably between 0.20 wt % and 1.00 wt % based on the amount of PET. The process according to claims 1-5, wherein the cycle time is lower than 25 seconds and the amount of nucleating agent is comprised between 0.30 wt % and 1.50 wt %, preferably between 0.40 and 1.20 wt % based on the amount of PET. The process according to claims 1-5, wherein the cycle time is higher than or equal to 25 seconds and the amount of nucleating agent is comprised between 0.10 wt % and 1.00 wt %, preferably between 0.20 and 0.80 wt % based on the amount of PET. A blow-molded PET container obtainable according to the process of claims 1-7. The PET container according to claim 8, having crystallinity equal to or higher than

31% and comprising an amount of nucleating agent comprised between 0.10 wt % and 1.50 wt % based on the amount of PET. The PET container according to claims 8 or 9, characterized by the fact that the container presents a multiplicity of layers, which layers have a nucleating agent content that varies from the internal layer to the external layer, being the content the highest in the external layer and the lowest in the internal layer. The PET container according to claims 8-10, having an oxygen permeability according to the method ASTM F 1307-20 equal to or lower than 0.010 cc/(pkg x 24 h x atm).