How to improve the heat resistance of PET hot-fill bottles

THE SHAPE SPECIFICATION AND HEAT RESISTING PROPERTY OPTIMIZATION OF PET HEAT SET BOTTLE

[Abstract] This paper analyzes some variables that need to be considered in the design and selection of bottles for PET hot-filling bottles, and puts forward some technological methods for improving the heat-resisting performance in the PET hot-filling bottle forming process.

[Key words] PET hot-fill bottle heat resistance

[Abstract] Analysis the key elements of bottle shape which must be considered when design and choose a PET heat set bottle. Several solutions to optimize the heat resisting properties are also been introduced.

[Key Words] PET; heat set; bottle shape; heat resisting property

I. Introduction

When producing low-acidity, neutral beverages, such as tea, fruit juices, fruity water, etc., the semi-finished products and bottles after filling must be sterilized in order to control the microbial contamination of the products. The available filling methods are:
1. Add preservatives;
2. Aseptic or cold sterilization filling;
3. Hot filling (except for gas drinks);
4. Pasteurization.

In recent years, hot filling technology has been widely adopted in the beverage industry in China because of its better safety and economy. The prospects are very broad. The following is a summary of the author's work in the actual work, through the working principle of one-step hot filling bottle machine and hot filling equipment.


two. Hot filling technology

In the hot filling process, the product is UHT ultra-high temperature sterilization (instantaneous heating to 1200C ~ 1400C, stay dozens of seconds), and then cooled to the filling temperature (850C ~ 900C). After filling and capping, the bottle is inverted or laid side by side for about 30 seconds so that the bottle cap and bottle neck can be sterilized at the same temperature as the bottle body. The bottle stays at a high temperature for a certain period of time (30~120 seconds), then it is sent into the cooling channel, and the bottle is cooled to 340C~380C in sections (the time for passing through the cooling channel is about 12~20 minutes), then the bottle is labeled, After packing and other packaging.


three. Heat-resistant bottle design points

When designing a thermos bottle type, the following factors must be considered:

1. Within 30 seconds after filling, the positive pressure in the bottle rises. This is because:
(1) The residual air temperature in the bottle after filling rises from about 300C to 800C~900C;
(2) The biaxially oriented PET bottle shrinks when heated and its volume decreases. At high temperatures, the bottle must be able to withstand a positive pressure of 0.1 to 0.3 Bar without permanent deformation.

2. The volumetric shrinkage of PET bottles at high temperatures. Ordinary PET bottles shrink at 20% at 850C. However, heat-resistant bottles blown with PET pellets for heat-resistant bottles typically have a shrinkage of between 1% and 1.5%;

3. The amount of shrinkage and filling points after filling. The higher the filling temperature. The greater the volumetric shrinkage. Experiments show that the volumetric shrinkage between 860C and 900C is particularly sensitive to temperature rise. The lower the height of the filling point, the greater the volume of residual air in the bottle after filling and the greater the contraction of the bottle. This is because the greater the volume of residual air in the bottle, the less resistance to contraction and deformation of the bottle. Typically, hot fill bottles are bottled at the bottle support ring.



Fig.1 Relationship between volumetric shrinkage and filling temperature of hot-fill bottles


4. The cycle blowing air cooling time after blow molding has an effect on the bottle volume, crystallinity and stiffness. The longer the circulating air cooling time, the greater the bottler volume. Therefore, when designing a bottle, it is important to consider using the lowest high-pressure air consumption to achieve the best performance of the bottle.

5. When the beverage after hot filling drops to room temperature, the change in the specific gravity of the beverage at different temperatures causes the volume of the liquid in the bottle to drop by about 2%. At the same time, the drop in temperature also leads to an increase in the solubility of residual air in the bottle in the liquid. All this causes the volume of residual air in the bottle to expand, resulting in a negative pressure of 0.2 to 0.3 times atmospheric pressure. The relationship between the pressure of the cavity (residual air) and the temperature of the bottle and the time after filling is shown in the figure below.



Fig.2 Curve of the relationship between the pressure of the cavity (residual air) and the temperature of the bottle and the time after filling

Therefore, heat-resistant bottles must meet the following requirements:
(1) Volumetric shrinkage between 1% and 1.5% at high temperatures (850C~900C);
(2) Crystallinity of more than 30% ensures good heat resistance and weak moisture adsorption capacity (water absorbed on the bottle wall reduces the mechanical properties of the bottle like lubricant between the molecular chains;
(3) Reasonable wall thickness distribution to avoid nonlinear shrinkage (deformation) after hot filling;
(4) The bottle body adopts a special frame-shaped structure design, and the bottom of the bottle is provided with concave reinforcing ribs to withstand the negative pressure in the bottle after the bottle is cooled to room temperature.


four. Measures to Improve Heat Resistance of Bottles in Bottle Making Process

(1) Reasonably design the preform. The optimized preform shape design helps to improve the wall thickness distribution of the bottle and avoid distortion or shrinkage in different areas of the bottle body.

(2) Strictly control injection and drawing-blow molding process parameters and temperature distribution in each area to avoid the residual stress being released under the glass transition temperature (>750C) of the PET and causing the bottle to deform;

(3) Bottle blank injection cooling time control. Strictly control the injection cooling time of the preforms and allow the preforms to be released as soon as possible. This shortens the molding cycle, increases bottle yield, and induces spherical crystals due to higher residual temperatures. The crystal diameter of the spherical crystals is very small, only 0.3 to 0.7 microns, and does not affect the transparency.





Fig. 3 Relationship between crystallinity, volume shrinkage and bottle injection cooling time



(4) The use of blow mold temperature control technology. The blow mold is usually warmed by hot oil circulation. There are three cycles for temperature control of blow molds:
- Bottle hot oil circulation. The blow mold is heated to 1200C to 1400C. In this way, the temperature difference between the preform and the blow mold cavity is reduced, which promotes further crystallization. Extend the blowing pressure holding time, make the bottle wall and the cavity contact for a long time, there is enough time to improve the crystallinity of the bottle, reaching about 35%, but without sacrificing transparency. The mold temperature below 1000C has little effect on the crystallinity of the bottle, because the crystallisation of the bottle occurs above 1000C.



Figure 4 The relationship between the crystallinity of heat-resistant bottle and the temperature of blow mold


- Cooling water circulation at the bottom of the bottle. The bottom of the bottle is kept at a low temperature (100C~300C), avoiding whitening due to excessive crystallization of the unstretched bottom of the bottle;

- bottleneck thermostat (optional). The non-crystallized bottle mouth part was completely cooled after it was released from the injection mold. Most of the non-crystallized bottle mouth adopts the reinforced bottle mouth design (increase the wall thickness of the bottle mouth), thereby improving the sealing performance and avoiding the deformation of the bottle mouth during the capping process. Usually, the ellipticity of the bottle after filling is controlled within 0.2 mm, and the shrinkage of the outside diameter of the thread is less than 0.6%.

(5) Recycled air blowing technology. When using hot blow molds, how to control the deformation of the bottles after demoulding is critical. Before the blow mold is opened, the air is blown into the air and the air is evacuated to cool and shape the bottle, thereby controlling the amount of deformation after the mold is released. The intake air of the circulating cooling air passes through the same passage as the initial blow and the secondary blow, but is exhausted from the draw bar head hole through the draw rod. The cycle blowing time is about 0.5~2 seconds. Therefore, the high pressure air consumption of the thermos bottle making machine is much higher than that of the ordinary bottle making machine.

With the further enrichment of practical experience, hot-fill bottle production technology is also developing rapidly. The main trends include the constant weight reduction of the bottle without reducing the filling temperature, and the wide use of non-crystallized bottle mouths. To meet the concept of environmental protection, new bottles with a stylish and concise appearance are increasingly being favored by consumers.

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