what is the hot fill technology for PET bottle?

1. Structure of PET

PET refers to polyethylene terephthalate, which is composed of three elements: carbon, hydrogen, and oxygen.  The molecular structure is as follows:PET is generally formed by direct esterification or transesterification of terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol (EG) to produce PET monomers, followed by solid-state polycondensation.The chemical reaction equation is as follows:

nHOOC-C6H4-COOH   nHOCH2CH2OH---CO-C6H4-COO CH2CH2-O-]n+nH2O

2. Physical properties of PET

PET is divided into homopolymerized and copolymerized PET according to polymerization methods, and is classified into textile PET, bottle-grade PET, and sheet-grade PET according to applications.PET used for blow molding can be divided into homopolymers and copolymers.  Homopolymers have lower viscosity and melting point than copolymers, with IV ranging from 0.75 to 0.83, while copolymers range from 0.75 to 0.90.  Generally, homopolymers or copolymers with lower IV are used as raw materials for the production of PET hot-fill bottles.

3. Chemical properties of PET

3.1 Hydrolysis reactionUnder high temperature, high pressure or alkaline conditions, PET is prone to hydrolysis, resulting in the breakage of the PET molecular polymer chain, a decrease in molecular weight (i.e., a decrease in IV), and a reduction in mechanical properties.

3.2 Thermal degradation reactionPET is prone to decomposition reactions at high temperatures, resulting in the formation of acetaldehyde and carbon dioxide.Therefore, during the high-temperature heat setting process of bottle blowing, care should be taken to adjust the temperature to avoid too high a concentration of acetaldehyde.

PET hot-filling bottles refer to bottles used for hot-filling production of beverages with a temperature of over 75°C.  Compared to ordinary PET bottles, they have the characteristics of high temperature resistance, shrink resistance, and resistance to vacuum pressure.At present, the most common method for producing heat-resistant PET bottles is high-temperature heat setting, which mainly increases the crystallization of PET bottles to enable them to withstand higher temperatures.In general, the higher the degree of crystallization, the better the heat resistance of PET bottles, and the worse their water absorption, resulting in better preservation.

PET exhibits different mechanical states at different temperatures.  Below 78°C, PET exhibits a glassy state, between 78°C and 245°C, it exhibits a high elastic state, and above 245°C, it exhibits a viscous flow state.In the glassy state, PET molecules are relatively active, and the molecular chains can move and orderly arrange to form a crystalline state.The required temperature for crystallization heating treatment of PET is generally between 78 and 220°C.

In production, the primary and secondary blowing methods are mainly used for heating treatment.The secondary blowing method first blows the preform to a larger volume, then heats it to around 200°C, and then shrinks it, and finally blows it into the final bottle shape.Its advantage is high degree of crystallization and better heat resistance than the primary blowing method.  However, due to the large and complex auxiliary equipment, the footprint is large, and the required heat energy is high, resulting in an increase in overall operating expenses.  Therefore, we currently use the primary blowing method, which uses the Vfine hot filling bottle blowing machine for production.The primary blowing method requires heating the mold to 90-170°C.When the preform is stretched to the shape of the hot blowing mold, the crystallization process will last for several seconds to more than 10 seconds.The bottle blown by the Vfine bottle blowing machine can reach a maximum heat resistance temperature of 93°C.

4.1 Improve the crystallization rate of the bottleDuring the production process, increasing the crystallization rate of the bottle mainly depends on increasing the heat setting temperature and extending the crystallization time.The crystallization rate has a significant impact on the water absorption rate of PET.  The higher the crystallization rate, the less water the bottle absorbs under the same temperature and humidity conditions.

4.2 Reduce the absorption of moisture in the airDue to its moisture absorption properties, PET (including chips, preforms, and bottles) placed in the air will absorb moisture from the air.  The longer it is placed, the more water it absorbs.The moisture content in PET directly affects its performance.  For hot-filling bottles, it affects the heat resistance temperature of the bottle.  The higher the moisture content, the lower the heat resistance temperature of the bottle.Generally speaking, for hot-filling bottles, from the production of preforms to the filling of beverages, the heat resistance of the bottle mainly depends on its moisture content.  Therefore, the storage conditions are very strict.

5. Production process of hot filling bottles

The production principle of PET hot-filling bottles is basically the same as that of ordinary PET bottles, but the production process and production conditions are much more complex.  Currently, our company mainly adopts a one-time blowing and biaxial stretching high-temperature heat setting production process.  The following is its main production process:The production of hot-filling bottles involves additional steps such as mold heating, air cooling and two-step stretching compared to ordinary PET bottles, which increases the complexity of the bottle blowing process.  At the same time, due to the longer crystallization time required for the bottles during the bottle blowing process, their blowing speed is also much slower than ordinary PET bottles.It should be noted that product performance is closely related to the bottle blowing production speed.  Generally speaking, the slower the speed, the better the product performance.

5.1 The main process parameters are as follows:

5.1.1 Blowing speed

Take 0.5L hot filling bottle as an example, the blowing speed of each bottle blowing machine is: 500-700 per cavity.

5.1.2 Mold temperature

Mold temperature: 110-160℃ (first generation) 130-170℃ (second generation) Bottom mold temperature: 70-120℃

5.1.3 Heating principle of preform

Try to increase the percentage of light tubes in the adjustable area of the heating furnace and reduce the number of working heating light tubes.  The increase in the temperature of the preform will help reduce the stress generated during bottle blowing and improve heat resistance.

5.1.4 Capacity adjustment

The size of the capacity is related to the mold temperature, bottle blowing time, air cooling time, and mold manufacturing.For example, if the capacity is too small, the mold temperature can be lowered, the bottle blowing termination cam can be moved forward, the air cooling time can be increased, and the capacity shims of the mold body can be removed.