PET Application and Brief Analysis of Manufacture

 

Introduction of features

PET

PET, with its scientific name polyethylene glycol terephthalatet, is also called Dacron, polyester. Featuring excellent resistance of gases, resistance to pressure, shock resistance, transparency and surface gloss, it is widely used in food containers with traits of odorlessness and fragrance conserve, the safety of the food hygiene, beautiful shape and easily-forming.

 

PET is a kind of crystallization resin, with bulk density of 1.4kg/L, melting temperature of 245 ℃, and molding temperature between 290 and 315 degrees Celsius. Its grain bulk density fluctuates between 0.8 to 0.85kg/L, sheet bulk density between 0.38 to 0.4 kg/L.

 

PET is mainly employed to produce synthetic fiber, PET bottle and thin sheet. With the development of economy in recent years, peoples awareness of environmental protection is under enhancement. Thus PET sheet, as a kind of packaging material shows the favorable development potential as the alternative to replace difficult-recyclable PVC packing material.

 

APET

APET (amorphous PE) sheet is made by T mould extrusion. Its glass transition temperature is 80℃. The molten PET finalizes the design into piece after being shock cooled while it is too late for resin to be crystallized, without fixed form. This process eventually produces the sheet with very good transparency which still retains PET’s other advantages.

 

CPET

Another kind of CPET sheet is crystalloid PET sheet, it is a PET sheet produced by T mould extrusion by adding nucleating agent. Because the PET molecule inside CPET sheet stays at high crystallized state, CPET come out opaquely and has milk-white color. The main advantages of CPET performance are the widely-used feature since it could be used between -40 and 240℃. It is the ideal base with microwave containers.

 

GPET

GPET sheet is also called PETG sheet, which is produced from amorphous PET resin modified by cyclohexanediol. The biggest advantage of GPET sheet is excellent performance of heat sealing that renders operation easy and sealing effect is favorable. Besides, its resistance to chemicals and optical property are very good. GPET sheet can be used in medicine sanitary equipment, products packaging of food and personal care, and electronic devices packaging. The unfavorable factor of promotion is high price.

 

Terminology

Glass Transition Temperature Tg

Macroscopic explanation: based on the characteristics that mechanical property of the samples would be changed complying with temperature, the amorphous high polymer can be divided into three mechanical states: glassy state, high-elastic mode and viscous state. The transition between glassy state and high-elastic is called the Glass Transition Temperature; the corresponding temperature change is referred to as the glass transition temperature which is usually indicated with Tg. The glass transition temperature of generally bottle-used PET is about 80℃, and glass transition temperature of engineering-used PET is about 160℃.

 

Crystallization Temperature Tc

When temperature keeps rising and achieve 160℃, molecules interacted with each other begin the partial rearrangement to produce globular crystallization, namely crystallization. PET’s maximal crystallized degree is about 55 percents that could generate orderly crystallization zone. The crystallization temperature range for PET is 120~220℃.

 

Drying Temperature

Drying treatment before process is necessary due to strong hygroscopicity of PET. Suggested drying condition is 120~165℃ and 4~6 hours of drying treatment. Moisture content should be less than 200ppm after drying if it is sheet extrusion line while moisture content should be less than 50ppm after drying if is pet preform production line. Moisture of common spinning is less than 50ppm while high-speed spinning is less than 35ppm.

 

Drying Features of PET

PET is a kind of hygroscopic material. If moisture content is too much, hydrolysis will happen under high temperature and cause PET molecular chain will be ruptured, producing acetaldehyde and lowering physical and mechanical property. Meanwhile, the speed of crystallization rate accelerates so some parts of pet preform will be crystallized during shooting, influencing the quality of the products. Before the pet preform being shot and formed, PET grain need to be dried with moisture content below 50ppm 0.005%, so good non-crystalline transparent pet preform could be produced. Recovery proportion generally should not exceed 25%. Recycled material shoud be completely dried.

 

Purpose of PET Drying

 Remove water

 

 

Drying principle of PET

 Moisture out

 

     Moisture=unrestrained moisture content + equilibrium moisture content
     Unrestrained moisture content is the moisture adhered to the surface of PET, which is easily removed.
     Equilibrium moisture content, which is hardly removed, is the moisture inside PET molecule. Vacuum drying or dehumidifying is usually used to lower its moisture.

 

 Crystallization

     Under high temperature, amorphous PET would be sticky. The aim of Crystallization is to prevent dry stick of PET under high temperature. Boiling PET by mixing with drying process and air flow is the commonly used method.

 

Process Control

 Temperature

     As usual, the higher drying temperature is, the faster speed of drying will be; the shorter drying time makes lower moisture in material after drying. Drying temperature of Stirring Crystallizer is normally between 120~160℃ and dehumidifying temperature between 160~180℃.

 

 Time

   Under general circumstance, PET pre-crystallizing time is 1~2 hours. Crystallizing time is inversely proportional to pre-crystallizing temperature. 

    4~6 hours of drying time would allow PET moisture to approach or reach the balance moisture. Drying time also has something to do with drying temperature, vacuum degree and dry air moisture content. The higher drying temperature, the shorter drying time; if that is vacuum drying, higher degree of vacuum will make shorter drying time; the lower dry air moisture content, the shorter drying time.

 

 Air speed

     Normally, the faster drying speed, the higher PET and airflow velocity. That shortens the drying time while powder generated by the system increases, which come to consider to remove excessive dusts.

 

     Wind speed of General Hopper Drying is 8~10m/s (that would avoid blowing hopper bed into mess and ensure material drops in plunger type evenly inside drying device.

 

 Air Humidity (Air dew-point)

    The lower hot air moisture percentage (dew-point), the faster drying rate of material, corresponding materials equilibrium water content becomes less. During the actual process, with considering the processing cost and power consumption for bottle, dew point value of less than -40℃ can satisfy material drying moisture percentage of 100ppm for preform and injection molding.

 

 Air Quantity

    When used for preform or injection molding, the unit drying air reference value needed for PET is 1.7~2.5Nm3/hr-kg; when used for extrusion molding, the corresponding value is 1.2Nm3/hr-kg. In fact, since values from the raw material suppliers are often various, the specific values is necessary for reference.

 

PET Preform System

  

A complete PET preform production system should contain equipments as follows:

 

 

Due to the feature of PET strong water absorbability, special attentions must be paid to the storage and moisture-proof processing is necessary. Drying conditions is often dehumidification in 160~180℃ lasting for 4~6 hours. To prevent plastics grain absorbing moisture again, it needs 140℃ above to maintain temperature level while preventing the plastic’s turning yellow, dehumidifying drier need to lower temperature to 100℃, preserving heat only but not dehumidifying.

 

Picture: SHINI PET Dehumidifying and Drying System

 

PET Sheets

Picture: PET Sheets Products

 

PET is usually a crystallized plastic and replaces traditional packaging materials due to advantages of light quality, low energy consumption, not easily broken. It has a wide application in food, beverage and cosmetics industries. To recycle PET for recovery is actually the process of renewing PET.

 

 Picture: SHINI PET Crystallization System

 

There are two PET bottles recycling methods: chemical recycling and physical recycling. Chemical recycling method is to deploymerize the PET waste in certain conditions to generate useful chemicals production, such as production of low-class fuels. Physical recycling method is the re-granulation through waste separation, destroy, washing and drying treatment.

 

PET Sheet Recycling Process Flow

PET recycling process of bottle flakes is to separate PET bottles from cover and labels, then shatter, clean and dry. Dimension of recycled material cannot be too big; otherwise it will generate bridge” in the feeding mouth, which may affect plasticizing. General advice for dimension is less then 10mm.