PET (resin) basics

Basic knowledge of Pet Resin

PET (also known as PETE) is a polyester material that can be drawn, injection molded, and blown. According to different requirements, special grades of resin can have different Rubber And Plastic applications.

PET is a long chain (long chain of molecules is shown by the number of basic units), a white or slightly blue thermoplastic plastic, made of polycondensation of terephthalic acid and ethylene glycol. Each of these raw material particles provided by the resin manufacturer is about 0.05 g. PET was an important textile material in the 1950s. Good mechanical strength, temperature resistance and abrasion resistance make PET an ideal material to replace naturally produced silk, cotton and wool.

PET also has good barrier properties, low water absorption and good toughness. The tensile strength of the PET film is similar to that of an aluminum sheet and is three times that of a PA and PC film. The PET film is transparent, and if it is stretched qualitatively, its sheet tensile strength can reach 1/3 to 1/2 of the steel sheet. He is the toughest thermoplastic film. It burns intensely with a yellow flame when there is an open flame, and it can continue to burn after leaving the fire.

Bottle grade PET

Today, Pet Resin Iv 0.8 is still widely used in these areas, but since the 1970s, the market needed a lightweight, easily broken bottle to package gas-containing beverages. PET has become the best choice. Unlike simple polymers such as PE, PET is not manufactured in a single process, but rather is derived from the chemical reaction of Pure Terephthalic Acid and ethylene glycol. In the past, the supply of PET was designated as the first to be practical, resulting in new production capacity to be put into production this year, ensuring that PET will continue to grow in the future. The related polyesters are PEN (polyethylene naphthalate) and PBT (polybutylene terephthalate, which is mainly cited in engineering plastics). PEN has exceeded the performance of PET, especially its performance in barrier properties and heat resistance. The use of PEN and PET for the manufacture of a series of alloys has made possible the application of special packaging.

As a transparent, wear-resistant, corrosion-resistant plastic, PET (bottle grade) has high strength and smoothness. PET bottles are widely used to package mineral water, fruits, edible oils, pharmaceuticals, and cosmetics.

Melting temperature: 254-256°C

Crystallization: ≥45%

Crystalline or non-crystalline PET products can be produced by controlling the crystallization temperature and the cooling rate. The higher the viscosity, the slower the crystallization speed. The key factors that affect the PET bottle molding and performance are the orientation of crystallization and crystallization. If the PET digital sweat Pvc Resin is high, hydrolysis will occur during the molding process, and the viscosity will decrease, implying that the quality of the product is affected.

Terminated carboxyl: ≤20mol/t

Acetaldehyde: ≤ 3ppm (not included in production, related to drying and molding temperature)

Carbonated beverage bottles must be ≤ 9ppm; mineral water bottles need ≤ 4ppm

Density: 1.38~1.40g/mm3

Glass transition temperature: 82°C

In the industrial production of bottles, the PET chain length is usually used to describe the viscosity of the resin. Bottles have a viscosity of 0.65 to 0.85 dl/g, or 100 to 150 repeating units per chain.

Most bottle grade PETs are copolymers and some of the modifications are part of the polymer chain. Copolymers Ethylene-Vinyl Acetate are more easily injection molded because the crystallization properties have been improved.

An unusual change occurs when injection molded PET is stretched to the proper length at the right temperature. The long chain is subjected to chain hardening and chain induction and crystallized. Under this suitable process, the PET bottle becomes transparent, resists internal pressure, has uniform wall thickness, has toughness and many other characteristics. In order to obtain these good properties, it is necessary to choose the right grade of resin, suitable semi-finished product, bottle design and good experience.

Some grades of PET have other modifications to improve the bottle's barrier properties, heat resistance (two-step system) or reduce the production of acetaldehyde (AA).

The main advantages of PET

Completely transparent

The product looks fresh, pure and healthy. Luminous PET bottles attract attention. The appearance of the product sparkles like a glass.

pure

The product tastes good. PET complies with international food-related regulations.

Safety

PET bottles are tough and do not break during the production, storage and transportation of products. Even if it breaks, it will only crack and will not break. It has high impact force and good tensile strength making it an ideal carbonated beverage packaging product.

Good barrier properties

PET has low permeability to oxygen, carbon dioxide, and water vapor, meaning it protects and maintains the integrity of the product, giving the product a good shelf life. PET also has good chemical stability.

Lightweight

Only 10% of the weight of the same glass packaging, PET bottles can reduce transport costs by 30%; and because of the thin wall of the product, can save 25% of the shelf space, compared with the same number of glass bottles. High-strength, low-weight PET bottles and glass bottles can be stacked at the same height.

No leakage

Because the injection molded bottle mouth makes absolute seal integrity possible. Seamlessly connected bottles cannot leak.

Flexible design

Suitable for all shapes, sizes, calibers, layouts and colors of containers.

Recyclable

Excellent environmental characteristics depend on the single material. Used PET bottles can be washed, repelletized and reshaped into PET bottles or re-used as raw materials for belts, blankets, fiberfills, etc. Specially designed rear wall bottles can be cleaned, refilled and reused. Like PET and paper, there are three elements (carbon, hydrogen, oxygen) and no toxic substances. Then only carbon dioxide and water are produced without any residue.

Good stability.

PET shows the best chemical stability in all the main polymers used for packaging.

Long shelf life

Very good shelf life performance, especially the new high barrier form.

Choose the right grade of resin to meet different packaging needs.

Although all grades are feasible, only one result from all walks of life tends to meet his needs.

In fact, PET is used in a wide variety of packaging, from soft drinks to cosmetics. However, the quality of the resin must meet the standards to protect the safety of products and distribution. Not only does the product need to maintain a good storage environment, it also has to be sold. In this case, the beverage food and its packaging must be attractive, sensible and convenient. The choice of resin for a certain PET application must meet the needs of the end product. Strength, transparency, barrier properties, and UV resistance are a few of the factors that are considered for packaging.

Both homopolymers and copolymers are applied to PET resins. In the PET homopolymer, only repeat units of terephthalic acid and ethylene glycol give the PET its highest melting point and best strength properties. The copolymer PET contains other monomers, such as isopeptide acids, which disturb the polymeric chains, lower the melting point, slow down crystallization, and increase transparency especially in thickness applications.

Different grades of resin are suitable for different end uses. These special features can create the ideal product in their design packaging. Which features of PET are the most important? According to the above, the characteristics of the most important ones are determined by the bottles that are packaged and packaged.

Carbonated soft drinks (CSD)

Strength: The resin used to package PET bottles for carbonated beverages has a definite quality requirement. This material must have very strong strength to control the pressure of carbon dioxide production without deformation or expansion. High-viscosity, low-polymerization resins can meet this demand.

Transparency: Transparency is an important factor in beverage packaging in terms of consumer acceptance. Low crystallinity can be made more transparent using PET.

Colour: Although one of the important properties of PET is a very clean material, it is not necessary for soft drinks because the drinks themselves are required to be coloured and they are packaged in coloured PET bottles.

water

Some water is packaged in the same bottle as soft drinks because there are many similar requirements, but there are some differences.

Strength: Unlike carbonated beverages, the bottle of water needs to be strong enough to maintain water requirements. In most cases the viscosity only requires 0.74 to 0.76.

Color and Transparency: Transparency is one of the most important reasons for packaging water. PET bottles for packaging water require a very sensation and a little bit of blue. Resins with high copolymerization levels increase transparency.

Pure: Water is an odorless product, so the requirements for plastics taste odorless, smelling odorless is necessary. Under normal circumstances, PET does not affect the taste of the product, so it is an important material for packaged water.

Juice - hot filling

Fruits and many other products, such as sports drinks, high-acid beverages, are hot filled in PET bottles. The packaging in this case is thermoset in order to provide the heat resistance of the container. PET resins with high glass transition temperatures and high crystallization rates are preferred. Low copolymerization levels are usually preferred, or a viscosity of 0.8 is acceptable.

Strength and Color/Transparency: This factor is not as important as water and carbonated beverages. The purity factor is very important. If the bottle is not pure enough, the juice may taste like plastic.

Beer ----- new demand for new markets

General beer is seen as an extension of the carbonated beverage market, but it does require hot filling and high barriers. The technology used in the manufacture of beer bottles makes beer bottles of many types and applications (cold filling or pasteurization) difficult to find common ground. Making production quality meet predetermined requirements has always been the most important factor. Typical types of beer used are: non-tunnel pasteurization, one-way tunnel sterilization and recyclable bottles.

Barrier properties: Beer bottles require a higher barrier to carbon dioxide and oxygen than PET's packaged carbonated beverages. The degree of demand depends on the type of beer, the size of the container, the distribution channels and the environmental conditions (storage time, temperature and humidity). Improved barrier properties can be obtained by coatings, multilayer structures, and deoxidizers.

Color, transparency, UV barrier: In addition to those barrier requirements, most beer needs to prevent UV light. This protection can be obtained by coloring or adding anti-UV additives, which can be added during the injection phase of PET preforms. Because of beer's various needs, resins must be selected to provide adequate barrier, UV resistance, and transparency.

Strength: Beer bottles require sufficient strength to maintain the pressure of carbon dioxide and pasteurize at a wide range of temperatures. Viscosities in the range of 0.8 to 0.84 are generally suitable. The beer prefers to use a champagne base rather than a petal-like base. Under all these conditions, PET is a resource that is effectively used in consumer beverages. An indispensable reason is that PET is lightweight. Not only does it benefit the consumer, but also the reduction in transportation costs and the reduction in energy consumption during turnover are all due to weight. Because PET is lighter than alternatives, a truck can hold more than 60% of the beverage, and reduce 80% of the packaging, fuel savings of 40% and reduce air pollution.

PET preform/bottle production

Conventional PET bottle forming machines are divided into two types, one-step "thermoforming" and two-step "cold forming".

One-step method: All the processes from particle to bottle are done on one machine. This means that the injected semi-finished preform is removed from the mold cavity when it is hot, and then blown into a bottle. Later, there is no need to store a large number of preforms for blow molding and no additional heating of the preforms; and there is no risk of damaging the surface of semi-finished products during transportation and storage, each time being the latest molding. One law is very suitable for small and medium-sized production lines.

The two-step method uses two separate machines. First of all injection molding semi-finished products, then the second step is to reheat and blow molding. The two-step system uses two separate machines: an injection molding machine to make semi-finished products, and a reheat blow molding machine to reheat semi-finished products from cold to semi-finished products and blow molded bottles. Heating semi-finished equipment systems means a lower thermal efficiency of the two-step process. This method is more suitable for large and medium-sized production.

A two-step process is usually used in the production of PET bottles.

Drying of PET

PET absorbs moisture from the air. Moisture must be removed by dehumidifying drying before production.

PET plasticization

The dried PET particles are squeezed and melted by the screw.

PET finished product injection molding

Dissolved PET is injected into the injection chamber and rapidly cooled to form a semi-finished product. (The semifinished product of the blow-molded bottle is in the form of a test tube)

Heated PET semi-finished products

The temperature of the semifinished product is adjusted by correcting the parameters of the blowing bottle.

Blow molding PET container

Hot semi-finished products are simultaneously pulled in the cavity to form a tough, lightweight container. The PET is heated to a temperature at which point the extended PET chain molecules are broken to fully flow and spread out, which can be directed to stretching. Stretching is applied in two orthogonal directions, and in the blow-blow molding method, a two-axis direction is given. In the stretching direction, the oriented PET is controlled closely together in a straight line. Because the individual molecules are linked together, the material strength is higher. The tensile strength of oriented PET is several times that of non-stretch, and the impact strength, barrier properties, and chemical stability are also well improved, so the bottle can be lighter without sacrificing performance.

Discharger for PET container

The finished container is discharged.

Acetaldehyde content

What is acetaldehyde? Acetaldehyde (AA) is a compound found in citrus fruits and other foods. It has a distinctive acidic taste. AA will also be produced during the production of PET bottles.

How does AA affect the product? With the popularity of PET containers, water companies are more concerned about AA and how it affects the taste of the product. In most cases, the smell of AA was barely noticeable during the consumption of beverages. For example, a very sweet and intense odor in gas soft drinks masks the slight taste of that plastic. However, the extremely weak taste of pure water is not enough to mask acetaldehyde, and it is easy for consumers to notice the appearance of AA. When people choose the brand of water, they always put the taste first. They look for cleanliness that cannot be found in tap water. Water Company retains the loyalty of consumers and fulfills its brand promise by providing products with consistently high data. Even slight differences in taste will compromise the promise of the brand and the impact of sales.

How does AA occur in PET bottles? In the bottle manufacturing process, resin particles are dried and melted at high temperatures (over 260°C) and injection molded into semi-finished products. The semifinished product is reheated and blown into a container. In the melting phase AA was generated.

Optimizing the molding conditions by minimizing the melting temperature and maintaining the minimum melting residence time helps to produce the least AA. Although not completely eliminated. AA produced in the production process is left in the polymer matrix of the bottle and the container. As AA will volatilize at room temperature, it will eventually migrate to the atmosphere and the contents of the package, affecting the taste. The migration of AA to quantity is directly related to storage time and temperature. There are many ways to effectively reduce the level of AA so that taste is not affected. First, select the right PET resin. The resin specifically adjusts the effective performance according to the needs of the water container. For example, Voridian Aqua PET 18696 and 20837 resins are considered representative of low levels of AA. Each has a low viscosity value between 0.72 and 0.76. The low viscosity value minimizes the production of AA during the melting process.

Because of the importance of the performance of the bottles, these resins are designed to not reduce the performance of the final bottle for low molecular weight.

Of course, the water market hopes that the level of AA sought will be even lower than the most advanced polymers allowed. In this case, the low AA resin combines with the AA clear agent. The AA scavenger is mixed in PET when the PET is melted during injection molding. The scavenger reaction, or combined with AA, locks it with the polymer matrix and cannot migrate into the product. The scavenger can further reduce the level of AA in the semifinished product and can reach 70-80%. Choosing the right PET to solve the reasonable balance between low AA and high performance is very important. This produces bottles with excellent styling and optical properties. Low AA resins can mix colors well, and dyed resins can produce many products with a look and feel to support the brand.

PET recycling

PET can be recycled completely, and it is convenient. Its recycling code is 1, and post-consumer recycled PET (PRC PET) can be used to make clothes and carpet fibers, or it can be used to make fibers, for example as a filler for pillows. Recycled PET can be used for non-food bottles, such as cleaning products. Containers for the manufacture of food and beverages do not have recycled PET. It must go through an approved process to ensure that there are no contaminants, and must retain sufficient raw properties to meet the final quality requirements.

A simple and popular method is to simply break the PET formed product and then mix the broken PET particles into the new PET resin, but the mixing ratio cannot exceed 20%. The high repolishing ratio will make the melt viscosity, color, and odor different. When using recycled materials, magnetic hoppers are recommended for injection molding machines.