Why Is PET Called Chips Instead Of Pellets? What Is The Difference Between Bottle-grade And Fiber-grade PET Chips?

Everyone knows that the common plastic shape is "granules", but some plastics, such as PA nylon and PET polyester, are not actually called granules.

They have a professional-sounding name called "slices". Why?

First of all, their similarities are that they are all "granules" with regular dimensions and uniform size. The dimensions are different from "powder" and "chips", and they have the following advantages:

Figure: Recycled plastic particles	Figure: PET slice

The material enters the screw from the granular form, is melted, plasticized, and then molded. Because the general plastic raw material transportation is mainly vacuum-based, the loss of powder during transportation and processing is quite large. Large, irregular chips often cause "bridging" of the feed port and affect continuous production. It's just that some products have to be processed with powder due to process restrictions.

Of course, granules are convenient for transportation. They are packed in a sack and then loaded onto a truck and transported to the destination. If it is in powder form, many materials are hygroscopic, and the surface area of ​​the powder itself is relatively large, which makes it easier to agglomerate and even degrade, which is not conducive to the processing itself. Large, irregular chips have the same weight and much larger volume, which is a headache to think about!

When using plastic granules, granules can make the material heat evenly, increase plasticization evenly, make subsequent mixing with other additives convenient, and make the product smooth.

The molding of polymer materials is often not as simple as physical action. Since the diffusion path of small molecular products in small particles is shorter than that in large particles, they are easy to exclude. In addition, the sample particles are small, the total surface area of ​​the particles increases, the heat transfer rate increases, and the reaction rate is also accelerated. Therefore, within a certain range, the reaction speed is inversely proportional to the size of the raw material particles. However, if the particles are too small, they are prone to bonding, which in turn affects the reaction rate. Therefore, when making materials, "granular" is generally selected.

Why is it called PET chips instead of PET pellets?

In English, chips are called chips and pellets are called particles. The biggest difference between the two is that they are formed in different shapes. Common "pellets" are cylindrical, and most plastics are formed into pellets, such as PE, ABS, PP, etc. They are generally made by strand granulation, so they are cut into cylindrical shapes.

Nylon (crystalline) and polyester are crystalline polycondensates. The melt strength of their resins is very low and cannot be pelletized by stranding. They can only be pelletized by underwater hot cutting or air cooling. The particles produced in this way are in the form of flakes, which are called slices. Therefore, slices generally refer to nylon, polyester and other resins produced by polymerization plants or resins that have been modified by a small amount of additives (such as glossy, semi-glossy and matte PET slices modified by adding titanium dioxide). Generally, nylon or polyester that has been modified downstream, such as adding GF, filling, and toughening, is no longer called a slice, but a particle.

Strand granulation	Air-cooled pelletizing

How to distinguish PET chips from fiber-grade PET chips?

At present, there are two main aspects of research on the recycling of bottle PET chips:

First, the research on bottle-to-bottle re-production technology, which converts cleaned bottle flakes into food-grade slices that can replace fresh materials and directly contact food;

Second, bottle flake PET recycled materials have been widely used in the production of staple fibers and non-woven fabrics, or qualified polyester filaments can be spun through the selection of appropriate processes. For example, bottle flake PET treated with patented crystallization technology can be directly used for spinning. The technical performance of recycled PET staple fibers processed by this process can basically reach the standard slice spinning quality. Pan Wanlian and others from Donghua University used bottle flake PET recycled materials imported from Japan to spin polyester filaments under appropriate process conditions.

However, what is the difference between bottle-grade PET slices and fiber-grade PET slices? The following is a discussion of their differences based on the four major performance parameters of PET.

Intrinsic viscosity is often used to characterize the molecular weight of polyester. The intrinsic viscosity test is based on the friction generated by the movement of macromolecules in the solvent. The greater the resistance of the polymer solution during the flow process, the greater the melt viscosity. Viscosity is a quality indicator that reflects the molecular weight of PET chips, and it is also the most important quality indicator of PET chips.

Both bottle-grade PET and fiber-grade PET melts are non-Newtonian pseudoplastic fluids, and the apparent viscosity decreases with the increase of shear rate and temperature. Under the same conditions, the apparent viscosity of bottle-grade PET melt is higher than that of fiber-grade PET, and the amplitude of apparent viscosity fluctuation is smaller than that of fiber-grade PET. The non-Newtonian index of both melts decreases with the increase of shear rate, and they deviate more and more from the flow characteristics of Newtonian fluids. Under the same temperature conditions, the elasticity of bottle-grade PET melt is greater than that of fiber-grade PET, the degree of structuring of the melt is higher than that of fiber-grade PET, and the spinnability is poor.

Generally speaking, the bottle strength corresponding to the slice with high viscosity is also higher, and it can withstand pressure and impact; the bottle strength corresponding to the slice with low viscosity is also lower, and the pressure and impact resistance are low.

The viscosity of the popular carbonated beverage bottle-grade slices (BG85) on the market is about 0.87dl/g, the viscosity of the hot-filled beverage bottle-grade slices (BG801) is about 0.78dl/g, the viscosity of the mineral water bottle-grade slices (BG80) is about 0.80dl/g, and the viscosity of the edible oil bottle-grade slices (BG802) is about 0.83dl/g. Since the melt temperature of high-viscosity slices is high during processing, the viscosity will also increase accordingly. Therefore, the viscosity of bottle-grade PET slices is not the higher the better, as long as it meets the strength requirements of the product.

The viscosity of PET slices is about 0.70dl/g, which can reach the level of spinning civilian silk. The most important thing is that the fluctuation of the characteristic viscosity should be as small as possible. Large fluctuations in viscosity are prone to poor molding, floating silk, twisting, and many defects, which seriously affect the spinnability. In addition, the viscosity of oil-free silk fluctuates greatly, and the fiber strength fluctuation will also increase.

The melting point characterizes the temperature at which the polymer chain moves freely. A high melting point means that the impurity content in the recycled PET is low, but a higher spinning temperature is required to make the melt flow better. If the melting point is too low, the impurity content is high, the tensile properties of the fiber are poor, and the strength and modulus are low.

The production process of bottle-grade chips and fiber-grade chips is basically the same, but in order to meet some properties of bottle-grade chips, a third monomer, isophthalic acid (IPA) and some additives are added. The addition of the third monomer reduces the melting point of the chips and the crystallization rate of the melt, improves the transparency of the bottle, and the average relative molecular weight of bottle-grade PET chips is generally more than 30,000, while that of fiber-grade PET is about 15,000 to 22,000.

Isophthalic acid is added to bottle-grade chips during the production process to achieve the purpose of copolymerization modification, which increases the flexibility of the bottle chips and reduces their crystallization properties, which is conducive to the processing and molding of bottle blanks. At the same time, for every 1% increase in the isophthalic acid content, the melting point of the chips will drop by 3~3.5℃. The effect of isophthalic acid on the intrinsic quality of slices is mainly to change their crystallization properties.

Different products have different requirements for the crystallization properties of bottle slices, so the content of isophthalic acid cannot be generalized. For example, the edible oil bottle preform is heavy and thick (generally around 110 grams), and the preform cooling speed is slow, so the crystallization properties of the bottle slices are required to be as low as possible, that is, the isophthalic acid content should be as high as possible; and hot-filled beverage bottles need to withstand 90°C.

There are currently two representative views on the relationship between the end carboxyl content and the quality of PET slices. One is that the end carboxyl content reflects the degree of thermal degradation of the slices, so the end carboxyl content should be appropriately lower; the other concept is that the end carboxyl content indirectly reflects the molecular weight distribution of the slices. Bottle-grade PET slices are used for plastic processing and require a slightly wider molecular weight distribution, so the end carboxyl content should be appropriately higher.

Combining the opinions of both parties and the actual end carboxyl content of domestic bottle-grade PET slices, the end carboxyl content of bottle-grade PET slices is generally between 20 and 35 mol/t.

A high content of terminal carboxyl groups indicates a wide molecular weight distribution. During the spinning process, the degradation of macromolecules is aggravated after heating, and the spinnability is poor. The terminal carboxyl content of fiber-grade PET chips is generally below 30 mol/t.

Ash content indicates the content of inorganic components in polyester resin chips. In addition to dust, its source mainly comes from various additives added during the polyester production process, such as catalysts, heat stabilizers, toners, and inorganic impurities in the chips. The ash content caused by inorganic components will reduce the thermal stability and thermal oxygen stability of the chips, which may lead to increased viscosity and reduced product quality during processing.

The ash content has a great influence on the performance of carbon fiber. The high ash content of carbon fiber will increase fiber defects and affect the ablation performance of the fiber. Therefore, the ash content is an important indicator in the performance test of carbon fiber. The ash content of fiber-grade PET should not be higher than 0.025%. In the production of PET bottles, ash is also the crystal nucleus of crystallization to a certain extent, so high ash content can easily cause the bottle blank to fog. Ash content will also affect the color of the bottle blank to a certain extent.