Raw materials for rotomizing processes

At present, the common rotoplastic raw materials on the market include the following:
Polyethylene (PE)
Polypropylene (PP)
Nylon (PA)
Polyvinyl chloride (PVC)
Polycarbonate (PC)
Not all of the plastics mentioned above can be used for rotomolding. Rotoplastic requires specially designed materials. Basic requirements include:
Easy to grind (or stay liquid). With the use of high performance room temperature mill and low temperature mill, we can already handle the common rotoplastic raw materials such as polyethylene and polypropylene, and the cost is constantly reduced.
The right liquidity. Taking commonly used polyethylene raw materials as an example, the fusion finger (MI, or MFI) range should generally be between 2 and 10 (g /10 minutes), and the optimized fusion finger range is 3-6 (g /10 minutes). The melting finger is too low, the product is difficult to form; If the melting finger is too high, the physical properties of the product will decline.
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Polyethylene (PE) raw material
PE is widely used in the rotomolding process for a reason:
PE has a wide processing window, which is suitable for a long time in a high temperature environment, reducing the requirements for rotomolding machinery;
Second, at room temperature, PE will not react with water, most fats, acids and alkaline substances, and has a wide range of application;
Third, PE raw materials have low cost and are easy to promote.
Because the orientation of the molecular structure of polyethylene is too strong, the performance in the vertical direction is relatively weak. In order to improve this situation, copolymeric monomers have been introduced into polyethylene production to improve the degree of branching of polyethylene. Common copolymeric monomers include butene (C4), hexene (C6) and octene (C8). With the increase of the carbon number, the length of the branch chain in the polyethylene molecule increases, and many properties will be significantly improved at the macro level, such as impact strength, toughness, and ESCR (environmental stress resistance, which refers to the failure of plastic products under the action of long-term external forces). In addition, with the increase of the proportion of copolymer, the overall density of polyethylene decreases.
On the other hand, the distribution of molecular weight of polyethylene also affects its performance. Polyethylene is a mixture of various molecular chains of different lengths. Generally speaking, the shorter the molecular chain length, the better the fluidity, the higher the melting finger; Otherwise, the melting finger decreases. Secondly, the wider the molecular weight distribution, the easier the raw material is to process (because the low molecular weight part can play the role of plasticizer), but the performance of the product is relatively weak.
The distribution of molecular weight is mainly determined by the polymerization device of polyethylene and the type of catalyst used.
Another important factor is the crystallinity of polyethylene. Crystallization is the process by which polyethylene molecular chains fold cambium crystals and then crystallize. The shape is spherical, so it is also called spherulite. Under a certain stress, spherulite is elastic and can be restored to its original state after the force is reduced. But to a certain strength, the spherulite will disintegrate into a fiber, this process is irreversible, this strength is the yield strength. The difference in the crystallinity of polyethylene will be reflected in the difference in density: the higher the crystallinity, the higher the density of polyethylene. At the same time, the melting point, tensile strength and other physical properties will be improved; Also, some attributes are reduced accordingly, such as ESCR.
Under the combined action of the above factors, linear polyethylene shows two key indicators - melting finger and density.
Fusion fingers can be used to evaluate the flow properties of raw materials. In general, the American Society for Testing Materials (ASTM) D-1238 standard, or the Organization for Standardization (ISO) 1133 standard, is often used to determine the fusion finger. The test conditions specified in the two standards are slightly different, but in general, they can be easily compared. The test conditions are: the weight of the raw material extrudated from a thin tube in grams per 10 minutes (g/10min) at a temperature of 190 degrees and a weight pressure of 2.16 kg over a period of 10 minutes.
The density is generic and measured under ASTM D1505 or ISO1183 in grams per cubic centimeter (g/cm^3).
At the same time, these factors also determine the other physical properties of polyethylene, such as melting point, tensile strength, tensile elongation, elastic modulus, and so on.
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Polypropylene (PP) raw material
In the consumption structure of synthetic resins, polypropylene is the second most common raw material after polyethylene. Compared with polyethylene, polypropylene has the following characteristics:
Low density: The density of PP is roughly between 0.85-0.93, while ordinary polyethylene is between 0.91-0.98. One of the reasons is that the crystallinity of PP is lower than that of PE;
Good mechanical properties: the tensile strength and elastic modulus of PP are generally higher than PE. At present, modified PP can even be comparable to PS (polystyrene) mechanical properties, widely used in electronic appliances and automotive fields;
Good optical performance: compared with PE, PP transparency is much higher;
High temperature resistance: the melting point of PP is about 160-170 degrees, which is much higher than the 100-130 degrees of PE. Therefore, it can be used in higher temperature environment;
Low temperature resistance: below zero, the impact strength of PP is low, not suitable for use in low temperature freezing environment;
Good tolerance: PP than PE water resistance, chemical corrosion resistance, acid resistance, alkali resistance are improved, more suitable for the production of chemical containers;
Poor aging performance: PP is easy to oxidize and degrade in the environment of sunlight (ultraviolet light, heat). Therefore, it is not suitable for long-term outdoor use.
The production of PP also requires the participation of catalysts, and the catalyst is still the ZN catalyst mentioned earlier. However, PP products produced with metallocene catalysts have also appeared on the market.
Like PE, PP obtained by polymerization of propylene monomer is called homopolymer polypropylene; The polypropylene obtained by polymerization with other monomers (commonly ethylene) is called copolymer polypropylene, and copolymerization is divided into block copolymerization and random copolymerization.
According to the arrangement of methyl groups in propylene, PP can be divided into three kinds: isotactic, intertactic and random. Atactic polypropylene cannot crystallize, so its transparency is the highest in PP.
In the rotundizing, the application of PP has not been expanded, mainly for the following reasons:
The low temperature embrittlement temperature limits many applications.
PP grinding is difficult and needs to be carried out in a low temperature environment, which is not conducive to the development of PP rotoplastic raw materials;
In order to improve the resistance of PP to high temperature and ultraviolet light, some special additives need to be added to PP to improve its performance.
The suitable PP processing temperature range is very narrow, which puts forward high requirements for process control.
Despite these unfavorable conditions, considering the advantages of PP in elastic modulus, chemical resistance and transparency, many suppliers are also trying to develop corresponding PP rolling plastics, and have been commercially available, such as TPS-D-0023 (high transparency type) and TPS-D-0026 (impact strength improvement type) launched by Total.
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The rapid development of the rotomolding industry not only requires product design to be creative, functional and systematic, processing equipment with large automation, precision and energy saving, will also drive the diversification and functional development of rotomolding raw materials. At present, functional polyolefin materials such as temperature resistant and high impact polyethylene for rotoplastic, polyethylene for steel lining coating and lightweight foamed polyethylene have developed rapidly in China, which greatly expands the development and application of rotoplastic products in the field.