What Are The Raw Materials That Can Be Used In The Rotational Molding Process?
Currently, common rotational molding 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 rotational molding. Rotational molding requires specially designed materials. Basic requirements include:
Easy to grind (or easy to keep in liquid form). Using high-performance normal temperature grinding mills and low-temperature grinding mills, we can already process commonly used rotational molding raw materials such as polyethylene and polypropylene, and the cost is also continuously reduced.
Proper liquidity. Taking commonly used polyethylene raw materials as an example, the melt index (MI, or MFI) range of a brand should generally be between 2 and 10 (g/10 minutes). The optimized melt index range is 3-6 (g/10 minutes). ). If the melt index is too low, the product will be difficult to form; if the melt index is too high, the physical properties of the product will decrease.
Polyethylene (PE) raw materials
PE is widely used in rotational molding processes for a reason:
, PE has a wide processing window and is suitable for long-term high-temperature environments, which reduces the requirements for rotational molding machinery;
Second, at room temperature, PE does not react with water, most greases, acids and alkaline substances, and has a wide range of applications;
Third, PE raw materials are low in cost and easy to promote.
Because the molecular structure of polyethylene is too oriented, its performance in the vertical direction is relatively weak. In order to improve this situation, comonomers are introduced into polyethylene production to improve the degree of branching of polyethylene. Commonly used comonomers include butene (C4), hexene (C6) and octene (C8). As the carbon number increases, the branch length in the polyethylene molecule increases, and many properties will be significantly improved macroscopically, such as impact strength, toughness, and ESCR (environmental stress resistance), which refers to the effect of plastic products on long-term external forces , damage may occur), etc. In addition, as the comonomer ratio increases, the overall density of polyethylene decreases.
On the other hand, the molecular weight distribution of polyethylene will also affect its properties. Polyethylene is a mixture of molecular chains of various lengths. Generally speaking, the shorter the molecular chain length, the better the fluidity and the higher the melt index; otherwise, the melt index decreases. Secondly, the wider the molecular weight distribution, the easier it is to process the raw material (because the low molecular weight part can act as a plasticizer), but the performance of the product is relatively weak.
The main determinants of molecular weight distribution are the polyethylene polymerization device and the type of catalyst used.
Another important factor is the crystallinity of polyethylene. Crystallization is the process in which polyethylene molecular chains fold to form layer crystals and then crystallize. The shape is spherical, so it is also called spherulite. Under a certain stress, the spherulites are elastic and can return to their original shape after the stress is reduced; but at a certain strength, the spherulites will disintegrate into fiber shapes. This process is irreversible, and this strength is the yield strength. The difference in 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, physical properties such as melting point and tensile strength will also increase; similarly, some properties will decrease accordingly, such as ESCR, etc.
Under the combined effect of the above factors, linear polyethylene exhibits two key indicators - melt index and density.
Melt index can be used to evaluate the flow properties of raw materials. In general, the American Society for Testing and Materials (ASTM) D-1238 standard or the Standardization Organization (ISO) 1133 standard is often used to determine the melt index. There are subtle differences in the test conditions specified by the two standards, but in general, they can be compared simply. The test conditions are: at a temperature of 190 degrees, under a weight pressure of 2.16 kg, the weight of the raw material extruded from a thin tube within 10 minutes, the unit is grams/10 minutes (g/10min).
Density is universal, measured under ASTM D1505 or ISO1183 conditions, in grams per cubic centimeter (g/cm^3).
At the same time, these factors also determine other physical properties of polyethylene, such as melting point, tensile strength, tensile elongation, elastic modulus, etc.
Polypropylene (PP) raw materials
In the consumption structure of synthetic resins, polypropylene is the 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 that of 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 that of PE. At present, the mechanical properties of modified PP can even be comparable to that of PS (polystyrene), and it is widely used in electronic appliances and automobile fields;Good optical properties: Compared with PE, PP has much higher transparency;
High temperature resistance: The melting point of PP is around 160-170 degrees, which is much higher than PE's 100-130 degrees. Therefore, it can be used in higher temperature environments;
Low temperature resistance: below zero, PP has low impact strength and is not suitable for use in low-temperature freezing environments;
Good tolerance: PP has better water resistance, chemical corrosion resistance, acid resistance and alkali resistance than PE, making it more suitable for the production of chemical containers;
Poor aging performance: PP is easily oxidized and degraded in an environment exposed to sunlight (ultraviolet, heat). Therefore, it is not suitable for long-term outdoor use.The production of PP also requires the participation of a catalyst, which is still the ZN catalyst mentioned earlier. However, PP products produced using metallocene catalysts have also appeared on the market.
Like PE, PP obtained based on the polymerization of propylene monomer is called homopolymerized polypropylene; while polypropylene obtained by polymerizing with other monomers (usually ethylene) is called copolymerized polypropylene. Copolymerization is divided into block copolymerization and Random gatherings.
According to the arrangement of methyl groups in propylene, PP can be divided into three types: isotactic, syndiotactic and random. Atactic polypropylene cannot crystallize, so its transparency is the highest among PP.
In rotational molding, the application of PP has not been expanded, mainly for the following reasons:
The low temperature embrittlement temperature is very low, which limits many applications;
PP grinding is difficult and needs to be carried out in a low-temperature environment, which is also not conducive to the development of PP rotational molding raw materials;In order to improve PP's resistance to high temperatures and ultraviolet rays, some special additives need to be added to PP to improve its performance;
The suitable PP processing temperature range is very narrow, which places high requirements on process control.Despite these unfavorable conditions, considering the advantages of PP in elastic modulus, chemical corrosion resistance, and transparency, many suppliers are also working hard to develop corresponding PP rolling plastics and have already commercialized them, such as TPS-D launched by Total -0023 (high transparency type) and TPS-D-0026 (impact strength improved type), etc.