Rotomolding processing knowledge of PE and composite materials

Polyethylene is a macromolecular compound obtained by addition polymerization of ethylene. Actual molecular weight varies from 10,000 to several million depending on the polymerization conditions. The first invented polyethylene was a low density polyethylene obtained by a high pressure method and had a specific gravity of 0.91 to 0.925 g/cm3. The specific gravity of the polyethylene obtained by the low-pressure and medium-pressure processes is then 0.94l to 0.965g/cm3, which is referred to as high-density polyethylene. Polyethylene is a white waxy translucent material that is soft and tough, slightly stretchable, non-toxic, flammable, melts and drips when burned, and emits odor when paraffin burns. The properties of polyethylene are related to its molecular weight and also to its crystallinity.

Many of the mechanical properties of polyethylene are determined by the material's density and melt index. From low density polyethylene to high density polyethylene, the density varies from 0.90 to 0.96 g/cm3. The melt index (melt flow index) of polyethylene varies widely and can range from 0.3 to 25.0. Many important properties of polyethylene vary with density and melt index.

Polyethylene has a low glass transition temperature of 125°C, but it maintains its mechanical properties over a wide temperature range. The equilibrium melting point of linear high molecular weight polyethylene is 137°C, but it is generally difficult to reach an equilibrium point, and the melting point during processing is usually 132-135°C. The ignition temperature of polyethylene is 340°C, the auto-ignition temperature is 349°C, and the dust ignition temperature is 450°C. The melt index of polyethylene is determined by its molecular weight. When the polyethylene materials of different molecular weights are mixed, the melt index also takes certain values ​​according to a certain rule.

Polyethylene is water resistant and does not change its physical properties in high humidity or water. Concentrated sulfuric acid, concentrated nitric acid and other oxidants can slowly attack polyethylene. In aliphatic, aromatic, and chlorinated hydrocarbons, polyethylene swells, but the swelling agent can recover its original performance after volatilization. Below 60°C, polyethylene is resistant to most solvents, but hydrocarbon solvents can quickly attack polyethylene at temperatures above 70°C. As the temperature continues to increase, the polyethylene dissolves in certain solvents. The polyethylene separated from the solution forms a paste or colloidal state upon cooling depending on the temperature.

Polyethylene is susceptible to photo-oxidation, thermal oxidation, ozonolysis, and halogenation reactions. Due to chemical inertness and non-polarity of the surface, polyethylene is difficult to bond and difficult to print. However, after oxidant, flame and corona discharge treatment, polyethylene will have good adhesion and printing performance.

When polyethylene undergoes irradiation, cross-linking, chain scission, and formation of unsaturated groups occur, but the main reaction is cross-linking. When polyethylene is irradiated in an inert gas, it will produce an overflow of hydrogen and lose weight. Irradiating polyethylene in air will increase the weight due to the addition of oxygen. Unsaturated groups are added to the polyethylene molecules after irradiation, resulting in a decrease in oxidation stability. The cross-linking reaction of polyethylene overcomes the effects of chain scission and formation of unsaturated groups, and the cross-linking reaction can improve the weatherability of polyethylene. Therefore, irradiated polyethylene products are higher than unirradiated polyethylene. Products have better weatherability.

Polyethylene is slowly degraded by the action of oxygen in the air. This process is accelerated by the effects of heat, ultraviolet light, and high-energy radiation. Degradation aging is characterized by the product discoloring, brittleness, and damage. Carbon black has a significant light shielding effect on polyethylene. Adding 2% of carbon black can effectively increase the service life of polyethylene products. In addition to carbon black, the addition of certain UV absorbers to polyethylene can also have an anti-aging effect.

Polyethylene plastics have poor thermal conductivity. In order to allow the heat to be introduced into the entire volume of the plastic powder particles quickly during the rotomoulding process, the particle size of the polyethylene powder used for rotomoulding should meet certain requirements. The smaller the particles, the more easily the heat is introduced and the easier the temperature of the material reaches its melting point. However, when the particles are too small, the material is easily hygroscopic and agglomeration is not conducive to the tumbling movement in the mold. Polyethylene plastics that are commercially available are often pellets that need to be ground and sieved to meet rotomoulding process requirements.

Polyethylene is a relatively tough plastic that, when processed using a conventional attrition mill, will have its granules pulled into a shape that is not conducive to regrind. The pulverization of polyethylene pellets requires special high-speed shredding equipment.

Key elements in the process of polyethylene rotomoulding

1, release agent

During the heating phase of the rotomoulding process, chemical or physical bonding occurs at the interface between the polyethylene powder or the melt and the inner surface of the mold due to surface oxidation. When the inner surface of the mold has local defects, the polyethylene melt will flow into these defects and form a partial inlay. This will make it difficult to remove the product after cooling from the mold. In order to avoid this, it is necessary to apply a layer of heat-stable material on the inner surface of the mold to prevent sticking. This type of material is called release agent. There are many types of industrial mold release agents. The rotomolding process of polyethylene has higher requirements for mold release agents, which are mainly requirements for heat resistance. Oils, waxes, and silicone oils are commonly used mold release agents, but they need to be applied one time before each addition. Therefore, they are called disposable release agents. Such mold release agents have low cost and good demoulding effect, but are easy to migrate to the surface of products and affect their surface properties. Cross-linked silicone is a semi-permanent release agent, it does not need frequent painting, migration will not occur, will not be affected by temperature changes, has a good effect of stripping, but the cost is higher.

A permanent layer of polytetrafluoroethylene (combined with a commercially available non-stick pan) is applied to the surface of the mold cavity to achieve a permanent release effect. PTFE is a permanent release agent.

2, temperature control

The rotomolding process of polyethylene has a special phenomenon: During the powder melting process, the air trapped between the powder particles forms bubbles, and these bubbles disappear as the heating process continues. Further studies have shown that the disappearance of these bubbles is not due to the fact that they move to the free surface of the melt under buoyancy, but because the air in the bubbles gradually fuses in the melted plastic melt. Experiments have shown that bubbles of different sizes are formed in the polyethylene melt when the temperature is raised to 150°C. Due to the large viscosity of the polyethylene melt, the buoyancy of the bubbles is not sufficient to push the bubbles towards the free surface. When the temperature rose to 200 °C, all the bubbles disappeared. Therefore, for the rotomolding of polyethylene, scientifically controlling the heating process has very important significance in eliminating air bubbles in polyethylene products and improving product quality. As the rotomoulding heating time is sometimes longer, especially when the product wall is thicker. It may last from half an hour to more than one hour. At this time, it is required to take measures to prevent the thermal oxidation and material properties of the material during the heating process. Generally, the addition of antioxidants to the polyethylene plastic can achieve the purpose of prevention. However, when the polyethylene material is heated to an excessively high temperature or the heating time is too long, the antioxidant does not prevent the oxidation of the material. When the thickness of the article needs to be heated for a long time, the heating temperature must be reduced. If the heating time is shortened by increasing the temperature, there is a possibility that the air bubbles remain because the air in the bubbles disappears. When the polyethylene plastic is heated to the molten state, the material will undergo a process of conversion from the crystalline state to the melt, which is exactly what happens when the polyethylene particles begin to melt and soften. It first appeared in a layer of material in contact with the inner wall of the mold, forming a uniform layer of molten material. Then, gradually expand to the inner layer until the entire cross section completely becomes a plastic melt. The next step is to continue heating and gradually disappear the bubbles. The temperature control and time control of this process need to be adjusted.

3, cooling process

During cooling, the temperature of the polyethylene melt will decrease from 200°C to near room temperature, and the molecules of the polyethylene will change from a disordered state to a more ordered crystalline state. The process of crystallization takes some time, and the speed of crystallization is related to the viscosity of the polyethylene melt. When the polyethylene melt is rapidly cooled, the viscosity of the polyethylene melt rapidly increases, so that the growth of crystal grains is hindered, thereby affecting the crystallinity of the polyethylene. When the degree of crystallinity is different, the density of polyethylene products will be different, and the physical properties will also be different. As a result, rapidly cooled polyethylene rotomoulded articles have a lower density, while slow-cooled articles have a higher density. Of course, the slower the product is cooled, the longer the production cycle and the higher the cost. The polyethylene powder used for rotomoulding production itself has a certain density, which is determined by the manufacturer of the material. However, after rotomolding production, the density of polyethylene rotomoulded products will change due to different cooling rates.