A Polyolefin Coating is a type of coating that is applied to a surface in order to protect it. This coating is typically applied to surfaces that are subject to wear and tear, such as floors and countertops. Another large application is in the field of industrial infrastructure. Polyolefin coatings are also used in a variety of other settings, including automotive and marine applications.
Polyolefin coatings are made from a variety of materials, including polyethylene, polypropylene, and polystyrene. These materials are combined together to create a tough, durable coating that is able to withstand a variety of conditions. Polyolefin coatings are also available in a wide range of colors, which allows for customization depending on the needs of the user.
One of the benefits of using a polyolefin coating is that it is environmentally friendly. These coatings are non-toxic and biodegradable, which means they can be safely used in environmental applications such as pipelines and bridges,
Different properties of polyolefin coatings
When making pipe coatings, we need to find a balance between the different properties of the PE resin. We have to make sure it is strong and easy to process. There are different grades of PE, classified as low density (LDPE), medium density (MDPE), and high density (HDPE).Low density PE (<0.930 g/cm³) is not advised for engineering use because it generally has low strength, fluid permeability, and resistance to stress cracking. Its molecular structure features many multiple branches off the main PE chain.
High density polyethylene has greater yield strength. This means it can hold more weight before breaking. Initially, catalytic polymerisation only formed long chains of molecules. But now we can make them shorter and wider. The shorter the chains, the lower the melt viscosity. The Melt Mass-Flow Rate (MFR) is a way to measure how thick the melt is (4,5). The test measures how much (grams) of molten PE at 190°C extrudes through a 2.1mm diameter hole in 10 minutes under a load of 2.16kg; though 5kg is sometimes used. At 2.16kg, MFR can be between 0.1 and 45 for PE.
How to determine different properties of Polyofin coatings
MFR is inversely proportional to melt viscosity. In other words, the lower the MFR, the higher the melt viscosity. The relationship isn’t linear, but exponential. So a small change in MFR can result in a large change in melt viscosity. Melt Index (MI) is also a way of measuring melt viscosity. It is the amount (in grams) of molten material that flows out of a die in 10 minutes at 190°C under a load of 5kg. The lower the MI, the higher the melt viscosity.
The difference between MFR and MI is that MFR is measured at 2.16kg, while MI is measured at 5kg. So MI gives a higher viscosity reading. This is why MFR is generally used to characterise polyolefins.
The main thing that affects the MFR of a PE is the degree of shortening and fattening (branching) of the chains. The longer and straighter the chains, the higher the MFR. The shorter and more branched the chains, the lower the MFR. So, to make a PE with a low MFR (high viscosity), we need to use a highly branched polymer.
The other main thing that affects MFR is molecular weight. In general, the higher the molecular weight, the lower the MFR. But there is an optimum molecular weight for each application. If the molecular weight is too low, the polymer will be too light and won’t have enough strength. If the molecular weight is too high, the polymer will be too heavy and will be difficult to process.
Polyolefin coatings can be applied to a variety of surfaces, including metals, plastics, glass, and even concrete. This type of coating is typically used in areas where there is high wear and tear, such as in industrial settings or in areas that see a lot of foot traffic. Polyolefin coatings are also often used in food processing facilities, as they are non-toxic and easy to clean.
Polyolefin coatings can be applied using a variety of methods, including spraying, roll coating, and dip coating. The most common method is spray coating, as it provides a uniform coating that is resistant to chipping and flaking.
The melt mass-flow rate (MFR) is important in the production of a polyofin coatings. This is because the melt viscosity should be high enough (low MFR) to have good melt strength. This will help maintain the extruded shape without distortion as it exits from the die.
If the viscosity is too high, it becomes more difficult to extrude because higher extrusion pressures are required. If there is a variation in the melt viscosity between batches of PE, this could cause problems during the coating process since different batch extrusions would require various modifications. Additionally, it’s possible that the variance in melt Visibility has an effect on both coating application and residual stress- which as we know, ultimately leads to stress cracks development over time.
Stress cracking is created by stress acting on a defect and initiating a crack that grows very slowly. The poorer the resistance, the faster the crack speed or the lower the stress necessary to initiate and grow the crack. The defect is often a sharp notch caused for example by interference damage but with poorly resistant grades normal surface scratches or intrinsic microscopic flaws within the material may generate it eventually.
Stress cracking is when a material cracks under stress. This can be caused by high temperatures or surfactants like detergents. In the 1970s, scientists found that adding co-monomers like butene, hexene, or octene to the polymerization of PE could produce co-polymers with slightly lower densities (about 0.940 g/cm³) but much better stress crack resistance.
The copolymer creates short side branches on the main PE molecular chain. This has been used with success for medium density gas pipes all over the world. It was only adopted after stress crack problems had occurred in O-die extruded coatings.OOne problem with gas pipes made from MDPE is that the manufacturer’s recommended value for viscosity (a measure of how thick the liquid is) is only 0.3. This means that some companies have had trouble making a pipe coating using this resin. Even so, these materials have excellent stress crack resistance, which is the most important thing, and so it is still recommended to use them even if a lower viscosity variant has to be used for the pipe coating.
Many different methods have been used to measure stress crack resistance over the years. The Bell Telephone Test, ASTM D1693 (6), initially bent a small sheet sample that had been razor-blade notched and then exposed it to a surfactant (10% Igepal) at 50°C. Stress cracks usually appeared after some time had passed. The time for 50% of the samples to crack (F50) was noted down.
This test is only useful for assessing the performance of stress crack materials that are not very good. If the material is reasonably resistant, the crack does not start because the PE gradually reduces the stresses at the artificial notch. Other constant load tests have been introduced at 80°C either on notched sheet samples or on notched pipes. In these types of test, the stresses at the notch tip remain constant and actually increase as the crack grows. The tests are harder. The PENT test is better than other tests because it is constant.
Pipe coatings need to be able to withstand bad weather and damage from the sun’s UV rays during storage and construction. PE resins that have enough antioxidant and UV stabilizer additives will work best. Carbon-black pigmented resins usually perform the best, but there are additive packages available for other colors too. Un-pigmented (natural) resins would not work well because the UV rays can penetrate deeper into the surface.
UV exposure can cause small surface cracks and crazes in the coating that could lead to stress cracking. It is a good idea to make PE coatings resistant to UV exposure. This requires an OIT time of at least 10 minutes. PE resins that are used for low pressure gas pipes should last for at least 30 hours. PENT test data from 1965 to 1993 shows that these resins can last anywhere from 1.4 hours to 45,000 hours.
Polyofin coatings need the following variables to be accurate:
Density
Density is measured using a density-gradient column according to the ISO 1183 / ASTM D 1505 standards.
Melt Mass Flow Rate (MFR)
The MFR test is a way to measure how easy it is to melt the polymer. A high viscosity means that the polymer is difficult to melt, and a low MFR means that it is easier to melt. This test method is defined in ISO 1133 / ASTM D 1238.
The thickness of the resin affects how easy it is to spread during the coating process. In general, thinner materials are easier to spread and result in lower residual stresses in the coating.
Oxidation Induction Time
The OIT is a measure of how well the polymer can withstand heat. The test method (ISO TR10837) measures the time it takes for a sample of the molten polymer to start to degrade in a pure oxygen atmosphere when held at a temperature of 200oC.
Anti-oxidant is added to the polymer to stop it from breaking down during processing and coating extrusion. The test checks if there has been any degradation during manufacture of the coating, and gives an estimate of whether there is enough anti-oxidant to resist further degradation by UV exposure or weathering.
The ISO/CEN standard for gas pipe material requires a minimum IOT of 20 minutes. This is because the pipe material might need to be fused together during construction on site. If the pipe is only going to be coated (not fused), then a value of 10 minutes would be acceptable, as long as care is taken during the application of heat shrink sleeves.
Infra Red Analysis
IR analysis is a way to check if there is any dirt or pollution on the PE resin before it is used.
Source: Mr. David Norman of the EPRG (European Pipeline Research Group)
