High-Density Polyethylene: Structure, Properties, Application

Ans.

1. Natural polymers: The polymers which occur in nature mostly in plants and animals are called natural polymers. A few common examples are starch, cellulose, proteins, rubber nucleic acids, etc. Among them, starch and cellulose are the polymers of glucose molecules. Proteins are formed from amino acids which may be linked in different ways. These have been discussed in detail in unit 15 on biomolecules. Natural rubber is yet another useful polymer which is obtained from the latex of the rubber tree. The monomer units are of the unsaturated hydrocarbon 2-methyl-i, 3-butadiene, also called isoprene.

Examples of natural polymers: Natural rubber, cellulose, nucleic acids, proteins etc.

2. Synthetic polymers: The polymers which are prepared in the laboratory are called synthetic polymers. These are also called man-made polymers and have been developed in the present century to meet the ever-increasing demand of modern civilization.

Examples of synthetic polymers: Dacron (or terylene), Bakelite, PVC, Nylon-66, Nylon-6 etc.

Ans. The molecular weight distribution of a given polymer of a given grade is determined by MFI, which is an important feature of HDPE. Mechanical strength qualities improve when the MFI is low, and the molecular weight distribution is narrow. Reprocessing and adding higher MFI grade material will eventually improve the MFI, but it is inadvisable as the final product's mechanical qualities are significantly weaker and of lower quality. Furthermore, it is unethical in the manufacturing industry.

Ans. PET is transparent because it is biaxially orientated and has a low degree of crystallinity. Most bottled water firms want to prove that their water is clear and colourless in smaller bottles (approximately 1000 mL or less), thus they'll utilize PET. Even so, many bottled water companies will continue to use HDPE gallon jugs for 1-gallon volumes (which are opaque due to the high degree of crystallinity and massive spherulitic crystallites that scatter visible light easily), much as milk and certain juice are offered in the same size HDPE jugs. [By the way, the even larger capacity bottles (5 gallons, for example) are likely made of polycarbonate and can be subjected to a lot of handling and damage.]

Ans. It is determined by the product's requirements. There are several factors to consider when packing a product efficiently and economically, the majority of them are dependent on the type of product. Food and non-food items are packaged differently; liquids and beverages are packaged differently than solids. Transparency is the main distinction between HDPE and PP, which distinguishes their applications. Because of its low density (0.902 g/cm³), PP packages are clear, whereas HDPE (0.94–0.965 g/cm³) has a distinct colour and is opaque. As a result, HDPE is commonly utilized in milk pouches, liners, and containers, whereas PP is commonly used in transparent cases and blow-moulded bottles.

Ans. Fusion welding is possible with HDPE. In most situations, the base material is machined down to a clean, void-free surface, then heated/melted and forced together under pressure to join the components. When the joint is cool, it has the potential to be stronger than the parent materials.

Electrofusion welding uses a resistance element in the fittings to melt the metal in the same way as butt fusion welding works.

Then there's HDPE welding, which involves melting the base stock using a heat gun and then adding a filler rod to strengthen the junction. It's like oxy-acetylene or TIG welding, but instead of a flame or an arc, hot air is used.

Ans. On the basis of structure, polymers are classified into three types. These are linear chain polymers, branched-chain polymers and cross-linked polymers.

1. Linear chain polymers: In this case, the monomer units are linked to one another to form long linear chains. These linear chains are placed one above the other and are closely packed in space. The close packing results in high densities, tensile strength and also high melting and boiling points. High-density polyethene is a very common example of this type. Nylon, polyesters and PVC are also linear chain polymers.

2. Branched-chain polymers: In this type of polymers, the monomer units are linked to form long chains which also have side chains or branched chains of different Lengths attached to them. As a result of branching, these polymers are not closely packed in space. They have low densities, low tensile strength as well as low melting and boiling points. Some common examples of such polymers are; low-density polyethene, amylopectin, starch, glycogen etc.

3. Cross-linked polymers: In these polymers, also called network polymers, the monomer units are linked together to form three dimensional networks as shown in the figure. These are expected to be quite hard, rigid and brittle. Examples of cross-linked polymers are bakelite, glyptal melamine-formaldehyde polymer etc.

Ans. No, natural HDPE is not UV resistant and will disintegrate if exposed to sunlight for an extended period (1 year or more). In the production process, UV absorbent materials might be used. HDPE will be more UV resistant depending on the amounts of UV absorbent compounds added. Under direct UV exposure, black HDPE with a carbon black addition can survive more than 50 years.

Ans. HDPE, or High-Density Polyethylene, is made from petrochemical wastes as raw ingredients. It is not biodegradable, and if properly cared for, a good grade HDPE pipe or other material can last at least 50 years. Then trashed HDPE may be recycled, and aged HDPE materials can be used to make other low-performance products.

Ans.  The functionality of a monomer implies the number of bonding sites present in it. For example, monomers like propene, styrene, acrylonitrile have the functionality of one which means that they have one bonding site.

Monomers such as ethylene glycol, hexamethylenediamine, adipic acid have the functionality of two which means that they have two bonding sites.

Ans. In comparison to HDPE, the polymer chains of LDPE have several branches. Such a structure prevents the chains from stacking neatly next to each other. The plastic becomes softer, more flexible, and less strong when the intermolecular forces of attraction are reduced. Because of its outstanding "formability," LDPE is better suited to hard items such as plastic bottles, buckets, and bowls.

When it comes to HDPE, it has the fewest number of polymer chain branching. This causes the linear molecules to pack together tightly during the crystallization process. HDPE becomes denser and stiffer as a result of this process. HDPE has a wide range of applications due to its increased tensile strength.

Ans. Focus on the solid line in this thermogravimetric examination of high-density polyethylene (HDPE). This diagram depicts when the polymer loses mass as a function of temperature. This is where harmful byproducts would form when the body loses mass. It's around 300 degrees Celsius, or 572 degrees Fahrenheit, here. As a result, the polymer would not decompose above 350 degrees Fahrenheit (177 degrees Celsius). I've melted HDPE for moulding/extrusion at temperatures as low as 130 degrees Celsius. When HDPE is melted, it becomes extremely sticky and difficult to remove after it cools.

Ans. HDPE is made using a process called coordination polymerization, which involves the production of coordination complexes. Ethylene, for example, donates pi electrons to form a coordination complex with the transition metal titanium. The pi complex that results reacts with a higher number of ethylene molecules in a stepwise manner, eventually forming a polymer. In comparison to LDPE, the polythene produced has a higher density (0.97 g/cm³) and a higher melting point (403 k).

Ans. In terms of lightweight and shatter resistance, the advantage of plastic bottles over glass and metal bottles is well established. Polyester (PET), Polyethylene (HDPE & LDPE), Polypropylene (PP), and PVC are the most common plastics used in bottles.

HDPE, LDPE, and PP are all polyolefins, meaning they are made up entirely of carbon and hydrogen atoms. Acids, alkalies, insecticides, and pesticides can all be packaged in them since they are highly resistant to chemical attacks. PVC and PET, on the other hand, are both made up of polar atoms, such as chlorine in PVC and oxygen in PET. Because these chemicals attack them, they are not ideal for packaging. PVC and PET, on the other hand, are mechanically stronger than polyolefins and have their applications.

HDPE is the best polyolefin for bottles and jars for food, cosmetics, pesticides, industrial chemicals, lubricants, and other items because:

• In terms of rigidity and impact resistance, this product has the highest mechanical strength.
• When exposed to the elements, this material has the highest resistance to weathering.
• Processability is simple.

Ans. Ethylene, which comes from fossil fuels, is used to make HDPE. So, by "eco-friendly" in the sense that it is made from natural resources, the answer is no. It's also not easily biodegradable, so it's not "eco-friendly" in that sense. It is, nevertheless, exceedingly inert, making it an excellent candidate for recycling. Recycled HDPE is used to manufacture long-lasting artificial timber, which eliminates the need to cut down trees for decking and railings along paths, making it "eco-friendly" in that situation. For Yellowstone or Yosemite, we can see that recycled HDPE "timber" is commonly used along trails for railings, trail markers, and trail bridges.