Polyethylene is a thermoplastic polymer consisting of long hydrocarbon chains. The term polyethylene describes a huge family of resins obtained by polymerizing ethylene gas(Fig1.9) CH2=CH2, and it is by far the largest volume commercial polymer.
This ingredient or monomer is ethylene, It has the formula C2H4, consisting of a pair of Ch2 groups connected by a double bond. PE(polyethylene) is a rather stable. molecule that polymerizes only upon contact with catalysts. The conversion is highly exothermic, that is process which releases a lot of heat.
This thermoplastic is available in a range of flexibilities and other properties depending on the production process, with high density materials being the most rigid. Polyethylene can be formed by a wide variety of thermoplastic processing methods, such as injection, extrusion and blowing, and is particularly useful where moisture resistance and low cost are required.
Polyethylene plastics are classified by density and the primary differences are in rigidity, heat resistance, chemical resistance, and ability to sustain loads.
(1)Classification Polyethylene is classified into several different categories based mostly on
s density and branching. For examples, HDPE(High-density polyethylene), LDPE (Low-density
polyethylene), LLDPE(Linear low-density polyethylene), MDPE (Medium-density polyethylene)
VLDPE (Very-low-density polyethylene), HMWPE(High-molecular-weight polyethylene)
UHMWPE (Ultra-high-molecular-weight polyethylene), ULMWPE(Ultra-low-molecular-weight
polyethylene), PEX or XLPE(Cross-linked polyethylene), HDXLPE(High-density cross-linked
polyethylene), CPE(Chlorinated polyethylene.
Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight.
(2) Physical and chemical properties Polyethylene plastic is characterized by low density,
good toughness, near-zero moisture absorption, excellent chemical resistance, excellent electrical
chemical resistance, excellent electrical insulating properties, low coefficient of friction, and ease of
processing, good processability, food contact acceptable, antioxidant, high ESCR(Environmental
Stress Crack Resistance), copolymer, high impact resistance, good stiffness
Low density polyethylene typically has a density value ranging from 0.91 to 0.925 g/cm,
linear low density polyethylene is in the range of 0.918 to 0.94 g/cm,, while high density
polyethylene ranges from 0.935 to 0.96 g/cm and above.
The temperature at which these occur varies strongly with the type of polyethylene. For
common comm
ercial grades of medium-density and high-density polyethylene the melting point
typically in the range 120C to 130C. The melting point for average, commercial, low-density
polyethylene is typically105℃to115℃.
HDPE exhibits less branching in its hydrocarbon chains and is therefore harder than LDPE. In
general, as density increases, hardness, heat resistance, stiffness, and resistance to permeability
increase. Low-density polyethylene plastics are quite flexible with high impact strength and
relatively low heat resistance
Most LDPE, MDPE and HDPE grades have excellent chemical resistance, meaning that it is
ot attacked by strong acids or strong bases. It is also resistant to gentle oxidants and reducing
agents. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of
paraffin. The material continues burning on removal of the flame source and produces a drip
BO. HDPE can be recycled but, like LDPE, it also loses some of its quality during the second time around.
(3)Applications Film, Packaging (especially in food packaging), Bags, Piping, Industrial
Applications, Containers.
Polyethylene is used in packaging, vibration damping. and insulation, as a barrier or buoyancy
en as a packaging material. Polyethylene foam is popular for nautical uses, and is used to wrap
fumiture, computer components, electronics, sporting goods, plants, frozen foods, clothing, bowling
balls, signs, metal products, and more. Polyethelyne, particularly HDPE is often used in pressure
pipe systems due to its inertness, strength and ease of assembly.
Low density and linear low density polyethylene are primarily used for packaging films, food and drink containers, outdoor furniture, floor tiles, shower curtains, extrusion of pipe and tube, wire caging and cable coating. low-density polyethylene is an important plastic. It is resilient, easy to weld and shape and flexible to the point of being almost unbreakable.
Polyethylene packaging can be broken into five main packaging applications:retail, food, fozen, medical and industrial.
Product packaging for retail often requires poly with high clarity, high gloss and excellent hea
seal ability. Low density polyethylene is commonly used to manufacture this type of poly bag
Frozen applications require a polyethylene film that can withstand freezing temperatures. Cold
crack is the freezing temperature at which the integrity of the film is compromised. Poly bags used
to package frozen products like ice also require excellent seal strength and puncture resistance
Medical supply and equipment packaging is used to protect products and equipment from dust,
moisture and other contaminants. Puncture strength and sealability are often crucial for this
application. Medical devices are typically manufactured in a sterile environment. The bags and film
surrounding these products must be free of punctures and remain sealed to maintain the sterile
environment.
Industrial Packaging is used for heavy industrial applications including transportation of goods
lining containers or covering products. Strength is the most important feature of this film. Strength
is measured in many ways depending on the approach. Tensile Yield measures the ability of the film
to return to its original shape after being stretched. Tensile Break measures the amount of stress the
film can hold before breaking. Tear Strength measures the protection against being torn apart.
Puncture resistance measures the protection against contents poking through the bag.
