What is the code number on plastic bottle HDPE?

Hint: There are some numbers which helps us identify the type of plastic used to make that particular plastic container and those numbers are generally called as code numbers. The code numbers vary from 1-7 for different types of plastics.

Complete step by step answer:
The addition polymerisation of ethene with hydrocarbon solvent in the presence of triethylaluminum and Ziegler Natta catalyst ( Titanium tetrachloride right) at a temperature of 333 K to 343 K under the pressure of 6 - 7 atm results in the formation of High Density Polyethylene or HDPE. HDPE thus consists of linear molecules and has a high density because of close packing. It is chemically inert and hard in nature and is used in making buckets, pipes, bottles, dustbins etc.
At the bottom of most of the plastic containers there is a small number written inside the three arrow triangle recycling symbol it is nothing but a code number. It generally refers to what type of plastic is used in making that particular plastic container
Where the code numbers range from
In 1 type, the plastic used is PET( Polyethylene Terephthalate right) , In2 HDPE( High density Polyethylene right), 3 PVC( PolyVinyl Chloride right), 4 LDPE( Low Density Polyethylene right), 5 PP( Polypropylene right), 6 PS( Polystyrene right), 7( BPA, Polycarbonate and LEXAN right)

So, the code number on plastic bottle HDPE is number 2.

Additional information:
The plastic recycling numbers 2, 4 and 5 are considered to be the safest and the ones with the number 1, 3, and 6 must be avoided. HDPE has an excellent moisture barrier and chemical resistant properties, permeable to gas and is hard to semi flexible. HDPE is found to release estrogenic chemicals in harsh conditions. But it is one of the safe plastics to use for food applications when compared to PET.

Note: HDPE is 90% crystalline when compared to LDPE. It has got a high strength to density ratio and is used in making buckets, corrosion resistant pipes, plastic bottles, shampoo, detergent bottles, toys and dustbins. HDPE is one of the easiest plastic polymers that could be recycled.