Polyethylene
PE types
We distinguish between:
- PE-LD: highly branched polymer chains, thus low density between 0.915 g/cm3 and 0.935 g/cm3, ("LD" stands for "low density").
- PE-HD: lower branched polymer chains, resulting in high density between 0.94 g/cm3 and 0.97 g/cm3, ("HD" stands for "high density").
- PE-LLD: linear low density polyethylene, the polymer molecule has only very few and short branches.
- PE-HMW: high molecular weight polyethylene. The polymer chains are longer than for HDPE, LDPE or LLDPE, the average molecular weight is 500 to 1000 kg / mol.
- PE-UHMW: ultra-high molecular weight polyethylene having an average molecular weight of up to 6000 kg / mol and a density of up to 0.99 g/cm3
Comparison of LD-PE versus HD-PE
| Comparison parameters | LD-PE | HD-PE |
| Density in g/cm³ | 0,915 to 0,935 | 0,94 to 0,97 |
| Melting point in °C | 105 to 110 | 130 to 135 |
| Chemical resistance | good | better |
Properties of PE
Polyethylene consists of only hydrogen and carbon, it can burn therefore in a waste incinerator in the ideal case to carbon dioxide and water vapor.
It burns with dripping, a bright flame and continues to burn even when you remove the flame. The ecological balance is good. Polyethylene has a high resistance to attack by acids, alkalis and other chemicals. Polyethylene is partially crystalline, with increasing degree of melting the density increases.
Higher crystallinity also increase the mechanical and chemical stability.
Polyethylene absorbs hardly any water and floats on water. The water absorption is less than 0.1%, density <1 g / cm ³ and practically does not swell in polar solvents. For water vapor it is impermeable while oxygen and carbon dioxide can permit through. Its properties can be changed by appropriate copolymerization adjustments. Basically, the chemical resistance of PE increases with the density.
Production of PE
PE-LD is produced at pressures of 1000- 3000 bar and at temperatures of 100 - 300 ° C using catalysts (oxygen or peroxides) from the monomer ethene.
PE-HD is produced industrially by the Ziegler-Natta process.
This process is characterized by it`s low pressure of 1-50 bar and low temperature of 20 - 150 ° C.
As catalysts, titanium acetate, titanium halides or aluminum alkyls are used.
Alternatively, PE-HD is obtainable also with the Phillips process at temperatures from 85 - 180 ° C and pressures of 30 – 40 bar.
Effect of the density and the melt flow index on the properties of the polyethylene
Increasing Density
| Melting Point | increases | |||
| Hardness | increases | |||
| Stiffness | increases | |||
| Chemical resistance | increases | |||
| Gas-and Aroma tightness | increases | |||
| Impact strength |
decreases | |||
| Transparency | decreases | |||
| Stress-crack resistance | decreases |
Decreasing Melting Point
| Impact strenght | increases | |||
| Resistance to progressive Deformation | increases | |||
| Stress-crack resistance | increases | |||
| Chemical resistance | increases | |||
| Abrasion resistance | increases | |||
| Processability | decreases |
|||
| Flowability | decreases | |||
| Distortion slope | decreases |
Application Areas of polyethylene-melt adhesives (PE-HD/- LD)
Polyethylene is used as hot melt adhesives substantially in two forms, which are caused by the production process. High-Density polyethylene (HDPE) has a melting range of 125 ° -140 ° C and seals more slowly than Low-Density polyethylene (LDPE).
Due to this fact HDPE tends to be less penetrating through the substrate during coating. The handle is harder than that of a LDPE coating. The applied HDPE hot melt adhesive is practically insoluble in perchlorethylene (dry cleaning). Coatings with HDPE are excellent wash-resistant. Therefore, they are preferably used in the washable garment industry sector for the manufacturing of collars and cuffs reinforcements.
Due to the high pressure production process the outcome is a branched, low molecular and low crystalline high-pressure polyethylene. It is preferably used in the fixation of small parts, where no high fastness properties are expected.
In the carpet industry LDPE is well established as a back-reinforcing layer due to its adjustable melt flow index. Also for floor assemblies and trunk liners for the automotive industry, HDPE and LDPE are used in an increasing manner.
The tendency to increasingly use polyethylene adhesives for other applications, partly through blends with other polymers or modifications in the polymerization are seen in medical technology and the construction industry.
Competence
We are happy to advise you further. Contact our experienced technicians.
ROWAK AG