The polyethylene (PE) has its molecular chain constituted, in most cases, only by carbon and hydrogen; and is a material translucent or milky, malleable and flammable. It is flexible, as it has glass transition temperature far below normal ambient temperatures. The polyethylene can be produced with different densities and with linear, branched or crosslinked structure, having diversified fields of commercial application. It is widely used for the production of bags, packaging and housewares such as pots and containers, for being a cheap and easy thermoplastic processing. Also, is lightweight, non-toxic, chemically resistant and can contact with food and pharmaceuticals without imparting taste or smell.
In the United States, polyethylene represents about half of the total plastic waste discarded.
Below are shown different types of polythylene available in market.
LOW DENSITY POLYETHYLENE (LDPE) - density from 0.91 to 0.925g/cm³
Molecular structure of LDPE
Produced under high pressures (1 to 2 kilobar) and high temperatures (210° to 570°F), the LDPE is caracterized by very branched molecules. These branching prevent the efficient and rapid arrangment of the molecules during crystallization, and as a result, it was observed that it prevents a percentage of variations in crystallinity between 40 and 65%.
Filme de PEBD
Features:
- Flexible
- Higher impact strength (as compared to HDPE)
- Translucent or transparent
- Higher viscosity (as compared to HDPE)
Applications: Masterbatches, films, bags, transparents parts, packaging and flexible caps, ballpoint pen ink tube, bubble wrap etc.
Melting point:
230 to 240°F (110 to 115°C)
Glass transition (Tg):
-184°F (-120°C)
LINEAR LOW DENSITY POLYETHYLENE (LLDPE) - density from 0,91 to 0,925g/cm³
Water tank made with LLDPE
The fact of being polymerized under low pressures makes the production of LLDPE more economical than the conventional LDPE, making this material a great alternative to applications requiring intermediate properties between LDPE and HDPE.
Features:
- Resistance to stress cracking
- Increased brightness (compared with conventional LDPE)
- Rigidity (compared with conventional LDPE)
- Low gas permeability (compared with conventional LDPE)
- Good resistance to tearing
- Good tensile strength
Applications:
Masterbatches, films, bags, caps with seal, water tanks, rotomolded parts in general.
Melting point:
250 to 265°F (120 to 130°C)
Glass transition (Tg):
-185°F (-120°C)
MEDIUM DENSITY POLYETHYLENE (MDPE) - density from 0,926 to 0,940g/cm³
Is obtained by mechanical mixing of LDPE and HDPE, producing a polyethylene with properties intermediate between both types. Their use has grown extensively in engineering applications such as plastic pipes for water distribution and gas systems.
HIGH DENSITY POLYETHYLENE (HDPE) - density from 0,945 to 0,96g/cm³
Molecular structure of the HDPE
Produced with an extremely active catalyst, low pressure (under 30 bar) and temperatures between 105 and 300°F is characterized by long linear molecules. HDPE has a crystallinity between 85 and 95% depending also on the molecular weight and its distribution, in addition to the conditions of the crystallization kinetics.
It has greater mobility and has no branches, and easier to move between other chains and participate in the formation of the ordered region (crystal).
Container made with HDPE
Features:
- Excellent chemical resistance
- Increased hardness (compared with LDPE)
- Lower viscosity (compared with LDPE)
Applications:
Pails, covers, chemical containers, jars, bottles, boxes, tanks, pipes etc.
Melting point:
265 to 285°F (130 to 140°C)
Glass transition (Tg):
-185°F (-120°C)
ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE (UHMWPE) - density from 0,93 to 0,94g/cm³
UHMWPE powder
Ultra high molecular weight polyethylene is one that has molar mass between three and six million, this material can replace aromatic polyaramid fibers (Kevlar) in bullet proof vests. It is widely used in applications that require abrasion resistance and self-lubricating, such as coating to dump of truck, skating rinks (instead of ice) and parts that come into contact with chemicals, taking advantage of the common chemical inertness of polyethylene.
The extremely high molar mass of UHMWPE provides such a high melt viscosity that its melt index approaches zero, being impossible to process it by conventional methods of injection, blow molding or extrusion. The UHMWPE can be processed by compression, thermopressing or piston extruder, whereby plates, blocks and semi-finished billets are obtained by cutting.
Following market trends, is starting the production of ultra-high bimodal, which optimizes performance in the fiber segment. Analyzing the tenacity in application of fibers, normal polyethylene has a tenacity between 4 and 6, polyamides between 10 and 12. Polyethylene of ultra high as the procedure of processing reaches 18 (if processed similarly to the extrusion of monofilaments and raffia, with sheet of 1 to 2mm sliced with subsequent stretching) and 30 (with the oil solubility of polyethylene and further injection in the matrix and solvent extraction with a very high orientation of the molecules). This second method allows the use of fibers obtained in the manufacture of bulletproof vests, helmets and some special applications of special cables. Another application is the cold compression, when it is too complicated to fill several cavities of the mold. It is the cold compression of the preforms and then mounts the final piece. Playing with heat and pressure can then come to a very complex piece and with very low defect rate.
UHMWPE fibers
Features:
- High abrasion resistance
- Good corrosion resistance
- High resistance to cyclic fatigue
- High resistance to impact fracture
- High resistance to surface-cracking
- High chemical resistance
- High hardness
- Low coefficient of friction
Applications:
Marine fenders, gears, rods, parts of surgical application, coatings, pumps, gaskets, bearings etc.
Melting point:
275°F (135°C)
Glass transition (Tg):
-150 to -190°F(-100 to -125°C)
Crystallinity:
~45%
Bibliography:
HARPER, Charles A.; PETRIE, Edward M. Plastics Materials and Process: A Concise Encyclopedia. Hoboken: John Wiley & Sons, Inc., 2003.
CANEVAROLO JR., Sebastião V. Ciência dos Polímeros: Um texto básico para tecnólogos e engenheiros. 2.ed. São Paulo: Artliber Editora, 2002.
Notes: the data of the properties listed below are taken from our database, thus, they are found in commercial resins. In the cases where the values are presented in interval form, these data represent the lowest and highest value found to the material type. In some cases, where the propertie doesn't have as much variation from resin to resin, we present only the arithmetic mean. It is also important to note that the existance of a certain value here is not representative of all resins, for example, the indication of classification V0 (flameproof) doesn't mean that all materials of the same type are classified V0, but that there are resins with this classification.
About the author: Daniel Tietz Roda is Plastics Technologist graduated from the FATEC/ZL and Mechanical Design Technician from ETEC Aprígio Gonzaga, in São Paulo, Brazil. Roda worked 5 years with technical assistance and development of plastics in industries and nowadays is the publisher of this website.
