 POLYETHYLENE
(PE)
Definition and Features
Polyethylene is one of the oldest polymers of the thermoplastic family.
Previously it was only manufactured in low density, however it has been
developed and three new types have been added to the family, which are high
density, linear and middle density. All of these four types include monomer
ethylene and different structures of polymer molecule have leaded these
three new types. Different ramification conditions of macro molecules in
chain shape forming the polymer molecule enables the variety of polyethylene.
For instance, ramification is very subtle in linear low density polyethylene,
it is a little more in high density and it is more than that in middle
density polyethylene. The fact that ramification is at maximum level, but
long and short in low density polyethylene affects the polymer features.
These four types of polyethylene are the basic polymeric materials of
industry and they have diverse usage fields.
General characteristics of polyethylene are as below:
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It
is durable, |
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It is resistant to acid,
base and solvents, |
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Its dielectric
features are superior, |
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It’s resistant to
environmental conditions, |
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They can be easily
processed. |
Ramifications in polymer chain determine
the degree of crystallite. Ramification in linear and high density
polyethylene is very little and its molecule structure is linear. In
molecule structures where the ramification is small, crystallite is
generally more. As the crystallite in polymer increases, hardness increase,
mechanical and chemical features improve and resistance to liquids and gases
increase. The best defining feature of polymers is the average molecule
weight of that polymer. Average molecule weight is approximately determined
by melting index. Melting index is inversely proportional to the molecule
weight. Hardness and durability of PEs with high molecule weight is higher,
their melting flow indexes (MFI) is lower. According to the rule which is
generally valid for all polymers high molecule weight is equivalent to low
melting flow index. Molecule weights of polymers are measured by Gel
Permeation Chromatography (GPC) device. This device is a state of art
technological device and its operation is very special. The chemicals below
are added into polyethyleneby melting with mixing method under pressure to
improve the features of polyethylene and increase its durability against
some effects.
Antioxidants:
Ultraviolet segment of sunlight affects every kind of carbon and carbon
bonds and it makes this bond break by time. In order to prevent this,
substances absorbing this affect of sunlight before polymer are added into
polymer. These chemicals are divided into two main groups; UV absorber and
UV quencher. Besides, mixtures of these two groups showing synergetic impact
are used in increasing the stability of UV. UV additives are generally
divided into three types which are benzophenon, nickel quencher and HALS.
These kinds of additives used in industry today are the above mentioned
types or their mixtures with different combinations. Substance known as HALS
is new and its chemical name is Hindered Amine Light Stabilizers. HALS is
very common nowadays and it is being manufactured by many companies. On the
other hand, colorful pigments also have significant effect on preventing UV.
Especially carbon black polymer is widely used in increasing the durability
of polymer against sunlight.
UV stabilizers: Güneş ışığının ültraviyole kesimi her türlü karbon- karbon
bağına etki eder ve bu bağı zaman içinde zayıflatarak kırılmasına neden
olur. Bunun önüne geçmek için polimer içine güneşin bu etkisini polimerden
önce soğuran kimyasal maddeler konur. Bu kimyasal maddeler UV absorber , UV
quencher olmak üzere iki ana gruba ayrılırlar. Ayrıca bu iki grubun
sinerjitik etki gösteren karışımları da UV kararlılığını
artırmada kullanılır. UV katkı maddeleri genelde benzofenon , nikel quencher
ve HALS olmak üzere üç cinstir. Şu anda sanayide kullanılan bu tür katkı
maddeleri yukarda verilen maddeler ya da bunların değişik kombinasyondaki
karışımlarıdır. HALS diye bilinen maddeler yeni bir ürün olup, kimyasal
adları .Hindered Amin Light Stabilizors. dür. HALS ler günümüzde çok
yaygınlaşmış olup birçok firma tarafından üretilmektedir.
Diğer yandan renkli pigmentler de UV önlemede önemli etkiye sahiptirler.
Özellikle karbon siyahi polimerin güneşe dayanımını artırmada yaygın olarak
kullanılmaktadır.
Sliders and anti-blockings:
Amines of oil acids (stearic acid, oleic acid, erusic acid) and inorganic
compounds including over 90% silicium dioxide are used to prevent blocking.
Distribution of granule size and oil absorbance of the substance is very
important in anti-blocking additives .Oil absorption in anti-blocking
substance used by slider is very important.
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