|
Nucleated or clarified Polypropylene (PP) is quite common since it enhances rigidity, improves productivity
by lower cycle time, and with clarifying agents enhances clarity. There are many applications of this type
of nucleation. Recently, nucleating agent that provides beta crystal form has also been commercially developed.
PP is semi crystalline with 40�60% crystallinity. It does not provide clarity. Increasing the crystals enhance
rigidity along with higher melting points. However none of commercial alpha nucleating agents enhances impact strength.
PP can develop different types of crystal forms but the alpha crystal form is most common and is also the most
stable form of crystal. All other crystal forms are less stable. Certain chemicals can cause formation of beta
crystals in PP preferentially of course under right processing conditions.
Beta crystal has lower melting point (about 12-15 degree C lower than alpha crystal). Besides, beta crystal exhibits
strikingly different behaviour in stretching, resulting in formation micro voids. It also has lower tensile yield
compared to alpha crystal. PP therefore becomes more ductile and therefore has better impact strength.
Typically alpha crystal melts at 164 degree C while beta crystal in PP melts at 151 degree C. This lowers the
processing temperature and hence energy can be saved or productivity can be increased. Beta crystal being more
ductile can thermo form before the crystals get melted and thus the processing window for thermoforming gets
widened compared to alpha crystal.
Micro voids created by beta crystals are quite tiny and therefore maintain mechanical properties. Voids created
by fillers typically used are about 10- 15% larger creating some loss of mechanical properties.
The micro porosity generated provides an advantage for good printability due to tiny crevices can help
in flow of printing ink. High level of micro porosity can also lead to provide higher transmission of moisture vapour. This leads to
breathable PP oriented film but is waterproof.
Some of the chemicals that can act as beta-nucleating agents are:
 |
 |
Mixing of dibasic acids with hydroxide or oxide of certain group II a metals |
|
|
Red quinacridone pigment |
Mayzo Corporation of USA has even developed a masterbatch based on PP carrier. This masterbatch is generally
used at 2-3% dosage in order to achieve beta crystallization
| | |
{{comment.DateTimeStampDisplay}}
{{comment.Comments}}