We often encounter the thorny problem of spherulites in film production. In fact, this is typical defect that is prevalent in the field of polymer films and is not limited to PE raw materials and blown film processes. According to statistics, spherulite defects reduce the qualified of film products by 15%-20%. This article provides a brief introduction to the nature, origin, and prevention of spherulites, hoping to provide an for everyone to understand this "nuisance" that often comes into contact with us.
The Nature and Causes of Spherulites
As the name implies, spulites are point-like defects related to the polymer cooling crystallization process. They do not refer to a specific type of substance but rather a localized optical abnormality that is to the naked eye. They may originate from the material itself or be caused by foreign matter contamination.
Spherulites caused by the material's own structure include: insufficient, uneven cooling rate leading to local differences in crystallization; insoluble particles formed by the crosslinking of polymer chains; uneven distribution of additives, resin, etc. leading to local differences in transparency, etc. These types of spherulites have the same composition as the normal film surface and show the characteristics of semi-transparency and strongraction under a microscope. Thermal analysis may reveal new melting points or crystallization peaks.
Spherulites formed by foreign matter contamination include: undispersed inorganic fill; dust, fibers, etc., in the operating environment; metal chips generated by equipment wear; carbonized materials accumulated in the screw for a long time, etc. These types spherulites have significant differences in composition from the matrix, usually showing deeper color or abnormal reflection. Under a microscope, clear boundaries can be seen, and the spherul may be raised.
How to systematically prevent and control spherulite problems in actual production?
At the "source," it is necessary to control from the source: the appropriate filter screen mesh and replace it regularly to ensure filtering accuracy; thoroughly clean the screw to prevent the formation of carbon deposits in blind spots, which can form non-melting particles; pay attention to the thermal stability of raw materials and the particle size of inorganic particles; ensure uniform dispersion of additives through high-speed pre-mixing; and environmental cleanliness.
At the "outflow end," it is necessary to establish an interception mechanism: closely monitor the pressure difference of the melt filtration system; detect and spherulites in time through online monitoring (such as optical cameras); carry out polarizing inspection or visual inspection on finished products; correlate the frequency, position, size, and information of spherulite occurrence, and seal the problem batch in time and trace it back to the source.
In summary, spherulites are not mysterious, but may be a coordinated problem of the entire process system: uneven composition, abnormal shear, insufficient dispersion, crosslinking, crystallization, foreign matter contamination, etc., may trigger phenomena. Therefore, we need to have a clear understanding of spherulites in our daily work so that we can deal with them calmly when problems arise and truly keep such out.
(Quality Department/Yang Xiangchun)
