A Brief Discussion on the Causes of Plastic Film Tackiness and Methods to Avoid It

Under the action of a certain temperature and pressure over a period of time, adjacent layers of plastic film may sometimes adhere to each other; if high winding resistance is observed, it indicates that the roll film has blocked. When the curing temperature in the curing room is too high, the roll film is also prone to blocking and must be rewound before normal use. These phenomena indicate that packaging materials have tackiness; in fact, this tackiness is caused by the thermal motion of chain segments.

Causes of Plastic Film Tackiness

If the ambient temperature is above the Tg of the polymer, single bonds on the molecular chain segments of the film material will rotate freely, and the conformation of the molecular chains will change. Because the distance between layers of the roll film is very small, and under the action of a certain shrinkage tension (pressure), this thermal motion of chain segments may even cause migration of chain segments; chain segments in one film layer will migrate into the depressions on the surface of the adjacent film layer, causing blocking. If the polymer film is amorphous, this blocking will be more pronounced; for amorphous polymers, the lower the Tg, the more likely blocking is to occur. If it is a highly crystalline polymer, most molecular chain segments are confined within the crystal lattice and cannot undergo thermal motion, so blocking occurs less frequently.

In addition to being related to the aggregation-state composition and Tg of the material, the blocking phenomenon is also related to the surface flatness of the film. For example, for a smooth film surface, the layers of the roll film are in close contact with almost no gap, and the local vacuum formed during separation can also cause blocking.

Applications of Plastic Film Tackiness

The cling film widely used in the market today is an example of utilizing the tackiness of film. In the past, PVC was often used as cling film. PVC has low crystallization ability and is a typical amorphous polymer; after adding 40%–50% plasticizer, its Tg drops to -30°C, so the film is very soft and highly tacky, making it very suitable for use as cling film. However, the plasticizer DOP is harmful to the human body, so PVC cling film has been eliminated and replaced by PE cling film. LDPE has a lower Tg and is suitable for use as cling film, but PE has a certain degree of crystallinity, so its use effect is not as good as that of PVC. However, if we can effectively prevent the crystallization of PE, we can prepare cling film with good self-tackiness.

Methods to Avoid Plastic Film Tackiness

In actual production, to avoid blocking, packaging materials need to have good opening properties. This requires adding small-molecule amide slip agents or silica-based anti-blocking agents during film preparation.