As the anti-blocking of BOPP film, it must have good anti-viscosity, and dispersion, be non-toxic, and have a close refractive coefficient.
Bidirectional polypropylene film, referred to as BOPP film, the production of BOPP film technology content is high, is a very complex work, and reasonable control of the film’s physical and mechanical properties at the same time, and constantly improves the optical properties of the film to enable enterprises to obtain the maximum economic and social benefits. In this paper, the effect of the anti-blocking masterbatch on the mist degree of the film is discussed.
BOPP film because of its high transparency and luster, non-toxic material, stable molecular structure, lightweight, low permeability, odor, and moisture have a certain barrier function and other advantages, is widely used in food, medicine, daily light industry, cigarettes, and other products packaging, and widely used as the base material of the composite film, two-way stretching polypropylene film has the reputation of “packaging queen”.
1. Influence of crystallization on the fog degree of BOPP film
The crystallinity and crystal size of polypropylene in BOPP films have important effects on the mechanical and optical properties of Bopp films. High crystallinity, high strength, poor toughness. The crystal size is small and uniform, which is conducive to improving the mechanical strength, wear resistance, heat resistance, transparency, and surface gloss of the film. The bidirectional stretching can break the larger particles which may be produced in polypropylene, thus reducing the crystal size, improving the transmittance, and reducing the fog degree. Such as polypropylene bidirectional stretching, fog decreased by 50%.
From the point of view of crystallization, to produce low fog BOPP film, the size of the polypropylene crystal should be reduced as far as possible, generally can adjust the process parameters, such as the cooling speed of each section, low processing temperature, large tensile ratio, so that the crystallization is fine and does not form large crystals so that the film can obtain good mechanical properties and reduce the film fog. Too low a crystallinity will also lead to the mechanical properties of the film decline, and stiffness is poor. Generally speaking, the crystallization of PP, a crystalline polymer with high isotropy, is closely related to the rate of cooling. The lower the surface temperature of the cold roll, the faster the heat conduction, or the tighter the sheet attached to the roll surface, the faster the melt cooling rate, at this time, the sheet crystallinity is small and conducive to tensile.
2. Influence of film slip masterbatch on BOPP fog
PP is the main component of the main material used in the industrial production of BOPP film. Isocanonical PP and inter-canonical PP have different crystal structures. Isocanonical PP is crystallized in a three-dimensional growth mode of homogeneous nucleation, while inter-canonical PP is mainly crystallized in a two-dimensional growth mode of homogeneous nucleation, forming small wafers with irregular appearance and size. Moreover, due to the low regularity of molecular structure of inter-canonical PP, inter-canonical PP has a lower crystallization rate and crystallinity. The results show that the higher the isophorone, the faster the crystallization rate, the higher the yield strength and surface hardness of the film products, and the random PP plays the role of internal lubricant in the polymer, and is conducive to polymer orientation, and helps to improve the optical properties of the film.
Different petrochemical manufacturers because of their different processes and technologies, the production of raw materials there are certain differences. The index that has the greatest influence on the film fog is the yellow index, which is a measure of the color of the raw material. The higher the yellow index, the yellower the color of the film roll produced, and the higher the fog of the corresponding film. The smaller the yellow index is, the greener the film roll is produced, and the smaller the fog of the corresponding film is.
3. Influence of film anti-blocking masterbatch on fog degree
(1) Action mechanism, type, and characteristics of BOPP film anti-blocking
In the production of BOPP film, both semi-finished products and finished products must be wound on the coiling core. Because the film is thin and the winding is tight, it will lead to the possibility of adhesion between the films. When the surface layer of the film (anti-viscosity agent is usually added to the surface layer of the film) adds a certain amount of anti-viscosity agent and polypropylene resin insoluble, due to its particle size is 3 ~ 5um, and the surface thickness of the film is generally 1um, convex exposed on the surface of the anti-viscosity agent between the film formed an air isolation layer, to play the role of anti-viscosity.
As the anti-blocking of BOPP film, it must have good anti-viscosity, and dispersion, be non-toxic, and have a close refractive coefficient with polypropylene resin. At present, there are inorganic and organic anti-blocking agents in the domestic and foreign BOPP market. Inorganic anti-blocking agents mainly include synthetic silicon dioxide, synthetic China clay, glass beads, calcium carbonate, and so on. The shape of this kind of anti-adhesive is irregular or round, the particle size is about 3 ~ 5um, transparent, sensitive to moisture, in addition to silicon dioxide is hard, another hardness is moderate, in addition, anti-blocking agents have a high surface area, conducive to the absorption of other additives, this aspect of silicon dioxide anti-adhesive performance is slightly obvious.
Organic anti-viscosity agents are new varieties developed in recent years, among which the more typical products are silicone polymers, such as polysiloxane polymers. This kind of anti-stick agent has the function of anti-stick and slippage, the particle size is about 4um, round, transparent, softening temperature or melting point is low, and is not sensitive to moisture and other additives adsorption.
According to the raw materials used in the product, BOPP anti-stick agent can be preprepared with polypropylene homopolymer, polypropylene copolymer, and even slippery agent to make masterbatch, the concentration is generally 2 ~ l5, with a low concentration of its better dispersion.
(2) The effect of the same type of anti-stick agent and the additional amount on the film fog degree
When the anti-blocking is added to the surface layer of the film, the anti-blocking property of the film is improved. In addition, it also greatly affects the friction coefficient and optical properties of the film.
For example, in the production of ABA-type ordinary heat seal film, the core layer is added with 2% anti-static masterbatch of the same model, and the surface pure copolymer is added with 500ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm of the following typical anti-viscosity agents.
The fog degree of the film is shown in Figure 1: As can be seen from the figure, the turbidity, and luster of the film become worse after the addition of an anti-viscosity agent, which is because the anti-viscosity agent, especially an inorganic anti-viscosity agent, is insoluble with polypropylene resin.
During the film stretching process, there will be holes around the anti-viscosity agent, and there is a slightly different refractive index interface between the two materials. In addition, the anti-viscosity agent protrudes from the surface of the film to make the surface of the film rough. This will result in poor optical properties of the film.
4. Closing remarks
The main influencing factors of fog degree of bidirectional tensile polypropylene film are crystallinity, yellow index of raw material, type of anti-viscosity agent, and the amount added. To produce a film with excellent optical properties, it is necessary to control the crystallinity as low as possible in the production, the yellow index of the selected raw material is negative, and the appropriate anti-viscosity agent with small particle size and the addition of moderate amount.