CPP film is made of polypropylene as the main raw material, extruded by T-shaped head, and its properties are mainly affected by processing conditions. Polypropylene is a semi-crystalline thermoplastic, with high impact resistance, strong mechanical strength, resistance to a variety of organic solvents and acid and alkali corrosion, has a wide range of applications.
Casting CPP film is a kind of non-tensile and non-directional flat extrusion film made of multilayer (3-5 layers) co-extruded polypropylene material by melting and condensing by casting roll.
CPP films usually use copolymerization or homopolymer polypropylene with a melt index of 5-12 as the main raw material, and are selected according to their different applications. CPP films are generally divided into two stages, namely cooking grade and non-cooking grade.
First, the impact of the choice of raw materials on product performance
1, high temperature cooking sterilization of composite substrate film RCPP, this kind of film is mainly made of copolymer polypropylene raw materials. When used for high temperature cooking (resistance to cooking sterilization above 120°C), the block copolymer polypropylene is used; General cooking (resistant to 100~120°C), made of random copolymer polypropylene.
2, ordinary CPP(non-cooking grade) composite substrate, can only be used for ordinary packaging film, this film is usually made of homopolymer.
Due to the different processing resin, the performance of the film has a certain difference, the homopolymer is usually higher than the melting point of the copolymer, the copolymer due to copolymerization modification, making the impact strength of the cooking grade is significantly higher than that of the non-cooking grade, therefore, the safety performance of the liquid and hard packaging is better. Anti-puncture cooking grade materials also need to add elastomer, according to the amount of elastomer added, can be prepared super soft RCPP, for zongzi with bone and meat packaging, the addition amount is generally about 3% to 8%.
Second, the effect of processing conditions on the performance of CPP films
In the case of a certain raw material, only by choosing a reasonable molding process can we get the expected product. In the production of CPP film, the main process conditions include processing temperature, cooling roller temperature, traction speed, winding tension and EDM treatment.
1, the effect of temperature 1) The impact of T-head temperature in the CPP film casting process, the primary parameter is the temperature, temperature selection and control is appropriate, directly affects the physical and mechanical properties of the film and chemical properties. In general, the processing temperature of homopolymer polypropylene is higher than that of copolymer polypropylene, below 250°C, and the copolymer is below 240°C. In actual production, depending on the performance of raw materials, the processing temperature should also be different. Due to the high toughness requirements of CPP film for packaging, especially for cooking food, CPP film has high impact resistance, so the control of crystallinity is extremely important. The increase of the processing temperature will increase the temperature difference between the melt and the cooling roll flowing from the T-shaped head, which can more effectively reduce the crystallization rate and reduce the degree of crystallinity, improve the transparency of the product, increase the impact strength and elongation, and decrease the heat sealing temperature, but at the same time the tensile strength will be reduced.
2) The temperature of the cooling roller affects the temperature of the cooling roller, which directly affects the transparency of the product. Under normal circumstances, the higher the temperature of the cooling roller, the greater the crystallinity and the worse the transparency of the product. In order to improve its transparency, the cooling effect should be strengthened and the temperature of the cooling roller reduced to reduce the crystallinity of the product.
Under normal circumstances, the temperature of the cooling roller is selected at about 30°C. While considering the temperature of the cooling roller, appropriately increasing the amount of air ventilation is also conducive to improving the transparency of the product. In addition, the temperature of the cooling roller also has a great impact on the strength of the product. The thickness of the product is directly related to the temperature of the cooling roll. When the temperature of the cooling roll is certain, increasing the thickness of the film will decrease the cooling speed, and the crystallinity in the film will increase, resulting in a decrease in the transparency of the product. At the same time, due to the increase in film thickness and insufficient cooling, the film will appear sticky roll phenomenon, limiting the production speed and output of the product, but increasing the thickness of the film can improve the impact strength of the product, so that the safety of the packaging is guaranteed.
2, the effect of traction speed In the production process of CPP films, traction speed is one of the most important reasons for molecular orientation. After the melt is extruded through the T-shaped head, there is only a small amount of orientation in the flow direction. Under the action of traction speed, the polymer molecules will produce a large orientation effect in the longitudinal direction, so that the tensile strength of the film along the longitudinal direction is increased, while the relative elongation is reduced. In order to reduce the greater difference between the vertical and horizontal directions and improve the application performance of the product, the traction speed should be properly controlled during processing. In addition, the size of the traction speed also has a certain impact on the thickness of the product. When the extrusion speed is certain, the acceleration of the traction speed will reduce the thickness of the product and affect the performance of its use.
3, the impact of winding tension in CPP film processing, winding tension directly affects the quality of the product. Under normal circumstances, the winding tension is too large, which is not conducive to the relaxation of the film, after the film is set, there will be cold pulling phenomenon, which seriously affects the performance of the product, and the winding tension is too small, which will bring trouble to the slitting, and the slitting tension is difficult to control. Therefore, in the production, the winding tension should be different according to the different materials. At the same time, but also according to the thickness of the film to make appropriate adjustments, if the film is thick, the winding tension should be selected larger, if the film is thin, should be appropriate to reduce the tension.
4, corona treatment When the CPP film is used as a composite film substrate, or for printing, electrical discharge treatment must be carried out to improve the adhesion fastness and composite strength of the ink. Under normal circumstances, products that can meet the printing and composite requirements must have a surface tension of more than 38dyn/cm, but after corona treatment, the surface tension of the film will decrease over time. Therefore, in the production, the surface tension of the product should be larger than the actual required tension value to offset the impact caused by the decline. The decrease of surface tension is mainly related to the content of low molecular additives in the raw material. Generally, the more the increase of low molecular matter, the greater the decrease of surface tension.