Development direction of high performance BOPP film——plastic film recycling machine, jwell machine
BOPP is generally a multi-layer co-extruded film, which is obtained by co-extruding polypropylene particles into a sheet, and then stretching in both vertical and horizontal directions. This film has good physical stability, mechanical strength, air tightness, high transparency and gloss, and is tough and wear-resistant. It is a widely used packaging film and is also the base film of BOPP tape.
On the basis of the material properties of the existing films,improving the important performance control indicators of BOPP films is an important mean to improve the market competitiveness of BOPP films.
During the use of BOPP film packaging, the static electricity of the film is composed of two parts:
The static electricity carried by the film itself and the static electricity generated by friction during the packaging process. Static electricity will cause it to produce electrostatic adhesion, which will have adverse effects on cutting, conveying, folding films, etc., and will cause malfunctions in the operation of the film machine. Therefore, if only the electrostatic value of the film itself is emphasized, and the electrostatic value generated during the packaging process is ignored, the detection performance of the film will be very good, but there will always be failures when running on the machine.
Antistatic properties are one of the basic conditions to ensure smooth packaging. Long-acting or permanent antistatic agents have been used industrially, but the amount added is too large, the cost is high, and the negative impact on optical properties is relatively large. On the basis of the current technology, it will be one of the main research directions to obtain ideal, stable and continuous antistatic properties with a small amount of antistatic agent. The in-depth study of BOPP film with long-term antistatic properties can be considered from two aspects: one is the polarization of the surface of BOPP film; the other is to get rid of the dependence of antistatic property on humidity and directly add conductive substances to the surface.
02. Differentiated friction performance
BOPP film is used for carton packaging. When the film is in contact with paper during the packaging process, its friction coefficient should be controlled at a slightly higher level, so as to facilitate the positioning of carton packaging and film in the forming wheel, improve the folding quality of the film, and obtain compactness. packaging effect. Further research in this direction can start from how to adjust the differential friction coefficient of the inner and outer layers to meet the needs of positioning and high-speed packaging.
In BOPP film, there are many factors that affect the coefficient of friction:
(1) The type of slip agent. Silicone oil and amide slip agents have good high and low temperature slip properties, while waxes have better slip properties at room temperature. Slip agent can significantly reduce the coefficient of friction and is the main factor affecting the friction performance of the film.
(2) Anti-blocking agent. The anti-blocking agent is generally a solid powder with a particle size of 2-5 μm. Adding it to the surface of the film can form many protrusions, which will reduce the actual contact area between the film layers and between the film and the external interface. Reduce its cohesive force, and it will be easier to slide each other, which is conducive to the reduction of the friction coefficient.
(3) Antistatic agent. Commonly used internally added antistatic agents are surfactants, which can reduce the surface tension of the film, thereby reducing the coefficient of friction.
In the production of enterprises, the requirements for the coefficient of friction of products are very different. For example, some require the coefficient of friction of the film to exceed 1, that is, to produce non-slip BOPP film; some require the coefficient of friction of the film to be less than 0.15, to produce super smooth BOPP film.
03. Low temperature heat sealing performance
The heat-sealing performance of BOPP film is characterized by heat-sealing temperature and heat-sealing strength, and the heat-sealing temperature should generally be controlled between 85 and 110°C. Different packaging machines have different heat-sealing conditions, and the same type of equipment requires different heat-sealing temperatures in different operating environments. Therefore, the wider heat-sealing temperature range makes the film have better heat-sealing adaptability, which can ensure its smooth operation on various packaging machines.
The heat-sealing strength of BOPP film is related to the thickness of the heat-sealing layer. In actual production, the thickness of the heat-sealing layer should be controlled according to the needs of use. In general, the thickness of the heat-sealing layer of a 22μm standard film is 0.8~1.2μm, which is suitable for packaging For films with slower speeds, the heat-sealing time of the film is slightly longer during use, so the thickness of the heat-sealing layer of the film can be adjusted appropriately. The selection and use of heat-sealing materials directly affect the packaging performance of the film on the machine. The polypropylene resin used for heat sealing must have a lower heat sealing temperature and a higher melting point, which is a trend in the development of high-speed heat sealing materials.
04. Adjustable heat shrinkage rate
Adjusting the heat shrinkage rate of BOPP film can start from the following aspects.
(1) Add stiffening masterbatch to the core layer. The stiffening masterbatch has a lower melting point and better compatibility with PP, and it is easy to cause a high degree of orientation of the polymer chain during the stretching process of the BOPP film. The amount of stiffening masterbatch can be adjusted between 0-30%, so the film can obtain a thermal shrinkage rate of 0-12%, but the stiffening masterbatch is also likely to cause a large after-shrinkage of the film.
(2) Adjust the stretching process. The longitudinal shrinkage of BOPP film is its main control index. In terms of stretching process, the thermal shrinkage of the film can be adjusted by adjusting the longitudinal stretch ratio, stretching preheating temperature and stretching temperature. For example, when the longitudinal stretch ratio is 6 times, the stretching preheating temperature is 115°C, and the stretching temperature is 95°C, the heat shrinkage rate that can be obtained is 4%~5%.
(3) Control the setting temperature. The thermal shrinkage rate is also related to the setting temperature after the film is stretched horizontally. The orientation and de-orientation of the film are a pair of contradictions. At a certain temperature, the de-orientation of the film proceeds spontaneously. The setting temperature after transverse stretching of the film is generally 135~155°C. At such a high temperature, the de-orientation of the film can occur rapidly. In actual production, the thermal shrinkage rate of the film can also be controlled by adjusting the setting temperature.
When adjusting the thermal shrinkage rate of the film, the post-shrinkage of the film should be prevented.
05. High gloss, low haze
In addition to ensuring that the film can be packaged on the machine normally, the most important function of BOPP packaging film is the bright packaging appearance. Starting from the basic principles of optics, two important quantitative indicators of the optical properties of BOPP films appear, namely gloss and haze.
Gloss is used to evaluate the visual appearance of a film surface. The more light reflected directly from the surface of the film, the higher the gloss. The light reflected by the high-gloss surface is highly concentrated, which can clearly reflect the image. Therefore, the surface of BOPP film should have high surface smoothness. Haze is inversely known as transparency, which is a measure of the percentage of transmitted light that deviates from the direction of incident light at a greater than a certain angle. When the scattering angle is small, the contents of the package are relatively clear; if the scattering angle is large and inconsistent, the contrast will be reduced, the contents of the package will be hazy, and the lower haze can show the clear and bright trademark pattern on the outer box of the product.
The pursuit of high gloss and high transparency has always been the research and development goal of BOPP film companies. Although the quality of domestic BOPP films has been greatly improved in recent years, there is still a large gap with foreign films in terms of optical properties.
06. Stiffness, abrasion resistance
In addition to the properties mentioned above, stiffness is also an important property of BOPP film. In the index control process of film products, the stiffness is not directly measured, but reflected by the elastic modulus. The greater the modulus of elasticity, the higher the stiffness. When the film stiffness is high, high folding quality can be obtained in the packaging forming process, and good heat sealing effect can be achieved in a short pause time. High stiffness is a prerequisite for increasing packaging speed, and it is suitable for high-speed packaging machines with automatic speed changes.
At present, a technical problem that BOPP films urgently need to solve is to improve the surface scratch resistance of the film. Although some research has done some work on improving the hardness of the substrate PP, the problem has not been fundamentally resolved. Some manufacturers have introduced anti-abrasion flower BOPP. The thin film, after comparative analysis, actually only reduces the surface scratches to a certain extent. In-depth study of the root cause of the easy abrasion of the film surface and the negative effect of anti-blocking agent particles on the surface anti-scratch is an important direction for the high performance of BOPP films.