The coefficient of friction is one of the basic physical properties of plastic films . When two objects in contact with each other have relative movement or relative movement tendency, the mechanical force generated by the contact surface of the two objects that hinders relative movement is friction. The friction properties of plastic films can be characterized by the dynamic and static friction coefficients of the materials. The static friction force is the maximum resistance at the beginning of the relative movement of the two contact surfaces, and the ratio to the normal force is the static friction coefficient; the dynamic friction force is the resistance when the two contact surfaces are relatively moved at a certain speed, and the ratio to the normal force is the dynamic friction coefficient. . The coefficient of friction is an index for measuring the sliding characteristics of packaging materials . The smooth surface of the film material and the appropriate coefficient of friction are very important for the film in printing, laminating, bag making, and packaging transportation.
The use of plastic films in different regions, environments, and different packaging material combinations will require different friction coefficients. Therefore, flexible packaging companies need films with different friction coefficients, and film companies also need to produce different friction coefficients according to the different needs of enterprises film.
First, the surface friction coefficient, roughness and smoothness of plastic film
The surface friction coefficient, roughness and smoothness of plastic film, the relationship of these three indicators are complementary.
The smoothness of the film can be evaluated by the friction coefficient. Evaluating slip properties and dynamic (static) friction coefficients have important reference values. The dynamic (static) friction coefficient generally has the same trend. When the slip is improved, the dynamic (static) friction coefficient will change. In addition, in order to be closer to the needs of downstream customers, the friction coefficient can be detected by simulating changes in different temperature (humidity) and different conditions.
1.Friction coefficient of plastic film surface
In production, the coefficient of friction of the film is an important technical indicator. On the one hand, it and the anti-blocking performance of the film together become a quantitative evaluation index of the openness of the plastic film. On the other hand, it can be used as one of the reference data for the automatic packaging machine's running speed, tension adjustment, and wear during film operation. In the process of printing, laminating, bag making, packaging and transportation, the surface friction coefficient of the film has a linear relationship with its surface roughness. Under certain conditions, the greater the surface roughness, the smaller the friction coefficient. In other words, reducing the friction coefficient of the film surface is beneficial to printing, compounding, and aluminizing. It can increase the bonding surface between them and the plastic film, and improve the adhesion between them. The coefficient of friction is adjusted by adding a dose.
2. Surface roughness of plastic film
The surface roughness is a comprehensive evaluation of the micro-geometric characteristics of the small peak-valley irregularities on the surface of the thin film. Proper surface roughness facilitates ink printing, lamination, and vacuum aluminum plating. Of course, if the roughness is too large, ink, glue or aluminum molecules cannot fill the recessed points on the film surface, forming voids and affecting the adhesion between the two. The film is delaminated and small dots are lost during printing.
Generally, the surface of the plastic film is very smooth, and the smooth surface will cause blocking when the film is rolled up, and cannot be rolled up normally, and it is not easy to unwind. At the same time, the smooth film surface is also very detrimental to ink printing, compounding and vacuum aluminum plating, because the smooth surface will greatly reduce the adhesion between ink, adhesive and film, including the adhesion between ink, adhesive and aluminum foil and film. . In order to make the film surface have a certain degree of roughness (except corona treatment) to increase the adhesion with other substances, a method of adding an anti-blocking agent to the film resin is usually used to form the film surface during the film formation process. Certain roughness. The surface roughness of the film is related to the type and quantity of additives (anti-blocking agents), the particle size and shape of the additives, the dispersion of the additives, the surface treatment and other factors. Different additives should be selected according to different types of films and applications. With the increase of the additive content in the film, the friction coefficient μs of the film decreases, and the surface roughness increases. Generally, it is more suitable to control the surface roughness of the plastic film at Ra = 0.08 ~ 0.16.
3.Smoothness of plastic film
The smoothness is mainly affected by the type and content of additives, and also by the corona treatment. In addition, the film use environment, downstream customer use process conditions, and production environment will also affect the smoothness. The smoothness of the film is important for the smooth production of the film and the use of downstream customers. This requires that the slip process be controlled in the film production process so as to be able to provide proper slip, without causing other impacts on the use of downstream customers due to the precipitation of additives.
Slip agents are also called lubricants. Lubricants are divided into internal lubrication and external lubrication. When processing CPP (IPP) and CPE (IPE) films, amide-based slip agents are usually added to the heat-seal layer. When processing BOPP films, slip agents are usually added to the core layer. The molecules of these lubricants migrate to the surface of the film ("outward migration") to form a layer of oily surface, which plays a lubricating role to improve the surface properties of the film and reduces the coefficient of friction. The addition amount should be determined according to the thickness of the film and the friction coefficient value required by the customer. Generally, amide-based slip agents are not required during the processing of BOPET and BOPA films.
Influence of friction coefficient on film gravure printing
1. Generally, a small inner friction coefficient and a suitable outer friction coefficient are required in printing.
The outer friction coefficient is too large, which will cause excessive resistance during the printing process, causing the material to stretch and deform to affect the printing effect. If it is too small, it may cause the drag mechanism to slip, causing the correction system to be inaccurate, and the printing fastness to be reduced. Affects packaging speed. In addition, in the printing process, the fineness value of the ink is low, and the pigment particles are coarse. In this way, the friction coefficient becomes larger during printing, and the printing resistance of the printing plate is low. It is also easy to produce offsets, ink accumulation and ink transfer failure during printing. Parity.
Take nylon film as an example: At normal temperature, the dynamic friction coefficient of BOPA film surface (non-treated surface) is generally required to be between 0.25 and 0.35 (non-moving national standard is ≤0.6). And in areas with high humidity such as Sichuan and Chongqing, or in high temperature and high humidity climate seasons, the friction coefficient of the surface of the BOPA film (non-treated surface) should be less than 0.25. Otherwise, the film surface adhesion state is likely to occur. In order to meet the requirements for fast coding of finished bags, BOPA film launched high-slip and super-slip products to meet the market demand. Nylon film is a film with high moisture absorption. Generally, under normal use environment, the film will absorb moisture to reach a saturated state within 20 minutes, and its friction coefficient will not change much after moisture absorption and saturation.
Table 1 Variation of non-non-friction coefficient of BOPA film under different humidity conditions
Since the original standard has been formulated for ten years, and this is the decade when domestic nylon membrane production and its rapid development have been in use, many physical indicators have not adapted to the current market and product development needs, so it should be based on different regions and products. Need to set the respective friction coefficient value.
2. Under the conditions of high temperature and high humidity, consider the influence of friction coefficient
In high-temperature and high-humidity environments, special attention should be paid to improving the binding fastness of the ink to the substrate and increasing the printing speed.
Generally in use, the metal parts of the machine are mostly operated at a high temperature of about 40 ° C. The increase in temperature has a great impact on the friction coefficient, because the thermal mobility of macromolecules of polymer materials under high temperature conditions is enhanced, the activity speed is accelerated, the molecular gap is enlarged, and the surface properties change will change the film friction coefficient. Generally speaking, as the ambient temperature increases, the friction coefficient of the surface of the material will change to a certain extent, but the magnitude of the change varies greatly due to the sensitivity of the material to temperature and the type and amount of additives. For example, before printing, not only the friction coefficient of the BOPA film at normal temperature should be measured, but also the change in the friction coefficient of the actual use environment temperature and humidity should be detected. The molecular arrangement gap of BOPA film itself is larger than that of other films. It is very beneficial to measure the change of friction coefficient under the condition of using temperature to stabilize and improve product quality.
Effect of friction coefficient on compounding and curing
1.Effect on film winding tension and quality
In actual use, plastic films are used as printing layers, composite interlayers, and PE and CPP as heat sealing layers. However, in the composite, the change in the friction coefficient of each layer of material will also affect the composite peeling fastness, and the type and content of the slip agent have the greatest effect on the friction coefficient.
The slip agents of PP and PE are mostly inorganic polymer materials (such as amines), which are not compatible with the resin. Usually from 30 ° C, the commonly used slip agents have reached their melting point and become sticky. The friction coefficient rises sharply, and the force applied during the test varies greatly, exhibiting an intermittent sliding or bonding effect.
After the composite winding, the pressure and distance between the layers of the composite film are also closely related to the thickness uniformity of each substrate constituting the composite film, the design of the printed pattern, and the thickness of the ink layer. The thicker the ink layer, the greater the hardness of the area after rewinding, which means that the greater the pressure between the layers of the composite film there, the smaller the distance between the layers, and the "smoothing" from the film is smooth The greater the chance that the agent will "migrate" to another film, the greater the quantity. As a result, the coefficient of friction of the surface of the heat seal layer corresponding to the area where the ink layer is thick is significantly larger than the coefficient of friction of the surface of the heat seal layer corresponding to the area where the ink layer is thin or no ink.
In addition, if the friction coefficient of the composite film is too small, the friction force between the inner surface layer and the outer surface layer of the composite film during winding is very small. Cannot enter the next process. Occasionally, unwinding may occur and the rewinding may fail. If the friction coefficient is too large during rewinding, it will bring other quality problems, such as severe wrinkling and film breakage.
2.Friction coefficient decreases after aging
Some flexible packaging companies have reported that the friction coefficient of the non-treated surface of the heat-seal layer film was originally required to be between 0.1 and 0.2, and the actual measured result of the friction coefficient of the film entering the factory may be greater than 0.3. If the film is placed in the curing chamber for a period of time, the friction coefficient can be reduced to 0.1 ~ 0.2. This is that the slip agent that has been added to the film requires a certain temperature and time to complete the "outward migration", thereby destroying the fastness of the composite layer. Therefore, enterprises should pay special attention to the changes in the compound peeling fastness of high-temperature cooking bag products and products with a heat seal layer over 60um.
3.Effect of friction coefficient on automatic packaging and bag making
In the actual packaging process, the friction force is often both drag force and resistance, so the size of the friction coefficient must be effectively controlled to make it within an appropriate range. If the coefficient of friction is too large, for example, the friction coefficient (movement) of the composite film exceeds 0.3 (contact surface with steel), then the roll film will not be pulled on some wide bags or automatic packaging machines. If the coefficient of friction (treatment surface) of the bag-making product exceeds 0.25, the bag with a smaller specification is prone to have a defective opening. The friction coefficient is not as small as possible. For example, the roll film for automatic packaging, the friction coefficient of the inner layer film is too small, which may cause the bag making finished product to be unstable and cause wrong side chaos. Causes the drag mechanism to slip, resulting in inaccurate tracking and cutting positioning of the electric eye.
The coefficient of surface friction of the heat-sealable layer of film products is an index of great concern for composite flexible packaging. Different applications of packaging materials have different requirements for friction coefficient. The terminal enterprises have also put forward increasingly strict requirements for the surface friction coefficient of the heat-sealing layer of the roll film packaging material.
At present, in the use of automatic packaging machines and bag making machines, with the continuous updating of equipment, the production speed continues to increase. The acceptance index of the surface friction coefficient (dynamic) of the automatic packaging material is basically around 0.15 (the coefficient of friction between the film and the steel plate). During the processing of composite materials, the surface friction coefficient of the heat seal layer will show a certain degree of change with different weather and process conditions. For example, the friction coefficient of the non-treated surface of the BOPA film needs to be smaller (about 0.18 ~ 0.22) when fast coding (bag making) (more than 250 pcs / min).
Some special process requirements or targeted products, for which requires a particularly low coefficient of friction, should be treated with caution. To control the coefficient of friction between film and metal within a certain range, the moisture absorption of the film in use should also be considered. For example, the coefficient of friction of nylon film and cellophane after moisture absorption (not reaching saturation) must be higher. The friction coefficient is too small, which may cause unstable feeding, slippage, dislocation, bag cutting bleeding, or different specifications of the finished product. The friction coefficient is too large, the friction between the film and the machine is increased, and the material is difficult to drag, which will cause the material to be pulled and move, which will cause the material to stretch or deform or break.
Generally, the thicker the film, the more additives are added, and the more serious the phenomenon of delamination. Especially for PE films with a thickness of 60um or more, the amount of additives added in production increases the chance of delamination. This is because at the same surface area, as the thickness of the film increases, the amount of additives contained in the film also increases. Increase accordingly. When the content of additives exceeds a certain amount, the fastness of the composite film will be affected. Therefore, when producing boiled and cooked products, special attention should be paid to the amount of additive contained in the heat-sealing layer material. Generally, films with little or no additives are selected.
In addition, adding too much dosage like PET, BOPA film will also increase the roughness of the film surface, and also form granular pits, which will affect the surface gloss and printing effect. Especially when using silica as an anti-binder, this kind of phenomenon will occur, and at the same time, the haze of the film will be increased. Therefore, to improve the transparency of the film, the content of silicon dioxide must be minimized. The amount of adhesive and slip agent, as well as the stiffness, smoothness, and smoothness of the film will affect the friction coefficient of the composite film. In general, the friction coefficient of the composite film is not implicitly related to the type of added slip agent, the amount added, the amount of migration, the direction of migration, and the temperature and humidity during storage.
Figure 1 Trends of different content additives, friction coefficient and haze
Fourth, the case analysis of the influence of friction coefficient changes on quality
1. PA // PE, poor peel strength of composite film
Some flexible packaging companies found that the peel strength was poor after compounding PA // PE. After peeling it off, a white "cream" -like substance can be seen on the PE film, and it can be removed by hand. Put the composite film back into the curing room or electric heating box at 80 ℃ for a period of time, and check the peel strength immediately after taking out, the peel strength value will be significantly improved, and the original white film will not be seen on the PE film. "Frost" -like material; but after the composite film is left at room temperature for two days, its peel strength will gradually decrease with time.
This is because after a period of time, the PE film slip agent will gradually migrate to the surface of the PE film, forming a very thin dense layer. This dense layer blocks the bonding of the adhesive to the surface of the PE film. The adhesion between the adhesive and the PE interface layer prevents the adhesive from contacting the PE molecules, which results in a significant decrease in the composite initial adhesion. Even with increasing curing time, the composite strength is always low. This problem indicates that for PE films with special requirements, high slip agents must be used, and generally should not be left for too long. If the PE film is used for an extended period of time due to some reasons, it is best to test it or put the PE film at a temperature of 60 to 70 ° C for more than 8 hours before lamination. At this time, the slip agent will partially fail, and then go to lamination. Will reduce the problem of low composite strength.
2.Effect of slip agent transfer between films on compound fastness
The inner surface of the PA film is bonded to the composite surface of the PE film through the action of an adhesive, and the outer surface of the PA film is tightly "closed" to the heat cover of the PE film under the effect of the winding tension. Together.
Therefore, the slip agent that has migrated to the surface of the heat seal layer may migrate to the surface of the PA film and the inside thereof. The slipping agent migrated to the composite surface of the PE film will lead to a decrease in peel strength, on the other hand, it will reduce the amount of effective slipping agent for the PE film hot cover, which will cause the friction coefficient of the hot cover to increase. The internal slip agent on the one hand will cause the amount of effective slip agent on the PE film heat cover to decrease, which will cause the friction coefficient of the PE film heat cover to increase, on the other hand, it will cause the friction coefficient of the outer surface of the PA film to decrease.
3.Effect of friction coefficient on BOPA film in solvent-free compounding
The types and components of different solventless adhesives are different, and the friction coefficient of the composite film is also greatly different. When performing solventless compounding, it is necessary to select a suitable solventless adhesive according to the specific requirements of the friction coefficient.
Increasing the sizing amount not only causes the friction coefficient to increase, but also has other adverse effects: After the composite film is compounded, its friction coefficient will increase by about 0.1 ~ 0.2, mainly due to the absorption of the slip agent by the adhesive and the wear or consumption during production. Remove some slip agent. Therefore, in production, the control of the sizing amount is also very important. Adding one gram of sizing amount per square meter will increase the friction coefficient by about 0.025.
The sizing amount should not be too small. If the sizing amount of solvent-free compounding is small, it will easily cause quality problems such as uneven coating, white point, and low strength. Especially white inks are prone to white point and bubble problems.
Different types of solventless composite adhesives have different components. As the aging temperature increases, the degree of increase in the friction coefficient also varies. For example, the mainstream polyester-based component and polyether-based component are the main factors that affect the friction coefficient. Under the same conditions, the curing temperature is increased, and the friction coefficient of the composite film is significantly different. On the one hand, as the temperature increases, the reaction of the adhesive to absorb the slip agent becomes faster, which makes the slip agent partially ineffective; on the other hand, some low-melting slip agents become stuck when the temperature increases, and lose their effect by themselves. Similarly, as the maturation time increases, the friction coefficient increases. In solvent-free compounding, it is generally recommended that the curing temperature does not exceed 40 ° C. Generally speaking, the friction coefficient of PE film is required to be about: 0.1 ~ 0.15 for bag-making products; friction coefficient for semi-finished products of rolls: about 0.2. Therefore, the size of the friction coefficient of the raw material film such as PE and CPP directly affects the friction coefficient of the composite film, which can only be controlled from the source to ensure that the final product has a suitable friction coefficient. In this way, in solvent-free compounding, quality assurance is only available for products containing BOPA // PE and BOPET // CPP combinations.