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Effect of friction coefficient on the application of BOPA film

Release time: 2020 / 06 / 28 10:05:07 reading volume: 1448 times


Friction coefficient is one of the basic physical properties of BOPA film. When there is relative motion or relative movement tendency between two objects in contact, the mechanical force that hinders the relative motion generated by the contact surface is friction force. BOPA The friction properties of the 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 relative movement of two contact surfaces, and the ratio of static friction force to reverse force is the static friction coefficient; the dynamic friction force is the resistance of two contact surfaces moving relative to each other at a certain speed, and the ratio of static friction force and reverse force is the dynamic friction coefficient.

The friction coefficient is an index to measure the sliding characteristics of packaging materials. The smooth surface of film materials with appropriate friction coefficient is very important for the film in printing, laminating, bag making, processing, packaging and transportation. The production of plastic films with different friction coefficients has different requirements and influences on the application range and packaging materials combination products in different regions and environments.

1、 The surface friction coefficient, roughness and smoothness of BOPA film are as follows

The surface friction coefficient, roughness and smoothness of BOPA film are closely related to each other.

The static friction coefficient is related to the force required to make the object move from static to moving. The dynamic friction coefficient refers to the force required to maintain the motion of the object. The smoothness of the film can be evaluated by the friction coefficient. These two parameters have important reference value in evaluating smoothness. The static and dynamic friction coefficients generally have the same change trend. When the smoothness is improved, the static and dynamic friction coefficients will decrease. In addition, in order to meet the needs of production and downstream customers, we can simulate the change of friction coefficient under different temperature and time conditions. A change in film wettability can also be caused by a change in film wettability due to changes in film wettability.

1、 BOPA The friction coefficient of the film surface is: 1

In the production of BOPA film, the friction coefficient is an important technical index. On the one hand, it can be used as the quantitative evaluation index of film opening together with the film anti adhesion performance, on the other hand, it can be used as one of the reference data of automatic packaging machine running speed, tension adjustment and film running wear. In the process of printing, compounding, bag making, processing, packaging and transportation, the friction coefficient of plastic film is also required. Under certain conditions, the smaller the surface roughness is. In other words, reducing the friction coefficient of the film surface is beneficial to printing, laminating and aluminum plating, and it is beneficial to increase the joint surface between them and plastic film, and to improve the adhesion between them. The friction coefficient is adjusted by adding dose.

2、 BOPA The surface roughness of the film is as follows     

Surface roughness refers to the comprehensive evaluation of the micro geometric size characteristics of the micro peak valley unevenness of the film surface at a small distance. It is certain that proper surface roughness is beneficial to ink printing, lamination and vacuum aluminizing. Of course, if the roughness is too large, it may cause that the ink, glue or aluminum molecules can't fill the surface depression of the film, and form voids, which will affect the adhesion between the two. In serious cases, it will cause the ink, glue or aluminum coating to separate from the film, and the small dot loss during printing.

Generally, the surface of BOPA film is very smooth, and the smooth surface will produce adhesion when the film is rewinding, which can not be rolled normally and is not easy to unwind. At the same time, smooth film surface is also very unfavorable to ink printing, lamination and vacuum aluminum plating, because 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 roughness (except corona treatment) and increase its adhesion with other substances, the method of adding some anti adhesion agent to the film resin is usually used to make the film surface form a certain roughness in the film forming process. The size of film surface roughness and additives ( Anti adhesion agent ) The type of additives, the amount of additives added, the particle size of additives are related to the shape, dispersibility and surface treatment of additives. Different additives are selected according to different types of films and applications. With the increase of additive content, the friction coefficient μ s The surface roughness increases with the decrease of film surface roughness Ra=0.08~0.16 It is more suitable.   

3、 BOPA Smoothness of films

The content of additives is also affected by the content of corona. In addition, the film using environment, downstream use process conditions, production environment will also affect the smoothness. The smoothness of the film is very important for the smooth operation of the film production process and downstream customers. This requires the film production process to control the smoothness, so as to provide the appropriate smoothness, and at the same time, it will not cause other faults to the downstream customers due to the precipitation of additives.

Lubricants are also known as lubricants. Lubricants are divided into internal lubrication and external lubrication. CPP in process( IPP )、 CPE IPE )Film, the smooth agent is usually added to the heat seal, in the processing BOPP In the case of thin film, the lubricant is usually added to the core layer. The molecules of these lubricants migrate to the surface of the film( " Emigration " )A layer of oily surface is formed, which can improve the performance of the film surface and reduce the friction coefficient. The addition amount depends on the thickness of the film and the friction coefficient required by the customer. stay BOPET and BOPA It is not necessary to use amides in the process of film processing.

2、 Influence of friction coefficient on BOPA film in gravure printing process

1. Generally, small inner friction coefficient and appropriate outer friction coefficient are required in printing. Too large outer friction coefficient will cause excessive resistance in printing process, cause material tensile deformation, and affect printing effect. If too small, it may cause sliding of drag mechanism, inaccurate deviation correction system, reduced printing fastness and packaging speed. In addition, the fineness of the ink is low, the pigment particles are coarse, and the friction coefficient is large in the printing process, so the printing plate's printing resistance is low, and it is easy to produce paste, ink accumulation and uneven ink transmission during printing. Take nylon film as an example: General requirements at room temperature BOPA The dynamic friction coefficient of the film surface (non treated surface) is 0.2~0.4 between. However, in areas with high humidity such as Sichuan and Chongqing, BOPA The friction coefficient of film surface (untreated surface) should be less than zero point two five In order to meet the requirements of quick coding of finished bags, therefore BOPA High smooth and super smooth products are introduced to meet the market demand.

For BOPA film under different humidity conditions / It can be seen from the table that the nylon film with high moisture absorption is generally in the twenty The results show that the moisture absorption reaches the saturation state within minutes, but the friction coefficient of the film changes little after it is saturated.

It should be noted that in the GB / t20218-2006 standard, the friction coefficient (dynamic) is not treated surface / The index of non treated surface is ≤ zero point six And here we propose 0.2~0.4 The reason is that the original standard has been formulated for ten years, and this decade is the rapid development of nylon film and its use. Many characteristics of the original indicators are far from meeting the needs of the current market and product development.

2. Under the condition of high temperature and high humidity, when considering the influence of friction coefficient, special attention should be paid to improving the combination fastness of ink and substrate and improving printing speed. Generally in use, most of the metal parts of the machine are in the fifty It operates at high temperature of about ℃. However, the increase of temperature has a great influence on the friction coefficient. Under high temperature conditions, the thermal mobility of macromolecules of polymer materials is enhanced, the activity speed is accelerated, the molecular gap is enlarged, and the surface properties change, which will change the friction coefficient of the film. Generally speaking, with the increase of the ambient temperature, the friction coefficient of the material surface will change to a certain extent, but the magnitude of the change depends on the sensitivity of the material to temperature and the type and amount of additives added. It's not just measuring before printing BOPA The friction coefficient of the film at room temperature should also be measured under the actual use of environmental temperature and humidity.

3、 Effect of friction coefficient on BOPA film in compounding and curing process :

1. The influence of film winding tension and quality :

Although BOPA film is used as printing layer and composite interlayer , There are also used as thermal seals PE and CPP However, due to the change of the friction coefficient of the heat seal material, the peel fastness of the composite is also affected, among which the type and content of the smooth agent are the most important.

The smooth property of the film is mainly achieved by adding a smooth agent. After the film is formed, the smooth agent migrates from the inside of the film to the surface and accumulates into a uniform thin layer, which can significantly reduce the friction coefficient of the film and make the film have a good smooth effect. PP、 PE Most of them are inorganic polymer materials ( Such as amines ) It has poor compatibility with resin, and is usually from thirty The friction coefficient of the film will rise sharply, and the applied force will fluctuate greatly during the test, showing an intermittent sliding or bonding effect. If the content of the lubricant is too high, it will affect the printability and compounding suitability of the film, which is very unfavorable to the printing and lamination. The pressure and distance between the composite film layers after composite winding are closely related to the thickness uniformity of each substrate, the design of printing pattern and the thickness of ink layer. The thicker the ink layer, the greater the hardness of the part after winding up, which means that the greater the pressure between layers and the smaller the distance between layers " Emigration " The smooth agent coming out " transfer " The probability of going to another film will be greater and the number will be more. The result is that the friction coefficient of the hot seal surface corresponding to the ink layer thickness area is significantly greater than that of the hot seal surface corresponding to the area with thin ink layer or no ink layer.

In addition, if the friction coefficient of the composite film is too small, the friction force between the inner surface layer and the outer layer of the composite film is very small. When the winding diameter is larger and larger, the friction force can not support the film winding end face to be neat, resulting in cross rolling, and can not enter the next production process. Sometimes the unwinding will occur and the winding will fail. If the friction coefficient is too large, it will lead to other quality problems, such as serious wrinkling, broken film and so on.

2. The friction coefficient decreased after aging :

Some flexible packaging enterprises reported that the friction coefficient of the non treated surface of the heat seal film was originally required to be between 0.1 and 0.2, and the measured friction coefficient of the film entering the factory may be greater than zero point four If the film is put into the curing chamber for a period of time, the friction coefficient can be reduced to 0.1~0.2 Between. This is the smooth agent added into the film, which needs a certain temperature and time to complete " Emigration " Therefore, the color fastness of the composite layer is destroyed, especially the high temperature cooking bag products and the heat sealing layer 60um The change of compound peel fastness of the above products.

4、 The influence of friction coefficient on automatic packaging and bag making process:

In the actual packaging process, the friction force is often both a drag force and a resistance, so the friction coefficient must be effectively controlled to make it in an appropriate range. If the friction coefficient is too large, such as the friction coefficient of composite film is more than 0.3( BOPA If the friction coefficient (processing surface) of the bag making products exceeds zero point two five Small size bags are prone to poor opening. The smaller the friction coefficient is, the better. For example, if the friction coefficient of inner film is too small, it may lead to unstable stacking of finished bag making products, resulting in dislocation and confusion. If the friction coefficient of outer layer is too small, it may cause sliding of drag mechanism, resulting in inaccurate tracking and cutting positioning of electric eye.

The surface friction coefficient of the heat seal layer of composite film products is a very concerned index for the enterprises engaged in the processing of composite flexible packaging materials. Different packaging materials have different requirements for friction coefficient. The end of the end of the hot film packaging material is also more and more stringent requirements of the surface coating. At present, in the use of automatic packaging machine and bag making machine, with the continuous updating of the equipment, the production speed is constantly improved. The acceptance index of the surface friction coefficient of the automatic packaging material is basically about 0.15 (friction coefficient value of film and steel plate). In the process of composite processing, the surface friction coefficient of heat seal layer will change with the climate and process conditions. BOPA Friction coefficient of non treated surface of membrane , In fast coding (bag making)( two hundred and fifty individual /min The requirements are smaller. ( about 0.15~0.20) For some special process requirements or targeted products, especially low friction coefficient should be treated with caution. The friction coefficient between the film and metal can be controlled in a certain range, and the moisture absorption of the film in use should also be considered, such as nylon film and cellophane after moisture absorption ( Not saturated ) The coefficient of friction is definitely higher. The friction coefficient is too small, the feeding is not stable, the sliding and dislocation are affected, the bag making and cutting bleeding or the length and length of finished products are different. If the friction coefficient is too large, the friction between the membrane and the machine will increase, and the material will be difficult to drag, which will cause the material to fail to move and stretch or break, so that the material can not be produced.

The more additives are added, the thicker the film will be. If the content of smoothness agent is too high, it will affect the printing and lamination of the film, which is very unfavorable for printing and lamination. especially 60um Over thickness PE Membrane, adding more dosage in production , The possibility of delamination also increases , This is because under the same surface area, with the increase of film thickness, the measurement of additives in the film also increases. When the additive content exceeds a certain amount, it will affect the fastness of the lamination. Therefore, the production of boiling, cooking products to pay special attention to the amount of heat seal material contained in the amount, generally choose less or no additive film.

In addition, like pet BOPA Too much film additive will increase the roughness of the film surface , It can also form granular pits and affect the surface gloss and printing effect. Especially when silica is used as anti binder, this phenomenon will occur , At the same time, it will increase the haze of the film , Therefore, in order to improve the transparency of the film, the content of silica must be reduced as much as possible. The friction coefficient of the composite film can be affected by the addition of anti adhesion agent and smooth agent of the inner layer material and the stiffness, smoothness and smoothness of the film. It is related to the amount of additive, the direction of friction and the type of film.

5、 Material combination of BOPA film as an example , The influence of the change of friction coefficient in use:

1) PA//PE The peeling strength of the composite film is poor

Some flexible packaging enterprises reported that the peeling strength of the processed packaging materials, such as PA / / PE structure, was poor PE A layer of white can be seen on the membrane " frost " And can be erased by hand. Put the composite film back into the curing chamber or eighty After taking it out of the electric heating box at ℃ for a period of time, the peeling strength value will be significantly improved PE There is no white color on the membrane " frost " However, the peel strength of the composite film decreased with the time after it was placed at room temperature for two days.

This is because: after a period of time, PE film smoothing agent will gradually migrate to PE On the surface of the film, a thin dense layer is formed, which blocks the adhesive and the adhesive PE The binding of the membrane surface (at PA membrane // Adhesives and PE As a result, the adhesive can not mix with PE The initial viscosity of the composite decreases obviously due to the contact of molecules. Even with the increase of curing time, the composite strength remains very low. For special requirements must use high smooth agent PE In general, the membrane should not be placed for too long. If for some reason PE For the extended use of membrane, it is better to make a small test before compounding PE Membrane placement 60~70 At the temperature of ℃ 8h At this time, the composite strength will be reduced.

2) PA//PE After a period of time, the peeling fastness of the composite became worse

With the increase of the solid content of the adhesive, the initial bonding strength of the composite is acceptable, but with the increase of the curing time, the composite strength becomes lower and lower, and a layer of white powder appears between the two layers. This is due to production PE High smoothness was used in the membrane PE The raw material, due to the high content of smooth agent in the film, is subjected to high temperature( 40~50 In the process of curing, a large number of these molecules migrate to both sides of the membrane. With the increase of time, the amount of migration to the surface gradually increases, which destroys the adhesive and the adhesion agent PE The physical bonding process between molecules will destroy the newly formed weak adhesive force. The longer the time, the stronger the destructive force and the lower the composite strength. In this case, the method of increasing the curing temperature is generally adopted to accelerate the crosslinking speed of the adhesive, so that the reaction speed exceeds the migration speed of the smooth agent, so as to make up for the PE The negative effect of too much synovium. The lubricant added into the film will migrate from the surface to the interior of the film at higher temperature ( Internal migration ) At lower temperature, it will migrate to the surface of the film again ( Emigration ) In other words, olefin materials have relatively good compatibility with oleic amide and erucic acid amide external lubricants at higher temperature, while oleic amide and Erucic amide external lubricants have relatively poor compatibility at lower temperature.

3) Smooth agent transfer between the film, the right BOPA The influence of composite fastness of composite roll film is as follows

The inner surface of PA film is influenced by adhesive PE The composite surfaces are bonded together, PA The outer surface of the film is affected by the winding tension PE The thermal surface of the film is tightly attached " by " Together.

Therefore, the lubricant migrating to the surface of hot seal may migrate to the surface and interior of PA film. Migrate to PE On the one hand, the smooth agent on the composite surface of the film will lead to the decrease of peel strength, on the other hand, it will lead to the decrease of peeling strength PE The results show that the amount of effective lubricant decreases, which leads to the increase of friction coefficient PA On the one hand, the smooth agent on the surface and inside of the film will lead to PE Thus, the amount of the effective film on the cover is reduced PE On the other hand, the friction coefficient of the film hot surface will increase PA The friction coefficient of the outer surface of the film decreases. The pressure and distance between the film layers after winding are closely related to the thickness uniformity of each substrate, the design of printing pattern and the thickness of ink layer. The thicker the ink layer, the greater the hardness of the part after winding, which means that the greater the pressure between the composite film layers and the smaller the distance between layers PE In the membrane " Emigration " The smooth agent coming out " transfer " reach PA The probability of going into the film will be greater and the amount will be more. The result is that the friction coefficient of the hot seal surface corresponding to the ink layer thickness area is significantly greater than that of the hot seal surface corresponding to the area with thin ink layer or no ink layer.

4) The friction coefficient in solvent-free composite BOPA The effect of membrane combination was as follows

There is a big difference between the types of composite adhesive and the friction coefficient of composite film. In the solvent-free composite, it is necessary to select the appropriate solvent-free composite adhesive according to the specific requirements of friction coefficient.

The friction coefficient of the composite film will increase by about 0.1 ~ 0.2 after the composite film is compounded, which is mainly due to the absorption of the lubricant by the composite adhesive and the wear or consumption of a part of the lubricant in the production. Therefore, in the production, the control of the amount of glue is also very important. If one gram of glue is added per square meter, the friction coefficient will increase zero point zero two five

The amount of glue can not be too small, solvent-free composite if the amount of glue is small, easy to cause glue coating uneven, white spots, low strength and other quality problems, especially white ink, easy to appear white spots, bubbles.

With the increase of curing temperature, the friction coefficient increases. On the other hand, with the increase of temperature, the friction coefficient of the composite film is obviously different. This is because, on the one hand, with the increase of temperature, the reaction of the adhesive adsorbing the smooth agent is faster, which makes part of the smooth agent lose its function; on the other hand, some low melting point smooth agent adhesion when the temperature increases, and lose its function. Similarly, the friction coefficient increases with the increase of aging time. In solvent-free composite, it is generally recommended that the curing temperature should not exceed 40 ℃. In general, PE The friction coefficient of the film is about: for bag making products, the friction coefficient requirements are as follows: 0.1~0.15 Requirements for friction coefficient of packaged products: zero point two Therefore, the raw film such as PE CPP The size of friction coefficient directly affects the friction coefficient of composite film, which can only be controlled from the source to ensure that the final product has the appropriate friction coefficient. In this way, the PE CPP Only the quality of the combined products can be guaranteed.

5) The friction coefficient of the composite coextrusion film was improved

The three-layer coextrusion composite film was used as the inner sealing material of washing powder extrusion and BOPA / inner printing was used as printing film, ECPA Polyethylene was used as the intermediate layer for extrusion and the inner layer was coextruded forty μ m It is found that the coefficient of friction is changed from the 0.2~0.3 Up to 0.7~0.8 Why?

The reason for the increase of friction coefficient of inner layer heat sealing film after extrusion is that the temperature is higher during extrusion. The inner sealing film is pressed with steel roller and silicone rubber roller with outer layer BOPP film at higher temperature, and the soft low melting point before lamination must be made( LLDPE The melting point is one hundred and twenty-five one hundred and thirty-five C) LLDPE LDPE(50:50) In addition, the friction coefficient will be greatly increased due to the friction caused by the traction force in production. (three layer coextrusion material is :LLDPE+LDPE(50:50)/LLDPE(100%)/LLDPE+LDPE(50:50)

6) Influence of environmental temperature and humidity , The surface of finished bag is sticky and the code spraying is not good

There is a phenomenon that the surface of the finished product does not stick to the surface of the bag for several times.

It is difficult to describe the phenomenon of sticking on the outside of the bag: 1; two The inner opening of the bag is good without hardening;

Judgment reason: the raw materials are damp, and a layer of sticky layer is formed on the surface of the printing film. The bags are put together to form a local vacuum between the bags, which makes it difficult to disperse.

This is because in the high temperature and high humidity season, the workshop environment humidity is high, raw materials are affected by moisture, the surface friction coefficient increases, and the friction force increases. Friction adsorption occurs between bags, clinging together, and forming a local low pressure state between bags instantaneously. Under the action of external air pressure, the bags adsorb each other. In the high humidity and high temperature season, the environment is relatively humid, and the humidity in the workshop is large. So it is helpful to improve the quality problems.

                                                                                                      Source: packaging garden soft packaging





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