Abstract: this article offers a detailed introduction to coefficient of friction as well as the test of film friction coefficient.

Coefficient of friction is one of the important indexes in evaluating film package. Acting as both dynamic force and resisting force in package process, frictional force should be controlled within a proper range. In the study of friction coefficient, it should be noted that temperature has a great influence on coefficient of friction. Therefore, not only coefficient of friction under normal temperature should be tested, that of actual using temperature should also be examined.

1 Coefficient of Friction

1.1   Introduction of Friction Coefficient 

Friction coefficient is one of the basic properties of various materials. Whenever there is relative movement or tendency of relative movement existing between two contacting objects, the motion- opposing force between the two surfaces is called frictional force. Frictional property of materials can be descried in terms of static and dynamic friction coefficient. Static friction is the value of the limiting friction just before slipping occurs, whose ratio to normal force is the coefficient of friction. Kinetic friction is the value of the limiting friction after slipping has occurred; whose ratio to normal force is the coefficient of kinetic friction. Friction coefficient aims at a group of rubbing pairs and will have no significance when spoken in relation to one single material. Therefore material of rubbing pairs, test conditions (environment temperature, load and speed) as well as the sliding materials must be simultaneously clarified in the description of friction coefficient.  

Many scholars believe that the nature of friction results from molecules cohesion of the contacting surfaces. In fact, only when there is micro convex between two surface does they contact each other. So actual contact area is far smaller than the apparent contact area (area of the specimen) because most of the place does not contact. The fact that friction resistance is in direct proportion to positive pressure is because friction resistance is in direction ratio to actual contact area, which is usually in direct ratio to positive force. Since molecule cohesion of different materials is not the same and can impose influence on frictional force of objects. Therefore friction coefficient of different materials also varies with each other.

1.2 Friction Coefficient of Plastic Film

High polymer gained a wide application in the industry of flexible package. Being one of the main influencing factors of the operation speed of packing machines and of the opening easiness of packing products, Friction coefficient of material surface can usually be adjusted through adding additive agent (such as antitack agent) in the process of manufacture.

Lubricant agent can be divided into inner type and external type in terms of function. Inner lubricant agent can accelerate the motion of macromolecule chains or segments of the polymer, thus improve fluidity of materials. The external lubricant agent is organic chemical with polarity and has lower compatibility with polymer radicals. Under the function of Brownian movement of polymer chains, molecules can transfer to film surface and then form a greasy surface, which can improve smoothness of film surface and reduce friction coefficient of material surface. 

The antitack agent is solid powder with the particle size usually between 2～4μm. When added into film surface, the convex formed by antitack agent can reduce the actual contact area of film layers and correspondingly reduce the tack strength, which makes the sliding easy. Therefore antitack agent is useful to reduce friction coefficient.

Moreover, static adorability not only can influence friction coefficient of film, it also can influence many physical properties of materials. Therefore antistatic agent is one of the common additive agents.

However, the compatibility of additives and film structure is not the same. The additive agents that can transfer to material surface and play certain role in material lubricating have time and environment dependence. Moreover, the functional uniformity of additive agent does not consistent, thus make friction coefficient of material surface inconsistency with each other. It should be noted that for comparison test of friction coefficient, unless it is research on material aging, the tests should be carried out simultaneously.

2 Friction Coefficient Test and Precautions

2.1 Friction Coefficient Test

Comparatively speaking, test method of friction coefficient is rather unitive: fix one specimen on test board with double sided adhesive bondage or in other measures and   another tailored specimen on special gliding block. Then put the sliding block on center of the first specimen according to operational instructions. Test orientation should be parallel to that of sliding without measuring system bearing any force. The following test structure is usually adopted:

 

 Figure 1. Test Method

Note: A sliding block B test board C supporting base E measuring system F constant speed driving system I nylon silk

The following points needs to be clarified in friction coefficient test:

First, film friction coefficient test takes ASTM D1894 and ISO 8295（GB 10006 is equal to ISO 8295）as the base. Among that, there are extremely higher demands on levelness and smoothness of board surface as well as nonmagnetic of material. Different test conditions are specified in different standards. For example, a test speed of 150±30mm/min is specified in ASTM D1894, while in ISO 8295（GB 10006 is equivalent to ISO 8295）, the required value is 100mm/min. Test speed has an obvious influence on test results.

Secondly, heating test can be realized. It should be noted that room temperature of gliding block should be secured and only test board needs heating. All this is clearly clarified in ASTM D1894.

Thirdly, one same test structure can be used to test friction coefficient of metal, paper and etc. however, to different test objects, parameters such as the weight of gliding lock, the stroke and the speed are different.

Forth, in this method, the influence of inertia on test should be taken into consideration.

Fifth, friction coefficient of material is usually less than one. However, the occasions where it is bigger than one is also mentioned in some document. For example, kinetic friction coefficient of rubber and metal is between 1 and 4.

2.2   Precautions

2.2.1 Temperature 

With the rising of temperature, friction coefficient of some film will present an increase tendency. One the one hand, this is determined by the nature of polymer. On the other hand, it is closely related with the lubricant agent adopted in film production (it is possible the lubricant agent has already approached its melting point and become sticky). After temperature increment, fluctuation rate of the measuring curve will increase until the ‘sticky slip’ phenomena appear. 

With Labthink FPT-F1 friction coefficient tester/peeling tester, the author carried out heating tests of kinetic and static friction coefficient to five specimens at a test speed of 100mm/min. the temperature range being tested is between 18℃～60℃. Some of the test data are listed in table 1.

Friction Coefficient under Different Temperature

Specimen	Thickness	Test data 1	Test temperature						Note
			18℃	23℃	30℃	40℃	50℃	60℃
Aluminum	25 μm	μs	0.384	0.465	0.45	0.4	0.446	0.432
		μk	0.371	0.456	0.399	0.381	0.429	0.416
BOPP	25 μm	μs	0.342	0.329	0.343	0.355	0.409	0.496
		μk	0.323	0.328	0.345	0.321	0.377	0.442
1＃	75 μm	μs	0.238	0.238	0.247	0.289	0.305	0.353	At 60℃, the heated specimen has deformed to some extent
		μk	0.189	0.198	0.192	0.234	0.22	0.252
Complex film 2＃	90 μm	μs	0.174	0.197	0.206	0.247	0.249		At 60℃, stick slip phenomenon appears At 50℃, the heated specimen deformed
		μk	0.106	0.117	0.104	0.136	0.141
PE monochrome film	100μm	μs	0.302	0.384	0.417	0.416	0.464	0.484	At 60℃and 50℃, the heated specimen deformed to certain extent
		μk	0.214	0.312	0.297	0.331	0.389	0.464

 Note: μ_refers to static friction coefficient，μ_k refers to kinetic friction coefficient

From table one we can see that , to specimen BOPP, complex film 1＃,complex film 2＃ and PE monochrome film, both kinetic and static friction coefficient of  materials presents certain increase with the rising of temperature. However, the extent of increase is not the same. Being plotted according to the data of table 1,Figure 2 are the friction coefficient-temperature curves of specimen AL, complex film 2#and PE monochrome film. Though the influence of temperature fluctuation is not obvious to all materials, it really influenced friction coefficient of polymer material to some extent, which is one of the characteristics of polymer materials, For example, complex fim2# is less influenced by temperature. Metal material is also less influenced by temperature. For example, friction coefficient of AL curve in figure 2 basically maintains stable.

Figure 2. Friction coefficient-temperature curve

In the process of automatic packing, for the reason of element radiating, actual temperature of the equipment is a little higher than room temperature. Therefore it is necessary to test film friction coefficient   under actual packing temperature. When cigarette is packed with cigarette film, external film slides on metal elements such as lower film channel, molding V-groove, folding board, soldering and guide. Friction coefficient of external cigarette film should be controlled in a lower level. Since most of these metal elements works at a temperature higher than 50℃, it becomes extremely important to control friction coefficient of metal under high temperature conditions. In this way, the smooth gliding of film on thermal metal elements can be secured. Therefore, friction coefficient of external film to metal should be test under high temperature conditions. It is not good to decide friction coefficient of higher temperature based on the data obtained under normal temperature. 

2.2.2 Specimen Preparation 

If friction property on two sides of the specimen is different, each side should be tested respectively.

Special attention should be paid to specimen orientation, which is strictly required in the standards, especially in data comparison. Otherwise, test result will be obviously influenced. Under normal cases, length direction of specimen (i.e., test direction) should parallel to vertical direction of specimen. Of course, the direction of specimen  (i.e., mechanical process direction) on sliding block should consistent with specimen orientation before fixed onto test board.

Such test method does not applicable to emplastic materials. Moreover, if ‘sticky-slip ‘phenomenon appears, in addition to the separate test of kinetic and static friction coefficient test, nylon silk should be replaced by metal silk.

In various standards, the requirement on specimens dimension is a little different. The operators should carry on specimen preparation preprocessing according to required standard or regulated requirement.

2.2.3 The Relationship of Friction Coefficient and Contact Period

It is stated in some literature that friction coefficient changes with contact period within the initial contacting period. This is because on the one hand as the contacting period prolonged, the actual contacting area of the two objects under certain positive force will increase. Moreover, the elastic deformation of contacting surfaces becomes greater. Some literature points out that contact period can influence plastic deformation. In fact, the increase of contact duration not only can influence friction coefficient of contact materials. If two layers of the commonly used film stick to each other for a long time, it is difficult to separate them. It is no doubt this is mainly caused by self-nature of material and the function of additive agent used in the process of production. However, the dependence relation of friction coefficient and contact period can be saturated. When both elastic deformation and plastic deformation achieved their maximum limit after certain contact duration, the biggest static friction as well as the static friction coefficient reaches corresponding saturation value and will not increase with contact period afterwards. Saturation contact period is closely related with contact duration and property of contact materials.

Since contact period will influence friction coefficient of materials and such influence changes with the property of materials, therefore time interval of sliding blocks after being placed on test board and before the test begins should be unified in the test. In standard ISO 8295(GB 10006 is equivalent to ISO 8295), the required time interval is 15s.

2.2.4 Kinetic Friction Coefficient and Static Friction Coefficient

Normally, the kinetic friction coefficient of our tested specimen is smaller than the static friction coefficient. However, to some plastic, its kinetic friction coefficient is approximately equal to its static friction coefficient and even bigger than static friction coefficient. Test result of specimen M3 (PE) is listed in TABLE1 in standard ASTM D1894, in which Kinetic friction coefficient is bigger than the static friction coefficient. However, the difference is only 0.01.

3 Conclusions

Under ideal situation, to certain rubbing pairs, both the kinetic and static friction coefficient are constant and has nothing to do with contact area and gliding speed. However, since actual test conditions do not usually consistent and the material is not uniform, it is natural that there is difference existing between test data of different labs.

Since the suitability of friction coefficient not only can influence the production and using of film, but also influence opening easiness of package products, friction coefficient has become one of the basic test items of packing materials. Though this test item has already popularized with its simple and easily understood test principle, the operators still needs to pay attention to the test precautions. Test method and precautions listed in this article are summarized based on the material testing of Labthink. For any other test problems, you can contact us for further discussion.