4.2Slip properties are generated by additives in some plasticfilms,for example,polyethylene.These additives have varying degrees of compatibility with thefilm matrix.Some of them bloom,or exude to the surface,lubricating it and making it more slippery.Because this blooming action may not always be uniform on all areas of thefilm surface,values from these tests may be limited in reproducibility.
4.3The frictional properties of plasticfilm and sheeting may be dependent on the uniformity of the rate of motion between the two surfaces.Care should be exercised to ensure that the rate of motion of the equipment is as carefully controlled as possible.
4.4Data obtained by these procedures may be extremely sensitive to the age of thefilm or sheet and the condition of the surfaces.The blooming action of many slip additives is time-dependent.For this reason,it is sometimes meaningless to compare slip and friction properties offilms or sheets produced at different times,unless it is desired to study this effect.
4.5Frictional and slip properties of plasticfilm and sheeting are based on measurements of surface phenomena.Where products have been made by different processes,or even on different machines by the same process,their surfaces may be dependent on the equipment or its running conditions.Such factors must be weighed in evaluating data from these meth-ods.
4.6The measurement of the static coefficient of friction is highly dependent on the rate of loading and on the amount of blocking occurring between the loaded sled and the platform due to variation in time before motion is initiated.
4.7Care should be exercised to make certain that the speed of response of the recorder,either electronic or mechanical,is not exceeded.
4.8For many materials,there may be a specification that requires the use of this test method,but with some procedural modifications that take precedence when adhering to the specification.Therefore,it is advisable to refer to that material specification before using this test method.Table1of Classi-fication System D4000lists the ASTM materials standards that currently exist.
5.Apparatus
5.1Sled—A metal block63.5mm(21⁄2in.)square by approximately6mm(0.25in.)thick with a suitable eye screw fastened in one end.When aflexiblefilm(see
6.2)is to be attached,the block shall be wrapped with a sponge rubber63.5 mm(21⁄2in.)in width and3.2mm(1⁄8in.)in thickness.The foam shall beflexible,smooth-faced,and have a nominal density of0.25g/cm3when measured in accordance with the Density Test of Methods D3574.The pressure required to compress the foam25%shall be85615kPa(12.562.5psi). The foam shall also have a high hysteresis when deformed.8 The rubber shall be wrapped snugly around the sled and held in place against the bottom and top of the sled with double-faced masking tape.When a sheet(see6.3)is to be attached, double-faced tape shall be used to attach the specimen.The total weight of the(wrapped)sled and specimen shall be2006 5g.
N OTE3—Round-robin testing9has shown that the physical properties of the backing can drastically affect both the coefficient of friction and stick-slip behavior of thefilm.
5.2Plane—A polished plastic,wood,or metal sheet,10 approximately150by300by1mm(6by12by0.040in.).A smooth,flat piece of glass may cover the upper surface of the plane.This provides a smooth support for the specimen.
5.2.1In some cases,it may be desirable to run tests at temperatures other than23°C.The plane must then have a heater or cooling unit capable of maintaining the temperature to within62°C of the desired temperature.A cover for the plane is also necessary to maintain the temperature during the test.
5.3Scissors or Cutter,suitable for cutting specimens to the desired dimensions.
5.4Adhesive Tape,cellophane or pressure-sensitive.
5.5Adhesive Tape,double-faced.11
5.6Nylon Monofilament,having a0.3360.05-mm(0.013 60.002-in.)diameter and capable of supporting a3.6-kg(8-lb) load.
5.7Beaded Chain,flexible metal cable,or equivalent, having a spring rate no less than600lbs per inch of stretch per inch of length(40lbs/in.(7000N/m)for a15-in.chain)in the range of50to150g of tension(such as beaded lampswitch pull chain).
5.8Low-Friction Pulleys—A phenolictype pulley mounted in hardened steel cone bearings on a metal fork.A ball-bearing type pulley may also be used.
5.9Force-Measuring Device,capable of measuring the frictional force to65%of its value.A spring gage12(Note3), universal testing machine,or strain gage may be used.
N OTE4—The capacity of the spring gage(Fig.1(a and b))needed will depend upon the range of values to be measured.For most plastic,a500-g capacity gage with10-g or smaller subdivisions will be satisfactory.This spring will measure coefficients of friction up to and including2.5. 5.10Supporting Base—A smooth wood or metal base approximately200by380mm(8by15in.)is necessary to support the plane.The supporting base may be a simple rectangular box.If a universal testing machine is used to pull a moving plane,a supporting base of sufficient structural strength and rigidity to maintain afirm position between the moving crosshead and the force-measuring device will be necessary.
5.11Driving or Pulling Device for Sled or Plane—The plane may be pulled by a driven pair of rubber-coated rolls not less than200mm(8in.)long,capable of maintaining a uniform surface speed150630mm/min(0.560.1ft/min) (Fig.1(b)),by the crosshead of a universal testing machine
8Sheet stock,available from Greene Rubber Co.,59Broadway,North Haven, CT06473,has been found satisfactory.
9Supporting data are available from ASTM Headquarters.Request RR:D20-1065.
10Acrylic or rigid poly(vinyl chloride)sheeting has been found satisfactory for this purpose.
11Available from Minnesota Mining and Manufacturing Co.,Inc.,or Permacel Tape Corp.
12Model L-500,available from Hunter Spring Co.,Lansdale,PA,has been found satisfactory for this
purpose.
(Fig.1(d ))(Note 6),or a worm drive driven with a synchronous
motor (Fig.1(e )).A constant-speed chain drive system has also
been found satisfactory (Fig.1(a )).A power-operated source
may be used for pulling the sled over the horizontally-mounted
specimen at a uniform speed of 150630mm/min (0.560.1
ft/min).A universal testing machine equipped with a load cell
in its upper crosshead and a constant rate-of-motion lower
crosshead has been found satisfactory (see Fig.1(c )).
N OTE 5—Where the moving crosshead of a universal testing machine is
used to pull the moving plane through a pulley system (Fig.1(d )),the
strain gage load cell,or other load-sensing instrument in the testing
machine,acts as the force-measuring device.6.Test Specimens 6.1The test specimen that is to be attached to the plane shall be cut approximately 250mm (10in.)in the machine direction and 130mm (5in.)in the transverse direction when such extrusion directions exist and are identifiable.6.2A film specimen that is to be attached to the sled shall be cut approximately 120mm (41⁄2in.)square.Film is defined as
sheeting having a nominal thickness of not greater than 0.254mm as indicated in Terminology D 883.6.3A sheeting specimen (greater than 0.254mm nominal thickness)or another substance that is to be attached to the sled shall be cut 63.5mm (21⁄2in.)square.6.4Sheeting specimens shall be flat and free of warpage.Edges of specimens shall be rounded smooth.6.5Five specimens shall be tested for e
ach sample unless otherwise specified.N OTE 6—Plastic films and sheeting may exhibit different frictional
properties in their respective principal directions due to anisotropy or
extrusion effects.Specimens may be tested with their long dimension in
either the machine or transverse direction of the sample,but it is more
common practice to test the specimen as described in 6.1with its long
dimension parallel to the machine direction.
N OTE 7—Caution:Extreme care must be taken in handling the speci-
mens.The test surface must be kept free of all dust,lint,finger prints,or
any foreign matter that might change the surface characteristics of the
specimens.A.Sled
H.Constant-speed drive rolls B.Plane
I.Nylon monofilament C.Supporting base
J.Low-friction pulley D.Gage
K.Worm screw E.Spring gage
L.Half nut F.Constant-speed chain drive
M.Hysteresis,synchronous motor
G.Constant-speed tensile tester crosshead FIG.1Five Methods of Assembly of Apparatus for Determination of Coefficients of Friction of Plastic
Film
7.Preparation of Apparatus
7.1Fig.1showsfive ways in which the apparatus may be assembled.The support bases for all apparatus assemblies shall be level.
7.2If the apparatus of Fig.1(a)or(b)is used,calibrate the scale of the spring gage as follows:
7.2.1Mount the low-friction pulley in front of the spring gage.
7.2.2Fasten one end of the nylonfilament to the spring gage,bring thefilament over the pulley,and suspend a known weight on the lower end of thefilament to act downward.
N OTE8—The reading on the scale shall correspond to the known weight within65%.The weight used for this calibration shall be between 50and75%of the scale range on the gage.
7.3The drive speed for the apparatus of Fig.1(a and b)shall be adjusted to150630mm/min(0.560.1ft/min).This speed may be checked by marking off a150-mm(0.5-ft)section beside the plane and determining the time required for the plane to travel150mm(0.5ft).
7.4If the apparatus of Fig.1(c and d)employing a universal testing machine is used,select the proper sp
eed setting for a crosshead motion of150630mm/min(0.560.1ft/min).A similar speed for the load-displacement recorder is desirable. However,the speed of the recorder can be adjusted to give the desired accuracy in reading the pen trace.
7.5When the apparatus of Fig.1(c)(moving sled-stationary plane)is used,wipe the support base free of foreign matter and lay down two strips of double-faced adhesive tape along the length of the supporting base so that they are approximately 100mm(4in.)between centers.
7.6Fix the plane in position on the tape strips andfirmly press in place.
8.Conditioning
8.1Conditioning—Condition the test specimens at236 2°C(73.463.6°F)and5065%relative humidity for not less than40h prior to test in accordance with Procedure A of Practice D618,for those tests where conditioning is required. In cases of disagreement,the tolerances shall be61°C (61.8°F)and62%relative humidity.
8.2Test Conditions—Conduct tests in the standard labora-tory atmosphere of2362°C(73.463.6°F)and5065% relative humidity,unless otherwise specified in the test meth-ods or in
this test method.In cases of disagreement,the tolerances shall be61°C(61.8°F)and62%relative humid-ity.In specific cases,such as control testing,where the conditioning requirements cannot be met and the data still may be of direct assistance to the operation,other conditioning procedures may be used and recorded in the report.Frictional properties should be measured only after sufficient time has been allowed for the specimens to reach essential equilibrium with the ambient atmosphere.
9.Procedure
9.1Tape the250by130-mm(10by5-in.)film or sheet specimen to the plane with the machine direction of the specimen in the250-mm direction.Smooth thefilm specimen to eliminate wrinkles if necessary,taking care not to alter the specimen surface throughfinger oils,etc.
N OTE9—For some samples it has been found necessary to tape only the leading edge of the specimen to the plane.In some cases the specimen has been pulled through the nip rolls apparatus of Fig.1(b)without the plane. However,should any dispute arise,taping of all four edges will be the referee method.
N OTE10—For the sake of uniformity and later comparison when testing a specimen sliding over itself,the specimens shall be mounted so that the same side of the specimen shall be used as the cont
act surface for both the moving and stationary specimens.
N OTE11—Coefficient of friction measurements may be made on afilm or sheeting specimen when sliding over itself or over other substance surfaces wherein the movement is made in the transverse direction of the specimen.However,the methods described here will be confined to movements in the machine direction of the specimens.
9.2Forfilm specimens,tape the edges of the120-mm (41⁄2-in.)squarefilm specimen to the back of the sled,using adhesive tape and pulling the specimen tight to eliminate wrinkles without stretching it.For sheet specimens,tape the 63.5-mm(21⁄2-in.)square sheet specimen or second substrate to the sled face with double faced tape.Keep the machine direction of the specimen parallel to the length of the sled (where such a direction exists and is identifiable).
9.3Attach the specimen-covered sled through its eye screw to the nylonfilament.If a universal testing machine is used (Fig.1(c and d)),pass thefilament through pulley(s)and upward to the bottom of the load-sensing device and attach securely.If a spring gage is used(Fig.1(a and b)),securely attach thefilament to it.The nylonfilament shall be of sufficient length to allow maximum sled or plane travel.With some slack in the nylonfilament,lightly place the sled in position on the horizontal plane(Not
e11).The positioning of the sled shall be such that the length of the sled,the adjacent length of nylonfilament,and the long dimension(machine direction)of the plane-mounted specimen are parallel.For material combinations found to have an excessive stick-slip tendency,wherein the kinetic portion of the test degenerates into a series of static tests interspersed by rapid jumps of the sled,it is advisable but not mandatory to substitute the metal tow line(5.7)for the nylon tow line to make kinetic measure-ments.This will necessitate making separate measurements for static and kinetic friction coefficients.Each laboratory will determine what level of stick-slip is considered excessive for its materials.In case of disagreement between testing labora-tories,the nylon tow line remains the referee procedure.
N OTE12—The purpose of using a nylonfilament for the static friction and sometimes a metallic tow line for kinetic friction is to avoid a faster force buildup in the static measurement than the recorder can respond to, and to allow time for the recorder to separate the buildup of static friction force in the nylonfilament from the mass acceleration force as the sled breaks loose.The opposite effect is needed from the metallic tow line during kinetic friction measurement to prevent the occurrence of repeated stick-slips instead of steady motion.
N OTE13—The sled must be placed very lightly and gently on the plane to prevent any unnatural bond
from developing.A high starting coefficient of friction may be caused by undue pressure on the sled when mounting it onto the plane.
9.4Start the driving mechanism(which has been adjusted previously to provide a speed of150630mm/min(0.560.1 ft/min)).As a result of the frictional force between the contacting surfaces,no immediate relative motion may take place between the sled and the moving plane until the pull
on
the sled is equal to,or exceeds,the static frictional force acting at the contact surfaces.Record this initial,maximum reading as the force component of the static coefficient of friction.
9.5Record the visual average reading during a run of approximately130mm(5in.)while the surfaces are sliding uniformly over one another.This is equivalent to the kinetic force required to sustain motion between the surfaces and normally is lower than the static force required to initiate motion.After the sled has traveled over130mm(5in.)stop the apparatus and return to the starting position.
9.6If a strain gage and load-displacement recorder are used, either draw the best straight line midway between the maxi-mum points and minimum points shown on the chart while the sled is in motion,or obtain the average load by integration of the recorder trace.The mean load is the kinetic friction force required to sustain motion on the sled.
9.7Remove thefilm or sheeting specimen from the sled and the horizontal plane.The apparatus is now ready for the next set of specimens.A new set of specimens shall be used for each run.No specimen surface(s)shall be tested more than once unless such tests constitute one of the variables to be studied. N OTE14—The maximum point at which initial motion takes place between the sled and the horizontal plane should be carefully examined with reference to the rate of loading and the speed of response of the sensing device.Failure to consider this factor can lead to meaningless results for the value of the static coefficient of friction.
10.Calculation
10.1Calculate the static coefficient of frictionµs,as follows:
µs5A s/B(1) where:
A s5initial motion scale reading,g,and
B5sled weight,g.
10.2Calculate the kinetic coefficient of friction,µk,as follows:
µk5A k/B(2) where:
A k5average scale reading obtained during uniform sliding
of thefilm surfaces,g,and
B5sled weight,g.
10.3Calculate the arithmetic mean of each set of observa-tions and report these values to three significantfigures. 10.4Calculate the standard deviation(estimated to be6 15%of the value of the coefficient of friction)as follows,and report it to two significantfigures:
s5=~(X22n X¯2!/~n21!(3) where:
s5sample standard deviation,
X5value of a single observation,
n5number of observations,and
X5arithmetic mean of the set of observations.
11.Report
11.1Report the following information:
11.1.1Complete description of the plastic sample,including manufacturer’s code designation,thickness,method of produc-tion,surfaces tested,principal directions tested,and approxi-mate age of sample after manufacture,
11.1.2Description of second substance if used,
11.1.3Apparatus used,
11.1.4Average static and kinetic coefficients of friction, together with the standard deviation,
11.1.5Number of specimens tested for each coefficient of friction,and
11.1.6Temperature at which the test was conducted.
12.Precision and Bias13
pulleys
12.1Table1is based on a round robin conducted in1986in accordance with Practice E691,involving four materials tested by seven laboratories.For each material,all of the samples were prepared at one source.Each laboratory obtained seven test results for each material.Each test result was one deter-mination per each material.S r and S R are based onfive determinations forfive materials in accordance with the test method.
N OTE15—Caution:The following explanations of I
r
and I
R
(12.2-12.2.3)are only intended to present a meaningful way of considering the approximate precision of this test method.The data in Table1should not be rigorously applied to acceptance or rejection of material,as those data are specific to the round robin and may not be representative of other lots, conditions,materials,or laboratories.Users of this test method should apply the principles outlined in Pr
actice E691to generate data specific to their laboratory and materials,or between specific laboratories.The principles of12.2-12.2.3would then be valid for such data.
12.2Concept of I r and I R—If S r and S R were calculated from a large enough body of data,and for test results that were averages from the number of determinations stated in12.1.
12.2.1Repeatability,I r(Comparing two test results for the same material,obtained by the same operator using the same equipment on the same day)—The samples represented by the
13Supporting data are available from ASTM Headquarters.Request RR:D20-1131.
TABLE1Precision Data
Static Coefficient of Friction
Material Avg S r A S R B I r C I R D Polyethylene,0.180.0180.0660.0500.186 (M3)
Polyethylene,0.190.0270.1350.0770.383 (M4)
Polyester,0.200.0090.0370.0250.104 (M1)
Polyester,0.700.0660.0940.1860.265 (M2)
Kinetic Coefficient of Friction
Material Avg S r A S R B I r C I R D Polyethylene,0.190.0070.0460.0190.131 (M3)
Polyethylene,0.120.0070.0250.0210.071 (M4)
Polyester,0.170.0050.0210.0150.059 (M1)
Polyester,0.660.0540.1230.1540.349 (M2)
A S
r
5within-laboratory standard deviation of the average,
B S
R
5between-laboratories standard deviation of the average,
C I
r
52.83S r,and
D I
R
52.83S R
.

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