NIST SPECIAL PUBLICATION 260-144

U. S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Standard Reference Materials

Certification of Standard Reference Material 1473b,

Low Density Polyethylene Resin

J.R. Maurey, C.R. Schultheisz, W.R. Blair, and C.M. Guttman
NIST Special Publication 260-144

Standard Reference Materials

Certification of Standard Reference Material 1473b,

Low Density Polyethylene Resin

J.R. Maurey, C.R. Schultheisz, W.R. Blair, and C.M. Guttman

Polymers Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8541

UNITED STATES OF AMERICA

DEPARTMENT OF COMMERCE

U.S. DEPARTMENT OF COMMERCE, Donald L. Evans, Secretary

TECHNOLOGY ADMINISTRATION, Phillip J. Bond, Under Secretary of Commerce for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arden L. Bement, Jr., Director

Issued October 2002
Certain commercial equipment, instruments, or materials are identified in this paper in

order to specify the experimental procedure adequately. Such identification is not intended

to imply recommendation or endorsement by the National Institute of Standards and

Technology, nor is it intended to imply that the materials or equipment identified are

necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 260-144

Natl. Inst. Stand. Technol. Spec. Publ. 260-144, 18 pages (October 2002)

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iii

CONTENTS

Abstract ...........................................................................................................................iv

1. Introduction ................................................................................................................. 1

2. Experimental Procedure......................................................................................................... 1

 2.1 Sampling and Randomization of Charge Sequence of SRM 1473b.................................. 1

 2.2 Instrument Calibrations and Alignment ............................................................................. 1

2.2.1 Temperature Indication.................................................................................... 1

2.2.2 Metering of Plastometer Components ......................................................... 2

2.2.3 Alignment of Plastometer............................................................................. 2

3. Melt Flow Rate of SRM 1473b..................................................................................... 3

4. Data Analysis on SRM 1473b ..................................................................................... 4

5. Comparison with SRM 1473a...................................................................................... 4

 5.1 Comparison of SRM 1473a with a Change in Measurement Protocol................... 5

6. Uncertainty Estimates of Melt Flow Rate Data ............................................................ 5

 6.1 Repeatability and Sampling Uncertainties............................................................ 5

 6.2 Charge-to-Charge Variability Within a Bottle........................................................ 6

 6.3 Bottle-to-Bottle Variability ..................................................................................... 6

 6.4 Day-to-Day Variability........................................................................................... 6

 6.5 Operator-to-Operator Variability ........................................................................... 6

 6.6 Systematic Uncertainties...................................................................................... 6

6.6.1 Instrument Variability ................................................................................... 6

6.6.2 Measurement Uncertainties......................................................................... 7

 6.6.2.1 Weighing Uncertainty...................................................................... 7

 6.6.2.2 Timing Uncertainty.......................................................................... 8

 6.6.2.3 Temperature Uncertainty ................................................................ 8

 6.7 Combined Standard Uncertainty .......................................................................... 9

 6.8 Expanded Uncertainty .......................................................................................... 9

7. Conclusions of Melt Flow Rate Study.......................................................................... 9

Acknowledgment........................................................................................................... 10

References.................................................................................................................... 10

Table 1 .......................................................................................................................... 11

Table 2 .......................................................................................................................... 12

Table 3 .......................................................................................................................... 12

AppendixóCertificate for Standard Reference MaterialÆ 1473b .................................. 13
iv

ABSTRACT

 The melt flow rate of Standard Reference Material (SRM) 1473b Low Density Polyethylene

Resin was determined to be 1.13 g/10 min at 190 OC under a load of 2.16 kg using the ASTM

Method D 1238-00. The average results from 48 determinations on samples with a standard

deviation of 0.013 g/10 min. The overall expanded uncertainty, including type A and type B

uncertainties, is estimated as 0.098 g/10 min.
1

1. Introduction

 Melt flow rate is widely used in polymer technology as a product specification since this

value, which includes a statement of the load and temperature under which it is obtained, gives an

indication of the processing properties of the polymer(1-4). The value of melt flow rate is

expressed as the mass of polymer melt pushed from the heated cylinder of the extrusion

plastometer through its precision bore orifice by its piston in a period of time, the standard units of

the value being grams per ten minutes (g/10 min).

 This is a report on the melt flow rate certification of SRM 1473b Low Density Polyethylene

Resin. SRM 1473b is the designated successor to SRM 1473 Low Density Polyethylene Resin

and SRM 1473a Low Density Polyethylene Resin. The characterization of these earlier SRMís was

described in earlier reports (5,6). SRM 1473a and SRM 1473b were obtained from different bags

of the same lot of the same resin from the same supplier.

2. Experimental Procedure

2.1 Sampling and Randomization of Charge Sequence of SRM 1473b

 Material for SRM 1473b came as pellets of polyethylene. The material was selected as a

result of a search for a polyethylene with a melt flow rate nearly equal to the melt flow rate of an

earlier SRM, SRM 1476 Branched Polyethylene. The supplier of this resin identifies the resin as a

low density polyethylene synthesized in an autoclave environment, which characteristically

generates a branched product. The supplier estimates the density to be 0.918 g/cm3

. The

Standard Reference Materials Program (SRMP) blended the material and divided the pellets into

356 units of 60 g each. Sixteen of these units were chosen by stratified random selection for

homogeneity and certification studies. Eight units of SRM 1473a that had been stored in the

Polymer Division were also used to compare SRM 1473a with SRM 1473b. Three charges were

taken for extrusion from each of the randomly chosen bottled samples of SRM 1473a and SRM

1473b during the course of the melt flow rate experiments. In preparation for the melt flow rate

determinations, the charges to be extruded were identified by ordinal numbers. Three such charge

numbers were assigned to each identified sample. The sequence of numbered charges taken

from the bottled samples for extrusion was randomized according to a procedure described by

Natrella (7).

2.2 Instrument Calibrations and Alignment

2.2.1 Temperature Indication

 The temperature of the extrusion plastometer cylinder was measured by a mercury column

thermometer of the form described in paragraph 5.7 of the ASTM method (8). Calibration of the

thermometer is traceable to the Thermometry Group of the NIST Process Measurements Division .

An iron-constantan thermocouple was calibrated by correlating its voltage with the scale readings

of an ASTM 68C thermometer at 10 points from 185 o

C to 194 oC, in a constant temperature oil

bath. The ASTM 68C thermometer had been calibrated at the ice point and at 190 OC in the

Thermometry Group of the NIST Process Measurements Division by means of a platinum

resistance thermometer. The temperature indication by the scale of the cylinder thermometer in
2

the extrusion plastometer was calibrated by correlating the thermometer scale readings with the

temperature measured by the emf from the calibrated thermocouple with its hot junction stationed

in a column of polyethylene melt in the bore, as described in paragraph 5.5.2 of the ASTM method.

Thermal conductivity between cylinder and thermometer was enhanced by adding Woodís metal to

the thermometer well in the cylinder. The standard uncertainty in the final temperature indication

may be regarded as equal to the nominal limit of resolution on the scales of the ASTM 68C

thermometer, and of the thermometer in the extrusion plastometer, 0.1 o

C. The effect of a 0.1 oC

standard uncertainty in temperature on the melt flow rate of SRM 1473b is described in the

subsequent section on uncertainty analysis.

2.2.2 Metering of Plastometer Components

 Many dies and piston feet were obtained from Tinius Olsen Testing Co.* so that during any

given day, a different die and piston foot was used for each melt flow determination made on that

day. Methods of cleaning the dies and piston feet were designed to avoid dimensional changes

(see section 3.0). The geometric dimensions of the plastometer cylinder, piston feet, and dies

were all measured and found to comply with the specifications described in the ASTM method.

 The diameter of the cylinder bore was determined by a Brown and Sharpe model 599-281

Intrimik inner diameter (ID) micrometer. The ID of the bore was measured from the top to the

bottom at one centimeter intervals. The top diameter measurement was made at a depth of 2 cm

into the bore; the bottom diameter measurement was made with the micrometer head resting on a

die at the bottom of the bore. The resulting measurements varied from 0.9543 cm to 0.9550 cm, in

compliance with the tolerance of this specification described in paragraph 5.2 of the ASTM

method. The apparent mass of the nominal 2060 g load was determined in the Mass Group of the

NIST Automated Production Technology Division, and found to be 2059.8 g. New detachable

piston feet were used to conduct the extrusions; a different foot was used on the piston for each

extrusion. The assembled piston varied in apparent mass from 100.027 g with the lightest foot

attached, to 100.071 g with the heaviest foot attached. Thus the calculated combined apparent

mass of piston and load varied from 2159.83 g with the lightest foot attached to the piston, to

2159.87 g with the heaviest foot attached to the piston, both limits well within the 0.5 % tolerance

described in paragraph 5.4.4 of the ASTM method (8).

2.2.3 Alignment of Plastometer

 The cylindrical axis of the bore was aligned with the gravity vector by the following plumbline procedure. First, a die was selected as the "target" die and stationed on the structural base

plate of the extrusion plastometer directly below the cylinder. Its position on the plane of the base

plate was adjusted to have the axis of its bore coincide with the projection from the axis of the

cylinder bore onto that plane. This was accomplished by viewing the target die through the bores

of two "sighting" dies, one stationed at its operational position in the bottom end of the cylinder

bore and the other stationed at the top end of the cylinder bore. The position of the target die on

the plane of the base plate was adjusted until it appeared centered in the view from above the

cylinder through the sighting dies in the cylinder bore.

* Certain commercial materials and equipment are identified in this paper in order to specify

adequately the experimental procedure. In no case does such identification imply

recommendation or endorsement by the National Institute of Standards and Technology, nor does

it imply necessarily the best available for the purpose.
3

 Next a plumb-bob was suspended by a plumb-line from the axis of a die supported in the top

end of the cylinder bore, with the pointer of the suspended bob extending down inside the bore of

the target die. The leveling screws were adjusted until the pointer of the plumb-bob appeared to

be centered inside the bore of the target die.

 The deviation of the plumb bob pointer was observed to be less than 1 mm from the point,

which would indicate ideally vertical orientation of the cylinder bore, at the end of a pendulum

length of 41 cm. Consequently, this procedure is considered to obtain alignment of the cylindrical

axis of the cylinder bore with the gravity vector, with a calculated uncertainty of (1 mm/41 cm).

Thus, the maximum uncertainty in terms of possible angle of displacement of the bore axis from

the gravity vector is estimated as arcsine(1 mm/41 cm)=0.14o

. It is estimated that such a

maximum placement in angle from the gravity vector would diminish the force of the loaded piston

to a level of its total mass multiplied by cos (0.14o

 ), or (2.16 kg)(0.999997) in the present case.

This factor would thus diminish the effective mass of the loaded piston by a decrement of

 3x10-4 %, incomparably less than the +0.5 % tolerance in combined mass of piston and load

described in paragraph 5.4.4 of the ASTM method. Standard uncertainties cited in this report are

considered equivalent to those corresponding to a 95 % level of confidence, as explained in the

subsequent section on uncertainty estimates in the melt flow rate data.

 The initial bore alignment was conducted at ambient temperature in preparation for the

characterization. The alignment was also occasionally tested, during the course of the

characterization extrusions while the cylinder was hot, with a circular level which can be mounted

atop the piston rod mounted in the bore, as described in the bore alignment section of the

ASTM method (8).

3. Melt Flow Rate of SRM 1473b

 The melt flow rates of SRM 1473b samples were determined by procedure A described in

Section 9 of ASTM Method D-1238-00 (8). Standard test condition 190/2.16 was used. Thus the

flow rate was determined at (190.0 ±0.1) OC using a load of 2.16 kg. (±0.1 OC is estimated to be

the standard uncertainty in the temperature measurement.) The flow rate of the melt was

measured by a manually operated extrusion plastometer obtained from the Tinius-Olsen Testing

Machine Co.

 A 3.4 g charge of pellets was used for each extrusion. The end of the 7.0 min preheat period

was marked as the beginning of timed test extrusion by making the initial extrudate cut at 7.0 min

and discarding the preheat segment. (The reader should note that in previous certifications of melt

flow a 6.0 min preheat was used. This time was allowed in ASTM D-1238 Methods with a date

before 1990. In the current methods a 7.0 min preheat is prescribed with a range of 0.5 min

allowed.) It was also observed that the 4 mm start section of the piston had always entered the top

of the guide collar part way at the moment of the initial cut to begin collecting timed test extrudate.

Three timed test extrudate segments were cut at 3.0 min intervals thereafter. After the third timed

test extrudate segment had been cut, the remaining melt in the cylinder was purged and

discarded. Following ASTM D1238-00 only the first timed extrudate was used in the final data

analysis for certification of SRM 1473b. The piston and bore were cleaned free of the polymer at

the end of each extrusion. Many dies and piston feet were obtained from Tinius Olsen Testing Co.

During a given day a different die and piston foot was used for each separate charge. The die and
4

piston feet were then cleaned in preparation for the next days run. Tools of brass and copper,

considerably less hard than steel, were applied in the cleaning process. The use of steel tools was

avoided in order to prevent changing dimensions of instrument components due to cleaning wear.

Cotton gun cleaning patches were used in the next step of the cleaning. The die and piston feet

were exposed to a vapor of boiling xylene for the final cleaning.

 Two operators were used in the operation of the equipment. Each operator made

measurements on eight extrusions (charges) on randomly chosen days.

4. Data Analysis on SRM 1473b

 Data from 48 charges were analyzed for the 2.16 kg load following the ASTM Method D1238-

00. The average melt flow rate of SRM 1473b was found to be 1.134 g/10 min with a standard

deviation of 0.013 g/10 min with 47 degrees of freedom. This standard deviation includes bottle to

bottle, charge to charge, operator to operator and day to day variability. The standard deviation of

the mean was calculated to be 0.0019 g/10 min. Our estimates of systematic uncertainties in the

measurement will be discussed in the following sections.

5. Comparison with SRM 1473a

 As noted earlier SRM 1473a and SRM 1473b are taken from different bags of the same lot

of the resin. Thus it was expected that the melt flow rates of these samples should be close to

each other. However as discussed in Section 3.0 the previous certifications of melt flow rate used

a 6.0 min preheat period. This time was allowed in ASTM D-1238 Methods dated before 1990.

This then was the preheat time used in the certification of SRM 1473a. In the current ASTM

Method of D-1238-00, a 7.0 min preheat is prescribed with a range of 0.5 min allowed. Thus, we

used a preheat time of 7.0 min for this certification. This change in preheat time required a change

in initial charge mass from 3.2 g with the 6.0 min preheat to a 3.4 g charge with the 7.0 min

preheat.

 As discussed in Section 3.0, as part of this certification we measure the melt flow of eight

samples of SRM 1473a under the new preheat conditions. For those samples using the 7.0 min

preheat and the 3.4 g initial charge, we obtained a melt flow of 1.151 g/10 min with a standard

deviation of 0.020 g/10 min with 23 degrees of freedom and a standard deviation of the mean

0.004 g/ 10 min. An ANOVA comparing these values with those from SRM 1473b suggests that

these samples are indeed different suggesting a slight bag-to-bag variation in the resin.

 Furthermore, the earlier (1995) measurements on SRM 1473a (6) gave a melt flow of 1.17

g/10min with a standard deviation of 0.015 g/10 min and a standard deviation of the mean of

0.0019 g/10 min. These measurements were made with the 6 min preheat and the 3.2 g initial

charge. A study of the data indicates these values differ (although only slightly so) from those

obtained under the conditions used in this certification.

 Generally speaking, changes in measurement protocol as described in the above paragraph

as well as normal sampling variations in materials can be expected to lead to variations in the melt

flow rate measurand. However, the offset observed here between the certified values for SRM

1473a and the new certified value for SRM 1473b (0.04 g/10 min) is contained well within the

expanded uncertainty of both the older and newer SRM (both near 0.10 g/10 min) and so

represent no significant shift between the certified melt flow rates of the two SRMs.
5

5.1 Comparison of SRM1473a with a Change in Measurement Protocol

 As mentioned above, we found the expanded uncertainty arising from changes in the

measurement protocol well within our overall expanded uncertainty. However, it was decided to

study the effect of protocol when operator variation and sample variation (bottle-to-bottle variation)

were held fixed. About 175 g of SRM 1473a obtained from many bottles were mixed together to

obtain a homogeneous mixture. (Little bottle-to-bottle variation has been found in SRM 1473a (5)

previously but the above was done as a precaution.)

 Over a two-day period, the melt flow of SRM 1473a was measured using either the protocol

with a 6 min preheat and 3.2 g charge or the protocol with a 7 min preheat and a 3.4 g charge. For

any day, 8 measurements were made four with each protocol. During any one day the order of

measuring the protocols was randomized.

 For the material using a 6 min preheat, we obtained 1.159 g/10 min melt flow rate with a

standard deviation of 0.0093 g/10 min and a standard deviation of the mean of 0.0033 g/10 min.

For the material using the 7 min preheat we obtained a melt flow rate of 1.144 g/10 min with a

standard deviation of 0.0269 g/10 min and 0.0095 g/10 min for the standard deviation of the mean.

ANOVA suggests that these values are different for a level of confidence of 95 percent. (10). It

should be noted this difference is indeed very small, well below the overall expanded uncertainty in

the measurement as noted in Section 6.8.

 Furthermore, the mean values for the melt flow of SRM 1473a obtained in 1995 with the

same protocol are within 2 standard deviations of the mean obtained in the above described work.

ANOVA on these data suggests there is little difference between the data obtained in 1995 and

those obtained with a 6 min preheat in this work.

 Also, the mean values for the melt flow of SRM 1473a obtained in the work described in

Section 5.0 with the 7 min preheat protocol is within 2 standard deviations of the mean of the value

obtained in the above-described work. ANOVA on these data suggests there is no difference

between these data .

6. Uncertainty Estimates of Melt Flow Rate Data

 The measurement uncertainties encountered in the process of determining the melt flow rate

were determined or estimated in compliance with the NIST policy governing the reporting of

uncertainties in measurement (10). The uncertainty due to instrument variability among the results

from a large population of laboratories was derived from expected precision limits tabulated in the

ASTM method description(8).

6.1 Repeatability and Sampling Uncertainties

 In this section we discuss the uncertainty in terms of variations of the measured melt flow rate

arising from sampling and bottling differences.

 For SRM 1473b, the standard deviation was 0.013 g/10 min for 48 melt flow rate

determinations in a range from 1.100 g/10 min to 1.166 g/10 min. The standard uncertainty arising

from the overall experimental extrusion repeatability is taken as the standard deviation of the mean

of all the melt flow rate determinations, u = 0.0019 g/10 min.
6

6.2 Charge-to-Charge Variability Within a Bottle

 As described in Section 2.1, 11 bottles were selected for measurement from the original

bottling. Three charges from each bottle were measured so a study of charge-to-charge variation

within an individual bottle was easily performed. The mean charge to charge standard deviation of

the melt flow value from charges removed from an individual bottle was 0.016 g/10 min, with a

range from (0.002 to 0.043) g/10 min. These data are not reported separately in Table 1, since

this uncertainty is included in the relative standard uncertainty reported in line 1 of Table 1.

6.3 Bottle to Bottle Variability

 The bottle to bottle variability was estimated from the population of all charges taken from all 16

bottles. The mean value of the melt flow rate for any bottle was found to lie within two standard

deviations of the mean melt flow rate for all the charges. The mean of any bottle was not found to

be significantly different at the 95 % confidence level from any other mean in the group. These

data are not reported separately in Table 1 since this uncertainty is included in the overall standard

uncertainty reported in line 1 of Table 1.

6.4 Day-to-Day Variability

 Eight melt flow rate experiments could be conducted in a single day. The day to day variability

was small compared to the charge to charge variability. These data are not reported separately in

Table 1, since this uncertainty is included in the overall standard uncertainty reported in line 1 of

Table 1.

6.5 Operator-to-Operator Variability

 Eight melt flow rate determinations could be conducted in a single day. On any one day a

single operator used the instrument. The operator for any given day was chosen at random. A

two-way ANOVA run comparing the two operators concluded the two operators were different. The

means from each operator differed from one another by less than 1 %. This uncertainty is not

reported separately in Table 1 since it is included in the overall standard uncertainty reported in

line 1 of Table 1.

6.6 Systematic Uncertainties

 Obtaining a systematic uncertainty analysis of the melt flow rate is a difficult matter since the

melt flow rate is not a fundamental property of the material and there is no simple relationship

describing its estimation. Nonetheless we shall make an effort in this section to estimate the

uncertainties from their possible contributing sources, and their contribution to the combined

uncertainty to be computed.

6.6.1 Instrument Variability

 As noted before, the estimates of our own repeatability are in line 1 of Table 1. These data

reflect the repeatability of our own experiments and do not reflect any instrument-to-instrument

variation since we used only one instrument.

 However, the results in Table 4 in ASTM D1238-00 (8) provide a means of estimating the

uncertainty among the results from a large population of instruments and operators applying

procedure A. For the purposes of this estimation for SRM 1473b with a melt flow rate of 1.13 g/10
7

min, the uncertainty data for the polyethylene using the same melt flow conditions with a melt flow

rate of 2.04 g/10 min was used. The tables of precision limits lists the within laboratory standard

deviation , Sr, and the between laboratory standard deviation, SR, in addition to other related

statistical parameters, all defined in ASTM Standard E 691-92, "Standard Practice for Conducting

an Interlaboratory Study to Determine the Precision of a Test Method" (9). The standard deviation

of the mean of an array of average melt flow rate results from a large population of laboratories

can be derived from the SR given in the precision tables of ASTM D 1238-00 by applying their

relationship with the more fundamental standard deviation of the mean described in ASTM E 691-

92. Thus a standard deviation of the mean of 0.079 g/10 min was obtained for the average melt

flow rate 2.04 g/10 min from its corresponding SR listed in Table 4 of ASTM D 1238-00. Assuming

our measurement has the same percent standard deviation of the mean arising from this

instrument variability , we estimate the uncertainty due to instrument variability in the results from

a large population of laboratories is 0.043 g/10min. This standard uncertainty due to instrument

variability is also listed in Table 1 of this report. Predicated on statistical analysis of

measurements from nine laboratories in a study coordinated by ASTM Committee D-20, this is a

Type A uncertainty in the nomenclature described in NIST Technical Note 1297.

6.6.2 Measurement Uncertainties

 An effort is made to estimate the intrinsic uncertainty in the measurement. We do this by

considering the uncertainties in the measured quantities (mass and time) as well as the

uncertainties in the controlled quantities (temperature). The melt flow rate, F, is given by

 F = Mass/Time

Thus the relative standard uncertainty in the melt flow rate, δF/F, is then obtained as the root-sumof-squares of the uncertainties in the systematic physical factors upon which the melt flow rate

depends (10)

 (δF/F)2

 = (δm/m)2 + (δt/t)2 + (cT δT/F)2.

where the relative standard uncertainty in mass of extrudate is δm/m, δt/t is the relative standard

uncertainty in timing of the extrudate cut and δT/T term arises from the uncertainty in the

temperature control and in the calibration of temperature indication. The factor cT is the sensitivity

coefficient, cT=(δF/δT). The causes of these uncertainties are discussed in the next few

paragraphs. These with the other uncertainties are given in Table 1 as well as an estimate of the

combined standard uncertainty resulting from all sources.

6.6.2.1 Weighing Uncertainty

 The extrudate segments were weighed on a balance with 0.01 mg resolution and an estimated

uncertainty of 0.05 mg. Replicate weighings of the segments always agreed to within 0.05 mg.

Paragraph 9.9 of ASTM Method D 1238-00 instructs the experimenter to "weigh the extrudate to

the nearest 1 mg when cool."

 The extrudate segments were routinely weighed within one hour after having been cut, in

compliance with the instruction in paragraph 9.9 of the ASTM method. Considering the

hydrophobic character of polyethylene it would not be anticipated that the extrudate would

accumulate moisture beyond the initial cooling stage prior to being weighed. On a few occasions

during the characterization of another polyolefin, extrudate segments, which had been weighed at

the end of a day, were weighed again on the following day without detecting any statistically valid
8

change of mass within the groups. All individual changes, either positive or negative, were much

smaller than 0.1 mg.

 The extrudate mass was about 350 mg. With an estimated overall weighing uncertainty of 0.15

mg from the above sources, the relative standard uncertainty

 δm/m = 0.04 %.

Since this weighing uncertainty appears negligible in comparison with larger uncertainties from

other sources, we take the weighing uncertainty to be zero.

6.6.2.2 Timing Uncertainty

 The 3 min interval (t = 180 s) between extrudate cuts for SRM 1473b was measured with a

battery powered stopwatch having a 0.01 s resolution in time indication and an uncertainty of less

than 0.05 s. Thus the extrudate cut was assumed to be timed to better than 0.1 s. Consequently,

we take 0.1 s as a practical estimate of the timing uncertainty. Hence, the relative standard

uncertainty in time interval may be expressed

 δt/t = ±0.1 s/180 s = 0.06 %

We take the timing uncertainty as zero, since it appears negligible in comparison with larger

uncertainties from other sources.

6.6.2.3 Temperature Uncertainty

 As described in Section 4.2.1, the extrusion cylinder temperature was measured by a mercury

column thermometer of the form described in paragraph 5.7 of the ASTM method D1238-00. The

uncertainty of the thermometer is certified to be within the tolerances of the ASTM method, by

comparison to standards traceable to NIST. The standard uncertainty in the temperature

indication calibration is less than 0.1 oC.

 Paragraph 5.7 in the ASTM method acknowledges that the temperature in the thermometer

well may not necessarily be the temperature of the polymer melt at the calibration point in the

bore. This is due to the steady state heat transfer gradients in the plastometer cylinder. Thus, the

thermal profile of an undisturbed column of polyethylene melt was scanned along the cylindrical

axis of the cylinder bore while the temperature was maintained at 190.0 oC at the calibration point

in the melt column. This experiment was conducted in another extrusion plastometer during an

earlier determination of the melt flow rate of SRM 1475. The column of melt was held stationary

by plugging the flow. The temperature in the stationary melt column was measured with a

thermocouple hot junction stationed at different heights above the top surface of the die, along the

cylindrical axis of the bore. Throughout the experiment the reading of the mercury column

thermometer remained at (190.0 (±0.1) o

C. The results are listed in Table 2.

 Inspection of the tabulated results indicates that the departure of melt temperature from the

measured cylinder temperature is within 0.1 o

C at any location in the melt column from 12 mm

above the die upward. There is a 0.7 o

C drop in temperature between the 12 mm and 1 mm levels

above the die. This temperature drop is probably at least partially erased by the

downward flow of melt during an extrusion. Thus we take only part of that total drop as the

uncertainty in the temperature. We take the 0.4 o

C temperature drop determined at the bottom

end of the melt column adjacent to the die, observed in Table 2, as an estimate of the temperature
9

uncertainty due to possible thermal gradients in the melt column. This uncertainty is combined

with the nominal standard uncertainty in temperature indication, 0.1 o

C, by root-sum-of-squares,

and the result rounded up to 0.5 oC, as an estimate of maximum expected standard uncertainty in

temperature, δT, due to temperature indication and to possible gradients in the thermal profile of

the melt column.

 The effect of small variations in temperature on the apparent melt flow rate was determined

during the characterization of the original SRM 1473 reported earlier (6). Thus, the effect of

temperature variation on melt flow rate was determined by conducting a set of five extrusions at

188.4 o

C, and another set of five extrusions at 191.5 oC. The two sets of extrusions at the different

temperatures were conducted with charges of polyethylene resin all taken from the same bottle of

SRM 1473.

 The resulting average melt flow rates from the first timed extrudate segments of those

extrusions, and from that at 190 o

C, are taken from Table 4 of the earlier report and listed in Table

3 of this report. Linear regression analysis of the tabulated melt flow rate versus temperature

provided the slope

 (δF/δT) = 0.040 g/10 min/ o

C

taken as the quantitative expression for the temperature dependence of the melt flow rate of SRM

1473 at temperatures near 190 o

C. A plot of melt flow rate versus temperature is seen in Figure

1 in reference 6.

 Since SRM 1473 and SRM 1473b are different lots of production of the same type of

polyethylene from the same manufacturer, this (δF/δT) for SRM 1473 is also taken as the

sensitivity coefficient, cT, of the flow rate to variation in temperature of the melt for SRM 1473b.

 Considering the standard uncertainty in temperature, δT = 0.5 o

C, this result yields an estimate

of 0.020 g/10 min for the uncertainty in melt flow rate of SRM 1473b due to uncertainty in

temperature.

6.7 Combined Standard Uncertainty

 The combined standard uncertainty (10) for the melt flow rate, uc, is obtained as the root-sumof-squares of component uncertainties from all sources. The combined standard uncertainty

computed thus from the sources discussed in the preceding paragraphs is uc = 0.049 g/10 min.

6.8 Expanded Uncertainty

 With the object of presenting an uncertainty consistent with the reproducibilityís tabulated in

the ASTM method, the combined standard uncertainty from Table 1 of this report is expanded to

the form of a 95 % confidence interval estimate. This is accomplished by applying a coverage

factor of 2 to the combined standard uncertainty (10) to obtain an expanded uncertainty. The

resulting expanded uncertainty is 0.098 g/min for the melt flow rate of SRM 1473b.

7. Conclusions of Melt Flow Rate Study

 The melt flow rate of SRM 1473b was found to be 1.13 g/10 min, with a standard deviation of

an average single measurement of 0.013 g/10 min, and a standard deviation of the mean of
10

0.0019 g/10 min. The combined standard uncertainty of 0.049 g/10 min, and the expanded

uncertainty is 0.098 g/10 min.

Acknowledgment

 The authors gratefully acknowledge Bruno Fanconi of the NIST Polymers Division for the

technical counsel and direction and Stephan Leigh of NIST Statistical Engineering Division for his

guidance in the statistical analysis of our results.

References

1.H. P. Frank: "Polypropylene," p.75, Gordon and Breach Science Publishers, N.Y. (l968).

2.S. Matsuoka and T. K. Kwei, in "Macromolecules, an Introduction to Polymer Science," p.346,

ed. by F. A. Bovey and F. H. Winslow, Academic Press. N.Y. (l979).

3.N. G. McCrum, C. P. Buckley, and C. B. Bucknall: "Principles of Polymer Engineering," pp. 279-

280, Oxford University Press, N.Y. (l988).

4.J. L. Throne: "Plastics Process Engineering," pp. 239-247, 288, Marcel Dekker, Inc., N.Y. (l979).

5. J. R. Maurey, W.R. Blair, and C. M. Guttman: "Studies on the Melt Flow Rate of the SRM

1473a, A Low Density Polyethylene Resin," NISTIR 5639, October 1995.

6. J. R. Maurey and C. M. Guttman: "Studies on the Melt Flow Rate of the SRM 1473, A Low

Density Polyethylene Resin," NISTIR 4627, April 1992.

7. M. G. Natrella: "Experimental Statistics," NBS Handbook 91, Section 1-4, pp. 1-6.

8. 2001 Annual Book of ASTM Standards, Vol. 08.01, D1238-00.

9. 1992 Annual Book of ASTM Standards, Vol. 14.02, pp. 491-510.

10. B. N. Taylor and C. E. Kuyatt: "Guidelines for Evaluating and Expressing the Uncertainty of

NIST Measurement Results," NIST Tech. Note 1297, January 1993.
11

Table 1

Estimates of Uncertainties in Melt Flow Rate of SRM 1473b Polyethylene

Under Condition 190/2.16

Sources of standard uncertainty Melt Flow Rate Uncertainty

 ui g/10 min Type c

1. Due to the repeatability

 of the experiment 0.002 A

2.Due to instrument variability 0.043 A

 as estimated from precision tables

 in ASTM D 1238-00

3. Due to weighing <0.001 B

4. Due to timing a < 0.001 B

5. Due to temperature 0.020 B

Combined standard uncertaintyb

, uc 0.049

 Expanded uncertainty, U = 2uc 0.098

 a. Sensitivity coefficient, cT = δF/δT, for variation of flow rate in response

 to small changes in melt temperature.

 b. The combined standard uncertainty is computed by root-sum-of-squares of the

 component uncertainties (10).

 c. Type of uncertainty (10)

 Type A uncertainties are evaluated by statistical methods.

 Type B uncertainties are evaluated by other means.
12

Table 2

Variation of Temperature with Height in

Undisturbed Melt in Cylinder Bore

 Height Above Melt Temp.

 Die, mm o

C

 ------------- ----------

 48 190.09

 36 189.93

 24 189.97

 12 189.94

 1 189.23

Table 3

Temperature Dependence of the Melt Flow Rate (F) of

SRM 1473 in the Vicinity of 190 o

C Under 2.16 Kg Load

(Reference 5)

 Temperature Melt Flow Rate

 o

C g/10 min.

 188.4 1.227

 190 1.287

 191.5 1.352

 estimated (δF/δT) = 0.040 g/10 min/ o

C
SRM 1473b Page 1 of 2

National Institute of Standards & Technology

Certificate of Analysis

Standard Reference Material® 1473b

Low Density Polyethylene Resin

This Standard Reference Material (SRM) is intended for use in calibration and performance evaluation of

instruments used in polymer technology and science for the determination of the Melt Flow Rate using ASTM

D 1238-00. The SRM is supplied as white pellets of polyethylene.

Certified Values and Uncertainties: This material is certified for melt flow rate using ASTM D 1238-00, Test

Method for Flow Rates of Thermoplastics by Extrusion Plastometer [1] Standard Test Condition 190/2.16. The flow

rate of the melt was determined at 190.0 °C ± 0.1 °C and a load of 2.16 kg by procedure A of the ASTM method. A

manually operated extrusion plastometer was used. Under these conditions [2], the certified melt flow rate for this

material is as follows:

Melt Flow Rate (FR) = 1.13 g/10 min ± 0.098 g/10 min

The uncertainty is the numerical value of an expanded uncertainty U = kuc, with U determined from a combined

standard uncertainty, uc, and coverage factor, k = 2, [3] at a level of confidence of 95 %. Type A and Type B

contributions to the expanded uncertainty include the standard deviation of the melt flow measurement, instrument–

to–instrument variation as discussed in ASTM D 1238-00, operator dependence of the measurement, and

temperature gradients in the apparatus [2]. The standard deviation for an average single measurement is 0.013 g/10 min,

with 47 degrees of freedom [2].

Expiration of Certification: The certification of SRM 1473b is valid until 01 January 2008, within the

measurement uncertainties specified, provided that the SRM is handled in accordance with the storage instructions

given in this certificate. This certification is nullified if the SRM is modified or contaminated.

Maintenance of SRM Certification: NIST will monitor this SRM over the period of its certification. If

substantive technical changes occur that affect the certification before expiration of this certificate, NIST will notify

the purchaser. Return of the attached registration card will facilitate notification.

Storage: The SRM should be stored in the original bottle with the lid tightly closed and under normal laboratory

conditions.

Homogeneity: The homogeneity of SRM 1473b was tested by melt flow measurements using ASTM D 1238-00.

The characterization of this polymer is described in Reference 2.

The technical coordination leading to certification of this SRM was provided by B.M. Fanconi of the NIST

Polymers Division. The technical measurement and data interpretation were provided by C.M. Guttman,

J.R. Maurey, C.R. Schultheisz, and W.R. Blair of the NIST Polymers Division.

Statistical analysis was provided by S.D. Leigh of the NIST Statistical Engineering Division.

The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

the NIST Standard Reference Materials Group by J.W.L. Thomas.

Eric J. Amis, Chief

Polymers Division

Gaithersburg, MD 20899 John Rumble, Jr., Chief

Certificate Issue Date: 11 July 2002 Measurement Services Division
SRM 1473b Page 2 of 2

REFERENCES

[1] ASTM D 1238-00; Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer; ASTM

Standards, Vol. 08.01, American Society for Testing and Materials, West Conshohocken, PA (2001).

[2] Maurey, J.R.; Schultheisz, C.; Blair, W.R.; Guttman, C.M.; Certification of Standard Reference Material

1473b, A Polyethylene Resin; NIST Special Publication Number SP 260-144.

[3] Guide to the Expression of Uncertainty in Measurement; ISBN 92-67-10188-9, 1st Ed., ISO, Geneva,

Switzerland, (1993); see also Taylor, B.N.; Kuyatt, C.E.; Guidelines for Evaluating and Expressing the

Uncertainty of NIST Measurement Results; NIST Technical Note 1297, U.S. Government Printing Office;

Washington, DC (1994); available at http://physics.nist.gov/Pubs/.

[4] Taylor, B.N.; Guide for the Use of the International System of Units (SI); NIST Special Publication 811; Ed.

(April 1995).

Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via

the Internet http://www.nist.gov/srm.