The characterization of linear polyethylene SRM 1475. III ...
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The Characterization of Linear Polyethylene SRM 1475. III. Density Determination James E. Brown Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (December 15,1971) T he d en sity of SRM 1475 at 23 °C wa s dete rm ined by ASTM method D 1505- 67 on samples pre- pared by Proce d ur e A of ASTM method D 19211- 68 , with the result 0.9784 g/c rn ". Key words: Crystalliza ti on; de gree of c rystallinit y; density; lin ea r po lye thylene; po l ymer; recr ys ta lli za ti on. 1. Introduction Th e density of polye th ylene crystallized fr om it s melt is se nsitive to a numb er offactors which influen ce th e d egr ee of crystallinit y. Among th ese fac tor s are: th e polymer 's pre vi ous th e rm al history a nd crystalliza- ti on tem pera tur e which affec t th e pe rfec tion and rate of crys tal gr owth [1 - 4] ,1 th e ave rage mol ec ular weight and its dis tributi on [3], a nd th e am ount of bran c hin g [5 - 6]. In thi s paper we report th e res ults of th e dens it y m eas ureme nt s on lin ear pol ye th ylene SRM 1475. The dens it y meas ur eme nt s and the sa mpl e pr eparation were perform ed using sta nd a rd me th ods of th e Am eric an Soc iety for T es ting a nd Materials (AS TM). It was n ecessary to sp ecif y closely th e preparation of th e s ampl es use d in this dete rmin ation sin ce th e density of polyethylene depe nd s to such an impo rt a nt e xt e nt on th e polyme r' s pre vi o us his tor y. From our e xp erien ce, it should be possi ble to dupli ca te th ese r es ult s closely provid ed that th e s ame methods are followed. 2. Experimental Procedure Three c omposit e samples, weighing about 100 g each, were made up separately as described in the first pap er of this series [7] by blend.ing equal amounts of th e SRM 1475 pe llets taken from the tops and bottoms of each of the original containers. These thr ee c ompo sit e s ampl es will subsequently be referred to as blend I, II , and III. T es t sa mpl es ( plaqu es ) we re prepar ed from bl e nd I by Proce dur e A of ASTM Designation: D 1928- 68 , "Standard Me thod for Preparation of Compr e ssion- Molded Polye thyl ene T es t Sampl es." 2 In acc ord an ce with this pro ce dur e, th e pellets of bl e nd I were made l Figures in brackets indicate th e lil t: ra ture refere nc es at t.h e e nd of this paper. 2 Avail a bl e. fr om the Ameri can Society for Testing and Materials. 19'16 Race Stree t, P hil adelp hi a. Pennsylva nia ] 9103. int o a crepe on a roll mill. Th e polye th ylene wa s milled for about fo ur minut es at a te mp era tur e s uffi- cie ntl y high to ca use th e polymer to flux but not to drip. Pi eces were c ut from th e cooled crepe a nd co m- pr essio n molded int o 3 mm thick plaqu es at 178 °C using pi ct ur e-frame-like ch as es . 3 The mold a sse mbl y co ntainin g th e molt en polyethylene was removed fr om th e press a nd qu enched to room te mp era tur e in a wat er ba th . Th e mold asse mbli es co nt a inin g th e qu enched pl aq u es we re th en pl aced in a h ea ted oven and main· tained at 170 °C, well abo ve th e crystal melting po int , fo r 1 h or more in or der to destroy th e polymer 's pre· vious the rm al history. Th e tempe ratur e of the oven wa s then lowered to 50 °C at a r at e of 5 °C/h r. After the pl a qu es had come to room t empe ra tur e, seve ral spec i· mens (2 -3 mm on a side) were cut from ea ch plaque for the de nsity meas urements. As a ch ec k on th e se nsitivity of th e AS TM pro- ce dur e to variation s in th e prepa rativ e t ec hnique , plaqu es of bl e nd s II and III were pre par ed so me what differently. Plaqu es of ble nd II were pr e par ed as des c ribed above exce pt that the milling time (ab o ut 2 minutes) was just long enou gh at about 125 °C to cause the pellets to fuse into a sh eet. Th e pla qu es of blend III were not molded from a milled crepe. Th ese plaques were compression-molded from th e pellets. Except for the milling pro ce ss , th e tes t s ampl es (plaques) of blend III wer e qu enched, r ecry stallized, and c ooled to room te mp e ratur e by th e same t ec h- niques de scrib ed for bl e nd s I and II. The density of th e thr ee ble nds was dete rmin ed by the proc edur e given in ASTM Des ignation : D1505- 67, " Stand a rd Me th od of T es t for Dens it y of Pl as ti cs by the Dens it y-Gradie nt T ec hniqu e." 2 Th e density of each sp ec imen tak en from th e r ec rystallized plaqu es was dete rmin ed by graphical int e rpolation of it s e quilibrium hei ght in th e density-gradi e nt liquid :t Although paragrap h 5.6 of the ASTM procedure ca ll s fo r a preheating period of 5 min, we have employed a lime of 15 s. 143
Transcript of The characterization of linear polyethylene SRM 1475. III ...
The Characterization of Linear Polyethylene SRM 1475. III. Density Determination
James E. Brown
Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234
(December 15,1971)
T he density of SRM 1475 at 23 °C was de te rm ined by ASTM meth od D 1505- 67 on sa mples prepared by Proced ure A of ASTM me thod D 19211- 68 , with the result 0.9784 g/c rn".
Key words : Crys talliza tion; degree of c rystallinit y; density; linear po lyeth ylene; polymer; recr ystalli zation.
1. Introduction
The de nsity of polyethylene crystalli zed from its melt is sensitive to a number offactors which influence the degree of cr ystallinity. Among these fac tors are : the polymer 's previous thermal his tory and crystallization tem pera ture which affect the perfection and rate of crystal growth [1- 4] ,1 the average molecular weight and its distribution [3], and the amount of branching [5- 6].
In this pa per we report the res ults of the density meas ure ments on linear polye th ylene SRM 1475. T he density measure ments and the sample preparation were performed using standard methods of the Americ an Society for T esting and Materi als (ASTM). It was necessary to specify closely the pre para tion of the samples used in this determination since the density of polyethyle ne depends to s uch an important extent on the polymer's previous his tory. From our experi e nce, it s hould be possible to duplicate these results closely provided that the same methods are followe d .
2. Experimental Procedure
Three composite samples, weighing about 100 g each , were made up separately as described in the first paper of this series [7] by blend.ing equal amounts of the SRM 1475 pellets taken from the tops and bottoms of each of the original containers. These three composite samples will subsequently be referred to as ble nd I , II , and III.
T est samples (plaques) were prepared from blend I by Procedure A of ASTM Designation: D 1928- 68, "Standard Method for Preparation of CompressionMolded P olye thylene Test Samples." 2 In accordance with this procedure , the pellets of blend I were made
l Figures in bracke ts indicate the lilt: ra ture re fere nces a t t.he e nd of thi s pa per. 2 Availabl e . from the America n Societ y for Tes ting and Materia ls. 19'16 Race S tree t,
Philadelphia. P ennsylvania ] 9103.
into a crepe on a roll mill. The polyethylene was milled for about four minutes a t a te mpe rature sufficiently high to cause the polymer to flux but not to drip. P ieces were cut from the cooled crepe and compression molded into 3 mm thick plaques at 178 °C us ing picture-fra me-like c hases.3 T he mold assembly containing the molten polyethylene was re moved from the press and quenched to roo m temperature in a water bath. The mold assemblies containing the quenc hed plaques were then pl aced in a heated ove n and main· tained a t 170 °C, well above the crys tal melting point , fo r 1 h or more in order to destroy the polymer 's pre· vious th erm al hi s tory. Th e te m perature of the oven was then lowered to 50 °C at a rate of 5 °C/hr. After the plaques had come to roo m temperature, severa l speci· mens (2- 3 mm on a side) were cut from each plaqu e for the density meas ureme nts.
As a check on the sensitivity of the ASTM procedure to variations in the preparative technique , plaques of blends II and III were prepared somewhat differently. Plaques of blend II were pre pared as described above except that the milling time (about 2 minutes) was just long enough at about 125 °C to cause the pellets to fuse into a sheet. The plaques of blend III were not molded from a milled crepe. These plaques were compression-molded from the pelle ts. Except for the milling process , the test samples (plaques) of blend III were quenched , recrystallized , and cooled to room te mperature by the same techniques described for blends I and II.
The density of the three ble nds was determined by the procedure given in ASTM Designation : D1505- 67, " Standard Method of T es t for Density of Plas ti cs by the Density-Gradie nt Technique." 2 The densi ty of each specimen taken from the recrystallized plaques was de termined by gra phical interpolation of its equilibrium height in the de nsity-gradient liquid
:t Although paragraph 5.6 of the ASTM procedure ca lls fo r a preheating period of 5 min , we have e mployed a lime of 15 s.
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maintained at 23 ± 0.1 0c. Isopropanol and water were used as the density-gradient liquids. The column was calibrated by a series of glass beads whose density was known to within ± 0.0001 g/cm3. The sensitivity of the column was found to be about 0.0002 g/cm3 per mm column height, and the position of beads and specimens could be read to about 1 mm.
3. Results and Discussion
The molding and recrystallization of the plaques and the density measurements were performed on several different days according to ASTM procedures for sample preparation and density measurement. No significant day-to-day variation was found in the results. It was concluded that the preparation and recrystallization of the samples were consistent from one experiment to the next to within the precision of the density determination. The average density of. twelve specimens taken from five samples (plaques) of blend I was found to be 0.97844 g/cm3 with a sample standard deviation of a single determination of 0.00014 g/cm3. The sample standard deviation of the mean of the twelve determinations is therefore 0.00004 g/cm3.
These values of the mean density and the sample standard deviation of the mean are reported on the certificate for SRM 1475. The range of the measurements was from 0.9784 to 0.9786 g/cm3•
The average of ten density measurements on blend II was found to be 0.97817 g/cm3 with a standard deviation of a single determination of 0.00010 g/cm3•
The density measurements for blend III gave a result of 0.97493 g/cm3 with a standard deviation of a single mpasurement of 0.00045 g/cm3, based on 21 measurements. These results are summarized in table 1. From the sample standard deviations and the systematic uncertainties associated with the calibrating beads,
TABLE 1. Density at 23 °C of Linear Polyethylene SRM 1475
Blend I Blend II Blend III
Average density, g/cm3 ........... *0.97844 0.97817 0.97493 Standard deviation of a single
measurement. .. . ..... . ... . ... . ... 0.00014 0.00010 0.00045 Number of measurements ........ 12 10 21
• Certificate value.
The samples of blend I were preparee from pellets of pOlyethylene which had been fluxed and milled twice as long (about 4 minutes) as blend II. The samples of blend III were molded from the polyethylene pellets.
we conclude that the overall accuracy of our measurements is well within the 0.05 percent limit cited in the ASTM procedure.
Although the 0.03 percent difference between the densities of blends I and II is statistically significant, for practical purposes this difference is unimportant in view of the 0.05 percent estimate of accuracy given in the ASTM procedure. On the other hand, the density of blend III (unmilled samples) is materially lower than that of blends I and II, and its standard deviation of a single measurement is much larger than those of blends I and II. These differences cannot be ascribed I
to differences in the crystallinity of the samples, for the parts of the procedure which might be expected to affect crystallinity (molding, quenching, recrystallization) were identical for all three blends. Nor can the differences be attributed to the lack of mixing of the individual pellets during the preparation of blend III, since pellet-to-pellet variation would be expected to yield a larger sample standard deviation for blend III than for blends I and II, but the same mean density. The most likely explanation of both the lower mean density of blend III and its larger sample standard deviation seems to be the presence of small voids resulting from the lack of milling and fluxing. This explanation is reinforced by the fact that specimens taken from plaques of blend III, even though carefully selected, appeared to contain small regions of entrapped air.
We conclude that the preparatory milling and fluxing stipulated in the ASTM procedure is essential if the stated precision of the procedure is to be realized.
4. References
[1] Mandelkern, L., in Growth and Perfection of Crystals, Ed. R. H. Doremus, B. W. Roberts, and D. Turnbull, p. 467 (John Wiley and Sons, Inc. , New York, N.Y., 1958).
[2] Mandelkern, L., Posner, A. S., Diorio, A. F., and Roberts, D. E. , J. App\. Phys. 32,1509 (1961).
[3] Chiang, R. and Flory, P. , J. Am. Chern. Soc. 83,2857 (1961). [4] Weeks , .T. J., Jr., J. Res. Nat. Bur. Stand. (U.S.), 67A (phys.
and Chern.), No.5, 441 - 451 (Sept.- Oct. 1963). [5] Stella, c., Compt. Rend. 248, 1819 (1959). [6] Sperati, C. A., Franta, W. G., and Starkweather, H. W., Jr.,
J. Am. Chern. Soc. 75,6157 (1953). [7] Hoeve, C. A . .T., Wagner, H. L., and Verdier, P. H.,]. Res. Nat.
Bur. Stand. (U.S.), 76A (phys. and Chern.) , No. 2, 137- 140 (Mar.-Apr.1972). Paper! of this series. I
(Paper 76A2- 708)
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