Processibility factors of polyolefins as determined on the Brabender plastograph

Author

James Thomas Ryan, New Jersey Institute of Technology

Document Type

Thesis

Date of Award

6-30-1962

Degree Name

Master of Science in Chemical Engineering - (M.S.)

Department

Chemical Engineering

First Advisor

C. L. Mantell

Second Advisor

Joseph Joffe

Third Advisor

Saul I. Kreps

Abstract

Some knowledge of the behavior of a plastic during processing is required if the materials best properties for a given application are to be retained. For thermoplastic polymers¹ changes in viscosity, a function of molecular weight, can be measured by various laboratory instruments, generally, under specialised conditions. One laboratory instrument, however, called the Brabender Plastograph will, it is claimed by the C. W. Brabender Company, measure the viscosity behavior of plastics under conditions found in production equipment. The Brabender plastograph measures changes in torque, a function of viscosity, which result from chain scission, shear thinning, temperature effect, density change and cross-linking. All of these are encountered during industrial processing.

Little information, other than that from the manufacturer, is available on the use of the Brabender plastograph for polymer evaluation. Except for some work with polyethylene by the C. W. Brabender Company no polyolefin² studies utilizing this apparatus have been published. The purpose of this paper is to investigate by means of the Brabender plastograph some of the factors that determine the processibility of polyolefins. These factors are temperature, time, shear rate, and varied molecular structure within the same polymer series.

The test results of this program show that the Brabender torque of the four polyolefins (low and high density polyethylenes, polypropylene, and polyisobutylene) is a function of temperature and shear rate as given by the respective equations

ω = Ae^2.303mT-1

ω = BSn

where w is the torque, A and B are constants, e is the base of natural logarithms, m is the slope d(log ω)/d(l/T), T is the absolute temperature, S is the shear rate, and n is the slope d(log ω)/d(log S). The exponents m and n and constants A and B differ for each material. Exponent n is independent of the shear rate and should be a measure of polymer molecular weight distribution. This slope could be called the Brabender flow-behavior index. Constant B expressed in the form log B is the viscosity of the polymer at a shear rate of 1 rpm and could be called the Brabender Consistency Index. For any of the four polyolefins the temperature rise at the end of 5 minutes at a shear rate of 50 rpm under nitrogen is given by the equation

ΔT₅ = 54.56T_B +20402/ log ω-0.091279T_B+29.04297 = T_B

where ΔT₅ is the temperature rise of the polymer in degrees centigrade, T_B is the absolute temperature of the Brabender plastograph °K and ω is the torque of the polymer in meter-grams.

Test results of polyolefin stability were not always in agreement with the results one would expect from just molecular structure. Little mechanical degradation of low and high density polyethylenes and polyisobutylene occurred at shear rates up to 50 rpm when mixed under nitrogen, but polypropylene was degraded at all shear rates. When the polymers were mixed under air the rate of degradation increased as expected and except for the low density polyethylene, was dependent upon the speed of mixing.

Additional work is necessary before it can be determined whether all polyolefins will behave in the manner described in this paper. Further studies could be made of the effect of molecular weight distribution on the flow-behavior index n. Other studies could determine if the equation of the temperature rise of a polyolefin after 5 minutes of mixing at a shear rate of 50 rpm could be expanded to cover any shear rate and a longer period of mixing. The test results, however, show that the Brabender plastograph is useful for studying on a batch basis polymer behavior over a range of temperature and shear rates which are found in production equipment.

¹ A thermoplastic polymer is a substance of high molecular weight which softens when heated, can then be forced into some shape, and solidifies when cooled. This can be repeated with the same material over and over again.

² polyolefins are polymerized from members of the ethylene (olefin) series of unsaturated hydrocarbons.

Recommended Citation

Ryan, James Thomas, "Processibility factors of polyolefins as determined on the Brabender plastograph" (1962). Theses. 3269.
https://digitalcommons.njit.edu/theses/3269