Shear viscosity of rubber modified thermoplastics: dynamically vulcanized thermoplastic elastomers and ABS resins at very low stress.
Rubber modified thermoplastics have a long history in the polymer industry. Two of the more successful products have been materials produced by (i) crosslinking elastomers in rubber-thermoplastic blends during mixing (dynamic vulcanization) (1) and (ii) free radical polymerization of monomer solution of elastomers such as polybutadiene (2). In this paper we study the rheological behavior of two types of polymer systems of materials in these categories. These are dynamically vulcanized polypropylene (PP)-ethylene propylene diene terpolymer (EPDM) blends and ABS resins.
There is a very large amount of literature on the rheological properties of rubber modified plastics. It is beyond the scope of this paper to review all of this material. We limit our considerations to the polypropylene-EPDM and ABS resin systems. The earliest study of rheological behavior of the PP-EPDM system was by Lee (3) on a high impact polypropylene long before the invention of dynamic vulcanization. The shear viscosity data indicated constant viscosity of PP at low shear rates, but for the PP-EPDM blend an increasing viscosity at low shear rates, which might indicate a yield stress below which there is no flow. More recent papers by Goettler et al. (4) and Han and White (5) have considered PP-EPDM dynamic vulcanizates. The shear viscosity data a both research group at low shear stresses continued in to increase sharply and suggest yield values. There have also been investigations of ABS resins (6-9). Studies by Zosel (7) and notably Munstedt (8) suggest the development of yield values in ABS resins at high rubber loadings. Investigations of the behavior of ABS resins in oscillating flows by Aoki (9, 10) show storage and loss moduli G[prime]([Omega]) and G[double prime]([Omega]), which appear to go asymptotically to finite values at zero frequency indicating solid and not fluid behavior.
In these studies, the data for the PP-EPDM dynamic vulcanizates and the ABS resins suggest but do not prove the existence of yield values. To prove the existence of yield values, it is necessary as pointed out, for example, by Nguyen and Boger (11) and Husband and Aksel (12) to make measurements at controlled stresses and find stresses below which there is no flow. Such experiments have been carried out in our laboratories for rubber with various small particles including carbon black, zinc oxide, CaC[O.sub.3] and silica (13-15) and thermoplastic-carbon black compounds (16).
It is our purpose in this paper to make measurements over a wide shear stress region including low stress creep on PP-EPDM dynamic vulcanizates and ABS resins.
Table 1. Materials Used. Designation Material Polypropylene Millennium Petrochemicals PP Petrothene PP 8000-OK AES TPE-1 Santoprene 203-40 hardness: 40D rubber content: low molecular weight of matrix: high AES TPE-2 Santoprene 201-55 hardness: 55A rubber content: high molecular weight of matrix: low ABS-1 Dow ABS 941, MFR = 2.0 ABS-2 Mitsubishi Chemical rubber content: 50 wt% particle size 0.60 [micro] - 20 wt% 0.25 [micro] - 80 wt% grafting degree: 48.6 wt%
Five materials were used in this study. These were (i) polypropylene, (ii) and (iii) two commercial PP/EPDM dynamically vulcanized thermoplastic elastomers designated TPE-1 and TPE-2, (iv) low rubber content ABS (ABS-1) and (v) high rubber content ABS (ABS-2). ABS-1 was a commercial sample. ABS-2 was an experimental sample. The characteristics of these five materials based on the information from the suppliers are summarized in Tab/e 1.
All measurements were made at 180 [degrees] C.
The measurement techniques used are very much as in our earlier papers (5, 13-15). A sandwich rheometer consisting of three parallel aluminum plates was used in this study for low shear rate measurements. The design and operation of this sandwich rheometer originally first used by Toki and White (17) has been modified in more recent studies. It was primarily used in the creep mode. The shear stresses were calculated from
[[Sigma].sub.12] F + W(inner member)/2A (1)
where A is the surface area of the plates in contact with the compressed polymer sheet F is the applied weight and W is the weight of inner member. Displacements were monitored as a function of time for each constant load attached. Values of the strain dependence on time were obtained by dividing displacements by sample thickness. The entire rheometer, except the displacement gauge, was enclosed in a 180 [degrees] C heat regulated chamber for temperature control. Nitrogen gas was used to preserve the material from oxidative degradation.
An Instron capillary rheometer with dies of diameter 1.5 mm and L/D ratio of 10, 20, 30 was used. Weissenberg corrections for the die swell shear rate and Bagley entrance corrections for the pressure were also performed. The maximum shear rate investigated was about 1000 [s.sup.-1].
At intermediate shear rates, a Rheometrics Mechanical Spectrometer with a coneplate fixture was used. Its diameter was 25 mm and cone angle was 0.1 rad. This machine was operated at constant shear rate. This instrument was used only for PP and TPE- 1. This was because of the difficulty of squeezing the samples into the 50 [[micro]meter] gap between the blunted tip of the cone and the plate.
The PP exhibited, except at the shortest times, a linearly increasing displacement as a function of time. More interesting results were found with the PP-EPDM dynamic vulcanizates and the ABS resins. We have plotted shear strain creep as a function of time at various shear stress levels for the PP-EPDM dynamic vulcanizates and the two ABS samples in Figs. 1a-1d. The ABS-1 exhibited typical linear viscoelastic behavior at low stresses. There are stresses for the TPE-2 and ABS-2 below which there is no flow. This indicates the occurrence of yield value. The TPE-1 still seems to flow but only very slowly at the lowest shear stresses we measured (796 Pa). The values of the measured yield stresses are summarized in Table 2.
Shear Viscosity Behavior
The shear viscosity obtained for the various materials in the sandwich, capillary and rotational rheometers is shown in Figs. 2a and 2b.The shear viscosities of the polypropylene and the PP-EPDM thermoplastic elastomers are plotted as a function of shear rate in Fig. 2a and the ABS resins in Fig. 2b. The data range over about ten decades of shear rate from [10.sup.-7] to[10.sup.3] [sec.sup.-1]. The data from all of the different instruments are quite consistent. The PP exhibits a zero shear viscosity at [10.sup.4] Pa s and the ABS- 1 at [10.sup.6] Pa s. In the low shear rate ([similar to][10.sup.-6] [sec.sup.-1]) range, the viscosity data of ABS-2 and TPE-2 exhibit plateaus in the range of [10.sup.9] to [10.sup.10] Pa s. The TPE-1 exhibits highly non-Newtonian behavior to the lowest shear rates studied. At high shear rates there is little difference between the various ABS and TPE samples.
We have plotted the shear viscosity as a function of shear stress in Fig. 3a and 3b. We may note the following:
(1) PP and ABS- 1 exhibit zero shear viscosities.
(2) TPE-2 and ABS-2 exhibit yield values. TPE-1 may have a yield stress below 800 Pa.
(3) ABS-2 and TPE-2 exhibit shear viscosity plateaus at very high viscosities [similar to] [10.sup.9] Pa s.
The large magnitude of the plateau in ABS-2 and TPE-2 cause extrapolations of the shear viscosity from higher stresses giving significantly larger yield values than the values measured by creep (60,000 Pa compared to 6500 Pa for ABS-2 and 20,000 Pa compared to 1770 Pa for TPE-2).
This paper contains the first proofs of the existence and measurements of yield values for dynamically vulcanized thermoplastic elastomers or ABS resins or any rubber modified plastics. Earlier papers have always given only extrapolated results. The occurrence of high shear viscosity ([similar to] [10.sup.9] Pa s) - intermediate shear stress plateau in high rubber content ABS and the higher rubber content thermoplastic elastomer has never been observed in the literature for rubber modified polymer melts. We carefully studied the previously published data on rubber-modified polymer melts. The two papers closest to the present studies are by Munstedt (8) for ABS and Han and White (5) for PP-EPDM TPE. We list the materials these investigators used in Table 3. We replot the data of the two above papers in Fig. 4 with our data. The data replotted [TABULAR DATA FOR TABLE 2 OMITTED] [TABULAR DATA FOR TABLE 3 OMITTED] from the investigations of Munstedt (8) and Hah and White (5) show viscosities as high as [10.sup.7]-[10.sup.8] Pa s, which suggest but do not prove yield values.
There are other studies similar to our own on other thermoplastic elastomers, most notably triblock copolymers with polystyrene end segments and a rubbery center segment. Studies of this type are given by Vinogradov et al. (18), Hansen and Williams (19) and Hall and White (20). We replot their data in Fig. 5 together with our data. Hansen and Williams (19) actually observed and measured yield values on a styrene-butadiene-styrene triblock and investigated the viscosity behavior beyond the yield value using a parallel plate constant-stress rheometer. They described the behavior at higher shear stresses finding viscosities gradually increase with increasing shear stress and shear rate (from [similar to] 3 x [10.sup.6] Pa s at 2 x [10.sup.2] Pa to 1 x [10.sup.7] Pa s at 5 x [10.sup.3] Pa). Their data also suggests a viscosity plateau. The data of Hah and White (20) for styrene-(ethylene-butene)-styrene triblock suggest a yield value but did not actually determine one. They determine a viscosity level of [10.sup.6] Pa s, which is higher than Vinogradov et al. (18) but lower than Hansen and Williams (19).
In our studies, the higher rubber content TPE (TPE-2) and ABS (ABS-2) exhibit yield values and high viscosity plateaus. The lower rubber content TPE (TPE-1) may have a yield value but does not exhibit a plateau. The yield value of TPE-2 is lower than that of ABS-2. The PP-EPDM vulcanizates and ABS have totally different morphologies and it is difficult to hypothesize a relationship between yield values and the difference in particle size or rubber content. However, if one compares our studies of ABS resIns with earlier work notably Munstedt (8) and Aoki (9, 10) it seems clear that high rubber levels, small rubber particles and large difference in composition between the rubber and matrix lead to agglomeration of the rubber particles and to yield values.
Yield values have long been discussed and measured for particle filled compounds including particles such as carbon black (13-17, 21, 22), calcium carbonate (15, 23) and talc (24, 25). This has now been verified for many of these compounds using creep measurements (13-16). It should be noted that yield values accompanied by the high viscosity plateaus have been described by other investigators in different systems such as the compounds of ethylene propylene terpolymer and polypropylene with carbon black (16).
The research described in this program was supported by Showa Denko K.K. Dr. Yuji Aoki of Mitsubishi Chemical supplied us with a special experimental ABS sample. Advanced Elastomer Systems and Dow Chemical supplied us with commercial samples.
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|Author:||Araki, Takumi; White, James L.|
|Publication:||Polymer Engineering and Science|
|Date:||Apr 1, 1998|
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