Ethylene copolymers as sintering enhancers and impact modifiers for rotational molding of polyethylene.
Rotational molding, also known as rotomolding, is a shear-free and pressure-free process used to manufacture hollow plastic parts with relatively low investment (1-7). Polyethylene accounts for more than 85% of the volume in rotomolding (1, 5) because of its acceptable processability, good thermal stability (4) and good mechanical properties. Because of recent technological changes and innovations in the polymer industry, rotational molders are seeking resins that can be processed at shorter cycle times or can provide better properties than incumbent polyethylenes. One objective of this work was to evaluate the effect of blending minor amounts of an ethylene styrene Interpolymer made by INSITE ** Technology, or ethylene vinyl acetate (EVA) copolymers differing in vinyl acetate contents, with polyethylene on the impact strength of rotomolded parts.
Cycle times in rotational molding are generally very long (about 25-30 minutes for thick parts). For the Interpolymer or EVA copolymers to be useful impact modifiers in rotational molding. an increase in cycle time would not be desirable. Hence, the effect of blending Interpolymer or EVA copolymers with polyethylene on sintering rates, bubble removal and cycle times in rotational molding was also studied in this investigation. Conventionally, cycle times have been reduced by using polyethylene of reduced molecular weight or melt viscosity. However, this can result in inferior impact strength. Blending small amounts of low molecular additives with polyethylene has also been shown to accelerate sintering and bubble removal, thereby resulting in shorter cycle times (7), but these additives do not have a beneficial effect on impact strength.
Minor amounts (10 weight percent) of ethylene copolymers were blended with linear low-density polyethylene (LLDPE) using a 30-mm twin-screw extruder. The ethylene copolymers used were one ethylene styrene Interpolymer and two ethylene vinyl acetate (EVA) copolymers. The characteristics of the polymers are given in Table 1. The thermal transitions of Interpolymers vary significantly with styrene content (8). The ES30 Interpolymer was selected primarily because it had a glass transition temperature of about -20[degrees]C. This Interpolymer had a styrene content of 30 weight percent and its glass transition temperature was lower than that of Interpolymers with higher styrene contents (8).
The ethylene copolymers were first tumble blended with LLDPE pellets and then melt blended on the twin-screw extruder. The LLDPE was also extruded to make a control formulation without the ethylene copolymers. Melt temperatures ranged from 204[degrees]C to 210[degrees]C. The extruded formulations were pelletized and subsequently pulverized (at identical grinding conditions).
Properties of the pelletized products were tested. Viscosity and Tan [delta] were measured at 190[degrees]C using a Rheometrics Mechanical Spectrometer, Model 800, equipped with 25-mm-diameter parallel plates. Flexural modulus was tested using a bar of rectangular cross section, three-point loading system and 4.5kilogram load cell. Environmental stress crack resistance (ESCR) was measured by immersing 10 notched specimens of each sample in a 10% Igepal solution and observing the time taken for 50% of the specimens to break. Thermal transitions were measured by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC-7 equipment. The sample size was approximately 5 mg. and the following procedure was used: The sample was heated rapidly in a sealed aluminum pan from ambient temperature to 180[degrees]C (at a rate of 105[degrees]C per minute); kept at 180[degrees]C for three minutes to ensure complete melting; cooled at a rate of 10[degrees]c/min to about -60[degrees]C (or 40 celsius degrees below the ex pected glass transition temperature); stabilized at this temperature for three minutes; and heated to 15000 at a rate of 10[degrees]C/min The peak crystallization temperature (Tc) was obtained from the cooling curve. The peak melting temperature (Tm) was obtained from the melting curve (second heat).
The pulverized samples were also characterized. Melt index (2.16 kg, 190[degrees]C) was measured in accordance with ASTM D-1238 and bulk density was measured in accordance with ASTM D-1895. The bulk density of a powder is the mass of powder that is held in a given volume without being packed. Sieve analysis was conducted in accordance with ASTM D 1921-89. A set of sieves ranging in opening size from 35 mesh (500 microns) to 100 mesh (149 microns) was used. The relationship between sieve-opening size in mesh and microns ([mu]m) is given in the published literature (9) for selected sieve sizes. The average particle size and mean particle diameter was computed using the method and equations described in a previous publication (7).
Sintering experiments, uniaxial rotational molding and biaxial rotomolding were conducted using the procedures described elsewhere (7). The molded parts were inspected, cut and prepared for impact tests. A dart drop tester was used to perform low-temperature dart impact tests in order to characterize the mechanical properties of rotomolded parts that had been conditioned in a freezer for 8 hours at -40[degrees]C. The technique used for determination of mean failure energy, MFE, has been described in a previous publication (7). The error in the measurements of MFE was 5%. Bubble content, thickness distribution and density of the molded parts were also determined using the procedures described elsewhere (7).
The novel blend compositions described in this paper were designed to enhance sintering and impact strength of parts made by rotational molding. The concept was to blend minor amounts of ethylene copolymers with polyethylene, with the former melting at relatively lower temperatures, such that sintering would be enhanced. The enhanced sintering would also be expected to lead to improvements in the impact strength of the fabricated articles. By using only small amounts of the copolymers, the flexural modulus would not be affected substantially.
Sintering is defined as the formation of a homogeneous melt from the coalescence of powder particles (7). As the sintering of two spherical particles progresses, material transport occurs, resulting in growth of the contact neck between the particles. The sintering rate can be expressed by the rate of the neck growth. The quantitative characterization of the process can be described by the following expression (10), which is valid at the initial stages only (11, 12):
y/a = [(3[GAMMA]t/2[eta]a).sup.1/2] (1)
where [GAMMA], [eta], y, a and t are the surface tension of the polymer, viscosity, neck radius, particle radius, and sintering time, respectively. The growth of the neck between the two particles is proportional to the square root of time. The coalescence of the two particles is completed when the two particles have adhered together to form a single particle (y/a = 1). Sintering time (ts) is defined as the time required for 99% completion of neck growth and can be calculated using the following empirical model:
y/a = 1 - [A.e.sup.-(t/ts)] (2)
The model can fit the measured rate of neck growth. The same curve fitting and nonlinear regression can be used to calculate neck growth at various intervals (say, 330 seconds).
RESULTS ABD DISCUSSION
The shear-dependent melt viscosity and Tan [delta] (loss modulus divided by storage modulus) of the Interpolymer and EVA resins are compared with that of LLDPE in Figs. 1 and 2 (at a temperature of 190[degrees]C). Tan [delta] is a measure of melt elasticity--the higher the Tan [delta], the less elastic is the melt. The corresponding melt index values are given in Table 1. The ethylene copolymers exhibited more shear thinning than LLDPE, but the Interpolymer did not shear-thin as much as the EVA9 resin of similar melt index (0.8 dg/min). However, for rotational molding, the more relevant rheological properties are melt viscosity and elasticity measured at zero shear rate. A shear rate of 0.1 [s.sup.-1] can be taken to approximate zero shear conditions. At very low shear rate (0.1 [s.sup.-1]), the viscosity of LLDPE and the EVA15 resin were identical (Table 1 and Fig. 1). In contrast, the Interpolymer and EVA9 resin (of lower melt index) exhibited significantly higher viscosity at this shear rate. From the Tan [delta] measurements, it is apparent that the Interpolymer and EVA resins were significantly more elastic than the LLDPE (Fig. 2 and Table 1).
Table 2 describes the properties of extruded LLDPE and blends with ethylene copolymers (before pulverization). Blending Interpolymer or EVA9 with LLDPE resulted in increased melt viscosity and elasticity at 0.1 [s.sup.-1] shear rate (Table 2). The trends were consistent with the differences in viscosity and Tan [delta] of the individual polymers. However, the increases in viscosity were not as high as expected from the viscosities of the individual blend components. This may indicate that these blends were considerably compatible. On the other hand, the blend containing EVA15 exhibited slightly higher melt viscosity, but its Tan [delta] was much higher than anticipated (even greater than that of the individual blend components). This suggests that this blend was not very elastic, probably because of poor compatibility of the blend components.
The blends containing Interpolymer or the EVA15 resin exhibited significantly improved environmental stress crack resistance (Table 2). Flexural modulus was decreased with the addition of Interpolymer or EVA resins, but the modulus was still greater than 800 MPa (which is preferable for rotomolded articles).
The properties of the materials, after pulverization, are shown in Table 3 and Fig. 3. The powder properties that significantly affect rotational molding are viscosity, bulk density and particle size distribution. The mean particle diameters and particle size distributions were only slightly different and were not expected to cause large variations in rotational molding. The bulk densities of the materials were similar (approximately 0.3 g/[cm.sup.3]). Bulk density gives information about the packability of the resin. If the bulk density is high, the resin packs well during rotational molding, resulting in fewer bubbles. The fact that the bulk densities were similar suggests that there were not large differences in the amounts of entrapped air in these materials.
The results of the sintering measurements are plotted in Fig. 4. Figure 5 shows the sintering sequences for the different samples. At t = 0 sec, below the melting point of the polymer, the particles were solid. Sintering commenced when the temperature reached the melting temperature of the polymer. The contact zone between the two cylindrical particles increased and neck formation started. As sintering progressed, neck growth increased until the two particles fused together. The coalescence of the particles was completed when the neck radius became equal to the particle radius. Blending Interpolymer or EVA with LLDPE resulted in significantly faster sintering, even when the viscosity and/or elasticity at low shear rate was increased (Table 2) and/or it took longer for the onset of sintering to occur (Fig. 4). This may be attributed to the relatively lower melting points and crystallization temperatures of the minor blend components (compared with that of LLDPE), which would have increased the onset of flow, a nd thus the rate of heat transfer between the particles. DSC measurements revealed that the blends were not miscible and that the peak melting points of the various polymers were as follows: 127[degrees]C for LLDPE; 65[degrees]C for ES30; 94[degrees]C for EVA9; and 88[degrees]C for EVA15. The corresponding crystallization temperatures were: 113[degrees]C for LLDPE; 46[degrees]C for ES3O; 81[degrees]C for EVA9; and 72[degrees]C for EVA15. Thus, sintering of the samples evaluated in this study was primarily influenced by the melting points of the minor blend components and not by the differences in melt viscosity and/or elasticity at low shear rate.
Because of the air trapped between powder particles, bubbles are formed at the later stages of sintering in rotational molding. The rheology, thermal properties and powder properties of the resin influence the number and initial size of the bubbles (13). The amount of entrapped air and initial bubble size, along with the surface tension, further affect the bubble dissolution or bubble removal process in the polymer melt (14). The bulk density, mean particle diameter and particle size distributions of the pulverized materials were similar, such that their performance in rotational molding could be compared.
A crucial process parameter influencing the properties of rotomolded parts is the air temperature inside the mold (2, 15-17). The air temperature profiles in the center of the mold recorded as a function of time during uniaxial rotational molding of the various polymer compositions are shown in Fig. 6. The inside air temperature rose to a peak temperature (maximum inside air temperature ([T.sub.max])) in the heating cycle and decreased in the cooling cycle until it reached room temperature. Four major stages can be identified: Induction (up to A), adherence (from A to B), melting-sintering (from B to C) and fusion or densification (C to D). These stages have been described elsewhere (2, 18). There were no significant differences in the duration of each stage.
As the layers of polymer build in rotational molding, some air is trapped in the melt, and bubbles are formed. The presence of bubbles in molded parts depends not only on material properties, but also on process conditions. If the cycle time is not long enough, the molded part will have lots of bubbles. Therefore, inspection of the part for bubble content can provide good information on the progress of the process. Figure 7 shows photographs of the cross sections parallel to the surface of parts made by uniaxial rotational molding at a maximum inside air temperature ([T.sub.max]) of 210[degrees]C. The parts made from the blends containing Interpolymer or EVA had fewer bubbles than those made from the reference LLDPE, indicating that densification occurred relatively faster. It was surprising that increased melt viscosity and/or elasticity at low shear rate (Table 2) did not have a deleterious effect on bubble removal during rotational molding. The densification or bubble removal process was almost completed i n the case of the sample containing 10 wt% of EVA9, but was somewhat incomplete for the samples made with Interpolymer or the EVA15 resin. The blend containing the EVA15 resin exhibited a significant number of bubbles, even though its melt elasticity was rather low (Table 2).
The properties of the boxes obtained from uniaxial rotomolding of the various samples are shown in Table 4. Measurement of the thickness distribution is a good test to check the quality of the molded parts. The thickness of parts is affected by surface leveling in the adherence stage. The degree of smoothness is measured by the standard deviation of the thickness. and depends not only on the properties of the polymer, but also on the process parameters. The heating time is one of the most important controlling factors in surface leveling. For example, if the heating time is not long enough, the surface of the molded part will be uneven. From Table 4, it can be seen that at a temperature ([T.sub.max]) of 210[degrees]C, the average thickness of molded parts produced from the samples containing Interpolymer or EVA was similar to that of the parts made from the reference LLDPE sample, but the standard deviation was less. The large standard deviation in the case of the reference sample indicates uneven surfaces a nd suggests that the parts were undercured. Increasing [T.sub.max] from 210[degrees]C to 230[degrees]C or 245[degrees]C improved the thickness uniformity of parts made from LLDPE and blends with EVA9, but generally did not affect the uniformity of parts made from blends comprising Interpolymer and EVA15. The most uniform thickness was obtained with the sample containing 10 wt% of EVA9. No warpage was observed with any of the parts.
Densification is one of the most important aspects of the rotational molding process and significantly affects the part properties. The degree of densification is characterized by measuring the density of the part. The density of the molded parts made at different maximum air temperatures is reported in Table 4. The samples containing Interpolymer or EVA started densifying earlier than the reference LLDPE. At [T.sub.max] of 210[degrees]C, densification or bubble removal occurred fastest in the sample containing 10 wt% of the EVA9 resin. In particular, the densification process was far from complete at this temperature for the reference LLDPE. These findings were consistent with the observations made on bubble content at 210[degrees]C,. After 210[degrees]C, the rates of densification varied significantly and the sample containing the EVA15 resin densified fastest, followed by the blends comprising EVA9 and Interpolymer.
Impact strength and type of failure are two of the most sensitive properties of the molded part and can be correlated to the process conditions. Any small change in the heating cycle can significantly affect the impact strength, especially that at low temperature (17). Therefore, this parameter can be used to determine the processing windows of the different samples. Table 4 shows the mean failure energy (MFE) and type of failure of parts molded at different maximum inside air temperatures in uniaxial rotation. The mean failure energy, which represents the impact strength of the material, varied with the maximum inside air temperature and the polymer composition. When the inside air temperature was not high enough, the resulting parts were under-fused with low MFE and large numbers of bubbles. On the other hand, when the heating cycle was too long (i.e., too high maximum inside air temperature), the resulting parts were overcooked with low or zero MFE (i.e., evidence of oxidative degradation). The under-fused or overcooked parts exhibited brittle failure instead of ductile failure. All the blends resulted in significantly improved ductility in parts fabricated at lower processing temperatures and similar or increased impact strength, compared with the reference LLDPE resin. This may be attributed to faster bubble removal and possibly increased adhesion between the particles due to the lower melting points of the minor blend components. The blend containing Interpolymer yielded the highest MFE with ductile failure. This was surprising given the fact that this resin has a glass transition temperature of only about -20[degrees]C, well above the impact test temperature of -40[degrees]C. As [T.sub.max] increased, the MFE generally increased to a maximum value and then dropped. The exception was the blend containing the EVA15 resin, which demonstrated increasing MFE with increasing [T.sub.max] in the selected temperature range (that is, the operating window was relatively wide). Cycle times necessary to achieve 9.5 m k g (or greater) mean failure energy were significantly reduced by blending 10 wt% of Interpolymer or EVA9 resin with polyethylene.
Biaxial rotational molding was also used to fabricate parts from LLDPE and the blends with Interpolymer or EVA. The properties of the molded parts are tabulated in Table 5. The top portions of the parts were thicker than the bottom, probably because of differences in the surface texture of the mold--the top of the mold produced a smooth surface while the bottom resulted in a rough surface. Consequently, the MFE of the bottom portions tended to decrease significantly as a function of oven time, suggesting that they were "overcooked" (or thermally degraded). Hence, only the data on the top parts were considered in the evaluation. The MFE of the part fabricated from the control LLDPE tended to increase with increasing oven time and the maximum MFE attained was 9.8 m kg. In comparison, the parts made from blends containing Interpolymer or the EVA15 resin exhibited maximum MFE at a significantly shorter oven time (1320 seconds versus 1440 seconds). The blend containing EVA9 did not yield as high an MFE as the othe r samples, but this may have been because bubble removal occurred much faster than in the other samples (as described earlier and also reflected in the shorter cycle times evident in uniaxial rotomolding). That is, it is possible that optimum oven time with the blend containing EVA9 may have been less than 1320 seconds, but this was not evaluated. Cycle times necessary to achieve (or exceed) MFE of 8.5 m kg were significantly reduced by blending 10 wt% of Interpolymer or EVA with the polyethylene. The blend containing Interpolymer resulted in higher peak MFE than the reference LLDPE sample. In all cases, ductile failure was observed at the peak MFE, and the blend containing the EVA15 resin exhibited a particularly broad processing window (i.e., achieved both high MFE and ductile failure over the entire range of cycle time). Thickness uniformity was generally improved with the blends, particularly those containing Interpolymer or EVA15. These observations correlate reasonably well with the results of the sinte ring and uniaxial roto-molding experiments.
Optical microscopy was conducted on the parts made from the blends comprising Interpolymer and the EVA15 resin (Fig. 8). The Interpolymer gave a finer dispersion than the EVA15 resin in LLDPE, suggesting that it was relatively more compatible (Fig. 8)--in accordance with the earlier observations from measurements of melt viscosity and Tan [delta]. This difference in morphology may account for the unexpectedly large increase in MFE obtained with the blend containing Interpolymer.
Blending minor amounts of Interpolymer or EVA9 with LLDPE resulted in significantly increased melt viscosity and elasticity at low shear rate. The blend containing EVA15 exhibited higher melt viscosity, but elasticity was decreased. Sintering rates were primarily influenced by the melting points of the minor blend components, and not by the differences in melt viscosity and/or elasticity at low shear rate, such that the blends sintered significantly faster than LLDPE. Blending Interpolymer or EVA with LLDPE also resulted in significantly improved ESCR and decreased flexural modulus.
The results of the rotational molding experiments correlated well with those obtained from the sintering experiments. Blending 10 wt% Interpolymer or EVA with LLDPE was generally observed to result in faster bubble removal, more uniform thickness and shorter cycle times. Ductile failure was generally observed at the peak mean failure energy (MFE). The operating window was relatively wide in the case of the blend containing the EVA15 resin. The blend containing Interpolymer yielded significantly increased MFE, possibly because the Interpolymer was compatible with LLDPE and was very well dispersed. No warpage was observed in any of the parts.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Fig. 4 Effect of Interpolymer and EVA on rate of neck growth. no 10 wt% 10 wt% 10 wt% additive ES30 ESA9 EVA15 Onset of Sintering (seconds) 95 105 90 85 Calculated Sintering Time (seconds) 850 572 600 622 y/a at 330 seconds--from Curve 0.84 0.93 0.94 0.93 Fitting Note: Table made from line graph
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Table 1 Properties of Polymers. Weight percent Weight vinyl percent Designation acetate styrene Test method LLDPE LLDPE not applicable not applicable Ethylene Strene ES30 or not applicable 30.0 Interpolymer Interpolymer ELVAX * 770 EVA9 9.5 not applicable ELVAX 550 EVA15 15.0 not applicable Melt Index (dg/min) -190[degrees]C, Density 2.16 kg (g/[cm.sup.3]) Test method ASTM D-1238 ASTM D-792 LLDPE 4.0 0.939 Ethylene Strene 0.8 0.937 Interpolymer ELVAX * 770 0.8 0.930 ELVAX 550 8.0 0.935 Viscosity Tan [delta] at (Pa-s) at 190[degrees]C 190[degrees]C and and 0.1 [s.sup.-1] and 0.1 [s.sup.-1] shear rate shear rate Test method LLDPE 2092 30 Ethylene Strene 13980 4 Interpolymer ELVAX * 770 17048 2 ELVAX 550 2062 10 * Trademark of Dupont. Table 2 Properties Before Pulverization. Ethylene Copolymers Blended with LLDPE Test Method none 10 wt% ES30 10 wt% EVA9 Density (g/[cm.sup.3]) ASTM D-792 0.9399 0.9402 0.9389 Viscosity (Pa-s) at 2092 2529 2223 190[degrees]C and 0.1 [s.sup.-1] shear rate Tan [delta] at 30 28 29 190[degrees]C and 0.1 [s.sup.-1] shear rate Flexural modulus ASTM D-790, 1015 829 998 (MPa) Method 1 ESCR (hours) ASTM D-1693 534 > 5000 not measured Ethylene Copolymers Blended with LLDPE 10 wt% EVA15 Density (g/[cm.sup.3]) 0.9396 Viscosity (Pa-s) at 2258 190[degrees]C and 0.1 [s.sup.-1] shear rate Tan [delta] at 45 190[degrees]C and 0.1 [s.sup.-1] shear rate Flexural modulus 838 (MPa) ESCR (hours) > 5000 Table 3 Powder Properties. no 10 wt% 10 wt% 10 wt% additive ES30 EVA9 EVA15 Melt index (dg/min) 4.2 4.1 4.3 4.3 Bulk density (g/[cm.sup.3] 0.296 0.289 0.293 0.288 Mean particle diameter ([mu]m) 234 245 260 245 Table 4 Results From Uniaxial Rotational Molding. Mean Failure Energy, MFE (m kg) [T.sub.max] No additive 10 wt% ES30 10 wt% EVA9 10 wt% EVA15 ([degrees]C) 210 7.2 7.3 8.0 7.0 220 -- 9.8 9.7 8.9 225 9.2 -- -- -- 230 9.5 12.8 0.0 9.7 245 2.4 0.0 -- 10.2 Type of Failure 210 brittle ductile ductile ductile 220 -- ductile brittle ductile 225 ductile -- -- -- 230 ductile ductile brittle ductile 245 brittle brittle -- mostly brittle Thickness (mm) 210 3.15 3.13 3.13 3.15 220 -- 3.03 3.11 3.05 225 3.05 -- -- -- 230 3.08 3.08 3.07 3.12 245 3.10 3.07 -- 3.12 Standard Deviation of Thickness (mm) 210 0.32 0.27 0.23 0.25 220 -- 0.33 0.18 0.25 225 0.25 -- -- -- 230 0.25 0.28 0.19 0.24 245 0.27 0.28 -- 0.25 Density (g/[cm.sup.3]) 210 0.9288 0.9308 0.9355 0.9310 220 -- 0.9400 0.9410 0.9413 225 0.9408 -- -- -- 230 0.9408 0.9445 0.9398 0.9413 245 0.9433 0.9448 -- 0.9393 Table 5. Results From Biaxial Rotational Molding. Mean Failure Energy, MFE (m kg) Time (seconds) No additive 10 wt% ES30 10 wt% EVA9 10 wt% EVA15 1320 8.3 10.4 8.5 9.8 1380 8.2 -- 8.7 9.4 1440 9.8 10.4 -- 9.8 Type of Failure 1320 ductile brittle some brittle ductile 1380 ductile -- ductile ductile 1440 ductile ductile -- ductile Thickness (mm) 1320 2.98 3.03 2.99 3.10 1380 2.90 -- 3.04 3.05 1440 2.96 3.01 -- 3.02 Standard Deviation of Thickness (mm) 1320 0.20 0.17 0.23 0.27 1380 0.24 -- 0.20 0.16 1440 0.17 0.15 -- 0.16
(**) Trademark of The Dow chemical Company.
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BHARAT INDU CHAUDHARY (*)
(*) Corresponding author.
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|Author:||Chaudhary, Bharat Indu; Takacs, Elizabeth; Vlachopoulos, John|
|Publication:||Polymer Engineering and Science|
|Article Type:||Statistical Data Included|
|Date:||Jun 1, 2002|
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