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Extrusion blow molding of long fiber reinforced polyolefins.

A 58% (by weight) long glass fiber reinforced (LGF]-HDPE master batch was blended with a typical blow molding HDPE grade. HDPE composites having between 5% and 20% (by weight) long fiber content were extruded at different processing conditions (extrusion speed, die gap, hang time). The parison swell (diameter and thickness) decreased with increasing fiber content. Although the HDPE exhibited significant shear rate dependence, the LGF/HDPE composites were shear rate insensitive. Both the diameter and weight swell results also indicated very different sagging behavior. The LGF/HDPE parisons did sag as a solid-body (equal speed at different axial locations) governed by the orientation caused by the flow in the die. Samples taken from blown bottles showed that fiber lengths decreased to 1-3 mm, from the original 11 mm fiber length fed to the extruder. No significant difference in fiber length distribution was found when samples for different regions of the bottle were analyzed. SEM micrographs corroborate the abs ence of fiber segregation and clustering or the occurrence of fiber bundles (homogeneous spatial fiber distribution) as well as a preferential fiber orientation with the direction of flow. The blowing step did not change the orientation of the fibers. Five-percent (5%) and 10% LGF/HDPE composites could be blown with very slight variations to the neat HDPE inflation conditions. However, 20% LGF/HDPE composites could not be consistently inflated. Problems related to blowouts and incomplete weldlines were the major source of problems.

INTRODUCTION

From the viewpoint of raw materials manufacturers, the automotive industry is the market segment in which the use of polymeric materials is increasing at an above average rate. For example, in 1970, the share of plastics in European vehicles was 27 kg/unit, while in 1998 this number grew to over 100 kg/unit (1). This increase in plastics use has been promoted by several factors: fuel economy required weight reduction; crash safety required stiffness and impact performance; environmental friendliness required cheap and simple recyclability; and finally cost reduction and faster model rotation required modularization and function integration (2). The biggest targets for replacing metals with plastics and composites can be grouped in four main categories: body frame and doors; bumpers, fascias and radiator grille panels; under-the-hood applications; and shafts, wheels and suspensions systems. Table 1 shows the estimated consumption of plastics and composites in the automotive industry in 2005 (3).

Plastics account for an average share of 9% of the overall weight of the vehicle, which corresponds, to an average volume content, in many cases, higher than 50%. Polyolefins represent about 50% of the plastic content with the remainder, split up between polyurethanes, engineering plastics and rubber. The current worldwide automotive consumption of poly-olefin thermoplastics, growing at 10% per year, is approaching one billion pounds (4). Of this, 160 million pounds correspond to HDPE, with 68 million pounds being used in the manufacture of plastic gas tanks. The other 92 million pounds are used for air ducts, fluid reservoirs, intake manifolds, dashboard components and various other under-the-hood parts.

In order to maintain the growth in automotive applications, plastics have to meet some important challenges. Among them we can cite the following: excellent mechanical performance under service conditions between -40[degrees]C and 210[degrees]; flow properties conducive to easier and repeatable processing: in-mold Class A surface finish; and repairability issues. Long glass fiber (LGF) /polyolefin composites are ideal materials for the development of new automotive applications since they can significantly improve the performance of the neat resins. In particular, they increase the rigidity and modulus of elasticity; the energy absorption level (important in failure and crash situations); the sound damping characteristics; the strength to weight ratio; the high temperature resistance (HDT); and the dimensional stability due to their lower coefficient of thermal expansion (lower shrinkage and warpage).

The use of LGF thermoplastics composites in the automotive industry is fairly recent. Some LGF injection molded parts already in use include radiator tanks, fans, fan brackets, gearbox covers, horn shells and heater housings. However, only recently, the possibility of having blow molded LGF reinforced parts has been explored. These parts will certainly benefit from the improved properties provided by the LGF reinforcement (5. 6) that include, among others, better melt strengths, dimensional stability and improved mechanical properties.

Only a couple of reports in the literature deal with the blow molding of LGF composites. Thielen (6) found that longer average fiber lengths (10 mm) lead to enhanced mechanical properties (dimensional stability, low shrinkage, and reduced warpage). As expected, the fiber length depended on screw shape and extrusion speed. Also it was found that the low blow pressures (< 200 psi) created difficulties in ensuring exact mold replication and rough surfaces with fibers on the surface and sunken spots. Bush et al (7, 8) studied the blow moldability of a number of polyolefins using a particular type of long glass fiber reinforcements and a patented system to introduce random orientation. However, no published literature has dealt, in detail, with the swelling and sagging behavior of LGF reinforced parisons. its dependence on processing parameters and its effect on final part properties.

In this paper we studied the material's flow behavior during the extrusion and blowing stages. Parison swell and sag; fiber content and distribution in the parison and bottle; and wall thickness distributions of a 51 bottle were measured and correlated to basic material properties and operating conditions. The feasibility of blowing a LGF/HDPE part was established based on the wall thickness distribution and on the impact strength measurements.

PARISON BEHAVIOR

A successful blow molding resin has to combine a number of properties that will guarantee an optimal processing (blow moldability) as well as good final properties (9). The control of parison behavior (swell and melt strength of the material) is one of the key factors in the blow molding of complex technical parts. Diameter ([B.sub.D]), thickness ([B.sub.Th]), and weight ([B.sub.weight])swells described the resin's parison behavior during extrusion and prior to inflation.

[B.sub.Th] = [h.sub.parison]/Die gap (1)

[B.sub.D] = [D.sub.parison]/Die diameter (2)

These parameters determine the diameter and thickness of the parison before blowing. Swell is a true material property dependent on the temperature and deformation history to which the material is subjected in the die. Since swell is a large deformation phenomenon, it can be best related to the non-linear viscoelastic properties of the material.

On the other hand, sag represents the stretching of the parison due to its own weight (10). Sag determines the diameter and thickness distribution along the parison length. The deformations involved during the sagging of the parison are fairly small and therefore can be related to linear viscoelastic properties. i.e., uniaxial elongational properties of the material. The sag phenomenon begins during the extrusion of the parison and continues up to the moment of parison clamping. Since swell and sag act in opposite directions, it is useful to establish their relative importance with a single measure. The combined effect of swell and sag can be quantified from the weight swell, [B.sub.weight]

[B.weight] = [B.sub.T] . [B.sub.D] [approximately equal to] weight of parison/theoretical weight of parison (3)

[W.sub.parison] represents the cumulative weight of the panson and is the weight of an annular segment having the dimensions of the die ([R.sub.o], [R.sub.i]) and the length of an individual pillow. Under isothermal extrusion conditions, the weight swell can be approximated by the area swell.

It is also important to establish the relationship between the weight and diameter swells. This relationship quantifies the type of deformations occurring in both the radial and circumferential directions.

[B.sub.D] = A . [([B.sub.W]).sup.[beta]] (4)

If the swelling is isotropic in the horizontal plane, diameter and thickness swells are equal, then A = 1 and [beta] = 0.5.

Finally, in order to determine beforehand if a material has the necessary characteristics to be blow molded and to define its optimal processing window, it would be advantageous to define a "blow moldability" index. This index should reflect the relative contribution of the swell and sag effects to the parison formation and the ranges in which they control the process. Let us define a relative area swell, (RAS), as (11)

RAS [([B.sub.weight]).sub.max] - [(B.sub.weight]).sub.min]/[(B.sub.weight]).sub.max] (5)

For a uniform parison (low sag), RAS [right arrow] 0, while for a highly deformed parison (low melt strength), RAS [right arrow] 1.

In the case of reinforced (short glass fibers, mica, etc) thermoplastic materials, very little has been done to ascertain their behavior during the different stages of extrusion blow molding. It is widely accepted that reinforced materials show negligible parison swell as well as significant sag (12, 13). On the other hand, polyolefins/talc composites (14) and poyolefins/ Ca[CO.sub.3] (15) show greatly reduced swell and very small sag with increasing talc or Ca[CO.sub.3] content and decreasing filler particle size. Furthermore, the thickness distribution of the talc filled thermoplastics was found to be more uniform. In a study of a PS/mineral whisker composite (16), the presence of whiskers increases the elongational viscosity and therefore reduces the sag. On the other hand, the die swell is reduced.

EXPERIMENTAL

Materials

The raw materials used in this study were a typical blow molding HDPE grade (Petromont 6200, MFI = 0.37 g/10 mm; [rho] = 0.956 g/[cm.sup.3]) and a long glass fiber/HDPE (58% fibers by weight) masterbatch (Ticona, Celstran-PEG58). The LGF compound is produced by the pultrusion process (17). Continuous fiber rovings are pulled through, spread and separated in an impregnation head and processing die. The molten resin impregnates the fibers. A smooth rod of resin and fibers is drawn out from the processing die and then cooled. This continuous rod is chopped into uniform pellets, typically 11 nun long and 3 mm in diameter. The individual glass fibers have an average diameter of 15 [micro]m and thus the initial aspect ratio of the fiber is ~700. The resulting pellets have the following characteristics: the glass fibers are totally encapsulated by the polymer; all the fibers are parallel and the fibers have the same length of the pellets.

The neat HDPE and the LGF masterbatch were dry blended to produce compounds having 5%, 10% and 20% LGF by weight. The rheological properties (G". [eta] & [[eta].sub.E]) of the LGF/HDPE compounds (samples taken from granulated blow molded bottles) were measured using small amplitude oscillatory shear (RMS) and dual-bore capillary rheometry (Rosand Precision Ltd.).

Processing

An intermittent extrusion blow molding machine (Placo 3XY-Placo Machinery) was used to process the material. The LGF/HDPE compounds were fed to a 2.5" extruder having a general purpose metering type screw (L/D = 24). This configuration has been used in order to minimize fiber break-up. The extruder output feeds into a "first in-first out" accumulator, which injects the material through the die/head assembly. Diverging dies [D.sub.o] = 25 & 70 mm; [alpha] = 45[degrees]) and constant die gaps of 1, 3 and 6 mm were used. The processing conditions are given in Table 2. Two blow molds were used to produce an axisymmetric part (51 round bottle).

Material/Process Characterization

A pinch-off mold (18) (Fig. .1) was used to determine the diameter, thickness and weight distributions of the parison. A video-recording technique was used to film the extrusion of the parison. The images were used to determine the parison diameter along the parison and its length (sag and/or recoil) as a function of time during and after extrusion (19). The diameter and the thickness of the parison were obtained from the pinch-off mold and the video technique using the following procedure:

From pillow width measurements (L), the parison diameter ([D.sub.parison]) can be obtained from

[D.sub.parison] 2 X L/[pi] (6)

From the mass of the pillow, the thickness, [h.sub.parison]' may be inferred by approximating the volume of the pillow to that of a cylinder

[h.sub.parison] = [D.sub.parison]/2 - 1/2 [square root of ([D.sub.parison.sup.2] - [4m.sub.pillow]/[[rho].sub.resin] . [pi] . [H.sub.pillow])] (7)

Since the pillow measurements are made in the solid state, a correction factor that takes the effect of shrink-age (due to solidification) into consideration has to be applied.

The shear rate in the die can be approximated as:

[gamma] = 5.37 . Q/[pi] . ([R.sub.o] + [R.sub.i]) . [(Gap).sup.2] (8)

where Q is the measured volumetric flow rate; [R.sub.o] and [R.sub.i] are the outer and inner radius of the die; and gap is the die opening.

Fiber Content, Length and Microstructure

The fiber length measurements were made with the help of a semiautomatic image analyzer. Parison and bottle samples were put in crucibles and the matrix was removed by burning off the polymer in a muffle furnace maintained at 500[degrees]C for a period of 4 h. The recovered fibers were dispersed in glycerin and then analyzed with a photo enlarger system with variable enlargement. Two hundred to 300 fibers were counted in order to ensure a representative value for the average fiber length. The microstructure investigation of the original and pinched parison, as well as the blown part was done using a Jeol (JSM 850) scanning electron microscope (SEM). All samples were prepared to expose cross sections representing the planes longitudinal ([parallel]) and perpendicular ([perpendicular to]) to the direction of flow during extrusion. In order to establish a reliable characterization of the fiber orientation, some samples were initially sectioned, encapsulated in epoxy and finally polished using metallographic tech niques. A second set of samples was prepared by the cryo-fracture method using liquid nitrogen and followed by sputter coating with a thin layer of Au:Pd (~15 nm).

Inflation Stage: Wall Thickness Distribution and Homogeneity

A 51 round bottle ([D.sub.top] = 12.5 cm; [D.sub.bottom] = 15 cm) was blown from the neat HDPE and the LGF/HDPE composites. The thickness of the part was measured using a Magna-Mike probe (Hall effect). In order to establish the wall thickness distribution, measurements were made at 13 points along the length of the bottle (1 = bottom; 13 = top) and also at 4 circumferential positions (45, 135, 225 and 315[degrees] from the parting line).

Mechanical Properties (Impact)

Impact tests were performed on an instrumented impact tester. Samples cut from the bottom of the bottle were clamped In a circular test jig located Inside an environmental chamber. A hemispherically shaped top (12 mm), dropped at a speed of 3.6 m/s, was used.

RESULTS

Rheological Properties

The rheological properties of the neat and LGF reinforced materials were measured on samples cut and granulated from blow molded bottles. Fiber length measurements in the parison and bottles showed that the fibers had been reduced to ~2 mm long fibers from the original 11 mm (see Microstructure section). The rheological measurements in these samples are relevant since they approximate the conditions In the die and also because they represent the case in which the best sample homogeneity of the composite material can be obtained. The shear and elongational viscosities were measured using the converging flow theory derived by Cogswell (20). The pressure drops were monitored over time to check for oscillations due to the inhomogeneity of the samples (21-23). It was found that the oscillations were fairly low and that the average pressure drop could be used with confidence. The shear and elongational viscosities for the neat and LGF reinforced HDPEs are shown In Fig. 2.

It is interesting to note that the shear viscosities are almost identical for the three materials. On the other hand, the HDPE and the 5% LGF composite exhibit similar entrance pressure drops, while the 20% LGF composite shows a much higher pressure drop. This is also true in the case of the elongational viscosity ([[eta].sub.E(20% LGF)] ~ 3[[eta].sub.(HDPE)]). The higher pressure drop required to flow in the capillary exhibited by the 20% LGF composite results in a higher flow orientation that generates a higher elongational viscosity and consequently a higher resistance to tensile deformations. The linear viscoelastic behavior, as given by tan [delta] = (G"/G'), of the three materials was also measured as shown in Fig. 3. As expected. the elasticity decreases while the crossover frequency (G' = G"] increases with increasing fiber content. From the rheological results described above, the HDPE/LGF composites should exhibit lower swell and sag during extrusion as compared to the HDPE.

Parison Swell and Sag

Figure 4 shows the clear differences in flow behavior exhibited by the HDPE and the 5% LGF composite. The HDPE shows significant swell and sag while the 5% LGF composite behaves almost as a solid body. Shear rate conditions varied between 20 and 450 1/s (70 mm die) and between 50 and 6500 1/s for the 25 mm die.

Figures 5 and 6 show the diameter and weight swell for the HDPE. 5%, 10% and 20% LGF composites as a function of distance from the die exit. As expected from the rheological behavior of these materials, the diameter swell decreases with increasing LGF content, with the LGF/HDPE composites approaching an almost newtonian behavior ([B.sub.D] ~ 1.13). Furthermore, while the HDPE exhibits significant shear rate dependence, the LGF/HDPE composites are shear rate insensitive. Both the diameter and weight swell results also indicate very different sagging behaviors. For the HDPE, the relative area swell (11). RAS. is ~ 0.33, implying that the sag plays an important role in determining the parison shape. In the case of the LGF/HDPE composites. the (RAS) is ~ 0.03. This situation describes a parison having uniform dimensions along its length (deformation as a solid body). The RAS results are given in Fig. 7. These results certainly confirm the fact that the fibers are oriented by the flow inside the die and that as soon as they come out of the die, they form an axial tubular structure that limits the radial expansion (diameter swell) of the parison while keeping its shape constant. The dimensions and shape of the reinforced parison do not seem to be strongly influenced by the die gap or the mandrel and die diameters. Sagging speeds probably determine solid body movement (equal speed at different intervals). These parisons do not sag in the classical way but rather in a solid body type translation governed by the orientation caused by the flow in the die. Figure S shows the weight swell for the 5% and 20% LGF content pansons extruded from die gaps of 3 and 6 mm. For comparative purposes the swell values for the HDPE are also shown. The shear rate is higher for the smaller gap (270 1/s vs 80 1/s) and therefore according to the regular viscoelastic behavior, the swell for the 3 mm gap should be higher. This is the case for the HDPE where the gap effect is more pronounced. It is obvious once again that the weight swell beha vior of the LGF/ HDPE composites is barely affected by gap size.

The remaining parameter that can influence the flow field inside the die and consequently the swelling and sagging characteristics of the parison is the die outside diameter. Figure 9 shows the diameter swell for the 5% and 20% LGF content parisons extruded from dies having outside diameters of 25 and 70 mm. In the case of the HDPE. the effects of the shear rate are evident for both the diameter and the thickness swells. The material undergoes higher deformations (both shear and extensional) inside the 25 mm die and therefore the energy stored in the die is higher as compared to the 70 mm die.

For the LGF/HDPE composites, the behavior is much different, with both diameter and weight swells decreasing with increasing die diameter. Although the shear rates for the larger die are lower and the parison is heavier (for the same length and die gap). it appears that the die diameter is the factor controlling the parison behavior. This result could possibly be explained by the magnitude of the hoop stress effect (24). At lower shear rates and higher percent LGF. the viscoelastic hoop stresses are smaller and a further reduction in parison dimensions is observed.

The anisotropy of the parison deformation can be inferred from a [B.sub.W] vs [B.sub.D] plot. If the deformation is isotropic, [B.sub.Th] = [B.sub.D] and therefore [B.sub.W] = [B.sub.D.sup.2]. Figure 10 shows that the power-law exponent, [beta], is a function of die diameter. For the 70 mm die, [beta] > 2 and therefore [B.sub.Th] > [B.sub.D]. For the 25 mm die, the behavior is the opposite. [beta] < 2 and therefore [B.sub.Th] < [B.sub.D]. This type of anisotropy should have been expected since the parison basically swells in diameter, not in thickness, and furthermore, it does not sag at all. Finally, Fig. 11 shows a 3D plot of the diameter swell (maximum) dependence on shear rate and LGF content. Data for four materials (HDPE, 5%, 10% & 20% LGF/HDPE); two dies (25 & 70 mm); and a large number of flow rates have been included. The data can be well represented by the following equation

[B.sub.[D.sub.max]] = [theta] [[gamma].sup.[xi]] exp[[chi](1 - [PHI])] (9)

where [theta], [xi] and [chi] are material dependent parameters. This plot could be used to determine an optimal processing window.

Fiber Content and Length Distribution

The fiber content measured in both a pinched-off parison and in a blow molded bottle is shown in Fig. 12. The samples were taken from random locations. It can be seen that the HDPE/LGF composites are homogeneous (~ same fiber content as the raw material). Furthermore, the effect of both piston speed and die gap seem to be minor.

As expected, the LGF length distribution was drastically changed during the flow in the extruder, accumulator and in the die (25). The fiber degradation is enhanced by surface abrasion due to fiber/fiber contact or by contact with the screw and the wall of the barrel. Figure 13 shows the fiber length distribution in the bottle (sample size = 700). Most fiber lengths fall between 1 & 3 mm. which is a significant reduction from the original 11 mm fiber length. The fiber length distribution presents a positive skew. i.e.. with the bulk of the probability toward small fiber length. Therefore, a log-normal distribution function could be fit to the experimental results. This type of behavior is typical for crushed particles size distributions and seams applicable to the case of fiber breakage. It is pertinent to mention that no significant difference in fiber length distribution was found when samples for different regions of the bottle were analyzed separately.

Fiber Orientation

The micrographs (Figs. 14 and 15) display the perpendicular plane of the polished samples (parison and blown bottle) using the backscattered electron (BSE) mode in the SEM. These images show a distinct contrast between the glass fibers and the HDPE matrix. The presence of circular and slightly elliptical fiber cross sections indicates the preferential fiber orientation with the direction of flow. These images also show the homogeneous spatial fiber distribution and corroborate the absence of fiber segregation and clustering or the occurrence of fiber bundles. Furthermore, it appears that the blowing step does not change the orientation of the fibers.

Figures 16 and 17 display the parallel plane of the polished samples (parison and blown bottle). Embedded fibers of different lengths having preferential orientation in the direction of flow can also be seen. Again, the blowing induced no appreciable change in fiber orientation.

Figure 18 shows the fiber orientation (along the direction of flow) in a pillow obtained from the pinch-off mold. It illustrates how the oriented fibers follow the deformation induced by the "pinching."

Figures 19 and 20 show the secondary electron mode images obtained from the cryo-fractured samples of bottles. The results also show the preferential fiber orientation along the direction of flow. However. Fig. 19 shows fibers with slight orientation fluctuations. In the top part of this image (exterior wall surface) the fibers emerging from the surface are clearly detected and are considered to be responsible for the rough texture of the final part.

In the past, pyrolized samples have been used to quantify fiber orientation. Figure 21 shows the results obtained using this technique for a 5% GF bottle sample. In this case an artificially induced random orientation was obtained. In view of the results of the polished and fractured specimens, it is clear that this is only an experimental artifact. This can be explained by the effect of removing the viscous polymer melt matrix at high temperatures and not to be confused with a possible mat structure generated during the extrusion and blowing stages of the parison.

Parison Inflation

The inflation parameters ([t.sub.blow], [P.sub.blow]) were set initially to obtaining a good neat HDPE part. The 5% and 10% LGF/HDPE composites could be blown with very slight variations to the neat HDPE inflation conditions. On the other hand, the 20% LGF/HDPE composite could not be consistently inflated. Problems related to blowouts and incomplete weldlines were the major source of problems. It should be said that the morphology and fiber orientation in the 20% LGF reinforced material was very similar to that of the 5% and 10% composites, i.e. the original long fibers broke and oriented themselves in the flow direction.

Figure 22 shows the bottle's wall thickness distribution for the neat HDPE and for the 5% LGF/HDPE composite respectively. The neat HDPE exhibits a broader wall thickness distribution mainly because of the parison weight swell behavior. The parison is wider and thicker at the bottom and therefore the bottles wall thickness at the bottom is higher. In the case of the 5% LGF/HDPE composite, the overall wall thickness is lower and the thickness distribution appears to be narrower. This behavior can be again explained based on the parison shape. Due to the presence of the fibers, the LGF/HDPE composite parison has more uniform (although smaller) diameter and thickness distributions. This produces. on one hand, a part with a more uniform thickness, and on the other, a thinner part since the blow-up ratio ([D.sub.bottle]/[D.sub.parison] is higher ([BUR.sub.HDPE] ~ 1.33, [BUR.sub.5% LGF/HDPE] ~ 1.6). The neat HDPE produces a bottle with a relatively constant wall thickness between locations 5 and 11, while the 5% LGF/HDPE composite bottle has a continuously increasing wall thickness in this region. Again, these results point out how the complex balance between swell and sag determines the final part dimensions. It is also important to note that the circumferential wall thickness distribution is not uniform and again broader for the neat HDPE. This fact can be explained by a misaligned die/mandrel or by the misalignment between the parison and the mold halves. Unfortunately, these effects cannot be picked up by the pinch-off mold technique and correlated with the final part wall thickness distribution.

Mechanical Properties

It is well known that in the case of fiber reinforced blow molded parts (6, 26) the pinch-off region is one of the weak areas of the blown object. In many cases this is attributed to the lack of a strong bond across the welded interface, particularly when the mold wall temperature is low. The presence of the fibers prevents an even distribution of the compressive stresses across the pinch-off line. As opposed to neat polymers the welded interface in a reinforced material is thinner. From a typical load and energy curve (Fig. 23) generated during the instrumented impact test, the peak load (corresponding to the collapse of the impact or resistance) decreases with LGF content. At -40[degrees]C, only the welded interface was cracked. The energy to propagate ([E.sub.p]) the crack is the difference between the total energy ([E.sub.t] = 71 J) and the energy necessary to initiate ([E.sub.I]) crack. ([E.sub.I]) corresponds to the energy at the maximum load. Comparison of the crack behavior of the samples shows that the resistance of the LGF/HDPE reflects the mean stiffness and decreases drastically when compared to the HDPE values.

CONCLUSIONS

In this paper we studied the flow behavior of long fiber reinforced (LFR) high density polyethylene during the extrusion and blowing stages of an intermittent blow molding process.

It was found that parison swell (diameter and thickness) decreased with increasing fiber content. Furthermore, while the HDPE exhibits significant shear rate dependence, the LGF/HDPE composites are shear rate insensitive. Both the diameter and weight swell results also indicate very different sagging behaviors. During the flow inside the die, the fibers are oriented in the axial direction causing a reduction in the hoop stress at the die exit. This situation favors the formation of a tubular envelope that limits the radial expansion (diameter swell) of the parison while keeping its shape constant. The dimensions and shape of the reinforced parison don't seem to be strongly influenced by the die gap or the mandrel and die diameters. These parisons do not sag in the classical way but more in a solid body type translation (equal speed at different axial locations) governed by the orientation caused by the flow in the die. It should also be mentioned that the parison behavior could only be correlated with the e xtensional viscosity values for the LFR/HDPE resins.

As expected, the LGF length distribution was drastically changed during the flow in the extruder, accumulator and in the die. Samples taken from the blown bottles showed that fiber lengths fall between 1 & 3 mm, which is a significant reduction from the original 11 mm fiber length. No significant difference in fiber length distribution was found when samples for different regions of the bottle were analyzed separately.

The SEM micrographs corroborate the absence of fiber segregation and clustering or the occurrence of fiber bundles (homogeneous spatial fiber distribution) as well as a preferential fiber orientation with the direction of flow. Furthermore, it appears that the blowing step does not change the orientation of the fibers.

The 5% and 10% LGF/HDPE composites could be blown with very slight variations to the neat HDPE inflation conditions. On the other hand, the 20% LGF/ HDPE composite could not be consistently inflated. Problems related to blowouts and incomplete weld-lines were the major source of problems.

As opposed to neat polymers. the welded interface in a reinforced material is generally thinner. Instrumented impact test results, at room temperature and at -40[degrees]C, showed that the impact resistance decreased with LGF content.

[Figure 2 omitted]

[Figure 3 omitted]

[Figure 5 omitted]

[Figure 6 omitted]

[Figure 7 omitted]

[Figure 8 omitted]

[Figure 9 omitted]

[Figure 10 omitted]

[Figure 11 omitted]

[Figure 12a omitted]

[Figure 12b omitted]

[Figure 13 omitted]

[Figure 22 omitted]

[Figure 23 omitted]
Table 1.

Estimated Consumption of Plastics and Composites in Autos, Minivans
and Trucks in 2005 (3).

 '000 tonnes/year
 North America Europe Asia-Pacific

Body frames 715 432 389
Bumpers, fascias and grilles 377 639 575
Under-the-hood 390 656 591
Total 1,482 1,727 1,555


 Rest of World Total

Body frames 173 1,709
Bumpers, fascias and grilles 255 1,846
Under-the-hood 263 1,900
Total 691 5,455
Table 2.

Processing Conditions for the Neat and Reinforced Resins.

Parameter Range

Extruder Temperatures (C)
Rear 200
Middle 205
Front 210
Die Temperature (C) 210
Screw speed (rpm) 10-60
Die Diameter (mm) 25, 40 & 70
Die Gap (mm) 1(flush), 3 & 6
Accumulator speed (%) 20-100
Volumetric Flowrate 15,000-160,000
([mm.sup.3]/S)
Extrusion time (s) 4-25
Parison length (cm) 30, 45, 60, 90 & 120


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Author:Garcia-Rejon, A.; Meddad, A.; Turcott, E.; Carmel, M.
Publication:Polymer Engineering and Science
Date:Feb 1, 2002
Words:5687
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