For several decades, the use of glass fiber reinforced thermoplastic (GFRT) composites by the automotive industry has been steadily increasing for standard performance applications.1 The values that GFRTs bring include intrinsically high specific stiffness, low cost, and the ability to produce parts quickly with minimal manufacturing complexity. Reinforced thermoplastics will remain an attractive material option as OEMS and the supply chain continue to innovate and reduce weight to reach the 2025 CAFE mileage requirements.
One of the most common resins considered for these GFRTs is polyamide due its balance of mechanical performance, chemical and heat resistance. For these reasons, glass fiber reinforced polyamide composites have been used to replace and lighten metallic components such as front end bolsters, seat pans, bumper beams, battery trays, gears, engine covers, intake manifolds, etc.2 Despite these successes and the potential for further weight savings, there is hesitancy to use polyamide based GFRTs in high-performance under-hood applications where high operating temperature is a requirement. In some applications where they are used, significant heat shielding is required, which has the negative effect of increased manufacturing complexity. Thus, to further incorporate GFRTs into vehicle construction and allow for additional weight savings, the thermal performance of these materials must be enhanced.
Nano-scale additives offer a path to improve the thermal performance of polyamide GFRTs. Since the beginning of the nano “boom” in the early 1990s, additive materials such as carbon nanotubes, nano-clay, nano-talc, etc. have been evaluated in thermoplastic resin systems.3-7 These additives have been reported to cause heat deflection temperature (HDT) to rise by as much as 140%, or as little as 20% in neat resin without fiber reinforcements.3,7 Clearly, if increases such as these could be realized in polyamide GFRTs, their use in under-hood applications would increase. As such, the objectives of this study are to determine the mechanical and thermal performance of parts fabricated with glass contents of 0, 30, and 50 wt% with nano-talc weight contents of 0 and 3 wt%.
Materials and Methods
Silane sized chopped e-glass* with a density of 2.63 g/cm3 was used as the fiber reinforcement throughout the study. This particular sizing/glass combination is marketed to be compatible with a variety of polyamide matrices. As such, polyamide-6 (PA6) homopolymer 8202 HS obtained from BASF was used as the polymer matrix for the specimen in this study. In addition, a 40 wt% nano-talc/PA6 masterbatch (N-200), supplied by AdMark Intl., was used to introduce the nanoparticles to the resin melt during the compounding of the requisite samples.
*CHOPVANTAGE® HP 3610 fiberglass obtained from PPG Industries.
Composite Compounding and Molding
A Coperion STS co-rotating twin screw extruder with a screw diameter of 35 mm and an L/D ratio of 40:1 was used to extrude the resin compound. Prior to extrusion, the polyamide resin and masterbatch was dried in a desiccant drier for 4 hrs at 120°C. Virgin polyamide 6 and the nano-talc masterbatch were fed through the hopper while the chopped glass was introduced into the polymer melt in zone #7 via a side stuffer.
The compounding throughput was held constant at a rate of 15 kg/hr. As such, the final glass content of the compound was controlled by varying both the resin and chopped glass feed rates to achieve the desired glass content levels of 0, 30 and 50 wt%. The extruder parameters used for compounding the polyamide GFRT in this work are outlined in Table 1.
GFRT Specimen Injection Molding
Before injection molding, any moisture present in the pellets was desorbed by drying in a convection oven at 100°C for 2 hrs. After drying, ISO tensile test specimen were molded via a Van Dorn 55-ton injection molding machine. The mold is a balanced design using fan gates at the specimen tabs and produces two tensile bars per injection cycle.
The pertinent molding parameters used are outlined in Tables 2 and 3. The impact specimen were fabricated by removing the tabs of the tensile bars. Care was taken to ensure that each specimen conformed to relevant ISO requirements.
Mechanical/Thermal Property Characterization
The heat deflection temperature (HDT) was determined using a Rheometric Scientific DMTA IV operating in the three-point bending mode using rectangular specimen (80 x 10 x 2 mm) as in References 8-10. The deflection of the specimen was measured from ambient to 200°C at a temperature scanning rate of 2°C/min at a constant stress of 8 MPa as specified by ISO 75 Method “C.” The HDT reported was defined as the temperature at which the outer surface strain of the evaluated specimen reached 0.2%.
For each of the samples, 10 ISO tensile bars were fabricated to allow for their associated tensile moduli and strengths to be determined. The tensile tests were performed via an Instron 5584 servo hydraulic test system using a constant cross-head rate of 2 mm/min. The strain data was captured through an extensometer with a gage length of 25 mm and the force data through a 100 kN load cell. The tensile moduli and strengths of the various specimen were calculated as recommended by ISO 527.
Charpy Notched and Un-Notched impact tests were carried out on 10 specimens per sample following the ISO 179 standard using a 18.5 N hammer with an impact energy of 21.6 J. The specimen outside dimensions for both the Notched and Un-Notched Charpy impact tests were the same at 80 x 10 x 4 mm. However, for the Notched specimen, a type “A” notch was cut in the center of the test specimen.
Results and Discussion
As previously discussed, the objective of this study is to evaluate the effect of a nano-talc additive on the mechanical and thermal performance of a GFRT with a polyamide-6 matrix. Based on the studies of References 3-7, the addition of nano-talc is expected to cause appreciable increases in thermal and mechanical performance of the matrix. However, it is unclear if the addition of glass fiber reinforcement, which has been shown to increase composite thermal and mechanical performance,11 will react synergistically with the nano-talc additive and as a result improve overall composite performance.
The HDT results for the samples in this study are illustrated in Figure 1. The figure demonstrates that the HDT of the samples not containing nano-talc increased by approximately 63% (i.e., 66 to 108°C) as glass content increased from 0 to 50 wt%. Correspondingly, the HDT of the samples containing nano-talc increased by about 61% (i.e., 73 to 118°C) with the same glass content increases. Clearly, the addition of glass fibers produced a significant improvement in HDT. In addition, the HDT performance of the samples containing nano-talc at each glass content level outperformed those without for all cases. In fact, at a glass content of 30 wt%, HDT was increased by approximately 29% (i.e., 79 to 102°C).
In Figure 2, the tensile moduli for the samples fabricated for this study using 0 and 3 wt% nano-talc as the glass content is increased from 0 to 50 wt% is shown. As expected, increasing glass content resulted in the moduli of the samples to increase whether nano-talc was used or not. Furthermore, in those samples that contained nano-talc, the measured moduli were found to be about 1,000 MPa greater (on average) than those without, regardless of the glass content level.
The tensile strength results for this study are illustrated in Figure 3. The figure indicates that the strength of those samples without the nano-talc additive increased by approximately 240% (i.e., 64 to 217 MPa) as glass content increased from 0 to 50 wt%. Similarly, the tensile strength of the samples containing nano-talc increased by about 130% (i.e., 77 to 176 MPa) as glass content was increased in the same manner. Clearly, the increases in strength with the incorporation of rising levels of glass fiber reinforcement were not equivalent in the samples with and without nano-talc.
Close examination of the data reveals that the addition of the nano-talc in the samples without fiber reinforcement (glass content = 0 wt%) caused a strength increase of approximately 20% (i.e., 64 to 77 MPa). However, this positive influence of the nano-talc transitioned into a negative influence with the incorporation of fiber reinforcement into the composite. This negative effect progressively worsened as the fiber reinforcement levels were increased from 30 to 50 wt%. In fact, the tensile strength of the samples with nano-talc were reduced by as much as 19% (i.e., 217 to 176 MPa) when compared to those without at the 50 wt% fiber reinforcement level. These results were surprising, as the increased tensile moduli of the nano-talc specimen was expected to result in similar increases in tensile strength.
These results could possibly be explained by the polyamide matrix becoming less tough (i.e., more brittle) due to the nano-talc additive acting as a nucleating agent, which has the effect of increasing crystallization temperature. An increase in crystallization temperature decreases the concentration of tie-molecules between the polyamide’s crystalline and amorphous regions, which can cause the junction of these regions to be susceptible to crack growth/propagation.12
If correct, the tensile strain to failure for the nano-talc specimen would be expected to be significantly less than those without. Inspection of the tensile failure strain results illustrated in Figure 4 appears to support this explanation, as the average failure strain of the nano-talc specimen was approximately 21% less than those specimen without (i.e., reduced from 3.3 to 2.6%).
Summaries of the un-notched and notched impact strength of the evaluated samples are illustrated in Figure 5a and 5b. The figures indicate the un-notched and notched impact strength of those samples without the nano-talc additive increased approximately 160% (i.e., 42 to 108 kJ/m2) and about 630% (i.e., 2.4 to 17.5 kJ/m2), respectively, as glass content increased from 0 to 50 wt%. For the samples containing nano-talc, the un-notched and notched impact strength increased around 74% (i.e., 39 to 68 kJ/m2) and 24% (i.e., 5.1 to 9.3 kJ/m2), respectively, with increasing glass content.
The improvement in impact strength with elevated glass content, as was observed in both the samples with and without the nano-talc additive, is expected as the glass fibers help to arrest and bridge the cracks as they propagate throughout the samples. However, the impact performance of the samples with nano-talc underperformed those without by approximately 37% for un-notched and 47% for notched impact strength. These results are attributed to the nano-talc causing a decrease in the tie-molecule concentration at the amorphous and crystalline phase junctions, which in turn caused the matrix to become brittle.12
The objectives of this study were to evaluate the tensile, impact, and thermal performance of a glass fiber reinforced thermoplastic (GFRT) composite with a nano-talc additive and compare them to those of a GFRT without. The results demonstrated that the addition of glass fiber reinforcements significantly improve material mechanical and thermal performance. Furthermore, the incorporation of nano-talc into the studied GFRTs was shown to increase the tensile modulus and heat deflection temperatures (HDT) by as much as 11% and 29%, respectively, with the addition of nano-talc. However, the nano-talc additive negatively influences their associated tensile and impact strengths due to matrix embrittlement.
Thus, for stiffness-dominated GFRT parts with elevated operating temperatures, such as those in automotive under-hood applications, in current production the use of a nano-talc additive may help to reduce/eliminate heat shielding needs. Moreover, the observed improvement in thermal performance with the use of nano-talc makes the replacement of additional metallic components in automotive applications more feasible, further enhancing the attractiveness of thermoplastic composites for vehicle lightweighting.
The authors would like to thank Tom McGee at AdMark Intl., Nanovation, Phil Luckadoo, Nathan Cormier, Jason Dobyne, Jose Mezey, and John Ruppe for their significant contributions to the manufacturing, preparation, and testing of the specimen used in this study.
Editor’s note: This paper was first published at the SPE ACCE in Novi (Detroit), Mich., September 7-9, 2016. Reprinted with permission.
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