Driving innovation with new elastomers for high performance automotive TPOs.
One of the clear messages from this exercise is an ever-increasing need in the automotive industry for higher TPO performance including higher stiffness (flexural modulus) and/or improved low-temperature ductility. New performance balances present an opportunity for light weighting, resulting in improved automotive fuel efficiency.
From an elastomer perspective, this provided an opportunity to develop a polyolefin elastomer with best in class TPO performance by providing an improved stiffness-toughness balance. Therefore, Dow embarked on a multi-disciplined investigation including re-examination of material science fundamentals and use of molecular architecture techniques. This effort resulted in the introduction of a new class of elastomer commercialized as Engage XLT polyolefin elastomer.
Starting with a very basic impact schematic (figure 1), an elastomer is dispersed within a crystalline polymer matrix. Upon part impact, energy is propagated by a fracture or crack through the matrix with the elastomer essentially helping dissipate that energy by elastic response and cavitation and, preferably, resulting in a ductile, versus a brittle, failure.
[FIGURE 1 OMITTED]
According to Wu (ref. 4) , there is a critical distance between elastomer impact modifier particles, called the critical ligand thickness ([L.sub.c]), which determines the TPO impact response from the brittle to ductile region. Wu proposed that [L.sub.c] is proportional to the critical elastomer particle diameter, dc, according to the following equation:
[L.sub.c] = [dc([pi]/6[psi]c).sup.1/3_] 1 (1)
where [psi]c is the critical elastomer volume
There is a growing consensus that Lc (ligand thickness) is a key parameter for determining impact toughness in semi-crystalline polymers (ref. 5). Since the impact modifier inter-particle distance is inversely proportional to rubber volume, but directly proportional to panicle size, as shown in equation 1, it is possible for an elastomer having a smaller particle size to have an equivalent [L.sub.c] at a lower volume giving improved efficiency.
New elastomer development, therefore, was focused on how to enhance elastomer dispersion and maximize impact performance. Of the many elements studied, there were both the type and level of the [alpha]-olefin comonomer which strongly influence the dispersion characteristics in a PP matrix, as shown in figure 2. Also, increasing comonomer level results in lower crystallinity and influences the low temperature glass transition temperature (Tg), as shown in figure 3.
These figures describe differential [alpha]-olefin comonomer efficiency (octene > butene > propylene) for adjusting interfacial tension, with a similar trend in low temperature performance at comparable crystallinity. These elastomer attributes also help explain the dramatic mid-1990s shift away from ethylene-propylene elastomers to higher [alpha]-olefin copolymers for TPO systems. These elastomers (including Dow's ENGAGE[TM] polyolefin ethylene-butene (EB) and ethylene-octene (EO) based elastomers produced using proprietary INSITE[TM] Technology) enable improved TPO toughness-stiffness balance, and reduced elastomer loadings, in the TPO compounds to achieve increasing modulus, higher melt flow and improving low temperature ductility performance. Furthermore, these new elastomers are delivered in free-flowing pellet form for compounder operational efficiency, including continuous compounding techniques (ref. 6).
Further material science understanding and use of advanced molecular architecture techniques recently have resulted in development and commercial production of a new class of polyolefin elastomer called ENGAGE[TM] XLT. This new product provides further optimization of elastomer dispersion for superior TPO performance while retaining sufficient crystallinity to deliver a free-flowing product.
This article discusses several grades of Engage XLT that are being investigated and/or commercialized as high performance impact modifiers for polypropylene (PP). XLT-EO1 provides excellent impact performance leading to improved toughening efficiency which better balances the need for stiffness and toughness in a rigid TPO. XLT-EO2 is a grade with bulk shipping and silo handling capabilities which maintains superior toughening capabilities. These specialized materials will be compared with current high performance ethylene/[alpha]-olefin copolymers (ref. 7).
A testing protocol has been previously established to screen candidate impact modifiers targeting high-end interior and/or exterior automotive TPOs (ref. 2 and 3). The testing methodology uses a basic TPO formulation and reducing the elastomer content to yield ductile behavior at room temperature, but brittle behavior at low temperature. To begin developing a higher performing modifier, the formulation standard was set by determining this threshold performance with a high performance TPO impact modifier, Engage 8180 (E8180), and comparing performance to 0.5 melt index grades of the new elastomer class.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The materials used in the current study included the following ingredients:
* Polypropylene C-700-35 medium impact copolymer (ICPPP) with a melt flow rate (2.16 kg @ 230[degrees]C) of 35 dg/min. from Dow Chemical.
* JetFil 700C--fine particle size talc from Luzenac;
* elastomers listed in Table 1; and
* Irganox B-225--antioxidant (AO) from Ciba.
This study consisted of TPO compounds outlined in table 2 which were prepared by melt blending on a Coperion WP-25 co-rotating, twin-screw extruder and molded on a 100 ton Toyo molding machine. Samples were aged for a minimum of 48 hours prior to ASTM and ISO testing. Additionally, samples of the molded plaques were scanned with a Digital Instruments Multimode AFM in tapping mode with phase detection and nano-sensor tips.
Results and discussion
Improved efficiency (XLT-EO1)
Previous studies have been completed to clearly demonstrate the improved efficiency of EO1 through dispersion in PP homopolymer, followed by measurements of rubber particle size, interparticle distance and the ductile-brittle transition temperature in both instrumented dart and notched impact tests (ref. 1). The control comparison of XLT-EO1 to E8180 in a more typical high performance TPO continues to demonstrate this high efficiency, as shown in the spider diagram in figure 4. Significant gains in both low-temperature dart and notched impact are illustrated with comparable stiffness, heat distortion and melt processability.
This higher efficiency also enables a compounder to reduce the XLT-EO1 loading and adjust the performance balance. As shown in figure 5, this adjustment can result in higher stiffness, improved flow and higher heat distortion temperature while maintaining comparable low temperature impact. This, in turn, can lead to the increased ability to thin-wall and lightweight a molded TPO part.
Bulk capable (XLT-EO2)
One issue facing TPO suppliers is the effect that storage temperature has on the incoming raw material, specifically the elastomers used to modify the polypropylene. Many of the higher performance elastomers are produced with very low crystallinity, lowering the glass transition temperature and helping to give improved low temperature impact performance. However, this same low crystallinity can lead to pellet blocking or massing. Pellet blocking limits the supplier's ability to deliver material in bulk and limits the storage options at the customer.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
XLT-EO2 was designed specifically to minimize blocking while delivering excellent low temperature impact performance. Figure 6 shows the results of a Dow proprietary test to measure the blocking performance of different talc dusted elastomers used in TPO applications. The test simulates storage conditions in a silo or bulk container over a period of time at elevated temperature. The massing index was calculated based on the ability of the pellets to resist massing after storage compared to an already established non-massing control using Engage 8157 (or E8157).
The threshold is the point at which the material becomes massed (blocking occurs) and a value above the threshold may require significant effort to remove the material from the container. The results showed XLT-EO2 was very close in performance to the control of E8157 (0.5 MI, 0.868 g/cc) which has well-established bulk handling capability. The lower density XLT-EO1 and E8180 both showed blocking indexes above the threshold and would not be suited for bulk container shipping and storage.
Mechanical properties and impact performance, XLT-EO1 and XLT-EO2
Engage XLT products join our current high performance Engage product line, with a differing balance of impact strength, stiffness, and handling capabilities. The following charts illustrate the varying balance of properties for the formulations listed in table 2. Comparative TPO properties are illustrated in the spider diagram shown in figure 7.
At the same loading, XLT-EO1 and XLT-EO2 exhibited noticeably higher notched impact ductility at sub-ambient temperatures compared to the E8180 control. There was a higher stiffness-toughness balance using XLT-EO1 at the lower loading versus E8180. The difference in performance between the XLT-EO1 at 17.7 wt. % and E8180 at 20.7 wt. % was most apparent in the dart impact results where a noticeable difference in the temperature where ductile versus brittle behavior began was observed. Table 3 lists the instrumented dart results for the E8180, XLT-EO1 and XLT-EO2.
[FIGURE 6 OMITTED]
These results confirm that XLT-EO1 exhibits about 15-20 % higher impact efficiency than E8180, and the onset temperature to brittle failure decreased almost 10[degrees]C. At the same 20.7 wt. % loading, the onset to brittle failure was not observed for the XLT-EO1, and the XLT-EO2 showed as good or better performance compared to the E8180 Control. These results place the XLT-EO2 in the high end of performance, with the added benefit of being bulk and silo capable. Previous studies (refs. 1 and 2) have shown TPOs made with ICP-PP and XLT-EO1 displayed smaller particle domains in the ICPPP matrix. Figure 8 shows AFM micrographs of TPOs containing E8180 and XLT-EO2 both at 20.7 wt. %.
As seen in figure 8, the XLT-EO2 particle size is noticeably smaller than that of the E8180 control. It is likely that the higher impact toughness measured for the XLT-EO2 versus E8180 at equivalent loading was due, in part, to the visually observed smaller particle size after compounding, and the lower Tg of the XLT-EO2 is also important.
Several other TPO characterization measurements were made in this study comparing the XLT grades to the ENGAGE 8180 and summarized below:
* Noticeable increase in tensile elongation at break.
* Comparable scratch and mar performance.
* Slightly higher TPO shrinkage at same formulation, which may limit ability for running change without some compound adjustments.
[FIGURE 7 OMITTED]
Further TPO formulary design of experiment studies have been conducted demonstrating other compounding options that might be considered for optimizing desired performance (ref. 8). This work also includes investigation of other polymer design variables, including higher melt index versions optimized for very high flow TPO compounds. Finally, these efforts are being supplemented with mold flow and finite element analysis to continue building a knowledge base for the influence of Engage XLT polyolefin elastomer on finished part performance.
Engage XLT polyolefin elastomer is being introduced as a new family ofelastomer with superior toughness-stiffness balance and delivered in flee-flowing product form. This product demonstrates:
* Superior toughness/flex balance in a high performance TPO compound with the ability to further optimize performance for improved stiffness, heat distortion and melt flow processability;
* Performance improvements attributed to polymer design and leading to optimized elastomer dispersion; and
* Ability to be delivered in free-flowing pellet form, including bulk capable XLT-EO2, recently commercialized as Engage XLT 8677
[FIGURE 8 OMITTED]
(1.) Jones, M.A., et al., "New polyolefin low temperature impact modifier" Proceedings of the SPE International Polyolefins Conference, Houston 2010.
(2.) Walton, K.L, et al, "New higher impact efficiency elastomers for high performance rigid TPOs," proceedings of the SPE-Automotive TPO Global Conference, Detroit 2009.
(3.) Walton, K.L., et al., "The case for higher impact efficiency elastomers for rigid TPOs, "proceedings of the SPE-Automotive TPO Global Conference, Detroit 2008.
(4.) Wu, S., "Phase structure and adhesion in polymer blends: A criterion for rubber toughening, "Polymer, Vol. 26, p. 1,855 (1985).
(5.) Corte, L., and Leibler, L., "A model for toughening of semicrystalline polymers," Macromolecules, Vol. 40, p. 5,056 (2007).
(6.) Hemphill, J.J., et al., "New advances in elastomer technology, "proceedings of the SPE-Automotive TPO Global Conference, Detroit 2003.
(7.) Kummer, K.G., et al., "New elastomers for high performance rigid TPOs--driving innovation," proceedings of the SPE Automotive TPO Global Conference, Detroit 2010.
(8.) Barry, R.P., et al., "From catalyst to end performance," proceedings of the SPE Automotive TPO Global Conference, Detroit 2010.
by Jim Hemphill, Kyle G. Kummer, Kim L. Walton, Russell Barry and Mary Ann Jones, Dow Chemical
Table 1--elastomers used for TPO studies Density Melt index ([I.sub.2]) Glass transition (9/cc) (dg/min, 190[degrees]C/ temperature (Tg 2.16 kg) [degrees]C) Engage 8180 0.863 0.5 -55 XLT--E01 0.868 0.5 -67 XLT--E02 0.870 0.5 -65 Table 2--formulations used for TPO study Study 1 E8180- XLT-E01 XLT-E01 XLT-E02 control control low control 705-35 59.3 59.3 62.3 59.3 E8180 control 20.7 -- -- -- XLT E01 -- 20.7 17.7 -- (high efficiency) XLTE02 -- -- -- 20.7 (bulk capable) Jetfil7000 20 20 20 20 Irganox B225 0.2 0.2 0.2 0.2 Table 3--instrumented dart impact results Dart impact Temperature E8180 XLT-E01 XLTE01 XLT-E02 6.7m/sec 20.7% 17.7% 20.7% 20.7% Number of -20[degrees]C 1 of 10 0 0 0 brittle -30[degrees]C 6 of 10 2 of 10 0 1 of 10 failures
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|Author:||Hemphill, Jim; Kummer, Kyle G.; Walton, Kim L.; Barry, Russell; Jones, Mary Ann|
|Date:||Oct 1, 2011|
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