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New higher impact efficiency elastomers for high performance rigid TPOs.

The automotive market has seen significant adoption of elastomers over the past couple of decades. Today, there are more than three billion pounds of elastomer used globally in the production of automobiles. The market is dominated by TPOs with 43% of the market, as shown in figure 1.

Talc-reinforced polypropylene (PP) impact modified with ethylene/[alpha]-olefins (EAO) grew rapidly in automotive applications during the 1990s and now is the dominant rigid plastic material used in bumper fascia. The use of these thermoplastic olefin (TPO) compounds continues to spread across other automotive applications in the interior as the raw material for instrument panels, door pillars and knee bolsters.

In a recent Voice of the Customer survey of the TPO compound value chain, the interviewees indicated that new rigid TPO specifications will call for an increase in flexural modulus with no change in low-temperature impact resistance (ref. 1). Thus, TPO suppliers face increasing challenges to find the fight combination of polypropylene, impact toughness and reinforcing agent to meet ever more demanding performance requirements. This is a well recognized issue and there have been numerous attempts to improve the stiffness-toughness behavior in TPO (refs. 2 and 3).

This article covers the development of a new class of polypropylene impact modifiers designed to improve stiffness-toughness in TPO. These materials will be compared with current ethylene/cc-olefin copolymers.

Experimental

Materials

* Polypropylene C-700-35--a 35 MFR polypropylene impact copolymer (ICP) from Dow Chemical;

* D221.00--a 35 MFR high isotactic content developmental polypropylene homopolymer (hPP) from Dow Chemical;

* JetFil 700C--fine particle talc from Luzenac, a wholly owned subsidiary of Rio Tinto;

* elastomers described in table 1 from Dow Chemical;

* Irganox B-225--antioxidant from Ciba Specialty Chemicals; and

* SEBS--Kraton G1657, 8 MFR (measured at 200[degrees]C/5 kg), 13 wt. % styrene, styrene-ethylene/butene-styrene triblock/diblock mixture from Kraton Polymers.

Compounding

The TPO compounds of this study were prepared by melt blending on a 25 mm, Coperion WP-25 ZSK, co-rotating, twin-screw extruder at a speed of 500 rpm. The polypropylene and elastomer were fed into the extruder using individual loss/ weight feeders. The AO additive was tumble blended with the elastomer ahead of compounding. Talc was fed through a side arm feeder that was introduced into the third zone of the extruder barrel, and vacuum was used during extrusion to remove volatiles. The compounding extruder rate was 0.38 kg/ minute (50 Ibs./hour) with a melt temperature of about 220[degrees]C/ 430[degrees]F. The extruded strand was water-cooled and chopped into pellets.

The TPO formulations were grouped into two sets:

* 56 parts D221.00 hPP, 24 parts elastomer, 20 parts talc, and 0.2 parts B-225; and

* varying parts Dow ICP C-700-35 and elastomer, 20 parts talc and 0.2 parts B-225.

Sample preparation

Samples were prepared by injection molding on a 100 ton Toyo molding machine. Molding temperatures for the barrel were set at 177[degrees]C/350[degrees]F (feed), 204[degrees]C/400[degrees]F, 216[degrees]C/420[degrees]F, 216[degrees]C/420[degrees]F, 216[degrees]C/420[degrees]F (barrel through nozzle), while the mold temperature was 38[degrees]C/100[degrees]F. Injection cycles were maintained at 1.8 sec. injection, 25 sec. hold and 25 sec. cool time. The injection/hold pressures were approximately 400 bars, which was adjusted to completely fill the mold cavities. Injection speed was maintained at 19 cc/sec. Samples for tensile testing and Izod impact were aged at room temperature for 48 hours prior to testing. For dart impact testing, samples were aged for one week at room temperature.

Testing

The following ASTM testing was performed on the injectionmolded specimens after at least 168 hours of ASTM conditioning:

* Instrumented dart impact (IDI) testing was performed according to ASTM D3763. A hemispherical dart with a 12.7 mm in diameter tip was used in a MTS 819 high rate test system. Prior to testing, the 10.1 cm diameter by 3.1 mm thick samples were allowed to age after molding, as described previously, then conditioned at the test temperature in a commercial chest freezer accurate to 2[degrees]C for fours hours. The clamped specimen was moved into the fixed dart at a constant 6.7 m/s velocity;

* Notched Izod impact test at various temperatures by ISO 180/1A;

* tensile properties were measured on single gate 80 mm by 10 mm by 4 mm thick specimen bars according to ISO 527-1; and

* flexural modulus by ISO 178. Chord modulus was determined measuring stress from 0.25 to 0.5% strain.

Morphology

Pieces from the center diameter and core of the bulk of the injection molded dart impact disks were cryogenically polished at -120[degrees]C. Polished blocks were mounted on standard 12 mm atomic force microscope (AFM) pucks with quick-set epoxy. The specimens were scanned with a Digital Instruments Multimode AFM in tapping mode with phase detection. Nano-sensor tips were used in all experiments. Post-processing of images was conducted with Adobe Photoshop v9.0. Image analysis data were generated from five 20 [micro][m.sup.2] images using Leica Qwin image analysis software.

[FIGURE 2 OMITTED]

Results and discussion

New impact modifier development--screening protocol A testing protocol was established to screen candidate impact modifiers. The methodology is outlined in figure 2. According to Wu (ref. 4), there is a critical distance between elastomer impact modifier particles, called the critical ligand thickness (Lc), which determines the TPO impact response from the brittle to ductile region. Wu proposed that Lc is proportional to the critical elastomer particle diameter, de, according to the following equation:

[L.sub.c] = [d.sub.c]([pi]/6[[PHI].sub.c).sup.1/3]-1 (1)

where [[PHI].sub.c] is the critical elastomer volume. As part of the testing protocol, a formulation was developed where the level of a high performance EO impact modifier (Engage 8180) was reduced to yield ductile behavior at room temperature, but brittle behavior at low temperature. Formulation 1, shown in table 2, was used as the template compound for the screening tests. Twelve sample runs using Engage 8180 in formulation 1 were undertaken to establish compounding and testing standard deviation.

After screening numerous candidate impact modifiers, the candidate EO1 was observed to exhibit -20[degrees]C dart impact characteristics well outside that of Engage 8180. Figure 3 compares the -20[degrees]C dart impact average breaks and standard deviation of formulation 1 containing Engage 8180 versus EO1. As shown, the compound containing EO1 exhibited significantly higher impact ductility than the compound containing Engage 8180 at an identical level.

[FIGURE 3 OMITTED]

EO1 was designed to exhibit higher polypropylene compatibility than more standard ethylene/octene copolymers. Figure 4 shows AFM comparisons of the elastomer particles in the TPO compounds from the injection molded samples containing EO1 and Engage 8180, respectively.

As seen in figure 4, the EO1 elastomer particle size is significantly smaller than that of Engage 8180. It is likely that the higher impact toughness observed for the EO1 compared with Engage 8180 at equivalent levels seen in figure 4 is due to the greater level of dispersion of this elastomer in polypropylene and the resultant smaller distance between rubber particles. There is a growing consensus that Lc is a key parameter for determining impact toughness in semicrystalline polymers (ref. 5). Since the impact modifier interparticle distance is inversely proportional to rubber volume, but directly proportional to particle size, as shown in equation 1, it follows that an elastomer having a smaller particle size than a comparative elastomer in polypropylene can have an equivalent interparticle distance at a lower volume level versus the comparative elastomer. This elastomer with greater dispersion characteristics would be expected to yield higher impact efficiency.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Level studies

The stiffness and impact characteristics of TPOs at varying elastomer levels were compared using compounds containing filled ICP.

As can be seen in figures 5 and 6, for the impact copolymer case, filled TPO compounds containing the EO1 underwent brittle-ductile transitions at higher moduli than Engage 8180.

There was a higher stiffness-toughness balance using EO1 versus Engage 8180. The differences between the EO1 and Engage 8180 were most apparent for the dart impact performance. A significant difference in ductile versus brittle behavior was observed between these two elastomers over the range tested. These results show that the EO1 exhibited about 1520% higher impact efficiency than Engage 8180.

Comparison with SEBS

Engage 8180 and EO1 were compared with SEBS in PP homopolymer at 24 wt. % elastomer in the compound (figure 7). Both the EO1 and SEBS exhibited higher ductility than the Engage 8180. However, there were striking differences between the responses of these two elastomers under different impact tests. As can be seen, for Izod impact testing at -20[degrees]C, SEBS exhibited the highest level of energy, followed by EO1, then Engage 8180. However, for high speed dart impact testing, the highest energy to break and highest level of ductile breaks was observed for EO 1, followed by SEBS, then Engage 8180. Clearly, the modes of failure for these two different tests are quite different. The Izod impact test, due to the induced notch, encourages crack propagation along one plane in the direction of the hammer strike. On the other hand, there is a much higher level of biaxial strain observed in the un-notched dart specimen when struck by the dart. Finally, it should be noted that the SEBS exhibited good hPP toughening, but with a significant loss in flexural modulus versus EO1.

The data from this study suggest that the newly designed impact modifier, EO 1, exhibited an improved balance in stiffness and toughness compared with either SEBS or high performance ethylene/octene copolymer.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Conclusions

* There is a significant shift in performance requirements occurring for TPO compounders.

* The increased performance requirements necessitate the development of a new, high efficiency polypropylene impact modifier.

* A higher efficiency ethylene/octene based impact modifier with improved polypropylene compatibility has been developed.

* The new modifier showed improved ductility in TPOs based on either polypropylene homopolymers or impact copolymers.

by Kim L. Walton, Mark Berard, Theresa Hermel-Davidock, Phillip Hustad, Jim Hemphill, Didem Oner-Deliormanli and Russell Barry, Dow Chemical

References

(1.) Walton, K.L., et al., "The Case for higher impact efficiency elastomers for rigid TPOs, "proceedings of the SPE-Automotive TPO Global Conference 2008.

(2.) Laughner, M.K., et al., "Modification of polypropylene by ethylene/a-olefin elastomers produced by single site constrained geometry catalyst," proceedings of the SPE-Automotive TPO Global Conference 1999.

(3.) Dharmarajan, D., "Elastomers with isotactic propylene crystallinity as components of TPO, "proceedings of the SPE-Automotive TPO Global Conference 2002.

(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).
Table 1--elastomers for impact modification

 Density Melt index
 Co- (g/cc) 190[degrees]C/2.16 kg
Elastomer monomer (g/10 min.)

Engage 8180 Octene 0.863 0.5
EO1 Octene 0.863 0.5

 DSC glass
 transition
Elastomer ([degrees]C)

Engage 8180 -55
EO1 -67

Table 2--formulation 1, outline used for
elastomer screening

 Parts
Ingredient
D221.00 56
Elastomer 24
JetFil 700C 20
Irganox B225 0.2

Figure 1--breakdown on use of elastomers
in the automotive industry

Fluoroelastomers 1%
PAEs 1%
CPE 1%
SBCs 1%
TPU 2%
COPE 3%
Silicone
 elastomers 5%
Polychloroprene 12%
EPDM 31%
TPOs 43%

Note: Table made from pie chart.
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Author:Walton, Kim L.; Berard, Mark; Hermel-Davidock, Theresa; Hustad, Phillip; Hemphill, Jim; Oner-Deliorm
Publication:Rubber World
Date:Apr 1, 2010
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