The advance of TPOs into new interior and exterior automotive applications was the theme of the latest annual Automotive TPO Global Conference, sponsored by the Society of Plastics Engineers in Dearborn, Mich. Examples included new platable TPOs designed to replace engineering thermoplastics, improved soft-skin grades that can outperform flexible PVC, and a developmental additive that reportedly provides unprecedented mar and scratch resistance.
TPOs with a 'bright' future
Solvay Engineered Polymers has developed a novel family of TPOs that could replace engineering thermoplastics in metal-plated decorative parts ranging from instrument-panel bezel trim to claddings, appliqués, and grilles. Designed specifically for electro-chemical metalization, new Platon TPOs include in their composition unsaturated olefins that can be oxidized by acid etching—the most crucial step for preparing a molded part for metalization.
Platon can be used in existing ABS plating processes. Chrome-plated Platon reportedly exhibits a surface finish with a DOI equivalent to plated ABS and PC/ABS. Metal peel strengths and heat-cycle resistance are also said to be similar. Platon TPOs can be tailored to withstand stone impingement in the same manner as PC/ABS, or softened to yield on impact. Also, Platon regrind can be used at levels of 5% to 10% without property degradation or surface defects.
For injection molding, Platon EXP 443 (14 MFR) has low-temperature ductility and medium rigidity. Plated parts exhibit ductile impact at 0° C. A 9-MFR grade, EXP 4-10, has a lower flexural modulus—650 MPa vs. 1000 MPa for 443—and plated parts remain ductile at -30 C. It absorbs maximum impact energy comparable to PC/ABS, Solvay says.
For extrusion, 1-MFR Platon SH001 is designed to be coextruded with a more rigid TPO substrate for profiles or thermoformable sheet.
Solvay has also developed a new colorable TPO for hard instrument panels that integrate the passenger-side airbag module into the IP to achieve a seamless or hidden appearance. General Motors has used this material in the IP covers, glove-box doors, and knee bolsters of the 2005 Chevrolet Colorado and GMC Canyon, and a third vehicle program is under way. These pick-up trucks are GM's first vehicles with all-TPO IPs. New Sequel 2380 is said to provide the required balance of stiffness and low-temperature impact, along with excellent surface durability and moldability.
This TPO is said to combine the properties of an IP material with those of an airbag cover. Its flexural modulus of 1600 MPa is closer to that of a typical IP TPO (2100 MPa) to provide the necessary stiffness and dimensional stability. It also has high-speed ductility at -30 C, similar to a typical airbag-cover TPO. A proprietary compatibilizer used with the talc reinforcement is key to the material's excellent scratch resistance of 15N with no stress whitening, according to the Ford five-finger scratch test.
For auto exteriors, Solvay has two new families of engineered TPOs that exhibit high stiffness, impact resistance, and enhanced surface durability comparable to engineering thermoplastics. One of these, the Sequel 1800 Series, is said to match the mold shrinkage of many amorphous and crystalline engineering TP alloys, such as PC/ABS, PC/PBT, and PPO/nylon. These TPOs are aimed at bumper systems, grilles, and vertical body panels.
For example, Sequel 1823 (20 MFR), replaced a PC/PBT in what is said to be the first TPO bumper energy absorber, which appears on the rear of Chrysler's 2005 Jeep Grand Cherokee. Compared with PC/PBT, it reportedly provides equivalent performance at lower cost. Not only does the resin cost less, but part weight is 5% to 15% lower and low viscosity allows use of a two-cavity mold. Sequel 1825 (25 MFR) is being evaluated as an alternative to PC/ASA blends for a 2005 car grille.
Solvay's Indure TPO family is aimed at grained and ungrained pigmented exterior parts that require good scratch resistance. Grade X-190 (11 MFR) and X-195 (14 MFR) have high stiffness, low CLTE, and good scratch resistance plus inherently lower gloss.
Three high-gloss Indure grades—EX-76 (35 MFR) and X-210 and X-225 (both 40 MFR )—boast superior scratch resistance, good stiffness/impact balance, and excellent colorability and processability. They are suited to wheel flares, claddings, and body-side moldings.
Better soft TPO skins
Basell Polyolefins has developed new soft-skin TPOs that are said to overcome the shortcomings of previous versions, in cluding fogging, color, and uv-stability issues. Hifax E 3274 is a new-generation thermoformable TPO with a proprietary elastomeric component developed especially for this application. Advantages cited include improved formability and reduced gloss as well as improved scratch resistance and chemical resistance against common organic materials and oils.Variants include: higher melt-strength E 3274LG/T, reduced-melt-strength E 3277, low-gloss E 3274LG, more scratch-resistant E3274ST, and lower-gloss, scratch-resistant E 3274LT/T/SR with improved thermoformability.
Suited for typical thermoforming of both supported and unsupported interior skins, these TPOs also show promise for the newer “negative forming” technology, in which the grain is imparted to ungrained sheet during forming in a female tool. While somewhat more expensive, these grades reportedly deliver skins with terrific grain definition.
DuPont Dow Elastomers discussed two of its new Engage high-melt-strength polyolefin elastomers (POEs) based on ethylene-butene. Combined with PP, they result in soft TPOs that are suited to automotive interior sheeting used in negative thermoforming. These simple, cost-effective blends outperform PVC and are similar to blends of ethylene/octene Engage and PP that are reactively modified with peroxide during compounding.
Engage ENR 7086.01 is a 3.9-MI, 0.9-g/cc grade that has a high level of long-chain branching and relatively broad MWD. ENR 7380 (2.0 MI, 0.870 g/cc) is a high-MW elastomer with a medium level of long-chain branching. TPO blends of these materials are suited to extrusion and calendering, and researchers say resulting sheet has been successfully thermoformed in “negative”-tool processes. Durability tests show that these soft TPOs outperform PVC in three ways: retained flexibility after heat aging at 120 C, better low-temperature impact, and improved resistance to common cleaning fluids.
Ciba Specialty Chemicals has a novel, proprietary additive that at 3% loading is said to improve scratch and mar properties of automotive TPO and PP. For use in both exterior and interior applications, it is said to tackle exudation, stickiness, poor paint adhesion, and lack of permanence associated with current technologies such as erucamide, wollastonite and nanoclays, siloxane additives, and coatings.
No special equipment is required to use this new product, which could potentially be added at the molding machine. In a PP impact copolymer filled with talc and carbon black, the developmental additive performed better than siloxane or erucamide, achieving up to 15N in Ford's five-finger scratch test.