By Jan H. Schut
Three new materials are being introduced at the SPE’s Automotive TPO Engineered Polyolefins Conference September 30 to October 3, 2012, in Troy, Mich. (speautomotive.com/tpo) that offer intriguing new surface options for thermoformed automotive parts. Two are tie layer materials for coextrusion of glossy scratch-resistant PMMA (polymethylmethacrilate) over TPOs, which hadn’t been commercially available for a while.
Nearly a decade ago Solvay Engineered Polymers developed a blended, non-olefinic tie layer material for acrylic over TPO, called Addhere 228, which was introduced commercially for coextrusion in 2007. Reportedly it worked, but in early 2008 Solvay Engineered Polymers was bought by LyondellBasell (www.lyondellbasell.com), and Addhere 228 was no longer actively marketed. LyondellBasell may not have entirely forgotten about the tie layer technology because it got a patent in 2010 on a tie layer for polyolefins and acrylates. The tie layer, described as a multi-component blend of vinyl cyanide and a styrenic block copolymer (U.S. Pat. # 7740951), was initially filed by Solvay, but issued in LyondellBasell’s name.
A third new material presented for the first time in the U.S. at the TPO conference is a soft interior automotive skin with about 40% bio content. Developed in Germany, the two-layer bio-TPO film achieves sharp grain without rupturing even in deep draw corners.
REACTIVE ‘BRUSH’ MOLECULES
One new tie-layer chemistry for acrylic over TPO is introduced by Macro-M S.A. De C.V. (www.macro-m.com) in Lerma, Mexico, a spinoff in 2008 from the corporate R&D center of Mexican automotive and chemical conglomerate Kuo Group (www.kuo.com). Leticia Flores Santos, technology manager of Macro-M and co-inventor of the technology, presents “Compatibilizers and Surface Modifiers for Polyolefins Using Copolymers with Brush Structures and Controlled Branch Molecular Weight.” The TPO conference is the first presentation of Macro-M’s new reactive “brush” copolymer for polyolefins, which can be used as a PMMA/TPO tie layer.
Macro-M makes a patented family of functional, or reactive, block copolymers (WO2010/140040; CN200680031216), which the company calls “living polymers.” Its functional blocks are made by combining monomers with a commercially available controlling agent in a reactor. The controlling agent (from Arkema) initiates chain addition of another monomer or monomers onto the polymer in a di- or tri-block structure and controls molecular weight by limiting the number of appended monomers in each chain.
These functional copolymers are tailor made for specific applications. A di-block has a miscible block and a reactive block. A tri-block has either a reactive mid-block and miscible end blocks or a miscible mid-block and reactive end blocks, depending on the desired application. The miscible block can be styrenic (PS, ABS, HIPS, SEBS, etc.), PP, polyester, or acrylic.
Commercial di-block products include MacroGran AM functionalized with anhydride groups to bond to nylon or fiberglass; MacroGran EP functionalized with epoxy groups to bond to PC or polyester; MacroGran AC functionalized with amino groups to bond to anionically charged fillers; MacroGran OH functionalized with hydroxyl groups for polar fillers and polyesters; and MacroGran AA functionalized with acrylic acid groups to interact with methacrylates. used as a tie layer for acrylic over TPO.
MacroGran additives can be used at low loadings of 3-5% as compatibilizers or at high loadings of 15-20% as impact modifiers. The MacroGran AA family includes MacroGran AA124 and AA117, which can be used as a tie layer for acrylics over PP. Both are di-block copolymers with a PP-miscible block and a block that has high affinity to methacrylates, but they differ in the amount of acrylic acid functional groups and molecular weight.
The newest MacroGran product is the MacroGran PO family, which is the first that is brush-structured. The brush structure is made by taking functional block “brushes” and grafting them in a secondary reaction in a twin-screw extruder onto different polymer backbones, which are miscible with polyolefins. PO AA grades, for example, are made by putting PP-miscible acrylic-acid-containing brushes onto a PP or TPO backbone in a twin-screw extruder. TPOs are rubbery blends of polypropylene, polyethylene, PP/rubber block copolymers, and any of several rubbers like EPDM (ethylene-propylene-diene).
PO AA grades, used as a tie layer for PMMA over TPO, have better rheological properties than the original AA124 and AA117, Macro-M says. They have the advantage of being similar in viscosity to TPO because they are assembled on a polyolefin backbone, whereas the original AA124 and AA117 additives can be more difficult to coextrude because their viscosity is different from TPO’s.
MacroGran PO EP has been available for about a month for testing and is now being produced in commercial quantities. Other MacroGran PO “brushes” can also be made by grafting other functional “brushes” onto PP. PO AC, PO OH, and PO AM are being produced at pilot plant scale and should be commercial later this year. These functional di- or tri-blocks could also be grafted onto other polymer backbones, such as EPDM, SBS, or SEBS. Macro-M has capacity for 200 ton/year of reactive polymers and 800 ton/year of nano master batches, in which reactive polymers are used to compatibilize nano materials for a specific polymer matrix.
NEW TIE LAYER FOR IMPACT-MODIFIED ACRYLIC
The Altuglas International division of Arkema Inc., Bristol, Pa. (www.altuglasint.com), has also developed a new tie layer material as part of a system for coextrusion of PMMA over TPO, called Solarkote T (thermoformable). Arkema “informally” introduced the Solarkote T system a year ago at the SPE Thermoforming Conference in September 2011 in Schaumburg, Ill., by exhibiting at its booth a thermoformed sample of a molded toy truck made of PMMA/TPO.
At that time Arkema gave out no data on the acrylic capped TPO. The first presentation of data on the new PMMA/TPO system will be at the TPO conference, when Thomas Richards, senior applications development engineer at Arkema, presents “Solarkote Acrylic Capstocks for TPO.”
Arkema plans to commercialize this as a system in which the tie layer material and Solarkote resins are sold as a package. But the developmental tie layer material is what’s new. It’s called PRD-940B (polymer R&D), and it’s a “functionalized olefin-acrylic blend” according to an Arkema patent application on a three-layer acrylic-tie-TPO composite sheet (U.S. Pat. Applic. # 2008022274). PRD-940B is available for limited customer sampling, but is still too new to have a commercial datasheet.
Arkema has tested the tie layer in coextrusion with two common TPO grades for automotive thermoforming, Metaform 7200 from Mytex Polymers (www.mytex.com) and E3400 from LyondellBasell. Test sheet was 0.110 inch thick with 0.006 inch acrylic capstock, 0.007 inch tie layer, and 0.097 inch TPO substrate. Specific gravity of the three-layer sheet was 1.09 g/cc. Specific gravity of unpigmented Solarkote capstock is 1.14-1.16 g/cc, TPO density is 0.92-1.1 g/cc, and PRD-940B has specific gravity of 0.94 g/cc.
Arkema has tested putting colorant in the tie layer, including metal flake pigments, with clear acrylic cap stock, which gives a high gloss “wet” look, Arkema says, and is testing for color matches and weathering. Thermoforming trials so far have been done only in Arkema’s labs on a lab scale Maac thermoformer, but even in lab trials Arkema (like Solvay earlier) found that acrylic’s higher HDT gives rigidity to the TPO, reducing sag during thermoforming.
The three-layer coex structure was tested with 40% regrind blended back into the substrate layer, typical of thermoforming processes, and showed only slight reduction in impact properties, Arkema reports. Arkema’s Richards says they have also tested sheet with up to 60% regrind without an impact strength problem. Target markets are automotive and truck after-market parts, parts for farm and recreational vehicles, kayaks, canoes, and boats.
|Impact Properties of 3-Layer Solarkote T Sheet With and Without Regrind|
|Sheet thickness, inches||0.110||0.110|
|Biaxial impact, J, @ 73 F||18||17|
|Biaxial impact, J, @ 32 F||21||20|
|Biaxial impact, J, @ 5 F||23||22|
FIRST BIO-CONTENT AUTOMOTIVE TPO SKIN
Automotive compounder Benecke-Kaliko AG in Hanover, Germany (www.benecke-kaliko.com) has developed what is believed to be the first bio-based interior automotive TPO skin, by compounding existing TPO formulations using bio-based polyethylene instead of petro-based. Benecke-Kaliko executive director of R&D, Juergen Buehring, presents these “New Sustainable Surface Materials for the Automotive Interior” for the first time in the U.S. at the TPO conference. The technology was introduced for the first time anywhere two weeks earlier at the “Automobile Cockpit 2012” conference in Sindelfingen, Germany, sponsored by VDI Group in Dusseldorf (www.vdi-wissensforum.de).
Benecke-Kaliko duplicated an existing two-layer TPO skin with a solid soft TPO surface layer 0.4-0.5 mm thick and a TPO substrate layer 0.4-1.0 mm thick. The soft TPO surface material contained up to 30% bio-based PE and 15% regrind. The TPO substrate material contained up to 50% bio-based PE and 30% regrind. So total bio content for the film was about 35-40%, depending on the thickness of the substrate, Benecke-Kaliko’s Buehring says.
The bio TPO compounds were developed for the requirements of several OEMs, he adds. Benecke-Kaliko offers the compounds commercially, though they are so new they still don’t have a product name. The two-layer skin has been tested in molds with in-mold graining for car door parts using serial equipment and tooling at a Tier 1 OEM.
The bio-based TPO skin can also be used with Benecke-Kaliko’s patent-applied-for “Tepeo 2” electron-beam cross-linking technology (U.S. Pat. Applic. # 2008019684). Tepeo 2 crosslinks a TPO sheet before vacuum forming into a female mold to improve grain appearance and prevent rupturing in deep draw corners.