Scratch and mar resistance (SMR) is particularly critical for interior parts such as instrument panels and other trim because customers perceive scratching and marring as signs of poor quality. J.D. Power and Associates, which conducts customer satisfaction studies, includes scratch and mar performance in its automobile survey. The effect of scratch performance on these ratings appears to be getting attention from OEMs, comments Johanne Wilson, NAFTA marketing manager for automotive plastics at Ciba Corporation. Another trend is the use of new grained surfaces, with some geometric or dual gloss grains showing scratches more readily. OEMs are looking at scratch-resistant additives as part of their quest for continually better SMR, say industry suppliers.
Dr. Ashutosh Sharma, director of technology at Axel Plastics Research Laboratories, agrees that the bar is being raised for scratch and mar resistance. He predicts that the industry’s minimal standard of 10 Newtons (N) ASTM scratch resistance is heading towards 15N. Dr. Hung-Jue Sue, director of the Polymer Technology Center and Scratch Consortium at Texas A&M University (TAMU), notes that many Japanese and U.S. automotive companies and their suppliers are conducting significant research in improving scratch resistance.“Once the economy stabilizes, you will see scratch research accelerating,” predicts Dr. Sue.
In the past, OEMs required short-term scratch-resistance (during assembly and at the dealer), but in the last year or two OEMs have begun demanding longer-term scratch resistance, because consumers want their vehicle to look good over time. Indeed, the importance of overall interior quality is increasing. Besides scratch and mar resistance, key properties include gloss, haptics (typically a soft-touch feel), and low fogging or emissions due to volatile organic compounds (VOCs).
While requirements are increasing, there is also a trend towards replacing more expensive, scratch-resistant resins with more scratch-susceptible polypropylene (PP) or PP/olefinic TPE (TPO) compounds. This is driving increasing use of anti-scratch additives, say suppliers. For example, one customer replaced glass-fibre filled PA with talc-filled PP containing an antiscratch additive in a door handle application, says Kathrin Lehmann, group leader of plastics technical service and development at Evonik.
The largest market for additives targeting improved scratch and mar resistance is moulded-in-colour (MIC) interior applications such as instrument panels, report industry sources. In many exterior applications, TPOs still face challenges like minimizing warpage and shrinkage, or matching body paint colours. While designers are working on these, scratch resistance will remain a secondary issue, say sources. However, MIC parts are already widely used for exterior applications such as bumpers and rocker panels, and these applications are beginning to use SMR additives, especially in the U.S., notes Ms. Lehmann. MIC-TPO is expanding into front-end modules, where SMR will be very important, adds Sara Robinson, business development manager for minerals at R.T. Vanderbilt Company.
Scratch and mar test methods
Scratch and mar properties are affected by a complex mix of variables including polymer type, mineral properties and content, additives, and the part’s surface texture. The industry uses several different scratch tests to measure performance. Formulations may perform differently depending on the test method used, notes Frederic Jouffret, director of polymer and paint development at Rio Tinto Minerals. The Erichsen method,commonly used in Europe, emphasizes mar or abrasion, while the Ford 5 finger test, prevalent in the U.S., emphasizes scratching. The Scratch Consortium, part of the Polymer Technology Center at Texas A&M University (TAMU), developed a test that was established in 2005 as ASTM standard D7027-05 and in November, 2008 as ISO standard 19252. Consortium director Dr. Sue predicts increasing use of the method now that the ISO standard has been published, noting that many companies are buying the TAMU Scratch-4 Surface Testing System. A key benefit of the new Surface Testing Machine is that the testing is straightforward and allows direct comparisons of scratch resistance between radically different materials or additive formulations, and can evaluate textured surfaces, which cannot be reliably achieved using earlier test methodology, says Dr. Sue. This system, currently the only commercial device for the ASTM and ISO scratch methods, is manufactured through an exclusive license by Surface Machine Systems, LLC. Industry members agree that the ASTM/ISO test is slowly gaining acceptance, but expect continued use of Erichsen, Ford, and other company-specific tests. In addition, long-term scratch resistance is being tested by putting a part through eat cycles to simulate the changes in temperature a vehicle might go through in years of use.
Additives improve scratch and mar resistance
Migratory additives such as erucamide have traditionally been used to improve scratch and mar performance. However, due to concerns about vehicle interior air quality (VIAQ), OEMs, particularly in Europe and Asia, are increasingly cautious about using additives that bloom to the surface and contain volatile organic compounds (VOCs) that contribute to fogging or malodours and can be measured as emissions. Automotive manufacturers, such as the Japanese Automobile Manufacturers Association (JAMA), now specify emission limits for interior materials. In addition to VOC concerns, erucamides can potentially interact with stabilizers and cause surface stickiness. Amides provide only short-term scratch protection, typically measuring 6-7 N in the ASTM test, says Dr. Sharma. Advanced scratch and mar resistant additives from several suppliers overcome some of these shortfalls with permanent scratch resistance approaching 10 N or more, low or no migration, no odour or fogging, and other advantages, say suppliers.
Advanced anti-scratch additives
Ciba® Irgasurf® SR100, an anti-scratch additive commercialized in 2005, is being used globally for both interior and exterior automotive parts as well as non-automotive PP/TPO applications like snowmobiles. Customers report that it shows improved SMR in filled TPO, high-gloss TPO, and soft TPE, says Ms. Wilson. Irgasurf can also boost the performance of recycled PP.
Multibase’s Dow Corning® MB50-001/ MB50-321siloxane masterbatch contains a non-migrating, anti-scratch additive that improves scratch and mar resistance in rigid, mineral-filled TPOs. The ultrahigh molecular weight siloxane offers long-term scratch and mar resistance with no fogging or emissions and very little effect on gloss, says Alexis Von Tschammer, global marketing manager for Multibase, a Dow Corning Company. MB50 masterbatches can be used in TPOs, TPVs, TPEs, and other engineered plastics and elastomers. For soft-touch overmoulding, Multibase offers siloxane-enhanced TPE (SiE-TPE) and cross-linked silicone thermoplastics (TPSiV) that give longterm scratch and mar protection with very low emissions.
Evonik introduced the organic modified siloxane (OMS) Tegomer® AntiScratch 100 (TAS100) additive, targeting improved scratch and mar resistance in talc-filled PP and PP/TPO parts (see Plastics Additives & Compounding March/April 2008). TAS100 does not migrate, yielding permanent scratch resistance and no fogging or gloss change, as well as no odour, reports Ms. Lehmann. Recently, TAS100 has been shown to boost scratch resistance of glass-filled PA, says Ms. Lehmann. She explains: “The scratch appearance in PA is different from that in PP. In PA, the scratch is seen as a change in gloss. TAS100 reduces the scratch depth, resulting in a smaller gloss difference.” In 2008, Evonik commercialized an anti-scratch additive for high gloss PC/ABS or PMMA/ABS parts. Tegomer M-Si2650, originally used in TPEE, improves scratch and mar resistance for these high-gloss parts. AXEL Plastics Research Laboratory’s non-migratory additives are a complex mix of polymeric materials and amides that have no fogging, odour, or interaction with UV stabilizers. “Axel’s scratch resistant additives have a double advantage – not only do they improve scratch resistance but also improve dispersion of rubber and talc within a TPO, leading to improved impact resistance,” comments Dr. Sharma. Since TPO rubber and talc content greatly affect scratch resistance, additive formulations should be designed for a given TPO formulation. In tests at Axel, MoldWiz® INT-35CPD showed best results for a high rubber content TPO with 20 per cent talc, and the new MoldWiz INT-701S was best for lower rubber content TPO with 13 per cent talc. MoldWiz INT-701S shows no scratch whitening up to 15N in the TPO compounds tested. Because of their affinity for the rubber and talc particles in a TPO, the additives can be used at low levels of 1-2 per cent. explains Dr. Sharma. These two additives are being used commercially in applications such as instrument panels, and a third, MoldWiz XP-I-702S, is in development.
Minerals affect scratch performance
Automotive TPO parts are typically filled with minerals like calcium carbonate, talc, kaolin, or wollastonite that have a significant effect on scratch performance. When a part is scratched, exposed mineral particles reflect light and make the scratch more visible. As the mineral loading increases, scratches become more easily visible. Morphology and particle size play a role, with better scratch resistance from finer particle sizes. Minerals with a surface treatment like silane show improved scratch and mar resistance over formulations with untreated grades.
Mineral coatings reduce absorption of slip or other anti-scratch additives onto the mineral, leaving more to function at the part surface. Surface treatments may improve dispersion, which improves scratch resistance, and create a better bond between polymer and mineral so that less of the bright mineral surface is revealed by scratching, adds Anssi Koikkalainen, sales manager at Nordkalk.
IMI Fabi recently introduced its NS100 micronized, coated talc for improved dispersion and scratch and mar resistance in TPO. Joint research with Evonik has optimized combinations of NS100 with Evonik’s TAS100 siloxane additive for scratch-resistant compounds.
Multibase recently introduced a new microtalc masterbatch to improve SMR in filled, rigid TPOs. The product combines a very fine particle size talc with proprietary processing technology to create excellent dispersion in the PP matrix, allowing lower filler levels, says Mr. Von Tschammer. He explains: “Processors typically use 12-20 per cent talc-filled, ready-to-use compounds. This microtalc masterbatch can achieve comparable physical properties with only 3-5 per cent microtalc. Lower talc levels help reduce weight as well as improve scratch and mar resistance. In addition, highly filled masterbatches are more cost-effective than compounds.” He says that the product is currently being tested by processors. Multibase is evaluating mixtures of microtalc and high-molecular weight siloxane for physical property and scratch and mar performance.
Rio Tinto Minerals’ Luzenac R-7 surface treated talc is used globally as a scratch and mar resistant, UV-stable, and high-impact talc, notes the company. Choosing a mineral depends on the physical properties required by the part as well as scratch resistance. For example, microcrystalline talc with round particles demonstrates better abrasion resistance than lamellar talcs, but lamellar talcs have better stiffness. Mr. Jouffret recommends Mistron® microcrystalline talcs for SMR performance in SEBS soft-touch TPEs, but lamellar talcs for PP/TPO applications that need stiffness and SMR. Ultrafine, lamellar Jetfine® talcs, with average particle sizes of 1 micron or less, give a balance of stiffness, toughness, and scratch and mar resistance, says Mr. Jouffret. Jetfine® talc has been processed in Europe, and in 2008 Rio Tinto Minerals added Jetfine® processing to a North American location; products from this facility are in the customer evaluation phase.
Wollastonite is used in both interior and exterior automotive engineering thermoplastics, although not as widely as talc in PP/TPO. Because it is a relatively hard mineral (about 5 Mohs hardness compared to talc with about 1 Mohs hardness), wollastonite has inherently better scratch and mar resistance than standard talcs. Wollastonite’s needle-shaped structure is not as easily breakable as talc’s lamellar, platy structure, resulting in lower scratch visibility, says Mr. Koikkalainen. While the needle structure is good for shrink control, platy minerals are better for warp control, notes R.T. Vanderbilt’s Ms. Robinson, who points out that binary systems combining talc and wollastonite can be beneficial.
Outlook
Despite the poor economy, industry members expect opportunities for additivesto play a big role in innovation and new product development. Ms. Robinson concludes: “Cars are still being built even in a downturn. A slow economy is a window of opportunity, and I am optimistic that companies will be competing aggressively and looking for ways to get their problems solved.”
Automotive glazing: a new opportunity for additives
Polycarbonate (PC) is taking off in automotive glazing applications because it brings greater design freedom and allows lightweighting. Suppliers use coatings to add necessary weatherability and a high level of scratch resistance to PC glazing parts. UV absorbers are an important part of the formulation, protecting PC from UV degradation and maintaining clarity. Colorants used in glazing are typically dyes that maintain high clarity and can meet 10 year weatherability requirements.
Additives are finding a new role in controlling solar-energy transmission. Exatec LLC, the glazing arm of SABIC Innovative Plastics, offers two grades of Lexan PC resins containing infrared (IR) absorbing additives that reduce the heat load inside the vehicle, improving both passenger comfort and fuel economy, explains Stephen Shuler, Exatec’s chief technology officer. He notes that solar-energy control is an ongoing development area, with both IR absorbing and IR reflecting additives being investigated for further improvement. Dr. Kerry Kirwan, PC glazing expert in the University of Warwick’s WMG (formerly the WarwickManufacturing Group), comments: “Polymer glazing is years behind the glass industry in incorporating thin metal oxide layers for IR-reflectance. The high temperature application methods used in glass do not work as well in plastics, so the industry is searching for solutions.”