Polypropylene (PP) resins and compounds are being widely used in various automotive interior plastic parts such as instrument panels, door trims, console, and pillar trims due to their versatility, wide range of physical and mechanical properties, excellent processability and recyclability, and relatively low material cost. PP compounds have been replacing engineering plastics such as ABS (acrylonitrile butadiene styrene) and PC (polycarbonate)/ABS for years.
Scratch resistance is one of the key requirements for materials in molded-in-color interior applications. This subject has been one of the major technical challenges for automotive industry engineers working with parts made of PP resins or compounds, and its importance has been increasing over the past decade for most global OEMs.
The scratch phenomenon of plastic materials is the surface deformation by external friction force and usually accompanies whitening on the damaged area. The level of scratch damage on a part depends on the speed, force, and geometry of the damaging object, the grain type and depth, surrounding temperature and humidity, and material properties.
There are several governing factors that determine the scratch characteristics of plastic materials—e.g., the crystallinity and mechanical strength of the base resins, impact modifiers, fillers, additives, pigments, and surface morphology. Engineers from Hyundai Motors’ Polymeric Materials Research Team investigated the contribution of each major component in PP compounds to the scratch phenomenon and established general guidelines to achieve a desirable level of scratch resistance in designing PP compounds. The researchers will present their findings at the SAE World Congress as part of the “Advances in Instrument Panels and Interiors” technical session on Wednesday, April 13.
The scratch phenomenon
Scratching is usually characterized by surface crazing and cracking, debonding, formation of cavities, and micro-voids—referred to as “micro-zones.” A scratched surface usually shows whitening. The damaged area looks brighter than other areas due to the light scattering by the micro-zones.
In general, the level of whitening increases with the number of phases in the base resins. PP block copolymers contain EPR (ethylene propylene rubber) domains in homo PP matrix phase and exhibit more visible white lines than homo PP. This is because more micro-zones are formed by debonding of the weak rubber domains from the matrix by the friction force during scratch of PP block copolymers.
SEM images on the scratched surface of a talc-filled PP material show that the talc particles exposed to the surface by the resin-rich skin layer being removed during scratch increase the level of whitening by light scattering. As adding rubbers or fillers into the PP matrix, the whitening becomes more severe due to the debonding during scratch. The bonding force between filler and PP matrix is generally not strong and is easily debonded by scratch, resulting in formation of micro-zones and whitening. If the adhesion between PP matrix and fillers can be enhanced, the formation of micro-zones and the severity of the whitening can be reduced.
Surface hardness is another factor in determining the scratch resistance. In general, higher surface hardness is achieved with lower molecular weight PP chains than high molecular weight chains. This is due to the faster crystallization with shorter chains, resulting in higher crystallinity after the polymer melt cools down after fabrication. Higher crystallinity accompanies a higher tensile strength as well.
Scratch resistance is better in a material with higher surface hardness and higher tensile strength. HIPP (High Isotacticity PP) is being used as base resin for PP compounds that require high stiffness and impact balance. HIPPs have higher hardness and tensile strength than general homo PPs and block PPs and exhibit a better performance in scratch resistance.
Guidelines for best results
It is important to choose appropriate PP base resins since they take the largest portion in PP compound formulations and are one of the most important factors determining the scratch resistance. The Hyundai researchers compared the scratch resistances of four different homo PPs and four different PP block copolymers with varying MFR (melt flow rate).
Among homo PP, PP block copolymer, and HIPP with a similar level of MFR, the order of scratch resistance was homo PP, then HIPP, then block PP. Even though homo PP exhibits a superior scratch resistance than the other types of PPs, its usage for automotive parts has been decreasing due to its poor impact/stiffness balance. Meanwhile, the usage of HIPP has been increasing due to the good impact/stiffness balance and good scratch resistance. PP block copolymer can provide good impact strength, but its scratch resistance is not as good as other PPs.
It also is important to choose appropriate filler and impact modifier rubber systems. The majority of automotive-application PPs requires adding certain amounts of fillers and rubbers to meet all market requirements. However, adding fillers and rubbers in PP hurts the scratch resistance; the material designer should try to use minimal amount of fillers and rubbers. Using a smaller size filler, a more compatible coated filler, and a high molecular weight rubber replacing some portion of talc with wollastonite should be helpful in minimizing the reduction of scratch resistance.
When the researchers compared the scratch resistance of PPs with different fillers, the order of scratch resistance was coated talc, then wollastonite, then uncoated talc. The talc coated with calcium carbonate (CaCo3) gives better dispersion and stronger bonding with PP matrix than other inorganic fillers and less debonding of talc particles during scratch. Because PP filled with coated talc has good impact strength, it needs a lesser amount of rubber impact modifiers, which also helps improve the overall scratch resistance.
Likewise, using the appropriate slip agent additives is important. Migratory-type additives such as erucamide can provide good scratch resistance at lower cost, but surface blooming can be an issue if it is used too much. Slip agents having good compatibility with PP and high molecular weight are desirable.
Another type of slip agent is “nonmigratory,” the most commonly used for PP being polysiloxane. A polysiloxane slip agent does not quickly migrate to the surface due to its high molecular weight and gives fewer problems in surface blooming and tackiness compared to migratory types. Since polysiloxane tends to be absorbed in talc and coat talc particles, it gives excellent scratch resistance.
Because nonmigratory slip agents generally provide a great improvement in scratch resistance but are more expensive compared to migratory additives, sometimes migratory and nonmigratory additives are used together. Since adding a nonmigratory additive can significantly affect the material mechanical properties, engineers should carefully choose the additive type and dosage amount.
This article is based on SAE International technical paper 2011-01-0461 written by Won-Jong Noh, Jung-Gyun Noh, Dae-Sik Kim, and Suk-Hwan Kim, Polymeric Materials Research Team, Hyundai Motor Co.