scratch and mar | Additives
Tackling testing SMR testing is somewhat unusual in the plastics world because there are many different test types, which are often designed to produce indicative results for a specific market. “The appearance of a scratch on the surface is difficult to characterise with just one test,” says Lehmann at Evonik. “Grain, colour, gloss, and viewing angle all change the true appearance of the surface.” In the automotive market, nearly every company has
its own standard, although some are well known such as the five-finger scratch test. The Erichsen scratch test, which is becoming more widely used, makes a grid of straight cuts with a needle or pin at a specific force. The difference in brightness between the scratched and unscratched plates is then measured. “The shinier the scratches as compared to the intact
surface, the more susceptible the tested surface is to scratches. For more extensive testing, the scratches can additionally be examined microscopically, for example to thoroughly study their contours, depth and roughness inside the groove,” says Klaus Pohmer at Wacker. Atomic force microscopy (AFM) can be used to quantify the deformation, crack, fracture and/or delami- nation for the scratch, explains Dr He at Inteva Products. However, he points out that although it is capable of providing morphological data more accu- rately after the scratch, it is not able to link that to how the scratch occurred. Scratch maps can be used to provide qualitative results of deformation mechanisms and establish material property–scratch performance relationships. And advanced technologies such as 3D-data analysis and modeling have the potential to accelerate development in these areas, he notes. Mar damage occurs through a different failure mechanism than scratch damage. Mar is related to degradation in micro-levels within the surface or near the surface (for example, stress/strain fatigue or ageing due to environmental, thermal, physical, mechanical, and/or chemical attacks), says Dr He.
SMR on the agenda at CWC 2016 Scratch resistance will be one of the topics on the agenda at the Compounding World Congress, which takes place in Cologne in Germany on 18-20 April 2016. Kaertner Montanindustrie’s Application Manager Dl. Veronica Mayer will discuss the specification and processing of functional minerals to improve scratch resistance, reduce warpage and add reinforcement to polymer compounds on the final day of the event. To find out more about attending the Compounding World Congress
contact conference organiser Kat Langner. Tel: +44 (0)117 314 8111; Email:
kl@amiplastics.com. Or download the brochure:
http://bit.ly/CWC16B
40 COMPOUNDING WORLD | February 2016 Current mar testing techniques are generally
qualitative ( a subjective evaluation of visual changes) and more accurate and direct measurements of the internal stress/strain are needed to establish a reliable and quantitative correlation between property and performance, he says. More fundamental understand- ing of failure mechanisms is also necessary to provide scientific guidance into developing new testing equip- ment and methods. Some in the industry are actively at work in this area, with just one example being quantita- tive modeling and other aspects under examination at Texas A&M University’s Polymer Technology Center (
http://ptc.tamu.edu/). Although abrasion and mar resistance testing methods are less standardised than scratch testing, some automotive OEMS are looking at abrasion-type testing as an important piece in addition to Erichsen scratch testing, according to Lehmann & Voss. One method that can be used, explains Pohmer, is a
round-edged test disc based on General Motors Europe (GME) specifications. “Similar to an upright coin, this disc bluntly scrapes over the plastic surface in straight, parallel lines. Since the lines are close together and the resultant marks overlap, the stress is on a rectangular section of the surface. This section has a different gloss than the rest of the surface. A surface that was originally matt will be shinier after the treatment than before, taking on a polished appearance, because the scraping levels out the microstructure that produced the matt surface character. Conversely, if the surface was originally smooth, its gloss is reduced,” he explains. “Another option is the crockmeter test, which
involves a test probe covered with a cotton cloth rubbing the same section of a test panel several times with a specific force. The frequency of the test probe’s back-and-forth movement during the test and its downward force can be adjusted on the test device. The test panel is subsequently inspected visually. If the surface of a dark test panel is very sensitive to the repeated rubbing, the stressed area takes on a shiny appearance; if it is not sensitive to the rubbing, its appearance is unchanged,” Pohmer says.
Click on the links for more information: ❙
www.excista.com ❙
www.intevaproducts.com ❙
www.imerys.com ❙
www.tenasitech.com ❙
www.crodapolymeradditives.com ❙
www.wacker.com ❙
www.dowcorning.com ❙
www.evonik.com ❙
www.basf.com
www.compoundingworld.com
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