SURFACE MODIFICATION | SCRATCH AND WEAR
Figure 1: Crockmaster abrasion results for PLA with various Vinnex and Genioplast additive content Source: Wacker
Figure 2: Erichsen scratch test results for PLA with Vinnex and Genioplast additives Source: Wacker
The use of silicone additives resulted in lower
energy cost in processing. In the compounding of filled PBS, addition of friction-lowering silicone pellets reduces the torque demand on the extruder during production of the compounds and leads to lower power consumption. The observed effects are small, but they can yield appreciable cost savings in large lines. The flow behaviour of the melted compounds was tested using the melt volume-flow rate specified in ISO 1133 and a spiral flow test. In the latter test, the combination of the additive systems results in a longer flow path. The additive combination generally leads to a substan- tial increase in the melt volume-flow rate, with combinations with Vinnex 2504 proving especially beneficial to the melt flow. A superior surface finish was also achieved for the biodegradable polyesters. When used in combination with Vinnex, the the Genioplast Pellet exerts a particularly strong influence on the surface finish of the plastics. The surface roughness of the polyesters is not improved by the use of Vinnex on its own, because Genioplast needs to be added to lessen the effects of friction. As a result, a coeffi- cient of sliding friction of less than 0.3 can be
18 COMPOUNDING WORLD | February 2022
achieved in the case of both PLA and calcium carbonate-filled PBS – a level required by plastics converters for many applications. In the case of PLA, this level is achieved with a combination of Vinnex 2504 and Genioplast Pellet. In the case of filled PBS, it can be achieved with a blend of Vinnex 2525 and Genioplast Pellet. The reduced surface friction provides the biopolyesters with greater scratch and abrasion resistance. This was confirmed by visual assessment of abrasion resistance using the Crockmeter test and gravimetric determination of abrasion by the abrasive disk method according to DIN 53754. The friction-reducing effect is especially clear from a confocal microscopic examination of scratch marks produced on smooth injection-moulded test panels using a scratch hardness tester (model 530 P-I from Erichsen). With Genioplast Pellet, the scratches are shallower, the scratch profile be- comes flatter, and the surface inside the scratch is less rough. This effect is particularly pronounced at an additive application rate of 2% (Figure 1 and 2). The Wacker researchers say that, with regard to mechanical properties, the study revealed a mixed picture. Combinations of the two additive types yielded improvements in some cases, but not in others. The effects depended on the grade of Vinnex employed and varied with the polyester matrix. For example, the combination of the two additives produced a higher elongation at break in the case of calcium carbonate-filled PBS but left unfilled PLA virtually unchanged. As expected, Vinnex 2504 acted as an impact modifier for PLA, whereas Vinnex 2525 did not. A combination of Vinnex 2504 and Genioplast Pellet increased the tear strength of PLA blown film in both longitudinal and transverse directions. However, combination with Vinnex 2525 lead to a marked improvement in the transverse direction. The tear strength of the films was determined using the Graves angle test (DIN 53515). The mechanical properties also improved, without impairment of heat resistance. This was confirmed through determination of the Vicat softening points using ISO 306 (methods A and B). The studies concluded that Vinnex and Geni-
oplast Pellet are complementary in their effects and exert a greater influence when employed in combination than separately. Both the processing properties and the material properties of the biodegradable polyesters benefited substantially from the additive combination. Such a combination proves especially advantageous in the production of blown film; Vinnex enhancing the flowability and melt strength and Genioplast Pellet increasing the
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