Influence of sample thickness on optical properties

Variations in sample thickness have an impact on the morphology of injection molded specimens. Fewer but larger spherulites are formed where the melt cools slowly, whereas faster cooling leads to more but small crystalline entities.^[106] As a results of the temperature gradient from the outside to the inside of the specimens the spherulite size increases from the skin to the core and is on average larger for higher mold thicknesses.^[107] As the transparency of objects is mainly determined by the amount of transmitted light, values for haze increase with thickness. The influence of the mold thickness on haze of PA6 comprising the 1,1-dimethylpropyl substituted bisurea 1k, the tert-butyl substituted bisurea 1p and talc is compared in Figure 79. As usual the haze values increase with the sample thickness.

As discussed in chapter 5.4.4 talc caused only a very modest reduction in haze. At a thickness of 0.5 mm a plateau in haze was reached at around 55 % for concentrations above 0.1 wt%.

For 1.1 mm the haze values steadily decreases with higher additive concentrations to a value of only 85 % at the highest concentration of 1.5 wt%. For specimens thicker than 1.1 mm the haze values remained at 100 % independent of the additive concentration.

In contrast both bisurea derivatives 1k and 1p were capable to enhance the optical properties of PA6 for all thicknesses. For a mold thickness of 0.5 mm both additives showed minimum haze values of around 8 % at a concentration of 1.3 wt%. In thicker samples, further addition of 1k resulted in a steady decrease in the haze values, whereas for 1p a plateau was reached for 1.1 mm and 2.0 mm samples. Thus it can be assumed an increase of the 1k concentration would even lead to lower haze values. In platelets with a thickness of 1.1 and 2.0 mm, 1p with tert-butyl substituent was found to be the more efficient clarifier for PA6 up to a concentration of 0.8 wt%. This trend is reversed for concentrations exceeding 1.0 wt%. Here 1k featured lowest haze values of 11 % at 1.1 mm and 21 % at 2.0 mm, whereas 1p displayed values at 19 % and 30 % respectively. At a mold thickness of 3.0 mm the lowest haze achieved is 44 % for 1p (1.5 wt%) and 53 % for 1k (1.3 wt%).

Remarkably even for very thick samples (5.0 mm), the lowest haze value obtained with 1k was 77 % (1.5 wt%) whereas 1p caused only a modest reduction in haze (93 %, 1.5 wt%).

Nucleation and clarification of polyamides 1,1-dimethylpropyl substituted bisurea 1k for different mold thicknesses (0.5 mm, ; 1.1 mm, ; 2.0 mm, ; 3.0 mm, and 5.0 mm, ) as function of the additive concentration.

5.4.8. Laser transparency

Laser transmittance of PA6 comprising the tert-butyl substituted trans-bisurea 1p and talc was determined at a wavelength of 1064 nm on injection molded plaques. The investigated thickness is varied from 1.1 mm, 2.0 mm, to 3.0 mm. In Figure 80 the values for laser transparency are plotted as function of the additive concentration. The LT values for neat PA6 were found to be 82 % for a thickness of 1.1 mm, 72 % for a thickness of 2.0 mm and 59

% for a thickness of 3.0 mm.

Similar to the values for haze presented in chapter 5.4.3.2 one would expect steady improvements in laser transparency with increasing amounts of additive. Surprisingly for all thicknesses at low concentrations in the range from 0.02 – 0.1 wt% the LT-values are lower compared to the neat material. For a thickness of 1.1 mm at a concentration of 0.02 wt% the LT-value is only 78 % and thus 4 % lower compared to the neat PA6. At concentrations exceeding 0.2 wt% 1p is capable to enhance the laser transparency of PA6. Here the transmittance is increased by 3 % to 86 % for a thickness of 1.1 mm at a concentration of 1.5 wt%. A very similar trend is observed for the 2.0 mm and 3.0 mm thick specimens. The lowest LT-values were 63% for a thickness of 2.0 mm at 0.02 wt% and thus 9 % lower compared to the neat PA6. For a thickness of 3.0 mm at an additive concentration of 0.02 wt% the LT-value is 53 %, 6 % lower as the neat polymer. Similar to 1.1 mm above 0.2 wt%

the LT increases. The increase for 2.0 mm is 10 % to a LT-value of 82 % at a concentration of 1 wt%. For 3.0 mm the LT increases by 13 % to 72 % at a concentration of 0.8 wt%. These results demonstrate that 1p is capable to distinctly increase the LT in PA6 especially in thick parts.

The different shapes of the curves for haze and laser transparency at low additive concentrations can be explained by the different measurement procedures for haze and laser transmittance. Haze is determined using an integrating sphere that collects the total hemispheric transmittance when the sample is placed directly on the entrance port of the sphere. Values for haze are obtained by dividing the diffuse by the total transmittance.^[59] In contrast, laser transparency is determined by a sensor located 1.1 mm below the polymer specimen and calculated as the ratio of the reference and the measurement beam.^[66] While haze is calculated with regard to the total transmittance the former can improve while the total transmittance decreases compared to the neat material.

Nucleation and clarification of polyamides

While talc caused only modest improvements in haze as already shown in chapter 5.4.4 laser transmittance is reduced throughout the whole investigated concentration range and for all mold thicknesses. It is interesting to note that the transmittance values reach a plateau above an additive concentration of 0.1 wt%. For a thickness of 1.1 mm the LT-value decreases by 2 % to values of 80 %. For a thickness of 2.0 mm the LT-plateau value is at 65 % and 12 % lower than the neat PA6. For 3.0 mm the plateau value is at 50 % and 9 % lower than neat PA6. The origin of the poor laser transparency of talc is irregular and due to insufficient distribution in the polyamide melt causing additional scattering of the irradiating laser.

0.0 0.2 0.4 0.6 0.8 1.0 45

50 55 60 65 70 75 80 85 90

3.0 mm 1.1 mm 2.0 mm

laser transparency [%]

concentration [wt%]

0.0 0.2 0.4 0.6 0.8 1.0 45

50 55 60 65 70 75 80 85 90

concentration [wt%]

3.0 mm 1.1 mm

2.0 mm

Figure 80. Laser transparency values of PA6 comprising 1p (left) and talc (right) for different mold thicknesses (1 mm, ; 2 mm, and 3 mm, ) as function of the additive concentration.