WCA March 2024

Technical Article

Accelerated weathering was conducted using a Q-Lab QUV/ SE accelerated weathering tester per ASTM D1248 Standard Specification for Polyethylene Plastics Extrusion Materials for Wire and Cable. ASTM D638 Type V dogbone specimens were exposed at 70°C for 20h and 0.7W/(m 2 nm), followed by 4h dark at 55°C with condensation. Samples were removed after 4,000h ageing for testing. Tensile strength and elongation (T&E) were tested according to ASTM D638 Standard Test Method for Tensile Properties of Plastics on an Instru-Met model 4201 tensile testing machine. The T&E was tested at 25.4mm per minute crosshead speed. Each specimen was cut per ASTM D638 Standard to a Type V dogbone with nominal 1.9mm thickness. Abrasion resistance was assessed using a Taber Abrader. The wheels were wrapped in sandpaper, a 1kg load was applied, and the wear was recorded after 100 cycles. 3. Results and discussion 3.1 Laser marking additives Due to the excellent mark durability, fast marking speeds, low maintenance costs, reduced scrap during cable manufacturing, and low environmental impact of laser marks, laser marking technology has started replacing inkjet prinking in many applications, including cable manufacturing. Standard black jacketing compounds are not laser markable, due to the presence of >2.3% carbon black. To achieve localised colour changes with sufficient mark contrast, additives, fillers, pigments or dyes are employed and the carbon black content is typically lowered to <1%. [3] Common laser marking additives include metal oxides, metal salts, and minerals such as mica. Selected additives were evaluated in jacket compound compositions. Figure 1 compares laser marking on compounds LP1 and LP2. Both compounds show good mark contrast over a range of pulse intensities and frequencies. LP2, with the addition of a laser marking additive, shows a somewhat enhanced marked contrast at lower pulse intensities and higher frequencies (bottom right), suggesting that laser-marking additives can broaden the range of printing conditions.

Figure 2 : SEM cross-sections of (a) BK1 and (b) LP1 on areas subjected to laser radiation. The scale bars are 100μm (a) and 50μm (b)

3.2 Effect of resin and printing conditions Polyethylene-based compounds are used in a range of fibre optic cable jacket applications; therefore it is desirable to have a robust laser-printing solution to meet multiple needs. The previous section and prior work [2] demonstrated laser printing on HDPE-based compounds. Here we demonstrate the solution is extendable to MDPE jacket compounds and the effect of print conditions is studied on both HDPE and MDPE. Laser settings can have a significant influence on mark quality. [6] For a given laser wavelength, the effectiveness of laser marking can be optimised through parameters including energy density, pulse frequency and marking speed. Energy density and beam spot size are controlled by adjusting the focal point of the beam and lens. Pulse frequency affects how the beam energy is dissipated by the polymeric compound. Lower pulse frequencies will vaporise the surface of the polymeric compound, due to the rapid temperature increase and low heat transfer. Higher frequencies will result in temperature rises near the exposed area. Marking speed also has an effect on print quality, as too high a marking speed can result in a non-continuous marked line, with marks appearing as a series of dots. Too low a marking speed may result in excessive mark depth or burning of the substrate. Figure 3 shows laser printing on an MDPE compound, at print speeds of 2,000 and 4,000mm/s, where the resin density is 0.938g/cm 3 . Figure 4 compares the effect of print conditions on HDPE, where the resin density is 0.948g/cm 3 . Both compounds are laser

Figure 3 : Laser printing on MDPE at (a) 2,000 and (b) 4,000mm/s

Figure 1 : Laser printing of (a) LP1 and (b) LP2 at 2,000mm/s

The presence of additives in laser markable compounds can absorb laser energy and convert it to heat. The heat is capable of vaporising the compound and generating gas bubbles that produce a foamed structure on the compound surface. [4] The foamed structure scatters incident light, producing light marks on a dark background. Figure 2 shows cross-sectional SEM micrographs on laser-printed compounds. [5] The conventional black jacket BK1 (a) shows the absence of the foamed structure, whereas LP1 (b) shows the foamed surface structure resulting in good mark contrast.

Figure 4 : Laser printing on HDPE at (a) 2,000 and (b) 4,000mm/s

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March 2024