Page 46 - Plastics News Issue May 2025
P. 46
FEATURE NEWS
mers produced via emulsion polymerization, de- mechanical energy transforms the polymer’s
signed to balance thermal stability (Tg >100 C) amorphous regions into ordered structures. This
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and miscibility with PVC. is particularly crucial for PVC, which requires
precise thermal and shear conditions to achieve
The Fundamental Challenge of Layer Bonding
optimal fusion.
Unlike traditional manufacturing methods that Laboratory rheometer studies demonstrated
produce uniform structures, FDM 3D printing this effect dramatically. At 170 C with 2 PHR load-
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builds objects by depositing molten polymer ing, acrylic-modified PVC formulations reached
strands one layer at a time. This method creates complete fusion in under three minutes at 45
weak spots at the layer boundaries because RPM, compared to prolonged times for unmodi-
rapid cooling and the polymer’s rheological fied PVC. Increasing rotor speed to 65 RPM to
properties restrict molecular diffusion and en- simulate higher shear conditions in printer noz-
tanglement. In PVC, a material with a notoriously zles further accelerated fusion by 30-40%, prov-
narrow processing window. thermal sensitivity ing the additives’ shear-thickening behavior.
and prolonged heat exposure further intensify
these issues. From Lab to Printer
Dr. Nerkar’s research shows that poor interlay- Transitioning from controlled lab experiments
er adhesion results from limited polymer chain to actual 3D printing revealed the harsh realities
mobility during the crucial bonding phase. When of PVC processing. Initial attempts using stand-
the printer deposits each layer, the brief ther- ard PVC filaments at 175-185 C resulted in cata-
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mal contact period prevents sufficient molecular strophic nozzle clogging and “mouse bite” de-
interpenetration. This limitation produces parts fects, a phenomenon where incomplete layer
with mechanical anisotropy, with Z-axis strength bonding causes material to tear during deposi-
dropping 50–70% compared to the X–Y plane, a tion.
critical problem for functional applications.
The research team tackled these issues with
Acrylic Additives: The Molecular Architects an iterative optimization process. They tried in-
creasing the temperature to 200–230 C, which
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Dow’s solution leverages the unique properties initially seemed promising. However, the higher
of high molecular weight MMA copolymers from temperatures caused new problems. PVC de-
their PARALOID additive series. These special- graded when exposed to temperatures above
TM
ized acrylic processing aids function as molecu- 180 C, leading to dark discoloration and blocked
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lar architects within the PVC matrix through sev- extruders that required disassembly for clean-
eral synergistic mechanisms.
ing.
The additives’ high glass transition temperature The solution emerged through careful formula-
creates a miscible blend with PVC, while their tion and process optimization. Incorporating 4
extended polymer chains function as physical PHR of PARALOID K-175 alongside optimized
TM
bridges between PVC molecules. During pro- stabilizers, the team achieved stable extrusion
cessing, these acrylic chains enhance shear- at 210 C, which stays below PVC’s degrada-
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induced crystallization, a phenomenon where
tion threshold while ensuring proper flow. They
46 PLASTICS NEWS May 2025
