The influence of some color matching pigments on the strength of injection molded parts
Color pigments are key auxiliary materials for adjusting the appearance of plastic parts during injection molding. However, their addition not only alters color but can also affect the mechanical properties of plastic parts through physical or chemical interactions. The chemical composition, particle size distribution, and dispersion of different pigments can interfere with the molecular structure and crystallization behavior of the plastic matrix, leading to fluctuations in properties such as strength and toughness. Therefore, understanding the effects of common pigments on the strength of plastic parts is crucial for balancing appearance and performance.
The impact of inorganic pigments on plastic part strength is primarily related to particle hardness and dispersibility. Titanium dioxide ( TiO₂ ), a commonly used white pigment, has minimal impact on the tensile strength of polyolefin materials like PE and PP (typically less than 5% ) if its particle size is controlled within 0.2-0.3μm and uniformly dispersed . This is because the particles can embed themselves between polymer chains without disrupting the crystal structure. However, when used at levels exceeding 5% , the rigid particles of TiO₂ become stress concentration points, resulting in a 10%-15% decrease in impact strength . Iron oxide red ( Fe₂O₃ ), due to its catalytic activity, may accelerate the degradation of PVC during high-temperature injection molding , reducing tensile strength by 8%-12% . Stabilizers are required to mitigate these negative effects. Carbon black, a black pigment, not only provides color, but also improves the aging resistance of HDPE by scavenging free radicals, increasing tensile strength retention by 5%-8%, if a highly structured variety is used at a level of ≤2%.
The impact of organic pigments on plastic part strength is primarily reflected in their compatibility with the resin. Azo pigments (such as Permanent Red) have good compatibility with polar resins like PS and ABS. At a usage level of up to 3%, the impact strength decreases by only 3%-5% due to their molecular chains forming weak hydrogen bonds with the resin matrix. However, in non-polar PP, azo pigments tend to aggregate into micron-sized particles, resulting in a 10%-15% decrease in tensile strength, necessitating the addition of a dispersant (such as calcium stearate) to improve dispersion. Copper phthalocyanine pigments, such as phthalocyanine blue and phthalocyanine green, exhibit excellent chemical stability. Even at a usage level of 5% in PA66, their impact on flexural strength is limited to 5%. Their flaky structure also slightly improves the material’s rigidity (increasing the elastic modulus by 2%-3%).
Pearlescent pigments and metallic pigments, due to their specific forms, have a dual impact on the strength of plastic parts. Mica-based pearlescent pigments are flake-like. When oriented in transparent plastics like PC and PMMA, they increase melt flow resistance and create tiny bubbles within the part, reducing impact strength by 8%-12%. However, tensile strength parallel to the flake direction can be increased by 5%-7%. Unsilane-treated metallic pigments like aluminum powder and copper powder can react with polar groups in POM and PVC, leading to material degradation. For example, adding 5% untreated aluminum powder to PVC can reduce tensile strength by 15%-20%, while treated metallic pigments can limit strength loss to less than 5%. Furthermore, the conductivity of metallic pigments can cause static electricity in the mold during the injection molding process, attracting plastic debris and forming defects, indirectly affecting the structural integrity of the part.
The synergistic effect of pigment dosage and processing technology can also affect the strength of plastic parts. Generally speaking, for every 1% increase in pigment dosage, the viscosity of the plastic melt increases by 2%-3%. If the injection pressure is not increased simultaneously, the plastic part is prone to underfilling, resulting in a 10%-20% decrease in local strength. For example, when adding 10% titanium dioxide to ABS, the injection pressure must be increased from 80MPa to 95-100MPa to ensure that the melt fills the mold cavity and avoid strength defects caused by material shortages. Excessively high processing temperatures can cause pigment decomposition. For example, azo pigments release toxic gases when subjected to temperatures exceeding 230°C, which can also cause PE molecular chains to break, reducing tensile strength by 15%-25%. Therefore, processing parameters must be adjusted based on the pigment’s heat resistance. For example, phthalocyanine blue has a heat resistance of up to 280°C, making it suitable for PC (processing temperature 260-300°C), while azo pigments are more suitable for materials like PS and PE, which have processing temperatures below 220°C.
The negative impact of pigments on strength can be mitigated through formulation optimization and process adjustments. Adding 0.5%-1% of a compatibilizer (such as maleic anhydride-grafted PP) to the formula can improve the interfacial bonding between the inorganic pigment and the non-polar resin, reducing impact strength loss by over 50%. Using nano-sized pigments (particle size <100nm) can reduce stress concentration effects. For example, when nano-titanium dioxide is added to PP at an 8% dosage, the tensile strength can still remain over 90% of its initial value. A two-step injection molding process—first injecting the uncolored substrate, then the pigmented melt—concentrates the pigment on the surface of the part (thickness <0.5mm), minimizing its impact on overall strength. Furthermore, pigment pretreatment (such as high-speed mixing or surface modification) can improve dispersion uniformity and avoid strength fluctuations caused by localized aggregation. These measures can maintain acceptable part appearance while keeping strength loss within an acceptable range (typically <10%).
