Acid value is a core indicator for assessing the polarity, purity, and stability of plastic additives, directly determining their functional direction and application suitability in processing, while also interacting with other technical parameters.
1. Influence of Acid Value on Different Additives
Oxidized Polyethylene Wax:
High acid value (25–30 mgKOH/g): Higher polarity enhances internal lubrication, significantly improving melt flow and accelerating PVC plastification. Ideal for highly filled rigid products like SPC flooring and pipes.
Low acid value (10–16 mgKOH/g): Emphasizes external lubrication, reducing mold adhesion and improving surface gloss and demolding performance. Suitable for films, shrink wraps, and calendered sheets where surface quality is critical.
EBS (Ethylene Bis-Stearamide):
Acid value should be controlled ≤7.0 mgKOH/g. Lower acid value indicates fewer free fatty acids, resulting in higher product purity, better thermal stability, and reduced risk of blooming or yellowing. Widely used in PVC formulations and color masterbatches to balance internal/external lubrication and improve pigment dispersion.
Oleamide and Erucamide:
Both require acid values ≤7.0 mgKOH/g. A lower acid value ensures better product stability and long-lasting slip performance.
Oleamide: Low melting point (72–76°C), fast surface migration, ideal for LDPE, LLDPE, and general-purpose films to improve slip and anti-blocking properties.
Erucamide: Higher melting point (82–85°C), superior heat resistance, slower migration, and stronger anti-blocking effect. Preferred for BOPP, CPP, and high-speed packaging films requiring stable low friction coefficients.
2. Key Factors Influencing Acid Value
While acid value primarily reflects the content of free carboxylic acid groups in the molecule, it is indirectly affected by several structural and environmental factors:
Molecular Weight Distribution: Lower molecular weight increases the proportion of chain-end carboxyl groups, potentially raising acid value. Broad distribution may lead to inconsistent acid value readings.
Oxidation Level: Oxidation during production or storage generates new carboxylic acid groups, directly increasing acid value—a common cause of quality degradation.
Raw Material Purity: Impurities or unreacted acidic components (e.g., in stearic acid or maleic anhydride) can elevate the final product’s acid value.
Storage Conditions: High temperature, humidity, and UV exposure promote hydrolysis or oxidation, causing acid value to rise over time—indicative of product aging.
3. Summary of Core Principles
Acid value is not an isolated parameter, but a systemic indicator closely linked to molecular structure, processing behavior, and end-use performance. Its impact on lubrication type (internal vs. external), compatibility with resins, and thermal stability must be evaluated in context.
In practice, selecting the optimal acid value requires aligning with specific processing conditions and performance goals, while considering interrelated factors such as molecular weight, oxidation resistance, and storage stability. Precise matching of acid value enables balanced optimization of processing efficiency, surface quality, and long-term product reliability.
