Rheology and Dispersibility of Masterbatch: Analysis of Correlation Patterns

On August 29, in the context of the polymer materials industry continuously advancing towards high-end and functional development, masterbatches serve as the key carriers that combine pigments/functional additives with the resin matrix. Their quality directly affects the performance and processing stability of downstream products. Throughout the entire process of masterbatch research, production, and application, rheological properties play an indispensable core role — they not only directly reflect the processing behavior of masterbatches but also serve as sensitive indicators of the dispersion state of pigments/functional additives and the compatibility between masterbatches and the base resin. A deep understanding and precise testing of masterbatch rheological properties have become crucial measures for enterprises to optimize formulations, improve product quality, and ensure processing stability.

From the perspective of industry practice, the importance of the rheological properties of masterbatches is mainly reflected in three dimensions. Firstly, they directly impact processing performance. Rheological data such as Melt Flow Rate (MFR/MVR) and apparent viscosity directly determine the ease of flow of the masterbatch in equipment like extruders and injection molding machines, as well as the filling effect and the required processing pressure and temperature. Abnormal rheological behavior can easily cause extrusion fluctuations, unstable output, surface defects in products, increased energy consumption, and even equipment damage. Additionally, through time sweep testing, the change in melt viscosity of the masterbatch over time at processing temperatures can be evaluated, effectively assessing its thermal degradation risk and providing a basis for setting processing parameters.

According to the Masterbatch Industry Network, the next aspect is the "probe" role of dispersion and microstructure. Poorly dispersed pigment or additive agglomerates can significantly increase melt viscosity like "fillers" and alter its viscoelastic behavior. If the pigment/additive has poor compatibility with the resin matrix and weak interfacial adhesion, the energy dissipation mode during melt flow will change, which can be directly reflected in the rheological curve. For special systems such as conductive carbon black masterbatches and high-content flame retardant masterbatches, rheology is an effective means to detect their percolation threshold and network structure formation, helping enterprises grasp the characteristics of the system's microstructure. Finally, rheological properties can also predict the performance of the final product — good rheological properties usually imply uniform dispersion of pigments/additives and good compatibility between the masterbatch and the matrix resin, which is the basis for downstream products to achieve excellent mechanical properties, appearance quality, and functional stability. Conversely, abnormal rheology is often an early warning signal of performance defects in the final product, helping companies identify problems in advance.

In the field of rheological performance testing of masterbatches, capillary rheometers and rotational rheometers are two core devices. Although their testing principles differ, they provide complementary key information. First, let's look at the capillary rheometer. Its testing principle involves forcing the molten masterbatch through a long, narrow capillary die under a set temperature using pressure. By measuring the pressure drop and volumetric flow rate, shear stress, shear rate, and apparent viscosity can be calculated. The core testing contents of this device include flow curve plotting, entrance pressure drop/Bagley correction, and determination of the melt fracture critical point. The flow curve (apparent viscosity η vs shear rate γ̇) reflects viscosity characteristics at high shear rates (simulating actual processing conditions such as extrusion and injection molding; overly high or low viscosity can affect processing and product strength), shear thinning index (indicating the melt's sensitivity to shear rate, poor dispersion can alter this behavior), and low shear rate viscosity/zero shear viscosity (η₀) trends (sensitive to fillers/agglomerates, excessively high η₀ can lead to difficulties in melt conveyance during early processing). Entrance pressure drop/Bagley correction can reflect the melt's elasticity and true shear stress; poor dispersion or compatibility can cause an abnormally high entrance pressure drop. The melt fracture critical point measures the critical shear stress or shear rate when surface distortions (such as shark skin or melt fracture) appear on extrudates; poor dispersion can lower the critical value, narrowing the processing window. However, capillary rheometers also have limitations, such as difficulty in conducting low-frequency/low shear rate tests and the inability to directly measure elastic modulus and other viscoelastic parameters.

The rotational rheometer operates on the principle of applying controlled strain or stress to a melt sample at a set temperature. By measuring the stress or strain response of the sample, rheological data is obtained, often using parallel plate or cone-and-plate fixtures. Core test items include dynamic frequency sweep, dynamic strain/stress sweep, steady-state rotational tests, and thixotropy tests. Dynamic frequency sweep (applying small-amplitude oscillatory strain/stress within the linear viscoelastic region) can measure the storage modulus (G'), loss modulus (G''), and loss factor (tanδ = G''/G') as they vary with angular frequency (ω). A plateau or reduced slope of G' in the low-frequency region may suggest the formation of a filler network structure. The plateau value of complex viscosity (η*) at low frequencies can more accurately estimate zero-shear viscosity (η₀), while tanδ reflects the ratio of viscosity to elasticity of the melt. Dynamic strain/stress sweep can determine the linear viscoelastic region (LAOS) range and yield stress/strain, where a narrowed linear region indicates a fragile system structure, and significant yield stress is a signal of poor dispersion and difficult processing initiation. Steady-state rotational tests, similar to the capillary method, can measure the curve of viscosity as it changes with shear rate, but with higher precision in the low shear region. Thixotropy tests assess the network structure's rebuilding ability by observing the recovery behavior of viscosity under constant shear; slow recovery may indicate poor compatibility or weak agglomeration. The advantages of the rotational rheometer lie in its ability to precisely measure rheological behavior at low shear rates/frequencies, directly obtain viscoelastic information, be sensitive to microstructural changes, and measure yield stress. However, at high shear rates, it is prone to edge effects and secondary flows, which can reduce test accuracy, and it requires stringent sample preparation.

It is worth noting that there is a clear correlation between the rheological properties of masterbatches and the dispersibility of pigments/additives. From the perspective of viscosity change, inadequately dispersed pigment/additive agglomerates are equivalent to additional solid particles, which can significantly increase the resistance to melt flow, resulting in a noticeable increase in low shear viscosity (η₀) and viscosity at low shear rates/frequencies. In some cases, the capillary high shear viscosity may also increase accordingly. From the perspective of elastic characteristics, agglomerates hinder the movement of polymer chains, enhancing the melt's elastic energy storage capacity, which is reflected in the increase of the storage modulus (G'), particularly more obvious in the low-frequency region. It may also lead to an increase in the entry pressure drop in capillary tests. From the perspective of yield behavior, if weak network structures form between agglomerates or between agglomerates and molecular chains, the masterbatch melt may exhibit or enhance yield behavior, requiring a certain stress to break the network and allow flow. The appearance of yield stress is a typical characteristic of severe poor dispersion. From the perspective of the linear viscoelastic region, network structures formed by agglomerates are usually fragile and can be destroyed by small strains, resulting in a narrower linear viscoelastic region. From the perspective of thixotropy, in poorly dispersed systems, the speed of weak agglomerate network reconstruction after destruction may slow down or speed up, directly reflecting changes in thixotropic recovery behavior. Additionally, the presence of agglomerates may alter the melt's sensitivity to shear response, leading to changes in shear thinning behavior. Understanding these correlation rules can help companies quickly assess masterbatch dispersibility through rheological testing, providing precise directions for formulation optimization and process adjustment.