Analysis of the Impact of KRL Testing on Polymer Additives in Lubricating Oils

I. Core Mechanism of the KRL Test

The KRL test (CEC L-45-A-99 standard) evaluates the shear stability of polymer additives (e.g., viscosity index improvers VIIs, anti-wear agents) in lubricants by simulating high-shear stress environments (e.g., gearboxes, hydraulic systems).

Its core mechanisms are as follows:

1. Mechanical Shear Degradation: - During the test, tapered roller bearings operate at 1500 rpm under a 5000 N load, generating shear rates as high as 10⁶~10⁷ s⁻¹, causing the fracture of long-chain polymer molecules (reducing molecular weight). For example, after the molecular chains of polymethacrylate (PMA)-based VIIs break, the viscosity index (VI) of the lubricant decreases significantly.

2. Quantification of Viscosity Loss: - The viscosity loss rate is calculated by measuring the change in the lubricant's kinematic viscosity before and after the test (ASTM D445 standard). Typical requirement: The viscosity loss rate for hydraulic oils must be ≤15% (SH/T 0845 standard).

II. Specific Impact on Polymer Additive Performance

1. Viscosity Index Improvers (VIIs): - Shear-sensitive polymers (e.g., polyisobutylene OCP) experience a 30%~50% reduction in molecular weight during the KRL test, leading to decreased high-temperature high-shear (HTHS) viscosity and affecting oil film thickness. Stable polymers (e.g., star-shaped PMA) exhibit stronger shear resistance due to their branched structure, with viscosity loss rates controllable within 8%.

2. Anti-Wear/Friction-Reducing Additives: - The adsorption films of some polymer additives (e.g., polyether esters) are disrupted under high shear, resulting in an increased friction coefficient (e.g., from 0.08 to 0.12). However, composite polymers containing nanoparticles (e.g., Zr-MOFs) maintain low wear rates (wear volume reduced by ≥90%) due to the "rolling bearing effect."

3. Dispersants and Pour Point Depressants: - Shear degradation may weaken the dispersant's ability to suspend soot, increasing the risk of sludge formation.

III. Application Significance and Improvement Directions

1. Additive Formulation Screening: The KRL test is a key method for screening highly stable VIIs. For example, by switching to hydrogenated styrene-isoprene copolymer (HSD) in a PAO-based full synthetic oil, the viscosity loss rate decreased from 18% to 7%.

2. Lubricant Life Prediction: Test data can be correlated with oil aging models under actual operating conditions. For instance, a wind turbine gear oil with a KRL test loss rate ≤10% corresponds to an extended service life of over 5 years.

3. Development of New Additives: Based on KRL test results, developing shear-stable polymers (e.g., hyperbranched structures, nanocomposite additives) has become a trend. For example, PMA additives modified with graphene show only a 5% viscosity loss rate and reduced friction coefficient.

IV. Industry Standards and Testing Recommendations

1. Core Standards: CEC L-45-A-99: Specifies the speed, load, and temperature parameters for the KRL test. ASTM D6278: A supplementary method for shear stability testing.

2. Testing Optimization Recommendations: Gradient Loading: Gradually increase the load (500N→5000N) to observe degradation thresholds under different shear intensities. Multi-Temperature Testing: Compare results at 40°C (conventional) and 100°C (extreme) to evaluate temperature-shear coupling effects.

The KRL test, by simulating extreme shear environments, directly reflects the shear resistance and functional durability of polymer additives in lubricants, making it a core evaluation tool for optimizing additive molecular design and enhancing oil performance. Future efforts should integrate molecular dynamics simulations with experimental data to develop smarter polymer systems