Views: 1 Author: Site Editor Publish Time: 2023-07-07 Origin: Site
Methods for Determining Plasticity:
This method includes the Williams method, the Wallace Rapid Plasticity method, and the De Mattia Plasticity method. These methods measure the plasticity of raw rubber and compounded rubber and are conducted at a constant temperature since plasticity varies with temperature.
a. Williams Method: It evaluates plasticity based on the change in height of a cylindrical sample (Φ=16mm, h0=10mm) after a specified time under a constant load at a controlled temperature (T=70±1°C or 100±1°C). The compression deformation and recovery deformation are measured to calculate the plasticity (P) using the formula.
b. Wallace Rapid Plasticity Method: Similar to the Williams method, it measures plasticity based on the change in thickness of a rubber sheet under constant temperature, load, and time.
c. De Mattia Method: Plasticity is represented by the required load when compressing a sample to a specified height under fixed temperature and time.
Mooney Viscosity Method (Rotational Shear Method):
This method determines plasticity based on the torque experienced by a sample during deformation between a rotating rotor and a stationary chamber. The Mooney viscosity, indicated by the Mooney Scorch value (MS), is measured using a Mooney viscometer. The test involves preheating the sample, rotating the rotor for a specified time at a controlled temperature (e.g., 100°C), and measuring the torque.
This method involves using a capillary rheometer to measure the extrusion rate of the rubber compound under specific temperature, pressure, and die conditions. The plasticity is expressed in terms of the milligrams of extruded material per minute. This method provides insights into the influence of plasticity on the extrusion performance.
The plasticizing processes for rubber include preparation processes and various machine-based plasticizing techniques such as open mill mixing, internal mixer mixing, and screw mixing.
Drying: Rubber is dried at specific temperatures and durations to remove moisture, typically at 50-60°C for NR and 24-40°C for CR.
Cutting: Raw rubber is cut into small pieces using a cutting machine, with weights varying based on the rubber type (e.g., 10-20kg for NR).
Breaking: Rubber blocks are broken using a breaking machine to facilitate plasticizing. The roller gap is generally set at 2-3mm, and the temperature is controlled below 45°C.
Machine-Based Plasticizing Processes:
Open Mill Mixing: In this process, the rubber compound is placed between two counter-rotating rolls, and the material is repeatedly sheared, mixed, and squeezed to achieve plasticizing. Different techniques such as the encased plasticizing method and thin-pass plasticizing method can be employed.
Internal Mixer Mixing: This method uses internal mixers with rotors to subject the rubber to intense mechanical shear, resulting in plasticizing. Different plasticizing methods like the one-stage plasticizing method, staged plasticizing method, and chemical plasticizing method can be utilized.
Screw Mixing: Screw-type plasticizing machines utilize the shearing and mixing action of rotating screws to achieve plasticizing. It is commonly used for large-scale production in tire manufacturing. However, it can generate higher temperatures and irregular extrusion, requiring additional equipment like compression molding machines or pelletizing machines for further processing.
After plasticizing, additional processing steps may include pressing or pelletizing, cooling and drying, storage, and quality inspection.
Plasticizing Characteristics of Common Rubbers:
Natural Rubber (NR): NR is relatively easy to plasticize. Different types of NR may have varying plasticizing characteristics, with some requiring plasticizing and others not. Plasticizing is commonly performed using open mill mixing at temperatures around 40-50°C or through thin-pass plasticizing. Chemical plasticizing agents can also be added to enhance plasticity.
Styrene-Butadiene Rubber (SBR): Soft SBR typically does not require plasticizing, as it has a relatively low initial Mooney viscosity (54-64). Mechanical plasticizing methods are less effective for SBR, and high-temperature plasticizing (130-140°C) is preferred, although excessive temperatures can lead to gelation.
Butadiene Rubber (BR): BR generally does not require plasticizing.
Chloroprene Rubber (CR): CR has low initial Mooney viscosity and does not require plasticizing. However, during storage, plasticity may decrease, necessitating plasticizing to obtain the desired properties. Open mill mixing at temperatures below 155°C, particularly the thin-pass plasticizing method, is suitable for CR.
Isobutylene-Isoprene Rubber (IIR): Some IIR varieties with Mooney viscosity between 38 and 75 may not require plasticizing. Mechanical plasticizing methods have limited effects, but using internal mixers at temperatures above 120°C with the addition of plasticizing agents can achieve favorable results.
Nitrile Rubber (NBR): NBR plasticizing is typically performed using open mill mixing at temperatures below 40°C, with a small roll gap (around 1mm) and reduced capacity compared to NR. However, open mill mixing is not preferred for NBR as it can easily generate gelation.
It's important to note that plasticizing processes and characteristics can vary depending on specific rubber formulations, additives, processing conditions, and desired properties.