There is a correlation between the hardness and viscosity of liquid silicone, but it's not a simple linear relationship. Changes in the hardness are influenced by multiple factors, including silicone type, composition adjustments, and processing techniques. The following analysis focuses on four dimensions: core correlation, type differences, composition adjustments, and processing influences:
Core Correlation: Viscosity and Hardness Show a Trending Connection
In most cases, the viscosity and hardness of liquid silicone exhibit a positive correlation. For example, for condensation-type mold silicone, as the hardness ranges from low to high, the viscosity initially increases and then decreases, forming an arched curve. This means that in the lower hardness range, the viscosity increases with increasing hardness. After reaching a certain critical point, the viscosity decreases with further increases in hardness. This trend reflects the changing arrangement of the silicone's molecular structure at different hardness levels: at low hardness, the molecular chains are looser, resulting in lower viscosity. As hardness increases, the molecular chains become more cross-linked, leading to higher viscosity. However, when cross-linking is excessive, causing the molecular chains to break, the viscosity decreases.
Type Differences: Viscosity-Hardness Characteristics of Addition- and Condensation-Type Silicones
Liquid silicones are primarily categorized into two types: addition- and condensation-type silicones. Their viscosity-hardness relationships differ significantly:
Addition-type silicones typically have lower viscosities (below 10,000 cP), and at the same hardness, their viscosity is significantly lower than that of condensation-type silicones. For example, the viscosity of food-grade addition-type silicones can be adjusted by adding silicone oil, but hardness adjustment primarily depends on the hydrogen content of the hydrosilicone oil-the higher the hydrogen content, the greater the hardness. Every 2% addition of dimethyl silicone oil can reduce hardness by 1 degree, but excessive addition can lead to surface stickiness and reduced performance after curing.
Condensation-type silicones exhibit an arc-shaped viscosity-to-hardness curve, and their overall viscosity is higher than that of addition-type silicones. For example, pad printing silicones require high hardness (approximately 50 degrees) and low fluidity, resulting in higher raw material viscosity and the need for dilution with silicone oil to adjust the hardness. Composition Adjustment: The Impact of Base, Fillers, and Additives
The viscosity and hardness of liquid silicone rubber can be precisely controlled by adjusting the composition:
Base Viscosity: 107 base rubber is the primary component of addition-forming silicone rubber. Its initial viscosity (e.g., 10,000, 20,000, or 50,000 cP) directly affects the viscosity of the final product. Bases of varying viscosities can be used to match silicone rubbers with varying performance requirements.
Filler Function: Silica (siloxane), as a performance filler, significantly improves the tear strength, tensile strength, and elongation of silicone rubber. However, excessive filler content can increase viscosity, necessitating adjustments to the formulation to balance performance and flowability.
Additive Adjustment: Dimethyl silicone oil is a commonly used viscosity modifier, generally not exceeding 10%. A moderate addition can reduce viscosity, but excessive amounts can disrupt the silicone rubber's cross-linking structure, resulting in decreased hardness and performance degradation.
Process Influence: The Impact of Curing Conditions and Processing Methods
The viscosity and hardness of liquid silicone rubber are also affected by curing conditions and processing methods:
Curing Time and Temperature: Heat curing can shorten the vulcanization time, but excessive temperatures may lead to excessive cross-linking, increasing hardness and decreasing viscosity. For example, automotive oil pipe seals must maintain stable performance within a temperature range of 40°C to 200°C, requiring a precise balance between hardness and viscosity through the curing process.
Processing Methods: Liquid silicone rubber can be molded into ultra-thin layers (e.g., contact lenses) as thin as 0.01mm through injection molding and spray molding. However, solid silicone rubber requires an open mixing process, with a minimum molding thickness of 0.3mm. Different processing methods require different silicone fluidity (viscosity) and hardness, necessitating tailored formulation adjustments.