The viscosity of liquid silicone rubber is influenced by a variety of factors, which regulate its flowability by altering its molecular structure, interactions, or physical state. The following is a detailed analysis of the main influencing factors:
1. Molecular Weight and Structure of the Base Polymer (Rubber Base)
Molecular Weight: A higher molecular weight of a rubber base results in longer molecular chains and more entanglements between chain segments, significantly increasing viscosity. For example, the viscosity of 107 rubber base ranges from 10,000 cP to 50,000 cP, with higher molecular weight indicating a higher initial viscosity.
Molecular Structure: Straight-chain silicone rubbers have lower viscosity, while branched or cross-linked structures hinder molecular flow and increase viscosity. For example, the viscosity of vinyl-containing addition-type silicone rubbers is more significantly affected by crosslink density due to their highly reactive molecular chains.
2. Filler Type and Content
Filler Types:
Silica (fumed or precipitated): As a reinforcing filler, it significantly improves the tear and tensile strength of silicone rubber, but significantly increases viscosity. Fumed silica has a larger surface area and a more pronounced viscosity-increasing effect. Calcium carbonate and silica powder: As inert fillers, they have little effect on viscosity, but excessive addition can increase viscosity due to particle accumulation.
Filler content: The higher the filler content, the greater the viscosity. For example, pad printing silicone requires a high hardness (around 50 degrees), and with a high filler content, the raw material viscosity is significantly higher than that of ordinary silicone.
3. Addition of plasticizers (diluents)
Silicone oil: Dimethyl silicone oil is a commonly used plasticizer that reduces silicone viscosity. The addition ratio should generally not exceed 10%. Excessive addition can lead to surface stickiness, decreased hardness, and deterioration of mechanical properties after curing.
Other diluents, such as low-viscosity vinyl silicone oil, can be used to specifically adjust the viscosity of addition-type silicone without affecting the curing reaction.
4. Crosslinkers and Curing Systems
Crosslinker Types:
Hydrogenated silicone oil (addition-type silicone): The higher the hydrogen content, the greater the crosslink density and the higher the hardness after curing. However, the uncured viscosity may increase due to the tendency of the molecular chains to crosslink.
Organotin catalyst (condensation-type silicone): The catalyst concentration affects the cure rate, but excessive addition may cause localized over-crosslinking, resulting in an abnormally high viscosity. Curing Conditions:
Temperature: Increasing the temperature can reduce viscosity (viscosity decreases by approximately half for every 10°C increase in temperature), but the curing temperature must be controlled to avoid excessive crosslinking.
Time: Excessive curing time may lead to further crosslinking of the molecular chains, increasing viscosity.
5. Molecular Chain End Groups and Reactivity
Reactive Groups: Silicones containing vinyl, hydroxyl, or hydrogen groups at the molecular chain ends have varying viscosities due to varying inter-segment interactions. For example, silicones with a high vinyl content have lower viscosity but higher hardness after curing.
Capping Agents: Using different capping agents (such as hexamethyldisiloxane) can adjust the activity of the molecular chain ends, thereby affecting viscosity.
6. Processing and Mixing Methods
Stirring Speed and Time: High-speed stirring can introduce air and disrupt the molecular chain alignment, causing a temporary increase in viscosity. Thorough mixing ensures uniform dispersion of the filler and avoids localized viscosity abnormalities.
Degassing: Undegassed silicone may have an apparent viscosity lower than the actual value due to the presence of bubbles, but these bubbles will weaken mechanical properties after curing. 7. External Factors
Temperature: Increasing ambient temperature decreases viscosity (in accordance with the Arrhenius equation), but the effect of temperature on the curing reaction should be considered.
Shear Force: Liquid silicone is a non-Newtonian fluid, and its viscosity decreases with increasing shear rate (shear thinning), making it suitable for high-shear processes such as injection molding and spraying.
Examples of Viscosity Control in Practical Applications
Low Viscosity Requirements: For example, in contact lens injection molding, an addition-type silicone with a viscosity below 10,000 cP is required. This is achieved by selecting a low molecular weight base and a small amount of silicone oil.
High Viscosity Requirements: For example, in electronic component potting compounds, high viscosity is required to prevent flow. This can be achieved by adding a high proportion of silica or using a branched base.
Viscosity-Hardness Balance: Pad printing silicone requires both high hardness (50 degrees) and medium viscosity. This is achieved by optimizing the ratio of silica to silicone oil.