Understanding the curing speed of silicone rubber can help you plan production distribution reasonably and meet customer demands. If you don’t control the curing time of silicone rubber, you may encounter problems with poor molding, resulting in customer loss. This article will introduce three curing systems and the factors that affect the curing speed.
3 Curing Systems and Curing Speed Features
Tin Catalyzed Curing
Tin catalyzed curing is a type of condensation curing. It is mainly used for RTV-1 and RTV-2 (room temperature vulcanization silicone). Moreover, tin catalyzed systems are primarily moisture-activated and are less dependent on elevated temperatures.
RTV systems can cure over a broad temperature range, but curing proceeds most efficiently under typical room-temperature conditions with adequate humidity.
RTV-1
RTV-1 mainly relies on water for curing and the curing process is relatively slow. If you use RTV-1 to produce silicone rubber products, it usually takes 24 hours or even longer for your products to fully cure and reach their performance level. This curing process occurs from the outside towards the inside.
Under the conditions of 25°C and 50% relative humidity, you will find that RTV-1 will cure approximately 2 to 3 millimeters every 24 hours.
RTV-2
Unlike RTV-1, RTV-2 requires a chemical reaction between two components to cure, and the curing process is faster. Under the correct ratio and at a room temperature of 25°C, RTV-2 systems typically become tack free within 1-4 hours and may require 8-72 hours to reach full cure,depending on the formulation and catalyst concentration.
However, you might have used the same ratio. Under a temperature of 10°C, it takes 3 to 5 hours for the surface of RTV-2 to be tack-free. And the complete curing time will also increase, reaching 24 hours or even longer.
Platinum Catalyzed Curing
Platinum curing is also known as addition curing, and it mainly uses platinum catalysts. Platinum catalyzed addition curing is widely used in LSR, HCR and RTV systems. During the entire process, there are no peroxide decomposition by-products, which can reduce the contamination risk of your silicone rubber products and reduce downtime.
Although platinum curing can also be carried out at room temperature, it takes a longer time and complete curing may require 8 to 24 hours. At curing temperatures ranging from 120°C to 200°C (248°F to 392°F), the silicone rubber product may be cured within 30 seconds to 5 minutes.
However, the curing speed will vary depending on the specific silicone rubber formula you use. So if you have an urgent order, you can increase the processing temperature during the production process, thereby enhancing the production efficiency.
UV Curing
Compared to the previous two curing systems, UV curing is a relatively new technology. UV-curable silicone can be used in specialized injection molding applications, although platinum-cured LSR remains the dominant technology in mainstream silicone injection molding.
Speed Features
This curing process mainly uses photo-sensitive catalysts. You can use it to increase the curing speed of silicone rubber. The silicone rubber formulations used in this curing process contain photoreactive functional groups that crosslink when exposed to ultraviolet light. The curing time you spend can be shortened to 0.5 seconds.
However, the specific time depends on your production line speed, process temperature and ultraviolet intensity. Compared with traditional curing methods, the final performance change rate of your UV curing silicone rubber products can be controlled within 5%, and the curing speed is not affected by the transparent or light-colored silicone rubber prouducts.
For example, for relatively thick and transparent silicone rubber products, the traditional curing method may take 30 minutes. With UV curing, it might only take you 5 minutes.
Factors Affecting the Curing Speed of Silicone Rubber
Types and Formulations of Silicone Rubber
The curing speed of different types of silicone rubber varies naturally. Moreover, factors such as the type, proportion and addition time of the curing agent in the formula you use will all affect the curing speed.
Humidity
Humidity has a certain impact on the curing speed of silicone rubber, especially for RTV-1. If during the curing process, the relative humidity of the environment is lower than 30%, you will notice a decrease in the curing speed.
Processing Temperature
Temperature plays a crucial role in the curing process of silicone rubber, and it significantly affects the curing speed of the silicone rubber. Generally speaking, the higher the temperature, the faster the curing speed; the lower the temperature, the slower the curing speed. And, temperature has an impact on different curing systems.
During the platinum curing process, for every 10°C increase in the curing processing temperature, the curing rate of your silicone rubber product can be approximately enhanced by 20% to 25%. During the curing process of RTV-2, as the temperature drops, you will notice that the curing speed slows down.
However, you should be aware that do not increase the amount of curing agent while raising the curing temperature. Doing so will prevent you from effectively controlling the temperature increase. Moreover, an excessively high temperature will increase the risk of degradation of the silicone rubber product.
Product Thickness
The thickness of your designed product will also affect the curing speed of the silicone rubber. Silicone rubber has poor thermal conductivity. If your silicone rubber component is relatively thick and the initial reaction temperature is low, the heating rate will also slow down accordingly.
However, after a period of reaction, the heat generated during crosslinking may accumulate inside thick sections. Therefore, when it comes to the addition curing, you need to consider the internal heat dissipation of the thick product to prevent the rapid surface curing from causing incomplete internal curing.
The curing time will still increase as the thickness increases. When using the condensation curing method for production, if the thickness exceeds 1/4 inch, curing of the RTV-1 will slow drastically, as the material cannot receive sufficient moisture for crosslinking.
Dosage of Catalyst or Crosslinker
Adding catalysts or crosslinkers can accelerate the curing process. Increasing catalyst concentration generally accelerates crosslinking reactions by increasing the reaction rate.
However, there is an upper limit to the amount of catalyst or crosslinking agent used. You cannot add too much blindly. Sometimes, excessive catalysts will cause the temperature to rise too high, and the silicone rubber will degrade.
This will result in a decline in the mechanical properties of your silicone rubber products, shortened product lifespan, and negatively impact the brand reputation.
So you also need to analyze the impact of temperature on the curing speed. Besides adjusting the dosage of the curing agent, in platinum curing you also need to balance the catalyst dosage with the temperature setting. It is not recommended to increase both at the same time. Avoiding over-control to affect the product quality.
How to Choose Silicone Rubber According to Its Curing Speed?
Applications
You can make a choice based on the different requirements for the curing speed of the silicone rubber for various applications.
Mold Making
If you plan to use silicone rubber for mold making, we recommend choosing platinum-cured RTV-2 silicone. Because platinum-cured silicone has a low shrinkage rate, it can precisely reproduce fine details and better meets the requirements of mold making.
As for the curing speed, in order to have sufficient time to complete all the details, we suggest that you can reduce the temperature or decrease the concentration of the catalyst to extend the curing time.
Medical Device Manufacturing
If you are looking to manufacture medical-related silicone rubber products such as tubes and seals, we still recommend using medical-grade silicone rubber that is platinum-cured. This type of silicone rubber can be quickly cured through injection molding. It also has certain advantages for creating complex geometries in medical devices.
In addition to being biocompatible, the low volatile content of platinum-cured silicone rubber is more conducive to meeting the requirements of medical-grade silicone rubber.
Electronics and Semiconductor Packaging
If you want to use silicone rubber for vibration reduction and moisture protection in electronics and semiconductors, we recommend using addition curing RTV-2.
Because addition curing RTV-2 has almost low shrinkage and no corrosive by-products, it will not damage electronic components and semiconductors. Moreover, you can reasonably control the curing time to ensure that the silicone rubber flows into every tiny cavity, avoiding air stagnation and dielectric failure.
Production Efficiency and Cost
The curing speed of silicone rubber also affects your production efficiency and energy consumption, and the costs vary as well. The cost you spend is related to the final profit, so choosing a silicone rubber with high production efficiency and relatively lower cost is also very important.
Generally speaking, a faster curing speed can reduce the demolding time you spend and thereby increase the daily output. Therefore, UV curing and platinum curing systems are more recommended.
The cost comparison of the three curing systems can be referred to in the following table:
| Cost Factor | Condensation (Tin Curing) | Addition (Platinum Curing) | UV Curing |
| Raw Material Cost | Low | High (platinum catalyst) | Medium-High (photoinitiators) |
| Capital Equipment | Low | Medium | Medium-High (UV lamps, conveyors) |
| Energy/Operating Cost | Low (room temp curing) | Medium (heating required for speed) | Low-Medium (electricity for lamps) |
| Process Sensitivity | Low (forgiving) | High (contamination risk) | Medium |
| Scrap/Rejection Risk | Low | High if contaminated | Low |
| Maintenance Cost | Low | Low | Medium (bulb replacement) |
FAQ
Why Won’t Platinum Curing Silicone Rubber Cure Even After Long Time?
Perhaps it is due to the inhibition of curing. Platinum catalysts are prone to being contaminated by sulfur compounds, nitrogen compounds, and heavy metals, so you need to keep the factory environment clean. Before conducting production and processing, all equipment, molds, and substrates should be thoroughly cleaned.
How Do Acetoxy and Alkoxy Condensation Silicones Differ in Curing Speed?
The complete curing time of Alkoxy condensation silicone is longer than that of Acetoxy condensation silicone. Generally speaking, the curing time of the Acetoxy system is 12-24 hours; while that of the Alkoxy system is 24-72 hours.
However, the curing of Acetoxy condensation silicone will produce acetic acid, which may corrode sensitive substrates such as copper or electronic components. Alkoxy condensation silicone is relatively safer.
Is Faster Curing Speed Always Better for Silicone Rubber?
No. Although the fast curing speed may increase your production efficiency, it could also lead to bubble residue, rushed pouring process and reduced detail quality, which would actually increase defects rather than improve efficiency.
Does Silicone Rubber With Faster Curing Speed Always Lower Per-unit Product Cost?
Not necessarily. It needs to be determined based on your order situation. The cost of the condensation curing raw materials is relatively low, and it may have an advantage in small batch orders. However, in large-scale applications, the prices of platinum or UV curing may decrease due to the improvement in production efficiency.
Why Is the Curing Time on Datasheets Different From Actual Production Time?
The curing time of the silicone rubber in the datasheet was measured under ideal laboratory conditions. If your actual curing conditions are the same as those in the laboratory, it is possible that your silicone rubber material has been left for too long, and the catalyst has degraded over time, resulting in a slower curing speed.
Final Thoughts
Legensilico is a professional manufacturer of silicone rubber. We can provide you with a variety of cured silicone rubber products.
If you are interested in our products or want to customize products with your own brand characteristics, please feel free to contact us. We can produce silicone rubber products according to your color, specifications, and logo requirements.
