How to Apply Silicone: A Complete Technical Guide

For the purchasers and manufacturers of the silicone industry, even a minor mistake during the application of silicone can lead to problems with the products and damage the brand’s reputation. We will provide a detailed description of a series of steps involving the application of silicone. This will help you avoid incorrect usage of silicone and increase the utilization rate of the material.

Different Types of Silicone

RTV Silicone (Room Temperature Vulcanizing)

If you are involved in the fields of building curtain walls, window sealing, electronic potting, or mold manufacturing, you must be familiar with RTV silicone. This silicone comes in two types: RTV-1 and RTV-2.

RTV-1: This one-component silicone can cure by coming into contact with moisture in the air. It is mainly used in glass adhesives, sealants, and electronic fixing adhesives.

RTV-2: This type of silicone needs to be mixed in two parts: part A and part B, and then cured. It is suitable for deep curing scenarios, such as mold silicone and electronic potting.

LSR (Liquid Silicone Rubber)

This type of silicone is catalyzed by platinum and undergoes an addition reaction for vulcanization. Moreover, no by-products are produced. LSR is highly suitable for you when manufacturing medical supplies, baby pacifiers, diving masks, precision seals, electronic accessories, and other products.

HTV/HCR

This type of silicone needs to be vulcanized in a high-temperature and high-pressure environment through a mold. It is usually used to make sealing rings, silicone tubes, silicone accessories, buttons, kitchenware. If you are an importer or factory of auto parts, electronics, home appliances, or hardware products, you should pay more attention to this type.

How to Prepare for Silicone Application?

Storage Conditions for Uncured Silicone

RTV-1 should be stored in a cool and dry environment (with a temperature no higher than 30 ℃), and it can usually be kept for 6 to 12 months. RTV-2 should be stored in a sealed container in a dry room environment and kept away from moisture. It can be stored for 12 months typically.

For LSR, you should be cautious. This type of silicone needs to be protected from light and stored at a temperature no higher than 25 ℃. Moreover, its shelf life is relatively short, only 180 days.

If your factory contains strong acids, especially hydrofluoric acid, be sure to keep the silicone away from it. Because hydrofluoric acid is able to dissolve silicone. And it is necessary to avoid coming into contact with compounds containing elements such as nitrogen, sulfur, phosphorus and tin. Otherwise, these may lead to incomplete sulfurization or poor results.

Surface Preparation for Silicone Bonding

If the surface energy of the base is too low, it may result in poor adhesion. To ensure a firm adhesion to metal, glass, plastic or concrete substrates, it is necessary to perform surface cleaning beforehand. If you want to save just $0.01 on surface cleaning costs, it could lead to $100,000 product recall.

First, perform chemical solvent cleaning work, which will remove most of the contaminants. Before you start large-scale production, please make sure to conduct compatibility tests to verify whether the solvent is compatible with your substrate. The table below lists the commonly used cleaning solvents for industrial silicone applications.

Second, you can use a clean cloth with no lint and dip into the cleaning solution. Then, wipe in one direction to prevent the dirt from reattaching to the surface. You need to wait for 2 to 5 minutes at room temperature for the solvent to completely evaporate. Please do not use compressed air to dry it.

When you find the surface is clean and no more water droplets appear after the spraying, it means the cleaning work will be finished.

Third, if you use smooth with low surface substrates like some plastic products (PP, PE, PTFE), you’d better sand the surface. It can make microscopic anchoring points for the silicone to embed into. This step is not necessary and can be determined based on your specific circumstances.

Primer Necessity for Silicone Adhesion

Primer is a thin chemical film coated between the cleaned substrate and the silicone. It is capable of forming chemical bonds with both surfaces, significantly enhancing the bonding strength. Whether the specific base material requires the use of primer, you can refer to the table below.

For non-safety-critical applications, you can leave it out to save costs; but if you manufacture components that require sealing or are capable of withstanding load, the primer is absolutely necessary.

Pre-purchase Sample Testing

We suggest that you request the supplier to provide samples before making a large-scale purchase, so as to conduct a small-scale test. This test not only enables comparisons of various silicone products, but also allows for the preservation of test samples, serving as the basis for batch tracing and preventing unnecessary losses.

Silicone Application: 5-Step Procedure

Step 1: Select an Appropriate Dispensing Equipment

When you are involved in the construction industry or conducting tests or maintenance in the R&D department, it is recommended to use a manual glue gun. This option is cost-effective and easy to carry.

If this is your first time introducing a dispensing machine, you can choose an air-powered dispensing machine. It has a moderate investment cost and can also reduce the waste rate. This air-powered dispensing machine is also suitable for general sealing and home appliance industries.

If you are involved in the field of automotive components or electronics industry, then we recommend the desktop 3-axis dispensing machine.

We recommend that importers with long-term and stable orders or those in the medical, precision electronics, or EV battery industries should use fully automated systems directly. This way, the overall cost over the long term will be lower.

Step 2: Apply

Continuous Gluing Technology: When using a manual glue gun, you should maintain an angle of 45°to 60°, move at a uniform speed of 5-10 cm/s, apply force evenly and retract the glue gun. Avoid pauses, stringing and dripping.

For the automatic glue dispenser, you can set the matching speed and glue output, decelerate at corners, activate the anti-drip and suction functions to guarantee the glue line is uniform and without accumulation or breakage.

Avoid Bubbles: Bubbles mainly originate from the interior of the rubber compound, air entrapped during the application of the coating, high-speed dispensing vortex, and the concave areas of the substrate. They can be removed through centrifugation, static settling, vacuum degassing tanks, or online degassing tubes.

Before your official production, you should test the glue line on the transparent substrate to check for bubbles. If bubbles continue to appear, it is necessary to verify whether there is a leakage in the feed system and address the issue promptly.

Control the Width of the Adhesive Line: Its width is decided by the inner diameter of the nozzle, the applied pressure and feed rate of dispensing, the height of the nozzle and the viscosity of the adhesive.

If manual control is required, an appropriate nozzle should be selected, a guiding tool should be used, and the nozzle should be kept vertical; while for automatic dispensing, precise control can be obtained through closed-loop control, laser detection, etc.

The measurement can be conducted using a vernier caliper, a microscope or a comparison card. If your factory needs to produce seals with a waterproof rating of IP67 or above. It is recommended to equip a visual inspection system.

Step 3: Join and Secure

If after applying the adhesive, the material does not stick to your hands and an elastic film has formed on the surface, it will be too late to assemble. It is recommended to assemble immediately after applying the adhesive (within 30 seconds), and then clamp it.

Clamping is done to eliminate air bubbles, but sufficient adhesive should be retained to fill the gaps. You can use limit fixtures (such as gaskets, stop blocks) to ensure uniform thickness of the adhesive layer and use multi-point uniform pressure.

Step 4: Cure

We suggest that for applications with a thickness greater than 5mm, RTV-1 should not be used as the internal part may not fully cure. Instead, RTV-2 should be adopted. You can refer to the following curing summary method:

 Step 5: Inspect

In addition, during the gluing process and the curing stage, you need to check and record the width of the glue line and the curing time. After completion, you also need to conduct early strength tests and final quality inspections.

Early Strength Tests: To check whether the fixture can be disassembled and proceed to the next process, after the product has been subjected to a 4-8 hour curing period, you should conduct a push-pull test (with a low force value) and a hardness test on the silicone.

Final Quality Inspections: After complete curing, usually within 24 hours to 7 days, you can conduct the final quality inspection of the silicone products. The inspection methods include: adhesion test (cross-cut method), peel test, shear strength test, compression permanent deformation test and accelerated aging test.

Common Silicone Application Mistakes to Avoid

Failure of RTV to Cure

If after curing, you find that some areas become soft or sticky, or do not cure at all, there are two possible scenarios:

First, if you are using RTV-1, it could be due to low temperature, low humidity in the curing environment, or too thick a coating. Please monitor the temperature and humidity in the curing area. If necessary, use heating or humidification. For thick silicone applications, you should use addition-cure silicone instead.

Second, when you use RTV-2 or the curing process fails, it might be due to an incorrect proportion or an uneven mixture. When mixing strictly in accordance with the proportions provided by the supplier, it is important to note that the mixing should be done based on the weight ratio, rather than the volume ratio.

The problem of uneven mixing is mostly caused by manual stirring. It is recommended to use an electric mixer or a static mixing tube. Many two-component silicone A/B products have different colors. You can check whether the colors are evenly mixed after the mixture is completed to determine if mixing is uniform.

Failure of Addition-Cure Silicone to Cure

When you notice that the mixed addition-cure silicone remains in a liquid or semi-cured state for a long time (> 24 hours), and the surface becomes sticky, it indicates that the curing process has failed. Due to the influence of substances such as nitrogen and sulfur, your factory should avoid using latex gloves and instead use nitrile gloves.

If you still have the production of condensation-cured RTV products, it should be carried out in another workshop. The by-products of condensation-cured RTV can also affect the curing effect of addition-cured silicone.

Quality Control and Compliance for Silicone

Food Contact Material Certification: If you are planning to produce silicone products such as baking molds and baby nipples, please pay attention to the different standards corresponding to different markets: FDA 21 CFR 177.2600 in the US, EU 10/2011 in the EU, GB 4806 series in China, and JFSL 370 in Japan.

It is necessary to conduct tests such as extraction and migration quantity, and you are required to provide third-party test reports from SGS and others, with the validity period within one year.

Electronic and Electrical Hazardous Substance Restriction Certification: This type is applicable to the electronic potting compound and PCB anti-corrosion coating industries. You need to obtain the conformity declaration or test report for each batch.

The following requirements must be met: RoHS (restricting 10 types of harmful substances), REACH (paying attention to the SVHC list), UL94 (flame retardant), IEC 61249 (halogen-free).

Medical-Grade Biocompatibility Certification: If you are involved in industries such as medical catheters or implantable silicone, you must pass the ISO 10993 series and USP Class VI tests, and comply with the ISO 13485 quality management system.

You are also required to provide the customized formula, biocompatibility test report and DMF document to assist in the completion of the medical device registration.

FAQ

Will White Powder or Oil on Cured Silicone Affect Performance? How to Deal with It?

If your RTV cures in a high-humidity environment, it may produce white powder. This does not affect the bonding and sealing performance. You can wipe it off with a dry cloth.

If it has light oil, you can wipe it with alcohol. However, if the situation is severe, it indicates that your curing conditions are improper or the formula quality is poor. In this case, you can adjust the mixing ratio, curing temperature, or change the supplier.

How to Store Opened RTV-1 for Long-Term Use?

Once the RTV is opened, it will start to come into contact with moisture. Even if the lid is resealed, the shelf life will be significantly shortened.You can immediately seal the nozzle with aluminium foil or plastic film after use, then screw on the cap and minimize the air inside the tube. Put it in a sealed bag or vacuum container and add desiccant. We recommend using it within 1-3 months.

How to Quickly Check Silicone-Plastic Compatibility without Testing Each Material?

We do not have a completely reliable non-testing judgment method available, because the bonding performance of the same type of plastic varies greatly depending on the additives, release agents, and fillers. Any new substrate must undergo actual testing before it can be used on a large scale.

Final Thoughts

Legensilico, as a professional manufacturer of silicone products, can offer you professional custom silicone services and application guidance. If you are interested in our silicone products, please feel free to contact us at any time.