You may know that silicone can be bonded to other surfaces and substrates. Do you know how easy it is to bond silicone to different surfaces? This article will explain the principles and performance of bonding with other surfaces. It helps you make a well-informed judgment and choice in selecting silicone adhesives.
What makes Silicone Stick to Different Surfaces
Chemical Structure and Cross-Linking
Silicone is a three-dimensional network of silicone-oxygen bonds, not a simple chain. After curing, it forms many cross-linking points on the surface, which allows the silicone to grab more molecules when it touches the substrate, spreading it out and sticking to it. It also remains soft and heat-resistant, and is not prone to cracking due to temperature changes or contraction.
The Role of Side Groups in Adhesion
In addition to the overall structure, the side groups on the molecule are also important. This affects the surface energy and wettability of the silicone, which in turn determines which materials it is suitable for.
The low polarity and surface energy of the methyl group make it easier to form stable adhesions with low surface energy materials such as polyethylene, polypropylene, and Teflon. It does not depend on a chemical reaction, just spreading and contact.
Phenyl has an aromatic ring, high flexibility, and high heat resistance. It enhances intermolecular van der Waals interaction on high surface energy materials such as glass, metal, and ceramics, making the adhesion more stable.
Amines are polar and hydrophilic, and can easily form hydrogen bonds with metals, glass, ceramics, wood, etc. sometimes participate in stronger interfacial bonding, so they stick more firmly on these materials.
Different types and combinations of side bases can make the same silicone adhesion behave differently on different materials. Some formulations are more stable on plastics, others are firmer on metals or ceramics. Understanding these differences will give you more direction when picking a silicone without having to try it blindly.
Factors that Affect Silicone Adhesion
The ability of silicone to stick to a surface is affected by several factors. From the silica gel itself, different types and formulations are suitable for different materials and environments; it is important to pick the right silica gel. It is important to clean the surface of the material; if there is dust, oil, or other residues, the silicone is difficult to spread and not firmly affixed. The curing temperature and time will also affect the adhesive force; if curing is incomplete will be easy to fall off. Daily inspection and maintenance will also affect its long-term performance.
Surface Energy
The ability of a material’s surface to adsorb outside substances is surface energy. Metals, glass, ceramics, and other materials with high surface energy can be firmly adhered to when the polar groups in the silicone molecule come into contact with them. It is this molecular-level interaction that allows silicone to spread out and stick firmly. Low surface energy materials are not so easy, such as polyethylene, PTFE, molecules, and the surface can not form close contact, the natural adhesion. When encountering such materials, it is usually necessary to do surface treatment or use a specially formulated silicone to stick.
Cleanliness of the Surface
A material surface with some attachments, such as oil, dust, moisture, or any other contaminants, can hinder the effect of silica gel molecules contacting the substrate. These adhesive substances occupy the surface space and reduce the area where the silicone spreads out, leading to a decrease in bonding strength. For the silicone to stick firmly, the surface needs to be cleaned first to make sure that there is nothing preventing contact.
Material Texture
The rough or porous surface of the substrate is more likely to make the silica gel adsorbent. Because the silica gel in the liquid state can penetrate the surface texture or pores, and when cured, the formation of a physical adsorption. This makes the overall bond stronger and less likely to fall off due to slight pulling or vibration. If the surface is smooth, there is not a lot of space, and the silicone can not go deep into it. In this case, the adhesion will be obviously weaker, but also more susceptible to external forces or temperature changes.
Curing Process
The curing process of the silicone has an effect on the bond strength and also affects the durability of the silicone for later use. Temperature and time directly affect the level of curing. Too high a temperature will make the silicone hard or become fragile; too low a temperature will result in incomplete curing. A short time will also result in poor adhesion. When silicone is not fully cured, it cannot form good adherence. Adhesion will be significantly reduced and may even lead to work failure. When used for a long time under these conditions, the performance of the silica gel decreases rapidly and gradually loses its effectiveness. These curing conditions are not suitable; the silicone will not be able to perform as it should. The service life and stability will be affected.
Compatibility of the Silicone
The type of silicone itself varies, as does the relative suitability for different materials and environments. Adaptability to different substrates is determined by the chemical composition of the silicone, the cross-linking method, and the curing mechanism. Even the same type of silicone will have different adhesion strengths depending on the formulation. Some silicones are suitable for bonding to hard surfaces such as metal and glass. Others are suitable for plastics and other flexible materials.
Acid-cured silicone adheres well to most surfaces, but it emits byproducts that may corrode some metals. In contrast, neutral curing silicone has neutral volatiles. It is more suitable for use on metals, plastics, and glass, and is less likely to damage the substrate.
Properties of Silone for Pressure-Sensitive Adhesive
Silicone has a stable molecular structure, strong silicone-oxygen bonds, and solid chemical properties. It can maintain its performance for a long time in a variety of environments and is not affected by oils, solvents, acids, or alkalis. It has strong UV resistance and will not fade or age when exposed to sunlight for long periods of time, making it ideal for outdoor use.
Due to the lack of hydrophilic groups in the molecule, silica gel has low water absorption and is not easy to absorb water even in humid environments. In terms of temperature, it adheres stably in the range of -50°C to 250°C, and can be used normally in low temperatures or extremely cold environments. These properties make the silicone very resistant to aging, high temperatures, and prolonged use.
In addition to these basic properties, Pressure-Sensitive Adhesive has some special advantages. It is easy and quick to use and does not require heating or curing. Only a slight pressure is required for a strong bond. This characteristic makes it easy to apply in a wide array of specialized environments.
Adhesion Strength
Pressure-Sensitive Adhesive’s adhesion strength is divided into initial adhesion and long-term adhesion. Initial adhesion is characterized by the ability to quickly form bonds with a wide range of material surfaces. It is especially suitable for materials with low surface energy. Long-term adhesion refers to the ability to maintain strong adhesion over a long period of time. Even at high temperatures or in harsh environments, it can still maintain a stable adhesion effect.
Flexibility and Cushioning
Pressure-Sensitive Adhesive resists vibration, cushions, and absorbs shock. Its low – modulus and flexibility allow it to adapt to changes in temperature and surface contraction without breaking or falling off easily. It can also withstand higher frequency vibrations than ordinary silicone, providing effective cushioning for longer periods of time. This allows it to form strong bonds on complex or irregular surfaces.
Repeatability and Adjustability
When using PSA, it is easy to bond with just a gentle pressure. It is also possible to adjust the position in time without damaging the surface and adhesion during the bonding process. This adhesion utilizes its physical adsorption properties on the contact surface. So there is no need to heat or wait for the curing time.PSA excels in labels, films, or applications that require precise docking.
Glue Adapted to Silicone
Silicone is a low surface energy, non-polar material. This makes it difficult for ordinary glues to stick to it effectively. Therefore, we need glue specially designed for silicone, or glue that is more compatible with silicone.
Silicone Glue
This kind of glue is designed for silicone and is widely used in various special environments. It is especially suitable for occasions that require long-term stable bonding. Specialized silicone glue can maintain good adhesion in a high-temperature environment. They have excellent adhesion and fluidity, can form very good contact with the surface of silicone, and can adapt to the flexibility of silicone and high-temperature characteristics. They can maintain a strong bond in extreme environments
Epoxy Resin Glue
Epoxy resin glue has very strong bonding power and provides high mechanical strength and high temperature resistance after curing. However, although it has a strong bond, it does not adhere as well to silicone surfaces as silicone-specific glues. If used where silicone is subjected to dynamic loading, de-bonding may occur. Epoxy resin glue is suitable for fixed silicone parts, such as electronic equipment packaging.
Hot-melt Adhesives
Hot-melt adhesives are often used in fast-bonding situations. It has good fluidity when heated. However, the adhesion of hot melt adhesive is less compatible with silicone, and it is necessary to sand or spray primer on the surface of the silicone when using it. This ensures a stable bond. This makes hot melt adhesives ideal for mass production and efficient production lines, but requires extra preparation.
Acrylic Glues
Acrylic glues have a medium level of bonding power. Although not as strong as silicone-specific glues, it still provides effective bonding in applications where strength is not required or as a secondary binder. It is suitable for lighter load silicone bonding tasks. It may be less suitable for applications requiring greater durability.
Silane Coupling Agents
Silane coupling agents chemically convert the surface of a silicone gel into a polar surface that is more suitable for bonding with other adhesives. Adhesion to materials such as metal, glass, or plastic can be significantly improved by coating the silicone surface with a silane coupling agent. It is a chemical additive used to enhance the bonding of silicone and is particularly suitable for those application scenarios where a strong bond with other materials is required.
How to Enhance the Adhesion of Glue to Silicone
Plasma Treatment
By plasma or gas discharge, some dirt and low surface energy can be removed from the surface of silicone. This makes it easier for the glue to adhere. Plasma treating a smooth silicone surface makes it more suitable for bonding and improves results.
Sanding
Sanding the surface of the silicone increases the roughness, which helps the glue to adhere to it more firmly. A rougher surface provides more contact points for the adhesive and improves bond strength.
Chemical Coating
Applying a specific primer or chemical coating to the surface of the silicone greatly enhances the adhesion of the silicone to the adhesive, resulting in a stronger bond. It is suitable for bonding of some complex materials to enhance stability and reliability.
Flame Method
Flame treatment slightly cauterizes the surface of the silicone gel through a short period of high-temperature contact, removing grease and contaminants and increasing the surface activity. This method can quickly improve the adhesion effect of the glue, especially suitable for mass production in need of rapid treatment.
FAQ
Can Silicone Stick to Metal Like Aluminum or Stainless Steel?
Yes, it can, but it needs to be suitable for surface treatment and product selection.
Metal has a high surface energy and can be easily bonded to silicone. However, if there is oil or oxidation on the metal surface, the bonding effect will be greatly reduced. Therefore, it is necessary to clean and degrease the metal surface before silicone adhesion. If the application is under high temperature, the bonding effect will be better if special high-temperature silicone is used or cured under higher temperature.
Can Silicone Stick to Plastic?
Whether silicone can stick to plastic depends on the type of plastic. There are plastics with high surface energy and plastics with low surface energy. Plastics with high surface energy can be easily stuck to silicone, such as ABS, PC, PVC, nylon, etc. Plastics with low surface energy are less likely to stick to other materials. Plastics with low surface energy, such as PE, PP, PTFE, etc., do not stick easily.
If you need to bond low surface energy plastics, you have to treat the surface of the plastics, such as plasma treatment, flame treatment, or use specially designed silicone formulations for plastics; it is easy to have the problem of debonding.
Can Silicone Stick to PVC?
PVC has a high surface energy and wets well to silicone. Ordinary neutral curing or high-temperature silicone can usually be adhered directly. As long as the surface is clean and free of oil, the bonding is good.
Why does Silicone Fail to Adhere Even though the Surface Looks Clean?
Just because it looks clean doesn’t mean it’s really cleaned up. Many materials will have residues on the surface that are unseen by the eye, such as mold release agents, silicone oils, processing lubricants, plasticizers, and even grease left by your fingers. These residues will greatly reduce the surface energy, resulting in silicone being unable to fully spread and wet, and adhesion decreases.
The surface energy of some materials is low and lacks the active sites for silicone to act, so even if the surface is clean, without some surface treatment, the physical adsorption of silicone is weak, and the stability is poor.
What Causes Silicone Adhesive to Peel off after Curing?
Usually, the treatment before bonding is not enough. This will make the bond of the silicone gel only barely last for a period of time, and after a period of force, heat, or use, the interfaces begin to gradually loosen and will eventually detach completely.
Curing conditions can also affect the effectiveness of silicone bonding. If the temperature, humidity, or curing time is not suitable, the bonding force between the silicone and the surface of the substrate will be reduced. In the later stages of use, high temperatures, water vapor, ultraviolet rays, and chemical agents can accelerate the removal of silicone that was not strong enough in the early days.
Can Silicone Maintain Adhesion under High Temperature or Thermal Cycling?
Silicone itself has a high bonding energy in its molecular structure, which makes it temperature-resistant and not easy to age at high temperatures. Most silicone adhesives can work stably and for a long time at high temperatures, 50-250°C. The silicone adhesive can be used in a wide range of applications. Its flexibility can effectively relieve the stress caused by thermal expansion and contraction.
High temperature resistance does not mean that adhesion can be maintained under all conditions. If the surface energy of the substrate is low and there is no pre-adhesive treatment, the silicone will gradually lose its adhesion under prolonged thermal cycling.
Final Thoughts
The degree of difficulty of silicone adhesion on different substrate surfaces is affected by many factors. It depends on the suitability of the substrate, the application environment, and whether the right products and processes have been selected.
Legensilicone provides you with more precise silicone formulations and technical support according to different substrates and application scenarios. Whether it is a high surface energy material such as metal, glass, PVC, or a low surface energy plastic such as PE or PP, we can recommend the right type of silicone, high temperature, or ambient curing solution according to the actual needs. In addition to primers, surface treatments, or silane systems, we ensure stable and reliable bonding results.
Our silicone products maintain stable performance under different extreme environments. Helping you reduce rework and maintenance costs. If you want to know more, please feel free to contact us.
