Biocompatible silicones within the medical industry

In this blog post, we will go through the different types of biocompatible silicones that we offer. The selection of silicone will most of all have to be adapted to which properties the end product should have and with which manufacturing method will be used in the final production.

Within each category, you will find a number of variations in the final properties of the silicone, e.g. hardness (durometer), strength and different process parameters such as cure time etc. Here is a guide to which category to start your search within.

Adhesives and sealants

Silicone adhesives and sealants are designed to bond silicone to silicone as well as other materials such as metals or plastics. They are formulated for the best possible adhesion and are available in 1- or 2-component versions as well as varying work life to fit the needs of every production. The viscosities range from flowing to thixotropic. Pressure Sensitive Adhesives (PSAs) are also available in this category.

Primers

Primers are used to improve the adhesion of a silicone adhesive to the substrate it will be bonded to. There are primers for every situation: condensation curing silicones, addition curing silicones and every type of substrate, e.g. metal or plastic.

LSR – Liquid Silicone Rubber

LSR stands for Liquid Silicone Rubber. This type of silicone is especielly developed for injection moulding and has a consistency of a petroleum jelly. LSRs are filled with silica for better mechanical properties (higher tear strength for instance) and is the type of silicone normally used for moulded parts such as o-rings, seals, valves, gaskets and other precision moulded parts within the medical industry.

LSRs can be quite difficult to work with manually due to their relatively high viscosity, but it is still recommended to use them if the final production method will be injection moulding. The mechaincal properties are significantly better than for the next group of biocompatible silicones – the low viscosity elastomers.

Low viscosity elastomers

Low viscosity elastomers are a version of LSR but with a low, flowable and pourable viscosity which makes them suitable for processes where the silicone will be poured into a mould. These types of silciones are mainly used when a low viscosity isnecessary and the biggst downside compared to an LSR is the lower tear strength. Low viscosity elastomers can be used for potting, coating or moulding of silicone details.

Silicone dispersions

Something being a dispersion means that the material (in our case a silicone) is diluted in some sort of solvent. Silicone dispersions often have a very low viscosity which is suitable for applications where a thin film or coating is needed. Silicone dispersions work excellent for dipping or spraying.

HCR – High Consistency Rubber

Silicones of the type HCR, High Consistency Rubber, have a consistency like a clay when they are uncured. HCR are often either platium curing or peroxide curing and consist of two components that a kneaded, for example through calendaring, then is shaped and cured with heat. Peroxide curing HCRs can be either pre-catalyzed or un-catalyzed. HCR silcione is suitable for processing methods such as extrusion, compression moulding and tranfer moulding to produce silicone parts like tubes, profiles, balloons and o-rings. This type of silicone has superior strength compared to other silicones.

Silicone gels

Silicone gels tend to be low viscous before curing and cure into very soft after curing. Gels are for example suitable for soft tissue implants or potting of sensitive electronics.

Silicone foams

With their low density and flexibility, silicone foams have multiple uses. The actual foam is formed as a result of a chemical reaction that happens during cure (air is not mixed in manually). Silicone foams can be used for low density sheeting, tubes or profiles for which shock and vibration absorption is important.

Masterbatches

Most biocompatible silicone materials are transparent or translucent. These can of course be pigmented to the color of your needs using biocompatible color masterbatches. The term ”masterbatch” refers to the fact that the pigment is dispersed in a silicone polymer with vinyl functionality to be able to avoid pigment contamination and dust from loose pigment. This is especially important in a clean room environment. The silicone that the pigment is provided in will, during cure, covalently bond into the silicone matrix. Masterbatches with barium sulphate or titanium dioxide are also available for radioopacity.

Masterbatches are compatible with LSRs, HCRs and low viscosity elastomers and have minimal impact on the final properties of the silicone.

Silicone inks

Looking for an ink for silicone surfaces? Nusil’s series of inks (MED-6613 and MED6608-X) are developed to have adhesion on silicone surfaces. They are available in a number of different colors and can be applied using pad printing and silk screen printing.

Silicone based lubricants

Biocompatible silicones are not only used as cured materials within the medical industry, but also as non-curing fluids. Silicone fluid, silicone grease, cpecially formulated caotings or self-lubricating silicones are available for lubrication needs within the medical industry. Applications range from reduction of friction on silicone surfaces, needle lubrication, lubrication of cutting tools etc.

Biocompatibility

All biocompatible silicones from Nusil have Master Files with the FDA that can be upon request. The products are tested and approved according to USP Class VI and/or ISO 10993.

Feel free to contact us if you want help finding the right biocompatible silicone for your application!


The glass transition temperature (Tg) for adhesives

What is Tg, or glass transition temperature?

The glass transition temperature, often shortened to Tg, is a temperature often given for thermoset plastics. When a material, such as an epoxy adhesive, is cured, its chemical structure is permanent. All the polymer chains of the epoxy are cross-linked and have formed a large network. This crosslinking means that the adhesive can neither be melted or dissolved. However, that it cannot be melted does not mean that its mechanical properties stay the same at different temperatures….

What happens is that at a specific temperature, the material softens. The plastic quite suddenly goes from being hard and glassy to a soft and rubber-like. If the temperature then decreases again, the plastic/rubber goes back to its previous state. This is true for all the curing adhesives that are commonly found within adhesive technologies, including epoxies, polyurethanes, silicone, SMP polymers and acrylates.

Tg values for a few adhesives

Silicone is at room temperature already rubber-like! This means that its Tg is somewhere below room temperature. To be more precise, it is usually somewhere around -115 to -40 °C. Epoxies, that tend to be quite hard at room temperature need to warm up to soften sligtly. This means that the Tg is above room temperature – usually somewhere between 30 and 100 °C

What happens at the glass transition temperature?

The glass transition temperature for an adhesive is the temperature where the modulus of elasticity (in other words, the flexibility) changes drastically. As the temperature reaches this point, the adhesive goes from being glassy to rubbery. With increased temperature, molecules become increasingly mobile and can move around with much more ease. Below Tg the polymers in the adhesive are ”frozen”, figuratively speaking. Only short segments of the polymers can move and within a very limited space. The low mobility of the polymers means that it cannot adapt as easily to distortion – the material is more brittle and will break instead of flexing.

Another important property that drastically changes at Tg is the coefficient of thermal expansion (CTE). Generally, the CTE is considerably higher above Tg than under.

Properties that change at Tg include flexibility, creep strength, density, hardness, cohesive strength, modulus, chemical resistance and volume.

Can an adhesive be used above its Tg?

Absolutely! In many cases, the changes in mechanical properties above Tg are only positive. The adhesive for example becomes more impact and vibration resistant. And as already mentioned: Tg is not a melting point – only a temperature at which some properties change.   

How is Tg measured?

Despite what it may sound like, Tg is not an exact point. Depending on the method of measuring Tg, you will get different answers and in addition, interpretation of curves (whether it is done by a person or software) can vary. Comparing values off of technical data sheets can therefore be difficult.

The most common methods for measuring Tg is DCS (Differential Scanning Calirometry), TMA (Thermogravimetric Analysis) and DMA (Dynamic Mechanical Analysis). DSC is considered the most reliable of the methods an also gives an idea of how the properties change above and below Tg. For exemple how elastic the material becomes. To sum up, different methods look at different properties of the material and at what temperature the change takes place:

  • DSC measures the heat flow.
  • DTA measures the coefficient of thermal expansion
  • DMA measures the modulus
  • Dilatometry measures the specific volume.

To give you an example of the difference that can result, let’s look at the 2-component epoxy Epo-Tek 353ND. When Tg is measured using DMA, Tg is 90 °C while using DSC gives us 124 °C – quite a difference!

Why care about Tg?

Why care about the Tg of an adhesive? Well, for someone designing a bond line within a device, it can be of interest to know how this bond (which is essentially like putting a little piece of plastic between your parts) behaves. Since the mechanical properties are different at different temperatures, it can be good to know whether the adhesive will be on the glassy or rubbery side of Tg.

Please keep in mind that the Tg value stated in the technical data sheet for a product represents a certain cure schedule and a 100 % cured material. Generally, heat curing will give a higher Tg and in some cases, epoxies that are cured in room temperature will not get a Tg of over 15 degrees Celsius over the curing temperature, i.e 35 °C if you cure at room temperature!

Seal against fuel with fluorosilicone

The unique properties of fluorosilicone (FMVQ) can solve issues in applications where both a wide operating temperature and resistance to fuel and other solvents is necessary. For these types of applications, such as when you need to seal against fuel, tradtitional silicone adhesives fall short.

Silicone adhesives are often an excellent choice for bonding or sealing against a number of fluids. They can even handle being immersed in said fluid during long persionds of time. But there are certin fluids that will make silicone swell, or even deteriorate it. Among these fluids are for example gasoline, organic solvents and fuels as well as silicone fluids. For these harsh conditions, the solution to the problem is fluorosilicone.

Chemical compatibility of fluorosilicone

Just like regular silicone elastomers, fluorosilicones offer a similarly wide operation temperature, oxidative stability and remarkable flexibility. They are also highly resistant to heat, ozone and sunlight (UV).

In other words, fluorosilicone works just lika a regular silicone (dimethyl silcione) would, but with increased resistance to fluids such as gasoline, diesel, acetone and alcohol. Additionally, fluorosilicone does not swell in regular silicone fluid and vice versa.

In the picture below, the abilities of fluorosilicones are displayed compared to a regular silicone. Details of the same dimensions have been immersed in jet fuel. After a certain time, the samples are taken out of the fuel and weighed to see how much they have absorbed. The difference in swelling is quite substantial – the winner being fluorosilicone.

Tätning som tål bensin
Swelling of regular silicone compared to fluorosilicone in jet fuel (source: Nusil).

Important to note, however, is that fluorosilicone is less resistant than regular silicone against polar fluids. Polar fluids include ketones, aldehydes, amines and break fluids that are not based on petroleum.

Fluorosilicone chemistry

The side group of fluorosilicone

The chemical difference between a regular silicone and a fluorosilicone is that the fluorosilicone has trifluoropropyl-groups (-CH2CH2CF3) covalently bonded to the main polymer chains. Flourosilicones can have different levels of these side groups depending on the desired properties. There are for example products with 100 mol% fluorosilicone and 50 mol% fluorosilicone. Sometimes, you can also run across the term FMVQ which esentially means fluorosilicone.

When to pick fluorosilicone

Fluorosilicone can be used to seal against fuel, gasoline, diesel, non-polar solvents and silicone fluids. They can be used as adhesives, sealants, moulded silicone parts or protective coatings that may come in contact with these types of fluids.

Common uses for fluorosilicone are within the automotive and aviation industries. A quite common application is the sealing of fuel tanks. Often in automotive applications, the combination of fuel and high temperature resistance is necessary in order to maintain the properties that would make you choose silicone to begin with. Tests show that fluorosilicones are superior to regular silicones for these types of challenges that you may encounter when you need to seal against fuel.

The biggest drawback of using a fluorosilicone is the price which is substantially higher than for a regular silicone adhesive. Fluorosilicone is therefore notheing you will want to choose unless there is a need for it.

Product recommendations

Nusil FS-3730 is a 1-component moisture curing fluorosilicone adhesive.

Nusil CF1-3510 is a 2-component, heat curing fluorosilicone.

There are many more options so please contact us for advice!

Also check out Nusil’s selector guide and fluorosilicone flyer with tons of product options to choose from.

UV lamp safety: what you need to know

Industrial UV-lamps are used when curing UV-adhesives and coatings. The lamps emit a high intensity of UV light and this light is important to protect yourself from. In this text we go though what you need to think about in regards to UV lamp safety.

UV light is a form of electromagnetic energy. UV light is found just below visible light on the electromagnetic spectrum. Hence, it is something that the human eye cannot see. The human body has a difficult time protecting itself from this type of light, since the natural mechanisms, i.e. shrinking of the pupils, is not triggered in the same way. It is therefore important to use the correct safety equipment when working with UV lamps, and not deactivate the safety mechanisms that the lamp is equipped with.

UV lamps tend to be very bright even to our eyes. But actually, it is not the light that we can see that is the dangerous type, but the light we cannot see.

Different types of UV light

UV light is normally divided into three categories: UV-A, UV-B and UV-C. All types have the potential to injure an operator if used incorrectly, but UV-C constitutes the biggest risk and can, among other things, burn your corneas. UV-B is the type of radiation that can cause sunburn. Luckily, neither UV-C or UV-B is used in industrial UV lamps, but rather UV-A, that is slightly better comparatively.

UV light is the biggest cause of melanoma and can damage the cornea of the eye.

Elektromagnetiskt spektrum för UV-lim
Electromagnetic spectrum (not to scale)

Built-in protection in UV-lamps

Most industrial UV lamps are designed with safety functions. Some examples are shielding, safety latches, intuitive design and light absorbing plastics that prevent the light from ever reaching an operator. Additional materials, such as safery shields, can also be purchased. In the cases where this protection is not sufficient, it is important that the operator in question has received a satisfactory training in how to protect themselves from exposure to minimize any safety risks.

Measuring the exposure

A radiometer is a small device that can measure the intensity of UV light in a certain location. Apart from the fact that it is always a good idea to keep track of the efficacy of the lamp, it can also be used to measure the exposure of an operator when working with the UV-lamp. Turn on the lamp and hold the sensor of the radiometer approximately where the operator’s exposed skin would be present during use. Compare this to what you would be exposed to during a normal day in the sun. Hopefully, you will see that the UV lamp is considerably lower than this. Remember to make sure that your radiometer matches the wavelengths emitted by your lamp.

The sun will emit a UV-A intensity of approximately 2-5 mW/cm2 on a sunny say here on earth.

Protection and recommendations

  • UV/Vis-blocking safety glasses should be worn. These are usually tinted which also helps protect you from bright visible light. With time, even visible light can result in headaches or sun spots.
  • Safety gloves: The part of your body most likely to be exposed to the light from a UV lamp is probably your hands. You should of course always be wearing gloves to protect your skin from the UV adhesive itself, whether you are working with an acrylic or epoxy, but the gloves will also protect you from radiation.
  • Protective clothing should mainly be worn to protect your own clothes and skin from any adhesive spills. It will also help protect you from any light.

Industrial UV lamps need special attention regarding work environment. However, when industrial UV lamps are used correctly they are both easty and safe to use.

Do you need advice?

Contact us for help!

Moisture curing silicone adhesives and sealants

How moisture curing silicones work

A 1-component RTV silicone is cured with moisture unlike a 2-component silicone adhesive which cures upon mixing. The moisture curing silicone will cure from the outside in which means that first, a skin will be formed on the surface. This is a relatively fast process, perhaps 10 minutes at normal temperature and humidity. What it means in reality is that this is the time you have after dispensing until your pieces should be bonded (otherwise known as the ”open time”, ”tack free time” or ”skin formation time”). After the skin has formed, the adhesive will continue to cure inwards which takes longer. A normal cure speed is approximately 2 mm/24 hours, counted from the surface of the adhesive that is in contact with air.

The great advantage of choosing a 1-component silicone adhesiva as compared to a 2-component one is not the cure speed, but the ease of dispensing.

Chemically, what happens during curing is that tiny water molecules creep into the silicone. There they chemically react with the components of the silicone adhesive and create a by-product. Depending on the type of adhesive, this by-product can be either an alcohol (alkoxy), acetic (acetoxy), an oxime or more uncommonly, an amine.

The curing speed of 1-component moisture curing silicones is directly dependent on the:

  1. Relative humidity.
  2. Temperature.
  3. Design and dimensions of the bond.

The influence of relative humidity should be clear by now – it is the water molecules in the air that are needed to kick off the chemical curing reaction at all. A higher temperature make the molecules in the adhesive more mobile which allows them to react more quickly. However, be careful not to increase the temperature too much (above approximately 60 ˚C). A too high temperature can degrade the components and either prevent cure altogether or impair the end physical properties.

Advantages of moisture curing silicone adhesives

  • Bonding, sealing and fixating.
  • Adhere to most surfaces.
  • Easy handling with no mixing step.
  • Soft and elastic, even at low temperatures down to -60 or in some cases even below -100 ˚C.
  • High temperature resistance, often between 200 ˚C till 300 ˚C continuosly.
  • Contain no solvents and are usually work environmentally friendly.
  • High chemical resistance.
  • Water resistant.
  • Bacteria and fungi do not thrive on silicone which make them suitable for use in humid environments.
  • Can withstand harsh outdoor conditions.
  • Are available in many colors (even transparent) and consistencies.
  • Good electrical properties.

Applications for moisture curing silicone adhesives

1-component moisture curing silicone adhesives have many areas of use in a number of industries: anything from consumer products such as the doors of ovens to applications within aerospace, aviation, denfense, electronics, medical and everything in between.

Tips & tricks

  • Do not use a 1-component moisture curing silicone adhesive if you plan on using it in deep sections. The depth is measured from the side of the silicone that is in contact with surrounding air. This means that moisture curing silicones are not suitable for closed-in areas either. In these cases, you will be better off with a 2-component silicone.
  • Ig the original packaging of the adhesive is ungainly, the adhesive can easily be re-packaged into smaller syringes.
  • Oximes can cause cancer. Please avoid this type of moisture curing silicone if possible and go for an alkoxy or acetoxy curing one instead.
  • Do you need some bond strength instantly? Look for a silicone adhesive with a high green strength (wet strength). Silicone adhesives with high green strength have a type of stickiness that ensures that the bond gets some strength even before curing has begun.
  • Keep in mind that many different qualities of moisture curing silciones are available. In a more sensitive application, a higher purity grade silicone, than the ones used in bathrooms for example, will probably be necessary. There are even low outgassing options for very sensitive applications such as in space and electronics as well as implant grade ones.

Contact us if you are interested in what moisture curing silicones we can offer for your application.

5 types of bicompatible lubricants

Biocompatible lubricants based on silicone are used to lower the friction between components in medical applications or between a component and skin. Lubricants from Nusil are biologically tested according to ISO 10993 and have FDA masterfiles.

Silicones are effective lubricants thanks to their long, linear polymer chains with high mobility. They are also resistant to oxidation, chemically inert as well as hydrophobic.

When you are trying to lower the friction of your medical component, the type of material, design of the component and the type of movement will contribute to your choice of lubricant.

Applications for biocompatible lubricants

  • Reduce insertion force, e.g. for needles, cannulas, trocars and cutting tools.
  • Reduce drag forces, e.g. for catheters, guidewires and cutting tools.
  • Minimize break-loose force, eg. for valves and stopcocks.
  • As a hydrohpbic coating, e.g. on the inside of syringe cylinders.

Types of biocompatible lubricants based on silicone

Silicone fluid

Biocompatible silicone fluids are available in a number of different viscosites. Higher viscoisty fluids will lubricate for longer than lower viscoisty fluids. You will also find both regular silicone fluids and fluorosilicone fluids as well as mixes of the two to be able to adapt them to different substrates and applications.

Silicone grease

Silicone greases are thixotropic (non-flowing) formulations that have little or no migration. This si the reason they are usually chosen when the lubrication needs to last for longer periods of time and on specific locations. They can also seal under vacuum.

Silicone dispersion

A silicone dispersion is a silicone diluted in solvent to achieve a lower viscosity. This helps to get a thinner and more even layer of lubricant applied on the detail compared to un-diluted silicone fluids or greases.

Self-lubricating silicone

A self-lubricating silicone is a silicone elastomer with built-in lubricant. The moulded silicone part will slowly and continually emit a small amount of lubricant from the silicone detail. This will reduce the tackiness as well as the friction of the silicone elastomer.

Nusil självsmörjande silikon / biocompatible lubricants

Common applications for self-lubricating silicone elastomers are injection moulding or compression moulding of valves, o-rings, seals and plugs.

Curable silicone coating

Biocompatible friction reducing coating.

Curable coatings from Nusil that lower the coefficient of friction are applied to the surface of a silicone detail and cured there to lower the surface friction. This will prevent tackiness and friction of the silicone detail as well as increase the abrasion resistance of moving silicone parts. In tests, the coefficient of friction has been shown to decrease by up to 74 %. The products are solvent-based and can be applied through spraying, dipping or brushing on moulded silicone parts. Efter application, the coating is allowed to cure to a dry coating which ensures that it will not migrate.

Do you want to know more about Nusil’s selection of biocompatible lubricants? Get in touch!

Choosing a biocompatible lubricant

Lubricant can be necessary within many typed of medical applications to reduce the friction on moving parts. Which factors are important to keep in mind when choosing a biocompatible lubricant for your medical application? Let’s go through some things to consider.

For more information about what types of lubricants are offered for medical applications, read our previous blog post describing these.

Type of application

The biocompatibility of lubricants from Nusil is tested according to ISO 10993. Within this specification, there are two levels of biocompatibility: for short-term implantation (less than 30 days) and for long-term implantation(over 30 days). Make sure to choose a lubricant with the right type of classification.

What type of friction reduction is needed? Hydrophobicity?

For example:

  • Reducing the insertion force into human tissue for needles, cannulas, trocars and cutting tools.
  • Reducing the drag force against other components or tissue for catheters, guidewires or cutting tools.
  • Minimizing the break loose force for valves and stopcocks.

What material is being lubricated?

Silicone

The surface of a cured silicone elastomer (silicone rubber) often has a high coefficient of friction and can feel tacky. This can be an obstacle when certain parts need to be kept mobile. Silicone elastomers also have a tendency to stick to each other in a process of chemical affinity called blocking.

When choosing a lubricant for silicones, it is important to consider the chemistry of the silicone. If the elastomer and fluid are too similar, the fluid lubricant can migrate into the elastomer, making it swell and losing its lubrication effect. Most silicone rubbers are based on dimethyl silicone polymers. Choosing a fluorosilicone-based lubricant will prevent migration. Another thing that can help is choosing a higher viscosity lubricant, such as a silicone grease. Apart from the more tradtitional lubricans such as fluids and greases, there are also curing coatings that can be applied on the surface of the elastomer. These products bind chemically to the surface and form a dry and flexible coating that will not influence the mechanical properties other than lowering the coefficient of friction. Another non-traditional option is using a self-lubricating silicone when manufacturing the silicone detail.

Metal

Metal surfaces on for example scalpels, needles and cannulas can have higher friction than we could like when they come in contact with our skin and tissue. The friction from emtal can cause unnecessary damage and of course, causes pain for the patient. To reduce friction, a lot of effort has been put into optimizing the design of the metal. Look for example on the tip of an injection needle and you will see that the needle tip has a certain shape meant to minimize the insertion force. The friction that remains can be reduced even more using a biocompatible silicone lubricant.

For metals, it is especially important to think about how long the lubricant needs to work for. Is it a product for one-time-use (e.g. injection needle) or for multiple use (e.g. a suture needle that will penetrate the skin multiple times).

A common choice for metals is to use a silicone fluid. For single-use items, a low viscosity fluid is suitable, but for multiple use a higher viscosity can be better, perhaps dispersed in solvent to get an asier and more even dispensing.

Glass

Silicones tend to have a very good adhesion to glass thanks to their quite similar chemical structure with Silicon-Oxygen polymer chains. Adding heat can actually make the otherwise so non-reactive silicone crosslink to the glass for even better long-term adhesion.

Often, in lubrications applications concerning glass, the purpose of it is to get a hydrophobic surface. For example inside syringe barrels to enable easier dispensing of the medicine inside. This type of applciation is usually referred to as siliconisation. A biocompatible silicone fluid or grease is a good choice for glass. Potentially, it can be dispersed in solvent for the sake of dispensing and adding heat is optional.

Plastics

There are many types of plastics commonly used within the medical industry. Friction points on plastics can require a lubricant. To get valves and plugs of different sorts to glide better, a biocompatible silicone grease is common since this type of lubricant tends to migrate less than a fluid. If you want to use a dispersion – make sure that the plastic is not damaged by the solvent.

What type of process is possible?

Thnak about whether your process has the ability to apply heat for heat curing products, if there is sufficient humidity levels for moisture curing and if you can consider using solvents.

How will the lubricant be applied?

Through wiping, brushing, spraying or dipping? Will the process be manual or automated?

How many cycles should the lubricant hold for?

If only one or a few, choose a low viscosity lubricant.

If more – choose a high viscosity lubricant or a curing system.

Selector guide for biocompatible lubricant from Nusil

Any questions? Don’t hesitate to ask!

The effect of plasma treatment on plastics

A requirement to achieve adhesion between an adhesive and a substrate is that the former wets the latter. If an adhesive does not wet the surface that you want to bond, there are two options: 1) Change adhesive, or 2) Pre-treat the substrate surface.

There are many pre-treatment methods for plastics, including roughening, primer, etching, Corona and plasma treatment. In the video below, you will see how a difficult-to-bond plastic (polypropylene, PP) is affected by plasma treatment.

Surface tension without plasma treatment
The surface tension of common plastics/adhesives.

Surface tension & wetting

Plasma treatment (in the featured video using a low-pressure plasma equipment from Diener) has several functions that you can read more about in our previous blog post about plasma. One of them is to increase the surface tension of plastics. The surface tension of an adhesive and the surface that is to be bonded determines if the adhesive will wet that surface.  The general rule is that an adhesive will wet a surface that has a higher surface tension than itself. In other words, plasma treatment makes a plastic surface easier to wet and therefore easier to bond!

To evaluate wetting, special testing fluids, or inks, can be used. These have a known surface tension and can thereby indicate the surface tension of the substrate. In the video, a fluid with a surface tension approximately equal to that of an epoxy adhesive is used.

Results from plasnma treatment of polypropylene

Before pre-treatment (only cleaning), this ink does not wet the polypropylene plastic at all, i.e. the plastic has a lower surface tension than the ink. After plasma treatment for 5 minutes, however, the result is the opposite. The ink now spontaneously wets the plastic which means that the surface tension of the polypropylene now exceeds that of the ink.

According to the results of this simple test, the polypropylene should now be significantly easier to bond.

How does a low pressure plasma work?

  1. Details are placed in the plasma chamber.
  2. Using a vacuum pump, the pressure in the chamber drops to approximately 0.2-0.4 mbar.
  3. A gas (in this case water vapor) enters the chamber and is transformed into plasma. The plasma, in turn, changes the properties of the surface exposed to it. The bright purple color is the plasma.
  4. After a few minutes, the pressure is released and the details can be taken out of the chamber and be bonded.

Are you interested in more information about plasma treatment?

Contact us!

4 ways to lower the friction of silicone and avoid blocking

Silicone generally has a high coefficient of friction and is inherently tacky. The softer the silicone, the tackier it will be. This means that silicone will tend to pick up dust, easily sticks to itself and other materials and has a high coefficient of friction. When silicone sticks to itself, it is usually called blocking. These properties can become problematic in applications requiring a silicone component that will need to be able to move or slide. Here are 4 ways to lower the friction of silicone, to avoid dust pick-up and blocking:

1.      Low Coefficient of Friction Coating

A nice and long-term way to lower the friction of your silicone, for example to reduce insertion force or drag force, is to apply a curable coating on the silicone elastomer. Coatings that lower the coefficient of frictions (CoF) are available from Nusil. These coating formulations are solvent based, so your process must tolerate the use of solvents. When cured, the coating is completely dry and non-migrating.

A low CoF coating will decrease the static coefficicient of friction by up to 74 % and the kinetic coefficient of friction by up to 54 % on a typical LSR. In addition, the coating will increase the resistance to abrasion.

There are two types of curing mechanisms available for the coatings: platinum catalyzed for platinum catalyzed rubbers (like most LSRs and HCRs) or moisture curable for Tin-containing silicones.

2.      Silicone fluid / oil

Nusil självsmörjande silikon

A silicone fluid can help minimize the break loose force and reduce the insertion force and drag force. When choosing a silicone fluid as a lubricant, it is important to choose one with low chemical affinity to the silicone rubber. If a regular silicone oil is used on regular silicone (PDMS), the fluid will migrate into the elastomer, making it swell, and of course, lose its lubricating properties. Instead, choose a fluorosilicone based lubricant and vice versa.

The silicone fluid can be applied to your silicone part as-is or be dispersed in a solvent for a thinner and more even application. Consider the viscosity when choosing your fluid. A higher viscosity will migrate more slowly.

3.      Silicone grease

A silicone grease will also minimize the break loose force. The advantage of choosing a grease over a fluid is that the higher viscosity makes it migrate less which in turn, will lubricate your silicone rubber for longer.

As for silicone fluids, a silicone grease should be chosen with the silicone chemistry in mind. A fluorosilicone grease should be used for regular silicone and a regular fluid for fluorosilicone, respectively. The grease can be applied as-is or dispersed in solvent.

4.      Pick a self-lubricating silicone

To reduce processing time and avoid adding extra manufacturing steps for lubrication, using a self-lubricating silicone for your molded parts can be an option. This technology means that your LSR or HCR already is loaded with lubricant. This lubricant will, after vulcanisation of the silicone elastomer, migrate out to the surface in an even pace. The silicone as well as the amount of lubricant can be adapted.

Contact us to discuss your specific application and get advice on which product to choose!

UL 94 Flammability rating

What is the UL 94 Flammability testing standard?

UL 94 is a flammability test for polymeric materials that are being sold to and used in electronic devices. UL 94 classifies plastic materials (including adhesives, potting resins, coatings etc) according to how they burn. This includes both how easily a material is set on flame and how quickly it self-extinguishes after the flame is removed.

If a material is UL certified, it is verified that the electronic device can operate safely under normal conditions without high risk of fire or electrical default.

How is the UL 94 test performed?

There are three types of tests performed under the UL 94 standard. In principle, all of them are similar; You expose a piece of the material to a flame and then measure how long it takes for it to self-extinguish and what happens to the piece itself.

Horizontal burn (HB)

One type of test is called Horizontal Burn. This test can give a material the flammability classification UL 94 HB if it passes the test (see table below). This is the lowest rating. The procedure of the test is to apply a flame for 30 seconds to one end of a horizontally held piece of material. After the flame is removed, the flame must self-extinguish, and the burning rate must be slower than specified for the thickness of the specimen to pass the test.

Vertical burn (VB)

The other method is called Vertical Burn. The sample specimen is placed vertically, and a flame is directed towards the bottom of it. After 10 seconds, the flame is removed and the time the piece continues to burn and glow are noted.The same procedure is repeated a second time on the same specimen and for five different specimens. The results from this test can give three levels of flammability rating.

Surface burn (SB)

Surface Burn is a version of the vertical burn but with a bigger flame and more repeated tests on the same specimen. This is the most aggressive test within the UL 94 standard and hence also gives the highest flammability rating. The test is performed similarly to VB but using a larger flame. After 5 seconds, the flame is removed, and times noted. This is repeated 5 times for each specimen and for 5 different specimens.

What ratings are there?

Here is a summary of the different UL 94 flammability ratings, arranged from highest to lowest.

UL 94 rating Method Requirements
5VA SB Highest rating. Self-extinguishes within 60 seconds. No holes may be present in the specimen.
5VB SB Self-extinguishes within 60 seconds but there may be holes in the specimen.
V-0 VB Self-extinguishes within 10 seconds and may not drip.
V-1 VB Self-extinguishes within 60 seconds and may not drip.
V-2 VB Self-extinguishes within 60 seconds and may drip.
HB HB Burns slower than specified in the standard for a certain thickness. Lowest rating.

Why do I need a material with UL 94 rating?

Fire resistant components give design engineers and customers trust that the electrical unit will be able to deliver the necessary performance. Flammability tested components also give a lower safety risk. UL 94 is the most common standard for adhesives, potting resins and coatings within the electronics industry.

Products with UL 94 ratings

Bear in mind that the UL approvals are specified to cerain dimensions of the specimens tested, size of the flame etc. This is most likely not the case in a real-life situation. It is therefore important to verify the flammability for your specific application.

A material can be tested according to the UL 94 standard but by a third-party laboratory. This would mean they are not registered with the UL (Underwriters’ Laboratory) but would likely pass if they were.

Här kommer ett urval av produkter som är UL 94-godkända. Hittar du ingen som passar, kontakta oss för fler valmöjligheter.

Silicones

Most silicones are by nature capable of at least an HB flammability rating. Fillers can improve the flame resistance further.

Silicone products from Nusil marked are flame retardant are not officially certified.

Nusil SFM-2350 Flame retardant silicone foam

Epoxy resin

The flammability of epoxies can also be improved with certain filler materials.

ALH Systems NP1003 UL 94 V-0
ALH Systems NP1025 UL 94 V-0 (not certified)

Polyurethane resin

Bectron PU 4537 UL 94 V-0
Bectron PU 4522 UL 94 V-0