Comparison of Acetoxy Curing and Neutral Curing Silicones

Tech Tip

One component condensation curing silicones are broadly classified into two main categories, acetoxy curing silicones and neutral curing silicones.

Acetoxy curing silicones derive their name because they release acetic acid upon curing. They typically cure fairly quickly, and relative to other silicones, tend to adhere better to various substrates. While the emission of acetic acid is acceptable in many industrial applications, if certain sensitive electronic components are present, the acetic acid emissions could potentially have an adverse effect on some of the components involved. While it is important to note that acetoxy curing silicones might be corrosive for certain substrates, they do offer many other advantages, and some of them are even biocompatible, making them useful for medical device manufacturing.

Some examples include:

Neutral curing systems include the oxime and alkoxy silicones. Unlike the acetoxy system, methyl ethyl ketoxime is released with the oxime curing systems, and an alcohol is released with the alkoxy curing systems, and as such they are non-corrosive. Other neutral curing silicones might release acetone upon curing. These neutral curing silicones are extensively used as go-to products in electronic applications. In fact, Master Bond has gone one step further and started offering NASA low outgassing approved silicones. These unique systems are widely used in aerospace, electronic, and optical applications. Also, there are other neutral curing silicones which have been tested for biocompatibility as well.

Some examples include:

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Featured Products in this Tech Tip

MasterSil 910Med

Acetoxy type system. Bonding, sealing, coating compound. Pass USP Class VI testing and ISO cytotoxicity specifications. Tack free time 10-20 minutes at 75°F. Shore A hardness 20-30. Elongation 400-600%. Paste viscosity. Service temperature range -75°F to +400°F.

MasterSil 415

Optically clear, room temperature curing, no mix acetoxy type system. Good bond strength and outstanding elasticity. Serviceable from -185°F to 400°F.

MasterSil 930

Acetoxy type fluorosilicone for bonding, sealing, coating. Enhanced resistance to fuels, oils, solvents, water, greases. Paste viscosity. Withstands thermal cycling. Serviceable up to +450°F. No mix system.

MasterSil 921-LO

Passes NASA low outgassing test requirements. Spreads evenly and smoothly. Bonds well to many substrates. Wide service temperature range of -175°F to +500°F. Neutral curing, non-corrosive type system.

MasterSil 800

Ultra high temperature resistance up to 300°C. Superb flexibility and elasticity. Non-corrosive. One component, room temperature curing. Flowable.

MasterSil 705TC

Thermally conductive and electrically isolating. Easy to use silicone for high performance bonding, sealing and coating. Non-corrosive and room temperature curable. Ideally suited for applications requiring flexibility and high temperature resistance.

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Latest Developments

Case Study

EP114: Utilized to produce nanocomposites via vacuum infiltration

Master Bond EP114 is a two-component nano silica filled epoxy system which offers excellent dimensional stability and a very high glass transition temperature (Tg > 200°C upon curing. Its ultra-low initial mixed viscosity (500–1500 cps at room temperature) can be further reduced by preheating to a slightly higher temperature, making EP114 ideal for vacuum infiltration to produce dense nanocomposites, as shown in the case study below.

EP42HT-2ND-2Med Black Two Part Epoxy System

Technical Data Sheet Excerpt

EP42HT-2ND-2Med Black Product Information

Two component epoxy paste adhesive. Resists sterilization exposure. USP Class VI approved. Outstanding thermal stability and electrical insulation properties. Cures at ambient temperatures. Withstands multiple cycles of autoclaving, radiation, EtO and chemical sterilants. Post curing is recommended to optimize physical properties. Serviceable from -60°F to +450°F.

White Paper

Epoxy Compounds Get Even Tougher

With adhesive products, high performance and rigidity are often thought to go hand in hand. And it is true that the very best strength, thermal, chemical and electrical properties tend to be found in rigid compounds, especially epoxies. Yet there is a growing class of adhesives, sealants and coatings that add ductility to the long list of desirable epoxy properties. Master Bond’s white paper examines the use of these toughened epoxies.

White Paper

Spotlight on Silicones

Silicone adhesive systems have several important performance characteristics that distinguish them from other adhesive families. Learn more about their unique combination of flexibility, high temperature resistance, and more specialized engineering properties.

White Paper

Electrically Conductive Adhesives Make Right Connections

As electronic circuits become more complex, engineers are finding new ways to assemble and package them. Electrically conductive adhesives provide durable bonds with conductive paths to suit a variety of electronics applications. They can be engineered to combine bond strength and electrical conductivity with other service-critical properties.

Technical Data Sheet Excerpt

EP37-3FLF Product Information

Highly flexible, low viscosity, optically clear adhesive. Resistant to severe thermal cycling and thermal shock. Bonds well to dissimilar substrates. Low exotherm system. Long working life. Superior electrical insulation properties. Service temperature range from 4K to +250°F. One to one mix ratio by weight or volume.

Lab Report

Testing Adhesives for Resistance to Xylene

Many Master Bond epoxy systems are formulated with superior chemical resistance. In the lab, we continually test our materials by exposing them to specific chemicals over a long period of time. A common way of testing the chemical compatibility of an epoxy is immersing a sample in a chemical and measuring its change in weight over time. A significant loss or gain in weight would indicate a decreased ability of a material to stand up to chemical exposure. These tests allow us to more accurately recommend the right adhesive based on specific application requirements.In this experiment, we focused on testing our epoxies for their resistance to xylene. The compounds Master Bond used for testing are a variety of two component epoxies and one component UV curing systems with good overall chemical resistance. For the first round of testing the products tested were EP41S-5LV, EP42HT-2AO-1, EP46HT-2AO Black, UV16, UV22DC80-1, and UV26. For the second round of testing the products tested were EP125, EP42HT-2 and UV25. For the final round the products tested were EP41S-5, EP41S-1HT, EP62-1HT and EP62-1. These were compared against the resistance of a generic or standard non-chemically resistant epoxy. A few thin castings, roughly 2 inches in diameter and around 0.125 inches thick, were made for each product and cured in accordance to their specifications. Once the cured samples were created and initial weight was recorded, we submerged them in xylene. Then we continued recording weekly weight measurements. We carried on these tests with frequent weight recording for varying lengths of time based on the products performance. Below you will see the results for soaking results for around 10 months in the first batch, 2 years for the next batch, and greater than 2.5 years for the third batch of products. The castings were weighed periodically and the graphs shown below demonstrate the percentage of weight change over time.For the purpose of comparison, please notice that for each testing timeframe, a casting of a standard epoxy was also tested under these same conditions which served as a reference. As can be seen in the graphs, the standard epoxy was not as resistant to xylene as the other epoxies tested. It demonstrated a significantly greater change in weight over time, and the casting ultimately dissolved in xylene after 15 weeks.In general, a swelling of less than 4-5% can be considered excellent, especially since this test may be more rigorous compared to actual service conditions. It is also worth noting that in the context of a bonded joint or a potted assembly the exposure to xylene might not be as severe or direct as in the above test conditions.Please note, when choosing an epoxy for an application where the resistance to xylene is critical, many other factors must be considered in addition to the chemical resistance. Each of the epoxies in the charts above offers a distinct set of performance properties. For example, if thermal conductivity and high temperature resistance are needed, EP46HT-2AO Black may be a good option. If a fast cure is required and there is full access to a light source with no shadows, UV26 can be considered. For longer term exposure to xylene over several months to a year or longer, the likes of UV25, EP125 EP41S-5, and EP62-1 are excellent candidates. They all need to be processed or cured differently and although some of them can be cured at room temperature alone, for enhancing/optimizing chemical resistance especially to xylene, it is critical to add heat.Disclaimer: The findings in this article are not meant to be used for specification purposes.

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