RTV Silicone Adhesive Sealants:
A Brief Introduction to Acetone Cure

Introduction
There are several types of curing mechanisms involved in the field of room temperature vulcanising (RTV) silicone sealants. They are normally described in terms of the small amount of the chemical produced during reaction with atmospheric moisture that allows the crosslinking mechanisms to occur, leading to the formation of an elastic and relatively tough silicone rubber.

Sealant Types
The most common types of silicone RTV sealants are:

Acetoxy
As the name suggest, the by-product of cure is acetic acid. Most people carrying out DIY work on showers, baths and kitchens are familiar with acetoxy cure silicone sealants that give off the characteristic odour resembling strong vinegar. This family of sealants dominates the world of silicone sealants because of their extensive use as construction sealants in buildings in such applications as expansion joints, window installations, etc. However, the formation of acetic acid places a restriction on their use in applications where non-corrosion is a prerequisite.

Oxime
The by-product of cure is called butan-2-one oxime or methylethylketoxime. This family of silicone sealants exhibits many attractive features such as excellent primerless adhesion to many substrates, and relatively non-pungent odour during cure (the odour is described as sweetly ammoniacal). However, there are indications that butan-2-one oxime may be a skin sensitiser. This is not a problem in assembly plants where adequate ventilation and good industrial hygiene are employed. Oxime sealants have their limitations in electronics/electrical applications because they can react with copper and some copper alloys. Nevertheless, this is a very important and useful family of silicone RTV sealants. High temperature operation of oxime sealants is limited to around 200°C, with only a few special grades operable at 240°C.

Alkoxy
The by-product of cure is either methanol (so-called "methoxy" sealants) or more commonly a mixture of methanol and ethanol. Both types come under the generic name of "Alkoxy" silicone sealants. Both methyl alcohol (methanol) and ethyl alcohol (ethanol) have faint, relatively pleasant odours. During the curing cycle the odour arising from the titanium catalyst and the mixed alcohol by-products is not unpleasant. However, alcohols - particularly methanol – can be quite hazardous at relatively low concentrations, and users are still advised to work in a well-ventilated area. Alkoxy sealants are completely neutral and do not react with copper and its alloys, or with other metals. They also exhibit excellent primerless adhesion to many types of substrate. Like oxime sealants, the thermal stability of Alkoxy sealants is limited to around 180 to 240°C.

Amine
This is a relatively new family of sealants. The by-product of cure in this case is called cyclohexylamine, a relatively hazardous and corrosive primary amine with a characteristic smell. The main applications for this group of sealants are in the construction industry.

A New Sealant Type

Acetone
This is a much less common family of silicone sealants. As the name suggests, the by-product of cure is acetone. Acetone is a chemical with relatively low toxicity. The following is offered as a means of explanation and putting this into context. The Occupational Exposure Limit is a measure of the safe working concentration of a chemical. This is divided into three categories:
MEL – Maximum Exposure Limit
STEL – Short Term Exposure Limit (15 minute period)
LTEL – Long Term Exposure Limit (8 hour period)
Concentrations are given in parts per million (ppm)

In addition each chemical may carry so-called "Risk Phrases" that describe other potential hazards associated with it.

The following table gives the values for each of the above taken from the official government figures published in EH40/2003.

Key to Risk Phrases
R21: Harmful in contact with skin
R21/22: harmful in contact with skin and if swallowed
R23/24/25: Toxic by inhalation, in contact with skin and if swallowed
R34: Causes burns
R35: Causes severe burns
R36: Irritating to eyes
R39: Danger of very serious irreversible effects
R40: Limited evidence of carcinogenic effects
R41: Risk of serious damage to eyes
R43: May cause sensitisation by skin contact
R66: Repeated exposure may cause skin dryness and cracking
R67: Vapour may cause drowsiness and dizziness

As one can see from the above exposure information, acetone sealants appear to be safer than most other silicone sealants.

ACC’s new range of Neutral Cure Acetone Sealants offers the following key features:

• Non-corrosive to copper and its alloys
• Excellent primerless adhesion to most substrates
• Adhesion improves with the age of the bond
• Relatively low level of toxicity to the operator
• Rate of cure comparable to many acetoxy sealants
• High temperature grades with performance up to 300°C
• Thermally conductive grades available
• Thixotropic and self-levelling grades available
• Excellent electrical properties

Under current legislation, there are no risk phrases associated with the final formulations of any of the new range of ACC Acetone Cure Sealants.