In this post we explore the factors that determine the durability of a silicone adhesive, as well as the respective deterioration processes. Additionally, we break down different types of silicone adhesives and their particularities such as: how they age, the risks of using them after the expiration date, and preventive measures that can be taken to extend their effective lifetimes.
Silicone adhesives have several specific properties and advantages compared to other organic elastomers and sealants. They are known for their low flammability and toxicity, excellent dielectric properties and very high degradation temperatures. In addition, they have excellent resistance to UV, weathering, high and low temperatures and chemicals. Generally, their effective lifetimes can reach two and a half years if their temperatures are maintained accurately.
1. What factors determine the durability of a silicone adhesive?
A silicone adhesive’s effective lifetime is calculated using factors in its immediate environment; the main ones being:
As a rule of thumb, when the physical properties of a product have degraded to 50% of their original values, the material is reaching the end of its useful lifetime and is no longer suitable for its initial purpose. To determine the durability of a silicone adhesive, it is necessary to demonstrate that the adhesive is still fit for purpose once its lifetime is finished.
2. The deterioration process of silicone adhesives
All silicone adhesives start to age from the day of their production. This aging implies a progressive loss of the physical properties over time due to numerous external factors. Each manufacturer guarantees correct functioning of the product according to the specifications that mark the limit of its durability. This is, however, only valid if the product has been kept tightly sealed in its original packaging, maintained within its specified temperatures, and is being used within its useful lifetime.
Next, we will explore the deterioration and durability of each type of silicone adhesive. By means of simple tests we will explain how the users can (always under their responsibility) judge the use of the product after the expiration date has been exceeded.
3. Can an expired silicone cartridge be used?
The primary way of determining if silicone adhesives are in useful condition is by checking whether they achieve satisfactory adhesion within a reasonable time period. This period would be seven days of curing for monocomponent adhesives and 24 hours for bicomponents. The consistency of the product must also be ‘solid’, without being sticky or soft in any case.
The peeling test can be done to check whether the adhesion is satisfactory, as shown below.
Peeling test: adhesive failure. Undesired as the adhesive peels off very easily.
Peeling test: cohesive failure. Desired, the adhesive breaks since there is good bond strength.
4. Types of silicone adhesives: aging, risks, recommendations and preventive measures
A. Monocomponent sealants or adhesives
As the adhesive deteriorates over time, a gradual curing takes place inside the package, which in the extreme case ends in complete curing. If we risk using it after its useful life, the viscosity will increase so that in the extreme case it will not be possible to extrude it further. Adhesion will deteriorate and cure time will vary. To confirm a good condition of the product we can perform the following tests:
- Peel adhesion test.
- Extrusion test
- Curing and surface tack test.
As a preventive measure it is imperative to store the adhesive sealed hermetically and under the temperatures prescribed by the manufacturer.
Neutral or Alcoxy curing
Curing begins to degrade and, in the worst case, ceases to take place as the adhesive ages. Keep in mind that silicone sealants of the Alcoxy type generally age faster than those of the acetic type. Once the product has expired, curing slows down so that in extreme cases the product will no longer cure. The surface will be sticky and the adhesion will deteriorate extremely.
To ensure that the product is still in good condition we can perform the following actions:
- Peel adhesion test.
- Extrusion test
- Cure and stickiness test.
To promote the durability of the product it is advisable to store it tightly sealed and under the temperatures prescribed by the manufacturer.
Neutral or Oxime curing
In the case of Oxime neutral curing monocomponent sealants or adhesives, all aging, risks, recommendations and preventive measures are very similar to those of the acetic monocomponents explained above.
B. Bicomponent liquid sealants, adhesives or silicones
RTV condensation (mold making or adhesives type Ex: EA-2626)
Component A forms the basis of the mixture. In this type of product, the viscosity increases gradually over time and in the extreme case it will be difficult to extrude. Keep in mind that the aging process is more marked in condensing bicomponent silicones than in those cured by addition. This is because the former contain much less water in the formulation. Once the components A and B have been mixed, the curing time may be faster. In case of manual mixing, it will be necessary to increase the vacuum evacuation time to eliminate the bubbles, which in effect reduces the fluidity of the mixture.
The following tests can be performed to ensure that the product retains its properties:
- Test to verify the fluidity of component A and the mixture. To do this, ideally the viscosity is measured; though if that is not possible, it is recommended to check the fluidity with the naked eye. Component A must flow freely without ‘elastic’ (thixotropic) behavior. Likewise, the mixture should flow freely until 90 seconds after mixing.
- Test to verify that the mixture performs the curing correctly, without sticking, within a period of 30 hours and at a temperature of 25 ° C.
As preventive measures it is necessary to:
- Maintain a storage temperature that does not exceed the specified maximum.
- Hermetically seal the adhesive containers to prevent contact with moisture.
Component B is the catalyst of the mixture. When the product ages there is a slowdown in curing and a progressive decantation of ingredients (phase separation). The entry of water through the seals of the drum creates a cured surface layer that needs to be removed. Curing time is usually faster once both components are mixed. If it is to be mixed manually, we will increase the vacuum evacuation time to avoid bubbles.
It is advisable to apply the following measures:
- Maintain an adequate storage temperature.
- Keep sealed containers without contact with moisture.
- Turn the drum regularly to homogenize the components well.
Products for addition cure molding
As the product ages, an increase in viscosity can be observed reducing the durability of the silicone adhesive.
Once the components A and B are mixed, a slight increase in viscosity will be observed, which will make it necessary to increase the evacuation time in vacuum to eliminate bubbles when we mix it manually. In addition, a possible slowdown of cure may occur.
To confirm that the product is in good condition we can perform the following tests:
- Test to verify the fluidity of component A and the mixture. Ideally, viscosity is measured, but if this is not possible it is advisable to visually assess fluidity. Component A must flow freely without ‘elastic’ (thixotropic) behavior. Likewise, the mixture should flow freely until 90 seconds after mixing.
- Test to verify that the mixture cures completely, without being sticky, within a time of 30 hours and a temperature of 25Cº.
In this case the product ages and may cause a slight slowdown in curing.
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