There is potentially a very long list of “things” that the cured adhesive must provide. It is worth noting that despite the technical values that appear on a data sheet, these usually apply only to the adhesive, and the adhesive will behave very differently when it is bonding two parts together. The complete assembly must be tested by the customer to ensure reliability. After all, one customer’s aluminium might be a very different (grade of) aluminium than another customer is using.
Here are some of these “things”, again in no particular order!
Bond strength – this is usually one of the first factors considered when choosing an adhesive! How strong must the join be? Most customers will reply “as strong as possible”. There are two elements to “bond strength”: cohesive bond strength, and adhesive bond strength.
Adhesive bond strength is defined as the bond strength of the adhesive to the materials being bonded. In every case, this has to be tested in the specific application under discussion. No two materials give the same bond strength, so an adhesive that bonds well to stainless steel might be terrible with aluminium. Even within a generic material such as Polycarbonate, care needs to be taken. There are in excess of 100 grades of Polycarbonate available, and each will probably bond differently! Generally, the technical data sheet will point in the right direction, so it’s a very good starting point, but only a starting point.
Cohesive bond strength is more straightforward, it is the inherent strength of the adhesive, and is independent of the materials that it is bonding. It will be specified on the data sheet, and is measured simply by curing a test coupon of the adhesive and then pulling it apart until it breaks! The force required to break it is the strength of the adhesive, called the tensile shear strength (sometime just the tensile strength).
Flexibility – there are different ways to indicate the flexibility of an adhesive: elongation, Young’s Modulus, sometimes shore hardness. Elongation is the easiest to both measure and visualise – it is calculated in the same test that determines the tensile strength of the adhesive. The percentage that the test coupon has strength when it breaks is the elongation, also specified in the data sheet – simple!
Temperature resistance – this will usually be specified on a data sheet, and depending on the application, will be more or less important to each customer. For those working in the military, aerospace and automotive fields, the temperature range of interest will be from -40°C or -55°C to +125ºC or +150ºC. For commercial applications, it will be much narrower, usually from -20°C to +80°C.
For tests such as temperature cycling and thermal shock, no data sheet will state “our adhesives survive” these tests. The temperature rating specified on the data sheet is a very good guideline (for example, an adhesive that is only rated to 80°C should not be considered for an application that must work up to 120°C.), but it not an absolute guarantee. Again, the complete assembly must be tested to ensure these requirements are met.
Chemical resistance – there are so many different chemicals that could possibly come in contact with the bonded assembly that it is absolutely impossible to test for all of them. Most application areas will have their own set of chemicals that the adhesive must survive. For example, in the automotive industry, this set usually consists of petrol, diesel, AdBlue, engine oil, brake fluid acetone, and maybe DI water. However, the mobile phone industry will have a completely different set of fluids, coffee, alcohol and water being the most frequent!
As a general guideline when choosing an adhesive for an application that needs a high chemical resistance, keep away from flexible adhesives. These tend to have a less rigid molecular chain once cured, so it is easier for chemicals to penetrate the adhesive and attack the bond. Hard, rigid adhesives generally offer the best chemical resistance.
Applications that will see changes in temperature over their lifetime need to consider how the adhesive expands and contracts with these temperature changes. Two very important characteristics of the adhesive need to be consider here – the glass transition temperature (Tg) of the adhesive, and the coefficient of thermal expansion (CTE) of the adhesive. These are directly related to each other.
The Tg is defined as the temperature at which the mechanical and physical properties of the adhesive start to change. Below the Tg, the adhesive network is in a solidified, relatively immobile condition. Above the Tg, the polymer chains of the adhesive may move slightly relative to each other, independent of the degree of cross-linking (i.e. curing). This may cause the adhesive to “soften” slightly above the Tg, but the degree of softening varies according to the adhesive.
In an ideal world, the chosen adhesive should have a Tg in excess of the operating temperature of the end application. As the Tg is exceeded, the adhesive softens, even if it is only very slightly. As the operating temperature comes back down below the Tg, the adhesive will harden slightly. Doing this thousands of times during the lifetime of the application will induce stress into the adhesive, causing the join to fail more quickly than it should.
CTE is defined as how the size of an object changes with changes in temperature, and is usually expressed in parts per million (ppm). Technical data sheets should contain two CTE values – ∝1 for the CTE below the Tg, and ∝2 for the CTE above the Tg. They will nearly always be significantly different values!
When bonding similar materials together, e.g. glass to glass, CTE is not an issue as both materials will expand and contract at the same rate. However, when bonding dissimilar materials such as glass and aluminium, the trick is to choose an adhesive with a CTE that is somewhere in between the CTE’s of the glass and the aluminium – around 4ppm for glass and 23ppm for aluminium. While it may not sound much, this can be a huge difference for large pieces.