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FEATURE


FASTENINGS & ADHESIVES STICKING WITH THE FACTS: A GU


Structural adhesive chemistries – such as polyurethanes (PUs), epoxies, and methyl methacrylates (MMAs) – often come as two components. These cure due to a chemical reaction that begins when the two parts are mixed, which creates some handling and processing requirements that manufacturers must consider to ensure a productive and safe process. Here, Kevin Brownsill, head of technical: learning and development at Intertronics, gives an overview of handling requirements for two-part structural adhesives


and disadvantages, which can be related to its processing requirements or technical properties. For the process to be suitable, it is important to consider storing, handling, dispensing and curing the adhesive, as well as its technical properties – such as how strongly it will bond the substrate. Importantly, the adhesives must be mixed sufficiently and in the correct ratio. So, what do manufacturers need to know about handling and processing these adhesives at the specification stage?


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STORAGE AND PACKAGING While an adhesive’s data sheet will list the specific storage requirements in detail, most two-part structural adhesives will last their whole shelf life if stored at room temperature in a cool dark place, provided they are resealed well. However, differences between the chemistries may impact how they are stored. For example, PUs are hygroscopic, so users may have to take care that the material does not come into contact with moisture, and that they are resealed in the original packaging. Some epoxies crystallise at lower temperatures


(usually, this is reversible). So, making sure that the adhesive is at room temperature before use is best practice – unless, of course, they are being used outside. As well as having simple storage requirements,


most structural adhesives come in packaging that makes handling straightforward. Two-part cartridges are common, and some are available in twin-pack sachets, or in separate bulk containers. Other than adhesives supplied in bulk, very little physical handling of the adhesive will be required – the adhesive, for example, can be applied directly from a cartridge, either manually or by incorporation into semi-automated or fully automated dispensing equipment. This brings benefits both to process efficiency and health and safety.


METER, MIX, AND DISPENSE Two part adhesives typically comprise of the resin (part A) and the curing agent (part B), and come in a range of different mix ratios, anywhere between 1:1 or 100:1. Achieving the


44 DESIGN SOLUTIONS MAY 2024


hen specifying a structural adhesive, there are many considerations. Each material will offer distinct advantages


desired performance requires a process that ensures the correct ratio and adequate mixing. Which mixing technology is appropriate will depend on the packaging the adhesive is supplied in and the volume of adhesive to be mixed, alongside other factors like viscosities and mix ratio. Some mixing technologies are able to meter, mix and dispense in one, whereas other technologies may mix only. To save an additional step decanting material, many manufacturers prefer to dispense directly from the original packaging. Structural adhesives can be dispensed using a vast range of dispensing technologies, from machines based on time/pressure to volumetric dispensing or precision jetting. It is worth contacting an adhesives specialist to advise on suitable equipment which will meet accuracy, repeatability, volume and speed requirements. If the adhesive is supplied in a two-part


cartridge, an appropriate static mixing nozzle is attached and the adhesive is extruded through it, mixing and dispensing both parts of the material in one go. Adhesives supplied in two-part twin packs can usually be mixed in the bag. For adhesives supplied in separate bulk


containers, manufacturers may choose to mix the material by hand or with specialist equipment. Mixing by hand will add an additional step to the process – weighing out the two components – and can lead to process inconsistencies if team members mix differently to one another. It can also introduce air to the mix that might be undesirable and need to be removed, adding another step to the process. There are several options available for those who choose to invest in metering and mixing equipment. One is the 2-K-DOS metering, mixing, and dispensing system, which can process ratios from 1:1 to 10:1, meters using gear pump technology, and can be set up to match the required reservoir size and dispensing hose length. For applications that require high precision,


however, manufactures may consider a volumetric system, such as one based on preeflow eco-DUO technology. These systems are based on positive displacement using a progressive cavity pump principle for accuracy ±1%, >99% of the time. Another option is a non-contact planetary


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