Pressure-sensitive adhesives (PSAs) are commonly used in industrial applications and in the manufacture of consumer electronics products. The most common form factor for these PSAs is an adhesive tape, and the advantages are obvious. Tape is always ready to use and bonds immediately to the surface. The instantaneous initial bond strength, also known as the tack, is built up by applying only a brief contact pressure.
This allows direct handling of the bonded components and potentially enables a short processing time, making PSA tape a highly attractive solution for industrial bonding applications. For these reasons, tape has become a standard in many of today’s manufacturing processes, and large/flat surfaces are well suited for tape application.
However, tapes have two drawbacks:
- When it comes to small and complex geometries, they can be difficult to handle due to their lack of stiffness; and
- The adhesive surface is sticky as soon as the protective liner is removed, meaning the tape can easily stick to other materials/surfaces before it even gets to where it needs to be.
These drawbacks make tape application difficult to automate (and for difficult, read ‘complex and expensive’). Accordingly, even in relatively high-volume manufacturing scenarios, tape processes are at best semi-automated, and often fully manual. Combined with the continuing trend of ever smaller and more complex structures, tapes face significant limitations in terms of processability and economic efficiency.
Production Runs
A liquid PSA does not have the above drawbacks. It can be dispensed on to a component of virtually any size and shape, and the process can be fully automated because it’s just like applying any other liquid adhesive.
After dispensing, the liquid PSA is cured with UV light and the surface becomes instantly tacky, and now has the properties of a classic tape. Indeed, adhesives with tape-like properties are suitable for almost any application and open up many new manufacturing process possibilities.
The benefits of a liquid PSA over traditional tape become evident when considering several aspects of the bonding procedure (see also table 1, below).
A liquid PSA can be handled like any other liquid adhesive during the dispensing process, especially when it comes to dealing with small features, corners and/or complex 3D geometries.
Depending on which liquid PSA is used (there are several on the market) as well as the structure/component to which it is to be applied, dispensing can be by needle, jetting, droplets, stencil printing, screen printing or slot-die coating. Automation of the dispensing process is straightforward. Also, the dispensing process can be paused at any time. Note: halting the placement of a tape is not as easy, and often requires a restart (a new piece of tape, for example).
Tape-like
As mentioned, after the liquid PSA has been dispensed, it is irradiated with UV light. This typically is of a wavelength of 360 or 400nm for most liquid PSAs on the market.
This irradiation causes the liquid adhesive to enter a ‘tape phase’ within seconds. This phase is marked by a significant increase in viscosity, thus preventing unwanted flowing of the dispensed pattern.
The tacky surface of the liquid PSA will easily adhere to a second substrate upon the application of moderate pressure. The resulting immediate functional strength allows for instant processing of the bonded components and enables short cycle times.
Despite its speed, the process is still robust, as the liquid PSA’s desired bonding properties are triggered only upon irradiation. Also, the tackiness is maintained during the tape phase over several minutes (or even several days, depending on the product).
Compared to conventional UV curing processes, where at least one substrate must be transparent for the irradiation stage, no such limitations exist with a liquid PSA, since the irradiation is performed before joining the components. Also, as no heat is required for final curing, the process is especially suited for applications where thermal stress must be kept at a minimum. After irradiation, liquid PSAs cure at room temperature without an additional process step.
Regarding Table 1, you may have noticed the inclusion of ‘CO2 footprint’ so let’s touch on that for a moment. Tapes need to be manufactured beforehand. Granted, liquid PSAs do too, but making a tape is a complex and energy-consuming process, one that involves applying an adhesive layer to the tape carrier material and the use of a solvent.
Also, tapes have a mandatory protective liner that, once removed, is just waste. There’s also the issue of wasted tape through trimming.
Chemistry
There are a variety of liquid PSA products on the market, all based on different chemistries to cover multiple requirement profiles. In general, they will be acrylate- or epoxy-based.
Acrylate products offer characteristics similar to common tapes. Their soft and highly flexible properties provide a good peel resistance on various substrates. Also, their Young’s modulus is typically low (usually less than 10MPa), and they are fully cured within seconds of irradiation. Just like the majority of acrylate-based tapes, these liquid alternatives offer a medium bond strength right after joining thanks to their high tackiness and they remain at a medium strength level after 24 hours, following which the characteristic physical post-crosslinking effects are completed.
Acrylate-based liquid PSAs, such as Delo Photobond PS, are therefore ideal for bonding tasks that require a short cycle time and only a moderate final strength. As an additional benefit, most products offer a basically unlimited tape phase, which allows components to be joined even weeks later; if components need to be shipped, for example. Owing to the excellent dampening properties and low outgassing values, acrylate-based liquid PSAs are, for example, used in the assembly of smartphone speakers on structures with sub-millimetre widths.
Epoxy-based liquid PSA products, on the other hand, provide a higher final bond strength, and their Young’s modulus can exceed 1GPa. They also have a higher temperature stability (up to +150°C for some products on the market) and a good chemical resistance, making them ideal for harsh environment applications, as often found in the automotive sector, for example.
Figure 2 provides an approximate illustration of the initial and final bond strength of two of Delo’s liquid PSAs. Delo Katiobond PS is an example of an epoxy-based PSA, and Delo Photobond PS is an example of an acrylate-based PSA product.
Summary
Liquid PSAs combine the properties of pressure-sensitive tapes, and even have a ‘tape phase’. Thanks to the tacky surface after irradiation, components can be joined by the application of moderate pressure. A bond forms immediately, but its strength will be governed by the PSA’s chemistry (acrylate-based or epoxy-based at a fundamental level, but there are other factors).
Compared to tapes, the dispensing process is easy to automate, and the size and shape of the component should present minimal challenges.
It would be wrong to say that liquid PSAs are better than tapes in all applications. However, liquid PSAs are undoubtedly a serious contender in volume manufacturing scenarios where automation makes sense and where tricky geometries need to be catered for.
