Motorsport Composite Fundamentals: Adhesives, Repair Fillers, Fairing Compounds
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Adhesives, Repair Fillers, Fairing Compounds
06.28
| 00:00 | Adhesives have been mentioned at various stages when discussing bonding parts together. |
| 00:05 | But we're yet to discuss in detail what adhesives we can use and for what jobs. |
| 00:10 | In this module we'll also be discussing repair and fairing fillers. |
| 00:15 | These are somewhat like body fillers and sit somewhere between coatings and adhesives. |
| 00:20 | We'll start back with adhesives though and these might be used to bond composite parts together or other components like captive hardware to our composite parts. |
| 00:29 | The easy go-to option for composite to composite or composite to metal bonding is an MMA or Methyl Methacrylate Adhesive. |
| 00:39 | These are acrylic resin based and like most resins are in two parts. |
| 00:44 | MMAs are usually sold in a dispenser that will automatically dispense the appropriate amount of resin and hardener together through a mixer nozzle ready for application. |
| 00:54 | MMAs are a fast curing adhesive with only a very short working time, usually around six minutes or so. |
| 01:01 | They will bond to a wide range of materials including carbon fiber, metals commonly used for automotive applications like aluminium, titanium and steel, as well as plastic. |
| 01:11 | They're easy to use in that they're tolerant of oily or unprepared surfaces, basically requiring less care for quick or difficult jobs. |
| 01:20 | With that said, we generally prepare all composite bonding jobs the same way, with a light keying of the surface using sandpaper and a thorough clean. |
| 01:28 | Even though MMAs are easy to use in this way, they still have great peel and shear strength, as well as being tough and durable for bonding cured composite parts together and to metal inserts and fittings. |
| 01:40 | They're also relatively cheap compared to the other adhesives we'll discuss shortly. |
| 01:45 | The other thing to note is that MMAs are often colored intentionally, which means that they might not be suitable for certain acidic applications where a clear adhesive is required. |
| 01:55 | The main alternative to MMAs is epoxy adhesives, which will usually be around twice the price. |
| 02:01 | While two part adhesives are the norm here, heat curable single component epoxies are available, although they don't usually provide the same strength and the parts must be clamped well to avoid movement with heat during curing. |
| 02:15 | Two part epoxies usually provide a stronger bond than MMAs, so they're generally the preferred option for bonding fixings like captive studs into composite parts. |
| 02:25 | It should be noted that not all epoxies are stronger, and there are so many different formulations for different purposes, like clear epoxies that prioritize clarity, or modified epoxies that provide a flexible joint or extremely high temperature resistance. |
| 02:41 | There's also a range of different curing times, from fast at around 10 minutes working time to slow at two or more hours, which can be a good option if we have specific processing requirements for our projects. |
| 02:53 | Lastly on the topic of adhesives, we mentioned in our earlier discussions in the resin section of the course, that polyurethane resins can be used as adhesives to join composite parts. |
| 03:04 | These aren't as high performance in terms of strength, but offer the best gap filling capability and are soft, flexible and tough. |
| 03:12 | If we're bonding a carbon fiber hood skin to an inner bracing for example, then a polyurethane adhesive would generally be a good choice, as there'll likely be some inaccuracies in a big part that we'll need to fill, and we'd also expect some amount of flex. |
| 03:28 | Okay, let's move on to cover repair fillers and fairing compounds. |
| 03:32 | Like we might fill small imperfections or repairs in our car's bodywork with filler before painting, we can do a similar thing with our composite panels. |
| 03:41 | Of course, the goal is always to avoid or minimize this, as it comes with some drawbacks including weight, but sometimes that's inevitable. |
| 03:50 | Larger filling of surface imperfections and general filling and shaping of low areas so they blend with the rest of the surface is referred to as fairing in composites, because we're making the surface fair to the eye and touch. |
| 04:03 | This is a term used more commonly in the marine industry with boat building, but it still has its place on fiberglass body panels. |
| 04:10 | An example of a fairing compound is the very popular West Systems MicroLite filler. |
| 04:16 | The process is somewhat like that of using a body filler with a few extra steps. |
| 04:21 | First we'll lightly sand and then clean the surface, then mix the resin and hardener to match the initial construction of the panel. |
| 04:29 | And then in some cases apply a thin layer of this to the surface as something of a primer. |
| 04:35 | We'll then thoroughly mix the filler and the resin to the desired consistency, which is usually similar to smooth peanut butter. |
| 04:43 | From here it's much like applying body filler to the surface, smoothing it to the desired shape. |
| 04:49 | After curing, the excess compound can be sanded smooth and flat for a fair surface ready for whatever coatings we'll use next. |
| 04:57 | On the other hand, repairs on the likes of small cracks in composites should be done with a specific repair filler that's compatible with the resin or gel coat we use to construct the part. |
| 05:07 | These are much thinner than a fairing compound and very thixotropic, so they can be teased into the crack but hold their place while curing. |
| 05:16 | Following which they're often sanded flat and then clear coated over to match the rest of the part. |
| 05:21 | We'll leave it there for now, as we don't want to be getting too deep into the practical skills at this point of the course. |
| 05:27 | In summary, bonding of composite parts to each other and to other materials is usually done with MMAs, epoxy adhesives or polyurethane adhesives. |
| 05:36 | MMA or acrylic based adhesives are relatively cheap and easy to use while still providing a strong and durable bond. |
| 05:44 | For ultimate strength and durability though, we'll usually turn to epoxies. |
| 05:48 | Although there's a wide range of formulations available for all different applications, so be sure to review the supplier's offerings. |
| 05:56 | Polyurethane adhesives are more suited to larger assemblies that will need some gap filling and will require some flexibility with movement. |
| 06:04 | Fairing compounds are similar to body fillers and are mixed with our resins before being applied to the surface and worked smooth. |
| 06:11 | For small cracks and composite parts, particularly carbon fibre components, we'll use a specific repair compound with the same base resin as used in the construction. |
| 06:21 | It can be applied and worked into the crack before being sanded smooth and finished. |
