Motorsport Composite Fundamentals: Polyester

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Polyester

07.17

00:00	Polyester resins are the workhorse of the composite industry.
00:03	This is the traditional and most widely used resin and fiber reinforced polymer composites.
00:09	Of all the resins we'll discuss in this section of the course, this is the lowest cost, but still offers reasonable mechanical properties and weathering resistance, essentially meaning it lasts well when exposed to the elements.
00:21	These are used extensively in the marine industry, specifically in boat building, where UV resistance is crucial, but also in tooling and mould making for high performance applications.
00:32	Polyester resin is most commonly used with chop strand matte fiberglass.
00:36	This is partially due to the way it dissolves the binder, allowing the reinforcement to be wet through more effectively.
00:43	It shouldn't be overlooked though that its cheaper price tag and less ideal mechanical properties simply match the compromises of chop strand matte fiberglass construction methods, where efficiency tends to rule over optimal performance.
00:56	Just before the construction process, the liquid polyester resin is mixed with another chemical catalyst, typically MEKP, which is short for Methyl Ethyl Ketone Peroxide.
01:08	This is generally at 1.5-2%, so for every 100mm of resin, there will be about 2mm of MEKP.
01:17	Mixing causes a chemical reaction which leads to the polymerisation of the resin, meaning it cures into a solid polymer, providing the structure for the final part and bonding to the fibers.
01:29	Generally, speaking, polyester resin cures faster than other resins, which naturally offers some benefits around the construction time.
01:37	However, it also comes with some disadvantages, such as increased shrinkage during curing, which causes internal stresses in the part, micro -cracking and sub-optimal bond to the fibers.
01:49	In other words, this makes it difficult to control the dimensional accuracy of the finished part and simply results in less strength.
01:57	In reality, there's a lot of different variations within polyester resins that lead to varying properties, one of the most common being whether the resin is iso or ortho.
02:07	This tells us the type of acid used in the resin, isothalic or orthothalic acid.
02:14	Again, we don't need to get too deep into the chemistry, all we need to know is that ortho polyester resins are more reactive, so they cure easily.
02:23	Because of this, we can be confident we've achieved a full cure, and this is why ortho resins are commonly used to construct food grade composites.
02:31	The downside of ortho resins is their mechanical properties.
02:35	Put short, they aren't very strong.
02:37	The resin is quite cheap though and can still be used for parts in less demanding applications.
02:42	Iso polyester resins offer much better mechanical properties and chemical resistance.
02:48	This isn't just good for parts but also allows us to make moulds with more stiffness and durability.
02:54	The term marine grade is often used for iso resins to communicate how they're fit for purpose.
03:01	As you'd expect however, they are more expensive than their ortho alternatives.
03:05	Circling back to the point we made on ortho resins being easier to cure than iso, it's important to understand that this is relative, and an iso polyester resin is still going to be much easier to laminate with and get a good cure on compared to the other resins that we'll discuss.
03:23	So, with that said, even though polyester resins can lack in mechanical properties compared to other resins, generally speaking, the results often turn out better for more enthusiast made parts, where there are some compromises in the process and not much experience.
03:39	There's another variation within polyester resins that we need to discuss before moving on to vinyl ester resins, and that's waxed and unwaxed resins.
03:47	Basically, polyester resin cure is air inhibited, which means that the surface layer in contact with air will not fully cure and remain tacky.
03:57	Put simply, when wax is added to the resin, it rises to the surface layer to make a barrier to the air.
04:03	This allows it to cure to a dry, non-tacky surface hard enough for sanding and painting.
04:10	The issue here is that we often work in stages in the lamination process.
04:15	Completing the first step with a thin layer of reinforcement that contours easily to the detailed areas, and then allowing this to partially cure before backing it up with a heavier layer.
04:27	When doing this, the wax will rise to the surface after we've completed that first layer and prevent a good bond to the following layers.
04:35	Using an unwaxed resin will prevent this, and then we could use a wax resin for the final layer to provide a good surface cure.
04:43	For this reason, a wax resin will often be referred to as a finishing resin.
04:47	However, be sure to double check when purchasing resins, as many laminating resins will still be waxed.
04:54	Finally, we have vinyl ester resins, which are like a combination of the polyester and epoxy resins that we'll be discussing next.
05:01	These offer better mechanical properties such as strength and impact resistance, sitting somewhere between polyester and epoxy on this scale, but also offer improved chemical resistance, making them suitable for parts like fuel tanks.
05:16	Lower shrinkage while curing helps retain dimensional accuracy in our parts as well.
05:21	They're more expensive than polyester resins and a bit more difficult to work with for hand laminations, but the step up in properties is generally worth it for our applications.
05:30	To summarise what we've discussed in this module, polyester resin is the most widely used resin in the fibre reinforced composites industry, specifically with fibreglass due to its relatively low cost while still having great weathering resistance and reasonable mechanical properties.
05:47	After being mixed with an MEKP catalyst, the pot life of the polyester resin is relatively short, which although lending itself to high production environments, can be detrimental to the dimensional quality and the strength in the finished product.
06:01	That being said, these resins can be formulated and chemically tailored to provide certain properties and process compatibility.
06:08	It's important to understand that while the theory points to lower mechanical properties than alternative resins, polyester resins are much easier to work with, so the practical realities often lead to better parts at the enthusiast level.
06:22	There's a lot of variations in the products available on the market, a key one being ISO or Ortho.
06:29	While Ortho resins are cheaper and easier to cure, for our applications we'll almost always use ISO polyester resins as they have much better mechanical properties.
06:39	Another variation we need to be aware of is waxed and unwaxed resins.
06:44	The wax rises to the surface sealing it from air and allowing a full cure to a hard, non-tacky surface.
06:51	This is great for a finishing coat, or if we're completing the entire lamination process in one step.
06:57	However, if we're ever laminating in stages, we should use an unwaxed resin to ensure we have a good bond between each stage.
07:06	Finelaster resins offer a step up in performance from polyester, finding a middle ground before moving on to epoxy resins.