Plastics Versus Weather: The Nature of a Losing Battle
Weather has always tested the mettle of human construction, and in all cases wins eventually. The sunlight that warms us also harbors a great hatred of plastics; the high heat of summer turns dark-hulled boats into furnaces, and can wreak havoc on paints, finishes, and fillers. The water that gives life to most creatures on earth is also among the most corrosive solvents that exist. Oxygen, ozone, acid rain, tannins, and a whole slew of other little nasties wait to turn our innocent polymeric creations into awful-looking maintenance hogs.
There are almost no boats today—even the most traditional wooden kind—without a few plastic parts exposed to the elements. Plastics have become an integral part of our everyday lives, and especially of our lives on the water—yet as resilient as they are, they do degrade. Nature wins.
In this article, we hope simply to provide some insight into the reasons for plastic degradation, and to recap some of our coverage of products that can help slow the process nearly to a halt, given diligence and elbow-grease.
Understanding the degradation of plastics requires a basic knowledge of what a plastic is. There are essentially two different types, known as thermosets and thermoplastics. Thermosets are represented by plastics like epoxy, polyester, and polyurethane. Once they're cured, they can't be re-melted or returned to a liquid state. A loaf of baked bread would be a good analogy. Thermoplastics are those that can be melted and become solid upon cooling. They may be re-melted again with the introduction of more heat. They would include plastics like polyethylene, polycarbonate, and PVC, to mention a few. In this case, think of a bar of chocolate.
Polyester is a good example of a thermoset. It's comprised of many polyester molecule chains that have a specific range of lengths, and styrene monomer, which is what smells so bad when you open the can. If you have a fiberglass boat, all or much of it is likely to be built with polyester, although many boats do now contain varying amounts of vinylester, a resin more resistant to water penetration.
Each individual chain is made up of repeating units of polyester molecules called mers. The mers are like the links in a chain, all the same and all hooked together by molecular bonds. Many (poly) of the links can be chemically assembled end to end (in a process called polymerization), and the results are referred to as polymers.
Polyester molecules may have in the range of 1,000 to 5,000 or so links or mers in their chain. When polyester is made, it's a very thick, resinous material comprised of many chains that are tangled or sitting close together like a mass of many millions of submicroscopic threads.
Uncured polyester resin is a thick liquid that results from the very thick resinous polymer being dissolved in a thinner styrene monomer ("mono" meaning "one" and defining a single unit of mer that could be assembled into a chain). Its consistency is similar to motor oil.
The Free Radical Cure
In chemistry, atoms and molecules strive to maintain a state where their electrons are paired. If one of a pair of electrons is missing from an atom or molecule, that atom or molecule becomes electrically unbalanced. It then goes on a hunt to find a replacement electron, either by stealing it or sharing it with another atom or molecule. Meanwhile, the roving electron, called a "free radical," can be highly reactive and destructive to some molecular species. (These days, free radicals are more commonly known as the stuff that ruins our bodies as we age. They are the reason why a whole new market of dietary supplements called antioxidants has sprung to life.)
Free radicals can also be harnessed and put to use. In the case of curing polyester resin (polyester polymer and styrene monomer), a free radical in the form of benzoyl peroxide is introduced as a catalyst that causes the styrene monomer molecules to link themselves between the polyester polymer chains. In millions of crosslinking reactions, the catalyst steals electrons from the styrene monomer molecules as well as the polyester molecules. In turn both the styrene and polyester molecules are now unstable and seek electrons to redeem their losses. They share electrons between them, thus forming the chemical crosslinks that hold the monomer and polymer together.
The result is a fully crosslinked network that is solid, similar to long lengths of chain that have single links of a different type built between them. The previously individual chains can no longer move independently of the now crosslinked adjacent chains, even with the addition of heat energy. They might vibrate and move, but they cannot separate.
Thermoplastics, on the other hand, are comprised of single chains of polymer (previously assembled when the polymer is made) that are not crosslinked together. The chains remain independent. If heat energy is applied to them they will eventually possess enough movement to become fluid-like and melt, just like ice becoming liquid. In this case, the chains can move independently of each other and the molten plastic can flow, but it remains viscous because the chains are long compared to individual molecules.
The lengths of thermoplastic polymer chains can vary widely depending on the type of plastic and how it's made. They may be relatively short like the polyester chains or as high as over a million repeating units, as in some thermoplastic elastomers.
The Range of the Chains
By rule of thumb, the longer the polymer chains, the more flexible the plastic. Many different types of elastomers or rubbers typically have chain lengths in the range of 200,000 repeating units. To complicate matters, rubbers can be based on many different kinds of polymer and can be either thermoset or thermoplastic. Polymers can be made up of one type of plastic molecular repeating unit, like polyethylene, or made up of different types of polymer in combination. These are known as copolymers (two types of polymer) like styrene butadiene (SBR rubber) or terpolymers (three types of polymer) like acrylonitrile butadiene styrene (ABS).
In fact, there are myriads of combinations and types of polymer that make up the vast realm we call plastics. You could think of this realm as a giant erector set with different kinds of pieces that can be used to create many different types of structures. The way that the environment treats each of these structures can be as unique as the structures themselves.
Polymer attack is typically either random along the length of its chains or involves the unzipping of chains from their ends—and may involve both at the same time. As chains unzip, monomer units may escape as gas—causing blistering in paints, for example. In many cases, the materials under the paints are responsible for the blisters, as opposed to the paints themselves.
If we envision polymer molecules as long erector-set components, we can also understand that the connectors and links have different properties. Some possess more energy than others, and in any case, since it's energy that binds them together, energy can break them apart.
In polymer degradation, the energy can come from applied heat or ultraviolet radiation. Other molecules like the free radicals we discussed earlier can also bring unstable energy into the scheme of molecular attack. For those of us who remember some high-school chemistry, oxygen exists as a pair of atoms (O2) that are happily stable. An ozone molecule (O3) on the other hand, is unstable, and is well-known for its ability to assist in the destruction of primary bonds (connectors) in polymers containing double bonds called dienes. Dienes are the root structure of a large group of elastomers. Elastomers are often referred to as rubbers and include natural rubber, neoprene, isoprene, butadiene, and all copolymeric materials including this root structure like ABS (butadiene).
Ozone is created by several means, one of which is the passing of an electrical discharge (such as lightning) through atmospheric oxygen. This is why a diver never hangs his wet suit near his furnace: The furnace is an ozone generator (it uses an electrical arc through air to burn oil).
UV radiation in conjunction with auto exhaust can also yield ozone. The O3 molecule breaks the double bond on the backbone or primary chain of the polymer, and, with the addition of moisture, will cleave the chain. Cumulative chain cleavage results in some polymers turning brown and losing their flexibility. Depending on what polymeric species is being considered, chemical fragments of the original chain can combine with other species like water to form acids, which can in turn attack polymer structures further.
Oxidation originally meant a reaction where oxygen combined with another substance. Today its broad meaning includes any reaction in which electrons are transferred from one molecule or atom to another — either borrowed (reduction) or added (oxidation). As we discussed earlier, the gain and loss of electrons creates instability in molecules and atoms. In the case of oxidation, chemical bonds in the chain are broken and form radicals which can grow themselves at the further expense of the polymer, introducing large amounts of oxygen in the form of other chemical species such as hydroxyl, acid or ester groups, and others. The instability equates to the breaking or stealing of a link in the erector chain. The same breakage in the chemical structure results in polymer degradation. A loss of mechanical properties and chemical resistances results. Metallic pigments like those in red and blue paints and gelcoats can oxidize and chalk badly. Rubber coatings and toughened paints or rubber-modified resins and polymers will yellow and become more brittle.
Hydrolysis is another form of degradation that occurs in polymers. In the hydrolysis process, water can attack a linkage and convert it into a variety of species, one of which may be acidic. This is typical of the process with polyesters and marine blisters. Again, the result is chain breakage or fragmenting. In the case of polyesters, it is the ester linkage that is attacked.
All plastics absorb moisture, but at a wide variety of rates. Some of the more moisture-absorbent polymers include polycarbonate, nylons, and polyesters. Most of these are also subject to hydrolysis. Urethanes are also susceptible, but do not absorb moisture as readily. For the sake of comparison, the ester linkages found in vinylesters give them superior resistance to breakdown by this mechanism.
Acids and bases have the ability to break bonds through electrochemical means. Depending on the strength of the entity, degradation may occur slowly or quickly. It's a well-known fact that boats located in freshwater areas of the northeast (lakes and waterways) develop plastic-related degradations more quickly than those in areas where the pH of the water is closer to neutral. Thanks to years of coal burning, steel smelting, and a high density of automobiles, people in the northeast are forced to work a bit harder to keep their freshwater boats in shape.
Microbial attack is another form of degradation that can occur in polymers. Although not as quick or severe as other degradative systems, they can lead to just as many maintenance headaches as others. Mildew growth on vinyls is among the most common. Today many paints and polymers (vinyl siding, for example) have antimicrobial additives to assist in combating these little beasts.
The products of the degradation of polymers can be harmful to the life expectancy of the materials surrounding them. They can include styrene, toluene, benzene, acids, esters, and many others. Like the classic degradation that causes gelcoat blistering in FRP boats, these materials can and will attack their parent polymers. Degradation usually occurs in stages where radicals, energy, water, or some other source causes breakage of the polymer chain or crosslinks.
As this process goes on, other molecular species arise out of the cleaved fragments of the polymer molecule. When combined with water or on their own, they begin to inflict new and different damage to the polymer, which results in loss of gloss, loss of color, peeling, embrittlement, cracking, crazing, tearing, and many other types of damage.
Ultimately, as with most choices we have in life, there are trade-offs. Some of the hardest, most weather-resistant polymers are brittle. We add rubber to them to make them more impact-resistant and durable, but in the process we reduce their ozone resistance and hence their weatherability.
The stabilization of polymers to improve their resistances to these attackers is a science unto itself. Most plastics aren't exclusively comprised of whatever they're called—they're either originally formulated or post-compounded by their suppliers to contain additives such as lubricants, heat stabilizers, colorants, flame retardants, mold releases, and plasticizers, to name a few. They can also contain a host of additives that are used to combat some of the degradation mechanisms we've discussed: UV inhibitors are very commonly used in plastics that will be used for outdoor applications. Antioxidants and radical scavengers may also be used.
Antioxidants inhibit atmospheric degradation and its negative effects on polymers during production, storage, and use. As oxidation generates radicals, the antioxidant or radical scavengers convert them to less reactive or completely unreactive radicals by building molecular groups around the radicals that hinders or shields approach by other molecules. Many of these belong to the hindered phenol family. Others act in a secondary way later in the degradation process by decomposing such radicals. These may include those found in the phosphite family. They're typically used in combination to yield a synergistic effect where the combination outperforms either component separately.
If We're So Smart...
Given all this available knowledge, why do we still wind up with plastic components on our boats that don't hold up and require constant maintenance or replacement? The answer is in the engineering. If you haven't heard it by now, it's time you knew that the marine industry is very weak in the area of engineering. This includes process, reliability/durability, structural, and, most of all, quality engineering.
Not all builders, but many production builders, especially in the powerboat area, fall into this category. A larger percentage of sailing yacht producers are stronger in this area because more sailboats are designed by naval architects who actually calculate loads. Some may be a bit weak in composites, but know where to go to get those areas treated properly.
Even when quality components are specified by engineering, the purchasing department will often step in with the goal of reducing the cost of the finished product, and beat up a supplier for a similar but less expensive material or component: The company that molds a plastic cleat gets pressure from the cleat supplier, who gets pressure from the builder, who may be trying to cut costs to the bone in order to stay in business. The result is the selection of a lower-cost, less durable molding resin to make the cleat. The engineers at the company that molds the cleats might not consider the ramifications of the change, or the cleat supplier might override their objections, or perhaps the purchasing manager at the builder might override them all above the objections of his own engineers. This is just one possible scenario. We wouldn't lay the blame exclusively on purchasing people, but this happens all too often.
Some companies don't compromise performance for cost, and rely on the intelligence of engineering to specify good stuff even if it's more expensive. The quality and reputation of their boats allows them to get their price and make their margins even with the more expensive parts and materials.
Just as we humans use sunscreen to protect our skin, we can use various waxes and conditioners to protect the plastic parts on our boats. In a marine environment, we have to apply them more regularly than on our cars, because the sun is stronger and the horizon is longer than on shore.
Waxes are the best therapy for most gelcoat finishes. This is even more true if they incorporate a UV screen. Wax yields a surface layer that insulates many plastic surfaces from moisture, ozone and oxygen. While it can't stop their permeation completely, it certainly can slow them down.
Other conditioners for plastics contain plasticizers. These materials are like oils that can condition and keep plastics soft and pliable. Beware of these types of materials— they shouldn't be used on some plastics such as polycarbonate (Lexan) and other crystalline systems (in many cases clear plastics). They can swell the surface and cause crazing, especially if they're low in molecular chain length. They are, however, good for other plastics like vinyl.
There are dozens of products to help prolong the life of various plastics in their battle against nature. Practical Sailor has tested and reviewed many of these products in the past (see sidebar) and we'll continue to do so. In the meantime, if you're up against a big plastic preservation or restoration project on board, call the tech service departments of the products you're considering and ask them questions: Is this product a good candidate for the job in question? How was it tested? What standards were used in the testing? Is there any published data?
Finally, it's important to remember two things that provide the sandwich for whatever material you use to preserve your plastic: First, for any paint or coating, surface preparation is of immense importance in making the application work correctly to begin with. Second, regular, steady maintenance is the key to staying ahead of the ravages of Mother Nature.
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