The primary purpose of a protective coating is to protect the substrate from the ravages of the elements. First we will select the correct chemical family for maximum moisture resistance with our eye on compatibility with the subsequent needed additives for resistance to ultraviolet light.
To combat the effects of water permeability, the chemical formulation choices of the coating consist of urethanes, epoxides, and the reaction products of certain natural oils (such as linseed oil or tung oil) or other kinds of resins called alkyds. These compounds are more resistant to water than acrylic resins and many of the polyesters.
As mentioned before, we can eliminate current technology epoxides as topcoats due to their lack of UV resistance and their incompatibility with UV absorbing additives. We can also eliminate conventional varnishes and paints due to their incompatibility with antioxidants.
This directs us toward the family of polyurethanes. First, to remove the confusion of terms, the term “urethane” actually refers to an element (or group) of the chemical structure which, when replicated in a chain, becomes a poly—urethane compound. But in common conversation the more accurate “polyurethane” often yields to the slang term “urethane.”
Polyurethane coating systems have a high degree of resistance to the damaging effects of the ultraviolet rays from the sun. With only a slight amount of ultraviolet inhibitor added polyurethane paints can be made nearly immune to ultraviolet ray damage. Polyurethane coatings also have the highest degree of chip, nick, and scratch resistance of all currently available paint systems when properly applied.
Polyurethane Chemical Families:
Polyurethanes are polymers containing urethane groups (-NH-CO-O-), created by reacting isocyanates with polyols and chain extenders. By varying the nature of these three components, we can create literally thousands of different combinations of properties.
Polyurethanes are divided into two fundamental chemical families; Aromatics and Aliphatics.
Aromatic molecules are composed of resonant benzene rings. The energy-absorbing molecular resonance frequency band lies within the visible light spectrum. As the molecule shakes at a certain frequency it absorbs energy thus changing color. Aromatics do not have to change very much to affect the nature of the reflected light thus appearing heavily discolored. Because of their chemical stability they tend to be excellent for applications requiring heat and/or chemical resistance. They do, however, suffer from poor light stability.
Aromatic urethanes are the less expensive of the polyurethanes and are often used for interior, lining or underground applications. Depending upon their formulation, aromatic polyurethanes will exhibit a certain degree of color change ("yellowing") over time with prolonged UV exposure.
Aromatic urethanes are based on aromatic isocyanates (e.g. MDI and toluene diisocyanate, TDI) and mostly polyether polyols. As a point of reference, TDI is also used to cure polyurethane foams. We know from experience that such foams will discolor and crumble with extended exposure to strong sunlight.
At the low end of the urethane product spectrum are the one-part urethane coatings typically sold in your neighborhood hardware store. These include products such as “Varathane.”
Next are the moisture-cured urethanes, also called latex-based, in which the curing reaction takes place in the presence of moisture. While these products have their place, most are simply not suitable for exterior applications.
Aliphatic urethanes are relatively high in cost but provide the best UV resistance and color stability among all types of industrial coatings. They are therefore often used for exterior applications and any other places where color stability is concerned. The term “aliphatic” refers to straight chains of carbon molecules in the backbone of the compound (i.e., linear). The "linear" term refers to a simple saturated line of carbon atoms rather than the alternating unsaturated bonds characteristic of aromatic molecules.
Aliphatic polyurethanes are compounds based on aliphatic isocyanates (e.g. HDI and IPDI) and mostly polyester and/or acrylic polyols.
When the curing agent is purely aliphatic (linear) it has much better UV resistance. A classic polyurethane curing agent is HMDI (hexamethylene diisocyanate) which is a ring but a saturated, non-resonant structure which does not turn into a resonant ring upon exposure to UV.
The aliphatics can be further classified as either acrylic-saturated or polyester-saturated.
Acrylic Aliphatic/Isocyanate (linear) Polyurethanes:
The acrylic-based polyurethanes are two-part systems typically used effectively for automotive finishes. (House of Kolors by Valspar, for example)
Acrylic urethane systems are easy to apply but have the disadvantage that they are not as flexible once applied to the finished surface. Acrylic urethanes require a high quantity of ultraviolet inhibitors to be added to achieve the natural ultraviolet resistance of polyurethane automotive paints. With respect to the sun, elements, and general toughness of the paint, acrylic polyurethanes are not the most durable over time.
This chemical family of urethanes represents the current top of the line technology for weather resistance. Examples include our line of mil-spec StratoChem aircraft paints, “Awlgrip” by US Paint, “Epifanes Polyurethane Clear Top Coat” and DuPont’s famous “Imron.” This system is typically used for aircraft paint, hanger/warehouse floors, and marine finishes. The addition of the polyester saturation improves upon the already excellent properties of linear (aliphatic) polyurethanes.
Top Coat Additives For UV Resistance
We now have selected not only the correct chemical family (polyurethanes) but we also now have a thorough understanding of the type of urethanes we want to use and why. We must now provide the correct additives to this matrix to protect both it and the underlying substrate from the ravages of ultraviolet light.
Resisting UV Light
All organic compounds, whether synthetic or natural, will eventually be attacked and broken down by ultraviolet light. Even some of the best urethane paints will lose about half their gloss in two years of outdoor exposure. It is not enough to make a clear coating which is not degraded by ultraviolet light. Such a coating would simply transmit the ultraviolet light through to the wood underneath. Thus, the need for another family of compounds called ultraviolet absorbers.
The most effective UV absorbers (“UVAs”) are chemical compounds that vibrate at the molecular level when impacted by photons of light. Through this vibration, the light energy is converted into heat and dissipated through the coating. All vibrating materials have a fatigue life. This molecular vibration can be sustained for only so long until it wears out. Eventually these absorbers no longer are able to vibrate and so stop absorbing the energy of UV radiation. The higher the concentration of UV absorbers the manufacturer puts into the coating formulation, the longer the coating will last. But, of course, UV absorbers add to the cost of the product.
Typical examples of UV absorbers include the chemical “Tetratriazole” found under the trade name “Tinuvin” manufactured by Ciba-Geigy.
Coatings experience an undesired degradation when exposed to sunlight. This starts first with loss of gloss and/or discoloration followed by cracking and blistering leading eventually to total destruction of the film.
Common UV Absorbers
UV absorbers have the ability to convert the energy absorbed from UV light into heat, via a mechanism called keto-enol tautomerism. This heat can then dissipate through the substrate.
Due to this cyclic mechanism, UV absorbers remain active during the life-time of the coating in which they reside. Obviously this mechanism favors thick coatings (high path length) with high concentrations of very effective UV absorbers. In practice, however, the surface of the coating cannot be 100% protected. This is why these UVAs are often used in concert with products having a different stabilization mechanism (e.g. HALS), thus complementing their stabilization and protection properties.
UV Reflectors & Refractors
In addition to UV absorbers, another kind of ultraviolet protection is derived from small particles of certain minerals. These particles are small enough to allow passage of most visible light but large enough to scatter and reflect most of the shorter-wavelength ultraviolet light. Unlike the absorbers, these microscopic reflectors never wear out. They do, however, have the disadvantage of adding to the haziness of the coating as their concentration or film thickness is increased.
Antioxidants & Light Stabilizers
Even the best absorbers and reflectors are not 100% effective, some UV enters the coating and initiates the photo-oxidation process. It is this process that creates the free radicals that want to cross-link with the coating thus reducing flexibility. A clearcoat stabilized by UV-absorbers still remains unprotected at its immediate surface. Even with the use of UVAs in high performance coatings, the coating surface is still vulnerable—the coatings are physically protected by UV-absorbers that stop UV-radiation reaching the object to be protected.
To minimize this effect, we must scavenge these free radical intermediates. This is done through the use of chemical compounds that act as antioxidants called light stabilizers or, more specifically, hindered amine light stabilizers (HALS). HALS are special chemicals that prevent the photolytic degradation of the binder as well as the substrate. They are designed to trap and neutralize free radicals in the coating before they can cause their crosslinking damage. These antioxidants (similar in chemical structure to vitamin-E) work in the same way antioxidant vitamins work in your body.
Hindered Amine Light Stabilizers (HALS) are extremely efficient stabilizers against light-induced degradation of most polymers. They do not absorb UV radiation, but act to inhibit degradation of the polymer, thus extending its durability. Significant levels of stabilization are achieved at relatively low concentrations. HALS’ high efficiency and longevity are due to a cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process. They also protect polymers from thermal degradation and can be used as thermal stabilizers.
Conventional varnishes cure by a chemical reaction between the oil and the oxygen in the air. This is called oxidation, so it is easy to see that any attempted addition of antioxidants to a conventional varnish would poison the curing reaction. It is therefore impossible to add antioxidants to varnish and is why any traditional varnish will lose its flexibility fairly rapidly with exposure to the sun.
Chemically cured coatings such as two-component polyurethanes are compatible with both ultraviolet absorbers and antioxidant light stabilizers, thus providing the best resistance to UV effects.
The HALS and the UVA additives are typically used in a 2 to 1 ratio together, where the overall formula might have, for example, around 2% by weight Tinuvin 292 (HALS) and 1% by weight of any of the 400 series of Tinuvin tetratraziole UV blockers.
These additives add an amber or yellow tint to the coating resin. Visually, you can generally conclude that the higher quality coating corresponds to a more yellow or amber color, and thus better UV protection. As one would expect, the Ciba UV additives add significantly to a product’s cost.
As with most things in life, some is good, but more is not necessarily better. A balance must be held between UV resistance and flexibility. You could formulate the most expensive, and most highly-loaded UV resistant formula but if the urethanes cross-link too tightly the product becomes brittle.
The effectiveness of the final coating is also dependent upon the film mil thickness—too thin, and the sunlight penetrates past even the best blockers. Too thick and you risk polymer shrinkage cracks.
Even the best UV protection in the best aliphatic urethane formula will eventually fatigue. This is why you want to apply the maximum safe mil thickness so that as the surface of the coating fatigues, the sub-layer is protected.
The Best Plasticizers
A high-quality top coat will also contain a low-volatility plasticizer which does not evaporate quickly with age or time. These plasticizers are generally more expensive and therefore not found in the low-end products.