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The Final Coat

by: Kent Schneider 09/14/09


When we talk about "greening" a room one of the first suggestions that will inevitably come up is the use of low VOC paints.  We agree with this, but there are a great number of unpainted surfaces in home.  Some of these will remain natural and have no top coat at all, but the vast majority will have some sort of final coat added for protection, decoration, or both. Unfortunately many of these coatings are laden with VOC's.  What follows is a look at the clear coats, and oils commonly used in homes, with an overview of the benefits and potential dangers of their use.

 

 

For those of you who are unaware, VOC is short for Volatile Organic Compound.  Volatile Organic Compounds are those chemical additives found in household products (such as paints, cleaners, and furniture) that can become gas at room temperature.  This gas can then slowly evaporate into the air over time.  Once released, these compounds often combine with other airborne pollutants to form a "household smog" or ground level ozone.  In low doses these pollutants may pass through our body with little or no effect, but over time the concentrations of these chemicals can increase.  Today, modern homes are made to be very energy efficient.  This means that they are good at keeping the cold air in the house during the summer and out during the winter.  To do this they are built more air tight, but this same energy saving benefit creates the unfriendly side effect of trapping pollution.  This wouldn't be an issue if "household smog" were harmless. However, according to the World Health Organization "there is a one thousand fold increase in potential human exposure to a typical pollutant released indoors versus the same release to outdoor air" and many of these finishes can give off a bevy of pollutants.  Children are particularly susceptible to these pollutants and scientists are now beginning to see a link between the increasing rates of childhood asthma and indoor air quality.

 

We know modern building practices are trapping increased concentrations VOC's in our homes but where are these VOC’s coming from?  As a home decor studio we are going to focus our discussion on common furniture finishes, but many of these same finishes will be used on floors, trim, decks and other parts of the home.  Clear wood finishes and stains are formulated to showcase or enhance the natural grain in wood and often serve to protect the wood.  Wood finishes can be broken into two general categories; film finishes and penetrating finishes.  While all wood finishes will "penetrate" to some extent the primary difference between the two is the ability of film finishes to be built up.  Regardless of the finish type, they will generally be made of three components; Pigment/Dye, Resin, and Solvent.  Any one or all three of the components may be leading to the VOC build up in a home.


Pigments & Dyes
The first component in most clear coats is a pigment or dye to add color.  Pigments and dyes are slightly different in how they work but for our purposes we will lump them together.  Early pigments were derived from biological and mineral sources.  Some of these early colors included Carmine Red created by crushing dried cochineal insects and Indian Yellow which was created by collecting the urine of cows that were fed mangos (honest).  While these early pigments and dyes were effective they were also difficult to acquire and lacked color consistency.  The industrial revolution and advances in chemistry led to a new breed of synthetic dyes and pigments.  Unfortunately, many of these relied on the use of heavy metal and other toxic substances such as chromium, mercury, and lead.  Modern pigments and dyes are constantly being developed to reduce toxicity while increasing consistency but some toxicity persists in most.

 

Resin
The second common component is a resin or binder.  Resin is needed to hold the finish to the surface of the wood.  For centuries resins were created from natural sources such as tree sap and insect secretions (a common insect secretion used is called "Lac".  This secretion was collected from tree branches and then mixed with natural oils and turpentine to create a clear protective finish.  This is where the names "shellac" and "lacquer" are derived from.). Most natural resins have since been replaced by synthetic counterparts which are often petroleum based.  Petroleum based synthetic resin had the initial effect of creating more durable finishes, but the chemicals used were prone to off-gassing and required more solvent (see below).  In the early 1970's environmentalists began to take note of this and increasing concern over human contact with these chemicals led to research into the development of low-voc (or high solid) water based resins (discussed in more detail below).

 

Solvent
The last component in most finishes is a solvent.  Solvents are used to maintain the finish in liquid form until it has been used.  Once the finish is applied the solvent begins to dissipate (into your home) and the finish will "cure".  Examples of common solvents include mineral spirits, toluene, naptha, and in some instances water.  Several standard wood finishes such as Polyurethane Varnish, Shellac, and Lacquer are notorious for having high solvent and VOC content.  In response to this, producers developed so called "water-based" finishes.  These products are actually made using microscopic droplets of solvent based finish suspended in water.  As the water evaporates the solvent based finish droplets come together and coalesce.  The use of water as a suspension medium allows for less solvent use and therefore reduces the overall VOC content (by as much as 20%).

 

So what is an eco friendly finish?
So now we know what goes into these finishes but what should you look for if you are concerned about the environment?  As with most things there is no hard and fast answer here.  Let’s consider the pros and cons of each:

  • Varnishes – All resin and solvent mixed wood finishes, such as Polyurethane, can technically be referred to under the umbrella name of “Varnish”.  Standard petroleum based varnishes are readily available and relatively inexpensive.  They are generally thought to be more resistant to water vapor and heat and are difficult to damage.  They can be applied easily and produce consistent predictable results.  On the down side the often require a moderate to heavy solvent content to work properly.  This solvent releases VOC’s while applying and takes a few days to dissipate as the finish cures.  Clean up of equipment also requires the use of additional solvents which must be properly disposed of.  The US EPA limits the VOC content of clear varnish at 450 grams per liter.  Third party certifiers such as GreenSeal will only certify Varnishes of 350 g/L or less.
  • Shellacs – Pure shellac is actually a mixture of a naturally occurring secretion from the lac insect and distilled alcohol.  Many craftsmen still prefer to mix their own shellac, which can make consistency and application a bit more difficult.  Shellac is capable of producing a beautiful sheen but is susceptible to damage from heat and liquid.  This weakness makes its use as a final coat somewhat limiting. Shellac is an evaporative finish using spirits as the solvent.  The solvent content is relatively high.  The US EPA limits the VOC content of clear shellac at 730 g/L.  This higher limit reflects that fact that the solvent base for shellac (distilled alcohol) is generally considered less toxic.
  • Lacquers – Lacquer is the finish of choice for many furniture makers as it is well suited for accenting wood grain.  Like Varnish, lacquer is widely available and relatively easy to apply.  It is more resistant to heat and water than shellac (although not as resistant as varnish) and can be built up and polished to a nice sheen.  Lacquer is an evaporative finish meaning that it cures as the solvent evaporates into the air.  For this reason is can have a very high and toxic solvent content.  The US EPA limits lacquer VOC’s to 680 g/L.  Third party certifiers such as GreenSeal will only certify Lacquers of 550 g/L or less.
  • Natural Oils – Natural oils are generally considered penetrating finishes which cure by absorbing oxygen from the air.  Typical examples of oil finishes are Linseed or Tung Oil.  Be aware that very few products labeled as “oils” are actually pure oils.  In fact many if not most are actually a type of whipping varnish, or polymerized oil that have had heavy metals and solvents added to hasten dry time (usually salts of cobalt, manganese, or zinc).  Oils used in a pure state (pure Tung Oil or Linseed Oil) have very long dry times and provide little resistance to water penetration and almost no protection from scratching.  Because these oils dry slowly with air exposure they require less solvent when it is introduced.  Oils finishes will have a lower VOC content as a result but are not suitable for all applications. 
  • Water Based – Water based finishes are created by taking microscopic particles if solvent based finishes and suspending them in water.  The use of water as a suspension medium reduces VOC content, but does not mean that they are completely benign.  In fact many water based finishes contain as much as 20% solvent by volume.  Water based finishes have the benefit of being non-yellowing and quick drying.  They are very scuff resistant and moderately resistant to heat and water damage.  While water based finishes are generally considered to be easier to clean up it is important to remember that it does contain solvents which when rinsed will be introduced into the water system.

 

Conclusions:

The best finish depends on the application.  At Verde Home we believe that when evaluating the “green-ness” of a product one must take into consideration not only personal health factors but also global impact and durability.  Table tops, coffee tables and other surfaces which have a high probability of water spillage and contact with heat are often best served with a moderate to highly protective surface coating such as Varnish, Lacquer, or WaterBase to ensure prolonged use.  If you are primarily concerned with indoor air quality then off gassing should be considered.  Look for lower VOC products such as WaterBase and Oil, but be aware that a low VOC label does not mean that it is necessarily a low "off gasser".  The EPA regulates VOC's that contribute to low level smog.  They do not however regulate an exhaustive list of all VOC's that may be harmful to humans if they have nothing to do with low level smog.  Many manufacturers of "low VOC" products are keenly aware of this and concentrate efforts on removing only the regulated VOC's.  Look for finishes that have full disclosure of their contents such as AFM Polyureseal and Acrylacq.  And as usual…when in doubt ask questions.

 

*Special Thanks to Jay Watts of AFM Safecoat who was very forthcoming and helpful in our research.


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