VOCs (volatile organic compounds) are blamed for damaging the earth's ozone layer and thereby contributing to global warming. These chemicals are also dangerous to human health. As a result of these concerns, the last twenty years have seen a concerted effort by governments to control their release into the environment.
The Importance of Control
The human body reacts to VOCs when breathed in. Some of them cause dermatitis and styrene is 'reasonably anticipated to be a human carcinogen' according to the US National Toxicology Program. (However, there is an abundance for reliable evidence that disputes this).
Most advanced countries require that any production facility which uses VOC-based resins in any significant quantity has to have an emission control system as well as providing clean breathing air and protective suiting, gloves and goggles for its workers.
Industry too is striving to minimize the use of these chemicals. Styrene and other esters can leach out into the atmosphere after the composite has been manufactured, and this is known as off gassing. Automobile manufacturers have recognized a condition which they call 'sick car syndrome' and construction firms too recognize 'sick house syndrome'. Customers don't like these fumes coming from the products they buy.
Scale of the Problem
VOCs make up a significant portion of traditional resin formulations used for composites. Typically, a lay-up which requires 10 gallons of conventional vinyl ester resin would put 30 lbs of VOCs into the air. A large bathtub manufacturer could produce 250,000 tons per year of styrene fumes.
In the US, since 2006, those firms that produce more than 10 tons of volatile organic compound (VOC) emissions per year have to meet the Environmental Protection Agency's Maximum Achievable Control Technology (MACT).
There are a number of solutions to the problem:
- Resins with reduced VOC content. These are known as low-HAP resins (low Hazardous Air Pollutant). Most of these resin formulations offer a range of 32% to 37% styrene content.
- New resins with zero VOC content. So-called green epoxy resins are versatile and are cited as a direct replacement for regular resins even in aerospace applications. These are available as one- and two-part formulations.
Physical Plant and Process Changes
- VOC abatement equipment requires significant capital investment. Typical solutions involve incineration of fumes. A bathtub manufacturer might incinerate their ¼ million tons of styrene fumes, but for the plant to do that is well beyond the resources of most small and medium sized composite manufacturers.
- Low emission production uses non-atomized spraying for open molding. Closed-mold resin transfer molding and closed die injection for pultrusion provide direct physical constraint of emissions.
Most smaller manufacturers combine low- or zero-VOC resins with the low emission production techniques.
Post Production Emissions
'Bake-out' is now being used by builders before occupancy of a new building. This process is designed to drive the VOCs out of the construction materials by increasing the temperature in the building to as much as 110 deg. F. Outdoor air exchange is maintained so that hazardous gases are emitted from the building. In commercial buildings this process is carried out with all furnishings in place as composites and plastics are heavily used in the interior. The procedure takes up to two weeks and is obviously performed prior to occupancy.
At present there do not appear to be any automobile manufacturers who use bake-out, though it would seem that it would be something that the component manufacturers (e.g. composite dashboard assemblies, carpeting and so on) could carry out.
VOC Controls - the Future
Unintended consequences are often a result of legislation, and now some larger manufacturers who have invested heavily in VOC abatement are finding that their systems are so efficient that they can use resin formulations with higher VOC content than they used before. That is certainly not what the legislators intended.
Demand is driving researchers to develop 'greener' resins because it is a certainty that controls will tighten. Resins based on recycled raw materials are now being marketed.
It is inevitable that low- and zero-VOC resins will become more widely used as production volumes increase and raw material prices fall.