New class of plastics promise better environmental characteristics
The first group is known as metallocene polyolefins. This group has been under development for several decades, but spent around 15 years of this time embroiled in lawsuits, resulting in very little progress until now. The advantage of the polymerisation process involved is that very clean polyolefins, made up of only carbon and hydrogen, can now be precisely engineered to have very specific properties. Polyolefins can now be viable replacements for PVC which, as we have seen, carries a high environmental burden. We should begin to see a wide range of metallocene polyolefin based building products, from window frames and roofing membranes to wall cladding and cable sheathing in the near future.

Plastics in the Building Industry Plastics are a large group of synthesised carbon based compounds produced by polymerisation. The building blocks of a plastic (monomers) are combined into a large variety of long, chain-like polymers. The type of plastic produced depends on the nature of the monomers used and their configuration in the polymer chain. Plastics can be as hard as stone or as soft as cotton, as strong as steel, clear as glass or as elastic as rubber.

The plastic industry is forever growing, with the USA now producing around 39 million tonnes per year, with around 60,000 different compounds being produced. In North America around 22% of plastic sales are for building products, making construction a very significant sector in the plastics industry. Apart from the obvious, such as cladding, flooring, pipework, cable sheathing and foam insulation, plastics are used in everything from the concrete foundations to the final coat of paint.

There are more than 50 families of plastics in existence, the most important for the building industry are listed in the table below. There are two broad groups, thermoplastics that can be repeatedly softened and reformed (recyclable) and thermosetting plastics which set permanently after polymerisation making plastics that tend to be durable and heat resistant. Plastics can also be segregated according to their production process, i.e. plastics can be moulded, extruded, pultruded, cast, foamed or spun. Plastics use in the Past Humans have used natural plastics such as horn and tortoiseshell for thousands of years, and natural shellac has been moulded into buttons and knobs for a long time. The first man made plastic came in 1862, called pyroxylin, it was derived from cellulose, a very common natural polymer. The following year work began on finding a man made substitute for the many tonnes of ivory used in making billiard balls each year. The first truly synthetic plastic (Bakelite) was produced in 1909. Plastics really took off in the 1930s, when petroleum based plastics were developed, DuPont and ICI leading the way with products made from polystyrene, PVC, nylon, neoprene and polyethylene. The use of polyethylene by the Allies during World War 2 knocked 270kg off the weight of aircraft radar systems.

The first major building plastic was a vinyl floor tile introduced in 1933 at an exhibition in Chicago. PVC window frames first appeared in Germany in the 1950s, and today PVC is by far the dominant plastic in building use (see Table 1).



Environmental Issues with construction Plastics



If you look at the issue of plastics in construction it is important to accept that it is not all bad news. Plastic products require very little maintenance, and their low weight reduces transport energy requirements. The best U-values are obtained by foamed plastic insulants, which may be important where insulation thickness is an issue. Plastics play an important role in sealing and caulking buildings to ensure air-tightness. Also polymer resins make engineered wood a possibility.

It is the very complexity of plastics which gives rise to their most troubling environmental impacts. The inability of organisms to break down plastics contributes to durability, but it also results in the persistence of these materials in landfills, and even in living systems.

Fossil fuels are the primary feedstocks for nearly all plastics, and the pollution associated with oil extraction and transport is an important consideration. If plastics are incinerated at end of use, the stored carbon will contribute to global warming. Plastics account for around 10% of fossil fuel consumption. Of the main plastics, PVC is the least dependant on fossil fuels, 40% of its content being chlorine derived from brine. Recycled content can be added to some feedstocks to reduce fossil fuel use.

Primary environmental concerns during manufacture are release of hazardous materials into the environment, and health of production workers. Many common plastics rely on hazardous constituents. As an example, vinyl chloride monomer (in PVC) and benzene (in polystyrene) are known carcinogens, and styrene itself is a possible cause of cancer. Plasticisers such as phthalates used in PVC are being studied as possible carcinogens and hormone disrupters. The greatest threats now come from plastics made in the Third World, as the developed world has done a lot to clean up its act.

Many foamed plastics were produced using ozone-depleting chemicals such as CFCs and HCFCs, but these are now largely phased out. Another consideration is the embodied energy of the various plastics, see Table 2.

Disposal of plastics presents possibly the greatest dilemma. They don’t break down easily, adding significantly to long-term solid waste disposal headaches, with around 17 million tonnes a year entering the waste stream in the USA alone. Recycling (thermoplastics only) generally leads to conversion into a lower-grade product, and there are major logistical problems associated with collection, transport and separation of such lightweight, low value materials. Only around 7% of plastics were recycled in the year 2000, with the figure for construction plastics being even lower.

Some plastics produce persistent organic pollutants (POPs) when disposed of. These compounds bioaccumulate in living systems, and become more concentrated at the top of the food chain. This is particularly the case for plastics containing chlorine or bromine, which can become sources for POP dioxins when burned, particularly at low temperature such as on a building site bonfire. PVC is of the greatest concern for dioxin generation, and should not be burnt under any circumstances. Unfortunately, some of the plastics being promoted as PVC alternatives contain brominated fire retardants. New Developments

There are two broad categories of plastic currently being developed that may offer significant environmental advantages over existing plastics.

The first group is known as metallocene polyolefins. This group has been under development for several decades, but spent around 15 years of this time embroiled in lawsuits, resulting in very little progress until now. The advantage of the polymerisation process involved is that very clean polyolefins, made up of only carbon and hydrogen, can now be precisely engineered to have very specific properties. Polyolefins can now be viable replacements for PVC which, as we have seen, carries a high environmental burden. We should begin to see a wide range of metallocene polyolefin based building products, from window frames and roofing membranes to wall cladding and cable sheathing in the near future.

The second promising group is bio-plastics. These show multiple environmental benefits as follows:

Because polymerisation is catalysed by enzymes in living organisms, the chemical bonds can also be broken down naturally – they are biodegradable.

Bio-plastics are renewable – fossil fuel may be used for processing but not as a feedstock.

Feedstocks produced agriculturally – supports hard hit farm economies.

Less risk of harmful effluent or emissions during manufacture.

There are several types of bio-plastic, some of which are produced from simple plant sugars, particularly from corn. A word of caution here about one type of plastic currently under development by our old friends Monsanto. They (and a few other companies) have genetically engineered plants such as mustard and corn to grow a plastic in their leaf tissue. Conclusions

It would be very difficult to build without any plastics, though most of us in green building prefer to use more natural non-fossil based materials. Plastics have important attributes which make total avoidance churlish, such as water resistance, flexibility, light weight, durability etc. However, plastics such as PVC, which result in eventual release of POPs, should be avoided if at all possible, particularly for major components such as window frames and rainwater goods. As metallocene polyolefin technology matures, more benign alternatives for many polluting plastics should appear – watch out for them! Acknowledgements

A large much of the information given above was adapted from an article entitled ‘Plastics in Construction’ published in Environmental Building News, Vol 10 No 7/8.