Solving the problem of solvent

What harm do they cause? If not managed properly VOCs can be dangerous. These risks include: Health risks: many VOCs act as irritants or carcinogens Environment: VOCs can also undergo chemical reactions contributing to the formation of low-level ozone, an air pollutant. High concentrations of ozone can be detrimental to human and animal health causing respiratory diseases, damage to crops and plant life, as well as damaging building materials, rubbers and plastics Financial: most solvents are very expensive to buy and to dispose of as they are classified as 'special waste' Why manage solvent use? To save money - with minimal costs, a small to medium sized company can save anything between £5,000 and £20,000 a year. For larger companies the savings can be enormous. Legislation demands the control of VOC emissions. Effective management of solvent emissions ensures that you aren't breaking the law or shelling out a fortune just to comply. Better solvent management could also lower your insurance premiums. A survey carried out for the UK government (www.envirowise.gov.uk) has identified the following areas where there is potential to reduce solvent usage: * Up to 20% can evaporate into the air * Up to 10% can be lost by inefficient mixing procedures * Up to 10% can leak away through faulty pipes and tanks * Up to 10% can be wasted through overspray in spray booths * Up to 5% can remain as residue in cans and drums So what about leather? Leather finishers have made a tremendous effort to reduce their solvent usage. The problem is that this is still not enough (according to the legislation) and the higher the percentage reduction achieved, the more difficult it becomes to eliminate the small remaining percentage contained in the formulation. This leads us to consider when is a water based finish really water based? For example, a water lacquer emulsion will usually contain about 50% solvent. When this is mixed 1:1 with water the resultant solution will have a solvent content of 25%. This should still, therefore, be considered to be a solvent based finish. What about the situation where a polyurethane resin (average solvent content of 5%) is used as part of a base coat? The total mix may well have a solvent content of less than 1%. Is this still a solvent based finish? The tanner would say 'no' but the local authority would say 'yes'. What are the pros and cons? The principle problems when using solvent are generally considered to be the smell, the flammability and any potential health hazards. Smell When a leather finish is applied a combination of solvents are used, some with fast evaporation rates and some with slow to assist with finish flow and film formation. Those with slow evaporation rates tend not to disperse quickly but stay in the vicinity of the buildings after extraction often leading to complaints - particularly in older tanneries situated in residential areas. Health Certain solvents have been proven to act as irritants or carcinogens and have been banned as a consequence. Current solvents used by the industry do not appear to have these health hazards providing threshold limits are met but this is an everchanging situation as scientific knowledge develops. Flammability Unfortunately, the most efficient solvents used in leather finishing are those with low flash points (lower than 32°C). The obvious disadvantages are the extra care required to store and handle these products. The less obvious disadvantages are the extra costs involved in insurance, transport and the modifications of machines and buildings to prevent any hazard from the electrical supply. When we look at the advantages of solvent based finishes the main factors are drying, flow out and price. Drying The energy input required to dry a water based finish is far higher - requiring higher temperature, improved air circulation and/or longer exposure time. One of the biggest problems when converting to water based finishes is that the drying equipment may require modifications to improve efficiency. Flow Out Solvent finishes flow out better and it is much easier to obtain a uniform film. The products added to water based finishes to improve the flow can actually reduce the physical properties. Price This may not actually be the advantage that many perceive it to be. Chemical suppliers are faced with increasingly higher legislative costs so the cost of solvent based products will inevitably rise much faster than their water based counterparts. This relates to the handling costs of the solvents and little investment in production capacity means that existing manufacturing equipment will become more inefficient which, in turn, will result in an increase in cost. Where are we now? Solvent useage in tanneries has dropped dramatically with the efforts made by tanners to reduce their environmental impact. The chart below illustrates the impressive performace of a Dutch tannery between 1994 and 1999 but also illustrates how difficult it is to make further reductions despite governmental pressure to do so. The main source of solvents for leathers finished with water based systems is in top coat formulations (5-10%). Usually this originates from the cross-linker that is required to give performance characteristics - particularly in automotive leather. Generally, as these materials are sensitive to water, they have to be carried in solvent until the point when they are added to the application mix. There is no simple solution to this problem. When higher solids cross-linkers are used, the solvent content is reduced but they are more difficult to disperse in the mix and require in-line blending at the point of application. Latest developments The major chemical companies in the leather sector are investing heavily in solving the solvent problem. While there will continue to be challenging times ahead progress is being made. One company who has taken this issue on board is Stahl. They have launched a new waterborne activated polycarbodiimide crosslinking agent Crosslinker XR-5592 that they are now able to produce as VOC free. It has the benefit of the following characteristics: * Used in aqueous basecoat formulations * VOC free * Improves wet rub fastness * Improves print retention * Excellent resistance to accelerated ageing It is obvious that there still remains a need for some solvent to be used in leather finishing and, at this time, total elimination is not possible - but the industry is getting there! Further information on legislation The regulations surrounding the control of VOC emissions is vast and often complicated. There are several pieces of legislation that affect solvent users and these will differ globally. Below are some key EU/UK regulations: * Environmental Protection Act 1990 * Pollution Prevention and Control Regulations 2000 * EU Solvents Directive (1999/13/EC) * Health and Safety at Work Act 1974 * Control of Substances Hazardous to Health (COSHH) regulations A company's requirements for compliance under these regulations is dependent on the volume of solvents used, the type of solvents used and the processes they are used in.