TechArchive

Original article date: April 1998

An advanced material in a mundane application? Aramid fibres have been trying to break into general manufacturing for many years. Now, an air conditioning system provides the opportunity.

Nomex has never been far from the limelight since its introduction in the late 1960s. However, to a large degree, the high awareness of the product has stemmed from adoptions for a number of high performance application, such as space suits, composites for aircraft and racing yachts and protective apparel for Formula 1.

In a more mundane context, Nomex in its honeycomb structure form is playing a major role in an innovative breakthrough in air conditioning. The Model DA 150 from Engelhard/ICC is a desiccant air conditioning unit which combines ventilation, humidity control, air cleaning and temperature regulation in one air handling system.

In conventional A-C systems, the processing of lowering humidity introduces a chill factor. Though welcome in some high temperature climatic zones, this is inappropriate in many other situations where hot/humid and cold/humid conditions co-exist. The new system can regulate humidity without further air cooling, even in situations where relatively low ambient temperatures prevail. And in situations of warm humidity, the DA 150 150Os dehumidifying process lowers the air temperature, often to the point that little or no auxiliary cooling is necessary. These factors give the unit a high efficiency index in terms of both performance and energy consumption.

The first stage of the DA150 system involves a cleaning process where the fresh or recycled air is sprayed with a fine mist to remove particulates. The supply air is then fed through a slowly revolving, humidistat-regulated rotor mechanism which uses a high efficiency desiccant to remove moisture, emerging at a constant relative humidity level of less than 70%. It then passes through a second heat exchange rotor, also thermostatically controlled, which cools the air to the appropriate level.

The efficiency of the rotor mechanisms lie at the heart o the system. Extremely strong, light and durable rotors are required which offer very high surface area, yet require minimal energy.

Conventional low-mass materials, such as corrugated paper, are too heavy and lack the necessary strength and durability for this critical component, which could see continuous operation for in excess of 20 years. Faced with this requirement, the designers opted for a honeycomb structure of Nomex, coated with Engelhard titanium silicate desiccant.

The new system combines the three functions of air-change ventilation, temperature adjustment and humidity control. In many traditional buildings, these functions are often performed by separate systems, leading to significant design and cost redundancies. For example, it is clearly bad practice to expend energy to cool down air for dehumidification purposes and then immediately expend further energy to heat it up again.

A further disadvantage of using conventional parallel systems relates to the problems of control. Unless a complex, expensive an effective building management system (BMS) is installed, one which can intelligently integrate the functions of the different systems, the overall control of internal air quality on a day-to-day basis is often, at best, a hit or miss affair.

NOMEX – A THUMBNAIL SKETCH

In chemists’ parlance, Nomex is an aromatic polyamide. Aromatic compounds, known as such for their distinctive odour, are the classification for organic molecules that comprise closed ring molecular structures of wholly or mainly carbon atoms (benzene rings). Although originally associated only with the coal-tar product benzene, the term aromatic is now applicable to about half of all organic compounds.

These cyclic compounds are renowned for their physical, chemical and thermal stability. Other, non-aromatic, organic substances are classified as aliphatic compounds. They are short, essentially linear carbon chains in irregular alignment. They are relatively weak and offer much less resistance to heat and chemical attack. Nylon is an example of an aliphatic compound.

The amide links between the rigid ring structures in an aromatic compound are very strong, owing to the fact that all the electrons are shared by the entire molecule, rather than being individually “owned” by discrete atoms. In Nomex, these aromatic amide chains are, more precisely, meta-aramid, signifying that the backbone linkages to the ring are at 120deg angles, rather than the normal 60deg.

In 1965, Du Pont development engineers in Richmond, Virginia produced the first samples of Nomex, offering outstanding heat, flame and electrical insulating properties, together with strength and durability. The material can be supplied in a wide range of formats, including staple, yarn, felt, paper and board.

The aerospace industry was quick to see the benefits and the first men on the moon wore suits made of Nomex fabric. Flying machines themselves hold many uses for the fibre in components and structural elements. Even modern hot air balloons, identical in principle to the Montgolfier original, use flame-retardant skirting made of Nomex to make them much safer than their 18^thcentury counterparts.

A standard Nomex sheet structure can be converted into a resin-impregnated structural honeycomb matrix by bonding and expansion. It is then surfaced with a suitable facing skin, such as its para-aramid cousin Kevlar, aluminium or plywood. Advantages of this type of structural panel material include an extremely good strength to weight ratio, inherent fire resistance, low smoke propagation, non-corrodability, noise and vibration damping characteristics, impact resistance and ease of repair. A special ply-faced version, Nomex Decore, is aimed at the marine industry, where low weight combined with a traditional timber appearance is required for interior furniture and fitments.

The properties of Nomex

Heat resistance:- engineered into the molecular structure, so are permanent, without chemical pre-treatments. Yet at cryogenic temperatures, such as 77K, sheet structures have tensile strengths in excess of room temperature values.

Self-extinguishing and non-melt:- in the absence of an external heat source, Nomex does not burn. It will not melt or drip even under extreme temperatures, starting to carbonise only when temperatures exceed 370degC.

Electrical performance:- in sheet structure form, Nomex is a highly versatile electrical insulating material. In fabric form, where anti-static properties are required, a blend with anti-static fibre can be used.

Dimensional stability:- At 177degC, the residual shrinkage potential is below 1%, whilst at 285degC, this residual shrinkage value is still below 2.5%.

Chemical and environmental resistance:- the polymer structure is inherently chemical resistant and proof against hydrolysis. Only the strongest of acids and bases are likely to lead to any significant degradation. UV light can affect Nomex, but any degradation is self-screening, with the underlying material remaining unaffected.

Forming:- sheet structures can be made to virtually any size and conformation. It is weldable with ultrasonics and can be formed into 3D shapes.

• Du Pont
• 00 41 22 717 5233

April 1998