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Working at various temperatures
21st February 2018 TFarrugia

Working at various temperatures

Posted in Uncategorised

Working in cold conditions:

A chemical process is speeded up by increasing the material temperature, and slowed down by reducing the temperature. All thermosetting plastic processes are essentially thermo-chemical: thus work with polyester, epoxy, polyurethane and similar resins are governed by this basic rule.

As a general rule, 15 to 20 degrees Centigrade is considered a reasonable general working temperature. With temperatures below 10 degrees C., the advice below becomes applicable. Whenever possible, consider the time of day. Start the resin work before midday if possible. Contamination of a partially set resin system by rain, or condensation overnight, is more likely in colder conditions. Freezing conditions will ‘kill’ an unset polyester system, whereas an epoxy system will generally recover its curing process if subsequently warmed. Avoid working in conditions liable to freeze or become wet, unless protection is available.

In cold conditions, try to warm the job/working area. When using heaters, ensure that the heat is ‘dry’. All direct combustion heaters, working without heat exchangers, produce large amounts of water vapour. This will often contaminate the resin system, producing a ‘bloom’, waxing, or more severely interfere with the setting process. It is important to note that the resin system will ‘read’ the temperature of the substrate – e.g. a boat hull. Any large moulding or assembly subjected to prolonged low temperatures, will be cooled. Working outside on such an item, during a ‘warm spell’ of weather must account for this factor. Remember that thick laminates will develop more exothermic heating than thin, aiding the setting process.

Always maintain the recommended mix ratios for any given resin system. Alteration of mix ratios will inevitably reduce the ultimate cure strength of the product. Speeding up the setting process should be by use of the ‘accelerator’ component of the system (if available!), not by adding more catalyst or hardener: epoxy systems will be particularly degraded by use of too much hardener: use ‘fast hardeners’ whenever possible, with careful use of a warm air gun.

Overnight chilling may be avoided by thermally insulating, or by tenting over the job, and using a low powered electrical heating source of a convection, or radiant type. Even a filament type light bulb may emit sufficient heat over a small area to prevent freezing. Be particularly careful in the use of radiant heaters: prolonged use at short range may well cook the area above acceptable limits. Above all, be aware of the fire risk of leaving any heat source unattended. It is desirable to switch any such device on for some time before leaving a job overnight, to check that the device only warms the area, and is never likely to get hot. As a guide – the human hand will not bear temperatures much above 40 Deg.,C., 50 Deg., C., is a burn! Always anticipate temperatures likely to by experienced by the setting resin system.


Working in ‘normal’ conditions:

It is only under ‘normal’ conditions that the performance data published by many manufacturers apply: in addition, thermosetting materials can become quite difficult to handle under conditions of extremes of temperature. It cannot be too strongly recommended that work with these materials should always, where possible, be carried out in warm and dry conditions.

As a general rule, 15 to 20 degrees Centigrade is considered a reasonable general working temperature. Most data published for the performance of resin systems assume work within this temperature range. Working under these dry conditions provides for the maximum freedom of choice of working styles.

Choice of working mixes, for example whether to use fast or slow hardeners, or use of accelerators, should be determined by the requirements of the job, not by the environment. This is the idea. In particular, work on large projects is possible, and full control over work schedules maybe obtained by correct use of curing agents. By way of example, epoxy systems are best used ‘wet’ on ‘wet’ or ‘tacky’ surface. This can be controlled by use of fast or standard hardeners during the day, and slow hardeners overnight to provide a wet surface for further work on the following day. For polyester systems, laminates may be governed purely by a thickness limited only by exothermic heating considerations. When using two part polyurethane foams, the likelihood of chilling on contact with the substrate, and loss of yield is avoided.

Alteration of mix ratios will inevitably reduce the ultimate cure strength of the product. Speeding up any setting process should be by use of the accelerator component of the system, not by adding more catalyst or hardener. Epoxy systems may be particularly degraded by use of too much hardener. As a general rule, all resin systems produce the strongest composites when used with slow hardeners; this provides for the longest working as well as curing time. Curing may be accelerated by ‘post curing’ by the application of the limited heat, which may also improve general properties of the laminate.


Working in hot conditions:

When temperatures exceed 25 degrees C., the advice below becomes applicable. Starting the resin work early in the morning may avoid high temperatures, the main problems being that thermosetting resin systems will overheat and gel, allowing insufficient time to carry out work, or the thicker laminates may exothermically overheat on setting.

Be careful not to mix up more resin and hardener than may be used before the mix becomes unusable. Working life is extended if the mixed resin system is decanted into a shallow tray. Work in shaded conditions and keep the resin system components cool if possible. In certain climates, very high day temperatures may be succeeded by very cold nights. In these conditions, protection from condensation may be necessary – see notes on ‘Working in cold conditions’.

Slowing down any setting process should be by use of a slow hardener, or by using an un-accelerated resin to which only a limited amount of accelerator may be added. Epoxy systems are particularly vulnerable to incorrect mixing.

Where direct and powerful sunlight is involved, effecting shade on the area will significantly reduce the amount of direct heat on the job. The resin will ‘read’ the temperature of the substrate – e.g. a car body. 40 Deg., C., is a temperature hard to bear on the human hand. The resin system will develop its own exothermic heat in setting, and may well ‘run away’ to very high temperatures indeed. Avoid thick laminates which may cook themselves.

With temperatures above 30 Deg., C., working with ‘standard’ resin systems in any bulk may not be possible, in which case special slow systems/hardeners may be required.