Effect of temperature and humidity on heat effect of commercial chemical warmers based on iron powder
Introduction
A cold environment are conditions in which the ambient temperature is less than 10 °C [1]. A cold can cause numbness, shivering, frostbite, hypothermia [2]. In order to maintain thermal comfort, clothing with an appropriate level of thermal insulation should be used [1], [3]. Hands and feet are much more sensitive to cold than other parts of the body [4], [5]. To prevent excessive cooling of the hands and feet the use of protective gloves and footwear is demanded [6]. Moreover, many tasks performed in a cold environment require manual dexterity. When the temperature of the fingers drops below 16 °C [7], their dexterity decreases what makes work more difficult or can be the cause of the accidents.
One of the way of heating the body is electrically heated garments (EHGs). The heating elements, such as flexible electrical wires, conductive rubber, graphite elements are built-in garments. In order to maintain thermal comfort of the hands, electrically heated gloves are used. They are an auxiliary heat source applied locally – directly on the hands. The disadvantage of this solution is that the construction elements make the gloves stiff, which reduces the fingers' dexterity. Moreover, the heat is not evenly distributed because it is generated along the implanted wires, and the battery efficiency is not satisfactory [8], [9].
An alternative to electrically heated gloves can be heating elements: heaters or warmers, commercially known as hot packs or hot pockets. They are a portable source of heat. There are disposable or reusable kinds of the heaters. The reusable heaters use phase-change-materials (PCMs), such as organic materials (e.g. paraffin or non-paraffin compounds) or inorganic materials (e.g. metals, salt hydrate) [10]. A sodium-acetate trihydrate (NaC2H3O2·3H2O) is hydrated salt commonly used in the commercial warmers. The reusable heaters use the properties of a supercooled liquid. When the metastable equilibrium state of the supercooled liquid is disturb, it undergoes a crystallization process. Disturbing the metastable equilibrium state is caused by a crystal seed from a small metallic clicker inside the pocket which is bended during the activation of the heater. The crystallization process is exothermic reaction and generates heat. In order to use the heater again, it should be heated in water until the crystallized liquid is completely dissolved [11].
The second type of hot pockets are disposable chemical warmers. These warmers consist of an exothermic composition closed in an air and water vapour permeable inner bag, which is hermitically sealed in an outer bag that is air and water vapour impermeable. Heat is generated during the exothermic reaction that occurs after the inner bag is removed from the package.
The initial compositions consisted of metal powder, electrolyte and in some cases an oxidizing agent or a depolarizer. Well mixed ingredients were stored in a dry form in a suitable container. Such a mixture was chemically inactive and release heat only when water was added to it. The used metals were iron, zinc, copper, aluminium or nickel. Iron, however, was the most commonly used metal due to high availability, low price and a very good thermal effect. As the electrolyte, ammonium chloride (NH4Cl) was initially used. However, due to the unpleasant odour of the ammonia vapours generated during heat production, it was replaced with sodium chloride (NaCl) or potassium chloride (KCl). As a depolarizer, for example, manganese dioxide (MnO2) was used, which has strong oxidizing properties. An example of such a composition was mixture of iron powder, manganese dioxide and sodium chloride in the amount of 84 wt%, 10 wt% and 6 wt%, respectively [12].
Further work concerned expanding the composition of water-absorbing, moisture-retaining and odours-absorbing ingredients. Initially, calcium chloride (CaCl2) was used as the absorbing material [13]. Also, charcoal, sawdust or wood flour was used. The absorbent materials prevent agglomeration of the mixture components. The inert absorbent materials (wood flour, sawdust, cellulose absorbent wadding,) control the temperature uniformity and prevent the location of the reaction due to the even distribution of moisture. They also prolong the reaction by keeping the moisture for a certain period of time. The active absorbing materials (charcoal, activated carbon) additionally accelerate the exothermic reaction. The addition of ingredients such as silicic acid, silica gel, pumice or diatomaceous earth increased the volume of the heater without deteriorating the heat-generating capacity per unit of mass [13], [14].
The container of the heaters was also improved. The elastic (fabric or paper) bags with a hole to add water were used. The bag after the addition of water was placed in a waterproof rubber container to protect the mixture against moisture loss. Further developments were related to the creation of more practical in use heaters that do not require water from the external source. The main change was the introduction of water into the mixture in the process of making heaters. One solution was to keep water and dry reactants separately until the heater was used. The liquid portion was placed in a flexible bag that was placed in the outer bag. In the outer bag there was also a powder part of the mixture. In order to use the heater, sufficient pressure was applied to the outer container in order to rupture the inner container and release the liquid [15].
Modern chemical warmers contain an exothermic powder, which is usually a mixture of iron powder, water, salt, active carbon and vermiculite. Some warmers also contain diatomite. Iron powder is the main heat-generating agent, as the oxidation reaction is an exothermic reaction [16], [17], [18]. The iron powder oxidizes when it comes into contact with the oxygen in the atmosphere after the outer bag is opened. The water and salt accelerate the oxidation of the iron powder [19]. Vermiculite and diatomite have insulating properties, so they act as warmth-retention agents and prevent the heat from dispersing rapidly. The active carbon has a high surface area and a porous structure, so it absorbs and retains the water and water vapour that is generated during the heat generation process. Thus, the active carbon ensures an appropriate level of water during the whole chemical reaction, and also absorbs the unpleasant odour that is generated during iron oxidation [16].
Warmers have a wide range of applications and can be used as poultices for therapeutic purposes (neuralgia, arthritis, rheumatism), as food or beverage warmers, as input for insulated bags to keep them warm at the appropriate level, or by winter sport enthusiasts (snowboarders, skiers). In this work, environmental tests in a climatic chamber were carried out to determine the effect of temperature and humidity on the heat effect of the active compounds in the warmers investigated. The morphology, structure and phase composition of the exothermic composition studied is also presented.
Section snippets
Material
The warmers investigated were produced by FarmTech Co. Ltd. Fig. 1 shows the inner bag of a warmer and the exothermic powder. Before oxidation, the exothermic powder is black in colour, turning reddish after activation. This is characteristic for corrosion products of iron. According to the manufacturer, the exothermic composition consists of a mixture of powders: iron, vermiculite, diatomite, active carbon and salt, with an appropriate amount of water.
Characterization of the exothermic powder
The morphology of the exothermic
Morphology of the exothermic powder
Fig. 2 shows the morphology of the exothermic powder in the initial state. Single particles and agglomerates of different sizes can be seen. These agglomerates are formed during the mixing of all the components, and are not desirable, as they partially limit the reactive surface of the iron powder. During mixing, fragile components such as vermiculite are divided. Subsequently, the vermiculite, which has a layered structure [22], appears in the form of flakes on the surface of the particles as
Conclusions
The heat effect of commercially available warmers based on an exothermic mixture of iron, sodium chloride, vermiculite, diatomite, active carbon and water was investigated. Based on the experimental results presented, the following conclusions can be drawn:
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The morphology of the exothermic mixture was described. The presence of all the mixture components as stated by the manufacturer was confirmed. During the oxidation reaction, the morphology and chemical composition of the exothermic
Acknowledgement
This work was financed by The National Center of Research and Development in the framework of the Applied Research Programme during 2015-2018. Project No. PBS3/A9/33/2015 “COLDPRO – The use of environmentally friendly, active mineral compounds in the manufacture of gloves and footwear for protection against cold”.
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