Introduction

The sorption abilities of the liquid phase are one of the most significant features required of materials used in the manufacture of medical dressings [1]. High-porous carbon materials play an especially important part in such applications. High-porous carbon in fibrous form [2,3,4,5], as active carbon combines the features of classical fibrous dressings, which absorb water on the external surface of the macro-porous fibres [19] and through the spaces formed between them, with the high absorbability of active carbon. This latter, in contrast also accumulate the substance adsorbed inside the porous fibres. The absorption features of carbon material result from the adsorption ability of their highly developed internal surface, which sometimes even exceeds 1000 m²/g. The high sorption of toxins and multicomponent physiological fluids which exude from wounds are of especially great importance [6,7]. These features predestine carbon materials for application in the new generation dressings. At the same time, carbon materials have for years been applied as implants in surgery, thanks to their high biocompatibility with the human body [8,9,10].

The properties of active carbon nonwovens depend, among other things, on the structure of the precursor nonwoven. Precursor nonwovens manufactured by the spun-lace technique belong to a new generation of materials, and their application possibilities have hitherto not been fully recognised. A viscose nonwoven spun-laced manufactured was used as the precursor material for our investigation. The influence of temperature established in the precursor carbonisation process, and of the amount of the activating medium (the set up metering intensity of the medium) on the sorption capacity, were determined in our investigation.

Experimental

1.7 dtex viscose fibres of 40 mm length, manufactured in Finland, was the raw material for our precursor nonwovens. A spun-lace precursor nonwoven with an area mass of 150 g/m² was manufactured from the above-mentioned fibres at the Lentex SA company, Poland.

Samples prepared from the obtained precursor nonwoven were thermally processed. The pyrolysis and carbonisation processes were carried out at three temperatures of 400°C, 600°C, and 800°C, and the activation took place at a temperature of 850°C. Steam with a metering intensity of 25gwather/gcarbon·h, and 60 gwather/gcarbon·h was used as an activating medium for carbon nonwovens. As the effect of such processing, we received the samples of activated fibres, of less and higher burned-off surface, which means of various process efficiencies.

Water sorption was measured by means of the sorption kinetic determination method with the use of the SORP-3 apparatus. Distilled water was used as the test liquid. A sorption curve, which visualises the liquid amount absorbed by the material as a function of time, was recorded during the measurements (Figure 1). A program developed by us, designed to analyse the sorption curve, allowed us to determine the following factors:

ƒ the sorption capacity S, in gwather/gcarbon;

ƒ the maximum velocity of liquid absorption Vmax, in gwather/gcarbon·s,

ƒ the delay time to characterising the wettability in s; and

ƒ the time of the total sorption tmax in s.

Figure 1. The sorption curve

Investigation of the porous structure of the active carbon fibres was based on the nitrogen adsorption under static conditions. The adsorption isotherms were determined with the use of the Micrometrics ASAP sorption apparatus at the temperature of 196°C. The following parameters of the porous structure were determined:

ƒ the total capacity of pores Vtotal,

ƒ the capacity of micropores Vmic,

ƒ the capacity of mesopores Vmes,

ƒ the pore capacity distribution (determined by the BJH method [17,18]), and

ƒ the specific pore surface SBET (determined by the BET method [19]).

Results and Discussion

We have stated that the water absorption in non-activated fibres of small porosity depends mainly on the surface tension at the boundary of the phases, as well as on the wetting angle and radii of the menisci curvatures which are formed in the spaces between the fibres and on the fibre surfaces (SEM photo on Figure 2).

The above-mentioned fibres had an oxygen-hydrophilic group on their surfaces, which resulted from their chemical structure [11,12,13]. The carbon fibres obtained from these non-activated fibres also had great amounts of hydrophilic groups, mainly carboxyl groups [14]. The activation process which proceeds as oxidation of the carbon surface (for example, by the use of steam), not only leads to the formation of a pore system. The mechanism of the oxidation process in the first stage is based on the chemisorption of oxygen on the fibre surface, in the form of surface oxygen groups with a hydrophilic character (carbonyl, carboxyl, and other groups) [6,15], which partly remain on the surface after the end of the process. Such a mechanism caused a very valuable increase in the hydrophilic features of the active fibres and the activated nonwovens (Table 1).

Figure 2. SEM photo of carbon fibre obtained from viscose precursor

Table 1. Sorption factors of the precursor nonwoven and of the active nonwovens ; D – sorption capacity, vmax - maximum velocity of liquid absorption, to - delay time, tmax - time of the total sorption

Table 2. Porous structure of the activated nonwovens; micropores are pores of diameters below

1.5 nm according to IUPAC

Denotation	Specific surface area of the transition pores in accordance with BET method	Pore volume			Capacity ratio of micropores to mezopores
		Total	of micropores	of mezopores
-	m²/g	cm³/g			-
	ACTIVATED NONWOVEN high burned-off (small efficiency)
AWH400,	1177	0,607	0,453	0,154	2,95
AWH600,	1129	0,589	0,431	0,156	2,74
AWH800,	1189	0,579	0,468	0,111	4,22
	ACTIVATED NONWOVEN low burned-off (great efficiency)
AWL400,	696	0,463	0,362	0,101	3,58
AWL600,	586	0,412	0,295	0,117	2,53
AWL800,	712	0,529	0,364	0,165	2,21

The processes of pyrolysis and activation caused the formation of a porous structure with a specific surface significantly greater than 1000m²/g (Table 2), and this occurred independently of the thermal decomposition (pyrolysis) temperature. The change in the pyrolysis temperature was a significant factor modifying the fibres’ porous structure, and also influenced the hydrophilic properties of the carbon fibres. It seems that the mesopore (pores of great width) capacity (Table 2) is the most important carbon fibre feature obtained by the processing which we carried out. Fibres which had the greatest amount of mesopores (Table 2) were characterised by the greatest sorption of water, and those which had the greatest ratio of micropores to mesopores manifested the smallest sorption at similar total pore capacities vtotal (Table 2). The differentiation of pyrolysis conditions at comparable conditions of activation allowed us to obtain nonwovens differentiated not only by the sorption capacity of water, but also by the sorption velocity. It can be stated that active carbon nonwovens have a significantly higher average sorption velocity (determined as the quotient of the sorption capacity S and the total sorption time tmax) compared to the precursor fibres, which nevertheless have a slightly longer wetting initial time (to).

Slit-pores in the shape of a wedge with a very irregular and jagged surface, which are easily accessible for liquids, are created upon the activation of carbon fibres obtained from viscose (Figure 3) [16]. This is conducive to the observed increase in wetting the surface and penetration of water between the porous fibre surfaces and inside the fibre, and to the formation of menisci with great radii [20]. At the same time, this results in the increase in water absorbed.

Figure 3. STM (scanning tunneling microdcope) photo of active carbon fibre obtained from viscose precursor, and the transverse cross-section along the x-axis of a fibre fragment

In contrast to the adsorption of moisture and water, which takes place in micro- and mesopores, the absorption of liquid only (and this in essential amounts) mainly takes place in great pores and in the spaces between fibres. Thus, the internal porous structure of the fibres, which was formed with a pore capacity of about 0.6 cm³/g, was not the only factor which caused the 2.5-time increase in the fibres’ water retention. Probably, such great and diversified sorption is connected with the advantageous changes in the electro-chemical properties of the fibres’ surfaces, which allow for the formation of multilayer water coats on the surface of carbon fibres. This phenomenon can be connected with the appearance of oxygen-carbon units (oxygen surface units), as well as with the high sorption potential of the active carbon surfaces. The most advantageous properties, regarding sorption ability, are shown by the nonwovens pyrolysed at temperatures of 600°C and 400°C (AWH-400, AWL-600, and AWH600), that is, those which are characterised by the greatest amount of surface units, and at the same time have a more acidic surface character compared with the nonwovens pyrolysed at the temperature of 800°C.

Fibres obtained at high pyrolysis temperatures, for example at 800⁰C, have a significantly better arranged structure of the fibre’s carbon matrix, and fewer of the pores with great inlets which hinder penetration of water. At the same time, these fibres have fewer active centres, which are able to connect oxygen and create surface oxygen groups. These factors disadvantageously influence the hydrophilic ability of the surface and the water absorption (AWH-800).

The above-mentioned considerations allow us to state that opposite factors mutually superimpose within the active fibre sorption systems obtained. One group of factors increases the hydrophilic tendency of the active fibres, whereas the second group decreases this tendency.

Conclusions

The nonwovens manufactured from viscose fibers by the spun-laced method allow us to obtain active nonwovens with high and diversified water retention abilities in dependence on the processing conditions; these nonwovens can be applied according to the requirements resulting from the various destinations of medical dressings. Their sorption capacity and water retention is over twice as high as the sorption capacity of the precursor nonwovens. However, it seems that this sorption capacity depends to a lesser degree on the micropore structure, and to a greater degree on the features of the fibre’s carbon surface. Active nonwovens obtained from a precursor which was carbonised at a temperature of 600⁰C or 400⁰C have the best sorption abilities.