Physico-chemical characterisation of carboxymethylated spun cellulose fibres

Abstract

A new fibrous material for the treatment of exudative wounds is characterised in terms of its physico-chemical properties. The chemistry of the material is analysed and its crystalline structure hypothesised. The influence of the structure on the hydration properties of the carboxymethylated cellulose fibre in fabric form is also demonstrated by physical test methods in comparison with fibrous alginate dressings. The methods illustrate the ability of the material to immobilise fluid by gel blocking and suggest how this can be beneficial in the treatment of chronic wounds by protecting the delicate peri-ulcer area from maceration by exudate.

Introduction

The management of wound exudate with fibrous materials has been common throughout the history of wound care. Cotton and viscose gauze although still much in use for acute wounds has largely been replaced for the treatment of chronic wounds by modern wound dressings made from alginate and more recently fibres made from carboxymethylated cellulose (NaCMC fibre), (Hydrofibers™, Aquacel™ ConvaTec Ltd). Wound dressings made from alginates have proved to have important advantages over gauze products in that upon hydration, ion exchange takes place resulting in the formation of a gel over the wound site [1]. This offers advantages to the patient and the carer in that the alginate dressing can be removed directly or by irrigation without damage to the newly forming tissue. NaCMC fibres also gel in contact with wound exudate but offer the carer additional advantages for fluid management. This paper examines the development of NaCMC fibre, their physico-chemical properties and suggests how these may contribute to dressing performance.

The most readily available natural fibre is cellulose, for example cotton. It is low cost and even in an unrefined state it has good tensile properties and is readily textiled. However, the use of cotton for exudate management is limited in this form since it has relatively poor fluid handling properties. It is possible to alter these properties of cotton by modifying the crystallinity within the structure. This can be achieved by swelling with strong acid or alkali solutions; a common example of this type is viscose [2]. Further improvements may be made by chemical derivatisation of some or all of the hydroxyl groups.

Ideally, in the care of chronic and exuding wounds a material in contact with the wound should have high fluid absorbency and retention for long wear times and should not disrupt or damage the newly formed tissues when removed. Sodium carboxymethylcellulose (NaCMC) fulfills these criteria, forming a soft gel or viscous solution with wound fluid. This gel-forming property observed for NaCMC has led to its use for wound care, in gel formulations and in hydrocolloid dressings that contain NaCMC in an insoluble matrix. Fibrous dressings made from alginate combine the properties of fluid absorbency and gel formation and some also retain their integrity so that they can be readily removed. A combination of the soft gel properties of NaCMC and the greater structural integrity of a needled alginate dressing would yield products with improved fluid handling and which would be easier to handle and remove from the wound. However, the production of a robust NaCMC fibre is not simple and until recently has not been a practical proposition.

Conventional NaCMC manufacture is an industrial bulk chemical process with the majority of output being used as a thickener or stabiliser for food and personal care products and as a viscosifier and suspending agent in household and industrial products such as detergents and drilling muds. A much smaller percentage of more refined material is used as a pharmaceutical excipient. The cellulose source is typically wood pulp or cotton linters slurried in a mixed aqueous/organic solvent such as water and propanol or acetone. The swollen alkali form is generated by the addition of a strong sodium hydroxide solution and conversion to NaCMC is achieved by addition of monochloroacetic acid. The reaction mixture is then neutralised with an acid such as glacial acetic. The by-products, sodium chloride and sodium acetate, are removed by washing with further volumes of mixed aqueous/organic solvent. The degree of conversion of hydroxyl groups commonly referred to as the degree of substitution (DS), is controlled by the concentration of monochloroacetic acid, temperature and reaction time [3]. The DS is a major factor in the water solubility of NaCMC, below approximately 0.4 the polymer is swellable but insoluble, above this the polymer is fully soluble with its hydroaffinity increasing with increasing DS. Commercially available products typically have a DS between 0.65 and 1.45.

The above reaction is heterogeneous and therefore final particle dimensions are dependent on the feed stock. Although often fibrous in nature they are typically only 50–300 μm in length, therefore too small for traditional yarn making processes or conversion into a nonwoven structure (Fig. 1).

An alternative method of manufacturing fibre is to produce monofilaments by forcing the polymer in a liquid form (dope) through a small aperture (spinneret) into an environment where it will rapidly solidify. This can be achieved from the molten state for thermoplastics (melt spinning), by using a volatile solvent which can be air dried (dry spinning) or by spinning into a non-solvent (wet spinning) [4]. An example of material manufactured by wet spinning is alginate; a solution of the sodium salt is spun into a bath containing a high concentration of calcium ions. Calcium alginate is insoluble and can be drawn directly as fibres. For NaCMC, melt spinning and dry spinning can be excluded as it is does not have significant thermoplastic properties and no suitable volatile solvent has been identified. Wet spinning has been demonstrated for water-soluble NaCMC, but these fibres have poor tensile properties and are too readily dispersible for use in wound care products. Low-DS water-insoluble NaCMC would be preferable but preparation of a suitable dope has not been achievable.

The novel process for carboxymethylating cellulose fibres has eliminated the problems inherent in manufacturing fibres from NaCMC. The major technological advance has been the development of wet-spun cellulose fibre (marketed under the trade names Tencel and Lyocell, Acordis Specialty Fibres). As the cellulose dope is a true solution, all the original crystalline structure, which restricts fluid absorption in the fibre, is lost. The spun monofilament has improved homogeneity and an altered pattern and distribution of crystalline regions but retains the requisite physical properties of strength and flexibility. The modified chemical and physical structure of the fibre enables further derivatisation of the cellulose backbone to be carried out in a more even and targeted way. Using this as source material and by careful control of reaction conditions, conversion to a low-DS but highly absorbent NaCMC fibre has been achieved. The molecular mass and the tensile properties of the parent fibre are preserved, making subsequent textiling processes possible. The resulting NaCMC fibres are washed, crimped, cut to the required staple length and air dried. In the nonwoven manufacturing process staple fibres are passed through a carding machine to align the fibres prior to the creation of a nonwoven web. The web is cross-lapped to increase its density before being needled to further increase its integrity.