Journal of Industrial and Engineering Chemistry
Formulation of new biodegradable lubricating greases using ethylated cellulose pulp as thickener agent
Introduction
In recent years, there has been a great interest in the use of renewable resources for different industrial applications due to its positive effect on the environment. The introduction of the ecolabelling has had significant impact in the use of biodegradable fluids with the aim of becoming more “environmentally friendly” and being applied in more sustainable industrial processes [1]. The EU ecolabel scheme establishes criteria for groups of products and services in order to meet high environmental and performance standards. A relatively new group of products created in this scheme is that regarding lubricants [2]. Lubricating greases constitute a group of lubricants with particular properties mainly related to their gel-like characteristics, which are mainly attributed to the thickener agent. Besides this, the lubricant industry in the European Community must fit the REACH regulation [3] dealing with the registration, evaluation, authorization, and restriction of chemical substances. Concerning the impact that lubricants exert on the environment, every year millions of tonnes of engine, industrial and hydraulic oils leak into the ground or waterways, or are disposed of into the environment. Mineral-based oils can contaminate groundwater for up to 100 year, which may inhibit trees growth and be toxic to aquatic life [4]. To minimise the contamination damage that petroleum-based products cause in the environment, there has been an increasing demand for green products suitable to be used as lubricants [5]. The substitution of traditional thickener agents in lubricating grease formulations (metallic soaps, phyllosilicates or polyurea compounds) by others more environmentally acceptable seems to present an additional difficulty, mainly due to the functional properties and effectiveness achieved with those compounds, especially metallic soaps. Operating conditions in current technologies impose high demands on lubricating greases [6], [7] and, consequently, alternative thickeners coming from natural resources must impart to the final product, apart from non-toxicity and biodegradability, gel-like characteristics, thermal resistance and efficiency to minimise friction and wear in the machinery similar to that achieved with the traditional non-renewable thickener agents [8]. Moreover, biopolymeric additives used in industrial formulations should also present stability against mechanical degradation [9]. In this sense, products derived from cellulose, which is the most abundant polymer in the organic world obtained from renewable resources, may play an important role as environmentally friendly substitutes to polymeric compounds and thickeners in general [10], [11].
Ethyl cellulose (EC) is an important commercially produced cellulose ether, widely used in paints for various products, such as wooden products, metal surfaces, paper, rubber, thermotropic products and integrated circuits; all kinds of precise membrane module used in industry and employed as dye printing slurry in textile industry [8], [9]. For EC, the solubility in water is dependent on the degree of substitution with ethyl groups (DS) on the cellulose backbone [12], [9]. The derivatization of cellulose fibers into ethyl cellulose in several substitution degrees in order to reduce the polarity of original cellulose, among other uses, may represent an interesting way to achieve a polymeric biodegradable material, which could result applicable as a thickener agent in lubricating greases. The role of ethyl cellulose to improve the physical stability of α-cellulose dispersions in oils has been previously reported [13].
The main goal of the present work is focused on the development of new renewable and biodegradable thickener agents based on ethylcellulose derived from industrial grade cellulose pulps in order to obtain green lubricating grease formulations. To achieve a suitable derivative for this use, the different variables affecting the ethylation reaction of kraft cellulose pulp have been studied. In particular, the effects of ethylation process temperature, ethylation time and ethyl/glucose molar ratio (E/G) have been investigated. Oleogels prepared with selected ethyl cellulose samples have been rheologically characterized.
Section snippets
Materials
Commercial grade kraft pulp of eucalyptus sheets were kindly supplied by ENCE, S.A. (Huelva factory, Spain). Chemical composition (oven-dry weight) and some physical properties of this pulp was: α-cellulose (72.49%); hemicelluloses (as xylan) (20.43%); lignin (0.28%); Tappi viscosity (688 cm3 g−1).
The lignin and α-cellulose contents were analyzed according to Tappi procedures T-222 and T-203-OS-61, respectively. Holocellulose contents were determined using the method described by Wise et al. [14]
Ethylation process optimization
It was observed that properties (average molecular weight, polydispersity index, number average molecular weight and substitution degree) of obtained ethyl cellulose under the experimental design were dependent on experimental operating conditions, which is shown in Table 1. In that form, the design of the experiment includes dependent variables or responses (Table 1) such as average molecular weight, polydispersity index, substitution degree and yield. Maximum average molecular weight (2.53 × 105
Conclusions
Ethylation process of cellulose pulp to obtain gel-like dispersions could be a suitable process to formulate renewable and biodegradable lubricating greases. Ethyl/glucose molar ratio has the higher positive influence on the cellulose ethylation process from commercial kraft pulp.
The rheology of ethyl cellulose/castor oil binary systems is mainly influenced by DSW. Ethylated cellulose pulp samples with substitution degrees between 25% and 60% provides suitable rheological response and physical
Acknowledgements
This work is part of two research projects (CTQ2007-60463 and CTQ2010-15338) sponsored by a MEC-FEDER Programme. The authors gratefully acknowledge their financial support. Thanks are also given to ENCE (Huelva, Spain) for the supply of raw cellulose samples.
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