Acetylating Natural Fibres Replacement for Glass by Acetylated Fibres Ltd
Ecological awareness has resulted in a renewed interest in natural materials and issues such as recyclables and environmental safety are becoming increasingly important for the introduction of new composite materials and products. Environmental legislation as well as consumer demand is increasing the pressure on manufacturers of materials and end-products to consider the environmental impact of their products at all stages of their life cycle, including recycling and ultimate disposal. Hence, a ‘cradle-to-grave’ approach emerges. These environmental issues have recently generated considerable interest in the development of composite materials based on renewable resources such as natural fibres as environmentally friendly and low-cost alternatives for glass fibres and the use of plastics based on renewable resources for the development of true bio composites. Currently a large number of interesting applications are emerging and especially the automotive industry is looking seriously into the use of eco-composites as a way to serve the environment and at the same time save weight and cost.
It has been known for some time that the acetylation of natural fibres would produce a material capable of matching or bettering the performance of glass fibre on a volume for volume basis as reinforcement for plastic composites. Unfortunately the traditional method used to achieve the acetylation using autoclaves has proved to slow and costly to be commercially viable. Acetylated Fibres Ltd has patents applied for, that overcome this problem using a semi-continuous process where the expected price per tonne would be competitive with that of glass, thereby reducing the cost of reinforcement by approximately 50%.
Fibre reinforced composites are not a new concept. Vegetable and insect derived gums and resins were used by advanced civilizations of the pre-Christian era to treat vegetable fibre fabrics for the preservation of human corpses (mummifying).
Straw has long been recognized as advantageous in the production of load bearing bricks from friable clay matrices; daub and wattle allowed large wall areas to be constructed. In more recent times, the inclusion of fibrous slag in iron was found to impart ductility to the purified metal and the in corporation of reinforcing bars of metal into concrete castings has allowed load bearing structures to be constructed, enhancing the science of civil engineering.
The advent of synthetic resins in the late 19th and early 20th centuries introduced a series of new materials to the use of man. Additives of particulate and fibrous nature are commonly used to make these synthetic resins into practical products. Composites of synthetic resins with fibres now occupy an important sector of the market for load bearing items.
Since the introduction of glass fibres with synthetic resins during the 1940’s, this fibre source has become the norm. Other high tech fibres such as carbon (made by carbonizing synthetic fibres) and synthetic organic fibres have been introduced for special applications. Natural fibres of various types have also been used, but generally for low cost applications where quality was not important - wood “flour” being the most widely employed
Natural fibres are widely used in textiles, although since the introduction of “synthetics” their importance has somewhat diminished. All natural fibres rot over a period of time, a process which is influenced largely by exposure to water, both by direct liquid contact and from moisture in the atmosphere. Not only does this exposure encourage biological degradation of the fibres, but absorption of water into the fibres initially causes swelling and dimensional instability.
A Review of Acetylation
During the latter half of the 19th century it was recognized that cotton fabric in particular was susceptible to deterioration over a period of time and this was further identified as being influenced by water. Following many investigations, it was found that cotton fibres could be dissolved in acetic acid and this solution, when passed through a fine orifice, could be converted back to fibre. This new fibre, cellulose tri acetate, exhibited different appearance to the original cotton and was popularly called artificial silk. What had happened was that the water-attracting hydroxyl groups (-OH) of the cellulose structure of the cotton had been chemically changed to acetyl groups (-OC-C2H5) which are water repellent. This process works very well with cotton, which has very high cellulose content (92%) but is inefficient with stalk fibres which have significant levels of hemicelluloses - typically 1 part hemi cellulose to 3 to 4 parts cellulose.
Partial acetylation of stalk fibres retains the original fibre structure whilst reacting with the “free” hydroxyl groups which are available, in the natural state, to biological attack. The acetylation of these free hydroxyl groups effectively prevents this and increases both molecular weight and volume of the fibre - typically 15% to 20% depending on the fibre used. This is very close to the volume expansion of untreated fibre in water; partial acetylation thus stabilizes the fibre against water induced dimensional changes. Acetylation of natural fibres to the level indicated only marginally affect the physical attributes of the fibre, whereas complete acetylation completely modifies the original fibre. Stalk (bast) fibres are stronger than seed fibres but less extensible; they approach glass in stiffness (modulus) and are significantly stiffer than synthetics.
Wood fibres are extensively used in the paper and board industries but are adversely affected by water and moisture. Many techniques have been developed over the last century to minimise this problem. Over the last 25 years chemical modification has been seriously considered and acetylation of the hydroxyl groups has been shown by many investigations to be technically the most efficient. However, a practical commercial process has not yet been developed.
The principles of Acetylated Fibres Ltd previously were involved with a company (Adtech Ltd) which started investigatory work on acetylation in 1997.
After consultations with British Petroleum, Bio Composites and Gothenburg University considerable testing was carried out using a small autoclave and the resultant acetylated fibre characteristics were very encouraging.
A pilot autoclave plant was built capable of acetylating fibre in half tonne batches and although the quality of the fibre could be maintained the process was too slow and therefore too expensive to produce commercially viable fibres. Further desk research showed that even if the operation were to be scaled up to a much larger plant the time taken to achieve the level of acetylation necessary would again make the process uncompetitive.
After four years research and some £1million spent the company ran out of research funds and the project was cancelled.
Acetylation is the principle of replacing reactive hydroxyl groups on cellulose fibres with acetyl groups. The result is a fibre which much less hydrophilic and hence lower moisture movement, has greater resistance to biodegradation and is more compatible with thermoplastic and thermo set resins. Acetylation produces fibre, which could replace glass fibre in plastic composite applications in industries such as the automotive industry. The advantages are reduced weight, environmental sustainability and less hazardous than glass fibre also cheaper if a more economical way of acetylating fibres can be found Acetylation is carried out by bringing the dried fibre into contact with acetic anhydride at elevated temperature and pressure. Generally an excess of anhydride has been used and a by-product, acetic acid, remained mixed with the excess anhydride leading to an expensive operation to recover unused anhydride. This process can be controlled such that the extent to which available hydroxyl groups are converted can be directed to reach a predetermined % replacement up to the end point of the reaction, which is approximately 26% acetylation. It is the acetylation per se which changes the physical properties of the fibres. Plates I and 2 are scanning electron micrographs of no acetylated and acetylated fibres respectively. Plate 2 clearly shows the degree to which the internal ‘channels’ in the fibre open up (and at the same time become straighter and confer greater rigidity upon the fibre).
A New Concept
Although no longer working for the now defunct Adtech Ltd, the principles of Acetylated Fibres Ltd continued to apply themselves to the problem and have recently invented a semi-continuous process for that acetylation of fibres that involves smaller quantities in the reactor at any one time but a larger production over a given period of time due to a reduced residence time in the reactor. Generally an excess of anhydride has been used and a by-product, acetic acid, remained mixed with the excess anhydride leading to an expensive operation to recover unused anhydride. The present proposal uses only the stochiometric requirement of anhydride, i.e. matched quantities resulting in virtually pure acetic acid as the by product, which can be sold on. This new method, which in principle should be able to produce a Tonne per day from a modular plant, would allow for more units to be easily added to increase capacity to a commercially viable level.
Potential Market Size
The current global composites market is estimated to be in the region of 5 million tonnes. Fibres are introduced into plastics as a reinforcement and are overwhelmingly man made with glass, Kevlar and carbon being the most common. Glass fibre reinforced plastics are amongst the cheapest and widely used composites based on man-made fibres. Glass fibre as a reinforcement in composites, has a global market of some 2 million tonnes with an annual growth rate approaching 6%, in Europe this figure is closer to 10%.
Industry sectors (by process) use of glass fibre as a reinforcement.
· Thermo sets - 54% o Sheet moulding Compound/Bulk Moulding Compound - 14% o Hand Lay Up - 12% o Spray Lay Up - 7% o Filament winding/Centrifugal casting - 7% o Resin Transfer moulding - 5% o Pultrusion - 4% o Other - 5% · Thermoplastics - 33% · Textiles (Weaving) - 13%
- Glass Fibre Consumption by End User
Development of the Process
Although the concept has been proved in very small scale trials, laboratory research is required before the building of even a small pilot plant
Stage 1 - Research
Examination of the fibre
It is important to have an appreciation of the surface properties, visually, of the fibre. The presence of finely divided residual pectin and other materials detracts from the composite action of fibres with a polymer matrix. A clean fibre is most important, of which there is abundance around the world very often a waste product, but initially we would use hemp which is grown in the uk and a very good fibre for acetylating producing a strong reinforcement. Microscopy will allow a detailed understanding of the fibre both on the outside surface and also the internal structure. Moisture content It is important for the success of acetylation to introduce dry fibre into the reactor. Establishing the moisture content will give an indication of the drying facilities needed. Also the degree of acetylation can be varied using the matched quantity principle and the optimum degree of acetylation needs to be established in order to achieve a fibre with the desired ambient moisture content and compatibility with polymer matrices.
Composite preparation and composite physicals determination Facilities for processing polymer composites are required in order to produce plaques to be prepared and tested to assess the strength and stiffness of composites using varying proportions of fibre at a range of degrees of acetylation. Degree of acetylation Acetylation involves the replacement of hydroxyl groups with acetyl groups which represents a weight increase. The degree of acetylation can therefore be assessed crudely by comparing dry samples before and after treatment. This would involve a fixed weight of initial fibre followed through the process and recovered. A better method is to digest samples of fibre using warm alcoholic sodium hydroxide solution. The degree of acetylation can then be determined by acidifying to excess and back titrating or alternatively passing samples of liquor through an HPLC set up for acetate. Initial phase of laboratory research is planned to be for nine months to carry out extensive testing of process, including the e building of a small scale lab plant capable of producing a few kilos of acetylated fibre, to establish the exact amounts of acetic anhydride related to the fibre get the timing right to achieve the degree of acetylation required so the desired material is produced consistently.
Stage 2 - Pilot Plant
Following the successful completion of the Laboratory research phase, a pilot plant capable of acetylating approximately 50 kg of fibre per day would be constructed. This would highlight any problems in the scaling up from a small laboratory plant which could then be addressed at this time. Once this pilot plant has proved that it can produce a consistent acetylated fibre with the physical characteristics required development would then move on to a full scale commercial machine capable of producing one tonne per day.
Building a full production plant Development Costs
Laboratory & Staff for nine months Dr John Murray £66,400
Mechanical Engineer - Tom Ryan £22,500
Project Co-ordinator - Brian Chandler £20,000 Equipment Purchases £25,000
Admin Expenses £5,000
Total for Lab Research £150.900
Factory Space £25.000
Build Plant £130,000
Ancillary Equipment £80,000
Mechanical Engineer + 1 £50,000
Dr John Murray £42,000
Project Co-ordinator £30,000
Admin Expenses £8,000
Contingencies at 10% £32,800
Total for Pilot Plant £382,800
It is estimated that a full scale production module could be built and set up in Luton for £300.000 to £350,000 capable of producing some 250 tonnes per year. The plants modular construction would allow for extra manufacturing to be easily added as demand rises.
This would produce an income of £500,000 per annum from a single module if we consider selling at the same price per tonne as glass with an operating cost of approximately £200,000 (including consumables) which would of course reduce with the addition of more modular units,
Later plants will be built near a fibre source in various countries in Europe; currently there is a big demand in Germany from the motor car industry.
Note we would propose to work with Imperial College London who have considerable amount of in depth knowledge on acetylating no allowance has been made for there help