‘TECH MEETING IN A TOOLBOX’
FIBREGLASS REINFORCED PLASTICS SIMPLIFIED
The July toolbox visit was to Fibreglass & Resin Sales where our host, Charlie Urwin, together with one of his staff, Laney (he calls them his girls), gave us a very informative presentation and demonstration of the full range of fibreglass materials and how they can be used in so many different ways to construct so many different things – including boats of course.
Charlie commenced with a bit of background on himself and the industry generally. He has been in the business of composite construction for most of his working life and has expertise in its application to racing cars, light planes and gliders and to boats. He was also licensed to work on wooden airframes in his earlier years. Over this time he has experienced the growth of this industry into a very varied market place. Whilst resin systems per se are not new – they date back to the 1914/18 war – they have moved on from the casines (milk derivative), tree based resins (similar to hot glue guns) and bakelites (still used in some fire situations) to the modern resin based systems of today. Charlie noted here that one of these, Araldite, was on the ‘secrets list’ until 1952. The use of fibreglass has also developed from simple two dimensional applications to quite complex three dimensional applications including artworks.
Charlie pointed out that the use of the terminology ‘fibreglass’ is erroneous as fibreglass is the fibrous reinforcement by itself. It needs the addition of the resin to form the matrix that then is correctly termed ‘fibreglass reinforced plastic’ or ‘FRP’. This then led to an explanation of the different reinforcements (includes fibreglass, Kevlar and carbon fibre) and the various resins including polyester, vinylester and epoxy. The reinforcements also come in different configurations. Chopped strand matt consists of a matt of random fibres held together by an emulsion binder which dissolves when the resin is applied (however needs a special binder for epoxy resin) and woven fabric which comes in numerous weaves and styles. The type of resin and reinforcement depends on the nature of the project – construction of a complete wooden boat, gluing components on a wooden boat or water proofing a wooden boat.
Charlie then explained in detail the difference between a polyester type resin and an epoxy. Polyester is a long chain resin and can be likened to a chain that becomes one when all the links are welded. The chemical reaction is started by the addition of a ’catalyst’ which disappears and takes no part in the final product. The disappearance of the catalyst as the reaction proceeds and the resin sets results in some shrinkage which is common in this type of resin. The speed of the reaction can be controlled with respect to varying temperature conditions by the addition of more or less catalyst. Hence, small variations in the amount of catalyst do not impact on the integrity of the final result. Vinylesters are very similar to polyesters but have better chemical resistance.
On the other hand, epoxy resins cure following the mixing of an exact proportion of resin and hardener. In this case, the hardener is an integral part of the final product and therefore epoxy resins do not shrink as they cure. BUT, the mix ration must be maintained and particularly in summer, the curing time can only be slowed by changing the temperature of the mix by use of some means such as an ice bath. The best advice is to work quickly as the epoxy resin will commence its ‘cross linking’ process very quickly in warm conditions. And for all these resins it is important to know the time required to gain full cure and full strength.
Next we moved to discussion on the various fabrics that provide reinforcement to the resins. Chopped strand matt is labelled by weight in oz/ft2 (typically 225, 450 and 600) and requires 2.5 times the resin of woven fabrics to wet it out. Woven fabrics come in various different weaves and are labelled by weight in gms/m2. The common woven fabrics are plain weave (one warp end weaves over one weft thread), crows foot (two or more warp ends weave over and under the same number of weft threads), twill weave (one or more warp ends weave over or under two or more weft threads in a regular fashion) and satin ( each warp and weft threads weaves under three or more and under one crossing thread). There are also woven rovings which are a much heavier fabric (630gms/m2) that will not curve, Dynel, a polyester (not fibreglass) woven fabric generally used with an epoxy resin, carbon fibre (high tensile/longitudinal strength) and Aromid (or Kevlar) which has a higher impact resistance but is harder to repair.
Similar material is also available in a non woven form in which the threads may be aligned in a number of different configurations including unidirectional, bidirectional and triaxial. These accommodate the need for designers to vary the directional strength in an FRP component dependent upon its use. In this regard also, the relative strength for carbon fibre is six times fibreglass and for Kevlar is 3 times fibreglass.
All these materials come in a wide variety of weights and varying widths off the roll. It was noted that the lighter fabrics are relatively more expensive because weaving the lighter material is more difficult than weaving the heavier ones. As a guide to cost, the range of normally used fibreglass woven fabrics are between approximately $5 and $45 per metre, Kevlar $45 to $85 per metre and carbon $82 to $140 per metre.
Various types of fillers can be used with resins for different purposes such as fairing, filling and coving of FRP projects. Charlie showed us three products that he stocks – 410 Microlight for fairing, 411 Microspheres for general fairing and 403 Microfibre for coving joints. There are also pre-filled systems available in which the filler is already added to the resin.
For all of these materials, Charlie pointed out that it is critical that surface preparation be completed properly. The surface needs to be sanded to break the surface fibres and the resin mix applied immediately. On oily timbers such as teak, there will also be a need to use acetone or isopropal alcohol to wash down the surface just prior to applying the glue. By way of example, when manufacturing finger jointed timber, Bunnings found that they achieved a 15% strength increase if the gluing was done immediately following preparation compared with joints where the preparation had been done some time before application of the glue. There was also the case of the two friends doing identical projects using identical materials and methods but achieving different results. The cause was traced to one of them who had a problem with flies and had sprayed fly spray near the job just before applying the glue. The invisible fly spray mist had landed on the prepared joint and affected the end result.
Core materials are generally used to separate two layers of FRP to create a very strong composite which can be used as a construction method in a wide variety of projects. It follows that there is a wide variety of core materials available to suit different purposes. The best way to select the most suitable of these is to make up a test piece and load test it before committing to the whole job.
The simplest of these is Divinycell, a foam sheet material that is available in thicknesses from around 6mm up to several hundred mm. When used with a fibreglass layer each side this is a very strong material for general construction such as building a dinghy or super structure. Another alternative is Nomex, a honeycomb type material which bends well in one direction but is very expensive. For more complex projects there is X-Panel made up with corrugated aluminium sandwiched between fibreglass panels each side. Also end grain balsa which has been used for a long time for this purpose is still available but not cheap. It is particularly good in compression.
Special hardeners are available for resins which will be intended to be the clear finish on the project. Top coats may be air dry single pack polyurethanes or a two pack polyurethane which is the recommended approach.
In closing, Charlie touched briefly on vacuum pumps and associated accessories which are all now available at a cost which may be economical for amateurs undertaking significant size projects.
This was a very interesting and informative ‘tech meeting in a toolbox’ and we thank Charlie and Laney for sparing their time on a Saturday afternoon to share their immense knowledge with us all.