Durability of Polymer Composites
All engineering materials are prone to degradation over time and FRP materials are no exception to this rule. However we will see in this chapter that they do offer some significant durability advantages over the more conventional construction materials, although they are not without their own problems.
The ageing of a polymer can be defined as a slow and irreversible variation of the material structure, morphology and/or composition, leading to a deleterious change. This slow degradation is caused by the interaction of the material with the environment into which it is placed. Clearly knowledge of the processes and the controlling factors is a prerequisite for successful composite design.
The most significant factors that cause polymer composites to degrade with time under various environments are:
- moisture and aqueous solutions, particularly alkaline environments
- thermal effects
- ultra-violet radiation.
All of these are considered in this article, but before doing so we will make some general points. Some field surveys undertaken on FRP composite materials throughout the world have been described by Hollaway (2007) and indicate the relative importance of degradation mechanisms.
For example, in-service factors that contribute to the degradation of GFRP composites include temperature, ultraviolet rays from the sun, moisture absorption and freeze– thaw cycles, the latter two factors being considered the most critical. Exposure to alkaline environments and UV radiation also affect long-term durability, but to a lesser extent.
Furthermore, UV degradation resistance of most composites is being improved by applying protective coats and additives during the manufacturing process. It is difficult to analyse the problems owing to their very slow progress (typically over a number of years), consequently accelerated testing is sometimes undertaken. This requires specimens being exposed to an accelerated test regime, which generally involves an environment many times more severe than that which would be experienced in practice.
Furthermore, these test samples are sometimes also exposed to elevated temperatures to further increase the degradation. This accelerated test regime to obtain durability data in one environmental situation will generally not be relevant to the more gradual degradation effect that would have taken place had the conditions been less rigorous.
An accelerated test programme can, however, be undertaken to build kinetic models that describe the changes over time of the behaviour of the material; these models are then used to predict the durability from a conventional lifetime criterion. It is then necessary to prove the pertinence of the choice of accelerated ageing conditions by a mathematical form of the kinetic model. Some investigators use empirical models but these are highly questionable because they have to be used in extrapolations for which they are not appropriate.
It is therefore important to treat the results of accelerated experimental testing with caution and care, particularly if the polymer is heated to increase the rate of degradation. Heating would change the characteristics of the specimen, and in addition the mechanical properties of polymers degenerate with increase in heat.
It is particularly important to appreciate the effects that heat has on thermosetting polymers and hence on thermosetting polymer composites. Therefore, before considering the durability factors it is necessary to discuss what happens as the temperature of a polymer rises above a certain temperature termed the glass transition temperature, Tg.
The glass transition temperature of a polymer (Tg)
The Tg is the temperature below which a wholly or a partially amorphous polymer behaves in a similar way to that of a solid phase (a glassy state) and above which it behaves in a manner similar to that of a liquid (a rubbery state).
The Tg is the mid-point of a temperature range of a few degrees in which the wholly or partially amorphous polymer gradually becomes less viscous and changes from being in a solid to being in a liquid state. The epoxy and vinylester polymers used in construction are generally in the amorphous state, with a small amount of crystalline structure.
Thermoplastic polymers are crystalline solids and have a melting temperature, Tm, which spans a range of a few degrees. Above this temperature they lose their crystalline structure. Furthermore, they have a Tg below the Tm value and at temperatures below the Tg they are rigid and brittle and can crack and shatter under stress.
Most crystalline polymers possess some degree of amorphous structure and most amorphous polymers have some degree of crystallinity, thus they can have both a glass transition temperature (the amorphous portion) and a heat distortion temperature (the crystalline portion).
In summary, all polymers below the Tg are rigid; they therefore have both stiffness and strength. Above the Tg, amorphous polymers are soft elastomers or viscous liquids and have no stiffness or strength. Crystalline polymers will range in properties from a soft to a rigid solid depending upon the degree of crystallinity.
Moisture and aqueous solutions
Polymers are not impervious to moisture and aqueous solutions, and if such solutions do penetrate the polymer they could do damage to the fibres. For example, all glass fibres are very susceptible to alkaline environments, and when incorporated as rebar reinforcement for concrete, the susceptibility is primarily due to the presence of silica in the glass fibres.
This problem does not affect carbon fibres, but aramid fibres do suffer some reduction in tensile strength when exposed to an alkaline environment (Balazs and Borosnyoi, 2001). However, there are now glass fibres that are more resistant to an alkaline environment and therefore will increase the durability of a GFRP composite.
Ownes Corning manufactures a glass fibre known as Advantex; when this is embedded in a polymer matrix and immersed in a simulated concrete pore-water solution and loaded to 30% of its ultimate load capacity, Benmokrane et al. (2002) have reported that it retains 100% of its strength after 140 days of immersion; conventional E-glass fibres exposed to the same conditions resulted in a 16% loss of tensile strength.
As the matrix protects the fibre from external influences, the long-term properties of the matrix of CFRP and AFRP composites are of importance to the overall properties of the composite. In construction there are many different polymers on the market, some of which have been modified by chemists over the years to improve their in-service performance.
Furthermore, additives are on occasion incorporated into cold-setting polymers to enhance curing. Each time these polymers are changed/modified the durability will be affected.
In comparison with other construction materials FRP composites do not ‘rust’; this makes them attractive in applications where corrosion is a concern. For example FRP composites are used in:
- rebars and grids for reinforcing concrete
- cables for pre- and post-tensioning concrete
- cable stays to bridges
- the upgrading of components of reinforced concrete and steel structures where the material is exposed to salt solutions, e.g. marine waterfront structures, cladding panels, pipe lines and walkways in harsh environments, and de-icing solution during winter snow storms.
A fully cured polymer exhibits good resistance to acidic and alkaline attack if selected and designed properly; resin manufacturers should be consulted when choosing a resin to be utilised for a specific corrosive environment.
Behaviour of polymer composites in fire
The polymer component of a composite is an organic material and is composed of carbon, hydrogen and nitrogen atoms; these materials are flammable to varying degrees. Consequently, a major concern for the building construction engineer using polymers is the problem associated with fire; the major health hazard derived from polymers and composites in a fire is from the toxic combustion products produced during burning.
Smoke toxicity plays an important role during fire accidents in buildings, where the majority of people who die do so from inhalation of smoke. Improved methods of assessment need to be developed if toxicity is to be included as part of the fire hazard risk identification. The degree of toxicity generated depends on:
- the phase of burning of the fire
- oxidative pre-ignition
- flaming combustion or fully developed combustion
- ventilation controlled fires.
When a composite material is specified, it must meet the appropriate standards of fire performance. It is usually possible to select a resin system that will meet the requirements of BS Specification BS 476. The UK Building Regulations require that, depending upon their use, building components or structures should conform to given standards of fire safety.
The fire tests as defined in BS 476: Parts 4–7 and in BS 476: Parts 3–8, respectively, fall into two categories:
- reaction to fire – tests on materials
- fire resistance – tests on structures.
Virtually all composites used in structural engineering will have high fibre volume fractions and thus the rate of progress of the fire through the composite is slow; carbon, glass or aramid fibres do not burn.
To enable the flame-retardant properties of the composites to be improved additives are incorporated into the resin formulations, but in so doing an impurity is added to the polymer and some mechanical and/ or in-service property of the polymer may be compromised. The chemical structure of the polymer could be altered, thereby modifying the burning behaviour and producing a composite with an enhanced fire property.
Aluminium trihydrate and antimony trioxide may be used as fillers for both lamination and gel-coated resins, but the use of flame retardants can affect the colour retention of the polymer; a pigment is then added to produce a particular colour in a structural component.
Het-acid-based resins can be used where flameretardant characteristics are required. Nano-clay particles will give some protection against fire and may be added to the pristine polymer, but the process is complicated and at present is expensive for the civil engineering industry (Hackman and Hollaway, 2006).
Nevertheless, modification of the polymer can only aid the fire resistance of the composite to a certain degree; eventually fire will damage composites and indeed all civil engineering materials.
Thermal effects can be divided into thermal expansion and thermal conductivity.
The coefficients of thermal expansion of polymers, which range from 50 to 100 × 10−6 /C degree, are much higher than those of the fibre component of the fibre/matrix composites, e.g. 8.6 × 10−6 /C degree for glass fibre and from 1.6 × 10−6 / to 2.1 × 10−6 /C degree for carbon fibres, depending on the fibre’s structural properties.
The thermal expansion of an FRP composite system is thus reduced from the high value of the polymer to a value near to that of conventional materials; this reduction is due to the stabilising effect that the fibres have on the polymer. The final value will depend upon:
- the type of fibre
- the fibre array
- the fibre volume fraction of the composite
- the temperature and the temperature range into which the composite is placed
- the degree of cross-linking of the polymer will also influence the rate of thermal expansion.
The thermal conductivity of polymers is low, consequently they are good heat insulators. This property is particularly important when FRP composites are exposed directly to the sun’s rays. An example where this effect is particularly relevant is in FRP bridge decks that are incorporated into the superstructure of a reinforced concrete bridge.
The effects of temperature on polymers can be separated into short-term and long-term effects. Short-term effects are generally physical and reversible when the temperature returns to ambient, while longterm effects are generally dominated by chemical change and are not reversible.
As the temperature varies both physical and mechanical properties of polymers change, therefore it will be necessary to fully characterise a material over a range of temperatures. These remarks on the selection of properties apply equally to measuring the ageing effects of long-term exposure. Certain short-term effects such as glass transition temperature, thermal expansion and melting point, are thought of as separate properties, although they are particular cases of the effects of temperature.
Constantly fluctuating temperatures have a greater deleterious effect on all composites but, particularly GFRP. At a micro scale, the difference in the coefficients of thermal expansion of the glass and of the resin may contribute to progressive de-bonding and weakening of the materials, although the extensibility of the resin system will usually accommodate differential movement.
When GFRP composites are exposed to high temperatures a discoloration of the resin may occur; this is noticed by the composite’s becoming yellow. Both polyester and epoxy show this effect and the problem will be aggravated if flame retardants are added to the resin during manufacture of the composite. Furthermore, as a result of the exposure to high temperatures, the composite will become brittle. These effects are not noticed when carbon fibre composites are used.
The ultraviolet (UV) component of sunlight degrades polymers and therefore composites to varying degrees by either causing discolouration of the material causing it to become brittle; the short wavelength band at 330 nm has the most effect upon polymers.
It is manifested by a discoloration of the polymer and a breakdown of the surface of the composite. Ultraviolet stabilisers are incorporated into polymer resin formulations to obviate this problem. The inclusion of stabilisers in epoxy resin formulations seems to have little effect regarding the discoloration but there is no evidence that continuous exposure to sunlight affects the mechanical properties of epoxy polymers. A gel coat surface coating can also be applied to the composite for increased UV and weather protection.
Design with composites
Designing with composites is an interactive process between the designer and the production engineer responsible for the manufacturing technique. It is essential that a design methodology is selected and rigorously used, because many different composite materials are on the market and they can be affected by the quality of their manufacture and the environment into which they are placed.
It is also important that the designer recognises the product cost, because the constituents of composite materials (the fibre and the matrix) can vary significantly in price and the manufacturing process can range from simple compact moulded units cured at room temperature to sophisticated high-temperature- and pressure-cured composites. The design process can be divided into five main phases:
- the design brief and an estimation of cost
- the structural, mechanical and in-service details
- the manufacturing processes and cost details
- the material testing and specification information
- the quality control and structural testing information.
The choice of design factors of safety is an important aspect of the work; these are likely to be given in the relevant code of practice. However, if the design is unique, it may be necessary for the designer/ analyst to select specific factors of safety, bearing in mind the exactness of the calculations, the manufacturing processes, the in-service environment, the life of the product and the loading.
The selection of these design factors follows the pattern for other materials but, with the variation in properties, owing to the anisotropic nature and the different manufacturing techniques of composites, a more involved calculation and a greater reliance upon the design factors will result.
In recent years a significant number of design guides, design codes and specifications have been published by technical organisations in several countries throughout the world; these provide guidance for design with FRP materials for civil engineering. As a considerable volume of FRP composites has been concerned with bridge work these design guides are mainly directed to bridge engineering.