Comparison of Resin Properties
The choice of resin system to be used in any component depends on a number of its properties, the following are probably the most important in most composite structures:
1. bonding properties
2. Mechanical properties
3. Microscopic crack resistance
4. Fatigue strength
5. Water corrosion degradation
Bonding Properties
It has been discussed how the adhesion properties of the resin system are important in achieving the full range of mechanical properties of the composite. In sandwich structures, the adhesion of the resin matrix to the fiber reinforcement or core material is important.
Polyester resins typically have the lowest adhesion properties of the three systems described here. Vinyl Lester resins exhibit better bonding properties than polyesters, but epoxy resin systems provide the best performance of all the adhesives and are therefore often found in many high-strength adhesives. This is due to their chemical composition and the presence of polar hydroxyl and ether groups. Since epoxy resins cure at a low shrinkage rate, the various surface contacts established between the liquid resin and the adherends are not disturbed during the curing process. The bonding properties of epoxy resins are particularly useful in the construction of honeycomb core laminates, where a small bonding surface area means that maximum bonding is required.
The strength of the bond between resin and fiber depends not only on the bonding properties of the resin system, but is also influenced by the surface coating of the reinforcing fibers.
Mechanical Properties
Two important mechanical properties of any resin system are tensile strength and stiffness. Figures 22 and 23 show the results of tests performed on commercially available polyester, vinyl Lester, and epoxy resin systems cured at 20°C and 80°C.
After a seven-day curing period at room temperature, it can be seen that typical epoxy resins offer higher performance in terms of both strength and stiffness than typical polyesters and vinyl esters.The beneficial effects of curing for five hours after 80°C can also be seen.
Equally important to composite designers and builders is the amount of shrinkage of the resin during and after curing. Shrinkage is due to the rearrangement and reorientation of resin molecules in the liquid and semi-gel phases. Polyesters and vinyl esters require a significant amount of molecular rearrangement to reach a cured state and can show shrinkage of up to 8%. However, the different nature of the epoxy reaction results in very little rearrangement and the absence of volatile bi-products, reducing typical epoxy shrinkage to about 2%. To some extent, the absence of shrinkage is responsible for the better mechanical properties of epoxy than polyester, as shrinkage is associated with internal stresses that can weaken the material.
In addition, shrinkage in the thickness of the laminate leads to "print-through" of the reinforcing fiber pattern, which is a difficult and expensive cosmetic defect to eliminate.
Microscopic cracks
The strength of a laminate is usually considered in terms of how much load it can withstand before it fails completely. This ultimate strength or breaking strength is the point at which the resin exhibits catastrophic fracture and the reinforcing fibers break.
However, before this ultimate strength is reached, the laminate will reach a stress level where the resin will begin to crack from the fiber reinforcement layers that are not aligned with the applied load, and these cracks will propagate through the resin matrix. This is known as "transverse microcracking" and although the laminate has not yet failed completely, the rupture process has already begun. Engineers who want a durable structure must therefore ensure that their laminates do not exceed this point under regular service loads.
The strain that a laminate can achieve before micro-cracking is very much dependent on the toughness and bonding properties of the resin system. For brittle resin systems, such as most polyesters, this occurs long before the laminate fails, and therefore does limit the strain that such laminates can withstand. For example, recent tests have shown that for polyester/glass woven roving laminates, microcracking typically occurs at about 0.2% strain, with final damage not occurring until 2.0% strain. This corresponds to a usable strength of only 10% of the ultimate strength. Since the ultimate strength of the laminate under tension is determined by the fiber strength, these resin microcracks do not immediately reduce the ultimate performance of the laminate.
However, in environments such as water or humid air, a microcracked laminate will absorb more water than an uncracked laminate. This will result in an increase in weight, moisture in the resin and fiber sizing agent, a decrease in stiffness, and a decrease in ultimate performance over time.
The increase in resin/fiber adhesion usually stems from the chemistry of the resin and its compatibility with the chemical surface treatment applied to the fibers. Here, the well-known properties of epoxy adhesives help the laminate achieve higher microcracking strains. As mentioned earlier, resin toughness is difficult to measure, but is widely indicated by its ultimate failure strain. A comparison of different resin systems is shown in Figure 25.
Fatigue strength
In general, composites have excellent fatigue resistance compared to most metals. However, since fatigue failure is often caused by the gradual accumulation of a small amount of damage, the fatigue behavior of any composite material will be affected by the toughness of the resin, its resistance to microcracking, and the number of voids and other defects that occur during the manufacturing process. For this reason, epoxy base laminates tend to exhibit very good fatigue resistance compared to polyester and vinyl ester, which is one of the main reasons for their use in aerostructures.
Water Corrosion Degradation
An important characteristic of any resin, especially in the marine environment, is its ability to withstand degradation by incoming water. All resins absorb some water, adding weight to the laminate, but more importantly how the absorbed water affects the resin and resin/fiber bond in the laminate, leading to a gradual and long-term loss of mechanical properties. Both polyester and vinyl ester resins are susceptible to water degradation due to the presence of hydrolyzable ester groups in the molecular structure.
As a result, thin polyester laminates can be expected to retain only 65% of their interlayer shear strength after one year of immersion in water, whereas epoxy resin laminates will retain approximately 90% after one year of immersion.
Figure 26 shows the effect of water on epoxy and polyester woven glass laminates that were immersed in water at 100°C. This high temperature immersion gives the immersed laminates accelerated degradation properties.
Permeability
All laminates in the marine environment allow very small amounts of water to pass through them in the form of vapor. As water passes through, it reacts with any hydrolyzable components within the laminate to form tiny cells of concentrated solution. Under the osmotic cycle, more water passes through the semipermeable membrane of the laminate in an attempt to dilute this solution. This water increases the fluid pressure inside the cell to 700 psi. Eventually, the pressure will deform or rupture the laminate or gelcoat and may result in a typical "chicken pox" surface. Hydrolyzable components in the laminate may include dirt and debris trapped during the manufacturing process, but may also include ester bonds in the cured polyester and, to a lesser extent, vinyl lath.
The use of a resin-rich layer next to a gel coat is essential for polyester resins to reduce this type of degradation, but usually the only cure once the process begins is to replace the affected material. In order to prevent infiltration from occurring in the first place, it is necessary to use a resin that has both low permeability and high resistance to water erosion. Blistering can be virtually eliminated when used in conjunction with a reinforcing material that has a similar resistance to surface treatment and is laminated to a very high standard. Polymer chains with epoxy-based chains resist the effects of water much better than many other resin systems. Such systems have been shown to have excellent chemical and water resistance, low water permeability and very good mechanical properties.
Summary of Comparative Resin Properties
The polyester, vinyl ester, and epoxy resins discussed here probably represent about 90% of all thermoset resin systems used in structural composites. In summary, the major advantages and disadvantages of these types are：
Other resins for composites
In addition to polyester, vinyl ester and epoxy resins, there are many other specialty resin systems used where their unique properties are needed.
Phenolic resins
Mainly used where high fire resistance is required, phenolic resins retain their properties well at high temperatures. For room temperature cured materials, the use of corrosive acids leads to unpleasant handling. The condensed nature of their curing process tends to result in the inclusion of many voids and surface defects, the resin tends to be brittle and has poor mechanical properties. Typical cost: £2-4/kg.
Phenyl Isocyanate
Mainly used in the aerospace industry. The excellent dielectric properties of the material make it ideal for use in low dielectric fibers such as quartz, which are used in the manufacture of radomes. The material is also temperature stable up to 200°C wet. Typical cost: £40/kg.
Silicone Resins
Synthetic resins with silicon as the base material rather than carbon as the organic polymer. Good resistance to fire and high temperatures. Requires high temperature curing. Used in missile applications. Typical cost: >£15/ kg.
Polyurethane
Highly ductile material, sometimes blended with other resins, due to relatively low laminate mechanical properties in compression. Uses hazardous isocyanates as curing agents. Typical cost: £2-8/kg.
Bismaleimide (BMI)
Mainly used in aircraft composites requiring higher temperatures (230°C wet/250°C dry). For example, engine inlets, high speed aircraft flight surfaces. Typical cost:>£50/kg.
Polyimide
Used where operation at higher temperatures is required than bismaleimide can withstand (use up to 250°C wet/300°C dry). Typical applications include missile and aircraft engine components. Extremely expensive resin (>£80/kg) with toxic raw materials used in the manufacturing process. Polyimides are difficult to work with due to the condensation reaction that releases water during the curing process, and are relatively brittle after curing.PMR15 and LaRC160 are the two most commonly used polyimides in composites.
Source "Frontiers in Composites"