What Materials are Used to Make Composites?
Polyester resins are a family of polymers used to produce a broad range of products. Thermoset polyesters are known as unsaturated polyester resins. They are available in a number of formulations for specific applications. These formulations are usually designated by the principle raw material that determines the performance characteristics of the resulting polymer backbone. Common unsaturated polyester resin formulations include:
- Bisphenol A Fumarate Polyesters
- Vinyl ester
On the most basic level these resins consist of an unsaturated polyester polymer dissolved in a crosslinking monomer and various additives that compromise a “resin system”. Technically, polyester resin is the product of reacting an unsaturated dibasic acid, usually maleic anhydride, with a glycol. The characteristics of the resin are varied by adding a saturated dibasic acid such as orthophthalic anhydride, isophthalic acid or adipic acid.
Polyester Resin Systems
The polyester backbone (orthophthalic, isophthalic, etc.) is combined with a crosslinking monomer to create resin in the commonly used liquid form. The most widely used monomer is styrene. Other common monomers include: Methylmethacrylate (MMA), vinyl tolulene (VT), alpha methyl styrene (AMS), para methyl styrene (PMS), and diallyl phthalate (DAP). These are often used in combination with styrene to optimize properties. Additional monomers continued to be explored to minimize environmental impact.
- Inhibitors – Because of the tendency of unsaturated polyesters to self-polymerize inhibitors are added to both the monomer and the formulated resin. Inhibitors slow the curing reaction by reacting with free-radicals before they cause crosslinking between the polymer and the monomer. Common inhibitors include Quinones (HQ, MTBHQ, PBQ, or THQ) and p-tert-butylcatechol (TBC).
- Promoters – Sometimes referred to as accelerators, these materials facilitate effective room temperature curing . The promoter reacts with the initiator at the time of molding. Promoters include cobalt compounds, used with MEKP initiators; and aniline compounds, used for co-promoters in a MEKP system and as the primary promoter in ambient conditions with BPO initiators. Polyester resins are available in pre-promoted form, where the resin manufacturer includes the promoter in the product; or in unpromoted formulations where the molder adds promoter prior to processing.
- Initiators – Sometimes incorrectly referred to as catalyst. When added to polyester resin the initiator reacts with the resin and decomposes, forming free radical molecules. These free radicals overcome the inhibitor and begin a cascading polymerization reaction, also known as curing.
- Functional additives – Resin characteristics and performance can be modified with the addition of a number of additives. These include: Fire retardant additives, thixotropes, pigments, vapor suppressants, and other additives.
Corrosion Product Resins
Isophthalic Polyester Resin
Isophthalic polyester resins are a broad class of resins formulated from isophthalic acid, glycols, and maleic anhydride. The specific resin specification is selected to impart desired properties and corrosion resistance. These resins can be used for moderate corrosion resistance applications to a temperature range around 180° F. Isophthalic resins exhibit good resistance to water, acids, weak bases, and hydrocarbons such as gasoline and oil.
Vinyl Ester Resin
Technically vinyl ester resins are a polyester resin, however they are normally classified separately from polyesters due to their enhanced mechanical properties and corrosion resistance. Bisphenol A based epoxy vinyl ester resins are methacrylated epoxy polyesters The Novolac epoxy vinyl ester resins enhance the epoxy component resulting in higher performance. Vinyl esters offer enhanced strength and generally better impact and thermal shock resistance than polyester resins. While the standard epoxy vinyl ester resins are limited to 220 – 250° F in most applications, other versions with higher-density crosslinking are suitable for temperatures above 250° F. These resins exhibit excellent resistance to acids, alkalis, hypochlorites, and many solvents.
Bisphenol A Fumarate Polyester Resin
Bisphenol A fumarate polyester resin is made by reacting bisphenol A with propylene oxide and fumaric acid to provide a resin that is particularly resistant to alkali environments. This resin is used primarily for applications involving hot caustic solutions. It is suitable for service with acids, selected organic solvents and salt solutions to a temperature range of about 250° F.
Chlorendic Polyester Resin
Chlorendic resins are unsaturated, halogenated polyester resins. They are particularly well suited for equipment operating at elevated temperatures or in highly oxidizing environments such as hot, wet chlorine. These resins are particularly well suited for chimney liners, flue gas duct, chrome plating tanks, pickling tanks, and chlorine headers.
The phenolic polymer represents one of the earliest commercialized thermoset resins and first appeared in the late 1800’s. Phenolic resins are formed by the reaction of phenol and formaldehyde It is typically a heat cured thermoset. There are two types of phenolic resins – resole (one-step) and novolac (two-step). Resoles are base-catalyzed thermosetting resins that are self-crosslinking. Novolacs are catalyzed with acids and require a hexamine crosslinker to become thermosetting. Phenolics offer high temperature resistance; excellent resistance to chlorinated solvents and salt water; excellent fire/smoke properties.
Furan Resin or Furfural Alcohol Resin
Furan resins also known as furfural alcohol resin are based on a polymer derivative of furfural alcohol. These resins are different from typical polyester or vinyl ester resin in that they are cured using an special acid catalyst blends supplied by the resin manufacturer. Because of this different precautions are required. The resin also cures by a condensation reaction which means water is given off as the resin cures. These resins have to be post cured slowly to avoid delamination during the heat up of the resin. These polymers provide excellent resistance to strong alkalis and acids containing chlorinated organics and are superior to polyesters and epoxy vinyl esters in solvent resistance. Because of this Furfural alcohol resins are used to make chemical storage tanks when vinyl ester resins or polyester resins can not be used. These resins are suitable for use up to about 250° F for many corrosive applications. Furfural alcohol based resin is not suitable for oxidizing chemicals and is not be used in environments containing chromic or nitric acids, peroxides or hypochlorites.
Epoxy resins are very similar to vinyl esters. However, while a polyester resin has double carbon to carbon bond sites, epoxies are characterized by the presence of an epoxy group (a three member ring, two carbon and one oxygen). There is another difference between epoxies and polyesters that involves the number of active sites along the polymer backbone. In epoxy resins, these sites are found only at the ends of each polymer or branch chain, while the carbon-carbon double bonds in the polyester backbone occur many times along the polymer chain. Epoxies provide excellent fiber bonding (matrix to fiber) which improves compressive strength, flexural strength, increases interlaminar shear ,and enhances toughness (improve impact strength, better damage tolerance).
Corrosion Product Reinforcements
Reinforcement materials are combined with polymer matrix systems in a variety of forms to create laminates used in corrosion applications. Different forms of reinforcement have been adapted to particular process methods, or the method has been built around the type of reinforcement. There are two major fibers used as reinforcements for the composites furnished to the corrosion industry: fiberglass and carbon.
Continuous glass filaments are formed by drawing molten glass resting on platinum/rhodium bushings through thousands of holes the appropriate fiber diameter (5 to 25 microns), quenched, sized, and wound into strands of either 102 or 204 filaments. The sizing acts as a coupling agent to bond the resin to the glass filament during resin impregnation.
Typical glass formulations used in corrosion applications:
- E-glass – (lime aluminum borosilicate) is the most commonly used reinforcement in the composites industry because of its good strength properties, resistance to water degradation and relative cost.
- S-Glass – High Strength
- C-Glass – Corrosion resistant
Carbon fibers are made from organic precursors, including PAN (polyacrylonitrile), rayon, and pitches. PAN is a commercially available fiber, while pitch is the residue from the distillation of coal tar or petroleum. It must be formed into a fiber prior to manufacturing carbon fiber. The process of making carbon fibers suitable for fabrication in corrosion composites begins with the “stabilization” of the fibers. Either PAN or Pitch fibers are drawn, under tension thru heated air (400ºF to 750ºF). This process crosslinks and rearranges the fibers so that they do not melt in subsequent processing. The material is then subjected to much higher temperatures (900ºF – 2700ºF) referred to as the carbonization or pyrolization stage in a Nitrogen atmosphere, still under tension. The fibers, still under tension and in a nitrogen atmosphere, are subjected to even higher temperatures (3600ºF – 6000ºF) during the graphitization stage. Finally, the fibers are cleaned in an electrolytic bath, and then a sizing (a polymeric finish for compatibility to the polymer matrix) applied, dried and wound onto spools.
Carbon fibers offer the highest strength and stiffness of all the reinforcement fibers. High temperature performance is particularly outstanding for carbon fibers. The major drawback to PAN-based fibers is their high relative cost, which is a result of the cost of the base material and an energy-intensive manufacturing process.
The importance of the reinforcement to the final end product performance is very critical to the composite fabricator. The properties of these textiles also play a very important role in the selection of the fabricating process. Products that may yield very good properties for a hand lay-up product may not be suitable for vacuum infusion, but readily adaptable to RTM.
There are numerous types and forms of reinforcements used in the fabrication of composites. Some are available direct from the fiber producers, others are converted from basic fiber to specialized products by textile manufacturers. The properties exhibited by these various forms of reinforcement will depend upon many factors. These factors may include the type of weave, the weights of the fibers per area, the thickness of the fabric, thread count, fiber diameter, and type of fiber.
Woven composite reinforcements are woven on a loom with the fibers typically aligned in the machine direction (warp). The weaving, over and under the warp fibers is done automatically at relatively high speeds. This weave of crossing fibers is called the fill. If an additional weave that cuts 45º across the fibers is utilized, it is referred to as the bias.
Woven composite reinforcements generally fall into the category of cloth or woven roving. The cloths are lighter in weight, typically from 6 to 10 ounces per square yard. This is the most common type of reinforcement used for large structures because it is available in fairly heavy weights (24 ounces per square yard is the most common), which enables a rapid buildup of thickness. Textile weavers have developed products that provide desired characteristics for the fabrication of composites. Openness refers to the space between the parallel fibers, and is a measure of the tightness of the weave. It is inversely related to warp and fill counts. The drape of a reinforcement refers to how well the material conforms to the shape of the mold. These and other characteristics are dependant not only upon the fiber properties, but also on the method of which they are woven.
Knitted fabrics are woven reinforcements in which the warp and fill fibers are looped to create a fabric with high drape and conformability. These reinforcements provide greater strength and stiffness per unit thickness as compared to woven roving.
Chopped Strand Mat and Chop Reinforcements
Mat reinforcements are roll stock products used in hand lay-up fabrication. Chopped strand mat consists of randomly oriented glass fiber strands 1 to 2 inches in length that are held together with a styrene soluble binder. Continuous strand mat is similar to chopped strand mat, except that the fiber is continuous and laid down in a swirl pattern. Both hand lay-up and spray-up produce plies with equal physical properties. This is a very economical way to build up thickness, especially with complex molds.
Continuous Roving Reinforcement
Glass fibers used for continuous roving generally range in diameter from 0.00035” to 0.00090” (9 to 23 microns). They start as molten glass (2500°F). These fibers are pulled thru platinum bushings at very high speeds (200 mph) There may be as many as 4,000 of these tiny fibers to make one filament. These filaments are then gathered into bundles, called strands. They are held together with a special binder or sizing.
Multi-end rovings consist of many individual strands which can be chopped and randomly deposited into a resin matrix. Processes such as spray-up sheet molding compound, perform use multi-end roving. Filament winding and pultrusion can also use multi-end rovings.
Processes that utilize a unidirectional reinforcement such as filament winding or pultrusion will use single-end roving. This product consists of many individual filaments wound into a single strand.
Other Corrosion Materials
A veil is a lightweight non-woven fabric that is used in the manufacture of Fiber Reinforced Plastics. Nonwoven veils (also known as mats or tissues) are often used for making corrosion barriers in composite tanks, pipes, ducts, flu stacks, fittings, and pump/valve housings. Nonwoven veils are typically made from C-glass fiber, carbon or other synthetic fiber.
The use of a veil inhibits the generation of micro-cracks in composite surfaces. For highly corrosive environments, these veils are usually made from C-glass or carbon fiber.
The veil can provide the following characteristics or properties to an FRP part:
- Improved surface appearance and profile
- improved corrosion resistance and service life
- Improved abrasion resistance and impact resistance
- Improved thermal shock properties
- Improved dye wears and reduced pull forces in pultrusion
- Prevent underlying Glass of weathered FRP parts from blooming to the surface
- Serve as Print Blocker of underlying reinforcements
- Provide electrical conductivity
Carbon veils provide increased chemical stability, with the added benefit of also being electrically conductive. Carbon veils are used in composite structures for grounding, to minimize the build-up of static electricity. Static dissipation is particularly important in composite tanks and pipes that handle explosive liquids and gasses.
A sandwich composite consists of a very lightweight material placed between two composite layers. The material in the center is referred to as the core, while the composite layers are called skins. In a sandwich construction (core material faced on each side with a reinforcing glass/resin skin) offers greater mechanical strength and stiffness, pound-for-pound, than any other structural option, regardless of the materials used. Further, the great variety of available core material types, ranging from balsa to foams to honeycombs, provides a broad spectrum of material densities, geometries, processing options, costs and physical attributes.
There four basic core materials widely used in corrosion applications: end-grain balsa, styrene acrylonitrile (SAN) foam, polyvinyl chloride (PVC) foam and polyethylene terephthalate (PET) foam. End-grain balsa is popular because it is inexpensive and because its end-grain “honeycomb” structure provides robust mechanical properties Balsa’s mechanical properties are derived mainly from its end-grain structure. Cores made of SAN and PVC are less dense than balsa (about 4 lb/ft³), but tend to be more expensive. Both SAN and PVC are thermoset formulations that provide structural support. PET as a structural foam requires a slightly higher density to match the mechanical strength and stiffness of SAN and PVC, and a substantially higher density to match balsa.
Some core materials provide better mechanical properties, but add additional cost to the blades. There are economical core materials that lack mechanical strength. These variables require an in depth review of the specifications and requirements of the part performance. Core materials can also be process specific, for example, honeycomb does not lend itself to infusion.