REINFORCEMENTS for FRP

Definition:

Reinforcements are strong, inert fibrous material incorporated in the polymeric mass to improve physical properties.

·         GLASS FIBERS:

It is the material originally and most widely used for low pressure moldings. It is used in several forms like chopped strands, rovings, reinforcing mats, yarns, surfacing and overlay mats, woven fabrics, woven rovings, non woven fabrics etc.

Preheating and finishing techniques:

All the glass fibers are treated and finished in a special manner to promote the maximum adhesion of the resin to fibers. First, the original lubricant, (or sizing) used to help in drawing the filament, is removed by the process known as heat cleaning. Some resins do not require a further finishing but for the other resins a finish known as a coupling agent is applied to the fiber, to result in increased adhesion, particularly under high humidity or moisture condition. Some finishes are best suited to particular resin but the most widely used finishes are the chrome complexes and the silanes. However, the type of finish should be determined before using any glass fiber as reinforcements.

·         ASBESTOS:

Six fibrous material considered as asbestos can be classified under two major groups as:

1.      Fibrous serpentine or chrysotile:

It is a hydrated magnesium silicate with double layers of silica tetrahedrons and magnesium hydroxide. Fibers are long and hence it is ideal for the use in laminated plastics.

2.      Amphiboles:

They are various combinations of Mg, Fe, Ca, and Na hydrated silicates with a basic structure of Si₄O₁₁ groups in a double chain configuration. Amphiboles include crocidolite, amosite, anthophillite, actinolite and tremolite. Crocidolite fibers impart high tensile strength, hence are used for general molding. Amosites have limited use in polymer industry because the moldings obtained from it are brittle. It is used in specialized case where acid resistant characteristic is required. Actinolite and tremolite are not significant in commercial plastics.

Mining of asbestos is a difficult process and results in a product having wide range of fiber lengths and includes impurities. Short fibers impart higher modulus and greater hardness to the finished product compared to unfilled system but do not improve other properties.

The moldings produced with asbestos as filler have excellent heat resistance, lower flexibility and tensile strength and increased specific gravity.

Asbestos can be wetted by almost any thermoplastic or thermoset resin. However, when longer fibers agglomerate, it prevents the development of homogenous mixture.

Health problem also results, when asbestos is used. It causes mesothelinoma and lung cancer to the workers; hence the exposure levels must be below the limits prescribed by OSHA. (Occupational safety and health act)

·         NYLON:

It is available as flock, filament and chopped fabric. It flows at many molding temperature causing considerable mold shrinkage. Hence it is used when special surface is required in the molding e.g. shower stall bases.

·         POLYESTER FIBERS:

They can reinforce both thermoplastic and thermoset. The fiber thermoplastic composite have unusually high strength because they serve as nucleating agents for the crystalline thermoplastic. Abrasion and impact resistance of these fibers can be increased by reinforcing it with woven roving of polyethylene terephthalate. These fibers are solvent resistant and high melting and can reinforce thermoplastic, phenolic and epoxy resin system.

·         EXOTIC FIBERS:

To prepare the composites using the exotic fibers, only boron, graphite and Kevlar are used.

·         BORON:

It is mainly used where the fibers can be laid in relatively straight configuration because these fibers are very stiff and brittle.

·         GRAPHITE FIBERS:

They are used in most of the high modulus applications because they are cheaper than boron, have lower specific gravity and are capable of being formed easily into 3-D curves and bent into small radii. Also the parts which are graphite reinforced have a very low coefficient of expansion. This makes such material very useful for high precision parts like optical parts, space antennas etc. In general it is used in articles where high stiffness and light weight are required.

·         KEVLAR (POLYARAMID FIBERS):

They can form composites with excellent strength and heat resistance. They also have low densities and hence show high specific strength values.

Three types of Kevlar aramid are available. The tensile strength of Kevlar is better than twice that of nylon or polyester and 15% better than E-type fiber glass. Modulus is 20 times greater than nylon, 10 times greater than Polyester and 2 times than E-type fiber glass. Elongation is low and density is 40% less than glass. They are as strong as steel at 1/5 the weight. Their thermal stability is also good and no embrittlement or degradation occurs during cryogenic exposure. Composites made with Kevlar have excellent dimensional stability because it has slightly negative coefficient of thermal expansion in the longitudinal direction. They have good chemicla resistance except to strong acids and alkalis. They have high stress rupture life and fracture toughness, good cyclic tensile fatigue resistance and fair damping characteristic. However they have high moisture absorption, low compressive strength and difficulty of machining.

GEL COAT for FRP

Functions/Significance/Applications:

When the parts are molded at 0-15 psi, their surface showed up a distinct 3-D fiber pattern due to the shrinkage of the resin during curing. This condition cannot be tolerated for aerodynamic and aesthetic reasons. Use of gel coat is developed as remedy.

The usual gel coat is polyester resin heavily filled with mineral filler and a pigment. This shows very little shrinkage due to its very high filler-to-resin ratio.

Also the gel coat, when properly applied and cured, results in a hard, smooth colored coating, 10-30 mils thick throughout. This minimizes the effect of scratches.

Sometimes, a clear gel coat is used to improve the stain resistance, gloss and depth of coating when applied over cultured marble.

A chemical resistant clear gel coat is used over chemical resistant laminates to improve the resistance to corrosive environments.

Epoxy gel coats are also used to improve the surface quality of the cured reinforced plastic part.

Polyester gel coat formulation:

Gel coat consists of

1.      Resin

2.      Fillers

3.      Thixotropic agents

4.      Pigments

5.      Solvents

RESIN:

A low viscosity resin is used so it can be easily and will allow entrapped air to escape. Usually a high impact grade is preferred to insure freedom from chipping caused by impact thermal stresses. Concentration of the resin in the complete formulation is 25-95% by weight with lower concentration preferable. 

FILLERS:

Usually CaCO₃ (calcium carbonate-marble) and hydrated Aluminum silicate are used. Other fillers are metal powders, carbides, oxides, silicates, sulfides, carbon and graphite depending on the physical, chemical or electrical properties desired. The fillers are used to reduce the resin shrinkage; lower the exothermal heat; increase the hardness and thermal conductivity; and change the density and opacity of the resin. Their concentration ranges from 5-75% by weight.

THIXOTROPIC AGENTS:

Colloidal silica (Cab-O-Sil) and Mg Al silicate clays (Bentonite) are used as thixotropic agents, to prevent sagging and running of the gel coat when applied to vertical surfaces. They also minimize filler settling and increase efficiency. Their concentration ranges from 2-15 % by weight.

PIGMENTS:

They are dispersed into the resin to act as coloring agents. Their concentration ranges from 2-10 % by weight.

SOLVENTS:

They are added to the formulation to thin the material to spraying consistency. Minimum amount of solvent should be used to prevent attack on the mold release which in turn results in part sticking, poor surface etc. Usually acetone is used as solvent.

TECHNIQUES FOR APPLICATION OF GEL COAT:

The catalyzed gel coats are applied to the release coated mold surface by brush, roller coat or spray. The spray coating technique is most widely used. The coat is allowed to gel and then the reinforcing fiber and resin are applied while it is still tacky. If done correctly, the bond between the gel coat and the reinforcing fiber laminate will be excellent but if the reinforcing mat is applied before gelation of the gel coat, fiber exposure or the impression of the fiber or fabric may result. However, most gel coats are made using air inhibited resin, so the gel coat will automatically cure with a tacky surface.

Gel coats are used in parts where a smooth, hard, tough and colored surface is required.

RESIN MATERIAL FOR FIBER REINFORCED PLASTIC

RESIN:

Function:

The resin in RP holds the fibers together and transmits the loads from fiber to fiber.

Requirements of an ideal resin in RP:

1.      The uncured resin must wet the reinforcement well and should be cured at room temperature or by heat. Also, when cured, it should have good adherence to the fibers, good heat resistance and good electrical and chemical properties.

2.      The resin should be available in a wide range of viscosities, should be compatible with pigments, filler material and if possible, must be cured at temperature ranging from ambient to 150⁰C.

Conventional resins:

1.      Polyester: It is the 1^st resin used in the low pressure process and is the most widely used polymer because it possesses almost all the requirements of an ideal resin.

a.       Polyester resins are available in liquid forms in many viscosities, so good fiber impregnation is relatively easy.

b.      Using different catalysts the resin can be cured at room temperature or at an elevated temperature or by ultra violet (uv) light. This allows a part to be worked on all day without its setting up and then be cured overnight in an oven, or by using uv radiation from a lamp (sunlight can also be used)

c.       Cured materials have excellent electrical and chemical properties.

d.      Cured materials have good strength to weight ratios.

e.       Costs are relatively low.

f.       Disadvantages of polyester resin: (1) Relatively high shrinkage occurs when curing takes place. (2) Under high humidity or moisture condition, less adherence (than ideal) to the fibers occurs hence fibers are specially treated to upgrade the performance of laminates. (3)For thin fiberglass reinforced parts, relatively high brittleness occurs.

g.      Special formulations are produced which show increased toughness. Vinyl ester resins are tougher and have higher temperature resistance than the conventional polyester resins.

2.      Epoxies: These materials have all good properties of the polyester. The epoxy resins also have some special properties of their own, which makes them superior to the polyesters like: Low shrinkages and excellent adhesion to most material which gives higher strength to weight ratio to the laminates.

3.       Other resins: Some of the older resins are modified so that they can be used with the new low pressure technique. Some new resin types are also developed. Today there are a number of thermoset resin each having unique characteristic e.g. Phenolics, Silicones, Melamines, Polyimides, Diallyl phthalates.

CURING TECHNIQUES:

The resins were cured at pressures ranging from 0-15 psi but now pressures up to 50-100 psi are used. The use of high pressures will result in more highly densified part because no volatiles are given off in the polymer. The densification takes place by compression of the dissolved air. This results in stronger parts.