RELEASE AGENTS for FRP

RELEASE AGENTS: (Parting agents or mold release)

Regardless of the type of reinforcement or the resin used, the parts have a tendency to stick to the mold surface. Hence some type of release agent is always used to facilitate removal of the part from the mold. The release agent functions by forming a barrier film between the mold surface and the part. Various materials used as release agents can be classified as:

1.      External release agents--applied to the mold surfaces.

2.      Internal release agents--combined with the resin.

·         EXTERNAL RELEASE AGENTS:

They can be further classified as--1) films, 2) film formers and 3) wipe-ONs.

FILMS:

It can be obtained in roll or sheet form. The film may be applied as a flat sheet cut to size or as thermoformed sheet made to conform to a 3-D contour e.g. Regenerated cellulose (Cellophane), PVA, Polyester, nylon, and Teflon films are usually used as flat sheets and PE, cellulose acetate, PVC and various fluorocarbon films can be easily thermoformed.

The film will satisfactorily prevent mold sticking and will usually stick to the part after curing so that it can be easily stripped off.

Selection Criteria:

Temperature at which the part is to be cured should be well below the softening temperature of the film material.

FILM FORMERS:

They can be applied either by brush or spry gun on the mold surface. Most common materials are: cellulose acetate lacquers, PVA and nylon solutions, heat cured fluorocarbon coatings and possibly varnishes and/or automobile type paint fillers which can be sanded to a high gloss finish.

These materials are generally used over porous surfaces like wood or plaster of paris. They are also used where the surface are to be filled and smoothed to remove scratches, pits, or the grain pattern of a wooden form. They are frequently used along with a wax, silicone, or fluorocarbon type wipe ON material.

WIPE-ONs:

They are applied with a wiping rag (for certain waxes), a brush (for oily material) or a spray, usually in the form of an aerosol can. In most cases after the initial application the mold is wiped with a rag to smoothen the mold surface. Most common materials are:

WAX: There are number of wax preparations ranging from pure carnauba wax to various soft paste waxes similar to automobile waxes. The pure carnauba wax is generally applied to hot molds.

Advantage:

1) High gloss can be obtained if the material is correctly applied and polished.

2) The mold can be used immediately with no drying or curing period required.

SILICONES:

Various Silicone materials are available in solution form and are wiped or brushed on. Some are air dried and others are heat cured. Silicones are also available in the form of aerosol solutions which are either air drying or heat curing.

FLUOROCARBONS:

The aerosol spray cans of fluorocarbon particles dispersed in a volatile solvent carrier have advantage like-- the excellent release properties and high temperature resistance. This allows them to be used with polymers which are curing at the highest temperature. However, the final gloss may not be as high as that obtained with paste wax.

In general, the use of wrong mold release can result in increased time for part removal or may result in damaged surface or parts due to mold sticking. A mold release should be chosen according to the post molding process being carried out.

·         INTERNAL RELEASE AGENTS:

The chemical compounds used as internal release agents are added to the resin by the manufacturer to minimize the mold sticking after cure. Metallic soaps like zinc or calcium stearate are used for polyester and phenolic resins. Silicone oils and high melting point waxes are used in epoxies. Organo-phosphates and waxes are used in polyester resin. The internal release agents work by being squeezed or exuded on the surface during cure of the resin.

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.