INTERFACE OF MATRIX AND FILLER in COMPOSITES

Post 180 ⇒   by Gautam Shah 

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Matrix-Filler Interface in Composites

A composite material is a complex entity. Understanding its constituents and their role helps in understanding of its strength and weaknesses. Here in very simple terminology this has been explained.

 

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Plasan Sand Cat light (5t) military vehicle featuring integrated composite armoured body

 

In a composite material, the Filler in the form of particles, fibres and sheets, is expected to take up the stresses in unison with the Matrix, due to the strong interface provided by the later.

 

Composite materials with weak interfaces have low strength and stiffness, but high resistance to fracture, On the other hand materials with strong interfaces have high strength and stiffness but are brittle. The bonding between the matrix and the filler is dependent on the atomic arrangement and chemical properties of filler and on the molecular conformation and chemical constitution of the matrix.

 

A crack that starts in a monolithic material generally continues to propagate until that material fails, whereas the filler-matrix combination reduces the potential for a complete fracture.

 

Bonding at the interface: In a simple system the bonding is due to adhesion between filler and the matrix.

 

Adhesion can be attributed to following FIVE main mechanisms:

 

1 Adsorption and wetting: When two electrically neutral surfaces are brought sufficiently close, there is a physical attraction. Most solids, have surfaces that are rarely perfectly in level, blemish, and without any contamination. So a wetting agent that substantially covers all hills and valleys, displaces all air, and overcome the effects of contamination, is required.

 

2 Interdiffusion: It is possible to form a bond between two polymer surfaces by the diffusion of the polymer molecules on one surface into a molecular network of the other surface. The bond strength will depend on the amount of molecular entanglement and the number of molecules involved. Interdiffusion may be promoted by the presence of solvents and Plasticizing agents.

 

3 Electrostatic attraction: When one surface carries a net positive charge and the other surface, a net negative charge, electrostatic attraction occurs (as in acid+base reaction). Electrostatic attraction has no major role in contribution of bond strength, but has importance on how things initially begin to get mixed.

 

4 Chemical bonding: It is formed between a chemical group of filler and a chemical group of a matrix. The bond formation or breakage usually involves thermal activity.

 

5 Mechanical adhesion: Some bonding occurs by the mechanical interlocking of two surfaces (e.g. fibre shape-section).

 

Shocks, impact, loadings or repeated cyclic stresses can cause the Individual fibres to separate from the matrix, e.g. a fibre pull-out. In case of laminated or layered construction there could be a separation at the interface between two layers, a condition known as de-lamination.

 

Composite material: Radar absorbent material for stealth air craft

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COMPOSITES – Part 1

Post 156   by Gautam Shah ➔

Natural and manufactured raw materials are invariably compounds, made of many materials. These raw materials are formed into products by a method of assimilation -the COMPOSITE or the raw materials are organized into a geometric form -the COMPOSITION.

Compo

1 Composite (Cloth+Gypsum) (Wikipedia image by JanSLWC)  2 Composition -geometric arrangement (Wikipedia image by Viapastrengo at En Wikipedia)

Many of the raw materials are naturally compounded materials, in the form of Composites. The composite materials come into being, by putting together natural and manufactured materials in such a special way that the strength and other qualities are different from the constituents, individually and cumulatively. The term different, is considered here as an improved quality, because man-made composites are designed and created towards specific performance requirements only.

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Howrah_Bridge,_Kolkota

Man-made composition -Howrah Bridge Kolkota

Similarly natural and man-made materials have an inherent organization of geometric arrangements which endow unique structural behaviour capacities. Some of the simplest examples are hives of honey bees or birds’ netted nests. At a complex level a truss, bicycle frame, a hull of a boat, are all Structural Compositions or geometric arrangements.

Natural and manufactured materials, and their composites, all are further shaped, re-formed and geometrically integrated to create secondary components as well as Structural Compositions.

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UNDERSTANDING THE DIFFERENCES BETWEEN COMPOSITES AND COMPOSITIONS

A composite is a natural or designed material entity with potential utility, but has no operational functionality. On the other hand, a component is a configuration of many materials into a utilitarian product. Component manufacturing involves processes that are many times similar to a composite formation. As a matter of fact for component manufacturing, the ‘composite formation’ and the ‘component creation’ both occur simultaneously. Structural compositions (trusses, bridges, buildings) are geometric-configuration of materials, often assisted by components (nuts, rivets, pins, bearings, etc.). Structural compositions use composites to form the constituent elements.

A Layered composite -Plywood

Various Definitions of Composites:

# Consisting of two or more physically distinct and conceptually separable or visually identifiable materials.

# Products that can be made by mixing separate materials, so the dispersion of one material in the other can be done in a controlled way to achieve optimum properties.

# Products with properties that are superior and possibly unique in some specific respects compared to the properties of their individual components.

Particulate composite

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Cement Aggregate composite -Mozaic tile

Classes of Composites:

Natural composites               Wood, Bamboo, Bone, Muscle and other tissues

Macro composites                 Galvanized steel,

Engineered products           Reinforced cement concrete (beams, etc.), Helicopter blades Skis, Tennis rackets.

Microscopic composites     Metallic alloys, Toughened plastic (impact polystyrene, ABS), Reinforced plastics.

Nano composites                   Electronics circuits, diodes, transistors

Toys Object Sport Ping Pong Ping-pong Table Tennis

Some natural composites are easy to identify, such as: wood, bamboo, bones, muscles, etc. First man-made composites related to the bronze, as man tried to fix natural stones and ceramic pieces by hammering into the bronze. Layered wood composites have been used by Egyptians. Mud bricks reinforced with hay, hair, and rice husk have been used in prehistoric times. Cow-dung is also reinforced with granular sand particles for wall plaster. Gypsum (Plaster of Paris) has been applied on a lattice of jute, papyrus and such other fibres.

1 - 640px-Textolite

Macro, Micro and Nano Composites: Composites can be categorized in terms of the size of constituent particulate matter. Ingredients of macro composites can be distinguished by naked eye, whereas one may need an electron microscope to understand the constituents of micro composites and nano composites. Nano composites are created by introducing nano-particulate, which drastically add to the electrical, thermal, and mechanical properties of the original material.

1 -Composite_laminate_specimen

Basic Constituents of a Composite:     MATRIX and FILLERS

The constituents of a composite are ordinarily classified as Matrix and Filler. It is the nature of relationship between the filler and matrix, or the Interface that defines the composite. Fillers serve to resist stresses, mainly tension, and the Matrix serves to resist the shear, and all materials present including any aggregates, serve to resist the compression.

Fibres -used as FILLER in composite

Categorization of Composites on the basis of strengthening mechanisms.

Composite materials can be distinguished into three categories based on the strengthening mechanism.

These categories are:

1. Dispersion strengthened,

2 Particle reinforced

3 Fiber reinforced.

Dispersion Strengthened Composites have a fine distribution of secondary particles (fillers) in the matrix of the material. These particles impede the mechanisms that allow a material to deform. Many Metal-matrix composites would fall into the dispersion strengthened composites’ category.

Particle reinforced composites have a large volume of particles dispersed in the matrix. The load is shared by the particles and the matrix. Most commercial ceramics and many filled polymers are particle-reinforced composites.

Fiber-reinforced composites have fibre as the main load-bearing component. Fiberglass and carbon fibre composites are examples of fiber-reinforced composites.

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WOOD SURFACE PREPARATIONS for CLEAR COATINGS

Post -by Gautam Shah

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Chinese Style Redwood Wood Bathroom Cabinet

Natural Woods are many different types, such as Soft, Medium and Hard grain. Woods are likely to be Fresh, Seasoned and Old woods. Woods have different types of faces such as the end grains, flat and side faces. The woods have problems of uneven colours, patterns. The variations in textures are both natural and tools or machine made.

Soft Woods Pinus sylvestris

Wood products have similar problems but an average quality due to the mass production processes. The variations however are between near-natural products (plywoods, veneers) and synthesized products (particle boards, MDFs etc.)

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For a clear coating application all wood surfaces require surface preparation treatments and post coating application treatments. The surface preparation treatments are over and above the nominal craft processes such as planing, sand papering, etc. These are mainly Filling, Sealing and Staining. The post coating application treatments are not always required, but could be Buffing, burnishing, waxing, etc.

Hardwood tops

Fillers

Fillers generally consist of an Extender, a Binder and occasionally a Colourant. Fillers are required to fill in the pores in the wood grains of Natural woods and cavities in case of wood products. A levelled surface provides better gloss and integrity of the coating (one continuous surface without breaks). The coating material does not sink into it, and provide an even finish. Transparent fillers (low body NC lacquer, shellac, etc.) are used not only to fill the pores, but provide a sealing coat to the decayable material in the grains and vessels filled with gum exudates.

Extenders

Extenders are low opacity -reflectivity, fine grade powder of materials like gypsum, chalk, china clay, precipitated calcium carbonate, lime, asbestine, colloidal silica, barytes and talc(unlike the Zinc oxide or Titanium dioxide -the white pigments that adds whiteness and opacity).

Binders

Binders are binding agents that hold together the extenders, temporarily or permanently, and also bind the extenders to the wood substrate. A binder could be water, gums, oils, alkyds and poly vinyl emulsions. Solvent bound binders are better compared to water bound binders. The later ones raise the grain or fibres off the surface.

Colourants

Colourants provide an equalizing tint to the wood surface and slightly colour the white extenders. Fillers, made of low opacity-reflectivity pigments serve a dual purpose, of filling as well as staining.

Wood Stain Brush Delete Planks Wood Wax Wood

Stains as colourants

Stains provide a correct transparent tinge to the surface. Stains are generally dye-material soluble in water, oil or solvents. Water soluble stains though of many different varieties, raise the fibres and are difficult to penetrate. Oil soluble stains are heavy bodied, take longer to dry out and interfere in the subsequent coating application. Solvent soluble stains are costly, dry out immediately and may bleed residual gums and other exudates.

Wood Staining

Stains have one important drawback that they darken the existing colour of the substrate. Where timber surfaces need to be of lighter colour, surfaces have to be bleached or toned with opaque materials.

Maple_wood_grain_(Sapwood_and_heartwood_on_the_same)

Bleaching

Bleaching process lightens the existing colour of the wood. It includes a treatment with hydrogen peroxide followed by an alkaline accelerator like lime, caustic soda, sodium silicate or ammonia. Bleaching affects adhesion and toughness of coating. It also provides an amber hue to the coating on aging. Staining is also done by micro spray guns, singeing, burning or carbon deposition from flames.

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Imposed patterns

On wood surfaces where there are very irregular grains or no patterns, these are screen-printed, pressed or embossed using stains. Such patterns may emulate a wood grain pattern or just very fine mesh or lines.

Wenge veneer imposed pattern

Post application treatments

Clear finishes often require some post application treatments. These are mainly burnishing and waxing. Burnishing is mainly done to NC lacquer, Acrylic and Melamine coatings, to provide a glossy surface. Burnishing is not done to slow drying coatings, because such coatings, though are dry on outer face, take days to thoroughly dry out. Burnishing is done with a Carborundum like rubbing material with a waxy or oily base. Waxing provides a dull sheen and a protective coating. Waxing is also done to renovate old coatings. Waxing compounds also include a small amount of oils and sometime silicone materials.

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