Natural Composite Material

Chapter 1 INTRODUCTION

Composite materials can be classified into synthetic and natural. Synthetic composites are based on metal/ceramic/polymer matrix. Some of the examples of natural composite materials are wood, concrete, bone, teeth, mud bricks.

1.1

NATURAL COMPOSITE MATERIALS

1.1.1 History Natural composite materials exist since first ancient builder used straw to reinforce mud bricks. In 12th century, Mongols made archery bow by using cattle tendons, horn, bamboo, silk, and natural pine resin. In late 1800s, canoe builders glued together layers of kraft paper with shellac to get paper laminates. Though the concept was successful but performance of materials was not good. Between 1870 and 1890, a revolution took place with the development of first synthetic resin, called polyester by American Cyanamid and DuPont. During same period, Owens-Illinois began weaving glass fiber into a textile fabric on a commercial basis. From 1934 and 1936, Ray Green, combined these two new products for making small boats and this was the beginning of composite era. Composites industry began in late 1940s and expanded rapidly through 1950-55s. Boats, car bodies, truck parts, aircraft components, underground storage tanks, buildings are some of the products which were made at that time. Composite material generally consists of two materials: one is the matrix and other is reinforcement, final product retains properties of each constituent. Wood is an example of natural composite material, made up of cellulose (reinforcement), and lignin (matrix). Other examples of natural composites are bones, teeth, plant leaves and bird feathers.

1.1.2 Examples of Natural Composite Materials Some of the examples of natural composite materials are given below:

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Wood: Wood is an excellent example of natural composite in which cellulose fibers are held together by lignin. Closely packed cellulose molecule provides higher density and strength in the composites. Presence of cellulose is responsible for adequate strength in plants and trees. Mud Bricks: The most primitive man made composite material is mud brick, made of straw and mud for construction use. Mud has lower tensile strength and fail upon bending. When straw is embedded into mud and allowed to dry, resulting mud brick resists both compressive and tensile forces, making an excellent building material. Individual bricks of mud can be made and then laid by using a mud mortar in a similar way to laying bricks or entire wall can be formed by continuous molding process so that a wall is one big brick effectively. Bone and Teeth: Bone is a naturally occurring composite material made of calcium phosphate (mineral) embedded in collagen (protein) matrix, recent attempts to grow artificial hydroxylapatite bone composite have proved successful. Bones contains short and soft collagen fibers i.e. inorganic calcium carbonate fibres dispersed in a mineral matrix called apatite. These fibers grow and get oriented in the direction of load. Tooth is a special type of bone consisting of flexible core and hard enamel surface, outer enamel have high compressive strength. Tooth has piezoelectric properties i.e. reinforcing cells are formed when pressure is applied. Concrete: Concrete is a useful engineering material where aggregate (small stones or gravel) and sand are bond together. Water is absorbed by the cement, and bonds other components together creating a stone like material. Applications of concrete are in pavements, building structures, foundations, motorways/roads, overpass, bridges, parking structures, brick/block walls and footings for gates, fences and poles.

1.2

CLASSIFICATION OF COMPOSITES Composite materials can be classified based on the matrix material (metal,

ceramic, and polymer). Metal Matrix Composite (MMC): Metal matrix composites are composed of a metallic matrix (aluminum, magnesium, iron, cobalt, copper) and a dispersed metal phase (lead, tungsten, molybdenum).

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Ceramic Matrix Composite (CMC): Ceramic matrix composites are composed of a ceramic (matrix) and embedded fibers of other ceramic material (dispersed phase). Polymer Matrix Composite (PMC): Polymer matrix composites are composed of polymer matrices: thermoset (unsaturated polyester, epoxy) or thermoplastic (poly vinyl chloride, polystyrene) and reinforcement in the form of fibers (glass, carbon, steel, kevlar fibers etc.) or fillers (talc, mica, CaCO3 etc.). They can further be classified into thermoset composites, thermoplastics composites and natural fiber composites.

1.2.1 Metal Matrix Composite (MMC) Metal matrix composite (MMC) consists of a metallic matrix and reinforcement of metal (lead, tungsten, molybdenum) or ceramic (oxides, carbides). Matrix materials used generally are aluminum, magnesium, titanium but for high temperature applications, cobalt and cobalt-nickel alloy are preferred. The reinforcement can be either continuous, or discontinuous. Discontinuous (whiskers, short fibers, or particles) reinforcement gives isotropic properties, whereas continuous reinforcement can be in the form of monofilament wires or silicon carbide fiber. Some of the common MMC are: 

Aluminum Matrix Composite



Magnesium Matrix Composite



Titanium Matrix Composite



Copper Matrix Composite

Aluminum Matrix Composite: This is the widest group of MMC, and matrix systems are alumina (Al2O3) or silicon carbide (SiC) particles up to 15-70 (vol%). Reinforcement can be in the form of continuous fibers of alumina, silicon carbide, graphite or discontinuous fibers of alumina. They can be processed by powder metallurgy (sintering), stir casting, and infiltration processes. Important properties of aluminum matrix composites are: 

Low density



High stiffness



High thermal conductivity 3



Excellent abrasion resistance



Good strength at elevated temperatures

Some of the important applications of aluminum matrix composites are automotive parts (pistons, push rods, brake components), brake rotors for high speed trains, bicycles, golf clubs, electronic substrates, cores for high voltage electrical cables. Magnesium Matrix Composite: Magnesium matrix composites are reinforced by silicon carbide particles generally and important properties are as follows: 

Low density



High stiffness



High wear resistance



Good creep resistance



Good strength at elevated temperatures

Magnesium matrix composites are used for manufacturing components for racing cars, automotive brake system, and aircraft parts (gearbox, transmission, compressor and engine). Titanium Matrix Composite: Titanium matrix composites are reinforced by continuous monofilament of silicon carbide fiber and titanium boride/titanium carbide particles. Powder metallurgy (sintering) is used for fabrication of titanium matrix composites, some of the important properties of these composite are enlisted below: 

High strength



High stiffness



High thermal stability



Good wear resistance



Good creep resistance

Important applications of titanium matrix composites are structural components of F-16 jet such as landing gear, turbine engine components (fan blade, actuator piston, synchronization ring, connecting link, shaft, disc), automotive engine components, drive train parts, and machine components.

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Copper Matrix Composite: Copper matrix composites are reinforced by continuous fibers of carbon, silicon carbon, tungsten, stainless steel and silicon carbide particles. Powder metallurgy (sintering) and infiltration technique are used for fabrication of these composites. Important features offered by copper matrix composite are: 

High stiffness



Good wear resistance



High thermal conductivity



Good electrical conductivity



Low coefficient of thermal expansion

Copper matrix composites are used for manufacturing hybrid modules, electronic relays, electrically conducting springs and other electrical/electronic components. Applications: Metal matrix composite are more expensive than conventional composites, therefore should be used where improved properties and better performance are required. Most common uses are aircraft components, space systems and sports equipments. They can be used in wide temperature range, do not absorb moisture, have better conductivity, good resistance to radiation damage, and do not display outgassing. Processing of these composites is difficult, and available experience in their usage is also limited. Carbide drills are made of tough cobalt matrix with hard tungsten carbide as reinforcement. Tank armors can be made of metal matrix composites (steel reinforced with boron nitride as reinforcement because it is very stiff and does not dissolve in molten steel). Copper silver

matrix containing diamond particles, is used as a

substrate for high power and high density multi-chip modules in electronics due to high thermal conductivity. Radio Frequency Quadrupoles (RFQ) or electron targets use copper based MMC for usage at high temperatures and under radiations. F-16 fighting Falcon uses monofilament silicon carbide fibers in titanium matrix for making structural component of jet's landing gear. Specialized bicycles have used aluminum based MMC compounds for bicycle frames, Griffen bicycles also use boron carbide aluminum bike frame. Automotive sector is an important area, where MMC are used widely. Automotive disc brakes are made of MMC. Lotus Elise used aluminum MMC rotors, 5

now rotors are made of MMC (silicon carbide matrix and carbon fiber) because of its high specific heat and thermal conductivity. Insert of aluminum matrix for strengthening cast aluminum disc brake calipers are used due to its light weight and high stiffness. MMC driveshaft is important for racers, allowing top speed to be increased far beyond the safe operating speeds of standard aluminum drive shaft. Honda has used cylinder liners made of aluminum metal matrix composite in engines. Toyota has used metal matrix composites in Yamaha designed 2ZZ-GE engine, whereas Porsche uses MMC to reinforce the engine's cylinder sleeves.

1.2.2 Ceramic Matrix Composite (CMC) Ceramic matrix composite consists of ceramic matrix combined with a ceramic dispersed phase (oxides, carbides). Matrix material for long fiber composite is silicon carbide, alumina, alumina-silica, and carbon. Ceramic matrix composites are designed to improve brittleness of conventional ceramics, which was a major disadvantage with ceramics. Reinforcement can be either continuous or discontinuous fibers. Important reinforcement can be whiskers of silicon carbide, titanium boride, and zirconium oxide. Silicon carbide fibers are very common due to high strength and good modulus of elasticity. Monofilament fibers produce stronger interfacial bonding with matrix material and improve the toughness. Some of the important properties of CMC are: 

High stiffness



Good thermal stability



Very high fracture toughness



High thermal shock resistance



Improved dynamical load capability



High corrosion resistance at high temperature



Good mechanical strength at high temperature

Following are the main steps in manufacturing of CMC: 

Layup and fixation of fibers



Introduction of matrix material



Machining and other treatments (coating or generation of porosity) if required

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First of all, fibers (in the form of roving) are arranged by layup, filament winding, braiding and knotting techniques to get preform. Matrix can be introduced by five different procedures in between fibers of perform: 

Deposing out of a gas mixture



Pyrolysis of a pre-ceramic polymer



Chemical reaction of elements



Sintering at low temperatures



Electrophoretic deposition of ceramic powder

Finally machining operations such as grinding, drilling, lapping or milling are performed with diamond tool if required. Applications: Some of the major applications of silicon carbide matrix composites are gas turbine engines parts, heat exchangers, rocket propulsion components, filters for hot liquids, burner parts, immersion burner tubes. Alumina-silica matrix composites are used in making heat exchangers, filters for hot liquids, thermophotovoltaic burners, and combustion liners of gas turbine. Carbon-carbon composites are used in high performance braking systems, refractory components, hot pressed dies, heating elements, and components of turbojet engine.

1.2.3 Polymer Matrix Composite (PMC) Polymer matrix composites can be classified into following groups: 

Thermoset composite



Thermoplastic composite

Thermoset Composites: Thermoset matrix dominant composite industry because of their reactive nature and ease of impregnation. Reaction takes place in a monomeric or oligomeric state, when matrix has low viscosity. Thermoset composites use glass, carbon, and aramid fibers which improve modulus, creep resistance, impact strength, and heat resistance properties. Some of the disadvantages of fiber addition are more cost, higher viscosity, anisotropy, and abrasiveness on mould and machineries. Orientation of fiber plays key role in the determining the properties of final composite.

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Matrices: More than 95% of thermoset composite are based on polyester and epoxy resins, other resins used can be phenolics, silicones, and poly imides. Some of the commonly used matrices in thermoset composite are discussed briefly: For common applications, polyesters is used that can be cured at room temperature and atmospheric pressure. Resin is economical, have good mechanical, electrical, chemical properties, and better dimensional stability. Polyesters can be compounded to flexible and resilient, hard or brittle material. It is also available in ready forms such as bulk moulding compound (BMC), and sheet moulding compound (SMC). Bulk moulding compound is a premixed material containing resin, filler, glass fibers and additives. Sheet moulding compound consists of resin, glass fiber, filler and additives, and processed in continuous sheet form for structural and load-bearing applications. Epoxy resins are low molecular weight syrup like liquids, cured with hardeners to cross link thermoset. Hardeners become the part of finished structure and determine the properties of final product (This is in contrast to polyester formulations where the function of catalyst is primarily to initiate cure). Epoxies can also be cured at room temperature but high temperature curing give superior properties. Epoxy resins have good electrical properties, high wear resistance and outstanding adhesive properties desired for laminated structures. Cured resins have better resistance to solvents and alkalis than polyesters, but resistant to acids is not good. Phenolics are the oldest of thermo setting resins, and have excellent insulating properties. They have good resistance to moisture and chemical resistance except with strong acids and alkalis. Thermal stability and dielectric properties are also excellent. Reinforced phenolics are processed by high pressure methods such as compression moulding and continuous lamination due to condensation of volatiles during molding. Recently developed injection mouldable grades have made the processing of phenolics competitive with thermoplastic materials. Common properties of some thermosetting resins are listed in Table 1.1.

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Table 1.1. Properties of Thermosetting Resins (Bahadur and Sastry, 2002) Property

Polyester

Vinylester

Epoxy

Bismaleimide

Density (Kg/m3)

1100-1500

1150

1100-1400

1320

Tensile Modulus (GPa)

1.2-4.5

3-4

2-6

3.6

Compressive Strength (MPa)

90-250

127

100-200

200

Elongation (%)

2-5

1-5

1-8.5

1-6.6

60-200

53

45-70

49

50-110

100-150

50-250

250-300

Coefficient of Expansion (10–6/C) Glass Transition Temperature (C)

Reinforcements: Glass is most widely used reinforcing material in thermoset composites, accounting for almost 90% of the reinforcements. Other reinforcing materials are carbon, graphite, boron, and aramid (kevlar) for high performance applications. Glass fibers are available in several forms: roving (continuous strand), chopped strand, woven fabric, continuous strand mat, chopped strand mat, and milled fibers (hammer milled through screen). Longer fiber gives greater strength but continuous fibers are best in this respect. E-glass fiber is used commonly, but high strength form such as S-glass is also available. Carbon fibers are available in long/continuous form or short/fragmented forms and can be directionally or randomly oriented. Short fibers are less costly but have inferior properties than continuous fibers. Milled fibers are the shortest carbon fibers used for reinforcement and length range from 30 to 3,000 microns. Short chopped fibers (with an L/D ratio of about 800) reinforced composites have better strength and modulus compared with milled fiber. Outstanding properties of carbon fiber composites are high strength to weight ratio, excellent fatigue resistance and good chemical resistance particularly in alkaline environments. Structures with very low linear and planar thermal expansion can be designed in precision instruments such as telescopes, aerospace antennas and similar critical parts. However, carbon fiber composites are relatively brittle and have no yielding.

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Glass reinforced polyester composites are used for automobile body panels, seats, panels for transit cars, boat hulls, bathroom shower tub structures, chairs, architectural panels, agricultural seed, fertilizer hopper, tanks and housings for number of consumer/industrial products. Wound pipes, tanks, and circuit boards can be made of epoxies. It is widely used in aircraft industry due to its light weight, high strength and excellent fatigue properties. Thermoplastics Composites: Thermoplastic composites uses thermoplastic polymer as matrix, and reinforcement as fibers (glass, carbon, aramid, metal), or particulate fillers. Depending upon the property required in composites, variety of fillers such as talc, CaCO3, mica can be used. Some of the advantages of filler addition are cost reduction, density control, higher thermal conductivity, reduced thermal expansion, and improved mechanical properties. Polypropylene is the main consumer of fillers and can be reinforced with talc platelets and asbestos fibers for better stiffness and heat resistance properties. Because of health problem by use of asbestos, use of calcium carbonate and mica flakes has increased. Mica is better than talc for stiffness and heat resistance, and calcium carbonate provide good impact strength. Fillers such as meta kaolinite is used in cable for better electrical properties and alumina tri hydrate (ATH) for improved fire retardancy. Properties of fillers are affected by particle size, shape and surface chemistry. The term reinforcing filler is used with discontinuous additive, modified with the aim of getting better mechanical properties. One of important parameter to check the effectiveness of filler is aspect ratio (surface area to volume), which should be as high as possible. Modification in process or material is carried out to increase the aspect ratio and adhesion/ compatibility of the filler. Some of the important features of filler addition are: 

Cost reduction



High melt viscosity



Property enhancement



Lower mold shrinkage and thermal expansion



Fast molding cycle (due to higher conductivity)

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Role of additives can not be underestimated in polymer composite. They have remarkable functions in some major properties. Few examples of fillers and their function are enlisted in Table 1.2 ( Mascia, 1974). Table 1.2. Fillers and their Functions Functions of Fillers Modification of Mechanical Properties

Examples of Fillers (Low aspect ratio): Talc, Kaolin, Glass sphere, Wood flour, Wollastonite , CaCO3 (High aspect ratio): Glass fiber, Mica, Nano clays, Carbon nano tube, Carbon fiber, Aramid fiber, Natural fiber

Enhancement of Fire Retardancy

Al (OH)3 , Mg (OH)2

Electrical and Magnetic Property

Metals, Carbon black/fibers, Nano tubes, Mica

Surface Property

Silica, Graphite, CaCO3, Poly tetra floro ethylene

Degradability

Starch, Cellulosics

Dimensional Stability

Particulate fillers, Glass beads, Mica

Control of Damping

Flake fillers, Glass, BaSO4

1.3

NATURAL FIBER COMPOSITES Today, there is renewed awareness for improving the properties of plastics

based components to meet engineering requirements to compete with nano materials. Longevity of such structures as compared with traditional routes has resulted potential savings of material, energy and other resources. Development of commercially viable green products such as wood plastic composites (WPC) based on natural sources for wide range of applications is increasing day by day. Lignocellulosic thermoplastic composite have been

filled

in use since 1980, wood plastic composites

(WPC) share the major market. Various lignocelulosic materials such as jute, sisal, bagasse, coconut, banana, rice husk are in use. These are light in weight, non-toxic, and have lower abrasive properties. Composite made of these materials are biodegradable and leaves no toxic residue or byproducts when combusted (Yang et al., 2007).

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Properties of natural fibres in term of physical and mechanical are dependent on the source from which they are obtained. Few properties like apparent density, tensile strength and Young modulus of major natural fibers (bast, leaf and seed) are given in the Table 1.3 (Baillie, 2004). Table 1.3. Physical and Mechanical Properties of Major Natural Fibers (bast, leaf, and seed) Apparent Density

Tensile Strength

Young Modulus

(kg/m )

(MPa)

(GPa)

Flex (b)

1500

500-900

50-70

Hemp (b)

1500

310-750

30-60

Jute (b)

1500

200-450

20-55

Kneaf (b)

1200

295-1191

22-60

Banana (l)

1350

529-914

27-32

Pineapple (l)

1440

413-1627

60-82

Sisal (l)

1450

80-840

9-22

Cotton (s)

1550

300-700

6-10

Coir (s)

1150

106-175

6

Fiber Type

3

Plastics such as high density polyethylene (HDPE), low density polyethylene (LDPE), polystyrene (PS), polypropylene (PP), poly methyl metha acrylate (PMMA), polyester, phenolics utilize most of the natural fibers. High moisture absorption and poor adhesion of lignocellulosic materials to hydrophobic polymer matrix results lower mechanical properties. Lower degradation temperature of lignocellulosic materials is also a limitation while using them as filler in composite making. Coupling agents and compatibilzers are used for improving bonding between polymer and fillers. Other challenges are seasonal quality variation, formation of agglomerates during processing, and reduced dimensional stability (Nachtigall et al., 2007).

1.4

RAW MATERIALS Commodity plastics such as polyethylene (low density and high density), poly

vinyl chloride, polystyrene, polypropylene are in wide use in domestic and industrial sectors. Low density and good mechanical properties of polypropylene makes it a material of choice for composite making. 12

1.4.1 Polypropylene (PP) Polypropylene is the commodity plastics used for various applications in domestic and industrial sectors. It is available in three forms such as atactic, isotactic and syndiotactic. Among these, isotactic form has maximum crystallinity. Main steps in the synthesis of polypropylene are: 

Monomer purification



Polymerization



Resin degassing



Resin pelletization

Polypropylene has outstanding physical, chemical, mechanical, thermal and electrical properties. Compared to low and high density polyethylene, it has better tensile strength but lower impact strength with superior working temperature. It is light in weight, have good resistance to staining, and low moisture absorption. Polypropylene does not present stress cracking problems and offers excellent electrical and chemical resistance at higher temperature. It has excellent resistance to organic solvents, degreasing agents and electrolytic attack. Polypropylene has good resistance to acids and alkalis, but not suitable in contact with aromatic, aliphatic and chlorinated solvents. In Table 1.4, important properties of polypropylene which was used in the study, are given. Table 1.4. Important Properties of Polypropylene (H110 MA) Property

ASTM

Value

MFI (gram/10 min)

ASTM D 1238

11

Tensile strength (MPa)

ASTM D 638

36

Impact strength (J/m)

ASTM D 256

27

Heat deflection temperature (ºC)

ASTM D 648

104

Polypropylene is widely used in consumer products such as houseware, furniture, appliance, luggage, toy, battery case and other durable items for home, garden or leisure uses. Polypropylene is used to make film, sheet, rolls, tape, rods, tubing, pipes, and tank. Other applications are hoods, orthopaedic devices, structural cover, light table, rinse and etch housing for electronics industry.

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Some of the important advantages of polypropylene in automobile are low coefficient of thermal expansion, low specific gravity, high chemical resistance, availability in attractive colors, good weatherability, and proper impact/stiffness balance. Some of the components made of polypropylene are automotive interiors, mono material dash board, bumpers, cladding and exterior trim. In flexible packaging, polypropylene is one of the leading materials and has replaced cellophane, metals and paper on account of its superior puncture resistance and price. Polypropylene film is used for food, confectioneries, tobacco and clothing applications. Reusable and collapsible/stackable crates can be made of polypropylene, which offers ease of transport and efficient storage of products. Polypropylene caps/closures are used for mineral water containments and edible oil container. Polypropylene bottles find wide range of applications in condiments, detergent and toiletries.

1.4.2 Rice Husk (Hull) Rice hull is hard protecting covering of grains of rice. During milling of paddy, about 78% of weight is rice, broken rice and bran, rest 22% of weight of paddy is rice husk. Main components of rice husk are cellulose, hemi cellulose, lignin, ash and crude protein.

Figure 1.1. Photographic Representation of Rice Husk To protect the seed during growing season, hull is made of hard material, including opaline silica and lignin. Hull is mostly indigestible to humans and removed from grain for making white rice.

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Some of the important characteristics of rice husk are: 

Rice husk has low bulk density (70-180 kg/m³), requires large volumes for storage and transport.



Angle of response is about 40-45° indicating poor flow ability, creating difficulty in feeding.



It has high average calorific value, making it a good renewable energy source.



Rice husk is difficult to ignite and does not burn easily with open flame unless air is blown through the husk. When burned, ash content is 17-26%, higher than most fuels (wood 0.2-2%, coal 12.2%).



It is highly resistant to moisture penetration and fungal decomposition making it a good insulation material.

Rice husk contains about 75% organic volatile matter and balance 25% is converted to rice husk ash (RHA) by firing process. Presence of RHA is a great environment threat to land and surrounding area where it is dumped. Important applications of rice husk ash are mentioned below: 

Rich source of silicon



Release agent in ceramics industry



Repellant in the form of vinegar tar



As an insulation powder in steel mill



As an absorbent for oils and chemicals



Economical substitute for micro silica/silica fume



Aggregates and fillers for concrete and board production

Rice husk ash is also used for insulation of molten metal in tundish and ladle in slab caster. Temperature of molten metal in ladle is around 1400ºC but when this metal flows from ladle to tundish, temperature drops to around 1250ºC. Reduction in temperature leads to choking, causing breakdown in slab caster. Coating of RHA over the molten metal in tundish and ladle serve a very good insulator and reduces breakdown time of casting.

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Another important use of RHA is in cement making. The particle size of cement is about 35 microns and there is void formation in concrete mixes due to improper curing resulting lower strength of concrete. Silpozz, which is made out of RHA, have small particle size of 25 microns, it fills the interstices resulting high strength and density when used in cement. Other possible uses are as fuel, and with proper technology it can be burned to power steam engines. Rice hull is a low cost material, and silicon carbide (SiC) whiskers can be made of it. SiC whiskers are used to reinforce ceramic cutting tools, and increase strength significantly. Rice hull is organic in nature and can be composted but its high lignin content makes the process slower. With vermi composting techniques, it can be converted to fertilizer in about four months. Rice husk can be used to produce mesoporous molecular sieves for uses in catalysts, support for drug delivery system and adsorbent in waste water treatment. Rice husk pillows are loosely stuffed and good for therapeutic use due to the ability of retaining shape of the head.

1.5

ANALYSIS OF RAW MATERIALS Properties of rice husk are dependent on the source form which it is obtained

as well as method of separation from rice. Following properties were studied to avoid quality variation of incoming materials. 

Bulk density



Moisture content



Ash content

1.5.1 Bulk Density (ASTM D1895) Bulk density is important test for powders and pellets. It measures the size of the material which affect the flow and packaging behavior of materials. It includes the void volume and can be measured for rice husk and thermosetting materials.

1.5.2 Moisture Content by Oven Drying Method (ASTM D789) Presence of the moisture is not desired in the plastics as it decreases mechanical, and insulation (electrical) properties. It has negative effect on the process 16

ability of plastics and gives surface imperfections in the part. Further it may enhance the degradation of plastics, therefore material needs to be dried before use.

1.5.3 Ash Content (ASTM D2584) It is the non-volatile inorganic matter which remains after subjecting the material to high decomposition temperature. This test is very useful for determining the filler content in the plastics but cannot be used to determine the percent of carbon fiber/black since carbon burns off during the test.

1.5.4 MELT FLOW INDEX (ASTM D1238) Melt flow index or melt flow rate is a measure of ease of flow of a thermoplastic polymer. It is important test for thermoplastics and gives idea about the processabilty of polymers. It is inversely related to molecular weight of polymer. Melt flow rate is generally measured at 190°C for polyethylene, polypropylene and at 230°C for engineering plastics. Ratios between two melt flow rate values for one material at different gravimetric weights are often used as a measure for the broadness of molecular weight distribution. The details of procedures for above tests are given in chapter 3.

1.6. PROCESSING TECHNIQUES FOR MAKING COMPOSITES 1.6.1 Single Screw Plastic Extruder (Granulator) Extrusion is a continuous process, for making pipe/tubing, window frames, weather stripping, wire insulation, and adhesive tape. Common plastic materials used in this process are polyethylene, polypropylene, nylon, polystyrene, polycarbonate, acetal, acrylic, and acrylonitrile butadiene styrene (ABS). Screw is the main part of the extruder as shown below in Figure 1.2.

Figure 1.2. Schematic Representation of Extruder Screw

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Screw design is specified by the L/D ratio, compression ratio and depends on the materials being processed. L/D is defined as the ratio of length of flighted part of screw to inside diameter of barrel. L/D ratio of 24:1 is common, but it can be up to 32:1 for better mixing and higher output with same screw diameter. Compression ratio is the flight depth of feed to metering zone and can vary from 2:1 to 3:1. It can be increased to create more back pressure on the material and have effect on the quality of the product.

1.6.2 Composite Sheet Preparation by Compression Molding Compression molding is generally used for thermosetting materials. Nowadays, advanced thermoplastics composites containing unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand can also be processed by this method. Compression molding process can be explained with the help of Figure 1.3.

Figure 1.3. Compression Molding Process Some of the important features of this process are enlisted below (MortonJones, 1989): 

Low flash



Low orientation



Low tooling cost



Not suitable for intricate shapes 18



Minimum damage to reinforcing fibers



Longer cycle time than injection molding process

Molding material used in this process can be in the form of powders, pellets, liquid, or preform. Preheating of charge is beneficial as it remove the moisture and shortens the cycle time. Machine is heated to reach the desired temperature. Mold is cleaned and lubricant is applied on it. After keeping the polymer in mold, it is kept between heated platens. After heating cycle, mold is cooled and product is removed (Sinha et al., 2011). Common defects and their remedies during production of compression molded parts are discussed in the Table 1.5. Table 1.5. Faults and Remedies in Compression Molding Process (Athalye, 1991) Faults

Remedies

Blistering

Preheat to eliminate moisture, Vent the mold, Increase pressure and cure, Close mold slowly at low pressure and apply high pressure to purge the gases

Porosity

Preheat material, Increase pressure, Lower mold temperature, Use the semi positive mold

Mold Sticking

Use lubrication, Polish the mold, Increase cure time, Remove undercuts, Check proper draft

Warpage

Heat mold slowly, Increase cure, Use low shrinkage material, Increase cooling time, Redesign mold

Cracking

after Increase thickness, Use more flexible material, Increase radii,

Molding

Provide ribs

Lower

Increase process temperature, Increase cure, Lower mold

Mechanical

temperature, Remove contamination in material, Improve design

Strength Burn Marks

Reduce preheat temperature and mold temperature, Avoid breathing

Flash

Reduce charge, Lower mold temperature, Ensure overflow grooves are clean, Close mold on low pressure and apply high pressure immediately 19

Some of the industrial products made by this process are hoods, fenders, scoops, spoilers, lift gates, electrical motor components, phonograph records, rubber tyres, and automotive end uses. Other applications can be dinnerware, telephones, television set frames, electrical circuit breakers, wall outlets, pot and pan handles, electric plugs, and sockets housing.

1.7

ANALYSIS OF RICE HUSK POLYPROPYLENE COMPOSITES Prepared rice husk polypropylene composite sheet needs to be characterized.

Characterization process can be divided into following headings: 

Mechanical properties



Thermal properties



Chemical properties

1.7.1 Mechanical Properties Study of mechanical properties such as tensile strength, impact strength, hardness, and abrasion resistance is important to meet the engineering requirements. Polymers in service may be subjected to various type of loadings such as stretching (pulling), shock, cycling etc., therefore detailed study of mechanical properties is required (Sinha et al., 2010). Importance/ significance of these properties and respective American society for testing and materials (ASTM) methods for their measurement are given in brief. 1.7.1.1 Tensile Strength (ASTM D638) Tensile strength is important mechanical property, useful for design purposes. Tensile modulus is an indication of relative stiffness of the material and can be obtained from stress strain diagram. Tensile properties may vary with specimen preparation, testing speed and environmental conditions. Important factors that affecting the tensile strength are discussed below (Shah, 1998): 

Molecular weight: Tensile strength increases with molecular weight.



Orientation: Load applied parallel to direction of orientation will yield higher tensile strength than load applied perpendicular to the orientation e.g. injection molded sample have higher tensile strength than compression molded product. 20



Rate of loading: With increase in strain rate, tensile strength and modulus, both increases.



Temperature: There is reduction in tensile strength and modulus with temperature but elongation increase with temperature.

1.7.1.2 Impact Strength (ASTM D256) Impact test study the shock bearing capacity of the materials when load is applied at high speed. To find out the impact strength, sample is generally notched to prompt the brittle fracture. Certain plastics such as polycarbonate, nylon, acrylonitrile butadiene styrene (ABS) have exceptional impact strength but it can be lowered by introducing sharp corners. Two types of the impact test for plastics are: falling weight for flexible materials and pendulum impact for rigid materials. In the pendulum impact test, kinetic energy consumed by pendulum to break the specimen is measured. This energy includes the energy required to deform, initiate, propagate fracture, and to throw the broken ends of the sample. Addition of additives such as impact modifiers, plasticizers, rubbers, fibrous fillers and acrylic have positive effect on the impact properties. Higher percentage of crystallinity enhances the chance of brittle failure and reduces the impact strength. Most of the mechanical properties decrease with temperature but impact strength increase with temperature (Shah, 1998). 1.7.1.3 Abrasion Resistance (ASTM D4060) Abrasion resistance of a material is complex property closely related with tensile strength and hardness of material. It is measured in term of weight loss (in milligram) in 1000 cycles. Sometimes, variation in haze can be taken as measure of abrasion loss. Abrasion resistance depends on frictional force, load and area of contact. All these factors tend to increase wear or abrasion. Abrasion resistance is also affected by test conditions, type of abradant, and dissipation of heat. Excessive heat generation due to abrasion can make the polymer surface oxidized (Shah, 1998).

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1.7.1.4 Hardness Test Hardness is defined as resistance of material to plastic deformation, usually by indentation. Greater is hardness of the material, greater is resistance to deformation. Two common methods for hardness measurement of plastics are: 

Rockwell hardness test



Durometer hardness test

Rockwell Hardness Test: Rockwell hardness is generally used for harder plastics such as nylon, polycarbonate, and acetal. It measures net increase in depth of impression as load on an indenter is increased from minor to major and then returned to minor load. Size of steel ball and load applied vary depending on the scale of hardness. Hardness scale in order of increasing hardness is R, L, M, E and K. Durometer Hardness Test (ASTM 2240) : Durometer is used to determine the relative hardness of soft materials. The test measures the penetration of material by indenter under specified conditions of force and time. Test conditions and temperature of measurement affect the hardness results. Surface finished samples have better hardness than machined samples. Addition of filler have positive impact on the hardness of plastics generally.

1.7.2 Thermal Properties Important

applications

of

polymers

at

high

temperature

are

in

aircraft/aerospace interiors, electrical components, under-hood parts, wire insulation for high temperature applications, cable couplings and connectors, medical tubing, products requiring sterilization, and monofilament for production of woven products for filters, belting, meshes. Therefore study of thermal properly becomes essential to explore new applications of composite materials. 1.7.2.1 Heat Deflection Temperature (ASTM 648) Heat deflection temperature (HDT) test measure the heat resistance ability of plastics for shorter duration. It distinguishes the materials that are able to sustain light loads at high temperatures and lose their rigidity over a narrow temperature range. Some of the factors that can affect test results are discussed below (Shah, 1998):

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

Method of preparation of sample: Compression molded samples have higher HDT than injection molded due to presence of less stresses.



Thickness: Thicker sample have higher HDT because it takes long time for heating.



Crystallinity: In amorphous polymers, HDT is nearly the same as the glass transition temperature (Tg) of material. HDT for crystalline polymers is nearer to melting point.



Glass fiber: Addition of glass fibers enhance the HDT of plastics, it has significant effect on crystalline polymers.

1.7.3 Chemical Properties Good chemical properties of polypropylene (PP) make it’s a successful candidate for various applications such as chemical tanks, industrial piping, automotive batteries, labware. Two basic types of chemical attack on plastics are solvation and chemical reaction. Solvation occurs when a chemical either dissolves the polymer or is absorbed causing swelling and/or softening of the polymers with measurable weight change. When a chemical reaction takes place, there is a change in chemical structure of polymer resulting change in molecular weight and loss in physical properties. 1.7.3.1 Chemical Resistance of Plastics (ASTM D543) This test evaluates the performance of plastic materials in various chemical reagents such as lubricants, cleaning agents, inks, foods items. When plastics are exposed to chemicals, changes in weight, dimension, appearance and strength may takes place. Exposure time, strain condition and temperature affect the properties of plastics significantly. 1.7.3.2 Swelling (Percentage Weight Gain ASTM D471) Various application of rubber such as gaskets, hoses and seals need to perform well in aggressive liquids in severe environments. Temperature and time of testing may vary depending on the application. Exposure of plastics to chemicals cause swelling phenomenon.

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1.7.4 Analysis of Composites by Instrumental Methods 1.7.4.1 Thermal Analysis Techniques These tests are very important for quality control and characterization of basic raw materials. These are group of techniques in which physical and chemical properties of a substance and its reaction products are measured as a function of temperature when material is subjected to a controlled temperature program. Some of important thermal analysis methods are discussed below: 1.7.4.1.1 Thermo Gravimetric Analysis (TGA) In TGA, mass of sample is recorded continuously as material is heated in controlled atmosphere. Plot of mass or mass percentage as a function of time is known as thermogram. Loss in weight results due to evaporation of residual moisture, solvent, or polymer decomposition at higher temperature. Some of the important applications of TGA are: 

Thermal and oxidative stability



Residual solvents in pharmaceutical



Analysis of residual monomers in polymers



Gas adsorption studies in zeolites and catalysts



Measurement of moisture and study of drying characteristics



Quantitative analysis of additives such as glass fibers, calcium carbonate, talc, kaolin, carbon black, inorganic pigments , plasticizers.

Thermogram is analyzed carefully to get the valuable information for different materials. This is very effective test for checking the thermal stability and identification of polymers. Each polymer has its characteristic thermogram. In electronics, TGA can be used to find out the resin content of printed circuit boards, volatilization curves for fluxes, thermal decomposition of components. In medical applications, water and volatiles content of solid dosage can be determined with the help of TGA.

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1.7.4.1.2 Differential Scanning Calorimetry (DSC) Differential scanning calorimetry DSC is another important thermal analysis technique and measure change in energy during melting, degradation etc. The percentage crystallinity of a polymer can be obtained from crystallization peak of the DSC graph and heat of fusion from the area under an absorption peak. Following important thermal transition in polymers can be studied with DSC analysis. Glass Transition Temperature (Tg): On heating the polymer, plot shifts downward suddenly, it means heat is absorbed by the sample. Polymers have a higher heat capacity above glass transition temperature and thermogram can be used to measure polymer's glass transition temperature. Crystallization: Above glass transition, polymer molecules have lot of mobility and never stay in one position for longer period. At particular temperature, they gain enough energy to move into ordered arrangements (crystallization, an exothermic reaction) and highest point recorded is crystallization temperature. Melting: On continuous heating of polymer beyond its crystallization temperature, there is attainment of another thermal transition called melting. When polymer crystals melt, they absorb heat (an endothermic transition) and this extra heat flow during melting shows a large dip in plot. 1.7.4.2 Fourier Transform Infrared Spectroscopy (FTIR) Infrared (IR) spectroscopy gives information about the vibration and rotational motion of a molecule; therefore it is an important technique for identification and characterization of a substance. IR spectrum of an organic compound provides a unique fingerprint, which is readily distinguished from the absorption patterns of all other compounds. Superior sensitivity, good resolution, rapid sample measurement, absolute wavelength accuracy, versatile spectra processing, and total automation are important reasons for increasing use of IR in research applications. Compared to IR, FTIR has high speed and gives better resolution. IR works on single spectrum where as FTIR uses an interferometer which makes several scans. FTIR spectroscopy does not require a vacuum, since neither oxygen nor nitrogen absorbs infrared rays. Minute quantities of materials can be used in FTIR and it can be in solid, liquid, or gaseous stage. 25

Individual peaks in the FTIR plot are used to yield information about the specimen. Organic contaminants in solvents can be analyzed by first separating the mixture into its components by gas chromatography, and then analyzing each component by FTIR. It can be used to study crystallinity, polymerization, vulcanization, phase change, hydrogen bonding, kinetics reactions, degradation studies, photochemistry, molecular orientation and interactions. It is also used in the rubber and polymer industries for both qualitative as well as quantitative analysis. 1.7.4.3 Scanning Electron Microscopy (SEM) Scanning electron microscopy (SEM) is a type of electron microscopy that images the sample surface by scanning it with high energy beam of electrons. It gives information about surface topography, composition and electrical conductivity. The types of signals produced by an SEM are secondary electrons, back scattered electrons (BSE), characteristic X-rays, light (cathodoluminescence), specimen current and transmitted electrons. Secondary electron detectors are common in all SEMs, but it is rare that a single machine would have detectors for all possible signals. SEM can give very high resolution images of a sample surface, revealing details about less than 1 to 5 nm in size. Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three dimensional appearance useful for understanding the surface structure of the sample. Back scattered electrons (BSE) are beam electrons that are reflected from the sample by scattering. Since the intensity of BSE signal is strongly related to the atomic number (Z) of the specimen, therefore BSE images can provide information about the distribution of different elements in the sample. BSE imaging can image colloidal gold immuno labels of 5 or 10 nm diameter which would otherwise be difficult to detect in secondary electron images in biological specimens. Characteristic X- rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher energy electron to fill the shell and release energy. These characteristic X- rays are used to identify the composition and measure the abundance of elements in the sample.

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1.8

COUPLING AGENT /COMPATIBILIZER Addition of fillers in the polymer decrease the mechanical properties due to

poor compatibility on account of hydrophobic nature of the polymer (filler has hydrophilic nature). Use of coupling agent and compatibilizers improve the interfacial adhesion between polymer and filler (rice husk) resulting higher tensile strength in the composites. Coupling agents are bifunctional molecules that bond chemically with filler surface and polymer matrix forming molecular bridge between the two. The strong interfacial bond not only helps in the mixing of two phases but also enhance properties of composite. The most commonly used coupling agents are organo tri alkoxy silanes, organo titanates and functionalized (especially acid functionalized) polymers. Other surface modifiers that provide a physical rather than a chemical bond between filler and resin are also available such as waxes or fatty acids. Important coupling agents used are silane, titanate and zirconate.

1.8.1 Silane Silane has the ability to react with the surface of most mineral fillers to impart required properties. Organic part of silane reacts with the polymer and this covalent bridging mechanism improves surface properties such as adhesion. Silanes have the generic structure of Y-R-Si-X3, where X is a hydrolysable alkoxy group (methoxy or ethoxy) and Y an organo functional group (amino, vinyl, epoxy, meth acryl etc.) attached to the silicon by an alkyl bridge R. The alkoxy groups react with the surface groups of inorganic fillers. The Si-OR bonds hydrolyze readily with water, even if only moisture is present on the surface, to form silanol Si-OH groups. These silanol groups then condense with each other to form polymeric structures with very stable Si-O-Si bonds. It can also condense with metal hydroxyl groups on the surface of glass, minerals or metals to form stable Si-O-M bonds (M: Si, Al, Fe etc.). This allows surface treatment, coupling and assembling of very dissimilar surfaces chemically between inorganic and organic materials. A tight polymeric siloxane network is created on the inorganic filler or metal surface, which becomes more diffused into the adjacent organic resin. Silane coupling

27

agents provide a strong, stable, water and chemical resistant bond between filler and resin, affecting the following properties: 

Lower shrinkage and viscosity



Better mechanical and electrical properties



Better surface appearance of processed parts



Good weather resistance with lower surface or internal defects

Silanes require active sites, preferably hydroxyl groups, on the filler surface for reaction to occur. Therefore it can be can used with silicate type fillers, inorganic metal oxides and hydroxides. Silane is effective with aluminum tri hydrate (ATH), alumina, chrome oxide, hydrous and calcined clays, glass fiber, magnesium hydroxides, mica, mineral wool, oxide pigments, quartz, silica, talc, titanium dioxide but do not interact with calcium carbonate, barium sulphate and carbon black to significant extent. Silanes are highly effective coupling agents with polar thermoplastics, thermosets and rubbers but have a slight interaction with nonpolar polymers such as polyolefins (where titanates are of bigger interest).

1.8.2 Titanate Organo titanates overcome many limitations of silanes as coupling agents for fillers. Like silanes they have four functional groups. Silanes have only one pendant organic functional Y group but titanates have three, therefore providing better bonding with polymer. These are suitable not only for fillers with surface hydroxyl groups but also for carbonates, carbon black and other fillers that do not respond to silanes. Titanate couplers also act as plasticizers facilitating higher filler loadings at lower cost compared with silanes. Titanates have the general structure: XO-Ti-(OY)3, where XO can be a mono alkoxy or neo alkoxy group capable of reacting with the inorganic substrate, and OY is the organo functional fragment. Y has several different groups to provide interactions between polar and non polar thermoplastics and thermosets. Unlike silanes, titanates do not require the presence of water to react. The simplest titanate is the mono alkoxy (e.g. iso propoxy) titanate. It reacts with the filler surface via solvolysis generating an alcohol by product. Free protons, unlike the hydroxyl groups needed for silane reaction, are present on almost all three 28

dimensional particulates, making titanates more reactive. The reaction with free protons generates an organic monomolecular layer at the inorganic surface in contrast to the poly molecular layers with other coupling agents. Other types of titanates are chelates (for greater stability in wet environments), and quats (water-soluble systems). Mechanism of titanate coupling agent is shown in Figure 1.4.

Figure 1.4. Titanate Coupling Agent Mechanism

1.8.3 Zirconate The chemical structure and application of alkoxy zirconates is completely analogous to those of alkoxy titanates. They neither discolor in presence of phenols nor interact with hindered amines. Titanate occupies intermediate position between fatty acid surface modifiers and silanes/titanates in terms of cost and functionality both. Zirconate can provide coupling between fluorinated polymers and metal subtract. Use of zirconate improve the thermal stability, surface smoothness and reduce filler loading. Better dispersion of filler help in getting improved mechanical properties with lower moisture absorption. Their main applications are in polyethylene, polypropylene, and rubbers.

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1.8.4 Compatibilizer Addition of lignocellulosic filler decreases the mechanical properties of the composites. Rough morphology indicates the poor interaction between them resulting lower mechanical strength. Maleic anhydride grafted polypropylene (MAPP) when used as compatibilizer improves interfacial adhesion between the filler and polymer matrix. The polar functional group present in these materials binds with the surface of fillers, while PP backbone mixes with the polymer. Result is better dispersion of the filler and enhancement in mechanical properties of the composites. When MAPP is used as a compatibilizer, particulate mineral no longer remains filler but reinforces the polymer matrix. The factors affecting the interfacial bonding between polymer and filler are: 

Method of incorporating of filler



Component properties of the blend: type, amount, and distribution



Processing conditions and techniques (injection molding, extrusion etc.)

Mechanism of compatibilizer is shown in Figure 1.5 (Yang et al., 2007).

Figure 1.5. Mechanism of Compatibilizing Agent between Hydrophilic Filler and Hydrophobic Matrix. 30

It is observed that tensile strength improve with increasing compatibilizing agent content. Most effective content of compatibilizing agent is 3 wt% . At 5 wt% of compatibilizing agent content, the tensile strength is almost the same level as 3 wt%. Use of MAPP reduce the voids sizes and make surface more homogeneous resulting better adhesion in the interfacial region.

1.9

OBJECTIVES Improper disposal and non biodegradable nature of polymers are major threat

to environment. Attempts are being made to degrade the polymers by various means such as de-polymerization, pyrolysis, and incineration. But these methods are not feasible as require extensive energy. Biodegradable polymers are the better candidate for resolution of such problems. Polymer can be made biodegradable by utilizing the lignocellulosic materials such as wood flour, bagasse, rice husk, banana fibers etc. Number of commodity plastics such as low density polyethylene, high density polyethylene, and polypropylene, are available for various uses. Polypropylene is a good candidate for making the composites due to its light weight and good mechanical properties. Among the lignocellulosic materials, rice husk is chosen due to its easy availability, low bulk density, high silica content and uniform properties. Some work on development and use of rice husk polypropylene composites was reported in the literature. Majority of the work was focused on use of maleic anhydride polypropylene compatibilzer, which improve tensile strength of composites. Thermal properties of compatibilizer modified RHPP composites were also studied in the past. To explore other applications/usages of these composite materials, detailed study of mechanical properties such as tensile strength, impact strength, hardness, and abrasion is to be carried out. Effect of processing parameters and additives on mechanical properties of RHPP composites is also studied to seek new applications. Chemical properties can be studied by exposing the composites to various chemicals. There is scope to study the effect of additives on melting and degradation behavior of RHPP composites by using thermal analysis techniques such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC).

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Keeping in view of above, research was focused on Development and Characterization of Rice Husk Polypropylene Composites. The main objectives of present study include: 1.

To characterize polypropylene by melt flow index and measurement of its mechanical properties (tensile strength, impact strength, abrasion resistance and hardness).

2.

To characterize the

rice husk (lignocellulosic

materials)

by bulk

density, moisture content and ash content tests. 3.

To formulate the rice husk polypropylene composites by blending rice husk and polypropylene and its characterization by measuring the mechanical properties.

4.

To analyze the effect of additives and process parameters on the mechanical properties of RHPP composite.

5.

To analyze the chemical properties of rice husk polypropylene composites.

6.

To characterize rice husk polypropylene composites by heat deflection temperature test and thermal analysis techniques.

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