68

CHAPTER - 3 FABRICATION OF COMPOSITES S. No. Name of the Sub-title

Page No.

3.0

Introduction

69

3.1

Materials

71

3.2

Polymers

72

3.2.1

Polyester Resin

73

3.2.2

Mixing of Resin

75

3.3

Fiber Preparation and Hybridization

76

3.4

Composite Specimen Preparation

77

3.5

Summary

82

69

3. FABRICATION OF COMPOSITES 3.0 INTRODUCTION Continuous advancements in the manufacturing methodologies and performance of fiber reinforced polymer (FRP) have led to a significant growth in its market acceptance. The fabrication of composites is a complex process and it requires the simultaneous consideration of various parameters, such as component geometry, layering sequence, production volume, reinforcement & matrix types, tooling requirements, and process economics. The availability of choices makes it imperative that the factors of economics, design and manufacturing be integrated during the development process itself. For composites to become competitive with metals, cost reduction is a necessity, besides durability, maintainability and reliability. The multitude of tasks involved in the manufacturing of composite laminates can be divided into two stages: (1) Fabrication/manufacturing techniques and (2) Processing methodologies. The fiber and matrix may be in a pre-impregnated form, or the fiber and matrix material may be combined for the first time, during this step of developing the structural form. Fabrication techniques for composites are not dependent on the type of matrix material. In fact, some metal forming techniques have been adapted to composites fabrication, whereas, the processing conditions are entirely dependent on the type of matrix material used. For instance, thermosets require long processing times, whereas

70 thermoplastics require relatively high pressures and temperatures. In the present experimental study, a brief introduction to the manufacture of composites by using the hand lay-up process, and the issues related to manufacturing are presented and discussed in detail. From the general perspective, manufacturing begins with the manufacture of structural components from layers or plies of preimpregnated material called prepreg. Specifically, layers of a material with the fibers in each layer aligned in a specific direction are used to form a laminate. The hand fabricated laminate will be processed in an autoclave, which is a pressurized oven that provides the proper levels of heat and pressure. In the early years of the development of fiberreinforced materials, structural components were fabricated by hand. Even today, in prototype development hand fabrication is common, and this is also the case for specialty manufacturing and in many university laboratories. However, as labor costs and the need for consistency have increased, engineers have been entrusted with the task of designing low cost automated manufacturing techniques. Now, automated techniques like robotic tow and tape placement methods, injection molding and pultrusion have dramatically reduced the cost of manufacturing composite structures. In all manufacturing methods, the use of tools like the die, mold and mandrel is common, and they provide the structural shape to the composite material. These tools are usually an inverse, of the desired

71 structural shape, and the design of the tool is a critical and expensive process. The cost of the tool often far exceeds the material and labor costs

to

produce

a

composite

structure.

Also

common

to

all

manufacturing methods, is the need to apply temperature and pressure to the structural component, after the fiber and matrix are brought together to the desired structural form. The pressure takes two forms: actual pressure, ideally hydrostatic, to consolidate the tows and layers; a vacuum to remove the air entrapped between the layers, and to reduce the amount of unwanted gases given off by the resin as it cures. The application of pressure can be in the form of closing both halves of the tool, or as with a flat structural component pressing the laminate in a hot press. Finally, the vacuum requirement is met by enclosing the structural component in a vacuum-tight bag and drawing a vacuum. 3.1 MATERIALS In this investigation Sisal (Agave sisalana), Jute (Corchorus oliotorus) and glass fibers are used for fabricating the composite specimen. The sisal and jute fibers were obtained from Dharmapuri District, Tamil Nadu, India. Polyester resin and the catalyst Methyl Ethyl Ketone Peroxide (MEKP) were purchased from M/s. Sakthi fiber glass Ltd., Chennai, India. The accelerator used for the investigation is Cobalt Napthanate, and 1% of it is added with the resin and the catalyst. The fibers used for the composite fabrication are presented in Fig. 3.1. The

72 resin,

accelerator,

catalyst,

and

thinner

used

for

processing

of

composites are given in Fig. 3.2.

(a) Jute fiber

(b) Sisal fiber

(c) Glass fiber

Fig. 3.1: Fibers used for composite fabrication

(a) Polyester resin

(b) Cobalt Napthanate

(c) Methyl Ethyl (d) Acetone Ketone Peroxide thinner (MEKP) Fig. 3.2: Materials used for composite fabrication

3.2 POLYMERS Polymers can be classified on the basis of their origin, that is, naturally occurring, or synthetic as given in Fig.3.3. Natural polymers are available in large quantities from renewable sources, while synthetic polymers are produced from non-renewable petroleum based resources. Nowadays, degradable polymers are used in various forms including films, moulded articles, sheets, etc.

73 Cellulose Plant

Polysaccharides

Starch Alginate

Polysaccharides Natural

Animal origin

Chitin (Chitosan) Hyaluronate

Collagen (Gelation)

Proteins

Albumin Poly (3- hydroxyalkanoate)

Polyesters

Poly (hydroxybutyrate)

Microbe origin

Poly (hydroxybutyrate-co-hydroxyvalerate) Hyaluronate

Polysaccharides

Starch

Polylactides

Alginate

PolyglycolidePGA Aliphatic Polyesters

Biodegradable polymers

Polycaprolactone

Poly (butylenes succinate)

Poly (alkenedicarboxylates)

Poly (ethylene succinate)

Poly (p-dioxanone)

Poly (butylene succinate -co-adipate)

Poly (ethylene adipate)

Poly (trimethylene carbonate)

Polycarbonate

Poly (propylene carbonate) Copolyester carbonate

Synthetic

Aliphatic- Aromatic Polyesters Polyurethanes Poly amides and poly ester amide Polyanhydrides

Poly (butylenes adipate -co-terephalate) Poly (tetramethylene adipate /terephthalate)

Poly (ortho esters) Poly (propylene fumarate) Polyphosphazenes Polyphosphoster

Fig. 3.3: Classification of bio-degradable polymers [143] 3.2.1 Polyester Resin Polyester, also known as thermoplastic polymer, is used in a wide variety of applications, including packaging and labelling, textiles (e.g., ropes, thermal underwear and carpets), stationery, plastic parts and reusable

containers

of

various

types,

laboratory

equipment,

loudspeakers, automotive components, and polymer banknotes.

74 Durable goods such as motor vehicles, appliances and carpets account for about 50% of polyester end uses in the industrialized areas of the world. Consumption in these markets is mostly dependent on economic cycles and consumer spending on hard goods. Packaging, another major market for polyester, is often considered recession-proof in comparison with other polyester end users, with regard to pullthrough

demand.

However,

inventory

swings

along

packaging's

substantial supply chain can play havoc with the production demand, particularly during long down-market swings, such as those experienced in 2000–2003 and again in 2008–2010. The packaging market has suffered the impact of legislation mandating an increasing use of recycled plastics, source reduction, and the reduction of disposables. Compared

with

other

large-volume

thermoplastics,

the

polyester

business continues to exhibit excellent growth. Over the past ten years, the world demand has grown annually by 4.8%. Since 2007–2008, massive capacity additions have forced older units to be rationalized, while the steep crude oil price increase has created some fundamental shifts in the light olefin markets, creating a long-term increase in the propylene-to-ethylene price ratio. Accordingly, polyester has become more expensive relative to polyethylene, promoting shifts from polyester to alternative products like HDPE (high density polyethylene), wherever possible.

75 While new applications continue to be developed for polyester, it is still largely a commodity thermoplastic subject to economic fluctuations, especially in the area of durable goods. Producers continue to struggle with sustaining profitability, where cost volatility often undercuts the ability to recover pricing in tightly supplied markets. Competitors have consolidated, formed joint ventures and alliances in order to secure feedstock, and have pursued broader commercial positions and/or technologies to streamline and hopefully reduce the profit volatility of their businesses. The properties of polyester resin are presented in Table3.1. Table 3.1 Properties of polyester resin [144] Property

Range

Property

Tensile strength (MPa)

15-20

Tensile modulus (GPa)

0.8-1.2 Poisson’s ratio

Density (g/cm3)

Range 1-1.3 0.45

Compressive strength (MPa) 80-220 Specific gravity

1-1.5

Flexural strength (MPa)

25-32

0.005-0.009

Flexural modulus (GPa)

1.1-1.6 Young’s modulus (GPa) 4-6

Shrinkage (%)

3.2.2 Mixing of Resin The process of resin preparation is presented in Fig. 3.4. The accelerator and catalyst are mixed with resin and stirred, using a stirrer as shown in Fig. 3.4(a). After mixing, the quantity of resin to be used for each layer is measured, and given in Fig. 3.4(b).

76

(a)Mixing of resin

(b) Resin ready to use

Fig. 3.4: Preparation of resin 3.3 FIBER PREPARATION AND HYBRIDIZATION Most natural fibers have low processing temperatures; they cannot be processed over 150ºC due to their biological nature and hence, fiber preparation below 80ºC gives better properties. The raw sisal and jute fibers are cut into equal lengths of 32±0.5cm, and the glass fiber of unidirectional mat with 300gsm is also cut into the same length, and used for the specimen preparation. The hybridization of fibers refers to the combination of conventional fibers with natural fibers, by using either synthetic or biopolymer matrix for improving the properties. In this experiment, sisal and jute fibers are hybridized with glass fibers by using polyester resin, with two different fiber orientations of 0º and 45º, and three different fiber ratios of sisal-jute-glass fiber in the order of 40:0:60, 0:40:60 and 20:20:60 respectively. The main reason for hybridization is

77 to reduce the weight, cost and environmental effects. The physical properties of fibers used for composites fabrication are given in Table 3.2. Table 3.2 Physical properties of the fibers [10] Physical property

Glass fiber

Sisal fiber

Jute fiber

2.5-2.7

1.3-1.6

1.3-1.5

1700-2500

540-720

610-780

70-75

30-40

15-35

3-5

2.2-3.3

1.0-1.9

Max. elongation (mm)

20-30

5-10

10-14

Tensile modulus (GPa)

68-75

10-40

12-60

29

18

32

Young’s modulus (GPa)

-

13

15-30

Cellulose content (%)

-

65-75

59-70

Hemicellulose content (%)

-

10-15

15-20

Lignin content (%)

-

7-13

11-15

Lumen size (mm)

-

11

13

Fiber length (mm)

-

10-150

120-900

Microfibrillar angle (deg)

-

11-20

8-9

Moisture absorption (%)

-

11

12

Density (g/cm3) Tensile strength (kN/mm2) Stiffness (kN/mm) Elongation at break (%)

Specific modulus (approx.)

3.4 COMPOSITE SPECIMEN PREPARATION The hand lay-up is one of the oldest, simplest and most commonly used methods for composite parts’ construction. The specimen is fabricated in layer stacking, and each layer is oriented to achieve the maximum utilization of its properties. Layers of different materials can be combined to further enhance the overall performance of the laminated composite samples. Resins are impregnated by hand into fibers, which

78 are in the form of woven, knitted, stitched or bonded fabrics. This is usually accomplished by rollers or brushes, with an increasing use of nip-roller type impregnators for forcing the resin into the fabrics, by means of rotating rollers. Then the composite laminates are allowed to cure under normal atmospheric conditions and dried under the hot sun for over 24 hours. The composite samples used for the present investigation consist of five layers, and fabricated by the hand layup method. In the five layers, the glass fiber layers are mounted on the top, middle and bottom of the specimen. The second and fourth layers of the specimen are filled by natural fibers, and the resin has to be poured on every layer. The layer sequence of the different composite samples is presented in Table 3.3. The fiber orientation, reinforcement arrangement and fiber content in volume percentage of the fibers used for the fabrication of composites are tabulated in Table 3.4. Table 3.3 The layer sequence of the composite samples Layer/Sample

S1 and S4

S2 and S5

S3 and S6

Layer 1

Glass fiber

Glass fiber

Glass fiber

Layer 2

Sisal fiber

Jute fiber

Sisal fiber

Layer 3

Glass fiber

Glass fiber

Glass fiber

Layer 4

Sisal fiber

Jute fiber

Jute fiber

Layer 5

Glass fiber

Glass fiber

Glass fiber

79 Table 3.4 Fiber orientation, reinforcement arrangement and fiber content of the composite samples Reinforcement Sample

Fiber content

Fiber

arrangement

orientation

(Number of layers)

(Deg.)

(Volume %)

Sisal

Jute

Glass

Sisal

Jute

Glass

fiber

fiber

fiber

fiber

fiber

fiber

S1

0

2

0

3

40

0

60

S2

0

0

2

3

0

40

60

S3

0

1

1

3

20

20

60

S4

45

2

0

3

40

0

60

S5

45

0

2

3

0

40

60

S6

45

1

1

3

20

20

60

Add 1% catalyst and 1–1.5% accelerator by weight with resin for quick setting, immediate mixing, and reduce the heat generated due to exothermic reaction. Before fabricating the composite specimen the sisal and jute fibers are dried in the hot air oven at 80ºC for 8 hours to remove the moisture completely. Initially, the releasing agent is coated over the plain horizontal table for easy removal of the specimen, and the first layer of the specimen, i.e., the glass fiber mat is placed over the coated surface after the releasing agent gets dried. Then the resin is applied over the glass fiber mat and the resin is evenly distributed on the entire surface by using a roller. The resin is allowed 10-20 minutes for getting completely mixed; after that, the second layer of the specimen, i.e., the natural fiber is placed over the glass fiber. The process is repeated for all

80 five layers of the sample as well as for all the samples. The air gaps formed between the layers during processing are gently squeezed out. Then these samples are taken to the hydraulic press to remove the air gap between the layers, and any excess air present in the resin, by applying a force of about 70 to 100N for 48 hours, to get perfect samples. After the samples get hardened completely, they are taken out from the hydraulic press, and the rough edges are neatly cut as per the required dimensions.

(a) Glass fiber layer

(c) Natural fiber layer

(b) Resin distribution

(d) Fabricated composite laminates

Fig. 3.5: Processing of the composite samples

81 The process involved in fabricating the composite specimen is explained in Fig. 3.5. In this experiment, the sisal, jute and glass fiber reinforced hybrid composites are prepared at 32±2ºC, and an average relative humidity of 65%.

(a) Tensile test (ASTM D638)

(b) Flexural test (ASTM D790)

(c) Impact test (ASTM A370) Fig. 3.6: Test specimen as per ASTM standards

82 After fabrication, the composite samples are prepared for various mechanical tests, such as tensile, flexural and impact tests as per ASTM standards. The standard followed for the tensile test is ASTM D 638 [145], for flexural test ASTM D 790 [146], and the charpy impact strength was conducted as per ASTM D 6110 [147]. Then the tests are conducted for three samples in each case, and the average values are used for discussion. The tensile, flexural and impact test samples according to ASTM standards, are presented in Fig. 3.6. 3.5 SUMMARY This chapter presents the details of the materials, and methodology for fabricating composite samples, polyester resin and its properties, resin preparation, fiber preparation and hybridization, reinforcement arrangement, layer sequence, fiber content, processing of composite samples by the hand lay-up method, and specimen preparation according to ASTM standards.