Wood fibres as reinforcements in natural fibre composites 25 (a) 0 HOC-C=C-C-O C-C-0-C- C-o-c C-C- C-C-OH ★★ n=3-6 Denotes reactive sites Ester groups (b) CHg CH2一CH-CH2-O C O-CH2-CH-CH2 CH3 (c) OH C OH c-c+c-oHc-c-c-o c-c-c-o-c-c-c ★★ ★★ n Denotes reactive sites Ester groups n=1-2 1.7 Chemical structure of thermoset matrices:(a)polyester;(b)epoxy; (c)vinyl ester. a (b) CH3 1.8 Chemical structure of thermoplastic matrices:(a)polypropylene; (b)polyethylene;(c)polystyrene;(d)poly(vinyl chloride). @Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 25 © Woodhead Publishing Limited, 2014 O (a) (b) (c) HOC C C * * * * * * C CC C CC CCCC CC O O O O O OH C O O O C CC CC C O C C C OCC OH O n C CCC * * O O CH2 CH2 OCO O CH2 CH CH2 CH3 CH3 OH CH O O OO n * Denotes reactive sites * Denotes reactive sites n = 3–6 n = 1–2 Ester groups Ester groups 1.7 Chemical structure of thermoset matrices: (a) polyester; (b) epoxy; (c) vinyl ester. n (a) (b) (c) (d) n n n CH3 Cl 1.8 Chemical structure of thermoplastic matrices: (a) polypropylene; (b) polyethylene; (c) polystyrene; (d) poly(vinyl chloride)
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© Woodhead Publishing Limited, 2014 Table 1.9 Advantages and disadvantages of synthetic matrices Resin Advantages Disadvantages Polyester Tough and rigid; processed by thermoplastic operations; recycled into useful products as basis of resins in such applications as sailboats Subject to attack by acids and bases; low thermal resistance, poor solvent resistance Epoxy High mechanical and thermal properties; high water resistance; long working times available; temperature resistance; low cure shrinkage More expensive than vinylesters, critical mixing, corrosive handling Vinyl ester Very high chemical and environmental resistance; higher mechanical properties than polyester Postcure generally required for high properties; high styrene content, higher cost than polyester; high cure shrinkage Polypropylene Low cost, low coeffi cient of friction; excellent fl exural, impact strength; excellent moisture fatigue and chemical resistance High thermal expansion; UV degradation; poor weathering resistance; fl ammable Polyethylene Lightweight; resistance to stain and organic solvents; low moisture absorption; resistance to electrolytic attack; good fatigue and wear resistance; excellent stress and crack resistance High thermal expansion; poor weathering resistance; subject to stress cracking; diffi cult to bond; fl ammable; poor temperature capability; low strength/ stiffness Polystyrene Low cost; non-hygroscopic; good optical clarity; easily processed; good thermal stability; good property retention; good creep resistance; easily decorated; easily bonded; good toughness Thick sooty smoke Poor weatherability Highly fl ammable Highly notch sensitive Poor resistance to petroleum solvents poly(vinyl chloride) Relatively inexpensive; very compatible with additive and plasticizers; good weatherability can be made to be extremely fl exible; easy to decorate; excellent abrasion, cut through, and solder resistance; heat shrinkable; fl ame retardant; lightweight compared with steel alternatives; good insulator Hydrochloric gas can be formed when burned; not good for food packaging because of; plasticizer leakage; non-biodegradable; unable to use in Europe (banned); can be highly reactive when mixed with acetal
Wood fibres as reinforcements in natural fibre composites 27 building materials where they have been successfully used as decking and window framing.Polyester,30-310 epoxy and vinyl esters(VE)3 are used as the thermoset matrices.The main chemical structures,advantages and disadvantages of each of these matrices are given in Figs 1.7,1.8 and Table 1.9 respectively.The thermoset matrix-based composites include furniture, housing panels,doors and windows,ceilings,flooring.household articles, containers,mechanical components and corrugated boards. 1.5 Process techniques of wood fibre composites The processing of wood fibre composites has a crucial influence on the prop- erties of the final products.Generally,the selection of process techniques is based on the matrices,the feed form of the raw ingredients,final product (e.g.form,application)and costs. The processes of compression,5312-316extrusion3463063-1 and injec- tion moulding291.313315.320-322 have been used for the inorganic compound,nat- ural high polymer,thermoplastic matrices.These methods are also the most popular processes within the wood-fibre composite industries.The sheet moulding compound(SMC)%23 and resin transfer moulding(RTM)315.324-326 are suitable for thermoset matrices. Moisture control,along with the specific mechanical energy (SME)gen- erated during processing,plays an important role in establishing the final composite properties.Maintaining the desired pressure and temperature throughout the component over the required period of time is the basic requirement of all composite manufacturing processes.327 The feature of low thermal stability limits the process temperature and the matrices for natural fibre-based composites.Maintaining the critical aspect ratio of the wood fibres is essential in order to obtain the reinforcing characteristic of the fibre. Compounding is the process of blending wood fibre with matrices.It is a critical step for the manufacture of wood fibre composites.The purpose of a compounding operation is to produce a pelletized feed stock that can be processed further,such as by injection moulding,extrusion or thermoform- ing.There are several types of compounding processes,including extrusion, kneading,and high-shear mixers.Manufacturing processes can be divided into open mould and closed mould.The open mould process comprises wet lay-up,bag moulding and autoclave moulding.The closed mould pro- cess comprises transfer moulding,compression moulding and injection moulding. Another main stream of wood fibre composites is to use resins as binder for the composites,such as MDF,PB,and oriented strand boards(OSB)and hardboard(HB).The composites generally consist of over 90%wood fibre or particles,and less than 10%of resin binding systems.Random distribution Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 27 © Woodhead Publishing Limited, 2014 building materials where they have been successfully used as decking and window framing. Polyester, 308–310 epoxy 82 and vinyl esters (VE) 311 are used as the thermoset matrices. The main chemical structures, advantages and disadvantages of each of these matrices are given in Figs 1.7, 1.8 and Table 1.9 respectively. The thermoset matrix-based composites include furniture, housing panels, doors and windows, ceilings, fl ooring, household articles, containers, mechanical components and corrugated boards. 1.5 Process techniques of wood fibre composites The processing of wood fi bre composites has a crucial infl uence on the properties of the fi nal products. Generally, the selection of process techniques is based on the matrices, the feed form of the raw ingredients, fi nal product (e.g. form, application) and costs. The processes of compression, 45,312–316 extrusion 289,293,294,296,306,317–319 and injection moulding 291,313,315, 320–322 have been used for the inorganic compound, natural high polymer, thermoplastic matrices. These methods are also the most popular processes within the wood‒fi bre composite industries. The sheet moulding compound (SMC) 323 and resin transfer moulding (RTM) 315,324–326 are suitable for thermoset matrices. Moisture control, along with the specifi c mechanical energy (SME) generated during processing, plays an important role in establishing the fi nal composite properties. Maintaining the desired pressure and temperature throughout the component over the required period of time is the basic requirement of all composite manufacturing processes. 327 The feature of low thermal stability limits the process temperature and the matrices for natural fi bre-based composites. Maintaining the critical aspect ratio of the wood fi bres is essential in order to obtain the reinforcing characteristic of the fi bre. Compounding is the process of blending wood fi bre with matrices. It is a critical step for the manufacture of wood fi bre composites. The purpose of a compounding operation is to produce a pelletized feed stock that can be processed further, such as by injection moulding, extrusion or thermoforming. There are several types of compounding processes, including extrusion, kneading, and high-shear mixers. Manufacturing processes can be divided into open mould and closed mould. The open mould process comprises wet lay-up, bag moulding and autoclave moulding. The closed mould process comprises transfer moulding, compression moulding and injection moulding. Another main stream of wood fi bre composites is to use resins as binder for the composites, such as MDF, PB, and oriented strand boards (OSB) and hardboard (HB). The composites generally consist of over 90% wood fi bre or particles, and less than 10% of resin binding systems. Random distribution
28 Natural fibre composites of resin over the surface of the fibre or particles governs the properties of final products.Hardboard,one of the fibreboards,can also be manufactured without any binder,using water as a medium. 1.5.1 Compression Compression (Fig.1.9)is a method of moulding in which the moulded mate- rial,generally preheated,is first placed in an open,heated mould cavity.328 The compression moulding technique has proved suitable for the pro- duction of profiles with any thermoplastic and thermoset composites.The mechanical properties and dimensional stability of compression moulded composites are influenced by the design of the mould cavity,location of the charge and processing parameters,e.g.compound temperature,mould cooling or heating rate,closing speeds and the filler loading.329 The com- pression method is common,popular in the manufacture of WPC.It can be connected with other processes(e.g.extrusion,150.33031 injection314)to form a new process.It also can be used as one component of the manufacturing process(e.g.SMC,bulk moulding compound(BMC)). 1.5.2 Extrusion Extrusion (Fig.1.10)is by far the most important processing method for WPC.Non-synthetic polymers,e.g.cement,32 starch233 and PLA,3 can be used as the matrices in this process.It involves two types of extruders:sin- gle-screw and twin-screw.Due to the modular character35 and other advan- tages (e.g.excellent compounding capability,good functional versatility) over single-screw extruders,twin-screw extruders dominate today's market for the manufacture of wood fibre composites.A suitable combination of processing variables including screw speed,screw configuration,throughput (a (b】 Moulding press Moulding press 2度 1.9 Schematic of compression moulding(a)and compression moulded wood fibre composite products(b). Woodhead Publishing Limited,2014
28 Natural fi bre composites © Woodhead Publishing Limited, 2014 of resin over the surface of the fi bre or particles governs the properties of fi nal products. Hardboard, one of the fi breboards, can also be manufactured without any binder, using water as a medium. 1.5.1 Compression Compression (Fig. 1.9) is a method of moulding in which the moulded material, generally preheated, is fi rst placed in an open, heated mould cavity. 328 The compression moulding technique has proved suitable for the production of profi les with any thermoplastic and thermoset composites. The mechanical properties and dimensional stability of compression moulded composites are infl uenced by the design of the mould cavity, location of the charge and processing parameters, e.g. compound temperature, mould cooling or heating rate, closing speeds and the fi ller loading. 329 The compression method is common, popular in the manufacture of WPC. It can be connected with other processes (e.g. extrusion, 150,330,331 injection 314 ) to form a new process. It also can be used as one component of the manufacturing process (e.g. SMC, bulk moulding compound (BMC)). 1.5.2 Extrusion Extrusion (Fig. 1.10) is by far the most important processing method for WPC. Non-synthetic polymers, e.g. cement, 317,332 starch 289,333 and PLA, 334 can be used as the matrices in this process. It involves two types of extruders: single-screw and twin-screw. Due to the modular character 335 and other advantages (e.g. excellent compounding capability, good functional versatility) over single-screw extruders, twin-screw extruders dominate today’s market for the manufacture of wood fi bre composites. A suitable combination of processing variables including screw speed, screw confi guration, throughput (a) (b) Moulding press Resin Moulding press Preform wood fibres sheet 1.9 Schematic of compression moulding (a) and compression moulded wood fi bre composite products (b)
Wood fibres as reinforcements in natural fibre composites 29 (a) Hopper Heaters Cold water in Cold water out Motor Screw (b) 1.10 Schematic of extrusion moulding(a)and extrusion moulded wood fibre composite products(b). rate and barrel temperatures is necessary to limit thermal degradation and darkening of the filler. Extrusion has overcome the shortcomings of Hatschek and other con- ventional processing systems of wood fibre cement,and has improved the mechanical performance.The extruded wood fibre cement products offer advantages in terms of the versatility of section profiles,end product per- formance characteristics,and production throughput.Wood fibres offer a desirable balance of performance and cost in this composite.A comparison between the extruded and cast fibreboard336 reveals that the extruded prod- ucts are better in terms of strength,stiffness,toughness,fibre distribution, fibre orientation,and bond of fibre with matrix,even in the presence of a higher percentage of air voids. 1.5.3 Injection moulding Injection moulding is a versatile process accounting for nearly one-third of polymeric materials processed,particularly where intricate shapes are needed in cyclic,high volume production.The benefits include excellent dimensional tolerance and short cycle times coupled with few post-pro- cessing operations(Fig.1.11).One of the challenges posed by injection moulding wood fire composites is to produce pellets of a consistent qual- ity.Injection moulding requires a polymer with a low molecular weight to @Woodhead Publishing Limited,2014
Wood fi bres as reinforcements in natural fi bre composites 29 © Woodhead Publishing Limited, 2014 rate and barrel temperatures is necessary to limit thermal degradation and darkening of the fi ller. Extrusion has overcome the shortcomings of Hatschek and other conventional processing systems of wood fi bre cement, and has improved the mechanical performance. The extruded wood fi bre cement products offer advantages in terms of the versatility of section profi les, end product performance characteristics, and production throughput. Wood fi bres offer a desirable balance of performance and cost in this composite. A comparison between the extruded and cast fi breboard 336 reveals that the extruded products are better in terms of strength, stiffness, toughness, fi bre distribution, fi bre orientation, and bond of fi bre with matrix, even in the presence of a higher percentage of air voids. 1.5.3 Injection moulding Injection moulding is a versatile process accounting for nearly one-third of polymeric materials processed, particularly where intricate shapes are needed in cyclic, high volume production. The benefi ts include excellent dimensional tolerance and short cycle times coupled with few post-processing operations (Fig. 1.11). One of the challenges posed by injection moulding wood fi re composites is to produce pellets of a consistent quality. Injection moulding requires a polymer with a low molecular weight to (b) (a) Heaters Hopper Motor Screw Cold water out Cold water in 1.10 Schematic of extrusion moulding (a) and extrusion moulded wood fi bre composite products (b)