Alternative Solutions for Reinforcement of Thermoplastic Composites

Authored by: Nadir Ayrilmis , Alireza Ashori

Natural Fiber Composites

Print publication date:  November  2015
Online publication date:  November  2015

Print ISBN: 9781482239003
eBook ISBN: 9781482239010
Adobe ISBN:

10.1201/b19062-4

 

Abstract

3.1 Introduction

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Alternative Solutions for Reinforcement of Thermoplastic Composites

3.1  Introduction

Over the past two decades, the thermoplastic industry has been attempting to decrease the dependence on petroleum-based fuels and products due to increased environmental consciousness. This leads to the need to investigate environmentally friendly, sustainable materials to replace existing ones. Currently, the most viable method of dealing with ecofriendly composites is the use of natural fibers as reinforcement. Natural fibers have received considerable attention as a substitute for synthetic fiber reinforcements in plastics. As replacements for conventional synthetic fibers such as aramid and glass fibers, natural fibers are increasingly used for reinforcement in thermoplastics due to their low density, good thermal insulation and mechanical properties, reduced tool wear, unlimited availability, low price, and problem-free disposal. Natural fibers also offer economical and environmental advantages compared with traditional inorganic reinforcements and fillers. As a result of these advantages, natural fiber-reinforced thermoplastic composites are gaining popularity in automotives, garden decking, fencing, railing, and nonstructural building applications, such as exterior window and door profiles, as well as siding.

The combination of interesting physical, mechanical, and thermal properties together with their sustainable nature has triggered various activities in the area of green composites. This chapter aims at providing a short review on developments in the area of natural fibers and their applications in fiber-based industries such as wood-plastic composites (WPCs).

3.2  Cellulose-Based Fibers

3.2.1  Natural Fibers

Fibers can be classified into two main groups: man-made and natural. The term “natural fibers” is used to designate various types of fibers, which naturally originate from plants, minerals, and animals [1]. All plant fibers are composed of cellulose, hemicelluloses, and lignin whereas animal fibers consist of proteins (hair, silk, and wool). To clarify our case, the word “plant” might be cited as “vegetable,” “cellulosic,” or “lignocellulosic.” Natural fibers offer the potential to deliver greater added value, sustainability, renewability, and lower costs compared with man-made fibers [2].

Plant fibers can be subdivided into nonwood fibers and wood fibers. Nonwood fibers can be classified according to which part of the plant they originate from. These include bast (stem or soft sclerenchyma) fibers, leaf, seed, fruit, root, grass, cereal straw, and wood [3]. Some of the important plant fibers are listed in Table 3.1 [4]. Agricultural residuals such as wheat straw, rice straw, bagasse, and corn stalks are also sources of natural fibers, although they have a lower cellulose content compared with wood [5]. Reddy and Yang [6] reported that velvet leaf (Abutilon theophrasti), which is currently considered a weed and an agricultural nuisance, could be used as a source for high-quality natural fibers. The fibers of the velvet leaf stem have properties similar to those of common bast fibers such as hemp and kenaf. The availability of large qualities of such fibers with well-defined mechanical properties is a general prerequisite for their successful use, namely in reinforcing plastics.

3.2.2  Wood Fibers

Wood is built up of cells, most of which are fibrous. Wood fiber is a composite material that is composed of a reinforcement of cellulose microfibril in a cementing matrix of hemicellulose and lignin [7]. Lignocellulosic materials contain cellulose, hemicellulose, lignin, and extractives in various amounts and chemical compositions. The chemical composition of different lignocellulosic fibers is displayed in Table 3.2 [8]. The properties of the wood flour are dependent not only on the main polymeric components but also on the structural arrangement of these components at the micro and macro scales. The chemical properties and behavior of wood or nonwood components during pulping and bleaching are very important. The proportions for wood are, on average, 40%–50% cellulose, 20%–30% lignin, and 25%–35% hemicellulose [9].

Table 3.1   List of Important Plant Fibers

Fiber Source

Species

Origin

Abaca

Musa textilis

Leaf

Bagasse

Grass

Bamboo

(>1250 species)

Grass

Banana

Musa indica

Leaf

Broom root

Muhlenbergia macroura

Root

Cantala

Agave cantala

Leaf

Caroa

Neoglaziovia variegate

Leaf

China jute

Abutilon theophrasti

Stem

Coir

Cocos nucifera

Fruit

Cotton

Gossypium sp.

Seed

Curaua

Ananas erectifolius

Leaf

Date palm

Phoenix dactyifera

Leaf

Flax

Linum usitatissimum

Leaf

Hemp

Cannabis sativa

Stem

Henequen

Agave foourcrocydes

Leaf

Isora

Helicteres isora

Stem

Istle

Samuela carnerosana

Leaf

Jute

Corchorus capsularis

Stem

Kapok

Ceiba pentranda

Fruit

Kenaf

Hibiscus cannabinus

Stem

Kudzu

Pueraria thunbergiana

Stem

Mauritius hemp

Furcraea gigantea

Leaf

Nettle

Urtica dioica

Stem

Oil palm

Elaeis guineensis

Fruit

Piassava

Attalea funifera

Leaf

Pineapple

Ananas comosus

Leaf

Phormium

Phormium tenas

Leaf

Roselle

Hibiscus sabdariffa

Stem

Ramie

Boehmeria nivea

Stem

Sansevieria

Sansevieria

Leaf

Sisal

Agave sisilana

Leaf

Sponge gourd

Luffa cylinderica

Fruit

Straw (cereal)

Stalk

Sunn hemp

Crorolaria juncea

Stem

Cadillo/urena

Urena lobata

Stem

Wood

(>10,000 species)

Stem

Source: John, M.J. and Thomas, S. 2008. Biofibers and biocomposites. Carbohydrate Polymers 71:343–64. With permission [4].

It should be noted that natural fibers are cylindrical in shape with length to diameter ratios commonly in the range of 10%–60%. Natural fibers are usually hollow with walls having a composite, multilayered structure of commonly three main layers (Figure 3.1 [10]): the middle lamella (ML), the primary layer (P), and the secondary layer (S1, S2, and S3) [9]. In living, undamaged plant tissue, the walls taper at the ends of the fiber to form a sealed envelope around the central cavity, or lumen. Each layer of the fiber cell wall is made up of millions of microfibrils that are wound in a semi-structured helical fashion around the main fiber axis with varying quantities of lignin and amorphous hemicellulose binding the microfibrils into bundles, called “fibrils” [10].

Table 3.2   Chemical Composition of Common Lignocellulosic Fibers

Fiber

Cellulose (%)

Hemicellulose (%)

Lignin (%)

Extractives (%)

Ash Content (%)

Water Soluble (%)

Cotton

82.7

5.7

6.3

1.0

Jute

64.4

12

11.8

0.7

1.1

Flax

64.1

16.7

2.0

1.5–3.3

3.9

Ramie

68.6

13.1

0.6

1.9–2.2

5.5

Sisal

65.8

12.0

9.9

0.8–0.11

1.2

Oil palm EFB

65

19.0

2.0

Oil palm frond

56.0

27.5

20.5

4.4

2.4

Abaca

56–63

20–25

7–9

3

1.4

Hemp

74.4

17.9

3.7

0.9–1.7

Kenaf

53.4

33.9

21.2

4.0

Coir

32–43

0.15–0.25

40–45

Banana

60–65

19

5–10

4.6

PALF

81.5

12.7

Sun hemp

41–48

8.3–13.0

22.7

Bamboo

73.9

12.5

10.2

3.2

Hardwood

31–64

25–40

14–34

0.1–7.7

<1

Softwood

30–60

20–30

21–37

0.2–8.5

<1

Source: Jawaid, M. and Abdul Khalil, H.P.S. 2011. Cellulosic/synthetic fiber reinforced polymer hybrid composites: A review. Carbohydrate Polymers 86:1–18. With permission [8].

Cellulose, the most abundant biopolymer on Earth, is also the main constituent of wood. It is located predominantly in the secondary wall of the wood fiber [11]. Approximately 45%–50% of extractive-free dry substance in most plant species is cellulose, and it is the single most important component in the fiber cell wall in terms of its volume and effect on the characteristics of wood [12]. Cellulose molecules consist of long linear chains of homo-polysaccharide, composed of β-d-glucopyranose units, which are linked by (1-4)-glycosidic bonds. Most of its chemical properties may be related to the hydroxyl groups in each monomer unit and the glycosidic bonds. The glycosidic bonds are not easily broken, thus causing cellulose to be stable under a wide range of conditions. However, the hydroxyl groups in cellulose can be readily oxidized, esterified, and converted to ethers. During compounding in an extruder, the thermal, chemical, and mechanical degradation of cellulosic fibers often deteriorate the mechanical properties of cellulosic fibers. Usually, cellulose is relatively insensitive to the effect of heating at moderate temperatures over short periods of time. However, thermal degradation begins to appear as the temperature and duration of heating are increased. At lower temperatures (below 300°C), thermal degradation of cellulosic fibers results in the decomposition of the glycosyl units of cellulose with the evolving of water, carbon dioxide, and carbon monoxide, when cellulosic fibers are exposed to the effects of heat, air, and moisture [13]. These reaction products can accelerate the degradation process.

Diagram of cell wall organization. (From Smook, G.A. 1992.

Figure 3.1   Diagram of cell wall organization. (From Smook, G.A. 1992. Handbook for Pulp and Paper Technologists. 2nd ed. Vancouver, BC: Angus Wilde. With permission [10].)

Cellulose has a strong tendency to form intra- and intermolecular hydrogen bonds that are responsible for much of the super-molecular structure, as well as for the physical and chemical behavior of cellulose [11]. Within the microfibrils in the cell wall, cellulose is arranged into zones that display varying degrees of crystallinity and a degree of polymerization (DP) of 7,000–10,000 in native (or unpulped) wood. Cellulose of various plant origins (wood and nonwood) exhibits a wide range of crystallinity (20%–90%), which greatly affects its resistance to chemical attack and utility [14]. In addition, the tensile strength of wood flour comes primarily from the crystalline cellulose microfibrils.

The lignin contents in different woods range between 20% and 30%, typically 26%–32% in softwoods and 20%–28% in hardwoods. Simultaneously, nonwood fibers contain between 5% and 23% lignin [11]. Lignin is a highly complex noncrystalline, heterogeneous polymer with nonrepeating monomeric units and chemical linkages. Lignin acts as the plastic matrix that, in combination with hemicellulose, binds wood fibers together and provides wood with its structural rigidity and resistance to moisture and microbial attack [13]. Lignin concentration varies in different morphological regions of the plant and in different types of plant cells. In general, lignin concentration is high in the ML and P wall, and it is low in the S wall (Figure 3.1).

Noncellulosic carbohydrates or hemicelluloses are heteropolysaccharides that contain hexosan and pentosan monomer units. The amount of hemicellulose in wood is usually between 20% and 30%. Softwood hemicellulose consists of both pentosans and hexosans, whereas hardwood hemicellulose consists mainly of pentosans. Usually, hemicellulose can be characterized as a thermoplastic polymer and is similar to cellulose in possessing high-backbone rigidity through intermolecular hydrogen bonding. The pentosan content in softwoods is about 7%–10%, and in hardwoods it is about 19%–25% [14]. Most hemicelluloses have a DP of about 100–200 [11], are branched, and generally do not form crystalline regions.

All species of wood and other plant tissues contain small to moderate quantities of chemical substances in addition to the macromolecules of cellulose, hemicellulose, and lignin [9]. To distinguish them from the major cell wall components, these additional materials are known as the “extractive components,” or simply “extractives.” Extractive contents in most temperate and tropical wood species are 4%–10% and 20% of the dry weight, respectively. A wide range of different substances is included under the following extractive headings: flavonoids, lignans, stilbenes, tannins, inorganic salts, fats, waxes, alkaloids, proteins, simple and complex phenolics, simple sugars, pectins, mucilages, gums, terpenes, starch, glycosides, saponins, and essential oils. Extractives occupy certain morphological sites in the wood structure [11]. Many woods contain extractives that are toxic to bacteria, fungi, and termites; other extractives can add color and odor to wood.

Of the wood fiber cell wall components, cellulose and hemicellulose are essentially linear polysaccharides, whereas lignin is a three-dimensional phenolic component. Cellulose, a semicrystalline polymer with a crystallinity of about 60%–70%, is the primary component of wood flour. The tensile strength of wood flour comes primarily from the crystalline cellulose microfibril. Hemicellulose is composed of noncellulosic polysaccharides that serve as a matrix for wood flour and are probably amorphous in their naturally occurring state [9].

3.2.3  Paper Fibers

Pulp technology deals with the liberation of fibers fixed in the wood or plant matrix. Pulp consists of fibers, usually acquired from wood (Figure 3.2). The pulping processes aim first and foremost to liberate the fibers from the wood matrix. In principle, this can be achieved either mechanically or chemically.

By grinding wood or wood chips, the fibers in the wood are released and a mechanical pulp is obtained. Mechanical methods demand a substantial amount of electric power; on the other hand, they make use of practically all of the wood material, that is, the yield of the process is high (>90%) [15]. Since the fibers in wood and plants are glued together with lignin, the chemical way in which pulp is produced is by removing most of the lignin, thereby releasing the fibers. The delignification of wood is achieved through degrading the lignin molecules by introducing chemically charged groups, keeping the lignin fragments in solution, and eventually removing them by washing. No pulping chemicals are entirely selective toward lignin; the carbohydrates of the wood are also, to a varying extent, lost. In chemical pulping, only approximately half of the wood becomes pulp, and the other half is dissolved [16].

The making of pulp and paper products from wood.

Figure 3.2   The making of pulp and paper products from wood.

Paper fibers can be obtained from thermo-mechanical pulps (TMP) and chemically modified mechanical pulps. The TMP are produced by the mechanical defibration of wood chips at about 160°C under steam pressure in refiners. The grinding under steam pressure, which should have succeeded in softening the lignin-rich layer between the fibers before the wood structure is broken, results in a greater retention of fiber length than in conventional grinding. Chemically modified mechanical pulps are produced by treating wood chips with sodium sulfite. Then, the treated wood chips are refined either at atmospheric pressure to produce a chemi-mechanical pulp (CMP) or at higher pressure to produce a chemi-thermo-mechanical pulp (CTMP). The chemical treatment preserves the lengths of fibers and causes the fiber surfaces to be richer in hydrophilic polymers [9].

3.2.4  Nanocellulose

Over the past decade, and particularly in the past 5 years, a growing number of research groups worldwide have reported the formation and utilization of celluloses with widths of the fibrils or crystals in the nanometer range. Engineering fiber and design of lignocellulosics or rod-like cellulose nanoparticles and microfibrils to get high value-added products with special performance can reach new markets through nanotechnology [17]. It has been shown that the cellulose microfibrils present in wood can be liberated by high-pressure homogenizer procedures. The product, microfibrillated cellulose (MFC), exhibits gel-like characteristics. A second type of nanocellulose, nanocrystalline cellulose (NCC), is generated by the removal of amorphous sections of partially crystalline cellulose by acid hydrolysis. The NCC suspensions have liquid–crystalline properties. In contrast to MFC and NCC, which are prepared from already biosynthesized cellulose sources, a third nanocellulose variant, bacterial nanocellulose (BNC), is prepared from low-molecular-weight resources, such as sugars, by using acetic acid bacteria of the genus luconacetobacter. The in situ biofabrication of BNC opens up unique possibilities for the control of shape and the structure of the nanofiber network [18]. The family of nanocellulose materials is presented in Table 3.3 [18].

Table 3.3   The Family of Nanocellulose Materials

Type of Nanocellulose

Selected Reference and Synonyms

Typical Sources

Formation and Average Size

Microfibrillated cellulose (MFC)

Microfibrillated cellulose, nanofibrils and microfibrils, and nanofibrillated cellulose

Wood, sugar beet, potato tuber, hemp, and flax

Delamination of wood pulp by mechanical pressure before and/or after chemical or enzymatic treatment

Diameter: 5–60 nm

Length: several micrometers

Nanocrystalline cellulose (NCC)

Cellulose nanocrystals, crystallites, whiskers, and rodlike cellulose microcrystals

Wood, cotton, hemp, flax, wheat straw, mulberry bark, ramie, Avicel, tunicin, cellulose from algae, and bacteria

Acid hydrolysis of cellulose from many sources

Diameter: 5–70 nm

Length: 100–250 nm (from plant celluloses); 100 nm to several micrometers (from celluloses of tunicates, algae, and bacteria)

Bacterial nanocellulose (BNC)

Bacterial cellulose, microbial cellulose, and biocellulose

Low-molecular-weight sugars and alcohols

Bacterial synthesis Diameter: 20–100 nm; different types of nanofiber networks

Source: Klemm, D., Kramer, F., Moritz, S., Lindström, T., Ankerfors, M., Gray, D., and Dorris, A. 2011. Nanocelluloses: A new family of nature-based materials. Angewandte Chemie International Edition 50:5438–66. With permission [18].

Micro/nanofibrils isolated from natural fibers have garnered much attention for their use in composites, coatings, resins, and film because of their high-specific surface areas, renewability, and unique mechanical properties in the past two decades [19]. Nanocrystals are rice-like, needle-shaped, and strong with diameters in the 5 to 10 nm range and lengths on the order of 100 to 200 nm, depending on the source. In contrast, nanofibrils, which tend to have roughly 5 nm diameters, are spaghetti-like because they are longer (a micrometer or more), flexible, and easily entangled [20]. The potential uses of natural fiber composite (NFC) and NCC are presented in Figure 3.3 [21]. Some of the attributes (and benefits) of nanocellulose are as follows [22]:

  • Abundant, sustainable, environmentally and biologically safe, and derived from plant material
  • Long fiber structure, providing composite material strength enhancements, such as in plastics, glass, or concrete
    Potential use of natural fiber composite and nanocrystalline cellulose (NNC). (From Noticias de Nanotecnología. 2014.

    Figure 3.3   Potential use of natural fiber composite and nanocrystalline cellulose (NNC). (From Noticias de Nanotecnología. 2014. http://www.tecnologianano.com [Accessed 20 June 2014]. With permission [21].)

  • High-tensile strength with a strength/weight ratio being eight times that of steel
  • Clear or transparent in water but, with manipulation, it possesses very unique and interesting optic properties due to its structure such that with modifications it can screen ultraviolet (UV) rays or enhance color brilliance
  • Benign to the human body and thus can be used as a unique carrier for drug delivery systems or to build tissue scaffolding, create microfilters for blood, or strengthen bones
  • Safe for human consumption, offering properties that can enhance the food and beverage markets, where it can be used for delivery of taste, or enhancing food textures such as in yogurts or salad dressings or as an additive in low-calorie products
  • Unique properties for aerogels or hydrogels
  • Easily functionalized, meaning it can be manipulated or combined with other polymers for many applications

Composites, such as glass or plastics, can benefit greatly with the addition of nanocellulose, with one study showing a 3,000 times strength gain by adding one ounce of nanocellulose to one pound of plastic. In this example, engineers can significantly reduce the weight of a plastic composite without sacrificing strength (benefiting auto manufacturing, for instance) or keep the mass of plastic the same while dramatically increasing strength. When properly aligned, nanocellulose offers an alternative to even stronger applications, such as replacing Kevlar® from DuPont. It is being investigated by the Department of Defense for its use in body armor and ballistic glass. Due to its transparency, strength, and optic properties, nanocellulose is being researched by Pioneer Electronics as a replacement for glass for flexible screens. Due to these optical and other properties, it is a unique additive for inks, paints, dyes, or glazing, thue adding strength while enhancing color brilliance. Combined with its safe attributes, it has applications in cosmetics and personal care products (for instance, as a sunscreen) [22].

3.2.4.1  Microfibrillated Cellulose

A cellulose fiber is composed of bundles of microfibrils where the cellulose chains are stabilized laterally by inter- and intramolecular hydrogen bonding. Cellulose fibers are turned into nanofibrous forms by chemical and mechanical treatments. One type of such nanofibers is called MFC, which can be obtained by a high-pressure homogenizing treatment [2324]. The properties of MFC have been previously reviewed by Siro and Plackett [25]. The MFC consists of moderately degraded long fibrils that have a greatly expanded surface area. Typically, the traditional MFC consists of cellulose microfibril aggregates with a diameter ranging from 20 to even 100 nm and a length of several micrometers, rather than single nanoscale microfibrils [26]. Microfibrils comprise elementary fibrils where monocrystalline domains are linked by amorphous domains. A photograph showing MFC and an optical micrograph showing the microstructure of the cellulose fibrils are presented in Figure 3.4 [27].

(a) Photograph showing microfibrillated cellulose (MFC) (5% MFC and 95% water) and (b) optical micrograph showing the microstructure of the cellulose fibrils. (From Turbak, A.F., Snyder, F.W., and Sandberg, K.R. 1983. Microfibrillated cellulose, a new cellulose product: Properties, uses and commercial potential.

Figure 3.4   (a) Photograph showing microfibrillated cellulose (MFC) (5% MFC and 95% water) and (b) optical micrograph showing the microstructure of the cellulose fibrils. (From Turbak, A.F., Snyder, F.W., and Sandberg, K.R. 1983. Microfibrillated cellulose, a new cellulose product: Properties, uses and commercial potential. Journal of Applied Applied Polymer Science 37:815–27. With permission [27].)

Regarding the production of MFC, several mechanical treatments have been used, such as a two-step process, including a refining and a high-pressure homogenization step, cryocrushing, and grinding methods. Developed in 1983 by Turbak et al. [27], the homogenization technology allows the production of a network of interconnected cellulose microfibrils. Without any cellulose pretreatment, the two-step mechanical process has usually led to MFC with the smallest diameters. In general, MFC is obtained from cellulose fibers after a two-step mechanical disintegration process, consisting of an initial refining step followed by a high-pressure homogenization step. More recently, there has been a focus on energy-efficient production methods, whereby fibers are pretreated by various physical, chemical, and enzymatic methods before homogenization to decrease the energy consumption. Pretreatments are alkaline pretreatment, oxidative pretreatment, and enzymatic pretreatment [28]. The most important characteristics of MFC are the dimensions and distribution of the fibrillar material, and the rheology of the resulting dispersion [18]. The scheme of interaction between cellulose molecular chains within the crystalline region of cellulose microfibrils is presented in Figure 3.5 [29]. These microfibrils have disordered (amorphous) regions and highly ordered (crystalline) regions. In the crystalline regions, cellulose chains are closely packed together by a strong and highly intricate intra- and intermolecular hydrogen-bond network (Figure 3.5), whereas the amorphous domains are regularly distributed along the microfibrils [29].

The hydrophilic nature of MFC constitutes a major obstacle for its use in composite applications. To tackle this problem, one strategy involves the chemical modification of surface hydroxyl groups of the MFC, to prevent hornification phenomena and/or decrease the nanofiber surface hydrophilicity. In past decades, the chemical modification of MFC has received significant interest from the scientific community. Thus, many reactions have already been performed to permanently modify the surface properties of the MFC (i.e., surface polarity), involving the use of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidative agent, silane reagents, carboxymethylation, acetylation, isocyanates, poly(ॉ-caprolactone), or anhydrides [30].

Scheme of interaction between cellulose molecular chains within the crystalline region of cellulose microfibrils. (From Zhou, C. and Wu, Q. 2012.

Figure 3.5   Scheme of interaction between cellulose molecular chains within the crystalline region of cellulose microfibrils. (From Zhou, C. and Wu, Q. 2012. Chapter 6: Recent development in applications of cellulose nanocrystals for advanced polymer-based nanocomposites by novel fabrication strategies. In: Nanotechnology and Nanomaterials: Nanocrystals—Synthesis, Characterization and Applications, ed. Nerella, S. Intec: Rijeka, Croatia. With permission [29].)

3.2.4.2  Nanocrystalline Cellulose

NCC, also known as “whiskers,” consists of rodlike cellulose crystals with widths and lengths of 5–70 nm and between 100 nm and several micrometers, respectively (Figure 3.6). They are generated by the removal of amorphous sections of a purified cellulose source by acid hydrolysis, often followed by ultrasonic treatment [18]. The commercialization of cellulose nanocrystals is still at an early stage, but appears very promising, as the strengthening effect and optical properties of NCC may find use in nanocomposites, paper making, coating additives, security papers, food packaging, and gas barriers. High aspect ratio, low density, low energy consumption, inherent renewability, biodegradability, and biocompatibility are the advantages of environmentally friendly crystalline nanocellulose (CNC) [29]. Bacterial cellulose, microbial cellulose, or biocellulose is formed by aerobic bacteria, such as acetic acid bacteria of the genus Gluconacetobacter and Acetobacter xylinum, which serves as a pure component of their biofilms [18].

Example of NCC.

Figure 3.6   Example of NCC.

During the past decade, CNC has attracted considerable attention, which could be attributed to its unique features. First, CNC has nanoscale dimensions and excellent mechanical properties. The theoretical value of Young’s modulus along the chain axis for perfect native CNC is estimated to be 167.5 GPa, which is even theoretically stronger than steel and similar to Kevlar, whereas the Young’s modulus of native CNC from cotton and tunicate reaches 105 and 143 GPa, respectively [29].

An excellent review on cellulose whiskers summarized the dimensional characteristics with their respective sources, description of isolation processes, hydrolysis conditions, and techniques of determination and performance of this material in suspension and in polymeric matrixes [17]. Comments on dispersity, related to their tendency toward agglomeration, and their compatibility with commercial hydrophobic polymers were also discussed. A recent review showed that NCC exhibited intriguing scientific and engineering discoveries and advancements [31]. However, the authors pointed out that the field is still in its infancy and open to opportunities for new advancements and discoveries. Other authors showed that cellulose nanocrystals are attractive material for incorporation into composites, because they can introduce additional strength gains with highly versatile chemical functionality [17].

3.3  Natural and Wood Fiber‑Reinforced Plastic Composites

WPCs refer to any composites that contain natural (including wood and nonwood) fibers and thermosets or thermoplastics [1]. Thermosets are plastics that, once cured, cannot be remelted and reshaped. These include resins such as epoxies and phenolics, and plastics that the forest products industry is most familiar with. Thermoplastics are polymers that can be repeatedly melted [15]. This property allows other materials, such as wood fibers, to be mixed with the plastic to form a composite product. Polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) are the most widely used thermoplastics for WPC and currently they are very common in buildings, construction, furniture, and automotive products. WPCs are used in nearly every field of application, but their main application is in construction engineering, with a main focus on decking for terraces and balconies. Cost effectiveness is very important for decking material. For this reason, mainly commodity polymers such as PE or PP are used [32].

Over the past two decades, natural fibers have received considerable attention as a substitute for synthetic fiber reinforcements in plastics. As replacements for conventional synthetic fibers such as aramid and glass fibers, natural fibers are increasingly used for reinforcement in thermoplastics due to their low density, good thermal insulation and mechanical properties, reduced tool wear, unlimited availability, low price, and problem-free disposal [33]. Several types of natural fibers such as kenaf [34], jute [35], sisal [36], flax [37], hemp [38], and coir fiber [39] were studied as reinforcement for thermoplastics such as PP and PE. Natural fibers also offer economical and environmental advantages compared with traditional inorganic reinforcements and fillers. As a result of these advantages, natural fiber-reinforced thermoplastic composites are gaining popularity in automotive and nonstructural construction applications. Natural fibers have attracted remarkable interest in the automotive industry owing to their hard-wearing quality and high hardness (not fragile such as glass fiber) and good acoustic resistance, in addition to, being moth-proof, nontoxic, resistant to microbial and fungi degradation, and not easily combustible [40]. The natural fiber-reinforced thermoplastic composites serve as a replacement for glass fiber in automotive components. They are used as trim parts in dashboards, door panels, parcel shelves, seat cushions, backrests, and cabin linings.

Forest product companies see WPCs as a way to increase the value-added utilization of waste wood and wood of low-commercial value. Plastic processors see wood as a readily available, relatively inexpensive filler that can lower resin costs, improve stiffness, increase profile extrusion rates, and act as an environmentally friendly method by which the use of petroleum-based plastics could be decreased. WPCs are resistant to moisture, insects, decay, and warping when compared with traditional pressure-treated lumber. They are stiffer, exhibit less creep, and are more dimensionally stable than unfilled plastic lumber. In addition, WPCs offer a “wood” look and feel with minimum maintenance [41].

WPCs are usually produced by mixing wood with polymers or by adding wood fiber as a filler in a polymer matrix, and then pressing or molding it under high pressure and suitable temperature. Additives such as colorants, coupling agents, stabilizers, blowing agents, reinforcing agents, fire retardants, biocides, foaming agents, and lubricants help tailor the end product to the target area of application [42]. The predominant manufacturing processes that are used to produce WPCs are usually extrusion and injection molding. In comparison to pure wood materials, more complex shapes can be formed by injection molding (Figure 3.7 [43]).

The characteristics and specifications of composites made from cellulose-based materials and thermoplastics (usually WPC or NFC) are given in the European Standard (EN)15534 [44] as follows:

  • Part 1: Test methods for characterization of compounds and products
  • Part 2: Load-bearing applications—Determination of modification factors for bending properties
  • Part 3: Specifications of materials
  • Part 4: Specifications for decking profiles and tiles
  • Part 5: Specifications for cladding profiles and tiles
  • Part 6: Specifications for fencing profiles
  • Part 7: Specifications for general purpose profiles in external applications (outdoor)
  • Part 8: Specifications for outdoor furniture
Injection-molded wood-plastic composite (WPC) samples. (From Ton-That, M.T. and Denault, J. 2007.

Figure 3.7   Injection-molded wood-plastic composite (WPC) samples. (From Ton-That, M.T. and Denault, J. 2007. Development of Composites Based on Natural Fibers. The Institute of Textile Science: Ottawa, ON. With permission [43].)

WPC lumber also tends to be quite dense compared with regular wood, and this means that several approaches have been adopted to reduce lumber weight (Figure 3.8 [45]). One approach deals with reducing section weight through the use of hollow profile cross-sections [46].

WPCs have been widely used for several years, and their market share is continuously growing. It is widely recognized that the use of a polymer and one or more solid fillers allows obtaining several advantages and, in particular, a combination of the main properties of the two (or more) solid phases. Among the fillers used, it is worth citing calcium carbonate, glass fibers, talc, kaolin, mica, wollastonite, dolomite, silica, graphite, synthetic fillers (e.g., polyethylene terephthalate (PET)- or polyvinyl alcohol (PVA)-based fibers), and high-performance fibers (carbon, aramidic, etc.) [47]. Major industries such as the aerospace, automotive, construction, or packaging have shown enormous interest in the development of new composite materials. One example of this is the replacement of inorganic fibers, such as glass or aramid fibers, by natural fibers that serve as fillers [48].

Extruded WPC samples. (From Web catalog. 2014. Greiner Tech. Profile GmbH, Austria,

Figure 3.8   Extruded WPC samples. (From Web catalog. 2014. Greiner Tech. Profile GmbH, Austria, http://www.greiner-techprofile.com [Accessed 16 June 2014]. With permission [45].)

In general, making composites with large particles is difficult with the extrusion process. In addition, the range of thickness and the width of the natural fiber-reinforced plastic composites made using the extrusion process are lower than those of wood-based panels such as fiberboard and particleboard. Another little explored possibility is to produce natural fiber-reinforced thermoplastic composites on a flatpress such as the traditional wood-based panels. Flat-pressing technology can be considered a promising alternative for manufacturing large dimensioned panels, as slit-die extrusion is limited in width, thickness, and output rate (Figure 3.9 [49]). The advantage of this technology is that only a relatively low pressure level is required, compared with extrusion and injection molding. The productivity of the pressing technology is much higher than that of extrusion and injection molding. Dimensions of flat-pressed WPC panels resemble more those of wood-based panels with a thermoset as an adhesive, such as particleboard and medium-density fiberboard (MDF), so that new application fields of WPC could be discovered in the future, particularly when elevated moisture resistance is required [5052].

Flat-pressed WPC panel samples. (Benthien, J.T., Ohlmeyer, M., and Fruhwald, A. 2011. Wood plastic composites (WPC) flat pressed and large-dimensioned. Poster article, Department of Wood Science and Technology, Hamburg University. With permission [

Figure 3.9   Flat-pressed WPC panel samples. (Benthien, J.T., Ohlmeyer, M., and Fruhwald, A. 2011. Wood plastic composites (WPC) flat pressed and large-dimensioned. Poster article, Department of Wood Science and Technology, Hamburg University. With permission [49].)

PVC provides the greatest strength and stiffness for WPC composites followed by PE and PP. It has an excellent cost/benefit ratio when compared with other polymer resins. WPC products made with PVC exhibit good impact strength, stiffness, and strength-to-weight ratio. PVC products offer good dimensional stability at ambient temperatures, resistance to chemicals and oils, durability, and a nonflammable nature [53]. However, PVC is the most toxic plastic for one’s health. It is also the most environmentally harmful plastic.

3.3.1  Modifiers and Additives for Composites

Natural fibers have a number of advantages and disadvantages compared with traditional synthetic fibers. Their ecological nature, biodegradability, low costs, nonabrasive nature, safe fiber handling, high possible filling levels, low energy consumption, high specific properties, low density, and wide variety of fiber types are very important factors for their acceptance in large volume markets, such as the automotive and construction industries. Furthermore, the public generally regards products made from renewable raw materials as environmentally friendly [2,54].

Wood flour is derived from various scrap wood from wood processors. High-quality wood flour must be of a specific species or species group and must be free from bark, dirt, and other foreign matter [55]. The most commonly used wood flours for plastic composites are made from pine, oak, maple, birch, spruce, fir, and poplar. Some species, such as red oak and chestnut, can contain low-molecular-weight phenolic compounds and tannins, which may cause stains if the composite is repeatedly wetted. Wood flour is typically about $0.10–0.30/kg. Most commercially manufactured wood flours used as fillers in thermoplastics are less than 425 μm (40 US standard mesh). Wood flour having a 40–60 mesh size is suitable for extrusion, whereas wood flour having a 80–100 mesh size is suitable for injection molding. Very fine wood flours can cost more and increase melt viscosity more than coarser wood flours, but composites made with them typically have more uniform appearance and a smoother finish [55].

However, certain drawbacks, such as a tendency to form aggregates during processing, low thermal stability, low resistance to moisture, and seasonal quality variations (even between individual plants in the same cultivation), greatly reduce the potential of natural fibers to be used as reinforcement for polymers [2,54]. The high moisture adsorption of natural fibers leads to swelling and the presence of voids at the interface, which results in poor mechanical properties and reduces the dimensional stability of composites [56]. It is, therefore, clear that chemical modification or use of adhesion promoters can be interesting paths to improve the overall mechanical properties.

The greater the wood content, the better the stiffness properties of the composite. However, there is a direct trade-off between wood content and the moisture-resistant properties of the WPC lumber. When the wood content of WPC increases beyond 65 wt.%, the resulting water absorption will increase accordingly because the wood is less likely to be fully encapsulated by the matrix polymer. With high-plastic percentages, WPCs are less likely to have absorbed much water from immersion tests [46].

Modification relies on physical and chemical techniques, which are mainly focused on grafting chemical groups that are capable of improving the interfacial interactions between filler particles and polymer matrix. The main techniques may be summarized as follows [5759]:

  • Thermal modification of wood fiber: Thermal modification is considered one of the most effective methods used to improve dimensional stability, durability, equilibrium moisture content, permeability, and surface quality of wood and wood-based composites [6065]. It is the oldest, the least expensive, and the most eco-friendly modification method that has been popularly used during the past decade [66]. Various thermal modification methods are used in the world and some of those methods have been registered, such as Thermowood (Finland), Perdure (France), Plato (Netherlands), and Menz Holz (Germany) [67]. All these methods have some major differences such as process conditions, wet or dry process, steering schedules, process steps, atmosphere (oxygen or nitrogen), steaming, and use of oil [68]. Having lower equilibrium moisture content and density along with increased wettability are also important advantages of heat-treated wood [69]. However, the adverse influence of heat treatment on the mechanical properties of wood is inevitable [70]. The physical and chemical properties of wood under heat treatment change at a temperature near 150°C and this continues with increasing temperature [66]. A typical heat treatment is applied at temperature levels and exposure times ranging from 120 to 250°C and from 15 minutes to 24 hours, respectively, depending on the process, species, sample size, moisture content, and target utilization [71]. Recent efforts on thermal treatment of wood fibers lead to an improvement in the dimensional stability of WPC. Ayrilmis et al. [62] reported that water absorption and thickness swelling of WPC containing Eucalyptus camaldulensis wood fibers treated at three different temperatures (120, 150, or 180°C) for 20 or 40 minutes under saturated steam in a laboratory autoclave was significantly lower than those of the WPC containing untreated wood fibers. Hot water extraction of wood chips is also effective in improving the dimensional stability of WPC [72].
  • Alkali treatment (also called mercerization): This is usually performed on short fibers, by heating at approximately 80°C in 10% NaOH aqueous solution for about 3–4 hours, washing, and drying in a ventilated oven. This process disrupts the formation of fiber clusters in order to obtain smaller and better-quality fibers. It should also improve fiber wetting.
  • Acetylation: The fibers are usually first immersed in glacial acetic acid for 1 hour, then immersed in a mixture of acetic anhydride and a few drops of concentrated sulfuric acid for a few minutes, filtrated, washed, and dried in a ventilated oven. This is an esterification method that should stabilize the cell walls, especially in terms of humidity absorption and consequent dimensional variation.
  • Treatment with stearic acid: The acid is added to an ethyl alcohol solution, up to 10% of the total weight of the fibers to be treated, and the obtained solution is thus added drop wise to the fibers, which are then dried in an oven. It is an esterification method as well.
  • Benzylation: The fibers are immersed in 10% NaOH, then stirred with benzoyl chloride for 1 hour, filtrated, washed, dried, immersed in ethanol for 1 hour, rinsed, and dried in an oven. This method allows for decreasing of the hydrophilicity of the fibers.
  • Toluene diisocyanate (TDI) treatment: The fibers are immersed in chloroform with a few drops of a catalyst (based on dibutyltin dilaurate) and stirred for 2 hours after adding toluene-2,4-diisocyanate. Finally, the fibers are rinsed in acetone and dried in an oven.
  • Peroxide treatment: The fibers are immersed in a solution of dicumyl (or benzoyl) peroxide in acetone for about half an hour, then decanted, and dried. Recent studies have highlighted significant improvements with regard to mechanical properties with this treatment.
  • Anhydride treatment: This is usually carried out by using maleic anhydride or maleated PP (or PE) in a toluene or xylene solution, where the fibers are immersed for impregnation and a reaction with the hydroxyl groups on the fiber surface. Literature reports a significant reduction of water absorption.
  • Permanganate treatment: The fibers are immersed in a solution of KMnO4 in acetone (typical concentrations may range between 0.005% and 0.205%) for 1 minute, then decanted and dried. Investigations have pointed out a decreased hydrophilic nature of the fibers on performing this treatment.
  • Silane treatment: The fibers are immersed in a 3:2 alcohol–water solution, containing a silane-based adhesion promoter for 2 hours at pH ≈ 4, rinsed in water, and dried in an oven. Silanes should react with the hydroxyl groups of the fibers and improve their surface quality.
  • Isocyanate treatment: The isocyanate group can react with the hydroxyl groups on the fiber surface, thus improving the interface adhesion with the polymer matrix. The treatment is typically performed with isocyanate compounds at intermediate temperatures (around 50°C) for approximately 1 hour.
  • Plasma treatment: This recent method allows a significant modification of the fiber surface. However, chemical and morphological modification can be very heterogeneous depending on the treatment conditions, and therefore it is not easy to generalize; process control is a critical aspect, and the final surface modifications strongly depend on it. More specifically, TDI, dicumylperoxide, and silane treatment seem to guarantee the best results with regard to mechanical properties, whereas alkali treatment and acetylation seem to give better improvements in thermal and dimensional stability.

One of the major disadvantages of natural fibers is the poor compatibility exhibited between the fibers and the polymeric matrices, which results in the nonuniform dispersion of fibers within the matrix and poor mechanical properties. Wood and plastic are similar to oil and water, and they do not mix well [73]. Most polymers, especially thermoplastics, are nonpolar (“hydrophobic,” repelling water) substances that are not compatible with polar (“hydrophilic,” absorbing water) wood fibers and, therefore, this can result in poor adhesion between polymers and fibers in WPC. To improve the affinity and adhesion between fibers and thermoplastic matrices in production, chemical “coupling” or “compatibilizing” agents have been used [54]. Chemical coupling agents are substances, typically polymers, that are used in small quantities to treat a surface so that bonding occurs between it and other surfaces, for example, wood and thermoplastics. The coupling forms include covalent bonds, secondary bonding (such as hydrogen bonding and van der Waals’ forces), polymer molecular entanglement, and mechanical interblocking [74]. Therefore, chemical treatments can be considered in modifying the properties of natural fibers. Some compounds are known to promote adhesion by chemically coupling the adhesive to the material, such as sodium hydroxide, silane, acetic acid, acrylic acid, isocyanates, potassium permanganate, peroxide, and so on. The mechanism of the compatibilizing agent is shown in Figure 3.10 [75].

The coupling agent chemically bonds with the hydrophilic fiber and blends by wetting in the polymer chain [76]. An example of the improvements that a maleated adhesion promoter can assure to a WPC based on PP and 30 wt.% wood flour is reported in Table 3.4 [77].

Another issue is the processing temperature, which restricts the choice of matrix material. Natural fibers are composed of various organic materials (primarily cellulose as well as hemicellulose and lignin), and, therefore, their thermal treatment leads to a variety of physical and chemical changes. Thermal degradation of those fibers leads to poor organoleptic properties, such as odor and colors, and moreover to the deterioration of their mechanical properties. It also results in the generation of gaseous products, when processing takes place at temperatures above 200°C, which can create high porosity, low density, and reduced mechanical properties. For the improvement of thermal stability, attempts have been made to coat and/or graft the fibers with monomers [4].

Interaction of silane with natural fibers by hydrolysis. (From Xie, Y., Hill, C.A.S., Xiao, Z., Militz, H., and Mai, C. 2010. Silane coupling agents used for natural fiber/polymer composites: A review.

Figure 3.10   Interaction of silane with natural fibers by hydrolysis. (From Xie, Y., Hill, C.A.S., Xiao, Z., Militz, H., and Mai, C. 2010. Silane coupling agents used for natural fiber/polymer composites: A review. Composites: Part A 41:806–19. With permission [75].)

3.3.2  Processing Techniques, Properties, and Applications of Composites

The term “WPC” usually means compounding equal amounts of wood flour or particulates with thermoplastic polymers such as PP, PE, polystyrene (PS), PVC, or acrylonitrile-butadiene styrene (ABS). Since the wood content in the polymer matrix is high, the appearance and odor of WPC are similar to those of natural wood [15]. WPCs can be produced by different processes depending on the consumer’s need. Since the major markets for WPC are decking, railing, fencing, and siding, 97% of WPCs are produced by profile extrusion [73]. Wood flour is well accepted by plastic processors as a filler, because it is cheap and readily available. The stiffness and strength of plastics can be increased by adding wood flour as a filler. Wood also causes less abrasion to an extruder than mineral fillers such as glass fiber and talc. Since wood cools faster than plastics, there is no need for “calibrating” to shape a part of WPC lumber [78]. The result was the introduction of WPC injection mold industries that produce construction-related products such as windows, door sills, railing spindles, railing post skirts, and caps, as well as nonconstruction-related products, including automotive parts and furniture components. The ability to use recycled plastics in WPCs is an economic incentive, and there is a general perception that WPCs are quite durable and resistant to decay since the wood particulates are expected to be completely encapsulated by plastic.

Table 3.4   Main Mechanical Properties of PP-Wood Flour/Fiber Composite with or without Maleated Adhesion Promoter

Property

Unit

PP

PP +40 wt.% Wood Flour

PP + 40% Wood Flour + 3% Coupling Agent

PP + 40% Wood Fiber

PP + 40% Wood Fiber + 3% Coupling Agent

Specific gravity

 0.90

1.05

1.05

1.03

1.03

Tensile strength

MPa

28.5

25.4

32.3

28.2

52.3

Tensile modulus

GPa

 1.53

3.87

4.10

4.20

4.23

Bending strength

MPa

38.30

44.20

53.10

47.90

72.40

Bending modulus

GPa

 1.19

3.03

3.08

3.25

3.22

Source: Stark, N.M. and Rowlands, R.E. 2003. Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites. Wood and Fiber Science 35:167–74. With permission [77].

Wood flour usually contains at least 4% moisture when delivered, which must be removed before or during processing with thermoplastics. Though moisture could potentially be used as a foaming agent to reduce density, this approach is difficult to control and is not common industrial practice. Commercially, moisture is removed from the wood flour (1) before processing using a dryer, (2) by using the first part of an extruder as a dryer in some in-line process, or (3) during a separate compounding step (or in the first extruder in a tandem process). Once dried, wood flour can still absorb moisture quickly. Depending on the ambient conditions, wood flour can absorb several weight percent of moisture within hours. Even compounded material often needs to be dried before further processing, especially if high weight percentages of wood flour are used. The hygroscopicity of wood flour can also affect the end composite. Absorbed moisture interferes with and reduces hydrogen bonding between cell wall polymers and alters the mechanical performance of the product [55]. The wood flour should be dried in a dryer at 100°C for 24 hours to reach 0%–1% moisture content and then be stored in sealed plastic bags until blending with the polymer matrix.

The properties, ability to process, and rate of production of WPC can be improved drastically if proper additives are used. Plastics, such as high-density PE (HPPE) and PP, tend to absorb much less moisture than wood in the natural environment. Therefore, WPCs are less affected by moisture and possess better dimensional stability and fungus/termite resistance than solid wood, because wood particulates are encapsulated by the polymer matrix. Since thermoplastic polymers such as PP and HDPE are nonpolar (hydrophobic) materials whereas wood particulates are polar (hydrophilic) materials, there is a high probability of obtaining poor adhesion between wood and polymer, resulting in low tensile and flexural strengths of the WPC [74]. To improve the adhesion between wood and plastics, maleic anhydride grafted polymers are generally introduced as a compatibilizer to improve the strength of WPC. In addition, when the adhesion of wood flour and plastics is enhanced, the rate of water adsorption also decreases in the presence of the coupling agent. The production rate of WPC can be substantially increased when lubricants such as zinc stearate or fatty acids are added in the compounding process. Lubricants help suppress edge tearing and melt fracture phenomena happening in the extrusion process. Generally, the density of WPC is higher than that of solid wood and this limits the applications of WPC [79]. The density of WPC can be reduced by as much as 30% by adding blowing agents that make the density of WPC similar to that of real wood. Foaming also helps a manufacturer save on material cost.

Fungal and insect resistances of WPC are improved by adding boron compounds such as zinc borate, boric acid, borax, or disodium octaborate tetrahydrate (DOT) [80]. The boron compounds can be incorporated into the wood particles by conventional methods such as dipping-diffusion or vacuum-pressure treatments. The WPC manufacturers usually use the boron compounds as dry powder in a blender due to its easy application. However, the dipping-diffusion or vacuum-pressure treatments of boron compounds are more effective than dry blending of powders of wood and boron compounds against wood-destroying insects and fungi. This is because dipping-diffusion or vacuum-pressure treatments enable the chemicals to be more deeply impregnated into the wood particles or wood flour as compared with their powder form. The key issue to expand boron use for wood protection appears to be their fixation into wood while allowing for sufficient mobility so they remain fungicidal. In a recent study, boric acid was fixed into wood with condensed tannins and hexamine through a non-formaldehyde emission polymer network. This treatment greatly reduced boron leaching [81].

WPCs can be colored easily by adding colorants. Adding a UV stabilizer can improve the UV stability and weatherability of WPC. Light stabilizers can be classified into two types according to their action mode: UV absorbers act by shielding the material from UV light, and hindered amine light stabilizers (HALS) act by scavenging the radical intermediates formed in the photooxidation process. The WPC manufacturers offer specific light stabilizers for each polymer. The amount of protection can be influenced by both photostabilizer concentration and exposure variables [82]. Antioxidants are also applied here to prevent the polymer from degrading during the compounding process. Finally, some fillers such as glass fiber, talc, calcium carbonate, and nanoclay are recommended as the second filler in the WPC to improve mechanical properties and creep resistance. Indeed, the desired properties of a WPC can be achieved with a combination of different additives [1]. Overall, a coupling agent, lubricant, and blowing agent are the additives that attract more attention in the WPC industry.

The WPC manufacturers are introducing new applications for the furniture industry. Further expansion into the residential construction industry and development of applications for the furniture industry require an understanding of the fire performance of WPCs. For some applications, it may be necessary to improve the fire performance. Therefore, a knowledge of the effect of fire retardants in WPCs is also critical. Polymers used in WPCs burn and drip in case of fire, resulting in a very risky scenario. Burning plastics may produce hazards such as the evolution of toxic gases, loss of physical integrity, and melting and dripping, thereby providing other ignition sources [41]. Thus, fire retardant agents must be used to improve fire behavior [83]. The most effective and widely used fire retardant chemicals for WPCs are ammonium polyphosphate and expandable graphite, decabromodiphenyl oxide, magnesium hydroxide, melamine polyphosphate, aluminum hydroxide, and boron compounds (zinc borate) [51,84–86]. In a previous study, Ayrilmis et al. [51] reported that higher levels of wood flour content resulted in significantly improved fire resistance of the WPC panels with and without fire retardant as measured in the cone calorimeter test.

These days, the applications of WPCs are limited by material performance. The reason for this is that the flexural strength of plastics used in compounding WPC is approximately 40–80 MPa, whereas the flexural modulus is only about 1.5–2.5 GPa; the corresponding values for natural wood can be as high as 80 MPa for strength and 9 GPa for the modulus [79]. Thus, any combination of wood flour and plastic will lead to WPC flexural and tensile moduli that are significantly lower than those of natural wood. A deck constructed out of WPC, therefore, will flex much more than an identical wood deck for the same load, and this is undesirable. In addition, more research needs to be done on the long-term material behavior such as weather durability and creep. The flame retardancy of WPC is especially important in some states such as California, which have regulations in this regard. Material performance of WPCs these days has already met builder acceptance as a nonstructural material for decking. With more and more research being done in this area, it may be possible to use WPCs as structural materials in the future. Therefore, product evaluation standards of WPCs have to be established to meet the consumer’s demand and safety requirements.

3.4  Conclusions

A new generation of composites is emerging as material behavior is better understood, processing and performance are improved, and new opportunities are identified. Recent trends such as the desire to decrease petroleum dependence, increase bio-content, commercial production of nanocellulose, and changing markets will play a major role in the future of these composites. For example, composite manufacturers are seeking to take advantage of the favorable balance of properties (e.g., low density, good mechanical properties) of bast fibers from plants in composite applications. Natural fiber-reinforced polymer composites have received much attention because of their low density, low cost, nonabrasive, combustible, nontoxic, and biodegradable properties. A new building material, WPC, has emerged.

One main challenge of natural fiber-reinforced thermoplastic composites is their inherent moisture sensitivity, a major cause for fungal decay, mold growth, and dimensional instability, resulting in decreased service life as well as costly maintenance. Another issue is to understand the critical issues of durability, color stability, and UV weathering of natural fiber-reinforced thermoplastic composites. Optimum content of additives such as flame retardants, antioxidants, UV stabilizers, absorbers, pigments, antioxidants, and biocidies significantly improves the properties of natural fiber-reinforced thermoplastic composites. A serious issue of natural fibers is their strong polar nature, which creates incompatibility with most polymer matrices. Surface treatments are potentially able to overcome the problem of incompatibility. Chemical treatments can increase the interface adhesion between the fiber and the matrix, and they can decrease the water absorption of fibers.

Nanocellulose-based reinforcements constitute another class of naturally sourced reinforcements of recent interest. Application of cellulose nanofibers in polymer reinforcement is a relatively new research field. The main reason for using cellulose nanofibers in composite materials is that one can potentially exploit the high stiffness of the cellulose crystal for reinforcement.

References

Ashori, A. 2008. Wood-plastic composites as promising green-composites for automotive industries. Bioresource Technology 99:4661–4747.
Bismarck, A. , Baltazar, Y.J. , and Sarlkakis, K. 2006. Green composites as Panacea? Socio-economic aspects of green materials. Environment, Development and Sustainability 8:445–463.
Peijs, T. , Cabrera, N. , Alcock, B. , Schimanski, T. , and Loos, J. 2002. Pure all-polypropylene composites for ultimate recyclability. Paper presented at the 9th International Conference on Fiber Reinforced Composites—FRC 2002, Newcastle.
John, M.J. and Thomas, S. 2008. Biofibers and biocomposites. Carbohydrate Polymers 71:343–364.
Panthapulakkal, S. , Zereshkian, A. , and Sain, M. 2006. Preparation and characterization of wheat straw fibers for reinforcing application in injection molded thermoplastic composites. Bioresource Technology 97:265–272.
Reddy, N. and Yan, Y. 2008. Characterizing natural cellulose fibers from velvet leaf (Abutilon theophrasti) stems. Bioresource Technology 99:2449–2454.
Ashori, A. , Jalaluddin, H. , Raverty, W.D. , and Mohd Nor, M.Y. 2006. Chemical and morphological characteristics of Malaysian cultivated kenaf (Hibiscus cannabinus) fiber. Polymer-Plastics Technology & Engineering 45:131–134.
Jawaid, M. and Abdul Khalil, H.P.S. 2011. Cellulosic/synthetic fiber reinforced polymer hybrid composites: A review. Carbohydrate Polymers 86:1–18.
Ashori, A. 2006. Non-wood fibers—A potential source of raw material in papermaking. Polymer-Plastics Technology & Engineering 45:1133–1136.
Smook, G.A. 1992. Handbook for Pulp and Paper Technologists. 2nd ed. Vancouver, BC: Angus Wilde.
Sjöström, E. 1993. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego, CA: Academic Publisher.
Rowell, R.M. , Young, R.A. , and Rowell, J.K. 1997. Paper and Composites from Agrobased Resources. Boca Raton, FL: CRC Press.
Rowell, R.M. and Clemons, C.M. 1992. Chemical modification of wood fiber for thermoplasticity, compatibilization with plastics and dimensional stability. Paper presented at the International Particleboard/Composite Materials Symposium, Pullman, WA.
TAPPI Test Methods. 2002. TAPPI Press: Atlanta, GA.
Nourbakhsh, A. , Ashori, A. , Ziaei Tabari, H. , and Rezaei, F. 2010. Mechanical and thermo-chemical properties of wood-flour polypropylene blends. Polymer Bulletin 65:691–700.
Ek, M. , Gellerstedt, G. , and Henriksson, G. 2009. Paper Chemistry and Technology. Walter de Gruyter GmbH and Co. KG: Germany.
Durán, N. , Lemes, A.P. , Durán, M. , Freer, J. , and Baeza, J. 2011. A mini review of cellulose nanocrystals and its potential integration as co-product in bioethanol production. Journal of the Chilean Chemical Society 56:672–677.
Klemm, D. , Kramer, F. , Moritz, S. , Lindström, T. , Ankerfors, M. , Gray, D. , and Dorris, A. 2011. Nanocelluloses: A new family of nature-based materials. Angewandte Chemie International Edition 50:5438–5466.
Spence, K.L. , Venditti, R.A. , Rojas, O.J. , Habibi, Y. , and Pawlak, J.J. 2011. A comparative study of energy consumption and physical properties of microfibrillated cellulose produced by different processing methods. Cellulose 18:1097–1111.
Jacoby, M. 2014. Nano from the forest. Chemical & Engineering News 92:9–12.
Noticias de Nanotecnología. 2014. http://www.tecnologianano.com [Accessed 20 June 2014].
. 2014. Silvantris: Primer on NanoFibers and NanoCellulose. Silvantris, LLC: Orem, UT, http://www.silvantris.com [Accessed 15 June 2014].
Iwamoto, S. , Yamamoto, S. , Lee, S.H. , and Endo, T. 2014. Mechanical properties of polypropylene composites reinforced by surface-coated microfibrillated cellulose. Composites: Part A 59: 26–29.
Jang, J.H. , Lee, S.H. , Endo, T. , and Kim, N.H. 2013. Characteristics of microfibrillated cellulosic fibers and paper sheets from Korean white pine. Wood Science and Technology 47:925–937.
Siro, I. and Plackett, D. 2010. Microfibrillated cellulose and new nanocomposite materials: A review. Cellulose 17:459–494.
Kettunen, M. 2013. Cellulose nanofibrils as a functional material. PhD diss., Aalto University, Helsinki.
Turbak, A.F. , Snyder, F.W. , and Sandberg, K.R. 1983. Microfibrillated cellulose, a new cellulose product: Properties, uses and commercial potential. Journal of Applied Applied Polymer Science 37:815–827.
Kwon, J.H. , Lee, S.H. , Ayrilmis, N. , and Han, T.H. 2014. Effect of microfibrillated cellulose content on the bonding performance of urea-formaldehyde resin. Paper presented at the 57th International Convention of Society of Wood Science and Technology, Zvolen.
Zhou, C. and Wu, Q. 2012. Chapter 6: Recent development in applications of cellulose nanocrystals for advanced polymer-based nanocomposites by novel fabrication strategies. In: Nanotechnology and Nanomaterials: Nanocrystals—Synthesis, Characterization and Applications, ed. Nerella, S. Intech: Rijeka, Croatia.
Tingaut, P. , Eyholzer, C. , and Zimmermann, T. 2011. Chapter 14: Functional polymer nanocomposite materials from microfibrillated cellulose. In: Advances in Nanocomposite Technology, ed. Hashim, A. Intech: Rijeka, Croatia.
Habibi, Y. , Lucia, L.A. , and Rojas, O.J. 2010. Cellulose nanocrystals: Chemistry, self-assembly, and applications. Chemical Reviews 110:3479–3500.
Seefeldt, H. 2012. Flame retardancy of wood-plastic composites. PhD diss., Technischen Universität Berlin.
Bledzki, A.K. and Gassan, J. 1999. Composites reinforced with cellulose based fibers. Progress in Polymer Science 24:221–274.
Chow, P. , Lambert, R.J. , Bowers, C.T. , McKenzie, N. , Youngquist, J.A. , Muehl, J.H. , and Kryzsik, A.M. 2000. Physical and mechanical properties of composite panels made from kenaf plant fibers and plastics. Paper presented at the 2000 International Kenaf Symposium, Hiroshima.
Rahman, R. , Hasan, M. , Huque, M. , and Islam, N. 2010. Physico-mechanical properties of jute fiber reinforced polypropylene composites. Journal Reinforced Plastics and Composites 29:445–455.
Joseph, P.V. , Mathew, G. , Joseph, K. , Thomas, S. , and Pradeep, P. 2003. Mechanical properties of short sisal fiber-reinforced polypropylene composites: comparison of experimental data with theoretical predictions. Journal of Applied Polymer Science 88:602–611.
Arbelaiz, A. , Cantero, B.G. , Llano-Ponte, R. , Valea, A. , and Mondragon, I. 2005. Mechanical properties of flax fiber/polypropylene composites. Influence of fiber/matrix modification and glass fiber hybridization. Composites Part A 36:1637–1644.
Schirp, A. and Stender, J. 2010. Properties of extruded wood-plastic composites based on refiner wood fibers (TMP fibers) and hemp fibers. European Journal of Wood and Wood Products 68:219–231.
Ayrilmis, N. , Jarusombuti, S. , Fueangvivat, V. , Bauchongkol, P. , and White, R.H. 2011. Coir fiber reinforced polypropylene composite panel for automotive interior applications. Fibers and Polymers 12:919–926.
Mohanty, A.K. , Misra, M. , and Drzal, L.T. 2005. Natural Fibers, Biopolymers and Biocomposites. Boca Ranton, FL: Taylor & Francis.
Stark, N.M. , White, R.H. , Mueller, S.A. , and Osswald, T.A. 2010. Evaluation of various fire retardants for use in wood flourepolyethylene composites. Polymer Degradation and Stability 95:1903–1910.
Tabari, H.Z. , Nourbakhsh, A. , and Ashori, A. 2011. Effects of nanoclay and coupling agent on the mechanical, morphological, and thermal properties of wood flour/polypropylene composites. Polymer Engineering & Science 51:272–277.
Ton-That, M.T. and Denault, J. 2007. Development of Composites Based on Natural Fibers. The Institute of Textile Science: Ottawa, ON.
EN 15534. 2014. Wood plastic composites (WPCs). Test methods for characterization of WPC materials and products. European Committee for Standardization, Brussels, Belgium.
Web catalog. 2014. Greiner Tech. Profile GmbH, Austria, http://www.greiner-techprofile.com [Accessed 16 June 2014].
Gardner, D.J. and Murdock, D. 2010. Extrusion of wood plastic composites. University of Maine, Orono.
La Mantia, F.P. and Morreale, M. 2008. Accelerated weathering of polypropylene/wood flour composites. Polymer Degradation and Stability 93:1252–1258.
Alemdar, A. and Sain, M. 2008. Isolation and characterization of nanofibers from agricultural residues—Wheat straw and soy hulls. Bioresource Technology 99:1664–1671.
Benthien, J.T. , Ohlmeyer, M. , and Frühwald, A. 2011. Wood plastic composites (WPC) flat pressed and large-dimensioned. Poster article, Department of Wood Science and Technology, Hamburg University.
Benthien, J.T. and Thoemen, H. 2013. Effects of agglomeration and pressing process on the properties of flat pressed WPC panels. Journal of Applied Polymer Science 129:3710–3777.
Ayrilmis, N. , Benthien, J.T. , Thoemen, H. , and White, R.H. 2012. Effects of fire retardants on physical, mechanical, and fire properties of flat-pressed WPCs. European Journal of Wood and Wood Products 70:215–224.
Ayrilmis, N. and Jarusombuti, S. 2011. Flat-pressed wood plastic composite as an alternative to conventional wood-based panels. Journal of Composite Materials 45:103–112.
Gomez, J.A. 2014. Application Profiles. Plastic Technology, Gardner Business Media, Inc: Cincinnati, OH.
Kim, J.P. , Yoon, T.H. , Mun, S.P. , Rhee, J.M. , and Lee, J.S. 2006. Wood–polyethylene composites using ethylene–vinyl alcohol copolymer as adhesion promoter. Bioresource Technology 97:494–499.
Clemons, C.M. and Caufield, D.F. 2005. Chapter 15: Wood flour. In: Functional Fillers for Plastics, ed. Xanthos, M. Weinheim: Wiley. VCH Verlag GmbH & Co.
Gassan, J. and Gutowski, V.S. 2000. Effects of corona discharge and UV treatment on the properties of jute–fiber epoxy composites. Composites Science and Technology 60:2857–2863.
Dipa, R. , Sarkar, B.K. , Rana, A.K. , and Bose, N.R. 2001. Effect of alkali treated jute fibers on composite properties. Bulletin of Materials Science 24:129–135.
Dominkovics, Z. , Dányádi, L. , and Pukánszky, B. 2007. Surface modification of wood flour and its effect on the properties of PP/wood composites. Composites: Part A 38:1893–18901.
Kalia, S. , Kaith, B.S. , and Kaura, I. 2009. Pretreatments of natural fibers and their application as reinforcing material in polymer composites—A review. Polymer Engineering and Science 49:1253–1272.
Unsal, O. and Ayrilmis, N. 2005. Variations in compression strength and surface roughness of heat-treated Turkish river red gum (Eucalyptus camaldulensis Dehn.) wood. Journal of Wood Science 51:405–409.
Jarusombuti, J. , Ayrilmis, N. , Bauchongkol, P. , and Fueangvivat, V. 2010. Surface characteristics and overlaying properties of MDF panels made from thermally treated rubberwood fibers. Bioresources 5:968–978.
Ayrilmis, N. , Jarusombuti, S. , Fueangvivat, V. , and Bauchongkol, P. 2011. Effect of thermal-treatment of wood fibers on properties of flat-pressed wood plastic composites. Polymer Degradation and Stability 96:818–822.
Kamdem, D.P. , Pizzi, A. , and Jermannaud, A. 2002. Durability of heat treated wood. Holz als Roh-und Werkstoff 60:1–6.
Nguyen, C.T. , Wagenfuhr, A. , Phuong, L.X. , Dai, V.H. , Bremer, M. , and Fischer, S. 2012. The effects of thermal modification on the properties of two Vietnamese bamboo species, Part I: Effects on physical properties. Bioresources 7:5355–5366.
Chen, Y. , Tshabalala, M.A. , Gao, J. , Stark, N.M. , Fan, Y. , and Ibach, R.E. 2014. Thermal behavior of extracted and delignified pine wood flour. Thermochimica Acta 591:40–44.
Salca, E.A. and Hiziroglu, S. 2014. Evaluation of hardness and surface quality of different wood species as function of heat treatment. Materials and Design 62:416–423.
Esteves, B. and Pereira, H.M. 2009. Wood modification by heat treatment. A review. Bioresources 4:370–404.
Militz, H. 2002. Thermal Treatment of Wood: European Processes and Their Background. IRG/WP 02-40241.
Candan, Z. , Buyuksari, U. , Korkut, S. , Unsal, O. , and Cakicier, N. 2012. Wettability and surface roughness of thermally modified plywood panels. Industrial Crops Products 36:434–436.
Kocaefe, D. , Poncsak, S. , Tang, J. , and Bouazara, M. 2010. Effect of heat treatment on the mechanical properties of North American jack pine: thermogravimetric study. Journal of Materials Science 45:681–687.
Priadi, T. and Hiziroglu, S. 2013. Characterization of heat treated wood species. Materials and Design 49:575–582.
Ozdemir, F. , Ayrilmis, N. , Kaymakci, A. , and Kwon, J.H. 2014. Improving dimensional stability of injection molded wood plastic composites using cold and hot water extraction methods. Maderas Ciencia y Tecnología 16:365–372.
Ziaei Tabari, H. , Nourbakhsh, A. , Ashori, A. 2011. Effects of nanoclay and coupling agent on the mechanical, morphological, and thermal properties of wood flour/polypropylene composites. Polymer Engineering and Science 51:272–277.
Lu, J.Z. , Wu, Q. , and McNabb, H.S. 2000. Chemical coupling in wood fiber and polymer composites: A review of coupling agents and treatments. Wood and Fiber Science 32:88–104.
Xie, Y. , Hill, C.A.S. , Xiao, Z. , Militz, H. , and Mai, C. 2010. Silane coupling agents used for natural fiber/polymer composites: A review. Composites: Part A 41:806–819.
Yan, S. , Yin, J. , Yang, Y. , Dai, Z. , Ma, J. , and Chen, X. 2007. Surface-grafted silica linked with L-lactic acid oligomer: A novel nanofiller to improve the performance of biodegradable poly(L-lactide). Polymer 48:1688–1694.
Stark, N.M. and Rowlands, R.E. 2003. Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites. Wood and Fiber Science 35:167–174.
Yeh, S.K. and Gupta, R.K. 2010. Nanoclay-reinforced, polypropylene-based wood-plastic composites. Polymer Engineering and Science 50:2013–2020.
Yeh, S.K. , Kim, K.J. , and Gupta, R.K. 2013. Synergistic effect of coupling agents on polypropylene-based wood-plastic composites. Journal of Applied Polymer Science 127:1047–1053.
Manning, M.J. 2008. Chapter 26: Borate wood preservatives: The current landscape. In: Development of Commercial Wood Preservatives: Efficacy, Environmental, and Health Issues, eds. Schultz, T.P. et al. ACS Symposium Series, Washington, DC.
Thévenon, M.F. , Tondi, G. , and Pizzi, A. 2010. Environmentally friendly wood preservative system based on polymerized tannin resin-boric acid for outdoor applications. Maderas Ciencia y Tecnología 12:253–257.
García, M. , Hidalgo, J. , Garmendia, I. , García-Jaca, J. 2009. Wood–plastics composites with better fire retardancy and durability performance. Composites: Part A: 40:1772–1776.
White, R.H. , Stark, N. , and Ayrilmis, N. 2011. Recent activities in flame retardancy of wood-plastic composites at the Forest Products Laboratory. Paper presented at the 22nd Annual Conference on Recent Advances in Flame Retardancy of Polymeric Materials, Stamford.
Ayrilmis, N. , Benthien, J.T. , Thoemen, H. , and White, R.H. 2011. Properties of flat-pressed wood plastic composites containing fire retardants. Journal of Applied Polymer Science 122:3201–3210.
Nikolaeva, M. and Karki, T. 2011. A review of fire retardant processes and chemistry, with discussion of the case of wood-plastic composites. Baltic Forestry 17:314–326.
Seefeldt, H. and Braun, U. 2012. A new flame retardant for wood materials tested in wood plastic composites. Macromolecular Materials and Engineering 297:814–820.
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