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ISSERTATIONS | TANELI VÄISÄNEN | EFFECTS OF THERMALLY EXTRACTED WOOD DISTILLATES... | No 222
TANELI VÄISÄNEN
Wood-plastic composites (WPCs) represent an ecological alternative to conventional
petroleum-derived materials. The wood distillates studied in this thesis displayed good
potential as bio-based additives for WPCs as they improved the water resistance and mechanical properties. It was also shown that proton-transfer-reaction time-of-flight
mass-spectrometry (PTR-TOF-MS) can be applied to study the release of volatile organic
compounds (VOCs) from WPCs.
TANELI VÄISÄNEN
Effects of Thermally
Extracted Wood Distillates on the Characteristics of Wood-Plastic Composites
Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences
Number 222
Academic Dissertation
To be presented by permission of the Faculty of Science and Forestry for public examination in the Auditorium SN201 in Snellmania Building at the University of
Eastern Finland, Kuopio, on June, 10, 2016, at 12 o’clock noon.
Department of Applied Physics
Editors: Prof. Pertti Pasanen,
Prof. Jukka Tuomela, Prof. Pekka Toivanen, Prof. Matti Vornanen
Distribution:
Eastern Finland University Library / Sales of publications P.O. Box 107, FI-80101 Joensuu, Finland
tel. +358-50-3058396 http://www.uef.fi/kirjasto
ISBN: 978-952-61-2123-9 (printed) ISBN: 978-952-61-2124-6 (PDF)
ISSNL: 1798-5668 ISSN: 1798-5668 ISSN: 1798-5676 (PDF)
70211 KUOPIO FINLAND
email: taneli.vaisanen@uef.fi Supervisors: Professor Reijo Lappalainen, Ph.D.
University of Eastern Finland Department of Applied Physics P.O. Box 1627
70211 KUOPIO FINLAND
email: reijo.lappalainen@uef.fi Laura Tomppo, Ph.D.
University of Eastern Finland Department of Applied Physics P.O. Box 1627
70211 KUOPIO FINLAND
email: laura.tomppo@uef.fi Reviewers: Professor Raimo Alén, Dr.Tech.
University of Jyväskylä Department of Chemistry P.O. Box 35
40014 JYVÄSKYLÄ FINLAND
email: raimo.j.alen@jyu.fi Professor Rupert Wimmer, Ph.D.
University of Natural Resources and Life Sciences, Vienna
Sustainable Biomaterials Group Institute for Natural Materials Technology Konrad Lorenz Strasse 20
3430 TULLN AUSTRIA
email: rupert.wimmer@boku.ac.at Opponent: Professor Jyrki Vuorinen, Dr.Tech.
Tampere University of Technology Department of Materials Science P.O. Box 589
33101 TAMPERE FINLAND
The use of raw materials derived from renewable sources is increasing due to the finiteness of crude oil reserves. In wood-plastic composites (WPCs), the plastic in a material is partially replaced by wood, which is an abundantly available and inexpensive raw material. WPCs are materials that encompass a wide range of performance levels such that they have diverse applications, e.g., in fencing and decking as well as in the manufacture of automobiles. The use of WPCs in indoor applications is also becoming increasingly popular. Despite the increasing popularity of WPCs, certain inherent limitations mean that these materials are unsuitable for some applications. Examples of the limitations associated with WPCs are their insufficient mechanical strength and their susceptibility to excess water absorption.
Furthermore, the VOC (volatile organic compound) characteristics of WPCs have not been widely studied and therefore, a better understanding of these properties of WPCs would be of great importance. The properties of WPCs and their constituents can be altered by incorporating additives. However, some additives are rather expensive and their incorporation into WPCs is not straightforward. There is a clear need for novel, affordable and effective filler materials, especially those that would minimize the use of expensive constituents.
Wood distillates are products originating from thermal processes where the components of wood are partly or completely decomposed into charcoal, condensable vapors, and non-condensable gases.
Although the liquid components of wood have many potential applications, large volumes of liquids are still being discarded and not exploited in industrial applications. Thus, the incorporation of more of wood distillates into WPCs would enhance the use of raw materials and secondary products from the wood-processing industries. This would be both economically valuable and environmentally friendly since it would represent sustainable development by making commercial use of a potentially hazardous waste product.
The main aim of this thesis was to investigate whether wood distillates could be used as WPC components. Another aim was to assess the possibility to improve the mechanical properties and water
spectrometry (PTR-TOF-MS) in determining the VOC emission characteristics of WPCs was studied. The effects of incorporating hardwood and softwood distillates into WPCs were examined by characterizing the mechanical properties, water resistance and VOC emissions of these WPCs modified with the distillates. The distillate content varied from 1 wt% to 20 wt%. The suitability of PTR-TOF-MS for analyzing VOC emissions from WPCs was assessed by measuring VOC emissions from a WPC deck during a 41-day trial and comparing VOC emission rates between seven different WPC decks.
Both hardwood and softwood distillates exerted positive effects on the water resistance of the WPC; the addition of hardwood distillate decreased the water absorption of the WPC by over 25% whereas at least a 16% decrease was observed for the WPC with the softwood distillate. Moreover, a 1 wt% addition of hardwood distillate into the WPC led to a highly significant increase (11.5%, p < 0.01) in the tensile modulus as well as achieving minor enhancements in some other mechanical properties. Similarly, when 2 wt% of softwood was added to the WPC, a highly significant increase in the tensile strength (5.0%, p < 0.01) was observed. Even though the addition of the distillates increased the total release of VOCs, the emission rates of harmful compounds, such as benzene, remained low. Nonetheless, the results from the VOC analyses indicated that some of the compounds investigated in this thesis may be smelled from the WPCs because their odor thresholds were exceeded.
Wood distillates displayed good potential as natural additives in WPCs as they improved the mechanical properties and water resistance. The results of this thesis provide a basis for the further development of wood distillates as bio-based additives in WPCs.
Library of Congress Subject Headings: Composite materials; Wood distillation;
Pyrolysis; Hardwoods; Softwood; Volatile organic compounds; Wood—Chemistry;
Wood—Mechanical properties
Yleinen suomalainen asiasanasto: komposiitit; puu; muovi; kuivatislaus; fysikaaliset ominaisuudet; vetolujuus; kosteudenkestävyys; haihtuvat orgaaniset yhdisteet;
puukemia; puuteknologia
This thesis summarizes the studies performed in the Department of Applied Physics at the University of Eastern Finland during the years 2014 and 2015. The studies were financially supported by European Regional Development Fund (ERDF, granted by the Finnish Funding Agency for Technology and Innovation, project 70049/2011), Centre for Economic Development, Transport and the Environment (North Savo, project S12261), the Academy of Finland (decision no. 252908) and Teollisuusneuvos Heikki Väänänen’s Fund.
First, it is my pleasure to thank my supervisors for their support and guidance during the thesis project. I express my thanks to my first supervisor Prof. Reijo Lappalainen, Ph.D., for his trust, advice and encouragement during this process. I owe my deepest gratitude to Laura Tomppo, Ph.D., for her friendship, patience and prompt assistance whenever it was needed. I also want to thank all my co- authors for their contributions, especially Jorma Heikkinen for his expertise in the preparation of the composite granules, and Pasi Yli- Pirilä, M.Sc., for the advice in PTR-TOF-MS analyses.
I am very grateful to the pre-examiners of this thesis, Prof. Raimo Alén, Dr.Tech., and Prof. Rupert Wimmer, Ph.D., for their comments and constructive feedback. Furthermore, I am grateful to Prof. Jyrki Vuorinen for accepting the role of my opponent at the defense of this thesis and thus being part in one of the most important events of my academic career. I also want to thank Ewen MacDonald, Pharm.D., for his linguistic advice.
I express my special thanks to the co-workers in Panos building and in the new premises. The good humor, inspiration, and support really helped me to do my best and gave me extra motivation. The support from my friends is also highly acknowledged.
I am thankful to my family, Raija, Matti, Jouni, Arja, Emma, Lauri, and Juuso, for the support throughout my life. Thank you for believing in me and for encouraging me to always follow my heart.
Kuopio, June 2016 Taneli Väisänen
ASTM American Society for Testing and Materials CIS Charpy’s impact strength
DMTA Dynamic mechanical thermal analysis EBS Ethylene-bis-stearamide
EDS Energy-dispersive x-ray spectroscopy FTIR Fourier transform infrared spectroscopy GC/MS Gas chromatography-mass spectrometry HDPE High-density polyethylene
HWD Hardwood distillate
ISO International Organization for Standardization L/D Barrel-length-to-diameter
LDPE Low-density polyethylene
LG LunaGrain
MAPE Maleated polyethylene MAPP Maleated polypropylene MFA Microfibril angle
MOE Modulus of elasticity
PAH Polycyclic aromatic hydrocarbon
PE Polyethylene
PEEK Polyether ether ketone
PLA Polylactide
PP Polypropylene
ppb Parts per billion
ppmv Parts per million by volume
PS Polystyrene
PTFE Polytetrafluoro-ethylene
PTR-MS Proton-transfer-reaction mass-spectrometry PTR-TOF-MS Proton-transfer-reaction time-of-flight mass-
spectrometry
PVC Polyvinyl chloride
SEM Scanning electron microscopy SWD Softwood distillate
RH Relative humidity
RMT Reinforced matrix theory
TOF Time-of-flight
UF UPM ForMi
VOC Volatile organic compound WPC Wood-plastic composite
XPS X-ray photoelectron spectroscopy
A Cross-sectional area
Asample Area of a sample
b Width
B Bending
c Water absorption
Creal room Real room air concentration of a VOC
Cvoc Concentration of a VOC
ds Thickness
d Deflection
E Modulus of elasticity
Evoc Emission rate of a VOC
� Strain
Fvoc Flow rate
F Load/force
FS Flexural strength
h Height
I�, I� Cellulose polymorphs k[r] Rate coefficient L0 Initial gauge length L Final length of the gauge
Ls Length of span
Lp Product loading factor Mvoc Molar mass of a VOC m1 Mass of a dried specimen
m2 Mass of a specimen after water immersion
n Air exchange rate
S1, S2, S3 Layers of secondary wall
S Maximum surface stress
� Stress
T Temperature
TM Tensile modulus
TS Tensile strength
�t Reaction time
This thesis is based on data presented in the following articles, referred to by the Roman numerals I–IV.
I Väisänen T, Tomppo L, Selenius M and Lappalainen R. Effects of slow pyrolysis derived birch distillate on the properties of wood- plastic composites.Eur J Wood Prod.74 (1), pp. 131-133, 2016.
II Väisänen T, Laitinen K, Yli-Pirilä P, Tomppo L, Joutsensaari J, Raatikainen O and Lappalainen R. Rapid technique for monitoring VOC emissions from wood-plastic composites.
Submitted for publication.
III Väisänen T, Heikkinen J, Tomppo L and Lappalainen R.
Improving the properties of wood-plastic composite through addition of hardwood pyrolysis liquid. J Thermoplast Compos Mater. DOI: 10.1177/0892705716632862.2016.In press.
IV Väisänen T, Heikkinen J, Tomppo L and Lappalainen R.
Softwood distillate as a bio-based additive in wood-plastic composites.J Wood Chem Tech.36 (4), pp. 278-287, 2016.
The original articles have been reproduced with permission of the copyright holders.
This dissertation is based on four publications that examined the treatment and testing of WPCs modified with two types of distillates, and the characterization of VOCs from various types of WPCs. Papers I, III, and IV were concerned with the characterization of WPCs treated with hardwood and softwood distillates. Paper II focused on the evaluation of the applicability of PTR-TOF-MS for monitoring VOC emissions from WPCs.
The original idea for the utilization of wood distillates in WPCs was presented by Prof. Reijo Lappalainen, who also treated the granules with the hardwood distillate in paper I. The author was mainly responsible for the sample preparation. Moreover, the sample characterization and result analyses were conducted by the author.
The author was also the main writer for the paper, with contributions from other authors.
In paper II, the comparative measurements of seven different WPC decks were conducted by the author with the kind help from Pasi Yli- Pirilä, M.Sc. Kimmo Laitinen, M.Sc., conducted the measurements for the 41-day trial and wrote a part of the materials and methods -section for the paper. The samples for the study were acquired by Dr. Laura Tomppo and Prof. Reijo Lappalainen. The author analyzed the results and wrote the majority of the research paper. Pasi Yli-Pirilä, M.Sc., Dr.
Laura Tomppo, Doc. Jorma Joutsensaari, Doc. Olavi Raatikainen, and Prof. Reijo Lappalainen gave constructive comments and suggestions for the paper.
The ideas for papers III and IV were devised by the author and Dr.
Laura Tomppo. Jorma Heikkinen processed the distillates and impregnated the granules. The author was mainly responsible for the preparation of the samples. In addition, the sample characterization and sample analyses were undertaken by the author. The papers were written by the author with contributions from Dr. Laura Tomppo and Prof. Reijo Lappalainen.
1 Introduction ... 19
2 Wood-plastic composites ... 23
2.1 Raw materials ... 24
2.1.1 Wood ... 25
2.1.2 Polymers ... 28
2.1.3 Additives ... 31
2.2 Properties ... 33
2.2.1 Mechanical properties ... 34
2.2.2 Water absorption ... 38
2.2.3 VOC emissions ... 40
2.3 Manufacturing technologies ... 42
2.3.1 Extrusion ... 42
2.3.2 Injection molding ... 44
2.3.3 Compression molding ... 45
2.3.4 Choosing appropriate manufacturing method ... 46
3 Characterization of wood-plastic composites ... 49
3.1 Mechanical properties ... 49
3.1.1 Tensile strength ... 50
3.1.2 Flexural strength and modulus ... 52
3.1.3 Impact strength ... 53
3.2 Water absorption ... 55
3.3 VOC emissions ... 56
3.3.1 TD-GC-FID/MS ... 56
3.3.2 PTR-MS ... 57
4 Thermal processing of wood ... 61
4.1 ThermoWood® process ... 63
4.2 Slow pyrolysis of wood ... 65
4.3 Products obtained from the processes ... 67
4.3.1 Charcoal... 67
4.3.2 Condensable vapors ... 68
4.3.3 Non-condensable gases ... 70
5 Aims and significance ... 71
6 Materials and methods ... 73
6.1 Sample preparation ... 73
6.1.1 Distillates ... 74
6.1.2 Impregnation of WPC granules ... 76
6.1.3 Injection molding ... 77
6.2 Mechanical properties ... 78
6.2.1 Tensile and flexural properties ... 79
6.2.2 Charpy’s impact strength ... 80
6.3 Water absorption ... 81
6.4 VOC emissions ... 81
6.5 Statistical analyses ... 85
7 Results ... 87
7.1 Mechanical properties ... 88
7.2 Water absorption ... 89
7.3 VOC emissions ... 91
8 Discussion ... 97
8.1 Impregnation of WPC granules with wood distillates ... 97
8.2 Mechanical properties ... 98
8.3 Water absorption ... 101
8.4 VOC emissions ... 103
8.5 Limitations and future prospects ... 109
9 Summary and conclusions ... 113
Bibliography ... 115
The finiteness of crude oil reserves is globally recognized, and therefore, new raw material alternatives are being sought from renewable sources (Najafi et al. 2010). Wood is an inexpensive and abundantly available material that possesses suitable characteristics for multiple applications, such as in the construction industry (Clemons 2008). On the other hand, the combination of wood with the commodity plastics, adhesives, and other substances provide unique properties that cannot be achieved with either wood or plastic products on their own.
Thus, wood-plastic composites (WPCs) are ecological, durable, and recyclable materials (Kim and Pal 2010). In WPCs, the wood fibers are surrounded by a continuous polymer matrix, and the compatibility between these two constituents is typically enhanced by incorporating coupling agents and other additives into the composite. WPCs can be created with a wide range of performance levels, and therefore, they have many applications not only in decking, and fencing, but also in more sophisticated manufacturing, e.g., in the car-making industry (Klyosov 2007, Faruk et al. 2012).
Even though the use of WPCs is becoming more and more common, at present, these materials cannot be used in applications where high mechanical strength is required. This is mainly due to the weak bonding between the hydrophilic wood fibers and the hydrophobic polymer matrix (Gao et al. 2008, Yuan et al. 2008, Butylina et al. 2011). Moreover, wood fibers contain a large amount of hydroxyl groups that can form hydrogen bonds with water molecules. Hence, WPCs are susceptible to water absorption that induces thickness swelling and the creation of microcracks in the material (Li et al. 2014), which increases the risk of mold growth.
Many different approaches have been examined to eliminate, or at least reduce, these limitations in the present generation of
WPCs. There are several ways to modify wood fibers, e.g., heat treatment (Ayrilmis et al. 2011), the extraction of hemicelluloses (Hosseinaei et al. 2012) and the treatment with coupling agents (Müller et al. 2012); these modifications can considerably increase the water resistance of the WPCs. On the other hand, in some instances, the mechanical properties of WPCs can be enhanced by using recycled polymers instead of virgin material (Adhikary et al. 2008a). Moreover, the mechanical durability of WPCs can be improved by incorporating additives, such as maleated polypropylene or polyethylene (MAPP or MAPE) (Pérez et al. 2012, Ndiaye et al. 2013), waste charcoal (Li et al.
2014, Das et al. 2015a), nanoclay (Abdolvahaba et al. 2014), or inorganic fillers (Gwon et al. 2012), into the composite.
WPCs are also increasingly used in indoor applications, such as window frames and furniture. However, the impact of WPCs on the quality of the indoor air has not been studied widely.
Volatile organic compounds (VOCs) are chemicals that have a high vapor pressure at room temperature, allowing a great number of molecules to evaporate from the material and enter the surrounding air. VOCs include chemical compounds that occur in nature, and compounds that originate from human activity. Some VOCs exert harmful effects on human health and the environment, and therefore, their release and maximum concentrations in indoor air are regulated.
Several studies have examined the effects of organic wastes and residues on WPCs (Ashori and Nourbakhsh 2010, Hamzeh et al. 2011, Madhoushi et al. 2014, Das et al. 2015b). When organic waste is added to WPCs, typically there is an emphasized need for coupling agents to improve the bonding between the fibers and the polymer matrix. The choice, acquisition and application of the right coupling agent for WPCs is neither straightforward nor inexpensive, and the whole process requires time. Therefore, there is a desire to minimize the use of coupling agents and other additives with similar challenges, especially if they can be substituted with more affordable bio-based filler materials that can confer similar benefits.
A new and environmentally friendly approach to improve the properties of WPCs is to add the thermal degradation products of wood into the composites (Das et al. 2015b). Wood can be converted into charcoal, liquids, and non-condensable gases in pyrolytic processes (Klass 1998). The yields of these products vary depending on the process type (Nachenius et al.
2013). Since it is the primary product which is sought, the secondary products of the processes are commonly considered simply as waste. The potential of biochar as an additive in WPCs has been investigated previously, and the positive effects of incorporation of biochar into WPCs were evident (Li et al. 2014, Das et al. 2015a). Even though the liquid components of wood have multiple applications, e.g., as biocides, pesticides, material coating, and medicine (Bridgwater 1996, Fagernäs et al. 2012a), their effects on the characteristics of WPCs have not been studied earlier. Nonetheless, since wood distillates have a rather versatile nature, one could speculate that they could be utilized in WPCs as ecological additives, coupling agents, lubricants, biocides or stabilizers after proper processing.
The present thesis project investigated the effects of liquid components of wood on the properties of WPCs. It was hypothesized that the utilization of liquid components of wood in WPCs could provide many advantages. First, the content of rather expensive and petroleum-derived polymers in WPCs could be reduced. Second, the raw materials would be exploited more effectively as these liquids would otherwise be treated as waste and therefore, not be further utilized. The focus of this thesis was also on the determination of the VOC characteristics of WPCs. Proton-transfer-reaction time-of-flight mass- spectrometry (PTR-TOF-MS) was used to analyze the levels of VOCs emerging from WPCs. The effects of hardwood and softwood distillates were evaluated in mechanical tests, water absorption studies, and VOC analyses. The working hypothesis was that WPC granules could be effectively impregnated with hardwood or softwood distillates to increase the water resistance of WPCs and to strengthen the materials. The VOC
emission rates were expected to increase due to incorporation of these types of distillates.
By definition, composite materials are formed by combining two or more constituent materials that have substantially different chemical or physical properties (Callister 2005). As a result, the individual components remain distinct within the finished material, and thus composites can possess properties that cannot be achieved with the individual constituent materials. Composites can be classified into particle-reinforced, fiber-reinforced, and structural composites. There are many well known composite materials, e.g., metal and ceramic composites, cements, concrete, and reinforced plastics.
In materials science, a fiber is commonly defined as a substance that has been drawn into a long and thin filament, i.e., the aspect ratio, which is defined as the ratio of fiber length to diameter, is at least 100 (Callister 2005). However, the term fiber may also refer to the spindle-shaped cells within the wood material (Clemons 2008), and in the case of natural fiber- polymer composites, fiber can be defined as an object with an aspect ratio greater than one (e.g. Stokke et al. 2014). Thus, from the viewpoint of materials science, some WPCs can be classified as particle-reinforced composites although they are commonly referred to as fiber-reinforced composites. In this thesis, the definition of a fiber is adopted from the terminology used for natural fiber-polymer composites.
WPCs are fiber-reinforced composites produced by mixing wood components and molten thermoplastics. In a WPC, a polymer forms a continuous matrix that surrounds the reinforcing wood components. The low price and high stiffness of wood makes it an attractive reinforcement for the commodity plastics. Since the processability of WPCs is similar to the plastic, there are several appropriate manufacturing technologies available for WPCs. Although the majority of WPC products are extruded, injection and compression molding are
other major technologies used in WPC production. (Godavarti 2005)
2.1 RAW MATERIALS
The properties of WPCs are mainly determined by the characteristics of their two main constituents. Even though both are polymer-based materials, wood and plastic exhibit distinctive properties and have different origins (Clemons 2008).
Matrix polymers are high-molecular-mass materials created by the polymerization of small repeating subunits, monomers.
Polymers can be either natural or synthetic and furthermore, virgin material or recycled based on their origin (Adhikary et al.
2008a, Adhikary et al. 2008b). Several polymers are used as the matrix material in WPCs, e.g., polyethene (PE), polypropene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polylactide (PLA). Due to the high molecular mass relative to the small molecule compounds, polymers possess unique physical properties, such as viscoelasticity and toughness.
Wood is a natural composite consisting primarily of three polymeric components: cellulose, hemicelluloses and lignin (Pettersen 1984). Cellulose constitutes 40–45%, hemicelluloses 25–35%, and lignin makes up much of the remaining 20–30% of wood. Wood is an attractive material to be incorporated in polymer composites not only because it is abundant but also due to its light weight in relation to its mechanical properties.
In WPCs, the wood components are surrounded by the continuous polymer matrix. In general, the development of high quality WPCs is limited by two physical factors (Godavarti 2005): the difference between the surface energy of the polymer matrix and wood components, and the upper temperature at which wood can be processed. There are several ways to offset or minimize these limitations and to improve the general performance of the WPC. The most common approach involves the incorporation of different types of additives. Examples of
additives used in WPCs are coupling agents, lubricants, stabilizers, inorganic fillers, biocides, and flame retardants.
2.1.1 Wood
Wood has unique and useful properties – it is a recyclable, biodegradable, renewable, bendable, and relatively stable material. In addition, wood has an important role in carbon sequestration; growing trees take up and store considerable amounts of atmospheric carbon dioxide (CO2) (Hill 2006a).
The reinforced matrix theory (RMT) is a concept which can help to understand the cell wall structure of wood fibers, and ultimately the properties of wood. In short, the RMT describes the cell wall structure as follows: the cell wall of a plant consists of the thermoplastic matrix (lignin) reinforced by the high tensile strength fibers (cellulose) and the hygroscopic material (hemicellulose). (Stokke et al. 2014)
Wood can be anatomically divided into two classes (Wiedenhoeft 2010, Wiedenhoeft 2012): softwoods (gymnosperms) and hardwoods (angiosperms). When examined in the microscope, wood can be observed to be a composite of many cell types. It is a complex biological structure whose parts act together to fulfill the needs of a living plant: to conduct water from the roots to the leaves, to provide mechanical support for the plant’s body, and to store and synthesize essential biochemicals. Both softwoods and hardwoods consist mainly of tracheids – these are elongated and hollow cells arranged in parallel to each other along the trunk of the tree. In general, softwoods have a simpler structure than hardwoods because softwoods have only two cell types and less variation in the structure within the cell types (Pettersen 1984, Godavarti 2005, Wiedenhoeft 2012). The most distinctive difference in the structure between hardwood and softwood is the presence of vessel elements in hardwoods; these elements are absent in softwoods. Generally, softwoods have longer (3–8 mm) wood fibers than hardwoods (0.2–1.2 mm), but the length of wood fibers varies between wood species (Wiedenhoeft 2010, Clemons et al. 2013).
The layered structure of wood fibers also explains the unique properties of wood. As shown in Figure 1, the cell wall of a wood fiber consists of two main parts: the primary and secondary wall. The secondary wall consists of three separate layers designated as S1, S2, and S3.
Figure 1. The layered structure of the cell wall of wood. The lines in the primary and secondary cell wall layers describe the orientation of microfibrils.
The middle lamella is a lignin-rich region that binds the fibers together. The primary cell wall is made up of a loose and thin (0.1 µm) network of randomly oriented cellulose microfibrils. It also consists of hemicelluloses, proteins, and pectin. The first layer of the secondary cell wall, S1, is approximately 0.2 µm thick with a relatively high microfibril angle (MFA). S2 is the thickest layer of the cell wall (up to 20 µm thick), and it primarily defines the mechanical properties of the fiber. S2 consists mainly of cellulose and hemicelluloses. S3 is a thin layer (0.1 µm) of cellulose microfibrils. (Pettersen 1984, Stokke et al.
2014)
The chemical composition of wood also varies from species to species. In general, dry wood has an elemental composition of approximately 50% carbon, 6% hydrogen, and 44% oxygen.
In addition, wood contains trace amounts of other elements such as calcium, potassium, sodium, magnesium, iron, manganese, sulfur, and phosphorous. (Rowell et al. 2013)
Cellulose is a linear and highly crystalline polymer of D- glucopyranose units linked together by �-(1�4)-glucosidic bonds (Pettersen 1984, Li 2011). The repeating unit in cellulose is a two-sugar unit, cellobiose. When randomly oriented cellulose molecules form intra- and intermolecular hydrogen bonds, the packing density of cellulose increases, leading to the formation of crystalline regions. For example, wood-derived cellulose may contain as much as 65% of crystalline regions that confer the strength and structural stability to the wood (Rowell et al. 2013, Stokke et al. 2014). There are several different crystalline structures of cellulose. Cellulose I is the form of cellulose found in nature (Thomas et al. 2011). It has structures I� and I�, of which I�is enriched in the cellulose produced by algae and bacteria, and I� in higher plants (Stokke et al. 2014).
Hemicelluloses are heteropolymers that include arabinoxylans, glucomannans, xyloglucans, glucuronoxylans and xylans (Rowell et al. 2013). In addition to glucose, hemicelluloses can be made of other sugar monomers, such as xylose, mannose, and galactose. They are present in plant cell walls along with cellulose and lignin. In contrast to the linear and crystalline structure of cellulose, hemicelluloses are branched and amorphous polymers with little strength.
Whereas cellulose consists of approximately 10 000 glucose molecules per polymer, hemicelluloses have shorter chains of about 2 000 sugar units (Pettersen 1984, Clemons 2008). In the cell walls of plants, hemicelluloses form a network of cross- linked fibers, thus endowing flexibility to the plant.
Lignin is a complex, amorphous and cross-linked polymer, consisting of aromatic alcohols known as monolignols (Pettersen 1984, Li 2011, Stokke et al. 2014). There are three monolignol monomers incorporated into lignin during its biosynthesis in the form of phenylpropanoids: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The chemical composition of lignin varies in the different wood species. For example, lignin in the softwoods consists almost entirely of guaiacyl moieties. In cell walls, lignin can be considered as a chemical adhesive that fills the gaps between hemicelluloses
and cellulose. Lignin is covalently linked to hemicellulose molecules, increasing the mechanical strength of the cell wall.
Lignin is a non-polar hydrophobic polymer whereas cellulose and hemicelluloses are hydrophilic (Thomas et al. 2011). In the pulp industry, lignin is normally removed from the pulp (chemical pulp) when manufacturing bleached writing paper, because lignin is responsible for the yellowing of paper with age (Ek et al. 2009). Since lignin yields a considerable amount of energy when burned, it is considered as a potential alternative to fuels derived from non-renewable sources. In addition, the pyrolysis of lignin yields chemical compounds that are thought to be potentially useful in many fields of applications (Lora and Glasser 2002). For instance, guaiacol, which is a thermal degradation product of lignin, has smoky sensory notes and it can be used as a flavorant (Goldstein 2002, Dorfner et al. 2003).
In addition to lignocellulose, wood contains small amounts (3–10%) of other organic components (Pettersen 1984, Rowell et al. 2013, Stokke et al. 2014). Wood extractives include simple sugars, fats, waxes, resins, proteins, terpenes, and gums. The extractive compounds are crucial components of the defense system of the tree, but they also act as energy reserves and support tree metabolism (Clemons 2008). There are also trace amounts (about 1%) of inorganic ash in wood (Rowell et al.
2013).
2.1.2 Polymers
A variety of thermoplastic or thermosetting polymers can be used as the matrix material in WPCs (Clemons 2008). However, the low thermal stability of wood limits the polymers to those that have adequately low processing temperatures. The thermal degradation of wood components begins at approximately 120 °C, and major changes take place at over 200 °C. Thus, polymers which have a processing temperature lower than 200 °C need to be used in WPCs (Godavarti 2005). The most common polymers include PE, PP, and PVC that are thermoplastics.
The properties of a polymer are primarily defined by its molecular structure. Homopolymers contain only one type of monomer whereas copolymers or terpolymers consist of several kinds of monomers. The branching of polymer chains has multiple effects on the polymer. For example, the highly branched low-density polyethylene (LDPE) is softer and has a lower density and poorer tensile strength than the more linear high-density polyethylene (HDPE). (Clemons et al. 2013)
The properties of polymers also depend on its tacticity – the arrangement of monomers along the polymer backbone.
Polymer tacticity can be divided into three classes: an isotactic polymer has all of its substituents on the same side of the backbone, and polymers with alternating placements of substituents along the backbone are called syndiotactic. Atactic polymers lack any consistent arrangement in their substituents.
(Clemons et al. 2013)
The monomers of a copolymer can also be organized in a variety of ways (Clemons 2008). An alternating copolymer consists of two different monomers arranged in an alternating sequence within the chain of the molecule (ABABAB…). The organization of monomers in random copolymers is not defined (ABAABBBA…). Statistical copolymers have monomers arranged according to a known statistical rule. Block copolymers are made up of polymerized monomer blocks. If a copolymer contains side chains that have a different composition compared with the main chain, the polymer is termed as a graft copolymer.
The crystallinity of the polymer affects its thermal and physical properties because the crystalline regions inside the polymer structure increase the interactions between the polymer chains. When the structure of the polymer is highly ordered, there are fewer possibilities for the polymer chains to move relative to one another. Thus, more energy is required to transform the polymer into an unordered fluid state, meaning that polymers with high crystallinity have higher melting points in comparison with their more amorphous counterparts. (Beyler and Hirschler 2001) High crystallinity also means that the
polymer will be strong but brittle, which accounts for the high modulus and low impact resistance (Galeski 2003).
Semicrystalline polymers have both crystalline and amorphous regions, i.e., these polymers combine the high strength of crystalline polymers with the flexibility of the amorphous types (Callister 2005). In composite materials, semicrystalline polymers are typically more efficiently reinforced by fibers than amorphous ones because the fibers act as nucleation sites for the crystallization process with the fiber becoming surrounded by a finely divided microcrystalline structure, which improves the modulus, especially the flexural modulus (Quan et al. 2005).
At the moment, PEs are the most commonly used plastics because they are easy to produce and modify. PE (Figure 2) is a semicrystalline polymer. The polymer chains in PE can branch in a different manner, resulting in polymers with different properties. HDPE has a variety of applications because of its excellent barrier properties and resistance to different solvents.
LDPE is commonly used in containers, bottles, films, and plastic bags because it is a flexible and tough polymer with a good resistance to chemicals. In addition, LDPE has good electrical properties. (Klyosov 2007, Kim and Pal 2010)
Figure 2. The chemical structure of PE.
The properties of PP resemble those of PE. PP is a semicrystalline polymer with a methyl group (CH3) attached to the polymer backbone (Figure 3), meaning that PP can be either isotactic, atactic or syndiotactic. However, over 90% of produced PP is isotactic. Like PE, PP finds its applications in packaging, containers and films. Furthermore, PP is used in the automotive industry and can be found in laboratory equipment and textiles. (Kim and Pal 2010)
Figure 3. The chemical structure of isotactic PP.
PVC (Figure 4) is commonly used in construction, packaging, and insulation because it has an excellent weather resistance and electrical properties. Additionally, PVC has good mechanical properties. However, the processing of PVC is problematic because it releases a toxic compound, hydrochloric acid (HCl), when burned or melted. Furthermore, the thermal stability of PVC is very poor but can be improved by adding heat stabilizers during processing. The presence of a chlorine group in PVC means that the polymer may have different tacticity. Unlike PP, PVC has mainly an atactic stereochemistry.
(Klyosov 2007, Kim and Pal 2010)
Figure 4. The chemical structure of atactic PVC.
2.1.3 Additives
Additives are introduced into a polymer to alter its processability or performance (Clemons 2008). When a polymeric material contains additives, it is usually referred to as a “plastic”. Examples of additives are plasticizers, pigments, biocides, UV stabilizers, and antioxidants. The additive content is usually low because these materials are often rather expensive.
Moreover, an excessive amount of the additive may deteriorate the properties of the material (Clemons et al. 2013). In WPCs, additives are used to improve the processability of the composites and especially to enhance the coupling between chemically different wood fibers and plastics. Furthermore,
additives provide WPCs with a better surface appearance and long-term durability (Sherman 2004).
Coupling agents are additives that improve the adhesion between wood and plastics, and their content in WPCs is typically less than 5%. Coupling agents can be classified into the surface-active agents and functional modifiers. Surface-active agents do not form covalent bonds with either the polymer matrix or the wood fiber; instead, they increase the interfacial adhesion of these constituents by acting as a solid surfactant. MAPP is one of the most commonly used coupling agents; its anhydride part forms ester bonds with wood’s hydroxyl groups and the long hydrophobic polymer incorporates into the polymer network. MAPP is therefore a functional modifier. Consequently, the wood fibers and the polymer matrix become bonded together, resulting in enhanced mechanical properties and reduced moisture absorption.
Organosilanes, acrylic-modified polytetrafluoro-ethylene (PTFE), epoxides, isocyanates, organic acids, inorganic agents, and titanates are some other examples of the coupling agents used in WPCs (Lu et al. 2000, Godavarti 2005).
Mineral additives are another major group of additives used in WPCs. They include talc (Mg3Si4O10(OH)2), calcium carbonate (CaCO3), kaolin clay (Al2Si2O5(OH)4), and silica sand (SiO2). In particular, talc and calcium carbonate are commonly utilized in WPCs because they are abundantly available, inexpensive, and they clearly enhance mechanical properties of the composite. In addition, talc has a natural affinity to oil, making it a good filler and lubricant for the mineral oil derived plastics (Klyosov 2007).
Due to its hydrophobicity and ability to close the pathways for water in the composite, the addition of talc into WPCs results in reduced moisture absorption and less swelling liability.
(Huuhilo et al. 2010)
The processability and surface appearance of WPCs can be improved using lubricants, such as zinc stearate, paraffin waxes, oxidized PE, and ethylene-bis-stearamide (EBS). However, the use of metal stearates together with maleated coupling agents can nullify the effects of both additives. Typically, the amount
of lubricants in WPCs is less than 5%, but it is also dependent on the type of the polymer matrix. For example, the lubricant content level for a HDPE-wood composite (wood content 50–60%) is usually 4–5% whereas a similar composite composed of PP as a polymer matrix instead of HDPE is typically manufactured with 1–2% of lubricant. (Sherman 2004)
Light stabilizers and colorants (pigments) are also added to WPCs to improve the resistance against color fade and UV degradation, and to provide the desired appearance (Sherman 2004, Clemons et al. 2013). The amount of pigments in WPCs must be 1–3% or even higher to avoid color staining from the wood. Biocides, such as zinc borate, protect composites against fungal and microbial attacks and maintain their surface appearance. They also reduce the moisture absorption. Flame retardants suppress the production of flames and therefore, prevent the spread of fire.
2.2 PROPERTIES
There are two common reasons to add wood to polymers: 1) to lower the price of the final product and 2) to reduce the dependency on mineral oil based products (Kim and Pal 2010).
This, however, means a compromise as the properties of wood and plastics are altered, i.e., WPCs possess rather different characteristics. For example, WPCs absorb less water than wood but have higher tensile strength than plastics. WPCs have therefore found use in multiple applications. The low density and good processability of WPCs are favored in automobile industry, for instance. On the other hand, WPCs are widely used in building products, such as siding and decking because of their low water absorption and good creep performance.
However, the properties of WPCs are highly dependent on the product formulation, manufacturing, and the quality of the raw materials.
2.2.1 Mechanical properties
Wood fibers are added to polymers to increase their stiffness and strength (Wolcott and Englund 1999). The presence of wood fibers in the polymer matrix typically increases the strength and modulus of the composite (Bhaskar et al. 2012, Li et al. 2014).
However, both the polymer matrix and the fiber reinforcement are responsible for the mechanical performance of the composite. Tensile strength is more sensitive to the properties of the polymer matrix whereas the modulus of elasticity of the composite is primarily dependent on the properties of the fiber.
In order to increase tensile strength, a strong fiber-matrix interface, oriented fibers, and low stress concentration are required whereas the maximization of the tensile modulus requires fiber wetting in the matrix phase, a high fiber concentration and fibers with a high aspect ratio. (Saheb and Jog 1999)
The fiber must have a certain minimum length, i.e., the critical fiber length, in order to achieve the fully stressed properties to the fiber in the polymer matrix (Stark and Rowlands 2003, Sain and Pervaiz 2008). The critical length depends on the fiber characteristics and shear strength of the fiber-matrix bond. The fiber-matrix interface is likely to fail due to the debonding at lower stresses if the length of the fiber is less than its critical strength (Stark and Rowlands 2003, Bourmaud and Baley 2007). By contrast, exceeding the critical fiber length may reduce the strength of the composite because the effective stress transfer may be impaired due to fiber curling and fiber bending (Sreekumar et al. 2007).
Interphase and interface are two important concepts in fiber- reinforced polymer composites. The interface is a two- dimensional surface between the fiber and the matrix whereas the interphase is the three-dimensional intermediate between the matrix phase and the fiber phase (Pilato and Michno 1994, Oksman Niska and Sanadi 2008, Jesson and Watts 2012). The interface in any fiber-polymer composite system is responsible for transmitting stresses from the matrix to the fibers, and the contribution of surfaces to stress transfer depends on both the
roughness and the surface chemistry of the constituents. The stress in WPCs is transferred not only by shear along the length of the fiber, but also by tension at the fiber-matrix interface. The stress transfer is limited by the fiber strength, the shear yield strength, and the tensile yield strength of the plastic matrix polymer. (Sretenovic et al. 2006, Sain and Pervaiz 2008) A composite failure can occur through several scenarios, and the uneven nature of the surfaces makes the process even more complex. However, in the simplest case, an adhesive failure can occur in the fiber-interphase interface or in the interphase- matrix interface. A cohesive failure of the interphase is also possible. The typical techniques to evaluate interfacial interactions and adhesion between the main constituents include surface analysis methods, such as X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR), microscopy, single fiber-pullout and microbond tests, and dynamic mechanical thermal analysis (DMTA). (Sretenovic et al. 2006, Oksman Niska and Sanadi 2008)
The mechanical properties of WPCs have been extensively investigated. Changes in the amount of the wood component exert multiple effects on the characteristics of WPCs. When the wood fiber content is increased, the tensile and flexural moduli tend to increase because wood, especially cellulose, is a highly crystalline material compared to PE, PP, and PVC (Bhaskar et al.
2012). However, the moduli of WPCs are highly dependent on the fiber type and source (Bouafif et al. 2009, Butylina et al. 2011, Ashori et al. 2011, Adhikari et al. 2012, Migneault et al. 2015).
Although an increase in the wood fiber content may also lead to the higher hardness, it tends to reduce impact and tensile strength (Bledzki et al. 2002, La Mantia et al. 2005, Ndiaye et al.
2013). In addition, the tensile strain at break decreases considerably. Huang and Zhang (2009) and Ashori et al. (2011) concluded that higher loadings of wood flour in WPCs induced the agglomeration of wood particles, which may impair the mechanical durability of WPCs.
Interestingly, Bledzki et al. (2002) showed that WPCs consisting of hardwood fibers had a higher elongation at break
and better impact strengths compared with WPCs containing softwood fibers. Their findings can be explained by the compositional differences between hardwoods and softwoods;
hardwoods contain more cellulose and hemicelluloses than softwoods. On the other hand, the higher lignin content in softwoods could explain the better stiffness of those WPCs consisting of softwood fibers. (Sain and Pervaiz 2008, Lai 2012)
The durability of WPCs can be considerably modified by altering the characteristics of the wood fiber surface, which changes the compatibility between wood fibers and coupling agents. For example, if WPCs are manufactured with wood fibers from bark, then the esterification reactions between reinforcing fibers and the coupling agent are inadequate and these WPCs are mechanically weaker. Conversely, the manufacture of WPCs with pure cellulose fibers leads to stronger WPCs because cellulose fibers and polymer matrix can be more extensively coupled through coupling agents. The underlying reason for this phenomenon is the difference between the surfaces of fibers; the surface of pure cellulose fiber is more polar than the surface of bark because cellulose contains more polar hydroxyl groups. In contrast, bark consists mainly of lignin and extractives that are chemically non-polar.
Furthermore, the coupling between wood fibers and polymers can be altered by treating wood fibers with coupling agents, acids or alkalis. (Balasuriya et al. 2002, Bouafif et al. 2009, Farsi 2010, Müller et al. 2012, Zhang et al. 2013, Migneault et al. 2015) Thermal treatment of wood fibers is another way to modify the properties of WPCs (Ayrilmis et al. 2011). In general, WPCs reinforced with thermally treated wood fibers are mechanically weaker than those reinforced with non-treated fibers. However, the thermal treatment of the wood fibers significantly increases the dimensional stability and water resistance of WPCs. The thermal degradation of hemicelluloses begins already at 120 °C.
As mentioned in section 2.1.1, hemicelluloses act as the connective bridges between cellulose fibers and lignin, leading to the stiffer wood material. The degradation of hemicelluloses, therefore, results in weakened mechanical properties.
Hosseinaei et al. (2012) showed that the extraction of hemicelluloses from wood fibers significantly improved the tensile properties of WPCs because the resulting wood fibers were more hydrophobic and less polar, enhancing the compatibility between wood fibers and thermoplastics.
Bouafif et al. (2009) demonstrated that wood fiber size also affected the mechanical properties of WPCs; increasing the fiber size improves the modulus of elasticity (MOE) and maximum strength in both flexural and tensile tests; the results have been confirmed by Kociszewski et al. (2012). Migneault et al. (2008) demonstrated that increasing fiber length and maintaining constant fiber diameter exerted beneficial effects on the tensile and flexural moduli and toughness of WPC.
The possibility of modifying the polymer matrix also results in the preparation of WPCs with distinctive characteristics. For example, using recycled polymers instead of virgin materials may improve mechanical properties of WPCs (Adhikary et al.
2008a). However, the use of recycled polymers in WPCs can be challenging since the post-consumer plastics waste may contain several grades, colors, and contaminants, leading to varying outcomes when the plastics are combined with wood fibers (Najafi 2013). Sobczak et al. (2013) showed that the flexural and impact strength of WPCs increase along with the mass-average molecular mass of the polymer. The polymer matrix of WPCs does not necessarily consist of one type of polymer; Gao et al.
(2008) used a PE/PP-blend as a polymer matrix. Clemons (2010) investigated WPCs with varying HDPE:PP ratios and observed that if the ratio was changed from 75:25 to 25:75, the tensile yield stress of the WPC increased considerably whereas the opposite effect was observed for impact energies and yield strain.
There are other ways to optimize further the mechanical properties of WPCs, e.g., the incorporation of additives can help to overcome an incompatibility between the wood and the polymers. The use of MAPP or MAPE is a well-established approach to improve the durability of WPCs. Several studies have confirmed the effectiveness of MAPP and MAPE (Nourbakhsh and Ashori 2009, Pérez et al. 2012, Bhaskar et al.
2012, Ndiaye et al. 2013). In addition, new types of additives have been recently introduced. For example, Li et al. (2014) and Das et al. (2015a) used waste charcoal as an additive in WPCs and noted improvements in the tensile and flexural properties.
Abdolvahaba et al. (2014) improved the durability of WPCs by using nanoclay as a filler. Gwon et al. (2012) added three different types of inorganic fillers (kaolin, talc, and zinc-borate) to WPCs and observed that those WPCs containing kaolin or talc had higher mechanical strengths than WPCs with zinc- borate. The mechanical performance of WPCs with kaolin filler was the highest because kaolin has a staked plate shape, small particle size and a surface with highly hydrophilic characteristics.
To summarize, the mechanical properties of WPCs are highly dependent on the product formulation. The incorporation of additives, such as coupling agents, is usually required to produce WPCs with adequate mechanical properties. Thus, new potential additives providing higher mechanical strength are constantly being discovered and developed.
2.2.2 Water absorption
A well-known disadvantage resulting from the addition of wood fibers in plastics is the consequent susceptibility to water absorption (Adhikary et al. 2008b). Moisture penetrates into the composite materials by three different mechanisms. The first and the most common process is the diffusion of water molecules inside the microgaps between the polymer chains.
The second mechanism is capillary transport into the gaps and flaws at the interfaces between the fibers and polymers.
Moisture transport by microcracks formed during the processing is another mechanism. In general, water absorption on natural fiber reinforced composites follows the kinetics of a Fickian diffusion process. (Espert et al. 2004)
Wang et al. (2006) studied moisture absorption in natural fiber-plastic composites. They proposed that moisture absorption occurred via two mechanisms depending on the fiber content of the composite. At higher fiber loadings, when
the accessible fiber ratio was high and the material more homogeneous, the diffusion process was the dominant mechanism. At low fiber loading, percolation is the dominant mechanism; the fiber loading threshold for percolation is about 50 wt%. Percolation was applicable for nonhomogeneous materials and it takes into account the randomness of the composite structure.
As WPCs absorb water, not only do they become more vulnerable to the dimensional changes and microbial attack, but they also become mechanically weaker (Espert et al. 2004, Sombatsompop and Chaochanchaikul 2004, Tamrakar and Lopez-Anido 2011). Several efforts have been made to improve the water resistance and dimensional stability of WPCs. Some manufacturers have attempted to reduce water absorption of WPCs by the addition of zinc borate, which also improves the fungal resistance. On the other hand, along with the improvements in mechanical properties, the addition of MAPP or MAPE also reduces moisture absorption (Adhikary et al.
2008a, Najafi et al. 2010). Li et al. (2014) improved the water resistance of WPCs by incorporating biochar into the composite, but other additives have also been proven to decrease water absorption of WPCs (Lee and Kim 2009, Huuhilo et al. 2010, Turku et al. 2014).
An increase in wood fiber content or fiber size leads to a higher water absorption but like the mechanical durability, this property is also highly dependent on the fiber type and source (Yang et al. 2006, Migneault et al. 2008, Bouafif et al. 2009, Migneault et al. 2009, Ayrilmis et al. 2011, Butylina et al. 2011).
In addition, the characteristics of the polymer matrix exert a considerable impact on water absorption (Adhikary et al. 2008a, Najafi et al. 2010, Sobczak et al. 2013); in general, water absorption of WPCs with PP as the polymer matrix is higher than of those with PE (Najafi et al. 2007). However, the water absorption of WPCs is also dependent on the temperature of the water, i.e., by increasing the temperature, then one also increases the amount of water absorbed (Najafi et al. 2007).
Water absorption of WPCs can be reduced by modifying the wood fibers. For instance, Dányádi et al. (2010) showed that the benzylation of wood fibers resulted in decreased water absorption. The wood modifications conducted by Müller et al.
(2012) had similar effects. Hosseinaei et al. (2012) extracted hemicelluloses from wood fibers, which also resulted in lower water absorption of their WPCs. Wei et al. (2013) modified poplar wood fibers chemically by esterification and noted that the esterified fibers were more hydrophobic than the unmodified fibers. Consequently, the compatibility between wood fibers and the plastic matrix increased, leading to lower water absorption. Thermal modification of wood also results in a considerably lower water absorption of WPCs (Ayrilmis et al.
2011, Butylina et al. 2011).
2.2.3 VOC emissions
VOCs have a low boiling point and therefore, a high vapor pressure at room temperature, leading to the evaporation of a large number of molecules into the surrounding air. VOCs are abundantly present in nature, for example, they play an important role in the communication between plants (Ueda et al. 2012). However, some VOCs exert adverse effects on human health and may cause harm to the environment. Therefore, legislative efforts have been made to diminish the release of harmful VOCs from commercial products. The regulation of indoor VOC emissions aims to limit VOC emissions from commercial products into indoor air where concentrations are the highest. The major difficulty in the research of VOCs and their effects is that their concentrations are usually low and the symptoms and illnesses they evoke tend to develop very slowly.
The VOC emission characteristics of WPCs have not been extensively studied because these materials are generally used outdoors. Nevertheless, WPCs are increasingly being used indoors (Kim and Pal 2010). Therefore, their effects on indoor air quality are becoming more relevant. As WPCs consist mainly of wood and plastics, it is often presumed that their VOC emissions are dominated by these two major constituents.
However, WPCs have usually been processed at high temperatures, which may change the VOC emission characteristics of the material. Furthermore, the incorporation of additives exert effects on the VOCs.
Schwarzinger et al. (2008) conducted an elemental analysis of different WPCs by two-stage pyrolysis-GC/MS (gas chromatography-mass spectrometry). In addition to the identification of marker compounds for different wood types, they identified pyrolysis products from polymers (PE, PP, and PVC), which is important with respect to VOCs. Furthermore, their analysis of WPCs with various lignocelluloses provided further insights into the fundamental differences between WPCs with different reinforcements.
Félix et al. (2013) examined the release of VOCs from WPCs made from landfill-derived plastic and sawdust. Their findings were in accordance with those of Schwarzinger et al. (2008); the key markers for WPCs were phenols and aldehydes. In general, the profile of VOCs displayed alkanes, alkenes, phenols, aldehydes, aromatic hydrocarbons, terpenes, carboxylic acids, esters, nitrogen compounds, ketones, and alcohols. The most abundant VOCs in WPCs were furfural, �-pinene, 2-ethyl-1- hexanol, 2-methoxyphenol, N-methylphthalimide, butylated hydroxytoluene, 2,4-di-tert-butylphenol, and diethylphthalate.
In addition, Félix et al. (2013) demonstrated that the incorporation of additives increased the release of certain VOCs.
Another important finding was that WPCs have the potential to emit off-odor compounds that cannot be completely masked by odorizing agents. The identified off-odor compounds in WPCs included acetylfuran, hexanal, 4-vinylguaiacol, acetic acid, and 2-methoxyphenol.
One way to control VOC emissions from WPCs is to incorporate odorants into the composites to mask at least part of the off-odor compounds (Félix et al. 2013). Moreover, the addition of other types of additives capable of affecting the odor characteristics along with the other properties of WPCs could provide a simple solution. Another approach, proposed by Yeh et al. (2009), is to cover the composite with a thin layer of virgin
polymer in a co-extrusion process. This layer should be able to delay the release of VOCs and therefore, alter the odor profiles of WPCs.
2.3 MANUFACTURING TECHNOLOGIES
There are several alternative methods for manufacturing WPCs.
Compounding is a process in which filler and additives are added to the molten polymer. The compounded material can be formed into pellets or granules prior to future processing, or they can be immediately shaped into the final product (in-line processing) (Clemons et al. 2013). The processability of WPCs is similar to plastics, which is an advantage since WPCs are can typically be processed with the same machinery. Extruders are the most commonly used systems for WPC compounding. Hot- cold mixers are also used but mainly for the processing of PVC- based WPCs (Schwendemann 2008).
The product manufacturing technologies for WPCs include sheet or profile extrusion, injection molding, and compression molding (Stokke et al. 2014). Profile extrusion is the most commonly used manufacturing method for a WPC, and it is used to produce composites with a continuous profile of the desired shape (Gonçalves et al. 2014). WPC panels can be produced by sheet extrusion. Injection and compression molding produce non-continuous pieces with a more complicated shape.
2.3.1 Extrusion
Extrusion produces continuous linear profiles by forcing a melted WPC through a die. Different types of extruders and processing strategies have been used to produce WPCs. For example, some processors manufacture WPCs in one step, using twin-screw extruders whereas some prefer to adopt several extruders in tandem to compound and finally form the desired profile of the WPC. (Clemons et al. 2013)
A typical screw extruder consists of feeders, modular barrels, screws, a gearbox, a heating, and cooling unit (Figure 5), and a centralized control unit to adjust the extrusion speed, feeding rate, temperature, and other process parameters (Schwendemann 2008). The extruding screw system, consisting of screws and barrels, mixes, devolatilizes, and performs the reactions for multiple applications.
Figure 5. The basic structure of an extruder.
The screws mix the components in order to produce a homogeneous blending fluid in the barrel. The screws are usually made up of three zones: the feeding, melting, and melt pumping zone. In the feeding zone, the raw materials for WPC are usually solid, but when they move to the melting zone, most polymers have melted while fillers and additives remain in a solid state. The melt pumping zone forms a continuous fiber- polymer blend, which is finally pumped to the pelletizer after cooling or extruded through the die. (Stokke et al. 2014)
A typical barrel-length-to-diameter (L/D) ratio of a single- screw extruder varies from 20 to 30. The screw builds up high pressure in the composite melt so that it can be extruded through the die. Even though the single-screw extruders are less expensive than those with twin-screw systems, they suffer from a limited mixing and self-cleaning ability as well as from the selective material intake. The twin-screw extruders, whether co- rotating or counter-rotating according to the screw rotation
directions, are used for compounding, mixing or reactive polymer materials. L/D ratios for the twin-screw extruders vary from 39 to 48. The advantages of the twin-screw extruders include the self-cleaning and high mixing ability. However, unlike the single-screw extruders, these systems cannot develop a up high pressure in the melt pumping zone. In addition, the twin-screw extruders are more expensive. (Schwendemann 2008, Stokke et al. 2014)
The barrels are divided into the sections heated with the individual control units (Stokke et al. 2014). The temperature of the barrel gradually increases from the rear to the front which allows the material to melt gradually and to prevent thermal degradation or overheating. Sometimes the friction and high pressure in the barrel provide the required heat for the system, and the heaters can be turned off.
Multi-layered WPC structures are produced by coextrusion.
This process utilizes multiple extruders (single or twin-screw) to melt and deliver different types of materials to a single extrusion die that will extrude the materials in the desired form (Stokke et al. 2014). In addition to the reduced material and production costs, coextrusion makes the properties of final products highly controllable, which is a significant advantage over other production technologies.
2.3.2 Injection molding
Injection molding is used for producing large quantities of WPC pieces with complex geometries (Migneault et al. 2009). The molding of WPCs begins by inserting the pelletized raw material into the hopper, which feeds the material into the heated barrel with a reciprocating screw (Figure 6). The majority of the injection molding machines are equipped with single screws. The increased thermal energy reduces the viscosity of the material, allowing the screw to push the material forward.
The simultaneous mixing and homogenizing increase the friction and heat within the barrel. The material is collected at the front of the screw and then injected at high pressure and velocity into the mold. The volume of the material that is used
to fill the mold is known as a shot (Stokke et al. 2014). The high packing pressure completes the mold filling and compensates for thermal shrinkage. Once the cavity entrance solidifies, no more material can enter the cavity. Consequently, the screw reciprocates and receives the new material for the next cycle.
Meanwhile, the material inside the mold is cooled to the preset temperature and ejected from the mold. After the prepared piece is demolded by an array of pins, the mold closes and the process is repeated.
Figure 6. The structure of an injection molding apparatus.
Most injection-molded WPCs are produced from pelletized raw materials. However, in-line compounding is also possible. It is a combination of a two-stage injection unit with a co-rotating twin screw. Once the raw materials are fed into the co-rotating twin screw, they are compounded and transferred to a shooting pot. The shooting pot pushes the material via the machine nozzle and hot runner into the mold. (Schwendemann 2008) 2.3.3 Compression molding
Compression molding of WPCs is utilized especially in the automotive industry due to its capability to produce large and complex parts. Moreover, it wastes relatively little raw material, and therefore, it is one of the least expensive molding methods.
However, the product quality is not always consistent and it can
