1 Introduction
2.3 Manufacturing technologies
2.3.4 Choosing appropriate manufacturing method
Processing methods have significant effects on the properties of WPCs. For instance, Migneault et al. (2009) observed that injection-molded WPCs have better physical and mechanical properties and reduced water absorption than extruded WPCs.
However, the extruded WPCs had higher densities. These investigators also discovered that wood fibers in injection-molded WPCs were aligned in the main flow direction whereas the fibers in extruded samples were more randomly oriented.
This resulted in a better stress transfer between wood fibers and the polymer matrix in the injection-molded samples.
Bledzki et al. (2005a) compared three different compounding processes (two-roll mill, high-speed mixer, and twin-screw extruder) and claimed that the composites compounded by extrusion had the best mechanical strength and lowest water
absorption. Yeh and Gupta (2008) observed that a change in the extrusion parameters did not exert any significant effect on the mechanical properties of WPCs, but it did influence the water absorption behavior. Namely, the longer residence time and higher screw speed resulted in a lower rate of water absorption as well as lower density. They speculated that the lower density and therefore, the lower rate of water absorption was caused by the loss of hydrophilic compounds in WPCs. Goncalves et al.
(2014) revealed that the design of the extrusion die had a considerable effect on the properties of a WPC deck. They used a computer system to design a die and then simulated experimental conditions, and the comparison between numerical and experimental results achieved good qualitative agreement.
Even though there may be considerable differences in the properties of WPCs produced with different manufacturing methods, it is also important to assess the strengths and limitations of the manufacturing processes in a production point of view. Table 1 presents a simple comparison between extrusion, injection molding and compression molding.
Table 1. A comparison between extrusion, injection molding, and compression molding.
Parameter Extrusion Injection molding
Compression molding
Setup costs Moderate High Low
Production costs Low Low Moderate
Production speed Moderate High Low
Product consistency High High Low
Product geometry Limited to parts of a
fixed cross section Complex Limited to flat or curved parts
plastic composites
Material testing is usually the last step in the manufacturing process, and its purpose is to ensure that the material meets the requirements of its applications. WPCs are commonly used in applications that require adequate mechanical strength and resistance to water absorption. The physical and mechanical properties of WPC products are typically evaluated with standard laboratory tests. In general, WPCs and plastics are tested with similar procedures because they are typically manufactured with the same technologies.
The British standard BS EN 15534-1 was validated as a standard in the EU in 2014. This standard specifies the procedures for the determination of physical and mechanical properties of WPCs. In addition, the test methods for durability, such as weathering and natural ageing, and thermal properties are defined.
The characterization of WPCs can also be carried out according to ISO (International Organization for Standardization) and ASTM (America Society for Testing and Materials) standards. ASTM standards are widely applied in the US whereas ISO standards are typically used in Europe.
3.1 MECHANICAL PROPERTIES
The mechanical properties of WPCs are characterized according to the standards originally developed for plastics since in most cases, these standards are suitable for WPCs. However, it is possible that the testing of WPCs according to these standards does not provide valid results. For example, the effect of the size
dimensions specified in the ISO and ASTM standards. In other words, it is possible that the stress required to fracture the WPC sample is relatively lower at smaller sample dimensions than it would be at the dimensions of the final applications because wood fibers are larger in relation to the cross-sectional area of the sample and thus undergo fractures more easily.
According to BS EN 15534-1, WPC samples should be conditioned in a standard atmosphere at 23 ± 2 °C and relative humidity (RH) of 50 ± 10% before mechanical testing. An atmosphere of 20 °C and 65% RH may also be used but those conditions should be declared.
When determining flexural, tensile, and impact strength of WPCs, the preferred test specimen according to ISO standards should be 80 ± 2 mm in length, 10.0 ± 0.2 mm in width, and 4.0 ± 0.2 mm in thickness. The corresponding specimen dimensions for WPCs in ASTM standards are 125 mm × 12.7 mm
× 3.2 mm. In total, a set of 10 specimens should be tested unless the coefficient of variation has a value less than 5%. In that case, a minimum number of five specimens may be sufficient.
3.1.1 Tensile strength
Tensile strength is the measure of the maximum amount of tensile stress that a material can withstand while being stretched before breaking. It is defined as a stress and expressed as force per unit area. The most important parameters for tensile testing include testing speed, force capacity, precision, and accuracy.
The testing speed is expressed as mm/min, and according to ISO 527-1, it can vary between 0.125–500 mm/min depending on the sample type.
At the start of the measurement, the machine slowly extends the sample until it breaks. The elongation of the sample is measured against the applied force. With the measured elongation, it is possible to calculate the strain, �, using the following equation:
� = L-LL 0
0 , (3.1)
whereL is the final length of the gauge andL0 is the initial gauge length. The applied force is used to calculate the stress,�, using the following equation:
� = AF , (3.2)
whereF is the tensile force (N) andA is the cross-sectional area of the sample. The relationship between the stress and strain of a material can be displayed on a stress-strain curve (Figure 8).
The curve also provides the fracture strength of a material, which is the final recorded point.
Figure 8. A typical stress-strain curve for WPCs.
Tensile properties of WPCs can be determined according to ISO 527-1 and ASTM D638-14 standards. Although the testing procedures presented in these standards are rather similar, there are some differences that can significantly influence the results obtained. In ISO 527-1, it is stated that the specimens must not be pre-stressed considerably prior to testing.
Pre-stresses can be induced when the specimen is centered in the grips or when the clamping pressure is applied. The maximum allowable pre-stress must be less than 1% of the measured stress results. The induced strain value must be less than 0.05%, accordingly. ASTM D638-14 does not contain these specifications, and therefore, it lacks the defined status of the stress after placing the specimen in the grips. Thus, the corrected strain zero point is defined as the point where the linear slope of the stress-strain curve crosses the strain axis. This correction can exert a significant effect on the measured tensile modulus. In ISO 527-1, the point where the tensile modulus is measured is precisely defined whereas the definition of tensile modulus in ASTM D638-14 is based on the corrected strain zero point. If the material lacks the linear region in the stress-strain curve, the modulus is determined from a secant modulus that is determined between the corrected strain zero point and a freely selected point on the curve. Consequently, this may result in significant variations between the results carried out according to ISO or ASTM standard.
Another difference between these standards is related to the test speed used in determining tensile modulus. In ISO 527-1, the tensile modulus is measured with the lower test speeds than tensile strength, but the same test speeds are allowed to be used throughout the test in ASTM D638-14. There are also differences in the requirements for extensometers; ISO 527-1 allows the lower measurement uncertainty than ASTM D638-14.
3.1.2 Flexural strength and modulus
The material’s ability to resist deformation under load is defined as the flexural strength, which is typically measured using a three- or four-point flexural test technique. During the test, the sample experiences many kinds of stresses throughout its depth.
At the outside of the bend, the stress is tensile in its nature whereas at the load-bearing side, the sample experiences compressive stress (Sain and Pervaiz 2008). Usually most of the materials fail under tensile stress rather than compressive stress, meaning that the maximum tensile stress value that the material
can withstand before breaking is its flexural strength.
Mathematically, the formula to calculate the maximum surface stress,S, for a rectangular sample in the three-point bending test is expressed as:
S = 2bd3FLs
s2 , (3.3)
where F is the bending load (N) at the given point, Ls is the length of span (mm),b is the width of the sample (mm), andds is the thickness of the sample (mm). In ISO 178, Ls is recommended to be 64 mm.
Flexural modulus,E, is the ratio of stress to strain within the elastic region. It is computed from the slope of a stress-strain curve (Figure 8) obtained from the flexural strength test. The flexural modulus for the three-point test of a rectangular sample can be expressed as:
E = 4bhL3sF3d , (3.4) whereh is the height of the sample (mm) andd is the deflection (mm).
The test parameters for flexural testing are defined differently in ASTM D790-15 and ISO 178 since the dimensions of the specimen are also different. In addition, the point where the test is stopped is not the same. In ASTM D790-15, the test is stopped when a 5% deflection is reached or if the specimen breaks this value. In ISO 178, the test continues until the specimen breaks. If the specimen does not break, the stress at 3.5% strain is reported. Consequently, these standards provide different results if the specimen strain is higher than 3.5%.
3.1.3 Impact strength
Impact strength can be determined using either the Charpy or Izod impact test. Although the principle of these tests is similar, there are some differences in the testing procedures. In Charpy impact test, a specimen with dimensions of 4.1 mm × 10.1 mm ×
A pendulum strikes the middle of the specimen that may be unnotched or have a U-notch or V-notch. In the Izod impact test, the dimensions of the specimen are 4.1 mm × 10.1 mm × 75 mm.
The specimen is placed vertically on the support, and it can be unnotched or have only a V-notch. In the Izod test, the notch is facing the pendulum whereas in the Charpy test the notch side of the specimen faces away from the pendulum.
The Charpy impact test determines the amount of energy absorbed by a material during the fracture. The measurement apparatus consists of a pendulum of a known mass and length.
The pendulum is dropped from a known height so that it strikes the specimen. The amount of energy absorbed by the material at the impact can be determined by comparing the heights of the hammer before and after the fracture. The energy absorbed in the breaking is expressed as impact energy (J/m). It is calculated by dividing the energy by the thickness of the sample. In ISO 179-1, the Charpy’s impact strength is reported in kJ/m2, which is derived by dividing the impact energy by the area under the notch. In ASTM D6110-10, the results are reported as J/m.
Notching has a considerable effect on the results of the impact test. Thus, the exact geometries and dimensions of notches have been determined in ISO 179-1. In addition, the size of the sample can affect the results.
The differences between ISO and ASTM impact tests are related to the type of pendulum hammer used in the tests. In ISO 179-1, the pendulum hammer may be used in the range from 10 to 80% of its nominal potential energy whereas the maximum value in ASTM D6110-10 is 85%. In addition, according to ISO 179-1, the largest possible hammer must be used because the speed loss at the impact must be kept as low as possible. ASTM D6110-10 defines that the standard pendulum hammer has a rated initial potential energy of 2.7 J, and the hammer size is increased by doubling its dimensions.
However, unlike ISO 179-1, the smallest hammer in the range has to be used in the tests.
3.2 WATER ABSORPTION
Water absorption of WPCs is typically evaluated using the standard methods developed for plastics, wood, and WPCs.
However, the time to reach the moisture equilibrium is longer for WPCs than for plastics or wood (Defoirdt et al. 2010); wood reaches an equilibrium in hours or weeks, plastics in weeks and WPCs in months. Therefore, the testing methods used for WPCs may not allow the material to reach the moisture equilibrium and this may affect the results obtained. This, however, does not mean that the test methods described in these standards could not be used to investigate the differences between various WPC types; considerable differences in the water resistance can be observed even with only two hours of water immersion.
Guidelines for the determination of water absorption of WPCs are given in BS EN 15534-1. ISO 62 can also be used because it applies to the reinforced plastics, to which WPCs belong. Similarly, ASTM D570-98(2010)E1 can also be used to determine the moisture absorption of WPCs. According to the standards, the water absorption of the material can be determined by completely immersing the samples in water at 23 °C or in boiling water. Before the immersions, the samples must be dried in an oven at 50 ± 2 °C for at least 24 h and then cooled to room temperature in a desiccator before weighing.
After the immersion, water absorption,c (%), of the materials can be determined by using the following formula:
c = m2m-m1
1 � 100%, (3.5)
wherem1 is the mass of the test specimen after initial drying and before the immersion, andm2 is the mass of the test specimen after the immersion.
There are some differences in these standards. In BS EN 15534-1, the immersion period in the water bath (23 °C) is 28 ± 1 days whereas the immersion period in ISO 62 should be at least 24 hours. In addition, the immersion time in boiling test is 5 h ± 10 min in BS EN 15534-1, but in ISO 62 the immersion
should last for at least 30 ± 2 min. ASTM D1037-12 specifies two methods to determine water absorption of WPCs. In method A, the WPCs are first immersed for two hours in fresh water (20 ± 1 °C) and then weighed. After weighing, the sample is submerged in water for an additional 22 hours and then weighed again. Method B is similar to the 24-hour immersion procedure described in ISO 62.
3.3 VOC EMISSIONS
There are several methods available for the determination of VOC emissions from solid products. If the product is primarily intended for indoors use, the emissions are commonly determined according to ISO 16000-6. In this standard, the emissions are analyzed by thermal desorption/gas chromatography with flame ionization detector and mass spectrometry (TD-GC-FID/MS) using a Tenax TA® absorbent tube as a collector for VOCs.
Proton-transfer-reaction mass-spectroscopy (PTR-MS) is another way to determine and compare VOC emissions between different samples. This is an online monitoring technique using gas phase hydronium ions as the ion source reagents. This technique is used in food science, medicine, and biological and environmental research. (Schripp et al. 2014) 3.3.1 TD-GC-FID/MS
The guidelines for determination of VOC emissions from building products using emissions test chamber system are given in ISO 16000-9. In this procedure, the air flow transfers the emitted compounds from the chamber to a Tenax TA®absorbent tube. After reaching the tube, the compounds of interest are adsorbed onto the surface of the material. When the compounds are being analyzed, the tube is heated to a temperature over 250–300 °C and the adsorbed compounds are released into the flow of carrier gas that transfers the compounds to the GC-MS.
The measurements are conducted under controlled conditions as defined in the standard. This states that the products should be tested at a temperature of 23 ± 2 °C and RH of 50 ± 5%, with an air velocity in the range 0.1–0.3 m/s. In addition, the temperature should not vary by more than ± 1.0 °C during the measurements, and RH and air flow rate can fluctuate by only ± 3%. Before the measurements, the test chamber must be cleaned with alkaline detergents and then rinsed twice with distilled water. In addition, cleaning by thermal desorption is also allowed. To eliminate the possible effects of background emissions, an air sample of the empty emission chamber is taken before the actual measurements.
When the sample is placed in the chamber, it should be positioned in the center of the chamber to ensure that the air flow is evenly distributed over the emitting surface. The measurements should be carried out at predefined sampling times that depend on the objective of the test. However, duplicate air samples should be taken at least at 72 ± 2 h and 28 ± 2 days after the start of the test.
For TD-GC-FID/MS, the analysis of VOCs is optimal for the range of VOCs eluting between and including n-hexane and n-hexadecane (Woolfenden 2009). However, when the tube is heated, the adsorbed compounds are released slowly from the tube. This may lead to low sensitivity and wide chromatographic peaks. In addition, this system is not capable of measuring the emissions of certain VOCs, such methane and formaldehyde. Modern TD-GC-MS systems avoid wide peaks by using cold traps to focus the samples before they reach the column, but the properties of column also affect the wideness of the peaks. Overall, the sensitivity of TD-GC-MS is highly dependent on the absorbent material, system parameters and the amount of the sample.
3.3.2 PTR-MS
PTR-MS is an easy-to-use online VOC monitoring system with high sensitivity and rapid time response. In PTR-MS, the VOC trace gases in the sampled air are ionized in proton-transfer
reactions using hydronium (H3O+) as the primary reagent ion (Lindinger et al. 1998, Lindinger et al. 2001). The concentrations of the product ion and the reagent are then measured in a mass spectrometer. The system consists of an ion source (hollow cathode), a drift tube reactor (reaction chamber) and a mass spectrometer. In the ion source, a hollow cathode discharge in water vapor produces H3O+ ions (de Gouw and Warneke 2007).
These ions are then injected into the drift tube where VOCs are ionized with H3O+ ions according to the following reaction:
H3O+ + R� RH+ + H2O (3.6) Next, a homogeneous electric field in the drift tube transports the reagent and the product ions into the second intermediate chamber. Most of the air is pumped away and a small fraction of ions is extracted for analysis in the mass spectrometer. The concentration of trace gas [RH+] is computed from the following equation:
[RH+] = [H3O+]0(1 – e-k[r]�t) � [H3O+]k[R]�t (3.7) The approximation of the equation is made assuming that [R] is small and, therefore, [H3O+] is equal to [H3O+]0. The rate coefficientk for the proton-transfer-reaction and reaction timet are predefined parameters, and the fraction of [RH+] and [H3O+] is obtained from the mass spectrometry.
The sensitivity of PTR-MS can be further improved by combining the PTR ion source with a time-of-flight mass spectrometer (TOF-MS). In this arrangement, the system can separate most atmospherically relevant protonated isobaric VOCs and identify their corresponding empirical formulas (Müller et al. 2010, Schripp et al. 2014). However, the maximum measurable concentration of PTR-MS is limited to approximately 10 ppmv (parts per million by volume). If the total concentration of VOCs is too high, the concentration calculation will be incorrect. In addition, the identification of
The sensitivity of PTR-MS can be further improved by combining the PTR ion source with a time-of-flight mass spectrometer (TOF-MS). In this arrangement, the system can separate most atmospherically relevant protonated isobaric VOCs and identify their corresponding empirical formulas (Müller et al. 2010, Schripp et al. 2014). However, the maximum measurable concentration of PTR-MS is limited to approximately 10 ppmv (parts per million by volume). If the total concentration of VOCs is too high, the concentration calculation will be incorrect. In addition, the identification of