Thermal Expansion-Contraction

The coefficient of linear expansion-contraction (CTE, for coefficient of thermal expansion) is a measure of the how much. In fact, the coefficient numerically describes a fraction of the board length that would be added to (expansion) or subtracted from (contraction) per 1°C temperature. If, for example, a 20-ft WPC board is elongated by 1/2 in. when the board surface temperature increased from 70 to 130°F, the coefficient of linear expansion is 0.57240760 deg = 3.47 X 10 1/deg. This, by the way, is in the neighborhood of a very typical value for expansion-contraction of WPC boards. 

Wait a minute, one would say—is not the 130°F a little bit too high a temperature, even on a hottest day  

it is not too high for some situations. Commonly, on a summer afternoon a deck surface temperature is higher than the air temperature. To be more specific, it 

For neat plastics, the CTE is about twice as much compared with WPC boards, which are about 50% filled with nonplastic materials, which is wood liber and sometimes minerals. As the coefficients of expansion-contraction of both wood fiber and minerals are about ten times lower than those for WPC materials, hence, the reduction in the coefficient s value for filled WPC. In reality, the picture is somewhat more complicated because it is the expansion-contraction of wood along the grain that is 10 times lower compared to common WPC. Expansion-contraction of wood across the grain is close to that of WPC. That is, an orientation of wood fiber in a WPC material can increase or decrease the coefficient of expansion-contraction. 

The longer the fiber (the higher is the fiber aspect ratio) and the more it is oriented longitudinally, along the deck board, the lower is the coefficient of thermal expansion-contraction. Overall, for different commercial WPC deck boards the coefficient is in the range of 2 X 10 to 5 X 10 1/°F. In other words, some commercial WPC boards can expand-contract by 250% higher than others. These overexpanded decks are very noticeable and sometimes cause complaints from the deck owners. 

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To minimize the stresses induced by differential thermal expansion/contraction one must (1) employ fastening techniques that allow relative movement between the component parts of the composite structure, (2) minimize the difference in coefficient of linear thermal expansion between the materials... 

Peak positions. Shifts in the positions of the peaks in an experimental powder XRD pattern may arise due to a number of instrumental factors. Furthermore, comparison of powder XRD patterns recorded at different temperatures may show differences in appearance (particularly in regions with significant peak overlap) as a result of anisotropic thermal expansion/contraction. This issue is particularly relevant when an experimental powder XRD pattern recorded at ambient temperature is compared with simulated powder XRD patterns for known crystal structures determined from single-crystal XRD data at low temperature. 

Overall, values of expansion-contraction of WPG boards are largely unpredictable and represent highly empirical values. To make composite deck boards with truly minimized coefficients of thermal expansion-contraction is a very challenging task, not resolved as yet in the industry. 

Compared to wood, polyethylene shows a higher coefficient of thermal expansion-contraction. A 20-ft. long unrestrained HDPE-based composite deck board expands or contacts (lengthwise) by 3/8" to one inch in the temperature range between 50 and 130°F, depending on the amount and type of fillers. Fasteners on a real deck often restrict that movement, though it is still typically larger than that of wood. 

Thermal expansion-contraction of plastics will be considered in detail in Chapter 10, Temperature-driven expansion-contraction of wood-plastic composites. Linear coefficient of thermal expansion-contraction. Here it can be briefly mentioned that this property is about the same with HDPE, polypropylene, PVC, ABS, and Nylons 6 and 6/6, and the respective coefficients of thermal expansion are all overlapping in the range of 2-7 X 10 1/°F (4-13 X 10 1/°C). Only with LDPE the coefficient is noticeably higher and equal to 6-12 X 10 1/°F (10-22 1/°C) [12]. 

In terms of thermal expansion-contraction, PVC is practically overlapping with other thermoplastics (HDPE, PP, ABS, Nylons) in the range of 2-7 X 10 l/° F (4-13 X 10 1/°C). This will be discussed in more detail in Chapter 10,... 

Temperature-driven expansion-contraction of wood-plastic composites. Linear coefficient of thermal expansion-contraction. ... 

Material Specific gravity (density) (g/cm ) Elexural properties Strength, psi Modulus, psi Compressive strength (psi) Water absorption after 24 h (%) Coefficient of thermal expansion-contraction, X 10-5 Weather stability... 

Thermal expansion-contraction of inorganic fillers is much lower compared with that of plastics. Therefore, the higher the filler content, the lower the coefficient of expansion-contraction of the composite material (see Chapter 10). Many inorganic nonmetallic fillers decrease thermal conductivity of the composite material. For example, compared with thermal conductivity of aluminum (204 W/deg Km) to that of talc is of 0.02, titanium dioxide of 0.065, glass fiber of 1, and calcium carbonate of 2-3. Therefore, nonmetallic mineral fillers are rather thermal insulators than thermal conductors. This property of the fillers effects flowability of filled plastics and plastic-based composite materials in the extruder. 

Effect of talc on the coefficient of linear thermal expansion-contraction was not pronounced (Table 4.10) and was actually superimposed with that of wood flour. In other words, the principal effect was just a displacement of plastic with a filler, regardless whether it was wood flour or talc. It certainly makes sense because both wood and mineral fillers have their own coefficients of thermal expansion-contraction by an order of magnitude lower than that of HDPE (see Chapter 10). 

TABLE 4.10 Effect of talc (Mistrofll P403, median particie size of 7.8 (im) on the coefficient of iinear thermal expansion-contraction (CTE) of an HDPE (MFI 0.4)-based composite material containing a 40-mesh pine wood flour (up to 55% content w/w) and the talc [8]... 

Under the microscope, Biodac appears as small rocks attached to cellulose fibers and bundled into granules. Because of short cellulose fibers on the surface of Biodac , the filler provides a mechanical adhesion (interlocking) with the plastic matrix without any surface treatment. The thermal expansion-contraction coefficient of Biodac appears to be much closer to that of plastics, compared to the coefficients of common mineral fillers. Hence, the bond between Biodac and the plastic after weathering of the composite material becomes loose at lesser extent compared to inorganic fillers. 

TEMPERATURE DRIVEN EXPANSION-CONTRACTION OF COMPOSITE DECK BOARDS LINEAR COEFFICIENT OF THERMAL EXPANSION-CONTRACTION... 

TABLE 10.2 Linear coefficients of thermal expansion-contraction of plastics, determined in accord with the procedure of ASTM D 696... 

TABLE 10.5 Linear coefficient of thermal expansion-contraction for WPCs, determined in accord with ASTM D 6341. Data were obtained by Dr. Tatyana Samoylova, LDI Composites... 

TABLE 10.8 The linear coefficient of thermal expansion-contraction for extruded WPCs, filled with rice hulls (20-80 mesh size) or 40 mesh sawdust, determined in accordance with ASTM D 6341. Biodac (granular porous filler having 50% cellulose fiber and 50% mineral, see Chapter 4) was 28% in all the cases. MFI for the HDPE was 0.3. Data by Dr. Tatyana Samoylova, LDI Composites... 

Linear coefficient of thermal expansion-contraction for wood is significantly lower compared to those for plastics and WPCs. It appears to be independent of species (hardwoods and softwoods) and specific gravity, and equal (along the grain) to... 

TABLE 10.9 The linear coefficient of thermal expansion-contraction for LDPE and HDPE filled with cellulose fiber (wastepaper). The published article [2] indicated only the absolute expansion values (in mm) per 1°C, without providing the size (length) of the samples. It was assumed that all the samples were of 60 mm in length, and the data were recalculated to CTE (1/°F)... 

TABLE 10.10 The linear coefficient of thermal expansion-contraction for HDPE-based WPC. The published article [4] did not provide details on the formulation of the composite material. MAPP — maleic anhydride derivative of polypropylene (see Chapter 5). 

Dry wood again, as with the shrinkage (see the preceding chapter), is a superior material compared to plastic and WPCs with respect to thermal expansion-contraction. The trouble is that in the real world wood is almost never dry, hence, snbject to shrinkage, microbial degradation, fading, and so on. 

Tensile strength, 62 thermal expansion-contraction, 62 tonghness, 61 water absorption, 62 Ahsolnte temperature, 634 Absorption, 612 Abstraction of hydrogen, 497 Acceptance Criteria 174 (AC 174), 14, 225, 236, 238, 242, 253, 259-261, 280, 303, 305-307... 

Bonds with plastics, 142 Calcium carbonate in, 112, 141 Cellulose in, 112 Chemical composition, 112 Cost of, 112 Granules, 141 Ingredients, 141 Kaolin clay in, 112 Mold shrinkage, effect on, 142 Oil absorption, 141 Porosity, 141 Shape of particles, 142 Speciflc gravity, 142 Tensile modulus, effect on, 142, 143 Thermal expansion-contraction coefficient, 142 Biodegradable plastics, 79 Biodegradable wood-plastic composites, 91 Bioresistance, 42 Biotite, 146 Black Algae, 426 Black mold, 29, 31, 424, 429 Black panel temperature, 41, 132 Black panel thermometer, 612 Black panel, 41, 132 Bleached cellulose, 11, 14, 180 Cost, 14... 

Lignosulfonate-based resin, 80 Limiting viscosity, 618, 619, 631 Linear coefficient of thermal expansion-contraction, 134, 356, 356-368 Across the grain, 365, 367 Along the grain, 362, 365-367 ASTM tests, 359 Calcium carbonate, 362 Calculation, 357 GeoDeck, 362 Fasalex, 363 Formula, 357... 

Coefficient of friction, 211 Coefficient of thermal expansion-contraction, 52 Crystallinity, 51 Degree of crystallinity, 53 Density, 54 Flexural strength, 54 Glass transition point, 51 Grades, 54... 

Specific gravity, 59 Syndiotactic, 59 Tensile modulus, 72 Tensile strength, 72 Thermal degradation, 60 Thermal expansion-contraction, 60 Thermal stability, 59 Water absorption, 60 Zig-zag stereoconfiguration, 58 Pooling of boards, 24 Poor durability, 61 Poor melt flowability, 617 Poor weatherability, 61 Porosity aid, 87... 

Talc, 14, 28, 88, 90, 98, 123, 125, 128-133, 137-139 Abrasion, 139 Chemical composition, 137 Chemical formula, 137 Coefficient of thermal expansion-contraction, effect om, 137 Cost, 14... 

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