Properties of WPC Products

There currently are no manufacturing standards for WPCs and the standard test methods for evaluating the mechanical and physical properties of WPC products are under development by the American Society for Testing and Materials (ASTM), committee D7 on Wood. The lack of manufacturing standards combined... 

Generally, there is a certain correlation between density, on the one hand, and flexural strength and modulus, on the other, for many other materials, and that correlation is not related to porosity. For example, there is a strong correlation (R = 0.984) between density of all 38 polyethylene materials, listed in Table 7.49 of Chapter 7, including LDPE, LLDPE, HDPE, and their flexural modulus (Figure 6.1). Besides, mineral fillers in WPC materials increase density of the final product and also increase its flexural modulus. However, this chapter is mainly concerned about relationships between density and properties of WPC having the same formulation but produced at different regimes. 

It should be mentioned that the relationships between average molecular weights, MWD, and the power-law index of the respective polymer melts are not clear and completely unexplored in case of wood-filled composites. For example, increasing viscosity does not always improve physical properties of products. It was found that the increase of MFI of polypropylene from 3 to 30 g/10 min did not alter the efficiency of wood fiber dispersion and did not result in an improvement of any measured property of WPC. On the contrary, a change of MFI for HDPE from 0.15 to 7.0 led to better wetting of wood fiber and superior mechanical properties of the WPC. 

Generally, WPC products have strength and stififiiess properties that are somewhere between both materials (1,2). They are stiffer than neat plastics. Nevertheless, composites based on commodity plastics [e.g., polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinylchloride (PVC)] and wood-fibers do not offer mechanical performance similar to that of solid wood (1). For example, the flexural strength of WPCs made with commodity plastics are about two to three times lower than that of natural pine (softwood) or oak (hardwood). While, flexural modulus of WPC products is about one-half that of natural pine or oak (1). This lowered stiffness implies that, for the same load, a deck constructed with WPC products will bend more than a similar wood deck. 

Water absorption of WPC was little affected by ionic strength. Fleming et al. (18] examined the effect of 5% NaCI on the water absorption of soy and sunflower flours, concentrates, and isolates. The water absorption of the soy and sunflower flours was higher in 5% NaCI than in water. Generally, salt decreased water absorption of the isolates the concentrates of both plant products were little affected by NaCI. Data reported by Fleming et al. reflect a response to NaCI however, the type of ion is known to affect the type of response of other properties (33] and possibly the same is true for water absorption. 

Whey protein concentrates (WPC), which are relatively new forms of milk protein products available for emulsification uses, have also been studied (4,28,29). WPC products prepared by gel filtration, ultrafiltration, metaphosphate precipitation and carboxymethyl cellulose precipitation all exhibited inferior emulsification properties compared to caseinate, both in model systems and in a simulated whipped topping formulation (2. However, additional work is proceeding on this topic and it is expected that WPC will be found to be capable of providing reasonable functionality in the emulsification area, especially if proper processing conditions are followed to minimize protein denaturation during their production. Such adverse effects on the functionality of WPC are undoubtedly due to their Irreversible interaction during heating processes which impair their ability to dissociate and unfold at the emulsion interface in order to function as an emulsifier (22). 

It is essential to consider the physico-chemical properties of each WPC and casein product in order to effectively evaluate their emulsification properties. Otherwise, results merely indicate the previous processing conditions rather than the inherent functional properties for these various products. Those processing treatments that promote protein denaturatlon, protein-protein Interaction via disulfide interchange, enzymatic modification and other basic alterations in the physico-chemical properties of the proteins will often result in protein products with unsatisfactory emulsification properties, since they would lack the ability to unfold at the emulsion interface and thus would be unable to function. It is recommended that those factors normally considered for production of protein products to be used in foam formation and foam stabilization be considered also, since both phenomena possess similar physico-chemical and functionality requirements (30,31). 

Concentration of whev proteins. As mentioned earlier, microfiltration can be used to remove bacterias. In addition, they are capable of separating phospholipids, fats and casein fines of sweet whey or sour (acidified) whey. Ultrafiltration of whey has been well proven to provide an array of protein products of diverse compositions and properties. Inorganic membrane filtration can be used at different stages of the process to make whey protein concentrates (WPC) in powder form with a protein content reaching 50%. 

Let us take a look at a generic wood-plastic composite (WPC) deck, preferably of a premium quality. What should be done in order to avoid the deck owner complaints and, god forbid, a lawsuit Which properties of the deck should we consider, in order to extend its lifetime as much as possible, preferably longer than that of a common pressure-treated lumber deck In other words, what is required to make a material that is both durable enough to meet the warranty guidelines and at the same time cost-efflcitive to be competitive in the marketplace What can happen to the WPC deck in use, and how to prevent it Which properties of the composite material should we aim at, what should we study in that regard, what shonld we test and how, what shonld we optimize in order to make a premium product, or, at least—for a less ambitious manufacturer—to pass the building code ... 

This mathematical exercise was aimed at optimizing a shape of WPC board shown in Figure 7.1. The principal goal was to minimize the cross-sectional area in such a way that the product would by reduced by at least 10% by weight, at the same time retaining (or improving) its mechanical properties exemplified by flexural strength and modulus, and impact resistance. 

The use of ion-exchange resins (Figure 4.41) offers an effective method for the preparation of high-quality whey protein products, referred to as whey protein isolates (WPI), containing 90-95% protein (see Marshall, 1982 Mulvihill, 1992). Although the functional properties of WPI are superior to those of WPCs on an equiprotein basis (due to lower levels of lipids, lactose and salts), their production is rather limited, due to higher production costs. 

By far, wood particles are the major raw material source used for manufacturing WPCs. Wood particles can originate from sawdust, planer shavings, short solid pieces of lumber, conventional wood composite scrap [6], and scrap pallets [12], Both softwoods and hardwoods can be used for WPC production. Currently, most WPCs using softwoods are made with southern yellow pine, while WPCs produced with hardwoods are made with oak, maple, or aspen. The anatomical features as well as physical, mechanical, and chemical properties of softwoods and hardwoods differ considerably among species, and may affect the wood-polymer interface, and, as a consequence, the composite s properties and performance. 

The mechanical properties of Wood-Plastic-Composites substantially depend on the quality of the raw material, the composite and the manufacturing process. The density of WPC lies between 0.9 and 1.4 g cm and is higher than that of one of the two components, plastic and wood, alone. The porous wood structure is densified during the production process and plastics and additives partly penetrate into the cavities. Thus, water absorption is made harder, and the material swells less and more slowly. ... 

Shebani et al. [20] noted that removing extractives improved the thermal stability of different wood species. Therefore, using extracted wood for the production of wood-plastic composite (WPCs) would improve the thermal stability of WPCs. Because wood and other bio-fibres easily undergo thermal degradation beyond 200°C, thermoplastic matrix used in the composites is mainly limited to low-melting-temperature commodity thermoplastics like polyethylene (PE) and polypropylene (PP). However, the inherently unfavourable thermomechanical and creep properties of the polyolefin matrix limit some structural applications of the materials. 

The compounding process is very important in the manufacture of WPCs. The performance of a compound has big influence on the properties or productivity of the WPC. So, in this section, not only the technique but also the purpose and some points of attention are included in the discussion of a compounding process. 

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