Unreinforced Plastics

We will start with a consideration of a plastic that has excellent resistance to gamma radiation relating to its physical properties, namely, polyimide. 

---

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

In this book no prior knowledge of plastics is assumed. Chapter 1 provides a brief introduction to the structure of plastics and it provides an insight to the way in which their unique structure affects their performance. There is a resume of the main types of plastics which are available. Chapter 2 deals with the mechanical properties of unreinforced and reinforced plastics under the general heading of deformation. The time dependent behaviour of the materials is introduced and simple design procedures are illustrated. Chapter 3 continues the discussion on properties but concentrates on fracture as caused by creep, fatigue and impact. The concepts of fracture mechanics are also introduced for reinforced and unreinforced plastics. 

Suppose we change our attention from structures in which the driver is functional consideration alone to something like an automobile where cost is also extremely important. We can get the functional job done with other materials, like steel and aluminum and fiberglass in certain places and unreinforced plastic in others. Then, the question becomes can we make a material substitution that will enable us to compete with the cost of these other materials to do a job that with all the other materials we cannot accomplish That is a different kind of question, and then cost becomes an extremely important driver. And, as cost of advanced composite structures goes down, we can expect to see more and more utilization of advanced composite materials. 

Reinforced plastic In common with metals and unreinforced plastics, RPs also is susceptible to fatigue. However, they provide high performance when compared to un-... 

Once a process is being used, recognize that if the compound additives or fillers are changed or added, bond performance can change or even not exist. As an example an unreinforced plastic can be welded to itself however with a certain amount of glass fiber fillers (they do not melt) added to the plastic, action in weld strength can be reduced or even eliminated. 

When compared to unreinforced plastics, the analysis and design of RPs is simpler in some respects and perhaps more complicated in others. Simplifications are possible since the stress-strain behavior of RPs is frequently... 

To meet fabricated dimensional tolerances different approaches are used. They include use of specific fillers and reinforcements and process control (Chapter 3). Popular filler used is short glass fibers (Chapter 15). Over 50wt% of all types of glass fibers used with different plastics and by different processes are used in injection molding compounds. Table 2.2 shows the shrinkage of different unreinforced plastics ad glass fiber reinforced plastics based on ASTM testing procedures. 

Polymer matrix composites (PMCs), or fiber-reinforced plastics (FRPs). provide a wide range of properties and behavior. Materials with discontinuous fibers are slightly stiffer than conventional unreinforced plastics, whereas the fully aligned continuous fiber systems can record exceptionally high specific properties (property divided by density), exeeeding those of competing materials such as steel and aluminum. There are a virtually infinite number of materials, and material formats that can be combined to form a composite material, as shown in Table 1. 

Following the success of ISO 10,350 [34] on comparable data for materials selection for plastics, it was agreed in 1996 to prepare two parts. Part 1 covers unreinforced plastics and plastics reinforced by fibers shorter than 7.5 mm. and Part 2 covers reinforced plastics w ith fibers greater than 7.5 mm. These database standards quote the appropriate international standard, normally ISO as discussed above. The original standard has been adopted widely leading to a greater use of the referenced ISO standards. The wide range of material properties found in composite materials requires that the correct method (e.g.. Part 4 or 5 of ISO... 

Like unreinforced plastics, reinforced plastics are also susceptible to fatigue. There is, however, no general rule concerning whether glass reinforcement enhances the fatigue endurance of the base material. In some cases the unreinforced plastic exhibits greater fatigue endurance than the reinforced material in other cases, the converse is true. 

Composites may be identified and classified many hundreds of ways. There are aggregate-cement matrix (concrete), aluminum film-plastic matrix, asbestos fiber-concrete matrix, carbon-carbon matrix, carbon fiber-carbon matrix, cellulose fiber-lignin/silicic matrix, ceramic fiber-matrix ceramic (CMC), ceramic fiber-metal matrix, ceramic-metal matrix (cermet), concrete-plastic matrix, fibrous-ceramic matrix, fibrous-metal matrix, fibrous-plastic matrix, flexible reinforced plastic, glass ceramic-amorphous glass matrix, laminar-layers of different metals, laminar-layer of glass-plastic (safety glass), laminar-layer of reinforced plastic, laminar-layers of unreinforced plastic. 

Both RTSs and RTFs can be characterized as high performance engineering plastics, competing with engineering unreinforced plastics. When comparing processability of RTSs and RTFs, the RTFs are usually easier to process and permit faster molding cycles. 

Such non-rectangular sections are common in many RP or unreinforced plastic components. Channels, T-sections, and hollow corner pillars are found in crates and stacking containers, and inverted U-sections and cantilevers that are common in parts such as street lamp housings to aircraft structural parts. 

There are different plastic matrixes (also called resin matrixes) used in RPs that provide different properties and processing procedures in fabricated products. Plastics are a family of materials such as ceramics and metals. Within this family of plastics about 30 wt% of them are RPs the others are referred to as unreinforced plastics (URPs). Factually, the quantity of RPs could be debatable since it depends on defining when a plastic is an URP they can include additives that can be used not as a reinforcement. As shown in Figure 3.1 the amount of resin in RPs influences properties. 

One-piece RP components replace multiple parts previously made both fi-om RPs and from metals or other materials including unreinforced plastics. The motivations for the design and the challenges for fabricating... 

The gradual growth of the total USA plastic [RP and URP (unreinforced plastic)] industry for over a century has been spectacular evolving into today s routine to sophisticated high performance products (parts). Examples of these products are in building and construction (34 wt%), transportation (33%), sports and appliances (14%), electrical and electronic (10%), and others (9%). Use is made of thermoplastic (TP) and thermoset (TS) plastic matrixes. Plastics data source, such as the Society of Plastics Industry s (SPI s) suite of economic studies and statistics, provide current, comprehensive data, and analysis of the domestic and international trade markets for USA plastic materials, processing, machinery, and moldmaking industries that follows the ups and downs of the overall local and worldwide economy (www.plasticsdatasource.org or tel. 800.541.0736). 

Buses and public transport in general, offer wide opportunities for the use of RPs. Unreinforced plastics (URPS) and reinforced plastics (RPs) have been used in different types of buses. In the world of buses during year 2000 Brunswick Technologies (Brunswick, ME) fabricated the so-called CompoBus. Its design incorporates oriented glass fiber satin woven tri-axial fabrics with TS polyester plastics in its chassis and body. 

Reinforced plastics (RPs) and unreinforced plastics (URPs) offer the opportunity to optimize plastic designs by focusing on material composition as well as product structural geometry to meet different product requirements (Figures 7.1 and 7.2). The minimum volume of plastic that will satisfy the structural, functional, appearance, and moldbility requirements of an application is usually the best choice. This is in sharp contrast to machining operations, where one starts with a solid block of material and machines away only until what one needs to make the part function remains. 

In structural applications for RPs, which generally include those in which the product has to resist substantial static and/or dynamic loads, it may appear that one of the problem design areas for many unreinforced plastics (URPs) is their low modulus of elasticity. Since shape integrity under load is a major consideration for structural products, low modulus type plastic products are designed shapewise for efficient use of the material to afford maximum stiffness and overcome their low modulus. These type plastics and products represent most of the plastic products produced worldwide. 

Underwriters Laboratories United Nations unsaturated polyester (TS) unreinforced plastics ultraviolet volt... 

When illustrating the characteristic mechanical behaviour of unreinforced plastics, we shall rely mainly on schematic diagrams rather than actual data, for clarity. It follows that the diagrams are often simplifications of real behaviour, although they represent real trends. 

---