Thermoset selected properties

The principle of heat aging tests is straightforward the material is held at an elevated temperature for a period and the changes in selected properties are measured. Because the change with time is not usually linear, and with thermosetting materials may even change direction, rather more information is obtained if measurements are made after a number of exposure times. For complete characterization, and particularly for predictive purposes, tests should be made at several temperatures. Unfortunately, such complete characterization is both time-consuming and expensive. 

This book focuses on the relationships between the chemical structure and the related physical characteristics of plastics, which determine appropriate material selection, design, and processing of plastic parts. The book also contains an in-depth presentation of the structure-property relationships of a wide range of plastics, including thermoplastics, thermosets, elastomers, and blends. 

The processing methods will be selected according to the importance of the series, part sizes and targeted properties. Table 6.1 suggests some processes, for thermoplastic and thermoset composites, according to the part sizes and production outputs. 

The function of the resin matrix material in filament-wound structures is to help distribute the load, maintain proper fiber position, control composite mechanical and chemical properties, and provide interlaminar shear strength. Either a thermosetting or a thermoplastic resin material may be selected. Thermosetting resins may be selected for application in a wetwinding process or as part of a prepreg resin system. 

From the practical point of view, the glass transition is a key property since it corresponds to the short-term ceiling temperature above which there is a catastrophic softening of the material. For amorphous polymers in general, and thus for thermosets, one can consider that the glass transition temperature, Tg, is related to the conventional heat deflection temperature (HDT) (usually, HDT is 10-15°C below Tg, depending on the applied stress and the criterion selected to define Tg). 

The most common advanced composites are made of thermosetting resins, such as epoxy polymers (the most popular singlematrix material), polyesters, vinyl esters, polyurethanes, polyimids, cianamids, bismaleimides, silicones, and melamine. Some of the most widely used thermoplastic polymers are polyvinyl chloride (PVC), PPE (poly[phenylene ether]), polypropylene, PEEK (poly [etheretherketone]), and ABS (acrylonitrile-butadiene-styrene). The precise matrix selected for any given product depends primarily on the physical properties desired for that product. Each type of resin has its own characteristic thermal properties (such as melting point... 

The decorative laminates described in the previous chapter are made with selected thermosetting resins while resins of this type can be moulded and extruded by methods similar to those outlined in the present and the next chapter the materials employed for these processes predominantly are thermoplastic. Many such plastics can be moulded and extruded under suitable conditions, the most important in terms of quantities used being those that combine properties satisfactory for the purpose with convenience in pro-cessing-especially the polyolefins (polyethylene and polypropylene), poly(vinyl chloride), and styrene polymers and blends. Other plastics with special qualities, such as better resistance to chemical attack, heat, impact, and wear, also are used—including acetals (polyformaldehyde or polyoxymethylene), polyamides, polycarbonates, thermoplastic polyesters like poly(ethylene terephtha-late) and poly(butylene terephthalate), and modified poly(phenylene oxide),... 

This Section presents a cross-referenced bibliography on the application of dielectric property measurements to thermosetting materials. Our literature search identified almost 200 papers with some relevance to the subject. Of these, we have selected about 70 for inclusion in this section. These papers provide particularly useful application examples, or provide data typical of the particular material or application which future investigators can use for comparisons with their own results. 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Properties of a thermoset can be varied by using different formulations such as fiber content, filler content, etc. this uniqueness makes it difficult to generalize the definition of properties of the material. Its characteristics are used as guidelines in part design and material selection to understand the effect of the change in formulation on mechanical properties. Table 6 shows static and impact properties of SMC, BMC, LPMC, and ZMC. Tensile and flexural properties are routinely... 

With advances in computer technology in recent years, simulation has often provided a very powerful tool to predict the end-use performance from the selected component properties and to establish structure-performance relations with a certain degree of confidence. This approach has been termed in silico design as opposed to in vitro or in vivo. " In silico experimentation - the ability to explore simulations rapidly reduces the need for in vivo experimentation slow, multiple iterations of trial and error for design of new composite structures. The combination of design tools and computer modeling will drive development of new thermosets, with improved performance. This includes new structures based on known materials and the development of new chemistries for selected applications. 

Table 3 summarizes the key properties of selected major types of thermosetting resins. Their cross-linking reactions can occur by radical chain addition... 

Since several reactive groups may exist at the respective monomer molecules A and B, net-like linked polymer molecules with thermoset adhesive layer properties occur in polyaddition. Due to the various selection possibilities regarding the chemical structure of the components A and B, curing behavior can be influenced as well. Differences are made between ... 

Binders. Thermosetting and thermoplastic binders are listed, together with their abbreviations, in Table 3.6. A broad range of raw materials is now available and it has become difficult to systematically classify binder properties. Outstanding properties are obtained with suitably selected systems and have contributed to increased... 

These assumptions are not always justifiable when applied to plastics unless modification has occurred. The classical equations cannot be used indiscriminately. Each case must be considered on its own merits, with account being taken of such factors as the mode of deformation, the service temperature and environment, the fabrication method, and so on. In particular, it should be noted that the past traditional equations that have been developed for other materials, principally steel, use the relationship that stress equals the modulus times strain, where the modulus is constant. Except for thermoset reinforced plastics and certain engineering plastics, many plastics do not generally have a constant modulus. Different approaches have been used for the nonconstant situation some are quite accurate. The drawback is that most of these methods are quite complex, involving numerical techniques that are not attractive to designers. One method that has been widely accepted is this so-called pseudoelastic design method. In this method appropriate values of such time-dependent properties as the modulus are selected and substituted into the standard equations. 

A second critical formulation area is the use of selective, reactive tougheners in combination with thermoset polymers, which results in a highly toughened adhesive that does not sacrifice mechanical properties. These avenues are far superior to the traditional method of softening the polymer matrix with thermoplastic materials that will degrade mechanical properties. Further development in this area continues. 

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