Brittle thermoplastics

At low strains there is an elastic region whereas at high strains there is a nonlinear relationship between stress and strain and there is a permanent element to the strain. In the absence of any specific information for a particular plastic, design strains should normally be limited to 1%. Lower values ( 0.5%) are recommended for the more brittle thermoplastics such as acrylic, polystyrene and values of 0.2-0.3% should be used for thermosets. 

Polymerised styrene, a hard, transparent, rather brittle thermoplastic material. 

A typical unsaturated unreinforced polyester resin has an extremely low notched Izod impact strength. The addition of fibrous glass can change this extremely brittle material into a high impact strength composite. The same phenomenon occurs with some brittle thermoplastics, such as polystyrene and styrene—acrylonitrile. 

Choice of Polymer. The ideal polymer is a tough, amophorous, but not brittle thermoplastic with a glass transition temperature more than 50 °C above the expected use temperature. A high molecular weight is important. Commercial polymers made for injection molding have molecular weights in the 30000... 

The generic name hydrocarbon resins designates several families of low molar mass polymers (M from 600 to 104) obtained by polymerization of petroleum, coal tar, and turpentine distillates [80-82], In most cases, these products are obtained by cationic polymerization of mixtures either of aliphatic and/or aromatic mono and diolefins present in the more or less enriched Cs and C9 feedstreams, or of pure aromatic monomers generally of the styrene type. They are complex mixtures of polymers ranging from viscous liquids and tacky fluids to hard, brittle thermoplastics, and are used as additives in adhesives, printing inks, rubbers, coatings, etc. [80-82], They are obviously amorphous and are characterized by their softening point (0 to —150° C), determined by standardized methods (i.e.,... 

The beginning of cellular polystyrene may be dated back to the time of the discovery of a material called styrene. Polystyrene is a clear, brittle, thermoplastic aromatic resin ... 

Polystyrene (PS) in its atactic and syndiotactic forms is a brittle thermoplastic, even in an orientated state [4]. To improve the toughness of aPS, impact modification has been practised for a long time, either by polymerizing the styrene in the presence of a polybutadiene rubber leading to high-impact polystyrene, commonly called HIPS, or by blending the polystyrene with multi-block copolymers, mainly of the styrene-butadiene-styrene (S-B-S) type. 

A more effective toughening of brittle thermoplastics is often achieved using rubber modification. This is discussed in more detail in Section 5. 

Shellac was in use by 1856. It is another natural polymer and is based on a secretion from an insect, Laccifer lacca - the lac insect - which is a plant parasite. To produce shellac, the secretion and the insects are scraped off plants and filtered, resulting in a hard, brittle thermoplastic material. It is mixed with wood flour or a mineral filler, pressed, and steamed, dyed, and rolled into sheets to be moulded. 

Vinsol [Aqualon], TM for a series of low-cost, dark, brittle, thermoplastic resins ruby-red by transmitted light, dark-brown by reflected light. Available in solid form, flakes, fine powder, and aqueous dispersion. 

Graft Copolymers of EPDM Elastomers as Reinforcing Agents for Brittle Thermoplastic Materials... 

The impact strength of brittle thermoplastic materials is generally improved by adding small amounts of rubber, either pure or modified by grafting with the monomer or monomers constituting the matrix to be reinforced (1, 2, 3, 4, 5). As a rule, modification is achieved by monomer polymerization in the presence of the reinforcing elastomer, which is usually a butadiene polymer or copolymer (6, 7). 

These materials have been chosen due to their economic expediency and the valuable combination of physical and chemical, physical and mechanical, and other properties of individual pol5miers-PHB is a brittle thermoplastic PIB an elastomer. Preparation of composite materials based on... 

Typical stress-strain curves for plastics are shown in Fig. 1.2. The figure shows qualitatively (a) the steep, but non-linear curve associated with amorphous, brittle thermoplastics such as unmodified polystyrene, (b) the equivalent graph for a similar brittle thermoplastics material to which rubber has been added to produce a high impact grade and (c) an intrinsically tough thermoplastics material, such as a nylon (polyamide). 

It is possible to produce orientation at normal temperatures in a brittle thermoplastic like polystyrene, by the application of a sufficiently high hydrostatic pressure during extension. The hydrostatic pressure prevents brittle failure at small strains, and can be regarded as reducing Ty, below ambient temperature. Similar results may be obtained by hydrostatic extrusion, where a billet of the polymer is forced through a reducing die under the pressure of a hydraulic fluid. An example of the properties achieved in this way has been given in Section 1.2 above. 

Due to creep experiments particularly at tensile loading are much easier to handle than stress relaxation measurements long-term relaxation data of polymers are rarely available in Uterature and databases, therefore. Compared to tensile loading three- or four-point flexural loading to determine the long-term creep and relaxation properties is often applied especially in case of thermosets, brittle thermoplastic and... 

Self-tapping inserts. Fig. 14.4(a), cut their own threads, are available in brass, case-hardened steel and stainless steel from M3 to M16 and are used in the more brittle thermoplastics and in thermosets. 

Hard, glassy, brittle thermoplastics such as polystyrene (PS) and polymethylmethacrylate (PMMA) have low attenuations, of order 6-10 dB/cm at 10 MHz, and in the case of PS, a low acoustic impedance. Ductile polymers such as polycarbonate (PC), many polyolefins and impact-modified thermoplastics generally have high absorption coefficients, in the range 20-40 dB/cm. The same molecular structures and mobiUty, which contribute to ductihty, may also contribute to absorption of ultrasonic energy. Not surprisingly, rubbers and, by extension, any polymer above its... 

Rubber matrices have commonly been used as a second phase to improve the toughness of brittle thermoplastic materials, such as polypropylene and polyethylene. These systems, commonly referred to as polyolefin thermoplastic elastomers (TPOs), are a special class of thermoplastic elastomers that combine the processing characteristic of plastics at elevated temperatures with the physical properties of conventional elastomers at service temperature, playing an increasingly important role in the polymer material industry. Polyolefin blends attract additional interest due to the possibility of recycling plastic wastes, avoiding the complex and expensive processes of separation of the different components. 

The thermo-chemical properties of PHA have been shown to be strongly related to their structure. Thus, PHAscl are mainly brittle thermoplastics with high melting and glass transition temperatures, whereas PHAscl are mainly elastomers. Biocompatibility is another interesting property of PHAs. In this case, PHAmcl have been shown to be more biocompatible than PHAscl, however, they are also more hydrophobic and the hydrophobicity may be an important drawback for such types of applications. Thus, postfermentation modifications of PHAs were also applied to modify their structure. Those modifications consist either of monomeric modifications,... 

Application at T < Tg polymer hard, stiff, brittle, thermoplastics, duromers. 

Lavach [50] discussed the factors that affect results obtained by the Gardner impact test. This test is used by plastics producers to approximate the mean failnre energy for many plastics. The test is inexpensive and easy to operate. The test eqnipment can be placed close to the manufacturing equipment, permitting fast and nearly online determination of the impact resistance of an article. The test is useful for finding the mean failure energy for brittle thermoplastics such as acrylic and high-impact polystyrene with standard deviations between 8% and 10%. 

Of the many NR modification reactions, the cyclization reaction is a simple way of modifying NR to obtain a hard, brittle, thermoplastic derivative of NR. Cyclized products are used in the formulation of adhesives, paints and inks, and also in the compounding of NR to improve its mechanical character-... 

NR and plastic blends have been reported for the preparation of thermoplastic NR blended systems. Many types of plastics have been blended with NR, such as polyethylene, polypropylene, polystyrene, " polyacrylates or polyacrylic acid, and poly(methyl methacrylate). These thermoplastic NRs provide new materials with different properties that range from a soft elastomer to a semi-rigid NR plastic for various industrial applications.In addition, NR can improve the toughness of brittle thermoplastic materials. The NR and thermoplastic blends express greater ductility and impact strength at low temperatures. The requirements for certain mechanical and physical specifications can be achieved by optimizing the blended compositions and selecting the most suitable thermoplastics. 

Although it is brittle in tension, polystyrene is ductile in compression, and the same is true of other apparently brittle thermoplastics, and also of lightly cross-linked thermosetting resins. Very tightly cross-linked resins show little... 

Addition of 30 wt% PEG to PLA with low stereoregularity decreases 7g from above ambient temperature to below ambient temperature and thereby decreases the modulus and increases the ductility of this relatively rigid, brittle thermoplastic. Immediately after cooling from the melt, blends with up to 30 wt% PEG are amorphous and homogeneous with a single Tg that depends on composition in accordance with the Fox relationship. However, the blends with 20 wt% or more PEG are not stable at ambient temperature. 

The method for obtaining an impact-resistant resin composition by introducing a rubber component into hard and brittle thermoplastic resin such as polystyrene, polymethyl methacrylate, styrene-acrylonitrile copolymer, polyvinyl chloride, or the like has been well known for many years. 

---