Advantages over Plastics

Within the polymer group, there may be certain plastics that have specific superior individual properties to polyurethanes. In the overall situation, the total requirements of the application must be considered. 

The ability to produce polyurethane parts with a large cross-section ( 30 mm) easily is a major advantage over other polymers. A conventional polymer such as polyethylene requires heat under pressure to form a thick cross-section with a long, slow cooling/annealing cycle to prevent stress and voids in the part. The production of other than simple-shaped articles is very difficult. After this molding process, postmachining also needs to be carried out. 

Tooling cost for the production of all polyurethane parts is much lower than that for making plastic parts by either compression or injection molding. The molds do not have to withstand the pressures involved when compression and injection molding is carried out. These lower molding costs can be used to prepare prototypes of a new concept for general evaluation prior to the expense of an injection molding die. 

The wear resistance is better than most plastics, except for UHMWPEin sliding wear applications such as chutes. Polyurethanes can be made in thick-sectioned intricate shapes. This makes them a very good choice in wear-resistant applications. 

The thermoset component in polyurethanes gives them better compression set than most thermoplastic polymers. They also have better cold flow properties. Polyurethanes are tough and more resilient than a large number of other plastics. 

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Metal foils have potential advantage over plastic. They tolerate high temperature processes in contrast to glass and even more to plastics. They also have much better dimensional stability because the metal foils have higher elastic modulus ( 200 GPa), and lower coefficient of thermal expansion ( 10 ppm/°C). 

There are now various expandable meshed metallic stents used in the ureter such as Wallstents (Schneider, Minneapolis, Minn.), Palmaz-Schatz (Johnson and Johnson Interventional Systems, Warren, N.J.) and Ultraflex (Boston Scientific, Boston, Mass.) stents. The Wallstent consists of multiple stainless steel filaments (0.14 mm in caliber), woven in a crisscross pattern to form a tubular mesh that self-expands to a predetermined diameter and length. The stent is compliant and flexible it moulds to the lumen of the ureter but leaves more than 75% of its surface free of metal. Expandable metal stents have received widespread recognition in the treatment of a variety of occlusive diseases, particularly in the arterial and biliary systems. The advantages over plastic polymers are their larger luminal diameter, the ease of placement, the ability to limit the length of the area the stent covers so that it closely... 

DADC HomopolymeriZation. Bulk polymerization of CR-39 monomer gives clear, colorless, abrasion-resistant polymer castings that offer advantages over glass and acryHc plastics in optical appHcations. Free-radical initiators are required for thermal or photochemical polymerization. 

Generally, plastics have excellent resistance to weak mineral acids and are unaffected by inorganic salt solutions—areas where metals are not entirely suitable. Since plastics do not corrode in the electrochemical sense, they offer another advantage over metals most metals are affected by slight changes in pH, or minor impurities, or oxygen content, while plastics will remain resistant to these same changes. 

Thermoplastic elastomers have now been available for over 30 years and the writer recalls organising a conference on these materials in 1969. In spite of considerable publicity since that time these materials still only comprise about 5-10% of the rubber market (equivalent to about 1-2% of total plastics consumption). It is important to appreciate that simply being a thermoplastic material (and hence being processed and reprocessed like a thermoplastic plastics material) is not enough to ensure widespread application. Crucially the material must have acceptable properties for a potential end-use and at a finished product price advantageous over other materials. 

No one steel exceeds the tensile modulus of mild steel. Therefore, in applications in which rigidity is a limiting factor for design (e.g., for storage tanks and distillation columns), high-strength steels have no advantage over mild steel. Stress concentrations in mild steel structures are relieved by plastic flow and are not as critical in other, less-ductile steels. 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

Thermoplastic elastomers are materials that have the properties of vulcanized rubbers but can be processed by techniques associated with thermoplastics. The commercial importance of TPEs is due to their superior processing properties and economic advantages over conventional rubbers and plastics. TPEs from rubber-plastic blends became important because they combine the superior processability of thermoplastics and the... 

Plastics, such as PE, PP, polystyrene (PS), polyester, and nylon, etc., and elastomers such as natural rubber, EPDM, butyl rubber, NR, and styrene butadiene rubber (SBR), etc., are usually used as blend components in making thermoplastic elastomers. Such blends have certain advantages over the other type of TPEs. The desired properties are achieved by suitable elasto-mers/plastic selection and their proportion in the blend. 

Wooden racks used in sea-water tests are likely to be subject to severe damage by marine borers. The wood used, therefore, must be treated with an effective preservative, for example creosote applied under pressure, if the test is to extend for several years. Organic copper compound preservatives may suffice for shorter tests, for example 2 or 3 years. Since the leaching of such preservatives may have some effects on corrosion, metal racks fitted with porcelain or plastics insulators have an advantage over wooden racks. 

These TPs have high inherent heat resistance and offer such other advantages over TSs as higher toughness and ease of processing. Some of these plastics are amorphous with a high Tg such as PES, and some like PEEK and... 

P.R.190 has a relatively small impact on the market. It is a specialty product for industrial paints, especially for automotive finishes, but offers no advantage over other members of its class. Its shade is dull and referred to as scarlet. In white reductions, the commercial type is very bluish and equally dull. The particle sizes of this product are too coarse for it to be used in metallic finishes. P.R.190 is very durable and very fast to organic solvents and to migration. The pigment is heat stable up to processing temperatures of 200°C. Despite its good heat stability, the pigment possesses only limited commercial value as a colorant for plastics. 

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