Cross Linked Polymer
A cross-linked polymer is a three-dimensional network of polymer chains that are linked together by chemical bonds. This structure provides the polymer with increased strength, rigidity, and resistance to solvents and heat. Cross-linked polymers are commonly used in a variety of applications, including in the production of rubber, adhesives, and coatings.
5 Key excerpts on "Cross Linked Polymer"
- Laurence W. McKeen(Author)
- 2009(Publication Date)
- William Andrew(Publisher)
...It is a polystyrene backbone with chains of polybutadiene grafted onto the backbone. The polystyrene gives the material strength, but the rubbery polybutadiene chains give it resilience to make it less brittle. 3.3. Linear, Branched and Cross-Linked Polymers Some polymers are linear, a long chain of connected monomers. Polyethylene, PVC, Nylon 66, and polymethyl methacrylate are some linear commercial examples found in this book. Branched polymers can be visualized as a linear polymer with side chains of the same polymer attached to the main chain. While the branches may in turn be branched, they do not connect to another polymer chain. The ends of the branches are not connected to anything. Cross-linked polymer, sometimes called network polymer, is one in which different chains are connected. Essentially the branches are connected to different polymer chains on the ends. These three polymer structures are shown in Figure 3.3. Figure 3.3 Linear, branched, and cross-linked polymers. A higher amount of cross-linking in plastics generally leads to higher fatigue crack propagation rates. This is shown in Figure 3.4. This figure shows the fatigue crack propagation rate increases as the amount of cross-linking increases in polymethyl methacrylate. The data to the left side of the plot has the highest amount of cross-linking. The uncross-linked data on the far right has the best performance. Figure 3.4 Fatigue crack propagation in polystyrene with different amounts of cross-linking agent. 3.4. Molecular Weight A polymer’s molecular weight is the sum of the atomic weights of individual atoms that comprise a molecule. It indicates the average length of the bulk resin’s polymer chains. All polymer molecules of a particular grade do not all have the exact same molecular weight. There is a range or distribution of molecular weights. The average molecular weight can be determined by several means, but this subject is beyond the scope of this book...
eBook - ePubUV Coatings
Basics, Recent Developments and New Applications
- Reinhold Schwalm(Author)
- 2006(Publication Date)
- Elsevier Science(Publisher)
...Thus, in order to get hard and flexible coatings, the crosslink density should be decreased to allow a T b -transition below room temperature and the glass transition has to be increased above RT by means of chemical composition and crosslinking during photopolymerization. Low crosslink densities are difficult to realize in 100% coatings without using large amounts of monofunctional reactive diluents, since viscosity also increases with molecular weight. Therefore, water-based systems, where the viscosity is independent of molecular weight of the resin, or systems applicable by melting, like hot melt or powder coatings, are option towards hard and flexible coatings. 11 6.2 EFFECT OF CROSSLINKING ON COATING PROPERTIES In the preceding section some structure–property relationships have been discussed influencing the mechanical properties of UV cured coatings. In daily life however, the application properties of the coatings, like abrasion-, scratch- or chemical resistance, are the crucial key factors. For linear polymers a lot of data exist characterizing the structural influences on mechanical properties, however much less data exist for crosslinked coatings. And even less data are available for correlating property–property relationships, like the influences of mechanical properties on scratch resistance, pendulum hardness, elasticity or Taber abrasion (Figure 6.11). FIG. 6.11 From structure–property to property–property relationships. The reason why such correlations hardly exist is based on the fact, that many of these application properties, like scratch resistance, are not a defined material property. The scratch resistance of a material is dependent on the scratch test, which means on the scratching material (hardness, shape, etc.) as well as on the impact (velocity, mass) affecting the coating...
eBook - ePub- Leslie H. Sperling(Author)
- 2015(Publication Date)
- Wiley-Interscience(Publisher)
...All proteins are actually copolymers of polyamide-2 (or nylon-2, old terminology). As made by plants and animals, however, the copolymers are highly ordered, and they have monodisperse molecular weights, meaning that all the chains have the same molecular weights. Cellulose and starch are both polysaccharides, being composed of chains of glucose-based rings but bonded differently. Their structures are discussed further in Appendix 2.1. Natural rubber, the hydrocarbon polyisoprene, more closely resembles chain polymerized materials. In fact synthetic polyisoprene can be made either by free radical polymerization or anionic polymerization. The natural and synthetic products compete commercially with each other. Pitch, a decomposition product, usually contains a variety of aliphatic and aromatic hydrocarbons, some of very high molecular weight. 1.5 CROSS-LINKING, PLASTICIZERS, AND FILLERS The above provides a brief introduction to simple homopolymers, as made pure. Only a few of these are finally sold as “pure” polymers, such as polystyrene drinking cups and polyethylene films. Much more often, polymers are sold with various additives. That the student may better recognize the polymers, the most important additives are briefly discussed. On heating, linear polymers flow and are termed thermoplastics. To prevent flow, polymers are sometimes cross-linked (): (1.16) The cross-linking of rubber with sulfur is called vulcanization. Cross-linking bonds the chains together to form a network. The resulting product is called a thermoset, because it does not flow on heating. Plasticizers are small molecules added to soften a polymer by lowering its glass transition temperature or reducing its crystallinity or melting temperature. The most widely plasticized polymer is poly(vinyl chloride). The distinctive odor of new “vinyl” shower curtains is caused by the plasticizer, for example. Fillers may be of two types, reinforcing and nonreinforcing...
eBook - ePubFlexible Polyurethane Foams
A Practical Guide
- Chris Defonseka(Author)
- 2019(Publication Date)
- De Gruyter(Publisher)
...The term ‘graft’ has been used to denote a controlled or engineered addition to the main polymer chain. AAAAAAAAAAAAAAAAA / / B B B B 5. Interpenetrating structures – these polymer networks are an entangled combination of two cross-linked polymers that are not bonded to each other. A ‘plastic alloy’ is formed if two or more different polymers are physically mixed together in the melt. The term ‘polyblend’ refers to a plastic that has been modified by the addition of an elastomer. The identity of the monomer residues (repeating units) forming a polymer is its first and most important attribute. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer. Polymers that contain only a single type of repeating unit are known as homopolymers, whereas polymers containing a mixture of repeating units are known as copolymers. Polystyrene, for example, is composed of only styrene monomer residues and is therefore classified as a homopolymer. Ethylene vinyl acetate contains more than one variety of monomer and is thus a copolymer. 3.2.2 Microstructure The microstructure of a polymer (sometimes called ‘configuration’) relates to the physical arrangement of monomers along the backbone of the chain. These are the elements of polymer structure that require breaking of a covalent bond in order to change. Structure has a strong influence on the other properties of a polymer. For example, two samples of natural rubber may exhibit different durabilities even though their molecules comprise the same monomers. An important microstructural feature of a polymer is its architecture, which relates to the way branch points lead to a deviation from a simple linear chain. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. Types of branched polymers include star polymers, comb polymers, brush polymers, dendronised polymers and dendrimers...
eBook - ePub- Ralf Janda(Author)
- 2021(Publication Date)
- tredition(Publisher)
...Linear polymer chains with many hydroxyl groups and free epoxy terminal groups are formed when performing the polyaddition in excess of epychlorohydrine. These polymers are thermoplastic and partially liquid and can be cross-linked by diamines, carboxylic anhydrides or their dicarboxylic acids. The cross-linking bridges are, therefore, amide or ester bridges. This cross-linking reaction is an addition reaction and is catalyzed by water or by the free hydroxyl groups of the EP. Cross-linked epoxides are mainly duromers. Their big advantage compared with other duromers manufactured via cross-linking reactions (e.g. aminoplasts, phenoplasts) is that their cross-linking is done by polyaddition that means no low molecular compounds are set free. The properties of epoxides after cross-linking depend strongly on the structures of the reactants as well as of the type of catalyst and, therefore, vary in a wide range. The higher the degree of cross-linking, the higher is the temperature resistance and the chemical resistance. The polyether impression materials are certainly the best known polyethers in dentistry. They are not “pure” polyethers but have elastic aliphatic polyether macromolecular segments with polyester groups bearing the reactive aziridine terminal groups which form C-C links during cross-linking (Fig. 59b) [ 110, 229 ]. PPE has been suggested in dentistry - for use in dental restorative prosthetics [ 230, 231 ] - as dental resin posts [ 232 ] - as resin caps for implants [ 233 ] There are several other patents that claims PPE for use as or in dental materials...
