Polymer Crosslinking

8 Key excerpts on "Polymer Crosslinking"

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  eBook - ePub

  Polymer Blends and Composites

  Chemistry and Technology

  • Muralisrinivasan Natamai Subramanian(Author)
  • 2017(Publication Date)
  • Wiley-Scrivener

    (Publisher)

  Crosslinking is a very useful technique for the modification of polymers. It can be initiated by heat, chemical agents, irradiation, or a combination of these [111]. Theoretically, any linear polymer can be converted into a crosslinked polymer with some modification in the molecule [112, 113]. Crosslinking is a method for improving the dimensional stability at higher temperatures and increasing resistance to mechanical stresses which produce deformation. Crosslinking is an effective way to improve its quality [114].

  Molecules have two or more groups capable of reacting with the functional groups of polymer chains, where such a reaction connects or links the chains 2-mercaptobenzothiazole, benzoyl peroxide, dicumyl peroxide, sulfur, and toluene diisocyanate. Crosslinking of a thermoplastic is normally a two-step procedure. First, compounding and forming are performed under conditions which should not activate the crosslinking reactions. The latter is instead performed in a second step, often at somewhat increased temperature.

  The rate of a thermally activated crosslinking reaction, e.g., a nucleophilic substitution, should thus be as high as possible. The reactivity should, on the other hand, not be high enough to prematurely cause molecular enlargement or even crosslinking. Wherever a branch radical combines with another branch radical, it forms a crosslink between the two macromolecules involved. Wherever the number of crosslinks thus formed exceeds a certain critical fraction of the number of chains, usually an infinite molecular weight crosslinked insoluble network may result.

  Crosslinking is a rather general term in composite chemistry. Crosslinking is used as a more general term to include curing, which stands for crosslinking by chemicals. Crosslinking is a process of forming a three-dimensional network structure from a linear polymer by a chemical or physical method. Chemical methods produce covalently bonded networks, i.e., chemical gels. The formation of network structure is one of the essential conditions for generating the physical properties. Crosslinking agents have no effect within the crystalline regions of a polymer. Chemical crosslinking is another method that has been used commercially for crosslinking polymers such as polyethylene.

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  Microwave-Assisted Polymerization

  • Anuradha Mishra, Tanvi Vats, James H Clark(Authors)
  • 2015(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  In this chapter we have included the work done in the area of material processing using microwaves with special reference to Polymer Crosslinking and curing; polymer composites; polymer blends, and processing of polymeric scaffolds and particles. 8.2 Polymer Crosslinking/Curing/Derivitization Curing is a process during which a chemical reaction like polymerization or a physical action like evaporation results in a harder, tougher or more stable linkage or substance. Some curing processes require maintenance of a cer-tain temperature and/or humidity level, others require a certain pressure. Crosslinking or curing results in the formation of insoluble and infusible polymers wherein the polymer chains are joined together to give a three-dimensional structure. The process is pictorially represented in Figure 8.1 below. The cure process at the molecular level involves several sequential stages. The first stage is the uncured stage, which consists of only a mixture of monomers or oligomers. On the commencement of the curing process, linear chain extension of the monomers and oligomers takes place. This is followed by chain crosslinking, which gives an effectively thermoplastic (flexible and moldable) phase. This process may take some time, and ini-tially any rise on either the percent conversion or T g may be detectable. Further heating would then result in chain crosslinking dominating the reaction to give a fully hardened solid (the commonly termed ‘‘C’’ stage) thermoset material, with significant shrinkage due to the molecular chains being pulled closer together by the crosslinks. 9 Curing is of utmost importance in applications which require higher performance of thermoset polymers, such as adhesives and encapsulating sealant for the microelectronics industries. These requirements have led to unacceptably long cure times. In case of conventional thermal curing methods the cure rates increase, and in turn the cure times become lower.

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  Atomic Radiation and Polymers

  International Series of Monographs on Radiation Effects in Materials, Vol. 1

  • A. Charlesby(Author)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 9 THE PROPERTIES OF A CROSSLINKED NETWORK MANY polymers, after exposure to small doses of high energy radiation, show an increase in viscosity, average molecular weight and degree of branching. These changes are due to dimerization—the linking of mole-cules together—and the reaction has also been observed in low molecular weight compounds such as «-paraffins. As the reaction proceeds, closed loops are formed and produce a three-dimensional network polymer, the properties of which are very different from those of the original linear or branched material. This transition in properties from a one- to a three-dimensional structure is also observed in the vulcanization of rubber, in the chemical crosslinking of polymer molecules such as copolymers of styrene and divinyl benzene, and in the gelation of polyesters. Polymers can be considered as consisting of an assembly of units (not necessarily identical) which are mono-functional, di-functional or poly-functional. The mono-functional units (—A) are needed to end the chain and therefore act as terminal units. Di-functional units (—A—) extend the chain but do not provide any means of linking to other molecules to form a network. This function is assumed by the tri-functional (—A—) I or higher functional units which also occur in branched polymers. The possibility of network formation depends on the degree of functionality, i.e. on the ratio of poly-functional units to mono-functional units. A small discrepancy arises from the possibility of intramolecular crosslinks, i.e. links of one poly-functional unit in a molecule with another such poly-functional unit in the same molecule. In the theory as given here, the term crosslink is taken to refer to a tetrafunctional link or junction point, binding two long molecules together side by side: —A—A—A—A—A— .

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  Atomic Radiation and Polymers

  International Series of Monographs on Radiation Effects in Materials

  • A. Charlesby(Author)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  C H A P T E R 9 THE PROPERTIES OF A CROSSLINKED NETWORK M A N Y polymers, after exposure to small doses of high energy radiation, show an increase in viscosity, average molecular weight and degree of branching. These changes are due to dimerization—the linking of mole-cules together—and the reaction has also been observed in low molecular weight compounds such as «-parafBns. As the reaction proceeds, closed loops are formed and produce a three-dimensional network polymer, the properties of which are very different from those of the original linear or branched material. This transition in properties from a one- to a three-dimensional structure is also observed in the vulcanization of rubber, in the chemical crosslinking of polymer molecules such as copolymers of styrene and divinyl benzene, and in the gelation of polyesters. Polymers can be considered as consisting of an assembly of units (not necessarily identical) which are mono-functional, di-functional or poly-functional. The mono-functional units (—A) are needed to end the chain and therefore act as terminal units. Di-functional units (—A—) extend the chain but do not provide any means of linking to other molecules to form a network. This function is assumed by the tri-functional (—A—) i or higher functional units which also occur in branched polymers. The possibility of network formation depends on the degree of functionality, i.e. on the ratio of poly-functional units to mono-functional units. A small discrepancy arises from the possibility of intramolecular crosslinks, i.e. links of one poly-functional unit in a molecule with another such poly-functional unit in the same molecule. In the theory as given here, the term crosslink is taken to refer to a tetrafunctional link or junction point, binding two long molecules together side by side: —A—A—A—A—A— .

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  Advances in Polymer Processing

  From Macro- To Nano- Scales

  • S Thomas, Weimin Yang(Authors)
  • 2009(Publication Date)
  • Woodhead Publishing

    (Publisher)

  Electron beam technology of polyethylene is a well-established technology, applied commercially for decades. Electron beam processing of crosslinkable plastics has yielded materials with improved dimensional stability, reduced stress cracking, higher service temperature, reduced solvent and water permeability, and significant improvement in other thermo-mechanical characteristics. The crosslinking process changes a linear molecule into a three-dimensional network and involves a radical modification of the characteristics of the material (Chapiro, 1962; Ivanov, 1992). Crosslinking increases the branching rate and the average molecular weight of the polymer. It also imparts properties like insolubility and infusibility to the material and a significant improvement of its dimensional stability in chemically aggressive and high-temperature conditions. Radiation crosslinking can be applied to a great number of plastics including thermoplastics, elastomers and thermoplastic elastomers (TPE). Some of them can crosslink on their own, others need to be formulated with a sensitizer or to be modified during their polymerization. Commercial crosslinkable resins are available. Some polymers, such as polytetrafluoroethylene (PTFE), polyoxymethylene (POM) or polypropylene (PP), degrade by irradiation if not formulated with the sensitizer. Radiation crosslinking influences the mechanical behaviour, chemical stability, thermal and flame resistance. Of all properties, the thermal properties are most strongly affected after radiation crosslinking. In particular, significant improvements can be observed in dimensional stability, heat distortion temperature (in the case of thermoplastics), creeping, glow wire resistance, flame resistance, and compression set (in the case of elastomers and TPE).

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  Molecular Characterization and Analysis of Polymers

  • John M. Chalmers, Robert J. Meier(Authors)
  • 2008(Publication Date)
  • Elsevier Science

    (Publisher)

  As a general rule, the acceptable melt viscosity for extrusion, etc., is 150°C above T g and 50°C above T m, where T g and T m are the glass transition and crystalline melt temperatures, respectively. Inter-chain attraction between the polymer chains determines the DP n necessary to give the polymer the required toughness, strength, etc. For polymers containing heteroatoms, e.g., polyesters, polyamides, DP n of 100–200 is adequate, whereas for polymers with little or no inter-chain attraction, e.g., PE and PP, then DP n >1,000 is recommended. Thermoset polymers (sometimes called network polymers) can be formed from either monomers or low MW macromers that have a functionality of three or more (only one of the reagents requires this), or a pre-formed polymer by extensive crosslinking (also called curing or vulcanisation; this latter term is only applied when sulfur is the vulcanising or crosslinking agent.) The crosslinks involve the formation of chemical bonds — covalent (e.g., carbon–carbon bonds) or ionic bonds. The act of crosslinking can either be an addition reaction (e.g., radical/radical) or a condensation reaction (e.g., alcohol/carboxyl). In the latter, the formation of volatile condensates is often not desirable because they may weaken the structure. Crosslinking reactions involve creating radicals along the polymer chain, which can couple to form a crosslink. A problem is that chain scission may occur. These radicals can be generated from added peroxide or, with more control, by high-energy radiation, e.g., X-rays (synchrotron), electron beams or γ -rays (Co-60). For example, crosslinked PE is much tougher, more environmental stress cracking resistant and is often used for wire coating. When cold stretched and then heated it returns to its original shape, hence its use for shrink-wrap films or tubes

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  Handbook of Thermoset Plastics

  • Hanna Dodiuk, Sydney H Goodman(Authors)
  • 2013(Publication Date)
  • William Andrew

    (Publisher)

  [2] , summarizes some of the major technological differences and the application range of the various cross-linking methods.

  Table 17.7

  Technological Comparison of Polymer Cross-Linking Methods [2 ,6]

  : In practical use; : Technically possible, but no practical example; ×: Hard to apply.

  Applications of Cross-Linked Thermoplastics

  Polyolefin Foams

  Polyolefin foams represent an important class of industrial materials. Although most polymeric foamed materials are either based on polyurethane, polystyrene, or poly(vinyl chloride), polyethylene foams are only ranked fourth in terms of sale, but the growth rates of foams based on cross-linked PE are impressive [61] . Polyolefins can be cross-linked by irradiation [62] as well as chemical means such as silane grafting [63] ; the typical manufacturing process comprises three steps: (a) sheet formation, (b) cross-linking, and (c) foaming [61] .

  Table 17.8 shows typical properties of a series of radiation cross-linked closed-cell polyethylene foams [64a] . The foams, ranging in density from 1.5 to 15 lb/ft3 , are characterized by excellent mechanical, thermal, and chemical properties, together with a fine-cell structure and an exceptionally smooth surface; they are available in thicknesses from 1/32 inch to more than 1 inch. A cross-linked polyethylene foam sheet with an integral skin is also available in the same range of densities and thicknesses. The skin offers increased abrasion resistance without reducing the foam’s flexibility. Table 17.8 shows VOLARA foam products with a Type E designation; Voltek literature describes them as cross-linked polyethylene copolymer foams specially formulated to provide more flexibility and resilience than their standard grade of Type A designation. The science and technology of polyethylene foams based on cross-linked PE was extensively reviewed by Rodriguez-Pérez [61]

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  Polymer Grafting and Crosslinking

  • Amit Bhattacharya, James W. Rawlins, Paramita Ray, Amit Bhattacharya, James W. Rawlins, Paramita Ray(Authors)
  • 2008(Publication Date)
  • Wiley

    (Publisher)

  Elastomer Crosslinking Elastomers are made of long chain molecules with molecular weight vary in the range of 1.5–5 × 10 5 . Uncrosslinked elastomers have limited commercial value because of their easy deformation under load and rapid dissolution in different solvents. To achieve the desired properties in the final products, it is necessary to crosslink the elastomers known as vulcanization. 40 BASIC FEATURES AND TECHNIQUES The term “vulcanization” refers to a chemical process in which the uncured long chain rubber molecules are tied together into a three-dimensional elastic network by the insertion of crosslinks [96]. Hence, by vulcanization a raw rubber is transformed from its thermoplastic state to an elastic rubbery or hard ebonitelike state. The three-dimensional network structure imparts strength, rigidity and elastiticity, improves solvent properties, and enhances the resis- tance to deformation of elastomers under heat and cold. Initially, vulcanization referred to the crosslinking of elastomer chains with the aid of sulfur but in due course different types of new crosslinking agents have been developed. The selection of crosslinking agents depends upon the type of elastomers and performance properties desired. Curing of the Rubber Stock and Stages of Vulcanization The curing and vul- canization of rubber are synonymous. By plotting the change in a specific property such as tensile strength or modulus with the length of curing, the cure curve is obtained. The rate of vulcanization is an extremely important param- eter in rubber industries as it controls the optimum cure time, which in turn determines the total batch cycles that may run per day in an industry. However, the rate of cure may be monitored by suitable choices of crosslinking agents and conditions. Vulcanization may occur in three different stages in an elasto- mer, which may be presented graphically by the generation of tensile modulus with time (Figure 2.6).

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