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Radiation Crosslinking of Pvc with Ethylene Glycol Dimethacrylate
| Content Provider | Semantic Scholar |
|---|---|
| Author | Dbo, J. A. Ns |
| Copyright Year | 2007 |
| Abstract | The crosslinking of PVC in the presence of ethylene glycol dimethacrylate was studied by microcalorimetry, ESR and by structure investigation. A substantial part of the monomer polymerizes already during processing and storage. High concentration of free radicals of the methacrylate type may be frozen in in the PVC matrix and become re-activated at higher temperature. The radicals disappear and reappear upon prolonged storage. The different stages of the grafting and crosslinking reaction are discussed. The radiation crosslinking of PVC coated wires has been recently established as the first radiation process of big volume. As the radiation crosslinking of PVC is performed in presence of monomeric additives, this process is both a grafting and a crosslinking reaction. For crosslinking additives, bifunctional esters of methacrylic acid are often used. These monomers exhibit some features which make their use advantageous. Firstly, they are effective plasticizers for PVC, giving good contact with the PVC chains and thus facilitating the grafting reaction. Secondly, they are sensitive to radiation: their free radical yield (G-value) upon irradiation is high. In the classical concept of graft copolymerization this is regarded as a disadvantage. If we use a monomer with a high G-value, the free radicals are generated mainly in the monomer, giving rise to homopolymerization rather than grafting. But, things stand differently with bifunctional monomers like the dimethacrylates. Aso has shown,1 that in these monomers the second double bond has about 75 times less reactivity, because of restricted mobility. The dimethacrylates polymerize therefore mainly in long chains, which are crosslinked at a relatively early stage, but in which a high concentration of pendant double bonds remains, in spite of considerable internal cyclization. The concentration of pendant double bonds does hardly change up till high conversions. Through these pendant double bonds may the poly-dimethacrylate homopolymer form graft copolymer and consequently crosslinks with the matrix polymer. The crosslinking of PVC in the presence of monomeric additives has been extensively studied by Salmon and Loan.2 50 parts of tetraethylene glycol dimethacrylate has been added to 100 parts of PVC and processed on a mill. The total gel content rises to almost 50% with a dose of only 100,000 rads. But this does not mean, that at this point the PVC has been effectively crosslinked. The crosslinked material consists mainly of poly-tetraethylene glycol dimethacrylate, while only 20% of the PVC is crosslinked. A substantial part of the original unsaturation is yet also present at this stage, in the form of pendant double bonds, leading, in the second stage, to the crosslinking of the greater part of the PVC, in this case of 81% of the original polymer at 5 Mrad. The taking of good kinetic curves of the polymerization in similar systems presents considerable difficulties. The dilatometer commonly used in chemical kinetics for taking continuous curves from the same sample, is here not applicable. Thus, the scatter between different samples has to be taken into account. In addition, the course of the kinetic curve is expectedly more complicated, than in homogenous kinetics. Therefore, we made use of microcalorimetry and followed the chemical kinetics through the thermokinetic curves. The microcalorimeter built for this purpose is of the Calvet-type (Fig. 1). It consists of two metallic cylinders situated in a block connected to a water thermostat. The thermal contact between the metallic cylinders and the block is secured through batteries of 120 thermoelements connected in series; any temperature difference between cell and block creates a proportional thermal flux through the wires and, at the same time, a proportional electromotive force. The two cells are connected in opposition. The time constant of the calorimeter is 3 mm; as the time scale of the experiments is much longer, the reading of the electromotive force may generally be taken as a measure of the instantaneous rate of polymerization. The construction of our calorimeter allows the introduction of the thermostated block into a cobalt-source of 500 Ci; for purely thermal measurements without radiation, we made use of a high sensitivity commercial microcalorimeter type Setaram. Figure 2 shows the thermokinetic curves of the polymerization of methyl methyacrylate at 55°C. The rate of polymerization is even in the stationary state steadily increasing, as often in radiation polymerization. One may see in the region of the gel-effect a maximal increase in rate by about 12 times. If we add PVC powder to the monomer, the increase in rate becomes much more conspicuous, the maximum being about 6 times higher, than in methylmethacrylate homopolymerization. In the mixture with only 20 parts of methylmethacrylate the reaction slows down at a very low conversion. The reason is, that at the comparatively low temperature of the experiment the monomer forms a Fig. 1. Microcalorimeter for in-source polymerization. (1) sample; (2)reference;(3)battery;(4)thermostatedblôck;(5)styrofoamcap. Microcalorimeter |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.iupac.org/publications/pac/1976/pdf/4601x0001.pdf |
| Language | English |
| Access Restriction | Open |
| Subject Keyword | Chemical Kinetics (discipline) Cobalt Cold Temperature Copolymer Copolymerization Cross link Cyclization Esters Ethylenes Experiment Free Radicals Glycol Dimethacrylate Grafting (decision trees) Kinetics Internet Protocol Limited stage (cancer stage) Mass Effect Trilogy Maximal set Megarad Methacrylates Methylmethacrylate Mobitz type II atrioventricular block Pendant Dosage Form Peroxisomal Bifunctional Enzyme Plasticizers Poly A Poly Adenosine Diphosphate Ribose Poly I-C Polyethylene Glycol 400 4 MG/ML / Propylene glycol 3 MG/ML Ophthalmic Solution Polyethylene Glycols Polymer Polyvinyl Chloride Radiation hardening Series and parallel circuits Situated Stage level 2 Stationary state The Matrix Thermostat Device Component Transplanted tissue ethylene dimethacrylate ethylene glycol methacrylic acid microcalorimetry monomer radiation absorbed dose |
| Content Type | Text |
| Resource Type | Article |
