M. Lazár

Efficiency of Chemical Cross-Linking of Polypropylene

Abstract The efficiency of the chemical cross-linking of polypropylene was found to be dependent on the peroxide used (dicumyl peroxide, benzoyl peroxide, t-butyl peroxide, and t-butyl peroxy-benzoate). The reason for different cross-linking efficiency of peroxide and enhancement of the cross-linking efficiency of peroxide in the presence of polyfunctional monomer-penta-erythritol tetrallyl ether are discussed.

Solid‐state polypropylene grafting as an effective chemical method of modification

The role of vinylidene groups, formed by the action of organic peroxides, on crosslinking reactions in isotactic polypropylene (iPP), is discussed. Grafting of polymerizable monomers on powdered iPP, performed below its melting temperature, occurs with high efficiency, depending on the structure of peroxide used.

PEROXIDE GRAFTING OF POWDERED POLYPROPYLENE BY BUTYL ACRYLATE

The efficiency of the grafting of powdered polypropylene (PP) by butyl acrylate (BA) initiated by tert.-butylperoxy-2-ethylhex-as a ratio of the grafted and polymerized monomer was relatively high during the whole course of the reaction and it does not decrease under 0.5. The relatively high polymerization rate and also grafting efficiency at the beginning of the reaction is refered to the gel effect - the monomer is absorbed in the amorphous part of the powdered PP. With increasing the content of monomer in the reaction feed, the content of unsoluble cross-linked polymer increased as the consequence of the radical cross-linking of polybutylacrylate chains. The mechanism of grafting of powdered PP by BA is discussed.

Oxidation of hydrocarbon polymers

Oxidation of hydrocarbon polymers which is the main process causing deterioration in the useful properties of polymeric products is discussed with emphasis on the elementary reactions of the process. The principal difference between the oxidation of low and high molecular weight hydrocarbons which consists of the effect of catalyst residues, structural defects as well as in the existence of heterogeneous oxidized microdomains and morphological heterogeneities in the polymer is outlined. Various pathways of initiation of free radical oxidation such as those caused by atmospheric pollutants, ozone, singlet oxygen, ultraviolet and cosmic radiation and different reactive groups inherently present in the polymer together with a large variety of propagation free radicals which differ each from other in reactivity and in mobility lead to a complex process the general features of which have been elucidated but particular steps are still the subject to correction. The present paper is a review of about one hundred references, of which about 60% are from last 5 years.

Degradation of Polypropylene Under the Effect of the Low-Molecular-Mass Organic Peroxides Below the Melting Temperature of the Polymer

Abstract The efficiency of five commercially available peroxides on the level of degradation of polypropylene (PP) at temperatures below its melting point has been studied. The efficiency extrapolated onto 100°C increases in the following order: tert. butylperoxy-2-ethyl-hexanoate ≈ tert. butylperoxy-isobutyrate <dilauroyl peroxide <1,1-bis(tert. butylperoxy)-3,3,5-trimethyl cyclohexane ≈ tert. butyl-perbenzoate. The various effects of peroxides on the degradation of PP is explained by different reactivity of primary radicals of the peroxides and their solubility.

Influence of Crosslinking on Surface Hardness of Poly(Methyl Methacrylate)

Abstract The influence of crosslinking on the surface hardness of poly-(methyl methacrylate) sheets examined by means of damping of standard pendulum oscillations as well as on the nonisothermal mass loss is discussed in this paper. Crosslinked poly(methyl methacrylate)s with different crosslink densities were prepared by copolymerization of methyl methacrylate with polyfunctional comonomers of the allyl and vinyl type and by additional crosslinking of poly(methyl methacrylate) by transamination with aliphatic α,ω-diamines. The highest increase in surface hardness, up to 52% of its value for a silicate plate glass standard, was observed for (PMMA-co-DAlP) sheets with 17 to 25 wt% of comonomer. For commercial noncrosslinked PMMA cast sheets, this value is only 27% of the above standard. Comonomers of the allylic type are more suitable than the methacrylic multifunctional crosslinkers. A large portion of the double bonds remains unreacted if a high concentration of multifunctional agent is copolymerized wit...

ESR studies of free radicals in crosslinked polyglycol methacrylate

Abstract The decay of free radicals in irradiated polyglycol methacrylate (PGMA) has been studied. It has been established that the degree of crosslinking of PGMA affects the rate of decay of free radicals. With increase in the degree of crosslinking the rate of decay of free radicals decreases. In the course of decay at the constant temperature of 66°, the structure of the ESR spectrum of free radicals changes rapidly. In this paper the interpretation of ESR spectra of free radicals and the effect of crosslinking on the rate of radical decay is discussed.

Oxidative electropolymerization of pyrrole in the presence of pyridinium chlorochromate

Abstract The addition of pyridinium chlorochromate during the electrochemical synthesis of polypyrrole (PPy) significantly accelerates the formation of insoluble polymer. The acceleration of electrochemical polymerization is due to chemical oligomerization of pyrrole by the oxidizing agent as well as the lower oxidation potential of the oligomer compared with the initial pyrrole. Compared with simple electrochemical synthesis the combined method of polypyrrole preparation produces a smaller amount of released gas which results in a doped polymer with a lower conductivity (ca. 10 S cm −1 ). The conductivity of polypyrrole prepared by chemical oxidation in the presence of pyridinium chlorochromate and tetraethylammonium tetrafluoroborate is about 10 9 times lower than the conductivity of PPy prepared by electrochemical reaction.

Study on the influence of water on the electrochemical preparation and conductivity of polypyrrole films and porous layers

Abstract The influence of water on the electrochemical preparation and conductivity of polypyrrole films and porous layers is discussed. Values from 0.5 to 1 wt% of water in electrolyte have been found to be optimum for obtaining polypyrrole films with maximum conductivity. Both films and layers have shown a high sensitivity to the water vapour present in the measuring column in a temperature range of 268–277 K.

Chemiluminescence of peroxides in macromolecular media

Abstract The decomposition of peroxidic initiators is accompanied by light emission in the visible part of the spectrum. This paper gives the results of a study of the decomposition of dibenzoyl peroxide in polymers, especially esters of polymethacrylic acid and copolymers with styrene. In polymer matrics, the peroxide decomposition is kinetically similar to decomposition in solvents, but there are some peculiarities due to the physical character of the macromolecular medium.