Polyethylene hydroperoxide groups

Thermal aging is another simple pretreatment process that can effectively improve adhesion properties of polymers. Polyethylene becomes wettable and bondable by exposing to a blast of hot ( 500°C) air [47]. Melt-extruded polyethylene gets oxidized and as a result, carbonyl, carboxyl, and hydroperoxide groups are introduced onto the surface [48]. 

The formation and role of hydroperoxide groups, particularly in the early stages of polymer oxidation is well discussed in the introduction to the next chapter and also features in many of the references cited in this chapter. Their detection and quantification is therefore important. Although this can be done directly or implicitly through many of the instrumentation techniques discussed in this chapter, there are several tests that have been developed, some of which are still widely used, that are based more on chemical methods, titration or staining. The majority have been applied to polyolefins, especially polyethylene. 

Breck at al (68) havs recently examined the reactivities of a number of saturated and unsaturated polymers towards singlet oxygen generated by microwave generator. The saturated ohain polymers as polystyrene,polyurethane and polyethylene were found to be inert within the experimental conditions with singlet oxygen,while the unsaturated polymers as poly-dienes were found to react quite readily.In this paper concentration of hydroperoxide groups has been measured. 

Introduction of hydroperoxide groups into polyethylene by coextrusion with air under pressure at 190°C in a 20-mm extruder with 3-min residence time. 

Both chain scission and cross-linking take place. The key intermediate is the hydroperoxide group, whose stability and decomposition has been the subject of much research. Hydroperoxides in polyethylene can have long lifetimes so if this mechanism is correct these groups should be detectable. The SO2 reaction is positive... 

Peroxyl radical reaction with the polymer. This is a generally much slower process, which is structure dependent. In saturated hydrocarbon polymers, e g. polyethylene (PE) and polypropylene (PP), it is exclusively a hydrogen atom abstraction. In this case. Per is a hydroperoxide group (POOH). The corresponding rate constant is very low = 10 -10 l.mor. s at ambient temperature (see Table 12.7). In polyenic elastomers, e g. polybutadiene (PBD) and polyisoprene (PIP), step 3 can also be an addition to double bonds. In this case. Per is a peroxide bridge (POOP). The corresponding rate constant is also very low typically = 10 -10 l.mor. s at ambient temperature for an intramolecular addition (see Table 12.7). 

Hydroperoxide groups formed at low temperatures in polyethylene (85-95°) and propylene (50-70 °C) are isolated from each other, and when UV irradiated very slowly, they photodecompose and have virtually no photo-initiating effect [145, 149, 778, 2277],... 

Hydroperoxide groups formed at high processing temperatures in polyethylene (160 °C) [125, 848, 1918, 2279] and polypropylene (135 °C) [443, 1879,1975,2339] are associated (adjacent) with each other, and are rapidly thermally- and/or photo-decomposed into free radicals which initiate the degradation process. 

In polyethylene the proportion of vicinal hydroperoxides is lower than in PP and random chain scission is initially the dominant process. However, a variety of low molar mass oxidation products are formed via vicinal hydroperoxides (Scheme 1). The alkoxyl radicals formed by decomposition of the hydroperoxides contain weak carbon-carbon bonds in the a positions to the hydroperoxide groups which lead to the formation of low molecular weight aldehydes and alcohols that further oxidise rapidly to abiotically stable carboxylic acids. These are biodegradable species, similar to those formed by hydrolysis of aliphatic polyesters and they provide an environment for rapid biofilm formation (Section 5.5). 

Another phenomenon specific for polymers is the cage effect in slow bimolecu-lar reactions. It is well known that the viscosity of liquids does not influence on the rate of slow bimolecular reaction, which occurs with an activation ener and is not controlled by the rate of diffusion of reactants. However, slow reactions in the polymer matrix occur more slowly than in the liquid under the same conditions. It was proved by comparison of the experimetal rate constants of the reaction of 2,4,6-tri-tert.butylphenoxyl radical with hydroperoxide groups of polypropylene (PP) and polyethylene (PE). 

Pobedimsky D.G., Kirpichnikov P.A., and Denisov E.T., About reactions of phosphorus-organic inhibitors with hydroperoxide groups and polyethylene peroxide radicals , Vysokomol. Soed., 1976, vol. A18, pp. 2650 - 2658. (Rus)... 

The products of thermal oxidation of polyethylene films can be characterized by C FTNMR furthermore, using the spin-lattice relaxation technique, quantitative estimates can be made of the oxidized functional groups. Observation of the development progress of the various functional groups leads to the postulation of hydroperoxides as the primary oxidation products, which undergo further transformations to the other derivatives in a complex scheme . 

The most Importeuit Initiation process involved in the early stages of the photo-oxidation of polyethylene is shown to be hydroperoxide photolysis eissociated with the decay of virylidene groups. This is followed by carbonyl photolysis occurring primarily by the Norrish type II process. 

Starch and cellulosic materials are frequently used as fillers in degradable materials. The addition of starch to LDPE in combination with a pro-oxidant increases the photooxidation rate and the formation of hydroperoxides and carbonyl groups. Starch alone does not increase the photooxidation rate. The addition of starch to LDPE increases its stability in 80°C water. Slower degradation in water is due to leaching out of the pro-oxidant. The addition of starch causes biodegradation process under soil burial conditions. Further increase in the degradation rate can be achieved by preheating polyethylene filled with starch. ... 

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