Aging of Polymers and Polymeric Materials Caused by Environmental Impact. Part 2


The Ministry of Science and Higher Education of the Russian Federation Kazan National Research Technological University











            AGING OF POLYMERS AND POLYMERIC MATERIALS CAUSED BY ENVIRONMENTAL IMPACT


            Part 2



        Tutorial












Kazan
KNRTU Press
2019

         UDK 541.64:66 (076.5)



Published by the decision of the Editorial Review Board of the Kazan National Research Technological University

Reviewers:
Professor, PhD in Chemistry A. Burilov
Associate Professor, PhD in Chemistry S. Egorova






         Authors: E. Cherezova, G. Nugumanova, G. Timirbaeva, Yu. Karaseva
         Aging of Polymers and Polymeric Materials Caused by Environmental Impact : tutorial : In 2 parts. Part 2 / E. Cherezova [et al.]; The Ministry of Science and Higher Education of the Russian Federation, Kazan National Research Technological University. - Kazan : KNRTU Press, 2019. - 112 p.

         ISBN 978-5-7882-2588-3
         ISBN 978-5-7882-2590-6 (p. 2)

       The main data on issues of polymer aging caused by various factors are summarized. The characteristics of macromolecular reactions in addition to thermal, thermal-oxidative degradation, degradation caused by the aggressive environment, high-energy radiation, etc. are considered.
       The tutorial is intended for students studying Master Degree Program 18.04.01 «Chemical Engineering» related to such study fields as Chemical Engineering of Polymers and Polymeric Materials as well as Chemical and Physical Modification of High Molecular Compounds, Chemical Engineering of Synthetic Rubber, Chemistry and Physics of High Molecular Compounds, Catalytic Engineering in Petroleum Chemistry and Polymer Chemistry.
       The tutorial is prepared by Technology of Synthetic Rubber Department.


UDK 541.64:66 (076.5)


ISBN 978-5-7882-2590-6 (p. 2)
ISBN 978-5-7882-2588-3

  © Cherezova E., Nugumanova G., Timirbaeva G., Karaseva Yu., 2019
                               © Kazan National Research Technological University, 2019

    1.     PRINCIPLES OF PROTECTING POLYMERS AGAINST THERMAL AGING. THERMAL STABILIZERS. CLASSIFICATION AND APPLICATION EXAMPLES



      The thermal resistance of polymers and polymeric materials determines their chemical stability when heated. The chemical stability of polymers under heating is characterized by the degradation temperature Td.
      The degradation temperature Td of polymers is increased by:
     -  using strong bonds in the macromolecule structure;
     -  polarizing covalent bonds forming macromolecules;
     -       accumulating conjugated regions with multiple and polarized bonds in the chains of macromolecules;
     -       introducing stable five- and (or) six-membered cycles, which are linked by strong bonds called hinges to improve technological effectiveness and crack resistance;
     -       introducing heterocycles in which atoms with varying electronegativities are present, that strengthens the cycles because of polarization and conjugation;
     -       increasing the number of bonds connecting the units of macromolecules (ladder, block-ladder, parquet structures);
     -       creating a network of chemical bonds to slow the chain processes of degradation;
     -  eliminating anomalies in arranging units (units diversity);
     -       introducing thermal stabilizers (blocking of active radicals, acceptance of low-molecular degradation products).
    The main mechanism for the reaction of thermal degradation of most polymers is a radical process, proceeding with the formation of alkyl radicals in the first stage. Radical scavengers are used as thermal stabilizers.

    1.1. Scavengers of Alkyl Radicals

1.1.1. Sterically Hindered Amines

        The mechanism of sterically hindered amine stabilizers (referred to as HAS, Hindered Amine Stabilizers) is explained by the reaction of alkyl radicals R with nitroxide radicals >N-O*.

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         Nitroxide radicals are formed during the oxidation of sterically hindered amines. Therefore, peroxide radicals in the system are necessary for the formation of key nitroxide radicals.
         Scheme 1.1 shows how sterically hindered amines based on tetramethylpiperidine derivatives go into nitroxide radicals:

(1.1)

This process proceeds as a cyclic one (fig. 1.1), the nitroxide radical > >N-O* is reduced until it is completely destroyed in the side reactions.

Fig. 1.1. The capture of radicals by nitroxide radicals

       It is known that the reactions of nitroxide radicals with peroxide and acylperoxide radicals (A, B and C) also contribute to the stabilization efficiency along with the capture of alkyl radicals by nitroxide ones, as it is shown in fig. 1.1.
       Sterically hindered amine stabilizers are also highly efficient light stabilizers and are designated simultaneously as HALS.

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1.1.2. Hydroxyl Amines


         Hydroxyl amines contribute to the stabilization of polymers according to the mechanism shown in fig. 1.2. The active substance is the nitron formed as an intermediate which can capture C-radicals.

Fig. 1.2. Capture of C-radicals by hydroxylamines



1.1.3. Benzofuranone Derivatives


      The powerful scavengers of R radicals are benzofuranone derivatives.
The assumed mechanism of action of benzofuranone is shown in fig. 1.3.

Fig. 1.3. Capture of C-radicals by benzofuranone derivatives



1.1.4. Phenols Modified by Acryloyl

       Acryloyl modified phenols are effective scavengers of C-radicals (fig. 1.4).


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Fig. 1.4. Capture of C-radicals by acryloyl modified phenols

       They are very effective in preventing the cross-linking or decomposition of styrene copolymers (e. g., styrene-butadiene-styrene or styrene-isoprene-styrene) during processing.

    1.2. Scavengers of Low Molecular Weight Degradation Products


      When a number of polymers are heated long before the main chain breaks, lateral substituents containing heteroatoms (which catalyze the further process of destruction) are separated. During the thermal aging of such polymers, scavengers of low-molecular degradation products (HCl, H2O, CH2O, etc.) are used. Bonding of degradation products increases the stability of the polymer.
    In particular, first the destruction of polyvinyl chloride (PVC) occurs due to dehydrochlorination, as a result of which hydrogen chloride (HCl) is released, and double bonds are formed in the main polymer chain (fig. 1.5).


                                t°c            .                 .
~CH₂ - CH - CH₂ - CH--------► ~CH₂ - CH - CH₂ - CH~ + ci

Cl Cl                                     Cl

~CH₂ - CH - CH₂ - CH~ + ci ---► -CH₂ - CH - CH - CH~----►
            L        L I           -HC1                 I

Cl                              Cl


-----► ~CH₂ - CH=CH - CH~

Cl
Fig. 1.5. Dehydrochlorination of polyvinyl chloride


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    The catalyst here is HCl, while the reaction proceeds more rapidly at adjacent sites with double bonds. As a result of dehydrochlorination, regions with alternating single and double bonds that are capable of changing the color of PVC are formed.



    1.3. Thermal Stabilizers for Polyvinyl Chloride

    The most commonly used PVC thermal stabilizers are organometallic compounds; salts of lead, cadmium, barium, zinc and tin; epoxy compounds; phosphites.
    Over 90 % of the total volume of used PVC thermal stabilizers are barium-cadmium, organotin and organolead compounds. The amount of using lead-containing stabilizers is constantly decreasing, which is explained by their high toxicity.
    A very important property of thermal stabilizers is the mutual enhancement of the action (synergism), so two or more kinds of thermal stabilizers are often used for thermal stabilization.
    The main effects of PVC thermal stabilizers are believed to be the following:
     -  neutralization of HC1;
     -  substitution of labile PVC chlorides by more stable ligands;
     -  addition to double bonds;
     -  prevention of oxidation;
     -  complex formation with promoters of destruction;
     -  deactivation of free radicals.
        The primary mechanism of stabilization is the substitution of labile chlorides, and the remaining mechanisms are secondary stabilization processes.

1.3.1. Metal-Containing Stabilizers for PVC

        Organotin compounds are widely used when stabilizing products from transparent rigid PVC. They are the most efficient (and most expensive per unit) of all industrial classes of primary thermal stabilizers. Organotin stabilizers can stabilize PVC by a variety of mechanisms. For example, organotin mercaptides can perform the following stabilizing functions:
    -   substitute labile chlorides - scheme (1.2);

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   -   neutralize HC1, thus forming mercaptans - scheme (1.3);
   -     mercaptan can subsequently decompose hydroperoxides -scheme (1.4) or be attached to polyene sequences - scheme (1.5).

 ~ CH2-CHC1~ + R2Sn(SR')2 ^ ~ CH2-CH(SR')~- + R2Sn(SR')Cl   (1.2)
 HCl + R2Sn(SR')2 ^ R2Sn(SRM)Cl + HS-R'                     (1.3)
 R''O-OH + HS-R' ^ R''OH + H(O)SR'                          (1.4)
 ~сн2-сна—сн=сн-сн2-сна~ + hs-r' ^
               ^ ~CH2-CHa-CH2-CH(SR')-CH2-CHCl~             (1.5)
    The organotin mercaptides are superior to organotin carboxylates in efficiency. To a large extent, this is due to the antioxidant effect of sulfur and the superiority of mercaptide ligands over carboxylate as nucleophiles.
    The mechanism of thermal stabilization of many salts described by the formula MY2 where M is a metal cation (Sn⁺², Ва⁺², Cd⁺², Zn⁺², Pb⁺²), and Y is an organic anion (thiolate or carboxylate) is linked to the reaction of labile halogen substitution according to the scheme:

 MY2 + R'Cl ^ R'Y + MYCl                                     (1.6)
 MYCI + R'Cl ^ R'Y + MCI2                                    (1.7)
      R'Cl is an unstable defect.


     The problem occurs when the resulting metal chloride MCl2 is a strong Lewis acid (e. g., CdCl2 or ZnCl2). Such particles can catalyze dehydrochlorination. This disadvantage can be overcome by using mixtures of metal salts, the combined action of which is synergistic. Typical “mixed metal” stabilizers are stearates or laureates of barium and zinc; calcium and zinc; barium and cadmium. A mechanism involving reactions (1.6) and (1.7) (where M is a metal with a higher Lewis acidity, i. e. Cd or Zn) accompanied by the reaction (1.8) can explain their synergism:


 MCl2 + M'Y2 ^ MY2      + M'Cl2

(1.8)

 where М' is Ba or Ca.
Barium chloride produces no effect on PVC.

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     The last process (1.8) has a double advantage, which is the possibility of regenerating MY2 stabilizer.
    For example, the synergism observed between carboxylates of barium and cadmium results from the reaction of barium carboxylate with cadmium chloride, as a result of which the cadmium carboxylate is reduced and barium chloride is formed, thereby preventing the accumulation of the cadmium chloride prodegradant:

(RC(O)O)2Ba + CdCl2 — (RC(O)O^Cd + ВаСЬ                    (1.9)
(RC(O)O) 2Ва + ~CH2-CHCl~ —— the reaction does not proceed (1.10)

    Soaps based on heavy metals, such as cadmium and zinc, can neutralize HC1 by the reaction:

(RC(O)O)2Cd + 2 HCl —    2 RC(O)OH + CdCl2 ,               (1.11)

but they can act as primary stabilizers due to the reaction of nucleophilic substitution of labile chlorides, which is even more important:

(RC(O)O) 2Cd + ~CH2-CHC1~                                  (1.12)
                    —         ~CH2-CH(O(O)CR)~        +
RC(O)OCdCl

    This mechanism is facilitated by the micellar structure of metallic soaps, which results from the approach of ion pairs of a metal mixture in layered micelles [(RC(O)O)2Ba-Cd(O(O)CR)2]n.
    Fig. 1.6 shows the synergistic effect of barium stearate and zinc stearate. The maximum synergistic effect in the common case is achieved at the equimolar ratio of two individual soaps.

1.3.2. Esters and Salts of Epoxy-carboxylic Acids

    The mechanism of the stabilizing effect of epoxy organic compounds is connected by the HC1-acceptor reaction. The reaction products of epoxy compounds with HCl have a chlorohydrin structure:


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--CH—CH— + HC1

(1.13)

--CH—CH—

OH Cl

Fig. 1.6. Synergism between barium and zinc carboxylates, fixed due
to the effects of color change (yellowness index) over time in the thermal stability test at 190 °C. Composition: 100pts. wt. PVC; 2.0 pts. wt. epoxidized soybean oil (ESBO); 50 pts. wt.. dioctyl phthalate; 1.0 pts. wt. barium and or zinc stearate. 1 - barium stearate 1.0pts. wt.; 2 - zinc stearate 1.0pts. wt.;
        3 - barium stearate 0.5 pts. wt. + zinc stearate 0.5 pts. wt.

       Esters of glycidic acid and its derivatives, for example, diethyl ester of phenylmethyl glycidic acid, diethyl ester of в-n-tolyl glycidic acid, alkyl ester of a-alkyl-e-(epoxyalkoxyphenyl) glycidic acid were suggested as the first epoxy stabilizers.
       Technically, esters of long chain epoxycarboxylic acids, such as oleates or linoleates, as well as natural oils, have become the most important for the stabilization of halogenated polymers.
       Epoxy esters are also used to stabilize polymers that do not contain chlorine. Esters of epoxidized higher fatty acids and С6-С16 alcohols, for example, 2-ethylhexyl epoxy stearate, can serve as thermal stabilizers of polyethylene; compounds with two or more epoxy groups, for example dicyclopentaiene diepoxide or 3,4-epoxy-6-methylcyclohexylmethyl ester of 3,4-epoxy-6-methylcyclohexane carboxylic acid, sometimes in combination with bisphenols they protect the homo- and copolymers of oxymethylene from heat:

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