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					In ternational Biodeterioration & Biodegradation 31 (1993) 161-170




Bunker Memorial Lecture

     Aspects of Biodeterioration of Inert and Degradable
                          Polymers


              Ann-Christine Albertsson & Sigbritt Karlsson
    Department of Polymer Technology, The Royal Institute of Technology, S-100 44
                               Stockholm, Sweden


                                      ABSTRACT

    Inert and degradable polymers are susceptible to deterioration to varying
    degrees. The process is slow for inert polymers but can be encouraged to take
    place quickly or with a predetermined rate by using degradable polymers.
    Degradable polymers can be inert polymers containing an additive, which
    thus accelerates degradation," a polymer with a backbone susceptible to, for
    example, photo-oxidation, hydrolysis, or biodegradation; or a native
    polymer (biopolymer). Performing biodegradation tests the selection of
    relevant micro-organisms, the creation of an optimal environment for
    biodegradation, and the avoidance of the accumulation of toxic compounds
    during the test, which could affect the viability of the micro-organisms



                                 INTRODUCTION

The term biodegradation is often used loosely. It is frequently used to
explain events occurring in both the natural environment and the living
body. Polymers utilised in the medical field are not necessarily degraded
biologically but are, in effect, hydrolysed (Albertsson & Ljungquist, 1988)
whereas polymers degraded in the environment suffer loss of mechanical
properties owing to ultra-violet radiation or are on occasions degraded by
the direct action of living organisms.
  Some authors define biodegradation as being a clear material/organism
                                           161
International Biodeterioration & Biodegradation 0964-8305/93/$06-00 © 1993 Elsevier
Science Publishers Ltd, England. Printed in Great Britain.
i 62                Ann-Christine Albertsson, Sigbritt Karlsson

 relationship in which specific enzymes produce a well-defined action on the
 material in question (Kumar et al., 1982, 1983). An alternative definition
 that excludes mechanisms of degradation other than biodegradation is:
 'transformation and deterioration of polymers solely by living organisms'
 (Albertsson & Karlsson, 1990).
    The ease with which polymers are degraded is entirely dependent upon
 their molecular composition and conformation. Complex molecules, such
 as lignin and asphalt, despite the fact that they are biopolymers, are very
 resistant to attack. On the other hand, synthetic polymers containing ester
 linkages (e.g. polyester polyurethanes) are readily biodegraded by the
 action of esterases, despite the fact that esterases are usually enzyme-
specific. In addition, uniform long-chain molecules similar to synthetic
polymers (e.g. polyethylene) certainly occur sporadically in nature as well
as aliphatic and aromatic macromolecules resembling those of higher
petroleum derivatives.
    Since the early 1930s,, one of the aims of polymer science has been the
development and synthesis of new inert polymers. The characteristics that
have been sought are resistance to UV radiation, moisture, organic
solvents, oxygen, etc. They have often been accomplished by the inclusion
of different additives, which can, for example, slow down photo-oxidation
and at the same time may prevent attack of the polymer by micro-
organisms. Additives can, of course, be themselves degraded, and in many
instances they diffuse into the surrounding milieu and thereby diminishing
the long-term resistance of the polymer. Low-density and high-density
polyethylene (LDPE and HDPE) are considered to be inert polymers.
However, the presence of hydroperoxides and carbonyl compounds
produced during manufacture increase their potential susceptibility to
degradation. The presence of hydrocarbons of shorter polymeric-chain
length can also be degraded, but initially no degradation of the longer
polymeric chains is observed.
    More recently, the focus has been towards the development of
degradable polymers, the most powerful reason being the increasing
problems of the disposal of plastics waste. Another important aspect is the
possibility of using degradable polymers in biomedical devices.
   There are several types of degradable polymers, some containing
chromatophoric groups that are subject to photo-oxidation and others
being subject to hydrolysis, which can occur in biotic or abiotic environ-
ments. Another method for changing polymers to make them degradable is
to add suitable inorganic or organic compounds. Degradable polymers can
also be of natural occurrence e.g. poly(fl-hydroxybutyrate) (PHB), which is
extracted from bacteria and fungi. The present paper discusses certain
aspects of the biodeterioration of both inert and degradable polymers.
                  Biodeterioration of inert and degradable polymers                     163

         MICRO-ORGANISMS AND BIODETERIORATION

Table 1 shows the degradation of paraffins on agar and in solution by two
species of fungi. The enhanced rate of degradation in solution can be
explained by the greater contact between the polymer and the micro-
organism. However, there is another factor to be considered, and that is
that the liquid has a better buffering effect than the solid substrate. It can
also be seen in Table 1 that Chrysosporium degrades these paraffins at a
greater rate than Cephalosporium.
   As the above results show, in degradation testing, it is important to
choose the relevant micro-organism to suit the polymer and the environ-
ment to which it is exposed. In general, either bacteria or fungi are utilised
or a combination of them both may be used. Older methods often
recommended using a cellulose-degrading organism as a reference in
studying biodegradation. However, this is not to be recommended because
cellulose, which is a naturally occurring polymer, has a quite different
chemical structure from most synthetic polymers. For example, it would be
very surprising if a cellulose-degrading organism attacked polyethylene.
   Nowadays, other organisms are recommended, e.g. the fungus Aureo-
basidium pullulans and the bacterium Pseudomonas aeruginosa. The
standards specify four or five different fungi and the same number of
bacteria for petri-dish testing. Pseudomonas spp. are especially useful, since
they are noted for their nutritional versatility; some are able to degrade
starch, cellulose, agar, chitin, phenols, naphthalene, hydrocarbons, and
resins.
   In some cases, it is desirable to use pure enzymes rather than a particular
organism in degradation testing, since many are endoenzymes (i.e. those
that can only degrade ingested material). In these cases, it is necessary to
follow degradation in a solution containing the polymer and the enzyme.


                                                 TABLE 1
               B i o d e g r a d a t i o n of Paraffins in A g a r a n d in Solution*

                           ngi       Cephalosporium               Chrysosproium

                                     Agar        Solution        Agar        Solution
             CzsHs8                   0.2           17             -            15
             C32H66                   0-5            2.4          10.5          25
             C36H74                   0-2            0.5           -             0-2
             L D P E + wax             -             1             7              6
             +UV 24h

             * D e g r a d a t i o n is expressed as % weight changes.
164                  Ann-Christine Albertsson, Sigbritt Karlsson

 However, enzymes are more difficult to handle than micro-organisms, since
 they are very susceptible to changes in temperature, pH, etc. Only minor
 changes are needed to render the enzyme inactive.
   Provided that the necessary nutrients and physical parameters are
 favourable, cultivation of micro-organisms is easily achieved. Naturally,
these conditions must also be met and controlled when challenging
polymers with these organisms. In order to initiate growth, inorganic
salts and sugars (e.g. glucose) are often employed. The amount must be
strictly controlled to prevent the organism from using this material as the
sole carbon source.
   Growth is confirmed by visual inspection, but absence of growth may be
due to an unsuccessful inoculation as well as to the fact that the organisms
are unable to utilise the polymer as a sole carbon source. In testing for
biodegradation, assessment also often involves calculation of the loss of
weight and microscopical examination of the polymer surface. Small
circular holes are sometimes observed when a polymer is incubated with
fungi.
   The hydrophobicity of most polymers is often a major obstacle to
obtaining good results from a biodegradation test. Consequently, it is
customary to include a surfactant (Tween) to enhance the growth
conditions. However, micro-organisms themselves also often produce
surfactants.


           MOLECULAR WEIGHT AND DEGRADATION

Albertsson and R~nby (1979) showed that the biodegradation of
polyethylene can be compared with the biodegradation of paraffins
(Table 1). Oligomers of PE C28H58, C32H66 and C36H74 lost 15%, 25%,
and 0.2%, respectively, over a four-month incubation period.
  Adding a paraffin (C32H66) to an H D P E does not increase the
degradation initially (see Fig. 1). This is because the paraffin is degraded
preferentially and the HDPE is not attacked until the paraffin has been
exhausted. Nevertheless, after four years, the percentage degradation of
H D P E with paraffin and that of H D P E without paraffin are practically the
same. When HDPE with and without oxidant is compared, the results are
quite different; the HDPE with oxidant is degraded to about one-third of
that of the HDPE without oxidant (Fig. 2).
  Low-molecular-weight compounds cause degradation of otherwise inert
PE, increase the brittleness, and form radicals that can attack the PE chain.
The degradation rate is also affected by factors such as accessibility,
                   Biodeterioration of inert and degradable polymers                                  165

                         0"6r-
                                    o 201
                         0-5~-      t 203                                -
                                    • 205
                     c" 0-4         {) 278               l'_~""

                         0.3

                         o2
                    £3
                         0.1
                                    I      1     L       L       I           L       J        I
                                         400        800      1200                            1600
                                                  Time, days

Fig. 1. Evolved 14CO2, expressed as percentage degradation of total amount of
polymeric material, as a function of degradation time (days) for high-density
polyethylene (HDPE) with and without C32H66 (FI, 201; O, 203; II, 205 = HDPE;
                       ~ , 271; O, 274 = HDPE + C32H66).




                          I
                         06
                         0.5
                     ~'0.4

                         0.3
                    "~L~ ~ |        ffc..~°'~j~)'~,~ ~                       0 203

                         0.1 ~                                               o 274
                               I~    I       I    L          I       I           I       J        J
                           0             400         800      1200                            1600
                                                  Time (days)

Fig. 2. Evolved 14CO2,expressed as percentage degradation of total amount of polymeric
material, as a function of degradation time (days) for HDPE with and without antioxidant
       (I-q, 201; O, 203; II, 205 = HDPE; O, 278; (3,279 = HDPE + antioxidant).



hydrophobicity, porosity, surface texture, and the presence of secondary
phases.


                  WEIGHT LOSS A N D D E G R A D A T I O N

Albertsson and Karlsson (1988) studied the degradation of 14C-labelled
LDPE in soil over a fifteen-year period. Half of the samples contained a
UV-sensitiser (photoaccelerator) to increase the possibility of photo-
oxidation. The percentage of 14CO2 evolved during a period of ten years
is shown in Table 2. The total degradation is low (2-15%) and is dependent
 166                 Ann-Christine Albertsson, Sigbritt Karlsson

                                     TABLE 2
             Biodegradation of ~4C-Labelled LDPE for Ten Years in Soil

            Sample                    Photo-oxidation       14C02
                                                                evolved
                                          (days)                   (%)
            LDPE                              0                    0-2
            LDPE + UV sensitiser              0                    1.0
            LDPE                             42                    5.7
            LDPE + UV sensitiser             42                    8-4



upon additives that aid photodegradation to varying degrees. However,
this is not the total degradation, since it reflects only the amount of 14CO2
absorbed into potassium hydroxide. Several samples of LDPE lost about
2% in weight during the initial photo-oxidation and others containing the
photoaccelerator lost nearly 16%. The loss is attributed to the evolution of
low-molecular-weight oxidation products.
   In the soil, carbon dioxide is not the only degradation product; typically,
the shorter hydrocarbons, alcohols, organic acids, ketones, aldehydes, etc.,
are formed (Carlsson & Wiles, 1969; Albertsson & Karlsson, 1990).
  Degradation products may remain in the system and some may provide
nutrients for microbial growth (Table3). About 10% of the 14C-value
obtained as 14CO2can be detected in the mycelium of the fungi used in the
degradation studies (Albertsson, 1988). Similar results were obtained by
Guilett et al. (1988), where up to 5% by weight of the carbon in a
photodegraded lac-labelled Ecolyte polystyrene was utilised by the fungi
over a six-month period.


               CHANGES IN POLYMER PROPERTIES

The first step in the degradation of PE is photo-oxidation, resulting in the
formation of several hydroperoxide groups. These eventually form
carboxylic groups, which undergo fl-oxidation (Albertsson et al., 1987)
and are totally degraded via the citric acid cycle, which results in the
formation of CO2 and H20. fl-Oxidation and the citric-acid cycle are
catalysed by micro-organisms.
  Changes in carbonyl groups, double bonds, and several other functional
groups can be monitored by using infra-red (IR) spectroscopy. This
method is convenient for monitoring the degradation of polymers. The
carbonyl index increases during abiotic degradation, but, under biotic
degradation, these moieties decrease.
  Monitoring the crystallinity of a polymer is another way of estimating
                                                                  TABLE3

                  Release of 14C from H D P E Film and Powder after Two Years' Storage in Biotic and Abiotic Environment*

Percentage                    Nutrient + soil               Nutrient +                   Nutrient + bactericide         Distilled water
14C released                                             Fusarium redolens
F r o m film
        in medium                  0.02                         0-05                             0.06                        0.05
        in mycelium                0-02                         0.04                               -                          _
        as 14CO 2                  0.36                         0.39                             0.12                        0.10

Total                              0.40                         0.48                             0.20                        0-15
F r o m powder
        in medium                  0.02                         0.05                             0.17                       0.08
        in mycelium                0-03                         0-04                              -                           _
        as 14CO 2                  0.51                         0.49                             0.12                       0.14

Total                              0.56                         0-58                             0.29                       0.22

*The amount of 14C found is expressed as a percentage of the total amount in the polymeric material.
168                 Ann-Chr&tine Albertsson, Sigbritt Karlsson

the degradation rate. Generally, degradation starts in the amorphous part
of the polymer and leads to an initial increase in the crystallinity as
observed by differential scanning calorimetry (DSC). An increase in
crystallinity can be attributed to weight loss, but, in fact, non-accelerated
hydrolytic degradation of poly(fl-propriolactone) showed that only 1% of
the 15% increase could be attributed to weight loss (Mathisen &
Albertsson, 1990; Mathisen et al., 1991). The other 14% increase was
due to the formation of new chain terminations during hydrolysis.


                     DEGRADATION PRODUCTS

In order to understand the mechanisms and kinetics of degradation, it is
important to monitor the products. Polyolefins degrade with a random
chain scission producing a complex of degradation products, such as
hydrocarbons, alcohols, ketones, organic acids, aldehydes, etc. (Albertsson
& Karlsson, 1990). Thermally degraded LDPE with MB gave several of the
same products (Albertsson et al., 1991). It must be remembered that the
more complex the material, the more difficult degradation products are to
detect.
   In designing degradable polymers, it is necessary to ensure that the
products are non-toxic. It is often stated that, by using naturally occurring
polymers, e.g. polyhydroxyalkanoates (PHA), the problem is avoided. This
is not always the case, since the biodegradation of caseins used as additives
in several materials produces degradation products that are not necessarily
harmless (Karlsson et al., 1988; Karlsson & Albertsson, 1990). These
products occur as metabolites in the anabolic and catabolic cycles
(Table4). Biodegradation of wood by fungi can also produce harmful
products, and several of the more potent toxins originate from fungi, e.g.
patulin, aflatoxin, etc.

                                    TABLE 4
                      The Degradation Products of Casein
Class of compound                                    Compound
Organic acid                  Acetic, propionic, butyric, valeric, caproic acids
Monoamine                     Triethylamine, n-pentylamine,di-iso-butylamine,
                              fl-phenylethylamine
Di- and polyamine             Putrescine, cadaverine, histamine, agrnatine, serotonine,
                              tyramine
                 Biodeterioration of inert and degradable polymers       169

              D E G R A D A B L E S T A R C H - F I L L E D LDPE

The most-discussed degradable polymers on the market today are those
containing biodegradable corn starch, an idea that was introduced by
Griffin, (1973, 1988). A more recent formulation (Griffin, 1988) contains
an autooxidant and a transition metal in addition to starch and gives a
material that should degrade more quickly in the environment. The
degradability is enhanced because it can take place by several routes, e.g.
photo-oxidation, biodegradation, etc.
   LDPE containing corn starch plus an additive (in a masterbatch (MB))
has been studied. LPDE with 20% MB showed the highest surface
oxidation when irradiated in a U V C O N Weatherometer (Albertsson et al.,
1991). Accelerated degradation at 100°C showed an initial low oxidation
rate, but after a while it increased considerably.
   Changes in molecular weight were monitored by using high-temperature
size-exclusion chromatography (ht-SEC). It was observed that thermal
ageing was very effective in decreasing molecular weight. Surprisingly, over
fifteen weeks, ageing at 60°C was more effective than ageing at 100°C
(Albertsson et al., 1992).
   Samples of lac-labelled LDPE with MB were inoculated with micro-
organisms without pretreatment, and others were heated to 100°C for six
days before inoculation. After approximately two months, liquid-scintilla-
tion counts showed that the heated samples were much more degraded than
the untreated ones. An initial period of heating appears to be necessary to
initiate biodegradation, as the present authors have shown in unpublished
work.



                              CONCLUSIONS

The biodegradation of natural polymers usually proceeds faster than that
of their synthetic homologues because they contain traces of amino acids,
vitamins, growth hormones, and enzymes. However, the degradation of
polymers usually depends on the molecular composition whether they are
naturally occurring or synthetic. Indeed, the conformation of some
polymers makes them very susceptible to degradation, whereas modifica-
tions to the polymer structure can often render them less vulnerable to
attack. In some biopolymers, the amino acids, etc., may make them
unsuitable because they are more readily degraded by hydrolysis or by
biodegradation.
170                   Ann-Christine Albertsson, Sigbritt Karlsson

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