1.2 Biopolymers
5
Poly(ε-caprolactone) (PCL) is a linear polyester synthesised via the ring-opening
polymerisation of ε-caprolactone in the presence of metal alkoxides [2]. It possesses
good flexibility and high elongation at break, which makes it attractive for various
applications such as polyvinyl chloride (PVC) plasticisation, drug release control
and soft compostable packaging [2]. The main drawback of PCL is its low melting
point occurring at 65 °C, which can be alleviated by polymer blending [52, 53] or by
modifications such as cross-linking [54]. PCL biodegradability can be assessed in
the presence of fungi, which is ready for enzymatic degradation [54]. Such similar
effect is also achieved in bacteria and yeast [55]. The low hydrolysis rate of PCL
homopolymer [56, 57] has been shown to be improved significantly when blended
with starch [52]. However, high production cost is considered as its main material
demerit.
In contrast to above-mentioned biopolymers, water-soluble polymers have a broad
range of applications such as food and pharmaceutical use, detergent builders, scale
inhibitors, adhesives and so on [58]. A majority of water-soluble polymers are
prepared from acrylic acid, maleic anhydride, methacrylic acid and various combinations of these monomers. Except their oligomers, these polymers are commonly
non-biodegradable. Therefore, conventional water-soluble polymers persist in water
depositories such as oceans and lakes and their effects are harder to be recognised by
consumers with potential problems [58]. Water-soluble biopolymers could be synthesised via the modification of starch and cellulose such as carboxymethyl cellulose
(CMC) and hydroxyethyl cellulose (HEC). However, the biodegradability of such
polysaccharide-derived polymer decreases when a high level of cellulose modification is used. Moreover, many of these polymers have not yet been studied by standard
testing methods to determine their biodegradability [58].
Poly(amino acids) with free carboxylic groups, such as poly(aspartic acid) and
poly-(glutamic acid), are considered as suitable material candidates for water-soluble
biopolymers. As such, those polymers based on aspartic acid can offer greater
commercial success. Poly(aspartic acid)s (thermal polyaspartate, TPAs) are functionally equivalent to poly(acrylic acid), and become fully biodegradable when they
are highly linear in chemical structures [58, 59]. TPAs used in a wide range of
applications such as performance chemicals, diapers and agriculture. Poly(malic
acid)s, the polyester equivalent of poly(aspartic acid)s, may be useful polymers for
biodegradable detergents despite their hydrolytically unstable characteristic [58].
To date, PVA is the only water-soluble polymer with exclusively carbon atoms
in main chains with good biodegradability. It is currently used in textiles, paper and
packaging industries as paper coatings, adhesives and films [58].
1.3 Poly(Vinyl Alcohol) (PVA)
PVA is a water-soluble polymer with 1, 3-diol units or 1, 2-diol units, which depends
on the hydrolysis of poly(vinyl acetate). In addition, the content of 1, 2-diol units
is reduced when the temperature of vinyl acetate polymerisation decreases. The
5
Poly(ε-caprolactone) (PCL) is a linear polyester synthesised via the ring-opening
polymerisation of ε-caprolactone in the presence of metal alkoxides [2]. It possesses
good flexibility and high elongation at break, which makes it attractive for various
applications such as polyvinyl chloride (PVC) plasticisation, drug release control
and soft compostable packaging [2]. The main drawback of PCL is its low melting
point occurring at 65 °C, which can be alleviated by polymer blending [52, 53] or by
modifications such as cross-linking [54]. PCL biodegradability can be assessed in
the presence of fungi, which is ready for enzymatic degradation [54]. Such similar
effect is also achieved in bacteria and yeast [55]. The low hydrolysis rate of PCL
homopolymer [56, 57] has been shown to be improved significantly when blended
with starch [52]. However, high production cost is considered as its main material
demerit.
In contrast to above-mentioned biopolymers, water-soluble polymers have a broad
range of applications such as food and pharmaceutical use, detergent builders, scale
inhibitors, adhesives and so on [58]. A majority of water-soluble polymers are
prepared from acrylic acid, maleic anhydride, methacrylic acid and various combinations of these monomers. Except their oligomers, these polymers are commonly
non-biodegradable. Therefore, conventional water-soluble polymers persist in water
depositories such as oceans and lakes and their effects are harder to be recognised by
consumers with potential problems [58]. Water-soluble biopolymers could be synthesised via the modification of starch and cellulose such as carboxymethyl cellulose
(CMC) and hydroxyethyl cellulose (HEC). However, the biodegradability of such
polysaccharide-derived polymer decreases when a high level of cellulose modification is used. Moreover, many of these polymers have not yet been studied by standard
testing methods to determine their biodegradability [58].
Poly(amino acids) with free carboxylic groups, such as poly(aspartic acid) and
poly-(glutamic acid), are considered as suitable material candidates for water-soluble
biopolymers. As such, those polymers based on aspartic acid can offer greater
commercial success. Poly(aspartic acid)s (thermal polyaspartate, TPAs) are functionally equivalent to poly(acrylic acid), and become fully biodegradable when they
are highly linear in chemical structures [58, 59]. TPAs used in a wide range of
applications such as performance chemicals, diapers and agriculture. Poly(malic
acid)s, the polyester equivalent of poly(aspartic acid)s, may be useful polymers for
biodegradable detergents despite their hydrolytically unstable characteristic [58].
To date, PVA is the only water-soluble polymer with exclusively carbon atoms
in main chains with good biodegradability. It is currently used in textiles, paper and
packaging industries as paper coatings, adhesives and films [58].
1.3 Poly(Vinyl Alcohol) (PVA)
PVA is a water-soluble polymer with 1, 3-diol units or 1, 2-diol units, which depends
on the hydrolysis of poly(vinyl acetate). In addition, the content of 1, 2-diol units
is reduced when the temperature of vinyl acetate polymerisation decreases. The