Plasticizers and Plasticization

Plasticization is a process in which the thermal and mechanical properties of a system or material

such as a polymer are changed (Immergut and Mark 1965). These changes involve the polymer

becoming more flexible at room temperature, increasing the amount of elongation possible before

breakage occurs (at room temperature), and increasing the mechanical strengths (tensile and tear) of the

polymer (Vieira et al. 2011; Bergo and Sobral 2007). Plasticizers are defined by the desired properties

they impart to a polymer system. For example, in a food packaging biopolymer, polyols are used as

plasticizers to increase flexibility.

1.3.1. Types of Plasticizers

There are two main types of plasticizers: external and internal. The first type, external plasticizers,

are plasticizers that interact with the polymer without chemically reacting (Vieira et al. 2011; Immergut and

Mark 1965). They typically have low vapor pressures and interact with polymers at increased

temperatures by way of their swelling (solvent) power. When external plasticizers do not completely enter

both the amorphous and crystalline regions of the polymer (often only the amorphous region is

penetrated), they are considered secondary plasticizers because they are non-solvent plasticizers. The

second type of plasticizers, internal plasticizers, are chemically bonded to the polymer. This is due to the

plasticizer being a second monomer that is copolymerized as an integral part of the polymer system. The

result of this copolymerization is that the polymer is more disordered, which reduces the glass transition

temperature of the material.

1.3.2. Opposition to Plasticization

Intermolecular forces including dispersion forces, induction forces, dipole-dipole interactions, and

hydrogen bonding must be overcome by a plasticizer in order for it to enter the intermolecular spaces

serve as a plasticizer (Vieira et al. 2011; Immergut and Mark 1965). All intermolecular forces must be of

the same magnitude in order for plasticization to take place: plasticizer-plasticizer, polymer-polymer, and

plasticizer-polymer. Crystallinity must be interrupted for a substance to be properly plasticized. This is

important because if the substrate is completely crystalline, the plasticizer cannot penetrate the ordered

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1.3.3. Requirements for Plasticization

The four main requirements for plasticization include solvent power, compatibility, efficiency, and

permanence (Immergut and Mark 1965). Firstly, plasticizers must be able to act as solvents in order to

penetrate the crystalline regions of polymers. When plasticizers are non-solvent, they act as softeners

and only enter the amorphous regions. The downside of disrupting the crystallinity of a polymer with a

plasticizer is that the mechanical properties of said polymer that depend upon its crystallinity, including

tensile strength and tear resistance, will change. Secondly, the optimal temperature range for the

plasticizer must match that of the polymer. This ensures that processing and use temperature ranges will

be optimal for all chemical species involved. In addition, the polarity, molecular weight, and shape of the

plasticizer should be complimentary to those of the polymer. Thirdly, when efficiency is used to describe a

plasticizer-polymer relationship, it refers to the balance between the levels of plasticizer and polymer

required to produce a desired polymeric system. This is determined by plasticizer size, shape, molecular weight, and rate of diffusion into the polymer’s amorphous and crystalline regions. For example, smaller molecules will diffuse more quickly than larger molecules which makes them more efficient. Fourthly,

plasticizers must be permanently ingrained within the polymeric matrix in order for an effective plasticizer-

polymer relationship to have taken place. One of the main factors in permanence is volatility of the

plasticizer. The larger the molecule, the lower the volatility and longer the permanence. The ability of the

plasticizer to diffuse into the polymer matrix also impacts permanence, so the greater the diffusion, the

greater the permanence. However, if a plasticizer can diffuse quickly into a polymeric matrix it can also

diffuse out of the matrix quickly. Thus molecular size must be well thought out when choosing a

plasticizer.

1.3.4. Polyols as Plasticizers

Films made from only AX can be brittle, causing them to crack when handled and have low

extensibility and flexibility. Plasticizers are small, non-volatile compounds added to reduce the brittleness

and increase the amount of flexibility of a film (Antoniou et al. 2014; Vieira et al. 2011). Polyols such as

sorbitol are commonly used as plasticizers in polysaccharide-based films (Antoniou et al. 2014). Sorbitol

is the common name for D-Glucitol, which is the fully reduced form of glucose (BeMiller 2007a). It

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2014). Sorbitol is a highly soluble compound that is available in both crystalline and syrup forms (BeMiller

2007a). Sorbitol is a compound that is generally recognized as safe that is non-cariogenic and a

humectant. Polyols are a form of dietary fiber, but they also have a sweet taste (the degree of sweetness

depends upon the polyol). Glycerol has a lower molecular weight than sorbitol, and as a result, has a

higher ratio of hydroxyl groups per unit mass than sorbitol (Antoniou et al. 2014).

The type of plasticizer used greatly influences the characteristics of a material. For example, films

that utilize glycerol as a plasticizer tend to have good mechanical properties including elongation and

resistance (Antoniou et al. 2014). However, films made with sorbitol have lower hydrophobicity and water

vapor transmission rates than those made with glycerol. Regardless of which polyol is chosen, both

increase the mobility present in the polymer chains and reduce the glass transition temperature.