**3. Mechanisms of plasticization**

142 Recent Advances in Plasticizers

Plasticizers are often inert organic compounds with low molecular weight, high boiling points and low vapor pressures that are used as polymer additives. The main role of the plasticizer is to improve mechanical properties of the polymers by increasing flexibility, decreasing tensile strength and lowering the second order transition temperature [6]. The International Union of Pure and Applied Chemistry (IUPAC) developed a definition for a plasticizer as a "substance or material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability, or distensibility" [7]. Attributes of a good plasticizer are good compatibility with polymer, which depends on polarity, solubility, structural configuration and molecular weight of plasticizer and results from a similar chemical structure of polymer and plasticizer. Other important factor is plasticizer permanence related to its resistance to migration. Therefore, a good plasticizer should have high boiling point and low volatility (low vapor pressure) to prevent or reduce its loss during processing. Plasticizers should also be aroma free and non-toxic. Another important feature is low rate of migration out of material to preserve desirable properties of plasticized polymer and avoid contamination of the materials from the point of potential health and environmental impacts in contact with it. The permanence of plasticizer in polymer is dependent on the size of the plasticizer molecule, thus the larger molecules, the greater permanence of the plasticizer. The higher diffusion rate of plasticizer in the polymer, the lower permanence due to the migration out of the polymer matrix [8, 9]. Plasticizers influence also processing of the polymers by changing various parameters: viscosity, filler incorporation, dispersion rate, flow, power demand and heat generation [7]. A good plasticizer should also be insensitive to solar UV radiation, stable in a wide temperature range and inexpensive [6]. The efficiency of a plasticizer is defined as the quantity of plasticizer required to provide desired mechanical properties of obtained material [8]. Taking into consideration that effective plasticization is depended on such factors as: chemical structure of the plasticizer, its compatibility and miscibility with the polymer, molecular weight and concentration of plasticizer, rate of diffusion of the plasticizer into the

polymer matrix, different polymers require different plasticizers [8].

There are two techniques for plasticization: external and internal. External plasticization is a method that provides plasticity through physical mixing. Thus, external plasticizers are not chemically bound to the polymer and can evaporate, migrate or exude from polymer products by liquid extraction [6]. Plasticization of polymers by incorporation of comonomers or reaction with the polymer, providing flexible chain units is called an internal plasticization. Internal plasticizers are groups (flexible segments) constituting a part of a basic polymer chain, which may be incorporated regularly or irregularly between inflexible monomers (hard segments) or grafted as side chains thus reducing intermolecular forces [7, 10-12]. According to the compatibility with the polymer, external plasticizers can be classified into two principal groups: primary and secondary ones, called also extenders. Primary plasticizers have a sufficient level of compatibility with polymer to be able to be used as sole plasticizer in all reasonable proportions, giving a desirable modifying effect. They interact directly with chains. Secondary plasticizers have limited compatibility and will exude from the polymer if used alone. They are used along with the primary plasticizer, as a part of plasticizer system, to meet a secondary performance requirements (cost, low-temperature properties, permanence). Extenders can

**2. Plasticizer classification** 

There are several theories that describe the effects of plasticizers and a combination of them allows to explain the concept of polymer plasticization [8, 10, 13-15]:


The Effect of Concentration and Type of Plasticizer

on the Mechanical Properties of Cellulose Acetate Butyrate Organic-Inorganic Hybrids 145

peroxisome proliferation, reproductive toxicity, carcinogenicity, allergic symptoms in children [6, 21, 22, 24, 25]. Public health concerns implied changes in legal provisions. Since 1999, the use of six phthalate plasticizers: DINP, DIDP, DEHP, DBP, BBP and DnOP (di-n-octyl phthalate) in childcare products and toys that can be placed in the mouth of children under the age of three in European Union is restricted. Further regulations in 2005 introduced directive that forbids the use of DEHP, DBP and BBP in any toys and childcare articles within European Union. DEHP, DBP and BBP are also forbidden to be used in cosmetic products and restricted in preparations such as paints and varnishes for end-consumers [18, 22]. The above mentioned reasons caused growing interest in less questioned substitutes of phthalate esters. Commercial used phthalates can be replaced by nontoxic alkyl esters of adipic and citric acids or natural-based plasticizers like epoxidized triglyceride vegetable oils from soybean oil, linseed oil, castor-oil, sunflower oil, and fatty acid esters [7]. The advantages of these alternative additives are good technical performance, processing ease and low toxicity. An important feature of alternative plasticizers is also biodegradability, due to the growing interest of materials obtained from degradable polymers and biopolymers from renewable resources [26, 27]. Other substitutes to phthalates are polymeric plasticizers (for example based on phthalic acid) and oligomers that exhibit low volatility and thus show low rate of migration out of the polymer and leaching tendency. Promising properties show also phenol alkyl sulfonate plasticizers which exhibit excellent gelling capacity thus reducing processing time and temperature. This class of additives shows also reduced leaching tendency and are predestined for medical applications such as polymeric materials exposed to warm, aqueous media for an extended period of time. An interesting, environmentally friendly alternative to phthalates (especially for PVC and poly(methyl methacrylate) are also ionic liquids, however they are still under research [6]. Among esters of bioderived citric acid tributyl citrate, acetyl tributyl citrate, triethyl citrate, acetyl triethyl citrate, and tri(2 ethylhexyl) citrate are of importance. Citric acid esters have been approved as plasticizers for medical plastics, personal care, and according to the U.S. Food and Drug Administration, as additives in food [9, 28]. Citrate esters have been used as effective plasticizers for environmental friendly polymers such as poly(lactic acid), cellulose acetate. However, besides enhanced processability, accelerated degradation rates were also observed [29]. Another class of plasticizers applied in biodegradable polymers are polyols. Among them glycerol, ethylene glycol (EG), propylene glycol (PG), diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol and polyethylene glycol (PEG) are the most often used as polymer additives [6, 7]. Glycerol, which have found application as effective plasticizer for starch or gelatin, and TEG are suitable for use in the

food industry as they are on the FDA's Generally Regarded As Safe (GRAS) list [6].

cellulose acetate butyrate plasticized with TEC [27, 30-33].

price.

In spite of a wide range of new plasticizers available for polymer industry it must be emphasized that alternative additives may replace traditional ones only in some specific applications due to the several requirements: compatibility, solvation, permanence and

There are numerous reports in the literature associated with polymer blends based on cellulose derivatives plasticized with conventional and alternative plasticizers: cellulose acetate plasticized with DEP, triethyl citrate (TEC), and poly(caprolactone triol) (PCL-T),

d. Mechanistic theory of plasticization considers that plasticizer molecules are not bound permanently to the polymer, but rather there is a dynamic exchange process whereby, a constant associations and disassociations of polymer-polymer, polymerplasticizer and plasticizer-plasticizer molecules form. Some plasticizers form stronger associations with polymer than others. At low plasticizer levels, the plasticizerpolymer interactions are the dominant interactions, what explains "antiplasticization". At high plasticizer loadings plasticizer-plasticizer associations predominate.

Plasticizers have been used as a polymer additives since 1800s [7]. The worldwide plasticizer demand in 2009 was about 5.7 million tons constituting 51.8% share of global polymer additives market [16]. About 100 plasticizers among 1200 different plasticizers produced worldwide are classified as commercially important [7]. Approximately 90% of all plasticizers are used in plasticized or flexible poly(vinyl chloride) (PVC) products [13, 16]. Plasticizers are also required in such polymer systems as poly(vinyl butyral), poly(vinyl acetate), acrylic polymers, poly(vinyldiene chloride), nylon, polyamides, cellulose molding compounds, polyolefins and certain fluoroplastics [7, 17]. The most significant and the largest group of PVC plasticizers is esters of phthalic acid with the share of 97% of all plasticizers used. Phthalate esters plasticizers are mostly based on carboxylic acid esters containing linear or branched aliphatic alcohols of chain lengths C6-C11. Phthalate esters have been used as plasticizers in plastic materials since the 1920s. Widely used phthalates are: di(2-ethylhexyl)phthalate (DEHP), also known as dioctyl phthalate (DOP), di-isononyl phthalate (DINP), di-isodecyl phthalate (DIDP), di-butyl phthalate (DBP) and butyl benzyl phthalate (BBP). The most broadly used since 1930s phthalate plasticizer has been DOP [6, 7]. Phthalate esters are usually added in concentrations up to 50% of the final weight of the products [18, 19]. According to Ceresana Research report, plasticizer market in 2010 was dominated by phthalate esters, with 54% share of DOP, as the most widely used. Ceresana Research forecasts that over the next years DOP will be increasingly replaced by alternative plasticizers due to worldwide growing concerns about the potential toxicity of phthalate esters to humans and the environment [20]. The application of phthalate plasticizers is being questioned because as low molecular weight compounds they migrate out of the polymer matrix. Since they are commonly used in a variety of products: flexible plastics, toys, flooring and car dashboards, food contact materials, packaging systems, synthetic leather, medical devices like blood transfusion bags and haemodialysis tubing, cosmetics, as a result, they have been found in terrestrial and aquatic ecosystems, in domestic foods and wastes, and also in animals and humans. Main human exposure pathways to phthalates include inhalation of air contaminated due to off-gassing from plasticized products, also food and drinking water containing plasticizers that exude from packaging materials designed for victuals or are extracted by the foodstuff [6, 18, 21]. Unfortunately, the exposure to a number of phthalates among the general population is wide, with the highest doses for infants and children, due to additional intake caused by the mouthing behavior of toys. Important exposure pathways of phthalates are food and intensive medical care [6, 22]. There are numerous reports showing that phthalates exert adverse effects on animals' liver, heart, kidney, lungs [23]. A number of studies have been also conducted to evaluate the potential toxicity of phthalate plasticizers on human health. The results showed several implications: hormonal disorders, inducing hepatic

d. Mechanistic theory of plasticization considers that plasticizer molecules are not bound permanently to the polymer, but rather there is a dynamic exchange process whereby, a constant associations and disassociations of polymer-polymer, polymerplasticizer and plasticizer-plasticizer molecules form. Some plasticizers form stronger associations with polymer than others. At low plasticizer levels, the plasticizerpolymer interactions are the dominant interactions, what explains "antiplasticization". At high plasticizer loadings plasticizer-plasticizer associations

Plasticizers have been used as a polymer additives since 1800s [7]. The worldwide plasticizer demand in 2009 was about 5.7 million tons constituting 51.8% share of global polymer additives market [16]. About 100 plasticizers among 1200 different plasticizers produced worldwide are classified as commercially important [7]. Approximately 90% of all plasticizers are used in plasticized or flexible poly(vinyl chloride) (PVC) products [13, 16]. Plasticizers are also required in such polymer systems as poly(vinyl butyral), poly(vinyl acetate), acrylic polymers, poly(vinyldiene chloride), nylon, polyamides, cellulose molding compounds, polyolefins and certain fluoroplastics [7, 17]. The most significant and the largest group of PVC plasticizers is esters of phthalic acid with the share of 97% of all plasticizers used. Phthalate esters plasticizers are mostly based on carboxylic acid esters containing linear or branched aliphatic alcohols of chain lengths C6-C11. Phthalate esters have been used as plasticizers in plastic materials since the 1920s. Widely used phthalates are: di(2-ethylhexyl)phthalate (DEHP), also known as dioctyl phthalate (DOP), di-isononyl phthalate (DINP), di-isodecyl phthalate (DIDP), di-butyl phthalate (DBP) and butyl benzyl phthalate (BBP). The most broadly used since 1930s phthalate plasticizer has been DOP [6, 7]. Phthalate esters are usually added in concentrations up to 50% of the final weight of the products [18, 19]. According to Ceresana Research report, plasticizer market in 2010 was dominated by phthalate esters, with 54% share of DOP, as the most widely used. Ceresana Research forecasts that over the next years DOP will be increasingly replaced by alternative plasticizers due to worldwide growing concerns about the potential toxicity of phthalate esters to humans and the environment [20]. The application of phthalate plasticizers is being questioned because as low molecular weight compounds they migrate out of the polymer matrix. Since they are commonly used in a variety of products: flexible plastics, toys, flooring and car dashboards, food contact materials, packaging systems, synthetic leather, medical devices like blood transfusion bags and haemodialysis tubing, cosmetics, as a result, they have been found in terrestrial and aquatic ecosystems, in domestic foods and wastes, and also in animals and humans. Main human exposure pathways to phthalates include inhalation of air contaminated due to off-gassing from plasticized products, also food and drinking water containing plasticizers that exude from packaging materials designed for victuals or are extracted by the foodstuff [6, 18, 21]. Unfortunately, the exposure to a number of phthalates among the general population is wide, with the highest doses for infants and children, due to additional intake caused by the mouthing behavior of toys. Important exposure pathways of phthalates are food and intensive medical care [6, 22]. There are numerous reports showing that phthalates exert adverse effects on animals' liver, heart, kidney, lungs [23]. A number of studies have been also conducted to evaluate the potential toxicity of phthalate plasticizers on human health. The results showed several implications: hormonal disorders, inducing hepatic

predominate.

peroxisome proliferation, reproductive toxicity, carcinogenicity, allergic symptoms in children [6, 21, 22, 24, 25]. Public health concerns implied changes in legal provisions. Since 1999, the use of six phthalate plasticizers: DINP, DIDP, DEHP, DBP, BBP and DnOP (di-n-octyl phthalate) in childcare products and toys that can be placed in the mouth of children under the age of three in European Union is restricted. Further regulations in 2005 introduced directive that forbids the use of DEHP, DBP and BBP in any toys and childcare articles within European Union. DEHP, DBP and BBP are also forbidden to be used in cosmetic products and restricted in preparations such as paints and varnishes for end-consumers [18, 22]. The above mentioned reasons caused growing interest in less questioned substitutes of phthalate esters. Commercial used phthalates can be replaced by nontoxic alkyl esters of adipic and citric acids or natural-based plasticizers like epoxidized triglyceride vegetable oils from soybean oil, linseed oil, castor-oil, sunflower oil, and fatty acid esters [7]. The advantages of these alternative additives are good technical performance, processing ease and low toxicity. An important feature of alternative plasticizers is also biodegradability, due to the growing interest of materials obtained from degradable polymers and biopolymers from renewable resources [26, 27]. Other substitutes to phthalates are polymeric plasticizers (for example based on phthalic acid) and oligomers that exhibit low volatility and thus show low rate of migration out of the polymer and leaching tendency. Promising properties show also phenol alkyl sulfonate plasticizers which exhibit excellent gelling capacity thus reducing processing time and temperature. This class of additives shows also reduced leaching tendency and are predestined for medical applications such as polymeric materials exposed to warm, aqueous media for an extended period of time. An interesting, environmentally friendly alternative to phthalates (especially for PVC and poly(methyl methacrylate) are also ionic liquids, however they are still under research [6]. Among esters of bioderived citric acid tributyl citrate, acetyl tributyl citrate, triethyl citrate, acetyl triethyl citrate, and tri(2 ethylhexyl) citrate are of importance. Citric acid esters have been approved as plasticizers for medical plastics, personal care, and according to the U.S. Food and Drug Administration, as additives in food [9, 28]. Citrate esters have been used as effective plasticizers for environmental friendly polymers such as poly(lactic acid), cellulose acetate. However, besides enhanced processability, accelerated degradation rates were also observed [29]. Another class of plasticizers applied in biodegradable polymers are polyols. Among them glycerol, ethylene glycol (EG), propylene glycol (PG), diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol and polyethylene glycol (PEG) are the most often used as polymer additives [6, 7]. Glycerol, which have found application as effective plasticizer for starch or gelatin, and TEG are suitable for use in the food industry as they are on the FDA's Generally Regarded As Safe (GRAS) list [6].

In spite of a wide range of new plasticizers available for polymer industry it must be emphasized that alternative additives may replace traditional ones only in some specific applications due to the several requirements: compatibility, solvation, permanence and price.

There are numerous reports in the literature associated with polymer blends based on cellulose derivatives plasticized with conventional and alternative plasticizers: cellulose acetate plasticized with DEP, triethyl citrate (TEC), and poly(caprolactone triol) (PCL-T), cellulose acetate butyrate plasticized with TEC [27, 30-33].

The Effect of Concentration and Type of Plasticizer

name

CAB

TEA

Diethyl phthalate DEP COOC2H5

Dibutyl phthalate DBP COOC4H9

Dioctyl phthalate DOP COOC8H17

Table 1. Properties of the materials used in this study.

Full name Short

Tetraethoxysilane TEOS

Triethyl citrate TEC

Tributyl citrate TBC

Acetyl triethyl

citrate

Cellulose acetate

butyrate

on the Mechanical Properties of Cellulose Acetate Butyrate Organic-Inorganic Hybrids 147

O

CH2OH

CH OCOC3H7 <sup>3</sup>COO

CH2COOC2H5

CH2COOC2H5

CH2COOC4H9

CH2COOC4H9

COOC4H9

COOC2H5

COOC4H9

COOC8H17

COOC2H5

CH2COOC2H5

CH2COOC2H5

COOC2H5

Si H5C2O OC2H5

H5C2O OC2H5

HO C

HO C

CH3OOC C

Chemical structure Molecular

O

weight

average Mn ~70,000

208.33 <1 mmHg

276.28 1 mmHg

318.32 not

360.44 not

222.24 1 mmHg

278.34 1 mmHg

390.56 1.2 mmHg

(20°C)

(107°C)

available

available

(100°C)

(147°C)

(93°C)

Vapor pressure


Boiling point

(melting range 150- 160°C)

168°C

235°C/ 150 mmHg

228- 229°C/ 100 mmHg

234°C / 17 mmHg

298-299°C

340°C

384°C

In our previous work we examined the effect of inorganic phase amount and diethyl phthalate and citrate plasticizer on the degradability of organic-inorganic cellulose acetate butyrate films in sea water [34]. The results of our study showed that the higher the amount of silica incorporated into the CAB with the DEP plasticizer, the higher degradability of the samples. The experiment also showed a synergistic effect of the applied plasticizer on the degradation rate of the CAB/silica hybrids. The CAB/silica hybrids with diethyl phthalate were degraded faster than the hybrids with tributyl citrate due to the higher brittleness of those samples. The aim of the present study is to examine the effects of six different plasticizers: citrate esters and phthalates, on the mechanical properties of cellulose acetate butyrate hybrids.
