Annex 2 Introduction to plasticiser families

Identification and assessment of alternatives to selected phthalates

This Annex provides some basic information on the different plasticiser substances families summarised in section 3.2.

Terephthalates
Terephthalic acid is identical to phthalic acid, except for the physical placement of one part of the molecule. It may be reacted with an appropriate alcohol to produce terephthalate esters which are used as plasticisers. In practice, terephthalates are more commonly used in the USA than elsewhere. An example of a commonly used terephthalate is DEHT, which in spite of its close chemical similarity with DEHP has distinctly different health and environment characteristics (TURA, 2006). According to Krauskopf and Godwin (2005), DEHT is commercially available at similar price as DEHP.

Benzoates
Benzoates are the esterification products of benzoic acid and selected glycols, usually diols. Preferred glycols are dipropylene glycol and butane diols. One commonly used benzoate is dipropylene glycol dibenzoate, DGD (commercially Benzoflex® 9-88). Its preferred use is in PVC flooring products, owing to its strong solvating strength, and it reportedly controls plasticiser bleeding into asphalt adhesives. In vinyl sheet flooring, the benzoate enhances processing, while the low molecular weight contributes a hardened, stain resistant surface, due to volatilization, similar to the effect of BBP in flooring. Benzoates are generally strong solvaters due to the high aromaticity, as are lower molecular weight phthalates such as dihexyl (DHP) and butyl, octyl (BOP), as well as butylbenzyl (BBP). Commercial practice includes the use of up to 10–20% of the plasticiser system as “strong solvating” type plasticisers, such as aryl-alkyl phthalates (e.g. BBP), benzoates, sulfonates, and so forth (Krauskopf and Godwin, 2005).

Citrates
Citrate plasticisers are tetraesters (Krauskopf and Godwin, 2005). Examples of citrate plasticiser esters include triethyl citrate, acetyl triethyl citrate, tributyl citrate, acetyl tributyl citrate and tri-(2-ethylhexyl)-citrate. They are produced from citric acid and are traditionally used to plasticise vinyl resins used in applications including medical equipment and packaging films. Of the approximately 230,000 tons of citric acid used annually in Western Europe, 9% are applied for industrial uses such as for plasticisers. Tributyl citrate is used in PVC, polyvinyl chloride/vinylidene chloride copolymers or polyvinyl chloride/vinyl acetate resins that are subsequently used for such items as food-wrapping film. Among the advantages of tributyl citrate is that it is heat-stable and does not discolour when processed in compounded resins. Acetyl tributyl citrate is used in electrical coatings and casings because of its solvating characteristics. It is also used in inks. Acetyl tributyl citrate and acetyl triethyl citrate are also used in hair sprays and aerosol bandages. Triethyl citrate has applications in the food industry as flavour and flavour emulsion (ECPI 2009).

Trimellitates
These materials are produced by the esterification of a range of alcohols with trimellitic anhydride (TMA), which is similar in structure to phthalic anhydride with the exception of a third carbonyl group on the aromatic ring. Consequently, esters are produced in the ratio of three moles of alcohol to one mole of anhydride. Common esters in this family are Tris-2-ethyhexyl trimellitate (Tri-octyl trimellitate - TOTM), L79TM (an ester of mixed semi-linear C7 and C9 alcohols, and L810TM, an ester of mixed C8 and C10 linear alcohols. The principle features of these esters, when processed with PVC, is their low volatility, and consequently large volumes of trimellitate esters are used in high specification electrical cable insulation and sheathing. The extraction and migration resistance of these materials are also significantly improved relative to the phthalates. The low volatile loss also results in usage in automotive interior applications where the issue of windscreen fogging is important. In this respect they often compete with the linear high molecular weight phthalates such as 911P (ECPI 2009).

Adipates
Alcohols of similar chain length to those used in phthalate manufacture can be esterified with adipic acid, rather than phthalic anhydride, to produce the family of adipate plasticisers which are a part of the "aliphatic dibasic esters" in the table above. For example, esterification of 2-ethylhexanol with adipic acid yields di-2-ethylhexyl adipate (DEHA), also known as di-octyl adipate (DOA), which is reported to be the most used adipate plasticiser (ECPI 2009; Krauskopf and Godwin 2005). The family of adipic acid esters used in PVC applications improves low temperature performance relative to phthalates and give significantly lower plastisol viscosities in plastisol applications, due to the lower inherent viscosities of the plasticisers themselves. Adipates used are typically based on alcohols in the C8 to C10 range (ECPI 2009). Typically, lower molecular weight alcohols are used with higher molecular weight acids, and vice versa, such that the total carbon content per molecule ranges between C₁₈ and C₂₆. This maintains the apolar/polar ratio required to provide PVC compatibility along with low temperature properties (Krauskopf and Godwin, 2005).

Relative to phthalates, adipates have higher volatilities and higher migration rates, and are generally higher priced. As a result, it is not uncommon for adipates to be used in blends with phthalates to produce a compromise of properties (ECPI, 2009). Diisononyl adipate (DINA) is used for low temperature applications requiring lower plasticiser volatility (Krauskopf and Godwin, 2005).

Sebacatesa and azelates
Esters produced from 2-ethylhexanol and higher alcohols with linear aliphatic acids are used in some demanding flexible PVC applications where superior low temperature performance is required. Di-2-ethylhexyl sebacate (DOS) and di-2-ethylhexyl azelate (DOZ) are the most common members of this group, but di-isodecyl Sebacate (DIDS) is also used. They give good low temperature performance in combination with adipates. Their usage has generally been limited to extremely demanding low temperature flexibility specifications (e.g. underground cable sheathing in arctic environments) (ECPI 2009).

Phosphates
Phosphate plasticisers may be considered as “inorganic esters”, where the phosphate plays the role otherwise played by carboxylic acids, and bonds with alcohols or phenols to form the desired plasticisers. An important feature of phosphate plasticisers is that they, in addition to plasticising the PVC, act as a flame retardant. While hard PVC is quite resistant to fire, the addition of plasticisers generally decreases the fire resistance, and additional fire resistance can be provided by adding a fire retarding plasticiser. Commercial phosphate plasticisers use combinations of aryl (aromatic) and C8 and C10 alkyl (carbon chain) groups to offer a balance of fire reduction, volatility, and efficiency. 2-ethyhexyl diphenyl phosphate has widespread use in flexible PVC applications due to its combination of properties of plasticising efficiency, low temperature performance, migration resistance and fire retardancy. Tris(2-ethylhexyl) phosphate, tricresyl phosphate (TCP) are other examples of phosphate plasticisers. Phosphate plasticisers may be combined with other plasticisers to reduce formulating costs (ECPI 2009; Krauskopf and Godwin 2005).

Sulfonates
Sulfonates exhibit strong solvency for PVC. One example is the phenyl cresyl esters of pentadecyl sulfonic acid. It is reportedly resistant to hydrolyses and diffusion controlled plasticiser losses (Krauskopf and Godwin, 2005). ASE (alkylsulfonic phenylester) is also an example of a sulfonate plasticiser.

Epoxides
Epoxy plasticisers enhance thermal and UV stability of PVC. They are the only class of plasticisers that undergo a chemical grafting (side-chain bonding) onto the PVC polymer. They are thus internal plasticiser, contrary to phthalates. This chemical family is composed of essentially two types of epoxidized natural products. Epoxidized oils are prepared from soybean oil (ESBO or ESO) and linseed oil (ELSO). These oils have molecular weights of approx. 1,000, causing them to perform as low volatility plasticisers. The primary performance attributes of epoxy plasticisers are their role in PVC stabilization, which is accomplished at low concentrations in the PVC.

Polymeric plasticisers
Polymeric plasticisers are typically polyesters, with a molecular weight range from 1,000 to 8,000. Polyester plasticisers often have the structure of combined (bonded) propylene glycol or butylene glycol with aliphatic dibasic acids such as adipates. The greater the plasticiser viscosity, or molecular weight, the greater its permanence (e.i. resistance to loss by diffusion and evaporation). Polymeric plasticisers composed of branched structures are more resistant to diffusivity losses than those based on linear isomeric structures; on the other hand they are more susceptible to oxidative attack (degradation). The polarity, or the oxygen-to-carbon ratio, also impacts extraction resistance of the polymerics. Lower polarity materials exhibit better extraction resistance towards polar extraction fluids such as soapy water (Krauskopf and Godwin, 2005).

Other plasticiser types
Other plasticisers exist, many of which are also (like phthalates, etc.) esters of alcohols and carboxylic acids.

Pentaerythritol esters are a type of “miscellaneous” plasticisers that impart both low volatility and diffusivity. Pentaerythritol is a tetra alcohol esterified with straight chain fatty acids to make plasticisers (Krauskopf and Godwin, 2005). Trimethyl pentanyl diisobutyrate (TXIB) is another example (Eastman, 2009).