Identification and assessment of alternatives to selected phthalates

48
Table 3.1
Selected parameters used to characterise plasticisers, and examples of decisive factors
influencing these parameters (primarily based on Krauskopf and Godwin, 2005)
Parameter Plasticiser characteristics influencing parameter
Solvency in polymer resin
(also called compatibility or
miscibility)
Efficiency
(defined as flexibility in polymer
compared to DEHP)
The good plasticiser for PVC has, within its molecule, a suitable mix of polar and
apolar functional groups. Oxygen (carbonyl groups) and aromatic rings impart
higher solvency. Plain hydrocarbon chain parts add apolarity, and thus lower solvency.
Good solubility is required for a plasticiser, but too high solubility dissolves
the crystalline parts of the polymer and thereby breaks the polymer apart.
Lower molecular weight phthalates such as BBP, and other high aromaticity substances
such as benzoate esters and tri(cresyl) phosphate, or more polar structures
such as sulfonates, are examples of high solvency plasticisers.
Branched hydrocarbon chains in the plasticiser increase flexibility amongst the
polymer chains, and thus the overall flexibility of the resulting material. Too many
hydrocarbon branches decrease solubility and resistance to hydrolysis (degradation
in contact with water).
Volatility Smaller, lower weight molecules tend to have higher volatility than larger, heavier
molecules. For example DBP is deemed to be too volatile for many polymer applications,
while a large molecule like ditridecyl phthalate has low volatility and can
therefore be used in polymers exposed to elevated temperatures. Large molecules
like trimellitates and polyesters have typically even lower volatility. In some cases
high volatility is desired. BBP is an example of as plasticiser which can contribute
to volatile fuming during processing and volatilization in end use applications,
and thus give a hardened, stain resistant surface, due to volatilization. The same
is the case for certain benzoates. Volatile losses of plasticiser are influenced by
vapour pressure, solvency strength for the polymer and oxidative degradation.
Diffusivity Movements of the plasticisers within the polymer matrix are ruled by diffusion.
Low diffusivity is contributed by high molecular weight and highly branched isomeric
structures. For example, DIDP and the polyester family impart improved
resistance to diffusion-controlled plasticiser losses. Plasticiser losses due to extraction
by oily media (in which plasticisers are highly soluble) are controlled by
diffusivity rates.
Low temperature performance Higher share of linear hydrocarbons (versus branched hydrocarbons) in the plasticiser
increase flexibility at low temperatures. The entire family of aliphatic dibasic
esters contributes exceptional low temperature properties. Di-2-ethylhexyl
adipate (DEHA, DOA) is the standard and most widely used plasticiser in this
class. Di-2-ethylhexyl azelate (DOZ), di-2-ethylhexyl sebacate (DOS), and
diisononyl adipate (DINA) are used for low temperature applications requiring
lower plasticiser volatility.
Another important factor for plasticiser selection is the ease of processing of
the resin-plasticiser system in the various steps involved in flexible polymer
manufacture. Both the polymer and the plasticiser characteristics influence the
processability. For a given polymer resin, the choice of plasticiser influences
the temperatures needed for gelling (absorption of the plasticiser in the resin)
and fusing (settling of the mixture in its final state), and the viscosity of the
hot PVC melt or the plastisol blend, etc. Solvency for the PVC resin plays a
role, and strong solvating plasticisers may be mixed into the general plasticiser
to enhance processability. The volatility of the plasticiser, on the other hand,
is typically the limiting factor on levels of strong solvating plasticisers used.
Higher molecular weight plasticisers typically decrease volatility, but also viscosity,
etc., with resulting constraints in processability (Krauskopf and
Godwin, 2005). BBP is an example of a plasticiser which can reduce the operating
temperatures in PVC processing.
3.2 Introduction to plasticiser substance families
This section focuses on the alternative plasticisers. The phthalates are described
in more detail in Chapter 2.