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What are the Phthalates?

Published: 31st May 2017
Author: H Proctor
A chemical that is often used in the tanning industry is phthalic acid. At the turn of the last century the industry was fascinated by the development of chromium tanning, that aspired to be a viable alternative to the vegetable tanning method (that was universally practiced).
During chromium tannage, masking salts can be added to increase the overall uptake of the chromium salts. Phthalic acid has been used to do this since the 1920s. But, phthalic acid is not the only phthalate used in the tanning industry and the chemical class does have a nefarious side.
Who are the bad guys?
Figure 1 shows the general structure of the phthalate esters. If the ester is connected to two hydrogen ions then the resulting chemicals are those shown in Table 1. Phthalic acid is a useful acid that is often used during deliming and chromium tannage (as either a masking agent, or as a chromium saver).  

Figure 1. The chemical structure of the phthalate esters.

The phthalic acid Chemical Abstracts Service (CAS) registry number is given in Table and this is the number that is used when the chemical companies are compiling technical data sheets (TDS). Most of the time the o-phthalic acid is used as it has the highest chelating power. In fact, when used during deliming it will try to take the bath pH to 5.51, so will be a good source of acidity, but not too excess like the other two forms of phthalic acid. 

Table 1. A table of properties of the three main phthalic acids.

Occasionally the chemical companies will sell you a deliming acid blend that includes phthalic acid, but will not use the CAS number associated with phthalic acid directly – not for any reason except that they will be using a catch-all number that will indicate a blend of dicarboxylic acids, which may include varying amounts of phthalic acid.
Phthalic acid is a chemical that has risk-phrases associated with it. It is listed as an irritant, so contact with the skin, like all tanning acids will need prevention and correct handling. It does not have special interest in terms of regulation or restriction if used in leather
The other esters of the phthalates are not treated with as much enthusiasm. There is a currently a consultation in the European Union (EU) for the transfer of some of the phthalate esters from the Candidate List to the Authorisation List (Annex XIV) these include the substances: 1,2-benzenedicarboxylic acid, dihexyl ester, branched and linear (CAS #68515-50-4) and dihexyl phthalate (CAS #84-75-3). 
These esters join a few other branched and linear phthalate esters that have already been severely restricted in the United States (US) and in the EU. The list of restricted phthalate esters includes: diisobutyl butyl phthalate (DIBP); dibutyl phthalate (DBP); diisononyl phthalate (DINP); and diisodecyl phthalate (DIDP), which are normally included as plasticisers.
Where are the esters used in the tanning industry?
These plasticisers are not generally added in the wet-end as they function as plasticisers. That is, they are useful to increase the softness and flexibility of finish films. In normal plastic production, the phthalate esters are often added into plastic containers (like bottles) to assist with softness of the plastic. A tanner would expect to see them in the manufacture of low-cost soft finishing resins, or in hard resins that companies are trying to make softer.
The problem comes from these plasticisers moving out of the plastic into the food, other contents, or liquid the container holds. The user will then consume them (or come into contact with them) and that is when the problems begin. The phthalate esters are oestrogen mimics, in that they will change aquatic organisms into females during embryo development, but they have also been implicated in causing breast cancers or cervical cancers (carcinogenic). They also disrupt general human reproduction, by lowering fertility.'
How do if I know if the leather contains these?
The easiest way to find out if you are going to have trouble from these substances, is to look at the material safety data sheets (MSDS). The law in most countries insists on the chemical labelling and packaging (CLP) of chemicals which includes regulations on the use of MSDS. In the MSDS, the tanner will find a list of restricted (or hazardous) chemicals and the quantity that they included.
The chemical companies are required by law to list the chemicals, or could face severe penalties for lacking to do so. If the restricted substances are not found there, then the tanner will use the product assuming it is not included. The leather will then be made into the final article, with possibly a sample of the unformed leather being sent to a testing house. 
The test house will send a test report back to the manufacturer, who will most likely forward the test report back to the tannery (if any detections of restricted substances, at concentrations above the permitted levels is found). The tannery then needs to know where it is likely that these chemicals are entering the process. In the case of the phthalates, then it is highly likely that it has entered through a finishing product.
The main point of this article was to draw a very clear distinction between the types of chemicals that are used in the tanning process. Not all phthalates that are used by tanners are restricted in the North American or European markets. The phthalate acids are not plasticisers and are not risk-phrased as anything more than irritants (like most tanning acids). The phthalate esters, however, do need the tanner’s attention and should be looked for in the MSDS that accompany all good chemical products.
Brown, H.C. Determination of organic structures by physical methods. (Edited by Baude, E.A. and Nachod, F.C). Academic Press, New York, 1955
In the next issue: A look at nonyl phenol ethoxylates. Why is this and the some of the other ethoxylate class banned in Europe? What are the viable alternatives? Are ethoxylates easy to understand and use?
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