Phenolic, non-staining antioxidants - a guide to select the right product.

Phenolic, non-staining antioxidants - a guide to select the right product

Phenolic antioxidants - a historical background

At the early beginning of manufacturing rubber goods, the producers were faced with rapid deterioration. It resulted in either hardening, brittleness or in sickness of the rubber goods. Around 1910 the first type of antioxidants were discovered, mainly naturally existing materials like creosote, asphalt and coal tar pitch. These were used until roughly 1925 when phenol, cresol, hydroquinone and aniline were used as antioxidants. Between 1925 and 1940 a big number of patents on antioxidants were issued, most of them based on secondary aromatic amine derivatives.

It was not until after 1940 when an increasing worldwide production of synthetic rubber, and consequently the development of tires based on synthetic rubber, called for a more demanding type of antioxidant, not only protecting the rubber goods and tires against oxygen and heat, but also against the severe cracking caused by ozone. The result was the development of a new class of antioxidants, p-phenylendiamine derivatives, which was soon after called antiozonant, due to their ability to protect rubber goods against cracking.

After 1945 the existing large synthetic rubber production had to be diverted to civilian use. The known and common used antioxidants and antiozonants were up to this stage staining types of chemicals. With the new approach to civilian use, there was a demand for non-staining antioxidants because synthetic rubber for civilian use included natural, white or light colored rubber goods, for example:

* Sports rubber goods;

* Surgical rubber goods;

* Latex products, including foam;

* White side-walls;

* Footwear;

* Sheetings;

* Flooring tiles;

* Sponge rubber; only to name a few. With the development of non staining antioxidants, it was possible to protect rubber against deterioration, caused by oxygen, heat, light and certain metals like copper and manganese, and still maintain the natural color of the rubber. These non staining antioxidants are more or less hindered phenolic antioxidants and are the subject of this article.

Antioxidants - economical background

Antioxidants are rubber chemicals added to the rubber in the range of 1 to 2%. Although the price of these antioxidants is ranging from $5 to $10/kg., the cost of protecting the rubber is only a few cents. It is estimated that the lifetime of rubber goods is three to five times longer when protected with antioxidants and one can get some idea on the indirect value of antioxidants. Adding to this indirect value, costs of failure like idle machinery, break down of cars, blown tires and so on, then it is easy to understand that the costs of antioxidants becomes insignificant compared to the value of the improved rubber goods.

Antioxidants - theory of oxidation

Oxygen is present in our atmosphere with appr. 20 (vol-) %. In addition, ozone is ranging from 1 to 20 pphm.

The first step of the oxidation attack is started by abstracting one hydrogen from the polymer chain by a starting-radical. [Mathematical Expression Omitted]

Then oxygen can react with the rubber radicals to form peroxides. [Mathematical Expression Omitted]

The peroxide again can react with a hydrogen from another polymer chain to form again a rubber radical and a hydroperoxide. [Mathematical Expression Omitted]

Furthermore, hydroperoxides can form a peroxide again and a rubber alkoxy radical. This radical can than take a hydrogen again from other chains and so the oxidation process and subsequent aging continue. [Mathematical Expression Omitted]

This aging effect can be seen, for example, as a softening with natural rubber and polyisoprene or a hardening in the case of SBR or NBR. The first step needs energy and oxygen. While energy is coming from light, motion and heat, the oxygen comes from 20% oxygen in the air and also from ozone which is in the air in the range of 1.0 - 2.0 pphm. The oxidation process occurs in a different mechanism. The aging effect can be accelerated by heavy metals, such as copper, manganese and iron which very often is part of the filler used.

Classification

Within the group of phenolic, nonstaining antioxidants there are basically three types of products:

* Monophenols - High volatility, discoloration tendency, medium performance;

* Bisphenols - Low volatility, discoloration tendency only when bridged in ortho position, high performance;

* Polyphenols - Extreme low volatility, no discoloration, resistant against leaching and extraction, very high performance, best AO for latex application.

All phenolic antioxidants have something in common:

* Bulky ortho groups;

* Para chain.

In case of bisphenolic antioxidants an extra factor is involved: The bridge linking the phenols is in either the ortho or the para position.

A bridge in the ortho position (22M46) gives excellent antioxidant performance, but a tendency to discoloration (pinking). In case of a para position (44B25, 44S36) the antioxidant performance is slightly reduced, but no discoloration occurs.

The only exceptions to this general characterization are the polyphenols, which are ortho-bridged and therefore have high antioxidant performance, but a lack of any discoloration.

Antioxidants - testing methods

Methods of testing rubber goods and the evaluation of antioxidants are generally well known. Phenolic non-staining antioxidants do not give any or very little protection against ozone aging. Due to this fact, accelerated aging tests are normally done in an air circulated over at varying temperatures.

The aging test is done at 70 [degrees] C to 120 [degrees] C, and after 7, 14 and 28 days of aging, samples are taken from the oven and the retained percentage of physical properties (elongation, tensile strength) are compared between different antioxidants and the blank.

In this testing procedure, aminic antioxidants always indicate a better performance, due to their ability to protect rubber against high temperature, with all disadvantages like extractability, discoloration and toxicological question.

This is one of the reasons why aminic staining types of antioxidants will be used in rubber goods containing carbon black.

The selection of a non-staining phenolic type of antioxidant depends on:

* Color properties;

* Physical form;

* Volatility;

* Performance;

* Price;

* Toxicology.

Especially for latex applications the extractability is an important factor. The performance of any antioxidants with high extractability is decreasing to such an extent that the required protection is not existing anymore.

The aminic antioxidant ACP (TMQ) is extracted to such a large extent, that after a normal aging procedure of 14 days no protection is existing anymore.

Cost and performance must be studied together. Since the curve indicating the level of protection is not linear, a doubling of the amount of antioxidant does not result in a doubling of the protection; but yet the cost will double.

Therefore it is important to test and evaluate cheaper priced (per kg) antioxidants and judge those "bargains" on their price/performance level.

An easy and very fast "screening test" for the evaluation of antioxidants is by DSC (differential scanning calorimetry). Tested at high temperature under pure oxygen it indicates by oxygen uptake the performance of antioxidant, but does not correlate well to practice.

Conclusions

The expected aging performance of any non-staining, phenolic antioxidant can be predicted by various properties.

The relation between structure and performance is best respected by simple testing as aging in an air circulated oven at higher temperatures or by DSC, however it does not take into account interferences between antioxidants and the vulcanization systems and very often does not correlate to normal lifetime at room temperature. In severe testing conditions, for example latex applications. It is necessary to consider the probability of volatilization or extraction (washing test) to predict the aging performance.

PHOTO : Figure 1-monophenols

PHOTO : Figure 2-bisphenols

PHOTO : Figure 3-polyphenols

PHOTO : Figure 4-testing at 110 [degrees] C, 1% AO

PHOTO : Figure 5-volatility at 135 [degrees]

PHOTO : Figure 6-antioxidant performance in NR

PHOTO : Figure 7-antioxidant performance in chloroprene rubber

PHOTO : Figure 8-air aging at 120 [degrees] C until brittleness (SBR latex)

PHOTO : Figure 9 antioxidant extractability in latex

PHOTO : Figure 10 Performance level