Processing Effects on Antioxidant Transformation ... - ACS Publications

27

Processing Effects

on Antioxidant Transformation

and Solutions to the Problem

Downloaded by UNIV LAVAL on September 28, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch027

of Antioxidant Migration

S. Al-Malaika* and S. Issenhuth Polymer Processing and Performance Group, Department of Chemical Engineering and Applied Chemistry, Aston University, Birmingham B4 7EΤ, England Polymer oxidation takes place inadvertently of polymers.

Antioxidants

are generally

throughout

incorporated

inhibit or minimize oxidative degradation.

Many of these

are depleted by undergoing chemical transformations their antioxidant function fabrication.

during processing and in service. Migration involving polymers

and human environment.

of antioxidants

and

polymers is a major

in direct contact with food

This concern is compounded

by the

realiza be

havior

two

of antioxidant

transformation

are advocated

of antioxidants

products.

logical antioxidant

W E L L - K N O W N

In this chapter

to minimize risk attached

and their transformation

products:

to the

migration

the use of the bio

vitamin Ε and the efficient grafting of reactive

tioxidants on polyolefin backbones during

E

performing

processing

tion that very little is known about the nature and the migration approaches

H

while

at various stages of polymer

to

antioxidants

In addition, antioxidants are subject to loss from

concern in applications

T

the life cycle

in polymers

FREE-RADICAL

an

processing.

OXIDATIVE

DEGRADATION

PROCESS

of

h y d r o c a r b o n p o l y m e r s (I) is accelerated, t o v a r y i n g extent, d u r i n g a l l phases o f t h e p o l y m e r life c y c l e . T h e stages o f c o n v e r t i n g m o n o m e r s (polymerization) a n d polymers to

finished

products

to

polymers

(processing a n d fabrica

tion) contribute significandy to accelerating the oxidative degradation

process

of polymers d u e to the presence o f adventitious chemical impurities. D u r i n g p o l y m e r i z a t i o n , f o r e x a m p l e , residues o f m e t a l ions f r o m catalysts p l a y a m a i n role i n accelerating oxidation (2). D u r i n g p r o c e s s i n g a n d fabrication, t h e b u i l d

C o r r e s p o n d i n g author 0065-2393/96/0249-0425$12.00/0 © 1 9 9 6 A m e r i c a n C h e m i c a l Society

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


426

POLYMER DURABILITY

up of small concentrations of hydroperoxides, unsaturation, and carbonyl com pounds, all due to the effect of high processing temperatures, shearing forces, and the inevitable presence of a small oxygen concentration, contribute to undesirable chemical changes in the polymer such as cross-Unking and chain scission (3, 4). These chemical changes, together with the effects of outdoor exposure to sunlight, heat, stresses, leaching solvents, and detergents, lead to premature oxidation of the product with concomitant loss of useful properties. The re processing of in-house rejects or fully pledged recycling operations for reuse of polymers in second and possibly subsequent lives, followed by reexposure to environmental conditions during reuse, contribute further to oxidative deg radation. These approaches may cause major changes to the additive system used in the polymer and lead to inferior performance of the recycled product. Figure 1 gives a schematic representation of oxidation during various steps of the life cycle of polymers.

The Role of Antioxidants and Problems of Their Migration Most polymers require the use of stabilizers and antioxidants to inhibit the oxidative degradation reactions that occur at the different stages of the poly mer life cycle. Antioxidants are normally incorporated into polymers during the high-temperature processing operation to serve as melt stabilizers or to provide protection during service as thermal and U V stabilizers. However, under the normal conditions of processing, most antioxidants undergo oxida tive transformation as a consequence of their antioxidant function (5-7). This transformation results in a chemical loss of antioxidants, which can occur dur ing processing, reprocessing, and recycling or during service life. The overall stabilization afforded to the polymer, therefore, depends not only on the in-

Figure 1. Oxidation during polymer life cycle.



27.

A L - M A L A I K A & ISSENHUTH

Antioxidant Transformations

421

itially added antioxidants but also on the behavior of the antioxidant transfor mation products. This behavior can either be beneficial (when transformation products are themselves antioxidants) or harmful (when products exert prooxidant effects) to the overall polymer stability. Physical losses of antioxidants occur because of volatilization (especially at the high processing tempera tures), poor solubility, diffusion, and leaehability when in contact with ag gressive solvents during service (8, 9). For antioxidants to be effective, therefore, they must be inherendy effi cient (based on their chemical structure and activity) and physically retained by polymers throughout their fife cycles. Migration of antioxidants and their chemical transformation products can occur from polymer articles into the surrounding medium. Even at small concentrations, this migration gives rise to major concerns when antioxidants are used in applications that involve di rect contact with the human environment (e.g., food-packaging plastics, chil dren's toys, textiles, packaging materials for pharmaceuticals and other medical applications). Migration of this sort can lead not only to premature failure of the plastics article but also to associated health hazards. In the case of foodpackaging applications migration of antioxidants and their transformation products into food represent a major source of contamination of the packaged food. Health authorities in Europe, the United States, and many other countries have strict regulations to control the use of additives in plastics used for food packaging {10,11). Existing regulations stipulate that packaging materials must not alter the quality of food and that additives must have toxicity clearance and approval for their use. Recent mechanistic studies on antioxidant action (12, 13), aided by advances in analytical techniques, have led to a better un derstanding of the antioxidancy role of parent antioxidants and their transfor mation products. Further, the problem of migration of antioxidants and transformation products has received greater attention. The shortcomings of existing regulations are that although parent antioxidants may have toxicity clearance, the chemical nature, migration behavior, and toxicity effects of their transformation products (mainly formed during processing) still remain either unknown or uncertain.

Effect of Processing on Antioxidant Performance and Antioxidant Transformations Two approaches are advocated to reduce risks associated with the migration of antioxidants and their transformation products: use of a fat-soluble biolog ical antioxidant such as vitamin Ε (α-tocopherol), which may be considered as an acceptable migrant in food packaging; and the immobilization of anti oxidants by tying them down to the polymer backbone to eliminate the prob lem of migration. In both cases the processing operation plays an important


428

POLYMER DURABILITY


role: in the use of vitamin E , processing is the major step that leads to the formation of transformation products (not only from vitamin Ε but from other antioxidants as well); whereas in the immobilization case, processing is used in a novel way to affect high levels of grafting of reactive antioxidants on polymer backbones. Use of Vitamin Ε i n Polyolefins. The structure of vitamin Ε is essentially based on a chroman nucleus and a phytyl chain containing 3 chiral carbon atoms on 2-, 4'-, and expositions. The hindered-phenol-type structure of the chroman nucleus is responsible for the intrinsic antioxidant activity of vitamin E ; the side chain plays only a minor role and acts to enhance solubility. Natural vitamin Ε is a mixture of tocopherols (α, β, 7, and δ) that differ only by the number and position of the aromatic methyl groups on the benzene ring. The most bioactive of these is 2R,4'R,8'R α-tocopherol, 1, which occurs in only one stereochemical form, the R,R,R configuration (14). Synthetic vi tamin E , on the other hand, is a cfl-a-tocopherol that is an all-racemic mixture of the eight possible stereoisomers.

The antioxidant action of synthetic hindered phenols such as Irganox 1076 (2), Irganox 1010 (3), and B H T (4) is due to their chain-breaking donor ac tivity; they donate Η to ROO* to give a stable phenoxyl radical (Scheme I) (12). Likewise, α-tocopherol was shown (15) to be a very effective alkyl-peroxyl radical trap leading to the formation of a very stable tocopheroxyl radical (Scheme II). The reactivity of α-tocopherol toward ROO" in styrene was



2 7 .

AL-MALAIKA &

I s s E N H U T H Antioxidant Transformations

429

Scheme II. found (15) to be 250 times greater than B H T . This finding was attributed to stereoelectronic effects exerted by the chroman structure giving additional stabilization to the tocopheroxyl radical (5) through interaction between porbitals of the two para-oxygens. Synthetic antioxidants containing a hindered phenol function such as Irganox 1076 and 1010 are known to be good melt stabilizers for polyolefins. Figure 2 shows the melt-stabilizing effect of 0.2% of Irganox 1010, Irganox 1076, and α-tocopherol in polypropylene (PP) and low-density polyethylene ( L D P E ) as functions of processing severity. PP undergoes chain scission dur ing processing (reflected in the increase in the melt-flow index [MFI]), whereas L D P E undergoes cross-Unking (evidenced by a decrease in M F I ) (3). Figure 2 shows clearly that, under these conditions, a-tocopherol-containing polyolefin samples exhibit higher melt stability than the best commercially available synthetic melt stabilizers such as Irganox 1010. The superiority of atocopherol is also evident at very low concentration, such as 0.01% in P P (16a), and under severe processing and reprocessing (multiple-extrusion) conditions (unpublished work). The effect of polyethylene extrusion and multiple extrusions (reprocessing at 180 °C) on the retention and chemical transformations of the hindered phenol antioxidants α-tocopherol, Irganox 1076, and Irganox 1010 was further



430

POLYMER DURABILITY

0.1 J

ι ι I i 1 2 3 4 Number of extrusion passes

Processing Effects on Antioxidant Transformation ... - ACS Publications

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