Stabilizers for Peroxydicarbonate Initiator Solutions

PETER FRENKEL andTED PETTIJOHN

Witco Corporation M c u s W ~ , T e x a ~ 75671 -1 439

Liquid dialkyl peroxydicarbonates are used as initiators in the W C industry. Because of the thermal reactivity of these initiators, they require very low tempera- ture storage, shipment, and handling. At temperatures above 10°C. most undergo autoaccelerated self-induced decomposition. In other words, their self-accelerating decomposition temperature (SADT) is exceeded. New additives have been discov- ered which increase the SADT of the initiators. These additives effectively stabilize the products, making them safer to handle, store, and ship. The proprietary addi- tives and a mechanism of stabilization are discussed. We also include a section concerning the implications these products have for future initiator formulation.

INTRODUCTION

ost commonly, vinyl chloride polymerization is M initiated by dialkyl peroxydicarbonates or their mixtures with peroxyesters. The initiation proceeds via a generation of free radicals as a result of thermol- ysis (in some cases photolysis or presence of a redox system) of organic peroxide.

0 0 0

ROCOOCOR ~ 2ROCO ~ T O C _

The heat sensitivity and rate of decomposition of a peroxide generally depend upon the structure of the R groups. Peroxydicarbonates are extremely reactive, decompose at moderate temperatures, and require a controlled temperature environment for shipping, han- dling, and storage.

Self-Accelerating Decomposition Temperature (SADT) is used to describe that critical ambient temperature above which the decomposition reaction proceeds with very rapid self-acceleration in a specific package. The SADT can be directly correlated with the onset tem- perature of decomposition of the peroxides as meas- ured by differential thermal analysis (M'A) (1).

Several groups of chemical compounds were reported as stabilizers for the peroxydicarbonates. Phenolics, hydroperoxides, and ethylenically unsaturated acety- lenes and nitriles are among them (2-4). The search for new stabilizers which provide enhanced safety for shipping and handling of peroxydicarbonates contin- ues to be an important task, because the known addi- tives are either toxic compounds or negatively affect the color stability of a polymer.

EQUIPMENT AND PROCESSES

Materials

Technically pure (min. 96%) di(2-ethylhexyl) peroxy- dicarbonate (EHP) commercially supplied by Witco as Espercarb 840 was used for the evaluation of new sta- bilizers which were purchased from Aldrich. Stabilized peroxydicarbonate samples were prepared by mixing the liquid peroxide compound or its solution in the desired amount of a liquid additive or a solution of an additive in a suitable solvent.

Stability at a Constant Temperature The storage stability of the peroxydicarbonate and

its stabilized composition was determined as an accel- erated aging test at 15°C constant temperature. Eight- gram samples of each composition were placed into a loosely closed (for gas release) test tube and kept in a constant temperature bath. The purity of the tested materials was measured by iodometric titration. The aliquots were sampled out weekly during a four-week period.

Differential Thermal Analysis A differential thermal analyzer (Radex Solo, Astra

Scientific International) was used to determine the onset temperatures of decomposition for a one-gram sample of the stabilized and nonstabilized peroxydi- carbonate in a sealed cell. The measurements were carried out by using an isothermal hold temperature of 30°C for 15 minutes and then a linear temperature increase of l"C/minute to 130°C. The onset tempera- ture was defined as the point where the rate of in- crease (AT) of the sample temperature reached 0.2"C/ minute.

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, SEPTEMBER 1999, Vol. 5, No. 3 165

Peter Renkel and Ted Petfijohn

SADT Measurement Thermal Analysis

The SADTs of stabilized EHP were determined for samples according to the UN recognized method (5) for 8-lb. samples in a one-gallon plastic container without outer package.

RESULTS & INTERPRETATION

Effective Retardants of Decomposition

Recently, Witco discovered several classes of both organic and inorganic compounds capable of stabiliz- ing peroxydicarbonates. They are cyclic a-diketones (3-ethyl- 1.2-cyclopentanedione; 1,2-~yclohexanedi- one), a-hydroxyal@l peroxides [hydroxymethyl-t-butyl peroxide (HMBP); 1,l-dihydroqdicyclohexyl peroxide], enolized P-diketones (2.4-pentanedione; 2-acetylcyclo- hexanone) and P-ketoesters (ethyl 2-cyclohexanone carboxylate (ECHC); ethyl 2-cyclopentanone carboxy- late], oximes (acetaldoxime; acetone oxime), and phos- phomolybdic acid (6- 10).

This paper focuses on 2.4-pentanedione (PD), ac- etaldoxime (AOX), and phosphomolybdic acid (PMA). All of these are relatively nontoxic, inexpensive, and commercially available.

Accelerated Aging Study

The accelerated aging tests were carried out at 15°C (noticeably higher than the storage temperature for the peroxydicarbonate) in order to observe sigmficant changes in the studied systems within the period of one month.

Experimental data showed that new stabilizers mark- edly retarded the rate of thermal decomposition of EHP at 15°C during all four weeks of testing (Fig. 1). PMA, AOX, and HMBP were the most effective. For example, at the end of the fourth week the purity of the nonsta- bilked sample dropped by 60%, while the purity of EHP stabilized with PMA was reduced by 20%.

100

80

60

40

20

By definition, onset temperature indicates the tem- perature at which decomposition becomes noticeable. A higher onset temperature corresponds to a more stable and safer system. The presence of stabilizers at the level of 0.1-3.0Yo shifted onset temperatures of de- composition from 36.3"C (for nonstabilized EHP) to 54.0"C (for the material stabilized with 3% of PD).

DTA data for both nonstabilized and stabilized EHP are included in Fig. 2 as a bar-graph. The improve- ment of the EHP stability with additives was signifi- cant.

Based on known values of SADTs for different per- oxides and their corresponding onset temperatures, the linearity of the two was confirmed (Fig. 3). and SADTs of the stabilized EHP were calculated for one- gallon packages.

The predicted SADTs for stabilized EHP were higher than for the nonstabilized material by up to 19°C (Table I), a result which demonstrated the enhanced safety of storage, shipping, and handling of the per- carbonate. Measured SADTs satisfactorily confi ied the calculated data (Table 1 ) .

The SADT is greater for a diluted product than for a neat material. However, stabilizers more than ade- quately compensate for the SADT of a pure product at its higher concentration. Therefore, nonstabilized di- luted peroxydicarbonates can, in many cases, be safely substituted by the same stabilized technically pure products, increasing the storage capability of a cus- tomer's site.

Structure of Stabilizers and Mechanism of Stabilization

The structures of both known and recently discov- ered stabilizers consist of two main fragments located next to each other (including cases where prototropic tautomerism is considered): a mobile proton (or pro- ton-donating functional group) and a center of exces- sive electron density.

I

. \

t

None 0.1% PMA

- - - - - . . - 2.9% 2,4-PD - I 3.0% ECHC - -3.2% HMBP 1

0 0 5 10 15 20 25 30

Days a. 1 . Storage stabiltty of EHP at 15°C.

166 JOURNAL OF VINYL i? ADDITIVE TECHNOLOGY, SEPTEMBER 1999, Vol. 5, No. 3

Stabilizers for Peraxydicatbonate Initiator Solutions

Q. 2. Onset tempemture of decomposition

40

35

30

25

20 0 6 ' 15 I= P a 10 v)

5

0 -p 5a.O 6c.O 7c

-1 0

Onset Temperature, deg. C

Q. 3. S A D T U S . onset tempemture.

+ 1 gallon SADT - Linear (1 gallon SADT)

.O

Table 1. Onset Temperatures and Predicted +Gallon SADTs for EHP.

Addltiw wt. % Onset

Temperature, "C Predicted SADT, "C

Measured SADT, "C

None HMBP AOX PMA PD AOX PD

~

0.0 3.0 1 .o 0.1 1 .o 3.0 3.0

36.3 43.7 48.2 48.7 50.1 51.4 54.0

1 .o 9.0

13.8 14.4 15.9 17.3 20.1

1 (3) - 20 20 20

JOURNAL OF VINYL & ADDmVE TECHNOLOGY, SEWEMBER 1999, Vol. 5, No. 3 1 67

Peter Frenkel and Ted Pettijohn

The combination of two factors, the electronic struc- ture of the oxygen atoms of the 00 bond and the meso- meric effect of carboxyl-groups. explains the low 00 bond-dissociation energy (- 120 kJ/mol) of peroxydi- carbonate molecules. Consequently, the retarding ac- tivity of stabilizers is targeted toward this bond. It is logical to assume that hydrogen-type bonding can be a part of structural organization between a peroxydi- carbonate and a stabilizer. As a result of that weak in- teraction, the excess electron density of the 00 group is “discharged” on the stabilizer molecule through the mobile proton, forming a corresponding complex of a peroxydicarbonate with a stabilizer.

Scavenging can be another possible mechanism of retardation of the autoaccelerated decomposition. How- ever, no inhibiting effect is observed.

The stabilizing effect is limited by self-association of the additive molecules, molecular symmetry of the peroxide, and temperature.

coNcLusIoNs 1. The newly discovered stabilizers increase SADT

and by this means enhance safety of storage, ship- ping, and handling of the least stable commercially available organic peroxides/initiators for the poly- mer industry.

2. Nonstabilized solutions of peroxydicarbonates can be safely substituted by the Same stabilized techni-

cally pure products, thus increasing the storage capability of a customer’s site.

3. Electron storage ability along with a protowc func- tional group of a retardant molecule are responsi- ble for the intermolecular mechanism of stabiliza- tion.

REFERENCES 1. T. Grewer. Thermal Hazards of Chemical Reactions,

Elsevier ( 1994). 2. D. J. Black and R. H. Tang, pemnde ’ Composition Con-

taining Phenolic Antiavidant U.S. Patent 4.552.682. 3. E. Boelema. M. C. Tammer, and J. Nuysink, Stabilized

F’eraxydiwrbonate Composition. US. Patent 5.155.192. 4. J. Sanchez, Stabilized DiauaJl Perawydicarbonate Compo-

sitions and7’heirUses. U.S. Patent 5.541.151. 5. Recornnwdah . ns on the lkmsp0r-t of Dcugerous coods.

Manual of Tes ts and Criteri~ Second revised edition, United Nations, N.Y. and Geneva, 1995. Part 11. pp.

6. C. Abma and P. Frenkel, Organic Peroxide Stabilization with a-Hydrawyalkyl Pemuides, US. Patent 5.654.463.

7. C. Abma. P. Frenkel. and L. Bock, Organic Peroxide Stabilization with Cyclic a-Diketone Compounds, US. Patent 5,654,464.

8. C. Abma. P. Frenkel. L. Bock. A. Andrews. and M. Wells. Organic Peroxide Stabilization with p-Dicarbonyl Compounds, U S . Patent 5.714.626.

9. P. Frenkel and C. Abma. Organic Peroxide Stabilization withPhosphDmoIybdkAcid. U.S. Patent 5.719.304.

10. P. Frenkel, Organic Peroxides Stabilization with Oximes, U.S. Patent Application No. 09/031903.

286-287.

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