NATIONAL BUREAU OF STANDARDS REPORT

10 456

Progress Report

on A STABILIZING COMONOMER

PART II STABILIZATION AND

POLYMERIZATION CHARACTERISTICS

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

311.05-11-3110189 July 29, 1971

NBS REPORT

1 0 456

Progress Report

on A STABILIZING COMONOMER: PART II STABILIZATION AND POLYMERIZATION CHARACTERISTICS

R. L. Bowen*, DDS and H. Argentar**, BS

* Associate Director, American Dental Association Research

Unit at the National Bureau of Standards, Washington, D. C.

20234.

** Research Associate, American Dental Association Research

Unit at the National Bureau of Standards, Washington, D. C. 20234.

This investigation was supported in part by Research Grant

DE02494-05 to the American Dental Association from the

National Institute of Dental Research and is part of the

dental research program conducted by the National Bureau

of Standards in cooperation with the American Dental

Association- the Dental Research Division of the United

States Army Medical Research and Development Command: the

Dental Sciences Division of the School of Aerospace

Medicine, USAF; the National Institute of Dental Research,

and the Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF STAN

for use within the Government. Be and review. For this reason, the p whole or in part, is not authorizei Bureau of Standards, Washington,

the Report has been specifically pri

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

accounting documents intended

biected to additional evaluation'

sting of this Report, either in

Office of the Director, National he Government agency for which

les for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

1

A STABILIZING COMONOMER: II. STABILIZATION

AND POLYMERIZATION CHARACTERISTICS

The hardening times of an experimental composite material and the gel times of n twoliquid monomer formulation increased with

higher concentrations of a stabilizing comonomer. The compound (BHM) polymerized

under anaerobic conditions when initiated

with 2-cyano-2-propyl radicals.
2

A STABILIZING COMONOMER: II. STABILIZATION

AND POLYMERIZATION CHARACTERISTICS

The compound BHM (3, 5-di - tert-bu tyl-4-hydroxybenzy 1 methacrylate)

reported in Part I, 1 was evaluated with regard to its effectiveness as a stabilizer (polymerization inhibitor). The present

paper describes these findings and verifies that the methacrylate

portion of the molecule can polymerize.

Mate^ri a Is and Methods *

The polymerization-inhibiting capability of BHM was measured

by two methods: (1) a "slide immobilization test"'' of hardening

time for a composite formulation and (2) a gel- time determination for an equal-volume mixture of two liquid formulations.

SLIDE IMMOBILIZATION TEST METHOD2

. —Liquid monomer formulations were prepared with Monomer X-980t ( tr ime thylolpropane

* Certain commercial materials and equipment are identified

in this paper to specify the experimental procedure. In

no instance does such identification imply recommendation

or endorsement by the National Bureau of Standards or that

the material or equipment identified is necessarily the

best available for the purpose.

+ Rohm and Haas
3

trimethacrylate, as supplied with 200 ppm of the monomethyl

ether of hydroquinone) , 0.586% of N,N-dimethyl-3 , 5-xylidme

,

and various concentrations of BHM. Fifteen drops of liquid

were mixed by hand spatulation with a powder containing 1.0%

benzoyl peroxide dispersed on spheroidal particles of fused

silica treated with 3-me thacry loxypropyl tr ime thoxysilane , as

described previously. 2 A homogeneous mix was prepared using

as much powder as possible.

Each mixture was transferred immediately onto a clean,

dry glass plate (3-1/4 x 4 inch) . Spacers, (microscope

slides, 1 mm thick) were placed around, but not touching, the

mix, and another glass plate (with its long axis perpendicular

to that of the bottom glass) was placed on the spacers, forcing

the plastic material to form a disc about 20 mm in diameter.

With the lower slide immobilized, the upper slide was gently

moved a short distance (no greater than 1 mm) by hand, back and

forth linearly every quarter minute. The time at which the upper

slide could no longer be moved easily in this fashion was termed

the "linear hardening time." Then a rotational force was
4 -

applied by twisting the upper slide about the approximate

center of the specimen. The force was applied in a clockwise and then counterclockwise direction and the distance

traversed was no more than about 3 degrees of an arc. The

time at which the upper slide could no longer be perceptibly

rotated relative to the lower slide, when considerable hand

force was applied, was recorded as the "rotational hardening

time .

GEL TIME DETERMINATION • ——A stock solution was prepared

with 47% methyl methacrylate* [as supplied with 35 ppm BHT

(2 , 6-di-_t-butyl-4-me thylphenol) ] and 53% "HEMA-phthalates " t

(as supplied with somewhat less than 35 ppm BHT) . The

phthalate monomers have been described previously M The

stock solution was divided into two parts. To one part was

added sufficient N,N-dimethyl-3 , 5-xylidi.ne to obtain a 1.03%

solution of this amine accelerator. To the other part was

added enough benzoyl peroxide* to give a 3.33% solution.

* Rohm and Haas

t The Epoxylite Corp.

* Lucidol -98, Pennwalt Corp.
5

The solution containing the peroxide was further subdivided

into portions into which were dissolved various amounts of

BHM or BHT

.

One cc of amine solution was injected, using a calibrated

syringe with needle, into a small test tube supported beneath

an automatic gel timer.* One cc from one of the peroxide

solutions was injected forcefully into the same tube so

as to mix the two solutions. The gel timer was immediately

started; it automatically raised and lowered a disposable

wooden cylindrical probe (2.2 to 2.3 mm in diameter) in the

liquid mixture, further stirring the combined solutions.

The probe had been adjusted to closely approach but not touch

the bottom of the test tube. After the induction period,

the length of which depended upon the inhibitor concentration

and effectiveness, the gelling or solidifying immobilized

the probe, automatically stopping the timer. The precision

of this measurement was about 0.1 minute.

* "Tecam, " Techne (Cambridge) Ltd., Duxford, England
6

POLYMERIZATION OF THE STABILIZING COMONOMER The colorless crystals of BHM (2.84 gm) were placed into a centrifuge

tube provided with a rubber stopper and two valved ports. One

port was connected to a low-pressure nitrogen source and the

other was attached to a trap cooled in dry ice (solid carbon

dioxide) and alcohol and thence to a vacuum pump. The container was evacuated and then refilled with nitrogen repeatedly

Meanwhile the crystals were melted in a steam bath. Since

polymerization did not occur the first hour with the sample

under vacuum at about 95-100°C , AIBN ( 0 , 0

- / -azo-bis -isobutyronitrile; 0.459%) was added and the pumping and heating processes

were resumed. Within 30 minutes, the compound had polymerized

to a hard, glassy, transparent and colorless solid. A portion

of this was dissolved in deuterated chloroform for NMR analysis

Re su its

SLIDE IMMOBILIZATION TEST*—-The effectiveness of BHM

in inhibiting polymerization is shown graphically in Figure 1.

The plots, obtained by least-squares fitting of the data,''

appear to show a linear increase of hardening time with increase of BHM over the concentration range investigated.
7

GEL TIME The results given in Table 1 also demonstrate

that BHM inhibits polymerization. The induction period increases with increasing concentration in a nearly linear

manner. The efficiency is not as high as that of BHT , as

the gel times are shorter for BHM at approximately equal

molalities (concentrations in moles/kg of solvent)

.

The hardening times for BHT did not obey a linear relationship with concentration according to statistical tests.

The hardening times increased linearly (at the 95% confidence level) with the concentrations of BHM whether expressed

in weight percentage or molality.' When weighting factors (the

reciprocals of the squares of the standard deviations of

the hardening time data for each concentration) 5 were used

in linear least-square fitting, then the following relationships

were obtained:

the gel time = 1.97 + 19.49 m,

the linear hardening time = 2.49 -f 9.08 m and

the rotational hardening time = 3.13 + 13.56 m,

where m is the concentration of BHM in moles per

kg of solvent (monomers)

.
8

The linear relationship that exists between the rotational

hardening times and the linear hardening times, was obtained

from the data, assuming experimental error in both sets of

measurements; 7 thus the relationship was: rotational hardening

time = 1.34 (linear hardening time) -0.22, in minutes.

The monomer solution containing benzoyl peroxide and

only about 30 ppm of BHT, the lowest concentration used,

polymerized during storage at room temperature after only

a few days, whereas none of the other solutions gelled during

a much longer period.

HOMOPOLYMERIZATION •—The NMR spectrum of the polymerized

compound (BHM) was consistent with homopolymer together with

residual monomeric BHM. A segment of the polymer is shown

in Figure 2.

Di^scus^ion

The effectiveness of BHM relative to BHT is in harmony

with the mechanism that had been postulated for similar

sterically-hindered phenols.' The methyl group on the 4-

position in BHT is more electron-releasing and less sterically

hindering, compared to the corresponding me thacryloxyme thy

1

group of BHM. Since the efficiency of 3 , 6-di-_t-buty 1-4-

substituted phenolic compounds is dependent upon the electronreleasing nature of, and lack of steric-hindrance by the 4-
9

substituent/ a somewhat higher molar concentration of BUM

(relative to BHT) would be required to obtain the same

storage stability and working time for a given acrylic liquid

or paste formulation.

The postulated mechanism of inhibition involves the

removal of the phenolic hydrogen atom from the inhibitor

by a molecule containing an oxygen atom with an unshared

valence, or odd electron (free radical) /

' ' The molecule

containing this oxygen atom might be a benzoyloxy initiator

radical 10or a peroxy radical [formed by the facile addition

of an atmospheric oxygen molecule (0 2 ) to a polymeric free

radical during polymerization.]1 x The inhibitor molecule then

becomes a conjugated free radical that cannot appreciably

initiate polymerization due to the delocalized nature of the

odd electron (resonance stability) 1 " but it can react with

and terminate another free radical.

Due to the di-radical nature of molecular oxygen, which

is present in any system from which it has not been specifi-
10

cally excluded, it reacts very readily with the carbon radicals

in the growing polymer chains. The resulting peroxy radicals

are relatively unreactive 1

?

and thus slow to initiate polymerization of methacrylate monomers. Although peroxy' 1 and benzoylox

radicals can react with phenolic inhibitors, these inhibitors

are ineffective in the absence of air. lr Therefore, BHM could

be polymerized in a vacuum when initiated with 2-cyano-2-propy

1

radicals derived from the thermal decomposition of A I BN

.

1

?

C onclus ions

A methacrylate monomer that contained a phenol- type polymerization inhibitor in the same molecule was evaluated. The

times between mixing and hardening of methacrylate monomers

containing small amounts of this compound increased linearly

with the concentrations. The pure compound (BHM) could be

polymerized by using an oxygen-free atmosphere, together with

a free radical initiator (AIBN) that did not contain oxygen

atoms.

Compounds such as this, which can stabilize monomers by

inhibiting premature polymerization and can take part in the

polymerization when the initiator system is activated, are

worthy of further investigation.
11

The authors are grateful to Mr . R. A. Thompson of the

National Bureau of Standards, for obtaining the NMR

spectra

.
12

TABLE 1

GEL TIMES FOR MONOMERS CONTAINING VARIOUS

CONCENTRATIONS OF BHT AND BHM STABILIZERS

Stabilizer concentration Gel time ^ ry "t* % molality* min

.

min

.

BHT

0.003 0.0001 2.0 0.06

0.12 0.0105 2.5 0.16

0.66 0.0612 3.7 0.62

1.94 0.183 7.2 0.54

3.06 0.296 9.7 3.46

BHM

0.22 0.0071 2.2 0.22

0.84 0.0279 2.7 0.22

4.16 0.143 4.7 0.36

5.95 0.208 5.8 0.77

* Average of 6 measurements made at room tempe

t Standard deviation

T(x-x) 2

\] n-l

* Moles per kg of solvent
6

FIGURE 1: Hardening times for a formulation of trimethylolpropane trimethacrylate (as supplied with 0.02% of the

monomethyl ether of hydroquinone) containing 0.586% N,Ndimethyl-3, 5-xylidine and varying concentrations of BHM. The

average linear hardening times (o) and rotational hardening

times (P), as defined in the text, increase with increasing

BHM concentrations. The length of the vertical line from each

symbol represents the standard deviation of three measurements,

rphe lines were obtained by a least— square fitting of the data.
14 -

-£

H

i CH3

C-CHCHo

0

C=0 n

CHo-C

CH.

FIGURE 2.—*—Structural formula of a segment of polymerized

or copolymerized BHM ( 3 , 5-di- ter

t

-buty 1-4-hydroxybenzyl methacrylate)
15

References.

1. Bowen, R. L., and Argentar, H.: A Stabilizing Comonomer:

I. Synthesis and Confirmation of Structure, (submitted to J Dent Res )

.

2. Bowen, R. L., and Argentar, H. : Amine Accelerators for

Methacrylate Resin Systems ( J Dent Res July-Aug 1971,

in press)

.

3. Bowen, R. L. : Crystalline Dimethacrylate Monomers

J Dent Res £9:810-815, 1970.

4. Bowen, R. L. : Ternary Eutectic Dimethacrylate Monomer

System and Restorative Dental Material Prepared Therefrom,

U. S. Patent 3,539,526, 1970.

5. Parratt, L. G.: Probablity and Experimental Errors in

Science , New York, John Wiley & Sons, Inc. 1961, pp 127-

129.

6. Natrella, M. G.: Experimental Statistics NBS Handbook 91,

Washington, D. C., U. S. Government Printing Office, 1963

6y7-21 and list of errata.

7. Mandel, J. : The Statistical Analysis of Experimental Data,

New York, John Wiley & Sons, Inc. 1961, pp 288-292.

8. Ingold, K. U.: Inhibition of Oil Oxidation by 2,6-Di-_tbutyl-4-subs tituted Phenols, J Phys Chem 64:1636-1642,

1960.

9. Lloyd, W. G., and Zimmerman, R. G.: Antioxidant Inhibition by Substituted Phenols, Ind and Eng Chem 4:180-185,

1965.
16

10. Kleiner, E. K. (to J. R. Geigy SA)

Polymeric Antioxidants

with Blocked Phenolic Groups, French Patent 2011412, 1970.

11. Georgieff, K. K. : Relative Inhibitory Effect of Various

Compounds on the Rate of Polymerization of Methyl Methacrylate,

J Appl Polymer Sci 9:2009-2018, 1965.

12. Walling, C.: Free Radicals in Solution , New York, John

Wiley & Sons, Inc. 1957, pp 25, 70, 168-170.

13. Pryor, W. A.: Free Radicals , New York, McGraw-Hill Book

Co. 1966, pp 290-295, 326.