Mechanism of Surface Discoloration of Polymer in Contact with Water

Mun Ho Kim, Doo-Jin Byun, Ji-Eun Yu, Kil-Yeong Choi, Se-Moon Shin

Reliability Assessment Center for Chemical Materials, Korea Research Institute of Chemical Technology (KRICT)

141 Gajeong-ro, Yuseong-gu, Daejeon 305-609, Republic of Korea

E-mail: kychoi@krict.re.kr

MoDeSt 2012 (P-I_18)

Introduction

Summary

The white-colored inner surface of a polypropylene (PP) material containing water discolored and turned yellow. The discoloration occurred selectively on the surface at the point of contact with the air-water interface. Since the polymer surface was exposed to water in darkness at room temperature, no sign of deterioration or degradation of the polymer at the discolored surface was confirmed. This study conducts an in-depth analysis of the discoloration mechanism of the polymer surface. A variety of technical approaches, including microscopic, spectroscopic, and chromatographic analysis techniques, were used to investigate the nature of discoloration and the root cause. From the analysis results, the discoloration was ascribed principally to a phenol transformation compound having the structure of a quinone methide, which was identified as a degradation product of a primary antioxidant. Based on the observations and experimental results, a plausible discoloration mechanism was proposed.

Results and Discussion

In the present study, we examined a discoloration problem of polymer surface involving the diffusion process as a representative case study. White-colored inner surface of a polypropylene (PP) material containing water discolored and turned yellow. The discoloration occurred selectively on the surface at the point of contact with the air-water interface. The polymer material was made from PP, and TiO2 pigment was added into the polymer material in order to lend white coloring. The purpose of this study is to conduct an in-depth analysis on the discoloration mechanism.

SEM surface images of

(A, B): discolored

(C, D): normal samples.

Schematic illustration of the discoloration

point on the polymer surface.

Surface images of (B) discolored and (C)

normal samples. (Scale bar: 100 m)

The white-colored inner surface of a polypropylene (PP) material containing water discolored and turned yellow. The discoloration occurred selectively on the surface at the point of contact with the air-water interface. In order to investigate the nature of the discoloration and the root cause, a variety of technical approaches were used. There was no sign of deterioration or degradation of the polymer at the discolored surface. From the observations and experimental results, the discoloration was ascribed principally to a phenol transformation compound having the structure of a quinone methide. It is expected that one of the additives, a primary antioxidant, was released into water and hydrolyzed into the phenol transformation compound. As the additive leaches into the water, the degradation compound would continuously be formed and accumulated on the polymer surface where the contact line with air and water formed.

B C

APP

surface

Water surfaceB

C

Continuous movement of water surface

by evaporation and supply

A B

C DThere is little difference in terms of surface morphology between the two samples.

ATR-FTIR spectra of (A) discolored and (B) normal polymer surfaces.

A

B

A

BThe appearance of an absorption band in

the 3600-3200 cm-1 region is assigned to the

hydrogen bond of alcohol (-OH); the peak

at 1720 cm-1 confirms the presence of

carbonyl (C=O) of –COOH, and the peak at

1645 cm-1 corresponds to a C=O group

adjacent to an olefinic double bond or

enolic C=O group present in the discolored

PP surface.

Polar groups were incorporated into the surface of the discolored polymer.

Microscopic Analysis (SEM)

A B

C D

Spectroscopic Analysis (ATR-FTIR)

Chromatographic Analysis (GC-MS)

A

B

A

BThe peak was identified as cyclohexa-1,4-dien-

1,5-bis(tert-butyl)-6-on-4-(2-carboxylethylidene),

which has the structure of a quinone methide.

Peaks at 9.46 min, 29.41 min, and 31.03 min were

also observed in the chromatogram of a normal

sample.

GC Chromatograms of (A) discolored and (B) normal polymer samples.

If the polymer material has been exposed to sufficient UV and heat to turn yellow, the antioxidant added to the formulation would be completely depleted during aging. The antioxidant depletion is commonly monitored by the oxidative induction time (OIT), measured by differential scanning calorimetry (DSC). OIT is closely related to the amount of antioxidants remaining in the material. However, our tests found no notable difference in OIT results between the discolored polymer sample and a reference sample (not shown here). Therefore, it is concluded that photochemical and heat-based degradation were not involved in the discoloration.

A

B

A

B

Chromatograms of (A) an acetone extract of the discolored polymer sample

and (B) pure acetone. (The inset in Figure A shows an image of discolored

acetone after extraction (left) compared to pure acetone (right)).

+ 4

Oxid

ation

Irganox 1010 (pentaerythritol-tetra-[-(3,5,-di-tert-butyl-4-hydroxyphenyl)-propionate])

3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoic acid

2,2-bis(hydroxyphenyl)-1,3-propanediol

Cyclohexa-1,4-dien-1,5-bis(tert-butyl)-6-on-4-(2-carboxylethylidene)

4

PP surface

Water surface

Water evaporation

Causative compount

Surface Discoloration

The proposed chemical reaction of the formation

of cyclohex-1,4-dien-1,5-bis(tert-butyl)-6-on-4-

(2-carboxylethylidene).

A schematic illustration of compound accumulation on the polymer surface

where the contact line with water forms.

Since the water evaporation rate is highest at the

edge of the contact line, water influx occurs

toward this region. The water influx transports

the compound in the water to the contact line,

which enables the compounds in the water to

accumulate continuously on the polymer surface

where the contact line with water formed.

Therefore, the discoloration occurred selectively

on the surface where the contact line with air and

water formed, as shown in Figure.

The discoloration was ascribed principally to a phenol transformation compound having the structure of a quinone methide, which was identified as a degradation product of a primary antioxidant.

To confirm our conclusion regarding the compound that leads to yellowing, an analysis of a solvent extract of the discolored polymer sample was conducted.

As the additive leaches into the water, the degradation compound would continuously be formed and accumulated on the polymer surface where the contact line with air and water formed.