SHORT-TERM NATURAL WEATHERING OF
POLY(VINYL CHLORIDE) PIPE
by
David S Ogunniyi* and Stephen I Ojebor
Department of Chemistry
University of Ilorin
Ilorin, Nigeria
Abstract
The effects of short-term weathering on commercial poly(vinyl chloride) pipes have been studied
in Ilorin. The parameters used to assess the effects of weathering are tensile strength,
elongation-at-break and intrinsic viscosity. It was found that all these properties deteriorated as
the period of weathering increased. The deterioration of properties was attributed to photooxidative degradation involving chain scission of polymer molecules.
Key words: weathering, poly(vinyl chloride), tensile strength, elongation-atbreak, intrinsic viscosity.
Introduction
Weathering is the deterioration of a material when such material is exposed outdoors to the
combined effects of oxidation, light, heat, humidity, rain, wind, dust and chemical pollutants [1].
From a chemical viewpoint, it may involve the breaking down of large molecules to smaller
molecules (i.e. scission) or the tying together of chain molecules (i.e. crosslinking) and a
consequent deterioration of properties. Generally, weathering is viewed as any undesirable
change in polymer properties due to exposure to any of the weathering agents [2] listed
previously.
*
Author to who correspondence should be addressed; e-mail: <dsogunniyi@hotmail.com>
Using gel permeation chromatography measurements, Mahmud and co-workers [3] found that an
increase in the period of weathering led to the lowering of the number average molecular weight;
this observation was attributed to the combined effect of thermal and photo-oxidation. Also,
White and his group [4] have reported observations made when injection-moulded bars were
weathered in the extreme hot weather of Jeddah, Saudi Arabia.
Rigid poly(vinyl chloride) (PVC) pipes are used in Nigeria mainly for the transportation of potable
water, domestic and industrial wastes and treated sewage effluent. Although these pipes are of
standard quality, their life times are limited and some do fail prematurely because of long
exposure to weather elements prior to use. Thus the aim of this work is to study the extent of
deterioration of mechanical properties when a rigid poly(vinyl chloride) pipe is subjected to natural
weathering.
Materials and Methods
Unexposed rigid poly(vinyl chloride) pressure pipe was obtained from commercial sources. The
composition of the pipe was given as: PVC – 100; Whiting filler –2; Lead-based stabiliser
/Lubricant – 2.6; Carbon black – 0.01. The pipe material was placed in an oven at a temperature
of 100oC for about 2 hours to soften. Thereafter, several dumbbell specimens were cut from the
pipe material. These dumbbell specimens were exposed on a rack for a period ranging from 0 to
14 months according to ASTM D 1435-75 [5]. During the period of exposure, samples were
exposed to temperatures ranging from 38 oC to 18 oC, humidity and solar radiation, which are
aggressive to plastic materials. Exposure was stopped at 14 months because a survey of PVC
pipe market showed that pipes would have been sold before that length of time. Furthermore, the
results of Qayyum and White [6] suggest that the most rapid change in properties takes place in
the first year of exposure. Samples were exposed on top of a university building that is free from
atmospheric pollutants and weathering started in the month of October.
As weathering progressed, samples were taken and tested for tensile strength and elongation-tobreak with the aid of a Hounsfield Tensometer at a crosshead speed of 20mm/min; testing was
according to guidelines of ASTM D – 882. These are the mechanical properties mostly affected
by weathering [7]. The mechanical properties of an unexposed sample were taken as control and
it corresponded to zero month of exposure.
Solution viscosity measurements were obtained and intrinsic viscosity was determined for each
weathered sample. In this regard, the upper surface of the weathered PVC was scraped and
0.2g dissolved in 100 ml cyclohexanone for solution viscosity measurements; measurements
were according to guidelines of ASTM D 2857 – 70. The thickness of the pipe material (6.5mm)
is such that weathering effect will be more concentrated on the surface. Even though solution
viscosity is usually measured for virgin, linear polymers, measurements were taken to give an
indication of relative changes in molecular size at the surface.
Results and Discussion
The effect of time of weathering on mechanical properties of pressure pipe is shown in Figures 1
and 2. In this case, the tensile strength and the elongation-at-break decrease as the time of
weathering increase. The results showed little but gradual decrease in tensile strength of PVC
during the period of weathering. On the other hand, the elongation-to-break reduced sharply from
80% for the unexposed sample, to 20% at the end of 14 months weathering period. This result is
in agreement with that of Qayyum and White [6] who found that the tensile strength of a
weathered polymer declined by only 5% in five years. Also, Yamasaki and Blaga [7] obtained
similar result in their studies and they attribute the reduction in the tensile stress and elongation –
to-failure to reduction in the molecular weight of the surface layer. The elongation-at-break,
however reduced more sharply than the tensile strength. It is possible that microcrack which
developed on the weathered surface as weathering progressed gave rise to sharply reduced
elongation-at-break. Also, cyclic changes in daytime and nighttime temperatures may be
responsible for the trend of results obtained. Generally, it has been reported [8] that natural
weathering leads to uptake of oxygen, complex chemical reactions, an increase in brittleness,
chain scission and the physical failure of the sample. White and Rapoport [9] reported that the
chemical reactions involved in thermo-oxidation and photo-oxidation are essentially the same.
The general reaction scheme of these workers is thought to be applicable in the present study
and it is shown below:
The degradative chain reaction comprises four main stages beginning with initiation in which the
long chain polymer molecule, PH is converted into a radical.
(i)
Initiation:
PH  O2  P  HO2 …………………………………..(1)
(ii)
Propagation:
P  O2  POO …………………………………………(2)
POO  PH  POOH  P ……………………………..(3)
(iii)
Chain branching:
POOH  PO  HO …………………………………...(4)
2POOH  POO  PO  H2O …………………………(5)
The radicals formed in steps 4 and 5 then re-enter the propagation phase via the
following reactions:
PO  PH  POH  P ………………………………….(6)
HO  PH  P  H2O …………………………………...(7)
(iii)
Termination:
P  P  inactive products …………………………….(8)
P  PO  inactive products …………………………..(9)
POO  POO  inactive products ……………………..(10)
Reaction Scheme 1: General reaction scheme of aliphatic polymer oxidation [Ref.9]
Overall, the inclusion of proprietary stabilizer/lubricant compound and carbon black could have
slowed the rate of deterioration of physical properties.
The intrinsic viscosity results (Figure 3) showed a wide scatter but in general, it appeared to
reduce with increase in outdoor exposure. This may imply scission of polymeric chains due to
exposure. Thus, it may be deduced that photo-oxidation causes chain scission in polymers under
natural conditions outdoors and it may be the main cause of property deterioration. However, it is
worth noting that during outdoor weathering, PVC is subjected not only to solar radiation, but also
to weathering factors such as temperature and moisture. Their role in the degradation process is
not clear. For all the samples, there was no change in the intrinsic viscosity in the interior of the
bulk material and the unexposed side. This is a further indication that weathering in this study
starts on the surface and subsequently lead to deterioration of overall bulk properties.
It was also observed that the exposed side of the pipe samples was discoloured. A gradual
whitening of exposed surface was obtained as the period of weathering increases. As expected,
the unexposed part did not undergo any colour change. It has been reported that at high UV
intensities, as it is present in solar radiation in hot climates, oxidation is very rapid and oxygen
depletion occurs, restricting degradation to a region near to the surface [6,9]. Also, Qayyum and
White [6,10] have deduced that cyclical changes in daytime and nighttime temperatures set up
temperature gradient on the surface of samples facing the sun, with consequent formation of
residual stresses in the plastic pipe. The stresses so formed are capable of accelerating
photochemical degradation and can also assist crack formation and propagation in the degraded
layer. Generally, the exposure of PVC results in thermal and photooxidative degradation; this
combination leads to complex reactions. It is likely that scission predominated and this resulted in
reduction of tensile properties, decrease in molecular weight and the discolouration of exposed
pipe surface.
Conclusion
As in other hot regions of the world, the exposure of PVC to weather elements in Ilorin leads to
deterioration of mechanical properties. Intrinsic viscosity measurements showed that weathering
was more prominent on the surface of the material. Complex reactions take place during
weathering and more long-term studies of natural weathering are needed before prediction can
be made of useful life of exposed materials.
References
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