Natural and artificial weathering of low‐density polyethylene (LDPE): Calorimetric analysis

Abstract

SYNOPSISStudies of the thermal and photodegradation of two commercial polymers, stabilized and unstabilized low-density polyethylene, show the effects of weathering on differential scanning calorimetry (DSC) curves. The shape and the size of the melting peak vary significantly in thermal aging and in accelerated photoirradiation, but no change was observed in natural weathering, implying that the crystallinity remains steady in outdoor exposures. The melting temperatures remain steady for all exposure tests. The fall of mechanical properties and the evolution of chemical structure, followed by IR spectroscopy results, especially near the carbonyl regions, were also observed. The resistance to UV light irradiation as probed by deformation at break was superior in stabilized LDPE compared to unstabilized LDPE. Simple correlations observed between the fall of mechanical properties, the rate of oxidation, and morphology. 0 1993 John Wiley & Sons, Inc. I NTRO DUCT10 NWhen the polyethylene films for greenhouse covering are exposed to heat, UV light in the presence of oxygen, they became brittle, and visual signs of deterioration are apparent. Degradation and embrittlement of those films can be related to the evolution of chemical structure and morphology (i.e., the crystallinity, size, and perfection of the crystallites, the deformation of amorphous regions, etc.) .It is now well established that the primary mechanism of degradation of polyethylene when exposed to heat and/or UV light involves the process of thermal and/or photooxidation.1'2 Those reactions lead to the buildup of a vinyl group and of a variety of oxygen-containing groups such as various carbony1 functions. The photochemical degradation of PE results from competing reactions of crosslinking and chain s c i~s i o n .~~~ At first stages of exposures, the crosslinking reactions may predominate, leading to an increase of the Young's m o d~l u s .~.~ Polyethylene is a semicrystalline polymer. Its * To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 47, 1897-1903 (1993) structure can be considered as an alternating crystalline and amorphous layers (two-phase system). Neighboring crystalline lamellae are connected by tie molecules, passing through the amorphous interlamellar regions.6 It was assumed that oxidation was restricted to the amorphous region^.^'^ Oxidation damage at the tie molecule region can be responsible of the sudden loss of mechanical properties, especially elongation at break.8*9 It was assumed that the crystallinity increase with oxidation. It seems that this effect is due to the replacement of C -H bonds by C = 0 bonds in the amorphous region^.^ Natural weathering studies were conducted by exposing stabilized LDPE It was concluded that the observed slight increase in crystallinity could not be attributed to the formation of crosslinked structure because of the complete solubility of the sample.During photooxidation, LDPE, is well known to undergo changes of molecular Other works on high pressure crystalli...

Degradation behavior of linear low‐density polyethylene films containing prooxidants under accelerated test conditions

“…13 Elemental analysis was performed by determination of the C, H, and O content with an elemental analyzer (Thermo Finnigan, Flash EA, Milan, Italy) to quantify the content of oxygen in the degraded samples. The MFIs of all resins were measured on a MFI tester (International Equipments, Mumbai, India) at 1908C as per ASTM D 1238.…”

Natural Ageing of Stabilized and Unustabilized Ldpe Films Used as Greenhouses Covering Materrials: Atr-Ftir and Sem Analysis

“…For the unstabilized LDPE samples (Fig-3) [25,26,27,28]. These unsaturations are also visible to 1641 cm -1 .…”

“…For the stabilized LDPE samples (Fig-2) we observe: -Two peaks with a very weak intensity: one at 888 cm -1 is assigned to vinylidene >C=CH 2 groups and the other at 831 cm -1 corresponding to the vinyl R 2 C=CHR groups [25,26,27,28]. These unsaturations are also visible to 1635 cm are attributed to the stretching vibration of C-O bond, this latter belongs to ether R-O-R and alcohol RCH 2 -OH groups [19,29].…”

Investigating the Degradation Behavior of LDPE-grafted Maleic Anhydride for Use as Compatibilizer in Environmentally Degradable Compositions

“…where DH f,(observed) is the melting enthalpy of sample (second heating run) and DH f,(100%crystalline) is the melting enthalpy of 100% crystalline polyethylene, which has been reported in the literature to be 285 J=g [20]. Thermal decomposition behavior was investigated by heating the sample at 10 C=min under nitrogen atmosphere from an initial temperature of 50 C to 700 C. The acid value of the grafted polymer was determined by wet chemical titration technique as per the procedure reported elsewhere [21].…”