Encapsulation of PV modules using ethylene vinyl acetate copolymer as a pottant: A critical review

Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

TG-FTIR analysis of PVC thermal degradation and HCl removal

This review treats mainly the initial transformation of the starting unoriented lamellar material into the final oriented microfibrillar structure and its drawing. If the starting material has partially oriented shish-kebab structure the drawing is a combination of transformation of lamellae into the microfibrils and the drawing of microfibrils. The review article is based on the literature from the last ten years and relys heavily on recent reviewing [1–3]. Older articles are only included if they have contributed to the basic ideas and experiments of drawing.

Drawing and extrusion of semi-crystalline polymers

Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements

Samples of low-density polyethylene, free from additives, were heated at temperatures between 284° and 355°C under high-purity nitrogen. Changes in molecular weight distribution (MWD), molecular weight averages, and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). At 284° and 315°C, the MWD's were shifted toward higher molecular weights and the Mw values increased. At 333° and 355°C, the MWD's shift toward lower molecular weight, but the high molecular weight, tail is largely retained. Mw decreases slowly at 333°C. At 355°C, Mw undergoes a rapid initial drop which levels off. Mw/Mn and the degree of LCB increase with heating time and temperature. Olefinic unsaturation increases. The vinyl groups show a larger relative increase than do the trans-vinylene and vinylidene groups. At 355°C, the peak of the unimodal DSC thermogram is shifted to ∼3°C higher temperature. A lower melting peak then develops, and after 72 and 90 min the two peaks are about equal in size. The density increases from 0.922 g/cm3 to 0.930 g/cm3 for samples heated at 355°C, and the weight loss was 1.5% after 90 min. A reaction scheme for the thermal degradation of polyethylene is discussed. Initiation is suggested to be accomplished by scission of allylic CC bonds. Propagation proceeds by both intra- and intermolecular hydrogen abstraction, followed by β-scission. Termination can occur by both combination and disproportionation. Combination reactions are suggested to account for the observed formation of LCB and high molecular weight material. Due to changes in the degree of LCB during the degradation, viscometry alone will not give a proper measure of the changes in molecular weight.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/app.1974.070181023

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Formation of anomalous structures in PVC and their influence on the thermal stability: 2. Branch structures and tertiary chlorine

Samples of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and tetratetracontane (n-C44H90) free from additives were heated in air at temperatures between 120 and 180°C. As a comparison, “as received” HDPE containing unspecified additives has also been included.



The structural changes have been studied with gel chromatography, viscometry, infrared spectroscopy, differential scanning calorimetry, and gravimetric measurements.



LDPE, HDPE, and n-C44H90 follow the same course of thermooxidative degradation when they are free from additives and present in the molten state. Both molecular-diminishing and enlargement reactions occur. At temperatures below 150°C molecular enlargement is not observed until after rather long exposure times, whereas at higher temperatures enlargement occurs immediately. The difference is because “peroxide curing” becomes increasingly important above 150°C, whereas ester formation is operating at all temperature levels. Degradation below Tm is restricted to the amorphous phase that results in a different degradation pattern.



In accelerated testing work extrapolations of the Arrhenius type in the prediction of structural change are thus not justified, even within the actual narrow temperature range. Neither are changes in commonly used standards like carbonyl content justified as a measure of the changes; for example, in mechanical properties.



The stabilizer in the unpurified HDPE not only influences the induction period but also the course of the thermooxidative degradation.

Thermooxidative degradation of polyethylene. I and II. Structural changes occurring in low‐density polyethylene, high‐density polyethylene, and tetratetracontane heated in air

This is the first in a series of papers in which structural changes during thermal degradation of ethylene-vinyl acetate (EVA) and ethylene-butyl acrylate (EBA) copolymers are compared. EVA, containing 11.4 mol% vinyl acetate (VA) and EBA, containing 5.4 mol% butyl acrylate (BA), were pyrolyzed at 280°C in nitrogen for 30 min. In another series of pyrolysis, EVA containing 1.2, 2.2, and 11.4 mol% VA were treated at 150–190°C for 3 h. The volatile decomposition products were collected in cooled traps respectively gas bags and then analysed with GC-MS and ion-chromatography. EVA is rather labile. The main volatile decomposition product is acetic acid. A linear decomposition rate was found already at the lowest investigated pyrolysis temperature, 150°C. After 30 min at 280°C every 15th of the acetate side groups had been eliminated. EBA is much more stable to pyrolysis. Thirty minutes at 280°C resulted in a decomposition of one out of 1500 BA groups. Butene is the main volatile decomposition product. Ester pyrolysis is supposed to account for the degradation of both types of polymers. The big difference in reactivity is presumably due to conformational differences. The ester pyrolysis mechanism will result in random unsaturations in EVA and carboxylic groups in EBA. To a minor extent acetaldehyde is formed when EVA is degraded. According to the mechanisms suggested, carbonyl groups remain in the main chain. Contrary to what is reported for poly(butyl acrylate), no alcohol was formed when pyrolysing EBA. This indicates that adjacent acrylate groups are needed for alcohol formation. For both types of polymer, scissions of the main chain results in hydrocarbon fragments mainly. In addition, acrylate containing fragments are observed when EBA is degraded. EVA, however, does not give any acetate-containing fragments.

Thermal degradation of EVA and EBA—A comparison. I. Volatile decomposition products

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.

Erling M. Sörvik

Papers

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Formation of anomalous structures in PVC and their influence on the thermal stability: 2. Branch structures and tertiary chlorine

Thermooxidative degradation of polyethylene. I and II. Structural changes occurring in low‐density polyethylene, high‐density polyethylene, and tetratetracontane heated in air

Thermal degradation of EVA and EBA—A comparison. I. Volatile decomposition products

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen