Arie Ram

Orientation and Shrinkability in Polymers

Abstract This work deals with uniaxial orientation and shrinkage of two grades of well-characterized low-density polyethylene (LDPE), and a general-purpose polystyrene (PS). The LDPE grades differed in molecular-weight distribution and degree of long chain branching. The polymer samples were stretched to different levels of deformation over a range of temperature, quenched, and subsequently annealed. The shrinkage and thermoelastic force during recovery were measured upon annealing. Molecular orientation resulting from the stretching process was characterized by birefringence measurements in the amorphous polymer and by X-ray diffraction in the semicrystalline polymer. Morphology changes in the latter were characterized by polarized optical and electron microscopy. The relationship between the amount, rate and temperature of stretching, to the resulting orientation and morphology, and the shrinkage behavior were investigated.

Durability of Polyethylene Films

Abstract This work deals with the weather-resistance of low-density polyethylene films. The performance under controlled laboratory conditions, of well-characterized polymer grades and various combinations of stabilizers was studied. The high molecular weight and narrowly-distributed grades seemed to have a better performance. While the unstabilized films were found to fail after a relatively short period, the synergistic combinations of UV absorbers, quenchers and anti-oxidants showed a marked increase in film duration. For the protected films, increasing film thickness improved performance. The increase of crystallinity upon ageing with the resulting decrease in oxygen permeability are highlighted. The role of oxygen diffusion into the film is significant both in thermal and in photo-oxidation.

Molecular-weight distribution and long-chain branching of low-density polyethylene

Abstract The molecular-weight distribution curves, indices and frequencies of long-chain branching for some samples of low-density polyethylenes were obtained. The study was based on intrinsic viscosity data of the whole polymer and on gel permeation chromatography distribution curves, both for 1,2,4-trichlorobenzene solutions at 130°C. A correlation between the index of long-chain branching and the first molecular-weight dispersion index (ratio between weight and number average molecular weights) has been postulated. Results are limited to two models for branching used in this work.

Prospects for application of post‐consumer used plastics in food packaging

The two most widely used polymers in packaging in recent years are polyethylene terephthalate (PET) and polyethylene (PE). The biggest fractions of these polymers are not re-utilized, in spite of the fact that they possess excellent properties even after their first application. The ban on using recycled polymers in food packaging applications and the lack of good value outlets for these materials causes them to end up in landfills. The high cost nylon, used in packaging primarily as high gas barrier laminates with PE, also finds its way to landfills. In this case, the reason is the difficulty of recycling different polymers that are incompatible. Thus, the Municipal Solid Waste (MSW) stream transferred to landfills contains many plastic packages. These packages are being blamed as a major pollutant of the environment in spite of the fact that all plastics contribute only a small percentage to the weight of the garbage in landfills. If proper and cost effective applications for the recycled polymers could be developed, the waste related to their disposal could be limited. In addition, the contribution of plastic packages to the environmental problem could be diminished. In the present paper, the possibility of sandwiching a contaminated PET layer between two layers of the virgin material was studied. The aim of the study was to determine whether such an operation could lower the migration level of contaminants from a multilayer structure (containing a recycled layer of PET) to values below the limits required by regulatory agencies. The diffusion coefficients (required to determine migration) of four organic liquids in PET were determined. As a result of the sandwiching operation, the amount of pollutant (toluene) migrating into the food simulant was reduced by two orders of magnitude. The properties of PE/nylon blends were also studied. It was found that the high gas barrier properties of nylon are preserved in the blend when proper processing conditions are used. Therefore, the recycled material could be used as a centre layer in a multilayer structure providing good gas barrier properties to this structure.

Behavior of Polymers

I n this chapter we will discuss extensively the behavior of polymers and their response to stress. This will allow us to better understand the physical and thermal properties—relating structure and performance.

Structure–Property Relationships in Low-Density Polyethylene

Abstract Modern techniques were adopted and utilized for molecular-weight characterization of commercial low-density (branched) polyethylenes. The index of long-chain branching was uniquely related to that of molecular-weight dispersion, Dn . Two groups of low-density polyethylenes were visualized, differing in width of MW distribution and in the branching index. An attempt was made to relate the flow properties, melt-flow index (MFI) and intrinsic viscosity [n], as well as some physical properties–density, tensile strength, ultimate elongation and resistance toenvironmental stress cracking–toatypical molecular parameter. In all correlations, the two groups are represented by two distinctly different curves. By combining the parameter g for long-chain branching with the weight average molecular weight, [Mbar]γ , the flow parameters are unified into single curves. The mechanical properties studied were found to improve increasing [Mbar]γ and with narrowing of the molecular-weight distribution.

Introduction to the World of Polymers

W hat is the meaning of polymers? This word steins from Greek: Poly = many, mers = particles. So this term describes a molecule composed of many identical parts, called mers. The large molecule is therefore termed: macromolecule. What is left for us, is the practical definition of the term “many”. The minimum considers hundreds of mers. However, there are no significant mechanical properties below about 30 mers, while the useful average reaches 200–2,000 mers. If one wants to speak about molecular weights (which is usually done in chemistry) a broad range between 5,000 up to 2 × 106 may be representative, while in some cases it may reach 107.

Description of Major Plastics: Structure, Properties and Utilization

T his chapter compiles the information available in the field of polymers and plastics regarding structure and properties, and indicates significant unique uses. Special developments within each family of polymers will be highlighted. About 40 families of polymers will be briefly described, appearing commercially in over than 13,000 grades. After a detailed description, comparative data on properties, utilities and prices will be provided. It is obvious that the data represent typical average values, as there appear many grades of each polymer differing by molecular weight, distribution, degree of crystallinity and so on, not to mention modifications, blends and copolymers. In addition, the strength, rigidity, and sometimes also toughness of the polymer can be surpassed by appropriate compounding, mainly by reinforcement. The order of presentation of the materials is ranked according to quantitative utilization.

Compounding and Processing of Plastics

T he major goal is to make durable products that are composed of polymers and suitable additives, namely plastic materials. This process is generally composed of two stages—a) preparation of the final compound (compounding) by homogeneous dispersion, and b) forming the final shape (shaping). In this chapter the various problems involved in making the final product will be illuminated.

Structure and Characterization of Polymers

T he chemical structure of the homopolymers consists of an exact repetition of the chemical structure of the mer unit. The chemical bonds are mostly primary covalent ones, mainly C—C and C—H, but include also C—O, C—N, and so forth. There appear also various isomers, double bonds (unsaturation), tertiary or quaternary carbons, ring-like structures, and others. There are also secondary bonds, albeit weaker than the primary ones. One typical isomer, found in polymers, is based on the presence of “head-to-tail” bonds as compared to “head-to-head” or “tail-to-tail” bonds. This is illustrated by the vinyl group, with the substitute radical —X.