Polypropylene and Polyethylene

Polyethylene and Polypropylene Acceptable bonds have been obtained between treated polyolefin surfaces with polar adhesives, such as epoxies, or solvent cements containing synthetic rubber or phenolic resin. The solvent adhesives are applied to both surfaces and the solvents allowed to evaporate before the parts are joined. Recommended epoxies are the anhydride-cured and amine-cured types. Also suitable is a two-component, polyamide-modified epoxy compound. Other adhesives that provide adequate bond strength to treated polyolefins include styrene-unsatmated polyester and solvent-type nitrile-phenolic (15). 

Early work on polyethylene and polypropylene has been reviewed by Madorsky[38] and Winslow and Hawkins [39]. Initiation by random scission or at weak links has been proposed. Since little monomer is evolved, chain depolymerization seems to be of minimal importance. The low molecular weight products formed are the result of inter- or intramolecular free radical transfer. 

The effect of structure on the mechanism of thermal decomposition of saturated hydrocarbon polymers has been studied more recently by Wall and Straus [40]. Linear and branched polyethylene, polypropylene and various copolymers have been investigated and the rates of volatilization compared. 

The linear materials behaved according to the theory of random degradation (Fig. 18) for 1/7 - 0, while the branched materials did not. The greater the branching, the greater was the rate of decomposition and 

The volatile products formed during degradation of polyethylene have been analysed by mass spectrometry by Madorsky [38]. Gas chromatographic analysis of the low molecular weight volatilization products was performed later by Moisev et al. [42,43], and still more recently by Tsuchiya and Sumi [44]. The very efficient separation and quantitative determination performed by these authors is shown in Fig. 22. 

The formation of products is explained on the basis of a free radical mechanism. Initiation consists of preferential scission of weak bonds or ordinary C—C bonds. The weak bonds may include oxygen atoms carbon—carbon bonds in the j3 position to double bonds or adjacent to tertiary carbons may also be involved. The mechanism proposed is 

The reactions of polyethylene and polypropylene in the presence of Lewis acids are proposed to be similar to those for Bronsted acids, i.e. carbenium ions are the active intermediates. However, the formation of the protons which initiate the degradation is a result of the interaction of the Lewis acid with polar impurities, e.g. water, as illustrated in eqn. (1), It is proposed that the charged complex formed interacts with the polymer chains to form a carbenium ion and its corresponding counter-anion. 

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Table 16. Hydrated Filler Systems for Polyethylene and Polypropylene ...

Synthesis of polyethylene and polypropylene Eischer-Tropsch synthesis Oxidation of SO2 to SO ... 

Other Films. Although commercially less important than polyethylenes and polypropylenes, a number of other plastic films are in commercial use or development for special appHcations, including ethylene—vinyl acetate, ionomer, and polyacrylonitrile [25014-41-9]. 

Petroleum resins are low molecular weight thermoplastic hydrocarbon resins synthesized from steam cracked petroleum distillates. These resins are differentiated from higher molecular weight polymers such as polyethylene and polypropylene, which are produced from essentially pure monomers. Petroleum resin feedstocks are composed of various reactive and nonreactive aliphatic and aromatic components. The resins are usually classified as C-5... 

Plastic. A plastic bag usuaUy consists of a single heavy waU of plastic film, woven sheets of plastic tape, or laminates. Principal materials of constmction are polyethylene and polypropylene (see Fibers, olefin). Both transparent and opaque sheeting are used, and printabUity usuaUy is exceUent. Plastic bags can be fiUed and closed with conventional equipment beat-sealing is essential for open-mouthed bags to effect a moisture barrier. 

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

Increasingly, plastics are being used as parenteral packaging (qv) materials. Plastics such as poly(vinyl chloride), polyethylene, and polypropylene are employed. However, plastics may contain various additives that could leach into the product, such as plasticizers (qv) and antioxidants. PermeabiUty of plastics to oxygen, carbon dioxide, and water vapor must be tested in the selection of plastic containers. Furthermore, the plastic should withstand sterilization. Flaking of plastic particles should not occur and clarity necessary for inspection should be present. 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

Typical substrates for siUcone release coatings are supercalendered kraft paper, glassines, and thermally sensitive films such as polyethylene and polypropylene. Ideal curing conditions are 150°C or lower, and line speeds are as fast as 460 m /min. Key properties for release coatings are cure speed, integrity of cure, and stable release values. 

Polyolefins. Low concentrations of stabilizers (<0.01%) are added to polyethylene and polypropylene after synthesis and prior to isolation to retard oxidation of the polymers exposed to air. 

Nonionic surfactants perform well in nonpolar polymers such as polyethylene and polypropylene. Examples of nonionic surfactants ate ethoxylated fatty amines, fatty diethanolamides, and mono- and diglycetides (see Amines, fatty amines Alkanolamines). Amphoteric surfactants find Httle use in plastics (134). 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

DlsaZO Pigments. The diaiylide yeUows and oranges also known as benzidines are derivatives of benzidine coupled to two moles of substituted acetoacetanilide. Benzidine Yellows AAMX, AAOT, AAOA, and HR (PY 13, 14, 17, and 83) ate examples (Fig. 1). Yellows AAMX and AAOT are used in flexible vinyls. AAOA also colors polyethylene and polypropylene. These three differ only slightly in shade. Benzidine YeUow HR is redder. 

Nonmetallic equipment normally is not used for ethyleneamine service. Ethyleneamines can permeate polyethylene and polypropylene, even at ambient temperature. However, certain grades of these materials may be acceptable in some storage appHcations. Baked phenolic-lined carbon steel is acceptable for storage of many pure ethyleneamines, except EDA. 

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

Figure 11.7. Comparison of oxidation rates of unstabilised polyethylene and polypropylene (After...

In broad tonnage terms the injection moulding markets for high-density polyethylene and polypropylene are very similar. The main reasons for selecting polypropylene have been given above. In favour of HDPE is the inherently better oxidation and ultraviolet resistance. Whilst these properties may be greatly improved in polypropylene by the use of additives these may increase the cost of polypropylene compounds to beyond that which is considered economically attractive. It is for this reason that HDPE has retained a substantial part of the crate market. 

This polymer is typical of the aliphatic polyolefins in its good electrical insulation and chemical resistance. It has a melting point and stiffness intermediate between high-density and low-density polyethylene and a thermal stability intermediate between polyethylene and polypropylene. 

It is less resistant to aliphatic hydrocarbons than polyethylene and polypropylene and in fact pipes may be solvent welded. At the same time the resistance to environmental stress cracking is excellent. 

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

Their physieal properties are essentially those of the alkanes. It is the unsaturated linkages that dominate the ehemistry and the main reaetion is one of addition (e.g. hydrogen, halogen, and hydrogen halides) aeross the double bond to produee saturated eompounds. This reaetivity is utilized in the manufaeture of long-ehain polymers, e.g. polyethylene and polypropylene. 

Papirer et al. used ATR, XPS, and SIMS to determine the effect of flame treatment on adhesion of polyethylene and polypropylene to styrene/butadiene (SBR) rubber [8]. Each flame treatment consisted of a 75-ms pass over a circular burner. The distance between the upper flame front and the polymer was kept fixed al 8 mm. A band was observed near 1720 cm" in the ATR spectra and assigned to carbonyl groups this band increased in intensity as the number of flame... 

Butyl latex can be used in packaging and as a tackifying and flexibilizing additive in higher strength adhesives for adhesion of polyethylene and polypropylene. 

Polyethylene and polypropylene are semitransparent plastics made by polymerization. They are produced from ethylene and propylene in a variety of grades. Their mechanical properties are determined mainly by density (degree of crystallinity) and molecular weight, characterized by the Melt Index (MI). 

The effect of these two parameters on mechanical and physical properties of polyethylene and polypropylene are shown in Tables 3.44 and 3.45. The copolymer grade is usually propylene with a little ethylene (5%), wliich considerably improves the impact strength while causing only a slight loss in stiffness. 

Rhodamine B vaseline [155] diphenyl, polyphenols [156] maleic and fu-maric acids [162] flavonoids [158] alcohols as 3,5-dinitrobenzoates [159, 160] gangliosides [161] 1-hydroxychlorden [162] carbamate pestiddes [163] para-thion and its metabolites [164] polyethylene and polypropylene glycols [165] terpene derivatives [166] menthol [167]... 

Deep fluonnation using the La-Mar technique was carried out on polymers such as polyethylene and polypropylene [M], on polyethers [19, 20, 21], and on polyesters subsequently treated with sulfur Cetrafluoride [22] Deep fluorinations carried out under conditions producing limited fragmentation produced oligomeric perlluoropolyethers from powdered polyethylene oxide [23] Deep fluorinations earned out in the limited presence of molecular oxygen result in the conversion of... 

Many high molecular weight synthetic polymers, such as polyethylene and polypropylene, have a large percentage of their molecules in the crystalline state. Prior to dissolution, these polymers must usually be heated almost to their melting points to break up the crystalline forces. Orthodichlorobenzene (ODCB) is a typical mobile phase for these polymers at 150°C. The accuracy and stability of the Zorbax PSM columns under such harsh conditions make them ideal for these analyses (Fig. 3.8). 

The temperature must be raised when there is no solvent that can dissolve samples at ambient temperature. For example, polyolefines such as polyethylene and polypropylene are usually analyzed at 130-140°C because no solvent can dissolve these polyolefines at lower temperatures. It is also preferable to perform analyses at elevated temperatures when the viscosity of the elution solvent is considerably higher at ambient temperature. However, a temperature around 25-40°C is recommended when good solvents having low viscosity are available at such a temperature. It is much more convenient to operate a GPC instrument at 25-40°C than to operate at higher temperatures. 

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