Cross polypropylene

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

Laboratory tests indicated that gamma radiation treatment and cross-linking using triaHylcyanurate or acetylene produced a flexible recycled plastic from mixtures of polyethylene, polypropylene, general-purpose polystyrene, and high impact grade PS (62). 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

Polyolefins. Interest has been shown in the plasticization of polyolefins (5) but plasticizer use generally results in a reduction of physical properties (12), and compatibiHty can be achieved only up to 2 wt %. Most polyolefins give adequate physical properties without plasticization. There has been use of plasticizers with polypropylene to improve its elongation at break (7) although the addition of plasticizer can lower T, room temperature strength, and flow temperature. This can be overcome by simultaneous plasticization (ca 15 wt % level) and cross-linking. Plasticizers used include DOA. 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

Corrosion. Aqueous solutions of citric acid are mildly corrosive toward carbon steels. At elevated temperatures, 304 stainless steel is corroded by citric acid, but 316 stainless steel is resistant to corrosion. Many aluminum, copper, and nickel alloys are mildly corroded by citric acid. In general, glass and plastics such as fiber glass reinforced polyester, polyethylene, polypropylene, poly(vinyl chloride), and cross-linked poly(vinyl chloride) are not corroded by citric acid. 

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

Other thermoplastic elastomer combiaations, ia which the elastomer phase may or may not be cross-linked, include blends of polypropylene with nitrile (30,31), butyl (33), and natural (34) mbbers, blends of PVC with nitrile mbber (35,36), and blends of halogenated polyolefins with ethylene interpolymers (29). Collectively, thermoplastic elastomers of this type ate referred to herein as hard polymer/elastomer combinations. Some of the more important examples of the various types are shown in Table 3. 

NOTE Maximum weigFt, 1 metric ton (2205 IL) cross-section dimensions, 890 by 890 mm (35 by 35 in) tare weight, 3 kg (7 lb) material of construction, woven polypropylene body and polyester lifting straps. 

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. 

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

Example 2.10 The polypropylene snap fit shown in Fig. 2.22 is to have a length of 10-30 mm. If the insertion force is not to exceed 4 N and the yield stress of the plastic is 30 MN/m, calculate suitable cross-sectional dimensions for the snap fit. The short-term modulus of the polypropylene is 900 MN/m and the coefficient of friction is 0.3. The safety factor on stress is to be 2. 

An extruded T-section beam in polypropylene has a cross-sectional area of 225 mm and a second moment of area, I, of 12.3 x lO mm. If it is to be built-in at both ends and its maximum deflection is not to exceed 4 mm after 1 week, estimate a suitable length for the beam. The central deflection, S, is given by... 

Figure 1. Temperature variation of the conductivity for a cross-section of polymer electrolytes. PESc, poly (ethylene succinate) PEO, polyethylene oxide) PPO, polypropylene oxide) PEI, poly(ethyleneimine) MEEP, poly(methoxyethoxy-ethoxyphosphazene) aPEO, amorphous methoxy-linked PEO PAN, polyacrylonitrile PC, propylene carbonate EC, ethylene carbonate.

Forming the hinge cross-section by using an extruder die results in a hinge with poor flex life. Because hinges are formed in the direction of the polymer flow, they cannot be sufficiently oriented when flexed. However, if an extruded hinge is formed by the take-off mechanism while the polypropylene... 

It has been patented as a cross-linking agent for polypropylene and poly butadiene (Ref 4)... 

---