Foamed EVA

Obviously, there exists severe interplastics competition, e.g. PP vs. ABS, clarified PP vs. PS, PA, PVC, HDPE and PS (Table 10.7). A wide range of cross-linked and thermoplastic elastomer applications, from footware to automotive parts and toothbrushes, are adopting new metallocene-catalysed polyolefin elastomers (POEs). These low-density copolymers of ethylene and octene were first accepted as impact modifiers for TPOs, but now displace EPDM, (foamed) EVA, flexible PVC, and olefinic thermoplastic vulcanisates (TPVs). Interpolymer competition may also result from... 

Uses Flame retardant for ABS, PE, PP, PS, PU rigid and flexible foam, EVA, TPE, TPR, rubber, acrylics, polyterephthalate, intumescent latex and solv.-based paints, intumescent mastic, caulks, and putties, building materials (wall coverings, ceiling tiles, roofing prods., wall panels, wood chip board)... 

Cross-linked and foamed EVA polymers, e.g. Evazote, are widely used as soft sheet for cushioning during packaging. 

Acoustic barriers consist of a low-density layer and a heavy (usually highly filled) layer. Glass fibers, foams, EVA, SEES compounds (injection molded), asphalt-ics, and thermoset polyurethanes are used. 

Can be crosslinked by peroxides and/or, may also be chemically blown to give cellular products the density of crosslinked, foamed EVA can be very low indeed and is far... 

Shoe adhesives. CR adhesives are used for the permanent attachment of shoe soles. For difficult-to-bond sole materials (plasticized PVC, EVA foaming soles, thermoplastic rubber, SBR) graft polymer solutions of Neoprene AD-G combined with a polyisocyanate provide a good adhesion. Another major area for CR contact adhesives is the manufacture of leather goods, particularly leather shoe sole bonding and belt lamination. 

Eaves (92) distinguished between polyolefin plastomers (POP) with density >910 kg m and polyolefin (POE) elastomers with densities <910 kg m-3. The density of a polyethylene at 20 °C is a linear function of the crystallinity, with limiting values of 854 kg m 2 for zero crystallinity and 1000 kg m for 100% crystallinity. The polyolefin elastomer foams compete with EVA copolymer foams. Metallocene chemistry also allows the production of copolymers with a larger comonomer content in the high molecular weight part than in the low molecular weight part this... 

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

The styrene content affects the crystallinity of ESI (131) for >50% styrene the copolymers are amorphous. As the styrene content is increased from 50 to 70% styrene the Tg increases from -15 °C to 20 °C. Low density foams were made (8) from a blend of 50% of various ESI polymers, 33% of EVA and 17% of azodicarbonamide blowing agent. Thermal analysis showed that the blends, with an ESI having approximately 70% styrene, had a Tg in the range 22 to 30 °C. Dynamic mechanical thermal analysis (DMTA) traces (see Section 5.1) show that these blends... 

If up to 40% of ESI is blended with LDPE then foamed, the foam properties are closer to those of LDPE foams. Ankrah and co-workers (33) showed that the ESI/LDPE blends have slightly lower initial compressive yield strengths than the LDPE alone, allowing for the density of the foam. The temperature dependence of the yield stress is similar to that of LDPE foam (Figure 3). Although the yield stress is higher than EVA foam of the same density, the compression set values are lower. The ESI/LDPE foams have improved impact properties, compared with EVA foams of similar density. Analysis of creep tests shows that air diffuses from the cells at a similar rate to EVA foams of a greater density. 

Variation of initial compressive yield stress of ESI/LDPE foams with temperature, compared with an EVA and a... 

Abnormally large cells, with >20 faces, formed by cell face collapse in EVA foam of density 150 kg m", with a background of normal cells (a.9)... 

At strains >10%, when the polymer structure has begun to collapse, gas loss, by diffusion through the cell faces of closed cell foams, may contribute to the creep. The effect of this on the creep of LDPE and EVA foams was determined (266). The foam diffusivity for air was predicted from the polymer permeability P and the foam density p using ... 

The creep stress was assumed to be shared between the polymer structure yield stress and the cell gas pressure. A finite difference model was used to model the gas loss rate, and thereby predict the creep curves. In this model the gas diffusion direction was assumed to be perpendicular to the line of action of the compressive stress, as the strain is uniform through the thickness, but the gas pressure varies from the side to the centre of the foam block. In a later variant of the model, the diffusion direction was taken to be parallel to the compressive stress axis. Figure 10 compares experimental creep curves with those predicted for an EVA foam of density 270 kg m used in nmning shoes (90), using the parameters ... 

Creep curves for EVA foam of density 275 kg m", and the modelling predictions (parameters in text) at the... 

For the EVA foam midsoles of running shoes, the durability for repeated impact appears to improve with increased density. However the polymer conhibution to the impact response reduces after simulated (and real) use (a. 18), since wrinkles and some holes appear in the cell walls. Consequently these applications require high density foams. 

Some foam products are subjected to up to 10 major impacts in their lives, therefore a foam may need to be selected so that the performance is still adequate. Products, subjected to hundreds of thousands of impacts as in running shoe midsoles, were dealt with under fatigue. It appears that the higher density HDPE (419) and PP foams loose a significant amount of their protection after several impacts, but nevertheless it may be possible to use sufficient foam thickness to provide the required protection. If low density EVA foams are used, the impact energy densities are much lower, as the majority of the resistance is from compressing the cell gas. Ankrah and co-workers (33) performed multiple impacts on LDPE/ESI blend foams, and found... 

Figure 12 shows the temperature variation of the in-phase Young s modulus E of an EVA foam (Verdejo, unpublished). E falls as the amorphous phase goes through a glass transition close to -10 °C, then again as the crystalline phase melts at 80 °C. The peaks in tan S at these temperatures are labelled a and p. 

DMTA graph of storage Young s modulus and damping (tan 5) versus temperature, for EVA foam of density... 

Dubois (76) described the use of crosslinked blends of PE and ESI in shoes. In blends with EVA, there is an improvement in the 23 °C compression set value alternatively a softer foam with greater resiliency can be obtained with the same compression set compared with EVA. 

Helmets for sports such as canoeing or cricket tend to have less severe impact requirements than for bicycle helmets, so may use smaller amounts of LDPE or EVA foam. As ventilation is important, there may be large holes in the foam and in the helmet shell. 

The construction of football shin guards (a.26) is a curved outer shell over layer(s) of EVA foam. The... 

Harden s (27) market survey of the growth of polyolefin foams production and sales shows that 114 x 10 kg of PE was used to make PE foam in 2001. The growth rate for the next 6 years was predicted as 5-6% per year, due to recovery in the US economy and to penetration of the automotive sector. In North America, 50% of the demand was for uncrosslinked foam, 24% for crosslinked PE foams, 15% for EPP, 6% for PP foams, 3% for EVA foams and 2% for polyethylene bead (EPE) foam. As protective packaging is the largest PE foam use sector, PE foam competes with a number of other packaging materials. Substitution of bead foam products (EPP, EPE, ARCEL copolymer) by extruded non-crosslinked PE foams, produced by the metallocene process was expected on the grounds of reduced costs. Compared with EPS foams the polyolefin foams have a lower yield stress for a given density. Compared with PU foams, the upper use temperature of polyolefin foams tends to be lower. Eor both these reasons, these foams are likely to coexist. 

Foams were prepared from blends of EVA and ethylene-styrene interpolymers having styrene contents ranging from 30 to 73 wt.% and Tgs ranging from -2 to 33C, in the presence of various amounts of dicumyl peroxide and their viscoelastic and mechanical properties investigated. The effects of styrene content and peroxide concentration on the morphology, degree of crosslinking and expansion ratio of the foams were assessed as was the influence of the type of ethylene-styrene interpolymer on the Tg of the foams. It was found that the Tg of the blends could be tailored by selection of the appropriate interpolymer. 19 refs. 

Journal of Cellular Plastics 38, No.6, Nov.2002, p.471-96 ELECTRON BEAM IRRADIATION OF NONCROSSLINKED LDPE-EVA FOAM... 

LDPE and EVA were blended in a 80/20 ratio and the non-crosslinked foam thereof was exposed to various doses of electron beam irradiation in air. The foam was made using isobutane as blowing agent. It was found that foam with uniform cell structure was not achieved in the case of pre-crosslinked foam with more than 40% gel fraction. Non-crosslinked foams were, however, easily crosslinked to a gel content of about 57% with electron beam irradiation under optimum conditions. The gel... 

Foamed blends of ethylene-styrene interpolymer and LDPE were subjected to a range of mechanical tests, including compressive impact testing, Instron compression and Poisson s ratio measurements, compressive creep measurements and compression set and recovery measurements. The data obtained were compared with those for EVA and the suitability of these foamed blends as replacements for EVA in the manufacture of soccer shin guards and midsoles for sports shoes was evaluated. 20 refs. 

Details are given of the E266-4 injection press developed by Main Gronp for the production of shoe soles from EVA and crosslinked EVA foams. 

The use of crosslinked expanded EVA and other crosslinked foamed polymers in shoe soles is discussed, and developments by Main Group in injection moulding machines for the production of soles from such materials are examined. 

No.2, 2001, p.79-100 MODELING THE GAS-LOSS CREEP MECHANISM IN EVA FOAM FROM RUNNING SHOES... 

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