Polypropylene foams

Polyolefin foams are manufactured in a variety of processes and in low density (25-250 kg/m ) or high density (250-700 kg/m ) versions. Foamed polyolefins have a long tradition in research, production and application. The consumption in 1995/96 all over the world was more than 190000 tonnes, including almost 18000 tonnes of expandable polyolefin beads. Polyolefin foams are expected to continue growing at a faster rate than other plastic foams. A major part of the polyolefin foams is based on polyethlyene (PE) and its copol)uners. Nearly 50% of the PE-based foam is partly cross-linked material. 

High viscosity of the polyolefin melt is necessary, especially in foaming equipment to produce sheets, tubes and shapes in batch and in continuous processes. On the contrary, polyolefin bead production is not so sensitive to melt viscosity. 

It is easy to understand that the solubility of volatile, or the dispersity of the nonsoluble, chemical blowing agents influences the homogeneity of the foamed material. 

The cell structure of the polyolefin foam can be further influenced by adding cell nucleation agents into the polymer melt. Highly dispersed fillers and primary gas generators which build small voids (cavities) in the interface, or in the homogeneous melt, act as nucleation agents for further cell formation. 

Polypropylene (PP) homo- and copolymers being linear chain molecules possess a narrow processing window in the melt. 

Polypropylene Foams Polypropylene foams are comparatively recent entries into the structural foam field, supplanting hi -density polyethylene foams, but their use is increasing rapidly because of the extreme range of grades and properties available, plus a favorable price advantage, compared with other thermoplastic foams. Glass-reinforced (30% glass) polypropylene foams are commonly used (13). 

Low-density flexible polypropylene foam film can be extruded in the 0.7 Ib/ft (11.2 kg/m ) range. In spite of its low density the fUm sheeting consists of a uniform matrix of small closed-cell gas-filled bubbles. This film has outstanding toughness and strength over a wide- 

---

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. 

Table 4.8 Examples of some polypropylene foam properties...

Figure 4.14. Tensile strength examples versus density of Polypropylene foams Examples of applications...

An interesting development initiated by Neste and Norsk Hydro is a multilayer coating for the insulation of deep-water pipelines. The steel pipe is protected by a layer of solid thermoplastic, a core of polypropylene foam and a coating of solid plastic, all of which are built up by cross head extrusion. 

The typical properties of polypropylene foams for sandwich technology are ... 

EFFECT OF NUCLEATING AGENTS ON POLYPROPYLENE FOAM MORPHOLOGY... 

FOAM EXTRUSION OF POLYPROPYLENE FOAMS - NEW DEVELOPMENTS AND APPPLICATIONS... 

Polymer Engineering and Science 43,No.l, Jan.2003,p.40-54 IMPORTANCE OF TIMELY POLYMER SINTERING WHILE PROCESSING POLYPROPYLENE FOAMS IN ROTATIONAL MOLDING... 

Brookfield, Ct., SPE, 2002, Interior Applications Session, p.97-117, 27cm, 012 SIMPLIFIED HIGH EFFICIENCY POLYPROPYLENE FOAM PART DESIGNS FOR AUTOMOTIVE HEADLINER COUNTERMEASURE APPLICATIONS... 

Brookfield, Ct., 2002, Session I, p.1-8. 27cm, 012 LOW DENSITY EXTRUDED POLYPROPYLENE FOAMS - ONE OF THE FINAL FRONTIERS Folland R Reichelt N Stadlbauer M Borealis Polymers NV Borealis GmbH (SPE,Thermoplastic Materials Foams Div. ... 

Polymer Engineering and Science 42, No.7, My 2002, p. 1481-92 STRATEGIES FOR ACHIEVING ULTRA LOW-DENSITY POLYPROPYLENE FOAMS... 

MELT COMPOUNDING BASED ROTATIONAL FOAM MOLDING TECHNOLOGY FOR MANUFACTURE OF POLYPROPYLENE FOAMS Pop-Iliev R Park C B Toronto.University... 

Shawbury, Rapra Technology Ltd., 2001, Paper 7. 012 STRATEGIES FOR ACHIEVING FINE-CELLED LOW-DENSITY POLYPROPYLENE FOAMS Naguib H E Park C B Hesse A Panzer U Reichelt N Toronto,University (Rapra Technology Ltd.)... 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Compositions and processing conditions are disclosed for the production by rotomoulding of polypropylene foams, hitherto satisfactorily produced only by injection... 

RECYCLING TECHNOLOGY FOR LAMINATES COMPOSED OF THERMOPLASTIC POLYOLEFIN ELASTOMER AND CROSSLINKED POLYPROPYLENE FOAM... 

Espoo, Technical Research of Finland, 1998, pp.l03. 10 ins. 15/4/99. VTT Publn. 361 NEW TECHNOLOGY TO MANUFACTURE POLYPROPYLENE FOAM SHEET AND BIAXIALLY ORIENTED FOAM FILM Raukola J 1... 

Patent Number US 5567742 A 19961022 DIMENSIONALLY-STABLE POLYPROPYLENE FOAM EXPANDED WITH INORGANIC BLOWING AGENTS... 

Patent Number US 5527573 A 19960618 EXTRUDED CLOSED-CELL POLYPROPYLENE FOAM... 

A closed-cell polypropylene foam, with at least 80% closed cells and a foamability characteristic of less than about 1.8 is disclosed. Also described are several methods for making such closed-cell polypropylene foams. 

Journal of Vinyl and Additive Technology 2, No. 4. Dec. 1996, p.349-357 EXTRUSION OF POLYPROPYLENE FOAMS WITH HYDROCEROL AND ISOPENTANE... 

Carbon dioxide gas was used as a physical blowing agent to produce medium density polypropylene foam sheets using a single screw extruder. The mechanical properties of the foam were similar in the machine direction and in the transverse direction. Abetter surface finish and a lower density was produced by using a commercial wrapping film as a cap layer. The process conditions and the die design data are presented and an attempt made to relate them to the product characteristics. 4 refs. 

---