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Why do particulate fillers tend to aggregate?
Additives for Polymers
Mav 1999
However, there may be an area of non-critical parts where a broad tolerance could be acceptable, and perhaps the: value of the development of natural fibre for rleinforcement of plastics is that it will produce a re-evaluation and valueanalysis of parts, identifying those which, with present materials and technology are, in fact, over-engineered. The main modification that K-M has made to its system is to redesign the processing head, in which the two liquid resin components are brought together with1 the continuous roving fibre reinforcement, and the latter is chopped to the required length. The different characteristics of natural fibres, whiich are not cut as easily as glass, required a difYerent approach and K-M has opted for a scissor-type method of chopping. The basic LFI-PUR system offers advantages over previous polyurethane reaction injection moulding methods and other long fibre moulding systems, in that it is possible to robotize the processing head, and distribute the reinforcement exactly where it is required in the mould, so avoiding costly preforms and also allowing lower moulding pressures to be employed. The system. with glass or natural fibre reinforcement, could be used for moulding relatively large but simple components, such as automobile door interior panels, dashboards and parcel shelves. K-M believes, however, that it also has good potential in the furniture industry. ??
A mixture of equal parts of flax and sisal fibre is used with a polyurethane binder in a process developed by Beckc:r Group Europe, Wuppertal, Germany: for production of door inner trim panels for Mercedes Benz. With a thickness of 1.7-1.X mm, and a unit weight of 1300-1600 grams/m’, they are claimed to cost less than similar polyurethane/glass fibre panels, because the natural fibres which make up 60% of the composite are less expensive than glass fibre. The polyurethane system comes from Bayer.
Contact. Krauss-MaJf& AG, Krauss-MaffeiStrasse 2, D-80997 Mtinchen, Germany; tel: + 49-89-88-99-3.5.51; 2230
01999
Elsevier Science
.fax: f 49-89-88-99-
TECHNICAL
BRIEFS
Why do particulate fillers tend to aggregate ? A continuing problem with particulate fillers is that they often will not flow smoothly, but tend to aggregate. This is a particular problem in producing a filled compound, leading to irregular distribution of the particles, with attendant processing problems, poor surface quality and reduction in mechanical properties. Some work in Hungary, reported recently, throws useful light on the problem. Eleven commercial calcium carbonate (CaC03) fillers, of differing particle sizes and specific area, were investigated with various techniques, by researchers at the Technical University and the Institute of Chemistry, Budapest. It is possible to identify many attractive forces that might lead to aggregation of particles. Dispersed in a viscous liquid, the viscous force hinders separation of the particles and, in the case of incomplete wetting, capillary forces act between particles connected by discrete liquid bridges. Several forces may also act between electrostatically charged particles. There IS a smaller number of separating forces. such as repulsion between particles having the same charge, but more significant is the hydrodynamic force acting on particles dispersed in a fluid, as in processing of polymer melts. Since the viscosity of the melt is usually high, this force may significantly hinder the development of aggregated structures. In polymer composites, approximate calculations suggest that adhesive and hydrodynamic forces are the most important” ?4scous force is also relatively large. but the value reduces with particle size. Analysis showed that aggregation is determined by the relative magnitude of attractive and separating forces, while the most important factors influencing the homogeneity of polymer composites are the size of the particles, their surface tension and the shear forces acting on them during homogenization.
Additives-for
Polymers
May 1999
Characteristics of CaCOJ tillers investigated
Evaluation of the different types of CaC03 was carried out by compounding each (at a loading of 25% by volume) in a polypropylene matrix in an internal mixer. The high filler content was chosen deliberately to promote aggregation, and the composite sample was then compression moulded into 1 mm thick plates at 19O”C, for subsequent studies.
composites, the most important attractive force is adhesion, while hydrodynamic forces (such as shear) lead to separation of particles. The size and surface tension of the particles strongly influence aggregation. Although the specific surface area tends to give a good indication of the aggregation tendency of a filler, the particle size distribution is more important, since individual particles tend to interact with each other. The results obtained also indicated that the properties of the powder and the
It was concluded that the extent of aggregation is always determined by the relative magnitude of the forces attracting and separating. In polymer
Forces acting on 20 pm large particles in a polymer (PP) composite Force Direction
Character
1o-4
Viscous(a)
1o-5-1 o-6
Adhesive forces
1
Electrostatic
Separating forces I^\_
_
-.__
Magnitude (N)
Adhesive
Capillary’a’
,
Type
Surface tension
1o-5-1 o-6
Hydrostatic
1o-5-10”
Coulomb
1o-7
Dipole
10”
Image change
1o‘7
Space change
1O-e
Hydrodynamic
10”
Electrostatic
1o-6
_.
(*IFor particles of 100 pm diameter Source: Polymers & Polymer Composites
8
0 1999 Elsevier Science
Additives for Polymers
May 1999
suspension may yield valuable indirect information about aggregation. The extent of aggregation is specific to the system being employed, since the adhesive, and especially the separating forces, depend very much on the type of polymer and filler, as well as on processing conditions. Aggregation of the filler will usually impair the properties of a composite, but it tends to have different effects on different properties. The extent of aggregation may be reduced by non-reactive surface treatment and increased shear. Polymers & Polymer 5, 1998, ~~313-322
Composites,
Vol. 6, No.
Styrene block copolymers as impact modifiers and polymer compatibilizers Styrenic block copolymers and their compounds have been in widespread commercial use for many years, with many applications. With the latest technology, they have become particularly interesting as impact modifiers for plastics, both thermoplastics and thermosets. Nice Marks, of Shell Chemicals Research, Louvain-La-Neuve, Belgium. gave a usefill up-date on recent developments at the first Engineering Thermoplastic World Congress, in Ziirich, Switzerland. Most polymers are thermodynamically incompatible with others polymers and mixtures tend to separate into two phases, even when they are part of the same molecule, as in block copolymers Poly(styrene-b-elastomer-b-styrene) co-
polymers, in which the elastomer is the main constituent, give a structure in which the polystyrene end-segments form separate spherical regions (‘domains’) dispersed in a continuous phase. At room temperature, these polystyrene segments are hard and act as physical crosslinks, tying the elastomers chain together in a threedimensional network, not unlike the network that formed by crosslinking of thermosetting rubber during vulcanization. A second-generation of these block copolymers (such as Shell’s Kraton G series) is manufactured by selective hydrogenation technology, allowing conversion of polybutadiene or polyisoprene into, respectively, polyethylene/butylene (S-ES-S) or polyethylene/propylene (S-EP-S) rubber. The thermodynamics leading to phase separation in these heterogeneous polymers and the resulting elasticity are well-known over many years. They can be compounded with other polymers. fillers, flame retardants and other additives, tolerating very high loadings, often with the S-EBS polymer comprising as little as 25% of the volume, so offering potential cost-savings. They can also be used in polymer modification of thermoplastics and SMC/BMC materials. Hydrogenated S-EB-S block-copolymers can be used with olefinic plastics such as PP and PE because of their higher temperature allowance, and can even be used with engineering plastics that usually require melt temperatures well above 275°C. But, for polar engineering plastics. such as PA 6 and 66, maleic anhydride func-
Kraton G as a compatibilizer for engineering thermoplastics Plastic type
PA 6 PA 66 -. PET PBT PC Mod PPO
Source: Shell Chemicals
0 1999 Elsevier Science
9
Mav 1999
However, there may be an area of non-critical parts where a broad tolerance could be acceptable, and perhaps the: value of the development of natural fibre for rleinforcement of plastics is that it will produce a re-evaluation and valueanalysis of parts, identifying those which, with present materials and technology are, in fact, over-engineered. The main modification that K-M has made to its system is to redesign the processing head, in which the two liquid resin components are brought together with1 the continuous roving fibre reinforcement, and the latter is chopped to the required length. The different characteristics of natural fibres, whiich are not cut as easily as glass, required a difYerent approach and K-M has opted for a scissor-type method of chopping. The basic LFI-PUR system offers advantages over previous polyurethane reaction injection moulding methods and other long fibre moulding systems, in that it is possible to robotize the processing head, and distribute the reinforcement exactly where it is required in the mould, so avoiding costly preforms and also allowing lower moulding pressures to be employed. The system. with glass or natural fibre reinforcement, could be used for moulding relatively large but simple components, such as automobile door interior panels, dashboards and parcel shelves. K-M believes, however, that it also has good potential in the furniture industry. ??
A mixture of equal parts of flax and sisal fibre is used with a polyurethane binder in a process developed by Beckc:r Group Europe, Wuppertal, Germany: for production of door inner trim panels for Mercedes Benz. With a thickness of 1.7-1.X mm, and a unit weight of 1300-1600 grams/m’, they are claimed to cost less than similar polyurethane/glass fibre panels, because the natural fibres which make up 60% of the composite are less expensive than glass fibre. The polyurethane system comes from Bayer.
Contact. Krauss-MaJf& AG, Krauss-MaffeiStrasse 2, D-80997 Mtinchen, Germany; tel: + 49-89-88-99-3.5.51; 2230
01999
Elsevier Science
.fax: f 49-89-88-99-
TECHNICAL
BRIEFS
Why do particulate fillers tend to aggregate ? A continuing problem with particulate fillers is that they often will not flow smoothly, but tend to aggregate. This is a particular problem in producing a filled compound, leading to irregular distribution of the particles, with attendant processing problems, poor surface quality and reduction in mechanical properties. Some work in Hungary, reported recently, throws useful light on the problem. Eleven commercial calcium carbonate (CaC03) fillers, of differing particle sizes and specific area, were investigated with various techniques, by researchers at the Technical University and the Institute of Chemistry, Budapest. It is possible to identify many attractive forces that might lead to aggregation of particles. Dispersed in a viscous liquid, the viscous force hinders separation of the particles and, in the case of incomplete wetting, capillary forces act between particles connected by discrete liquid bridges. Several forces may also act between electrostatically charged particles. There IS a smaller number of separating forces. such as repulsion between particles having the same charge, but more significant is the hydrodynamic force acting on particles dispersed in a fluid, as in processing of polymer melts. Since the viscosity of the melt is usually high, this force may significantly hinder the development of aggregated structures. In polymer composites, approximate calculations suggest that adhesive and hydrodynamic forces are the most important” ?4scous force is also relatively large. but the value reduces with particle size. Analysis showed that aggregation is determined by the relative magnitude of attractive and separating forces, while the most important factors influencing the homogeneity of polymer composites are the size of the particles, their surface tension and the shear forces acting on them during homogenization.
Additives-for
Polymers
May 1999
Characteristics of CaCOJ tillers investigated
Evaluation of the different types of CaC03 was carried out by compounding each (at a loading of 25% by volume) in a polypropylene matrix in an internal mixer. The high filler content was chosen deliberately to promote aggregation, and the composite sample was then compression moulded into 1 mm thick plates at 19O”C, for subsequent studies.
composites, the most important attractive force is adhesion, while hydrodynamic forces (such as shear) lead to separation of particles. The size and surface tension of the particles strongly influence aggregation. Although the specific surface area tends to give a good indication of the aggregation tendency of a filler, the particle size distribution is more important, since individual particles tend to interact with each other. The results obtained also indicated that the properties of the powder and the
It was concluded that the extent of aggregation is always determined by the relative magnitude of the forces attracting and separating. In polymer
Forces acting on 20 pm large particles in a polymer (PP) composite Force Direction
Character
1o-4
Viscous(a)
1o-5-1 o-6
Adhesive forces
1
Electrostatic
Separating forces I^\_
_
-.__
Magnitude (N)
Adhesive
Capillary’a’
,
Type
Surface tension
1o-5-1 o-6
Hydrostatic
1o-5-10”
Coulomb
1o-7
Dipole
10”
Image change
1o‘7
Space change
1O-e
Hydrodynamic
10”
Electrostatic
1o-6
_.
(*IFor particles of 100 pm diameter Source: Polymers & Polymer Composites
8
0 1999 Elsevier Science
Additives for Polymers
May 1999
suspension may yield valuable indirect information about aggregation. The extent of aggregation is specific to the system being employed, since the adhesive, and especially the separating forces, depend very much on the type of polymer and filler, as well as on processing conditions. Aggregation of the filler will usually impair the properties of a composite, but it tends to have different effects on different properties. The extent of aggregation may be reduced by non-reactive surface treatment and increased shear. Polymers & Polymer 5, 1998, ~~313-322
Composites,
Vol. 6, No.
Styrene block copolymers as impact modifiers and polymer compatibilizers Styrenic block copolymers and their compounds have been in widespread commercial use for many years, with many applications. With the latest technology, they have become particularly interesting as impact modifiers for plastics, both thermoplastics and thermosets. Nice Marks, of Shell Chemicals Research, Louvain-La-Neuve, Belgium. gave a usefill up-date on recent developments at the first Engineering Thermoplastic World Congress, in Ziirich, Switzerland. Most polymers are thermodynamically incompatible with others polymers and mixtures tend to separate into two phases, even when they are part of the same molecule, as in block copolymers Poly(styrene-b-elastomer-b-styrene) co-
polymers, in which the elastomer is the main constituent, give a structure in which the polystyrene end-segments form separate spherical regions (‘domains’) dispersed in a continuous phase. At room temperature, these polystyrene segments are hard and act as physical crosslinks, tying the elastomers chain together in a threedimensional network, not unlike the network that formed by crosslinking of thermosetting rubber during vulcanization. A second-generation of these block copolymers (such as Shell’s Kraton G series) is manufactured by selective hydrogenation technology, allowing conversion of polybutadiene or polyisoprene into, respectively, polyethylene/butylene (S-ES-S) or polyethylene/propylene (S-EP-S) rubber. The thermodynamics leading to phase separation in these heterogeneous polymers and the resulting elasticity are well-known over many years. They can be compounded with other polymers. fillers, flame retardants and other additives, tolerating very high loadings, often with the S-EBS polymer comprising as little as 25% of the volume, so offering potential cost-savings. They can also be used in polymer modification of thermoplastics and SMC/BMC materials. Hydrogenated S-EB-S block-copolymers can be used with olefinic plastics such as PP and PE because of their higher temperature allowance, and can even be used with engineering plastics that usually require melt temperatures well above 275°C. But, for polar engineering plastics. such as PA 6 and 66, maleic anhydride func-
Kraton G as a compatibilizer for engineering thermoplastics Plastic type
PA 6 PA 66 -. PET PBT PC Mod PPO
Source: Shell Chemicals
0 1999 Elsevier Science
9