Plastics Additives & Compounding July/August 2005

28

Fancy fillersFillers are no longer just added to polymers to reduce costs,but are increasingly enhancing properties of the compound.Calcium carbonate, talc,clay,mica,wollastonite and glassfibre are all widely used in polymer compounding operations.However, there are some lesser well known fillers that canprovide some useful properties in polymers.Dr.ChrisDeArmitt and Professor Roger Rothon explain.

Fillers have long been used to modify theproperties of polymers, including fibres,coatings , sealants and adhesives. To manypeople, the word filler is associated with

cost reduction, but in recent years it hasbecome more widely recognized that fillersalter all properties of the material in whichthey are mixed. This means that even when

the main goal is cost reduction, one mustunderstand the effects of adding fillers interms of altered processing and finalproperties. Similarly, when the primary aimis to enhance properties, one must alsounderstand the effects of the filler, bothpositive and otherwise. Previous articles inPlastics Additives & Compounding haveshown how fillers can be used effectively toalter polymer properties. Using fillers canallow a performance/cost ratio not foundfor unfilled polymers, or in other instances,the filler imparts properties not possible inunfilled polymers. Most commonly, fillersare used to assist processing and to upgradethe mechanical properties of the hostpolymer. Several well-known fillers are usedin that capacity; such as calcium carbonate,talc, clay, mica, wollastonite and glass fibre.Additionally, there are many other fillersthat are used, not to change mechanicalproperties, but rather to impart some otherproperty such as conductivity (electrical orthermal), sound and vibration damping,density changes, magnetism, reducedwarpage and so on. This article introducessome of these less well-known fillers.

Expandable microspheresMost fillers are minerals and as such theyare inorganic and relatively inert. Forexample, they maintain their shape bothduring processing and afterwards. In sharpcontrast, expandable microspheres areorganic and change their shape duringprocessing. The particles are composed of apolymeric shell, which contains aninnocuous hydrocarbon liquid. As thesebeads are heated, the shell softens and the

ISSN1464-391X/05 © 2005 Elsevier Ltd.All rights reserved.

Figure 1:Expancel expansion schematic.

Figure 2:Tunable expansion temperature.

F i l l e r s

Plastics Additives & Compounding July/August 2005

29

trapped liquid boils, the pressure from thetrapped vapour forces the particles toexpand to many times their originalvolume. By tuning the chemicalcomposition of the polymeric shell and theboiling point of the liquid, one can tune theexpansion temperature range of theparticles over a wide range.Such particles find uses in diverseapplications, including polymers, paperboard and printing ink. One application isin printing paper for Braille books. Byadding the particles to the paper it becomesthermally sensitive. When such paper isused with a thermal printer, the heat fromthe printer causes the particles to expandonly in specified locations, i.e: where thepaper is heated by the printing head. Thiscauses elevated dots on the paper that canbe read by visually impaired people.In polymers, these particles can beexpanded during processing and then as thepolymer cools the bubbles created arelocked into the polymer to give a materialwith lowered density, i.e: for flotation aidsand in filled thermosets, a material withincreased stress relaxing ability. In paperboard, additional bulk is achieved by theuse of low temperature expandingmicrospheres added to the pulp.

Diatomaceous EarthDiatomacous Earth has long been used asan anti-block for polymer films. Theparticles impart a microscopic roughness tothe film and the refractive index of the filler

Figure 4:Diatomaceous Earth with one predominant type of Diatom [Micrograph courtesy of World Minerals]

Figure 3:Diatomaceous Earth from freshwater deposit in Mexico [Micrograph courtesy of World Minerals]

Table 1:Diatomaceous Earth in Nylon 6[Courtesy of World Minerals]

Filler Level % Tensile Flexural Tensile Elongation Impact

modulus modulus strength at yield strength

w/w /MPa /MPa /MPa /% Un-notched /kJ/m2

None 0 2750 1420 67 3.7 >100

Glass beads 35 5265 2640 58 2.5 22

Silane treated 35 5700 2990 79 3.0 62calcined clay

Untreated DE 30 4900 2445 70 3.0 36

Treated DE 30 4760 2465 72 3.7 >100

Plastics Additives & Compounding July/August 2005

30

(1.45) is close to that of manythermoplastics so transparency ismaintained. More recently work has beendone to develop this mineral as a filler,particularly in nylons and in rubber. Theparticles are actually the skeletal remains ofsea algae and are composed of silica and sothe specific shape can vary widelydepending upon the prevalent type ofdiatom where the deposit was laid down.

In order to develop the DiatomaceousEarth (DE) as a filler, work was done toclassify the product to give an appropriatesize distribution and also to identify theoptimal surface treatment for the DE ineach matrix material. The results belowshow the properties given by DE in nylon6 as compared to those from othercommon fillers for that polymer. Thesurface treatment aids wetting of the

micro-voids in the DE by the polymermelt, giving rise to the improved impactresistance of the treated DE over that ofthe untreated grade.

MagnetiteMost mineral fillers have a density in therange 2.5-3 g/cm3 whereas polymers lie inthe range 0.9-1.4 g/cm3, therefore fillersusually lead to increased density. Addeddensity leads to added transportationcosts and in many applications is viewedas an unwelcome side effect of addingmineral fillers to polymers.However, there are some instances whereit is favourable to increase density. Forexample, it is well-known that sounddeadening can be accomplished by usingdense materials. Such formulations arecomposed of a dense filler in a soft,gummy polymer (such as EVA, PVB,bitumen).Barium sulphate (density of 4.48 g/cm3)is by far the most well-known dense fillerfor such applications. In recent yearsmagnetite, Fe3O4 (density of 5.1 g/cm3)has been introduced into the market andgained acceptance in automotive andother applications. It has an interestingcombination of properties. Firstly, thehigh density makes it useful, but it alsohas some electrical and thermalconductivity, which can be useful forelectric motor housings. The thermalconductivity aids in heat dissipation andthe electrical conductivity contributes toelectromagnetic shielding. There are somepotential drawbacks though. Therelatively high hardness (Mohs' 5.5-6)could lead to undesirably rapid wear onprocessing equipment, but this has notbeen reported to be a problem, probablybecause machine wear depends on otherfactors like particle size and shape, nothardness alone.Additionally, the high iron content mightbe a concern as it is well documented thattransition metals like iron can lead torapid polymer degradation, especially inpolyolefins like PE and PP. In reality itturns out that the magnetite does notcause big stability problems as thechemical form of the iron is far moreimportant than the amount.

Figures 5 and 6:Magnetite is becoming increasingly used as filler in automotiveapplications. [Photographs courtesy of Minelco]

F i l l e r s

F i l l e r s

Plastics Additives & Compounding July/August 2005

31

Beta Spodumene -Lithium Feldspar,Li2O.Al2O3.4SiO2

There is a specialized market in very lowthermal expansion fillers. BetaSpodemene is one such product. It isproduced from the naturally occurringmineral Spodumene. Upon heating to1082°C, Spodumene undergoes a phasetransition and expands going from adensity of 3.2 g/cm3 for Spodumene to2.4 g/cm3 for the Beta Spodumene. TheBeta Spodumene is notable for having anexceptionally low coefficient of thermalexpansion, ~1x10-6/°C over the range 25-1000°C. This property has been used forceramic oven-to-table ware as thematerial is resistant to thermal shock due

to the low coefficient of thermalexpansion. Minerals are often added topolymers to reduce the coefficient ofthermal expansion. Polymers typicallyhave values in the range ~ 10x10-5

mm.mm-1 °C-1 whereas many mineralsare a factor of ten lower ~ 10x10-6

mm.mm-1 °C-1. The use of mineral fillersgreatly reduces the coefficient of thermalexpansion according to the linear law ofmixtures in proportion to the volumepercentage of polymer replaced bymineral. For applications demandingexceptionally low thermal expansion,Beta Spodumene may be used.

Authors:

Dr. Chris DeArmitt FRSC CChem

BASF AGE-mail: chris.dearmitt@basf-ag.deTel: +49 621 60 97307

Professor Roger RothonRothon ConsultantsE-mail: rothon@virtual-chester.netTel: +44 1244 300 798

For further information:

Expandable microspheresWebsite: www.expancel.comDiatomacous EarthWebsite: www.worldminerals.comMagnetiteWebsite: www.minelco.comBeta SpodemeneWebsite: www.spodumene.com