Filler Treatment with Famasil Silanes and Titanatesfamastechnology.com/pdf/filler_treatment_with_famasil_silanes_and... · Filler Treatment with Silanes and Titanates Ð Famas Technology

Filler Treatment with Silanes and Titanates – Famas Technology Version 04/07

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Fil ler Treatment with Famasil Silanes and Titanates Introduction:

The global market of silane surface treatments for fillers and reinforcements has been growing significantly during the past 20 years, Reasons for employing silanes

were first to improve abrasion resistance and electrical properties of synthetic

rubbers filled with kaolins and clays, later to improve stiffness and heat resistance

of engineering thermoplastics with fillers like talc, wollastonite and mica. In recent years, it has been the use of precipitated silica to improve dynamic

properties and rolling resistance of automotive tires while replacing carbon black.

This latter segment alone generated silane sales of over !uro 100 MM in 2006.

All these developments were only possible, due to silanes ability to react with the

surface of most mineral fillers to impart designed properties. One design feature is also the fact that the organic part of the silane is reacted with the polymer and

that this covalent bridging mechanism controls surface properties like adhesion.

A large proportion of all thermoplastics, thermosets and elastomers worldwide are

compounded and reinforced with fillers and fibers. Their function is to provide

specific properties to the final product and to reduce costs. The majority of these fillers are incompatible with the polymer matrix, particularly the inorganic minerals

that account for a large majority of the fillers used. Embrittlement, degradation of

mechanical properties and increased moisture pickup of the composite can result.""To overcome this problem, coupling agents and other surface modifications

have been developed.

Coupling agents tend to be bifunctional molecules able to bond chemically with both the filler surface and the polymer matrix, which forms a 'molecular bridge'

between the two. The strong interfacial bond not only aids the mixing of the two

phases but also benefits the overall properties of the composite. The most

commonly used coupling agents are organotrialkoxysilanes, organotitanates and functionalized (especially acid functionalized) polymers. A worldwide total of

about 16,000 tonnes - worth around !uro 300 millions - of coupling agents are

consumed annually in the treatment of around 3 million tonnes of fillers.

Other surface modifiers that provide a physical rather than a chemical bond

between filler and resin are also available for applications where the performance

levels provided by coupling agents are not necessary. These are mostly waxes or fatty acids such as stearic acid. These yield improved filler incorporation and

dispersion for around a tenth of the cost of the average silane. Their role in

polymer composites is far from trivial: of the 3 million or so tonnes of fillers


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surface treated each year some 1.75 million tonnes are calcium carbonates

treated with fatty acids for incorporation in PVC, PP and various elastomers.

Usage of treated fi l lers

Mineral fillers, such as calcium carbonate (CaCO3), clays, silicas, mica, talc, alumina

trihydrate (ATH) and titanium dioxide, account for the lion's share (about 90%) of

the demand for fillers and extenders, with CaCO3 being by far the most commonly used filler.

Non-mineral fillers and extenders include carbon black, glass beads and various

organic materials.""Among the thermoplastics, PVC accounts for about 70% of the

demand for fillers, with PP, nylon and polyester together accounting for another 20%. Natural calcium carbonate (treated with fatty acid) accounts for more than

80% of that total.

In terms of volume, the main fillers treated with coupling agents for incorporation in thermoplastics are ATH, calcined clays, wollastonite (calcium metasilicate) and

mica, together consuming about 2,500 tonnes of coupling agents per year.""The

annual requirement for treated fillers in elastomers is reported to be in excess of 500,000 tonnes, of which about 70% employ coupling agents. More than 10,000

tonnes / year of coupling agents of various types are used, with silanes

predominating. "

For thermosets the picture is slightly more complicated due to in-situ coatings and the use of mineral fillers in conjunction with glass fiber.

Silanes

Organofunctional silanes were introduced over 50 years ago as coupling agents

for fibreglass and have proved equally successful in treating mineral fillers. They are the dominant materials in the coupling agent market, their success is due to

their ability to react with a broad range of fillers and resins, the fact that they can

be produced in readily dispersible form, with stably attached organic functionalities.""


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Coupling mechanism

Silanes have the generic structure:

Y-R-Si-X3,

where X is a hydrolysable alkoxy group (methoxy or ethoxy) and Y an organofunctional group (amino-, vinyl-, epoxy-, methacryl- etc.) attached to the

silicon by an alkyl bridge, R.""The alkoxy groups are able to react with the surface

groups of many inorganic fillers.

They first react with water to produce the silane triol, releasing alcohol as a by-

product. The silanol groups then condense with oxide or hydroxyl groups on the filler surface. Neighboring siloxane chains can interact further to produce a

polysiloxane layer at the surface.

Silanes require active sites, preferably hydroxyl groups, on the filler surface for

reaction to occur. They can therefore be used to treat all silicate-type fillers,

inorganic metal oxides and hydroxides. Materials successfully treated with these coupling agents include: ATH, alumina, chrome oxide, hydrous and calcined clays,

glass fiber, magnesium hydroxides, mica, mineral wool, oxide pigments, quartz,

silicas, talc, titanium dioxide.

However, silanes do not interact to any significant degree with calcium carbonate,

or with barium sulphate or carbon black.""Once coupled at the filler interface the

reactive Y component allows bonding to the polymer matrix by chemical reaction (grafting, addition, substitution) with active groups on the polymer and / or by

physicochemical interactions.


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Y groups are selected to maximize compatibility with particular resin formulations.

For example, methacrylate-functional silanes are most used in

unsaturated polyesters, while amino-functional silanes are found more frequently in polyamides as well as epoxys.

In general, silanes are highly effective coupling agents for polar thermoplastics, thermosets and rubbers but have only a slight interaction with non-polar

polymers such as polyolefins (where titanates are of bigger interest).

Organic Group - Silane

Base Polymer Amino Epoxy Sulfur Mercapto Methacryl Vinyl Ureido Fluorine

Acrylic

Acrylic latex

Butyl

Cellulose

Epoxy

Fluoro

Melamine

Neoprene

Nitrile

Phenolic

Polyamide

Polyester

Polyether

Polyolefin

Polysulfide

Polyurethane

PUD

Silicone

SBR emulsion

SBS

Vinyl

Generally Effective / Best Choice

Alternative / 2nd Choice

Only effective with specific silanes

Unsuitable


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Fil ler treatment

There are several commercially established methods of treating fillers with silane

coupling agents.

1. Fillers can be pre-treated before compounding with the polymer by spraying

neat silane onto dry, well-agitated filler. Moisture on the filler surface is

typically sufficient to initiate the hydrolysis reaction.

2. Integral blending methods can also be used. The silane can be blended with the polymer and filler during compounding either in the form of a dry

concentrate (absorbed on a carrier), or as neat silane added before or

together with the filler during compounding, followed by intensive mixing. This in-situ method is widely use in resin-filler systems because of its

simplicity.""

Silane loading depends on surface area of the mineral filler usually about 1 wt.% of silane on filler is required for fillers with a surface area of less than 20 m2/g.

Higher surface area fillers require a higher dosage.

Property enhancement

Silane coupling agents provide a strong, stable, water- and chemical-

resistant bond between fi l ler and resin, typically improving:

- mechanical and electrical properties,

- reduces shrinkage, - increases weather resistance, and lessens or eliminates surface or internal

defects,

- surface appearance of processed parts.

The immediate benefit that can be observed when blending a silane-treated filler

with a polymer is usually improved wettabil ity - the filler adsorbs the polymer more completely because the silane treatment reduces the interfacial

tension with organic l iquids.

Another result of improving the interfacial tension with the polymer, are reductions in viscosity of the fi l ler / polymer system.

Silane treatments make their surface hydrophobic and the interfacial tension is increased. As a result of increased interfacial tension with water, the penetration

of the liquid into the interphase is hampered and a better resistance

against humidity is achieved.


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Note that this is almost independent of the inorganic group of the silane, as long as the treatment conditions are set to ensure complete reaction between the

silane and the polymer. The major factor governing the hydrophobic

effect is thus the silane organic group - best results are usually obtained with octyl silanes - however, the benefit would disappear with time if the silane

treatment itself was not chemically stable in a wet environment.

Chemical interaction between fi l ler and polymer.

The reactions between the silane, the filler and the polymer take place in presence of surface water : The first reaction is the hydrolysis reaction of the

silane. For example the reaction of a trimethoxysilane releases methanol and

consumes water:

R-Si(OMe)3 + 3/2 H2O ! R-Si (OH)3 + 3 MeOH

Then the resulting silanol reacts with a surface hydroxyl group of the filler surface

(condensation).

In the equilibrium state, the amount of OH-groups on the filler surface is the most

important parameter governing the quantity of silane-filler covalent bonds.

Separately, the reaction kinetics are related to surface catalysis.

The rate-determining step for the chemisorption process is under most conditions

the condensation reaction. Hydrolysis and condensation reactions are pH-

dependant and are catalyzed by both acidic and basic conditions.

2 3 4 5 6 7 8 9 10

Rate of Condensation Rate of Hydrolysis

-1

-5


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Table below shows the improvement in flexural strength of filled polyester with

0.5% of Methacryl silane (Famasil ME-TMO) after 4 hours in boiling water.

Filler

OH

Groups

Filler

pH

Filler

loading, phr

Flex Str.

MPa, Untreated

Flex. Str.

MPa, Silane-

treated

% Chg.

ATH high 10 160 30 60 +100%

Amorphous Silica high 7 120 60 100 + 67%

Hydrous Clay high 9 90 30 45 + 50%

Calcined Clay low 5 100 60 100 + 67%

Mica low 8 50 20 30 + 50%

Talc low 9 100 30 50 + 67%

To increase the reactivity of silanes one may also use the inherent basic

nature of aminosilanes. Combining aminosilanes with other compatible silanes or titanates can help achieving faster chemisorption on difficult substrates.

After silanes built covalent bridges between filler and polymer, this bridge needs to be resistant and stable.

The data below illustrates the stability of silylated surfaces towards hydrolysis.

One reason for this is that the silane / water reaction is reversible, allowing rearrangements with the overall effect, that the trifunctional silanes are able to

re-adsorb on the filler surface after partial hydrolysis.

50% Kaolin-filled PA 6. Effect of AM-TEO silane and AM-TITAN blend treatment

after wet ageing

Unfilled

Untreated

kaolin

AM-TEO

Treated

AM-TITAN

Treated

Flexural strength,

MPainitial 85 120 150 155

16 hours in 50°C

water 39 69 100 138

Flexural Modulus,

MPaInitial 1800 5730 6050 6240

16 hrs in 50°C

water 570 2350 2620 2970

Tensile Strength,

MPaInitial 60 70 80 90

16 hrs in 50°C

water 40 40 60 80


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On the other hand the silane organic group must react with the polymer to complete the bridging process.

Guidelines for determining silane amount to be applied on fi l lers

Average particle size of mineral filler (microns)

Amount of silane (%w/w)

< 1 1.0 - 5.0

1 to 10 1.0 – 2.0

10 to 20 0.75 – 1.0

20 to 100 < 0.1

Solvents can be used in the preparation of a suitable silane solution (ask your local Famas representative for guidelines). The benefit is that the process allows for

complete silane reaction and elimination of alcohols as hydrolysis by-products. In

some cases water can be used, but only aminopropylsilanes can be dispersed well in water at elevated concentrations.

Titanates

Organotitanates overcome many of the limitations of silanes as coupling agents

for fillers. Like silanes they have four functional groups, but where silanes have only one pendant organic functional Y group, titanates have three, providing more

effective coupling to the resin. In addition, the mechanism by which they couple

to inorganic surfaces differs, which means that they are suitable not only for fillers with surface hydroxyl groups but also for carbonates, carbon black and other

fillers that do not respond to silanes.

In addition to improving filler dispersion and enhancing the properties and

processing of the composite as with silanes, titanate couplers also act as

plasticizers facilitating higher filler loadings, and as catalysts for a number of reactions in the polymer matrix. Costs for titanate treatment are slightly lower

than silanes.""


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Structures and mechanisms

Titanates have the general structure:

XO-Ti-(OY)3,

where XO- can be a monoalkoxy or neoalkoxy group capable of reacting with the inorganic substrate, and -OY is the organofunctional fragment.

The Y portion can typically contain several different groups to provide interactions between polar and non-polar thermoplastics (e.g. benzyl, butyl) and thermosets

(e.g. amino, methacryl), as well as pyrophosphato or carboxylic groups that can

introduce additional functions.

Unlike silanes titanates do not require the presence of water to react." "Titanates

fall into several classes. The simplest are the monoalkoxy (e.g. isopropoxy) titanates introduced in the Mid 70s. These react with the filler surface via

solvolysis generating an alcohol by-product.""

The neoalkoxy titanates have a more complex but more thermally stable

structure. They were developed for high-temperature applications (above 200°C in

the absence of water) such as in-situ addition during thermoplastics compounding

and the production of urethane composites. They react via a coordination mechanism with free protons on the filler surface, generating no by-product or

leaving group.

Free protons, unlike the hydroxyl groups needed for silane reaction, are present on almost all three-dimensional particulates, which is claimed to make titanates

more universally reactive. The reaction with free protons generates an organic

monomolecular layer at the inorganic surface - in contrast to the polymolecular layers typical with other coupling agents - which in combination with the chemical

structure of the titanates creates surface energy modifications and polymer phase

interactions. In addition to their higher thermal stability, neoalkoxy titanates offer somewhat enhanced final properties compared to their monoalkoxy counterparts.

Other types of titanates are chelates (for greater stability in wet environments), and quats (water-soluble systems).


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Fil ler treatment

In-situ coupling is recommended but requires good compounding techniques to

avoid localization and inconsistent coupling. Uniform distribution of the titanate before the polymer melt phase is essential. The recommended dosage for most

fillers is typically 0.2-2% by weight of filler.

Applications

A large body of published research is available demonstrating the ability of titanate coupling agents to enhance the properties of composites when combining

a wide range of fillers and polymers.

In addition to silane-reactive fillers, titanates are effective with carbonates, carbon

black and other fillers. With appropriate organofunctional groups, titanates can

also bond successfully with polypropylene (and other polyolefins) as well as PVC, two of the largest consumers of fillers.

High loadings of other common mineral fillers such as wollastonite and talc have

also been achieved in PP with the aid of titanate coupling agents – especially organofunctional titanates such as NDZ 130 are suitable candidates in these

applications.

Several wollastonite producers report that both silanes and titanates are used to surface treat its products, though these markets for organotitanate-treated

wollastonite are relatively small and specialized.

The reactivity of the TiO bond can cause problems with discoloration in the

presence of phenols.

Zirconates

The chemical structure and applications of alkoxy zirconates are completely analogous to those of alkoxy titanates.

Zirconates' main advantage is their greater stability; unlike titanates they neither discolor in the presence of phenols nor do they interact with hindered amines

(HALS).


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Functionalized polymers

Functionilzed Polymers are the most class of coupling agents. They account for

about 6% of the market for coupling agents by value, or about $10 million / 3000

tonnes annually, and acid functional polyolefins are intermediate in price between fatty acids and silanes / titanates.""

The coupling concept here is to have substrate reactive groups on molecules of the host polymer itself, or of another polymer compatible with it. This removes

the problem of finding polymer reactive functionalities, and is particularly

attractive for thermoplastic polyolefins. The problem to date seems to have been in producing effective functionalized polymers. This is partly due to

the prevalence of siliceous fillers in composite materials. These are most

effectively bonded with alkoxysilanes, but such groups are difficult and expensive to introduce into polymer chains.""

The easiest materials to produce are probably acid functionalized polymers, especially those with grafted or copolymerized anhydride groups. Examples are

carboxylated polyethylene and polypropylene and maleinized polybutadienes.

The main l imitation of these acid functionalized additives is that they are most effective with basic or amphoteric substrates, while the

majority of substrates where true coupling is required are sil iceous in

nature and generally not directly responsive to them.

One option is to pre-treat the siliceous filler with an aminosilane, which can

then react with the acid functionalized polymer to form an amide

linkage. Where functionalized polymers can be used, the effects achieved are typically those of coupling (improved heat distortion temperature, strength,

stiffness and abrasion resistance, for example).


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Conclusion

Appropriately used, silane and titanate coupling agents create significant

improvements in the properties and processing of filled plastics. Silanes are well

established and enjoy widespread use. Titanates can have broader functionality but often suffer due to incorrect use.

Functionalized polymers occupy an intermediate position between fatty acid

surface modifiers and the silanes / titanates in terms of both cost and functionality."

Surface treatment of fillers using organofunctional silanes has a strong influence on several parameters :

- Physical interactions between filler and polymer can be modified to control processing properties like wetting speed and efficiency, viscosity during

compounding and filler dispersion.

- Chemical interactions between filler and polymer leading to the formation of

stable covalent bonds. The formation of a chemisorbed layers protect the

filler/polymer interface from hydrolysis and improves ultimate mechanical properties and ageing.

- Physical interaction between filler particles can be designed to control

rheological and dynamic mechanical properties.

- Chemical modification of the polymer in the interphase can be introduced

through silane crosslinkers or reactive plasticizers to minimize the impact of large property differences between filler and polymer.

The treatment of fillers is a relatively simple process if attention is payed on the reactivity between silane / titanate and filler. The surface reactivity should be

measured case by case.

For more information on products and applications also visit our webpage:

http://www.famastechnology.com

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