What are the benefits of using a surface treated functional filler vs. an untreated one?

Blog Archive | 10 minutes  | Author: Koen Nickmans , Ph.D.

Surface treated fillers for improved polymer composite performance

Mineral fillers are employed in a diverse range of polymeric systems, including thermoplastics, thermosets, and elastomers. Furthermore, they also find use in protective coatings, 2k adhesives and many other types of polymer-based systems.

In many industries the primary function of these solid additives has been to volume fill and to reduce overall cost. Highly cost-effective composite materials can be obtained by filling a polymer system with a commoditised mineral filler, such as talc or calcium carbonate. This ability of mineral fillers to ‘extend’ a polymer matrix remains one of the primary reasons for their use today.

However, it is also true that the incorporation of inorganic fillers into polymers can also improve performance and add new functionalities. More is expected from polymer materials than ever before. For example, performance aspects can be related to mechanical properties, density, and processability. Examples of functionality include fire retardance, thermal conductivity and electrical insulation. This multifunctional nature of what we now term ‘functional fillers’ is increasingly challenging the perception of mineral fillers as purely extending materials.

The constant innovation and research into mineral fillers, together with a rapidly expanding range of novel polymeric matrices, is resulting in an increasing number of possible material combinations and resultant properties. One innovation in particular has enabled a paradigm shift in composite design: the surface modification of mineral fillers.

HPF Quarzwerke are one of the leading producers of surface treated fillers and are a key supply partner to Lawrence Industries, who promote these materials into the UK and Irish markets. In this article you will learn how the surface treatment is carried out, what difference this makes when compared to an untreated filler and how to choose the correct treatment for the polymer type you are working with. Our technical sales team are here to assist you with your enquiry so please call us on 01827 314151 to discuss your needs.

 

Surface treated fillers – why would you want to use them?

Historically, progress in the field of polymer/mineral composites was stifled by the intrinsic incompatibility of inorganic fillers with organic polymer compounds. This incompatibility stems from the polar hydrophilic surfaces of inorganic fillers, in contrast to the non-polar and hydrophobic surfaces of many polymer matrices (eg. Polyolefins). As a result there can be difficulties in processing and a degradation of mechanical properties, in proportion to the loading levels.

However, the surface chemistry of the filler can be modified by applying a surface coating during a treatment process. By choosing the correct surface modification, the surface modified mineral filler can provide a strong, favourable, interaction between the filler and the polymer matrix. This greatly improves the processing and dispersion of the mineral fillers, as can be seen in the SEM images shown in Figure 1.

SEM image showing better filler dispersion in polymer after surface treatment

Figure 1. SEM images comparing the dispersion of (left) untreated and (right) treated wollastonite fibres (TREMIN®) in a polymer matrix. Appropriate surface treatment greatly improves the wettability and dispersion of functional fillers into polymeric systems.

 

Moreover, the interfacial adhesion at the boundary between the filler and polymer matrix is optimised, improving mechanical strength of the resulting composite. An example of this can be seen in Table 1, where a comparison is drawn of the resulting mechanical properties displayed by a polyamide composite – containing untreated vs. aminosilane treated wollastonite. It is clear that significant improvements of elongation at break and Izod impact strength are achieved when switching to a surface treated filler. All other filler properties are otherwise the same e.g. particle size distribution.

 

Functional Filler

Ultimate Tensile Strength (MPa)

Elongation at break (%)

Elastic Modulus (MPa)

Izod Impact Strength (kJ/m2)

HDT (°C)

TREMIN® 283-600

82

4.5

4170

55

85

TREMIN® 283-600 AST

88

7.2

4210

73

84

 

Table 1. A comparison of the mechanical properties exhibited when an untreated wollastonite (TREMIN® 283-600) and an aminosilane treated wollastonite (of the same specification, TREMIN® 283-600 AST), are included into a polyamide.

 

How are mineral fillers surface treated?

Surface modified fillers are prepared by the reaction of a mineral filler with a coupling agent. The most prevalent coupling agents are organosilanes. These molecules usually consist of a silicon atom attached to three reactive alkoxy (-OR) groups and to an additional stable organic functional group (‘R), which is selected for its interaction with the relevant polymer matrix.

The coupling agent is reacted (i.e. covalently bonded) with the filler surface, in a process that involves hydrolysis of the alkoxy groups into silanol groups (-OR à -OH), followed by a subsequent condensation reaction with surface hydroxyl/silanol groups of the filler particle (Figure 2).

Schematic showing how surface treated fillers are produced

Figure 2. Schematic of the reaction between organosilane and surface silanol groups found on many commonly used fillers.

 

Small amounts of alcohol are the by-product of the hydrolysis reaction, which are evaporated during the surface treatment process. Water is the by-product of the condensation reaction, but it is also stripped during thermal treatment. This offers an advantage from a formulator’s perspective, as an alternative to this would be to perform a silanisation in-situ; preventing the remaining water by-product from escaping beforehand and thus affecting the properties of the polymeric system and causing process issues.

Silane grafting can be applied to any inorganic particle that has hydroxyl/silanol groups at the surface. Fillers that can be treated in this way include quartz, silica, micawollastonite, feldspar, aluminium trihydroxide, aluminium oxide and talc.

For pigments and fillers with no silane-receptive groups on the surface, such as carbon black and certain types of glass, alternative coupling agent chemistries can be utilised. Lawrence Industries offer the TYTAN® CP range for such cases, which are based on organotitanate chemistry. These are extremely reactive molecules which can couple with free protons, as well as more polar groups.

 

Find the right surface treated filler for your system

When dealing with surface treated fillers the only chemistry of real consideration to the formulator is that of the organic functional group (‘R), with which the mineral filler has been treated. Since this chemical functionality is present across the surface of the filler, it is the chosen functional group that is solely responsible for the chemical interaction of the particles with the polymer matrix. It is the functional group that will interact with the polymer matrix - affecting the dispersion, mechanical strength, viscosity, etc.

Several combinations of surface treatments with specific polymer matrices are already well established. An overview of these guidelines can be found in Table 2.

Table offering guidelines on proper filler surface treatment selection by polymer type

Table 2. Guidelines on the correct organosilane surface treatment to choose, based on the nature of the polymer that the filler is incorporated into.

 

The appropriate selection of organosilane treatment is determined by the chemical nature of the polymer matrix. If the silane treatment is designed to provide surface hydrophobicity, then a silane with a hydrophobic group, such as butyl, octyl, fluorocarbon, or phenyl, should be chosen. As the surface treated fillers are more hydrophobic - compared to their untreated counterparts - the presence of water on the filler surface is also avoided.

If the silane treatment is designed to provide compatibility of the mineral in a specific polymer matrix, then the nature of the organic group on the silane should be similar to the chemical structure of the polymer (i.e., an octyl or longer-chain alkyl group will help provide compatibility and dispersibility of the mineral in a polyolefin matrix). Since the chosen surface treatment interacts favourably with the polymer system, it is generally easier to incorporate the surface treated fillers into the polymer matrix compared to the untreated ones. As a result, higher loadings can be achieved, with positive effects on performance and less of an impact on viscosity build.

Should you be formulating in a reactive polymer system, then it is desirable to covalently bond the filler into the polymer matrix. In this case, an organosilane with a reactive functionality should be chosen so that it can cross-link into the polymer during curing. Examples of reactive functionalities include epoxy, amino, and vinyl silanes. As a result of the chemical reaction (system specific and under the right conditions), the mechanical strength can be greatly improved and viscosity/unit filler also reduced when compared to an untreated filler of the same specification.

 

Interested in enhancing your polymer formulation? Get in touch with Lawrence Industries

Selecting the right mineral fillers depends on a range of criteria, such as particle size distribution, morphology, surface chemistry and cost. Get in touch with our technical sales team to discuss your project (01827 314151) or request a sample online.

Author: Koen Nickmans , Ph.D.

Koen studied chemistry at the Catholic University of Leuven in Belgium with a specialisation in polymers. He subsequently obtained a PhD in the field of responsive polymeric coatings from the Eindhoven University of Technology in the Netherlands. He has been with Lawrence Industries since 2019 as a technical sales manager, covering all areas involving polymers: thermoplastics, thermosets, and elastomers.