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HGMTx™ – Hollow Glass Microsphere Technology

ULTRA HIGH STRENGTH MICROSPHERES ARE USED TO CREATE HIGH COMPRESSION AND SHEAR STRENGTH COMPOSITE CORES AS WELL AS A WIDE RANGE OF OTHER PRODUCTS

Hollow Glass Microspheres (HGMs) are starting to open up whole new possibilities in the composite industry. HGM cores enable the faces and cores in sandwich constructions to be optimised and fabrication of lightweight, high strength, thin walled complicated shapes a reality.

Rapid development in the field of deep sea exploration in the middle of the 20th century was one main reasons for development of hollow glass microsphere (HGM) technology. Development engineers of deep submergence vehicles required new structural materials with densities less than that of water but of high compression strength and water resistance. SYNTACTIC composites based on HGMs were able to meet these requirements. Structural elements made using these materials are capable of withstanding water pressure down to 6,000 m(Fig. 1).

Figure 1

Hollow glass microspheres form a white colour powder consisting of tiny bubbles with diameters ranging between 20150 µm with walls thickness less than 1 µm (Fig. 2). Glass composition and a near perfect spherical shape of the microspheres provides high compressive strength. The main distinction between high and low grade HGMs is their shape and structure. Lower quality HGMs fail under less load, less predictably, as shown Figure 3 compared to high grade HGMs. Other key properties include low water absorption, low heat conductivity, high chemical resistance and radio transparency.

Hollow Glass Microspheres The Fragment of Failed Microspheres

Figure 2 Figure 3

Good adhesion of HGMs towards polymer binders makes them ideal for composites giving a unique combination of properties. All the above mentioned factors define a wide variety of applications for HGM.

The technology for HGM manufacture is a combination of complex hydrodynamic and chemical processes that take place in the course of forming of hollow bubbles blown from microparticles of glass melt. An exact dosage of gas into the melted powder blows microspheres with the required diameter and wall thickness. With such a complex technological process it is impossible to make microspheres with a strictly identical predetermined diameter (Fig. 4). Therefore calibration of microspheres is performed according to their dimensions and density. The strength of the microspheres is established by testing the hydrostatic pressure at which not more than 10% of the HGMs fail. It is natural that microspheres with a greater density and thus with thicker walls are stronger (Fig. 5).

Figure 4 Figure 5

Applications for HGMs are constantly being developed, for example incorporation of HGMs into;

- explosives increase their yield considerably, whilst the incorporation of HGMs into armour provides a very effective blast attenuation layer.

- drilling mud intensifies the drilling process whilst greatly increasing the life cycle of the drilling equipment.

- cement mortar makes it possible to produce shrinkage free, quick hardening, thermal insulating materials.

- SMC and BMC materials enables their density to be reduced considerably, for example from 1.8 g/cm3 to 1.3 g/cm3 without detectable reduction of elasticity or strength of the composites.

Today HGM are widely used for making special mastics and compounds. But the most interesting field of application for them is in the creation of super strong sandwich composites.

New Possibilities

Outer surfaces of composites are the areas under maximum load, further from the surface of a multilayer composite, where tensions are less, lower strength, lower density, materials can be used. Multi layer composites provide an appreciably increase in thickness of the structure and, as a result, to drastically build up its strength and especially its rigidity without a vast augmentation of weight. (See Reinforced Plastics, March, 2002 , p. 3224).

Traditional multilayer sandwich structures can conditionally be classified into two main categories; those that use honeycomb core materials and secondly those that use lightweight foam cores. Each has its advantage and disadvantage and we must choose the most efficient field of application for them. Foam and honeycomb cores allow us to make sandwich composites with very high specific characteristics of rigidity but the low shear strength and rigidity of these traditional cores does not allow us to use the strength of the faces to maximum effect and as a result, the full potential of present day sandwich composites are not realised.

This drawback is clearly seen and more pronounced when making thin sandwich structures. In this case to obtain high strength and rigidity of thin sandwiches one has to use faces made of materials with high elasticity and strength characteristics. In so doing core thickness is comparable with face thickness demanding greater shear properties of the core material. This is exactly the reason why up to now sandwiches on the basis of honeycomb and foam are not widely used when making relatively thin and at the same time strong structures.

We’ll illustrate this taking as an example an ordinary board. If one board does not secure necessary strength and rigidity, the simplest way to ensure this is to put one board on another one. As a result we create a two layers sandwich structure (Fig.6a), and when it is bent with the layers (boards) not fastened with each other, the boards simply displace reference one another. In this case each board works independently, and we have ordinary double strength and rigidity of the structure. Having bonded the boards together with a glue, or just with nails, in order to prevent shear it’s possible for them to work together and form a considerably stronger (2 times) and more rigid (4 times) structure (Fig.6b). It is essential that the strength of the connection between the layers has to be high enough to have the boards working together, the stronger and the more rigid the material the boards are made from, the higher the shear tension that occurs in the connecting layer. This example can be used to illustrate a three layer sandwich in which the functions of the boards are fulfilled by the faces and the glue – by the lightweight core. The difference is that in this case the boards (faces) are relatively thin and the glue layer (the core) in a foam or honeycomb sandwich composite is thicker. So, it becomes clear that to make all the layers of the sandwich structure work together it is necessary to have a certain minimum shear strength in the core.

a)

b)

Figure 6

Failure of the sandwich composite to bend may happen due to two basic reasons. Firstly, failure of the faces when the load exceeds the materials ultimate strength, secondly, failure of the core when subjected to ultimate shear tension. Ideally in an optimum sandwich composite structure, failure of the faces and the core should be simultaneous, but usually it does not work like this. Commonly failure of the multilayer structure happens when one or more of the layers has not exhausted its strength potential, or one layer fails before the others.

The majority of present day composites are manufactured with relatively thin faces made of materials with relatively low elasticity and strength characteristics because, as it was mentioned above, low shear properties of traditional cores (honey comb and foam plastics) do not allow the full potential of the high strength faces to be used. In another words, when limited shear stresses for a given material are achieved in the core, increase of strength and rigidity of the faces does not result into an augmentation of strength of the whole sandwich structure.

To make all the sandwich layers work, it is necessary to have a certain minimum shear strength in the core. It is also true that the higher strength and rigidity of the faces the greater the importance is to have high shear characteristics in the core. Today mathematical models describing behaviour of multilayer loaded composites have been developed. It allows us to formulate requirements as to the elasticity and strength characteristics of the layers in a multilayer composite structure depending upon its geometry and loading conditions. Table 1 shows comparable properties of foam plastics and composites based on HGM. The vast superiority of elasticity and strength properties of Sintactic cored sandwich composites to foam plastic cored composites opens up whole new physical possibilities in the designing of sandwich composites.


Table 1

Quality

Foam

Syntactic

Density ( ү ), kg\m

30 400

400 – 650

Compressive strength ( σ ), MPa

0,2 12

25 – 95

Specific compressive strength, ( σ / ү ), 10 m

0,8 – 3,0

6,2 – 15

Compressive modulus ( E ), MPa

20 600

1000 – 3000

Specific compressive modulus ( E / ү ), 10 m

0,7– 15

25 – 46

Shear strength (τ), MPa

0,3 – 6,5

6 – 22

Specific shear strength ( τ / ү ), 10 m

1,0 – 1,6

1,5 – 3,3

Shear modulus ( G ), MPa

11 – 200

500 – 1300

Specific shear modulus ( G / ү ), 10 m

3,7 – 5

12 20

We’ll illustrate it as follows: two types of samples are prepared representing sandwich composite plates. Dimensions of the samples and thickness of the core were the same (Fig.7). Facing materials and their thickness were also the same. The difference was that in one case the core was made of polyurethane foam and in another there was a composite based on hollow glass microspheres (Sintactic).

Table 2

Quality

Core

Faces

Foam

Syntactic

Density, kg\m3

Сompressive

Strength, MPa

Shear

Strength, MPa

300

4,9

2,9

510

62

21

2100

700

55,0

Figure 7

a)

b)

Table 2 represents material properties of the faces and core. Fig. 8a shows clearly that in the first case the sample failed due to exceeding the maximum shear strength in the core material (2.9 Mpa).The faces remained damage free in this case, as their tensions accounted for 101 Mpa only. In the second alternative (Fig. 8b) the core and the faces failed simultaneously that means that the potential strength of the faces (700 Mpa) was realized in full and the shear stresses in the Sintactic core have reached their maximum values (20 Mpa).

Figure 8

A dramatic increase in specific strength at bend of the sandwich composite with the Hollow Glass Microsphere core resulted see (Fig. 9).

These experimental results are well correlated with theoretical calculations of these sandwich structures (Fig.10). So, even in spite of the higher density of Syntactics, their use allows, in a number of cases, to dramatically increase specific strength and rigidity of the sandwich structures. When making thin wall sandwich composites with high strength faces, composites based on HGM cores are unrivalled.

Figure 9

Figure 10

High mechanical properties of Syntactics are far from being their only merit as a material used as a core when manufacturing sandwich composites. Resistance towards local static and impact loads is always a big problem in multilayer composites based on honey comb and foam plastic cores. Even light impacts on such kind of structures often destroys the integrity of the faces of the composite sandwich. Use of Sintactics as the core based on HGM not only solves this problem only but increases significantly the cyclic and longterm strength of the sandwich composites. Besides, high load bearing stress strength allows the use of not only bolted connections but also riveted joints for such kind of materials.

FABRICATION

Development of sandwich composites is not always a simple technological task especially when manufacturing multilayer items of complicated geometric shapes, with variable thickness and local reinforcements. Composites based on HGM have opened up new possibilities for designers and industrial engineers developing sandwich structures. A break through in this field became possible owing to the creation of a highly workable half finished sheet product based on HGM for manufacturing sandwich composites. In Russia this material was named SYNLAY.

In essence, SYNLAY is similar to sheet SMC materials. The difference is that the core is not chopped glass fibres and mineral fillers but HGMs. In outward appearance SYNLAY looks like rolled pastry which does not stick to the skin. Depending upon operational requirements of the composite being created the content of the microspheres in the material can theoretically achieve a probable limit equal to 70% of the whole volume. The HGMs are encapsulated within a specially designed polymer binder allowing the SYNLAY material to be elastic even with such a high percentage of HGMs. SYNLAY sheets can be supplied in thicknesses from 0.7mm to 50+ mm. Shelf life of the SYNLAY material can be up 3 months at + 200C and up to two years at – 50С.

A major technological advantage of SYNLAY is its high elasticity in the uncured state allowing the moulding of items of any shape with use of many well known technological techniques (press method, vacuum and autoclave moulding, winding, blow moulding etc..

Fig.12

Fig.11

Thanks to the good flow of the materials, manufacture of the sandwich structures of various thickness does not create technological problems either. One more advantage of the material is that as a core for sandwich structures it does not need glues, as the amount of polymer binder is quite enough in the material to be bonded to the faces. Thus, multilayer structure technology with use of SYNLAY is practically identical to that of well mastered mould technology from pre-pregs (Figure 11). SYNLAY based on HGM allows the fabrication of very thin, down to 1 mm, sandwich structures. Production of such thin composites using foam plastics or honey comb cores is not possible from a technological point of view and, above all, it is senseless from the point of view of realising the strength capabilities of the faces, the thickness of which is comparable with the thickness of the core in thin sandwich composites (Figure 12).

CONCLUSION

It can be seen that the vast potential of HGMs is far from being realised today. In the near future whole new fields of HGM applications in the composite industry will be opened up. SYNLAY cores enable sandwich constructions to be optimised and in the manufacture of lightweight, high strength, thin walled complicated shaped structures SYNLAY cored compostes are unrivalled.