Glass Bonding

 This post may help with choosing high performance fastenings to glass objects.  Glass bonding with silane and polymer coatings.

Although glasses are often valued for their chemical inertness, this property also presents challenges when attempting to form strong chemical bonds with other materials. Silanes and polymer coatings offer effective solutions by enhancing the bond between a glass and other materials in a composite.

The Challenge

The challenge of bonding glass to polymers spans across several industries, including industrial, automotive, and healthcare. In biomaterial applications, polymer carriers are often used to deliver glass or ceramic particles to specific treatment sites. However, bioactive glasses, commonly used in treatment delivery and bone regeneration, face a similar issue: the mismatch in critical surface tension and adhesion properties between the materials in the composite.¹ This mismatch is primarily driven by differences in hydrophobicity and hydrophilicity, making it difficult to create strong, stable bonds.

The key question, then, is how to improve and strengthen the bonds between glass and polymer materials to create stable composites that benefit from the properties of both material types.

The Solution

Fortunately, there are several approaches to improving the bonding characteristics of glass. One such solution is the use of silane or polymer materials as adhesive treatments. These materials help bypass the challenges of forming direct chemical bonds between the surface oxide groups on the glass and the substrate of interest.

Silanes

Silanes are particularly effective in improving the bonding between glass and other materials due to their ability to form highly stable siloxane bonds. These strong covalent bonds enhance the compatibility between glass and various organic or inorganic surfaces, creating a stronger interface than unmodified glass. Silanes are an excellent choice for composites, retaining the properties of glass while significantly improving surface chemistry and wettability.

One challenge in forming glass-bonded materials is the introduction of a new surface type, which can create potential regions of weakness and shear. Silanes act as effective coupling agents by forming strong covalent bonds with both the glass and the substrate, reducing these weak points and enhancing the overall stability and durability of the material.²

While physical abrasion and etching with hydrofluoric acid can improve adhesion by roughening the surface, chemical modification using silanes is often preferable. Chemical bonds offer superior grafting properties compared to physical methods, resulting in stronger, more durable connections between glass and other substrates.

Polymers

Polymer materials are widely used for bonding to glass due to their flexibility, which is highly advantageous in adhesive applications. While silicone-based materials can bond to glass, they are generally not as strong as many polymer adhesive options.

One such polymer adhesive, polyurethane, is a popular choice for bonding glass in various industries. This popularity is due to its flexibility, which helps absorb and mitigate vibrations induced by movement, enhancing the durability and integrity of the bonded structure. Similarly, acrylic adhesives are the ideal choice for oily or corrosive environments or for use in high-temperature applications. Epoxy adhesives also offer similar benefits, with excellent chemical and electrical resistance.

While many polymers show good adhesion to glass surfaces, their bonding interactions tend to be weaker than the covalent bonds formed with silanes, relying instead on intermolecular forces.³

Polymer adhesion is sufficient for many applications, especially where motion or substrate deformation is likely, as flexibility in the bonding is beneficial. However, for applications that demand the highest levels of adhesion, combining silane treatment with polymer bonding provides a superior solution. This approach significantly enhances bond strength, making it ideal for situations requiring both flexibility and durability.

MO SCI Solutions

MO SCI has a long history of developing custom glass solutions for even the most challenging applications. We can help you find innovative and effective ways to overcome the challenges of glass bonding and adherence to create devices that not only have the properties for peak application performance but are also stable and resistant to environmental degradation.

Contact us today to discuss your application.

References and Further Reading

1. Brauer, D. S. (2015). Bioactive Glasses — Structure and Properties Angewandte. Angewandte Chemie – International Edition, 54, 4160–4181. https://doi.org/10.1002/anie.201405310
2. Yavuz, T., & Eraslan, O. (2016). The effect of silane applied to glass ceramics on surface structure and bonding strength at different temperatures. Journal of Advced Prosthodontics, 75–84. https://doi.org/10.4047%2Fjap.2016.8.2.75
3. Park, H., & Lee, S. H. (2021). Review on Interfacial Bonding Mechanism of Functional Polymer Coating on Glass in Atomistic Modeling Perspective. Polymers, 13, 2244. https://doi.org/10.3390/polym13142244

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Firing silicone mastic

Several people have asked over time about the consequences of firing glass with some silicone mastic (or caulking) still attached to the glass. 

I can say with confidence that it does not break the glass.

I can also say, that it really is not a good idea.  Take as much as possible off with knives, then use silicone disolvers to remove the remainder.  These photos show the results of firing silicone residue.

Where there were pieces of silicone, a divot appeared with the black combustion product from the mastic/caulking.  Where there were strips of silicone, a small valley occurred.  The smallest amount of silicone appeared as a dark divot in the glass.  

It is possible to remove the silicone residue with sandblasting.  Other abrasive methods are possible, but much more time consuming.   Once the silicone has been sandblasted away, the glass needs to be cleaned of all the dusts, and covered with a fine layer of fine frit, or if prefered, powder.  But I find fine frit works better, although it requires a full fuse to form a smooth surface.

Glass Frit Seals for Microelectronics

 Glass frit sealing technology provides a superior solution for achieving reliable hermetic seals in precise applications like micro electromechanical systems manufacturing and packaging.

Image credit: Mo-Sci, Llc

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Radiation Shields

 Glass has a use as a radiation shield in medicine, industry, and aerospace.

Image credit: Mo-Sci,Llc

Using Ceramic to Drape

Characteristics

Before choosing a ceramic shape to use in draping of glass, you need to consider the characteristics of the two materials.  This is one circumstance where CoE is actually useful. 

The expansion of the two materials is different. 

• Soda lime glass typically has an expansion rate - in the 0°C to 300°C range - of 81 to 104.  
• Ceramic has an expansion rate - in the 0°C to 400°C range - of 30 to 64.  

This is important in the final cooling of the project.  As the glass expands more than the ceramic on the heat-up, so it also contracts more during the cool.  This means that the glass will shrink enough to trap the ceramic or even break if the stress on the glass is too much. 

Shape

The shape of the ceramic form will have a big effect on the usability of it as a mould.  Ceramics with right angles between the flat surface and the sides will not be suitable for draping without modifications or cushioning.  The forms suitable for draping need to have a significant draft to work well.

Ceramic forms such as rectangles, cubes, and cylinders do not have any draft in their form.  

A cube shape unsuitable for draping

Ceramic cylinders with straight sides

Although rounded at the base, the sides are too straight to be a draping mould

The glass will contract around these forms until they are stuck to the ceramic or break from the force of the contraction around the ceramic.

You can experience this trapping effect in a stack of drinking glasses.  Sometimes one glass sticks inside another even though there is a slope (i.e., a draft) on the sides of the glasses. This happens mostly when you put a cold glass inside a warm one.  On cooling the warm glass contracts to trap the cooler one. You can separate these by running hot water on the bottom glass, so that it expands and releases the inner, now cool, one. 

Effect of Shape

The ceramic contracts at about half the rate the glass contracts (on average), unlike steel which contracts faster than the glass. This means steel contracts away from the glass, while the glass contracts against the ceramic, on the cooling.

Because the glass is in its brittle or solid phase during the last 300°C to 400°C, this contraction tightens the glass against the ceramic, causing stress in the glass, even to the point of breaking.

However, if you choose ceramic forms with significant draft, you can drape over ceramic.  This is possible when the slope is great enough and the form is coated with enough separator, to allow the glass to slip upwards as it contracts more than the form. Experience with different draft forms will give you a feel for the degree of slope required. 

 

These pyramid shapes have sufficient draft to allow the glass to move up the mould during cooling.

Compensation for Lack of Draft

You can compensate for the insufficient draft of ceramic forms by increasing the thickness of the separators for the form.  The hot glass will conform to the hot ceramic, so there needs to be a means of keeping the glass from compressing the form while cooling.  This can most easily be done by wrapping the form that has little or no draft with 3mm ceramic fibre paper.  It is possible to get by with as little as 1mm fibre paper, but I like the assurance of the thicker material.

Kiln post wrapped in 3mm fibre paper with cap over the post's hole.

The fibre paper can be held to the form by thin wire wrapped around the outside of the fibre paper. The advantage of the 3mm fibre paper is that the wire will sink below the surface of the paper.  You can tie off the wire with a couple of twists.  Cut off the ends and push the twist flat to the fibre paper to keep the glass from catching onto the wire.  If you want further assurance, you can put a bit of kiln wash onto the wire.

Conclusion

The choice of ceramic shapes to drape glass over is very important.  It needs to have sufficient draft and separator to allow the glass to slip upwards as it contracts more than the ceramic during the cooling.  You often can use items with no draft if you wrap fibre paper around the sides of the form.

Heat Shielding Glass

Glass coatings have exhibited remarkable bonding capabilities with various metals and alloys in aerospace applications to shield materials from heat.

Image source: iStock

Testing for Stress

Testing for stress is one of the most important elements in kilnforming.  It may not look like there is stress when there is considerable amounts.  The non-destructive tests are outlined in this Power Point presentation, prepared some time ago, to describe why and how stress testing can be conducted.  There is no commentary.

 

Bone Grafts with Glass

 The use of glass in bone grafts.

image credit: Mo-Sci, Llc

 

Bioactive glass

 A description of bioactive glass from Mo-Sci,Llc

Image credit: Mo=Sci, Llc

Bubbles on Single Layer Fusing

“I'm making 3mm French Vanilla sconce covers; …

·        [initially they were] fine, but now 1.5" bubbles form during the full fuse.

·        I pop the bubbles and fill the holes with frit and refire,

·        [The]… edges draw in and distort the design…

·        The shelf is flat,

·        I fire on Bullseye paper, and

·        the 13.5 hour long firing schedule [in F] is:

200 to 1150, hold 30 minutes.

50 to 1225, hold 30 minutes.

300 to 1490, hold 30 minutes.

9999 to 990, hold 60 minutes.

100 to 750, hold 1 minute.

Does anyone know what I can do to avoid the large bubbles? 

A critique of the schedule. 

 This is for a single sheet of 3mm glass, so the hold at 621˚C/1150˚F is unnecessary as is the slow rise to and hold at 663˚C/1225˚F, because it is a single sheet and does not need the traditional bubble squeeze. 

 The hold of 30 minutes at 810˚C/1490˚F is excessive. 

·        The temperature may be too high.

·        Ten minutes at top temperature is sufficient in most cases. 

·        A soak of 1 minute would be enough. 

·        The anneal soak at 990˚F is most probably a misprint for                          516˚C/960˚F. 

·        The anneal soak is longer than the half hour necessary, but not a             bubble creating problem.

 It means the schedule could have been:

111˚C/200˚F to 796˚C/1465˚F for 5 minutes

AFAP to 516˚C/960˚F for 30 minutes

83˚C/150F˚ to 370F˚/700F˚, 0 minutes

Off

 

Different firing strategies are possible.

•         Reduce the time at top temperature to no more than 10 minutes. 
•         Reduce top temperature by 55˚C/100˚F or more and extend the soak to 20 minutes, if necessary.  Peek frequently to see when the kiln work is complete.
•         Fire on fibre paper covered with Thinfire to allow air out from under the glass.

These strategies can be mixed as desired, and the reasoning for the strategies is:

• Excessive time at the top temperature allows the glass to thin as it migrates to form thicker areas/edges. This makes the glass too thin to resist the air pressure from below.
• Reducing the top temperature will increase the viscosity, so              resisting the migration of the glass, and maintain the original            thickness. 
• Also, single layers are prone to dog boning, but there are ways of reducing it.

Ways to reduce the risk of bubbles appearing in general are:

•    Reduce the time at the top temperature,
•    Reduce the top temperature,
•    Provide ways for the expanding air to migrate from under the glass.

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Dog Boning in Slumps

I have done a few experiments on rectangular moulds with 3mm and 6mm thickness. I could not eliminate dog boning with larger rims, slower rates, or lower temperatures in any combination - although they did reduce the effect.

Square single layers dog boned even with increased rim width, and reduction of slumping depth made little difference in the amount of dog boning. 

Rectangular single layers shapes persisted in dog boning on the long side regardless of the rim dimension, and exhibited more dog boning on the long side than in the equivalent single layer square.  Two layer slumping had a decrease in dog boning with increased rim width, but with less effect on the long side.

In general, glass slumped in rectangular moulds is more sensitive the shape of the rectangle than the size of the rim, and very sensitive to symmetrical placing on the mould.  The depth of the mould has less influence than the size of the rim, especially for single layers.  The wider the rim, the less dog boning, in general terms.

Deeper moulds, higher temperatures, longer holds, narrower rims, all increased the dog boning. I conclude slumped square glass looks better because the dog boning is symmetrical.

My solution is to make bigger rims and cut the piece square after slumping. This approach needs cold work to the edges, of course.

The reason rectangular slumps dog bone is because the glass at the sides is drawn into the mould more easily than the corners, because there is more glass to draw in, just as in flat dog boning.

An alternative to the cold working is to round the corners of the rectangles to reduce the amount to draw-in.  A 1cm/0.375” radius curve will reduce the extent of the dog boning, but does not eliminate the effect.

Pressing glass

I have been looking for a different way than flows or melts to mix colours and thought glass pressing might be a promising way to achieve what I wanted.

Weight vs Temperature

I conducted some experiments attempting to thin 1.25 kg/2.75 pounds of glass to 3-4mm.  One and then two 40x40cmx15mm thick shelves were placed on top of the glass cullet with 3mm spacers at the corners. The glass was fired at 220ºC/396ºF to 825ºC/1517ºF and initially held for 30 minutes, later extended to 90 minutes.  The thickness stubbornly remained between 5 and 7mm. 

A few other attempts with different times and temperatures gave inconsistent results.  Perhaps the uneven piling of cullet had an influence on the outcomes, but I was still looking for a flow and mixing of colours different to that obtained by melts.

Other experiments were being conducted in parallel, relating to viscosity. These indicated that glass became thinner than 6-7mm at higher temperatures without pressing.  These experiments lead me to think there are four elements controllable by kilnformers in pressing: size, weight, time, temperature.

The same weight of press with the same temperature and time will make small amounts thinner than large amounts, and this is not surprising.  More time with the same temperature, weight, and amount allows some slight decrease in thickness. 

Higher temperatures with the same weight, and time will allow thinner pressings of the same amount of glass.   Viscosity decreases with temperature, so higher temperatures make glass easier to thin.

More weight is required get the same thickness when pressing a greater volume of glass.  Of course, more time and temperature can be added to increase the effect of the weight.

However, the main factor in pressing large amounts of glass is higher temperatures, which results in reducing the viscosity and the resistance to thinning. 

 

Annealing and Cooling

An important aspect of pressing is the annealing requirements.  It is sensible to anneal for a longer time than normal for thick glass, because of the heat retention of the pressing weights. 

This image shows the stress in an 8mm/0.3” (or 5/16”) after annealing as for 16mm/0.63” (5/8”).  There is widespread low level stress with 30mm thick pressing weight.

Indications are that extending the annealing to at least 3 times the target thickness is a minimum annealing soak requirement.  Alternatively, if it is possible to remove some, or all, of the weight from the glass at the beginning of the anneal soak, the annealing time can be reduced.

 

Veiling

The stress picture above shows there is visual element too.  This veiling is most apparent in clear glass, and less obvious in coloured and opalescent glass.  Small volume stacks, which are pressed thin will exhibit less of the veiling.

 

 

Four factors that kilnformers can control in pressing glass to less than 6mm are weight, size, time, and temperature.  The main one is temperature.