Glass-to-acrylic and acrylic-to-acrylic cylindrical adhesive bonds
Freek BOS
Ph.D. researcher
TU Delft, Faculty of Architecture
Delft, the Netherlands
Summary
A specific problem in the development of tubular glass structural members, is their connection to
the external structure. The option of glueing in an acrylic part at the tube ends has been
investigated. The cylindrical bond shape poses special difficulties with regard to the application and
curing of the adhesive as well as the differences in thermal expansion of the glass and the acrylic.
The first part of the research aimed at the proper joint design and adhesive application method,
while the second part consisted of investigating the joint shear strength. As a variant, a joint with an
acrylic part glued into an acrylic tube was also investigated. In both cases, joint strengths of over 10
kN were reached, which was enough for application in an experimental structure the author is
currently working on.
Keywords: glass, acrylic, cylindrical, adhesive.
1. Introduction
Despite their unmistakable aesthetical quality and structural advantages, glass tubes are seldom
used as structural members. One of the difficulties in realising such members is the connection
between the glass tubes and the external structure. The tubular shape makes drilling holes even
more complex than in flat glass, and friction grips are also hard to establish. In the only project
realized in practice with such components, the hanging façade of the Tower Place office building in
London, this was solved by using contact pressure between stainless steel heads and the edge of the
glass tube, generated by a central post-tensioning rod [1].
The author is currently working on similar transparent façade struts in which a glass tube has to be
connected to the external structure. To optimize transparency, an acrylic (PMMA) joint component
will be glued into the tube. Such a cylindrical adhesive bond, however, is hard to produce with high
optical and mechanical quality. Most of the difficulties are related to the bond shape [2].
2. Difficulties of a cylindrical glass-PMMA adhesive bond
Contrary to an ordinary flat adhesive bond, it is quite difficult to obtain a proper adhesive bond in a
cylindrical shape. Several shape specific problems emerge when trying to produce such a bond. In
the first place, the application of the adhesive (how to get adhesive between the substrates) is not at
all obvious. Secondly, curing of the adhesive involves shrinkage. The cylindrical bond shape does
not allow for this. An adhesive with controlled curing (e.g. by heat or UV-light), is therefore
preferable. This problem is similar to that encountered during the production of resin laminated
glass tubes [3]. Furthermore, the coefficient of thermal expansion α of borosilicate glass (of which
the tubes are made) is around 1/30th of that of PMMA (3.3x10-6 mm/mmºK vs approximately 100
mm/mmºK). Thus the PMMA pin will expand much more at an elevated temperature than the glass
tube. The joint needs to be able to accommodate the expansion caused by a temperature change of
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approximately ± 30 ºC with regard to room temperature. An air temperature of 50 ºC is certainly
possible in a glass structure.
Besides these shape-specific problems, the adhesive also has to provide enough strength. The
specimens in the presented research where tested directly and the calculable load on the structural
joints (once in 50-year load) is 4 kN, but since fatigue can degrade the long term bond strength, it
seems wise to require a direct test to exceed at least 10 kN.
To solve these difficulties, four basic parameters are available, all of which interact with each other:
- Adhesive selection,
- Bond shape design,
- Adhesive application method,
- Adhesive curing method.
3. Adhesive bond parameters
3.1. Provisional adhesive selection
The UV-curing acrylate GB368 of DELO was initially selected for the adhesive bond. According to
DELO product information [4], glass-PMMA bonds can reach shear strengths of up to 16 MPa.
Furthermore, the cured bond is completely transparent and the curing process can be controlled
tightly by controlling the specimen exposure to UV-light.
3.2. Bond shape design and application method
PMMA pin;
D = 60 mm.
Adhesive layer.
Tube. Initially, a PMMA
tube of Dxt = 60x3 was
used, PMMA pin. Outer
diameter = 60 mm. Later, a
glass tube of Dxt = 50x2.5
mm was applied.
The bond shape design and application method
are interrelated and they are described here
simultaneously. The specimen design was
developed through a process of trial-and-error.
The overall joint geometry is given in fig. 1. At
this stage, syrup was used instead of an
adhesive. Syrup has a viscosity comparable to
the adhesive, but it is cheaper and easy to clean
with warm water. Furthermore, it’s colour
makes the distribution of the fluid better
visible.
Also, instead of using glass tubes, specimens
were prepared with PMMA tube (hence
resulting in a PMMA-PMMA cylindrical
bond). The application problems are roughly
the same, but because PMMA is easier to
Fig. 1: Section of the overall joint geometry, machine, it was more appropriate for this stage
starting point for the adhesive application of the research in which the bond design had to
method. In principal, the joint axisymmetric be altered continuously. The problems of
combining two different materials, with
around the central axis.
different coefficients of thermal expansion were
thus also temporarily eliminated. Additionally, the behaviour of PMMA-PMMA adhesive bonds is
interesting for an alternative strut design with PMMA tubes.
2
Throughout the development of the design and application method, care was taken to distribute the
adhesive from one mass as much as possible. Research on the fatigue strength of adhesive bonds [5]
has shown the number of flow fronts in an adhesive layers significantly influences the fatigue
strength as fatigue cracks are likely to starts from areas where different flow fronts meet.
In the first design/application combination, both the PMMA tube end and the PMMA pin had
tapered edges (fig. 2). The pin was placed slightly into the tube. The tapered edge was then filled
with syrup and the pin was lowered slowly. It was expected that the capillary effect might suck the
adhesive down into the cavity between the pin and the tube. However, this proved not to work. If
the cavity is too small (t S 0.1 mm), the syrup will not distribute in the cavity, but it will flow
through by gravity when the cavity is bigger (t V 0.1 mm), see fig. 3. This will not result in a
homogeneous distribution.
●●
●
Fig. 2: First joint
design geometry.
●
●●
Fig. 3: First joint design.
The syrup flows through
because the cavity is too
wide.
Fig. 4: Second joint
design geometry.
Fig. 5: Second joint
design.
Therefore, a second combination was devised. Again, the PMMA cylinder had a tapered edge. The
pin however had a straight edge and a varying diameter (fig. 4). The last 5 mm of the pin fits
exactly in the PMMA cylinder, while the rest of the shaft had a slightly smaller diameter, resulting
in a 0.2 mm wide cavity. The pin was then fitted into the cylinder. The tapered edge of the cylinder
was again filled with syrup, relying on gravity for the distribution (fig. 5). The slightly thicker end
of the pin shaft did block the syrup from flowing through. However, the distribution was still not
very regular even when the pin was moved up and down and/or twisted. Furthermore, the tight
fitting end of the pin would be impossible in the glass-PMMA variant because of the thermal
expansion of the PMMA.
It was thus concluded that the adhesive is best applied under pressure. The consecutive design
featured a vertical canal in the PMMA pin (fig. 6). The syrup was injected using a syringe
connected to a small hose which was placed in the canal (fig. 7). A transparent silicone rubber ring
in a circular canal at the bottom of the bond prevents syrup from dripping out at that side. This
setup resulted in a smooth distribution of the adhesive, although it would not go around the cylinder
completely.
3
Syringe
Hose
A
A’
Silicone ring
Vertical canal
for hose.
Section A-A’
Fig. 6: Third joint design geometry.
Fig. 7: Application of syrup in third design joint
(here, a glass tube was used).
The specimen development was continued by producing a specimen with a glass cylinder and two
vertical canals. After a satisfactory result with syrup, the specimen (I) was produced with GB368.
The fluid distribution was somewhat worse than with the syrup, hence a specimen design with three
vertical canals was finally settled for (fig. 8).
Fig. 8: The third joint design developed from having
one vertical canal to having three in the end.
Fig. 9: Crack pattern in
glass tube caused by thermal
expansion of the PMMA pin.
3.3. Curing method
For curing, specimen A was placed under
UV-light, at a distance of approximately 15
cm. During curing, several delamination
spots appeared. The shrinkage of the
adhesive can apparently not be
compensated by flow of extensively
applied adhesive. Therefore, the
consequent specimens were placed on a
rotating disc at 2.8 m distance from the UV
source, to ensure slow and even curing.
4. Specimen testing
4.1. Subjection to elevated
temperatures.
Now that the design for application was
satisfactory, six PMMA-PMMA specimens were produced. However, before it was decided to
produce more glass-PMMA specimens, specimen A was placed in an oven at 50 ºC for
approximately one week (the air temperature in which the specimen was produced, was
approximately 22 ºC). Sometime during its stay in the oven, the glass cylinder of the specimen
cracked due to differences in thermal expansion (fig. 9). Judging from the pattern, the cracks started
at the top of the glass cylinder, which can be explained from the fact that the glass edge quality is
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inferior to the surface and the stresses are higher than at the bottom edge. The inner diameter of the
cylinder is 45 mm. Thus, the inner diameter of the borosilicate glass tube increases only 4.16x10-3
mm, while the acrylic pin wants to expand 0.13 mm. The solution for this problem was sought by
applying an adhesive with a lower stiffness. Thus, part of the volume change may be
accommodated by the adhesive and the stresses that will develop in the glass should be lower. One
specimen was produced with the UV-curing DELO acrylate PB437 and three were produced with
the experimental GBVE55789 adhesive which is based on PB437.
Unfortunately, the glass cylinder of these specimens broke as well after some time in the oven. The
crack pattern was identical to the one observed in the previous specimen. Nevertheless, it was
decided to try to test their strength in their damaged state.
4.2. Compression-shear test
The specimens were subjected to a compression-shear test to assess their strength and failure
behaviour. The loading on the adhesive bond (shear) is not principally different if the specimens
would be loaded in tension. Compression was chosen because it was easier to apply. The tests were
carried out on a universal Zwick test rig with a 250 kN load cell and Test Expert software. The test
results as well as some specimen data are listed in table 1. The table includes specimens II, III, and
IV which are introduced in the paragraphs below.
The PMMA-PMMA specimens 1-6 show average strength of 10.7 kN, but a large standard
deviation of 43.1 %. For a significant part this is caused by specimens 2 and 3. Visual inspection of
specimen 2 prior to testing showed opaque spots which were probably caused by contamination.
This explains the low strength compared to the other specimens. Specimen 3 was tested at a higher
speed (2.0 mm/min instead of 0.5 mm/min). Visually, no significant inferiority to the other
specimens was established. The loading speed has significant influence on the specimen strength.
Spec.
1
2
3
4
5
6
7
8
9
10
Materials
PMMA-PMMA
PMMA-PMMA
PMMA-PMMA
PMMA-PMMA
PMMA-PMMA
PMMA-PMMA
Glass-PMMA
Glass-PMMA
Glass-PMMA
Glass-PMMA
Adhesive
GB368
GB368
GB368
GB368
GB368
GB368
PB437
GBVE55789
GBVE55789
GBVE55789
Design
As fig. 6
As fig. 6
As fig. 6
As fig. 6
As fig. 6
As fig. 6
As fig. 6
As fig. 6
As fig. 6
As fig. 6
Ful [kN]
17.54
6.80
5.16*
10.32
14.19
10.00
1.30
0.29
0.55
0.36
I
II
III
IV
Glass-PMMA
Glass-PMMA
PMMA-PMMA
PMMA-PMMA
GB368
GB368
Tensol-6
Tensol-6
As fig. 6
As fig. 11
As fig. 12
As fig. 12
17.54
9.42
31.58
34.34
Without specimens 2 and 3, the average
strength is 31.01 mm (SA = 27.4 %). The
average shear stress at that load is 1.92
MPa. This is considerably lower than the
value of 16 MPa listed by [4]. However,
[2] concludes that the linear relation
between (shear) stress, load and surface
area is not valid for adhesive bonds,
especially not for cylindrical ones.
Rather, the peak stresses will govern.
These are not only dependent on the
global load, but also on the joint design
and quality of the application.
Table 1: Specimen data and test results. All specimens Remarkably, all specimens showed a kind
were tested at 0.5 mm/min, except (*) which was
of post-peak behaviour which was not
tested at 2.0 mm/min.
expected because the adhesive is
essentially brittle. Fig. 10 shows the combined load-displacment curve for the first and a second test
on specimen 5, which is typical for all the specimens. In the first test a peak load of 14.2 kN is
reached, and in the second test, a trajectory of crumbling of the adhesive layer is found with a
5
steadily increasing displacement at a more or less constant load. This behaviour is only partially
explained by the friction in the specimen caused by the silicone rubber ring (because the PMMA
pins could be pressed in by hand during specimen preparation, while the loads in this trajectory
could be as high as 1.4 kN). Apparently, the adhesive layer does not break all at once. The fact that
the post-peak trajectory is caused by a form of crumbling is supported by the rugged appearance of
the graph, when zoomed in (fig. 10, top right).
When considering structural safety, this can be regarded as safe behaviour (because after initial
failure an significant amount of post breakage strength remains). However, further investigations
into this phenomenon are required before this can be relied upon.
Failure of specimens 7 through 10 (of which the glass was already broken in the oven) is caused by
a combination of glass crumbling and adhesive bond failure. It was hard to establish what was
causing what. Inspection of the specimens afterwards did show that glass pieces were dislodged
from the PMMA, so failure was probably caused not only by crumbling of the already broken glass.
Specimen A (which also had cracked glass) was also tested, with much better results. The glass
continued to crack from approximately 13 kN onwards, until it finally shattered at 23.5 kN. This
indicates that failure was predominantly caused by glass breakage, while a specimen with
undamaged glass would have shown higher strength.
4.2.1. Solution to the problem of thermal expansion
The results of GB368 were far better than those with PB437 and GBVE55789. Even though also
with those adhesives better results should be obtainable, GB368 is clearly a better choice for this
connection. However, the problem breakage through thermal expansion remained. Therefore,
another specimen (II) was produced with a 2 mm slit though the middle of the PMMA part (fig. 11).
The slit was filled with flexible silicone, so the adhesive would not flow in. This design allows the
PMMA to expand inwards at elevated temperatures. Furthermore it allows slight movement of the
PMMA part during curing of the adhesive. Visual inspection of specimen II indeed showed no large
defects. The specimen was put in the oven like the other, but the glass did not break. It was
subjected to a compression shear test like the other specimens. The glass broke at 9.42 kN, but the
adhesive layer stayed unscaved. No further attempts were made to test this specimen, but it is very
likely the adhesive layer strength will be at least up to that of specimen I, and thus well over the
required joint strength of 10 kN.
4.2.2. Alternative adhesive for PMMA-PMMA variants
Two alternative specimens (III and IV) for the PMMA-PMMA adhesive joint were also produced.
Instead of GB368, a solvent adhesive Tensol-6 was used. This is basically a liquid variant of
PMMA. It dissolves the top layer of the acrylic substrates and solidifies with them to a bond that
virtually can not be distinguished from the substrate. Because this adhesive had a much lower
viscosity, the joint geometry could also be simplified (fig. 12). This resulted in an optically perfect
connection between both PMMA parts. They were also tested in a compression shear test. Both
specimen failed differently, the load-displacement curves are given in fig. 13. Specimen III broke
by shear failure in the PMMA part (fig. 14). Just before complete failure, a crack through the
specimen appeared. Specimen IV on the other hand failed on the adhesive layer (fig. 15), at once in
a brittle manner. Both specimens failed at loads much higher than achieved with the other adhesive.
The lack of post-peak behaviour may be a disadvantage with regard to safety, but the superior
strength and transparency will make this method preferable if the consequences of structural failure
of this joint are limited.
6
15
14001,98
Compressive displacement in mm
1,99
2,00
2,01
2,02
1410
Load in N
Load in kN
10
1420
1430
1440
5
0
0
1st run
2
2nd run
4
6
8
Compressive displacement in mm
Fig. 10: Combined load-displacement curves of specimen 5 (1st and 2nd run). Top right is the loaddisplacement behaviour zoomed in around 2 mm displacement in the 2nd run (= approx. 4 mm
overall).
5. Conclusion
Methods have been presented to produce cylindrical adhesive bonds acrylic-to-acrylic and glass-toacrylic. It is concluded that, for the acrylic-to-acrylic bonds:
- The method based on the use of GB368 applied under pressure gives reasonable results,
although they are highly dependent on execution quality.
- This method results in remarkable post-peak behaviour, which provides a degree of
structural safety, although further research is necessary to assess this phenomenon in detail.
- The method based on a gravity applied solvent adhesive gives superior strength and optical
results, although it does not provide the post-peak behaviour of the former method. Better
fatigue characteristics are also expected.
For the glass-to-acrylic bonds, it is concluded that:
- The developed application method based on the use of GB368 applied under pressure gives
fine results in terms of strength and transparency.
- Using PB437 and GBVE55789 resulted in significantly inferior strength.
- A slit through the PMMA part should be added to improve the curing of the adhesive and
avoid glass breakage at elevated temperatures through thermal expansion of the PMMA.
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-
The final strength of the GB368 bonds could not be established because failure in the
specimens was always governed by glass fracture. Thus, an adhesive crumbling
phenomenon as observed in the PMMA-PMMA specimens, could not be encountered.
Nonetheless, the specimens far exceeded the preset strength requirement of 10 kN.
Fig. 11: Added slit
in specimen II.
First crack
30
Specimen III
Force in kN
Fig. 12: PMMAPMMA Joint
design with Tensol6 adhesive
20
Specimen IV
10
0
0
Fig. 14: Failure
through PMMA pin
in specimen III.
Fig. 15: Failure of
adhesive layer in
specimen IV.
2
Compressive displacement in mm
4
Fig. 13: Load-displacement curves of specimens III
and IV.
6. Literature
[1]
[2]
[3]
[4]
[5]
ACHENBACH. BEHLING, DOENITZ, JUNG, Konstruktive Elemente aus Glasrohrprofilen,
Glas: Architektur und Technik, nr. 4, 2002, pp.5-10.
POULIS, J.A., Small cylindrical adhesive bonds, Delft, The Netherlands, 1993.
NIEUWENHUIJZEN, BOS, VEER, The column for the All Transparent Pavilion, 9th GPD,
Tampere, Finland, 2005.
DELO, Glassbond selection chart, www.delo.de.
VEER, ZUIDEMA, RIEMSLAG, van, KRANENBURG, van, Mode II fatigue crack growth
of transparent adhesive joints, IVOV 2001.
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