Leak Repair in Kite Surfing Bladders

1

Massachusetts Institute of Technology

Department of Chemical Engineering

10.26 Chemical Engineering Projects Laboratory

Spring 2019

Proposal: Team 1

Leak Repair in Kite Surfing Bladders

May 2019

Team Members

James Cao

Jennifer Nwenyi

Johan Villanueva

Project Sponsors

Jean Dunoyer

Staff

B.S. Johnston, instructor

E. Powers, teaching assistant

Team 1 10.26 Chemical Engineering Projects Laboratory Spring 2019

Final Report 2

COVER LETTER

Deepti Dhir gave some very helpful feedback regarding the final paper. Mostly, we felt the

delivery of the material was well done, particularly in our alignments in the technical

approach and the discussion section. It all seemed to flow relatively well. However, as Deepti

pointed out, we felt that we did not properly articulate our deliverable method as well. The

goal of the project was to develop a novel transport method that could deliver an adhesive

repair solution (that was known to work in normal conditions) without having the need to

remove the TPU bladder. We made some adjustments to really emphasize that. Likewise, with

the findings from the past few weeks, we have rewritten the discussion and concluding

sections to reflect on this (mostly the fact that we found a better adhesive to use).

In the Abstract section, we sought to clarify the fact that the internal repair delivery was done

using syringe needle and the fact that our main ways of determining success was to compare

the pressure drop over time in various TPU bladders (undamaged, damaged and sealed). We

also decided to reemphasize our objective (and our deliverable) in the Introduction section.

We wanted to clarify to the reading that our goal was to develop the frameworks for internal

repair based on an adhesive that could seal TPU damages. Similarly, we also reworked our

Background Research to emphasize this point as well as reflect on the recent discoveries

during our project (particularly about choice in adhesives). The Technical Section was left

relatively untouched, but we did add some more specificity and clarifications as Deepti had

suggested. Lastly, for our discussion session, we tried to reduce some of our redundancies (for

the information that was already explained in the Technical Approach).


Final Report 3

ABSTRACT

The extreme water sport, kite surfing, involves harnessing wind currents with a large inflatable

kite that is structurally supported by rigidly inflated bladders made of thermoplastic

polyurethane (TPU). Due to its rigidity, these TPU bladders can puncture and leak, which is

detrimental to the kite’s performance when surfing. Current methods of repairing holes in these

bladders involves a lengthy and difficult removal of the bladder from the kite apparatus, which

is inefficient and costly for kite surfers. This project proposes a novel method of repairing kite

bladder holes without the inconvenience of removing the kite bladder. We have developed an

injectable repair method (via syringe needle) that penetrates externally into the kite and delivers

an adhesive repair solution internally to seal bladder holes. From our comparisons to the

pressure change in undamaged and leaking TPU bladders, the delivery of 80 µL Loctite™

Liquid Super Glue via internal repair delivery was determined to be able to completely repair

damaged lab scale bladders and maintain desired kite operating pressures. With lab scale repair

of the damaged TPU bladders, we have established a framework for exploration into alternative

kite repair via internal repair delivery.


Final Report 4

TABLE OF CONTENTS

COVER LETTER..................................................................................................................2

ABSTRACT ..........................................................................................................................3

TABLE OF CONTENTS.......................................................................................................4

LIST OF FIGURES ...............................................................................................................5

LIST OF TABLES ................................................................................................................6

1. INTRODUCTION .............................................................................................................7 1.1. Objectives ................................................................................................................8 1.2. Method of Approach ................................................................................................9

2. BACKGROUND ............................................................................................................. 10 2.1. Kite Materials ........................................................................................................ 10 2.2. Conceptualization of Leaks .................................................................................... 11

2.3. Current Methods of Repair ..................................................................................... 12 2.4. Repair Materials .................................................................................................... 13

2.5. Determined Repair Method: Internal Adhesive Injection ........................................ 13

3. TECHNICAL APPROACH ............................................................................................. 14 3.1. Pressure Gasket Chamber Design ........................................................................... 14

3.2. Bladder Hole Formation ......................................................................................... 15 3.3. Adhesive Material Selection .................................................................................. 16

3.4. Internal Repair Execution ...................................................................................... 17

4. BLADDER HOLE FORMATION AND CHARACTERIZATION .................................. 19 4.1. Undamaged TPU Leak Test ................................................................................... 19

4.2. Damaged TPU Leak Test ....................................................................................... 19

5. ADHESIVE MATERIAL SELECTION AND OPTIMIZATION .................................... 22 5.1. Infinite Cure Time Tests ........................................................................................ 22

5.2. Adhesive Cure Time Optimization ......................................................................... 23 5.3. Baseline Integrity ................................................................................................... 24

5.4. Effective Adhesive Spread Area............................................................................. 29

6. INTERNAL REPAIR METHOD EXECUTION .............................................................. 31 6.1. Blind Delivery Tests .............................................................................................. 31

7. CONCLUSIONS ............................................................................................................. 35

8. RECOMMENDATIONS ................................................................................................. 36

9. REFERENCES ................................................................................................................ 38

10. APPENDIX A ............................................................................................................... 39

11. APPENDIX B................................................................................................................ 40

12. APPENDIX C................................................................................................................ 41


Final Report 5

LIST OF FIGURES

Figure 1-1. Primary Components of the Kite .........................................................................7

Figure 1-2. Internal Repair Delivery Schematic. ...................................................................8

Figure 2-1. Schematic of Thermoplastic Polyurethane ........................................................ 11

Figure 3-1. Pressure Gasket Chamber Loaded with TPU Sample ........................................ 14

Figure 3-2. Design for Pressure Leak Testing ..................................................................... 15

Figure 3-3. Schematic for Pressure Leak Testing ................................................................ 16

Figure 3-4. Schematic for Bladder Hole Repair ................................................................... 17

Figure 3-5. Internal Adhesive Delivery against Gravity ...................................................... 18

Figure 4-1. Plot of Normalized Pressure Drop in an Undamaged TPU Bladder ................... 19

Figure 4-2. Plot of Normalized Pressure Drop in a Damaged TPU Bladder ......................... 20

Figure 4-3. Natural TPU Hole vs Punctured TPU Hole ....................................................... 21

Figure 5-1. Normalized Pressure Drop for FlexSeal™ Repaired TPU ................................. 22

Figure 5-2. Normalized Pressure Drop for Loctite™ Repaired TPU .................................... 23

Figure 5-3. Internal Dryness of FlexSeal™ Droplet ............................................................ 24

Figure 5-4. Normalized Pressure Drop for 100 µL Loctite™ Repaired TPU ....................... 25

Figure 5-5. Loctite™ Tearing ............................................................................................. 25

Figure 5-6. Rechecking Normalized Pressure Drop for 100 µL Loctite™ Repaired TPU .... 26

Figure 5-7. Normalized Pressure Drop for Loctite™ Repair Variations ............................... 27

Figure 5-8. Rechecking Normalized Pressure Drop for Loctite™ Repair Variations ........... 28

Figure 5-9. Normalized Pressure Drop for 80 µL Loctite™ Repaired TPU ......................... 28

Figure 5-10. Rechecking Normalized Pressure Drop for 80 µL Loctite™ Repaired TPU .... 29

Figure 6-1. Normalized Pressure Drop for Loctite™ Blind Testing .................................... 31

Figure 6-2. Comparison of Successful and Unsuccessful Interval Delivery ......................... 32

Figure 6-3. Inaccurate Internal Delivery of 80 µL Loctite™ Liquid Super Glue .................. 33

Figure 6-4. Successful of Interval Repair Delivery .............................................................. 34

Figure A-1. Microscope Calibration Slide ........................................................................... 39

Figure B-1. 30 G Needle (310 µm) Puncture Imaging ......................................................... 40

Figure C-1. Loctite™ Liquid Super Glue Spreading Area ................................................... 41


Final Report 6

LIST OF TABLES

Table 3-1. Repair Material Desired Properties .................................................................... 16

Table 5-1. Dryness across Different FlexSeal™ Volumes ................................................... 23

Table 5-2. Dryness across Different Loctite™ Volumes ..................................................... 24

Table 5-3. Spreading Area of 80 µL Loctite™ Liquid Super Glue ...................................... 30

Table B-1. Summary of Needle Puncturing Success at Various Pressures ........................... 40


Final Report 7

1. INTRODUCTION

Kite surfing is an extreme water sport that utilizes a large inflatable kite to manipulate wind

currents to precisely maneuver across the water, achieving speeds of up to 50 miles per hour.

In order to ensure the safety of the kite surfer and the integrity of the sport, manufacturers have

paid close attention to the production of these kites, particularly improving the kite’s overall

durability. As shown by the 2D and 3D cross sections in Figure 1-1, kites are composed of two

major components: a canopy made of Ripstop nylon and a leading edge made of woven

polyester, commonly known as Dacron, which is modified with a resin to create a water-

resistant fabric. These components are structurally supported by thermoplastic polyurethane

(TPU) bladders, which are located within the struts and leading edge of the kite. When these

bladders are pumped up to about 8-12 psig, the kite becomes rigid and keeps shape. Typically,

when inflated to this operating pressure range, the TPU bladder is stiffly pressed against the

Dacron fabric. This rigidity is necessary to steer the kite, but the high pressure and stiffness of

the TPU bladder makes them more susceptible to be puncture or worn down. Holes can often

form on the leading edge TPU bladder, causing the bladder to leak air and lose its preferred

rigidity. These bladders holes must be fixed in order for the kite to be operational once more.

Figure 1-1. Primary Components of the Kite. As demonstrated in the top figure (2D cross

section), the canopy is supported by the Dacron Leading Edge and struts, which contain

inflatable thermoplastic polyurethane (TPU) bladders. When inflated to the operating pressure

(8-12 psig), the TPU is firmly pressed against the Dacron fabric, giving the kite a rigid shape

as shown by the bottom figure (3D cross section).


Final Report 8

Current methods to repair these kite bladders are lengthy and may be frustrating for the user.

Bladder holes are typically on the micron scale, making them difficult to visually identify. Kite

surfers often resort to spraying down the entire length of the kite with soapy water in order to

observe soapy bubbling that the leaking air would create. Once the location of the leak is

identified and measured, the kite surfer must deflate the kite and remove the TPU bladder from

the kite. An adhesive nylon rubber patch is then carefully placed onto the hole before the bladder

is precisely pushed back into the leading edge fabric. Throughout this whole three hour process,

the TPU bladder can potentially twist and tear, creating further areas for leakage once pumped

back up. Additionally, having a professional repair these types of damages can often cost the

kite surfer up to about $100. A repair method that circumvents the need to physically remove

the TPU bladder would cut down on repair time and repair costs, allowing the kite surfer to

focus more on enjoying the sport.

1.1. Objectives

The objective of the project was to develop the framework for a deliverable method of repairing

holes in kite surfing that would bypass current difficulties and to provide kite surfers with a

cheap and efficient alternative means of repairing their kite bladders. As shown in Figure 1-2,

we have developed an injectable method that penetrates through the Dacron kite fabric into the

TPU bladder—via syringe needle—to deliver an adhesive repair internally to precisely seal

bladder holes. By ignoring the need to physically remove the bladder and directly stopping

leakage, this approach establishes the basis for alternative repairs that heavily reduce repair

time, repair costs, and user frustration.

Figure 1-2. Internal Repair Delivery Schematic. By penetrating past the Dacron fabric and

into the TPU bladder via syringe needle, the proposed repair method circumvents the need to

remove the TPU bladder from the kite, which would provide kite fliers with a simplified and

accurate means of sealing bladder holes.


Final Report 9

1.2. Method of Approach

To demonstrate the effectiveness of the proposed delivery, the project was separated into three

main phases: TPU leak replication, adhesive material selection and internal repair execution.

The first phase, TPU leak replication, involved a lab scale reproduction of an inflated TPU

bladder and a streamlined process for creating and identifying bladder holes, as a means of

creating a standard to measure the success of the proposed repair method. Next, based on the

desires for an adhesive material that could quickly dry and seal holes, various commercial

adhesive materials were tested for their capability with internal delivery, their relative cure

times and how well adhesive sealed bladders could hold pressure relative to the undamaged

TPU bladders. After choosing the adhesive that best fulfilled these criteria, the third phase

involved the internal delivery of this adhesive to a lab scale model of a leaking TPU bladder, to

determine the success and replicability of the proposed repair.


Final Report 10

2. BACKGROUND

Although kite bladders are designed for durability, the high operating pressure required for

kite surfing can often make the TPU kite bladders more susceptible to punctures and other

types of damage. In order to address the issue of kite bladder repair, a clear understanding of

the kite’s makeup was necessary in order to understand how bladder holes may form and how

it may affect the performance of the kite. From there, a viable alternative repair method was

determined based on current commercial repairs as well as the findings from previous teams

that have worked on the project.

Current commercial methods involve the removal of the TPU bladder from the kite apparatus,

which can often be costly and inefficient. Predecessors of this project attempted to circumvent

the need to remove the TPU bladder by applying an aerosolized solution of TPU dissolved in

DMF through the nozzles of the kite. However, this method was overall deemed as unusable

due to the hazardous solvent used as well as the failure to effectively locate the damaged area

in the TPU. The goal of this project is to build upon the previous team’s findings and develop

the basis for an ideal repair delivery that can effectively seal a bladder hole, maintain safety as

well as be affordable by the common kite surfer. The team has suggested an internal repair

method via needle syringe that directly deliver an adhesive material at the site of a leak.

2.1. Kite Materials

The rigidness of the kite canopy and leading edge is attributed to a series of inflated bladders,

which are primarily composed of thermoplastic polyurethane (TPU). These bladders are fitted

into the sleeves of the kite’s leading edge and struts, which are commonly composed of

woven polyester, commonly known as Dacron. An understanding of the composition of these

two primary materials needed to be established in order to effectively approach the proposed

intern

TPU is widely used for kite bladders due its incomparable mechanical properties. This

material is resilient, highly elastic, and has high resistance to weather, abrasion, and impact

[4]. These properties can be attributed to TPU’s segmented block copolymer structure, which

allows the polymer to possess hard and soft segments [3]. The hard segments of the polymer

provide tensile strength while the soft segments provide a degree of elasticity [4]. A diagram

of the hard and soft segments is depicted in Figure 2.1. The physical properties of this

polymer can be disrupted by solvation and high temperatures but can easily recovered by

reverting these changes such as evaporating the solvent and cooling the polymer [5].


Final Report 11

Figure 2-1. Schematic of Thermoplastic Polyurethane. This schematic shows the structure

of the TPU polymer chains. The function of TPU is dependent on the ratio of the hard

segments to soft segments. Because the bladder is expected to be able to expand to fit into a

kite sleeve, it is suspected that the ratio leans more towards soft segments.

The grade of TPU commonly used in kite bladders may differ between manufacturers as

certain additives may be included into their TPU material. However, it is suspected that the

kite bladder mainly consists of polyether-based TPU because of the need to have hydrolysis-

resisting abilities due to the nature of kitesurfing [7].

Despite its purpose to hold pressure in the kite, air can still diffuse out of the TPU bladder

over time, even in the presence of no hole. Due to low diffusivity of air through TPU, the

pressure drop experience is considered to be negligible during kite surfing sessions. However,

over an extended period of time (overnight), a pressurized TPU bladder will be noticeably

more deflated [5]. For the purposes of testing TPU bladder to confirm the efficacy of the

proposed internal repair delivery, these changers were accounted for.

The kite fabric provides the inflated TPU bladder with its structure. This fabric is primarily

comprised of Dacron, a woven polyester fabric [8]. Dacron is made from high-strength textile

polyethylene terephthalate fibers. The mechanical properties that this material has that allows

it to provide a shape for the TPU include high tensile strength, high resistance to stretching

(when wet and dry) and high abrasion resistance [9]. Dacron is treated with resin and other

additives to make the fabric water resistant; however, the nature of these additives differs

among manufacturers.

2.2. Conceptualization of Leaks

Despite the flexibility granted by TPU’s mechanical properties, TPU kite bladders can still be

damaged significantly. Since the valves on the leading edge kite are used to inflate the kite,

the leading edge TPU bladder is often suspect to the formation of bladder holes. For example,

sand particles can often accumulate within the leading edge sleeve and wear down on the

bladder. Likewise, the fabric stitching on these sleeves can often poke the TPU bladder. Over

extensive use of the kite, these abrasions and punctures can form significant holes in the kite

[2].

Once a hole forms on the bladder, air will begin to leak out the bladder, creating a dynamic

pressure drop over time. Eventually, the overall kite pressure will drop below desired

operating conditions, preventing the kite surfer from safely using their kite. The geometry and

size of these holes are not well documented, but are speculated to be pinhole-sized according

to various kite surfers [2].


Final Report 12

The previous team, however, analyzed naturally occurring holes in kite bladders for their

shape and size. The particular bladder hole studied was about 90 µm and showed signs of

abrasion [5]. Although this hole was characteristic of a possible bladder hole, this particular

instance did not completely encompass all types of leaks that a kite surfer may encounter.

However, for the purpose of testing their repair method, it was appropriate to simply recreate

a bladder hole of that size.

2.3. Current Methods of Repair

There are several commercially available approaches to repairing a leaking kite that are

commonly utilized by kite surfers, but most approaches are relatively similar in execution.

First, in order to identify the source of leak, kite surfers often spray down the length of the

kite with soap and water. If air is leaking out of the kite, the sprayed soap and water will start

to bubble. Once the leak is found, its location along the kite is noted and the kite bladder is

removed from the Dacron fabric. This can prove to be relatively challenging as all bladders in

the kite are connected through a valve system. These connections must be blocked off before

the bladder can removed from the Dacron kite sleeve [2].

Once removed, a kite surfer must then be able to match up the location of the bladder hole

(which can often be on the micron scale in diameter) along the length of the kite, which are

often about 15 m in length. The located hole is then patched with an adhesive elastomer that is

similar in mechanical property to that of TPU. Once applied, the kite surfer must reinsert the

flaccid TPU bladder back into the kite sleeve and confirm that the kite can hold pressure.

Throughout this whole process, the TPU bladder can be further damaged, due to potential

twisting and tearing as the bladder is inserted and removed from the kite sleeve.

To circumvent the difficulty of repair, the previous team proposed a delivering an aerosolized

adhesive through the valves of the kite. Regarding the creation of this aerosolized adhesive,

the team focused on utilizing solvent cementing. Solvent cementing focuses on utilizing a

compatible solvent to weak the bonds between polymers chains, to allow for more mobility

within various parts in a polymer [6]. Once the solvent evaporated completely, the polymer

loses this mobility, creating a permanent welding [2, 6]. Although this method was proven to

be viable by the previous team, the possible solvents that were compatible with TPU—such as

DMF—were too hazardous for commercial use. Likewise, the suggestion method of pumping

aerosolized solution was deemed to be ineffective. Since TPU bladders can achieve up to

lengths of 15 m, it would be very difficult to pump repair solution through the kite valves and

selectively and accurately seal the bladder hole. In addition, significant pressure would need

to be applied for holes that are far from the valve openings. From the previous team’s

findings, their proposed aerosolized delivery was

The method of repair that the previous team used is a process known as solvent cementing.

Solvent cementing is a process in which a dissolved polymer in a solvent is used to create a

joint between thermoplastic parts [6]. In this process, the addition of the solvent weakens the

bonds between the polymer chains which allows for more freedom of movement which allows

the softened parts to meld. After a short period of time, most of the solvent diffuses away

from the joint which creates a permanent weld [2, 6]. Although this is a viable method of

repair, it would be difficult to use since TPU is only easily dissolved in hazardous solvents,

such as DMF, that are not appropriate for consumer use only able to generate a coating of 3

microns around 10 to 35 cm from the spray gun nozzle, a thickness less than 25% of the

necessary thickness of 16 microns to fix the hole [5]. The current team’s proposed internal

repair delivery sought to improve on this method, providing a more accurate and direct repair.


Final Report 13

2.4. Repair Materials

In order to demonstrate the efficacy of the proposed internal delivery method, a compatible

adhesive with certain operating properties was desired. In order to be considered more

successful and efficient than the timely and difficult commercial repair, the chosen adhesive

material that would be chosen would need to have high elasticity, high melting point, low cure

time, high binding strength with the TPU bladder (mechanically and chemically) and

relatively low viscosity. The repair chosen would need to have similar properties to that of

TPU so that bladder function would remain relatively unchanged. The material would also

need to be relatively resilient at typical kite surfing temperatures (50 – 80 °F) [2]. This project

explored adhesives that are both chemically binding and mechanically binding. Lastly, in

order to be injectable via needle syringe, the adhesive needed to have a relatively low

viscosity. Likewise, this low viscosity would be able to achieve a greater spread once applied,

slightly mitigating the issue of accuracy.

There were five potential adhesives that were tested throughout this process. These include:

TPU dissolved in DMF, Scotch™ Weld Epoxy Adhesive, Scotch™ Super Glue, Loctite™

Liquid Super Glue, and FlexSeal™ Spray. Each adhesive has its advantages and

disadvantages that were considered.

Although, TPU dissolved in DMF was proven to be an effective repair due to solvent

cementing and chemical compatibility with the TPU bladder, the repair was too dangerous to

be utilized in practical settings. Scotch™ Weld Epoxy Adhesive was considered to be viable

adhesive due to its high long-term resistance, high temperature resistance and low cure time.

Due to the presence of cyanoacrylate (ECA) groups, both Scotch™ Super Glue and Loctite™

Liquid Super Glue were also potential candidates for internal repair delivery. Cyanoacrylates

are fast acting adhesives that are used in a variety of commercial products. They undergo

anionic polymerization in the presence of a weak base, most commonly water, to form strong

polymer chains [10]. Due to the high moisture levels in kite surfing environments, these super

glues would only improve in effective performance. Lastly, FlexSeal™ Spray, an aerosolized

liquid rubber, was considered for its low cure time, low viscosity, high elasticity, and ability

to form durable polymer repairs. The durability is primarily attributed to its emulsified

bitumen base.[11]. Although the presence of water does not necessarily enhance its adhesion

performance, FlexSeal™’s resistance to hydrolysis suggested that it may be resilient adhesive.

2.5. Determined Repair Method: Internal Adhesive Injection

The concept of external repaid deliver was considered, in which adhesive would be pumped

through the Dacron fabric in order to reach the damaged areas of the TPU below. However,

due to manufacturing modifications, the Dacron fabric and TPU bladder had relatively similar

chemical compositions, which could lead to the desired adhesive to selectively spread to the

Dacron fabric or adhere the Dacron to the TPU bladder. Aside from potentially failing as a

repair, users would be unable to remove the TPU bladder from the kite sleeve if they needed

to.

Alternatively, the team has suggested avoiding contact with Dacron with an internal repair

delivery. This method would utilize a syringe needle to puncture past the Dacron layer and

into the TPU bladder. This would allow for the selective deposition of adhesive material.

Likewise, this method would not be limited by bladder hole location as the syringe would be

able to be inserted anywhere along the length of the kite. As the kite bladder would not have

to be removed from the Dacron fabric, this proposed method, along with the chosen adhesive

would significantly ease delivery and shorten repair time.


Final Report 14

3. TECHNICAL APPROACH

Our proposed internal delivery method can be broken up into three major phases: TPU leak

replication, adhesive material selection and internal repair selection. In order to begin the first

phase, a lab scale model of an inflated TPU bladder was first constructed, using a pressure

gasket chamber connected in a closed system. Using this setup, the change in pressure over

time for various types of TPU samples were characterized as a means of comparing the

effectiveness of the repaired TPU sample. More specifically, the leaking behaviors of

undamaged TPU and damaged TPU were quantified. These baselines were used to determine

the viability of the various adhesive materials that were used for internal repair. These adhesive

materials, however, were first categorized by various desired parameters such as their relative

viscosities and cure times. The desired repair material needed to be injectable through a syringe

needle, quick-curing, and able to successfully seal a hole in the TPU bladder. Once this was

established, internal repair was deployed onto damaged samples of TPU, in attempt to seal a

fabricated hole. After curing, a repair would be deemed successful if it was able to hold pressure

similarly to that of the undamaged TPU bladder, not the damaged TPU bladder.

3.1. Pressure Gasket Chamber Design

In order to properly determine the effectiveness of the proposed repair delivery, an

understanding of the TPU bladder and its behavior when leaking was first established. A lab

scale model was developed to simulate an inflated TPU bladder under operating pressure (8-

12 psig). As shown in Figure 3-1, the previous team built a pressure gasket chamber that

could be loaded with samples of TPU and inflated under a closed system. More specifically,

the TPU sample was mechanically sealed with rubber gasket layers and topped with a

stainless steel wired mesh to prevent the TPU from over-expanding, similar to the Dacron

fabric in the kite.

Figure 3-1. Pressure Gasket Chamber Loaded with TPU Sample. TPU samples were

mechanically sealed in this pressure gasket chamber, allowing for the TPU sample to inflate

to high pressures. A piece of Dacron fabric or wire mesh was typically used to hold down the

TPU and prevent it from over-expansion (and possible bursting). This allowed for a relatively

accurate lab scale of an actual inflated kite for testing.


Final Report 15

Using this pressure chamber as a means of simulating the inflated kite, the change in pressure

over a period of time was measured for various samples of TPU (undamaged, leaking, and

sealed), based on the following schematic in Figure 3-2. Compressed air—which is adjusted

by a manually controlled valve—enters the system, increasing the pressure in the gasket

chamber. Monitoring the system’s pressure with a pressure transducer, the system was closed

off from the surroundings once the desired operating pressure (8-12 psig) was achieved. Aside

from the sealing properties of the gasket chamber, the system’s exhaust valve was shut to ensure

that air could not rapidly escape unless the TPU sample was damaged or if the pressure chamber

was not properly assembled. With this closed system, pressure was dynamically measured over

time using the pressure transducer. To ease comparisons between various trials, these measured

pressures were normalized to the maximum pressure that the system was inflated to.

Figure 3-2. Design for Pressure Leak Testing. Pressurized air enters the system and begins

to inflate the TPU sample in the pressure chamber to its desired operating pressure of 8-12

psig. Once filled, the system is clamped, closing the system off from the atmosphere. A

pressure transducer is then used to measure the change in pressure over time within the

system. Aside from the exhaust valve and the natural diffusion of air through the TPU, air can

only exit the system through damages in the TPU sample. This would be indicative of a leak

or an unsuccessful repair.

In order to ensure the integrity of the experimental setup, soap and water was sprayed onto the

chamber and its various connections. Rapid bubbling would be observed, indicating loose

connections or a damaged sample of TPU.

3.2. Bladder Hole Formation

Using this experimental setup, the pressure change over time in an undamaged sample of TPU

was measured with the pressure transducer. When first loaded onto the pressure chamber, the

sample was inflated to operating pressure and sprayed with soap and water to test for air

leakage. Once ensured that the sample was undamaged, the TPU sample was inflated to

operating pressure and sealed off from its surroundings via clamping. The dynamic pressure

change in the undamaged samples was then measured over a fixed period of time and then

normalized to the maximum pressure measured. Due to being undamaged, these samples served

as a baseline of comparison for the subsequent experiments involving creating bladder holes

and sealing holes.


Final Report 16

As explained in Figure 3-3, undamaged samples of TPU were then punctured in order to

simulate a damaged kite bladder. With the TPU sample pressurized in the chamber, a syringe

needle of a known gauge—the inner diameter of the needle tip—, was inserted into the sample,

creating a hole of known diameter and geometry. This newly damaged TPU sample was then

re-inflated to operating pressure and its leakage was observed by measuring the pressure change

over time. Similar to the undamaged bladder, the punctured TPU sample served as a means of

measuring the success of the sealed TPU sample as a sealed sample that decreased in pressure

to a similar rate of a damaged bladder would be deemed non-repairing.

Figure 3-3. Schematic for Pressure Leak Testing. First, the TPU sample was tested for

possible damages in order to ensure that the only leak in the TPU would be the one that was

intentionally produced. Once it was confirmed that the sample was completely undamaged, it

was punctured with a syringe needle of known gauge (inner diameter of needle tip) at

pressurized conditions. Lastly, the TPU sample was re-inflated to operating pressure and

allowed to leak air.

3.3. Adhesive Material Selection

Material selection was approached by looking at various commercially available adhesive

products. FlexSeal™, Scotch™ Weld Epoxy, Scotch™ Super Glue, Loctite™ Liquid Super

Glue were all seen as viable candidates and experimented, in addition to the prior’s team

proposed solution of TPU in dimethylformamide (DMF). In order for these adhesives to be

viable for internal delivery repair, the material had to have a relatively low viscosity such that

it would be injectable through the syringe needle. For a given adhesive, if there was difficulty

in pushing the syringe plunger, it was retired as a plausible option for repair. Similarly, it was

desired for the repair material to be quick drying to outcompete the three hour mark conceived

from the current repair method used for kite bladders. The average cure time was extrapolated

by testing the drying capabilities of different adhesives at various volumes. Table 3-1 reflects

all the data collected throughout running all the experiments meant to determine the best

material selection.

Table 3-1. Repair Material Desired Properties


Final Report 17

Based on the adhesive’s abilities to cure, relative viscosity, and injectability through the syringe,

Loctite™ Liquid Super Glue and FlexSeal™ were chosen as possible candidates for the internal

repair delivery due to their low average cure times and low viscosity. These two adhesives were

tested for their sealing capabilities. To a damaged sample of TPU, fixed volumes of adhesive

material was added onto the sample and allowed to cure for an extended period of time (“infinite

cure time test”). This was then pressurized to 8-12 psig in the pressure chamber and tested for

leaking. If the adhesive was able to successfully seal the hole, it was then considered for internal

repair delivery. However, in order to maximize the adhesive’s efficiency, the relative cure time

for given volumes—from 100 to 750 µL—was determined. By measuring a volume of repair

solution to one droplet area, allowing it a set amount of time to dry, and examining if there was

liquid by using a needle throughout the entire droplet, the collected data would give greater

insight on the volume required to seal holes of varying sizes and geometries.

For volumes of adhesive material that were able to cure completely for a given time, damaged

TPU samples were penetrated with a syringe against gravity, sealed directly at the site of

leakage and tested for its integrity by measuring the pressure change over time. Fine tuning of

this volume as well as the injection technique (such as rotating the syringe or adding incremental

volumes) were done based on these initial findings. Lastly, the area of how each adhesive would

spread was measured in order to gauge the effective range for a specific volume of adhesive.

These measurements would serve as a baseline approximation for how effective repairs would

be for internal repair delivery for a given volume and cure time.

3.4. Internal Repair Execution

After determining the appropriate repair material and optimizing the amount of volume needed

for an effective cure, internal repair delivery was conducted as dictated by Figure 3-4.

Figure 3-4. Schematic for Bladder Hole Repair. With the artificially damaged sample of

TPU, internal repair delivery was deployed at kite surfing operating pressures. The syringe

needle was inserted into the pressure chamber against gravity. Once the material was fully

deposited and cured for the desired duration of time, the sealed sample was re-inflated to

operating pressure and tested for structural integrity.

Damaged samples of TPU—that were punctured by a syringe needle of known gauge—were

re-inflated to operating pressure condition of 8-12 psig in the pressure chamber. By spraying

soap and water onto the surface of the damaged TPU, the general location of the bladder hole

was identified. The apparatus was then flipped such that the base of the apparatus faced

upwards. Pumping the apparatus to about 1-2 psig (which would assist in applying repair onto

the hole), a needle syringe filled with the desired volume of repair material was then inserted

halfway vertically into the TPU film. Working against gravity—which would help the injected

materials to flow through the bladder hole—, the repair material was slowly added into the TPU

interior as depicted in Figure 3-5. To ensure an even application, the syringe was rotated along

its axis, allowing the deposited material to envelop the damaged area. Once completely

deposited, the needle was carefully removed from the apparatus and the repair material was

allowed to cure for the desired amount of time. Lastly, the apparatus was flipped over once


Final Report 18

more and the sealed TPU sample was re-inflated back to operating pressure. By measuring the

dynamic pressure over time, the repair was compared the behaviors of the undamaged and the

leaking TPU bladder measurements. A successfully sealed TPU bladder will behave similarly

to the undamaged TPU bladder such that the pressure will remain relatively unchanged over a

long period of time.

Figure 3-5. Internal Adhesive Delivery against Gravity. The pressure apparatus was

flipped upside such that the chamber base was facing upwards. The syringe was inserted

vertically, penetrating past the wire mesh/Dacron fabric and into the interior of the TPU

sample. The repair material was then injected slowly into interior. To ensure even sealing, the

syringe was rotated as it was injecting material. Similarly, the injection was performed against

gravity as the force of gravity and the pressure differential between the chamber interior and

the atmosphere would force the repair material through the hole.


Final Report 19

4. BLADDER HOLE FORMATION AND CHARACTERIZATION

4.1. Undamaged TPU Leak Test

The pressure change over time in undamaged samples of TPU was first determined as a means

of forming a baseline of comparison with damaged and sealed samples of TPU. After inflating

to an operating pressure of about 8 psig, the pressure within the chamber was measured over a

20 hour period. As depicted in Figure 4-1, the normalized pressure dropped about 11.7% over

this timespan. Since each sample of TPU was tested for initial leaks with soap and water, this

slight pressure drop is attributed to the natural diffusion of air through the TPU layer. In

consideration of the fact that the previous group’s leaking samples rapidly dropped pressure

within 3 minutes, the undamaged TPU samples were considered to be at relatively steady state

pressure in comparison to the damaged and sealed samples.

Figure 4-1. Plot of Normalized Pressure Drop in an Undamaged TPU Bladder. The

normalized pressure change over 20 hours was measured for an undamaged sample of TPU

bladder. The pressure within the chamber dropped by about 11.7% over this timespan (due to

diffusion), indicating that the undamaged TPU bladder was relatively at steady state.

4.2. Damaged TPU Leak Test

With the baseline pressure drop in an undamaged TPU sample established, undamaged samples

were then punctured with a 21 Gauge (approximately 770 µm inner diameter tip) needle at

about 1 psig. As mentioned previously, the puncturing was performed at a slight pressurization

in order to simulate more realistic conditions in which holes may form. Once punctured, the

damaged TPU sample was brought back to an operating pressure of about 8 psig and allowed

to deflate. As shown in Figure 4-2, the three trials of damaged TPU samples yielded rapid

pressure drops within a few minutes, indicating air was leaking from the punctured hole due to

the pressure difference between the chamber and the atmosphere.


Final Report 20

Figure 4-2. Plot of Normalized Pressure Drop in a Damaged TPU Bladder. The

normalized pressure drop over for three samples of TPU was measured over a 10 minute time

period. Compared to the undamaged TPU sample, the pressure dropped rapidly within

minutes before arriving at a nonzero steady state.

Since the bladder hole created an open system in the pressure chamber, it was expected for

these samples to continue to leak until 0 psig. However, each sample seemed to approach a non-

zero steady state pressure. This seemingly indicated that punctured TPU samples could hold

pressure despite being damaged. To further explore this phenomena, punctured samples were

imaged under a microscope and compared with the previous group’s naturally formed bladder

hole. As shown in Figure 4-3, the holes formed by the 21 Gauge syringe needle had a vastly

different geometry from the natural hole. It was expected that puncturing the TPU with a

relatively cylindrical needle would create a circular hole. However, the imaging has instead

shown a slit with thicknesses of approximately 770 µm. It is speculated that this particular

geometry may explain why damaged TPU samples were able to reach a non-zero steady state

pressure. At high pressures— where there is higher pressure driving force for air to exit the

chamber—the slits flap back and forth, allowing air to leak out of the bladder hole. As the

pressure begins to drop, these slits will begin to flap less. Eventually, the pressure driving is

unable to overcome and push the slit open. This would allow for pressure to remain within the

chamber, leading to a nonzero steady state.

Since the pressure does rapidly decrease over time, the punctured hole provides a reasonable

model for a damaged bladder. However, this type of damage does not encompass all types of

holes that a kite surfer may encounter, as seen in the left image in Figure 4-3. On the other hand,

the punctured TPU is characteristic of holes produced via internal repair delivery. As mentioned

previously, part of the process for repair involved puncturing the Dacron fabric and TPU, which

would create additional holes within the TPU bladder. Based on the findings in Figure 4-2 and

4-3, it seems that the damages produced would be relatively minor as the formulated slits are

thin geometry and can hold nonzero pressures to some extent. It is reasonable to assume that

the internal delivery repair will be able to easily seal this slit.


Final Report 21

Figure 4-3. Natural TPU Hole vs Punctured TPU Hole. The image on the left depicts a

TPU bladder hole that was naturally created due to abrasion. The image on the right depicts

the artificial hole produced from a 770 µm needle syringe. The two geometries are vastly

different, indicating that the needle generated hole was not completely indicative of natural

bladder holes. The slit-like geometry, however, may be able to hold low pressures, which

explains the non-zero steady state that the damaged TPU samples achieved. Additionally,

these artificial holes are representative of the holes that will be produced during internal repair

delivery.


Final Report 22

5. ADHESIVE MATERIAL SELECTION AND OPTIMIZATION

5.1. Infinite Cure Time Tests

Due to their low cure times and ease of injection, Loctite™ Liquid Super Glue and FlexSeal™

were investigated for their sealing properties on TPU. To ascertain this property, 500 µL of

each adhesive was applied directly to individual samples of damaged TPU bladders and left to

cure for approximately one week (“infinite time”) in a fume hood. Once cured, the structural

integrity and success of the repair was gauged by measuring the dynamic pressure over time in

the pressure gasket chamber.

Figure 5-1. Normalized Pressure Drop for FlexSeal™ Repaired TPU. The normalized

pressure drop of a FlexSeal™ Repaired TPU sample (that was given a week to cure) was

measured over 2 minutes. Compared to an undamaged sample of TPU, which was able to

maintain a relatively stable pressure, the repaired sample began to drop pressure rapidly

within minutes. This was indicative of an unsuccessful repair.

As shown in Figure 5-1, the FlexSeal™ sealed TPU sample rapidly drops in pressure, indicating

that the repair was unsuccessful. Since the repair was directly applied onto the hole of the

damaged TPU bladder, the FlexSeal™ repair was determined to be non-sealing for holes on the

TPU bladder. Similarly, it was observed that the applied FlexSeal™ material had relatively

poor adhesion to the TPU bladder. Upon removal from the pressure chamber apparatus, the

fully cured FlexSeal™ material easily slid off the TPU, showing no signs of adherence.

In contrast, the Loctite™ Liquid Super Glue repaired TPU sample was able maintain a constant

pressure over an extended duration of time as shown in Figure 5-2. Although pressure was not

held as efficiently as an undamaged sample of TPU, Loctite™ Liquid Super Glue seemed to be

a promising choice in adhesive to use for internal repair delivery for its ability to seal as well

as its curing and viscous properties.


Final Report 23

Figure 5-2. Normalized Pressure Drop for Loctite™ Repaired TPU. The normalized

pressure drop of Loctite™ Liquid Super Glue repaired TPU (that was allowed a week to cure)

was measured over an hour. The sealing abilities of the Loctite™ Liquid Super Glue was

relatively effective as the pressure drop experience was minimal and similar in behavior to

undamaged TPU. With its low cure time and low viscosity, Loctite™ Liquid Super Glue was

a promising candidate for internal delivery repair.

5.2. Adhesive Cure Time Optimization

Given the two candidates, FlexSeal™ and Loctite™ Liquid Super Glue, the optimal cure time

for various specified volumes was determined. From initial tests to determine relative viscosity,

it was determined that the more viscous material, FlexSeal™, required a larger applied

volume—compared to Loctite™ Liquid Super Glue—in order to evenly spread around an area

of repair of the TPU. For that reason, volumes of 250, 500, and 750 µL of FlexSeal™ were

tested for intervals of 15 minutes for an hour. More particularly, an applied volume sample was

considered to be dry if no liquid was observed when probed. If any amount of liquid was

encountered, the sample was considered wet and therefore not fully cured. Table 5-1

summarizes the success of curing for each given volume and time interval.

Table 5-1. Dryness across Different FlexSeal™ Volumes.


Final Report 24

For 250 µL, the droplet was completely dried after 30 minutes of allocated cure time. This

creates a lower bound to the amount of time needed to fully cure FlexSeal™ at 250 µL. For

larger volumes, more time was required. For the 500 and 750 µL samples, FlexSeal™ was

unable to be fully cured within the hour. However, it was noted that as the allocated cure time

increased, the applied sample would retain less and less liquid as depicted in Figure 5-3. More

specifically, the base of the droplet seemed to dry unevenly. In contrast, it was observed that

the exterior of larger volume samples had properly solidified into its expected rubber structure.

These findings show concerns for the usage of larger volumes of FlexSeal™ in the internal

repair delivery.

Figure 5-3. Internal Dryness of FlexSeal™ Droplet. The cross section of a droplet of

FlexSeal™. The darker red indicates the rubberized sections of the droplet. As allocated cure

time increase, the FlexSeal™ droplet would uniformly dry.

Due to its lower relative viscosity (thus ease of spread), volumes of 100, 200, 300, and 400 µL

of Loctite™ Liquid Super Glue was checked in intervals of 15 minutes for a 45 minute duration.

As depicted by Table 5-2, all samples with an applied volume greater than 100 µL were wet,

thus not fully cured. However, for the 100 µL volume sample, a fully cured sample was

observed starting at cure times of 30 minutes. This became the lower bound of the amount of

time needed to fully cure Loctite™ Liquid Super Glue at 100 µL.

Table 5-2. Dryness across Different Loctite™ Volumes.

5.3. Baseline Integrity

Despite its relatively quick cure time, FlexSeal™ was dismissed as a potential choice for

internal repair delivery due to its lack of adhesion to TPU and its inability to seal a damaged

bladder properly. The baseline integrity of Loctite™ Liquid Super Glue, however, was tested

at 100 µL for its optimized cure time of 30 minutes and loaded with Dacron fabric onto the

pressure apparatus. Figure 5-4 indicates that for the designated volume and cure time for

Loctite™ Liquid Super Glue, the damaged TPU bladder was able to be properly sealed and

behaved similarly to the undamaged TPU sample in terms of relative pressure drop. These two

factors demonstrated that the proposed choice in adhesive and the specified repair parameters

could possibly be applied to internal repair delivery.


Final Report 25

Figure 5-4. Normalized Pressure Drop for 100 µL Loctite™ Repaired TPU. 100 µL of

Loctite™ Liquid Super Glue was applied directly to a damaged sample of TPU, in which the

location of the bladder hole was known. After curing for 30 minutes, the repaired sample was

placed onto the pressure apparatus and the normalized pressure change over time was

measured. Bladders that were sealed in this matter were able to hold pressure within the

bladder and behave almost exactly the same as an undamaged sample.

Upon removal, however, it was observed that the TPU and Dacron were stuck tightly together

likely due to contact with slightly wet Loctite™ Liquid Super Glue. Separating the two pieces

appeared to have torn off some of the repair as shown by the red circles in Figure 5-5. To verify

if the Loctite™ Liquid Super Glue repair was durable and long lasting, the integrity of the torn

Loctite™ Liquid Super Glue repair was tested once more in the pressure gasket chamber.

Figure 5-5. Loctite™ Tearing. The TPU and Dacron fabric were stuck together likely due to

some trace amounts of wet Loctite™ Liquid Super Glue as shown by the red circling. The

removal of the Dacron layer created a slight tear in the repair. The durability and integrity of

the applied repair after this tear was of concern.


Final Report 26

As demonstrated in Figure 5-6, repeating the experiment indicated that tearing could potentially

ruin the Loctite™ Liquid Super Glue sealing on the damaged TPU. Two out of the three samples

began to drop in pressure rapidly as compared to its behavior before tearing as shown in Figure

5-4. However, one sample was still able to maintain its integrity after being torn. It is speculated

that that the amount of force and the angle of separation of the TPU and Dacron fabric could

alter how significant the tear is.

Regardless, in order to ensure that the internal repair delivery was consistent, the injection

technique and the amount of volume was altered in attempt to reduce the possibility of wet

Loctite™ Liquid Super Glue adhering the TPU to the Dacron fabric. First, various injection

techniques were attempted such as revolving the syringe needle and adding incremental

volumes at various times. Revolving the needle would allow for the Loctite™ Liquid Super

Glue spread over a wider area, but with a thinner coating. This was theorized to reduce the

amount of cure time required. Likewise, by adding incremental volumes at various time

increments would allow for a layer by layer curing, which could improve the overall dryness of

the cure. Lastly, the amount of volume added was reduced from 100 µL to 80 µL. Since Table

5-2 demonstrated that smaller volumes were able to dry faster, a smaller volume could lead to

a more consistent and complete cure. These three tests were applied separately to damaged

samples of TPU and the pressure change over time was measured and rechecked after removing

the TPU from the Dacron Fabric.

Figure 5-6. Rechecking Normalized Pressure Drop for 100 µL Loctite™ Repaired TPU.

After removing the TPU from the Dacron fabric, the torn samples were tested for structural

integrity. Compared to the previous baseline test, two out of three samples were unable to

hold pressure, indicating that the TPU sample was leaking and that the repair was not durable.

This motivated further testing to the volume and injection of wet Loctite™ Liquid Super Glue

added in order to avoid this type of tearing.

The three different methods were all able to hold pressure and behave similarly like the

undamaged TPU bladder sample as demonstrated in Figure 5-7. In fact, there seemed to be no

noticeable differences between the methods as the differential change in the normalized


Final Report 27

pressure over time was approximately equivalent. However, there were noticeable changes

when these samples were rechecked after separating the TPU from the Dacron. Similarly, to the

previous 100 µL samples, the 100 µL samples for the incremental addition of volume and the

revolution of the syringe needle both experienced some adhesion between TPU and Dacron.

This created tears in the repair which caused the pressure to drop rapidly as demonstrated in

Figure 5-8. However, the 80 µL sample—which did not stick the TPU and Dacron together—

was able to hold pressure as well as it did previously in Figure 5-7. The success of sealing and

lack of tearing motivated further baseline experimentation with the application of 80 µL of

Loctite™ Liquid Super Glue.

Figure 5-7. Normalized Pressure Drop for Loctite™ Repair Variations. In order to

improve the consistency of Loctite™ Liquid Super Glue delivered repair, variations of the

100 µL repair were explored as summarized in the legend above. These variations were able

to seal a damaged bladder with virtually no differences between each other. This indicates that

these variations may not affect the repair as heavily as previously assumed.


Final Report 28

Figure 5-8. Rechecking Normalized Pressure Drop for Loctite™ Repair Variations. To

ensure the durability of the repaired TPU bladders, the TPU and Dacron were separated and

then tested once more for its integrity. Due to the tearing in the 100 µL samples (revolving

and incremental), rapid pressure drops were measured. In contrast, the 80 µL sample, which

did not tear, was able to maintain a steady operating pressure. These results suggest that a

smaller volume of Loctite™ Liquid Super Glue could lead to more consistent and durable

repairs.

Upon repetition of the 80 µL of Loctite™ Liquid Super Glue baseline test, the sealing

capabilities were determined to be relatively consistent. As demonstrated in Figure 5-9, 80 µL

of Loctite™ Liquid Super Glue samples with a 30 minute cure time were able to maintain a

steady operating pressure that was characteristic of an undamaged TPU bladder sample.

Figure 5-9. Normalized Pressure Drop for 80 µL Loctite™ Repaired TPU. To confirm

the effectiveness of the 80 µL Loctite™ Liquid Super Glue sealing, the repair was done three

times on separate samples of damaged TPU. All three samples were able to maintain a steady

pressure after the 30 minute cure time, indicating a successful repair.


Final Report 29

However, once the baseline integrity was rechecked, two of the three samples had a slight

pressure drop before reaching a steady state pressure as shown in Figure 5-10. It is speculated

that this slight pressure drop is due to a systemic relaxation in the pressure apparatus between

when the chamber is being clamped off at the desired pressure and the TPU bladder’s expansion

due to the influx of pressurized air. This phenomena indicates that the system could potentially

drop below the desired pressure if the system was inflated to its minimum operating pressure

of 8 psig. Regardless, the differential change in the normalized pressure over time for these two

samples after this systemic relaxation was similar in nature to that of the undamaged TPU

bladder, indicating that the 80 µL of Loctite™ Liquid Super Glue was able to maintain its

integrity even after removal.

Figure 5-10. Rechecking Normalized Pressure Drop for 80 µL Loctite™ Repaired TPU.

The successfully sealed 80 µL Loctite™ Liquid Super Glue samples were retested after being

separated from the Dacron. Similarly to before, there was no adhesion between the Dacron

and TPU observed. The pressure in Trial 2 and Trial 3 appeared to drop slightly before

achieving a steady state behavior similar to that of an undamaged TPU bladder. The pressure

apparatus was clamped as soon as it reached the desired operating pressure, but the TPU

sample itself was still inflating due to the flow of air into the system. This may have caused

this rapid pressure drop. Overall, the steady state behaviors of each of the three trials indicated

that the 80 µL Loctite™ Liquid Super Glue repair was consistent and durable.

5.4. Effective Adhesive Spread Area

Given the success of the 80 µL Loctite™ Liquid Super Glue trials, its spreading was explored

in order to understand its effective area of repair. Table 5-3 depicts the relative effective areas

in which 80 µL of Loctite™ Liquid Super Glue can successful seal a leaking TPU bladder.

Understanding the area of spreading provides insight onto the ranges of hole sizes and

geometries that can be repaired by 80 µL of Loctite™ Liquid Super Glue. For the self-punctured

holes (770 µm in diameter), the 80 µL repair was successful when applied directly to the leaking

hole in the TPU bladder. However, in consideration of the fact that the interval repair delivery

will be done with only the general area of leakage known, the success of the repair may suffer


Final Report 30

from inconsistencies. More specifically, difficulty in assessing the location of damaged

significantly reduces the likelihood of 80 µL of Loctite™ Liquid Super Glue being effective.

Table 5-3. Spreading Area of 80 µL Loctite™ Liquid Super Glue


Final Report 31

6. INTERNAL REPAIR METHOD EXECUTION

6.1. Blind Delivery Tests

With the desired adhesive material selected and its volume and delivery optimized for sealing

and quick cure time, the 80 µL Loctite™ Liquid Super Glue adhesive was utilized in internal

repair delivery. Differing from the baseline feasibility tests, the leaking TPU sample was

covered by Dacron and loaded onto the pressure apparatus. With only its general location

known—via soap and water spraying—the syringe punctured through the Dacron and TPU and

injected 80 µL of the Loctite™ Liquid Super Glue against gravity. After allowing the applied

Loctite Liquid Super Glue to cure within the apparatus for 30 minutes, the apparatus was

inflated to operating pressure and the normalized pressure drop in the system was measured

over time. As demonstrated in Figure 6-1, the applied Loctite™ Liquid Super Glue was able to

properly cure within the pressure apparatus and seal the damages despite only being able to

approximately locate the area of leakage in the TPU bladder. This indicates that for the given

volume, the cure time was optimal to seal the leak and the spread was wide enough to cover the

damaged hole and the punctured hole from the syringe delivering the repair. Likewise, this

confirmed the success of the proposed internal delivery method for repair.

Figure 6-1. Normalized Pressure Drop for Loctite™ Blind Testing. After optimizing the

volume and cure time of Loctite™ Liquid Super Glue, the adhesive was utilized in an internal

repair delivery, in which a syringe penetrated past the Dacron fabric and the repair was

applied internally onto TPU while against gravity. After curing for 30 minutes, the resulting

repair was able to properly seal the damaged TPU bladder and hold a steady operating

pressure similar to an undamaged sample.

However, with consecutive trial of this internal delivery repair, it was observed that the repaired

TPU bladder began to rapidly drop in pressure within minutes as demonstrated by Trial 2 in

Figure 6-2. Understanding that the 80 µL Loctite™ Liquid Super Glue would be able to

properly cure and seal a hole by the 30 minute mark, it was reasoned that the applied Loctite™

Liquid Super Glue adhesive did not spread enough onto the hole of the damaged TPU.


Final Report 32

Figure 6-2. Comparison of Successful and Unsuccessful Interval Delivery. Comparing

Trial 1 and Trial 2, the sealed TPU in Trial 2 began to rapidly drop in pressure despite being

sealed with the same technique and quantity of Loctite™ Liquid Super Glue as in Trial 1.

This was indicative of an unsuccessful repair and implied that the Loctite™ Liquid Super

Glue may not been able to properly spread onto the hole of the TPU.

Upon further inspection of the TPU bladder after it was removed from the apparatus, it was

observed that the applied Loctite™ Liquid Super Glue had completely missed the hole as shown

in Figure 6-3. When producing the hole in the TPU bladder, a 21G syringe was used to puncture

the TPU bladder within the black circle. The cured Loctite™ Liquid Super Glue was unable to

spread and seal that hole, explaining why the sealed TPU bladder still leaked after repair in

Trial 2. This raises two concerns regarding the interval repair delivery. Although 80 µL avoided

any tearing between the TPU and Dacron fabric, its spread may not always be enough to seal

the damaged TPU if the user is unable to accurately determine the location of the hole.

Likewise, the method of soap and water can only be used to determine a general location for

damages in the TPU, which creates difficulties in trying to deliver such small volumes

accurately. Aside from the issue of accuracy, the method of puncturing the Dacron fabric and

the TPU and delivering an adhesive material against gravity was able to seal any damages and

allow the TPU bladder to behave as if it was undamaged.


Final Report 33

Figure 6-3. Inaccurate Internal Delivery of 80 µL Loctite™ Liquid Super Glue. The TPU

Bladder was punctured with a 21G needle within the black circle. As this circle was covered

by the Dacron when the blind test was performed, the general area of leakage was determined

using soap and water. However, the Loctite™ Liquid Super Glue repair was unable to

properly seal the hole as the delivery had missed the hole completely. Although the interval

repair delivery method may be able to properly seal holes in damaged TPU, the issue of

accuracy with the user remains a major concern.

The efficacy of the internal repair delivery is summarized in Figure 6-4. In comparison to

undamaged TPU and damaged TPU, the 80 µL Loctite™ Liquid Super Glue repaired TPU was

able to repair the damages in the bladder and effectively prevent future leakage. The dynamic

pressure behavior was almost identical to that of the undamaged bladder, which signifies the

success of the repair. Likewise, if the repair was unsuccessful, it would be expected for pressure

to drop relatively quickly as shown in the damaged TPU bladder. If the consistency of the

delivery can be improved, these results suggest that internal repair delivery may be a viable

repair method for kite surfers to adopt.


Final Report 34

Figure 6-4. Success of Internal Repair Delivery. Comparing the undamaged TPU and

damaged TPU, the 80 µL Loctite™ Liquid Super Glue TPU behaved similarly to the

undamaged TPU, indicating the success of the repair. Given the optimized cure time and

volume, it was expected for the repair to be able to repair damages in the TPU if applied

accurately. The results confirm the efficacy of internal repair delivery and suggests that

further research into this method may lead to an efficient method of repair for kite surfers.


Final Report 35

7. CONCLUSIONS

(1) Based on the success of sealing, the team’s proposed internal repair delivery—which

involved penetrating through layers of the kite and injecting adhesive onto the interior of

the TPU bladder—serves as a novel precursor for alternative kite repair techniques that

avoids expensive and difficult repair methods that are currently utilized by kite surfers. By

standardizing the delivery technique and operations parameters such as adhesive volume

and cure time, the team’s findings have formed the foundation for future development.

(2) The adhesive chosen to demonstrate the effectivity of internal repair delivery, LoctiteTM

Liquid Super Glue, was determined to be quick curing, easily injectable (relatively low

viscosity) and able to seal damages in the TPU. The optimal quantities of the LoctiteTM

Liquid Super Glue was determined to be 80 µL cured for 30 minutes.

(3) Despite the confirmations from the Baseline Integrity tests, LoctiteTM Liquid Super Glue

was unable to seal TPU damages during internal repair delivery in the pressure apparatus.

This issue was attributed to a lack of accuracy and confidence in the location of bladder

holes. Soap and water tests could be only be used to obtain a general location of leaking air,

but it could not specifically identify where damages in the TPU bladder would be.

(4) The ability for an adhesive to seal is dependent on its allocated cure time. Likewise, smaller

volumes would take less time to fully cure. If a delivered repair was wet—such that it was

not fully cured—the repair would be ineffective as the adhesive could potentially adhere to

the Dacron fabric. More importantly, an uncured delivery will be unable to properly seal

the holes in the Dacron fabric, thus the bladder will continue to leak.

(5) Punctures created by the syringe needle formed a slit-like geometry as opposite to the

expected circular geometry demonstrated by the previous team. These punctures were found

to maintain a nonzero steady state gauge pressure. Although this puncture cannot be

representative of all potential holes native to kite surfing, the observed geometry indicates

that the puncture created by the internal repair delivery will be slit-like in nature. Due to its

thinness, it was speculated that these slits would be easily sealed up by the adhesive. This

theory is backed up by the coverage of spreading of LoctiteTM Liquid Super Glue that was

measuring during the Baseline Integrity Tests.


Final Report 36

8. RECOMMENDATIONS

(1) For the given parameters for the LoctiteTM Liquid Super Glue, internal repair delivery via

needle syringe was able to successfully seal kite bladder holes and allow the bladder to

function as well as an undamaged TPU bladder. However, the consistency of the success

indicates that there is room for improvement in the repair process. A major flaw in the

current repair process is the procedure in which leaks are identified in a leaking TPU

bladder. Soapy water is sprayed onto the surface of the Dacron fabric and the subsequent

bubbling would indicate the location of the bladder hole. However, this is only provides a

general approximation of where the syringe should penetrate through. Given the small

volumes of adhesive—due to the difficulties of cure time—the repair will likely not

consistently work as expected. Likewise, the efficacy of the repair must be examined under

various types of stresses. As the kite surfer needed maneuver the kite in order to move in

the water, the TPU bladder may be subjected to various types of stresses and strains. The

durability of the internal repair delivery must be tested under these conditions before

commercialization can be considered.

(2) Following up on the need to further improve the repair process, the current choice in

LoctiteTM Liquid Super Glue requires a small volume of 80 µL and cure time of 30 minutes.

In consideration of the kite surfer, this cure time may still be considered to be too long. The

idea of internal repair delivery is to avoid the need to remove the bladder from the Dacron

leading edge, but also provide a quick method of repair that kite surfers can utilize while on

the beach. Likewise, the small volume may be difficult for the user to work with. Aside

from accuracy, the technique also requires the user to rotate the needle along its axis to

promote an even spreading. Given the amount of volume, a user could potentially apply too

much force and fail to spread the adhesive properly. Explorations into other types of quick

curing adhesives may be useful in fixing these major issues. In these regards, the chemical

safety of the adhesive must also be take into consideration. The previous team’s selected

repair involved using TPU dissolved in DMF. While this would be the optimal choice of

repair material to seal holes in a TPU bladder, the choice in solvent cannot be used in a

commercial setting. Lastly, adding automations to the adhesive injection may also assist in

ensuring that the desired repair material would be able to properly spread and seal holes in

the TPU bladder.

(3) The current proposed method uses a needle syringe to puncture the Dacron fabric and the

TPU bladder in order to deliver the LoctiteTM Liquid Super Glue to the desired area of

repair. There are concerns regarding the safety of using syringe needles commercially. A

kite surfer can easily be stabbed by the sharp point on the needle and possibly inject the

adhesive material into their bodies. In order ensure the safety of the user, swapping out the

syringe needle with a blunter needle may be a viable alternative. Based on puncture

calibrations done at various operating pressures (See Appendix B), blunt needles may also

be able to properly puncture the Dacron fabric and the TPU bladder. If this is done, the

geometry of the punctures must be re-characterized to determine whether or not nonzero

pressure steady states can be still observed.

(4) Lastly, although the possibility of internal repair delivery has been established by the

project’s findings, all testing has been done under a laboratory scale model of the kite

bladder. Upon scale up, the properties of bladder leaking and repair may differ. Based on

the previous team’s measurements of the pressure apparatus, the effective volume within

the chamber was approximately 88.25 cm2. Although the size of the TPU bladder differs


Final Report 37

between manufacturers and type of kite, the effective volume is approximately on the scale

of 2000 cm2. Assuming the ideal gas law, the dynamic change in pressure over time in the

larger TPU bladder will be less prominent than in the pressure apparatus. As a result, it may

be difficult to observe bubbling when using the soap and water test. Likewise, minor holes

in TPU bladders may not contribute as much to the drops in the operating pressure as

originally suggested by the project’s consultant. Similarly, holes that can be identified with

the soap and water test may be much larger in size than what was tested on the lab scale

(approximately 770µm). There may be benefit in exploring the limits of damaged TPU hole

size on the efficacy of internal repair delivery. Although a larger volume can be used to

induce greater spreading of the repair, the success of this larger volume is greatly hindered

by the cure time. In order to streamline the process for the user, optimal volume and cure

times should be determined for various types of hole sizes and geometries.


Final Report 38

9. REFERENCES

[1] Events Committee, ISAF Kiteboarding Format Trials; Santander, Spain, 2012.

[2] Dunoyer, J., ODL, MIT Video Production (MVP), personal communication, 15 February

2019.

[3] The Lubrizol Corporation. “What Is Thermoplastic Polyurethane (TPU).” Lubrizol,

www.lubrizol.com/Engineered-Polymers/Overview/What-is-TPU. (n.d.)

[4] American Chemistry Council. Introduction to Polyurethanes: Thermoplastic

Polyurethanes. polyurethane.americanchemistry.com/polyurethanes/Introduction-

toPolyurethanes/Applications/Thermoplastic-Polyurethane/. (n.d.)

[5] Klee, D.; Samuel, A.; Stinn, C. Team 1 Final Report: Leak Repair in Kite Surfing

Bladders, 10.26 Chemical Engineering Projects Laboratory, May 2017.

[6] Troughton, M. J. Chapter 16 – Solvent Welding, Handbook of Plastics Joining (Second

Edition), May 2009, 139–143.

[7] Huntsman Corporation. A Guide to Thermoplastic Polyurethanes. Huntsman,

http://www.huntsman.com/portal/page/portal/polyurethanes/Media%20Library/global/files/gu

ide_tpu.pdf. (n.d.)

[8] Maramenides, D., Epic Kites Kiteboarding, personal communication, 25 February 2019.

[9] Encyclopedia Britannica, Inc. Polyethylene Terephthalate: Structure, Properties, & Uses,

https://www.britannica.com/science/polyethylene-terephthalate, 28 April 2019.

[10] Ellsworth Adhesives. Loctite Design Guide for Bonding Plastics, vol 6, 2011.

https://www.ellsworth.com/globalassets/literature-library/manufacturer/henkel-loctite/henkel-

loctite-design-guide-plastic-bonding.pdf, 15 May 2019.

[11] Home Effect. Flex Seal Products Review: Flex Shot, Flex Glue, Flex Tape, Flex Liquid,

15 Nov. 2018, homeffect.com/flex-seal/.


Final Report 39

10. APPENDIX A

In order to properly measure the size of the punctured holes, a calibration slide of known size

was utilized to determine a scaling of the punctures. Every image taken by the microscope

was done at a x10 zoom.

Figure A-1. Microscope Calibration Slide. By determining the scaling of the microscope,

an aspect ratio was determined for images seen under the microscope compared to the

sample’s actual size. This allowed for the determination of size of holes produced by

puncturing.


Final Report 40

11. APPENDIX B

Various needle gauges (inner diameter of needle tip) were used to puncture the TPU bladder

at the upper and lower limits of kite surfing operating pressure (8-12 psig). The smallest

needle gauge (310 µm) was able to penetrate the TPU bladder at 8 and 12 psig. It was then

assumed that since the smallest gauge needle—which would be the most brittle and

susceptible to breaking—was able to successfully penetrate the TPU, larger gauge needles

would be able to achieve the same. Provided below is a summary of the findings as well as

microscope imaging of the punctured TPU samples. There is virtually no different between

the two pressures, indicating the success of using the smallest gauge needle.

Table B-1. Summary of Needle Puncturing Success at Various Pressures

Figure B-1. 30 G Needle (310 µm) Puncture Imaging. The image on the left depicts a

puncture at 8 psig. The image on the right depicts a puncture at 12 psig. The 30 G needle was

able to puncture the TPU at varying operating pressures with ease. Likewise, there are

minimal differences in the geometry of the hole produced.


Final Report 41

12. APPENDIX C

In order to calculate the effective spreading of LoctiteTM Liquid Super Glue, the dried LoctiteTM

Liquid Super Glue area (after inject for the Baseline Integrity tests) was imaged. To simplify

area calculations, the dried area was determined using rectangles as their approximate shape.

Figure C-1. LoctiteTM Liquid Super Glue Spreading Area. Using a metric ruler as our

scaling factor, the area of spreading for LoctiteTM Liquid Super Glue was determined using

approximate rectangles. This area provided a reasonable estimate for how effective and

accurate the LoctiteTM Liquid Super Glue would be during internal repair delivery.

Documents

Leak Repair in Kite Surfing Bladders