6.4 Bonding and sealing

Bonding and sealing are crucial in soft robotics, enabling the creation of functional structures by joining different components and materials. The choice of bonding method depends on the materials, required strength, flexibility, and operating environment of the soft robot.

Chemical, physical, and adhesive bonding are key techniques used in soft robotics. Proper surface preparation, adhesive selection, and joint design are essential for creating strong, durable bonds. Sealing techniques and leak testing methods ensure the integrity of pneumatic actuators and fluidic systems.

Types of bonding

• Bonding is a critical aspect of soft robotics, enabling the joining of different components and materials to create functional structures
• The type of bonding used depends on the materials being joined, the required strength and flexibility of the bond, and the operating environment of the soft robot

Chemical bonding

Top images from around the web for Chemical bonding

• 

  Chemically induced repair, adhesion, and recycling of polymers made by inverse vulcanization ... View original

  Is this image relevant?

• 

  Frontiers | Soft Self-Healing Nanocomposites View original

  Is this image relevant?

• 

  Frontiers | Covalently Linked Organic Networks View original

  Is this image relevant?

• 

  Chemically induced repair, adhesion, and recycling of polymers made by inverse vulcanization ... View original

  Is this image relevant?

• 

  Frontiers | Soft Self-Healing Nanocomposites View original

  Is this image relevant?

1 of 3

Top images from around the web for Chemical bonding

• 

  Chemically induced repair, adhesion, and recycling of polymers made by inverse vulcanization ... View original

  Is this image relevant?

• 

  Frontiers | Soft Self-Healing Nanocomposites View original

  Is this image relevant?

• 

  Frontiers | Covalently Linked Organic Networks View original

  Is this image relevant?

• 

  Chemically induced repair, adhesion, and recycling of polymers made by inverse vulcanization ... View original

  Is this image relevant?

• 

  Frontiers | Soft Self-Healing Nanocomposites View original

  Is this image relevant?

1 of 3

• Involves the formation of covalent, ionic, or metallic bonds between atoms or molecules of the materials being joined
• Typically results in strong, permanent bonds that are resistant to separation
• Examples of chemical bonding in soft robotics include vulcanization of elastomers (sulfur crosslinking of rubber) and curing of thermoset polymers (epoxy, silicone)

Physical bonding

• Relies on intermolecular forces such as van der Waals forces, hydrogen bonding, or electrostatic interactions to hold materials together
• Generally weaker than chemical bonds but can still provide sufficient strength for many soft robotic applications
• Examples include the use of thermoplastic elastomers (TPEs) that can be heat-sealed or welded together, and the use of gecko-inspired dry adhesives that rely on van der Waals forces

Adhesive bonding

• Uses a separate adhesive material to join two surfaces together
• Adhesive bonds can be either chemical (reactive adhesives that cure) or physical (pressure-sensitive adhesives that rely on surface interactions)
• Widely used in soft robotics due to the ability to bond dissimilar materials, distribute loads evenly, and provide flexible joints
• Examples include the use of silicone adhesives to bond silicone elastomers, and the use of polyurethane adhesives to join various polymers and fabrics

Bonding materials

• The selection of bonding materials is crucial for creating strong, durable, and functional bonds in soft robotic systems
• Bonding materials must be compatible with the substrates being joined, provide the necessary strength and flexibility, and withstand the operating environment

Elastomers for bonding

• Elastomers are polymers with high elasticity and the ability to recover from large deformations, making them ideal for flexible bonds in soft robotics
• Can be bonded to themselves or other materials using adhesives, heat sealing, or chemical crosslinking
• Examples include natural rubber, synthetic rubbers (nitrile, neoprene), and thermoplastic elastomers (TPEs) such as thermoplastic polyurethanes (TPUs) and styrenic block copolymers (SBCs)

Silicone rubbers for bonding

• Silicone rubbers are synthetic elastomers with excellent heat resistance, chemical stability, and biocompatibility
• Can be formulated with various curing chemistries (platinum-catalyzed, condensation, peroxide) and hardnesses (shore A durometer) to suit different bonding requirements
• Commonly used in soft robotics for bonding, sealing, and encapsulation of components
• Examples include room temperature vulcanizing (RTV) silicones, high consistency rubber (HCR) silicones, and liquid silicone rubbers (LSRs)

Polyurethanes for bonding

• Polyurethanes are versatile polymers that can be formulated as elastomers, adhesives, coatings, and foams
• Offer good balance of strength, flexibility, and adhesion to various substrates
• Can be two-component (polyol and isocyanate) or one-component (moisture-curing) systems
• Examples in soft robotics include polyurethane adhesives for bonding elastomers and fabrics, and polyurethane coatings for creating flexible, abrasion-resistant surfaces

Surface preparation techniques

• Proper surface preparation is essential for achieving strong, durable bonds in soft robotic systems
• The goal of surface preparation is to remove contaminants, create a suitable surface texture, and promote adhesion between the bonding material and the substrate

Cleaning surfaces before bonding

• Removing dirt, oils, and other contaminants from surfaces is critical for achieving good adhesion
• Methods include solvent cleaning (isopropyl alcohol, acetone), detergent washing, and plasma cleaning
• The choice of cleaning method depends on the type of contamination and the sensitivity of the substrate to various cleaning agents

Roughening surfaces for bonding

• Creating a roughened surface can increase the surface area for bonding and provide mechanical interlocking between the adhesive and substrate
• Methods include sanding, abrasive blasting (sand, glass beads), and chemical etching (acid etching for metals, plasma etching for polymers)
• The degree of roughness required depends on the bonding material and the desired bond strength

Priming surfaces for adhesion

• Applying a primer to the surface can improve adhesion by creating a compatible interface between the substrate and the bonding material
• Primers can be chemical (silane coupling agents, titanate coupling agents) or physical (plasma treatment, corona discharge treatment)
• The choice of primer depends on the chemical nature of the substrate and the bonding material, as well as the environmental conditions the bond will be exposed to

Adhesive selection criteria

• Choosing the right adhesive is critical for creating strong, durable, and functional bonds in soft robotic systems
• Factors to consider when selecting an adhesive include the substrates being bonded, the required strength and flexibility of the bond, and the operating environment

Strength requirements for bonded joints

• The adhesive must provide sufficient strength to withstand the loads and stresses the bonded joint will experience during operation
• Different types of strength to consider include shear strength, tensile strength, and peel strength
• The required strength will depend on the specific application and the design of the bonded joint (lap shear, butt joint, etc.)

Flexibility needs for bonded interfaces

• In soft robotics, bonded interfaces often need to be flexible to allow for the desired motion and deformation of the system
• The adhesive should have a suitable modulus (stiffness) and elongation to match the flexibility of the substrates being bonded
• Examples of flexible adhesives include silicone adhesives, polyurethane adhesives, and some epoxies formulated with flexibilizers

Environmental resistance of adhesives

• The adhesive must be able to withstand the environmental conditions the bonded joint will be exposed to, such as temperature, humidity, chemicals, and UV radiation
• Different adhesives have varying levels of resistance to these factors, and the choice of adhesive will depend on the specific operating environment
• Examples of environmentally resistant adhesives include silicone adhesives (high temperature and chemical resistance), epoxies (chemical and moisture resistance), and UV-curable adhesives (for applications exposed to sunlight)

Adhesive application methods

• The method used to apply an adhesive can impact the quality and consistency of the bonded joint
• Factors to consider when selecting an application method include the viscosity of the adhesive, the size and geometry of the parts being bonded, and the production volume

Dispensing adhesives

• Dispensing involves applying the adhesive using a syringe, cartridge, or automated dispensing system
• Suitable for low to medium viscosity adhesives and for applications requiring precise control over the amount and placement of the adhesive
• Examples include time-pressure dispensing, positive displacement dispensing, and jet dispensing

Spraying adhesives

• Spraying involves atomizing the adhesive into a fine mist and applying it to the surface using a spray gun or automated spraying system
• Suitable for low viscosity adhesives and for covering large surface areas quickly and evenly
• Examples include conventional air spray, airless spray, and high-volume low-pressure (HVLP) spray

Brushing on adhesives

• Brushing involves applying the adhesive using a brush, roller, or other manual applicator
• Suitable for high viscosity adhesives and for small-scale or prototype applications where automation is not necessary
• Examples include brushing on paste adhesives, applying adhesive tapes, and using adhesive films or preforms

Curing processes

• The curing process is the transformation of an adhesive from a liquid or semi-solid state to a solid state through chemical reactions or physical changes
• The choice of curing process depends on the type of adhesive, the required curing time and temperature, and the production environment

Room temperature curing

• Some adhesives, such as silicones and cyanoacrylates, can cure at room temperature through exposure to moisture or other chemical activators
• Room temperature curing is convenient for many soft robotic applications, as it does not require specialized equipment or high temperatures
• However, room temperature curing can be slower than other methods and may be sensitive to environmental conditions such as humidity

Heat curing adhesives

• Many adhesives, such as epoxies and polyurethanes, require heat to initiate and accelerate the curing reaction
• Heat curing can be done using ovens, heating blankets, or other specialized equipment
• Heat curing allows for faster curing times and can improve the strength and chemical resistance of the bonded joint
• However, heat curing may not be suitable for temperature-sensitive substrates or large, complex structures

UV curing adhesives

• UV curing adhesives contain photoinitiators that trigger the curing reaction when exposed to ultraviolet light
• UV curing is fast (often seconds to minutes) and can be done at room temperature, making it suitable for high-volume production and temperature-sensitive substrates
• Examples of UV curing adhesives include acrylics, epoxies, and silicones
• However, UV curing requires access to the bondline for the UV light to penetrate, which may not be possible in some joint designs

Joint design considerations

• The design of the bonded joint can have a significant impact on the strength, durability, and performance of the soft robotic system
• Factors to consider when designing bonded joints include the type of joint, the load distribution, and the failure modes

Lap shear joints

• Lap shear joints involve overlapping the two substrates being bonded, with the adhesive applied between the overlapping surfaces
• Lap shear joints are commonly used in soft robotics due to their simplicity, high shear strength, and ability to distribute loads evenly
• The strength of a lap shear joint depends on the overlap length, the adhesive thickness, and the properties of the substrates and adhesive
• Examples of lap shear joints in soft robotics include bonding of pneumatic actuators, attachment of sensors and electronics, and joining of modular components

Butt joints

• Butt joints involve joining two substrates end-to-end, with the adhesive applied to the mating surfaces
• Butt joints are less common in soft robotics due to their lower strength compared to lap shear joints, as the load is concentrated at the bondline
• However, butt joints may be necessary in some applications where overlapping of substrates is not possible or desirable
• The strength of a butt joint can be improved by using a higher-strength adhesive, increasing the bonding surface area, or using mechanical aids such as pins or reinforcements

Peel strength of joints

• Peel strength refers to the resistance of a bonded joint to peeling forces, which can cause the adhesive to separate from one of the substrates
• Peel forces are common in soft robotic applications where the bonded joint undergoes bending or flexing, such as in hinges or flexible connections
• The peel strength of a joint depends on the adhesive properties (peel adhesion, cohesive strength), the substrate properties (surface energy, flexibility), and the joint design (angle of peel, thickness of substrates)
• Improving peel strength can involve using a more flexible adhesive, optimizing the joint geometry to minimize peel forces, or using mechanical interlocking features to resist peeling

Sealing techniques

• Sealing is the process of creating a barrier to prevent the passage of fluids, gases, or other contaminants between two surfaces or compartments
• In soft robotics, sealing is critical for creating airtight or watertight structures, such as pneumatic actuators, fluidic channels, and protective enclosures

Gaskets for sealing

• Gaskets are deformable materials (rubber, silicone, foam) that are compressed between two surfaces to create a seal
• The effectiveness of a gasket seal depends on the gasket material, the compressive force applied, and the surface finish and flatness of the mating surfaces
• Examples of gasket seals in soft robotics include sealing of pneumatic fittings, creating watertight housings for electronics, and sealing modular components

O-rings for sealing

• O-rings are circular cross-section elastomeric seals that are compressed between two surfaces to create a seal
• O-rings are commonly used in soft robotics for sealing moving parts (pistons, shafts) and static connections (tubes, fittings)
• The effectiveness of an O-ring seal depends on the O-ring material, the compression ratio, the surface finish of the mating surfaces, and the presence of lubricants
• Examples of O-ring seals in soft robotics include sealing of pneumatic cylinders, creating rotary joints, and sealing fluidic connectors

Liquid sealants

• Liquid sealants are flowable materials (silicone, polyurethane, anaerobic) that are applied to a joint or gap and cure to form a solid, flexible seal
• Liquid sealants are useful for sealing complex geometries, filling gaps, and bonding dissimilar materials
• The effectiveness of a liquid sealant depends on the sealant material, the curing mechanism, the surface preparation, and the joint design
• Examples of liquid sealants in soft robotics include sealing of wires and cables, creating waterproof enclosures, and bonding and sealing in a single step

Leak testing methods

• Leak testing is the process of detecting and measuring the rate of fluid or gas leakage from a sealed system
• In soft robotics, leak testing is important for ensuring the integrity and performance of pneumatic actuators, fluidic channels, and other sealed components

Pressure decay leak testing

• Pressure decay leak testing involves pressurizing the system with air or another gas and monitoring the pressure over time
• A decrease in pressure indicates a leak, and the rate of pressure decay can be used to quantify the leak rate
• Pressure decay testing is a simple and widely used method, but it may not be suitable for detecting very small leaks or leaks in large-volume systems

Bubble emission leak detection

• Bubble emission leak detection involves submerging the pressurized system in water or another liquid and observing for bubbles emerging from the leak site
• This method is useful for pinpointing the location of leaks and detecting small leaks that may not be detectable by pressure decay testing
• However, bubble emission testing requires access to the entire system and may not be practical for large or complex structures

Tracer gas leak testing

• Tracer gas leak testing involves filling the system with a tracer gas (helium, hydrogen) and using a gas detector to locate and measure leaks
• Tracer gas testing is highly sensitive and can detect very small leaks, making it suitable for critical applications such as medical devices and aerospace components
• However, tracer gas testing requires specialized equipment and may be more expensive and time-consuming than other methods

Troubleshooting bonding issues

• Bonding issues can arise due to various factors, such as improper surface preparation, incompatible materials, incorrect adhesive selection, or poor joint design
• Troubleshooting bonding issues requires a systematic approach to identify the root cause and implement corrective actions

Adhesive failure modes

• Adhesive failures can occur in different modes, depending on the location and mechanism of the failure
• Cohesive failure occurs within the bulk of the adhesive, indicating that the adhesive strength is the limiting factor
• Adhesive failure occurs at the interface between the adhesive and the substrate, indicating poor surface preparation or incompatible materials
• Mixed-mode failure involves a combination of cohesive and adhesive failure, suggesting a complex interplay of factors

Cohesive failure vs adhesive failure

• Distinguishing between cohesive and adhesive failure is important for identifying the root cause of the bonding issue
• Cohesive failure suggests that the adhesive itself is the weak link, and may require a different adhesive formulation or curing process
• Adhesive failure suggests that the bond between the adhesive and the substrate is the problem, and may require improved surface preparation, priming, or material selection
• Mixed-mode failure suggests that multiple factors may be contributing to the bonding issue, and may require a combination of corrective actions

Identifying root causes of failures

• Identifying the root cause of a bonding failure requires a thorough analysis of the failed joint, the materials involved, and the processing conditions
• Techniques for failure analysis include visual inspection, microscopy, spectroscopy, and mechanical testing
• Common root causes of bonding failures include contamination, moisture, thermal stresses, inadequate curing, and design flaws
• Once the root cause is identified, corrective actions can be implemented to improve the bonding process and prevent future failures
• Examples of corrective actions include optimizing surface cleaning and priming procedures, selecting a more suitable adhesive or substrate material, modifying the joint design to minimize stress concentrations, and implementing better process controls and quality assurance measures