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Adhesives in the Marine Industry
adhesion& bonding Adhesives in the Marine Industry
A
dhesive bonding is becoming a significant joint process in the marine industry, although it has been slower to develop than in other transportation sectors. Much of this advance is due to the use of newer substrates especially in powerboats and recreational boating and the realization that adhesive bonding is a lightweight and corrosion resistant means of joining these materials. This article is an overview of the use of structural adhesives in the marine industry. The effects of substrate materials, processing requirements, and environments common to the marine industry are also addressed.
HISTORICAL PERSPECTIVE With certain exceptions (e.g., pleasure boat manufacturers, high-performance sailboats), the marine sector has been relatively conservative when it comes to adopting adhesives. The large shipbuilding industry has employed adhesives for many years with success in non-load bearing applications. Interest is now shifting toward using adhesive bonding for more structural functions. Traditionally, welding is the primary process for joining the different structural parts in a large ship. Some of this conservatism is a consequence of the reluctance of the world’s military naval forces to use adhesives because of: (1) flammability in war time situations, even when fully cured and (2) potential toxic outgassing in enclosed areas (e.g., submarines). This conservatism extends to most polymeric materials, including composites. But the greatest influence on the unwillingness to www.metalfinishing.com
tion of small marine craft. This is mainly because lightweight materials, such as composites and aluminum, are used instead of steel, and adhesives are a reliable and efficient method of joining these materials.
use adhesives in marine applications has been the recognized degenerative effect that moisture has on the strength of an adhesively bonded joint.1 Yet, another factor for the slow growth of adhesives in the marine industry is the unique manufacturing environment that exists at many shipyards and plants. Chemical substrate treatments are commonly used in the aerospace and even automotive industries to improve the durability of adhesive joints. However, often the shipbuilding industry cannot afford such high-quality surface preparation, and substrate processing has to remain basic. Another manufacturing problem is the hostile nature of the shipyard. Curing temperatures may vary significantly with climatic temperatures, and there are many sources for contamination and weak boundary layer to develop. Adhesive bonding has generally been better accepted in the construc-
VALUE PROPOSITION Even with this historical perspective, the advantages of adhesive bonding are beginning to be noticed across the breadth of the marine industry. Adhesive bonding is being adopted generally in the following applications: • Joining of metals for increased fatigue life • Joining of polymer composites • Joining of dissimilar substrates • Reducing weight to improve speed and fuel performance • Improving shop safety • Lowering manufacturing costs and improving production efficiency Adhesive bonding is expected to provide both improved strength properties and save time and costs. The major property advantages of adhesive bonded joints (Table 1) have always
ADVANTAGES
DISADVANTAGES
• Provides attractive strength-to-weight ratio • Provides large stress-bearing areas • Provides excellent fatigue resistance • Heat, if required, is too low to affect metal part • Damps vibration and absorbs shock • Minimizes or prevents galvanic corrosion between dissimilar metals • Joins all shapes and thicknesses and any combination of similar or dissimilar materials • Provides aero- and hydro-dynamically smooth contours • Seals joints
• • • • •
Surfaces must be cleaned Long cure times may be necessary Heat and pressure may be required Jigs and fixtures may be required Rigid process control usually necessary; special training sometimes required • Inspection of finished joints is generally difficult • Useful life depends on the environment
Table 1. Advantages and Disadvantages of Adhesive Bonding in Marine Structures November/December 2013 | metalfinishing | 47
adhesion& bonding TEST TYPE
CONDITION
REQUIREMENT
Lap shear testing
Control (room temperature)
Manufacturer’s values achieved
Elevated temperatures Cold temperatures Cleavage peel testing
Control (room temperature)
Manufacturer’s values achieved
Elevated temperatures Cold temperatures Fatigue testing
ASTM D3166
Target value achieved (e.g., 107 cycles)
Immersion tests
Water
Minimum percentage retention achieved
Fuel Glass transition temperature
ASTM E1356 (calorimetry)
Durability: lap shear per ASTM D1002 or D3163
Loaded to manufacturer’s minimum specified value and weathered per ASTM D1183: • 28 days • 56 days
• Mean > 90% reference • Mean >95% 28 day value
Loaded to manufacturer’s minimum specified value and weathered per ASTM D1183: 28 days 56 days
• Mean > 90% reference • Mean >95% 28 day value
Durability: cleavage fracture per ASTM D433
Above minimum value (e.g., 70°C for military ships)
Table 2. Typical Requirements for Approval of Marine Adhesives
been visible to ship designers through the historical development of adhesives in the aerospace and automotive industry, but it has only been recently that the production advantages of adhesive bonding are also becoming evident to the shipbuilder. Adhesive bonding offers shipbuilders the opportunity to replace welding of steel structures to reduce distortion, effectively eliminate residual stress, and improve fatigue performance. Avoidance of high welding temperatures leads to safer construction in hazardous environments. Adhesive bonding of composites and aluminum also provides well-distributed loading and maximizes the utilization of the adherend material. It has been estimated that adhesive bonding could provide a cost saving
of at least 20 percent for fastening supports, stiffeners, and other attachments in outfitting of large passenger ships.2 For a patrol craft, reductions in building cost through the use of adhesive bonding in the superstructure can be expected to be around 25 to 30 percent. For fast ferries, an important benefit is a weight reduction of the structure by about 4.5 to 9 tons. Over a period of time these weight reductions can save significant amounts of diesel fuel. An added benefit of adhesive bonding is that it will also make positive contributions to the preservation and improvement of the quality of the environment by reducing the amount of welding slag created. A mediumsized shipyard generates about 60 tons per year of welding slag, which must
48 | metalfinishing | November/December 2013
be disposed of as a controlled waste. It also should be noted that “marine adhesives” do not only encompass water transportation vehicles. Offshore structures such as drilling platforms and wind turbines and near-to-shore structures such as piers, piping, and other miscellaneous construction are also viable candidates for adhesive bonding. These applications often have the same substrates and performance requirements as do marine vessels.
REQUIREMENTS OF A MARINE ADHESIVE The marine industry presents design engineers with what are often extremely diverse bonding challenges. One application may require the ultimate in adhesion performance, and another application may require aesthetic requirements and good aero- or hydro-dynamic design. Yet another application may require vibration damping and the ability to take repeatedly high impact forces. The adhesive must also, of course, last the life of the ship, and not degrade in the aggressive marine environments. When choosing an adhesive for any particular marine application, a number of factors need to be considered. The primary factors include the type of materials to be bonded, production conditions available for cure of the adhesive, and the required performance in the service environment. All marine structures require certification by societies such as the American Bureau of Shipping (ABS), Bureau Veritas, Lloyds, Det Norske Veritas (DNV), etc. Several of these societies have requirements for adhesives that are used in assembly. Many adhesive suppliers indicate certification authority approval for marine use in their product data sheets. Table 2 shows typical test requirements. For small boats up to 24m long structural requirements are provided by ISO 12215-6. STRUCTURAL MARINE ADHESIVES Two-part, room temperature reactive adhesive systems such as epoxy, polyurethane, and thermosetting acrylic are popular in structural marine applications. The popularity is mainly because www.metalfinishing.com
adhesion& bonding the formulations can be tailored to suit particular working conditions or application requirements. Often the resin and hardener ratios can be tailored to meet differing application conditions or to meet simple mechanical properties, such as tensile-shear strength, toughness, or peel strength. Epoxy adhesives are well-known two-part materials with wide applicability. They cure at room temperature or with a mild heat bake cycle. Heat curing generally improves the properties of the resulting bond. Depending on the type of epoxy base polymer and the formulation, epoxy adhesives have good resistance to creep, moisture, and most chemicals common to the marine environment. They also have a low degree of shrinkage on cure and excellent high temperature properties. The major disadvantage of epoxy adhesives is that they generally require vigorous surface preparation. Mix ratio and degree of mixing also have to be carefully controlled. Polyurethane adhesives are some-
what less well known than epoxies in the marine industry, but they have similar properties. Polyurethane adhesives can be cured at room temperature and are available in a broad range of cure rates. They have a high degree of adhesion to composites and most high surface energy thermoplastics, but often require a primer to bond to metal substrates. The major difference of polyurethanes relative to epoxy adhesives is that they can be formulated to range from hard and rigid to flexible and tough. Because of their flexibility, polyurethane adhesives are generally the best for bonding two different materials. They are also generally the adhesives of choice when a great deal of flexibility or impact resistance is required. Polyurethane adhesives provide a very high degree of bond strength to wood because of their reactivity to cellulose molecules and because of their easy penetration into porous substrates. Among the several disadvantages of polyurethane adhesives is that they
BASE POLYMER
CHARACTERISTICS
Polysulfide
Excellent adhesion and flexibility. Can take up to 25% elongation. Allows for movement associated with stress and temperature change. Slow curing; often takes a week or more to fully cure. Once cured, it can be sanded and painted.
Silicone
Excellent adhesion and flexibility. Ideal electrical insulator and waterproofing agent for wiring, windshields, ports, and emergency gaskets. Temperature resistance to 200°C. Do not use in areas that require sanding or painting.
Butyl
Cures fast and adheres well to most materials including hard-to-bond plastics. Not sandable but can be painted. Does not have the elongation that polysulfide or silicone sealants have. Of the five types of sealants, butyls have the shortest service life.
Acrylic
Ideal for bedding on wood, fiberglass, and metals. Skins over quickly and can be painted about 30 mins after application. Water dispersed formulations are available, but these generally have poorer moisture resistance than solvent formulations. Not generally considered as long lasting as polysulfide, polyurethane, or acrylic.
Polyurethane
Often thought of as an adhesive rather than a sealant because of its high tensile strength and outstanding adhesion especially to wood and composites. Resistance to high humidity is not as good as silicone or polysulfide. Cure time is shorter than polysulfide, but still may require up to a week.
Table 3. Characteristics of Common Marine Sealants www.metalfinishing.com
have relatively poor resistance to elevated temperatures. For example, they are not suitable for applications in the engine compartment. Polyurethanes, depending on the formulation, also have less hydrolytic stability and UV resistance than most epoxy adhesives. Thermosetting acrylic adhesives are less common than epoxy or polyurethane adhesives but they offer several unique advantages. Acrylic adhesives offer the fastest room temperature cure while maintaining adequate working time. They are available either as two-component systems or as a one-component product with a reactive primer. The nature of their curing mechanism provides great tolerance to variations in mix ratio and climatic conditions during cure. Acrylics have excellent resistance to UV, moisture, and general outdoor conditions. They bond very well to difficult substrates with a minimum amount of surface preparation. Thus, acrylic adhesives are often used to bond thermoplastics, metals, and composites. The main disadvantages of acrylic adhesives are that they are flammable in the uncured state and have a characteristic acrylic monomer odor. Some thermosetting acrylics may have cure rates that are too fast and do not allow accurate positioning of parts before the material begins to gel.
REFERENCES 1 Petrie, E.M., “How Moisture Effects Adhesives, Sealants, and Coatings”, Metal Finishing, October 2011. 2 “European Shipyards Can Become Competitive and More Environmentally Friendly”, Innovations Report, www.innovations-report.de, August 2005. 3 Davis, P., “Marine Industry”, Chapter 48 in Handbook of Adhesion Technology, da Silva, Ochsner, and Adams (eds.), Springer-Verlag, Heidelberg, 2011, pp. 1239-1240. BIO Edward M. Petrie is the sole proprietor of EMP Solutions, a Cary, N.C.–based consulting firm focused on solving problems in the adhesives and sealants industry. Visit www.specialchem4adhesives.com.
November/December 2013 | metalfinishing | 49
A
dhesive bonding is becoming a significant joint process in the marine industry, although it has been slower to develop than in other transportation sectors. Much of this advance is due to the use of newer substrates especially in powerboats and recreational boating and the realization that adhesive bonding is a lightweight and corrosion resistant means of joining these materials. This article is an overview of the use of structural adhesives in the marine industry. The effects of substrate materials, processing requirements, and environments common to the marine industry are also addressed.
HISTORICAL PERSPECTIVE With certain exceptions (e.g., pleasure boat manufacturers, high-performance sailboats), the marine sector has been relatively conservative when it comes to adopting adhesives. The large shipbuilding industry has employed adhesives for many years with success in non-load bearing applications. Interest is now shifting toward using adhesive bonding for more structural functions. Traditionally, welding is the primary process for joining the different structural parts in a large ship. Some of this conservatism is a consequence of the reluctance of the world’s military naval forces to use adhesives because of: (1) flammability in war time situations, even when fully cured and (2) potential toxic outgassing in enclosed areas (e.g., submarines). This conservatism extends to most polymeric materials, including composites. But the greatest influence on the unwillingness to www.metalfinishing.com
tion of small marine craft. This is mainly because lightweight materials, such as composites and aluminum, are used instead of steel, and adhesives are a reliable and efficient method of joining these materials.
use adhesives in marine applications has been the recognized degenerative effect that moisture has on the strength of an adhesively bonded joint.1 Yet, another factor for the slow growth of adhesives in the marine industry is the unique manufacturing environment that exists at many shipyards and plants. Chemical substrate treatments are commonly used in the aerospace and even automotive industries to improve the durability of adhesive joints. However, often the shipbuilding industry cannot afford such high-quality surface preparation, and substrate processing has to remain basic. Another manufacturing problem is the hostile nature of the shipyard. Curing temperatures may vary significantly with climatic temperatures, and there are many sources for contamination and weak boundary layer to develop. Adhesive bonding has generally been better accepted in the construc-
VALUE PROPOSITION Even with this historical perspective, the advantages of adhesive bonding are beginning to be noticed across the breadth of the marine industry. Adhesive bonding is being adopted generally in the following applications: • Joining of metals for increased fatigue life • Joining of polymer composites • Joining of dissimilar substrates • Reducing weight to improve speed and fuel performance • Improving shop safety • Lowering manufacturing costs and improving production efficiency Adhesive bonding is expected to provide both improved strength properties and save time and costs. The major property advantages of adhesive bonded joints (Table 1) have always
ADVANTAGES
DISADVANTAGES
• Provides attractive strength-to-weight ratio • Provides large stress-bearing areas • Provides excellent fatigue resistance • Heat, if required, is too low to affect metal part • Damps vibration and absorbs shock • Minimizes or prevents galvanic corrosion between dissimilar metals • Joins all shapes and thicknesses and any combination of similar or dissimilar materials • Provides aero- and hydro-dynamically smooth contours • Seals joints
• • • • •
Surfaces must be cleaned Long cure times may be necessary Heat and pressure may be required Jigs and fixtures may be required Rigid process control usually necessary; special training sometimes required • Inspection of finished joints is generally difficult • Useful life depends on the environment
Table 1. Advantages and Disadvantages of Adhesive Bonding in Marine Structures November/December 2013 | metalfinishing | 47
adhesion& bonding TEST TYPE
CONDITION
REQUIREMENT
Lap shear testing
Control (room temperature)
Manufacturer’s values achieved
Elevated temperatures Cold temperatures Cleavage peel testing
Control (room temperature)
Manufacturer’s values achieved
Elevated temperatures Cold temperatures Fatigue testing
ASTM D3166
Target value achieved (e.g., 107 cycles)
Immersion tests
Water
Minimum percentage retention achieved
Fuel Glass transition temperature
ASTM E1356 (calorimetry)
Durability: lap shear per ASTM D1002 or D3163
Loaded to manufacturer’s minimum specified value and weathered per ASTM D1183: • 28 days • 56 days
• Mean > 90% reference • Mean >95% 28 day value
Loaded to manufacturer’s minimum specified value and weathered per ASTM D1183: 28 days 56 days
• Mean > 90% reference • Mean >95% 28 day value
Durability: cleavage fracture per ASTM D433
Above minimum value (e.g., 70°C for military ships)
Table 2. Typical Requirements for Approval of Marine Adhesives
been visible to ship designers through the historical development of adhesives in the aerospace and automotive industry, but it has only been recently that the production advantages of adhesive bonding are also becoming evident to the shipbuilder. Adhesive bonding offers shipbuilders the opportunity to replace welding of steel structures to reduce distortion, effectively eliminate residual stress, and improve fatigue performance. Avoidance of high welding temperatures leads to safer construction in hazardous environments. Adhesive bonding of composites and aluminum also provides well-distributed loading and maximizes the utilization of the adherend material. It has been estimated that adhesive bonding could provide a cost saving
of at least 20 percent for fastening supports, stiffeners, and other attachments in outfitting of large passenger ships.2 For a patrol craft, reductions in building cost through the use of adhesive bonding in the superstructure can be expected to be around 25 to 30 percent. For fast ferries, an important benefit is a weight reduction of the structure by about 4.5 to 9 tons. Over a period of time these weight reductions can save significant amounts of diesel fuel. An added benefit of adhesive bonding is that it will also make positive contributions to the preservation and improvement of the quality of the environment by reducing the amount of welding slag created. A mediumsized shipyard generates about 60 tons per year of welding slag, which must
48 | metalfinishing | November/December 2013
be disposed of as a controlled waste. It also should be noted that “marine adhesives” do not only encompass water transportation vehicles. Offshore structures such as drilling platforms and wind turbines and near-to-shore structures such as piers, piping, and other miscellaneous construction are also viable candidates for adhesive bonding. These applications often have the same substrates and performance requirements as do marine vessels.
REQUIREMENTS OF A MARINE ADHESIVE The marine industry presents design engineers with what are often extremely diverse bonding challenges. One application may require the ultimate in adhesion performance, and another application may require aesthetic requirements and good aero- or hydro-dynamic design. Yet another application may require vibration damping and the ability to take repeatedly high impact forces. The adhesive must also, of course, last the life of the ship, and not degrade in the aggressive marine environments. When choosing an adhesive for any particular marine application, a number of factors need to be considered. The primary factors include the type of materials to be bonded, production conditions available for cure of the adhesive, and the required performance in the service environment. All marine structures require certification by societies such as the American Bureau of Shipping (ABS), Bureau Veritas, Lloyds, Det Norske Veritas (DNV), etc. Several of these societies have requirements for adhesives that are used in assembly. Many adhesive suppliers indicate certification authority approval for marine use in their product data sheets. Table 2 shows typical test requirements. For small boats up to 24m long structural requirements are provided by ISO 12215-6. STRUCTURAL MARINE ADHESIVES Two-part, room temperature reactive adhesive systems such as epoxy, polyurethane, and thermosetting acrylic are popular in structural marine applications. The popularity is mainly because www.metalfinishing.com
adhesion& bonding the formulations can be tailored to suit particular working conditions or application requirements. Often the resin and hardener ratios can be tailored to meet differing application conditions or to meet simple mechanical properties, such as tensile-shear strength, toughness, or peel strength. Epoxy adhesives are well-known two-part materials with wide applicability. They cure at room temperature or with a mild heat bake cycle. Heat curing generally improves the properties of the resulting bond. Depending on the type of epoxy base polymer and the formulation, epoxy adhesives have good resistance to creep, moisture, and most chemicals common to the marine environment. They also have a low degree of shrinkage on cure and excellent high temperature properties. The major disadvantage of epoxy adhesives is that they generally require vigorous surface preparation. Mix ratio and degree of mixing also have to be carefully controlled. Polyurethane adhesives are some-
what less well known than epoxies in the marine industry, but they have similar properties. Polyurethane adhesives can be cured at room temperature and are available in a broad range of cure rates. They have a high degree of adhesion to composites and most high surface energy thermoplastics, but often require a primer to bond to metal substrates. The major difference of polyurethanes relative to epoxy adhesives is that they can be formulated to range from hard and rigid to flexible and tough. Because of their flexibility, polyurethane adhesives are generally the best for bonding two different materials. They are also generally the adhesives of choice when a great deal of flexibility or impact resistance is required. Polyurethane adhesives provide a very high degree of bond strength to wood because of their reactivity to cellulose molecules and because of their easy penetration into porous substrates. Among the several disadvantages of polyurethane adhesives is that they
BASE POLYMER
CHARACTERISTICS
Polysulfide
Excellent adhesion and flexibility. Can take up to 25% elongation. Allows for movement associated with stress and temperature change. Slow curing; often takes a week or more to fully cure. Once cured, it can be sanded and painted.
Silicone
Excellent adhesion and flexibility. Ideal electrical insulator and waterproofing agent for wiring, windshields, ports, and emergency gaskets. Temperature resistance to 200°C. Do not use in areas that require sanding or painting.
Butyl
Cures fast and adheres well to most materials including hard-to-bond plastics. Not sandable but can be painted. Does not have the elongation that polysulfide or silicone sealants have. Of the five types of sealants, butyls have the shortest service life.
Acrylic
Ideal for bedding on wood, fiberglass, and metals. Skins over quickly and can be painted about 30 mins after application. Water dispersed formulations are available, but these generally have poorer moisture resistance than solvent formulations. Not generally considered as long lasting as polysulfide, polyurethane, or acrylic.
Polyurethane
Often thought of as an adhesive rather than a sealant because of its high tensile strength and outstanding adhesion especially to wood and composites. Resistance to high humidity is not as good as silicone or polysulfide. Cure time is shorter than polysulfide, but still may require up to a week.
Table 3. Characteristics of Common Marine Sealants www.metalfinishing.com
have relatively poor resistance to elevated temperatures. For example, they are not suitable for applications in the engine compartment. Polyurethanes, depending on the formulation, also have less hydrolytic stability and UV resistance than most epoxy adhesives. Thermosetting acrylic adhesives are less common than epoxy or polyurethane adhesives but they offer several unique advantages. Acrylic adhesives offer the fastest room temperature cure while maintaining adequate working time. They are available either as two-component systems or as a one-component product with a reactive primer. The nature of their curing mechanism provides great tolerance to variations in mix ratio and climatic conditions during cure. Acrylics have excellent resistance to UV, moisture, and general outdoor conditions. They bond very well to difficult substrates with a minimum amount of surface preparation. Thus, acrylic adhesives are often used to bond thermoplastics, metals, and composites. The main disadvantages of acrylic adhesives are that they are flammable in the uncured state and have a characteristic acrylic monomer odor. Some thermosetting acrylics may have cure rates that are too fast and do not allow accurate positioning of parts before the material begins to gel.
REFERENCES 1 Petrie, E.M., “How Moisture Effects Adhesives, Sealants, and Coatings”, Metal Finishing, October 2011. 2 “European Shipyards Can Become Competitive and More Environmentally Friendly”, Innovations Report, www.innovations-report.de, August 2005. 3 Davis, P., “Marine Industry”, Chapter 48 in Handbook of Adhesion Technology, da Silva, Ochsner, and Adams (eds.), Springer-Verlag, Heidelberg, 2011, pp. 1239-1240. BIO Edward M. Petrie is the sole proprietor of EMP Solutions, a Cary, N.C.–based consulting firm focused on solving problems in the adhesives and sealants industry. Visit www.specialchem4adhesives.com.
November/December 2013 | metalfinishing | 49