Unit 2 - Adhesives Flashcards
What do adhesives depend on?
Adhesives, or glues, depend on close molecular contact between surfaces - adhesion forces operate over distances of less than one nanometre
Two perfectly smooth surfaces would naturally bond together, however these do not exist and so a liquid is needed to fill the gaps between the surfaces
Define adhesive
- An adhesive is defined as a liquid which sets and prevents the vertical or lateral movement of two surfaces
- The two surfaces are known as substrates or adherends
Name the 4 theories of adhesion
- Mechanical
- Chemical
- Diffusive
- Electrostatic
Describe mechnical adhesion
- Works by physically interlocking of the substrates by the adhesive
- This is the primary form of bonding where one substrate is porous such as paper or board
- The adhesive fills the gaps between the substrates and penetrates their surface by capillary action, interlocking with the fibres
- This creates mechanical links between the adhesive and the two substrates
- This type of adhesive tends to have a low viscosity to allow them to flow into the surface features
- Sometimes also called interlocking adhesion and takes place on a macroscopic level
Describe chemical adhesion
- Occurs between much smoother surfaces and involve the atoms of the adhesive forming chemical bonds with the atoms on the surface of the substrate
- Sometimes called specific adhesion and takes place on an atomic level
Describe diffusive adhesion
- Diffusion is the movement of atoms from an area of high to low concentration
- This type of adhesion occurs when molecules from one substrate migrate into the other and vice versa - typically occurs in polymeric materials
Describe electrostatic adhesion
- Applies to metallic films, deposited on polymer substrates, which carry an electrostatic charge
- When films with opposite charges are brought together, the charges attract and therefore bring the two surfaces together
What is vital to produce a good bond using adhesive?
- In order to produce a strong bond, it is vital that a continuous film of adhesive can be applied - this is known as wetting
- The substrates must be thoroughly cleaned
- Some surfaces are non-polar and so need to be treated to they can accept the adhesive
Name the 2 types of adhesive failure
- Cohesive failure
- Adhesive failure
Describe cohesive failure
Occurs within the adhesive layer and is usually due to faulty adhesive
Remember, cohesive means between particles of the same substrate
Name 3 causes of cohesive failure
- Too much adhesive
- Too little adhesive
- Wrong type of adhesive
Name adhesive failure
- Occurs at the boundary between the adhesive and the substrate and is usually caused by incorrect application
- Remember, adhesive means between particles of different substrates
Describe adhesive viscosity
Viscosity refers to the resistance of the adhesive to flow
Why is it important to choose an adhesive with the correct viscosity?
- Important to choose one that has correct viscosity so its wets the surface properly but doesn’t run off the substrate
- Some machines are only designed for a specific viscosity range
Name 2 things that affect viscosity and how
- Temperature dependant and is generally higher at lower temperatures
- If too low - insufficient adhesive dispensed
- If too high - charring, gelling, stringing, and smoking may occur
What is viscosity measured in?
Measured in centipoise (cps) or millipascal seconds (MPas)
Describe adhesive tack
Refers to the stickiness of the adhesive, or its resistance for being pulled apart when wet
What happens is adhesive tack is too high or too low?
- Too low - joint can come apart during handling
- Too high - joint can set in the machine before it comes into contact with the substrate
What types of products require a high adhesive tack and why?
- Carton sealing operations require a high tacks because there is little time to hold the joint while the adhesive sets
- Remoistening adhesives on envelopes also have a high tack as you don’t want to hold the envelope closed for ages while it sets
What type of adhesives are permanently tacky?
Pressure sensitive adhesives are permanently tacky and have different degrees depending on application - e.g. post-it notes have low tack
Describe ‘open-time’ of an adhesive
The ‘window of stick’, or the maximum amount of time between the application of the adhesive and the achievement of an effective bond
Name 3 things that affect the open time of an adhesive
- Temperature
- Substrate porosity
- Amount of adhesive applied
Describe solid content of adhesive and how it is usually expressed
- When the liquid carrier medium, which could be water or solvent, has evaporated the solid component remains
- Generally given as a percentage
State the solids content of hot melt adhesives, emulsions, and starch
Hot melt adhesives are 100% solids, emulsions are 50%, and starch is 18-30%
Name 4 types of adhesive
- Starch
- Casein
- Synthetic emulsions
- Solvent-based
How are starch adhesives made?
Made by the chemical treatment of vegetable starches - starch is formed into a slurry with water; when this is heated, the starch granules burst, absorb water, and produce a tacky adhesive
What substrate is starch adhesive used for?
Used with corrugated board
What type of substrate can starch adhesive not be used for and why?
Not used for triple-wall boards as the heat cannot penetrate the inner plies (PVA is used for this - it is a type of thermoplastic)
What effect does the low solid content of starch adhesives have?
High tack and long open time
Give 3 advantages of unmodified starch adhesives
- Low cost
- Good adhesion
- Easy to recycle
What are the three ways starch adhesives can be modified?
- Mechanical
- Thermal
- Chemical (acidification to break starch down into dextrin or glucose)
Give 2 advantages of modified starch adhesives
Increases the stability and produces a high wet tack
Give 2 uses of modified starch adhesives
- Used to apply labels to glass - needs a high viscosity and high wet tacks (high wet tack overcomes the issue of slow drying times)
- Used to glue paper bags which requires a paste-like consistency which is not absorbed into the paper
What can modified starch adhesives be mixed with to improve tack?
Borax
What effect does alkaline treatment of modified starch adhesives have?
Alkaline treatment will produce jelly-like glue with high tack and internal cohesion - this dries to a tough, colourless, flexible films
What are casein adhesives derived from?
Derived from phosphoprotein in acidified milk
What is the main characteristic of casein adhesives?
Main characteristic is that they can absorb a lot of water without a change in viscosity
Name the major use of casein adhesives and why
One major application is labels to bottles in cold and wet conditions as they have very high tack
Give an advantage of using casein adhesives
Soluble enough to allow for easy removal with returnable bottles
Name the 3 polymers that synthetic emulsions can be based on
- Polyvinyl acetates
- Acrylates
- Maleates
Describe the formulation of synthetic emulsions and why
- The solid adhesive polymeric particles are surrounded by a protective colloid (such as PVA), which are then suspended in water
- A polymer adhesive added to water would separate into layers (a bit like oil and water) so to prevent this, the adhesive is coated in a stabilising system to form a stable emulsion
- The stabilising system has one hydrophilic side and one hydrophobic side to create a film around the polymer particles
What is the solids content of synthetic emulsions?
Have a high solids content of around 50-70% and good heat-resistance and better water resistance than starch
What products are synthetic emulsion adhesives used for?
Used for back and sack making, and gluing folded cartons
Describe how synthetic emulsion adhesives work for board
- When the adhesive is applied to the board, the emulsion ‘sets’ through the loss of water
- Suspension breaks down and the polymer units begin to combine to form an adhesive film, the second substrate is then applied
- Initially, a partial green bond is formed, but as pressure is applied and the board continues to draw out water, particles of adhesive are brought closer together and the stabilising coating breaks down
- Bond strength increases as water further evaporates and eventually only the polymer adhesive remains
What must one substrate be for synthetic emulsion adhesives to be suitable for use
One substrate must be porous to absorb the adhesive and allow the carrier solution to evaporate; the lamination of the second substrate must also take place while the adhesive is still wet
Name 7 storage conditions that must be adhered to for synthetic emulsion adhesives
- Must be stored above 7°C to maintain emulsion
- Must not be allowed to freeze
- Rotate stock to ensure shelf-life is not exceeded
- Container should be kept closed while not in use to prevent contamination
What types of substrates are solvent-based adhesives used for?
Plastic films
Give an advantage of solvent-based adhesives
Faster drying than water-based
Why is use of solvent-based adhesives decreasing?
Environmental concerns
Describe the use of solvent-based adhesives
Adhesive is applied to one surface then dried to leave a tacky layer, after which the second substrate is laminated to it
Give 2 disadvantages of solvent-based adhesives
- Risk of retained solvent can result in blistering of the structure and can result in unacceptable taint or odour
- Also need a solvent-recovery system or incineration plant
Give an example of a solvent-based adhesive and describe its composition
One example is polyurethane adhesive - a mixture of di-isocyanate (solid) and di-alcohol (solvent) which mix together and cure to form polyurethane
What form do pressure sensitive label adhesives come in?
Too look at, they are solid and are delivered in a block, but they are actually classed as a very high viscosity liquid
Name the 3 things that pressure sensitive label adhesive formulation can be based on
- Acrylic
- Styrene butadiene
- Rubber / resin blends
Give an advantage of pressure sensitive label adhesive
Advantage is that they form an instant bond which can be peelable or permanent
What are PSL adhesives used in conjunction with and why?
When used on self-adhesive labels, they are used in conjunction with silicone / fluoropolymer coated backing paper for easy release
Give a disadvantage of using PSLs
One of the only packaging systems where 50% of the purpose needs to be thrown away - the backing
What are cold seal adhesives based on?
Natural rubber latex
Give a unique property of cold seal adhesives
Usually based on natural rubber latex and have a unique property in that they only adhere to themselves
How do cold seal adhesives work?
They are applied to the reverse of the film and only need pressure to form a bond
Give a disadvantage of cold seal adhesives
The seal is not air tight
Give 3 reasons why cold seal adhesives are used for confectionery
- Water-based adhesives bond poorly with plastic film
- Solvent-based adhesives affect chocolate taste
- Plastic film low melting point unsuitable for heat sealing
Name 5 substances that hot melt adhesives are based on and state what each does
- Polymers - for strength
- Resins - for tack and adhesion
- Wax - as a dilutant to give flow properties
- Stabilizers
- Antioxidants - important at application temperatures to prevent charring
What type of plastic is a hot melt adhesives, and what does this mean?
Adhesive is a thermoplastic - softens on heating and solidifies on cooling
What % solids are hot melt adhesives?
100
Is the open-time of a hot melt adhesive short or long? What is the implication of this?
Have a short open time so are fast setting
Are holt melt adhesives best for smooth or rough surfaces
Rough as they fill gaps
Why can hot melt adhesives be an issue in warmer climates?
Have limited heat resistance and can melt again (which can be an issue in warmer climates)
Why are hot melt adhesives energy intensive?
Require constant heating to stay in their liquid form in machinery, therefore are relatively energy intense
Why were warm melt adhesives developed?
Hot melts may be a burn hazard
Name 3 formats that hot melt adhesives can be delivered in
Slates, pastilles, or blocks
Name 4 causes of hot melt temperature being too low
- Incorrect set temperature
- High adhesive level in tank
- Too much adhesive added
- External temperatures
Name 4 causes of hot melt temperature being too high
- Incorrect set temperature
- Low adhesive level in tank
- External temperature
Name the 4 effects of hot melt adhesive temperature being too low
- Reduced open time
- Increased viscosity
- Dirty running (stringing)
- Poor penetration
Name the 5 effects of hot melt temperature being too high
- Increased open time
- Decreased viscosity
- Charring
- Dirty running
- High fume level
Why must hot melt temperature be related to the speed of the machine?
Application temperature must be related to the speed of the machine so that cooling of the adhesive is sufficient to prevent failure of the joint after leaving the machine
Name 5 disadvantages of using hot melt adhesives
- Application pressure is important for accurate and even application to reduce stringing and splashing. Affected by the nozzle size, temperature, and distance to the application point
- Lids must be kept on to prevent contamination which could block application nozzles
- Cannot be dropped into tanks to avoid splashes - they cannot be removed from things until they have cooled and solidifies which is problematic if it falls on a person
- Operating level and temperature is critical for good performance
- Recycling issues need to be taken into account - they cannot be easily dispersed or removed in paper recycling mills
