Adhesives and fillers

Adhesives are usually divided into mineral, synthetic, animal and plant products (Table 17.2). Fillers are produced in the same way as ordinary adhesives and mixed in with powdered stone, fossil meal, wood dust, chalk, perlite and similar substances.

Archaeological exploration indicates that animal glue adhesives were in use as far back as 3000 to 4000 BC. In China and Egypt casein glue was used in finer joinery. Somehow this knowledge disappeared, but

Table 17.2 Adhesives used in building

CO

Type of adhesive

Main constituents

Moisture

Areas of use

cc <

resistance1

Mineral adhesives:

Û.

Cement adhesive

Portland cement; stone dust, possibly acrylates; water

5

Fixing of ceramic tiles and aerated concrete blocks

Waterglass adhesive

Waterglass; lime; stone dust; possibly acrylates; water

4

Fixing of ceramic tiles; bonding of wood fibreboards, chipboards and fillers

Synthetic resins:

Acrylate adhesives

Acrylates (butylacrylate etc.); water; organic solvents2

5

Fixing of timber; plastics; ceramics; carpets; linoleum; fibreglass; bonding of fillers

Chloroprene rubber CR

Chloroprene; organic solvents; softeners

5

Fixing of plastics

Epoxy adhesive EP

Epoxy; amines; polyamide

5

Fixing of stone; glass; metal; plastic; ceramic tiles; binder in fillers

Ethylene vinyl acetate

Ethylene; vinyl acetate; water;

5

Fixing of plastic sheeting

adhesive EVA

organic solvents2; softeners

and linoleum

Isocyanate adhesive EPI

Isocyanates; styrene-butadiene rubber or polyvinyl-acetate; water

5

Bonding of timber, plywood; fixing of metals

Melamin-urea-formaldehyde MUF3

Melamin; urea; formaldehyde

4

Bonding of timber and wood fibreboards

Melamin-urea-phenol-

Melamin; urea; formaldehyde

4

Bonding of timber, wood

Formaldehyde MUPF3

fibreboards and particle boards

Methylene-diphenyl-isocyanate MDI

Methylene-diphenyl diisocyanate

4

Bonding of particle boards

Phenol-formaldehyde PF3

Phenol; formaldehyde

5

Bonding of mineral wool, wood fibreboards, chipboards, plywood and cork tiles

Phenol-resorcinol-

Phenol; resorcinol;

5

Bonding of timber and

formaldehyde PRF3

formaldehyde

plywood

Polyurethane adhesive

Polyurethane or isocyanates

4

Bonding of timber, wood

PUR (1 or 2 component)

and polyols

fibreboards, particle boards, cork tiles, strawboards; fixing of wood, metal and plastics

Polyvinyl-acetate adhesive (PVAC)4

Vinylacetate; water; organic solvents2; softeners

3

Bonding of timber and fillers; fixing of soft floor coverings, wallpaper, wood products

Silicone rubber Si

Silicone; organic solvents

4

Fixing of timber, metals, plastics

Styrene-butadiene rubber SBR

Butadiene; styrene; organic solvents; softeners

5

Fixing of timber, mineral sheeting; bonding of carpets

Urea-formaldehyde UF

Urea; formaldehyde; water

3

Bonding of chipboard

Table 17.2 (Continued)

Plant-based adhesives:

Cellulose adhesive

Derivatives of cellulose; organic solvents

5

Fixing of linoleum

Cellulose paste

Methyl cellulose; water

S

Fixing of wallpapers, hessian and linoleum; bonding of fillers

Colophony resin

Colophony; organic solvents

4

Bonding of wood fibreboards

Latex adhesive

Natural rubber; organic solvents

4

Bonding of paper sheeting; fixing of linoleum and timber

Potato flour paste

Potato starch; water

2

Fixing of wallpaper

Rye flour paste

Rye flour starch; water

2

Fixing of wallpaper and linoleum

Soya adhesive

Soya protein; water

S

Bonding of plywood

Sulphite lye adhesive

Lye from waste; water

S

Bonding of wood fibreboards and paper sheeting

Animal glues:

Casein glue

Milk protein; lime; water

4

Bonding of timber and plywood; fixing of timber

Collagen glue

Protein from tissue; water

S

Bonding of timber and plywood

Blood albumin glue

Blood protein; ammonia; calcium hydrate; water

4

Bonding of plywood

1 Sensitivity to moisture is divided into a scale from 1 to 5, meaning: 5: For outdoor use. 4: Outdoor use, but sheltered from rain. 3: Indoor use, in relatively dry places. 2: Indoor use, in permanently dry situations.

2 Secondary solvent.

3 As these adhesives are most often used as 'hot melts' solvents are less needed. Cold versions may depend on substantial amounts of added solvents.

4 An alternative version for outdoor use have chromium compounds added.

1 Sensitivity to moisture is divided into a scale from 1 to 5, meaning: 5: For outdoor use. 4: Outdoor use, but sheltered from rain. 3: Indoor use, in relatively dry places. 2: Indoor use, in permanently dry situations.

2 Secondary solvent.

3 As these adhesives are most often used as 'hot melts' solvents are less needed. Cold versions may depend on substantial amounts of added solvents.

4 An alternative version for outdoor use have chromium compounds added.

was rediscovered in Europe around the sixteenth century. The first glue factory was built in the Netherlands in 1690. Around 1875 the manufacture of plywood started, and at the turn of the century laminated timber construction began. Synthetic adhesives came into production around 1930 and today are used across the whole industry. There are now between 100 and 300 different building adhesives available on the market. A normal Swedish home contains about 700 litres of adhesive, when furniture is included.

Fillers only came into use well into the twentieth century when smooth, even surfaces were required.

Mineral adhesives are usually based on Portland cement or water-glass (see page 90) and are used mainly for ceramic tiles, but have also become an adhesive for masonry. They are then used for precision components with accurate dimensions, such as blocks of aerated concrete. The adhesive used is usually a cement glue with a large proportion of acrylates mixed in. The joint is so strong that attempts at dismantling the wall may be difficult without destroying the blocks. Waterglass glue can also provide the base for a filler by mixing it with clay powder.

Mineral adhesives are based on resources with rich reserves. Both the energy used and pollution caused during production are moderate compared to commercial alternatives. Inside a building, products containing acrylates can cause problems for the indoor environment during their curing process. Pure waterglass products create no problems at all. As waste, waterglass glue is considered to be inert, whilst cement-based glue containing acrylates has to be deposited at special tips.

Synthetic adhesives are usually divided into thermosetting and thermoplastic adhesives.

The thermosetting adhesives are often called 'synthetic resins' and the binders are usually based on one or more of the compounds urea, formaldehyde, phenol, melamine, resorcinol, acrylic acid, epoxy and polyurethane. Epoxy is composed of epichlorohydrin and bisphenol A, while polyurethane is manufactured from isocyanates. In several products a separate hardener is added in order to complete the gluing process, usually amines, ammonium chloride or polyoxymethylene. Some thermosetting adhesives are soluble in water while others need organic solvents like ethanol and ethylene glycol.

Important thermosetting adhesives are phenol-resorcinol-formalde-hyde (PRF) and melamine-urea-formaldehyde (MUF), often used for the laminating of timber. Polyurethane (PUR) and urea-formaldehyde (UF) dominate in the production of chipboards and plywood. Phenolformaldehyde (PF) is used in the production of matting of mineral wool, and epoxy adhesives to glue a wide range of materials from stone to glass and metals.

Synthetic rubber adhesives produced from chloroprene (CR) and styrene-butadiene (SBR) represent a subgroup of thermosetting adhesives with almost permanent elasticity. They require large amounts of organic solvents that can include aromatics and esters.

Thermoplastic adhesives are delivered ready-made from the factory, often emulsified in a solvent. Important adhesives of this type are polyvinyl-acetate (PVAC), ethylene-vinyl-acetate (EVA) and methyl-metacrylate (MMA). Thermoplastic adhesives are partly soluble in water, partly soluble in organic solvents, most often ethanol or toluene. The dispersion agent nonylphenol was widely used earlier, but is probably substituted in most modern products. Water-based products always have fungicides added, usually isothiazolinone or bronopol. Softeners are also used, most often as butyldiglycolacetate or phthalates.

So-called 'thermal bonding' can be achieved by melting thermoplastic fibres, usually of polypropylene (PP) and polyethylene terephthalate (PET). This is generally not considered as conventional gluing. Thermal bonding is much used for fixing natural fibres in insulation matting.

Thermosetting adhesives are very widespread in the building industry, but they are less relevant as building adhesives on site, except when gluing external components where high strength and resistance to moisture is needed. Thermoplastic adhesives are the most common glues used on site. Fillers for indoor use are mainly based on PVAC adhesive.

The synthetic adhesives are based on fossil resources. The production is intensive in energy and a wide range of serious pollutants can be emitted. Within buildings, these products can create problems for the indoor climate through the emission of solvents and other volatile

Table 17.3 Synthetic adhesives and essential hazardous substances that may be found in production and/or in finished product

CO

Product

Substances1

Icc <

Chloroprene rubber CR

Butadiene; chloroprene2

Epoxy

Epichlorohydrin; bisphenol A; amines2

Û.

Isocyanate EPI

Isocyanates2; styrene; xylene; butadiene; hexane2; toluene2

Melamin-urea-formaldehyde MUF

Formaldehyde

Melamin-urea-phenol-formaldehyde MUPF

Phenol; formaldehyde

Methylene-diphenyl diisocyanate MDI

Methylene-diphenyl diisocyanate2

Phenol-formaldehyde PF

Phenol; formaldehyde2

Phenol-formaldehyde-resorcinol PFR

Phenol; formaldehyde; resorcinol

Polyurethane PUR

Isocyanates2; chlorinated hydrocarbons; amines2

Silicone Si

Chlorinated hydrocarbons; xylene2; amines2; siloxanes2

Styrene-butadiene rubber SBR

Styrene2; xylene2; butadiene; hexane2; toluene2; amines2

Urea-formaldehyde UF

Formaldehyde2

See risk potential in Table 2.5.

1 Additional substances may well occur. Production of basic ingredients is not included.

2 Risk of emissions from cured products.

See risk potential in Table 2.5.

1 Additional substances may well occur. Production of basic ingredients is not included.

2 Risk of emissions from cured products.

compounds during the curing phase, and sometimes for a longer period, in some cases as a result of ageing (Table 17.3). Waste from hardened and non-hardened adhesives usually requires disposal at special tips, as do glued products, depending upon which adhesive is used and in what quantity. As a whole, PVAC and EVA-glues are the least problematic.

Animal glues are based on substances rich in protein such as milk, blood and tissues, and are divided into three main types: collagen glue, blood albumin glue and casein glue. These are all soluble in water. They are all good glues for wood, and can be used on everything from veneers and furniture to large laminated timber structures. Some glues have fungicides added.

Collagen glues are mostly based on waste from abattoirs and fisheries. Casein glue is produced from milk. Under dry conditions in buildings, the products cause no problem. However, in combination with damp cement, they can emit ammonia that irritates respiratory passages. In continuous damp conditions, there is a chance of mould or other bacteria developing and the rotting products can cause bad odours, irritation and allergies. This can also lead to the deterioration of the building structure. Wastes from the glues are highly eutrophicating, but this risk is insignificant since the amount is usually small. Glues containing potentially damaging fungicides must be deposited on special tips.

Materials glued with animal glue can normally be energy recycled in ordinary incinerators, or can be dumped without any particular restrictions.

COLLAGEN GLUE

Collagen glue is made from the tissues of animals containing collagen, a protein. By boiling it in an evacuated vessel it turns into glue. This is then dried into a granulated powder or made into small bars. Gelatine is collagen glue that has been cleaned of colour, smell and taste.

There are three basic types of collagen glue: bone glue, hide glue and fish glue.The first two are often called 'glutin glue'. Bone glue is madefrom bones and knuckles. Hide glue is madefrom waste hidesfrom places such astanneries. Fish glue is madefrom fish bones and other fish waste. All of these glues are strong, but hide glue is considered the best.

Collagen glue bars or powder can be placed in cold water to soften up and then dissolved in waterat 50-60 °C using about two tothreetimes as much water asthe weight of softened glue.The powder can also be released directly into warm water.Temperatures above 60 °C decrease the quality of the glue. Bone glue and hide glue have to be used warm and the pieces to be glued must be put under pressure before it stiffens. The glue cures quickly when cooling. Fish glue can be used cold, as can the other collagen glues when calcium chloride is added.

To make a good collagen glue filler for timber surfaces, saw dust or wood flour is mixed in. Colour pigments can also be added. Adding gypsum makes the filler white.

Collagen glues based on bones and hides have been used a great deal for gluing plywood. Right up to the Second World War, these were also dominant in furniture making, and there are still craftsmen who say that the quality then was much higher than that achieved today with adhesives such as urea-formaldehyde (Brenna, 1989).

BLOOD ALBUMIN GLUE

Blood albumin glue is soluble in water. It is prepared from fresh blood or from blood serum which is allowed to swell in water. The glue is made by adding ammonia and a calcium hydrate solution in certain proportions. The objects must be warmed up during the actual gluing. At certain temperatures the protein coagulates, and the glue joint becomes totally watertight.The joints have a riskof being attacked by mould and should be kept dry, if fungicides are not added.

CASEIN GLUE

Casein glue wasused bycraftsmen in ancient China. Itis madefromskimmed milk.The milk is warmed and rennet is added to separate out the casein.The casein is then dried and mixed with 2.5% (by weight) lime.The powder is mixed with three times as much water, so that the lime is slaked. A glue is then produced which, after setting, tolerates humidity better than collagen glue. However, in permanently damp surroundings, and with timber at more than 18% moisture content, the glue can be attacked by micro-organisms.

Casein glue can be used for internal loadbearing structures, stairs, plywood, laminated timber, etc. without fungicide. However, it is seldom used nowadays. Producers of laminated timber prefer adhesives that can be used in all situations, and therefore choose synthetic resins which have a higher resistance to moisture. Strengthwise, casein glue is as good, and there is proof of its long-lasting qualities in structures that have kepttheir strengthfor more than 60 years ( Raknes,1987). Avery impressive example can be seen in Stockholm Central Station, where enormous laminated timber arches have been put together with casein glue. During the Second World War, casein glue was used in the manufacture of fighter planes ('Mosquito').

There is a need for the renaissance for environmentally-friendly casein glue. This does not necessarily conflict with economic considerations: it has been shown that casein glue can be produced for less than 25% of the cost of synthetic alternatives.

Plant glues include soya glue, natural resin glue and cellulose glues as well as glues based on rye flour and potato flour.

Lignin is a natural resin that is found in some quantity in all plant material. It separates out under heating, normally with the help of small amounts of aluminium sulphate. In many hard and semi-hard plant-based board materials it is therefore unnecessary to add other adhe-sives. The addition of laccase enhances this process to the extent that a wider spectre of product types can also be made without added glues (Lund, 2003). Another natural resin is colophony which is extracted from coniferous trees and used with additional solvents, mostly turpentine distilled from wood tar.

Soya glue is a water-based protein glue taken from the waste products of cooking oil production. Glues produced from cellulose are available in both water and solvent-based variations. The organic solvents used are most often turpentine or pure alcohol, the latter up to as much as 70%. The water-based cellulose glue is usually called paste, and is used for putting up wallpaper. Paste can also be made from potato starch or rye flour in water-based solutions. Cellulose adhesives are not attacked by micro-organisms, even in damp conditions. Soya glue and flour paste are more vulnerable and should be restricted to use in dry places.

Plant glues are all based on renewable resources. The products usually cause little pollution in their manufacture, one exception being cellulose glues which are based on methylcellulose. Their production involves the use of chlorinated hydrocarbons such as methyl chloride, methyl iodide and dimethyl sulphate. Another exception is products containing colophony, which is a well-known allergenic substance.

During building use, plant glues do not generally cause problems. Waste from glue can cause the growth of algae in water systems, but this risk is insignificant as the amount of glue in question is usually small. Glues with highly toxic fungicides added are an exception to this. Materials glued together with plant glue can usually be energy recycled in normal incinerators or deposited without special restrictions.

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