TEXTILE CHAIN
2.11 Carpet back-coating
The backing process is an important production step which is applied to improve the stability of textile floor-coverings. Moreover, backing may have a positive influence on properties such as sound-proofing, stepping elasticity and heat insulation.
One can distinguish the following types of coatings:
• pre-coating
• foam coating
• textile back-coating
• heavy coating
• reinforcement
• back finish.
Pre-coating
A common feature of tufted carpets is that they are pre-coated after tufting to permanently anchor the needled pile loops in the carrier layer (Figure 2.28). The pre-coating material used consists of:
• x-SBR latex, which is a dispersion containing a copolymer produced from styrene, butadiene and carbonic acid
• fillers
• water
• additives (e.g. thickeners, anti-foam, foam-stabilisers, etc.).
Figure 2.28: Pre-coated tufted carpet [63, GuT/ ECA, 2000]
The pre-coating can be applied:
• unfoamed, by means of slop-padding (Figure 2.29)
• foamed, by means of the doctor-blade technique (Figure 2.30).
Figure 2.29: Pre-coating application by slop-padding [63, GuT/ ECA, 2000]
Figure 2.30: Pre-coating application by doctor-blade technique [63, GuT/ ECA, 2000]
During the subsequent drying stage, thanks to the formation of hydrogen bonds, the polymer chains are netted into a three-dimensional web and an elastic plastic layer is produced.
SBR foam coating
Foam coating methods consist in the application of a foam layer onto a pre-coated carpet, as the following figure shows.
Figure 2.31: Foam-coated tufted carpet
The foam finish is carried out in two steps: foam application and foam solidification through drying. The lattice is foamed with air and then applied by means of a doctor-blade onto the pre-coated carpet.
The SBR foam must be stabilised until it is solidified in the vulcanisation oven. For this stabilisation, two methods are used:
• the non-gel process, which uses surfactants as foam stabilisers
• the gel process, which uses ammonium acetate (AA gel system) or silicium fluoride (SF gel system) as gelling agents.
The overall process is schematised in Figure 2.32.
Figure 2.32: Representation of the SBR foam coating process [63, GuT/ ECA, 2000]
The foam is composed of:
• the SBR colloidal dispersion
• a paste, which contains a number of active additives
• inactive fillers (mainly chalk, which is added to the ready-compounded paste)
• water
• thickeners (e.g. polyvinyl alcohol, methyl cellulose, polyacrylates)
• colourants and pigments
• anti-oxidants and ozone stabilisers.
Some of the active components of the paste are responsible for the environmental impact of this coating method. In order to identify better the emission sources they can be divided as follows:
Polymerisation additives:
Remarks - Foam stabilisers
- Cross-linkers Usually sulphur, but also peroxides - Vulcanisation
accelerators
- Mercaptobenzothiazoles (e.g. zinc mercaptobenzothiazole) - Dithiocarbamates, such as zinc diethyldithiocarbamate or zinc
dibenzyldithiocarbamate or zinc dibutyldithiocarbamate (the most commonly used one)
- Activators Usually combination of ZnO and stearic acid (one source reports that ZnO is not necessary for non-gel and some SF applications [281, Belgium, 2002])
Processing additives:
Remarks - Foaming agents and
stabilisers
Surfactants
- Gelling agent e.g. Ammonium acetate (AA gel system) or silicium fluoride (SF gel system)
- Hydrophobic substances in order to improve the foam surface and the water-repellent properties
Paraffin dispersions and silicon emulsions
- Complexing agents, to chelate metal ions (they behave as catalysts for ageing the foam layer)
e.g. EDTA, DTPA, polyphosphates
- Antioxidants
- Thickeners Organic polymers based on polyacrylates and cellulose (e.g.CMC) Functional additives:
- UV stabilisers - antistatic agents
- flame retarding agents (e.g. Al2O3).
PU foam coating
Polyurethane is another method for foam coating. The ICI polyurethane coating process is the most commonly applied. The carpet is prepared by steaming and then reaches the spray chamber where the components of the polyurethane (diisocyanate and an alcohol) are sprayed.
The CO2 produced during the chemical reaction is embedded into the foam. The coating is reinforced in an infrared heating field and in a subsequent reaction field. The process is schematically represented in the following diagram.
Preparation of material (Steaming)
Aftertreatment (cutting off edges embossing device,
rolling up) Reinforcement of the
PU (IR heating field
reaction field) Coating of the
material in the spray chamber
Figure 2.33: PU foam coating [63, GuT/ ECA, 2000]
Textile back coating
Textile backing consists in the application of a textile fabric onto the pre-coated carpet. The connection between the carpet and the textile fabric is obtained through the application of a layer of:
• laminating glue
• melting glue.
Figure 2.34: Textile backing [63, GuT/ ECA, 2000]
Laminating glue
In this process an x-SBR latex is applied to the carpet by slop-padding. After the application of the textile fabric, the final reinforcement of the latex is carried out by means of heat treatment (Figure 2.35). The latex composition is similar to that used for pre-coating, with a higher share of polymer dispersion in order to allow a higher adhesive power.
Figure 2.35: Textile backing by means of the laminating glue process [63, GuT/ ECA, 2000]
Melting glue
This system uses thermoplastic polymers (mainly polyethylene) which are meltable by means of heat. In powder lamination (and in particualr in powder scattering lamination) polyethylene powder is evenly sprinkled onto the back of the carpet. Subsequently the polymer is melted in an infrared field. In the next stage the fabric is pressed into the melting glue. Through subsequent cooling, the melting glue becomes permanently connected between the textile fabric and the bottom side of the carpet. The process is represented in Figure 2.36.
Figure 2.36: Textile backing by means of the powder lamination (melting glue) [63, GuT/ ECA, 2000]
Another textile backing process by means of melting glue is the so-called AdBac process. In this case the carpet is constructed using a primary cloth (carrier layer) with low melting point additives. In the next stage the secondary cloth (also with a low melting point) is brought into contact with the back of the carpet before this enters the heating zone. The higher temperature melts the cloths, which are then forced together by nip rolls at the exit of the heating zone. The carpet is then cooled. A scheme of a carpet produced with the AdBac process is reported in Figure 2.37.
Figure 2.37: Carpet manufactured with the AdBac process [63, GuT/ ECA, 2000]
Heavy coating
Heavy coating is mainly used for the coating of self-lying (SL) tiles. The coating process consists in the application of the coating material by means of slop-padding or doctor blade and subsequent reinforcement. In most cases the coating material is applied into layers (two-coat technique). After the first layer, which may also serve as a pre-coating layer, a glass-fibre web may be added. The second coating application follows. The following coating materials are used:
• APO (abbreviation for “atactical polyolefin”)
• bitumen (enriched with inorganic and organic additives)
• PVC (polyvinylchloride)
• EVA (ethylen vinyl acetate).
The process principle is schematised in Figure 2.38.
Figure 2.38: Representation of the heavy coating process [63, GuT/ ECA, 2000]