Sound deadening, thermal insulatingand undersealing material

The type of material used for sound damping or deadening depends on whether or not it is also required to provide undersealing. A material required for sound deadening only will normally be applied to the interior of a vehicle,whereas one required to provide sound deadening and undersealing properties will be applied to the underside of the vehicle. Thus the former need not be fully water resistant,whereas the latter must be water resistant in addition to many other necessary requirements.
The sound deadening properties of a material are related to its ability to damp out panel vibrations, and this in turn is related to some extent (but not solely) to its weight per unit volume. Thus the cheapest sound deadening materials are based on mixtures of sand and bitumen,although these tend to be brittle. A better material is bitumen filled with asbestos; although this is probably less effective as a panel damper than sand-filled bitumen,it is nevertheless more suitable owing to its better ductility. In general,those sound deadeners applied to the interior of the vehicle are water-based bitumen emulsions with fillers,whereas sound deadener/ sealers applied externally should be solvent-based materials. A more effective sound deadener than asbestos-filled bitumen is a clay-filled water dis- persed polyvinyl acetate (PVA) resin emulsion; this has damping characteristics approximately three times better than the bitumen-based material,but naturally it is more expensive.

Other sound absorbing materials are now used for insulation in the automotive industry. Needle felts are blends of natural and manmade fibres locked together by needle punching. These are used in diecut flat sheet forms for attachment to moulded carpets,floor boot mats and as anti-rattle pads. Bonded, fully cured felts are similar blends of fibres bonded together with synthetic resins. All the binder is cured during the felt making process. As with needled materials,these are mainly used for flat products, especially where low density is required such as in sound absorption pads and floor mats. In moulded felts and moulded glass wool the binder is only partially cured during the felt making process. The curing sequence is completed under the action of temperature and pressure in matched die compression moulded tools to produce components which have three-dimensional form and a controllable degree of rigidity. Fully cured and needled products can be given various surface treatments including abrasion-resistant and waterproof coatings such as latex,PVC or rubber,or they can be combined with bitumen to improve the sound insulation properties. Moulded felt can be supplied covered with a range of woven textile covers or with various grades of PVC and heavy-layer bitumen EVA products.
Polyurethane flexible moulded foam can be modified or filled to meet different insulating requirements in the vehicle. The foam can also be moulded directly on to the hard layer,allowing simple tailoring of insulation thickness.
Table 4.12 indicates the use of various undersealing materials.