An adhesive can be defined as a substance capable of holding materials together by surface attachment.  Sealants provide a load bearing elastic joint between two surfaces, which can also act as a barrier to dust, dirt, moisture and chemicals.  Sealants reduce noise, vibration and harshness (known as NVH in the automotive industry) and also act as insulators.  There is a considerable overlap between adhesives and sealants so they are generally classified together.  The materials used in adhesives and sealants are often similar if not the same.  

In both adhesives and sealants, the individual components of a formulation are determined by the end use properties sought.  The basic component is the binding substance that provides the adhesive and cohesive strength of the bond.  This is usually a polymer system although it can be an inorganic compound or a natural product.

Fillers are generally added to an adhesive or sealant to improve the working properties, permanence or strength of the end product.  Extenders are used to reduce the proportion of expensive active ingredients in an adhesive or sealant and reduce the cost of the finished product.  Industrial minerals are usually used as fillers or extenders in adhesives and sealants but in some inorganic adhesives they form the material base of the compound.

Kaolin, mica and talc are used as reinforcing fillers in adhesives and sealants because of their platey structure.  Reinforcing fillers increase the tear resistance, tensile strength, high compression set in adhesives and also aid the permanent set in sealants.  Fillers that do not have reinforcement properties include calcium carbonate, silica, barytes and aluminium hydroxide.  For certain types of product, fillers with an ultrafine particle size are required such as coated PCC, fumed silica or carbon black.

Kaolin is used as a filler to improve a variety of processing characteristics in adhesives in addition to reinforcement and thixotropy.  These modifications to adhesive products include:

Improve and accelerate wet tack

Provide opacity and whiteness where desired

Control penetration

Reduce premature sealing of dried adhesive

Raise solid content of adhesives and so reduce drying time

Increase viscosity and improve consistency

Reduce stringing of adhesive and aid spreading

Reduce unit cost of product

Not all adhesive and sealant products contain fillers as the higher the mineral loading of a product the lower its performance characteristics.  Filler:binder ratios tend to be in the region of 10:1 to 12:1 but some can be as high as 30:1.  The difference in mineral loadings between products can be pronounced.  For example, putties can contain up to 85% filler compared to none in specialist high performance adhesives.

Kaolin is also used as a source of aluminium silicate pigments (ASPs) in adhesives and sealants.  Specifications can vary widely with particle sizes of between 0.55m and 4.8m.  Water washed and airfloated forms of kaolin are equally important in this application but water washed grades have been preferred in the past due to their lower abrasiveness.  This is important in some applications, such as plywood adhesives, where reductions in general wear and tear on machinery are a useful selling point for products.  Kaolin can also be surface treated with stearates to render it hydrophobic but still dispersible in organic products.