Polymer softeners versus conventional fatliquoring

The composition of specially modified softening polymers and their homogenous behaviour make them less sensitive to structural differences within the leather than conventional fatliquors. These polymers ensure a highly uniform particle size as an emulsion, something of great importance when entering and softening the very dense corium structure. It is because of this high relaxation softness that a piece of leather that is lifted-up will fall gently and flat onto a surface such as a table.
The polymers used for these specially modified softening agents have an open and flexible structure with a low molecular weight between 3,000-10,000. Inter chain interaction is low, resulting in polymers with a relatively low viscosity. The highly flexible and mobile backbone gives the polymers the possibility to adopt low energy configurations with a good softening capability and a good penetration into the collagen fibres during wet-end application.
Because of the unique backbone structures of these polymers, the interaction with the polypeptide chains of the collagen and/or the chrome in the wet-blue is excellent. The specially designed side groups are hydrophobic and have a high lubricating power. They provide low levels of friction between the fibres and facilitate the opening up of the fibre structure in the leather resulting in a durable self-recovering soft and spongy feeling.
Another remarkable property of these polymers is that the interaction with other chemicals is good which makes it possible to make combinations with other functional leather chemicals (such as silicones, lecithin) for the optimal result. With these polymers it is, therefore, possible to design polymer softeners with well-balanced properties. By having good lightfastness, heat resistance and weather resistance these polymer softeners are suitable for a wide range of leather types.
These polymer based softeners are used as a main softening agent with smaller percentages of regular fatliquors added to them. The polymer acts upon the oils and activates their dormant properties. Leathers made in such manner are ‘upgraded’ compared to those processed with conventional fatliquors alone. They also have an even distribution and high uptake, even under difficult conditions.

Fixation
Upon acidifying the polymer side-chain searches for a full and low energy configuration fixation. Conventional fatliquors are only partly fixed, as a large portion is merely deposited between and around the fibres, uneven distribution is the result.
Softening polymers, on the other hand, do not deposit in such a manner. To fix, they need to actually find a binding partner. If one partner is already occupied, they search for another until all have found one point to react with, like pearls neatly tied on a string. As a result softening, polymers enable the tanner to achieve better overall softness from neck to butt (see Figure 1).
By controlling this process it is necessary to approach the fixation pH with care. Adding the acid in smaller quantities in 3 to 4 steps supports this. At a pH of around 4.0-4.2 milky floats
visibly begin to exhaust until the bath becomes clear.

Looseness
Loose grain is caused by tensions within the hide, mainly between the grain and main corium layers. One layer does not like to bend the same way as the other, thus creating creases when bent. Uneven fatliquoring can cause this. It is this capacity for high uniformity of fatliquoring, which makes these polymer softeners outstanding for tight shoe upper leathers. Although never used alone, they help a combination of oils to create softness and relaxation instead of tension within the structure.
A specific composition in the polymer treatment prevents both the deeper layers and the area just below the grain from loosening up. Milled leathers thus have a fine and flat grain, elastic enough to withstand long milling times.

Polymer properties
The properties of polymer softeners can be summed up as follows:
• A very smooth and fine feel over the entire leather surface
• A very pleasant light weight and relaxed handle
• Permanent softness after a prolonged period of time
• Total bath exhaustion and permanent fixation. No subsequent migration
• Improved uptake from chrome-free leathers due to their chemical composition
• Ideal for hard-grained hides/skins (eg buffalo) in combination with sulphited oils
• Drying in a hot chamber at higher temperatures than usual
• Better resistance to loose grain after toggle drying
• Odour-free, lightfast and heat-resistant

Measurement of softness
Softness was assessed with the BLC-softness tester. The softness in different corresponding points on a straight line from neck to butt has been measured. Higher values mean softer leathers while lower values correspond to a firmer handle. The graph demonstrates the more regular progression of the leather’s softness when a polymer softener is used.

Grain tightness
Grain tightness was assessed by using the test method Satra STD 174. The grain appearance in different corresponding areas has been compared to the scale. Higher values mean a looser grain while lower values correspond to better grain tightness. The graph demonstrates that the leather made with a polymer softener, even though softer, shows a better grain tightness.