Tetra-hydroxymethyl phosphonium chloride(THPC)has been considered as an important chrome-free tanning agent.To understand the THPC tanning mechanism

the structure

charge distribution

activity and tanning ability of each phosphorous compound in THPC tanning system were studied

by

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P NMR

FT-IR spectroscopy

differential scanning calorimetry(DSC)and computational chemistry method

etc.When pH raised to 6.0

the decomposition of THPC would take place

which results in a production of free formaldehyde

tri-hydroxymethyl phosphonium(TrHP)and tri-hydroxymethyl phosphine oxide(TrHPO).At pH 9.0

THPC will be converted completely to TrHP and most TrHP is further oxidized into TrHPO.It is possible that

in reaction of phosphorous compounds and collagens

both P—C and C—O bonds would break simultaneously or individually.From molecular charge distribution and bond polar properties

it is deduced that

if P C bonds break

the activity is in order of TrHPOTHPCTrHP

whereas if C—O bonds break

the order is TrHPTHPCTrHPO.It is more possible that P—C bonds will break in reaction with collagen

and TrHPO may be more active in the THPC tanning system.The results of tanning and DSC also prove the above conclusion.Furthermore

the fact that the shrinkage temperature of THPC tanned leather was below 70℃ when basified to pH 5.0 or lower suggests that the hydroxymethyl groups of THPC and TrHP are less possible to combine directly with amino groups of collagen.