Properties of Diphyl


General chemical and physical properties


Diphyl is a clear, pale yellow liquid with a low viscosity. It is an eutectic mixture consisting of approx. 73.5% by weight diphenyl oxide and approx. 26.5 % by
weight diphenyl. There is no significant rise in viscosity before the fluid solidifies at approx. 12.3C.


In contrast to water there is a reduction in volume when Diphyl changes from the fluid to the solid state which means that there is no risk of plant or pipelines being
damaged if Diphyl solidifies.


Diphyl has an application range from +13C  to +400C and a high boiling point of approx.257 C at a pressure of 1.013 bar, thus permitting the pressureless operation of standard heat transfer systems at temperatures up to 250 C. At a maximum application temperature of 400C, the pressure amounts to only 10.7 bar. This means that simple plant designs can be used, even in high temperature regions. The plant should be designed so that the temperature at the heating surface
is limited to 410 C.


The water content of Diphyl is in equilibrium with the atmospheric humidity and amounts to a maximum of 0.02% by weight on delivery. Diphyl is not hygroscopic
and is virtually immiscible with water. Even at 30C and 100% relative humidity,
a maximum of only 0.07 % by weight water dissolves in Diphyl. As with all
combustible fluids, there is also the possibility of Diphyl forming an explosive
mixture with air. However the explosive range between 1.0 and 3.5 % by volume
for the Diphyl vapour is very small.


Diphyl will not ignite spontaneously before reaching an autoignition temperature
above 615C. Diphyl thus belongs to Temperature Class T1.

The typical, inherent odour of Diphyl enables leaks to be identified quickly.
The level of production‑related impurities, such as chlorine and sulphur compounds,
is so low in Diphyl that corrosive damage caused by the heat transfer fluid coming
into contact with the materials of the plant can be ruled out.


Detailed references in:

VDE 0170/0171.



Diphyl - specification and characteristic data



Specification                                    Test method              Unit          Diphyl

Diphenyl oxide content (GC)                               Internal 2011-0270101 91   %                   72 - 75

Biphenyl Content (GC)                                        Internal 2011-0270101 91   %                   25 - 28

Chlorine content                                                  Internal 2011-0380801 93   mg/kg             max. 10

Solidification point                                               Internal 65073/161/1           C                  min. 12.2

                                                                            ISO 1392
                                                                            ASTM D 1493/67

Water                                                                  Internal 2011-0332301 92   mg/kg             max. 200
                                                                            IEC 814
                                                                            ASTM D 1744-83

Compliance with the Specification is subject to constant monitoring.

Characteristic data

Density (20C)                                                    DIN 51757                            kg/m3                    approx. 1062
                                                                            ASTM D 4052-91

Kinematic viscosity (20C)                                  DIN 51561                            mm2/s            approx.. 4.12
                                                                            ISO 3104/3105
                                                                            ASTM D 445/446

Neutralization number (acidic)                            DIN 51558                            mg KOH/g      approx. 0.01
                                                                            ASTM D 974-64

Boiling temperature (1.013 bar)                          ISO 918                               C                  approx. 257
                                                                            OECD-Test Spec. 103

Flash point                                                          EN 22719                             C                  approx. 115
                                                                            ASTM D 93-75

Autoignition temperature                                     DIN 51794                            C                  approx. 615
                                                                            ASTM D 286-58T

Lower explosion limit (108,5 C)                         -                                           % by vol.       approx. 1.01

Upper explosion limit (138,5C)                           -                                           % by vol.       approx. 3.47

Solubility in water (20C)
Diphenyl oxide                                                    Quentin method                   mg/l                11.3
Biphenyl                                                                                                          mg/l                3.9

Surface tension (20C)                                       OECD-ring method               N/m                approx. 0.04

Thermal conductivity (20C)                              -                                           W/m  . K         approx. 0.138

Mean specific heat (20C)                                  -                                           kJ/kg  .K         approx. 1.55

Max. film temperature                                         -                                           C                  approx. 410

Gross calorific value,  H0                                   -                                           MJ/kg             approx. 37.6

Net calorific value,  HU                                       -                                           MJ/kg             ca. 36.3

Temperature class                                              VDE 0171                            -                     T1

Hazard class                                                      -                                           -                     None

Coefficient of thermal expansion (20 - 300C)   -                                           K-1                 10,6 . 10 -4

Characteristic data provide further information of the product.
and are not subject to constant monitoring.