Direct bonding is a special technique for bonding rubber and framework materials. It can not only improve the bonding between rubber and fabric, but also can be used for bonding between rubber and metal.

The direct bonding method is referred to as HRH bonding system, H(accelerator H, Hexamethylenetetramine) R(resorcinol) and H (the initials of the silica Hisil produced by Harwick, USA).

HRH binding system of resorcinol, formaldehyde and silica. Rubber and metal can be bonded directly when vulcanized by adding into rubber compound. It is suitable for tire, rubber hose, tape and other rubber-metal products. The HRH binding system can effectively improve the initial bonding strength between rubber and framework materials such as fabric and steel cord.

Currently, accelerator H and resorcinol are no longer used for the direct bonding of natural rubber and metals. They produce a lot of smoke in the processing process and seriously pollute the environment and affect human health. Various methylene donors and methylene acceptor have been developed to replace the bonding system, which can obtain satisfactory bonding effect under reasonable allocation.

Rubber adhesive HMMM

(1) Methylene acceptor

  1. RS is a mixture of resorcinol and stearic acid 2:1, with a light brown flake appearance and a softening point of 60℃. It is usually used in combination with HMMM or RH as a methylene donor. In this formulation, the amount of stearic acid should be considered to deduct the amount in RS. To ensure uniform dispersion, RS should be added in the mixing process.
  2. RE is a low molecular weight condensation of resorcinol and acetaldehyde (mole ratio: 1:0.5). Its appearance is translucent amber solid with a softening point of about 70℃. It is easy to disperse in the adhesive and has a softening effect on the rubber compound. It is also usually used with binder HMMM or RH.

(2) Methylene donor

  1. Adhesive HMMM is hexahydroxy methyl melamine, by the effect of melamine and formaldehyde by etherification, the product is white paste, at 50℃ or so can become a flowing body, easy to disperse in the rubber, slightly softening effect, no obvious effect on the vulcanization rate. Usually used in combination with the methylene acceptor RE or RS, it should be added in the late mixing stage under the condition that the rubber temperature is not higher than 90℃.
  2. RH is a complex with 1∶1 molar ratio of resorcinol and hexamethylenetetramine, the appearance is pink powder, nitrogen content is about 22%, in 110℃ above thermal decomposition into resorcinol, hexamethylenetetramine, ammonia and amino-phenolic resin, etc. It has the effect of promoting the vulcanization of rubber, so it is necessary to adjust the varieties and dosage of accelerator appropriately. Usually used in combination with RS or RE. RH should be added at the late mixing stage under the condition that the glue temperature is not higher than 90℃, otherwise it will lose the bonding effect due to premature decomposition.
Rubber Adhesive

Material selection

  1. Raw rubber is suitable for natural rubber and most synthetic rubber. NR/BR compound (such as tire cord compound) has the best effect.
  2. The adhesion strength increases with the increase of sulfur dosage, vulcanization temperature and vulcanization time.
  3. In order to make the adhesive flow well enough to facilitate penetration into the holes of the fabric, priority should be given to aftereffect accelerators, with the best effect of accelerator DZ and accelerator CZ.

Key points of process:

  1. Mixing is usually completed by adding the methylene acceptor, zinc oxide mixing and reaction at a higher temperature (such as 140 ° C), then cooling to 100 ° C and adding the methylene donor.
  2. Vulcanization, increase of vulcanization temperature and lengthen of vulcanization time are conducive to direct bonding.