Diphenyl Guanidine

Why Does DPG Increase Cure-State Modulus in Silica-Filled Tread Compounds?

In silica-filled passenger-car radial tread compounds formulated with solution SBR and BR at glass transition temperatures between −30 °C and −45 °C, 1,3-diphenylguanidine (CAS 102-06-7) functions primarily as an alkaline surface-blocking agent rather than as a standalone vulcanisation accelerator. The compound is compounded with zinc oxide at 3.0–5.0 phr, stearic acid at 1.0–2.0 phr, antidegradants 6PPD and TMQ, a primary sulfenamide accelerator such as CBS or TBBS at 1.1–1.7 phr, and diphenyl guanidine at 0.15–0.5 phr. Sulfur is charged at 1.2–2.0 phr according to the target crosslink density. Diphenyl guanidine is introduced because the silanol groups on precipitated silica with BET nitrogen surface area from 150 m²/g to 190 m²/g adsorb sulfenamide accelerators and reduce their effective concentration; diphenyl guanidine competes for silanol binding sites and prevents adsorption of CBS or TBBS onto the silica surface. It also neutralises acidic residual hydrogen donors that would otherwise shorten scorch delay and lower cure rate. At 0.4–0.5 phr, diphenyl guanidine produces a higher torque difference in moving-die rheometer testing conducted under ASTM D5289-19a at 160 °C, 0.5° arc, and 1.67 Hz, and an elevated 300% modulus in tensile testing under ISO 37:2017. The magnitude depends on silica loading and silane coupling agent efficiency; no universal percentage improvement applies. Above 0.8 phr, excess diphenyl guanidine no longer remains adsorbed and migrates to the surface as a waxy bloom on green tread strip and cured sidewall until final finishing removes it. High addition also increases heat build-up and can narrow the reversion resistance window at cure temperatures above 170 °C. For 60–70 phr silica, 0.15–0.3 phr diphenyl guanidine is typical; for 80–90 phr silica in low rolling-resistance treads, 0.4–0.5 phr is used, with the upper limit confirmed by a 100 °C, 24 h bloom test on a 2 mm press-cured slab. Compliance of the finished tread compound is demonstrated by ISO 37:2017 tensile properties, ISO 289-1:2015 Mooney scorch at 121 °C, ISO 815-1:2014 compression set, ISO 4649:2017 abrasion resistance, and EU tyre labelling Regulation (EC) No 1222/2009 for rolling resistance, wet grip, and exterior noise. REACH Regulation (EC) No 1907/2006, Annex II applies to the extended safety data sheet for diphenyl guanidine, and workplace exposure during weighing and mixing is controlled under Directive 98/24/EC with local exhaust ventilation at the bag-slitting station and internal mixer feed chute. Mixing is performed in a two-pass cycle in an intermeshing or tangential internal mixer with net chamber volume of 250–650 L and fill factor of 0.72–0.78. In the first non-productive pass, elastomer, silica, silane coupling agent, zinc oxide, stearic acid, and antidegradants are charged, and dump temperature reaches 150–160 °C to complete silanisation. The batch is sheeted on a two-roll mill at 70–90 °C and cooled. In the productive pass, masterbatch is returned at 40–50 °C, and the cure system containing diphenyl guanidine, CBS or TBBS, and sulfur is added with dump temperature maintained below 100–105 °C. Tread extrusion uses a pin-barrel cold-feed extruder with screw diameter of 200–250 mm and L/D 16:1, head temperature 90–110 °C, and screw speed 20–45 min⁻¹. The specified t5 at 121 °C should remain above 18 min to avoid scorch in the head or die. Extruded tread profiles are cut to length and cured in platen presses at 150–160 °C for 10–15 min or in continuous tyre presses. End product types include passenger-car radial tyre tread strips, tyre sidewall profiles, and retread cushion gum compounds requiring silica-compatible cure activation.

Conveyor Cover Stocks and the Scorch-Control Boundary During Continuous Vulcanisation

For fabric-reinforced and steel-cord conveyor belts used in mining, cement plants, and bulk material transport, the cover stock is based on NR or NR/SBR/BR blends and requires a cure system that develops sufficient tensile strength and tear resistance without scorch during calendering and continuous vulcanisation. Diphenyl guanidine is used as a secondary accelerator with CBS or MBT primary accelerators at addition ratios of 0.2–0.5 phr. Primary accelerator loading is typically 0.8–1.2 phr CBS for NR-rich covers, with sulfur at 2.0–2.5 phr for abrasive cut-and-chip service. Diphenyl guanidine is selected because it shortens induction time and raises final crosslink density, but the same basic accelerator effect reduces scorch safety in a continuous rotocure or double-band press running at 160–170 °C. A lower limit of 20 min for Mooney scorch t5 at 121 °C, measured according to ISO 289-1:2015, is typically imposed before the calender stage. When diphenyl guanidine is increased from 0.2 phr to 0.5 phr, t5 decreases in a formulation-dependent manner; production control charts establish the exact relationship for each cover compound because carbon black loading, limestone filler level, and moisture content alter the response. At diphenyl guanidine levels above 0.55 phr, industrial mixing records show surface roughness on calender rolls and pre-cure particles in the cover sheet when limestone-filled NR/SBR covers are processed at 95–105 °C calender roll temperatures. For food-grade conveyor belts, diphenyl guanidine is not preferred where low extractables are required under EC 1935/2004 or FDA 21 CFR 177.2600, because positive bloom potential and fatty-simulant extraction must be validated. Compliance of industrial cover compounds is demonstrated by ISO 14890:2013 for textile conveyor belts, ISO 15236-1:2016 for steel-cord conveyor belts, ISO 283:2015 for full-thickness tensile strength and elongation, ISO 4649:2017 for abrasion resistance, ISO 815-1:2014 for compression set, and ISO 4589-2:2017 oxygen index for flame-retardant covers. European Machinery Directive 2006/42/EC applies to the complete conveyor installation rather than to the rubber cover compound alone. The cover compound is mixed in a tangential internal mixer with 250–400 L net chamber volume, 0.75–0.80 fill factor, dump temperature 130–140 °C for the non-productive stage, and final cure system addition at 90–100 °C. The mixed batch is sheeted out on an open mill, cooled with anti-tack solution, and fed to a three-roll or four-roll calendering line. The cover sheet is laminated onto polyester/nylon fabric plies or steel cord layers at roll pressures of 30–80 kN/m, then conveyed into a continuous hot-air tunnel, rotocure drum, or double-band press. Curing at 160 °C for 20–35 min is common for 8–20 mm thick covers. The control point for diphenyl guanidine is the calender exit stock temperature and scorch coupon; if exit temperature exceeds 105 °C, the masterbatch is cooled and the diphenyl guanidine addition is reduced by 0.05 phr in the next batch. End product types include abrasion-resistant mining conveyor belt covers, grain elevator belts, cement plant conveyor belts, and power transmission belt covers.

When NBR-Based Hydraulic Hose Liners Are Extruded Through Pin-Barrel Cold-Feed Lines

In hydraulic hose inner tubes and covers based on NBR with acrylonitrile content of 34–41%, diphenyl guanidine is used as a secondary accelerator at very low addition ratios of 0.1–0.3 phr in the inner tube and 0.2–0.4 phr in the extruded CR/SBR cover. The cure system for NBR hose liners frequently contains 0.8–1.2 phr MBTS or 0.6–1.0 phr CBS, 0.1–0.3 phr diphenyl guanidine, 1.5–2.0 phr sulfur, and 5.0 phr zinc oxide. The low addition is required because NBR compounds processed through pin-barrel cold-feed extruders have a short thermal history between the barrel and the crosshead die; a basic secondary accelerator reduces scorch time rapidly when the extruder head is held at 85–95 °C for continuous production runs. At 0.4 phr diphenyl guanidine and above, scorch pre-cure particles can form in the screen pack and create surface defects on the tube. Diphenyl guanidine is preferably added as a pre-dispersed masterbatch in NBR or as an 80% active dispersion in EPDM binder to improve weighing accuracy at low addition ratios and reduce dust exposure. Direct powder addition at this scale can create batch-to-batch variation if the material remains in the hopper because diphenyl guanidine tends to compact under humid plant conditions. Compliance of hydraulic hose liners is qualified by ISO 18752:2014 for pressure classes, ISO 1817:2015 for volume change in IRM 903 oil after 72 h at 100 °C, ISO 188:2011 for hot-air ageing at 100 °C for 72 h, ISO 289-1:2015 for Mooney scorch, and ISO 37:2017 for tensile elongation. SAE J517:2021 may be referenced for hose assemblies in mobile equipment. REACH Annex II and EU CLP Regulation (EC) No 1272/2008 apply to the diphenyl guanidine raw material as supplied. The NBR inner tube is compounded in an internal mixer or continuous kneader, then extruded through a pin-barrel cold-feed extruder with L/D 16:1 and screw diameter 60–120 mm. Barrel temperatures are maintained from 40 °C in the feed throat to 70–80 °C in the barrel and 85–95 °C in the head; die temperature is held at 95–100 °C. After extrusion, the tube is cooled, braided with high-tensile steel wire or synthetic fibre, covered with a CR/SBR cover compound, and cured in a continuous steam tube at 150–170 °C under 15–25 bar external pressure. The low diphenyl guanidine level is validated by measuring t5 at 121 °C in the head; t5 should remain above 14–18 min for the inner tube compound to prevent pre-cure in the screen pack during a 4 h run. End product types include braided hydraulic hose for construction machinery, mining hydraulic hose, oil-resistant industrial hose, and medium-pressure fluid transfer hose.

For compressed or injection-moulded vulcanised rubber outsoles used in safety footwear and casual shoes, diphenyl guanidine is applied as a secondary accelerator to MBTS or CBS at 0.3–0.8 phr in NR/SBR/BR blends containing high-styrene resin. A typical outsole formulation contains NR 40–60 phr, SBR 1502 30–50 phr, BR 15–25 phr, sulfur 1.8–2.2 phr, MBTS 0.8–1.2 phr, diphenyl guanidine 0.3–0.8 phr, zinc oxide 3.0–5.0 phr, stearic acid 1.0–2.0 phr, and precipitated silica or whiting filler at 30–60 phr. Diphenyl guanidine contributes to higher hardness and improved abrasion resistance measured under ISO 4649:2017, but at the upper 0.8 phr limit the unreacted material can bloom to the outsole surface during warehouse storage in tropical humidity. This bloom produces a greyish haze that interferes with subsequent cementing or polyurethane direct-attach bonding. Shoe outsole manufacturers using direct vulcanisation or cold-cement construction therefore maintain diphenyl guanidine below 0.5 phr when the outsole is bonded to a leather or textile upper with chloroprene or polyurethane adhesive, because surface bloom reduces peel adhesion unless a halogenated surface treatment is applied. Compliance of safety footwear outsoles is evaluated under ISO 20345:2021 for slip resistance, abrasion, and fuel-oil resistance, ISO 20871:2018 for abrasion resistance of vulcanised rubber, ISO 17707:2005 for flex cracking resistance, and REACH Annex XVII entry 50 for polycyclic aromatic hydrocarbons in the rubber outsole. Diphenyl guanidine itself is supplied under REACH Regulation (EC) No 1907/2006, with exposure scenarios in the extended safety data sheet. The outsole compound is prepared in an internal mixer with a 75–110 L tangential chamber, dumped at 140–150 °C in the non-productive stage, and the cure system containing diphenyl guanidine is added on a two-roll mill at 80–90 °C. The sheeted stock is cooled to 30–40 °C, passed through a stock blender, and cut into blanks for compression moulding at 150–160 °C and 12–18 min, or fed to a screw-injection moulding machine with injection pressure 80–120 bar and mould temperature 155–165 °C for injection-moulded soles. For direct injection onto the upper, the outsole compound must have a Mooney scorch t5 at 121 °C above 12 min and a minimum mould flow spiral of 55–70 cm at 150 °C. End product types include vulcanised rubber outsoles for safety boots, fashion wedge soles, dual-density casual outsoles, and rubber sheet sandals.

Medium-voltage EPDM insulation and chloroprene sheathing compounds for submersible pump cable and trailing cable use diphenyl guanidine only in sulfur-donor or mixed cure systems where the low unsaturation of EPDM demands an additional nitrogen-base activator to raise cure rate without increasing sulfur dosage. Addition ratios are 0.1–0.4 phr diphenyl guanidine in EPDM insulation, with a primary accelerator such as TMTD at 0.5–0.8 phr, MBT at 0.4–0.6 phr, and zinc oxide at 5.0 phr. Sulfur is kept at 1.2–1.8 phr for low-voltage insulation and 1.5–2.5 phr for chloroprene sheathing. Diphenyl guanidine at 0.2–0.4 phr raises modulus and hot-set resistance of EPDM insulation by promoting higher crosslink density, but when the continuous vulcanisation line uses nitrogen gas at 220–240 °C, excess diphenyl guanidine can volatilise and leave deposits on the curing tube walls. The upper addition limit is therefore set at 0.4 phr for nitrogen-cured EPDM insulation and 0.5 phr for steam-cured CR sheathing. Cable compounds are evaluated under IEC 60502-1:2021 for power cables up to 1 kV, IEC 60811-501:2012 for hot-set and elongation testing, IEC 60811-401:2012 for thermal ageing in air, ISO 37:2017 for tensile properties, and IEC 60332-1-2:2015 for flame propagation where required. Low-smoke zero-halogen variants replace diphenyl guanidine-containing elastomers only when the entire formulation is halogen-free; diphenyl guanidine itself is halogen-free, but the total sheath formulation may contain chloroprene rubber. The EPDM insulation compound is mixed in an intermeshing internal mixer with 120 L net chamber volume, compounded with fillers and plasticiser, and the cure system is added at 80–90 °C. The compound is extruded through a three-zone extruder with head temperature 75–85 °C onto the conductor, then passed through a continuous vulcanisation tube at 200–240 °C for nitrogen-cured EPDM or at 160–190 °C for steam-cured CR sheathing. Line speed is controlled so that the insulation reaches a tensile strength of at least 5 MPa and hot-set elongation below 20% under 0.2 MPa load at 200 °C for 15 min in accordance with IEC 60811-501:2012. End product types include submersible pump cable insulation, EPDM trailing cable sheaths, welding cable jackets, and medium-voltage flexible cable insulation.

Anti-Vibration Mount Compounds and the Adhesion Threshold in Injection Moulding

NR/BR compounds for automotive engine mounts, suspension bushings, and exhaust hangers use diphenyl guanidine as a secondary accelerator at 0.2–0.5 phr with CBS or MBS primary accelerators and 2.0–2.5 phr sulfur. The addition accelerates cure onset in thick sections and produces a flatter cure plateau, which is desirable for moulding cycle times of 6–10 min at 150–160 °C. When diphenyl guanidine is raised above 0.5 phr, rubber-to-metal adhesion in bonded mounts can deteriorate because unreacted diphenyl guanidine migrates toward the adhesive interface during the pre-cure injection stage and interferes with bond formation. Adhesion peel data determined by ASTM D429-16 Method B are used to establish the upper addition limit for each production control plan. The processing window is therefore bounded by the required adhesion strength and the mould fill time. Automotive anti-vibration components are validated under IATF 16949 control plans, while the rubber compound itself is tested by ISO 7743:2017 for compression stress-strain properties, ISO 1817:2015 for fluid resistance after immersion in ASTM oil No. 1 and No. 3, ISO 188:2011 for heat ageing at 100 °C for 72 h, and ISO 48-4:2018 for hardness. Diphenyl guanidine raw material is controlled under REACH and the supplier safety data sheet. The compound is mixed in a tangential mixer of 160–270 L with dump temperature 140–150 °C for the masterbatch and 90–100 °C for the final mix. Injection moulding is performed on vertical or horizontal injection moulding machines with clamp force of 200–500 t, screw compression ratio of 1.25:1–1.35:1, injection pressure 90–120 bar, and mould temperature 150–165 °C. The diphenyl guanidine-containing stock is conditioned at 25–30 °C and 50–60% relative humidity before moulding; if the stock is allowed to equilibrate above 60% relative humidity, moisture absorption can produce porosity at the adhesive interface when the mould is opened. The moulded mounts are post-cured for 2–4 h at 70–80 °C when high adhesion and low compression set are required. End product types include automotive engine mounts, suspension strut mounts, exhaust hangers, and industrial vibration isolators.